ABSTRACT
Maintaining high conversion under the premise of high oxygenates selectivity in syngas conversion is important but a formidable challenge in Rh catalysis. Monometallic Rh catalysts provide poor oxygenate conversion efficiency, and efforts have been focused on constructing adjacent polymetallic sites; however, the one-pass yields of C2+ oxygenates over the reported Rh-based catalysts were mostly <20 %. In this study, we constructed a monometallic Rh catalyst encapsulated in UiO-67 (Rh/UiO-67) with enhanced proximity to dual-site Rh1,2-Rhn ensembles. Unexpectedly, this catalyst exhibited high efficacy for oxygenate synthesis from syngas, giving a high oxygenate selectivity of 72.0 % with a remarkable CO conversion of 50.4 %, and the one-pass yield of C2+ oxygenates exceeded 25 %. The state-of-the-art characterizations further revealed the spontaneous formation of an ensemble of Rh single atoms/dimers (Rh1,2) in the proximity of ultrasmall Rh clusters (Rhn) confined within the nanocavity of UiO-67, providing adjacent Rh+-Rh0 dual sites dynamically during the reaction that promote the relay of the undissociated CHO species to the CHx species. Thus, our results open a new route for designing highly efficient Rh catalysts for the conversion of syngas to oxygenates by precisely tuning the ensemble and proximity of the dual active sites in a confined space.
ABSTRACT
A quantitative nuclear magnetic resonance spectroscopy (qNMR) method was established for determining the total phenol and six polyphenolic components in the polyphenol extract of Cinnamomi cortex. The qNMR approach utilized DMSO-d6 as the deuterated solvent and potassium hydrogen phthalate as the internal standard for quantifying the total phenolic content, expressed as epicatechin equivalence in the sample. Two complementary qNMR methods with DMSO-d6 or D2O as solvent were established to simultaneously determine 6 polyphenol components in the cinnamon polyphenol extract, including epigallocatechin gallate (EGCG), epicatechingallate (ECG), epicatechin (EC), epigallocatechin (EGC), gallocatechin gallate (GCG) and gallic acid (GA). Method validation demonstrated excellent precision with intraday relative standard deviation (RSD) below 1.08% and interday RSD below 1.48%. The linear correlation coefficient (r) exceeded 0.999, and the limits of detection (LOD) were from 0.01 to 0.14 mg mL-1, while the limits of quantification (LOQ) were from 0.07 to 0.69 mg mL-1. Recovery rates for this method fell within the range of 98.2% to 101.7%. Furthermore, the method has been successfully applied for determining the polyphenolic content in authentic cinnamon polyphenol extracts obtained from different sources.
