Turnover Rate, Reaction Order, and Elementary Steps for the Hydrodechlorination of Chlorofluorocarbon Compounds on Palladium Catalysts

Abstract

The rates of hydrodechlorination catalyzed by Pd supported on carbon for four chlorofluorocarbons spanned a range of 7 orders of magnitude. The rates scaled up to the bond strength of the carbon−chlorine bond for the gas-phase reactant. This finding demonstrates that the rate-determining step involves the scission of the C−Cl bond and suggests, through Polanyi and linear free-energy relationships, that rates for other compounds can be estimated if the C−Cl bond strength is known. The reaction orders for the most abundant products are approximately first-order for the chlorine-containing compound, half-order in H2, and inverse first-order in HCl. The reaction steps consistent with these orders include a rate-determining step involving the adsorption of the chlorofluorocarbon to a single site (which could be a single surface palladium atom) and equilibrated steps between gas-phase H2, gas-phase HCl, and adsorbed hydrogen and chlorine atoms. The rates on the supported catalysts are comparable to the ones reported before on a Pd foil, indicating that the support does not play a role in the reaction. The product distribution is independent of conversion, implying that the various products are formed from a single visit of the reactant on the surface and not from readsorption of gas-phase products. The four compounds studied were chloropentafluoroethane (CF3−CF2Cl), 2-chloro-1,1,1,2-tetrafluoroethane (CF3−CFClH), 1,1-dichlorotetrafluoroethane (CF3−CFCl2), and 1,1,1-trichloro-2,2,2-trifluoroethane (CF3−CCl3).

Catalytic dechlorination of gas-phase perchloroethylene under mixed redox conditions

“…In the literature, two major reaction mechanisms are postulated for HDC reactions [32,52]: a sequential/serial mechanism and a concerted/parallel mechanism. A sequential mechanism can be represented as follows: (8) where * refers to an active site on the catalytic surface or an adsorbed species that is activated.…”

Modifier effects on Pt/C, Pd/C, and Raney-Ni catalysts in multiphase catalytic hydrogenation systems

“…The poisoning of the catalyst by HCl in the HDCl reactions is one of the main drawbacks in most HDCl systems [38][39][40]; however, under the multiphase conditions in the presence of onium salt (e.g. A336) we have not observed any pronounced rate inhibition due to this effect.…”

Chlorofluorocarbon dechlorination on Pd(111): effect of chlorine stoichiometry

“…For instance, on Pt/Al 2 O 3 catalysts the rate of hydrodechlorination of CCl 4 is 100 times faster than that of CH 3 Cl under identical conditions [8]. On supported Pd catalysts, a systematic study of the hydrodechlorination of four different CFCs reported by Thompson et al revealed that the ratio of the turnover rates for hydrodechlorination of CF 3 CCl 3 :CF 3 CFCl 2 :CF 3 CHFCl:CF 3 CF 2 Cl is 10 7 :10 3 :3:1 [5]. In that work a correlation was established between the C-Cl bond strengths of the four CFCs and the turnover rate.…”

“…A recent study suggested that the ratedetermining step involves C-Cl bond cleavage possibly via oxidative addition [4,5]. Although there is not complete agreement on the rate-determining step, previous studies have shown that rates of CFC dechlorination on the Pd(111) surface correlate quite well with rates of hydrodechlorination on Pd catalysts [6,7].…”

“…The agreement in the trend between rates of dechlorination and rates of catalytic hydrodechlorination implies that C-Cl cleavage must be kinetically significant in the hydrodechlorination reaction. Previous studies of the hydrodechlorination of CFCs over several catalysts have revealed that as the number of chlorine atoms attached to a carbon atom increases, the rate of hydrodechlorination also increases [4,5,7,8]. For instance, on Pt/Al 2 O 3 catalysts the rate of hydrodechlorination of CCl 4 is 100 times faster than that of CH 3 Cl under identical conditions [8].…”