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建材化学品和燃料。将获得基本见解，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。该项目侧重于与城市垃圾处理厂规模相适应的技术，从而避免了垃圾在大型集中化学处理厂进行处理的长途运输。除了提高美国在循环和环保化学工艺方面的竞争力外，该项目还将包括教育和推广工作，以培养下一代科学家和工程师，同时提高社会对塑料污染问题的认识，并努力解决这一问题。研究者实验室的初步努力已经确定，非催化的热自由基反应机制极大地限制了催化材料在将聚烯烃催化转化为高价值的中长链烷烃和烯烃方面的效率和作用。此外，先前的工作已经揭示了催化与热量和质量传递效应之间的联系，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。该项目将研究高粘性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过程中质量传递的影响。采用一种新的微波能量传递机制，将热能直接传递给催化剂颗粒，以避免或大大减少非催化热自由基反应机制的作用。研究了具有酸性和金属反应位点的双功能催化剂的基本表面反应动力学，从而平衡和优化聚烯烃脱氢、C-C裂解和加氢的组成反应动力学。碳化硅（SiC）是一种机械上和化学上都很强大的微波感受器，将用于同时提供可调的固体酸反应位点，淬灭热自由基，并将微波能量转化为局部热能。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结合，提供类似金属的表面化学，以实现有效的氢化。此外，通过SiC微波吸收产生的热声子的作用将被研究，以加速动力学困难的反应步骤。为了明确体催化和表面催化之间的联系，催化剂将使用前和原位x射线衍射、高分辨率能量色散x射线、x射线光电子和高灵敏度低能离子散射光谱进行研究。利用一系列反应条件，提供一系列的热量和质量传递环境，清晰地了解非催化反应机制的作用，中间化学势，以及催化剂表面附近的成分极化。链动力学和分支点在催化裂解机制中的作用也将被确定，从而可以开发出一种更强大和通用的催化过程，用于实际的混合聚烯烃废料。还将确定水含量在激发最佳催化剂配方中的作用，以确保将科学开发应用于实际的聚烯烃升级循环工作。除了聚烯烃上循环之外，该项目还将产生与热量和质量传递以及反应器设计相关的可转移知识，这可能会大大改善催化上循环难降解、固体或高粘性材料的工作。教育部分将加强诺克斯维尔田纳西大学的化学工程和材料科学课程，并提高当地社区对塑料污染问题的认识。该奖项反映了美国国家科学基金会的法定使命，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.