Disordered, Sub-Nanometer Ru Structures on CeO₂ are Highly Efficient and Selective Catalysts in Polymer Upcycling by Hydrogenolysis

Abstract

Nondegradable polyolefin plastics pose severe environmental threats and thus demand efficient upcycling technologies. In this work, we discovered that low-loading (≤0.25 wt %) Ru/CeO2 exhibits remarkable catalytic performance in the hydrogenolysis of polypropylene (PP), polyethylene (PE), and n-C16H34 that is superior to high-loading (≥0.5 wt %) Ru/CeO2. They possess high PP conversion efficiency (sevenfold increase over current literature reports), low selectivity toward undesired CH4, and good isomerization ability. In the low-loading range, the intrinsic activity of Ru in PP hydrogenolysis increases as the particle size decreases, opposite of the trend in the high-loading range. Detailed characterization revealed that the abrupt changes in catalytic behaviors coincide with Ru species transitioning from well-defined to highly disordered structures in the low-loading domain. The disordered Ru species were shown to be sub-nanometer in size and cationic. Mechanistically, the regioselectivity and the rate dependence on hydrogen pressure of C–C bond cleavage are different on low- and high-loading Ru/CeO2, both explained by the higher coverage of adsorbed hydrogen (*H) on low-loading Ru/CeO2. This work reveals the remarkable catalytic performance of highly disordered, sub-nanometer, cationic Ru species in polyolefin hydrogenolysis, opening immense opportunities to develop effective, selective, and versatile catalysts for plastic upcycling.

Rapid atom-efficient polyolefin plastics hydrogenolysis mediated by a well-defined single-site electrophilic/cationic organo-zirconium catalyst

“…The combined mass of the volatiles and DCM extract fractions as a percentage of the initial reactant polyolefin mass is taken as the percent conversion for a given polyolefin hydrogenolysis reaction. These analytical methods for handling polyolefin hydrogenolysis products are similar to those used in the recent polyolefin hydrogenolysis literature 12 , 14 , 28 .…”

Dual-Pathway Polyolefin Upcycling over an Unexpectedly Bifunctional Low-Loading Ru/TiO2-Anatase Catalyst

“…10 Thus, to test the hypothesis, we impregnated Ni and Pt onto TiO2-A-SG (0.07 M/nm 2 ), both of which are inert for C−C bond cleavage, i.e., hydrogenolysis, under these conditions. 12,13,26 Figure 2a shows that both Ni and Pt convert PP on TiO2-A-SG more efficiently than Ru, yielding similar product distribution (Figures 2d and S3b): no CH4, much more C4-5 than C2-3, narrow liquid distribution around C10, and yellow-colored products (Figure S2f). In addition, the TiO2-A-SG support alone, without any metal, can convert ~5% PP (Figure 2a), also with similar product distribution (Figure S3c).…”

“…For hydrogenolysis, most studies used catalysts with high noble metal (Ru or Pt) content, 12,13,[15][16][17][18][19][20][21][22][23][24][25] and the selectivity needs to be steered away from low-valued gas alkanes, especially CH4, to high-valued liquid alkanes. We have shown that disordered Ru clusters have better activity and selectivity in the reaction than rigid particles, 26 but the structure-function relationship needs further understanding for the rational design of active sites. For hydrocracking, most studies used expensive, scarce Pt as the metal, [27][28][29][30][31][32][33][34] and zeolites as the acidic support, [30][31][32][33][34][35] the microporous structure of which creates difficulty for the diffusion of long-chain polymers and thus limits active site accessibility.…”

Plastic Waste Valorization by Leveraging Multidisciplinary Catalytic Technologies

“…The advantages and disadvantages of these technologies are summarized in Table . In the available chemical approaches, the well-developed catalysts, processes, and infrastructures in traditional industries can readily be adopted for the transformation of waste plastic into valuable products. ,,,,,,, In particular, there are pilot- or industrial-scale processes for recycling monomers from postconsumer plastics . However, many chemical methods still suffer from harsh reaction conditions (e.g., high temperature and pressure), limiting their application in an environmentally benign fashion.…”