Hydrochlorination of ethyne

The heterogeneously catalysed conversion of eth3me (acetylene) into vinyl chloride (chloroethene) by the addition of hydrogen chloride is a reaction important for the manufacture of pol3rvinyl chloride (PVC) [18, 213, 248, 249]. Mercuric chloride, supported on activated carbon, is often used as a catalyst commercially [18, 213, 248-252] but, unfortunately, this catalyst rapidly deactivates. The high rate of deactivation, together with the toxicity associated with mercury compounds, has prompted a search for alternative catalysts [213,248,249,251,252]. 

It was shown that the rate-determining step is the addition of HCl to a surface ethyne complex [18,213,248,253]  

Active catalysts should therefore be able to form surface metal-ethyne and metal-HCl complexes. One of the most extensive studies of metal chloride catalysts was carried out by Shinoda [254] 20 metal chlorides supported on carbon were investigated for ethyne hydrochlorination and it was proposed that a correlation existed between the catal3dic activity and the electron affinity of the metal cation, divided by the metal valence. The correlation consists of two straight lines and, for this reason, it cannot be used predictively. However, electron affinity is, necessarily, a one-electron process, whereas the hydrochlorination of ethyne is more likely to be a two-electron process, involving 27t electrons of ethyne. Many of the metal cations investigated in the original study of Shinoda [254] are divalent and, consequently, the standard electrode potential was proposed as a more suitable correlation parameter. 

On the basis of this correlation, Hutchings predicted that gold would be the most active catalyst for ethyne hydrochlorination [255], and his subsequent research confirmed this prediction [252,253,256-259]. Gold catalysts were found to be about three times more active than the commercial mercuric chloride catalysts (see Fig. 6.7). 

Indeed, under appropriate conditions, supported catalysts are considerably more active than mercuric chloride and the best catalyst is gold on activated carbon, obtained using an IW method using HAuCU dissolved in aqua regia [252,258,259], 

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Gold(III) was identified as the most active catalyst for that process in 1985, when Hutchings recognized that the efficiency in catalyzing the hydrochlorination of ethyne to vinyl chloride (a very important industrial process that previously used mercury salts as catalysts) correlated with the standard reduction potential of the supported metal cation. That meant that the metal could be found as a transient species in the reaction [10]. 

FIGURE 4.1. Correlation of activity for ethyne hydrochlorination with the electron affinity of the cation divided by the metal valence. ... 

FIGURE 4.3. Deactivation rate of Au/carbon catalysts for ethyne hydrochlorination as a function of temperature (0.0005 mol Au/IOO g catalyst, C2H2 HQ = 1 1.2). ... 

Interestingly, the deactivation with An catalysts for ethyne hydrochlorination was found to be temperature dependent, and high rates of deactivation were observed at both high and low temperatures. The low-temperature deactivation was caused by coke deposition [252,258], probably the result of surface polymerisation reactions of vinyl chloride and ethyne. The high-temperature deactivation was probed using Au Mossbauer spectroscopy [252,258] and it was found that this was due to the reduction of Au + to An. 

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