Pyrolysis Of Lignin Research Articles

Unraveling the synergistic development of carbon skeleton and pore networks involved in lignin pyrolysis

To gain new insights into lignin pyrolysis, the synergistic development of carbon skeleton and pore networks in a wide temperature range of 200–800 °C was unraveled. The relative abundance of aromatic carbon structures increased exponentially with increasing temperature, as determined by 13C NMR deconvolution. Additionally, the g-factor type of radicals changed from O-centered (g-factor <2.0030) to C-centered (g-factor >2.0040) radicals. The evolution of the char macromolecules structure from disorder to order was principally characterized by a decrease in aromatic spacing (d002, 3.36→3.37 Å) and an increase in stacking degree (Lc, 7.89→8.67 Å) and aromaticity (fa, 75.19%→96.94%). fa has a strong negative linear correlation with d002 (R2 = 0.965). Ternary quadratic polynomials can relate specific surface area, total pore volume, and relative carbon content to the pore fractal dimension. The hypothesized chemical reaction networks and synergistic development of carbon skeleton and pore networks are essential for lignin effective valorization by a pyrolysis-based biorefinery strategies.

Simulating the pyrolysis interactions among hemicellulose, cellulose and lignin in wood waste under real conditions to find the proper way to prepare bio-oil

The distribution of cellulose, hemicellulose, and lignin in the biomass structure was uneven. To explore the interactions of three components under real conditions during pyrolysis and find the proper way to prepare bio-oil, chemical methods were used to remove them in poplar wood and the remainders were pyrolyzed separately. The coke produced by hemicellulose pyrolysis inhibited the pyrolysis of lignin and cellulose, while the promoting synergistic effects between cellulose and lignin reduced the pyrolysis temperature (up to 23.2 °C) and activation energy (up to 15.07%). The content of light oil in the pyrolysis oil was higher and more conducive to combustion. XPS analysis showed that during the pyrolysis of lignin and cellulose, oxygen atoms were effectively fixed into the biochar and more benzene ring substances were in the bio-oil, thereby improving the quality of the pyrolysis oil. Finally, the most suitable artificial neural network (ANN) model was selected, and the pyrolysis behavior of the remaining substances after the removal of hemicellulose was studied in depth. It also showed that removing part of hemicellulose was more beneficial to the preparation of high-quality bio-oil.

Characterization of the degradation products of lignocellulosic biomass by using tandem mass spectrometry experiments, model compounds, and quantum chemical calculations.

Biomass-derived degraded lignin and cellulose serve as possible alternatives to fossil fuels for energy and chemical resources. Fast pyrolysis of lignocellulosic biomass generates bio-oil that needs further refinement. However, as pyrolysis causes massive degradation to lignin and cellulose, this process produces very complex mixtures. The same applies to degradation methods other than fast pyrolysis. The ability to identify the degradation products of lignocellulosic biomass is of great importance to be able to optimize methodologies for the conversion of these mixtures to transportation fuels and valuable chemicals. Studies utilizing tandem mass spectrometry have provided invaluable, molecular-level information regarding the identities of compounds in degraded biomass. This review focuses on the molecular-level characterization of fast pyrolysis and other degradation products of lignin and cellulose via tandem mass spectrometry based on collision-activated dissociation (CAD). Many studies discussed here used model compounds to better understand both the ionization chemistry of the degradation products of lignin and cellulose and their ions' CAD reactions in mass spectrometers to develop methods for the structural characterization of the degradation products of lignocellulosic biomass. Further, model compound studies were also carried out to delineate the mechanisms of the fast pyrolysis reactions of lignocellulosic biomass. The above knowledge was used to assign likely structures to many degradation products of lignocellulosic biomass.

Comparison of the NiAl2O4 derived catalyst deactivation in the steam reforming and sorption enhanced steam reforming of raw bio-oil in packed and fluidized-bed reactors

The choice of appropriate reactors and reforming strategies is key to make progresses on scaling up H2 production processes from raw bio-oil. This work compares the performance (conversion, product yields and deactivation) of packed-bed and fluidized-bed reactors (PBR and FBR, respectively) using a NiAl2O4 spinel derived catalyst for the H2 production from raw bio-oil via steam reforming (SR) and sorption enhanced SR (SESR, with dolomite to capture CO2). The experiments were carried out at 600 °C; steam/carbon ratio, 3.4; space time, 0.15 h; time on stream, 5 h; dolomite/catalyst ratio, 10 (SESR runs); and with prior thermal separation of the pyrolytic lignin from the raw bio-oil. The initial H2 yields are 80 % and 69 % in the SR runs with PBR and FBR, respectively, and 99 % and 92 % in the CO2 capture period (of 30 min duration) of the SESR runs in the PBR and FBR, respectively. The lower H2 yield in the FBR is due to the less efficient gas–solid contact (bubbling or slugging phenomena). Based on the analysis of the spent catalysts with varied techniques the catalyst deactivation is related to the coke deposition, whose quantity and nature (amorphous or structured) depends on the reactor type and reforming strategy. The catalyst deactivation is slower in the FBR due to the rejuvenation of the catalyst surface by the moving particles that favor external coke gasification. The presence of dolomite prolongs the period of stable catalyst activity in both reactors with different effects on the coke quantity and nature. The results are of interest to advance on scaling up the SESR process that would require a FBR integrated with a regeneration unit for the catalyst and sorbent.

Co-Hydrotreatment of Yellow Greases and the Water-Insoluble Fraction of Pyrolysis Oil. Part I: Experimental Design to Increase Kerosene Yield and Reduce Coke Formation

This paper reports the co-hydrotreatment of Biomass Technology Group commercial pyrolysis oil water-insoluble (WIS) phase (also known as pyrolytic lignin) and yellow greases (waste cooking oil), aiming to produce sustainable aviation fuels (SAFs). We use a sulfided NiMo/Al2O3 blend with 16 wt % WIS and a central composite experimental design to identify processing conditions increasing kerosene yield and reducing coke formation. The input variables were: (1) reaction temperature (320, 350, and 380 °C), (2) initial hydrogen pressure (5, 6, and 7 MPa), and (3) amount of catalyst (0.7, 1.0, and 1.3 g). The hydrotreated oily phases were distilled to obtain gasoline (<150 °C), kerosene (150–250 °C), diesel (250–350 °C), and residual oil (>350 °C). The reaction temperature is the main factor affecting the yield of gaseous, solid, and liquid products. Meanwhile, a higher initial hydrogen pressure and catalyst loading increased the yield of kerosene and other distillates and decreased the coke formation. A high temperature correlated with a lower content of oxygenates in kerosene cuts. Based on our experimental results, we propose to conduct hydrotreatment studies at 380 °C, initial H2 pressure of 7 MPa, and 1.3 g of catalyst. Under the identified conditions, it was possible to improve the kerosene yield to more than 20 wt % and reduce the yield of coke to close to 2.0 wt %. The chemical composition and fuel properties of the gasoline, kerosene, and diesel cuts were thoroughly analyzed. The content of aromatics and phenols in the kerosene fraction produced at the conditions identified in this project exceeded the recommended values for SAFs. New strategies (such as blending and more intense hydrotreatment to remove oxygenated compounds) need to be implemented to reduce the content of these molecules in our final product.

Preparation of Ru/N-doped carbon catalysts by induction of different nitrogen source precursors for the hydroprocessing of lignin oil.

The lignin oil produced by rapid pyrolysis of lignin is considered a promising liquid fuel source. Hydrodeoxygenation (HDO) is a kind of efficient method to upgrade the lignin oil, and a high-performance catalyst is key to the hydrodeoxygenation of lignin oil. In this study, a high dispersion and small size Ru nanoparticle loaded N-doped carbon catalyst was derived by the direct pyrolysis of a mixture of ruthenium trichloride and melamine, and it could efficiently convert lignin oil. The lignin oil was completely transformed at 240 °C and 1 MPa H2, and 36.58% cyclohexane was obtained. The formation, surface area, and nitrogen species of the catalyst could be controlled by changing the precursor of the nitrogen-doped carbon support. The percentage of pyridine nitrogen possessed with melamine as a nitrogen-carbon precursor (31.35%) was much higher than that with urea (16.47%) and dicyandiamide (8.20%) as nitrogen-carbon precursors. The presence of pyridine nitrogen could not only serve as the coordination site for even dispersity and stability of Ru nanoparticles but also regulated the electron density of Ru nanoparticles (NPs) and increased the active site Ru0 through electron transfer.

Catalytic pyrolysis of enzymatic hydrolysis lignin by transition-metal modified HZSM-5/MCM-41 core–shell catalyst for the enhancement of monocyclic aromatic hydrocarbons

Enzymatic hydrolysis lignin (EHL) is produced in large quantities during the fermentation of lignocellulosic biomass to produce ethanol, resulting in environmental pollution and resource waste. HZSM-5/MCM-41 core–shell zeolites (H/M) loaded with transition metal Mn, Fe, Co, Ni, Cu, and Zn at different loading levels (2, 4, 6, 8, and 10 wt%) were prepared as catalysts. Py-GC/MS was employed to investigate their effects on the selectivity for and relative abundance of monocyclic aromatic hydrocarbons (MAHs) in the catalytic pyrolysis products of EHL. On this basis, bimetal-loaded H/M catalysts were constructed, namely Fe-Zn@H/M, Fe-Co@H/M, Fe-Ni@H/M, Co-Zn@H/M, Co-Ni@H/M, and Zn-Ni@H/M. Co-Ni@H/M increased the selectivity for and relative abundance of MAHs by 11.68% and 28.01%, respectively, compared with the H/M catalyst. Moreover, Co-Ni@H/M induced a further increase in the selectivity for and abundance of MAHs compared with the corresponding monometal-loaded catalysts. The characterization results indicated that bimetallic Co and Ni loading were uniformly dispersed on the catalyst surface and did not change the structure of the parent catalyst but affected the pore characteristics and acidic site distribution of the catalyst. Further analysis of the pyrolysis products suggested that bimetallic Co and Ni loading enhanced the dealkylation capacity of the catalyst for furans, aldehydes and phenol derivatives. Oligomerization, cyclization and aromatization of short alkyl chains were promoted in the MAHs by Co-Ni@H/M. Meanwhile, bimetallic Co and Ni loading inhibited the polymerization of phenols and other oxygen-containing compounds by promoting dehydration, decarboxylation and decarbonylation, which indirectly reduced the formation of coke.

Production of Fuel Range Hydrocarbons from Pyrolysis of Lignin over Zeolite Y, Hydrogen

In the current study, plain and lignin loaded with Zeolite Y, hydrogen was decomposed in a pyrolysis chamber. The reaction parameters were optimized and 390 °C, 3% catalyst with a reaction time of 40 min were observed as the most suitable conditions for better oil yield. The bio-oil collected from the catalyzed and non-catalyzed pyrolytic reactions was analyzed by gas chromatography mass spectrometry (GCMS). Catalytic pyrolysis resulted in the production of bio-oil consisting of 15 components ranging from C3 to C18 with a high percentage of fuel range benzene derivatives. Non-catalytic pyrolysis produced bio-oil that consists of 58 components ranging from C3 to C24; however, the number and quantity of fuel range hydrocarbons were lower than in the catalyzed products. The pyrolysis reaction was studied kinetically for both samples using thermogravimetry at heating rates of 5, 10, 15 and 20 °C/min in the temperature range 20–600 °C. The activation energies and pre-exponential factors were calculated using the Kissinger equation for both non-catalytic and catalytic decomposition and found to be 157.96 kJ/mol, 141.33 kJ/mol, 2.66 × 1013 min−1 and 2.17 × 1010 min−1, respectively. It was concluded that Zeolite Y, hydrogen worked well as a catalyst to decrease activation energy and enhance the quality of the bio-oil generated.

Synergistic effects during pyrolysis of binary mixtures of biomass components using microwave-assisted heating coupled with iron base tip-metal

The interaction between microwave and tip-metal can cause microwave discharge under appropriate conditions, showing promotion effect on pyrolysis of actual biomass. However, the fundamental studies of biomass component pyrolysis in terms of this method is lack, leading to synergistic effect during pyrolysis of binary mixtures of biomass components unclear. Therefore, the work studied pyrolysis characteristics of individual component (cellulose, lignin, and xylan) and binary mixtures of these components using microwave-assisted heating coupled with iron base tip-metal, and focused on the synergistic effect during the pyrolysis of binary mixtures. It was found microwave-assisted heating coupled with tip-metal had a triggering effect on the pyrolysis of lignin and xylan, reducing initial pyrolysis temperature by half. The maximum pyrolysis rate of the binary mixtures (cellulose-lignin and lignin-xylan) improved with the increase of lignin. The initial pyrolysis of binary mixture comprised by cellulose and xylan was also promoted. Synergistic effect of binary component pyrolysis was strengthened with this method. The pyrolysis synergistic effect of cellulose and lignin was centralized before 350 °C, and the synergistic effect obtained with the binary mixture of lignin and xylan was benefited most from this method. The work provides valuable information on biomass pyrolysis using this method.

Toward Native Hardwood Lignin Pyrolysis: Insights into Reaction Energetics from Density Functional Theory

Lignin is one of the three structural components of lignocellulosic biomass and the only renewable source for sustainably producing aromatic chemicals. Despite lignin’s promise for targeted valorization to fine chemicals, fuels, and polymer composites, a major roadblock is an ambiguity associated with its molecular structure. Due to the lack of an established molecular structure, all types of computational studies from reaction mechanism to reaction energetics are performed with model lignin compounds. Among several thermochemical conversion techniques, lignin pyrolysis has been an active research area for both experimental and computational investigations. Historically, computational studies on the pyrolysis of lignin have mainly been limited to dimeric or trimeric models using electronic structure methods. Very recently, model oligomers up to the size of decamers have been studied with density functional theory (DFT) calculations. While these studies used very simplified models to study lignin’s pyrolysis behavior, theoretical investigations on a more realistic lignin structure are warranted to advance its state-of-the-art. To address this, we have modeled a native hardwood lignin polymer consisting of 20 repeating units, including all three monolignols, i.e., guaiacyl (G), syringyl (S), and p-hydroxy coumaryl (H) units, and multiple types of linkages, i.e., β-O-4, 4-O-5, β–β, and β-5. This more realistic molecular model for the wild-type poplar lignin is based on a proposed lignin structure reported in a previous experimental study. The initial stage in lignin pyrolysis involves the homolytic cleavage of β-O-4 linkages present in lignin. The activation energy for homolysis of this linkage is considered to be slightly greater than the bond dissociation enthalpy (BDE) required to cleave it. We used a composite method using molecular mechanics-based conformational sampling and quantum mechanically based density functional theory (DFT) calculations to determine the reaction energetics for this reaction, which indicates the activation energy required for the early stage of lignin pyrolysis. In addition, we calculated standard thermodynamic properties for all species, including enthalpy of formation, heat capacity, entropy, and Gibbs free energy of formation as a function of temperature. This study provides the reaction energetics and standard thermodynamic quantities that could be used in kinetic and reactor modeling for biomass conversion. Additionally, these predictions would be particularly helpful in advanced-generation biorefinery, where hardwoods can be used as a potential feedstock. Moreover, the predictions reported in this study will benefit further computational studies and cross-validation with pyrolysis experiments, ultimately contributing to solving the puzzle of the structural ambiguity of lignin.

Catalytic Hydropyrolysis of Lignin for the Preparation of Cyclic Hydrocarbon-Based Biofuels

The demand for biomass utilization is increasing because of the depletion of fossil resources that are non-renewable in nature. Lignin is the second most renewable organic carbon source, but currently it has limited scope for application in the chemical and fuel industries. Lignin is a side product of the paper and pulp, sugar, and 2G bioethanol industries. Many research groups are working on the value-addition of lignin. Among the lignin depolymerization methods, catalytic hydropyrolysis is gaining attention and is playing a crucial role in developing biorefinery. The hydropyrolysis of lignin was conducted at a higher temperature in the presence of H2. The hydropyrolysis of lignin results in the selective formation of non-oxygenated cyclic hydrocarbons in a shorter reaction time. It is possible to use the cyclic hydrocarbons directly as a fuel or they can be blended with conventional gasoline. This review focuses on the prior art of pyrolysis and hydropyrolysis of lignin. Possible products of lignin hydropyrolysis and suitable synthetic routes to obtain non-oxygenated cyclic hydrocarbons are also discussed. The influence of various process parameters, such as type of reactor, metal catalyst, nature of catalytic supports, reaction temperature, and H2 pressure are discussed with regard to the hydropyrolysis of lignin to achieve good selectivity of cyclic hydrocarbons.

Pyrolysis of dealkaline lignin to phenols by loading grinding beads in a rotary kiln reactor

The pyrolysis of dealkaline lignin to phenols was performed in a rotary kiln reactor using grinding beads as filler. The effects of pyrolysis temperature and the amount and material of grinding beads on the liquid yield and composition were investigated. The results showed that the use of the rotary kiln reactor loaded with grinding beads could improve the efficiency of lignin pyrolysis. Temperature had a considerable influence on the pyrolysis process. The maximum liquid yield of 52.10 % was obtained at 500 °C and carrier gas flow rate of 300 mL min−1. Compared with other grinding beads, γ-alumina spheres could result in higher liquid yield and selectivity for guaiacols and alkylphenols. The liquid yield increased significantly with the use of grinding beads and increased with the increase in the number of grinding beads. However, both liquid yield and selectivity for guaiacol decreased when the volume ratio of grinding beads to reaction chamber increased from 1:2–2:3. The present study confirms the facilitative effect of grinding beads on the pyrolysis reaction, with the maximum increase of 86.87 % liquid yield and 12.58 % selectivity for guaiacol and alkyl phenols.

Ex-situ catalytic microwave pyrolysis of alkali lignin facilitates the production of monophenols and monoaromatics under the application of LaFe1-xCuxO3 perovskites

Two perovskites, LaFe1-xCuxO3 (x = 0, 0.20), were synthesized by co-precipitation approach, are applied for the ex-situ catalytic pyrolysis of alkali lignin (AL) under microwave-assisted. The results showed that the as-constructed LaFe1-xCuxO3 perovskites exhibited excellent activity towards the production of monophenols and monoaromatics compared with non-catalysis. Especially, after Cu doping exerted stronger deoxygenation effect in comparison to LaFeO3, which was more conducive to forming monocyclic aromatic hydrocarbons (MAHs) with a selectivity of 21.11 %. Interestingly, although the oxygen vacancy concentration in LaCu0.2Fe0.8O3 was higher than that in LaFeO3, the activity of the former towards monophenols (78.72 % selectivity) was basically the same as the latter (79.08 % selectivity), which was associated with their approximately equal surface area, manifesting that the surface area of LaFe1-xCuxO3 perovskites played a more dominant role in ex-situ catalytic pyrolysis of AL relative to oxygen vacancy. Furthermore, air-TG results declared the LaFe1-xCuxO3 catalysts possessed good anti-coking performance (coke amount ∼ 5 %). Notably, the presence of oxygen vacancy (Fe3+-□-Fe2+) in LaFeO3 while Cu doping resulted in forming Cu2+-□-Cu+, which were responsible for combining with oxygenated volatiles to yield the active sites FeOx and CuOx respectively, dominating the ex-situ catalytic pyrolysis of AL.

Investigation of the reaction mechanism regarding thermal treatment of biomass using molten salts: A kinetic study of small molecule gases formation

The conversion of biomass to produce high-valued fuel is an important way to improve the utilization rate of biomass energy. For the coupled utilization of solar energy and biomass, molten salts are used as heat storage and catalytic medium to convert biomass into combustible gases. To balance the supply of energy storage with the energy consumption for biomass conversion, the reaction mechanism regarding thermal treatment of biomass using molten salts is studied in the present research. The thermal treatment reactor combined with an online mass spectrometry is employed to characterize the gases release behavior of typical biomass components and actual biomass. In addition, a kinetic model of gas released from beech wood pyrolysis in molten salts was constructed by pyrolysis unit reactions of three typical components. The experimental results showed that molten salts can enhance the yields of small molecule gases compared to conventional pyrolysis process. For the relevance of gas release kinetics between biomass and three components, the results showed a good agreement between simulated curves and experimental data. The release of CO and H2 was mainly from the pyrolysis of lignin, CO2 was mainly from the pyrolysis of cellulose, and CH4 was mainly from the pyrolysis of both hemicellulose and cellulose. In addition, the gas release characteristics of beech wood were similar to those of the lignin, which played a leading role in the gas release of actual biomass pyrolysis.

Preparation of carbon-based microwave absorbing materials by hybrid activation pyrolysis of lignin and coal

• Preparation of carbon materials by potassium hydroxide activation. • Lignin and coal synergistically improved the microwave absorption performance. • The best reflection loss of the prepared carbon-based material was -61.02 dB. In this work, the cheap and eco-friendly microwave absorber materials were prepared by the activation pyrolysis of lignin and coal blends. The microwave absorption performance of the carbon materials prepared by mixing and activation of lignin and coal is more than twice as high as that of pure coal and lignin carbon-based materials. The optimum reflection loss of -61.02 dB at 6.16 GHz and less than -10 dB in the frequency range of 4.72 GHz to 7.6 GHz, with an effective absorption bandwidth of 2.88 GHz. The results indicate that the lignin and coal mixture have a synergistic effect during pyrolysis, which can effectively improve the pore structure and graphitization degree of the carbon material, thus improving the microwave absorption performance of the material. This study provides a low-cost and effective way to produce lightweight and efficient microwave absorbing materials.

Catalytic conversion mechanism of guaiacol as the intermediate of lignin catalytic pyrolysis on MgO surface: Density functional theory calculation

MgO has better catalytic effect on the alkylation reactions of methoxy phenols in the pyrolysis of lignin to form alkylated phenols, thus, the research on the conversion mechanism of methoxy phenols into alkylated phenols on the MgO surface is necessary. In this study, the catalytic conversion mechanism of guaiacol as a model compound of methoxy phenols on the MgO surface is investigated by density functional theory calculations. The possible reactions for the conversion of guaiacol on the clean MgO(100) and the H atom pre-adsorbed MgO(100) surfaces are discussed. The adsorption energies, adsorption structures, electronic structures, and energy barriers of guaiacol on the MgO(100) surfaces are obtained to illustrate the impact of pre-adsorbed H atom on the adsorption of guaiacol and the conversion mechanism of guaiacol. The results indicated that a hydrogen bond is generated between the O1 atom of guaiacol molecule and the pre-adsorbed H atom, resulting in the adsorption energy of guaiacol molecule on the MgO(100) surface is lower than that on the H atom pre-adsorbed MgO(100) surface. The energy barriers for the conversion of guaiacol molecule to form o-cresol molecule on the clean MgO(100) surface is higher than generate catechol molecule on the H atom pre-adsorbed MgO(100) surface. Thus, the results indicate that the presence of hydrogen source has a promotion effect on the conversion of methoxy phenols over MgO catalyst into alkylated phenols.

Thermodynamic study on energy crops thermochemical conversion to increase the efficiency of energy production

The actual paper analyses the performance of different energy crop biomasses, Miscanthus x giganteus Greef et Deu (EC-1) and Arundo donax L. (EC-2) stems, during slow pyrolysis process monitored by simultaneous TG-DTG-MS techniques, through chemical exergy analysis. In addition to considering the physical and chemical characteristics of given feedstocks for their efficient thermo-chemical conversion into pyrolytic gas, in this study, a theoretical simulation for their implementation use in the gasification process was also performed. The performed thermodynamic study with detailed exergy analysis showed that the large contribution of exergy in syngas components such as CO and H2 originates primarily from cellulose pyrolysis of EC-1, while large exergy contribution in syngas component as CH4 originates from lignin pyrolysis of EC-2. It was founded that the exergy efficiency of syngas for EC-1 equals 19.04%, which is lower than the exergy efficiency of syngas for EC-2 (20.46%), as a result of higher ash content in EC-1. Also, it was reported that higher carbon (C) and hydrogen (H) contents present in the EC-2 sample generate higher gaseous energy and exergy values, i.e. the increment of exergy efficiency of syngas, by both approaches (pyrolysis and gasification exergy analysis), but results in a lower biomass chemical exergy (18.28 MJ kg−1). The methodology applied to the gasification process was shown a higher exergy efficiency for EC-2 (∼36 – 42%) than for EC-1 (∼33 – 39%), dependant on the equivalence ratio (ER).

Biomass pyrolysis mechanism for carbon-based high-value products

The utilization of biomass, the only carbon-bearing renewable energy source, is critical for environmental and energy sustainability as well as carbon neutrality. Pyrolysis is the efficient conversion of biomass to fuel gas, liquid oil, and solid char. It is necessary to understand the mechanism of biomass pyrolysis for valuable and efficient utilization. In this review, recent progress with respect to biomass pyrolysis properties, the pyrolysis process, and product upgrading strategies focusing on carbon-based high-value liquid fuels and carbon-bearing materials are summarized. First, the correlations between the pyrolysis behavior and biomass structure and process conditions are considered. The mechanisms of the biomass pyrolysis of hemicellulose, cellulose, and lignin as well as the potential effect of inorganic matter on biomass pyrolysis are discussed. Furthermore, the regulation and controlling strategies for the biomass pyrolysis of high-value products are outlined. Finally, the challenges and prospects are discussed in order to provide a comprehensive picture of biomass pyrolysis development, which is significant for the understanding of biomass pyrolysis and the development of biomass utilization technology.

Comparison of hydrocracking and cracking of pyrolytic lignin over different Ni-based catalysts for light aromatics production

This work compared hydrocracking and cracking of pyrolytic lignin (PL) for light (single-ring and double-ring) aromatics production over Ni-based catalysts. Due to the supports' original acidities and the interactions between metal and supports, these Ni-based catalysts had different amounts of available Ni and acid sites, which affected depolymerization and deoxygenation in both cracking and hydrocracking. During cracking, the PL depolymerization could be promoted by increasing Ni and acid sites but this promotion was limited by the hydrogen inside PL, whereas both Ni and acid sites exhibited poor performance in deoxygenation. The cooperation of Ni and acid sites achieved efficient hydrodepolymerization and hydrodeoxygenation in hydrocracking. Acid sites benefitted the cracking of linkages in PL and the resulting intermediates could be converted into monophenols through Ni-catalyzed hydrogenation. In the selective hydrodeoxygenation mechanism of monophenols to monoaromatic hydrocarbons, acid sites facilitated the initial tautomerization and the final dehydration reactions, while the middle hydrogenation step was catalyzed by Ni. Additionally the micropore structure of catalyst also favored the generation of light aromatics. Therefore, hydrocracking over Ni/HZSM-5 with sufficient available Ni and acid sites and the micropore structure, realized the most effective conversion of PL to light aromatics, especially aromatic hydrocarbons.

In-situ synthesis of micro/mesoporous HZSM-5 zeolite for catalytic pyrolysis of lignin to produce monocyclic aromatics

Catalytic pyrolysis of lignin is a promising way for lignin upgrading. To date, commercial zeolites have been demonstrated to be active in lignin catalytic pyrolysis for monocyclic aromatics production, but usually exhibit low selectivity and fast deactivation during reaction. These are mainly attributed to the inadequate of mesoporous structure and improper acidic property of commercial zeolites. Herein, HZSM-5 zeolites with different micro/mesoporous morphology were synthesized via the green and energy-efficient in-situ synthesis method. The porous structure of the zeolites was regulated by adjusting the synthesis conditions, i.e., the concentration of tetrapropylammonium hydroxide (TPAOH, used as the microporous templating agent), the synthesis temperature, and synthesis duration. With the HZSM-5 sample synthesized under the optimal condition (S8HZ), the relative content of monocyclic aromatics achieved during lignin catalytic pyrolysis reached 39.3 % at 500 °C, which was much higher than that attained over the commercial HZSM-5 (17.3 %). In addition, the acidic property and pore structure of the S8HZ catalyst was further modulated by cobalt impregnation. The introduction of cobalt, rendered a good balance between the structural property and acidity of the catalyst, which eventually facilitated the generation of monocyclic aromatics by both improving mass transfer and reducing coke formation. As it turned out, 1 wt% of cobalt introduction to the S8HZ boosted the generation of monocyclic aromatics to 46.3 % at 550 °C. Moreover, the catalyst retained good stability in catalytic lignin pyrolysis after 10 reaction cycles. Overall, this work demonstrates the feasibility of using in-situ synthesized HZSM-5 catalyst with rich micro/mesoporous structure as well as Co modification for catalytic lignin upgrading to monocyclic aromatics, which offers new insights for lignin upgrading via catalytic pyrolysis.