Spherical nanocrystalline cellulose from agro-industrial biomass: production, optimization and thermo-kinetic evaluation
Creators: name: Ahmed Mohamed, Mohamed Rashid
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400499
subject: Chemical engineering not elsewhere classified
classificationCode: 401609
subject: Polymers and plastics

description: In recent years, research activities involved in the production of nanocellulosic materials have grown substantially, rapidly stimulating the development of innovative production techniques. In particular, attention has focussed on spherical nanocellulose production, in view of increasing momentum in applying these nanomaterials as sustainable value-added nano-additives for many modern industrial and medical applications. These nanomaterials are chemically extracted by acid-catalyzed hydrolysis of the renewable and widely available cellulosic biomass. Sulfuric acid-catalyzed hydrolysis of cellulosic biomass is a commonly known method for nanostructured cellulose production. However, this method can result in many disadvantages, to include: short catalyst-lifetime, corrosion of the reactor materials and issues managing the spent sulfuric acid used in the production process. This dictates the implementation of an eco-industrial alternative for spherical nanocrystalline cellulose (SNCC) catalytic production. A viable and practical alternative is the application of heterogeneous (solid acids) catalysts, which can be more conducive to providing favorable platforms for efficient cellulose hydrolysis. Within this context, this study investigates the feasibility and efficiency of applying heterogeneous acid-catalyzed hydrolysis in exploiting the plentiful cellulosic biopolymer, such as rice husk (RH) and cotton linter (CL), as sustainable and renewable resources for SNCC production.

Microcrystalline cellulose (MCC) has been considered in this study as intermediate material (precursor). Here, two types of MCC have undergone the heterogeneous catalysis process. Rice husk-based microcrystalline cellulose (MCC-RH) and cotton linter-based microcrystalline cellulose (MCC-CL) were used as precursors for SNCCs production. Unlike the commercially sourced and readily available MCC-CL, RH cellulosic pulp was utilized in the laboratory-scale production of MCC-RH.

In the first stage, MCC-RH was prepared by valorizing the abundant RH, as nonwood agro-industrial residue, via consecutive soda delignification and non-acidic catalytic oxidation. The resultant MCC-RH, with a 31 wt% yield, has granular microstructured particles with an average diameter of 42 ± 4 ¿m. The porous morphology has been observed in MCC-RH micrographs, using scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) and TGA/DTG thermal analysis indicated the presence of typical cellulose characteristics. While, the presence of carbonyl spectral peak signalized the efficacy of the D-RH oxidation process. X-ray diffraction (XRD) confirmed the coexistence of cellulosic crystalline forms of cellulose I and cellulose II with 85% crystallinity index. Significant growth of crystal size was observed from 1.7 nm (RH) to 9.4 nm (MCC-RH). The maximum decomposition temperature (Tmax) was found to be 324 °C, indicating good thermal stability. 

For the second precursor, MCC-CL has shown morphological characteristics of rod-shaped microparticles with irregular and faceted surfaces. MCC-CL diffractogram revealed typical cellulose I diffraction peaks with crystal size measured at 4.7 nm. Similarly, typical cellulose functional groups were also observed from the FTIR analysis of MCC-CL. In contrast to MCC-RH, Tmax was determined to be 13 °C higher from the MCC-CL thermogram, which indicates better thermal stability. Overall, both MCC-RH and MCC-CL have shown attractive thermal and physicochemical properties for SNCCs production.

In the heterogeneous catalysis production of SNCCs, three different heterogeneous catalysts were assessed for their suitability and catalytic activity. Only Amberlite IR 120 was stable under the mild reaction conditions; hence it was the most suitable heterogeneous catalyst for SNCCs production. As a result, two SNCCs were prepared, in a single-step reaction, using heterogeneous acid-catalyzed hydrolysis. The produced SNCCs were coded SNCC-RH and SNCC-CL, based on the origin of their precursors, i.e., MCC-RH and MCC-CL. High yields of 95% and 68% for SNCC-CL and SNCC-RH respectively, are clear evidence of successful SNCCs production. The heterogeneous catalysis reaction was observed to follow first-order kinetics with inverse correlation associated with the reaction medium, evident between the concentrations of the precursor and hydronium ion. The morphology observations of both NCCs, using transmission electron microscopy (TEM), have revealed spherical shaped nanoparticles. SNCC-CL was found to be highly monodispersed nanospheres with an average diameter of 36 ± 8 nm. By comparison, SNCC-RH appeared as agglomerated nanospheres with an average diameter of 25 ± 4 nm. The TGA/DTG and XRD curves demonstrated significant improvements in SNCC-CL crystallinity and thermal properties. Increased resistance to thermal degradation was observed for SNCC-CL compared to its precursor MCC-CL. Heterogeneous catalysis was less effective in improving SNCC-RH crystallinity and thermal properties, compared to SNCC-CL. This research not only confirmed the applicability of heterogeneous catalysis in producing nanocellulosic materials, but also provided a high selective production technique for creating cellulose nanospheres. This technique has further demonstrated an effective and rapid approach to improve crystallinity, microscopic morphology and thermal properties of the resulting products.

Since SNCC-CL has exhibited better morphological, crystallinity and thermal properties, it was therefore used to optimize the production process. Moreover, in optimizing and predicting optimal production conditions, a set of experimental data was generated using a full factorial experimental design. It was found that the lowest experimental inputs of reaction time (30 mins), Amberlite IR120 concentration (0.5 M) and MCC amount (2 g in 250 mL Mili-Q water) and their interactions have the most contributive factors in producing the maximum SNCC yield and production rate. Coats-Redfern model-fitting approach was used to model SNCC-CL thermal decomposition. The kinetic results showed that SNCC-CL decomposition mechanism was well described by the First-order reaction model (F1), with its activation energy determined at 312 kJ/mol.