Acid sites requirements in the gas phase methylation of imidazole

NICOLÁS VANOY-VILLAMIL; CARLOS R. APESTEGUÍA; CRISTINA L. PADRÓ

Munich

Congreso; 15th International Congress on Catalysis; 2012

Dechema e.V.

The N-alkylation of heterocyclic compounds, such as imidazole or pirazole, is a useful reaction for the preparation of fungicidal, anticonvulsant, bactericidal and protozoacidal drugs. 1-Methylimidazole, in particular, can be used as cocatalyst for the synthesis of cyclic carbonates from carbon dioxide and epoxides, also as raw material in the manufacturing of ionic liquids, which can replace conventional solvents for catalytic and organic reactions. The gas phase methylation of imidazole has been studied on acid zeolites such as HY, HZSM5 and HMOR and mixed oxides of Al and Mg. However, further analysis of the reaction including the identification of the catalytic active sites and catalyst stability is still required. The catalysts based on Y zeolite (ZnY and HY) exhibited the greatest concentrations of acid sites. The acid strength of HPA/SiO2 was clearly higher with a desorption peak at 900 K (TPD profile not shown here) but the acid sites density per gram was the lowest. HY and HBEA zeolites showed a similar contribution of both Brønsted and Lewis acidity. The incorporation of Zn2+ in the Y zeolite generated essentially strong Lewis sites that after evacuation at 573 K retained 75% of pyridine remaining after evacuation at 423 K (Table1). The acidity of HPA/SiO2 and HMCM22 was predominantly Brønsted, with a Brønsted/Lewis (B/L) ratio of 6 and 3 (Tdes=423 K).2 was clearly higher with a desorption peak at 900 K (TPD profile not shown here) but the acid sites density per gram was the lowest. HY and HBEA zeolites showed a similar contribution of both Brønsted and Lewis acidity. The incorporation of Zn2+ in the Y zeolite generated essentially strong Lewis sites that after evacuation at 573 K retained 75% of pyridine remaining after evacuation at 423 K (Table1). The acidity of HPA/SiO2 and HMCM22 was predominantly Brønsted, with a Brønsted/Lewis (B/L) ratio of 6 and 3 (Tdes=423 K).2+ in the Y zeolite generated essentially strong Lewis sites that after evacuation at 573 K retained 75% of pyridine remaining after evacuation at 423 K (Table1). The acidity of HPA/SiO2 and HMCM22 was predominantly Brønsted, with a Brønsted/Lewis (B/L) ratio of 6 and 3 (Tdes=423 K).2 and HMCM22 was predominantly Brønsted, with a Brønsted/Lewis (B/L) ratio of 6 and 3 (Tdes=423 K).des=423 K). The selectivity to the desired product was always higher than 95%. It is noteworthy that all catalysts deactivated during the course of the reaction, although it was more significant on catalysts which posses higher density of strong Brønsted acid sites such as HY and HMCM22 (Figure 2 and Table1). Solids that have strong acid sites of Lewis or Brønsted nature almost exclusively were less active. Indeed, the yields at t=0 h on ZnY and HPA/SiO2, obtained by extrapolating the respective curves to the origin, were 20% and 14% respectively. On zeolites HY and HBEA, which have B/L ratio closed to 1, the highest yields at t=0h (100% and 74%) were obtained, denoting that the simultaneous presence of Brønsted and Lewis acid sites promoted efficiently the N-alkylation reaction.2, obtained by extrapolating the respective curves to the origin, were 20% and 14% respectively. On zeolites HY and HBEA, which have B/L ratio closed to 1, the highest yields at t=0h (100% and 74%) were obtained, denoting that the simultaneous presence of Brønsted and Lewis acid sites promoted efficiently the N-alkylation reaction.