Diffusion neopentane

It has been shown by Mozumder and Tachiya (1975) that, within the context of the diffusion model, the probability of generation of free ions is independent of postthermal electron scavenging, both in the absence and presence of an external field. Thus, the experimental finding—that the free-ion yield is reduced in neopentane (NP) by the addition of electron attaching solutes SF6,... 

After obtaining from the measured value of kl by this procedure, one can determine the attachment efficiency in the quasi-free state, rj = fe1f/fed.ff, by the same procedure as for scavenging reactions (see Eq. 10.11 et seq.). Mozumder (1996) classifies the attachment reactions somewhat arbitrarily as nearly diffusion-controlled, partially diffusion-controlled, and not diffusion-controlled depending on whether the efficiency p > 0.5, 0.5 > r > 0.2, or r < 0.2, respectively. By this criterion, the attachment reaction efficiency generally falls with electron mobility. Nearly diffusion-controlled reactions can only be seen in the liquids of the lowest mobility. Typical values of r] are (1) 0.65 and 0.72 respectively for styrene and p-C6H4F2 in n-hexane (2) 0.14 and 0.053 respectively for a-methylstyrene and naphthalene in isooctane (3) 1.8 X 10-3 for C02 in neopentane and (4) 0.043 and 0.024 respectively for triphenylene and naphthalene in TMS. 

No single model can exactly describe molecular reorientation in plastic crystals. Models which include features of the different models described above have been considered. For example, diffusion motion interrupted by orientation jumps has been considered to be responsible for molecular reorientation. This model has been somewhat successful in the case of cyclohexane and neopentane (Lechner, 1972 De Graaf Sciesinski, 1970). What is not completely clear is whether the reorientational motion is cooperative. There appears to be some evidence for coupling between the reorientational motion and the motions of neighbouring molecules. Comparative experimental studies employing complementary techniques which are sensitive to autocorrelation and monomolecular correlation would be of interest. 

The diffusion coefficients of the ion are usually estimated from their mobilities (or conductances), which can be measured independently. Diffusion coefficients (or mobilities) vary over very large ranges for the solvated electron in different solvents (neopentane, D 2 x 10 4 m2 s 1 and n 7 x 10 3 m2 V-1 s 1 hexane, D 2 x 10 7 m2 s 1 and ju 8x 10 6 m2 V-1 s 1) [320]. There is considerable evidence that the mobility of electrons is not constant, but on the contrary, the mobility depends on the applied electric field, increasing approximately proportionately with electric field at high fields in solvents where n is small and decreasing with electric field in solvents where n is large. If the solvated electron mobility depends on the electric field, then the diffusion coefficient may also depend on the electric field. The implications of these complications are discussed in Sect. 2.2 and in Chap. 8, Sect. 2.7. 

Among the chemical reactions of interest catalyzed by zeolites, those involving alkanes are specially important from the technological point of view. Thus, some alkane molecules were selected and a systematic study was conducted, on the various steps of the process (diffusion, adsorption and chemical reaction), in order to develop adequate methodologies to investigate such catalytic reactions. Linear alkanes, from methane to n-butane, as well as isobutane and neopentane, chosen as prototypes for branched alkanes, were considered in the diffusion and adsorption studies. Since the chemical step requires the use of the more time demanding quantum-mechanical techniques, only methane, ethane, propane and isobutane were considered. 

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. 

In the temperature range of 373-573 K no transformation of the C1-C5 alkanes occurred on the H-ZSM 5 samples. The adsorption of the small methane molecules was very weak, while the large neopentane molecules could not enter the narrow zeolitic channels. Thus, the response to the applied pressure modulation was too small to record meaningful FR spectra with these molecules. For propane and isobutane the FR results suggest that diffusion in the micropores is the rate limiting process of transport over the entire temperature range. 

Counter-Diffusion of n-Hexane vs Neopentane and Neopentane vs n-Hexane by "Micro"-FTIR... 

Fig. 31 Counter-diffusion of neopentane (preloaded) versus n-hexane (subsequently admitted) in H-ZSM-5...

Fig. 32 Counter-diffusion of n-hexane (preloaded) versus neopentane (subsequently admitted)...

The first studies of catalytic processes which confirm the relevance of singlefile diffusion in zeoHte catalysis were carried out by Sachtler and co-workers. During investigation of palladium-catalyzed conversion of neopentane in supports of different topologies, characteristic differences were observed [117]. With zeolite HY, the process was foimd to proceed at substantially lower... 

FTIR experiments on diffusion were also carried out with single crystals of H-ZSM-5 as adsorbent and paraffin molecules (n-hexane, n-heptane, n-octane, neopentane, n-nonane) as diffusing species [944,945]. In these experiments an IR microscope (cf. Sect. 4.1) was employed and a cell adopted from Lercher and coworkers [862,894] after slight modifications. 

G. D. Lei, B. T. Carvill, W. M. H. Sachtler, Single file diffusion in mordenite channels neopentane conversion and H/D exchange as catalytic probes, Appl Catal A, 1996, 142, 347-359. 

In addition, experiments of isobutane diffusion reveal that the effective diffusivity is three times higher over a carbon-templated hierarchical zeolite [143]. Similar conclusions were drawn by Groen et al. [144] with hierarchical zeolites synthesized using the desilication method, where a two-order-magnitude improvement was observed in the diffusion of neopentane inside desiUcated ZSM-5 because of the shorter diffusion path length and presence of an accessible network of mesopores. 

C NMR spectra of CH4 in an AIPO4-II molecular sieve have revealed exchange effects between adsorbed and nonadsorbed methane gas. The diffusion of a mixture of methane and xenon in the zeoHte siUcahte has been studied by pulsed field gradient NMR spectroscopy. In situ H MAS NMR studies of the H/D exchange of deuterated propane adsorbed on H-ZSM-5 have been reported. C MAS NMR spectroscopy has been used to study the initial stages of propane activation over H-ZSM-5, and sorption properties of hnear alkanes in ferrierite. Protonated sites on sulfate-promoted zirconium oxide catalysts have been studied using H NMR spectroscopy. NMR relaxation and self-diffusion of pentane, neopentane, dodecane, benzene, cyclohexane, and... 

In some solvents such as hexane, the mean free path is small and the diffusion model can be reliably used to model the system. However, in other hydrocarbons such as neopentane the escaped electrons are in a quasi-free state vide infra), and the motion is not fully diffusive due to trapping by the solvent molecules. 

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