Preparation of Formaldehyde

Formaldehyde is an important chemical in the plastics industry, being a vital intermediate in the manufacture of phenolic and amino resins. It was also used by Reppe during World War II as an important starting point for the preparation of a wide range of organic chemicals. Consumption of formaldehyde in acetal resins is still a minor outlet for the material but exceptionally pure material is required for this purpose. 

The most important route for the production of formaldehyde is from methanol, this normally being prepared by interaction of carbon monoxide and hydrogen. 

The two gases involved can be obtained by the water-gas reaction which involves passing water vapour over hot coke. 

Methanol is converted into formaldehyde by catalytic vapour phase oxidation over a metal oxide catalyst. In one variation of the process methanol is vaporised, mixed with air and then passed over the catalyst at 300-600°C. The formaldehyde produced is absorbed in water and then fed to a fractionating column. A 37% solution of formaldehyde in water is removed from the bottom of the column with some methanol as a stabiliser whilst excess methanol is taken from the top of the column and recycled. 

Formaldehyde is also produced by the oxidation of light petroleum gases, a process which also yields methanol and acetaldehyde. This process is currently used in the Celanese Corporation plant for the production of Celcon. 

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This is a chloromethylation reaction and is consequently listed at 11-24. However, in the course of the reaction formaldehyde is generated from the acetal. This reaction is not listed at 10-6 (hydrolysis of acetals), because it is not really a preparation of formaldehyde. 

Finally, reaction of primary, secondary, or tertiary alcohols 11 with Me3SiCl 14 in the presence of equivalent amounts of DMSO leads via 789 and 790 to the chloro compounds 791 [13]. n-Pentanol, benzyl alcohol, yS-phenylefhanol or tert-butanol are readily converted, after 10 min reaction time, into their chloro compounds, in 89-95% yield, yet cyclohexanol affords after reflux for 4 h cyclohexyl chloride 784 in only 6% yield [13] (Scheme 6.5). 1,4-Butanediol is cyclized to tetrahydrofuran (THF) [13a], whereas other primary alcohols are converted in 90-95% yield into formaldehyde acetals on heating with TCS 14 and DMSO in benzene [13b] (cf also the preparation of formaldehyde di(n-butyl)acetal 1280 in Section 8.2.1). 

The industrial preparation of formaldehyde has occurred since the late 1800s and involves the catalytic oxidation of methanol 2CH,OH,. + 0 ,. —> 2CH 0,.. The oxidation takes place at temperatures between 400°C and 700°C in the presence of metal catalysts. Metals include silver, copper, molybdenum, platinum, and alloys of these metals. Formaldehyde is commonly used as an aqueous solution called formalin. Commercial formalin solutions vary between 37% and 50% formaldehyde. When formalin is prepared, it must be heated and a methanol must be added to prevent polymerization the final formalin solution contains between 5% and 15% alcohol. 

The method can also be employed in the preparation of formaldehyde, since the methyl alcohol on further oxidation is converted into formaldehyde (see p. 58). 

When a few percent of formic acid was added to gaseous formaldehyde at about 500 mm pressure a rapid polymerization was observed, the velocity was some hundredfold greater than with pure formaldehyde. It appeared that formic acid was a powerful initiator of formaldehyde polymerization under these conditions. The polymerization was confined to the surface of the vessel and the kinetics were those of a heterogeneous system. Because of the much faster formaldehyde polymerization promoted by formic acid the purity of the formaldehyde became less important. The erratic results of earlier investigators were best explained by varying degrees of purity of earlier preparations of formaldehyde monomer. 

Bauer, H. Organic compounds in chemotherapy. 11. The preparation of formaldehyde sulfoxylate derivatives of sulfanilamide and of amino compounds. J.Attu ChettL Soc. 1939, 61, 617-618. 

However, no data as to optimum temperatures or times of contact were given. Loew 8 in continuing the study of the oxidation of methanol confirmed Tollens in the feasibility of substituting copper for platinum as a catalyst and obtained somewhat higher yields of aldehydes. Kablukow 10 is also to be associated with Tollens and Loew in attempting to modify Hofmann s experiments in such a way as to facilitate the preparations of formaldehyde in relatively large quantities. 

The synthesis and decomposition of fonnates has been the subject o a number of other investigations and it has long been known that the fonnates of different metals decompose differently and that even one anc the same formate may be made to yield a variety of products undei different conditions. The salts of the alkaline earth metals have in par ticular found application in organic chemistry in connection with the preparation of formaldehyde and its higher homologs. Systematic re search in this field lias been attempted by Bredig and Carter 121 and b Hofmann in collaboration with others.122 The former investigators weri... 

Preparation of formaldehyde by photochemical condensation of carbon monoxide and tritiated hydrogen, and the synthesis of a tritiated morphine derivative Lane, A. C. McCoubrey, Arthur Peaker, R. 

With the proper catalyst and the exclusion of sulfur and iron, methanol of a purity of 99 per cent or better can be produced. As an alternative, methanol may result as part of the synthesis of other valuable higher alcohols, where the exclusion of iron is no longer necessary. The largest single use of methanol is for the preparation of formaldehyde, this product consuming some 45 per cent of the production. About 40 per cent of the production is used as an antifreeze, about 5 per cent as a denaturant, about 5 per cent in the manufacture of dimethylanUine and dimethylamine, and about 5 per cent as a solvent and for miscellaneous Uses. 

Methanol is used for methylation, and is the alcohol most easily esterified. It is used in the preparation of formaldehyde and methyl esters (e.g., dimethyl terephtha-late, methyl methacrylate, methyl formate). It is also employed as a solvent for cellulose nitrate, colophony, shellac, and urea resins in the explosives and paint industries. Furthermore, it is used as an antifreeze, fuel, and extracting agent. Methanol is toxic although cases of poisoning are extremely rare if it is correctly used. 

Commercial preparations of formaldehyde (a gas that dissolves in liquid) are stabilized by additives such as methanol, or are packaged in ampoules under nitrogen and are thus fairly costly. For most fixation procedures, it is preferable to make formaldehyde fresh from paraformaldehyde on the day it will be used. 

Methanol is used as an automobile fuel and in the preparation of formaldehyde, a starting material for certain plastics. 

The major uses of methanol are in the preparation of formaldehyde and tert-hwiy methyl ether (known commercially as MTBE). Formaldehyde is a starting material for various resins and plastics, including the first completely synthetic plastic Bakelite. MTBE is an effective gasoline additive, but problems with it leaking from underground tanks and contaminating groundwater make it unsuitable for continued use. 

The most common route for the preparation of formaldehyde is the following ... 

The major use of methanol is in the preparation of formaldehyde as a starting material for various resins and plastics. 

In 1910, O, Blank patented the use of a silver catalyst in Germany.. A. year later Le Blanc and Plaschke- reported that formaldehyde yields obtained with sih er catah sts yere higher than those ith copper. Thoma reported laboratory-scale results on the preparation of formaldehyde by oxidizing methanol in the presence of copper, silver, and gold catalysts. For the sih er catalyst these results approach those reported by Homer in In 1913, the silver catalyst -was introduced in U. S. operations with the patent of Kusneiiow h... 

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