Studies on porous organic polymers and their nanocomposites for electrocatalytic and biological applications
Creators: name: Venkatachalam, Rajagopal
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340604
subject: Electrochemistry

description: Porous organic polymers (POPs) are a promising class of porous materials which contain organic building blocks that are interwoven via strong covalent bonds. In the recent decade, there has been rising attention in POPs as nanoreactors for heterogeneous catalysis (electrochemical, biological, thermal, and photochemical) due to their structural diversity which implies large surface areas, high chemical, and thermal stabilities, tuneable pore size distributions, and adjustable chemical functionalities. These salient structural features have allowed work on different synthetic strategies for constructing various POP materials to influence their functionality and study their impressive application as future generation nanoreactors. The above-mentioned characteristics of POP-based materials makes them useful as electrocatalysts, as they offer the potential for preparation of either metal-free or metallated polymer catalysts that favour electrocatalytic reduction of CO2 into value-added products, and production of clean H2 and O2 from water. In addition, antibacterial applications have been studies for POPs, as physical damage of bacterial cell walls can be caused by their structure. 

This thesis emphasizes the integration of different synthetic methods, and the application of POPs as a catalyst that offers opportunities in next generation for the synthesis of functional materials with vital effect in both applied and basic research areas. In this present work, the POP reactivity is tailored for specific applications by fine starting materials in the synthetic procedures. 

Initially, we focused on the synthesis and electrochemical applications of phloroglucinol, 1,4-phenylenediamine and triazine (PPT-POP) based covalent triazine frameworks (CTF). The skeletal structure of synthesized CTF has a large amount of ‘N’ content that results in excellent stability (chemical and thermal) and catalytic activity. Due to their large surface areas, they are often used as electrocatalysts as well as in energy storage applications. Furthermore, the carbonization of CTF at high temperatures under an inert atmosphere lead to the decomposition of the framework structure. It is noted that an increase in carbonization temperature improves the degree of graphitization, thereby yielding N-doped carbon materials with high porosity and active sites which are essential for improved catalytic activity. In this aspect, we reported the synthesis and application of a new carbonized porous organic polymer materials based on triazine and its electrocatalytic performance towards electrocarboxylation, hydrogen-evolution, and oxygen-evolution reactions. 

As a next step, an o-tolidine based triazine polymer (TTP) was synthesized by adopting the above synthetic route with o-tolidine as a starting material instead of 1,4-phenylenediamine for validating and comparing the catalytic activity with PPT-POP. The synthesized TTP had a large surface area for electro-catalysis applications, which combined with high ‘N' content, and excellent chemical and thermal stability, allowing exploration of catalytic and energy storage applications. Also, the structural flexibility and tunability allow precise modulation for the desired applications. The triazine interconnects allowed the introduction of a metal ion/metal oxide via coordination, which is known to further enhance the conductivity and catalytic properties. It was thus demonstrated that the synthesis of o-tolidine based triazine (TTP) covalent organic frameworks and modification of the surface with Cu, Ni and Cu-Ni alloy, allowed electrocatalysis of the electrocarboxylation of epoxide to cyclic carbonate, and water splitting reactions. 

In the next step, we introduced metal ions into the 1,4-phenylenediamine based covalent organic polymer (PD-COP) to investigate the electrochemical behaviour. Metal ion decoration on PD-COP polymers allows the synthesis a range of new types of micro-and mesoporous (2–50 nm) materials, involving amorphous organic polymers and crystalline COPs as active and selective electrocatalysts. Au nanoparticles (NPs) decorated on COP have been frequently used by electrochemists due to their catalytic activity. For electrocatalysis towards total water splitting (TWS) in an alkaline medium, to date, there is no report on Au NPs decorated on COP. Herein, we attempted to use Au NPs modified COP as an electrocatalyst for water splitting in an alkaline medium. We proposed a facetious route for the incorporation of Au NPs on COP involving sodium borohydride (NaBH4) reduction. It is noteworthy to mention that the excess amine sites on the COP act as a stabilizing agent. In this step, we reported the development of an Au NPs decorated polymer catalyst with for total water splitting, wherein the as-prepared catalysts show excellent catalytic activity. 

Finally, we focused on a biological application of the initially synthesized COP which are scarcely studied and reported in the literature. The COF has a high specific surface area and porosity and hence finds applications in gas storage/CO2 reduction, battery materials, chemical sensors, gas/chemical separation, catalysis processes, and light-emitting materials. To date, many of these reported applications are based on their physical and chemical properties, and very little has been explored on their biological properties/applications. While the physical and chemical properties highly depend on the specific surface area, nature of interactions in the pores, and pore size, the biological applications completely rely on aspects such as biocompatibility and anti-fungal/antiviral behavior. In the present work, we reported the synthesis of o-tolidine and triazine interlinked covalent organic polymer (oTT-COP) and its antibacterial activity towards Staphylococcus aureus and Pseudomonas aeruginosa (gram-positive and gram-negative) respectively. The antibacterial activity of PPT-COP (phenylene diamine, phloroglucinol interlinked triazine covalent organic polymer) was evaluated to understand the role of the amine group on the polymer network towards antibacterial activity. 

Overall, we have successfully synthesized and characterized various porous organic polymers (phenylene diamine, phloroglucinol and o-tolidine-based POPs) and their nanocomposites containing various metal ions such as Cu, Ni, Cu-Ni and Au. The catalytic properties were studied for electrocarboxylation, and water splitting (HER and OER). Furthermore, the synthesised phenylene diamine, phloroglucinol and o-tolidine-based metal free POPs were used in biological studies for the first time.