Ind. Eng. Chem. Res. 1997, 36, 4821-4826
4821
Tin/Platinum on Alumina as Catalyst for Dehydrogenation of Isobutane. Influence of the Preparation Procedure and of the Addition of Lithium on the Catalytic Properties Guillermo J. Siri,†,‡ Mo´ nica L. Casella,‡ Gerardo F. Santori,‡ and Osmar A. Ferretti*,†,‡ Departamento de Ingenierı´a Quı´mica, Universidad Nacional de La Plata, and CINDECA, Universidad Nacional de La Plata and CONICET, 47 No. 257, CC 59, 1900 La Plata, Argentina
A series of monometallic and bimetallic Pt and PtSn catalysts, modified or not by the addition of Li, was studied in the dehydrogenation of isobutane to isobutene reaction. Special emphasis was devoted to the behavior of catalysts obtained following different preparation procedures. In relation to the addition of lithium, a strong dependence of the catalytic properties on the order in which Li was added was observed. Two alternatives were analyzed: (a) the support was modified by adding lithium onto it, and then it was impregnated with the metallic precursors; (b) the support was impregnated with the metallic precursors and afterward lithium was added. The application of surface organometallic chemistry on metals (SOMC/M) techniques for the preparation of PtSn bimetallic catalysts led to solids having a better performance than when conventional techniques (successive impregnations with inorganic precursors) were used. Selectivities close to 100% at a high level of activity were obtained when lithium was added on PtSn catalyst (alternative b) prepared from SOMC/M techniques. These systems also showed a good performance in deactivation-regeneration cycles. 1. Introduction Supported Pt-based catalytic systems are widely employed in industrial processes of great importance, such as naphtha re-forming and alkane dehydrogenation (Sittig, 1978; Loc et al., 1988). There is an important difference between these two processes: in the first, the γ-Al2O3 used as support must have a true acidic character (chlorinated γ-Al2O3) to be able to produce aromatization and isomerization reactions; on the other hand, isomerization should be avoided in the dehydrogenation process. However, they have some aspects in common: the operating conditions imposed by thermodynamics to carry out the above mentioned processes also favor coke formation, the main cause of catalyst deactivation; besides, C-C bond splitting is favored in the presence of group VIII metals, such as Pt. For these reasons, it is necessary to decrease the rate of coke formation and/or increase catalyst deactivation resistance, limiting the effects of the hydrogenolysis reactions. It has become a common practice to use bimetallic catalysts, obtained by the addition of a second metal catalytically less active or even completely inactive to the monometallic one. This combination leads to the suppression of the destructive C-C bond splitting in favor of nondestructive reactions, such as dehydrogenation and aromatization. Moreover, the deactivation by coke formation is retarded. Supported bimetallic catalysts have gained unquestionable importance in subjects such as refining, petrochemistry and fine chemistry since their earliest use in naphtha re-forming during the 1950s (Sinfelt, 1983). Dehydrogenation reactions are used in the industrial production of propene, butene, butadiene, isobutene, isoprene, and R-C11-C15 olefins from the corresponding alkanes. In the last decade various situations in the * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Universidad Nacional de La Plata. ‡ CINDECA. S0888-5885(96)00674-4 CCC: $14.00
market of chemicals have focused interest in the direct catalytic dehydrogenation of light alkanes to light olefins. As it was mentioned, selectivity, stability, and regeneration efficiency are very important features of the catalysts for these types of reactions (Sanfilippo et al., 1992). Tin is the most widely used modifier of platinum in catalysts for direct alkane dehydrogenation, leading to a system with higher selectivity to olefins and higher stability. Considering the state of the art obtained from the literature (Yining et al., 1991; Zhou and Davis, 1992; Ferretti and Casella, 1995), the most important effects of tin on these catalysts may be as follows: (i) Strong inhibition of cracking due to a geometrical effect. Tin decreases the size of platinum ensembles necessary to generate the active site for cracking reactions. However, effects of electronic nature cannot be discarded. (ii) Decrease of deactivation rate, either for a decrease of coke formation rate or a modification of the nature of the coke formed. This fact is normally assigned to a diminution of the adsorption energy of coke precursors (effect of electronic nature), which have more mobility and can migrate easily to the support, increasing the available metallic surface. The accurate nature of PtSn systems is very complicated and still a matter of debate. The complexity of the system appears logical, having in mind that supported tin may exist as Sn4+, Sn2+, and Sn0, besides forming five stable alloys with Pt (PtSn, PtSn2, PtSn4, Pt2Sn3, Pt3Sn) (Massalski, 1986). Numerous factors affect the characteristics of this kind of catalyst, such as the acidity of the support, preparation procedure, nature of the precursor compounds, sequence of impregnation, metal loading, activation of the solids, etc. Preparation techniques play a significant role in controlling the type of interactions that occur between tin and platinum and between them and the support. The influence of different impregnation conditions on the final characteristics of the metallic phase has been mentioned by several authors; for instance, Baronetti and co-workers have shown that different conventional © 1997 American Chemical Society
4822 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
techniques (coimpregnation and successive impregnations) resulted in the formation of different adsorbed species on the support after the impregnation step, when aqueous solutions of H2PtCl6 and SnCl2 were used as metallic precursors (Baronetti et al., 1986). Surface organometallic chemistry on metals (SOMC/M) procedures applied to the synthesis of bimetallic catalysts lead to stable and highly dispersed systems, as has been reported in the literature for rhodium, palladium, and nickel catalysts (Ferretti et al., 1987, 1991). These techniques involve the interaction of organometallic compounds of tin, such as tetra-n-butyltin (SnBu4), with a transition metal (Rh, Ni, Pd) supported on SiO2 or Al2O3. An example to illustrate the influence of the preparation procedure can be found in a work of Ve´rtes et al. (1991), who carried out a series of Mo¨ssbauer experiments over bimetallic PtSn/γ-Al2O3 catalysts obtained by reaction between Sn(C2H5)4 and Pt/γ-Al2O3 and by impregnating SnCl2 on Pt/γ-Al2O3. These studies showed that while in the first case tin alloyed the platinum to form a supported PtSnx alloy, in the second case most of the tin was in an ionic state. In order to limit the acidity of the support, the γ-Al2O3 is generally poisoned by adding alkali metal ions, for instance Li or K, as has been previously published (de Miguel et al., 1995). The way in which these promoters are added could influence the final properties, not only of the support but also of the metallic phase. This paper is focused on the dehydrogenation of isobutane to isobutene, using PtSn/γ-Al2O3(Li) catalysts, prepared from SOMC/M and conventional techniques, with emphasis on the behavior of the different catalytic materials related to selectivity, deactivation due to coke formation, and regeneration. Preparation procedures for bimetallic systems are compared; the effect of lithium and the way in which it is added is also studied. 2. Experimental Section Preparation of Catalysts. A commercial γ-Al2O3 with a BET surface area of 191.5 m2/g, crushed to a size of 60-100 mesh, has been used as support. Before impregnation, alumina was calcined at 823 K for 3 h. (a) Monometallic System Pt/γ-Al2O3. The samples were prepared at room temperature, impregnating alumina with an aqueous solution of H2PtCl6 (Aldrich Chemical Co., 99.995%) of a concentration such as to obtain 1% w/w Pt in the solid. After washing, the catalysts were dried at 378 K, calcined in air at 773 K, and reduced under H2 flow at the same temperature. (b) Bimetallic System PtSn/γ-Al2O3, Successive Impregnations (SI). The samples were obtained by impregnating the monometallic catalyst with an aqueous solution of SnCl2 (Merck, p.a.). After drying, the solids were reduced under H2 flow at 773 K. (c) Bimetallic System PtSn/γ-Al2O3, Organometallic (OM). The solids were prepared by employing SOMC/M techniques (Ferretti et al., 1995). The monometallic catalyst Pt/γ-Al2O3 was made to react with tetra-n-butyltin (SnBu4) in n-heptane solution under a H2 atmosphere. The reaction temperature was progressively raised to 363 K; after approximately 6 h, the solid was filtrated out, washed with several portions of n-heptane, and dried. The bimetallic phase PtSn/γAl2O3 was obtained by eliminating all the organic moieties with a reduction treatment under H2 flow at 773 K.
(d) Li-Containing Catalysts. Two series of Licontaining catalysts were prepared. In one case, γ-Al2O3 was impregnated with an aqueous solution of the alkali metal precursor (LiOH‚H2O, Anedra p.a.) having a concentration so as to obtain 1% w/w Li in the resulting solid. After impregnation, samples were dried at 378 K and calcined in air at 773 K, and then H2PtCl6 was added by following the same procedure as in (a) (-Li catalysts). In the other series, the inverse procedure was used; i.e., alumina was first impregnated with the metallic precursors (Pt or Pt and Sn), and the resultant catalyst was then calcined and reduced under the conditions previously mentioned; afterward, lithium was added (also at a concentration of 1% w/w Li) (+Li catalysts). After the reduction step, all the catalysts (monometallic, bimetallic, with and without Li) were washed several times with NH3 solution (0.1 M) at room temperature, in order to obtain a chlorine concentration under 0.1% in the resulting solids. Hydrogen Chemisorption. The measurements were performed in pulse dynamic equipment with catharometric detection. The catalysts samples (500 mg) were placed into a quartz tubular reactor, treated under H2 flow at 773 K for 2 h and then 3 h under Ar flow at the same temperature (gas flow 40 cm3/min). After cooling to room temperature under Ar flow, measured H2 pulses were injected in the Ar stream (pulse volume 0.1 cm3) until the saturation value was reached. From the amount of hydrogen consumed, H/Pt values were calculated as a measure for the average Pt dispersion of the monometallic catalyst. Transmission Electron Microscopy (TEM). The size of the metallic particles was examined on a JEOL 100CX transmission electron microscope, operated at an acceleration voltage of 100 kV, using a magnification of 80000×, 100000×, and 140000×. Catalyst samples, previously reduced in H2 at 773 K for 2 h, were ground into a fine powder, dispersed in acetone and finally dropped onto a grid, where the solvent was evaporated. Catalytic Tests. The test reaction was the dehydrogenation of isobutane to isobutene. Activity tests were performed by placing the sample (100 mg) into a glass tubular reactor provided with a chromel-alumel thermocouple located in the center of the catalytic bed. The temperature of the reactor was raised to 823 K with a heating rate of 10 K/min, under H2 flow. This condition was maintained for 2 h, and then the reactor was fed with a mixture of H2 and isobutane (iso-C4) with a ratio H2/iso-C4 ) 3 and total gas flow of 50 cm3/min. The composition of the feed flow and of the reaction products was analyzed by gas chromatography, using a Carlo Erba Fractovap Series 2150 gas chromatograph on line with the reactor outlet. A 1/4 in. o.d. stainless steel column, 6 m in length, packed with tricresyl phosphate on Chromosorb W, operated at 353 K was employed in this work. The coke formed during the course of the catalytic tests was quantitatively determined at the end of the run by means of a Coulomat 702 (Stro¨hlein Instruments) apparatus. Regeneration. A series of coking and regeneration treatments were performed sequentially in order to evaluate the stability of the catalysts toward regeneration. Carbon deposition on initially fresh samples was carried out by running the isobutane dehydrogenation reaction for 2 h under the experimental conditions previously mentioned. Then H2 was eliminated from the feed flow and the deactivation of the catalysts was
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4823 Table 1. Composition and Method of Preparation of Studied Catalysts catalyst
% Pt
Pt Pt-Li Pt+Li PtSn OM PtSn IS PtSn-Li IS PtSn+Li OM PtSn-Li OM
0.96 0.90 0.96 0.96 0.96 0.90 0.96 0.90
% Sn
% Li 1.0 1.0
0.32 0.27 0.30 0.32 0.27
1.0 1.0 1.0
accelerated. Regeneration of coked catalysts was performed by heating the catalysts under an air flow of 50 cm3/min with a rate of 4 K/min up to 723 K. The samples were kept under these conditions for approximately 70 min and then were cooled to room temperature overnight. The regenerated catalysts were submitted to a new cycle. After two regeneration cycles, an activity test was performed on the catalysts. 3. Results and Discussion Table 1 shows the composition of the catalysts studied as they were determined by atomic absorption spectroscopy. According to what is known for similar systems such as Rh/SiO2, the reaction between Pt/γ-Al2O3 and SnBu4 can be described by the following global equation:
Pt/γ-Al2O3 + ySnBu4 + xy/2H2 w Pt[SnBu4-x]y/γ-Al2O3 + xyBuH (1) where x and y have values of ca. 1.5 and 0.3, respectively. These values were obtained from chromatographic analysis of n-butane evolved during the reaction and the consumption of SnBu4 in the impregnation solution. After several washings with n-heptane, tin was still bonded to Pt/γ-Al2O3, indicating strong interaction between both precursors. With the aim of studying the specificity of the interaction between the organometallic precursor (SnBu4) and the monometallic catalyst (Pt/γ-Al2O3), a series of experiments were conducted: (i) SnBu4 was made to react with γ-Al2O3 and γ-Al2O3 modified with 1% of lithium at 298 and 363 K under the same experimental conditions used during the preparation of the bimetallic catalysts. The quantity of tin fixed was found to be less than 150 ppm, provided that the chlorine concentration was under 0.1%. (ii) SnBu4 was made to react with a mechanical mixture of equal parts of Pt/γ-Al2O3 and γ-Al2O3 in conditions identical to those previously stated. When the reaction temperature was 298 K, there was not an appreciable deposition of tin on the γ-Al2O3 particles. As the temperature rose to 363 K, the tin concentration found was ca. 350 ppm. Taking into account the concentration of tin present in PtSn OM catalyst, this value is no more than 10% of total tin fixed. This fact could indicate that there is not an appreciable deposition of tin on the support, in spite of the presence of the metallic phase. The results obtained in these experiments are gathered in Table 2. Bimetallic catalysts are obtained when the organometallic supported phase Pt[SnBu4-x]y/γ-Al2O3 is treated under hydrogen at 773 K for 2 h, according to the following global reaction:
Pt[SnBu4-x]y/γ-Al2O3 + y(4 - x)/2H2 w PtSny/γ-Al2O3 + y(4 - x)BuH (2)
method of preparation H2PtCl6 aqueous solution on γ-Al2O3 H2PtCl6 aqueous solution on lithium-impregnated γ-Al2O3 LiOH aqueous solution on Pt catalyst SnBu4 in n-C7 solution (SOMC/M) on Pt catalyst SnCl2 aqueous solution on Pt catalyst SnCl2 aqueous solution on Pt-Li catalyst LiOH aqueous solution on PtSn OM catalyst SnBu4 in n-C7 solution (SOMC/M) on Pt-Li catalyst Table 2. Tin Deposited (Snd) on Support Obtained from Its Reaction with SnBu4
a
sample
temp (K)
Snd (ppm)
γ-Al2O3 γ-Al2O3 γ-Al2O3-Li γ-Al2O3-Li Pt/γ-Al2O3 + γ-Al2O3 Pt/γ-Al2O3 + γ-Al2O3
298 363 298 363 298 363
nda
