Hydrogenation of Citral over Activated Carbon Cloth Catalyst

Jeannette Aumo, Susanna Oksanen, Jyri-Pekka Mikkola, Tapio Salmi, and. Dmitry Yu. Murzin*. Laboratory of Industrial Chemistry, Process Chemistry Centr...
2 downloads 0 Views 346KB Size
Download PDF
Ind. Eng. Chem. Res. 2005, 44, 5285-5290

5285

Hydrogenation of Citral over Activated Carbon Cloth Catalyst† Jeannette Aumo, Susanna Oksanen, Jyri-Pekka Mikkola, Tapio Salmi, and Dmitry Yu. Murzin* Laboratory of Industrial Chemistry, Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FIN-20500 Åbo/Turku, Finland

Catalytic hydrogenation of an R,β-unsaturated aldehyde, citral, over different Ni and Pt fiber catalysts as well as a commercial Pt powder catalyst was performed in a batch reactor under pressurized conditions. Active carbon was used as a support material. The influence of the important variables, such as temperature and pressure, was investigated in the batch reactor. The metal loading in the catalyst was varied as well. The catalysts were characterized with scanning electron microscopy, atomic force microscopy, N2-physisorption, temperatureprogrammed hydrogen desorption, and inductively coupled plasma mass spectroscopy. Four consecutive experiments were carried out with each fiber catalyst in the batch reactor to elucidate eventual catalyst deactivation. The decrease in activity was fairly notable. The results of the kinetic experiments demonstrated the feasibility of utilizing woven active carbon cloths as supports in three-phase metal-catalyzed hydrogenation of citral. The Ni on active carbon cloth (ACC) yielded citronellal, menthol, and isopulegol. Pt/ACC catalyst favored hydrogenation of the CdO group, i.e., the formation of geraniol and nerol, whereas the commercial Pt/AC powder catalyst led to citronellal and various other products of its hydrogenation. Possible limitations for application of Pt/ACC catalyst are associated with deteriorating activity as a pronounced deactivation was observed for the fiber catalyst. Introduction New approaches to catalytic reaction engineering, like process intensification, require that catalyst development and reactor selection now proceed hand-in-hand; thus the catalyst, its preparation, morphology, activity, active centers, kinetics, and deactivation are inseparable from the choice of a catalytic reactor.1,2 Process intensification aims at transforming current practices in chemical engineering and bringing forth new developments in equipment, processing techniques, and operational methods. The goal is more compact, safer, energy efficient, and environmentally friendly processes.3 Application of structured catalysts is one of the ways to sustain the product quality by means of continuous operations and is thus considered as process intensification. Replacement of conventional catalysts by new, structured catalytic materials is an active research and development area. Among various alternative solutions, monolith structures,4 packed elements (i.e., Sulzer Katapak),5 microreactors,6 and catalytic fibers7 and various woven cloths8,9 could be mentioned. The interest lies in the fact that one wants to overcome the disadvantages of slurry catalysts and large catalyst pellets used in classical fixed beds. The need to separate the product from the catalyst after the reaction, catalyst attrition, and internal diffusion are associated with slurry catalysts. When using a fixed bed with pellets, diffusion and a high pressure drop are significant. By utilizing structured catalysts, these problems can be avoided. Active carbon cloths (ACCs) possess a number of advantages over powder catalysts, including greatly improved contact efficiency with the media leading to * To whom correspondence should be addressed. Tel.: +358 2 2154985. Fax: +358 2 2154479. E-mail: Dmitry.Murzin@ abo.fi. † Dedicated to Professor M. P. Dudukovic.

higher rates of adsorption, much higher surface areas (up to 3000 m2/g), and the potential for greatly simplified in situ regeneration. Activated carbon, which is commonly used as a support for precious metals, is a difficult material to specify, mainly due to its natural origin.10 However, a synthetically made precursor such as Kynol novoloid fibers, which are infusible and insoluble and possess physical and chemical properties that clearly distinguish them from natural fibers, can be used. Kynol novoloid fibers are cured phenolaldehyde fibers made by acid-catalyzed cross-linking of melt-spun novolac resin to form a fully cross-linked, three-dimensional amorphous structure similar to that of thermosetting phenolic resins. The pores of this activated carbon cloth fiber are generally straight and uniform in size.11 The pore configuration and high surface-to-volume ratio of the fibers permit extremely rapid and efficient adsorption and desorption.12 Generally there are several applications of activated carbon fibers (ACFs), such as purification of water and service solutions, purification of air, and medical applications in addition to the use of ACFs or ACCs as catalyst supports.12,13 Citral hydrogenation was chosen as a model reaction to investigate the potential of the active carbon cloths. Citral is an interesting model molecule for hydrogenation since it contains an isolated and a conjugated double bond, as well as a carbonyl group. The multifunctionality of the citral molecule enables chemoselective hydrogenation, provided that a suitable catalyst is found.14,15 Furthermore, citral is the name of the mixture of the geranial (the trans isomer) and the neral (the cis isomer), and therefore the reactivity of the two stereoisomers can also be studied. The reaction scheme of citral hydrogenation is displayed in Figure 1. The hydrogenation products geraniol, nerol, citronellal, and citronellol are used as ingredients in the flavoring and

10.1021/ie0492000 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/10/2005

5286

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005

Figure 1. Hydrogenation of citral, the mixture of geranial ((E)-3,7-dimethylocta-2,6-dienal or trans-citral) and neral ((Z)-3,7-dimethylocta2,6-dienal or cis-citral) into geraniol ((E)-3,7-dimethylocta-2,6-dien-1-ol), nerol ((Z)-3,7-dimethylocta-2,6-dien-1-ol), citronellal (3,7dimethyloct-6-enal), citronellol (3,7-dimethyloct-6-en-1-ol), isopulegol ((1R*,2S*,5R*)-5-methyl-2-(propen-2-il)cyclohexan-1-ol), menthol ((1R*,2S*,5R*)-5-methyl-2-(1-methylethyl)cyclohexan-1-ol), 3,7-dimethyloctan-1-ol, and 3,7-dimethyloctanal.

Figure 2. SEM image of the woven ACC catalyst.

perfumery industries.16 Furthermore, geraniol is used as the main ingredient in insect repellent products.17,18 The isomerization and hydrogenation products isopulegol and menthol are used for instance in cosmetics.19 Within the current work, we have investigated the potential of woven ACCs (Figure 2) impregnated with either Pt or Ni in a special arrangement of a batch reactor and compared it to a conventional Pt/active carbon powder catalyst. Experimental Section Catalyst Preparation and Characterization. Woven ACC from Kynol Europe was utilized as a support upon preparation of the catalyst. Due to the shape of the cloths they can easily be used, bent, and rolled. Also the spaces between the fibers in the cloth allow gas or liquid to pass through the cloth more easily than through a granulated carbon bed. Suitable dimensions (68 mm × 39 mm × 0.5 mm) of ACC-507-15 (surface area, 1500 m2/g; fiber thickness, 9 µm) were cut according to the dimensions of the

integrated gadget of a combined stirrer and catalyst holder. ACC catalysts were characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM), N2-physisorption, temperature-programmed hydrogen desorption (H2-TPD), and inductively coupled plasma mass spectroscopy (ICP-MS). Monometallic Pt (5.9 and 10.9 wt % Pt) catalysts, Pt/ ACC, were prepared by immersing ACC, which had been dried overnight at 60 °C, in a water solution of H2PtCl6 (Degussa), for approximately 4 h. The impregnated samples were washed with deionized water. After drying, the cloths were attached to the stirrer (Figure 3) and reduced in situ at 200 °C for 2 h under hydrogen flow. For comparison, a commercial (Johnson Matthey) 4.6 wt % Pt/active carbon (AC) powder catalyst (mean particle size, 16 µm) was used and characterized with N2-physisorption and ICP-MS. The preparation of the Ni catalysts principally comprised three stages: acidification, impregnation, and reduction. Ni (0.2, 0.5, and 0.8 wt %)/ACC samples were prepared as follows: the ACCs were dried overnight at 60 °C, after which they were acid treated with 1 M HCl (37%, J. T. Baker). Thereafter the cloths were washed with deionized water and dried overnight at 60 °C, upon which they were impregnated in a water solution of Ni(NO3)2‚6H2O (98%, Fluka) for approximately 4 h and washed with deionized water. After the ACCs were dried overnight at 60 °C, they were chemically reduced with a 0.26 M aqueous solution of NaBH4 (J. T. Baker). After the chemical reduction, the cloths were washed and attached to the stirrer. Finally, prior to the reaction, the Ni catalysts were additionally reduced in situ at 200 °C for 2 h under hydrogen flow. The hydrogen used had a purity of 99.999% (AGA). Experimental Setup. The hydrogenation of citral was carried out in a laboratory autoclave (600 mL, Parr Inc., USA) equipped with a heating jacket. The reactor was equipped with an integrated gadget of a combined propeller stirrer and catalyst holder (Figure 3), coupled to a Rushton turbine for effective gas distribution. Due to this special integrated stirrer and catalyst holder, the

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5287

Figure 4. SEM image of 5.9 wt % Pt/ACC.

Figure 3. Integrated gadget of a combined stirrer and catalyst holder.

stirring rate was 500 rpm in the experiments performed with the ACC catalysts, whereas it was 1500 rpm with the AC powder catalyst. The temperature and stirrer controllers used were Parr 4843 (Watlow Controls Series 982). Prior to each experiment, the catalyst (1.5 g) and reactor vessel were preheated under a hydrogen atmosphere to the desired temperature. Citral (Lancaster 5460) with a 1.95 geranial-to-neral (or trans-to-cis) ratio was used as received. Citral (1.1 g, 0.017 M) was dissolved in 435 mL of hexane (Merck, analytical grade, 99%), and the solution was saturated with hydrogen before being injected into the reactor. The total pressures of the experiments were between 6 and 51 bar. The partial pressure of hydrogen was calculated taking into account the vapor pressure of hexane at different temperatures.20 The hydrogen used had a purity of 99.999% (AGA). An amount of 500 µL of internal standard (0.021 M cyclohexanone in cyclohexane) was added to the samples withdrawn from the reactor. The samples were analyzed by gas chromatography (GC). The column was DB-1 with length 30 m, inner diameter 0.25 mm, and film thickness 0.5 µm. The following temperature programming was used: 70 °C for 1 min, 13 °C/min to 120 °C and continued with 1 °C/min to 125 °C, and 0.5 °C/min to 130 °C. At the end, the temperature was increased 10 °C/min to 160 °C and kept constant for 5 min. Some of the products were identified with GC-mass spectrometry (GC-MS). Results and Discussion Catalyst Characterization. The aim of the catalyst preparation was to produce 5 and 10 wt % Pt/ACC and 4.5, 9, and 20 wt % Ni/ACC. However, as the Pt and Ni contents on the ACC catalysts were verified by ICP-MS, it became clear that the applied impregnation procedure of Ni had been only partially successful yielding just 0.2, 0.5, and 0.8 wt % Ni, while the preparation method

Figure 5. SEM image of a Pt agglomerate in a defected site of a Pt/ACC catalyst.

for Pt/ACC catalysts was efficient, resulting in somewhat higher values of Pt than expected, namely, 5.9 and 10.9 wt %. Furthermore, the Pt content on the commercial powder catalyst was detected to be 4.6 wt %, which is in reasonable agreement with the manufacturer’s data claiming a Pt loading of 5 wt %. The SEM images revealed the morphology of the 5.9 wt % Pt/ACC catalyst. Figure 4 illustrates the distribution of Pt particles observed on the surface of the activated carbon fibers. Furthermore, some defected sites on the fiber structure were noticed. Big agglomerates of Pt particles were trapped in the defected sites as demonstrated by Figure 5. It was difficult to detect Ni on the Ni/ACC catalysts due to their low metal contents. The topographies of the ACC catalysts were analyzed with AFM. The surface was very fuzzy, and that complicated the surface analysis. However, it was confirmed that the Pt particles on the surface appeared as small particles as well as agglomerates of these. The Ni particles were not observed on the surface. Nitrogen physisorption was measured on the fresh ACC support material, fresh and spent Pt/ACC, fresh and spent Ni/ACC, and fresh Pt/AC powder catalyst. The specific surface areas and pore volumes are listed in Table 1. A clear decrease in the surface areas with an increasing metal content was observed. This is most likely due to metal particles blocking the pores of the catalyst structure. Furthermore, a decrease of surface area and pore volume was detected, when comparing the fresh catalyst to spent ones. Apparently this explains catalyst deactivation by pore blocking. H2-TPD was measured on the fresh 5.9 and 10.9 wt % Pt/ACC (Figure 6). The desorption of hydrogen starts at a much lower temperature for the 10.9 wt % Pt/ACC

5288

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005

Figure 6. H2-TPD diagram of 5.9 wt % Pt/ACC and 10.9 wt % Pt/ACC. Table 1. Specific Surface Areas and Pore Volumes for the Catalysts Used BET

Dubinin

specific specific specific microsurface pore surface pore area volume area volume (m2/g) (cm3/g) (m2/g) (cm3/g)

catalyst Kynol ACC support 4.6 wt % Pt/C powder (Johnson Matthey) 5.9 wt % Pt/ACC 10.9 wt % Pt/ACC 10.9 wt % Pt/ACC 0.2 wt % Ni/ACC 0.5 wt % Ni/ACC 0.5 wt % Ni/ACC 0.8 wt % Ni/ACC

Figure 7. The relative concentration of citral as a function of reaction time in the hydrogenation of citral over a 0.5 wt % Ni/ ACC catalyst at 100 °C and 5 bar ([), 100 °C and 10 bar (2), 120 °C and 10 bar (4), and 100 °C and 5 bar (]) (consecutive experiments were performed in this order).

fresh

954 797

0.63 0.76

1348 999

0.48 0.36

spent fresh spent spent fresh spent spent

630 804 734 643 946 493 580

0.35 0.45 0.49 0.41 0.51 0.31 0.38

865 1228 997 990 1319 747 932

0.31 0.44 0.35 0.35 0.47 0.27 0.33

catalyst, partly explaining the difference in activity in favor of the 10.9 wt % Pt/ACC catalyst. A second peak detected at 480 °C might indicate spillover hydrogen. Hydrogenation with Ni/ACC. Three to four consecutive experiments were performed with each Ni catalyst. Experiments were carried out in the following order at 100 °C and 5 bar, 100 °C and 10 bar, 120 °C and 10 bar, and 100 °C and 5 bar using 0.5 and 0.8 wt % Ni/ACC. With 0.2 wt % Ni/ACC the first two experiments were performed at the conditions mentioned above, but the third experiment was performed at 120 °C and 50 bar. The citral-to-Ni ratios were 382, 152, and 96 for 0.2, 0.5, and 0.8 wt % Ni/ACC, respectively. The reaction products obtained, when using Ni/ACC, were menthol and isopulegol. In the literature it has recently been shown that acid sites in combination with metal sites on the catalyst efficiently isomerize citronellal to isopulegols with subsequent hydrogenation and yields of 90% menthols directly from citral over a Ni(3%)/AlMCM-41 catalyst.15 Similarly synthesis of (-)-menthol from (+)-citronellal was reported on Ru/SiO2 and Ru/ C.21,22 However, in general, one-pot synthesis of menthol from citral is a difficult task to achieve. Due to the insufficient impregnation procedure of Ni onto the ACC, the obtained conversions were low. The highest conversion value, 13% at 415 min, was achieved with 0.5 wt % Ni/ACC at 120 °C and 10 bar (Figure 7). Independent of the Ni loading, increasing the pressure from 5 to 10 bar at 100 °C did not improve the conversion, but elevating the temperature from 100 °C to 120 °C at 10 bar significantly increased the conversion. When repeating the first experiment under exactly the same conditions, profound catalyst deactivation was

Figure 8. The relative citral concentration as a function of reaction time in the hydrogenation of citral over a 5.9 wt % Pt/ ACC catalyst at 80 °C and 5 bar ([), 80 °C and 10 bar (2), 100 °C and 5 bar (4), and 80 °C and 5 bar (]) (consecutive experiments were performed in this order).

observed. With 0.5 wt % Ni/ACC at 5 h, the conversion decreased from 7% to 3%. With 0.8 wt % Ni/ACC at 6 h, the conversion decreased from 4% to 2%. A lumped productivity value of 2.1 × 10-5 mol/(dm3 s gNi) for menthol and isopulegol was obtained with 0.2 wt % Ni/ ACC at 120 °C and 50 bar. Hydrogenation with Pt Catalysts. Four experiments (at the following conditions: 80 °C and 5 bar, 80 °C and 10 bar, 100 °C and 5 bar, and back to 80 °C and 5 bar) were performed with 5.9 wt % Pt/ACC in a consecutive manner with the citral-to-Pt ratio equal to 13. Production of geraniol, nerol, and citronellal was favored over the Pt/ACC catalyst. Note that geraniol and nerol are a result of CdO carbonyl group reduction, which is thermodynamically less preferential than hydrogenation of the CdC bond leading to citronellal. The highest productivity value of 1.9 × 10-6 mol/(dm3 s gPt) for the unsaturated alcohols geraniol and nerol was achieved in the first experiment. The geraniol-to-nerol ratio was not affected by the catalyst deactivation and was independent of conversion being equal to 2.3. The highest selectivity to geraniol (70%) at 80 °C and 10 bar was achieved at 10% conversion. At 80 °C/5 bar and at 100 °C/5 bar the selectivity to geraniol at 15% conversion decreased from the initial level of 70% to 50% due to citronellal formation. At this conversion level the overall selectivity for the reduction of the CdO group (nerol + geraniol) was 80-100%. Figure 8 illustrates the changes of activity in the series of consecutive experiments, which decreased with each experiment;

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5289

Figure 9. The relative citral concentration as a function of reaction time in the hydrogenation of citral over a 10.9 wt % Pt/ ACC catalyst at 80 °C and 5 bar ([), 80 °C and 10 bar (2), 100 °C and 5 bar (4), and 80 °C and 5 bar (]) (consecutive experiments were performed in this order).

namely, in four experiments conversions of 23%, 13%, 11%, and 3% were achieved after 7 h. Four consecutive experiments under different experimental conditions were performed with 10.9 wt % Pt/ ACC: 80 °C and 5 bar, 80 °C and 10 bar, 100 °C and 5 bar, and 80 °C and 5 bar. The citral-to-Pt ratio was 7. The highest productivity value of geraniol and nerol was 3.9 × 10-6 mol/(dm3 s gPt) obtained in the first experiment. This is a 2-fold increase compared with the value for the 5.9 wt % Pt/ACC. The product ratio geraniol to nerol was 2.3 (as it was with 5.9 wt % Pt loading) in the three first experiments, whereas it obtained a value of 2.6 in the last experiment at 80 °C and 5 bar. The overall selectivity to CdO group reduction was 80100% at low conversion. In the first experiment, where the conversion reached a value of 58% after 7 h, the selectivity to CdO group reduction was 73% due to the formation of citronellal and the secondary hydrogenation product citronellol. In the following consecutive experiments the conversion decreased being 24%, 19%, and 6%, respectively (Figure 9). The following consecutive experiments were performed with the 4.6 wt % Pt/AC powder catalyst (citralto-Pt ) 7): 80 °C and 5 bar, 100 °C and 5 bar, 100 °C and 10 bar, and back to 80 °C and 5 bar. The respective conversion values after 6.5 h were 49%, 49%, 62%, and 13%. Not surprisingly, conversion was improved by increasing both temperature and pressure. Comparison between the first and fourth experiments performed under the same experimental conditions clearly revealed catalyst deactivation. Production of citronellal with a selectivity of 35-40% in each experiment, independent of pressure and temperature, was favored over the Pt/ AC powder catalyst. Broader product distribution for the powder catalyst compared to the fiber catalyst was noticeable. Selectivity toward 3,7-dimethyl-2,7-octadien1-ol (trans) was 12%. Among other products, 3,7dimethyl-7-octenol, 3,7-dimethyloctanol, and citronellol with selectivities in the range of 5-8% were observed. The highest overall selectivity to geraniol and nerol (44%) was achieved at 8% conversion in the fourth experiment, at 80 °C and 5 bar, while in the other experiments, the overall selectivity to geraniol and nerol was approximately 14%. Even though the selectivity to geraniol and nerol is lower for the Pt/AC powder catalyst compared to Pt/ACC catalysts, the productivity value for geraniol and nerol was 10 times higher, being 5.4 × 10-5 mol/(dm3 s gPt). The average of the geraniol-to-nerol ratio was 1.15. At 100 °C the ratio decreased from 1.4

Figure 10. The trans-to-cis ratios as a function of conversion for 5.9 wt % Pt/ACC and 4.6 wt % Pt/AC powder: solid symbols, transto-cis ratio of citral; open symbols, geraniol-to-nerol ratio.

to 0.9 with increasing conversion. Figure 10 illustrates the dependence of the geraniol-to-nerol ratio as well as the citral trans-to-cis ratio on conversion for both ACC and AC powder catalysts at 100 °C and 5 bar. In the starting citral solution, the trans-to-cis ratio is 1.95. The citral trans-to-cis ratio remains at the initial value over the Pt/AC powder catalyst, while it increases from 2 to 2.8 following the same trend as the geraniol-to-nerol ratio for Pt/ACC. An explanation for the difference between the ratios could be a difference in the Pt particle size; another reason might be that strong metal support interactions (SMSI) prevail in the case of ACCs affecting the stereoselectivity. SMSI has previously been shown to affect the product distribution of citral hydrogenation over Pt/TiO2 compared to Pt/SiO2.23 Other factors affecting the selectivity in the hydrogenation of R,β-unsaturated aldehydes, such as citral, have been extensively reviewed.24,25 Further characterization of the catalysts is needed for deeper insight into the role of the support and the active metal in the ratios of geraniol to nerol and the citral trans-to-cis ratio. Conclusions. Woven active carbon cloths were used as catalyst supports in three-phase metal-catalyzed hydrogenation of the R,β-unsaturated aldehyde, citral, demonstrating the potential of these structured materials. The Ni/ACC yielded citronellal, menthol, and isopulegol, but as the impregnation of Ni was incomplete, the conversions were low (2-13%). Hydrogenation of the CdO group leading geraniol and nerol was favored over the Pt/ACC catalyst, whereas the CdC hydrogenation resulting in citronellal and various other products of its hydrogenation was predominant over the commercial Pt/AC powder catalyst. The ratio of geraniol to nerol was also different, being an average of 2.3 for the 5.9 wt % Pt/ACC catalyst and roughly 1.15 for 4.6 wt % Pt/AC powder catalyst. The trans-to-cis ratios of citral were in the same range as the geraniol-to-nerol ratios for the Pt/ACC catalysts, whereas the trans-to-cis ratio of citral maintained the initial value of 1.95 even though the ratio of geraniol to nerol decreased drastically on the Pt/AC powder catalyst. Pronounced catalyst deactivation was observed for the fiber catalyst, which could limit possible applications of the Pt/ACC catalyst. Clearly the Pt/ACC catalysts cannot compete with the Pt/AC powder catalyst in productivity under these experimental laboratory conditions since the productivity values are 10 times lower. At the same time, Pt/ACC afforded high selectivity in reduction of the CdO bond, which is thermodynamically unfavorable and is difficult to achieve over conventional catalysts. One has to keep in mind that mass transfer

5290

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005

could influence the results with carbon cloths due to the difference in the stirring rate connected with the limitations of the integrated gadget of a combined catalyst holder and stirrer. Obviously the catalyst preparation, as well as the experimental equipment setup, should be further optimized to improve the performance of the promising Pt/ACC catalysts. Acknowledgment The authors are grateful to colleagues from Åbo Akademi University for their analytical assistance: Clifford Ekholm, for the SEM analysis, Paul Ek and Virpi Va¨a¨na¨nen, for the ICP-MS analysis, Mikael Bergelin, for the AFM analysis, and Markku Reunanen, for the GC-MS analysis. The financial support from the Finnish Graduate School in Chemical Engineering (GSCE) is gratefully acknowledged. This work is part of the activities at the Åbo Akademi Process Chemistry Centre (PCC) within the Finnish Centre of Excellence Programme (2000-2005) by the Academy of Finland. Literature Cited (1) Dudukovic, M. P. Trends in catalytic reaction engineering. Catal. Today 1999, 48, 5. (2) Dudukovic, M. P.; Larachi, F.; Mills, P. L. Multiphase reactors-revisited Chem. Eng. Sci. 1999, 54, 1975. (3) Charpentier, J. C.; McKenna, T. F. Managing complex systems: Some trends for the future of the chemical and process engineering. Chem. Eng. Sci. 2004, 59, 1617. (4) Boger, T.; Heibel, A. K.; Sorensen, C. M. Monolithic catalysts for the chemical industry. Ind. Eng. Chem. Res. 2004, 43, 4602. (5) Steinigeweg, S.; Gmehling, J. n-Butyl acetate synthesis via reactive distillation: Thermodynamic aspects, reaction kinetics, pilot-plant experiments, and simulation studies. Ind. Eng. Chem. Res. 2002, 41, 5483. (6) Kolb, G.; Hessel, V. Micro-structured reactors for gas-phase reactions. Chem. Eng. J. 2004, 98, 1. (7) Aumo, J.; Lilja, J.; Ma¨ki-Arvela, P.; Salmi, T.; Sundell, M.; Vainio, H.; Murzin, D. Yu. Hydrogenation of citral over a polymer fibre catalyst. Catal. Lett. 2002, 84, 219. (8) Salmi, T.; Ma¨ki-Arvela, P.; Toukoniitty, E.; Kalantar Neyestanaki, A.; Tiainen, L.-P.; Lindfors, L.-E.; Sjo¨holm, R.; Laine, E. Liquid-phase hydrogenation of citral over an immobile silica fibre catalyst. Appl. Catal., A 2000, 196, 93. (9) Matatov-Meytal, Yu.; Scheintuch, M. Catalytic fibers and cloths. Appl. Catal., A 2002, 231, 1. (10) Stiles, A. B.; Koch, T. A. Catalyst manufacture; Marcel Dekker: New York, 1995.

(11) www.kynol.com. (12) Ermolenko, I. N.; Lyubliner, I. P.; Gulko, N. V. Chemically modified carbon fibers and their applications; VCH: Weinheim, 1990. (13) Suzuki, M. Activated carbon fiber: Fundamentals and applications. Carbon 1994, 32, 577. (14) Ma¨ki-Arvela, P.; Tiainen, L.-P.; Lindblad, M.; Demirkan, K.; Kumar, N.; Sjo¨holm, R.; Ollonqvist, T.; Va¨yrynen, J.; Salmi, T.; Murzin, D. Yu. Liquid-phase hydrogenation of citral for production of citronellol: Catalyst selection. Appl. Catal., A 2003, 241, 271. (15) Trasarti, A. F.; Marchi, A. J.; Apesteguı´a, C. R. Highly selective synthesis of menthols from citral in a one-step process. J. Catal. 2004, 224, 484. (16) De Simone, R. S.; Gradeff, P. S. Process for the hydrogenation of citral to citronellal and of citronellal to citronellol using chromium-promoted Raney-nickel catalyst. U.S. Patent 4,029,709, 1977. (17) Marin, A. B.; Butler, J. F. Method for repelling fire ants and horn flies and compositions for repelling fire ants and horn flies and acting as anti-feedants for fire ants and horn flies. U.S. Patent 5,753,686, 1998. (18) Warren, C. B.; Butler, J. F.; Wilson, R. A.; Mookherjee, B. D.; Smith, L. C.; Marin, A. B.; Narula, A. P. S.; Boden, R. M. Insect repellent compositions and methods for using same. U.S. Patent 5,633,236, 1997. (19) Mohammadi, F.; Vargas, A. Cosmetic composition for stressed skin under extreme conditions. U.S. Patent 6,649,178, 2003. (20) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The properties of gases and liquids, 4th ed.; Mc-Graw-Hill: New York, 1988. (21) Milone, C.; Gangemi, C.; Neri, G.; Pistone, A.; Galvagno, S. Selective one step synthesis of (-)menthol from (+)citronellal on Ru supported on modified SiO2. Appl. Catal., A 2000, 199, 239. (22) Milone, C.; Gangemi, C.; Ingoglia, R.; Neri, G.; Galvagno, S. Role of the support in the hydrogenation of citronellal on ruthenium catalysts. Appl. Catal., A 1999, 184, 89. (23) Singh, U. K.; Vannice, M. A. Influence of metal-support interactions on the kinetics of liquid-phase citral hydrogenation. J. Mol. Catal. A: Chem. 2000, 163, 233. (24) Gallezot, P.; Richard, D. Selective hydrogenation of R,βunsaturated aldehydes. Catal. Rev.sSci. Eng. 1998, 40, 81. (25) Zsigmond, A.; Balatoni, I.; Notheisz, F.; Joo´, F. J. Catal. 2004, 227, 428.

Received for review August 30, 2004 Revised manuscript received December 2, 2004 Accepted December 7, 2004 IE0492000