Fischer kinetic scheme

In this work, a detailed kinetic model for the Fischer-Tropsch synthesis (FTS) has been developed. Based on the analysis of the literature data concerning the FT reaction mechanism and on the results we obtained from chemical enrichment experiments, we have first defined a detailed FT mechanism for a cobalt-based catalyst, explaining the synthesis of each product through the evolution of adsorbed reaction intermediates. Moreover, appropriate rate laws have been attributed to each reaction step and the resulting kinetic scheme fitted to a comprehensive set of FT data describing the effect of process conditions on catalyst activity and selectivity in the range of process conditions typical of industrial operations. 

Fig. 7. Kinetic scheme of Fischer Tropsch surface polymerization regarding chain branching and product desorption as paraffins, olefins and alcohols (formation of dimethyl branched chains omitted).

The kinetic scheme of the ideal Fischer Tropsch polymerization model is given in Fig. 8. Chain growth is possible only with one type of reaction step and this is assumed to be independent of chain length. Only one sort of... 

Fig. 12. Kinetic scheme of Fischer Tropsch surface polymerization assuming two different catalyst sites.

Another proposal for explaining the two slope distributions is very consistent with the peculiarities of the Fischer Tropsch system The products of Fischer Tropsch synthesis do usually provide a liquid phase and a gaseous phase under reaction conditions.The gaseous compounds leave the reactor normally within a few seconds. The liquid does need a day or more until it elutes from the catalyst bed. Solubility of paraffinic hydrocarbon vapours in a paraffinic hydrocarbon liquid increases by a factor of about 2 for each carbon number of the product (ref. 27). Thus it needs only an increase of a very few carbon numbers of the product molecules to have them leaving the reactor mainly with the gas phase or with the liquid phase. With increasing residence time in the reactor the chance of readsorption increases and correspondingly the probability of chain prolongation increases. The kinetic scheme of this model is shown in Fig. 14. This model is very consistent with the experimental distributions. 

Tertiary amine catalyzed reactions were also studied by Tanaka and Kakiuchi (8), who essentially supported the Fischer mechanism, but disagreed on the kinetic order. The copolymerization kinetic scheme proposed by both Fischer and Tanaka postulate three rates, R, R2, and R, as follows ... 

A typical kinetic scheme for homogeneous systems is considered, which includes radical initiation (ki), monomer propagation (fep), bimolecular termination by radical combination (k,), and RAFT reaction, i.e., addition reaction to a dormant chain ( add) fragmentation of the radical intermediate (kfrag)- In addition, bimolecular termination by combination of radicals with the radical intermediates (fe ,) has been included. The methodology first proposed by Fischer to study the persistent radical effect in NMLP is used to find an analytical solution for the mass balances on the different species (radicals, R", intermediate radicals, T", and dormant chains, D). In particular, by plotting the solution in a log-log scale, it has been shown that it becomes possible to identify distinct time intervals or regions where the different... 

This study explores the potential of periodic operation for the Fischer-Tropsch synthesis aiming at Diesel range products. The approach followed is modeling the process in a dynamic form using a simple CSTR reactor configuration. The kinetic scheme is based on steady-state data reported in literature. The steady-state behavior is in agreement with experimental observations reported earlier by various research groups. 

Figure 6.11.1 Kinetic scheme of chain growth and termination during Fischer-Tropsch synthesis (S = surface species, P = desorbed product).

A possible mechanistic pathway for the M3(CO)i2 (M = Ru or Os) Fischer-Tropsch catalysts is presented in Scheme 3. It should be emphasized that most of the ideas outlined above are extremely speculative. However, it is to be hoped that with the advent of homogeneous catalyst systems, detailed kinetic and mechanistic studies will lead to a clarification of the situation in the not-too-distant future. 

The observation by Fischer et al.18 that the 4,1-addition of dimethylamine to compound la is thermodynamically controlled at 20°C, whereas 2,1-addition/elimination is kinetically controlled at -115°C, turned out to be limited to few cases.20 It has been shown9a 9b 42 112 113 that for most cases, three competing reaction paths must be considered (i) 2,1-addition/elimina-tion with formation of (l-amino)alkynylcarbene complexes (= 2-amino-l-metalla-l-en-3-ynes) 98 (ii) 4,1-addition to give [(2-amino)alkenyl]carbene complexes (= 4-amino-l-metalla-l,3-butadienes) 96 and (iii) 4,1-addition/ elimination to (3-amino)allenylidene complexes (= 4-amino-l-metalla-1,2,3-butatrienes) 99 (Scheme 33, M = Cr, W). The product ratio 96 98 99 depends on the bulk of substituents R and R1, as well as on the reaction conditions. Addition of lithium amides instead of amines leads to predominant formation of allenylidene complexes 99.112 Furthermore, compounds 99 also can be generated by elimination of ethanol from complexes 96 with BF3 or AlEt3114 and A1C13,113 respectively. 

Arasappan and Fraser-Reid described the preparation of w-pentenyl galactofuranosides and evaluated their prospects as glycosyl donors (O Scheme 21) [81]. Fischer glycosidation of D-galactose under kinetic conditions using -pentenyl alcohol and DMSO as co-solvent [82]... 

The results for nickel and iron catalysts imply that the reaction scheme given in Figures 48 and 49 do not represent the FTS adequately. A hybrid mechanism of both CO and CH insertion into growing chains has been proposed for nickel catalysts. On iron catalysts the monomer building blocks are proposed to be heterogeneous. Thus, isotopic transient kinetic studies have provided a more detailed understanding of the Fischer-Tropsch reaction. 

The power law kinetic equation could be a simplified form of a mechanistic scheme. A summary of some of the reported reaction orders for the partial pressure of hydrogen and carbon monoxide which have been obtained from power law fits by different groups are listed in Table 9. The partial pressure dependencies vary rather widely. The power law fits were obtained for different cobalt catalysts prepared using different supports and methods. The data in Table 9 show that there is not one best power law equation that would provide a good fit for all cobalt catalysts. Brotz [10], Yang et al. [12] and Pannell et al. [13] defined the Fischer-Tropsch rate as the moles of hydrogen plus carbon monoxide converted per time per mass of catalyst (r g+Hj) Wang... 

Class I aldolase-like catalysis of the intermolecular aldol reaction with amines and amino acids in aqueous solution has been studied sporadically throughout the last century. Fischer and Marschall showed in 1931 that alanine and a few primary and secondary amines in neutral, buffered aqueous solutions catalyze the self-aldolization of acetaldehyde to give aldol (11) and crotonaldehyde (12) (Scheme 4.3, Eq. (1)) [41]. In 1941 Langenbeck et al. found that secondary amino acids such as sarcosine also catalyze this reaction [42]. Independently, Westheimer et al. and other groups showed that amines, amino acids, and certain diamines catalyze the retro-aldolization of diacetone alcohol (13) and other aldols (Scheme 4.3, Eq. (2)) [43-47]. More recently Reymond et al. [48] studied the aqueous amine catalysis of cross-aldolizations of acetone with aliphatic aldehydes furnishing aldols 16 (Scheme 4.3, Eq. (3)) and obtained direct kinetic evidence for the involvement of enamine intermediates. 

Hanns Fischer elaborated kinetics describing at least some CRPs in terms of persistent radical effect (PRE) (the term coined by Finke). This is an application of the theory explaining the phenomena of self-regulation of the radical reactions that are related to Scheme 21. The principle, as Fischer says, is simple. 

Studer and Schulte" and Solomon. NMP is also discussed by Fischer and Goto and Fukuda in their reviews of polymerization kinetics and is mentioned in most reviews on RDRP. A simplified mechanism for NMP is shown in Scheme 68. 

PRE explains the low level of termination observed in NMP and other SRMP s as well as in ATRP. Reversible homolysis of alkoxyamines, the key step of NMP, is a typical example of a reaction governed by the PRE. The kinetic equations describing this process were experimentally verified in 1998 by Fischer and coworkers with the model alkoxyamine 8 (Scheme 4.6). 

In the course of NMP, the persistent radical effect (PRE) leads to a steady increase in excess nitroxide. This slows the polymerization rate down and leads to longer polymerization times. As shown by Matyjaszewski, Fukuda and Miura/ introduction of a conventional radical initiator which slowly decomposes under the reaction conditions considerably enhances the conversion rate. Even a low rate of external initiation ( 1% of the initial internal initiation, reaction 1, Scheme 4.5) leads to a considerable reduction in the polymerization time while the livingness, polydispersity and controlled degree of polymerization remain virtually unchanged. The extra radicals reduce the concentration of persistent nitroxides rather than initiating new chains. The kinetic aspects of additional initiation were studied by Fukuda et alP and Fischer et In summary, if the rate R of generation of additional... 

Scheme 16.1 Lumped microkinetics model of Fischer-Tropsch reaction kinetics, where (-Q represents adsorbed CO, 0,. adsorbed chains of / carbon atoms, vacant surface sites, and P, desorbed alkanes of length .

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