Development of modified ZrO2 catalysts for the production of levulinic acid esters

WOx/ZrO2 catalysts have been synthesized with different tungsten loadings and calcination temperatures via one-pot evaporation induced self-assembly (EISA) technique. The tungsten contents and calcination temperatures greatly influence the textural properties of the material. The crystallinity and surface state of materials are confirmed by X-ray diffraction and Laser Raman spectroscopy. The surface acidity of the materials are evaluated by NH3-temperature programmed desorption (NH3-TPD) and diffuse reflectance Fourier transform (DRIFT) with adsorbed pyridine. The catalysts containing 20-25 wt%. WO3 demonstrate the highest surface acidity and population of Brönsted acid centers. The activity of catalyst was tested for Levulinic acid (LA) esterification with 1-butanol (1-BuOH) with mole ratio 2 to 1. The only products after the reaction are pseudo- (p-BL) and normal-butyl levulinate (n-BL). The highest total yield of esters (67%) and selectivity towards n-BL (97%) can be attained over 20 wt.% WOx/ZrO2 catalyst calcined at 800 °C. A strong correlation between the population of Brönsted acidity and catalytic activity could be affirmed.The second part of this work elaborates the production of levulinic esters via addition reaction of alpha-angelica lactone (a-AL) with 1-BuOH over Amberlyst-36. Full conversion with 97% selectivity towards the ester can be achieved by performing the reaction with stoichiometric amount of the reactants at a milder condition. The reaction mechanism for the ester formation has been confirmed revealing that p-BL is the intermediate during the transformation of a-AL to n-BL. Different alcohols have also been studied suggesting the effect of linearity and the length of alkyl chain influence the reaction. This study eventually indicates that a-AL can be used as a better candidate to substitute LA for the synthesis of levulinic acid esters. Different type solid acid catalysts have been tested suggesting that strong acidity is required to allow the addition reaction.Despite possessing strong acid sites, not all acid catalyst can apparently catalyze the addition reaction. The third part of this work is intended to examine the influence of acid species on the addition of a-AL. To realize the investigation, WOx/ZrO2 with adjustable Brönsted population was prepared via EISA and impregnation route. Both methods result in the catalyst with similar textural properties. Raman spectroscopy and X-ray diffraction demonstrated that catalysts prepared by EISA are able to retain meta-stable tetragonal phase of ZrO2 as well as delay the development of WO3 nanoparticles. NH3-TPD and DRIFT with adsorbed pyridine were employed to evaluate the surface acidity. The spectra from pyridine adsorbed at 150°C reveal that the highest Brönsted acid concentration occurs at 15 and 20 wt.% WO3 for the catalysts prepared by impregnation and EISA respectively. The catalysts were employed in the addition reaction of a-AL with 1-BuOH. The conversion of a-AL and selectivity towards n-BL depend strongly on the concentration of Brönsted acid sites. Finally the materials prepared by impregnation method shows higher catalytic activity