The hydroformylation of terminal alkenes is an important reaction in the chemical industry and sees on the wayside of 800,000 tons produced per year1, an important cornerstone to a plethora of chemical syntheses2.
Biphasic catalysis was theorized by Manassen et al.4, implemented by Joó et al. and realised by Kuntz, where was suggested the utility of a "liquid support"; two immiscible liquid phases would separate the catalyst and the reactant. This would effectively heterogenize the catalyst by "separating" it from the reactant phase, facilitating species separation, whilst retaining the crucial rate-defining properties of homogeneous catalysis.
Eventually removed from its roots as a homogeneous process in its evolutionary chain5, the industrial hydroformylation of 1-propene took towards the aqueous biphasic direction and featured wholly improved throughput and selectivity (near exclusivity of aldehydes, linear/branched ratios of up to 98%6) as well as easy product separation via decantation7 and minimalization of catalyst leaching via higher recyclability6, all facilitated via agency of water and its unique characteristics8.
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Since its inception, a wave of propositions as regards improvements and variations on the RCH/RP model had been instigated, though many falling short in terms of practical application(REF), with a single-minded determination to bolster the activity and selectivity of the catalytic process and to push the boundaries of biphasic catalysis as far as possible.
Review - Methods and Systems in Biphasic Hydroformylation
The RCH/RP Process - Aqueous "Biphase" Catalysis
In response to the requirement that the catalyst be water-soluble, sulphonated triphenylphosphine ligands were acknowledged as affording rigidity and high solubility in polar media, i.e. water. Meta-triphenylphosphino monosulfonate(m-TPPMS), the first known water-soluble sulphonated aryl phosphine of its kind( from book), was used due to its, indeed, high water solubility and high thermal stability(REF).
Figure.2 -Mono-, Di- and Tri-sulfonated aryl phosphines3.
It was soon discovered that despite the relative ease of synthesis and its putative water solubility, m-TPPMS-modified Rhodium was prone to catalyst leaching, a sign that the mono-sulfonate substituent was not lipophobic enough9. It emerged that further sulfonation of the remaining aryls, as can be seen in figure 2, should bring about a higher degree of thermal stability as well as kinetic stability, such as to phosphine oxidation due to steric protection, but more importantly it should increase hydrophilicity and as such prevent considerable expense from catalyst loss from leaching; it is notably immune to leaching in 1-butanal, the major product in the RCH/RP process9 .
Since its conception in 1974 and documented by Kuntz in 197510, the trisulfonated ligand TPPTS had been effectively synthesised by direct sulfonation using "fuming sulfuric acid" (oleum) at 40oC followed by hydrolysis and neutralisation via NaOH, leaving an immiscible aqueous layer of sodium sulfate. The resulting mixture contained primarily the tri-sulfonated aryl phosphine, TPPTS and its unwanted oxygenated form OTPPTS, in an expected ratio of 55:45%, as well as other minor related phosphines9. Separation and purification from the oxide was then achieved by precipitation via aqueous methanol, for purities reportedly up to 95% TPPTS. By 199311, ratios of 94:6 were being observed, with adjustments to the conditions in the production of TPPTS. As of now, a number of academic efforts (find some refs =) are being focussed in finding pertinent alternatives to TPPTS via use of alternative functionalities that have notable strengths that bolster one or more aspects of the hydroformylation process. Up until this point, there has been no industrially viable replacement that has been found.
Formation of the water-soluble catalyst itself is achieved through the reaction between [Rh(CO)2X2] and TPPTS under an atmosphere of (CO/H2). In-depth discussion by Arhancet et.al [38 book] surrounded the formation of the resting state of the active Rhodium catalyst of the form, [HRh(CO)(TPPTS)3], using the acetylacetonate analogue of the Rhodium precursor, [Rh(CO)2(acac)]; here, a solution of the ligand is added to [Rh(CO)2(acac)] to yield the precatalyic solution, which is exposed to a syngas atmosphere at rtp for a time of 6hrs. Afterwards, the catalyst is purified by removal of residual Rhodium metal by filtration under N2 and extraction via saturated (CO/H2) absolute ethanol, which washes the unreacted/oxygenated TPPTS, as both are much more soluble in ethanol[38book].
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Aside the merit of this process, it is unanimously noted(REF) that in general, the hydroformylation of alkenes which are longer than 1-pentene (C>5) under the conditions prescribed by this process become increasingly difficult, to the point where the rate of hydroformylation becomes so laborious that potential side reactions, for example alkene isomerisation, become much more frequent, so that hydroformylation is rendered industrially unviable. The cause of this inverse proportionality between rate and length of n-alkene chain is an increase in the hydrophobicity of the reactant alkene. This results in a sharp decrease in the miscibility of the reactant phase with the aqueous catalyst phase, and thus a drop in activity.
Modification of the phosphine ligand such that the overall hydrophobicity is increased
Commercially, therefore, hydroformylation of higher alkenes (C>6) are catalyzed by cobalt-carbonyl species(7-9 saviour 1), which give an overall higher rate than that afforded by the current Rh equivalent, despite issues with catalyst stability, leaching and expensive recovery procedures(FIND THAT REF).
The most pertinent endeavours fall on not those which aim to design extravagant processes in which to propagate the already industrially sufficient transformations of lower alkenes, but those which focus on the hydroformylation of higher, asymmetric and functionalised alkenes, which present many unknowns on which light can be shed.
Variations on the TPPTS ligand
The hydroformylation of alkenes which possess C>5 becomes difficult as was discussed earlier. One answer documented by Fell et.al[11book] is to increase the hydrophobicity of the catalyst itself. By adding methylenes in between the aryl phosphine and the para-substituted sulfonate as below, the surfactant ability of the catalyst increases and effectively creates a higher catalytic surface area for which to accommodate more hydrophobic alkenes.
Figure.3 - methylated arylphosphinosulphonate
This is reflected in light-scattering experiments[43book] performed on the aqueous catalyst solution containing the above ligand derived from [Rh(CO)2(acac)], which elucidate the presence of surfaced inverse micellar species of approximate hydrodynamic radius 19 Å, caused by aggregation of the catalyst at the phase boundary, providing concentrated hydrophobic spots that can accommodate alkene. In the hydroformylation of 1-octene, the turnover rate for conversion to 1-nonanal is reportedly 160 h-1 for the methylated phosphine and 90 h-1 for TPPTS; selectivity was also superior, (look up [14book]).
Another take on the hydroformylation of 1-octene sees an adaptation of the idea of "interfacial" catalysis, using the species [Rh(cod)Cl]2TPPTS] (cod=cyclooctadiene) which alone sees only a marginal increase in activity over that of TPPTS. It is after the addition of a so-called "promoter ligand", which in this case is PPh3, that interesting results arise. The whilst lipophilic phosphine, has a very strong affinity for the catalytic species, and thus acts as a sort of magnet, holding the aqueous catalyst at the interface. In the elucidation of the origin of this effect, Chaudhari et.al12 devised an experiment whereby promoter-free reaction mixture was recycled with the subsequent inclusion of promoter ligand, resulting in a high increase in catalytic activity. The participation of the suspected modified phosphino-rhodium complexes in this effect was ruled out via analysis via a monophase using methanol, yielding marginally different turnover rates between the unmodified complex and the phosphinated complex.
Chelating Phosphine Ligands and Di-Rhodium catalysts - the Cooperativity effect
Expansion of the repertoire of phosphine ligands has unearthed a new concept in their use. In the Rhodium-catalyzed organic phase hydroformylation, it was discovered that diphosphines possessed superior selectivity of linear aldehydes over their monophosphine cousins13, thanks to the rigid cis-arrangement of the diphosphine, which directs the kinetic attack of carbon monoxide on the terminal of the alkene, giving a terminal aldehyde14. However, there is a corresponding drop in activity seen in the catalysis of these species15. The corresponding sulfonation of the diphosphine is somewhat less facile than for the single species, such that only a small number of examples are known.
Two such examples are seen in the study carried out by Herrmann et.al of sulfonated-diphosphine ligands of Rhodium in the hydroformylation of 1-propene:-
i) BINAS(8) and ii) NORBOS(2) are sulfonated ligands from novel procedures14:-
Figure.4 - Scheme of novel sulfonation of NAPHOS (7) and DMTPPNOR (1)
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It appears that the increased steric crowding experienced by the multiple aromatic groups is para-directing in the case of DMTPPNOR. Similarly, there is reasonable control in the degree of sulfonation, which is otherwise rather difficult
Phosphanorbornadienes, of which NORBOS is one, have displayed a noteworthy effectiveness in hydroformylation, such as that of ethyl acrylate[4herrmann]. In this report, it was observed that the catalytic activity of 1 did not diminish as temperature was dropped, as is the case with many other mono-phosphines. In addition, selectivities greater than 99% were being returned for the linear product, formyl propanoate. Hydroformylations of 1-hexene and styrene were also reportedly up to ten times quicker than otherwise presented by standard rhodium triphenylphoshine.
Following the hydroformylation of 1-propene, it was noted that starting ligand/metal ratios were as follows:- BINAS:- 6.8:1, NORBOS 13.5:1, TPPTS 80:1. The following chart depicts the result:-
Figure.5 - Relative catalytic activities and n/i selectivities
The results were in spite of the apparent handicap in favour of TPPTS. The startling result is the Rh-BINAS, which not only surpasses the TPPTS activity by a factor of twelve, but also ensures on average 4 more linear molecules per 100. The phosphanorbornadiene species NORBOS sees a large increase in activity relative to
Despite these seemingly systematic improvements on the otherwise limited industrial application with regards to alkene length, there still seems to be a degree of mass transfer complications that would otherwise be less taxing were the reaction to take place in a purely homogeneous environment with respect to both catalyst and substrate.
Further developments of the biphasic model begin to include ideas of changing the constitution of the "aqueous" phase, which, despite its many advantages, is the main culprit in the retardation that arises in catalysis of larger organic molecules.
Of all the processes encountered, it can be said that in the interests of science, there is a lot of unexplored ground. Though many of these concepts have been extensively researched and thus stagnant, there are those which are relatively new, or show promise, but have not been properly implemented. It goes to show that seemingly promising concepts that were discovered, refined and eventually industrialised and remain to this day, are not the be all and end all of catalysis. Although the hydroformylation of alkenes, which has been touched upon in this review, is one of the most industrially important, biphasic catalysis has documented many other types of reactions that show a teasing versatility that remains attractive to research. Analogously to the time when this form of catalysis was discovered, technological and thus synthetic standards will improve, bringing with it the platform to more capably explore this area of catalysis and bring out its true potential.