Search

As one of the major components of lignocellulosic biomass, the current understanding on pyrolysis mechanism of hemicellulose is relatively limited compared to that of cellulose due to its random and complicated structure. Here, we reported an in-depth study on hemicellulose pyrolysis by combining advanced experimental techniques and quantum chemical calculation, using xylose, xylobiose and xylan as the model compounds. The structural evolution during pyrolysis was firstly characterized via in-situ diffuse reflectance infrared Fourier transform spectroscopy. The corresponding two-dimensional perturbation correlation infrared spectroscopy analysis shows that the dissociation of side chains dominates in the initial stage of xylan pyrolysis, followed by the cleavage of glycosidic bond and the opening of xylopyranose ring. The release behaviors of condensable vapors from hemicellulose pyrolysis obtained by thermogravimetric analyzer coupled with photoionization mass spectrometry indicate that the pyrolysis of xylan polymer with more side chains is prone to produce small compounds via ring rupture reactions compared to that of xylose and xylobiose. The possible reaction pathways of xylan pyrolysis were proposed based on the experimental results and were further confirmed via quantum chemical calculation. This work expands the size of model compounds from common monomers and dimers without side chains to larger tetramers with side chains. It was found that the dissociation of side chains and the cleavage of glycosidic bonds are competitive reactions. The backbone of xylan is decomposed following the dissociation of side chains to yield furfural, dianhydro xylose and other small compounds. [ABSTRACT FROM AUTHOR]

The selective conversion of biomass into high‐value carbon materials has recently been attracting increasing attraction. In this study, a microalga with a high nitrogen content, namely Spirulina platensis, was used as both the carbon and nitrogen precursors in the synthesis of sponge‐like nitrogen‐enriched carbon materials through a one‐step activation method with NaHCO3 as a green activator. The sponge‐like nitrogen‐enriched carbon materials obtained at different activation temperatures were systematically characterized by using SEM, N2 adsorption‐desorption, XRD, Raman, and XPS techniques. A series of tests were performed to investigate the electrochemical characteristics of the obtained carbon materials to determine their potential use as supercapacitors. The results show that sponge‐like nitrogen‐enriched carbon activated at 700 °C, which has a large BET surface area of 865 m2/g and a high nitrogen content of 7.5 wt%, gave the largest specific capacitance of 234.0 F/g at a current density of 1 A/g in a 6 M KOH electrolyte. High carb: A one‐step activation method is applied to convert nitrogen‐rich microalgae into nitrogen‐containing carbon materials, where NaHCO3 is used as a green activator. The sponge‐like nitrogen‐enriched carbon activated at 700 °C exhibits a large BET surface area of 865 m2/g and a high nitrogen content of 7.5 wt%. It shows a large specific capacitance of 234.0 F/g at a current density of 1 A/g in a 6 M KOH electrolyte when used as a supercapacitor electrode. [ABSTRACT FROM AUTHOR]

Metal modification on hierarchical HZSM-5 produced by alkaline desilication was adopted to improve aromatics production from catalytic pyrolysis of cellulose. The influence of alkali concentration in the desilication on catalytic performance of zeolites was first studied. Mild alkaline treatment preserved the micropore structure and acid sites of zeolites when introducing mesopores, while further increase of alkali concentration destroyed the micropores and acidity. The HZSM-5 desilicated by 0.3 M NaOH solution had the best catalytic performance, which increased the aromatics production and BTX selectivity. Then, metal (Fe and Ga) modification on the optimal zeolite was used to further improve the catalytic performance. Fe modification showed negative effect on aromatics production while Ga modification had the positive. Both Fe and Ga modification increased the selectivity of monoaromatics at the expense of polyaromatics. Furthermore, Fe modification mainly enhanced the production of BTX, while Ga modification promoted the formation of both BTX and alkylbenzenes. [ABSTRACT FROM AUTHOR]

In this study, a β-O-4 dimer with abundant oxygen substituents was successfully synthesized, and was used as the model compound for lignin to study the pyrolysis mechanism. Py-GC/MS (micro pyrolyzer coupled with gas chromatography/mass spectrometry) was employed to identify the distribution of pyrolytic products under temperatures of 150–850 °C. It was found that the yields of methoxylated monoaromatics underwent tendencies of first increase and then decrease at high temperature. Polyaromatics and benzofuran were only detected at high temperature. Based on experimental analysis, a detailed pyrolysis kinetic model was developed by combining the density functional theory (DFT) and the transition state theory (TST). The homolysis of C β -O was the most favorable route for the initial depolymerization of dimer rather than the concerted retro-ene fragmentation. For the evolution of intermediate products, the breakage of ether bond and the keto-enol tautomerization of enols were the most favorable routes, while intramolecular cyclization and group dissociation exhibited high energy barriers and low reaction rates. The methoxyl in guaiacol preferred to undergo demethylation leading to the formation of catechol. [ABSTRACT FROM AUTHOR]

The Front Cover shows sponge‐like nitrogen‐containing carbon materials prepared by using a one‐step activation method from nitrogen‐rich microalgae biomass. A large Brunauer–Emmett–Teller surface area of 865 m2/g is obtained with a high nitrogen content of 7.5 wt %. This research provides a convenient and ecofriendly way to prepare high‐performance nitrogen‐containing carbons. More information can be found in the Article by Y. Liu et al. on page 646 in Issue 3, 2019 (DOI: 10.1002/celc.201801272). [ABSTRACT FROM AUTHOR]

The influence mechanism of torrefaction on biomass pyrolysis was studied in detail. 13 C CP/MAS NMR was employed to characterize the evolution of functional groups during torrefaction, indicating that hemicellulose was extensively deacetylated and depolymerized; the crystalline structure of cellulose was destroyed; and the aryl ether linkages and propyl side branches in lignin were cleaved. A distributed activation energy model with three combined Gaussian functions was introduced to analyze the pyrolysis kinetics of biomass. The mean activation energies for three parallel reactions remained unchanged after torrefaction, while the weighing factors proved that their contributions to devolatilization largely changed. Acetic acid yield significantly decreased after torrefaction, with reduction of the yields of 5-hydroxymethylfurfural and levoglucosan because of the depolymerization of carbohydrates during torrefaction. Due to the dissociation of propyl branches and the demethylation of methoxyls in lignin, pyrolysis of torrefied biomass yielded more guaiacol and catechol and less phenols with C4-propyl groups. [ABSTRACT FROM AUTHOR]