Microhoneycomb Monoliths Prepared by the Unidirectional Freeze-drying of Cellulose Nanofiber Based Sols: Method and Extensions

Summary

Here, we present a general protocol to prepare a variety of microhoneycomb monoliths (MHMs) in which fluid can pass through with an extremely low pressure drop. MHMs obtained are expected to be used as filters, catalyst supports, flow-type electrodes, sensors and scaffolds for biomaterials.

Abstract

Monolithic honeycomb structures have been attractive to multidisciplinary fields due to their high strength-to-weight ratio. Particularly, microhoneycomb monoliths (MHMs) with micrometer-scale channels are expected as efficient platforms for reactions and separations because of their large surface areas. Up to now, MHMs have been prepared by a unidirectional freeze-drying (UDF) method only from very limited precursors. Herein, we report a protocol from which a series of MHMs consisting of different components can be obtained. Recently, we found that cellulose nanofibers function as a distinct structure-directing agent towards the formation of MHMs through the UDF process. By mixing the cellulose nanofibers with water soluble substances which do not yield MHMs, a variety of composite MHMs can be prepared. This significantly enriches the chemical constitution of MHMs towards versatile applications.

Introduction

As a brand-new material, microhoneycomb monolith (denoted MHM) has recently attracted tremendous attention from multidisciplinary fields^{1,2,3,4,5,6,7,8}. The MHM was first prepared by S. Mukai et al. through a modified unidirectional freeze-drying (UDF) approach as a monolith with an array of straight microchannels with honeycomb-like cross sections⁹. MHM possesses the gen....

Protocol

1. Preparation of 1 wt% 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO)-mediated Oxidized Cellulose Nanofiber (TOCN) Sol

NOTE: The sol is defined as a colloidal suspension of very small solid particles in a continuous liquid medium.

1. Suspend 66.7 g of Nadelholz Bleached Kraft Pulp (NBKP, containing 12 g of cellulose) in 700 mL of deionized (DI) water with a mechanical agitator at 300 rpm for 20 min.
2. Add 20 mL of aqueous TEMPO solution (containing 0.15 g of TEMPO) and 20 mL of aqueous NaBr solution (containing 1.5 g of NaBr) slowly to the above NBKP suspension^14,15.<....

Results

The morphologies for different positions of the MHM-TOCN along the direction of unidirectional freezing are investigated and shown in Figure 2. With the position being further away from the bottom part of the MHM-TOCN, a gradual morphology change was revealed (Figure 2, Discussion). By introducing a second component into the TOCN sol to form a homogeneous mixture sol, it is possible to prepare various kinds of co.......

Discussion

The most critical step for achieving the MHMs is the unidirectional freezing step, during which water solidifies to form columnar ice crystals and push the dispersoid aside to form the framework. The unidirectional freezing process basically involves thermal transfer between the precursor sol and the coolant. In our setup, a dipping machine was used to insert a PP tube containing a precursor sol into the coolant (liquid nitrogen) with a constant velocity. Since liquid nitrogen keeps evaporating all the time, a fluctuant .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Nadelholz Bleached Kraft Pulp	Seioko PMC company	CSF=600
TEMPO	Macklin Inc.	T819129	98%
NaBr	Macklin Inc.	S818075	AR, 99%
NaClO	Aladin Inc.	S101636	6-14 wt% active chlorine basis
SBR colloid	JSR corp.	TRD102A	48.5 wt%
TiO2	Sinopharm Chemical Reagent Co., Ltd.	A63725402	crystalline anatase phase
carbon fiber	Shenzhen Xian’gu Ltd.	XGCP-300
Nitric acid	Huada Reagent Ltd.	7697-37-2	65-68 wt%
Mixer	Scientific Industries, Inc	G-560	the mixer
Mechanical blender	Waring Lab Ltd.	MX1000XTX	For disintegrating cellulose bundles into nanofibers.
Homogenizer	Scientz Ltd.	HXF-DY	For dispersing TiO2 nanoparticles
pH meter	Horiba Ltd.	F-74BW