1. Introduction

Cellulose is the most produced and utilized bio-synthesized material on earth. This β-1,4 glucoside-bonded biopolymer maintains chain structure despite of various synthesizing mechanisms. Cellulose arises in many polymorphs in terms of chain direction and hydrogen bonding arrangement, basically including native and regenerated forms. The intercalation and allomorphic conversion of cellulose with ammonia and amines have been reported by many groups, helping us better understand the mechanism of dissolution and enzymatic biolysis.

As the simplest diamine, hydrazine (N2H4) is recently highlighted because of the development of direct hydrazine fuel cell system, not only exhibiting a fascinating power output, but achieving zero-CO2-emission as well. Unfortunately, to overcome the chemical toxicity and biological hazardousness, which considered as the major drawback of hydrazine as power source, promising storage material was sought after.

In this research, the interaction between cellulose and hydrazine, including complexation and physical adsorption behavior, was thoroughly investigated and discussed.

2. Stoichiometry of cellulose-hydrazine complexes

2.1 Experiments

Green alga (Cladophora sp.) sample was collected as the source of cellulose I, treated in 4% NaOH and 0.3 % NaClO2, followed by water washing. The resulting cellulose material was homogenized and made into uniaxially oriented film by the rotational shear flow technique. Ramie fiber were neatly aligned, immersed in NaOH and washed in water to prepared oriented mercerized fibers. After repeated 3 times, the fibers were vacuum-dried over night.

2.2 Results and discussion

Analysis of Fig. 1 allowed determination of a two-chain monoclinic unit cell of cellulose I-hydrazine complex as: a = 8.57 A, b = 9.22 A, c = 10.38 A and γ = 94.4°, with unit cell volume of 819 A3 . The meridional reflections 00l with odd l were absent, indicating that the space group was P21. Moreover, the hydrazine complex of mercerized ramie (not shown) also shows a high degree of crystalline order. With all the peaks refined, the unit cell parameters were determined as: a = 10.42 A, b = 10.46 A, c = 10.38 A, γ = 129.7 °, with a two-chain monoclinic unit cell, with space group P21 and unit cell volume of 870 A3. The expansion of the unit cells leads to the stoichiometry of approximately 1 hydrazine molecule incorporated versus 1 glucose residue.

Another evaluation of stoichiometry was performed by thermogravimetry of the cellulose-hydrazine complexes as in Fig. 2. The weight decrease of cellulose I hydrazine complex was 15.0%, translated into molecular ratio of 0.89 hydrazine molecule per glucose.

3. Stability and allomorphic conversion

3.1 Experiments

The hydrazine complexes were treated in the excess water, methanol and ethanol. Synchrotron X-ray diffraction analysis was carried out to monitor the allomorphic conversion of the subjects.

3.2 Results and discussion

Fig. 3 exhibits the X-ray diffraction equatorial profiles of the starting cellulose, hydrazine complex, and cellulose recovered by water or alcohol treatments (cellulose II profiles not shown). The complexation disrupts hydrogen bonds between cellulose and hydrazine with alcohol treatments, leading to the formation of cellulose III-type structure, i.e. the non-staggered mutual arrangement of cellulose chains. When hydrazine is extracted by water, cellulose recovers its original structure, cellulose I; but the resultant crystal has somewhat reduced crystallinity, and crystal type was changed from cellulose Iα to cellulose Iβ. This change is indicated by the slightly narrower interval between (1-10) and (110) peaks.

On the other hand, extraction of hydrazine by alcohols resulted in the formation of mixtures of cellulose IIII and cellulose Iβ. The ratio of the two forms was about 7:3 for both alcohols (Figs. 3d and 3e). The overall behavior is similar to those of alkylamine complexation of cellulose, giving cellulose IIII of higher purity, meaning that the action of hydrazine is intermediate between alkylamines and water.

4. Complexation and crystal size

4.1 Experiments

Cellulose samples, including Funacel (microcrystal powder), CF11 (microcrystal fiber) and tunicate cellulose (purified, homogenized and dried) were treated in aqueous hydrazine with various concentrations and the formation of complex was monitored by X-ray diffraction.

4.2 Results and discussion

According to Table. 1, tunicate cellulose exhibits the largest crystal size, approximately 100 A, followed by CF11, 50-70 A. Funacel is the least crystalline samples of all, the crystal size is only 30-45 A. Fig. 4 indicates that the complex formation is achieved within a narrow concentration range of 30-45%. Furthermore, it is an obvious result that cellulose with smaller crystal size tends to form complex easily.

5. Adsorption behavior of hydrazine on cellulose

5.1 Experiments

Adsorption isotherm of hydrazine onto cellulose in liquid phase is plotted in Fig. 5. Concentration difference before and after cellulose treatment was measured through absorbance of hydrazine aqueous.

5.2 Results and discussion

Considering one hydrazine molecule is uptake per glucose unit (discussed above), the incorporated hydrazine molecules within cellulose unit cell also contributed to the concentration decreasing. Thus, some 0.2 (molecular weight of hydrazine/glucopyranose unit: 32/162) of ratio of adsorption was not actually attributed to physical adsorption occurring on the surface of cellulose fibrils. With this portion being subtracted, the ratio of adsorption ascribed to pure physical adsorption is determined as about 0 for Funacel, 0.15 for CF11 and 0.75 for tunicate cellulose, respectively.

Assisted with electronic scanning microscopy and nitrogen adsorption analysis (data not shown), it is an evident result that cellulosic materials with fiber network structure and high pore volume could make a good storage material of hydrazine.

Fig. 1 X-ray diffraction pattern of cellulose I-hydrazine complex

Fig. 2 Thermogravimetric curves of cellulose I and its hydrazine complex

Fig.3 Equatorial X-ray profiles of cellulose I (a), complex (b), complex treated by water (c), methanol (d) and ethanol (e).

Table. 1 Crystal size (A) of cellulose samples, calculated by Scherrer's equation.

Fig. 4 Content of complexes with increasing concentration of aqueous hydrazine.

Fig. 5 Adsorption isotherm of hydrazine onto cellulose in aqueous hydrazine.

本論文は、ヒドラジン分子がセルロース結晶の分子鎖中に取り込まれ、セルロース/ヒドラジン複合体を形成するという特異な現象に注目し、その複合体の形成挙動、構造、安定性を明らかにすることを目的としたものである。

第1章では、序論として、セルロースの構造と物性、特にセルロースとアミンとの相互作用に関する既往の研究が紹介されるとともに、近年開発されたヒドラジンを燃料とする燃料電池のためのヒドラジン吸蔵物質としてセルロースが使用できるのでないかとの可能性が示された。

第2章ではセルロース/ヒドラジン複合体の化学量論について検討した。天然由来のセルロースI試料とそれをマーセル化してセルロースIIへ改変した試料から、高結晶性かつ一軸配向したセルロースI/ヒドラジン複合体とセルロースII/ヒドラジン複合体をそれぞれ調製した。そして、放射光繊維X線回折を行い、記録した繊維図(Figure 1)からそれぞれの複合体の単位格子を精密化した。これらの単位格子体積とセルロースI、IIの単位格子体積を比較することによって、両複合体はグルコース1残基あたりヒドラジン1分子を含んだ構造であることを明らかにした。また、両複合体の熱重量分析を行い、室温から180℃までにおよそ15%の重量減少があることから、熱重量分析によっても両複合体がグルコース1残基あたりヒドラジン1分子を含んだ構造であると分かった。

第3章ではセルロース/ヒドラジン複合体の安定性とヒドラジンの放出に伴う結晶転移について検討した。窒素雰囲気下180℃での加熱処理により複合体はヒドラジンを放出し、複合体形成前のセルロースI、IIへそれぞれ転移した。また、複合体を水に浸漬した場合はセルロースI、IIへ戻るが、メタノールやエタノールなどのアルコールに浸漬した場合は別の結晶形であるセルロースIIIへ転移した。

第4章では、セルロースへのヒドラジン吸着とセルロース/ヒドラジン複合体の形成挙動について検討した。まず、結晶サイズの異なる3種類のセルロースI試料を様々な濃度のヒドラジン水溶液、ヒドラジン/トルエン溶液に浸漬し、乾燥させることなくX線回折に供した。結晶サイズが小さいほど低濃度で複合体が形成されることが分かった。ヒドラジン水溶液ではヒドラジン濃度40~50 wt%で形成が始まったが、ヒドラジン/トルエン溶液ではより低濃度の10~30 wt%で始まった。すなわち、セルロースと相互作用のないトルエン溶液中では容易にヒドラジンを取り込んで複合体が形成されることが分かった。

ヒドラジン水溶液中におけるセルロースへのヒドラジンの吸着等温線を測定したところ、シグモイド曲線で表され、タイプVに分類されるものであった(Figure 2)。吸着等温線の解析により、ヒドラジンの吸着量はセルロースの結晶サイズとは関係なく、セルロースの比表面積と関係していることが明らかになった。すなわち、比表面積の大きなセルロースはヒドラジンを多く吸着することが分かった。そして、比表面積65.7 m2/gのセルロースI試料において、セルロース1 gあたり最大0.9 gヒドラジンを吸着したことから、セルロースをヒドラジン吸蔵物質として使用することの可能性が示された。

以上のように、本論文はセルロース/ヒドラジン複合体の形成挙動、構造、特性を明らかにしたもので、セルロースの利用を考える上での重要な知見を与えることから、審査委員一同は本論文が博士(農学)の学位論文として価値あるものと認めた。