# Liquid Phase Catalytic Transfer Hydrogenation of Crotonaldehyde over ReOx-Supported Catalysts Using Formic Acid as In Situ Hydrogen Donor

## TL;DR

This paper explores using rhenium-based catalysts with formic acid to selectively hydrogenate crotonaldehyde into crotyl alcohol, achieving high selectivity.

## Contribution

The study introduces ReOx-supported catalysts with tailored acid-base properties for selective hydrogenation using formic acid as a hydrogen donor.

## Key findings

- ReOx/ZrO2 and ReOx/g-C3N4 showed crotyl alcohol selectivity above 57% at 25% conversion.
- Formic acid outperformed molecular hydrogen in activating crotonaldehyde and improving selectivity.
- Support material influenced rhenium dispersion, oxidation state, and surface acidity.

## Abstract

The selective hydrogenation of the C=O bond over the C=C bond in α,β-unsaturated aldehydes remains a well-known challenge. This work investigates the liquid-phase catalytic transfer hydrogenation of crotonaldehyde to crotyl alcohol over ReOx-based catalysts, using formic acid (FA) as an in situ hydrogen donor. A series of 10 wt% Re catalysts supported on G200, g-C3N4, TiO2, and ZrO2 were synthesized and tested in a batch reactor at 20 bar and temperatures of 140–180 °C. Catalysts were characterized by XRD, BET, NH3-TPD, and XPS to correlate their physicochemical properties with catalytic behavior. Among the studied materials, ReOx/ZrO2 and ReOx/g-C3N4 exhibited the highest crotyl alcohol selectivity above 57% for all reaction temperatures, evaluated at crotonaldehyde conversion of 25%. The nature of the support strongly influenced the dispersion and oxidation state of Re species, as well as the surface acidity, which governed the activation of both the carbonyl group and the FA decomposition. Compared with molecular hydrogen, FA improved both conversion and selectivity due to its superior hydrogen-donating ability in the aqueous phase. These findings demonstrate that tailoring the acid–base characteristics of ReOx catalysts and employing biomass-derived hydrogen donors represent an effective strategy for selective hydrogenation of α,β-unsaturated aldehydes.