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Acidic sites

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... [Pg.581]

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... [Pg.718]

Fig. XVIII-20. Spectra of pyridine adsorbed on a water-containing molybdenum oxide (IV)-Al203 catalyst L and B indicate features attributed to pyridine adsorbed on Lewis and Brpnsted acid sites, respectively. (Reprinted with permission from Ref. 191. Copyright 1976 American Chemical Society.)... Fig. XVIII-20. Spectra of pyridine adsorbed on a water-containing molybdenum oxide (IV)-Al203 catalyst L and B indicate features attributed to pyridine adsorbed on Lewis and Brpnsted acid sites, respectively. (Reprinted with permission from Ref. 191. Copyright 1976 American Chemical Society.)...
Figure C2.12.2. Fonnation of Br0nsted acid sites in zeolites. Aqueous exchange of cation M witli an ammonium salt yields tlie ammonium fonn of tlie zeolite. Upon tliennal decomposition ammonia is released and tire proton remains as charge-balancing species. Direct ion-exchange of M witli acidic solutions is feasible for high-silica zeolites. Figure C2.12.2. Fonnation of Br0nsted acid sites in zeolites. Aqueous exchange of cation M witli an ammonium salt yields tlie ammonium fonn of tlie zeolite. Upon tliennal decomposition ammonia is released and tire proton remains as charge-balancing species. Direct ion-exchange of M witli acidic solutions is feasible for high-silica zeolites.
Factors other tlian tire Si/Al ratio are also important. The alkali-fonn of zeolites, for instance, is per se not susceptible to hydrolysis of tire Al-0 bond by steam or acid attack. The concurrent ion exchange for protons, however, creates Bronsted acid sites whose AlO tetraliedron can be hydrolysed (e.g. leading to complete dissolution of NaA zeolite in acidic aqueous solutions). [Pg.2787]

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidie sites of such solid acids must be eon-sidered. Nafion-H contains acidic -SO3H groups in clustered pockets. In the acidic zeolite H-ZSM-5 the active Bronsted and Tewis acid sites are in close proximity (—2.5 A). [Pg.201]

Lewis acid sites Lewis-base donors Lewis bases Lewisite... [Pg.563]

Fig. 11. Receptor molecules (cryptands) having hetero (nonoxygen) donor atoms (7) or endo-functional acidic sites (8) in the framework. Fig. 11. Receptor molecules (cryptands) having hetero (nonoxygen) donor atoms (7) or endo-functional acidic sites (8) in the framework.
An additional effect of the use of an organic medium in the catalyst preparation is creation of mote defects in the crystalline lattice when compared to a catalyst made by the aqueous route (123). These defects persist in the active phase and are thought to result in creation of strong Lewis acid sites on the surface of the catalysts (123,127). These sites ate viewed as being responsible for the activation of butane on the catalyst surface by means of abstraction of a hydrogen atom. [Pg.454]

Acid Sites. Acidic zeoHtes have outstanding catalytic activity. The hydrogen form may be produced by ammonium ion exchange, foUowed by thermal deammoniation. The unsolvated proton forms an OH group with a bridging O ... [Pg.449]

In a reversal of the reaction with SiCl, aluminum can be introduced into the framework by reaction of the hydrogen or ammonium form with gaseous AlCl (36). Similarly, reaction with aqueous ammonium fluoroaluminates replaces framework-Si with Al (37). When alumina-bound high siUca 2eohtes are hydrothermaHy treated, aluminum migrates into framework positions and generates catalyticaHy active acid sites (38). The reaction can be accelerated by raising the pH of the aqueous phase. [Pg.451]

The catalyst for the second stage is also a bifimctional catalyst containing hydrogenating and acidic components. Metals such as nickel, molybdenum, tungsten, or palladium are used in various combinations and dispersed on sofid acidic supports such as synthetic amorphous or crystalline sihca—alumina, eg, zeofites. These supports contain strongly acidic sites and sometimes are enhanced by the incorporation of a small amount of fluorine. [Pg.206]

Because soHd acid catalyst systems offer advantages with respect to their handling and noncorrosive nature, research on the development of a commercially practical soHd acid system to replace the Hquid acids will continue. A major hurdle for soHd systems is the relatively rapid catalyst deactivation caused by fouling of the acid sites by heavy reaction intermediates and by-products. [Pg.47]

The mineral talc is extremely soft (Mohs hardness = 1), has good sHp, a density of 2.7 to 2.8 g/cm, and a refractive index of 1.58. It is relatively inert and nonreactive with conventional acids and bases. It is soluble in hydroduoric acid. Although it has a pH in water of 9.0 to 9.5, talc has Lewis acid sites on its surface and at elevated temperatures is a mild catalyst for oxidation, depolymerization, and cross-linking of polymers. [Pg.301]

Possible role of the induced acidity and basicity in catalysis and environmental chemistry is discussed. The suggested mechanism explains the earlier reported promotive effect of some gases in the reactions catalyzed by Bronsted acid sites. Interaction between the weakly adsorbed air pollutants could lead to the enhancement of their uptake by aerosol particles as compared with separate adsoi ption, thus favoring air purification. [Pg.56]

Spectral studies at low temperatures enable us to broaden the number of test molecules for surface acidic sites and besides ammonia pyridine and nitriles, to use CO, NO and that do not adsorb at 300 K. [Pg.431]

The effect of pH is rarely of use for pK measurement it is more often of use in identifying the site of protonation/deprotonation when several basic or acidic sites are present. Knowing the incremental substitutent effects Z of amino and ammonium groups on benzene ring shifts in aniline and in the anilinium ion (40), one can decide which of the nitrogen atoms is protonated in procaine hydrochloride (problem 24). [Pg.61]

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... [Pg.261]

The most common use of curing agents is with carboxylic latices. Isocyanates and melamines can be used but zinc oxide is the most common curing agent. Zinc oxide cross-links carboxylated latices and improves bond strength by ionomer formation [78]. Carboxylated polychloroprene reacts slowly with zinc oxide in dispersed form, causing a gradual increase in adhesive gel content. This can lead to restricted adhesive shelf life. Resin acid sites compete with the polymer acid sites for Zn(II). The more resin acid sites, the more stable the adhesive. [Pg.669]

The isomerization reaction, which is acid-site controlled, includes the conversion of alkylcyclopentanes into alkylcyclohexanes, which, in turn, are quickly converted to aromatics by dehydrogenation. In addition, isomerization also includes the conversion of feed n-paraffms into higher octane I-paraffins. [Pg.49]


See other pages where Acidic sites is mentioned: [Pg.161]    [Pg.584]    [Pg.584]    [Pg.734]    [Pg.735]    [Pg.735]    [Pg.2777]    [Pg.2787]    [Pg.2788]    [Pg.2789]    [Pg.2793]    [Pg.163]    [Pg.201]    [Pg.202]    [Pg.1118]    [Pg.311]    [Pg.9]    [Pg.565]    [Pg.372]    [Pg.323]    [Pg.449]    [Pg.48]    [Pg.54]    [Pg.152]    [Pg.196]    [Pg.111]    [Pg.495]    [Pg.49]    [Pg.556]   
See also in sourсe #XX -- [ Pg.365 ]

See also in sourсe #XX -- [ Pg.145 ]

See also in sourсe #XX -- [ Pg.38 , Pg.40 , Pg.103 , Pg.493 , Pg.494 , Pg.496 , Pg.638 ]




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Acetonitrile acid sites

Acid Site Type, Concentration and Strength

Acid Sites and Extra-Framework Aluminum

Acid site concentration

Acid site concentration measurement

Acid site concentration measurement observation

Acid site from reaction rates

Acid site generation

Acid site number

Acid site structure

Acid site titrations

Acid sites

Acid sites accessibility

Acid sites cracking Zeolite catalysts

Acid sites strength

Acid sites, BrSnsted

Acid sites, coke deposition effect

Acid sites, neutralization

Acid-base catalysis, site-directed mutagenesis

Acid-base pair site

Acid-base sites

Acid-base sites selective molecular probes

Acid-base surface sites, adsorption

Acid-base surface sites, adsorption organic molecules

Acidic site characterization

Acidic sites iron oxides

Acidic sites on alumina

Acidic sites on solid surfaces

Acidic sites pigments

Acidic sites silica adsorption

Acidic sites, NiSMM catalysts

Acidic surface sites

Acidic zeolite sites

Acrylic acid Catalytic active sites

Active site amino acid decarboxylases

Adsorption microcalorimetry acid sites strength

Adsorption, acidic sites

Altering Lewis acidic site

Aluminas Lewis acid sites

Aluminum acid sites

Amine-Catalyzed Reactions Enhanced by Acid Site on Silica-Alumina

Amino Acid Information site

Amino Acid Uptake Sites

Amino acid active site

Amino acid residue, modification site

Amino acid residues chemical modification sites

Amino acids active site, /3-galactosidase

Ammonia adsorption acid site characterization

Ammonia adsorption, Lewis acid sites

Antigenic site amino acid composition

Arachidonic acid sites

Benzene and Phenol as Probes for Acid Sites

Binding sites Lewis acidic

Boronic acid-Nucleophile Complex Formed in the Enzyme Active Site as a way to Improve Potency and Selectivity

Boronic acid-containing binding sites

Br0nsted acid sites

BrOnsted acid sites in zeolites

Brdnsted acid sites

Bronsted acid site density

Bronsted acid sites, reactions catalyzed

Bronsted acid sites, reactions catalyzed hydrocarbons

Bronsted acid sites, reactions catalyzed zeolites

Bronsted acidic sites

Bronsted acidity site concentration measurement

Bronsted acidity site interaction with probe

Brpnsted acid sites

Brpnsted acid sites in zeolites

Brpnsted acid sites interaction

Carboxylic acids protonation site

Carboxylic acids site of protonation

Catalyst acid-base sites

Catalysts Bronsted acid sites

Catalysts Lewis acid sites

Characterization of Acid-Base Sites in Oxides

Characterization of Acid-Base Sites in Zeolites

Characterization surface acid sites

Clay active sites Acid-activated

Clay properties Bronsted acid sites

Clay properties Lewis acid sites

Conductometric titrations, acid site

Density acid sites

Diffusivities strong interaction with acid sites

Direct Observation of Bronsted Acid Sites

Enzymes/nucleic acids, reactive sites

Exchangeable acid sites

Fatty acid cellular site

Fatty acids metabolism sites

Faujasite acidic sites

Framework Lewis Acid Sites

Frameworks and acid sites

Generation of acid site

HETEROGENEITY OF THE ACID SITES

Heterogeneous acidic sites

Heterogeneous catalyst Bronsted acid sites

Humic acid complexing sites

Hydride Lewis acid sites

Hydroxamic acids acid sites

Infrared spectroscopy acid site concentration

Lewis acid sites

Lewis acid sites adsorbing basic probes

Lewis acid sites commercial processes

Lewis acid sites in zeolites

Lewis acid sites molecular sieves

Lewis acid sites pyridine adsorbed

Lewis acid sites spectra

Lewis acid sites weak acidity

Lewis acidic site catalysts

Lewis acidic sites

Lewis acidity strong sites

Lewis acidity surface sites

Lewis acidity weak sites

Lewis and Bronsted acid sites

Measurement of Acid Site Concentration and Strength in Microporous Solids

Medium pore zeolites influence of crystal size and acid site density

Metal and acid sites

Migration of an Acid Plume at a Uranium Mill Tailings Site

NO Lewis acid sites

Nitriles as Probes for Acid Sites

Nuclear magnetic resonance acid site concentration

Nucleic acids multiple metal-binding sites

Nucleic acids site-directed mutagenesis

Number of acid sites

Oligomerization reactions, Bronsted acid sites

Oligomerization reactions, Bronsted acid sites catalyzing

Oligosialic Acid Binding Sites

Pigments acid-base sites

Pillared clays Bronsted acid sites

Poisoning of acid sites

Potassium acid sites, neutralization

Protein acidic sites

Protonic acid sites

Protonic acid sites zeolite catalysis

Protonic acid sites zeolite structures

Protonic zeolites Bronsted acidic sites

Purple acid phosphatases active sites

Pyridine probing Bronsted acid sites

Pyridine probing Lewis acid cation sites

Pyridine, Ammonia and Amines as Probes for Acid Sites

Sialic acid-binding site

Silica Lewis acid sites

Silica-aluminas acidic sites

Site-Directed Mutagenesis Substituting Individual Amino Acids in Proteins

Site-Selective Reactions of Malic Acid Derivatives

Site-Specific Internal Functionalization of Nucleic Acids with Transition-Metal Ligands and Other Moieties

Sites, Bronsted acid

Solid acids acid site type

Specific Amino Acids at the Active-Site Involved in Catalysis and Substrate Binding

Spectroscopic Detection of Surface Bronsted Acid Sites

Strength of acid sites

Subject acid sites

Surface acid sites, characterization spectroscopic methods

Surface functional group Lewis acid site

Surface: active oxygen alumina, silica acid sites

Target sites amino acid biosynthesis inhibiting

The Lewis Acid Sites of Aluminas and SAs

Transition metals sites with Lewis acidic properties

Two Different Sites Amino Acids

Weaker acid sites

Zeolite Bronsted acid sites

Zeolite catalysis Brpnsted acid sites

Zeolite catalyst acidic sites

Zeolites Brdnsted acid sites

Zeolites Brpnsted acid sites

Zeolites acid sites

Zeolites surface Bronsted acid sites

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