40 Search Results

Supported platinum nanoparticles are known to convert polyolefins to high-quality liquid hydrocarbons with hydrogen under relatively mild conditions. However, no systematic study has been undertaken using bimetallic catalysts for polyethylene upcycling. Specifically, a total of 98 monometallic and bimetallic combinations (Ag, Cr, Co, Cu, Fe, Ga, In, Mn, Ni, Pd, Pt, Rh, Ru, Zr) on alumina were synthesized utilizing surface organometallic chemistry (SOMC) technique via robotic platform. These were investigated at a small scale (10 mg of catalyst and 50 mg of polyethylene) for their activity for the hydrogenolysis of polyethylene in a high-throughput batch reactor. Combinations of Ni andmore » Co were selected as candidates with high activity toward conversion into paraffin oils. Reaction conditions were optimized with Ni/Co/Al₂O₃ catalyst at a larger scale (300 mg catalyst and 3 g polyethylene) to obtain a high yield (93.1%) of paraffin wax with desired properties (M_n = 380 Da) and low polydispersity (Đ = 1.2). Ni/Co/Al₂O₃ was compared against Co/Ni/Al₂O₃ to understand the role of the deposition sequence. When Co is deposited before Ni, a layer of cobalt aluminate is formed upon reduction, stabilizing the deposition of 5 nm metallic Ni particles. When nickel is deposited before Co, particles are larger (average >20 nm) and more oxidized (Ni^δ+ in NiAl₂O₄), decreasing the availability of the catalytically active metallic Ni. In conclusion, the difference in electronic environments was also described by DFT calculations, which revealed that smaller 3D clusters of Ni are preferred on CoAl2O4 over the 3D clusters on NiAl₂O₄ and that these smaller clusters are more reducible, as confirmed experimentally.« less

Surface grafted organozirconium catalyzes C─H/Et─Al exchange reactions, involving saturated hydrocarbons and AlEt₃, to afford organoaluminum compounds and ethane. The Zr(O^tBu)₃@SiO₂-Al₂O_3–700 (1) catalyst contains monopodal ≡SiO─Zr(O^tBu)₃ and only a few residual silanols (<5%). Nonetheless, these silanols are the Achille's heel of 1, providing a pathway for surface and catalyst degradation during catalysis, limiting the alkylaluminum yield and catalyst turnover. Support degradation, involving the cleavage of Si─O bonds by activated surface organometallics, is inhibited by capping silanols with ─SiMe₃. Residual silanols in 1 react with allyltrimethylsilane, as determined by solid-state ¹³C and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, andmore » reaction stoichiometry, to form Zr(O^tBu)₃/SiMe₃@SiO₂-Al₂O_3–700 (2), which is resistant to degradation by AlEt₃. C─H alumination of dodecane catalyzed by 2 produces higher yields of the 1-dodecylaluminum product in comparison to 1, and in >95% selectivity. Additionally, methane undergoes 2-catalyzed C─H alumination, providing a route to AlMe₃.« less

Here, the rotation frequencies of amido ligands are highly sensitive to the electronic structure of d⁰ transition metal complexes and have been used to study ligand donor properties. While attempting to study the donor properties of silanolate ligands in a silica-supported Cr complex, we observed highly restricted motions due to the added steric hindrance from the support, with only approximately half of the amides rotating on a 50 ms time scale. Surprisingly, when the same species is grafted into narrow 2.2 nm pores, all amido ligands are able to rotate. Density functional theory calculations suggest that confinement may limit themore » possible coordination sites and the configuration of the formed surface species, potentially enabling the formation of conformationally homogeneous surface site populations.« less

AlMe3-treatment of the borylation precatalyst La(BH₄)₂(THF)_2.5^-Ph₃Si-HY₃₀ affords La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀, which is the superior lanthanum-based precatalyst for benzene borylation with pinacolborane (HBpin), giving higher turnovers (>285) and improved yields (up to 42%) of phenylpinacolborane (PhBpin). Solid-state NMR spectroscopy, X-ray adsorption spectroscopy, and theoretical studies characterized the precatalytic sites in La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ as κ²-O,O-{≡SiO(=Al)≡SiO}La(BH₄)₂(AlMe₃), revealing that AlMe₃ had displaced the THF ligands. In contrast to the expected high reactivity of THF-free organolanthanum, the turnover frequency (TOF) for PhBpin formation catalyzed by La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ (2.7 h^–1) is slightly lower than that of untreated La(BH₄)₂(THF)_2.5^-Ph₃Si-HY₃₀ (3.6 h^–1), implying that AlMe₃ is a stronger inhibitor than THF formore » lanthanum. On the other hand, AlMe₃-treatment of inactive La(BH₄)₂(THF)_2.2^-SiO₂ generates an active benzene borylation catalyst. AlMe₃ also desorbs surface–O-B_xH_y species and quenches residual Brønsted acid sites (BAS) and silanols. Alumination of the BAS inhibits HBpin degradation, while alumination of silanols creates sites for that reaction. The 8-fold inhibition of the BAS-catalyzed HBpin decomposition rate by AlMe₃ treatment gives a kinetic advantage to the lanthanum-catalyzed C–H borylation, leading to increased yields and turnovers. Knowledge of the competing roles of sites in La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ and the catalytic rate law law enables identification of favorable conditions of low [HBpin] to maximize turnovers or PhBpin yield. Sterics affect the selectivity in borylation of substituted arenes and heteroarenes, which can proceed without the precoordination of a donor. These steric effects, as well as the AlMe₃ treatment having an opposite effect on the activity of lanthanum in HY vs SiO₂, point to confinement-activated sites.« less

Biomass-derived ethanol (EtOH) and acetaldehyde (AcH) conversion to 1,3-butadiene (1,3-BD) is an alternative process for 1,3-BD production. The present investigation reports the preparation and characterization of isolated La sites introduced into the silanol nests in DeAlBEA as well as detailed studies of the mechanism and kinetics for the conversion of an EtOH-AcH mixture to 1,3-BD. La sites supported on DeAlBEA are found to be present as (≡SiO)₂La-OH groups that are H-bonded with adjacent Si-OH groups, possessing high C-C coupling activity and stability, superior to state-of-the-art Y-DeAlBEA. La sites supported on silica (La-SiO₂) with a similar chemical structure but no H-bondingmore » interaction with Si-OH groups were prepared for comparison. Lewis acid La sites promote AcH aldol condensation, and the activity of such sites is nearly identical for both La-DeAlBEA and La-SiO₂. Further, the rate of C₄ product formation increases by a factor of 4.8 upon addition of EtOH to the feed of AcH over La-DeAlBEA, whereas that over La/SiO₂ remains unchanged. Investigation of the mechanism and kinetics of AcH aldol condensation and EtOH-AcH conversion to 1,3-BD revealed two C-C bond forming pathways-AcH aldol condensation by Lewis acid La sites and direct coupling of EtOH-AcH over H-bonded (≡SiO)₂La-OH···HO-Si≡ sites. This study provides important information about the role of the local environment of isolated Lewis acid sites and their effects on the direct coupling of EtOH and AcH to form 1,3-BD.« less

Solid-state NMR experiments were used to investigate the dynamics of supported complexes grafted to a series of silica gel materials of varied pore sizes. Through dipolar recoupling measurements, we found that ligand dynamics were impeded in the more confined environments, as would be expected. A new form of motion involving the complex as a whole, however, appeared in the most restricted environment consisting of 22 Å diameter pores. These motions persisted down to –100 °C at which point the ligands were frozen on the NMR timescale. The newly observed dynamics could only result from the breaking of secondary dative metal–siloxanemore » interactions that otherwise lock the complex in a preferred orientation on the surface. Crucially, these results show that confinement effects alone can be sufficient to reduce a grafted metal's effective coordination number in direct analogy to the synthesis of undercoordinated complexes using bulky ligands. Finally, this finding could have important implications in the synthesis of more active heterogeneous catalysts.« less

Scandium borohydride grafted into the micropores of the faujasite zeolite HY30 catalyzes the C–H borylation of benzene, whereas silica-grafted species are inactive. This catalytic activity may originate from grafting at a Brønsted acid site leading to a more electron-deficient rare earth center. Herein, we apply multinuclear double-resonance nuclear magnetic resonance (NMR) experiments to probe the structure and dynamics of zeolite- and silica-bound scandium borohydride complexes. The experiments reveal that scandium centers located within the zeolite micropores, in proximity to Al-created Brønsted sites, are more dynamic than rigid scandium sites grafted on silanols. Through a combination of NMR and molecular dynamicsmore » simulations, we show that the coordination of the scandium in the zeolite is labile, with the metal exchanging between two binding sites. As a result, the weak electron donation from the support that enables the movement of the Sc center leads to the formation of an undercoordinated metal center that cannot exist on silica, ultimately leading to the new catalytic activity of the species.« less

Electrification to reduce or eliminate greenhouse gas emissions is essential to mitigate climate change. However, a substantial portion of our manufacturing and transportation infrastructure will be difficult to electrify and/or will continue to use carbon as a key component, including areas in aviation, heavy-duty and marine transportation, and the chemical industry. In this Roadmap, we explore how multidisciplinary approaches will enable us to close the carbon cycle and create a circular economy by defossilizing these difficult-to-electrify areas and those that will continue to need carbon. Here, we discuss two approaches for this: developing carbon alternatives and improving our ability tomore » reuse carbon, enabled by separations. Furthermore, we posit that co-design and use-driven fundamental science are essential to reach aggressive greenhouse gas reduction targets.« less

Supported platinum nanoparticle catalysts are known to convert polyolefins to high-quality liquid hydrocarbons using hydrogen under relatively mild conditions. To date, few studies using platinum grafted onto various metal oxide (M_xO_y) supports have been undertaken to understand the role of the acidity of the oxide support in the carbon-carbon bond cleavage of polyethylene under consistent catalytic conditions. Specifically, two Pt/M_xO_y catalysts (M_xO_y = SrTiO₃ and SiO₂-Al₂O₃; Al = 3.0 wt %, target Pt loading 2 wt % Pt similar to 1.5 nm), under identical catalytic polyethylene hydrogenolysis conditions (T = 300 degree celsius, P(H₂) = 170 psi, t = 24more » h; M_w = similar to 3,800 g/mol, M_n = similar to 1,100 g/mol, D = 3.45, N_branch/100C = 1.0), yielded a narrow distribution of hydrocarbons with molecular weights in the range of lubricants (M_w = < 600 g/mol; M_n < 400 g/mol; D = 1.5). While Pt/SrTiO₃ formed saturated hydrocarbons with negligible branching, Pt/SiO₂-Al₂O₃ formed partially unsaturated hydrocarbons (<1 mol % alkenes and similar to 4 mol % alkyl aromatics) with increased branch density (N_branch/100C = 5.5). Further investigations suggest evidence for a competitive hydrocracking mechanism occurring alongside hydrogenolysis, stemming from the increased acidity of Pt/SiO₂-Al₂O₃ compared to Pt/SrTiO₃. Additionally, the products of these polymer deconstruction reactions were found to be independent of the polyethylene feedstock, allowing the potential to upcycle polyethylenes with various properties into a value-added product.« less

Deconstruction of polyolefins by catalytic hydrogenolysis is typically accompanied by the generation of undesired light gases. At reaction temperatures, the desired liquid products also tend to be volatile. Secondary cleavage of these liquid products contributes to light gas formation. The latter process was mitigated by reactive separation, continuously separating the liquid products from the catalyst throughout the experiment. At equivalent conversion, the yield and selectivity for oligomeric liquid species are increased under reactive separation, even though the carbon–carbon bond cleavage rate is slower than that in sealed experiments. More light gas is formed in the sealed reactor. Under 1 atmmore » H₂ partial pressure, alkenes accompany the typical alkane hydrogenolysis products. Further, the alkene yield is higher, with greater selectivity for valuable α-olefins under reactive separation. These results provide the mechanistic insight that terminal alkenes are primary products of carbon–carbon bond cleavage during hydrogenolysis under experimental conditions, and secondary deconstruction of these species produces light gases.« less