Howard L. Greene

DECOMPOSITION CHARACTERISTICS AND REACTION MECHANISMS OF METHYLENE CHLORIDE AND CARBON TETRACHLORIDE USING METAL-LOADED ZEOLITE CATALYSTS

Abstract The catalytic activities and selectivities of four zeolite-Y catalysts (HY, CoY, NaY and Co-z.sbnd;Y/CA) for the oxidation of methylene chloride and carbon tetrachloride were compared. Reactor experiments were carried out in a fixed bed reactor with temperatures ranging from 150 to 350°C and a space velocity of 2400 h−1, at atmospheric pressure. Other catalyst characteristics, including oxygen adsorption capacities, surface area, acidity and catalyst composition were measured and compared. The CoY catalyst showed excellent activity compared to the other catalysts with complete conversion of both feeds at temperatures as low as 200°C, and therefore was used as a model catalyst for reaction mechanistic investigations. Variable space velocity reactor runs (100 to 47000 h−1, were conducted at 350°C with this catalyst to distinguish series/parallel reaction mechanisms. It was found that CO was the predominant deep oxidation product (> 95% selectivity) and that the formations of CO and CO2 occurred by parallel reactions in the oxidation of methylene chloride. Also, during the oxidation of carbon tetrachloride, phosgene was found to be a reaction intermediate. To elucidate the surface reaction mechanisms, in situ FTIR experiments were performed using a transmission reaction cell at 300°C. The results indicated that the CVOCs adsorbed on the Bronsted acid sites of the zeolite. The FTIR results also suggested the formation of an unstable intermediate (COHCI) during the oxidation of methylene chloride. The formation of phosgene as a reaction intermediate during the formation of CO2 was observed for the oxidation of carbon tetrachloride, consistent with the variable space velocity experiments. Based on these results and the current literature, deep oxidation reaction mechanisms have been proposed for these two systems.

Comparison of modified transition metal-exchanged zeolite catalysts for oxidation of chlorinated hydrocarbons

Complete catalytic oxidation of low-molecular-weight chlorinated hydrocarbons, such as methylene chloride, trichloroethylene, and carbon tetrachloride, was studied in air over several cation-exchanged Y zeolite catalysts. The reactions were carried out under atmospheric pressure with temperatures varying from 150 to 400°C and in presence of ≌13000 ppm of water. Primarily, three different cordierite-supported (washcoated) zeolite catalysts (CoY, CrY, and MnY) were prepared and their activities and selectivities were investigated for the above reactions, with CoY appearing to be superior. Although complete conversions of methylene chloride and carbon tetrachloride could be obtained at temperatures of 350 and 200°C, respectively, no significant conversion of trichloroethylene was noted at temperatures below 400°C. Incorporation of a transition metal oxide within the zeolite matrix by Cr2O3 impregnation of the cation-exchanged zeolites produced a substantial improvement in trichloroethylene conversion with over 90% destruction obtained at only 325°C. An unsupported chromia-impregnated 116-in. pelletized CoY catalyst showed even better activity results for chlorocarbon oxidation than the supported catalysts. No detectable partial oxidation products were noted in the product spectra, with HCl, CO, and CO2 being the only major products. Catalytic behavior was explained in the view of catalyst composition and properties such as acidity and oxygen adsorption capacity. Probable mechanistic details based on the activity and selectivity data were suggested.

Adsorption and catalytic destruction of trichloroethylene in hydrophobic zeolites

Several chromium exchanged ZSM-5 zeolites of varyingSiO2/Al2O3 ratio were prepared and investigated ambient (23°C) adsorption and subsequent oxidative destruction (250–400°C) of gaseous trichloroethylene (TCE, Cl2C=CHCl) in a humid air stream. With an increase in theSiO2/Al2O3 ratio from 30 to 120, the TCE saturation capacity of these dual-function sorbent/catalyst (S/C) media was found to increase from 6.0 to 10.1 wt% in a humid air stream. This phenomenon was attributed to an increase in hydrophobicity coupled with reduced steric hindrance and site competition for the adsorption of TCE molecules in the competitive adsorption of TCE and water. Ambient TCE adsorption experiments carried out in dry air showed the same trend, which was attributed to increasing organophilicity of the S/C media with an increase in the SiO2/Al2O3 ratio. In order to gain knowledge of physisorption sites for TCE molecules in the ZSM-5 structure, temperature-programmed desorption over a temperature range of 30–300°C and in-situ FT-IR studies at ambient conditions were also carried out. These studies revealed that in all zeolites (except for Cr-ZSM-5 withSiO2/Al2O3 ratio of 120) TCE interacted with terminal silanol (SiOH) and AlOH groups. At temperatures ≥ 300°C (with the exception of Cr-ZSM-5 withSiO2/Al2O3 ratio of 120), allS/C media showed >95% TCE destruction efficiency. Based on its high adsorption capacity and high activity for oxidative destruction of TCE, it is concluded that the Cr-ZSM-5S/C medium with aSiO2/Al2O3 ratio of 80 gives preferred performance both as a sorbent and a catalyst.

Oxidative destruction of chlorofluorocarbons (CFC11 and CFC12) by zeolite catalysts

Abstract The catalytic oxidations of CFC1 I and CFC12 were studied over four Y-zeolite catalysts (HY, CoY, CeY, and CrY) in a fixed-bed reactor at temperatures ranging from 150 to 400°C and space velocity of 10,500 h−1. Initial oxidation activities and selectivities for each catalyst were measured and compared. Two catalysts (HY and CrY) were investigated further at a temperature of 300°C and space velocity of 5000 h−1 for longer term stability-deactivation characteristics. Results showed initial activities and selectivities of all four catalysts to be comparatively similar, with substantially complete conversion (>90%) obtained at temperatures >-/ 250°C for CFC1 I and >-/ 400°C for CFC12. Initial selectivity at higher temperatures (>-/ 250°C) was predominantly to CO2; F2 and Cl, were largely adsorbed/reacted on the catalysts and could not be quantified. The longer term catalyst deactivation tests carried out on HY and CrY showed a drop in catalytic activity and deep oxidation selectivity (partial oxidation products detected were CCl4, COCI,, and other CFCs) with time due to the deactivation of the catalysts. The CrY catalyst was found to be much more resistant than the HY, probably because the Cr3+ cations were able to catalyze and subsequently desorb a substantial portion of the corrosive halogens that were formed.

Decomposition characteristics of chlorinated ethylenes on metal loaded zeolite Y and γ-Al2O3

Complete catalytic oxidation of 1075 ppm of vinyl chloride, trichloroethylene and perchloroethylene in humid (∼11 500 ppm water) air was studied over the temperature range of 250 to 400°C. Three different catalysts, chromium exchanged zeolite Y (CrY), cobalt exchanged zeolite Y (CoY) and cobalt impregnated γ-Al2O3 (Co/Al2O3) were prepared and their activities investigated for the above reaction. Conversions varied from 1.6 to 100% with catalytic activities decreasing in the order: CrY > CoY>Co/Al2O3. The adsorption capacity of the reactants also followed the same trend. The ability of chromium cation to exhibit multiple oxidation states coupled with its high ionization potential probably led to the higher activity of CrY. The catalytic activity was found to diminish with increasing chlorine content of the reactant. Therefore, the catalytic activity decreased in the order: vinyl chloride > trichloroethylene > perchloroethylene. This phenomenon was attributed to the inductive and resonance effects of chlorine atom(s) on the electron density at the CC bond. The zeolitic support produced higher activity than non-zeolitic because of the differences in the cation dispersion, acidity and redox potential of the cation.

Deactivation of metal exchanged zeolite catalysts during exposure to chlorinated hydrocarbons under oxidizing conditions

Abstract Deactivation of modified cation exchanged zeolite catalysts was studied during complete oxidation of methylene chloride, trichloroethylene and carbon tetrachloride over a temperature range of 175 to 400°C. Coking was found to be the cause of deactivation. However, the catalysts could be completely regenerated by oxidation in air at 450°C. Two different formulations of modified cobalt exchanged Y zeolite catalysts were tested to determine the coking and deactivation rates. Increased cation content increased deactivation and coking over a period of about 1000 hours. Changing the type of zeolite from larger pore Y to medium pore mordenite increased deactivation. The type of chlorinated feed also affected coking and deactivation with the rate of deactivation increasing in the order of trichloroethylene > methylene chloride > carbon tetrachloride. Both coking and deactivation increased with decreasing temperature. Higher space velocity produced more deactivation for trichloroethylene oxidation at 275°C. Based on these results a mechanism for coking is proposed with CO as the possible reaction intermediate that leads to the formation of coke.

Effects of catalyst composition on dual site zeolite catalysts used in chlorinated hydrocarbon oxidation

Abstract Cobalt exchanged and chromia impregnated Y zeolite catalysts (Co-Y/CA ) have been found to show good activity and selectivity for low-molecular-weight chlorohydrocarbon (CH 2 Cl 2 , C 2 HCl 3 , CCl 4 , etc.) oxidation in the range of 250–350°C. For such dual site catalysts, the effects of cobalt and chromia composition on the catalytic properties were investigated. Higher cobalt exchange increased the acidity and the oxygen adsorption capacity of the Co-Y zeolite without affecting the surface area, whereas increasing Cr 2 O 3 impregnation caused increased dealumination and consequent loss of acidity. Higher loading of Cr 2 O 3 also reduced catalyst surface area by combination of structural loss and pore/channel blockage. Increasing cobalt exchange produced improved conversion of methylene chloride (CH 2 Cl 2 ) while higher loading of impregnated Cr 2 O 3 was detrimental for methylene chloride conversion due to the associated loss of surface area and acidity. Methylene chloride conversion appeared to be primarily controlled by carbonium ion formation at the acidic sites followed by oxidation with the oxygen adsorbed at the cationic sites. Conversely, trichloroethylene (C 2 HCl 3 ) oxidation was primarily determined by the impregnated Cr 2 O 3 sites; low or no Cr 2 O 3 produced poor conversion while increasing Cr 2 O 3 loading increased the conversion. However, an optimum impregnation level (⋍5%) seemed to be necessary to avoid poor activity either due to lack of sites or due to structural damage by overloading. Additionally, increasing cobalt exchange level in the Co-Y/CA catalysts also showed improved TCE conversion probably due to the increasing acidity. Therefore, TCE conversion seemed to be controlled by the initial adsorption at the Cr 2 O 3 sites with subsequent reactions which may involve the acidic and/or cationic sites.

COMBINED SORBENT/CATALYST MEDIA FOR DESTRUCTION OF HALOGENATED VOCS

Abstract Several chromium modified zeolites have been developed and tested for their ability to physisorb chlorinated VOCs (CVOCs) at ambient and then catalytically destroy them at elevated temperatures (ca. 300°C). These dual function materials, which act as both sorbents and catalysts, are believed to be the key to implementing a new energy efficient process for the destruction of low concentration level CVOC streams. Data showing catalytic activity, sorptivity and other physical properties for Y and ZSM-5 zeolites based media are presented. A chromium exchanged ZSM-5 (Cr-ZSM-5) medium which showed superior performance catalytically (> 95% TCE and MeCl2 destruction at 300°C) and adequate sorption capacity (0.074 g TCE/g sorbent, 0.064 g MeCl2/g sorbent at 23°C in the presence of humid air) was chosen for subsequent dual function tests. These tests showed that ambient temperature fixed bed sorption of CVOCs followed by periodic heating of the upper portion of the bed to catalytic temperatures (ca. 35°C) with slow temperature ramping of the lower portion to desorb trapped CVOCs, produced a highly energy efficient cyclic process for their destruction. A Cr-ZSM-5 bed, which alternately stored and then destroyed CVOC from a humid 110 ppm TCE stream at a space velocity of 2400 h−1, was shown to operate over a 1460 min cycle which required heating for only 7% of the time.

Energy efficient dual-function sorbent/catalyst media for chlorinated VOC destruction

Abstract Several exchanged zeolites (modified Y and ZSM-5) have been developed which are capable of both ambient sorption and elevated temperature catalytic destruction of common chlorinated CVOCs giving rise to an energy efficient process for first storing and later destroying environmentally sensitive solvents.

Kinetic modeling for the catalytic oxidation of benzo(a)pyrene

Abstract A microcatalytic flow reactor system was used to elucidate the kinetics of benzo( a )pyrene (BAP) oxidation over a mixed vanadium pentoxide-molybdenum oxide catalyst. Experimental conditions covered the following ranges: reaction temperature 275–345 °C; BAP concentration 0.002-0.05 g mol/m 3 ; and oxygen concentration 4.0–26.0 g mol/m 3 . Current statistical methodology was utilized in conjunction with the data obtained to discriminate among rival kinetic models, as well as to provide model parameter estimates. Results show that the oxidation reaction follows a classical redox mechanism.