Study of Catalytic Hydrodechlorination of Chlorophenols using Cocurrent Downflow Contactor Reactor

Abstract

Chlorophenols are toxic, hazardous and persistent organic pollutants. These compounds are widely used during production of insecticides, pesticides, herbicides etc. and are also originated from landfill sites, incineration and chlorination of water etc. The structure of chlorophenol consists on a benzene ring, hydroxyl ion and chlorine atom(s) ranging from one to five. These chemicals are on the priority list of hazardous substances and strong precursors of known mutagens and carcinogens i.e. polychlorinated dibenzo dioxins and polychlorinated dibenzo furans. Toxicity of chlorophenols increases with degree of chlorination on phenol and affects all layers of atmosphere, the ecosystem and ecological relationships. Cohort studies confirmed that exposure to chlorophenols, resulted in higher risk of development of inter alia following diseases i.e. cardiovascular diseases, mesenchymal tumor, non-Hodgkin lymphoma, embryonic palatal mesenchymal and leukemia etc. A number of different techniques have been evolved during course of time for degradation of organohalides and most significant and widely used are electrochemical, biological treatment, advanced oxidation process and catalytic hydrodechlorination. Catalytic hydrodechlorination is a promising environmental friendly technique where through hydrogenation of C – Cl bond, highly toxic compounds are converted into lesser or nontoxic counterparts. In comparison to other techniques, the catalytic hydrodechlorination offers, lower energy costs, mild operating conditions, treatment of higher concentration of pollutants, non-production of more toxic intermediates due to sub stoichiometric quantities of reactants, flexible mechanism for monitoring of effluents and zero formation of dioxins and furans. Type and performance of any reactor plays a very fundamental role during a reaction. Role of chemical reactor has been very scarcely addressed in earlier large number of studies using catalytic hydrodechlorination technique. Continuous stirred tank reactor has largely been used during such studies. As compared to conventional reactors, bubble column reactors provide, attractive heat and mass transfer rates, facilitation while processing of high throughputs and capacities, a straightforward control and ease of operations. Cocurrent downflow contactor reactor, a bubble column reactor, has evolved as a promising intervention for 5 hydrogenation, biochemical reactions and depollution processes. Catalytic hydrodechlorination reactions for chlorophenols had been carried out in the cocurrent downflow contactor with 5 % Palladium/carbon catalyst. 2,4-dichlorophenol as a model pollutant was selected as it is being produced in largest quantities as compared to other chlorophenols. 2,4-dichlorophenol was degraded in the reactor using hydrogen gas under mild reaction conditions of temperature and pressure. Reaction of 2,4-dichlorophenol proceeded in a step wise mechanism through production of 2-chlorophenol and 4-chlorophenol as intermediate products and phenol and cyclohexanone as an end products. Optimum range of operating parameters for the reaction including temperature, pressure, initial concentration and catalyst loading was evaluated. The energy of activation for catalytic hydrodechlorination of 2,4-dichlorophenol was 43 KJ mol-1 indicating that reaction occurred under surface reaction rate controlled conditions with negligible mass transfer resistances. The reactor substantially degraded highly toxic 2,4-dichlorophenol (having acute oral LD50 of 580 mg kg-1 of body weight of rat) into lesser or nontoxic compounds like phenol (with LD50 of 650 mg kg-1 of body weight of rat) and cyclohexanone (with LD50 of 2375 mg kg-1 of rat). The rate of catalytic hydrodechlorination reaction invariably depends on many factors including the nature of base, solvent and hydrogen source. The efficacy of this reaction in cocurrent downflow contactor reactor was evaluated by using different bases, mixtures of solvent and hydrogen source. The catalytic hydrodechlorination reaction worked well with organic, inorganic and even in the absence of base in cocurrent downflow contactor reactor. Reaction was completed with faster reaction rate in the presence of potassium hydroxide. Sodium acetate trihydrate had lowest rate of reaction and potassium hydroxide had the highest rate of reaction whereas ammonium hydroxide, sodium hydroxide and triethylamine had intermediate reaction rates. 100 % water as a solvent produced highest rate of hydrodechlorination of 2,4-dichlorophenol in the range of 0.0815 min-1 and initial rate of reaction 2.405 x 10-4. Lowest hydrodechlorination rate and initial rate of reaction was observed for 12 % toluene and 88 % water mixture i.e. 0.0077 min-1 and 7.965 x 10-6 respectively. Liquid hydrogen donor compounds like formic acid and isopropanol have not demonstrated higher rate of reaction for treatment of 2,4-dichlorophenol as compared to hydrogen gas. During hydrodechlorination of 2,4-dichlorophenol in cocurrent downflow contactor reactor, 6 the hydrogen sources followed the sequence i.e. molecular hydrogen > formic acid > isopropanol. The hydrodechlorination rate (min-1) for molecular hydrogen, formic acid and isopropanol was 0.0815, 0.0110 and 0.0016 respectively. Cocurrent downflow contactor reactor was used for treatment of higher initial concentration of 2,4-dichlorophenol. In 180 min, 30 mmol dm-3 and 47.5 mmol dm-3 of 2,4-dichlorophenol was degraded to 95 % and 85 % respectively. This conversion value is much greater than the continuous stirred tank reactor. Cocurrent downflow contactor reactor has appreciably 100 % degraded, 2.53 mmol of 2,4,6-trichlorophenol in 45 min with 5% Pd/C using catalyst loading of 0.2 g dm-3, at 0.1 MPa and 303 K. This reactor has shown better results during catalytic hydrodechlorination of higher initial concentrations of 2,4-dichlorophenol and 2,4,6-trichlorophenol at mild reaction conditions of temperature and pressure.