Isobutane activation energy

The nature of the methanol-zeolite interaction has been shown to be sensitive to a number of parameters and as such has proved to be a good benchmark for judging the reliability of quantum chemical methods. Not only are there a number of possible modes whereby one and two molecules interact with an acidic site (245), the barrier to proton transfer is small and sensitive to calculation details. Recent first-principles simulations (236-238) suggest that the nature of adsorbed methanol may be sensitive to the topology of the zeolite pore. The activation and reaction of methane, ethane, and isobutane have been characterized by using reliable methods and models, and realistic activation energies for catalytic reactions have been obtained. 

Use the bond-dissociation enthalpies in Table 4-2 (page 143) to calculate the heats of reaction for the two possible first propagation steps in the chlorination of isobutane. Use this information to draw a reaction-energy diagram like Figure 4-8, comparing the activation energies for formation of the two radicals. 

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

If we accept the mean value reported for the activation energies Ei as the heat of reaction 1, then, together with some thermochemical data on heats of combination and a value of 87.5 Kcal for the heat of dissociation of the tertiary H atom in isobutane, we arrive at the thermal data shown in Table XIII.12. 

The processes are the monomolecular reaction through a protonated cyclopropane produced by the abstraction of H" over Lewis acid sites and the bimolecular mechanism where an olefin takes part in the reaction. The olefin is produced over Bronsted acid sites, in the case of butane in the monomolecular mechanism, isobutane is formed through protonated methylcyclopropane with an activation energy of 8.4kcalmoT followed by the formation of the primary isobutyl cation with high energy [134]. 

The dynamics of methane, propane, isobutane, neopentane and acetylene transport was studied in zeolites H-ZSM-5 and Na-X by the batch frequency response (FR) method. In the applied temperature range of 273-473 K no catalytic conversion of the hydrocarbons occurred. Texturally homogeneous zeolite samples of close to uniform particle shape and size were used. The rate of diffusion in the zeolitic micropores determined the transport rate of alkanes. In contrast, acetylene is a suitable sorptive for probing the acid sites. The diffusion coefficients and the activation energy of isobutane diffusion in H-ZSM-5 were determined. 

Figure 5. The FR spectra of the isobutane/H-ZSM-5 diameter. The apparent activation energy of diffusion (E.) of systems at 373 K and 133 isobutane obtained from the Arrhenius plot was 21 kJ mol Pa (A) Z57, (B) Z34 and (Figure 6, open symbols). This value is about three times higher (C) Z15. The sample than that obtained for the diffusion n-butane [12]. At 373 K and amount was 50 mg.

The selectivity problem has hardly been treated by kinetic analysis. Recently Bourne published a detailed kinetic selectivity study on the aldehyde formation [106b]. The activation energy required for the formation of n- and isobutanal was determined to be 54 and 82 kJ/mol, respectively. The reaction rate models r and /"iso (eqs. (9) and (10)) explained the observed n/i ratios at different reaction conditions within 8 % standard deviation from the experimental results. 

Calculated Activation Energies (E ) for Some Elementary Reaction Steps of Isobutane Compared with Experimental Numbers Extracted from Kinetic Data [17,113]... 

It should be pointed out that this mechanism accounts for the observed first order kinetics (based on the sum of all deuterated products), since the rate of the hydrogen transfer step (8) depends on the first power of the concentration of isobutane in the gas phase, i.e., the rate of adsorption of isobutane. Probably the hydrogen transfer step (8) is the slow step in the overall reaction, since the activation energy of the overall deuterium exchange reaction is not affected by the initiating step involving olefin. 

Experiments [38,39] show that kp is only a third as large as for isobutane (per hydrogen) which suggests that there is some steric hindrance to abstraction in dimethylpentane. On the other hand, the difference in activation energy for external and internal hydrogen abstraction, eqns. (53) and (54), is only one kcal mole-1 (equivalent to a kjkp ratio of 3.9 at 100°C), so the ratio of hydroperoxides is mostly dependent on the ratio of A-factors for the unimolecular and bimolecular processes and on the concentration of dimethylpentane (12.1 M in f-CH at 100°C). For the internal six-center process, Ar 1011-5 s-1 [40] and for the bimolecular process, Ap 109,0 1 mole-1 s-1 [39], Therefore the ratio of rate coefficients at 100°C is... 

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