Conversion of Polyolefin Waste to Liquid Alkanes with Ru-Based Catalysts under Mild Conditions

Abstract

<p>Chemical upcycling of waste polyolefins via hydrogenolysis offers unique opportunities for selective depolymerization compared to high temperature thermal deconstruction. Here, we demonstrate the hydrogenolysis of polyethylene into liquid alkanes under mild conditions using ruthenium nanoparticles sup-ported on carbon (Ru/C). Reactivity studies on a model <i>n</i>-octadecane substrate showed that Ru/C catalysts are highly active and se-lective for the hydrogenolysis of C(sp<sup>3</sup>)-C(sp<sup>3</sup>) bonds at temperatures ranging from 200-250°C. Under optimal conditions of 200°C in 20 bar H2, polyethylene (average Mw ~4,000) was converted into liquid <i>n</i>-alkanes with yields of up to 45% by mass after 16 h using a 5 wt% Ru/C catalyst, with the remaining products comprising light alkane gases (C1-C6). At 250°C, nearly stoichiometric yields of CH4 were obtained from polyethylene over the catalyst. The hy-drogenolysis of long chain, low-density polyethylene (LDPE) and a post-consumer LDPE plastic bottle to produce C7-C45 alkanes was also achieved over Ru/C, demonstrating the feasibility of this reac-tion for the valorization of realistic post-consumer plastic waste. By identifying Ru-based catalysts as a class of active materials for the hydrogenolysis of polyethene, this study opens new avenues for the valorization of plastic waste under mild conditions.<br></p>

Polyethylene Hydrogenolysis at Mild Conditions over Ruthenium on Tungstated Zirconia

“… 48 − 50 The proposed cascade mechanism (iv–vii) must be invoked in the decomposition to rationalize the disproportionally high methane yields in both LDPE and model hexane reactions consistent with recent contributions. 24 , 29 …”

“… 6 , 8 , 23 − 26 For example, the hydrogenolysis of polyethylene over an ordered mesoporous shell/active site/core, Pt/SiO 2 catalyst was showcased to demonstrate a processive mechanism. 27 Ruthenium-supported catalysts, e.g., Ru/C, 24 , 26 , 28 Ru/zeolites, 29 and Ru/CeO 2 , 23 possess higher activity than Pt 8 , 25 and are potentially promising materials. Unfortunately, Ru generates significant methane, which is an undesirable, low-value product.…”

“…Ultralow molecular weight plastics are commonly employed as substrates to enable high conversions at low temperatures. 23 , 24 While exciting initial results have been obtained, there is a clear need for more active and selective catalysts to enable practical implementation.…”

Breaking C–C Bonds via Electrochemically Mediated Hydrogen Atom Transfer Reactions

“…[1][2][3][4][5][6] However, the inertness of the Csp3-Csp3 linkages hinders the selective and energy-efficient bond scission in these substrates. [7][8][9] Indeed, Csp3-Csp3 bond activation is difficult due to several thermodynamic and kinetic constraints, [10][11][12][13][14] including high bond dissociation energies (BDEs) of ~90 kcal mol −1 , 10 steric inaccessibility caused by surrounding C-H bonds, and unfavorable orbital directionality towards cleavage which requires the rotation of two carbon sp 3 orbitals. 15 In some cases, C-C bond scission is facile, for example, for three-or four-membered cycloalkanes with high ring strain, for structures that introduce aromaticity upon cleavage (e.g., the elimination of a methyl group of ergosterol converts its cyclohexadiene ring into a phenyl ring), or for reactants that coordinate strongly to an active site (e.g., pincer-type compounds chelate to the catalyst metal center to direct the C-C bond cleavage).…”

“…7 Reductive catalytic depolymerization strategies, such as hydrogenolysis and metathesis, improve the energy efficiency by ameliorating reactions conditions (temperatures > 200°C), but require reductants such as high-pressure H2 and/or high-cost noble metal catalysts. 8,[16][17][18][19] Oxidative C-C bond cleavage generates oxygenated products as valuable chemicals, 3 but the traditional approach requires bromine as a co-catalyst, 20 impacting on the environment. These challenges together underscore the need to develop new strategies to selectively depolymerize plastic waste.…”