DOI: 10.1002/chem.201405921
Full Paper
& Heterogeneous Catalysis
Controlled Synthesis of Fe3O4/ZIF-8 Nanoparticles for Magnetically Separable Nanocatalysts Fei Pang, Mingyuan He, and Jianping Ge*[a]
external field during magnetic separation and disperse back into the solution after withdrawal of the magnetic field. For the Knoevenagel reaction, which is catalyzed by alkaline active sites on external surface of catalyst, small Fe3O4/ZIF-8 nanoparticles show a higher catalytic activity. At the same time, the nanocatalysts can be continuously used in multiple catalytic reactions through magnetic separation, activation, and redispersion with little loss of activity.
Abstract: Fe3O4/ZIF-8 nanoparticles were synthesized through a room-temperature reaction between 2-methylimidazolate and zinc nitrate in the presence of Fe3O4 nanocrystals. The particle size, surface charge, and magnetic loading can be conveniently controlled by the dosage of Zn(NO3)2 and Fe3O4 nanocrystals. The as-prepared particles show both good thermal stability (stable to 550 8C) and large surface area (1174 m2g 1). The nanoparticles also have a superparamagnetic response, so that they can strongly respond to an
Introduction
systematic studies of their catalytic performance are seldom reported in the literature. The nanocatalysts can be regarded as a quasi-homogeneous catalyst, which combines the advantages of traditional homoand heterogeneous catalysts.[13] Similar to the homogenous catalyst, the nanocatalyst is usually well dispersed in the reaction system and fully accessible to the reactants, so that it shows high reactivity in catalysis. It can also be separated from the reaction for recycling usage similar to a heterogeneous catalyst. Currently, a remaining problem for the generation of promising nanocatalysts is that good dispersion and fast separation are hard to achieve at the same time. It is difficult to separate the nanoparticles from the reaction solution for redispersion in the next round of the reaction by using traditional methods such as sedimentation and filtration. A possible solution to this problem is the introduction of magnetic materials to produce magnetically separable nanoparticles (MNPs).[14] Typically, the magnetic content should be firmly anchored to the surface of the composite particle or embedded inside that particle. At the same time, the composite particle should have superparamagnetic characteristics, so that the particles can not only strongly respond to the external field during magnetic separation, but also disperse back into the solution after withdrawal of the magnetic field.[15] Reported magnetic nanocomposites include bimetallic MNPs,[16] core– shell MNPs,[17] MNPs decorated on silica/polymer beads,[18] MNPs in encapsulated mesoporous solids,[19] multinuclei core– shell MNPs,[20] and MNP–silica nanocomposites.[14a] For instance, bimetallic core–shell CoPt nanoparticles were synthesized by reacting Co nanoparticles with a solution of [Pt(hfac)2] (hfac = hexafluroacetylacetonate) in nonane and the formation of CoPt alloy was explained by a redox transmetalation reaction between Co0 and Pt2 + .[16] Lu and co-workers introduced ferromagnetic cobalt nanoparticles into ordered mesoporous car-
In recent years, metal–organic frameworks (MOFs) have attracted great attention in various research fields due to their potential applications in gas storage and separation,[1] optical devices,[2] sensors,[3] drug delivery,[4] and heterogeneous catalysis.[5] Among all the MOF structures, zeolite imidazolate frameworks (ZIFs) possess merits of conventional MOFs and zeolites because of their large specific surface area, high thermal stability, and intersecting three-dimensional structure of the aluminosilicate zeolite.[6] Meanwhile, the ZIF compound is a bifunctional substance with both acidic and basic active site on the surface, which are induced by cations (Zn2 + , Cu2 + , Co2 + , etc.) and imidazole groups, respectively.[7] Therefore, the ZIF compounds could be promising catalysts for many heterogeneous reactions. Several successful demonstrations of using ZIFs as catalysts include Friedel–Crafts alkylation,[8] Knoevenagel condensation,[9] cycloaddition reactions, the synthesis of styrene carbonate from CO2 and styrene oxide,[7a, 10] and so forth.[11] Some of them may take advantage of the abundant surface area within the nanopores, whereas for reactions such as Knoevenagel condensation the active sites are generally located at the external surface of the ZIF particle,[8, 12] so that it is necessary to reduce the catalyst particle size as much as possible to effectively increase the number of active sites on the surface. However, the syntheses of nanoscale, crystalline ZIF structures and [a] F. Pang, Prof. M. Y. He, Prof. J. P. Ge Shanghai Key Laboratory of Green Chemistry and Chemical Processes School of Chemistry and Molecular Engineering East China Normal University Shanghai, 200062 (P.R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405921. Chem. Eur. J. 2015, 21, 1 – 10
These are not the final page numbers! ÞÞ
1
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper bons to obtain a surface-grafted magnetic catalyst for octane hydrogenation.[21] Tsang et al. utilized sequential spraying, chemical precipitation, and controlled pyrolysis to prepare carbon-encapsulated, iron-based magnetic nanoparticles (Fe M; M = Ni, Ca, Mn, Zn, Cu, Co) in large quantities and these particles were used as carriers for catalytically active species such as platinum nanoparticles.[22] Kotani et al. synthesized the magnetically separable heterogeneous catalyst Ru(OH)x/Fe3O4 for liquid-phase oxidation and reduction; most of the catalyst could be recycled for the next round of the reaction.[23] Overall, the involved magnetic content can be metals (Fe,[24] Co,[21] Ni[25]), alloys (FeNi, FeCu, FeCo,[22] FePt[24]), metal oxides (FeO, Fe2O3,[26] Fe3O4[27]), or ferrites (NiFe2O4,[28] CoFe2O4,[29] MnFe2O4,[29b] ZnFe2O4[30]). Among them, magnetic iron oxides have received most attention because of their chemical stability in air and ease of synthesis.[13b] Herein, the Fe3O4/ZIF-8 nanoparticles were synthesized through the room-temperature reaction between 2-methylimidazolate (MeIM) and zinc nitrate in the presence of Fe3O4 nanocrystals. Due to electrostatic attraction between negatively charged Fe3O4 nanocrystals and positively charged ZIF-8 species, Fe3O4/ZIF-8 composites were produced through the encapsulation of magnetic nanocrystals inside the ZIF-8 nanoparticles. The particle size, surface charge, and magnetic loading of Fe3O4/ZIF-8 nanoparticles were tuned by variations in the synthetic parameters, and their thermal stability, specific surface area, and magnetic properties were investigated. The Knoevenagel reaction between benzaldehyde and cyanoacetamide in methanol was used as a model reaction to evaluate the catalytic performance and magnetic separation efficiency of the as-prepared Fe3O4/ZIF-8 catalysts. It is believed that the magnetic ZIF nanostructures will find potential applications in practical catalytic processes and be convenient for recycling of the nanocatalysts.
Figure 1. Schematic illustration for the formation mechanism of Fe3O4/ZIF-8 particles. PAA = polyacrylic acid.
a negative surface charge (Figure 2 b and Figure S1 in the Supporting Information) are prepared in advance through a hightemperature polyol reaction by using PAA as a surfactant.[15] Because the Fe3O4 nanocrystals are grafted with highly charged carboxylate groups, they are extremely stable against aggregation in an aqueous solution with a pH value higher than five, and thereby, are also stable in an aqueous solution of MeIM. When Zn(NO3)2 was injected into the solution of MeIM and Fe3O4 nanocrystals, a composite nanostructure with magnetic nanocrystals anchored on the surface of ZIF-8 or embedded within the ZIF-8 nanocrystal could be produced (Figure 2 c and d).
Results and Discussion Synthesis and formation mechanism of Fe3O4/ZIF-8 nanoparticles The Fe3O4/ZIF-8 composite nanoparticles were synthesized through a room-temperature, aqueous reaction between MeIM and zinc nitrate in the presence of premade negatively charged Fe3O4 nanocrystals (Figure 1). The synthesis was developed from a previously reported preparation of pure ZIF-8 nanoparticles, which utilized the injection of small amount of Zn2 + into a concentrated solution of MeIM to initiate fast nucleation and limited growth of ZIF nanocrystals.[31] In ZIF-8 crystals, the Zn atoms are connected through the N atoms from MeIM to form a MOF structure, which possesses nanosized pores formed by four-, six-, eight-, and twelve-membered rings of ZnN4 tetrahedral clusters.[6b] As shown in Figure 2 a, pure ZIF-8 nanoparticles with an average diameter from 50 to 100 nm can be prepared by using this method, and the XRD patterns of these particles are consistent with reports in the literature.[31] To introduce magnetic content into the MOF structures, Fe3O4 nanocrystals with &
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
Figure 2. TEM images of: a) ZIF-8, b) Fe3O4, and c, d) Fe3O4/ZIF-8 nanoparticles.
The XRD patterns of Fe3O4/ZIF-8 nanoparticles were almost same as those of pure ZIF-8 nanoparticles, except a peak was observed at around 368, which indicated the inclusion of Fe3O4 content inside the hetero-nanostructures (Figure 3). Generally, the XRD intensity of a MOF material is stronger than that of nanocrystals by several orders of magnitude; thus the diffraction peaks attributed to inorganic nanocrystals can barely be observed for hetero-nanostructures. It should be mentioned that, although excess MeIM was used in the synthesis, after separation of Fe3O4/ZIF-8 nanoparticles the reaction solution can be used for the next round of the reaction by supplementing with consumed MeIM, which makes the synthesis a green and economic process. 2
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
Figure 3. XRD patterns of ZIF-8, Fe3O4/ZIF-8, and Fe3O4 nanoparticles.
In synthesis, the particle size of Fe3O4/ZIF-8 composite nanostructures can be finely tuned through continuous addition of zinc ions into the reaction solution. It was not a surprise to find that the average particle size increased from 58.3, 82.4, to 106 nm when 0.7, 1.4, and 2.1 mmol, respectively, of zinc nitrate was injected into the solution (Figure 4); this can be regarded as a typical seeding growth process. A common characteristic for these Fe3O4/ZIF-8 particles is that all of the Fe3O4 nanocrystals have been combined with the ZIF-8 particles and almost no isolated magnetic particles were observed. However, the location of the magnetic content was different for the three samples. For the smallest particle with a diameter of 58.3 nm, the Fe3O4 nanocrystals were attached to the surface of ZIF-8 particles. Some small pits on the particle surface may be caused by the detachment of Fe3O4 nanocrystals during synthesis or post-treatment, which suggests that the combination may not be firm enough in this stage. The exposed magnetic nanocrystals can still be observed when the overall particle size is increased to 82.4 nm. As the particle size further increases to 106 nm, almost all of the magnetic nanocrystals have been embedded inside the surface layer of the particle, producing stable and firmly encapsulated hetero-nanostructures. Apparently, there is an affinity between the ZIF-8 particles and Fe3O4 nanocrystals; however, no Fe3O4@ZIF-8 core– shell structures were produced, which suggested that ZIF-8 particles originated from homogeneous nucleation at the beginning of the reaction and the heterostructures were formed when the ZIF-8 particles grew to a critical value. In addition to tuning of the amount of Zn(NO3)2 in a single synthesis, the composite particle size can be further increased by multiple seeding growth. The molar amount of Zn(NO3)2 added in each round equals the molar amount of zinc atoms in Fe3O4@ZIF-8 seeds. After separation by centrifugation, half of the as-made Fe3O4/ZIF-8 particles could be redispersed in a solution of MeIM for the next round of seeding growth (Figure 5). The Fe3O4/ZIF-8 particles will gradually grow to submicron or 1– 2 mm sizes after 3 or 4 cycles; this proves that the current synthesis is an effective method to produce magnetic ZIF-8 with controllable sizes from the nano- to submicron range. Upon changing the distribution of magnetic nanocrystals within the Fe3O4/ZIF-8 particles, the surface charge of the entire particle changed accordingly. It is known that the Fe3O4 Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
Figure 4. TEM images and size distributions of Fe3O4/ZIF-8 nanoparticles with average diameters of 58.3 (a–c), 82.4 (d–f), and 106 nm (g–i), when 0.7, 1.4, and 2.1 mmol Zn(NO3)2, respectively (samples 1, 2, and 3), were used in the reaction. Scale bars: 200 (a, d, g) and 50 nm (b, e, h).
Figure 5. SEM images of Fe3O4/ZIF-8 particles with average diameters of 0.18, 0.38, 0.92, and 1.49 mm prepared by: a) 1, b) 2, c) 3, and d) 4 cycles, respectively, of seeding growth with continuous addition of Zn(NO3)2 (samples 4, 5, 6, and 7).
nanocrystals are negatively charged because of surface carboxylate groups, and the pure ZIF-8 nanoparticles are positively charged due to imidazole acting as self-stabilizing ligands. Taking the samples in Figure 5 as examples, one can find that the zeta potential changes from negative to positive as the particles grow larger (Figure 6 a). For nanoparticles with average sizes of around 100 nm, the surface charge was largely contributed to by the Fe3O4 nanocrystals in the surface layer, and the whole heterostructure was negatively charged. As more and more ZIF-8 grows into the Fe3O4/ZIF-8 particles in subsequent cycles, Fe3O4 nanocrystals are deeply embedded inside the composite, so that the whole particle behaves like pure ZIF-8 with a positive surface charge. We found a similar 3
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper reversal of surface charge for samples with different magnetic loadings (Figure 6 b and Figure S2 in the Supporting Information). Here, the production of ZIF-8 was fixed by adding the same amount of Zn(NO3)2 (1.4 mmol) and the addition of Fe3O4 nanocrystals was changed from 0.017 to 0.667 mmol (Table 1).
According to the magnetic distribution in hetero-nanostructures and the surface charge of the particles, the growth of Fe3O4/ZIF-8 composite particles can be attributed to a nanocrystal encapsulation process driven by electrostatic attraction between colloidal particles. At the beginning of the reaction, small ZIF-8 crystal seeds quickly formed through homogeneous nucleation after injection of an aqueous solution of Zn(NO3)2, even in the presence of Fe3O4 nanocrystals. As the ZIF-8 nanocrystal grew larger, its positive surface charge was gradually enhanced, so that the negatively charged Fe3O4 nanocrystals would be attracted to the surface of ZIF-8 due to electrostatic interactions. The attachment of Fe3O4 nanocrystals to the ZIF-8 particle and growth of the ZIF-8 particle may proceed simultaneously for some time, but the Fe3O4 nanocrystals will eventually be encapsulated inside the ZIF-8 matrix when excess Zn(NO3)2 is added and all magnetic nanocrystals are consumed. The aforementioned reaction mechanism explained some experimental phenomenon observed in our synthesis. For example, no composite particles were obtained if the negatively charged Fe3O4 nanocrystals were replaced by positively charged ones (polyethylenimine (PEI)-grafted Fe3O4 nanocrystals) because they repelled each other in solution. This proved that electrostatic attraction rather than coordination between imidazolate and Fe3O4 was the driving force to form composite structures. Second, no Fe3O4@ZIF-8 core–shell structures were produced during growth because attraction between the Fe3O4 nanocrystals and small ZIF-8 seeds was weak and homogeneous growth of ZIF-8 was dominant at the beginning of the reaction. Third, agglomeration of Fe3O4/ZIF-8 nanoparticles was observed (Figure 5) when their sizes increased to several hundred nanometers. Aggregation takes place because the surface charge can be zero as it changes from negative to positive in the third cycle of addition of Zn2 + , and the particle cannot be stabilized in solution according to colloidal stability theory.[32] However, the growth of ZIF-8 continues regardless of whether ZIF-8 particles are well dispersed or not, so that the aggregates are encapsulated inside to form submicron particles eventually (Figures 5 and 6 a).
Table 1. Synthetic parameters for the preparation of Fe3O4/ZIF-8 nanoparticles. Sample
n[Zn(NO3)2] [mmol]
n[Fe3O4] [mmol]
n[Zn(NO3)2]/n[Fe3O4]
1 2 3 4 5 6 7 8 9 10 11
0.7 1.4 2.1 2.8 5.6 11.2 22.4 1.4 1.4 1.4 1.4
0.167 0.167 0.167 0.167 0.167 0.167 0.167 0.017 0.067 0.167 0.667
4.19 8.38 12.6 16.7 33.5 67.1 134.1 82.4 20.9 8.38 2.01
Thermostability, porosity, and magnetization of Fe3O4/ZIF-8 particles ZIF-8 was perceived as a promising catalyst because it not only has abundant ordered micropores similar to zeolite, but also possesses better thermal stability than most of the other MOF structures. The microporous structure of ZIF-8 can be maintained below 500 8C in N2.[6b] Thermogravimetric analysis (TGA) was performed on as-made Fe3O4/ZIF-8 (Figure 7). The TGA curve shows a gradual weight loss of 3.4 % when the temperature rises from 25 to 500 8C, which corresponds to the loss of guest species such as water and other gases adsorbed inside the micropores. A larger weight loss is detected when the temperature is raised above 500 8C, which could be caused by collapse of the MOF structures. Through a comparison of the TGA curves of Fe3O4/ZIF-8 and ZIF-8, one can find that the thermal stability is determined by the ZIF content, and the introduction
Figure 6. Zeta potentials of various Fe3O4/ZIF-8 particles prepared by: a) continuous seeding growth (samples 4, 5, 6, and 7), and b) with different magnetic loadings (samples 8, 9, 10, and 11).
The particle surface charge became negative upon increasing the Fe3O4/ZIF-8 ratio because the magnetic nanocrystals would inevitably attach to the surface layer of the composite particles in this case (Figure S1 in the Supporting Information). These two groups of parallel experiments showed that the charge of the particle was determined by its surface constitution. In other words, one can estimate whether the magnetic particles are embedded in the surface layer or deeply inside the particle through zeta potential measurements. &
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
4
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper ior for micropores. The Langmuir and BET surface areas of ZIF8 are calculated to be 1932 and 1363 m2 g 1, respectively, according to the adsorption branch with relative pressure (P/P0) ranging from 0.05 to 0.28. As expected, the Langmuir and BET surface areas for Fe3O4/ZIF-8 (1703 and 1174 m2g 1, respectively) are smaller than those of pure ZIF-8 because the introduction of Fe3O4 nanocrystals makes no contribution to the microporous structures and increasing surface area. On the other hand, the retention of a large surface area suggests that the addition of magnetic content does not interfere with the formation of ordered micropores. A hysteresis loop appearing in the sorption isotherm of Fe3O4/ZIF-8 with P/P0 from 0.4 to 0.75 shows the formation of large pores, which could be induced by the uncompacted combination between Fe3O4 nanocrystals and ZIF-8 during particle growth. Briefly, the introduction of magnetic nanocrystals in the growth of ZIF-8 will generate a highly porous, magnetically separable structure, which is useful in recycling heterogeneous catalysis. In addition to the porous characteristics inherited from ZIF8, the Fe3O4/ZIF-8 composite also possesses a superparamagnetic response due to the encapsulation of Fe3O4 nanocrystals, which is useful for fast separation of the catalyst in liquidphase reactions. Figure 9 a shows the magnetic hysteresis loops of Fe3O4/ZIF-8 particles at room temperature. The absence of remanence and coercivity indicates that the Fe3O4/ ZIF-8 particles have a typical superparamagnetic response. This means that the particles can be readily separated from a liquid suspension by using a magnetic field and they can be redispersed into the suspension without any agglomeration after
Figure 7. TGA results for Fe3O4/ZIF-8 and ZIF-8 particles.
of inorganic content has little contribution to the enhancement of thermal stability. The long plateau in the temperature range of 25–500 8C proved the high thermal stability of the Fe3O4/ZIF-8 composite particles; this makes them an excellent heterogeneous catalyst for reactions at relatively high temperatures. Fe3O4/ZIF-8 nanoparticles have rich, ordered micropores and a large specific surface area. For ZIF-8 framework topologies, the imidazolate unit bridges the zinc tetrahedra to make a ZnIM-Zn angle of around 1458, which is close to the Si-O-Si angle in many zeolites. The expanded sod framework exhibits an interesting structure that a large sod cage (11.6 ) is accessible through a narrow six-ring pore (3.4 ).[6b] The porous structure of ZIF-8 and Fe3O4/ZIF-8 was analyzed by a nitrogen adsorption–desorption isotherm swept at 77 K (Figure 8), which revealed their microporous characteristics because both of the isotherms showed typical type I adsorption–desorption behav-
Figure 9. a) Magnetic hysteresis loops, and b) separation efficiency of Fe3O4/ ZIF-8 nanoparticles with different Fe3O4 loadings. c) Separation of Fe3O4/ZIF8 nanoparticles by using a magnet.
Figure 8. Nitrogen adsorption–desorption isotherms of the as-synthesized: a) ZIF-8 nanocrystals, and b) Fe3O4/ZIF-8 nanoparticles at 77 K. Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
5
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper removal of that magnetic field. The magnetic loading of Fe3O4/ ZIF-8 particles can be controlled by the addition of Fe3O4 nanocrystals in the synthesis. As the amount of Fe3O4 nanocrystals increased from 0.017, 0.067, 0.167, to 0.667 mol, its saturated magnetization was enhanced from 0.56, 1.98, 4.29, to 4.35 emu g 1, accordingly. It should be noted that the magnetic loading and response become saturated as the addition of Fe3O4 nanocrystals is increased from 0.167 to 0.667 mol, probably because the positive ZIF-8 particles can barely absorb more negative Fe3O4 nanocrystals during particle growth. To quantify the magnetic separation efficiency of Fe3O4/ZIF-8 particles, an in situ transmittance measurement system is established for real-time evaluations. Typically, 2 mL of Fe3O4/ZIF8 solution (c = 1.8 mg mL 1) was placed in a 1 1 cm optical cuvette, and its transmission spectrum was measured by means of a UV/Vis spectrometer. When the solution is diluted from 1.0c, 0.8c, 0.6c, 0.4c, 0.2c, to 0c, it can be regarded as the original solution with 0, 20, 40, 60, 80, and 100 %, respectively, of the particle separation. Therefore, a working curve can be plotted by recording the transmittance of these diluted solutions. In the formal test, a permanent magnet was placed beside the cuvette, and the cuvette center could sense a magnetic field of 600 gauss. In the magnetic field, the Fe3O4/ZIF-8 particles gradually moved to the cuvette wall along the magnetic field gradient, and the transmittance of the solution, as well as the magnetic separation efficiency, increased accordingly. Herein, we recorded the separation time as every 20 % of Fe3O4/ZIF-8 particles separated from the solution, according to the transmittance working curve. The experimental results are consistent with magnetization hysteresis loops, and higher magnetic loading leads to better separation efficiency. It takes 2.33, 3.75, 14.5, and 53.2 min for samples 7–10 to reach 80 % separation (Figure 9 b). Magnetic separation under specific conditions is seldom reported in the literature. The current results are repeatable in different laboratories and comparable with other magnetic particles. The short separation time indicates that magnetic separation of the catalyst is feasible in practical operations, even in a relatively weak magnetic field.
the Knoevenagel reaction between benzaldehyde and cyanoacetamide was selected to evaluate the activity of Fe3O4/ZIF-8 catalysts and demonstrate the importance of catalyst synthesis for high efficiency and reusable catalytic processes. Some reaction conditions, including solvent, catalyst pretreatment, dosage, and size, were first optimized for a higher conversion rate of substrate molecules. Typically, 2 mol % of Fe3O4/ZIF-8 catalysts was used to catalyze the condensation between benzaldehyde and cyanoacetamide in methanol at room temperature (Figure 10). Aliquots were withdrawn from the reaction mixture at different time intervals and the principal product, benzylidene cyanoacetamide, was analyzed by GC; this gave kinetic data throughout the whole reaction. The catalyst is highly active in protic solvent, such as methanol, probably because the solvent facilitates the formation of a nucleophilic carbanion and promotes condensation.[33]
Figure 10. Knoevenagel reaction between benzaldehyde and cyanoacetamide catalyzed by Fe3O4/ZIF-8 catalysts.
Various solvents, such as DMF, DMSO, acetonitrile, and methanol, were investigated as potential solvents for the Knoevenagel reaction (Figure 11 a), and methanol was found to be the most effective solvent for condensation, which suggested that the polarity had little influence upon the activity. The solvent experiment was also consistent with previously reported Knoevenagel reactions, in which ethanol and methanol were selected as most suitable solvents.[5c, 34] Furthermore, a baking pretreatment of Fe3O4/ZIF-8 catalysts was essential for high catalytic activity (Figure 11 b). Thanks to the high thermal stability, the catalysts could be baked at 300 8C for 2 h under the protection of nitrogen, so that magnetite would not be oxidized to hematite during calcination. It was observed that 72.3 % of benzaldehyde converted after 6 h with the baked catalyst, whereas only 63.2 % was converted with the unbaked catalyst. The catalytic activity improved because the surface active sites passivated by surfactant molecules were reactivated by annealing in nitrogen, and the crystallinity of ZIF-8 might also have been improved after calcination.[35] The optimal catalyst dosage was determined by comparing the conversion of benzaldehyde in the corresponding reactions. The catalyst concentration, in units of molar percentage of the substrate benzaldehyde, changed from 0 to 5 mol % for comparison (Figure 11 c). It was observed that condensation occurred even without the Fe3O4/ZIF-8 particles, but the conversion was only 7.7 % after 6 h and it increased slowly thereafter. When the catalyst dosage increased from 0.5 to 1.5 mol %, the conversion sharply increased from 25.1 to 65.2 % for the same reaction over 6 h. However, as the catalyst concentration was further raised to 2.0 or 5.0 mol %, the con-
Nanocatalysts for Knoevenagel reaction The Knoevenagel reaction is a nucleophilic aldol condensation between aromatic aldehydes, aromatic ketones, or benzal bromide and malonic ester, malonic acid, or cyanoacetic ester containing an activated methylene group, and it is usually catalyzed by a weak basic amine. Chizallet and co-workers investigated the position of acidic/basic sites in ZIF-8 based on CO adsorption monitored by FTIR spectroscopy and DFT calculations.[7b] Their study suggested that there were some Lewis acid sites from zinc(II) species, some Brønsted acid sites from NH groups, and some basic sites due to N moieties and OH groups on the surface of the catalyst. They also demonstrated that all active sites were located at the external surface, but not in the micropores of the material. Therefore, the Knoevenagel reaction is an ideal model reaction to evaluate nanoparticle-based catalysts, since their activities are closely related to particle size, but not porosity. For the above considerations, &
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
6
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
Figure 11. The influence of: a) solvent, b) baking treatment, c) catalyst concentration, and d) particle size on the conversion of benzaldehyde.
Figure 12. a) Kinetic curves for four continuous condensation reactions with Fe3O4/ZIF-8 particles as catalysts, and b) conversion of benzaldehyde in 3 h for each reaction.
version of substrate increased to 72.3 or 85.6 %, respectively. Therefore, the optimal catalyst dosage could be 1.5–2 mol % because further addition of catalyst would lower the conversion efficiency. Because the Knoevenagel reaction is mostly catalyzed on the external surface of ZIF-8, a smaller particle size can create more contacts between reactants and active sites, leading to a higher catalytic activity for Fe3O4/ZIF-8 particles. Here, catalyst particles with average diameters of 58.6, 72.2, 100.3, and 128.6 nm were used to show the influence of particle size upon activity (Figure 11 d). Upon decreasing the particle size from 128.6 to 58.6 nm, the conversion increased from 55.1 to 79.7 %. This proves that the active sites on the external surface of ZIF-8 were responsible for condensation, which was consistent with the transesterification reaction reported by Chizallet et al.[7b] Although the abundant active sites inside the micropores may not be utilized in this reaction, the nanosized MOF structures are still promising catalytic materials for other reactions. Compared with most homogeneous catalysts for organic reactions, the heterogeneous catalysts have intrinsic advantages in catalyst recycling and repeated usage. Because the nanocatalysts are usually well dispersed in solution as a quasi-homogeneous system, it is difficult to separate the catalyst from the reaction medium by convenient methods. Therefore, the introduction of Fe3O4 nanocrystals into ZIF-8 made the composite structure magnetic, so that separation could be realized by applying a magnetic field. To demonstrate the convenience of magnetic separation for continuous reactions, 10 mol % of Fe3O4/ZIF-8 particles were used as catalysts for condensation at room temperature. In each reaction, condensation proceeded for 3 h and the corresponding conversions are recorded in Figure 12. After separation by means of a magnetic field and Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
activation in vacuum at 250 8C for 2 h, the Fe3O4/ZIF-8 catalysts could be used again for the next round of the reaction. The conversion of benzaldehyde after 3 h reaction reached 85 % for the first round of catalysis, and gradually decreased to about 75 % for the fourth round of catalysis, which proved that the Fe3O4/ZIF-8 catalysts could be recovered and reused without significant loss in activity.
Conclusion Fe3O4/ZIF-8 nanoparticles were synthesized through a roomtemperature reaction between MeIM and zinc nitrate in the presence of Fe3O4 nanocrystals. Growth of the Fe3O4/ZIF-8 composite particles could be considered as a nanocrystal encapsulation process driven by electrostatic attraction between positively charged ZIF-8 particles and negatively charged Fe3O4 nanocrystals. Through the adjustment of the amount of Zn(NO3)2 and Fe3O4 nanocrystals, the particle size could be conveniently controlled, along with the surface charge and magnetic loading. The as-prepared Fe3O4/ZIF-8 nanoparticles could be used as a magnetically separable nanocatalyst in heterogeneous catalysis due to their high thermal stability, large surface area, and strong magnetic response. Herein, the Knoevenagel condensation reaction between benzaldehyde and cyanoacetamide was used to evaluate the catalytic performance of Fe3O4/ZIF-8. The reaction solvent, catalyst pretreatment, dosage, and size were optimized for the best conversion. Typically, the conversion reached 72.3 % in 6 h when condensation was catalyzed by 2 mol % of baked catalyst in methanol. Because condensation was catalyzed at the active sites on the external surface of catalyst, smaller Fe3O4/ZIF-8 particles usually presented a higher catalytic activity. At the same time, the 7
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper and products in the Knoevenagel reaction were analyzed by using an Agilent 7890A gas chromatograph equipped with an HP-5MS capillary column and using N2 as the carrier gas. The magnetic properties of the Fe3O4/ZIF-8 nanoparticles were measured by using a Lakeshore 7312 vibration sample magnetometer.
nanocatalysts could be continuously used in multiple catalytic reactions without loss of activity through magnetic separation, activation, and redispersion into the reaction solution.
Experimental Section Knoevenagel reaction and evaluation of catalytic activity
Chemicals
The Knoevenagel reaction between benzaldehyde and cyanoacetamide could be catalyzed by the ZIF-8 nanoparticles. Typically, cyanoacetamide (1.28 g, 15.2 mmol) and Fe3O4/ZIF-8 nanoparticles (36 mg) were first dispersed in methanol (20 mL) to form a homogeneous suspension. Benzaldehyde (0.8 mL, 7.6 mmol) was added to the above solution under stirring and the reaction was allowed to proceed for 6 h at room temperature. It should be noted that cyanoacetamide was used in excess to ensure complete conversion of benzaldehyde, and the molar ratio between catalyst (Fe3O4/ZIF8) and substrate (benzaldehyde) was 2:100. A small amount of anisole (0.4 mL) was added to the reaction solution as an internal standard for chromatographic analysis. During the 6 h reaction, samples (0.1 mL) were taken from the solution every 1 h, and then directly injected into the GC instrument to obtain the conversion of benzaldehyde. After the reaction, the Fe3O4/ZIF-8 nanocatalysts were purified by washing with methanol and magnetic separation (10 min) several times, and annealed under vacuum for 2 h at 250 8C to regenerate the catalytic activity.
FeCl3 (97 %), PAA (Mw = 1800), and diethylene glycol (DEG, 99 %) were purchased from Sigma-Aldrich. Zn(NO3)2·6 H2O (99 %), MeIM (98 %), cyanoacetamide (98 %), iron(III) acetylacetonate ([Fe(acac)3]; 98 %), and PEI (Mw = 1800, 99 %) were purchased form Aladdin Co. Ltd. Benzaldehyde (98 %) and anisole (99 %) were obtained from J&K Chemical Co. Ltd. All chemicals were used as received.
Synthesis of Fe3O4 nanocrystals Negatively charged PAA-grafted Fe3O4 nanoparticles were prepared according to a previously published procedure.[15] PAA (4 mmol) and FeCl3 (2 mmol) were first dissolved in DEG (15 mL) to form a transparent homogenous solution at 120 8C. The mixture was heated to 220 8C, and a solution of NaOH (2.5 mmol mL 1, 4 mL) in DEG was quickly injected to the above solution under vigorous stirring. After reacting at 220 8C for 2 h, the solution was cooled to room temperature and the nanocrystals were separated by centrifugation, rinsed with deionized water and ethanol three times, and finally dispersed in deionized water (10 mL) to form a brownish red transparent solution. The positively charged PEI-grafted Fe3O4 nanocrystals were prepared in a similar way, in which a solution of [Fe(acac)3] (2 mmol) and PEI (1 g) in DEG (15 mL) was directly heated at 220 8C for 2 h without the addition of NaOH.
Acknowledgements J.G. thanks the Major State Basic Research Development Program of China (2011CB932404), the National Science Foundation of China (21222107, 21471058), the Shanghai Rising-Star Program (13A1401400), and the Youth Talent Plan (Organization Department of the Central Committee of the CPC) for support.
Synthesis of Fe3O4/ZIF-8 nanoparticles In a typical process, an aqueous solution of MeIM (3.45 mol/L, 20 mL) was first mixed with the aforementioned aqueous solution of Fe3O4 nanocrystals (2.5 mL) by sonication, and stirred at a speed of 400 rpm at room temperature. Then the aqueous solution of Zn(NO3)2 (0.35 mol L 1, 4 mL) was loaded in a syringe and gradually added to the above solution over 20 min by means of a syringe pump to produce Fe3O4/ZIF-8 particles. After the addition of Zn(NO3)2, the products were separated by centrifugation, washed with deionized water twice, and dried at 300 8C under the protection of N2 flow for 2 h. After complete separation of Fe3O4/ZIF-8 particles and replacement of consumed MeIM, the reaction solution could be recycled for the next synthetic cycle.
Keywords: heterogeneous catalysis · magnetic properties · metal–organic frameworks · nanoparticles · synthesis design [1] a) J. R. Li, R. J. Kuppler, H. C. Zhou, Chem. Soc. Rev. 2009, 38, 1477 – 1504; b) R. Q. Zou, A. I. Abdel-Fattah, H. W. Xu, Y. S. Zhao, D. D. Hickmott, CrystEngComm 2010, 12, 1337 – 1353; c) N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe, O. M. Yaghi, Science 2003, 300, 1127 – 1129; d) M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe, O. M. Yaghi, Science 2002, 295, 469 – 472. [2] S. T. Meek, J. A. Greathouse, M. D. Allendorf, Adv. Mater. 2011, 23, 249 – 267. [3] a) O. Shekhah, J. Liu, R. A. Fischer, C. Woll, Chem. Soc. Rev. 2011, 40, 1081 – 1106; b) B. Liu, J. Mater. Chem. 2012, 22, 10094 – 10101; c) Y. X. Xu, Y. Q. Wen, W. Zhu, Y. N. Wu, C. X. Lin, G. T. Li, Mater. Lett. 2012, 87, 20 – 23. [4] a) R. C. Huxford, J. Della Rocca, W. B. Lin, Curr. Opin. Chem. Biol. 2010, 14, 262 – 268; b) C. Wang, D. M. Liu, W. B. Lin, J. Am. Chem. Soc. 2013, 135, 13222 – 13234. [5] a) H. L. Jiang, B. Liu, T. Akita, M. Haruta, H. Sakurai, Q. Xu, J. Am. Chem. Soc. 2009, 131, 11302 – 11303; b) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450 – 1459; c) J. Gascon, U. Aktay, M. D. Hernandez-Alonso, G. P. M. van Klink, F. Kapteijn, J. Catal. 2009, 261, 75 – 87; d) A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 2012, 41, 5262 – 5284; e) C. Hou, G. Zhao, Y. Ji, Z. Niu, D. Wang, Y. Li, Nano Res. 2014, 7, 1364 – 1369. [6] a) A. Phan, C. J. Doonan, F. J. Uribe-Romo, C. B. Knobler, M. O’Keeffe, O. M. Yaghi, Acc. Chem. Res. 2010, 43, 58 – 67; b) K. S. Park, Z. Ni, A. P. Cote, J. Y. Choi, R. D. Huang, F. J. Uribe-Romo, H. K. Chae, M. O’Keeffe,
Characterization The morphologies and sizes of the Fe3O4/ZIF-8 nanoparticles were investigated by means of a JEOL JEM-2100 transmission electron microscope operated at 200 kV. Powder XRD patterns of ZIF-8, Fe3O4, and Fe3O4/ZIF-8 nanoparticles were obtained by using a Rigaku Ultima IV X-ray diffractometer operated at 35 kV and 40 mA with Ni-filtered CuKa radiation. TGA was measured in the range of 20–980 8C by using a NETZSCH STA449-3F instrument under the protection of N2. Nitrogen adsorption–desorption isotherms, BET surface areas, and Barret–Joyner–Halenda (BJH) pore diameters were measured by using a JW-BK122 analyzer at 77 K. Dynamic light scattering (DLS) behavior and zeta potentials of nanoparticles were characterized by means of a Malvern Instruments ZS-90 analyzer equipped with a multipurpose autotitrator (MPT-2) at 25 8C. The absorption spectra were captured by using an Ocean Optics Maya 2000 pro UV/Vis spectrometer. The reactants
&
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
8
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
[7]
[8] [9]
[10] [11]
[12] [13]
[14]
[15] [16] [17]
[18] [19]
O. M. Yaghi, Proc. Natl. Acad. Sci. USA 2006, 103, 10186 – 10191; c) J. F. Yao, H. T. Wang, Chem. Soc. Rev. 2014, 43, 4470 – 4493; d) S. R. Venna, J. B. Jasinski, M. A. Carreon, J. Am. Chem. Soc. 2010, 132, 18030 – 18033; e) D. Fairen-Jimenez, S. A. Moggach, M. T. Wharmby, P. A. Wright, S. Parsons, T. Duren, J. Am. Chem. Soc. 2011, 133, 8900 – 8902. a) M. Q. Zhu, D. Srinivas, S. Bhogeswararao, P. Ratnasamy, M. A. Carreon, Catal. Commun. 2013, 32, 36 – 40; b) C. Chizallet, S. Lazare, D. BazerBachi, F. Bonnier, V. Lecocq, E. Soyer, A. A. Quoineaud, N. Bats, J. Am. Chem. Soc. 2010, 132, 12365 – 12377. L. T. L. Nguyen, K. K. A. Le, N. T. S. Phan, J. Catal. 2012, 33, 688 – 696. a) U. P. N. Tran, K. K. A. Le, N. T. S. Phan, ACS Catal. 2011, 1, 120 – 127; b) R. Z. Jin, Z. Bian, J. Z. Li, M. X. Ding, L. X. Gao, Dalton Trans. 2013, 42, 3936 – 3940; c) L. T. L. Nguyen, K. K. A. Le, H. X. Truong, N. T. S. Phan, Catal. Sci. Technol. 2012, 2, 521 – 528. C. M. Miralda, E. E. Macias, M. Q. Zhu, P. Ratnasamy, M. A. Carreon, ACS Catal. 2012, 2, 180 – 183. a) C. H. Kuo, Y. Tang, L. Y. Chou, B. T. Sneed, C. N. Brodsky, Z. P. Zhao, C. K. Tsung, J. Am. Chem. Soc. 2012, 134, 14345 – 14348; b) P. Z. Li, K. Aranishi, Q. Xu, Chem. Commun. 2012, 48, 3173 – 3175; c) F. Alonso, F. Foubelo, J. C. Gonzalez-Gomez, R. Martinez, D. J. Ramon, P. Riente, M. Yus, Mol. Diversity 2010, 14, 411 – 424. R. Selvin, H. L. Hsu, T. M. Her, Catal. Commun. 2008, 10, 169 – 172. a) V. Polshettiwar, R. S. Varma, Green Chem. 2010, 12, 743 – 754; b) Y. Li, Q. Y. Liu, W. J. Shen, Dalton Trans. 2011, 40, 5811 – 5826; c) C. P. Vinod, Catal. Today 2010, 154, 113 – 117; d) S. B. Kalidindi, B. R. Jagirdar, ChemSusChem 2012, 5, 65 – 75; e) R. Z. Chen, Y. Du, Q. Q. Wang, W. H. Xing, W. Q. Jin, N. P. Xu, Ind. Eng. Chem. Res. 2009, 48, 6600 – 6607. a) S. Shylesh, V. Schunemann, W. R. Thiel, Angew. Chem. Int. Ed. 2010, 49, 3428 – 3459; Angew. Chem. 2010, 122, 3504 – 3537; b) V. Polshettiwar, R. Luque, A. Fihri, H. B. Zhu, M. Bouhrara, J. M. Bassett, Chem. Rev. 2011, 111, 3036 – 3075. J. P. Ge, Y. X. Hu, M. Biasini, W. P. Beyermann, Y. D. Yin, Angew. Chem. Int. Ed. 2007, 46, 4342 – 4345; Angew. Chem. 2007, 119, 4420 – 4423. J. I. Park, J. Cheon, J. Am. Chem. Soc. 2001, 123, 5743 – 5746. a) D. S. Wang, J. B. He, N. Rosenzweig, Z. Rosenzweig, Nano Lett. 2004, 4, 409 – 413; b) Y. Deng, D. Qi, C. Deng, X. Zhang, D. Zhao, J. Am. Chem. Soc. 2008, 130, 28 – 29. P. G. Wu, J. H. Zhu, Z. H. Xu, Adv. Funct. Mater. 2004, 14, 345 – 351. W. R. Zhao, J. L. Gu, L. X. Zhang, H. R. Chen, J. L. Shi, J. Am. Chem. Soc. 2005, 127, 8916 – 8917.
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
[20] F. Caruso, M. Spasova, A. Susha, M. Giersig, R. A. Caruso, Chem. Mater. 2001, 13, 109 – 116. [21] A. H. Lu, W. Schmidt, N. Matoussevitch, H. Bonnemann, B. Spliethoff, B. Tesche, E. Bill, W. Kiefer, F. Schuth, Angew. Chem. Int. Ed. 2004, 43, 4303 – 4306; Angew. Chem. 2004, 116, 4403 – 4406. [22] S. C. Tsang, V. Caps, I. Paraskevas, D. Chadwick, D. Thompsett, Angew. Chem. Int. Ed. 2004, 43, 5645 – 5649; Angew. Chem. 2004, 116, 5763 – 5767. [23] M. Kotani, T. Koike, K. Yamaguchi, N. Mizuno, Green Chem. 2006, 8, 735 – 741. [24] K. Mori, N. Yoshioka, Y. Kondo, T. Takeuchi, H. Yamashita, Green Chem. 2009, 11, 1337 – 1342. [25] Y. W. Ma, B. Yue, L. S. Yu, X. Z. Wang, Z. Hu, Y. N. Fan, Y. Chen, W. W. Lin, Y. N. Lu, J. H. Hu, J. Phys. Chem. C 2008, 112, 472 – 475. [26] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chem. Commun. 2012, 48, 11275 – 11288. [27] a) J. P. Ge, Q. Zhang, T. R. Zhang, Y. D. Yin, Angew. Chem. Int. Ed. 2008, 47, 8924 – 8928; Angew. Chem. 2008, 120, 9056 – 9060; b) C. Li, R. Wei, Y. Xu, A. Sun, L. Wei, Nano Res. 2014, 7, 536 – 543. [28] D. Guin, B. Baruwati, S. V. Manorama, Org. Lett. 2007, 9, 1419 – 1421. [29] a) Y. H. Wang, J. K. Lee, J. Mol. Catal. A 2007, 263, 163 – 168; b) C. R. Vestal, Z. J. Zhang, Nano Lett. 2003, 3, 1739 – 1743. [30] S. H. Xu, D. L. Feng, W. F. Shangguan, J. Phys. Chem. C 2009, 113, 2463 – 2467. [31] Y. C. Pan, Y. Y. Liu, G. F. Zeng, L. Zhao, Z. P. Lai, Chem. Commun. 2011, 47, 2071 – 2073. [32] a) K. Ueno, M. Watanabe, Langmuir 2011, 27, 9105 – 9115; b) E. Tombcz, Soil Sci. 1999, 164, 814 – 824. [33] C. Reichardt, in Solvents and Solvent Effects in Organic Chemistry, WileyVCH, Weinheim, 2004, pp. 147 – 328. [34] J. Juan-AlcaÇiz, E. V. Ramos-Fernandez, U. Lafont, J. Gascon, F. Kapteijn, J. Catal. 2010, 269, 229 – 241. [35] Z. Q. Niu, Y. D. Li, Chem. Mater. 2014, 26, 72 – 83.
Received: November 2, 2014 Revised: February 2, 2015 Published online on && &&, 0000
9
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
FULL PAPER & Heterogeneous Catalysis
Increasing attraction: Fe3O4/zeolite imidazolate framework (ZIF-8) nanoparticles were synthesized through a roomtemperature reaction between 2-methylimidazolate and zinc nitrate in the presence of Fe3O4 nanocrystals (see figure). The particles have a superparamagnetic response and can be separated by means of a magnetic field and redispersed into the solution after withdrawal of the field.
F. Pang, M. Y. He, J. P. Ge* && – && Controlled Synthesis of Fe3O4/ZIF-8 Nanoparticles for Magnetically Separable Nanocatalysts
&
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
10
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
& Heterogeneous Catalysis
Controlled Synthesis of Fe3O4/ZIF-8 Nanoparticles for Magnetically Separable Nanocatalysts Fei Pang, Mingyuan He, and Jianping Ge*[a]
external field during magnetic separation and disperse back into the solution after withdrawal of the magnetic field. For the Knoevenagel reaction, which is catalyzed by alkaline active sites on external surface of catalyst, small Fe3O4/ZIF-8 nanoparticles show a higher catalytic activity. At the same time, the nanocatalysts can be continuously used in multiple catalytic reactions through magnetic separation, activation, and redispersion with little loss of activity.
Abstract: Fe3O4/ZIF-8 nanoparticles were synthesized through a room-temperature reaction between 2-methylimidazolate and zinc nitrate in the presence of Fe3O4 nanocrystals. The particle size, surface charge, and magnetic loading can be conveniently controlled by the dosage of Zn(NO3)2 and Fe3O4 nanocrystals. The as-prepared particles show both good thermal stability (stable to 550 8C) and large surface area (1174 m2g 1). The nanoparticles also have a superparamagnetic response, so that they can strongly respond to an
Introduction
systematic studies of their catalytic performance are seldom reported in the literature. The nanocatalysts can be regarded as a quasi-homogeneous catalyst, which combines the advantages of traditional homoand heterogeneous catalysts.[13] Similar to the homogenous catalyst, the nanocatalyst is usually well dispersed in the reaction system and fully accessible to the reactants, so that it shows high reactivity in catalysis. It can also be separated from the reaction for recycling usage similar to a heterogeneous catalyst. Currently, a remaining problem for the generation of promising nanocatalysts is that good dispersion and fast separation are hard to achieve at the same time. It is difficult to separate the nanoparticles from the reaction solution for redispersion in the next round of the reaction by using traditional methods such as sedimentation and filtration. A possible solution to this problem is the introduction of magnetic materials to produce magnetically separable nanoparticles (MNPs).[14] Typically, the magnetic content should be firmly anchored to the surface of the composite particle or embedded inside that particle. At the same time, the composite particle should have superparamagnetic characteristics, so that the particles can not only strongly respond to the external field during magnetic separation, but also disperse back into the solution after withdrawal of the magnetic field.[15] Reported magnetic nanocomposites include bimetallic MNPs,[16] core– shell MNPs,[17] MNPs decorated on silica/polymer beads,[18] MNPs in encapsulated mesoporous solids,[19] multinuclei core– shell MNPs,[20] and MNP–silica nanocomposites.[14a] For instance, bimetallic core–shell CoPt nanoparticles were synthesized by reacting Co nanoparticles with a solution of [Pt(hfac)2] (hfac = hexafluroacetylacetonate) in nonane and the formation of CoPt alloy was explained by a redox transmetalation reaction between Co0 and Pt2 + .[16] Lu and co-workers introduced ferromagnetic cobalt nanoparticles into ordered mesoporous car-
In recent years, metal–organic frameworks (MOFs) have attracted great attention in various research fields due to their potential applications in gas storage and separation,[1] optical devices,[2] sensors,[3] drug delivery,[4] and heterogeneous catalysis.[5] Among all the MOF structures, zeolite imidazolate frameworks (ZIFs) possess merits of conventional MOFs and zeolites because of their large specific surface area, high thermal stability, and intersecting three-dimensional structure of the aluminosilicate zeolite.[6] Meanwhile, the ZIF compound is a bifunctional substance with both acidic and basic active site on the surface, which are induced by cations (Zn2 + , Cu2 + , Co2 + , etc.) and imidazole groups, respectively.[7] Therefore, the ZIF compounds could be promising catalysts for many heterogeneous reactions. Several successful demonstrations of using ZIFs as catalysts include Friedel–Crafts alkylation,[8] Knoevenagel condensation,[9] cycloaddition reactions, the synthesis of styrene carbonate from CO2 and styrene oxide,[7a, 10] and so forth.[11] Some of them may take advantage of the abundant surface area within the nanopores, whereas for reactions such as Knoevenagel condensation the active sites are generally located at the external surface of the ZIF particle,[8, 12] so that it is necessary to reduce the catalyst particle size as much as possible to effectively increase the number of active sites on the surface. However, the syntheses of nanoscale, crystalline ZIF structures and [a] F. Pang, Prof. M. Y. He, Prof. J. P. Ge Shanghai Key Laboratory of Green Chemistry and Chemical Processes School of Chemistry and Molecular Engineering East China Normal University Shanghai, 200062 (P.R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405921. Chem. Eur. J. 2015, 21, 1 – 10
These are not the final page numbers! ÞÞ
1
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper bons to obtain a surface-grafted magnetic catalyst for octane hydrogenation.[21] Tsang et al. utilized sequential spraying, chemical precipitation, and controlled pyrolysis to prepare carbon-encapsulated, iron-based magnetic nanoparticles (Fe M; M = Ni, Ca, Mn, Zn, Cu, Co) in large quantities and these particles were used as carriers for catalytically active species such as platinum nanoparticles.[22] Kotani et al. synthesized the magnetically separable heterogeneous catalyst Ru(OH)x/Fe3O4 for liquid-phase oxidation and reduction; most of the catalyst could be recycled for the next round of the reaction.[23] Overall, the involved magnetic content can be metals (Fe,[24] Co,[21] Ni[25]), alloys (FeNi, FeCu, FeCo,[22] FePt[24]), metal oxides (FeO, Fe2O3,[26] Fe3O4[27]), or ferrites (NiFe2O4,[28] CoFe2O4,[29] MnFe2O4,[29b] ZnFe2O4[30]). Among them, magnetic iron oxides have received most attention because of their chemical stability in air and ease of synthesis.[13b] Herein, the Fe3O4/ZIF-8 nanoparticles were synthesized through the room-temperature reaction between 2-methylimidazolate (MeIM) and zinc nitrate in the presence of Fe3O4 nanocrystals. Due to electrostatic attraction between negatively charged Fe3O4 nanocrystals and positively charged ZIF-8 species, Fe3O4/ZIF-8 composites were produced through the encapsulation of magnetic nanocrystals inside the ZIF-8 nanoparticles. The particle size, surface charge, and magnetic loading of Fe3O4/ZIF-8 nanoparticles were tuned by variations in the synthetic parameters, and their thermal stability, specific surface area, and magnetic properties were investigated. The Knoevenagel reaction between benzaldehyde and cyanoacetamide in methanol was used as a model reaction to evaluate the catalytic performance and magnetic separation efficiency of the as-prepared Fe3O4/ZIF-8 catalysts. It is believed that the magnetic ZIF nanostructures will find potential applications in practical catalytic processes and be convenient for recycling of the nanocatalysts.
Figure 1. Schematic illustration for the formation mechanism of Fe3O4/ZIF-8 particles. PAA = polyacrylic acid.
a negative surface charge (Figure 2 b and Figure S1 in the Supporting Information) are prepared in advance through a hightemperature polyol reaction by using PAA as a surfactant.[15] Because the Fe3O4 nanocrystals are grafted with highly charged carboxylate groups, they are extremely stable against aggregation in an aqueous solution with a pH value higher than five, and thereby, are also stable in an aqueous solution of MeIM. When Zn(NO3)2 was injected into the solution of MeIM and Fe3O4 nanocrystals, a composite nanostructure with magnetic nanocrystals anchored on the surface of ZIF-8 or embedded within the ZIF-8 nanocrystal could be produced (Figure 2 c and d).
Results and Discussion Synthesis and formation mechanism of Fe3O4/ZIF-8 nanoparticles The Fe3O4/ZIF-8 composite nanoparticles were synthesized through a room-temperature, aqueous reaction between MeIM and zinc nitrate in the presence of premade negatively charged Fe3O4 nanocrystals (Figure 1). The synthesis was developed from a previously reported preparation of pure ZIF-8 nanoparticles, which utilized the injection of small amount of Zn2 + into a concentrated solution of MeIM to initiate fast nucleation and limited growth of ZIF nanocrystals.[31] In ZIF-8 crystals, the Zn atoms are connected through the N atoms from MeIM to form a MOF structure, which possesses nanosized pores formed by four-, six-, eight-, and twelve-membered rings of ZnN4 tetrahedral clusters.[6b] As shown in Figure 2 a, pure ZIF-8 nanoparticles with an average diameter from 50 to 100 nm can be prepared by using this method, and the XRD patterns of these particles are consistent with reports in the literature.[31] To introduce magnetic content into the MOF structures, Fe3O4 nanocrystals with &
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
Figure 2. TEM images of: a) ZIF-8, b) Fe3O4, and c, d) Fe3O4/ZIF-8 nanoparticles.
The XRD patterns of Fe3O4/ZIF-8 nanoparticles were almost same as those of pure ZIF-8 nanoparticles, except a peak was observed at around 368, which indicated the inclusion of Fe3O4 content inside the hetero-nanostructures (Figure 3). Generally, the XRD intensity of a MOF material is stronger than that of nanocrystals by several orders of magnitude; thus the diffraction peaks attributed to inorganic nanocrystals can barely be observed for hetero-nanostructures. It should be mentioned that, although excess MeIM was used in the synthesis, after separation of Fe3O4/ZIF-8 nanoparticles the reaction solution can be used for the next round of the reaction by supplementing with consumed MeIM, which makes the synthesis a green and economic process. 2
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
Figure 3. XRD patterns of ZIF-8, Fe3O4/ZIF-8, and Fe3O4 nanoparticles.
In synthesis, the particle size of Fe3O4/ZIF-8 composite nanostructures can be finely tuned through continuous addition of zinc ions into the reaction solution. It was not a surprise to find that the average particle size increased from 58.3, 82.4, to 106 nm when 0.7, 1.4, and 2.1 mmol, respectively, of zinc nitrate was injected into the solution (Figure 4); this can be regarded as a typical seeding growth process. A common characteristic for these Fe3O4/ZIF-8 particles is that all of the Fe3O4 nanocrystals have been combined with the ZIF-8 particles and almost no isolated magnetic particles were observed. However, the location of the magnetic content was different for the three samples. For the smallest particle with a diameter of 58.3 nm, the Fe3O4 nanocrystals were attached to the surface of ZIF-8 particles. Some small pits on the particle surface may be caused by the detachment of Fe3O4 nanocrystals during synthesis or post-treatment, which suggests that the combination may not be firm enough in this stage. The exposed magnetic nanocrystals can still be observed when the overall particle size is increased to 82.4 nm. As the particle size further increases to 106 nm, almost all of the magnetic nanocrystals have been embedded inside the surface layer of the particle, producing stable and firmly encapsulated hetero-nanostructures. Apparently, there is an affinity between the ZIF-8 particles and Fe3O4 nanocrystals; however, no Fe3O4@ZIF-8 core– shell structures were produced, which suggested that ZIF-8 particles originated from homogeneous nucleation at the beginning of the reaction and the heterostructures were formed when the ZIF-8 particles grew to a critical value. In addition to tuning of the amount of Zn(NO3)2 in a single synthesis, the composite particle size can be further increased by multiple seeding growth. The molar amount of Zn(NO3)2 added in each round equals the molar amount of zinc atoms in Fe3O4@ZIF-8 seeds. After separation by centrifugation, half of the as-made Fe3O4/ZIF-8 particles could be redispersed in a solution of MeIM for the next round of seeding growth (Figure 5). The Fe3O4/ZIF-8 particles will gradually grow to submicron or 1– 2 mm sizes after 3 or 4 cycles; this proves that the current synthesis is an effective method to produce magnetic ZIF-8 with controllable sizes from the nano- to submicron range. Upon changing the distribution of magnetic nanocrystals within the Fe3O4/ZIF-8 particles, the surface charge of the entire particle changed accordingly. It is known that the Fe3O4 Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
Figure 4. TEM images and size distributions of Fe3O4/ZIF-8 nanoparticles with average diameters of 58.3 (a–c), 82.4 (d–f), and 106 nm (g–i), when 0.7, 1.4, and 2.1 mmol Zn(NO3)2, respectively (samples 1, 2, and 3), were used in the reaction. Scale bars: 200 (a, d, g) and 50 nm (b, e, h).
Figure 5. SEM images of Fe3O4/ZIF-8 particles with average diameters of 0.18, 0.38, 0.92, and 1.49 mm prepared by: a) 1, b) 2, c) 3, and d) 4 cycles, respectively, of seeding growth with continuous addition of Zn(NO3)2 (samples 4, 5, 6, and 7).
nanocrystals are negatively charged because of surface carboxylate groups, and the pure ZIF-8 nanoparticles are positively charged due to imidazole acting as self-stabilizing ligands. Taking the samples in Figure 5 as examples, one can find that the zeta potential changes from negative to positive as the particles grow larger (Figure 6 a). For nanoparticles with average sizes of around 100 nm, the surface charge was largely contributed to by the Fe3O4 nanocrystals in the surface layer, and the whole heterostructure was negatively charged. As more and more ZIF-8 grows into the Fe3O4/ZIF-8 particles in subsequent cycles, Fe3O4 nanocrystals are deeply embedded inside the composite, so that the whole particle behaves like pure ZIF-8 with a positive surface charge. We found a similar 3
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper reversal of surface charge for samples with different magnetic loadings (Figure 6 b and Figure S2 in the Supporting Information). Here, the production of ZIF-8 was fixed by adding the same amount of Zn(NO3)2 (1.4 mmol) and the addition of Fe3O4 nanocrystals was changed from 0.017 to 0.667 mmol (Table 1).
According to the magnetic distribution in hetero-nanostructures and the surface charge of the particles, the growth of Fe3O4/ZIF-8 composite particles can be attributed to a nanocrystal encapsulation process driven by electrostatic attraction between colloidal particles. At the beginning of the reaction, small ZIF-8 crystal seeds quickly formed through homogeneous nucleation after injection of an aqueous solution of Zn(NO3)2, even in the presence of Fe3O4 nanocrystals. As the ZIF-8 nanocrystal grew larger, its positive surface charge was gradually enhanced, so that the negatively charged Fe3O4 nanocrystals would be attracted to the surface of ZIF-8 due to electrostatic interactions. The attachment of Fe3O4 nanocrystals to the ZIF-8 particle and growth of the ZIF-8 particle may proceed simultaneously for some time, but the Fe3O4 nanocrystals will eventually be encapsulated inside the ZIF-8 matrix when excess Zn(NO3)2 is added and all magnetic nanocrystals are consumed. The aforementioned reaction mechanism explained some experimental phenomenon observed in our synthesis. For example, no composite particles were obtained if the negatively charged Fe3O4 nanocrystals were replaced by positively charged ones (polyethylenimine (PEI)-grafted Fe3O4 nanocrystals) because they repelled each other in solution. This proved that electrostatic attraction rather than coordination between imidazolate and Fe3O4 was the driving force to form composite structures. Second, no Fe3O4@ZIF-8 core–shell structures were produced during growth because attraction between the Fe3O4 nanocrystals and small ZIF-8 seeds was weak and homogeneous growth of ZIF-8 was dominant at the beginning of the reaction. Third, agglomeration of Fe3O4/ZIF-8 nanoparticles was observed (Figure 5) when their sizes increased to several hundred nanometers. Aggregation takes place because the surface charge can be zero as it changes from negative to positive in the third cycle of addition of Zn2 + , and the particle cannot be stabilized in solution according to colloidal stability theory.[32] However, the growth of ZIF-8 continues regardless of whether ZIF-8 particles are well dispersed or not, so that the aggregates are encapsulated inside to form submicron particles eventually (Figures 5 and 6 a).
Table 1. Synthetic parameters for the preparation of Fe3O4/ZIF-8 nanoparticles. Sample
n[Zn(NO3)2] [mmol]
n[Fe3O4] [mmol]
n[Zn(NO3)2]/n[Fe3O4]
1 2 3 4 5 6 7 8 9 10 11
0.7 1.4 2.1 2.8 5.6 11.2 22.4 1.4 1.4 1.4 1.4
0.167 0.167 0.167 0.167 0.167 0.167 0.167 0.017 0.067 0.167 0.667
4.19 8.38 12.6 16.7 33.5 67.1 134.1 82.4 20.9 8.38 2.01
Thermostability, porosity, and magnetization of Fe3O4/ZIF-8 particles ZIF-8 was perceived as a promising catalyst because it not only has abundant ordered micropores similar to zeolite, but also possesses better thermal stability than most of the other MOF structures. The microporous structure of ZIF-8 can be maintained below 500 8C in N2.[6b] Thermogravimetric analysis (TGA) was performed on as-made Fe3O4/ZIF-8 (Figure 7). The TGA curve shows a gradual weight loss of 3.4 % when the temperature rises from 25 to 500 8C, which corresponds to the loss of guest species such as water and other gases adsorbed inside the micropores. A larger weight loss is detected when the temperature is raised above 500 8C, which could be caused by collapse of the MOF structures. Through a comparison of the TGA curves of Fe3O4/ZIF-8 and ZIF-8, one can find that the thermal stability is determined by the ZIF content, and the introduction
Figure 6. Zeta potentials of various Fe3O4/ZIF-8 particles prepared by: a) continuous seeding growth (samples 4, 5, 6, and 7), and b) with different magnetic loadings (samples 8, 9, 10, and 11).
The particle surface charge became negative upon increasing the Fe3O4/ZIF-8 ratio because the magnetic nanocrystals would inevitably attach to the surface layer of the composite particles in this case (Figure S1 in the Supporting Information). These two groups of parallel experiments showed that the charge of the particle was determined by its surface constitution. In other words, one can estimate whether the magnetic particles are embedded in the surface layer or deeply inside the particle through zeta potential measurements. &
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
4
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper ior for micropores. The Langmuir and BET surface areas of ZIF8 are calculated to be 1932 and 1363 m2 g 1, respectively, according to the adsorption branch with relative pressure (P/P0) ranging from 0.05 to 0.28. As expected, the Langmuir and BET surface areas for Fe3O4/ZIF-8 (1703 and 1174 m2g 1, respectively) are smaller than those of pure ZIF-8 because the introduction of Fe3O4 nanocrystals makes no contribution to the microporous structures and increasing surface area. On the other hand, the retention of a large surface area suggests that the addition of magnetic content does not interfere with the formation of ordered micropores. A hysteresis loop appearing in the sorption isotherm of Fe3O4/ZIF-8 with P/P0 from 0.4 to 0.75 shows the formation of large pores, which could be induced by the uncompacted combination between Fe3O4 nanocrystals and ZIF-8 during particle growth. Briefly, the introduction of magnetic nanocrystals in the growth of ZIF-8 will generate a highly porous, magnetically separable structure, which is useful in recycling heterogeneous catalysis. In addition to the porous characteristics inherited from ZIF8, the Fe3O4/ZIF-8 composite also possesses a superparamagnetic response due to the encapsulation of Fe3O4 nanocrystals, which is useful for fast separation of the catalyst in liquidphase reactions. Figure 9 a shows the magnetic hysteresis loops of Fe3O4/ZIF-8 particles at room temperature. The absence of remanence and coercivity indicates that the Fe3O4/ ZIF-8 particles have a typical superparamagnetic response. This means that the particles can be readily separated from a liquid suspension by using a magnetic field and they can be redispersed into the suspension without any agglomeration after
Figure 7. TGA results for Fe3O4/ZIF-8 and ZIF-8 particles.
of inorganic content has little contribution to the enhancement of thermal stability. The long plateau in the temperature range of 25–500 8C proved the high thermal stability of the Fe3O4/ZIF-8 composite particles; this makes them an excellent heterogeneous catalyst for reactions at relatively high temperatures. Fe3O4/ZIF-8 nanoparticles have rich, ordered micropores and a large specific surface area. For ZIF-8 framework topologies, the imidazolate unit bridges the zinc tetrahedra to make a ZnIM-Zn angle of around 1458, which is close to the Si-O-Si angle in many zeolites. The expanded sod framework exhibits an interesting structure that a large sod cage (11.6 ) is accessible through a narrow six-ring pore (3.4 ).[6b] The porous structure of ZIF-8 and Fe3O4/ZIF-8 was analyzed by a nitrogen adsorption–desorption isotherm swept at 77 K (Figure 8), which revealed their microporous characteristics because both of the isotherms showed typical type I adsorption–desorption behav-
Figure 9. a) Magnetic hysteresis loops, and b) separation efficiency of Fe3O4/ ZIF-8 nanoparticles with different Fe3O4 loadings. c) Separation of Fe3O4/ZIF8 nanoparticles by using a magnet.
Figure 8. Nitrogen adsorption–desorption isotherms of the as-synthesized: a) ZIF-8 nanocrystals, and b) Fe3O4/ZIF-8 nanoparticles at 77 K. Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
5
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper removal of that magnetic field. The magnetic loading of Fe3O4/ ZIF-8 particles can be controlled by the addition of Fe3O4 nanocrystals in the synthesis. As the amount of Fe3O4 nanocrystals increased from 0.017, 0.067, 0.167, to 0.667 mol, its saturated magnetization was enhanced from 0.56, 1.98, 4.29, to 4.35 emu g 1, accordingly. It should be noted that the magnetic loading and response become saturated as the addition of Fe3O4 nanocrystals is increased from 0.167 to 0.667 mol, probably because the positive ZIF-8 particles can barely absorb more negative Fe3O4 nanocrystals during particle growth. To quantify the magnetic separation efficiency of Fe3O4/ZIF-8 particles, an in situ transmittance measurement system is established for real-time evaluations. Typically, 2 mL of Fe3O4/ZIF8 solution (c = 1.8 mg mL 1) was placed in a 1 1 cm optical cuvette, and its transmission spectrum was measured by means of a UV/Vis spectrometer. When the solution is diluted from 1.0c, 0.8c, 0.6c, 0.4c, 0.2c, to 0c, it can be regarded as the original solution with 0, 20, 40, 60, 80, and 100 %, respectively, of the particle separation. Therefore, a working curve can be plotted by recording the transmittance of these diluted solutions. In the formal test, a permanent magnet was placed beside the cuvette, and the cuvette center could sense a magnetic field of 600 gauss. In the magnetic field, the Fe3O4/ZIF-8 particles gradually moved to the cuvette wall along the magnetic field gradient, and the transmittance of the solution, as well as the magnetic separation efficiency, increased accordingly. Herein, we recorded the separation time as every 20 % of Fe3O4/ZIF-8 particles separated from the solution, according to the transmittance working curve. The experimental results are consistent with magnetization hysteresis loops, and higher magnetic loading leads to better separation efficiency. It takes 2.33, 3.75, 14.5, and 53.2 min for samples 7–10 to reach 80 % separation (Figure 9 b). Magnetic separation under specific conditions is seldom reported in the literature. The current results are repeatable in different laboratories and comparable with other magnetic particles. The short separation time indicates that magnetic separation of the catalyst is feasible in practical operations, even in a relatively weak magnetic field.
the Knoevenagel reaction between benzaldehyde and cyanoacetamide was selected to evaluate the activity of Fe3O4/ZIF-8 catalysts and demonstrate the importance of catalyst synthesis for high efficiency and reusable catalytic processes. Some reaction conditions, including solvent, catalyst pretreatment, dosage, and size, were first optimized for a higher conversion rate of substrate molecules. Typically, 2 mol % of Fe3O4/ZIF-8 catalysts was used to catalyze the condensation between benzaldehyde and cyanoacetamide in methanol at room temperature (Figure 10). Aliquots were withdrawn from the reaction mixture at different time intervals and the principal product, benzylidene cyanoacetamide, was analyzed by GC; this gave kinetic data throughout the whole reaction. The catalyst is highly active in protic solvent, such as methanol, probably because the solvent facilitates the formation of a nucleophilic carbanion and promotes condensation.[33]
Figure 10. Knoevenagel reaction between benzaldehyde and cyanoacetamide catalyzed by Fe3O4/ZIF-8 catalysts.
Various solvents, such as DMF, DMSO, acetonitrile, and methanol, were investigated as potential solvents for the Knoevenagel reaction (Figure 11 a), and methanol was found to be the most effective solvent for condensation, which suggested that the polarity had little influence upon the activity. The solvent experiment was also consistent with previously reported Knoevenagel reactions, in which ethanol and methanol were selected as most suitable solvents.[5c, 34] Furthermore, a baking pretreatment of Fe3O4/ZIF-8 catalysts was essential for high catalytic activity (Figure 11 b). Thanks to the high thermal stability, the catalysts could be baked at 300 8C for 2 h under the protection of nitrogen, so that magnetite would not be oxidized to hematite during calcination. It was observed that 72.3 % of benzaldehyde converted after 6 h with the baked catalyst, whereas only 63.2 % was converted with the unbaked catalyst. The catalytic activity improved because the surface active sites passivated by surfactant molecules were reactivated by annealing in nitrogen, and the crystallinity of ZIF-8 might also have been improved after calcination.[35] The optimal catalyst dosage was determined by comparing the conversion of benzaldehyde in the corresponding reactions. The catalyst concentration, in units of molar percentage of the substrate benzaldehyde, changed from 0 to 5 mol % for comparison (Figure 11 c). It was observed that condensation occurred even without the Fe3O4/ZIF-8 particles, but the conversion was only 7.7 % after 6 h and it increased slowly thereafter. When the catalyst dosage increased from 0.5 to 1.5 mol %, the conversion sharply increased from 25.1 to 65.2 % for the same reaction over 6 h. However, as the catalyst concentration was further raised to 2.0 or 5.0 mol %, the con-
Nanocatalysts for Knoevenagel reaction The Knoevenagel reaction is a nucleophilic aldol condensation between aromatic aldehydes, aromatic ketones, or benzal bromide and malonic ester, malonic acid, or cyanoacetic ester containing an activated methylene group, and it is usually catalyzed by a weak basic amine. Chizallet and co-workers investigated the position of acidic/basic sites in ZIF-8 based on CO adsorption monitored by FTIR spectroscopy and DFT calculations.[7b] Their study suggested that there were some Lewis acid sites from zinc(II) species, some Brønsted acid sites from NH groups, and some basic sites due to N moieties and OH groups on the surface of the catalyst. They also demonstrated that all active sites were located at the external surface, but not in the micropores of the material. Therefore, the Knoevenagel reaction is an ideal model reaction to evaluate nanoparticle-based catalysts, since their activities are closely related to particle size, but not porosity. For the above considerations, &
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
6
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
Figure 11. The influence of: a) solvent, b) baking treatment, c) catalyst concentration, and d) particle size on the conversion of benzaldehyde.
Figure 12. a) Kinetic curves for four continuous condensation reactions with Fe3O4/ZIF-8 particles as catalysts, and b) conversion of benzaldehyde in 3 h for each reaction.
version of substrate increased to 72.3 or 85.6 %, respectively. Therefore, the optimal catalyst dosage could be 1.5–2 mol % because further addition of catalyst would lower the conversion efficiency. Because the Knoevenagel reaction is mostly catalyzed on the external surface of ZIF-8, a smaller particle size can create more contacts between reactants and active sites, leading to a higher catalytic activity for Fe3O4/ZIF-8 particles. Here, catalyst particles with average diameters of 58.6, 72.2, 100.3, and 128.6 nm were used to show the influence of particle size upon activity (Figure 11 d). Upon decreasing the particle size from 128.6 to 58.6 nm, the conversion increased from 55.1 to 79.7 %. This proves that the active sites on the external surface of ZIF-8 were responsible for condensation, which was consistent with the transesterification reaction reported by Chizallet et al.[7b] Although the abundant active sites inside the micropores may not be utilized in this reaction, the nanosized MOF structures are still promising catalytic materials for other reactions. Compared with most homogeneous catalysts for organic reactions, the heterogeneous catalysts have intrinsic advantages in catalyst recycling and repeated usage. Because the nanocatalysts are usually well dispersed in solution as a quasi-homogeneous system, it is difficult to separate the catalyst from the reaction medium by convenient methods. Therefore, the introduction of Fe3O4 nanocrystals into ZIF-8 made the composite structure magnetic, so that separation could be realized by applying a magnetic field. To demonstrate the convenience of magnetic separation for continuous reactions, 10 mol % of Fe3O4/ZIF-8 particles were used as catalysts for condensation at room temperature. In each reaction, condensation proceeded for 3 h and the corresponding conversions are recorded in Figure 12. After separation by means of a magnetic field and Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
activation in vacuum at 250 8C for 2 h, the Fe3O4/ZIF-8 catalysts could be used again for the next round of the reaction. The conversion of benzaldehyde after 3 h reaction reached 85 % for the first round of catalysis, and gradually decreased to about 75 % for the fourth round of catalysis, which proved that the Fe3O4/ZIF-8 catalysts could be recovered and reused without significant loss in activity.
Conclusion Fe3O4/ZIF-8 nanoparticles were synthesized through a roomtemperature reaction between MeIM and zinc nitrate in the presence of Fe3O4 nanocrystals. Growth of the Fe3O4/ZIF-8 composite particles could be considered as a nanocrystal encapsulation process driven by electrostatic attraction between positively charged ZIF-8 particles and negatively charged Fe3O4 nanocrystals. Through the adjustment of the amount of Zn(NO3)2 and Fe3O4 nanocrystals, the particle size could be conveniently controlled, along with the surface charge and magnetic loading. The as-prepared Fe3O4/ZIF-8 nanoparticles could be used as a magnetically separable nanocatalyst in heterogeneous catalysis due to their high thermal stability, large surface area, and strong magnetic response. Herein, the Knoevenagel condensation reaction between benzaldehyde and cyanoacetamide was used to evaluate the catalytic performance of Fe3O4/ZIF-8. The reaction solvent, catalyst pretreatment, dosage, and size were optimized for the best conversion. Typically, the conversion reached 72.3 % in 6 h when condensation was catalyzed by 2 mol % of baked catalyst in methanol. Because condensation was catalyzed at the active sites on the external surface of catalyst, smaller Fe3O4/ZIF-8 particles usually presented a higher catalytic activity. At the same time, the 7
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper and products in the Knoevenagel reaction were analyzed by using an Agilent 7890A gas chromatograph equipped with an HP-5MS capillary column and using N2 as the carrier gas. The magnetic properties of the Fe3O4/ZIF-8 nanoparticles were measured by using a Lakeshore 7312 vibration sample magnetometer.
nanocatalysts could be continuously used in multiple catalytic reactions without loss of activity through magnetic separation, activation, and redispersion into the reaction solution.
Experimental Section Knoevenagel reaction and evaluation of catalytic activity
Chemicals
The Knoevenagel reaction between benzaldehyde and cyanoacetamide could be catalyzed by the ZIF-8 nanoparticles. Typically, cyanoacetamide (1.28 g, 15.2 mmol) and Fe3O4/ZIF-8 nanoparticles (36 mg) were first dispersed in methanol (20 mL) to form a homogeneous suspension. Benzaldehyde (0.8 mL, 7.6 mmol) was added to the above solution under stirring and the reaction was allowed to proceed for 6 h at room temperature. It should be noted that cyanoacetamide was used in excess to ensure complete conversion of benzaldehyde, and the molar ratio between catalyst (Fe3O4/ZIF8) and substrate (benzaldehyde) was 2:100. A small amount of anisole (0.4 mL) was added to the reaction solution as an internal standard for chromatographic analysis. During the 6 h reaction, samples (0.1 mL) were taken from the solution every 1 h, and then directly injected into the GC instrument to obtain the conversion of benzaldehyde. After the reaction, the Fe3O4/ZIF-8 nanocatalysts were purified by washing with methanol and magnetic separation (10 min) several times, and annealed under vacuum for 2 h at 250 8C to regenerate the catalytic activity.
FeCl3 (97 %), PAA (Mw = 1800), and diethylene glycol (DEG, 99 %) were purchased from Sigma-Aldrich. Zn(NO3)2·6 H2O (99 %), MeIM (98 %), cyanoacetamide (98 %), iron(III) acetylacetonate ([Fe(acac)3]; 98 %), and PEI (Mw = 1800, 99 %) were purchased form Aladdin Co. Ltd. Benzaldehyde (98 %) and anisole (99 %) were obtained from J&K Chemical Co. Ltd. All chemicals were used as received.
Synthesis of Fe3O4 nanocrystals Negatively charged PAA-grafted Fe3O4 nanoparticles were prepared according to a previously published procedure.[15] PAA (4 mmol) and FeCl3 (2 mmol) were first dissolved in DEG (15 mL) to form a transparent homogenous solution at 120 8C. The mixture was heated to 220 8C, and a solution of NaOH (2.5 mmol mL 1, 4 mL) in DEG was quickly injected to the above solution under vigorous stirring. After reacting at 220 8C for 2 h, the solution was cooled to room temperature and the nanocrystals were separated by centrifugation, rinsed with deionized water and ethanol three times, and finally dispersed in deionized water (10 mL) to form a brownish red transparent solution. The positively charged PEI-grafted Fe3O4 nanocrystals were prepared in a similar way, in which a solution of [Fe(acac)3] (2 mmol) and PEI (1 g) in DEG (15 mL) was directly heated at 220 8C for 2 h without the addition of NaOH.
Acknowledgements J.G. thanks the Major State Basic Research Development Program of China (2011CB932404), the National Science Foundation of China (21222107, 21471058), the Shanghai Rising-Star Program (13A1401400), and the Youth Talent Plan (Organization Department of the Central Committee of the CPC) for support.
Synthesis of Fe3O4/ZIF-8 nanoparticles In a typical process, an aqueous solution of MeIM (3.45 mol/L, 20 mL) was first mixed with the aforementioned aqueous solution of Fe3O4 nanocrystals (2.5 mL) by sonication, and stirred at a speed of 400 rpm at room temperature. Then the aqueous solution of Zn(NO3)2 (0.35 mol L 1, 4 mL) was loaded in a syringe and gradually added to the above solution over 20 min by means of a syringe pump to produce Fe3O4/ZIF-8 particles. After the addition of Zn(NO3)2, the products were separated by centrifugation, washed with deionized water twice, and dried at 300 8C under the protection of N2 flow for 2 h. After complete separation of Fe3O4/ZIF-8 particles and replacement of consumed MeIM, the reaction solution could be recycled for the next synthetic cycle.
Keywords: heterogeneous catalysis · magnetic properties · metal–organic frameworks · nanoparticles · synthesis design [1] a) J. R. Li, R. J. Kuppler, H. C. Zhou, Chem. Soc. Rev. 2009, 38, 1477 – 1504; b) R. Q. Zou, A. I. Abdel-Fattah, H. W. Xu, Y. S. Zhao, D. D. Hickmott, CrystEngComm 2010, 12, 1337 – 1353; c) N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe, O. M. Yaghi, Science 2003, 300, 1127 – 1129; d) M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe, O. M. Yaghi, Science 2002, 295, 469 – 472. [2] S. T. Meek, J. A. Greathouse, M. D. Allendorf, Adv. Mater. 2011, 23, 249 – 267. [3] a) O. Shekhah, J. Liu, R. A. Fischer, C. Woll, Chem. Soc. Rev. 2011, 40, 1081 – 1106; b) B. Liu, J. Mater. Chem. 2012, 22, 10094 – 10101; c) Y. X. Xu, Y. Q. Wen, W. Zhu, Y. N. Wu, C. X. Lin, G. T. Li, Mater. Lett. 2012, 87, 20 – 23. [4] a) R. C. Huxford, J. Della Rocca, W. B. Lin, Curr. Opin. Chem. Biol. 2010, 14, 262 – 268; b) C. Wang, D. M. Liu, W. B. Lin, J. Am. Chem. Soc. 2013, 135, 13222 – 13234. [5] a) H. L. Jiang, B. Liu, T. Akita, M. Haruta, H. Sakurai, Q. Xu, J. Am. Chem. Soc. 2009, 131, 11302 – 11303; b) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450 – 1459; c) J. Gascon, U. Aktay, M. D. Hernandez-Alonso, G. P. M. van Klink, F. Kapteijn, J. Catal. 2009, 261, 75 – 87; d) A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 2012, 41, 5262 – 5284; e) C. Hou, G. Zhao, Y. Ji, Z. Niu, D. Wang, Y. Li, Nano Res. 2014, 7, 1364 – 1369. [6] a) A. Phan, C. J. Doonan, F. J. Uribe-Romo, C. B. Knobler, M. O’Keeffe, O. M. Yaghi, Acc. Chem. Res. 2010, 43, 58 – 67; b) K. S. Park, Z. Ni, A. P. Cote, J. Y. Choi, R. D. Huang, F. J. Uribe-Romo, H. K. Chae, M. O’Keeffe,
Characterization The morphologies and sizes of the Fe3O4/ZIF-8 nanoparticles were investigated by means of a JEOL JEM-2100 transmission electron microscope operated at 200 kV. Powder XRD patterns of ZIF-8, Fe3O4, and Fe3O4/ZIF-8 nanoparticles were obtained by using a Rigaku Ultima IV X-ray diffractometer operated at 35 kV and 40 mA with Ni-filtered CuKa radiation. TGA was measured in the range of 20–980 8C by using a NETZSCH STA449-3F instrument under the protection of N2. Nitrogen adsorption–desorption isotherms, BET surface areas, and Barret–Joyner–Halenda (BJH) pore diameters were measured by using a JW-BK122 analyzer at 77 K. Dynamic light scattering (DLS) behavior and zeta potentials of nanoparticles were characterized by means of a Malvern Instruments ZS-90 analyzer equipped with a multipurpose autotitrator (MPT-2) at 25 8C. The absorption spectra were captured by using an Ocean Optics Maya 2000 pro UV/Vis spectrometer. The reactants
&
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
8
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
[7]
[8] [9]
[10] [11]
[12] [13]
[14]
[15] [16] [17]
[18] [19]
O. M. Yaghi, Proc. Natl. Acad. Sci. USA 2006, 103, 10186 – 10191; c) J. F. Yao, H. T. Wang, Chem. Soc. Rev. 2014, 43, 4470 – 4493; d) S. R. Venna, J. B. Jasinski, M. A. Carreon, J. Am. Chem. Soc. 2010, 132, 18030 – 18033; e) D. Fairen-Jimenez, S. A. Moggach, M. T. Wharmby, P. A. Wright, S. Parsons, T. Duren, J. Am. Chem. Soc. 2011, 133, 8900 – 8902. a) M. Q. Zhu, D. Srinivas, S. Bhogeswararao, P. Ratnasamy, M. A. Carreon, Catal. Commun. 2013, 32, 36 – 40; b) C. Chizallet, S. Lazare, D. BazerBachi, F. Bonnier, V. Lecocq, E. Soyer, A. A. Quoineaud, N. Bats, J. Am. Chem. Soc. 2010, 132, 12365 – 12377. L. T. L. Nguyen, K. K. A. Le, N. T. S. Phan, J. Catal. 2012, 33, 688 – 696. a) U. P. N. Tran, K. K. A. Le, N. T. S. Phan, ACS Catal. 2011, 1, 120 – 127; b) R. Z. Jin, Z. Bian, J. Z. Li, M. X. Ding, L. X. Gao, Dalton Trans. 2013, 42, 3936 – 3940; c) L. T. L. Nguyen, K. K. A. Le, H. X. Truong, N. T. S. Phan, Catal. Sci. Technol. 2012, 2, 521 – 528. C. M. Miralda, E. E. Macias, M. Q. Zhu, P. Ratnasamy, M. A. Carreon, ACS Catal. 2012, 2, 180 – 183. a) C. H. Kuo, Y. Tang, L. Y. Chou, B. T. Sneed, C. N. Brodsky, Z. P. Zhao, C. K. Tsung, J. Am. Chem. Soc. 2012, 134, 14345 – 14348; b) P. Z. Li, K. Aranishi, Q. Xu, Chem. Commun. 2012, 48, 3173 – 3175; c) F. Alonso, F. Foubelo, J. C. Gonzalez-Gomez, R. Martinez, D. J. Ramon, P. Riente, M. Yus, Mol. Diversity 2010, 14, 411 – 424. R. Selvin, H. L. Hsu, T. M. Her, Catal. Commun. 2008, 10, 169 – 172. a) V. Polshettiwar, R. S. Varma, Green Chem. 2010, 12, 743 – 754; b) Y. Li, Q. Y. Liu, W. J. Shen, Dalton Trans. 2011, 40, 5811 – 5826; c) C. P. Vinod, Catal. Today 2010, 154, 113 – 117; d) S. B. Kalidindi, B. R. Jagirdar, ChemSusChem 2012, 5, 65 – 75; e) R. Z. Chen, Y. Du, Q. Q. Wang, W. H. Xing, W. Q. Jin, N. P. Xu, Ind. Eng. Chem. Res. 2009, 48, 6600 – 6607. a) S. Shylesh, V. Schunemann, W. R. Thiel, Angew. Chem. Int. Ed. 2010, 49, 3428 – 3459; Angew. Chem. 2010, 122, 3504 – 3537; b) V. Polshettiwar, R. Luque, A. Fihri, H. B. Zhu, M. Bouhrara, J. M. Bassett, Chem. Rev. 2011, 111, 3036 – 3075. J. P. Ge, Y. X. Hu, M. Biasini, W. P. Beyermann, Y. D. Yin, Angew. Chem. Int. Ed. 2007, 46, 4342 – 4345; Angew. Chem. 2007, 119, 4420 – 4423. J. I. Park, J. Cheon, J. Am. Chem. Soc. 2001, 123, 5743 – 5746. a) D. S. Wang, J. B. He, N. Rosenzweig, Z. Rosenzweig, Nano Lett. 2004, 4, 409 – 413; b) Y. Deng, D. Qi, C. Deng, X. Zhang, D. Zhao, J. Am. Chem. Soc. 2008, 130, 28 – 29. P. G. Wu, J. H. Zhu, Z. H. Xu, Adv. Funct. Mater. 2004, 14, 345 – 351. W. R. Zhao, J. L. Gu, L. X. Zhang, H. R. Chen, J. L. Shi, J. Am. Chem. Soc. 2005, 127, 8916 – 8917.
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
[20] F. Caruso, M. Spasova, A. Susha, M. Giersig, R. A. Caruso, Chem. Mater. 2001, 13, 109 – 116. [21] A. H. Lu, W. Schmidt, N. Matoussevitch, H. Bonnemann, B. Spliethoff, B. Tesche, E. Bill, W. Kiefer, F. Schuth, Angew. Chem. Int. Ed. 2004, 43, 4303 – 4306; Angew. Chem. 2004, 116, 4403 – 4406. [22] S. C. Tsang, V. Caps, I. Paraskevas, D. Chadwick, D. Thompsett, Angew. Chem. Int. Ed. 2004, 43, 5645 – 5649; Angew. Chem. 2004, 116, 5763 – 5767. [23] M. Kotani, T. Koike, K. Yamaguchi, N. Mizuno, Green Chem. 2006, 8, 735 – 741. [24] K. Mori, N. Yoshioka, Y. Kondo, T. Takeuchi, H. Yamashita, Green Chem. 2009, 11, 1337 – 1342. [25] Y. W. Ma, B. Yue, L. S. Yu, X. Z. Wang, Z. Hu, Y. N. Fan, Y. Chen, W. W. Lin, Y. N. Lu, J. H. Hu, J. Phys. Chem. C 2008, 112, 472 – 475. [26] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chem. Commun. 2012, 48, 11275 – 11288. [27] a) J. P. Ge, Q. Zhang, T. R. Zhang, Y. D. Yin, Angew. Chem. Int. Ed. 2008, 47, 8924 – 8928; Angew. Chem. 2008, 120, 9056 – 9060; b) C. Li, R. Wei, Y. Xu, A. Sun, L. Wei, Nano Res. 2014, 7, 536 – 543. [28] D. Guin, B. Baruwati, S. V. Manorama, Org. Lett. 2007, 9, 1419 – 1421. [29] a) Y. H. Wang, J. K. Lee, J. Mol. Catal. A 2007, 263, 163 – 168; b) C. R. Vestal, Z. J. Zhang, Nano Lett. 2003, 3, 1739 – 1743. [30] S. H. Xu, D. L. Feng, W. F. Shangguan, J. Phys. Chem. C 2009, 113, 2463 – 2467. [31] Y. C. Pan, Y. Y. Liu, G. F. Zeng, L. Zhao, Z. P. Lai, Chem. Commun. 2011, 47, 2071 – 2073. [32] a) K. Ueno, M. Watanabe, Langmuir 2011, 27, 9105 – 9115; b) E. Tombcz, Soil Sci. 1999, 164, 814 – 824. [33] C. Reichardt, in Solvents and Solvent Effects in Organic Chemistry, WileyVCH, Weinheim, 2004, pp. 147 – 328. [34] J. Juan-AlcaÇiz, E. V. Ramos-Fernandez, U. Lafont, J. Gascon, F. Kapteijn, J. Catal. 2010, 269, 229 – 241. [35] Z. Q. Niu, Y. D. Li, Chem. Mater. 2014, 26, 72 – 83.
Received: November 2, 2014 Revised: February 2, 2015 Published online on && &&, 0000
9
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
FULL PAPER & Heterogeneous Catalysis
Increasing attraction: Fe3O4/zeolite imidazolate framework (ZIF-8) nanoparticles were synthesized through a roomtemperature reaction between 2-methylimidazolate and zinc nitrate in the presence of Fe3O4 nanocrystals (see figure). The particles have a superparamagnetic response and can be separated by means of a magnetic field and redispersed into the solution after withdrawal of the field.
F. Pang, M. Y. He, J. P. Ge* && – && Controlled Synthesis of Fe3O4/ZIF-8 Nanoparticles for Magnetically Separable Nanocatalysts
&
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
10
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
