Characterization of acylating intermediates formed on H-ZSM-5

The adsorption of typical acylating agents (acetic acid, acetic anhydride and acetyl chloride) on the Bronsted-acid sites in H-ZSM-5 has been investigated with temperature-programmed-desorption (TPD) and thermogravimetric analysis (TGA), infrared spectroscopy and C-13 NMR. Acetic acid forms a hydrogen-bonded complex with the Bronsted sites that is characterized by v(C-H) stretching frequencies of 1740 and 1750 cm(-1). Both acetyl chloride and acetic anhydride react with the Bronsted sites below 400 K, forming an acetyl-zeolite intermediate and either HCI or acetic acid, characterized by a v(C-H) stretching frequency of 1705 cm(-1). The acetyl-zeolite intermediate reacts readily at room temperature with either water (forming acetic acid) or ammonia (forming acetamide). C-13 NMR isotropic shifts for the C-2 carbon of the acetic-acid adsorption complex and the acetyl-zeolite intermediate occur at similar frequencies, making them difficult to distinguish by this technique. In TPD, the acetyl-zeolite intermediate decomposes, with CO2 and acetone being the primary initial products. Most of acetic acid desorbs unreacted, although a fraction also forms CO2 and acetone, suggesting the formation of an acetate intermediate at higher temperatures. The implications of results for understanding acylation reactions on zeolites are discussed. (C) 2003 Elsevier B.V. All rights reserved.