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...

The aim of this study is to identify the reaction mechanism of methanol dehydrogenation on sodium carbonate catalyst. Quantitative analyses of the products of methanol dehydrogenation on sodium carbonate catalyst at 963 K indicate that methane is formed in parallel with formaldehyde, while carbon monoxide is mainly produced from further decomposition of formaldehyde. In a specially designed fixedbed reactor, more than half of the methanol conversion takes place in the post-catalytic space, where the selectivity for formaldehyde is in the same range as for the reaction in the catalyst bed. It is therefore suggested that free radicals produced on the catalyst surface play an important role in methanol dehydrogenation. Temperature-programmed desorption of methanol on sodium carbonate and transient isotope experiments show that a hydrogen species is strongly adsorbed on the catalyst, but carbon-containing species are weakly adsorbed. Temperature-programmed reaction experiments indicate that noncatalytic thermal decomposition of formaldehyde is more significant than the surface reaction at high temperatures. Based on these facts, it is proposed that chemisorbed methanol is dissociated on the catalyst surface into adsorbed hydrogen and a gas-phase *CH,OH radical. Recombination and desorption of the former is rate-determining, and the latter initiates a series of homogeneous reactions that result in the final reaction products. The proposed mechanism is useful for further improving the catalyst.

It has been shown that, after a partial deactivation, y-alumina ( y-A1203) is a good catalyst for selective methylation of 1,2-benzenediol in the temperature range of 260 -310 "C. The main products are the desired 3-methyl-I ,2-benzenediol and 2-methoxyphenol, which may be converted in another step into 3-methyl-I ,2-benzenediol, giving an overall selectivity for the desired products of 80 to 90%. The catechol forms a strongly chemisorbed surface species on y-alumina and its steric adsorption model correlates with the kinetic data.

Methanol dehydrogenation to formaldehyde was conducted in a fixed-bed flow reactor with sodium carbonate catalyst mixed with active carbons or transition metals. The additives promoted the reaction rate at 880-970 K without modifying formaldehyde selectivity. This effect increases with increasing carbon content in the carbon-carbonate mixture. Activation energy of methanol conversion is the same for the mixture and the carbonate alone. Temperature-programmed desorption experiments showed that hydrogen adsorption resulting from dissociative methanol chemisorption was enhanced by adding active carbon to the carbonate. Also, the carbon facilitates hydrogen desorption in comparison to the carbonate. It is suggested that atomic hydrogen produced on sodium carbonate during methanol dehydrogenation spills over onto active carbons (or metals) and recombines to form hydrogen gas. Hydrogen desorption from sodium carbonate, the rate-determining step, is thus accelerated.

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