Article pubs.acs.org/IECR
Synthesis of Sulfonic Acid Functionalized Silica Honeycombs Yoshitaka Satoh, Yuya Yokoyama, Isao Ogino, and Shin R. Mukai* Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo 060-8628, Japan ABSTRACT: A honeycomb-type silica monolith was functionalized with sulfonic acid groups to synthesize a solid acid catalyst for flow reaction systems. The sulfonic acid groups were introduced via anchoring of 3-mercaptopropyltrimethoxysilane (MPTS) on the silica surface and subsequent oxidation using a H2O2 solution. The honeycomb-type monolith used as the substrate was prepared using ice crystals as a template and was found to have a high open frontal area of ≈85% and flow-through macropores (≈27 μm in diameter) that are surrounded by silica walls (≈2 μm in thickness). The catalyst was tested by the liquid phase esterification of ethanol with acetic acid at 333 K in a batch reactor as well as a flow reactor. The catalyst was found to exhibit about 2-fold higher catalytic activity than Amberlyst-15 in a batch reactor. The catalyst also showed a stable catalytic activity for 24 h in a flow reactor. techniques.12−15 Such functionalized silicas were found to effectively catalyze acetylation,12 alkylation,13 esterification,14 and transesterification15 reactions. Mesoporous silicas such as MCM-41 or SBA-15 are typically used as the substrate of such catalysts as they have an ordered porous structure. However, the resulting catalysts are not so stable, as both the mechanical strength and hydrothermal stability of the mesoporous silicas used as the support are not so high.16 Moreover, the surface density of the sulfonic acid groups introduced in them also tends to be low, as the density of hydroxyl groups, the anchoring points, within such mesoporous silicas is low. The low density of hydroxyl group is thought to be mostly due to the template removal process,17 which is inevitable to obtain mesoporous silicas. SMHs possess a fairly high mechanical strength, and their hydrothermal stability is also high. Moreover, they can be made to have a high hydroxyl group density, as the ice templates used to synthesize them can be removed through thawing followed by drying. If the drying process is avoided by removing the water generated during the thawing of the ice templates by different methods, for example by extraction, they can be made to have a higher hydroxyl group density. Therefore in this work, attempts were made to introduce sulfonic acid groups into SMHs, to obtain an active solid acid catalyst, with easy accessible acid sites, and which only causes minimal hydraulic resistances. First 3-mercaptopropyltrimethoxysilane (MPTS) was anchored to the hydroxyl groups of SMHs, and then the introduced thiol groups were oxidized using H2O2. The catalytic performance of the resulting samples was estimated using the esterification of ethanol with acetic acid as the test reaction. Evaluations were conducted in both batch and flow systems.
1. INTRODUCTION Acid catalysts are widely used in industrial chemical processes.1 Although liquid acid catalysts such as H2SO4 and HF are typically used in liquid-phase reactions, they are highly toxic and are difficult to remove from the reaction media. These problems can be avoided by using solid acid catalysts such as ion-exchange resins2−4 or zeolites5−7 instead. Such solid catalysts are typically packed in a flow reactor as beads or pellets in many industrial chemical processes.1 The apparent activity of such catalysts is affected by its size, as intraparticle transport resistance becomes large in large particles. However, small particles cause a large hydraulic resistance, which indicates that a low hydraulic resistance and short diffusion path lengths within the particles are not compatible. Therefore size of the catalyst is determined so as to provide a proper balance between activity and hydraulic resistance. Recently, it was reported that monoliths having a honeycomb structure cause minimal resistance to fluid flows.8 However, the honeycomb wall thickness of conventional monolithic honeycombs range from several dozen micrometers to several hundred mircrometers, meaning that only the outer surface of the walls can be utilized in most case. To efficiently utilize the inner parts of the honeycomb walls, the thickness of the walls have to be reduced. However, such monoliths cannot be obtained via conventional honeycomb synthesis methods. Previously, we reported the synthesis of a silica monolith that has a honeycomb structure (silica microhoneycomb, SMH) using numerous micrometer-sized ice crystals as a template.9−11 The single micrometer-sized honeycomb walls synthesized using this method, which we named the ice templating method, are expected to show extremely low intrawall resistance toward reactants. As the fact that the structure of an SMH can be represented by a bundle of straight identical capillaries,11 indicates the resistance it causes toward fluid flows is lower than that of a particle bed. Therefore, if sites which show catalytic activities can be introduced into them, an extremely efficient catalyst can be obtained. There are many reports about attempts to functionalize porous silica with sulfonic acid groups, mostly via postgrafting © 2013 American Chemical Society
Special Issue: NASCRE 3 Received: Revised: Accepted: Published: 15293
February April 22, April 30, April 30,
19, 2013 2013 2013 2013
dx.doi.org/10.1021/ie400541d | Ind. Eng. Chem. Res. 2013, 52, 15293−15297
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2. MATERIALS AND METHODS 2.1. Sample Preparation. Silica microhoneycomb (SMH) functionalized with sulfonic acid groups was synthesized through MPTS incorporation and subsequent oxidation. An SMH was prepared according to the literature.9 A commercial sodium silicate solution (Wako Pure Chemical Industries, Ltd., Wako first grade) was diluted with distilled and deionized water, and the SiO2 concentration in them was adjusted to 1.9 mol L−1. Next ion-exchange resin particles (Amberlite IR 120B H AG, Organo, Japan) were immersed into the solution and were filtered out when the pH of the solutions reached ≈2.8. The obtained sol was poured into a polypropylene tube. The sol in the tube was aged at 303 K, and after it transformed to gel, the tube was dipped at a constant rate of 6 cm h−1 into a cold bath maintained at 77 K. After the gel in the tube was completely frozen, the tube was taken out from the bath. The frozen gel was released from the tube and was cut into portions 10−30 mm in height. In order to protect the porous structure of the gels, the resulting hydrogels were immersed into 2methyl-2-propanol18 and were freeze-dried by maintaining them under vacuum at 263 K. After freeze-drying, the obtained dry gels were pretreated at 423 K in a nitrogen flow (20 mL min−1) for 10 h and cooled down to room temperature. Next the gels were immersed into dry toluene which was refluxed at 383 K under a nitrogen atmosphere and were treated in it for 1 h. Then they were immersed into 3 times their weight of MPTS, the precursor of the sulfonic acid group, and were treated in it at 383 K for 24 h in a nitrogen atmosphere. During treatment, MPTS was refluxed. The treated gels were cooled down to room temperature and were thoroughly washed with toluene. Then they were immersed into 2-methyl-2-propanol and were freeze-dried under vacuum at 263 K. The finally obtained samples will be denoted as Pr-SMH-a hereafter. A part of the gels were functionalized using MPTS prior to freeze-drying. Hydrogels, which were synthesized following the method described above, were immersed into dry toluene and were treated for 7 days. During this period the toluene solvent was exchanged three times a day. Then, MPTS was anchored to the resulting samples using the same procedure described above without pretreatment in a nitrogen flow. After MPTS anchoring and freeze-drying, the obtained samples were treated with 30% H2O2 at room temperature for 24 h to oxidize the introduced functional groups. The resulting samples were filtered out, washed with 2-methyl-2-propanol and freeze-dried at 263 K. The samples obtained through this method will be denoted as Pr-SMH-p hereafter. 2.2. Characterization. The morphology of the synthesized samples was observed using a scanning electron microscope (SEM, JEOL Japan Inc., JSM-5410), and the average size of the macropores of the samples was estimated using the obtained micrographs. The hydraulic resistance the samples cause was evaluated by measuring the pressure drop that occurs when water was passed through it at a flow rate set between 0−10 mL min−1 using an HPLC pump (JASCO Co., PU-2080). Measurements were conducted at 293 K. The sulfur content of the samples prior to and after H2O2 treatment was estimated using ion chromatography. In addition, the density of sulfonic acid groups formed in the samples after H2O2 treatment was determined through titration. Typically, a sample was immersed into a 0.1 mol L−1 NaCl aqueous solution. Then, the resulting suspension was stirred for several hours and was titrated with a 0.01 mol L−1
aqueous NaOH solution. The porous properties of the obtained samples were evaluated through nitrogen adsorption experiments conducted at 77 K (BEL Japan Inc., BELSORPmini). Prior to experiments the samples were pretreated at 523 K in a nitrogen flow (20 mL min−1, 99.999% purity) for 4 h. The apparent surface area of the samples was calculated through the Brunauer−Emmett−Teller (BET) method, and their micropore volume was calculated using the t-plot method. The total pore volume of the materials was obtained from the volume of nitrogen adsorbed at a relative pressure of 0.95 by applying the Gurvich rule on the resulting adsorption data.19 2.3. Evaluation of Catalytic Activity. The catalytic activity of the samples was evaluated using the esterification of acetic acid with ethanol as a test reaction. Reaction experiments were also conducted using sulfuric acid or an ion-exchange resin (Amberlyst-15). Typically, 100 mmol each of ethanol and acetic acid were charged to a 50 mL glass vial equipped with a stir bar and a condenser. Then, after the catalyst was loaded to the reactor, the reactor was immediately immersed into a water bath set at 333 K. This moment was taken as the start of the reaction. Aliquots of the liquid sample were withdrawn periodically and analyzed using a gas chromatograph (Shimadzu Co., GC-17A) equipped with a capillary column (Shinwa Chemical Industries Ltd.; HR-1) and a flame ionization detector (FID). Experiments were also conducted in a flow system. A sample with ≈2 cm length introduced into a heat shrinkable tube was used as the reactor. The reactor was immersed into a water bath set at 333 K and an equimolar mixture of ethanol and acetic acid was passed through it at a liquid hourly space velocity of 1.4 h−1 using HPLC pumps. The moment the mixture first reached the outlet of the reactor was taken as the start the reaction. Liquid samples were collected at the outlet of the reactor periodically and were analyzed using the same gas chromatograph described above. The turnover number of the sample was calculated from the ethanol conversion before the reaction experiment was terminated. For comparison, experiments were conducted without using a catalyst. In such experiments, a tube 9 mm in diameter and 20 mm in length was used as the reactor.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Sulfonic AcidFunctionalized SMHs. As shown in Figure 1 both Pr-SMH-a
Figure 1. Cross-sectional SEM image of a nontreated SMH (a), PrSMH-a (b), and Pr-SMH-p (c).
and Pr-SMH-p had a microhoneycomb structure like a nontreated SMH, indicating that the microhoneycomb structure did not collapse during the anchoring of MPTS and subsequent H2O2 treatment. From SEM images, Pr-SMH-a was found to have an average honeycomb wall thickness and macropore size of ≈2 and ≈26 μm, respectively. These values are nearly equal to those of Pr-SMH-p (≈ 2 and ≈28 μm, respectively). These results indicate that functionalization does not affect the morphology of SMH. 15294
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The sulfur contents of Pr-SMH-a prior to and after H2O2 treatment estimated by ion chromatography were 1.35 mmol (g-sample)−1 and 1.03 mmol (g-sample)−1, respectively. The ≈24% loss of sulfur may be attributed to a partial leaching of thiol groups during the oxidation treatment.14 The amount of accessible sulfuric acid groups estimated by titration was 1.01 mmol g−1, and this value is consistent with the sulfur content determined by ion chromatography analysis, indicating that the oxidation of thiol groups to sulfonic acid group was complete and all sulfonic acid groups anchored in SMH may function as active sites in acid catalytic reactions. Because the incorporation of thiol groups is achieved through the condensation reaction of a surface silanol group of SMH with an alkoxy group of the precursor MPTS, a silica having a high density of surface silanol groups is inevitable to obtain a catalyst with a high acid density. It is reported that the number of surface silanol groups is proportional to the surface area of the silica.23 As the surface area of a freshly formed silica hydrogel is likely to decrease when it is dried,17 silica gels not experiencing drying is thought to be preferable for MPTS anchoring. Indeed, the sulfur content of Pr-SMH-p prior to H2O2 treatment (3.44 mmol g−1) was ≈2.5-fold higher than that of Pr-SMH-a, and the sulfur content of the Pr-SMH-p after treatment (2.10 mmol g−1) was ≈2.0-fold higher than that of Pr-SMH-a. The amount of sulfuric acid groups of Pr-SMH-p after H2O2 treatment was 1.81 mmol g−1. The value was ≈0.30 mmol g−1 lower than the total sulfur content in Pr-SMH-p, indicating that the oxidation of thiol groups was not completed or disulfide species were produced during oxidation.14 Nevertheless, by anchoring prior to drying, a large amount of active site can be introduced into SMHs. As shown in Figure 3, nitrogen adsorption isotherms of all of the synthesized samples exhibit I type in the IUPAC
Figure 2 summarizes results of the pressure drop measurements. The pressure drop of Pr-SMH-a increased linearly with
Figure 2. Hydraulic resistance of Pr-SMH-a (●). The solid line indicates the values calculated by the Hagen−Poiseuille equation.
the increase in the water flow rate. As the fluid flow in the macropores was expected to be laminar, the data was analyzed by the following Hagen−Poiseuille equation that describes a laminar flow in a tube:20 32μLu ΔP = (1) D2 where ΔP is the pressure drop, μ is the viscosity of the water at experimental temperature, L is the length of the Pr-SMH, u is the average velocity, and D is the average macropore diameter of the Pr-SMH. The data for Pr-SMH-a was ≈1.7-fold higher than the calculated values of a tube with ≈26 μm diameter, indicating that there are some irregularities in the channels of the microhoneycomb. From SEM image (Figure 1), the void ratio (frequently called open frontal area) of Pr-SMHs was ≈85%. The high void ratio of Pr-SMH relative to the porosity of a conventional packed bed of spherical particles ≈45% expects to lead to a low fluid resistance when a fluid is passed through it.8 Due to this unique morphology, facile mass transfer of reactant(s) and product(s) is expected in Pr-SMH. To compare the pressure drop caused by Pr-SMH-a with that caused by a conventional bed of particles, the pressure drop a bed of spherical particles causes was estimated using the following Kozeny−Carman equation:21 ΔP = K
(1 − ε)2 μLu ε3 d2
(2)
Figure 3. Nitrogen gas adsorption isotherms of SMH (⧫), Pr-SMH-a (▲), and Pr-SMH-p (●).
where K (= 180) is the Kozeny constant, ε is the porosity of the column packed with particles, and d is the spherical particles diameter. In this study, the calculation was performed for a bed of incompressible spherical particles with a column porosity of 0.45.22 As Pr-SMH-a has an average pore wall thickness of ≈2 μm, the calculation was performed for a column packed with spherical silica functionalized with MPTS particles with a particle diameter of 2 μm so that the diffusion path length in the silica matrix is same in both cases. Under this condition the column height of the bed of spherical particles was ≈70% shorter than the length of the monolith with the same mass of silica functionalized with MPTS. However, the calculation shows that the higher pressure drop the packed bed causes is ≈550-fold than that Pr-SMH-a causes. This result indicates that Pr-SMH causes substantially lower pressure drops than a conventional bed of particles having the same diameter as the honeycomb wall.
classification. In addition, the micropore volumes of these samples are close to their total pore volume (see Table 1). These results indicate that all of the synthesized samples were microporous. The mean micropore size of the functionalized samples estimated using the t-plot method was close to that of SMH (≈0.95 ± 0.5 nm). These results indicate that the introduction of thiol groups and subsequent oxidation hardly affected the porous structure. The BET surface area of SMH synthesized via the ice templating method was ≈1200 m2 g−1 and was comparable with mesoporous silicas such as MCM-41 and MCM-48.13 Although the surface area of functionalized samples was 30−40% lower than that of their parent SMH, the surface area was compared with mesoporous silica functionalized by sulfonic acid groups.13 These results show that the synthesized SMH functionalized with sulfonic acid groups have 15295
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Amberlyst-15 at 333 K was 56 kJ mol−1 and was close to the reported value (58 kJ mol−1).6 This result indicates that the sulfonic groups on Pr-SMH function similarly to those in Amberlyst-15 and that the reaction was not disguised by mass transfer limitation under the adopted reaction conditions. In a flow system, which is a preferable mode of reaction for the production of bulk chemicals in industry, the conversion of ethanol in the presence of Pr-SMH-p was stable at ≈48.3% over 24 h as shown in Figure 5, indicating that the leaching of
Table 1. Surface Areas and Pore Volumes of the Synthesized Materials pore volume cm3·g−1
cm3·g−1
sample
SBET (m2·g−1)
Vmicroa
Vtotalb
mean pore sizec (nm)
SMH Pr-SMH-a Pr-SMH-p
1205 914 798
0.59 0.42 0.38
0.61 0.43 0.39
0.99 0.97 0.91
a
Micropore volume determined by the t-plot method. bTotal pore volume. cMean micropore size determined by the t-plot method.
suitable textual properties for a solid catalyst due to easy reactant accessibility. 3.2. Catalytic Activity. In order to compare the catalytic activity between Pr-SMH-p and Pr-SMH-a, the esterification of ethanol with acetic acid was conducted in a batch reactor. The results are summarized in Figure 4. Note that the concentration
Figure 5. Esterification reaction of acetic acid with ethanol using PrSMH-p (●) in a flow system. Reaction conditions: temperature = 333 K, liquid hourly space velocity (LHSV) = 1.4 h−1, molar ratio between ethanol and acetic acid in feed = 1:1. A control experiment was performed without a catalyst under otherwise identical conditions (■).
sulfonic acid group is negligible under the reaction conditions. The total turnover number of Pr-SMH-p was ≈600 mol-ester (mol-ethanol total number of acid-sites)−1 before the reaction experiment was terminated, indicating that Pr-SMH-p can efficiently catalyze this esterification reaction in a flow system.
Figure 4. Catalytic activities of Pr-SMH-p (●), Pr-SMH-a (■), and Amberlyst 15 (▲). Reaction conditions: temperature = 333 K, initial molar ratios between acetic acid, ethanol, and protons provided from the catalysts = 1:1:1 × 10−3). A control experiment was performed without a catalyst under otherwise identical conditions (▼).
4. CONCLUSIONS We succeeded in anchoring sulfonic acid groups to SMHs having an average macropore size ≈27 μm. SMH which did not experience drying prior to anchoring was found to be preferable to achieve a high sulfonic acid group loading. As the functionalized SMHs have ≈85% of void ratio, they cause a ≈550-fold less fluid resistance when compared with a bed of functionalized silica beads. In a batch reactor the catalytic activity of these materials was 2-fold higher than that of Amberlyst-15, a conventional solid acid catalyst for esterification. In addition, in a flow reactor the catalytic activity of a typical sample was constant for 24 h indicating its high stability.
of protons provided from the catalysts was adjusted to be the same in experiments. The equilibrium conversion of this esterification reaction is reported to be about 63% at 333 K.24 Therefore in this study, to eliminate the influence of reverse hydrolysis, data corresponding to ethanol conversions below 10% were used to calculate the catalytic activities of the samples toward this esterification reaction. The catalytic activity of PrSMH-a or Pr-SMH-p was nearly equal, and the ethanol conversion in their presence after 1 h of reaction time was ≈10%. This value was ≈5-fold higher than the ethanol conversion achieved in the absence of a catalyst (≈2%) as also shown in Figure 4. These results show that both Pr-SMH-a and Pr-SMH-p are usable as catalysts for acid catalyzed reactions. In addition, the result that the catalytic activity of PrSMH-p or Pr-SMH-a was nearly equal indicates that the availability of sulfonic acid groups anchored prior to drying was similar to that of acid groups anchored after drying (conventional method), indicating that Pr-SMH-p is preferable as a catalyst as it has a higher sulfonic acid density (1.81 mmol g−1) than Pr-SMH-a (1.03 mmol g−1). Next, the catalytic activity of Pr-SMHs was compared with Amberlyst-15 which is a common ion-exchange resin used as a solid acid catalyst for esterification. Under the same sulfonic acid group loading, the initial ethanol conversion achieved after 1 h of reaction time using Pr-SMHs was ≈2-fold higher than that of Amberlyst-15 (≈5%). The apparent activation energy of
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Industrial Technology Research Grant Program in 2006, 06B44702a, from New Energy and Industrial Technology Development Organization (NEDO) of Japan, the Japan Society for the Promotion of Science (JSPS), Grant-in-Aid for Scientific Research (B) 21360384, and the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Ion Chromatography analysis was meas15296
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(21) Leinweber, F. C.; Lubda, D.; Cabrera, K.; Tallarek, U. Characterization of Silica-Based Monoliths with Bimodal Pore Size Distribution. Anal. Chem. 2002, 74, 2470−2477. (22) Holdich, R. G. Fundamentals of Particle Technology; Midland Information Technology and Publishing: UK, 2002. (23) Zhuravlev, L. T. The Surface Chemistry of Amorphous Silica. Zhuravlev model. Collid Surf. A 2000, 173, 1−38. (24) Blanchard, L. A.; Brennecke, J. F. Esterification of acetic acid with ethanol in carbon dioxide. Green Chem. 2001, 17−19.
ured by Ms. Kiuchi and Ms. Hattori at Instrumental Analysis Division, Equipment Management Center, Creative Research Institution, Hokkaido University.
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