skip to main content
DOE PAGES title logo U.S. Department of Energy
Office of Scientific and Technical Information

This content will become publicly available on April 8, 2020

Title: Dynamic Reorganization and Confinement of Ti IV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes

Abstract

The effect of dynamic reorganization and confinement of isolated Ti IV catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calix[4]arene–Ti IV centers or their calcined counterparts. Their location is synthetically controlled to be either unconfined at terminal T-atom positions (denoted as type-(i)) or within confining 12-MR pockets (denoted as type-(ii); diameter ~7 Å, volume ~185 Å 3) composed of hemispherical cavities on the external surface of zeotypes with *-SVY topology. Electronic structure calculations (density functional theory) indicate that active sites consist of cooperative assemblies of TiIV centers and silanols. When active sites are located at unconfined type-(i) environments, the rate constants for cyclohexene epoxidation (323 K, 0.05 mM TiIV, 160 mM cyclohexene, 24 mM tert-butyl hydroperoxide) are 9 ± 2 M –2 s –1; whereas within confining type-(ii) 12-MR pockets, there is a ~5-fold enhancement to 48 ± 8 M –2 s –1. When a mixture of both environments is initially present in the catalyst resting state, the rate constants reflect confining environments exclusively (40 ± 11 M–2 s–1), indicating that dynamic reorganization processes lead to the preferential location of active sites within 12-MR pockets. While activation enthalpies are ΔH app =more » 43 ± 1 kJ mol–1 irrespective of active site location, confining environments exhibit diminished entropic barriers (ΔS app = -68 J mol –1 K –1 for unconfined type-(i) vs -56 J mol –1 K –1 for confining type-(ii)), indicating that confinement leads to more facile association of reactants at active sites to form transition state structures (volume ~ 225 Å 3). These results open new opportunities for controlling reactivity on surfaces through partial confinement on shallow external-surface pockets, which are accessible to molecules that are too bulky to benefit from traditional confinement within micropores.« less

Authors:
ORCiD logo [1];  [2];  [2];  [1]; ORCiD logo [2];  [3]; ORCiD logo [1]
  1. Univ. of California, Berkeley, CA (United States)
  2. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  3. Chevron Energy Technology Company, Richmond, CA (United States)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1529380
Grant/Contract Number:  
AC02-76SF00515
Resource Type:
Accepted Manuscript
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 141; Journal Issue: 17; Journal ID: ISSN 0002-7863
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Grosso-Giordano, Nicolás A., Hoffman, Adam S., Boubnov, Alexey, Small, David W., Bare, Simon R., Zones, Stacey I., and Katz, Alexander. Dynamic Reorganization and Confinement of Ti IV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes. United States: N. p., 2019. Web. doi:10.1021/jacs.9b02160.
Grosso-Giordano, Nicolás A., Hoffman, Adam S., Boubnov, Alexey, Small, David W., Bare, Simon R., Zones, Stacey I., & Katz, Alexander. Dynamic Reorganization and Confinement of Ti IV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes. United States. doi:10.1021/jacs.9b02160.
Grosso-Giordano, Nicolás A., Hoffman, Adam S., Boubnov, Alexey, Small, David W., Bare, Simon R., Zones, Stacey I., and Katz, Alexander. Mon . "Dynamic Reorganization and Confinement of Ti IV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes". United States. doi:10.1021/jacs.9b02160.
@article{osti_1529380,
title = {Dynamic Reorganization and Confinement of Ti IV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes},
author = {Grosso-Giordano, Nicolás A. and Hoffman, Adam S. and Boubnov, Alexey and Small, David W. and Bare, Simon R. and Zones, Stacey I. and Katz, Alexander},
abstractNote = {The effect of dynamic reorganization and confinement of isolated TiIV catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calix[4]arene–TiIV centers or their calcined counterparts. Their location is synthetically controlled to be either unconfined at terminal T-atom positions (denoted as type-(i)) or within confining 12-MR pockets (denoted as type-(ii); diameter ~7 Å, volume ~185 Å3) composed of hemispherical cavities on the external surface of zeotypes with *-SVY topology. Electronic structure calculations (density functional theory) indicate that active sites consist of cooperative assemblies of TiIV centers and silanols. When active sites are located at unconfined type-(i) environments, the rate constants for cyclohexene epoxidation (323 K, 0.05 mM TiIV, 160 mM cyclohexene, 24 mM tert-butyl hydroperoxide) are 9 ± 2 M–2 s–1; whereas within confining type-(ii) 12-MR pockets, there is a ~5-fold enhancement to 48 ± 8 M–2 s–1. When a mixture of both environments is initially present in the catalyst resting state, the rate constants reflect confining environments exclusively (40 ± 11 M–2 s–1), indicating that dynamic reorganization processes lead to the preferential location of active sites within 12-MR pockets. While activation enthalpies are ΔH‡app = 43 ± 1 kJ mol–1 irrespective of active site location, confining environments exhibit diminished entropic barriers (ΔS‡app = -68 J mol–1 K–1 for unconfined type-(i) vs -56 J mol–1 K–1 for confining type-(ii)), indicating that confinement leads to more facile association of reactants at active sites to form transition state structures (volume ~ 225 Å3). These results open new opportunities for controlling reactivity on surfaces through partial confinement on shallow external-surface pockets, which are accessible to molecules that are too bulky to benefit from traditional confinement within micropores.},
doi = {10.1021/jacs.9b02160},
journal = {Journal of the American Chemical Society},
number = 17,
volume = 141,
place = {United States},
year = {2019},
month = {4}
}

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on April 8, 2020
Publisher's Version of Record

Save / Share: