Liquid Phase Hydrogenation of Maleic Anhydride and Intermediates

Aug 4, 1997 - In general, patent literature provides the coverage of a special invention ... The copper and zinc contents in this catalyst were detect...
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Ind. Eng. Chem. Res. 1997, 36, 2885-2896

2885

Liquid Phase Hydrogenation of Maleic Anhydride and Intermediates on Copper-Based and Noble Metal Catalysts Uwe Herrmann and Gerhard Emig* Lehrstuhl fu¨ r Technische Chemie I der Universita¨ t Erlangen-Nu¨ rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany

The liquid phase hydrogenation of maleic anhydride and intermediates was investigated using copper-based and noble metal catalysts. The experiments were performed in a stirred tank slurry reactor in discontinuous as well as continuous operation. Copper chromites and noble metal catalysts were found to be suitable for the hydrogenation of maleic anhydride. However, the hydrogenation of succinic anhydride proceeded with high selectivity to γ-butyrolactone and 1,4-butanediol on copper zinc catalysts, whereas other copper catalysts revealed no activity in the formation of 1,4-butanediol. Selective sorption interactions of succinic anhydride with the zinc surface were assumed to be responsible for this effect. Starting from γ-butyrolactone all copper catalysts were active in the formation of 1,4-butanediol while noble metal catalysts showed no or little activity. Kinetic models have been proposed for the hydrogenation of maleic anhydride and intermediates on the basis of experimental data obtained in a continuously operated stirred tank slurry reactor. Introduction 1,4-Butanediol is of major interest as a starting material for the production of important polymers. Examples are polyurethanes and poly(butylene terephthalate) based on 1,4-butanediol, which are mainly used for engineering plastics as well as for the synthesis of fibers, films, and adhesives. Furthermore, 1,4butanediol is a major feedstock for the production of tetrahydrofuran, which is synthesized by homogeneous dehydration. In 1992 in the U.S. above 50% of 1,4butanediol was converted to tetrahydrofuran (Weissermel and Arpe, 1994). Tetrahydrofuran is applied as a solvent for PVC and as a monomer for the formation of polytetramethylene glycol, which is used as intermediate for Spandex fibers and polyurethanes (Weissermel and Arpe, 1994; Harris and Tuck, 1990). Dehydrogenation of 1,4-butanediol on copper catalysts leads to the formation of γ-butyrolactone, which is another major solvent. The reaction of γ-butyrolactone with methylamine or ammonia results in the formation of Nmethylpyrrolidone and 2-pyrrolidone. Both chemicals are used as solvents, and additionally 2-pyrrolidone is in demand as a raw material for pharmaceuticals (Mitsubishi, 1988; Minoda and Miyajima, 1970). Currently the majority of 1,4-butanediol is produced by the Reppe process applied by BASF, Hu¨ls, DuPont, and other several major companies (Brownstein, 1991). Acetylene and formaldehyde are used as starting materials. In the U.S. in 1992 nearly 90% of the 1,4butanediol synthesized was produced according to the Reppe process (Weissermel and Arpe, 1994). Several disadvantages within the process led to the development of alternative routes for the synthesis of 1,4-butanediol. Disadvantages are, e.g., severe reaction conditions, the use of acetylene (implying explosion hazard) and formaldehyde (with possible cancerogenic effects), and the necessity to perform a multistep reaction pathway. The most important alternatives to the Reppe process are the hydrogenation of maleic anhydride or its esters, the conversion of propylene oxide (Arco process), and the * To whom correspondence should be addressed. E-mail: [email protected]. S0888-5885(96)00229-1 CCC: $14.00

acetoxylation of 1,3-butadiene (Mitsubishi process) (Tamura and Kumano, 1980; Tanabe, 1981). Maleic anhydride has become an interesting feedstock for the synthesis of γ-butyrolactone, 1,4-butanediol, and tetrahydrofuran in the past. A major reason is the improved and therefore more economic maleic anhydride production with n-butane as starting material (Contractor and Sleight, 1987). Competitive maleic anhydride processes were established based on fluidized bed processes used by, e.g., Lummus/Alusuisse (ALMA), BP/UCB, and DuPont companies (Contractor, 1995). A technical application of the maleic anhydride hydrogenation to 1,4-butanediol is currently performed in Japan using a two-step process on NiRe and NiCo catalysts (Weissermel and Arpe, 1994). The reaction pathway of the hydrogenation of maleic anhydride to 1,4-butanediol and tetrahydrofuran is illustrated in Figure 1. The reaction proceeds via succinic anhydride which is converted to γ-butyrolactone. The hydrogenation of γ-butyrolactone to 1,4-butanediol is followed by an acidcatalyzed dehydration to tetrahydrofuran. As one can see from Figure 1, the use of maleic anhydride results in the production of the above mentioned high value products within a single reaction path. Furthermore, the composition of the product mixture can be influenced substantially by a variation of the reaction conditions (Brownstein and List, 1977). Most publications regarding the hydrogenation of maleic anhydride are patent literature. In general, patent literature provides the coverage of a special invention without giving comprehensive informations on catalyst activities or kinetics. In most cases copperbased catalysts are mentioned regarding the gas phase hydrogenation of maleic anhydride to 1,4-butanediol and tetrahydrofuran (Budge, 1990; De Thomas et al., 1992). Liquid phase hydrogenation of maleic anhydride is performed using copper-, cobalt-, or nickel-based catalysts (Eggersdorfer et al., 1989; Budge et al., 1993) as well as noble metal catalysts containing, e.g., Pd, Re, or Ru (Freudenberger and Wunder, 1978; Attig and Graham, 1989). Previous studies on the hydrogenation of maleic anhydride were carried out mainly using copper chromite catalysts. Experiments were performed to investigate the influence of the catalyst composition © 1997 American Chemical Society

2886 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 1. Composition and Structure of Catalysts Used in Screening Experiments type G 66 R 3-12 G 13 G22 G 99

R 3-11 T 4489 PdRe/C PdReCuZn/C

Pd/C Ru/C

composition (wt %)

composition (atom %)

26 Cu 53 Zn 32 Cu 32 Zn 40 Cu 26 Cr 33 Cu 27 Cr 11 Ba 36 Cu 32 Cr 2 Ba 2.5 Mn 30 Cu 40 Cu 15 Al 7 Mn 1 Pd 4 Re 1 Pd 4 Re 15 Cu 15 Zn 1.5 Pd 1.5 Ru

33 Cu 67 Zn 50 Cu 50 Zn 61 Cu 39 Cr 46 Cu 38 Cr 16 Ba 50 Cu 44 Cr 3 Ba 3 Mn 100 Cu 65 Cu 24 Al 11 Mn 20 Pd 80 Re 3 Pd 11 Re 43 Cu 43 Zn 100 Pd 100 Ru

Figure 1. Reaction scheme of the hydrogenation of maleic anhydride to 1,4-butanediol/tetrahydrofuran.

and structure on the catalytic activity of maleic anhydride (Castiglioni et al., 1995; Messori and Vaccari, 1994). No emphasis was placed on the formation of 1,4butanediol. The present paper includes an investigation of the activity of different copper-based catalysts on each reaction step, illustrated in Figure 1. Additional experiments using noble metal catalysts provided a useful comparison between both types of catalysts. Catalytic activity toward the formation of 1,4-butanediol starting from different substrates was stressed. On the basis of the results obtained, the performance of different types of catalysts was investigated in the continuous way of operation, taking into account the industrial relevance of a “single-step” hydrogenation to 1,4-butanediol. In order to obtain quantitative data, kinetic measurements and a kinetic modeling were performed for each reaction step using noble metal and copper zinc catalysts. Experimental Section Catalysts and Reaction Procedure. The catalysts used for hydrogenation of maleic anhydride, succinic anhydride, and γ-butyrolactone are commercially available from companies BASF, Su¨d-Chemie, and Heraeus, Germany. An exception is PdRe/C, which was synthesized by Degussa, Germany, for laboratory use only. The chemical composition and the designation of these catalysts are illustrated in Table 1. As all catalysts were supplied in the form of tablets, the material was crushed and sieved to obtain the desired fraction of 80150 µm. The experiments were performed using this fraction unless otherwise mentioned. PdRe/C was supplied as a wet powder of 25 µm and used in this fraction.

carrier/structure mixed oxide without carrier mixed oxide on Al2O3 spinel-type

surface area (m2/g) 30.8 118.7

supplier Su¨d-Chemie AG BASF AG

46.7

Su¨d-Chemie AG

spinel-type

44.3

Su¨d-Chemie AG

spinel-type

41.7

Su¨d-Chemie AG

oxide on Mg2SiO4 mixed oxide without carrier

211.6 49.5

BASF AG Su¨d-Chemie AG

mixed oxide on activated carbon mixed oxide on activated carbon

1088.0

Degussa AG

oxide on activated carbon oxide on activated carbon

806.8 1220.9

Heraeus GmbH Heraeus GmbH

Furthermore, experiments were carried out on a modified PdRe/C catalyst impregnated with a copper zinc acetate solution (PdReCuZn/C). The copper and zinc contents in this catalyst were detected to be 15 wt % each. Before use, the catalysts were dried and activated in a fluidized bed in nitrogen and a mixture of nitrogen and hydrogen, respectively. Drying was performed at 423 K for 1 h with pure nitrogen. The activation was performed subsequently with 6 vol % hydrogen at 473 K for 1.5 h and finally at 473 K with 12 vol % hydrogen for 15 h. After this procedure, which was applied to noble metal based and copper-based catalysts, the material was kept under nitrogen atmosphere to avoid partial oxidation and to guarantee identical starting conditions for all experimental runs. Hydrogen, nitrogen, and methane were supplied by Linde, Germany, with >99.99% purity and used directly from the cylinder. The components maleic anhydride, succinic anhydride, and γ-butyrolactone as well as n-tetradecane were delivered by Fluka AG, Germany, with >99% purity. Methane and n-tetradecane were used as internal standards for the gaseous and liquid phase. 1,4Dioxane, which proved to be inert, was used as a solvent in both discontinuous and continuous reactor operation (>99% purity supplied by Merck, Germany). The reaction conditions applied for discontinuous experiments are illustrated in Table 2. Continuously performed experiments were carried out at T ) 543 K and p ) 7.5 MPa using a catalyst loading of 17.4 kg/ m3. The feed rates of liquid and gas were 4.2 mL/min and 1000 normal mL/min, respectively. The feed concentration of maleic anhydride was 0.5 mol/L. Kinetic measurements were performed by varying the feed concentration and reactor pressure at constant feed rates. Table 5 illustrates the range of operating conditions. The liquid and gas feed rates were 6.5 mL/min and 1000 normal mL/min. The particle size used was 80-150 µm. The temperatures were kept constant at T ) 418 K for maleic anhydride hydrogenation, T ) 543 K for succinic anhydride hydrogenation, and T ) 483 K for the hydrogenation of γ-butyrolactone. Activation energies were determined by performing runs in temperature ranges outlined in Table 5.

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2887 Table 2. Operating Conditions for Discontinuous Catalyst Testing substrate

ci,0 (mol/L)

pressure (MPa)

temp (K)

catalyst loading (kg/m3)

particle size (µm)

maleic anhydride succinic anhydride γ-butyrolactone

1.0 0.56 0.56

5.0 7.5 7.5

463 513 513

12 12 12

80-150 (PdRe/C: 25) 80-150 (PdRe/C: 25) 80-140 (PdRe/C: 25)

Figure 2. Reconstruction of the reactor head for continuous operation: PI, pressure indicator.

Apparatus. Discontinuously Operated Reactor. All experiments were carried out in a stainless steel (SS316) stirred tank reactor with 63.5 mm internal diameter and 152 mm height (Parr Instruments, Model 4562). To provide a high gas-liquid interfacial area, the turbine stirrer was replaced by a gas entrainment impeller. The impeller speed was variable and could be increased up to 1700 rpm. Liquid samples were drawn by means of a sampling tube equipped with a sinter metal filter to avoid the loss of suspended catalyst. The liquid samples could be either injected into the port of a gas chromatograph or fed into an continuously operating evaporizer. Analysis of the gaseous phase was performed by means of the sampling loop of the gas chromatograph. Liquid samples were analyzed by mixing the substrate with a defined amount of n-tetradecane, which was inert under the applied reaction conditions. Methane was used as internal standard for the gaseous phase. Discontinuous experiments were carried out by charging the reactor with the desired amount of substrate, internal standard, and solvent. Then catalyst was added under nitrogen atmosphere. Finally, the reactor was pressurized and heated up to the desired temperature. According to the gas consumed, hydrogen was added discontinuously to maintain a constant pressure. Apparatus. Continuously Operated Reactor. In order to carry out continuous experiments, some reconstructions of the equipment had to be carried out. The problem of maintaining a constant pressure and liquid level inside of the reactor vessel during continuous runs could be solved by adding an additional dip tube into the reactor. As all outlets of the reactor head were occupied, the tube was installed coaxially to the sampling line as illustrated in Figure 2. The sampling line was connected to a regulating valve which controls the sampling frequency. The dip tube was fixed to an overflow valve. This overflow valve simultaneously controlled the pressure in the reactor and the level of the liquid. To avoid loss of catalyst during continuous operation, both the sample line and the dip tube were provided with a sinter metal filter. A change in reactor pressure could be easily achieved by the adjustment of the overflow valve. The liquid level was varied by replacing the dip tube. To carry out automatically performed runs, all essential functions were controlled

by a personal computer. The liquid phase was fed by a Kontron 422 HPLC pump with a flow rate up to 20 mL/ min. The gas feed was provided by thermal mass flow controllers (Brooks 5850 E) in the range 0-1000 normal mL/min. Figure 3 shows the flow chart of the apparatus. Analytical Setup. Analysis of gaseous and liquid reactants was performed by means of gas chromatography (Hewlett-Packard 5890 Series II with flame ionization detector and thermal conductivity detector). The gas chromatograph was equipped with a medium polar HP-5 column (cross-linked phenyl silicone) and a polar HP-FFAP column (cross-linked polyethylene glycol). Both columns had 0.53 mm internal diameter and were 30 m long. The analysis was optimized by using temperature and pressure programs in the range from 353 to 503 K and 40 to 180 kPa. Quantitative analysis of the compounds in the product mixture was performed using a Hewlett-Packard 3396 Series II integrator. Peak areas were converted into concentrations using the internal standard method (Schomburg, 1987). Evaluation of quantitative amounts of substrate and products can be easily performed by means of the internal standard in both discontinuous and continuous operation. The mass balance for any species i in the reactor is

VR

dci dt

N

) V˙ L,0ci,0 - V˙ Lci + VR

νijrj ∑ j)1

(1)

Preliminary measurements of the residence time distribution showed that the continuous stirred tank reactor could be considered to be free from dead spaces or bypass effects under the conditions used in the present work. This results in uniform concentrations in the reactor vessel and allows the use of the above equation. The application of this equation for an internal standard shows that in the case of chemical inertness (rj ) 0) and in a stationary state of the reactor (dci/dt ) 0) the concentration is constant assuming V˙ L,0 ) V˙ L. Accordingly, the concentration of the standard in the product mixture is known and can be used as a reference for calculating the concentrations of the reactants. On the basis of the concentrations of the reactants the conversion Xi, yield Yk,i, and selectivity Sk,i were calculated as follows:

Xi )

ci,0 - ci ci,0

Yk,i )

||

ck νi ci,0 νk

Sk,i )

Yk,i Xi

Results and Discussion The main subjects which should be answered by the present work are as follows: (i) Finding suitable catalysts and reaction conditions for the hydrogenation of maleic anhydride, succinic anhydride, and γ-butyrolactone in the liquid phase. (ii) Determination of a suitable subdivision of the multistep reaction into appropriate steps using results achieved in discontinuously as well as continuously performed experiments. (iii) Evaluation of kinetics of the reaction steps outlined in Figure 1 in

2888 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997

Figure 3. Flow chart of the apparatus: V, valve; manually actuated marked by a rectangle, automatically operated marked by a circle; EV, evacuator; EP, evaporizer; S1, S2, sampling manually and automatically; M, stirring motor; PI, pressure indicator; PIC, pressure indicator and controller; TI, temperature indicator; TIC, temperature indicator and controller; WI, weight indicator; FIC, flow indicator and controller; LC, level controller.

Figure 4. Conversion of maleic anhydride vs reaction time on noble metal catalysts and copper chromites (left) and copper-based catalysts (right) (T ) 463 K, p ) 5.0 MPa).

a continuously operated stirred tank slurry reactor considering mass transfer resistances. Hydrogenation in Discontinuous Operation. The hydrogenation of maleic anhydride to succinic anhydride is known to proceed rapidly (Kanetaka et al., 1970a). Therefore mild reaction conditions were applied for this step (see Table 2). Figure 4 illustrates the obtained results using the catalysts mentioned above. The most active catalysts for the hydrogenation of maleic anhydride were found to be noble metals. No remarkable differences in conversion can be discerned between PdRe/C, Pd/C, and Ru/C catalysts. Copper chromites, which provide an almost total conversion of the substrate after 5 h reaction time, were less active. The copper chromite G 22 unveils the lowest activity compared to G 13 and G 99, which can be possibly reduced to the smaller copper content. Copper-based catalysts without spinel structure proved to be less active for the

hydrogenation of maleic anhydride. Among these catalysts the copper zinc containing G 66 showed the smallest activity, probably due to the absence of a carrier material. A comparison between the copper catalyst T 4489 and the copper chromite G 13, which both contain 40 wt % of copper, unveils a higher activity of the latter regarding the hydrogenation of maleic anhydride. This effect illustrates the influence either of the additional component chromium or of the presence of a spinel structure on the activity. The hydrogenation of maleic anhydride results mainly in the formation of succinic anhydride. The formation of γ-butyrolactone is low under mild reaction conditions using copper-based catalysts. The yield of γ-butyrolactone for copper-based catalysts was below 5%. Regarding noble metal catalysts, yields were 5% (PdRe/C), 13.5% (Ru/C), and 54% (Pd/C). The only byproduct formed was tetrahydrofuran (YTHF,MA ≈ 1.2%) when

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2889

Figure 5. Conversion of succinic anhydride vs reaction time on noble metal catalysts and copper chromites (left) and copper-based catalysts (right) (T ) 513 K, p ) 7.5 MPa).

Figure 6. Product selectivities on the hydrogenation of succinic anhydride (SA) after 5 h (T ) 513 K, p ) 7.5 MPa).

using copper zinc catalysts. The formation of tetrahydrofuran on copper zinc catalysts proceeded simultaneously to γ-butyrolactone, which indicates a direct reaction pathway from γ-butyrolactone to tetrahydrofuran without 1,4-butanediol being formed as an intermediate. The direct hydrogenation of γ-butyrolactone to tetrahydrofuran is no novelty and is described by Turek et al. (1994) on the gas-phase hydrogenolysis of dimethyl succinate. With none of the catalysts, 1,4butanediol was produced under the conditions applied for the hydrogenation of maleic anhydride. As outlined above, the hydrogenation of succinic anhydride proceeded slowly at the reaction conditions applied for the maleic anhydride conversion. For this reason the investigation of the hydrogenation of succinic anhydride was carried out at a higher temperature using a higher pressure (see Table 2). Figure 5 shows the conversion of succinic anhydride versus operation time. First of all, it can be seen that the order of activity observed on the hydrogenation of maleic anhydride has changed regarding the conversion of succinic anhydride. Copper zinc catalysts, which unveiled only a low activity for the hydrogenation of maleic anhydride, were more active than copper chromites or other zinc absent copper

catalysts. The rate of conversion on copper zinc catalysts is comparable with noble metal catalysts, which were active for both the hydrogenation of maleic anhydride and succinic anhydride. The presence of zinc is responsible for an increase of the activity in the hydrogenation of succinic anhydride resulting in a high rate of conversion. Castiglioni et al. (1993) found a positive effect of zinc toward the conversion of succinic anhydride in the gas phase hydrogenation of maleic anhydride using modified copper chromite catalysts. The copper chromites investigated in the present work showed a low activity in the hydrogenation of succinic anhydride compared to copper zinc catalysts. The copper catalysts R 3-11 and T 4489 as well as copper chromites showed no activity in the formation of 1,4-butanediol. We found a conversion of succinic anhydride to γ-butyrolactone of 100% and 70% on R 3-11 and T 4489, respectively, after finishing the experiment. Figure 6 illustrates the product selectivities related to succinic anhydride after 5 h of reaction time. Besides the enhancement of the conversion rate of succinic anhydride outlined above, zinc has a marked influence on the product selectivities. The use of copper zinc catalysts (G 66, R 3-12) results in the formation of a mixture of γ-butyrolactone and 1,4-

2890 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997

Figure 7. Conversion of γ-butyrolactone vs reaction time on copper-based and noble metal catalysts (T ) 513 K, p ) 7.5 MPa) (left). Selectivity to 1,4-butanediol vs conversion of γ-butyrolactone on different catalysts (T ) 513 K, p ) 7.5 MPz) (right). Table 3. Product Selectivities on the Hydrogenation of γ-Butyrolactone after 5 h (%) (T ) 513 K, p ) 7.5 MPa) 1,4-butanediol tetrahydrofuran 1-butanol propionic acid

G 66

R 3-12

G 13

G 22

G 99

R 3-11

T 4489

PdRe/C

Ru/C

Pd/C

95.0 4.1 2.0 0

77.8 16.5 4.5 0

76.3 14.5 2.1 0

87.9 4.0 8.8 0

81.4 6.2 4.4 0

58.6 22.5 4.2 0

71.2 14.5 5.6 0

20.7 20.5 31.6 0

0 16.7 0 0

0 4.8 0 0

butanediol. Both copper zinc catalysts convert succinic anhydride to valuable products with a selectivity of more than 90%. The absence of zinc results in the formation of γ-butyrolactone and small amounts of tetrahydrofuran (R 3-11) and 1-butanol (T 4489). No 1,4-butanediol is formed using these catalysts. The same refers to copper chromites, which convert succinic anhydride to γ-butyrolactone with poor selectivities below 80%. Among noble metal containing catalysts, PdRe/C exclusively showed an activity in the formation of 1,4butanediol. Pd/C and Ru/C are converting the substrate to γ-butyrolactone and a small amount of propionic acid. Other byproducts using PdRe/C are 1-butanol and tetrahydrofuran. As illustrated in Figure 6 the presence of zinc promotes the formation of 1,4-butanediol. Possibly zinc provides active sites for the hydrogenation of γ-butyrolactone or enables the adsorption of γ-butyrolactone on the copper surface. Catalytic tests using γ-butyrolactone as a starting material should give further information on the role of zinc and copper on the hydrogenation activities forming 1,4-butanediol. Figure 7(left) shows the conversion of γ-butyrolactone versus reaction time on the catalysts outlined in Table 1. Copper-containing catalysts unveil a comparable activity independent of additional elements as, e.g., chromium, zinc, and aluminum. The horizontal line in Figure 7(left) indicates the value for the equilibrium conversion of γ-butyrolactone. It is exceeded due to the formation of byproducts tetrahydrofuran and 1-butanol. Table 3 illustrates the product selectivities referred to γ-butyrolactone after 5 h of reaction time. A comparison of Figures 5 and 7(left) indicates that the conversion of γ-butyrolactone to 1,4-butanediol is possible on copper catalysts in principle. Starting from succinic anhydride the formation of 1,4-butanediol fails to proceed in the absence of zinc even if high yields of γ-butyrolactone were observed. It is likely that the presence of a small amount of succinic anhydride prevents the adsorption of γ-butyrolactone on copper-based, zinc-absent catalysts. It is possible that the presence of zinc provides a reduction of the coverage of the copper surface due to selective adsorption of succinic anhydride. A similar

interaction of dimethyl succinate with zinc oxide on the gas phase hydrogenolysis of dimethyl succinate is reported by Turek et al. (1994) as well. No higher catalytic activity is observed on the hydrogenation of γ-butyrolactone on copper zinc catalysts compared to copper catalysts in which zinc is absent. This indicates that zinc or zinc oxide does not provide additional sites for the chemical reaction of γ-butyrolactone to 1,4-butanediol but for the selective adsorption of succinic anhydride. Noble metal catalysts which showed a high rate of conversion for maleic and succinic anhydride were little effective on the hydrogenation of γ-butyrolactone. PdRe/C revealed an approximate 70% conversion of the substrate which was unselective to 1,4butanediol as shown in Table 3. The use of Pd/C and Ru/C resulted in little conversion of γ-butyrolactone. Unknown byproducts formed on Pd/C resulted in a loss of identified organic species of approximately 25% after 5 h of reaction. Figure 7(right) illustrates the selectivity to 1,4-butanediol versus the conversion of γ-butyrolactone. On PdRe/C the selectivity which is above 50% at a 15% conversion of γ-butyrolactone decreases due to the formation of tetrahydrofuran, 1-butanol and unknown byproducts. The selectivities obtained with copper-based catalysts are far better and remain at a high level as illustrated for the copper zinc catalyst G 66. The selectivity to 1,4-butanediol obtained on R 3-12 decreases from approximately 100% due to the formation of tetrahydrofuran probably induced by the acid carrier material Al2O3. The results of discontinuously operated screening experiments give reason for the assumption that a hydrogenation of maleic anhydride to the target product 1,4-butanediol can be achieved using different types of catalysts and/or reaction conditions. One possibility may be the use of a noble metal catalyst for the conversion of maleic anhydride to γ-butyrolactone in one reactor unit. The formed γ-butyrolactone should be easily converted in a second reactor using copper-based catalysts. Another path may be the hydrogenation of maleic anhydride to succinic anhydride followed by the

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2891 Table 4. Conversion and Product Yields on Continuous Hydrogenation of Maleic Anhydride (%) (T ) 543 K, p ) 7.5 MPa) maleic anhydride conversion succinic anhydride γ-butyrolactone 1,4-butanediol tetrahydrofuran 1-butanol propionic acid

G 66

R 3-12

G 99

R 3-11

T 4489

PdRe/C

Ru/C

Ru/C + R 3-12

PdReCuZn/C

96.6 0 61.1 16.9 6.0 0 0

100 0 64.5 26.0 4.9 0.8 0

99.5 68.6 27.5 0 0.2 0 0

93.5 66.5 25.3 0 0.6 0 0

89.8 92.7 7.3 0 0 0 0

100 18.3 73.3 0 0 0.7 0

100 52.2 24.2 0 0 0 15.6

100 54.0 22.0 0 2.7 0.9 0

100 34.1 52.2 0 0.8 0 3.0

Figure 8. Conversion of maleic anhydride and product yields on continuous hydrogenation vs time-on-stream: MA, maleic anhydride; SA, succinic anhydride; g-BL, γ-butyrolactone; 1,4-BD, 1,4butanediol; THF, tetrahydrofuran; n-BuOH, 1-butanol (T ) 543 K, p ) 7.5 MPa).

conversion of succinic anhydride to 1,4-butanediol over copper zinc catalysts as illustrated in Figure 6. Hydrogenation in Continuous Operation. Table 4 illustrates the results of continuously performed experiments. In addition to the experiments outlined above, a mechanical mixture of a noble metal catalyst and a copper zinc catalyst (Ru/C + R 3-12) as well as a modified PdRe/C catalyst (PdReCuZn/C) was used. Experiments on these catalysts were carried out to focus attention on the role of zinc on the hydrogenation of maleic and succinic anhydride. Pd/C is not listed in Table 4 due to marked deactivation found in the continuous hydrogenation of maleic anhydride. The catalyst G 99 was chosen as a representative for copper chromites. Table 4 illustrates that no succinic anhydride was formed using copper zinc catalysts (G 66 and R 3-12). Similar to the results obtained in discontinuous hydrogenation of succinic anhydride, a mixture of γ-butyrolactone and 1,4-butanediol was formed. Byproducts were tetrahydrofuran and 1-butanol. The hydrogenation of maleic anhydride seems to proceed directly to γ-butyrolactone and 1,4-butanediol. Figure 8 illustrates the conversion of maleic anhydride and the yields of the products formed over a copper zinc on alumina catalyst (R 3-12) versus time-on-stream. A noticeable formation of succinic anhydride occurs only at the beginning of the reaction. In a steady state, no desorption of succinic anhydride into the solution could be observed. Similar results were obtained using a copper zinc catalyst without a carrier (G 66). In continuous operation 1,4butanediol is formed exclusively in the case of copper zinc catalysts, thus supporting the influence of zinc discussed above. Figure 8 indicates that the effect of zinc is not based on an adsorption mechanism only because no saturation could be observed after 9 h of time-on-stream. In the stationary state of the reactor the mass balance was found to be approximately 100%,

indicating that succinic anhydride is not adsorbed irreversibly on the zinc surface. It seems that the presence of zinc also affects the kinetics of the hydrogenation which is currently investigated in further studies. The use of copper catalysts in which zinc is absent resulted in the formation of succinic anhydride and γ-butyrolactone. Consistent with the results found in the discontinuous hydrogenation of succinic anhydride, no formation of 1,4-butanediol was observed. Similar results were obtained using the noble metal catalysts PdRe/C and Ru/C. The hydrogenation of maleic anhydride led to the formation of a mixture mainly containing succinic anhydride and γ-butyrolactone. Byproducts were tetrahydrofuran, 1-butanol, and propionic acid, no 1,4-butanediol was formed. The 1,4butanediol selectivity for the mechanical mixture of Ru/C and R 3-12 was found to be zero as well. This result is surprising regarding the fact that the sole use of R 3-12 with the same loading as in the mixture resulted in the formation of 1,4-butanediol without succinic anhydride being formed. PdReCuZn/C was not active for the formation of 1,4-butanediol as well. The activity toward the conversion of succinic anhydride decreased compared to the unmodified catalyst. The impregnation with copper and zinc acetate may have resulted in partial pore blockage. Both catalyst systems (Ru/C + R 3-12 and PdReCuZn/C) indicate that the presence of copper and zinc is not sufficient for the synthesis of 1,4-butanediol. The rapid formation of succinic anhydride on the noble metal active sites combined with an increase in the concentration of succinic anhydride in the liquid phase may be a reasonable explanation as discussed above (see Figure 4). The coverage of the active copper sites with succinic anhydride at a high concentration inhibits the adsorption of γ-butyrolactone and the formation of 1,4-butanediol. In support of this assumption the influence of succinic anhydride on the hydrogenation of maleic anhydride was investigated separately. For this reason a continuous run was performed using a copper zinc catalyst (R 3-12) as outlined in Figure 8. After a time-on-stream of approximately 100 min the maleic anhydride feed was replaced by a mixture of succinic and maleic anhydrides. The concentration of maleic anhydride remained at 0.5 mol/L, the succinic anhydride concentration was 0.3 mol/ L. Figure 9 illustrates the conversion of maleic anhydride and the yields of the products before and after the addition of succinic anhydride versus time-on-stream. The yields of the products are related to the feed concentration of maleic anhydride, before as well as after the addition of succinic anhydride. The vertical line marks the time the feed was replaced. As supposed above, the yield of 1,4-butanediol decreased immediately, indicating the coverage of the active copper surface with succinic anhydride. The decrease in the yield of γ-butyrolactone illustrates that succinic anhydride, which is adsorbed from the bulk phase, is less

2892 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997

Figure 9. Influence of succinic anhydride on the continuous hydrogenation of maleic anhydride: MA, maleic anhydride; SA, succinic anhydride; g-BL, γ-butyrolactone; 1,4-BD, 1,4-butanediol; THF, tetrahydrofuran; n-BuOH, 1-butanol (T ) 543 K, p ) 7.5 MPa). Table 5. Range of Operating Conditions for Kinetic Measurements

substrate

ci,0 (mol/L)

catalyst pressure loading (MPa) temp (K) (kg/m3) catalyst

maleic anhydride max 0.57 2.5-7.5 393-463 succinic anhydride max 0.57 6.0-9.0 503-543 γ-butyrolactone max 2.5 3.0-9.0 433-493

0.5 10 10

Ru/C Ru/C CuZnOx/ Al2O3

active than succinic anhydride formed on the hydrogenation of maleic anhydride. Kinetic Analysis. Mass-Transfer Limitations. On the basis of the results obtained above, kinetic measurements were carried out in the continuous way of operation. Starting from maleic anhydride and succinic anhydride a Ru/C catalyst was used. The hydrogenation of γ-butyrolactone was investigated quantitatively over a copper zinc catalyst (R 3-12). Table 5 illustrates the reaction conditions applied in kinetic experiments. The analysis of mass transfer effects was performed both experimentally and theoretically based on suitable criteria (see Appendix). Due to the existence of a three-phase reaction, gas-liquid, liquid-solid, and intraparticle diffusional resistances may influence the rate and selectivity of the chemical reaction (Ramachandran and Chaudhari, 1983; Rode and Chaudhari, 1994). The influence of the gas-liquid mass transfer was investigated experimentally by varying the agitation speed at constant reaction conditions. The variation of the agitation speed did not result in a significant change in the reaction rate above 800 rpm. The evaluation of the criterion regarding the gas-liquid mass transfer, calculating the parameter R1, resulted in values below 0.01 for all reaction steps investigated. This fact indicates the absence of gas-liquid mass transfer resistances, confirming the experimental results. The analysis of restrictions due to the liquid-solid mass transfer was performed theoretically by calculating the parameter R2, which represents the ratio of the observed rate of reaction to the maximum rate of the liquid-solid mass transfer. The analysis showed that even at high reaction rates the value of R2 was below 0.07, indicating the absence of liquid-solid mass transfer limitations as well. For the investigation of pore diffusional resistances the particle size was varied while other reaction parameters were kept constant. The decrease of the reaction rate was significant for particle diameters

Figure 10. Evaluation of pore diffusional resistances for different reaction steps using eq 6: MA, maleic anhydride; SA, succinic anhydride; g-BL, γ-butyrolactone.

above 150 µm only in the case of the hydrogenation of maleic anhydride. No clear tendency could be observed for the catalyst fraction in the range of 80-150 µm used for kinetic measurements. However, no decrease in the reaction rate could be discerned on the hydrogenation of succinic anhydride and γ-butyrolactone when varying the particle diameter from 56 to 350 µm. This indicated that intraparticle diffusion limitations were absent during kinetic measurements of these reaction steps. The evaluation of a theoretical criterion (see Appendix) quantifying intraparticle diffusional resistances is illustrated in Figure 10 for typical runs on different reaction steps. It can be discerned that the threshold value of 0.2 is exceeded in the case of the hydrogenation of maleic anhydride, indicating that the kinetic measurements for this reaction step could be affected by pore diffusional resistances. The hydrogenation of succinic anhydride and γ-butyrolactone can be considered to be free from this kind of limitation, which supports the experimental observations. Kinetic Analysis. Kinetic Measurements and Modeling. The discrimination of different rate equations was performed by means of parameter estimation using a gradient method with the log-likelihood function (LLF) as objective function (Steiner et al., 1989). The value of LLF and the weighted sum of residual squares were chosen as a criterion for the quality of the approximation as well as parity plots. Figure 11 represents the effective reaction rate of the hydrogenation of maleic anhydride to succinic anhydride versus concentration of the substrate. The rate shows an approximately linear increase with the concentration of maleic anhydride, while the influence of the pressure, i.e., the hydrogenation concentration in the solution, is less significant. The analysis of the fit of different models including a power law model as well as mechanistically deduced equations showed that the smallest lack of fit was obtained using the following model:

r ) kcMAmcH2n

(2)

A further simplification of this model can be achieved regarding the results for any pressure separately cH2n ) constant, leading to the equation

r ) k′cMAm

(3)

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2893

Figure 11. Rate of maleic anhydride hydrogenation vs substrate concentration: points; experimental; line, model prediction (model M1) (T ) 418 K).

Table 6 shows the parameter values n, m, k, and k′ as well as the 95% confidential intervals regarding both a separate and a total consideration of the reactor pressure. The order for maleic anhydride proved to be m ≈ 0.8-1.0. The rate dependency on the hydrogen concentration results in a value for the exponent n of approximately 0.33. Similarly results were obtained by Kanetaka et al. (1970b), who found a first-order reaction mechanism for the hydrogenation of maleic anhydride on a NiRe catalyst using a power law model. The model prediction (model M1) is marked by the line in Figure 11, the experimental values are represented by single points. The activation energy for the hydrogenation of maleic anhydride was calculated to be 39.5 kJ/mol by varying the reaction temperature in the range from 418 to 463 K. The results of the hydrogenation of succinic anhydride to γ-butyrolactone are illustrated in Figure 12. An influence of the hydrogen concentration could not be discerned while the reactor pressure was increased from 6.0 to 7.5 and 9.0 MPa, resulting in a zero-order reaction for hydrogen. The dependency of the reaction rate on the succinic anhydride concentration is pronounced only for low values, indicating a saturation of substrate for high succinic anhydride concentrations. Table 7 reveals a power law model (B1) and a simple mechanistically deduced model (B2) applied for the hydrogenation of succinic anhydride. On the basis of the absence of an influence of hydrogen, other mechanistical models were deduced without taking the corresponding hydrogen concentration in the potential term into consideration. Rate equations including the adsorption of succinic anhydride or the desorption of γ-butyrolactone as the rate-determining step did not comprise significant parameters. The evaluation of different kinds of models showed that no significant values could be calculated for the adsorption constants of water, hydrogen, and γ-butyrolactone. Model B2 comprises both facts, the surface reaction as the rate-determining step and the adsorption of succinic anhydride. The variation of the reaction temperature and evaluation of the results based on model B2 led to the calculation of the activation energy for the hydrogenation of succinic anhydride of 117.96 kJ/mol. The model prediction according to model B2 is marked by the line in Figure 12; the experimental values are represented by single points. The hydrogenation of γ-butyrolactone to 1,4-butanediol has to be regarded as reversible in the range of

Figure 12. Rate of succinic anhydride hydrogenation vs substrate concentration: points, experimental; line, model prediction (model B2) (T ) 543 K).

reaction conditions used in the present work. The calculation of an equilibrium constant for the reaction of γ-butyrolactone and hydrogen to form 1,4-butanediol was performed via integration of the van’t Hoff equation using an increment method (Reid and Sherwood, 1988). The equilibrium constant was calculated to be Ke ) 17.78 L2/mol2 at T ) 483.15 K, which is in good agreement with the experimental data obtained in the discontinuous stirred tank reactor. The evaluation of different models including adsorption or desorption as a rate-determining step did not result in convergent solutions. The same applies for rate equations, which include an adsorption of 1,4-butanediol in the adsorption term. The remaining models L1-L3 differ in the kind of adsorption of hydrogen on the active sites. Model L1 includes a four-step mechanism, which comprises (a) the adsorption of γ-butyrolactone and (b) the adsorption of hydrogen on the same kind of active site according to the Langmuir adsorption theory. Step c is the formation of an adsorbed intermediate (I-s) in the reaction of γ-butyrolactone and hydrogen and is considered to be rate determining. Finally step d represents the formation of 1,4-butanediol by the reaction of the adsorbed intermediate and hydrogen. The reaction scheme for model L1 is proposed to be as follows: K1

(a)

γ-BL + s

\ y z

(b)

H2 + s

\ y z

(c)

γ-BL-s + H2-s

y\z

(d)

I-s + H2-s

y\z

K2

K3

K4

γ-BL-s H2-s I-s + s 1,4-BD + 2s

Model L2 takes into account the presence of two different kinds of active sites. Except for this, the reaction scheme corresponds completely to model L1. The fit of the experimental data for the models L1 and L2 showed no significant differences, which indicates that no clear statement can be given regarding the kind of active sites. Model L3 includes an Eley Rideal mechanism which embodies the reaction of adsorbed γ-butyrolactone with hydrogen from the bulk phase. The formation of an intermediate by adsorbed γ-butyrolactone and bulk phase hydrogen was supposed to be the rate-determining step. The evaluation of model L3

2894 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 6. Rate Equations and Parameters with 95% Confidence Intervals on the Hydrogenation of Maleic Anhydride; k, k′, Rate Constants; m, n, Reaction Orders for Maleic Anhydride and Hydrogen (T ) 418 K) model

rate equation

pressure (MPa)

k, k′ (Lm+n-1/(molm+n-1 s))

m

n

M1 M2 M3 M4

r ) kcMAmcH2n r ) k′cMAm r ) k′cMAm r ) k′cMAm

2.5-7.5 2.5 5.0 7.5

2.77 × 10-4 ( 4.67 × 10-5 2.15 × 10-4 ( 8.67 × 10-5 2.50 × 10-4 ( 1.80 × 10-4 2.00 × 10-4 ( 5.33 × 10-5

0.79 ( 0.12 0.99 ( 0.17 0.97 ( 0.62 0.69 ( 0.20

0.33 ( 0.10

Table 7. Rate Equations and Parameters with 95% Confidence Intervals on the Hydrogenation of Succinic Anhydride; k1, k2, Rate Constants; KSA, Sorption Constant for Succinic Anhydride; m, n, Reaction Orders for Succinic Anhydride and Hydrogen (T ) 543 K) model

k1 (Lm+n-1/(molm+n-1 s))

rate equation

2.61 ×

k1cSAmcH2n

B1

r)

B2

r ) k2cSA/(1 + KSAcSA)

10-4

( 4.50 ×

10-5

m

n

0.48 ( 0.11

0.56 × 10-5 ( 0.07

k2 (1/s)

KSA (L/mol)

1.61 × 10-3 ( 4.22 × 10-4

7.70 ( 3.18

Table 8. Rate Equations on the Equilibrium Reaction on the Hydrogenation of γ-Butyrolactone L1

(

k3K1K2 cBLcH2 r)

(

L2 cBD KecH2

)

cBD 1 + K1cBL + K2cH2 + cH2K2K4

( (

k3K1K2 cBLcH2 -

)

2

r)

L3 cBD KecH2

)

(

k2K1 cBLcH2 -

)

cBD (1 + K1cBL) 1 + K2cH2 + cH2K2K4

r)

(

) )

cBD KecH2

cBD 1 + K1cBL + K3cH2

Table 9. Parameters with 95% Confidence Intervals on the Hydrogenation of γ-Butyrolactone for Model L1a model L1

k3 (mol/(L s)) 3.20 ×

10-3

( 2.10 ×

10-3

K1 (L/mol)

K2 (L/mol)

Ke (L2/mol2)

K4 (mol/L)

0.82 ( 0.58

1.12 ( 1.29

207.5 ( 3325.6

143.0 ( 10442.2

a k , rate constant for the rate-determining step; K , K , sorption constants for γ-butyrolactone and hydrogen; K , equilibrium constant 3 1 2 e for the overall reaction of γ-butyrolactone and hydrogen to 1,4-butanediol; K4, equilibrium constant for the reaction of hydrogen and intermediate forming 1,4-butanediol (T ) 483 K).

be confirmed for a high concentration, e.g., starting from pure γ-butyrolactone. Conclusions

Figure 13. Rate of γ-butyrolactone hydrogenation vs substrate concentration: points, experimental; line, model prediction (model L1) (T ) 483 K).

resulted in a worse fit of the experimental data compared to the models discussed above. Table 8 illustrates the rate equations on the equilibrium reaction on the hydrogenation to γ-butyrolactone for the models L1L3. The parameter values with the corresponding (95% confidence intervals for model L1 are shown in Table 9. Figure 13 illustrates the experimental and calculated values based on model L1 for the hydrogenation of γ-butyrolactone. The fit is satisfactory for higher concentrations of γ-butyrolactone, while calculations for small concentrations result in remarkable deviations. The activation energy calculated for this step is 39.1 kJ/ mol. The zero-order dependence of γ-butyrolactone found in literature (Kanetaka et al., 1970b) could only

(i) Catalyst testing in a discontinuous stirred tank reactor showed that the hydrogenation of maleic anhydride, succinic anhydride, and γ-butyrolactone is possible in principle using copper catalysts. If succinic anhydride was used as a substrate, only copper zinc catalysts revealed activity in the formation of 1,4butanediol. No differences between different copper catalysts could be observed starting with γ-butyrolactone. We propose that zinc is not participating in the hydrogenation reaction but provides vacant active copper sites due to selective adsorption of succinic anhydride. This effect results in a formation of 1,4butanediol on the hydrogenation of succinic anhydride on copper zinc catalysts. However, by use of copper catalysts in which zinc is absent, no formation of 1,4butanediol was observed, which could be due to the coverage of the active copper sites even with a small amount of unreacted succinic anhydride. Noble metal catalysts were active for the hydrogenation of maleic and succinic anhydrides. No or little activity was observed on the hydrogenation of γ-butyrolactone to 1,4butanediol depending on the active component. (ii) With continuously operated experiments, the target product 1,4-butanediol was formed using copper zinc catalysts only. It is noticeable that the continuous hydrogenation of maleic anhydride on copper zinc catalysts proceeded without succinic anhydride present in the bulk phase of the liquid. The selective adsorption on zinc combined with a positive effect on the reaction kinetics makes copper zinc catalysts suitable for the

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2895

“one-step” hydrogenation to γ-butyrolactone/1,4-butanediol starting from maleic anhydride. A subdivision into different reaction units becomes unnecessary. (iii) Kinetic measurements were performed in a continuously stirred tank reactor. A power law model was suitable for the quantitative modeling of the hydrogenation of maleic anhydride. The kinetic model established for the succinic anhydride hydrogenation is based on a hyperbolic rate equation. Hydrogenation of γ-butyrolactone can be described using a LangmuirHinshelwood type kinetic based on the reversible character of the reaction. Nomenclature ci ) concentration of reactant i (mol/L) ci,0 ) concentration of reactant i for t ) 0/feed concentration (mol/L) cMA ) concentration of maleic anhydride (mol/L) cSA ) concentration of succinic anhydride (mol/L) cγ-BL, cBL ) concentration of γ-butyrolactone (mol/L) c1,4-BD, cBD ) concentration of 1,4-butanediol (mol/L) cH2 ) concentration of hydrogen (mol/L) cH2* ) concentration of hydrogen at the gas-liquid interface (mol/L) dP ) diameter of a catalyst particle (m) D ) molecular diffusion coefficient (m2/s) Deff ) effective molecular diffusion coefficient (m2/s) k, k′, k1, k2, k3 ) rate constants [depending on the model; see Tables 6-9] K1, K2, K3, K4 ) sorption constants [depending on the model; see Tables 8 and 9] KSA ) sorption constant for succinic anhydride (L/mol) Ke ) equilibrium constant for the hydrogenation of γ-butyrolactone (L2/mol2) kLaB ) volumetric gas-liquid mass transfer coefficient (1/ s) kSaP ) volumetric liquid-solid mass transfer coefficient (1/ s) LLF ) log-likelihood function m ) reaction order for maleic and succinic anhydride n ) reaction order for hydrogen p ) pressure (MPa) reff ) effective rate of reaction (mol/(L s)) rj ) rate of reaction j (mol/(L s)) Sk,i ) selectivity of product k referred to substrate i t ) time (s, min) T ) temperature (K) VR ) reaction volume (m3) V˙ L,0 ) volumetric rate of liquid feed (m3/s) V˙ L ) volumetric rate of product liquid (m3/s) Xi ) conversion of substrate i Yk,i ) yield of product k referred to substrate i Greek Letters R1 ) gas-liquid mass transfer parameter defined by eq 4 R2 ) liquid-solid mass transfer parameter defined by eq 5  ) porosity of the catalyst particle νij ) stoichiometric coefficient of species i in reaction j Φexp ) parameter for the estimation of internal diffusion resistance defined by eq 6 FP ) density of the catalyst particle (kg/m3) τ ) tortuosity factor of the catalyst particle

Acknowledgment These results were obtained within the framework of SFB 222. We thank the Deutsche Forschungsgemeinschaft for their financial support of this work. We also

wish to express our thanks to the above mentioned companies for supplying the catalysts used. Appendix For determination of the contributions of the mass transfer steps present in a three-phase catalytic system, the following criteria were used (Ramachandran and Chaudhari, 1983): 1. The limitation due to the gas-liquid mass transfer is negligible if

R1 )

reff < 0.1 kLaBcH2*

(4)

2. The limitation due to the liquid-solid mass transfer is negligible if

R2 )

reff < 0.1 kSaPci

(5)

3. Internal diffusion resistances can be neglected if

Φexp )

x

dP 6

FPreff < 0.2 wDeffci

(6)

The mass transfer coefficients kLaB and kSaP were calculated using the correlations of Calderbank and Moo-Young (1961) and Levins and Glastonbury (1972). The effective diffusion coefficient Deff was calculated as Deff ) (/τ)D. The ratio /τ was assumed to be 0.1. The molecular diffusion coefficients for organic substrates and for hydrogen were calculated as proposed by Wilke and Chang (1955) and Akgerman and Gainer (1972). The solubility of hydrogen in 1,4-dioxane was calculated by using the regular solution theory (Hildebrand et al., 1970). Literature Cited Akgerman, A.; Gainer, J. L. Predicting Gas-Liquid Diffusities. J. Chem. Eng. Data 1972, 17, 372. Attig, T. G.; Graham, A. M. Preparation of γ-Butyrolactone and 1,4-Butanediol by Catalytic Hydrogenation of Maleic Acid. U.S. Patent 4,827,001, 1989. Brownstein, A. M.; List, H. L. Which Route to 1,4-Butanediol? Hydrocarbon Process. 1977, 9, 159. Brownstein, A. M. 1,4-Butanediol and Tetrahydrofuran: Examplary Small-Volume Commodities. CHEMTECH 1991, 8, 506. Budge, J. R. Coated Catalysts Useful for the Hydrogenation of Maleic Anhydride to Tetrahydrofuran and Gammabutyrolactone. European Patent 0 404 408, 1990. Budge, J. R.; Attig, T. G.; Graham, A. M. Two-Stage Maleic Anhydride Hydrogenation Process for 1,4-Butanediol Synthesis. U.S. Patent 5,196,602, 1993. Calderbank, P. H.; Moo-Young, M. B. The Continuous Phase Heat and Mass-Transfer Properties of Dispersions. Chem. Eng. Sci. 1961, 16, 39. Castiglioni, G. L.; Gazzano, M.; Stefani, G.; Vaccari, A. Selective Hydrogenation of Maleic Anhydride by Modified Copper Chromite Catalysts. In Heterogeneous Catalysis and Fine Chemicals III; Guisnet, M., Barbier, J., Barrault, J., Bouchoule, C., Duprez, D., Perot, G., Montassier, A., Eds.; Elsevier Science Publishers: Amsterdam, 1993. Castiglioni, G. L.; Vaccari, A.; Fierro, G.; Inversi, M.; Lo Jacono, M.; Minelli, G.; Pettiti, I.; Porta, P.; Gazzano, M. Structure and Reactivity of Copper-Zinc-Cadmium Chromite Catalysts. Appl. Catal. A 1995, 123, 123. Contractor, R. M.; Sleight, A. W. Maleic Anhydride from C4Feedstocks using Fluidized Bed Reactors. Catal. Today 1987, 1, 587. Contractor, R. M. DuPont’s New Process for n-Butane to Tetrahydrofuran. Appl. Catal. B 1995, 6, N3.

2896 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 De Thomas, W.; Taylor, P. D.; Tomfohrde, H. F. Process for the Production of γ-Butyrolactone, THF in Predetermined Amounts. U.S. Patent 5,149,836, 1992. Eggersdorfer, M.; Franz, L.; Zimmermann, H.; Brenner, K.; Halbritter, K.; Sauer, W.; Scheiper, H.-J. Verfahren zur Herstellung von Tetrahydrofuran oder von Gemischen, die neben Tetrahydrofuran γ-Butyrolacton und/oder 1,4-Butanediol enthalten. Offenlegungsschrift DE 3726510, 1989. Freudenberger, D.; Wunder, F. Verfahren zur Herstellung von Butandiol-(1.4). Offenlegungsschrift DE 2715667, 1978. Harris, N.; Tuck, M. W. Butanediol via Maleic Anhydride. Hydrocarbon Process. 1990, 5, 79. Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and Related Solutions; Van Nostrand Reinhold Co.: New York, 1970. Kanetaka, J.; Asano, T.; Masamune, S. New Process for Production of Tetrahydrofuran. Ind. Eng. Chem. 1970a, 62, 24. Kanetaka, J.; Kiryu, S.; Asano, T.; Masamune, S. Hydrogenation of Maleic Anhydride and Intermediates by Nickel-Rhenium Catalyst Supported on Kieselguhr. Bull. Jpn. Pet. Inst. 1970b, 12, 89. Levins, D. M.; Glastonbury, J. R. Particle-Liquid Hydrodynamics and Mass Transfer in a Stirred Vessel. Trans. Inst. Chem. Eng. 1972, 50, 132. Messori, M.; Vaccari, A. Reaction Pathway in the Vapour Phase Hydrogenation of Maleic Anhydride and its Esters to γ-Butyrolactone. J. Catal. 1994, 150, 177. Minoda, S.; Miyajima, M. Make γ-BL and THF from Maleic. Hydrocarbon Process. 1970, 11, 176. Mitsubishi Kasei Corporation. 1,4-Butanediol/Tetrahydrofuran Production Technology. CHEMTECH 1988, 12, 759. Ramachandran, P. A.; Chaudhari, R. V. Three Phase Catalytic Reactors; Gordon and Breach Science Publishers: New York, 1983.

Reid, R. C.; Sherwood, Th. K. The Properties of Gases and Liquids; McGraw Hill: New York, 1988. Rode, C. V.; Chaudhari, R. V. Hydrogenation of m-Nitrochlorobenzene to m-Chloroaniline: Reaction Kinetics and Modeling of a Non-Isothermal Slurry Reactor. Ind. Eng. Chem. Res. 1994, 33, 1645. Schomburg, G. Gaschromatographie; VCH: Weinheim, 1987. Steiner, E. C.; Blau, G. E.; Agin, G. L. SimusolvsModeling and Simulation Software; The Dow Chemical Co.: Midland, MI, 1989. Tamura, M.; Kumano, S. New Process for 1,4-Butanediol via Allylalcohol. Chem. Econ. Eng. Rev. 1980, 12, 32. Tanabe, Y. New Route to 14-BG and THF. Hydrocarbon Process. 1981, 9, 187. Turek, T.; Trimm, D. L.; Black, D. St C.; Cant, N. W. Hydrogenolysis of Dimethyl Succinate on Copper-Based Catalysts. Appl. Catal., A 1994, 116, 137. Weissermel, K.; Arpe, H.-J. Industrielle Organische Chemie; VCH: Weinheim, 1994. Wilke, C. R.; Chang, P. Correlation of Diffusion Coefficients in Dilute Solutions. AIChE J. 1955, 1, 264.

Received for review April 22, 1996 Revised manuscript received July 23, 1996 Accepted July 25, 1996X IE960229G

X Abstract published in Advance ACS Abstracts, June 15, 1997.