Silica-alumina catalyst titration acidity

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

The work of Misono et al. (55) illustrates how acid strength distributions for silica-alumina catalyst can be deduced from catalytic titration measurements by use of an appropriate series of reactants. Surface concentration of amine, pyridine in this case, was adjusted by proper choice of amine partial pressure and desorption temperature while carrier gas flowed over the catalyst sample. At each level of chemisorbed pyridine, pulses of the reactants were passed over silica-alumina at 200°C and the products analyzed. The reactants were t-butylbenzene, diisobutylene, butenes, and f-butanol. It was concluded that skeletal transformations require the presence of very strong acid sites, that double-bond isomerization occurs over moderately strong acid sites, and that alcohol dehydration can occur on weak acid sites. 

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. 

Belatedly good evidence for the strong acidity of suitably dehydrated catalyst was obtained. Tamele (9 ), reporting work done in collaboration with 0. Johnson, L. B. Ryland, and E. E. Roper, noted that evidence for acidity was obtained for silica-alumina from ammonia adsorption, and that the catalyst acidity could be titrated with butylamine in benzene. Still later, Benesi (10), using the Hammett indicators, showed that silica-alumina catalyst had an acid strength greater than that of 90% sulfuric acid. 

Another correlation is shown in Fig. 11 where activity of a number of silica-alumina catalysts for cracking isopropyl benzene is plotted against acidity by titration by anhydrous n-butylamine in nonaqueous medium (Tamele, 9b). Cracking was conducted at 500° at various space velocities. Since the conversion is not a linear function of activity, and the reaction constants could not be calculated from conversions without knowledge of the kinetics of the reaction, values of fc were calculated... 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Selective chemisorption methods have been used with success for the determination of metal surface area and particle size in supported catalysts, and for titration of acid sites on silica-alumina and zeolite catalysts. The chemisorption methods are sometimes neglected in the quest for a more physical description of the catalyst surface, possibly with the penalty of missing an important and quantitative piece of information about the catalyst surface. 

Bakshi and Gavalas investigated a number of aluminas, silica-aluminas, and clays for ethanol dehydration, for which they determined acidity and basicity distributions via n-butylamine and trichloroacetic acid titrations, respectively. Catalyst activity was presumed to be given by the sum of the various group contributions, so that overall rate of reaction was given by equation 1 ... 

Silicates which have been calcined may show an acid reaction when placed in an aqueous medium. Gayer (14) reported, many years ago, that his silica-alumina polymerization catalyst, when tested with an indicator in an aqueous medium, was acid in reaction. He reported no connection between this observation and catalyst activity. Such a catalyst if placed in water can be titrated with a base. Long periods (18 hours) may be allowed for reaction with a base, and the results can... 

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-... 

Table 1 summarizes the information required for a detailed characterization of a supported metal catalyst for supported bimetallics there are additional questions, e.g., the distribution of atoms in bimetallic clusters and the surface composition of larger alloy crystallites. For the support and the prepared catalyst, the total surface area, pore size distribution, and surface acidity are routinely measured, if required, while other characteristics, e.g., thermal and chemical stability, will have been assessed when selecting the support. The surface structure of alumina, silica, charcoal, and other adsorbents used as catalyst supports has been reviewed. Undoubtedly, the most commonly measured property is the metal dispersion, often expressed in terms of the specific metal area and determined by selective chemisorption or titration but, as discussed (Section 2), there is the recurring problem of deciding the correct adsorption stoicheiometry. 

The titration of a weak base in an anhydrous medium has been recently reported by Tamele (9 ). The catalyst, suspended in benzene, was titrated with n-butylamine, using p-dimethylaminoazobenzene as indicator. This indicator does not color pure silica gel or pure alumina. A number of solid acids of known strength in aqueous solutions were titrated in the same system. It was found that this method detects acids whose dissociation constants are about 10 or more in aqueous... 

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