Structural stability and surface, acidic, and catalytic properties of magnesium aluminophosphate of type 36: Influence of [formula omitted] ratios and of thermal and hydrothermal treatments

Abstract

Effects of the thermal and hydrothermal treatments on the structural stability and physical properties (viz., surface concentrations of Mg, Al and P-, N 2-sorption capacity) of MAPO-36 have been studied. MAPO-36 has a higher thermal stability and a lower hydrothermal stability. MAPO-36 structure is thermally stable up to 823 K, and above the calcination temperature of 908 K, tridymite phase formation was observed. The partial conversion of MAPO-36 to amorphous material occurs due to the hydrothermal treatment at 398 K. The surface concentration of Mg decreases with the calcination temperature and also due to the hydrothermal treatment; however, the decrease is more significant with the latter. Changes in the lattice vibration region are observed due to the thermal and the hydrothermal treatments. In situ IR spectroscopic investigations of chemisorbed pyridine on the MAPO-36 catalysts revealed that the concentration of Brønsted and Lewis acid sites are strongly affected by the Mg-content in the AIPO 4-36 framework, thermal and hydrothermal treatments. The ratio of Brønsted to Lewis acid sites decreases with the decrease in the concentration of framework-substituted Mg and also by the thermal and hydrothermal treatments. MAPO-36 materials with different Mg-content and the hydrothermally treated MAPO-36 possesses a broad array of acid strength distributions. The number of weak and strong acid sites are influenced by the concentration of framework-substituted Mg in the AIPO 4-36 and by the thermal and the hydrothermal treatments. The catalytic activities of MAPO-36 (with different Mg P ratios ) and the thermally and hydrothermally treated MAPO-36 catalysts in n-hexane, isooctane, and toluene conversion reactions were studied. In n-hexane and isooctane reactions, the conversion and the aromatic formation decrease with the decrease in the Mg-content in the AIPO-36 framework. This is in agreement with the increase in the strong acid sites with framework Mg-concentration. The thermal treatment causes a pronounced decrease in the catalytic activity of MAPO-36 in these reactions than does the hydrothermal treatment. The aliphatic and aromatic distributions are different in these reactions over the MAPO-36 materials. The toluene disproportionation activity of the MAPO-36 is affected by the Mg-content and the thermal and hydrothermal treatments.