Inhibition effect of Na+ form in ZSM-5 zeolite on hydrogen transfer reaction via 1-butene cracking

Abstract

The Na+ and H+ of ZSM-5 having three Si/Al molar ratios (20, 35, and 50) were synthesized to investigate and illustrate 1-butene cracking mechanism. The physical and chemical properties of ZSM-5 were described by XRD, SEM-EDX, N2 physicsorption, FT-IR, pyridine-IR, 1H MAS NMR, TPO, and NH3-TPD techniques. For the characterization, the Na-ZSM-5 consisted of two main types of acid sites (weak and medium acid sites), whereas strong, medium, and weak acid sites were detected on H-ZSM-5. For H-ZSM-5, the Si-(OH)-Al site (strong Brønsted site) was an active site for 1-butene cracking reaction, while the active site of Na-ZSM-5 in the reaction was silanol nest (moderate Brønsted site). The 1-butene cracking reaction on H-ZSM-5 and Na-ZSM-5 was studied under the operating condition of temperature =500 °C, pressure =1 atm, and WHSV =3 h−1. The oligomerization, cracking, and hydrogen transfer were three main reactions in the 1-butene cracking mechanism. The alkenes excluding propylene occurred on weak acid site, while the formation of propylene underwent on medium acid site of ZSM-5 catalyst. The presence of strong acid site on ZSM-5 catalyst promoted the hydrogen transfer reaction, which provoked the alkane formation. The hydrogen transfer index in 1-butene reaction of Na-ZSM-5 was less than H-ZSM-5 in all Si/Al molar ratios because of the absence of strong acid. The Na-ZSM-5 (Si/Al molar ratio = 20) reached the highest propylene selectivity at 31.73 C-wt.% and the Na+ ion could block the strong acid site. At low Si/Al molar ratio, the stability of Na-ZSM-5 was higher than H-ZSM-5 because of the lower coke formation. Therefore, the Na-ZSM-5 was more suitable catalyst than H-ZSM-5 for propylene formation in 1-butene cracking reaction.