Mono- and tri-ester hydrogenolysis using tandem catalysis. Scope and mechanism

Tracy L. Lohr, Zhi Li, Rajeev S. Assary, Larry A. Curtiss, Tobin J. Marks

Research output: Contribution to journalArticle

21 Citations (Scopus)

Abstract

The scope and mechanism of thermodynamically leveraged ester RC(O)O-R′ bond hydrogenolysis by tandem metal triflate + supported Pd catalysts are investigated both experimentally and theoretically by DFT and energy span analysis. This catalytic system has a broad scope, with relative cleavage rates scaling as, tertiary > secondary > primary ester at 1 bar H2, yielding alkanes and carboxylic acids with high conversion and selectivity. Benzylic and allylic esters display the highest activity. The rate law is ν = k[M(OTf)n]1[ester]0[H2]0 with an H/D kinetic isotope effect = 6.5 ± 0.5, implying turnover-limiting C-H scission following C-O cleavage, in agreement with theory. Intermediate alkene products are then rapidly hydrogenated. Applying this approach with the very active Hf(OTf)4 catalyst to bio-derived triglycerides affords near-quantitative yields of C3 hydrocarbons rather than glycerol. From model substrates, it is found that RC(O)O-R′ cleavage rates are very sensitive to steric congestion and metal triflate identity. For triglycerides, primary/external glyceryl CH2-O cleavage predominates over secondary/internal CH-O cleavage, with the latter favored by less acidic or smaller ionic radius metal triflates, raising the diester selectivity to as high as 48% with Ce(OTf)3.

Original languageEnglish
Pages (from-to)550-564
Number of pages15
JournalEnergy and Environmental Science
Volume9
Issue number2
DOIs
Publication statusPublished - Feb 1 2016

ASJC Scopus subject areas

  • Renewable Energy, Sustainability and the Environment
  • Environmental Chemistry
  • Pollution
  • Nuclear Energy and Engineering

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