Modification of acid sites in ZSM-5 by ion-exchange: An in-situ FTIR study

Abstract

A study of the acid sites in M n+ -ZSM-5 zeolites (M n+ = H + , Al 3+ , Ca 2+ , and Ba 2+ ), synthesized by ion-exchange from the NH 4 + form, has been carried out using X-ray diffraction, 27 Al MAS NMR, and in-situ FTIR spectroscopy. X-ray diffraction data indicate that the crystalline structure of ZSM-5 is maintained on ion-exchange. 27 Al MAS NMR spectroscopy confirms that Lewis acid sites in all of the M n+ -ZSM-5 zeolites are mainly located on the tetrahedral aluminum atoms in the zeolite framework. However, octahedral extra-framework aluminum is another source of Lewis acid sites in H + -ZSM-5 and Al 3+ -ZSM-5. Brønsted acid sites are identified as originating from the hydroxyl groups that bridge Al and Si atoms. The acid sites in M n+ -ZSM-5 have been characterized by in-situ FTIR spectroscopy with pyridine as the probe molecule. FTIR spectroscopy demonstrated that the number of accessible acid sites and ratio of Lewis to Brønsted acid sites in M n+ -ZSM-5 can be effectively modified by ion-exchange. The number of acid sites in M n+ -ZSM-5 increases in the same order as the acidity of cations with Ba 2+ < Ca 2+ < NH 4 + < Al 3+ < H + . Though the strength of both Lewis and Brønsted acid sites is virtually identical for all M n+ -ZSM-5 zeolites, the ratio of Brønsted to Lewis acid sites varies as a result of the loaded cation. In addition, modification of acid sites in M n+ -ZMS-5 by loading different cations is discussed in terms of the accessibility of the surface of the zeolite channels and the reactivity of the cations with Brønsted acid hydroxyls. Weconclude that larger cations can effectively constrict the zeolite channels and impede the ability of pyridine to access acid sites on the surface of the channels.

Effect of addition of a second metal in Mo/ZSM-5 catalyst for methane aromatization reaction under elevated pressures

“…Accordingly, most of the research was focused on finding the ways to suppress coke formation. The different approaches has been applied such as using different zeolite frameworks [8,9], or catalyst modification [10,11], or addition of co-reactant [12,13] or increasing the reaction pressure [14], modification of the reaction conditions [15], and addition of promoters.…”

Catalytic Cracking of Methyl Ester from Used Cooking Oil with Ni-Ion-Exchanged ZSM-5 Catalyst

“…Therefore, a high catalyst concentration led to a high Ni content in ZSM-5and an increase in number of active sites of catalysts. After the active site increases, the gasoline yield increases [1,6,26].…”

“…Cracking has been accomplished with solid catalysts, such as H-ZSM-5, metal-impregnated MCM-41, and Y zeolite, which have good selectivity and appropriate pore size. Metal catalysts also support solid carriers, such as silica, alumina, carbon, zeolite, and zeotype structures [6,[14][15][16][24][25][26][27][28][29][30][31].…”

Synthesis of micro-mesoporous materials ZSM-5/FDU-12 and the performance of dibenzothiophene hydrodesulfurization

“…Thus, aluminosilicate zeolites, such as ZSM-5, beta and Y, are considered as supports in the HDS process due to their high hydrothermal stability, favourable isomerization properties and particularly abundant B acid sites. [5][6][7] Wu and co-workers prepared a series of metal cationfunctionalized M + -ZSM-5 (M + ¼ H + , Al 3+ , Ca 2+ and Ba 2+ ) materials and the characterization results showed that H + -ZSM-5 possessed the most accessible acid sites. 6 The B acids were derived from the bridge hydroxyl groups, which connected with Al and Si atoms, and the varying ratios of B acids to L acids coincided well with the different amounts and types of metal cations.…”