Alkylation of thiophenic compounds over heteropoly acid H3PW12O40 supported on MgF2

The alkylation of sulfur-containing compounds (benzothiophene and thiophene) with 2-methyl-1-pentene was investigated over MgF2-supported heteropoly acid (H3PW12O40: HPW) as catalysts under mild conditions (85 °C, atmospheric pressure). The magnesium fluoride used as support was prepared by a sol–gel method and exhibited a high specific surface area (186 m2 g−1). Several solids containing various amounts of HPW (in the range of 5 wt% to 46 wt%) were prepared by incipient wetness impregnation and fully characterized (ICP–OES, XRD, N2 adsorption, TEM, IR, adsorption of pyridine followed by IR, 19F and 31P MAS NMR). For all solids, it was observed that the Keggin structure of HPW was preserved and that the heteropoly acid was finely dispersed on the MgF2 support. A significant modification of the textural properties of the sample containing the higher amount of HPW (46 wt%) was observed and attributed to a pore blocking phenomenon by HPW. As expected, the HPW loading in the range of 5–30 wt% allowed an increase of the Brønsted acidity. Whatever the catalyst, benzothiophene was only transformed by mono-alkylation, whereas the olefin underwent alkylation, oligomerization and isomerization, the latter being largely predominant and yielded 2-methyl-2-pentene. The increase of the HPW loading until 30 wt% led to an increase of the catalytic activity, the selectivity being not modified. In addition, a significant deactivation of catalysts containing 30 wt% and 40 wt% of HPW was observed and attributed to a strong adsorption of the alkylated products. Thiophene presented a poor reactivity through alkylation compared to benzothiophene.