Selective acetylene hydrogenation catalyst development

KLP [Dow K Catalyst liquid phase] A selective hydrogenation process for removing acetylenes from cmde C4 hydrocarbons from ethylene cracking, with no loss of butadiene. The catalyst is based on either copper metal or alumina. Developed by Dow Chemical Company and first commercialized at its plant in Temeuzen, The Netherlands. The KLP licensing business was sold to UOP in 1991. 

Figure 16.32 represents the alkenylation of an acetylene according to the procedure developed by Cacchi. The coupling partners are an alkenyl triflate (prepared in analogy to Figure 13.23) and an alkyne. The coupling product of these compounds is a 1,3-enyne—in this case conjugated with a cyclohexenone. The C=C triple bond of such an enyne can be hydrogenated in a cis-selective fashion by using Lindlar s Pd catalyst (cf. Figure 17.81). In this way 1,3-dienes are formed that contain at least one cij-configured C=C double bond.

Catalysts suitable for selective hydrogenation of acetylenic compounds in cracked gas streams contain elements of group VI and VIII of the periodic table. An early catalyst was molybdenum sulfide supported on activated alumina (Key and Eastwood, 1946). This was followed by the development of cobalt molybdate and nickel based catalysts (Giaro, 1956 Barry, 1950). Modem catalysts for impure (sulfur-bearing) cracked gas streams typically contain nickel, cobalt, and chromium on a silica-alumina base (United Catalysts, 1993). 

A supported cobalt/molybdate catalyst, probably based on the ones developed in the 1930s, was one of the first types to be used in modem ethylene plants. The front-end reactor was located in the compressor train after heavy hydrocarbons were removed but before sulfur removal or gas drying. The catalyst was, therefore, partly sulfided. Careful temperature control was required to limit ethylene loss. About 10% steam was added to cracked gas, which limited the temperature rise and improved selectivity. An unusual feature of operation was that a significant proportion of the acetylene was removed as a polymer. This decreased the potential temperature rise but meant that catalyst regeneration and subsequent reactivation was a routine procedure at intervals of 2-4 weeks and that a spare reactor was needed. To compensate for loss of activity the gas temperature was continuously increased throughout the operating cycle. Acetylene levels were reduced to about 10-20 ppm with 1-3% ethylene loss. Up to 50% of any butadiene present in the gas was also hydrogenated. The catalyst was replaced after 1-2 years. 

Development of the fiont-end catalyst revealed the most important factor in achieving selectivity carbon monoxide is adsorbed by the catalyst surface so that acetylene, metltyl acetylene, and, less strongly, propadiene are adsorbed in preference to ethylene. Consequently, only small amounts of ethylene are hydrogenated at the bottom of the final bed when most of the acetylenes have been removed. 

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