Catalysts For Depolymerization Research Articles

Pyrolytic Depolymerization of Polyolefins Catalysed by Zirconium‐based UiO‐66 Metal–Organic Frameworks

AbstractPolyolefins such as polyethylenes and polypropylenes are the most‐produced plastic waste globally, yet are difficult to convert into useful products due to their unreactivity. Pyrolysis is a practical method for large‐scale treatment of mixed, contaminated plastic, allowing for their conversion into industrially‐relevant petrochemicals. Metal–organic frameworks (MOFs), despite their tremendous utility in heterogeneous catalysis, have been overlooked for polyolefin depolymerization due to their perceived thermal instabilities and inability of polyethylenes and polypropylenes to penetrate their pores. Herein, we demonstrate the viability of UiO‐66 MOFs containing coordinatively‐unsaturated zirconium nodes, as effective catalysts for pyrolysis that significantly enhances the yields of valuable liquid and gas hydrocarbons, whilst halving the amounts of residual solids produced. Reactions occur on the Lewis‐acidic UiO‐66 nodes, without the need for noble metals, and yield aliphatic product distributions distinctly different from the aromatic‐rich hydrocarbons that can be obtained from zeolite catalysis. We also demonstrate the first unambiguous characterization of polyolefin penetration into UiO‐66 pores at pyrolytic temperatures, allowing access to the abundant Zr‐oxo nodes within the MOF interior for efficient C−C cleavage. Our work highlights the potential of MOFs as highly‐designable heterogeneous catalysts for depolymerisation of plastics, which can complement conventional catalysts in reactivity.

Pyrolytic Depolymerization of Polyolefins Catalysed by Zirconium-based UiO-66 Metal-Organic Frameworks.

Polyolefins such as polyethylenes and polypropylenes are the most-produced plastic waste globally, yet are difficult to convert into useful products due to their unreactivity. Pyrolysis is a practical method for large-scale treatment of mixed, contaminated plastic, allowing for their conversion into industrially-relevant petrochemicals. Metal-organic frameworks (MOFs), despite their tremendous utility in heterogeneous catalysis, have been overlooked for polyolefin depolymerization due to their perceived thermal instabilities and inability of polyethylenes and polypropylenes to penetrate their pores. Herein, we demonstrate the viability of UiO-66 MOFs containing coordinatively-unsaturated zirconium nodes, as effective catalysts for pyrolysis that significantly enhances the yields of valuable liquid and gas hydrocarbons, whilst halving the amounts of residual solids produced. Reactions occur on the Lewis-acidic UiO-66 nodes, without the need for noble metals, and yield aliphatic product distributions distinctly different from the aromatic-rich hydrocarbons that can be obtained from zeolite catalysis. We also demonstrate the first unambiguous characterization of polyolefin penetration into UiO-66 pores at pyrolytic temperatures, allowing access to the abundant Zr-oxo nodes within the MOF interior for efficient C-C cleavage. Our work highlights the potential of MOFs as highly-designable heterogeneous catalysts for depolymerisation of plastics, which can complement conventional catalysts in reactivity.

Depolymerization of Native Lignin over Thiol Capped Ultrathin ZnIn2S4 Microbelts Mediated by Photogenerated Thiyl Radical

AbstractThe valorization of native lignin to functionalized aromatic compounds under visible light is appealing yet challenging. In this communication, colloidal mercaptoalkanoic acid capped ultrathin ZnIn2S4 (ZIS) microbelts was successfully fabricated, which was used as a superior catalyst for depolymerization of native lignin in birch woodmeal under visible light, with an optimum yield of 28.8 wt % to functionalized aromatic monomers achieved in 8 h. The capped mercaptoalkanoic acid not only enables a solvent modulated reversible interchange of ZIS between the colloidal state for efficient reaction and the aggregated state for facile separation, but also serves as a precursor for light initiated generation of reactive thiyl radical for highly selective cleavage of β‐O‐4 bond in native lignin. This work provides a green and efficient strategy for the depolymerization of native lignin to functionalized aromatic monomers under mild conditions, which involves a new mechanism for the cleavage of β‐O‐4 bonds in native lignin. The capability of cleavage of β‐O‐4 bonds in native lignin by photogenerated thiyl radicals also demonstrates the great potential of using photogenerated thiyl radicals in organics transformations.

Depolymerization of Native Lignin over Thiol Capped Ultrathin ZnIn2S4 Microbelts Mediated by Photogenerated Thiyl Radical.

The valorization of native lignin to functionalized aromatic compounds under visible light is appealing yet challenging. In this communication, colloidal mercaptoalkanoic acid capped ultrathin ZnIn2S4 (ZIS) microbelts was successfully fabricated, which was used as a superior catalyst for depolymerization of native lignin in birch woodmeal under visible light, with an optimum yield of 28.8 wt % to functionalized aromatic monomers achieved in 8 h. The capped mercaptoalkanoic acid not only enables a solvent modulated reversible interchange of ZIS between the colloidal state for efficient reaction and the aggregated state for facile separation, but also serves as a precursor for light initiated generation of reactive thiyl radical for highly selective cleavage of β-O-4 bond in native lignin. This work provides a green and efficient strategy for the depolymerization of native lignin to functionalized aromatic monomers under mild conditions, which involves a new mechanism for the cleavage of β-O-4 bonds in native lignin. The capability of cleavage of β-O-4 bonds in native lignin by photogenerated thiyl radicals also demonstrates the great potential of using photogenerated thiyl radicals in organics transformations.

Developing sustainable and energy efficient process to obtain high purity cellulose and its value-added chemicals from rice straw

This study addresses the environmental impact of traditional rice straw burning practices in agriculture and explores sustainable alternatives. Utilizing lignocellulose waste, the research developed an energy-efficient process involving Alkali Pretreatment and Acid & Bleaching Pretreatment to obtain highly purified cellulose, after that analysed by qualitative and quantitative analysis, FT-IR and the Van Soest method were used, respectively. Thermal stability and crystallinity of acid-treated rice straw and bleached-treated rice straw were evaluated via DSC. The outcomes showed that different pretreatments produced purified cellulose and removed 93 % of the lignin. By using transition metal catalysts for depolymerization, value-added chemicals were produced, as measured by HPLC, generated glucose, Sorbitol, Arabinose, Ribose, and organic acids (HMF, THF). Overall, this study provides insights into the potential use of rice straw as a substrate for extraction of high-purity cellulose and value-added chemicals production, offering a reliable and energy-efficient process.

Dual Recycling of Depolymerization Catalyst and Biodegradable Polyester that Markedly Outperforms Polyolefins.

Chemically recyclable, circular polymers continue to attract increasing attention, but rendering both catalysts for depolymerization and high-performance polymers recyclable is a more sustainable yet challenging goal. Here we introduce a dual catalyst/polymer recycling system in that recyclable inorganic phosphomolybdic acid catalyzes selective depolymerization of high-ceiling-temperature biodegradable poly(δ-valerolactone) in bulk phase, which, upon reaching suitable molecular weight, exhibits outstanding mechanical performance with a high tensile strength of ≈66.6 MPa, fracture strain of ≈904 %, and toughness of ≈308 MJ m-3 , and thus markedly outperforms commodity polyolefins, recovering its monomer in pure state and quantitative yield at only 100 °C. In sharp contrast, the uncatalyzed depolymerization not only requires a high temperature of >310 °C but is also low yielding and non-selective. Importantly, the recovered monomer can be repolymerized as is to reproduce the same polymer, thereby closing the circular loop, and the recycled catalyst can be reused repeatedly for depolymerization runs without loss of its catalytic activity and efficiency.

Dual Recycling of Depolymerization Catalyst and Biodegradable Polyester that Markedly Outperforms Polyolefins

AbstractChemically recyclable, circular polymers continue to attract increasing attention, but rendering both catalysts for depolymerization and high‐performance polymers recyclable is a more sustainable yet challenging goal. Here we introduce a dual catalyst/polymer recycling system in that recyclable inorganic phosphomolybdic acid catalyzes selective depolymerization of high‐ceiling‐temperature biodegradable poly(δ‐valerolactone) in bulk phase, which, upon reaching suitable molecular weight, exhibits outstanding mechanical performance with a high tensile strength of ≈66.6 MPa, fracture strain of ≈904 %, and toughness of ≈308 MJ m−3, and thus markedly outperforms commodity polyolefins, recovering its monomer in pure state and quantitative yield at only 100 °C. In sharp contrast, the uncatalyzed depolymerization not only requires a high temperature of >310 °C but is also low yielding and non‐selective. Importantly, the recovered monomer can be repolymerized as is to reproduce the same polymer, thereby closing the circular loop, and the recycled catalyst can be reused repeatedly for depolymerization runs without loss of its catalytic activity and efficiency.

Catalytic oxidation of lignin and model compounds over nano europium oxide

The heterogeneous catalyst for depolymerization of lignin has a good application prospect because the catalyst is easy to recover. Although heterogeneous noble metal catalysts exhibit excellent activity under mild conditions, high prices restrict their wide use. In recent reports, nano rare earth oxide catalysts have shown good performance in the catalytic oxidation of lignin, and their prices are relatively low, which has good research value. Our study found that rare earth oxide nanocatalysts have obvious structure-activity relationship in the catalytic oxidation of lignin. To investigate theses structure-dependent behaviour and the reaction mechanism, a series of shape-specific Eu2O3 nanocrystal catalysts (spheres, spindles, sheets and rods) with well-defined crystal facets were synthesized and investigated. The four catalysts show increasing conversions from 54.6% to 91.2% in the sequence of nanospindles, nanospheres, nanosheets and nanorods, under condition of 1 MPa O2, 493 K and 8 h. These results demonstrate that the Eu2O3 nanocrystal is structure-sensitive. It is found that the surface crystal facets of Eu2O3 nanocrystals play crucial roles in the catalytic properties. Among the nanocrystals, Eu2O3 nanorods with well-defined (220) crystal facets displayed superior catalytic activity. It is deduced that the high reactivity of Eu2O3 nanorods is related to the abundant lattice oxygen which is benefit to active oxygen species. A plausible reaction pathway that involves the Mars-van Krevelen mechanism was proposed.

Selective hydrogenolysis of catechyl lignin into propenylcatechol over an atomically dispersed ruthenium catalyst

C-lignin is a homo-biopolymer, being made up of caffeyl alcohol exclusively. There is significant interest in developing efficient and selective catalyst for depolymerization of C-lignin, as it represents an ideal feedstock for producing catechol derivatives. Here we report an atomically dispersed Ru catalyst, which can serve as an efficient catalyst for the hydrogenolysis of C-lignin via the cleavage of C−O bonds in benzodioxane linkages, giving catechols in high yields with TONs up to 345. A unique selectivity to propenylcatechol (77%) is obtained, which is otherwise hard to achieve, because this catalyst is capable of hydrogenolysis rather than hydrogenation. This catalyst also demonstrates good reusability in C-lignin depolymerization. Detailed investigations by model compounds concluded that the pathways involving dehydration and/or dehydrogenation reactions are incompatible routes; we deduced that caffeyl alcohol generated via concurrent C−O bonds cleavage of benzodioxane unit may act as an intermediate in the C-lignin hydrogenolysis. Current demonstration validates that atomically dispersed metals can not only catalyze small molecules reactions, but also drive the transformation of abundant and renewable biopolymer.

Kaolinite: A natural and stable catalyst for depolymerization of cellulose to reducing sugars in water

Kaolinite, a natural, abundant, low cost, and eco-friendly material, has been widely used as filler, additives, adsorbent and coatings. In the study, kaolinite was evaluated for the performance of the hydrolysis of cellulose in water. The effects of mass ratio, reaction temperature, reaction time and water amount were investigated in a batch reactor. The fresh and regenerated catalysts were characterized by XRD, FT-IR and TG-DTG. The catalytic results showed that the highest TRS yield of 38.8% was obtained on the kaolinite at 200 °C with the mass ratio of catalyst to cellulose of 0.2 for 3 h. Moreover, kaolinite was a stable catalyst after regeneration and the yield of TRS decreased from 38.8% to 38.1% after five times reuse. Kaolinite contains abundant surface Al-OH groups, and the high TRS yield for kaolinite should be attributed to the H+ ions released from the surface OH groups. This work provides a promising strategy for efficient cellulose hydrolysis into fine chemicals by natural clay minerals.

Ni–Mg–Al Catalysts Effectively Promote Depolymerization of Rice Husk Lignin to Bio-Oil

Catalytic depolymerization of lignin to produce bio-oil, liquid fuels, and aromatic chemicals in high yields is important for a biorefinery to remain competitive. In this study, undoped and Ni-doped catalysts for depolymerization of rice husk lignin (RHL) were developed and analyzed. The results showed that the catalysts had hydrotalcite-like structures with lamellar morphology. With suitable Ni-doping amount, the catalysts had high activity, thus better promoted the depolymerization of RHL. Among all catalysts, the Ni1/4MgAl catalyst could best promote the cleavage of the β-O-4 aryl ether bond in RHL molecule and thus could substantially increase the yields of bio-oil. The depolymerization catalyzed by this catalyst was also found to be temperature-, time-, and solvent-dependent. This study provides information that can be beneficial to the biorefinery industries and may help promote the valorization of lignin.

Novel in-situ preparation of nano sized Ni (0) catalyst for depolymerization of lignin-rich waste from industrial biorefinery

Lignin-rich waste are valuable sources of chemicals and other materials but the depolymerisation processes are still unworkable on industrial scale. One of the problems ongoing is the catalyst preparation which often requires laborious and time-consuming processes.In this paper, a first example of easy preparation of in-situ Ni(0) nano sized catalyst for lignin-rich industrial waste hydrogenolysis is reported. Ni(0) particles were produced by reduction of Ni(II) acetate dissolved in alcoholic suspension of lignin. Soda lignin was isolated from the solid stream of a second generation bioethanol industrial plant and was depolymerized to low molecular weight chemicals by Ni(0) and/or Pd/C in alcoholic media. In 1,2-propandiol at 210 °C, up to 70% of starting material was converted into soluble compounds with an overall yield of monolignol derivatives of about 18%. The remaining material is constituted by lignin fragments with molecular weight between 400 and 1400 Da. The oligomers formation and distribution was discussed.

A Phosphotungstic Acid Catalyst for Depolymerization in Bulrush Lignin

Obtaining renewable fuels and chemicals from lignin is an important challenge in the use of biomass to achieve sustainability and energy goals. At present, acid-based catalysts for lignin depolymerization are considered to be a potential but challenging way to produce low-molecular-mass aromatic chemicals. The main concerns with the use of Lewis acids and zeolite catalysts are the corrosive nature of the acids, the possible formation of unwanted byproducts, and the possible formation of harsh reaction conditions. We achieved high-yield conversion using phosphotungstic acid (PTA) polyoxometalate catalysts in ethanol/water under different reaction conditions with little formation of bio-char. The monomeric products were mainly composed of various types of aromatic compounds. Our method does not require the use of precious metals and harsh reaction conditions—it only requires relatively mild reaction conditions and homogeneous catalysis—thereby greatly reducing operating costs and increasing the yields. Therefore, this PTA catalyst, which has excellent performance in bulrush lignin catalysis, would be a good alternative to the traditional catalysts used in lignin depolymerization and have wide application in biomass use.

Carbon Modification of Nickel Catalyst for Depolymerization of Oxidized Lignin to Aromatics

Catalytic valorization of lignin is a sustainable way to provide aromatics for the human society, which depends on the electronic structure of catalytic sites. We herein report the preparation of a carbon-modified nickel catalyst via carbothermal reduction of Ni-doped layered double hydroxides. Lignosulfonate (LS), a lignin resource from the pulp industry, was used as a renewable carbon precursor. The carbon residues in the nickel surface layer changed the 3d electron distribution of nickel, which was highly selective for the C–O bond hydrogenolysis of lignin into aromatics, and 22 wt % total mass yields of aromatics were achieved from hydrogenolysis of oxidized birch lignin.

Oxidative Depolymerization of Lignin Using Supported Niobium Catalysts

Valorization of lignin into aromatics has driven researchers for decades. In this research, niobium was deposited on oyster shells (OSNC) and carbon rods (CRNC) and tested as a catalyst for depolymerization of lignin. Catalysts (2%, 5%, and 8% loading) were synthesized via wet impregnation. Batch experiments were performed at 95 °C, using 8 g of lignin, and 1 g of catalyst. Our research indicates that niobium supported catalysts are effective in partial oxidation of lignin. Maximum vanillin concentration for OSNC was found to be 86.25 mg L−1 (0.1 wt%) at 5% niobium whereas, as for CRNC, maximum vanillin concentration was found to be 139.40 mg L−1 (0.17 wt%) at 2% niobium loading. Addition of hydrogen peroxide into the batch reactor decreased the concentration of vanillin production.

Titanate Nanotubes as New Nanostrutured Catalyst for Depolymerization of PET by Glycolysis Reaction

The final destination of PET packaging is creating economic and environmental concerns. One of the alternatives to minimize this problem would be making use of chemical recycling of this material through glycolysis with the aim to produce bis(hydroxiethyl) terephthalate, BHET monomer. This reaction is well known, but it still presents problems as BHET purity since it makes necessary the development of new catalysts highly selective. In this context, the present work studied the catalytic activity of a nanostructured material, titanate nanotubes (TNT), and compared it to a commercial catalyst (zinc acetate), which is the most used for this glycolysis reaction according to literature researches, and analyzed the influence of PET type (virgin and post-consumer) in the depolymerization for reaction times of 2, 3 and 4 hours. Using TNT as catalyst, BHET production yield and values of turnover number for the evaluated reaction times were higher than the results using Zn(OAc)2 for virgin PET, proving itself as a promising catalyst.

Reductive de-polymerization of kraft lignin with formic acid at low temperatures using inexpensive supported Ni-based catalysts

This work aimed to explore inexpensive supported Ni-based catalysts for de-polymerization of kraft lignin with formic acid at lower temperatures. In this study, kraft lignin was de-polymerized in water–ethanol mixture 50/50 (v/v) with formic acid as an in-situ hydrogen source in the presence of Ni-based catalysts (Ni10%/Zeolite and FHUDS-2), compared with an expensive Ru-based reference catalysts (Ru5%/C). At 200–300°C for 1–3h with or without catalyst, Kraft lignin (KL, Mw ∼10,000g/mol) was effectively de-polymerized to low Mw (Mw 1000–3000g/mol) de-polymerized lignin (DL) products at a very high yield (85–95wt%). Generally, at all temperatures tested, the presence of a supported metal catalyst could effectively reduce the Mw of the DL product although it did not yield significant effects on the product yield. The effects of catalyst were more evident at a lower temperature. For instance, Ni10%/Zeolite exhibited excellent activity for de-polymerization of KL, producing DL of Mw=3150g/mol at 93.5(±4.1) wt.% yield at a low temperature (200°C) for 1h. Among all catalysts examined, the two inexpensive Ni-based catalysts (although at a loading double that of the expensive Ru catalysts), i.e., FHUDS-2 (a W-Mo-Ni industrial catalyst) and Ni10%/Zeolite demonstrated to be the most effective catalysts for sulfur-removal, reducing the sulfur content from 1.52wt% in KL to 0.47wt% in DL with Ni10%/Zeolite and to ∼0wt% in DL with FHUDS-2 catalyst at 300°C.

Transition metal (Ti, Mo, Nb, W) nitride catalysts for lignin depolymerisation

Metal nitrides are promising catalysts for depolymerisation of lignin in supercritical ethanol; cheap and abundant titanium nitride affords an aromatic monomer yield of 19 wt% from soda lignin. The reaction mechanism is discussed on the basis of the products and a guaiacol model compound study.

Catalytic Depolymerisation of Polystyrene

Polystyrene waste was depolymerised by using various acidic and basic catalysts. Among the metal oxides, calcium oxide was found to be the cheapest and most effective catalyst for the depolymerization of waste polystyrene into styrene. Metal carbonates provided pure styrene, but in low yields. Acids were found to be poor catalysts for depolymerisation, since they led to further reactions leading to side products.

A Density Functional Theory Study of a Silica-Supported Zirconium Monohydride Catalyst for Depolymerization of Polyethylene

A silica-supported zirconium hydride catalyst for depolymerization of polyethylene is studied using density functional theory (DFT) together with a generalized gradient approximation (GGA) for the exchange and correlation energy. The (100) and (111) surfaces of β-cristobalite are used as two possible models of a silica surface. Based on the experimental surface structure determined by J. Corker et al. (Science 1996, 271, 966), we propose a detailed atomic model of the zirconium monohydride that is believed to be the active site for depolymerization of polyolefins. Our model of the zirconium monohydride on the (100) surface is found to be very stable and the structure is in good agreement with extended X-ray absorption fine structure (EXAFS) measurements. We have carried out depolymerization of a small polyolefin chain (C3H8) to give CH4 and C2H6 by addition of H2. The rate-limiting step is a β-methyl transfer to the zirconium atom, and the activation energy is 29 kcal/mol on the (100) surface.