Formaldehyde copper/silver catalysts

Introduction.—The oxidative dehydrogenation of alcohols to aldehydes and ketones over various catalysts, including copper and particularly silver, is a well-established industrial process. The conversion of methanol to formaldehyde over silver catalysts is the most common process, with reaction at 750—900 K under conditions of excess methanol and at high oxygen conversion selectivities are in the region 80—95%. Isopropanol and isobutanol are also oxidized commercially in a similar manner. By-products from these reactions include carbon dioxide, carbon monoxide, hydrogen, carboxylic acids, alkenes, and alkanes. 

Monosubstituted acetylenes add formaldehyde in the presence of copper, silver, and mercury acetyUde catalysts to give acetylenic alcohols (58) (Reppe reaction). Acetylene itself adds two molecules (see Acetylene-DERIVED chemicals). 

Silver Catalyst Process. In early formaldehyde plants methanol was oxidized over a copper catalyst, but this has been almost completely replaced with silver (75). The silver-catalyzed reactions occur at essentially atmospheric pressure and 600 to 650°C (76) and can be represented by two simultaneous reactions ... 

Formaldehyde is nowadays one of the major produced chemicals due to its uses in many fields of chemical industry [1]. The commercial production started in 1890 using metallic copper catalysts. In 1910 copper catalysts were replaced by silver catalysts with higher yields [2]. Although the first report of the excellent catalytic behavior of iron molybdates in selective oxidation of methanol to formaldehyde is of 1931, the related industrial process based on them only went into operation in 1940-50 [1]. A recent report [3] shows that iron molybdates and silver catalysts are nowadays equally used as industrial catalysts for formaldehyde production. 

A comparison of these three metal catalysts shows that on the basis of total methanol reacted the silver catalyst was more active than the copper which was more active than the gold. Since the silver catalyst was also least active toward the decomposition of formaldehyde to hydrogen and carbon monoxide, it was recommended as the most desirable. 

To overcome the objectionable reoxidation of formaldehyde and decomposition at the temperature of the reaction zone in the oxidation of methane, it has been proposed to react the formaldehyde as fast as formed with some substance to give a compound more stable under the conditions of the reaction and thus to increase the yields obtainable. It is claimed 101 that a reaction between the newly formed formaldehyde and annnonia to form a more stable compound, hexamethylene-tetramine, is possible under certain conditions, so that the formaldehyde is saved from destruction and can be obtained in a technically satisfactory yield. The hexamethylenetetramine is prepared by oxidizing methane with air in the presence of ammonia gas. A mixture consisting of six volumes of methane, twelve volumes of oxygen, and four volumes of ammonia gas is passed through a constricted metal tube which is heated at the constriction. The tube is made of such a metal as copper, silver, nickel, steel, iron, or alloys of iron with tin, zinc, aluminum, or silicon or of iron coated with one of these metals. Contact material to act as a catalyst when non-catalytic tubes are used in the form of wire or sheets of silver, copper, tin, or alloys may be introduced in the tube. At atmospheric pressure a tube temperature... 

Methanol oxidation to formaldehyde (copper or silver catalysts). 

Several commercial formaldehyde plants were operating after 1900 using both copper and silver catalysts. These included the Formal plant, Cote d Or, France, which used long copper tubes, and theFHMeyer plant, Hanover-Heinholz, based on Orlov s work, with a copper reactor and copper or silver gauzes operating at 450-500°C. 

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... 

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

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. 

Methyl alcohol in fact is made in this manner from the carbon dioxide which results from certain industrial fermentations. When methyl alcohol vapor is mixed with air and passed over an initially heated catalyst of metallic copper or silver gauze, oxidation occurs, sufficient heat is evolved to maintain the catalyst at a bright red, and formaldehyde is formed. 

Two primary methods of manufacturing formaldehyde from methanol are used today. The first uses silver as a metal catalyst in its reactions. In earlier years, facilities used a copper catalyst in this process. The simultaneous reactions involved in the metal catalyst process occur at essentially atmospheric pressure and 600-650 °C (Gerberich et al. 1980). Approximately 50-60% of the formaldehyde produced using the metal catalyst process is formed during an exothermic reaction the remainder is formed from an endothermic reaction. The overall yield for this process is 86-90% formaldehyde. The domestic licensors for this process include Borden Chemical Company and Davy Powergas, Inc. (Gerberich et al. 1980). 

In industry, formaldehyde is obtained by heating a methanol and air mixture. This reaction is the dehydrogenation of methanol (Hoffman method). The process is carried out by using the oxygen from the air with a catalyst of copper and silver. 

The decomposition is endothermic and to dehydrogenate the alcohol, heat must be supplied. However, if oxygen, usually in the form of air, is supplied to the reaction and the process conducted in the presence of certain metallic catalysts, such as finely divided copper or silver, high yields of formaldehyde may be obtained and the reaction made exothermic. It is by this oxidation process that practically all of the formaldehyde is produced. This process, although apparently simple, requires close temperature control and regulation of the extent of reaction to prevent undue losses of raw material. 

A fairly large number of other processes which describe the conversion of methanol into formaldehyde are to be found in the literature. The use of silver deposited on copper wire or gauze and of a silver spiral alone has already been referred to in the preceding chapter.17 26 In this work Plaschke obtained yields of 58 per cent with silver and of 55.4 per cent with copper at temperatures of 455° C. and with a space velocity of almost 4000. Various adaptations of this form of catalyst have been described and patented. For example, Bobrov 20 found that when vapors of methanol and air were passed over copper gauze spirals, yields of formaldehyde which varied between 37 and 42 per cent were obtained, while if these were... 

Formaldehyde occurs naturally in the atmosphere at a concentration of about 10 parts per billion (0.000 001%) partly as a by-product of plant and animal metabolism, and partly as a product of the reaction of sunlight with methane (CH4), a much more abundant component of the air. At such low concentrations, it is not a natural source of the compound for commercial or industrial uses and is produced instead by the oxidation of methanol (methyl alcohol CH3OH) or gases extracted from petroleum (such as methane) over a catalyst of silver, copper, or iron with molybdenum oxide. 

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