Olefins surface chain initiation

Internal olefins are much less reactive than a-olefins in chain initiation and secondary hydrogenation reactions. The reactivity of added a-olefins in chain initiation reactions increased in the order ethylene > propylene s 1 -butene C5+ olefins it becomes almost independent of chain size for C5+ a-olefins. The higher reactivity of ethylene and propylene leads to C2 and C3 selectivity below those of the rest of the distribution in Flory plots (Fig. 5) and to the low termination probabilities measured for C2 and C3 surface chains (Fig. 8), as proposed previously by others (115). 

Different explanations have been proposed to explain this effect, including the influence of water on the adsorbed carbon species on the surface and the reduction of secondary hydrogenation of primary olefins by water, thereby facilitating olefin readsorption and chain initiation.8... 

The effective diffusivity Dn decreases rapidly as carbon number increases. The readsorption rate constant kr n depends on the intrinsic chemistry of the catalytic site and on experimental conditions but not on chain size. The rest of the equation contains only structural catalyst properties pellet size (L), porosity (e), active site density (0), and pore radius (Rp). High values of the Damkohler number lead to transport-enhanced a-olefin readsorption and chain initiation. The structural parameters in the Damkohler number account for two phenomena that control the extent of an intrapellet secondary reaction the intrapellet residence time of a-olefins and the number of readsorption sites (0) that they encounter as they diffuse through a catalyst particle. For example, high site densities can compensate for low catalyst surface areas, small pellets, and large pores by increasing the probability of readsorption even at short residence times. This is the case, for example, for unsupported Ru, Co, and Fe powders. 

We conclude that linear and branched olefins and paraffins can be formed after one surface sojourn by termination of growing surface chains. Therefore, they are primary Fischer-Tropsch products (4,14). a-Olefins readsorb and initiate chains in a secondary reaction. Thus, olefins reenter the primary chain growth process and continue to grow. These chains ultimately terminate as olefins or paraffins, in a step that can resemble a secondary hydrogenation reaction because it leads to the net consumption of olefins and to the net formation of paraffins, but which proceeds via primary FT synthesis pathways. 

At lower reactant pressures, olefins are more effectively intercepted by secondary hydrogenation reactions that prevent them from initiating surface chains. Primary chain growth pathways and olefin readsorption and chain initiation rates are not influenced strongly by reactant pressure thus, total... 

Higher intrapellet residence times increase the contribution of chain initiation by a-olefins to chain growth pathways. This intrapellet delay, caused by the slow diffusion of large hydrocarbons, leads to non-Flory carbon number distributions and to increasingly paraffinic long hydrocarbon chains during FT synthesis. But intrapellet residence time also depends on the effective diameter and on the physical structure (porosity and tortuosity) of the support pellets. The severity of transport restrictions and the probability that a-olefins initiate a surface chain as they diffuse out of a pellet also de-... 

Returning now to the question of the initiation step of the cationic hydrocarbon reactions, there are essentially two schools of thought on the subject. One is that the catalyst, itself, has the intrinsic ability to extract a hydride ion from the hydrocarbon or to distort a carbon-hydrogen bond to such an extent that the net result is the same (Milliken et al., 39). The other mechanism is the formation of a cationic complex by the simple addition of a proton to olefinic impurities, either initially present or formed in small amounts by thermal cracking or oxidation. This complex, however small its concentration on the surface of the catalyst, can then start a chain reaction through the hydrogen transfer mechanism, i.e., it can extract a hydride ion from a saturated hydrocarbon, which then becomes the propagating complex (Hansford, 43, 47 Thomas, 18). 

With respect to theories of wall heterogeneous effects in hydrocarbon pyrolysis reactions, the literature is almost void. Rice and Herzfeld (1951) have presented some theoretical arguments but with some severely simplifying assumptions Polotrak, et al. (1959) proposed mechanisms involving both chain initiation and termination as heterogeneous processes. More elaborate theoretical work on the interaction between hydrocarbons (paraffins and olefins) and metal oxide surfaces was done by Semenov (1958) and Kasansky and Pariisky (1965) in which the authors tried to explain the heterogeneous effects (activity) of the surfaces in terms of electronic conductivity. 

Olefin selectivities also decrease with increasing bed residence time and chain size on Ru catalysts (4,14). For example, propylene selectivity decreases with increasing bed residence time without a corresponding increase in propane selectivity, leading to a net decrease in the fraction of the converted CO that appears as C3 molecules (Fig. 7b). Readsorbed olefins initiate chains that continue to grow and ultimately desorb as larger olefins or paraffins, ( alitative trends are similar on all supports and on both Ru and Co catalysts. The selectivity details depend on the support physical structure, on the density of exposed surface metal atoms, and on the intrinsic readsorption properties (j8r) of Co and Ru surfaces. 

D(C1-C02R) = 56 kcal.mole" would satisfy the observed kinetics. This value (which seems too low by about 5-lOkcal.mole" ) is certainly a lower limit because surface initiation reactions are undoubtedly also important. The Arrhenius /4-factors observed for the normal elimination reactions to olefin, HCl, and CO2 fluctuate around the transition state estimates and do so probably as a result of experimental errors and reaction complexities. Note that the chloroformic acid, which is the primary elimination product, is very unstable at reaction temperatures and rapidly decomposes, probably by a 4-center transition state, to give HCl -I- CO2 (ref. 159). The experimental reaction rates of the chloroformate ester eliminations are two powers of ten faster than those for the corresponding formate and acetate esters. This is reasonable since electron withdrawing substituents at the (C-1) position accelerate the decompositions. It seems likely, then, that the normal uni-molecular eliminations and the free radical chain decompositions are competitive processes in these chloroformate ester reactions. 

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