Coupling of Acetaldehyde to Crotonaldehyde on CeO 2-x (111): Bifunctional Mechanism and Role of Oxygen Vacancies

Chuanlin Zhao, Charles Watt, Paul R. Kent, Steven H. Overbury, David R. Mullins, Florencia C. Calaza, Aditya Savara, Ye Xu

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21 Scopus citations

Abstract

Selective C-C coupling of oxygenates is pertinent to the manufacture of fuel and chemical products from biomass and from derivatives of C 1 compounds (i.e., oxygenates produced from methane and CO 2 ). Here we report a combined experimental and theoretical study on the temperature-programmed reaction (TPR) of acetaldehyde (AcH) on a partially reduced CeO 2-x (111) thin film surface. The experiments have been carried out under ultra-high-vacuum conditions without continuous gas exposure, allowing better isolation of active sites and reactive intermediates than in flow reaction conditions. AcH does not undergo aldol condensation in a typical TPR procedure, even though the enolate form of AcH (CH 2 CHO) is readily produced on CeO 2-x (111) with oxygen vacancies. We find however that a tailored "double-ramp" TPR procedure is able to successfully produce an aldol adduct, crotonaldehyde (CrA). Using density functional theory calculations and microkinetic modeling we explore several possible C-C coupling pathways. We conclude that the double-ramp procedure allows surface oxygen vacancy dimers, stabilized by adsorbate occupation, to form dynamically during the TPR. The vacancy dimers in turn enable C-C coupling to occur between an enolate and an adjacent AcH molecule via a bifunctional enolate-keto mechanism that is distinct from conventional acid-or base-catalyzed aldol condensation reactions. The proposed mechanism indicates that CrA desorption is rate-limiting while C-C coupling is facile.

Original languageEnglish
Pages (from-to)8273-8286
Number of pages14
JournalJournal of Physical Chemistry C
Volume123
Issue number13
DOIs
StatePublished - Apr 4 2019

Funding

The work at Oak Ridge National Laboratory (experiments mass spectrometry analysis) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences Geosciences, and Biosciences Division, under contract DE-AC05-00OR22725. Initial calculations by P.R.K. were conducted at the Center for Nanophase Materials Sciences, which is a US-DOE Office of Science User Facility. C.W. was supported by the US-DOE, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS), under the Science Undergraduate Laboratory Internships Program (SULI) program at Oak Ridge National Laboratory, administered by the Oak Ridge Institute for Science and Education. The work at Louisiana State University (computational modeling, theoretical analysis) was supported by the U.S. National Science Foundation under Grant CHE-1664984, and used high-performance computational resources provided by LSU (https://hpc.lsu.edu), by the Centerfor Nanophase Materials Sciences, which is a US-DOE Office of Science User Facility, and by the National Energy Research Scientific Computing Center, which is supported by the Office of Science of US-DOE under Contract DE-AC02-05CH11231. The work at Oak Ridge National Laboratory (experiments, mass spectrometry analysis) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under contract DE-AC05-00OR22725. Initial calculations by P.R.K. were conducted at the Center for Nanophase Materials Sciences, which is a US-DOE Office of Science User Facility. C.W. was supported by the US-DOE, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS), under the Science Undergraduate Laboratory Internships Program (SULI) program at Oak Ridge National Laboratory, administered by the Oak Ridge Institute for Science and Education. The work at Louisiana State University (computational modeling, theoretical analysis) was supported by the U.S. National Science Foundation under Grant CHE-1664984, and used high-performance computational resources provided by LSU (https://hpc.lsu.edu), by the Center for Nanophase Materials Sciences, which is a US-DOE Office of Science User Facility, and by the National Energy Research Scientific Computing Center, which is supported by the Office of Science of US-DOE under Contract DE-AC02-05CH11231.

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