Adapting Transition Metal-Based Heterogeneous and Homogeneous Catalysts for Polymer Disassembly

1. Adapting Transition Metal-Based Heterogeneous and Homogeneous Catalysts for Polymer Disassembly Susannah Scott Department of Chemical Engineering University of California, Santa Barbara Chemical Sciences Roundtable Workshop Closing the Loop on the Plastics Dilemma National Academies of Science, Engineering, and Medicine May 9, 2019

2. Tertiary (chemical) recycling of plastics Recovery of chemical components (monomers, or other valuable molecules) Condensation polymers, made by repeated elimination of a small molecule reversible by reaction with the same molecule Addition polymers, made by repeated formation of new, covalent bonds reversible thermally terephthalic acid ethylene glycol PET n + heat methyl methacrylate PMMA, ceiling temperature 198 °C 2

3. Kinetics and thermodynamics Even when depolymerization is thermodynamically favorable, it is usually not fast. catalysts speed up the reaction If depolymerization is not thermodynamically favorable, catalysts do not help. Making olefins (high temperature): C2H6, C3H8, i-C4H10 + heat  C2H4, C3H6, i-C4H8 + H2 Making polyolefins (low temperature): C2H4, C3H6, i-C4H8  polyolefin + heat Depolymerization (high temperature): polyolefin + heat  C2H4, C3H6, i-C4H8 3

4. Catalytic pyrolysis Ni-ZSM-5 catalyst 550 °C Wong, Ngadi, Abdullahand Inuwa, Ind. Eng. Chem. Res. 2016, 55, 2543 4

5. Transesterification of reclaimed polyester transesterification Ti(OBu)4 catalyst 220 °C Rorrer, Nicholson, Carpenter, Biddy, Grundl, and Beckham, Joule 2019, 3, 1006 5

6. Dehydrogenative polymerization of α,ω-diols Milstein’s Ru catalyst hydrogenates esters to alcohols (and dehydrogenates alcohols to esters) n ≥ 6 removed under vacuum • Mechanism: • dehydrogenate alcohol to aldehyde • couple aldehyde with alcohol to hemiacetal • dehydrogenate hemiacetal to ester Hunsicker, Dauphinais, Mc Ilrath, and Robertson, Macromol. Rapid Commun. 2012, 33, 232

7. Running the reaction backwards a,w-diols formed under 14 atmH2 at ca. 150 °C in THF/anisole Krall, Klein, Andersen, Nett, Glasgow, Reader, Dauphinais, Mc Ilrath, Fischer, Carney, Hudson and Robertson, Chem. Commun., 2014, 50, 4884 7

8. Catalytic hydrogenolysis of PET as little as 0.01 mol% Ru TON up to 104 Westhues, Idel, and Klankermayer, Sci. Adv. 2018;4: eaat9669 8

9. Westhues, Idel, and Klankermayer, Sci. Adv. 2018;4: eaat9669 9

10. Reactive separation Westhues, Idel, and Klankermayer, Sci. Adv. 2018;4: eaat9669 10

11. Olefin polymerization and alkane hydrogenolysis ZrNp4 + silica Lecuyer, Quignard, Choplin, Olivier, Basset, Angew. Chem. Int. Ed. Engl. 1991, 30, 1660 mechanism: s-bond metathesis linear alkanes (odd/even) diesel-range, light gases n C2H4 polyethylene b-alkyl transfer H2 150 °C, 15 h solvent-free, or in decalin Dufaud and Basset, Angew. Chem. Int. Ed. 1998, 37, 806 11

12. Converting LDPE to HDPE Pelletier and Basset, Acc. Chem. Res. 2016, 49, 664 12

13. Mechanistic switch: Alkane metathesis TaNp3(=Np) + silica at full conversion, only methane, no ethane Vidal, Théolier, Thivolle-Cazat and Basset, Science, 1997, 276, 99 Chabanas, Vidal, Copéret, Thivolle-Cazat and Basset, AngewChemInt Ed, 2000, 39, 1962 13

14. Catalytic polyolefin dehydrogenation ~ 1 h, 150 C, in p-xylene requires sacrificial olefin no change in molecular weight polyethylene much harder than poly-1-hexene Ray, Zhu, Kissin, Cherian, Coates and Goldman, Chem. Commun., 2005, 3388 14

15. Tandem alkane metathesis GC of products from metathesis of n-decane after 9 d at 175 °C Re2O7/Al2O3 Goldman, Roy, Huang, Ahuja, Schinski, and Brookhart, Science 2006, 312, 257 15

16. Cycloalkane metathesis polymerization Some Ir catalysts give cyclo-oligomers in decreasing yields with increasing carbon numbers (t-Bu4PCP)Ir also gives a significant amount of polyethylene: ring-opening metathesis polymerization of cyclooctane 12 h, 125 °C Ahuja, Kundu, Goldman, Brookhart, Vicente, and Scott, Chem. Commun. 2008, 253 16

17. Tandem catalytic cross-metathesis of polyethylene Jia, Qin, Friedberger, Guan, Huang, Sci. Adv. 2016; 2 : e1501591 17

18. 150 °C, several days hydrocarbon solvent as reactant diesel range oil, up to 98% yield no aromatics Jia, Qin, Friedberger, Guan, and Huang, Sci. Adv. 2016; 2 : e1501591 18

19. 423 K cyclohexane + H2 n-hexane Hydrocracking Pt/Al2O3 catalyzes paraffin hydrogenolysis H2 activation occurs on step sites alkane hydrogenolysis occurs on kink sites Pt becomes covered with carbon under reaction conditions Somorjai and Blakely, Nature, 1975, 258, 580 19

20. Distance-Selective Hydrocracking Ordered catalyst support Ordered Pt nanoparticle array Atomic layer deposition MeCpPtMe3 / O3 5 cycles at 200 °C average inter-particle spacing ~ 10 nm ~ C100 (√13x√13)R33.7° TiO2 double-layer SrTiO3(001) surface reconstruction, viewed from above (001) facet Poeppelmeier et al. Chem. Mater. 2018, 30, 841 Can distance between nanoparticles create a lower limit for product chain lengths? 20

21. Precision polyethylene chopping Pt nanoparticle array on strontium titanate Institute for Cooperative Upcycling of Polymers solvent-free Patent Pending Delferro et al., manuscript in preparation see Poster #1 “Catalytic upcycling of single-use polyethylene”

22. Prospects for catalytic re(up)cycling Currently no cost-competitive catalytic routes for chemical re(up)cycling of plastics. Future technologies will need: • low energy and solvent requirements • easy separations • robust catalysts, preferably not precious metals • value-added targets (recovered monomers generally not competitive with fossil-derived) olefin+ H2 + heat (cat) cat energy cat cat cat light alkane oligomer + H2 polyolefin + H2 22