Chemicals from Methane

Chapter 12 Chemicals from Methane In previous discussions we studied a large percentage of important chemicals derived from methane. Those in the top 50 are listed in Table 12.1 and their syntheses are summarized in Fig. 12.1. As we learned in Chapters 3 and 4, many inorganic compounds, not just ammonia, are derived from synthesis gas, made from methane by steamreforming. In the top 50 this would include carbon dioxide, ammonia, nitric acid, ammonium nitrate, and urea. No further mention need be made of these important processes. We discussed MTBE in Chapter 7, Section 4, and Chapter 10, Section 9, since it is an important gasoline additive and C 4 derivative. In Chapter 10, Section 6, we presented w-butyraldehyde, made by the oxo process with propylene and synthesis gas, which is made from methane. In Chapter 11, Section 8, we discussed dimethyl terephthalate. Review these pertinent sections. That leaves only two chemicals, methanol and formaldehyde, as derivatives of methane that have not been discussed. We will take up the carbonylation of methanol to acetic acid, now the most important process for making this acid. Vinyl acetate is made from acetic Table 12.1 Methane Derivatives in the Top 50 Ammonia Derivatives Methyl /-butyl ether /7-Butyraldehyde Dimethyl terephthalate Methanol Formaldehyde Acetic acid Vinyl acetate

catalyst methanol methane synthesis gas ammonia and derivatives w-butyraldehyde formaldehyde acetic acid vinyl acetate MTBE Figure 12.1 Synthesis of methane derivatives.

acid as seen in Chapter 9, Section 4. We will also discuss an important class of compounds, chlorofluorocarbons, some of which are derived from methane. Although not in the top 50, these have important uses and are making headlines these days. 1. METHANOL (WOOD ALCOHOL, METHYL ALCOHOL) CH 3 OH Before 1926 all methanol was made by distillation of wood. Now it is all synthetic. Methanol is obtained from synthesis gas under appropriate conditions. This includes zinc, chromium, manganese, or aluminum oxides as catalysts, 30O 0 C, 250-300 atm (3000-5000 psi), and most importantly a 1:2 ratio of COiH 2. Newer copper oxide catalysts require lower temperatures and pressures, usually 200-30O 0 C and 50-100 atm (750-1500 psi). A 60% yield of methanol is realized. As seen in Chapter 3, many synthesis gas systems are set to maximize the amount of hydrogen in the mixture so that more ammonia can be made from the hydrogen reacting with nitrogen. The shift conversion reaction aids the attainment of this goal. When synthesis gas is to be used for methanol manufacture, a 1:2 ratio CO:H 2 ratio is obtained by adding carbon dioxide to the methane and water. 3CH 4 + 2H 2 O + CO 2 ^ 4CO + 8H 2 CO + 2H 2 A v CH 3 OH pressure metal oxides Thus methanol and ammonia plants are sometimes combined since carbon dioxide, which must be removed from hydrogen to use it for ammonia production, can in turn be used as feed to adjust the CO:H 2 ratio to 1:2 for efficient methanol synthesis. The methanol can be condensed and purified by distillation, bp 65 0 C. Unreacted synthesis gas is recycled. Other products include higher boiling alcohols and dimethyl ether. Table 12.2 gives the uses for methanol. The percentage of methanol used in the manufacture of formaldehyde has been fluctuating. It was 42% in 1981. It has decreased in part because of recent toxicity scares of formaldehyde. The percentage of methanol used in acetic acid manufacture is up from 7% in 1981 because the carbonylation of methanol has become the preferred acetic acid manufacturing method. MTBE is the octane enhancer and is synthesized directly from isobutylene and methanol. It was

Table 12.2 Uses of Methanol MTBE 40% Formaldehyde 24 Acetic acid 12 Solvents 6 Chloromethanes 3 Methyl methacrylate 3 Methylamines 2 Dimethyl terephthalate 2 Miscellaneous 8 Source: Chemical Profiles the fastest growing use for methanol for many years but it will drop dramatically if MTBE is banned. Many other important chemicals are made from methanol, although they do not quite make the top 50 list. Some of these can be found in Chapter 13 where the second 50 chemicals are summarized. With a U.S. production of 2.9 billion gal and a price of 470/gal, the commercial value of methanol is $1.4 billion. Not mentioned in the table is the direct use of methanol as fuel for automobiles. It is added in small amounts to gasoline, sometimes as a blend with other alcohols such as f-butyl alcohol, to increase octane ratings and lower the price of the gasoline. Experimentation is even being done on vehicles that burn pure methanol. This fuel use is usually captive but a good estimate is that it may account for almost 10% of the methanol produced. 2. FORMALDEHYDE (METHANAL) O Il H-C-H Formaldehyde is produced solely from methanol. The process can be air oxidation or simple dehydrogenation. Since the oxidation is exothermic and the dehydrogenation is endothermic, usually a combination is employed where the heat of reaction of oxidation is used for the dehydrogenation. oxidation dehydrogenation

Table 12.3 Uses of Formaldehyde Urea-formaldehyde resins 23% Phenol-formaldehyde resins 19 Acetylene chemicals 12 Polyacetal resins 11 MDI 6 Pentaerythritol 5 Urea-formaldehyde concentrates 4 HMTA 4 Melamine resins 4 Miscellaneous 12 Source: Chemical Profiles Various metal oxides or silver metal are used as catalysts. Temperatures range from 450-90O 0 C and there is a short contact time of 0.01 sec. Formaldehyde is stable only in water solution, commonly 37-56% formaldehyde by weight. Methanol (3-15%) may be present as a stabilizer. Formaldehyde in the pure form is a gas with a bp of - 21 0 C but is unstable and readily trimerizes to trioxane or polymerizes to paraformaldehyde. trioxane paraformaldehyde Table 12.3 summarizes the uses of formaldehyde. Two important thermosetting plastics, urea- and phenol-copolymers, take nearly one half the formaldehyde manufactured. Urea-formaldehyde resins are used in particleboard, phenol-formaldehyde resins in plywood. 1,4-Butanediol is made for some polyesters and is an example of acetylene chemistry that has not yet been replaced. Tetrahydrofuran (THF) is a common solvent that is made by dehydration of 1,4-butanediol. THF

Polyacetal resins have a repeating unit of -Q-CH 2 -. They are strong, stiff polymers for valves, hoses, and tube connectors. Pentaerythritol finds end-uses in alkyd resins and explosives (pentaerythritol tetranitrate). To appreciate this synthesis, the student should review two condensation reactions, the crossed aldol and the crossed Cannizzaro. Acetaldehyde reacts with 3 mol of formaldehyde in three successive aldol condensations. This product then undergoes a Cannizzaro reaction with formaldehyde. pentaerythritol Hexamethylenetetramine (HMTA) has important uses in modifying phenolic resin manufacture and is an intermediate in explosive manufacture. Although it is a complex three-dimensional structure, it is easily made by the condensation of formaldehyde and ammonia. Debate is continuing on the safety and toxicity of formaldehyde and its products, especially urea-formaldehyde foam used as insulation in construction and phenol-formaldehyde as a plywood adhesive. Presently the TLV-STEL of formaldehyde is 0.3 ppm. Formaldehyde is on the "Reasonably Anticipated to Be Human Carcinogens" list. 3. ACETIC ACID Two manufacturing methods and the uses of acetic acid were discussed in Chapter 9, Section 3, since it is made from ethylene and the C 4 stream.

However, since 1970 the preferred method of acetic acid manufacture is carbonylation of methanol (Monsanto process), involving reaction of methanol and carbon monoxide (both derived from methane) with rhodium and iodine as catalysts at 175 0 C and 1 atm. The yield of acetic acid is 99% based on methanol and 90% based on carbon monoxide. The mechanism is well understood, involving complexation of the rhodium with iodine and carbon monoxide, reaction with methyl iodide (formed from the methanol with hydrogen iodide), insertion of CO in the rhodium-carbon bond, and hydrolysis to give product with regeneration of the complex and more hydrogen iodide. Since acetic acid is used to make vinyl acetate (Chapter 9, Section 4) in large amounts, this top 50 chemical is also dependent on methanol as a major raw material. 4. CHLOROFLUOROCARBONS (CFCs AND HCFCs) AND FLUOROCARBONS (HFCs) Because of the growing importance of CFCs in environmental chemistry, a basic understanding of the chemistry and uses of this diverse chemical family is necessary. Together they represent a production of over 800 million Ib/yr that, at $1.00/lb, is a large commercial value. This industry segment uses common abbreviations and a numbering system for CFCs and related compounds. The original nomenclature

developed in the 1930s at Du Pont is still employed and uses three digits. When the first digit is O, it is dropped. The first digit is the number of carbons minus 1, the second digit is the number of hydrogens plus 1, and the third digit is the number of fluorines. All other atoms filling the four valences of each carbon are chlorines. Important nonhydrogen-containing CFCs are given below. Originally these were called Freons. CCl 2 F 2 CCl 3 F CCl 2 FCClF 2 CFC-12 CFC-Il CFC-113 When some of the chlorines are replaced by hydrogens, CFCs become HCFCs, the now more common nomenclature for those chlorofluorocarbons containing hydrogen. The numbering is the same. When more than one isomer is possible, the most symmetrically substituted compound has only a number; letters a and b are added to designate less symmetrical isomers. CHClF 2 CF 3 CHCl 2 CF 3 CHClF HCFC-22 HCFC-123 HCFC-124 CCl 2 FCH 3 CClF 2 CH 3 HCFC-141b HCFC-142b When there is no chlorine and the chemical contains only hydrogen, fluorine, and carbon, they are called HFCs. CF 3 CHF 2 CH 2 FCF 3 CH 3 CHF 2 HFC-125 HFC-134a HFC-152a Halons, a closely related type of chemical that also contain bromine, are used as fire retardants. Numbering here is more straightforward: first digit, no. of carbons; second digit, no. of fluorines; third digit, no. of chlorines; and fourth digit, no. of bromines. Common Halons are the following: Halon 1211, CF 2 BrCl; Halon 1301, CF 3 Br; and Halon 2402, C 2 F 4 Br 2. Most CFCs are manufactured by combining hydrogen fluoride and either carbon tetrachloride or chloroform. The hydrogen fluoride comes from fluorspar, CaF 2, reacting with sulfuric acid. The chlorinated methanes are manufactured from methane. Important reactions in the manufacture of CFC-11 and -12 and HCFC-22 are given in Fig. 12.2. The current use pattern of CFCs is shown in Table 12.4. The classic CFCs that have been used for refrigeration and air conditioning are mostly CFC-11 and -12, with some -114 and -115. A large portion of this usage is

Figure 12.2 Manufacture of Chlorofluorocarbons. now for automobile air conditioning. Refrigerants and home air conditioning are switching to HCFC-22. Foam blowing agents use CFC-11 and -12. Solvent use, especially for cleaning of electronic circuit boards, employs CFC-113. A large previous use of CFCs was in aerosols and propellants. This has been outlawed. An estimated 3 billion aerosol cans/yr used CFCs in the early 1970s. What are the properties of CFCs that make them unique for certain applications? Propellants for aerosols need high volatility and low boiling points. Interestingly, compared to the same size hydrocarbons, fluorocarbons have higher volatility and lower boiling points, unusual for halides. They are less reactive, more compressible, and more thermally stable than hydrocarbons. They also have low flammability, toxicity, and odor. They are used in air conditioners and refrigerators because they have high specific heats, high thermal conductivities, and low viscosities. Their nonflammability and low toxicity are also attractive in these applications. What's the problem with CFCs? In the mid-1970s CFCs were determined to photodissociate in the stratosphere to form chlorine atoms. These chlorine atoms then react with ozone to deplete this protective layer in Table 12.4 Uses of Chlorofluorocarbons Refrigerants/air-conditioning 46% Fluoropolymers 28 Foam blowing agents 20 Solvent cleaning 3 Miscellaneous 3 Source: Chemical Profiles

our atmosphere. The mechanism is a typical free radical chain process. Initiation in step (1) involves breaking a carbon-chlorine bond, weaker than a carbon-fluorine bond. Two propagation steps then can rapidly deplete ozone by reaction with the chlorine atoms. (1) CCl 2 F 2 hv» Cl- + -CClF 2 or CCl 3 F -J^ el- +.CCl 2 F (2) Cl- + O 3 *> ClO- + O 2 (3) ClO- + O * Cl- + O 2 then (2), (3), (2), (3), etc. Net reaction, (2) + (3): O 3 + O ^ 2O 2 Long-range effects of having less ozone in the stratosphere involve greater ultraviolet sunlight transmission, alteration of weather, and an increased risk of skin cancer. The ozone depletion potential for CFCs and other fluorocarbons have been measured and are given below relative to CFC-Il and -12. Notice that the HCFCs with lower chlorine content have lower depletion potentials than the CFCs, and the one HFC studied shows no depletion potential because it contains no chlorine. CFC-Il 1.0 HCFC-123 0.016 CFC-12 1.0 HCFC-HIb 0.081 CFC-113 0.8 HCFC-22 0.053 CFC-114 1.0 HFC-134a O CFC-115 0.6 HCFCs and HFCs, because of the hydrogen in the molecule, react with hydroxyl groups in the lower atmosphere. The HCFCs are being pushed as possible temporary replacements in some applications of CFCs, though HCFCs will be phased out early in the 21st century. The manufacturing picture and the recent past and future of CFCs are rapidly changing. In 1988 annual CFC consumption was 2.5 billion Ib. In the U.S. about 5,000 businesses at 375,000 locations produced goods and services valued in excess of $28 billion. More than 700,000 jobs were supported by these businesses. In 1999 the consumption of fluorocarbons was only 800 million Ib. Obviously the CFC phaseout must be done

properly to minimize the effects on these businesses and individuals. The following brief chronology will give the student an idea of the situation as of this writing. 1978 The EPA outlawed CFC-Il and -12 in aerosol and propellant applications because of fear of ozone depletion. They were replaced by propane and butane, highly flammable hydrocarbons. 1984 An ozone hole over Antarctica was discovered with especially low concentrations of ozone above that continent in their spring (Northern Hemisphere's fall). This was linked to CFCs. 1988 Du Pont, the largest producer of CFCs, called for a total CFC production phaseout. A possible arctic ozone hole was studied. The EPA called for a total ban of CFCs. 1989 The Montreal Protocol was completed. This asked for a worldwide production freeze at the 1986 levels, a 20% cut by 1993, and another 30% lowering of production by 1998 for CFC-Il, -12, -113, -114, and-115. 1992 At a meeting in Copenhagen 80 nations set HCFC deadlines including a production freeze by 1996, a 35% reduction by 2004, 90% by 2015, and 100% by 2030. 1993 HCFC-HIb replaced CFC-Il as the blowing agent in insulating foams. 1994 HFC-134a replaced CFC-12 in auto air-conditioners for all new cars. CFC-12 was to be used in old cars because of the high cost to switch. HFC-134a and HCFC-123 replaced CFC-12 and CFC-Il in large building cooling systems. 1995 The Nobel Prize in Chemistry went to Rowland, Mokina, and Crutzen for linking ozone depletion with NO x and CFCs. 1997 Large doses of HCFC-123 were found to possibly cause liver damage. 1998 HFCs were found to possibly have an effect as long-lived greenhouse gases, increasing the average atmospheric temperature. Suggested Readings Chemical Profiles in Chemical Marketing Reporter, 6-22-98, 10-11-99, 7-31-00, and 8-28-00. Kent, Riegel's Handbook of Industrial Chemistry, pp. 800-809. Szmant, Organic Building Blocks of the Chemical Industry, pp. 61-187. Wiseman, Petrochemicals, pp. 148-155. Wittcoff and Reuben, Industrial Organic Chemicals, pp. 294-337.