Surface acidity titration

Alkalinity (Soluble Soda) Determination. The surface alkalinity or soluble or leachable soda is determined by making a fixed weight percent slurry in water and determining the alkalinity of the solution by pH measurement or acid titration. Sodium ion-sensitive electrodes have been investigated. 

Example 2.1 Evaluation of Surface Charge from Alkalimetric and Acidimetric Titration Curves and Determination of Surface Acidity Constants... 

Fatty acid, MRNi hydrogenation, 32 243-245 Faujasites, 34 160-183 acidic sites, 27 151-154 alkaline and rare earth forms, 27 160-165 amine titration, 27 163 infrared smdies, 27 160-163 surface acidity and catalytic activity, 27 163-165... 

The effect of fluoridation with hydrofluoric acid on the. surface acidity of catalytic alumina was also studied by Ballou e< al. (340), About 1 % of fluoride was taken up by the catalyst. The surface acidity was estimated using a titration technique developed by Benesi (341a). After addition of varying amounts of butylamine in benzene solution, the color changes... 

The picture has been clarified by surface acidity and basicity distribution measurements for several catalysts using thermometric titrations with... 

The detailed synthesis procedure and textural properties (surface area, Sggy in m2 g-1 pore volume, V in ml g"1 and main pore diameter, d in nm), determined by nitrogen adsorption from 8.E.T. method have been published elsewhere (refs. 13-18) and are summarized in Table 1, where the surface acidity and basicity of supports are also collected. These values were determined by a spectro-photometric method described elsewhere (ref. 19), that allows titration of the amount (in tunol g 1) of irreversibly adsorbed benzoic acid (BA, pKa> 4.19), pyridine (PY, pka= 5.25) or 2,6-diterbutyl-4-methylpyridine (DTMPY, pKa 7.5) employed as titrant agents of basic and acid sites, respectively. Furthermore, the apparent rate constant values of different supports in the gas-phase skeletal isomerization of cyclohexene (CHSI), in Mmol atm"1 g"1 s-1, at 673 K, are also collected in Table 1, because these values are another way of measuring the stronger acid sites of supports (ref. 19). 

In essence, aqueous titration of surface acidity is an ion-exchange process in which hydrated surface protons are replaced by other hydrated cations (e.g., Na+, NIV") during the course of the titration. The procedure is straightforward. It usually consists of the direct titration of an aqueous suspension of the sample of powdered solid with a dilute base (e.g., sodium hydroxide) to a neutral endpoint. Another commonly used procedure consists of noting the pH of an appropriate salt solution (e.g., ammonium acetate), adding the sample, and measuring the amount of di-... 

Determination of acid strengths by means of fluorescent indicators (21) and still another class of indicators, the arylcarbinols (24), has also been reported. Comparison of these indicators for the titration of surface acidity will be discussed in the following section. 

After the acid strength of a catalyst surface has been bracketed by means of colors of adsorbed indicators, the next logical step in the determination of surface acidity is the measurement of the number of acidic groups. This is generally done by titrating a suspension of the catalyst with a solution of a suitable amine in an inert solvent the previously described indicators are used to determine endpoints. 

Fig. 2. Surface acidity of silica-alumina visual and spectrophotometric titrations (21, 24, 26). , A, Hammett indicators O, Fluorescent indicators , arylcarbinol indicators.

Lewis acid sites can coordinate with a given indicator molecule to produce an adsorption band identical in position with that produced through proton addition. Even if the indicators used are responsive only to Brpn-sted acids, most basic reagents used to titrate surface acidity (e.g., n-butylamine, pyridine) are strongly adsorbed on surface sites other than Br0nsted acid sites. In this connection, a recent study indicates that adsorption equilibrium is not fully established during titration of silica-alumina with n-butylamine because of the irreversible attachment of amine molecules by adsorption sites at which they first arrive (31). 

If the objective is to fingerprint surface acidity (so that one acidic solid can be distinguished more clearly from another), the nonaqueous titration method (26) would be appropriate. The original method could be made more selective for titration of Br0nsted acids by choosing recently recommended indicators (21) and using a sterically hindered amine (54) as the basic reagent. However, even an improved indicator method may have its drawbacks a sterically hindered amine is not perfectly selective,... 

Fluoride addition promotes the cracking (108-110) and isomerization (108, 111) activity of alumina, presumably, because of the formation of Brdnsted acid sites. In a comprehensive study of fluorided aluminas, Antipina et al. (110) demonstrated that there is a close parallelism between generation of cumene cracking activity at 430°C and surface acidity when fluoride content is increased from 0 to 7% wt (see Fig. 15). Acidity was measured by n-butylamine titration endpoints were determined by means of an arylcarbinol indicator that detected Br0nsted acids stronger than those corresponding to a pKa of — 13.3. In a separate article (112), Anti-... 

Take et al. (69) extended the acid strength range of the n-butylamine titration method. They employed indicators such as 4-nitrotoluene (pKa = - 10.5) and 2,4-dinitrotoluene (pKa = - 12.8), which are considerably less basic than the other Hammett indicators used to measure surface acidity. Endpoints were determined spectrophotometrically. These authors found that the acid sites on silica-alumina catalyst had an acid strength corresponding to an H0 between - 10.5 and - 12.8 a few sites had even higher acid strengths (H0 < 12.8). Strong acid sites were eliminated when silica-alumina was poisoned with sodium ions. 

The remaining studies reviewed in this section deal with binary oxides that contain titania. Shibata and Kiyoura (140) measured surface acidities by the n-butylamine titration method of the Ti02-Zr02 system as a function of composition and method of preparation. Samples were prepared by calcination of coprecipitated mixtures of titanium and zirconium hydroxides that were made by ammonia or urea addition. The products had... 

Johnson (9) had proposed that one could titrate the acidity of colored solids by adding white solid acids and then titrate while observing the color changes on the white solid acids. This idea is based on the assumption that the colored and white solids do not interact with each other but only with the titrant and dyes. This assumption has been used for many years without challenge, but T. Fabish (14) has just recently shown that carbon blacks do indeed interact with silica-alumina particles and can appreciably change their surface acidity. 

Aqueous titrations of the phenolic and carboxylic surface acids are successful routine laboratory techniques involving agitation of slurried carbon black in alkaline aqueous solutions for hours or even days, followed by back-titration. Figure 4... 

For the characterization of the nature of surface sites, probe molecules are needed. In the following, however, we confine ourselves to those probe molecules that can principally be used as poisons under catalytic conditions. Thus, for example, the indicator molecules usually used for the titration of surface acidity and basicity will not be treated. The conditions under which these measurements are carried out differ greatly from those applied during actual catalysis, and the molecular size of the probe molecules is unfortunately usually very large.2 These methods have been reviewed very recently by Tanabe (20) and by Fomi (42). 

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