Insights into the nature of the active sites of pt-wox/al2o3 catalysts for glycerol hydrogenolysis into 1, 3-propanediol

The chemo-selective hydrogenolysis of secondary hydroxyls is an important reaction for the production of biomass-derived a, ¿-diols. This is the case for 1, 3-propanediol production from glycerol. Supported Pt-WOx materials are effective catalysts for this transformation, and their activity is often related to the tungsten surface density and Brönsted acidity, although there are discrepancies in this regard. In this work, a series of Pt-WOx/¿-Al2O3 catalysts were prepared by modifying the pH of the solutions used in the active metal impregnation step. The activity–structure relation-ships, together with the results from the addition of in situ titrants, i.e., 2, 6-di-tert-butyl-pyridine or pyridine, helped in elucidating the nature of the bifunctional active sites for the selective production of 1, 3-propanediol. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Valorisation of Biowastes for the Production of Green Materials Using Chemical Methods

With crude oil reserves dwindling, the hunt for a sustainable alternative feedstock for fuels and materials for our society continues to expand. The biorefinery concept has enjoyed both a surge in popularity and also vocal opposition to the idea of diverting food-grade land and crops for this purpose. The idea of using the inevitable wastes arising from biomass processing, particularly farming and food production, is, therefore, gaining more attention as the feedstock for the biorefinery. For the three main components of biomass—carbohydrates, lipids, and proteins—there are long-established processes for using some of these by-products. However, the recent advances in chemical technologies are expanding both the feedstocks available for processing and the products that be obtained. Herein, this review presents some of the more recent developments in processing these molecules for green materials, as well as case studies that bring these technologies and materials together into final products for applied usage

New mechanistic insights into the role of water in the dehydration of ethanol into ethylene over ZSM-5 catalysts at low temperature

International audienceThe low-temperature dehydration of bioethanol-to-ethylene is of great interest to reduce energy consumption and achieve high product purities in the biorefinery and olefin industry. Thermokinetic constraints, however, lead to low ethylene selectivity at low temperature. In this work, we integrate a new approach that combines a hierarchical acid H-form ZSM-5 (HZSM-5) with systematic catalytic testing to study how the physicochemical modification of the surface and intermediate catalytic species affect the ethanol-to-ethylene route at 225 °C. Four HZSM-5 zeolites were treated with OH species under basic conditions (OH−) or solely with H2O. Kinetic evidence coupled to 27Al-nuclear magnetic resonance, NH3-temperature-programmed desorption and N2 adsorption, as well as density-functional theory calculations, correlate ethylene selectivity with the appearance of new extra-framework Al(V) and Al(VI) species, acting as Lewis acid-sites. The adopted approach allows us to experimentally unveil the cooperative effect between Brønsted- and Lewis-acid sites that seem to play a key role in ethylene formation from ethanol at low-temperature via (i) a primary route via ethanol dimerization on neighboring Brønsted-acid sites to diethylether, which subsequently cracked on Lewis-acid sites to ethylene; (ii) a secondary route via the direct ethanol dehydration on Brønsted-acid sites. Theoretical calculations support the proposed catalytic cycle. These new insights shed light on the mechanism of ethanol-to-ethylene at low temperature, and on how the precise control over the strength of acid-sites and their population in HZSM-5 affects catalysis. This work progresses towards more active and stable catalysts, advancing into more mature low-temperature technologies for the dehydration of bioethanol into sustainable ethylene

Tuning the Acid Nature of the ZSM‑5 Surface for Selective Production of Ethylene from Ethanol at Low Temperatures

Solid-acid ZSM-5 catalysts stand out as highly reactive for ethanol dehydration, but the selective production of ethylene at low temperatures, however, is still a challenge. Herein, two ZSM-5 zeolites with a distinct Si/Al ratio have been modified with Ce, La, or P species or treated with H2O or NH3 to get a better understanding on the contribution of acid sites to the ethanol-to-ethylene catalysis. The doping of ZSM-5 affects both the number and strength of acid sites, of which the Ce content inversely increases the population of weak acid sites (WAS). Atomically dispersed oxygen vacancy-rich CeOx synergistically contribute to the dissociation of H2O during the synthesis of Al-rich ZSM-5 to modify the local Al environment by forming new Al–OH bonds, acting as WAS. This significantly enhances the conversion of ethanol into intermediates and ultimately into ethylene (selectivity up to 92%). Further poisoning of strong acid sites (SAS) by in situ NH3-titration confirms that ethanol-to-ethylene catalysis at low temperatures occurs mostly over WAS, while the contribution of SAS is minor. The underpinning insights can serve as a basis for further developments in the combination of other multivalence species with ex situ water treatments or in situ cofeeding to design robust catalysts that are able to efficiently dehydrate bioethanol into ethylene at low temperatures

Acid-catalyzed conversion of Xylose in 20 solvents: Insight into interactions of the solvents with Xylose, Furfural, and the Acid Catalyst

In this study, the acid-catalyzed conversion of xylose to furfural was investigated in 20 solvents ranging from water, alcohol, ketones, furans, ethers, esters, hydrocarbons, and aromatics with the aim to understand their involvement in each step from xylose to furfural. Compared with water, alcohols can stabilize the reactive intermediates, promote the formation of furfural, and slow its degradation with prolonged reaction times. Iso-propanol and 2-butanol can direct the conversion of xylose to levulinic esters via transfer hydrogenation catalyzed by a Brønsted acid catalyst. The other solvents with the carbonyl groups (i.e., ketones) or conjugated π bonds (e.g., furan) react with both xylose and furfural. Either xylose cannot make its way to furfural or furfural cannot survive for long in these solvents. In ethers, hydrocarbons, and aromatics, the formation of furfural is quick but so is the degradation of furfural due to the aprotic properties of these solvents. In an ester like methyl formate, xylose can be converted to furfural selectively and efficiently. Approximately 70% yields of furfural were achieved at 150 °C in a very short time, and more importantly, methyl formate is highly volatile (boiling point: 32 °C). It can be distilled from furfural very easily, making it a promising solvent for furfural production. The solvents also interact with the acidic resin catalysts in varied ways due to their different polarities and molecular size/shape, determining the availability of the acidic sites on the inner surfaces of the catalysts