Article pubs.acs.org/IECR
Microencapsulation of Silicotungstic Acid To Retain Catalytic Activity Levent Degirmenci† and Nese Orbey* Chemical Engineering Department, University of Massachusetts, Lowell, 1 University Avenue, Lowell, Massachusetts 01854, United States ABSTRACT: Microencapsulation is used as a means to retain the activity of silicotungstic acid (STA), an industrially important catalyst, during repeated use in ethyl acetate synthesis reaction. Standard sol−gel microencapsulation procedures were modified to microencapsulate STA inside a silica shell. The microcapsules were characterized to determine their morphology, the presence and amount of the active material, and the pore structure. The activity of the catalyst was tested in an esterification reaction between acetic acid and ethanol. The catalyst was used multiple times after being washed and dried after each run. Results indicate no leaching of the active material and no significant loss in catalyst activity after repeated runs.
1. BACKGROUND The two most important properties of any catalyst are high catalytic activity in a reaction and the ability to retain this activity in repeated use. Silicotungstic acid (STA) is a heteropoly acid catalyst with superior acidic characteristics enabling its use in a variety of industrially important reactions, including esterification with activities higher than other catalysts currently being used in industry.1,2 STA has very low surface area and is highly soluble in water and other polar solvents. Hence, its activity is not retained in repeated use. Research efforts have been focused on increasing the activity and stability of STA-based catalysts. One method is to immobilize STA on a support material. Degirmenci et al.3 studied vapor-phase ethyl and tert-butyl ether production using activated carbon supported silicotungstic acid (Ac-STA) and activated carbon supported cesium salts of silicotungstic acid (AC-Cs-STA) as catalysts. Their results showed higher catalytic activities with supported STA but some loss of activity and leaching of STA in repeated use.3 Varisli et al.4 immobilized STA inside an aluminosilicate structure and used them in ethanol dehydration to yield diethyl ether and ethylene reactions between 250 and 400 °C. They obtained good catalytic activity with high conversions and yields favoring either diethyl ether or ethylene depending on the temperature; however, the activity was not retained over repeated use. Although immobilization maintains an interaction between the support material and the active compound (STA) by means of hydrogen bonding; it does not prevent leaching of the active material, especially in reactions that involve the use of polar solvents.3−5 In the present work, STA is microencapsulated in silica to prevent leaching and enable the use of catalyst in repeated runs without loss of activity. Microencapsulation is the process where a valuable material is coated with a continuous film of a polymeric material to form microcapsules with sizes ranging from micrometer to millimeter.6 Microencapsulation has gained increasing attention due to enhanced convenience, increased stability, and adjustable release rate of the active material after encapsulation.7 Coating materials used in microencapsulation are generally linear polymers, but silica-based compounds gained popularity as shell components in microencapsulation due to silica’s © 2013 American Chemical Society
unique properties, such as chemical inertness, mechanical and thermal stability, and the ease of incorporating various functional groups.8−10 Encapsulating catalysts is one viable method to retain catalyst activity during repeated use. In the literature, microencapsulation has been used with metal species as the active material and polymer compounds as shell components. Artner et al.11 encapsulated molybdenum, palladium, and rhodium complexes in thermoseting epoxy resins. The metal-doped catalysts were tested in the Suzuki coupling reaction of iodobenzene with phenylboronic acid and resulted in a favorable selectivity of 98%. The catalysts were easily removed from the reaction medium by means of filtration and reused in repeated runs without further treatment retaining their activity and selectivity in repeated runs. Cornejo et al.12 encapsulated chiral Pybox−Ru catalyst in linear polystyrene and used it in the cyclopropanation reaction between styrene and ethyl diazoacetate with high conversions (68%) and without loss of catalyst activity. In the present study, STA is encapsulated in silica through consecutive hydrolysis and polycondensation of TEOS. Catalytic performance of microencapsulated STA is evaluated by studying the conversion of liquid-phase acetic acid to ethyl acetate in repeated runs. Ethyl acetate is a solvent and is produced in large scale in industry from ethanol and acetic acid in the presence of catalyst.13,14 The synthesis was traditionally conducted using homogeneous catalysts that could not be separated and reused.13 The use of heterogeneous catalysts enabled separation from the reaction media by filtration. Gurav et al.14 studied ethyl acetate synthesis in the presence of dodecatungstophosphoric acid supported on K10 montmorillonite with amounts of the active material ranging between 10 and 30%. The reaction was carried out at 110 °C at elevated pressures resulting in conversions of 90%. However, they observed loss of catalytic activity during repeated use.14 Received: Revised: Accepted: Published: 16714
July 16, 2013 September 19, 2013 October 22, 2013 October 22, 2013 dx.doi.org/10.1021/ie4022754 | Ind. Eng. Chem. Res. 2013, 52, 16714−16718
Industrial & Engineering Chemistry Research
Article
Table 1. Synthesis Conditions Applied during Microencapsulation of STA cat.
HCl (mL)
STA (g)
CA (mL)
Tween 80 (g)
water (mL)
TEOS (mL)
WP1
WP2
A B C D E F G
1.2 − − − − − −
− 0.117 0.117 0.117 0.234 0.234 0.234
− − − 1 − − 2
− 0.25 − − − 0.25 −
1 4.7 1 − 2 4.7 −
5 5 5 5 5 5 5
TEOS + HCl STA + W + Tween 80 STA + W + TEOS STA + CA + TEOS STA + W + Tween 80 STA + W + Tween 80 STA + CA + TEOS
− TEOS − − − TEOS −
2. EXPERIMENTAL WORK 2.1. Chemicals. Silicotungstic acid (STA), Tween 80 (surfactant), tetraethyl orthosilicate (TEOS), ethanol, acetic acid, and ethyl acetate were purchased from Sigma Aldrich. Citric acid was purchased from Hemco. 2.2. Preparation of Microencapsulated STA Catalysts. A standard sol−gel microencapsulation procedure7 for hydrophilic compounds was used in the present study with modifications to encapsulate STA. The encapsulation procedure consists of two main stages. In the first stage, the hydrophilic active compound (STA) and the hydrophilic encapsulation precursor TEOS are emulsified. This emulsion constitutes the water phase (WP). In the second stage, the WP is added to an oil phase (OP). A vegetable oil (VO) is used as the oil phase in the present study. In conventional microencapsulation, the emulsions in liquid state are precipitated by adding gelling agents (acid or base) into the systems.7 However, in the present study, the use of acid or base is eliminated because the hydrophilic, active compound STA also acted as the gelling agent. TEOS is converted into silica by the following successive hydrolysis and polycondensation reactions:15
emulsion VO VO VO VO VO VO VO
+ + + + + + +
WP1 WP1 + WP2 WP1 WP1 WP1 +WP2 WP1 +WP2 WP1
In the synthesis, the water phase (WP) included a solution of STA, citric acid (CA), and the surfactant Tween 80 in water (W). The chemicals used and their respective amounts varied for different runs. The sequence of TEOS addition also differed from run to run: in some experiments the solution was directly added to TEOS to initiate hydrolysis and in others it was added during synthesis. The WP is added dropwise to vegetable oil to complete the hydrolysis and polycondensation of TEOS. Catalyst C is selected as the base catalyst because it is prepared with the least amount of chemicals during its synthesis: 5 mL of TEOS is mixed with 0.117 g of STA and 1 mL of water in an ultrasonicated bath for 40 min, forming the water phase (WP1). The resulting solution is then added to 100 mL of vegetable oil and the emulsion is mixed with a propeller at 220 rpm for 4 h to complete the polycondensation of TEOS. The mixing rate was optimized to obtain particle diameters between 210 and 710 μm and a uniform distribution.16 Catalyst E was prepared with the same procedure but with twice the STA amount. A similar procedure is followed in the synthesis of remaining catalysts with differences in the contents and numbers of WP used. For catalysts B and C, Tween 80 was used as surfactant, and two water phases were prepared. The first water phase (WP1) consisted of STA, Tween 80, and water, and this was added to the vegetable oil (VO). TEOS solution is then added to the emulsion as the second water phase (WP2) to initiate hydrolysis. For catalysts D and G, 1 M citric acid was used to increase the pore volume. In the literature, researchers report the use of citric acid to promote mesopore formation in aluminophospahates and zeolites.17,18 In all experiments 100 mL of vegetable oil was used as the oil phase and the emulsion was mixed with a propeller at 220 rpm for 4 h. 2.3. Characterization of Microcapsules. SEM images of catalysts were obtained with a JEOL JSM-7401F-A FE-SEM instrument to determine the formation of the microspheres. Presence of STA in the microspheres was determined by titration and by instrumental neutron activation analysis (INAA). Titration is the simplest method for visual determination of STA in the catalyst structure. Five milliliters of water and 1 drop of methyl red were added to 0.1 g of catalyst particles, and the resulting suspension was titrated with 0.1 M NaOH. Catalyst particles were titrated as is and also after being crushed to determine whether STA was inside or outside the microspheres. γ-Ray spectroscopy INAA was used to determine the amount of tungsten present in the molecular structure of STA. Samples of 100 mg were weighed in an acid-cleaned polyethylene vial, and the material was subjected to a neutron flux for several hours. The neutrons were captured by various stable isotopes forming radioactive isotopes. The γ-rays emitted by isotopes
(RO)3 Si−OR + H 2O hydrolysis/esterification
⇐=========⇒ RO3Si−OH + ROH (RO)3 Si−OR + RO−Si(RO)3 alcohol condensation/alcoholysis
⇐==============⇒ (RO)3 Si−O−Si(OR3) + ROH (RO)3 Si−OH + HO−Si(OR)3 water condensation / hydrolysis
⇐============= ⇒ (RO)3 Si−O−Si(OR3) + H 2O
In literature, two different routes for hydrolysis and polycondensation reactions of TEOS are reported: the basic route and the acidic route.15 The use of a base during microencapsulation increases the rate of polycondensation, resulting in the formation of dense silica particles without porosity. The use of an acid, on the other hand, provides formation of porous particles with homogeneous particle size distribution.15 Since the objective of the present study is to use microencapsulated STA as a catalyst, the acidic route leading to a porous structure is selected. Six microencapsulated STA catalysts were synthesized under different conditions in the present work. These catalysts are designated using letters A−G and the synthesis procedures employed are detailed below and summarized in Table 1. Catalyst A is used as a control and does not contain any of the active material STA. Therefore, an acid (0.1 M HCl) had to be used in its synthesis. In all other synthesis, STA serves the function of the acid. 16715
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during their decay to stable forms were detected by means of a broad energy germanium detector. Barret−Joyner−Halenda (BJH) desorption pore size distributions, single point Brunauer−Emmett−Teller (BET) surface area, and average pore size of the catalysts were determined using a Qunatachromo Autosorb 3B instrument. 2.4. Reaction Experiments with the Microencapsulated STA Catalyst. The activity of catalyst C was evaluated in ethyl acetate synthesis reaction. The reaction was conducted for 6 h in a three neck round bottomed, jacketed batch reactor equipped with a magnetic stirrer and a reflux condenser. The reaction was carried out at 343 K. A 1:1 molar ratio of ethanol:acetic acid and 0.75 g of catalyst were used in the experiments. The reaction mixture was stirred at 1000 rpm to eliminate internal or external mass transport limitations. The reactor was initially charged with acetic acid and catalyst and was brought to 343 K. Ethanol was then added. Samples were obtained every 2 h from the reaction mixture to monitor the conversion of acetic acid to ethyl acetate using an Agilent 5890 GC-MS instrument equipped with a DB-Wax column and a FID detector. The instrument is calibrated with known concentrations of ethyl acetate. At the end of 6 h, the catalyst was filtered, washed with water, and dried at 40 °C for 2 days in an oven before being reused. The reaction was also conducted in the absence of any catalyst for comparison.
Table 2. Tungsten Amounts Determined by INAA Analyses catalyst
tungsten (% by wt)
catalyst
tungsten (% by wt)
A (control, no STA) B B2 C C2
0.0 3.8 3.2 4.6 4.2
D E F G
4.4 4.5 4.9 6.5
experiments was checked by duplicate runs for catalysts B and C, and these results are also presented in Table 2. Catalyst A was the control where no STA was present during synthesis. The maximum amount of tungsten that would be encapsulated in microspheres can be calculated by assuming that all TEOS is used to encapsulate the STA and all of the water is removed during drying. For catalysts B−D this amount is approximately 1.8% (by weight) and it is 3.6% for catalysts E−G. The amount of tungsten in all the catalysts is higher than this maximum amount, suggesting that not all the TEOS polycondenses to form the silica microencapsules but some remained in the solution. The starting amounts of STA in the synthesis of catalysts D−F is double those of catalysts B−D, and the results indicate that higher amounts of STA result in higher conversion of TEOS to silica. The use of surfactant did not seem to affect the TEOS to silica conversion. Physical properties of the catalysts are tabulated in Table 3. The surface area measurements were made with the BET
3. RESULTS AND DISCUSSION 3.1. Characterization Studies. All catalysts synthesized were analyzed under SEM to confirm the formation of microspheres. In all cases, solid microspheres with smooth external surfaces were obtained. A typical result is shown for catalyst C in Figure 1.
Table 3. Physical Properties of the Catalysts sample catalyst catalyst catalyst catalyst catalyst
A C D E G
surface area (m2/g)
av pore diameter (nm)
1.89 1.50 1.46 2.00 1.83
2.4 2.0 2.4 2.2 2.4
method, and desorption pore size distributions were estimated using the BJH method. The pore size distribution of catalysts is given in Figure 2. The results indicate that the catalysts have low surface area. Average pore diameter values indicate that the catalysts have both microspores and mesopores. 3.2. Reaction Experiments. The performance of microencapsulated STA as a catalyst was tested in the reaction between acetic acid and ethanol to produce ethyl acetate. The reaction was also carried out without any catalyst for comparison. The first run (run 1) was conducted with fresh catalyst C. At the end of the reaction, the catalyst was filtered, washed, and dried at 40 °C to remove any water and was used again in a new ethyl acetate synthesis (run 2). This procedure was repeated for a second time (run 3). Conversion of acetic acid as a function of time for the three runs as well as the run where no catalyst was used are shown in Figure 3a,b. An increase of about 2.5-fold in conversion is observed with the usage of STA as the catalyst. The results show only a 10% decrease in catalyst activity at the end of the third run. This decrease is relatively small, showing that microencapsulation was an effective method to retain the catalytic activity of STA in multiple uses. In order to determine whether there was leaching of STA during repeated reactions, fresh and reused catalysts were analyzed for their tungsten content using INAA, and the results are presented in Table 4. Tungsten amounts for fresh and spent catalyst C were
Figure 1. SEM images of catalyst C microsphere obtained at 200×.
The microspheres were filtered, washed, dried and then titrated to determine whether there was any STA on the surface. The microspheres were also crushed and titrated to determine the presence of STA in the microspheres. STA is found only in crushed samples. No presence of STA was observed in the uncrushed samples. Quantitative analysis of STA encapsulation was made by determining the amount of elemental tungsten using INAA. The weight percents of tungsten detected in the catalysts are given in Table 2. The reproducibility of the microencapsulation 16716
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Table 4. Tungsten Amounts of Fresh and Spent Catalyst Determined by INAA catalyst
tungsten (% by wt)
C2 (fresh) C2 (after run 3)
4.2 4.2
of the catalysts is thought to prevent leaching of STA, resulting in preservation of the activity in repeated runs. As discussed in the previous section, the catalysts have micropores and low surface areas. However, no essential diffusion limitation was observed during the reaction. This is believed to be due to the swelling of the silica enabling interaction of the reactants with STA. The catalyst structure needs to be studied further to understand the mechanism of diffusion of reactants and products in and out of the microspheres.
4. CONCLUSIONS Microencapsulation was found to be an effective method to prevent the leaching of the active catalyst material (STA). The amount of STA was determined initially and after repeated use in three runs using INAA. No significant change in STA amount was observed. The conversion of acetic acid to ethyl acetate in three consecutive runs with the same catalyst decreased only by 10%. During microencapsulation, STA acted both as the hydrophilic compound and the gelling agent, thus eliminating the use of an additional acid. Elimination of the surfactant during microcapsulation did not adversely effect formation of microspheres and the amount of STA encapsulated.
Figure 2. BJH desorption pore size distributions of (i) catalysts A, C, E, and G and (ii) catalysts E and G.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: (978) 934 3141. Present Address †
Assistant Professor, Chemical and Process Engineering, Bilecik University, Gulumbe Campus, 11210, Bilecik, Turkey. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the Scientific and Technical Research Council of Turkey (TUBITAK) for providing financial support to L.D. under the Science Fellowships and Grant Program (BIDEB). The authors wish to extend their sincere thanks to Prof. Nelson Ebby (University of MassachusettsLowell) for his help and expertise with INAA analysis, to Prof. Daniel Schmidt (University of MassachusettsLowell) for his help in BET, and to Dr. Earl Eda (University of MassachusettsLowell) for his help in SEM analysis.
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Figure 3. (a) Effect of catalyst activity on acetic acid conversion and catalyst stability for repeated runs and (b) percent activity change for repeated runs.
ABBREVIATIONS BET, Brunauer−Emmett−Teller; BJH, Barret−Joyner−Halenda; INAA, instrumental neutron activation analysis; OP, oil phase; SEM, scanning electron microscope; STA, silicotungstic acid; TEOS, tetraethyl orthosilicate; VO, vegetable oil; W, water; WP1, WP water phase 1; WP2, WP water phase 2.
found to be the same. This result indicates that there is no leaching of active material during reactions. This was the primary aim for encapsulating STA. The microporous structure 16717
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Article
REFERENCES
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