Dehalogenation of 1,1,2-Trichloro-1-fluoroethane over r-Cr 2 O 3 (101 h2) Steven C. York and David F. Cox* Department of Chemical Engineering, Virginia Polytechnic Institute & State UniVersity, Blacksburg, Virginia 24061 ReceiVed: October 1, 2002; In Final Form: March 31, 2003 The reaction of CFCl 2 CH 2 Cl over the stoichiometric Cr 2 O 3 (101 h2) surface yields CFCldCH 2 , HCtCH, and surface halogen. The 1,2-dihalo-elimination reaction to CFCldCH 2 is initiated via C-Cl bond cleavage at the CFCl 2 -end of the molecule to give a -CFClCH 2 Cl haloalkyl surface intermediate. A rate-limiting -chlorine elimination from the surface alkyl gives rise to the CFCldCH 2 product. Acetylene is formed by the subsequent reaction of CFCldCH 2 in series. The chlorine liberated from CFCl 2 CH 2 Cl binds at the five-coordinate surface Cr 3+ sites on the stoichiometric surface and shuts down the dehalogenation chemistry by site blocking. No carbon buildup is observed on deactivated surfaces, and no evidence is seen for the replacement of surface lattice oxygen by halogen under the conditions of this study. At elevated temperatures, the thermal removal of surface chlorine is observed, and it is attributed to migration into the sample bulk. I. Introduction The reactions of halogen-substituted alkanes with chromia have been much studied because of their relevance to the vapor- phase manufacture of hydrofluorocarbon compounds (environ- mentally friendly refrigerants with low ozone-depleting poten- tial) and because of interest in the destruction and conversion of chlorine-containing hydrochlorofluorocarbon (HCFC) ana- logues. 1-3 Typically, these reactions are conducted in the presence of HF where fluorination (for example, fluorine-for- chlorine exchange) is an expected result. The halocarbon reactions most commonly reported over Cr 2 O 3 are summarized in Table 1. Rowley, Webb, Winfield, and co-workers 4-7 have studied the interaction of halogenated compounds with Cr 2 O 3 microcrys- talline powders using radiolabeled HF, HCl, and halocarbon compounds. They unambiguously demonstrated the uptake of halogen by Cr 2 O 3 during the initial “activation” phase of reaction, and found that 36 Cl and 18 F could be deposited on both untreated and pre-halogenated Cr 2 O 3 catalysts by exposure to HX (X ) halogen) or halocarbon compounds at 623 K. It was also reported that Cl and F are capable of replacing one another on the surface, and that halogen exchange occurs between the surface and various CFC compounds. 4,6,7 Direct halogen ex- change with surface (HF) n -HX oligomers was also proposed. 4,6 The resulting halogen exchange sequences yield a mix of halocarbon compounds over both untreated and pretreated (halogenated) Cr 2 O 3 catalyst. In studies of the trichlorotrifluoroethane/Cr 2 O 3 system, Blan- chard and co-workers focused on the disproportionation and isomerization of CFCl 2 CF 2 Cl to CCl 3 CF 3 over Cr 2 O 3 powder, supported Cr 2 O 3 , and AlF 3 . 8-10 They found that disproportion- ation and isomerization reactions dominate over Cr 2 O 3 in the absence of HF. They also observed that pretreating the surface with pyridine, a Lewis base, inhibits surface activity, indicating that Lewis acid sites (cation centers) are important to the reaction mechanism. 8 The presence of small amounts of the unsaturated compounds CCl 2 dCCl 2 and CFCldCCl 2 were also reported in the product mix. Later work from Blanchard and co-workers focused a broader spectrum of halocarbon surface reactions including dispropor- tionation, isomerization, dehalogenation, dehydrohalogenation, and halogen exchange. 11 They reported that halogen exchange dominates in the presence of HF, but that disproportionation, isomerization, and dehydrochlorination occurs without HF. It was suggested that fluorination, in the presence of HF, was governed by the acidity of the catalyst, 11 and that the catalytic activity for the fluorination of CF 3 CH 2 Cl to CF 3 CH 2 F is a function of the number of reversibly oxidizable chromium sites on the catalyst surface. 12,13 XPS measurements of fluorine uptake by the catalyst following exposures to HF and CF 3 CH 2 Cl yielded a maximum F/Cr ratio of 0.36, regardless of the treatment gas used. 13 Other authors have postulated that the oxidation state of surface ions plays a crucial role in determining activity. On the basis of product distributions and thermal desorption data, Coulson et al. 14 propose that the reduction of high-valent chromium ions to Cr 3+ is necessary to create active sites for the disproportionation of CHF 2 Cl over powdered Cr 2 O 3 micro- crystalline catalyst. They argue that the active site is a coordinately unsaturated Cr-X center, which acts as a strong Lewis acid. Following exposures at 773 K, Coulson and co- workers 14 report XPS atomic percentages that correspond to F/Cr ratios of 0.22 and 0.30 for CHF 3 and HF, respectively. Kohne and Kemnitz have studied the reactions of several halocarbons with chromia, and provided evidence that both direct halogen exchange and HX abstraction/addition reactions * Corresponding author. Fax: (540) 231-5022. E-mail: dfcox@vt.edu. TABLE 1: Halocarbon Reactions Reported To Occur over Cr 2 O 3 Observed Reactions Reaction Conditions HF addition to CdC double bond Dominant reaction in the presence of HF Halogen exchange Replacement of Cl by F is usually the desired reaction and is favored in the presence of HF. Dehydrohalogenation (HF and HCl elimination) Reaction may occur in the presence and absence of HF. Disproportionation (dismutation) Presence of HF not required. Isomerization Presence of HF not required. 5182 J. Phys. Chem. B 2003, 107, 5182-5189 10.1021/jp022172j CCC: $25.00 © 2003 American Chemical Society Published on Web 05/13/2003