Additional acid site on HZSM-5 treated with basic and acidic solutions as detected by temperature-programmed desorption of ammonia

Abstract

Through the temperature-programmed desorption of ammonia, an additional acid site (named h +-peak) was identified on an HZSM-5 treated with ammonia water and acidic solutions. The h +-peak appeared at ca. 780 K, higher than that for a normal acid site. The strength of the additional acid site Δ H was calculated to be 185–192 kJ mol −1 on the basis of a theoretical equation. A small amount of nitrogen appeared simultaneously with the h +-peak. The strong acid sites were characterized using infrared spectroscopy, nuclear magnetic resonance (NMR), and catalytic cracking of octane. Quantitative measurements revealed that the acid site on the HZSM-5 treated with ammonia water consisted of two aluminum atoms, one of which was the extra-framework Al. On the other hand, the acid site on the HZSM-5 treated with acidic solutions consisted of one to two Al. On the HZSM-5 treated with ammonia water, the strength of Brønsted acid site is enhanced by the attached Al-containing species, e.g., Al-(OH) 2+, and the enhanced Brønsted acid site played a prominent role in the catalytic cracking. Under the treatment with an acidic solution, however, the very strong acid sites that were inactive in catalytic reaction were formed. Discussion was made on the structure of the very strong acid sites.