Enhancement of the production of bio-aromatics from renewable lignin by combined approach of torrefaction deoxygenation pretreatment and shape selective catalytic fast pyrolysis using metal modified zeolites

Catalytic fast pyrolysis of waste pine sawdust over solid base, acid and base-acid tandem catalysts

Catalytic pyrolysis is an effective means for high-value utilization of biomass. This study investigated the effect of solid base catalysts (CaO, calcium aluminate catalysts CaAl-1, CaAl-2, CaAl-3), acid zeolite catalysts (ZSM-5, Fe/ZSM-5, Co/ZSM-5, Ni/ZSM-5, Cu/ZSM-5, Zn/ZSM-5) and base-acid tandem catalysts on pine sawdust pyrolysis using Py-GC/MS. Acid zeolite catalysts exhibited robust deoxidation and aromatization capabilities, favoring aromatics, while solid base catalysts yielded more phenols and ketones. Among the solid base catalysts, CaAl-3 (CaO-Ca12Al14O33) showed comparable deoxygenation activity to CaO and optimal aromatic selectivity with structural stability. Zn/ZSM-5 excelled in deoxygenation and aromatic selectivity (70.42%) among metal-modified ZSM-5 catalysts. Base-acid tandem catalysis promoted the formation of aliphatics and BTX (benzene, toluene, xylene) while suppressing polycyclic aromatics. The highest BTX content (44.35%) was achieved with CaO-Ca12Al14O33&Zn/ZSM-5 tandem catalysts in a 1:3 ratio. This work demonstrates base-acid tandem catalysis as a promising approach for converting pine sawdust into valuable chemicals.

Hydrothermal carbonization coupled with fast pyrolysis of almond shells: Valorization and production of valuable chemicals

In this study, it was found that hydrothermal carbonization (HTC) can be an effective method for almond shell (AS) valorization. The severity of HTC treatment had a significant effect on hydrochar yields, with higher severity promoting carbonization but reducing yields. Furthermore, the work found that HTC treatment effectively demineralized biomass samples by removing inorganic material that could catalyze carbonization. As residence time or temperature increased, the amount of carbon increased, while the amount of oxygen decreased. An acceleration in thermal degradation was detected for hydrochars after pretreating for 4 h. The hydrochars showed they had a higher volatile content than untreated biomass, making them potentially useful for producing quality bio-oil through fast pyrolysis. Finally, HTC treatment led to the production of valuable chemicals such as guaiacol and syringol. For syringol production, HTC residence time had more effect than HTC temperature. However, high HTC temperatures benefited levoglucosan production. Overall, the results demonstrated the potential for HTC treatment to be an effective method for valorizing agricultural waste, offering the possibility of producing valuable chemicals.

Efficient Conversion of Lignin to Aromatics via Catalytic Fast Pyrolysis over Niobium-Doped HZSM-5

A niobium-doped HZSM-5 (H[Nb]ZSM-5) was prepared by a hydrothermal synthesis method. The morphology, phase structure, composition, pore structure, and acid content of the catalyst were characterized using a series of analysis techniques such as scanning electron microscope (SEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption, and temperature programmed desorption measurements (NH₃-TPD). The H[Nb]ZSM-5 catalyst fully remained within the crystal framework and pore structure of HZSM-5. Meanwhile, introduction of niobium (V) endowed the catalyst with both Lewis acid and Bronsted acid sites. Catalytic fast pyrolysis (CFP) of alkali lignin was carried out through a pyrolysis and gas chromatography-mass spectrometry (Py-GC/MS) at 650 °C and atmospheric pressure. The results indicated that H[Nb]ZSM-5 can efficiently and selectively convert lignin into monoaromatic hydrocarbons (MAHs), compared to the control HZSM-5. Catalyzed by H[Nb]ZSM-5, the content of MAHs and aliphatic hydrocarbons reached 43.4% and 20.8%, respectively; while under the catalysis of HZSM-5, these values were 35.5% and 3.2%, respectively. H[Nb]ZSM-5 remarkably lowered the phenol content to approximately 2.8%, which is far lower than the content (24.9%) obtained under HZSM-5 catalysis.

Critical Techniques for Overcoming the Diffusion Limitations in Heterogeneously Catalytic Depolymerization of Lignin

Heterogeneously catalyzed depolymerization of lignin to value-added chemicals is increasingly attractive but highly challengeable. Particularly, the diffusion limitation of lignin macromolecule to the solid catalyst surface is a big barrier, which significantly decreases the yield of monomer while increasing char formation. Therefore, for the potential industrial utilization of lignin, new knowledge focused on the size of lignin particles is of great importance to offer guidance for promoting lignin depolymerization and suppressing condensation in the heterogeneously catalytic systems. In this Review, the size of lignin particles and macromolecules are summarized. Previous approaches for improving the mass diffusion including enhancing the solubility of lignin and exploitation of hierarchical and "solubilized" materials are also discussed. Based on these, a constructive perspective is proposed. Thus, this work provides a new insight on the rational design of heterogeneous catalytic techniques for efficient utilization of the aromatic polymer of lignin.

Lignin and spent bleaching clay into mono-aromatic hydrocarbons by a cascade dual catalytic pyrolysis system: Critical role of spent bleaching clay

In the present study, a cascade dual catalytic system was used for the co-pyrolysis of lignin with spent bleaching clay (SBC) to efficiently produce mono-aromatic hydrocarbon (MAHs). The cascade dual catalytic system is composed of calcined SBC (CSBC) and HZSM-5. In this system, SBC not only acts as a hydrogen donor and catalyst in the co-pyrolysis process, but is also used as a primary catalyst in the cascade dual catalytic system after recycling the pyrolysis residues. The effects of different influencing factors (i.e., temperature, CSBC-to-HZSM-5 ratio, and raw materials-to-catalyst ratio) on the system were explored. It was observed that, when the temperature was 550 °C, the CSBC-to-HZSM-5 ratio was 1:1, and when the raw materials-to-catalyst ratio was 1:2, the highest bio-oil yield was 21.35 wt%. The relative MAHs content in bio-oil was 73.34 %, whereas the relative polycyclic aromatic hydrocarbons (PAHs) content was 23.01 %. Meanwhile, the introduction of CSBC inhibited the generation of graphite-like coke as indicated by HZSM-5. This study realizes the full resource utilization of spent bleaching clay and reveals the environmental hazards caused by spent bleaching clay and lignin waste.

Elucidating radical-mediated pyrolysis behaviors of preoxidized lignins

Effect of lignin preoxidation on subsequent radical-mediated pyrolysis was discussed in this study. Technical hot-water-extracted lignin was preoxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in diverse degrees and pyrolyzed under different temperatures. Characterizations indicated that preoxidation increased lignin oxygen contents and converted α-hydroxyls to α-carbonyls. These structural modifications caused by preoxidation reduced the thermal stability and pyrolysis reactivity of lignin, shifting lignin thermal decomposition to the low temperature region and inhibiting lignin pyrolysis into bio-oil fractions. However, recognition of species and yields of specific compounds via analytical pyrolysis declared that although preoxidation reduced product yields, it did not alter the reaction pathways. The fixed bed experiments proved the above findings and gave the gas compositions, mainly CO₂ derived through decarbonylation. Both radicals in chars and bio-oils were monitored, and char radical concentrations were proportional to the preoxidation degrees. This work sorted out the performances of lignin pyrolysis after preoxidation and determined their negative effects.

Co-pyrolysis of microalgae and other biomass wastes for the production of high-quality bio-oil: Progress and prospective

Microalgae are the most prospective raw materials for the production of biofuels, pyrolysis is an effective method to convert biomass into bioenergy. However, biofuels derived from the pyrolysis of microalgae exhibit poor fuel properties due to high content of moisture and protein. Co-pyrolysis is a simple and efficient method to produce high-quality bio-oil from two or more materials. Tires, plastics, and bamboo waste are the optimal co-feedstocks based on the improvement of yield and quality of bio-oil. Moreover, adding catalysts, especially CaO and Cu/HZSM-5, can enhance the quality of bio-oil by increasing aromatics content and decreasing oxygenated and nitrogenous compounds. Consequently, this paper provides a critical review of the production of bio-oil from co-pyrolysis of microalgae with other biomass wastes. Meanwhile, the underlying mechanism of synergistic effects and the catalytic effect on co-pyrolysis are discussed. Finally, the economic viability and prospects of microalgae co-pyrolysis are summarized.

Unmasking radical-mediated lignin pyrolysis after benzyl hydroxyl shielding

Whether lignin benzyl hydroxyl shielding could promote its pyrolysis to phenolic compounds was investigated in this paper. Lignin benzyl hydroxyl was first preoxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and stabilized by propionaldehyde respectively, then pyrolysis was conducted with milled wood lignin as a control. Organic stable radicals in pyrolytic chars were further detected to reveal lignin pyrolysis chemistry. Results showed that benzyl hydroxyl shielding process weakened lignin thermal stability, and decreased liquid yields regardless of the frequency of lignin β-O-4 linkages. In addition, char yield grew after benzyl hydroxyl shielding. Radical concentration was inversely proportional to β-O-4 content which indicated the non-negligible impact of shielded benzyl hydroxyl on lignin pyrolysis. Furthermore, gases from propionaldehyde stabilized lignin quenched its radicals. This work confirmed that lignin β-O-4 linkages and shielded benzyl hydroxyl both played the great role in radical-mediated pyrolysis, but the enhancement of liquid products could not be achieved via benzyl hydroxyl shielding.

Fates of heavy organics of bio-oil in hydrotreatment: The key challenge in the way from biomass to biofuel

Heavy organics in bio-oil generally refer to the sugar oligomers and lignin-derivatives. They are important fractions in bio-oil and their effective conversion in hydrotreatment determines carbon yield from biomass or bio-oil to biofuel. Fates of the heavy organics largely determine intrinsic reaction behaviors of bio-oil during hydrotreatment. The heavy organics in bio-oil have high tendency towards polymerization upon thermal treatment, which is one of the main precursors for coke formation and catalyst deactivation. Furthermore, the heavy organics have some other unique characteristics in hydrotreatment such as the steric hindrance for contacting active sites on surface of catalyst. How to effectively convert the heavy organics has been regarded as the bottle-neck issue in hydrotreatment of bio-oil and the key barrier in the roadmap from biomass to biofuels. Thus, this review particularly focuses on the progress in understanding reaction behaviors of the heavy organics in hydrotreatment of bio-oil, a central challenge to be resolved. The results indicated that coke formation from heavy organics in bio-oil remains main obstacle in hydrotreatment and further fundamental studies are required to develop suitable catalyst and process to stabilize the heavy organics in bio-oil. In particular, the mechanism for coke formation from the heavy species of varied chemical family should be clarified and corresponding measures should be developed to tackle high tendency of coking. Techno-economic feasibility should be considered in the first place in development of catalysts or process for tackling the heavy fractions of bio-oil.

Catalytic upgrading of penicillin fermentation residue bio-oil by metal-supported HZSM-5

Antibiotic fermentation residue (AR) is composed of hazardous organic waste produced by the pharmaceutical industry. AR can be effectively converted into bio-oil by fast pyrolysis, but its high nitrogen content limits the prospect of bio-oil as a fuel resource. In order to further reduce the nitrogen content of AR bio-oil, we have examined the catalytic removal of N and O from penicillin fermentation residue (PR) bio-oil under fast pyrolysis conditions. We have used M/HZSM-5 (M = Fe, Co, Ni, Cu, Zn, Zr, Mo, Ag and Ce) metal catalysts, with a metal oxide content of 10%. Additionally, the effect of mixed and separated catalytic forms on catalytic upgrading were analyzed, and changes in the catalyst itself before and after pyrolysis under separated catalytic conditions were specifically investigated. Our results show that the metal elements in the fresh catalyst will exist in the form of oxides, ions and simple metals. In-situ reduction caused by pyrolysis gas in the catalytic pyrolysis process makes some ionic metals (e.g., Co²⁺, Cu²⁺ and Ag⁺) in the catalyst transform into oxides, and some metal oxides are reduced to simple metals or suboxides (including Fe, Ni, Cu and Mo). The N content in the mixed catalytic bio-oil decreased from 10.09 wt% to Zn/HZSM-5 (6.98 wt%), Co/HZSM-5 (7.1 wt%), Cu/HZSM-5 (7.18 wt%) and Ce/HZSM-5 (7.18 wt%). We also observed significant reduction in the O content (9.77 wt%) with Ag/HZSM-5 (3.75 wt%), Mo/HZSM-5 (6.86 wt%), Ce/HZSM-5 (8.39 wt%) and Fe/HZSM-5 (8.54 wt%) in the separated catalytic bio-oil. The Ni/HZSM-5 catalystcan reduce the organic acid content in bio-oil from 22.9% to 10.8%. The separated catalysis methodology also promoted an increase of hydrocarbons in the bio-oil: Zn/HZSM-5, Ag/HZSM-5, Mo/HZSM-5, Zr/HZSM-5 and Ce/HZSM-5 reached 11.6%, 11.5%, 11.1%, 10.1%, and 8.8%, respectively. Carbon deposition formed by aromatic carbon/graphite carbon, pyrrole and pyridine compounds leads to deactivation of the catalyst.

Optimization of the nitrogen and oxygen element distribution in microalgae by ammonia torrefaction pretreatment and subsequent fast pyrolysis process for the production of N-containing chemicals

In this work, ammonia (NH₃) torrefaction pretreatment (ATP) was developed to optimize the nitrogen and oxygen element distribution of microalgae via the N-doping and oxygen removal reaction, which could obviously improve the potential use of microalgae as a feedstock for the production of N-heterocyclic chemicals through fast pyrolysis technology. The nitrogen content increased from 8.3% of raw microalgae to 11.51% at 300 °C of ATP, while the oxygen content decreased from 35.96% to 21.61%, because of the Maillard reactions. In addition, the nitrogen-doping ratio and oxygen removal ratio of ATP was much higher than the conventional nitrogen torrefaction pretreatment (NTP). With the increase of ATP torrefaction temperature or the pyrolysis temperature, the relative content of the N-containing compounds increased, while the O-containing compounds decreased. For the N-heterocyclic chemicals, higher pyrolysis temperature favored the formation of pyrroles, while inhibited the formation of pyridines and indoles.

Microwave-assisted catalytic fast pyrolysis of rice husk over a hierarchical HZSM-5/MCM-41 catalyst prepared by organic base alkaline solutions

This paper reports the results obtained for microwave-assisted catalytic fast pyrolysis (MACFP) of rice husk. The MACFP process employed a hierarchical catalyst prepared via a combination of organic alkali treatment (TPAOH) and the generation of an external layer of MCM-41-type mesoporous channels. We propose this catalyst which is used for the first time for pyrolysis of lignocellulosic biomass, as a tool to reduce coke agglomeration and increase hydrocarbon yields. Our results indicate that during catalyst preparation the mass fraction of cetyltrimethylammonium bromide (CTAB) has a direct effect on the content of MCM-41 formed on top of the HZSM-5 core. For MACFP, we hypothesize that the small molecules generated from thermal decomposition of rice husk react further to form aromatic and aliphatic hydrocarbons by decarbonylation, decarboxylation, oligomerization and aromatization. The highest hydrocarbon yield (60.5%) was obtained for a catalyst modified by a 2.0 mol/L TPAOH solution, with 10 wt% of CTAB (template for producing MCM-41), as well as with digestion and crystallization at 110 °C for 24 h. In addition, the highest liquid yield (47.6 wt%) was obtained at 550 °C. The relative content of hydrocarbons goes through a maximum of 60.5% with CTAB mass fraction which was higher than values obtained with MCM-41 (3.2%) and HZSM-5 (36.0%). Characterization and catalytic testing results suggest that the digestion temperature plays a more important role in the catalyst synthesis than the crystallization temperature. High digestion temperature (120 °C) decreases the overall hydrocarbon selectivity from 60.5% (110 °C) to 39.2%. The relative content of oxygenates reached the lowest value of 35.9% at the digestion and crystallization temperature of 110 °C. The synergistic effect of the MCM-41 shell and the HZSM-5 core promotes the catalytic activity, leading to outstanding deoxygenation capabilities and excellent selectivity to BTEX (52.7%).

The influence of torrefaction on pyrolysed biomass: The relationship of bio-oil composition with the torrefaction severity

In this study, three types of biomass were torrefied at different times (0.5, 1, 1.5 h) and temperature (200, 240, 280, 320 °C), which were further pyrolyzed at 550 °C after torrefaction. CEI (carbon element index), which was established based on the carbon content of the torrefied biomass, was chosen as an indicator for reflecting torrefaction severity. The results showed that there was a curvilinear relationship between CEI and the physicochemical characteristics, energy recovery of torrefied biomass, which obtained an average goodness of fit was higher than 0.93. Moreover, the goodness of fit between CEI and pyrolysis carbon and bio-oil yield was higher than 0.95 and 0.91, respectively. Especially, the bio-oil composition and CEI were fitted by a quadratic function (y = a + bx + cx²). Based on the function, the yield of phenols could be predicted based on the CEI value, which would benefit for the preparation of higher quality bio-oil directionally.