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TEMPO-oxidized cellulose nanofibers (TOCNs) from waste of oil palm empty fruit bunches (OPEFB) were integrated into an alginate matrix to increase the capacity of the alginate membrane for water-ethanol separation. The membrane composed of the alginate matrix and TOCNs was characterized in terms of its morphological, physical-mechanical properties and performance in the separation of water-ethanol suspensions, with ethanol concentrations in the suspension of 10% and 20%. Other alginate membranes integrated with commercial TOCNs from wood were also prepared and tested for comparison. The results showed that the addition of TOCNs (made from wood and OPEFB waste) to the alginate matrix improved the water adsorption capacity of the membrane. The water adsorption capacity of the alginate membranes with wood-derived TOCNs, OPEFB-derived TOCNs and alginate only was 78%, 87% and 66%, respectively. The flux capacity of the alginate membrane, integrated with OPEFB-derived TOCNs, was higher than that of the alginate membrane alone, but lower than that of the alginate membrane integrated with wood-derived TOCNs. This study showed the utilization of nanocellulose from palm oil biomass waste can be considered to improve the physical-mechanical properties of alginate-based membranes used for various applications, including filtration.

Cellulose nanofibers obtained from wood pulp by TEMPO-mediated oxidation acted as a chiral enhancer in direct aldol reactions of 4-nitrobenzaldehyde and cyclopentanone with (S)-proline as an organocatalyst. Surprisingly, catalytically inactive TEMPO-oxidized cellulose nanofibers enriched the (R,R)-enantiomer in this reaction, affording 89% ee in the syn form with a very high yield (99%). Conversely, nanocellulose-free (S)-proline catalysis resulted in poor selectivity (64% ee, syn form) with a low yield (18%). Green organocatalysis occurring on nanocellulose solid surfaces bearing regularly aligned chiral carbons on hydrophobic crystalline facets will provide new insight into asymmetric synthesis strategies for interfacial catalysis.

A chitosan nanofiber (CsNF)-catalyzed Knoevenagel reaction in green solvent, namely aqueous methanol, was investigated. CsNFs solely catalyzed the desired C–C bond formations in high yield with high selectivity, while conventional small-molecule amines, such as n-hexylamine and triethylamine, inevitably promoted transesterification to produce a large amount of solvolysis byproducts. Structural and chemical analyses of CsNFs suggested that the unique nanoarchitecture, in which chitosan molecules were bundled to ensure the high accessibility of substrates to catalytic sites, was critical to the highly efficient Knoevenagel condensation. The products were obtained in high purity without solvent-consuming purification, and the CsNF catalyst was easily removed and recycled. This study highlights a novel and promising function of CsNFs in green catalysis as emerging polysaccharide-based nanofibers.

Cellulose nanofibers (CNFs) are finding a wide range of applications in the forthcoming sustainable society because of their carbon-neutral renewability and superior physicochemical properties. Here, we first show a cooperative organocatalysis by combining TEMPO-oxidized cellulose nanofiber (TOCN) and proline to enhance the catalytic efficiency in a direct aldol reaction. The yields of proline-catalyzed aldol products drastically increased in the presence of catalytically-inactive TOCN. This effect was also achieved by simply adding the TOCN to the reaction conditions where various proline analogues including structurally simple pyrrolidine and piperidine were used instead of proline. TOCN was superior to physically-pulverized CNF in the organocatalytic efficiency, and the nanofibrillation of cellulose microfibrils in reaction media was essential to induce the drastic enhancement in catalytic activity. The present finding will bring a new entry in the applications of CNFs, and open up a new phase in developing highly efficient molecular transformations in green chemical industries.

Cellulose nanofibers (CNFs) have recently attracted much attention as catalysts in various reactions. Organocatalysts have emerged as sustainable alternatives to metal-based catalysts in green organic synthesis, with concerted systems containing CNFs that are expected to provide next-generation catalysis. Herein, for the first time, we report that a representative organocatalyst comprising an unexpected combination of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-oxidized CNFs and proline shows significantly enhanced catalytic activity in an asymmetric Michael addition.

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs), which have a high-density of exposed carboxylic acid groups on their crystalline surfaces, effectively act as acid catalysts in acetal hydrolysis. Carboxy-free cellulose nanofibers, polymeric carboxylic acids, and homogeneous acetic acid do not show significant catalytic activity under the same reaction conditions. Mercerized TOCNs differing from the original TOCNs in a crystalline structure were also ineffective, which suggests that the unique nanoarchitectural features of TOCNs, such as regularly aligned carboxylic acid groups, large specific surface areas, and structural rigidity, must be major factors in the acceleration of acetal hydrolysis. Kinetic analysis suggested that substrates and/or acid catalyst species were concentrated on the TOCN crystalline surfaces, which significantly enhanced the catalytic activity.

An enantioselective Friedel–Crafts reaction with aliphatic ketimines generated in situ from hemiaminal ethers afforded products with high enantioselectivity under the influence of a chiral phosphoric acid catalyst., An enantioselective Friedel–Crafts reaction with aliphatic ketimines generated in situ from hemiaminal ethers catalyzed by a chiral Brønsted acid was investigated. The reaction of 2-methoxyfuran with (thio)hydantoin-derived hemiaminal methyl ether proceeded under the influence of a chiral phosphoric acid catalyst to afford the corresponding adduct possessing a quaternary stereogenic center in high yield with high enantioselectivity. Theoretical studies were also conducted to clarify the mechanism of the stereochemical outcome and the major factors contributing to the efficient enantioselection.

Nanocellulose produced by 2, 2, 6, 6-tetramethylpiperidine-1-oxyl (TEMPO)-catalyzed oxidation, described as TEMPO-oxidized cellulose nanofibers (TOCNs), has a high density of negative charges on its surface. Its use in microbial fermentation systems is expected to be beneficial. In particular, microbial stability is required in acetone–butanol–ethanol (ABE) fermentation. Here, TOCNs derived from oil palm empty fruit bunches pulp were added to extractive ABE fermentation media, followed by microbial fermentation. The results showed that the presence of TOCNs induced higher total butanol production in broth by improving the growth environment of Clostridium saccharoperbutylacetonicum N1-4, which was used as the butanol-producing strain. Electrostatic repulsion between anionic surface carboxylate groups of TOCNs and negatively-charged bacteria made a positive contribution to the microenvironment for bacterial growth. The addition of TOCNs to fermentation media had significant positive effects on the total butanol yield density cell weight (DCW).

The use of oil palm waste as a source of raw materials for nanocellulose production is a promising approach for turning agricultural waste into valuable products. This study aimed to characterize cellulose nanofibers (CNFs) derived from oil palm empty fruit bunches (OPEFB) produced by 2, 2, 6, 6-tetramethylpiperidine 1-oxyl (TEMPO)/NaBr/NaClO. TEMPO-mediated oxidation was carried out at pH 10 by applying different amounts of oxidant: 10, 15 and 20 mmol/g-cellulose at room temperature for 2 h, where the resultant CNFs were denoted as T-10, T-15 and T-20, respectively. Characterization results revealed that the carbon content of each CNF decreased as compared to the starting pulp, from 40% to ca. 37%. Microscopic analysis showed that the aspect ratios of obtained CNFs remained almost unchanged by increasing the amounts of oxidant, which were ca. 40. The crystallinity indices (CrIs) ranged from 34 to 55%. The carboxy contents of CNFs increased by adding higher amounts of oxidant, but the thermal resistance properties were slightly decreased. High-performance nanomaterials such as TEMPO-oxidized CNFs were successfully prepared to show good characteristics, from low-quality biomass waste such as OPEFB.

Chiral phosphoric acids have emerged as promising asymmetric Bronsted acid catalysts that harness hydrogen bonding interactions as key stereocontrolling elements. A new approach to chiral phosphoric acid catalysis through ion-pairing interactions between the anionic conjugate base of the catalyst and a cationic electrophile has recently attracted attention. However, the mechanism of stereocontrol through ion-pairing interactions is still elusive. As a probe reaction for studying the stereocontrolling element involved in such catalytic reactions, we investigated the Petasis–Ferrier-type rearrangement of a 7-membered cyclic vinyl acetal catalyzed by chiral phosphoric acids. DFT calculations suggested that non-classical C–H⋯O hydrogen bonds between the catalyst and the substrate play an important role in determining the stereoselectivity. In addition, π–π stacking interactions were found to be a key factor for stereocontrol when using a 9-anthryl group-bearing catalyst.