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Dehydroisomerization of n-butane to isobutene in a chromatographic reactor
~ ELSEVIER
APPLIED CATALYSS I A:GENERAL
Applied Catalysis A: General 146 (1996) 131 - 143
Dehydroisomerization of n-butane to isobutene in a chromatographic reactor M.R. Sad, C.A. Querini, R.A. Comelli, N.S. F~goli, J.M. Parera * lnstituto de lnvestigaciones en Catdlisis y Petroqulmica - - 1NCAPE, F1Q, UNL - - CON1CET, Santiago del Estero 2654, 3000 Santa Fe, Argentina
Abstract The dehydroisomerization of n-butane to isobutene in only one step was studied in a chromatographic pulse reactor at atmospheric pressure and different temperatures and space velocities. Dehydrogenation, isomerization and alkene/hydrogen chromatographic separation functions were present. Cd-exchanged 4A and 13X zeolites were used as base materials able to separate alkenes from alkanes at high temperatures. They were loaded with Pt or Pt + Sn. Composite catalysts containing the three functions mentioned above were used as well. It was found that under chromatographic conditions, alkene amounts above 35 wt.-% were achieved at 380°C using a P t / 4 A C d catalyst, which are higher than the thermodynamic equilibrium values. The use of wider pore 13X zeolite as base material improves isobutene selectivity; the incorporation of NH~- or La 3+, which improves acidity, adversely affects the chromatographic effect. Composite catalysts have good performance: 80% alkenes selectivity at 35% total conversion and up to 22% isobutene selectivity were obtained at 380°C. The presence of isobutane in products could be ascribed to isobutene rehydrogenation. Keywords: n-Butane dehydroisomerization; Chromatographic pulse reactor; Exchanged zeolites; Composite
catalysts
1. Introduction Isobutene is an important raw material for the production of gasoline additives like MTBE and alkylates, both of them increasing the octane number without polluting effects. Isobutene is normally obtained from catalytic cracking of petroleum, but the amounts produced are not sufficient due to the continuous growth in additives production, n-Butane, from natural gas and LPG, is an * Corresponding author. Tel.: (+ 54-42) 533858; fax: (+54-42) 531068/550944. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 1 1 9 - 6
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131 143
132
abundant raw material; it can be transformed into isobutene by isomerization and dehydrogenation. The actual commercial processes include two plants, one for isomerization and the other for dehydrogenation, making the process not economically attractive. A process in which isobutene can be produced by the use of two catalysts arranged in sequence in a single reactor at 400-600°C, was recently patented [ 1]. Isomerization and dehydrogenation have quite different requirements. While the former is thermodynamically favored at low temperatures, dehydrogenation is favored at high temperatures. The convenient catalysts for the two reactions are different: metal supported catalysts for dehydrogenation and high acidic ones for isomerization. There is no change in the number of moles in isomerization, while there is a change in dehydrogenation. Dehydrogenation can be favored if the products are separated allowing the reaction to continue, thus overcoming the thermodynamic equilibrium. Membrane or chromatographic pulse reactors [2,3] can be useful to achieve such a separation. The concept of a chromatographic pulse reactor is essentially the following: in a catalytic reaction, reactant and products have different adsorptive behavior on the catalyst. Thus, if a long thin tube is filled with catalyst, and a carrier gas flows through it, it is expected that the reaction components will become separated as in a chromatographic column. If the reaction is limited by equilibrium, the chromatographic action should destroy equilibrium conditions by continuous separation of the products, so that the reverse reaction is impossible. After a sufficient length of time the reaction will be essentially complete. The chromatographic pulse reactor can be advantageous when: the equilibrium constant for the reaction is small; reaction rates are high enough so that separation of products rather than rate of reaction limits the extent of reaction; - - at least two products, which are chromatographically separated in the reactor, are formed; reactants are not separated in the reactor; this is limited to a single reactant, or two reactants where one also serves as the carrier gas. All these requirements are fulfilled by some dehydrogenation reactions. According to the literature [4] Cd exchanged zeolites are effective in the separation of alkenes from alkanes. They also have catalytic dehydrogenation activity at high temperature and were used in chromatographic conditions to achieve conversions beyond equilibrium in ethane dehydrogenation [5]. From the former considerations it should be convenient to carry out the simultaneous dehydrogenation and isomerization of n-butane under conditions in which both reactions are favored: isomerization, working at low temperatures and dehydrogenation by the chromatographic separation of the products. The objective of this paper is to search for materials which allow, in only one step, the transformation of n-butane to isobutene, overcoming the thermodynamic limitations using chromatographic pulse reactors. Dehydrogenating and -
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-
-
-
-
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
133
isomerizing functions as well as alkene/alkane chromatographic separation capacity should be present.
2. Experimental 2.1. Catalysts Zeolites Linde 4A and 13X, originally in 1/8" extrudates (ca. 20% binder clay content), grounded and sieved to obtain the 3 5 / 8 0 mesh fraction were used as base materials. Both zeolites were exchanged with C d 2+ by batchwise contacting with an excess of CdC12 o r C d S O 4 0.5 M solutions, as described elsewhere [4]. Three successive contacting steps, each one with the volume required for 100% exchange and lasting 24 h were performed at room temperature. Exchanged materials were filtered, washed with distilled water until no chloride or sulfate was detected in liquids, and dried at 120°C overnight. Chemical analysis of Cd 2+ in exchanging solutions revealed that more than 80% of total exchange capacity was achieved. The materials obtained were designated as 4ACd and 13XCd. 13XCdLa and 13XCdLaNH 4 were prepared at lower exchange levels (10% of exchange capacity for each cation) in one simultaneous exchange step, using the volumes of 0.5 M solutions of each CdCI 2 , La(NO3)3, and NH4C1 required for such exchange level, assuming total exchange. This was verified by chemical analysis of the exchanging solution. Pt loading of 4ACd, 13XCd, 13XCdLa and 13XCdLaNH 4 was made by impregnation with H2PtC16.6H20 aqueous solutions using the incipient wetness technique, to avoid C d 2+ losses. Pt deposition on 13X was achieved either by ion exchange with Pt(NH3)4C12 aqueous solution or by the impregnation procedure. Nominal Pt content of 4A and 13X catalysts was 2% and 1%, respectively. Pt(2%)Sn(2%)/4ACd catalyst was prepared by successive impregnation of 4ACd with HzPtC16.6H20 and SnC12 solutions. Other catalytic materials used w e r e S i O 2 / A I 2 0 3 (87% SiO2, 13% AI203 from Strem Chemicals) and Pt(l%)Sn(1%)/SiO 2 prepared by impregnation of silicagel (Alfa, 300 m 2 g-~), with a solution of a HzPtC16-SnC12 complex. All Pt containing catalysts were calcined in air at 500°C (3 h) to decompose the metallic precursor and reduced in flowing H 2 at 350°C (3 h) prior to use.
2.2. Reaction system and product analysis Chromatographic reactors were 1/8" O.D. stainless steel columns filled with 2 g of 35-80 mesh catalysts, their lengths ranging between 100 and 150 cm. They were coiled and had 15 cm of 3 5 / 8 0 mesh quartz packing at both ends; they were kept in an oven controlled to + I°C. Carrier gas (N 2 99.5% from
134
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
AGA) flow was controlled using the pneumatic system of a chromatograph (Shimadzu GC-8) and measured by a soap bubble flowmeter. Pulses of pure n-butane (99.5% from AGA) were introduced using a conventional six port sampling valve (Valco, loop volume 0.5 cm3), and the effluent from the reactor was either sent directly to the FID or collected over water in glass samplers (ca. 250 ml capacity) having a rubber septum. Direct injection to FID was used either to determine hydrocarbon retention times or to select the length of sample collection periods (pulse intervals). In the collecting mode, samples were injected to the chromatograph for detailed hydrocarbon analysis using a 7 m long, 1,/8" O.D. column packed with dimethylsulfolane on Chromosorb P, operated at 3 k g / c m 2 inlet pressure, at room temperature and using nitrogen as carrier gas. Complete resolution of all hydrocarbons from C] to C 4 was achieved; typically three pulses were collected together in each sampler. Complementary chromatographic analysis were made in the pulse coming out from the reactor to determine hydrogen and hydrocarbons composition at different emergence times. In this case, an additional sampling valve was installed downstream the reactor, and samples were analyzed in a chromatograph (SRI 8610) with TCD and FID, using the same column as before. Some conventional flow experiments were performed on 13X based catalysts by flowing pure n-butane at the same flowrates used for carrier in pulse experiments, in order to study deactivation effects or to compare n-butane conversions with those obtained in the chromatographic mode. Other flow experiments to assess the skeletal isomerization capacity of 13X based catalysts were also carried out using 1-butene (Matheson, 99.5%) as feed, at atmospheric pressure, 380°C and 0.1 h-~ WHSV in a microcatalytic reactor with on line GC analysis (Shimadzu 14APT), and using a 2GS-alumina plot GC column. TPR and TPO analysis of fresh and deactivated catalyst samples, respectively, were performed in an specially designed equipment described elsewhere [6].
3. Results and discussion
3.1. Equilibrium calculations Fig. 1 shows the equilibrium product distribution for n-butane dehydrogenation calculated from available thermodynamic data [7]. Hydrogen is not included, the percentages being referred only to the hydrocarbon fraction. The C j - C 3 hydrocarbons were not considered to better reflect dehydrogenation equilibrium. The total amount of monoalkenes is about 10% at 427°C, while butadiene can be obtained only above 527°C (Fig. 1A). The product distribution within the monoalkenes fraction is shown in Fig. lB. The lower the temperature, the higher the isobutene fraction. About 50% of the monoalkenes fraction corresponds to isobutene at 427°C.
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
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Fig. 1. Equilibrium values corresponding to n-butane dehydrogenation. P = 1 atm; H 2 / n C 4 = 0; N 2 / n C 4 = 0. (A) Product distribution. (B) Alkenes distribution in the C 4 monoalkenes fraction, iC4: isobutene: t2C4: trans-2-butene: c2C~ : cis-2-butene; 1C~ : l-butene.
3.2. Zeolite based catalysts A reactor containing 2 g of 4A zeolite, was used to measure the retention times of n-butane and 1-butene. In Table 1 it can be seen that retention times are very small, in the same order of residence time (estimated from column void space to be ca. 0.09 min at 15 ml min-1). The difference among n-butane and 1-butene retention times is less than 15%. Table 1 also shows retention times in a reactor containing 2 g of 4ACd. In this case l-butene has a much higher retention time than n-butane, the difference being significant even at temperatures as high as 390°C. Metallic cadmium has a high vapor pressure and some references indicate that it can be lost in reducing atmospheres at high tempera-
Table 1 Chromatographic effect of different materials. Carrier flow rate: 15 ml min- ] Material
T (°C)
4A 4ACd
350 300 350 390 300 350 380 380 380 380 380
13X
13XCd Pt(imp)/13X Pt(exc)/13X Pt(imp)/13XCdLaNH 4 Not affected by reduction at 500°C.
Retention time (min) n-butane
1-butene
0.14 2.19 a 1.29 a 0.97 a 1.58 1.10 0.66 0.45 0.64 0.96 0.64
0.17 20.94 10.02 6.47 4.10 2.00 1.50 18.00 0.71 1.81 0.75
136
M.R. Sad et al. /Applied Catalysis A: General 146 (1996) 131-143
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tures [8]. Therefore the influence of high temperature reduction in retention capacity of the 4ACd was studied. It was found that retention times are not modified, even when reduction is carried out at 500°C. Fig. 2 shows the TPR of 4A, 4ACd and P t / 4 A C d . 4A zeolite does not show any reduction peak below 520°C, while 4ACd has two reduction peaks, at 400 and 550°C, that could be due to Cd 2+ at different positions in the zeolite framework [9]. Two consecutive TPR analysis on the same sample with intermediate reoxidation are shown in Fig. 2 (lst TPR and 2nd TPR). Peak locations are not modified, but peak areas are lower for the second TPR, and some metallic Cd deposition was observed on the TPR reactor walls. In the case of P t / 4 A C d , Cd 2+ reduction temperature is lower (shoulder at 280°C) than in 4ACd due to the presence of Pt. The peak at 100°C corresponds to Pt reduction. These results indicate that, although retention times for 4ACd are not modified by a 2 h reduction; at very high temperatures P t / 4 A C d could lose its retention capacity in reducing atmospheres. In order to avoid this problem, the dehydrogenation reaction should be carried out at temperatures lower than 450°C when using this catalyst. Fig. 3 shows the weight percentage of each C 4 alkene in the hydrocarbon fraction obtained using a chromatographic reactor loaded with P t / 4 A C d catalyst at 380°C as a function of carrier flow rate. The total amount of alkenes is above 35% when using 15 ml rain ~ carrier flow rate, which is well above the thermodynamic equilibrium. The highest isobutene percentage obtained with this catalyst was 8% at 10 ml min I cartier flow rate. After the catalyst was used under a continuous flow of n-butane, the amount of alkenes produced decreased to 10%, without isobutene production. It must be noted that n-butane conversions obtained in normal flow operating mode were below 5% (i.e. 4.7% at 380°C and 15 ml n C 4 min-~). After the catalyst is regenerated by burning off coke at 380°C, alkene production capacity is partially
M.R. Sad et al. / Applied Catalysis A: General 146 (1996) 131-143 n C 4 Retention times: 3.2 1.71 0.80
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recovered. The amount of alkenes depends on the cartier flow rate. Fig. 3 shows that as the carrier flow rate increases, the amount of alkenes goes through a maximum. This is because at very low flow rates alkenes are either cracked to lower molecular weight products or oligomerized and the oligomers cracked into fragments. As residence time decreases, selectivity to alkenes increases. But at higher flow rates, such as 30 ml min-1, the amount of alkenes decreases because residence time is not enough to achieve a high conversion. Isobutene production is not fully recovered after regeneration. In the upper part of Fig. 3 n-butane retention times are shown. When the catalyst is deactivated its retention capacity decreases, which is partially recovered after regeneration Fig. 4 shows alkene composition ratios that correspond to results presented in Fig. 3. The equilibrium ratios are 1 C 4 / i C 4 = 0.18, 1 C 4 / t 2 C 4 = 0.39 and 1 C 2 / c 2 C 4 =0.61 at 380°C. It can be seen that C 4 alkenes are not in thermodynamic equilibrium (1-butene is in excess with respect to the other alkenes), which is not surprising taking into account that the acidity brought by Cd 2+ exchange may be inadequate or insufficient. But linear alkenes are nearer equilibrium than iC 4 in all cases, independently of flow rates or if the catalyst is coked or regenerated. 1-Butene/isobutene ratio is lower (nearer to equilibrium value) at the initial state of the catalyst. Pore openings of 4ACd zeolite are expected to be wider (similar to the 5A calcium exchanged zeolite) than the original sodic 4A form, but they are nevertheless too small. Branched molecules like isobutene that could be produced at internal acid sites would have little chance to get out to the gas phase and the production of this alkene is certainly due to external acid sites. These acid sites are the first to be deactivated. As the
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
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catalyst deactivates, there are not enough acid sites to equilibrate the isobutene with linear C 4 alkenes; this is the reason why after flow experiments isobutene production strongly decreases (Fig. 3). The carrier flow rate also affects isobutene production, the higher the flow rate, the lower the amount and the fraction of isobutene in the monoalkenes fraction. Fig. 5 shows the TPO of the catalyst, after being used with a continuous flow of n-butane during 1.5 h; it indicates that 380°C is not enough to remove all the coke from this catalyst, and this may be the reason of the lower retention times obtained with the regenerated catalyst. The amount of carbon on the coked catalyst was 1.47 wt.-%. Fig. 6 shows the composition along the pulse coming out of the chromato-
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graphic reactor with P t / 4 A C d . Hydrogen shows a maximum concentration (measured as hydrogen peak height in TCD analysis) at the front of the pulse and an slow release afterwards. The tail of hydrogen elution peak overlaps with the alkenes and some rehydrogenation of them occurs, as indicated by the presence of isobutane (up to 12 wt.-%) in the products. Hydrocarbon compositions plotted are mass percentages in the hydrocarbon fraction as detected by FID. C 4 alkenes show a maximum at 3 rain after the pulse started coming out of the reactor, n-Butane concentration decreases continuously from a sharp maximum at the beginning. After 6 min, the main products are C1-C 3 hydrocarbons, that come from the cracking of the alkenes a n d / o r from the oligomers formed from alkenes that were retained inside the reactor. Since the total amount of hydrocarbons that comes out of the reactor at this time is small, the contribution of this light compounds to the selectivity in the pulse is not important.
3.3. 13X Zeolite based catalysts In order to increase the surface area accessible for isobutene formation through wider pore openings, 13X zeolite was used as base material. Retention
140
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
Table 2 n-Butane conversion (X), monoalkenes selectivity (Smo,1), C 4 alkenes ratios (R) and isobutene selectivity (Sic,). Carrier flow rate: 15 ml rain t Material
13X 13XCd Pt(imp)/13XCd Pt(exc)/13XCd
Pt/13XCdLa Pt/13XCdLaNH 4 PtSn/4ACd + SiO 2/A1203 PtSn/SiO 2 + 4 A C d + SiO 2 / A I 2 0 3
T (°C) X (%) S ...... ~ (%) R
380 380 420 380 350 380 420 380 420 380 420 380 420 380
0 3.8 12.0 38.8 31.5 45.3 58.8 22.5 36.0 25.4 35.7 23.7 32.3 35.0
71.1 50.7 56.4 84.6 76.5 60.8 72.5 64.7 65.3 57.7 74.9 56.1 80.5
Sic4 b (%)
IC4=/iC4
1C4 / t 2 C 4 -
1C4=/c2C4
0.97 1.49 0.83 4.15 2.9t 1.80 2.61 1.55 1.70 1.10 0.55 0.55 0.77
0.67 0.58 0.52 0.48 0.54 0.60 0.49 0.58 0.54 0.60 0.51 0.59 0.52
0.96 0.82 0.78 0.71 0.77 0.84 0.72 0.83 0.78 0.86 0.75 0.83 0.76
15.6 7.4 12.5 8.9 5.8 7.7 5.8 9.1 8.1 11.1 22.4 17.8 18.9
a % butenes produced/% n-butane transformed. b % isobutene produced/% n-butane transformed.
times of n-butane and l-butene for these materials can be seen in Table 1. Table 2 shows n-butane conversion and monoalkenes selectivity for unmodified and modified 13X zeolite, at different temperatures and 15 ml min-J carrier flow rate. The original sodic form of 13X does not convert n-butane, while 13XCd, which has an important retention capacity for alkenes, produces a low n-butane conversion. The incorporation of Pt improves the catalytic behavior, showing higher both n-butane conversion and monoalkenes selectivity. Pt addition by ion exchange allows to obtain a material with somewhat better conversion and C 4 alkenes selectivity levels than those obtained with the Pt containing catalyst prepared by the incipient wetness technique. Better isobutene selectivity and 1 C 4 / i C 4 ratio closer to equilibrium for the impregnated catalyst may be ascribed to a higher acidity due to the preparation method. As butene distribution was far from isomerization equilibrium, some modification of catalyst acidity was attempted by La 3+ or NH~- exchange. Exchanged zeolites were separately tested for 1-butene isomerization in a microcatalytic reactor and results obtained are shown in Table 3. An improvement with respect to 13XCd was achieved. As it can be seen in Table 2, the addition of La 3+ and NH~-, which was expected to improve the acidity, does not enhance the catalytic performance. This might be due to the poor chromatographic effect of these materials (see Table 1). In all cases, n-butane total conversion increases with temperature while monoalkenes selectivity decreases. Regarding isomerization activity, Table 2
M.R. Sad et al. /Applied Catalysis A: General 146 (1996) 131-143
141
Table 3 Selectivities for l-butene skeletal isomerization (microcatalytic reactor, 380°C, WHSV = 0.1 h i ) Catalyst
Sic (mol iC 4 = / m o l nC 4 converted)
13X 13XCd 13XCdLaNH 4 13XCdLa
0.01 0.03 0.14 0.10
also shows butenes ratios: the linear butenes ratios are nearer equilibrium values than the 1-butene/isobutene ratio since double bond isomerization is a much easier reaction than skeletal isomerization.
3.4. Composite catalysts In order to avoid the adverse effect of the incorporation of acidity modifiers on the zeolite capacity to selectively adsorb alkenes, two chromatographic reactors, using mechanical mixtures of the materials, were made. One of them was loaded with S i O J A I 2 0 3 and PtSn/4ACd (1 g each), thus separating the acid component from the dehydrogenating/adsorbing material. The other one was loaded with SiO2/AI203, PtSn/SiO 2, and 4ACd (1 g each), thus separating the three functions, namely acid, dehydrogenating and chromatographic ones. Some typical results are shown in Table 2. Total conversions are somewhat lower than those obtained with some of the catalysts previously mentioned, but selectivities to monoalkenes (except at the higher temperatures) were high. Conversions lower than those obtained with P t / 4 A C d (Fig. 3) may be due to the lower amount of Pt catalyst used and also to a lower dehydrogenation activity of the PtSn metallic function. The selectivity to isobutene is also better, reflecting the skeletal isomerizing effect of SiO2/AI203. The incorporation of Sn as a Pt modifier, improves the selectivity to C 4 through a lower formation of light hydrogenolysis products.
4. Concluding remarks n-Butane dehydrogenation can effectively be boosted over equilibrium by operation in a chromatographic (pulse) reactor. The subsequent skeletal isomerization to isobutene is not improved because chromatographic separation has no influence on this type of reversible reactions [10]. Consequently the ultimate isobutene selectivity achievable is thermodynamically limited and one can expect a theoretical limiting value of around 50% at 380°C, as pointed out before. A maximum value of 22.4% was achieved in this work (see Table 2). Zeolites seem to be adequate as base materials for catalysts having good
142
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
performance in chromatographic regime. The three functions (dehydrogenation, selective adsorption of one of the reaction products, and isomerization) must be present and regulated. Some difficulties can arise from the resulting pore structure, deactivation effects and interaction between the functions, that require further study, n-Butane conversions lower than 50% were obtained in this work, except at high temperatures with a loss in C 4 alkenes selectivity. It is apparent from our results, that the chromatographic separation was not good enough and that the hydrogen elution peak overlaps alkenes eluted so rehydrogenation can occur in some extent. It must be pointed out that isobutane is produced in considerable amounts (typically 3-8%) which can be reasonably attributed to isobutene rehydrogenation and hydrogen transfer from coke deposited on the catalyst. The chromatographic effect is nevertheless present as can be seen from C a olefins percentages well beyond dehydrogenation equilibrium and the presence of butadiene which is much more thermodynamically unfavoured at the experimental conditions used in this work. Composite catalysts are also an interesting alternative to be extensively explored, avoiding direct interaction between catalytic and adsorptive functions. The incorporation of Sn as Pt modifier and the use of SiO 2 as support improves the selectivity of the dehydrogenating function. The presence of higher amounts of isobutane (5-12%) in the product using these catalysts indicates some lack of chromatographic resolution due to an insufficient proportion of the selective adsorption material. Finally it may be envisaged from our results that a technology for one-step isobutene production at low temperatures is possible using reactor operating modes that adequately exploit the chromatographic effect (i.e. pulse, moving bed or pressure swing reactors).
Acknowledgements The financial assistance of CAI + D (UNL) is acknowledged. The work was made under a JICA (Japan International Cooperation Agency)-CENACA (National Catalysis Center) project.
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G. Bellussi, A. Giusti and L. Zanibelli, UK Patent, 2 246 524 (1992). A. Ray, R. Carr and R. Aris, Chem. Eng. Sci., 49 (1994) 469. E.M. Magee, Ind. Eng. Chem. Fund., 2 (1963) 32. P.E. Eberly, J. Phys. Chem., 66 (1962) 812. P. Antonucci, N. Giordano and J.C.J. Bart, J. Chrom., 150 (1978) 309. S.C. Fung and C.A. Querini, J. Catal., 138 (1992) 240.
M.R. Sad et al. /Applied Catalysis A: General 146 (1996) 131-143
143
[7] D. Stull, E. Westrum and G. Sinke, The Chemical Thermodynamics of Organic Compounds, John Wiley, New York, 1969, Chap. 3. [8] D.J.C. Yates, J. Phys. Chem., 69 (1965) 1676. [9] D.W Breck, Zeolite Molecular Sieves, John Wiley, New York, 1974, Chap. 2. 10] T. Paryjczak, Gas Chromatography in Adsorption and Catalysis, Ellis Horwood Limited, Chichester - PWN Polish Scientific Publishers, Warsaw, 1986, Chap. ll.
APPLIED CATALYSS I A:GENERAL
Applied Catalysis A: General 146 (1996) 131 - 143
Dehydroisomerization of n-butane to isobutene in a chromatographic reactor M.R. Sad, C.A. Querini, R.A. Comelli, N.S. F~goli, J.M. Parera * lnstituto de lnvestigaciones en Catdlisis y Petroqulmica - - 1NCAPE, F1Q, UNL - - CON1CET, Santiago del Estero 2654, 3000 Santa Fe, Argentina
Abstract The dehydroisomerization of n-butane to isobutene in only one step was studied in a chromatographic pulse reactor at atmospheric pressure and different temperatures and space velocities. Dehydrogenation, isomerization and alkene/hydrogen chromatographic separation functions were present. Cd-exchanged 4A and 13X zeolites were used as base materials able to separate alkenes from alkanes at high temperatures. They were loaded with Pt or Pt + Sn. Composite catalysts containing the three functions mentioned above were used as well. It was found that under chromatographic conditions, alkene amounts above 35 wt.-% were achieved at 380°C using a P t / 4 A C d catalyst, which are higher than the thermodynamic equilibrium values. The use of wider pore 13X zeolite as base material improves isobutene selectivity; the incorporation of NH~- or La 3+, which improves acidity, adversely affects the chromatographic effect. Composite catalysts have good performance: 80% alkenes selectivity at 35% total conversion and up to 22% isobutene selectivity were obtained at 380°C. The presence of isobutane in products could be ascribed to isobutene rehydrogenation. Keywords: n-Butane dehydroisomerization; Chromatographic pulse reactor; Exchanged zeolites; Composite
catalysts
1. Introduction Isobutene is an important raw material for the production of gasoline additives like MTBE and alkylates, both of them increasing the octane number without polluting effects. Isobutene is normally obtained from catalytic cracking of petroleum, but the amounts produced are not sufficient due to the continuous growth in additives production, n-Butane, from natural gas and LPG, is an * Corresponding author. Tel.: (+ 54-42) 533858; fax: (+54-42) 531068/550944. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 1 1 9 - 6
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131 143
132
abundant raw material; it can be transformed into isobutene by isomerization and dehydrogenation. The actual commercial processes include two plants, one for isomerization and the other for dehydrogenation, making the process not economically attractive. A process in which isobutene can be produced by the use of two catalysts arranged in sequence in a single reactor at 400-600°C, was recently patented [ 1]. Isomerization and dehydrogenation have quite different requirements. While the former is thermodynamically favored at low temperatures, dehydrogenation is favored at high temperatures. The convenient catalysts for the two reactions are different: metal supported catalysts for dehydrogenation and high acidic ones for isomerization. There is no change in the number of moles in isomerization, while there is a change in dehydrogenation. Dehydrogenation can be favored if the products are separated allowing the reaction to continue, thus overcoming the thermodynamic equilibrium. Membrane or chromatographic pulse reactors [2,3] can be useful to achieve such a separation. The concept of a chromatographic pulse reactor is essentially the following: in a catalytic reaction, reactant and products have different adsorptive behavior on the catalyst. Thus, if a long thin tube is filled with catalyst, and a carrier gas flows through it, it is expected that the reaction components will become separated as in a chromatographic column. If the reaction is limited by equilibrium, the chromatographic action should destroy equilibrium conditions by continuous separation of the products, so that the reverse reaction is impossible. After a sufficient length of time the reaction will be essentially complete. The chromatographic pulse reactor can be advantageous when: the equilibrium constant for the reaction is small; reaction rates are high enough so that separation of products rather than rate of reaction limits the extent of reaction; - - at least two products, which are chromatographically separated in the reactor, are formed; reactants are not separated in the reactor; this is limited to a single reactant, or two reactants where one also serves as the carrier gas. All these requirements are fulfilled by some dehydrogenation reactions. According to the literature [4] Cd exchanged zeolites are effective in the separation of alkenes from alkanes. They also have catalytic dehydrogenation activity at high temperature and were used in chromatographic conditions to achieve conversions beyond equilibrium in ethane dehydrogenation [5]. From the former considerations it should be convenient to carry out the simultaneous dehydrogenation and isomerization of n-butane under conditions in which both reactions are favored: isomerization, working at low temperatures and dehydrogenation by the chromatographic separation of the products. The objective of this paper is to search for materials which allow, in only one step, the transformation of n-butane to isobutene, overcoming the thermodynamic limitations using chromatographic pulse reactors. Dehydrogenating and -
-
-
-
-
-
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
133
isomerizing functions as well as alkene/alkane chromatographic separation capacity should be present.
2. Experimental 2.1. Catalysts Zeolites Linde 4A and 13X, originally in 1/8" extrudates (ca. 20% binder clay content), grounded and sieved to obtain the 3 5 / 8 0 mesh fraction were used as base materials. Both zeolites were exchanged with C d 2+ by batchwise contacting with an excess of CdC12 o r C d S O 4 0.5 M solutions, as described elsewhere [4]. Three successive contacting steps, each one with the volume required for 100% exchange and lasting 24 h were performed at room temperature. Exchanged materials were filtered, washed with distilled water until no chloride or sulfate was detected in liquids, and dried at 120°C overnight. Chemical analysis of Cd 2+ in exchanging solutions revealed that more than 80% of total exchange capacity was achieved. The materials obtained were designated as 4ACd and 13XCd. 13XCdLa and 13XCdLaNH 4 were prepared at lower exchange levels (10% of exchange capacity for each cation) in one simultaneous exchange step, using the volumes of 0.5 M solutions of each CdCI 2 , La(NO3)3, and NH4C1 required for such exchange level, assuming total exchange. This was verified by chemical analysis of the exchanging solution. Pt loading of 4ACd, 13XCd, 13XCdLa and 13XCdLaNH 4 was made by impregnation with H2PtC16.6H20 aqueous solutions using the incipient wetness technique, to avoid C d 2+ losses. Pt deposition on 13X was achieved either by ion exchange with Pt(NH3)4C12 aqueous solution or by the impregnation procedure. Nominal Pt content of 4A and 13X catalysts was 2% and 1%, respectively. Pt(2%)Sn(2%)/4ACd catalyst was prepared by successive impregnation of 4ACd with HzPtC16.6H20 and SnC12 solutions. Other catalytic materials used w e r e S i O 2 / A I 2 0 3 (87% SiO2, 13% AI203 from Strem Chemicals) and Pt(l%)Sn(1%)/SiO 2 prepared by impregnation of silicagel (Alfa, 300 m 2 g-~), with a solution of a HzPtC16-SnC12 complex. All Pt containing catalysts were calcined in air at 500°C (3 h) to decompose the metallic precursor and reduced in flowing H 2 at 350°C (3 h) prior to use.
2.2. Reaction system and product analysis Chromatographic reactors were 1/8" O.D. stainless steel columns filled with 2 g of 35-80 mesh catalysts, their lengths ranging between 100 and 150 cm. They were coiled and had 15 cm of 3 5 / 8 0 mesh quartz packing at both ends; they were kept in an oven controlled to + I°C. Carrier gas (N 2 99.5% from
134
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
AGA) flow was controlled using the pneumatic system of a chromatograph (Shimadzu GC-8) and measured by a soap bubble flowmeter. Pulses of pure n-butane (99.5% from AGA) were introduced using a conventional six port sampling valve (Valco, loop volume 0.5 cm3), and the effluent from the reactor was either sent directly to the FID or collected over water in glass samplers (ca. 250 ml capacity) having a rubber septum. Direct injection to FID was used either to determine hydrocarbon retention times or to select the length of sample collection periods (pulse intervals). In the collecting mode, samples were injected to the chromatograph for detailed hydrocarbon analysis using a 7 m long, 1,/8" O.D. column packed with dimethylsulfolane on Chromosorb P, operated at 3 k g / c m 2 inlet pressure, at room temperature and using nitrogen as carrier gas. Complete resolution of all hydrocarbons from C] to C 4 was achieved; typically three pulses were collected together in each sampler. Complementary chromatographic analysis were made in the pulse coming out from the reactor to determine hydrogen and hydrocarbons composition at different emergence times. In this case, an additional sampling valve was installed downstream the reactor, and samples were analyzed in a chromatograph (SRI 8610) with TCD and FID, using the same column as before. Some conventional flow experiments were performed on 13X based catalysts by flowing pure n-butane at the same flowrates used for carrier in pulse experiments, in order to study deactivation effects or to compare n-butane conversions with those obtained in the chromatographic mode. Other flow experiments to assess the skeletal isomerization capacity of 13X based catalysts were also carried out using 1-butene (Matheson, 99.5%) as feed, at atmospheric pressure, 380°C and 0.1 h-~ WHSV in a microcatalytic reactor with on line GC analysis (Shimadzu 14APT), and using a 2GS-alumina plot GC column. TPR and TPO analysis of fresh and deactivated catalyst samples, respectively, were performed in an specially designed equipment described elsewhere [6].
3. Results and discussion
3.1. Equilibrium calculations Fig. 1 shows the equilibrium product distribution for n-butane dehydrogenation calculated from available thermodynamic data [7]. Hydrogen is not included, the percentages being referred only to the hydrocarbon fraction. The C j - C 3 hydrocarbons were not considered to better reflect dehydrogenation equilibrium. The total amount of monoalkenes is about 10% at 427°C, while butadiene can be obtained only above 527°C (Fig. 1A). The product distribution within the monoalkenes fraction is shown in Fig. lB. The lower the temperature, the higher the isobutene fraction. About 50% of the monoalkenes fraction corresponds to isobutene at 427°C.
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
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Fig. 1. Equilibrium values corresponding to n-butane dehydrogenation. P = 1 atm; H 2 / n C 4 = 0; N 2 / n C 4 = 0. (A) Product distribution. (B) Alkenes distribution in the C 4 monoalkenes fraction, iC4: isobutene: t2C4: trans-2-butene: c2C~ : cis-2-butene; 1C~ : l-butene.
3.2. Zeolite based catalysts A reactor containing 2 g of 4A zeolite, was used to measure the retention times of n-butane and 1-butene. In Table 1 it can be seen that retention times are very small, in the same order of residence time (estimated from column void space to be ca. 0.09 min at 15 ml min-1). The difference among n-butane and 1-butene retention times is less than 15%. Table 1 also shows retention times in a reactor containing 2 g of 4ACd. In this case l-butene has a much higher retention time than n-butane, the difference being significant even at temperatures as high as 390°C. Metallic cadmium has a high vapor pressure and some references indicate that it can be lost in reducing atmospheres at high tempera-
Table 1 Chromatographic effect of different materials. Carrier flow rate: 15 ml min- ] Material
T (°C)
4A 4ACd
350 300 350 390 300 350 380 380 380 380 380
13X
13XCd Pt(imp)/13X Pt(exc)/13X Pt(imp)/13XCdLaNH 4 Not affected by reduction at 500°C.
Retention time (min) n-butane
1-butene
0.14 2.19 a 1.29 a 0.97 a 1.58 1.10 0.66 0.45 0.64 0.96 0.64
0.17 20.94 10.02 6.47 4.10 2.00 1.50 18.00 0.71 1.81 0.75
136
M.R. Sad et al. /Applied Catalysis A: General 146 (1996) 131-143
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tures [8]. Therefore the influence of high temperature reduction in retention capacity of the 4ACd was studied. It was found that retention times are not modified, even when reduction is carried out at 500°C. Fig. 2 shows the TPR of 4A, 4ACd and P t / 4 A C d . 4A zeolite does not show any reduction peak below 520°C, while 4ACd has two reduction peaks, at 400 and 550°C, that could be due to Cd 2+ at different positions in the zeolite framework [9]. Two consecutive TPR analysis on the same sample with intermediate reoxidation are shown in Fig. 2 (lst TPR and 2nd TPR). Peak locations are not modified, but peak areas are lower for the second TPR, and some metallic Cd deposition was observed on the TPR reactor walls. In the case of P t / 4 A C d , Cd 2+ reduction temperature is lower (shoulder at 280°C) than in 4ACd due to the presence of Pt. The peak at 100°C corresponds to Pt reduction. These results indicate that, although retention times for 4ACd are not modified by a 2 h reduction; at very high temperatures P t / 4 A C d could lose its retention capacity in reducing atmospheres. In order to avoid this problem, the dehydrogenation reaction should be carried out at temperatures lower than 450°C when using this catalyst. Fig. 3 shows the weight percentage of each C 4 alkene in the hydrocarbon fraction obtained using a chromatographic reactor loaded with P t / 4 A C d catalyst at 380°C as a function of carrier flow rate. The total amount of alkenes is above 35% when using 15 ml rain ~ carrier flow rate, which is well above the thermodynamic equilibrium. The highest isobutene percentage obtained with this catalyst was 8% at 10 ml min I cartier flow rate. After the catalyst was used under a continuous flow of n-butane, the amount of alkenes produced decreased to 10%, without isobutene production. It must be noted that n-butane conversions obtained in normal flow operating mode were below 5% (i.e. 4.7% at 380°C and 15 ml n C 4 min-~). After the catalyst is regenerated by burning off coke at 380°C, alkene production capacity is partially
M.R. Sad et al. / Applied Catalysis A: General 146 (1996) 131-143 n C 4 Retention times: 3.2 1.71 0.80
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recovered. The amount of alkenes depends on the cartier flow rate. Fig. 3 shows that as the carrier flow rate increases, the amount of alkenes goes through a maximum. This is because at very low flow rates alkenes are either cracked to lower molecular weight products or oligomerized and the oligomers cracked into fragments. As residence time decreases, selectivity to alkenes increases. But at higher flow rates, such as 30 ml min-1, the amount of alkenes decreases because residence time is not enough to achieve a high conversion. Isobutene production is not fully recovered after regeneration. In the upper part of Fig. 3 n-butane retention times are shown. When the catalyst is deactivated its retention capacity decreases, which is partially recovered after regeneration Fig. 4 shows alkene composition ratios that correspond to results presented in Fig. 3. The equilibrium ratios are 1 C 4 / i C 4 = 0.18, 1 C 4 / t 2 C 4 = 0.39 and 1 C 2 / c 2 C 4 =0.61 at 380°C. It can be seen that C 4 alkenes are not in thermodynamic equilibrium (1-butene is in excess with respect to the other alkenes), which is not surprising taking into account that the acidity brought by Cd 2+ exchange may be inadequate or insufficient. But linear alkenes are nearer equilibrium than iC 4 in all cases, independently of flow rates or if the catalyst is coked or regenerated. 1-Butene/isobutene ratio is lower (nearer to equilibrium value) at the initial state of the catalyst. Pore openings of 4ACd zeolite are expected to be wider (similar to the 5A calcium exchanged zeolite) than the original sodic 4A form, but they are nevertheless too small. Branched molecules like isobutene that could be produced at internal acid sites would have little chance to get out to the gas phase and the production of this alkene is certainly due to external acid sites. These acid sites are the first to be deactivated. As the
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
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Fig. 4. Alkene composition ratios as a function of the carrier flow rate. Catalyst: P t / 4 A C d . T = 380°C. I C 4 : l-butene; iC 4 : isobutene; t2C 4 : trans-2-butene; c 2 C ~ : cis-2-butene; BD: butadiene.
catalyst deactivates, there are not enough acid sites to equilibrate the isobutene with linear C 4 alkenes; this is the reason why after flow experiments isobutene production strongly decreases (Fig. 3). The carrier flow rate also affects isobutene production, the higher the flow rate, the lower the amount and the fraction of isobutene in the monoalkenes fraction. Fig. 5 shows the TPO of the catalyst, after being used with a continuous flow of n-butane during 1.5 h; it indicates that 380°C is not enough to remove all the coke from this catalyst, and this may be the reason of the lower retention times obtained with the regenerated catalyst. The amount of carbon on the coked catalyst was 1.47 wt.-%. Fig. 6 shows the composition along the pulse coming out of the chromato-
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Fig. 6. Product distribution of the pulse coming out from the chromatographic reactor as a function of time. T = 380°C, carrier flow rate: 13 ml min - I . Catalyst P t / 4 A C d .
graphic reactor with P t / 4 A C d . Hydrogen shows a maximum concentration (measured as hydrogen peak height in TCD analysis) at the front of the pulse and an slow release afterwards. The tail of hydrogen elution peak overlaps with the alkenes and some rehydrogenation of them occurs, as indicated by the presence of isobutane (up to 12 wt.-%) in the products. Hydrocarbon compositions plotted are mass percentages in the hydrocarbon fraction as detected by FID. C 4 alkenes show a maximum at 3 rain after the pulse started coming out of the reactor, n-Butane concentration decreases continuously from a sharp maximum at the beginning. After 6 min, the main products are C1-C 3 hydrocarbons, that come from the cracking of the alkenes a n d / o r from the oligomers formed from alkenes that were retained inside the reactor. Since the total amount of hydrocarbons that comes out of the reactor at this time is small, the contribution of this light compounds to the selectivity in the pulse is not important.
3.3. 13X Zeolite based catalysts In order to increase the surface area accessible for isobutene formation through wider pore openings, 13X zeolite was used as base material. Retention
140
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
Table 2 n-Butane conversion (X), monoalkenes selectivity (Smo,1), C 4 alkenes ratios (R) and isobutene selectivity (Sic,). Carrier flow rate: 15 ml rain t Material
13X 13XCd Pt(imp)/13XCd Pt(exc)/13XCd
Pt/13XCdLa Pt/13XCdLaNH 4 PtSn/4ACd + SiO 2/A1203 PtSn/SiO 2 + 4 A C d + SiO 2 / A I 2 0 3
T (°C) X (%) S ...... ~ (%) R
380 380 420 380 350 380 420 380 420 380 420 380 420 380
0 3.8 12.0 38.8 31.5 45.3 58.8 22.5 36.0 25.4 35.7 23.7 32.3 35.0
71.1 50.7 56.4 84.6 76.5 60.8 72.5 64.7 65.3 57.7 74.9 56.1 80.5
Sic4 b (%)
IC4=/iC4
1C4 / t 2 C 4 -
1C4=/c2C4
0.97 1.49 0.83 4.15 2.9t 1.80 2.61 1.55 1.70 1.10 0.55 0.55 0.77
0.67 0.58 0.52 0.48 0.54 0.60 0.49 0.58 0.54 0.60 0.51 0.59 0.52
0.96 0.82 0.78 0.71 0.77 0.84 0.72 0.83 0.78 0.86 0.75 0.83 0.76
15.6 7.4 12.5 8.9 5.8 7.7 5.8 9.1 8.1 11.1 22.4 17.8 18.9
a % butenes produced/% n-butane transformed. b % isobutene produced/% n-butane transformed.
times of n-butane and l-butene for these materials can be seen in Table 1. Table 2 shows n-butane conversion and monoalkenes selectivity for unmodified and modified 13X zeolite, at different temperatures and 15 ml min-J carrier flow rate. The original sodic form of 13X does not convert n-butane, while 13XCd, which has an important retention capacity for alkenes, produces a low n-butane conversion. The incorporation of Pt improves the catalytic behavior, showing higher both n-butane conversion and monoalkenes selectivity. Pt addition by ion exchange allows to obtain a material with somewhat better conversion and C 4 alkenes selectivity levels than those obtained with the Pt containing catalyst prepared by the incipient wetness technique. Better isobutene selectivity and 1 C 4 / i C 4 ratio closer to equilibrium for the impregnated catalyst may be ascribed to a higher acidity due to the preparation method. As butene distribution was far from isomerization equilibrium, some modification of catalyst acidity was attempted by La 3+ or NH~- exchange. Exchanged zeolites were separately tested for 1-butene isomerization in a microcatalytic reactor and results obtained are shown in Table 3. An improvement with respect to 13XCd was achieved. As it can be seen in Table 2, the addition of La 3+ and NH~-, which was expected to improve the acidity, does not enhance the catalytic performance. This might be due to the poor chromatographic effect of these materials (see Table 1). In all cases, n-butane total conversion increases with temperature while monoalkenes selectivity decreases. Regarding isomerization activity, Table 2
M.R. Sad et al. /Applied Catalysis A: General 146 (1996) 131-143
141
Table 3 Selectivities for l-butene skeletal isomerization (microcatalytic reactor, 380°C, WHSV = 0.1 h i ) Catalyst
Sic (mol iC 4 = / m o l nC 4 converted)
13X 13XCd 13XCdLaNH 4 13XCdLa
0.01 0.03 0.14 0.10
also shows butenes ratios: the linear butenes ratios are nearer equilibrium values than the 1-butene/isobutene ratio since double bond isomerization is a much easier reaction than skeletal isomerization.
3.4. Composite catalysts In order to avoid the adverse effect of the incorporation of acidity modifiers on the zeolite capacity to selectively adsorb alkenes, two chromatographic reactors, using mechanical mixtures of the materials, were made. One of them was loaded with S i O J A I 2 0 3 and PtSn/4ACd (1 g each), thus separating the acid component from the dehydrogenating/adsorbing material. The other one was loaded with SiO2/AI203, PtSn/SiO 2, and 4ACd (1 g each), thus separating the three functions, namely acid, dehydrogenating and chromatographic ones. Some typical results are shown in Table 2. Total conversions are somewhat lower than those obtained with some of the catalysts previously mentioned, but selectivities to monoalkenes (except at the higher temperatures) were high. Conversions lower than those obtained with P t / 4 A C d (Fig. 3) may be due to the lower amount of Pt catalyst used and also to a lower dehydrogenation activity of the PtSn metallic function. The selectivity to isobutene is also better, reflecting the skeletal isomerizing effect of SiO2/AI203. The incorporation of Sn as a Pt modifier, improves the selectivity to C 4 through a lower formation of light hydrogenolysis products.
4. Concluding remarks n-Butane dehydrogenation can effectively be boosted over equilibrium by operation in a chromatographic (pulse) reactor. The subsequent skeletal isomerization to isobutene is not improved because chromatographic separation has no influence on this type of reversible reactions [10]. Consequently the ultimate isobutene selectivity achievable is thermodynamically limited and one can expect a theoretical limiting value of around 50% at 380°C, as pointed out before. A maximum value of 22.4% was achieved in this work (see Table 2). Zeolites seem to be adequate as base materials for catalysts having good
142
M.R. Sad et al./Applied Catalysis A: General 146 (1996) 131-143
performance in chromatographic regime. The three functions (dehydrogenation, selective adsorption of one of the reaction products, and isomerization) must be present and regulated. Some difficulties can arise from the resulting pore structure, deactivation effects and interaction between the functions, that require further study, n-Butane conversions lower than 50% were obtained in this work, except at high temperatures with a loss in C 4 alkenes selectivity. It is apparent from our results, that the chromatographic separation was not good enough and that the hydrogen elution peak overlaps alkenes eluted so rehydrogenation can occur in some extent. It must be pointed out that isobutane is produced in considerable amounts (typically 3-8%) which can be reasonably attributed to isobutene rehydrogenation and hydrogen transfer from coke deposited on the catalyst. The chromatographic effect is nevertheless present as can be seen from C a olefins percentages well beyond dehydrogenation equilibrium and the presence of butadiene which is much more thermodynamically unfavoured at the experimental conditions used in this work. Composite catalysts are also an interesting alternative to be extensively explored, avoiding direct interaction between catalytic and adsorptive functions. The incorporation of Sn as Pt modifier and the use of SiO 2 as support improves the selectivity of the dehydrogenating function. The presence of higher amounts of isobutane (5-12%) in the product using these catalysts indicates some lack of chromatographic resolution due to an insufficient proportion of the selective adsorption material. Finally it may be envisaged from our results that a technology for one-step isobutene production at low temperatures is possible using reactor operating modes that adequately exploit the chromatographic effect (i.e. pulse, moving bed or pressure swing reactors).
Acknowledgements The financial assistance of CAI + D (UNL) is acknowledged. The work was made under a JICA (Japan International Cooperation Agency)-CENACA (National Catalysis Center) project.
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G. Bellussi, A. Giusti and L. Zanibelli, UK Patent, 2 246 524 (1992). A. Ray, R. Carr and R. Aris, Chem. Eng. Sci., 49 (1994) 469. E.M. Magee, Ind. Eng. Chem. Fund., 2 (1963) 32. P.E. Eberly, J. Phys. Chem., 66 (1962) 812. P. Antonucci, N. Giordano and J.C.J. Bart, J. Chrom., 150 (1978) 309. S.C. Fung and C.A. Querini, J. Catal., 138 (1992) 240.
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[7] D. Stull, E. Westrum and G. Sinke, The Chemical Thermodynamics of Organic Compounds, John Wiley, New York, 1969, Chap. 3. [8] D.J.C. Yates, J. Phys. Chem., 69 (1965) 1676. [9] D.W Breck, Zeolite Molecular Sieves, John Wiley, New York, 1974, Chap. 2. 10] T. Paryjczak, Gas Chromatography in Adsorption and Catalysis, Ellis Horwood Limited, Chichester - PWN Polish Scientific Publishers, Warsaw, 1986, Chap. ll.