Monomethyl-branching of long n-alkanes in the range from decane to tetracosane on Pt/H-ZSM-22 bifunctional catalyst

Single long n-alkanes in the range n-C-10-n-C-24 were hydroisomerized at 23 3 degrees C in a fixed-bed down-how vapor phase reactor loaded with Pt/H-ZS M-22 catalyst. The conversion was varied by varying the contact time. Up to ca. 60% conversion, the n-alkane molecules undergo almost exclusively a si ngle methyl branching. In this conversion range, the distribution of positi onal methylbranched isomers remains constant. The distributions of the posi tional methylbranched isomerization products obtained with n-C-12 and longe r n-alkanes are typically bimodal. The first maximum in the methylalkane pr oduct distribution occurs at the 2-methyl-branched isomer. There is a minim um in the distribution at the 4-methylbranched isomer. The second maximum i s broad and occurs at methyl positions at C-5-C-11, depending on the carbon number. These peculiar product distributions can be explained by the diffe rent pore mouth and key lock modes of physisorption of these long n-alkanes in pore openings of the zeolite. The physisorption enthalpies and entropie s were estimated from molecular models of the positional methyluncosane iso mers 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 m outh, the n-alkane penetrates partially into one pore opening only. The por e mouth mode favors branching at C-2 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 pene trate each into a different pore opening. This mode favors more central bra nching of the chain. The contribution of the key lock mode increases with i ncreasing carbon number. (C) 2000 Academic Press.