Effect of tungsten loading on zirconia impregnated MCM-41 and its catalytic activity on transesterification reaction

Abstract

WO₃/ZrO₂ weight % (WO₃/ZrO₂ = wt% = 4/22, 8/22, 15/22, 15/30 and 15/40) was impregnated on Si-MCM-41. The crystalinity, textural property, surface feature and surface acidity of the materials were characterized by XRD, N₂-physisorption isotherm (BET), NH₃-TPD, Laser-Raman and XRF. The XRD and BET results revealed that the materials were hexagonally ordered mesoporous (pore size = 2.58–3.07 nm) materials. High angle XRD showed the presence of tetragonal ZrO₂ (crystallite size L = 14.88 nm) and monoclinic ZrO₂. The catalytically active t-ZrO₂ was stabilized by increasing wt% of WO₃. Raman spectra confirmed the presences of crystalline WO₃ along with t-ZrO₂ and m-ZrO₂ which was not detected by high angle XRD. WO₃ restrict the phase transfer from metastable t-ZrO₂ to thermodynamically favoured m-ZrO₂. NH₃-TPD results indicated the presence of weak acid sites and it increased with an increase in tungsten loading on mesoporous materials. There was no appreciable increase in acid sites by increasing ZrO₂. The catalytic activity was studied on transesterification of diethyl oxalate diester with various alcohols in liquid phase system under autogeneous pressure. When the t-ZrO₂ phase increases the selectivity of diester also increases. However the optimum metal oxide loading on Si-MCM-41 was with 15 % WO₃/22 % ZrO₂/Si-MCM-41. Hence the materials can be a convenient and suitable acid catalyst for transesterification.