Direct Synthesis of Full-Biobased Cellulose Esters from Essential Oil Component α,β-Unsaturated Aldehydes

Abstract

To become independent of petroleum resources, societies have to develop methods to synthesize full-biobased polymers from natural products directly. In this study, full-biobased cellulose esters were synthesized by the oxidative esterification process between cellulose and natural α,β-unsaturated aldehydes in an ionic liquid working as solvent and catalyst. The role of the ionic liquid in this cellulose-modification system was systematically investigated, and a newly designed noncarboxylate-type ionic liquid, 1-ethyl-3-methylimidazolium 2-pyridinolate (EmimOPy), was developed to prevent a serious side reaction in which the carboxylate anion of the carboxylate-type ionic liquid is introduced into the hydroxyl group of cellulose as an ester group on the products. Under optimized conditions, this oxidative esterification-reaction system in EmimOPy afforded fully substituted cellulose ester (up to the degree of substitution = 3.0). Furthermore, cellulose reacted with several types of aldehyde substrates, even biobased low-reactive α- or β-substituted unsaturated aldehydes (e.g., citral and perillaldehyde). Thus, this synthesis demonstrates excellent atom economy without using any additional catalysts or activators under metal-free conditions.

Aryloxy Ionic Liquid-Catalyzed Homogenous Esterification of Cellulose with Low-Reactive Acyl Donors

“…To improve cellulose solvation, EmimOPh 5 (the pKa value of the conjugated acid is 9.86), which has a smaller size, higher basicity anion, and is a better acceptor of hydrogen bonds, was synthesized. However, the synthesized 5 was not sufficiently stable for cellulose modification [ 26 , 34 , 35 ]. To stabilize the negative charge on the anions of ILs, we tested EmimOPys 6–8 which have a nitrogen atom at the 2-4 position of the aromatic ring.…”

“…Recently, we reported a synthetic method for fully bio-based cellulose esters using bio-based unsaturated aldehydes such as cinnamon-based cinnamaldehyde 12 as acyl donors based on different mechanisms of transesterification [ 26 , 28 ]. This reaction proceeds via an oxidative esterification mechanism in which the carbon–carbon double bond (C=C) works as an intramolecular oxidant and is an environmentally benign method because there is no leaving group, unlike conventional esterification methods, including transesterification, that use acid chloride, anhydride, and condensation reagents.…”

“…This reaction proceeds via an oxidative esterification mechanism in which the carbon–carbon double bond (C=C) works as an intramolecular oxidant and is an environmentally benign method because there is no leaving group, unlike conventional esterification methods, including transesterification, that use acid chloride, anhydride, and condensation reagents. Under mild reaction conditions, oxidative esterification hardly proceeds without N -heterocyclic carbene (NHC)-based catalytic ability of the ionic liquid [ 26 , 28 , 44 ]. Therefore, aldehydes are regarded as inactive acyl donors in conventional cellulose esterification without an ionic liquid.…”

“…EmimOAn 2 [ 24 ], 1-ethyl-3-methylimidazolium phenoxide (EmimOPh 5 ) [ 26 ] and 1-ethyl-3-methylimidazolium 2-pyridinolate (Emim2OPy 6 ) [ 26 ] were prepared using the reported method from our group. 1-Ethyl-3-methylimidazolium 3-pyridinolate (Emim3OPy 7 ), 1-ethyl-3-methylimidazolium 4-pyridinolate (Emim4OPy 8 ), 1-Ethyl-3-methylimidazolium theophylinate (EmimTEO 4 ) and 1-ethyl-3-methylimidazolium 4,6-dimethyl-2-hydroxypyrimidinolate (Emim2OPm(diMe) 3 ) were synthesized according to conventional protocols, as shown below [ 26 ].…”

“…The synthesis of cellulose esters via oxidative esterification was conducted using a previously reported method [ 26 , 28 ]. As a typical example, cellulose (120 mg, [AGU] = 0.740 mmol, and [OH] = 2.22 mmol) was mixed with the Emim2OPy 6 (455 mg, 2.22 mmol) and dried for three hours in vacuo at 80 °C.…”