Revisiting the phosphonium salt chemistry for P-doped carbon synthesis: toward high phosphorus contents and beyond the phosphate environment†
*a
Christel
Gervais,
b
Hannes
Zschiesche,
a
Teodor
Jianu,
a
Nieves
López-Salas,
ac
Markus
Antonietti
*a
and
Mateusz
Odziomek
*a
Abstract
The introduction of phosphorus and nitrogen atoms in carbo-catalysts is a common way to tune the electronic density, and thereby the reactivity, of the material, as well as to introduce surface reactive sites. Numerous environments are reported for the N atoms, but the P-doping chemistry is less explored and focuses on surface POx groups. A one-step synthesis of P/N-doped carbonaceous materials is presented here, using affordable and industrially available urea and tetrakis(hydroxymethyl)phosphonium chloride (THPC) as the N and P sources, respectively. In contrast to most of the synthetic pathways toward P-doped carbonaceous materials, the THPC precursor only displays P–C bonds along the carbon backbone. This resulted in unusual phosphorus environments for the materials obtained from direct thermal treatment of THPC–urea, presumably of type C–P–N according to 31P NMR and XPS. Alternatively, the in situ polymerization and calcination of the precursors were run in calcium chloride hydrate, used as a combined reaction medium and porogen agent. Following this salt-templating strategy led to particularly high phosphorus contents (up to 18 wt%), associated with porosities up to 600 m2 g−1. The so-formed P/N-doped porous materials were employed as metal-free catalysts for the mild oxidative dehydrogenation of N-heterocycles to N-heteroarenes at room temperature and in air.
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