Prog. Catal, Vol. 13, No. 1-2, pp. 35-41 (24) Prog. Catal. Zn/H-ZSM-5 zeolite as catalyst for benzene alkylation with isobutane Adriana Urdă *, Ioan Săndulescu, Ioan-Cezar Marcu University of Bucharest, Department of Chemical Technology and Catalysis, 4-12 Bd. Elisabeta, 318 Bucharest, Romania Received Abstract Benzene alkylation with isobutane was performed on a zinc-modified ZSM-5 catalyst. The influence of the temperature and molar ratio between reactants on the performances of the catalysts was investigated. At low temperature only the alkylation of benzene with alkenes formed from isobutane dehydrogenation takes place, while at high temperatures isobutane also forms aromatic hydrocarbons by aromatization reactions. The process does not consist only of a direct alkylation of benzene with butenes resulted from isobutane, but also with fragments resulted from isobutane cracking on the acid function of the catalyst. The main alkylaromatic hydrocarbon obtained is isopropylbenzene. Key words: alkylation, Zn/H-ZSM-5, isobutane 1. Introduction Benzene alkylation with alkenes or alcohols is a large-scale process, used to obtain valuable organic intermediates in petrochemistry. Alkylation with propene, producing cumene, uses either kieselguhr-supported phosphoric acid or Friedel-Crafts catalysts, but these catalytic systems have environmental and corrosion drawbacks [1]. Light alkanes are abundant and relatively inexpensive compared to alkenes, and finding new processes for their functionalization represents a way of increasing their value. There are studies in literature concerning alkylation of benzene with propane [1-8], butanes [5] or ethane [5, 9] on modified ZSM-5 zeolites, using Ga or Pt as modifiers. Knowing that Zn-modified ZSM-5 zeolites have similar catalytic activity to Gamodified ones in other processes involving the activation of light alkanes [1-12], we have studied benzene alkylation with isobutane on a Zn/H-ZSM-5 zeolite. The effects of temperature, benzene : isobutane molar ratio, and the conversion of isobutane and benzene as individual reactants were examined. * Corresponding author: Univ. Bucharest, Faculty of Chemistry, Dept. Chemical Technology and Catalysis, Bd. Regina Elisabeta No. 4-12, sector 3, Bucharest, Romania, Fax: -4-21-3159249; e-mail: aurda@chem.unibuc.ro; urda.adriana@unibuc.ro
36 A. Urdă et al. 2. Experimental 2.1. Catalyst The catalyst sample was Zn/H-ZSM-5, with a Si/Al atomic ratio of 92 and 2.% (wt) Zn, prepared by impregnation with Zn(NO 3 ) 2 and subsequent calcination in steps up to 55 o C. The zeolite sample was characterized for crystalinity, surface area, porosity, acidity and zinc content (total and on the surface). Results were presented elsewhere [13, 14]. 2.2. Materials Benzene (Chimopar, p.a. grade) and an industrial i-butane cut ( 95.5% isobutane) were used as reagents. 2.3. Catalytic tests Catalytic reactions were performed in a fixed bed reactor (2 mm ID), at atmospheric pressure, with 7 cm 3 of catalyst. The catalyst sample was pelleted, then crushed and sieved (1-1.4 mm). It was placed in the reactor between a wool glass plug and a bed of inert material. The reactor was heated with an electric oven. When benzene and isobutane were used as individual reactants, they were mixed with the corresponding N 2 volume so that VHSV values to remain unchanged. In alkylation tests, benzene was continuously introduced with a micro dosing pump, and mixed with i-butane in the upper part of the reactor, in the bed of inert material. Reaction temperatures were between 2 o C and 5 o C, and different molar ratios between benzene and isobutane were used. Reaction products were cooled, liquid and gaseous fractions separately collected and analyzed on a ThermoQuest gas chromatograph, equipped with a FID detector and a 3-m GS-alumina column, using N 2 as carrier gas. Conversion was calculated as the amount of raw material transformed in reaction divided by the amount that was fed to the reactor. Selectivity to alkylated products was calculated as the amount of alkylated aromatics divided by the amount of benzene that was transformed. 3. Results and discussion Benzene and isobutane were first tested as individual reactants on the zeolite sample, because they can be transformed by other reactions besides the alkylation process. Results are shown in figures 1 and 2.
Selectivity (%) Conversion (%) Zn/H-ZSM-5 zeolite as catalyst for benzene alkylation with isobutane 37 7 6 5 4 3 2 1 2 3 4 5 Fig. 1. Influence of temperature on the conversion of benzene and isobutane on 2% Zn/H-ZSM-5; benzene conversion; isobutane conversion As the temperature increases conversion values grow but, for benzene, they are small over the whole temperature range, not exceeding 1% even at 5 o C, and the main reaction products are coke and small cyclohexane amounts. For isobutane, conversion values are low up to 4 o C, but above this temperature, they increase rapidly, reaching 62% at 5 o C. 5 45 4 35 3 25 2 15 1 5 2 3 4 5 Fig. 2. Influence of temperature on the selectivity for products in isobutane conversion on 2% Zn/H-ZSM-5; selectivity for dehydrogenation; selectivity for aromatics; selectivity for cracking At low temperatures, isobutane is mainly dehydrogenated to butenes, but as temperature increases, cracking becomes more important (figure 2), leading mainly to propene and methane. The catalyst proves to be bifunctional, with the zinc modifier acting as dehydrogenating function at low temperatures, and the acid zeolite at higher
Conversion (%) 38 A. Urdă et al. temperatures. Above 3 o C aromatics are formed also, their selectivity increasing up to 28% at 5 o C. Among aromatic hydrocarbons, toluene is formed in larger amounts, followed by xylenes and benzene. Conversion to aromatics takes place by oligomerization of the alkenes formed in early steps of the process (dehydrogenation or cracking), and then dehydrocyclization [13]. This pathway is proved by the presence of oligomers in the reaction products on the entire temperature range. The alkylation process was studied on the same temperature range (2-5 o C), and the results are shown in figures 3 and 4, for benzene VHSV of 2 h -1 and a benzene-to-isobutane molar ratio of 2:1. Conversion values have a similar evolution with that shown in figure 1, except for lower values for isobutane and higher for benzene at high temperatures. The higher values for benzene conversion are due to alkylation reactions leading to aromatic hydrocarbons. Lower conversion values for the starting alkane were also reported in literature for alkylation with propane [1], and were explained by a stronger adsorption of benzene with respect to propane, that prevent direct contact of propane molecules with the active sites of the zeolite. We can assume that it is also the case in the reaction with isobutane. 25 2 15 1 5 2 3 4 5 Fig. 3. Influence of temperature on the conversion of benzene and isobutane in alkylation on 2% Zn/H-ZSM-5; benzene VHSV = 2 h -1 ; benzene-to-isobutane molar ratio = 2:1; benzene conversion; isobutane conversion The distribution of reaction products strongly depends on temperature (figure 4), with the selectivity for alkylaromatics passing through a minimum value at 35-4 o C, and the selectivity for C 9+ aromatics continuously decreasing with temperature. This behavior can be explained by the nature of the products that are formed. At low temperatures, C 9+ are the main aromatic hydrocarbons formed in the process, with a high selectivity (almost 7%). Isopropylbenzene and n-propylbenzene are observed on the entire temperature range, followed by sec-butylbenzene (which is the main C 9+ hydrocarbon at 2 o C), n-propylbenzene, n-butylbenzene and pentylbenzenes (in low concentrations, and only at low temperatures). The
Selectivity (%) Zn/H-ZSM-5 zeolite as catalyst for benzene alkylation with isobutane 39 distribution of these alkylaromatics in reaction products at different temperatures is shown in table 1. 8 6 4 2 2 3 4 5 Fig. 4. Influence of temperature on the selectivity for products in benzene alkylation with isobutane on 2% Zn/H-ZSM- 5; benzene VHSV = 2 h -1 ; benzene-to-isobutane molar ratio = 2:1; selectivity for alkylaromatics; selectivity for C 9+ aromatics Table 1. Distribution of main alkylaromatics in the reaction products at different reaction temperatures; benzene VHSV = 2 h -1 ; benzene-to-isobutane molar ratio 2:1 Compound (wt %) 2 o C 25 o C 3 o C 35 o C 4 o C 45 o C 5 o C Isopropylbenzene 25.2 23.1 18.6 5.6 3.2 4..8 n-propylbenzene - 3.5 9.8 6.3 5.7 4.4 1.1 sec-butylbenzene 38.9 13.9 6.6 - - - - n-butylbenzene - 1.2 - - - - - Pentylbenzenes 5.6 12.7 3.8 - - - - All these products are formed by alkylation with alkenes formed either by dehydrogenation or cracking, or with oligomers (that are also formed in reaction) in the case of pentylbenzenes. As temperatures increase, the selectivity for C 9+ aromatics decreases due to cracking reactions in the side chain, leading to toluene and ethylbenzene. Simultaneously, cracking of isobutane intensifies (figure 2), generating C 1 -C 3 hydrocarbons that are reacting with benzene. At high temperatures, isomerization and transalkylation of initially formed alkylaromatics also intensifies, all these pathways leading to a very complex product distribution, dominated above 45 o C by toluene and xylenes. The alkylaromatic hydrocarbons formed below 3 o C are formed entirely by alkylation of benzene with alkenes obtained from isobutane. At higher temperatures, a part of the aromatics is formed only from isobutane, as proved by the reactions performed with isobutane-n 2 mixture. (figure 2). As we were interested in the alkylation process, following tests were performed at 25 o C in order to avoid intense cracking side reactions.
Selectivity (%) Conversion (%) 4 A. Urdă et al. The influence of the benzene-to-isobutane molar ratio was studied in the range.3-2.2 (molar ratios from 1:3 up to 2.2:1). The results are shown in figures 5 and 6. While conversion values for benzene and isobutane remained practically unchanged, the selectivity to alkylaromatics and that for C 9+ aromatics passed through a maximum at a molar ratio of 1.1. As the ratio increases above this value, the amounts of isopropylbenzene and n-propylbenzene decrease abruptly, and heavier C 9+ aromatics (butylbenzenes and pentylbenzenes) are formed. 9 8 7 6 5 4 3 2 1 1 2 3 Benzene : isobutane molar ratio Fig. 5. Influence of benzene-to isobutane molar ratio on the conversion of benzene and isobutane in alkylation on 2% Zn/H-ZSM-5; T = 25 o C; benzene conversion; isobutane conversion 1 9 8 7 6 5 4 3 2 1 1 2 3 Benzene : isobutane molar ratio Fig. 6. Influence of benzene-to isobutane molar ratio on the selectivity to products in benzene alkylation with isobutane on 2% Zn/H-ZSM-5; T = 25 o C; selectivity for alkylaromatics; selectivity for C 9+ aromatics 4. Conclusions The Zn/H-ZSM-5 catalyst is a bifunctional one, with the zeolite acid function promoting the cracking and alkylation reactions, and zinc modifier promoting the dehydrogenation reactions. At low temperatures isobutane dehydrogenation reactions
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