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Title: Monomolecular Siloxane Film as a Model of Single Site Catalysts

Achieving structurally well-defined catalytic species requires a fundamental understanding of surface chemistry. Detailed structural characterization of the catalyst binding sites in situ, such as single site catalysts on silica supports, is technically challenging or even unattainable. Octadecyltrioxysilane (OTOS) monolayers formed from octadecyltrimethoxysilane (OTMS) at the air-liquid interface after hydrolysis and condensation at low pH were chosen as a model system of surface binding sites in silica-supported Zn 2+ catalysts. We characterize the system by grazing incidence X-ray diffraction, X-ray reflectivity (XR), and X-ray fluorescence spectroscopy (XFS). Previous X-ray and infrared surface studies of OTMS/OTOS films at the airliquid interface proposed the formation of polymer OTOS structures. According to our analysis, polymer formation is inconsistent with the X-ray observations and structural properties of siloxanes; it is energetically unfavorable and thus highly unlikely. We suggest an alternative mechanism of hydrolysis/condensation in OTMS leading to the formation of structurally allowed cyclic trimers with the six-membered siloxane rings, which explain well both the X-ray and infrared results. XR and XFS consistently demonstrate that tetrahedral [Zn(NH 3) 4] 2+ ions bind to hydroxyl groups of the film at a stoichiometric ratio of OTOS:Zn ~ 2:1. The high binding affinity of zinc ions to OTOS trimersmore » suggests that the six-membered siloxane rings are binding locations for single site Zn/SiO 2 catalysts. Finally, our results show that OTOS monolayers may serve as a platform for studying silica surface chemistry or hydroxyl-mediated reactions.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [1] ;  [5]
  1. Illinois Institute of Technology, Chicago, IL (United States); Argonne National Lab. (ANL), Lemont, IL (United States)
  2. Illinois Institute of Technology, Chicago, IL (United States); Georgia Inst. of Technology, Atlanta, GA (United States)
  3. Argonne National Lab. (ANL), Lemont, IL (United States)
  4. Univ. of Chicago, Chicago, IL (United States)
  5. Illinois Institute of Technology, Chicago, IL (United States)
Publication Date:
Grant/Contract Number:
AC02-06CH11357
Type:
Accepted Manuscript
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 138; Journal Issue: 38; Journal ID: ISSN 0002-7863
Publisher:
American Chemical Society (ACS)
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
OSTI Identifier:
1375032

Martynowycz, Michael W., Hu, Bo, Kuzmenko, Ivan, Bu, Wei, Hock, Adam S., and Gidalevitz, David. Monomolecular Siloxane Film as a Model of Single Site Catalysts. United States: N. p., Web. doi:10.1021/jacs.6b05711.
Martynowycz, Michael W., Hu, Bo, Kuzmenko, Ivan, Bu, Wei, Hock, Adam S., & Gidalevitz, David. Monomolecular Siloxane Film as a Model of Single Site Catalysts. United States. doi:10.1021/jacs.6b05711.
Martynowycz, Michael W., Hu, Bo, Kuzmenko, Ivan, Bu, Wei, Hock, Adam S., and Gidalevitz, David. 2016. "Monomolecular Siloxane Film as a Model of Single Site Catalysts". United States. doi:10.1021/jacs.6b05711. https://www.osti.gov/servlets/purl/1375032.
@article{osti_1375032,
title = {Monomolecular Siloxane Film as a Model of Single Site Catalysts},
author = {Martynowycz, Michael W. and Hu, Bo and Kuzmenko, Ivan and Bu, Wei and Hock, Adam S. and Gidalevitz, David},
abstractNote = {Achieving structurally well-defined catalytic species requires a fundamental understanding of surface chemistry. Detailed structural characterization of the catalyst binding sites in situ, such as single site catalysts on silica supports, is technically challenging or even unattainable. Octadecyltrioxysilane (OTOS) monolayers formed from octadecyltrimethoxysilane (OTMS) at the air-liquid interface after hydrolysis and condensation at low pH were chosen as a model system of surface binding sites in silica-supported Zn2+ catalysts. We characterize the system by grazing incidence X-ray diffraction, X-ray reflectivity (XR), and X-ray fluorescence spectroscopy (XFS). Previous X-ray and infrared surface studies of OTMS/OTOS films at the airliquid interface proposed the formation of polymer OTOS structures. According to our analysis, polymer formation is inconsistent with the X-ray observations and structural properties of siloxanes; it is energetically unfavorable and thus highly unlikely. We suggest an alternative mechanism of hydrolysis/condensation in OTMS leading to the formation of structurally allowed cyclic trimers with the six-membered siloxane rings, which explain well both the X-ray and infrared results. XR and XFS consistently demonstrate that tetrahedral [Zn(NH3)4]2+ ions bind to hydroxyl groups of the film at a stoichiometric ratio of OTOS:Zn ~ 2:1. The high binding affinity of zinc ions to OTOS trimers suggests that the six-membered siloxane rings are binding locations for single site Zn/SiO2 catalysts. Finally, our results show that OTOS monolayers may serve as a platform for studying silica surface chemistry or hydroxyl-mediated reactions.},
doi = {10.1021/jacs.6b05711},
journal = {Journal of the American Chemical Society},
number = 38,
volume = 138,
place = {United States},
year = {2016},
month = {9}
}