Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass Pyrolysis

Experiments in pulse-injected flow reactors show that gas-phase pyrolysis of alcohols makes a transition from dehydrogenation dominance for mono-alcohols to dehydration and fragmentation for tested diols and a triol. These dehydration and dehydrogenation reactions are proposed to occur by unimolecul...

Full description

Bibliographic Details
Main Authors: P. Westmoreland, P. Fahey
Format: Article
Language:English
Published: AIDIC Servizi S.r.l. 2016-06-01
Series:Chemical Engineering Transactions
Online Access:https://www.cetjournal.it/index.php/cet/article/view/3170
_version_ 1818871275302420480
author P. Westmoreland
P. Fahey
author_facet P. Westmoreland
P. Fahey
author_sort P. Westmoreland
collection DOAJ
description Experiments in pulse-injected flow reactors show that gas-phase pyrolysis of alcohols makes a transition from dehydrogenation dominance for mono-alcohols to dehydration and fragmentation for tested diols and a triol. These dehydration and dehydrogenation reactions are proposed to occur by unimolecular and bimolecular pericyclic reactions, as has been used to explain glucose and cellulose pyrolysis in the absence of ions (Seshadri and Westmoreland, 2012, 2015). Vaporizable mono-alcohols (ethanol, propanol, butan-2-ol, t-butyl alcohol, and neopentyl alcohol), diols (ethan-1,2-diol, propan-1,2-diol and propan-1,3-diol), and a triol (propan-1,2,3-triol) were pulse-injected into a helium flow, pyrolyzed at 400°C in a tubular flow reactor, and swept directly into a two-dimensional gas chromatograph with time-of-flight mass-spectrometric detection (Pegasus 4D, LECO Corp.). Methanol and 2- propanol were pyrolyzed in a tubular quartz reactor with solely mass-spectrometric analysis. All the mono-alcohols dehydrogenated except t-butyl alcohol, which has no H on the alcohol carbon. Calculations at a CBS-QB3 level of theory showed that the energetically favored transition states for dehydrogenation were six-centered pericyclic reactions with a hydrogen-bonded ROH molecule. These transition-state enthalpies were about 55 kcal/mol, compared to about 85 kcal/mol for four-centered H2 elimination. Non-hydrogen-bonded six-centered transition states can occur, but their enthalpies were 109 kcal/mol and higher. The diols displayed dehydrogenation, dehydration, and cyclic Grob fragmentation, and the triol yielded only dehydration and fragmentation products. Details of the analyses are presented, and the findings are compared and contrasted to other findings in the literature.
first_indexed 2024-12-19T12:20:20Z
format Article
id doaj.art-048cb9e884b84b9c8c14fea781acd55c
institution Directory Open Access Journal
issn 2283-9216
language English
last_indexed 2024-12-19T12:20:20Z
publishDate 2016-06-01
publisher AIDIC Servizi S.r.l.
record_format Article
series Chemical Engineering Transactions
spelling doaj.art-048cb9e884b84b9c8c14fea781acd55c2022-12-21T20:21:47ZengAIDIC Servizi S.r.l.Chemical Engineering Transactions2283-92162016-06-015010.3303/CET1650013Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass PyrolysisP. WestmorelandP. FaheyExperiments in pulse-injected flow reactors show that gas-phase pyrolysis of alcohols makes a transition from dehydrogenation dominance for mono-alcohols to dehydration and fragmentation for tested diols and a triol. These dehydration and dehydrogenation reactions are proposed to occur by unimolecular and bimolecular pericyclic reactions, as has been used to explain glucose and cellulose pyrolysis in the absence of ions (Seshadri and Westmoreland, 2012, 2015). Vaporizable mono-alcohols (ethanol, propanol, butan-2-ol, t-butyl alcohol, and neopentyl alcohol), diols (ethan-1,2-diol, propan-1,2-diol and propan-1,3-diol), and a triol (propan-1,2,3-triol) were pulse-injected into a helium flow, pyrolyzed at 400°C in a tubular flow reactor, and swept directly into a two-dimensional gas chromatograph with time-of-flight mass-spectrometric detection (Pegasus 4D, LECO Corp.). Methanol and 2- propanol were pyrolyzed in a tubular quartz reactor with solely mass-spectrometric analysis. All the mono-alcohols dehydrogenated except t-butyl alcohol, which has no H on the alcohol carbon. Calculations at a CBS-QB3 level of theory showed that the energetically favored transition states for dehydrogenation were six-centered pericyclic reactions with a hydrogen-bonded ROH molecule. These transition-state enthalpies were about 55 kcal/mol, compared to about 85 kcal/mol for four-centered H2 elimination. Non-hydrogen-bonded six-centered transition states can occur, but their enthalpies were 109 kcal/mol and higher. The diols displayed dehydrogenation, dehydration, and cyclic Grob fragmentation, and the triol yielded only dehydration and fragmentation products. Details of the analyses are presented, and the findings are compared and contrasted to other findings in the literature.https://www.cetjournal.it/index.php/cet/article/view/3170
spellingShingle P. Westmoreland
P. Fahey
Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass Pyrolysis
Chemical Engineering Transactions
title Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass Pyrolysis
title_full Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass Pyrolysis
title_fullStr Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass Pyrolysis
title_full_unstemmed Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass Pyrolysis
title_short Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass Pyrolysis
title_sort dehydration and dehydrogenation kinetics of oh groups in biomass pyrolysis
url https://www.cetjournal.it/index.php/cet/article/view/3170
work_keys_str_mv AT pwestmoreland dehydrationanddehydrogenationkineticsofohgroupsinbiomasspyrolysis
AT pfahey dehydrationanddehydrogenationkineticsofohgroupsinbiomasspyrolysis