Reactive hydrogen-driven dehydroxylation of hydroxyapatite enables anti-Ostwald ripening of Ag nanoparticles for ethanol valorization to aromatics
Abstract
The selective conversion of biomass-derived ethanol to high-value chemicals remains challenging due to the complex reaction pathways involving dehydrogenation, C-C coupling, dehydration, and particularly for aromatic products, aromatization of reactive intermediates. Herein, we designed a silver-containing hydroxyapatite catalyst (Ag/HAP) enabling ethanol upgrading to methylbenzyl alcohols and methylbenzaldehydes with a record-high aromatics selectivity of 77.8% and a yield of 33.3%. Notably, the average Ag particle size underwent a reduction from 1.27 nm in the fresh catalyst to 0.85 nm after five catalytic cycles, which represented a clear departure from the typical sintering tendency of Ag nanoparticles during reaction. In situ diffuse reflectance infrared Fourier transform spectroscopy combined with isotopic labeling mass spectrometry revealed that reactive hydrogen species (H*), generated either from ethanol dehydrogenation or external hydrogen donors, induced dehydroxylation of HAP to form the oxygen vacancies. Density functional theory calculations confirmed that the abundant oxygen vacancies in Ag/HAP favored the dispersion of Ag nanoparticles, manifesting as anti-Ostwald ripening under reaction conditions and resulting in a size reduction of Ag nanoparticles. X-ray photoelectron spectroscopy indicated the coexistence of metallic (Ag0) and cationic (Ag+) silver species, wherein Ag0 promoted ethanol dehydrogenation to acetaldehyde, while Ag+ facilitated aldehyde coupling and subsequent aromatization. Their cooperative interplay with an oxygen-vacancy-rich HAP accounted for the high selectivity toward aromatic products. This study provides valuable insights and a transferable approach for the rational design of active and durable catalysts resistant to Ostwald ripening.
