21 Search Results

Polyurethanes (PUs) are the sixth most commonly utilized plastic class, yet ∼80% of commodity material is landfilled or incinerated at the end of life. Disposal of thermosets is particularly problematic as cross-linking prevents the repurposing of disposed material. Thus, there is considerable interest in the development of PUs derived from inexpensive feedstocks that can be inherently chemically deconstructed. Ring opening metathesis polymerization (ROMP) of the naturally occurring sesquiterpene β-caryophyllene in the presence of dihydroxy chain terminators afforded the polyol hydroxy-terminated polycaryophyllene (HTPCR). Incorporation of HTPCR into PUs through reaction with polyisocyanates produced polymers with thermal and rheological properties comparable tomore » commodity materials. The feasibility of chemical degradation of both thermoplastic and thermoset materials was also demonstrated through ruthenium-mediated metathesis, utilizing the metathesis-active olefins within the repeat caryophyllene monomer unit. Overall, this work highlights the value of biorenewable, chemically reprocessable polysesquiterpenes in the PU space.« less

The molecular structure and activity of supported MoO_x olefin metathesis catalysts are heavily impacted by the choice of catalyst support. In this study, surface modification of the SiO₂ support with AlO_x and selective anchoring of the MoO_x on the surface AlO_x sites were used to tune the structure, activation, and reactivity of the resulting surface MoO_x sites. Extensive in situ molecular characterization, chemical probe studies, and density functional theory (DFT) calculations reveal that the enhanced activity of the supported MoO_x/AlO_x/SiO₂ catalyst over the MoO_x/ SiO₂ catalyst is associated with more favorable activation and kinetics of surface MoO_x anchored at AlO_xmore » sites.« less

Traditional chemical recycling approaches for condensation polymers suffer compounding energy losses and CO₂ emissions across multiple polymerization and depolymerization cycles. Entropic recycling can address these energy losses by entrapping free energy within the deconstruction products. Entropic recycling involves depolymerization to macrocyclic monomers, but such processes have not been feasible due to the high dilutions typically required to generate macrocyclic compounds. Here, we leverage selective catalysis to allow entropic recycling at concentrations 20–2000× higher than typical for macrocyclization reactions. We find that Ru-based olefin metathesis catalysts containing bulky iodine ligands significantly bias the ring–chain kinetic product distribution during ring-closing metathesis (RCM)more » toward the formation of oligomeric cycloalkenes. Further improvements in reaction concentration and macrocycle yield are obtained by using high catalyst loadings and by predisposing the alkene substrates to undergo favorable macrocyclization. These RCM optimizations translate effectively to cyclodepolymerization (CDP) of an olefin-containing polymer, with RCM and CDP affording similar macrocycle product distributions under identical reaction conditions. Macrocycle polymerization by entropy-driven ring-opening metathesis provides much higher molecular weight polymers than condensation polymerization of linear analogues, reducing the time to achieve high molecular weight from hours to minutes and enabling polymerization at room temperature. Finally, our findings re-emphasize the importance of energy consumption during a polymer’s lifecycle and provide a framework for the design of efficient entropic recycling systems.« less

Metathesis reactions can synthesize a semiconductor Zn ₃ WN ₄ from Li ₆ WN ₄ combined with a ZnX ₂ salt (where X = Br, Cl, F).

Abstract Dehydrogenation of alkanes is of increasing importance in fulfilling global demand for olefins and offers a potential source of carbon‐neutral hydrogen as a co‐product. Currently commercial dehydrogenation processes occur at high‐temperatures (500–900 °C) which is energy intensive and results in side reactions and rapid coking of the catalysts. In addition, the hydrogen produced is often burned to maintain temperature and to inhibit the back reaction. Here, pressure is utilized as a parameter to enable novel chemical catalytic processes, and ambient‐temperature dehydrogenation of alkanes by palladium is observed at 50–100 MPa, with both hydrogen gas and olefins recovered on decompression.more » This reaction follows a fundamentally different path to current commercial high‐temperature low‐pressure dehydrogenation processes with the palladium catalyst reversibly forming a hydride intermediate.« less

The first Blechert-type ruthenium complexes containing cyclic(alkyl)(amino)carbene (CAAC) ligands are reported. Furthermore, these catalysts demonstrate remarkable thermal stability in solution and excellent catalytic performances at low catalytic loading (up to 0.005 mol%) in ring-closing metathesis (RCM), macro-RCM, ring-closing enyne metathesis (RCEYM), cross-metathesis (CM), ethenolysis and ring-opening cross metathesis (ROCM). Moreover, up to 95% ee was obtained in asymmetric ring-opening cross metathesis (AROCM) and 57% ee asymmetric cross-metathesis (ACM)

A series of supported 3% MoOx catalysts were synthesized by incipient-wetness impregnation of 5%-15% TaO_x surface modified γ-Al₂O₃ support. The catalysts were characterized by in situ spectroscopies (DRIFTS, Raman, UV-vis, XAS) and multiple chemical probes (C₂H₄/C₄H₈ titration, C₃H₆-TPSR, steady state propylene metathesis, NH₃-IR adsorption). The supported tantalum oxide phase was present as surface TaO_x sites on the γ-Al₂O₃ support that capped that Al₂O₃ surface hydroxyls. The change in available surface hydroxyls caused the subsequent anchoring of MoOx species to occur at different surface hydroxyls. This shifted the anchoring of MoO_x species from basic (Al-OH) to neutral (Al₂-OH) to more acidicmore » (Al₃-OH) surface hydroxyls as well as perturbation of the remaining alumina surface hydroxyls by the surface TaO_x sites. The TaO_x surface modified γ-Al₂O₃ support increased the number of activated surface MoO_x sites (Ns) by ~6x and the TOF by ~10x resulting in an increased activity of ~60x. In conclusion, It was found that the specific anchoring surface hydroxyls rather than the extent of oligomerization of the surface MoO_x sites control the number of activated MoO_x sites and TOF for propylene metathesis. No relationship between the nature of the surface Lewis/Brønsted acid sites and Ns and TOF were found to be present.« less

A series of supported ReO_x catalysts were synthesized by incipient-wetness impregnation of perrhenic acid onto one component (Al₂O₃ and SiO₂) and surface modified mixed oxide supports (SiO₂/Al₂O₃, Al₂O₃/SiO₂, and ZSM-5 (Si/Al=15)), characterized with in situ molecular spectroscopy (Raman, DRIFTS, UV-vis and XAS) and chemically probed (ammonia chemisorption, C₂H₄/C₄H₈-titration, C₃H₆-TPSR and steady-state propylene self-metathesis). The initial dehydrated surface rhenia species were coordinated to the oxide supports as isolated Re⁷⁺O₄ sites. For the Al-containing supports, dioxo surface (O=)₂Re(-O)₂ sites appear to be the preferred coordination. The number of activated surface ReO_x sites during metathesis is determined by the oxide support ligands (3%ReO_x/ZSM-5more » > 3%ReO_x/5%AlO_x/SiO₂ > 3%ReO_x/5%SiO_x/Al₂O₃ > 3%ReO_x/Al₂O₃ ~ 3%ReO_x/SiO₂). The specific activity (TOF) is also controlled by the oxide support ligands (3%ReO_x/Al₂O₃ > 3%ReO_x/5%SiO_x/Al₂O₃ >> 3%ReO_x/ZSM-5 ~ 3%ReO_x/5%AlO_x/SiO₂ >> 3%ReO_x/SiO₂). The overall propylene metathesis activity (N × TOF), however is dominated by the number of activated sites (Ns). Consequently, the enhanced overall activity of surface ReO_x supported on SiO₂-Al₂O₃ mixed oxide supports is related to the greater number of activated surface ReO_x sites. Furthermore, the overall propylene metathesis activity was not related to the local surface ReO₄ molecular structure or strength of the Brønsted acid site since the same rhenia structures appeared to be present on all of the active catalysts and the strength of the Brønsted acid sites were comparable for all of the active catalysts, respectively.« less