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118 Ralf Steudel et al. 1 Introduction Elemental sulfur is produced on a huge scale, partly by mining (Frasch pro- cess) but mainly by desulfurization of “sour” natural gas (Claus process) and of crude oil (hydrodesulfurization in combination with the Claus pro- cess) [1]. Approximately 1000 Claus plants are operating worldwide. The to- tal amount of elemental sulfur recovered annually from these three sources was 43 million tons in 1999 [2]. The sulfur is obtained in the liquid form and is often also shipped as a liquid. On the other hand, sulfur is the element with the largest number of solid allotropes. At least 30 crystalline sulfur phases have been prepared and characterized by X-ray diffraction [3]. Most of these allotropes consist of homocyclic molecules with ring sizes ranging from S6 to S20 but in some allotropes polymeric chain-like molecules are present [3–5]. The molecular structures of the sulfur homocycles S6 [6], S7 [7], S8 [8], S9 [9], and S10 [10] are known from accurate X-ray diffraction studies on single crystals, usually carried out at low sample temperatures. At very high pressures three-dimensional networks are formed and the various phases of sulfur eventually become metallic conductors. Below 10–17 K sul- fur is even superconducting under high pressure [3, 11, 12]. All sulfur rings from S6 to S28 have been detected in liquid sulfur by high- pressure liquid chromatography of extracts of quenched sulfur melts which had been equilibrated before at temperatures of between 116 and 250 C [13]. Raman spectra of hot sulfur melts also showed the presence of cyclo-S7 besides S8 [14]. In addition, the sulfur melt contains varying concentrations of polymeric insoluble sulfur [13–15]. Above 150 C trace amounts of sulfur radicals have been detected in liquid sulfur by ESR spectroscopy [16, 17] as well as by magnetic measurements [18]. It can be expected that the equilib- rium sulfur vapor exhibits a similarly complex molecular composition as the melt although the smaller molecules will be enriched in the gas-phase, of course. 2 Sulfur Vapor 2.1 General Sulfur vapor, saturated and unsaturated, has been investigated by UV-Vis spectroscopy [19], by mass spectrometry (see below), by resonance Raman spectroscopy [20–23], as well as by a sophisticated mathematical analysis [24] of carefully performed pressure measurements as a function of temper- ature and total sulfur concentration [25–27]. It was concluded that the vapor consists of all molecules from S2 to S8 in temperature- and pressure-depen- dent equilibria of the following type:PDF Image | Topics in Current Chemistry
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