Hydrodechlorination over Zeolite Supported Catalysts—Clarification of Reaction Mechanism—

Abstract

Chlorinated compounds emitted into the atmosphere are responsible for the current thinning and shrinking of the ozone layer in the stratosphere. Two main potential methods have been suggested for their transformation: oxidative destruction to form environmentally safe products or hydrodechlorination to form potentially valuable chemical compounds. In this review, performances of halogenated compounds over the zeolites and the noble metal-containing zeolite catalysts are characterized by different spectroscopic methods (IR, NMR, etc.) to clarify the reaction mechanism of hydrodechlorination over the catalysts. NMR studies provided evidence that chlorofl uorocarbon reacts more aggressively with zeolites at elevated temperatures. The extent and nature of reaction depended on the zeolite type and the exchanged cation. Infrared spectra of the gas phase showed that HCl, CO2 and COCl2 were detected as the reaction products in the hydrodechlorination reaction of CCl2F2 over HY. The phosgene intermediate is believed to be the key to the structural damage of the zeolite. Pt-containing zeolite catalyst was more active in the hydrodechlorination of carbon tetrachloride than the Pd-containing catalyst, and beside the main products (methane and HCl) chloroform as intermediate product was formed. On Pd-containing zeolite no chloroform, but high amounts of ethane as fi nal products together with methane were formed. The activity and selectivity of the hydrodechlorination reaction of various chlorine-containing compounds (C1, C2, with and without fl uorine content) on different metal-containing zeolites of various types is summarized.

Promoting Light Hydrocarbons Yield by Catalytic Hydrodechlorination of Residual Chloromethanes Using Palladium Supported on Zeolite Catalysts

“…As the NPs are isolated from others NPs due to their encapsulation on the zeolite cages and different channels, the catalytic activity can be improved, as well as the stability [20]. Due to this, zeolites are commonly used in liquid phase hydrodechlorination reactions [13,21,22], however, fewer studies have been applied to gas phase HDC [23][24][25][26]. In those gas-phase studies, conversion usually decreased after a few hours on stream, high selectivities to methane were obtained, and/or very low reactant inlet concentrations (below 100 ppm) were used [23,27,28].…”

“…Nevertheless, the stability of the catalysts was poor, and the conversion decreased down to 6% after a few hours on stream, despite chloroethane molecules being usually more reactive than chloromethanes in HDC reaction. Pd and Pt supported on zeolites were used as catalysts in the HDC of carbon tetrachloride and dichlorodifluoromethane [25,26]. Pd-zeolites achieved higher selectivities to C2-C3 hydrocarbons than the Pt ones.…”

“…It was concluded that the bond between carbon tetrachloride molecules and Pd atoms is stronger than with Pt. So, the probability of hydrogen insertion into the same carbon tetrachloride fragment is higher when using Pd than with Pt, resulting in more methane with Pt, and more ethane with Pd [26]. We have previously reported two studies about trichloromethane (TCM) conversion into C2-C3 hydrocarbons by catalytic hydrodechlorination with Pd supported on activated carbons obtained by the chemical activation of lignin [29,30].…”

Selectivity to Olefins in the Hydrodechlorination of Chloroform with Activated Carbon-Supported Palladium Catalysts

“…Other studies presented good results on the dechlorination of monochloromethane to olefins using molecular sieves like ZSM-5, HZSM-5, SAPO-34, and HSAPO-34 as catalysts, − but they suffered a rapid deactivation, which is a limiting factor for the application of this technology. The catalysts most commonly used in gas-phase HDC are composed of a metallic active phase (such as, Pt, Pd, and Rh) supported on different porous materials, such as activated carbons, zeolites, or zirconia, − among others. We recently studied the behavior of Pd, Pt, Rh, and Ru catalysts in the hydrodechlorination of chloromethanes to yield ethane and ethylene.…”

IR spectroscopic study of the hydrodechlorination of 2-chlorophenol over nobel metal containing zeolites

“…E-mail: hannus@chem.u-szeged.hu friendly ways. The Pt-and Pd-containing zeolites could be the real applicants as catalysts in these processes, if the lifetime of the catalyst is acceptable, and the spent catalyst can be regenerated [2].…”