Stabilized amorphous sulfur

Stabilized insoluble sulfur Insoluble sulfur Amorphous sulfur Stabilized amorphous sulfur Polymeric sulfur Stabilized polymeric sulfur... 

Owing to its excellent thermal and mechanical stability and its rich chemistry, alumina is the most widely used support in catalysis. Although aluminium oxide exists in various structures, only three phases are of interest, namely the nonporous, crys-tallographically ordered a-Al203, and the porous amorphous t]- and y-Al203. The latter is also used as a catalyst by itself, for example in the production of elemental sulfur from H2S (the Claus process), the alkylation of phenol or the dehydration of formic acid. 

The crystal structure of M0S2 (hP6). Mering and Levialdi ( ) have shown that nonstoichiometric sulfur-rich modifications (which have essentially the hP6 structure with expansion and distortion of the lattice) can be prepared as metastable compositions up to M0S2 gg amorphous metastable phases are obtained with higher sulfur contents. For information on other metastable phases, see (1 ) and ( ). According to Brewer s (1 ) phase diagram the compositional range of stability decreases as the temperature increases. 

Irradiation of PP in air leads to oxidative degradation, evidenced by discoloration and embrittlement. The extent of the degradation depends on crystallinity, MW, MWD, and chain mobility [Kadir et al., 1989 Kashiwabara and Seguchi, 1992 Williams, 1992]. Neat PP does not discolor on irradiation up to 100 kGy [Williams, 1992]. The antioxidants should be selected so as not to cause the discoloration. However, most commercial preparations containing phenolic antioxidants turn yellow on irradiation. Phenolic antioxidants produce stable phenoxyl radicals that convert into colored quinonoids. Other stabilizers and antioxidants are compounds that contain either phosphorous [Bentrude, 1965 de Paolo and Smith, 1968], sulfur [Jirackova and Pospisil, 1979], or hindered piperidine derivatives [Carlsson, et al., 1980 Felder et al., 1980 Allen et al., 1981]. A comprehensive list of stabilizers and their mode of action was given by Dexter [1992]. It is noteworthy that antioxidants and stabilizers are excluded from the crystalline regions [Winslow et al., 1966] thus they would provide protection only within the amorphous domains. 

Polybenzimidazole (PBI) is an amorphous thermoplastic with a high glass transition temperature (T ) of 425°C-436°C (Li et al. 2003c) and demonstrates excellent thermal, chemical, and mechanical stabilities (Zhang et al. 2008.). Moreover, PBI exhibits a good protonic conductivity even at low relative humidity (Bouchet and Siebert 1999). PBI has to be doped with several acids, such as sulfuric acid and phosphoric acid, in order to improve its conductivity. The acid acts as both donor and acceptor in proton transfer and therefore allows for proton migration along the anionic chain. PBI also showed lower methanol permeability (Pivovar et al. 1999). 

TPO materials are defined as compounds (mixtures) of various polyolefin polymers, semicrystalline thermoplastics, and amorphous elastomers. Most TPOs are composed of polypropylene and a copolymer of ethylene and propylene called ethylene—propylene rubber (EPR) [2]. A common rubber of this type is called ethylene propylene diene monomer rubber (EPDM), which has a small amount of a third monomer, a diene (two carbon-carbon double bonds in it). The diene monomer leaves a small amount of unsaturation in the polymer chain that can be used for sulfur cross-linking. Like most TPEs, TPO products are composed of hard and soft segments. TPO compounds include fillers, reinforcements, lubricants, heat stabilizers, antioxidants, UV stabilizers, colorants, and processing aids. They are characterized by high impact strength, low density, and good chemical resistance they are used when durability and reliability are primary concerns. 

It is worth noting that both Pd-aUoy and sUica-based membranes present some problem about material instability in the WGS environment. The Pd-aUoy membranes can be negatively affected by surface carbonization, sulfur poisoning, and hydrothermal embrittlement, whereas the amorphous silica-based membranes can show some degradation caused by the condensation reaction of sUanol in hydrothermal conditions (Tang et al., 2010). In particular, the siliceous MFI-type zeolite membranes, constituted by a crystalline microporous zeolite membrane, in recent years have been seen as attractive candidates for the WGS reaction because of the high-temperature hydrogen separation and for their intrinsic sulfur tolerance and hydrothermal stability. 

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