Brief Description



Oxidation based catalytic technologies are among these key technologies applied in petrochemicals, power sources, pharmaceuticals, fine chemicals and environmental protections, and, particularly, exploring novel catalytic technologies for both utilizing renewable resources and eliminating pollutants has been attracting much more attentions than ever. However, in the fundamental research of redox chemistry, the great challenges still exist in understanding the oxidative relationships of the key active intermediates including metal oxo (Mn+=O), hydroxo (Mn+-OH) and hydroperoxide (Mn+-OOH) occurring in oxidation processes, and in clarifying how the co-catalysts modulate the reactivity of the redox catalysts; these unveiled puzzles substantially retards the designing of catalysts and elucidating of those elusive oxidation phenomena. Our catalytic oxidation & mechanism group mainly focuses on addressing these fundamental issues in oxidations, meanwhile developing novel catalytic technologies for biomass valorization, pollutant treatments, and industrial applications.

 

1) Understand the oxidative relationships of the active intermediates in oxidations and the synergetic effect in the components of the redox catalysts

In biological and chemical oxidations, there are three major categories of key active intermediates including metal oxo (Mn+=O), hydroxo (Mn+-OH) and hydroperoxide (Mn+-OOH); the enzyme’s selections on them are not randomly but definitely based on their distinct reactivity differences. However, the knowledge of their relationships in oxidations is very limited up to now, mainly due to that the platform to address these issues is very scarce. It is worth to note that the viable differences between Mn+=O and Mn+-OH species are their metal-oxygen bond order (M=O vs M-O) and protonation state, which lead to the occurrence of their differences in physicochemical properties such as the net charge, redox potential and the oxidative reactivity. Clarifying their oxidative relationships would benefit the understanding of the enzyme’s selections on them and thus promote the understanding of the enzymatic mechanisms. Meanwhile, it may also provide clues to the fundamental understanding of those elusive oxidation phenomena, and provide the foundations for the rational design of oxidation catalysts. One of our on-going projects is to clarify the oxidative relationships of these active intermediates by investigating their reactivity similarities and differences in hydrogen abstraction, oxygen transfer and electron transfer processes using different chemical models.

In addition to those redox metal ions which directly participate in oxidation, there still have certain other components in redox catalysts which can modulate the stability and/or reactivity of the catalysts. In heterogeneous catalysts, many inactive metal ions are used as the additives in catalyst to modify its stability and reactivity, however, their roles in catalysis are not fully understood until now. Similar phenomena may also exist in the biological system. For example, the oxygen evolution center in Photosystem II contains a Mn4CaO5 cluster; while the manganese elements are proposed to directly catalyze water oxidation, the role of Ca2+ in cluster is still elusive. Accordingly, one of our on-going projects is to investigate how these redox inactive metal ions modulate the reactivity of the redox catalysts. Our findings on this issue would provide clues to understand their roles in biological and chemical oxidations, benefit the design of the selective oxidation catalysts and control of their reactivities.

 

Acc. Chem. Res., 2013, 46, 483-492


2)Pd(II)/LA catalyst for C-H bond activations in organic synthesis

    It was found that increasing the net positive charge of the redox metal ions can increase its redox potential, and its electron transfer ability increases as well. Increasing the net charge can be achieved through protonation by Br?nsted acid, or interaction with Lewis acid. Inspired by this, we found that adding non-redox metal ions to the redox active metal ions can greatly promote their capability in stoichiometric and catalytic oxidations. Following up these findings, we further found that non-redox Sc3+ cation can accelerate Pd(II)-catalyzed Wacker-type oxidations better than Cu2+, indicating that the Lewis acidity of the Cu2+ cation plays the significant roles in Wacker-type oxidations as well as its redox properties. Inspired by this, we has coined Pd(II)/LA catalyst for versatile Pd(II)-catalyzed C-H bond activations in organic synthesis.

Dalton Trans., 2015, 44, 17508-17515.

 

3) Explore new catalytic oxidation technologies for biomass valorization

With the diminishing of the fossil resources, developing new technologies to utilize versatile biomass as the alternative feedstock of energy and chemicals has been attracting more and more attentions than ever. In exploring new chemicals from polysaccharides, C6 based resources including glucose and fructose have been fully recognized. For example, synthesis of 5-hydromethyl furfural (HMF) and its sub-products have been extensively explored, whereas the studies on C5 resources are still very limited. Compared with C6 based HMF, C5 based furfural is now industrially produced in large scale, and its raw resources come from the agricultural raw materials including corncobs, oat, wheat bran, and sawdust etc. Particularly, these materials are annually renewable, and not competitive with human beings, moreover, the furfural production is an on-going industrial process. Consequently, exploring the sub-products from furfural as the replacements of the fossil resources is apparently attractive.

Direct oxidation of furfural may provide maleic acid which can be further transformed to fumaric acid by isomerization or maleic acid by dehydration. Currently, maleic acid and its sub-products, which are in more than million tons scale annually, are dominantly manufactured from the fossil resources including benzene, butane and butene. Because of the large availability of raw and renewable materials for furfural manufacture as well as its on-going industrial processes, exploring novel technologies for maleic acid and its sub-products synthesis from furfural not only provides an alternative and renewable route to the fossil feedstock, but also provides a new route for valorization of these raw materials which are in large scale and not competitive with human being. One of our on-going projects is to explore new chemicals from the renewable furfural, for example, developing new catalytic technologies to synthesize maleic acid and its alternatives by oxidations using dioxygen as the terminal  oxidant.

 

J. Phys. Chem., C, 2011, 115, 17516-17522


4) Develop new catalytic technologies for wastewater treatments

The scarcity of water resource has been the global problem for a long while, while efficient treatments of wastewater and its reuse can not only eliminate the environmental pollution, but also relieve the pressure of water scarcity. Even that versatile biological, physical and chemical technologies have been applied in pollutant treatments, they are apparently not enough to solve the problems. In a variety of chemical technologies, the supported metal oxide catalyst based technologies have attracted extensive attentions, and it has been recognized as one of the most promising technologies in pollutant treatments. However, drawbacks still exist for this kind of technologies: with the degradation of pollutants proceeding, the reaction media may gradually become acidic; as a result, the toxic metal ions may leach from the supported catalysts, which would cause the second pollution; furthermore, under the elevated temperature, for example, in catalytic wet oxidation technologies, the situation becomes even worse. The leaching of toxic metal ions may not only cause the second heavy metal ion pollution, but also decrease the lifetime of catalyst and meanwhile increase the cost, which substantially impede the broad applications of these technologies. One of our on-going projects is to develop bicarbonate activated hydrogen peroxide system for solving the above mentioned metal ion leaching problem in wastewater treatments. The promising feature of this technology is that, during the pollutant degradation, the pH of water is always in the range of neutral to weak basic conditions due to the presence of bicarbonate, which would greatly prevent the metal ions leaching from the supported catalyst. In addition, profited from our understanding of oxidation mechanisms, we are also engaged in improving the well-known Fenton technologies, and developing new generation of the Fenton technologies for wastewater treatments.   


Environ. Sci. & Technol., 2013,47, 3833-3839