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

Journal Article · · Journal of the American Chemical Society
DOI:https://doi.org/10.1021/jacs.6b05711· OSTI ID:1375032
 [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)
+ Show Author Affiliations

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.

Research Organization:
Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC)
Grant/Contract Number:
AC02-06CH11357
OSTI ID:
1375032
Journal Information:
Journal of the American Chemical Society, Vol. 138, Issue 38; ISSN 0002-7863
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 4 works
Citation information provided by
Web of Science

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY