Olefin Polymerization from the Gas Phase

大多数烯烃聚合过程是催化的，通常涉及多相催化剂。聚合反应发生在催化剂表面或孔内的活性位点。这意味着单体必须与催化剂接触，聚合物要么生长在催化剂表面，要么生长在催化剂的孔隙中。所涉及的传输过程包括单体从本体相通过聚合物层扩散到活性部位，以及反应的热（反应是放热的）通过同一聚合物层扩散出去。因此，这些反应与温度有关，并受运输限制。因此产物聚合物的性质是由催化剂的孔结构控制的。随着反应的进行，在催化剂内部孔隙中产生的聚合物越来越多地阻碍运输路径，产生单体浓度和温度的梯度。聚合速率降低，产物聚合物分子量变化较大。催化剂最终破碎，暴露出更大的表面积，以及更多的进入以前无法进入的孔隙的通道。新的位点暴露，传输路径长度减少，然后聚合加速。这种断裂是由于聚合物链在狭窄、易碎的催化剂通道中受限生长和膨胀而发生的，是一种自催化现象。这种断裂速率取决于催化剂的孔结构，因此孔结构对聚烯烃的分子量分布和收率具有重要意义。气相聚合的优点是：(1)所需的资本投资少；(2)它是一个无溶剂的过程（即不需要蒸馏、干燥和溶剂回收）；(3)因此操作成本和能耗低。pi计划继续实验研究三维孔隙结构对烯烃气相聚合的影响。孔隙结构将通过物理吸附、x射线断层扫描和孔隙率测定来表征。扩展氮气吸附和129 XeNMR将用于表征催化剂/聚合物体系的微孔隙度和单体扩散到活性表面的速率。二维和三维x射线成像将被开发，以确定催化剂在生长的聚合物中的空间分布。这些结果将有助于我们更好地理解动力学和输运现象之间的相互作用，以及设计更好的催化剂和整个反应器。

The majority of olefin polymerization processes are cataytic, most often involving a heterogeneous catalyst. The polymerization reaction occurs at some active site on the catalyst either on its surface or within its pores. This means that monomer has to contact the catalyst and that the polymer grows either on the surface or within the pores of the catalyst. The transport processes involved include monomer from the bulk phase diffusing through a polymer layer to an acitve site and the heat of reaction(the reactions are exothermic) diffusing out through this same polymer layer. The ractions are thus temperature related and transport limited. The properties of the product polymer are therefore controlled by the pore structure of the catalyst. As the reaction proceeds, polymer created in the internal pores of the catalyst increasingly blocks transport paths generating gradients in monomer concentration and temperature. Polymerization rate decreases and the molecular weight of product polymers vary greatly. The catalyst ultimately fragments, exposing more surface area as well as more accesss to previoulsy inaccessible pores. New sites are exposed, transport path length is reduced, and polymerization is then accelerated. The facture occurs due to confined growth and expansion of polymer chains in narrrow, friable channels of the catalysts and is an autocatalytic phenomena. This fracture rate depends on the catalyst pore structure which is thus of significant importance in the molecular weight distribution and yield of polyolefins. Gas phase polymerization has the advantage that: (1) low capital investment is required, (2) it is a solvent free process (i.e. distillation, drying and solvent rcovery are not necessary), and (3) therefore operating costs and energy consumption are low. The PIs plan to continue to experimentally study the effects of thethree-dimensional pore structure on the gas-phase polymerization of olefins. Pore structure will be characterized by physical adsorption, X-ray tomography and porosimetry. Extended N2 adsorption and 129 XeNMR will be employed to charactized the microporosity of the catalyst/polymer system and the rate by which the monomer diffuses to the active surface. X-ray imaging in two and three demensions will be developed to determine the spatial distribution of the catalyst within the growing polymer. The results should yield a better fundamental understanding of the interaction of kinetics and transport phenomena and the design of not only better catalysts, but whole reactors as well.