posted on 2023-11-27, 06:00authored byNiklas Stegmann, Cristina Ochoa-Hernández, Khai-Nghi Truong, Hilke Petersen, Claudia Weidenthaler, Wolfgang Schmidt
The partial selective
oxidation of n-butane to
maleic anhydride with molecular oxygen is commercially well-established
and strongly associated with the vanadium phosphorus oxide (VPO) catalyst.
We report that also titanium phosphate (TiPO) exhibits the rare feature
of accomplishing the most demanding complex selective oxidation reaction
industrially applied. A facile molten salt method was used to prepare
TiPO catalysts from mixtures of (NH4)2HPO4 and TiO2 (P25). In a continuous flow process under
industrially relevant conditions with TiPO, conversions above 50%
resulted in 20% overall selectivity for maleic anhydride with 90%
oxygenate selectivity. Due to a high tendency to total oxidation (>60%),
the performance of TiPO catalysts cannot yet compete with the industrial
VPO catalyst. However, herein we want to highlight our studies on
the reaction pathway and mechanism for the complex multistep conversion
of n-butane to maleic anhydride, which is still under
debate for the VPO catalyst after more than four decades of research.
A complete chain of reaction intermediates was identified via online
mass spectroscopy, under industrially relevant conditions, and in
pulse experiments, including the consecutive formation of 2-butene,
1,3-butadiene, furan, and 2-furanone as C4 intermediates. Cyclic pulse
experiments complemented with EPR measurements revealed a combined
mechanism involving carbocation chemistry via Brønsted acid sites
and a redox mechanism according to Mars van Krevelen.