Methylacetylene, selective hydrogenation

C4 Hydrorefining. The main components of typical C4 raw cuts of steam crackers are butanes (4-6%), butenes (40-65%), and 1,3-butadiene (30-50%). Additionally, they contain vinylacetylene and 1-butyne (up to 5%) and also some methylacetylene and propadiene. Selective hydrogenations are applied to transform vinylacetylene to 1,3-butadiene in the C4 raw cut or the acetylenic cut (which is a fraction recovered by solvent extraction containing 20-40% vinylacetylene), and to hydrogenate residual 1,3-butadiene in butene cuts. Hydrogenating vinylacetylene in these cracked products increases 1,3-butadiene recovery ratio and improves purity necessary for polymerization.308... 

C4 raw cuts of stream crackers typically contain butanes (4-6%), butenes (40-65%) and 1,3-butadiene (30-50%), as well as some vinylacetylene, 1-butyne, propadiene and methylacetylene. First, acetylenes are selectively hydrogenated and the 1,3-butadiene is extracted resulting in butene cut (or raffinate I). Isobutylene is next removed to produce raffinate II which contains linear butenes and some residual 1,3-butadiene. The latter needs to be removed to achieve maximum butene yields. The methods and catalysts for this process are chosen according to the final use of butenes. The demand for polymer-grade... 

Linnett o gives a discussion of the use of valence force fieid with the addition ol selected cross terms. One method of reducing the number of constants to Tdc determined from the frequencies is to carry over from molecule to molecule certain force constants for squared terms and even for cross terms. Linnett mentions in this connection the work of Crawford and Brinkley who studied acetylene, ethane, methylacetylene, dimethylacetylene, hydrogen cyanide, methyl cyanide and the methyl halides in this way, and were able, for all the molecules, to account for 84 frequencies with 31 constants. Linnetttreated some of these compounds using a different force field. He was able to account satisfactorily for 25 frequencies using 11 force constants. From our point of view the trouble with these results is that Linnett obtained a value for the C - C force constant in these acetylene derivatives which was different from that obtained by Crawford and Brinkley. For C - C in methyl cyanide for example, Linnett obtained... 

It was found that the selective hydrogenation of methylacetylene over ion-exchanged Cu/SiOj produced only oligomeric material (referred to as green oil or foulant) and propylene, but no propane [11]. At temperatures > 160 C the selectivity to propylene Increased drastically and reached 80 % at 280 C. The selectivity increased even more (>99.3 %) when high amounts of propylene were introduced [11]. In order to obtain reliable data for deactivation, operations were carried out under sub optimal reaction conditions at low temperatures (130 "C - 160, where about 20 % olefines and 80 % green oil are produced. 

Kinetics, activation energies and selectivity observed in the hydrogenation of methylacetylene... 

Many features of these reactions can be illustrated by the data in Fig. 5.8. One can see (left part of Fig. 5.8) that the reaction has a slower phase and a faster one. The first one corresponds to the reaction which runs prevailingly to propene (this happens with a selectivity shown by horizontal lines in the right part of Fig. 5.8). When methylacetylene disappears from the gas phase, a subsequent faster hydrogenation to propane takes place. Such behaviour is explained by the following scheme comprising two parallel reaction pathways ... 

Fig. 5.8. Hydrogenation of methylacetylene in a static closed system with various metals as catalysts. Left Pressure drop as a function of time. Right Selectivity to propene as a function of conversion (100% conversion corresponds with the reaction to propene [15]).

The behavior of the EDA complexes of graphite with potassium or sodium is also of interest, as the complexes not only chemisorb hydrogen, but also catalyze the hydrogenation of unsaturated hydrocarbons at room temperature. The hydrogenation of methylacetylene proceeded selectively to propylene, and then to propane. 

Acetylenes are hydrogenated over many metals, but the most frequently used is Pd, usually on carriers, and then Ni. Other metal catalysts include Pt, Ru, Rh, Ir, Fe, Co, and Os-on-alumina for the vapor phase hydrogenation of 2-butyne to cw-2-butene at 80-150°C . Selectivity of unsupported metals for the conversion of methylacetylene to propene decreases Pd (98%) > Pt (92%) > Rh (87%) > Ni (76%) > R (44%) > Ir (29%), whereas their stereoselectivity to cis-olefins lies between 91-98%. The efficiency of the metal is also dependent on the support charcoal, alumina, BaS04 and, more widely, CaC03 are used. 

Product distributions are plotted in Figures 7 to 12. These data suggest that the primary products are ethylene, methane, hydrogen, butenes, butadiene, methylcyclopentene, hexadienes, acetylene, and ethane, and the secondary products are cyclopentene, cyclopentadiene, benzene, toluene, and polycyclic aromatics. All C4 olefins were present, with 1-butene the major component. Isobutylene, trans- and cis-2-butene were also identified. Selectivity of polycyclic aromatic hydrocarbon formation was calculated in terms of naphthalene because the main part of these aromatics was found to be naphthalene. Trace amounts of propane, allene, methylacetylene, cyclohexane, cyclohexadiene, 4-methylcyclohex-ene, xylenes, and styrene were also identified. The amount of carbon deposit on the reactor wall was very small and was neglected. 

Production of this monomer amounts to 20 miUion tons/year. Traditionally, production was based on cheap and toxic raw materials acetone, a side product of the production of phenol, and hydrogen cyanide, which is highly toxic, a side product of the production of acrylonitrile. The process has now been abandoned because it produces 2.5 kg of ammonium hydrogen sulfate/kg of methyl methacrylate and is characterized by -factor 2.5. The catalytic process, introduced by chemists of the Shell Co., is based on the methoxycarbonylation of methy-lacetylene (propyne). Besides 100 % atom utihzation. this process is characterized by high chemical yield and selectivity, both over 99 % [12]. For the economy of any industrial process, the turnover is a particularly important technological parameter. Approximately 100,000 mols (ca. 4000 kg) of methylacetylene is... 

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