Gas-Phase Hydrogenolysis of Dimethyl Maleate to 1, 4-Butanediol

maleate isomerize to dimethyl fumarate. The reaction then ... dimethyl succinate, until 1,4-butanediol and γ-butyrolactone are formed together with m...
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Ind. Eng. Chem. Res. 1999, 38, 1264-1270

Gas-Phase Hydrogenolysis of Dimethyl Maleate to 1,4-Butanediol and γ-Butyrolactone over Copper/Zinc Oxide Catalysts Jan H. Schlander and Thomas Turek* Institut fu¨ r Chemische Verfahrenstechnik, Universita¨ t Karlsruhe (TH), D-76128 Karlsruhe, Germany

The gas-phase hydrogenolysis of dimethyl maleate has been studied over copper/zinc oxide catalysts at temperatures between 473 and 513 K in the pressure range from 2 to 35 bar. A Cu/ZnO catalyst with a low copper content of 15 mol % was found to be the most active for the conversion of dimethyl maleate. At very low residence times, significant amounts of dimethyl maleate isomerize to dimethyl fumarate. The reaction then proceeds via the saturated ester, dimethyl succinate, until 1,4-butanediol and γ-butyrolactone are formed together with methanol. Furthermore, the chosen catalyst produces low amounts of tetrahydrofuran and water and only traces of undesirable butanol and CO2. It was found that the production of high amounts of 1,4-butanediol in one step is not possible, because the simultaneous presence of diesters and butanediol gives rise to the formation of a polymeric material via transesterification. Therefore, a two-step process including the complete conversion of dimethyl maleate to γ-butyrolactone, carried out at high temperature and moderate pressure, without formation of butanediol followed by the hydrogenation of γ-butyrolactone to 1,4-butanediol at low temperature and high pressure was proposed. It could be shown by systematic investigation of the hydrogenation of γ-butyrolactone that the amount of 1,4-butanediol formed is limited by thermodynamic constraints. The dependence of the equilibrium constant for the reaction between γ-butyrolactone and 1,4butanediol was determined as a function of temperature and pressure. 1. Introduction The demand for 1,4-butanediol (BDL) is steadily growing because of its major uses as a starting material for polymers and solvents. According to Weissermel and Arpe,1 more than 400 000 tons of 1,4-butanediol were manufactured in 1992, roughly 50% of which were used for the preparation of polybutylene terephthalate and polyurethanes. The majority of the remaining amount was converted to tetrahydrofuran (THF) and γ-butyrolactone (GBL). In the USA and in Europe, the major fraction of 1,4-butanediol is currently being produced via the classical Reppe process based on acetylene and formaldehyde, whereas in Japan, the production is mainly based on the acetoxylation of 1,3-butadiene (the Mitsubishi process). Another interesting possibility is the preparation of 1,4-butanediol from maleic anhydride. Butane-based maleic anhydride is available in sufficient amounts at reasonable prices. The direct hydrogenation of maleic anhydride in the liquid phase has now again been commercialized in Japan,1 after smaller amounts had already been produced there in the 1970s.2 Recently, Herrmann and Emig3-5 have carried out an extensive study of the hydrogenation of maleic anhydride over different noble metal and copper catalysts. It was shown that 1,4-butanediol and γ-butyrolactone can be obtained in one step if copper catalysts promoted with zinc oxide are used. Alternatively, maleic anhydride may be converted to a dialkyl ester before reacting to the desired products. * To whom correspondence is addressed. Telephone: +49(0)721 608 4267. Fax: +49(0)721 608 6118. E-mail: [email protected].

During this hydrogenolysis step, which is a versatile reaction applicable for a wide variety of substrates,6 1,4butanediol and its derivatives are formed together with the alkanol, which is recycled after separation to the esterification of maleic anhydride. This route can be carried out in the gas phase at especially mild conditions. Davy McKee was the first to commercialize this process, which is carried out at temperatures of around 473 K, mild pressures from 30 to 45 bar, and high molar hydrogen-to-ester ratios to prevent condensation of reactants.7-9 The reaction is believed to proceed via dialkyl succinate and γ-butyrolactone, while butanediol can react further to tetrahydrofuran (Figure 1). The relative amounts of γ-butyrolactone and 1,4-butanediol can be controlled by choice of the reaction conditions, with high pressures and low temperatures favoring the formation of 1,4-butanediol. The information available in the open literature regarding reactions of maleic esters or intermediates is limited. Thomas et al.10,11 studied the reaction of dimethyl succinate using a Raney copper catalyst. At 513 K and the low pressures used (5 bar), only γ-butyrolactone and tetrahydrofuran, but no 1,4-butanediol, were formed. Zhang and Wu12 converted diethyl succinate over copper-based catalysts at temperatures between 473 and 503 K and pressures ranging from 20 to 40 bar. It could be clearly shown that butanediol formation was enhanced at low temperatures and high pressures. Turek et al.13 studied the hydrogenolysis of dimethyl succinate over several copper-based catalysts. Coprecipitated copper/zinc oxide catalysts were shown to have much higher catalytic activity per unit copper surface area than other copper catalysts. In the recent patent literature, further examples of processes employing the

10.1021/ie980606k CCC: $18.00 © 1999 American Chemical Society Published on Web 02/17/1999

Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1265

Figure 1. Reaction scheme for hydrogenolysis of dialkyl maleates.

hydrogenolysis of maleic esters to 1,4-butanediol and derivatives over copper/zinc oxide catalysts can be found.14,15 In the present contribution, the activity of copper/zinc oxide catalysts with different compositions was compared during the gas-phase hydrogenolysis of dimethyl maleate (DMM) to 1,4-butanediol and its derivatives, γ-butyrolactone and tetrahydrofuran. Over the most active catalyst, a systematic study of the reaction system including the influence of pressure, temperature, and space velocity was carried out. 2. Experimental Section Catalysts. The copper/zinc oxide catalysts were prepared by a method proposed by Herman et al.16 A 1 M sodium carbonate solution was added dropwise (1.7 cm3/ min) to 1 M solutions of zinc and copper nitrate under vigorous stirring at 359 K. After the pH had been raised to 7.0, the suspension was allowed to cool for 1.5 h. The precipitate was filtered, washed thoroughly with deionized water, and dried overnight at 353 K. The subsequent calcination was carried out in air by heating the precursors within 2 h to 623 K, maintaining the maximum temperature for 1 h. The resulting oxide powders were pressed to tablets, crushed, and sieved to obtain a fraction between 315 and 500 µm with an apparent density of 1.0-1.8 g/cm3. The catalysts were characterized by mercury porosimetry and by measurement of the BET and copper surface areas, with the latter being determined after reduction with hydrogen. The reduction was started with 3% H2 in He at 413 K using a volume flow rate of 300 cm3/min (STP). The temperature was raised to 513 K at 20 K/h, and finally the diluted hydrogen stream was replaced by pure hydrogen. After cooling to 333 K in flowing He, the reduced samples were contacted with a mixture of 0.1 vol % N2O in He at a volume flow rate of 100 cm3/min (STP). The copper surface areas were calculated from the consumed amount of nitrous oxide according to Chinchen et al.17 Catalytic Measurements. The experiments were carried out in an electrically heated stainless steel tube reactor with 13 mm inner diameter and 200 mm length in the temperature range from 473 to 513 K. In the reactor, the gaseous reactants were preheated in a bed of glass beads and then contacted with up to 8.0 g of catalyst. Prior to the experiments, the fresh catalysts were reduced in hydrogen as described above. The axial temperature profile in the reactor was measured with a thermocouple in a central tube with an outer diameter of 2 mm. Additionally, the temperature of the reactor wall was monitored by use of a second thermocouple. Hydrogen and the internal standard nitrogen were supplied by thermal mass flow controllers (Bronkhorst HI-TEC, FA-201C). The liquid reactants, dimethyl maleate (Fluka, >95%), γ-butyrolactone (Merck, >99%), and 1,4-butanediol (Merck, >98%), were used without further purification. The liquids were stored in a vessel of 500 cm3 at 3 bar above the pressure in the reactor

and supplied by means of a thermal mass flow controller (Bronkhorst HI-TEC, FA-11-0). The evaporation was carried out using a commercial unit (Bronkhorst HITEC, W-002-119-P/W-100) in the hydrogen stream. The reactor pressure of 2-35 bar was maintained by means of a unit consisting of a pressure sensor (Bronkhorst HI-TEC, P-502C) and a valve (Ka¨mmer, 80157). All lines were maintained at 473 K to prevent condensation of reactants. Bypassing the reactor allowed for measurement of the inlet concentrations. On-line analysis of the reactant streams was performed after pressure release and admixture of the internal standard by a gas chromatograph (HewlettPackard, 5890 II series) equipped with a thermal conductivity detector and a capillary column (WCOT fused silica, CP-Wax 52 CB) temperature-programmed from 323 to 483 K. Hydrogen was used as the carrier gas at a flow rate of 100 cm3/min (STP). The peak areas were converted to molar flow rates, n˘ i, using relative molar response factors related to nitrogen, the flow rate of which was precisely known. The following components could be analyzed (relative molar response factors are given in parentheses): carbon dioxide (1.49), water (0.70), methanol (1.20), tetrahydrofuran (2.11), n-butanol (2.64), dimethyl fumarate (3.74), dimethyl succinate (4.14), dimethyl maleate (3.72), γ-butyrolactone (2.39), and 1,4-butanediol (2.46). When mole fractions were calculated,

xi )

n˘ i,out

∑i n˘ i,out

(1)

the amount of hydrogen, always being in excess, was not taken into account. The modified residence time, tv, is defined as the ratio of catalyst mass, m, and the total volume flow rate, V˙ , at standard temperature (298 K) and pressure (1 bar). Because the flow rate of educt i at the reactor inlet is known, the total carbon fraction at the reactor outlet,

YC )

∑i n˘ i,outi n˘ i,ini

(2)

where i is the number of carbon atoms in component i, can be calculated, allowing for assessment of the carbon balance. 3. Results and Discussion Selection of Catalyst. A series of catalysts with different compositions was prepared and tested during the hydrogenolysis of dimethyl maleate at a temperature of 513 K and a molar hydrogen-to-ester ratio of 25. In these preliminary experiments, a maximum temperature difference of 6 K between the center of the catalyst bed and the reactor wall, mainly caused by the exother-

1266 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999

Figure 3. Product composition as a function of the modified residence time at 513 K and 15 bar (molar hydrogen/ester ratio ) 60, catalyst with 15 mol % copper).

Figure 2. Dimethyl succinate conversion during reaction of dimethyl maleate (upper part) and BET and copper surface area (lower part) over Cu/ZnO catalysts with different compositions (T ) 513 K, m ) 8.0 g, V˙ ) 500 cm3/min (STP), molar hydrogen/ ester ratio ) 25).

mic reaction of dimethyl maleate to dimethyl succinate (DMS), was observed. Steady-state product compositions were obtained after 5 to 20 h, depending on the composition of the catalyst used. The starting material, dimethyl maleate, was completely converted in all cases, i.e., the conversion of dimethyl maleate to dimethyl succinate is rapid under the given conditions. For this reason the conversion of dimethyl succinate to further products

XDMS )

n˘ DMM,in - n˘ DMS,out n˘ DMM,in

(3)

was used as a measure for the catalytic activity of the different samples (Figure 2). Repeated experiments showed that the conversions can be reproduced at a relative deviation of less than 5%. It can be seen that at the lowest pressure of 2 bar the catalyst composition has little effect on the achievable conversions. At higher pressures, copper catalysts with a low copper content exhibit the highest activities. In all cases, a product mixture of methanol, γ-butyrolactone, tetrahydrofuran, water, and 1,4-butanediol together with traces of butanol and carbon dioxide was obtained. However, only at a pressure of 15 bar could significant amounts of 1,4butanediol be observed. Carbon balances accurate to (2% were obtained during these measurements. The lower part of Figure 2 reveals that the admixture of zinc oxide gives rise to higher BET surface areas while the specific copper surface area increases until a maximum at around 30% copper is reached. This maximum can possibly be explained by the fact that the copper content in the catalysts with less than 30% Cu is too low to further increase the copper surface area, while on the other hand the surface of the ZnO becomes progressively smaller at higher copper contents. The influence of ZnO

on the catalytic activity of copper/zinc oxide catalysts is complex. Low amounts of ZnO appear to have a slightly detrimetal effect, whereas at high ZnO contents an increase of the activity at higher pressures can be observed. It appears that both high copper surface areas and high ZnO contents are required for high dimethyl succinate conversions. It was shown earlier that the addition of zinc oxide, which is not active for the reactions studied, to copper catalysts increases the rate of the hydrogenolysis of dimethyl succinate.13 A positive effect of zinc oxide was also found in the direct hydrogenation of maleic anhydride to γ-butyrolactone and tetrahydrofuran18 as well as during the hydrogenolysis of methyl acetate19 over copper-based catalysts. The nature of the interaction between copper and zinc oxide is not yet clear and requires further research effort. Hydrogenolysis of Dimethyl Maleate to γ-Butyrolactone and 1,4-Butanediol. For further measurements, the Cu/ZnO catalyst with a copper content of 15% was selected. The experiments described in the following were aimed at determining the product composition as a function of the residence time in the reactor. Because high pressures are obviously needed to obtain the most desirable product, 1,4-butanediol, reactor pressures of 15, 25, and 35 bar were selected at 493 and 513 K. A hydrogen/dimethyl maleate ratio of 60 was used to prevent condensation of reactants and products. The overtemperature in the catalyst bed was less than 3 K in any case. Figure 3 shows a typical result for the product composition in mole fractions at 15 bar and 513 K versus the modified residence time. Variation of the residence time was achieved by changing the flow rate or the catalyst mass. Identical results were obtained at different flow rates and the same modified residence time. Figure 3 shows that dimethyl succinate, which is again rapidly formed from the reactant dimethyl maleate, mainly reacts to methanol and γ-butyrolactone until complete DMS conversion is reached at a modified residence time of ca. 2.5 g‚s‚cm-3. The ratio of methanol and γ-butyrolactone is slightly lower than that expected from the stoichiometry of the reaction of DMS with hydrogen (cf. Figure 1), because further products are formed in consecutive steps. The main byproducts are tetrahydrofuran and water (not shown) in approximately the expected equimolar ratio. At 15 bar, the mole fraction of 1,4-butanediol remains below 1.7%, while only traces of butanol and carbon

Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1267 Table 1. Product Compositions over 15 mol % Copper Catalyst at Different Conditions (m ) 8.0 g, Molar Hydrogen/Ester Ratio ) 60) p/bar T/K XDMS tv/(g‚s‚cm-3) xBDL xGBL xTHF xH2O xMeOH xBuOH xCO2 YC

15 513 0.99 2.4 0.017 0.233 0.047 0.048 0.632 0.007 0.015 0.97

25 513 1.00 2.4 0.040 0.205 0.049 0.050 0.632 0.008 0.015 0.96

35 513 1.00 2.4 0.066 0.167 0.044 0.049 0.646 0.009 0.018 0.94

25 493 0.92 2.4 0.030 0.192 0.040 0.041 0.652 0.002 0.010 0.83

35 493 0.54 0.46 0.018 0.125 0.018 0.023 0.637 0 0 0.72

dioxide, which is believed to be formed by the steam reforming of methanol,13 can be observed. These results reveal that dimethyl maleate can be completely converted to γ-butyrolactone and other valuable products by gas-phase reaction with hydrogen while only minor amounts of undesirable components are being formed. It is evident that only very little 1,4-butanediol is obtained at the chosen reaction conditions, 15 bar and 513 K. Because higher pressures and lower temperatures were reported to favor butanediol formation, a series of further experiments, the results of which are summarized in Table 1, was conducted. The results at 513 K reveal as expected that the achievable butanediol yield at nearly complete ester conversion rises at the expense of γ-butyrolactone formation when the pressure is increased. It can also be seen that the amounts of the coproducts tetrahydrofuran and water are almost independent of the reactor pressure. Furthermore, approximately the same small amounts of undesirable byproducts (butanol and carbon dioxide) are being formed at 513 K. Note that the mole fractions calculated according to eq 1 reflect the content of species i in the stream at the reactor outlet, while the total carbon fraction, YC, also given in Table 1 relates the detected amount of carbon at the reactor outlet to the amount at the inlet, present as dimethyl maleate. These values show that only a minor carbon deficit occurs at a reactor temperature of 513 K. Nevertheless, a tendency toward a lower value of YC at increasing pressure can be seen. On the other hand, a dramatic carbon loss, the extent of which is increased at the higher pressure of 35 bar, is observed at the lower reactor temperature of 493 K. This is accompanied by unexpectedly low mole fractions of butanediol. The continuous loss of carbon-containing species was caused by the formation of a polymer that could be found in the catalyst bed and at the colder parts of the reactor and the lines after the experiments at 493 K. Surprisingly, no catalyst deactivation could be observed during these runs. The composition of the polymer could not yet be determined; however, it is highly probable that it was formed by reaction of 1,4-butanediol with dimethyl maleate, fumarate, or succinate. According to the scheme presented in eq 4 for the example of dimethyl maleate, the bifunctional monomers can react to polymers while forming methanol.

H3COOC(CH)2COOCH3 + HO(CH2)4OH f H3COOC(CH)2COO(CH2)4OH + CH3OH (4) The formation of polyesters has also been reported to take place during the hydrogenation of maleic anhydride in the liquid phase,2 in that case by reaction of

Figure 4. Dimethyl succinate conversion, total carbon fraction at the reactor outlet, and mole fractions of GBL and BDL versus reactor pressure (T ) 513 K, m ) 8.0 g, V˙ ) 500 cm3/min (STP), molar hydrogen/ester ratio ) 60).

1,4-butanediol with succinic acid. The above assumption could explain why the carbon balance is better fulfilled the less 1,4-butanediol is expected from the thermodynamics of the reaction between GBL and BDL. Further evidence for the proposed polymer formation by transesterification of 1,4-butanediol with a diester is gained from the fact that no carbon deficit could be observed even at 473 K and 35 bar when γ-butyrolactone was used as the reactant. The results of these measurements will be described in the following sections. Other possible reasons for the carbon deficit at lower temperature are less likely. Polycondensation reactions are of minor importance because water is only in slight excess compared to the low THF concentrations observed at 493 K. Furthermore, the highest partial pressure of 1,4butanediol, the component with the highest boiling point (503 K at 1 bar), in all experiments during the hydrogenolysis of dimethyl maleate was only about 40 mbar. Thus, condensation of liquid reactants is highly improbable, even if one takes into account the possibility of capillary condensation. The mean diameter of the pores of the catalyst used was found to be 20 nm. It remains an open question whether polymer formation during the hydrogenolysis of dimethyl maleate to 1,4-butanediol and γ-butyrolactone can be avoided when other catalysts or reaction conditions are employed. However, an obvious strategy to prevent polymer formation appears to be the complete conversion of dimethyl maleate at high temperature and low pressure, where no or only very little formation of 1,4-butanediol takes place. In the second step carried out at low temperature and high pressure, γ-butyrolactone produced in the first step reacts then to 1,4-butanediol. Conversion of Dimethyl Maleate to γ-Butyrolactone. A temperature of 513 K was selected as the reactor temperature for the conversion of dimethyl maleate to γ-butyrolactone, because high temperatures enhance the rate of reaction while, on the other hand, the amount of 1,4-butanediol formed remains low. The results of a series of measurements at 5-35 bar were employed to select the most suitable reactor pressure. Figure 4 shows that the achievable conversion is significantly increasing, most probably because of the higher reactant concentrations at ascending pressures. Again, it can be seen that the mole fraction of 1,4butanediol is increasing when less γ-butyrolactone is

1268 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999

Figure 5. Product composition as a function of the modified residence time at 513 K and 10 bar (molar hydrogen/ester ratio ) 60).

being formed. Significant carbon deficits can only be observed in the presence of detectable amounts of 1,4butanediol in the reactant stream. For these reasons, a reactor pressure of 10 bar was selected as suitable for the conversion of dimethyl maleate to γ-butyrolactone without the danger of carbon losses due to polymer formation. The hydrogenolysis of dimethyl maleate was then studied in more detail at 513 K and 10 bar. Figure 5 shows the product composition versus the modified residence time. Dimethyl succinate is rapidly being formed from dimethyl maleate at very low residence times followed by conversion to γ-butyrolactone and methanol (upper part). The mole fraction of water (lower part) is slightly higher than the respective value for tetrahydrofuran, because small amounts of butanol are also formed. Carbon dioxide occurs especially at high residence times, and 1,4-butanediol concentrations are lower than those at a reactor pressure of 15 bar. The product compositions measured at very low residence times (Figure 6) reveal that dimethyl maleate is not exclusively converted to dimethyl succinate. The isomerization to dimethyl fumarate also takes place to a certain extent. However, at a residence time of 0.15 g‚s‚cm-3, both unsaturated diesters have completely reacted to dimethyl succinate and the formation of subsequent products has started. Conversion of γ-Butyrolactone to 1,4-Butanediol. Figure 7 shows the reactant concentrations during the reaction of γ-butyrolactone or 1,4-butanediol at 493 K and 35 bar as a function of the modified residence time. A hydrogen-to-reactant ratio of 90 was used. During these measurements, a maximum butanediol partial pressure of 175 mbar was reached, while the

Figure 6. Product composition at low residence times (T ) 513 K, p ) 10 bar, molar hydrogen/ester ratio ) 60).

total carbon fraction at the reactor outlet was always equal to unity within the experimental error ((3%). This shows clearly that the condensation of liquid 1,4butanediol cannot be responsible for the carbon deficit observed during the hydrogenolysis of dimethyl maleate and that the formation of the polymer requires the simultaneous presence of a diester and butanediol. From the upper part of Figure 7 it can be seen that the equilibrium between γ-butyrolactone and 1,4-butanediol is attained after a residence time of ca. 3 g‚s/cm3. At higher residence times, the concentrations of both components decrease because of the formation of the subsequent products tetrahydrofuran, butanol, and water (lower part of Figure 7). Carbon dioxide could not be found in any case. The concentrations of byproducts during the experiments with 1,4-butanediol as the starting material not shown in Figure 7 were slightly higher than those observed with γ-butyrolactone. However, because the mutual interconversion of γ-butyrolactone and 1,4-butanediol takes place with a relatively high rate, it cannot be concluded via which reaction paths tetrahydrofuran and butanol are formed. In Figure 8, the ratio of the mole fractions of 1,4butanediol and γ-butyrolactone normalized to the maximum ratio observed at each series of measurements is depicted as a function of the modified residence time for several series of experiments carried out in the temperature range 473-513 K at 25 and 35 bar reactor pressure. It can be seen that the achievable amount of butanediol increases at higher pressure, while the residence time necessary for reaching the equilibrium is strongly decreasing with ascending temperature. The amounts of byproducts, tetrahydrofuran, butanol, and water, increased with ascending temperature, while the pressure had no effect on the mole fractions of these components. From the maximum ratio of mole fractions

Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1269

Figure 9. Equilibrium constant for reaction (5) versus reciprocal temperature obtained from measurements with different starting materials. The solid line was calculated with ∆H ) -65 kJ/mol.

Figure 7. Upper part: Mole fractions of γ-butyrolactone (squares) and 1,4-butanediol (circles) versus modified residence time for measurements starting with pure GBL (open symbols) or pure BDL (closed symbols). Lower part: Mole fractions of butanol, tetrahydrofuran, and water during the GBL hydrogenation (T ) 493 K, p ) 35 bar, molar hydrogen/reactant ratio ) 90).

Figure 8. Ratio of 1,4-butanediol and γ-butyrolactone mole fractions, normalized to the maximum ratio for each series of measurements, as a function of modified residence time (molar hydrogen/γ-butyrolactone ratio ) 90).

of 1,4-butanediol and γ-butyrolactone, the equilibrium constant for the reaction

(CH2)3COO + 2H2 h HO(CH2)4OH

(5)

can be calculated. Because hydrogen is always in large excess (pH2 ≈ p), Kp can be obtained from

Kp )

( )( ) xBDL pQ xGBL p

2

(6)

Figure 9 shows the equilibrium constants calculated using the 1,4-butanediol/γ-butyrolactone ratios during measurements with different starting materials as a function of the reciprocal temperature. It can be seen that, not only with 1,4-butanediol and γ-butyrolactone but also during the hydrogenolysis of dimethyl maleate, equilibrium is reached after sufficient residence times in the reactor. The 1,4-butanediol/γ-butyrolactone ratio follows the expected pressure dependence as expressed in eq 5. Moreover, the dependence of Kp on temperature can be described with the van’t Hoff equation:

d ln Kp ∆H ) dT RT2

(7)

The enthalpy of reaction was found to be ∆H ) -65 ( 5 kJ/mol (95% confidence interval), which is in quite good agreement with the value reported by Zhang and Wu12 (-58 kJ/mol). Thus, it is evident that the amount of 1,4-butanediol formed in the hydrogenation of γ-butyrolactone is limited by thermodynamic constraints. It is therefore not possible to obtain pure butanediol, except at extremely low temperature and high pressure. The optimum conditions for the hydrogenation of γ-butyrolactone have to be determined in further studies. The achievable amount of 1,4-butanediol increases with descending temperature, while at the same time the rate of GBL hydrogenation strongly decreases. Moreover, it has to be noted that the Cu/ZnO catalyst with 15 mol % copper had been selected because of its performance in the hydrogenolysis of dimethyl maleate, whereas catalysts with different compositions have not yet been compared during the hydrogenation of γ-butyrolactone. 4. Conclusions (1) Comparison of a series of copper/zinc oxide catalysts during the hydrogenolysis of dimethyl maleate in the gas phase showed that the performance of such catalysts is improved by addition of zinc oxide. ZnO increases the copper surface area, but an additional promoting effect, the nature of which is not yet known, was also observed especially at higher reactor pressures. (2) Dimethyl maleate can be completely converted to γ-butyrolactone and other valuable products by gas-

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phase reaction with hydrogen while only minor amounts of undesirable products are being formed. (3) If diesters are present together with significant amounts of 1,4-butanediol, a polymeric material is formed, most probably via transesterification reactions. To prevent polymer formation, it was proposed to produce 1,4-butanediol in two steps. In the first step, dimethyl maleate is converted to γ-butyrolactone at high temperature and moderate pressure (e.g., 513 K and 10 bar) without formation of 1,4-butanediol. In the second step, γ-butyrolactone reacts to 1,4-butanediol at low temperature and high pressure. (4) The amount of 1,4-butanediol formed during the hydrogenation of γ-butyrolactone is limited by thermodynamic constraints. The enthalpy of reaction determined from the development of the product composition at thermodynamic equilibrium with temperature was found to be ∆H ) -65 ( 5 kJ/mol.

(8) Tuck, M. W. Conversion of Butane to Petrochemicals: Maleic Anhydride and Butanediol. EUROGAS 90, Proc. Eur. Appl. Res. Conf. Nat. Gas. 1990, 165.

Acknowledgment

(9) Harris, N.; Tuck, M. W. Butanediol via Maleic Anhydride. Hydrocarbon Process. 1990, 5, 79.

The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft. Nomenclature ∆H ) enthalpy of reaction [kJ/mol] Kp ) equilibrium constant m ) catalyst mass [g] n˘ ) molar flow rate [mol/s] p ) pressure [bar] pQ ) standard pressure [1 bar] R ) gas constant [8.4134 J/(mol‚K)] SBET ) BET surface area [m2/g] SCu ) copper surface area [m2/g] tv ) modified residence time [g‚s/cm3] T ) temperature [K] V˙ ) volume flow rate [cm3/s] xi ) mole fraction of species i Xi ) conversion of species i YC ) total carbon fraction Abbreviations BDL ) 1,4-butanediol BuOH ) butanol DMF ) dimethyl fumarate DMM ) dimethyl maleate DMS ) dimethyl succinate GBL ) γ-butyrolactone MeOH ) methanol THF ) tetrahydrofuran Greek Symbol i ) number of carbon atoms in species i

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Received for review September 24, 1998 Revised manuscript received December 22, 1998 Accepted January 6, 1999 IE980606K