Cyclohexane, isomerization

From stochastic molecnlar dynamics calcnlations on the same system, in the viscosity regime covered by the experiment, it appears that intra- and intennolecnlar energy flow occur on comparable time scales, which leads to the conclnsion that cyclohexane isomerization in liquid CS2 is an activated process [99]. Classical molecnlar dynamics calcnlations [104] also reprodnce the observed non-monotonic viscosity dependence of ic. Furthennore, they also yield a solvent contribntion to the free energy of activation for tlie isomerization reaction which in liquid CS, increases by abont 0.4 kJ moC when the solvent density is increased from 1.3 to 1.5 g cm T Tims the molecnlar dynamics calcnlations support the conclnsion that the high-pressure limit of this unimolecular reaction is not attained in liquid solntion at ambient pressure. It has to be remembered, though, that the analysis of the measnred isomerization rates depends critically on the estimated valne of... 

Catalysts Based on Mordenite. Isomerization of paraffins over H-mordenite based catalysts has been described (6, 7,14, 0, 21). Minachev (7) reports that cyclohexane isomerization activity of Na-H-mordenite catalysts increases linearly with H+ concentration in the zeolite for 25-94% exchange. He further observed that H-mordenite is deactivated by other cations such as Li, K, Mg, Cd, Zn, and Al. This agrees with Bryant s work (6) he reported that, compared with Pd-H-mordenite, samples in which hydrogen was partly replaced by Ca or Zn had an appreciably lower n-pentane isomerization activity. 

The mechanism and kinetics of pentane, hexane, and cyclohexane isomerization over Pd-H-mordenite have been extensively investigated by Bryant (6), Hopper (21), and Beecher (20). They assume a conventional dual function mechanism as described earlier. Bryant (6) pointed out that H-mordenite itself has a high activity for pentane isomerization and that impregnation of a noble metal does not change the rate of the isomerization reaction. This exceptional activity of mordenite has since been reported by Benesi (14) and Minachev (7) as well. In Mina-chev s paper the reaction mechanism of n-pentane isomerization over H-mordenite is discussed in some detail. The rate of reaction is inversely proportional to the hydrogen pressure, and it is concluded that the reaction proceeds according to the following scheme ... 

Polar solvents also may affect the geometrical isomerization of ethylenes by affecting the efficiency of intersystem crossing [96,97]. Thus, even in the absence of polar substituents, solvent polarity can be of importance in the photochemical isomerization of aryl-substituted ethylenes. For example, cis-l-(9-anthryl)-2-(l-naphthyl)ethylene 74 in cyclohexane isomerizes to the trans-isomer 75 with a quantum yield of 0.05, while the quantum yield in acetonitrile solution is 0.31. Any trans->cis isomerization 75—74 in cyclohexane has not been observed, but the reaction does proceed with a preparatively useful quantum yield of 0.05 in acetonitrile [98],... 

In a study of the deactivation by coking of an atmospheric residue HDM catalyst, we have been able to obtain coked catalysts almost free from metal deposits in batch reactor and coked catalysts containing small amounts of metal sulfide deposits in continuous flow reactor using a Safaniya atmospheric residue under similar experimental conditions (30). We report in this paper a study of the deactivating effects of the deposits using toluene hydrogenation, cyclohexane isomerization and thiophene hydrodesulfurization reactions. 

Toluene hydrogenation (HYD), cyclohexane isomerization (ISOM) and thiophene hydrodesulfurization (HDS) tests have been performed in a high pressure fixed bed continuous flow "Catalest" unit. 

Toluene hydrogenation and cyclohexane isomerization were determined simultaneously at 6 MPa, 350°C, LHSV = 2 h 1, H2/HC - 450N1/1 and 40 cm3 Qf catalyst. The liquid feed was composed of toluene (20 wt %), thiophene (2 wt %) and cyclohexane (78 wt %). Under these conditions thiophene was completely hydrogenated to butane and H2S as determined by GC analysis. 

Some samples of the TS and P series have been chosen for tests using model compounds in order to evaluate the deactivation. Conversions obtained for toluene hydrogenation at 3509C, cyclohexane isomerization at 380 and 400°C and thiophene hydrodesulfurization at 220°C are reported in Table 1. 

To analyse the variation of catalytic activities of the used catalysts, carbon contents measured on the used samples removed after tests using model molecule have been used. As can be noted in Table 1, the cyclohexane isomerization activity at 380°C or at 400°C is not measurable compared to the activities of the fresh catalyst. This indicates that the isomerization sites are strongly poisoned by the deposits. It is dear in Table 1 that even the samples containing less than 200 ppm V are strongly poisoned indicating that the catalyst acidity has been considerably neutralized by the carbon deposit. 

A palladium-hydrogen-mordenite catalyst with a 10.8/1 silica/alumina mole ratio was evaluated for the hydroisomerization of cyclohexane. The rate of reaction followed a first-order, reversible reaction between cyclohexane and methylcyclopentane. The energy of activation for this reaction between 400° and 500°F was 35.5 it 2.4 kcal/mole. Cyclohexane isomerization rates decreased with increasing hydrogen and cyclohexane-plus-methylcyclopentane partial pressure. These effects are compatible with a dual-site adsorption model. The change of the model constants with temperature was qualitatively in agreement with the expected physical behavior for the constants. 

Cyclohexane and w-Pentane Isomerization. The data on catalytic properties of cation forms of mordenite in cyclohexane isomerization are presented in Table I. 

Al-mordenite exhibits a considerably smaller activity. The activity of two-valent forms is still lower (Zn- and Cd-forms are the most active of these, but they cause no cyclohexane isomerization at 300°C) while univalent modifications show no activity at all. 

Changes of activity with temperature in mordenite samples, with Na" ions replaced by to various extents, can be seen in Figure 3. With increase of exchange rate, the temperature decreases. Thus, the same degree of cyclohexane isomerization can be attained with samples of maximum and minimum rate of exchange at 270° and 420 °C, respectively. 

As indicated above (Table I), 2- and 3-valent cation forms of mordenite are considerably less active in cyclohexane isomerization than that of hydrogen. It is this property of mordenite-supported catalysts that distinguishes them sharply from zeolites of the faujasite type. Thus, in the case of faujasites, the activity of H-form (decationized form) and 2-valent modifications (such as Ca and Mg ) in acid-base reactions (cracking, isomerization, alkylation) is the same, while H-mordenite is many times as active as any cation form under study. 

Dehydrogenation of cyclohexanes Isomerization/dehydrogenation of cyclopentanes Dehydrocyclization of paraffins... 

Relative toluene hydrogenation activity per Mo atom, as well as relative cyclohexane isomerization per catalyst volume unit, are reported on Figure 4 as a function of the amount of TMSi grafted on the support. No clear trend is seen for isomerization, as may be expected from such a low silicium loading [15]. On the contrary, an almost linear increase is observed for toluene hydrogenation. Indeed, when taking catalyst prepared on alumina as a reference (100 %), CoMo catalyst prepared on 0.9-TMSi-alumina has a hydrogenation activity of 123 %. 

Kuharski R A, Chandler D, Montgomery J, Rabii F and Singer S J 1988 Stochastic molecular dynamics study of cyclohexane isomerization J. Phys. Chem. 92 3261... 

Figure 5 Time-dependent transmission coefficients K t) for (a) cyclohexane isomerization in CS2 (adapted from ref. 62) (b) the Cl + CH3CI 5 2 reaction in water (adapted from ref. 35). Note the more complicated structure in the cyclohexane K(r) at longer times.

Figure 6 Representative trajectories for cyclohexane isomerization in C 2. Of particular interest here are the significant number of transition state recrossings that occur several hundred femtoseconds into the trajectory. Adapted from ref. 62.

Chandler and co-workers successfully used stochastic dynamics in their studies of -alkanes and cyclohexane isomerization in solution. The method used is based on the BGK theory. The assumption is that the primary form of interaction between the solvent molecules and the isomerizing system is in the form of hard collisions. These collisions, when they occur, randomize the velocity of one of the isomerizing molecule s atoms. The computational implementation of this is quite simple at random times, based on the collision frequency (which is taken to be proportional to the solvent viscosity), instantaneously change one random atom s velocity to one selected from the Boltzmann distribution at the temperature of interest. Then continue running dynamics until the next collision occurs, at which time another... 

Results of FTIR spectroscopic study are in good agreement with the model acid-catalyzed reactions of n-heptane and cyclohexane isomerization. The introduction of alumina into Pt/SZ decreases the total catalyst activity in n-CzHie isomerization, which shows up as increase of the temperature of 50% n-heptane conversion from 112 to 266 (Table 9). For isomerization of cyclohexane to methylcydopentane, higher operating temperatures are thermodynamically more favorable (Tsai et al., 2011). As a result, Pt/SZA catalyst is more efficient for cyclohexane isomerization due to higher selectivity at higher temperatures (Table 10). 

Bi-Sr-Ca-Fe-CuOx, having a paramagnetic phase, exhibited catalytic activity in the dehydration of tert-BuOH to isobutylene, another ceramic superconductor Bi-Sr-Co-Fe-CuO having a diamagnetic phase, inhibited the reaction. The paramagnetic poly(aniline disulfide), when used as a modifier for LaHY Zeolite (20%)-Al2O3 ( %) catalysts for the cyclohexane isomerization at 350 °C, increased the selectivity for methylcyclopentane and methylcyclopen-tenes [194],... 

---