Activation energy dehydration processes

Table II lists the thermodynamic parameters for the conduction process. For the Na+ samples the activation energies are on the average 3.5 kcal lower than those for the conduction process of the corresponding dehydrated zeolites (<8). For K+-zeolites this difference averages 2.1 kcal. NaF69.8 is not included because of experimental difficulties in pellet preparation. The activation entropies are negative for the X-type zeolites and positive for the Y-type. The activation entropies are higher than those of the dehydrated samples (8) except for KF86.5. The effect of AS on E...

The position of absorption B on the temperature scale depends on the nature and on the number of exchangeable cations. We conclude that absorption B is a cationic relaxation process. The activation energy of absorption B for NaF86.5 agrees with that reported by Matron (10) for their -relaxation process. It is in line with the increase of activation energy for relaxation after adsorption of water on dehydrated Na+-zeolites. Thus, absorption B is caused by the local migration of site III cations over two neighboring sites III, as observed on dehydrated X-type zeolites (8) and on partially hydrated NaX (10). 

The nature of the exchanged ion in a zeolite determines the amount of equilibrated adsorbed water, decreasing from 16.3% for NaL to 12.5% for CsL. Activation energies of the dehydration process were calculated for these zeolites by the method given in Ref. 25. Their values for sodium, potassium, and cesium forms are 4.52, 2.31, and 1.85 kcal/mole. The results show that the smaller the cation radius is (i.e., the stronger its field), the higher the activation energy is. 

Alternative mechanisms for the OH transfer process in enzyme-coenzyme B12-catalysed dehydration of 1,2-dihydroxyethane, to give acetaldehyde and water, have been explored using ah initio MO calculations.75 Transfer within an (HOCH— CH2OH) radical was ruled out because the activation energy is too high, and no intermediate bridge structure could be found to facilitate conversion of 1,2-dihydroxyethyl cation (if it could be formed from the radical) to 2,2-dihydroxyethyl cation. The radical cation (HOCH—CH20H2)+ transformed rapidly to a stable... 

Extensive kinetic investigations have also indicated that the activation energy of the overall thermal-decomposition process is substantially lowered by the addition of sodium chloride and sodium carbonate. Madorsky and coworkers have, therefore, proposed that these salts catalyze the dehydration of cellulose by scission of the C —O bonds (bonds a, b, and c in 1 see p. 438), and that this results in destruction of the hexose units and increases the yield of water and char at the expense of levoglucosan. This theory has found substantial support in subsequent experiments and publications however, it may be noted here that Golova and associates" consider that inorganic salts promote the cleavage of C—C, rather than C—O, bonds in the macromolecule. 

The dehydration and rehydration reactions of calcium sulfate dihydrate (gypsiun) are of considerable technological importance and have been the subject of many studies. On heating, CaS04.2H20 may yield the hemihydrate or the anhydrous salt and both the product formed and the kinetics of the reaction are markedly dependent upon the temperature and the water vapour pressure. At low temperatures (i.e. < 383 K) the process fits the Avrami-Erofeev equation (n = 2) [75]. The apparent activation energy for nucleation varies between 250 and 140 kJ mol in 4.6 and 17.0 Torr water v our pressure, respectively. Reactions yielding the anhydrous salt (< 10 Torr) and the hemihydrate ( (HjO) >17 Torr) proceeded by an interface mechanism, for which the values of E, were 80 to 90 kJ mol. At temperatures > 383 K the reaction was controlled by diffusion with E, = 40 to 50 kJ mol. ... 

No significant effect of either water or sucrose content was observed on the activation energy of the relaxation E 480 kj/mol) in the concentration range studied. This could be due to the actual merging of sub-Tg processes, dehydration artifacts, and a relaxation features, which caused some difficulties in the accurate determination of the peak maximum at different frequencies. 

The adsorption process is usually fast on evaporated films. However, on bulk solids, e.g. porous catalyst carriers, adsorption rates are usually slow and activated with activation energies typically in the range 10-40 kcal/g mole (Hayward and Trapnell, 1964). Some activation energies for typical activated chemisorption process are given in Table 5.9. In their investigation of the catalytic dehydration of methylcyclohexane, Sinfelt et al. (1960) found that to obtain a suitable kinetic expression finite rates of the adsorption-desorption process must be taken into consideration. In this section allowance is made for finite rates of adsorption and both activated and non-activated adsorption are considered ... 

This equation is first order in [Ca ], [P04 ] and s. The average rate constant kf was determined to be 173 11 mof m sec The apparent Arrhenius activation energy was determined over a temperature range of 10 to 40°C to be 186 15 kJ mol Inskeep and Silvertooth (1988) speculated that the rate-limiting step is a surface process, specifically that it is the surface diffusion of Ca and PO4 ions and dehydration commensurate with binding at surface sites of incorporation. Using a constant-composition method and seeded solutions with Sha = 3.05-5.58 Koutsopoulos (2001) found hydroxylapatite formed without a precursor phase. The spiral growth model was found to best fit their kinetic data. 

The metastable intermediate which is closely related to the appearance of acidic sites is formed by the process of heating. It is interesting to see from this aspect that the activation energy of isothermal dehydration increases as dehydration progresses 21) ... 

---