Isobutene oligomerization

In a system of two parallel reactions (Fig. 10.4), selectivity is an important parameter to monitor. The measure of selectivity is defined here as the ratio of isobutene reacted to ethers / total amount of isobutene reacted . Glycerol is consumed only in the etherification reaction, so the selectivity is calculated with respect to isobutene. Depending on the extent of the reaction, the formation of an ether molecule consumes from one to three isobutene molecules, and oligomerization consumes from two to four isobutene molecules [8],... 

In the dimerization of isobutene, tertiary-butyl alcohol (TBA, 2-methyl-2-propanol) has a strong role in modifying the selectivity of the reaction to Cg hydrocarbons and limits further oligomerization to C12 and Ci6 hydrocarbons [34]. Also, in the etherification of glycerol with isobutene the addition of TBA has a clear effect on the selectivity and on hydrocarbon distribution. The selectivity to ethers increased and the fraction of the Cu and Ci6 hydrocarbons decreased while the concentration of TBA was increased from 0 to 2.6 mol.%. As a conclusion, the formation of C12 and C16 hydrocarbons can be prevented in two ways either TBA should be added to the reaction mixture or the reaction should be carried out at high temperatures [8]. 

These selectivities obtained with Amberlyst-15 seem to be rather optimistic in view of recent work, where only with overstoichiometric ratios high di-+tri-ether selectivity is obtained. It is also not settled why addition of tert-butanol to the reaction mixture suppresses isobutene oligomerization selectivity [34, 35], At this stage it should also be stressed that the tert-butylglycerol ethers (TBGE) mentioned are excellent substitutes for MTBE (ETBE) as gasoline octane boosting components [36]. 

A more recent raw material for plasticizer alcohols is crack-C4 as a byproduct of steamcrackers in ethene/propene production. After extraction of butadiene for use and etherification of isobutene with methanol to methyl-tertiary-butylether MTBE as an octane enhancer, a stream is left containing 1-butene, 2-butene, and butanes, so-called raffinate II. Oligomerization of the butenes yields C8 olefin mixtures ( dibutene ) as the main product and the corresponding C12 olefins as the main byproduct (tributene). They are the... 

Several authors suggest the use of the NiY zeolite and Ni/Si-Al as catalysts for the oligomerization of isobutene [7-9]. The aim of this work is to compare the performance of two different green acid catalysts in situ sulfated Ti02 synthesized by the sol-gel method [22] with NiY zeolite, in the gas phase trimerization of isobutene at mild pressure and temperature conditions atmospheric pressure and 40°C, in terms of activity, selectivity and stability. 

The oligomerization of isobutene in function of the time is shown in Figures 1 and 2 for the sultated Ti02 and NiY zeolite catalysts respectively. After a stabilization time of 2 hours on stream, the sulfated Ti02 catalyst showed initial conversion values after 3 h in stream of 100, 70, 65 and 42% at GHSV of 8, 16, 32 and 64 h, respectively (Fig. 1). For the NiY zeolite catalyst the activities were 56, 43, 33 and 17% for the previous GHSV values respectively (Fig. 2). 

The isobutene oligomerization is a highly exothermic reaction, carried out via the carbenium ion mechanism, which is thermodynamically favoured at low temperature. The kind of products obtained as well as the conversion and stability at constant temperature and pressure will depend on the reaction GHSV, which determine the intermediate carbenio ion formed during the first steps. 

Figure 4 shows the reaction pathway for the isobutene oligomerization. After the dimer formation, the addition of another one butene molecule will depend mainly of Brbnsted acidity to stabilize the formation of the carbocation. The oligomerization to heavier olefins will be favored on catalyst showing a low L/B acids sites ratio [12]. 

In the process (Fig. 1), the crude C4 fraction is extracted with acetone, furfural, or other solvents to remove alkanes such as n-butanc, iso-butane, and small amounts of pentanes, leaving only 1- and 2-butenes and isobutene. The isobutene is removed by extraction with sulfuric acid because it oligomerizes more easily. 

An important intermediate for synthetic p-ionone (36) is the C8 building block methyl heptenone (37). In addition to the synthesis shown above, two further processes are known for its industrial production. In the process of Rhodia INC 36), the starting material is isoprene, and methyl heptenone (37) is obtained via prenyl chloride. At BASF, methyl heptenone (37) is produced, for economic reasons, in the form of its double bond isomer (37 a) by thermal condensation of isobutylene, formaldehyde and acetone 37) (see page 13). By suitable choice of the reaction conditions, various side-reactions, such as the Cannizzarro reaction of formaldehyde, the oligomerization of isobutene and aldol condensation between formaldehyde and acetone, can largely be suppressed. 

Two methods are available for the selective oligomerization of isobutene. They are practised by VOP and IFP in particular, and apply to the following situations ... 

A detailed study of the reactions of trichlorogermane with unsaturated compounds was performed . It became clear that among selected olefins only 1-heptene forms the anti-Markovnikov adduct in the reaction with trichlorogermane. In contrast to the generally accepted opinion, in the reactions with 1-methylcyclohexene (equation 11), styrene (equation 12), 2,3-dimethyl-l-butene (equation 13) and isobutene (equation 14) both regioisomers 4 and 5, 6 and 7, 8 and 9, and 10 and 11 appear in commensurable amounts (together with oligomeric products see later, equation 16). 

The second stage in the process is required because the MTBE formation is an equilibrium reaction. The temperature needed ( 100°C) to achieve a sufficiently high rate of conversion means a decrease in isobutene equilibrium conversion (XiB = 0.9 at 65°C, Xjb = -0.75 at 100°C). The main side reaction in the MTBE process is the dimerization of isobutene towards di-isobutene (two isomers). Side reactions with essentially no significance are the formation of f-butyl alcohol (due to the presence of water as feed impurity), the formation of dimethyl ether from methyl alcohol, and the oligomerization of isobutene towards tri- and tetramers. A (three stage) process is also in operation which tolerates butadiene. The butadiene/ methyl alcohol reaction is faster than that of the n-butenes but consider-... 

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