Carbon number olefin readsorption

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

We previously proposed that intrapellet (pore) diffusion within liquid-filled catalyst pores decreases the rate of a-olefin removal. This increases the residence time and the fugacity of a-olefins within catalyst pellets and increases the probability that they will readsorb onto FT chain growth sites and initiate new chains. This occurs even for small catalyst particles ("0.1 mm pellet diameter) at normal FT conditions. Larger a-olefins remain longer within catalyst particles because diffusivity decreases markedly with increasing molecular size (carbon number). As a result, readsorption rates increase with increasing carbon number. 

Each time an a-olefin readsorbs, there is a chance that it will desorb as a larger paraffin. Desorption as a paraffin is an irreversible termination step. At high carbon numbers, pore diffusion effects dominate and a-olefins do not exit the catalyst particles unreacted because of enhanced readsorption only unreactive paraffins are observed. As a result, the olefin/paraffin ratio decreases asymptotically to zero as carbon number increases. 

Iron. Fe-Cu-K data are shown in Table 2. Carbon number distribution (Flory plots) and a-olefin/paraffin data are shown in Figures 3 and 4, respectively. As on Ru catalysts, the Flory plot is curved and the a-olefin/paraffin ratio decreases to zero as carbon number increases. The experimental conditions are different for the Ru and Fe systems therefore, we cannot make direct comparisons. Such comparisons will be made in a later publication (14). However, we comment on three important findings. First, C2 and C3 hydrocarbons fall close to the Flory curve (Figure 3) for Fe, in clear contrast with the results on Ru. This suggests that the high rate of ethylene readsorption that leads to low C2 concentrations on Ru (7,8) does not occur on Fe. Secondly, both a-olefins and / -olefins persist at higher carbon numbers than on Ru the Flory plot for Fe shows a more pronounced curvature and the asymptotic value of a is reached at higher carbon number than on Ru. Finally, Ru catalysts produce about 40 wt% C20+ product whereas the Fe... 

The trends in carbon number distribution and in a-olefin/paraffin ratio on Ru, Fe, and Co, three very different catalytic surfaces, are remarkably similar. All catalysts show a curved Flory plot and an a-olefin/paraffin ratio that decreases with increasing carbon number until only paraffins are observed at high carbon numbers. In each case, diffusion-enhanced olefin readsorption accounts for such trends. Its contribution depends on the catalytic surface, its physical structure, and reaction conditions. 

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. 

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. 

Chain termination probabilities are readily separated into the individual contributions from each type of chain termination [Eq. (4)]. The net termination to olefins becomes the difference between jSo, the probability of forming an a-olefin, and j8r, the probability that it will readsorb before leaving the catalyst bed. On both Co and Ru catalysts, the net olefin chain termination probability (j8 - j8r) decreases rapidly with increasing carbon number (Figs. 9a and 10a). This termination probability becomes zero for hydrocarbon chains larger than about C25 because olefins disappear from the reactor effluent, even at very short bed residence times. Bed residence times shorter than 2 s (and CO conversions less than about 10%) do not affect chain termination probabilities even for short hydrocarbon chains, because fast convective transport removes a-olefins from the catalyst bed before readsorption occurs. Yet, readsorption within pellets occurs even at short bed residence times, because intrapellet residence times do not depend on space velocity consequently, chain termination to olefins and the olefin content in products still decrease strongly with increasing hydrocarbon size. 

Ethylene and propylene intrinsic readsorption rates exceed those of larger olefins (4). This leads to the low apparent chain termination probabilities for C2 and C3 molecules shown in Fig. 8. These lower /8t values reflect a more effective reversal of chain termination to olefins (/3o) for these smaller but more reactive molecules (Figs. 9 and 10). The high rate of ethylene and propylene readsorption accounts for their appearance below the rest of the distribution in carbon number distribution plots on both Co and Ru catalysts (Fig. 5). 

The diffusion-enhanced olefin readsorption model described in Section III,C was used to predict the effect of carbon number on chain growth probability and paraffin selectivity. The model requires only one adjustable parameter the exponent c in a hydrocarbon diffusivity equation that depends on molecular size ( ), but that is identical for paraffins and olefins of equal size ... 

The high asymptotic value of C5+ selectivity at large values of occurs on pellets that restrict the removal of reactive a-olefins and allow many readsorption events in the time required for intrapellet olefin removal by diffusion. Yet, transport restrictions within these pellets must not significantly hinder the rate of arrival of CO and H2 reactants to the active sites. Carbon number distributions also obey Flory kinetics for high values of because even the smaller olefins significantly react within a catalyst pellet. 

Our readsorption model shows that carbon number distributions can be accurately described using Flory kinetics as long as olefin readsorption does not occur (/3r = 0), because primary chain termination rate constants are independent of chain size (Fig. 24). The resulting constant value of the chain termination probability equals the sum of the intrinsic rates of chain termination to olefins and paraffins (j8o + Ph)- As a result, FT synthesis products become much lighter than those formed on Co catalysts at our reaction conditions (Fig. 24, jSr = 1.2), where chain termination probabilities are much lower than jS -I- Ph for most hydrocarbon chains. The product distribution for /3r = 12 corresponds to the intermediate olefin readsorption rates experimentally observed on Co/Ti02 catalysts, where intrapellet transport restrictions limit the rate of removal of larger olefins, enhance their secondary chain initiation reactions, and increase the average chain size of FT synthesis products. 

Fig. 24. Effect of enhanced a-olefin readsorption rates on carbon number distributions (simulations experimental/model parameters as in Fig. 16).

Carbon number distribution plots also become linear when olefins readsorb very rapidly (large /3r) or when severe intrapellet transport restrictions (large ) prevent their removal from catalysts pellets before they convert to paraffins during chain termination (Fig. 24, jSr = 100). In this case, chain termination to olefins is totally reversed by fast readsorption, even for light olefins. Chain termination occurs only by hydrogen addition to form paraffins, a step that is not affected by secondary reactions and for which intrinsic kinetics depend only on the nature of the catalytic surface. The product distribution again obeys Flory kinetics, but the constant chain termination probability is given by )8h, instead of po + pH- Clearly, bed and pellet residence times above those required to convert all olefins cannot affect the extent of readsorption or the net chain termination rates and lead to Flory distributions that become independent of bed residence time. 

Diffiisional restrictions increase the effectiveness of olefin interception sites placed within catalyst pellets. Very high olefin hydrogenation turnover rates or site densities within pellets prevent olefin readsorption and lead to Flory distributions of lighter and more paraffinic hydrocarbons. Identical results can be obtained by introducing a double-bond isomerization function into FT catalyst pellets because internal olefins, like paraffins, are much less reactive than a-olefins in chain initiation reactions. However, light paraffins and internal olefins are not particularly useful end-products in many applications of FT synthesis. Yet, similar concepts can be used to intercept reactive olefins and convert them into more useful products (e.g., alcohols) and to shift the carbon number distribution into a more useful range. In the next section, olefin readsorption model simulations are used to explore these options in the control of FT synthesis selectivity. 

The carbon number dependence reflects the relative reactivity of carbon—carbon bonds in cracking reactions (133,134). In this kinetic scheme, cracking does not occur at the three C—C bonds nearest the end of a molecule but occurs with equal probability at all other C—C bonds. Such reactive trends are qualitatively similar to those reported for carbenium ion-type cracking reactions on acid catalysts (56,133,134). We express the cracking probability as a ratio (j8c//8r) in order to describe the competition between cracking and readsorption pathways for available a-olefins. Also,... 

In contrast with readsorption reactions, which broaden the carbon number distribution of FT synthesis products, cracking reactions narrow such distributions, within the constraints imposed by the random nature of C—C bond cleavage in carbenium ion reactions of large olefins. Cracking of n-paraffins can also occur on intrapellet acid sites, but acid-catalyzed paraffin reactions are much slower than those of corresponding olefins of equal size. As in all secondary reactions, cracking sites are used most efficiently when... 

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. 

The Fischer Tropsch kinetics of product formations are best understood as a non trivial surface polymerization. The basic kinetic interrelations are well described by an ideal model with carbon number independent probability of chain propagation. The pecularities of real Fischer Tropsch systems are then described as deviations from the ideal model. In this paper (because of space limitations) only the Anderson two slope distributions are discussed and explained by a readsorption extension of the ideal model. The full model including chain branching and formation of olefins, alcohols and aldehydes is being published shortly (ref. 28). 

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