Green and Low-cost Production of Thermally Stable and Carboxylated Cellulose Nanocrystals and Nanofibrils Using Highly Recyclable Dicarboxylic Acids

Summary

Here we demonstrate a novel method for green and sustainable productions of highly thermally stable and carboxylated cellulose nanocrystals (CNC) and nanofibrils (CNF) using highly recyclable solid dicarboxylic acids.

Abstract

Here we demonstrate potentially low cost and green productions of high thermally stable and carboxylated cellulose nanocrystals (CNCs) and nanofibrils (CNF) from bleached eucalyptus pulp (BEP) and unbleached mixed hardwood kraft pulp (UMHP) fibers using highly recyclable dicarboxylic solid acids. Typical operating conditions were acid concentrations of 50 – 70 wt% at 100 °C for 60 min and 120 °C (no boiling at atmospheric pressure) for 120 min, for BEP and UMHP, respectively. The resultant CNCs have a higher thermal degradation temperature than their corresponding feed fibers and carboxylic acid group content from 0.2 – 0.4 mmol/g. The low strength (high pKa of 1.0 – 3.0) of organic acids also resulted in CNCs with both longer lengths of approximately 239 – 336 nm and higher crystallinity than CNCs produced using mineral acids. Cellulose loss to sugar was minimal. Fibrous cellulosic solid residue (FCSR) from the dicarboxylic acid hydrolysis was used to produce carboxylated CNFs through subsequent mechanical fibrillation with low energy input.

Introduction

Sustainable economic development requires not only using feedstocks that are renewable and biodegradable but also uses green and environmental friendly manufacturing technologies to produce a variety of bioproducts and biochemicals from these renewable feedstocks. Cellulose nanomaterials, such as cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), produced from renewable lignocelluloses are biodegradable and have unique mechanical and optical properties suitable for developing a range of bioproducts ^{1, 2}. Unfortunately, existing technologies for producing cellulose nanomaterials are either energy intensive when using pure mechanical fibrillation or environmentally unsustainable due to non-recycling or insufficient recycling of processing chemicals, such as when using the concentrated mineral acid hydrolysis process ^3-8 or oxidation methods ^9-11. Furthermore, oxidation methods may also produce environmentally toxic compounds by reacting with lignocelluloses. Therefore, developing green manufacturing technologies for producing cellulose nanomaterials is critically important to make full use of the abundant and renewable material – lignocelluloses.

Using acid hydrolysis to dissolve hemicellulose and depolymerize cellulose is an effective approach for producing cellulose nanomaterials. Solid acids have been used for sugar production from cellulose with the advantage of easing acid recovery ^{12, 13}. Previous studies using concentrated mineral acids indicated that a lower acid concentration improved CNC yield and crystallinity ^{3, 5}. This suggests that a strong acid may damage cellulose crystals while a milder acid hydrolysis might improve the properties and yield of cellulose nanomaterials through the approach of integrated production and CNC with CNF ^{3, 14}. Here we document a method using concentrated solid dicarboxylic acids hydrolysis to produce CNC along with CNF ¹⁵. These dicarboxylic acids have low solubility at low or ambient temperatures, and therefore they can be easily recovered through the mature crystallization technology. They also have good solubility at elevated temperatures which facilitates concentrated acid hydrolysis without boiling or using pressure vessels. Since these acids also have a higher pKa than typical mineral acids used for CNC production, their use results in good CNC crystallinity, and despite lower CNC yields, with a substantial amount of fibrous cellulosic solid residue (FCSR or partially hydrolyzed fibers) remaining due to incomplete cellulose depolymerization. The FCSR can be used to produce CNF through subsequent mechanical fibrillation using low energy inputs. Therefore, cellulose loss to sugars is minimal as compared to using mineral acids.

It is well known that carboxylic acids can esterify cellulose through Fisher-Speier esterification ¹⁶. Applying dicarboxylic acids to cellulose can result in semi-acid un-crosslinked esters ¹⁷ (or carboxylation), to produce carboxylated CNC and CNF as we demonstrated ¹⁵ previously. The method documented here can produce carboxylated and thermally stable CNF and CNC that is also highly crystalline from either bleached or unbleached pulps while having relatively simple and high chemical recovery and using low energy inputs.

Protocol

NOTE: Bleached eucalyptus kraft pulp (BEP) and unbleached mixed hardwood kraft pulp (UMHP) fibers from commercial sources were used as feedstock for producing CNC and CNF. Commercial maleic acids purchased were used for hydrolysis. Hydrolysis conditions were acid concentrations of 60 wt% at 100 °C for 60 min and 120 °C (no boiling at atmospheric pressure) for 120 min, for BEP and UMHP, respectively. 1. Preparation of Concentrated Dicarboxylic Acid Solution Heat 40 mL deio…

Representative Results

Typical AFM images of the CNC and CNF from BEP and UMHP along with corresponding SEM images of the feed acid hydrolyzed fibers are shown in Figures 1 and 2. The images clearly show the substantial reductions in fiber length by acid hydrolysis with minimal change in fiber diameters (comparing Figure 1a with 1b, and 2a with 2b). The shortened fiber length was also reflected by the measured cellulose degree of polymerizatio…

Discussion

The thicker CNC diameters of the CNC samples from maleic acid hydrolysis resulted in a moderate average aspect ratio 7.24 and 8.53, for the CNCs from BEP and UMHP, respectively, despite their long lengths as discussed above. The CNFs had a longer length and a thinner diameter, which resulted in a large aspect ratio of 13.9 and 19.0, for the CNCs from BEP and UMHP, respectively, both greater than their respective CNCs. It is possible to use severe mechanical fibrillation to reduce CNF diameter to improve the aspect ratio …

Materials

Bleached eucalypus pulp	Aracruz Cellulose
Unbleached mixed hardwood kraft pulp	International Paper
Maleic acid	Sigma-Aldrich	M0375-1KG/CAS110-16-7	Powder; assay: 99.0%(HPLC)
Glycerol	Sigma-Aldrich	G5516-4L/CAS56-81-5
Sodium hydroxide	Fisher Scientific	S318-500/CAS1310-73-2, 497-19-8	Certified ACS
Sodium chloride	Mallinckrodt	7581-12/CAS7647-14-5	Crystal,AR
Cupriethylenediamine solution	GFS Chemicals	E32103-1L/CAS14552-35-3	1M, for determination of solution viscosity of pulps
Acetone	Fisher Scientific	A18-500/CAS67-64-1	Certified ACS
Accu-TestTM Vials for COD Testing	Bioscience,Inc.	01-215-28	COD testing for 20 to 900mg/L standard range concentration
Heating plate	IKA		Mode: C-MAD HS7 digital
Magnetic stir bar	ACE Glass
Pyrex three-neck round-bottom flask	Sigma-Aldrich	CLS4965B500-1EA
Dialysis tubing cellulose membrane	Sigma-Aldrich	D9402-100FT	Typical molecular weight cut-off = 14000
Disposable aluminum dishes	Sigma-Aldrich	Z154857-1PAK	Circles, 60mm
Disintegrator	Testing Machines Inc.(TMI)
Microfluidizer	Microfluidics Corporation
Sonicator	Qsonica LLC.		Mode: 3510R-MT, 50-60 Hz,180 W
Zeta potential analyzer	Brookhaven Instruments Corporation
FTIR	PerkinElmer
Conductometric titrator	Yellow Springs Instrument (YSI)
TGA analyzer	PerkinElmer
X-ray diffractometer	Bruker Corporation
AFM imging	AFM Workshop
SEM imaging	Carl Zeiss