Article pubs.acs.org/Biomac
Nanofibrillar Chitin Aerogels as Renewable Base Catalysts Yoshiyuki Tsutsumi,† Hirotaka Koga,†,‡ Zi-Dong Qi,† Tsuguyuki Saito,† and Akira Isogai*,† †
Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan ‡ The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki Osaka, 567−0047, Japan S Supporting Information *
ABSTRACT: We demonstrate the fabrication of chitin nanofibril aerogels and their successful application as base catalysts for the production of useful chemicals. Squid-pen chitin nanofibrils (ChNF) with primary C2-amine groups on their crystalline surfaces were fabricated into highly porous aerogels with high specific surface areas up to 289 m2 g−1 using freeze-drying or a supercritical drying process. The prepared ChNF aerogel was used in the aqueous Knoevenagelcondensation reaction and acted as a highly efficient base catalyst, suggesting that the combination of the nanofibrous aerogel structure and primary C2-amines exposed on the crystalline ChNF surface was effective for continuous flow catalysis. Because the ChNF aerogel can be easily prepared from abundant and renewable chitin present in nature, this strategy is a gateway to promoting and conducting green and sustainable chemistry.
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INTRODUCTION The environment-friendly conversion technologies of various chemicals using catalysts have played a key role in a wide range of industrial processes such as useful chemicals production, environment purification, and energy conversion.1 Recently, metal nanoparticles have received much interest as highly active catalysts owing to their large surface area-to-volume ratio.1 However, metal nanocatalysts are prone to aggregation, thus leading to reduced specific surface area (SSA) values, and subsequent reduced catalytic activities. To circumvent this issue, they have generally been immobilized on supporting materials such as silica,2 alumina,3 and polymers.4 However, one of the present challenges is the structural design of supporting materials capable of providing effective access of the reactants to the catalysts.5 Aerogels are promising catalyst supports because they have low densities, large inner surface areas, and interconnected porous structures. These characteristics can not only provide high catalytic reaction efficiencies, but are also applicable to flow processes employed in continuous synthesis of target products.6,7 Recent trends toward sustainable development demonstrate the importance of active and effective use of renewable biopolymers as catalyst supports.8,9 In previous studies, highly crystalline nanofibrils were extracted from cellulose,10 which is the most abundant biopolymer in nature, and were applied as supporting materials for metal nanocatalysts.11,12 The highly dispersed and exposed metal nanocatalysts on the crystalline surface of the cellulose nanofibrils allowed effective contact with the reactants and efficient catalytic reactions. Moreover, the metal-decorated cellulose nanofibrils were successfully fabricated into aerogels by a simple freeze-drying treatment, resulting in good catalytic efficiency and reusability.11,12 Despite © XXXX American Chemical Society
these achievements, the limited availability of metal resources has led to growing requirements for reducing dependence on metal catalysts and for developing catalytic material substituents using sustainable chemistry. Besides cellulose, chitin is another abundant biopolymer present as highly crystalline nanofibrils in nature. It can be obtained in large quantities from food waste, including the exoskeletons of crabs, lobsters, and shrimps, and the internal shells of squids. In our previous report, individual chitin nanofibrils (ChNF) with widths of 3−4 nm and aspect ratios of more than 500 were successfully extracted from squid-pen by simple mechanical treatment under mild acidic conditions at pH 3−4.13 ChNFs with large SSAs are considered to have functional primary C2-amine groups densely distributed on the crystalline fibril surface,13−17 and can be developed into aerogels.18−20 These features would provide great potential applicability to the fabrication of green, sustainable, and effective base catalysts directly derived from natural bioresources. It has been reported that primary C2-amines act as base catalysts in various reactions such as the Knoevenagel condensation reaction.21−35 Here we show the fabrication and direct use of naturally derived ChNF aerogels as base catalysts for effective production of useful chemicals. Crystalline ChNF were fabricated into highly porous aerogels with large SSAs by either freeze-drying or supercritical drying method, and were then examined as catalysts in a typical Knoevenagel condensation reaction at room temperature. The ChNF aerogels demonstrated an Received: September 5, 2014 Revised: October 6, 2014
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Figure 1. Photographs of (a) ChNF hydrogel and (b) AG1. Cross-sectional SEM images of (c,d) AG1, (e,f) AG2, and (g,h) AG3. (i) Nitrogen adsorption−desorption isotherms of the ChNF aerogels.
excellent catalytic performance in a continuous flow reaction system, a breakthrough in green and sustainable catalytic processes.
NaOH solution (2 M) using a 5 mL syringe to prepare hydrogel beads. The beads were stored in the alkaline solution for 2 h, then recovered and washed with distilled water. Hydrogel beads were directly freeze-dried to prepare chitosan aerogel beads (diameter of ∼3 mm, density of 0.04 g cm−3). An aerogel sample with no amine groups but a SSA comparable with that of AG2 was fabricated using cellulose as a control. Cellulose single nanofibrils, with a similar morphology to that of ChNF, were prepared according to our previous report.13 Cellulose nanofibril aerogels were prepared from 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized wood cellulose nanofibrils by freeze-drying of their alcogels with tbutyl alcohol, according to our previous paper.37 Characterization. Scanning electron microscopy (SEM) observations of the aerogel cross sections were performed with a Hitachi S-4800 field-emission scanning electron microscope (Tokyo, Japan), operating at 1.0 kV. Prior to SEM observation, the samples were coated with osmium using a Meiwafosis Neo osmium coater. The SSA of the aerogels was determined from nitrogen adsorption measurements using a Quantachrome NOVA 4200e (Kanagawa, Japan). Catalytic Performance Test. Knoevenagel condensation reaction was carried out at 23 °C in a flow mode. The reactants, benzaldehyde (1.0 mmol) and ethyl cyanoacetate (0.85 mmol), and an internal standard for gas chromatography (GC), ntetradecane (0.1 mmol), were dissolved in an ethanol/water mixture (5 mL; 9/1 v/v). The solution was then injected into a 5 mL syringe (Terumo SS-05LZ, internal diameter of 13 mm), securely loaded with the ChNF aerogel (15 mg, 3 cm3, amine content of 7.5 μmol) at a constant flow rate between 0.01 and 0.06 mL min−1 using a syringe pump (Econoflo, Harvard Apparatus, Physio-Tech Co., Ltd., Tokyo, Japan). Chitosan aerogel beads and cellulose nanofibril aerogel were also investigated as control samples. The reaction solution was analyzed to determine the concentration of the target product, α-cyanocinnamic acid ethyl ester, using GC equipped with a
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EXPERIMENTAL SECTION Materials. All the reagents, including benzaldehyde and ethyl cyanoacetate, were of laboratory grade and purchased from Wako Pure Chemicals, Ltd., Tokyo, Japan, and used without further purification. Chitin [degree of N-acetylation (DNAc) of 0.90, crystallite size of 3.1 nm, Figure S1a and b in the Supporting Information] was obtained from squid-pen (Sepioteuthis lessoniana) according to our previous report.13 Preparation of ChNF Aerogels. An aqueous dispersion of ChNF (0.05% w/w in dilute acetic acid at pH 3.5) with an optical transmittance of ∼90% at a wavelength of 600 nm was obtained via the same procedure as that reported in a previous paper (Figure S1c in the Supporting Information).13 The dispersion pH was adjusted to 10−11 by adding sodium hydroxide to produce a hydrogel. Subsequently, the ChNF hydrogel was redispersed in distilled water by thorough shaking and then recovered by centrifugation at 12,000 G for 20 min. This desalting procedure was repeated three times. For preparation of aerogel AG1, the resulting hydrogel (∼0.5% w/w) was fabricated into an aerogel directly by freeze-drying. For preparation of aerogel AG2, the ChNF hydrogels first underwent solvent exchange from water ×2 to water/ethanol (1/1, v/v) × 1, ethanol ×2, and ethanol/tert-butyl alcohol (1/1, v/v.) × 1 then to tert-butyl alcohol ×2. The resulting alcogel was freeze-dried to obtain ChNF aerogel AG2. Aerogel AG3 was obtained following supercritical carbon dioxide (CO2) drying (40 °C, 100 atm, 1 h) of the ChNF alcogel (in ethanol) using a JEOL JCPD-5 supercritical drying apparatus (Tokyo, Japan). Preparation of Control Samples. Chitosan powder (0.5 g, Mw = 416 000, DNAc = 0.13)36 (Chitosan-100, Wako Pure Chemicals Co., Tokyo, Japan) was dissolved in acetic acid (40 mL, 0.05 M). The solution was added dropwise to aqueous B
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flame ionization detector and a TC-1 column (0.25 mm × 30 m, GL Sciences Inc., Tokyo, Japan). To investigate the recyclability of the aerogel catalyst, a fivecycle test was performed on AG1, as follows. Aerogels were recovered from the syringe after the reaction, washed successively with ethanol and distilled water, followed by dispersion in dilute acetic acid. The ChNF dispersion obtained was refabricated into an aerogel, using the same process as mentioned previously, then subjected to the catalytic performance test in the flow system. This procedure was repeated five times.
physisorption.38 Such a hysteresis loop is observed in typical mesoporous materials such as mesoporous silica. Owing to the homogeneously distributed nanofibrillar networks (Figure 1f), AG2 exhibited a higher SSA (212 m2 g−1) than that of AG1 (24 m2 g−1). These results indicate that the use of tert-butyl alcohol with a low surface tension allowed the preparation of high-SSA aerogels following its removal from the alcogel while maintaining homogeneously distributed nanofibrillar network structures.18−20 Supercritical CO2 drying of the ChNF alcogel resulted in further improvement in the SSA of the aerogel. Theoretically, aggregation of ChNF, owing to the surface tension of solvent during conventional drying process, can be avoided in supercritical drying because the liquid−gas transition of the solvents is accomplished without crossing any phase boundaries. As shown in Figure 1g and h, the as-prepared ChNF aerogel, AG3, featured a reduced aggregated nanofibrillar structure when compared with that of AG2, resulting in a higher SSA (289 m2 g−1). Thus, crystalline ChNF aerogel with a high SSA was successfully prepared using successive solvent exchange and supercritical drying. Nevertheless, almost no difference in visual appearance was observed between AG1 (Figure 1b), AG2, and AG3. Catalytic Performance of the ChNF Aerogels. The Knoevenagel condensation between aldehydes or ketones and active methylene compounds has been one of the most important and convenient reactions for C−C bond formation.39 In this study, the catalytic performance of the ChNF aerogel was evaluated toward the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate to produce α-cyanocinnamic acid ethyl ester (Figure 2a), which is an important intermediate in medicine production. The reaction was carried out at 23 °C in a simple flow system (Figure 2b).
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RESULTS AND DISCUSSION Structural Design of ChNF Aerogels. ChNF formed a stable dispersion in mild acidic water owing to repulsion between the fibrils upon ionization of the surface C2-amine groups (Figure S1c in the Supporting Information). When the pH value of the ChNF dispersion was adjusted to 10−11, the ChNF dispersion immediately turned into a freestanding hydrogel even at a low concentration of 0.5% w/w; the transparency of the original dispersion was retained (Figure 1a). The onset of gelation was due to the elimination of repulsive interactions between the ChNF elements under basic conditions, and the partial assembly of nonionized ChNF elements, subsequently generating physical hydrogels. The hydrogel was then fabricated into an aerogel with a low density of 0.0048 g cm−3 by freeze-drying (Figure 1b). X-ray diffraction (XRD) analysis of the as-prepared aerogel, AG1, showed that the crystal structure of ChNF remained unchanged even after aerogel fabrication (Figure S1a in the Supporting Information). As shown in the cross-sectional SEM images (Figure 1c), the crystalline AG1 featured a polygonal macroporous structures with pore sizes ranging between several and several tens of micrometers owing to ice crystallization during the freezing process in liquid nitrogen.18 Although fine networks of ChNF could be observed inside the macropores (Figure 1d), most of the nanofibrils were densely embedded in the walls of the macropores. As observed in Figure 1i, AG1 featured at Type II nitrogen adsorption−desorption isotherm that is consistent with its macroporous nature. The SSA of AG1 calculated according to the Brunauer−Emmett−Teller (BET) theory was 24 m2 g−1. Based on the width of the ChNF (3.1 nm), obtained from XRD analysis, the calculated upper limit of the SSA of the ChNF aerogels was ∼900 m2 g−1 using a matchstick model, which is considerably higher than the measured SSA of AG1. The low observed SSA of AG1 was possibly because of the formation of a large amount of bundles in the macropore walls of ChNF. Nevertheless, a crystalline ChNF aerogel was successfully prepared by simple freeze-drying of the corresponding hydrogel, with a SSA that was significantly lower than the theoretical SSA value. For improvement of the SSA of the ChNF aerogels, the hydrogels were solvent exchanged from water to solvents with lower surface tension values, and the resulting aerogels were investigated. Figure 1e,f show the cross-sectional SEM images of the ChNF aerogel, AG2, prepared by freeze-drying of the alcogel filled with tert-butyl alcohol. Macropores observed in AG1 were no longer present in AG2; AG2 possessed a mesoporous structure derived from the ChNF networks and some localized bundles and aggregation. As shown in Figure 1i, the nitrogen adsorption−desorption isotherm of AG2 showed hysteresis loops within the high relative pressure region indicating a Type IV pattern according to the IUPAC classification of
Figure 2. (a) Scheme of the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate in the presence of primary amines, and (b) experimental setup in flow mode.
Figure 3 shows the yields of the target product obtained using different aerogel catalysts at a flow rate of 0.04 mL min−1. The reaction proceeded in the absence of a catalysts; however, the yield of the product was low (12%). In the presence of AG1, the yield increased to 30%. In the presence of ChNF aerogels with higher SSAs, the product yield significantly improved to 43 and 54%, for AG2 and AG3, respectively. The cellulose aerogel, which has no base groups, however, similar structure and SSA (227 m2 g−1) to those of AG2 (212 m2 g−1) (Figure S2 in Supporting Information), showed poor performance comparable with that obtained in the aerogel-free system. Neither volume shrinkage nor expansion was visually observed for ChNF or C
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Figure 3. Yields of α-cyanocinnamic acid ethyl ester obtained in the absence of catalyst (catalyst-free system) and the presence of different aerogel catalysts using a flow rate of 0.04 mL min−1. The amino group contents were 7.5 μmol in both the ChNF aerogels and chitosan aerogel beads. Cellulose aerogel does not contain any amine groups.
cellulose aerogels during the reaction with a 10% v/v aqueous ethanol used as the flow solvent. These results demonstrated that the C2-amine residues of ChNF served as base catalysts in the Knoevenagel reaction. Furthermore, the catalytic performance of the ChNF aerogels is expected to be closely related to their surface areasincreasing amounts of primary C2-amines are expected to be more accessible to the reactants in aerogels with larger SSAs. It should be noted that all ChNF aerogels exhibited higher catalytic efficiencies than that of the chitosan aerogel, although the latter had a higher SSA (93 m2 g−1) than that of AG1 (24 m2 g−1) (Figure S2 in the Supporting Information). In each case study, the C2-amine group contents were set at 7.5 μmol. Thus, the actual amount of accessible primary amines in the ChNF aerogels is larger than that in the chitosan aerogel owing to the surface-concentrated distribution of primary C2-amines on the crystalline surface of ChNF.13−17 Based on these results, the crystalline ChNF aerogel catalysts improved the efficiency of the flow-mode Knoevenagel condensation reaction. The extent of such improvement was enhanced by the structural design of the ChNF aerogels, i.e., the increase in SSA. Figure 4a shows the yields of the target product obtained in the absence catalyst and the presence of AG1 and AG2 as a function of flow rate. Here, AG1 and AG2 were used in the experiment, because they were easer in preparation than AG3, for which the supercritical CO2 drying of the ChNF alcogel was required. In all cases, the product yield increased with decreasing flow rates owing to an increase in the contact time between the aerogel and reactants. In the absence of catalyst, the product yield was very low: 19.8% and 5.9% at 0.01 and 0.06 mL min−1, respectively. The yield was improved by nearly a factor of 3 in the presence of AG1:57.9 and 14.9% at 0.01 and 0.06 mL min−1, respectively. The use of AG2 resulted in further improvement of the reaction efficiency. The product yield reached ∼100% when the flow rate was reduced to 0.01 mL min−1. The markedly improved product yield at low flow rates suggests that the mesoporous network structures of the high-SSA ChNF aerogels are particularly effective in low-flow rate reactions. However, for ensuring high productivity, a balance between the product yield and reaction time should be taken into account. Figure 4b shows changes in the productivity as a
Figure 4. (a) Yields of the Knoevenagel reaction in the absence of catalyst or in the presence of different ChNF aerogels, as functions of flow rate. (b) Productivities of AG1 and AG2 as functions of flow rate. Amino group contents were 7.5 μmol in AG1 and AG2.
function of flow rate. The productivity (h−1) is defined as the yield of the product (mol) per hour per mole of amine groups. As observed, AG2 showed higher productivity than AG1 under all examined flow rates. When either AG1 or AG2 was used as the catalyst, the productivity increased as the flow rate increased up to 0.03 mL min−1. When the flow rate was higher than 0.04 mL min−1, the productivity measured in the presence of either AG1 or AG2 decreased. The use of AG2 resulted in the highest productivity of 22 h−1, that is, ∼1.3 times as much as that of AG1 within a flow rate range of 0.03−0.04 mL min−1. To our knowledge, the latter value obtained is higher than those obtained in the presence of heterogeneous base catalysts using identical model reactions, although the conditions differ to some extent.40−44 The recyclability of the ChNF aerogels as catalysts was confirmed as evidence by their sustained catalytic performance, as discussed. After AG1 was used in the reaction, it was redispersed in dilute acetic acid, and then refabricated into an aerogel. As shown in Figure 5, the recycled AG1 maintained its catalytic performance even when the aerogel was recycled four times. This result showed that the crystal structure and surface C2-amine groups of ChNF was not damaged or modified during the reaction. D
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Figure 5. Catalytic performance of the recycled AG1 at a flow rate of 0.04 mL min−1.
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CONCLUSIONS We have demonstrated the fabrication of high-SSA aerogels, from naturally derived crystalline chitin, and their high catalytic performance in the Knoevenagel condensation reaction. Both the mesoporous aerogel structure and the surface-exposed C2amines on the crystalline ChNF surface were effective for use in continuous flow catalysis. Chitin is stable to most solvents and has both hydrophilic and lipophilic properties. These specific features are desirable attributes in a wide range of catalytic reactions. Thus, the naturally derived ChNF aerogels are promising renewable and recyclable catalytic materials for realizing a green and sustainable chemistry.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray diffraction patterns of squid pen chitin and ChNF, DNAc determination of ChNF, light transmittance of ChNF dispersion, and SEM images and nitrogen adsorption/ desorption isotherms of cellulose and chitosan aerogels are given in the Supporting Information file. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +81 3 5841 5538; fax: +81 3 5841 5269; E-mail: aisogai@ mail.ecc.u-tokyo.ac.jp. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Grants-in-Aid for Scientific Research (Grant Nos. 21228007 and 23688020) from the Japan Society for the Promotion of Science (JSPS), and by the Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Agency (JST). We thank Dainichiseika Color & Chemicals Mfg. Co. Ltd., Osaka, Japan, for providing the squid pen specimen.
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Nanofibrillar Chitin Aerogels as Renewable Base Catalysts Yoshiyuki Tsutsumi,† Hirotaka Koga,†,‡ Zi-Dong Qi,† Tsuguyuki Saito,† and Akira Isogai*,† †
Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan ‡ The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki Osaka, 567−0047, Japan S Supporting Information *
ABSTRACT: We demonstrate the fabrication of chitin nanofibril aerogels and their successful application as base catalysts for the production of useful chemicals. Squid-pen chitin nanofibrils (ChNF) with primary C2-amine groups on their crystalline surfaces were fabricated into highly porous aerogels with high specific surface areas up to 289 m2 g−1 using freeze-drying or a supercritical drying process. The prepared ChNF aerogel was used in the aqueous Knoevenagelcondensation reaction and acted as a highly efficient base catalyst, suggesting that the combination of the nanofibrous aerogel structure and primary C2-amines exposed on the crystalline ChNF surface was effective for continuous flow catalysis. Because the ChNF aerogel can be easily prepared from abundant and renewable chitin present in nature, this strategy is a gateway to promoting and conducting green and sustainable chemistry.
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INTRODUCTION The environment-friendly conversion technologies of various chemicals using catalysts have played a key role in a wide range of industrial processes such as useful chemicals production, environment purification, and energy conversion.1 Recently, metal nanoparticles have received much interest as highly active catalysts owing to their large surface area-to-volume ratio.1 However, metal nanocatalysts are prone to aggregation, thus leading to reduced specific surface area (SSA) values, and subsequent reduced catalytic activities. To circumvent this issue, they have generally been immobilized on supporting materials such as silica,2 alumina,3 and polymers.4 However, one of the present challenges is the structural design of supporting materials capable of providing effective access of the reactants to the catalysts.5 Aerogels are promising catalyst supports because they have low densities, large inner surface areas, and interconnected porous structures. These characteristics can not only provide high catalytic reaction efficiencies, but are also applicable to flow processes employed in continuous synthesis of target products.6,7 Recent trends toward sustainable development demonstrate the importance of active and effective use of renewable biopolymers as catalyst supports.8,9 In previous studies, highly crystalline nanofibrils were extracted from cellulose,10 which is the most abundant biopolymer in nature, and were applied as supporting materials for metal nanocatalysts.11,12 The highly dispersed and exposed metal nanocatalysts on the crystalline surface of the cellulose nanofibrils allowed effective contact with the reactants and efficient catalytic reactions. Moreover, the metal-decorated cellulose nanofibrils were successfully fabricated into aerogels by a simple freeze-drying treatment, resulting in good catalytic efficiency and reusability.11,12 Despite © XXXX American Chemical Society
these achievements, the limited availability of metal resources has led to growing requirements for reducing dependence on metal catalysts and for developing catalytic material substituents using sustainable chemistry. Besides cellulose, chitin is another abundant biopolymer present as highly crystalline nanofibrils in nature. It can be obtained in large quantities from food waste, including the exoskeletons of crabs, lobsters, and shrimps, and the internal shells of squids. In our previous report, individual chitin nanofibrils (ChNF) with widths of 3−4 nm and aspect ratios of more than 500 were successfully extracted from squid-pen by simple mechanical treatment under mild acidic conditions at pH 3−4.13 ChNFs with large SSAs are considered to have functional primary C2-amine groups densely distributed on the crystalline fibril surface,13−17 and can be developed into aerogels.18−20 These features would provide great potential applicability to the fabrication of green, sustainable, and effective base catalysts directly derived from natural bioresources. It has been reported that primary C2-amines act as base catalysts in various reactions such as the Knoevenagel condensation reaction.21−35 Here we show the fabrication and direct use of naturally derived ChNF aerogels as base catalysts for effective production of useful chemicals. Crystalline ChNF were fabricated into highly porous aerogels with large SSAs by either freeze-drying or supercritical drying method, and were then examined as catalysts in a typical Knoevenagel condensation reaction at room temperature. The ChNF aerogels demonstrated an Received: September 5, 2014 Revised: October 6, 2014
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Figure 1. Photographs of (a) ChNF hydrogel and (b) AG1. Cross-sectional SEM images of (c,d) AG1, (e,f) AG2, and (g,h) AG3. (i) Nitrogen adsorption−desorption isotherms of the ChNF aerogels.
excellent catalytic performance in a continuous flow reaction system, a breakthrough in green and sustainable catalytic processes.
NaOH solution (2 M) using a 5 mL syringe to prepare hydrogel beads. The beads were stored in the alkaline solution for 2 h, then recovered and washed with distilled water. Hydrogel beads were directly freeze-dried to prepare chitosan aerogel beads (diameter of ∼3 mm, density of 0.04 g cm−3). An aerogel sample with no amine groups but a SSA comparable with that of AG2 was fabricated using cellulose as a control. Cellulose single nanofibrils, with a similar morphology to that of ChNF, were prepared according to our previous report.13 Cellulose nanofibril aerogels were prepared from 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized wood cellulose nanofibrils by freeze-drying of their alcogels with tbutyl alcohol, according to our previous paper.37 Characterization. Scanning electron microscopy (SEM) observations of the aerogel cross sections were performed with a Hitachi S-4800 field-emission scanning electron microscope (Tokyo, Japan), operating at 1.0 kV. Prior to SEM observation, the samples were coated with osmium using a Meiwafosis Neo osmium coater. The SSA of the aerogels was determined from nitrogen adsorption measurements using a Quantachrome NOVA 4200e (Kanagawa, Japan). Catalytic Performance Test. Knoevenagel condensation reaction was carried out at 23 °C in a flow mode. The reactants, benzaldehyde (1.0 mmol) and ethyl cyanoacetate (0.85 mmol), and an internal standard for gas chromatography (GC), ntetradecane (0.1 mmol), were dissolved in an ethanol/water mixture (5 mL; 9/1 v/v). The solution was then injected into a 5 mL syringe (Terumo SS-05LZ, internal diameter of 13 mm), securely loaded with the ChNF aerogel (15 mg, 3 cm3, amine content of 7.5 μmol) at a constant flow rate between 0.01 and 0.06 mL min−1 using a syringe pump (Econoflo, Harvard Apparatus, Physio-Tech Co., Ltd., Tokyo, Japan). Chitosan aerogel beads and cellulose nanofibril aerogel were also investigated as control samples. The reaction solution was analyzed to determine the concentration of the target product, α-cyanocinnamic acid ethyl ester, using GC equipped with a
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EXPERIMENTAL SECTION Materials. All the reagents, including benzaldehyde and ethyl cyanoacetate, were of laboratory grade and purchased from Wako Pure Chemicals, Ltd., Tokyo, Japan, and used without further purification. Chitin [degree of N-acetylation (DNAc) of 0.90, crystallite size of 3.1 nm, Figure S1a and b in the Supporting Information] was obtained from squid-pen (Sepioteuthis lessoniana) according to our previous report.13 Preparation of ChNF Aerogels. An aqueous dispersion of ChNF (0.05% w/w in dilute acetic acid at pH 3.5) with an optical transmittance of ∼90% at a wavelength of 600 nm was obtained via the same procedure as that reported in a previous paper (Figure S1c in the Supporting Information).13 The dispersion pH was adjusted to 10−11 by adding sodium hydroxide to produce a hydrogel. Subsequently, the ChNF hydrogel was redispersed in distilled water by thorough shaking and then recovered by centrifugation at 12,000 G for 20 min. This desalting procedure was repeated three times. For preparation of aerogel AG1, the resulting hydrogel (∼0.5% w/w) was fabricated into an aerogel directly by freeze-drying. For preparation of aerogel AG2, the ChNF hydrogels first underwent solvent exchange from water ×2 to water/ethanol (1/1, v/v) × 1, ethanol ×2, and ethanol/tert-butyl alcohol (1/1, v/v.) × 1 then to tert-butyl alcohol ×2. The resulting alcogel was freeze-dried to obtain ChNF aerogel AG2. Aerogel AG3 was obtained following supercritical carbon dioxide (CO2) drying (40 °C, 100 atm, 1 h) of the ChNF alcogel (in ethanol) using a JEOL JCPD-5 supercritical drying apparatus (Tokyo, Japan). Preparation of Control Samples. Chitosan powder (0.5 g, Mw = 416 000, DNAc = 0.13)36 (Chitosan-100, Wako Pure Chemicals Co., Tokyo, Japan) was dissolved in acetic acid (40 mL, 0.05 M). The solution was added dropwise to aqueous B
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flame ionization detector and a TC-1 column (0.25 mm × 30 m, GL Sciences Inc., Tokyo, Japan). To investigate the recyclability of the aerogel catalyst, a fivecycle test was performed on AG1, as follows. Aerogels were recovered from the syringe after the reaction, washed successively with ethanol and distilled water, followed by dispersion in dilute acetic acid. The ChNF dispersion obtained was refabricated into an aerogel, using the same process as mentioned previously, then subjected to the catalytic performance test in the flow system. This procedure was repeated five times.
physisorption.38 Such a hysteresis loop is observed in typical mesoporous materials such as mesoporous silica. Owing to the homogeneously distributed nanofibrillar networks (Figure 1f), AG2 exhibited a higher SSA (212 m2 g−1) than that of AG1 (24 m2 g−1). These results indicate that the use of tert-butyl alcohol with a low surface tension allowed the preparation of high-SSA aerogels following its removal from the alcogel while maintaining homogeneously distributed nanofibrillar network structures.18−20 Supercritical CO2 drying of the ChNF alcogel resulted in further improvement in the SSA of the aerogel. Theoretically, aggregation of ChNF, owing to the surface tension of solvent during conventional drying process, can be avoided in supercritical drying because the liquid−gas transition of the solvents is accomplished without crossing any phase boundaries. As shown in Figure 1g and h, the as-prepared ChNF aerogel, AG3, featured a reduced aggregated nanofibrillar structure when compared with that of AG2, resulting in a higher SSA (289 m2 g−1). Thus, crystalline ChNF aerogel with a high SSA was successfully prepared using successive solvent exchange and supercritical drying. Nevertheless, almost no difference in visual appearance was observed between AG1 (Figure 1b), AG2, and AG3. Catalytic Performance of the ChNF Aerogels. The Knoevenagel condensation between aldehydes or ketones and active methylene compounds has been one of the most important and convenient reactions for C−C bond formation.39 In this study, the catalytic performance of the ChNF aerogel was evaluated toward the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate to produce α-cyanocinnamic acid ethyl ester (Figure 2a), which is an important intermediate in medicine production. The reaction was carried out at 23 °C in a simple flow system (Figure 2b).
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RESULTS AND DISCUSSION Structural Design of ChNF Aerogels. ChNF formed a stable dispersion in mild acidic water owing to repulsion between the fibrils upon ionization of the surface C2-amine groups (Figure S1c in the Supporting Information). When the pH value of the ChNF dispersion was adjusted to 10−11, the ChNF dispersion immediately turned into a freestanding hydrogel even at a low concentration of 0.5% w/w; the transparency of the original dispersion was retained (Figure 1a). The onset of gelation was due to the elimination of repulsive interactions between the ChNF elements under basic conditions, and the partial assembly of nonionized ChNF elements, subsequently generating physical hydrogels. The hydrogel was then fabricated into an aerogel with a low density of 0.0048 g cm−3 by freeze-drying (Figure 1b). X-ray diffraction (XRD) analysis of the as-prepared aerogel, AG1, showed that the crystal structure of ChNF remained unchanged even after aerogel fabrication (Figure S1a in the Supporting Information). As shown in the cross-sectional SEM images (Figure 1c), the crystalline AG1 featured a polygonal macroporous structures with pore sizes ranging between several and several tens of micrometers owing to ice crystallization during the freezing process in liquid nitrogen.18 Although fine networks of ChNF could be observed inside the macropores (Figure 1d), most of the nanofibrils were densely embedded in the walls of the macropores. As observed in Figure 1i, AG1 featured at Type II nitrogen adsorption−desorption isotherm that is consistent with its macroporous nature. The SSA of AG1 calculated according to the Brunauer−Emmett−Teller (BET) theory was 24 m2 g−1. Based on the width of the ChNF (3.1 nm), obtained from XRD analysis, the calculated upper limit of the SSA of the ChNF aerogels was ∼900 m2 g−1 using a matchstick model, which is considerably higher than the measured SSA of AG1. The low observed SSA of AG1 was possibly because of the formation of a large amount of bundles in the macropore walls of ChNF. Nevertheless, a crystalline ChNF aerogel was successfully prepared by simple freeze-drying of the corresponding hydrogel, with a SSA that was significantly lower than the theoretical SSA value. For improvement of the SSA of the ChNF aerogels, the hydrogels were solvent exchanged from water to solvents with lower surface tension values, and the resulting aerogels were investigated. Figure 1e,f show the cross-sectional SEM images of the ChNF aerogel, AG2, prepared by freeze-drying of the alcogel filled with tert-butyl alcohol. Macropores observed in AG1 were no longer present in AG2; AG2 possessed a mesoporous structure derived from the ChNF networks and some localized bundles and aggregation. As shown in Figure 1i, the nitrogen adsorption−desorption isotherm of AG2 showed hysteresis loops within the high relative pressure region indicating a Type IV pattern according to the IUPAC classification of
Figure 2. (a) Scheme of the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate in the presence of primary amines, and (b) experimental setup in flow mode.
Figure 3 shows the yields of the target product obtained using different aerogel catalysts at a flow rate of 0.04 mL min−1. The reaction proceeded in the absence of a catalysts; however, the yield of the product was low (12%). In the presence of AG1, the yield increased to 30%. In the presence of ChNF aerogels with higher SSAs, the product yield significantly improved to 43 and 54%, for AG2 and AG3, respectively. The cellulose aerogel, which has no base groups, however, similar structure and SSA (227 m2 g−1) to those of AG2 (212 m2 g−1) (Figure S2 in Supporting Information), showed poor performance comparable with that obtained in the aerogel-free system. Neither volume shrinkage nor expansion was visually observed for ChNF or C
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Figure 3. Yields of α-cyanocinnamic acid ethyl ester obtained in the absence of catalyst (catalyst-free system) and the presence of different aerogel catalysts using a flow rate of 0.04 mL min−1. The amino group contents were 7.5 μmol in both the ChNF aerogels and chitosan aerogel beads. Cellulose aerogel does not contain any amine groups.
cellulose aerogels during the reaction with a 10% v/v aqueous ethanol used as the flow solvent. These results demonstrated that the C2-amine residues of ChNF served as base catalysts in the Knoevenagel reaction. Furthermore, the catalytic performance of the ChNF aerogels is expected to be closely related to their surface areasincreasing amounts of primary C2-amines are expected to be more accessible to the reactants in aerogels with larger SSAs. It should be noted that all ChNF aerogels exhibited higher catalytic efficiencies than that of the chitosan aerogel, although the latter had a higher SSA (93 m2 g−1) than that of AG1 (24 m2 g−1) (Figure S2 in the Supporting Information). In each case study, the C2-amine group contents were set at 7.5 μmol. Thus, the actual amount of accessible primary amines in the ChNF aerogels is larger than that in the chitosan aerogel owing to the surface-concentrated distribution of primary C2-amines on the crystalline surface of ChNF.13−17 Based on these results, the crystalline ChNF aerogel catalysts improved the efficiency of the flow-mode Knoevenagel condensation reaction. The extent of such improvement was enhanced by the structural design of the ChNF aerogels, i.e., the increase in SSA. Figure 4a shows the yields of the target product obtained in the absence catalyst and the presence of AG1 and AG2 as a function of flow rate. Here, AG1 and AG2 were used in the experiment, because they were easer in preparation than AG3, for which the supercritical CO2 drying of the ChNF alcogel was required. In all cases, the product yield increased with decreasing flow rates owing to an increase in the contact time between the aerogel and reactants. In the absence of catalyst, the product yield was very low: 19.8% and 5.9% at 0.01 and 0.06 mL min−1, respectively. The yield was improved by nearly a factor of 3 in the presence of AG1:57.9 and 14.9% at 0.01 and 0.06 mL min−1, respectively. The use of AG2 resulted in further improvement of the reaction efficiency. The product yield reached ∼100% when the flow rate was reduced to 0.01 mL min−1. The markedly improved product yield at low flow rates suggests that the mesoporous network structures of the high-SSA ChNF aerogels are particularly effective in low-flow rate reactions. However, for ensuring high productivity, a balance between the product yield and reaction time should be taken into account. Figure 4b shows changes in the productivity as a
Figure 4. (a) Yields of the Knoevenagel reaction in the absence of catalyst or in the presence of different ChNF aerogels, as functions of flow rate. (b) Productivities of AG1 and AG2 as functions of flow rate. Amino group contents were 7.5 μmol in AG1 and AG2.
function of flow rate. The productivity (h−1) is defined as the yield of the product (mol) per hour per mole of amine groups. As observed, AG2 showed higher productivity than AG1 under all examined flow rates. When either AG1 or AG2 was used as the catalyst, the productivity increased as the flow rate increased up to 0.03 mL min−1. When the flow rate was higher than 0.04 mL min−1, the productivity measured in the presence of either AG1 or AG2 decreased. The use of AG2 resulted in the highest productivity of 22 h−1, that is, ∼1.3 times as much as that of AG1 within a flow rate range of 0.03−0.04 mL min−1. To our knowledge, the latter value obtained is higher than those obtained in the presence of heterogeneous base catalysts using identical model reactions, although the conditions differ to some extent.40−44 The recyclability of the ChNF aerogels as catalysts was confirmed as evidence by their sustained catalytic performance, as discussed. After AG1 was used in the reaction, it was redispersed in dilute acetic acid, and then refabricated into an aerogel. As shown in Figure 5, the recycled AG1 maintained its catalytic performance even when the aerogel was recycled four times. This result showed that the crystal structure and surface C2-amine groups of ChNF was not damaged or modified during the reaction. D
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(3) Katayama, T.; Kamata, K.; Yamaguchi, K.; Mizuno, N. A supported copper hydroxide as an efficient, ligand-free, and heterogeneous precatalyst for 1,3-dipolar cycloadditions of organic zzides to terminal alkynes. ChemSusChem 2009, 2, 59−62. (4) Ishida, T.; Haruta, M. Gold catalysts: towards sustainable chemistry. Angew. Chem., Int. Ed. 2007, 46, 7154−7156. (5) Kuroda, K.; Ishida, T.; Haruta, M. Reduction of 4-nitrophenol to 4-aminophenol over Au nanoparticles deposited on PMMA. J. Mol. Catal. A: Chem. 2009, 298, 7−11. (6) Tai, Y.; Murakami, J.; Tajiri, K.; Ohashi, F.; Date, M.; Tsubota, S. Oxidation of carbon monoxide on Au nanoparticles in titania and titania-coated silica aerogels. Appl. Catal. A: General 2004, 268, 183− 187. (7) King, J. S.; Wittstock, A.; Biener, J.; Kucheyev, S. O.; Wang, Y. M.; Baumann, T. F.; Giri, S. K.; Hamza, A. V.; Baeumer, M.; Bent, S. F. Ultralow loading Pt nanocatalysts prepared by atomic layer deposition on carbon aerogels. Nano Lett. 2008, 8, 2405−2409. (8) Guibal, E. Heterogeneous catalysis on chitosan-based materials: A review. Prog. Polym. Sci. 2005, 30, 71−109. (9) Macquarrie, D. J.; Hardy, J. J. E. Applications of functionalized chitosan in catalysis. Ind. Eng. Chem. Res. 2005, 44, 8499−8520. (10) Isogai, A.; Saito, T.; Fukuzumi, H. TEMPO-oxidized cellulose nanofibers. Nanoscale 2011, 3, 71−85. (11) Koga, H.; Tokunaga, E.; Hidaka, M.; Umemura, Y.; Saito, T.; Isogai, A.; Kitaoka, T. Topochemical synthesis and catalysis of metal nanoparticles exposed on crystalline cellulose nanofibers. Chem. Commun. 2010, 46, 8567−8569. (12) Koga, H.; Azetsu, A.; Tokunaga, E.; Saito, T.; Isogai, A.; Kitaoka, T. Topological loading of Cu (I) catalysts onto crystalline cellulose nanofibrils for the Huisgen click reaction. J. Mater. Chem. 2012, 22, 5538−5542. (13) Fan, Y.; Saito, T.; Isogai, A. Preparation of chitin nanofibers from squid pen β-chitin by simple mechanical treatment under acid conditions. Biomacromolecules 2008, 9, 1919−1923. (14) Hackman, R. H. Studies on chitin. IV. The occurrence of complexes in with chitin and protein are covlently linked. Aust. J. Biol. Sci. 1960, 13, 568−577. (15) Lavall, R. L.; Assis, O. B. G.; Campana-Filho, S. P. β-Chitin from the pens of Loligo sp.: extraction and characterization. Bioresour. Technol. 2007, 98, 2465−2472. (16) Kim, S. S.; Kim, S. H.; Lee, Y. M. Preparation, characterization and properties of β-chitin and N-acetylated β-chitin. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 2367−2374. (17) Ifuku, S.; Saimoto, H. Chitin nanofibers: Preparations, modifications, and applications. Nanoscale 2012, 4, 3308−3318. (18) Päak̈ kö, M.; Vapaavuori, J.; Silvennoinen, R.; Kosonen, H.; Ankerfors, M.; Lindström, T.; Berglund, L. A.; Ikkala, O. Long and entangled native cellulose I nanofibers allow flexible aerogels and hierarchically porous templates for functionalities. Soft Matter 2008, 4, 2492−2499. (19) Nogi, M.; Kurosaki, F.; Yano, H.; Takano, M. Preparation of nanofibrillar carbon from chitin nanofibers. Carbohydr. Polym. 2010, 81, 919−924. (20) Heath, L.; Zhu, L.; Thielemans, W. Chitin nanowhisker aerogels. ChemSusChem 2013, 6, 537−544. (21) Valentin, R.; Molvinger, K.; Quignard, F.; Brunel, D. Supercritical CO2 dried chitosan: An efficient intrinsic heterogeneous catalyst in fine chemistry. New J. Chem. 2003, 27, 1690−1692. (22) Reddy, K. R.; Rajgopal, K.; Maheswari, C. U.; Kantam, M. L. Chitosan hydrogel: A green and recyclable biopolymer catalyst for aldol and Knoevenagel reactions. New J. Chem. 2006, 30, 1549−1552. (23) Ricci, A.; Bernardi, L.; Gioia, C.; Vierucci, S.; Robitzer, M.; Quignard, F. Chitosan aerogel: A recyclable, heterogeneous organocatalyst for the asymmetric direct aldol reaction in water. Chem. Commun. 2010, 46, 6288−6290. (24) Kühbeck, D.; Saidulu, G.; Reddy, K. R.; Díaz, D. D. Critical assessment of the efficiency of chitosan biohydrogel beads as recyclable and heterogeneous organocatalyst for C−C bond formation. Green Chem. 2012, 14, 378−392.
Figure 5. Catalytic performance of the recycled AG1 at a flow rate of 0.04 mL min−1.
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CONCLUSIONS We have demonstrated the fabrication of high-SSA aerogels, from naturally derived crystalline chitin, and their high catalytic performance in the Knoevenagel condensation reaction. Both the mesoporous aerogel structure and the surface-exposed C2amines on the crystalline ChNF surface were effective for use in continuous flow catalysis. Chitin is stable to most solvents and has both hydrophilic and lipophilic properties. These specific features are desirable attributes in a wide range of catalytic reactions. Thus, the naturally derived ChNF aerogels are promising renewable and recyclable catalytic materials for realizing a green and sustainable chemistry.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray diffraction patterns of squid pen chitin and ChNF, DNAc determination of ChNF, light transmittance of ChNF dispersion, and SEM images and nitrogen adsorption/ desorption isotherms of cellulose and chitosan aerogels are given in the Supporting Information file. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +81 3 5841 5538; fax: +81 3 5841 5269; E-mail: aisogai@ mail.ecc.u-tokyo.ac.jp. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Grants-in-Aid for Scientific Research (Grant Nos. 21228007 and 23688020) from the Japan Society for the Promotion of Science (JSPS), and by the Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Agency (JST). We thank Dainichiseika Color & Chemicals Mfg. Co. Ltd., Osaka, Japan, for providing the squid pen specimen.
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