Abstract
The quest for new, cheap, and alternative carbon sources for carbon nanotubes (CNTs) production are explored daily to meet the rising demand of CNTs and CNTs products as a result of their remarkable properties such as high mechanical strength, high surface area, excellent chemical inertness and thermal stability, high electrical conductivity and outstanding water-transport properties which has contributed to their versatile applications in various fields. Waste plastics are viable resource with very high carbon content are deemed encouraging step towards recycling and reusing technology for creating “wealth from wastes” and maintaining a circular economy. Utilizing plastics wastes as starting materials for CNTs synthesis is likely to simultaneously reduce the quantity of plastic solid wastes (PSW) and lower CNTs production cost.
Currently, one of the challenges of applying plastics derived CNTs in most field of study is whether the produced CNTs can be used in purity sensitive applications due its poor quality and lack of uniformity in structure and property. In this study, waste polypropylene (PP) plastics was upcycled into value-added CNTs of high quality and yield and further infused into polymeric polysulfone (PSF) membrane to form mixed matrix membrane. The fabricated membrane was utilized in membrane separation for removing heavy metals from modelled gold acid mine drainage (AMD). To achieve this, different metal oxides (MgO, CaO, TiO2), mixed-metal oxide (CaTiO3) and carbon-based (biochar; corn-cob derived char) materials were employed as supporting catalyst for bimetallic NiMo catalyst. All catalysts were prepared using sol-gel method, calcined in air, and further reduced in-situ during CNTs synthesis under H2/Ar gas mixture. Furthermore, carbon-based catalyst support obtained from waste corncob was pyrolyzed and processed into char and further activated using chemical and steam activation methods before incorporation into NiMo catalyst for CNTs synthesis. The structures and properties of all prepared catalysts were expansively characterized for phase structure, textural
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and surface properties, morphology, reducibility using X-ray diffraction (XRD), N2 physisorption analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and temperature-programmed reduction (TPR) respectively. In this study, synthesis of CNTs was carried out using single-stage catalytic chemical vapour deposition (CVD) reactor where waste PP was pyrolyzed at 700 ℃ at a heating rate of 10 ℃/min and released gases during pyrolysis was deposited over catalyst under similar conditions for the synthesis of CNTs. Deposited CNTs were characterized using thermo-gravimetric analysis (TGA), XRD, Raman spectroscopy and TEM.
This study also investigated for the first time using artificial neural network (ANN) to develop a model that described the production of CNTs in a single-stage CVD (chemical vapour deposition) reactor using waste PP plastics. The quality and yield of the PP derived CNTs deposited over the best performing catalyst (NiMo/CaTiO3) was predicted and modelled based on the best validation performance of mean square error (MSE) and correlation coefficient (R). Experimental dataset was initiated using user-specific design with four numeric factors: synthesis temperature (600-800 ℃), carrier gas flow rate (70-120 ml/min), furnace heating rate (5-15 ℃/min), and residence time (20-30 min). Levenberg-Marquardt algorithm was utilized in training, validating and testing these experimental dataset.
In this study also, synthesized waste PP plastic derived multi-walled carbon nanotubes (MWCNTs) was applied as filler in mixed matrix membrane (MMM) for high performance membrane separation of toxic metals from gold mine wastewater. Different loadings (0, 0.05, 0.10 and 0.15 wt. %) of MWCNTs was infused into polysulfone (PSF) membrane via phase inversion method. Morphology, thermal stability and hydrophilicity properties of all fabricated and unmodified membranes were evaluated using SEM, TGA and water contact angle, respectively. Cations in the permeate stream was quantitatively analysed using Atomic Absorption Spectrophotometry (AAS).
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Results from this study showed that supports play crucial role in minimizing agglomeration of active metal particles, improve the stability of catalysts and enhanced metal-support interaction (MSI) even at elevated temperature compared to the control catalyst (unsupported NiMo catalyst). Multiwalled carbon nanotubes (MWCNTs) deposited over NiMo/CaTiO3 catalyst showed highest yield of 40% and better quality (Peak intensity ratio of G-band IG to intensity of D-band ID; IG/ID = 1.25) with average outer diameter of 24.01 nm ± 4.89 with better aligned graphite walls. Similarly, result obtained from NiMo supported char catalysts showed that different activation procedures resulted in different types and microstructures of CNTs (bamboo-type and filamentous CNTs). Although good quality (IG/ID = 1.44) filamentous CNTs was obtained over NiMo supported steam activated biochar catalyst (NiMo/ACT), however, obtained CNTs yield was very low (about 7%).
Result from the predicted model obtained using ANN showed how synthesis temperature, heating rate, residence time and inert gas flow rate can affect the yield and quality of plastic derived CNTs. Based on the best validation performance of mean square error (MSE) and correlation coefficient (R), developed model which could be used on similar dataset to predict the yield and quality of plastic derived CNTs are given as follows: 𝑦1=𝑛𝑒𝑡𝑋1 where, 𝑋1 is the set of matrix input variables and, net is the neural network model that would be called using the code produced from experimental data training, validation and testing (TVT) used to estimate the possible values of y (quality and yield).
Regarding the application of synthesized MWCNTs, result obtained in this study showed and confirmed the potentials of plastic derived CNTs in membrane separation over pristine polysulfone (PSF) membrane. Consequently, fabricated PSF infused waste PP derived-MWCNTs mixed matrix membranes (MWCNTs/PSF MMM) showed improved surface and mechanical properties, high flux, and increased metal rejection in the range of 81 - 91% and 64 - 74% for iron and nickel, respectively. Therefore, the application of plastic derived.
MWCNTs/PSF MMM could significantly improve the economy of membrane technology while expanding CNTs market, lower CNTs cost and provide revolutionary sustainable plastic waste management option.