Thermodynamic Analysis and Experimental Study of Selective Dehydrogenation of 1,2-cyclohexanediol over Cu2+1O/MgO Catalysts

Abstract

The dehydrogenation of 1,2-cyclohexanediol (CHD) helps in the effective utilization of not only fossil derivatives but also vicinal diols and polyols from sustainable biomass-derived resources. A thermodynamic analysis of CHD dehydrogenation was computed with density functional theory (DFT) calculation using Gaussian 09. The result indicates that CHD can be converted to 2-hydroxy cyclohexanone (HCO), 2-hydroxy-2-cyclohexen-1-one (HCEO) and pyrocatechol depending on the degree of dehydrogenation. HCO and HCEO are the stable products of the primary and secondary dehydrogenation. Experimentally, Cu/MgO catalysts were prepared using glucose as a reductant, and were characterized by SEM, TEM, XRD, XPS, TPR, BET and ICP. Furthermore, their catalytic performance regarding the oxygen-free dehydrogenation of CHD was investigated. The results indicate that the primary active crystalline phase of Cu/MgO was Cu 2+1 O, and that the dehydrogenation products were mainly HCO and HCEO, in accordance with thermodynamic predictions. Upon optimizing the reaction conditions, the total selectivity of HCO and HCEO exceeded 90% and the conversion of CHD was approximately 95%.Sustainability 2019, 11, 902 2 of 14 in a variety of byproducts, but also recycled excessive oxidants difficultly. Lastly, an excessive number of oxidants increased the likelihood of an explosion via a massive exothermic process.A possible solution for mitigating this setback was to undergo oxygen-free dehydrogenation. Research into the oxygen-free dehydrogenation of CHD has mainly focused on nickel-based [8] and palladium-based [9] catalysts, while the main product, pyrocatechol, was formed by the deep dehydrogenation of CHD. Few papers have reported the oxygen-free dehydrogenation of CHD to HCO. Sato et al. studied the synthesis of HCO from CHD by oxygen-free dehydrogenation over an Ir-ReOx/SiO 2 catalyst with an HCO yield of 24.7% at 493 K for 1.0 h [22]. Although the Cu-based catalyst was also an important catalyst, which has been widely used in the dehydrogenation of alcohols [23], it has not been reported in the oxygen-free dehydrogenation of CHD to HCO or 2-hydroxy-2-cyclohexen-1-one (HCEO). The main active components of Cu-based catalysts are Cu 0 and Cu + , the latter possessing better activity and selectivity. Especially at higher temperatures, Cu 0 will increase the yield of the phenol byproduct [24][25][26][27]. In addition, both dehydrogenated intermediates of CHD, CHO and CHEO are not only used as raw material for adipic acid synthesis but are also common building blocks for asymmetric and heteropoly cyclic compounds [28][29][30][31][32]. Thus, it is crucial to investigate the controlled dehydrogenation of CHD to HCO and HCEO over Cu-based catalysts. Furthermore, the dehydrogenation of CHD as a model compound can play an indispensable role for the effective utilization of environmentally-friendly and sustainable resources, since vicinal diols and polyols are common moieties in biomass-derived molecules.In this work, the reaction thermodynamics of CHD o...