Efficient cofactor regeneration and reuse are highly desired for many important biotransformation applications. Here we show for the first time that cofactor NAD(H) covalently attached to micro particles, which can be easily recovered and reused, effectively mediated multistep reactions catalyzed by enzymes that were also immobilized with the micro particles. Such an immobilized enzyme-cofactor catalytic system was examined for the production of methanol from CO(2) with in situ cofactor regeneration. Four enzymes including formate, formaldehyde, alcohol, and glutamate dehydrogenases were coimmobilized using the same particles as that used for cofactor immobilization (enzymes and cofactor were immobilized separately). Reactions were performed by bubbling CO(2) in a suspension solution of the particle-attached enzymes and cofactor. It appeared that the collision among the particles afforded sufficient interactions between the cofactor and enzymes, and thus enabled the sequential transformation of CO(2) to methanol along with cofactor regeneration. For a 30-min batch reaction, a productivity of 0.02 micromol methanol/h/g-enzyme was achieved. That was lower than but comparable to the 0.04 micromol methanol/h/g-enzyme observed for free enzymes and cofactor at the same reaction conditions. The immobilized system showed fairly good stabilities in reusing. Over 80% of their original productivity was retained after 11 reusing cycles, with a cumulative methanol yield based on the amount of cofactor reached 127%. That was a promising enhancement in cofactor utilization as compared to the single-batch yield of 12% observed with free enzymes and free cofactor.
Structural modifications of molecular cobalt catalysts have provided important insights into the structure-function relationship for the hydrogen evolution reaction. We have shown that replacement of equatorial pyridines with more basic and conjugate isoquinoline groups of a pentadentate ligand results in lower overpotential and higher catalytic activity for electro- and photolytic H production in aqueous solutions. To fully understand the electronic and steric effects of the axial group that lies trans to the proposed cobalt hydride intermediate, isoquinoline groups were introduced in two new pentadentate ligands, N, N-bis(2-pyridinylmethyl)[3-(2-pyridinyl)isoquinoline)]-1-methanamine (DPA-1-MPI) and N, N-bis(2-pyridinylmethyl)[1-(2-pyridinyl)-isoquinoline)]-3-methanamine (DPA-3-MPI). Despite a slight structural difference of the introduced isoquinoline group, the resulting cobalt complexes display drastic changes in their electro- and photochemical properties. There are positive shifts of 290 and 260 mV, respectively, for the Co/Co and Co-H/Co-H couples from [Co(DPA-1-MPI)(HO)](PF) to [Co(DPA-3-MPI)(HO)](PF), with the former being ∼32 times as active as the latter in photocatalytic H production. Density functional theory (DFT) calculations show that the protonation of Co to yield the Co-H species is energetically more favorable for [Co(DPA-1-MPI)(HO)](PF) than that of [Co(DPA-3-MPI)(HO)](PF). Both experimental results and DFT computations suggest that the presence of a planar conjugate bipyridyl unit or its isoquinoline derivative is a key feature for stabilizing low valent Co species toward proton binding. The incorporation of an electron-donating group trans to the proposed Co-H species also facilitates proton binding and H-H bond formation, which is proposed to occur by the heterolytic coupling of Co-H species. The overall catalytic H evolution is presented as the modified electron transfer (E)-proton transfer (C)-electron transfer (E)-proton transfer (C) (mod-ECEC) pathway. This study provides important new insight into the electronic and steric factors controlling catalytic H production by Co complexes with pentadentate ligands.
Ammonia borane (NH(3)BH(3), AB) is an intriguing molecular crystal with an extremely high hydrogen capacity and moderate thermal stability. In the present study, we show a simple but effective approach for destabilizing AB for promoted hydrogen release at moderate temperatures. It is found that mechanically milling with magnesium hydride (MgH(2)) can dramatically improve the dehydrogenation properties of AB, on both the kinetic and thermochemical aspects. For the mechanically milled AB/0.5MgH(2) material, over 8 wt% hydrogen can be released from AB within 4 h at around 100 degrees C without undesired volatile by-products. Moreover, the dehydrogenation reaction of the AB/0.5MgH(2) sample becomes significantly less exothermic than that of neat AB. In situ X-ray diffraction results demonstrate that the MgH(2) additive well maintains its phase stability during the ball-milling and the subsequent heating processes. Meanwhile, Raman spectroscopy and in situ(11)B NMR studies show that the MgH(2) additive exerts considerable influence on the chemical bonding state and decomposition process/products of AB. Combined phase/structure analyses results suggest that MgH(2) exerts effect via developing solid phase interaction with AB.
Inheritance and transformation: an in situ topological transformed NiCoFe-MOF nanosheet electrocatalyst exhibits highly efficient activity for water oxidation in an anion exchange membrane water electrolyzer.
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