Coupling of Consecutive Reactions in a Two-Layer, Flow-Through

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Coupling of Consecutive Reactions in a Two-Layer, Flow-Through Catalytic Membrane M.J. Alfonso, M. Mene´ ndez, and J. Santamarıá* Department of Chemical Engineering, University of Zaragoza, 50009 Zaragoza, Spain

A flow-through catalytic membrane has been used to carry out consecutively the oxidative and non-oxidative dehydrogenation of butane, with the aim of achieving thermal and chemical coupling of both processes. To this end, two distinct zones were created across the membrane radius: the first, containing V supported on MgO, was prepared by sequential impregnation, and the second, containing Pt-Sn on γ-Al2O3, was prepared by by sol-gel procedures. The investigation highlights some of the problems encountered during membrane preparation, which are discussed and related to the reaction results observed. When the preparation procedures are tailored to achieve segregation of the catalytic materials in both layers, and when the operating conditions are such that complete oxygen consumption within the oxidative dehydrogenation layer is obtained, the use of the flow-through, two-layer membrane allows stable operation and, at the same time, gives rise to increased conversion and selectivity. Introduction The coupling of different reactions in a single reactor is attractive because, in principle, it allows an efficient use of the heat and/or products generated in situ. In a general way, chemical coupling can be defined as the transfer of species from one reaction to another. This obviously includes unused reactants, but also products from one reaction that either become reactants in another or are used in a different way, e.g., to inhibit deactivation reactions, as illustrated in this work. Membrane reactors are particularly well suited to carry out reaction coupling (see, for instance, the reviews by Dixon1 and by Coronas and Santamaria2). Gryaznov3 was among the first to propose the coupling of reactions in a membrane reactor. He used a Pd membrane separating packed beds of catalyst where dehydrogenation and hydrogenation reactions were carried out. This was a case of chemical coupling, i.e., the product from one reaction (H2) was used as a reactant in the other. This is efficient for equilibrium displacement, as H2 removal increases the conversion of the dehydrogenation reaction. Although hydrogenation reactions are exothermic and dehydrogenation processes are endothermic, thermal coupling (i.e., the transfer of reaction heat from one reaction to another) is not possible because dehydrogenation reactions tend to run at temperatures that are higher than those of hydrogenation reactions. Thermal + chemical coupling has been achieved by use of a combustion catalyst packed on the permeate side to burn the hydrogen produced in the dehydrogenation,4-6 although this can only be contemplated if H2 is not the desired reaction product. Coupling through a Pd membrane also presents other problems, the main ones being the fouling of the membrane and the need to match permeation and reaction rates, something that can require very large Pd areas. The above examples imply the selective transfer of one product from one reaction zone to another. An alternative approach to reaction coupling that does not * Corresponding author. Fax: +34 976 762142. E-mail: [email protected].

require permselectivity involves the flow of premixed reactants through multilayered catalytic membranes, where different reactions are carried out. There are several potential benefits of the flow-through mode of operation: On one hand, it allows simultaneous reaction and filtration7,8 and enhances the contact efficiency of the operation by reducing the internal and external mass transfer resistances.9-11 On the other hand, the flow-through operation mode has also been advocated as a means of attaining higher selectivities. Thus, Binkerd et al.12 demonstrated that a higher selectivity (compared to that of a fixed-bed reactor loaded with the same catalyst) was achieved when methane oxidative coupling was carried out in a radial flow-through reactor operating in the Knudsen permeation regime, where gas-phase reactions were minimized. In the same work, it was also shown that selectivity increases could be achieved by controlling the residence time of products in the catalytic layer. None of the above works has explored one of the potential advantages of the flowthrough mode of contact: the fact that heat and/or mass coupling of consecutive reactions becomes possible if a membrane with more than one catalytic layer is used. Thus, the close proximity of the catalytic layers makes it easy to transfer the heat produced in exothermic reactions to endothermic ones. Also, all or part of the products generated in the first reaction can be used in subsequent ones as reactants, inhibitors, or diluents. Multilayered porous catalytic membranes have been proposed and/or used in the past, usually in singlereaction systems and often with segregated, rather than premixed, feed. Thus, for instance, Harold and Lee13 simulated a two-layer (inactive or diffusion layer plus active layer) membrane in segregated-feed operation and found that, by adjusting the feed configuration and contact times in the support and reaction sides, significant improvements in the yield to the intermediate product could be obtained. Tsai et al.14 considered a three-layer membrane in their simulation study of syngas production: a first, large-pore, support layer was followed by a dense, oxygen-permeable, perovskite layer and then by a porous catalytic layer. Oxygen would be

10.1021/ie000643n CCC: $20.00 © 2001 American Chemical Society Published on Web 01/18/2001

Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1059

Figure 1. Reactor concept.

separated from air fed to the support side, while the permeated oxygen, methane, and reaction products would be in contact with the catalytic layer. Alfonso et al.15 carried out an experimental study of the oxidative dehydrogenation of propane in segregated-feed systems using two-layered membranes containing the same active layer (V/γ-Al2O3) and different diffusion layers. As in the work of Harold and Lee,13 the heterogeneity of the system (i.e., the presence of the inactive layer) was key to the results obtained. Finally, precise control of the thicknesses of the active (Pt/γ-Al2O3) and inert layers can be obtained by successive slip-casting steps, as demonstrated by Yeung et al.16 This work addresses the use of the flow-through mode of contact in multilayered membranes for the coupling of mass and heat in successive reactions. More specifically, we have carried out the oxidative and nonoxidative dehydrogenations of butane as a two-step process, using a double-layered membrane. The concept is illustrated in Figure 1. A premixed butane/oxygen/ inert feed enters the membrane and undergoes two consecutive reactions at the two different catalytic layers. Oxidative dehydrogenation (ODH) of butane takes place in the first layer, which contains a V/MgO catalyst, consuming most or all of the oxygen in the feed. The product stream of the first reaction then enters the second catalytic layer (Pt-Sn/γ-Al2O3), where nonoxidative dehydrogenation (DH) takes place. It is expected that the non-oxidative dehydrogenation step uses the heat generated in the first reaction (thermal coupling), the unreacted butane and the butene formed (both of which would then dehydrogenate non-oxidatively), and the CO2 and steam that are the products (unwanted and wanted, respectively) of the oxidative dehydrogenation stage. Both CO2 and steam are expected to act effectively as inhibitors of coking in the Pt-Sn/γ-Al2O3 catalytic layer. Experimental Section Membrane Preparation and Characterization. The first layer was prepared by filling most of the volume in the pores of a commercial microfiltration membrane (SCT, R-Al2O3, pores ranging from 200 nm in the separation layer to 12 µm in the support) with a

catalyst containing V supported on MgO (hereinafter termed VMgO catalyst). After this, a thin second layer containing Pt and Sn supported on alumina (termed PtSnAl catalyst) was deposited via sol-gel procedures as described below. To create the VMgO-containing layer, MgO was first deposited inside the pores of the support. To this end, the tube was impregnated at room temperature with a saturated solution of Mg(NO3)2 in water. The membrane was dried at 110 °C and then calcined at 700 °C. This procedure was repeated until the desired weight increase was obtained. To impregnate vanadium, the tube was immersed in a saturated solution of NH4VO3 for 30 min. After impregnation, the tube was dried at 110 °C and calcined at 700 °C. This procedure was repeated until a V2O5 load of between 10 and 25% (relative to the weight of VMgO catalyst) was obtained. The actual values of the V concentration are shown in Table 1. To prepare the PtSnAl sol, a bohemite sol was first prepared by dispersing hydrated aluminum hydroxide as pseudo-boehmite (CONDEA Chemie) in deionized water at 80 °C under constant stirring, to give a stable sol containing 10% of Al2O3 by weight. A solution of poly(vinyl alcohol) (PVA) was prepared by dissolving 3.5 g of PVA in 100 mL of deionized water to which 0.3 mL of 15.5 M HNO3 was added. The PVA solution was kept under reflux for 3.5 h. After being cooled to room temperature, 10 wt % of the PVA solution (referred to the weight of Al2O3) was added to the alumina sol. In parallel, aqueous solutions of Pt (0.11 M) and Sn (0.27 M) were prepared by dissolving, respectively, H2PtCl6 in deionized water and SnCl2 in an aqueous solution containing 3 wt % of nitric acid. The appropriate amounts of both solutions were added to the aluminaPVA sol to give a concentration (by weight, referred to Al2O3) of 0.5% Pt and 0.3% Sn, i.e., a Pt/Sn atomic ratio of 1:1. This was the ratio used by Szegner,17 who found that this composition yielded catalysts with optimum stability and selectivity. ICP (inductive plasma coupling) analysis of the sol particles yielded 0.5 and 0.25 wt % of Pt and Sn, respectively, which is very close to the desired composition. The PtSnAl sol was then slip-casted onto the membranes. According to the scheme in Figure 1, the goal was to have two well-defined zones across the membrane radius. This implies that the penetration of the PtSnAl sol into the oxidative dehydrogenation layer must be minimized. Otherwise, in the presence of oxygen, PtSnAl would act as an effective combustion catalyst, giving rise to a low selectivity to dehydrogenation products. To avoid or at least minimize the penetration of the PtSnAl sol into the outer layer of Figure 1, two procedures were explored: (i) block the membrane pores by filling them with liquid (water or n-hexane) prior to slip-casting and (ii) use PtSnAl sols of higher viscosity, which were obtained by aging the sol before slip-casting. In both cases, the casting of the PtSnAl sol was carried out by plugging one end of the tubular membrane, filling the inside with the sol, and emptying it immediately, to reduce the time for penetration of the PtSnAl sol into the VMgO layer. After predrying, the process was repeated with the membrane in an inverted position, to achieve a more homogeneous loading. The membrane was predried again and pretreated before reaction. Two alternative predrying procedures were used. In method 1, a wet air stream at room temperature was passed through the membrane

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Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001

Table 1. Characteristics of the Membranes Used in This Work membrane

VMgO load (% of V)a

AlSnPt load

blocking liquid

AlSnPt sol

drying/pretreatment procedure

0.9 g (12) 1.1 g (25) 1g (15)

0.17 g

n-hexane

aged

2/1

0.20 g

water

fresh

1/1

0.21 g

n-hexane

aged

2/2

M1 M2 M3

Q1,b Q2c Q1 ) 4.2 + 5Pav Q2 ) 5.2 + 0.4Pav Q1 ) 3 + 3Pav Q2 ) 1.5 + 2.5Pav Q1 ) 3.5 + 2.5Pav Q2 ) 2.4 + 0.1Pav

a Expressed as V O , referred to the total weight of the VMgO catalyst. b N permeation flux at 580 °C (in mL min-1 bar-1 cm-2) after 2 5 2 the membrane is loaded with VMgO. c N2 permeation flux at 580 °C (in mL min-1 bar-1 cm-2) after the membrane is loaded with VMgO and PtSnAl.

Table 2. Characteristics of the Pretreatment Methods method 1 temperature room temp to 200 °C at 200 °C 200 to 600 °C at 600 °C at 600 °C at 600 °C 600 °C to room temp

time (h) 3 3 10 5 0.25 5 5

method 2 gasa

temperature

N2

room temp to 200 °C at 200 °C 200 to 350 °C at 350 °C at 350 °C at 350 °C 350 to 600 °C 600 °C to room temp

N2 N2 O2 N2 H2 N2

time (h)

gasa

3

N2

3 3.5 5 0.25 5 6 5

N2 N2 O2 N2 H2 N2 N2

a Gas passed through the inside (PtSnAl side) of the tubular membrane.

tube (inner) side for 12 h, to favor the drying of the outer (VMgO) layer. In method 2, the outside of the membrane was wrapped with Teflon tape, and a dry air stream at room temperature was passed through the inside (PtSnAl side) of the membrane. Two methods of pretreatment were also employed, which are given in Table 2. The membranes were characterized by scanning electron microscopy (SEM) and by permeation measurements. SEM measurements (JEOL JSM 6400) were carried out on transversal cuts on the membrane tube, where the radial distribution catalytic material could be determined by electron-probe microanalysis (EPMA). The permeation characteristics of the membrane were determined by measuring nitrogen permeation rates at the reaction temperature (580 °C) before and after the PtSnAl layer was deposited. As in previous works,11 the permeation flux was fit to a function of the average pressure in the membrane according to

r r2 F ) 1.06 + 0.125 P ) R + βPav LτµRT av LτxMRT where F is the permeation flux normalized per unit of time, area, and pressure difference (mL min-1 bar-1 cm-2); Pav is the average pressure across the membrane (bar); µ is the viscosity; T is the absolute temperature; and R is the universal gas constant. L, , τ, and r are the membrane thickness, porosity, tortuosity, and pore radius, respectively. R and β indicate, respectively, the Knudsen and laminar contributions to the permeation flux. A good linear fit of F vs Pav was obtained for the three membranes tested in this work. Reaction Experiments. The reaction system has been described elsewhere.15 The main difference is the feed arrangement used, as a flow-through configuration was employed in the present work. A premixed, massflow-controlled feed stream containing butane, oxygen, and He was fed to the outer (VMgO) side of the membrane, as indicated in Figure 1. In this first layer, oxidative dehydrogenation of butane took place, ideally

consuming all of the oxygen feed and generating dehydrogenation products, carbon oxides, steam, and heat in the process. The reacting stream then entered the second (PtSnAl) layer, where non-oxidative catalytic dehydrogenation took place. The catalytic membranes described above were placed inside a stainless steel outer shell, using graphite gaskets, and the ensemble was heated by means of a two-zone furnace. An on-line gas chromatograph (HP 5890) analyzed the exit stream. The main products were butene isomers, butadiene, carbon monoxide, and carbon dioxide; the exit gases were also analyzed for unreacted butane and oxygen and for propane, propene, ethane, ethylene, and methane. Carbon mass balance closures were always better than 5% and usually better than 3%. Preliminary experiments were also run with the PtSnAl sol in powder form to determine its activity and selectivity under reaction conditions. These experiments were run in a quartz microreactor loaded with 1 g of PtSnAl catalyst, which was obtained from the abovedescribed PtSnAl sol using pretreatment 2. The catalyst had a moderate activity: When a feed containing 40% butane in He at 580 °C was passed over the catalyst at 15 mL (STP)/min, 11% conversion was obtained, with 70.4% selectivity to butenes and 20.2% to butadiene, other products being propene, ethylene, and methane. Results and Discussion Membranes M1and M2. The permeation data given in Table 1 show different qualitative and quantitative changes of the permeation fluxes upon casting the PtSnAl sol upon membranes M1 and M2. As could be predicted, in both cases, the formation of the PtSnAl layer results in a reduction of the total permeation flux; also as expected, a strong increase in the Knudsen contribution (from 46 to 96% when the average pressure across the membrane is 1 bar) is observed for membrane M1. However, in membrane M2, the relative proportion of the laminar contribution increases, which indicates the development of cracks on the membrane. On a first analysis, these cracks could be the result of mechanical stresses due to oxidation and reduction treatments at elevated temperature (600 °C, see Table 2). However, membrane M1 was subjected to the same treatments, and in this case, it is the relative Knudsen contribution that increases after deposition of the PtSnAl sol. On the other hand, it must also be noticed that membrane M2 was the only one using water as the blocking liquid, and the drying procedure used (method 1) keeps most of the moisture in contact with the PtSnAl layer. Because water has a large heat capacity and latent heat of evaporation compared to n-hexane, during drying at 200 °C, pronounced temperature gradients are possible,


Figure 3. Butane and oxygen conversions and selectivities to butene, butadiene, and carbon oxides as a function of the C4H10/ O2 ratio. Membrane M2, 580 °C, 15 mL (STP)/min.


giving rise to thermal stress and possibly to the development of cracks. Figures 2 and 3 show the reaction results obtained with membranes M1 and M2, respectively, comparing the performance before (single-layer membrane) and after (two-layer membrane) the incorporation of the PtSnAl layer. Unexpectedly, with membrane M1 (Figure 2), the conversions of butane and oxygen decrease considerably (7-8%) for the two-layered membrane throughout the interval explored. At the same time, an increase in the selectivity is only observed at C4H10/O2 feed ratios of 1.5 and above. The decrease in conversion can probably be attributed to the severity of the reduction treatment used to activate the PtSnAl catalyst (reduction in H2 at 600 °C). Blasco and co-workers,18 in temperature-programmed-reduction experiments with V/MgO catalyst, found that two reduction peaks appeared, at 350 and 550-600 °C, with a total oxygen consumption corresponding to the reduction of V+5 to V+3. Therefore, after the reduction pretreatment used with membrane M1, a significant proportion of the highly active V+5 will very likely be reduced to V3+, which explains the decrease in catalytic activity observed for the two-layer membrane. In addition, at the flow rates used for the experiments shown in Figure 2,

the amount of unconverted oxygen is substantial (between 15 and 25%), i.e., oxygen breaks through the VMgO layer into the PtSnAl layer, which is detrimental to selectivity. Consequently, at C4H10/O2 ratios of 0.75 and 1, the selectivity obtained with the two-layer membrane is lower than that obtained with the singlelayer (VMgO) membrane because of the combustion of butane and hydrocarbon products that is taking place in the PtSnAl layer. At high values of the C4H10/O2 ratio only, the trend is reversed, and the selectivity obtained with the two layer membrane is higher. Under these conditions, the hydrocarbon-to-oxygen ratio in the PtSnAl layer is high (e.g., for a C4H10/O2 feed ratio of 2, using the conversion and selectivity data given in Figure 2, the total hydrocarbon-to-oxygen ratio for the product stream leaving the PtSnAl layer would be 7.6; this ratio increases to 17.3 when the C4H10/O2 feed ratio is 4). The reaction results obtained with membrane M2 are shown in Figure 3. In this case, a lower feed flow rate was used, and it can be seen that, before the introduction of the PtSnAl layer (i.e., when the membrane contained only V/MgO as a catalyst), the conversion of oxygen was essentially complete (98%) at any of the feed concentrations employed. The selectivities to butene and butadiene follow reverse trends, indicating that butadiene is mainly formed from butene. As the C4H10/O2 ratio increases from 0.75 to 2.7, the total selectivity to C4 dehydrogenation products increases from 33 to 40%; at the same time, the butane conversion decreases from ca. 40 to 25%, which causes the yield to C4 products to decrease. All of these results are consistent with the trends normally observed in conventional (e.g., in a fixed-bed reactor) oxidative dehydrogenation of butane.

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Figure 5. Variation of the Pt/Al and V/Mg atomic ratios along the membrane radius. Membrane M2.

Figure 4. SEM micrograph of a transversal cut of membrane M2. The “support”, “intermediate”, and “separation” layers of the original microfiltration membrane, with pore sizes decreasing from 12 µm to 200 nm, can be seen. These pores were partly filled with V/MgO catalyst. The PtSnAl layer can be seen on top of the original “separation” layer.

The introduction of the PtSnAl layer increases the conversion of oxygen slightly (by 1%) and the conversion of butane by a larger margin (up to 10%). At the same time, the selectivities to the desired products (butene and butadiene) decrease, and the selectivity to carbon oxides increases. Given the results just discussed, lower butane and oxygen conversions could be expected after the introduction of the PtSnAl layer, as the reduction treatment is the same for membranes M1 and M2. However, in the case of membrane M2, other factors come into play. Thus, unlike membranes M1 and M3, the pore-filling liquid is water, and a fresh PtSnAl sol was used. The presence of water in the pores facilitates the penetration of the PtSnAl sol and the dissolution of Pt and Sn components. Penetration into the pores is also aided by the much lower viscosity of the freshly prepared sol. Thus, it seems likely that penetration of Pt and Sn components into the oxidative dehydrogenation (VMgO) layer has occurred. To test this hypothesis, after the experiments that have been described were performed, membrane M2 was cut transversally and observed by SEM. In Figure 4, it is apparent that the thickness of the PtSnAl layer is irregular. In some cases (not shown), the layer was absent at places, probably having peeled off during use. The Pt/Al, Sn/Al, and V/Mg radial atomic profiles determined by EPMA are shown in Figure 5. With some scatter, the V/Mg ratio stays around 1.4 throughout the membrane radius, which is markedly above the stoichiometric value for this catalyst (0.15). This reflects the fact that the V salt was impregnated after deposition of MgO and, thus, vanadium partly covers the MgO particles. The Pt/Al ratio rapidly decreases, from a high atomic ratio of 0.3% close to the inner surface to 0 after ca. 500 µm, and the evolution of the Sn/Al ratio is very similar. This means that penetration of the PtSnAl sol into the oxidative dehydrogenation layer has not been avoided; on the contrary, Pt and Sn deposits extend to about one-third of the total membrane thickness. The high values of the Pt/Al and Sn/Al ratios at the inner

membrane surface of the membrane (between 2 and 3 times the stoichiometric bulk value for the sol) indicate that Pt and Sn are preferentially deposited on the outside of the γ-Al2O3 particles. The EPMA results just presented explain the increase in conversion observed upon casting the PtSnAl sol. Because of the penetration of the sol inside the oxidative dehydrogenation layer, Pt and Sn will be in contact with a gas stream containing hydrocarbons and oxygen, leading to a higher yield to combustion products. In addition to a higher selectivity to CO and CO2, the presence of the acid alumina gives rise to an increase in cracking products (C3, C2, and methane; not shown in the selectivity graph), which make up the rest of the product distribution. As the C4H10/O2 ratio increases, there is less oxygen available in the membrane zone penetrated by the PtSnAl sol, and the reaction results obtained before and after deposition of the PtSnAl layer converge (Figure 3). Membrane M3. The results obtained with membranes M1 and M2 have highlighted several problems: the appearance of cracks in the membrane depending on the procedure used to fill the membrane pores with liquid and for subsequent drying, deactivation of the VMgO catalyst due to the activation method employed for PtSnAl, and loss of selectivity due to penetration of the PtSnAl sol into the oxidative dehydrogenation layer. Therefore, a new membrane (M3) was synthesized taking into account all of these issues: n-hexane was used as the pore-blocking liquid, an aged PtSnAl sol with a high viscosity was used for slip-casting, and the temperature for activation of PtSnAl was reduced to 350 °C (pretreatment procedure 2). In addition, the PtSnAl sol was slip-casted twice, to obtain a higher loading of dehydrogenation catalyst. The permeation results of Table 1 show that the total permeation flux is reduced after deposition of PtSnAl and that the Knudsen contribution strongly increases (58/96% before/after deposition of PtSnAl). This indicates that a relatively homogeneous PtSnAl layer has been formed on the membrane, without major layerwide cracks. The SEM micrograph in Figure 6 shows a transversal cut of membrane M3. A good-quality first layer of PtSnAl (thickness of 15-17 µm) was formed, which was probably responsible for the change in permeation properties just discussed. The adherence of the second PtSnAl layer (thickness of ∼10 µm) is poor, and cracks are clearly visible. EPMA measurements of the Pt and Sn radial profiles (Figure 7) indicate that the preparation procedure was effective in preventing the penetration of the sol inside


Figure 6. SEM micrograph of a transversal cut of membrane M3. See caption of Figure 4.


Figure 7. Variation of the Pt/Al, Sn/Al, and V/Mg atomic ratios along the membrane radius. Membrane M3.

the VMgO layer. The atom percentages of both Pt and Sn quickly decrease, reaching 0 after 30 µm, in good agreement with the SEM observations regarding the thickness of the PtSnAl layers. This is reflected in the reaction results given in Figure 8. The conversion of oxygen is now essentially complete (98.5-99.5%), and it is approximately the same before or after the introduction of the PtSnAl layer, indicating that (except at low values of the C4H10/O2 ratio) only non-oxidative dehydrogenation takes place on the PtSnAl layer. The presence of the PtSnAl layer increases the conversion of butane by between 2 and 13%, while the selectivity clearly increases, except at C4H10/O2 feed ratios equal to 1 and lower, where a small decrease is observed. This is the expected result, since, at low values of the C4H10/ O2 feed ratio, it is expected that some oxygen (around 1.5% according to the results in Figure 8) will break through the VMgO layer, giving rise to a small increase in the selectivity to combustion products. As shown in Figure 8, at higher values of the C4H10/O2 feed ratio, the conversion of oxygen is unchanged by the PtSnAl layer, but the butane conversion and the selectivity to dehydrogenation products increase. This is especially significant, as a trade-off between conversion and selectivity normally exists in oxidative dehydrogenation reactions. The fact that both increase simultaneously

Figure 9. Evolution of conversion and selectivity with time on stream. Membrane M3, 580 °C, 15 mL (STP)/min.

indicates that non-oxidative dehydrogenation is taking place in the PtSnAl layer. Finally, extended experiments were also carried out to determine the ability of the membrane to sustain stable values of conversion and selectivity. In a conventional reactor, the PtSnAl catalyst is expected to deactivate under conditions of non-oxidative dehydrogenation, as coke builds up on the active sites of the catalyst. However, as indicated above, the flow-through membrane used is expected to reduce coke formation through the presence of steam and CO2 in the dehydrogenation layer. Figure 9 shows that, in the two-layer membrane, some deactivation is initially observed, with the conversion dropping from ca. 19.5 to 17.5% while the selectivity increases from ca. 33 to 38%. An analysis of the product distribution (not shown) indicates that the increase in selectivity to dehydrogenation products is mainly due to a reduction of the cracking reactions. These are expected to take place mainly on the most acid sites of the catalyst used (γ-Al2O3), which are rapidly deacti-

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vated. In any case, after about 3 h on-stream, the behavior of the catalyst remains stable (both in conversion and selectivity), indicating that further coking is effectively inhibited in the flow-through configuration used. Conclusions The two-layer membrane used in this work is capable of effectively coupling oxidative and non-oxidative dehydrogenation reactions taking place consecutively on VMgO and PtSnAl catalytic layers. Several conditions hold the key to the success of the two-layer membrane: (i) Good segregation of the catalysts (VMgO and PtSnAl) in the two layers must be obtained. This can be achieved by filling the pores of the membrane with a suitable liquid before slip-casting the PtSnAl sol and by adjustment of the sol viscosity. (ii) The absence of interference between the preparation/activation procedures of both catalytic layers must be ensured by selection of the pretreatment conditions. (iii) Essentially complete oxygen consumption must be achieved in the oxidative dehydrogenation layer, as oxygen breakthrough into the PtSnAl layer produces a decrease of selectivity; this entails adjustment of the operating conditions (feed flow rate, temperature) and/or the catalyst loading in the oxidative dehydrogenation layer. When these requisites are fulfilled, the two-layer membrane is capable of increasing simultaneously the conversion and the selectivity with respect to the results attained in the VMgO single-layer membrane. Additional advantages of the flow-through operation mode with the double-layer membrane concern the achievement of stable operation by reduction or avoidance of coking in the PtSnAl layer and the thermal coupling of the oxidative dehydrogenation (exothermic) and dehydrogenation (endothermic) processes, which take place in close physical proximity in the system investigated. Literature Cited (1) Dixon, A. G. Innovations in Catalytic Inorganic Membrane Reactors. Catalysis 1999, 14, 40-92. (2) Coronas, J.; Santamarıá, J. Catalytic reactors based on porous ceramic membranes. Catal. Today 1999, 51, 377-389. (3) Gryaznov, V. M.; Verdernikov, V. I.; Gul′yanova, S. G. Hydrogen permeable palladium membrane catalysts. Kinet. Catal. 1986, 26, 129. (4) Itoh, N. Simultaneous operation of reaction and separation by a membrane reactor. Stud. Surf. Sci. Catal. 1990, 54 (3-4), 268-283.

(5) Gobina, E.; Hughes, R. Reaction coupling in catalytic membrane reactors. Chem. Eng. Sci. 1996, 51, 3045. (6) Itoh, N.; Wu, T. H. An adiabatic type of palladium membrane reactor for coupling endothermic reactions. J. Membr. Sci. 1997, 124, 213-222. (7) Saracco, G.; Specchia, V. Catalytic Ceramic Filters for FlueGas Cleaning. 2. Catalytic Performance and Modeling Thereof. Ind. Eng. Chem. Res. 1995, 34, 1480-87. (8) Saracco, G.; Specchia, S.; Specchia, V. Catalytically modified fly ash filters for NOx reduction with NH3. Chem. Eng. Sci. 1996, 51, 5289-97. (9) Pina, M. P.; Meneńdez, M.; Santamarıá, J. The KnudsenDiffusion Catalytic Membrane Reactor: An Efficient Contactor for the Combustion of Volatile Organic Compounds. Appl. Catal. B: Environ. 1996, 11, L19-27. (10) Pina, M. P.; Irusta, S.; Meneńdez, M.; Santamarıá, J.; Hughes, R.; Boag, N. Combustion of Volatile Organic Compounds over Pt-Based Catalytic Membrane Reactors. Ind. Eng. Chem. Res. 1997, 36, 4557-66. (11) Irusta, S.; Pina, M. P.; Meneńdez, M.; Santamarıá, J. Development and application of perovskite-based catalytic membrane reactors. Catal. Lett. 1998, 54, 69-78. (12) Binkerd, C. R.; Ma, Y. H.; Moser, W. R.; Dixon, A.G. An Experimental Study of the Oxidative Coupling of Methane in Porous Ceramic Radial-Flow Catalytic Membrane Reactors; Paper presented at the 4th International Conference on Inorganic Membranes, ICIM4-96, Gatlinburg, TN, 1996; pp 441-450. (13) Harold, M. P.; Lee, C. Intermediate product yield enhancement with a catalytic inorganic membrane. II. Nonisothermal and integral operation in a back-mixed reactor. Chem. Eng. Sci. 1997, 52, 1923-40. (14) Tsai, C. Y.; Ma, Y. H.; Moser, W. R.; Dixon, A. G. Modeling and simulation of a nonisothermal catalytic membrane reactor. Chem. Eng. Commun. 1995, 134, 107-32. (15) Alfonso, M. J.; Julbe, A.; Farruseng, D.; Meneńdez, M.; Santamarıá, J. Oxidative dehydrogenation of propane on V/Al2O3 catalytic membranes. Effect of the type of membrane and reactant feed configuration. Chem. Eng. Sci. 1999, 54, 1265-72. (16) Yeung, K. L.; Aravind, R.; Zawada, R. J. X.; Szegner, J.; Cao, G.; Varma, A. Nonuniform catalyst distribution for inorganic membrane reactors: Theoretical considerations and preparation techniques. Chem. Eng. Sci. 1994, 49, 4823-38. (17) Szegner, J. Effects of nonuniform catalyst distribution on inorganic membrane reactor performance: experiments and model. Ph. D Thesis, University of Notre Dame, South Bend, IN, 1997. (18) Blasco, T.; Lo´pez Nieto, J. M.; Dejoz, A.; Vazquez, M. I. Influence of the acid-base character of supported vanadium catalysts on their catalytic properties for the oxidative dehydrogenation of n-butane. J. Catal. 1995, 157, 271-282.

Received for review July 6, 2000 Revised manuscript received November 28, 2000 Accepted December 1, 2000 IE000643N

Coupling of Consecutive Reactions in a Two-Layer, Flow-Through

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