Unveiling the Mechanism of Glycerol Oxidation to Lactic Acid on Pt/Sn-MFI Zeolite:an In situ Solid-state NMR Study

Heterogeneous glycerol(GLY)oxidation offers a promising route for the production of lactic acid(LA);a key monomer in biodegradable bioplastics.However,the specific reaction pathways remain poorly understood.This study presents a mechanistic investigation of GLY oxidation to LA using Pt/Sn-MFI catalysts.Characterizations via DR-UV-Vis spectroscopy,119Sn NMR,and TEM reveal the formation of zeolite framework Sn and well-dispersed Pt nanoparticles in Pt/Sn-MFI.The Lewis acidity of framework Sn in MFI zeolite is confirmed through 31P NMR probe techniques.GLY conversion and LA selectivity correlate strongly with framework Sn concentration and the presence of Pt nanoparticles.In situ 13C solid-state NMR experiments,complemented by two-dimensional 13C correlation NMR,allow real-time monitoring of GLY conversion and identification of various mobile and rigid(surface-adsorbed)species.Results indicate that GLY preferentially transforms to LA via a dihydroxyacetone(DHA)intermediate,facilitated by the Pt-Sn synergistic effect.However,accumulation of surface-adsorbed LA on Sn sites promotes consecutive oxidation of GLY to glyceric acid,tartaric acid,and ultimately CO_(2).

Mechanistic Insight into Ethanol Dehydration over SAPO-34 Zeolite by Solid-state NMR Spectroscopy

The reaction mechanism of ethanol dehydration over SAPO-34 zeolite is investigated by using solid-state NMR spectroscopy. SAPO-34 zeolites with different Si contents are prepared and their acidities are characterized by NMR experiments. The higher content of stronger Brønsted acid sites is correlated to the higher Si content. The adsorption of ethanol on the Brønsted acid sites in SAPO-34 leads to the formation of frustrated Lewis pairs(FLPs). Surface ethoxy species is observed by the dehydration of the FLP sites at room temperature, which can be further converted into ethene products. The decomposing of diethyl ether over Brønsted acid sites is responsible for the formation of ethoxy species at higher reaction temperatures. Triethyloxonium ions are formed in the reaction. A plausible reaction mechanism is proposed for the dehydration of ethanol over SAPO-34.

Challenges in Analysis of Hydrophilic Metabolites Using Chromatography Coupled with Mass Spectrometry

Hydrophilic metabolites play important roles in cellular energy metabolism,signal transduction,immunity.However,there are challenges in both identification and quantification of the hydrophilic metabolites due to their weak interactions with C18-reversed-phase liquid chromatography(RPLC),leading to poor retention of hydrophilic metabolites on the columns.Many strategies have been put forward to increase the retention behavior of hydrophilic metabolites in the RPLC system.Non-derivatization methods are mainly focused on the development of new chromatographic techniques with different separation mechanisms,such as capillary electrophoresis,ion-pairing RPLC etc.Derivatization methods improve the hydrophobicity of metabolites and can enhance the MS response.This review mainly focused on the illustration of challenges of LCMS in the analysis of hydrophilic metabolomics field,and summarized the non-derivatization and derivatization strategies,with the intention of providing multiple choices for analysis of hydrophilic metabolites.