Ind. Eng. Chem. Fundam. 1988, 25,337-343
effective diffusion coefficient of solute effective diffusion coefficient of reactant in partial pore effectiveness factor of partial pore linear velocity in catalyst bed pore area fraction, defined by eq 8 catalyst activity for asphaltene cracking intrinsic reaction rate constant per unit surface area catalyst activity for vanadium removal length of packed bed liquid hourly space velocity molecular weight of standard polystyrene molecular diameter of reactant reaction order pore diameter total pore volume per unit weight of catalyst partial pore volume per unit weight of catalyst average concentration of solute within porous material radius of catalyst reaction rate in catalyst bed reaction rate in a single catalyst particle reaction rate in partial pore reaction rate defined by eq 6 radius of porous material radial distance total surface area per unit weight of catalyst partial surface area per unit weight of catalyst time linear velocity in packed bed axial position in packed bed axial position in catalyst bed total porosity void fraction of column partial porosity volume loss in partial pore due to coke deposit volume loss in partial pore due to vanadium sulfide deposits
337
OTj(r,z,t) volume loss in partial pore due to vanadium sulfide and coke deposits bulk density Pcat. catalyst particle density Pvs density of vanadium sulfide Registry No. V, 7440-62-2; Co, 7440-48-4; Mo, 7439-98-1; polystyrene, 9003-53-6. L i t e r a t u r e Cited Anderson, J. L.; Quinn, J. A. Bbchem. Blophys. Acta 1974, 255, 273. Asaoka. S.; Nakata, S.; Shiroto, Y.; Takeuchi. C. Ind. Eng. Chem. process Des. Dev. 1983, 2 2 , 242. Baltus, R. E.; Anderson, J. L. Chem. Eng. Sci. 1983, 38, 1959. Beck, E. M.; Schultz, J. S. Blochem. Blophys. Acta 1972, 255, 273. Beuther. J.; Schmid, B. K. I n proceedings of the 6th W o M Petroleum Congress, Frankfurt, June 7963; Verein zur Ftirderung des 6.Welt-ErdaCKongress: Hanburg; section 3,paper 20. p 297. Beverkige, 0. S. 0.; Schechter, R. S. Optimizatbn: Theory and fractkx; McGraw-HiII: New York, 1970;p 51. Cannell, D. S.;Rondeier, F. Macromolecules 1980. 13, 1599. Deutzenberg, F. M.; Van Kllnken, J.; Pronk, K. M. A.; Sie. S. T.; Wijffeis, J. E. ACS Symp. Ser. 1978, No. 65, 254. Hughes, C. C.; Mann. R. ACS Symp. Ser. 1978, No. 65, 201. Inoguchl. M.; Kagaya, H.; Daigo, K.; Sakwada, S.; Satoml, Y.; Inaba, K.; Tate, K.; Nishiyama, R.; Onlshl, S.; Nagai, T. BUN. Jpn. Pet. Inst. 1971, 13, 153. Newson, E. J. Ind. Chem. RocessDes. Dev. 1975, 14, 27. Oth, J.; Desreux, V. Bull. Soc. Chim. Be&. 1954, 63, 285. Prasher, B. D.; Ma. Y. H. AIChE J . 1977, 23, 303. Rajagopalan, K.; Luss, D. Ind. Chem. process Des. Dev. 1979, 78. 459. Renkin, E. M. J . e n . fhysiol. 1954, 38, 225. Salterfield, C. N.; Colton, C. K.; Pitcher, W. H. A I C M J . 1973, 79, 628. Spry, J. C.; Sawyer, W. H. Abstracts of Papers, 68th AIChE Annual Meeting, Los Angeles, CA; AIChE: New York, 1975;paper 30c. Stein, W. D. The Movement of Mokcuks Across Cell Membranes ; Academic: New York, 1967;p 112. Tamm, P. W.; Hernsberger, H. F.; Brldge. A. 0. Abstracts of Papers, 72nd AIChE Annual Meeting, San Francisco, CA; AIChE: New York, 1979; paper 23b. Thiele, E. W. Ind. Eng. Chem. 1939, 37, 916.
Received for review March 15, 1984 Revised manuscript received May 31, 1985 Accepted July 22, 1985
Silicated Aluminas Prepared from Tetraethoxysilane: Catalysts for Skeletal Isomerization of Butenes BJorrn P. Nllsen, Julia H. Onuferko, and Bruce C. Gates" Center for Cata&t/c Sclence and Technology, Department of Chemical Englneerlng, Unlverslty of Delaware, Newark. Delaware 79716
Catalysts were prepared by the condensation reaction of tetraethoxysilane with the surface of alumina and characterized by X-ray photoelectron spectroscopy to determine surface compositions and by infrared spectroscopy with adsorbed pyridine to determine surface acidity. Silicon-containing groups formed on the alumina surface at less than monolayer coverage, inducing strong acidity; both Brernsted and Lewis acid groups were detected. The catalytic activity for skeletal isomerization of 1-butene was determined in flow reactor experiments at atmospheric pressure and 450-525 'C. The reaction is firstorder In the reactant; the activity increased with increasing surface acidity, Le., with increasing Si content of the surface and apparently with increasing Br~nstedacidity. The surface is comparable in structure and catalytic activity to that of silica-alumina, and the catalytic sites are suggested to be Si-OH groups positioned next to coordinatively unsaturated AI3+ sites.
Introduction
Supported catalysts with simple, stable structures can be prepared by reaction of organometallic compounds with surface functional groups of metal oxides (Yermakov, 1983). There are excellent prospects for tailoring of catalytic properties of these materials by appropriate com0196-4313/86/1025-0337$01.50/0
binations of the precursor metal complex and the support. Industrial catalysts in this class include silica-supported complexes derived from chromocene, used for ethylene polymerization (Karol et al., 1978);related polymerization catalysts have been prepared from complexes of Cr, Zr,and T i (Yermakov e t al., 1981). There are numerous other 0 1986 American Chemical Society
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Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986
Table I. Properties of the Alumina Supportn grade loss on ignition (1 h at 1000 "C, wt % wet base) composition (wt '70dry basis) A1203
NazO
so4
SOz Fe physical properties surface area, m2/g pore volume (H,O), mL/g apparent bulk density, g/mL particle size distribution (micromesh sieve, wt '70) 149 pm 105 pm 14 pm 40 pm 20 pm average particle size, pm a
D 25 balance 0.03 0.3 0.2 0.03 250 0.55 0.8 99 92 70 30 5 55
Data provided by the supplier.
examples of catalysts in this group, and the literature is growing rapidly (Boehm and Knozinger, 1983);the examples include silica-supported olefin metathesis catalysts prepared from [Mo(OC,H,),] and [Re(OC,H,),] (Kuznetsov et al., 1980). One of the intriguing and potentially useful examples is akin to silica-alumina, which, with zeolites, constitutes one of the most important groups of mixed-metal oxide catalysts (Tanabe, 1970). The starting point of this research was a set of patents (Buonomo et al., 1977a,b; Manara et al., 1977) describing catalysts prepared by the reaction of tetraethoxysilane, [Si(OC2H,)4],with the surface of A1203. Hydrolysis of the new surface structures results in groups resembling the Si-OH groups on Si02. The patents provide evidence that these are strongly acidic solids, which are stable, selective, and highly active catalysts for skeletal isomerization of butenes; they are also attrition resistant. The principal objectives of the research reported here were to evaluate this new class of catalyst and to lay a foundation for catalyst design. We have used [Si(OC,H,),] to prepare alumina-supported catalysts with systematically varied submonolayer surface concentrations of the Sicontaining functional groups. The samples have been characterized by elemental analysis and X-ray photoelectron spectroscopy to determine surface compositions, by infrared spectroscopy of chemisorbed pyridine to characterize the acidity, and by kinetics of a catalytic test reaction requiring strong acidity, the skeletal isomerization of 1-butene. This reaction is known to be catalyzed by strongly acidic solids (Choudhary, 1974; Choudhary and Doraiswamy, 1975), proceeding via carbenium ion intermediates. It is potentially valuable industrially for the production of isobutene from straight-chain butenes. This reaction may take on increased importance in the manufacture of lead-free gasoline, since the conversion of isobutene with methanol to give methyl tert-butyl ether, a high-octane blending component, is a successful commercial process (Sherwin, 1981).
Experimental Section A. Catalyst Preparation. The alumina used as the catalyst support was a fluid-bed quality Ketjen/Akzo y-alumina grade D, supplied by Armak Catalysts Division. The physical properties and a typical analysis are given in Table I. The tetraethoxysilane (99%) was supplied by Alfa. It was degassed and stored under nitrogen prior to use.
Reagent grade n-hexane was distilled from sodium prior to use. All catalysts were synthesized under an inert atmosphere with a Schlenk line, Schlenk flasks, and ordinary flasks sealed with serum bottle caps. The glassware was held at 150-200 "C overnight prior to use and cooled under nitrogen. The alumina support was pretreated by heating in helium for 18 h in a quartz tube held vertically in a Lindberg combustion furnace. The particles of alumina were fluidized in the helium stream. The alumina used for catalyst samples 1-5 was pretreated at 300 "C and that used for samples 6-9 at 320, 440, 520, and 660 "C, respectively. The pretreated samples of alumina were brought in contact with solutions of tetraethoxysilane, which was used with n-hexane solvent, except for sample 1, for which no solvent was used. In a typical preparation, n-hexane (20 mL) was added to about 10 g of alumina; tetraethoxysilane was added and the flask swirled occasionally. After 3-4 h, the solvent was vaporized and removed with a stream of nitrogen, and the solid was then transferred to a quartz tube, with the handling done under an inert atmosphere. The sample was heated from room temperature to 200 "C in a period of 6 h and then maintained at this temperature for 12 h in a flow of helium at about 10 mL/min. After this period, the helium stream was introduced through a one-stage saturator containing water at room temperature, and the flow rate was increased to about 100 mL/min, which was sufficient to fluidize the particles. The sample was hydrolyzed in the fluidized bed at 200 "C for 24 h. After this treatment, the saturator was bypassed, and the sample was dried in helium for 1h at 200 "C. It was then placed in a quartz boat and dried in nitrogen for 18 h at 600 "C in a horizontal Lindberg furnace. B. Catalyst Characterization. Elemental Analysis. The analysis of the catalyst samples for Si was done at Schwarzkopf Microanalytical Laboratory, Woodside, NY, and at the Central Institute for Industrial Research, Oslo, Norway. No experiments were done to determine the distribution of the Si in the particles. Infrared Spectroscopy. For characterization by infrared spectroscopy, samples were ground to a fine powder with an agate mortar and pestle. The powder was pressed into thin wafers, 13 mm in diameter. Spectra of the semitransparent wafers were recorded with a Nicolet 7199 Fourier transform instrument equipped with a highly sensitive liquid-nitrogen-cooleddetector. The infrared cells and the vacuum/gas handling system are described elsewhere (Barth et al., 1983). Before a wafer was placed in the cell, the background spectrum was recorded with the cell empty. Sample pretreatment and dosing with pyridine were carried out with the cell mounted in the spectrometer and attached to the vacuum system. The cell was connected to the vacuum system through one port and to the pyridine reservoir through another. Before the valve in the line connecting the cell to the reservoir was opened, the pyridine was frozen with liquid nitrogen. After a sample was placed in the cell, the cell was evacuated, and the temperature was raised to 500 "C and maintained for 3 h. The sample was then cooled to room temperature and the spectrum recorded. Excess pyridine was then adsorbed on the catalyst by opening the Teflon valve to the pyridine reservoir, which was kept a t room temperature. After 30 min, the excess pyridine was desorbed by evacuation for 30 min at room temperature, and the spectrum was recorded. The temperature was then
Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986 339
raised to 150 "C and maintained for 1 h to desorb hydrogen-bonded pyridine, whereupon the spectrum was again recorded. The sample was cooled to room temperature, and the final spectrum was recorded. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectra characterizing the catalysts were acquired with a Physical Electronics Model 550 instrument, comprising a high-vacuum pretreatment chamber and an ultrahigh-vacuum analysis chamber, separated by a gate valve. Each sample was prepared as a wafer (as for the infrared experiments), mounted on the specimen holder, and inserted into the pretreatment chamber, where it was held for 1h at 300 "C under vacuum. Prior to transfer to the ultrahigh-vacuum chamber, the sample was cooled to 20 "C. Mg Kar X-rays (1253.6 eV) were used to irradiate the samples. The energy resolution was 0.5 eV, and the binding energies are estimated to be accurate to within f0.2 eV. All binding energies were determined on the basis of the reference value of 284.6 eV for the adventitious carbon. A commercially available software package (Physical Electronics, Version 5)) executed on a PDP-11 computer, facilitated data accumulation by repetitive scanning, data averaging, and data manipulation. Relative atomic concentrations of Si and Al were obtained from the peak areas and appropriate peak area sensitivities of the Si 2p and Al2p peaks (Physical Electronics, 1979). X-ray satellites were removed with a mathematical subtraction routine prior to peak area measurement. Relative atomic concentrations determined in this manner are estimated to be accurate within &lo%. Catalytic Reaction Experiments. The catalyst performance was measured with a fixed-bed tubular flow reactor and an on-line gas chromatograph. The reactant streams, 1-butene and helium, flowed through Brooks diaphragm valves (Model 8744)) which maintained constant flow rates even with varying downstream pressures. Typical flow rates were 25 and 35 mL/min for helium and 1-butene, respectively. The pressure in the reactor was atmospheric. Stainless-steel tubing (l/g in.) and Swagelok fittings were used throughout. The reactor tube (40 cm in length) was 'l2-in. stainless steel, with a Swagelok fitting at the inlet (top) and a Cajon fitting with an O-ring at the outlet. The feed lines downstream of the flow metering devices and the transfer line connecting the reactor to the gas sampling valve were heated with electrical heating tape. The reactor tube was heated by an aluminum block in which four electrical cartridge heaters (Hotwatt, Inc.) were mounted axially. The temperature was regulated with a proportional controller; the sensing device was a thermocouple mounted in the aluminum block at the interface with the reactor. Reaction temperatures were varied from 450 to 525 "C. The reactor was packed with a plug of quartz wool at the lower (downstream) end and then with Pyrex beads, added to allow placement of the catalyst bed in the central position in the heating zone. Pyrex beads filled with space between the catalyst bed and the reactor inlet; the catalyst bed was separated from the Pyrex beads with quartz wool. The mass of catalyst was varied from 0.125 to 1.00 g. The products were analyzed with an Antek gas chromatograph equipped with a heated gas sampling valve, a flame ionization detector, and an electronic integrator. The column, a 6.6-ft X '/8-in.-o.d. stainless-steel tube packed with 80/100-mesh Carbopack C/O.19% picric acid, was used a t room temperature. Product identifications were based on retention times of standards and coinjection of standards with reaction products. The major components of the products were eluted in the order 1-butene, iso-
Table 11. Analysis of Catalysts for Si sample no. lb 2 3 4 5 6 7 8 9
calcination temp of A1203,OC 300 300 300 300 300 320 440 520 660
Si content, wt%
3.40; 3.25,d 3.33e 2.49' 1.69; 1.61: 1.6P 1.12; 1.11: 1.12e 1.02; 1.06: 1.04e 3-16? 3.14; 3.67," 3.61; 4.18; 3.91: 2.95; 3.84; 4.35: 3.58,' 4.10; 3.87:
3.32O 3.90e 3.71e 3.88e
no. of Si groups per nm2a 2.9 2.1 1.5 0.96 0.87 2.9 3.3 3.2 3.3
"Calculated on the basis of a surface area of 250 m2/g. Prepared without n-hexane solvent; excess [ Si(OC2H,),] was used neat. 'Schwarzkopf Microanalytical Laboratory. Central Institute for Industrial Research, Oslo, Norway. e Average.
butene, cis-2-butene, and trans-Zbutene. Traces of lowmolecular-weight products were observed but not determined quantitatively. To begin a catalysis experiment, helium flow was started and the temperature was raised to 525 OC and maintained for 1 h, or in some experiments, overnight. Then the temperature was decreased to the desired initial value, usually 450 "C. The flow of 1-butene (Phillips, CP) in helium was then started. After 30 min, the product stream was analyzed, and after an additional 30 min, the analysis was repeated to confirm that a virtual steady state had been attained; catalyst deactivation was negligible over the period of the repeated analyses. The catalyst temperature was then changed and the analysis sequence repeated, usually until data had been collected at 450,475,500, and 525 OC. Since the repeat analyses consistently provided confirmation of the earlier analyses, they were omitted in the majority of experiments. In some series, the partial pressure of butene was varied systematically to allow determination of a rate equation. In all experiments, flow rates were chosen to give differential conversions of 1butene (0.4-4%). Results Surface Analyses. The compositions of the catalysts determined by elemental analysis are summarized in Table 11. The uptake of tetraethoxysilane from the hexane solution ranged from 80% to 90%. These uptakes gave surface densities ranging from 0.87 to 2.9 Si atoms/nm2, calculated from the elemental analyses and the initial A1203 surface area of 250 m2/g. Changes in the surface area resulting from heating of A1203 and incorporation of the Si-containing groups were not measured; they could have been significant. The surface density of Si-containing groups on A1203 (calculated on the basis of a surface area of 250 m2/g) depended slightly on the calcination temperature of A1203, as indicated by the results of Table 11. There appears to be a pattern of increasing surface density of Si-containing groups with increasing calcination temperature from about 300 to 400 "C, whereupon a plateau is reached. Representative X-ray photoelectron spectra for Si are shown in Figure 1. The areas under the peaks, combined with similar results for Al, determined ratios of Si to Al, which are plotted in Figure 2 as a function of the surface Si content determined by elemental analysis. The dependence confirms that the Si was bonded at the A1203 surface and demonstrates that X-ray photoelectron spectroscopy provides a convenient and useful measure of the surface Si content. A surface Si/A1 atomic ratio of 0.038 was measured for the sample with no added Si. This represents the Si impurity in the alumina; according to the
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supplier of the alumina, the bulk average content of Si was 0.09 wt 5%. Infrared Spectra. Infrared spectra of pyridine adsorbed on the samples were determined in the range 1400-1700 cm-' (Figures 3 and 4). The catalysts characterized in detail with the infrared technique are samples 1, 2, 6, and 9, listed in Table 11; these include samples prepared from A1203calcined a t different temperatures and samples calcined a t 300 "C and having Si contents systematically varied by the amount of added tetraethoxysilane. Infrared spectra were also obtained with the A1203support alone, with a standard commercial silicaalumina (Davison 970-13,13w t % A1203,-100 m2/g) and with a commercial sample designated as "silicated alumina" (Harshaw, purchased from Strem, 6 wt ?& SO2, 210-240 m2/g). Catalytic Activity for Butene Isomerization. The catalysts were active for isomerization of 1-butene, giving 2-butene (too rapidly to measure) and isobutene. Trace amounts of cracking products were also observed. The catalysts underwent deactivation during the flow reactor experiments, presumably as a result of coke formation. Data illustrating the deactivation are shown in Figure 5; normally, the less severe conditions typified by the lower curves were used to minimize the deactivation and allow estimates of activities of the fresh catalysts. The con-
versions used to determine these activities were differential, as illustrated by the results of Figure 6; the slopes of the curves determined reaction rates directly, and the activity data are represented as reaction rates. Rates were determined as a function of the 1-butene partial pressure for two of the catalysts. The data, plotted in Figure 7, demonstrate that the isomerization to give isobutene is first-order in the reactant partial pressure. The dependence of the catalytic activity on the surface density of Si is shown in Figure 8. We caution that these data are calculated on the basis of the assumption that the surface area did not change during the catalyst preparation steps. The catalytic reaction rate increased with the loading of Si up to about 1.5 Si groups/nm2, becoming nearly constant in the range from 1.5 to 2.0 Si groups/nm2, and increasing sharply at higher loadings. The data of Figure 8 were determined with catalyst samples which had been held at 525 OC for only 1 h prior to the rate measurements. The same pattern, with slightly higher rates, was observed when the catalyst samples were held at 525 "C overnight prior to the rate measurements. The data of Figure 9 show that there is a strong dependence of the catalytic activity on the calcination temperature of the A1203 support; the small variations in Si contents of the catalysts in the series are not sufficient to explain the significant effects observed. The calcination temperature giving the maximum catalytic activity is about 400 O C . The activities and deactivation behavior of the commercial silica-alumina catalysts are compared in Figure 10 with those of catalysts prepared from tetraethoxysilane.
Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986 341
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The activation energies are summarized in Table 111. Estimates by standard methods indicated that there was no significant influence of intraparticle transport processes on the observed rates.
Discussion The analyses of the catalysts combined with the X-ray photoelectron spectra demonstrate clearly that tetraethoxysilane reacted with the A1203surface, giving submonolayer coverages of Si-containing groups. Infrared
"The support was calcined at 600 "C.
spectroscopy, with adsorbed pyridine used as a probe molecule, provides evidence of the nature of acidic groups on surfaces such as these. The spectrum of pyridine adsorbed on silica-alumina (Figure 3), which is in agreement with published results (Tanabe, 1970),includes a broad band located in the region 1540-1550 cm-' and a band located at 1455 cm-'. The former is attributed to the pyridinium ion, providing evidence of Bransted acid sites on the surface, and the latter is attributed to pyridine coordinated at Lewis acid sites (coordinatively unsaturated A13+ions) (Parry, 1963;Basila et al., 1964;Bourne et al., 1970).
Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986
342
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Figure 8. Dependence of Catalytic activity for 1-butene isomerization on the surface concentration of Si-containing groups. The catalysts were pretreated at 525 "C for 1h. Data were obtained with a partial pressure of 1-butene equal to 0.56 atm.
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Spectra obtained with pyridine adsorbed on the A1203 support alone (Figure 3) provide evidence of Lewis acid sites but not of Brernsted acid sites. Incorporation of Sicontaining groups (2.1 groups/nm2) (sample 2, Figure 4) did not lead to the formation of detectable Brernsted acid groups, but Lewis acid groups were again present, as evidenced by the band at 1450 cm-'. When a larger amount of Si was bonded to the surface (2.9 groups/nm2) (sample l),new bands appeared in the spectrum at 1548 and 1639 cm-' (Figure 4), indicative of Bransted acid groups. The
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Ind. Eng. Chem. Fundam., Vol. 25, No. 3, 1986
Alternatively, single -OH groups might react, forming a single A1-0-Si link. Hydrolysis is expected to have occurred during the treatment of the samples with water, e.g., as follows: C2H5O
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