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
A small amount of acrylate comonomers are routinely used in PMMA grades destined for heat processing, since this stabilizes the polymer to depolymerization ("unzipping") during processing 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
Polyolefins are insulating, hard to heat
60% of plastic is PE or PP
PE has no reactive sites, thermally stable and UV stable 3

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

5. Transesterification of reclaimed polyester
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)
removed under vacuum
n ≥ 6
Mechanism:
dehydrogenate alcohol to aldehyde
couple aldehyde with alcohol to hemiacetal
dehydrogenate hemiacetal to ester
Driving force for high molecular weight polymer ( Mn up to g mol-1) is H2 removal; Any H2 kept in the system prevents polymerization
Harder for condensation polymerization to remove byproduct, achieve high Mw, achieve narrow D
Analogous to acyclic diene metathesis (ADMET) polymerization (removal of ethylene)
Hunsicker, Dauphinais, Mc Ilrath, and Robertson, Macromol. Rapid Commun. 2012, 33, 232

7. Running the reaction backwards
Solvent choice important for solubility
See also Z. Han, L. Rong, J. Wu, L. Zhang, Z. Wang and K. Ding, Angew. Chem., Int. Ed., 2012, 51,
a,w-diols formed under 14 atm H2 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
Change title
Westhues, Idel, and Klankermayer, Sci. Adv. 2018;4: eaat9669 8

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

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 H2
b-alkyl transfer
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, Angew Chem Int 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 : e 17

18. diesel range oil, up to 98% yield no aromatics
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 : e 18

19. cyclohexane + H2  n-hexane
Hydrocracking
423 K
cyclohexane + H2  n-hexane
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
energy
light alkane
olefin+ H2
polyolefin
+ H2
oligomer
+ heat
(cat)
cat
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)
cat 22