Autocatalytic reaction acceleration

As suggested by the name, the products of an autocatalytic reaction accelerate the rate of the reaction. For example, an acid-catalyzed reaction may produce... 

Of particular interest are changes in the chain length occurring in connection with the autocatalytic reaction acceleration in TS-6. Numerous thermal polymerization studies showed that the activation energy is EJl = 1.00 + 0.02 eV, independent of conversion. Consequently the autocatalytic reaction enhancement cannot be the result of an increase of the Boltzmann factor. Instead, an increase in the number of monomers consumed per primary chain initiation event has been postulated. Experimentally n(X = 0.5)/n(X = 0) = 200 is found... 

The products of an autocatalytic reaction accelerate the rate of the reaction. For example, an acid-catalyzed reaction may produce additional acid. The rate of most reactions has an initial maximum and then decreases as reaction proceeds. Autocatalytic reactions have an initially increasing rate, although the rate must eventually decline as the reaction goes to completion. A model reaction frequently used to represent autocatalytic behavior is... 

A second type of negative feedback is double autocatalysis. The essence of this pattern is that a species generated autocatalytically, often explosively, in the primary autocatalysis is consumed by a second autocatalytic reaction in which it serves as the precursor. As the primary autocatalyst builds up, the second autocatalytic reaction accelerates, consuming the primary species at an ever-faster rate and ultimately terminating the explosion. Franck (1985) constructed a modified version of the original Lotka-Volterra model that nicely illustrates double autocatalysis ... 

There are many reactions in which the products formed often act as catalysts for the reaction. The reaction rate accelerates as the reaction continues, and this process is referred to as autocatalysis. The reaction rate is proportional to a product concentration raised to a positive exponent for an autocatalytic reaction. Examples of this type of reaction are the hydrolysis of several esters. This is because the acids formed by the reaction give rise to hydrogen ions that act as catalysts for subsequent reactions. The fermentation reaction that involves the action of a micro-organism on an organic feedstock is a significant autocatalytic reaction. 

Autocatalysis happens when a reaction product, formed during reaction, acts as a catalyst which accelerates the progress of the reaction even at constant temperature. An example is the acid-catalysed saponification of various esters and related compounds. Autocatalytic reactions can be easily experimentally identified by means of differential thermal analysis methods. 

Processes are called autocatalytic if the products of a reaction accelerate their own formation. Autocatalytic reactions get faster as the reaction proceeds, sometimes dramatically, sometimes slowly and steadily. Exponential growth is a very basic non-chemical example. Of course the acceleration cannot be permanent the reaction will slow down and eventually come to an end once the starting materials have been used up. Only economists believe in sustainable growth. 

An extreme example of an autocatalytic reaction is an explosion. In this case, it is not directly a chemical product that accelerates the reaction, it is the heat generated by the reaction. The more heat produced, the faster the reaction the faster the reaction, the more heat, etc. 

This type of autocatalytic reaction is a simplification of many biological reactions such as fermentation, where the reaction produces products (species B in the previous example), which accelerates the rate. In fermentation, yeast cells in the solution produce enzymes that catalyze the decomposition of sugar to produce ethanol as a byproduct of yeast reproduction. Since the yeast population increases as the reaction proceeds, the enzyme concentration increases, and the process appears to be autocatalytic. A highly simplified description of fermentation might be... 

The oxidation is first order with respect to catalyst and alcohol, while the order with respect to NMO is fractional. A rate expression was derived and formation of a catalyst snbstrate complex proposed [500]. Oxidation of 2-propanol to acetone (and other secondary alcohols) by stoich. TPAP/CH Cl may be anto-catalytic the initial redaction product (RuO ) may form an adduct [Ru0. nRu02] with [RuO ]. The initially slow rate of oxidation by TRAP accelerated sharply as the concentration of product built up and then decreased near the end of the reaction because of the lower concentration of reactants, giving a bell-shaped curve typical of autocatalytic reactions [501]. 

The reactions when ferric chloride or ferric acetylacetonate is present occur in three stages (1) an initial fast reaction decreasing in rate, (2) an arrest during which the oxidation rate may be very low, and (3) a "sigmoid phase in which an autocatalytic reaction is finally overtaken by reactant consumption. With the ferric (bissalen) chloride complex, the reaction was accelerated but Stages 1 and 2 did not appear (curve 340R, Figure 2). 

This is due to the fact that under isothermal conditions, the nth-order reaction presents its maximum heat release rate at the beginning of the exposure to initial temperature, whereas the autocatalytic reaction presents no heat release rate at this time. Thus, temperature increase is delayed and only detected later after an induction period, as the reaction rate becomes sufficiently fast. Hence acceleration, due to both product concentration and temperature increase, becomes very sharp. 

This has essential consequences for the design of emergency measures. A technical measure to prevent a runaway could be a temperature alarm set at, for example, 10 K above the process temperature. This works well with nth-order reactions, where the alarm is activated at approximately half of the TMRld. However, autocatalytic reactions are not only accelerated by temperature, but also by time. This can lead to a sharp temperature increase. In the case shown in Figure 12.2, a temperature alarm is not effective, because there is no time left to take measures in the example given, only a few minutes are left before runaway. Therefore, it is important to know if a decomposition reaction is of autocatalytic nature or not that is, the safety measures must be adapted to this type of reaction. 

Thanks to its versatility, this model has proved to describe a great number of autocatalytic reaction systems [5]. Systems with a slow initiation reaction are called strong autocatalytic. Because the rate of the initiation reaction is low, product is formed slowly, leading to a long induction time under isothermal conditions. For such systems, the initial heat release rate is low or practically zero. Consequently, the reaction may remain undetected for a relatively long period of time (Figure 12.4). When the reaction accelerates, such an acceleration appears suddenly and may lead to runaway. A strong autocatalytic reaction is formally equivalent to a Prout-Tompkins mechanism. 

In some autocatalytic reactions where the products accelerate the reaction, see Section 6.14, the concentration of intermediates switches abruptly from a steady value to one which oscillates violently. 

The autocatalytic reaction of A to form Q is one that accelerates with conversion. An example of this is shown below ... 

During the induction period propionaldehyde and hydrogen peroxide are the main products, although a little acetaldehyde is also formed. At the end of the induction period the reaction accelerates autocatalytically and formaldehyde, methanol, carbon monoxide, methane and ethane together with acids and peroxides are formed. Hydrogen peroxide is the most abundant peroxidic product, but there are also appreciable amounts of organic peroxides and peroxyacids [17]. These reach concentrations (as does formaldehyde, and to a less marked extent the higher aldehydes) at the time of maximum rate. 

At all events, however, once the concentration of an autocatalyst attains to the threshold value in a high explosive of the true AC type confined in the closed cell, the explosive starts the autocatalytic reaction, and thereafter, the reaction accelerates gradually, even if the test is performed under isothermal conditions. It is, therefore, possible also in the isothermal storage test to locate without... 

Temperature at which the self-accelerating decomposition reaction, or the autocatalytic reaction, or the quasi-autocatalytic reaction, that may lead to the ultimate thermal explosion of a chemical of the AC type, including every powdery chemical of the quasi-AC type, having an arbitrary shape and an arbitrary size, confined in an arbitrary closed container of the corresponding shape and size, and placed in the atmosphere under isothermal conditions, starts just 7 d after the placement of the container in the atmosphere maintained at the temperature [K]. 

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. 

Many studies of the thermal stability of this reactant have been completed, due to its technical use as a primary explosive. Four polymorphic forms exist [12] of which the orthorhombic a-form is the most stable. The other forms can be prepared under specific conditions [12], At high temperatures (above 613 K) the reaction accelerates to detonation. This autocatalytic process has been attributed [54] to an increased concentration of defect sites, rather than a mechanism controlled primarily by temperature increases due to self-heating. 

The self-accelerating oxidation of hydrocarbons is called autoxidation. Its initial stage is characterised by a slow reaction with oxygen followed by a phase of increased conversion until the process comes to a standstill. The degradation is driven by an autocatalytic reaction described by the well-established free radical mechanism [1, 2], consisting of four distinct stages ... 

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