57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The failure of direct methanol C-C coupling to explain prolonged induction periods with increasing methanol purity, product isotopologue distributions in labeling experiments, and high activation barriers for ylide formation via computational methods supports an indirect "hydrocarbon pool mechanism" for the conversion of methanol to hydrocarbons. This mechanism states that unsaturated hydrocarbons trapped within the zeolite framework undergo methylation, dealkylation, and hydrogen transfer reactions to generate the observed MTH products over zeolite and zeotype materials, in effect, acting as co-catalysts. Olefins and aromatics have been shown to be present within the zeolite framework under MTH reaction conditions through spectroscopic and analytical techniques. Our work to date has focused on quantifying the rate and mechanism for olefin methylation and cracking reactions involved in the preferential synthesis of branched hydrocarbons from methanol.

The kinetics of ethylene, propylene, and butene (1-butene, cis-2-butene, trans-2-butene, iso-butene) were measured on MFI and BEA zeolites and for ethylene, propylene, and 1-butene on FER and MOR zeolites at low olefin chemical conversion (<0.2%) and high DME: olefin ratio (15-60:1). Measured kinetics of olefin methylation normalized per active proton center show that (i) rates and consistently higher on MFI and BEA zeolites compared to FER and MOR materials, and (ii) the rate of methylation is directly related to the degree of substitution about the double bond, which stabilizes intermediate carbocations through inductive electron donation. Comparing rate constants in Table 1 and the carbenium ion-like transition states formed, compounds with similar bond order about the carbocation have similar rate constants toward methylation with an order of magnitude gap between subsequent groups.

Table 1. Rate parameters for C₂-C₄ olefin methylation over zeolites. All k values reported are in h^-1 bar^-1 and normalized to 373 K. Activation energies are reported in kJ mol^-1

Concurrently, our focus has evolved to study cracking reactions of higher olefins which combined with olefin chain growth comprise the two elementary steps that make up the olefin methylation/dealkylation cycle in the conversion of methanol-to-hydrocarbons. Our results for cracking of 2-methyl butene and 2-pentene on proton form MFI and BEA zeolites show that each olefin undergoes facile isomerization reactions under reaction conditions (T = 723-773 K; P_olefin =0.1 to 1 kPa) to form an equilibrated mixture that subsequently cracks to form ethylene and propylene and that reaction rates are first order in the pressure of the pentene isomer. A comparison of measured rate parameters for 2-pentene and 2-methyl-butene β-scission on H-ZSM-5 is shown in Table 2. Consistent with our observation that equilibrium is established among C₅ olefins, nearly invariant rate parameters are obtained from these independent experiments concerning cracking pathways of pentene isomers. Zeolite BEA is noted to have a rate constant for cracking that is at least 20-fold higher than that for MFI under similar reaction conditions. These experimental observations explain why MFI is a better methanol-to-gasoline catalyst than BEA; the comparatively lower rate constant for cracking of higher olefins results in a higher production of C₅-C₈ gasoline-range hydrocarbons in the effluent when using MFI.

Table 2. Kinetic parameters for 1-pentene and 2-methyl-2-butene β-scission as inferred from measured ethylene and propylene synthesis rates in the effluent. At low chemical conversions (<0.5%) only ethylene and propylene are formed as products in a 1:1 ratio.

In summary, our work has shown that across FER, MFI, BEA, and MOR zeolite materials, olefin methylation rate constants increase and activation barriers systematically decrease with increasing olefin size, indicating that the relative stability of reaction intermediates increases with increasing carbon chain length. Our current work focused on assessing olefin methylation and cracking rates will enable us to compare the propagation of olefin methylation cycles across zeolites and result in a quantitative assessment of zeolite structure on catalyst function for the synthesis of highly branched C₄-C₇ alkanes.