Non-Flory product distributions

Non-Flory Product Distributions in Fischer— Tropsch Synthesis Catalyzed by Ruthenium, Cobalt, and Iron... 

Carbon number distributions are similar on all Co catalysts. As on Ru catalysts, termination probabilities decrease with increasing chain size, leading to non-Flory product distributions. The modest effects of support and dispersion on product molecular weight and C5+ selectivity (Table III) reflect differences in readsorption site density and in support pore structure (4,5,14,40,41), which control the contributions of olefin readsorption to chain growth. Carbon number distributions obey Flory kinetics for C30+ hydrocarbons the chain growth probability reaches a constant value (a ) as olefins disappear from the product stream. This constant value reflects the intrinsic probability of chain termination to paraffins by hydrogen addition it is independent of support and metal dispersion in the crystallite size range studied. 

TI Non-Flory product distributions in Fischer - Tropsch synthesis catalyzed by ruthenium, cobalt. and iron KW Fischer Tropsch synthesis hydrocarbon distribution. Flory kinetics carbon monoxide hydrogenation, chain growth carbon monoxide hydrogenation, ruthenium catalyst carbon monoxide hydrogenation, cobalt iron catalyst hydrocarbon distribution IT Hydrocarbons, preparation... 

In recent years, catalysts have been developed which give a non-Schulz-Flory product distribution in fixed beds. Such catalysts should be tested in slurry phase operation. 

Recently, we have shown that non-Flory distributions cannot arise from the higher solubility of larger olefins because thermodynamic equilibrium between the two phases requires that the fugacity, chemical potential, and kinetic driving force for each component be the same in the two phases (4,5,14,40,41,44). Transport restrictions, however, can lead to higher intrapellet concentrations and residence times of a-olefins, a feature of FT chemistry that accounts for the non-Flory distribution of reaction products and for the increasing paraffin content of larger hydrocarbons (4,5,14,40,... 

Non-Flory molecular weight distributions have also been attributed to the presence of several types of active sites with different probabilities for chain growth and for chain termination to olefins and paraffins (45). Two-site models have been used to explain the sharp changes in chain growth probability that occur for intermediate-size hydrocarbons on Fe-based catalysts (46,47). Many of these reports of non-Flory distributions may instead reflect ineffective dispersal of alkali promoters on Fe catalysts or inadequate mass balances and product collection protocols. Recently, we have shown that multisite models alone cannot explain the selectivity changes that occur with increasing chain size, bed residence time, and site density on Ru and Co catalysts (4,5,40,44). 

We have shown previously that non-Flory distributions often reflect the transport-limited removal of reactive olefins from catalyst pellets on Ru and Co catalysts (4,5,14,40,41,44). This proposal is consistent with the similar effects of bed residence time and of molecular size on chain growth probability and product functionality. It accounts for the observed effects of convective and diffusive rates of reactive olefins and for the non-Flory distribution of highly paraffinic hydrocarbons formed from synthesis gas on Co and Ru catalysts. 

Chain termination probabilities initially decrease with increasing chain size (Fig. 2b) product distributions are non-Flory on all catalysts. This reflects an increase in readsorption rate as larger a-olefins become increasingly difficult to remove from liquid-filled catalyst pellets (4,5,14,40,41,44). Large olefins readsorb extensively and leave catalyst pellets predominantly after they form n-paraffins in sequential chain initiation and termination steps. As larger olefins (n > 30) disappear from the products, the chain termination probability reaches a constant value and product distributions become predominantly paraffinic and obey Flory kinetics (Fig. 2b). The asymptotic termination probability (/3=o) reflects the intrinsic probability of... 

Many of the chain growth pathways and transport effects described above also occur on Fe-based FT synthesis catalysts. As on Co and Ru catalysts, FT synthesis on Fe often yields non-Flory carbon number distributions of products, where the chain growth probability and the paraffin content increase with hydrocarbon chain size (38-40). These effects were previously... 

Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive a-olefins. These secondary reactions reverse chain termination steps that form these olefins and lead to heavier products, higher chain growth probabilities, and more paraffinic products. Diffusion-enhanced readsorption of a-olefins accounts for the non-Flory carbon number distributions frequently observed during FT synthesis on Co and Ru catalysts. Diffusion-limited reactant (H2/CO) arrival leads instead to lower selectivity to higher hydrocarbons. Consequently, intermediate levels of transport restrictions lead to highest selectiv-ities to C5+ products. A structural parameter containing the pellet diameter, the average pore size, and the density of metal sites within pellets, determines the severity of transport restrictions and the FT synthesis selectivity on supported Ru and Co catalysts. 

Product distributions can be evaluated for reaction probabilities of elemental surface reaction steps with the model of non trivial surface polymerisation [2]. Specific inhibition of desorption of a chemisorbed organic species has been postulated to be the intrinsic principle of the FT-synthesis [5]. A chemisorbed species can react further by linear chain prolongation or chain branching or it can desorb as a paraffin, olefin or an organic oxygen compound. Growth probabilities pg, that contain a similar information as the Anderson-Schulz-Flory parameter a. 

FTS produces a wide distribution of products. Conventional catalysts typically give an Anderson-Schultz-Flory distribution of products, hence they are generally non-selective for specific products. Therefore, the development of highly active and selective catalysts has been a key goal. 

Under ideal condition, when an isothermal CSTR is operating at steady state (SS) in a totally micro-mixing condition, the PS product will be homogeneous having a perfect ZO polydispersity (the Schultz-Flory distribution). Deviations from ideality have been theoretically studied by operating CSTR processes under non-steady state conditions with fort peritxlic operation [70]. The effects of an independent sinusoidal forcing of the monomer and the initiator feed concentrations results in theoretical control of the polydispersity. 

Metal carbonyl dusters (e.g. of Rh) can be used as precursors to form catalysts for reactions involving CO, induding the water gas shift reaction, alkene hy o-formylation, and CO hydrogenation. Although the catalysts exhibit some unusual selectivities, such as in the hydrogenation of CO to give non Schulz-Flory distributions in their hydrocarbon products, they are not highly active relative to some of the more conventional catalysts. The spedes in the zeolites that are formed from the cluster precursors and which are the actual catalytically active spedes have not yet been eluddated. 

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