Abstract
In this work, the results of gas phase cyclohexane photocatalytic oxidative dehydrogenation on MoOx/SO4/TiO2 catalysts with DRIFTS analysis are presented. Analysis of products in the gas-phase discharge of a fixed bed photoreactor was coupled with in situ monitoring of the photocatalyst surface during irradiation with an IR probe. An interaction between cyclohexane and surface sulfates was found by DRIFTS analysis in the absence of UV irradiation, showing evidence of the formation of an organo-sulfur compound. In particular, in the absence of irradiation, sulfate species initiate a redox reaction through hydrogen abstraction of cyclohexane and formation of sulfate (IV) species. In previous studies, it was concluded that reduction of the sulfate (IV) species via hydrogen abstraction during UV irradiation may produce gas phase SO2 and thereby loss of surface sulfur species. Gas phase analysis showed that the presence of MoOx species, at same sulfate loading, changes the selectivity of the photoreaction, promoting the formation of benzene. The amount of surface sulfate influenced benzene yield, which decreases when the sulfate coverage is lower. During irradiation, a strong deactivation was observed due to the poisoning of the surface by carbon deposits strongly adsorbed on catalyst surface.
Highlights
Photocatalytic oxidation reactions have been widely used in processes such as the decontamination of water and air [1,2,3,4,5,6,7,8]
With the aim to confirm if carbonylic compounds are the responsible of catalyst deactivation, a photocatalytic test on 8MoDT2 was carried out by feeding cyclohexanone with the same operating conditions used for cyclohexane
From coupling of gas phase and DRIFTS analysis, a deeper knowledge of phenomena occurring during photocatalytic selective oxidation of cyclohexane on MoOx/TiO2 was obtained
Summary
Photocatalytic oxidation reactions have been widely used in processes such as the decontamination of water and air [1,2,3,4,5,6,7,8]. Cyclohexene or benzene are selectively obtained through gas phase oxidative dehydrogenation of cyclohexane on MoOx/TiO2 photocatalysts, with UV illumination both in fixed [21] and fluidized bed reactors [22,23,24]. MoOx/TiO2 was recently proposed by analyzing the gas-phase coming from a photoreactor [21] This mechanism involves dehydrogenation of cyclohexane to cyclohexene followed by oxy-dehydrogenation to benzene on molybdenum oxide active sites via a detailed sequence of elementary steps [21]. In this study we present the results of gas phase cyclohexane photocatalytic oxidative dehydrogenation on MoOx/TiO2 through the analysis of products present in gas-phase exit of the reactor, complemented by in situ DRIFTS analysis, to monitor the photocatalyst surface in the absence and during UV irradiation
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