MOF co-catalyst allows selectivity of branched aldehydes of up to 90%

February 26, 2020

Efforts to develop heterogeneous catalysts that appeal to the fine chemical industry have been limited by underwhelming results. Although some approaches have shown promising catalytic activity, "heterogenization" itself is not enough. To be adopted by industry, heterogeneous catalysts must promote selectivity that is difficult or even impossible to obtain with existing catalytic systems--the chemical properties of any proposed heterogeneous catalysts must go beyond easier separation and recycling.

The chemical-flexibility, tuneable pore size and chemical and structural stability of metal-organic frameworks (MOFs) makes them ideal for designing active sites at the molecular level. Able to selectively adsorb different molecules depending on their structure, they can direct selectivity and reaction performance. Many promising catalytic applications using MOFs as precursors for novel materials as well as model systems for understanding heterogeneous catalysis processes have been described. The field of catalysis by MOFs is still in its infancy though since most examples are proof-of-concepts and do not offer attractive advantages to existing catalysts.

In the paper Metal-organic frameworks as kinetic modulators for branched selectivity in hydroformylation, researchers from the Paul Scherrer Institute's Syncat Group, led by Marco Ranocchiari, and EPFL's Laboratory of Molecular Simulation, a computational group led by Berend Smit, used the example of hydroformylation to show how adsorption properties of MOFs can be exploited in catalysis to get results that have otherwise been inaccessible. The methods presented can be used to predict the effect of such microporous co-catalysts in increasing selectivity in any homogeneous or heterogeneous catalytic reaction.

Hydroformylation, or oxo synthesis, is an industrial process for obtaining aldehydes from olefins. Current catalytic processes yield both linear aldehydes, which are key intermediates for the detergent and polymer industry, and branched ones, which are considered a powerful tool for the fine chemical industry because of their possible use in producing enantioenriched products, that is, products featuring a greater proportion of a given enantiomer of a chiral substance.

The linear isomers are often formed with rhodium catalysts. Branched aldehydes are formed from rhodium catalysts with bidentate ligands with directing groups to enhance selectivity. Producing the sought-after branched isomers without these directing groups is still a challenge though and can only be achieved through complex Rh catalysts. They have been shown to result, for instance, in a selectivity for 2-methylhexanale from 1-hexene up to 75% and up to 86% for 2-methylbutanale from 1-butene.

The researchers first screened several catalytic conditions to maximize the yield of the branched product that could be obtained with homogeneous catalysis. They then showed how they could go beyond this limit and achieve much higher branched selectivity by adding MOFs to the reaction mixture. They also tested different MOF topologies to understand the role of the MOF environment in such a change in selectivity.

The group was able to show that the micropores of MOFs push the cobalt-catalyzed hydroformylation of olefins to kinetic regimes that favor high branched selectivity, without the use of any directing groups. The addition of MOFs allowed branched selectivity of up to 90% in these cases, a feat that cannot be achieved with existing catalysts. Monte Carlo and density functional theory simulations combined with kinetic models show that the micropores of MOFs with certain topologies increase the density of the olefins while partially preventing the adsorption of the synthesis gas--this is what leads to the high branched selectivity.

Though the research focused on aldehydes, the methods presented can be used to predict the effect of microporous co-catalysts in increasing selectivity in any homogeneous or heterogeneous catalytic reaction. Researchers can determine the microporous material that has the best chances of increasing selectivity by first choosing those that can adsorb the catalyst while being inert under reaction conditions, and by then using simulations to determine how the microporous materials might change the local concentration of the selectivity determining reactant(s) within the micropores.
-end-


National Centre of Competence in Research (NCCR) MARVEL

Related Aldehydes Articles from Brightsurf:

NASA's Terra highlights aerosols from western fires in danger zone
The year 2020 will be remembered for being a very trying year and western wildfires have just added to the year's woes.

NOAA-NASA Suomi NPP captures fires and aerosols across America
On Sep. 07, 2020, NOAA/NASA's Suomi NPP satellite provided two different views of how fires are affecting the US.

Organocatalyst that controls radical reactions for complex and bulky compound synthesis
In catalytic reactions with organocatalysts, it is difficult to control radical reactions.

Difference in a single gene may boost stress on heart from e-cigarettes in people of East Asian descent
A genetic difference in one gene common in people of East Asian descent may make them more susceptible to rapid heart rates and chemical toxicity if they use e-cigarettes.

IKBFU and Chinese scientists invented a new way of creating meat analogs
In recent years, high-humidity extrusion technology has become widely used as it makes it possible to obtain a fibrous meat-like structure from vegetable proteins.

MOF co-catalyst allows selectivity of branched aldehydes of up to 90%
Heterogeneous catalysts are often preferred because of their robustness and lower operating costs, but homogenous catalysts still dominate when high selectivity is needed -- finding superior heterogeneous catalysts has been a challenge.

Novel biotechnological route developed to obtain fine chemicals from agricultural waste
Preliminary calculations show that the new biotechnological route can increase the value of sugarcane bagasse and wheat straw up to 5,000-fold and multiply the price of ferulic acid by a factor of up to 75 when these residues are converted into coniferol.

Brewing beer that tastes fresh longer
Unlike wine, which generally improves with time, beer does not age well.

Vicinal reaction: A radical strategy for linking three organic groups together
A Japanese research team at Kanazawa University developed a reaction for creating functionalized ketones.

Exposure to second-hand e-cigarettes increasing among young people
A growing number of middle- and high-school students are being exposed to second-hand aerosols from e-cigarettes by living with or being around individuals who are vaping, according to data from a national survey.

Read More: Aldehydes News and Aldehydes Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.