Synthesis of Propylene Carbonate from Urea and 1

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Synthesis of Propylene Carbonate from Urea and 1,2-Propylene Glycol in a Monolithic Stirrer Reactor Dongfang Wu,* Yali Guo, Shu Geng, and Yinghao Xia School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing 211189, P. R. China

ABSTRACT: A monolithic stirrer reactor, in which the cordierite monolith-supported metal oxide and mixed-metal oxide catalysts are used as stirrer blades, is applied for the alcoholysis of urea with 1,2-propylene glycol to synthesize propylene carbonate. It is shown that the mixed-metal oxides give much higher urea alcoholysis performances than the metal oxides, arising from the existence of a strong synergetic effect in the mixed-metal oxides. The zinc−chromium mixed oxide not only shows an excellent catalytic performance but also has extremely strong adhesion strength on the monolithic substrate. For the monolith- supported zinc−chromium mixed oxide catalyst, the monolithic stirrer reactor performs very well, and the highest yield of propylene carbonate reaches 97.8%. The performance is comparable to that of a conventional mechanically agitated slurry reactor, revealing the monolithic stirrer reactor can be an attractive alternative to the slurry reactor for heterogeneously catalyzed liquid−liquid reactions.

1. INTRODUCTION toward the urea alcoholysis and the yield of PC reached 96.5% 9,10 Propylene carbonate (PC) is widely used as a polar, aprotic and 92.4%, respectively. Zhao et al. performed the solvent in organic syntheses, cosmetics, gas separation, battery alcoholysis of urea with PG over homogeneous zinc acetate, 1,2 − electrolytes, metal extraction, etc. Several methods were supported zinc acetate, and zinc iron double oxide catalysts, reported for the synthesis of PC, such as the phosgenation of and the yield of PC reached 94%, 78%, and 78.4%, respectively. 1,2-propylene glycol (PG) with phosgene,2 transesterification A serious loss of zinc acetate was observed for the supported of alkyl carbonate with PG,3 reaction of carbon dioxide with o- zinc acetate.9 Zhou et al.11 examined the catalytic activities of chloropropanol,4 cycloaddition of propylene oxide with carbon several single-metal carbonates and Pb−Zn mixed-metal − dioxide,5 and alcoholysis of urea with PG.6 11 Urea alcoholysis carbonates in the urea alcoholysis, and the highest yield of is a new route and shows many advantages, such as cheap and PC reached 96.3%. easily available feedstock, mild reaction conditions, and safe Although an amount of information is already at hand − operation.6 9 Furthermore, it is the first step in the two-step concerning the alcoholysis of urea to PC, previous studies were process for the production of dimethyl carbonate (DMC) from all carried out in slurry reactors filled with powdered catalysts, urea and methanol,12,13 shown in Figure 1. The second step is and no effort has been made to examine the applicability of new reactor types. As is well-known, slurry reactor suffers from a few problems associated with catalyst attrition, agglomeration, recovery, and reuse. An alternative is to combine the mixing with catalytic function by fixing the catalyst to a stirrer. Thus, a novel monolithic stirrer reactor (MSR) was proposed as a promising replacement for a conventional slurry reactor in − multiphase reactions.14 17 In this reactor, monolithic catalysts are used as stirrer blades, creating a catalytic stirrer. The most important advantage of the MSR is the easy catalyst handling. Figure 1. Two-step process for the production of dimethyl carbonate This reactor is thought to be especially useful in the production from urea and methanol. of fine chemicals and in biochemistry and biotechnology. Albers et al.14 showed that the MSR works for low viscosity liquids, fi and hydrodynamic aspects of the reactor type were analyzed. the transesteri cation of PC with methanol and its byproduct, 15 fi Hoek et al. examined the applicability of the MSR in a PG, is recycled as raw material in the rst step. Thus, the overall − process forms a green chemical cycle, which increases the heterogeneously catalyzed gas liquid reaction. The open efficiency of utilization of raw material and greatly lowers the structure of the monolithic stirrer blade enables a high throughput of reactants and a large geometrical area. de cost for the production of DMC. 16,17 Li et al.6,7 used a series of metal-oxide catalysts to produce Lathouder et al. presented the use of the MSR for carrying PC from urea and PG and found that amphoteric ZnO showed the highest catalytic activity and the yield of PC reached 98.9%, Received: August 6, 2012 while the activity decreased for basic oxides with strong basic Revised: December 8, 2012 sites, and acidic oxides had no activity. Gao et al.8 reported that Accepted: December 11, 2012 both MgCl2 and ZnCl2 showed good catalytic performances Published: December 11, 2012

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out enzyme-catalyzed reactions in organic media and 2.2. Catalytic Test. Catalytic activity of powdered catalysts considered that the MSR is a convenient tool to compare in the synthesis of PC was measured at atmospheric pressure in monolithic catalysts in the absence of external mass transfer the MASR, a 100 mL four-necked round-bottom flask equipped limitations. with a mechanical agitator, cycle reflux condenser, thermo- Development of monolithic catalysts and reactors has been couple, and nitrogen inlet. A monolithic catalyst activity test one of the major achievements in the field of heterogeneous was carried out in the MSR shown in Figure 2. Two monolithic − catalysis and catalytic reaction engineering.18 29 For more than 30 years, monolithic catalyst manufacturers have been successful in stationary and automotive exhaust gas treatments, where the gas phase detoxification must be fast with contact time less than a second, since large volumes of gas have to be − treated.19,21 23 In recent years, monolithic catalysts and reactors are expected to have increasing applications in chemical and biochemical processes, such as in mass production of chemicals, in the treatment of fuel and flue − gases, and in other multiphase processes.18,24 29 The MSR is a new type of monolithic reactor. In this Article, we will demonstrate its industrial possibilities in a heterogeneously catalyzed liquid−liquid reaction, i.e., alcoholysis of urea with PG. Monolith-supported metal oxide and mixed-metal oxide catalysts were prepared and used in the MSR. The performance was compared with that of a conventional mechanically agitated slurry reactor (MASR). The MSR process optimization was also performed in order to improve the yield of PC.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. All chemicals used were of analytical-reagent (AR) grade and were purchased from Sinopharm Chemical Reagent Co., Ltd., China. They were used as received without further purification. Cylindrically shaped monolithic structures (Φ16 × 24 mm) were obtained Figure 2. Schematic representation of the monolithic stirrer reactor. by cutting commercial cordierite straight-channel monoliths with a square cell density of 400 cpsi (cells per square inch) and catalysts were mounted on a stirring shaft, replacing conven- a wall thickness of 0.21 mm. Each mini-monolith weighed tional impeller blades. The vertical location of the stirring shaft about 2.30 g. Before use, they were pretreated in an acetic acid was adjusted to ensure that the monolithic catalysts were solution for 1 h and then calcined at 600 °C in air for 2 h to located in the middle of reaction liquid. remove adsorbed impurities. In order to facilitate comparison, the amount of the Powdered metal oxide and mixed-metal oxide catalysts were powdered catalyst loaded in the MASR was 0.40 g, which prepared from aqueous metal nitrate solutions and from mixed approximated the total active phase amount of the two aqueous solutions of two metal nitrates in a molar ratio of 1:1, monolithic catalysts used in the MSR. After addition of 24.0 ° g (0.4 mol) of urea and 45.0 mL (0.6 mol) of PG, the MASR or respectively. These solutions evaporated at 110 C in air to ° dryness and then were calcined at 450 °C in air for 3 h. MSR was heated under stirring (300 rpm) to 170 C in an oil Monolith-supported metal oxide and mixed-metal oxide bath, and the reaction proceeded at this temperature for 4 h. catalysts were prepared by the following procedure. Pseudo- Nitrogen gas passed continuously through the reactor during boehmite, urea, and deionized water in a weight ratio of 2:1:5 reaction to protect the reaction system and to drive out the were mixed and vigorously stirred at room temperature for 1 h. byproduct NH3. The resultant mixture was cooled to room temperature, filtered to remove solid material (powdered In this period, a small amount of nitric acid was added slowly to ff maintain pH at 1.5. As a result, a stable alumina sol was catalyst in the MASR or catalyst powder peeled o from the obtained. A dried monolith was dipped for approximately 1 min monolithic catalysts in the MSR), and then analyzed by gas chromatography equipped with a FID detector and a packed in this sol. The monolith was then shaken to remove the excess fi liquid remaining in the channels, followed by drying column lled with organic support 402. The yield of PC was horizontally at 120 °C in air for 1 h and then calcination at calculated as follows: 500 °C in air for 3 h. Afterward, the washcoated monolith was moles of PC produced yield (%) =×100 immersed in an aqueous solution of metal nitrate or in a mixed moles of urea in feed aqueous solution of two metal nitrates in a molar ratio of 1:1. weight of reaction liquid obtained× weight percent of PC After the monolith was removed and shaken, the monolith was = molecular weight of PC× moles of urea in feed left horizontally at room temperature overnight, dried × 100 horizontally at 120 °C in air for 1 h, and then calcined at 450 °C in air for 3 h. For each metal oxide and each mixed- 2.3. Process Optimization in the Monolithic Stirrer metal oxide, two monolithic catalyst samples were prepared. Reactor. In the urea alcoholysis for the PC synthesis, there are The obtained monolithic catalysts contain 0.08−0.09 g of many process factors which have possible effects on the yield of washcoat (γ-alumina) and 0.20−0.21 g of active phase (metal PC, e.g., molar ratio of PG to urea, reaction temperature, oxide or mixed-metal oxide) per monolith. reaction time, amount of active phase, stirrer speed, monolith

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cell density, and monolith length. In this study, the PG/urea molar ratio was fixed at 1.5:1 and the other factors were selected as controllable factors to optimize the synthetic process to improve the yield of PC. For this purpose, monolithic catalysts with different active phase loadings, different cell densities, and different lengths were prepared, and urea alcoholysis experiments were performed at different reaction temperatures, different reaction times, and different stirrer speeds. The detailed catalyst preparation and PC synthetic process were similar to those mentioned above. 3. RESULTS AND DISCUSSION 3.1. Comparisons among Catalysts and between Reactors. Figures 3 and 4 show the urea alcoholysis

Figure 5. Blank experiments.

MASR test with powdered γ-alumina, MSR test with bare cordierite monolith, and MSR test with cordierite monolith coated with γ-alumina give approximately equal yields of PC. They are all close to the result of the MASR blank test without catalyst, revealing that the cordierite substrate, γ-alumina washcoat, and powdered γ-alumina have no marked catalytic activity in the urea alcoholysis. Nevertheless, comparing Figures 3 and 4 with Figure 5, it is clear that all the metal oxides and mixed-metal oxides examined are active in the PC synthesis. As can be seen from Figure 3, there is the same catalytic activity order of the metal oxides between the MASR and MSR. Irrespective of reactor type, lead oxide leads to the highest yield Figure 3. Catalytic activities of the metal oxides. of PC, followed by magnesium oxide and then zinc oxide. However, for the mixed-metal oxides, an abnormal sample is given in Figure 4. The activity of magnesium−lead mixed oxide ranks second to last in the MASR, whereas its activity ranks second in the MSR. Except this sample, the mixed-metal oxides also give the same catalytic activity order in the two reactors. For example, zinc−lead mixed oxide leads to the highest yield of PC, followed by zinc−chromium mixed oxide. Furthermore, the mixed-metal oxides all give much higher yields of PC than the metal oxides, irrespective of reactor type, which provides support for the existence of a strong synergetic effect in the mixed-metal oxides. For monolithic catalysts, catalytically active phase is loaded inside monolith channels and its contact with reaction mixture may be inadequate under the experimental conditions used. It results in a poor mass transfer in the MSR; therefore, for any oxide, the MSR always gives lower yields of PC than the MASR. It should be mentioned that, among all the oxides examined, only three lead-containing oxides, i.e., lead oxide, zinc−lead mixed oxide, and magnesium−lead mixed oxide, were found to be remarkably peeled off from their respective monolithic substrates in the MSR. It is probably due to the large bulk Figure 4. Catalytic activities of the mixed-metal oxides. density of lead which results in poor adhesion strengths of these oxides on their monolithic structures. The spalling of catalyst powder increases mass transfer and hence improves the performances of various metal oxides and mixed-metal oxides PC yield, which is the cause of the MSR activity abnormality of in the two reactors. To confirm their catalytic activities, several magnesium−lead mixed oxide. blank tests were previously carried out under the same In this study, only the reaction temperature of 170 °C was conditions and the results are presented in Figure 5. The used to compare catalysts and reactors. If the temperature

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changes, the catalytic activity orders in Figures 3 and 4 could be different since different catalysts have different activation energies and therefore selectivity ratios vary with temperature. As discussed below, 170 °C is, however, close to the optimum reaction temperature, and much higher or lower temperatures are not suitable for the urea alcoholysis reaction. Therefore, it has actual meanings to compare the catalytic activities at the selected temperature of 170 °C. 3.2. Optimization of the PC Synthetic Process. As shown above, zinc−chromium mixed oxide not only shows a good catalytic performance but also has extremely strong adhesion strength on the monolithic substrate. It was therefore used to prepare monolithic catalysts to optimize the PC synthetic process in the MSR. 3.2.1. Influence of the Reaction Temperature. Four trials were performed to examine the effect of the MSR reaction temperature on the yield of PC, and the results are shown in Figure 6. In each trial, two monolithic catalysts are used and

Figure 7. Influence of the reaction time on the yield of PC (Monolithic catalysts: Φ16 × 24 mm, 400 cpsi; Reaction conditions: 0.4 mol of urea and 0.6 mol of PG, reaction temperature of 180 °C, stirrer speed of 300 rpm).

becomes comparatively high in the reactor and side reactions such as the polymerization of PC will proceed predominantly. Therefore, a 6 h reaction is optimum for the MSR PC synthesis. 3.2.3. Influence of the Amount of Active Phase. As shown in Figure 8, the PC yield increases significantly at first with the

Figure 6. Influence of the reaction temperature on the yield of PC (Monolithic catalysts: Φ16 × 24 mm, 400 cpsi; Reaction conditions: 0.4 mol of urea and 0.6 mol of PG, reaction time of 4 h, stirrer speed of 300 rpm).

their mean composition is plotted. It can be seen that the catalysts used in different trials have approximately equal loadings of γ-alumina and zinc−chromium mixed oxide, though they were not prepared in a batch. The PC yield increases with the elevation of the reaction temperature. When the temperature is 180 °C, the yield of PC reaches its maximum value of 71.1%. Thermodynamic analysis Figure 8. Influence of the amount of active phase on the yield of PC reveals that the urea alcoholysis is endothermic,9 and the (Monolithic catalysts: Φ16 × 24 mm, 400 cpsi; Reaction conditions: increase in temperature promotes the synthesis of PC. 0.4 mol of urea and 0.6 mol of PG, reaction temperature of 180 °C, However, the normal boiling point of PG is about 188 °C, reaction time 6 of h, stirrer speed of 300 rpm). and an exorbitant temperature will lead to its fast evaporation. The elevation of temperature may also cause side reactions such as the polymerization of PC.30 The upper temperature increase of the amount of zinc−chromium mixed oxide. When limit is therefore confined at 180 °C. the mixed oxide amounts to about 0.21 g/monolith, the PC 3.2.2. Influence of the Reaction Time. Figure 7 indicates yield reaches its maximum value of 95.0%. The increasing of the that, after a 6 h reaction, the yield of PC reaches its maximum mixed oxide loading gives much more catalytically active sites value of 95.0% and then the PC yield gradually decreases with and hence leads to the improvement of the urea alcoholysis further prolonging of the reaction time. One of the possible performance. However, when it is increased to about 0.30 g/ reasons is that, after a 6 h reaction, the PC concentration monolith, the PC yield begins to decline slightly.

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The most probable reason for the reduction of activity is that Figure 9 indicates that the yield of PC increases at first, there exists a strong interaction effect between the amount of reaches its maximum value at 300 rpm, and then decreases with active phase and the reaction time. The optimum conversion is increasing stirrer speed. Within the range of the low stirrer attained earlier for trial 9 since a larger amount of active phase speeds, an increase in the stirrer speed results in an increasing is in the reactor; that is, the optimum reaction time for trial 9 is liquid flow through monolith channels, which improves the probably less than 6 h. Therefore, compared with trial 6, the mass transfer and hence increases the catalytic activity. side reaction time is longer which results in a lower observed However, if the stirrer speed is excessively fast, the whirlpool activity of trial 9. The reduction of activity is also probably comes forth, and less reaction liquid flows through monolith connected with the great decrease in the open frontal area of channels. Therefore, a moderate stirrer speed, 300 rpm, is the monolith owing to the extremely large loading of active desired to increase the urea alcoholysis activity. phase. For a given stirrer speed, with the decrease of the open 3.2.5. Influence of the Monolith Cell Density. In Figure 10, frontal area of the monolith, the contact between the reactant the urea alcoholysis performances of the monolithic catalysts mixture and the catalyst active phase becomes difficult, and thus a severe mass transfer limitation arises. Moreover, to increase the active phase loading, a highly concentrated impregnating solution should be used. It has been reported that the impregnation using a concentrated solution not only gives a large crystalline size of active phase but also gives rise to the migration of metal precursor salts during drying and thus the accumulation of activity phase at the outer part of the monolith where the water evaporates fastest, resulting in an inefficient monolith with only the outer part of the structure having a significant catalytic activity.31,32 This might be another reason for low activity of the monolithic catalysts with high active phase loadings. The optimum amount of active phase is therefore 0.21 g/monolith for the PC synthesis. 3.2.4. Influence of the Stirrer Speed. Trials 10−17 were done to investigate the effect of the stirrer speed. As shown in Figure 9, the used monolithic catalysts contain 0.24−0.25 g of

Figure 10. Influence of the monolith cell density on the yield of PC (Monolithic catalysts: Φ16 × 24 mm; Reaction conditions: 0.4 mol of urea and 0.6 mol of PG, reaction temperature of 180 °C, reaction time of 6 h, stirrer speed of 300 rpm).

with different cell densities are presented. These catalysts were prepared by dipping in the same alumina sol and then impregnating in the same mixed nitrate solution; however, their loadings of washcoat and active phase are different. As the monolith cell density is increased from 300 to 400 cpsi, the washcoat and active phase loadings increase, which arises from the increasing surface area of the monolith channel walls. Nevertheless, further increase in the cell density probably leads to the monolith channels being inaccessible to alumina sol and impregnating solution due to a great, small open frontal area. The 600 cpsi catalysts, therefore, have lower washcoat and Figure 9. Influence of the stirrer speed on the yield of PC (Monolithic active phase loadings than the 400 cpsi catalysts. catalysts: Φ16 × 24 mm, 400 cpsi; Reaction conditions: 0.4 mol of It is also seen that the changing of the yield of PC has the urea and 0.6 mol of PG, reaction temperature of 180 °C, reaction time same behavior as the changing of the active phase loading. of 6 h). When the cell density is increased from 400 to 600 cpsi, the decrease in the yield of PC is larger than the decrease in the active phase loading. It reveals that, besides the effect of the zinc−chromium mixed oxide per monolith, slightly higher than amount of active phase, a remarkable mass transfer limitation the optimum loading of active phase. Trial 12 was conducted at takes place in trial 19, resulting in a rapid reduction of the yield the stirrer speed of 300 rpm, and thus, it has the same urea of PC. alcoholysis conditions as trial 6. As a repeated test, trial 12 gives 3.2.6. Influence of the Length of the Monolithic Structures. the PC yield of 94.8%, which is fairly close to the result of trial Figure 11 shows that the washcoat and active phase loadings 6, though they have a little difference in the loading of always increase with increasing monolith length and that the catalytically active phase. yield of PC increases at first, reaches its maximum value of

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It is shown that a single-factor-at-a-time method was used in this work for doing experiments to analyze the effect of the process factors. For the monolith-supported zinc−chromium mixed oxide catalyst, the optimum MSR reaction conditions are obtained as follows: the reaction temperature at 180 °C, the reaction time at 6 h, the amount of active phase at 0.21 g/ monolith, the stirrer speed at 300 rpm, the monolith cell density at 400 cpsi, and the monolith length at 20 mm, resulting in a PC yield of 97.8%. It should be pointed out that these optimum results are perhaps tentative, because the single- factor-at-a-time method is unable to detect the presence of the interaction and combined effect of factors. For instance, the lower observed activity of the monolithic catalysts with larger amount of active phase, shown in Figure 8, probably indicates a strong interaction effect between the amount of active phase and the reaction time, resulting from the existence of side reactions such as the polymerization of PC. This interaction leads to different optimum reaction times required for different loadings of active phase, which is, however, completely ignored fl Figure 11. In uence of the monolith length on the yield of PC in the single-factor-at-a-time method. Furthermore, there is a (Monolithic catalysts: Φ16 mm, 400 cpsi; Reaction conditions: 0.4 ° probable interaction between the monolith length and the mol of urea and 0.6 mol of PG, reaction temperature of 180 C, ff reaction time of 6 h, stirrer speed of 300 rpm). monolith cell density. For di erent monolith lengths, the optimum monolith cell densities could be different due to variation of significance of the monolith entrance effect. 97.8% at the monolith length of 20 mm, and then decreases Therefore, a precise optimization should be carried out by with increasing monolith length. studying all the factors collectively by, for example, statistical Several aspects should be taken into consideration to 33 examine the effect of the monolith length on the MSR reaction experimental designs such as response surface methodology. performance. Clearly, the increasing amount of active phase Despite all this, a satisfactory PC yield has been reached using with increasing the monolith length improves the MSR reaction the rough single-factor-at-a-time optimization method, and performance. However, as mentioned above, the increasing there is enough support for the applicability of the MSR to the amount of active phase also leads to a longer side reaction time alcoholysis of urea. − and hence to a lower observed activity due to the interaction It is also worth mentioning that, in previous studies,6 10 the between the amount of active phase and the reaction time. In PC synthesis was mainly carried out under reduced pressures at addition, with the increase of the length of the monolithic large molar ratios of PG to urea, e.g., 4:1 and 5:1. The reduced fi ff structures, a signi cant entrance e ect appears which limits pressure leads not only to an extra consumption of energy mass transfer and hence reduces the MSR reaction perform- owing to assembling of vacuum equipment but also to loss of ance. In the MSR, reaction liquid enters into monolith channels raw material and then change of raw material proportioning. A with an angle, φ, as shown in Figure 12. In the case of short greater excess of PG increases the yield of PC but also leads to monoliths, the inlet angle is close to a right angle and the liquid flows easily through monolith channels. As the monolith length trouble for the product separation process and is therefore not increases, the inlet angle increases. That is to say, the entrance expected in industry. Nevertheless, in this study, the alcoholysis of reaction liquid into monolith channels becomes difficult and of urea proceeds in the MASR and MSR reactors at the mass transfer limitation gets prominent. Therefore, a atmospheric pressure at a very low PG/urea ratio of 1.5:1, moderate monolith length is beneficial to the improvement of which provides a great convenience for actual industrial the MSR reaction performance. production.

Figure 12. Schematic drawing of the entrance effect.

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4. CONCLUSIONS (11) Zhou, J. C.; Wu, D. F.; Zhang, B. R.; Guo, Y. L. Synthesis of propylene carbonate from urea and 1,2-propylene glycol over metal It has been shown that all the metal oxides and mixed-metal carbonates. Chem. Ind. Chem. Eng. Q. 2011, 17, 323−331. oxides examined are active in the alcoholysis of urea to PC and (12) Bhanage, B. M.; Fujita, S.; Ikushima, Y.; Arai, M. Trans- that their catalytic activity order does not vary with reactor esterification of urea and ethylene glycol to ethylene carbonate as an type. Irrespective of reactor type, the mixed-metal oxides give important step for urea based dimethyl carbonate synthesis. Green much higher urea alcoholysis performances than the metal Chem. 2003, 5, 429−432. oxides, providing support for the existence of a strong (13) Lin, H. Y.; Yang, B. L.; Sun, J. J.; Wang, X. P.; Wang, D. P. synergetic effect in the mixed-metal oxides. It is also found Kinetics studies for the synthesis of dimethyl carbonate from urea and that the lead-containing oxides are easily peeled off from the methanol. Chem. Eng. J. 2004, 103,21−27. monolithic substrates, while the zinc−chromium mixed oxide (14) Albers, R. K. E.; Houterman, M. J. J.; Vergunst, T.; Grolman, E.; Moulijn, J. A. Novel monolithic stirred reactor. AIChE J. 1998, 44, not only shows an excellent catalytic performance but also has − extremely strong adhesion strength on the monolithic substrate. 2459 2464. (15) Hoek, I.; Nijhuis, T. A.; Stankiewicz, A. I.; Moulijn, J. A. Furthermore, the MSR process optimization indicates that the Performance of the monolithic stirrer reactor: Applicability in multi- MSR performance increases with increasing reaction temper- phase processes. Chem. Eng. Sci. 2004, 59, 4975−4981. ature and that moderate-level reaction time, amount of active (16) de Lathouder, K. M.; Flo, T. M.; Kapteijn, F.; Moulijn, J. A. A phase, stirrer speed, monolith cell density, and monolith length novel structured bioreactor: Development of a monolithic stirrer prove to be beneficial to the performance of the MSR. For the reactor with immobilized lipase. Catal. Today 2005, 105, 443−447. monolith-supported zinc−chromium mixed oxide catalyst, the (17) de Lathouder, K. M.; Bakker, J. J. W.; Kreutzer, M. T.; Wallin, S. MSR performs very well and the highest yield of PC reaches A.; Kapteijn, F.; Moulijn, J. A. Structured reactors for enzyme 97.8%. From this case study, it is revealed that the MSR can be immobilization: A monolithic stirrer reactor for application in organic − an attractive alternative to the MASR for heterogeneously media. Chem. Eng. Res. Des. 2006, 84, 390 398. − (18) Boger, T.; Heibel, A. K.; Sorensen, C. M. Monolithic catalysts catalyzed liquid liquid reactions, e.g., the alcoholysis of urea to − PC. for the chemical industry. Ind. Eng. Chem. Res. 2004, 43, 4602 4611. (19) Tomasic, V.; Jovic, F. State-of-the-art in the monolithic catalysts/reactors. Appl. Catal. A: Gen. 2006, 311, 112−121. ■ AUTHOR INFORMATION (20) Nijhuis, T. A.; Beers, A. E. W.; Vergunst, T.; Hoek, I.; Kapteijn, Corresponding Author F.; Moulijn, J. A. Preparation of monolithic catalysts. Catal. Rev.-Sci. − *E-mail: [email protected]. Eng. 2001, 43, 345 380. (21) Ochonska,́ J.; McClymont, D.; Jodłowski, P. J.; Knapik, A.; Gil, Notes B.; Makowski, W.; Łasocha, W.; Kołodziej, A.; Kolaczkowski, S. T.; The authors declare no competing financial interest. Łojewska, J. Copper exchanged ultrastable zeolite Y-A catalyst for NH3-SCR of NOx from stationary biogas engines. Catal. Today 2012, ■ ACKNOWLEDGMENTS 191,6−11. Financial support from the National Natural Science (22) Tomasic, V. Application of the monoliths in DeNOx catalysis. Catal. Today 2007, 119, 106−113. Foundation of China, under Grant No. 21176048, is gratefully (23) Metkar, P. S.; Harold, M. P.; Balakotaiah, V. 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