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

Abstract

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.

Highly Efficient Polyolefin Hydrogenolysis Enabled by a Single-Site Organo-Tantalum Catalyst on a Super-Acidic Support

“…The high-resolution roomtemperature spectrum reveals two signals at δ 1.0 and 1.3 ppm (Figure S24), both assignable to multiple Ta-hydride/alkyl species generated during hydrogenolysis of the Ta-Np ligands, similar to AlS/ZrNp2. [12][13] Note that the Ta-H moiety is not observable in the room temperature 1 H MAS NMR spectrum, most likely due to the accelerated decoherence with the increased mobility of the complex molecule at the higher temperature. [23] Ta-H formation is further confirmed by the DRIFTS observation of a new feature at 1849 cm -1 , characteristic of a terminal νTa-H mode (Figure 2D).…”

“…Considering that surface organo-Ta complexes alkane metathesis catalysts, [9, 16, 19b] we propose that a Ta-H intermediate catalyzes σ-bond metathesis via a concerted four-centered transition state yielding a Ta-Me bond and methane (Figure 3B, path a). In contrast, the analogous AlS/ZrNp2-derived catalyst does not mediate ethane hydrogenolysis since the σ-bond metathesis barrier is prohibitively high, and only β alkyl transfer is operative for larger alkanes [13] . In the next step, as seen in the DRIFTS and SSNMR data in Figure 2, the Ta-Me moiety readily reacts with another H2 molecule to regenerate the Ta-H intermediate and methane.…”

“…of volatiles + DCM extract)•(mol Ta) -1 •(h) -1 , respectively (Table 3, entries 1 and 2) --~3x faster than for the previously reported AlS/ZrNp2 catalyst. [13] The next challenge was to determine whether "real-world" post-consumer plastics could be hydrogenolyzed with catalyst 1. Interestingly, when a milk jug, sandwich bag, or water bottle cap were subjected to hydrogenolysis in the glass reactor, low conversions of 35-53% were obtained at 200˚C after 21h, possibly reflecting mass transfer effects due to mixing difficulties (Figure S4).…”

“…This laboratory recently reported that ZrNp4 chemisorption on Brønsted acidic sulfated alumina (AlS) [12] yields a highly active cationic electrophilic Zr polyolefin hydrogenolysis catalyst (AlS/ZrNp2) having ≥ 10 2 x the activity of silica-alumina /ZrNp4 (Figure 1A). [10b], [13], [14] Experimental data and DFT computation argue that the rate-determining step in C-C bond hydrogenolysis is a β-alkyl transfer. Despite the high activity of this Zr catalyst, challenges such as limited thermal stability and viscous polymer mass transfer limitations remain.…”

Polypropylene Degradation Catalyzed by Tantalum Hydrides Supported on Sulfated Alumina

“…The low mass balances obtained from hydrogenolysis reactions at elevated pressures indicate that 1 favors the formation of light gases over the cleavage of iPP under these conditions. The origin of this observation is not clear, but this reactivity pattern is common in hydrogenolysis reactions of polymers with well-defined metal hydrides supported on oxides. , If Ta–R + intermediates are resting states, either those formed after C–H bond activation or those formed after chain cleavage, successive β-alkyl elimination to form low-molecular-weight products from a single iPP chain is plausible. Similar processive type mechanisms were proposed in hydrogenolysis reactions of PE .…”