Hydrothermal and photocatalytic conversion of glucose with commercial titanium dioxide catalysts (TiO2)

SSCI-VIDE+CARE:CDFA+IBD:FDA:MET:CGU:NESInternational audienceIn order to find alternatives to the fossil carbon, catalytic gasification, pyrolysis, and liquefaction processes have been developed for the production of high-value chemicals from sustainable carbon sources. However, the development of efficient and cheap processes remains the major challenge and photocatalysis presents a high potential to face this issue [1]. In this regard, the hydrothermal catalytic conversion (high temperature and high pressure) and the photocatalytic conversion (under ambient temperature) of glucose in aqueous solutions were studied with the objective of subsequently combining these two processes which would allow to decrease the reaction temperature (hence energy expenditure) and limit the formation of side products. Under hydrothermal condition (glucose/catalyst=1, 0.5 g/L, 120 °C, inert atmosphere), the catalytic performances of different commercial titanium dioxide catalysts (UV100, PC105, MPT-625, Rut 160, P25, P90) were compared. A large number of platform molecules such as fructose, 5-hydroxymethylfurfural (HMF) and lactic acid were produced in the liquid phase. The main ones were: fructose with UV100, PC105, P25 and P90; HMF with MPT-625 and Rut160, without evident correlation with the TiO2 phase structure. The catalytic performance of hydrothermal glucose conversion was also investigated under air (oxidant) atmosphere and was found similar to that under inert (neutral) atmosphere. The photocatalytic conversion of glucose in aqueous solutions (glucose/catalyst=1, 0.5 g/L, room temperature, under air, UV (365 nm, 6.7 mW/cm²)) was also investigated in the presence of the above catalysts. Numerous different platform molecules such as formic acid, glyceraldehyde, gluconic acid, erythrose, arabinose, and lactic acid were produced, accounting for the occurrence of a quite different mechanism pathway. P25 and P90, containing 80% anatase and 20% rutile phases, were the most selective for gluconic acid and erythrose. Whereas rutile phase, MPT-625 and Rut160 were the most selective for glyceraldehyde. Using PC105 under neutral environment (argon), the glucose conversion rate and the products yields were low in both liquid and gas phases. The search for potential relationships between the acid-base properties of the catalysts and their activity was also investigated using: 1) the simple model reaction of dihydroxyacetone (DHA) conversion in water [2] and 2) the adsorption of pyridine monitored by FTIR (identification of Bronsted and Lewis sites) and 3) calorimetry (to determine the number of acid and basic sites and their strength by adsorbing NH3 and CO2). The model reaction showed that while Rutile catalysts (MPT-625, Rut 160) do not promote lactic acid formation explained by the absence of no Lewis acid sites, the Anatase-Rutile catalysts (P25, P90) would have more of active Lewis acid sites than the Anatase catalysts (UV100, PC105) based on the extent of Lactic acid formation. This is in agreement with the adsorption of pyridine monitored by FTIR which shows that Rut160 has the least Lewis acid sites.It may be concluded that despite the differences between the mechanisms and the products of the two processes ((hydrothermal and photocatalysis), their combination might decrease the reaction temperature and limit the formation of unfavorable side products.References[1] L.I.Granone, F.Sieland, N.Zheng, R.Dillert, D.W.bahnemann Green Chem 20 2018, 1169.[2] E. Jolimaitre, D. Delcroix, N. Essayem, C. Pinel, and M. Besson, Catal. Sci. Technol. 8 2018, 1349