Shape selectivity temperature

As shown in Figure 1, the equiHbrium concentration is affected slightly by temperature (11). The actual concentration is affected by the reaction rate and the initial concentration of each isomer. Deviations beyond equiHbrium can be achieved when zeoHtes are used, owing to shape selectivity (see Molecularsieves). The thermal isomerization of the three xylenes has been studied at 1000°C (12). Side reactions predominated, and only a small percentage of xylenes was interconverted. 

Besides di- and poly-saccharides, zeolites have been applied for hydrolysis of simple glycosides as described by Le Strat and Morreau.132 Methyl a- and /i-D-glucopyrano-sides were treated with water in the presence of dealuminated HY faujasite with an Si/Al ratio of 15, at temperatures ranging between 100 and 150 °C. It was observed that the reaction rate for the (i glycoside was about 5-6 times higher than that for the oc anomer, a result that might arise from the shape-selective properties of the zeolite and stereoelectronic effects on the surface of the solid. 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

FIGURE 5.14 Column shape selectivity (ajEN/BaP) plohed as a function of temperature for monomeric and polymeric Cig columns. (Adapted from Sander, L.C. and Wise, S.A., Anal. Chem., 61, 1749, 1989. With permission.)... 

Analytical shape computation techniques were applied for the detection of cavities and the calculation of molecular surface properties of isolated cavity features and other ordered formations within these resultant alkyl stationary-phase simulation models [227]. Deep cavities (8-10 A wide) within the alkyl chains were identified for Cig polymeric models representing shape selective stationary phases (Figure 5.23). Similar-structure cavities with significant alkyl-chain ordered regions (>11 A) were isolated from two independent Cig models (differing in temperature,... 

When developing a liquid phase adsorptive separation process, a laboratory pulse test is typically used as a tool to search for a suitable adsorbent and desorbent combination for a particular separation. The properties of the suitable adsorbent, such as type of zeolite, exchange cation and adsorbent water content, are a critical part of the study. The desorbent, temperature and liquid flow circulation are also critical parameters that can be obtained from the pulse test. The pulse test is not only a critical tool for developing the equilibrium-selective adsorption process it is also an essential tool for other separation process developments such as rate-selective adsorption, shape-selective adsorption, ion exchange and reactive adsorption. 

The results on olefin isomers (Table Vll) can also be explained by the observation that the constraint index of ZSM-5 is approximately unity under the conditions of this study. Shape selectivity or preferential conversion of straight chain olefins by ZSM-5 cannot be expected at 500 C. Thus, under the conditions of this study, olefin isomer distribution was not significantly affected by deactivated ZSM-5. At temperatures lower than that employed in the present study, it is conceivable that distribution of olefin isomers could be altered by steam deactivated ZSM-5. 

This investigation of time resolved selectivity of methanol conversion on HZSM5 (and on HUSY for comparison) in the temperature range 250 to 500 °C reveals new shape-selectivity-controlled regimes of instationarity and changes in activity with reaction time and reaction temperature firstly autocatalysis and secondly consecutive retardation which are dominant at low temperature (250... 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

In practice, short-chain alkanes and alkenes are normally used as feedstock for shape-selective catalytic formation of isooctanes at relatively low temperatures. Until the 1980s, lead alkyls (Section 18.1) were added to most automotive fuels to help suppress engine knock, but they have been phased out in North America because of the chronic toxicity of lead and lead compounds. The most commonly used nonlead antiknock additive is now methyl tert-butyl ether [MTBE CH30C(CH3)3], which is made by the reaction of methanol with 2-methylpropene, (CHs C—CH2 (see Section 7.4). The latter is obtained by catalytic cracking of petroleum fractions to give 1-butene, which is then shape-selectively isomerized on zeolitic catalysts. 

Selective mono- or dicyclohexylation of naphthalene with cyclohexene (and cyclohexyl bromide) was achieved over HY zeolites.170 Monocyclohexylnaphtha-lenes are formed with 99% selectivity at 80°C. The amount of dicyclohexyl-naphthalenes increases dramatically with increasing temperature. Due to zeolite shape selectivity, mainly the p,p-disubstituted derivatives (2,6- and 2,7-dicyclo-hexylnaphthalene) are formed at 200° C. 

Sorption Properties. Sorption isotherms were determined of n-hexane and 2,3-dimethylbutane on variously pretreated samples of zeolite by a gravimetric method using a Cahn electrobalance. No shape-selective sorption was observed for these sorbates, which bespeaks a pore size greater than about 0.5 nm. The sorption capacity of S2 was appreciably lower than that of zeolite X, Y, or mordenite. Routine sorption capacities were determined by a simple procedure of pore filling with benzene at room temperature after calcination of the samples at various temperatures. 

Both H2O2 and hydroperoxides are industrially important oxidants. An accurate evaluation of advantages and disadvantages requires an accurate analysis of every specific case, in view of the different technical problems and economic constraints that the use of one or the other entails. The reactivity of H202 is so high that it can easily oxidize many primary reaction products, and these reactions become more likely as the reaction temperature is increased. Some of these reactions are influenced by reactant shape selectivity and by restricted transition-state shape selectivity. 

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