Catalytic titration

Catalytic titration, 27 114-118 gas-liquid chromatography, 27 115, 116 pulse reactors, 27 115, 116 Catalyzed reactions, mechanisms, 38 234-236 Catenanes, 24 135 Cathodes... 

Number and Strength of Acid Sites from Catalytic Titrations 114... 

An apparently straightforward method for the determination of number and strength of acid sites consists of the determination of the amount of base required to poison catalytic activity for a model reaction. By means of plots of activity versus amount of added base, the number of acid sites is obtained from the threshold amount of base required to remove catalytic activity acid strength is gauged from the slope of the titration curve. This method can therefore be called a catalytic titration. 

The work by Mills et al. (32) includes an early example of catalytic titration behavior. Figure 10 taken from their study shows that cumene cracking at 425°C drops sharply as nitrogen bases are chemisorbed in increasing amounts on silica-alumina catalyst. Base effectiveness decreases in the order quinaldine > quinoline > pyrrole > piperidine > decylamine > aniline. 

A different type of catalytic titration was carried out by Stright and Danforth (48), who added varying amounts of lithium or potassium hydroxide to aqueous suspensions of cracking catalysts. The products were dried, calcined, and tested for cetane cracking at 500°C by means of a flow reactor. Plots of cetane conversion versus amount of added lithium hydroxide were used to determine titers for a variety of cracking catalysts (Fig. 11). This type of catalytic titration is not recommended for quantitative studies because it given high acidity values. In aqueous media, alkali... 

The advent of gas-liquid chromatography and the use of pulse reactors (49) led to the independent development in two laboratories (50, 51) of a simple, straightforward technique for carrying out catalytic titrations. It... 

Special care has to be taken, however, that the quinoline titer truly represents the minimum amount of catalyst poison. In most cases this type of base is adsorbed by inactive as well as active sites. Demonstration of indiscriminate adsorption is furnished by the titration results of Roman-ovskii et al. (52). These authors (Fig. 13) showed that introduction of a given dose of quinoline at 430°C in a stream of carrier gas caused the activity of Y-zeolite catalyst (as measured by cumene conversion) to drop with time, reach a minimum value, then slowly rise as quinoline was desorbed. The decrease in catalytic activity with time is direct evidence for the redistribution of initially adsorbed quinoline from inactive to active centers. We have observed similar behavior in carrying out catalytic titrations of amorphous and crystalline aluminosilicates with pyridine, quinoline, and lutidine isomers. In most cases, we found that the poisoning effectiveness of a given amine can be increased either by lengthening the time interval between pulse additions or by raising the sample temperature for a few minutes after each pulse addition. 

An interesting type of catalytic titration procedure was reported by Goldstein and Morgan (55). They employed a flow technique in which... 

Br0nsted acid sites depends on the structure of the amine. Chemisorption data for amorphous oxides (54) show that 2,6-dimethylpyridine (which contains methyl groups that hinder coordination of the nitrogen atom with Lewis acids) is a more selective reagent for the determination of Br0nsted acidity than an unhindered amine such as pyridine. Jacobs and Heylen (44) arrived at a similar conclusion on the basis of an infrared study of amines chemisorbed on Y zeolite. They also found that the poisoning effectiveness of 2,6-dimethylpyridine is much greater than that of pyridine for the catalytic titration of Y zeolite. 

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. 

If the objective is a pragmatic study of the effects of variables such as composition, preparation, and pretreatment on the acidic properties of a solid catalyst, the catalytic titration method would be the most relevant way to determine number and strength of the acid sites that form the seat of catalytic activity. Further understanding could then be gained by complementing the catalytic method with an independent study using a tool such as infrared spectroscopy. 

As already indicated, catalytic titrations are carried out by monitoring conversion of a model reactant while the surface concentration of adsorbed reagent (usually an amine) is increased in measured steps. Conversion of the reactant is determined by means of a pulse reactor or a conventional flow reactor, depending on the needs of the investigator. In order to obtain the most meaningful titration results, we make the following recommendations. 

Marquez M, Silva M, Perez-Bendito D. 1988. Semi-automatic analysis of mercury in pharmaceuticals by catalytic titration. J Pharm Biomed Anal 6(3) 307-312. 

The Memotitrator can also be fitted with conventional optical detectors. Thus, a Perkln-Elmer 650-10S spectrofluorlmeter has allowed the development of the direct catalytic titration of EDTA based on its Inhibitory effect on the oxidation of 2-hydroxybenzaldehyde thlosemlcarbazone by hydrogen peroxide, catalysed by Iron(III), and the indirect determination of this and some alkaline-earth metal ions [61]. 

The catalytic transfer reductions of ethyl methyl ketone, isopropyl methyl ketone, and 4-methylacetophenone were studied over the wide series of basic, acidic and semiconducting oxides supported on Si02. Most of them exhibited remarkable activity in the studied transformations. The existence of the strong oxide oxide interaction between deposited phases and Si02 was noted. The nature of catalytic active sites was identified using catalytic titration with Hammett indicators and tetracyanoethylene. An unforeseen modifying effect of n-propylamine, o-nitroaniline, and TCNE onto AI2O3 was observed which led to the enhancement of catalyst activity. 

The catalytic titration experiments were performed introducing various amounts of corresponding poison into the reactant feed. The amount of poison was of the range from 20 to 1500 pmoles per 1 g of a catalyst. 

The unforeseen results were obtained during the catalytic titration of the alumina on silica system. All the used poisons caused the large enhancement of its activity in... 

Alumina supported on silica undergoes unforeseen modifying effect during catalytic titration with organic acid, base and electron acceptor. The observed phenomenon seems to be interesting from the practical point of view, however it would be an essential limitation for the catalytic titration method using the mentioned poisons. 

The similarity of FIA and classical batch titrations is useful to recognize, because such recognition turns our attention to the wealth of chemistries exploited by classical titrations that are now accessible to FIA adaptation. Indeed, all traditional titrations, that is, acid-base, com-pleximetric, redox, and precipitation, can be performed in the FIA mode. Catalytic titrations and titrations in nonaqueous solutions, including Karl... 

One of the major applications of inhibitory effects lies in the determination of the inhibitor using catalytic titration. This technique combines kinetic and... 

Both the cerium(IV)/arsenic(III) and the azide/io-dine systems have also been used for the indirect determination of metal ions that interact with the catalyst, thus inhibiting the catalytic cycle, by use of a kinetic method or catalytic titration. 

Several anions such as cyanide, fluoride, and sulfide can also be determined kinetically at concentrations between 0.1 and 4 rgml through their inhibitory effects on various metal-catalyzed reactions or, at somewhat higher concentrations, by direct catalytic titration. 

Aminopolycarboxylic acids (ethylenediaminetetra-acetic acid (EDTA), 1,2-diaminocyclohexanetetra-acetic acid, bis-(aminoethyl)glycolether-N,N,N, N -tetraaceticacid, diethylenetriaminepentaacetic acid and nitroacetic acid) can be quantified through their inhibitory effects on metal-catalyzed reactions, whether by kinetic methods (at concentrations between 10 and 10 mol 1 or by direct catalytic titration... 

Pharmaceuticals Bismuth Catalytic titration with potentiometric detection... 

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