Small Aromatic Ring Systems Research Articles

Changes in Char Structure during the Gasification of Mallee Wood: Effects of Particle Size and Steam Supply

The purpose of this study is to investigate the effects of the biomass particle size on the changes in char structure during the gasification in steam of mallee woody biomass. The experiments with continuous or sudden supply of steam were conducted using a fluidized-bed/fixed-bed quartz reactor. The structural features of chars were then characterized using Fourier transform (FT)-Raman spectroscopy. Our results indicate that the effects of the biomass particle size on char structure were minimal when the biomass was heated slowly while being in continuous contact with steam. On the contrary, when the biomass was first pyrolyzed in argon and then suddenly came into contact with steam, the biomass particle size could have a profound effect on the subsequent evolution of char structure during gasification. For the large biomass particles (>5 mm), the ratio between small and big aromatic ring systems in the char showed a maximum value after around 30 s of contact with steam.

Evaluation of structural features of chars from pyrolysis of biomass of different particle sizes

The structural features of chars derived from pyrolysis of mallee wood of different particle sizes in a novel fluidized-bed/fixed-bed reactor have been investigated. Raman spectroscopy was used for structural evaluation of chars. Spectra were curve-fitted with 10 Gaussian bands representing typical structural features of the chars. The temperature had a significant influence on the evolution of char structure and thus the total Raman peak area between 800 and 1800 cm − 1 is seen to decrease significantly with increasing pyrolysis temperature for all chars. On the other hand, the ratio I D/ I (Gr + Vl + Vr) between the band intensities of condensed aromatic ring systems (> 6 rings) and amorphous char structures with small aromatic ring (3–5 rings) systems is seen to increase with increasing temperature. The particle size of biomass has a great role in char structure at fast heating rate (> 1000 °C/s) pyrolysis although it has no effect on char structure at slow heating rate pyrolysis (0.17 °C/s). However, in the bigger biomass particle, the structure of char prepared under fast heating rate pyrolysis is similar to that of the structure of char prepared under slow heating rate pyrolysis.

Heteroaromatic Rings of the Future

Small aromatic ring systems are of central importance in the development of novel synthetic protein ligands. Here we generate a complete list of 24,847 such ring systems. We call this list and associated annotations VEHICLe, which stands for virtual exploratory heterocyclic library. Searches of literature and compound databases, using this list as substructure queries, identified only 1701 as synthesized. Using a carefully validated machine learning approach, we were able to estimate that the number of unpublished, but synthetically tractable, VEHICLe rings could be over 3000. However, analysis also shows that the rate of publication of novel examples to be as low as 5-10 per year. With this work, we aim to provide fresh stimulus to creative organic chemists by highlighting a small set of apparently simple ring systems that are predicted to be tractable but are, to the best of our knowledge, unconquered.

Fates and roles of alkali and alkaline earth metals during the pyrolysis of a Victorian brown coal

H-, Na- and Ca-form coal samples were prepared from a sample of Loy Yang brown coal and pyrolysed in a wire-mesh reactor. The tars were characterised with UV-absorption and UV-fluorescence spectroscopies. Increases in heating rate (1 to 2000Ks−1) and temperature (up to 700°C) were found to facilitate the release of larger (“equivalently” larger than naphthalene) aromatic ring systems from coal during pyrolysis. The presence of alkali and alkaline earth metallic (AAEM) species in the coal samples greatly hindered the release of the larger aromatic ring systems during pyrolysis. The AAEM species also reduced the effects of heating rate on the release on aromatic ring systems at lower temperatures. However, the hindering effect was not proportional to the contents of AAEM species in the coal. In addition, the ion-exchange processes caused irreversible changes to coal structure. Significant proportions of the AAEM species in the coal samples were volatilised during pyrolysis even at temperatures as low as 300°C. The volatilisation of AAEM species was not sensitive to changes in heating rate but was intensified with increasing temperature. The monovalent species (Na) was always volatilised to a much larger extent that the divalent species (Mg and Ca) under similar pyrolysis conditions. At high temperatures (900–1200°C), the drastic volatilisation of Na (up to 80%) and of Ca (up to 40%) was accompanied by the increases in tar yield during the pyrolysis of the Na-form and Ca-form samples. The fates and roles of the AAEM species during pyrolysis are thought to be related to their transformation during pyrolysis. The AAEM species might have been involved in a repeated bond-forming and bond-breaking process between the AAEM species and the coal/char matrix. During this process, tar precursors were repeatedly linked to the coal/char matrix and were thermally cracked. Some of the more aliphatic components and/or smaller aromatic ring systems in a tar precursor were cracked to gas and some of the larger aromatic ring systems were charred.

Structural characterization of tar from a coal gasification plant: Comparison with a coke oven tar and a crude oil flash-column residue

Residual tar from a coal gasification plant was characterized using a wide array of analytical techniques. Structural comparisons were made with a coke oven tar and a crude oil flash-column residue. The gasification tar residue was found to have a carbon aromaticity >94%, with extensively dealkylated aromatic structures and only small amounts of oxygenates, compared with the 56% carbon aromaticity and significant occurrence of alkyl substitution in the coke oven tar. The proportion of material boiling > 450°C as well as the elemental carbon and ‘fixed carbon’ contents of the gasification tar residue were also higher than those of the other samples. As expected, the petroleum flash-column residue was mainly aliphatic ( f a = 0.12), with small aromatic ring systems attached to long alkyl chains. For all three samples, MALDI mass spectra showed a signal at molecular masses > 20 000 u, with the gasification tar residue showing the narrowest range of molecular masses. Taken together, structural differences between the gasification tar and coke oven tar reflect the more severe thermal treatment and chemical history of the gasification tar, leading to the survival of low amounts (∼ 1 wt% of coal feed) of an apparently, chemically, very stable residue.