Introduction

Cellulose arises in many forms including native and regenerated ones. The native cellulose, cellulose Iα and cellulose Iβ, occurs in many organisms with varying crystallinity (Atalla and VanderHart 1984; Sugiyama et al. 1991a, b; Nishiyama et al. 2003), while the regenerated cellulose, cellulose II, is formed through crystal swelling or solubilization of native cellulose followed by regeneration or coagulation (Philipp 1993; Zhou and Zhang 2000; Fink et al. 2001; Zugenmaier 2001; Chen et al. 2006). It has been reported that ammonia and amines can penetrate into cellulose crystals, forming crystalline complexes (Clark and Parker 1937; Davis et al. 1943; Klenkova 1967). These complexes are characterized by the formation of new hydrogen bonds between guest molecules and glucan chains, leading to modified unit cells (Lee and Blackwell 1981; Lee et al. 1983, 1984; Wada 2001; Wada et al. 2003, 2006, 2008). The interaction between these guest molecules and cellulose has been utilized in many industrial processes to enhance accessibility and chemical (Yanai and Shimizu 2006) and enzymatic (Igarashi et al. 2007) susceptibility of cellulose.

Hydrazine (N2H4), the simplest diamine with ammonia-like odor, is known to dissolve cellulose at high temperature and pressure (Kolpak et al. 1977), but it also forms stable complexes with cellulose I and II under ambient temperature and pressure (Lee and Blackwell 1981; Lee et al. 1983). Hydrazine is recently highlighted as the fuel for a new-type fuel cell, which may contribute to reducing carbon dioxide emission (Asazawa et al. 2007). The major drawbacks of hydrazine are its toxicity and volatility. Therefore, the hydrazine-based energy needs development of safe and efficient storage method. While a potentially useful method has been proposed based on chemical binding to a solid substrate (Asazawa et al. 2007), an alternative physical approach also attracts attention, in which complex formation of hydrazine serves as stabilizing storage. In the cellulose-amine complexes, the hydrogen bonding of amino group to cellulose’s hydroxyls is the obvious driving force of complexation. Such an interaction may provide a convenient way of hydrazine storage with minimal processing of materials.

Based on this practical aspect as well as from crystallographic interests, we here investigated this behavior of cellulose-hydrazine complexation for native and regenerated (mercerized) cellulose samples in terms of crystal structures and their stability. One of our focuses is the stoichiometry of the complexes. Blackwell et al. reported the hydrazine/glucopyranoside ratio of 0.5 for cellulose I (Lee and Blackwell 1981), and 1.0 for cellulose II (Lee and Blackwell 1981; Lee et al. 1983). Reexamination of these values was attempted by using highly crystalline cellulose samples.

Experiments

Preparation of high crystalline cellulose I sample

Green alga (Cladophora sp.) sample was collected as the source of cellulose I at Chikura coast, Chiba, Japan. After washed with excess amount of water, the plant body was treated successively by 4% (w/v) NaOH and 0.3% (w/v) NaClO2, followed by water washing. Such process was repeated three times (Wada et al. 2004). Then the cellulose sample was treated in 0.1 M HCl at 80 °C, 30 min. The resulting cellulose material was then homogenized by a stator-rotor type homogenizer (Physcotron, Microtech Nichion, Tokyo, Japan) at 7,500 rpm for 30 min. The homogenized suspension was hydrolyzed with 64% H2SO4 for 4 h. The resulting cellulose microcrystal suspension was used for preparing uniaxially oriented film by the rotational shear flow technique (Nishiyama et al. 1997).

Preparation of high crystalline cellulose II sample

Ramie fiber sample was a gift of Teikoku Boseki Co. (Japan). The fibers were carefully aligned to form a bundle and clamped at both ends in a metal-frame stretcher. The sample was immersed in 5 M NaOH for 30 min, washed by water at 0 °C with stretching for 20 min. This procedure was repeated three times with 3 M NaOH. After water washing, the mercerized ramie sample was vacuum-dried overnight (Langan et al. 2001; Hori and Wada 2006).

Preparation of cellulose-hydrazine complexes

The dry cellulose samples were immersed in anhydrous hydrazine (98%) overnight, and vacuum dried overnight at room temperature.

Synchrotron fiber diffraction analysis

The hydrazine complexes of highly oriented cellulose were used for fiber diffraction analysis at the beamline BL38B1 of SPring-8 (Hyogo, Japan). Sample mounted on a goniometer head was irradiated for 60 s with synchrotron radiation (λ = 0.1 nm). The fiber patterns were recorded by a camera system equipped with a flat IP (R-Axis V, Rigaku). The sample-to-IP distance was calibrated by using Si powder (d = 0.31355 nm). The positions of all the visible reflections of the patterns were measured by Rigaku R-Axis software.

Thermogravimetric analysis

Cellulose I and II samples and their hydrazine complexes were subject to thermogravimetric analysis by a TGD-9600 (Ulvac-Riko, Japan). Vacuum dried samples (~30 mg) were loaded in a platinum cell, heated from room temperature to 600 °C at 5 °C/min in nitrogen flow of 100 ml/min.

Recovery of cellulose by removal of hydrazine

The hydrazine complexes were treated in the excess water, methanol or ethanol. Synchrotron X-ray diffraction analysis was carried out at the beamline BL40B2 of SPring-8 same as the method described in the preceding section.

Results and discussion

The cellulose-hydrazine complexation as reported previously (Lee and Blackwell 1981; Lee et al. 1983, 1984) was reproduced with the oriented films of Cladophora cellulose and repeatedly mercerized ramie fibers. Figure 1 shows the synchrotron fiber diffraction patterns of anhydrous hydrazine (98%)-treated Cladophora cellulose (cellulose I) and mercerized ramie (cellulose II). The sharpness and degree of orientation were similar to those of the original cellulose (not shown), indicating preservation of high crystalline orders.

Fig. 1
figure 1

Synchrotron X-ray diffraction patterns of cellulose I-hydrazine complex (a) and cellulose II-hydrazine complex (b)

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Analysis of Fig. 1a allowed determination of a two-chain monoclinic unit cell of cellulose I-hydrazine complex as: a = 0.879 nm, b = 1.076 nm, c = 1.038 nm and γ = 122.0°, with the unit cell volume of 0.833 nm3 (Table 1). The meridional reflections 00 l with odd ls were absent, indicating that the space group was P21, with the 21-axis along the fiber axis. However, the reflection near the meridian on the third layer line could not be indexed to a monoclinic unit cell. Similar reflections were reported in the fiber patterns of cellulose IIIII, showing that they are not true Bragg reflections but resulting from diffuse scattering (Wada et al. 2009b).

Table 1 Observed and calculated d-spacings for cellulose I-hydrazine complex
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The pair of strong 002 and weak 004 reflections indicates the cellulose chains do not have staggering, a structure analogous to cellulose IIII, cellulose I-ammonia complex, and cellulose I-ethylenediamine complex (Wada et al. 2004, 2006, 2009a). By assuming the same densities for hydrazine and cellulose as those of bulk materials, 1.01 g/cm3, and comparing to the volume of unit cell of cellulose Iβ (0.658 nm3, Nishiyama et al. 2002), this unit cell volume of the complex leads to the number of hydrazine molecules in unit cell as 3.28, i.e. 0.82 hydrazine/glucopyranoside.

The hydrazine complex of mercerized ramie (Fig. 1b) also shows a high degree of crystalline order. With all the peaks refined, the unit cell parameters were determined as: a = 1.042 nm, b = 1.046 nm, c = 1.038 nm, γ = 129.7°, also with a two-chain monoclinic unit cell, with space group P21 and the volume of 0.870 nm3 (Table 2). The intensity distribution on the meridian also indicates that the cellulose II-hydrazine complex is non-staggered structure. By comparison with the unit cell of cellulose II (a = 0.810 nm, b = 0.903 nm, c = 1.031 nm, γ = 117.1°, unit cell volume 0.671 nm3, Langan et al. 2001), the ratio of hydrazine/glucopyranoside was estimated to be 0.93.

Table 2 Observed and calculated d-spacings for cellulose II-hydrazine complex
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Another evaluation of stoichiometry was done by thermogravimetry of the cellulose-hydrazine complexes as in Fig. 2. The weight decrease of cellulose I hydrazine complex between 50 and 230 °C, before the onset of cellulose pyrolysis, was 15.0%. This value translates to molecular ratio of 0.89 hydrazine molecule per glucopyranoside. Together with the results of X-ray diffraction analysis above, the most likely stoichiometric value for cellulose I-hydrazine complex should be 1.0. This is twice of the value, 0.5, reported by Lee and Blackwell (1981). This discrepancy may have resulted from the differences in crystallinity of cellulose samples and the method of complex preparation.

Fig. 2
figure 2

Thermogravimetric analysis of cellulose samples and the corresponding hydrazine complexes: cellulose I and its complex (a) and cellulose II and its complex (b). Embedded were the XRD patterns of the cellulose samples heated up to 180 °C, indicating the total releasing of hydrazine

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The thermogravimetric analysis of mercerized ramie-hydrazine complex is shown in Fig. 2b. Again, the weight loss was 15%, giving the hydrazine/glucopyranoside ratio of 0.89. Together with the crystallographic data above, the most likely stoichiometry for cellulose II-hydrazine complexation is hydrazine/glucopyranoside ratio of 1.0. Complete thermal release of hydrazine was confirmed by the recovery of original X-ray diffraction patterns of cellulose I and II, respectively, after 180 °C treatment of the complexes for both Cladophora and mercerized ramie (Fig. 3a, b). All the updated stoichiometric data are listed in Table 3, compared with the previous data (Lee and Blackwell 1981; Lee et al. 1983).

Fig. 3
figure 3

Equatorial profiles of synchrotron XRD diffraction of original Cladophora cellulose (a), hydrazine complex (b), together with complexes treated by water (c), methanol (d), and ethanol (e). Q is the scattering vector (2π/d)

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Table 3 Hydrazine/glucopyranoside ratios together with previous data (Lee and Blackwell 1981; Lee et al. 1983)
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Figure 3 shows the X-ray diffraction equatorial profiles of the starting cellulose I, its hydrazine complex, and cellulose recovered by water or alcohol treatments. The hydrazine 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 (Fig. 3b), 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. The latter change is detected by the slightly narrower interval between (10) and (110) peaks (Wada et al. 1997) as marked in Fig. 3c. 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 (Fig. 3d, e). The overall behavior is similar to those of alkylamine complexation of cellulose studied by Wada et al. (2008) in detail. The latter cases, however, gave cellulose IIII of higher purity, meaning that the action of hydrazine is intermediate between alkylamines and water. The reason of the difference among the extracting agents needs further clarification.

Figure 4 shows the changes in crystallinity for cellulose II. Again, extraction of hydrazine by water lead to recovery of cellulose II (Fig. 4a, c), but its crystallinity was significantly lowered as seen by near disappearance of the 200 peak (only a slight shoulder on the higher angle side of 110, marked by asterisk). In contrast, extraction by alcohols gave an allomorph more like cellulose IIIII, but the exact structure of cellulose IIIII is still elusive (Wada et al. 2009b), which also has the non-staggered chain arrangement (Fig. 4d, e. Note here the displacement of the innermost peak from that of original cellulose II). Thus the action of alcohols on cellulose II-hydrazine complex is similar to the case of alkylamine complexes.

Fig. 4
figure 4

Equatorial profiles of synchrotron XRD diffraction of original mercerized ramie cellulose (a), hydrazine complex (b), together with complexes treated by water (c), methanol (d), and ethanol (e). Q is the scattering vector (2π/d). Asterisk in (c) indicates 110 peak as a shoulder

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Based on the behavior elucidated here, the release of hydrazine can be achieved readily by extraction with water or alcohol, or by heating above approx. 50 °C. Together with the moderate stability of the complexes at room temperature, these features make cellulose a potentially useful storage medium for hydrazine with capability of repeated loading and releasing.

Conclusion

The stoichiometry of cellulose-hydrazine complexes has been determined by synchrotron X-ray diffraction assisted with thermogravimetric analysis. The hydrazine/glucopyranoside ratio is 1:1 for both cellulose I- and II-hydrazine complexes. Hydrazine molecules incorporated can be easily extracted by immersing in water or alcohol, leading to formation of different cellulose allomorphs, cellulose I and III, respectively. Further study is in progress to determine crystal structure of these hydrazine complexes based on solid-state 13C NMR and synchrotron X-ray fiber diffraction.