March 6, 2023

Abstract

Polyolefins, including polyethylene (PE) and polypropylene (PP), are among the most common single-use plastics consumed worldwide, and are also among the most pervasive in landfills and the environment. In this presentation I will show two avenues for the chemical recycling of polyolefin waste into hydrocarbons that involve the selective cleavage of C-C bonds in the plastic backbone. Our initial studies identified Ru-based catalysts as highly active for the hydrogenolysis of C-C bonds in PE and PP under relatively mild conditions (200-250ºC, 20-40 bar H₂, 2-16 h). Despite their high activity, one of the remaining challenges with Ru-catalyzed hydrogenolysis over supports such as carbon is the production of methane, resulting from cleavage of terminal C-C bonds. First, I’ll show that supporting Ru nanoparticles on a Brønsted-acidic support improves selectivity by promoting a bifunctional acid/metal-catalyzed hydrocracking mechanism that suppresses methane formation. Operando EXAFS with model polyolefin hydrogenolysis revealed that the support affects the reducibility of the nanoparticles. Next, we used this mechanistic insight to encapsulate cobalt nanoparticles in different zeolite topologies, ultimately showing that a 5 wt% Co-ZSM catalysts converts PE and PP into propane with a selectivity >80 % under mild conditions.

A second strategy to deconstruct polyolefins involves tandem dehydrogenation-metathesis-hydrogenation using a “sacrificial” short-chain olefin that breaks the long plastic backbone. While olefin metathesis is a very robust strategy to interconvert olefins, the mechanism remains poorly understood for heterogeneous catalysts.  We recently demonstrated that co-feeding substituted olefins (e.g., 2,3-dimethyl-1-butene, i-4ME) that are generally unreactive towards cross-metathesis, the steady state rate of propylene self-metathesis can be promoted by orders of magnitude. I will show kinetic and spectroscopic data in support of bifunctional metal-acid active sites that operate via a 1,2-proton shift mechanism that is drastically enhanced by the presence of highly substituted olefins. Our work provides mechanistic insights into the catalytic promotion for heterogeneous olefin metathesis.

Bio

Prof. Yuriy Román obtained his Bachelor of Science degree in Chemical Engineering at the University of Pennsylvania in 2002 and completed his Ph.D. at the University of Wisconsin-Madison, also in Chemical Engineering, under the guidance of Prof. James Dumesic in 2008. Next, he completed a two-year postdoc at Caltech, working with Prof. Mark E. Davis on the synthesis of zeolites. Prof. Román joined the department of Chemical Engineering at MIT as an Assistant Professor in 2010, where he was promoted to Associate Professor in 2014 and Full Professor in 2020. The core of his research lies at the interface of heterogeneous catalysis and materials science, where he combines catalyst design, kinetic studies, and reaction engineering study the chemical transformation of molecules within reactive microenvironments. His research portfolio includes projects in biorefining, hydrocarbon conversion, porous materials, and electrocatalysis In addition to receiving an NSF Career Award in 2014, he also received two inaugural young investigator awards: the ACS Early Career in Catalysis Award and the AICHE Catalysis and Reaction Engineering Division Young Investigator Award. In 2018, he also received the Rutherford Aris Award granted by NASCRE and the Robert Augustine Award by ORCS. In 2022, he was selected as a finalist for the National Blavatnik Award in Chemistry, and in 2023 he received the Paul H. Emmett Award in Fundamental Catalysis.