Title: Effect of Siloxane Ring Strain and Cation Charge Density on the Formation of Coordinately Unsaturated Metal Sites on Silica: Insights from Density Functional Theory (DFT) Studies

Abstract

Amorphous silica (SiO₂) is commonly used as a support in heterogeneous catalysis. However, due to the structural disorder and temperature induced change of surface morphology, the structures of silica supported metal catalysts are difficult to determine. Most studies are primarily focused on understanding the interactions of different types of surface hydroxyl groups with metal ions. In comparison, the effect of siloxane ring size on the structure of silica supported metal catalysts and how it affects catalytic activity is poorly understood. Here, we have used density functional theory calculations to understand the effect of siloxane ring strain on structure and activity of different monomeric Lewis acid metal sites on silica. In particular, we have found that large siloxane rings favor strong dative bonding interaction between metal ion and surface hydroxyls, leading to the formation of high-coordinate metal sites. In comparison, metal-silanol interaction is weak in small siloxane rings, resulting in low-coordinate metal sites. The physical origin of this size dependence is associated with siloxane ring strain, and, a correlation between metal-silanol interaction energy and ring strain energy has been observed. In addition to ring strain, the strength of the metal-silanol interaction also depends on the positive charge density of the cations.more » In fact, a correlation also exists between metal-silanol interaction energy and charge density of several first-row transition and post-transition metals. The theoretical results are compared with the EXAFS data of monomeric Zn(II) and Ga(III) ions grafted on silica. In conclusion, the molecular level insights of how metal ion coordination on silica depends on siloxane ring strain and cation charge density will be useful in the synthesis of new catalysts.« less

Authors:

Das, Ujjal ^[1]; Zhang, Guanghui ^[2]; Hu, Bo ^[2]; Hock, Adam S. ^[2]; Redfern, Paul C. ^[3]; Miller, Jeffrey T. ^[3]; Curtiss, Larry A. ^[1]

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+ Show Author Affiliations

1. Argonne National Lab. (ANL), Argonne, IL (United States). Materials Science Division
2. Argonne National Lab. (ANL), Argonne, IL (United States). Chemical Sciences and Engineering Division; Illinois Inst. of Technology, Chicago, IL (United States). Dept. of Chemistry
3. Argonne National Lab. (ANL), Argonne, IL (United States). Chemical Sciences and Engineering Division

Publication Date:

Wed Oct 28 00:00:00 EDT 2015

Research Org.:

Argonne National Laboratory (ANL), Argonne, IL (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1236505

Grant/Contract Number:  

AC02-06CH11357

Resource Type:

Accepted Manuscript

Journal Name:

ACS Catalysis

Additional Journal Information:

Journal Volume: 5; Journal Issue: 12; Journal ID: ISSN 2155-5435

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE; DFT; EXAFS; Lewis acids; charge density; coordination number; ring strain; silica surface; single site catalysts