1. Field of the Invention
This invention relates to a method of hydroformylating olefinic compounds. More particularly, the invention relates to a method of hydroformylating an olefinic compound into the corresponding aldehyde in an organic solvent and in the presence of a rhodium complex and a trisubstituted phosphine, there being added to the reaction system at least one diphosphino compound of general formula (I) ##STR3## wherein A.sup.1 and A.sup.2, respectively, are aryl groups; R.sup.1 and R.sup.2, respectively, are an aryl group or a saturated hydrocarbon residue of 1 or more carbon atoms; and ##STR4## represents a substituted or unsubstituted alicyclic hydrocarbon group of 3 to 6 carbon atoms in the main ring, in a proportion of 0.20 to 5.0 equivalents per rhodium atom in said rhodium complex to thereby achieve a substantial prolongation of catalyst activity and, consequently, achieve a more advantageous hydroformylation of the olefinic compound.
2. Description of the Prior Art
There is known a hydroformylation reaction in which an olefin, exemplified by ethylene, propylene and butene, is reacted with a gaseous hydrogen-carbon monoxide mixture in an organic solvent and in the presence of a rhodium complex and a trisubstituted phosphine to obtain an aldehyde containing one more carbon atom than the starting olefin. The reaction has been commercially utilized, for example, in the production of butyraldehyde from propylene.
The rhodium complex as used for catalyzing the hydroformylation reaction is suited for industrial practice in that it helps perform the reaction under considerably milder conditions (lower temperature and pressure) than does a cobalt catalyst and that it contributes to a higher selectivity for normal-aldehyde. However, since the rhodium complex is quite expensive, the industrial value of a hydroformylation reaction with this complex depends largely on the catalyst life of the complex. Therefore, much research has heretofore been done and many proposals made in connection with means of maintaining the activity of the catalyst for an extended time under hydroformylating conditions. These methods may be roughly classified into three categories:
(1) A method in which the contemplated reaction is carried out while various reaction conditions such as the concentrations of the rhodium catalyst and trisubstituted phosphine, the partial pressure of carbon monoxide and the reaction temperature are controlled, each within a defined range, so as to suppress thermal degradation of the rhodium complex and formation of an inactive highly-carbonylated rhodium complex, e.g. see, German Patent Application (abbreviated as DTOS) 2,715,685;
(2) A method in which a small amount of oxygen is allowed to be present in the reaction system, e.g. see, DTOS 2,730,527; ;P (3) A method in which the reaction is carried out while the concentration of poisonous high-boiling byproducts in the reaction system is maintained below a certain level, e.g., see British Pat. No. 1,338,237 and British Pat. No. 1,545,706.
These hitherto-proposed methods, however, have room for improvement when industrial applications are envisaged. The first method (1) is commercially disadvantageous in that any drop in reaction temperature and any increase in concentration of the trisubstituted phosphine result in a reduced reaction rate which would require use of the expensive rhodium catalyst in an increased concentration in order to compensate for the reduction of reaction rate. With respect to the second method (2), the trisubstituted phosphine and the end product aldehyde are unstable against oxygen and tend to be converted to the substituted phosphine oxide and organic carboxylic acid, respectively, with the result that not only is the catalyst activity reduced but there are induced undesirable secondary reactions of the product aldehyde. The third method (3) is disadvantageous in that maintaining the concentration of high-boiling byproducts acting as catalyst poisons below a certain level is industrially equivalent to frequent regeneration, activation and recovery of the rhodium catalyst which are, of necessity, accompanied by losses of the rhodium catalyst and trisubstituted phosphine. Even by the above methods, a depression of catalyst activity is frequently encountered during the reaction and it has been inevitable to carry out the regeneration, activation and recovery of the rhodium catalyst with a fair frequency. This not only means a complicated procedure but also entails losses of the rhodium catalyst and trisubstituted phosphine in the regeneration step. Thus, the conventional methods for maintaining the activity of the rhodium catalyst leave much room for improvement.