Optimization of low-temperature catalytic cracking of polyolefin waste in open-batch reactors using zeolite beta with controlled intrinsic properties

Environmental problems are worsening due to the complexity in managing plastic waste. Chemical recycling emerges as a pivotal technology that can suppress carbon introduction into the carbon cycle and provide petroleum alternatives for current petrochemical processes. The utilization of zeolites can reduce energy consumption by lowering the operation temperature for pyrolysis. Here, we demonstrate low-temperature catalytic cracking of polyethylene (PE) utilizing an open-batch reactor configuration and *BEA-type zeolite catalysts. With the optimized open-batch setup and zeolites, high PE conversion (~80%) and liquid selectivity (~70%) were achieved at 330 °C. We systematically explored the effects of aluminum (Al) site density and crystal size, revealing that zeolite crystal size is another critical factor determining the liquid production. This work not only demonstrates that an effective combination and optimization of reactor and catalysts can enhance the overall catalytic activity but also offers insights into designing catalysis systems for effective recycling of polyolefin wastes.

Characterization of polyolefins-based pyrolysis oils: A comparison between one-dimensional gas chromatography and two-dimensional gas chromatography

The characterization of pyrolysis products is essential for the development of precise and accurate kinetic models. The resolution of pyrolysis products analyzed using traditional analytical methods may be compromised, resulting in inaccurate product identification and overlapped peaks. This study investigated the product distribution of pyrolysis oil derived from plastics using two analytical techniques: one-dimensional gas chromatography coupled with mass spectrometry (GC-MS) and two-dimensional gas chromatography coupled with flame ionization detector and time-of-flight mass spectrometer (GC×GC-FID/TOF-MS). Pyrolysis experiments of polypropylene (PP), high-density polyethylene (HDPE), polystyrene (PS), as well as co-pyrolysis of PP-PS and HDPE-PS, were conducted at 500 °C using a micropyrolyzer coupled with GC and GC×GC. GC×GC enabled the peak identification 2.8, 5.3, 4.8, 3, and 4.1 times more compared to GC in the pyrolysis of PP, HDPE, PS, PP-PS, and HDPE-PS, respectively. The yield of new chemical classes was found to be 1.3, 5.6, 0.2, 0.8, and 1.5 wt.% for pyrolysis of PP, HDPE, PS, PP-PS, and HDPE-PS, respectively. Furthermore, new products were detected in the co-pyrolysis of PP-PS (1:1), with a total yield of 0.5 ± 0.07 wt.%. Additionally, interaction effects were observed in the co-pyrolysis experiments, such as a 29.7% and 51.3 % increase in the experimental yield of toluene compared to the additive yield for PP-PS and HDPE-PS co-pyrolysis, respectively, which can be attributed to secondary reactions involving the consumption of styrene.

Conversion of Polypropylene into Light Hydrocarbons and Aromatics by Metal Exchanged Zeolite Catalysts

Polyolefins can be converted into C2-C5 hydrocarbons and benzene-toluene-xylene (BTX) aromatics as high-demand petrochemical feedstocks via catalytic pyrolysis on acidic zeolites. Bro̷nsted and Lewis acid sites are responsible for cracking polyolefins into olefins and subsequent aromatic formation. In this study, we have subjected the parent HZSM-5 zeolite to postsynthetic partial metal exchange with Fe, Co, Ni, Cu, and Ce cations to perturb Bro̷nsted/Lewis acidity. We have investigated these metal-modified HZSM-5 on the catalytic pyrolysis of polypropylene (PP) in a micropyrolyzer connected to a two-dimensional gas chromatograph coupled to a time-of-flight mass spectrometer and flame ionization detector (Tandem Pyrolyzer-GC × GC-TOF-MS/FID setup). Whereas Fe-, Co-, Cu-, and Ce-exchanged zeolites (with 2.5, 2.3, 1.9, and 0.8 wt % metal, respectively) had comparable product yields with the parent zeolite, Ni-exchanged zeolites with Ni content of 0.5 to 2 wt % were associated with enhanced BTX formation (28-38 wt %) compared to that of the parent zeolite (22 wt %). Pyridine-FTIR indicated that the Bro̷nsted/Lewis acid ratio of the parent zeolite decreased upon metal ion exchange. According to Pyridine-TPD, the parent zeolite's medium-strength acid sites were redistributed into weak and strong acid sites in Ni-exchanged zeolites. The higher amount of carbon deposits on Ni-exchanged zeolites compared to the parent and other metal ion exchanged zeolites was attributed to the enhanced aromatization activity by the simultaneous decrease in the Bro̷nsted/Lewis acid ratio and emergence of strong acid sites.

Chemical catalytic upgrading of polyethylene terephthalate plastic waste into value-added materials, fuels and chemicals

The substantial production of polyethylene terephthalate (PET) products, coupled with high abandonment rates, results in significant environmental pollution and resource wastage. This has prompted global attention to the development of rational strategies for PET waste treatment. In the context of renewability and sustainability, catalytic chemical technology provides an effective means to recycle and upcycle PET waste into valuable resources. In this review, we initially provide an overview of strategies employed in the thermocatalytic process to recycle PET waste into valuable carbon materials, fuels and typical refined chemicals. The effect of catalysts on the quality and quantity of specific products is highlighted. Next, we introduce the development of renewable-energy-driven electrocatalytic and photocatalytic systems for sustainable PET waste upcycling, focusing on rational catalysts, innovative catalytic system design, and corresponding underlying catalytic mechanisms. Moreover, we discuss advantages and disadvantages of three chemical catalytic strategies. Finally, existing limitations and outlook toward controllable selectivity and yield enhancement of value-added products and PET upvaluing technology for scale-up applications are proposed. This review aims to inspire the exploration of waste-to-treasure technologies for renewable-energy-driven waste management toward a circular economy.

Catalytic upcycling of post-consumer multilayered plastic packaging wastes for the selective production of monoaromatic hydrocarbons

In order to obtain extended storage life of food-grade materials and better barrier properties against environmental factors, a multilayer plastic packaging (MLP) is often used. The multilayer packaging plastics are labelled as "other" (SPI#7) category, and are manufactured with a combination of barrier plastics, rigid plastics and printing surface. Owing to their complex composition and difficulty in separating the layers of MLP, its mechanical recycling is challenging. In this study, MLP wastes (MLPWs) were collected from zero-waste garbage collection center of IIT Madras, India, and thoroughly characterized to determine their composition and plastic types. MLPWs were characterized using various physico-chemical methods such as thermogravimetric/differential scanning calorimetric analysis, Fourier transform infrared spectroscopy, bomb calorimetry, and proximate and ultimate analyses. The MLPWs were mainly made up of polyethylene (PE) and polyethylene terephthalate (PET). Further, the non-catalytic and zeolite-catalyzed fast pyrolysis of these MLPWs were studied using analytical pyrolysis coupled with gas chromatograph/mass spectrometer (Py-GC/MS). The non-catalytic fast pyrolysis of MLPWs primarily produced a mixture of aliphatic and alicyclic hydrocarbons, while zeolite catalyzed fast pyrolysis resulted in the formation of mono-aromatic hydrocarbons (MAHs). The activity of HZSM-5, zeolite Y (HY) and zeolite beta (Hβ) catalysts were evaluated, and the salient products were quantified. The yields of MAHs like benzene, toluene, ethylbenzene and xylene using the zeolites followed the trend: HZSM-5 (14.9 wt%) > HY (8.1 wt%) > Hβ (7.8 wt%), at 650 °C. The use of HZSM-5 resulted in highest yield of MAHs, viz. 16.1 wt%, at the optimum temperature of 550 °C and MLPW-to-catalyst ratio of 1:15 (w/w). The superior activity of HZSM-5 is due to its nominal acidity and larger pore size of 4.24 nm, as compared to HY and Hβ. The MAHs yield from three other types of MLPWs varied in the range of 9-16 wt%. The present study demonstrates a promising pathway for the catalytic upcycling of highly heterogeneous MLPWs in the context of circular economy.

Analysis of Fuel Alternative Products Obtained by the Pyrolysis of Diverse Types of Plastic Materials Isolated from a Dumpsite Origin in Pakistan

The current energy crisis and waste management problems have compelled people to find alternatives to conventional non-renewable fuels and utilize waste to recover energy. Pyrolysis of plastics, which make up a considerable portion of municipal and industrial waste, has emerged as a feasible resolution to both satisfy our energy needs and mitigate the issue of plastic waste. This study was therefore conducted to find a solution for plastic waste management problems, as well as to find an alternative to mitigate the current energy crisis. Pyrolysis of five of the most commonly used plastics, polyethylene terephthalate (PET), high- and low-density polyethylene (HDPE, LDPE), polypropylene (PP), and polystyrene (PS), was executed in a pyrolytic reactor designed utilizing a cylindrical shaped stainless steel container with pressure and temperature gauges and a condenser to cool down the hydrocarbons produced. The liquid products collected were highly flammable and their chemical properties revealed them as fuel alternatives. Among them, the highest yield of fuel conversion (82%) was observed for HDPE followed by PP, PS, LDPE, PS, and PET (61.8%, 58.0%, 50.0%, and 11.0%, respectively). The calorific values of the products, 46.2, 46.2, 45.9, 42.8 and 42.4 MJ/kg for LPDE, PP, HPDE, PS, and PET, respectively, were comparable to those of diesel and gasoline. Spectroscopic and chromatographic analysis proved the presence of alkanes and alkenes with carbon number ranges of C₉-C₁₅, C₉-C₂₄, C₁₀-C₂₁, C₁₀-C₂₈, and C₉-C₁₇ for PP, PET, HDPE, LDPE, and PS, respectively. If implemented, the study will prove to be beneficial and contribute to mitigating the major energy and environmental issues of developing countries, as well as enhance entrepreneurship opportunities by replicating the process at small-scale and industrial levels.