1. Introduction

First isolated in 1977 in the form of a translucent hydrogel at a USA-based paper and pulp company by repeated treatment of a 3% slurry of chopped wood pulp fibers with a high-pressure milk homogenizer, the new biomaterial later called nanocellulose was termed “microfibrillated cellulose” (MFC).1 “At the high temperature the high forces (pressure/cavitation/shear/impact) of the homogenizer acting in tandem or sequentially” wrote Turbak in 2015 “had broken down the cell walls of the microfibers and liberated the desired nanofibrils”.2 
Nanocellulose, namely cellulose fibrils with widths in the nanometer range,3 was born. Explicitly mentioning its “uses and commercial potential” in early research reports,1 chemists understood that the material had numerous potential applications.
In the subsequent four decades, scientists from across the world discovered the exceptional mechanical, optical, chemical, biological and thermal properties of the new bionanomaterial.3,4 Coupled to full recyclability, biodegradability, and lack of toxicity, said properties make nanocellulose and nanocellulose-based composites suitable for application in a vast array of industrial fields. Composites, furthermore, are easily obtained through the straightforward combination of highly hydrophilic nanocellulose with inorganic and organic substances.5
Numerous excellent books6,7,8 describe the structure and the preparation routes of different nanocelluloses, with recent monographs devoted to specific applications of nanocellulose  for instance in electronics9 or in water treatment.10
From nanocellulose-based aerogels,11 through energy storage systems,12 applications in biomedicine,13 environmental remediation,14 and catalysis,15 numerous review articles on nanocellulose uses have been published. Hence, rather than adding a new review in a rapidly evolving research field, this study provides a critical outlook on the emerging green production routes of this “ageless bionanomaterial”.16
To put the discussion in context, we first briefly discuss the current production routes, some of which have been commercialized, from an economic viewpoint. We then present the main emerging green chemistry production methods. The study concludes suggesting new avenues to tackle environmental issues using nanocelluloses sourced from cellulosic biowaste17 based on these new environmentally friendly production routes.

2. Current production routes

Nanocellulose is generally sourced from wood pulp as cellulose nanofiber (CNF) or cellulose nanocrystal (CNC). In addition, highly pure (and expensive) bacterial nanocellulose (BNC) is industrially synthesized on small scale from glucose using Gluconoacetobacter xylinus bacteria. “The way to produce bacterial nanocellulose today”, however, “is expensive and the methods are inefficient”.18