Cracking Of Polyolefins Research Articles

Cr6+ Loaded Lewis Acidic Sn‐Beta Zeolites as Reusable Catalysts for Selective Production of Light Olefins via Polyolefin Cracking

AbstractCatalysts for the selective recovery of light olefins from polyolefins should be developed to save limited fossil resources. It is previously revealed that Brønsted acid‐free Sn‐Beta zeolites selectively produced light olefins from polyolefin cracking. Brønsted acid sites ionize olefins by protonation and the generated carbenium ions are converted into other hydrocarbons via various reactions, thereby the elimination of their effects led to the improvement of light olefin production ability. However, the deactivation of zeolites by coke deposition is a critical problem for industrialization. Inhibition of coking deactivation is previously achieved by doping Cr6+ species connected to silanol groups in zeolites. This work aimed to inhibit coke deposition and realize selective and continuous production of light olefins through repetitive polyolefin catalytic cracking. Sn‐Beta zeolites are prepared with various crystallinity by changing the feed fluoride ions content in their synthesis and found that their activity on low‐density polyethylene (LDPE) cracking strongly depended on the feed F/Si ratios, implying the active sites for polyolefin cracking are Lewis acidic open‐Sn sites. Moreover, Cr6+‐doped Sn‐Beta zeolites showed over 45% light olefin yields even in the third LDPE cracking test, although excess Cr adding led to reduce the catalytic activity of Sn‐Beta zeolites.

Exploring the Morphological Effect of Zeolite Beta on Catalytic Cracking of Polyethylene.

Two types of zeolite catalysts, namely, nanosized Beta-N and micrometer-sized Beta-M, were used to crack low-density polyethylene (LDPE) with three different molecular weights: 4000, 200,000, and 3,000,000. The structural and acidic properties were analyzed by N2 physisorption, transmission electron microscopy, X-ray diffraction, temperature-programmed desorption of isopropylamine (IPA-TPD), and pyridine-adsorbed FTIR. The catalytic activity was tested at 623 K and 3.5 N2 MPa in an autoclave batch reactor for PE cracking. High Mw PE required higher decomposition activation energy due to transfer limitation. Beta-N showed better activity in PE cracking than Beta-M, with PE conversion of 82.7 and 62.0% for Beta-N and Beta-M, respectively. In addition, the nanosized Beta-N exhibited quite lower activation energy of catalytic PE decomposition than Beta-M, obtained by the Kissinger method in TGA measurement. The characterization results demonstrated that the Beta-N has abundant interparticulate mesopores to provide better dispersion for the catalysts into PE melt and the proximity of the cracking active sites. These results revealed that the Beta-N catalyst shows superior activity for the cracking of polyolefins.

CO2-facilitated upcycling of polyolefin plastics to aromatics at low temperature.

Plastics are one of the most produced synthetic materials and largest commodities, used in numerous sectors of human life. To upcycle waste plastics into value-added chemicals is a global challenge. Despite significant progress in pyrolysis and hydrocracking, which mainly leads to the formation of pyrolysis oil, catalytic upcycling to value-added aromatics, including benzene, toluene and xylene (BTX), in one step, is still limited by high reaction temperatures (>500°C) and a low yield. We report herein CO2-facilitated upcycling of polyolefins and their plastic products to aromatics below 300°C, enabled by a bifunctional Pt/MnOx-ZSM-5 catalyst. ZSM-5 catalyzes cracking of polyolefins and aromatization, generating hydrogen at the same time, while Pt/MnOx catalyzes the reaction of hydrogen with CO2, consequently driving the reaction towards aromatization. Isotope experiments reveal that 0.2kg CO2 is consumed per 1.0kg polyethylene and 90% of the consumed CO2 is incorporated into the aromatic products. Furthermore, this new process yields 0.63kg aromatics (BTX accounting for 60%), comparing favorably with the conventional pyrolysis or hydrocracking processes, which produce only 0.33kg aromatics. In this way, both plastic waste and the greenhouse gas CO2 are turned into carbon resources, providing a new strategy for combined waste plastics upcycling and carbon dioxide utilization.

Elucidating the structure-performance relationship of typical commercial zeolites in catalytic cracking of low-density polyethylene

Zeolite exhibits excellent catalytic performance in the catalytic cracking of polyolefins, but the reaction results are often controversial due to the differences in the lab-scale synthesized zeolites and the types of reactors selected. In this work, in order to eliminate the differences of catalysts synthesized by individual researchers, we studied the catalytic performance of four typical commercial zeolites (HZSM-5, Hβ, HY and SAPO-11) in the cracking of low-density polyethylene (LDPE) by self-designed batch reactor. By comparing the reaction performance of four zeolites under the same condition and correlating with the pore structure and acidity of the zeolites, it was found that the acid strength of zeolites was positively correlated with the cracking conversion. HZSM-5 and Hβ zeolites with high acid strength achieved a LDPE conversion of above 94%. Large micropore is beneficial to the formation of isomerized products, but promotes the formation of coke precursors, resulting in serious coking. By analyzing the product distribution over different zeolites, it was observed that Hβ achieved the highest yield of gasoline components. HZSM-5 and HY exhibited high yields of aromatization products. The high aromatization activity of HZSM-5 was attributed to its strong acidity which enhanced the hydrogen transfer process, while HY possesses high concentrations of both Brönsted and Lewis acid sites, and the synergistic effect of these two acid sites promotes the aromatization process.

Influence of the Feedstock on the Process Parameters, Product Composition and Pilot-Scale Cracking of Plastics.

Chemical recycling of polymers can lead to many different products and play a significant role in the circular economy through the use of plastic waste as a feedstock in the production of valuable materials. The polyolefins: polyethylene (PE) and polypropylene (PP), together with polystyrene (PS), can be chemically recycled by the thermal cracking (pyrolysis) process. In this study, continuous cracking of polyolefins and polystyrene in different proportions and with the addition of other polymers, like polyethylene terephthalate (PET) and polyvinyl chloride (PVC), was investigated at the pilot scale in terms of the process parameters and product yields. Gas chromatography with mass spectrometry (GC-MS) was used for the detailed analysis of the products’ compositions. The boiling temperature distribution and the bromine number were used for additional characterization of products. It was found that an increase of PP share caused a decrease in the process temperature, an increase of the product yield and a shift of the boiling range towards lighter products, increasing the content levels for unsaturates and branched hydrocarbons. It was observed that the addition of 5% PS, PET and PVC reduced the overall product yield, resulting in the creation of a lower-boiling product and increasing the conversion of polyethylene. An addition of 10% polystyrene increased the PP conversion and resulted in a higher product yield, without significant change in the boiling temperatures distribution.

Obtainment of hierarchical ZSM-5 zeolites by alkaline treatment for the polyethylene catalytic cracking

Great efforts have been applied in order to achieve the synthesis of hierarchical zeolites through post-synthesis alkaline treatment. In this case, the use of hierarchical ZSM-5 zeolites as acid catalysts in the catalytic cracking of polyolefins overcomes some diffusional limitations and expands the selectivity of products. Thus, the purpose of work was to promote a secondary porosity in ZSM-5 zeolite by NaOH treatment (using conventional electric and microwave heating methods), applying cetyltrimethylammonium bromide as pore-directing agent. Both heating methods generated additional secondary porosity, with microwave heating promoting a narrow pore size distribution, when compared to conventional. All hierarchical ZSM-5 have been showed low-density polyethylene conversions superior to the purely microporous ZSM-5, however the hierarchical zeolite processed by conventional electric heating reached the higher temperature reduction (42.6 K). It was evidenced through measurements of morphological and textural properties, ion exchange capacity and catalytic activity in the LDPE cracking that the CTAB cations exerts a protective effect at alkaline treatment avoiding the desilication process. The additional secondary porosity promoting the generation of compounds of lower molecular mass in LDPE catalytic cracking, by thermogravimetric analysis, proving that the alkaline treatment increases the accessibility to the active acidic sites of zeolitic catalysts for bulky molecules.

High Olefin Yield in Pyrolysis of Heavier Hydrocarbon Liquids Using Microwave as Heat Supplier

In order to expand the scope of C2H4 pyrolysis material and recycle the waste polyolefin, this study has employed microwave pyrolysis of heavier hydrocarbon liquids. Microwave pyrolysis not only maintains a stable energy supply to the reaction materials for obtaining a high conversion rate, but also avoids coking caused by local overheating. Steam may serve a dual role as diluent and heat carrier. Thus, this study has obtained higher olefin yield, higher olefin selectivity, and better operational continuity than conventional steam cracking. Light olefins (C2═–C4═) reached 50.65–68.64 mol% in the pyrolysis gas. The yields of C2H4 and C3H6 for heavier hydrocarbons pyrolysis were 42.80–44.76 and 12.98–13.91 wt% of the feedstock, respectively, while the maximum yields of C2H4 and C3H6 for pyrolysis of liquid products of polyolefin cracking were 47.00 and 22.83 wt%, respectively. Analysis of the pyrolysis mechanism indicates that the β-scission reactions of free radicals are predominant, while the secondary re...

Cocracking Kinetics of PE/PP and PE/Hydrocarbon Mixtures (I) PE/PP Mixtures

Cocracking behaviors of polyethylene/polypropylene and polyethylene/hydrocarbon mixtures have been investigated using a closed batch reactor/tube reactor, followed by Thermogravimetric Analysis (TGA) at certain heating rate from ambient temperature to 873 K. In part I, the cocracking of HDPE, LDPE, and PP was divided into two stages for intensive study: polyolefin cracking at closed batch/tube reactor was performed using nonisothermal/isothermal method, and further, the thermogravimetric study of polyolefin and the cracked products from polyolefin cracking was realized using nonisothermal method. Presence of synergistic effect is observed for cocracking of polyolefin mixtures, and such synergistic effect not only exists in mild cracking using closed batch reactor/tube reactor but also exists in the thermal degradation process of thermogravimetric analysis. The kinetic studies were performed according to Coats and Redfern method for first-order reaction. It is found that the pyrolysis process can be descri...

Study on mild cracking of polyolefins to liquid hydrocarbons in a closed batch reactor for subsequent olefin recovery

The polyolefins mild cracking experiments have been carried out in a closed batch reactor under inert nitrogen atmosphere over a temperature range from 370 °C to 420 °C and the characterization of the mild cracking products has been investigated. The objective of this work is to study the cracking mechanism and reaction path for the mild cracking of polyolefins with lower heating rate (the isothermal and non-isothermal process), and to find the appropriate operating conditions (temperature and residence time) for optimal yield of liquid hydrocarbons (oils/waxes) as the suitable feedstock of olefin recovery. The advantages of this work can be summarized as high yield of light fraction, low aromatics content and mild cracking conditions. More viscosity-reducing and less volatiles-loss can be simultaneously achieved in mild cracking of polyolefins by selecting the reasonable residence time-temperature combination. The results confirm that chain scission reactions are significantly predominant in these experimental conditions. Intramolecular hydrogen transfer and intermolecular hydrogen transfer, which are responsible for the reduction in molecular weight of liquid products, can be improved in lower temperature and longer residence time, and intermolecular hydrogen transfer can also be enhanced in the PE/PP mixture cracking. For polyolefins mixture cracking, the cracking temperature can be reduced about 20 °C, or the cracking time can be shortened significantly, and so utilizing the whole polyolefins mixture without a need of further separation is very desirable in the chemical recycling.

ChemInform Abstract: Developing Advanced Catalysts for the Conversion of Polyolefinic Waste Plastics into Fuels and Chemicals

AbstractReview: 132 refs.

Developing Advanced Catalysts for the Conversion of Polyolefinic Waste Plastics into Fuels and Chemicals

The present review is aimed at exploring the field of the catalytic cracking of polyolefins over solid acids, focusing on the role played by the catalysts toward the synthesis of fuels and chemicals as well as on the reaction systems currently used. Initially, conventional solid acids, such as micrometer sized crystal zeolites and silica–alumina, were used to establish the relationship among their activity, selectivity, and deactivation in the polyolefin cracking and the inherent properties of the catalysts (acidity, pore structure); however, the occurrence of steric and diffusional hindrances for entering the zeolite micropores posed by the bulky nature of the polyolefins highlighted the importance of having easily accessible acid sites, either through mesopores or by a high external surface area. This fact led toward the investigation of mesoporous materials (Al-MCM-41, Al-SBA-15) and nanozeolites, which allowed increasing the catalytic activities, especially for the case of polypropylene. Further advan...

Continuous Polyolefin Cracking on an HZSM-5 Zeolite Catalyst in a Conical Spouted Bed Reactor

The effect of operating conditions on product yields and compositions has been studied in the catalytic cracking of polyolefins (high- and low-density polyethylene and polypropylene) in a conical spouted bed reactor. The catalyst has been prepared by agglomerating an HZSM-5 zeolite (SiO2/Al2O3 = 30) with bentonite and alumina. The study has been carried out in continuous mode, in the 450–570 °C range, with polyolefin flow rates between 1 and 2 g min–1 and feeding up to 1 kg of polyolefin into a 30 g catalyst bed. The result is an excellent performance of the reactor–catalyst pairing for continuous catalytic pyrolysis, given that the yields obtained in the pyrolysis of high-density polyethylene at 500 °C without defluidization problems and very low catalyst deactivation are as follows: 60 wt % C2–C4 olefins, 15 wt % nonaromatic C5–C11 hydrocarbons, 10 wt % single-ring aromatics, and 14 wt % C4- alkanes. Only small differences are observed in the pyrolysis of the different polyolefins.

Catalytic properties in polyolefin cracking of hierarchical nanocrystalline HZSM-5 samples prepared according to different strategies

Two series of hierarchical nanocrystalline ZSM-5 zeolites prepared by different synthesis strategies (at low temperature and from silanized seeds) and with external surface areas ranging from 150 to 250 m 2 g −1 were tested in the cracking of pure LDPE and HDPE at 340 °C and of waste polyethylene at 360 °C. Hierarchical zeolites showed quite higher activity, with values even six times higher than a standard nanocrystalline sample used as reference ( n-HZSM-5). The activity values decreased from LDPE to HDPE due to the occurrence of some degree of branching in the former polymer, which act as preferential cracking sites. The major products were C 1–C 4 hydrocarbons (in the range 30–70%, mostly C 3–C 4 olefins) and C 5–C 12 hydrocarbons (20–60%), whose share depends on both the polyolefin and the catalyst. The amount of C 13–C 40 hydrocarbons was practically negligible (<1%) due to the high acid strength of the zeolites which promotes end-chain cracking reactions. Likewise, hierarchical nanocrystalline HZSM-5 zeolites prepared from silanized protozeolitic units showed higher activities than the hierarchical nanocrystalline HZSM-5 samples synthesized at low temperature and atmospheric pressure. The differences were especially remarkable in the case of waste polyethylene cracking. These results were ascribed to the stronger acidity of the hierarchical zeolite samples prepared from silanized seeds.

Catalytical and thermal pyrolysis of polyolefins

Polyolefins have a high potential for alternative oil production since they contain only carbon and hydrogen atoms. By pyrolysis of these materials up to 95% can be obtained as oil and gas. Upgrading the products by catalytic cracking of polyolefins is a subject of growing interest in the last years as less energy is needed for the pyrolysis and more valuable products are formed. Numerous studies have been reported in which a variety of catalysts such as zeolites, silica-alumina, mesoporous MCM-41, solid acids and reaction conditions have been investigated. In our studies we used Lewis acids and mixtures of Ziegler–Natta catalyst such as TiCl 4, AlCl 3 to pyrolyse polypropylene. Experiments were carried out in a batch reactor as well as in a fluidized bed process. The pyrolysis temperature can be decreased by 100 °C compared to runs without catalysts. A drastic increase in the amount of low boiling compounds (C4 hydrocarbons) can be observed by the use of the catalysts instead of longer chained hydrocarbons.

00/03733 Requirements for, and benefits of, environmentally sound and economically viable management of battery recycling in the Philippines in the wak

Polyolefins have a high potential for alternative oil production since they contain only carbon and hydrogen atoms. By pyrolysis of these materials up to 95% can be obtained as oil and gas. Upgrading the products by catalytic cracking of polyolefins is a subject of growing interest in the last years as less energy is needed for the pyrolysis and more valuable products are formed. Numerous studies have been reported in which a variety of catalysts such as zeolites, silica-alumina, mesoporous MCM-41, solid acids and reaction conditions have been investigated. In our studies we used Lewis acids and mixtures of Ziegler–Natta catalyst such as TiCl4, AlCl3 to pyrolyse polypropylene. Experiments were carried out in a batch reactor as well as in a fluidized bed process. The pyrolysis temperature can be decreased by 100 °C compared to runs without catalysts. A drastic increase in the amount of low boiling compounds (C4 hydrocarbons) can be observed by the use of the catalysts instead of longer chained hydrocarbons.

Catalytic conversion of polyolefins into fuels over zeolite beta

The catalytic degradation of polypropylene and both low- and high-density polyethylene over different samples of zeolite beta has been investigated. Zeolite beta synthesized by the fluoride method has a low activity for the cracking of polyolefins, which is related to its large crystal size (12 μm) and poor aluminium incorporation. In contrast, zeolite beta synthesized from amorphous xerogels consists of small crystallites (⩽200 nm) and allows all the polyolefins to be degraded with high conversion (40–60%) and good selectivity towards C 5–C 12 hydrocarbons (60–70%), heavier products being hardly obtained (<6%). The incorporation of titanium into the BEA structure has been proved to enhance the catalytic activity, as denoted by the highest conversion per Al atom obtained with the Ti–Al-beta sample compared to Al-beta.