Preparation And Performance Of Catalysts Supported By MOFs For Ethylene Oligomerization/Polymerization

As important raw materials in chemical industry, a-olefins have different properties and uses according to their different carbon chain structures. Thereinto, C4-C8 a-olefins are most widely used, so their demand is also the biggest. The cracking technology of paraffins is gradually being eliminated due to its poor controllability on molecular structure and product distribution, and the ethylene oligomerization technology is becoming more and perfect. Catalyst is essential in the process of ethylene oligomerization, and heterogeneous catalysts have many advantages, such as moderate operating pressure, easy speperation of products, easy regeneration of catalyst and friendly to environment, so they are paid much attention from reserachers. However, the side effect of the carrier on the catalytic system is relatively large. The activity is often reduced, and the selectivity is not improved obviously. To overcome this problem, the tranditional ethylene oligomerization and emering Metal-Organic Frameworks were combined in this thesis. Namely, a new kind of heterogeneous catalyst whose active centres were located in the holes of Metal-Organic Fraworks was developed.The effect of MOFs’ structure on catalytic performance was firstly studied with the mixed-ligand MOFs to synthesize ethylene oligomerization catalysts. It is found that the mixed-ligand method can afford a series of MixMOFs with different amount of amino groups. All the materials were highly crystalline, and the amount of amino groups only influenced the specific surface area of the parent materials. The less of amino groups, the bigger of specific surface area. As a result, IRMOF-3 gave the smallest specific surface area. The mixed-ligand MixMOFs are much more stable than IRMOF-3, and they can introduce more metal centers without the damage of structural integrity and porosity. On the contrary, the IRMOF-3 was severely damaged during the process of modification, and the porous structure almost completely disappeared. Even with the prolonged reaction time, the amount of nickel also had an upper limit. Two kinds of MOFs-Ni can catalyze ethylene oligomerization and gave similar activity, but due to their different integrity, MixMOFs-Ni whose active centers were located in the holes produced mainly smaller butenes and the products of severely damaged IRMOF-3 had no obvious selectivity. Considering the synergistic effect of optimal specific surface area and metal content, MixMOFs-b-Ni gave the highest activity.In the second part, a more stable mixed ligand MOF (MixUiO) was synthesized. One-step modification method and two-step modification method were used to generate two kinds of MixUiO-Ni complexes with different structures. As expected, the specific surface area and porosity of MixUiO-67 using diphenic acid were greater than that of MixUiO-66 using single benzene dicarboxylic acid. These MOFs with Zr as vertex metals were very stable. After modification, their structural integrity and porosity were retained. For one-step modification method, the specific surface area of MixUiO-67-Ni was larger than that of MixUiO-66-Ni, but for the two-step modification method, the specific surface area of MixUiO-66-SI-Ni was close to MixUiO-67-SI-Ni. In addition, the two-step modification method can also introduce more metal centers. Because of the larger molecular weight of MixUiO-67, the amount of amino groups was lower, and the corresponding Ni loading was also lower, so the distribution of active centers was more uniform. Under the same reaction conditions, the activity of MixUiO-67-Ni was higher than that of MixUiO-66-Ni, and the selectivity of hexenes and Octenes was also higher.Furthermore, a series of MOF-5-Ni catalysts were synthesized by ion exchange method, which had no influence on the properties of the matrix, and the structural characteristics of MOF-5 were completely preserved. The doping amount of Ni2+ ions can be controlled by reaction time. These MOF-5-Ni materials can be directly used as catalysts for ethylene oligomerization without co-catalyst. In the Ni catalyzed ethylene reaction, oligomerization plays a main role, so the products were only butenes. There were no any other olefins or solid polymers formed which may block the channel of MOFs. And the catalytic activity was sensitive to temperature and pressure, but the selectivity of butenes was consistent. When these MOF-5-Ni catalysts were treated with co-catalyst, their catalytic activity sightly decreased, but the selectivity of the system changed greatly, and the hexanes were generated except for butenes.Finally, a MOF-containing chromium catalyst was attempted to be used in the ethylene polymerization, in which the Cr center was introduced into the MOFs hole by post-synthetic modification. Although the homogeneous chromium complexes without any substituents were inactive for ethylene polymerization, when it was introduced into the MOF’s holes, it could catalyze ethylene polymerization with good activity. As the active center was located in the holes, the monomer and the co-catalyst had to diffuse into the holes, so the the diffusion had a great influence on catalytic performances. The activity with the order of TIBA>TEA>TMA =MAO, the molecular weight of obtained polyethylenes with the order of TIBA>TEA> TMA=MAO, and the molecular weight distribution (PDI) with the order of TMA=MAO> TEA>TIBA were found. For the larger co-catalysts, the increase in temperature could strengthen the diffusion, so the activity increased and molecular weight diminished. For the smaller co-catalysts, the deactivation still preoccupied at high temperature. The activity decreased with ethylene pressure, but Mn of PEs firstly increased and then remained unchanged.