Oxide-free carbon surfaces, reactions

Reactions of Olefins with Oxide-Free Carbon Surfaces... 

We undertook to investigate the reaction of olefins with oxide-free carbon surfaces in the hopes that chemisorption would occur in ways which could be related to the known chemistry of aromatic free radicals and dehydroaromatics, and that suitable manipulation might provide routes to homogeneously functionalized surfaces. High surface-area carbon fibers were heated to approximately 1000°C under vacuum to remove the surface oxides (evolution of H2O, C02, and CO). The samples were cooled to room temperature and exposed to vapors of various different substrates. The quantity of substrate adsorbed was determined and corrected for the quantity of physisorbed material which could be pumped off at room temperature. Some typical results are reported in Table II. In certain cases the reactivity towards oxygen was redetermined after exposure to the organic substrate. 

The number of these sites, as measured by Fink (246, 262), vary between 1.2 and 1.8 X 1013/cm2. This value nicely coincides with the number of sites that convert pyridine to the pyridone species (see Section IV.D.l). Thus, the X-sites certainly contain reactive and strongly basic OH groups and they may be identical with the sites responsible for the pyridone formation and the hydrolysis of ketones and nitriles. The highest-frequency OH group vanishes preferentially in all these surface reactions. Because the X-sites are Al-OH pair sites created by the formation of oxide vacancies in the immediate vicinity of the reactive OH groups, the above result lends some support to the interpretation of Dunken and Fink (116) that the reactive OH groups (3800 cm-1) are surrounded by four oxide vacancies rather than Peri s (120) assumption that they are surrounded by four O2- ions (see Section IV.A.l). Rosynek (267a) thinks that a free carbonate ion also exists on the surface and contributes to the band at 1480 cm-1. 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

One of the few cases in which hydrogenated surface complexes have been suggested to be active involves the decomposition of nitrous oxide.The reaction was suggested to involve hydrogen in the carbon, although the reaction mechanism was written in terms of gas phase hydrogen involved in a free radical chain reaction. As a result, the importance of hydrogenated complexes is open to question. 

---