Catalyst Systems Containing Alkali Metals

Alkali metals, their hydrides, and amides have been proven as catalysts for the hydroamination of olefins under various conditions. Some typical examples for the hydroamination of ethylene as the fundamental reaction, being also of considerable industrial interest, are given in Table 2. 

Ethylene and ammonia were found to react in the presence of metallic sodium or sodium hydride at temperatures above 200 °C and at 80-100 MPa pressure in an inert hydrocarbon medium such as heptane to produce approximately equimolar quantities of ethyl-, diethyl-, and triethylamines. The total conversion, based on ammonia, was about 40 % and the yield about 70 % a portion of the ammonia appeared in the products as sodium amide, to which the sodium was quantitatively converted. 

Lithium, potassium, and their hydrides were similarily active, and propene or other a-olefins also reacted with ammonia but with lower conversion to the corresponding mixtures of alkylamines, in which the addition followed Markovni-kov s rule. 

Amines could also be added, e. g., n-butylamine and ethylene produced diethyl-n-butylamine in about 50 % conversion besides a small amount of a complex mixture of high-boiling nitrogen-containing materials. The increase of chain length could result from the nucleophilic attack of ethylene by the y9-aminoethyl anion in competition with the protolytic reaction with the amine. 

The amides of rubidium or cesium have been proven as the best catalysts for the hydroamination of ethylene with ammonia. In liquid ammonia below the critical temperature (132.5 °C) at 100 °C and an initial pressure of only 11 MPa a turnover number (TON) of about 4 mol C2H4/(mol CSNH2) per h could be reached [6]. 

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Table 2. Catalyst systems containing alkali metals for the hydroamination of ethylene. 

One of the drawbacks of the systems containing alkali metals is their resistance under hydrothermal conditions and the presence of sulfur. In the first case the activity of soot oxidation drops dramatically for K/CeOs catalysts under water at higher temperatures due to the loss of potassium and a change in the K/Ce optimal ratio, while the formation of sulfates is responsible for activity loss under SOg. 

The chemical reactivity of the catalyst support may make important contributions to the catalytic chemistry of the material. We noted earlier that the catalyst support contains acidic and basic hydroxyls. The chemical nature of these hydroxyls will be described in detail in Chapter 5. Whereas the number of basic hydroxyls dominates in alumina, the few highly acidic hydroxyl groups also present on the alumina surface can also dramatically affect catalytic reactions. An example is the selective oxidation of ethylene catalyzed by silver supported by alumina. The epoxide, which is produced by the catalytic reaction of oxygen and ethylene over Ag, can be isomerized to acetaldehyde via the acidic protons present on the surface of the alumina support. The acetaldehyde can then be rapidly oxidized over Ag to COg and H2O. This total combustion reaction system is an example of bifunctional catalysis. This example provides an opportunity to describe the role of promoting compounds added in small amounts to a catalyst to enhance its selectivity or activity by altering the properties of the catalyst support. To suppress the total combustion reaction of ethylene, alkali metal ions such as Cs+ or K+ are typically added to the catalyst support. The alkali metal ions can exchange with the acidic support protons, thus suppressing the isomerization reaction of epoxide to acetaldehyde. This decreases the total combustion and improves the overall catalytic selectivity. 

The most common catalysts in order of decreasing reactivity are haUdes of aluminum, boron, zinc, and kon (76). Alkali metals and thek alcoholates, amines, nitriles, and tetraalkylureas have been used (77—80). The largest commercial processes use a resin—catalyst system (81). Trichlorosilane refluxes in a bed of anion-exchange resin containing tertiary amino or quaternary ammonium groups. Contact time can be used to control disproportionation to dichlorosilane, monochlorosilane, or silane. 

The production of sulphuric acid by the contact process, introduced in about 1875, was the first process of industrial significance to utilize heterogeneous catalysts. In this process, SO2 was oxidized on a platinum catalyst to S03, which was subsequently absorbed in aqueous sulphuric acid. Later, the platinum catalyst was superseded by a catalyst containing vanadium oxide and alkali-metal sulphates on a silica carrier, which was cheaper and less prone to poisoning. Further development of the vanadium catalysts over the last decades has led to highly optimized modem sulphuric acid catalysts, which are all based on the vanadium-alkali sulphate system. 

Haloalkenes can be prepared by dehydrohalogenating saturated hydrogen-containing polyhalocarbons using liquid alkali metal acid fluoride and/or alkali metal fluoride compositions [75], HCFC-133a can be converted to CF2=CHC1 using these catalyst systems as shown in eq 21. 

Formation of the mixed cement-containing systems within the range of low copper concentrations with addition of alkali metal dopants as well as catalytical properties of these systems in the ethane oxidative chlorination process have been investigated. Based on the obtained data the efficient and stable copper-cement catalyst has been worked out. This catalyst will assist in the development of a new technology of the vinyl chloride production from ethane. The basic peirameters of the ethane oxychlorination process have been determined at 623-673K, time-on-stream 3-5s and reactant ratio of C2H6 HCI 02 = 1 2 1 the conversion of ethane is more than 90% and the total selectivity to ethylene and vinyl chloride is 85-90%. 

The analysis of the known and our own experimental data indicated that the properties required may be offered by a copper-containing cement-based catalytic system modified with alkali metals. In this catalyst, copper-containing active sites catalyze the oxidation of hydrogen chloride, whereas the activity of the catalyst in the dehydrochlorination reaction is determined by the acid—base surface properties, which are inherent to cements with different phase compositions. 

In the presence of the sodium-containing heterobimetallic catalyst (R)-LSB (10 mol%), the reaction of enone 52 with TBHP (2 equiv) was found to give the desired epoxide with 83% ee and in 92% yield [56]. Unfortunately LSB as well as other bimetallic catalysts were not useful for many other enones. Interestingly, in marked contrast to LSB an alkali metal free lanthanoid BINOL complex, which was prepared from Ln(0- -Pr)3 and (R)-BINOL or a derivative thereof (1 or 1.25 molar equiv) in the presence of MS 4A (Scheme 17), was found to be applicable to a range of enone substrates. Regarding enones with an aryl-substitu-ent in the a-keto position, the most effective catalytic system was revealed when using a lanthanum-(.R/)-3-hydroxymethyl-BINOL complex La-51 (l-5mol%) and cumene hydroperoxide (CMHP) as oxidant. The asymmetric epoxidation proceeded with excellent enantioselectivities (ees between 85 and 94%) and yields up to 95%. 

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