Fabrication of magnetic carbon nanocomposites using rice husk via one pot solvothermal carbonization coprecipitation synthesis
Creators: name: SIDDIQUI, Muhammad Tahir Huss~
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400408
subject: Reaction engineering (excl. nuclear reactions)
classificationCode: 400410
subject: Wastewater treatment processes
classificationCode: 410306
subject: Environmental nanotechnology and nanometrology

description: Recent developments in hybrid nanomaterials have led to renewed interest in the fabrication of engineered material. Such as magnetic carbon hybrid nanocomposites that have significantly enhanced functionality and efficiency. The properties of the integration allow the usage in multi-dimensional areas such as adsorption, super capacitance, catalysis, biosensing devices, enzymes immobilization, CO2 capture. However, the overall fabrication cost, convenient processing, environmental and energy footprints, are the major concerns with the synthesis. To resolve such issues biomass from agricultural waste is currently being considered as a preferred green material to produce advanced carbon materials. Considering processing techniques, hydrothermal carbonization and pyrolysis are the major two thermochemical processes that have shown great promise for the synthesis of complex hybrid morphologies. However, hydrothermal synthesis is preferred as it requires lower energy and facilitates convenience in processing as it can handle wet precursors. In this study, the main objective is to produce hybrid magnetic nanostructures and formulate a scalable one-pot hydro/solvothermal method that can be utilized for energy-efficient processing and be able to facilitate scale-up with minimum up-gradation in the industry. In this work, hybrid magnetic carbon nanocomposites have been produced using one pot solvothermal carbonization coprecipitation (STCC) technique for graphene and chitosan supported carbon-based magnetic nanocomposites as well as bio-oil as a by-product. The novelty of the work comes with a single pot SSTC process integrating solvothermal and coprecipitation synthesis in a single route. Allowing incorporation of biomass, magnetic nanoparticles, and carbon functional nanomaterial (novel low-cost graphene followed by Patent) in a single step having convenient processing. Furthermore, it facilitated significant enhancement in key properties such as high BET surface area. The magnetic carbon nanocomposites produced via STCC has exclusive application in multiple fields of adsorption of heavy metals, EMI shielding, and as a catalyst that makes this research more significant.

In this study, several process factors were investigated to optimize the process to explore both quantitative and qualitative effects on magnetic carbon nanocomposites and bio-oil. The factors include solvents (ethanol, water, and a mixture of ethanol-water), additives  (graphene and chitosan), heating mechanisms (microwave-assisted and classical/conventional heating), process temperature (140, 180, 220, 260 °C), and process time (60, 90 and 120 minutes). Furthermore, statistical analyses were also performed to analyze the yield of products and the significance of each process variable. Finally, the magnetic carbon nanocomposites produced via STCC were explored for their thermal, structural, chemical, magnetic properties. Furthermore, bio-oil has been characterized for its composition and its higher heating value (HHV) content.

The heat transfer methods have a substantial effect on both products. It was found that microwave-assisted heating where microwaves facilitate homogenized heating resulting in reduced heat loss. This facilitated hollow structure for nanocomposite achieved shown by SEM micrographs and higher bio-oil yield. On the other hand, classical solvothermal synthesis impacted the surface properties of the composite and therefore porous structure was observed from SEM micrographs.
 
It was found from statistical optimization that process temperature was the most influential factor that impacted the yield of products (both magnetic carbon nanocomposite and bio-oil). The BET surface area of rice husk is 3.1 m2/g however, the highest BET surface area of magnetic carbon nanocomposite achieved by STCC was 280.04 and 194.62 m2/g for graphene and chitosan supported magnetic carbon nanocomposites. This significant improvement in surface area is due to the destruction of the fibrous structure of rice husk by STCC, leading to the creation of pores and with the increment of the temperature, the pores start to enlarge leading to an increase in the porosity. Also, BET surface area was enhanced due to functionalization with high porous additives such as graphene or chitosan. FESEM and XPS confirmed the presence of FeO and its functionalization with carbonaceous groups which were confirmed by HRTEM images. STCC facilitated dispersion of approximately 5 to 20 nm of iron oxide nanoparticles with the carbonaceous matrix in the hybrid magnetic carbon nanocomposites. While, water/ethanol (50:50) mixture reduced the critical point of the mixture by allowing enhanced diffusivity, miscibility, and unique transport properties that enabled the highest yield of bio-oil with the lowest O/C and H/C ratio and having an energy density of bio-oil to be equivalent to commercial bioethanol.

The application of magnetic carbon nanocomposites was considered for EMI shielding. Melt processing was used to mix magnetic carbon nanocomposite (MCNC) as a filler varying MCNC (5%, 10%, 15%) with polypropylene (PP) have been explored for its structural, thermal, mechanical, chemical, electrical, and electromagnetic interference performance Polypropylene/magnetic carbon nanocomposite (PP-MCNC). Rheological outcomes revealed from the values of G′, G″, tan δ, G* that more solid-like behavior was seen for PP-MCNC composites as of raw PP. Moreover, electrical conductivity was significantly affected with the integration of MCNC in the nanocomposite from 1E-15 to 3.91 S/m and overall EMI effectiveness increased.
 
Furthermore, chitosan and graphene-supported magnetic carbon nanocomposites were utilized for the adsorption of multi-heavy metal adsorption from the electroplating industry including Cd, Ni, Cu, and Cr. Also, the hybrid magnetic nanocomposites were applied as catalysts to produce bio-oil via catalytic liquefaction in the presence of supercritical ethanol. As an adsorbent, all the materials showed outstanding performance for multi-heavy metal adsorption. Isotherms kinetics revealed the mechanism was favorable for mono-layer adsorption. Furthermore, as a catalyst, catalytic liquefaction confirmed that higher bio-oil and biomass conversion were achieved with catalyst 36.8% and 60.81% as compared to without catalyst 29.5% and 48.11%  respectively. Also, fuel quality was observed with the catalyst having lower H/C and O/C ratios, reduction of acids, and significant enhancement in esters, phenols components that show enhancement in fuel properties.

To conclude STCC offers convenient and energy-efficient processing for the synthesis of magnetic carbon nanocomposites that showed exclusive properties such as higher porosity and higher thermal stability and can be utilized in multiple areas including catalysis, adsorption, and EMI shielding.