G = 119–165 D = 900–2300
Figure 2.1 Relative abundance of elements in the Earth’s upper crust. Source: Haxel et al. 2002 [2]. Reproduced with permission of United States Geological Survey.
The element carbon achieves in the form of graphite the highest sublimation point among all elements. In the allotropic form of diamond, carbon has the highest hardness. These outstanding properties are due to the formation of covalent bonds between the carbon atoms. In the case of diamond, four covalent bonds (sp3 hybridization) form a face‐centered cubic lattice with an interatomic distance of 0.154 nm. In the case of graphite, three covalent bonds form a hexagonal planar network with a bond length of 0.142 nm in the plane and an interplane distance of 0.335 nm. The fourth bonding p‐orbital overlaps with other adjacent p‐orbitals and creates the energy band for dislocated electrons (sp2 hybridization). The weak bonding between the layers is described as being of metallic character on the order of magnitude of van der Waals forces [7]. This results in a high electrical conductivity comparable with those of metals.
Figure 2.2 Allotropic modifications of the element carbon.
Carbon is the sine qua non condition for life on Earth. The capability to form complex three‐dimensional molecules by single or double bonds between carbon atoms and the incorporation of heteroatoms such as nitrogen, oxygen, phosphorus, sulfur, and last but not least hydrogen open a tremendous biological diversity. Some examples of these complex molecules are proteins, vitamins, and the genetic makeup in DNA, RNA, and adenosine triphosphate (ATP), the most important molecule for energy transfer in living organisms. Others are carbohydrates like starch or sugar. The research on this group of molecules has once initiated the separation into organic and inorganic chemistry. Meanwhile also synthetic macromolecules, such as polymers, are considered to be organic molecules, formed by covalent bonds between carbon atoms and by incorporating heteroatoms. Fossils like crude oil and coals are ranked as organic substances metamorphosed from once‐living matter. By far the majority of carbon compounds are classified as organic molecules. Only a few ones are located under the group of carbon‐containing inorganic compounds. Examples are carbon monoxide (CO) and carbon dioxide (CO2), cyanides (CN−), all carbides, and carbolic acid. The largest sources of inorganic carbon are limestone and dolomite.
The boundary between organic and inorganic substances is not of clear evidence. Diamond, graphite, and fullerenes are widely considered as inorganic substances. Inagaki and Feiyu [8] provide an excellent schematic about the molecular path from organic sp1‐, sp2‐, and sp3‐bonded molecules to the inorganic allotropes diamond, graphite, fullerenes, and carbines (Figure 2.5). The aggregation of carbon atoms to huge crystalline “supermolecules” in diamond and graphite goes along with the reduction of the hydrogen content to traces that are positioned at the fringes of the “supermolecules” for saturation purpose. Hydrogen has lost here its chemical dominance compared with hydrocarbons. This is as well the case in fullerenes and carbynes. Hence it is justified to classify these substances as inorganic.
Figure 2.3 The four most important allotropic forms of the solid element carbon and their main derivatives [5, 6]. Source: Adapted from Marsh and Rodriguez‐Reinoso 2000 [5] and Borkos et al. 1973 [6].
Figure 2.4 Bonding hybridization and corresponding crystal structure of carbon allotropes. Source: Borkos et al. 1973 [6]. Reproduced with permission of Taylor & Francis.
But how is the situation for the big number of transition forms so‐called non‐graphitic carbon materials? The hydrogen content and its importance for the substance of relevance could be the criteria for the classification as organic or inorganic substance. An overview of the hydrogen content for various hydrocarbons and their pyrolysis products are given in Figure 2.6.
Figure 2.5 C—C bonds and the formation of hydrocarbons and extension to carbon allotropes. Source: Borkos et al. 1973 [6]. Reproduced with permission of Taylor & Francis.
The range of hydrogen content is rather broad and reaches from 25% to traces in carbon materials heat‐treated at high temperatures. Green petroleum coke, a conversion product from crude oil refining, still contains about 4% hydrogen as some coal types do. It is dominantly used as a fuel. Thus the hydrogen content and its use classify green petroleum coke as organic substance. With further heat treatment the hydrogen content deceases to about 0.04% in calcined needle cokes. The material becomes inflammable and does not burn and hence may be classified as an inorganic substance. The same would then be valid for other carbon materials. An oxidized polyacrylonitrile (PAN) precursor with 5% hydrogen would be an organic substance. A high tensile carbon fiber would be of inorganic nature.
Figure 2.6 Hydrogen content of various hydrocarbons and heat‐treated residues.
This lengthy discussion is not of purely academic interest but has an influence on how carbon substances are classified by authorities in regard to working place and environmental exposure regulations. It would be helpful for academia and industry to approve clear rules for carbon materials whether they are belonging to the group of organic or inorganic substances.
2.2 Diamond
Diamond, despite its high hardness, is the metastable form of the element carbon. By exceeding temperatures of above 2000 K [9], diamond transfers into the stable modification of graphite (Figure 2.7). The retransformation of graphite is possible under high temperature and high pressure [10].
The sp3 hybridization with four bondings arranges the adjacent C atoms in a tetrahedral lattice with a bonding angle of 109°, 47° (Figure
