Influence of Acid Strength

The active site for the Beckmann rearrangement is still the subject of controversy. What is the acid strength suitable for lactam production-strong, weak or intermediate It has even been suggested that neutral sites are responsible for the rearrangement. For the zeolite catalysts, does the reaction occur inside the pore or on the external surface If the reaction occurs on the external surface, what is the role of the zeolite structure  

Takamiya et al. reported that B2O3-based binary metal oxide catalysts with a large number of acid sites of —8.2 —3.0 gave caprolactam preferentially 

The apparent relationship between external surface area and the activity and selectivity of the rearrangement can be interpreted differently. If the reaction occurs in the channels near the pore openings to the external surface, the effectiveness of the catalyst could be increased in line with the external surface area. Another reason for the claim that the Beckmann rearrangement of cyclohexanone oxime occurs on the external surface of the MFI structure is the similar sizes of f-caprolac-tam and the pore diameter. It should, however, be noted that the diameter values 

It is well known that silanol groups are formed upon dealumination of Y zeolites. The excellent catalytic performance of moderately dealuminated ultrastable 

Y zeolites [36], and of FSM-16 and MCM-41 [16], which contain many hydrogen-bonded silanol groups, for this rearrangement in 1-hexanol diluent is consistent with these findings, although it should also be noted that highly dealuminated 

---

Figure 13.5 The influence of acid strength on the nature of the reaction products during the isohutane/2-hutene alkylation.

A. E. Hirschler (Sun Oil Co., Marcus Hook, Pa. 19061) I should like to make a comment relating to your statement that activity is not a simple function of acidity. This statement appears to contain the implicit assumption that all acid sites able to protonate pyridine would have the same catalytic activity. Actually, one might expect an intensity factor (acid strength) to be operative in acid catalysis as well as an extensive factor, and the apparent lack of correlation you observed may result from an influence of acid strength on catalytic activity. Pyridine adsorption data alone do not completely define Bronsted acidity. One may need also some measure of acid strength. Acidity measurement with Hr indicators, such as we and others have reported, may supply useful information relative to acid strength as well as number of acid sites. 

Table 9. The influence of acid strength of proton acids on the acidity of proton acid-carrier systems.

Figure 17.11 Influence of acid strength on the shape of the curve for titration with NaOH. Each curve represents titration of 50.0 mL of 0.10 M acid with 0.10 M NaOH. 

Influence of ionic strength on the reaction rate constant. The influence of the ionic strength on the reaction rate constant was studied using KCl as electrolyte. The results obtained in this study are listed in Table 4, where we can see that the reaction rate constant for N-Br-alanine decomposition undergoes an increment of 40 % upon changing the ionic strength from 0.27M to IM, while in the case of N-Bromoaminoisobutyric acid the increment of the reaction rate constant is of about 12 %. This is an evidence of a non ionic mechanism in the case of the decomposition of N-Br-aminoisobutyric acid, as it is expected for a concerted decarboxylation mechanism. For the decomposition of N-Br-proline the increase on the reaction rate constant is about 23 % approximately, an intermediate value. This is due to the fact both paths (concerted decarboxylation and elimination) have an important contribution to the total decomposition process. 

Further substitution of alkyl groups in ethanoic acid has much less effect than this first introduction and, being now essentially a second-order effect, the influence on acid strength is not always regular, steric... 

The oxidative behaviour of glycolaldehyde towards hexacyanoferrate(III) in alkaline media has been investigated and a mechanism proposed, which involves an intermediate alkoxide ion. Reactions of tetranitromethane with the luminol and luminol-peroxide radical anions have been shown to contribute substantially to the tetranitromethane reduction in luminol oxidation with hexacyanoferrate(III) in aerated aqueous alkali solutions. The retarding effect of crown ethers on the oxidation of triethylamine by hexacyanoferrate(III) ion has been noted. The influence of ionic strength on the rate constant of oxidation of ascorbic acid by hexacyanofer-rate(III) in acidic media has been investigated. The oxidations of CH2=CHX (where X = CN, CONH2, and C02 ) by alkaline hexacyanoferrate(III) to diols have been studied. ... 

Although these effects are often collectively referred to as salt effects, lUPAC regards that term as too restrictive. If the effect observed is due solely to the influence of ionic strength on the activity coefficients of reactants and transition states, then the effect is referred to as a primary kinetic electrolyte effect or a primary salt effect. If the observed effect arises from the influence of ionic strength on pre-equilibrium concentrations of ionic species prior to any rate-determining step, then the effect is termed a secondary kinetic electrolyte effect or a secondary salt effect. An example of such a phenomenon would be the influence of ionic strength on the dissociation of weak acids and bases. See Ionic Strength... 

It is known that the activation temperature can influence the acid strength distribution. For example, measurements of the differential heats of ammonia adsorbed at 150°C for a HY zeolite have led to the conclusion that stronger acid sites, in the 150-180 kJ/mol range, are formed upon increasing the activation temperature from 300 to 650°C. Dehydroxylation at high temperature resulted in the formation of strong Lewis acid sites and the disappearance of intermediate and weak Brpnsted sites [62]. 

It is interesting that the acid strength of the (nta)Cr(OH)2Cr(nta)2 ion is comparable to that of the tetrapositive 1, 10-phen complex, for example, a fact which is in keeping with the above suggestion that factors other than the overall charge strongly influence the acid strength of these complexes. 

No rigorous practical definition of acid strength exists outside of very dilute aqueous solutions where activity coefficients can be assumed to be unity. Each method of measuring the acid strength of surface sites is liable to give somewhat different answers, the results being qualitative at best. The dielectric constant of the medium, in which the acid or base probe is used influences the measured acid-base strength. 

The acid and base sensitivity of condensation polymers whether or not under stress, e.g. polycarbonate, polyesters, polyamides and polysilanes under influence of acid or base the condensation bonds are hydrolysed under the cooperative action of mechanical stresses and the environment. A striking example is shown in Fig. 26.11, where the strength retention of PpPTA fibres is plotted versus pH after an exposure of 3 months at room temperature (Van den Heuvel and Klop). The hydrolysis of the polyamide is acid or alkali catalysed, in particular below pH = 3 and above pH = 9. 

Acidity has long been recognized as a major factor contributing to the deterioration of cellulose-containing materials. In an effort to combat the harmful influence of acidity, researchers have developed a variety of deacidification techniques capable of decreasing the acid content of most paper-containing objects that are found in museums and libraries. These techniques often are used by conservators in the care of books and works of art on paper (I). Nevertheless, the nature of the chemical processes that cause papers to yellow and to lose strength remains somewhat obscure, and the role of acidity in these processes also is not well understood. 

In this chapter, empirical rate constants were determined by following the loss of tensile strength and the decrease in diffuse reflectance of a rag and a newsprint paper buffered over a range of pH values. From the results of these experiments, the influence of acidity on both the atmospheric oxidation process and the oxygen-independent process was determined. Before describing the results of these experiments, the methodology employed to obtain the empirical rate constants will be discussed. 

Influence of Ionic Strength. It should be noted that eorrections to take aecount of ionic strength as discussed in Section 3.2.1.2 apply not only to the acid-base equilibrium constants but also to the stability constants for complex formation. 

XANES, particularly the edge position and white line intensity, can, however, give a good indication of the electron density of the aluminum. The electron density on the aluminum atom then in turn influences the electron density on the oxygen, which can have an influence on acid strength of the solid. 

Figure 9.33. (a) Schematic description of the effects of ionic strength (I) and pH on the conformations of a humic molecule in solution and at a surface. Rh denotes the hydrodynamic radius of the molecule in solution and 6h denotes the hydrodynamic thickness of the adsorbed anionic poly electrolyte. (Adapted from Yokoyama et al., 1989 and O Melia, 1991). (b) The influence of ionic strength of pH on diffusion coefficient, Dl, and on Stokes-Einstein radius of a humic acid fraction of 50,000-100,000 Dalton. (From Cornel et al., 1986). 

The catalytic performance of 03 to produce carbon oxides in MSR can, to some extent, be rationalized using a concept of Tatibouet et al. [35], which was originally applied to the selective oxidation of methanol and is based on the influence of the strength of acidic and basic oxide surface sites on product formation. According to this concept, the formation of carbon oxides (and preceding formate species) reqnires the presence of redox-active surface centers as well as both weak acidic and rather strong basic sites. 363 is a reducible/amphoteric oxide with... 

The disagreement with the results of Ohyoshi et al (3) can be ascribed to the differences in concentrations of Am (III) used in the two works (10 7 M against 10 3 m). The influence of ionic strength on oxidation rate was studied in order to confirm the suggested explanation of phosphate and sulfate effect on the kinetic rates."Figure 4"shows how the Am (III) oxidation rate varies with the ionic strength with and without phosphoric acid. We attempted to adjust these results to the Bronsted equation (7), which correlates the rate constants of a reaction with the reagent... 

Monolayer Studies. Surface pressure vs. area curves for OMS spread on sodium chloride solutions are given in Figure 5. The range of ionic strengths corresponds with those used in the film measurements. However there is no apparent significant influence of ionic strength on the form of the curve. The films formed are condensed, indicating, the predominance of cohesion between the octadecyl chains. The curves of surface pressure (tr) vs. area per molecule (A) resemble those obtained with stearic acid (7). The data at the lower surface pressures extrapolate to an area per... 

ZSM-5 zeolites having three types of extra lattice alumina, namely acid insoluble extra framework alumina (AIEFAL), amorphous alumina (AMAL) and binder alumina (BNAL) were studied. Microcalorimetric acid strength distribution studies indicated the presence of very strong acid sites in the sample containing AIEFAL. Presence of amorphous alumina did not show any significant influence on acid strength distribution. But, the amount of poly aromatic compounds formed in n-heptane aromatization reaction over this catalyst is more, indicates the occurrence of non-shape selective surface reactions on this alumina. Presence of binder alumina showed the positive effect on stability in activity of n-heptane aromatization reaction. A common phenomenon observed with all the catalysts is the fast deactivation of strong acid sites. 

Basically, the dediazoniation of diazoacetate (7.28) and diazoacetone (7.29) follows the same mechanism. The intermediate diazonium ions are not stable, but are steady state intermediates (ki < k- k- > ki)- McCauley and King (1952), Albery and Bell (1961), and Albery et al. (1968) found that the measured overall rate constants k of these dediazoniations are proportional to the acid strength in aqueous solution (Hammett acidity function Hq) reflecting the expected influence of acid on k, but the constants k also increase linearly with addition of salts of the acid used. The influence of these acid anions is a function of their nucleophilicity (Cl < Br- < I-). ... 

---