Isomerization of Alkanes on Sulfated Zirconia: Promotion by Pt and by Adamantyl Hydride Transfer Species

Abstract

Sulfated zirconia is a strong solid acid that catalyzes isomerization of alkanes when metal crystallites are also present. Isomerization of C 7 and higher alkanes leads to high cracking selectivities on Pt/ZrO 2-SO 4 and other solid and liquid acids, limiting industrial isomerization practice to C 4-C 6 feeds. For example, n-hexane isomerizes to isohexanes at 473 K with 99% selectivity on Pt/ZrO 2-SO 4 but n-heptane isomerization selectivities are only about 50% even at low conversions (<20%). Our work shows that hydride transfer species, such as adamantane, increase isomerization rates and inhibit CC scission reactions. n-Heptane isomerization rates show positive hydrogen kinetic orders, suggesting that the reaction proceeds on Pt/ZrO 2-SO 4 via chain transfer pathways, in which carbenium ions propagate, after a chain initiation step involving loss of hydrogen from alkanes, by hydride transfer from neutral species to carbocations. These pathways contrast with those involved in the bifunctional (metal-acid) catalytic sequences usually required for alkane isomerization, in which metal sites catalyze alkane dehydrogenation and acid sites catalyze skeletal rearrangements of alkenes. Rate-limiting hydride transfer steps are consistent with the strong influence of molecular hydride transfer agents such as adamantane, which act as co-catalysts and increase isomerization rate and selectivity. The addition of small amounts of adamantane (0.1-0.8 wt%) to n-heptane increases isomerization rates by a factor of 3 and inhibits undesirable cracking reactions. Adamantane increases hydride transfer and carbenium ion termination rates, thus reducing the surface residence time required for a catalytic turnover. As a result, desorption occurs before secondary cracking of isomerized carbenium ions. Less effective hydride transfer agents ( n-alkanes, isoalkanes) also increase n-alkane isomerization rate and selectivity, but require much higher concentrations than adamantane. Dihydrogen also acts as a hydride source in alkane isomerization catalysis, but it requires the additional presence of metals or reducible oxides, which catalyze H 2 dissociation and the formation of hydridic and protonic forms of hydrogen.