Effect of framework single and close (pairs and un-pairs) aluminum atoms on phosphorous modification of HZSM-5 in cracking of liquefied petroleum gas to ethylene and propylene

Abstract

To investigate the effects of framework single and close Al atoms on modification of HZSM-5 with phosphorus and steam in liquefied petroleum gas (LPG) cracking to light olefins, HZSM-5 zeolites were synthesized with two Al sources including Al(NO3)3 and Al(OH)3. The unmodified, P-modified HZSM-5 and steam-treated P/HZSM-5 zeolites were characterized by XRD, BET, 27Al MAS NMR and NH3-TPD techniques. The Al locations in the HZSM-5 zeolites were determined by combining the results of XRF and diffuse reflectance UV–visible spectroscopy. The catalytic performance showed that the olefin yields and catalytic stability of HZSM-5(1) synthesized with higher single Al atoms and lower Al atoms at channel intersections (β sites) was greater than that of HZSM-5(2) synthesized with higher close and pair Al atoms. The reduction of acid site strength and increment of weak to strong (W/S) acid ratio after P modifications and steam treatment resulted in a significant increase of olefin yields and a decrease of C5+ yields and coke amount. The catalytic stability and propylene to ethylene (P/E) ratio for P/HZSM-5(1) with lower strong acid sites was greater than that of P/HZSM-5(2). The P/E ratio, deactivation rate, propylene yield and olefin yields were found to be 1.005, 13.58%, 23.42% and 46.72% for P/St/HZSM-5(1) and 0.959, 15.49%, 20.09% and 41.04% for P/St/HZSM-5(1) after 240 min. The catalytic results showed that the highest and lowest catalytic stability were related to the P/St/HZSM-5(1) and HZSM-5(2) catalysts, respectively.