Surface functional group Lewis acid site

Abstracting from the complexity of the real systems, there is one common property of all natural particles. Their surfaces contain functional groups which can interact with H+, OH and metal ions and - if Lewis acid sites, e.g., =AI and =Fe, are available on the surface - with ligands. Many inorganic solids (oxidesQand silicates) contain hydroxo groups carbonates and sulfides expose -C-oh,-c oh, MeOH and -SH groups, respectively. While the interaction of alkaline and earth-alkaline ions... 

The concept of active sites has helped explain catalysis by enzymes and coenzymes. Although surface functional groups are less specific than enzymes, they form an array of surface complexes whose reactivities determine the mechanism of many surface-controlled processes. Many mechanisms can be described readily in terms of Br0nsted acid sites or Lewis acid sites. Of course, the properties of the surfaces are influenced by the properties and conditions of the bulk structure, and the action of special surface structural entities will be influenced by the properties of both surface and bulk. List I gives an overview of the major concepts and important applications. 

If one can prepare PCP nanochannels with surface functional Lewis acidic or basic moiety [51], they can be applied to catalytic pores for a variety of polymerization reactions. In this regard, most PCPs have pore surfaces with coordinatively saturated functional groups. The introduction of Lewis-acidic sites into the pore however, requires coordinatively unsaturated metal cation(s) (open metal site(s)) in the pore surface [35, 36, 52-54], Taking into account that the coordinative saturation of metal ions in PCPs tends to occur in usual frameworks, a clue to create open... 

It should be noted that minerals, and oxide minerals in particular, have different types of OH groups, depending on the coordination of the O atoms, as revealed by spectroscopic studies. Goethite (a-FeOOH) has four types of surface hydroxyls whose reactivities are a function of the coordination environment of the O in the FeOH group (Sposito 1984 Sparks 2002). The FeOH groups are A-, B-, or C-type sites, depending on whether the O is coordinated with 1,3, or 2 adjacent Fe(III) ions. The fourth type of site is a Lewis acid-type site, which results from chemisorption of a water molecule on a bare Fe(III) ion. Only A-type sites are basic (can bind H+), and, on the contrary, A-type and Lewis acid sites can release a proton. The B- and C-type sites are considered unreactive. Thus, A-type sites can be either a proton acceptor or a proton donor (i.e., they are amphoteric). Other spectroscopic studies have shown that boehmite (y-AlOOH) and lepidocrocite (y-FeOOH) have two types of OH, presumably associated with different crystal faces (Lewis and Farmer 1986). 

On account of the high calcination temperature the surface bears Bronsted as well as Lewis sites. Five distinct acid-base sites were observed by Steyrer [58] (see Fig. 4-7 [58]). The total concentration of surface hydroxyl groups on y-alumina activated at 470 K was 10 pmol/m, the amount of Lewis acid sites was assessed by means of pyridine adsorption as 0.6 pmol/m. The adsorption activity of alumina depends on the amount of adsorbed water [59, 60]. Alumina is suitable for separation of not too polar substances differing in steric arrangement or in the type of functional... 

The mineral surface may be considered as a solid source of Lewis and/or BrfSnsted acidity and the reactive sites S as localized acidic or basic functional groups. Reactions involving such sites may be understood in terms of Lewis acid/base or BrfSnsted acid/base interactions ( 1, 5, 6, 8, 38). As the acidity of the reactive sites increases, increasingly weak bases are neutralized and reactive surface complexes (A S) may be formed. The term "acidity" is often used in the broad sense of the word, including both BrjSnsted and... 

The solid chosen for their work on reaction (XLVIII) was alumina whose surface possesses several types of Bronsted and Lewis acidic and basic functional groups (-Al+-, -OHa+, -O - H, -0 , and defect sites) [175, 219] which could be potential catalysts for the mutarotation of glucose. Woelm... 

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