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A plausible reaction mechanism for propylene (C(3)H(6)) production from ethylene (C(2)H(4)) was investigated, based on the amounts of effluent hydrocarbons and hydrocarbons produced in the pores of SAPO-34. Propylene was produced via an oligomerization-cracking mechanism. On the basis of this mechanism, the conversions of C(2)H(4), pentenes, and hexenes were examined. The catalytic performance was compared, in order to investigate the role of the pore volume of zeolites with 8-, 10-, and 12-membered rings in the selective production of C(3)H(6). The selectivity for C(3)H(6) was crucially dependent upon the pore volume of the zeolite. Highly selective production of C(3)H(6) from olefins (C(2)H(4), pentenes, and hexenes) can be accomplished by employing a new concept: adjusting the pore volume of a zeolite to accommodate the volume of an olefin and/or its carbenium cations, as opposed to a conventional molecular sieve approach. For example, an unimolecular cracking of pentenes into C(3)H(6) and C(2)H(4) involving primary cations can be controlled by the pore volume of a zeolite.

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The role of the zeolite cavity in the production of C 3 H 6 from butene was investigated using various zeolites with pore structures of 8-, 10-, and 12-membered rings (MRs). The reaction mechanism is discussed on the basis of which octyl carbocations produced C 3 H 6 at low conversions of butene. The selectivity for C 3 H 6 was highly dependent on the volume of the zeolite cavity but not on the entrance pore diameter. The optimum cavity volumes of zeolites with 8-, 10-, and 12-MR entrance pore structures were similar, while the highest C 3 H 6 selectivity among the zeolites was different. The most selective production of C 3 H 6 can be accomplished by matching the volume of the specific octyl carbocation to that of the zeolite cavity. This concept can be employed to explain the selective production of C 3 H 6 according to a proposed reaction model. Furthermore, the reaction mechanism for the production of C 3 H 6 from C 2 H 4 was also investigated at low conversion of C 2 H 4 . C 3 H 6 was produced by the β-scission of the same specific octyl carbocations in the conversions of both butene and C 2 H 4 .

To investigate the effect of SAPO-34 particle size (with a fixed Si mole fraction in its framework) and that of the Si mole fraction (in a SAPO-34 framework with fixed particle size) on propylene selectivity and production rate for the conversion of ethylene to propylene, SAPO-34 was prepared by hydrothermal synthesis using tetraethyl ammonium hydroxide or morpholine as a structural agent. The conversion of ethylene was carried out at 473 K using SAPO-34. The selectivity for propylene, the rate of propylene production, and the lifetime of the catalyst were strongly influenced by the catalyst crystal size. The SAPO-34 with a approximately 2.5 microm particle size had the highest selectivity for propylene (approximately 80%) up to a high conversion of ethylene (approximately 70%), while SAPO-34 with smaller particles had a longer catalyst lifetime, implying that catalyst deactivation was suppressed. The mole fraction of Si in the SAPO-34 framework with fixed particle size had little influence on the selectivity for propylene, indicating that the acid strength of SAPO-34 is independent of the Si mole fraction and all protons in SAPO-34 behave equivalently. Furthermore, the acid strength of protons determined by the measurements of NH(3)-TPD (temperature-programmed desorption) spectra did not depend on either the Si mole fraction or the SAPO-34 particle size. This result was also evident in the cracking rate of n-butane, which increased proportionally with increasing number of protons in SAPO-34.The number of protons generated by the incorporation of Si4+ into the SAPO-34 lattice increased proportionally, up to one Si atom introduced into every cage of SAPO-34, but did not continue to increase with further introduction of Si4+ into the lattice.