Cocatalyst Optimization And Its Influence On Iron-catalyzed Ethylene Oligomerization

Linear a-olefins(LAOs)are widely used as the precursors in the manufacturing of petrochemical products such as polyolefin,detergent,plasticizer,synthetic lubricating oil and oil additives,etc.Ethylene oligomerization is of considerable academic and industrial interest as one of the major processes for the production of linear higher a-olefins.Iron complexes bearing 2,6-bis(imino)pyridine(BIP)ligands with small substituents on the ortho-position of the N-aryl rings,have been demonstrated to be excellent catalysts for ethylene oligomerization to linear a-olefins.These complexes exhibit very high catalytic activity and their substituents on the aryl rings and skeleton of the ligand can be modified to regulate molecular weight of the oligomerization products.It is now known that those complexes bearing a single ortho-alkyl substituent(especially methyl)on the aryl rings,upon activation by methylaluminoxane(MAO),exhibit the highest activity,to the best of our knowledge,in affording a-olefins with the Schulz-Flory distribution.However,the resulting a-olefins are usually featured with a very broad distribution ranging from C4 to C40+,and almost inevitably,accompanied by undesirable insoluble polyethylene with a content as high as 40%or more.Besides the waste of ethylene,the formation of the insoluble polyethylene particulates may also lead to the clogging/blockage reactor pipelines during the oligomerization process,rendering process issues.Moreover,large excess of expensive methylaluminoxane(MAO)of the catalytic system is another problem remaining to be solved.The ion pairs formed with precatalyst and cocatalyst determine the performance of the catalytic system.It is reasoned that cocatalyst with different structure and composition can make a difference in the reaction behavior of the oligomerization system especially in retarding the polymer share of the production.It is of great significance to systematically study the influence of the cocatalyst on the BIP-Fe-catalyzed ethylene oligomerization system in order to solve the problems discussed above.The interaction of the ion pairs formed by catalytic system can be a decisive factor for its catalytic behaviors and product distributions.It is thus of great importance to mediate the microstructure of the active species by changing the composition and structure of the cocatalyst.Aiming at such modulation of catalytic performance and product distribution,several methods,e.g.,alkylaluminum(TRA),modification of MAO by introducing modifiers(Mod-MAO),synthesis of aryloxy-aluminoxane(P-MAO)or alkylaluminoxane(RAO)as alternative activators and combination of alkylaluminum and the organic boron compounds would be discussed in our thesis.Moreover,the effect mechanism of the cotalyst structure during ethylene oligomerization would be studied.The main work and results are as follows:(1)Based on the good effect of Mod-MAO on insoluble polymers inhibition in iron-catalyzed ethylene oligomerization in laboratory study,ethylene high pressure oligomerization was continued in a 1.4 L reactor to further investigate catalytic performance of L-Fe(acac)3/Mod-MAO under the following conditions:P=0.3?1.5 MPa,T-=30?70 0C,[Al]/[Fe]=500?2000,[-OH]/[Al]=0-0.7.It is found that the polymer share in the total products is largely reduced(<lwt%)and the activity remains high(>107g·(mol-Fe)-1·h-1)with the use of Mod-MAO.Moreover,the introduction of p-BrPhOH promoted the high-temperature stability of the catalytic system,leading to the enhanced oligomerization activity as the catalytic system can catalyze ethylene oligomerization at higher temperatures.27A1-NMR test of the modified MAO manifests that its microenvironment of electric clound has been changed and its ability to disperse charges has also been weakened.A characterization of the catalytic system with electron paramagnetic resonance shows that introduction of p-BrPhOH significantly inhibits the formation of ferric ions which can be the main active centers responsible for generating undesired insoluble polymers,thus this can largely retard the production of insoluble polymers during ethylene oligomerization.(2)Based on the excellent regulation of iron-catalyzed ethylene oligomerization by introducing phenoxy structures in phenolic modified MAOs,a series of P-MAOS originated directly from the hydrolysis of reaction products of AlMe3 and phenols are synthesized for the first time,which can serve as effective polymer-retarding activators for the iron-catalyzed ethylene oligomerization.The molar ratios of[PhOH]/[AlMe3]and[H2O]/[Al]during the preparation are explored and their impacts on the oligomerization activity and product distribution are discussed.To obtain the effective activators with good polymer-retarding effect and relatively high activity,the optimized conditions are proposed to be[PhOH]/[AlMe3]= 0.5 and[H2O]/[Al]= 0.7.Various aluminoxanes with different[-OH]sources confirm the importance of using phenols in preparing the effective polymer-retarding activators.By utilizing these P-MAOs,the mass fraction of polymers in the total products could be reduced to lower than 1.0 wt%,which is much lower than that of the MAO-activated systems(>30 wt%),and without significant loss of activity.1H and 27Al NMR analyses confirmed the formation of the desired aluminoxane structures and ethylene oligomerization results manifest that P-MAOs can serve as effective polymer-retarding activators for the bis(imino)pyridine iron-catalyzed system.(3)Different structures and Lewis acidic RAOs can be obtained based on the hydrolysis of different aluminum alkyls,and the better cocatalyst structure can be screened by comparing their different effects on iron-catalyzed ethylene oligomerization.Under different molar ratios of[H2O]/[Al],methylaluminoxane(MAO-S),ethylaluminoxane(EAO),isobutylaluminoxane(i-BAO),methyl/isobutylaluminoxane(MBAO)and ethyl/isobutylaluminoxane(EBAO)are synthesized and employed in iron-catalyzed ethylene oligomerization.The results show that MAO-S renders the highest catalytic activity but with rather high undesired polymer share,and i-BAO is the most optimum cocatalyst rendering the predominant production of a-olefins with significantly reduced polymer share while at high catalytic activity.On the contrary,EAO shows the rather poor catalytic activity.The use of mixed aluminoxanes,MBAO and EBAO as cocatalysts also render different catalytic performance,indicating that the composition/structure of mixed aluminoxanes has pronounced effects on the catalytic activity and selectivity to linear?-olefins.The 27Al-NMR spectra of different aluminoxanes manifests that the introduction of iso-butyl groups on i-BAO changes the structure and reduces the Lewis acidity of their aluminium atoms which help retard polymer share in ethylene oligomerization as a result.(4)For the purpose of verifying the effect of the Lewis acid of co-catalyst on the inhibition of insoluble polymers in iron-catalyzed ethylene oligomerization,organic boron compounds(B)with different Lewis acidic(BArF3 and Ph3CBArF4)and different alkylaluminiums(TRA)are employed as the cocatalyst.The results show that the L-Fe(acac)3/B/TRA catalyst system has the comparative activity wtih the L-Fe(acac)3/MAO catalyst system at atmospheric pressure(>106g·(mol-Fe)-1·h-1)and better inhibition effect on insoluble polymers(<1 wt%).With the increase of reaction temperature,the activity of L-Fe(acac)3/B/TRA catalytic system is decreased,and the content of insoluble polymers in the product is decreased significantly.Under the same reaction conditions,L-Fe(acac)3/B/TiBA catalyst system has lower activity but better insoluble polymers inhibition than that of L-Fe(acac)3/1B/TMA.This can be mainly attributed to the more negatively charged of isobutyl groups which is not easily dispersed by the Lewis acid-based borides in the ionization process,so the co-catalyst is more closely bound to the active sites which is more conducive to the occurrence of the P-H elimination reaction.L-Fe(acac)3/Ph3CBArF4/TRA catalyst system has higher activity but worse insoluble polymers inhibition than that of L-Fe(acac)3/BArF3/TRA catalyst system because of higher Lewis acidity of Ph3CBArF4than BArF3.(5)The results of ethylene oligomerization catalyzed by L-Fe(acac)3/TRA indicate that TRA can also be used as the cocatalyst under specific reaction conditions(T<50°C;[Al]/[Fe]<1000).The activity and the insoluble polymer content of the product fluctuate greatly with the reaction conditions.It is speculated that the active sites may form an unstable ion pair structure with activation of TRA.The specific reaction mechanism remains to be further studied.