Role of hydrogen on the formation of molybdenum oxyhydride for cumene hydrocracking over platinum loaded molybdenum oxide

Abstract

The acidic nature of platinum supported molybdenum oxide (Pt/MoO3) catalyst has been found to have effects on cumene hydrocracking. The molybdenum oxide (MoO3) sample was prepared by calcination of molybdic acid (H2MoO4) at 673 K for 3 h. The Pt/MoO3 was prepared by impregnation of MoO3 with an aqueous solution of chloroplatinic acid (H2PtCl6) followed by calcination at 673 K in air. The catalyst was characterized by x-ray diffraction (XRD), field emission scanning electron microscopy-energy dispersive x-ray (FESEM-EDX), fourier transformation infra red (FTIR) and electron spin resonance (ESR). The hydrogen influence on surface of Pt/MoO3 was studied by in-situ XRD, 2,6-lutidine preadsorbed FTIR, ESR spectroscopy and hydrogen uptake capacity. 2,6-lutidine preadsorbed FTIR showed that both catalysts possess doublet adsorption bands ascribed to Lewis acid sites, and duo-doublet bands ascribed to hydroxyl groups; these indicate an -OH defect structure of MoO3 and Mo-OH Brönsted acidic sites. The XRD result confirmed the formation of molybdenum oxyhydride (MoOx)–(Hy)+ on the hydrogen treated Pt/MoO3, whereas the hydrogen adsorption on 2,6-lutidine preadsorbed catalysts showed the formation of protonic acid sites over Pt/MoO3. These results strongly suggested that the interaction of molecular hydrogen with Pt/MoO3 formed acidic Brönsted (MoOx)–(Hy)+ via a hydrogen spillover mechanism. In fact, no (MoOx)–(Hy)+ and protonic acid sites were observed on Pt-free MoO3. Hydrogen adsorption of Pt/MoO3 was studied at the temperature range of 373 - 573 K and at the initial hydrogen pressure of 6.7 kPa. The hydrogen uptake exceeded the H/Pt ratio of unity for adsorption at and above 423 K, indicating that hydrogen adsorption processes involves dissociative adsorption of hydrogen on Pt sites, hydrogen atom spillover and surface diffusion of the spiltover hydrogen atom over the bulk surface of MoO3 followed by formation of (MoOx)–(Hy)+. The rate controlling step of the hydrogen adsorption on Pt/MoO3 was the surface diffusion of the spiltover hydrogen with the activation energy of 83.1 kJ/ mol. The presence of hydrogen enhanced the activity of Pt/MoO3 in the cumene hydrocracking in which the rate conversion of cumene increased by about 30%, while the apparent activation energy decreased by approximately 28 kJ/mol. From the Response Surface Methodology (RSM), the optimum conditions for cumene hydrocracking for Pt/MoO3 were at treatment temperature of 673 K, treatment time of 4 h, reaction temperature of 573 K and flow of hydrogen over weight of catalyst (F/W) of 375 ml g-1 min-1, which the predicted value of propylene yield was 16.7% while the experimental value gave 17.1%.