Model compound reaction pathways and kinetics

These models require an extensive data base. Often this will be compiled from pure component or model compound reaction pathways and kinetics. Model compound experiments allow for the quantitative deduction of intrinsic reaction kinetics and, in favorable circumstances, reaction mechanism information. 

Such stochastic modelling was advanced by Klein and Virk Q) as a probabilistic, model compound-based prediction of lignin pyrolysis. Lignin structure was not considered explicitly. Their approach was extended by Petrocelli (4) to include Kraft lignins and catalysis. Squire and coworkers ( ) introduced the Monte Carlo computational technique as a means of following and predicting coal pyrolysis routes. Recently, McDermott ( used model compound reaction pathways and kinetics to determine Markov Chain states and transition probabilities, respectively, in a rigorous, kinetics-oriented Monte Carlo simulation of the reactions of a linear polymer. Herein we extend the Monte Carlo... 

With models for catalyst decay and effectiveness now in hand, the simulation of lignin liquefaction could be achieved given the initial lignin structure (as described earlier) and model compound reaction pathways and kinetics, both thermal and catalytic. Construction of a random polymer, as outlined earlier, began the simulation. This structural information combined with the simulated process conditions to allow calculation of the reaction rate constants, selectivities and associated transition probabilities. The largest rate constant then specified the upper limit of the reaction time step size. 

Table III, Model compound reaction pathways and kinetics...

Kinetic models which consider demetallation as a complex reaction network of consecutive and parallel reactions taught by model compound studies have been recognized with real feedstocks. Tamm et al. (1981) suggest a sequential mechanism where the metal compounds are activated by H2S. Model compound reaction pathway studies in the absence of H2S, discussed in Section IV,A,1, and experiments in which H2S was present in excess (Pazos et al., 1983) indicate that sequential reactions are inherent to the chemistry of the metal compounds irrespective of the presence of H2S. However, it is possible that both mechanisms contribute to metal removal. 

Application of these ideas to lignin liquefaction required definition of allowable lignin states and the probabilities associated with transitions from one state to another. The random construction just considered provided the initial t = 0) states. The model compound reaction pathways provided the set of allowable states for t > 0, and the model compound kinetics and selectivities provided the transition probabilities. 

The second step assumes that the reactivity of the ensemble is dominated by a few selected functionalities. The task then is to determine the reaction kinetics for each of the functional groups. Here the art of lumping applies in order to keep the number of kinetic lumps small. Information on reaction pathways and kinetics can be independently obtained from experiments using representative model compounds. For example, butyl benzene pyrolysis may serve as a model system for the pyrolysis of alkyl aromatics moieties in resids. 

In Section IV, the kinetics and mechanisms of catalytic HDM reactions are presented. Reaction pathways and the interplay of kinetic rate processes and molecular diffusion processes are discussed and compared for demetallation of nickel and vanadium species. Model compound HDM studies are reviewed first to provide fundamental insight into the complex processes occurring with petroleum residua. The effects of feed composition, competitive reactions, and reaction conditions are discussed. Since development of an understanding of the kinetics of metal removal is important from the standpoint of catalyst lifetime, the effect of catalyst properties on reaction kinetics and on the resulting metal deposition profiles in hydroprocessing catalysts are discussed. 

Using the thermochemical estimates given above, along with the considerable body of available thermochemical and kinetic data, several plausible reaction pathways in coal and model compound reactions will now be examined. This analysis is intended to discriminate between feasible and unlikely reaction mechanisms. It should be kept in mind that absolute rate constant estimates are often only very approximate, and we are testing ideas, not proving them. 

The question is now Which reaction pathways arc Followed, and to what extent This asks for a detailed modeling of the kinetics of the individual reaction steps of this network. This can be achieved on the basis of the half-lives of four s-triazinc herbicides in soil [17]. Figure 10.3-13 shows the four compounds For which data were Found in the literature. 

Model Compounds. Many of the complexities associated with practical polymers and the many simultaneous reaction pathways can be avoided by using model compounds. A typical one is eicosane (n-C20Hi 2) which forms single crystals similar to those in the crystalline regions of polyethylene (58-60). More work is needed on similar compounds and on low molecular weight oligomers to establish both the species produced and the kinetics of their reactions (61,62). 

Reviewed previous SCWO research with model pollutants and demonstrated that phenolic compounds are the model pollutants studied most extensively under SCWO conditions Studied supercritical water oxidation of aqueous waste Explored reaction pathways in SCWO of phenol Studied catalytic oxidation in supercritical water Explored metal oxides as catalysts in SCWO Studied decomposition of municipal sludge by SCWO Investigated the SCWO kinetics, products, and pathways for CH3- and CHO-substituted phenols Determined oxidation rates of common organic compounds in SCWO... 

The exact catalytic cycle is still under debate [89]. Studies of model compounds provide insight into the mechanism, but these model reactions differ from the actual catalytic cycle, and so may follow a different mechanistic pathway [90-92]. Kinetic studies show that the conversion of ethene follows the rate law in Eq. (3.1). This supports a pre-equilibrium that involves the dissociation of two chloride ions and one proton, thus explaining the sensitivity of the reaction to the presence of chloride ions. 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

The mechanisms by which manganese complexes and manganese superoxide dismutase react with superoxide radicals are of interest as knowledge of the kinetic parameters and the reaction pathways may allow the synthesis of model compounds with specific chemical features. These compounds may then have clinical application or may allow the control of specific redox chemistry in catalytic processes. 

The use and importance of aromatic compounds in fuels sharply contrasts the limited kinetic data available in the literature, regarding their combustion kinetics and reaction pathways. A number of experimental and modelling studies on benzene [153, 154, 155, 156, 157, 158], toluene [159, 160] and phenol [161] oxidation exist in the literature, but it would still be helpful to have more data on initial product and species concentration profiles to understand or evaluate important reaction paths and to validate detailed mechanisms. The above studies show that phenyl and phenoxy radicals are key intermediates in the gas phase thermal oxidation of aromatics. The formation of the phenyl radical usually involves abstraction of a strong (111 to 114 kcal mof ) aromatic—H bond by the radical pool. These abstraction reactions are often endothermic and usually involve a 6 - 8 kcal mol barrier above the endothermicity but they still occur readily under moderate or high temperature combustion or pyrolysis conditions. The phenoxy radical in aromatic oxidation can result from an exothermic process involving several steps, (i) formation of phenol by OH addition to the aromatic ring with subsequent H or R elimination from the addition site [162] (ii) the phenoxy radical is then easily formed via abstraction of the weak (ca. 86 kcal moT ) phenolic hydrogen atom. 

A kinetic model of polyolefins copolymerisation by Ziegler-Natta catalysts has been proposed in [81]. The model is based on a mechanism assuming a reaction pathway with different types of AC in the catalyst particle. The kinetic chart describes the formation and deactivation of AC, as well as the spontaneous reactions of chain transfer to hydrogen, monomer, or metal-organic compound. The model is suitable for calculations of copolymerisation rate, composition, and MWD of copolymers. 

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