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Esterification catalysis using functionalized membranes
Applied Catalysis A: General 296 (2005) 12–20 www.elsevier.com/locate/apcata
Esterification catalysis using functionalized membranes T.N. Shah, S.M.C. Ritchie * Department of Chemical and Biological Engineering, University of Alabama, 201 7th Avenue, Tuscaloosa, AL 35487-0203, USA Received 4 March 2005; received in revised form 24 June 2005; accepted 30 June 2005 Available online 11 October 2005
Abstract Novel functionalized microfiltration membranes have been developed for use as a heterogeneous, solid-phase, flowthrough catalytic structure for the esterification reaction between ethanol and acetic acid. Catalytic sites are located on each repeat unit of sulfonated polystyrene chains grafted in the flow pathways of the membrane. Batch studies showed that the activity of the catalytic membrane was comparable to the standard ionexchange resin (IER) for the same acid capacity. Significant improvement in reaction kinetics was observed in flowthrough studies; a residence time of 20 s gave the same conversion as 11 h in batch reaction. The activation energy in flowthrough reaction was at least 20% lower than for the conventional IER. Some loss of grafted polystyrene (25%) in the reaction permeate was observed during flowthrough experiments, resulting in partial loss of activity. However, an increase in the graft chain length reduced graft loss by 60%, and there was essentially no graft loss when the grafts were covalently bound. # 2005 Elsevier B.V. All rights reserved. Keywords: Microfiltration membrane; Polystyrene grafts; Membrane reactor; Catalysis
1. Introduction Acid catalysis covers a wide range of industrially important reactions. One of the broadly understood and applied reactions is esterification. Homogeneous catalysts such as sulfuric acid, and heterogeneous catalysts such as ion-exchange resins (IERs), are generally employed in conventional esterification processes. Homogeneous acid catalysis is effective; however, it suffers from severe drawbacks such as equipment corrosion, neutralization of the reaction mass, and separation of catalyst from the product stream. In general, these complications have been overcome through the use of heterogeneous catalysts such as strong acid ion-exchange resins [1]. IERs have been used commercially and studied extensively over the past few decades for a variety of esterification and other acid catalyzed reactions [1–10]. IERs are non-corrosive and easy to separate from the reaction mixture. Moreover, they can be reused without handling and storage concerns [7,8]. These catalysts are usually styrene-based sulfonic acid or persulfonic acid-based polymeric catalysts. Several other Abbreviations: A.A., acetic acid; CMR, catalytic membrane reactor; IEC, ion-exchange capacity; IER, ion-exchange resin; PES, polyethersulfone; RPES, raw polyethersulfone * Corresponding author. Tel.: +1 205 348 2712; fax: +1 205 348 7558. E-mail address: [email protected] (S.M.C. Ritchie). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.06.034
heterogeneous catalysts, such as acid treated clays, heteropolyacids, fibrous polymer-supported sulfonic acids, and zeolites (Si/Al), are reported in the literature with similar prominence as IERs for solid acid catalysis [2,11–16]. For example, more than 40% of all solid acid–base catalyst processes are catalyzed by zeolites [2]. However, particle attrition, high pressure drop, and accessibility to the reactants are considered to be major issues of concern for these catalysts. Recently, organosulfonic acid-functionalized mesoporous silicas have shown potential for use as a catalyst for methyl ester formation [22]. Methyl esters are intermediates for value added products such as biodiesel fuel. The mesoporous materials provide relatively high surface areas of 540– 820 m2/g and a range of pore sizes (2–5 nm median pore diameter) that permit improved site accessibility compared to the standard acid resins. The number of active sites in these catalysts is typically in the range of 0.6–1.44 meq/g. These mesoporous catalysts demonstrated almost 1.5 times the activity of Nafion resin (H+ capacity = 0.8 meq/g) and double the activity of Amberlyst-15 (IER, H+ capacity = 4.7 meq/g). However, the activation energy data (55–85 kJ/mol) of different acid-functionalized silicas suggest that the reaction was still limited by activated diffusion. Catalytic flowthrough membrane structures are gaining importance due to their considerably low mass transfer
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limitations [23]. Catalytically active microfiltration (0.1–1 mm pore size) membranes containing sulfonated polystyrene grafts have demonstrated potential as heterogeneous strong acid catalysts [26]. The acid sites located on the polymer grafts are rapidly accessible to the reactive species in a convective flow type configuration. Catalytic membrane reactors (CMRs) of this type are classified as flowthrough contactors [25]. In contrast to the extractor type CMRs, the membrane itself is catalytic. The membrane does not possess any molecular separation properties. Rather, it facilitates contact between the reactants and catalytic sites located in the membrane flow paths. In contrast, extractor type CMRs use pervaporation or vapor permeation membrane to selectively remove one product [17– 21]. The objective of this work was to apply novel membrane catalysts, developed in our laboratory [26], to acid catalyzed reaction systems. The esterification of ethanol and acetic acid (A.A.) to form ethyl acetate was used as the test reaction, and comparisons were made with standard available acid catalysts. The results are discussed in terms of batch reaction kinetics and in a flowthrough reaction system, where vast improvements in reaction kinetics were observed. Results on stability of the grafted membranes, as well as preliminary work to improve membrane stability, are discussed. 2. Experimental 2.1. Materials Commercially available polyethersulfone (PES) microfiltration membranes from Millipore (Bedford, MA) served as the substrate. These membranes were modified with sulfonated polystyrene grafts to impart catalytic activity. All necessary chemicals including styrene, toluene, concentrated sulfuric acid (99 wt%), methanol, and 1N sodium hydroxide for modification and analysis were purchased from Fisher Scientific (Pittsburgh, PA). The monomer-inhibited styrene (99.8% purity) was made inhibitor free by adsorption with alumina and stored at 4 8C until needed [27]. All the other chemicals for catalytic membrane preparation were used as received without further purification. The catalytic activity of the functionalized membranes was compared with conventional homogeneous catalyst (99 wt% concentrated sulfuric acid) and the standard polymeric strong acid resin (Amberlyst-36). The ion-exchange resin was purchased from Aldrich (St. Louis, MO). The resin particles (average Dp = 130 mm) have a reported average pore size of 12 nm and 30% porosity. The reagents used for the catalytic tests included glacial acetic acid and pure ethanol. The reagents, gas chromatography solvents, and standards were obtained from Fisher Scientific. The reagents and solvents used for the reaction studies had purity degrees better than 99% and were used as received. 2.2. Catalytic membrane preparation The membranes were prepared by cationic polymerization of styrene in the pores of the raw PES (RPES) membrane. The
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RPES membrane was modified into catalytic membrane by a simple three step procedure. First, the internal surface of the membrane was mildly sulfonated by contact with dilute (0.5N) sulfuric acid [26]. The immobilized sulfonic acid groups act as the initiator for cationic polymerization of styrene. In the second step, the styrene was polymerized by treatment with styrene/ toluene solution in convective fashion. Finally, the polystyrene grafts were formed in the membrane pores, and were activated by treatment with sulfuric acid (0.5N) solution. A more detailed description of the preparation conditions appears in a previous paper [26]. The results presented in this paper for the membrane catalyst are accomplished through the cationic route. The grafted membranes were also prepared by free-radical polymerization technique to demonstrate the improvement in graft stability due to covalent binding. The RPES membrane was pretreated by an electron beam processor (Energy Sciences Inc.) to produce free radicals. The irradiation was performed at 170 kV with a dosage of 100 kGy for 45 s. The polymer grafts were then formed and activated in a similar fashion as in the first technique. 2.3. Catalytic tests 2.3.1. Reactions in batch mode Batch experiments were carried out in 20 ml glass vials at atmospheric conditions. The ethanol was taken in far molar excess over acetic acid to drive the equilibrium towards ester formation (typically 10:1 mole ratio). The batch reactor volume was 6 cm3. The time at which the catalyst was added to the alcohol/acid mixture was considered as zero-time. The dry membrane was cut into pieces to achieve better contact. Vials were then agitated using a wrist action shaker (Burrell, Model 75) to minimize external mass transfer resistance. The catalytic activity of the membrane was compared with other catalysts by performing the reaction under the identical conditions and equivalent catalyst (H+ meq) capacity. Initial samples were withdrawn frequently to characterize any short-term change in activity. Samples were also collected at longer times to characterize the stability of the catalytic membrane. All catalytic tests were performed at least twice in order to ensure reproducible results. The error bars on all the graphs are generated by taking one standard deviation on the data from at least three repeated experiments. 2.3.2. Reactions in flowthrough process The flowthrough experiments were carried out by permeating a solution of ethanol and glacial acetic acid (10:1 mole ratio) through the catalytic membrane under a typical pressure drop of 28 psi. The reactor volume was considered as a porous membrane volume of 0.152 cm3. The convective flow of reagents through the dry membrane was maintained throughout the experiment. The complete experimental setup has been described in our previous work [26]. The feed tank was washed with water (pH 7) and dried completely before any convective run. The permeate was collected in fractions starting at zero reaction time with no recycle. The reaction temperature was controlled by external preheating of the feed tank with heating
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tape (115 V, Electrothermal), and the feed temperature was measured using a digital thermometer with stainless-steel probe (3 mm 18 cm, Fisher Scientific). The feed tank and the membrane holder were insulated to prevent heat loss. The system was pressurized and brought to temperature within 1 8C before any reactant permeation. Sample analysis was accomplished in similar fashion to the batch studies.
Table 1 Physical properties of heterogeneous catalysts used in this work
Average pore size (nm) Porosity (%) Density (g/cm3) Average BET surface area (m2/g) IEC (meq/g)
Catalytic membrane
Amberlyst-36
220 70 0.41 14.8 2.05
12 30 1.2 25 4.5
2.4. Analytical procedure All reaction samples were analyzed using gas chromatography (Shimadzu GC-14 A) equipped with a flame ionization detector. A 30 m 0.53 mm i.d. capillary column (Restek Corporation) containing a polyethylene–glycol-based stationary phase was used for the analysis. The temperature of the oven was programmed as 40 8C for 1 min, followed by ramping up at 10 8C/min for 16 min to 200 8C. Helium was used as the carrier gas. The analysis was conducted only with respect to ethyl acetate formation. Extraction of the polystyrene grafts into the esterification reaction permeate was studied by means of UV–vis spectrometry (Shimadzu UV-2401). The presence of sulfonated polystyrene was indicated by the absorbance peak corresponding to a band at 291 nm. The absorbance was directly proportional to the aromatic ring concentration, and hence UV–vis data provided quantitative information of extracted grafts as a function of time for flowthrough studies. 2.5. Catalyst characterization 2.5.1. Surface area measurements The internal surface area of the catalytic membrane applied in this work was measured by the Brunauer–Emmett–Teller (BET) method (ChemBET-3000, Quantachrome) using at least two cycles of nitrogen adsorption and desorption. Samples of known weight were placed in a glass column, dried, and degassed by heating in a mantle at 80 8C for 3 h. The membrane was cut into long strips in order to place it in the column. The average surface area was determined using the single point BET method.
dried membrane sample was coated by gold-sputtering to provide electrical conductivity. The micrograph was taken with a Philips XL 30 scanning electron microscope (SEM) operating at 5 kV. 3. Results and discussion 3.1. Catalyst characteristics The physical properties of the catalytic membrane and Amberlyst-36 are summarized in Table 1. Although the pore sizes of the catalytic membrane and the resin were very different, the higher porosity of the catalytic membrane resulted in very similar BET surface areas (14.8–25 m2/g) for the two materials. Polymer chains were grafted to the internal surface area of the membrane, which represents greater than 99.9% of the total membrane surface area. The characteristic length scales for the synthesized catalytic membrane and Amberlyst36 were measured to be 165 and 130 mm, respectively, resulting in comparable diffusion lengths for each material. The catalytic membrane is made of thermally stable polyethersulfone (Tg = 225 8C), and has a highly open structure, as shown by the cross-sectional view in Fig. 1. The chemical stability of the catalytic membranes has been verified using FTIR spectroscopy, which showed negligible change in the characteristic absorption band of sulfonic acid sites after treatment in dilute sulfuric acid and sodium hydroxide [26]. The acid strength of the membrane catalyst
2.5.2. Acid capacity determination The acid (H+ meq/g) capacities of the catalytic membrane and the wet Amberlyst-36 were quantified by elemental analysis of regenerated sodium ions in acid solution using atomic absorption spectroscopy (Varian 220 FS). A material balance on sodium ion gave the ion-exchange capacity (IEC) of each material. The exchange capacity of the resin was confirmed by simple acid–base titration. A known weight of resin sample (acid form) was added to 10 ml of the standard 1N sodium hydroxide solution and allowed to equilibrate for a day. Thereafter, the sodium ions from the base treated IER solution were quantified by titration. The mixture was titrated with drop wise addition of 0.1N HCl solution (Fisher Scientific). 2.5.3. Electron microscopy Scanning electron micrographs were obtained for the crosssection of a catalytic polymer (asymmetric) membrane. The
Fig. 1. Scanning electron micrograph of the catalytic membrane cross-section.
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was found to be 2.05 meq H+/g. The acid capacity of the IER (Amberlyst-36 wet) was measured to be 4.5 meq/g and was found to be in good agreement with the commercial reported value. 3.2. Batch reaction studies 3.2.1. Comparison with conventional catalysts Batch mode experiments on different catalysts were conducted under similar conditions of reactant concentration (mole ratio), agitation speed, and temperature. However, heterogeneous catalysts with a different number of active sites were considered. Consequently, the comparison of reaction performance of catalysts was made based on acid capacity, rather than mass-normalized loading [7,9,15]. The results for reaction studies performed at 25 8C with an ethanol to acetic acid ratio of 10:1 (mole ratio) are shown in Fig. 2. An acid capacity of 0.18 meq was used in each case. As expected, H2SO4 displayed the highest reaction rate with an overall conversion of 43% in 11 h at room temperature. A control run with no catalyst revealed negligible reactivity in contrast to the catalyzed reactions. As shown in the figure, the membrane demonstrated comparable catalytic activity to that of the Amberlyst-36. The results after 11 h showed 16 and 13% conversion to ethyl acetate for the catalytic membrane and Amberlyst-36, respectively. The reason for the difference in the activities could be attributed to the different pore architecture of the catalyst materials. The result suggests that the active sites of the porous membrane are somewhat more accessible in comparison to the resins under similar batch reaction conditions. A pseudo-first-order reaction model with respect to acetic acid was assumed because ethanol was in excess. The assumption is supported by the reasonable fits between the trend lines and the experimental data shown in Fig. 3. The slopes of the trend lines for different catalysts yield the rate
Fig. 2. Catalytic results for the esterification of acetic acid with ethanol in batch reaction.
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constants. In all cases, the trend line intercepts reduce to the logarithm of the initial acid concentration. The dominance of the H2SO4 activity was due to the internal mass transfer limitations encountered with the heterogeneous catalysts. The role of internal mass transfer resistance inside the catalyst material was reflected by calculation of the effectiveness factors. The molecular diffusion coefficient is obtained by Wilke–Chang equation (1.82 109 m2/s) and the effective diffusion coefficients (De) of catalysts are determined by setting the tortuosity to 2.1 (packed beds). The membrane was considered as a slab with an effective diffusion path length (L) of half of the membrane thickness (see Eq. (3.2)). The IER particles were considered as sphere with radius R for the effectiveness factor calculations (Eqs. (3.1) and (3.2)).
hmembrane ¼
hresin
3
pffiffiffiffiffiffiffiffiffiffiffiffi De =k1 L
pffiffiffiffiffiffiffiffiffiffiffiffi De =k1 ¼ R 9
(3.1)
(3.2)
The specific rate constants (k1) were calculated from the assumed pseudo-first-order kinetics with respect to acetic acid. Calculations showed that the effectiveness factors for catalytic membrane and the resin were 0.83 and 0.77, respectively. This observation gives secondary confirmation of the slightly better site accessibilities in case of membrane catalyst relative to the IER for the same number of acid sites. 3.2.2. Effect of reactant molar ratio It has been observed that the reactant molar ratio also has an effect on the reaction rate for pseudo-first-order esterification kinetics [4,5,7]. The initial molar ratio of ethanol to acetic acid was varied from 5:1 to 10:1 to 15:1 at room temperature. The
Fig. 3. Pseudo-first-order kinetics for the esterification of acetic acid with ethanol in batch reaction.
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Fig. 4. Effect of EtOH:A.A. mole ratio on ethyl acetate formation in batch reaction.
results are shown in Fig. 4, and reflect that the initial rate of the reaction is directly related to the ethanol concentration. As the mole ratio of ethanol to acetic was increased, the conversion of acetic acid increased due to increased excess ethanol. This resulted in a corresponding increase in the pseudo-first-order constant k1. The kinetic behavior of the catalytic membrane was similar to IERs in batch studies. The true kinetic improvements of the reaction using these catalytic membrane structures were revealed by the experiments in flowthrough configuration. The initial rate of reaction obtained from Fig. 4 was plotted against the ethanol concentration to deduce the order of ethanol. The slope of the graph (see Fig. 5) gives the order of ethanol which was found to be 1. Thus, the batch kinetics of esterification reaction suggests that the reaction is first-order
Fig. 5. Determination of order of ethanol based on initial rate of reaction.
Fig. 6. Effect of catalyst loading on the esterification of acetic acid with ethanol in batch reaction.
with respect to each reactant. The apparent rate constants based on the second-order kinetics are derived from Fig. 2. 3.2.3. Effect of catalyst loading A comparison of different IERs and other heterogeneous catalysts in terms of catalyst loading has been previously offered in the literature [8,15]. It was observed that the conversion of acetic acid increased with a proportional increase in the acid groups of the solid heterogeneous catalyst. Similar behavior was observed in this work for batch reactions at different catalyst loadings based on the same total volume of reactants under similar conditions. Fig. 6 shows that the apparent first-order rate constants based on the initial rate of reaction were observed to be directly proportional to the
Fig. 7. Homogeneous batch reaction for identical acid site density (per unit reaction volume) as the catalytic membrane flowthrough reactor.
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catalyst loading. In both cases, the rate of reaction is a linear function of the catalyst mass, with values of Amberlyst-36 approximately 20% higher than for catalytic membrane for the same catalyst mass. This is mainly because of the higher number of acid sites available in Amberlyst-36 per gram of catalyst material. However, theoretically one would expect the rate constants for the Amberlyst-36 to be much higher than membrane catalyst since the acid capacity of Amberlyst is about 4.5 meq/g compared to 2.0 meq/g for the membrane catalyst. This behavior could be explained by the fact that the experimentally determined IEC (<3 meq/g) at 200 min was less than the maximum theoretical acid capacity at equilibrium (4.5 meq/g). The membrane catalyst performance was slightly better than Amberlyst-36 on a site basis (Fig. 2). This suggests that the modified membranes are catalytically more efficient than resins for the same number of sites, most likely due to improved site accessibility. 3.3. Flowthrough reaction studies Although external mass transfer resistance can be minimized for resins, reactants are transported inside the particles by diffusion. A key difference between the catalytic membranes developed in this research and IERs is that transport of reactants to catalyst sites can achieved by convection. These are microfiltration membranes, and therefore only a small pressure drop (1–2 bar) was needed for membrane permeation. Catalyst sites were located on polystyrene grafts in the membrane flow pathways, and therefore internal mass transfer resistance was minimized. 3.3.1. Comparison to batch reaction kinetics The room temperature conversion of acetic acid was 16% in a residence time of less than 20 s. In comparison, the homogeneous and heterogeneous batch reactions took 360 and 2000 times longer to reach the same conversion (recall Fig. 2). Notice that the density of acid sites in flowthrough reaction is comparatively higher (0.18 meq H+ per 0.152 cm3 membrane reactor volume) than the batch reaction (0.18 meq H+ per 6 cm3 reactor volume). In other words, the local acid concentration in the membrane is 1.18N, which is 65 times higher than the acid concentration in the homogeneous batch reaction. The improved kinetic performance of the catalytic membrane is due to the fact that the transport of the reactants by convection greatly enhances reactant access to catalytic sites located in the membrane pores. As a result, the behavior of the catalytic membrane in flowthrough reaction should resemble that of a homogeneous catalyst where there is optimal site accessibility. This was demonstrated by comparing the apparent rate constants normalized with the number of acid sites for the flowthrough reaction and homogeneous batch reaction. A batch reaction using sulfuric acid as the catalyst was conducted with the same catalyst concentration of 1.18N. The outcome of the acetic acid conversion at room temperature over a longer period of time (10–40 min) is shown in Fig. 7. The
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experiment could not be conducted at very low times (<1 min) because of the presence of sulfuric acid in the product that requires neutralization. Acetic acid conversions at lower times were determined by applying a Langmuir–Hinshelwood model to correlate the experimental kinetic data according to the following equation: n¼
neq kt 1 þ kt
(3.3)
where n is the gmol of ethyl acetate formed at time t, k the specific rate constant (s1), and neq is the gmol of ethyl acetate formed at equilibrium. The equation can be rewritten in a linearized form as follows: t 1 1 ¼ (3.4) tþ n neq neq k The slope and the intercept of the trend lines yield the equilibrium gmol of ethyl acetate and the specific rate constant. Using these data, it was observed that the experimental results fit the model very well (Fig. 7). Based on the model, for the homogeneous batch case, the initial rate constant at 20 s residence time was calculated to be 0.0041 s1. The initial rate constant from the flowthrough esterification reaction at 20 s residence time using the catalytic membrane was calculated to be 0.0037 s1. This result shows that the flowthrough reaction approaches the behavior of the homogeneous catalyst for the same number of acid sites in a fixed reactor volume. 3.3.2. Effect of residence time In order to underline the role of residence time, the reaction was performed at different reactant permeation rates with the same initial feed concentration in each case. Other conditions such as temperature and catalyst loading were kept constant. Increased residence time should result in an increase in the conversion. The data in Fig. 8 clearly indicate that conversion in the flowthrough system was directly proportional to the
Fig. 8. Effect of residence time on ethyl acetate formation in flowthrough reaction.
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Fig. 9. Arrhenius relationship for esterification to form ethyl acetate in a catalytic membrane with flowthrough reaction.
Fig. 10. Catalytic membrane performance over time for the esterification of acetic acid with ethanol in flowthrough reaction.
residence time. This correlation would be expected to diminish upon approaching the equilibrium conversion. However, higher temperature studies showed a similar linear relationship up to at least 65% conversion, which may provide further evidence for the benefit of improved accessibility.
experiment where a reactant flow rate of 80 ml/h (residence time 10 s) was maintained to give an overall average conversion of 8%. The unmodified membrane exhibited insignificant activity under similar reaction conditions. The initial membrane activity was very high, but this decreased sharply in the first 2 h, followed by a leveling off at around 8%. This was most likely due to a loss of functionalized polymeric grafts, resulting in a decreased number of active sites. The membranes were reasonably stable during formation, with no loss of grafted chains during activation with 0.5N sulfuric acid and titration with 0.4N NaOH solution [26]. However, there was some loss of membrane grafts in the flowthrough reaction at higher feed ethanol concentrations, as indicated by the absorbance peak at 291 nm in Fig. 11. The amount of styrene
3.3.3. Effect of temperature The effect of temperature on conversion was studied in the range of 298–343 K, and an Arrhenius plot is shown in Fig. 9. The parameters of feed flow rate, catalyst loading, reactor volume, and reactant mole ratio were kept constant. The conversion increased from 16 to 65% over an increment of 458 at a residence time of less than 20 s. The activation energy was determined to be 31.26 kJ/mol, and the corresponding frequency factor was 18.81 1012 (m3)2 kmol2 s1. Kinetic data from literature show that the activation energy using IERs for the same reaction was 45–120 kJ/mol [28,29]. Rapid site accessibility and very low internal diffusion resistance during convective flow led to high reaction rate constants. As a result, the activation energy of the reaction would be lower for catalytic functionalized membranes compared to IERs. 3.3.4. Catalytic membrane stability The advantage of using cationic polymerization techniques is that it offers excellent range of internal surface modification [26]. In addition, this chemical method would be very conducive for scale-up in industry. The catalytic membrane stability can be enhanced by improving the molecular weight of the grafts that would provide better entanglement. UV analysis of the reaction volume for the batch process revealed no leaching of the functionalized polymer. Therefore, there was no deactivation of active sites in the batch process. Changes in the membrane activity during flowthrough reaction were observed as a function of time. Fig. 10 shows the average conversion of acetic acid over the course of a typical
Fig. 11. UV spectra of esterification reaction permeate to characterize loss of sulfonated polystyrene grafts.
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4. Conclusions
Fig. 12. Improvement in graft stability by varying the polymerization procedure during catalytic membrane preparation.
that underwent polymerization during the catalytic membrane preparation was known, and therefore the loss of polymer chains could be quantified. It was determined that approximately 25% of the grafted polystyrene chains were extracted in the reaction permeate. In the case of cationic polymerization, the graft interaction with the membrane pore walls was by an ionic route. There are two potential explanations for the loss of electrostatically bound polystyrene in the reaction permeates. First, an excess of acetic acid might result in masking of the electrostatic interaction between the polystyrene graft and the immobilized sulfonic acid. However, no graft loss was observed when excess acetic acid was used. The second possibility might be ionization suppression of the carbocation present on the polymeric grafts by excess ethanol. The UV-spectrum of pure ethanol permeated through the catalytic membrane showed the presence of grafted polymer, verifying at least partial graft loss by suppression of ionization. In terms of practicality of catalytic membranes, graft loss would be a key issue. The bar graph in Fig. 12 shows an overall picture of how graft stability was improved by changing the procedure for polystyrene grafting. An increase in the average number of repeat units per graft may prevent polymer loss through better entanglement with the membrane structure. This was achieved by increasing the styrene concentration to 20% (volume) during the cationic polymerization step of membrane modification. The graft length was increased from approximately 100 repeat units to 170 repeat units, resulting in a 60% decrease in graft loss. The catalytic membranes prepared by electron beam radiation grafting had an average chain length of 300 repeat units. Preliminary studies on the esterification reaction using such grafted membranes showed only 2% graft loss. This was attributed to covalent bonding of the grafted chains to the membrane during free-radical polymerization resulting in negligible loss of functionalized polymer.
Polyethersulfone microfiltration membranes containing sulfonated polystyrene grafts have been shown to exhibit clear benefits compared to the commercially available ion-exchange resins and homogeneous catalysts for a simple esterification reaction between acetic acid and ethanol. The catalytic membranes possess good site accessibility with no separation and corrosion problems. The catalytic membrane reactivity was on par with Amberlyst-36, and two to three times lower than the homogeneous catalyst (sulfuric acid) in batch processes. This was mainly due to internal mass transfer resistance in heterogeneous catalysts. The catalytic membranes performed very well in flowthrough mode, showing substantially higher catalytic performance compared to the batch process under similar reaction conditions. This observation was predominantly due to the virtual elimination of internal mass transfer resistance, and was confirmed by modeling of the homogeneous system at equivalent catalyst concentration. The catalytic performance was governed by the residence time provided to the reactive species. Although there was no observed loss of grafts in batch reaction, excess ethanol in flowthrough reaction played a vital role in suppressing carbocation ionization leading to a moderate loss (25%) of non-entangled grafts. An increase in the molecular weight of the sulfonated polystyrene grafts decreased the graft loss by 60%. When the membrane catalyst was prepared by free-radical polymerization, graft loss was negligible. The kinetic parameters reveal that the activation energy using membrane catalyst (32 kJ/mol) in flowthrough configuration was lower than the conventional IERs (>45 kJ/ mol). Acknowledgement This work is partially supported by the University of Alabama Graduate Council Research and Creative Activity Fellowship program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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Esterification catalysis using functionalized membranes T.N. Shah, S.M.C. Ritchie * Department of Chemical and Biological Engineering, University of Alabama, 201 7th Avenue, Tuscaloosa, AL 35487-0203, USA Received 4 March 2005; received in revised form 24 June 2005; accepted 30 June 2005 Available online 11 October 2005
Abstract Novel functionalized microfiltration membranes have been developed for use as a heterogeneous, solid-phase, flowthrough catalytic structure for the esterification reaction between ethanol and acetic acid. Catalytic sites are located on each repeat unit of sulfonated polystyrene chains grafted in the flow pathways of the membrane. Batch studies showed that the activity of the catalytic membrane was comparable to the standard ionexchange resin (IER) for the same acid capacity. Significant improvement in reaction kinetics was observed in flowthrough studies; a residence time of 20 s gave the same conversion as 11 h in batch reaction. The activation energy in flowthrough reaction was at least 20% lower than for the conventional IER. Some loss of grafted polystyrene (25%) in the reaction permeate was observed during flowthrough experiments, resulting in partial loss of activity. However, an increase in the graft chain length reduced graft loss by 60%, and there was essentially no graft loss when the grafts were covalently bound. # 2005 Elsevier B.V. All rights reserved. Keywords: Microfiltration membrane; Polystyrene grafts; Membrane reactor; Catalysis
1. Introduction Acid catalysis covers a wide range of industrially important reactions. One of the broadly understood and applied reactions is esterification. Homogeneous catalysts such as sulfuric acid, and heterogeneous catalysts such as ion-exchange resins (IERs), are generally employed in conventional esterification processes. Homogeneous acid catalysis is effective; however, it suffers from severe drawbacks such as equipment corrosion, neutralization of the reaction mass, and separation of catalyst from the product stream. In general, these complications have been overcome through the use of heterogeneous catalysts such as strong acid ion-exchange resins [1]. IERs have been used commercially and studied extensively over the past few decades for a variety of esterification and other acid catalyzed reactions [1–10]. IERs are non-corrosive and easy to separate from the reaction mixture. Moreover, they can be reused without handling and storage concerns [7,8]. These catalysts are usually styrene-based sulfonic acid or persulfonic acid-based polymeric catalysts. Several other Abbreviations: A.A., acetic acid; CMR, catalytic membrane reactor; IEC, ion-exchange capacity; IER, ion-exchange resin; PES, polyethersulfone; RPES, raw polyethersulfone * Corresponding author. Tel.: +1 205 348 2712; fax: +1 205 348 7558. E-mail address: [email protected] (S.M.C. Ritchie). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.06.034
heterogeneous catalysts, such as acid treated clays, heteropolyacids, fibrous polymer-supported sulfonic acids, and zeolites (Si/Al), are reported in the literature with similar prominence as IERs for solid acid catalysis [2,11–16]. For example, more than 40% of all solid acid–base catalyst processes are catalyzed by zeolites [2]. However, particle attrition, high pressure drop, and accessibility to the reactants are considered to be major issues of concern for these catalysts. Recently, organosulfonic acid-functionalized mesoporous silicas have shown potential for use as a catalyst for methyl ester formation [22]. Methyl esters are intermediates for value added products such as biodiesel fuel. The mesoporous materials provide relatively high surface areas of 540– 820 m2/g and a range of pore sizes (2–5 nm median pore diameter) that permit improved site accessibility compared to the standard acid resins. The number of active sites in these catalysts is typically in the range of 0.6–1.44 meq/g. These mesoporous catalysts demonstrated almost 1.5 times the activity of Nafion resin (H+ capacity = 0.8 meq/g) and double the activity of Amberlyst-15 (IER, H+ capacity = 4.7 meq/g). However, the activation energy data (55–85 kJ/mol) of different acid-functionalized silicas suggest that the reaction was still limited by activated diffusion. Catalytic flowthrough membrane structures are gaining importance due to their considerably low mass transfer
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limitations [23]. Catalytically active microfiltration (0.1–1 mm pore size) membranes containing sulfonated polystyrene grafts have demonstrated potential as heterogeneous strong acid catalysts [26]. The acid sites located on the polymer grafts are rapidly accessible to the reactive species in a convective flow type configuration. Catalytic membrane reactors (CMRs) of this type are classified as flowthrough contactors [25]. In contrast to the extractor type CMRs, the membrane itself is catalytic. The membrane does not possess any molecular separation properties. Rather, it facilitates contact between the reactants and catalytic sites located in the membrane flow paths. In contrast, extractor type CMRs use pervaporation or vapor permeation membrane to selectively remove one product [17– 21]. The objective of this work was to apply novel membrane catalysts, developed in our laboratory [26], to acid catalyzed reaction systems. The esterification of ethanol and acetic acid (A.A.) to form ethyl acetate was used as the test reaction, and comparisons were made with standard available acid catalysts. The results are discussed in terms of batch reaction kinetics and in a flowthrough reaction system, where vast improvements in reaction kinetics were observed. Results on stability of the grafted membranes, as well as preliminary work to improve membrane stability, are discussed. 2. Experimental 2.1. Materials Commercially available polyethersulfone (PES) microfiltration membranes from Millipore (Bedford, MA) served as the substrate. These membranes were modified with sulfonated polystyrene grafts to impart catalytic activity. All necessary chemicals including styrene, toluene, concentrated sulfuric acid (99 wt%), methanol, and 1N sodium hydroxide for modification and analysis were purchased from Fisher Scientific (Pittsburgh, PA). The monomer-inhibited styrene (99.8% purity) was made inhibitor free by adsorption with alumina and stored at 4 8C until needed [27]. All the other chemicals for catalytic membrane preparation were used as received without further purification. The catalytic activity of the functionalized membranes was compared with conventional homogeneous catalyst (99 wt% concentrated sulfuric acid) and the standard polymeric strong acid resin (Amberlyst-36). The ion-exchange resin was purchased from Aldrich (St. Louis, MO). The resin particles (average Dp = 130 mm) have a reported average pore size of 12 nm and 30% porosity. The reagents used for the catalytic tests included glacial acetic acid and pure ethanol. The reagents, gas chromatography solvents, and standards were obtained from Fisher Scientific. The reagents and solvents used for the reaction studies had purity degrees better than 99% and were used as received. 2.2. Catalytic membrane preparation The membranes were prepared by cationic polymerization of styrene in the pores of the raw PES (RPES) membrane. The
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RPES membrane was modified into catalytic membrane by a simple three step procedure. First, the internal surface of the membrane was mildly sulfonated by contact with dilute (0.5N) sulfuric acid [26]. The immobilized sulfonic acid groups act as the initiator for cationic polymerization of styrene. In the second step, the styrene was polymerized by treatment with styrene/ toluene solution in convective fashion. Finally, the polystyrene grafts were formed in the membrane pores, and were activated by treatment with sulfuric acid (0.5N) solution. A more detailed description of the preparation conditions appears in a previous paper [26]. The results presented in this paper for the membrane catalyst are accomplished through the cationic route. The grafted membranes were also prepared by free-radical polymerization technique to demonstrate the improvement in graft stability due to covalent binding. The RPES membrane was pretreated by an electron beam processor (Energy Sciences Inc.) to produce free radicals. The irradiation was performed at 170 kV with a dosage of 100 kGy for 45 s. The polymer grafts were then formed and activated in a similar fashion as in the first technique. 2.3. Catalytic tests 2.3.1. Reactions in batch mode Batch experiments were carried out in 20 ml glass vials at atmospheric conditions. The ethanol was taken in far molar excess over acetic acid to drive the equilibrium towards ester formation (typically 10:1 mole ratio). The batch reactor volume was 6 cm3. The time at which the catalyst was added to the alcohol/acid mixture was considered as zero-time. The dry membrane was cut into pieces to achieve better contact. Vials were then agitated using a wrist action shaker (Burrell, Model 75) to minimize external mass transfer resistance. The catalytic activity of the membrane was compared with other catalysts by performing the reaction under the identical conditions and equivalent catalyst (H+ meq) capacity. Initial samples were withdrawn frequently to characterize any short-term change in activity. Samples were also collected at longer times to characterize the stability of the catalytic membrane. All catalytic tests were performed at least twice in order to ensure reproducible results. The error bars on all the graphs are generated by taking one standard deviation on the data from at least three repeated experiments. 2.3.2. Reactions in flowthrough process The flowthrough experiments were carried out by permeating a solution of ethanol and glacial acetic acid (10:1 mole ratio) through the catalytic membrane under a typical pressure drop of 28 psi. The reactor volume was considered as a porous membrane volume of 0.152 cm3. The convective flow of reagents through the dry membrane was maintained throughout the experiment. The complete experimental setup has been described in our previous work [26]. The feed tank was washed with water (pH 7) and dried completely before any convective run. The permeate was collected in fractions starting at zero reaction time with no recycle. The reaction temperature was controlled by external preheating of the feed tank with heating
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tape (115 V, Electrothermal), and the feed temperature was measured using a digital thermometer with stainless-steel probe (3 mm 18 cm, Fisher Scientific). The feed tank and the membrane holder were insulated to prevent heat loss. The system was pressurized and brought to temperature within 1 8C before any reactant permeation. Sample analysis was accomplished in similar fashion to the batch studies.
Table 1 Physical properties of heterogeneous catalysts used in this work
Average pore size (nm) Porosity (%) Density (g/cm3) Average BET surface area (m2/g) IEC (meq/g)
Catalytic membrane
Amberlyst-36
220 70 0.41 14.8 2.05
12 30 1.2 25 4.5
2.4. Analytical procedure All reaction samples were analyzed using gas chromatography (Shimadzu GC-14 A) equipped with a flame ionization detector. A 30 m 0.53 mm i.d. capillary column (Restek Corporation) containing a polyethylene–glycol-based stationary phase was used for the analysis. The temperature of the oven was programmed as 40 8C for 1 min, followed by ramping up at 10 8C/min for 16 min to 200 8C. Helium was used as the carrier gas. The analysis was conducted only with respect to ethyl acetate formation. Extraction of the polystyrene grafts into the esterification reaction permeate was studied by means of UV–vis spectrometry (Shimadzu UV-2401). The presence of sulfonated polystyrene was indicated by the absorbance peak corresponding to a band at 291 nm. The absorbance was directly proportional to the aromatic ring concentration, and hence UV–vis data provided quantitative information of extracted grafts as a function of time for flowthrough studies. 2.5. Catalyst characterization 2.5.1. Surface area measurements The internal surface area of the catalytic membrane applied in this work was measured by the Brunauer–Emmett–Teller (BET) method (ChemBET-3000, Quantachrome) using at least two cycles of nitrogen adsorption and desorption. Samples of known weight were placed in a glass column, dried, and degassed by heating in a mantle at 80 8C for 3 h. The membrane was cut into long strips in order to place it in the column. The average surface area was determined using the single point BET method.
dried membrane sample was coated by gold-sputtering to provide electrical conductivity. The micrograph was taken with a Philips XL 30 scanning electron microscope (SEM) operating at 5 kV. 3. Results and discussion 3.1. Catalyst characteristics The physical properties of the catalytic membrane and Amberlyst-36 are summarized in Table 1. Although the pore sizes of the catalytic membrane and the resin were very different, the higher porosity of the catalytic membrane resulted in very similar BET surface areas (14.8–25 m2/g) for the two materials. Polymer chains were grafted to the internal surface area of the membrane, which represents greater than 99.9% of the total membrane surface area. The characteristic length scales for the synthesized catalytic membrane and Amberlyst36 were measured to be 165 and 130 mm, respectively, resulting in comparable diffusion lengths for each material. The catalytic membrane is made of thermally stable polyethersulfone (Tg = 225 8C), and has a highly open structure, as shown by the cross-sectional view in Fig. 1. The chemical stability of the catalytic membranes has been verified using FTIR spectroscopy, which showed negligible change in the characteristic absorption band of sulfonic acid sites after treatment in dilute sulfuric acid and sodium hydroxide [26]. The acid strength of the membrane catalyst
2.5.2. Acid capacity determination The acid (H+ meq/g) capacities of the catalytic membrane and the wet Amberlyst-36 were quantified by elemental analysis of regenerated sodium ions in acid solution using atomic absorption spectroscopy (Varian 220 FS). A material balance on sodium ion gave the ion-exchange capacity (IEC) of each material. The exchange capacity of the resin was confirmed by simple acid–base titration. A known weight of resin sample (acid form) was added to 10 ml of the standard 1N sodium hydroxide solution and allowed to equilibrate for a day. Thereafter, the sodium ions from the base treated IER solution were quantified by titration. The mixture was titrated with drop wise addition of 0.1N HCl solution (Fisher Scientific). 2.5.3. Electron microscopy Scanning electron micrographs were obtained for the crosssection of a catalytic polymer (asymmetric) membrane. The
Fig. 1. Scanning electron micrograph of the catalytic membrane cross-section.
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was found to be 2.05 meq H+/g. The acid capacity of the IER (Amberlyst-36 wet) was measured to be 4.5 meq/g and was found to be in good agreement with the commercial reported value. 3.2. Batch reaction studies 3.2.1. Comparison with conventional catalysts Batch mode experiments on different catalysts were conducted under similar conditions of reactant concentration (mole ratio), agitation speed, and temperature. However, heterogeneous catalysts with a different number of active sites were considered. Consequently, the comparison of reaction performance of catalysts was made based on acid capacity, rather than mass-normalized loading [7,9,15]. The results for reaction studies performed at 25 8C with an ethanol to acetic acid ratio of 10:1 (mole ratio) are shown in Fig. 2. An acid capacity of 0.18 meq was used in each case. As expected, H2SO4 displayed the highest reaction rate with an overall conversion of 43% in 11 h at room temperature. A control run with no catalyst revealed negligible reactivity in contrast to the catalyzed reactions. As shown in the figure, the membrane demonstrated comparable catalytic activity to that of the Amberlyst-36. The results after 11 h showed 16 and 13% conversion to ethyl acetate for the catalytic membrane and Amberlyst-36, respectively. The reason for the difference in the activities could be attributed to the different pore architecture of the catalyst materials. The result suggests that the active sites of the porous membrane are somewhat more accessible in comparison to the resins under similar batch reaction conditions. A pseudo-first-order reaction model with respect to acetic acid was assumed because ethanol was in excess. The assumption is supported by the reasonable fits between the trend lines and the experimental data shown in Fig. 3. The slopes of the trend lines for different catalysts yield the rate
Fig. 2. Catalytic results for the esterification of acetic acid with ethanol in batch reaction.
15
constants. In all cases, the trend line intercepts reduce to the logarithm of the initial acid concentration. The dominance of the H2SO4 activity was due to the internal mass transfer limitations encountered with the heterogeneous catalysts. The role of internal mass transfer resistance inside the catalyst material was reflected by calculation of the effectiveness factors. The molecular diffusion coefficient is obtained by Wilke–Chang equation (1.82 109 m2/s) and the effective diffusion coefficients (De) of catalysts are determined by setting the tortuosity to 2.1 (packed beds). The membrane was considered as a slab with an effective diffusion path length (L) of half of the membrane thickness (see Eq. (3.2)). The IER particles were considered as sphere with radius R for the effectiveness factor calculations (Eqs. (3.1) and (3.2)).
hmembrane ¼
hresin
3
pffiffiffiffiffiffiffiffiffiffiffiffi De =k1 L
pffiffiffiffiffiffiffiffiffiffiffiffi De =k1 ¼ R 9
(3.1)
(3.2)
The specific rate constants (k1) were calculated from the assumed pseudo-first-order kinetics with respect to acetic acid. Calculations showed that the effectiveness factors for catalytic membrane and the resin were 0.83 and 0.77, respectively. This observation gives secondary confirmation of the slightly better site accessibilities in case of membrane catalyst relative to the IER for the same number of acid sites. 3.2.2. Effect of reactant molar ratio It has been observed that the reactant molar ratio also has an effect on the reaction rate for pseudo-first-order esterification kinetics [4,5,7]. The initial molar ratio of ethanol to acetic acid was varied from 5:1 to 10:1 to 15:1 at room temperature. The
Fig. 3. Pseudo-first-order kinetics for the esterification of acetic acid with ethanol in batch reaction.
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Fig. 4. Effect of EtOH:A.A. mole ratio on ethyl acetate formation in batch reaction.
results are shown in Fig. 4, and reflect that the initial rate of the reaction is directly related to the ethanol concentration. As the mole ratio of ethanol to acetic was increased, the conversion of acetic acid increased due to increased excess ethanol. This resulted in a corresponding increase in the pseudo-first-order constant k1. The kinetic behavior of the catalytic membrane was similar to IERs in batch studies. The true kinetic improvements of the reaction using these catalytic membrane structures were revealed by the experiments in flowthrough configuration. The initial rate of reaction obtained from Fig. 4 was plotted against the ethanol concentration to deduce the order of ethanol. The slope of the graph (see Fig. 5) gives the order of ethanol which was found to be 1. Thus, the batch kinetics of esterification reaction suggests that the reaction is first-order
Fig. 5. Determination of order of ethanol based on initial rate of reaction.
Fig. 6. Effect of catalyst loading on the esterification of acetic acid with ethanol in batch reaction.
with respect to each reactant. The apparent rate constants based on the second-order kinetics are derived from Fig. 2. 3.2.3. Effect of catalyst loading A comparison of different IERs and other heterogeneous catalysts in terms of catalyst loading has been previously offered in the literature [8,15]. It was observed that the conversion of acetic acid increased with a proportional increase in the acid groups of the solid heterogeneous catalyst. Similar behavior was observed in this work for batch reactions at different catalyst loadings based on the same total volume of reactants under similar conditions. Fig. 6 shows that the apparent first-order rate constants based on the initial rate of reaction were observed to be directly proportional to the
Fig. 7. Homogeneous batch reaction for identical acid site density (per unit reaction volume) as the catalytic membrane flowthrough reactor.
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catalyst loading. In both cases, the rate of reaction is a linear function of the catalyst mass, with values of Amberlyst-36 approximately 20% higher than for catalytic membrane for the same catalyst mass. This is mainly because of the higher number of acid sites available in Amberlyst-36 per gram of catalyst material. However, theoretically one would expect the rate constants for the Amberlyst-36 to be much higher than membrane catalyst since the acid capacity of Amberlyst is about 4.5 meq/g compared to 2.0 meq/g for the membrane catalyst. This behavior could be explained by the fact that the experimentally determined IEC (<3 meq/g) at 200 min was less than the maximum theoretical acid capacity at equilibrium (4.5 meq/g). The membrane catalyst performance was slightly better than Amberlyst-36 on a site basis (Fig. 2). This suggests that the modified membranes are catalytically more efficient than resins for the same number of sites, most likely due to improved site accessibility. 3.3. Flowthrough reaction studies Although external mass transfer resistance can be minimized for resins, reactants are transported inside the particles by diffusion. A key difference between the catalytic membranes developed in this research and IERs is that transport of reactants to catalyst sites can achieved by convection. These are microfiltration membranes, and therefore only a small pressure drop (1–2 bar) was needed for membrane permeation. Catalyst sites were located on polystyrene grafts in the membrane flow pathways, and therefore internal mass transfer resistance was minimized. 3.3.1. Comparison to batch reaction kinetics The room temperature conversion of acetic acid was 16% in a residence time of less than 20 s. In comparison, the homogeneous and heterogeneous batch reactions took 360 and 2000 times longer to reach the same conversion (recall Fig. 2). Notice that the density of acid sites in flowthrough reaction is comparatively higher (0.18 meq H+ per 0.152 cm3 membrane reactor volume) than the batch reaction (0.18 meq H+ per 6 cm3 reactor volume). In other words, the local acid concentration in the membrane is 1.18N, which is 65 times higher than the acid concentration in the homogeneous batch reaction. The improved kinetic performance of the catalytic membrane is due to the fact that the transport of the reactants by convection greatly enhances reactant access to catalytic sites located in the membrane pores. As a result, the behavior of the catalytic membrane in flowthrough reaction should resemble that of a homogeneous catalyst where there is optimal site accessibility. This was demonstrated by comparing the apparent rate constants normalized with the number of acid sites for the flowthrough reaction and homogeneous batch reaction. A batch reaction using sulfuric acid as the catalyst was conducted with the same catalyst concentration of 1.18N. The outcome of the acetic acid conversion at room temperature over a longer period of time (10–40 min) is shown in Fig. 7. The
17
experiment could not be conducted at very low times (<1 min) because of the presence of sulfuric acid in the product that requires neutralization. Acetic acid conversions at lower times were determined by applying a Langmuir–Hinshelwood model to correlate the experimental kinetic data according to the following equation: n¼
neq kt 1 þ kt
(3.3)
where n is the gmol of ethyl acetate formed at time t, k the specific rate constant (s1), and neq is the gmol of ethyl acetate formed at equilibrium. The equation can be rewritten in a linearized form as follows: t 1 1 ¼ (3.4) tþ n neq neq k The slope and the intercept of the trend lines yield the equilibrium gmol of ethyl acetate and the specific rate constant. Using these data, it was observed that the experimental results fit the model very well (Fig. 7). Based on the model, for the homogeneous batch case, the initial rate constant at 20 s residence time was calculated to be 0.0041 s1. The initial rate constant from the flowthrough esterification reaction at 20 s residence time using the catalytic membrane was calculated to be 0.0037 s1. This result shows that the flowthrough reaction approaches the behavior of the homogeneous catalyst for the same number of acid sites in a fixed reactor volume. 3.3.2. Effect of residence time In order to underline the role of residence time, the reaction was performed at different reactant permeation rates with the same initial feed concentration in each case. Other conditions such as temperature and catalyst loading were kept constant. Increased residence time should result in an increase in the conversion. The data in Fig. 8 clearly indicate that conversion in the flowthrough system was directly proportional to the
Fig. 8. Effect of residence time on ethyl acetate formation in flowthrough reaction.
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Fig. 9. Arrhenius relationship for esterification to form ethyl acetate in a catalytic membrane with flowthrough reaction.
Fig. 10. Catalytic membrane performance over time for the esterification of acetic acid with ethanol in flowthrough reaction.
residence time. This correlation would be expected to diminish upon approaching the equilibrium conversion. However, higher temperature studies showed a similar linear relationship up to at least 65% conversion, which may provide further evidence for the benefit of improved accessibility.
experiment where a reactant flow rate of 80 ml/h (residence time 10 s) was maintained to give an overall average conversion of 8%. The unmodified membrane exhibited insignificant activity under similar reaction conditions. The initial membrane activity was very high, but this decreased sharply in the first 2 h, followed by a leveling off at around 8%. This was most likely due to a loss of functionalized polymeric grafts, resulting in a decreased number of active sites. The membranes were reasonably stable during formation, with no loss of grafted chains during activation with 0.5N sulfuric acid and titration with 0.4N NaOH solution [26]. However, there was some loss of membrane grafts in the flowthrough reaction at higher feed ethanol concentrations, as indicated by the absorbance peak at 291 nm in Fig. 11. The amount of styrene
3.3.3. Effect of temperature The effect of temperature on conversion was studied in the range of 298–343 K, and an Arrhenius plot is shown in Fig. 9. The parameters of feed flow rate, catalyst loading, reactor volume, and reactant mole ratio were kept constant. The conversion increased from 16 to 65% over an increment of 458 at a residence time of less than 20 s. The activation energy was determined to be 31.26 kJ/mol, and the corresponding frequency factor was 18.81 1012 (m3)2 kmol2 s1. Kinetic data from literature show that the activation energy using IERs for the same reaction was 45–120 kJ/mol [28,29]. Rapid site accessibility and very low internal diffusion resistance during convective flow led to high reaction rate constants. As a result, the activation energy of the reaction would be lower for catalytic functionalized membranes compared to IERs. 3.3.4. Catalytic membrane stability The advantage of using cationic polymerization techniques is that it offers excellent range of internal surface modification [26]. In addition, this chemical method would be very conducive for scale-up in industry. The catalytic membrane stability can be enhanced by improving the molecular weight of the grafts that would provide better entanglement. UV analysis of the reaction volume for the batch process revealed no leaching of the functionalized polymer. Therefore, there was no deactivation of active sites in the batch process. Changes in the membrane activity during flowthrough reaction were observed as a function of time. Fig. 10 shows the average conversion of acetic acid over the course of a typical
Fig. 11. UV spectra of esterification reaction permeate to characterize loss of sulfonated polystyrene grafts.
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4. Conclusions
Fig. 12. Improvement in graft stability by varying the polymerization procedure during catalytic membrane preparation.
that underwent polymerization during the catalytic membrane preparation was known, and therefore the loss of polymer chains could be quantified. It was determined that approximately 25% of the grafted polystyrene chains were extracted in the reaction permeate. In the case of cationic polymerization, the graft interaction with the membrane pore walls was by an ionic route. There are two potential explanations for the loss of electrostatically bound polystyrene in the reaction permeates. First, an excess of acetic acid might result in masking of the electrostatic interaction between the polystyrene graft and the immobilized sulfonic acid. However, no graft loss was observed when excess acetic acid was used. The second possibility might be ionization suppression of the carbocation present on the polymeric grafts by excess ethanol. The UV-spectrum of pure ethanol permeated through the catalytic membrane showed the presence of grafted polymer, verifying at least partial graft loss by suppression of ionization. In terms of practicality of catalytic membranes, graft loss would be a key issue. The bar graph in Fig. 12 shows an overall picture of how graft stability was improved by changing the procedure for polystyrene grafting. An increase in the average number of repeat units per graft may prevent polymer loss through better entanglement with the membrane structure. This was achieved by increasing the styrene concentration to 20% (volume) during the cationic polymerization step of membrane modification. The graft length was increased from approximately 100 repeat units to 170 repeat units, resulting in a 60% decrease in graft loss. The catalytic membranes prepared by electron beam radiation grafting had an average chain length of 300 repeat units. Preliminary studies on the esterification reaction using such grafted membranes showed only 2% graft loss. This was attributed to covalent bonding of the grafted chains to the membrane during free-radical polymerization resulting in negligible loss of functionalized polymer.
Polyethersulfone microfiltration membranes containing sulfonated polystyrene grafts have been shown to exhibit clear benefits compared to the commercially available ion-exchange resins and homogeneous catalysts for a simple esterification reaction between acetic acid and ethanol. The catalytic membranes possess good site accessibility with no separation and corrosion problems. The catalytic membrane reactivity was on par with Amberlyst-36, and two to three times lower than the homogeneous catalyst (sulfuric acid) in batch processes. This was mainly due to internal mass transfer resistance in heterogeneous catalysts. The catalytic membranes performed very well in flowthrough mode, showing substantially higher catalytic performance compared to the batch process under similar reaction conditions. This observation was predominantly due to the virtual elimination of internal mass transfer resistance, and was confirmed by modeling of the homogeneous system at equivalent catalyst concentration. The catalytic performance was governed by the residence time provided to the reactive species. Although there was no observed loss of grafts in batch reaction, excess ethanol in flowthrough reaction played a vital role in suppressing carbocation ionization leading to a moderate loss (25%) of non-entangled grafts. An increase in the molecular weight of the sulfonated polystyrene grafts decreased the graft loss by 60%. When the membrane catalyst was prepared by free-radical polymerization, graft loss was negligible. The kinetic parameters reveal that the activation energy using membrane catalyst (32 kJ/mol) in flowthrough configuration was lower than the conventional IERs (>45 kJ/ mol). Acknowledgement This work is partially supported by the University of Alabama Graduate Council Research and Creative Activity Fellowship program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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