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Post-synthesis deposition of mesoporous niobic acid with improved thermal stability by kinetically controlled sol-gel overcoating

Abstract

Niobia is a well-known solid acid catalyst owing to its intrinsic Brønsted and Lewis acidity. However, catalyst development with this oxide has been limited by our ability to control its pore structure and thermal stability. Here we report a novel post-synthetic approach for preparing mesoporous niobia catalysts. This method relies on controlling the kinetics of niobium (V) ethoxide to deposit conformal Nb2O5 overcoats on SBA-15 in a typical Stöber solution. Full Nb2O5 coverage over the mesopores of SBA-15 was achieved by adding 4 monolayer equivalents of precursor (4Nb2O5@SBA-15), which was verified by X-Ray Photoelectron Spectroscopy. This overcoated SBA-15 had a high surface area and retained a narrow as well as ordered pore size distribution. Importantly, the typical structural transition from amorphous to pseudo-hexagonal phase of Nb2O5 did not occur with the overcoat after calcination at 773 K. Due to the unprecedented thermal stability of our niobia overcoat, which did not undergo structural rearrangement, the acid sites were preserved after regenerations by calcination. Furthermore, 4Nb2O5@SBA-15 showed higher yields than commercial niobia (HY-340) and lab-synthesized bulk niobia in two probe reactions: xylose dehydration to furfural and Friedel-Crafts alkylation. In both cases, the improvement could be explained by the unique structural features of the niobia overcoat, including a favorable ratio of Brønsted and Lewis acid sites in the case of xylose dehydration and a high proportion of isolated Nb-OH groups for the alkylation reaction. Such structural features and unprecedented thermal stability provide additional tools for synthetizing unique solid acid catalysts using a simple post-synthesis deposition method.

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Publication details

The article was received on 07 Feb 2019, accepted on 11 Apr 2019 and first published on 11 Apr 2019


Article type: Paper
DOI: 10.1039/C9TA01459D
Citation: J. Mater. Chem. A, 2019, Accepted Manuscript

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    Post-synthesis deposition of mesoporous niobic acid with improved thermal stability by kinetically controlled sol-gel overcoating

    Y. Du, F. Héroguel, T. X. Nguyen and J. Luterbacher, J. Mater. Chem. A, 2019, Accepted Manuscript , DOI: 10.1039/C9TA01459D

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