Crystalline structure of cellulose

Major obstacles in the hydrolysis of cellulose are the interference of lignin (which cements cellulosic fibers together) and the highly ordered crystalline structure of cellulose. These obstacles necessitate a costly pretreatment step in which elementary cellulosic fibrils are exposed and separated. 

The crystalline structure of cellulose has been characterized by X-ray diffraction analysis and by methods based on the absorption of polarized infrared radiation. The unit cell of native cellulose (cellulose I) consists of four glucose residues (Figs. 3-6 and 3-7). In the chain direction (c), the repeating unit is a cellobiose residue (1.03 nm), and every glucose residue is accordingly displaced 180° with respect to its neighbors, giving cellulose a... 

Gao, P. J., Chen, G. J., Wang, T. H., Zhang, Y. S., and Liu, J. 2001. Non-hydrolytic disruption of crystalline structure of cellulose by cellulose binding domain and linker sequence of cellobiohydrolase I from Penicillium janthinellum. Shengwu Huaxue Yu Shengwu Wuli Xuebao, 33,13-18. 

Wood is about 65—75% carbohydrate and has been considered as a potential source of ethanol for fuel. The carbohydrate material can be hydrolyzed to monomer sugars, which in turn can be fermented to produce ethanol. However, wood carbohydrates are expensive to hydrolyze. Hydrolysis with acids and enzymes is impeded by the crystalline structure of cellulose. Lignin interferes with processing, and hydrolytic by-products such as furfural, acetic acid, and derivatives of lignin and extractives can inhibit fermentation. Research is still being conducted on wood hydrolysis to develop a process that is economically sound. Furfural is a useful chemical feedstock and results from the dehydration of pentose sugars. It can be obtained in 9 to 10% yield from the dilute acid hydrolysis of hardwoods (75). 

Figure 9-4. Influence of NaOH concentration on the crystalline structure of cellulose fibres [27].

A. Sarko, What is the crystalline structure of cellulose Tappi, 61 (1978) 59-61. 

E. Roche, H. Chanzy, M. Boudeulle, and R. H. Marchessault, Three-dimensional crystalline structure of cellulose triacetate II, Macromolecules, 11 (1978) 86-94. 

Lignocellulosic substrates usually require significant pretreatment to disrupt the structure of cellulose and lignin molecules within the substrate. Substrates are often ground to particle sizes of 1-2 mm to increase the surface area for attack and to disrupt cell walls. In addition, the crystalline structure of cellulose may be disrupted by steaming under pressine. Sometimes the steaming is carried out in conjunction with chemical pretreatment with acid or alkali. 

Figure 3. Schematic representation of the cholesteric liquid-crystalline structure of cellulosics P=Xjn where P represents the pitch, A the reflection wavelength, and n the mean refractive index of a sheet. P>0 for a right-handed twist, and P<0 for a left-handed twist.

Detailed studies have been made of the effects of cellulose source and conditions of acetylation on the crystalline structure of cellulose triacetate. The unit cell dimensions for cellulose triacetates I and II compared with those for celluloses I and II are shown in Table... 

Two interesting points are the number of cellobiose units per cell for cellulose triacetates I and II is 4, versus 2 for celluloses I and II and the measured density for cellulose triacetate II was 1.315 g/ cc, which is less than the calculated density of 1.348 g/cc as expected because cellulose triacetate is not 100% crystalline. The above studies on the crystalline structure of cellulose triacetate lead to the conclusion that commercial heat-treated cellulose triacetate is expected to have the cellulose triacetate II crystalline structure. Analysis of the crystal structure of cellulose triacetate continues [55]. 

The crystalline structures of cellulose I and II differ in the unit cell dimension and the polarity of the chains. Cellulose IIIl and Hill [31, 32] are formed by treating cellulose I and II with liquid ammonia or ethylene diamine [33-35]. Polymorphs IV1 and IV11 [36, 37] may be prepared by processing cellulose IIIl and mil respectively. These processes consist in heating celluloses in a suitable liquid such as glycerol at high temperatures and under tension. 

Pretreatment is required to make cellulose more accessible to the enzymes that convert the carbohydrate polymers into fermentable sugars. The goal is to break the lignin seal and disrupt the crystalline structure of cellulose. The ideal pretreatment process should be economic and include (1) high hemicellulose and cellulose conversion, (2) low sugar decomposition, and (3) reduced by-products (Sun and Cheng 2002). 

Antiparallel arrangement Parallel arrangement FIGURE 1 Chemical and crystalline structures of cellulose and chitin. 

In an alternative way of isolating lignin from wood, pre-extracted spruce wood meal has been treated with a commercial mono-component endoglucanase [27, 41]. Thereby, the crystalline structure of cellulose is destroyed and the now amorphous wood can be homogeneously swollen in aqueous urea. By subsequent fractionation of the material in aqueous alkali, all lignin can be recovered as various LCCs (Table 9.5). Also in this case, however, the wood... 

Chemical modification to improve compatibility of composite components is very often connected with the effect of lack of TCL. The effect of filler modification on TCL formation inhibition was presented by numerous studies. Quillin et al. [29] explained the lack of TCL as a result of fiber modification by covering the ordered crystalline structure of cellulose chains by particles of modifiers. Gray [31], on the other hand, said that crystalline cellulose, called the cellulose 11, unlike the cellulose 1, causes no formation of TCLs. He explained that by the differences in crystalline structures of both types of cellulose. The results contradictory to those presented earUer were presented by Son et al. [32] - the cellulose 11 did initiate the transcrystaUization in polypropylene matrix. 

Compared to starch, natural cellulose is much more crystalline and therefore more difficult to breakdown by any means. Some enzymes can catalyse oxidation reactions of either cellulose itself or the lower molecular weight oligomers produced from the enzymatic hydrolysis of cellulose (Aubert et al., 1988). By incorporating esters groups in cellulose molecular chains, the crystalline structure of cellulose is disrupted as a result, cellulose esters show much lower crystallinity in comparison with the original cellulose and can be more readily degraded in active biological environments. 

Figure 6.1 The crystalline structure of cellulose, (a) The unit cell of native ceiiuiose, or ceiiu-lose I, as determined by X-ray analysis (2,10,11). Cellobiose units are not shown on all diagonals tor clarity. The volume of the cell is given by l = abc sin (b) Unit cell dimensions of the tour forms of cellulose, (c) Known pathways to change the crystalline stmcture of cellulose. The lesser known form of cellulose x is also included (10).

Cellulose 11 as the most important from a technical and commercial point of view is formed from cellulose I by precipitating cellulose firom solution into an aqueous medium at room or slightly elevated temperature, i.e., in technical spinning processes for man-made cellulose fibers. It is also obtained in the large-scale mer-cerization process of cotton, which proceeds via the formation of sodium cellulose by interaction of the polymer with aqueous sodium hydroxide and subsequent decomposition of this intermediate by neutralization or washing out of the sodium hydroxide. It is not yet understood how the parallel chain arrangement of cellulose I undergoes transition into the antiparallel orientation of cellulose II without an intermediate dispersion of cellulose molecules. The crystalline structure of cellulose I and cellulose 11 are shown in Fig. 2. 

A facile preparation of the composites composed of cellulose and the polyacrylate-type polymeric ionic liquids was carried out by the in-situ polymerization of the polymerizable ionic liquids [66]. Ionic liquids, 2a and 2b were prepared by the reaction of 3-bromopropyl acrylate with 1-methylimidazole and 1-vinylimidazole which had one polymerizable group (acrylate) and two polymerizable groups (acrylate and vinyl) respectively. First, the pretreatment of cellulose in the mixtures of the two ionic liquids were performed, resulting in disruption of most of the crystalline structure of cellulose. Then, the radical polymerization of the ionic liquids in the pretreated mixtures was carried out by AIBN to obtain the composites, shown in Figure 5.15. 

The crystalline structures of cellulose in the composites were mostly disrupted indicating good compatibility between cellulose and the polymeric ionic liquids. The mechanical properties of the composites were changeable by changing the weight ratios of the two polymerizable ionic liquids [66]. 

In addition, I cellulose is reported to be the dominant polymorph in bacterial and alga celluloses, while Ip cellulose is predominant in higher plants such as cotton and wood [30]. Therefore, the crystalline structure of cellulose affects the physical, thermal and mechanical properties of the natural fibers. The degree of crystallinity of cellulose is one of the most important crystalline structure parameters. The rigidity of cellulose fibers increases while the flexibility decreases by increasing the ratio of crystalline to amorphous regions [58]. 

This modification of the crystalline structure of cellulose under the effect of soda increases its reactivity and is industrially exploited to chemically modily cellulose. The difference in reactivity between original and mercerized celluloses lies in the extent of hydrafion, with the mercerized cellulose fixing 100% more water than... 

The change in crystalline structures of cellulose can also be observed in the high-resolution solid-state NMR spectra [64, 67, 68]. As shown in Fig. 5.7, the C-4 signals are particularly well separated with the chemical shift at 91 ppm corresponding to the crystalline phase while that at 85 ppm due to the amorphous phase in cellulose. The two resonances of C-6 at 60-70 ppm are also related to the crystalline and amorphous phases of cellulose ... 

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