Tuning acidity in zirconium-based metal organic frameworks catalysts for enhanced production of butyl butyrate

Abstract

Three isostructural zirconium-based metal organic frameworks (MOFs), UiO-66, UiO-66(COOH)2 and UiO-66(NH2) were synthesized, fully characterized and efficiently used as active and recyclable catalysts for the esterification reaction of butyric acid to produce a green fuel additive, butyl butyrate. The catalytic activities of the used structures were comparable, and mostly better, than other heterogeneous acid catalyst reported in the literature. Moreover, 90% conversion was achieved by employing the most acidic member, UiO-66(COOH)2, which is close to the 95% conversion obtained by the conventional liquid catalyst H2SO4. Using the synthesized MOFs, large variations in the conversion to butyl butyrate were obtained which was the base of a detailed investigation on the origin of their catalytic activities. The analysis of the TGA results helped estimate the number of structural defects in each studied MOF. Interestingly, it was concluded that, for the MOFs with different organic linkers, the catalytic activity was not directly related to the number of defects. Further analysis was done to investigate the alternative parameters that could be behind this difference in catalytic activity, and the parameters included but were not limited to the surface area of the MOFs, their particle size, the linkers’ active sites, and their accessibility through effective mass transfer. Although a combination of these factors were found to contribute to the superior catalytic activity of UiO-66(COOH)2, however, its exceptional conversion was mainly attributed to the effect of the additional active acid functional groups grafted onto its organic linker, along with its smaller particle size which allowed for better mass transfer and accessibility of the active site Furthermore, two kinetic models were successfully developed and used to determine the different kinetic parameters of the esterification reaction and to study their dependence on the different characteristics of the MOFs. With this knowledge, catalytic activity of MOFs can be engineered from a laboratory prototype and optimized by tuning the functional groups of the organic linkers to serve as effective catalysts for the production of fine chemicals such as biofuels.