Valeraldehyde, raffinate

Because the thermal separation of products has been substituted by a liquid-liquid separation, the two phase technology should be best suited for hydroformylation of longer chain olefins. But with rising chain length of the olefins the solubility in the aqueous catalyst phase drops dramatically and as a consequence the reaction rate. Only the hydroformylation of 1-butene proceeds with bearable space-time yield. This is applied on a small scale for production of valeraldehyde starting from raffinate II. Because the sulfonated triphenylphosphane/rhodium catalyst exhibits only slow isomerization and virtually no hydroformylation of internal double bonds, only 1-butene is converted. The remaining raffinate III, with some unconverted 1-butene and the unconverted 2-butene, is used in a subsequent hydroformy-lation/hydrogenation for production of technical amylalcohol, a mixture of linear and branched C5-alcohols. 

Raffinate-II typically consists of40 % 1-butene, 40 % 2-butene and 20 % butane isomers. [RhH(CO)(TPPTS)3] does not catalyze the hydroformylation of internal olefins, neither their isomerization to terminal alkenes. It follows, that in addition to the 20 % butane in the feed, the 2-butene content will not react either. Following separation of the aqueous catalyts phase and the organic phase of aldehydes, the latter is freed from dissolved 2-butene and butane with a counter flow of synthesis gas. The crude aldehyde mixture is fractionated to yield n-valeraldehyde (95 %) and isovaleraldehyde (5 %) which are then oxidized to valeric add. Esters of n-valeric acid are used as lubricants. Unreacted butenes (mostly 2-butene) are hydroformylated and hydrogenated in a high pressure cobalt-catalyzed process to a mixture of isomeric amyl alcohols, while the remaining unreactive components (mostly butane) are used for power generation. Production of valeraldehydes was 12.000 t in 1995 [8] and was expected to increase later. 

A further development of this successful technology was achieved to take advantage of the available feedstock base of butene isomers (raffinate II) for the preparation of n-C5 products (n-valeraldehyde, n-isoamyl alcohol, and n-valeric acid). In December 1995 production of n-valeraldehyde was started up in a new plant at Hoechst/Ruhrchemie (138). Generally, there are strong restrictions in the application of the two-phase catalytic processes to higher alkenes (Section IV.B.l), but the adaptation to butenes was possible with little modification of the process developed for propene. 

The main application for w-valeraldehyde is its transformation into 2-propyl-heptanol (2-PH) by aldol condensation and subsequent hydrogenation of the product (Scheme 4.4) [33]. Oxeno (now Evonik Industries) combined n-regioselective hydroformylation of Raffinate II with aldol condensation in a two-phase system [34]. Similar to 2-EH, 2-PH is also an important plasticizer alcohol. M-Valeraldehyde is also used as an ingredient in flavoring mixtures. For iso-regioselective hydroformylation of 1-butene, which is of minor economic importance, Dow suggested the use of calixarene-based diphosphites as ligands for rhodium [35]. 

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