Metathesis, molecular redistribution of alkanes, and the chemical upgrading of low-density polyethylene

https://doi.org/10.1016/j.apcatb.2022.121873Get rights and content

Highlights

  • Supported tungsten oxide (WOx/SiO2) is shown to be an effective metathesis catalyst for the molecular redistribution of olefin/alkane feedstocks.

  • Addition of zeolite 4 A is essential to achieve high yields in olefin and alkane metathesis reactions.

  • Pretreatment temperature and chain length of alkane solvent have significant effects on catalytic conversion and selectivity.

  • Catalytic molecular redistribution of n-alkanes is a useful reaction in the chemical upgrading of polyethylene.

Abstract

A new strategy for polyethylene (PE) deconstruction via alkane metathesis is presented, in which WOx/SiO2 catalyzes the olefin metathesis reaction step. Zeolite 4 A is an essential component to protect the metathesis active sites from poisoning by in-situ-generated oxygenates in a batch reaction system. High conversion of 1-hexadecene (96%) and n-hexadecane (92%)—surrogates of long-chain molecules—demonstrates the high reactivity of WOx/SiO2 metathesis catalyst for olefin and alkane metathesis reactions, respectively, at moderate reaction temperatures of 300 °C for 2–3 h. Pretreatment temperature and length of the short n-alkane-chain solvent significantly affect the metathesis reactivity and selectivity. Results for the deconstruction of low-density PE (LDPE) in n-decane demonstrate a remarkable potential for PE upgrading with the advantages of short reaction times (3 h), the low mass ratio of solvent to LDPE, and the production of solid products with narrow molecular weight distributions.

Introduction

The negative environmental impact of plastic waste requires the urgent development of effective and economical plastic recycling and upgrading processes [1]. Polyethylene (PE) is the largest portion of the plastics waste stream and presents major challenges to chemical recycling due to the stability of the polymers’ C-C bonds [2], [3]. Numerous investigations utilizing various catalytic reactions for PE upgrading have been reported recently [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Catalytic alkane metathesis chemistry—comprising tandem (de)hydrogenation and olefin metathesis—has been explored as an alternative to direct deconstruction methods (e.g., pyrolysis or hydrocracking) due to its moderate operating temperature (~200 °C) and absence of reactive gases (i.e., H2) in the process, two factors favorable for the economic viability of a metathesis-based chemical recycling process [18], [19].

While alkane metathesis-based PE deconstruction has been demonstrated in the literature, the approach typically relies on a rhenium oxide catalyst for the olefin metathesis reaction [18], [19]. Rhenium oxide is expensive, cannot be applied at the high temperatures at which reaction kinetics are most favorable (due to the high volatility of surface rhenium oxide species), and can be complex to regenerate [18], [19], [20], [21], [22], [23], [24]. We sought to substitute this catalyst with a silica-supported tungsten oxide (WOx/SiO2), a relatively inexpensive material that addresses the adverse features of rhenium oxide (cost ratio of rhenium/tungsten = 106 (w/w) [25]). Olefin metathesis [20], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44] and alkane metathesis [45], [46] using WOx/SiO2 have been applied in plug-flow reactor systems. They are generally known as molecular weight redistribution processes. However, there have been no reports of the application of this catalyst for either olefin or alkane metathesis reactions in a batch reactor system or for PE deconstruction. A batch reactor may be preferable to a flow reactor for PE upgrading to address the challenges of converting mixed plastics waste; therefore, understanding the conditions needed for catalyst operation in a closed system is key to developing successful upgrading processes.

A critical barrier to implementing closed-system alkane metathesis is the generation of aldehydes and ketones during catalyst activation because these species poison catalytic active sites—tungsten alkylidenes—on the surface of WOx/SiO2 [26], [27], [40], [47]. Furthermore, given the molecular weight redistribution features of alkane metathesis chemistry—which not only shortens but also lengthens the chains of the initial reactants—a thorough investigation of reaction variables, such as dilution ratio and catalyst pretreatment conditions, is needed to design processes that can control product distribution toward higher-value components. The development of a model reaction system that utilizes polymer surrogates to screen and optimize the most crucial reaction conditions must precede widespread adoption of polymer metathesis upgrading processes.

Herein, we develop model reactions of catalytic olefin and alkane metathesis using a WOx/SiO2 catalyst in a batch system. Using these model reactions, we evaluate the role of zeolite 4 A (abbreviated 4 A) adsorbent in facilitating catalyst activation and reaction in a closed/batch system. We assess the factors that affect the catalytic reactivity and selectivity of the system. We demonstrate that because WOx/SiO2 can operate in the range of temperatures inaccessible to rhenium oxide, shorter reaction times can produce high conversions of the surrogate reactant. Finally, we show that the reaction system described herein has significant potential for LDPE upgrading with remarkable efficiency gains in comparison to reports on the deconstruction of PE using catalytic metathesis via rhenium oxide [18], [19]. To the best of our knowledge, this report is the first study in which WOx/SiO2 has been successfully implemented in the closed-batch reactor and, specifically, for the chemical upgrading of PE.

Section snippets

Experimental

Please see Appendix for the list of acronyms and abbreviations.

Characterization of the catalysts

Given that the surface-isolated tungsten alkylidene species are the active sites for heterogeneous olefin metathesis [20], [26], [27], [49], [50], the loading of W was set to 2 wt% to minimize waste metal when synthesizing catalyst. The Raman spectrum (Fig. S3) of a WOx/SiO2 olefin metathesis catalyst revealed that tungsten oxide was present in dehydrated monomeric form with peaks at 905, 985, and 1015 cm−1 [51], [52], [53]. Raman features for bulk WO3 nanoparticles (270, 720, and 805 cm−1) and

Conclusions

This report establishes a new alkane metathesis reaction system comprised of WOx/SiO2 conducted in batch mode for PE upgrading. Given the oxophilic properties of tungsten oxide, the presence of zeolites in the reaction vessel facilitates the activation of the catalyst and stabilizes active sites, which are otherwise poisoned by oxygenates generated in situ during the activation step. The pretreatment temperature of catalysts and the length of short-chain alkane are vital factors that

CRediT authorship contribution statement

Doyoung Kim: Conceptualization, Investigation, Methodology, Writing – original draft. Zachary Hinton: Investigation, Formal analysis, Writing – review & editing. Peng Bai: Writing – review & editing, Funding acquisition. LaShanda T. J. Korley: Writing – review & editing, Funding acquisition. Thomas H. Epps, III: Writing – review & editing, Funding acquisition. Raul F. Lobo: Supervision, Project administration, Writing – review & editing, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported as part of the Center for Plastics Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0021166.

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