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Title: Olefin Adsorption on Silica-Supported Silver Salts - A DFT Study

Abstract

Recent experiments have shown that silver salts supported on mesoporous silicas display excellent adsorption selectivities of ethylene over ethane and propylene over propane. Employing the techniques of density functional theory, we have investigated the fundamental bases of this separation process by examining silver salts dispersed on model silica surfaces. Our model system includes Ag+ cations, their counteranions, silica supports, and surface silanols. Both adsorption geometries and energetics of ethylene and propylene were explored. Our results indicate that the nature of the Ag-olefin interaction is predominantly hybridization between Ag d and olefin states, which is supported by analyses of electron density difference plots and density of states. The counteranions, such as NO3 -, were found to interact strongly with surface silanols through multiple hydrogen bonds but have limited effect on the adsorption energy of olefins on the Ag+ cations. The current work supports recent experiments, which indicate that Ag-salt/silica may be a very promising adsorbent for olefin/paraffin separation.

Authors:
 [1];  [1];  [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States). Center for Computational Sciences
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
930796
DOE Contract Number:
AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Langmuir; Journal Volume: 22; Journal Issue: 13
Country of Publication:
United States
Language:
English

Citation Formats

Sumpter, Bobby G, Dai, Sheng, and Jiang, Deen. Olefin Adsorption on Silica-Supported Silver Salts - A DFT Study. United States: N. p., 2006. Web.
Sumpter, Bobby G, Dai, Sheng, & Jiang, Deen. Olefin Adsorption on Silica-Supported Silver Salts - A DFT Study. United States.
Sumpter, Bobby G, Dai, Sheng, and Jiang, Deen. Sun . "Olefin Adsorption on Silica-Supported Silver Salts - A DFT Study". United States. doi:.
@article{osti_930796,
title = {Olefin Adsorption on Silica-Supported Silver Salts - A DFT Study},
author = {Sumpter, Bobby G and Dai, Sheng and Jiang, Deen},
abstractNote = {Recent experiments have shown that silver salts supported on mesoporous silicas display excellent adsorption selectivities of ethylene over ethane and propylene over propane. Employing the techniques of density functional theory, we have investigated the fundamental bases of this separation process by examining silver salts dispersed on model silica surfaces. Our model system includes Ag+ cations, their counteranions, silica supports, and surface silanols. Both adsorption geometries and energetics of ethylene and propylene were explored. Our results indicate that the nature of the Ag-olefin interaction is predominantly hybridization between Ag d and olefin states, which is supported by analyses of electron density difference plots and density of states. The counteranions, such as NO3 -, were found to interact strongly with surface silanols through multiple hydrogen bonds but have limited effect on the adsorption energy of olefins on the Ag+ cations. The current work supports recent experiments, which indicate that Ag-salt/silica may be a very promising adsorbent for olefin/paraffin separation.},
doi = {},
journal = {Langmuir},
number = 13,
volume = 22,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • Recent experiments have shown that silver salts supported on mesoporous silicas display excellent adsorption selectivities of ethylene over ethane and propylene over propane. Employing the techniques of density functional theory, we have investigated the fundamental bases of this separation process by examining silver salts dispersed on model silica surfaces. Our model system includes Ag+ cations, their counter anions, silica supports, and surface silanols. Both adsorption geometries and energetics of ethylene and propylene were explored. Our results indicate that the nature of the Ag-olefin interaction is predominantly hybridization between Ag d and olefin states, which is supported by analyses of electronmore » density difference plots and density of states. The counter anions, such as NO3-, were found to interact strongly with surface silanols through multiple hydrogen bonds but have limited effect on the adsorption energy of olefins on the Ag+ cations. The current work supports recent experiments, which indicate that Ag-salt/silica may be a very promising adsorbent for olefin/paraffin separation.« less
  • The simultaneous adsorption and reaction of hydrogen with preadsorbed carbon monoxide was investigated over a series of silica-supported ruthenium and silver-ruthenium bimetallic catalysts to elucidate the role of site sensitivity on hydrogen adsorption and methane formation. The specific rate of methane synthesis, which is a direct measure of the catalyst activity toward the Fischer-Tropsch synthesis, measured at 460 Torr and temperatures from 400 to 500 K, varied from 0.01 x 10{sup {minus}3} to 3 x 10{sup {minus}3}S{sup {minus}1}. As little as 3% Ag reduced the methanation rate of Ru by 80%. The temperature dependence of the turnover frequency showed thatmore » the apparent activation energy for methanation dropped from 24 kcal/mol for the monometallic ruthenium case to 18 kcal/mol for the bimetallic case. Since silver does not adsorb or react with either hydrogen or carbon monoxide, these results show that silver does not act merely as a dilutant in the bimetallic system. These observations are elucidated in terms of a new model for surface-sensitive hydrogen adsorption, termed portal site mediated adsorption, where low-coordination edge and corner sites on the catalyst surface act as sinks for rapid, dissociative adsorption of weakly bound, highly mobile surface hydrogen. In situ {sup 1}H-NMR measurements of surface hydrogen coverages during reaction were used to confirm the kinetic model treatment of the reaction data.« less
  • A {sup 13}C CP-MAS NMR and elemental analysis study of adsorption and interaction of silyl ethers, RSi(OMe){sub 3} (R = Et, Ph, OMe), as internal and external electron donors with MgCl{sub 2}-supported Ziegler-Natta catalyst has been carried out. A chemical activation of anhydrous MgCl{sub 2} with EtOH and AlEt{sub 3} produces a high-surface-area support stabilized by an organoaluminum compound, AlEt{sub 2}(OEt). In a treatment of the aluminum-modified MgCl{sub 2} support with silyl ether, the aluminum surface complex is retained and silyl ether is almost totally incorporated into the support. {sup 13}C CP-MAS NMR data of the methoxy region indicate thatmore » a mobile liquid-like silyl ether species dominates, except in the case of Si(OMe){sub 4}, where a more strongly bound species is also present on the support. TiCl{sub 4} treatment removes the weakly adsorbed silyl ether species, leaving a species which is attributed to an aluminum-bound silyl ether surface complex. No evidence of titanium-bound silyl ether species was found in the solid state or in solution where TiCl{sub 4} undergoes with silyl ethers an exchange reaction forming a yellow solid identified as (TiCl{sub 2}(OMe){sub 2}){sub x}. Activation of the catalyst with AlEt{sub 3} at a high Al:Ti ratio produces a material with a low silyl ether coverage showing a weak methoxy signal in {sup 13}C CP-MAS. The linewidths of the observed signals in {sup 13}C CP-MAS NMR are in the range 5-10 ppm at every stage of preparation of the catalyst, indicating heterogeneity of the coordination sites on the surface of chemically activated MgCl{sub 2}.« less