Characterization of Autocatalytic Reactions

Wendler R, GraneB G., Heinze X, Characterization of autocatalytic reactions in modified ceUulose/NMMO solutions by thermal analysis and UV/VIS spectroscopy. Cellulose, 12, 2005, 411-422. 

The first section of this chapter is an introduction of basic definitions, describing the behavior of autocatalytic reactions, their reaction mechanism, and a phenomenological study. The second section is devoted to their characterization and the last section gives some hints on mastering this category of reaction in the industrial environment. 

The above-mentioned phenomenological model is usually used to describe the kinetics of autocatalytic reactions that are characterized by a maximum reaction rate between 20 and 40% conversion. The shape of the photoinitiated polymerization rate curves... 

It is further stated by Reitzner in Ref 37b, p 13f, that Arrhenius simplification does not take into account that a large number of expls follow an autocatalytic reaction mechanism, fn the case of the inorganic azides, for example, the metal is considered to be the autocatalyst. The pressure-time curves for such autocatalytic reactions are characterized by an induction period, followed by acceleratory and decay periods. 

Fig. 26. STM images of the oxygen pre-covered platinum(l 1 1) surface during reaction with hydrogen. Images were recorded at a temperature of T = 111 K with a time interval of 625 K. The white ring in the upper right corner is associated with a reaction front of OH intermediates from the autocatalytic reaction. The outside is characterized by an oxygen-terminated surface, whereas water molecules from the reaction are identified inside the ring. Adapted with permission from Reference (757).

From this study it follows that such systems are characterized by a pronounced kinetic effect of the reaction inhibition and the resulting product is composed of clusters distributed in the reaction system non-homogeneously in space. The more autocatalytic the reaction is and the lower the mobility of the products, the stronger these two affects are. 

Since a reaction product catalyses the reaction, the initial concentration of product also has a strong effect on the TMRad. In the case illustrated in (Figure 12.6), an initial conversion of 10% leads to a reduction of the TMRad by a factor of 2. This also has direct implications for process safety the thermal history of the substance, that is, its exposure to temperature for a certain time increases initial product concentration, leading to effects comparable to those illustrated in Figure 12.5. Hence it becomes obvious that substances showing an autocatalytic decomposition are very sensitive to external effects, such as contaminations and previous thermal treatments. This is important for industrial applications as well as during the experimental characterization of such decompositions the sample chosen must be representative of the industrial situation, or several samples must be analysed. 

The key problems in a polymerization CSTR are the determination and characterization of micro- and macromixing, and the possibility of multiple steady states due to the exothermic nature of the reactions. Recent studies of CSTRs for bulk or solution free-radical polymerization indicate the possibility of multiple steady states due to the large heat evolution and difficult heat transfer that are characteristic of the reactors. Furthermore, even in simple solution polymerization (for example, in methyl methacrylate polymerization in ethyl acetate solvent), autocatalytic kinetics can lead to runaway conditions even with perfect temperature control for certain combinations of solvent concentration and reactor residence time. In practice, the heat evolution can be an additional source of autocatalytic behavior. 

The behavior of 1-alkylpyrroles to autoxidation was studied by Smith and Jensen12 with 1-methyl-, 1-isopropyl-, and 1 -w-butylpyrrole. It was found that N-alkylpyrroles reacted much more slowly with oxygen than C-alkylpyrroles. The reactions were characterized by an induction period, during which the colorless liquid turned yellow and no oxygen uptake was detected successively an autocatalytic reaction took place. The simple oxidation products formed in the case of 1-methylpyrrole were isolated and the structures 10-13 assigned. 

These autocatalytic instabilities in reaction system which we have described are characterized by a rate of reaction which starts off from some very low value, increases during the entire course of the reaction (which is usually of the order of magnitude of seconds or less) and reaches a maximum only because of the depletion of reactants. This behavior differs from a normal, slow reaction in that the half-life of the reaction is remarkably short and of the order of magnitude of the time required to develop the quasi-stationary concentrations of intermediates or temperature. ... 

In the mid 1970s, Falconer and Madix observed a surface- kinetic explosion for the decomposition of formic acid (HCOOH) [23] and acetic acid (CH COOH) [24] on the Ni(llO) surface, characterized by very narrow product desorption peaks in TPRS. Such autocatalytic reactions have also been observed in the decomposition of acetic acid on Pd(llO), Rh(llO), Rh(lll), and even supported Rh catalyst by Bowker et al. [70-75]. In general, these reactions exhibit accelerations in rate as the reaction proceeds to completion. Earlier work hypothesized that decomposition of the carboxylate species formed following adsorption of the acids on the surface was initiated at vacancies (i.e. bare metal sites) and propagated by the further creation of vacancies as the products desorbed from the surface [23, 24]. The rate of decomposition was well described by the rate equation r = -k(C / Cj )(Cj - c+/Cj), in which C is the instantaneous surface concentration of carboxylate, C, is the initial surface concentration, and/is the density of initiation sites. Since the decomposition produced an ever-increasing concentration of vacant sites, a kinetic explosion occurred. 

Biochemical transformations that are mediated by microorganisms are characterized by autocatalytic behavior. The fact that the rates of these reactions increase as the concentration of the organism increases provides opportunities for engineers to consider a variety of modes of operation to enhance the performance (and productivity) of a CSTBR facility. One fruitful approach is to do a partial separation and concentration of the cells contained in the efQnent from the CSTBR (see Figure 13.8) and recycle the resnlting process stream back to a point where it is mixed with the contents of the CSTBR. 

In 1972, Field, Koros, and Noyes (FKN group investigated the kinetic details which characterized the entire aspects of BZ reaction, illuminating the essential constiments of reaction and their roles in oscillations [31, 53]. The kinetic model composed three consecutive reaction processes as, (A), (B), and (C). The process A is a fast reaction step, the process (B) is an autocatalytic set of reaction, and C is the process where (Br ) ions are consumed. The oxidation of metal catalyst ions has also been taken place in the processes (A) and (B), respectively. A recovery step (process C) involves for the reduction of metal catalysts and regenerates the necessary reactant (Br ) ions for re-initiating the oscillatory-phase reaction from beginning. A schematic model for description of the chemistry of BZ reaction is shown in Fig. 1.3. 

Autocatalysis, with n = 1, characterizes biological population growth, for example, since the number of offspring born is proportional to the number of individuals in the population. It leads to the Malthusian population explosion. In chemical systems, where autocatalysis is less common, it can also result in explosion, since the solution to Eq. (2.1) is an exponentially growing concentration. Of relevance to polymer systems is the fact that any exothermic reaction is inherently autocatalytic, since an increase in the product concentration corresponds to production of heat, which leads to an increase in the rate constant of the reaction via the Arrhenius factor. If the reaction in question is lengthening a polymer chain by addition of the monomer, the rate should increase as the chain grows if the heat produced is not rapidly removed from the system. 

Based on kinetic data of the investigated aniline oxidation reaction as obtained with a variety of methods, including spectro-electrochemical ones, numerous researchers [252-270] have proposed an autocatalytic mechanism of oxidation and growth. In this scheme, polymer growth occurs without further electrooxidation of aniline monomers. The polymer film in its oxidized form contains oxidized aniline imits, most likely also at the ends of polymer chains. These units react basically like monomeric radical cations with further monomer molecules from solution (radical cation-parent molecule coupling). Subsequently, the chain has to be reoxidized, that is, one electron has to be transferred per monomer unit. An alternative proposal is that pemigraniline sites act as oxidants for monomer units [271]. The characterization of this pro-... 

Reactions with Carbonyl Compounds. TMSCF3 reacts with aldehydes in the presence of a catalytic amount of tetra-w-butylammonium fluoride (TBAF) in THF to form the corresponding trifluoromethylated carbinols in good to excellent yields following aqueous hydrolysis of the silyl ethers (eq 3) 2.3.6,10 The reaction also works very well for ketones under the same conditions, with the exception of extremely hindered ones such as l,7,7-trimethylbicyclo[2.2.1]heptan-2-one, di-l-adamantyl ketone, and fenchone. The reaction has been characterized as a fluorideinduced autocatalytic reaction. Other initiators such as tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF), potassium fluoride, Ph3SnF2, andRO can also be used for these reactions. ForthereactionsofTMSCFs and perfluorinated ketones and pentafluorobenzaldehyde, excess of KF is needed. 

The oxidation of Pb(C2H5)4 by oxygen is autocatalytic and characterized by an induction period [22, 32, 37, 46] that is different in different hydrocarbons [22]. The temperature coefficient of the reaction rate is fairly high [46]. 

Inhibited autoxidation is often characterized by critical phenomena reasoned by the autocatalytic character of the reaction and mentioned above feedback. Since hydroperoxide decomposes during oxidation, two different regime of inhibited oxidation appear non-stationary and quasi-stationary with respect to hydroperoxide. 

In general, an autocatalytic reaction is characterized by a curve of extent (or concentration of a product formed in a closed enviromnent) versus time that shows the so-called S shape, as shown in Figure 13.3. 

Reduction by Fe(ll) results in an increase in the amount of iron oxides, which favor further reaction. Such autocatalytic behavior characterizes the oxidation of Fe(II) by and explains C Cl NO reduction by Fe(ll) in the absence of an iron mineral phase. Generalizing this behavior, it can be assumed that Fe(III) colloids derived from Fe(ll) oxidation in subsurface anoxic systems, together with other colloids, affect the environmental persistence of nitroaromatic contaminants. Colon et al. (2006), for example, elucidate factors controlling the transformation of nitrosobenzenes and N-hydroxylanilines, which are the two intermediate... 

Because of all these minor components (e.g., catalysts and inhibitors, added to major ones) the cure of vinyl ester resins is very complex, involving many competitive reactions. There are some new variables to account for, such as the inhibitor and initiator concentrations and induction time. Several papers [81,96,200,201] use the mechanistic approach, claiming that the phenomenological models do not explicitly include these facts, resulting in a new parameter characterization after each change in resin formulation [96]. Despite these arguments, the phenomenological approach is the most widely used and is based on an autocatalytic model which has been successfully applied to epoxy resins. Many authors [30,34,74,199,202,203] proposed the Equation 2.30 to describe the cure kinetic of unsaturated polyesters ... 

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