Novel Fibrous Catalyst in the Decomposition of Airborne Chemical Agents at Moderate Temperatures
Volatile organic compounds (VOCs) are an important source of air pollution. The reduction of VOCs concentration in air, including their catalytic oxidation is becoming increasingly important, not only because of their toxicity, and typical or unpleasant odour, but also because of their environmental impact (smog, greenhouse gases, acid rain, etc). Many effective catalysts (mostly noble metals or metal oxides deposited on different supports) have been developed and tested for the catalytic decomposition of different classes of VOCs. The main disadvantage of these catalysts is the high working temperatures (most of them above 500 °C) and sometimes high operating pressures. The need of fast, efficient and economical decontamination technologies was also recognized in the security sector, especially for the use in the clean-up phase after a chemical, biological, radiological or nuclear (CBRN) incident (deliberate or accidental) that affects the quality of air and water. The aim of this research was to develop an alternative technology for the decomposition of airborne VOCs, based on the use of a novel heterogeneous modified polyacrylonitrile (PAN) fibrous catalyst, used as yarn or as knitted mesh, with or without the addition of H2O2 solution, and operating under mild conditions (moderate temperature and ambient pressure), contrary to most catalytic oxidation processes reported to date. The efficiency of the modified PAN catalyst was first tested in static mode, for the decomposition of a variety of chemical agents. This study provided the first comprehensive experimental evaluation of the potential use of the novel catalyst for gas phase reactions. A fixed bed catalytic reactor (44 cm3 volume) with temperature control and on-line analysis of gas samples was designed and constructed, enabling dynamic mode tests (at 45 °C and atmospheric pressure) on the efficiency of the fibrous catalyst in the decomposition of sulphur VOCs. The reaction products for the catalytic decomposition of ethyl mercaptan (EtSH) were identified and quantified and the conversion of reactant and turnover frequency for the catalyst were evaluated. The main product of this reaction was diethyldisulphide. It was determined that the physical form, but mainly the packing of the catalyst in reactor was responsible for differences in breakthrough time, which increased from 2.5 hours for catalyst used as threads to 29 hours for catalyst used as mesh. Further, a novel laboratory scale decontamination unit comprising of an absorption column and a three-phase (gas-liquid-solid) catalytic reactor with temperature control and the possibility of online analysis of gas samples was developed. The designed unit ensured the simultaneous and continuous presence of organic pollutant, fibrous catalyst and H2O2 solution in the system, being suitable for the decontamination of a continuous flow of polluted air, at moderate or ambient temperature and atmospheric pressure. The operating conditions were optimized in order to achieve the best performance for the unit. The efficiency of this system was evaluated for the decomposition of EtSH (93.3 mg/m3 air) at 45 °C and at ambient temperature, and for a mixture of EtSH (93.3 mg/m3 air) and dimethyl sulphide (DMS, 100 mg/m3 air) at ambient temperature. Ethane sulphonic acid was identified and quantified for the oxidation of EtSH both at 45 °C and at ambient temperature and the mixture of ethane and methane sulphonic acids was identified for the oxidation of the mixture of EtSH and DMS. The system was operated in steady state, with the main reaction products being the sulphonic acids. An amount of 20.1 g catalytic mesh (50% catalytic threads, [Fe] = 0.52 mmol Fe/g threads) was used in the presence of 100 mg/L H2O2 solution (2.4 mL/min) for a total of 1504 hours (about 62 days) as following: 415 hours in the decomposition of EtSH at 45 °C, 309 hours in the decomposition of EtSH at ambient temperature and 780 hours in the decomposition of the mixture of EtSH and DMS. A total of 45.12 L of air were treated, the total amount of VOCs passing through the unit was 252.67 mg EtSH and 140.4 mg DMS.
- PhD