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Effect of Ultrasound on Catalytic Hydrogenation of D-Fructose to D-Mannitol Blanka Toukoniitty, Jyrki Kuusisto, Jyri-Pekka Mikkola, Tapio Salmi, and Dmitry Yu. Murzin* Laboratory of Industrial Chemistry, Process Chemistry Centre, Åbo Akademi, FIN-20500 Turku/Åbo, Finland
The effect of sonification on D-fructose hydrogenation over several heterogeneous catalysts was investigated. The reaction rate and selectivity were studied at different temperatures, pressures, and ultrasonic power inputs. The ultrasonic effects were clearly catalyst dependent. The sonification during the hydrogenation reaction significantly enhanced reaction rate over Cu/ SiO2. In the case of Raney-Ni, just moderate improvements of reaction rates were observed under ultrasound and a slight decrease of catalyst activity by sonification was obtained over Cu/ZnO/ Al2O3. Although the choice of catalyst significantly influenced selectivity, no positive influence of acoustic irradiation on selectivity was achieved. The influence of pressure and temperature over Raney-Ni type catalyst was studied under sonification as well. High pressures and temperatures brought just moderate enhancements of reaction rates, while the variation of nominal ultrasonic power input significantly affected the catalyst activity. A catalyst deactivation study was conducted by recycling the catalyst. Acoustic irradiation significantly prevented catalyst deactivation compared to experiments in silent conditions. The effect of ultrasound on catalyst surface was confirmed by means of scanning electron microscopy (SEM). 1. Introduction The interest toward speciality sugar alcohols has considerably increased during the last years. The beneficial functional properties of some products, e.g., xylitol, have inspired the alimentary and pharmaceutical industry to increase application and research in the field of these speciality chemicals. The hydrogenation of D-fructose is an interesting reaction in view of low caloric sweetener D-mannitol.1 D-Mannitol is a sweet-tasting hexavalent sugar alcohol widely distributed in nature, found in olive trees, plane trees, fruits, and vegetables (e.g., strawberry, pumpkin, celery, and onion). However, production of mannitol by extraction of plant raw materials is no longer economical. Instead, the three-phase catalytic hydrogenation of fructose, glucose, or a mixture of those is the industrially dominating procedure.2 The hydrogenation process is carried out at high pressures and temperatures applying metal catalysts and dissolved hydrogen gas. In aqueous solutions, acyclic D-fructose is in equilibrium with its different cyclic forms: β-D-fructopyranose, R-D-fructopyranose, β-D-fructofuranose, and R-D-fructofuranose. As fructose is hydrogenated, β-fructose is transformed to mannitol while R-fructose gives sorbitol.3,4 The mutarotation equilibrium and hydrogenation scheme of Dfructose is displayed in Figure 1. At 80 °C, D-fructose solution contains β-D-fructopyranose (53%), β-D-fructofuranose (32%), R-D-fructofuranose (10%), R-D-fructopyranose (2%), and open keto-form (2%). Glucose and other unknown products may also be obtained in minor amounts as fructose is hydrogenated. Temperature, pH, mass transfer of hydrogen, and catalyst used influence the formation of these byproducts. Acoustic irradiation can alter the reactivity during heterogeneous catalysis of a variety of reactions.5-14 The * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +358 2 215 4985. Fax: +358 2 215 4479.
most heavily studied systems for hydrogenation involve nickel catalysts and have a long history.15 For example, the hydrogenation of alkenes by ordinary Ni powder is enormously enhanced (>105 -fold) by ultrasonic irradiation.16 In addition, cinnamaldehyde hydrogenation over Raney nickel type catalyst resulted in 20-fold activity improvement by sonification compared to stirred experiment.17 Acoustic irradiation was also found to retard the catalyst deactivation (Raney-Ni) notably in xyloseto-xylitol hydrogenation.18 Successful attempts reported in the literature involving heterogeneous Ni, respectively Raney-Ni, catalysts encouraged us to widen these studies to systematic investigation of the influence of acoustic irradiation in fructose hydrogenation over Raney-Ni catalyst. The enhancement of catalyst activity by ultrasonic irradiation is due to the improvement of mass transfer between the liquid and the catalyst surface, avoiding catalyst deactivation by cleaning the catalyst surface and exposing fresh, highly active surface, as well as due to the reduction of diffusion length in the catalyst pores by catalyst grinding. Two different modes of action of ultrasound can exist at different reaction conditions (intensity, pressure, etc.): cavitating and noncavitating ultrasound (acoustic streaming). Cavitating ultrasound forms cavitation bubbles, which violently collapse on or near the catalyst surface and direct jets of liquids toward it. These shock waves cause localized deformation of catalyst surface involving intensive cleaning. Acoustic streaming is the movement of the liquid induced by the sonic wave, which can be considered to be simply the conversion of sound to the kinetic energy, and is not a cavitation effect.19 2. Experimental Section 2.1. Experimental and Reactor Setup. The fructose (Danisco Sweeteners) was hydrogenated in a pres-
10.1021/ie050190s CCC: $30.25 © 2005 American Chemical Society Published on Web 07/13/2005
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Figure 1. Mutarotation and hydrogenation of D-fructose.
surized autoclave reactor (600 mL) in the presence and absence of ultrasound. Three different catalysts were tested: 20 wt % Cu/SiO2 (BASF H3-11), Raney nickel (Raney-Ni) type (Acticat, D ≈ 20 µm), 70 wt % CuO, 25 wt % ZnO, 5 wt % Al2O3 (BASF R3-30 SPKZ, D ≈ 100 µm). The catalysts were reduced under hydrogen flow (10 bar) at 220 °C for 2 h and cooled to reaction temperature, except in the case of Raney-Ni where no activation was needed. The catalyst amount was 10 wt % (prior to reduction) of fructose weight in the case of Cu/SiO2 and CuO/ZnO/Al2O3 and 5 wt % in the case of Raney-Ni catalyst. The ultrasonic irradiation was obtained by an ultrasonic horn (AEA Technologies Inc.). The apparatus consisted of a generator (nominal operational frequency 20 kHz, adjustable power output 0-100 W), a piezoelectric stack transducer and titanium horn (displacement 12 µm ( 1 µm peak to peak) with a tailormade connection to the bottom of a laboratory-scale pressure autoclave. A schematic drawing and a photograph of the experimental apparatus are presented in
(Figure 2). The effective liquid volume of the reactor was ca. 250 mL. The setup was equipped with a rushton turbine. The existence of external mass transfer limitations was investigated by varying the stirring rate within 1200-1800 rpm. The conversion and the selectivity were unaffected by changing of stirring rate. The stirring rate was 1800 rpm in all of the experiments to operate at the kinetically controlled regime, i.e., in the absence of external mass-transfer limitations. Internal mass transfer inside the catalyst particles was determined by calculation of the catalyst effectiveness factor ηeff.20 Calculation of ηeff under reaction conditions gave ηeff f 1, indicating that hydrogen diffusion inside the catalyst pores does not affect the reaction rate. A 30 wt % aqueous fructose solution was hydrogenated in all of the experiments (representing an industrially relevant concentration regime). The deoxygenated hydrogenation solution was injected into the reactor and the reaction was commenced. The partial pressure of hydrogen was varied from 10 to 50 bar and reaction temperature was
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Figure 3. Catalyst effect on conversion after 20 min of reaction time at 110 °C and 50 bar.
Figure 2. High-pressure autoclave with in situ ultrasonic irradiation system.
varied from 70 to 110 °C. The samples drawn out from the reactor were analyzed by a high-performance liquid chromatograph (HP 1100 LC series), equipped with an RI detector HP 1047 A, a Biorad Aminex HPX-87C carbohydrate column, a precolumn, a degasser, a binary pump with two channels, and an autosampler. The pH of the samples was analyzed off-line using a PHM 83 Autocal pH meter and a combined glass pH electrode pHC 2401-7, both from Radiometer Copenhagen. The selectivity to the desired product D-mannitol is defined as follows (eq 1):
selectivity ) Mannitol/[Mannitol+Sorbitol+Glucose+other] (1) 2.2. Catalyst Characterization. The catalyst particles were characterized by scanning electron microscopy (SEM). A 360 (LEO Electron Microscopy LTD) scanning electron microscope equipped with a secondary and backscattered electron detector was used for imaging of spent catalysts treated either in the presence or absence of ultrasound. 3. Results and Discussion 3.1. Cavitating and Noncavitating Processing Conditions. In sonochemistry two different domains exist: e.g., cavitating and noncavitating ultrasound regimes. Bath systems (low-intensity ultrasound) are usually noncavitating, whereas probe systems can be either cavitating or noncavitating.13 In our reaction system we assume that noncavitating ultrasound regime persists since high ambient H2 pressures (10, 30, and 50 bar) are applied. Increase of ambient pressure leads to increase of the cavitation threshold and thus cavitation bubbles cannot be created.19 Noncavitating ultrasound can lead to the enhancement of the reaction rate and prevent catalyst deactivation by catalyst surface cleaning. 3.2. Effect of Different Catalysts. Three different catalysts were tested in fructose hydrogenation. The experiments were carried out at the reaction temperature of 110 °C and 50 bar of hydrogen partial pressure. The nominal ultrasonic power input was maintained at 50 W during the experiment. Cu/SiO2 and Cu/ZnO/Al2O3
Figure 4. Catalyst effect on mannitol selectivity (0, Raney-Ni, silent; 9, Raney-Ni, sono; O, Cu/SiO2, silent; b, Cu/SiO2, sono; ], Cu/ZnO/Al2O3, silent; [, Cu/ZnO/Al2O3, sono).
catalysts were reduced prior the reaction in situ under H2 flow (10 bar) at 220 °C. The selection of catalyst had essential impact on ultrasonic effects. The sonification during the reaction significantly enhanced the catalyst activity over Cu/ SiO2, moderately over Raney-Ni, and slight decrease of the catalyst activity by sonification was observed over Cu/ZnO/Al2O3 (Figure 3). Choice of catalyst also significantly influenced the selectivity. The highest value of selectivity was observed over Cu/ZnO/Al2O3 (66%) and Cu/SiO2 (63%), while with Raney-Ni catalyst only 50% mannitol selectivity was obtained (Figure 4). From the literature it is known that a Cu-containing catalyst could enhance selectivity toward mannitol up to 70%.2 However, no remarkable influence of acoustic irradiation on selectivity was noticed (Figure 4). Although the most significant enhancement of catalyst activity by sonification compared to silent conditions was observed in the case of Cu/SiO2, the further study is focused on the catalyst with the highest activity, Raney-Ni, which is also the most industrially relevant. 3.3. Effect of Ambient Temperatures. The effects of three different temperatures (70, 90, and 110 °C) were studied at 50 bar of H2 partial pressure and 50 W of ultrasound power input over a Raney-Ni catalyst. The vapor pressure values were determined for different experimental temperatures: 70 °C (pvp ) 0.3119 bar), 90 °C (pvp ) 0.7015 bar), and 110 °C (pvp ) 1.4334 bar). In the present study the ambient reaction temperature had significant influence on the reaction rate and mannitol selectivity, e.g., leading to higher reaction rates (Table 1) and selectivity (Figure 5). The increase of the reaction rates at higher temperatures is caused by the fact the sugar mutarotation equilibrium is temperature dependent. At lower reaction temperature (70 °C) the pyranose form is more strongly absorbed to the catalyst surface, which might cause decrease of catalyst activity. However, the beneficial ultrasonic
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9373 Table 1. Temperature, Pressure, and Intensity Effect on Conversion and Selectivity over Raney-Ni Catalyst t1/2a (min)
selectivity (%)b
silent
ultrasound
silent
ultrasound
70 °C 90 °C 110 °C
25 17 15
temperature 27 13 12
43 46 53
43 44 51
10 bar 30 bar 50 bar
36 16 15
pressure 39 16 13
50 48 51
50 49 50
0 W/cm2 26 W/cm2 78 W/cm2 130 W/cm2
15
intensity 53 15 9 12
50 47 51
a Time required to obtain 50% conversion. b Selectivity at 50% conversion.
no significant enhancement of reaction rate and selectivity was observed by sonification. However, a moderate enhancement of the reaction rate under ultrasound was observed in the reaction carried out at 50 bar (Table 1). 3.5. Effect of Ultrasonic Power. The effect of nominal ultrasonic power input (0-50 W) was investigated in the model reaction over the Raney-Ni catalyst at the reaction temperature 110 °C and 50 bar of hydrogen partial pressure. The intensity of acoustic irradiation was defined for nominal power input, 10, 30, and 50 W, and related to the tip area of probe (0.38 cm2), 26, 78, and 130 W/cm2, respectively. Initially, the reaction rate was increasing, reaching a maximum (30 W) and then decreasing with additional power increase (Table 1). This is in agreement with the observation of Henglein and Gutierrez for iodine formation, when initially the rate was increasing linearly, reaching a plateau, and sharply dropping with additional power increase.21 However, the nominal power input does not correspond to the power dissipated (Pdiss) in a reaction mixture. One of the most common approaches to determine Pdiss to the system is calorimetry, which assumes that all the energy delivered to the system is dissipated as heat, as shown by the following:22
Pdiss )
( ) dT dt
t)0
(msolventcp,solvent) +
( ) dTv dt
Figure 5. Temperature effect on manittol selectivity (O, 110 °C, silent; b, 110 °C, sono; 0, 90 °C, silent; 9, 90 °C, sono, 4, 70 °C, silent; 2, 70 °C, sono).
effects on the reaction rate were just moderate. The slight improvement of reaction rate was obtained at higher ambient temperatures, 90 and 110 °C) (Table 1). Ultrasonic irradiation had only minor influence on mannitol selectivity (Figure 5). 3.4. Effect of Hydrogen Pressure. The effect of sonification on model reaction at three different hydrogen pressures, 9.6, 29.6, and 49.6 bar (which correspond to reactor over pressures 11, 31, and 51 bar, respectively), was investigated at the reaction temperature of 110 °C and 50 W of nominal ultrasonic power over Raney-Ni catalyst. Application of an external pressure to a reaction system, which increases the hydrostatic pressure of the liquid, leads to an increase in the energy required to initiate cavitation. Sufficiently large increase in the intensity (I) of the applied ultrasonic field could produce cavitation even at high overpressures and lead to more rapid and violent collapse.19 However, as mentioned above, due to the high H2 pressures and relatively low intensity (I ) 130 W/cm2) applied in our reaction system we assume that noncavitating ultrasound treatment conditions persist. The value of hydrogen partial pressure significantly influenced catalyst activity, the highest reaction rate was observed at more superior pressures of 30 and 50 bar (Table 1). The selectivity remained unaffected by different ambient pressures. The reaction order in H2 was determined and it is approximately 0.5. Generally,
(Awsxw)Fvesselcp,vessel (2)
t)0
where m and cp are the mass and heat capacity of the solvent, respectively, and (dT/dt)t)0 is the initial slope of the temperature rise of the reaction mixture versus time of exposure to the ultrasonic irradiation. Tv is the temperature of the inner vessel wall, Aws is the area of wetted surface of the vessel, and xw is the thickness of the inner wall. On the basis of this equation Hagenson and Doraiswamy estimated that 57.5% of the delivered power was dissipated as heat, while 42.5% was lost in the transfer process.22 The comprehensive study of power flow and dissipation was investigated by Disselkamp et al., determining 29% of power lost.23 However, these observations cannot be directly applied to our systems, since the amount of power dissipated into the system strongly depends on the system used. The selectivity was negligibly lower for the reaction carried out at 30 W (47%) compared to the silent experiment (51%) and experiments with 10 W (50%) and 50 W (50%). 3.6. Effect of Ultrasound on Catalyst Deactivation. A catalyst deactivation study was conducted by recycling the Raney-Ni catalyst. Four experiments under silent and sonic conditions were performed. The temperature of 110 °C, pressure of 30 bar, and nominal power input of 50 W were applied. Sonification during the reaction had a significant effect on catalyst deactivation. In experiments carried out under silent conditions, notable catalyst deactivation was observed (Figure 6). However, acoustic irradiation was found to retard deactivation significantly (Figure 7). No differences in the reaction rates were observed between experiments with 1st reused and 2nd reused catalyst under both silent and ultrasonic conditions. This is probably due to the catalysts’ reactivation in water, where they were stored for several hours between the experiments. The selectivity was essentially unaf-
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Figure 6. Catalyst deactivation series under silent conditions (], fresh; O, 1st reuse; 4, 2nd reuse; 0, 3rd reuse). Figure 9. Ultrasonic-treated spent Raney-Ni catalyst (SEM image).
Figure 7. Catalyst deactivation series under sonification ([, fresh; b, 1st reuse; 2, 2nd reuse; 9, 3rd reuse).
Figure 10. Spent Cu/SiO2 catalyst treated in the absence of ultrasound (SEM image, clusters are indicated in circles)
Figure 8. Spent Raney-Ni catalyst treated in the absence of ultrasound (SEM image).
fected by catalyst deactivation and sonification and remained in all experiments between 47 and 51%. The pH of the reaction mixture decreased from 7.0 (fresh) to 5.5 (1st, 2nd, and 3rd reuse) under silent conditions and from 7.5 (fresh) to 6.0 (1st, 2nd, and 3rd reuse) under ultrasound. 4. Catalyst Characterization Spent Cu/SiO2, Cu/ZnO/Al2O3, and Raney-Ni catalysts treated in the presence or absence of ultrasound were studied by SEM (scanning electron microscopy). In the Raney-Ni catalyst, acoustic irradiation seems to provide additional deformation of the catalyst surface, exposing fresh, highly active surface, and reduces dif-
Figure 11. Ultrasonic-treated spent Cu/SiO2 catalyst (SEM image, clusters are indicated in circles).
fusion length in the catalyst pores (Figures 8 and 9). Deformation and cleaning of the catalyst surface could be an explanation for the observed suppression of catalyst deactivation by sonification. For Cu/SiO2 catalyst, no significant differences in catalyst particles due to the ultrasonic treatment were observed (Figures 10 and 11). However, Cu clusters
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with the size of up to 10 µm are clearly visible at the SEM images. No observable differences in catalyst particles size distribution could be seen from SEM images of Cu/ZnO/ Al2O3. 5. Conclusions Utilization of ultrasound in hydrogenation of fructose over Cu/SiO2, Cu/ZnO/Al2O3, and Raney-Ni catalysts was investigated. The choice of the catalyst had a significant influence on ultrasonic effect. With the Cu/ ZnO/Al2O3 catalyst, ultrasound moderately decreased the reaction rate. In the case of Raney-Ni catalyst, a slight increase in the reaction rate could be observed by sonification. The essential improvement of the catalyst activity under acoustic irradiation was observed using Cu/SiO2. However, selectivity remained unaffected by sonification. Generally, no influence of the ambient reaction temperature and H2 pressure on catalytic activity and selectivity was observed by sonification, however, at higher temperatures (90 and 110 °C) moderate improvement of catalyst activity was observed under acoustic irradiation. Variation of the nominal ultrasonic power input (050 W) in D-fructose hydrogenation over Raney-Ni catalyst resulted in the maximum enhancement of catalyst activity at a moderate nominal power input 30 W and a further decrease with additional power increase. The acoustic irradiation resulted in the deformation of the catalyst surface; therefore retarding catalyst deactivation in D-fructose hydrogenation over RaneyNi catalyst, while without sonification profound deactivation was notable. Acknowledgment This work is part of the activities at the Åbo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Program (2000-2005) by the Academy of Finland. Financial support from Finnish Graduate School in Chemical Engineering (GSCE) is gratefully acknowledged. We are grateful to BASF AG for providing catalyst samples. Literature Cited (1) Makkee, M.; Kieboom, A. P. G.; van Bekkum, H. Production methods of D-mannitol. Starch 1985, 37, 136-141. (2) Makkee, M.; Kieboom, A. P. G.; van Bekkum, H. Hydrogenation of D-fructose and D-fructose/D-glucose mixtures. Carbohydr. Res. 1985, 138, 225-236. (3) Heinen, A. W.; Peters, A. J.; van Bekkum, H. Hydrogenation of fructose on Ru/C catalysts. Carbohydr. Res. 2001, 330, 381390. (4) Pijnenburg, H. C. M.; Kuster, B. F. M.; van der Baan, H. S. Kinetics of the hydrogenation of fructose with Raney nickel. Starch 1978, 30, 352-355.
(5) To¨ro¨k, B.; Bala´zsik, K.; Felfo¨dldi, K.; Barto´k, M. Asymmetric reactions in sonochemistry. Ultrason. Sonochem. 2001, 8, 191200. (6) Szo¨llo¨si, G.; To¨ro¨k, B.; Szakonyi, G.; Kun, I.; Barto´k, M. Ultrasonic irradiation as activity and selectivity improving factor in the hydrogenation of cinnamaldehyde over Pt/SiO2 catalysts. Appl. Catal. A 1998, 172, 225-232. (7) Kun, I.; To¨ro¨k, B.; Felfo¨dldi, K.; Barto´k, M. Heterogeneous asymmetric reactions Part 17. Asymmetric hydrogenation of 2-methyl-2-pentenoic acid over cinchona modified Pd/Al2O3 catalysts. Appl. Catal. A 2000, 203, 71-79. (8) Crum, A. L. Comments on the evolving field of sonochemistry by a cavitation physicist. Ultrason. Sonochem. 1995, 2, 147152. (9) Sulman, M. G. Effect of ultrasound on catalytic processes. Russ. Chem. Rev. 2000, 69, 165-177. (10) Gonzalo Rodrı´guez, J.; Lafuente, A. A new advanced method for heterogeneous catalysed dechlorination of polychlorinated biphenyls (PCBs) in hydrocarbon solvent. Tetrahedron Lett. 2002, 43, 9581-9583. (11) Fuentes, A.; Marinas, J. M.; Sinisterra, J. V. Catalyzed synthesis of chalcones under interfacial solid-liquid conditions with ultrasound. Tetrahedron Lett. 1987 28, 4541-4544. (12) Mikkola, J.-P.; Toukoniitty, B.; Toukoniitty, E.; Aumo, J.; Salmi, T. Utilisation of on-line acoustic irradiation as a means to counter-effect catalyst deactivation in heterogeneous catalysis. Ultrason. Sonochem. 2004, 11, 233-239. (13) Disselkamp, R. S.; Chin, Y.-H.; Peden, C. H. F. The effect of cavitating ultrasound on the heterogeneous aqueous hydrogenation of 3-buten-2-ol on Pd-black J. Catal. 2004, 227, 552-555. (14) Toukoniitty, B.; Mikkola J.-P.; Murzin, D. Yu.; Salmi, T. Utilization of electromagnetic and acoustic irradiation in enhancing heterogeneous catalytic reactions. Appl. Catal., A 2005, 279, 1-22. (15) Suslick, K. S. Sonocatalysis, Handbook of Heterogeneous Catalysis; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1997. (16) Suslick, K. S.; Casadonte, D. J. Heterogeneous sonocatalysis with nickel powder. J. Am. Chem. Soc. 1987, 109, 3459-3461. (17) Disselkamp, R. S.; Hart, T. R.; Williams, A. M.; White, J. F.; Peden, C. H. F. Ultrasound-assisted hydrogenation of cinnamaldehyde. Ultrason. Sonochem. 2005, 12, 319-324. (18) Mikkola, J.-P.; Salmi, T. In-situ ultrasonic catalyst rejuvenation in three-phase hydrogenation of xylose. Chem. Eng. Sci. 1999, 54, 1583-1588. (19) Mason, T. J.; Lorimer, J. P. Applied Sonochemistry; WileyVCH Verlag GmbH: Weinheim, Germany, 2002. (20) Ha´jek, J.; Murzin, D. Yu. Liquid-phase hydrogenation of cinnamaldehyde over Ru-Sn sol-gel catalyst. 1. Evaluation of mass transfer via combined experimental/theoretical approach. Ing. Eng. Chem. Res. 2004, 43, 2030-2038. (21) Henglein, A.; Gutierrez, M. Chemical effects of continuous and pulsed ultrasound: a comparative study of polymer degradation and iodide oxidation. J. Phys. Chem. 1990, 94, 5169-5172. (22) Hagenson, L. C.; Doraiswamy, L. K. Comparison of the effects of ultrasound and mechanical agitation on a reacting solidliquid system. Chem. Eng. Sci. 1998, 53, 131-148. (23) Disselkamp, R. S.; Judd, K. M.; Hart, T. R.; Peden, C. H. F.; Posakony, G. J.; Bond, L. J. J. Catal. 2004, 221, 347-353.
Received for review February 17, 2005 Revised manuscript received April 20, 2005 Accepted May 31, 2005 IE050190S
