Cobalt based catalyst converts carboxylic acids to alcohols
Publication in Science on new clean and sustainable conversion method
A team of chemists from the University of Amsterdam (UvA) has developed a cheap and effective catalyst for the production of alcohols. In the current issue of Science the chemists describe how their new soluble cobalt based catalyst converts a wide range of esters and carboxylic acids to alcohols under mild conditions. The new catalyst outperforms existing precious metal-based catalysts, increases the sustainability of the production of alcohols and enables the 'green synthesis' of intermediates for plastics and fibers.
The new catalyst is highly relevant for both fine chemical transformations and biomass conversion. It enables pharmaceutical and fine-chemical industries to increase the sustainability of their conversion technology. In the production of high-value chemicals from biomass, the new catalyst facilitates the chemical utilisation of abundantly available oxygen-containing functional groups.
The catalyst was developed at the UvA’s Van 't Hoff Institute for Molecular Sciences by Ties Korstanje, Jarl Ivar van der Vlugt, Kees Elsevier and Bas de Bruin. Their research was part of the UvA research priority area Sustainable Chemistry. The chemists partnered with chemical company DSM in a project for CatchBio, an ambitious 8-year Dutch research program in the field of catalytic biomass conversion.
Essentially, converting esters and carboxylic acids to alcohols means adding hydrogen atoms. The chemicals industry traditionally provides these through so-called stoichiometric reagents (such as aluminum- or borohydrides). However, these pose an inherent safety risk and produce undesirably large amounts of waste material.
Such disadvantages can be eliminated by using molecular hydrogen (H 2), but this requires appropriate catalysts - auxiliary compounds that facilitate reactions without being consumed themselves. This has turned out to be quite a struggle, in particular with the carboxylic acids, which often destroy the catalyst developed for their transformation. ‘Only a few solid, heterogeneous catalysts have been successfully employed’, says Amsterdam researcher Bas de Bruin. ‘However, these require very high temperatures and pressures.’
According to De Bruin the use of soluble (homogeneous) catalysts could in principle enable far milder and more sustainable reaction conditions. A few research laboratories have confirmed this approach, but all reported examples of such catalysts have been based on expensive, toxic and scarce metals such as ruthenium and iridium. De Bruin: ‘In a chemical sense, the desired transformation can be compared to producing whiskey on the rocks by starting out with lemon juice. This is a rather difficult task, as you can imagine.’
Tuning catalytic performance
The novel soluble catalyst De Bruin and co-workers now present in Science more than adequately performs this task. It is not based on precious metals but on the cheap and abundantly available metal cobalt. The researchers cleverly 'tuned' the catalytic performance of single cobalt atoms by equipping them with a sophisticated molecular surrounding: the resulting catalyst consists of Co(BF 4) 2·6H 2O paired with a 'TriPhos' tridentate phosphine ligand.
Experiments performed in cooperation with DSM Innovative Synthesis (the chemical synthesis department of DSM, located in Geleen) confirmed that the new catalyst converts a wide range of esters and carboxylic acids to their corresponding alcohols under mild reaction conditions (100 °C, 80 bar H 2). The system outperforms all earlier developed precious metal-based catalysts for this conversion, with turnover numbers up to 8000, indicating that industrial application is within reach.
The selective catalytic hydrogenation of esters and carboxylic acids to the corresponding alcohols could enable the 'green' synthesis of many products in various sectors of the chemical industry. One example is the conversion of (bio-based) succinic acid into the bulk chemicals 1,4-butanediol, THF or γ-butyrolactone, which can be used as solvents and as intermediates for the production of plastics and fibers. Also various pharmaceuticals require a selective hydrogenation step of either an ester or carboxylic acid moiety in their synthetic route.
Ties J. Korstanje, Jarl Ivar van der Vlugt, Cornelis J. Elsevier, Bas de Bruin: Hydrogenation of carboxylic acids with a homogeneous cobalt catalyst Science, 16 October 2015. DOI: 10.1126/science.aaa8938