CONVERSION OF METHANOL AND LIGHT OLEFINS TO GASOLINE OVER A SHAPE SELECTIVE CATALYST ZSM-5
HWU, FRANK SHAW-SHYONG
Doctor of Philosophy
Reactions of methanol and some light olefins (including ethylene, propylene, butenes, and 3-methylbutene-1) were studied over Ni-ZSM-5 zoelite catalyst. The product distribution over a broad range of space velocities indicates the reaction sequence: dimethyl paraffins methanol (,ether) C(,2)-C(,5) olefins (,aromatics) Monomethyl aliphatics predominate over both straight-chain and dimethyl hydrocarbons. Aromatics consist mainly of toluene and xylene mixtures. At 368(DEGREES)C, compositions of butene isomers are in thermodynamic equilibrium, over the range of residence times investigated (i.e. (tau) = 22 to 648 mg-sec/cc). Equilibrium composition of xylenes is not established until (tau) = 1440 mg-sec/cc; p-xylene is kinetically favored at short residence times. The effect of temperature on the reaction of propylene shows that a maximum in conversion is achieved at 310(DEGREES)C. As the temperature goes above 310(DEGREES)C, conversion decreases, indicating the occurrence of reversible cracking reactions. Temperature is also a crucial factor in determining the product distribution, for no aromatics are observed for reaction temperatures below 310(DEGREES)C. In both propylene and cis-butene-2 conversion experiments, the effect of temperature on isomer compositions (for butenes and pentenes) suggests that positional and/or configurational isomerization is an easier reaction than skeletal isomerization whose rate increases rapidly at higher temperatures. Comparative sorption studies for a number of hydrocarbons show the following order of rate of sorption: monomethyl dimethyl n-paraffins (,paraffins) aromatics (,paraffins) Isosteric heats of sorption obtained from adsorption isotherms are 11.5 kcal/mole for n-butane and 13.6 kcal/mole for n-pentane. The activation energy for diffusion of benzene in the Ni-ZSM-5 catalyst is 7 kcal/mole. All isotherms investigated can be represented by the Langmuir equation. ('14)C-labeled tracer studies confirm the important role that ethylene, propylene and butenes play as active intermediates in the over all methanol-to-gasoline reaction, although ethylene shows less reactivity than propylene and butenes. Isobutane, n-hexane and n-heptane are relatively stable final products, but the latter two paraffins can undergo some cracking reactions. Benzene and toluene can alkylate with methanol to form toluene and to a lesser extent the xylenes. No cracking of the benzene ring is observed at 368(DEGREES)C.