Hydroformylation - Part 1 The chemistry

Industrial Chemistry

Hydroformylation or Oxo Synthesis

Synthesis of aldehydes and alcohols from alkenes.

– Several million tons per year of oxo products are produced worldwide!

– Catalysts are carbonyl complexes of cobalt or rhodium.

– Most important process that uses a transition metal carbonyl complex.

– Linear aldehydes are more desirable than branched ones.

R2 + 2 CO + 2 H2

Co or Rh

catalyst RC

H

O+

RO

CH3

H

Thermodynamics of hydroformylation

Hydroformylation

Hydrogenation

The hydroformylation reaction is highly exothermic but less exergonic due to the decreasing entropy.

The thermodynamically favored product in hydroformylation is the hydrogenation product.

CH3CH=CH2 + H2 + CO CH3CH2CH2CHO ΔG = –42 kJ/mol

CH3CH=CH2 + H2 CH3CH2CH3ΔG = –88 kJ/mol

Cobalt Catalyst (1938, von Roelen)As the most desired products are linear aldehydes, considerable attention has been devoted to increasing the linear:branched selectivity. This focused attention on the mechanism, especially the step where the propene inserts into the Co–H bond, as this can be either Markovnikov or anti-Markovnikov.

The observed rate law for the catalytic reaction (above a minimum threshold of CO pressure) is:

From the above rate laws, a simplified cycle based on [CoH(CO)4] as pre-catalyst is generally accepted (see below).

The inverse dependence on CO pressure suggests a step involving CO dissociation from the catalyst.

The dependence on alkene concentration and on H2 pressure suggests that alkene coordination and hydrogen activation occur either before or during the rate determining step.

d[aldehyde]dt

= kobs[alkene][H2 ][Co][CO]−1

Catalytic mechanism (1st step)

Formation of the Catalyst from the Precatalyst

[Co2(CO)8] 2 [CoH(CO)4]

[CoH(CO)4] [CoH(CO)3] active species (CO dissociation: negative order in CO)

H2! →!!

– CO+ CO! ⇀!!!!↽ !!!!!

Catalytic mechanism (2nd-3rd step)Olefin Coordination and Migratory Insertion

Primary insertion → linear alkyl (anti-Markovnikov) Secondary insertion → branched alkyl (Markovnikov)

As the olefin insertion into the Co–H bond is fast and reversible, [CoH(CO)4] also catalyzes – besides hydroformylation – both olefin isomerization and H / D isotopic exchange in the olefin. However, because a vacant coordination site is needed for elimination, these side reactions are inhibited at higher partial pressures of CO.

The linear-to-branched ratio is determined by the kind of insertion (1ary vs. 2ary, see preceding slide) and by the rate of CO-insertion into the Co-alkyl bond (see next slide).

R[CoH(CO)4] +– CO

+ COCo

OCOC

CO

H

R

(CO)3Co

(CO)3Co

R

CH3

Rfast

linear

branched

faster

both equilibria are reversible

Catalytic mechanism (4th-5th step)CO coordination and insertion into the Cobalt-Alkyl Bond

Observation: High CO pressure increases the n : i aldehyde ratio.Explanation: CO scavenges the 16-electron alkyl complex B to give C. Thus, the inverse reaction (β-elimination to A) is inhibited: the insertion is under kinetic (not thermodynamic) control. At lower CO pressure, the coordinatively unsaturated, 16-electron complex [Co(R)(CO)3] (B) will have a long enough lifetime to undergo β-hydrogen elimination and alkene reinsertion to give the branched alkyls, which are slightly favored thermodynamically (why?).

The formyl complexes D are the only detectable species (resting species) when the steady-state reaction is examined by IR spectroscopy. Under standard catalytic conditions (linear olefins such as 1-octene, [Co2(CO)8], 130-175°C, 250 atm)

(CO)4Co

(CO)4Co

R

CH3

R

(CO)4Co

(CO)4Co

O

O

+ CO

– CO

+ CO

– COR

CH3R

CoOCOC

CO

H

R

(CO)3Co

(CO)3Co

R

CH3

Rfast

faster

+ CO

– CO

+ CO

– COA

B

B'

C

C'

D

D'

Catalytic mechanism (6th step)

C–H-Bond Formation (Rate Determining Step)

Two Possibilities:

(CO)4CoO

R'H2

[CoH(CO)4]

– CO(CO)3Co

O

R'

RH

O+ [Co2(CO)7]

RH

O+ [CoH(CO)3]

Catalytic cycle

Final comments on Co hydroformylation

Disadvantage: high CO partial pressure decreases the hydroformylation reaction rateAdvantage: high pCO increases linear-:branched-ratio Advantage: high pCO decreases alkene isomerization

→ Compromise between rate and regioselectivity!

Higher temperatures increase the rate but decrease the selectivity for the linear product and increase side reactions.

Typical side reactions are isomerization, alkene hydrogenation (typically ca. 1 %), and aldehyde hydrogenation to alcohol (typically 5–12 %). The latter is not unwelcome, as aldehydes are usually later hydrogenated to alcohols.

Drawbacks of Co-Catalysts:

– High temperatures (140 – 175°C) and pressures (200 bar).

– Branched aldehydes are the major product, but linear ones are the desired ones.

Rh catalysts for hydroformylationBinary rhodium carbonyls are not useful because of cluster aggregation:

[RhH(CO)4] is a very active hydroformylation catalyst, but gives olefin hydrogenation and isomerization, and lower linear : branched ratio than cobalt carbonyl catalysts.

Phosphine Ligands are used:

– Stabilize mononuclear complexes by inhibiting cluster formation → higher activity– Suppress olefin hydrogenation and isomerization– Increases the linear : branched ratio (up to 30 : 1)– Are active at ambient temperature and pressure

Therefore, rhodium catalysts have been used in commercial production since 1976.

Union Carbide Process: Propene Hydroformylation with [RhH(CO)(PPh3)3] as catalyst

Molten PPh3 (m. p. 79°C) as solvent, 100°C, 50 atm pressure → 92 % linear aldeyhde, negligible hydrogenation / isomerization.

Problems: Cost of rhodium, degradation of PPh3.

Mechanism

Under standard conditions, CO intercepts the coordinatively unsaturated alkyl complex and the insertion is irreversible → kinetic control. The primary insertion (antimarkovnikov) is favored because the corresponding transition state is less crowded. Under process conditions, the linear:branched ratio is typically of 8–9 : 1 (the stereochemistry of insertion (1ary/2ary) is not shown).

Support for the Proposed Mechanism Effect of [PPh3]: – reduces reaction rate (because the precatalyst must dissociate a PPh3):

– increases linear : branched product ratio

– suppresses olefin hydrogenation and isomerization

Effect of P(CO): – higher CO partial pressures cause high linear:branched ratio up to a limit, too high CO partial pressures lowers linear:branched ratio.

Effect of P(H2): – the rate law is first-order in P(H2) → step (d) is rate-determining.

Effect of [olefin]:– the rate law is zero-order in olefin at high olefin concentration

– at lower [olefin], the rate law becomes first order in [olefin], a step before step (d) becomes rate-determining.

Two-Phase Rhodium Hydroformylation Catalysts

Introduced by Kuntz at Rhône-Poulenc in 1981. Celanese-Ruhrchemie currently operates several hydroformylation plants based on this technology (C. W. Kohlpaintner, R. W. Fischer, B. Cornils, Applied Catalysis A: General, 2001, 221, 219.) Water-soluble sulfonated triphenylphosphine ligand P(C6H4-m-SO3Na)3 (TPPTS) Water-soluble catalyst [RhH(CO)(TPPTS)3], very high (9–) formal charge, totally insoluble in all but the most polar solvents.

A two-phase catalytic system results, in which the butanal product is essentially in the organic phase and can be easily separated. Similarly, the recovery of the catalyst is straightforward as it stays in the aqueous phase.

An excess of the phosphine is required for good linear/branched selectivitities, as with conventional Rh/PPh3 catalysts, but lower concentration is required because the dissociation equilibrium of TPPTS in water is shifted towards the rhodium complex.

The solubility in water of shorter chain alkenes (C2–C4) is high enough to allow hydroformylation.

→ Rather high linear-to-branched regioselectivities (16–18:1) can be obtained for propylene → Rates are slower than with conventional Rh/PPh3 catalysts due to lower alkene concentration in the water phase. → Alkenes higher than 1-pentene are not soluble in water and cannot be used.