Exfoliated Pd/HNb₃O₈ nanosheet as highly efficient bifunctional catalyst for one-pot cascade reaction

Graphical abstract: - Highlights: • Ultrathin and highly acidic HNb₃O₈ nanosheet was prepared by exfoliation of layered niobium oxide. • Pd/HNb₃O₈ nanosheet showed excellent catalytic performance for one-pot cascade reaction. • The reaction performance of Pd/HNb₃O₈ nanosheet is 7.1 and 1.2 times higher than that of layered Pd/KNb₃O₈ or Pd/HNb₃O₈, respectively. • Significant promoting effect of support acidity on the reaction performance was observed. - Abstract: Ultrathin two-dimensional metal oxide nanosheets have drawn attention as potential solid acid catalysts owing to their strong acidity, attributed to the bridged OH groups formed on the nanosheets. In this study, a new class of bifunctional acid–metal catalyst was realized by the deposition of Pd on layered niobium oxide (KNb₃O₈ and HNb₃O₈) or its exfoliated nanosheet (Pd/HNb₃O₈-NS) and applied to one-pot cascade deacetalization and hydrogenation. It was found that the acid strength of the support exerted a large influence not only on the promotion of the first deacetalization step, but also on the acceleration of the subsequent hydrogenation step. Comparative experiments using a series of Pd/HZSM-5 catalysts with different acidities reconfirmed the crucial role of acid strength on hydrogenation. However, the superior catalytic activity of Pd/HNb₃O₈-NS for hydrogenation compared to that of Pd/HZSM-5 of similar acidity suggests a more efficient ensemble effect of the strong acid sites with the nearby metal sites on the nanosheet surface. Among the catalysts used, Pd/HNb₃O₈-NS showed the best catalytic performance for one-pot cascade reaction affording the desired product (benzyl alcohol) in approximately 92% yield, which was 7.1 and 1.2 times higher than that of layered Pd/KNb₃O₈ or Pd/HNb₃O₈, respectively. The excellent catalytic performance of Pd/HNb₃O₈-NS may result from the characteristic features of nanosheets: (i) the synergistic cooperation between the bifunctional active sites and (ii) the two-dimensional open surface offering easier access of the reactants to the active sites. Although the use of NaBH₄ as hydrogen source was effective in improving the initial reaction performance, the basic nature of NaBH₄ adversely resulted in weakening the acid strength of the catalyst, and consequently led to a reduction in catalytic activity.