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

doi:10.1039/c5ee03256c

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^*

^*Corresponding author for this work

Chemistry

Research output: Contribution to journal › Article › peer-review

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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 language	English (US)
Pages (from-to)	550-564
Number of pages	15
Journal	Energy and Environmental Science
Volume	9
Issue number	2
DOIs	https://doi.org/10.1039/c5ee03256c
State	Published - Feb 2016

Funding

This material is based upon work supported as part of the Institute of Atom-efficient Chemical Transformation (IACT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Sciences, and Office of Basic Energy Sciences (contract DE-AC0206CH11357), which supported Z. L., R. S. A., and L. A. C. NSF grants CHE-1213235 and CHE-1464488 supported T. L. L. and provided reactor equipment. The Pd/ TiO2 catalyst was provided by Mr M. S. Liu. We thank Dr K.-L. Ding for the composition analysis of gaseous product mixtures. We gratefully acknowledge the computing resources provided on ‘‘Blues’’, a computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This research used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

ASJC Scopus subject areas

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

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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.",

author = "Lohr, {Tracy L.} and Zhi Li and Assary, {Rajeev S.} and Curtiss, {Larry A.} and Marks, {Tobin J.}",

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AB - 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.

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

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UN SDGs

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