Effect of iron and phosphorus on HZSM-5 in catalytic cracking of 1-butene

Abstract

HZSM-5 zeolites modified by phosphorus and/or iron were prepared. The physicochemical features of catalysts were characterized by XRD, N2 adsorption, 27Al MAS NMR, UV–vis, NH3-TPD, FTIR of adsorbed pyridine and CO2-TPD, and their performance for the cracking of 1-butene was measured. It was found that phosphorus modification significantly reduced the concentration of acid sites, especially strong ones. This suppressed hydrogen transfer reactions and enhanced the selectivity to propylene, while decreasing the butene conversion. Phosphorus modification also promoted the stability of the catalyst. Iron modification led to relatively little change on the acidity of the catalyst. However, the basicity increased after the introduction of iron. Accordingly, iron modification also suppressed hydrogen transfer reactions. After adding iron into phosphorus modified HZSM-5, the interaction between iron and phosphorus released some Brønsted acid sites which were previously neutralized by phosphate, increasing butene conversion and propylene selectivity compared with phosphorus modified HZSM-5. The presence of phosphorus promoted the dispersion of iron cations. Highest selectivity to propylene was observed on the catalyst co-modified by iron and phosphorus.