**Pd Nanoparticles and Mixture of CO2**/**CO**/**O2 Applied in the Carbonylation of Aniline**

### **Dominik Madej, Adrian Konopko, Piotr Piotrowski and Agnieszka Krogul-Sobczak \***

Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland; dmadej@connect.ust.hk (D.M.); akonopko@chem.uw.edu.pl (A.K.); ppiotrowski@chem.uw.edu.pl (P.P.)

**\*** Correspondence: akrogul@chem.uw.edu.pl; Tel.: +48-22-552-6289; Fax: +48-22-822-5996

Received: 19 June 2020; Accepted: 28 July 2020; Published: 4 August 2020

**Abstract:** CO2 is a compound of high stability which proves useful in some organic syntheses as a solvent or component decreasing explosivity of gases. It is also a good carbonylating agen<sup>t</sup> for aliphatic amines although not for aromatic ones, the latter being carbonylated with phosgene or, as in our previous works, with CO/O2 in the presence of Pd(II) complexes. In this work we have used the mixture of CO/O2 and CO2 for carbonylation of aniline to N,N'-diphenylurea. After optimization of the reaction conditions (56% of CO2 in CO2/CO mixture) we studied the activity of three kinds of pre-catalysts: (a) Pd(II) complexes, (b) Pdblack, and (c) palladium nanoparticles (PdNPs) in the presence of derivatives of pyridine (XnPy). The highest conversion of aniline (with selectivity towards N,N-diphenylurea ca. 90%) was observed for PdNPs. The results show that catalytic cycle involves Pd(0) stabilized by pyridine ligand as active species. Basing on this observation, we put the hypothesis that application of PdNPs instead of Pd(II) complex can efficiently reduce the reaction time.

**Keywords:** Palladium; nanoparticles; carbonylation; aniline; carbon monoxide; carbon dioxide
