Higher Linear Alkanes

With the introduction of the third and further carbon atoms into the linear alkane chain, the number of conceivable mono- and di-adsorbed structures increases rapidly (see Tables 4.3. 5), and quantitative modelling of the distribution of exchanged molecules becomes progressively more difficult. Thus exchange may proceed not only by the easy afi mechanism so favoured by ethane (process 6.J), but also (e.g. on rhodium) through reversible formation ofay species (i.e. 1-propyl o- 1,3-diadsorbed propane (Table 4.5)) further possibilities exist when there are four or more carbon atoms present (e.g. an a process ). In the case of propane, early work indicated that on nickel film the hydrogen atoms on the secondary 

TABLE 6A. Apparent Arrhenius Parameters (E, In A), Rates at 423 K and Multiplicities (M) for the Exchange of Higher Linear Alkanes with Deuterium 

Alkane Metal Form D2/HC /kJmoi In A Inr 3 Matr/K References 

Notes Exchange of secondary hydrogen atoms only. See also footnotes to Table 6.1. 

Variously structured nickel films also produced some very peculiar product distributions, as they did with ethane at 393-433 K on (111) and (100) oriented films gave propane-r/g and (usually) -d2 as the major products, but randomly orientated but sintered film at 400-435 K yielded propane-di and -d%, and an unsintered film at 273 K gave only the -d isomer. These results closely resemble those obtained with ethane, and lend stress to the importance of surface structure in deciding what intermediates are formed and how they react. The absence of results for single crystal surfaces of nickel is sorely felt. 

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At increasing reaction temperatures (230-350°C) the product selectivity is shifted towards C -C. The alkene to alkane ratio declines at higher reaction temperatures whereas the branched to linear alkane ratio increases as well as CO2 formation. These observations are entirely consistent with the behaviour of classical F-T catalysts (Table 1). 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

The selectivity to each product is defined on the basis of CO consumed. The oxide precursor W03 gives mainly linear alkanes (68%) but also methanol and ethanol (20% alcohols). WC leads to higher alkanes (up to C13) with a selectivity of about 80% to hydrocarbons and the formation of light alkanes is lower than on W03. The selectivity to hydrocarbons and alcohols of WC resembles more that of W03 than W2C. The catalytic behavior of W2C is very different. It did not produce any alcohol and its selectivity to alkanes is larger than for WC (87%). 

Another application of atomistic simulations is reported by De Pablo, Laso, and Suter. Novel simulations for the calculation of the chemical potential and for the simulation of phase equilibrium in systems of chain molecules are reported. The methods are applied to simulate Henry s constants and solubility of linear alkanes in polyethylene. The results seem to be in good agreement with experiment. At moderate pressures, however, the solubility of an alkane in polyethylene exhibits strong deviations from ideal behavior. Henry s law becomes inapplicable in these cases. Solubility simulations reproduce the experimentally observed saturation of polyethylene by the alkane. For low concentrations of the solute, the simulations reveal the presence of pockets in the polymer in which solubility occurs preferentially. At higher concentrations, the distribution of the solute in the polymer becomes relatively uniform. 

Related to their similar pore diameter and pore structure, unsurprisingly the Henry adsorption constants for linear alkanes are very close to each other on zeolite ZSM-22 and ZSM-23 (Table I). Somewhat higher constants are obtained for 2- and 3-methylbranched alkanes on ZSM-23 compared to zeolite ZSM-22. The adsorption constants of linear alkanes are obviously hi er than branched alkanes on the two cases. The separation power of a zeolite between a linear and a branched hydrocarbon may be given by the separation factor (a), which is the ratio of Henry consteints of linear and branched molecules at a certain temperature, a values at 523 K are given for both zeolites in Table 1. For comparison, values for ZSM-5 are also included, which is one of the most popular shape selective catalyst used in isomerization reactions. From this table it can be seen that both ZSM-22 and ZSM-23 have higher separation constants compared to ZSM-5. The zeolites can be listed in the following order with respect to their separation capacity between linear and 2- and 3-methylbranched alkanes ZSM-22 > ZSM-23 > ZSM-5. In narrow pore structures such as zeolites ZSM-22 and ZSM-23 it is very probable that linear alkanes with smaller kinetic diameters have more access to the available adsorption sites compared to the more bulky branched molecules. This may be regarded as the first... 

The pre-exponential factors on ZSM-22 and ZSM-23 are given in Table 1. Similar values are obtained for the C5-C8 range linear and branched hydrocarbons. Values of linear alkanes on ZSM-23 are slightly higher than on ZSM-22, and values of branched molecules are slightly lower. Pre-exponential factors of the isomers are significantly... 

Alkane oxidation to alcohols, aldehydes and ketones has been extensively studied and it was found that the smaller linear alkanes show higher turnovers than the longer linear, branched and cyclic alkanes [81]. Although the turnover numbers are found to increase with the addition of methanol as a co-solvent, the general role of the co-solvent in selectivity is still not clear. Catalytic epoxidations of relatively inert alkenes such as propylene and allyl chloride were found to be facile under mild... 

However, the treatment used by Cameron and his co-workers is physically incorrect. Equations 10-2 and 10-6 apply to a Langmuir-type adsorption equilibrium in which the solvent is merely the inert carrier for the solute and does not compete with it for the surface of the adsorbent. We have examined evidence in this chapter that demonstrates this is not always the case, particularly if the solvent has the linear long-chain structure of the higher n-alkanes used by Cameron zt al. The equilibria for the two-component adsorption are given by... 

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