Silver, methanol dehydrogenation

Methanol undergoes reactions that are typical of alcohols as a chemical class (3). Dehydrogenation and oxidative dehydrogenation to formaldehyde over silver or molybdenum oxide catalysts are of particular industrial importance. 

In the presence of metallic copper, metallic silver, or a copper-silver alloy used in the form of gauze or as metal deposited on a low surface area inert support, methanol can be dehydrogenated to formaldehyde at 400—500°C. 

Formaldehyde, produced by dehydrogenation of methanol, is used almost exclusively in die syndiesis of phenolic resins (Fig. 7.2). Iron oxide, molybdenum oxide, or silver catalysts are typically used for preparing formaldehyde. Air is a safe source of oxygen for this oxidation process. 

The oxidative dehydrogenation of methanol to formaldehyde was choosen as model reaction by BASF for performance evaluation of micro reactors [1, 49-51, 108]. In the industrial process a methanol-air mixture of equimolecular ratio of methanol and oxygen is guided through a shallow catalyst bed of silver at 150 °C feed temperature, 600-650 °C exit temperature, atmospheric pressure and a contact time of 10 ms or less. Conversion amounts to 60-70% at a selectivity of about 90%. 

Both processes - referring to the non-substituted and substituted methanol reactant- utilize elemental silver catalyst by means of oxidative dehydrogenation. Production is carried out in a pan-like reactor with a 2 cm thick catalyst layer placed on a gas-permeable plate. A selectivity of 95% is obtained at nearly complete conversion. This performance is achieved independent of the size of the reactor, so both at laboratory and production scale, with diameters of 5 cm and 7 m respectively. 

In order to recognize the pattern of reactivity of alcohols on modified nickel surfaces, it is essential to know the reaction pathways exhibited by less reactive surfaces. Initially the dehydrogenation of CH3OH was studied on copper (4 ) and silver (5 ) single crystal surfaces. On Cu(110), following the preadsorption of submonolayer quantities of atomic oxygen, methanol reacted via the following sequence (4,6) ... 

Oxidative dehydrogenation of methanol. Silver catalyst deposited in the pores of the membrane (66 wt% Ag). 

Lefferts, L., J. G, van Ommen and J. R. H. Ross, 1986, The oxidative dehydrogenation of methanol to formaldehyde over silver catalysts in relation to the oxygen-silver interaction. Appl. Catal. 23 385-401... 

A fundamental issue in selective oxidation is the activation of C—H bonds that is always required for ODH (oxidative dehydrogenation) and oxo-functionalization and is detrimental for epoxidation. A particular case is silver [70] as catalyst, which can achieve highly selective epoxidation of ethene as well as highly selective dehydrogenation of methanol to formaldehyde although it is notably in both cases only the same metallic catalyst. We will return to this case in the next section, which deals with the multiplicity of active oxygen species. 

Formaldehyde (methanal, melting point -92°C, boiling point -21°C) is produced solely from methanol by using a silver catalyst (Fig. 1) or a metal oxide catalyst (Fig. 2). Either process can be air oxidation or simple dehydrogenation. 

Formaldehyde production by dehydrogenation of methanol over a silver catalyst,... 

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. 

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. 

Dehydrogenation of methanol to formaldehyde methanol, air and steam are passed over a 5-10 cm high bed of silver crystals. 

Silver gauze for dehydrogenation or oxidative dehydrogenation of methanol to formaldehyde. 

The other component necessary for the preparation of novolacs is methanal that is produced by catalytic dehydrogenation of methanol as shown in equation 6. Typical catalysts are a combination of iron oxide and molybdenum oxide or silver [30],... 

Two process variants are used for the manufacture of formaldehyde from methanol, partial oxidative dehydrogenation or oxidation. In the first, methanol vapour is mixed with a stoichiometrically deficient quantity of air and passed over a silver catalyst at temperatures of 400-600°C. The two reactions... 

The conversion of ethanol to acetaldehyde can be effected by dehydrogenation over copper at 250-300°C or by (partially) oxidative dehydrogenation over silver at 450-500°C. However, this route was largely superseded by the Wacker process for the direct oxidation of ethylene in aqueous solutions of Pd/Cu chlorides. Rhone-Poulenc and BP have also patented potential processes for the homologation of methanol to acetaldehyde ... 

In contrast to methanol oxidation, oxidative dehydrogenation of methanol [route (b) in Topic 5.3.2] is an endothermic reaction (AH = -F84kJ mol ). Methanol is contacted at normal pressure with a heterogeneous silver at 500-700 °C. Owing to the kinetic instability of the formaldehyde product under the applied reaction conditions, the contact time at the catalyst is very short (t < 0.01 s). This is realized by high flow rates in the reactor and a very effective quenching of the product flow leaving the reactor. The product gas is contacted with water to produce an aqueous... 

Formaldehyde can be formed from methanol by oxidative [Eq. (11)] or nonoxidative dehydrogenation [Eq. (12)]. Table 1 shows the thermodynamic data for these two reactions. For the lower temperature mixed-oxidecatalyzed process, the oxidative pathway is the only thermodynamically favorable reaction. However, for the higher temperature silver-catalyzed process, the nonoxidative process also becomes thermodynamically favorable. Thus both nonoxidative and oxidative pathways contribute in this process. 

This oxide-catalyzed process operates at a much lower temperature of 270400°C than the silver-catalyzed process, and its feed has a lower methanol-air ratio, which is below the lower flammability limit (6.7 vol% methanol in air). The methanol concentration can be increased without danger of explosion if the oxygen concentration is reduced to below 10 mol% by diluting with recycled off-gas [24]. The amount of air used is in excess of the stoichiometric ratio. Methanol is produced by the highly exothermic oxidative dehydrogenation reaction [Eq. 

The metal oxide catalysts used for hydrogenation reactions are reduced to an active form of the metal before use. Apart from metallic platinum and silver, which are used to oxidize ammonia and methanol, respectively, oxidation catalysts are usually transition metal oxides. Acidic oxides, such as alumina, sihca alumina, and zeolites are used in cracking, isomerization, and dehydrogenation reactions. These are only a few examples of the catalysts now being widely used. A more detailed list is given in Table 1.4. 

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