Catalytic Performance and Kinetic Modeling of n-Butane Isomerization over Metal Bearing Silicotungstic Acid Supported on Mesoporous KIT-6

Abstract

In this study, mesoporous KIT-6 support was synthesized hydrothermally. Its structure was improved by impregnation with phosphotungstic acid (HPW), silicotungstic acid (HSiW), and some metals such as Pt, Pd, and Ce. This work converts mesoporous KIT-6 into a bifunctional catalyst containing acid and metal sites. The structures of the synthesized catalysts were characterized by various analyses such as XRD, XRF, FTIR, SEM, EDS, BET, NH3-TPD, and TGA. Furthermore, the catalytic performances of heteropoly acids (HPW and HSiW) and KIT-6 modified samples were investigated in an n-butane isomerization process. Impregnation of HSiW on mesoporous support resulted in higher initial n-butane conversion and isobutane selectivity and improved catalyst stability compared to the unsupported sample. The introduction of 0.5 wt % platinum on the 60%HSiW/KIT-6 led to the highest n-butane conversion (72%) and isobutane yield (56.8%) at 250 °C and weight hourly space velocity (WHSV) of 0.15 h–1, at the beginning of the reaction. The 0.5%Pt/60%HSiW/KIT-6 sample was the most stable catalyst against deactivation compared to other modified samples. Its conversion and its selectivity to isobutane reached 51.8 and 73.8%, respectively, after 7 h of reaction. Also, the influence of different process variables such as reaction temperature, WHSV, and carrier gas on the catalyst activity (n-butane conversion, isobutane selectivity, and yield) was investigated. In addition, the classical bifunctional model was used to describe the kinetic behavior of 0.5%Pt/60%HSiW/KIT-6 in the isomerization reaction. This model inhibits the hydrogen and n-butane pressure effects on the reaction rate and agrees with the ideal bifunctional metal–acid mechanism. The activation energy for this model was found to be 29.64 kJ mol–1. The classical bifunctional model provided a reasonable fit for the experimental data and allowed to specify the kinetic parameters.