Characterization of solid acid catalysts for isobutane/butene alkylation

by 1972- Platon, Alexandru

By Alexandru Platon, Ph.D. Washington State University December 2004 Chair: William J. Thomson The extinction coefficient ratio (ECR) of coordinatively bonded pyridine (Lpy) and protonated pyridine (Bpy) was determined by diffuse-reflectance FTIR spectroscopy (DRIFTS), by exposing sulfated zirconia (SZ) containing chemisorbed pyridine to water vapor. The previously suggested portability of published ECR values between different IR spectroscopy techniques or different types of materials was found questionable. For the SZ samples analyzed, an ECR value of 2.05 was determined, which allowed the measurement of an initial Lewis/Brönsted acidity ratio of 1.1 in the freshly activated material. This ECR determination method should be applicable to other similar solid acids. In a separate study, a new model test reaction was proposed for the estimation of lowtemperature hydride transfer (HT) activity of solid acids. The reaction of cyclohexene with isobutane on zeolites Beta having SiO2/Al2O3 ratios of 25 (25BEA) and 75 (75BEA), ZSM-5 and SZ, distinguished between disproportionation/hydrogen transfer (DHGT) and HT. HT was enhanced in 25BEA due to its higher acid density. On the other hand, the very high acid density SZ was less active than expected for HT due to its low isobutane adsorption capacity. ZSM-5 completely lacked HT activity although it showed significant DHGT activity. Finally, the catalytic behavior of the mentioned catalysts was studied in gas-phase, batch alkylation experiments employing isobutane and 1-butene at 80 °C. The observed alkylation iv performance was compared to their low-temperature HT activity, total acidity, adsorption capacity and surface area. The measured HT activity correlated with the amount of trimethylpentane produced per acid site for all materials with 25BEA being the most active and ZSM-5 being totally inactive for alkylation. Although of moderate alkylation activity, SZ had a higher cracking activity than other materials. Modification by water vapor exposure of 25BEA and SZ did not noticeably change their Brönsted acidity, but selectively lowered their alkylation activity 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. Evidence indicates that competitive adsorption with butene limits isobutane access to the active sites in all studied materials, resulting in limited hydride transfer. v