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Gas‐Phase Models for the Nickel‐ and Palladium‐Catalyzed Deoxygenation of Fatty Acids
Author(s) -
Parker Kevin,
Weragoda Geethika K.,
Pho Victoria,
Canty Allan J.,
Polyzos Anastasios,
O'Hair Richard A. J.,
Ryzhov Victor
Publication year - 2020
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.202000908
Subject(s) - chemistry , deoxygenation , catalysis , palladium , medicinal chemistry , electrospray ionization , carboxylate , dissociation (chemistry) , mass spectrometry , stereochemistry , organic chemistry , chromatography
Using fatty acids as renewable sources of biofuels requires deoxygenation. While a number of promising catalysts have been developed to achieve this, their operating mechanisms are poorly understood. Here, model molecular systems are studied in the gas phase using mass spectrometry experiments and DFT calculations. The coordinated metal complexes [(phen)M(O 2 CR)] + (where phen=1,10‐phenanthroline; M=Ni or Pd; R=C n H 2n+1 , n≥2) are formed via electrospray ionization. Their collision‐induced dissociation (CID) initiates deoxygenation via loss of CO 2 and [C,H 2 ,O 2 ]. The CID spectrum of the stearate complexes (R=C 17 H 35 ) also shows a series of cations [(phen)M(R’)] + (where R’ < C 17 ) separated by 14 Da (CH 2 ) corresponding to losses of C 2 H 4 ‐C 16 H 32 (cracking products). Sequential CID of [(phen)M(R’)] + ultimately leads to [(phen)M(H)] + and [(phen)M(CH 3 )] + , both of which react with volatile carboxylic acids, RCO 2 H, (acetic, propionic, and butyric) to reform the coordinated carboxylate complexes [(phen)M(O 2 CR)] + . In contrast, cracking products with longer carbon chains, [(phen)M(R)] + (R>C 2 ), were unreactive towards these carboxylic acids. DFT calculations are consistent with these results and reveal that the approach of the carboxylic acid to the “free” coordination site is blocked by agostic interactions for R > CH 3 .