HZSM-5 Zeolites with Different SiO₂/Al₂O₃Ratios. Characterization and NH₃Desorption Kinetics

Abstract

This article reports the influence of the SiO2/Al2O3 ratio (30, 80, and 280) on HZSM-5 zeolite properties. XRD patterns show a decrease in the XRD peak intensity at low 2θ angles for low SiO2/Al2O3 ratio zeolites. N2 adsorption and desorption isotherms display hysteresis with both micropores and mesopores likely contributing to the particles network. The NLDFT cylindrical model confirms the characteristic 5.5 Å micropores in the zeolite pore framework. Furthermore, specific surface area, pore volume, and pore size distributions demonstrate that changes in the SiO2/Al2O3 ratio do not influence HZSM-5 structural properties. NH3-TPD shows both weak and strong acid sites. Pyridine-FTIR also confirms that, in HZSM-5, the weak acidity encompasses Lewis and hydrogen-bonded sites, while the strong acidity mainly involves the Brönsted acid sites. It is proven that the ratio of weak to strong acid sites as measured by NH3-TPD and Pyridine-FTIR displays similar changes to the SiO2/Al2O3 ratio. Furthermore, NH3 desorption kinetics and numerical regression allow the establishment of energies of activation for both strong and weak acid sites. It is also found that the kinetic desorption rate constants vary in agreement with the changes in weak and strong acid sites as observed with NH3-TPD and Pyridine-FTIR.

Selective production of valuable hydrocarbons from waste motorbike engine oils via catalytic fast pyrolysis using zeolites

“…This is in agreement with earlier studies that showed an increasing trend of zeolite acidity with Al/Si ratio [36][37][38]. It is seen that even the ammonium form of zeolites possess reasonable surface acidities without conversion to hydrogen forms.…”

Effect of Fe-zeolite on formation of N2O in selective reduction of NO by NH3 over V2O5–WO3/TiO2 catalyst

“…4, in which the original diffraction intensity of FeZ has been reduced by 60 % for readier comparison. Peaks at 2h = 7.87°, 8.85°, and 23.07°appeared for FeZ; these positions are similar to those reported for as-synthesized H-ZSM-5 zeolite [27], indicating that all Fe ions are atomically isolated at extraframework sites. A sample of VWT gave strong reflection at 2h = 25.26°with substantial weak ones at higher 2h values.…”

Controlling coke deactivation and cracking selectivity of MFI zeolite by H3PO4 or KOH modification

“…The temperature of maximum desorption-cracking of t-BA (T TBA ) is linked to the acid strength of the catalyst, as stronger acid sites are active for the cracking of t-BA at lower temperatures. Considering this, the lower the temperature of the peak (lower T TBA ), the higher the acid strength of the sites, which is the opposite effect to that observed in the TPD analysis of commonly used NH 3 [6]. Besides, a small fraction of t-BA (signal at m/z = 58) is desorbed without cracking, corresponding to the base physisorbed on weak acid sites, which are inactive for the cracking of t-BA, and consequently also for the reactions involved in the transformation of 1-butene [25,45].…”

“…Rahimi and Karimzadeh [4] have reviewed the main modifications of MFI zeolites for obtaining and intensifying light olefin production, since the structure and acidity of the MFI zeolite could be tuned for each specific application by properly selecting the SiO 2 /Al 2 O 3 ratio [5][6][7], calcination with different protocols [8], using vapor at high temperatures namely steaming [9,10], incorporating rare earth or transition metals [4,11,12], protons by cations like Na + [13], selecting a proper matrix of binders and fillers [14], and by treatment with acids or bases [15,16], among others.…”