Chlorofluorocarbons reduction

Global hazards can be mitigated through targeted molecular design. Uncovering the mechanism of ozone reduction by chlorine, and specifically by chlorofluorocarbons, has led to research chemicals that had minimal impact on stratospheric ozone. 

The Montreal Protocol was initiated in 1987 which required the reduction of production and consumption of chlorofluorocarbons. 

Other gases with significant anthropogenic sources contribute to acid rain (NO and SO ), the reduction of the ozone layer (chlorofluorocarbons), nutrient transport (NHj... 

The magnitude of the effect of this scheduled phaseout on total carbon tetrachloride production is uncertain, since the Halogenated Solvents Industry Alliance estimates that only about 3% of the chemical manufactured in the U.S. is used in nonfeedstock applications (EPA 1991). Nevertheless, since the major current use of carbon tetrachloride is in the production of chlorofluorocarbons (see Section 4.3), and production of these chemicals has dropped significantly in recent years (C EN 1993) and is also scheduled for phase-out by 1996, a significant reduction in carbon tetrachloride production is expected during this decade. 

Nitrogen dioxide is about 20 to 50% of the total nitrogen oxides NO, (NO, NOz, HN03, N2Os), while CIO represents about 10 to 15% of the total chlorine species CIO, (Cl, CIO, HCI) at 25 to 30 km. Hence, the rate of ozone removal by CIO, is about equal to that by NO, if the amounts of NO, are equal to those of CIO,. According to a calculation by Turco and Whitten (981), the reduction of ozone in the stratosphere in the year 2022 with a continuous use of chlorofluoromethanes at present levels would be 7%. Rowland and Molina (843) conclude that the ozone depletion level at present is about 1%, but it would increase up to 15 to 20% ifthechlorofluoromethane injection were to continue indefinitely at the present rates. Even if release of chlorofluorocarbons were stopped after a large reduction of ozone were found, it would take 100 or more years for full recovery, since diffusion of chlorofluorocarbons to the stratosphere from the troposphere is a slow process. The only loss mechanism of chlorofluorocarbons is the photolysis in the stratosphere, production of HCI, diffusion back to the troposphere, and rainout. 

Rigid foams are based primarily on polyfunctional, low molecular weight alcohols and amines. Most global applications conventionally use polymeric isocyanates, TDI, or an undistilled grade of mixed TDI isomers. TDI prepolymers which have hydroxyl and isocyanate groups have been marketed as a low vapor pressure alternative to undistilled TDI. Density reduction is effected via the addition of chlorofluorocarbons, low molecular weight alkanes, or via the in situ generation of carbon dioxide. The resultant closed cell foams find applications as insulators in construction, appliance, transportation, pipeline, and tank end uses. 

This section is devoted to the application of cathodic reduction for treating aliphatic and aromatic pollutants at low concentrations, and also to the dechlorination of chlorofluorocarbons (CFCs) in aqueous media. The... 

Casado J, Cabot PL. Process for the reduction of chlorofluorocarbons and production of derivatives thereof in an electrolytic cell, cell for carrying out said reduction and process for removing the byproducts formed within the cell. PCT Int Appl WO 9724162 Al, 1997. 

Many of the commonly used solvents for precision cleaning are being eliminated due to their suspected involvement in reduction of the earth s ozone layer. Production of these chemicals, known as ozone depleting substances (ODS), is being eliminated by an international treaty known as the Montreal Protocol. This is an international agreement, first proposed in 1987 and entered into force in 1989, which limits production of chlorofluorocarbons (CFCs) and halons due to concerns that these substances were damaging the earth s ozone layer. The Montreal Protocol was modified in 1990 and again in 1992 to completely eliminate the production of chlorofluorocarbons, carbon tetrachloride, methyl chloroform (1,1,1 -trichloroethane) and halons by 1996. 

Whether reduction in use of these chlorofluorocarbons will be sufficient to prevent serious worldwide results caused by destruction of the ozone layer remains to be seen. Predictions based on the materials already in the atmosphere indicate that the damage will be significant, even if production could be stopped immediately, but such predictions are based on untested computer models and are subject to considerable error. Production has stopped or declined drastically in most countries, but the compounds proposed as substitutes are primarily those containing C, H, Cl, and F with lower stability. Whether they really reduce the effects is still uncertain, and complete replacement... 

Catalysis are classified into two types homogeneous and heterogeneous. In homogeneous catalysis the catalyst is present in the same phase as the reactants, as when a gas-phase catalyst speeds up a gas-phase reaction, or a species dissolved in solution speeds up a reaction in solution. Chlorofluorocarbons and oxides of nitrogen are homogeneous catalysts responsible for the destruction of ozone in the stratosphere. These reactions are examined in more detail in Section 20.5. A second example is the catalysis of the oxidation-reduction reaction... 

Fluoride is trapped as NaF and under certain conditions the yield of per-florotetralin can be maximized. Lewis acidic sites in crystal surface defects are thought to assist in the C-F activation process and at 470°C the system is active for demineralization of chlorofluorocarbons to afford NaF, NaCl and carbon [76]. Detailed electrochemical studies of the reduction of C-F bonds in aryltrif-luoromethanes and fluoroalkoxyarenes have been reported [77]. Free radicals cleave C-F bonds of fluorinated self-assembled monolayer surfaces [78]. 

Ozone (O3) exists in the atmosphere, 20-40 km above sea level. Ozone absorbs UV radiation which would otherwise harm living things. The effects of ozone loss could include increased human cataracts and skin cancer, reduction of plankton in ocean waters and destruction of plants, including crops. Ozone layer destruction in the Antarctic was reported in 1985 a major cause of this was believed to be the release of chlorofluorocarbon compounds, such as CCI2F2 (CFCs). These compounds are chemically unreactive, non-toxic and odourless, properties which at one time caused them to be used as solvents, aerosol propellants, refrigerant fluids and blowing agents for expanded plastic foams. They are so stable, however, that they persist in the atmosphere for years and eventually enter its upper layers, where they are broken down by the powerful UV radiation emitted by the sun. Their decomposition products can then destroy ozone ... 

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