The Experts below are selected from a list of 4008 Experts worldwide ranked by ideXlab platform
Jukka Vaari - One of the best experts on this subject based on the ideXlab platform.
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High-temperature decomposition of the Cellulose Molecule: a stochastic molecular dynamics study
Cellulose, 2017Co-Authors: Antti Paajanen, Jukka VaariAbstract:The kinetics and products of Cellulose pyrolysis can be studied using large-scale molecular dynamics simulations at high temperatures, where the reaction rates are high enough to make the simulation times practical. We carried out molecular dynamics simulations employing the ReaxFF reactive force field to study the initial step of the thermal decomposition process. We gathered statistics of simulated reactive events at temperatures ranging from 1400 to 2200 K, considering Cellulose Molecules with different molecular weights and initial conformations. Our simulations suggest that, in gas-phase conditions at these high temperatures, the decomposition occurs primarily through random cleavage of the β(1 → 4)-glycosidic bonds, for which we obtained an activation energy of (171 ± 2) kJ mol−1 and a frequency factor of $$\left( {1.07 \pm 0.12} \right) \times 10^{15}$$1.07±0.12×1015 s−1. We did not observe dependency of the kinetic parameters on the molecular weight or initial conformation. Some of the decomposition reactions involved the release of low-molecular-weight products. Excluding radicals, the most commonly observed species were glycolaldehyde, water, formaldehyde and formic acid. Many of our observations are supported by the existing experimental and theoretical knowledge. We did not, however, observe the formation of levoglucosan, which is the dominant product in conventional pyrolysis experiments at much lower temperatures. This is understandable, since the high temperatures can force the dominance of radical reactions over pericyclic reactions. Nevertheless, our results support further use of ReaxFF-based molecular dynamics simulations in the study of Cellulose pyrolysis.
Lata Gautam - One of the best experts on this subject based on the ideXlab platform.
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position specific measurement of oxygen isotope ratios in Cellulose isotopic exchange during heterotrophic Cellulose synthesis
Geochimica et Cosmochimica Acta, 2013Co-Authors: John S Waterhouse, Shuying Cheng, Dieter Juchelka, Neil J Loader, Danny Mccarroll, Roy V Switsur, Lata GautamAbstract:We describe the first reported method for the measurement of oxygen isotope ratios at each position in the glucose units of the Cellulose Molecule. The overall process comprises a series of synthetic organic sequences, by which α-Cellulose is hydrolysed to glucose, and oxygen atoms at specific positions in the glucose Molecule are removed in samples of benzoic acid for measurement of δ18O. Values of δ18O at specific positions in Cellulose are calculated from these δ18O values and the overall δ18O value of the Cellulose. We apply the method to determine the degree to which oxygen atoms at each position undergo isotopic exchange with water during heterotrophic Cellulose synthesis, such as occurs in the cambium of trees. To do this we extract α-Cellulose from wheat seedlings germinated in the dark in aqueous media of differing oxygen isotope ratios. Results indicate that oxygen atoms at positions 5 and 6 (O-5 and O-6 respectively) undergo around 80% exchange with medium water, O-3 undergoes around 50% exchange, and O-2 and O-4 do not undergo isotopic exchange. The results have important implications for extracting palaeoclimatic records from oxygen isotope time series obtained from tree ring Cellulose. As O-5 and O-6 undergo significant exchange with medium water during heterotrophic Cellulose synthesis, oxygen isotopes at these positions in tree ring Cellulose should carry a predominantly trunk (source) water signal. On the other hand, O-2 and O-4 should retain the isotopic signature of leaf water in tree ring Cellulose. Our method therefore potentially enables the separate reconstruction of past temperature and humidity data from oxygen isotope ratios of tree ring Cellulose – something that has hitherto not been possible. The measured degrees of isotopic exchange are to some extent unexpected and cannot be fully explained using current biochemical mechanisms, suggesting that knowledge of these processes is incomplete.
Shuying Cheng - One of the best experts on this subject based on the ideXlab platform.
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position specific measurement of oxygen isotope ratios in Cellulose isotopic exchange during heterotrophic Cellulose synthesis
Geochimica et Cosmochimica Acta, 2013Co-Authors: John S Waterhouse, Shuying Cheng, Dieter Juchelka, Neil J Loader, Danny Mccarroll, Roy V Switsur, Lata GautamAbstract:We describe the first reported method for the measurement of oxygen isotope ratios at each position in the glucose units of the Cellulose Molecule. The overall process comprises a series of synthetic organic sequences, by which α-Cellulose is hydrolysed to glucose, and oxygen atoms at specific positions in the glucose Molecule are removed in samples of benzoic acid for measurement of δ18O. Values of δ18O at specific positions in Cellulose are calculated from these δ18O values and the overall δ18O value of the Cellulose. We apply the method to determine the degree to which oxygen atoms at each position undergo isotopic exchange with water during heterotrophic Cellulose synthesis, such as occurs in the cambium of trees. To do this we extract α-Cellulose from wheat seedlings germinated in the dark in aqueous media of differing oxygen isotope ratios. Results indicate that oxygen atoms at positions 5 and 6 (O-5 and O-6 respectively) undergo around 80% exchange with medium water, O-3 undergoes around 50% exchange, and O-2 and O-4 do not undergo isotopic exchange. The results have important implications for extracting palaeoclimatic records from oxygen isotope time series obtained from tree ring Cellulose. As O-5 and O-6 undergo significant exchange with medium water during heterotrophic Cellulose synthesis, oxygen isotopes at these positions in tree ring Cellulose should carry a predominantly trunk (source) water signal. On the other hand, O-2 and O-4 should retain the isotopic signature of leaf water in tree ring Cellulose. Our method therefore potentially enables the separate reconstruction of past temperature and humidity data from oxygen isotope ratios of tree ring Cellulose – something that has hitherto not been possible. The measured degrees of isotopic exchange are to some extent unexpected and cannot be fully explained using current biochemical mechanisms, suggesting that knowledge of these processes is incomplete.
Antti Paajanen - One of the best experts on this subject based on the ideXlab platform.
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High-temperature decomposition of the Cellulose Molecule: a stochastic molecular dynamics study
Cellulose, 2017Co-Authors: Antti Paajanen, Jukka VaariAbstract:The kinetics and products of Cellulose pyrolysis can be studied using large-scale molecular dynamics simulations at high temperatures, where the reaction rates are high enough to make the simulation times practical. We carried out molecular dynamics simulations employing the ReaxFF reactive force field to study the initial step of the thermal decomposition process. We gathered statistics of simulated reactive events at temperatures ranging from 1400 to 2200 K, considering Cellulose Molecules with different molecular weights and initial conformations. Our simulations suggest that, in gas-phase conditions at these high temperatures, the decomposition occurs primarily through random cleavage of the β(1 → 4)-glycosidic bonds, for which we obtained an activation energy of (171 ± 2) kJ mol−1 and a frequency factor of $$\left( {1.07 \pm 0.12} \right) \times 10^{15}$$1.07±0.12×1015 s−1. We did not observe dependency of the kinetic parameters on the molecular weight or initial conformation. Some of the decomposition reactions involved the release of low-molecular-weight products. Excluding radicals, the most commonly observed species were glycolaldehyde, water, formaldehyde and formic acid. Many of our observations are supported by the existing experimental and theoretical knowledge. We did not, however, observe the formation of levoglucosan, which is the dominant product in conventional pyrolysis experiments at much lower temperatures. This is understandable, since the high temperatures can force the dominance of radical reactions over pericyclic reactions. Nevertheless, our results support further use of ReaxFF-based molecular dynamics simulations in the study of Cellulose pyrolysis.
John S Waterhouse - One of the best experts on this subject based on the ideXlab platform.
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position specific measurement of oxygen isotope ratios in Cellulose isotopic exchange during heterotrophic Cellulose synthesis
Geochimica et Cosmochimica Acta, 2013Co-Authors: John S Waterhouse, Shuying Cheng, Dieter Juchelka, Neil J Loader, Danny Mccarroll, Roy V Switsur, Lata GautamAbstract:We describe the first reported method for the measurement of oxygen isotope ratios at each position in the glucose units of the Cellulose Molecule. The overall process comprises a series of synthetic organic sequences, by which α-Cellulose is hydrolysed to glucose, and oxygen atoms at specific positions in the glucose Molecule are removed in samples of benzoic acid for measurement of δ18O. Values of δ18O at specific positions in Cellulose are calculated from these δ18O values and the overall δ18O value of the Cellulose. We apply the method to determine the degree to which oxygen atoms at each position undergo isotopic exchange with water during heterotrophic Cellulose synthesis, such as occurs in the cambium of trees. To do this we extract α-Cellulose from wheat seedlings germinated in the dark in aqueous media of differing oxygen isotope ratios. Results indicate that oxygen atoms at positions 5 and 6 (O-5 and O-6 respectively) undergo around 80% exchange with medium water, O-3 undergoes around 50% exchange, and O-2 and O-4 do not undergo isotopic exchange. The results have important implications for extracting palaeoclimatic records from oxygen isotope time series obtained from tree ring Cellulose. As O-5 and O-6 undergo significant exchange with medium water during heterotrophic Cellulose synthesis, oxygen isotopes at these positions in tree ring Cellulose should carry a predominantly trunk (source) water signal. On the other hand, O-2 and O-4 should retain the isotopic signature of leaf water in tree ring Cellulose. Our method therefore potentially enables the separate reconstruction of past temperature and humidity data from oxygen isotope ratios of tree ring Cellulose – something that has hitherto not been possible. The measured degrees of isotopic exchange are to some extent unexpected and cannot be fully explained using current biochemical mechanisms, suggesting that knowledge of these processes is incomplete.