Acid-base sites

Triprotic Acids and Bases, and Beyond The treatment of a diprotic acid or base is easily extended to acids and bases having three or more acid-base sites. For a triprotic weak acid such as H3PO4, for example, we can treat H3PO4 as if it was a mono-protic weak acid, H2P04 and HP04 as if they were intermediate forms of diprotic weak acids, and P04 as if it was a monoprotic weak base. 

The first pH indicators studied possessed the acid-base site (phenol, aniline, or carboxylic acid) as an integral part of the fluorophore. Structurally, in the most general sense, pH sensitivity is due to a reconfiguration of the fluorophorets re-electron system that occurs on protonation. Consequently, the acid and the base forms often show absorption shifts and also, when the two forms fluoresce, emission shifts or at least, when only one form emits, a pH-dependent fluorescence intensity. This class of compounds has been reviewed 112 and the best structures have to be designed according to the medium probed and the technique used. After a short consideration of physiological pH indicators we will describe the main photophysical processes sensible to protonation. 

The interaction of the probe and acid/base site should have a unequivocal spectral response. 

Kotanigawa and co-workers [7-9] studied the adsorption behavior of phenol and methanol on a Zn0-Fe203 system and highlighted the importance of acid-base sites for the selective ortho methylation. However, detailed studies of the adsorption behavior of possible products and reactants of phenol methylation on catalytic systems and their interaction among them are not available widely. 

High activity associated with x = 0.5 composition demonstrates an optimum concentration of acid-base sites is needed for phenol adsorption and subsequent polarization of both phenol and isobutene as in the ease of other alkylations. It was proposed that in the phenol t-butylation, t-butyl carbocation ean attaek phenol from the adsorbed as well as from the gaseous state resulted in the formation of para t-butylated products such as 4-tBP and 2,4-tBP. The steric hindrance of t-butyl group prevents the sequential attack of t-butyl cation at ortho position for dialkylation and that demonstrated the negligible formation of 2,6-di-t-butyl phenol. 

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

Some observations indicate that the zeolite framework is a prerequisite for good catalyst performance via physical constraints and proximity of acid-base sites within the molecular sieves environment.431 Other observations show that rather microporosity plays an important role in the alkylation.432... 

With minor adjustments, the treatment presented above for the reactions of Feaq3+/FeaqOH2+ is applicable to any reaction that involves two hydrolytic forms of one or more reactants and is easily extended to cases with more than one protonation/deprotonation site at a single center. The experimental rate law will depend on the pH range used and relative reactivities of acidic and basic form(s). The two general cases in Figure 8.3 schematically cover various possibilities for a reaction with a single acid-base site. The two curves differ only in relative reactivities of the acidic and basic forms. 

Therefore, according to this idea, two differently shaped hydrogen atoms (proton and hydride-ion) are transferred simultaneously from the hydrogen peroxide molecule. Primarily, hydride-ion formation and participation of acid-base sites in the catalysis act were noted in the works [82,110, 111], where the key role of hydride ion in the mechanism of enzymatic H202 dissociation was suggested. 

Figure 6.9 The formation mechanism of complex VIII (Chance), (a) Active complex with distal histidine and asparagine as acid-base sites, (b) Active intermediate—complex VIII (Chance).

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

The adsorption of H2 at RT on high-surface-area MgO sintered at 1073 K leads to heterolytic dissociation of the molecule on a few acid-base sites,... 

G. M. Zhidomirov, and V. B. Kazansky, Quantum-chemical cluster models of acid-base sites of oxide catalysts, Adv. Cat. 34, 131-201 (1986). 

Quantum-Chemical Cluster Models of Acid-Base Sites of Oxide Catalysts... 

Fig. 11. Cluster models of the adsorption complexes of H20 molecules on the acid base sites of a structure unit containing a bridged hydroxyl group, showing one-center (I, II) and two-center (III) adsorption forms.

Results of Calculations (CTP, CNDO/BW) of the Energetic Characteristics for Different Forms of Water Molecule Adsorption on Acid-Base Sites of H Forms of Zeolites as a Function of the Strength of the Acid site"... 

Some preliminary runs were made on the Al O support in the absence of Pt. Heating this alumina without CO pulsing showed a small peak appearing near 100°C. This was identified as N adsorbed as an impurity from the carrier gas. CO completely displaces the on pulsing at room temperature and, as previously reported, probably results from the presence of Lewis acid-base sites gn the alumina (8). The amount of this peak was approximately 10 moles per g of Al O which represents less than 5% of the CO desorbing from a well dispersed Pt catalyst of 0.4 wt% loading. Runs without the in-situ H O and 0 trap had shown that C0 desorbed with the CO. However, with the In-situ trap minimal CO was observed. This was confirmed by a pair of runs shown in Figure 1 with and without Al O (CO2 trap) placed at the exit of the desorption tube. 

Organo-gew-dimetallic compounds that possess two nucleophilic see Lewis Acids Bases) sites on one carbon atom... 

A commonly used technique is TPD of adsorbed bases such as ammonia and pyridine [84—86]. However, there are critical questions in this method [87-89]. The NH3 molecule is well known to interact with both the acidic OH group and the basic oxygen [90]. Thus, NH3 gives information on dual acid-base sites. 

Most of these advantaaes relate to the fact that acid/base sites of dense or... 

Acid-base properties strength and number of acid-base sites... 

Group analysis was carried out to see which combination of acid-base groups gave rate and selectivity in best agreement with experimental results. For ethylene formation, the best group association appeared to be an acid and a weak base site, with dissociative ethanol adsorption. For ether formation, two acid-base site-pairs were required, allowing a dissociatively adsorbed... 

Siddan and Narayan also employed 7-AI2O3 and Th02 for the dehydration of a number of model alcohols and observed that if the basicity of the alumina was increased by Na -ion doping, 7-elimination was enhanced using both neopentyl and pinacolyl alcohol. It appeared that as the alumina became less acidic and more basic, there was a shift from El/E2-like behaviour to an ElcB-type mechanism, which manifested itself in a concerted 7-elimination (Scheme 10 for neopentyl alcohol A,B = acid, base sites respectively reproduced by permission from J. Catal, 1979, 59, 405). This tendency was also observed by use of erythrof f/ireo)-3-methylpentan-2-ol. 

Reactions with Lewis acid—base sites... 

The following mechanism is proposed The catalytic entity is a dual site consisting of a carbenium ion locked up in the zeolite close to an aluminium ion on T-site, and an induced basic site on one of the oxide ions surrounding the A1 ion. This provides a dual acid/base site which is necessary for the carbene mechanism. The absence of basic sites on fresh H-ZSM-5 has been a major argument for rejecting the carbene mechanism. A formal description of the mechanism is given in schemes 1 and 2. 

To assess the surface acid/base site modification due to and Li incorporation into the lattice, FTIR spectroscopy of adsorbed methanol as a probe molecule was used. Methanol is known to interact with the acid-base pair O ") of the oxide surface [19] and to form methoxy and hydroxyl groups by dissociation of its O-H bond. The Vco (C-O stretching) and VcH3 (C-H stretching of the methyl group) bands of the methoxy species are measured since they are known to be sensitive to Lewis acid site nature [20]. Fig. 3 shows spectra of methanol adsorbed on y-Al203 and Mg -modified Y-AI2O3... 

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