Theoretical and Applied Aspects of Hydrodechlorination Processes—Catalysts and Technologies

Abstract

The commercial implementation of hydrodechlorination processes will be an essential step in resolving the problem of environmentally sound organochlorine wastes processing. By now, there is a number of fundamental, applied, and process elaborations of such processes, in which chlorine is almost completely removed from wastes. The review article contains a significant number of results including basic regularities of thermal hydrodechlorination, comprehensive and selective catalytic hydrodechlorination. It discusses thermodynamics, kinetics, and catalysts of gas and liquid phase processes. Considerable attention is paid to hydrodechlorination of vinyl chloride production wastes and utilization of tetrachloromethane, which is the ozone-depleting substance. It also discusses hydrodechlorination of mono-and (poly)chlorobenzenes. The important examples of liquid phase data include hydrogenation using complex hydrides of elements. It also includes several flow sheets of hydrodechlorination processes.Various methods for C-Cl bonds reduction in non-cyclic organochlorine compounds can be divided into the following groups:Catalysts 2020, 10, 216 3 of 52 one should note the reviews [4,10] published in the 1990s. It is noted that catalytic methods are more widely used for hydrodechlorination. Electrochemical ReductionElectrochemical reduction is a complex process in which the speed and direction of reactions depend on many factors. It is characterized by the following features. Two electrons are spent for breaking the C-Cl bond; the half-wave potential in most cases does not depend on the pH of the medium; in general, the electrode reaction is irreversible [1]. There is an approximate linear relationship between the half-wave potential of the reactionsIn [12], a scheme was proposed describing the mechanism of electrochemical reduction of the C-Cl bondRClHowever, there is another point of view, according to which there is a simultaneous transfer of a pair of electrons, similar to that for nucleophilic substitution reactions [1].Electrochemical reduction is carried out on cathodes of copper, zinc, amalgamated zinc, palladium, cadmium, mercury, and lead [13]. The cathodic dechlorination proceeds in an alkaline environment more efficiently than in an acidic one; however, in most cases, acidic electrolytes are used to avoid such side reactions as saponification [13,14]. The recovery is usually carried out in inert solvents, in particular in alcohols. In [13], the influence of the reduction conditions-the cathode material, current density, the nature of electrolyte and solvent, the concentration of the reducible substance, and temperature-on the course of the reaction is described.The recovery of monochlorohydrocarbons is difficult and results mainly in the replacement of chlorine by hydrogen in accordance with the overall equationIn some cases, dimeric compounds (R-R) are formed according to the schemeThe recovery of vinyl chlorine is difficult, and that of the allyl one is facilitated in comparison with chlorine in saturated com...

Two Scenarios of Dechlorination of the Chlorinated Hydrocarbons over Nickel-Alumina Catalyst

“…The value of 1,2-DCE conversion under these conditions was 90%. The obtained results are in good agreement with the literature data described for similar conditions [9].…”

“…The value of 1,2-DCE conversion under these conditions was 90%. The obtained results are in good agreement with the literature data described for similar conditions [9]. It should be noted that these results were obtained at a contact time (τ) of 0.4 s. It was found experimentally that this value was close to optimum: both the increase and reduction of the contact time resulted in a decrease in the ethylene yield.…”

“…There are a lot of different methods used for the abatement of the chlorinated organic compounds, such as high-temperature thermal incineration, catalytic combustion, electrochemical reduction, and radiolysis, thermal and catalytic hydrodechlorination, etc. Catalytic hydrodechlorination still attracts much interest due to the comparatively mild conditions of the process, absence of highly toxic by-products such as phosgene and dioxins, and the opportunity to adjust the selectivity towards the desired valuable products [8,9]. In addition, catalytic gas-phase hydrodechlorination can achieve complete dechlorination of the ClHCs and waste, which makes it possible to return the released olefins and HCl into the manufacturing cycle or utilize the obtained mixed hydrocarbons as a fuel [9].…”

Pd/Alumina Catalysts for Beneficial Transformation of Harmful Freon R-22

“…Huge stocks of R-22 must be destroyed or, preferably, converted to other valuable chemicals. The hydrodechlorination (HDC), a catalytic exchange of chlorine by hydrogen appears to be one of the most prospective technologies for the beneficial utilization of harmful chlorine-containing wastes by transforming them into useful, nontoxic products [6][7][8][9][10][11][12][13].…”

The Effect of Shape-Controlled Pt and Pd Nanoparticles on Selective Catalytic Hydrodechlorination of Trichloroethylene

“…In this respect, catalytic hydrodechlorination (HDCl) is one of the most suitable reactions, involving noble metals such as platinum or palladium converting chlorinated organic compounds to fine chemicals [21][22][23][24][25][26][27][28][29]. The selectivity and stability of the catalysts are the key features and many groups have devoted significant efforts on these topics [30,31].…”