Catalytic dehydrogenation of methanol to water‐free formaldehyde

Abstract

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...

Fate of methanol under one-pot artificial photosynthesis condition with metal-loaded TiO2 as photocatalysts

“…As have been discussed (vide supra), some of the fifteen reactions described in Scheme 1 can be rephrased as methanol steam reforming [70][71][72], dehydrogenation of methanol to formaldehyde or formic acid [73,82,83,96], autooxidation of formaldehyde to carbon dioxide and water [54,75,76], decomposition of formic acid to hydrogen and carbon dioxide [78][79][80], oxidative coupling of methanol to methyl formate (MF) [74,[97][98][99][100][101][102][103], and dimethoxymethane (DMM) [104,105], hydrolysis of MF to methanol and formic acid [106], decomposition of dry MF to formaldehyde and other compounds [68,69], and hydrolysis of DMM to methanol and formaldehyde [107,108]. These reactions are in fact currently receiving great attention because they are related to the production of hydrogen from methanol and formic acid, the removal of harmful formaldehyde vapor from the indoor and outdoor atmospheres, and the production of useful compounds such as MF, dry formaldehyde, and DMM from methanol.…”

Using microkinetic analysis to search for novel anhydrous formaldehyde production catalysts

“…Similarly, the non-oxidative CH 3 OH dehydrogenation processes have also been reported with 62% CH 2 O selectivity on mica-or zeolite-supported ZnO catalysts, where CH 2 O production rates decreased with time due to the reduction of active ZnO to inactive Zn metal [17,18]. Generally, the reductive environment inherent to CH 3 OH dehydrogenation causes oxygen-containing catalysts to deactivate quickly [5]; therefore, a highly capable catalyst for the dehydrogenation of CH 3 OH is needed.…”

“…However, in this process a significant fraction of the reactant stream is wasted in producing by-products: large amounts of H 2 O, CO, and dimethyl ether, as well as small amounts of CO 2 and formic acid [5]. This is further complicated by the formation of an azeotrope, which makes it costly to remove H 2 O from the aqueous CH 2 O product stream [6].…”

Alternative Liquid Fuels from Renewable Resources

“…The dehydrogenation of methanol was discussed as a direct and highly efficient synthesis pathway for monomeric formaldehyde. Su et al and Usachev et al published comprehensive reviews on the state of the art of catalytic methanol dehydrogenation. Monomeric formaldehyde would be an advantageous reactant but OME synthesis from DME and monomeric formaldehyde has not been demonstrated yet.…”