Characterization of solid acid catalysts for isobutane/butene alkylation
By Alexandru Platon, Ph.D.
Washington State University
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
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.
School:Washington State University
School Location:USA - Washington
Source Type:Master's Thesis
Date of Publication: