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 general advantages of honeycomb structures, i.e., efficient tessellation, high strength-to-weight ratio, and low pressure drop. Moreover, compared with the honeycomb monolith with a larger channel size, the MHM has a much larger specific surface area. The UDF method involves the unidirectional growth of ice crystals and simultaneous phase separation upon freezing. After the removal of the ice crystals, a solid component molded by the ice crystal is obtained. The morphology formed upon the phase separation depends on the intrinsic nature of the precursor (sol or gel), and in most of cases, lamella¹⁰, fiber¹¹, and fishbone¹² structures are likely to form rather than the MHMs. As a result, the formation of MHMs has been reported only in limited precursors, and this has significantly hampered the diversity of their chemical property. We have recently found that cellulose nanofibers have a strong structure-directing function toward forming the MHM structure through the UDF process¹³. Simply by mixing the cellulose nanofibers with other water-dispersible components, it is possible to prepare a variety of MHMs with different chemical properties. Moreover, their exterior shapes and channel sizes are flexibly and easily controlled¹³. Thus, MHMs are expected to be used as filters, catalyst supports, flow-type electrodes, sensors and scaffolds for biomaterials.

In this paper, we first explain the basic preparation technique of MHMs from the aqueous dispersion of cellulose nanofibers through the UDF process in detail. Moreover, we demonstrate the preparation of several different types of composite MHMs.

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.
3. Adjust the pH of the above suspension to approximately 10.5 (measured with a pH meter) with slowly adding 3.0 M NaOH solution.
4. Slowly add approximately 63.8 g of aqueous NaClO solution (with 6-14 wt% active chlorine) with a pipette to the above mixture to start TEMPO-mediated oxidation.
5. While adding NaClO, continue adding the NaOH solution to keep the pH of the system within the range of 10.0 to ~10.5. This process takes about 2.5 h.
6. Rinse the TEMPO-mediated oxidized cellulose fibers with DI water 3 times (1,200 mL of DI water each time) to remove the residual NaClO, NaOH and other chemicals.
7. Treat the paste with a powerful mechanical blender to disintegrate the oxidized cellulose fibers into nanofibers. Carry out the mechanical treatment carefully several times accompanied with an addition of equal amount of water. Finally, a 1 wt% TEMPO-mediated oxidized cellulose nanofiber (TOCN) sol is obtained. The TOCNs have a diameter of 4 to ~6 nm, and a length of 0.5 to ~2 μm.
8. Store the 1 wt% TOCN sol at 4 °C (cellulose nanofibers tend to rot at an ambient temperature).

2. Preparation of TOCN-styrene Butadiene Rubber (SBR) Mixed Sol

1. Add 0.21 g of SBR colloid (48.5 wt%) into 10 g of 1 wt% TOCN sol (step 11.7.) in a 20 mL glass vessel.
2. Agitate the above mixture for 3 min with a vortex mixer at the power level of 6 to achieve an evenly dispersed sol. Store the above mixture sol at 4 °C before use.

3. Preparation of TOCN-TiO₂ Mixed Sol

1. Add 0.1 g of TiO₂ nanoparticles (with an average particle size of 20 nm) into 10 g of 1 wt% TOCN sol in a 20 mL glass vessel.
2. Agitate the above mixture with a homogenizer for 10 min to achieve an evenly mixed sol. Perform this 10 min-process intermittently, since a significant amount of heat is generated in the process and results in the rise of temperature, which may degrade TOCNs. Store the mixture sol at 4 °C before use.

4. Preparation of TOCN-surface Oxidized Carbon Fiber (SOCF) Mixed Sols

1. Reflux 1.7 g of carbon fiber (300 mesh, with a diameter of 5.5 to ~6.0 μm and a length of approximately 50 μm) in 150 mL of concentrated nitric acid at 60 °C for 6 h to achieve SOCF¹⁶. Add 0.01 g of the above SOCF into 10 g of 1 wt% TOCN sol inside a 20 mL glass vessel.
2. Shake-mix the above mixture, and ultra-sonicate the mixture for 5 min to achieve an evenly mixed sol. Store the mixture sol at 4 °C before use.

5. Preparation of Microhoneycomb Monolith from the 1 wt% TOCN Sol (Denoted MHM-TOCN)

1. Load a polypropylene (PP) tube (with an inner diameter of 13 mm, an outer diameter of 15 mm, and a length of 150 mm) with glass beads and fill the bottom 5 cm part of the tube¹³.
2. Load a certain amount (the amount is not controlled for each time, but it is normally larger than 2.7 mL to ensure the subsequent cutting process) of the 1 wt% TOCN sol into the above PP tube containing glass beads.
   NOTE: TOCN sol was directly filled into the PP tube without pouring in glass beads for the study of the distance effect that has been involved in the unidirectional freezing process. In this case, the amount of TOCN sol was 11 mL.
3. Carefully remove the bubbles that may have been generated during the sol loading. Keep the PP tube containing the TOCN sol at 4 °C overnight before use.
4. Attach the above PP tube containing the TOCN sol to the dipping machine that is used for unidirectional freezing. Set the relevant parameters and start dipping the PP tube into a thermo jug containing liquid nitrogen (-196 °C) at a constant speed of 50 cm h^-1 (Figure 1).
5. Cut the PP tube part with a saw, and crack the frozen TOCN sol part into several sections. Freeze dry these sections with a freeze-drying machine at -10 °C for 1 day, then at -5 °C for 1 day, and finally at 0 °C for 1 day. The MHM-TOCN was obtained as white-colored monoliths (Figure 1).

6. Preparation of Microhoneycomb Monolith from the TOCN-SBR Mixed Sol (Denoted MHM-TOCN/SBR) and TOCN-TiO₂ Mixed Sol (Denoted MHM-TOCN/TiO₂ )

1. Load polypropylene (PP) tubes (with an inner diameter of 13 mm, an outer diameter of 15 mm, and a length of 150 mm) with glass beads, filling the bottom 5 cm part of the tubes.
   NOTE: The glass beads are used to cover the area where unsteady ice crystal growth occurs, to achieve uniform morphology of the resulting sample. Both the size and surface property of the glass beads do not affect the morphology of the resulting sample.
2. Load certain amount (the amount is not controlled for each time, but it is normally larger than 2.7 mL to ensure the subsequent cutting process) of the TOCN-SBR mixed sol or TOCN-TiO₂ mixed sol into the PP tubes containing glass beads.
3. Carefully remove the bubbles that may have been generated during the sol loading. Keep the PP tubes containing the above mixed sol at 4 °C overnight before use.
4. Attach the PP tubes containing the above mixed sols to the dipping machine that is used for unidirectional freezing. Set the relevant parameters and start dipping the PP tube into a tank containing liquid nitrogen (-196 °C) at a constant speed of 20 cm h^-1.
5. Cut the PP tube part with a saw, and crack the frozen TOCN-SBR mixed sol part into several sections.
6. Freeze dry these sections with a freeze-drying machine at -10 °C for 1 day, then at -5 °C for 1 day, and finally at 0 °C for 1 day. The MHM-TOCN/SBR and MHM-TOCN/TiO₂ were obtained as white monoliths.

7. Preparation of Microhoneycomb Monolith from the TOCN-SOCF Mixed Sol (Denoted MHM-TOCN/SOCF)

1. Load a polypropylene (PP) tube (with an inner diameter of 13 mm and an outer diameter of 15 mm, and a length of 150 mm) with glass beads, filling the bottom 5 cm part of the tube.
2. Load certain amount (the amount is not controlled for each time, but it is normally larger than 2.7 mL to ensure the subsequent cutting process) of the TOCN-SOCF mixed sol into the above PP tube containing glass beads.
3. Carefully remove the bubbles that may have been generated during the sol loading. Keep the PP tube containing the above mixed sol at 4 °C overnight before use.
4. Attach the PP tube containing the above mixed sol to the dipping machine that is used for unidirectional freezing. Set the relevant parameters and start dipping the PP tube into a tank containing liquid nitrogen (-196 °C) at a constant speed of 20 cm h^-1.
5. Cut the PP tube part with a saw, and crack the frozen TOCN-SOCF sol part into several sections. Freeze dry these sections with a freeze-drying machine at -10 °C for 1 day, then at -5 °C for 1 day, and finally at 0 °C for 1 day. The MHM-TOCN/SOCF was obtained as a white-grey monolith.

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