Alkeenmetatese met behulp van 'n reniumkatalisator
Van Rensburg, Alta Jansen
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From literature t is well known that the 3 % Re2O7/SiO2.Al2O3-catalyst system is very effective as a metathesis catalyst system for normal alkenes. The addition of a cocatalyst to this system not only makes the system suitable for the metathesis of alkenes containing functionalised groups. In this investigation the metathesis activity of the 3 % Re2O7/SiO2.Al2O3/SnMe4-catalyst system was determined. Factors that can influence the activity, selectivity and stability of the system were investigated in order to obtain optimum conditions for the metathesis activity of the 3 % Re2O7/SiO2.Al2O3/SnMe4-catalyst system. From the investigation into the influence of catalyst composition it was concluded that the maximum acidity and metathesis activity could be found on SiO2.Al2O3, with an Al2O3-content of 22-25 %. It was also found that a low percentage of Re2O7, (~ 3%) on SiO2.Al2O3 produces a high quantity of homo-metathesis products as a result of the preference of ReO4-ions for the more electropositive bridge OH-groups. With an increasing Re2O7-loading on SiO2.Al2O3, double bond isomerization and the resulting cross metathesis products increased as a result of the increase in the acidity of the system The addition of Cs+, PO43-, MoO3, and B2O3 to the 3 % Re2O7/SiO2.Al2O3/SnMe4-catalyst system led to the observation that small amounts (~ 2 %) of the different compounds led to a significant increase in the metathesis activity of the 3 % Re2O7/SiO2.Al2O3/SnMe4-system. The addition of Cs+, PO43- or a combination of the two ions led to the highest homo-metathesis- and the lowest cross metathesis production with the 2 % Cs+/3 % 3 % Re2O7/SiO2.Al2O3/SnMe4- (53 % homo- and just 16 % cross metathesis products) and the 2 % Cs+/2 % PO43-/3 % Re2O7/SiO2.Al2O3/SnMe4- system (54 % homo- and 15 % cross metathesis products) producing the best results. The addition of MoO3 or B2O3 led to high double bond isomerization and cross metathesis (45 % cross metathesis products for the 2 % MoO3/3 % Re2O7-calalyst system compared to the 16 % of the 2 % Cs+ system). The type of compound and the change in acidity that accompanies the addition of the compound play an important role in the metathesis. The addition of PO43-ions lead to the formation of medium and strong acid centres that results in higher cross metathesis in contrast to the addition of Cs+-ions that produce strong acid centres on the support together with lower isomerization and cross metathesis activity. Another important role of Cs+ is to replace the H+-ions that play a role during isomerization. The addition of B2O3 and MoO3 lead to high acidity and isomerization activity. SnMe4 also shows metathesis activity in combination with SiO2.Al2O3. From the investigation into the metathesis activity of this system it was concluded that two types of active metathesis sites exist on the Re2O7/SiO2.Al2O3/SnMe4-catalyst system, one formed between Re2O7, SnMe4, and the support and the other between SnMe4 and the support. The amount of SnMe4 required for maximum metathesis activity in both the SnMe4- and Re2O7-systems, is directly related to the maximum amount of active metathesis sites that can be formed between SnMe4 and the support. It was further concluded that some of the SnMe4 is lost during the reaction due to a useless reaction between the SnMe4 and Si-bound OH-groups and that the active Sn-carbenes are probably formed via a reaction between the SnMe4 and bridged OH-groups on the SiO2.Al2O3. It was also found that inactive metathesis sites could not be reactivated by the addition of extra SnMe4 and that the presence of SnMe4 as cocatalyst in the Re2O7-system led to a decrease in metathesis activity during reactivation of the catalyst due to the deposition of SnO2 on the catalyst system. The reaction conditions whereby the SnMe4/SiO2.Al2O3-catalyst system reach its maximum metathesis activity can be summarized as follow: 1) Reaction temperature should not exceed 60’C. 2) Low activation temperatures (about 500’C) are required for maximum metathesis activity. 3) For both the SnMe4/Si2.Al2O3- and 2 % PO43-/SnMe4/SiO2.Al2O3-catalyst systems a minimum activation time required. For the PO43- system this is about 1.5 to 2h and for the SnMe4/SiO2.Al2O3-system about 4h. 4) There is no difference in the metathesis activity as a result of activation in either N2 or O2. 5) The maximum metathesis activity is obtained by activating the catalyst for about 3h in O2 followed by 1 to 3h in N2. Small amounts of the additives water, BuOH, HOAc, MeCOPr and BuOAc (< 2 %) added to the Re2O7-catalyst system increase the metathesis activity of the system while larger amounts ( > 2 %) lead to permanent deactivation due to a reaction with the carbene structure. A higher IR stretching vibration of the C=O- and O-H-bonds of the different additives corresponds to a weaker deactivation of the catalyst system as the additive is less likely to break the C=O- or OH-bond to react with the active metal carbene. This relationship is found true for C-C-chain lengths ≤ 4 but in the caw of longer chain lengths sterical hindrance plays an important role in preventing the additive born reacting with the carbene.