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Bronsted acid-base properties

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. [Pg.308]

Scheme 1 Comparison between charges and Bronsted acid-base properties of the oligopeptide angiotensin II and a PVFA-co-PVAm/silica interphase... Scheme 1 Comparison between charges and Bronsted acid-base properties of the oligopeptide angiotensin II and a PVFA-co-PVAm/silica interphase...
BrOnsted acid—base properties, Lewis acid—base propeties (EPA—EPD) etc. Such classifications make it possible to select a type of solvent which we believe will fit the reaction. Often our choice is based on assumptions as to the reaction mechanism. [Pg.374]

The solid/gas interface was traditionally studied with respect to adsorption and catalysis. Here the assertion that the Bronsted definition of acidity is a particular case of the Lewis definition is neither obvious nor even helpful. It suffices to say that many reactions in heterogeneous catalysis require specifically the presence of either Bronsted or Lewis acidic (or basic) sites, and the reaction mechanisms depend on the nature of the surface site. A long-term goal of surface studies for the characterization of solid catalysts was to distinguish and quantify the number of Bronsted or Lewis sites with potential catalytic activity for gas-phase reactants. For that reason, when discussing the acid-base behavior of solid surfaces, it is no longer possible, nor desirable, to adopt the viewpoint that subsumes Bronsted acid-base properties in the more general Lewis definition. [Pg.75]

As a result of these suggested differences in isomorphic substitution mechanism, SAPO and MCM materials should have different ion-exchange and catalytic properties. SAPO s are cation exchangers and potential BrCnsted acid catalysts. MCM s are expected to be cation and/or anion exchangers and are potential Bronsted acid, Brensted base or Bronsted acid/base catalysts. [Pg.306]

Ampholyte — A substance that can react both as an acid and as abase is called an ampholyte, or amphoteric compound. Usually this property refers to the - Bronsted acid-base theory. An example is HCOj which can act as a proton acceptor and as a proton donator. An ampholyte can be a zwitterion, as in case of amino acids in the range between pH = pJCai and pH = pfCa2, they exist as [+(H3N)HRC-COO-]. [Pg.29]

The Bronsted acid/base pair pyridinium/pyridine is unusual because redox potentials are available for both partners in an accessible range [35]. This allows detailed anticipation of the sensory behaviour of systems containing pyridine receptors. (49), whose fluorescence properties have received some attention before [113], is a member of a family of PET sensors for protons whose excitation and emission wavelengths spread over a substantial fraction of the accessible optical spectrum without compromising sensory action [114]. [Pg.246]

Different approaches are used to explain acid-base properties of substances. According to Bronsted and Lowry [16,17], an acid is a substance able to act as proton donor, while a base is a substance able to act as proton acceptor. According to Lewis [18], acids are the compounds that are able to accept electron pairs forming covalent bond, while bases are the compounds that act as donors of electron pair. [Pg.20]

This ability to interact strongly with positively and negatively charged ions is consistent with water s nnique amphoteric (acid-base) properties, and is why it is fundamental to the Bronsted acid-base theory (equations 4 and 5). [Pg.3454]

The acid-base properties of decationated ZSM-5 zeolite have been studied in some detail using adsorption microcalorimetry, as shown in Table VIII (169-173). As the calcination temperature for HZSM-5 zeolites was increased from room temperature to 1073 K, a maximum in acidity was observed while the initial differential heat of ammonia adsorption increased continuously. Vedrine et al. (92) also found a maximum in the intensity of the IR hydroxyl bands (169) of HZSM-5 at 673 K. The IR absorption band of pyridine adsorbed on Brpnsted sites followed the same trend as that found for the hydroxyl stretching bands, confirming that above 673 K the Bronsted acidity decreased as the dehydration temperature increased. [Pg.199]

Drummond et al. [54] synthesized protic ionic liquids by combining Bronsted acid/base pairs where the primary amine cations were of the form RNH3 and R(OH) NH3 combined with organic anions of the form RCOO, R(OH)COO or with an inorganic anion. They studied physicochemical properties of synthesized PILs at nominally equimolar ratio of anion and cation (1 1 stoichiometry) and in the presence... [Pg.376]

Most ions constituting ionic liquids can be categorized according to their Lewis acid/base properties (i.e., their capability to accept or to donate an electron pair) nevertheless, some ions may be considered according to the Bronsted definition, i.e., on the basis of their ability to accept or donate a proton. Typical ionic liquids are those based on neutral or very weakly basic anions (BF4, PF, NOf, CHsSO, 4 f2N ) and neutral (tetraalkylammonium, dialkyl-pyrrolidium, trialkylsulfonium) or weakly acidic cations (1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium) (Figure 4.1). [Pg.158]

Interactions between the precious metal and support influence the performance of the catalyst. Beil (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section Catalyst characterization for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support... [Pg.471]

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. [Pg.303]

II. ACID-BASE PROPERTIES OF SOLID SURFACES A. General Definitions 1. Bronsted-Lowry... [Pg.71]

What are the factors that determine the acid-base properties of solid surfaces such as metal oxides On the basis of the discussion thus far it seems appropriate to relate the appearance of Lewis acidity and disappearance of Bronsted acidity to the increase in the degree of dehydroxylation. Indeed, the interconversion of Lewis and Bronsted acid sites has been demonstrated for some oxides, such as ZnO or supported Mo03 Cr203, or WO3, by IR studies of pyridine or ammonia adsorption [59]. But which factors determine the strength of acid sites ... [Pg.87]

Oxides, especially those of Al, Si, Fe, and Mg, are essential components of the earth s minerals their surface sites may have either Bronsted or Lewis acid-base properties. The weathering of rocks and the formation of soils are processes at the solid/liquid interface between minerals and natural waters or solutes from the medium in which solid phases are either formed, altered, or dissolved. Dissolution of solid minerals is controlled by slow surface reactions rather than... [Pg.112]

Give Arrhenius s and Bronsted s definitions of an acid and a base. Why are Bronsted s definitions more useful in describing acid-base properties ... [Pg.126]

This reaction is sometimes called the autoionization of water. To describe the acid-base properties of water in the Bronsted framework, we e q)ress its autoionization as follows (also shown in Figure 16.1) ... [Pg.531]

Acid-base properties so far have been discussed in terms of the Bronsted theory. To behave as a Bronsted base, for example, a substance must be able to accept protons. By this definition both the hydroxide ion and ammonia are bases ... [Pg.565]

Compensation of charge, when a trivalent atom substitutes for the tetravalent Si, requires the presence either of exchangeable cations (as with zeolites) or of acidic protons. The former type of samples may in principle show basic behavior, the latter shows invariably acidity, either Lewis or Bronsted acidity. These acid/base properties, thou much studied for their practical importance, are not well imderstood, and debate is going on. For this reason, a review on this subject is timely. We present here an accoimt on the possible sources for acidic behavior of MTS, being aware that the interpretations proposed may be considered as subjective. [Pg.218]

The oxide catalysts are microporous or mesoporous materials or materials containing both types of pores. In the latter case, the applicability is larger in terms of the molecular size of the reactants. Acid-base properties of these materials depend on the covalent/ionic character of the metal-oxygen bonds. These sites are involved in several steps of the catalytic oxidation reactions. The acid sites participate with the cation redox properties in determining the selective/unselective catalyst behavior [30,31]. Thus, many studies agree that partial oxidation of organic compounds almost exclusively involves redox cycles and acid-base properties of transition metal oxides and some authors have attempted to relate these properties with activity or selectivity in oxidation reactions [31,42]. The presence of both Bronsted and Lewis acid sites was evidenced, for example, in the case of the metal-modified mesoporous sihcas [30,39,43]. For the bimetallic (V-Ti, Nb-Ti) ions-modified MCM-41 mesoporous silica, the incorporation of the second metal led to the increase of the Lewis sites population [44]. This increased concentration of the acid sites was well correlated with the increased conversion in oxidation of unsaturated molecules such as cyclohexene or styrene [26,44] and functionalized compounds such as alcohols [31,42] or phenols [45]. [Pg.477]


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See also in sourсe #XX -- [ Pg.558 , Pg.562 ]




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Bases acid-base properties

Bronsted acid

Bronsted acid/base

Bronsted acidity

Bronsted acids properties

Properties based

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