Sulfonated foam catalysts for the continuous dehydration of xylose to furfural in biphasic media

This paper demonstrates the use of sulfonated foam structures, acting both as catalyst and liquid-liquid contactor, during the continuous dehydration of xylose to furfural in biphasic media. First, we develop and optimize a coating procedure comprising a two-step polymerization technique (polypropylene and polystyrene-divinylbenzene), followed by swelling and sulfonation. The method was highly reproducible and led to a stable, well-adhered, 12–50 μm layer of sulfonic resin with an ion exchange capacity of 0.1 meq/cmfoam3. The catalytic foams showed the same activity than H2SO4 in terms of conversion and selectivity versus residence time and temperature. The enhanced mass transfer properties of the foam-based reactor facilitated rapid furfural extraction, thus allowing for higher temperature operations (ca. 20–50 °C higher) and shorter residence times (ca. 10 min vs. 4–5 h) than conventionally reported in the literature, while preserving high furfural selectivity (ca. 70–80%). Finally, the stability of the sulfonated foam catalyst during operation was demonstrated up to 170 °C, although higher temperatures led to a visible decay in activity. We conclude that the sulfonated foams show great potential for this application

Kinetic study of propene oxide and water formation in hydro-epoxidation of propene on Au/Ti-SiO2 catalyst

A highly stable and active, Au/TiSiO2 catalyst was used to conduct an extensive kinetic study for the direct epoxidation of propene using hydrogen and oxygen in the non-explosive regime, with an aim to study the combined formation of propene oxide and water. A reaction mechanism was proposed which assumes that there are two types of Au sites: isolated, which forms only water and the ones in the vicinity of Ti which forms propene oxide as well as water. The active intermediate on both was assumed to be the hydro peroxide species, as widely accepted. Based on fundamental reaction steps obtained from this mechanism, rate equations were derived and fitted with the experimental observations. The kinetic model was found to describe well the simultaneous formation of propene oxide and water, thus emphasizing on the importance of Au-Ti synergy to obtain catalysts with higher activity and H2 utilization ability

Gas-Phase Epoxidation of Propene with Hydrogen Peroxide Vapor

A study on the gas-phase epoxidation of propene with vapor hydrogen peroxide has been carried out. The main purpose was to understand the key factors in the reaction and the relationship between epoxidation of propene and decomposition of hydrogen peroxide, which is the main side reaction. The decomposition was highly influenced by the materials used, being higher in metals than in polytetrafluoroethylene (PTFE) and glass, and it was complete when the epoxidation catalyst, TS-1, was introduced in the system. However, when propene was added, the peroxide was preferentially used for the epoxidation, even with amounts of catalyst as small as 10 mg, reaching productivities of 10.5 kg_PO·kg_cat^–1·h^–1 for a gas hourly space velocity (GHSV) of 450 000 mL·g_cat^–1·h^–1. The hydrogen peroxide was converted completely in all the experiments conducted, with a selectivity to PO of around 40% for all peroxide concentrations. Finally, if concentrations of propene higher than the stoichiometrically required amounts were used, the selectivity to PO increased to almost 90%

Selective Propylene Oxidation to Acrolein by Gold Dispersed on MgCuCr₂O₄ Spinel

Gold nanoparticles supported on a MgCuCr₂O₄ spinel catalyze the aerobic oxidation of propylene to acrolein. At 200 °C, the selectivity is 83% at a propylene conversion of 1.6%. At temperatures above 220 °C, propylene combustion dominates. The good performance of Au/MgCuCr₂O₄ in selective propylene oxidation is due to the synergy between metallic Au and surface Cu⁺ sites. Kinetic experiments (H₂ addition, N₂O replacing O₂) show that the reaction involves molecular oxygen. DFT calculations help to identify the reaction mechanism that leads to acrolein. Propylene adsorbs on a single Au atom. The adsorption of propylene via its π-bond on gold is very strong and can lead to the dissociation of the involved Au atom from the initial Au cluster. This is, however, not essential to the reaction mechanism. The oxidation of propylene to acrolein involves the oxidation of an allylic C–H bond in adsorbed propylene by adsorbed O₂. It results in OOH formation. The resulting CH₂–CH–CH₂ intermediate coordinates to the Au atom and a support O atom. A second C–H oxidation step by a surface O atom yields adsorbed acrolein and an OH group. The hydrogen atom of the OH group recombines with OOH to form water and a lattice O atom. The desorption of acrolein is the most difficult step in the reaction mechanism. It results in a surface oxygen vacancy in which O₂ can adsorb. The role of Cu in the support surface is to lower the desorption energy of acrolein