Wells-Dawson heteropolyacid supported on silica:isobutane alkylation with C4 olefins

摘要

The alkylation of isobutane with C4 olefins is studied, using heteropolyacids (HPA) with Wells-Dawson (WD) structure supported on silica (WD/SiO_2). The catalytic performance of these catalysts is cotnpared with a lanthanum-exchanged Y-zeolite catalyst. The loading of the HPA on silica was varied between 9 and 28 wt.%. These catalysts have activity for trimethylpentanes (TMP) production. The selectivity towards these products is not as high as in the case of the lanthanum containing Y-zeolite. The acidity of WD/SiO_2 catalysts increases as the loading increases, as seen by tH MAS-NMR. Correspondingly, a hetter TMP production is observed. The increase both in acidity and in the TMP production as a function of the WD content is more noticeable at low loading. The coke formed during thc rcaction requires high temperatures, 550 deg C approximately, in order to be fully removed with an oxygen containing carrier gas. The temperature-programmed oxidation (TPO) profile of this coke displays two peaks, the first one between 80 and 300 deg C associated with hydrocarbons that are released upon heating, and the second between 300 and 550 deg C, associated with coke that changed its structure during the heating. A regeneration at intermediate temperatures, e.g. 300 deg C, removes the coke that corresponds to the first peak, but does not restore the initial activity. If the regeneration is carried out at higher temperatures, e.g .500 deg C, most of the coke is removed. but this treatment leads to changes in the structure of the HPA. as indicated by FTIR and MAS-NMR analysis. Regeneration with O_3 at low temperature (125 deg C) is effective both for coke removal and to recover the catalytic activity.