Butyraldehyde from propene

Figure 8-5. The Hoechst AG and Rhone Poulenc process for producing butyraldehydes from propene (1) reactor, (2) catalyst separation, (3) stripper (using fresh syngas to strip unreacted propylene to recycle), (4) distillation.

This type of process represents the ideal biphasic method as long as the product can be extracted without contamination from the catalyst and catalyst immobilization solvent. This technique is employed commercially for the production of butyraldehyde from propene, carbon monoxide and hydrogen which is described in detail in Chapter 11 [3],... 

Following earlier contacts Ruhrchemie AG (RCH), now a part of Celanese AG, and Rhone-Poulenc joined forces in 1982 to develop a continuous biphase hydrofor-mylation process for the production of n-butyraldehyde from propene. 

Test runs with low P/Rh ratios and Rh concentrations while hydroformylating DCP showed excellent membrane separation results but decreasing activity data. This failure in the optimization approach of the hydroformylation and membrane separation step without regard to long-term stability again underlines a basic problem in catalyst development, the coincidental consideration of different contradictory circumstances. A second series with a high P/Rh ratio of 100 was performed with the same catalyst system and butyraldehyde from the hydroformylation of propene as feed. 

Today the installed hydroformylation capacity worldwide is more than 7.5 Mio tons per year (Baerns et al., 2006). The most important feedstock is propene, with the products n-butyraldehyde and iso-butyraldehyde (Scheme 6.14.3). The most important single product from propene hydroformylation is 2-ethyl-l-hexanol (>50% of the n-butyraldehyde production), the aldol condensation product obtained from n-butanal, which is an important plasticizer alcohol. After esterification with phthalic anhydride, dioctyl phthalates plasticizers are obtained that are used mainly in poly(vinyl chloride) plastics. 

Homogeneous rhodium-catalyzed hydroformylation (135,136) of propene to -butyraldehyde (qv) was commercialized in 1976. -Butyraldehyde is a key intermediate in the synthesis of 2-ethyIhexanol, an important plasticizer alcohol. Hydroformylation is carried out at <2 MPa (<290 psi) at 100°C. A large excess of triphenyl phosphine contributes to catalyst life and high selectivity for -butyraldehyde (>10 1) yielding few side products (137). Normally, product separation from the catalyst [Rh(P(C2H2)3)3(CO)H] [17185-29-4] is achieved by distillation. 

Hydroformylation is an important industrial process carried out using rhodium phosphine or cobalt carbonyl catalysts. The major industrial process using the rhodium catalyst is hydroformylation of propene with synthesis gas (potentially obtainable from a renewable resource, see Chapter 6). The product, butyraldehyde, is formed as a mixture of n- and iso- isomers the n-isomer is the most desired product, being used for conversion to butanol via hydrogenation) and 2-ethylhexanol via aldol condensation and hydrogenation). Butanol is a valuable solvent in many surface coating formulations whilst 2-ethylhexanol is widely used in the production of phthalate plasticizers. 

Since butyraldehyde has a low boiling point (75 °C) separation of catalyst from both reactants and product is straightforward. Most of the rhodium remains in the reactor but prior to recovery of propene and distillation of crude product the gaseous effluents from the reactor are passed through a demister to remove trace amounts of catalyst carried over in the vapour. This ensures virtually complete rhodium recovery. 

The use of other metals for this process in aqueous medium is rare. One example involves the use of a cobalt catalyst, [Co(CO)3(Ph2PCH2CH2N CH3 3)2]2(PF6)2, which was found to effectively hydroformylate 1-hexene in water [59]. Unfortunately, little selectivity was observed as only a 4.6 1 n/i ratio was obtained. Hydro-formylation of ethene and propene was also achieved through the use of the ruthenium catalyst, [Ru3(CO)9(TPPMS)3] [60]. In the hydroformylation of propene, this catalyst exhibited TONs on the order of 490 in water. Also, increasing the temperature from 100 to 120 °C favored the formation of n-butyraldehyde. Other catalytic systems such as Fe(CO)4(TPPMS) were nearly inactive, while activity increased slightly with the use of Co2(CO)8/TPPMS. 

Hydroformylation conditions feed, propene, 270 bar temperature, 125 °C P/Rh ratio, 2 1 reaction time, 2 h Rh concn., 20 ppm membrane separation conditions feed, butyraldehyde membrane type, UF-PA-5/PET 100 from the former Hoechst AG pressure, 15 bar temperature, 40°C amount of permeate, 1st stage, 91-84% 2nd stage, 95-92%. 

Rhodium Catalysts. - The hydroformylation of propene with a Rh/triphenyl-phosphine catalyst is now an established industrial process which will consume over a million tonnes per annum of propene when all licensed plants are operational. Most of the product n-butyraldehyde is converted to 2-ethylhexanol for plasticiser applications. The process is also applicable to the hydroformylation of C2, C4, and C5 alkenes. The process is remarkable for the long lifetime of the Rh catalyst but by-products are formed which deactivate the catalyst and have to be removed. The formation of triphenyl-phosphine oxide, benzaldehyde, and propyldiphenylphosphine under hydroformylation conditions has been investigated where benzaldehyde is produced by or /zo-metallation of triphenylphosphine followed by CO insertion and P-C bond cleavage and propyldiphenylphosphine was assumed to result from reaction of propene with the co-ordinated diphenylphosphine group remaining after benzaldehyde formation. The same authors have also studied the kinetics of the formation of heavy by-products which are dependent on... 

MPa was 0.08. The apparent activation energy was 79.1kJmor. Selectivity to n-butyraldehyde was independent of H2 and propene pressure. However, on decreasing the partial pressure of CO from 520 to 50kPa, the selectivity increased from 10 to 30. Increasing the temperature also caused an increase in selectivity. 

As expected and as shown in Table 2, this environmental quotient for conventional 0X0 processes (using cobalt catalysts) and the production of the bulk chemical tv-butyraldehyde is actually about 0.6-0.9, depending on the definition of the term target product . This range indicates that the main byproduct of propene hydro-formylation, isobutyraldehyde, is processed by a number of producers (e.g., to isobutanol, isobutyric acid, or neopentyl glycol) so that isobutyraldehyde is not always and everywhere a byproduct thus when it becomes a target product the E factor falls from 0.9 to 0.6. 

For example, in the case of propene hydroformylation from n-butyraldehyde, 2-ethylhexenal, 2-ethylhexanal, 2-ethylhexanol, various trimer aldol, Tishchenko, and ether-type products have been identified (27). 

Propene and syngas are fed to the reactor, where the gases are intimately contacted with the ligand-modified rhodium catalyst in solution. The reaction exotherm is removed by a dedicated heat exchanger. The liquid effluent from the reactor passes to a degassing column where unreacted propylene and syngas is evaporated from the catalyst/product solution and recycled back to the reactor. In the fourth column the hydroformylation products are separated from the Rh-catalyst by distillation. While butyraldehydes leave the column over the top the catalyst remains at the bottom of the column dissolved in liquid heavy products of the process to be recycled back to the reactor. The crude aldehyde products undergo a further purification step in the crude aldehyde column prior to their transfer to the n/iso-butyraldehyde splitter column. 

In order to address the problems of cost and rarity of rhodium, Kunz, at Rhone-Poulenc, adjusted a water-soluble phosphine, P(/w-C6H4S03 Na )3 (TPPTS), obtained by sulfonation of [HPPh3] in meta. This phosphine solubilizes the rhodium catalyst in water without loss of activity for the hydroformylation of propene to butyraldehyde. In such a biphasic system (under vigorous stirring), the catalyst in the aqueous phase is easily separated from the organic phase by decantation at the end of the reaction, recovered and recycled. 

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