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Subject: search.filters.subject.Acidic Properties

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    Acidic Properties of Mixed Tin-i-Antimony Oxide Catalysts
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (1), 1978) Irving, Elizabeth A.; Taylor, Duncan
    Mixed oxides of tin and antimony have been used as catalysts in a static system and outgassed both at room temperature and at 698 K in a study of the approximately zero order stages of the isomerization of 3,3-dimethylbut-1-ene (373 K), cyclopropane (411 K), but-1-ene (293 K) and cis-but-2-ene (293 K) and of the dehydration of isopropanol (343 and 408 K). With catalysts outgassed at room temperature, weakly acidic sites are present, and all the reactions probably occur by a carbonium ion type of mechanism with Brönsted acid sites as a source of protons. Rates increase to a maximum as the antimony content increases from zero to ≈ 50 atomic %, and then decline with further increase in the antimony content. Outgassing of the catalysts at 698 K increased the rate of isomerization of 3,3-dimethylbut-1-ene, but for cyclopropane and isopropanol decreased rates were observed due to poisoning by the propene product. For but-1-ene and cis-but-2-ene, the higher temperature outgassing procedure changed the rate against catalyst composition pattern considerably in that only catalysts with less than 50 % Sb were active, and a mechanism involving an allyl intermediate is proposed. Catalyst activity could be poisoned by treatment with bases or with sodium acetate. It is concluded on the basis of a proposed correlation between rates and acidity, that the catalyst composition corresponding to maximum acidity is different from that for maximum selective oxidation activity.
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    Acidic Properties of Silica-Alumina Gels as a Function of Chemical Composition: Titration and Catalytic Activity Measurements
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (2), 1977) Damon, Jean-Pierre; Delmon, Bernard Delmon; Bonnier, Jane-Marie
    The strength and number per unit surface area of acid sites on a series of silica—aluminas of composition ranging from 0 to 100 % in alumina have been investigated. The results of titration by n-butylamine in the presence of six different HR indicators and of activity and selectivity in the transformation of 4-methylpentan-2-ol are compared. An overall parallelism between the amount of acidity of different strengths and the activity in the different reactions (i.e., dehydration, cis-trans-isomerization, double-bond shift and isomerization of the hydrocarbon skeleton, which set in successively with increasing acidity of the catalyst) is observed. Acidity in all samples arises only from strongly acidic (HR⩽–13.3) and intermediate strength sites (–6.63 ⩽HR⩽+ 0.82). Comparison of the results with those of cation exchange capacity measurements is not satisfactory, because of the inherent limitations of the latter method. Results for strong acid sites agree perfectly with those reported in infrared studies using adsorbed pyridine and CD3CN. Results concerning intermediate strength sites suggest that i.r. studies, titration and catalytic acitivity measurements may complement each other.
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    Evolution of the Acidic Properties of Silica—Alumina Gels as a Function of Chemical Composition: Infrared Approach
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (2), 1977) Scokart, Paul O.; Declerck, Francois D.; Sempels, Raymond E.; Rouxhet, Paul G.
    The adsorption of pyridine and CD3CN on silica–alumina gels covering the whole composition range (0–100 % Al2O3) has been studied by infrared spectroscopy, the amount adsorbed being measured by an electrobalance. The measurement of the maximum intensity reached by the bands of strongly adsorbed species has allowed us to sketch the evolution of the surface Brönsted sites and various types of Lewis sites as a function of the chemical composition of the adsorbant. The overall concentration of Lewis sites increases smoothly from 0 to 75 % Al2O3; the sharper increase in the high Al2O3 range is mainly due to weak sites which are revealed by a pyridine band at 1615 cm–1 while pyridine coordinated with strong Lewis sites gives a band at 1625 cm–1. The concentration of acid sites associated with the mixed silica–alumina phase corresponds to the more acidic sites observed by titration in non aqueous media. A high proportion of them are strongly acidic hydroxlys which are detected by absorption of H bond acceptors. The mixed phase develops as the Al2O3 content increases from 0 to 25 %; above 50 % Al2O3 it is progressively diluted in a mixture with other phases and is no longer observed above 85 % Al2O3.