Oxidative dehydrogenation of lower paraffins with ZSM-5 zeolite catalysts

Oxidative dehydrogenation of lower paraffins with ZSM-5 zeolite catalysts Kaoru Fujimoto, Ikusei Nakamura, and Kohshiroh Yokota

Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Tokyo, Japan It was found that the oxidative dehydro-aromatization of lower paraffins was effectively catalysed by either a protonic or metal ion-exchanged ZSM-5 zeolite at temperatures from 450 to 550°C. Oxygen reacted selectively with surface hydrogen to promote its removal as water, to decrease its surface concentration, and, thus, to enhance the formation of aromatic hydrocarbons. Keywords: ZSM-5 zeolite; aromatization; oxidative dehydrogenation; lower paraffins

INTRODUCTION Dehydro-aromatization of lower paraffins has been attracting great attention as a new route from LPG to aromatic compounds. For this reaction, a protonic ZSM-5 zeolite exhibits an excellent activity. I-s Chen et al. has claimed that the formation of aromatics is composed of acid-catalyzed reactions including (1) conversion of feed paraffins to small olefins, (2) formation of higher olefins via their oligomerization and other reactions, and (3) the formation of aromatics and small paraffins via cyclizationdehydrogenation and hydrogen transfer. It has also been found that ZSM-5-type zeolites containing Ga or Zn exhibit higher selectivity of aromatics in the title reaction than do rionmetallized ZSM-5. 4-9 This reaction is going to be commercialized as Cyclar process. 10 Gallium or zinc are effective in the impregnated form and the ion-exchanged form or even when they are incorporated in the zeolite framework. 9 In the present study, the oxidative dehydrogenation of propane and butane were tried on an HZSM-5 and a Gasupported ZSM-5 zeolite to promote the formation of aromatic hydrocarbons. While the dehydrogenation of lower paraffins is highly endothermic and thus needs a heat supply to keep the reaction temperature at the suitable level, the oxidative d e h y d r o aromatization is an exothermic reaction, whose temperature rises as the reaction proceeds.

EXPERIMENTAL The ZSM-5 zeolite was a commercially available one (TSZ-850NAA, SiO2/A12Oa = 49.8) supplied by Toso Chemical Industry. The zeolite was protonated by

repeated ion exchange (three times) with a 1 N aqueous ammonium nitrate solution at around 60°C, which was followed by air calcination at 500°C for 3 h. Gallium was supported on it by an ion-exchange method from its aqueous nitrate solution (1 N) at 60°C. The loading was 15 mg Ga/g, which was determined by measuring the concentrations of remaining metal cations in the solutions. Reactions were conducted with a fixed-bed flow-type reaction apparatus under atmospheric pressure. T h e concentration of paraffins in the feed were 20% by volume. Products were analyzed by gas chromatography. RESULTS AND DISCUSSION Results of n-butane conversion on a variety of catalysts are demonstrated in Table 1. Material balance was measured only for carbon (because the amount of water was not determined), which revealed that the carbon balance was excellent (> 97% in every case). The product was composed of lower paraffins, whose major part was propane with a small amount of Table 1 Conversion of n-butane on ZSM-5 catalysts a Catalyst O= (mol% in feed) n-C4 converted (%) Hzlconverted n-C4

Hongo, Bunkyo-ku, Tokyo 113, Japan. Received 12 January 1988; revised 16 August 1988. (~ 1989 Butterworth Publishers 120

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0.0 39.5 0.40

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Product distribution (carbon %) C~ + C2 paraffins C3He C2-C4 olefines Cs+ aliphatics Aromatics

CO + CO2 Address reprint requests to Dr. Fujimoto, Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo,

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Oxidative dehydrogenation of lower paraffins: K. Fujimoto et al.

isomerized butane, Cz--C4 olefins, and aromatic hydrocarbons. The aromatics were composed of benzene, toluene, ethyl benzene, xylenes, and C9+ aromatics, among which toluene and xylenes were predominant. Although the protonic ZSM-5 (HZSM-5) was effective for cracking n-butane, the main product was not aromatic hydrocarbons but propane and C2-C 4 olefins under the conditions employed. It should be noted that the amount of propane was much higher than that of methane on the carbon basis, which means that propane was not produced through the simple cracking of butane as it has been already pointed out by Chen et al. s On the other hand, a GaZSM-5 catalyst showed a slightly higher conversion and gave an aromatic selectivity that was three times higher than that on HZSM-5. At the same time, the selectivity of Cz--C4 olefins was lower than that of HZSM-5. T h e phenomena can be apparently interpreted based on the assumption that the supported gallium acts as a promoter for aromatizing lower olefins to aromatic hydrocarbons, s However, when 5 mol% oxygen was added to the feed gas, the butane conversion on the HZSM-5 catalyst increased slightly from a value that corresponded to that on the GaZSM-5 catalyst and the selectivity of the aromatic hydrocarbons increased drastically from 4.1 to 16.4%. The new value was even higher than that from the GaZSM-5 catalyzed-run without 02 (13.1%). The formation of aromatics is promoted to either by added Ga, whose role has not been clarified yet, and added oxygen, which will be discussed below. Thus, the oxidative dehydrogenation on the GaZSM-5 gave the highest selectivity of aromatics. It should be also noted that the formation ratio of hydrogen which is defined as the mole ratio of hydrogen produced to the consumed n-butane, is much lower in the presence of oxygen in spite of much higher aromatic selectivity than in the absence of oxygen. The selectivities of carbon oxides (CO + CO2) were around 2% on carbon base for both catalysts. Figure I shows the results of propane conversion at 550°C as a function of oxygen content in the feed gas. It is clear from the data that both the conversion of propane and the selectivity of aromatic hydrocarbons increased with an increase in the oxygen content. Howevei', the formation ratio of hydrogen, which is defined as the molar ratio of product hydrogen to reacted propane, decreased with increasing oxygen content. The selectivity of carbon oxides, which are mostly composed of CO2, was 1.5% based on carbon at the highest point. The conversion of n-butane levels off above 5 vol% of oxygen. T h e promotional effect of oxygen could be attributed to the following three possible reasons: (1) the oxidative dehydrogenation of n-butane to butenes, (2) the in situ removal of coke on active sites (acid site) for aromatics formation with added oxygen, and (3) the removal of surface hydrogen on zeolite as water that regenerates the active site of hydrogen abstraction from hydrocarbons. Vapor-phase noncatalytic oxidation of butane was also conducted at 450°C to estimate the contribution

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of noncatalytic oxidative dehydrogenation of butane to butenes, revealing that its contribution was less than 0.5% of the total conversion. Therefore, the possibility of reason (1) is ruled out. Reason (2) could possibly be ruled out because the rate of coke deposition was even higher in the presence of oxygen than in the absence of oxygen. Thus, the most plausible reason for the promotional effect of oxygen is the removal of surface hydrogen, which will be discussed below. Based on reason (3), the leveling off of the butane conversion should be attributed to the shift of the rate-determining step from the removal of hydrogen from the active site to the abstraction of hydrogen from n-butane by active site. Figure 2 shows the catalytic performances of HZSM-5 for n-butane conversion as a function of reaction temperature. Both butane conversion and oxygen conversion increased monotonically with increasing temperature. The level of butane conversion was always higher in the presence of oxygen. The selectivity of aromatic hydrocarbons decreased with a rise in temperature to the minimum level at 450°C. The lowest aromatic selectivity at 450°C should be attributed to the secondary aromatization of propane, which was the main byproduct of butane cracking on HZSM-5, at higher temperature. The formation of carbon oxide was quite low at any temperature. The

ZEOLITES, 1989, Vol 9, March

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Oxidative dehydrogenation of lower paraffins: K. Fujimoto et al.

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122

ZEOLITES, 1989, Vol 9, March

selectivity of carbon oxides was about 9% based on carbon and about 10% based on oxygen below 450°C. All the above-mentioned phenomena suggest that hydrogen reacted with oxygen to form H20; namely, oxidative dehydrogenation of butane or propane proceeded on the HZSM-5. In fact, a substantial amount of water (not determined) was detected in the effluent gas. It should also be noted that a marked increase in the aromatic formation on the HZSM-5 occurred in the presence of oxygen, which means that HZSM-5 itself exhibits a high ability of aromatic formation if the product hydrogen is effectively removed from the reaction system. It is possible that hydrogen was oxidized in the gas phase to water. In fact, noncatalytic conversion of hydrogen to water u n d e r present conditions was about 80% for either the noncatalytic or catalytic reaction. The fact that gaseous hydrogen had little effect on the formation of aromatics on HZSM-5 (Table 1) suggests that the oxidation of hydrogen proceeded on the zeolite surface. If hydrogen is oxidized in the gas phase, the selectivity of aromatic hydrocarbons should never be affected by the added oxygen, because it merely lowers the hydrogen concentration in the gas phase. If the oxidation of hydrogen occurred on the zeolite, the surface concentration of hydrogen is decreased by added oxygen and thus the chance of dehydrogenation of hydrocarbons including adsorbed hydrocarbon species is increased. In the absence of oxygen, on the contrary, hydrogen atoms transferred from the hydrocarbons should stay longer on the zeolite surface to react with the hydrocarbon species, which are the intermediates from paraffin to aromatics to form olefins or paraffins ~ and thus suppress the formation of aromatic hydrocarbons.

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