Solid acid characteristics and isobutane/butene alkylation

Catalytic behavior during gas-phase, batch alkylation experiments employing isobutane and 1-butene at 80 °C was studied for Beta zeolites with SiO2/Al2O3 (SAR) ratios of 25 (25BEA) and 75 (75BEA), ZSM-5 and sulfated zirconia (SZ). The observed alkylation performance was compared to the low-temperature hydride transfer (HT) activity of the studied catalysts and with their acidity, adsorption capacity and surface area measurements. For all materials studied, a correlation was observed between the measured HT activity and the amount of trimethylpentane (TMP) produced per acid site. The 25BEA catalyst produced the highest amount of trimethylpentane (TMP) and also had the most elevated HT activity. ZSM-5 was found to be inactive for both HT and alkylation, and although SZ had moderate alkylation activity, it also had a higher cracking activity than other materials. Modification by water vapor exposure of 25BEA and SZ did not create any noticeable change in Brönsted acidity, but a dramatic decrease in TMP production was detected on the modified catalysts and explained by competitive adsorption between water and isobutane. The apparent butene conversion correlated well with the total catalyst surface area rather than with the total amount of acid sites. In all the catalysts studied a much stronger interaction with butene than with isobutane was observed, suggesting that competitive adsorption between the two reactants limits isobutane access to the active sites, resulting in limited hydride transfer.