Selectivity liquid-phase reactions

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. 

Table 3.2 Overview of combined in situ techniques and selected liquid-phase reactions.

For liquid-phase reactions, the effect of pressure on the selectivity and reactor volume is less pronounced, and the pressure is likely to be chosen to... 

In pulp and paper processing, anthraquinone (AQ) accelerates the delignification of wood and improves liquor selectivity. The kinetics of the liquid-phase oxidation of anthracene (AN) to AQ with NO2 in acetic acid as solvent has been studied by Rodriguez and Tijero (1989) in a semibatch reactor (batch with respect to the liquid phase), under conditions such that the kinetics of the overall gas-liquid process is controlled by the rate of the liquid-phase reaction. This reaction proceeds through the formation of the intermediate compound anthrone (ANT) ... 

The epoxidation of propene is analogous to that of ethylene catalyzed by silver. However, the selectivity is much lower. Due to the pronounced oxidation sensitivity of the allyl CH3-group, excessive combustion occurs as a side reaction. The heterogeneous process has no practical significance, therefore, as it has to compete with a highly selective liquid phase epoxidation process. 

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. 

Figure 1 compares the selectivities of hydrogenation products as a function of UAL conversion for liquid-phase reaction over the two Ru catalysts. Both catalysts produced significant amounts of SAL. However, selectivity towards UOL, the desired product, increased threefold for the K-exchanged catalyst compared to Ru/NaY. 

The three-step cumene process, including the liquid-phase reactions, is energyconsuming, environmentally unfavorable and disadvantageous for practical operation. The process also produces the unnecessary by-product acetone stoichio-metrically. Furthermore, the intermediate, cumene hydroperoxide, is explosive and cannot be concentrated in the final step, resulting in low phenol yield ( 5%, based on the amount of benzene initially used). Thus, direct phenol synthesis from benzene in a one-step reaction with high benzene conversion and high phenol selectivity is most desirable from the viewpoints of environment-friendly green process and economical efficiency. 

Coke oven gas consists mainly of a mixture of carbon monoxide, hydrogen, methane, and carbon dioxide. It is contaminated with a variety of organic and inorganic compounds that have to be separated in absorption columns before its further use as a synthesis gas. The selective absorption of coke plant gas contamination results from a complex system of parallel liquid-phase reactions. Instantaneous reversible reactions ... 

Enzymes are proteins that catalyze reactions. Thousands of enzymes have been classified and there is no clear limit as to the number that exists in nature or that can be created artificially. Enzymes have one or more catalytic sites that are similar in principle to the active sites on a solid catalyst that are discussed in Chapter 10, but there are major differences in the nature of the sites and in the nature of the reactions they catalyze. Mass transport to the active site of an enzyme is usually done in the liquid phase. Reaction rates in moles per volume per time are several orders of magnitude lower than rates typical of solid-catalyzed gas reactions. Optimal temperatures for enzymatic reactions span the range typical of living organisms, from about 4°C for cold-water fish, to about 40°C for birds and mammals, to over 100°C for thermophilic bacteria. Enzymatic reactions require very specific molecular orientations before they can proceed. As compensation for the lower reaction rates, enzymatic reactions are highly selective. They often require specific stereoisomers as the reactant (termed the substrate in the jargon of biochemistry) and can generate stereospecific products. Enzymes are subject to inhibition and deactivation like other forms of catalysis. 

Nickel (0)-allene complexes are characterized by configurational instability and a propensity to assume a high coordination number. It may not be surprising to find that the Ni(0) species is the most catalytically active of the triad. The cyclic dimers, 1,3- and 1,2-dimethylenecyclobutane, are formed only in the vapor phase reaction of allene with Ni(CO)2(Ph2PC6-H4PPh2) (143). The liquid phase reaction with Ni(0) complexes selectively produces the trimer (I), tetramer (II), andpentamer (III) (Table VIII) 123). Several intermediate Ni(0) complexes (IV-VI) were isolated. 

Radical reactions could be classified in different ways. First of all, they could be selected according to the phase they are studied in. This review deals almost entirely with liquid-phase reactions. Certain important comparisons can actually be made between reactions proceeding in gas and liquid phases. In discussing Arrhenius parameters, the limited liquid phase data are complemented by those taken from gas phase reactions. On the other hand, special attention is given to solvation effects, in general, and to hydrogen bond formation, in particular. 

The use of the catalyst in continuous liquid phase reactions avoids such handling problems. Here the advantages of the heterogeneous system is obvious compared to homogeneous discontinuous systems. The reactions had to be carried out at lower temperatures than in the batch reactors to stay below the boiling point of the starting materials. Nevertheless, even at room temperature conversions of about 20 % and a selectivity towards the monoalkylated product of more than 95 % could be achieved (Figure 14). 

The key to the success of the oxidation examples cited above is the ability of the catalysts used to exert proper kinetic control on the possible side reactions. Without it, thermodynamically favorable but undesired products such as CO2 and H2O are made instead. Controlling oxidation kinetics to stop at the desired oxygenated products is quite difficult, and has yet to be solved for many other systems. For instance, although many attempts have been made to develop a commercial process for the oxidation of propylene to propylene oxide, both the activity and the selectivity of the systems proposed to date, mostly based on silver catalysts, are still too low to be of industrial interest " propylene oxide is presently manufactured by processes based on chlorohydrin or hydrogen peroxide instead. In spite of these difficulties, though, recent advances in selective liquid phase oxidation of fine chemicals on supported metal catalysts have shown some promise, offering high yields (close to 100%) under mild reaction conditions." ... 

Both in vapor and in liquid phase reaction conditions, nonane is more reactive than heptane. The reactivity difference is however much more pronounced in the vapor phase. In USY zeolite micropores exposed to the vapor of the two n-alkanes, the heaviest alkane molecule is preferentially adsorbed, resulting in a higher apparent reaction rate. When the alkane mixture is fed in the liquid phase, the competing alkanes are adsorbed in a non-selective manner in the micro- and mesopores of USY. Consequently, in liquid phase conditions the relative reactivity of the n-alkanes corresponds to the relative intrinsic reactivities. 

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