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

Polyolefins comprise a major fraction of single-use plastics, yet their catalytic deconstruction/recycling has proven challenging due to their inert saturated hydrocarbon connectivities. Here a very electrophilic, formally cationic earth-abundant single-site organozirconium catalyst chemisorbed on a highly Brønsted acidic sulfated alumina support and characterized by a broad array of experimental and theoretical techniques, is shown to mediate the rapid hydrogenolytic cleavage of molecular and macromolecular saturated hydrocarbons under mild conditions, with catalytic onset as low as 90 °C/0.5 atm H2 with 0.02 mol% catalyst loading. For polyethylene, quantitative hydrogenolysis to light hydrocarbons proceeds within 48 min with an activity of > 4000 mol(CH2 units)·mol(Zr)−1·h−1 at 200 °C/2 atm H2 pressure. Under similar solventless conditions, polyethylene-co−1-octene, isotactic polypropylene, and a post-consumer food container cap are rapidly hydrogenolyzed to low molecular mass hydrocarbons. Regarding mechanism, theory and experiment identify a turnover-limiting C-C scission pathway involving ß-alkyl transfer rather than the more common σ-bond metathesis.

H 2 as a cost-effective deconstruction agent, and the overall process is thermodynamically favorable. Currently, the most common approaches involve using heterogeneous precious-metal catalysts such as Ru and Pt to transform polyolefins into wax-range and lighter hydrocarbons. Such processes typically employ high temperatures/pressures and extended reaction times [12][13][14][15][16][17] .
Regarding earth-abundant metal polyolefin hydrogenolysis catalysts, the only example to our knowledge employed a formally neutrally charged d 0 Zr alkyl bound to silica-alumina, which mediates relatively slow hydrogenolysis (Fig. 1a). This formally neutrally charged Zr alkyl was reported by Basset et al. to have ca. 3% of the activity now found for the catalyst reported here (vide infra) 18 . We hypothesized that the protonolytic chemisorption of Zr alkyls on very different, highly Brønsted acidic surfaces would create weak/weakly coordinating conjugate Brønsted base counteranions 19,20 , and would thereby create d 0 catalysts sufficiently electrophilic/cationic to rapidly cleave polyolefin C-C bonds. Supporting this high activity hypothesis, note that chemisorbing Cp*Zr(CH 3 ) 3 (Cp*=η 5 -Me 5 C 5 ) 21-23 on such Brønsted acidic oxides yields methane via Zr-CH 3 bond protonolysis and an electrostatically bound, formally cationic organozirconium adsorbate (Fig. 1b). This surface-bound formally cationic species catalyzes very rapidly, facially selective arene hydrogenation and olefin polymerization with nearly 100% of the Zr sites catalytically significant [21][22][23] . Such catalysts operate via mechanisms differing substantially from later transition metals, raising the intriguing question of whether they might rapidly activate/hydrogenolyze polyolefins via well-defined but unconventional pathways.

Catalytic polyolefin hydrogenolysis
Linear polyethylene (PE) homopolymer, isotactic polypropylene (i-PP), polyethylene-co−1-octene (PECO), and a post-consumer high-density polyethylene (HDPE) blue-dyed food container cap were initially investigated. Thus, 1.5 g of PE was heated with AlS/ZrNp 2 (0.03 mol%) at 200°C under 2.0 atm H 2 (Fig. 2, Eq. (1)). Remarkably, in <48 min, the starting PE is entirely consumed and converted into liquid and volatile products. To our knowledge, this is the most rapid catalytic PE hydrogenolysis process reported to date in the open literature and is surprising considering the low catalyst loading, mild reaction conditions, and simple reactor setup (high-pressure glass flask + magnetic stirring bar). The reaction is extremely fast, with a calculated activity of 4000 mol(CH 2 units) mol(Zr) −1 h −1 ( Table 1, Entry 1).
Polyolefin hydrogenolysis products were analyzed as follows. At the end of the reaction, the reactor is cooled to 25°C in a water bath, and the reactor headspace contents analyzed by GC/FID. Next, the remaining volatiles are vented, the residual liquid + higher molecular mass solid saturated hydrocarbon products are stirred with DCM, and the resulting suspension is filtered. The reactor components are then rinsed several times with DCM to ensure the collection of all solid and DCM-soluble liquid products. The filtrate is collected, the DCM is evaporated under~1 Torr reduced pressure, and the resulting hydrocarbons are weighed. The mass of this product gives the yield of the DCM extract fraction, which is analyzed by GC/MS (gas chromatography/mass spectrometry). We find that the DCM extraction and evaporation under reduced pressure results in <5 wt% loss of the volatile liquid hydrocarbons. After isolation via filtration, the DCM-insoluble higher molecular mass saturated hydrocarbon fraction is collected, dried, and weighed. The mass of this fraction, corrected for the weight of the catalyst, gives the yield of the solids fraction. The solid product contents are then analyzed by gel permeation chromatography (GPC). The volatiles fraction mass is the mass not accounted for after measuring the solids and DCM extract fractions, and its composition is readily analyzed via headspace gas chromatography/flame ionization detection (GC/FID). 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 .
To analyze the reaction products in more detail, the reaction was slowed by decreasing the temperature to 150°C and monitored at varied time intervals, with reactions halted periodically by cooling to 25°C (Eq. (1)). The time dependence of PE hydrogenolysis with 0.03 mol% Zr is shown in Fig. 2a and Table 1, entries 2 and 3. After only 10 min at 150°C, a 16% yield of volatile and DCM-soluble products is obtained (Fig. 2a-c), and the compositions were analyzed by GC/MS as noted above. For the starting PE, the number average molecular mass (determined using GPC) of the starting PE (M n = 11.62 kg mol −1 ) decreased by 33% to M n = 7.8 kg mol −1 (Fig. 2d) Entry 3), the volatile and liquid products were obtained in 86% and 96% yields, respectively. The PE solids comprise only a minor fraction of the products with M n = 2.5 kg mol −1 (for 50 min) and M n < 0.86 kg mol −1 (for 2 h) (Fig. 2d). Note also the gradual decline in the most probable chain length of the DCM extracts with conversion, falling from~18 carbons (10 min) to 14 carbons (30 min), 12 carbons (50 min), and 11 carbons (2 h) (Fig. 2c). We hypothesize that minor alkane branching observed in hydrogenolysis products of linear PE may be caused by the reinsertion of eliminated olefin. (vide infra). Note that PE hydrogenolysis catalyzed by ZrNp 4 supported on silica-alumina proceeds at~3% of the rate of the present reactions catalyzed by AlS/ZrNp 2 (Table 1, Entry 4).
While linear PE homopolymers are an informative substrate due to their relatively low melting points and simple structures, i-PP and PECO are of greater technological significance 29 . The AlS/ZrNp 2 catalyst was next applied to these materials at 190°C to lower the melt viscosity for ease of stirring (Fig. 3, Eq. (2)). Reacting 1.0 g i-PP (M n = 36 kg mol −1 ) over 0.04 mol% AlS/ZrNp 2 under 2 atm H 2 effects quantitative conversion to 68% low-molecular-weight (M w < C30) products and 28% C1-C6 volatiles in only 1 h (Table 1, entry 5, Fig. 3a, b). The i-PP DCM extract GC/MS chromatogram has greater complexity than that of PE since the product alkanes are not only linear and have significant ethyland methyl-branching, reflecting the different PP chain cleavage points (Fig. 3c). The DCM extract number average molecular mass (M n ) also falls with increasing conversion, and the distribution narrows as for PE (Fig. 3c). Stirring PECO (M n = 7.0 kg mol −1 , 2.5% 1-octene incorporation) reaction mixtures was challenged by the substantial melt viscosity. Nevertheless, hydrogenolysis proceeds rapidly to afford 85% volatile and 15% DCM soluble/low-M w alkane yields within 1 h (Fig. 3b). The 1 and 2 h GC/MS data are similar, probably reflecting H 2 starvation and yielding a most probable chain length of~15 carbons (Fig. 3d). Lowlevel alkane branching (~1%) is present, probably from the enchained 1-octene comonomer. Comparable hydrogenolysis rates with postconsumer HDPE (M n = 9.9 kg mol −1 ) are achieved at 200°C/2.0 atm H 2 /

Reaction mechanism
Early transition metal d 0 reaction pathways differ distinctly from those of most later metal systems and frequently involve combinations of four-center σ-bond metathesis and/or C = C/X = X insertion/extrusion. For the challenging cleavage of polyolefin C-C bonds, as mediated by the present very unusual surface catalysts, two turnover-limiting scenarios were examined: (1) σ-bond metathesis 26 (1) and (2) using n-dodecane as a model, in Fig. 6a, b, respectively 19,20 . As for catalyst choice, note that a multitude of AlS/ZrH(R) species (R = alkyl or H) of similar energies are doubtless rapidly equilibrating via C-H σ-bond metathesis processes (Fig. 6c). AlS/ZrH 2 was selected for simplicity; however, similar pathways are conceivable for other AlS/ZrH(R) species. For C-C scission via σ-bond metathesis (Fig. 6a), this pathway has a remarkably prohibitive 76 kcal/mol computed barrier. Indeed, an experiment with a 1:1 ratio of ethane and H 2 (1 atm total pressure) over a relatively high AlS/ZrH(Np) loading (0.9% mol Zr) reveals negligible hydrogenolysis at 150°C/1 h (Eq. (4)).
In contrast, a Zr-sec-dodecyl complex is readily accessed via C-H σ-bond activation/metathesis of n-dodecane (H 2 elimination step in Fig. 6b) and is the lowest energy intermediate found on the reaction coordinate (catalyst resting state), in agreement with the experimental zero-order rate law dependence on alkane concentration. Primary or secondary C-H activation is comparably exergonic with ΔG ≈ −5 kcal/mol. Secondary activation on a polyolefin chain seems statistically more probable, with the intermediate then undergoing intramolecular ß-alkyl transfer, yielding a Zr-alkyl and olefin ( Fig. 6b; ΔG = +14.5 kcal/mol). Here ΔG ‡ = 26.1 kcal/mol barrier and is ratelimiting for chain shortening. In principle, this process is reversible, and the olefin can reinsert, explaining the product composition of 1% methyl branches. The next step in this sequence is Zr-C bond hydrogenolysis with a computed 11.0 kcal/mol barrier to yield a Zr dihydride. This step is slightly endergonic (ΔG = 0.6 kcal/mol) and yields shorter alkanes. Note that the barrier for Zr-C bond hydrogenolysis is slightly lower than the ß-alkyl transfer step and therefore is not expected to be rate-limiting, in agreement with the experimental zero-order rate law dependence on H 2 concentration. Experimentally, alkenes are not detected at any stage in the reaction, and the DFT modeling shows that any alkene produced from ß-alkyl transfer rapidly inserts into a Zr-H bond in a barrierless, strongly exergonic step (ΔG = −24.2 kcal/mol), and the product is then hydrogenolyzed. Therefore, overall alkane hydrogenolysis is computed to be exergonic by ΔG = −14.3 kcal/mol. While this reverse of single-site polymerization producing smaller alkenes from a longer polyalkane chain is endergonic, the coupled olefin hydrogenation renders the overall alkane, and by inference, polyethylene deconstruction, decidedly exergonic. Note that identifying AlS/ZrH(Np)catalyzed β-alkyl transfer as the key turnover-limiting step is consistent with the experiment and has implications for other polymers and other electrophilic transition metal catalysts.
In conclusion, an earth-abundant single-site formally cationic Zr-alkyl/hydride catalyst was synthesized on highly Brønsted acidic sulfated alumina and characterized by solid-state 1   consumer polyethylene are rapidly hydrogenolyzed to low molecular mass hydrocarbons under mild conditions (150-200°C/2 atm H 2 ) at low catalyst loadings (0.02-0.04 mol% Zr). Experimental results combined with DFT computation reveal that the turnover-limiting step in alkane/polyolefin C-C scission/chain shortening is intramolecular ß-alkyl transfer, in contrast to the σ-bond metathesis process common in much early transition metal catalytic chemistry. The catalytic species, a supported electrophilic Zr-hydride, is readily formed during the hydrogenolysis process. These results convey implications for deconstructing other polymers and inventing new catalysts to achieve this.

Methods
All procedures for air-and moisture-sensitive compounds were carried out with rigorous exclusion of O 2 and moisture in flameor oven-dried Schlenk-type glassware interfaced to a high-vacuum (10 −5 -10 −6 Torr) line or in an argon-filled M-Braun glovebox with a high capacity recirculator (<1 ppm O 2 ). Argon used on high-vacuum lines (Airgas, UHP-grade) was purified by passage through MnO/vermiculite and activated Davidson 4 Å molecular sieve columns. All were solvents dispensed from activated alumina/CuO columns prior to use. n-pentane (Sigma) was further purified by drying over Na/K alloy followed by passage through a fiberglass filter in an argon glovebox. Aluminum oxide was purchased from Nanostructured and Amorphous Materials (gamma, nanopowder 20-30 nm). Sulfuric acid (98%) was purchased from Fisher. n-hexadecane (C16) was purchased from Sigma and was purified by heating at 120°C over Na for 48 h, followed by degassing at room temperature, and was further purified by passage through a 0.22 µm PTFE syringe filter immediately prior to use. All components containing plastic directly contacting C16 prior to hydrogenolysis experiments (i.e., syringes, syringe filters, needles, Teflon reactor caps) were pumped in the glove box inlet chamber overnight prior to use. Oxygen (UHP grade) used for calcination was purchased from Airgas and used without further purification. Deuterium (Sigma) and hydrogen (Airgas, UHP) were purified by passage through an oxygen/ moisture trap (Matheson, model MTRP-0042-XX). Zirconium(IV) chloride and neopentylmagnesium chloride (1.0 M solution in Et 2 O) were purchased from Sigma and used without further purification. Tetrakis(neopentyl)zirconium (ZrNp 4 ) was synthesized according to a modification of a literature procedure 43 . In a typical synthesis, neopentlymagnesium chloride (4.4 mmol) was added dropwise to a suspension of ZrCl 4 (1.0 mmol) in ether at −78°C. The mixture was allowed to stir for 2 h, slowly warmed to room temperature, then stirred for an additional 2 h at room temperature. The solvent was removed in vacuo, and the resulting solids were extracted with pentane. The pentane filtrate was isolated, and the solvent was removed in vacuo to yield a colorless solid. The crude product was purified by sublimation at 70°C and~10 −6 Torr, yielding a colorless microcrystalline solid with 83% yield. Polymers were dried in the melt (130-165°C) under a high vacuum for 48 h before use in the hydrogenolysis reactions.

Physical and analytical measurements
Inductively coupled plasma (ICP) analysis was performed by Galbraith Laboratories Inc., Knoxville, TN, USA. 1 H (500 MHz) and 13 C (125 MHz) NMR spectra of hydrogenolysis products were obtained with a Bruker Avance III system equipped with a DCH Cryoprobe. 1 H MAS (400 MHz) and 13 C CP-MAS (100 MHz) Solid State NMR measurements were obtained with a Bruker Avance III system equipped with a 4 mm Bruker HX probe. The rotor was charged with the sample in an Ar glove box. The rotor speed was set to 14 kHz for all spectra. Gas chromatography/ mass spectrometry (GC/MS) analysis of hydrogenolysis product mixtures was carried out on an Agilent GCMSD equipped with a DB5 column (oven program: 1. 2 min. 50°C hold 2. 30°C/min ramp 3. 2 min hold at 300°C). Split mode injection at 2 µL/injection and a 100:1 split ratio was used. For GC-MS quantification of n-hexadecane, a fourpoint calibration (0.1, 0.2, 0.3, 0.4 mg/mL) was carried out for each group of samples analyzed by GC/MS with a target sample concentration of~0.2 mg/mL. Calibration standards were prepared in 1,1,2,2-tetrachloroethane (TCE) and stored in airtight Teflon-valved glassware. Diffuse reflectance infrared spectroscopy (DRIFTS) measurements were taken under Ar on Thermo 6700 infrared spectrometer equipped with a Harrick Praying Mantis DRIFTS attachment. ZnS windows were used for the DRIFTS cell. Anhydrous KBr with an Ar glovebox atmosphere in the cell was used as a background. BET surface area measurements were carried out with a Micromeritics 3Flex Surface Characterization Analyzer.
X-ray absorption spectroscopy X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements at Zr K-edge (17,998 eV) were performed at the 5 BM-D beamline of the DND-CAT at the Advanced Photon Source. A double Si (111) monochromator was used for energy selection with an energy resolution of ΔE/E = 1.4 × 10 −4 . The X-ray energy was calibrated using a metallic Zr foil. The incident X-ray intensity was measured by a spectroscopygrade ionization chamber (FMB-Oxford) filled with 600 He/100 N 2 (Torr) and was detuned to 60% of its maximum for harmonic rejection. EXAFS spectra were collected in fluorescence mode using a passivated implanted planar silicon (PIPS) detector (Canberra). The sample and the detector were positioned 45°and 90°, respectively, to the X-ray beam direction. Energy scans were executed from 250 eV below to 550 eV above the Zr K edge, which produces the EXAFS spectra. The catalyst AlS/ZrNp 2 with a Zr loading of 1.40 wt% was pressed into a sample holder for the EXAFS measurements. The samples were sealed airtight in a THMS600 Linkam cell inside the glove box, which was pressured with ultrapure Ar gas. Positive Ar pressure was maintained throughout the measurement. After measuring the AlS/ZrNp 2 sample, in-situ catalyst hydrogenolysis was carried out by flowing H 2 gas with a flow rate of 50 sccm at room temperature with EXAFS data collected every 15 min for 2 h until no further changes were observed. This was followed by heating the sample to 150°C with continued H 2 flow. Data were collected every 15 min for up to 2 h. No changes were observed after this step of the treatment. There was a change in the structure during the hydrogenolysis reaction at room temperature, and these data are shown in the manuscript (Fig. 5e). XANES data extraction, normalization, and background subtraction was performed using Demeter:Athena. EXAFS data analysis was carried out using the software Demeter:-Artemis. The bond lengths (R) and coordination number (N) were obtained by a least-square fit in the R-space of the nearest neighbors using k 2 -weighted Fourier transform fitting parameter. For data analyses, a standard reference model compound was used: powder 2,6-t Bu 2 PhOZrBn 3 , which was measured in fluorescence mode.

Computational details
DFT-based simulations were performed with the/Quickstep package, using a hybrid Gaussian and plane wave method 44 . A double-quality DZVP Gaussian basis set was employed for the Al and Zr atoms, and a triple-quality TZVP Gaussian basis set was employed for all the other atoms 45 . The Goedecker-Teter-Hutter pseudopotentials 46 together with a 400 Ry plane wave cutoff were used to expand the densities obtained with the Perdew-Burke-Ernzerhof (PBE) 47 exchangecorrelation density functional, and vdW forces are taken in account with the Grimme D3 Method 48 . Only the gamma point was considered in a supercell approach. Periodic boundary conditions are applied in all directions of space. Molecular graphics were produced by the CHEMCRAFT graphical package 49 . Enthalpic and entropic contributions along the reaction pathway were evaluated by performing the frequency calculation of the molecular species at 298.15 K and 1 atm as implemented in the G16 code 50 . In this context, adsorbed catalysts were modeled by simple molecular species, and only the entropic contribution related to vibrational motion is considered. G16 calculations were performed at the level of the B3LYP hybrid functional. The standard all-electron 6-311G** basis set was used for all atoms. The enthalpic and entropic contributions were then "appended" to the SCF energy profile to obtain the Gibbs free energy profile.

General hexadecane hydrogenolysis procedure
In the glovebox, C16 (typically 0.50 g) was passed through a 0.

General polyolefin hydrogenolysis procedure
In the glovebox, AlS/ZrNp 2 and the desired amount of polymer were loaded into a dry heavy-walled glass pressure reactor (350 mL volume) containing a 10 mm ovoid stir bar, typically with~15% mass loading of catalyst (0.02-0.04 mol% Zr). The easily implemented and versatile hydrogenolysis reactor and analytical procedures serve four functions: (i) for constant polymer and catalyst, to screen products as a function of reaction conditions such as temperature, H 2 pressure, reaction time, and stirring rate. (ii) For constant polymer and reaction conditions, to screen different catalysts in terms of relative activity and product mix selectivity. (iii) For constant catalyst and reaction conditions, to screen different polymers in terms of relative activity and product mix selectivity. (iv) The optically transparent heavy-wall glass reactor can be filled in the glove box, is high-vacuum line compatible, and allows ready visualization of the reaction progress (solid → liquid → gas) as well as stirrer malfunction or possible coke formation (never observed). The polymers were prepared by shaving from a larger puck of pre-melted stock. The reactor was sealed with a threaded Teflon cap having an installed NPT valve. The vessel was carefully removed from the glovebox and interfaced with a high-pressure/high-vacuum line. The reactor was degassed at room temperature (30 s), then charged to 2 atm H 2 . The reactor was then placed in an oil bath set to the desired temperature (polyethylene 150°C, polypropylene, polyethylene-co−1octene, HDPE food container cap 190 or 200°C). Once the polymer melted and contacted the catalyst in the melt, the time interval was started. Stirring was typically set to 300 rpm initially, then increased tõ 800 rpm after a sufficient decrease in polymer viscosity. At the end of the time interval, the reactor was removed from the oil bath, then aircooled to room temperature. Headspace samples were taken at this point. Headspace samples were collected by the expansion of the reactor contents into an evacuated 500 mL Teflon-valved glass bulb. Individual samples for analysis were taken via septum and gastight headspace syringe (1 mL). The reactor was vented through the NPT valve and opened to air. Approx. 5 mL DCM was used to wash the Teflon cap and the interior of the NPT valve. These washings were added to the reactor. Solids were suspended in the DCM washings and then filtered to isolate the solids. The reactor was washed enough times to remove all residue. The solids were washed~3× with 5 mL portions of DCM, then dried at~1 Torr overnight. This product was assigned the "solids fraction". GPC analysis was carried out for this product fraction. The DCM washings were collected, and the DCM was removed under reduced pressure overnight (~1 Torr). These DCMsoluble hydrocarbons are assigned as "DCM extract". Note that some volatile liquid hydrocarbons (C5-C7) are lost during DCM removal, however, we find that <5 wt% of the total product mass is lost. Approximately 10 mg of the DCM extract fraction was added to~1 mL DCM for GC/MS analysis. Unaccounted-for mass is assigned to the "volatiles" fraction, and the presence of light hydrocarbons is confirmed via GC-FID. The percent conversion of a polyolefin hydrogenolysis reaction is defined as the mass of "volatiles" and "DCM extract" produced as a percentage of the initial polyolefin mass. Uncertainty in polymer hydrogenolysis products mass was estimated to be 20 mg (1-2% of initial polymer mass) and was carried over into calculating polymer hydrogenolysis activities.
Synthesis of AlS/ZrH(Np), AlS/ZrD(Np) and AlS/Zr(alkyl) 2 (Pentane-treated AlS/ZrH(Np)) In the glovebox, AlS/ZrNp 2 (200 mg) was added to a dry 75 mL heavywalled glass pressure reactor. The reactor was sealed, interfaced to a high pressure/high vacuum line, evacuated, then charged with 1 atm H 2 . The reactor was heated to 150°C for 5 min, then evacuated. A color change from pale yellow to colorless was observed. This cycle was repeated once more. The solid was then used as needed for further reactions or measurements. AlS/ZrD(Np) was synthesized in an analogous fashion as AlS/ZrH(Np), using D 2 in place of H 2 . To synthesize AlS/Zr(alkyl) 2

Data availability
Computational data and code used in this study have been deposited in the iochem-BD database under the following https://doi.org/10. 19061/iochem-bd-6-182. All additional information is available in the supplemetary materials.
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