The oxidised form of nicotinamide adenine dinucleotide (NAD+) is a natural cofactor participating in dehydrogenase-catalysed bio-oxidations. The stoichiometric use and unaffordable cost of the cofactor demand an efficient in situ regeneration system to ensure the whole process economically viable. Considering the drawbacks of the existing regeneration methods, this research has established a novel cofactor regeneration strategy, i.e., heterogeneous catalytic regeneration by using supported metal catalysts.
A series of mono metal catalysts supported on metal oxides (SiO2, MgO and CeO2) were firstly examined for NAD+ regeneration, where Pt outperformed Pd, Au, Ni, Co and Cu. The difference in metal activity was correlated with hydrogen binding energy (HBE), while Pt displayed highest reaction rate due to its moderate HBE. For the first time, proton-driven NAD+ regeneration with hydrogen (H2) formation (H+ + NADH = H2 + NAD+) over supported metal catalysts was demonstrated. To further understand and improve the catalytic performance, the effects of electronic state and structure sensitivity of Pt nanoparticles were then investigated. An obvious increase in reaction turnover frequency (TOF) was obtained on electron-deficient Pt (Pt/CeO2) than the electron-enriched one (Pt/MgO), with reaction mechanisms being proposed.
Moreover, the structure sensitivity of the NAD+ regeneration on supported Pt catalysts was investigated. A volcano-like TOF trend were determined with particle size ranging in 2.2-7.1 nm. It was concluded that the optimal particle size originated from the synergetic effect of terrace and corner/edge site. These findings provided in-depth knowledge into the subject and practical information for catalyst design. Heterogeneous catalysis-mediated NAD+ regeneration avoids the hazard of oxygen (typically used to oxidise NADH) to enzyme lifetime and ease the separation difficulty in downstream processes. It also offers an opportunity to combine the synthesis of value-added chemicals (via NAD+-dependant enzymes) with concurrent H2 production, achieving 100% atom efficiency.
Dehydrogenase-catalysed Glycerol and glucose bio-oxidation coupled with multiwall carbon nanotube (MWCNT) supported Pt catalyst has been studied as model systems and showed excellent compatibility, where 1,3-dihydroacetone and gluconate were continuously produced for 58 and 48 hours with simultaneous H2 formation. The MWCNT support served to accommodate Pt nanoparticles and to immobilise both the enzyme and cofactor. Such formed “nanoreactor” displayed desirable stability, retaining equivalent activity after three recycles. It demonstrated the significant potential of using supported metal catalyst for NAD+ regeneration and subsequently biocatalytic chemical and H2 co-production.
Based on the above findings, it can be argued that supported Pt catalysts were indeed functionally acting as certain enzymes, i.e., hydrogenase. Therefore, at the end of this research, we were also wondering if heterogeneous catalysts could also mimic and replace other important enzymes, e.g., formate dehydrogenase (FDH) to achieve CO2 utilization. Consequently, Pd, Pt and Cu catalysts were examined as FDH mimic for CO2 transformation to formate (with NADH as the reductant). The high yield/selectivity of CO2 to HCOOH (~90% at pH 9) indicated the successful mimicking of FDH by the artificial Cu/MWCNT catalyst.