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Handbook Of Metathesis

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) have led to numerous discoveries in the field of ruthenium olefin metathesis, until then largely dominated by the well-known N-heterocyclic...

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Cyclic (Alkyl)(amino)carbenes (CAACs) in Ruthenium Olefin Metathesis

The conversion of polyolefins to monomers would create a valuable carbon feedstock from the largest fraction of waste plastic. However, breakdown of the main chains in these polymers requires the cleavage of carbon–carbon bonds that tend to resist selective chemical transformations. Here, we report the production of propylene by partial dehydrogenation of polyethylene and tandem isomerizing ethenolysis of the desaturated chain. Dehydrogenation of high-density polyethylene with either an iridium-pincer complex or platinum/zinc supported on silica as catalysts yielded dehydrogenated material containing up to 3.2% internal olefins; the combination of a second-generation Hoveyda-Grubbs metathesis catalyst and [PdP(tBu)3(μ-Br)]2 as an isomerization catalyst selectively degraded this unsaturated polymer to propylene in yields exceeding 80%. These results show promise for the application of mild catalysis to deconstruct otherwise stable polyolefins. Description Making propylene from polyethylene Polyolefins are the most abundantly produced plastics but are unfortunately also among the hardest to break back down into their building blocks. Conk et al. developed a preliminary method to produce propylene from a sequence of reactions between waste polyethylene and fresh ethylene. Specifically, the process involves a small extent of desaturation of each polyethylene chain by a dehydrogenation catalyst, followed by steady breakdown of the chains into propylene through catalytic isomerization and metathesis reactions with ethylene. Subject to cost and process optimization, the method shows promise for generating a valuable feedstock chemical from waste plastic. —JSY A catalytic desaturation-isomerization-metathesis sequence produces propylene from polyethylene and added ethylene.

Catalytic deconstruction of waste polyethylene with ethylene to form propylene

Although polyethylene (PE) and polypropylene (PP) are by far the world's largest volume plastics, only a tiny fraction of these energy-rich polyolefins are currently recycled. Depolymerization of PE to its constituent monomer, ethylene, is highly endothermic and conventionally accessible only through unselective, high-temperature pyrolysis. Here, we provide experimental demonstrations of our recently proposed tandem catalysis strategy, which uses ethylene to convert PE to propylene, the commodity monomer used to make PP. The approach combines rapid olefin metathesis with rate-limiting isomerization. Monounsaturated PE is progressively disassembled at modest temperatures via many consecutive ethenolysis events, resulting selectively in propylene. Fully saturated PE can be converted to unsaturated PE starting with a single transfer dehydrogenation to ethylene, which produces a small amount of ethane (1 equiv per dehydrogenation event). These principles are demonstrated using both homogeneous and heterogeneous catalysts. While selectivity under batch conditions is limited at high conversion by the formation of an equilibrium mixture of olefins, high selectivity to propylene (≥94%) is achieved in a semicontinuous process due to the continuous removal of propylene from the reaction mixture.

Chemical Recycling of Polyethylene by Tandem Catalytic Conversion to Propylene.

This review is a critical presentation of catalysts based on palladium and ruthenium bearing N-heterocyclic carbene ligands that have enabled a more sustainable approach to catalysis and to catalyst uses. Aspects of sustainability associated with these in terms of catalytic uses or synthetic access are reviewed.

Sustainability in Ru- and Pd-based catalytic systems using N-heterocyclic carbenes as ligands.

New generations of Hoveyda and bis‐carbene type of ruthenium‐based olefin metathesis catalysts (10 and 12), containing cationic cyclic alkyl amino carbene (CAAC) ligands, have been synthetized. The catalysts show exceptional stability and activity in environmentally benign, protic media. Various olefin metatheses reactions of OH functionalized feedstock (e. g. RCM, ROMP CM) can be carried out at as low as 0.05 mol % catalyst loading in methanol, isopropanol, water or methanol/water solvent mixture, accomplishing the lowest applied catalyst loading reported so far in these media. The facile olefin metathesis of renewable feedstocks including phospholipids (23) and vegetable oils (20) in protic media has also been demonstrated.

Towards Sustainable Catalysis ‐ Highly Efficient Olefin Metathesis in Protic Media Using Phase Labelled Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Catalysts

Ammonia borane (AB) has received extensive attention in recent years as an emerging hydrogen storage material due to its high hydrogen density (19.6 wt %), nontoxicity, stability, and water solubil...

Highly Efficient Ammonia Borane Hydrolytic Dehydrogenation in Neat Water Using Phase-Labeled CAAC-Ru Catalysts

Abstract One of the most exciting scientific challenges today is the catalytic degradation of non‐biodegradable polymers into value‐added chemical feedstocks. The mild pyrolysis of polyolefins, including high‐density polyethylene (HDPE), results in pyrolysis oils containing long‐chain olefins as major products. In this paper, novel bicyclic (alkyl)(amino)carbene ruthenium (BICAAC−Ru) temperature‐activated latent olefin metathesis catalysts, which can be used for catalytic decomposition of long‐chain olefins to propylene are reported. These thermally stable catalysts show significantly higher selectivity to propylene at a reaction temperature of 75 °C compared to second generation Hoveyda–Grubbs or CAAC−Ru catalysts under ethenolysis conditions. The conversion of long‐chain olefins (e.g., 1‐octadecene or methyl oleate) to propylene via isomerization‐metathesis is performed by using a (RuHCl)(CO)(PPh3)3 isomerization co‐catalyst. The reactions can be carried out at a BICAAC−Ru catalyst loading as low as 1 ppm at elevated reaction temperature (75 °C). The observed turnover number and turnover frequency are as high as 55 000 and 10 000 molpropylene molcatalyst −1 h−1, respectively.

Catalytic Decomposition of Long‐Chain Olefins to Propylene via Isomerization‐Metathesis Using Latent Bicyclic (Alkyl)(Amino)Carbene‐Ruthenium Olefin Metathesis Catalysts

Márton Nagyházi

Papers

Towards Sustainable Catalysis ‐ Highly Efficient Olefin Metathesis in Protic Media Using Phase Labelled Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Catalysts

Highly Efficient Ammonia Borane Hydrolytic Dehydrogenation in Neat Water Using Phase-Labeled CAAC-Ru Catalysts

Catalytic Decomposition of Long‐Chain Olefins to Propylene via Isomerization‐Metathesis Using Latent Bicyclic (Alkyl)(Amino)Carbene‐Ruthenium Olefin Metathesis Catalysts

Cyclative MCRs of Azines and Azinium Salts

Synthesis and Characterization of Novel PEPPSI Type Bicyclic (Alkyl)(Amino)Carbene (BICAAC)-Pd Complexes