New Approaches to Alkene Hydroformylation
Since the advent of hydroformylation as an industrially important process seventy years ago, this method of converting alkenes into aldehydes has become one of the most important metal catalysed process of all. The first systems used cobalt complexes and despite the higher activity and greater selectivity of rhodium complexes, petrochemical giants such as BASF, Shell, Nissan and Exxon still use processes involving cobalt. The reason for this centres on the lower expense, stability and robust nature of cobalt catalysts modified with tertiary phosphines.
One of the main challenges in the industrial application of homogenous catalysis is the problem of catalyst separation and recovery. This has led to many innovative approaches such as polymer supports, biphasic mixtures, supercritical gases and ionic liquids. However, the equation of synthesis, activity, stability, energy and cost is a difficult one to square even without taking into account the (re)design of the vast reactors required. Hence, systems that can be used in existing reactors and are tolerant of the poisons present in the feedstock are very favourable. The hydroformylation reaction is shown below:
A major advantage of tertiary phosphines is the potential to tune the steric, electronic and physical properties of the ligand and thus influence the reactivity at the metal to which they are attached. Recent reports of a new phosphine derived from limonene prompted us to explore the possibility of using a modified version of the ligand to modify the hydroformylation of 1-octene by [Co2(CO)8]. The electronic and steric properties proved to be very encouraging, providing the right amount of electron density to the metal centre and enough steric bulk to promote linear product formation over that of the branched product (without slowing the rate of reaction unduly). Additionally, the modification we carried out sought to address the problem of catalyst recovery. This involved attaching a C-18 chain to the phosphine rendering the physical properties of the complex such that the product could be separated by distillation. A further advantage of cobalt is that, depending on the conditions, the products are alcohols rather than aldehydes which are often the ultimate target compound. The process is shown below:
In addition to the formation of branched products, two other processes that lower conversion to the desired product need to be minimised. These are octane formation (direct hydrogenation of the 1-octene feedstock) and aldol condensation product formation. The latter reaction, often termed 'heavy' formation, involves the reaction of the aldehyde concentration that builds up during the hydroformylation process. Therefore, rapid conversion of the 1-nonanal formed to the desired 1-nonanol is clearly beneficial and would minimise loss of aldehyde through formation of 'heavy' products.
The new ligand bearing the C-18 chain is a mixture of R and S isomers as shown below. This subtle stereochemical difference and its manifestation in terms of basicity and steric profile leads to differences in reactivity between the two isomers, as shown in complexation, protonation and methylation studies. However, there is relatively little difference in catalytic activity between the isomers and so a mixture can be used in the hydroformylation process. Our investigation demonstrated that the use of this ligand led to an effective and selective hydroformylation system for 1-octene with the advantage that the products could be separated by distillation from the catalytic species. This work was done in conjunction with Sasol and has now been patented.
The LIM-18 ligand
A cobalt complex containing two LIM-18 ligands (A. Polas)
Further details of this work can be found in the following article:
A. Polas, J. D. E. T. Wilton-Ely, A. M. Z. Slawin, D. F. Foster, P. J. Steynberg, M. J. Green and D. J. Cole-Hamilton, Limonene derived phosphines in the cobalt catalysed hydroformylation of alkenes. J. Chem., Soc., Dalton Trans., 2003, 4669.
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