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Ind. Eng. Chem. Res. 1998, 37, 3888-3895
Ethanol Dehydrogenation with a Palladium Membrane Reactor: An Alternative to Wacker Chemistry B. A. Raich and Henry C. Foley* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19707
The use of a membrane reactor offers the possibility of shifting the extent of reaction beyond the normal equilibrium position by continuously depleting one of the products. One such case is the dehydrogenation of ethanol to acetaldehyde. Formation of acetaldehyde is endothermic and overall unfavorable, but the secondary formation of ethyl acetate from the product and reactant is exothermic and favorable. Therefore, it is observed that the secondary product forms under catalytic reaction conditions, thereby dropping the yield of the desired aldehyde product. Herein, we show how the use of a palladium membrane to remove hydrogen in conjunction with a catalyst leads to a shift further to the right in the acetaldehyde-forming step before the product can react deleteriously with ethanol, and thereby increase the yield of acetaldehyde substantially over the conventional reactor case. With the membrane, ethanol conversion increased from 60% to nearly 90% with a commensurate rise in selectivity to acetaldehyde from 35% to 70%, moving the yield from 21% to 63%. Introduction The chemical process industry is currently seeking to alleviate the environmental impact of many of its operations through environmentally benign process design. The goal is to design processes which minimize both energy consumption and the quantity of byproducts. The Wacker process, which produces acetaldehyde through the oxidation of ethylene,1 is one process which requires modification or replacement by a cleaner, more modern alternative that does not produce chlorinated wastes. In 1989 greater than 98% of the world’s 2.5Mt capacity for acetaldehyde used the Wacker chemistry.2 Alternatives to ethylene oxidation which eliminate chlorinated waste formation include oxidation of ethanol, addition of water to acetylene, partial oxidation of hydrocarbons, and dehydrogenation of ethanol to acetaldehyde and hydrogen.2 Ethanol dehydrogenation is attractive because it produces no waste, utilizes a renewable resource, and provides needed hydrogen. Despite these benefits, ethanol dehydrogenation is not economically feasible due to the fact that ethanol is approximately twice as expensive as ethylene, and the reaction is unfavorable. For the last reason ethanol dehydrogenation requires higher operating temperatures than ethylene oxidation with higher attendant operating costs. The offsetting value of the hydrogen coproduct and the lack of chlorinated wastes could improve the economics, if overall process efficiencies could be raised. To establish a lower bound on the dehydrogenation of ethanol, we examined the equilibrium conversion levels for reaction 1:
C2H5OH T CH3CHO + H2
{
∆H°rx ) 68.9 kJ/mol 298 K ∆S°r ) 0.11 kJ/mol ∆G°r ) 33 kJ/mol
In addition to simple dehydrogenation of ethanol, dehydrogenative coupling between the product acetaldehyde and ethanol also occurs to produce ethyl acetate and hydrogen. Unlike the first step, this dehydrogenation reaction is exothermic and favorable overall.
C2H5OH + CH3CHO T CH3COOC2H5 + H2
{
∆H°rxn ) -43.45 kJ/mol ∆S°rxn ) 0.054 kJ/mol ∆G°r ) -27.4 kJ/mol
(2)
The equilibrium conversion levels of ethanol for reaction 1 are shown in Figure 1 as a function of temperature. The saturated vapor pressure of ethanol at 25 °C is 0.077 atm, and this is taken as the partial pressure of ethanol in the feed for the calculations. The computed equilibrium distributions of products for reaction 2 are given in Figure 2; similar trends arise for the other ethanol partial pressures. It is clear that ethyl acetate is the thermodynamically favored product at low temperatures and that higher temperatures favor the aldehyde. At 200 °C, if the dehydrogenation alone took place, the level of ethanol conversion at equilibrium would be 50%. The inclusion of the coupling reaction to ethyl acetate raises the equilibrium conversion to 90% at the same condition. The ethyl acetate product would also dominate over acetaldehyde with nearly an 8:1 ratio on a mole basis. The use of an effective catalyst with a palladium membrane reactor could lead to the requisite ethanol conversion, acetaldehyde yield, and coproduction of a valuable pure hydrogen byproduct stream. Hence, the object of this research was to examine the catalytic dehydrogenation of ethanol within a palladium membrane reactor. Background
(1)
Many different kinds of catalyst materials have been studied for the dehydrogenation of ethanol, ranging
S0888-5885(97)00942-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/29/1998
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3889
Figure 1. Equilibrium conversion of ethanol by dehydrogenation, including reaction both to acetaldehyde and to ethyl acetate.
Figure 2. Equilibrium yields of acetaldehyde, ethyl acetate, and hydrogen via ethanol dehydrogenation (P ) 0.077 atm).
from molecular sieves3 and zeolites such as silicalite4 to more complex catalysts such as a zinc-modified, silicapillared rectorite.5 However, reduced copper catalysts appear to be the most frequently employed catalysts due to their high activity and selectivity. The first experiments using reduced copper for the dehydrogenation of ethanol were carried out by Sabatier and Senderens.6 This was extended by Palmer,7-9 Rideal,10 and Neish.11 In 1951 Church and Joshi12 proposed an industial process for acetaldehyde production based on ethanol dehydrogenation using an asbestos fiber-supported copper catalyst containing 5% cobalt oxide and 2% chromium oxide as promoters. More recent investigations have focused on the effect of support material on secondary product selectivity. Iwasa and Takezawa13 examined SiO2, ZrO2, Al2O3, MgO, and ZnO as support media for copper catalysts. Importantly, they found that the dehydrogenation step to acetaldehyde, though unfavorable, (eq 1), was much faster than the favorable step to ethyl acetate (eq 2). Ethanol was rapidly dehydrogenated to acetaldehyde and reached equilibrium within the residence times that they had examined, but the reaction to form ethyl acetate was much slower and did not achieve equilibrium at the same conditions. Membrane reactors have been considered for methanol dehydrogenationsa closely related reaction. Zaspalis et al.14 studied the dehydrogenation of methanol to formaldehyde using a ZnO catalyst in a reactor tube
made from a porous alumina. It suffered low selectivities to formaldehyde due to further dehydrogenation (decomposition) of the formaldehyde to carbon monoxide and hydrogen. More recently, Deng and Wu15 studied the dehydrogenation of methanol in three different types of inorganic membrane reactors, all of which involved the modification of a commercially available porous ceramic membrane which had an average pore size of 0.5 µm. The first membrane was a palladium/ceramic composite membrane synthesized by electroless plating; the thickness of the resulting palladium layer was 20 µm. The second membrane was a porous alumina membrane in which the pores of the ceramic tube were narrowed to approximately 4 nm by impregnating the porous ceramic tube with a γ-AlOOH solution. The third membrane was labeled as a catalytic inorganic membrane and was prepared by adding copper and phosphorus as catalytic components to the silica solution used to impregnate the porous ceramic tube. A Cu-P/ SiO2 catalyst, prepared using an ion-exchange method, was used with the palladium/ceramic membrane and the porous alumina membrane. The yield of formaldehyde was found to increase for all three of these membranes when compared to that in a conventional reactor. At moderate reaction conditions, they found that the yield increased by 15% with the palladium ceramic membrane, 8% with the catalytic inorganic membrane, and 5% with the porous alumina membrane. From previous work with the palladium membrane reactor, we knew that we would require catalysts that were highly active and stable at low hydrogen fugactities.16 We set out to examine catalysts similar to those studied by Iwasa and Takezawa, since they reported fast rates for acetaldehyde formation with copper on silica. Thus, we chose to examine three silica-based catalysts. Two of the three catalysts were loaded with copper. One was synthesized by aqueous impregnation, duplicating Iwasa and Takezawa’s synthesis technique, while the other was prepared by ion exchange17 which is proposed to lead to a catalyst that is more resistant to deactivation by coking. The third catalyst was a platinum catalyst, modified with tin, and is more typical of the catalysts used for dehydrogenation of alkanes. Experimental Section The catalysts used in the present experiments were Pt-Sn/SiO2 (0.89% Pt and 0.65% Sn) and Cu/SiO2-Aq (1.62% Cu), both prepared by incipient wetness, and Cu/ SiO2-IE (2.89% Cu), synthesized by ion exchange. The calcining protocol of the SiO2 support (Davisil) for all three catalysts involved ramping the temperature from room temperature to 700 °C in four stages over a 4-h period in air. The first two stages were in increments of 150°, while the last two stages were in increments of 200°. The final temperature of 700 °C was maintained for 17 h. The silica was ground to 60/120 mesh and stored in air at 120 °C until use. Pt-Sn/SiO2. On the basis of a method developed by Cortright and Dumesic,18 a solution consisting of 0.496 g of tetraamine platinum nitrate, dissolved in 31 mL of water, was used to impregnate 25.00 g of the silica support which was dried at 120 °C in an oven for 8 h. A solution of 0.739 g of tri-n-butyl tin acetate in 31 mL of methanol was added to the solid, and then it was dried in air for 90 h at 120 °C. Finally, the solid temperature was raised 25 °C every 15 min to 300 °C, held for 2 h, and then cooled to room temperature.
3890 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998
Cu/SiO2-Aq. A solution consisting of 1.168 g of Cu(NO3)2‚2.5H2O, dissolved in 26.6 mL of water, was used to impregnate 19.58 g of the silica support using aqueous impregnation. The powder was dried in an oven at 140 °C and then calcined in air at 500 °C for 1 h. Cu/SiO2-IE. The support was prewashed by stirring 35 g of the calcined silica gel in a beaker with 500 mL of 5 N nitric acid for 1 h at room temperature. The acid was decanted, replaced with fresh acid, and stirred for an additional hour. The solids were rinsed thoroughly with deionized water, placed in an oven at 110 °C for 16 h, and sieved to 150/235 mesh. To 28.00 g of the nitric acid wash was added a solution consisting of 3.754 g of Cu(NO3)2‚2.5 H2O in 800 mL of 1 N ammonium hydroxide. This mixture was stirred for 24 h, filtered, dried at 110 °C, and calcined in air at 500 °C for 24 h. The reaction of nitrous oxide (N2O) has been found to be a suitable means of measuring copper metal surface areas.17,19-24
N2O(g) + 2Cu(s) f N2(g) + Cu2O(s)
(3)
The nitrous oxide used as the adsorbate was 99.999% pure (semiconductor grade) as supplied by Matheson. A 0.35-g sample of catalyst was used for each measurement. The nitrous oxide adsorption protocol consisted of these steps: (1) Heat the catalyst sample to 400 °C, under vacuum. (2) Reduce with 50 cm3m hydrogen at 400 °C for 10 h. (3) Evacuate for 12 h at 400 °C (
