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Cite this: Chem. Commun., 2014, 50, 14658 Received 10th August 2014, Accepted 30th September 2014
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Synthesis of hierarchical zeolites using an inexpensive mono-quaternary ammonium surfactant as mesoporogen† Xiaochun Zhu,a Roderigh Rohling,a Georgy Filonenko,a Brahim Mezari,a Jan P. Hofmann,a Shunsuke Asahinab and Emiel J. M. Hensen*a
DOI: 10.1039/c4cc06267a www.rsc.org/chemcomm
A simple amphiphilic surfactant containing a mono-quaternary ammonium head group (N-methylpiperidine) is effective in imparting substantial mesoporosity during synthesis of SSZ-13 and ZSM-5 zeolites. Highly mesoporous SSZ-13 prepared in this manner shows greatly improved catalytic performance in the methanol-to-olefins reaction compared to bulk SSZ-13.
Zeolites are crystalline microporous aluminosilicates built up from corner-sharing oxygen tetrahedra occupied by Si4+ or Al3+ ions. They combine tunable surface acidity with favorable textural properties for catalysis such as high surface area, high thermal stability and high connectivity of uniformly-sized micropores. Accordingly, they are widely used as acid catalysts in the petrochemical and oil refining industry.1 Although the microporous texture is advantageous in terms of shape selectivity in catalysis, transport of bulky reactant and product molecules may be hindered in the subnanometer micropores.2 These diffusion limitations may lower reaction rate, because of inefficient use of the internal regions of zeolite crystals.3 Related to this, side reactions such as coking may seriously lower the lifetime of zeolites.4 Accordingly, the development of mesoporous zeolites with sufficiently large voids between microporous domains has been widely explored.5 A common approach to obtain mesoporous zeolites involves post-synthesis methods such as chemical, steaming or leaching treatments of zeolites.6 Although some of these methods are relatively cheap and amenable to scale-up, they usually allow little control over textural and acidic properties of the final zeolite. The alternative bottom-up strategy has the advantage of better control over the pore hierarchy and the final acidity.7 a
Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: [email protected] b JEOL Ltd., Akishima, Tokyo 196-8558, Japan † Electronic supplementary information (ESI) available: Experimental details, preparation and characterization of the surfactants and catalysts, simulations results. See DOI: 10.1039/c4cc06267a
14658 | Chem. Commun., 2014, 50, 14658--14661
A drawback is, however, that complex and expensive templates have to be employed to bring about the additional porosity.7a,8 Ryoo and co-workers synthesized multifunctional structuredirecting agents (SDAs) that are able to direct formation of highly accessible zeolites (e.g., MFI, BEA) in the form of nanosheets, nanocrystals, and nanosponges.5c,7a,8c Preparation of these SDAs requires multiple steps, making them expensive. It has also been shown that these multifunctional SDAs can be used in combination with conventional zeolite-forming SDAs to obtain mesoporous zeolites.7h,i,8c These studies constitute a first step towards the use of mesoporogen additives in zeolite synthesis gel mixtures to prepare mesoporous zeolites in one step. Earlier attempts in this direction with mono-quaternary ammonium amphiphilic surfactants have not been successful.9 The interaction of the zeolite with monofunctional surfactants such as cetyltrimethylammonium bromide (CTAB) is not strong enough to compete with the zeolite-forming SDA (e.g., tetrapropylammonium for MFI zeolite),7h resulting at best in intimate mixtures of bulk zeolite and amorphous mesoporous silica.9c Thus, we hypothesized that surfactants with sufficiently strong interaction with the growing zeolite surface to effectively compete with the zeolite SDA need to be developed to allow for successful dual-template synthesis of hierarchical zeolites.5e,6a,7f Herein we report about a simple surfactant containing a bulky mono-quaternary ammonium head group in the form of N-methylpiperidine and a cetyl (C16) tail group. This versatile mesoporogen is capable of introducing mesoporosity in SSZ-13 and ZSM-5 zeolites. We simply replaced part of N,N,N-trimethyl-1adamantanammonium hydroxide (TMAdaOH) as the SDA for SSZ-13 formation by C16H33-[N+-methylpiperidine] (further denoted by C16MP) in a conventional SSZ-13 synthesis gel. The small-pore zeolite SSZ-13 with the chabazite (CHA) topology contains large 6.7 Å 10.9 Å-sized cavities interconnected by 8-membered rings with pore apertures of 3.8 Å. Its silicoaluminophosphate counterpart, SAPO-34, is the preferred catalyst in the industrial-scale methanol-to-olefins (MTO) process.11 Although the use of the more acidic aluminosilicate SSZ-13 would lower capital investment for the regenerator section in the MTO process, the higher acidity
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causes more rapid coke formation leading to premature catalyst deactivation and underutilization of its micropore volume.7i,10 The procedure for the synthesis of the amphiphilic surfactant C16MP is given in the ESI.† The C16MP template can be prepared in a single step from cheap starting chemicals, that is by reacting 1-bromohexadecane with N-methylpiperidine. The successful synthesis of C16MP was confirmed by ESI-MS, 1H and 13C NMR (ESI†). C16MP was used to synthesize mesoporous SSZ-13 (SSZ13-C16MP) in a gel containing N,N,N-trimethyl-1-adamantanammonium hydroxide (TMAdaOH) as the SDA (molar gel composition: 15TMAdaOH : 5C16MP : 7.5Na2O : 2.5Al2O3 : 100 SiO2 : 4400H2O, autogenous pressure; 160 1C; 8 days). Bulk SSZ-13 zeolite (SSZ-13-bulk) was prepared from the same gel without mesoporogen. For comparison, mono- and diquarternary ammonium surfactants C16H33-N+(CH3)2-C4H9 (C16N1) and C16H33-N+(CH3)2-C4H8-N+(CH3)2-C4H9 (C16N2) were also used to prepare SSZ-13-C16N1 and SSZ-13-C16N2, respectively. The XRD pattern of as-synthesized SSZ-13-C16MP (Fig. S1, ESI†) confirms the formation of SSZ-13 zeolite without any detectable impurity phases. Low-voltage scanning electron microscopy (LV-SEM) images show the decreased size of the microporous domains in SSZ-13-C16MP (Fig. 1 and Fig. S2, ESI†). The zeolite consists of particles with dimensions in the range of 200–500 nm made up from stacked three-dimensional intergrowths of cubic crystals with sizes much smaller than 100 nm. These images also show mesoporous voids between the primary particles. The mesopores can also been seen by TEM (Fig. 1c). On contrary, SSZ-13-bulk consists of micrometersized crystals with very smooth surfaces. Thermogravimetric analysis (Fig. S5, ESI†) showed higher weight loss due to template removal for SSZ-13-C16MP (B37%) than for SSZ-13bulk (B22%). The difference is due to the presence of the C16MP in the mesopores. The weight loss for SSZ-13-C16N2 (B33%) is comparable with that of SSZ-13-C16MP. Ar physisorption isotherms and pore size distribution curves for the calcined SSZ-13 zeolites are shown in Fig. 2. The isotherm of SSZ-13-bulk has the usual type I shape of microporous materials. The additional presence of mesoporosity in SSZ-13-C16MP and SSZ-13-C16N2 is evident from the type IV shape of their isotherms. Contrary to SSZ-13-C16N2, the mesopore size in SSZ-13-C16MP is well-defined and centers B3.8 nm. Table S1 (ESI†) further shows that this hierarchical zeolite has very favorable textural properties and, most notably, a high mesopore volume. The micropore volume of SSZ-13-C16MP (0.18 cm3 g 1) is comparable to that of
Fig. 1 LV-SEM images of as-synthesized SSZ-13-bulk (a) and SSZ-13C16MP (b). The TEM image (c) shows the presence of mesopores in SSZ-13-C16MP.
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Fig. 2 Ar physisorption isotherms (left) and pore size distributions (right) of calcined SSZ-13-bulk (m), SSZ-13-C16MP ( ) and SSZ-13-C16N2 ( ).
its bulk counterpart (0.22 cm3 g 1). The mesoporous SSZ-13 zeolite prepared with C16N2 also has a high Langmuir surface area, but its mesopore volume is two times lower than that of SSZ-13-C16MP. As the molar mesoporogen to silicon ratios were similar in the starting gels of the mesoporous zeolites, we conclude that C16MP is more effective in generating mesoporosity than the diquarternary ammonium surfactant C16N2. The use of C16N1 with an ordinary mono-quaternary ammonium group led to bulk SSZ-13. 27 Al MAS NMR spectroscopy shows that the majority of Al atoms reside in the framework of SSZ-13-C16MP and SSZ-13-bulk (Fig. S6a and Table S2, ESI†). By ICP-OES elemental analysis it was found that both samples have similar Si/Al ratio. Furthermore, the nature of hydroxyl groups of SSZ-13 zeolites was investigated by 1H MAS NMR and FT-IR spectroscopy. 1H MAS NMR spectra (Fig. S6b, ESI†) shows that the intensity of the silanol groups is much higher for SSZ-13-C16MP than for SSZ-13-bulk, which correlates to the higher external surface area of the zeolite crystals of SSZ-13-C16MP. Quantification of the Brønsted acid sites (BAS) by 1H NMR spectroscopy (Table S2, ESI†) shows that these materials contain similar amounts of BAS. FTIR spectra of adsorbed CO (Fig. S7, ESI†) and pyridine (Fig. S7e and f, ESI†) confirm that (i) the BAS in SSZ-13-C16MP and SSZ-13-bulk are of similar strength and (ii) the concentration of BAS at the external crystal surface of bulk SSZ-13 is negligible, whereas about 14% of the BAS reside at the external crystal surface in SSZ-13-C16MP. The mechanical stability of SSZ-13-C16MP was judged from the loss in surface area and mesopore volume due to pressing (150 MPa or 300 MPa, 2 min) followed by crushing (Fig. S4, ESI†). The resulting textural properties are also given in Table S1 (ESI†) and show that the mesopores of SSZ-13-C16MP were slightly damaged by the application of increasing pressure. Nevertheless, the loss in mesopore volume (12%) and Langmuir surface area (5%) was very small, showing the excellent mechanical stability of this material. To better understand the way these surfactants interact with the zeolite and compete with the zeolite-forming SDA, we performed molecular simulations using well-established force fields (Table 1 and Table S3, ESI†). The interaction energy of the N-methylpiperidine head group in C16MP with the CHA cage ( 496 kJ mol 1) is comparable with the interaction energy
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Table 1 Interaction energies of structure directing agents and mesoporogens with a SSZ-13 zeolite slab. Templates are given in their cationic form. The cationic charge of template is balanced by deprotonation of an external silanol group
Template
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Interaction energy (kJ mol 1) a
TMAda+
C16N1+
373
478
C16N22+ 1015a
C16MP+ 496
Two ammonium groups of C16N2 placed in two adjacent zeolite cages.
between TMAda+ and the CHA cage ( 373 kJ mol 1). In comparison, the interaction energy of C16N2 with its two quaternary ammonium groups placed in two adjacent cages is much higher ( 1015 kJ mol 1). C16N1 has comparable interaction energy with the zeolite ( 478 kJ mol 1) as C16MP and TMAda+. The reason that the use of C16N1 only results in bulk zeolite is that it can easily leave the cavities of SSZ-13. This implies that an important feature of C16MP is its bulky N-methylpiperidine head group, which is too large to traverse the pore openings of CHA zeolite. As a consequence, C16MP remains entrapped in the zeolite structure. While C16N2 can in principle also leave the micropores, this is prevented by its much stronger interaction with the zeolite. Given our finding that C16MP is more effective in imparting mesoporosity than C16N2, we compared synthesis of SSZ-13C16N2 and SSZ-13-C16MP from gels with a higher Si/Al ratio of 50. Crystallization at lower Al content in the synthesis gel is more difficult and usually results in formation of competing zeolite phases such as ZSM-5. This problem is augmented in the presence of mesoporogens.12 With C16N2 as the mesoporogen ZSM-5 zeolite was formed as the main product (Fig. S8, ESI†). On contrary, the use of the weaker interacting C16MP exclusively results in mesoporous SSZ-13 zeolite at a Si/Al ratio of 50 without any impurities. The same result was obtained when SSZ-13-bulk was prepared with Si/Al = 50. To explore the generality of our approach, we employed C16MP as an additive to a synthesis gel containing tetrapropylammonium hydroxide (TPAOH), 1,6-diaminohexane (DAH) or diethylamine (DEA), which are known SDAs for formation of the important medium-pore zeolite ZSM-5 (MFI topology).13 The combination of TPAOH with C16MP afforded only bulk ZSM-5 (Fig. S12a, ESI†). When DAH was the SDA, a highly mesoporous zeolite with broad XRD reflections of MFI zeolite (Fig. 3 and Fig. S11c, ESI†) was obtained. Evidence for formation
Fig. 3
SEM (a) and TEM image (b) of ZSM-5-C16MP-DAH.
14660 | Chem. Commun., 2014, 50, 14658--14661
of mesoporous ZSM-5 follows from the type IV Ar adsorption isotherm shown in Fig. S13 (ESI†). ZSM-5-C16MP-DAH has a mesopore volume of 0.22 cm3 g 1 (Table S1, ESI†). Although more detailed investigations are needed, the EM image in Fig. 3b show that the C16MP mesoporogen preferentially aligns in the straight channels of ZSM-5, resulting in formation of the sheet-like morphology of ZSM-5 that was also observed using di-quaternary ammonium SDAs developed by Ryoo and co-workers. Furthermore, in combination with DEA the use of C16MP resulted in formation of ZSM-5 with the less structured, yet highly mesoporous nanosponge morphology (Fig. S12d and Table S1, ESI†). The micropore volumes of mesoporous ZSM-5 zeolites are close to that of their bulk counterpart, indicating that they are highly crystalline. Molecular modeling results show that the interaction energies of C16MP and TPA+ with the zeolite framework are comparable ( 1095 kJ mol 1 and 1047 kJ mol 1 respectively, Table S4, ESI†). The N-methylpiperidine head group is located at the intersections of the straight and zig-zag channels of MFI. This is also the preferred location of TPA+. The interaction energies are dominated by electrostatic interactions of the positive charge of the quaternary ammonium group with the zeolite framework. On the contrary, the interaction of the non-charged DAH with the zeolite framework is much weaker because of the absence of quaternary ammonium ions. Whilst TPA+ gets entrapped at the intersections during zeolite synthesis, C16MP can freely move through the channels of MFI zeolite. We speculate that the repulsive electrostatic interactions between C16MP and TPA will lead to preferential occlusion of TPA+ driven by the lower surface energy of the growing zeolite. These repulsive interactions are weaker when C16MP is combined with DAH, so that the mesoporogen effectively competes with DAH and limits zeolite growth. The potential of mesoporous SSZ-13-C16MP in acid catalysis was evaluated by determining its catalytic performance in the MTO reaction. The versatility of the MTO process to produce light olefins derives from the feedstock flexibility for manufacture of syngas, which is used to synthesize methanol.14 The reaction tests were carried out at a temperature of 350 1C and a WHSV of 0.8 h 1. Fig. 4 shows methanol conversion as function of time on stream. The catalyst lifetime is defined as the time to reach 50% of methanol conversion. Initially, all catalysts convert the methanol feed completely. Deactivation of bulk SSZ-13 was rapid and its lifetime was only 5 h. For SSZ-13-C16MP methanol conversion only started decreasing after 9 h. The rate of deactivation was much slower and the lifetime of SSZ-13C16MP was 17 h. The greatly improved performance is attributed to the smaller microporous domains in the hierarchical zeolite, effectively reducing molecular trafficking distances. Catalyst lifetime of SSZ-13-C16MP was also substantially better than for other mesoporous SSZ-13 reported earlier by us.7h The total methanol conversion capacity as defined by Bjørgen et al.15 improved from 6 to 24 gmethanol gcatalyst 1 for SSZ-13bulk and SSZ-13-C16MP (Fig. S14, ESI†). This improvement is ascribed to effective utilization of the zeolite micropores in hierarchical SSZ-13. Fig. 4 also shows that almost similar
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Fig. 4 Methanol conversion as function of time on stream for the MTO reaction for SSZ-13-bulk (m), SSZ-13-C16MP ( ) and SSZ-13-C16MP (5%) ( ) (reaction conditions: WHSV = 0.8 h 1, T = 350 1C).
7
catalytic performance was obtained when a much smaller amount of TMAdaOH (5%) was replaced by C16MP (Ar physisorption and the corresponding textural data are shown in Fig. S3, ESI†). This finding and the much lower cost of C16MP compared to other mesoporogens (C16MP: 133 h kg 1; C16N2: 6363 h kg 1; NDC monoliths:16 748 h kg 1; TPOAC:7j 1468 h kg 1) render our approach promising for the practical use of mesoporous zeolites. Summarizing, hierarchically structured SSZ-13 and ZSM-5 zeolites have been successfully synthesized using the novel mono-quaternary ammonium surfactant as mesoporogen, which combines proper interaction with the zeolite and competition with the conventional SDAs. The resulting zeolite is highly mesoporous and its acid sites are comparable in number and density to those in a reference bulk SSZ-13 zeolite. The main advantage over previous approaches is that this mesoporogen is cheap and is already effective when it replaces a small amount of the conventional zeolite-forming SDA. Thus synthesized mesoporous SSZ-13 exhibits the highest catalytic performance in the MTO reaction reported so far. Our approach should be generally applicable to other zeolite topologies, as shown in this study for the synthesis of hierarchical ZSM-5. XZ acknowledges financial support from the China Scholarship Council. We thank the Cryo-TEM Research Unit of Eindhoven University of Technology for access to TEM facilities.
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Cite this: Chem. Commun., 2014, 50, 14658 Received 10th August 2014, Accepted 30th September 2014
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Synthesis of hierarchical zeolites using an inexpensive mono-quaternary ammonium surfactant as mesoporogen† Xiaochun Zhu,a Roderigh Rohling,a Georgy Filonenko,a Brahim Mezari,a Jan P. Hofmann,a Shunsuke Asahinab and Emiel J. M. Hensen*a
DOI: 10.1039/c4cc06267a www.rsc.org/chemcomm
A simple amphiphilic surfactant containing a mono-quaternary ammonium head group (N-methylpiperidine) is effective in imparting substantial mesoporosity during synthesis of SSZ-13 and ZSM-5 zeolites. Highly mesoporous SSZ-13 prepared in this manner shows greatly improved catalytic performance in the methanol-to-olefins reaction compared to bulk SSZ-13.
Zeolites are crystalline microporous aluminosilicates built up from corner-sharing oxygen tetrahedra occupied by Si4+ or Al3+ ions. They combine tunable surface acidity with favorable textural properties for catalysis such as high surface area, high thermal stability and high connectivity of uniformly-sized micropores. Accordingly, they are widely used as acid catalysts in the petrochemical and oil refining industry.1 Although the microporous texture is advantageous in terms of shape selectivity in catalysis, transport of bulky reactant and product molecules may be hindered in the subnanometer micropores.2 These diffusion limitations may lower reaction rate, because of inefficient use of the internal regions of zeolite crystals.3 Related to this, side reactions such as coking may seriously lower the lifetime of zeolites.4 Accordingly, the development of mesoporous zeolites with sufficiently large voids between microporous domains has been widely explored.5 A common approach to obtain mesoporous zeolites involves post-synthesis methods such as chemical, steaming or leaching treatments of zeolites.6 Although some of these methods are relatively cheap and amenable to scale-up, they usually allow little control over textural and acidic properties of the final zeolite. The alternative bottom-up strategy has the advantage of better control over the pore hierarchy and the final acidity.7 a
Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: [email protected] b JEOL Ltd., Akishima, Tokyo 196-8558, Japan † Electronic supplementary information (ESI) available: Experimental details, preparation and characterization of the surfactants and catalysts, simulations results. See DOI: 10.1039/c4cc06267a
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A drawback is, however, that complex and expensive templates have to be employed to bring about the additional porosity.7a,8 Ryoo and co-workers synthesized multifunctional structuredirecting agents (SDAs) that are able to direct formation of highly accessible zeolites (e.g., MFI, BEA) in the form of nanosheets, nanocrystals, and nanosponges.5c,7a,8c Preparation of these SDAs requires multiple steps, making them expensive. It has also been shown that these multifunctional SDAs can be used in combination with conventional zeolite-forming SDAs to obtain mesoporous zeolites.7h,i,8c These studies constitute a first step towards the use of mesoporogen additives in zeolite synthesis gel mixtures to prepare mesoporous zeolites in one step. Earlier attempts in this direction with mono-quaternary ammonium amphiphilic surfactants have not been successful.9 The interaction of the zeolite with monofunctional surfactants such as cetyltrimethylammonium bromide (CTAB) is not strong enough to compete with the zeolite-forming SDA (e.g., tetrapropylammonium for MFI zeolite),7h resulting at best in intimate mixtures of bulk zeolite and amorphous mesoporous silica.9c Thus, we hypothesized that surfactants with sufficiently strong interaction with the growing zeolite surface to effectively compete with the zeolite SDA need to be developed to allow for successful dual-template synthesis of hierarchical zeolites.5e,6a,7f Herein we report about a simple surfactant containing a bulky mono-quaternary ammonium head group in the form of N-methylpiperidine and a cetyl (C16) tail group. This versatile mesoporogen is capable of introducing mesoporosity in SSZ-13 and ZSM-5 zeolites. We simply replaced part of N,N,N-trimethyl-1adamantanammonium hydroxide (TMAdaOH) as the SDA for SSZ-13 formation by C16H33-[N+-methylpiperidine] (further denoted by C16MP) in a conventional SSZ-13 synthesis gel. The small-pore zeolite SSZ-13 with the chabazite (CHA) topology contains large 6.7 Å 10.9 Å-sized cavities interconnected by 8-membered rings with pore apertures of 3.8 Å. Its silicoaluminophosphate counterpart, SAPO-34, is the preferred catalyst in the industrial-scale methanol-to-olefins (MTO) process.11 Although the use of the more acidic aluminosilicate SSZ-13 would lower capital investment for the regenerator section in the MTO process, the higher acidity
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causes more rapid coke formation leading to premature catalyst deactivation and underutilization of its micropore volume.7i,10 The procedure for the synthesis of the amphiphilic surfactant C16MP is given in the ESI.† The C16MP template can be prepared in a single step from cheap starting chemicals, that is by reacting 1-bromohexadecane with N-methylpiperidine. The successful synthesis of C16MP was confirmed by ESI-MS, 1H and 13C NMR (ESI†). C16MP was used to synthesize mesoporous SSZ-13 (SSZ13-C16MP) in a gel containing N,N,N-trimethyl-1-adamantanammonium hydroxide (TMAdaOH) as the SDA (molar gel composition: 15TMAdaOH : 5C16MP : 7.5Na2O : 2.5Al2O3 : 100 SiO2 : 4400H2O, autogenous pressure; 160 1C; 8 days). Bulk SSZ-13 zeolite (SSZ-13-bulk) was prepared from the same gel without mesoporogen. For comparison, mono- and diquarternary ammonium surfactants C16H33-N+(CH3)2-C4H9 (C16N1) and C16H33-N+(CH3)2-C4H8-N+(CH3)2-C4H9 (C16N2) were also used to prepare SSZ-13-C16N1 and SSZ-13-C16N2, respectively. The XRD pattern of as-synthesized SSZ-13-C16MP (Fig. S1, ESI†) confirms the formation of SSZ-13 zeolite without any detectable impurity phases. Low-voltage scanning electron microscopy (LV-SEM) images show the decreased size of the microporous domains in SSZ-13-C16MP (Fig. 1 and Fig. S2, ESI†). The zeolite consists of particles with dimensions in the range of 200–500 nm made up from stacked three-dimensional intergrowths of cubic crystals with sizes much smaller than 100 nm. These images also show mesoporous voids between the primary particles. The mesopores can also been seen by TEM (Fig. 1c). On contrary, SSZ-13-bulk consists of micrometersized crystals with very smooth surfaces. Thermogravimetric analysis (Fig. S5, ESI†) showed higher weight loss due to template removal for SSZ-13-C16MP (B37%) than for SSZ-13bulk (B22%). The difference is due to the presence of the C16MP in the mesopores. The weight loss for SSZ-13-C16N2 (B33%) is comparable with that of SSZ-13-C16MP. Ar physisorption isotherms and pore size distribution curves for the calcined SSZ-13 zeolites are shown in Fig. 2. The isotherm of SSZ-13-bulk has the usual type I shape of microporous materials. The additional presence of mesoporosity in SSZ-13-C16MP and SSZ-13-C16N2 is evident from the type IV shape of their isotherms. Contrary to SSZ-13-C16N2, the mesopore size in SSZ-13-C16MP is well-defined and centers B3.8 nm. Table S1 (ESI†) further shows that this hierarchical zeolite has very favorable textural properties and, most notably, a high mesopore volume. The micropore volume of SSZ-13-C16MP (0.18 cm3 g 1) is comparable to that of
Fig. 1 LV-SEM images of as-synthesized SSZ-13-bulk (a) and SSZ-13C16MP (b). The TEM image (c) shows the presence of mesopores in SSZ-13-C16MP.
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Fig. 2 Ar physisorption isotherms (left) and pore size distributions (right) of calcined SSZ-13-bulk (m), SSZ-13-C16MP ( ) and SSZ-13-C16N2 ( ).
its bulk counterpart (0.22 cm3 g 1). The mesoporous SSZ-13 zeolite prepared with C16N2 also has a high Langmuir surface area, but its mesopore volume is two times lower than that of SSZ-13-C16MP. As the molar mesoporogen to silicon ratios were similar in the starting gels of the mesoporous zeolites, we conclude that C16MP is more effective in generating mesoporosity than the diquarternary ammonium surfactant C16N2. The use of C16N1 with an ordinary mono-quaternary ammonium group led to bulk SSZ-13. 27 Al MAS NMR spectroscopy shows that the majority of Al atoms reside in the framework of SSZ-13-C16MP and SSZ-13-bulk (Fig. S6a and Table S2, ESI†). By ICP-OES elemental analysis it was found that both samples have similar Si/Al ratio. Furthermore, the nature of hydroxyl groups of SSZ-13 zeolites was investigated by 1H MAS NMR and FT-IR spectroscopy. 1H MAS NMR spectra (Fig. S6b, ESI†) shows that the intensity of the silanol groups is much higher for SSZ-13-C16MP than for SSZ-13-bulk, which correlates to the higher external surface area of the zeolite crystals of SSZ-13-C16MP. Quantification of the Brønsted acid sites (BAS) by 1H NMR spectroscopy (Table S2, ESI†) shows that these materials contain similar amounts of BAS. FTIR spectra of adsorbed CO (Fig. S7, ESI†) and pyridine (Fig. S7e and f, ESI†) confirm that (i) the BAS in SSZ-13-C16MP and SSZ-13-bulk are of similar strength and (ii) the concentration of BAS at the external crystal surface of bulk SSZ-13 is negligible, whereas about 14% of the BAS reside at the external crystal surface in SSZ-13-C16MP. The mechanical stability of SSZ-13-C16MP was judged from the loss in surface area and mesopore volume due to pressing (150 MPa or 300 MPa, 2 min) followed by crushing (Fig. S4, ESI†). The resulting textural properties are also given in Table S1 (ESI†) and show that the mesopores of SSZ-13-C16MP were slightly damaged by the application of increasing pressure. Nevertheless, the loss in mesopore volume (12%) and Langmuir surface area (5%) was very small, showing the excellent mechanical stability of this material. To better understand the way these surfactants interact with the zeolite and compete with the zeolite-forming SDA, we performed molecular simulations using well-established force fields (Table 1 and Table S3, ESI†). The interaction energy of the N-methylpiperidine head group in C16MP with the CHA cage ( 496 kJ mol 1) is comparable with the interaction energy
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Table 1 Interaction energies of structure directing agents and mesoporogens with a SSZ-13 zeolite slab. Templates are given in their cationic form. The cationic charge of template is balanced by deprotonation of an external silanol group
Template
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Interaction energy (kJ mol 1) a
TMAda+
C16N1+
373
478
C16N22+ 1015a
C16MP+ 496
Two ammonium groups of C16N2 placed in two adjacent zeolite cages.
between TMAda+ and the CHA cage ( 373 kJ mol 1). In comparison, the interaction energy of C16N2 with its two quaternary ammonium groups placed in two adjacent cages is much higher ( 1015 kJ mol 1). C16N1 has comparable interaction energy with the zeolite ( 478 kJ mol 1) as C16MP and TMAda+. The reason that the use of C16N1 only results in bulk zeolite is that it can easily leave the cavities of SSZ-13. This implies that an important feature of C16MP is its bulky N-methylpiperidine head group, which is too large to traverse the pore openings of CHA zeolite. As a consequence, C16MP remains entrapped in the zeolite structure. While C16N2 can in principle also leave the micropores, this is prevented by its much stronger interaction with the zeolite. Given our finding that C16MP is more effective in imparting mesoporosity than C16N2, we compared synthesis of SSZ-13C16N2 and SSZ-13-C16MP from gels with a higher Si/Al ratio of 50. Crystallization at lower Al content in the synthesis gel is more difficult and usually results in formation of competing zeolite phases such as ZSM-5. This problem is augmented in the presence of mesoporogens.12 With C16N2 as the mesoporogen ZSM-5 zeolite was formed as the main product (Fig. S8, ESI†). On contrary, the use of the weaker interacting C16MP exclusively results in mesoporous SSZ-13 zeolite at a Si/Al ratio of 50 without any impurities. The same result was obtained when SSZ-13-bulk was prepared with Si/Al = 50. To explore the generality of our approach, we employed C16MP as an additive to a synthesis gel containing tetrapropylammonium hydroxide (TPAOH), 1,6-diaminohexane (DAH) or diethylamine (DEA), which are known SDAs for formation of the important medium-pore zeolite ZSM-5 (MFI topology).13 The combination of TPAOH with C16MP afforded only bulk ZSM-5 (Fig. S12a, ESI†). When DAH was the SDA, a highly mesoporous zeolite with broad XRD reflections of MFI zeolite (Fig. 3 and Fig. S11c, ESI†) was obtained. Evidence for formation
Fig. 3
SEM (a) and TEM image (b) of ZSM-5-C16MP-DAH.
14660 | Chem. Commun., 2014, 50, 14658--14661
of mesoporous ZSM-5 follows from the type IV Ar adsorption isotherm shown in Fig. S13 (ESI†). ZSM-5-C16MP-DAH has a mesopore volume of 0.22 cm3 g 1 (Table S1, ESI†). Although more detailed investigations are needed, the EM image in Fig. 3b show that the C16MP mesoporogen preferentially aligns in the straight channels of ZSM-5, resulting in formation of the sheet-like morphology of ZSM-5 that was also observed using di-quaternary ammonium SDAs developed by Ryoo and co-workers. Furthermore, in combination with DEA the use of C16MP resulted in formation of ZSM-5 with the less structured, yet highly mesoporous nanosponge morphology (Fig. S12d and Table S1, ESI†). The micropore volumes of mesoporous ZSM-5 zeolites are close to that of their bulk counterpart, indicating that they are highly crystalline. Molecular modeling results show that the interaction energies of C16MP and TPA+ with the zeolite framework are comparable ( 1095 kJ mol 1 and 1047 kJ mol 1 respectively, Table S4, ESI†). The N-methylpiperidine head group is located at the intersections of the straight and zig-zag channels of MFI. This is also the preferred location of TPA+. The interaction energies are dominated by electrostatic interactions of the positive charge of the quaternary ammonium group with the zeolite framework. On the contrary, the interaction of the non-charged DAH with the zeolite framework is much weaker because of the absence of quaternary ammonium ions. Whilst TPA+ gets entrapped at the intersections during zeolite synthesis, C16MP can freely move through the channels of MFI zeolite. We speculate that the repulsive electrostatic interactions between C16MP and TPA will lead to preferential occlusion of TPA+ driven by the lower surface energy of the growing zeolite. These repulsive interactions are weaker when C16MP is combined with DAH, so that the mesoporogen effectively competes with DAH and limits zeolite growth. The potential of mesoporous SSZ-13-C16MP in acid catalysis was evaluated by determining its catalytic performance in the MTO reaction. The versatility of the MTO process to produce light olefins derives from the feedstock flexibility for manufacture of syngas, which is used to synthesize methanol.14 The reaction tests were carried out at a temperature of 350 1C and a WHSV of 0.8 h 1. Fig. 4 shows methanol conversion as function of time on stream. The catalyst lifetime is defined as the time to reach 50% of methanol conversion. Initially, all catalysts convert the methanol feed completely. Deactivation of bulk SSZ-13 was rapid and its lifetime was only 5 h. For SSZ-13-C16MP methanol conversion only started decreasing after 9 h. The rate of deactivation was much slower and the lifetime of SSZ-13C16MP was 17 h. The greatly improved performance is attributed to the smaller microporous domains in the hierarchical zeolite, effectively reducing molecular trafficking distances. Catalyst lifetime of SSZ-13-C16MP was also substantially better than for other mesoporous SSZ-13 reported earlier by us.7h The total methanol conversion capacity as defined by Bjørgen et al.15 improved from 6 to 24 gmethanol gcatalyst 1 for SSZ-13bulk and SSZ-13-C16MP (Fig. S14, ESI†). This improvement is ascribed to effective utilization of the zeolite micropores in hierarchical SSZ-13. Fig. 4 also shows that almost similar
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Fig. 4 Methanol conversion as function of time on stream for the MTO reaction for SSZ-13-bulk (m), SSZ-13-C16MP ( ) and SSZ-13-C16MP (5%) ( ) (reaction conditions: WHSV = 0.8 h 1, T = 350 1C).
7
catalytic performance was obtained when a much smaller amount of TMAdaOH (5%) was replaced by C16MP (Ar physisorption and the corresponding textural data are shown in Fig. S3, ESI†). This finding and the much lower cost of C16MP compared to other mesoporogens (C16MP: 133 h kg 1; C16N2: 6363 h kg 1; NDC monoliths:16 748 h kg 1; TPOAC:7j 1468 h kg 1) render our approach promising for the practical use of mesoporous zeolites. Summarizing, hierarchically structured SSZ-13 and ZSM-5 zeolites have been successfully synthesized using the novel mono-quaternary ammonium surfactant as mesoporogen, which combines proper interaction with the zeolite and competition with the conventional SDAs. The resulting zeolite is highly mesoporous and its acid sites are comparable in number and density to those in a reference bulk SSZ-13 zeolite. The main advantage over previous approaches is that this mesoporogen is cheap and is already effective when it replaces a small amount of the conventional zeolite-forming SDA. Thus synthesized mesoporous SSZ-13 exhibits the highest catalytic performance in the MTO reaction reported so far. Our approach should be generally applicable to other zeolite topologies, as shown in this study for the synthesis of hierarchical ZSM-5. XZ acknowledges financial support from the China Scholarship Council. We thank the Cryo-TEM Research Unit of Eindhoven University of Technology for access to TEM facilities.
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