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. 2021 Apr 21;7(17):eabf8283.
doi: 10.1126/sciadv.abf8283. Print 2021 Apr.

Plastic waste to fuels by hydrocracking at mild conditions

Sibao Liu  1 , Pavel A Kots  1 , Brandon C Vance  1   2   3 , Andrew Danielson  1   2 , Dionisios G Vlachos  4   2   3
Affiliations

Plastic waste to fuels by hydrocracking at mild conditions

Sibao Liu et al. Sci Adv. .

Abstract

Single-use plastics impose an enormous environmental threat, but their recycling, especially of polyolefins, has been proven challenging. We report a direct method to selectively convert polyolefins to branched, liquid fuels including diesel, jet, and gasoline-range hydrocarbons, with high yield up to 85% over Pt/WO3/ZrO2 and HY zeolite in hydrogen at temperatures as low as 225°C. The process proceeds via tandem catalysis with initial activation of the polymer primarily over Pt, with subsequent cracking over the acid sites of WO3/ZrO2 and HY zeolite, isomerization over WO3/ZrO2 sites, and hydrogenation of olefin intermediates over Pt. The process can be tuned to convert different common plastic wastes, including low- and high-density polyethylene, polypropylene, polystyrene, everyday polyethylene bottles and bags, and composite plastics to desirable fuels and light lubricants.

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Figures

Fig. 1
Fig. 1. Current and proposed chemical conversion of plastic-waste to fuels.
(A) Polyolefin pyrolysis (23) and hydrogenolysis (28) require harsh reaction conditions and produce low amounts of fuel-range hydrocarbons. The proposed approach exhibits a high yield to gasoline at low temperatures without solvents or diluents. (B) Initial chopped PE bag before reaction and liquid product after reaction. Photo credit: Pavel Kots, University of Delaware.
Fig. 2
Fig. 2. Catalytic data at 250°C.
(A) Depolymerization of LDPE over Pt/WO3/ZrO2 with various solid acid catalysts for 2 hours. (B) Product yields and degree of isomerization by carbon number for pure Pt/WO3/ZrO2 (black) and Pt/WO3/ZrO2 mixed with HY(30) in a 1:1 mass ratio (yellow). (C) Influence of HY zeolite acidity on product yields in LDPE hydrocracking in a 1:1 mixture with Pt/WO3/ZrO2 for 4 hours. (D) Effect of the HY(30) zeolite mass fraction in a mixture with Pt/WO3/ZrO2 on the selectivity of the main product groups and solid yield for 2 hours. (E) Reaction network highlighting the role of the HY zeolite in accelerating deep cracking.
Fig. 3
Fig. 3. Product yield distribution by carbon number over Pt/WO3/ZrO2 mixed with different solid acids.
Reaction conditions: 250°C, 30-bar H2, 2.0-g LDPE, 0.1-g Pt/WO3/ZrO2, 0.1-g solid acid, and reaction time of 2 hours.
Fig. 4
Fig. 4. Selective poisoning of Pt/WO3/ZrO2.
(A) Effect of selective poisoning of Pt/WO3/ZrO2 or HY(30) zeolite by pyridine on reaction performance. Conditions: 275°C, 1 hour, and 30-bar H2. Data marked with 1 are collected at 250°C, 4 hours of reaction time. (B) Depiction of main intermediates diffusing over Pt/WO3/ZrO2 + HY(30) catalyst. (C) Reaction selectivity in case of intimate contact between Pt particles and zeolite acid sites. Feedstock and catalyst weights are as in Table 1 with HY(30) as an acid catalyst.
Fig. 5
Fig. 5. Effect of hydrogen pressure on LDPE conversion over Pt/WO3/ZrO2 mixed with HY(30).
Conditions: 250°C, 1 hour. Weights are in Table 1.
Fig. 6
Fig. 6. Pt/WO3/ZrO2 mixed with HY(30) recyclability.
Conditions: 275°C, 1 hour, and 30 bar H2. Regeneration conditions: 500°C and 3 hours in static air.
Fig. 7
Fig. 7. Hydrocracking of different feedstocks.
(A) (I) PP granules, (II) HDPE granules, (III) LDPE bottle, (IV) HDPE bottle, (V) HDPE bag, and (VI) 45 volume % of PP to 45 volume % of PE to 10 volume % of PS composite tape. (B): Product distribution by carbon number for different polyolefin feedstocks over Pt/WO3/ZrO2 mixed with HY(30). Reaction conditions: 250°C and 30 bar H2; reaction time: (A) 2 hours for (I) to (IV), 8 hours for (V), and 4 hours for (VI); (B) 2 hours. Photo credit: Brandon Vance, University of Delaware.
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