Deactivation of supported platinum catalysts for the hydrodechlorination of 1,1,1-trichloroethane

One barrier to the development of a catalytic hydrodechlorination (HDC) process is rapid deactivation. To understand the deactivation associated with catalytic HDC, fixed bed microreactor tests were carried out using a bed divided into three segments separated by glass wool plugs. Standard operating conditions were 523 K, atmospheric pressure, {dollar}rm Hsb2/111{dollar} TCA/He ratio of 10/1/89, and a space velocity of 24 L/gcat-h.; Experiments were carried out on {dollar}etadelta{dollar}-alumina and {dollar}alpha{dollar}-alumina support with and without Pt and on {dollar}rm Pt/eta{dollar}-alumina. Results showed that Pt was necessary for the removal of the three Cl atoms from 1,1,1-trichloroethane (111 TCA). 1,1-Dichloroethylene (11 DCE) was the only product with the {dollar}etadelta{dollar}-alumina support. Ethane was formed with Pt added to this support. However, the conversion of 111 TCA decreased more rapidly for {dollar}rm Pt/etadelta{dollar}-alumina than for {dollar}rm etadelta{dollar}-alumina. A comparison of the deactivated {dollar}rm Pt/etadelta{dollar}-alumina catalyst to the deactivated {dollar}etadelta{dollar}-alumina support showed that large quantities of coke were only formed on the Pt catalyst.; A conceptual reaction scheme was hypothesized to explain the cause of catalyst deactivation. For the Pt catalysts, it is proposed that the Pt metal catalyzes the polymerization of an adsorbed chlorinated ethyl species {dollar}rm (sp* Csb2Hsb3Clsb2){dollar} and that the coke migrates onto the support, or the Pt metal initiates the polymerization of {dollar}rmsp* Csb2Hsb3Clsb2{dollar} on the support. The formation of {dollar}rmsp* Csb2Hsb3Clsb2{dollar} occurs by the elimination of one chlorine atom from 111 TCA; as suggested by Bozzelli et al. (12) for the HDC of 1,2-dichloroethane (12 DCA) over a {dollar}rm Rh/SiOsb2{dollar} catalyst. {dollar}rmsp* Csb2Hsb3Clsb2{dollar} is the proposed coke precursor because at standard conditions large quantities of coke were only observed for the HDC of 111 TCA, the Pt catalyst was apparently stable for the HDC of 11 DCE, and 111 TCA is a saturated compound and unlikely to polymerize.; A mathematical model was developed for a simplified conceptual reaction network. In the mathematical model it is proposed that a reaction intermediate (11 DCE) was the coke precursor. In this model, it is suggested that 11 DCE can either polymerize to form coke, desorb off the catalyst, or react further to form ethane. The model is useful for predicting the concentration and coke content profile in the reactor bed with respect to time.