Various processes for producing aldehyde and/or alcohol compounds by the reaction of a compound having at least one olefinic carbon-to-carbon bond with carbon monoxide and hydrogen in the presence of a catalyst are known. Typically, these reactions are performed at elevated temperatures and pressures. The aldehyde and alcohol compounds that are produced generally correspond to compounds obtained by the addition of a carbonyl or carbinol group, respectively, to an olefinically unsaturated carbon atom in the starting material with simultaneous saturation of the olefin bond. Isomerization of the olefin bond may take place to varying degrees under certain conditions with the consequent variation of the products obtained. These processes are typically known as hydroformylation reactions and involve reactions which may be shown in the general case by the following equation: 
In the above equation, each group R1 to R4 may independently represent an organic radical, for example a hydrocarbyl group, or a suitable atom such as a hydrogen or halogen atom, or a hydroxyl group. The above reaction may also be applied to a cycloaliphatic ring having an olefinic linkage, for example cyclohexene.
The catalyst employed in a hydroformylation reaction typically comprises a transition metal, such as cobalt, rhodium or ruthenium, in complex combination with carbon monoxide and ligand(s) such as an organophosphine.
Representative of the earlier hydroformylation methods which use transition metal catalysts having organophosphine ligands are U.S. Pat. No. 3,420,898, U.S. Pat. No. 3,501,515, U.S. Pat. No. 3,448,157, U.S. Pat. No. 3,440,291, U.S. Pat. No. 3,369,050 and U.S. Pat. No. 3,448,158.
In attempts to improve the efficiency of a hydroformylation process, attention has typically focussed on developing novel catalysts and novel processes for recovering and re-using the catalyst. In particular, novel catalysts have been developed which may exhibit improved stability at the required high reaction temperatures. Catalysts have also been developed which may permit the single-stage production of alcohols rather than a two-step procedure involving separate hydrogenation of the intermediate aldehyde. Moreover, homogeneous catalysts have been developed which may permit improved reaction rates whilst providing acceptable yields of the desired products.
Although steps have been taken to develop improved catalysts, we have detected that some of these catalysts suffer from problems. In particular we have detected that cobalt catalysts comprising cobalt in complex combination with carbon monoxide and a ligand may decompose during the reaction to produce cobalt and/or cobalt carbide (a compound of cobalt and carbon, empirical formula CoxC, where x is 2 or 3). Cobalt carbide is catalytically inactive in hydroformylation reactions, thereby resulting in an increased rate of catalyst usage. The cobalt carbide is not only catalytically inactive in hydroformylation reactions but also has a relatively bulky, porous structure and is insoluble in the reaction medium. This represents a significant disadvantage, particularly for homogeneous cobalt catalysts, because the cobalt carbide typically tends to agglomerate and form detrimental deposits on the internal surfaces of the production facility. The deposition of cobalt carbide and cobalt solids impedes the running of a hydroformylation production facility with optimal efficiency. The presence of these cobalt or cobalt carbide solids also increases the rate of catalyst degradation.
Furthermore, cobalt and cobalt carbide solids present in “dry” spots of the reactor section including the pipework, as is the case in reactors which have a “gas cap” (also known as reactors which are not “liquid-full”) may catalyze the following methanation reaction:CO+3H2→CH4+H2O
Due to the highly exothermic nature of this reaction, this can lead to an undesirable increase in temperature, which, if left unchecked, can result in a potential safety hazard.
Co-pending U.S. application Ser. No. 10/294,320, filed Nov. 14, 2002, discloses a hydroformylation process comprising reacting a compound having at least one olefinic carbon-to-carbon bond with hydrogen and carbon monoxide in the presence of a cobalt catalyst and a sulfur-containing additive, wherein the additive suppresses the formation of cobalt carbide in the reaction mixture. However there is no disclosure in this application of the problem of cobalt-catalyzed methanation or of any technical solutions to solve the problem of methanation.