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Cubane (C8H8) and fullerene (C60) are famous cage molecules with shapes of platonic or archimedean solids. Their remarkable chemical and solid-state properties have induced great scientific interest. Both materials form polymorphic crystals of molecules with variable orientational ordering. The idea of intercalating fullerene with cubane was raised several years ago but no attempts at preparation have been reported. Here we show that C60 and similarly C70 form high-symmetry molecular crystals with cubane owing to topological molecular recognition between the convex surface of fullerenes and the concave cubane. Static cubane occupies the octahedral voids of the face-centred-cubic structures and acts as a bearing between the rotating fullerene molecules. The smooth contact of the rotor and stator molecules decreases significantly the temperature of orientational ordering. These materials have great topochemical importance: at elevated temperatures they transform to high-stability covalent derivatives although preserving their crystalline appearance. The size-dependent molecular recognition promises selective formation of related structures with higher fullerenes and/or substituted cubanes.

In the same thermogravimetric analyzer (TGA) under identical conditions, samples of pure, ash-free cellulose (i.e. Avicel PH-105, Whatman CF-11, Millipore ash-free filter pulp, and Whatman #42) obtained from different manufacturers undergo pyrolysis at temperatures which differ by as much as 30 C. Thus the pyrolysis chemistry of a sample of pure cellulose is not governed by a universal rate law, as is the case with a pure hydrocarbon gas (for example). Nevertheless, the pyrolytic weight loss of all the samples studied in this work is well represented by a high activation energy (228 kJ/mol), first order rate law at both low and high heating rates. These results do not corroborate the recent findings of Milosavljevic and Suuberg (1995). For a particular cellulose sample (for example Avicel PH-105), variations in the pre-exponential constant determined at different heating rates reflect uncontrolled, systematic errors in the dynamic sample temperature measurement (thermal lag).

We have prepared hydrogenated single-wall and multiwall carbon nanotubes, as well as graphite, via a dissolved metal reduction method in liquid ammonia. The hydrogenated derivatives are thermally stable up to 400°C. Above 400°C, a characteristic decomposition takes place accompanied with the simultaneous formations of hydrogen and a small amount of methane. Transmission electron micrographs show corrugation and disorder of the nanotube walls and the graphite layers due to hydrogenation. The average hydrogen contents determined from the yield of evolved hydrogen correspond to the compositions of C 11 H for both types of nanotubes and C 5 H for graphite. Hydrogenation occurred even on the inner tubes of multiwall nanotubes as shown by the chemical composition and the overall corrugation. The thermal stability and structural results suggest the formation of C-H bonds in nanotubes and graphite.

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The widely accepted Broido-Shafizadeh model describes cellulose pyrolysis kinetics in terms of two parallel (competing) reactions preceded by an initiation step. In spite of the fact that many recent experimental results seem to contradict the predictions of the model, its validity has not been seriously questioned. In this paper we report thermogravimetric analyses of Avicel cellulose involving prolonged thermal pretreatments of small samples (0.5 to 3 mg). The weight loss curves were simulated by modern numerical techniques using the Broido-Safizadeh and other related models. Results were not consistent with the presence of an initiation reaction, but they did strongly confirm the role of parallel reactions in the decomposition chemistry. A subsequent, high temperature (370 °C), pyrolytic degradation of solid intermediates formed below 300 °C was also detected. In the absence of a prolonged thermal Várhegyi et al.: Is the Broido -Shafizadeh model for cellulose pyrolysis true? Page 2 of 20 pretreatment, only one of the two parallel reactions can be observed. This reaction is first order, irreversible, and manifests a high activation energy (238 kJ/mol). The kinetic parameters of this reaction are not influenced by the large quantity of solid intermediates formed during prolonged, low-temperature thermal pretreatments, indicating that chemical processes are much more significant than the physical structure of the sample during pyrolysis.

ABSTRACT. The pyrolysis of four biomasses (corn stalk, rice husk, sorghum straw and wheat straw) was studied at different temperature -time functions in inert gas flow by thermogravimetric analysis (TGA). Linear and stepwise heating programs were employed. A distributed activation energy model (DAEM) with three pools of reactant (three pseudocomponents) was used due to the complexity of the biomass samples of agricultural origin. Compensation effects were observed between the kinetic parameters similarly to the works of other investigators. The compensation effects result in ambiguous parameter values hence they were eliminated by decreasing the number of the unknown parameters. For this purpose part of the kinetic parameters was assumed to be the same for the four biomasses. This approach also helps to express the similarities of the samples in the model. The sixteen experiments were evaluated simultaneously by the method of least squares to obtain dependable kinetic parameters. 2The resulting models described well the experimental data and were suitable for predicting experiments at higher heating rates. The checks on the prediction capabilities were considered to be an essential part of the model verification.