Understanding Advanced Heat and Mass Transport Control and Non-Noble Metal Catalyst Designs for Low Temperature Polyolefin Up-Cycling

该项目将研究廉价的催化材料和新的能源输送机制，以有效地将废塑料转化为有价值的化学品和燃料。 将获得基本的见解，以开发连续和模块化的化学转化过程，使当地市政当局能够利用废塑料（以及其他类型的城市废物）获得经济收益。该项目的重点是与城市废物处理厂规模相适应的技术，从而避免长途运输废物材料供大型集中化学加工厂加工。 除了提高美国在循环和环保化学工艺方面的竞争力外，该项目还将包括培训下一代科学家和工程师的教育和推广工作，同时也提高了社会对塑料污染问题的认识和解决这一问题的努力。研究人员实验室的初步努力已经确定，未催化的热自由基反应机制极大地限制了塑料的效率和作用。在聚烯烃催化转化为更高价值的中链和长链烷烃和烯烃中的催化材料。此外，先前的工作已经揭示了催化与热和传质效应之间的联系，从而抑制了催化剂和反应环境设计所需的基本机理见解的发展。该项目将研究专门用于高粘度聚合物熔体混合的先进反应器几何形状的设计和使用，以了解和限制聚烯烃催化裂解中的传质效应。一种新的微波能量传递机制，将热能直接传递到催化剂颗粒，以避免或显着减少非催化的热自由基反应机制的作用。 将研究呈现酸和金属反应位点的双功能催化剂的基本表面反应能量学，使得可以平衡和优化聚烯烃脱氢、C-C裂解和氢化的组分反应动力学。一种机械和化学稳定的微波感受器，碳化硅（SiC），将被用来同时提供可调的固体酸反应位点，淬火热自由基，并将微波能量转换为局部热能。SiC酸催化剂将与定义明确的非贵金属金属间化合物纳米颗粒催化剂组合，该催化剂将提供类似金属的表面化学，以实现有效的氢化。此外，热声子的作用，通过微波吸收SiC产生的，将研究加速动力学困难的反应步骤。为了获得本体和表面催化之间的明确联系，催化剂将使用前和原位x射线衍射和高分辨率能量色散x射线，x射线光电子，和高灵敏度低能量离子散射光谱进行研究。利用一套反应条件，提供了一系列的热量和质量传输环境，明确的见解，未催化的反应机制，中间化学势，和组合物极化附近的催化剂表面的作用将被开发。还将确定链动力学和分支点在催化裂解机理中的作用，从而可以开发出更稳健和通用的催化方法来操作实际的混合聚烯烃废物。还将确定水含量在促进最佳催化剂配方中的作用，以确保将开发的科学应用于现实世界的聚烯烃升级循环工作。除了聚烯烃的向上循环，该项目将产生与热量和质量传输和反应器设计相关的可转移的理解，这可能会大大提高催化向上循环难降解，固体或高粘度材料的努力。教育部分将加强田纳西大学诺克斯维尔的化学工程和材料科学项目，并提高当地社区对塑料污染问题的认识。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The project will investigate inexpensive catalytic materials and new energy delivery mechanisms to efficiently transform waste plastics into valuable building-block chemicals and fuels. Fundamental insights will be obtained to develop continuous and modular chemical transformation processes that will enable local municipalities to utilize waste plastics (as well as other types of municipal waste) for economic gain. The project focuses on technologies compatible with the scale of municipal waste treatment plants, thus avoiding long-distance transport of waste material for processing in large centralized chemical processing plants. Beyond promoting U.S. competitiveness in cyclic and environmentally friendly chemical processes, the project will include education and outreach efforts to train next-generation scientists and engineers, while also increasing societal awareness of the plastics pollution problem and efforts to solve the issue.Preliminary efforts in the investigator's laboratory have determined that uncatalyzed thermal radical reaction mechanisms dramatically limit the efficiency and role of catalytic materials in the catalytic conversion of polyolefins to higher-value medium and long-chain alkanes and alkenes. Additionally, the prior work has revealed linkages between the catalysis and heat and mass transport effects, thus inhibiting the development of fundamental mechanistic insights needed for catalyst and reaction environment design. The project will investigate the design and use of advanced reactor geometries specific for highly viscous polymer melt mixing to understand and limit the effect of mass transport in the catalytic cleavage of polyolefins. A new microwave energy delivery mechanism that delivers heat energy directly to the catalyst particles will be employed to avoid or dramatically reduce the role of uncatalyzed thermal radical reaction mechanisms. The fundamental surface reaction energetics of a bifunctional catalyst that presents both acid and metallic reaction sites will be investigated such that constituent reaction kinetics for polyolefin dehydrogenation, C-C cleavage, and hydrogenation may be balanced and optimized. A mechanically and chemically-robust microwave susceptor, silicon carbide (SiC), will be used to simultaneously provide tunable solid acid reaction sites, quench thermal radicals, and convert microwave energy to localized heat energy. The SiC acid catalyst will be combined with well-defined non-noble metal intermetallic compound nanoparticle catalysts that will provide metal-like surface chemistry to achieve efficient hydrogenation. Additionally, the role of hot phonons, produced through microwave absorption by SiC, will be investigated for accelerating kinetically-difficult reaction steps. To derive clear connections between bulk and surface catalysis, the catalysts will be investigated using ex- and in-situ x-ray diffraction and high-resolution energy-dispersive x-ray, x-ray photoelectron, and high-sensitivity low-energy ion scattering spectroscopies. Utilizing a suite of reaction conditions that provide a range of heat and mass transport environments, clear insights into the role of uncatalyzed reaction mechanisms, intermediate chemical potential, and composition polarization near the catalyst surface will be developed. The role of chain dynamics and branch points in the catalytic cleavage mechanism will also be determined such that a more robust and general catalytic process may be developed to operate on practical mixed polyolefin waste. The role of water content in motivating optimal catalyst formulations will also be determined to ensure the application of the science developed to real-world polyolefin up-cycling efforts. Beyond polyolefin up-cycling, this project will produce transferable understanding associated with heat and mass transport and reactor design that may greatly improve efforts to catalytically up-cycle recalcitrant, solid, or highly viscous materials. The educational component will enhance the chemical engineering and materials science programs of the University of Tennessee, Knoxville, and raise awareness of the plastics pollution problem in the local community.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.