2.2. Catalytic Hydropyrolysis
The oil obtained through catalytic pyrolysis is dark in color, heavy in odor, has a wide distribution of hydrocarbon products, and inevitably undergoes carbonization, and cooking, which requires further purification to improve the quality of the Pyrolysis fuel oil. Introducing hydrogen into catalytic cracking can reduce cooking and catalyst carbon deposition while increasing the yield and selectivity of gasoline and diesel components. In recent years, hydrocracking of plastics has gained widespread interest. Hydrocracking conditions are 200-400 oC and 15-30 MPa hydrogen pressure, Hydrocracking follows the same random bond cleavage mechanism as high temperature pyrolysis, and the free radicals generated by C-C bond cleavage are saturated with hydrogen. The distribution range of hydrocracking products is relatively narrow, consisting mostly of short-chain saturated hydrocarbons, with a tiny amount of olefins, aromatic hydrocarbons, and coke. Moreover, the addition of hydrogen has an effect on the removal of sulfur, chlorine, bromine and other heteroatoms contained in plastics.
High temperatures and hydrogen pressures can accomplish hydrocracking, but a catalyst facilitates the reaction’s efficiency. Plastic hydrocracking catalysts usually have two functions: cracking and hydrogenation-dehydrogenation. A typical hydrocracking catalyst has an acidic support with metal impregnated over it[
50]. The metal functions is hydrogenation/dehydrogenation in the hydrogenolysis reaction, hydrogenating olefinic intermediates that have been released from the acid sites and dehydrogenating saturated reactant molecules into olefins. The acidic support is responsible for cracking and isomerization reaction. In the process of carbon-carbon bond cleavage and rearrangement, the heat release of hydrogenation and heat absorption of cracking are the key factors for the mild reaction conditions of hydrocracking. The metal catalyst hydrogenates the intermediates and prevents catalyst coking, the carrier has an effect on the product comparable to catalytic cracking.
Lee et al. plotted the different catalysts in a coordinate system, with the horizontal and vertical coordinates indicating the activity of the acid sites and metal sites. As illustrated in
Figure 3, comparing the activity of different catalysts can quickly estimate the product distribution of depolymerization[
51]. The figure clearly demonstrates that Ru-based metal catalysts have much higher hydrogenolysis activity compared with other metal-based catalysts. Kots et al. reported the conversion of PP waste plastics to lubricating oils using a Ru/TiO
2 catalyst at a low temperature of 250
oC under moderate H
2 pressure with yields up to 80 %. Ru catalysts have significant hydrolytic activity on carbon-carbon bonds. significant yields of C
1–C
4 hydrocarbon gases were produced by Ru catalysts loaded on various carriers for PP cracking tests. Ru/C> Ru/CeO
2> Ru/SiO
2> Ru/Al
2O
3 produced the highest methane yield, which reached 83%[
52]. Further investigations tuned the catalyst’s electronic properties while holding the Ru particle size and physical characteristics constant by modifying the synthesis using ammonia. Ammonia dehydroxylates the TiO
2 surface reducing the density of Ti-OH groups. This, in turn, increases the Lewis acid strength of Ti
4+ sites. For the NH
3 treated sample, spillover reduces TiO
2 more extensively and forms delocalized ē in the titania conduction band in addition to the shallow traps. Ru particles bind more hydrogen, not only covalently but also through weak interactions involving Ru conduction electrons. This accelerates the rate of hydrogenolysis and consequently leads to a higher gas output when there is a significant amount of hydrogen [
53]. Subsequently, Rorrer et al. reported a Ru/C catalyst for hydrocracking of polyethylene at low temperature of 200
oC and 20 bar H
2. At a reaction time of 16 hours, the product is mostly gas up to 55% and the remaining 45% is composed of liquid n-alkanes [
54].
Hydrocracking also makes extensive use of metal Pt in addition to Ru-based catalysts. Celik et al. developed a Pt/SrTiO
3 nano cube catalyst with supported Pt nanoparticles to catalyze polyethylene. They observed that Pt exhibits excellent adsorption capability for polyethylene, and Pt/SrTiO
3 selectively binds to longer hydrocarbon chains at the catalytic sites of Pt for hydrolysis reaction. The process converted polyethylene into lubricants and waxes at 1.17 MPa H
2 and 300
oC for 96 h [
55]. A Pt-loaded ordered mesoporous SiO
2 catalyst that packs Pt nanoparticles as active sites at the bottom of a SiO
2 mesoporous shell layer was described by Tennakoon et al. Mesoporous SiO
2 can cause polyethylene chains to adopt a lengthy sawtooth structure, making it challenging for the long chains to leave until they connect with the Pt active site at the bottom of the pore and the large molecules are cut into micromolecules. The cracking process is shown in
Figure 4, mSiO
2/Pt/ SiO
2, compared with Pt/ SiO
2, can decompose polyethylene into small molecular alkanes with a concentrated carbon number distribution at 300
oC for 24 h and 1.72 MPa H
2 pressure condition. This regular spaced and selective cleavage from the end produces a controlled product distribution, which points to a solution to the problem of macromolecules’ limited access to the microporous structure and catalyst deactivation due to pore plugging [
56].
Metal-loaded zeolites are also used as catalysts for hydrocracking. It facilitates the production of isomerization reaction, hence improving the octane number of the gasoline.
Table 2 compares the performance of molecular sieves load with different metals for the hydrocracking of plastics. Bin Jumah et al. used PT-impregnated USY and beta zeolites to hydrocrack LDPE and squalane. Because the pore shape and the catalyst’s proper acidity, it provides good selectivity for C
4-C
6 alkane isomers[
57]. Liu et al. compared HY-zeolite cracking plastics with Pt-loaded WO
3/ZrO
3 and HY-shared hydrogenated cracking plastics. The conversion rate of Pt loaded WO
3/ZrO
3 and HY can reach 93% in 2 h, and predominantly yielding light olefins. A synergistic catalytic effect occurs when a metal-supported catalyst and zeolite work together. Furthermore, the acidity of HY in catalysis significantly influences the efficacy and selectivity of the reaction. Decreasing the acidity leads to reducing light olefins and alkanes in the liquid product[
58].
Catalytic hydrocracking is typically performed in a fixed-bed reactor. The process is influenced by temperature, hydrogen pressure, and reaction time. Rising the reaction temperature tends to increase the conversion rate of plastics. The temperature range for plastic hydrocracking generally does not exceed 400 oC, excessive temperatures can result in increased gas and coke. The H2 pressure must be controlled within the appropriate range during the reaction Hydrogen pressure has an impact on product selectivity. Increasing the hydrogen pressure leads to increasing in the light hydrocarbons. This suggests that the pressure increase facilitates hydrogen blended in molten polymer and rearranged the alkanes to get short chain hydrocarbons. Growing up for applications in industry is hindered by the extreme pressure conditions, which must reach tens of Bar for the hydrocracking reaction to take place. Hydrogen is very expensive as a non-renewable fossil fuel resource, high hydrogen pressure also necessitates increased hydrogen consumption, which raises the cost of this method and poses a significant obstacle to the hydrocracking of polymers.
2.3. Solvolysis
Solvolysis is extensively employed for the degradation of oxygenated plastics. This approach breakdown the plastics into individual monomers through nucleophilic assault on the C=O link. Furthermore, studies have demonstrated that solvents can serve as hydrogen during the breakdown of polymers and may also serve as a uniform medium for heat transfer in the pyrolysis process [
62].
Solvolysis often involves hydrolysis and alcoholysis. Hydrolysis can be categorized as acidic hydrolysis, neutral hydrolysis, and alkaline hydrolysis based on the acidity level of the aqueous solution. PET plastic may be broken down into high purity terephthalic acid (TPA) and ethylene glycol (EG) in tests by hydrolyzing ester groups at 200-300
oC and 1-4 MPa [
63]. Solvent hydrolysis of PET also requires metal centers with high hydrogenation properties and acidic carriers with strong C-O and C-C bond activation capabilities. Both Jing and Lu reported the hydrolysis of PET with Ru/Nb
2O
5 catalyst. Jin Y et al. used Ru/ Nb
2O
5 catalysts for the degradation of PET. The major Ru species on Nb
2O
5 are those with low coordination numbers (C.N. = 5~6), the small Ru clusters restrain the co-adsorption of H
2 and aromatic rings due to the increased barrier for aromatic ring adsorption on metal clusters compared to flat surfaces. Meanwhile, the adsorption of H
2 is less affected by the particle size effect and therefore Ru sub-nano particles maintain sufficient capability of H
2 dissociation to assist C–O/C–C cleavage. The cooperative effects of different components of catalyst are highlighted in
Figure 5. The carrier of Nb
2O
5 has a low coordination number of sub-nanoparticles, which prevents the benzene ring from hydrogenating. The poor performance of the Pd and Pt catalysts is due to the rapid hydrogenation of the aryl ring resulting in a stronger chemical bond C-O, which makes the monomer yields of both Pd/Nb
2O
5 and Pt/Nb
2O
5 in the process lower than Ru/ Nb
2O
5[
64]. Lu et al. also believe that decarboxylation and hydrolysis are the decisive steps for the conversion of PET to BTX. In the experiment, it was found that the use of Ru/Nb
2O
5, which is conducive to hydrolysis, can obtain better depolymerization than that of Ru/NiAl
2O
4 which tend to decarboxylate [
65]. Kang et al. conducted a study where he used modified ZSM-5 to hydrolyze PET into aromatic hydrocarbons. He found that the quantity of B acid sites and the acidity level of ZSM-5 are crucial factors in the breakdown of PET in water-based solvents water and hydrogen ions cause the carbonyl group of PET to be transformed into a hydroxyl group when B acid is present. This transformation reduces the thermodynamic activation energy and facilitates the hydrolysis process of esters [
66].
Alcoholysis is generally transesterification reaction carried out under the condition of organometallic catalyst. The solvents commonly used for alcoholysis are methanol, ethylene glycol, propylene glycol, etc. Studies on catalysts for alcoholysis have focused on alcohol dehydrogenation catalysts and hydrodeoxygenated catalysts. Using methanol as a solvent and hydrogen source. Gao et al. proposed a low-cost method for the one-step quantitative conversion of PET to paraxylene (PX) and EG at 210
oC with modified Cu/SiO
2 catalysts. The generation of PX occurs through the tandemed methanolysis of PET and the selective hydrodeoxygenation (HDO) of DMT, then CuNa/SiO
2 catalyzed transformed the 100% yield of dimethyl terephthalate (DMT) into 93.6% PX [
67]. Tang et al. divided the alcoholysis of PET into three steps, starting with the dissolution of PET and methanol at 200
oC. Subsequently, the dissolved product DMT was hydrogenated to obtain cyclohexane-1,4-dicarboxylic acid diol ester (DMCD) under the action of Pt/ C as a catalyst and 5 MPa H
2. Finally, the final product was obtained by hydrogen deoxygenation (HDO) catalyzed by DMCD under solvent-free conditions with Ru-Cu/SiO
2 [
68]. The mechanism of three-step alcoholysis is similar to the one-step reaction, Although the process is cumbersome, obtained a large proportion of C
7-C
8 cycloalkanes and aromatic that can be used as gasoline or additives to improve the densities and sealabilities of current bio-jet fuels.
The utilization of solvent decomposition techniques incurs higher costs, mostly attributable to their longer reaction times and the constraints faced with catalysts based on precious metals. In order to achieve widespread use, it is imperative to discover more cost-effective catalysts. Additionally, the decomposition of solvents poses a significant obstacle for particular kinds of plastics. It is crucial to conduct screening for inexpensive solvents that have the ability to dissolve various polymers while also eliminating colorants and other additives.
2.4. Supercritical Water Liquefaction
Supercritical fluid means the state of a substance when it is under conditions that exceed both its critical pressure and critical temperature. Supercritical fluids exhibit characteristics that both gases and liquids, such as dielectric constants, ionic strengths, and material densities undergo fast changes. It displays increases in solubility, molecular diffusion capacity, viscosity, and other molecular properties, resulting in a significant enhancement of heat and mass transfer efficiency in chemical reactions.
When plastics are dissolved in solvents for pyrolysis, the efficiency of solvent dissolution is enhanced when the system is in a supercritical state. This leads to a decrease in the reaction time and reaction temperature, and increase the conversion rate of waste plastics pyrolysis [
69]. In the early stages of researching supercritical liquefaction of plastics, scientists utilized solvents like methanol and ethanol to breakdown plastics such as PET and PC into chemical monomers [
70]. Recently, it has been shown that degradation of PS in a mixture of supercritical water and CO
2 yields the products hydrogen, methane and carbon dioxide [
71]. Achieving a supercritical state is the main goal of the supercritical water thermal liquefaction process, and water as a solvent is an ideal plastic cycle path. The development of supercritical water technology stemmed from disposing sewage treatment requirements, and now its application has expanded to fossil fuels as well. Because supercritical water has the common characteristics of gaseous and liquid, it can provide homogeneous reaction medium and high reaction rate for the decomposition of hydrocarbons, supercritical water is also suitable for free radical reaction, which can effectively inhibit condensation reaction and coking reaction in the experiment of plastic oil production to prevent excessive by-products, and finally obtain efficient and clean fuel.
There are two methods for breakdown waste plastics in supercritical water. One is to convert plastic water into gas fuel, primarily consisting of hydrogen and methane gas [
72,
73]. And the other is to liquify plastic water into liquid fuel, specifically short-chain hydrocarbons. The hydrothermal liquefaction degradation process includes reactions, such as depolymerization, gasification, aromatization, and others. The difference in reaction conditions between the two techniques refers to temperature and pressure, with hydrothermal gasification requiring a higher experimental temperature.
The hydrothermal liquefaction degradation process includes reactions, such as depolymerization, gasification, aromatization, and others. The difference in reaction conditions between the two techniques refers to temperature and pressure, with hydrothermal gasification requiring a higher experimental temperature [
74]. Onwudili et al. employed RuO
2/Al
2O
3 as a catalyst for supercritical hydrothermal gasification of plastics under conditions of 45MPa, 450
oC, and 1 hour. The plastic achieved a carbon gasification efficiency of 95%, while the hydrogen gasification efficiency surpassed 100%, suggesting that water contributed hydrogen during the experiment. Simultaneously, the generation of carbon monoxide (CO) and carbon dioxide (CO
2) in the oxygen-free low-density polyethylene (LDPE) also confirmed the participation of water in the reaction [
75]. Chen et al., reacted in supercritical water at 425
oC for 2-4 hours, PP was converted into oil up to 91%, and proposed that supercritical water serves as both an efficient reaction media and a catalyst [
76].
The supercritical water liquefaction is applicable to a wide range of polymers, including PP and PE. And high-density plastics which are not suitable for pyrolysis, such as PC and PET can be decomposed into useable monomers and high-value compounds. Xu et al. conducted comparative experiments on supercritical hydrothermal liquefaction products of different kinds of plastics, Generally, the calorific value of oil produced by oxygenated plastics is low. PC can produce the oil production rate can reach 50% under mild conditions [
77].
The ability of supercritical hydrothermal liquefaction is influenced by several parameters, with temperature, residence time, and water-carbon ratio. Higher temperatures promote the gasification rather than liquefaction in the reaction. Consequently, a greater amount of gas is obtained when the temperature is high [
60]. The isomerization of PP and PS and the dehydration reaction of PET and PC were promoted by increasing the temperature in the supercritical water plastic degradation experiment [
78]. Residence time refers to the time elapsed after the reactor temperature reaches a set temperature. With prolonging reaction time, the yield of gases and solids will increase [
79]. The composition and product yield are both impacted by the water-carbon ratio, while the gasoline distillate production will decline as the water/PE ratio increases. On the contrary, the water/PE ratio increase will result in an increase in diesel and heavy oil distillate production. Bai, B. Suggested that surpassing the hydrolysis capacity, increasing the water-carbon ratio will decrease the carbon conversion rate [
80]. The findings demonstrated that an appropriate water-carbon ratio could enhance the efficiency of plastic liquefaction. Additionally, optimizing the amount of plastic added to match the carbon required for the reaction of H+ ions in the solution produce the most favorable outcome in terms of plastic conversion.
The concept of supercritical water convert plastics to fuel was introduced in the previous century. Many of the studies conducted on the degradation and recycling of high-density polymers, such as PC and PET. The plastic recycling process faces significant obstacles in dealing with the mixture of plastic components and contains with organic materials. The exceptional solubility of supercritical water proves advantageous in addressing this issue. Supercritical extraction shows promise in the manufacture of waste plastic oil due to its high extraction efficiency, sustainable consumption, and environmental protection advantages.
2.5. Tandem technology of degradation
The industrialization of plastic pyrolysis technology has consistently encountered the reaction conditions that require high temperature and high pressure. Hydropyrolysis and solvolysis lower the temperature which plastic convert into fuel oil, however the approach is expensive. Zhang et al. proposed tandem cracking-alkylation method which degradation of PE and PP plastics into C
6-C
10 liquid iso-alkanes at a low temperature of less than 100
oC and in a highly ionized reaction environment without adding hydrogen and solvents.
Figure 6 depicts the reaction process. This reaction involves the sequential coupling of endothermic polymer cracking with exothermic alkylation. Ionic liquids containing metal salts have a polar environment, which can stabilize carbocation transition states by increasing the reactivity and standard chemical potential of non-ionic reactants. And allows the degradation of waste polymers at low temperatures and a lower free energy barrier[
81].
To broaden the scope of solvolysis applications. Jia et al. reported a series alkane metathesis (CAM) method for the degradation of PE plastics with C-C bonds, The method uses light alkanes as solvent and two catalysts for the dehydrogenation of alkanes and the cross olefins metathesis in the degradation process, respectively. The reaction employs iridium complex as catalyst for alkane dehydrogenation, aimed to remove the hydrogen from PE and light alkane (petroleum ether). This process results in the formation of unsaturated molecules and Ir-H
2. And the Re
2O
7/r-Al
2O
3 is employed to facilitate disruption of the olefin and the cleavage of the PE chain. Subsequently, and the final olefin is hydrogenated with Ir-H
2 to produce saturated alkanes. The polyolefin was completely transformed to low molecular weight oil and wax at 175
oC for 24h. Light alkanes in this process dissolve PE as a solvent, create a diluted solution with low viscosity, and serve as a source of hydrogen for further hydrogenation, Another advantage of this strategy is that the oil and wax in the product can be controlled by the reaction time, which allows the PE to be eventually converted to short hydrocarbons [
82].
Tandem hydrogenolysis/aromatization which is a low temperature and pressure was proposed by Zhang et al.
Figure 7 illustrates the chemical pathway, and polymers degrade without solvent and hydrogen. This approach combination of an exothermic hydrogenation reaction and an endothermic aromatization reaction. PE was degraded to linear alkyl aromatics mixture by using Pt-supported alumina as catalyst at a low temperature of 280
oC, and the conversion rate reached 80% [
83].