Chlorine Cycles

Subsequent photolysis of Cl2 and HOC1, respectively, brings Cl back ready to destroy ozone. At this altitude range, the most important chlorine cycle responsible for 60% to 75% of the ozone loss is... 

Scheme 7.1 The chlorine cycle for DMC synthesis via the phosgene route.

Oberg G (2002) The Natural Chlorine Cycle - Fitting the Scattered Pieces. Appl Microbiol Biotechnol 58 565... 

Oberg G, Holm M, Sanden P, Svensson T, Parikka M (2005) The Role of Organic-Matter-Bound Chlorine in the Chlorine Cycle A Case Study of the Stubbetorp Catchment, Sweden. Biogeochem 75 241... 

Naterer, G.F., et al. (2008), Thermochemical Hydrogen Production with a Copper-chlorine Cycle, I Oxygen Release from Copper Oxychloride Decomposition , International Journal of Hydrogen Energy, 33, 5439-5450. 

Orhan, M., I. Dincer, M.A. Rosen (2008), Energy and Exergy Assessments of the Hydrogen Production Step of a Copper-chlorine Cycle Driven by Nuclear-based Heat , International Journal of Hydrogen Energy,... 

Figure 7. The proposed chlorination cycle for alkenes hy Mn (IV) (salpn) Cl2.

Some economy can be gained in the chlorine cycle by using method 7 for preparing the methyl chloride from by-product hydrochloric acid coming out of the process. This could be accomplished by a catalyzed reaction of the concentrated acid with methanol2 and would eliminate half of the requirement of free chlorine given in equation 11. The rest of the free chlorine still is necessary for reaction with the silicon, however. The amended summary of the process then becomes... 

An important aspect of stratospheric bromine chemistry is the possibility of synergistic interactions between bromine and chlorine cycles via the following reaction,... 

Philippot P., Agrinier P., and Scambelluri M. (1998) Chlorine cycling during subduction of altered oceanic crust. Earth Planet. Sci. Lett. 161, 33—44. 

Figure 3 A conceptual model of the chlorine cycle in soils, with dominant processes indicated (after Oherg, 2002).

The existence of a chlorine cycle and the scattered evidence of biogeochemical cycles for halogenated hydrocarbons involve a wide range of environmentally relevant reaction mechanisms and pathways leading to their widespread distribution and matrix-dependent profiles. The extent to which biotic and abiotic reactions influence the chlorine (halogen) cycle depends on complex interactions between the intrinsic molecular properties of these compounds and characteristics of the environment. 

Oberg G. (2002) The natural chlorine cycle—fitting the scattered pieces. Appl. Microbiol. Biotechnol. 58, 565-581. 

Reductive carbonylation of nitro compounds, especially nitroaromatic compounds according to eq. (1), has been the subject of thorough industrial research starting in 1962 and continuing until the beginning of the 1990s due to the demand for a new, phosgene-free method for the production of isocyanates [1] and the discussions on the chlorine cycle in industry. 

Fluorine chemistry in the stratosphere has also been considered and attention has been drawn to the atmospheric chemistry of the FOx radicals. The compounds with O-F bonds have gained interest in connection with the ozone depletion problem. It has been suggested that FO and F02 radicals formed in the atmospheric degradation of hydrofluorocarbons (HFCs) could destroy ozone in chain reaction processes. Experimental studies of this hypothesis led to the conclusion that catalytic cycles involving F, FO, and F02 are irrelevant with respect to the chlorine cycle.8 However, kinetic investigations of the reactions of fluorine atoms with 02 and NOx provide useful information on the fluorine chemistry in the polluted atmosphere. 

DMC is classified as a non-toxic and environmentally compatible chemical [69]. In addition, the photochemical ozone creation potential of DMC is the lowest among common VOCs (2.5 ethylene = 100). The areas in which DMC acts, or can act, as a potential phosgene substitute correspond to the main areas of phosgene industrial exploitation, that is, production of aromatic polycarbonates and isocyanates, leading the production of these important chemicals out of the chlorine cycle. 

In particular iodine-sulphur reaction results quite attractive [68]. It consists of three steps at different operation temperatures, which involve the H2SO4 and HI dissociation and the re-production of both acids starting from I, SO2 and H2O. Particular interest is also focused on CeO2/Ce2O3 cycle, cerium-chlorine cycle (Ce-Cl), Zinc-zinc-oxide cycle (Zn/ZnO), but also on a Cu-Cl cycle, which is a cycle with an electrochemical step [69]. 

Table III. Catalytic Chlorine Cycle in Venus atmosphere...

It turns out that the HO NO, and CIO, cycles are all coupled to one another, and their interrelationships strongly govern stratospheric ozone chemistry. The NO, and CIO, cycles are coupled by reactions 4.34 and 4.35. For example, increased emissions of N2O would lead to increased stratospheric concentrations of NO and hence increased ozone depletion by the NO, catalytic cycle. Likewise, increasing CFC levels will lead to increased ozone depletion by the CIO, cycle. However, increased NO, will lead to an increased level of the CIONO2 reservoir and a mitigation of the chlorine cycle. Thus the net effect on ozone of si-... 

Another recyclable chlorinating reagent, 3-(dichloroiodo)benzoic acid (60), can be conveniently prepared by the chlorination of commercially available 3-iodobenzoic acid (59). A reduced form of reagent 60, 3-iodobenzoic acid (59), can also be easily separated as a solid from the products of chlorination by basic aqueous work-up followed by acidification with HCl [68,69]. Scheme 5.24 shows an example of a chlorination cycle using reagent 60 [69]. Alternatively, 59 can be separated from the reaction products by treatment with anionic exchange resin Amberlite IRA 900 (Section 5.3.2). 

Fig. 2.29 The global inorganic chlorine cycle (data from Moller 1990, 2003). S - source, W - wet deposition, D - dry deposition.

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