Cellulose precursors

The incorporation of mercury into cellulose has been accomplished via treatment of cellulose aniline ether derivatives with mercuric acetate [112]. Arsenic-containing cellulose derivatives have been obtained from sodium arsenate and diazotized cellulose precursors [113]. Platinum-containing polysaccharide derivatives have also been reported [114]. By an in situ method, cellulosics can be used to produce ferrites in the nanoscale size range, producing a cellulosic nanocomposite to be used as superparamagnetic materials... 

Katoh et al. [145] showed that H2S, methanethiol, and dimethylsulfide could be removed simultaneously from gaseous streams at room temperature over wet activated carbon fibers, while Dalai et al. [142] reported on the oxidation of methanethiol over an activated carbon. More recently, an activated carbon prepared from a cellulosic precursor by CO2 activation was found to exhibit outstanding performance in the oxidation of H2S (1000 ppm) in a hydrogen stream at 423 K namely, 100% conversion for more than 10 hours, and 100% selectivity to sulfur [146]. The selectivity aspects in the oxidation of H2S to sulfur were addressed in a paper by Bashkova et al. [147]. These authors reported... 

The lipid carrier was postulated as a mechanism necessary for crossing the cell membrane, and a cellulose precursor isolated from... 

CELLULOSE-PRECURSOR SYNTHESIS OF ELECTROCAT ALYTICALLY ACTIVE COMPONENTS OF SOFCs AND MIXED-CONDUCTING MEMBRANE REACTORS... 

Figure 1. SEMimages of CGO (A andB), Cu-CGO (C), Ni-CGO (D), metallic Ni (E) and SEA (F) fibers prepared by the cellulose-precursor technique.

The present work was focused on the synthesis of nanocrystalline components for electrochemical cells via the cellulose-precursor technique. This method was used to prepare nanostructured intermediate-temperature (IT) SOFC anodes made of a series of cermets comprising gadolinia-doped ceria (CGO), yttria-stabilized zirconia (YSZ), Gdi.86Cao.i4Ti207.5 (GCTO) pyrochlore, metallic nickel and copper. Perovskite-type SrFcojAlo.sOs.s (SFA) powder, also obtained via the cellulose precursor, was applied onto membranes of the same composition to enhance specific surface area and electrocatalytic activity in the reactors for methane conversion [3]. 

Figure 2. Dark- and bright-field TEM images and electron diffraction patterns of CGO, Cu-CGO cermet and SFA (F) powders prepared by cellulose-precursor technique.

In summary, the cellulose-precursor technique is relatively simple and low-cost synthesis route, enabling to fabricate nanostructured single-phase and composite layers for the electrochemical applications. 

Hopp, H.E., Romero, P.A., Daleo, G.R. Pont-Lezica, R. (1978) Synthesis of Cellulose Precursors. The Involvement of Lipid-linked Sugars , European Journal of Biochemistry, 84, 561-71... 

A cellulosic precursor (C6Hio05) has a carbon content of 44.4% but, unfortunately, in practice, the reaction is more complicated than just simple dehydration and the carbon yield is only of the order of 25-30%. 

The history of cellulosic precursors is interesting because of its association with the early forms of carbon fiber used for electric lamp filaments. 

Several books and reviews have been published which detail the conversion of viscose rayon to carbon fibers [1-7], Chapter 3 has described how carbon fiber first came onto the scene, way back in 1880, with the introduction of Thomas Edison s electric lamp filaments made from cellulosic precursors. Almost 80 years later, in 1959, the National Carbon Company (a division of Union Carbide) introduced a carbon cloth from a rayon precursor, to be followed by a carbon yarn in 1961. These products are described by Cranch [8]. The best grade on offer was WYB cloth, which was processed at 2200°C and although called graphite, was a form of carbon that was non-graphitizing. 

There remained a niche market for carbon fibers made from a cellulosic precursor and production was maintained by Polycarbon Inc. in the USA (acquired by SGL Technic in 1980) and RK Carbon Fibers (who also became part of SGL in 1997, when production was transferred to Polycarbon Inc. in the USA). Demand and availability of a suitable precursor has added to production problems. 

At elevated temperatures, a cellulosic precursor will break down into highly volatile gases, a tarry distillate and a carbon char. Initially, the product releases absorbed water up to about 120°C, followed by a dehydration process up to about 300°C, to form dehydrocellulose. This is accompanied by depolymerization, starting at about 250°C, forming predominantly l,6-anhydro-(3-D-glucopyranose (levoglucosan), the process running... 

As mentioned before, the early fiber made from a cellulosic precursor had a low modulus and manufacturers had to resort to hot stretching to orient the fine structure of the fiber and obtain a significant improvement of modulus, albeit at a cost. This approach is not specific to carbon fiber and stretching is widely used to improve properties of many manmade fibers e.g. viscose rayon. 

Figure 12.22 X-ray diffraction curve for graphitized carbon made from a cellulosic precursor.

A typical X-ray diffraction chart is shown in Figure 12.23 for carbon fiber made from a cellulosic precursor using a 40 kV source with Cu K radiation (0.1540598 nm) and a computer controlled stepping motor travelling at 0.02° per step every 2 s. 

The first commercial carbon fibers were based on viscose rayon, a cellulosic precursor, but Polycarbon is now the only current producer of this type of carbon fiber. The properties of rayon-based carbon fibers are listed in Table 20.1. A difficult fiber to produce with a low yield, its main use is in existing space programs. 

Union Carbide, Danbury, CT, USA—pioneered the use of cellulosic precursors to make Thornel carbon fibers and entered the market in 1965 but, unfortunately, had to resort to hot stretching to obtain properties to be able to compete with PAN based products. In 1973, started the manufacture of carbon fibers from a pitch base. Entered into a technical licensing agreement with Toray in 1978 to produce carbon fiber in the USA based on Toray s PAN technology. The carbon fiber facility was acquired by Amoco and the carbon fibers are distributed by EMI Composites in the UK. 

In order to prepare carbon aerogels from cellulose precursors Ishida and co-workers [17] used classical methods to dissolve microcrystalline cellulose in sulphuric acid or sodium hydroxide water solutions. The resultant aqueous suspension of cellulose with... 

Several cellulosic precursors such as viscose rayon yam, viscose tire cord, and coconut shells after modification have also been used for the preparation of carbon... 

Rayon-based carbon fibers are carbon fibers made from rayon (cellulose) precursor fibers. 

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