Selective production of phenols from lignin via microwave pyrolysis using different carbonaceous susceptors

Extraction and Depolymerization of Lignin from Different Agricultural and Forestry Wastes to Obtain Building Blocks in a Circular Economy Framework

Large amounts of agri-food waste are generated and discarded annually, but they have the potential to become highly profitable sources of value-added compounds. Many of these are lignin-rich residues. Lignin, one of the most abundant biopolymers in nature, offers numerous possibilities as a raw material or renewable resource for the production of chemical products. This study aims to explore the potential revalorization of agricultural by-products through the extraction of lignin and subsequent depolymerization. Different residues were studied; river cane, rice husks, broccoli stems, wheat straw, and olive stone are investigated (all local wastes that are typically incinerated). Traditional soda extraction, enhanced by ultrasound, is applied, comparing two different sonication methods. The extraction yields from different residues were as follows: river cane (28.21%), rice husks (24.27%), broccoli (6.48%), wheat straw (17.66%), and olive stones (24.29%). Once lignin is extracted, depolymerization is performed by three different methods: high-pressure reactor, ultrasound-assisted solvent depolymerization, and microwave solvolysis. As a result, a new microwave depolymerization method has been developed and patented, using for the first time graphene nanoplatelets (GNPs) as new promising carbonaceous catalyst, achieving a 90.89% depolymerization rate of river cane lignin and yielding several building blocks, including guaiacol, vanillin, ferulic acid, or acetovanillone.

Development of a Microwave-Assisted Bench Reactor for Biomass Pyrolysis Using Hybrid Heating

Microwave-assisted pyrolysis (MAP) is a cutting-edge technology that converts biomass into fuels, chemicals, and materials. In this study, an Arduino was used to control and automate a MAP system built from a microwave oven with a cordierite chamber filled with silicon carbide. Sugar cane bagasse was pyrolyzed at 250, 350, 450, and 550 °C to validate the MAP system and obtain pyrolytic products with different yields and chemical compositions. Lower temperatures led to high biochar yields, but the highest surface area of 25.14 m2 g-1 was only achieved at 550 °C. By contrast, higher temperatures favored the recovery of pyrolysis liquids. BET and scanning electron microscopy analyses revealed a porous biochar structure, while Fourier transform infrared spectroscopy analysis showed that the availability of functional groups on the biochar surface decreased with an increase in pyrolysis temperature. GC-MS analysis quantified valuable low molecular mass compounds in pyrolysis liquids, including aldehydes, ketones, phenols, and alcohols. With its unprecedented hybrid heating device, the MAP system promoted suitable heating rates (31.9 °C min-1) and precise temperature control (only 19 °C of set point variation), generating pyrolysis products devoid of microwave susceptor interferences. Therefore, MAP provided a rapid, safe, and efficient means of depolymerizing biomass, thus holding promise for biorefinery applications.

A review on the role of susceptors in the recovery of valuable renewable carbon products from microwave-assisted pyrolysis of lignocellulosic and algal biomasses: Prospects and challenges

Sustainable bio-economics can be achieved by the processing of renewable biomass resources. Hence, this review article presents a detailed analysis of the effect of susceptors on microwave-assisted pyrolysis (MAP) of biomass. Biomass is categorized as lignocellulosic and algal biomass based on available sources. Selected seminal works reporting the MAP of pure biomasses are reviewed thoroughly. Focus is given to understanding the role of the susceptor used for pyrolysis on the characteristics of products produced. The goal is to curate the literature and report variation in the product characteristics for the combinations of the biomass and susceptor. The review explores the factors such as the susceptor to feed-stock ratio and its implications on the product compositions. The process parameters including microwave power, reaction temperature, heating rate, feedstock composition, and product formation are discussed in detail. A repository of such information would enable researchers to glance through the closest possible susceptors they should use for a chosen biomass of their interest for better oil yields. Further, a list of potential applications of MAP products of biomasses, along with the susceptor used, are reported. To this end, this review presents the possible opportunities and challenges for tapping valuable carbon resources from the MAP of biomass for sustainable energy needs.

Vacuum Low-Temperature Microwave-Assisted Pyrolysis of Technical Lignins

Cleavage by microwave-assisted pyrolysis is a way to obtain higher-value organic chemicals from technical lignins. In this report, pine kraft lignin (PKL), spruce and beech organosolv lignin (SOSL and BOSL), and calcium lignosulfonates from spruce wood (LS) were pyrolyzed at temperatures between 30 and 280 °C using vacuum low-temperature, microwave-assisted pyrolysis. The mass balance, energy consumption, condensation rate, and pressure changes of the products during the pyrolysis process were recorded. Phenolic condensates obtained at different temperatures during pyrolysis were collected, and their chemical composition was determined by GC-MS and GC-FID. The origin of the technical lignin had a significant influence on the pyrolysis products. Phenolic condensates were obtained in yields of approximately 15% (PKL and SOSL) as well as in lower yields of 4.5% (BOSL) or even 1.7% (LS). The main production of the phenolic condensates for the PKL and SOSL occurred at temperatures of approximately 140 and 180 °C, respectively. The main components of the phenolic fraction of the three softwood lignins were guaiacol, 4-methylguaiacol, 4-ethylguaiacol, and other guaiacol derivatives; however, the quantity varied significantly depending on the lignin source. Due to the low cleavage temperature vacuum, low-temperature, microwave-assisted pyrolysis could be an interesting approach to lignin conversion.

Simple microwave pyrolysis kinetics of lignocellulosic biomass (oil palm shell) with activated carbon and palm oil fuel ash catalysts

The important parameters characterizing microwave pyrolysis kinetics, namely the activation energy (E a) and the rate constant pre-exponential factor (A), were investigated for oil palm shell mixed with activated carbon and palm oil fuel ash as microwave absorbers, using simple lab-scale equipment. These parameters were estimated for the Kissinger model. The estimates for E a ranged within 31.55–58.04 kJ mol−1 and for A within 6.40E0–6.84E+1 s−1, in good agreement with prior studies that employed standard techniques: Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). The E a and A were used with the Arrhenius reaction rate equation, solved by the 4th order Runge-Kutta method. The statistical parameters coefficient of determination (R 2) and root mean square error (RMSE) were used to verify the good fit of simulation to the experimental results. The best fit had R 2 = 0.900 and RMSE = 4.438, respectively, for MW pyrolysis at power 440 W for OPS with AC as MW absorber.

A Review of Bio-Oil Production through Microwave-Assisted Pyrolysis

The issue of sustainability is a growing concern and has led to many environmentally friendly chemical productions through a great intensification of the use of biomass conversion processes. Thermal conversion of biomass is one of the most attractive tools currently used, and pyrolytic treatments represent the most flexible approach to biomass conversion. In this scenario, microwave-assisted pyrolysis could be a solid choice for the production of multi-chemical mixtures known as bio-oils. Bio-oils could represent a promising new source of high-value species ranging from bioactive chemicals to green solvents. In this review, we have summarized the most recent developments regarding bio-oil production through microwave-induced pyrolytic degradation of biomasses.

Novel insight into pyrolysis behaviors of lignin using in-situ pyrolysis-double ionization time-of-flight mass spectrometry combined with electron paramagnetic resonance spectroscopy

In-situ detection on primary volatiles and stable radicals is of great importance for better understanding of lignin pyrolysis mechanisms and utilization. In this study, a novel in-situ pyrolysis time-of-flight mass spectrometry with double ionization sources was taken to in-situ detect primary volatiles and gas products, and the evolution of stable radicals in lignin pyrolysis residues was explored by EPR spectroscopy. The results show that the cleavage of β-O-4 linkage is mainly responsible for lignin depolymerization at 100-300 °C, releasing the G-type compounds. And these G-type compounds can further undergo O-CH₃, C_ar-OCH₃ and C_ar-OH bonds cleavage to form biphenolic hydroxyl compounds, phenols and aromatic hydrocarbons. According to the EPR analysis, the radical concentration increased from 10¹⁷ to 10¹⁹ spins/g with the temperature, and stable free-radical species are mainly composed of the o-methoxy and hydroxyl substituted phenoxy radicals and carbon-centered aromatic radicals, which can well interpret the demethylation, demethoxylation and dehydroxylation mechanisms.

Analytical and microwave pyrolysis of empty oil palm fruit bunch: Kinetics and product characterization

This study is focused on kinetics and product distribution from untreated empty oil palm fruit bunch (EOPFB) biomass and treated EOPFB using analytical pyrolysis combined with gas chromatograph/mass spectrometer and Fourier transform infrared spectrometer, and microwave pyrolysis. Industrial water wash led to significant reduction in ash content of EOPFB from 5.9 wt% to 0.7 wt%. Isothermal mass loss data collected in the temperature range of 400-700 °C showed that fast pyrolysis in the Pyroprobe® reactor followed diffusion-controlled kinetics with apparent activation energies of 30.4 and 39.6 kJ mol^-1 for untreated and treated EOPFB, respectively. Analytical pyrolysis of untreated EOPFB resulted in high selectivity to fatty acids, while phenolics dominated the pyrolysates from treated EOPFB. The selectivities to phenolic compounds were 74% and 57% from treated and untreated EOPFB, respectively, via microwave pyrolysis. The higher heating values of bio-crude from microwave pyrolysis of untreated and treated EOPFB were 30.1 and 29.7 MJ kg^-1, respectively.

Effective deoxygenation for the production of liquid biofuels via microwave assisted co-pyrolysis of agro residues and waste plastics combined with catalytic upgradation

Rice straw and sugarcane bagasse were co-pyrolyzed with polypropylene and polystyrene using microwaves, and the pyrolysis vapors were catalytically upgraded using HZSM-5 catalyst. The product yields, composition and properties of bio-oil from pyrolysis of individual feedstocks and equal composition mixtures before and after catalytic upgradation were thoroughly investigated. The pyrolysis oil yields from polypropylene (82 wt%) and polystyrene (98 wt%) were high compared to that from rice straw (26 wt%) and bagasse (29 wt%). Catalytic upgradation at weight hourly space velocity of 11 h^-1 resulted in higher selectivity to unsaturated aliphatics and aromatic hydrocarbons. Properties of upgraded bio-oil from biomass-polypropylene mixtures were similar to that of light fuel oil with high calorific value (43 MJ/kg), low viscosity (1 cP), optimum density (0.850 g/cm³) and flash point (70 °C). Oxygen content in catalytically upgraded co-pyrolysis bio-oil was low (<5%) as compared to upgraded pyrolysis bio-oil (14-18%), and pyrolysis bio-oil without upgradation (20-24%).