Cracking Of Low-density Polyethylene Research Articles

Preparation of Two- and Three-layered Hierarchical Catalysts Using Gel Skeletal Reinforcement and Properties for Catalytic Cracking of Low Density Polyethylene

Preparation of Two- and Three-layered Hierarchical Catalysts Using Gel Skeletal Reinforcement and Properties for Catalytic Cracking of Low Density Polyethylene

Coking Reduction of Cr‐loaded Beta Zeolite during Polymer Cracking: Hydrocracking of Aromatics by Synergistic Effect of Cr6+ and Zeolitic Acid Sites

AbstractZeolite deactivation induced by coke deposition is a critical problem in the industrialization of the chemical recycling (CR) of plastics. We previously demonstrated that Cr6+ species bonded to silanol groups in the zeolite framework inhibit coke deposition in low‐density polyethylene (LDPE) cracking. In this study, the role of Cr6+ species and the mechanism of coke reduction were determined experimentally. During LDPE cracking, hydrogen was generated from the alkane products at the Cr6+ sites. Furthermore, the hydrogenation and decomposition of aromatics, which are coke precursors, occurred at the zeolitic acid sites, resulting in the reduction of coke deposition. Owing to the synergistic effect of the Cr6+ and zeolitic acid sites, coke reduction was achieved without reducing cracking activity.

Systematic study of low temperature cracking of low-density polyethylene with ZSM-5

The viability of cracking low-density polyethylene (LDPE) over H-ZSM-5 catalysts at reaction temperatures below 300 °C was demonstrated using ZSM-5 catalysts with varying textual properties and acid site content. While ZSM-5 made from standard syntheses could catalyze LDPE cracking it was found that increasing the external surface area of H-ZSM-5 from 24 m2/g to 325 m2/g resulted in a dramatic increase in LDPE conversion. For all catalysts the products were permanent hydrocarbon gas products in the C1–C5 range. H-ZSM-5 prepared in nanosheet and nanosphere morphologies achieved 64 wt% and 58 wt% LDPE conversion, respectively, compared to a standard H-ZSM-5 sample with micron-sized crystals that achieved 28 wt% conversion after 8 h at 250 °C under inert conditions. It was also found that strong Brønsted acid sites were necessary for high conversion of LDPE in this temperature regime as shown by the relatively weaker solid acid catalysts Al-MCM-41 and B-MFI converting negligible LDPE under the same reaction conditions. Lastly, a low molecular weight model molecule (n-tetracosane) was tested to gain insight into the cracking mechanism of LDPE over MFI catalysts. The findings in this work point to new approaches for catalyst design to achieve low temperature LDPE depolymerization.

Hierarchical zeolites with high hydrothermal stability prepared via desilication of OSDA-occluded zeolites

Zeolites have been served as multifunctional microporous materials. However, diffusion limitation often exists within zeolitic micropores, limiting their performances. Hierarchical zeolites with meso- or macropores have been synthesized by an effective approach to solving the above problem. Herein, we synthesized hierarchical zeolites via an alkaline treatment for organic structure-directing agents (OSDA)-occluded zeolites. In this method, desilication occurred only on the external surfaces of zeolites thanks to the presence of OSDA, leading to selective formation of mesopores only on the external surfaces of zeolites. The zeolite with mesopores only on external surfaces showed much higher catalytic activity on cracking of low-density polyethylene than a pristine zeolite. The catalytic performance was the same as a conventional hierarchical zeolite with mesopores in overall particles. These results demonstrated that mesopores on the external surfaces of zeolites played important roles in improving mass transfers of the reactants. In addition, the zeolites with mesopores only on the external surfaces showed higher hydrothermal stability than the conventional hierarchical zeolite due to its fewer defects consisting of SiOH or AlOH. This work presented a new strategy for controlling the pore structures of zeolites.

Conversion of low-density polyethylene into monocyclic aromatic hydrocarbons by catalytic pyrolysis: Comparison of HZSM-5, Hβ, HY and MCM-41

The catalytic properties of different catalytic materials on the pyrolysis of plastics have been extensively investigated. In order to better understand the relevant mechanism of catalytic pyrolysis of plastic waste, the effect of different acidity and pore size of catalytic material on its pyrolysis kinetic parameters and the distribution of pyrolysis products needs to be studied to provide a stronger theoretical basis for its resource utilization. This study aims to study the effects of HZSM-5, Hβ, HY and MCM-41 on the thermodynamic properties , kinetic parameters and pyrolysis characteristics of low-density polyethylene (LDPE), analyze the relationship between the specific catalytic pyrolysis products and the properties of the catalyst. Among the four catalysts, HZSM-5 obtained the most concentrated product distribution and the highest yield of monocyclic aromatic hydrocarbons , and it was found that the cracking of LDPE mainly occurred in its pores. Due to the large pore size and low concentration of acid sites, the main product type obtained by MCM-41 was olefins . In addition, it was found that HY reduced the pyrolysis temperature of LDPE and the activation energy of the reaction to the greatest extent. The different catalytic effects were mainly caused by different pore structures and acid site concentration. Moreover, it was found that the addition of catalyst changed the cracking mechanism of LDPE. Under the comprehensive consideration with the goal of producing monocyclic aromatic hydrocarbons, HZSM-5 is the most suitable catalyst among them. • The catalytic effects of HZSM-5, Hβ, HY and MCM-41 were studied. • HZSM-5 is the most suitable catalyst for the production of MAHs from LDPE. • Different pore structures and acid sites cause the changes of catalytic effects. • Partial LDPE scission sites changed from random-chain to end-chain due to catalysis. • The cracking reaction of LDPE occurs in the pores of HZSM-5.

Thermal Catalytic-Cracking Low-Density Polyethylene Waste by Metakaolin-Based Geopolymer NaA Microsphere.

Metakaolin-based geopolymer microspheres (MGM) with hierarchical pore structures were prepared by suspension dispersion method in dimethicone at 80 °C. The hydrothermal modification of MGM was carried out at a lower temperature of 80 °C, and a NaA molecular sieve converted from metakaolin-based geopolymer (NMGM) with good crystal structure was prepared and applied in thermal catalytic cracking of low-density polyethylene (LDPE) reaction. The one-pot two-stage thermal catalytic cracking of LDPE was carried out in a 100 mL micro-autoclave under normal pressure. In this work, the optimal proportions and optimal reaction conditions of catalysts for NMGM thermal catalytic cracking of LDPE waste to fuel oil were investigated. The NMGM catalyst showed high selectivity to the liquid product of thermal catalytic cracking of waste LDPE. Under the reaction conditions of reaction time of 1 h and reaction temperature of 400 °C, the liquid-phase yield of thermal catalytic cracking of LDPE reached a high of 88.45%, of which the content of gasoline components was 10.14% and the content of diesel components was 80.97%.

High-Silica Layer-like Zeolites Y from Seeding-Free Synthesis and Their Catalytic Performance in Low-Density Polyethylene Cracking.

Layer-like FAU-type zeolite Y was synthesized by an organosilane-assisted low-temperature hydrothermal method and its catalytic activity was verified in the low-density polyethylene (LDPE) cracking process. The synthesis procedure of high-silica layer-like zeolite Y was based on organosilane as a growth modifier, and for the first time, the seeding step was successfully avoided. The X-ray diffraction and electron microscopy studies, scanning electron microscopy and transmission electron microscopy confirmed the formation of pure FAU structure and zeolite particles of plate-like morphology arranged in the manner of the skeleton of a cuboctahedron. The in situ Fourier transform infrared (FT-IR) spectroscopic studies, low-temperature nitrogen sorption, and electron microscopy results provided detailed information on the obtained layer-like zeolite Y. The acidic and textural properties of layer-like zeolites Y were faced with the catalytic activity and selectivity in the cracking of LDPE. The quantitative assessment of catalyst selectivity performed in FT-IR/GC–MS operando studies pointed out that LDPE cracking over the layer-like material yielded value-added C3–C4 gases and C5–C6 liquid fraction at the expense of C7+ fraction. The detailed analysis of coke residue on the catalyst was also performed by means of FT-IR spectroscopy, thermogravimetric analysis, and thermoprogrammed oxidation coupled with mass spectrometry for the detection of oxidation products. The acidic and textural properties gave a foundation for the catalytic performance and coking of catalysts.

LDPE cracking over mono- and divalent metal-doped beta zeolites

This study evaluates the effect of loading various mono and divalent metals in Beta zeolite on low-density polyethylene (LDPE) cracking. We revealed that Tl and Ba ions enhanced Lewis acidity, leading to higher catalytic activity on LDPE cracking.

Systematic Study of Low Temperature Cracking of Low-Density Polyethylene with Zsm-5

Systematic Study of Low Temperature Cracking of Low-Density Polyethylene with Zsm-5

Defect engineering to boost catalytic activity of Beta zeolite on low-density polyethylene cracking

Catalytic cracking is of considerable interest as a sustainable, renewable, efficient, and eco-friendly chemical conversion method of plastic wastes. In catalytic cracking, plastic wastes can be converted into valuable products such as monomers or petrochemical feedstock. Solid acid catalysts, especially zeolites, significantly lower the thermal decomposition temperature of plastics. However, the catalysts have not shown enough performance to be implemented in society. Therefore, major efforts have recently been devoted to solving this problem by appropriately designing zeolite catalysts. Herein, we report a new defect-rich Beta zeolite synthesized by a dry gel conversion method using methyltriethoxysilane. The obtained catalyst exhibited outstanding activity on low-density polyethylene cracking. We measured Fourier-transform infrared spectra using pyridine as a probe molecule and found that this high activity is contributed by the increase of the amount of Lewis acid sites. It probably arises from the ratios of extra-framework hexacoordinated aluminum and silanol groups higher than the conventional Beta zeolite shown in 27Al and 1H magic angle spinning nuclear magnetic resonance spectra. We can say that the increase of defects of zeolite Beta further accelerated polymer cracking. This work provides a new strategy to improve the catalytic activity of zeolites on polymer cracking.

Opening up ZSM-5 Hierarchical Zeolite's Porosity through Sequential Treatments for Improved Low-Density Polyethylene Cracking.

An adequately tuned acid wash of hierarchical ZSM-5 zeolites offers a levelling up in the catalytic cracking of low-density polyethylene. Identification of crucial and limiting factors governing the activity of the zeolite was extended with studies about the accessibility of acid sites, nature of the realuminated layer and role of Lewis acid sites. The sequential treatment of a ZSM-5 zeolite offered enhanced activity in low-density polyethylene (LDPE) cracking at low and high conversions, as confirmed by a decrease in the temperatures needed to reach 20% and 80% conversion (T20 and T80, respectively). A linear dependence of the T80 on the coupled IHF (indexed hierarchy factor) and AFB (accessibility factor) highlighted the importance of the textural and acidic parameters in the catalytic cracking of LDPE. Operando FT-IR-GC studies confirmed a higher fraction of short-chain hydrocarbons (C3–C5) in the product distribution of hierarchical catalysts resulting from the effective polymer cracking in easily accessible pores.

Pd-modified beta zeolite for modulated hydro-cracking of low-density polyethylene into a paraffinic-rich hydrocarbon fuel

The catalytic properties of a microporous beta zeolite and Pd-loaded analogue were assessed in the cracking of low-density polyethylene (LDPE). The idea behind this study was to investigate the potential of zeolite beta from a different perspective, aiming at producing a hydrocarbon fuel, i.e. formed by saturated hydrocarbons, without the addition of external H2 but using the self-generated hydrogen, or hydrogen-transfer reactions. For this, a catalytic system was carefully prepared and compared to the bare beta zeolite; a protonic Pd-containing beta zeolite has a similar texture, structure and acidity features as in the base zeolite. The characterisation showed that this catalyst was achieved. The operando FTIR catalytic results indicated that the Pd-beta zeolite reaches a high selectivity towards the C2-C5 and aromatics fractions, with better control of the composition of the C2-C5 fraction leading to high isoparaffinic content, and a relatively low share of olefinic compounds. Additionally, the retardation in the coke forming reactions was evidenced, due to the high Pd-hydrogenation efficiency. Further insights were rationalized about the coke content and the catalyst auto-regeneration. From the process point of view, it was found that the use of a beta zeolite having well-dispersed Pd is an effective way to upgrade the LDPE into a paraffinic-rich hydrocarbon fuel.

Evaluation of the Textural Parameters of Zeolite Beta in LDPE Catalytic Degradation: Thermogravimetric Analysis Coupled with FTIR Operando Studies

Zeolite-based catalysts are globally employed in many industrial processes, such as crude-oil refining and bulk chemical production. In this work, the cracking of low-density polyethylene (LDPE) was thoroughly followed in a FTIR operando study to examine the catalytic efficiency of purely microporous zeolites β of various textural characteristics. To provide complementary and valuable information on the catalytic activity of the zeolite β studied, the thermogravimetric analysis results were compared with yields of the products generated under operating conditions. The reaction products were analyzed via GC–MS to determine the hydrocarbon chain distribution in terms of paraffin, olefins, and aromatics. The individual impact of textural and acidic parameters on catalytic parameters was assessed. The accumulation of bridging hydroxyls of high strength in the zeolite β benefited the decrease in polymer decomposition temperature. Through a strategic comparison of purely microporous zeolites, we showed that the catalytic cracking of LDPE is dominated by the acidic feature inherent to the microporous environment.

Improved Catalytic Technology for Waste Plastic Processing: Toward Novel Remediation and Emission Control Measures

A mesoporous zeolite USY to process waste plastic, illustrated on low-density polyethylene, is presented. The technology is mobile and can be used as a remediation and control measure to the emissions in the open environment. The core of this approach lays on the catalytic material employed. It is a zeolite USY that possesses both micropores (up to 2 nm) and mesopores (between 2 and 50 nm) that was prepared by desilication using a mixture of mesopore inducing agents, tetra-butyl ammonium hydroxide and sodium hydroxide. This gives rise to a well-defined zeolite USY with double intracrystalline mesoporosity and higher intrinsic acidity. Those improvements had a positive impact on the cracking of low-density polyethylene, with a lowering of the cracking temperature, ascribed to the enhanced mass transfer of the reactant into the acid sites. The lowering of the reaction temperature reduces the energy requirements with operational savings of 0.50 MW with respect to the thermal process and 80 kW with respect to the untreated zeolite for a plant of 50 kt per year, though the scale of operation can be adjusted to the local requirements. The lower operation temperature triggered by the catalyst has also benefits in terms of lower capital investment since low-cost construction materials would be required. The zeolite preparation is industrially scalable. All these features make the deployment of this technology a realistic option.

The catalytic cracking of sterically challenging plastic feedstocks over high acid density Al-SBA-15 catalysts

The catalytic cracking of polyolefinic waste materials over solid acid catalysts, such as zeolites, is a promising process for the production of useful fuels and chemicals. However, the inherent diffusional constraints of the microporous zeolites restrict the access of bulky polyolefin molecules to the active site, therefore limiting their effectiveness. To address this, a simple yet effective method of producing mesoporous Al-SBA-15 materials with a high density of Brønsted acid sites has been employed. These catalysts are shown to be very active for the catalytic cracking of low density polyethylene (LDPE), a common waste plastic. The acidic and textural properties of the catalysts were characterised by ICP-OES, XPS, XRD, N2 physisorption, propylamine-TPD, pyridine-FTIR and STEM and have been correlated with their catalytic activity. The product distribution from the catalytic cracking of LDPE has been shown to depend strongly on both the pore architecture and the Al content of the SBA-15 and thus the density and strength of Brønsted acid sites. Fine-tuning the Al content of the SBA-15 materials can direct the product distribution of the hydrocarbons. The Al-SBA-15 materials display increased cracking orientated towards aliphatic hydrocarbons compared to ZSM-5, attributed to the mesoporous nature of SBA-15, overcoming diffusional limitations.

Catalytic cracking of polyethylene plastic waste using synthesised zeolite Y from Nigerian kaolin deposit

The increasing rate of accumulation of plastic waste (PW) is quite disturbing to the world, particularly in developing nations due to its non-biodegradable nature and inadequate waste management practices. The need to properly manage this waste and utilize the potential and chemical energy value that can be derived from this waste justifies the encouragement and employment of newer and better recycling methods and technology of these wastes. Therefore, this has led us to explore the catalytic pyrolysis of plastic waste using zeolite Y synthesized from kaolin deposit in Covenant University, Sango Ota, Ogun state of Nigeria. A stainless steel packed bed reactor was used in the cracking of low-density polyethylene (LDPE) plastic wastes into liquid fuel components at a temperature of 300 ℃ using zeolite Y catalyst. The liquid fuel obtained from the catalytic pyrolysis was analyzed using GC–MS. Fifty compounds were identified, which revealed the presence of mostly alkenes and aromatics in the hydrocarbons range of C8–C29. This is made up of 56% of gasoline fractions range of C6–C12, 26% of diesel and kerosene fractions range C13–C18, and 10% of fuel oil range C18–C23, while 8% is residual fuel range greater than C24.

Conversion of low density polyethylene (LDPE) over ZSM-5 zeolite to liquid fuel

Catalytic cracking of plastics waste to produce fuel is increasingly gaining the attention of researchers worldwide. The objective of this study is to assess the effects of temperature and liquid hourly space velocity (LHSV) on composition of liquid products from catalytic cracking of LDPE dissolved in benzene. A plausible reaction mechanism was also proposed. The catalytic cracking produced C1C8 hydrocarbons in all runs due to the effective heat exchange between polymer chains and catalysts. Analysis of products composition indicated that catalytic cracking of LDPE generally produced high amount of aliphatic branched-chain compounds, together with moderate amount of cyclic compounds. The reaction conditions also led to alkylation of benzene by the cracking products from LDPE. Hence, it is concluded that the catalytic cracking of LDPE is dominated by free radical mechanism, while the influence of carbenium ion is less pronounced due to low acidity of the catalyst. Overall, it is shown that catalytic cracking of LDPE solution in fixed bed reactor produced higher polymer conversion and liquid yield than conventional reactors and has the potential to extract hydrocarbon energy from recycled polymer waste.

Catalytic Cracking of LDPE Dissolved in Benzene Using Nickel-Impregnated Zeolites

Among the various processes and reactor designs in polymer cracking, catalytic cracking of polymer dissolved in solvent offers interesting features. In the present research, catalytic cracking of low-density polyethylene (LDPE) dissolved in benzene in a fixed-bed reactor was studied. The catalysts used included three zeolites in original form and nickel (Ni)-impregnated forms. The LDPE conversions achieved were high (>98%) despite the short retention times. The catalytic cracking of LDPE produced liquid in the gasoline range, gases ranged from C1 to C4, and hydrogen gas. The increase in catalyst acidity improved gas yield at the expense of the liquid yield. Although high catalyst acidity had less influence toward liquid product composition, it led to higher degree of cracking of the gaseous products. The dissolution of LDPE in benzene led to high cracking rate despite short retention time and produced liquid products that can be used as fuels.

Catalytic cracking of LDPE over nanocrystalline HZSM-5 zeolite prepared by seed-assisted synthesis from an organic-template-free system

The catalytic cracking of low density polyethylene (LDPE) was accomplished using nanocrystalline HZSM-5 zeolites. Nanocrystalline ZSM-5 zeolites were synthesized by a seed-assisted method from an organic-template-free system employing three different seeding strategies: two of these strategies based on aqueous clear solutions containing the ZSM-5 precursors (silanized and non-silanized) and the third strategy based on a commercial ZSM-5 zeolite in powder form. The plastic cracking reactions were carried out at 380–420°C, employing really mild conditions (plastic to catalyst mass ratio of 100). Nanocrystalline HZSM-5 zeolites prepared by seed-induced synthesis based on liquid solutions exhibit a superior catalytic activity than the reference microcrystalline zeolite. Conversion values very close to 100% are achieved at 420°C, by using these catalysts, while significant conversion values (>50%) are still attained at 380°C. Moreover, these nanocrystalline HZSM-5 zeolites promote the production of a high proportion of light olefins (>60% at 380°C), propylene, 1-butene, isobutene, being the main products (∼50%). Nanocrystalline HZSM-5 synthesized from a clear solution containing an organic structure-directing agent (TPA+) exhibits a 100% conversion value at 380°C. However, the economic and environmental advantages of the seed-assisted synthesis from a TPA+ free system and the remarkable activity showed by the nanocrystalline zeolites obtained, makes of these materials interesting catalysts in the LDPE catalytic cracking.

Mesoporous materials with enhanced porosity and acidity to obtain clean fuels from low-density polyethylene (LDPE) cracking

Mesoporous materials with Si/Al ratio 7, 19 and 23 have been synthesized from ZSM-5 zeolite precursors prepared from tetrapropylammonium cations-containing clear aluminosilicate solutions, in the presence of the surfactant cetyltrimethylammonium bromide, and by employing different precursors of secondary mesoporosity or mesoporogen agents, such as nanometric carbon particles (BP700 and BP2000) and the biopolymer chitosan. The synthesized materials were characterized by X-ray diffraction, FT infrared spectroscopy, thermogravimetric analysis, electron microscopy, chemical analysis and nitrogen adsorption. It has been found that the effect of the pore-forming agents depends on the Si/Al ratio of the material, it is very small for Si/Al = 7, while for Si/Al above 19, there is a substantial increase of the secondary mesoporosity due to the mesoporogen agent, being this effect more evident for the sample with Si/Al = 23. For this material, in addition to the porosity associated to pores with a narrow size distribution centered at 3.7 nm due to the surfactant, secondary mesopores with a size distribution centered at 20–30 nm for carbon nanoparticles and in the range 4–10 nm for chitosan, are also detected. The acidity of the samples increases with their Si/Al ratio, and no significant effect of the porogen agent on the acidity has been found. The catalytic properties of these materials were tested in the conversion of m-xylene and in the cracking of low-density polyethylene (LDPE). Noticeable differences in the activity and selectivity of the gas/liquid products obtained in the catalytic cracking of LDPE were observed.