Gas‐Phase Models for the Nickel‐ and Palladium‐Catalyzed Deoxygenation of Fatty Acids

• Published in: ChemCatChem Vol. 12; no. 21; pp. 5476 - 5485
• Main Authors: Parker, Kevin, Weragoda, Geethika K., Pho, Victoria, Canty, Allan J., Polyzos, Anastasios, O'Hair, Richard A. J., Ryzhov, Victor
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 05.11.2020
• Subjects: Biofuels; Biomass conversion; Carboxylic acids; Coordination compounds; Cracking (chemical engineering); Density Functional Theory calculation; Deoxygenation; Fatty acids; gas-phase reactions; homogeneous catalysis; Ions; Mass spectrometry; Molecular chains; Nickel; Palladium; Vapor phases
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.202000908

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(O2CR)]+ (where phen=1,10‐phenanthroline; M=Ni or Pd; R=CnH2n+1, n≥2) are formed via electrospray ionization. Their collision‐induced dissociation (CID) initiates deoxygenation via loss of CO2 and [C,H2,O2]. The CID spectrum of the stearate complexes (R=C17H35) also shows a series of cations [(phen)M(R’)]+ (where R’ < C17) separated by 14 Da (CH2) corresponding to losses of C2H4‐C16H32 (cracking products). Sequential CID of [(phen)M(R’)]+ ultimately leads to [(phen)M(H)]+ and [(phen)M(CH3)]+, both of which react with volatile carboxylic acids, RCO2H, (acetic, propionic, and butyric) to reform the coordinated carboxylate complexes [(phen)M(O2CR)]+. In contrast, cracking products with longer carbon chains, [(phen)M(R)]+ (R>C2), 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 > CH3. Fatty acid deoxygenation/cracking in the gas phase: Fatty acids can be catalytically deoxygenated and “cracked” to produce a mixture of hydrocarbons via complexing with Ni2+ or Pd2+ and 1,10‐phenanthroline inside a mass spectrometer. The detailed deoxygenation mechanism is described for propionic acid as a model system.