Structure-activity relationship in hydrogenolysis of polyolefins over Ru/support catalysts
Graphical Abstract
Introduction
Plastic wastes are causing serious environmental and ecological problems in recent years, and the reduction and/or effective use of plastic wastes are highly required. Chemical recycling or upcycling of plastic wastes to valuable chemicals is one of the promising methods. Among various plastics, polyethylenes and polypropylenes account for 57% of non-fiber plastic production [1], and therefore, the development of effective transformation methods of the polyolefins is essential.
Up to now, various transformation methods including (catalytic) thermal degradation into gas and mixed oils at high temperatures (≥623 K) have been developed [2], and the catalytic and selective transformation of polyethylene to more valuable chemicals, namely chemical upcycling, has recently attracted much attention [3], [4]. Some catalyst systems with homogeneous catalysts were reported to be effective for the transformation of polyolefins by reverse of Ziegler-Natta polymerization, cross alkane metathesis, and C-C bond alumination [5], [6], [7], however, considering the catalyst stability and reusability, heterogeneous catalysts are preferable to homogeneous ones. Catalytic hydrogenolysis (hydrocracking) of polyolefins can directly provide valuable chemicals such as waxes, lubricants, jet fuels, gasoline and naphtha, and recently the tandem hydrogenolysis and aromatization of polyethylene by Pt/γ-Al2O3 catalyst without H2 was also demonstrated to be the effective method for the direct synthesis of alkylbenzenes [8]. As for the hydrogenolysis of polyolefins, various effective heterogeneous catalysts have been developed, such as Ru-based [9], [10], [11], [12], [13], [14], [15], [16], Pt-based [17], [18], [19], [20], [21], [22], [23], [24] and Ni-based [25], [26] catalysts and other catalysts [27], [28], [29], and among these catalysts Ru-based catalysts have higher activity for the reaction than other metal-based catalysts. Our group reported that CeO2-supported Ru (Ru/CeO2) catalyst was an effective heterogeneous catalyst for the hydrogenolysis of polyolefins such as LDPE, HDPE, PP and waste PEs to provide valuable linear alkanes (≥C5, in the range of waxes and liquid fuels) in high yields (~90%) at mild reaction conditions (473–513 K, 2 MPa H2)[9], [10], which is based on our finding that Ru-based catalysts including Ru/CeO2 had high activity for the selective dissociate the C-C bond in the linear short alkanes and algae-derived branched alkanes [30], [31], [32], [33], [34]. Ru/C, Ru/TiO2 and Ru/WO3-ZrO2 were reported by other groups to be effective heterogeneous catalysts for the hydrogenolysis of PE and PP to valuable chemicals such as liquid fuels and waxes at comparatively low reaction temperatures (473–498 K) [11], [12], [13], [14], [15]. However, in the case of hydrogenolysis of polyethylene over Ru/C and Ru/WO3-ZrO2, the selectivity to the valuable alkanes (≥C5) is lower than that over Ru/CeO2 and the selectivity to cheap gas products (C1-C4) (15–60%) is higher than that of Ru/CeO2 (~10%), which may be related to acid-derived cracking via β-scission over the supports [23]. In terms of activity, Ru-based catalysts are promising for the transformation of polyolefins, however, there are no reports on the detailed studies of support effect and relationship between catalytic performance and catalyst structure.
In this context, we continued to investigate Ru-based catalysts for the hydrogenolysis of polyolefins, and found that zirconia-supported Ru (Ru/ZrO2) with high-temperature-calcined ZrO2 (1073 K) showed higher activity than Ru/CeO2 with suppression of gas production (a similar selectivity to the case of Ru/CeO2). Moreover, based on the activity tests and catalyst characterizations with various Ru/ZrO2 catalysts (calcination temperatures of ZrO2, different Ru loading amount, and preparation methods), the relationship between the catalytic performance and catalyst structure was clarified.
Section snippets
Catalyst preparation
Ru/support catalysts (Ru: 5 wt%) were prepared with various supports calcined at different calcination temperatures by impregnation method, and the detailed method is shown in SI. After the impregnation of Ru(NO)(NO3)3−x(OH)x (in diluted nitric acid, Sigma Aldrich, Ru: 1.5 wt%) on the supports, the solvent was evaporated. All the prepared catalysts were dried at 383 K for 12 h. The obtained dried catalysts were heated in the N2 flow or air at desired temperatures (typically 573 K) for 1 h as
Catalytic performance of various Ru catalysts
Supports are one of the important components of supported metal catalysts, and the calcination temperature of the supports can change the surface area, acid/base property and redox property, which influence the size and electronic state of the supported metal species. The effect of support types and the calcination temperatures of the supports in Ru/support catalysts was investigated in the hydrogenolysis of low-density polyethylene (LDPE, Mn: ~1700, Mw: ~4000) as a model reaction (Table 1). In
Conclusion
We investigated the effect of supports (type and calcination temperatures), Ru loading amount and heating conditions in the catalyst preparation in detail by using the hydrogenolysis of LDPE as a model reaction. Ru/ZrO2 with ZrO2 calcinated at 1073 K was an effective heterogeneous catalyst for the reaction under mild reaction conditions of 473 K and 2 MPa H2, showing about 3-fold higher activity than the standard catalyst of Ru/CeO2 with CeO2 calcined at 873 K and similar selectivity to the
CRediT authorship contribution statement
Masazumi Tamura: Funding acquisition, Conceptualization, Supervision, Formal analysis, Writing – review & editing. Shuhei Miyaoka: Investigation, Methodology, Formal analysis. Yosuke Nakaji: Investigation, Methodology, Formal analysis. Mifumi Tanji: Investigation, Methodology, Formal analysis. Shogo Kumagai: Formal analysis, Writing – review & editing. Yoshinao Nakagawa: Formal analysis, Writing – review & editing. Toshiaki Yoshioka: Formal analysis, Writing – review & editing. Keiichi
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 by the Environment Research and Technology Development Fund (3RF-1803) of the Environmental Restoration and Conservation Agency of Japan.
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