PROCESS DESIGN AND ENZYME IMMOBILIZATION TECHNOLOGIES ENABLING BIOCATALYTIC CO2 CAPTURE AND UTILILZATION

Advanced technologies for industrial utilization of CO2 as a carbon source are necessary to face the annual CO2 anthropogenic emissions (~9 Gton CO2/y). The deployment of carbon capture and utilization (CCU) processes in the industry is related to the availability of environmentally and energy-sustainable technologies that enable the use of CO2 streams from several sources (e.g. biorefineries) and bridging the gap with the biomass refinery chain. This way can lead to the production of biomass-derived and CO2-based commodity chemicals as well. Solvent-based CO2 capture from point sources (e.g. with amines-based solvents) is the most advanced CO2 capture process and it is ready for deployment in power plants. Its diffusion in other industrial sectors is limited by several factors, among these the mandatory thermal solvent regeneration is made possible by the low-heat steam available at the power plants. In addition, solvent-based capture provides an almost pure gaseous CO2 stream ready for geological or deep ocean storage as well as for enhanced oil recovery. Biocatalytic processes open new routes for CO2 capture and utilization thanks to the mild conditions provided by enzyme catalysis whenever proper and sustainable substrates are available. CO2 capture by absorption promoted by carbonic anhydrase has been deeply studied and developed up to a demonstration scale in the last twenty years [1]. The immobilization of carbonic anhydrase has been developed through several techniques based on covalent binding on support surfaces or into other solid matrices [2]. These techniques allowed not only the stabilization of the enzyme at the alkaline pH and temperatures typically used in the capture process but also the confinement of the biocatalyst in the absorption unit. The latter aspect is crucial to envisage new CCU strategies based on direct CO2 fixation in aqueous solvents made possible by biocatalytic or hybrid processes [2]. Indeed, enzymatic reactive absorption of CO2 into potassium carbonate solutions provides carbonate/bicarbonates solutions as intermediate products that could be used for the biocatalytic/biological fixation of CO2 as an alternative to thermal desorption. Cofactor-free de-carboxylases from Rhizobium sp. and Aspergillus oryzae have been proposed and characterized as biocatalysts for the carboxylation of phenolic compounds into valuable carboxylic acids (e.g. salicylic acid) [3]. The regioselective carboxylation catalyzed by de-carboxylases makes possible the fixation of CO2 to the electron-rich phenolic substrate provided that a sufficient excess of CO2/bicarbonate is dissolved in the liquid medium [4] (Fig. 1). Phenolic compounds (e.g. catechol, guaiacol) can be produced by fast catalytic pyrolysis of lignocellulosic biomasses and bio-oil fractionation. This process [5] provides a sustainable source of electron-rich substrates for CO2 fixation into valuable bulk chemicals. Preliminary studies have been carried out to assess the feasibility of the integration of the enzymatic reactive absorption and the enzymatic carboxylation of phenols as a fully biocatalytic CCU strategy [6]. Results encouraged to exploit the bicarbonate-rich solvent (1.8 M) produced in an enzymatic reactive absorption unit with immobilized carbonic anhydrase as a feedstock for an enzymatic carboxylation unit converting phenols (catechol, resorcinol, and orcinol) into carboxylic acid. The joined research activity is currently focused on the immobilization of 2,6-dihydroxybenzoic acid decarboxylases (2,6-DHBD) to obtain stable a biocatalyst under the conditions highlighted by the preliminary design study. The morphology of the biocatalyst was selected to allow the design of a continuous flow bioreactor. The reversible nature of the carboxylation reaction asks for segregated reactors (e.g. packed beds) thus, as a reference case solid microparticles were selected first. Commercial epoxy-activated beads (Immobead®) and silica beads were used upon specific amine and glutaraldehyde activation to obtain the covalent immobilization of the enzyme. Immobilization yield (about 97%) provided an enzyme loading close to 120 mg/g in the case of modified commercial Immobead® that was selected for further optimization and scale-up carboxylation tests. About 30% of the specific activity was retained by 2,6-DHDB against catechol upon immobilization. Ongoing studies on the operational and storage stability of the immobilized biocatalyst are planned to scale-up the biocatalyst preparation and set-up the continuous flow bioreactor for enzymatic carboxylation of reference phenolic substrates.