Methylacetylene, hydrogenation

Quench column 6 Gas purification 7 Drier 8 Low-temperature cooler 9 Hydrogen/ methane separation 10 Demethanization column 11 Deethanization column 12 Acetylene hydrogenation 13 Ethylene column 14 Depropanization column 15 Methylacetylene hydrogenation 16 Propylene column 17 Debutanization column 18 Depentanization column 19 Residue column... 

The overhead of the depropanizer is sent to the propylene fractionator. The methylacetylene (MA) and propadiene (PD) are usually hydrogenated before entering the tower. An MAPD converter is similar to an acetylene converter, but operates at a lower temperature and in the Hquid phase. Due to recent advances in catalysis, the hydrogenation is performed at low temperatures (50—90°C) in trickle bed reactors (69). Ordy rarely are methylacetylene and propadiene recovered. 

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

C3 Hydrorefining. The aim of C3 hydrorefining is to hydrogenate methylacetylene and propadiene present in the cut. Efficient liquid-phase processes were developed by Bayer314-316 (cold hydrogenation process carried out at 10-20°C) and IFP,317 but hydrogenation in the gas phase is also practiced. 

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... 

In comparison with the extensive studies of acetylene itself, the hydrogenation of monoalkylacetylenes has received much less attention. The hydrogenation of methylacetylene over pumice-supported metals [170, 181,184,185] and over metal powders [181—185], has been studied. No studies of the reaction of propyne with deuterium have been reported. 

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

In the absence of oxygen, about 82 peaks of hydrocarbon products were observed in the gas chromatogram, which showed that the main products consisted of C6 hydrocarbons (Table IV), hydrogen, methane, acetylene, ethylene, ethane, methylacetylene, allene, propane, 1-butene, and butadiene. 

Scheme 16. Intermediates of the hydrogenation reactions of methylacetylene and allene.

Other groups of Arrhenius parameters obtained for reactions on two or more metals and which have been shown to exhibit compensation effects include para-hydrogen conversion (on Cu, Ag and Au) (144), hydrogenation of methylacetylene (7 8) and of allene (245), and the inverse compensation behavior in decomposition of acetylene on metal films (26). 

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... 

Comparison of the electronic structures of acetonitrile and propyne (methylacetylene). In both compounds, the atoms at the ends of the triple bonds are sp hybridized, and the bond angles are 180°. In place of the acetylenic hydrogen atom, the nitrile has a lone pair of electrons in the sp orbital of nitrogen,... 

Scheme 4.1 Heats of hydrogenation of acetylene, methylacetylene, and dimethylacetylene (82°C).

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]).

It should be indicated that the methylacetylene and propylene " are more complex reactants than the nonsubstituted counterparts and depict nonequivalent hydrogen atoms at the acetylenic and methyl group (methylacetylene) and at the vinyl and methyl group (propylene). Therefore, even the detection of the atomic hydrogen loss makes it difficult to elucidate if the hydrogen atoms are lost from the methyl group, the acetylenic/vinyl units, or from both positions. In these cases, it is very useful to conduct experiments with partially deuterated reactants J3-... 

Here, the phenyl radical once again attacks the unsamrated bond. However, the steric effect and larger cone of acceptance (the methyl group screens the p carbon atom and makes it less accessible to addition) direct the addition process of the radical center of the phenyl radical to the a carbon atoms of methylacetylene and propylene (the carbon atom holding the acetylenic hydrogen atom). Consequently, crossed beam reactions with complex hydrocarbon molecules can be conducted and valuable information on the reaction pathways can be derived if (partially) deuterated reactions are utilized. 

Scheme 5 is a summary of the experimental frequencies of the v(CH), v(C = Q, and v(OH) modes for the 1 1 7i-bonded acetylene and methylacetylene complexes. Notice that the shift of v(C = C) and v(OH) bands is always negative with respect to the free hydrocarbon molecules (v(C = C) = 1974 and 2142cm for acetylene and methylacetylene, respectively) or Bronsted acid group, respectively. The shift of the v(OH) is a normal consequence of hydrogen-bonding and has been abundantly documented for many bases adsorbed molecularly in zeolites with proton affinities in the range of 420—840kJmol (20,21,156-161). Furthermore, the v(C = C)... 

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... 

Hydrogen-bonded precursor species were invoked in the interaction of acetylene, methylacetylene and ethylacetylene with HZSM-5. 3 These precursors... 

The photolysis of propene has received attention in several recent studies. McNesby et and Ausloos et have investigated the photolysis at 1470 and 1236 A, while Arai et have determined the product distribution at 1849 A. The major products of the photolysis are acetylene, propane, ethylene, ethane, hydrogen and methane in decreasing order of importance. Smaller quantities of allene, methylacetylene, butene and isobutane have been observed. The following reactions have been suggested. 

A brief study by Jackson et at 1470 A gave results very different from those reported above. The photolysis was conducted at pressures of 200 n and 10 torr of benzene. No hydrogen, allene, cyclohexadienes, biphenyl or dihydrobiphenyls was observed, while acetylene, ethylene, methylacetylene and vinyl acetylene were found. These authors conclude that neither the atomic nor molecular elimination of hydrogen occurs, while Hentz and Rzad conclude that both maybe operative. Thus the situation is confused at present, but both studies agree that polymer formation is extensive at 1470 A. 

As all pyrolysis reactions, the decomposition of furfural to furan and carbon monoxide is accompanied by other minor reactions. Of the gases formed, only 80 percent is carbon monoxide, other gases identified being 10 percent hydrogen as well as small quantities of carbon dioxide, butadiene, propadiene, ethylene, propylene, acetylene, methylacetylene, and cyclopropene. As the quantities of carbon dioxide and butadiene are roughly equal on a molar basis, it is believed that a part of the furfural reacts with pyrolytically liberated hydrogen ... 

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