how will you prepare 1,3-butadiene from butane-1,4-diol
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The essential raw material for PBT is butane-1,4-diol. It is still made commercially by the Reppe process dating back to pre-World War II Germany. One mole of acetylene reacts with two moles of formaldehyde to give 2-butyne-1,4-diol (HOCH2–C ≡ C–CH2–OH). This is then hydrogenated to butane-1,4-diol. While the chemistry is simple, a successful process depends on the details. The catalyst for the formaldehyde reaction is a bismuth/cuprous acetylide mixture (47) and the reduction step is better done in two stages with two different catalysts, via the intermediate 2-butene-1,4-diol (HOCH2–CH = CH–CH2OH) rather than proceeding directly to the end-product (48). Another route has been introduced by ARCO (49) based on the hydroformylation of allyl alcohol over a rhodium catalyst to give 4-hydroxy-butyraldehyde, HOCH2CH2CH2CHO. This is reduced to butanediol, but the process needs precise control to prevent side-reactions (50). Allyl alcohol can be made by rearrangement of propylene oxide, made by air oxidation of propylene. A third butanediol route has been piloted by Mitsubishi Chemical Industries (51). They react butadiene with acetic acid and oxygen over a palladium catalyst to form 1,4-diacetoxy-but-2-ene. This diester is hydrogenated (52) to 1,4-diacetoxy-butane and subsequently saponified to butanediol. Whereas ethylene glycol was once used as an antifreeze, butanediol crystallizes at 20 °C, so precautions must be taken to heat trace and insulate bulk storage and shipping facilities outdoors to prevent it freezing solid in cold weather.
Table 6.1. Physical parameters for PET
Crystal habit Triclinic, one chain per unit cell
α 100°
β 117°
γ 112°
a 0.444 nm
b 0.591 nm
c 1.067 nm
Cell density, calculated 1.52 g/cm3
Tm (DSC) 260–265°C
ØHf 140 J/g; 33.5 cal/g
Tg (DSC) solid chip 78°C
Tg drawn fiber 120°C
Table 6.2. Mark–Houwink coefficients for PET in various solvents (45)
Solvent K(10‒4 dL/g) a
OCP 6.32 0.658
P/TCE 7.44 0.648
HFIP 5.20 0.723
PFP/HFIP 4.50 0.705
6.4.1 Manufacture of PBT
PBT is nearly always made using an ester interchange (EI) route from DMT, because heating TA directly with butanediol causes dehydration to the volatile cyclic ether, tetrahydrofuran (THF). Both batch and continuous polymerization processes (53–55) are used. Because vinyl ester ends are not formed by thermal degradation, titanium catalysts can be used without the yellowing problem. The final reaction temperature is around 250 °C, so thermal degradation of PBT is reduced compared with PET and higher molecular weight polymers can be obtained by melt polymerization. PBT polymerizations are ‘cleaner’ than PET ones because the major thermal degradation product is THF, which simply boils off. No colorforming reactions are present, although excessive loss of THF can upset the reaction stoichiometry. PBT gives a very white polymer. As with PET, cyclic oligomers are formed in PBT polymer to about the same level, 1.4–1.8% w/w. In this case, the oligomers are an equal mixture of dimer and trimer (56) together with traces of higher cyclic oligomers.
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