Recent Strategies in Preparation of Cellulose Nanocrystals and Cellulose Nanofibrils Derived from Raw Cellulose Materials

Table 1

Introduction of various preparation methods of CNCs including major chemical reagents, typical conditions, yield, thermal stability, and some other advantages/disadvantages.

Method	Major chemical reagents	Typical conditions	(%)	(°C)	Other advantages/disadvantages
Mineral acid hydrolysis	H2SO4, HCl, H3PO4, HBr	64 wt.% H2SO4, at 45°C for 30 min [30].	15.7	253.2	It is difficult to treat waste acid. H2SO4 has a serious corrosion problem to equipment. The CNCs have excellent suspension stability.	[35–75]
		6 M HCl, 110°C, 3 h [58].	80	347.2	It is difficult to treat waste acid. HCl has a serious corrosion problem to equipment. The CNCs have bad suspension stability.
Solid acid hydrolysis	Cation exchange resin (NKC-9), Phosphotungstic acid	75 wt.% Phosphotungstic acid, at 90°C for 30 h [77].	60	350	Phosphotungstic acid could be recovered through extraction with ethyl ether and is low corrosion to equipment.	[76–79]
Organic acid hydrolysis	Formic acid, oxalic acid, Maleic acid, -toluenesulfonic acid	0.015 M FeCl3, 88 wt.% HCO2H, at 95°C for 6 h [63].	75	355	Formic acid could be recovered by reduced pressure distillation and is low corrosion to equipment.	[63, 80, 91–101]
		Melt oxalate dihydrate, at 110°C for 2 h, then, sonicated for 18 min [91, 93].	80.6	360	Oxalate dihydrate could be recovered by crystallization and is low corrosion to equipment.
Enzymatic hydrolysis	Cellulase	20 u/mL complex enzymes (cellulase : xylanase = 9 : 1), at 50°C for 5 h [104].	13.6	--	This method is environment-friendly, but the cost is very high.	[81, 102–105]
Oxidation degradation	(NH4)2SO5, NaIO4, NaClO2	1 mol/L (NH4)2SO5, at 80°C for 16 h [110].	35	330.9	Carboxylation CNCs could be produced. A large number of oxidants were consumed in the process.	[106–117]
Ionic liquid	[BMIM]HSO4, [BMIM]Cl, [EMIM][OAc], TBAA/DMAc	[BMIM]HSO4, at 70–90°C for 1 h [119].	--	260	Ionic liquid could be reused. It has advantage to preparing functional CNCs. However, ionic liquid is very expensive at present.	[82, 118–125]
Transition metal catalysis	Cr(III), Fe(III), Al(III), Ni(II), Co(II), Mn(II)	Solid-liquid ratio was 1 : 30 and 0.8 M Cr(NO3)3 was adopted at 80°C for 1.5 h [126].	83.6	344	Transition metal could be used to enhance the hydrolysis reactivity and reduce the amount of acid.	[68, 126–129]
Subcritical water	--	H2O, 120°C and 20.3 MPa for 60 min [130].	21.9	345	The method is environment-friendly. The cost of preparing CNCs was estimated to be 0.02 $/kg. The CNCs have bad suspension stability.	[83, 130]
DES	Choline chloride/Oxalic acid	Choline chloride/Oxalic acid dihydrate = 1 : 1 (Molar ratios), at 100°C for 6 h, then Mechanical disintegration with microfluidizer [131].	78	300	DES is recoverable, biodegradable, and environmentally friendly. However, the reaction degree is low, so it needs follow-up mechanical treatment to get the CNCs products.	[131]
AVAP	Sulfur dioxide/Ethanol	SO2/EtOH/H2O, 130–165°C [84].	24	--	AVAP method could produce hydrophobicity and morphology adjustable CNCs. The method is low cost and has been used for industrial production of CNCs.	[84, 132]

: the reference of typical conditions; : the details come from the reference of corresponding typical condition; : the max decomposition temperature, the details come from the reference of corresponding typical condition; : the reference of corresponding method of preparing CNCs.