Design of Base Zeolite Catalysts by Alkali-Metal Grafting in Alcoholic Media

This study investigates the synthesis of base catalysts through the postsynthetic grafting of alkali cations (Li, Na, K, Rb, or Cs) onto USY zeolites in alcoholic solutions of the corresponding metal hydroxides. In contrast to previous studies conducted in aqueous media, the utilization of alcohols (MeOH, EtOH, or iPrOH) offers increased control over the metalation process while simultaneously averting degradation and dissolution of the crystalline framework. The achievement of close to an atomic dispersion of the alkali metals in the zeolite is confirmed by in-depth characterization combining 23Na MAS NMR, microscopy, elemental mapping, and CO2 chemisorption. Both the size of the alkali cation and the carbon number of the alcohol influence the incorporation efficiency, which is found to correlate with the expected basic strength of the corresponding metal alkoxide. The presence of framework aluminum results in higher sodium loadings due to the parallel incorporation by both ion exchange and metalation. However, cations exchanged to the Al sites do not provide the distinct basic properties of the grafted metals, and thus, high-silica zeolites attain the highest basicity. Catalytic testing in the self-condensation of propanal, a model reaction for the deoxygenation of bio-oil, demonstrates excellent activity, with a > 90% selectivity to the aldol reaction and a stable performance. The selective character arising from the distinctive strength coupled with the isolated nature and high accessibility of the grafted basic sites holds a large potential for the development of superior zeolite catalysts for base-catalyzed applications.