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.
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.
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.
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 …
The authors have nothing to disclose.
This work was conducted while Bian, Chen, and Wang were visiting Ph.D. students at the US Forest Service, Forest Products Laboratory (FPL), Madison, WI, and on official government time of Zhu. This work was partially supported by the USDA Agriculture and Food Research Initiative (AFRI) Competitive Grant (No. 2011-67009-20056), the Chinese State Forestry Administration (Project No. 2015-4-54), the National Natural Science Foundation of China (Project No. 31470599), Guangzhou Elite Project of China, and China Scholarship Fund. Funding from these programs made the visiting appointments of Bian, Chen, and Wang at FPL possible.
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 |