H.C. Zeng, Ph.D., Professor
Department of Chemical and Biomolecular Engineering
Faculty of Engineering, National University of Singapore, Singapore 117585

Design and Synthesis of Integrated Nanocatalysts


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Related Research Thrusts

• Design and synthesis of nanomaterials for energy and sustainability
• Development and integration of novel catalytic materials
• Synthetic architecture of advanced functional materials

Program

In recent years, “nanocatalysts” has become a term often used by the materials chemistry and catalysis community [1]. By controlling particle composition, structure, shape, and dimension, researchers can move into the next phase of catalyst development if they are able to bridge old and new technologies. In this regard, one way seems to be to integrate active catalytic components with boundary-defined supports, which therefore retains the essence of traditional “catalyst-plus-support” configuration. The resultant catalysts have advantages for material engineering that often involves high level designs and integrations in their fabrication. Besides this, the active components in this new type of catalysts are in nanoscale and are easy to integrate into supporting material phases. For these reasons, we name such catalytic devices as integrated nanocatalysts (INCs) [1]. In our group, we carry out this type of materials research and develop new synthetic strategies and architectural designs for INCs with increasing compositional and structural complexities in order to meet new challenges of heterogeneous catalysis in energy and sustainability applcations.


Background

Integrated nanocatalysts (INCs) with multicomponent and hierarchically complex structures have recently drawn extensive research attention in terms of their fundamental sciences and industrial applications, and many innovative strategies have been established [1]. In pursuing this type of research, we are developing the state-of-the-art of INCs with different porous materials for catalyst technology and heterogeneous catalysis. In our research, we synthesize different nanoparticles or clusters (namely, metal, metal oxide, and hybrid nanoparticles) that are catalytically active in INCs [2]. Combining such active components with various porous materials provides us a huge array of architectural designs for INCs that can be used in chemical reactions. In choosing host materials for INCs, we particularly focus on shape-controlled mesoporous siliceous materials (e.g., silica and metal silicates) and microporous metal-organic frameworks (MOFs), since they represent the two important classes of porous materials known today. In our research program, therefore, we develop INCs which comprise these mesoporous and microporous solids and active catalytic components. A special emphasis is placed on their roles as supports, encapsulating shells and metal sources for targeted applications of energy and sustainability.


Methodologies

Basically, two types of materials are used in construction of these catalytic devices: active catalyst components and their supporting carriers [3-8]. In preparing INCs, various synthetic methods have been developed in recent years [5, 6]. Normally, the active components are synthesized into monodisperse nanoparticles through wet chemical routes. On the other hand, the catalyst carriers or hosts are often prepared as hollow or porous materials with desired shape-controls [3, 4]. In general, integration of the above two types of catalytic materials can be achieved in a step-by-step manner [7-9]. Both top-down (e.g., dissolution and regrowth) and bottom-up (e.g., self-assembly and deposition) strategies have been employed in the synthetic architecture of INCs (Figure 1) [5, 6].


Examples of Research Activities

In heterogeneous catalysis, structure and shape of solid catalysts play an important role in their interactions with reaction fluids and thus catalytic performance. For instance, intrinsic nanostructure of catalysts and related packing in fixed bed reactors will determine the retention time of gaseous reactants and reaction intermediates on catalyst surfaces, which in turn enhances the conversion and selectivity if more travelling routes and longer travelling paths for reactants can be created within a catalyst bed, as reported in our recent investigation on gas-phase hydrogenation of carbon dioxide [3]. Additionally, geometric shape of particulate catalysts also impose significant impacts on their performance, because an optimal configuration of catalyst can promote transport processes and thus increase catalytic activity in fluid-related reactions or environments (Figure 2) [4]. In this regard, a streamline body may represent a superior geometry since it experiences minimum fluid resistance. In this latter example, we developed a new class of integrated nanocatalysts with a streamline body and tunable chemical compositions. Advantages of these shape-controlled catalysts were further demonstrated with some model reactions such as liquid-phase alcohol oxidation, olefin hydrogenation, and Suzuki-Miyaura coupling, in comparison with their commonly used counterparts [4].

Future Direction

Departing from their conventional counterparts, the state-of-the-art catalysts for heterogeneous catalysis can be designed and prepared in a controllable fashion owing to rapid development of nanoscience and nanotechnology as well as the maturing chemistry of materials over the past two decades. It is anticipated that compositional and structural requirements of such catalysts can be met with a higher level of sophistication and precision but at a lower cost. As most of commercial catalysts use expensive metal elements (such as noble metals), INCs represent a future form of catalysts to boost atom economy of these precious metals. However, realization of this paradigmatic shift for modern catalyst manufacturing would still require collective efforts from the research community. We envision that the synthetic architecture of nanomaterials will become an important field in future development of state-of-the-art catalytic materials. We also believe that further integrations of a single type or multiple types of INCs could lead to even more powerful “supracatalysts”, achieving industrial scale applications [1].


Figure 1. Design and synthesis of 12 representative reactor-like integrated nanocatalysts [6].
 Figure 2. Streamline-shaped INCs made from noble metals/organic-inorganic hybrids [4].

References

[1] H.C. Zeng, Integrated Nanocatalysts, Acc. Chem. Res. 46, 226-235 (2013).
[2] G.W. Zhan, H.C. Zeng, Integrated Nanocatalysts with Mesoporous Silica/Silicate and Microporous MOF Materials, Coord. Chem. Rev. 320-321, 181-192 (2016).
[3] G.W. Zhan and H.C. Zeng, ZIF-67 Derived Nanoreactors for Controlling Product Selectivity in CO2 Hydrogenation, ACS Catalysis 7, 7509-7519 (2017).
[4] G. Zhan, H.C. Zeng, Smart Nanocatalysts with Streamline Shapes, ACS Cent. Sci. 3, 794-799 (2017).
[5] G.W. Zhan, P. Li and H.C. Zeng, Architectural Designs and Synthetic Strategies of Advanced Nanocatalysts, Adv. Mater. 30, 1802094 (2018).
[6] B.W. Li and H.C. Zeng, Architecture and Preparation of Hollow Catalytic Devices, Adv. Mater. 30, 1801104 (2018).
[7] G.W. Zhan and H.C. Zeng, Hydrogen Spillover through Matryoshka-Type (ZIFs@)n-1ZIFs Nanocubes, Nat. Commun. 9, 3778 (2018).
[8] Y.C. Tan and H.C. Zeng, Defect Creation in HKUST-1 via Molecular Imprinting: Attaining Anionic Framework Property and Mesoporosity for Cation Exchange Applications, Adv. Funct. Mater. 27, 1703765-73 (2017).
[9] Y.C. Tan and H.C. Zeng, Lewis Basicity Generated by Localised Charge Imbalance in Noble Metal Nanoparticle-Embedded Defective Metal–Organic Frameworks, Nat. Commun. 9, 4236 (2018).