The team dubbed “CytroCell” the new micronized cellulose. Both new CytroCell materials consist of cellulose of low crystallinity index (0.33 for lemon and 0.36 for grapefruit). Lemon “CytroCell” consists of 0.5-3 micron long microfibrils whose section varies between about 110 and 420 nm. Grapefruit CytroCell is comprised of ramified microfibrils whose diameter varies from 500 nm to 1 μm (Figure 2).46
The one-pot process requires no subsequent mechanical treatment of the fibrillated nanofibers. In brief, the highly efficient HC creates cavitation bubbles which fibrillate and promote decrystallization of the citrus peel microcrystalline cellulose fibers. The effect is intensified by the presence of the residual citric acid relatively abundant in the wet CPW used as cellulose source, such as in the case of acoustic cavitation applied to microcystalline cellulose in the presence of 0.2 M citric acid.47
4. Outlook and perspectives
Comparing four routes to wood pulp-derived nanocellulose (TEMPO-oxidation followed by sonication or homogenization and chloroacetic etherification followed by sonication or homogenization), Renneckar and co-workers recently found that TEMPO oxidation followed by homogenization is the lowest impact nanocellulose process.48 For comparison, the energy and environmental impact factors for nanocellulose industrial production using the latter optimal route are, respectively, 4 and 20 times larger than the kraft pulp production.
Whether using concentrated H2SO4 to hydrolyze the amorphous regions of cellulose and isolate CNC,22 or relying on the TEMPO-catalyzed process with bleach as primary oxidant and bromide as co-catalyst followed by mechanical treatment to afford CNF,25,26 current nanocellulose production methods are capital-intensive and have high operating costs. In the case of CNC, feedstock cost and capital investment are the major cost drivers, with sulfuric acid and lime consumption alone accounting for 25% of production costs for the lowest plant configuration (without acid recovery).22
In the case of CNF, high manufacturing costs are due to the large amount of electrical energy required for homogenization, followed by the high cost of the TEMPO catalyst.49 Furthermore, a large amount of solvent is needed to separate the CNF from genotoxic TEMPO, while the expensive TEMPO catalyst is lost in the reaction effluents. The method to recover the TEMPO catalyst in a mixture of a water-miscible organic solvent and water,30 could not be industrialized in the cellulose oxidation process carried out in water only likely due to high cost.50
It is perhaps not surprising to find that the first large nanocellulose production plants are operated by paper and pulp companies.26 Paper companies indeed are facing a dramatic fall in paper demand following the advent of the internet and other digital technologies.51 Beyond the aforementioned paper and pulp company producing CNF at two production sites in Japan,26 another paper and pulp company based in Norway since late 2016 produces MFC at a production site with a 1,000 t/a capacity. The ingredient is shipped to different industrial customers as 2% or 10% aqueous formulation.52
Comparing CNC chiefly extracted from different producers via hydrolysis with concentrated H2SO4 and purified according to different procedures, Cranston and Reid lately found that all products share the same basic chemical structure and physicochemical properties of laboratory-made CNC.53
Still, both main nanocellulose production methods need to be made less energy and capital intensive. Beyond enzymatic extraction,39 three methods are ready for optimization, scale up and commercialization: the heterogeneously catalyzed oxidation over new generation solid TEMPO catalysts,40 (including magnetically recoverable Karimi’s nanocatalyst);54 ball-milling in the presence of water of cellulose;37 and (for applications of nanocellulose of low crystallinity) acoustic and hydrodynamic cavitation.41,44,45 All are promising considering that many of the aforementioned catalysts are now commercial (Karimi’s nanocatalyst in the form of 50 nm beads easily recovered with a magnet);55 whereas the safe and robust HC process for the extraction of natural products such as waste orange peel is easily scaled-up.56
Enzymatic extraction, too, is an eminently clean nanocellulose production process.38,39 Dr Farinas in Brazil is one of the leading scholars in the field. Asked whether companies are using enzymatic extraction, she noted that:
“The use of enzymes to obtain nanocellulose is very challenging, but I believe it is a very promising alternative route. I believe that there are few companies using enzymes to obtain nanocellulose, but mostly to facilitate the production of nanofibrillated cellulose, as far as I know. One of them is VTT, using the HefCel concept in which enzymes facilitate the mechanical process”.57
“HefCel” stands for “high-consistency enzymatic fibrillation technology”, a process developed at VTT Technical Research Centre of Finland, in which cellulose fibrils are produced at high consistency (20-40%) by stirring concentrated cellulose pulp (20-40%) in the presence of a tailored cellulase enzyme mixture.58 The process results in 90% yield in fibrillated cellulose, while the low water content lowers the cost of drying.
Though not providing a timeline, market projections dating back to 2014 estimated a potential volume of nanocellulose exceeding 6.4 million tonnes/year only in the USA (5.9 million tons/year only in the high volume category, and 0.48 tonnes/year in the low volume category).59 Four years later, however, the nanocellulose market did not reach 40,000 tonnes (39,600 tonnes), with the large majority of being produced at pulp mills in form of MFC used in their own paper and paperboard products.60
In 2020, the nanocellulose market value amounted to about $300 million globally, with dried bacterial nanocellulose selling at $50/g,61 and the world’s largest nanocellulose production plant having a capacity of 2.5 t/day.60
It is instructive to learn that, besides use of MFC on paper and paperboard products, the main uses of nanocellulose currently concern low volume, and high value applications of CNF. For example, in Japan, the world’s leading CNF manufacturing country, products using CNF include biodegradable cutlery of enhanced strength; foam materials for the midsole of sport shoes; CNF-coated diaphragm for speakers and television sets; coating agent to prevent concrete adhesion; and undercoat paint preventing discoloration of the base material and cracks in the paint film.62
For nanocellulose-enabled technologies 5-10 to find practical utilization and large industrial uptake there is a need for green production technologies through which scale up production, lower production costs and make nanocellulose available to industrial customers at affordable price. This, inter alia, requires to switch from expensive wood pulp (priced at $907/tonne as of April 2021)63 to low cost and abundant biowaste as cellulose source.
Poor in lignin, and available in over 31 million tonnes/year, citrus biowaste whose other main component, pectin, is in high and increasing demand since more than a decade,64 is an ideal nanocellulose source in a integrated biorefinery.65
As put it by Jessop and Reyes, in scientific publications that use the “green chemistry” terminology, “there is an inversely proportional relationship between the number of papers published in an area and its associated environmental impact”.66 This is true also for nanocellulose production where the raw material originates from agrifood and forestal (paper and pulp, citrus, sugarcane, etc.) companies which are slowly, but inevitably, transforming into bioeconomy firms.26,52
Though originating from apparently distant areas such as heterogeneous catalysis, enzymatic catalysis and acoustic or hydrodynamic cavitation, the new green chemistry technologies to manufacture nanocellulose are ready to be scaled up and, if found technically and economically viable, uptaken by industry. To accelerate this progress, the new bioeconomy industry needs young researchers trained in the aforementioned chemical and physicochemical technologies, and with a critical knowledge of the field.67
This study offers such critical outlook en route to mass uptake of this versatile, robust and safe bionanomaterial sustainably produced and preferably sourced from low cost cellulosic biowaste.
Acknowledgements
We thank Dr. Cristiane Sanchez Farinas, Empresa Brasileira de Pesquisa Agropecuária (Embrapa), Brazil, for helpful correspondence. This study is dedicated to the memory of Professor Franco Piozzi (1928-2022), for nearly 40 years eminent Professor of organic chemistry at the University of Palermo.
Conflict of interest
The Authors declare no conflict of interest.
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