Acidity Characterization of Amorphous Silica鈥揂lumina

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• 作者：Emiel J.M. Hensen ; Dilip G. Poduval ; Volkan Degirmenci ; D.A J. Michel Ligthart ; Wenbin Chen ; Fran莽oise Maug茅 ; Marcello S. Rigutto ; J.A. Rob van Veen
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：October 11, 2012
• 年：2012
• 卷：116
• 期：40
• 页码：21416-21429
• 全文大小：518K
• 年卷期：v.116,no.40(October 11, 2012)
• ISSN：1932-7455

文摘

Surface characterization of amorphous silica鈥揳lumina (ASA) by CO_ads IR, pyridine_ads IR, alkylamine temperature-programmed desorption (TPD), Cs⁺ and Cu(EDA)₂²⁺ exchange, ¹H NMR, and m-xylene isomerization points to the presence of a broad range of Br酶nsted and Lewis acid sites. Careful interpretation of IR spectra of adsorbed CO or pyridine confirms the presence of a few very strong Br酶nsted acid sites (BAS), typically at concentrations lower than 10 渭mol/g. The general procedure for alkylamine TPD, which probes both Br酶nsted and Lewis acidity, is modified to increase the selectivity to strong Br酶nsted acid sites. Poisoning of the m-xylene isomerization reaction by a base is presented as a novel method to quantify strong BAS. The surface also contains a weaker form of BAS, in concentrations between 50 and 150 渭mol/g, which can be quantified by CO_ads IR. Cu(EDA)₂²⁺ exchange also probes these sites. The structure of these sites remains unclear, but they might arise from the interaction of silanol groups with strong Lewis acid Al³⁺ sites. The surface also contains nonacidic aluminol and silanol sites (200鈥?00 渭mol/g) and two forms of Lewis acid sites: (i) a weaker form associated with segregated alumina domains containing five-coordinated Al, which make up the interface between these domains and the ASA phase and (ii) a stronger form, which are undercoordinated Al sites grafted onto the silica surface. The acid catalytic activity in bifunctional n-heptane hydroconversion correlates with the concentration of strong BAS. The influence of the support electronegativity on the neopentane hydrogenolysis activity of supported Pt catalysts is considerably larger than that of the support Br酶nsted acidity. It is argued that strong Lewis acid sites, which are present in ASA but not in 纬-alumina, are essential to transmit the Sanderson electronegativity of the oxide support to the active Pt phase.