Depolymerization of waste polyethylene to linear alkenes via sequential dehydrogenation and metathesis

Abstract

Polyethylene (PE) is the most abundantly sourced plastic and significant efforts are needed for its end-of-life management. The deconstruction of PE is an uphill task and requires the breaking of highly stable C–C bonds. Here we demonstrate that PE can be deconstructed to value-added dodecene, along with other long-chain alkenes. The PCP–iridium complex catalyzes the dehydrogenation of commercial and post-consumer polyethylene waste to produce dehydrogenated polyethylene (DHP) with 0.5–1.0% unsaturation. The DHP was subjected to an ethylene cross-metathesis reaction in the presence of suitable catalysts. Through meticulous optimization of reaction parameters, 63% selectivity toward dodecene, with 26% overall yield, was achieved. The practical significance of our method has been demonstrated by subjecting post-consumer plastic waste to dehydrogenation followed by ethylene metathesis to produce dodecene as a major product, together with long-chain alkenes. The PE deconstruction has been confirmed by recording molar mass before and after depolymerization using high-temperature gel permeation chromatography. The existence of dodecene has been unambiguously ascertained using GC, GC-MS, NMR, and IR spectroscopy. Thus, these results demonstrate the conversion of waste PE to value-added dodecene and long-chain alkenes under mild reaction conditions.

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Introduction

The global polymer production stands at about 400 million tons per annum and is projected to grow further in the near future.^1–3 Polyolefins [such as polyethylene (PE) and polypropylene (PP)] constitute about 60% of the total polymers produced and have reached every corner of the planet Earth.⁴ Polyethylene (PE) finds widespread applications in daily life as it offers properties such as chemical robustness, long life (durability), high hydrophobicity, light stability, thermal stability, and mechanical flexibility. These properties that make PE a wonder material become a cause of concern for the end-of-life management. By one estimate, PE will take about 1200 years to degrade.⁵ Additionally, the recycling rate of PE and PP is only about 8–10% of the total production.^6,7 This is because a considerable portion of PE is used for packaging applications and there are no sustainable recycling technologies available to recycle waste PE. Therefore, large volumes of these polyolefins are either landfilled or escape into water bodies. The leakage of polyolefins pollutes the environment and adversely affects the ecosystem.^8,9 PE is neither prone to chemical recycling nor biodegradable due to the strong bond dissociation enthalpies of the C–C bond (362–369 kJ mol⁻¹) and the C–H bond (416.7 kJ mol⁻¹).¹⁰ As a result, PE exhibits remarkable resistance to various chemical transformations and it is hard to control product selectivity.¹¹ Various chemical and mechanical recycling methods, such as pyrolysis, fluid catalytic cracking, and hydrocracking, have been developed for the recycling of waste polyolefins.¹² Organo-catalysts have also been recognized for their efficacy in depolymerizing various functional polymers.^13–15 However, there are a few reports on their use in depolymerizing polyethylene (PE) and polypropylene (PP). These methods suffer from product selectivity and require very high operating temperatures (>500 °C) and pressure. As a result, any progress/improvement in this aspect is much appreciated. There have been a few reviews that have debated the challenges involved in the chemical recycling of waste plastic and the path forward.^16–20

In recent years significant research efforts have been directed towards the chemical recycling or upcycling of PE, as an innovative approach to treat post-consumer waste PE.^21–26 A handful of catalytic systems such as iridium, platinum, ruthenium, cobalt, copper, and ionic liquid-based catalysts have been shown to achieve PE recycling.^27–37 Among these various methods, dehydrogenation and tandem catalytic approaches have recently attracted considerable attention in the depolymerization or upcycling of PE. The very first report by Huang et al. (Fig. 1A)²¹ paved the way for subsequent innovations by Hartwig, Guironnet and Scott, demonstrating the depolymerization of PE to propylene (Fig. 1B and C).^38,39 Both these reports use (i) dehydrogenation, (ii) isomerization, and (iii) metathesis steps to obtain propylene. We hypothesized that value-added long-chain olefins can be obtained from waste PE in the absence of an isomerization catalyst. The catalytic depolymerization of PE waste into feedstock chemicals such as linear long-chain alkenes would not just address the plastic menace but would be a bonus (Fig. 1D). These long-chain alkenes can be upcycled to value-added fatty acids, fragrances, ionic surfactants, detergents, lubricants, etc.^40,41

Fig. 1 Metal-catalysed depolymerization of PE.

Herein, we report a two-step process for the selective production of dodecene and long-chain alkenes from waste PE. Dehydrogenation of waste PE is carried out using a PCP–Ir complex, leading to the production of dehydrogenated PE (DHP). The DHP is subjected to cross-metathesis with ethylene to yield dodecene as the major product. The existence of dodecene is confirmed by GC and GC-MS, IR and NMR spectroscopy. Dodecene and its derivatives are valuable building blocks for manufacturing various lubricants, surfactants, detergents, and plasticizers.

Results and discussion

Synthesis of Cat. 1 and Cat. 4 and dodecane dehydrogenation

Polyethylene is non-functional and therefore, introducing either functionality or unsaturation is necessary before one can think of breaking it down. Dehydrogenation of PE has been reported by Guan, Hartwig, Coates, Guironnet and Scott using POCOP-type pincer iridium catalysts to introduce unsaturation (Fig. 1B and C).^21,38,39,42 The POCOP ligands are relatively moisture-sensitive and have to be handled under an inert atmosphere, while PCP-type pincer ligands are relatively stable and have been rarely used in the dehydrogenation of PE. Therefore, we envisioned the application of PCP-type ligand-derived iridium complexes as catalysts in dehydrogenating PE.

Our research commenced with the synthesis of the reported PCP–iridium dihydride complex (IrH₂(^tBuPCP)) (Cat. 1). The ¹H (ESI, Fig. S1 and S3†) and ³¹P NMR (ESI, Fig. S2 and S4†) chemical shifts confirmed the formation of the desired iridium complex,⁴³ albeit with a moderate yield (see ESI† for details). To check the effect of the catalyst on dehydrogenation selectivity, we synthesized a POCOP type of ligand (ESI, section 2.3 and 2.4†) and the corresponding iridium hydride complex (Cat. 4). ³¹P (ESI, Fig. S4C†) and ¹H NMR (ESI, Fig. S4D†) confirmed the formation of Cat.4. Once the iridium dihydride (Cat.1) was prepared, we conducted the dehydrogenation of a model substrate, dodecane. The dehydrogenation takes place at 150 °C within 24 hours. ¹H-NMR spectroscopy (Fig. S5†) verified the formation of unsaturated dodecene, predominantly comprising internal double bonds. This outcome suggests the successful dehydrogenation of dodecane to dodecene and implies the potential application of the same strategy for longer-chain alkanes such as polyethylene (PE).

Dehydrogenation of polyethylene

In preliminary investigations, we explored the dehydrogenation of commercial polyethylene (PE) with an average molar mass of ∼4000 Da (M_w) (M_n = 1700 Da). Before starting the actual experiments, we re-precipitated both the commercial and post-consumer polyethylene to remove additives, antioxidants, etc. We then characterized the precipitated polymer using IR (ESI, Fig. S6A and S6B†), elemental analysis (Table S7†), and TGA (ESI, Fig. S73 and S74†) to confirm the absence of other impurities. The PCP–Ir complex Cat.1 and POCOP–Ir complex Cat.4 catalyzed the dehydrogenation of PE in the presence of ^tert-butyl ethylene (hydrogen acceptor or sacrificing agent, see the ESI† for details) at 180 °C and 200 °C respectively. The high-temperature ¹H-NMR spectrum of the above reaction mixture revealed a distinct peak at 5.42 ppm, indicating the presence of internal double bonds in the dehydrogenated polyethylene (DHP). This observation confirmed the successful dehydrogenation of PE (ESI, Fig. S7A, S7B and S7C†), which originally lacked double bonds (ESI, Fig. S6†). The reaction parameters were screened and Table 1 summarizes the most important entries. Under the optimized conditions, an unsaturation of 1% could be achieved (Table 1, entry 4). The molar mass of the PE before and after dehydrogenation remains the same (ESI, Fig. S8†). Apart from the PCP derived Cat.1, a POCOP derived Cat.4 was examined (ESI section 3.3†) in the dehydrogenation of PE. Under the optimized conditions, only 0.4% olefin content was achieved.

Table 1 Dehydrogenation of PE at different time intervals

Entry	PE (mg)	Cat. (mg)	Toluene (mL)	Time (h)	% olefin^a
Conditions: PE (40 mg), Cat. 1 (6 mg), TBE (^tertbutyl ethylene) (0.1 mL), toluene (2 mL), time (1–4 days), and temperature (180 °C).a Calculated from the ^1H-NMR spectrum.
1	40	6	2	24	0.3
2	40	6	2	48	0.5
3	40	6	2	72	0.6
4	40	6	2	96	1.0

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Cross-metathesis of dehydrogenated PE

The cross-metathesis of DHP with ethylene is known to produce propylene.^38,39,44 Therefore, we pondered if the same reaction can produce long-chain olefins in the absence of an isomerization catalyst. To ensure precision and reproducibility, we meticulously adhered to a carefully crafted experimental protocol, drawing upon well-established procedures documented in the literature. Scheme 1 depicts the reaction and Table 2 summarizes the important experiments. The DHP was suspended in toluene in a high-pressure reactor along with metathesis catalyst and the reactor was pressurized with ethylene gas. This mixture was heated to a suitable temperature for about 2–16 hours (Table 2). In the cross-alkene metathesis reaction, it is expected that ethylene gas reacts and some of it is consumed. We pressurized the reactor to the desired pressure (25 bars), but it was difficult to notice the pressure drop. This could be due to a small amount of sample (50 mg) with only ∼1% unsaturation in a large reactor (450 mL) with 25 bar pressure. Excess ethylene was released and collected into a Tedlar bag. Analysis of gases collected in the Tedlar bag displayed the presence of only ethylene gas (ESI, Fig. S38C†) and no other small gaseous products could be observed. The reactor content (liquid part) was passed through a silica bed. The thus obtained filtrate was injected into a gas chromatography (GC) system and GC-MS was performed to identify the compounds generated. When we performed the reaction for 2 hours, we observed some degree of selectivity in the metathesis product (Table 2, entry 3). The GC chromatogram revealed a dominant peak at approximately 11.9 ± 0.1 minutes, which constituted approximately 37% of the total area as determined by GC chromatogram analysis (Fig. 2A, middle). To identify the compound responsible for this peak, we injected the standards into the GC system (Fig. 2A, bottom) and purified the product (Fig. 2A, top). These experiments revealed that the retention time of the observed peak matched exactly with that of dodecene (Fig. 2A). The GC-MS analysis confirmed the mass of the generated dodecene product, corroborating our findings (ESI Fig. S40†). These results suggest a noteworthy degree of selectivity in the cross-metathesis reaction, leading to the predominant formation of dodecene.

Scheme 1 Ruthenium-catalyzed metathesis of DHP in the presence of ethylene. Conditions: DHP (50 mg), HG-II (7.5 mg), ethylene pressure (25 bar), toluene (2 mL), temperature (130 °C), and time (0.5–16 h).

Fig. 2 (A) GC chromatogram of isolated dodecene along with the GC chromatogram of alkene standards; (B) IR spectrum of dodecene; (C) ¹H NMR of the metathesis reaction mixture in CDCl₃; (D) screening of metathesis catalysts and selectivity towards dodecene.

Table 2 Optimization of metathesis reaction parameters

Entry	Met. Cat.	Time (h)	Temp. (°C)	Ter : Int	Selectivity^k	% of C12^l	Conv.^m (%)
Reaction conditions: DHP (50 mg), catalyst loading: 15 wt%.a 1 wt%.b 5 wt%.c 10 wt%, solvent: toluene, ethylene: 25 bars.d p-Xylene.e Chlorobenzene.f 5 bar.g 15 bar.h 30 bar.i Without ethylene pressure and in the absence of a catalyst.j DHP obtained using Cat. 4.k In % of terminal olefin, calculated using ^1H-NMR.l Calculated from the GC chromatogram.m See the ESI for details.†
1	G-II	0.5	130	—	—	00	—
2	G-II	1	130	1 : 1	50	06	26
3	G-II	2	130	1 : 0.7	59	36	26
4	G-II	4	130	1 : 0.9	53	32	26
5	G-II	8	130	1 : 0.8	48	08	26
6	G-II	16	130	1 : 2.6	27	02	26
7	G-II	2	80	1 : 2.2	31	02	10
8	G-II	2	90	1 : 2.0	33	05	10
9	G-II	2	100	1 : 1.7	37	07	22
10	G-II	2	110	1 : 1.4	42	08	22
11	G-II	2	120	1 : 1.3	43	29	24
12	G-II	2	140	0 : 1	100	07	08
13	G-I	2	130	1 : 0	100	04	12
14	HG-I	2	130	1 : 0.6	62	55	14
15	HG-II	2	130	1 : 0.6	63	63	26
16^a	HG-II	2	130	—	—	00	00
17^b	HG-II	2	130	1 : 2	33	35	10
18^c	HG-II	2	130	1 : 1.5	40	32	10
19^d	HG-II	2	130	1 : 0.8	56	04	10
20^e	HG-II	2	130	1 : 3	25	13	26
21^f	HG-II	2	130	0 : 1	00	03	04
22^g	HG-II	2	130	1 : 3	22	41	06
23^h	HG-II	2	130	1 : 4	20	33	14
24^i	—	2	130	—	—	00	00
25	Re2O7	2	130	—	—	00	00
27^j	HG-II	2	130	1 : 0.6	62	15	22

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The proton NMR spectrum of the crude reaction mixture revealed the existence of C12 alkene in 63 : 37% (terminal to internal double bonds) (Fig. 2C). Additionally, an IR spectrum displayed the existence of unsaturation (IR bands at 2915, 2848, 1719, 1464, and 722 cm⁻¹) in the product (Fig. 2B). These observations confirm that the PE was deconstructed mainly to long-chain alkenes such as dodecene. Please note that metathesis is an equilibrium-driven reaction and therefore it is important to know the time taken to reach this state during the reaction (Scheme 1). Therefore, to increase the yield of the product, the metathesis reaction was carried out for different time intervals (Table 2, entries 1–6).

Upon closer examination of the reaction conditions, we observed that the reaction time has a major influence on the long-chain alkenes. To address this issue, we further embarked on a systematic investigation of the reaction time, ranging from 30 minutes to 16 hours. The obtained experimental results demonstrate that the formation of internal olefins increased with longer reaction times. Peters et al. studied the kinetic model of isomerizing metathesis reactions, and disclosed that long-chain alkenes convert into short-chain alkenes with increasing time. If this is true, dodecene should convert into smaller alkenes over time when isomerization is favoured. Although we did not use any isomerization catalyst, the formation of C12 was found to reduce with time. This suggests that the metathesis catalyst might also be active in olefin isomerization,⁴⁵ but when the reaction time is short enough, the formation of long-chain terminal olefins is observed. This careful analysis suggests that the optimal time to obtain maximum conversion to long-chain olefins (with terminal double bonds) is 2 hours (Table 2 entry 3). Under these specific parameters, we achieved a noteworthy 59% yield of terminal double bonds (calculated based on NMR analysis) (ESI, Fig. S9†).

To check the effect of temperature, we initiated our screening process within the temperature range of 80 °C to 140 °C (Table 2 entries 7–12). The results from this screening revealed that a temperature of 130 °C yielded superior conversion and selectivity toward dodecene. It is worth mentioning that as the metathesis temperature increases, the solubility of ethylene in toluene decreases. This phenomenon was particularly evident at more than 130 °C, leading to a reduction in the formation of metathesis products (Table 2, entry 12; ESI, Fig. S15 and S16†).

Subsequently, we screened various metathesis catalysts and examined their activity and selectivity. Grubbs-I (G-I), Grubbs-II (G-II), Hoveyda Grubbs-I (HG-I), and Hoveyda Grubbs-II (HG-II) exhibited relatively increasing selectivity towards the desired products (Fig. 2D). The heterogeneous catalyst Re₂O₇/Al₂O₃ was synthesized using a literature procedure and was examined in the metathesis reaction under optimized conditions, but we could not observe any product (C12) formation.⁴⁶ However, HG-II displayed a higher conversion than other catalysts (Table 2, entry 15; ESI, Fig. S23 and S24†). Based on these findings, HG-II was selected for further optimization. Its ability to maintain competitive selectivity while providing a higher conversion makes it the catalyst of choice.

In our attempts to reduce catalyst loading, we screened the catalyst at different concentrations, specifically at 1 wt%, 5 wt%, 10 wt%, and 15 wt% (Table 2 entries 15–18). These experiments suggested that the catalyst loading of 15 wt% produced better results as compared to the others (Table 2, entry 15; ESI, Fig. S31 and S32†). Following extensive optimization efforts, we proceeded to screen solvents for the cross-metathesis reaction. In this screening, we evaluated the performance of different high-boiling solvents, specifically, toluene, p-xylene, and chlorobenzene (Table 2 entries 19 and 20). The results revealed that toluene is the preferred solvent for our experiments (Table 2, entry 15; ESI, Fig. S33 and Fig. S34†). Toluene provided the optimal conditions for our cross-metathesis reaction, likely offering a combination of good solubility for the reactants and catalyst, as well as favourable reaction kinetics. To investigate the effect of ethylene pressure on metathesis reaction, experiments were conducted at 5, 15, and 30 bar (entries 21–23). It was noted that lower ethylene pressure resulted in reduced conversion, whereas at 30 bar, moderate yields were achieved with decreased selectivity (entry 23). Therefore, based on these observations, the optimal pressure appears to be 25 bars. After optimizing the reaction conditions, catalyst recyclability was examined. As listed in ESI section 9,† though the catalyst could be recycled, the activity dropped to a significant level after 3 cycles (see ESI Fig. S59†).

To address the ambiguity of low molar mass polyethylene getting dissolved in toluene and affecting the overall yield, a control experiment was conducted without the addition of a HG-II catalyst in the absence of ethylene. After allowing the reaction to proceed for 2 hours, a standard workup procedure was followed, which revealed no weight loss (entry 24). These observations suggest that there is no low molecular weight soluble fraction in PE and the weight loss is due to the conversion of PE to long-chain alkenes. A complete mass balance of the reaction, along with quantification of other long-chain olefins, is presented in ESI section 7.†

During the cross-metathesis of DHP with ethylene, GC, GC-MS, NMR, and IR methods were used to establish the depolymerization product. In addition to that, we employed High-Temperature Gel Permeation Chromatography (HT-GPC) to determine the average molar mass of polyethylene (PE) before and after depolymerization. As is evident from Fig. 3A, the commercial polyethylene displays a weight-average molar mass (Mw) of ∼4000 Da, accompanied by a polydispersity index (PDI) of 1.65 (Fig. 3A). HT-GPC analysis of samples collected (soluble fraction) after the depolymerization revealed two distinct peaks, with the weight-average molar mass decreasing to 100–1300 Da (Fig. 3B). The molar mass of 100–1300 Da reaches the lower detection limit of HT-GPC and should be treated with care. Nevertheless, HT-GPC analysis provides direct evidence for a significant reduction in molar mass and underscores the considerable deconstruction of polyethylene.

Fig. 3 GPC chromatogram of (A) commercial PE and (B) PE after depolymerization.

In the above sections, our primary focus was on the in-depth analysis of liquid products and substances soluble in the reaction solvent. However, there was also a solid left in the reaction container after the metathesis. Utilizing advanced Differential Scanning Calorimetry (DSC) techniques, we examined the composition of the solid products left in the flask after metathesis (Fig. 4). Our findings revealed notable differences in the melting temperatures of commercial polyethylene, dehydrogenated polyethylene, and the solid material post-metathesis. The commercial polyethylene exhibited a melting temperature of 101.47 °C, whereas the DHP showed a T_m of 94.18 °C. This shift (decrease in melting temperature) in melting temperature signifies the incorporation of double bonds in the PE chains. After the metathesis reaction, we subjected the remaining solid to DSC analysis. We observed two distinct melting temperatures, 102.07 and 110.38 °C (Fig. 4, top). The former peak can be assigned to starting/commercial PE, while the latter melting peak cannot be fully assigned.

Fig. 4 DSC thermogram of (A) commercial polyethylene (bottom), (B) dehydrogenated polyethylene (middle) and (C) solid after metathesis (top).

Dodecene selectivity

As stated in previous reports, the unsaturation in DHP is random with a maximum unsaturation of ∼1% (Table 1, entry 4).³⁸ As the number of double bonds is low, the overall conversion is 26%. Ethylene metathesis with such DHP is anticipated to yield a range of alkenes. Indeed, our initial screening resulted in C10–C20 alkenes (see ESI Fig. S10, S12, S16, S18, and S20†). The carbon chain versus % area is plotted in Fig. S49 (ESI).† From these data, it is clear that odd and even carbon chains from C10 to C19 are formed in the DHP metathesis step, but with better selectivity towards dodecene. Meticulous optimization produced dodecene with 63% selectivity and 37% other alkenes. Such selectivity towards dodecene could arise due to the following factors. (I) Early experimental work by Goodman and others and theoretical calculations suggest chain folding between carbon numbers C12 and C18.^47–52 As the length of unbranched alkane chains reaches some critical length, intramolecular dispersion forces cause a self-solvation effect in which the chains assume a folded conformation. This folding impedes further isomerization of a double bond or makes it less favourable, resulting in a predominance of double bonds in the C12 to C16 range (ESI, Fig. S50†). (II) The metathesis catalyst too can induce selectivity due to ligand steric effects (ESI, Fig. S51†). (III) The optimized reaction conditions (short time, temperature, metathesis catalyst, ethylene pressure, etc.) enable isomerization of the double bond between C12 and C16 and selectivity towards dodecene increases (see ESI sections 8 and 9†). (IV) Peters et al. studied a simple kinetic model for a similar type of reaction involving polyethylene upcycling.⁴⁵ They demonstrated that over time, high molar mass or long-chain alkenes are converted into smaller chains or molecules (such as propene) through the isomerizing metathesis process. It was observed that double bonds are formed at internal positions regardless of their initial position, with the most likely positions being in the C12 to C16 range. Once isomerizing metathesis begins, these C16 or other long-chain alkenes are eventually converted into C12 chains. Due to other reaction parameters, after 2 hours of reaction time, the system stabilizes. If the reaction time is further increased, these C12 chains are converted into smaller alkenes and gaseous products are formed.

To support this hypothesis and provide direct evidence for C12 selectivity, we conducted a series of control experiments (Fig. 5). First, we synthesized a poly(cyclooctene) (PCO) using Ring Opening Metathesis of Polymerization (ROMP) of cyclooctene. PCO inherently contains 25% olefin content (ESI, Fig. S60†), incorporating a double bond every eight carbons. Cross-metathesis of PCO with ethylene predominantly produced octadiene, with a product range from C8 to C12 and a small amount of C16 (ESI, Fig. S62†). Subsequently, PCO was hydrogenated to obtain DHP-1 (ESI, Fig. S64†) with a 14% olefin content with random distribution of double bonds. Ethanolysis of DHP-1 (14% olefin content) yielded products ranging from C8 to ∼C30, with 8% of C12 and 9% of C16 chains.

Fig. 5 Ring opening metathesis polymerization of cyclooctene, partial hydrogenation of polycyclooctene and metathesis of partially hydrogenated polycyclooctene.

Subsequently, we hydrogenated the PCO under different conditions to obtain DHP-2 with a 10% olefin content. Under optimized conditions, metathesis of DHP-2 predominantly formed a C12 fraction (20%). Further hydrogenation of DHP-1 under identical conditions resulted in DHP-3, with an approximately 2.4% olefin content. Cross-metathesis of DHP-3 with ethylene disclosed selective formation of C12 (72%), with 20% yield. These findings and control experiments provide direct evidence that a polyethylene with a 1–2% olefin content shows better selectivity towards the C12 product.

Depolymerization of post-consumer waste PE

Having successfully conducted the experiments using commercially available PE, we embarked on a new endeavor aimed at tackling post-consumer plastic waste (PCPW). For this purpose, we selected a Dove shampoo bottle as our PCPW source material. Two different strategies were designed to address the PCPW (Scheme 2). In the first method (A), the Dove shampoo bottle was cut into small pieces and subjected to heat treatment at 160 °C. Subsequently, the polymer was precipitated by adding methanol. This precipitated PE was subjected to dehydrogenation. In the second method (B), small plastic pieces of Dove shampoo bottle were directly subjected to dehydrogenation, circumventing the prior dissolution–re-precipitation step.

Scheme 2 Depolymerization of waste polyethylene. Reaction conditions: step I: HDPE (50 mg), ^tbutyl ethylene (TBE) (100 μL), catalyst (10.5 mg), toluene (1 mL), and time (4 days) and step-II: DHP (50 mg), HG-II (7.5 mg), 2 h, toluene (2 mL), and C₂H₄ pressure (25 bar).

Following a standardized protocol, we conducted the dehydrogenation of both samples derived from the Dove shampoo bottle (Scheme 2) using the PCP-Ir catalyst under optimized conditions. Notably, the DHP obtained from both methods (A and B) was found to contain similar double bond incorporation of about 0.5% (ESI, Fig. S43 and S44†). These observations suggest that the dehydrogenation process is averse to additives, etc. and PCPW can be directly subjected to dehydrogenation without dissolution and re-precipitation.

Subsequently, we carried out the cross-metathesis of DHP prepared via methods A and B with ethylene under optimized conditions. The DHP prepared via method A showed 26% conversion and 40% selectivity towards dodecene (Fig. S45 and S46†), while cross-metathesis of DHP prepared via method B displayed 22% conversion and 34% selectivity (Fig. S47 and S48†). These observations suggest that the PCPW can be directly converted to value-added long-chain olefins such as dodecene, without the need for purification. The solid obtained after metathesis was analyzed by DSC, and it displayed lower melting temperature than the original HDPE, but the T_m was close to that of DHP (Fig. 6). The molar mass of the solid left was determined using HT-GPC and it was found to be 67 kDa (Fig. 7).

Fig. 6 DSC analysis of a Dove bottle i.e. real-world waste (top), dehydrogenated polyethylene (middle) and solid left after metathesis (bottom).

Fig. 7 HT-GPC analysis of the Dove bottle i.e. real-world waste (black), dehydrogenated polyethylene (DHP) (red) and solid left after metathesis (blue).

Our findings highlight the potential of our methodology to produce value-added chemicals from post-consumer plastic waste and may contribute to addressing the waste plastic menace in the near future. Our group is striving to improve the overall conversion and selectivity toward long-chain alkenes.

Conclusions

In summary, we have been able to demonstrate that the largest volume polymer on this planet, PE, can be deconstructed to value-added dodecene and long-chain alkenes. The PCP-Ir pincer catalyst can dehydrogenate PE (both commercial and post-consumer PE) to DHP with 0.5 to 1.0% unsaturation. The resultant DHP can be subjected to cross-metathesis with ethylene in the presence of a metathesis catalyst. Meticulous screening of the cross-metathesis reaction suggested 25 bar ethylene pressure, a temperature of 130 °C, Hoveyda–Grubbs-II catalyst, and 2 hours as optimal reaction conditions. Notably, under the optimized conditions, 63% selectivity toward dodecene with an overall conversion of 26% was observed for a commercial PE sample. The existence of dodecene was unambiguously ascertained using a combination of spectroscopic and analytical methods such as GC, GC-MS, NMR, and IR. The reduction in the molar mass of the PE was demonstrated by comparing the HT-GPC trace of PE before the reaction and PE after the depolymerization. The molar mass drops and reaches the lower detection limit of the HT-GPC. In our attempts to demonstrate the relevance of our strategy to mitigate waste plastic, a post-consumer plastic waste (PCWP), Dove shampoo bottle, was subjected to dehydrogenation followed by ethylene metathesis. To our delight, the PCPW produced dodecene as the major product after ethylene metathesis. The C12 selectivity has been demonstrated by preparing PCO and subsequent control experiments. Thus, our results demonstrate the conversion of waste PE to value-added dodecene and other alkenes under mild reaction conditions. We are currently focusing on achieving higher conversion and better selectivity in the metathesis step.

Author contributions

KVK executed the dehydrogenation and metathesis of commercial PE/PCPW and drafted the manuscript. NSR prepared PCP–Ir complexes and assisted in PE dehydrogenation and RR isolated and characterized the metathesis products. NB assisted in planning experiments and drafting the manuscript. SHC conceived the idea, planned it, got it executed, and corrected the manuscript.

Data availability

The data to reproduce the results, along with experimental details, are available through “ESI† on the journal cite. See https://doi.org/10.1039/x0xx00000x.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

N. B. gratefully acknowledges the Science and Engineering Research Board (SERB) (RJF/2021/000150) India and CSIR (HCP46), India, for research funding and CSIRNCL, India, for infrastructure. S. H. C. thanks DST-SERB (CRG/2021/005385), India, DSIR (for supporting a CRTDH@NCL), India, CSIR (HCP46), and CSIR-National Chemical Laboratory, India, for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4gc03646h

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