Monomethyl-Branching of Long n-Alkanes in the Range from Decane to Tetracosane on Pt/H-ZSM-22 Bifunctional Catalyst

Abstract

Single long n-alkanes in the range n-C10–n-C24 were hydroisomerized at 233° C in a fixed-bed down-flow vapor phase reactor loaded with Pt/H-ZSM-22 catalyst. The conversion was varied by varying the contact time. Up to ca. 60% conversion, the n-alkane molecules undergo almost exclusively a single methyl branching. In this conversion range, the distribution of positional methyl-branched isomers remains constant. The distributions of the positional methyl-branched isomerization products obtained with n-C12 and longer n-alkanes are typically bimodal. The first maximum in the methylalkane product distribution occurs at the 2-methyl-branched isomer. There is a minimum in the distribution at the 4-methylbranched isomer. The second maximum is broad and occurs at methyl positions at C5–C11, depending on the carbon number. These peculiar product distributions can be explained by the different pore mouth and key lock modes of physisorption of these long n-alkanes in pore openings of the zeolite. The physisorption enthalpies and entropies were estimated from molecular models of the positional methyluncosane isomers and a giant cluster of ZSM-22 framework having 1300 oxygen atoms. The physisorption energies are very large and dominate the reaction coordinate. The branching is always formed in that part of the chain that is residing in a pore mouth. In one favorable adsorption mode, designated as the pore mouth, the n-alkane penetrates partially into one pore opening only. The pore mouth mode favors branching at C2 and is less favorable at more central positions along the carbon chain. In the second favorable adsorption mode, designated as the key lock type, the two ends of the hydrocarbon chain penetrate each into a different pore opening. This mode favors more central branching of the chain. The contribution of the key lock mode increases with increasing carbon number.