Effect of phosphorus on HZSM-5 catalyst for C4-olefin cracking reactions to produce propylene

A series of HZSM-5 catalysts with various phosphorus (P) loadings (0-2.1 wt%) were prepared by impregnation of phosphoric acid. The effect of phosphorus on the structure and acidity of HZSM-5 catalyst and its catalytic performance for C-4-olefin cracking reactions were studied by XRD, N-2 adsorption, NMR, NH3-TPD, FTIR spectra of adsorbed pyridine, TGA, and catalytic tests. The results revealed that the structure of HZSM-5 samples was scarcely damaged with increasing P loadings, whereas strong acidity of them was weakened mainly due to the dealumination of tetrahedral framework aluminum (TFAL) after P modification. The results of C-4-olefin cracking reactions showed a great increment of the propylene selectivity for the P-modified HZSM-5 with the optimal propylene yield at a P-loading of 1.5 wt%. It was found that both coke deposition and dealumination were the crucial factors in the deactivation of HZSM-5. Due to the dealumination and thus elimination of part of the strong acid sites, P-modified HZSM-5 showed excellent anticoking ability. TFAL of P-modified HZSM-5 changed mainly to distorted tetrahedral aluminum (DTAL) or pentacoordinated aluminum (PAL) species in the process of C-4-olefin cracking reaction, which can still act as stable active centers for C-4-olefin cracking to produce propylene. Thus, P-modified HZSM-5 shows excellent stability. A possible reaction pathway for C-4-olefin cracking reactions has been proposed based on the reaction results. (C) 2007 Elsevier Inc. All rights reserved.