Catalytic Decomposition of Long-Chain Olefins to Propylene via Isomerization-Metathesis Using Latent Bicyclic (Alkyl)(Amino)Carbene-Ruthenium Olefin Metathesis 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 mol_propylene mol_catalyst ^-1 h^-1 , respectively.

Keywords: BICAAC; Hydrocarbon Decomposition; ISOMET; Metathesis; Propylene; Ruthenium.

Scheme 1

Tentative procedure for propylene production from high‐molecular‐weight mono‐olefins.

Scheme 2

Representative examples for carbene ligands used for homogeneous catalysis.

Scheme 3

Representative examples for ruthenium‐based olefin metathesis catalysts (PCy₃=tricyclohexylphosphine; Mes=mesityl).

Scheme 4

A representative example for the synthesis of BICAAC carbene precursor salts 10–13. Conditions: (i) DCM, room temperature, molecular sieves (3 Å); (ii) 2 equiv LDA, dry THF, 2 equiv methyl iodide, 0 °C, (iii) 3–6 equiv HCl in dioxane (3 M), 80 °C, NH₄BF₄.

Scheme 5

Synthesis of BICAAC ruthenium carbene complexes. Conditions: (i) 1.5 equiv of carbene precursors 10–13, 1.5 equiv LiHMDS, THF, 25 °C; (ii) 1.1 equiv of MeOTf, DCM, −30 °C; (iii) 3.0 equiv of carbene precursors 10 and 11, 3.3 equiv LiHMDS, THF, 25 °C.

Figure 1

X‐ray crystal structures of complexes 5, 14, 18 and 20.

Scheme 6

Ring‐closing metathesis (RCM) model reaction using catalyst 14, 18–20.

Figure 2

Comparison of the latent activity of BICAAC−Ru complexes. Mono‐carbene complexes: 14 (red), 18 (green), 19 (blue), bis‐carbene complex: 20 (orange). 50 °C for 60 min, then 75 °C for 300 min; toluene‐d₈; [21]=0.20 M; catalyst loading: 0.25 mol %.

Scheme 7

ISOMET of methyl oleate (23), purity 96.5 %; 75 °C; toluene; 24 h; 2 mol % RuH; 1 mol % HG2/1–3/14; [23]=0.25 M; p _Et(3.0)=10 bar, ethylene purity 99.9 %.

Scheme 8

ISOMET of 1‐octadecene (25). 75 °C; toluene; [25]=1.95 M; p _Et=10 bar; 24 h; [RuH]=200 ppm; ethylene purity 99.9 %.