No abstract

The direct oxidative dehydrogenation of lactates with molecular oxygen is a “greener” alternative for producing pyruvates. Here we report a one-pot synthesis of mesoporous vanadia–titania (VTN), acting as highly efficient and recyclable catalysts for the conversion of ethyl lactate to ethyl pyruvate. These VTN materials feature high surface areas, large pore volumes, and high densities of isolated vanadium species, which can expose the active sites and facilitate the mass transport. In comparison to homogeneous vanadium complexes and VOx/TiO2 prepared by impregnation, the meso-VTN catalysts showed superior activity, selectivity, and stability in the aerobic oxidation of ethyl lactate to ethyl pyruvate. We also studied the effect of various vanadium precursors, which revealed that the vanadium-induced phase transition of meso-VTN from anatase to rutile depends strongly on the vanadium precursor. NH4VO3 was found to be the optimal vanadium precursor, forming more monomeric vanadium species. V4+ as the major valence state was incorporated into the lattice of the NH4VO3-derived VTN material, yielding more V4+–O–Ti bonds in the anatase-dominant structure. In situ DRIFT spectroscopy and density functional theory calculations show that V4+–O–Ti bonds are responsible for the dissociation of ethyl lactate over VTN catalysts and for further activation of the deprotonation of β-hydrogen. Molecular oxygen can replenish the surface oxygen to regenerate the V4+–O–Ti bonds.

Direct air capture (DAC) processes for extraction of CO 2 from ambient air are unique among chemical processes in that they operate outdoors with minimal feed pretreatments. Here, the impact of humidity on the oxidative degradation of a prototypical solid supported amine sorbent, poly(ethylenimine) (PEI) supported on Al 2 O 3 , is explored in detail. By combining CO 2 adsorption measurements, oxidative degradation rates, elemental analyses, solid-state NMR and in situ IR spectroscopic analysis in conjunction with 18 O labeling of water, a comprehensive picture of sorbent oxidation is achieved under accelerated conditions. We demonstrated that the presence of water vapor can play an important role in accelerating the degradation reactions. From the study we inferred the identity and kinetics of formation of the major oxidative products, and the role(s) of humidity. Our data are consistent with a radical mediated autooxidative degradation mechanism.

Aqueous‐phase conversion of glyceraldehyde to lactic acid was investigated over Nb2O5, TiO2, ZrO2 and SnO2 in a fixed‐bed up‐flow reactor. Special attention was given to the catalysts acidity regarding the type, amount, strength and tolerance to water of surface acid sites. These sites were assessed by infrared spectroscopy of pyridine adsorbed on dehydrated and hydrated catalysts as well as by isopropanol decomposition. It was found that Nb2O5 and TiO2 have the highest fraction of water‐tolerant Lewis acid sites (40 and 47 %), while only 6 % was estimated for ZrO2. No relevant Lewis acidity was observed on SnO2, but it was noticed the presence of strong base sites. The transformation of glyceraldehyde into lactic acid proceeded via a cascade reaction in which glyceraldehyde is firstly dehydrated to pyruvaldehyde, followed by its rearrangement to lactic acid with the addition of a water molecule. The dehydration step occurs on Brønsted acid sites and/or on water‐tolerant Lewis acid sites. These latter sites also determine the selectivity to lactic acid. Strong base sites promote glyceraldehyde fragmentation leading to formaldehyde with high selectivity.

We studied the vapour-phase oxidative dehydrogenation of ethyl lactate to ethyl pyruvate over V2O5/TiO2 catalysts in a fixed-bed reactor.

The effects of lanthanum addition in Ni/CeO 2 catalysts were investigated. The influence of synthetic procedures, namely, impregnation or coprecipitation of lanthanum and cerium oxide, were evaluated. Materials were analyzed by BET, AAS, DRIFT-MS, TPR, OSC, XRD, and SEM-EDX. Samples were tested in ethanol steam reforming (ESR). Both lanthanumpromoted samples exhibited a higher stability in time than nonpromoted catalyst. Nonetheless, catalytic behavior is strongly affected by the preparation method. TPR, OSC, and XRD analyses showed that the coprecipitation method allowed the best interaction between ceria and lanthana, leading to an increased redox ability and best catalytic performances as a result. A catalyst with a support prepared via the coprecipitation method showed ethanol conversion of 90% and hydrogen selectivity higher than 70% even after 60 h of reaction.

Understanding surface reactions of biomass-derived oxygenates on metal oxides is important for designing catalysts for valorization of biomass. This work elucidated the effect of different pretreatments on molybdenum trioxide (MoO3) to understand how surface reactivity is controlled by the surface oxidation state. The catalyst was pretreated in oxidative, inert, and reducing environments. The inert and reducing pretreatments created oxygen vacancies on the catalyst surface that acted as active sites for the adsorption of oxygenated molecules, with the reducing pretreatment yielding a higher density of these active sites. Exposing the catalyst to an alcoholic solvent such as methanol also led to a partial reduction similar to the inert pretreatment. After pretreatment, the catalyst was exposed to ethanol, acetaldehyde, and crotonaldehyde with subsequent characterization by diffuse reflectance infrared spectroscopy (DRIFTS), temperature-programmed desorption (TPD), X-ray absorption near edge spectroscopy (XANES), and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) was also used to determine adsorption configurations and energies of ethanol, acetaldehyde, and crotonaldehyde. Reduced surfaces were shown to have a stronger affinity for carbonyls, leading to a higher activity for the aldol condensation of acetaldehyde and ethanol to C4 molecules. Catalysts pretreated in an oxidative environment were completely inactive toward chemisorption and reaction of acetaldehyde.

The performance of Brønsted-and Lewis-acidic La, Nb, and Zr phosphates (LaPO, NbPO, and ZrPO) during the aqueous phase conversion of dihydroxyacetone (DHA) to lactic acid (LA) is investigated using a fixed-bed reactor. Mass-transfer phenomena are thoroughly investigated, and the masstransfer coefficient is deconvoluted from the intrinsic kinetic constant for each catalyst to enable the quantitative assessment of both. NbPO is found to be masstransfer-limited. Despite this limitation, NbPO shows the highest yield of LA at 36%. The reaction over ZrPO is not transport-limited, allowing for a rigorous analysis of intrinsic kinetics. This analysis shows that the conversion of DHA into pyruvaldehyde (PVA) follows a second-order reaction mechanism via a dimeric intermediate, which consolidates previous reports in the literature. Additionally, a correlation between LA production and the carbon missing from the carbon balance (carbon loss) is observed. Finally, NbPO and ZrPO show stable performance up to 10 h on stream at 150 °C. After 15 h of reaction, the PVA yield increases at the expense of LA with NbPO. This is ascribed to the deactivation of the active sites necessary to produce LA, which are different from the sites that produce PVA. This hypothesis is supported by the characterization of the spent catalyst with 13 C magic-angle spinning nuclear magnetic resonance and attenuated total reflectance infrared spectroscopy.