40 Search Results

Due to the plastic waste crisis, selective chemical upcycling of polyolefins into value-added products is a topic of intense interest, demanding polymer deconstruction processes that afford control over the product chain lengths. Recently, a catalytic architecture was synthesized in which a polyolefin melt infiltrates a porous support, and its chains are cleaved by a metal nanoparticle catalyst at the bottom of the pores, yielding a narrow distribution of alkane products. Although the influence of various parameters of these catalytic materials, including the effects of the nanoparticle size and pore diameter on product chain length, has been examined before, here, wemore » investigate the role of the pore length in the cleavage process through the first study that combines catalytic hydrogenolysis and coarse-grained modeling to gain insights not available by experiment alone. We show that the pore length can permit control over the average product length with qualitative agreement between experiment and simulation. In conclusion, we go beyond this observation to uncover the dynamic phenomenon responsible for the pore-length dependence of the cleavage products.« less

Catalytic hydrogenolysis of polyolefins into valuable liquid, oil, or wax-like hydrocarbon chains for second-life applications is typically accompanied by the hydrogen-wasting co-formation of low value volatiles, notably methane, that increase greenhouse gas emissions. Catalytic sites confined at the bottom of mesoporous wells, under conditions in which the pore exerts the greatest influence over the mechanism, are capable of producing less gases than unconfined sites. A new architecture was designed to emphasize this pore effect, with the active platinum nanoparticles embedded between linear, hexagonal mesoporous silica and gyroidal cubic MCM-48 silica (mSiO₂/Pt/MCM-48). This catalyst deconstructs polyolefins selectively into ~C₂₀–C₄₀ paraffins andmore » cleaves C–C bonds at a rate (TOF = 4.2 ± 0.3 s^–1) exceeding that of materials lacking these combined features while generating negligible volatile side products including methane. The time-independent product distribution is consistent with a processive mechanism for polymer deconstruction. In contrast to time- and polymer length-dependent products obtained from non-porous catalysts, mSiO₂/Pt/MCM-48 yields a C₂₈-centered Gaussian distribution of waxy hydrocarbons from polyolefins of varying molecular weight, composition, and physical properties, including low-density polyethylene, isotactic polypropylene, ultrahigh-molecular-weight polyethylene, and mixtures of multiple, post-industrial polyolefins. Here, coarse-grained simulation reveals that the porous-core architecture enables the paraffins to diffuse away from the active platinum site, preventing secondary reactions that produce gases.« less

Six new solvent-free, homoleptic paramagnetic tris(alkyl)lanthanides Ln{C(SiHMe₂)₃}₃ (1Ln) and Ln{C(SiHMe₂)₂Ph}₃ (2Ln) (Ln = Gd, Dy, and Er) were synthesized to investigate the magnetic properties of 4f organometallic compounds stabilized by secondary Ln←H–Si and benzylic interactions. The unit cell of 1Gd contains one independent molecule (Z = 2), while 1Dy and 1Er crystallize with four independent isostructural molecules per unit cell (Z = 16). In all molecules, as in other 1Ln compounds, the three tris(dimethylsilyl)methyl ligands form a trigonal planar LnC₃ core, and six secondary interactions involving Ln←H–Si bonding in Ln{C(SiHMe₂)₃}₃ form above and below the equatorial plane. Two and fivemore » crystallographically independent molecules of each 2Ln (2Gd, Z = 8; 2Dy, Z = 20) form with three π-coordinated phenyl groups in addition to either one or two secondary Ln←H–Si interactions per molecule. The packing of these midseries organolanthanide compounds contrasts the single crystallographically unique molecules in previously reported La{C(SiHMe₂)₃}₃ (1La, Z = 2, Z' = 1) and La{C(SiHMe₂)₂Ph}₃ (2La, Z = 2, Z' = 1/3). 2La doped with 2Dy can adopt the crystallographic structure of 2La, which promotes magnetic properties, namely a higher χ_mT value at low temperatures as well as stronger magnetic anisotropy. The ac susceptibility data for 10% 2Dy doped into 2La suggests slow relaxation at low temperatures with a relaxation barrier of ~45 K. The computed saturated magnetization of 1Er (M ≈ 4.5 μ_B) and 1Dy (M ≈ 6 μ_B) matches the experimental values, while the computed value for 2Dy better matches the value measured for 2Dy diluted in 2La (M ≈ 5 μ_B). Gas-phase calculations predict that the ground-state and first excited-state multiplet separations are larger for 1Er than 2Er, while the ordering for dysprosium is 1Dy > 2Dy.« less

Carbon–carbon bond cleavage reactions, adapted to deconstruct aliphatic hydrocarbon polymers and recover the intrinsic energy and carbon value in plastic waste, have typically been catalysed by metal nanoparticles or air-sensitive organometallics. Metal oxides that serve as supports for these catalysts are typically considered to be inert. Here we show that Earth-abundant, non-reducible zirconia catalyses the hydrogenolysis of polyolefins with activity rivalling that of precious metal nanoparticles. To harness this unusual reactivity, our catalytic architecture localizes ultrasmall amorphous zirconia nanoparticles between two fused platelets of mesoporous silica. Macromolecules translocate from bulk through radial mesopores to the highly active zirconia particles, wheremore » the chains undergo selective hydrogenolytic cleavage into a narrow, C18-centred distribution. Calculations indicated that C–H bond heterolysis across a Zr–O bond of a Zr(O)₂ adatom model for unsaturated surface sites gives a zirconium hydrocarbyl, which cleaves a C–C bond via β-alkyl elimination.« less

Pt/SrTiO₃ (Pt/STO), prepared on small scale by atomic layer deposition (ALD), is a capable heterogeneous catalyst for the selective hydrogenolysis of polyolefins to hydrocarbon oils, providing a promising approach for upcycling plastic waste. However, because deposition by ALD is costly and resource-intensive, a new synthesis of Pt/STO is needed to effectively scale catalyst production and pursue the commercialization of upcycling processes. To that effect, this work details a scalable deposition method for Pt/STO made by surface organometallic chemistry (SOMC) techniques using Pt(II) acetylacetonate or and trimethyl(methylcyclopentadienyl)platinum. The STO support was calcined (550 °C), treated with ozone (200 °C), and finallymore » steamed (200 °C) to afford a clean STO surface populated with only hydroxyl groups. Pt precursors were dissolved in toluene and deposited onto STO. After reduction at 300 °C, the STO support was decorated with 1.0–1.5 nm Pt nanoparticles. The size and loading of these nanoparticles were varied by employing a multi-cycle deposition and oxidation and/or reduction process designed to ALD techniques. In conclusion, these Pt/STO catalysts hydrogenolyzed isotactic polypropylene into liquid products (>95% yield) with average molecular weights of 200–300 Da (~25 carbon atoms) and narrow size distributions at 300 °C and 180 psi H₂.« less

Abstract The zeolite‐supported lanthanide La(BH ₄ ) _x ‐HY ₃₀ catalyzes C−H borylation of benzene with pinacolborane (HBpin), providing a complementary approach to precious, late transition metal‐catalyzed borylations. The reactive catalytic species are generated from La grafted at the Brønsted acid sites (BAS) in micropores of the zeolite, whereas silanoate‐ and aluminoate‐grafted sites are inactive under the reaction conditions. During typical catalytic borylations, conversion to phenyl pinacolborane (PhBpin) is zero‐order in HBpin concentration. A turnover number (TON) of 167 is accessed by capping external silanols, selectively grafting at BAS sites, and adding HBpin slowly to the reaction.

Abstract The zeolite‐supported lanthanide La(BH ₄ ) _x ‐HY ₃₀ catalyzes C−H borylation of benzene with pinacolborane (HBpin), providing a complementary approach to precious, late transition metal‐catalyzed borylations. The reactive catalytic species are generated from La grafted at the Brønsted acid sites (BAS) in micropores of the zeolite, whereas silanoate‐ and aluminoate‐grafted sites are inactive under the reaction conditions. During typical catalytic borylations, conversion to phenyl pinacolborane (PhBpin) is zero‐order in HBpin concentration. A turnover number (TON) of 167 is accessed by capping external silanols, selectively grafting at BAS sites, and adding HBpin slowly to the reaction.

Surface ligands control the electronic properties of supported Au nanoparticles and thereby regulate their catalytic activity for the selective aerobic oxidation of amines to amides.

The proligands C₅H₅CMe₂CHPhOx^R (R = Me₂, CHMe₂, and CMe₃) react with M(NMe₂)₄ (M = Ti, Zr, and Hf) to give monodeprotonated (Ox^RCHPhCMe₂C₅H₄)M(NMe₂)₃, doubly deprotonated (C₅H₄CMe₂CPhOx^R)M(NMe₂)₂, or a mixture of both. The observed products depend on reaction conditions, the oxazoline substituent, and the metal center, with 4-t-butyl-oxazoline or Ti giving only the monodeprotonated (Ox^RCHPhCMe₂C₅H₄)M(NMe₂)₃. Amine elimination from (Ox^RCHPhCMe₂C₅H₄)M(NMe₂)₃ to give (C₅H₄CMe₂CPhOx^R)M(NMe₂)₂ is reversible for 4,4-dimethyl- and 4-isopropyl-oxazoline-based ligands in Zr or Hf complexes. Temperature-dependent kinetic studies of the equilibration of (C₅H₄CMe₂CPhOx^Me2)Zr(NMe₂)₂, HNMe₂ and (Ox^Me2CHPhCMe₂C₅H₄)Zr(NMe₂)₃ provide the experimental thermodynamic parameters ΔS° = -17.4 ± 2.6 cal·mol^–1K^–1 and ΔH° = -6.8 ± 0.8more » kcal·mol^–1. An Eyring plot of the rate constants, determined from the system as it approaches equilibrium, gives the activation entropy and activation enthalpy for the addition of (C₅H₄CMe₂CPhOx^Me2)Zr(NMe₂)₂ and HNMe₂ as -36 ± 4 cal·mol^–1 K^–1 and 8.9 ± 1.3 kcal·mol^–1, respectively; the elimination of HNMe₂ from (Ox^Me2CHPhCMe₂C₅H₄)Zr(NMe₂)₃ is characterized by ΔS^‡ = -19 ± 5 cal·mol^–1 K^–1 and ΔH^‡ = 15.7 ± 1.5 kcal·mol^–1. DFT computational models indicate a single-step, nonlinear transfer of the H between the benzylic position of the noninnocent, oxazoline-coordinated ligand and NMe₂. Computations also confirm the negative activation entropy and the trends in the barriers support the experimental results. Together, these studies indicate the importance of steric effects from the oxazoline ligand, metal center, ancillary ligands, and leaving group on the shuttling of the proton between HNMe₂ and the noninnocent ligand. Finally, these effects suggest that coordination of oxazoline to the metal center is a key part of the benzylic deprotonation and noninnocent behavior of the cyclopentadienyl-oxazoline ligand.« less