Other Liquid-Phase Reactions

Apart from acid-base catalysis, homogeneous catalysis occurs for other liquid-phase reactions. An example is the decomposition of H202 in aqueous solution catalyzed by iodide ion (II). The overall reaction is 

This reaction can be catalyzed in other ways by the enzyme catalase (see enzyme catalysis in Chapter 10), in which EA is 50 kJ mol-1, and by colloidal Pt, in which EA is even lower, at 25 kJ mol-1. 

Another example of homogeneous catalysis in aqueous solution is the dimerization of benzaldehyde catalyzed by cyanide ion, CN- (Wilkinson, 1980, p.28)  

Redox cycles involving metal cations are used in some industrial oxidations. 

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Production of acetic acid and other carbonylation products is one of the most feasible ways to improve the selectivity of the partial oxidation of methane by using, along with oxygenates, carbon monoxide formed for the carbonylation of methanol and other liquid-phase reaction products. The KANMET Company (Canada) proposed a method for the production of acetic acid on the basis of the direct oxidation of methane, b qjassing the step of s)mthesis gas production [282]. Methane is oxidized in the gas phase to methanol, which is then separated and carbonylated on a supported catalyst with carbon monoxide formed dining oxidation ... 

For the ethanol production process (including the CHP), the non-random two liquid (NRTL) property method with Henry components was used, which was also recommended by the Aspen Plus guidelines, as it is suitable for, among others, liquid-phase reactions and azeotropic alcohol separation. Some compounds involved in the ethanol production do not exist in the conventional Aspen Plus database. Therefore, physical properties of these components were taken from a database developed by the National Renewable Energy Laboratory (NREL) for biofuel components (Wooley et al., 1999 Aspentech, 2011). 

Inerts concentration. The reaction might be carried out in the presence of an inert material. This could be a solvent in a liquid-phase reaction or an inert gas in a gas-phase reaction. Figure 2.96 shows that if the reaction involves an increase in the number of moles, then adding inert material will increase equilibrium conversion. On the other hand, if the reaction involves a decrease in the number of moles, then inert concentration should be decreased (see Fig. 2.96). If there is no change in the number of moles during reaction, then inert material has no effect on equilibrium conversion. 

Solvent molecules may play a variety of roles in liquid phase reactions. In some cases they merely provide a physical environment in which encounters between reactant molecules take place much as they do in gas phase reactions. Thus they may act merely as space fillers and have negligible influence on the observed reaction rate. At the other extreme, the solvent molecules may act as reactants in the sequence of elementary reactions constituting the mechanism. Although a thorough discussion of these effects would be beyond the scope of this textbook, the paragraphs that follow indicate some important aspects with which the budding ki-neticist should be familiar. 

The effect of external pressure on the rates of liquid phase reactions is normally quite small and, unless one goes to pressures of several hundred atmospheres, the effect is difficult to observe. In terms of the transition state approach to reactions in solution, the equilibrium existing between reactants and activated complexes may be analyzed in terms of Le Chatelier s principle or other theorems of moderation. The concentration of activated complex species (and hence the reaction rate) will be increased by an increase in hydrostatic pressure if the volume of the activated complex is less than the sum of the volumes of the reactant molecules. The rate of reaction will be decreased by an increase in external pressure if the volume of the activated complex molecules is greater than the sum of the volumes of the reactant molecules. For a decrease in external pressure, the opposite would be true. In most cases the rates of liquid phase reactions are enhanced by increased pressure, but there are also many cases where the converse situation prevails. 

Other advantages of the tubular reactor relative to stirred tanks include suitability for use at higher pressures and temperatures, and the fact that severe energy transfer constraints may be readily surmounted using this configuration. The tubular reactor is usually employed for liquid phase reactions when relatively short residence times are needed to effect the desired chemical transformation. It is the reactor of choice for continuous gas phase operations. 

In the more general case for liquid phase reactions or other cases where S = 0, the autocatalytic term in the reaction rate expression can be written as... 

Ordinary or bulk diffusion is primarily responsible for molecular transport when the mean free path of a molecule is small compared with the diameter of the pore. At 1 atm the mean free path of typical gaseous species is of the order of 10 5 cm or 103 A. In pores larger than 1CT4 cm the mean free path is much smaller than the pore dimension, and collisions with other gas phase molecules will occur much more often than collisions with the pore walls. Under these circumstances the effective diffusivity will be independent of the pore diameter and, within a given catalyst pore, ordinary bulk diffusion coefficients may be used in Fick s first law to evaluate the rate of mass transfer and the concentration profile in the pore. In industrial practice there are three general classes of reaction conditions for which the bulk value of the diffusion coefficient is appropriate. For all catalysts these include liquid phase reactions... 

In stirred chemical reactors, unlike in combustion and with other gas-phase reactions, these closure terms should take into account that for liquids the Schmidt number (Sc = v/D) is in the order 100-1,000, and, hence, the role of species diffusion at scales within the Kolmogorov eddies should explicitly be taken into account (Kresta and Brodkey, 2004). Essential is that diffusion of chemical species is governed by the Batchelor length scale rjB which obeys to... 

Reactions in solution proceed in a similar manner, by elementary steps, to those in the gas phase. Many of the concepts, such as reaction coordinates and energy barriers, are the same. The two theories for elementary reactions have also been extended to liquid-phase reactions. The TST naturally extends to the liquid phase, since the transition state is treated as a thermodynamic entity. Features not present in gas-phase reactions, such as solvent effects and activity coefficients of ionic species in polar media, are treated as for stable species. Molecules in a liquid are in an almost constant state of collision so that the collision-based rate theories require modification to be used quantitatively. The energy distributions in the jostling motion in a liquid are similar to those in gas-phase collisions, but any reaction trajectory is modified by interaction with neighboring molecules. Furthermore, the frequency with which reaction partners approach each other is governed by diffusion rather than by random collisions, and, once together, multiple encounters between a reactant pair occur in this molecular traffic jam. This can modify the rate constants for individual reaction steps significantly. Thus, several aspects of reaction in a condensed phase differ from those in the gas phase ... 

Three special cases of equation 9.2-18 arise, depending on the relative magnitudes of the two mass-transfer terms in comparison with each other and with the reaction term (which is always present for reaction in bulk liquid only). In the extreme, if all mass-transfer resistance is negligible, the situation is the same as that for a homogeneous liquid-phase reaction, ( rA)im = kAcAcB. 

Two stirred tanks are available, one 100 m3 in volume, the other 30 m3 in volume. It is suggested that these tanks be used as a two-stage CSTR for carrying out a liquid phase reaction A + B product. The two reactants will be present in the feed stream in equimolar proportions, the concentration of each being 1.5 kmol/m3. The volumetric flowrate of the feed stream will be 0.3 x 10-3 m3/s. The reaction is irreversible and is of first order with respect to each of the reactants A and B, i.e. second order overall, with a rate constant 1.8 x 10-4 m3/kmols. 

A parallel reactor system for liquid-liquid phase reactions such as oxidation reactions with H202 at ambient pressure was reported from hte Aktiengesellschaft. If compared with other chemistries, rather mild-reaction conditions (ambient pressure, moderate temperature) are often applied in liquid-phase oxidation for fine chemical production with terminal oxidants that can be dosed as liquids (e.g., aqueous H202 or organic peroxides). The reaction that was investigated was the partial oxidation of... 

In general, liquid-phase reactions (Sc > 1) and fast chemistry are beyond the range of DNS. The treatment of inhomogeneous flows (e.g., a chemical reactor) adds further restrictions. Thus, although DNS is a valuable tool for studying fundamentals,4 it is not a useful tool for chemical-reactor modeling. Nonetheless, much can be learned about scalar transport in turbulent flows from DNS. For example, valuable information about the effect of molecular diffusion on the joint scalar PDF can be easily extracted from a DNS simulation and used to validate the micromixing closures needed in other scalar transport models. 

On the other hand, the mechanochemical solid-state reaction was found to be the most suitable for this purpose. Thus, when the solid-state reaction was conducted for Cgo in the presence of one equivalent or less of KCN under the HSVM conditions for 30 min, a clean reaction took place to give the [2-1-2] fullerene dimer C120 (3) in 30% yield while 70% of Cgo was recovered unchanged (Scheme 2) [20]. It is to be noted that no cyanated fullerene such as 4 was obtained in comparison to the result of a liquid-phase reaction in o-dichloroben-zene (ODCB)-DMF [21]. This is apparently ascribed to the difference in reactivity of the initially formed cyanated Cgo anion with or without solvation. 

Since the hydrogenation of MAA with unmodified RNi did not proceed as mentioned in the previous section, the kinetic parameters of the liquid-phase reaction with MRNi under atmospheric pressure could not be compared with those of RNi. However, it can be expected that the modification does not change the nature of hydrogenation with RNi since the activation energies of MRNis were exactly the same as each other and independent of the sort of modifying reagent. This expectation was confirmed by the results... 

Equation (24) is, in fact, a Langmuir—Hinshelwood-type equation. Similar models with a single site surface reaction as the rate-determining step were used for other liquid phase esterifications [448,451]. Experimental data for the l-butanol-x>leic acid system were best fitted by eqn. (24) [452] or eqn. (25) [451]... 

Table 2 reports the catalytic activities of the catalysts prepared for 2.6-DTBP oxidation. All the titanium grafted materials were active as catalysts for liquid phase oxidation of 2.6-DTBP, and catalytic activity decreased in the order of MCM-48 (24.5% conversion) > HMS (22.8%) > KIT-1 (16.0%) > MCM-41 (14.3%) > SBA-1 (5%). Apparently. 3 dimensional channel system of MCM —48, and HMS with small particle size and textual mesoporosity proved to be useful in liquid phase reaction [1,2,3], Chemical analysis of the titanium-grafted SBA-1 by EDX showed far less titanium at the surface than the others it seems surface nature of SBA-1 synthesized in acidic medium is different from the rest. All Ti-grafted samples suffered from titanium leaching during the liquid phase oxidation HMS host resulted in over 4 % loss in metal content while the rest showed 2%. 

Generally, the same methods can be used to measure reaction rates at high pressure as at low pressure. Some of them are more suitable than others for use at high pressure. The selection depends whether a homogeneous or a heterogeneous reaction should be investigated, whether it is a gas- or liquid-phase reaction, or a catalyst is used. 

Zeolites and clays are extensively used as catalysts in the bulk chemicals and petrochemicals industries. Typically, reactions are carried out in the gas phase at high temperatures. Such conditions become unreasonable when finer chemicals are involved. Instead, low temperatures and liquid phase reactions become necessary. The costs of the reagents and catalyst are also less important as the product values increase, but product selectivity and good yields are more important. Thus, there is considerable interest in the development of mild, selective, liquid phase reactions such as those described here. Although this review has concentrated particularly on the work of our own group, many others are working in the field and their contributions are referred to in the review articles cited. 

A liquid-phase isophorone process is depicted in Figure 4 (83). A mixture of acetone, water, and potassium hydroxide (0.1%) are fed to a pressure column which operates at head conditions of 205°C and 3.5 MPa ( 500 psi). Acetone condensation reactions occur on the upper trays, high boiling products move down the column, and unreacted acetone is distilled overhead in a water—acetone azeotrope which is recycled to the column as reflux. In the lower section of the column, water and alkali promote hydrolysis of reaction by-products to produce both isophorone and recyclable acetone. Acetone conversion is typically in the range 6—10% and about 70% yield of isophorone is obtained. Condensation—hydrolysis technology (195—198), and other liquid-phase production processes have been reported (199—205). 

We use the very simple case of a first-order irreversible liquid-phase reaction A — B where the rate of reaction is given by r = k Ca in mol/(l sec), k is the reaction rate constant in sec-1 and Ca is the concentration of component A in mol/l. Later we will show how the same principles can be applied to distributed system and also for other rates like rate of mass transfer for heterogeneous systems with multiple phases. 

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