Design of Functional Nanoporous Materials and their Catalytic Applications

The objective of our research is to develop new class of nanoporous materials or develop synthesis strategies that enable us to significantly improve the physico-chemical properties of existing materials. In order to obtain the desired activity for appointed catalytic applications, multifunctional solid catalysts with isolated and/or well defined catalytic active sites are developed. Consequently, the synthesis, characterization and catalytic applications of microporous/mesoporous crystalline materials such as zeolites, metal oxides, polyaniline, hybrid organo-inorganic materials such as MOF, carbon materials, and nano-particles are of significant interest to us. Catalysts with mono or multifunctional active sites allow us to use them in wide range of applications that includes, heterogeneous catalysis in chemicals synthesis, photo-assisted chemical synthesis, metal-free catalysis, plastic waste management, CO₂ conversion, and production of alternative energy resources such as biofuels etc.

Material Synthesis

Nanocrystalline/Hierarchical zeolites

Zeolites are highly accomplished catalytic materials for petrochemicals industry. It is also finding extensive use in gas separation and purification. These microporous materials shows significant deactivation phenomenon in petrochemical chemical and fine-chemical industry. Thus, it is essential to prepare zeolites having inter-crystalline/intracrystalline mesopores. Our group has demonstrated capability to develop a synthesis strategy for the preparation of mesoporous zeolites required for the 21st-century petrochemical/fine-chemical industry. Some of our contributions in this direction are as follows. (A) The development of unique structure-directing agents (SDAs) for preparing mesoporous zeolite Beta in just 36-48 h that exhibited large surface area and pore volume. (B) In general, mesoporous zeolites are disordered materials. It was challenging to achieve the order in both length scales (micropore and mesopore) in the mesoporous zeolite. Zeolite ZSM-5 nanosheets were synthesized using specially designed multi-quaternary ammonium surfactants-based SDAs. (C) It was extremely difficult but important to develop a synthesis strategy for the preparation of mesoporous zeolites of different framework structures using one SDA. A one-step, direct synthesis strategy was developed for the synthesis of mesoporous ZSM-5, MOR, and SOD zeolite nanocrystals using only one SDA under different synthesis conditions. (D) A highly challenging micro-meso-macroporous tri-level porous ZSM-5 zeolites using economical starch as structure-directing agents was prepared. This work was published and featured on the cover page of the journal ACS Sustainable Chemistry and Engineering.

Metal oxides

Metal oxides are another class of inorganic materials widely employed in catalysis due to their ease of synthesis and tunability. Their redox properties and acid-base characteristics can be finely tailored depending of appointed applications. The activity can also be enhanced by modulating morphology, particle size, exposed facets, defects, oxygen vacancies, etc. These properties can be incorporated into the metal oxide when a suitable synthesis strategy is employed. They can act as a catalyst and also support matric to decorate metal nanoparticles. Moreover, metal oxides are semiconductors, and their optical and electronic properties make them valuable for photocatalysis and electrocatalysis. Spinels, a class of metal oxide, are more exciting due to the incorporation of different metals in their framework structure. Spinels possess a remarkable characteristic in catalysis as they possess dual functionality. We developed different synthesis strategies to prepare metal oxides, and spinels and explore their potential in conventional thermal catalysis, electrocatalysis, and photocatalysis.

Metal organic framework

MOF is another microporous material initially developed for gas adsorptions, mainly H₂, CO₂, etc. Due to unsaturated metal sites, defects, and functionalization ability, the applications of MOF can be extended to catalytic applications that are being explored in our group. The functional nature of MOF allows the engineering of the active sites to form a wide range of high-scope catalytic materials useful in several heterogeneous catalytic transformations. The high catalytic activities of these engineered or modified MOFs can be directly correlated with the induced structure-activity relationship due to the active site engineering phenomena in MOFs. The tailorable properties of the MOF structure through modulating the pore size and chemical environment of the pore wall, altering the chemical connectivity and tuning the electronic surrounding of the metal node, and finally modifying the functionalities, size, and ligating sites of the organic linker. The emphasis is also being made on various engineering strategies and necessary precautions needed to take care during the modification process. The overall electronic properties of the engineered MOF framework dictate the catalytic efficacy provided the stability issue is taken into consideration. One of the critical issues attributed to the MOF framework is its thermal and chemical stability; this issue is even more pronounced after the active site engineering of the MOF structure. Therefore, a meticulous reaction procedure needs to be followed while modifying the MOF structure. Moreover, our group also uses the disadvantages feature of its thermal instability to advantageous one by carbonizing MOFs, which results in metal/metal oxide decorated carbon materials, useful for unique catalysis that are generally carried out using noble metal-based costly catalysts.

Catalytic Applications

Biomass Conversion

Biomass is renewable, abundant, and available in India at no cost. The valorization of biomass presents a significant challenge in the field. Lignocellulosic biomass, derived from agricultural and forestry waste, is made up of three main components: cellulose (40-60%), hemicellulose (20-30%), and lignin (15-40%). All three components are being converted to valuable chemicals and fuels. It is possible to prepare all those chemicals that are conventionally prepared from fossil fuel. From cellulose/hemicellulose-derived chemicals, several furanic compounds, alcohols, aliphatic acids, polymeric monomers, and N-containing heterocycles have been prepared. Similarly, Lignin is a potential source of aromatic platform chemicals and comprises three-dimensional cross-linked monolignols. Lignin is a complex biopolymer composed of diverse aromatic units, potentially producing value-added chemicals. However, the multiple functional groups within the monomeric units of the lignin polymer provide a challenge to develop a suitable catalyst for producing selective value-added chemicals. To overcome this challenge, lignin-derived monomeric phenolic units are utilized as model compounds to produce value-added chemicals through hydrogenation and deoxygenation. The transformation of lignin into bio-oils and the resulting bio-oils serve as platform chemicals for catalytic transformation, enabling the production of valuable chemicals and transportation fuel and their additives.

Plastic upcycling

Plastics have been widely applied since the 1950s and are ubiquitous in our daily lives. The present
estimate indicates that global plastic production is around 385 million tons per year and the estimated production of plastic in 2050 will be 30,000 million tons and generating a huge amount of plastic waste every year, which is hard to handle and manage. Nowadays, more than 300 million tons of plastic waste end up in landfilling and incineration processes. In 2018, 79 % of plastic waste accumulated in landfilling, and 12 % was incinerated whereas, only 9 % of plastic waste was recycled. The accumulated plastic waste in the natural environment by landfilling causes several harmful effects by entering microplastic into the ecosystem. In addition, the incineration of plastic waste leads to huge amounts of CO₂ emission accelerating environmental threats. In contrast to mechanical recycling, chemical upcycling of plastic waste emerges as a viable alternative solution to foster a waste-free and carbon-neutral society. Rajendra’s group focuses on developing heterogeneous catalytic on chemical upcycling processes, including hydrogenolysis, hydrodeoxygenation, solvolysis, cracking, and light-driven processes for sustainable upgradation of plastic waste into value-added products, fuel, and their parent monomers within a circular economy framework.

Photocatalysis

Our group aims to develop a sustainable visible light photocatalytic process for selective organic transformations, biomass valorisation, waste management, and atmospheric nitrogen fixation. We are determined to make a difference in biomass valorisation by harnessing visible light to create eco-friendly pathways for transforming biomass into essential chemicals and fuels. This approach reduces our dependence on non-renewable resources and minimizes environmental impacts. Also, addressing the global plastic waste crisis is another vital aspect of our work. Through visible light photocatalysis, we are breaking down complex plastic polymers into useful chemicals and fuels, contributing to a cleaner and greener planet. Moreover, we recognize the significance of atmospheric nitrogen fixation in sustaining life. By developing efficient photocatalysts, we can directly convert atmospheric nitrogen into valuable nitrogen compounds, offering a more sustainable alternative to energy-intensive industrial processes. Our research group envisions a world where visible light photocatalysis plays a pivotal role in biomass valorisation, selective organic transformations, plastic waste management, and atmospheric nitrogen fixation. Through our collaborative efforts and commitment to innovation, we strive to pave the way for a cleaner, brighter, and more sustainable future.

Metal-Free Catalysis

We are dedicated to exploring and advancing the fascinating realm of metal-free heterogeneous catalysis and its profound implications for sustainable chemical transformations. Our mission is to unravel the potential of non-metal catalysts in driving diverse reactions, reducing environmental impact, and revolutionizing the field of catalysis. Traditional catalysis heavily relies on metal-based catalysts, which often come with challenges such as resource depletion, high cost, and potential toxicity. In contrast, metal-free heterogeneous catalysis offers a promising alternative by utilizing non-metallic materials as catalysts. These materials can exhibit remarkable catalytic properties while avoiding the limitations associated with metal-based counterparts. Our group is at the forefront of developing innovative metal-free catalysts for organic transformations, biomass valorization, and investigating light-driven metal-free catalytic processes which opens up new avenues for harnessing solar energy to drive chemical reactions.

CO₂ Conversion

To combat global warming and the associated natural changes on Earth, it is imperative to reduce the carbon footprint and subsequently lower the concentration of CO₂ in the atmosphere. One approach to achieve this is through effectively mitigating CO₂ using various methods. Notably, capturing and storing CO₂, as well as utilizing it as a reactant in chemical synthesis, have shown promise as viable pathways for CO₂ reduction. In our group, we are dedicated to synthesizing valuable chemicals like cyclic carbonates, carbamates, and urethanes that find extensive application in the agrochemical and pharmaceutical industries. Our ongoing research focuses on exploring two promising avenues for CO₂ conversion: thermocatalytic and photocatalytic processes. We have also explored CO₂ as a carbonylation source to prepare carbamates instead of taking hazardous CO. To achieve this, we utilize the potential of metal oxides, MOFs, and carbon-based materials as catalysts, which have shown great promise in transforming CO₂ into valuable chemicals and fuels. We are also developing materials that can be utilized for direct air CO₂ capture and flue-gas CO₂ capture.