A critical review on metal-based catalysts used in the pyrolysis of lignocellulosic biomass materials

Contemporary avenues of the Hydrogen industry: Opportunities and challenges in the eco-friendly approach

Hydrogen (H₂) is a possible energy transporter and feedstock for energy decarbonization, transportation, and chemical sectors while reducing global warming's consequences. The predominant commercial method for producing H₂ today is steam methane reforming (SMR). However, there is still room for development in process intensification, energy optimization, and environmental concerns related to CO₂ emissions. Reactors using metallic membranes (MRs) can handle both problems. Compared to traditional reactors, MRs operates at substantially lower pressures and temperatures. As a result, capital and operational costs may be significantly cheaper than traditional reactors. Furthermore, metallic membranes (MMs), particularly Pd and its alloys, naturally permit only H₂ permeability, enabling the production of a stream with a purity of up to 99.999%. This review describes several methods for H₂ production based on the energy sources utilized. SRM with CO₂ capture and storage (CCUS), pyrolysis of methane, and water electrolysis are all investigated as process technologies. A debate based on a color code was also created to classify the purity of H₂ generation. Although producing H₂ using fossil fuels is presently the least expensive method, green H₂ generation has the potential to become an affordable alternative in the future. From 2030 onward, green H₂ is anticipated to be less costly than blue hydrogen. Green H₂ is more expensive than fossil-based H₂ since it uses more energy. Blue H₂ has several tempting qualities, but the CCUS technology is pricey, and blue H₂ contains carbon. At this time, almost 80-95% of CO₂ can be stored and captured by the CCUS technology. Nanomaterials are becoming more significant in solving problems with H₂ generation and storage. Sustainable nanoparticles, such as photocatalysts and bio-derived particles, have been emphasized for H₂ synthesis. New directions in H₂ synthesis and nanomaterials for H₂ storage have also been discussed. Further, an overview of the H₂ value chain is provided at the end, emphasizing the financial implications and outlook for 2050, i.e., carbon-free H₂ and zero-emission H₂.

Investigations into pyrolytic behaviour of spent citronella waste: Slow and flash pyrolysis study

Slow and flash pyrolysis of spent citronella biomass has been studied at varying temperatures. It is aimed to understand the pyrolytic behavior of spent citronella aromatic biomass with temperatures. Maximum bio-oil yield of 37.7 wt% was obtained with conversion of 71 wt% at 450 °C through slow pyrolysis. GC/MS, ¹H NMR, and FTIR analysis of pyrolytic liquid (bio-oil) was done which indicated various functionalities with maximum area% for phenolics. However, flash pyrolysis at high heating rate of 20 °C/ms resulted into maximum area% for carbonyls at all temperatures. In addition, an increasing trend for phenolics with temperature was also observed. The properties of obtained biochar are analysed by CHNS, FTIR, TOC, XRD, and SEM, which confirmed the significant decomposition of biomass constituents. The characterisation results revealed the potential usage of pyrolytic liquid i.e., bio-oil and pyrolytic residue i.e., biochar for different applications.

New trends on green energy and environmental technologies, with special focus on biomass valorization, water and waste recycling: editorial of the special issue

In this editorial piece, the Editors of the Virtual Special Issue (VSI) "New Trends on Green Energy and Environmental Technologies, with Special Focus on Biomass Valorization, Water and Waste Recycling", present summarized data corresponding to the accepted submissions, as well as additional comments regarding the thematic of the VSI. Overall, 83 manuscripts were received, with final publication of those having the highest quality, accepted after peer-reviewing. The Editors think that the result is a set of very interesting papers that increase the knowledge on the matter, and which would be useful for researchers and the whole society.

Novel study on catalytic pyrolysis of chitin biomass using waste cathode material recovered from spent Li-ion battery

Waste cathode (BC) from spent lithium-ion battery (LIB) was preliminarily studied for the catalytic pyrolysis of chitin biomass using thermogravimetric and pyrolysis-gas chromatography/mass spectrometry analysis. Compared with the dry-mixing method (BC1), the wet-impregnation method (BC2) significantly increased the decomposition rate of chitin pyrolysis and decreased the apparent activation energy from 85 kJ/mol to 76 kJ/mol. BC2 had a superior catalytic effect on the conversion of heavy components into light fractions. In particular, the selectivities for acetamides and acetonitrile were improved. Furthermore, BC2 enhanced the cleavage of glucosidic, C-O, and C-C bonds, thereby improving the thermolysis of chitin to acetamido acetaldehyde. The production of acetamides, acetonitriles, and other light components (e.g., ketones) was further enhanced via deoxygenation and hydrogenation reactions. Additionally, the selectivity of N-heterocycles (pyridine and pyrrole) and their derivatives (tar-N surrogates) decreased, indicating that tar was significantly reduced during the catalytic pyrolysis and gasification of chitin biomass.

Highly efficient C(CO)-C(alkyl) bond cleavage in ketones to access esters over ultrathin N-doped carbon nanosheets

Selective oxidative cleavage of the C(CO)–C bond in ketones to access esters is a highly attractive strategy for upgrading ketones. However, it remains a great challenge to realize this important transformation over heterogeneous metal-free catalysts. Herein, we designed a series of porous and ultrathin N-doped carbon nanosheets (denoted as CN-X, where X represents the pyrolysis temperature) as heterogeneous metal-free catalysts. It was observed that the fabricated CN-800 could efficiently catalyze the oxidative cleavage of the C(CO)–C bond in various ketones to generate the corresponding methyl esters at 130 °C without using any additional base. Detailed investigations revealed that the higher content and electron density of the graphitic-N species contributed to the excellent performance of CN-800. Besides, the high surface area, affording active sites that are more easily accessed, could also enhance the catalytic activity. Notably, the catalysts have great potential for practical applications because of some obvious advantages, such as low cost, neutral reaction conditions, heterogeneous nature, high efficiency, and broad ketone scope. To the best of our knowledge, this is the first work on efficient synthesis of methyl esters via oxidative esterification of ketones over heterogeneous metal-free catalysts.

Ultrathin and metal-free N-doped carbon nanosheets showed high activity and selectivity for oxidative esterification of ketones via C(CO)–C bond cleavage to access methyl esters.

Hydro-deoxygenation at atmospheric pressure converts the phenolic-rich pyrolysis liquid fraction into aromatics

Ambient pressure hydro-deoxygenation (HDO) of the phenolic-rich pyrolysis liquid fraction is a complex task due to the presence multiple phenolic compounds and light oxygenates. The phenolic-rich fraction differs from the overall pyrolysis liquid, known to be prone to re-polymerization and coking in the reactor or of the catalyst. In the present research, hydro-deoxygenation of oxygen-containing compounds in the phenolic fraction over Mo-based catalysts was carried out for the first time. It was found that Mo-based catalysts can successfully upgrade the phenolics into aromatics, the conversion rate was nearly 100%. The small amount of light oxygenates in the phenolic-rich fraction had no obvious effect on the hydro-deoxygenation reaction, the phenolic conversion was more than 95%. After assessing the performance for a representative phenolic model compound, the reaction was also successfully carried out on the phenolic fraction of the real pyrolysis liquid. It can be concluded that the catalysts can also be used for the HDO of the real pyrolysis liquid fraction at atmospheric pressure.

Pyrolysis of Methyl Ricinoleate: Distribution and Characteristics of Fast and Slow Pyrolysis Products

A stable temperature site and the speed of heating the feedstocks play a key role in pyrolysis processes. In this study, the product distribution arising from pyrolysis of methyl ricinoleate (MR) at 550 °C with low and high heating rates was first studied by pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). The results show that fast pyrolysis of MR favored the production of undecylenic acid methyl ester (UAME) and heptanal (HEP). Density functional theory (DFT) calculations were employed to reveal the UAME and HEP formation process from pyrolysis of MR. The bond dissociation energies (BDEs) of C–C bonds in MR showed that the C11–C12 bond is the weakest. This suggests that UAME and HEP are two major products. The process of slow and fast MR pyrolysis was the dehydration-first and the pyrolysis-first trend, respectively. The calculated activation energies of MR pyrolysis to UAME and HEP and MR dehydration to 9,12-octadecadienoic acid methyl ester were 287.72 and 238.29 kJ/mol, respectively. The much higher product yields obtained in the fast pyrolysis reactors than those from conventional tubular reactors confirmed the proposed process.

Batch Pyrolysis and Co-Pyrolysis of Beet Pulp and Wheat Straw

Granulated beet pulp and wheat straw, first separately and then mixed in a weight ratio of 50/50%, underwent a pyrolysis process in a laboratory batch generator with process temperatures of 400 and 500 °C. The feedstock’s chemical composition and the pyrolysis products’ chemical composition (biochar and pyrolysis gas) were analysed. A synergistic effect was observed in the co-pyrolysis of the combined feedstock, which occurred as an increase the content of the arising gas in relation to the total weight of the products. and as a reduction of bio-oil content. The maximum gas proportion was 21.8% at 500 °C and the minimum between 12.6% and 18.4% for the pyrolysis of individual substrates at 400 °C. The proportions of the gases, including CO, CO₂, CH₄, H₂, and O₂, present in the resulting synthesis gases were also analysed. The usage of a higher pyrolysis final temperature strongly affected the increase of the CH₄ and H₂ concentration and the decrease of CO₂ and CO concentration in the pyrolysis gas. The highest percentage of hydrogen in the synthesis gas, around 33%_vol, occurred at 500 °C during co-pyrolysis.