Zeolite-Catalyzed Isomerizations of Conjugated Double Bonds: Muconic Acid and Friends

Muconic acid (MA) is a value-added product with conjugated double bonds and reactive dicarboxylic groups. The MA market is growing, due to its potential as a precursor for value-added derivatives such as terephthalic and adipic acids and as a monomer for specialty polymers [1,2]. The transformation and polymerization of MA strongly depend on its isomeric form. Of 3 possible isomers: cis,cis-MA (ccMA), cis,trans-MA (ctMA) and trans,trans-MA (ttMA), only the latter was found to be an efficient precursor in polymerizations or in Diels-Alder cycloadditions to terephthalic acid.

Chemical ways for producing MA use petroleum-based feedstock (phenol, ...) in the presence of acids or heavy metal catalysts. Now, biotechnological pathways are being developed with decent titers (85 g/L) from bio-based feeds (e.g. catechol, glucose, ...). However, only ccMA is obtained, which requires an additional isomerization into the trans,trans prior to its valorization. Most known isomerization processes (see our recent review for an overview [1]), rely on palladium or the use of sacrificial reagents, such as iodine radicals under UV irradiation [3]. Safety, cost and limitations in feedstock concentration (<10 wt% for iodine system), force the development of new heterogeneous catalytic isomerization routes, with a focus on moderate conditions and productivity.

Here, we present a novel Ru-zeolite based catalytic system without sacrificial reagents that works at a mild 100 °C. The system allows high concentrations to be used and mixtures in excess of ttMA are obtained. An investigation of zeolite host, ester feedstock, Lewis acid characteristics and the nature of the Ru mechanism will be presented.

References

[1] Khalil, I., Quintens, G., Junker, T., Dusselier, M. ‘Muconic acid isomers as platform chemicals and monomers in the biobased economy’ Green Chemistry, 22, 1517 (2020)

[2] Carraher et al. Green Chemistry 19, 13 (2017).

[3] Settle et al. ChemSusChem 11, 1768 (2018)