Abstract
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.
Highlights
Cellulose is the key material to unlocking the gate toward a forthcoming sustainable society, because of its huge abundance and carbon-neutral renewability, and its fascinating physicochemical properties, which are equivalent or sometimes superior to those of artificial and synthetic materials
Among various Cellulose nanofibers (CNFs) prepared by several different methods, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofiber (TOCN) was focused on in this work for the initial screening because it provides a well-defined nanostructure with the narrowest fiber width (3–5 nm), and the largest specific surface area among those ever reported; carboxylate groups introduced to the surfaces of crystalline CNFs can facilitate disintegration of bundled nanofibers by electrostatic repulsion and osmotic effects in solvents[29]
Characterization of CNF and TEMPO-oxidized cellulose nanofiber (TOCN) used in this study is shown in Figures S1–S3 in the Supplementary Information (SI)
Summary
Cellulose is the key material to unlocking the gate toward a forthcoming sustainable society, because of its huge abundance and carbon-neutral renewability, and its fascinating physicochemical properties, which are equivalent or sometimes superior to those of artificial and synthetic materials. The applications of CNFs have been intensively studied for composite fillers[4], electronic devices[5,6], gas barrier films[7], and various gels in wet and aero forms[8,9]. These applications have been aimed to design structural materials using the superior physical properties of CNFs. The next-generation trends of applying CNFs have occupied much interest in catalytic transformations, through harnessing their characteristic and large surface areas via functional tuning. We first report a unique combination of CNF and proline to enhance the efficiency of organocatalytic aldol reaction significantly (Fig. 1)
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