Kinetic Study of the Thermal Decomposition of Cellulose Nanocrystals with Different Crystal Structures and Morphologies

Abstract

In this study, cellulose nanocrystals with cellulose crystal structure I (CNCs I) and with coexisting cellulose crystalline structures I and II (CNCs I-II) were isolated from pine cellulose using acid hydrolysis with H 2 SO 4 . It was possible to obtain CNCs with different morphologies, crystallinities and crystalline structures adjusting only the reaction conditions. The thermal stability is directly related to the crystalline structure and the crystallinity. It was observed that CNCs composed mainly of CNC II have an initial degradation temperature higher than the CNCs I when comparing samples with similar crystallinity. The kinetic results allow us to conclude that activation energy (E a ) not only depends on the crystalline structure and crystallinity of the CNCs but may also be related to the presence of sulfate groups. Understanding the influence of crystallinity and crystalline structure on the thermal stability of CNCs can direct the studies of future applications for these materials.

The Role of Eucalyptus Species on the Structural and Thermal Performance of Cellulose Nanocrystals (CNCs) Isolated by Acid Hydrolysis

“…Four main reflection peaks were identified at 2 θ = 15.0°, 16.5°, 22.6° and 34.5° due to the (101), ( ), (002) and (040) crystallographic planes of the monoclinic cellulose I-β lattice [ 65 ]. The highly crystalline structure of the obtained CNCs was thus corroborated and results were consistent with cellulose crystal structure I [ 31 ].…”

“…Although temperatures around 350 °C have been reported for the thermal decomposition of cellulose, the presence of sulphate groups in the surface of CNCs replacing the hydroxy groups may have catalyzed the decomposition and, therefore, decreased the stability of the CNCs [ 29 ]. Nevertheless, the low-temperature decomposition stage may be attributed to the degradation of amorphous regions that are more sulphated and accessible, and at higher temperatures the degradation of the non-sulphated CNCs may occur [ 31 , 74 ]. Similar results have been reported for CNCs isolated from corn stalk [ 33 ], kenaf [ 63 ], potato [ 75 ], and coconut husk [ 54 ].…”

“…The most sulphated CNCs showed lower thermal stability due to the catalytic contribution of the sulfuric acid on the thermal decomposition process [ 70 , 71 ]. The presence of the sulphate groups enabled the formation of carbon residues at the end of the pyrolytic decomposition [ 31 ]. However, these residues, ( R ), could not be correlated.…”

“…However, differences can be established for CNCs with crystal structures I and II. Whereas CNCs with crystal structure II usually show an increasing trend for the activation energy as a function of the conversion degree, the CNCs with crystal structure I have been reported with a first increase in the activation energy for conversions around 0.3 followed by a decrease in conversions close to 0.6 with an increase again for higher conversions [ 31 ]. Such is the case of the CNCs assessed in this study, which all have a cellulose crystal structure I. Analogous behavior was observed with crystal structure I obtained from bleached Kraft cellulose through acid hydrolysis with sulfuric acid [ 31 , 32 ].…”

“…Specifically, the crystalline arrangement and the chemical composition are in turn the most influencing parameters during thermal decomposition and may be subsequently affected by the cellulose source and also by the extracting method [ 29 ]. Although the pyrolytic decomposition of CNCs extracted from mango seeds, Kraft pulp, cellophane, corn stalk or curaua fibers has been reported in the literature [ 30 , 31 , 32 , 33 ], the comparative evaluation of the apparent activation energy ( Ea ) of CNCs from different eucalyptus species during thermal decomposition has not been yet reported. Therefore, Kissinger, Flynn–Wall–Ozawa (FWO), and Kissinger–Akahira–Sunose (KAS) methods were used to assess the apparent activation energy ( Ea ) of such CNCs during thermal decomposition.…”