Epoxidation electrochemical

Selectivity of propylene oxide from propylene has been reported as high as 97% (222). Use of a gas cathode where oxygen is the gas, reduces required voltage and eliminates the formation of hydrogen (223). Addition of carbonate and bicarbonate salts to the electrolyte enhances ceU performance and product selectivity (224). Reference 225 shows that use of alternating current results in reduced current efficiencies, especiaHy as the frequency is increased. Electrochemical epoxidation of propylene is also accompHshed by using anolyte-containing silver—pyridine complexes (226) or thallium acetate complexes (227,228). 

Maltol. Otsuka Chemical Co. in Japan has operated several electroorganic processes on a small commercial scale. It has used plate and frame and aimular cells at currents in the range of 4500—6000 A (133). The process for the synthesis of maltol [118-71 -8], a food additive and flavor enhancer, starts from furfural [98-01-1] (see Food additives Flavors and spices). The electrochemical step is the oxidation of a-methylfurfural to give a cycHc acetal. The remaining reaction sequence is acid-catalyzed ring expansion, epoxidation with hydrogen peroxide, and then acid-catalyzed rearrangement to yield maltol, ie ... 

There are, however, numerous cases where electronegative additives can act as promoters for catalytic reactions. Typical examples are the use of Cl to enhance the selectivity of Ag epoxidation catalysts and the plethora of electrochemical promotion studies utilizing O2 as the promoting ion, surveyed in Chapters 4 and 8 of this book. The use of O, O8 or O2 as a promoter on metal catalyst surfaces is a new development which surfaced after the discovery of electrochemical promotion where a solid O2 conductor interfaced with the metal catalyst acts as a constant source of promoting O8 ions under the influence of an applied voltage. Without such a constant supply of O2 onto the catalyst surface, the promoting O8 species would soon be consumed via desorption or side reactions. This is why promotion with O2 was not possible in classical promotion, i.e. before the discovery of electrochemical promotion. 

C. Karavasilis, S. Bebelis, and C.G. Vayenas, Non-Faradaic Electrochemical Modification of Catalytic Activity 10. Ethylene epoxidation on Ag deposited on stabilized Zr02 in presence of chlorine moderators, J. Catal. 160, 190-204 (1996). 

Figure 8.38. Steady state effect of current on the increase in the rates of ethylene epoxidation (rj) and deep oxidation to CO2 (r2) of C2H4 on Ag and comparison with the rate Go2=I/4F of electrochemical oxygen supply42 pC2H4=l-6 kPa, pO2=10 kPa, T=400°C intrinsic (1=0) selectivity 0.5, Reprinted with permission from Academic Press.

Electrochemical promotion has also been used to determine the optimal alkali promoter coverage on Ag epoxidation catalysts as a function of chlorinated hydrocarbon moderator level in the gas phase (Chapter 8). 

M. Stoukides The name of M. Stoukides is associated with the first electrochemical promotion studies and publications in 1981 (Chapter 1) when he as a graduate student of C. Vayenas at MIT was investigating ethylene epoxidation on Ag/YSZ. In recent years the group of Professor M. Stoukides in Thessaloniki has made interesting electrochemical promotion studies ofH2S decomposition and C2H4 and NH3 synthesis at elevated temperatures near the border of electrochemical promotion and electrocatalysis. 

Electrochemical promotion, or non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) came as a rather unexpected discovery in 1980 when with my student Mike Stoukides at MIT we were trying to influence in situ the rate and selectivity of ethylene epoxidation by fixing the oxygen activity on a Ag catalyst film deposited on a ceramic O2 conductor via electrical potential application between the catalyst and a counter electrode. 

In the following, some selected classes of reactions and applications of Ni11 polyazamacrocyclic complexes are discussed. Other applications include, inter alia, the electrochemical reduction of allyl ethers1613 and the epoxidation of alkenes.1614... 

Phenylthio-l-trimethylsilylalkanes are easily prepared by the alkylation of (phenylthioXtrimethylsilyl)mcthane as shown in Scheme 10 [40], The treatment of (phenylthio)(trimethylsilyl)methane with butyllithium/tetramethylethylene-diamine (TMEDA) in hexane followed by the addition of alkyl halides or epoxides produces alkylation products which can be oxidized electrochemically to yield the acetals. Since acetals are readily hydrolyzed to aldehydes, (phenylthioXtrimethylsilyl)methane provides a synthon of the formyl anion. This is an alternative to the oxidative transformation of a-thiosilanes to aldehydes via Sila-Pummerer rearrangement under application of MCPBA as oxidant [40, 41]. 

Electrochemical epoxidation of olefins has been developed for the production of ethylene and propylene oxides in aqueous sodium chloride or bromide solution. However, associated with these electrolyses are difficulties in achieving product selectivity as well as in obtaining high yields of the epoxides. Recently, a regiose-lective )-epoxidation of polyisoprenoids (23) to (24), promoted by electrooxidation in an MeCN/THF/H20-NaBr-(Pt) system, has been achieved (Scheme 10) [52]. 

The epoxidation of alkenes is one of the most impoi4ant oxidation methods. Electrochemical epoxidation of electron-poor olefins such as enoates (154 155) and enones has been accomplished by using silver(III)oxo bis(2,2 -bipyridine) and similar complexes (Scheme 61) [241], )-Dimethyl glutaconate is electrolyzed in an MeCN-LiCl04/Ag0Ac)(bpy)-(Pt) system to give the trans-epoxide in 90% yield. 

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