Morphological control of cellulose nanocrystals via sulfuric acid hydrolysis based on sustainability considerations: An overview of the governing factors and potential challenges

Structural morphology of cellulose nanocrystal (CNC) is a major determinant of its properties. However, the lack of comprehensive understanding on the morphological control of CNCs, commonly produced via sulfuric acid hydrolysis, potentially results in unnecessarily high consumption of acid, water, and energy in order to achieve the desired morphology. Yet, fine control of morphology remains challenging due to complex process dynamics and elusive characterizations. Here, we aim to provide an overview of the relative influences of key factors involved in sulfuric acid hydrolysis on CNC morphology: (1) the types of cellulosic sources (wood, plants, cotton, algae, bacterial cellulose, tunicates, and agro-industrial wastes), and (2) parameters of sulfuric acid hydrolysis (acid concentration, temperature, and hydrolysis time). Our analysis reveals that acid concentration is the most dominant factor, followed by temperature and duration of hydrolysis. Additionally, 60-62 wt% sulfuric acid at 40-43 °C for 40-60 mins would allow for effective control of CNC aspect ratio. Based on our analysis, we provide systematic guidelines to attain the desired CNC morphology while minimizing the impact of the process on the environment. Lastly, we also explore unresolved challenges pertaining to CNC processing and characterization which hinder the control of CNC morphology.