Journal
ACS CATALYSIS
Volume 8, Issue 5, Pages 4558-4568Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.7b03961
Keywords
direct amidation reaction mechanism; heterogeneous catalysis; surface chemistry; prebiotic chemistry; IR spectroscopy; cluster and periodic DFT simulations
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Funding
- MICINN [CTQ2014-59544-P, CTQ2014-60119-P, CTQ2017-89132-P]
- DIUE [2014SGR482, 2017SGR1320]
- ICREA Award
- Catalonia Supercomputer Centre (CESCA)
- University of Torino (Ricerca Locale)
- Italian MIUR (Minister dell'Istruzione, dell'Universita della Ricerca)
- Scuola Normale Superiore (project PRIN, STARS in the CAOS - Simulation Tools for Astrochemical Reactivity and Spectroscopy in the Cyberinfrastructure for Astrochemical Organic Species) [2015F59J3R]
- Programa Banco de Santander
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The mechanism of the amide bond formation between nonactivated carboxylic acids and amines catalyzed by the surface of amorphous silica under dry conditions is elucidated by combining spectroscopic measurements and quantum chemical simulations. The results suggest a plausible explanation of the catalytic role of silica in the reaction. Both experiment and theory identify very weakly interacting SiOH surface group pairs (ca. 5 angstrom apart) as key specific sites for simultaneously hosting, in the proper orientation, ionic and canonical pairs of the reactants. An atomistic interpretation of the experiments indicates that this coexistence is crucial for the occurrence of the reaction, since the components of the canonical pair are those undergoing the amidation reaction while the ionic pair directly participates in the final dehydration step. Transition state theory based on quantum mechanical free energy potential energy shows the silica-catalyzed amide formation as being relatively fast. The work also points out that the presence of the specific SiOH group pairs is not exclusive of the adopted silica sample, as they can also be present in natural forms of silica, for instance as hydroxylation defects on alpha-quartz, so that they could exhibit similar catalytic activity toward the amide bond formation.
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