Catalytic dehydrogenation of methanol is a promising process of producing water-free formaldehyde. The present paper reviews research in this field. As effective catalysts mainly transition metal compounds as well as oxides and salts containing sodium have been reported. Several catalysts exhibit high activity and high selectivity, for formaldehyde at low conversions while further efforts have to be made to improve catalyst stability and selectivity at high conversions. Catalytic dehydrogenation of methanol to formaldehyde is compared with methanol oxidation.
Current Production and Uses of FormaldehydeSince its initial discovery in 1859 by Butlerov, formaldehyde has become one of the most important chemicals in industry. In 1985 its world production amounted to about five million tons [l]. At room temperature, formaldehyde is a colourless gas with a pungent odour. It is unstable and polymerizes easily to form polyoxymethylenes both in the gas phase and in solutions. One of its properties of practical importance is the condensation reaction with compounds containing active hydrogen to produce water and compounds with -CH20H or -CH2 groups. Apart from being soluble in most organic solvents, in water it is strongly solvated or in the form of low molar mass polymers. Therefore, its vapour pressure is very low over aqueous solutions and it forms azeotropes with water [2]. This makes its separation from water difficult.Although oxidation of hydrocarbons is used to obtain formaldehyde, more than 90% of the world's formaldehyde production is at present achieved through catalytic oxidation of methanol [3]. Two processes, namely the silver catalyst process and the formox process, are commonly employed [3 -51. In the former, a methanol-rich methanolair-steam mixture (36 -45% methanol) is passed through the catalyst bed at 870 -970 K. The following reactions occur:CH30H + CH20 + H2 , AH900 = + 92 kJ/mol , (1) The conversion may either be complete or, alternatively, incomplete with recycling of methanol. The selectivity is about go%, with carbon oxides, methyl formate, methane and formic acid as by-products. The formox process differs from the silver catalyst process in the nature of the catalyst (iron-molybdenum oxides), methanol concentration (5 -10% methanol) and oxidation temperature (570 -670 K). Only reactions 2 and 3 take place under these conditions. In both processes, aqueous solutions are obtained and marketed as final products. Formaldehyde is also available commercially in its solid form, i.e., paraformaldehyde, which consists of polyoxymethylenes and is easily decomposed to formaldehyde. It is produced by distilling formaldehyde solutions under vacuum.Formaldehyde finds a wide application in industry. Although it is used to make other chemicals, most of its production is used to synthesize aminophenol and polyacetal resins through condensation reactions. Water in the aqueous solutions often interferes with these processes. When a small quantity of water is tolerable, paraformaldehyde can be used instead. Occasion...