Brønsted Acid Sites (Brønst + acid_site)

Distribution by Scientific Domains


Selected Abstracts


Significant Influence of Zn on Activation of the C-H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite,

CHEMPHYSCHEM, Issue 17 2008
Alexander G. Stepanov Prof.
Abstract Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C1,n -C4 alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of CH bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n- butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C1,n -C4 alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites. [source]


Incorporation of Aluminium and Iron into the Zeolite MCM-58

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2005
Gabriela Ko
Abstract The hydrothermal synthesis of zeolite MCM-58 is investigated with N -benzylquinuclidinium bromide as a structure-directing agent in order to isomorphously substitute aluminium or iron for silicon. Al-MCM-58 was synthesised in a wide range of nSi/nAl ratios (from 19 to 56), and Fe-MCM-58 was successfully prepared in a narrower range of nSi/nFe ratios (from 18 to 36). The obtained products were characterised by XRD, SEM, 27Al MAS NMR, 29Si MAS NMR, FTIR, and ESR spectroscopy. Two different calcination procedures, viz. in a stream of nitrogen and air or in a stream of ammonia, were used in order to modify the acid sites in the zeolite. FTIR spectroscopy before and after the adsorption of [D3]acetonitrile and pyridine was employed to determine the concentration and type of the acid sites. The acid forms of Al(Fe)-MCM-58 are characterised by the vibrations of bridging Si,OH,Al(Fe) groups at 3628 cm,1 and 3564 cm,1 (Al-MCM-58) or 3646 cm,1 and 3520 cm,1 (Fe-MCM-58). The acid sites of both zeolites Al-MCM-58 and Fe-MCM-58 are accessible for [D3]acetonitrile and pyridine, and all materials contain substantial numbers of Lewis sites (Al-MCM-58: 50,% of the total acid sites; Fe-MCM-58: 90,%) over the whole range of nSi/nAl (nSi/nFe) ratios studied. Only a small increase in the concentration of Brønsted acid sites was achieved after calcination of the as-synthesised samples in a flow of ammonia. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]


Properties of methanol bound to a defect of zeolitic structure

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2005
Nikolai F. Stepanov
Abstract Different mechanisms of Brønsted acidity formation upon the adsorption of a methanol molecule on a model Lewis acid site within zeolitic structure are evaluated by means of density functional theory in cluster approximation. The properties of the Brønsted acid sites formed are examined through studying their interaction with such probe molecules as methanol, ammonia, and ethylene. The results obtained are used to comment on a possible role of Lewis acid sites in methanol reactions catalyzed by zeolites. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source]


Catalytic performance of Brønsted acid sites during esterification of acetic acid with ethyl alcohol over phosphotungestic acid supported on silica

JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 6 2007
Abd El-Aziz A Said
Abstract Different ratios of phosphotungestic acid supported on silica gel were prepared by an impregnation method with PWA loadings ranging from 1 to 30% w/w and calcined at 350 and 500 °C for 4 h in a static air atmosphere. The catalysts were characterized by thermogravimety (TG), differential thermal analysis (DTA), X-ray diffraction, FT-IR spectroscopy and N2 adsorption measurements. The surface acidity and basicity of the catalyst were investigated by the dehydration,dehydrogenation of isopropanol and the adsorption of pyridine (PY) and 2,6-dimethyl pyridine (DMPY). The gas-phase estrification of acetic acid with ethanol was carried out at 185 °C in a conventional fixed-bed reactor at 1 atm using air as carrier gas. The results clearly revealed that the catalyst containing 10% w/w PWA/SiO2 is the most active and delivers reaction selectively to ester with 85% yield. The Brønsted acid site resulting from hydroxylation of tungsten oxide plays the main role in the formation of ester. Copyright © 2007 Society of Chemical Industry [source]


Pt Nanoparticles Supported on Highly Dispersed Alumina Coated inside SBA-15 for Enantioselective Hydrogenation

CHEMCATCHEM, Issue 10 2010
Hongna Wang
Abstract A series of Al2O3@SBA-15 composites with different alumina loadings have been prepared using a solid-state grinding method. Various techniques, such as X-ray diffraction, N2,sorption, FT-IR spectroscopy, and NMR spectroscopy, are used to characterize their structures and acidity. The Al2O3@SBA-15 composites retain the mesostructure of the SBA-15 host and the alumina is highly dispersed and uniformly coated inside the mesopores. The Al2O3@SBA-15 composites serve as remarkable supports for Pt nanoparticles, which can be used as catalysts in the enantioselective hydrogenation of ethyl pyruvate. Both the Brønsted acid sites that originate from the interface of Al2O3 and SBA-15 in the Al2O3@SBA-15 composites and the appropriate interaction of Pt nanoparticles with alumina coated inside the mesopores of SBA-15 can improve the catalytic behavior of Pt/Al2O3@SBA-15 catalysts. [source]


Significant Influence of Zn on Activation of the C-H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite,

CHEMPHYSCHEM, Issue 17 2008
Alexander G. Stepanov Prof.
Abstract Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C1,n -C4 alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of CH bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n- butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C1,n -C4 alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites. [source]