Kinetics of Liquid-Phase Hydrogenation of Furfuraldehyde to Furfuryl

Ind. Eng. Chem. Res. 2003, 42, 3881-3885

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of Liquid-Phase Hydrogenation of Furfuraldehyde to Furfuryl Alcohol over a Pt/C Catalyst Prakash D. Vaidya and Vijaykumar V. Mahajani* Chemical Engineering Department, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai-400019. India

The kinetics of the liquid-phase hydrogenation of furfuraldehyde to furfuryl alcohol over a 5% Pt/C catalyst was studied in a slurry reactor. The solvent used was a mixture of 2-propanol and water (12.4% w/w). At the temperatures (403-448 K) and pressures (1.03-2.06 MPa) studied, all mass-transfer resistances (gas-liquid, liquid-solid, and intraparticle diffusion) were absent. The initial rate data were analyzed using a power-law model. The initial rates increased almost linearly with hydrogen concentration at all temperatures. The reaction showed a zeroth-order dependence with respect to furfuraldehyde above a feed concentration of 0.13 kmol m-3. Below this value, the order was found to be 0.86. Therefore, a Langmuir-Hinshelwood-type model for a dual-site mechanism with molecular adsorption of all species was proposed, and this model provided the best fit of the experimental data. Catalyst reusability studies showed that the catalyst could be reused without any adverse effects. Introduction Furan chemicals will assume importance in the future because they can be manufactured from renewable sources. For example, furfuryl alcohol (FA) is an important chemical of “furan” class. It is mainly used in the manufacture of resins. Furfuryl alcohol resins find wide application in chemical resistance construction as foundry binders in foundries and as adhesives in the manufacture of plywoods and furniture.1 FA is also the starting material for the manufacture of tetrahydrofurfuryl alcohol (THFA). Furfuryl alcohol is prepared industrially by the catalytic reduction of furfuraldehyde using Ni and Cu/CrO catalysts. The liquid-phase catalytic hydrogenation of furfuraldehyde has been widely studied using Raney Ni catalyst either alone or promoted by Ru, Pd, and Pt.2,3 Supported noble metal catalysts, especially Ru and Pd, also seem to be promising.4,5 Merat et al.4 also studied furfural hydrogenation over Pt supported catalysts under mild operating conditions and reported that no reaction takes place at a temperature of 50 °C. In general, studies on Pt-based catalysts have been fewer.6-8 Little information is available in the published literature on insights into the hydrogenation of furfuraldehyde to furfuryl alcohol using Pt as the catalyst. It was therefore thought desirable to study the hydrogenation of furfuraldehyde over Pt/C as the catalyst. During the manufacture of furfural, the product containing 6% water is first obtained. Pure furfural is obtained by breaking this azeotrope. In the present investigation, attention was focused on using furfural containing water as the feedstock. This approach would eliminate one unit operation of breaking the azeotrope, thereby gaining * To whom correspondence should be addressed. Tel.: 9122-24145616. Fax: 91-22-24145614. E-mail: [email protected].

economic advantage. Further, conducting exothermic reactions using water as the medium or one of the media enhances the intrinsic safety of the process. We have addressed this issue in the present investigation. Experimental Section Materials. The furfuraldehyde used in all experiments was of analytical reagent grade and was purchased from S.D. Fine Chemicals, Mumbai, India. The 5% Pt/C supported catalyst was obtained from Arora Matthey Ltd., Kolkata, India, and was used as such. Hydrogen from a cylinder with a minimum stated purity of 99.9% was obtained from Industrial Oxygen Company Ltd., Mysore, India. Analytical reagent-grade 2-propanol was obtained from a local vendor and used as the reaction medium. The catalyst had a BET surface area of 948 m2 g-1, a pore volume of 0.43 cm3 g-1, and an average pore diameter of 6.5 nm. The particle size range was 23-69 µm, and the mean diameter was 36.6 µm. Catalyst characterization was done using a Micromeritics ASAP 2010 analyzer by nitrogen adsorption. The particle size range was determined using a particle size analyzer (Coulter LS 230). Experimental Setup. A schematic diagram of the experimental setup is shown in Figure 1. All experiments were conducted in a SS-316 Parr high-pressure reactor of capacity 0.1 dm3. The reactor was equipped with an electrically heated jacket, a turbine agitator, and a variable-speed magnetic drive. The temperature and the speed of agitation were controlled by means of a Parr 4842 controller. The gas inlet, gas release valve, cooling water feed line, pressure gauge, and rupture disk were situated on top of the reaction vessel. The liquid sample line and the thermocouple well were

10.1021/ie030055k CCC: $25.00 © 2003 American Chemical Society Published on Web 07/19/2003

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Figure 1. Schematic diagram of experimental setup for hydrogenation (PI, pressure indicator; TI, temperature indicator; SI speed indicator; R, reaction vessel/autoclave; T, thermocouple; H, electric heater; RD rupture disk; I, impeller; GS, gas sparger; SC, sample condenser; CY, gas cylinder).

immersed in the reaction mixture. A chilled water condenser was fitted on the sample valve exit line to avoid flashing of the sample. The entire assembly was leak proof. Experimental Procedure. The reactor was first charged with 0.05 dm3 of the reaction mixture and the catalyst. It was purged with nitrogen to ensure that the entire assembly was leak-proof under nitrogen pressure. The residual nitrogen left over near atmospheric pressure also ensured an inert atmosphere. Then, hydrogen was used to purge the nitrogen. All lines were closed, the speed of agitation was adjusted to a predetermined value, and the reaction temperature was set. The contents were heated to the desired temperature, and a sample was withdrawn. This was considered “zero” time for the reaction. Hydrogen from the cylinder was then sparged into the liquid phase directly beneath the impeller at the desired partial pressure of hydrogen. The amount of hydrogen charged was far in excess of that theoretically required. Samples were withdrawn periodically after sufficient flushing of the sample line. The decrease in pressure was monitored by a pressure gauge. Additional hydrogen was charged from the cylinder through a manually operated control valve to make up for the hydrogen consumed during the reaction, thus maintaining a constant total pressure. (The entire system was in semi-batch mode.) The reaction was allowed to proceed for a prescribed time, after which the autoclave was allowed to cool. Samples of the remaining reaction mixture were analyzed thereafter. Product Analysis. Analysis of the reaction mixture was carried out using gas chromatography. A GC 1000 unit (Chemito Instruments) equipped with a flame ionization detector was used for this purpose. Furfuraldehyde, furfuryl alcohol, and other byproducts formed, if any, were separated using a 2.5-m-long SE 30 column. The samples of the reaction mixture showed that no

THFA was formed and furfuryl alcohol was the major product. A high-boiling byproduct was formed in minor amounts (4%). However, it could not be identified. The reproducibility of the results was checked, and the error in all experimental measurements was less than 2%. General Considerations. In all experiments, furfural containing water (6%) was used as the feedstock. A few experiments were carried out at 423 K and 2.06 MPa hydrogen pressure to study product distribution. The results showed that, for a furfural feed concentration of 0.17 kmol m-3, the conversion was 58% after 1.5 h, and the selectivity to furfuryl alcohol was 96%. The only byproduct formed, which remained unknown, might be acetal between aldehyde and alcohol. The active acidic site on the catalyst might lead to the formation of acetal. Merat et al.4 observed the formation of furfural dimethyl acetal during hydrogenation in the presence of Ni supported catalyst using methanol as the reaction medium. Broekhuis et al.9 also reported cyclic acetalization of diols with aldehydes in the presence of an acid catalyst. The addition of small quantities of monoethanolamine (base) at 500 ppm almost eliminated this product (selectivity > 99%), thereby indirectly confirming the possible formation of acetal. 2-Propanol forms an azeotrope with water. If this azeotropic mixture could be used as the reaction medium, an economic advantage would be gained. Our results showed an increase in the selectivity to furfuryl alcohol (>98.5%) in the presence of water. Although no remarkable difference in conversion was observed, this approach would also increase furfuryl alcohol yields and hence prove profitable. All further experiments were conducted using 2-propanol containing water (12.4% w/w) as the reaction medium. It is assumed that the conversions and yields are the same for all practical purposes. At these reaction conditions, experiments were carried out to test the suitability of this catalyst for reuse.

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Figure 2. Effect of catalyst loading on initial rates (423 K, 2.06 MPa H2 pressure, feed furfural concentration 0.17 kmol m-3).

Starting with the fresh catalyst, the same catalyst was reused three times after separation from the reaction medium. The catalyst activity remained the same throughout, thus indicating the high stability of the catalyst under these conditions. In all experiments, reactant concentrations were measured at different time intervals. From these data, initial rates were calculated by the well-known procedure of fitting a polynomial followed by differentiation at time t ) 0 to obtain initial rates.

Figure 3. Effect of hydrogen pressure on initial rates at various temperatures (feed furfural concentration ) 0.17 kmol m-3, catalyst loading ) 0.96 kg m-3): [, 403 K; 9, 423 K; 2, 448 K.

Results and Discussion The catalytic hydrogenation of furfuraldehyde is a heterogeneous gas-liquid-solid catalyzed reaction involving the following transfer processes:10 transfer of hydrogen from the gas phase to the gas-liquid interface; instantaneous saturation of the interface; diffusion of hydrogen through the interface to the bulk liquid; transfer of dissolved hydrogen and furfuraldehyde present in the liquid phase to the catalyst surface through the solid-liquid interface; intraparticle diffusion followed by chemical reaction at the active centers and diffusion of the products. Any of these mass-transfer processes (gas-liquid, liquid-solid, and intraparticle diffusion) can influence the rates of reaction. Thus, to study the true kinetics, these diffusional resistances should be absent. The partial pressure of hydrogen was 2.06 MPa, whereas the total system pressure was 3.45 MPa. At this pressure of hydrogen, gas-side mass-transfer resistance could be ignored. The liquid-side mass-transfer coefficient and liquid-solid mass-transfer coefficient depend on the intensity of turbulence in the liquid phase. By varying the speed of agitation, we effectively varied the intensity of turbulence and hence the masstransfer coefficients. Experiments performed at 448 K and 2.06 MPa hydrogen pressure using a furfural feed concentration of 0.17 kmol m-3 showed that the rate of reaction was independent of the speed of agitation above an impeller speed of 15 rps, thus indicating the absence of these resistances. All experiments were conducted at lesser temperatures and speeds equal to 20 rps. The catalyst particle size was varied to test the significance of intraparticle mass-transfer resistance. Two experiments were carried out at 448 K using particle sizes in

Figure 4. Effect of initial furfural concentration on initial rates at various temperatures (H2 pressure ) 2.06 MPa, catalyst loading ) 0.96 kg m-3): [, 403 K; 9, 423 K; 2, 448 K.

the ranges 25-60 and 120-180 µm. It was observed that the catalyst particle size had no effect on the rate of reaction, and hence, pore diffusion was deemed to be absent. From this result, it is seen that various diffusional resistances associated with the transfer of hydrogen and furfural were absent, and true kinetics could be inferred from experimental data. Initial Rate Data. The kinetics of this reaction was studied between 403 and 448 K with initial concentrations of furfural in the range 0.04-0.17 kmol m-3 and a hydrogen partial pressure range of 1.03-2.06 MPa. The initial rate was found to vary linearly with the catalyst loading in the range of 0.19-0.96 kg m-3 (Figure 2). The effect of the hydrogen partial pressure on the initial rates at various temperatures is shown in Figure 3. These results indicate an approximately linear dependence on the hydrogen concentration at all temperatures. The dependence of the rate on the furfural feed concentration at various temperatures is shown in Figure 4. The reaction is zeroth-order with respect to furfural at feed concentrations higher than

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Table 1. Estimated Orders of the Hydrogenation Reaction: Initial Rate Data

Table 2. Residual Sum of Squares and Variance for the Model Proposed (Eq 3)

temperature (K)

k

na

m

403 403b 423 448

0.063 ( 0.02 0.069 ( 0.02 0.101 ( 0.05 0.140 ( 0.07

0.82 ( 0.02 0.81 ( 0.02 0.86 ( 0.03 0.91 ( 0.07

1.12 ( 0.02 1.12 ( 0.02 1.15 ( 0.02 1.16 ( 0.03

Valid up to a furfuraldehyde concentration of 0.13 kmol m-3. Kinetic parameters based on rates at different times from concentration vs time profile. a

b

Figure 5. Temperature dependence of the reaction rate constant.

0.13 kmol m-3. Below this value, the order with respect to furfural is slightly less than unity. This observation is very important from a process development point of view. A power-law model was used to fit the initial rate data for furfural concentrations less than 0.13 kmol m-3. The initial rate of reaction could be expressed as

r0 ) k[A]0m[B]0n

(1)

The orders of the reaction, m and n, with respect to hydrogen and furfuraldehyde at different temperatures were obtained by linear regression analysis. Although the reaction mixture is 2-propanol containing water (12.4%), the solubility of hydrogen in 2-propanol at the given partial pressure was used.11 The error due to this approximation can be neglected. The values of kinetic parameters with 95% confidence intervals are reported in Table 1. To investigate the effect of deactivation by the product, it was thought desirable to obtain kinetic parameters by having rates at different times from concentration vs time profile

r)

-d[B] ) k[A]m[B]n dt

(2)

The data were correlated with eq 2 via linear regression analysis. The parameters obtained (Table 1) indicate that their values are similar to those obtained via initial rates, thereby confirming the hypothesis of no deactivation of the catalyst by the product formed. The variation of k with temperature is shown in Figure 5. The energy of activation for this reaction was found to be 28 kJ mol-1, which indirectly confirms the observation of the absence of diffusional resistances.

temperature (K)

residual sum of squares

variance

403 423 448

2.57 × 10-7 4.21 × 10-7 5.10 × 10-7

6.42 × 10-8 1.05 × 10-7 1.27 × 10-7

Table 3. Rate Parameters with 95% Confidence Intervals for the L-H Model temperature (K)

k2 (m6 kmol-1 kgcat-1 min-1)

KA

KB

403 423 448

0.13 ( 0.07 0.24 ( 0.13 0.40 ( 0.28

7.30 ( 5.10 4.51 ( 2.40 2.20 ( 1.27

0.19 ( 0.09 0.14 ( 0.04 0.10 ( 0.02

Because the order with respect to furfuraldehyde is fractional, it is desirable to obtain insight into the kinetics via Langmuir-Hinshelwood models. Langmuir-Hinshelwood Models. Various Langmuir-Hinshelwood-type models were proposed to gain further insight into the reaction mechanism. Two singlesite mechanisms with and without dissociation of hydrogen were proposed. Further, in each of these models, rate expressions were developed for the case in which either the adsorption of furfuraldehyde or hydrogen or the surface reaction was the rate-controlling step. All these models gave negative parameters and hence were rejected. Another dual-site mechanism was also proposed in which hydrogen was molecularly adsorbed on active sites different from those involved in the adsorption of furfuraldehyde and furfuryl alcohol and surface reaction between adsorbed furfuraldehyde and adsorbed hydrogen was assumed to be rate-controlling. This model was found to give positive parameters. The rate expression derived for this case is

r)

k2KAKB[A][B] (1 + kA[A])(1 + kB[B])

(3)

The influence of the product furfuryl alcohol on the rate of reaction was studied at 423 K and 2.06 MPa hydrogen partial pressure to ascertain any deactivation of the catalyst. For this purpose, furfuryl alcohol (10% w/w of the reactant furfuraldehyde) was added initially before the reaction started. It was observed that, under these conditions, the presence of furfuryl alcohol had no effect on the rate of reaction. This finding was also indirectly confirmed by the kinetic analysis using powerlaw model, as stated earlier. Hence, the product term in the denominator of eq 3 was omitted. The kinetic data at various temperatures were correlated to this rate expression using MathCad. Table 2 shows the values of the residual sum of squares and the variance at various temperatures for this model. It was found that the initial rate data, as well as the kinetic data at all reactant concentrations in the range studied, could be fitted well by this model. The various parameters and the 95% confidence intervals for this model are given in Table 3. A comparison of the rates predicted by this model and the experimentally observed rates is shown in Figure 6. The predicted values are in fair agreement with each other. The energy of activation for the surface reaction was found to be 60.5 kJ mol-1. From the van’t Hoff equation, the heats of adsorption of furfuraldehyde and hydrogen were found to be 39.8 and 21.3 kJ mol-1, respectively.

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Nomenclature A ) hydrogen B ) furfuraldehyde [A] ) concentration of hydrogen in the bulk liquid phase, kmol m-3 [B] ) concentration of furfuraldehyde in the liquid phase, kmol m-3 [A]0 ) initial hydrogen concentration, kmol m-3 [B]0 ) initial furfuraldehyde concentration, kmol m-3 k ) reaction rate constant in eq 1, (m3)m+n kmol1-m-n kgcat-1 min-1 k2 ) surface reaction rate constant in eq 3, m6 kmol-1 kgcat-1 min-1 KA ) adsorption equilibrium constant for A KB ) adsorption equilibrium constant for B m ) order with respect to hydrogen n ) order with respect to furfuraldehyde r ) overall rate of reaction, kmol kgcat-1 min-1 r0 ) initial rate of reaction, kmol kgcat-1 min-1 t ) time, min Figure 6. Comparison of predicted rates (eq 3) to experimental rates at various temperatures and reactant concentrations (feed furfural concentration ) 0.043-0.17 kmol m-3, 95% confidence intervals for slope ) 1.02 ( 0.08).

Conclusions The kinetics of the liquid-phase hydrogenation of furfuraldehyde to furfuryl alcohol over a 5 % Pt/C catalyst was studied in the range of temperatures from 403 to 448 K and in the range of pressures from 1.03 to 2.06 MPa. The solvent used was 2-propanol containing 12.4 % (w/w) water. The initial rate of reaction varied linearly with the catalyst loading, whereas the impeller speed had no influence on the rates above a speed of 15 rps, thus indicating the absence of gas-liquid and liquid-solid mass-transfer resistances. Similarly, under the conditions employed, the catalyst particle size has no effect on the rates, and hence, the intraparticle masstransfer resistance was negligible. The initial rate data were analyzed using a simple power-law model. The rate of reaction was almost linearly dependent on the hydrogen concentration. Also, for concentrations less than 0.13 kmol m-3, the order with respect to furfuraldehyde was 0.86. However, the reaction was found to be of zeroth order with respect to furfuraldehyde above this concentration. The experimental data could also be explained using Langmuir-Hinshelwood kinetics. A dual-site mechanism with nondissociative adsorption of hydrogen and surface reaction as the rate-controlling step provided the best fit of the experimental data. Acknowledgment P.D.V. is grateful to the University Grants Commission, New Delhi, Government of India, for financial support.

Literature Cited (1) McKetta, J. J.; Cunningham, W. A. Encyclopedia of Chemical Processing and Design; Marcel Dekker: New York, 1986; Vol. 24, p 63. (2) Singh, P. Development of Furan Chemicals; Ph.D. Thesis, University of Mumbai, Mumbai, India, 1978. (3) Erzhanova, M. S.; Beisekov, T.; Sokolskii, D. V. Hydrogenation of furfural in mixed solvents on Raney nickel catalysts promoted by platinum group metals. Khim. Khim. Tekhnol. 1971, 11, 122; cf Chem. Abstr. 1971, 78, 84151w. (4) Merat, N.; Godawa, C.; Gaset, A. High Selective Production of Tetrahydrofurfuryl Alcohol: Catalytic Hydrogenation of Furfural and Furfuryl Alcohol. J. Chem. Technol. Biotechnol. 1990, 48, 145. (5) Beisekov, T. B.; Zhurkhabaeva, L. A.; Bitemirova, A. E. Hydrogenation of furfural on supported Palladium catalysts under hydrogen pressure. Zh. Prikl. Khim. 1994, 67 (6), 1041; cf Chem. Abstr. 1994, 122, 55836v. (6) Kijenski, J.; Winiarek, P. Selective hydrogenation of R,βunsaturated aldehydes over platinum catalysts deposited on monolayer supports. Appl. Catal. A 2000, 193 (1, 2), L1-L4. (7) Kijenski, J.; Winiarek, P.; Paryjczak, T. Metal on Oxide MonolayersNew Concept of Selective Catalyst for R,β-Unsaturated Aldehydes Hydrogenation. Pol. J. Chem. Technol. 1999, 1, 16. (8) Marinelli, T.; Ponec, V.; Raab, C. G.; Lercher, J. A. Furfural-Hydrogen Reactions, Manipulation of Activity and Selectivity of the Catalyst. Stud. Surf. Sci. Catal. 1993, 78, 195. (9) Broekhuis, R. R.; Lynn, S.; King, C. J. Recovery of Propylene Glycol from Dilute Aqueous Solutions via Reversible Reaction with Aldehydes. Ind. Eng. Chem. Res. 1994, 33, 3230. (10) Doraiswamy, L. K.; Sharma, M. M. Heterogeneous Reactions: Analysis, Examples and Reactor Design; John Wiley and Sons: New York, 1984; Vol. 2. (11) Shaw, J. M. A Correlation for Hydrogen Solubility in Aliphatic and Aromatic Solvents. Can. J. Chem. Eng. 1987, 65, 293.

Received for review January 21, 2003 Revised manuscript received June 9, 2003 Accepted June 11, 2003 IE030055K