Surface acidity and catalytic activity of sulfated zirconia.

摘要

A faster, more precise method (TPD/IR) is presented for quantitative measurements of Lewis and Bronsted acidity on solid acid catalysts (SAC), using temperature programmed desorption of pyridine (TPD), with simultaneous diffuse reflectance FTIR measurement (DRIFTS). By correcting IR data in the reflectance domain, before converting to the Kubelka-Munk domain, good repeatability is obtained. The method is applied to {dollar}rm ZrOsb2{dollar} and {dollar}rm SOsb4/ZrOsb2{dollar} (SZ) solid acids, clearly resolving bimodal Lewis strength distributions on {dollar}rm ZrOsb2{dollar} and SZ. Bronsted acidity is present only on SZ, and a net gain of pyridine on SZ Bronsted sites between 25 and {dollar}rm 150spcirc C{dollar} is attributed to surface migration from Lewis sites. A simple model of pyridine desorption and surface migration from 2 types of Lewis sites and one type of Bronsted sites produces excellent agreement with the experimental data, and indicates that sulfation increases the average strength of the Lewis sites that desorb above {dollar}rm 175spcirc C.{dollar}; A pulsed feed alkylation study of gas-phase 2-butene with isobutane was conducted over sulfated zirconia (SZ) to determine the effect of temperature and feed composition on the dominant reactions and deactivation mechanisms. For all conditions, high initial conversion of 2-butene to {dollar}rm Csb5{dollar} was observed, which is attributed to alkylation followed by rapid cracking on the strongest acid sites. These sites deactivated quickly, and then the steady state catalyst behavior was observed to be different for different temperatures. During alkylation at {dollar}rm 50spcirc C,{dollar} the catalyst deactivated after 40 pulses, which is attributed to the build-up of trimethylpentane (TMP) products which do not readily desorb from the catalyst surface at this temperature, and was verified when pulses of pure 2,3,4-TMP over SZ produced similar cracking results. At {dollar}rm 100spcirc C,{dollar} the adsorbed alkylation and/or oligomerization products were very selectively cracked to form 2-butene, resulting in a net dehydrogenation of isobutane to 2-butene. At temperatures between {dollar}rm 150spcirc C{dollar} and {dollar}rm 250spcirc C,{dollar} the product selection was dominated by high cracking rates, but the catalytic activity remained stable with nearly complete conversion of 2-butene at the highest temperatures. It was also found that the deactivated catalyst at {dollar}rm 50spcirc C{dollar} could be regenerated by simply heating to {dollar}rm 150spcirc C,{dollar} which apparently removed the adsorbed hydrocarbons by either volatilization or cracking.