HZSM-5 Catalysts

characterize the catalysts. After agglomeration, some zeolite protons are neutralized by clay sodium and, consequently, a lower n-butane conversion is...
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Ind. Eng. Chem. Res. 2001, 40, 3428-3434

KINETICS, CATALYSIS, AND REACTION ENGINEERING Influence of Clay Binders on the Performance of Pd/HZSM-5 Catalysts for the Hydroisomerization of n-Butane Fernando Dorado,* Rubı´ Romero, and Pablo Can ˜ izares Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, Universidad de Castilla-La Mancha, Campus Universitario s/n, 13004 Ciudad Real, Spain

The influence of the amount of two clay binders (montmorillonite and bentonite) on the acid properties and performance of Pd/HZSM-5 zeolite with different Si/Al ratios for the hydroisomerization of n-butane has been studied. Temperature-programmed desorption of ammonia, atomic absorption spectroscopy, chemisorption, and surface area measurements were used to characterize the catalysts. After agglomeration, some zeolite protons are neutralized by clay sodium and, consequently, a lower n-butane conversion is obtained. The product selectivity is also strongly influenced by the binder due to the fact that zeolite hydrogen transfer activity, metal/acid site balance, and diffusion of products are modified. If the appropriate binder is selected, the decrease in conversion will be compensated by a much higher isobutane selectivity. Thus, a catalyst based on Pd/HZSM-5, with a zeolite Si/Al ratio of 25, and agglomerated with bentonite in a zeolite/clay ratio of 35/65 wt/wt, showed not only an adequate mechanical resistance but also high isobutane selectivity and isobutane yield (87.1 and 23.9 mol %, respectively). It should be remarked that, under the same reaction conditions, the parent catalyst without binder showed worse isomerization activity (18.5 mol % isobutane yield with 47.3% isobutane selectivity). 1. Introduction Most industrial zeolite catalysts require the zeolite to be pelletized with a binder to obtain larger and more resistant particles and to avoid an extremely highpressure drop in fixed-bed reactors. Although binders are not active as catalysts, binder-zeolite interactions can have a marked influence on the activity, selectivity, and stability of the zeolite.1,2 The presence of binder can affect the acidic properties of a zeolite as a result of changes in the proton-exchange efficiency, trapping by the binder of coke precursors, and/or blocking of zeolite channels during the pelletization process. The binder can also influence the catalytic performance of a zeolite by trapping metal poisons such as nickel and vanadium from feedstock of a petrochemical origin.3,4 Information regarding the influence of binder on the acidity and catalytic performance of zeolites is, therefore, very important for the development of industrial zeolite catalysts. However, despite the large number of papers devoted to explaining the characteristics and industrial applications of zeolites, catalysts based on an agglomerated zeolite and the effect of the binder on the catalytic performance are rarely studied. Bentonite and montmorillonite are laminar and expandable clays with wet binding properties and are widely available throughout the world. The dispersability of clays in aqueous suspensions is the reason for their agglomeration properties: zeolite particles are sur* To whom correspondence should be addressed. E-mail: [email protected]. Phone: +34-926-295300. Fax: +34926-295318.

rounded by clay laminae, and when the water is removed, a solid phase is achieved in which the zeolite particles are bound by the clay. It has been shown that the acidic forms of clays do not have binding properties and that their sodium forms exhibit better properties.5 Because zeolites are mainly used as acid catalysts, further transformation of the bound zeolite to the acid form is required. In the coming years, the reformulation of gasoline imposed by environmental requirements will involve a reduction in the amount of aromatics, with a subsequent depletion of the octane level of the gasoline. On the other hand, reformulation will also call for a reduction in the amount of butanes in the gasoline pool to reduce the Reid Vapor Pressure. n-Butane can then be converted into high-octane gasoline components through isomerization to isobutane, which can be fed into alkylation units or dehydrogenated to yield isobutylene, which is used in the production of MTBE. Consequently, the isomerization of n-butane is an attractive industrial process. One possible route for the isomerization of n-butane is the use of zeolites as catalysts and, consequently, it is of great interest for studying the influence of binder on the catalytic performance in this reaction. In the work described here, several acid catalysts based on ZSM-5 zeolite with different Si/Al ratios, and agglomerated with a clay (montmorillonite or bentonite), were prepared. The aim of this work is to select a suitable catalyst and to study the modifications, the influence that the pelletization process can induce on the catalyst acidity (acid site density and strength), and how these factors affect the catalytic activity and selectivity.

10.1021/ie001133w CCC: $20.00 © 2001 American Chemical Society Published on Web 07/04/2001

Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3429 Table 1. Characterization Data for the Raw Materials

catalyst montmorillonite bentonite NaZSM-5 HZSM-5 HZSM-5 HZSM-5 PdZ25a a

acidity data

zeolite Si/Al ratio

Na (wt %)

total acidity (mmol of NH3/g)

weak acidity (mmol of NH3/g)

Td (°C)

strong acidity (mmol of NH3/g)

Td (°C)

25 15 25 40 25

1.969 0.480 1.982 0.040 0.040 0.040 0.038

0.086 0.038 0.393 0.707 0.602 0.467 0.609

0.086 0.038 0.393 0.031 0.012 0.018 0.010

270 274 275 300 295 300 290

0 0 0 0.676 0.590 0.449 0.599

419 400 403 400

Pd dispersion ) 21.3%.

2. Experimental Section 2.1. Catalyst Preparation. The bifunctional catalyst used in this work consists of H-ZSM-5 as the acid part, Pd as the metal part, and a binder (sodium montmorillonite or sodium bentonite). NH4-ZSM-5 was supplied in the ammonium form by Zeolyst International (Si/Al ratio ) 15, 25, and 40). A commercial montmorillonite, Gador Gel, was supplied by Minas de Gador (Almerı´a, Spain). Bentonite was supplied by Aldrich Chemical Co. Basic characterization data of the raw materials (clays and zeolite) are summarized in Table 1. The method of preparation consisted of three steps: agglomeration of ZSM-5, incorporation of functions (acid and metallic), and activation of the metal part. The ammonium forms of zeolite were calcined at 550 °C for 15 h to obtain the protonic form. For the binding process, zeolite and clay were mixed together and suspended in water at 60 °C for 2 h. The suspension was then dried at 120 °C overnight. After grinding and sieving, particles with an average particle size of 0.75 mm were obtained. Finally, the bound material was air-calcined at 550 °C for 15 h. After the agglomeration process, ion exchange with a mineral acid must be carried out to avoid a possible activity decrease due to partial exchange of the strongest protonic zeolite acid sites with alkaline cations from the binder during the preparation.2 Thus, the agglomerated catalysts were ion-exchanged with 35 mL/g of 0.6 N HCl under agitation at room temperature for 2 h. Metal incorporation was carried out by an impregnation technique: the sample was placed in a glass vessel and kept under vacuum at room temperature for 14 h to remove water and other compounds adsorbed on the zeolite. A known volume of an aqueous Pd(NO3)2 solution (the minimum amount required to wet the solid) was then poured over the zeolite. The solvent was then removed by evaporation under vacuum. The metal concentration of the impregnating solution was calculated to always yield a final Pd content of 0.82 wt %. After metal incorporation, the catalysts were aircalcined at 400 °C for 4 h and reduced in situ under a hydrogen flow of 190 mL/(min g) at 450 °C for 4 h. Catalysts used during this investigation are named as follows: first, the symbol of the metal is shown (Pd) and this is followed by a character representing the zeolite (Z). The subsequent number indicates the zeolite Si/Al ratio. For nonagglomerated samples, further characters are not included. For bound catalysts, however, there is another character related to the binder name (M for montmorillonite or B for bentonite) and finally a number that represents the amount of binder in the catalyst (wt %). For instance, PdZ25 is a catalyst based on Pd/HZSM-5 with a zeolite Si/Al ratio of 25 and with

no binder; PdZ15B65 is a catalyst based on Pd/HZSM5, with a zeolite Si/Al ratio of 15, agglomerated with bentonite, and with a proportion of clay in the final catalyst of 65 wt %. 2.2. Catalyst Characterization. Surface areas were determined from nitrogen adsorption and desorption data acquired on a Micromeritics Asap 2010 apparatus. Each sample was pretreated overnight at 350 °C under a vacuum of 5 × 10-6 Torr. Surface area measurements have an error of (3%. To quantify the total amount of palladium incorporated into the catalyst and the sodium content of each sample, atomic absorption measurements (AA) were performed using a SpectrAA 220FS spectrophotometer. Prior to measurement the samples were dissolved in hydrofluoric acid and diluted to the interval measurement. The atomic absorption measurements have an error of (1%. Total acid site density and acid strength distribution of each of the catalysts were measured by temperatureprogrammed desorption of ammonia (TPDA) using a Micromeritics TPD/TPR 2900 analyzer with a TCD. The samples were housed in a quartz tubular reactor and pretreated in a flow of helium (99.9990%) while heating at 15 °C/min up to the calcination temperature of the sample. After a period of 30 min at this temperature, the samples were cooled to 180 °C and saturated for 15 min in a stream of ammonia (99.9990%). The catalyst was then allowed to equilibrate in a helium flow at 180 °C for 1 h. The ammonia was then desorbed using a linear heating rate of 15 °C/min up to 600 °C. Temperature and detector signals were simultaneously recorded. The total acidity is defined as the total acid site density, which is obtained by integration of the area under the curve. To obtain the strength distribution, the desorption profiles were fitted using two peaks, the maxima and widths of these peaks being held as constant as possible while fitting each profile (see Figure 1, where three TPDA curves are shown as an example). Weak and strong acidities are defined as the concentration of weak and strong acid sites, respectively, obtained by integration of the area under the peaks at the lowest and the highest temperatures, respectively. The use of these peaks to fit the profiles was not based on any peak assignment to a specific acid site (Bro¨nsted or Lewis), but it was a convenient way to categorize the acid strength distribution obtained by this method. The average relative error in the acidity determination is PdZ25B65 > PdZ40B65). The hydrocarbon selectivities at ≈25 mol % conversion for these samples are given in Table 6. In agreement with the bimolecular mechanism described above, it is again observed that the higher the strong acid site density, the higher the disproportionation (formation of propane and pentanes)/ isomerization ratio. Thus, the catalyst PdZ15B65 showed the lowest selectivity to isobutane (Figure 5). Methane formation was higher for PdZ40B65 than for the two other catalysts, a situation that again suggests a low strong acid site density, which favors hydrogenolysis, as explained above. This effect is more marked at higher conversion levels, so that the isomerization decrease at

Figure 5. Isobutane selectivity as a function of n-butane conversion for PdZ15B65, PdZ25B65, and PdZ40B65 catalysts.

a conversion g30 mol % (see Figure 5) is precisely due to higher hydrogenolysis activity. Finally, it is interesting to compare the catalysts PdZ25 and PdZ15B65. These two systems had very similar strong acid site densities per gram of zeolite (see Table 3) and, indeed, they showed equal n-butane conversion (=39.5 mol %). However, isomerization activity was much higher for the latter catalyst than for the former, as can be easily seen by comparing hydrocarbon selectivity data in Tables 4 and 6, or isobutane selectivity vs conversion data in Figures 3 and 5. The differences in isomerization selectivity between both samples are likely due to changes in diffusional limitations. The reactant molecules can undergo many successive intermolecular reactions as they diffuse through the zeolite channels, with a consequential preferential formation of propane.17 Pentanes and butanes are very reactive compared to propane and can be transformed into this product. It should be noted that, for all the catalysts studied in this work, the propane/pentanes molar ratio is greater than 1, which indicates that the transformation of pentanes into propane always takes place. However, the C3/C5 ratio is particularly high for PdZ25, suggesting that this transformation is especially favored with this sample. First of all, the zeolite crystal size for these samples was measured, as the length of the intracrystalline diffusion depends on it. However, very similar results were obtained for both samples (about 4 µm). Second, it was considered a partial blockage of the zeolite micropore mouths, which would lead to an increase in the length of the effective diffusional pathway. For the

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Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001

PdZ25 sample, the metal particles are mainly located on the external zeolite surface, as stated above. Thus, partial blocking of the zeolite micropore mouths by the metal is expected for this sample. However, for the PdZ15B65 catalyst, the big metal particles are likely located not on the zeolite surface but into the meso- and macropores provided by the binder (see Table 3). Zeolite mouth pore partial blockage is then avoided, with consequently a lower diffusional constraint. Thus, the diffusion pathway for the PdZ25 sample would be clearly much longer than that for the agglomerated catalyst, which strongly favors propane formation, as observed. Even though the binder might also block partially the zeolite micropore mouths, the experimental results suggest that this effect is quite less important than the blocking by the metal particles. Further experimental work will be carried out to better confirm this point. Conclusions The effect of clay binders on the acid properties and performance of Pd/HZSM-5-based catalysts in the hydroisomerization of n-butane has been studied. The presence of binder decreases the zeolite strong acid site density due to solid ion exchange between zeolite protons and clay sodium. The extent of the solid ion exchange can be related to the initial clay sodium content and to the initial zeolite acid site density. The immediate consequence of the neutralization of some zeolite protons by the clay is a lower n-butane conversion. However, this negative effect can be compensated by a higher selectivity to isobutane, which is due to both a lower disproportionation activity and easier product diffusion. The catalyst based on Pd/HZSM-5, with a zeolite Si/Al ratio of 25, and agglomerated with bentonite in a zeolite/clay ratio of 35/65 wt/wt, showed not only adequate mechanical resistance but also high isobutane selectivity and yield. Nomenclature DH2: metal dispersion measured by using H2 chemisorption Si-C4: selectivity to isobutane X: conversion of n-butane

Literature Cited (1) Choudhary, V. R.; Devadas, P.; Kinage, A. K.; Guisnet, M. Influence of binder on the acidity and performance of H-gallasilicate (MFI) zeolite in propane aromatization. Appl. Catal. 1997, 162, 223.

(2) Fougerit, J. M.; Gnep, N. S.; Guisnet, M.; Amigues, P.; Duplan, J. L.; Hugues, F. Effect of the binder on the properties of a mordenite catalyst for the selective conversion of methanol into light olefins. Stud. Surf. Sci. Catal. 1994, 84, 1723. (3) Scherzer, J. Corrrelation between catalyst formulation and catalytic properties. Stud. Surf. Sci. Catal. 1993, 76, 145. (4) Mitchell J. M. M.; Hoffman, J. F.; Moore, H. F. Residual feed cracking catalysts. Stud. Surf. Sci. Catal. 1993, 76, 293. (5) Uguina, M. A.; Sotelo, J. L.; Serrano, D. P. Toluene disproportionation over ZSM-5 zeolite: effects of crystal size, silicon-to-aluminum ratio, activation method and pelletization. Appl. Catal. 1991, 76, 183. (6) Rodrı´guez, F.; Rodrı´guez, I.; Moreno, C.; Guerrero, A.; Lo´pez, J. D. Pt catalysts supported on activated carbons (I). Preparation and characterization. J. Catal. 1986, 99, 171. (7) Bond, G. C.; Mallat, T. Studies of Hydrogen Spillover. J. Chem. Soc., Faraday Trans. 1981, 77, 1743. (8) Can˜izares, P.; de Lucas, A.; Dorado, F.; Dura´n, A.; Asencio, I. Characterization of Ni and Pd supported on H-mordenite catalysts: Influence of the metal loading method. Appl. Catal. 1998, 169, 137. (9) Can˜izares, P.; de Lucas, A.; Valverde, J. L.; Dorado, F. n-Butane hydroisomerization over Pd/HZSM-5 catalysts. Palladium loaded by impregnation. Ind. Eng. Chem. Res. 1998, 37, 2592. (10) Romero, M. D.; Calles, J. A.; Rodrı´guez, A.; de Lucas, A. Acidity modification during the agglomeration of ZSM-5 with montmorillonite. Microporous Mesoporous Mater. 1997, 9, 221. (11) Choudhary, V. R.; Nayak, V. S. Effect of degree of proton exchange on the acidity distribution of HNa-ZSM-5. Zeolites 1985, 5, 15. (12) Kranniha, H.; Haag, W. O.; Gates, B. C. Monomolecular and bimolecular mechanisms of paraffin cracking: n-Butane cracking catalyzed by HZSM-5. J. Catal. 1992, 135, 115. (13) Asuquo, R. A.; Eder-Mirth G.; Lercher, J. A. n-Butane isomerization over acidic mordenite. J. Catal. 1995, 155, 376. (14) Shigeishi, R.; Garforth, A.; Harris, I.; Dweyer, J. Conversion of butanes in HZSM-5. J. Catal. 1991, 130, 423. (15) Weitkamp, J. Hydrocracking, cracking and isomerization of hydrocarbons. Erdoel, Kohle-Erdgas-Petrochem. 1978, 31, 13. (16) Giannetto, G. E.; Perot, G. R.; Guisnet, M.. Hydroisomerization and hydrocracking of n-alkanes. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 481. (17) Tran, M.-T.; Gnep, N. S.; Szabo, G.; Guisnet, M. Isomerization of n-butane over H-mordenites under nitrogen and hydrogen: Influence of the acid site density. J. Catal. 1998, 174, 185.

Received for review December 30, 2000 Revised manuscript received May 7, 2001 Accepted May 8, 2001 IE001133W