Efficient biofuel production by MTV-UiO-66 based catalysts

Abstract In this study, highly defected and functionalized metal-organic framework (MOF) structures are developed and exploited as catalysts for an esterification reaction for biofuel production. Two systems of multivariate UiO-66 series, namely MTV-UiO-66(COOH)2 and MTV-UiO-66(OH)2 incorporating dicarboxylate and dihydroxy groups, respectively, along with the single component structures, are thus explored for butyl butyrate production. Ratios of functionalized linkers to terephthalic acid are varied and a modulation synthesis approach is employed allowing for high levels of structural defects. The synthesized MOF structures are fully characterized using Powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), proton nuclear magnetic resonance (1H NMR), Brunauer–Emmett–Teller (BET), and scanning electron microscopy (SEM), and the results confirmed the homogeneous incorporation of the functionalized linkers in the structures. The combination of multivariate approach along with modulation synthesis yields structures with catalytic activity higher than those of highly defective fully functionalized structures and close to the homogeneous conventional catalyst used in esterification reactions. Moreover, the ratio of functionalized linkers to terephthalic acid is shown to be very important since not all MTV-UiO-66 performed better than the single-component structure which can be attributed to a combination of factors related to the density of active sites and their accessibility. The most active MTV-UiO-66(OH)2 member, incorporating 52% of functionalized linkers, a defects number of 1.9, and a surface area of 761 m2/g, yielded 92% conversion to butyl butyrate, compared to 95% for H2SO4, and its activity and stability is maintained over 4 consecutive cycles. Furthermore, by using the data of 33 different UiO-66 based catalysts for butyl butyrate production, a weighted linear regression model is suggested to predict the conversion based on the parameters that are concluded to mostly govern the catalytic activity of MOF catalysts. These parameters include the surface area, the catalytic loading, the defects number, and the level of incorporation of BDC, and the functionalized linkers. The weights calculated for each of these parameters indicate that there is a more pronounced effect of active sites density on the conversion when compared to the surface area or the catalyst loading. These conclusions help pave the way for the engineering of MOF-based catalysts in the path of bridging the gap between homogeneous and heterogeneous catalysis for efficient biofuel production.