A study of the dehydrochlorination of 1,2-dichloropropane over silica-alumina catalysts

The conversions of 1,2-dichloropropane (12dcp), 1,3-dichloropropane (13dcp) , and allyl chloride (3-chloropropene, 3cp=) over a silica-alumina bed have been investigated. Both 12dcp and 13dcp undergo dehydrochlorination above 500 K, mainly resulting in the two 1-chloropropene (1cp=) geometric isomers , which, above 650 K, further eliminate HCl to give propyne and burn in the presence of oxygen. Small amounts of 3cp= were also observed. 12dcp was fo und to react more rapidly than 13dcp. The conversion of 3cp= gives essentia lly COx above 700 K and small amounts of benzene in the range 600-700 K. Th e reaction order is zero. The activation energy for the conversion of 12dcp was found to be 14.4 kcal/mol (60.6 kJ/mol). IR data show that a reversibl e adsorption step is followed by a nucleophilic substitution by an active s urface oxide species, giving rise to an alkoxide species. The slow step is likely the El elimination of the alkoxide. The conversion of the primary 3- chloro-1-propyl carbenium ion to a 1-chloro-2-propyl carbenium ion plays a relevant role. The catalyst deactivates progressively at 523 K, its activit y being nearly halved after 4 h, but it attains a plateau after 7 h. At hig her temperatures, the stable total conversion of 12dcp was obtained after 1 0 h.