Alkylation of thiophenic compounds over heteropoly acid H3PW12O40 supported on MgF2
Résumé
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 degrees C, atmospheric pressure). The magnesium fluoride used as support was prepared by a sal-gel method and exhibited a high specific surface area (186 m(2) 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, N-2 adsorption, TEM, IR, adsorption of pyridine followed by IR, F-19 and P-31 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 Bronsted 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.