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Title: Nanoconfinement Effects in Catalysis

Abstract

In this investigation, the unique properties that stem from the constrained environment and enforced proximity of functional groups at the active site were demonstrated for a number of systems. The first system is a nanocage structure with silicon-based, atom-thick shells and molecular-size cavities. The shell imparts the expected size exclusion for access to the interior cavity, and the confined space together with the hydrophobic shell strongly influences the stability of charged groups. One consequence is that the interior amine groups in a siloxane nanocage exhibit a shift in their protonation ability that is equivalent to about 4 pH units. In another nanocage structure designed to possess a core-shell structure in which the core periphery is decorated with carboxylic acid groups and the shell interior is populated with silanol groups, the restricted motion of the core results in limiting the stoichiometry of reaction between carboxylic acid and a Co 2CO 8 complex, which leads to formation and stabilization of Co(I) ions in the nanocage. The second designed catalytic structure is a supported, isolated, Lewis acid Sn-oxide unit derived from a (POSS)-Sn-(POSS) molecular complex (POSS = incompletely condensed silsesquioxane). The Sn center in the (POSS)-Sn-(POSS) complex is present in a tetrahedral coordination,more » as confirmed by single crystal x-ray crystallography and Sn NMR, and its Lewis acid character is demonstrated with its binding to amines. The retention of the tetrahedral coordination of Sn after heterogenization and mild oxidative treatment is confirmed by characterization using EXAFS, NMR, UV-vis, and DRIFT, and its Lewis acid character is confirmed by stoichiometric binding with pyridine. This Sn-catalyst is active in hydride transfer reactions as a typical solid Lewis acid. In addition, the Sn centers can also create Brønsted acidity with alcohol by binding the alcohol strongly as alkoxide and transferring the hydroxyl H to the neighboring Sn-O-Si bond. The resulting acidic silanol is active in epoxide ring opening and acetalization reactions. The open structure of the Sn center makes it accessible to larger molecules, including cellobiose which can be converted to 5-(hydroxymethyl)-furfural. The third structure is a support planted with functional group pairing of a known separation distance. Using a precursor molecule that contains a hydrolysable silyl ester bond, and making use of known chemistry to convert silanol groups into amino/pyridyl and phosphinyl groups, silica surfaces with carboxylic acid/silanol, carboxylic acid/amine, carboxylic acid/pyridine, and carboxylic acid/phosphine pairs can be constructed. The amino groups paired with carboxylic acid on such a surface is more active in the Henry reaction of 4-nitobenzaldehyde with nitromethane.« less

Authors:
 [1]
  1. Northwestern Univ., Evanston, IL (United States)
Publication Date:
Research Org.:
Northwestern Univ., Evanston, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1325204
Report Number(s):
DE-FG02-01ER15184-final
DOE Contract Number:
FG02-01ER15184
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; catalysis; nanoscience

Citation Formats

Kung, Harold H. Nanoconfinement Effects in Catalysis. United States: N. p., 2016. Web. doi:10.2172/1325204.
Kung, Harold H. Nanoconfinement Effects in Catalysis. United States. doi:10.2172/1325204.
Kung, Harold H. 2016. "Nanoconfinement Effects in Catalysis". United States. doi:10.2172/1325204. https://www.osti.gov/servlets/purl/1325204.
@article{osti_1325204,
title = {Nanoconfinement Effects in Catalysis},
author = {Kung, Harold H.},
abstractNote = {In this investigation, the unique properties that stem from the constrained environment and enforced proximity of functional groups at the active site were demonstrated for a number of systems. The first system is a nanocage structure with silicon-based, atom-thick shells and molecular-size cavities. The shell imparts the expected size exclusion for access to the interior cavity, and the confined space together with the hydrophobic shell strongly influences the stability of charged groups. One consequence is that the interior amine groups in a siloxane nanocage exhibit a shift in their protonation ability that is equivalent to about 4 pH units. In another nanocage structure designed to possess a core-shell structure in which the core periphery is decorated with carboxylic acid groups and the shell interior is populated with silanol groups, the restricted motion of the core results in limiting the stoichiometry of reaction between carboxylic acid and a Co2CO8 complex, which leads to formation and stabilization of Co(I) ions in the nanocage. The second designed catalytic structure is a supported, isolated, Lewis acid Sn-oxide unit derived from a (POSS)-Sn-(POSS) molecular complex (POSS = incompletely condensed silsesquioxane). The Sn center in the (POSS)-Sn-(POSS) complex is present in a tetrahedral coordination, as confirmed by single crystal x-ray crystallography and Sn NMR, and its Lewis acid character is demonstrated with its binding to amines. The retention of the tetrahedral coordination of Sn after heterogenization and mild oxidative treatment is confirmed by characterization using EXAFS, NMR, UV-vis, and DRIFT, and its Lewis acid character is confirmed by stoichiometric binding with pyridine. This Sn-catalyst is active in hydride transfer reactions as a typical solid Lewis acid. In addition, the Sn centers can also create Brønsted acidity with alcohol by binding the alcohol strongly as alkoxide and transferring the hydroxyl H to the neighboring Sn-O-Si bond. The resulting acidic silanol is active in epoxide ring opening and acetalization reactions. The open structure of the Sn center makes it accessible to larger molecules, including cellobiose which can be converted to 5-(hydroxymethyl)-furfural. The third structure is a support planted with functional group pairing of a known separation distance. Using a precursor molecule that contains a hydrolysable silyl ester bond, and making use of known chemistry to convert silanol groups into amino/pyridyl and phosphinyl groups, silica surfaces with carboxylic acid/silanol, carboxylic acid/amine, carboxylic acid/pyridine, and carboxylic acid/phosphine pairs can be constructed. The amino groups paired with carboxylic acid on such a surface is more active in the Henry reaction of 4-nitobenzaldehyde with nitromethane.},
doi = {10.2172/1325204},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 9
}

Technical Report:

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  • The dehydrogenation of cyclohexene and cyclohexane, and the hydrogenation of cyclohexene were studied on the clean and preoxidized surfaces of three platinum single crystals: a Pt(111), a stepped Pt(S) - (6(111) x (100)) and a kinked Pt(S) - (7(111) x (310)). The choice of reaction conditions is discussed with respect to detection limits and the variation of the catalyst reactivity with reagent pressures and catalyst temperature; the reactions were carried out using flow conditions at low pressure (10/sup -6/ to 10/sup -5/ torr total pressure), at a platinum temperature of 150/sup 0/C. Oxygen coverages were monitored by Auger electron spectroscopymore » (AES), an Auger peak ratio of O/sub 510//Pt/sub 237/ = 0.5 having been determined to correspond to approximately 5 x 10/sup 14/ oxygen atoms/cm/sup 2/. The surface structures of the clean and oxidized platinum crystals were determined by low energy electron diffraction (LEED): after high temperature (800/sup 0/C) oxygen treatment, the predominant oxygen structure observed on the Pt(111) was a (2 x 2); the predominant oxygen structure observed on both the Pt(S) - (6(111) x (100)) and the Pt(S) - (7(111) x (310)) was a (radical 3 x radical 3) - R30/sup 0/. Low coverages of strongly bound oxygen enhanced the rates of the dehydrogenation and hydrogenation reactions, and changed the selectivity of cyclohexene dehydrogenation to benzene over hydrogenation to cyclohexane. These effects of preoxidation on catalytic rates and selectivity were found to be sensitive to the structure of the platinum surface, kink sites playing a particularly active role in the enhancement of dehydrogenation and hydrogenation activity by strongly bound oxygen. Three models are discussed which relate the oxidation of platinum surfaces to the observed effects on catalytic reactivity and the structure sensitivity. A change in the electronic structure of the platinum surface through oxidation provides the best general model for explaining the oxygen effects.« less
  • Accelerated aging at high temperatures (149 C) for short aging times (28 days) is effective in screening the compatibility of different materials in refrigeration systems. However, in actual applications temperatures are usually lower and operating times much longer. Therefore plots to allow for interpolation or extrapolation of experimental data to actual operating conditions are needed. In the current study, aging of refrigerant/lubricant/desiccant/metal systems was conducted at five different temperatures, and for each temperature at four different aging times. The data collected from this study provided plots relating refrigerant or lubricant decomposition to aging time, aging temperature, and type of desiccant,more » which can be used for interpolation or extrapolation.« less
  • The dissociation of anhydrous ethyl alcohol was studied over the temperature range 330 to 430 deg C with the employment of catalystg that were previously exposed to a range of estimated integrated'' neutron doses of up to 96.6 x 10/sup 18/ n/cm/sup 2/. The catalysts were ZnO, an n-type semiconductor, prepared by decomposition of the carbonate, and Cr/sub 2/O/sub 3/, a p-type semiconductor, prepared by dehydration of the hydroxide. Measurable changes in yield and decomposition mechanism were observed as compared with the unirradiated catalysts. The variation in the behavior of the catalysts was related to the various neutron doses received,more » and the results are discussed in the framework of the electronic theory of catalysis on semiconductors. (auth)« less
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  • This is the third quarterly report on the research program entitled Investigations of Coal Gasification Reaction Mechanisms being performed under Contract DE-AC21-80MC14592. Coal char, with and without catalysts consisting of alkali metal carbonates and alkali and alkaline earth hydroxides, will be gasified with H/sub 2/O, H/sub 2/, CO/sub 2/, and mixtures of gases. Physical measurements including optical microscopy, SEM, NMR, IR, ESR, and XPS will be conducted on the char residues following varying degrees of gasification. These measurements will be used to define reaction mechanisms consistent with the experimental data and provide a basis for developing mathematical models of themore » reaction kinetics. This report presents the data obtained in the gasification of K/sub 2/CO/sub 3/-catalyzed and noncatalyzed coal chars with CO/sub 2/ and the results of analyses and physical measurements made on the char residues of the gasification experiments. Gasification rate tests have been conducted at temperatures ranging from 650 to 850/sup 0/C and at pressures varying from 0.1 to 3.5 MPa. The presence of the catalyst accelerates CO/sub 2/ gasification by an order of magnitude or more. The relationship between the gasification rates and temperature and pressure differ for catalyzed and noncatalyzed chars. Physical and chemical analyses of the char residues indicate that the conversion of K/sub 2/CO/sub 3/ catalyst to K/sub 2/SO/sub 4/, initiated during the charring process, continues throughout the gasification step. The presence of carbonyl groups in partially gasified char residues has been shown by /sup 13/C NMR and FTIR spectra.« less