SYNTHESIS OF CARBONYL COMPOUNDS

Abstract: In a process for vapour-phase synthesis of carbonyl compounds from alcohols a mixture of the alcohol and air or oxygen is passed over a silver catalyst. The concentration of alcohol:air or alcohol:oxygen is below the explosive limits. The process is conducted at a temper­ ature between 200° and 450°C. Previously the production of carbonyl compounds required a temperature of 450°-700°C which necessitated the use of heat exchangers using fused salt baths. The silver catalyst may be supported.

Description: This invention relates to a process for the vapour-phase synthesis of carbonyl compounds from alcohols. One particular industrially-important carbonyl compound is cyclohexanone, used as a solvent and chemical intermediate, for example in the production of nylon. Cyclohexanone may be produced from cyclohexanol by dehydrogenation over a zinc, nickel or copper-based catalyst, but the reaction is endothermic and therefore commercially unfavourable. The reaction may, however, be rendered overall exothermic, and therefore commercially favourable, by adding oxygen (in the form of air or oxygen gas) to the cyclohexanol feed to react with the product hydrogen to form water. This reaction is known from, for example, European Patent Application No. 0008013, in which alcohols are oxidatively dehydrogenated to form carbonyl compounds in the presence of a silver and/or copper catalyst of specific particle size at a temperature of 450 to 700°C. In particular, cyclohexanol is reacted with air over a catalyst consisting of silver grains at a temperature of 600°C, to produce cyclohexanone at 88% of the theoretical yield. In this reaction, the concentration of cyclohexanol in the gases is 53% by volume, considerably above the explosive limits for cyclohexanol-air mixtures, which are believed to be 1-8% by volume. Furthermore, heat exchange at these high temperatures often necessitates the use of a heat transfer medium such as a fused salt bath. Such baths, however, are disadvantageous in that they tend to cause high levels of plant corrosion. US-A 2015751 and US-A 4097535 disclose similar processes, in both cases at alcohol:oxygen (or alcohol:air) concentrations considerably above the upper explosive limit. We have now found that oxidative dehydrogenation of alcohols to produce carbonyl compounds may be carried out at alcohol:oxygen (including alcohol:air) concentrations below the explosive limits, and at lower temperatures which avoid the use of fused salt baths. According to the present invention, therefore, a process for the vapour-phase synthesis of carbonyl compounds from alcohols having from 6 to 10 carbon atoms comprises passing a gaseous mixture containing the alcohol and oxygen over a catalyst comprising silver metal, in which the concentration of alcohol:oxygen is below the lower explosive limit and the reaction temperature is in the range 200-450°C. Throughout this specification, references to "oxygen" are to be taken to include oxygen in the form of air or oxygen gas, and oxygen in admixture with another gas such as nitrogen, except where the context specifies otherwise. By "explosive limits" is meant the experimentally-determined values, as these obviously vary according to the particular circumstances of the reaction, including the temperature and pressure at which the reaction is carried out and the concentration of oxygen used in the gas. For example, it is known that the lower explosive limit for a reaction in pure oxygen is slightly higher than that in air and the corresponding upper limit is substantially higher. Thus, for example, for methane the explosive limits in air are 5% (lower) and 15% (upper) whereas in oxygen they are 5.4% and 59.2%, respectively. Explosive limits for the corresponding alcohol in air and those for some other alcohols and cyclohexanone are shown below as examples: Alcohol Explosive Limits (air) (by volume CH₃OH 6.7 - 36.5% C₂H₅OH 3.28 - 18.95% n-C₃H₇OH 2.15 - 13.5% i-C₃H₇OH 2.02 - 11.8% n-C₄H₉OH 1.45 - 11.25% cyclohexanone 1.1 - 8.1% The process according to the invention proceeds via oxidative dehydrogenation but at a markedly lower temperature than with prior art vapour phase processes. In particular, for the conversion of cyclohexanol to cyclohexanone, at a concentration of <1% by volume cyclohexanol in air, the reaction temperature is in the range 200-450°C, preferably 250-400°C, to give up to about 90% by weight conversion of cyclohexanol, at a selectivity to cyclohexanone of not less than about 97% by weight. The principal loss is by conversion to CO₂, which typically is present at only very low levels, for example not greater than 0.2% by volume, in the gas stream, above the input concentration. In contrast, at a concentration of cyclohexanol in air in excess of the explosive limits, only low conversions of cyclohexanol are obtained at a temperature of 320°C, temperature of about 600°C being required for economic conversion rates at these higher concentrations. The ability to operate the process according to the invention at lower temperatures means that inert heat-exchange media such as "Dowtherm" (Registered Trade Mark) may be used. The process according to the invention may be operated at a very slightly super-atmospheric pressure, for example from 0.01-0.35 kg cm⁻² above atmospheric pressure, preferably 0.03-0.14 kg cm⁻² above atmospheric pressure. The silver catalyst may be silver metal in any convenient form, for example in the form of powder, grain, wire or gauze, or for reasons of economy may comprise silver or a silver-containing moiety dispersed on an inert support. The silver catalyst may optionally include a promoter material. The support is catalytically inert to prevent catalytic dehydration (for example of cyclohexanol to cyclohexene) from taking place. For this reason, high surface area catalyst supports are preferably avoided. More preferably, supports are selected from those having a surface area <200m²/g. Examples of suitable support materials include glass, α-alumina, silica, zinc oxide, tin oxide, chromia and metallic materials such as steel, copper, nickel and the like. As promoter material, we prefer to use an alkali metal or alkaline earth metal, or certain transition metals and metalloids. Examples of promoter materials include lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium, barium, copper, gold, iron, tin, nickel, manganese, molybdenum, zinc, palladium, antimony, bismuth and cobalt. The process temperature may be controlled by pre-heating the inlet gas stream. The invention will now be described by way of example for the conversion of cyclohexanol to cyclohexanone and of 1-octanol to 1-octanal, although a wide variety of alcohols or hydroxy-containing compounds are suitable for conversion to the corresponding carbonyl compound by the process according to the invention, such as alcohols having up to 10 carbon atoms, preferably alcohols having from 6 to 10 carbon atoms, but any alcohol which is in the vapour phase at the temperature of the reaction can be used. In the examples, cyclohexanol feed was technical grade cyclohexanol containing, typically, 89% cyclohexanol, 6% cyclohexanone, balance inert constituents. T max is the maximum reactor temperature in °C, ΔT is the temperature difference between T max and the minimum temperature in the bed. A negative sign for ΔT implies an endotherm. The system was allowed to equilibrate at the respective temperature before being sampled. Conversion in moles % is calculated from the gas chromatogram of the liquid product stream by Selectivity in moles % is calculated from the same chromatogram by In the examples, the reaction pressure was in the range 0.01-0.35 kg cm⁻² above atmospheric pressure, the precise value depending on the feed flow rate. Example 1A Cyclohexanol was reacted with air over a catalyst consisting of 96.5g silver grain of 3-7 mm size under the following conditions, the Table giving results at different temperatures:- Bed volume: 41.7 cm³ Gas flow rate: 40 ℓ min⁻¹ Feed flow rate: 75 g hr⁻¹ Gas composition: air Vapour:gas ratio 0.6% by volume At a higher temperature of 380°C the system was unstable and equilibration was not achieved. Example 1B (Comparative example at higher feed concentration outside the scope of this invention). Feed: cyclohexanol Catalyst type: silver grain 3-7 mm Catalyst weight: 98.8 g Bed volume: 41.7 cm³ Gas flow rate: 2.5 ℓ min⁻¹ Feed flow rate: 75 g hr⁻¹ Gas composition: air Vapour:gas ratio 10% by volume At the temperatures of 349°C and 410°C the system was not stable. The temperature continued gradually to rise and the figures were calculated from spot samples taken at those temperatures. It can be seen that, at this vapour:gas ratio above the explosive limits, the conversion of alcohol to product is substantially less favourable than at vapour:gas ratios below the explosive limits. Example 2 (Low density silver ribbon) Feed: cyclohexanol Catalyst type: melt-spun silver Catalyst weight: 21.08 g Bed volume: 41.7 cm³ Gas flow rate: 25 ℓ min⁻¹ Feed flow rate: 75 g hr⁻¹ Gas composition: air Vapour:gas ratio: 1% by volume At a higher temperature of 390°C the system was unstable and did not equilibrate. Examples 3 to 5 give results for silver catalysts deposited on a variety of different support materials. Example 3 Feed: cyclohexanol Catalyst type: 0.25% wt/wt Ag on 4 mm diameter glass beads Catalyst weight: 71.4 g Bed volume: 41.7 cm³ Gas flow rate: 25 ℓ min⁻¹ Feed flow rate: 75 g hr⁻¹ Gas Composition: 4% O₂ in N₂ Vapour:gas ratio: 1% by volume The system was unstable at a temperature of 410°C. Example 4 Feed: cyclohexanol Catalyst type: 0.26% wt/wt Ag on 1-2 mm α-Al₂O₃ (Alcoa T60) Catalyst weight: 83.84 g Bed volume: 41.7 cm³ Gas flow rate: 25 ℓ min⁻¹ Feed flow rate: 75 g hr⁻¹ Gas composition: air Vapour:gas ratio: 1% by volume At temperatures above 355°C the % conversion began to decrease and the system was unstable at a temperature of 405°C. Example 5 Feed: cyclohexanol Catalyst type: 3.5% wt/wt Ag on "304" stainless steel pad. Catalyst weight: 28.4 g Bed volume: 41.7 cm³ Gas flow rate: 25 ℓ min⁻¹ Feed flow rate: 75 g hr⁻¹ Gas composition: air Vapour:gas ratio: 1% by volume The system was unstable at a temperature of 410°C. Example 6 (Use of Octanol as Feed). Feed: 1-octanol (G.P.R. grade) Catalyst type: silver grain 3-7 mm Catalyst weight: 98.8 g Bed volume: 41.7 cm³ Gas flow rate: 25 ℓ min⁻¹ Feed flow rate: 75 g hr⁻¹ Gas composition: air Vapour:gas ratio: 0.8% by volume The system was unstable at a temperature of 358°C.

Claims: A process for the vapour-phase synthesis of carbonyl compounds from alcohols having from 6 to 10 carbon atoms, the process comprising passing a gaseous mixture containing the alcohol and oxygen over a catalyst comprising silver metal, in which the concentration of alcohol:oxygen is below the lower explosive limit and the reaction temperature is in the range 200-450°C. A process according to Claim 1, in which the reaction temperature is in the range 250-400°C. A process according to Claim 1 or Claim 2, in which the reaction pressure is in the range 0.01-0.35 kg cm⁻² above atmospheric pressure. A process according to any preceding claim in which the silver metal is present in the form of silver gauze. A process according to any of Claims 1 to 3 in which the catalyst is dispersed on an inert support. A process according to any preceding claim in which the alcohol comprises cyclohexanol and the carbonyl compound comprises cyclohexanone.

Inventors: PLAYER, PHILLIP JOHN, WEBSTER, DENNIS EDWARD
Applicants: JOHNSON MATTHEY PUBLIC LIMITED COMPANY
Application year: 1987
Main classification: C07C45
IPC codes: C07B61, C07C49, C07C45, C07C67, B01J23