energies
Article Dehydration Leads to Hydrocarbon Gas Formation in Thermal Degradation of Gas-Phase Polyalcohols
Asuka Fukutome and Haruo Kawamoto * Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan; [email protected] * Correspondence: [email protected]
Received: 30 April 2020; Accepted: 10 July 2020; Published: 20 July 2020
Abstract: To understand the molecular mechanisms of hydrocarbon gas formation in biomass gasification, gasification of simple polyalcohols (glycerol, propylene glycol, and ethylene glycol) were studied at 400, 600, and 800 ◦C (residence times: 0.9–1.4 s) from the viewpoint of dehydration reactions that form aldehydes with various substituents as intermediates to produce hydrocarbon gases. The results were also compared with those of glyceraldehyde and dihydroxyacetone, which are reported to produce syngas (H2 and CO) selectively. All polyalcohols became reactive at 600 ◦C to form condensable products in 15.7–24.7% yields (C-based), corresponding to 33.9–38.4% based on the amounts of reacted polyalcohols. These condensable products, mostly aldehydes, act as gas-forming intermediates, because the polyalcohols were completely gasified at 800 ◦C (hydrocarbon gas contents: 20.3–35.3%, C-based). Yields of the intermediates bearing alkyl groups at 600 ◦C were proportionally correlated to the yields of hydrocarbon gases at 800 ◦C, suggesting that the alkyl groups are further converted into hydrocarbon gases via the fragmentation of acyl radicals. Dehydration reactions were suggested to occur in both heterolytic and radical mechanisms by theoretical calculations. Glyceraldehyde tended to fragment directly into CO and H2, instead of forming a dehydration intermediate. These results are informative for controlling the product gas composition in biomass gasification.
Keywords: biomass gasification mechanism; polyalcohol; hydrocarbon gas; dehydration; density functional theory
1. Introduction Biomass gasification, which produces oxygenated (OX) (carbon monoxide (CO) and carbon dioxide (CO2)) and hydrocarbon (HC) (methane (CH4), ethylene (C2H4) and acetylene (C2H2)) gases along with H2, is a potential method for the sustainable production of biofuels and biochemicals. The producer gas can be used by gas turbines/engines to generate electricity. Petroleum and various chemicals can be produced over Fischer–Tropsch catalysts via syngas (CO + H2). Contamination from hydrocarbon gases, however, should be eliminated for the Fischer–Tropsch process, because of the poisoning problem on the catalysts. Ethylene is an important industrial chemical currently produced from petroleum, and the improvement of the production in biomass gasification would promote replacing petroleum with biomass as the source of ethylene. Therefore, controlling the gas selectivity of biomass gasification is important to expand its usability. For controlling the gas selectivity, the water–gas shift reaction [1,2] converts CO and H2O into H2 and CO2 under steam gasification conditions. These processes increase the H2 content instead of CO from biomass. However, reports are limited for the reactions that produce hydrocarbon gases from biomass gasification. However, molecular mechanisms that produce hydrocarbon gases in biomass components gasification have not been well clarified.
Energies 2020, 13, 3726; doi:10.3390/en13143726 www.mdpi.com/journal/energies Energies 2020, 13, 3726 2 of 18 Energies 2020, 13, x FOR PEER REVIEW 2 of 17
Stein et al. [[3]3] reported the ethylene formation from acrolein, and they explained the formation via vinylvinyl radicalradical formedformed by by the theα α-scission-scission of of the the acrolein acrolein radical. radical. Fukutome Fukutome et al.et [al.4] compared[4] compared the gasthe andgas cokeand coke forming forming reactions reactions of eight of cellulose-derived eight cellulose-derived volatile volatile intermediates intermediates in an ampoule in an reactorampoule at 600reactor◦C. Theyat 600 showed °C. They that showed the intermediates that the intermediates bearing methyl bearing groups methyl (acetic acidgroups and hydroxyacetone)(acetic acid and producehydroxyacetone) more hydrocarbon produce gasmore (mainly hydrocarbon methane) gas than (mainly that formed methane) using glycolaldehyde, than that formed formic using acid, orglycolaldehyde, furanic compounds, formic whichacid, or do furanic not have compounds, any methyl which groups. do Theynot have also any reported methyl that groups. glyceraldehyde They also (Gald;reported an that aldose) glyceraldehyde and 1,3-dihydroxyacetone (Gald; an aldose (DHA;) and a ketose) 1,3-dihydroxyacetone were selectively gasified(DHA; a into ketose) syngas were [5]. Therefore,selectively thegasified gas composition into syngas from[5]. Therefore, cellulose gasification the gas composition varies depending from cellulose on the chemicalgasification structure varies ofdepending the volatile on intermediatesthe chemical structure during cellulose of the volatile gasification. intermediates during cellulose gasification. Levoglucosan (1,6-anhydro- β-D-glucopyranose)-D-glucopyranose) is is a a major volatile intermediate in cellulosecellulose gasification,gasification, because of the large amounts (69.3% and 52.7%, C-based under nitrogen and 7% oxygenoxygen/nitrogen/nitrogen conditions, respectively) detected detected during during the the pyrolysis pyrolysis in in a a flow-type flow-type reactor reactor [6]. [6]. The gaseous levoglucosan is stable up to around 500 ◦°CC and starts to fragment at 600 ◦°CC into smaller compounds andand finally finally into into non-condensable non-condensable gases [7gase], althoughs [7], italthough readily degrades it readily into polymerizationdegrades into andpolymerization dehydration and products, dehydration including products, coke, including in the molten coke, statein the by molten cooling state [8]. by High cooling reactivity [8]. High of thereactivity molten of levoglucosan the molten levoglucosan has been explained has been by theexplained intermolecular by the intermolecular hydrogen bonding hydrogen acting bonding as acid andacting base as acid catalysts and [base9–11 catalysts]. [9–11]. The kinetic analysis of the decomposition of gaseousgaseous levoglucosan indicated the radical chain reactions acting as as the the rate-determining rate-determining step step [12]. [12]. In In analyzing analyzing the the fragmentation fragmentation pathways pathways of the of the C- C-and and O-centered O-centered radicals radicals of levoglucosan of levoglucosan from from the theperspective perspective of organic of organic reaction reaction mechanisms, mechanisms, an aninteresting interesting hypothesis hypothesis has has been been proposed; proposed; hydrocar hydrocarbonbon gases gases (methane, (methane, ethylene, ethylene, and and acetylene) acetylene) are produced byby thethe cleavage cleavage of of the the substituents substituents of aldehydes,of aldehydes, produced produced by theby dehydrationthe dehydration reactions reactions from thefrom levoglucosan the levoglucosan radicals radicals [12]. This [12]. hypothesis This hypothesis suggests suggests that the that hydrocarbon the hydrocarbon gas formation gas formation is directly is relateddirectly to related the extent to the of theextent progression of the progression of dehydration of dehydration reactions that reactions can occur that in both can heterolysisoccur in both and βheterolysis-scission of and radical β-scission intermediates. of radical intermediates. However, contributions However, of contributions these mechanisms of these have mechanisms not been have fully clarifiednot been duefully to clarified the complexity due to ofthe the complexity gasification of reactions the gasification of levoglucosan reactions as of an levoglucosan intermediate as from an celluloseintermediate and hemicellulosefrom cellulose in biomassand hemicellulose gasification. in Consequently, biomass gasification simple polyalcohols:. Consequently, glycerol simple (Gly), propylenepolyalcohols: glycol glycerol (PG), (Gly), and ethylenepropylene glycol glycol (EG) (PG), in and Figure ethylene1 were glycol used in(EG) this in study. Figure It 1 is were easier used to identifyin this study. the intermediate It is easier products to identify and the to discussintermediate their gas-formingproducts and reactions to discuss by e fftheirectively gas-forming utilizing thereactions theoretical by effectively calculation utilizing results obtainedthe theoretical by using calculation density functional results obtained theory (DFT) by using conducted density at MP4(sdq)functional/ Aug-cc-pVTZtheory (DFT)// DFT(M06-2X)conducted at/ 6-31MP4(sdq)/Aug-cc-pVTZ//DFT(M06-2X)/6-31+G+G (p,d) levels. The results are also compared (p,d) with levels. those reportedThe results for are Gald also and compared DHA [5 ],with which those selectively reported producefor Gald synthesisand DHA gas [5], (CO which and selectively H2) instead produce of the hydrocarbonsynthesis gas gases.(CO and H2) instead of the hydrocarbon gases.
OH OH OH OH O
OH OH OH OH OOH OH OH
Glycerol Propylene glycol Ethylene glycol Glyceraldehyde 1,3-Dihydroxyacetone (Gly) (PG) (EG) (Gald) (DHA) Figure 1. Polyalcohols used in the present investigation.
2. Materials and Methods 2.1. Materials 2.1. Materials Ethylene glycol, propylene glycol, and glycerol, which were used for pyrolysis trials, Ethylene glycol, propylene glycol, and glycerol, which were used for pyrolysis trials, were were purchased from Nacalai Tesque, Inc. (Kyoto, Japan). Table1 summarizes some physicochemical purchased from Nacalai Tesque, Inc. (Kyoto, Japan). Table 1 summarizes some physicochemical data data of these polyalcohols. These chemicals were used as received, without further purification. of these polyalcohols. These chemicals were used as received, without further purification.
Table 1. Physicochemical data of polyalcohols.
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3 Energies 2020 , 13, 3726 Molecular Weight (Da) Density (g/cm ) Melting Point (°C) Boiling Point3 of 18 (°C) Glycerol 92.09 1.261 17.8 290 Propylene glycol 76.09 1.036 −59 188 Ethylene glycol Table 62.07 1. Physicochemical 1.113 data of polyalcohols. −12.9 197
Glyceraldehyde 90.08 1.455 3 145 228* Molecular Weight (Da) Density (g/cm ) Melting Point (◦C) Boiling Point (◦C) 1,3-Dihydroxyacetone 90.08 1.283 90 214* Glycerol 92.09 1.261 17.8 290 Propylene glycol 76.09* predicted value 1.036 in SciFinder. 59 188 − Ethylene glycol 62.07 1.113 12.9 197 − Glyceraldehyde 90.08 1.455 145 228 * 2.2. Pyrolysis1,3-Dihydroxyacetone 90.08 1.283 90 214 * * predicted value in SciFinder. A flow-type two-stage tubular reactor (Figure 2) used in this study is fully described in our previous2.2. paper Pyrolysis [5]. Sample (15 mg) put in a ceramic boat was placed at the center of the evaporator, and the air insideA flow-type the reactor two-stage made tubular of reactor quartz (Figure glass2) tube used inwas this studyreplaced is fully with described a nitrogen in our previous flow at 400 mL min−1. Thepaper nitrogen [5]. Sample flow (15 was mg) continuously put in a ceramic supplied boat was placed during at thethe center pyrolysis of the and evaporator, for additional and the 2 min 1 after theair pyrolysis inside the to reactor recover made the of quartzvolatile glass produc tube wasts. replacedAfter the with pyrolyzer a nitrogen temperature flow at 400 mL was min− pre-set. to 400, 600,The or nitrogen800 °C, flowthe wasevaporator continuously was supplied heated during to 120 the °C pyrolysis and held and for for additional5 min, then 2 min further after the heated to pyrolysis to recover the volatile products. After the pyrolyzer temperature was pre-set to 400, 600, 200 °C at 16 °C min−1 and held for 5 min. This process completely volatilized the sample and led it to or 800 ◦C, the evaporator was heated to 120 ◦C and held for 5 min, then further heated to 200 ◦C the pyrolyzer. The residence1 times of gaseous sample in the pyrolyzer were 1.4 s (400 °C), 1.2 s (600), at 16 ◦C min− and held for 5 min. This process completely volatilized the sample and led it to the and 0.9 spyrolyzer. (800 °C), The which residence were times settled of gaseousto be near sample resi indence the pyrolyzer times of were actual 1.4 sgasifications (400 ◦C), 1.2 s [13,14]. (600), After the pyrolysis,and 0.9 the s (800 furnace◦C), which cover were was settled open to be and near residencecooled by times an ofair actual flow. gasifications The volatile [13,14 products]. After were passed throughthe pyrolysis, a gas the washing furnace coverbottle was containing open and cooledan oximation by an air reagent flow. The (hydroxylammonim volatile products were chloride passed through a gas washing bottle containing an oximation reagent (hydroxylammonim chloride (NH2OH•HCl), 20 mg) in dimethylsulfoxide (DMSO)-d6 (2.0 mL), where aldehydes and ketones were (NH OH HCl), 20 mg) in dimethylsulfoxide (DMSO)-d (2.0 mL), where aldehydes and ketones were 2 • 6 convertedconverted into the into corresponding the corresponding oxime oxime derivatives. derivatives. The The non-condensable non-condensable gases weregases recovered were recovered into a into a gas bag.gas bag.
Figure 2.Figure A flow-type 2. A flow-type two-stage two-stage tubular tubular reactor consisting consisti anng evaporator an evaporator connected connected to a pyrolyzer to a and pyrolyzer and productproduct recovery recovery unit. unit. 2.3. Product Analysis 2.3. Product Analysis The condensed substance attached on the reactor tube was washed with the DMSO-d6 solution in 1 Thethe condensed gas washing substance bottle, and attached then the resulting on the solutionreactor was tube directly was measuredwashed bywith proton the (DMSO-H) nucleard6 solution in the gasmagnetic washing resonance bottle, (NMR) and spectroscopythen the resultin using a Bruckerg solution AC-400 was (400 directly MHz) spectrometer. measured Yieldsby proton of (1H) the products were determined by the comparison of the peak areas with those of the internal standard nuclear magnetic(2-furoic acid) resonance which was (NMR) added to spectroscopy the NMR sample us solution.ing a Brucker AC-400 (400 MHz) spectrometer. Yields of theExcept products for acetylene,were determined non-condensable by the gasescomparison were determined of the peak by micro areas GC with (Varian those CP-4900) of the internal standardwith (2-furoic the two acid) columns which system was (MS5A added (10 m)to the and NMR PoraPLOT sample Q (10 solution. m)). Acetylene was determined by Except for acetylene, non-condensable gases were determined by micro GC (Varian CP-4900) with the two columns system (MS5A (10 m) and PoraPLOT Q (10 m)). Acetylene was determined by GC (Shimadzu GC-14B) with the column: RESTEC, Rt@-Alumina BOND/N2SO4 (30 m, 0.53 mm Ø). The details of the chromatographic conditions are described in our previous paper [5].
2.4. Computational Methods Calculations were made with Gaussian 09 software package [15]. The geometry optimizations and vibrational frequency calculations were conducted with DFT (M06-2X) and 6-31+G(d,p) basis
Energies 2020, 13, 3726 4 of 18
GC (Shimadzu GC-14B) with the column: RESTEC, Rt@-Alumina BOND/N2SO4 (30 m, 0.53 mm Ø). The details of the chromatographic conditions are described in our previous paper [5].
2.4. Computational Methods Calculations were made with Gaussian 09 software package [15]. The geometry optimizations and vibrational frequency calculations were conducted with DFT (M06-2X) and 6-31+G(d,p) basis sets. The energies of optimized geometries were calculated with MP4(SDQ). Single point energy calculation to estimate bond dissociation energy (BDE) were conducted with the CCSD(T) method and the AUG-cc-pVTZ basis sets. Activation energy (Ea) and BDE include zero-point energies. In frequency analysis, one imaginary mode was observed for transition state, but no imaginary modes for the reactants, products, and precomplexes. The intrinsic reaction coordinate calculations were also conducted to ascertain the transition state connecting the reactant and product.
3. Results and Discussion
3.1. Experimental Study of Gas Formation via Fragmentation of Intermediates
Gly, PG, and EG were recovered almost quantitatively at 400 ◦C and became reactive at 600 ◦C, where the recoveries were 32.0%, 43.2%, and 53.7%, respectively. The 24.7%, 21.8%, and 15.7% (C-based) of the condensable products were identified for the pyrolyzates from Gly, PG, and EG at 600 ◦C, respectively, which correspond to 36.3%, 38.4%, and 33.9% (C-based) against the amounts of degraded polyalcohols, respectively. Most of the remaining components were non-condensable gases. Because polyalcohols were completely converted into the non-condensable gases by increasing the temperature to 800 ◦C, these condensable products are the gas-forming intermediates from the polyalcohols. Therefore, a comparison of the chemical structures of condensable intermediates (600 ◦C) with the final gas composition (800 ◦C) would be valuable to understand the gasification pathways of polyalcohols. The compositions of the non-condensable gases produced from Gly, PG, and EG at 800 ◦C are summarized in Figure3, compared with those reported for Gald-DHA (61 /36, mol/mol) and DHA [5]. The gas yields (82.2–97.4%, C-based) are also shown in the figure. Although the major gaseous products were syngas, the hydrocarbon gas yields were different depending on the chemical structure of the polyalcohol. Gald-DHA (61/36) and DHA produced syngas selectively with the limited amounts of HC gases (2.9–3.0%, C-based) and CO2, whereas the contents of the HC gases (mainly methane and ethylene) reached 20.3%, 35.3%, and 22.8% (C-based) from Gly, PG, and EG, respectively.
Figure 3. Composition (mol%) of non-condensable (gaseous) products from glyceraldehyde (Gald)/1,3-dihydroxyacetone (DHA) (61/36, mol/mol), DHA, glycerol (Gly), propylene glycol (PG), 1 and ethylene glycol (EG) at 800 ◦C (residence time of 0.9 s under N2 flow of 400 mL min− ). * Yield (%, C-based) of the non-condensable product. The data of Gald/DHA, DHA and Gly are from literature [5].
1
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Discussion of the non-condensable gas formation with the composition formulas of polyalcohols provides insight into the gas-forming reactions. As shown in Figure4, the CHO ratio of Gald Energies 2020, 13, x FOR PEER REVIEW 5 of 17 and DHA (C1H2O1) is ideal for the production of equimolar amounts of CO and H2. However, PG (C1H2.7O0.6) has insufficient oxygen for the production of oxygenated (OX) gases (CO and CO2); also produced reasonable amounts of HC gases at 20.3% and 22.8% (C-based), respectively, although therefore, PG produced larger amounts of HC gases (35.3%, C-based). Gly (C1H2.7O1) and EG (C1H3O1) thesealso polyalcohols produced reasonable have sufficient amounts oxygen of HC gasesto produce at 20.3% CO. and Because 22.8% (C-based), the yields respectively, of CO2 were although low from Gly and EG, some of the oxygen atoms must be lost as water from these polyalcohols. Consequently, these polyalcohols have sufficient oxygen to produce CO. Because the yields of CO2 were low from gasGly composition and EG, some is ofnot the merely oxygen determined atoms must by be lostthe aselemental water from composition, these polyalcohols. indicating Consequently, that the gas forminggas composition reactions is are not merelynot equilibrated determined but by thecontroll elementaled kinetically composition, under indicating the present that the gasexperimental forming conditions.reactions areDehydration not equilibrated may comp but controlledete with kineticallythe fragmentation under the reac presenttions experimentalto form non-condensable conditions. gases.Dehydration may compete with the fragmentation reactions to form non-condensable gases.
Molecular Composition formula formula
Glycolaldehyde (GA) C2H4O2 C1H2O1
Glyceraldehyde (Gald) C3H6O3 C1H2O1
Suitable for CO + H2 Glycerol (Gly) C3H8O3 C1H2.7O1
Etylene glycol (EG) C2H6O2 C1H3O1
Propylene glycol (PG C3H8O2 C1H2.7O0.67 Oxygen deficient for CO + H 2 Figure 4. Production of syngas (CO + H ) from glyceraldehyde, 1,3-dihydroxyacetone, glycerol, Figure 4. Production of syngas (CO + H22) from glyceraldehyde, 1,3-dihydroxyacetone, glycerol, ethyleneethylene glycol, glycol, and and propylene propylene glycol glycol as as discussed withwith thethe composition composition formula. formula. Occurrence of dehydration reaction converts polyalcohols into enols, which are further rearranged Occurrence of dehydration reaction converts polyalcohols into enols, which are further into carbonyl compounds (Figure5A). As summarized in Figure6, carbonyl compounds, particularly rearranged into carbonyl compounds (Figure 5A). As summarized in Figure 6, carbonyl compounds, aldehydes, are the major components of condensable products. These results support the occurrence particularlyof dehydration aldehydes, reactions are duringthe major polyalcohol components gasification, of condensable although products. the chemical These compositionsresults support are the occurrencedifferent depending of dehydration on the polyalcohol reactions type. during Gald-DHA polyalcohol (61/36, molgasification,/mol) and DHA although gave formaldehyde the chemical compositions(FA), glycolaldehyde are different (GA), depending glyoxal (GO), on the and polyalcohol methylglyoxal type. (MeGO) Gald-DHA as the (61/36, major mol/mol) components. and GlyDHA gaveand formaldehyde PG produced larger(FA), amountsglycolaldehyde of C3 products, (GA), glyoxal which include (GO), hydroxyacetoneand methylglyoxal (HA) (MeGO) and acrolein as the major(ACR) components. from Gly, and propionaldehydeGly and PG produced (Pald) and allyllarger alcohol amounts (AllyOH) of fromC3 products, PG. EG, a Cwhich2 polyalcohol, include hydroxyacetoneformed acetaldehyde (HA) and (AA) acrolein and FA selectively.(ACR) from Gly, and propionaldehyde (Pald) and allyl alcohol (AllyOH)Aldehydes from PG. are EG, known a C2 polyalcohol, to fragment via formed acyl radicals acetaldehyde (Figure 5(AA)B) [ 16 and]. Aldehydic FA selectively. hydrogens have been suggested as the predominant sites for hydrogen abstraction by OH, from experimental and • theoretical investigations [17–19]. Pre-reactive complex formation between aldehyde and OH has • been proposed for this selective abstraction [19]. Although the reactivities of alkyl radicals such as CH remain unclear, the resulting acyl radical ( CR(=O)) is fragmented into CO and R through the • 3 • • α-scission mechanism. Therefore, the final gas compositions vary depending on the chemical structure of R, which is stabilized as R-H by abstraction of hydrogen or degraded further (Figure5C). • When R is H (case of FA), H is produced through hydrogen abstraction reaction. As in the • • 2 case of GO, the resulting R( CH=O) fragments into CO and H through similar β-scission reaction. • • • When R is hydroxymethyl or hydroxymethyne group (case of GA and Gald), the resulting C(R”)–OH • would be converted into CO and H2 via the aldehyde and enol intermediates. Accordingly, these intermediate aldehydes selectively produce syngas (CO and H2). Selective syngas formation from Gald and DHA is explainable with these reactions, since DHA is suggested to be rearranged into Gald in the gas phase [5]. On the other hand, the acyl radicals with alkyl groups can form HC gases via the stabilization of the alkyl radical intermediates. Methyl radical ( CH ) formed from AA would produce methane, • 3
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and ethyl ( CH –CH ) and vinyl ( CH =CH ) radicals formed from Pald and ACR, respectively, can • 3 3 • 2 3 Energiesbe 2020 converted, 13, x FOR into PEER ethane REVIEW and unsaturated hydrocarbons such as ethylene and acetylene. 6 of 17
(A) Aldehyde/ketone formation through dehydration of polyalcohol
HO OH HO Keto-enol OH Dehydration tautomerization -H O H 2
Enol Aldehyde/kenone
(B) Fragmentation of aldehyde
R' H H-Abstraction α-Scission R R R
O - R' H O CO
(C) Further conversion of R
to syngas to hydrocarbon gas
R = H (FA) H2 R = CH3 (AA) CH4
H C2H6 R = C (GO) CO + H2 R = H2CCH3 (Pald) H CCH O 2 2
H H H2CCH2 R = C (GA) C CO + H2 R = HC CH2 (ACR) H HC CH OH O H FigureFigure 5. Gas-forming 5. Gas-forming reactions reactions considered considered for for polyalcohols, including including fragmentation fragmentation of aldehyde of aldehyde intermediatesintermediates into intosyngas syngas and and hydrocarbon hydrocarbon gases. gases.
C3 C2 49.7* 44.1* 24.7* 21.8* 15.7* Propionaldehyde Acetic acid 100 (Pald) C3 Allyl alcohol Acetaldehyde (AllyOH) (AA) 80 Acetone Glyoxal (ACE) (GO) C2 Methylglyoxal Glycolaldehyde 60 (MeGO) (GA) Hydroxyacetone Ketene (HA) 40 Acrolein (ACR)
C1 20 C1 Methanol Formic acid 0 Formaldehyde (FA) Composition / %, C-basedproducts on condensable/ %, Composition Gald/DHA DHA Gly PG EG (61/36) FigureFigure 6. Composition 6. Composition (%, (%,C-based) C-based) of of condensable condensable productsproducts fromfrom glyceraldehyde glyceraldehyde (Gald) (Gald)/1,3-/1,3- dihydroxyacetonedihydroxyacetone (DHA) (DHA) (61/36, (61/36, mol/mol), mol/mol), DHA, DHA, glycerolglycerol (Gly), (Gly), propylene propylene glycol glycol (PG), (PG), and ethyleneand ethylene 1 glycol (EG) at 600 ◦C (residence time of 0.9 s under N2 flow of 400 mL min− ). * Yield (%, C-based) of glycol (EG) at 600 °C (residence time of 0.9 s under N2 flow of 400 mL min−1). * Yield (%, C-based) of the condensable product. The data of Gald/DHA, DHA and Gly are from literature [5]. the condensable product. The data of Gald/DHA, DHA and Gly are from literature [5].
Aldehydes are known to fragment via acyl radicals (Figure 5B) [16]. Aldehydic hydrogens have been suggested as the predominant sites for hydrogen abstraction by •OH, from experimental and theoretical investigations [17–19]. Pre-reactive complex formation between aldehyde and •OH has been proposed for this selective abstraction [19]. Although the reactivities of alkyl radicals such as •CH3 remain unclear, the resulting acyl radical (•CR(=O)) is fragmented into CO and •R through the α-scission mechanism. Therefore, the final gas compositions vary depending on the chemical structure of •R, which is stabilized as R-H by abstraction of hydrogen or degraded further (Figure 5C).
1
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When •R is •H (case of FA), H2 is produced through hydrogen abstraction reaction. As in the case of GO, the resulting •R (•CH=O) fragments into CO and •H through similar -scission reaction. When R is hydroxymethyl or hydroxymethyne group (case of GA and Gald), the resulting •C(R’’)– OH would be converted into CO and H2 via the aldehyde and enol intermediates. Accordingly, these intermediate aldehydes selectively produce syngas (CO and H2). Selective syngas formation from Gald and DHA is explainable with these reactions, since DHA is suggested to be rearranged into Gald in the gas phase [5]. On the other hand, the acyl radicals with alkyl groups can form HC gases via the stabilization of the alkyl radical intermediates. Methyl radical (•CH3) formed from AA would produce methane, and ethyl (•CH3–CH3) and vinyl (•CH2=CH3) radicals formed from Pald and ACR, respectively, can be converted into ethane and unsaturated hydrocarbons such as ethylene and acetylene. Alkyl groups attached on ketonic (>C=O) and carboxylic (-COOH) groups are also reported to produce HC gases [4]. Based on such information, acetone (ACE), MeGO, HA, acetic acid, AA, methanol, Pald, ACR, AllyOH and ketene are picked up as the intermediates bearing alkyl groups, and the yields of these compounds from polyalcohols (%, C-based on the condensable products) are compared in Figure 7. The contributions of these intermediates are small for Gald-DHA (61/36) (16.0%) and DHA (8.7%), but greater for Gly (62.4%), PG (88.1%), and EG (34.9%). It is noted that these yields are proportionally correlated to the contents of the HC gases in Figure 3, supporting the hypothesis; the intermediates bearing alkyl groups produce hydrocarbon gases. Many of such intermediates can be produced by dehydration reactions as discussed below. Comparatively large amount of methane formation from EG is explainable with the higher yield Energies 2020, 13, 3726 7 of 18 of AA, and the greater yields of ethylene from PG and Gly are coincide with the greater yields of Pald and ACR, respectively. Although the HC gas yields are quite small from Gald and DHA, high Alkyl groups attached on ketonic (>C=O) and carboxylic (-COOH) groups are also reported contributionsto produce of methane HC gases in [4 ].HC Based gas onare such expl information,ainable with acetone the AA (ACE), and MeGO, MeGO HA, formation. acetic acid, AA, Galdmethanol, is reactive Pald, ACR,for AllyOHgasification and ketene and aredirectly picked upconverted as the intermediates into CO bearing and H alkyl2 with groups, a smaller contributionand the of yields dehydration of these compounds reaction fromto form polyalcohols MeGO (%, leading C-based to on thethe condensable methane products)production. are Ethane would becompared produced in Figure through7. The coupling contributions of oftwo these •CH intermediates3. are small for Gald-DHA (61 /36) (16.0%) Therefore,and DHA the (8.7%), progression but greater forof Glythe (62.4%), dehydration PG (88.1%), reactions, and EG (34.9%). which It lowers is noted thatthe theseO/C yieldsratios of the are proportionally correlated to the contents of the HC gases in Figure3, supporting the hypothesis; intermediatesthe intermediates from polyalcohols, bearing alkyl is groups suggested produce to hydrocarbon be a key process gases. Many for the of such formation intermediates of hydrocarbon can gases, evenbe produced if the elemental by dehydration compositions reactions as of discussed polyalcohols below. are suitable for the production of syngas.
100
80
60
40
20 %,C-based on condensable products Content of the products with alkyl groups / groups with alkyl products the of Content 0 Gald/DHA DHA Gly PG EG (61/36) Figure 7.Figure Contents 7. Contents(%, C-based) (%, C-based) of the products of the products with saturated with saturated and andunsaturated unsaturated alkyl alkyl groups in groups in condensable products obtained from glyceraldehyde (Gald)/1,3-dihydroxyacetone (DHA) condensable products obtained from glyceraldehyde (Gald)/1,3-dihydroxyacetone (DHA) (61/36, (61/36, mol/mol), DHA, glycerol (Gly), propylene glycol (PG), and ethylene glycol (EG) at 600 ◦C. mol/mol),The DHA, data of glycerol Gald/DHA, (Gly), DHA propylene and Gly are fromglycol literature (PG), [and5]. ethylene glycol (EG) at 600 °C. The data of Gald/DHA, DHA and Gly are from literature [5]. Comparatively large amount of methane formation from EG is explainable with the higher yield 3.2. Theoreticalof AA, andStudy the of greater Intermediate yields of Formation ethylene from PG and Gly are coincide with the greater yields of Pald and ACR, respectively. Although the HC gas yields are quite small from Gald and DHA, high Dehydrationcontributions of of the methane alcohols in HC could gas are proceed explainable in withthe thegas AA phase and MeGO through formation. heterolytic dehydration, homolysis ofGald the is reactiveC–OH forbond, gasification and andβ-scission directly converted of C-centered into CO andradicals H2 with as a smaller illustrated contribution in Figure 8. of dehydration reaction to form MeGO leading to the methane production. Ethane would be produced However, very high bond dissociation energies of the C–OH bonds in polyalcohols (93.5–95.6 kcal through coupling of two CH3. −1 • mol for GlyTherefore, [5]) indicate the progression that the homolysis of the dehydration pathway reactions, is improbable which lowers even the at O600/C ratios°C, where of the Gly, PG, and EG intermediatesdegraded. Therefore, from polyalcohols, heterolysis is suggested and toradical be a key pathways process for were the formation considered. of hydrocarbon Along with the gases, even if the elemental compositions of polyalcohols are suitable for the production of syngas.
3.2. Theoretical Study of Intermediate Formation Dehydration of the alcohols could proceed in the gas phase through heterolytic dehydration, homolysis of the C–OH bond, and β-scission of C-centered radicals as illustrated in Figure8. However, 1 very high bond dissociation energies of the C–OH bonds in polyalcohols (93.5–95.6 kcal mol− for Gly [5]) indicate that the homolysis pathway is improbable even at 600 ◦C, where Gly, PG, and EG degraded. Therefore, heterolysis and radical pathways were considered. Along with the direct dehydration, cyclic Grob fragmentation and pinacol rearrangement were considered for the heterolysis reactions that release water [20]. For the radical pathways, the reactions from the C- and O-centered radicals were considered, which are formed through hydrogen abstraction from the -C–H and -O–H moieties of polyalcohols, respectively [21]. Various radical species such as OH and CH existing in the pyrolysis environment • • 3 can abstract these hydrogens [22]. Energies 2020, 13, x FOR PEER REVIEW 8 of 17 direct dehydration, cyclic Grob fragmentation and pinacol rearrangement were considered for the Energies 2020, 13, 3726 8 of 18 heterolysis reactions that release water [20].
HO H
Heterolysis
HO H H HO Homolysis Radical chain
FigureFigure 8. Three 8. Three possible possible dehydration dehydration pathwayspathways for for polyalcohols. polyalcohols.
For theIn radical this section, pathways, heterolytic the dehydration reactions reactionsfrom the were C- theoreticallyand O-centered evaluated radicals and discussed were considered, with the pyrolysis products obtained at 600 C. Then, the products that could not be explained with the which are formed through hydrogen abstraction◦ from the -C–H and -O–H moieties of polyalcohols, heterolytic reactions are discussed with the radical pathways that are reasonably drawn from the C- respectivelyand O-centered [21]. Various radicals. radical species such as •OH and •CH3 existing in the pyrolysis environmentBimolecular can abstract reactions these are hydrogens also considered, [22]. since bimolecular keto–enol tautomerization have been Insuggested this section, for DHA heterolyticenol dehydrationGald in our previous reactions paper were [5]. Intheoretically this paper, pyrolytic evaluated degradation and discussed of ↔ ↔ with theGald pyrolysis into MeGO products+ AA and obtained GA + FA at at 600 400 °C.◦C wereThen, explained the products with the that unimolecular could not reactions,be explained that with 1 the heterolyticis, dehydration reactions via the are six-membered discussed with cyclic the TS radi of enolcal intermediatepathways that of Gald are (reasonablyEa 35.5 kcal moldrawn− ) and from the 1 C- andretro-aldol O-centered fragmentation radicals. of Gald (Ea 40.9 kcal mol− ), respectively. DHA was suggested to degrade via the enol and Gald. However, the values of Ea of the enolization steps of Gald and DHA were calculated Bimolecular reactions are also considered,1 since bimolecular keto–enol tautomerization have to be too high (Ea 72.5 and 69.3 kcal mol− , respectively) to explain why these steps proceed at such been suggested for DHA ↔ enol ↔ Gald in our previous paper [5]. In this paper, pyrolytic a low temperature of 400 ◦C. In explaining the contradiction between experimental and theoretical degradationcalculation of Gald results, into bimolecular MeGO + AA mechanisms and GA have + FA been at 400 proposed °C were for explained decreasing thewith values the unimolecular of Ea to 1 reactions,46.4 that and 48.7is, dehydration kcal mol− , respectively. via the six-membered cyclic TS of enol intermediate of Gald (Ea 35.5 kcal mol−1)Nevertheless, and retro-aldol bimolecular fragmentation mechanism of should Gald be(Ea treated 40.9 kcal carefully, mol− because1), respectively. the meeting DHA of was two molecules are necessary in the gas phase, which is not effective due to the increasing entropy. suggested to degrade via the enol and Gald. However, the values of Ea of the enolization steps of Accordingly, to conclude the bimolecular mechanisms, more systematic studies should be necessary. Gald and DHA were calculated to be too high (Ea 72.5 and 69.3 kcal mol−1, respectively) to explain However, we consider that the bimolecular reactions would be possible under some special conditions why these steps proceed at such a low temperature of 400 °C. In explaining the contradiction between such as in cluster and a product–water complex formed just after the dehydration reaction even in the experimentalgas phase. and theoretical calculation results, bimolecular mechanisms have been proposed for decreasing Somethe values molecules of E sucha to 46.4 as carboxylic and 48.7 acidskcal mol and− alcohols,1, respectively. both of which have the ability of the Nevertheless,hydrogen-bonded bimolecular complex formation, mechanism are knownshould to be exist treated as a cluster carefully, in the gas because phase. the For example,meeting it of two moleculesis suggested are necessary that methanol in the exists gas as phase, dimers, trimers,which andis not oligomers effective in addition due to to the the monomerincreasing in theentropy. Accordingly,gas phase to [conclude23–29]. the bimolecular mechanisms, more systematic studies should be necessary. However,3.2.1. we Ethylene consider Glycol that (EG) the bimolecular reactions would be possible under some special conditions such as in cluster and a product–water complex formed just after the dehydration reaction EG produced AA and FA as the major condensable products. As shown in Figure9, both even in the gas phase. 1 1 direct dehydration (E-i, Ea 74.1 kcal mol− ) and pinacol rearrangement (E-iii, Ea 73.0 kcal mol− ) Somepathways molecules can explain such the as formation carboxylic of AA acids from and EG, althoughalcohols, the both energy of barriers which are have not small.the ability The Ea of the 1 1 hydrogen-bonded(74.1 kcal mol complex− ) of the formation, unimolecular are dehydration known to decreasedexist as a tocluster 64.0 kcal in the mol gas− by phase. assuming For theexample, it is suggestedbimolecular that mechanism methanol involving exists as two dimers, EG molecules. trimers, and oligomers in addition to the monomer in the gas phaseTounderstand [23–29]. the natures of the bimolecular mechanisms in direct dehydration (E-i), the ∆G‡, ∆H‡, and T∆S‡ values were calculated for EG (unimolecular), EG-EG, and EG-H2O at 25, 400, and 600 ◦C 3.2.1. Ethylene(Table2), and Glycol compared (EG) with those calculated for the keto–enol tautomerization of DHA (unimolecular), DHA-DHA, and DHA-H2O (Table3). The ∆G‡ of the enolization reaction from DHA was reduced 1 1 EGfrom produced 66.4 kcal AA mol and− (unimolecular) FA as the major to 45.4 condensable (DHA-DHA) products. and 52.3 (DHA-H As shown2O) kcalin Figure mol− at9, 25both◦C direct 1 dehydrationdue to the(E-i, significant Ea 74.1 kcal decrease mol− in1) theand∆ Hpinacol‡ values rearrangement (65.5–65.7 to 42.3–46.6 (E-iii, kcalEa 73.0 mol −kcal). The mol low−1) ∆pathwaysG‡ values (400 C) of 48.4 and 52.3 kcal mol 1 for DHA-DHA and DHA-H O, respectively, explain the can explain the formation◦ of AA from EG,− although the energy barriers2 are not small. The Ea (74.1 conversion from DHA to enol occurring at 400 C. kcal mol−1) of the unimolecular dehydration decreased◦ to 64.0 kcal mol−1 by assuming the bimolecular mechanism involving two EG molecules.
Energies 2020, 13, x FOR PEER REVIEW 9 of 17
Direct dehydration H E-i E-ii OH HO 74.1 (uni) HO O EG 64.0 (bi) AA
Pinacol rearrangement
HO H E-iii O O 73.0 AA Energies 2020, 13, 3726 H 9 of 18 Energies 2020, Figure13, x FOR 9. Possible PEER REVIEWheterolytic dehydration pathways for ethylene glycol (EG). The values next to the 9 of 17 arrows represent the activation energies (kcal mol−1), as calculated at the MP4(SDQ)//DFT(M06-2X) Direct dehydration level. AA, acetaldehyde; uni, unimolecular mechanism; and bi, bimolecular mechanism (EG-EG). H E-i E-ii To understand the natures of the OHbimolecular mechanisms in direct dehydration (E-i), the ∆G‡, HO 74.1 (uni) HO ∆H‡, and T∆S‡ values were calculated for EG (unimolecular), EG-EG, and EG-HO 2O at 25, 400, and 600 EG 64.0 (bi) °C (Table 2), and compared with those calculated for the keto–enol tautomerizationAA of DHA (unimolecular), DHA-DHA, and DHA-H2O (Table 3). The ∆G‡ of the enolization reaction from DHA was reduced from 66.4Pinacol kcal mol rearrangement−1 (unimolecular) to 45.4 (DHA-DHA) and 52.3 (DHA-H2O) kcal mol−1 ‡ −1 at 25 °C due to the significant decreaseHO H in the ∆H values (65.5–65.7 to 42.3–46.6 kcal mol ). The low E-iii ∆G‡ values (400 °C) of 48.4 and 52.3 kcal mol−1 for DHA-DHAO and DHA-H2O, respectively, explain O the conversion from DHA to enol occurring at73.0 400 °C. The influences of the bimolecular mechanisms on theAA dehydration reaction were much lower, H because of the smaller reduction of the ∆H‡ values (71.6–72.2 to 59.3–65.3 kcal mol −1); hence, the ∆G‡ FigurevaluesFigure 9. atPossible 400 9. Possible and heterolytic 600 heterolytic °C were dehy dehydration indration the range pathways pathways of 63.1–69.1 forfor ethylene ethylene kcal glycolmol glyc−1, (EG).oleven (EG). The with valuesThe the values nextbimolecular to next the to the 1 arrowsmechanisms.arrows represent represent This the result theactivation activationis concordant energies energies with (kcal (kcal the mol lowe−−1),r pyrolytic asas calculated calculated reactivity at theat the MP4(SDQ) of MP4(SDQ)//DFT(M06-2X)EG than//DFT(M06-2X) those of Gald level.and AA, level.DHA. acetaldehyde; AA, acetaldehyde; uni, uni, unimolecular unimolecular mech mechanism;anism; and and bi, bi, bimolecular bimolecular mechanism mechanism (EG-EG). (EG-EG). Table 2. The ∆G , ∆H , and T∆S values (kcal mol 1) calculated for the dehydration of ethylene Table 2. The ∆G‡,‡ ∆H‡, and‡ T∆S‡ values‡ (kcal mol−1) calculated− for the dehydration of ethylene glycol glycol (EG) by assuming the unimolecular (uni) and bimolecular (bi, EG-EG, and EG-H20) mechanisms ‡ To understand(EG) by assuming the natures the unimolecular of the bimolecular(uni) and bimolecular mechanisms (bi, EG-EG, in and direct EG-H 2dehydration0) mechanisms at (E-i), the ∆G , at 25, 400, and 600 C from the minimum energies of the transition states, as calculated at the ∆H‡, and T∆25,S ‡ 400,values and were600 °C ◦calculated from the minimum for EG energies(unimolecular), of the transition EG-EG, states, and as EG-Hcalculated2O atat 25,the 400, and 600 MP4(SDQ)//DFT(M06-2X) level. °C (Table MP4(SDQ)//DFT(M06-2X)2), and compared level.with those calculated for the keto–enol tautomerization of DHA 1 (unimolecular), DHA-DHA, and DHA-Hkcal2O kcalmol (Table-1 mol− 3). The ∆G‡ of the enolizationH reaction from DHA
−1 ΔG‡ ΔH‡ TΔS‡ −1 was reduced from 66.4 kcal mol (unimolecular) ∆G‡ ∆H‡ to 45.4 T ∆(DHA-DHA)S‡ and 52.3 OH(DHA-H2O) kcal mol HO EG (uni) 25°C 71.3 71.6 ‡ 0.3 −1 at 25 °C due to the significant25 ◦ decreaseC 71.3 in the 71.6∆H values 0.3 (65.5–65.7 to 42.3–46.6 kcal mol ). The low 400°C 70.7 72.1 1.4 ‡ EG (uni) 400 ◦C 70.7− 72.11 1.4 ∆G values (400 °C) of 48.4600°C and 52.370.3 kcal mol72.2 for 1.9DHA-DHA and DHA-HEG (uni)2O, respectively, explain 600 ◦C 70.3 72.2 1.9 the conversionEG-EG from DHA25°C to enol61.0 occurring 59.3 at 400 -0.7°C. 25 ◦C 61.0 59.3 0.7 (bi) 400°C 63.1 59.3 -2.8 − The influencesEG-EG (bi)of the bimolecular400 C 63.1 mechanisms 59.3 on 2.8the dehydration reaction were much lower, 600°C ◦ 64.3 59.4 -3.9 − 600 C 64.3‡ 59.4 3.9 −1 ‡ because of the smaller reduction ◦ of the ∆H values (71.6–72.2 − to 59.3–65.3 kcal mol ); hence, the ∆G EG + H2O 25°C 66.5 65.3 -1.2 −1 values at 400 and 600 °C 25were◦C in the 66.5 range 65.3 of 63.1–69.11.2 kcal mol , even with the bimolecular (bi) 400°C 68.2 65.1 -3.1 − EG + H2O (bi) 400 ◦C 68.2 65.1 3.1 mechanisms. This result600°C is concordant 69.1 with65.2 the lowe-3.9 r −pyrolytic reactivityEG-EG (bi) of EG than those of Gald 600 C 69.1 65.2 3.9 EG + H2O (bi) ◦ − and DHA.Energies 2020, 13, x FOR PEER REVIEW 10 of 17 1 Table 3. The ∆G‡, ∆H‡, and T∆S‡ values (kcal mol− ) calculated for the keto–enol tautomerization of ‡ ‡ ‡ −1 TableTable 1,3-dihydroxyacetone2. 3.The The ∆ G∆‡G, ∆, H∆H‡, and, and (DHA) T T∆∆SS‡ by valuesvalues assuming (kcal the molmol unimolecular−)1 )calculated calculated (uni) for for the and the keto–enol bimolecular dehydration tautomerization (bi, of DHA-DHA, ethylene of glycol 1,3-dihydroxyacetone (DHA) by assuming the unimolecular (uni) and bimolecular (bi, DHA-DHA, (EG) byand assuming DHA-H2O) the mechanisms unimolecular at 25, 400,(uni and) and 600 bimolecular◦C from the minimum (bi, EG-EG, energies and of EG-H the transition20) mechanisms states, at and DHA-H2O) mechanisms at 25, 400, and 600 °C from the minimum energies of the transition states, 25, 400,as calculatedand 600 at°C the from MP4(SDQ) the //minimumDFT(M06-2X) energies level. of the transition states, as calculated at the as calculated at the MP4(SDQ)//DFT(M06-2X) level. MP4(SDQ)//DFT(M06-2X) level. 1 kcal mol− kcal mol-1
ΔG∆‡G ‡ kcalΔH ∆mol‡ H‡ -1 TΔST‡∆ S‡ H
DHA (uni) 25°C25 C ΔG 66.4‡ 66.4 65.7ΔH 65.7 ‡ -0.7T Δ0.7S‡ ◦ − OH DHA (uni) 400°C400 C67.2 67.2 65.6 65.6 -1.6 1.6 HDHAO (uni) EG (uni) 25°C ◦ 71.3 71.6 − 0.3 600°C600 ◦ C67.8 67.8 65.5 65.5 -2.3 2.3 400°C 70.7 72.1 −1.4 DHA-DHA 600°C25°C25 ◦C 70.345.4 45.4 43.372.2 43.3 -2.1 1.92.1 EG (uni) (bi) 400°C 48.4 42.6 -5.8− DHA-DHA (bi) 400 ◦C 48.4 42.6 5.8 600°C 50.2 42.4 -7.8− 25°C600 ◦C61.0 50.2 59.3 42.4 -0.77.8 EG-EG − (bi) DHA + H2O 400°C25°C25 ◦C 63.148.9 48.9 46.6 59.3 46.6 -2.3 -2.82.3 DHA +(bi)H 2O 400°C 52.3 45.6 -6.7− 600°C400 ◦C64.3 52.3 59.4 45.6 -3.96.7 (bi) 600°C 54.3 45.4 -8.9− 600 ◦C 54.3 45.4 8.9 DHA-DHA (bi) DHA + H2O (bi) EG + H2O 25°C 66.5 65.3 − -1.2 (bi) 400°C 68.2 65.1 -3.1 The influences of the bimolecular mechanisms on the dehydration reaction were much lower, 600°C 69.1 65.2 -3.9 EG-EG (bi) EG + H O (bi) because of the smaller reduction of the ∆H values (71.6–72.2 to 59.3–65.3 kcal mol 1); hence, the ∆2 G Formation of FA and other minor products,‡ that is, GA, GO, and MeOH, cannot− be explained ‡ 1 values at 400 and 600 ◦C were in the range of 63.1–69.1 kcal mol− , even with the bimolecular with the heterolysis reactions, but with the radical pathways from the C- and O-centered radicals of EG as illustrated in Figure 10. FA and MeOH can form via the β-scission of the O-centered radical of EG (E-e). The formations of GA and GO are possible by the radical pathways E-a, E-d, and E-f and pathway E-g, respectively. Thus, radical pathways are necessary to explain the product formation from EG. AA can also form by pathway E-b.
C-centered radical
O OH
GA a E-a H OOH HO O b E-b AA c H O E-c HO OH E-f GA OH enol (cis) HO O OO E-g
GO O-centered radical O OH E-d OOH GA O e d H2C H E-e O OH FA + CH2 FA CH3OH Figure 10. Possible radical chain pathways from the C- and O-centered radicals of ethylene glycol (EG). The bond with a bold line, which is located at the β-position to the radical, is expected to be cleaved through β-scission reaction. GA, glycolaldehyde; GO, glyoxal; AA, acetaldehyde; and FA, formaldehyde.
The radical intermediates are fragmented mainly through the β-scission reactions, which require the radical p-orbital and the σ-orbital of the C–X bond that is cleaved to lie in the same plane [30].
Energies 2020, 13, x FOR PEER REVIEW 10 of 17
Table 3. The ∆G‡, ∆H‡, and T∆S‡ values (kcal mol−1) calculated for the keto–enol tautomerization of 1,3-dihydroxyacetone (DHA) by assuming the unimolecular (uni) and bimolecular (bi, DHA-DHA, and DHA-H2O) mechanisms at 25, 400, and 600 °C from the minimum energies of the transition states, as calculated at the MP4(SDQ)//DFT(M06-2X) level.
kcal mol-1
ΔG‡ ΔH‡ TΔS‡
DHA (uni) 25°C 66.4 65.7 -0.7 400°C 67.2 65.6 -1.6 DHA (uni) 600°C 67.8 65.5 -2.3
DHA-DHA 25°C 45.4 43.3 -2.1 (bi) 400°C 48.4 42.6 -5.8 600°C 50.2 42.4 -7.8
DHA + H2O 25°C 48.9 46.6 -2.3 Energies 2020(bi), 13 , 3726 400°C 52.3 45.6 -6.7 10 of 18 600°C 54.3 45.4 -8.9 DHA-DHA (bi) DHA + H2O (bi) mechanisms. This result is concordant with the lower pyrolytic reactivity of EG than those of Gald and DHA. Formation of of FA FA and other minor products, that is, GA, GO, and MeOH, cannot be explained with the heterolysis reactions, but with the radical pathways from the C- and O-centered radicals of EG as illustrated in Figure 10.10. FA FA and MeOH can form via the β-scission-scission of of the the O-centered O-centered radical radical of of EG (E-e). The The formations formations of of GA GA and and GO GO are are possib possiblele by by the radical pathways E-a, E-a, E-d, and E-f and pathway E-g,E-g, respectively.respectively. Thus, Thus, radical radical pathways pathways are are necessary necessary to explainto explain the productthe product formation formation from fromEG. AA EG. can AA also canform also form by pathway by pathway E-b. E-b.
C-centered radical
O OH
GA a E-a H OOH HO O b E-b AA c H O E-c HO OH E-f GA OH enol (cis) HO O OO E-g
GO O-centered radical O OH E-d OOH GA O e d H2C H E-e O OH FA + CH2 FA CH3OH
Figure 10. PossiblePossible radical radical chain chain pathways pathways from from the the C- and O-centered radicals of ethylene glycol (EG). The The bond bond with with a a bold bold line, line, which which is is located located at atthe the β-positionβ-position to tothe the radical, radical, is expected is expected to be to cleavedbe cleaved through through β-scissionβ-scission reaction. reaction. GA, GA,glycolaldehyde; glycolaldehyde; GO, GO,glyoxal; glyoxal; AA, AA,acetaldehyde; acetaldehyde; and andFA, formaldehyde.FA, formaldehyde.
The radical radical intermediates intermediates are fragmented mainly through the β-scission-scission reactions, reactions, which which require require thethe radical p-orbitalp-orbital andand the theσ σ-orbital-orbital of of the the C–X C–X bond bond that that is cleavedis cleaved to lieto inlie the in samethe same plane plane [30]. [30]. This pathway leads to the stereoselective formation of cis enol from EG (reaction E-c in Figure 10). Therefore, the influences of these stereoisomers during the keto–enol tautomerization were studied further 1 (Figure 11). The cis enol (Ea 74.6 kcal mol− ) was more stable than the corresponding trans isomer (Ea 1 58.6 kcal mol− ) by assuming the unimolecular mechanisms. In the gas phase, -O–H would associate with the double bond π electron to accomplish the proton-transfer required for tautomerization to the aldehyde isomer. The hydrogen bond formed between the two OH groups in the cis enol make this transition difficult. Consequently, pathway E-f would compete with the hydrogen abstraction pathway E-g reading to the formation of GO. The intramolecular hydrogen bonding lowers the BDE of the -O–H 1 bond in the cis isomer to 70.6 kcal mol− . The trans enol, which is not formed from the radical reactions of EG, may be transformed to GA through the unimolecular keto–enol tautomerization, because of the 1 low Ea of 58.6 kcal mol− . Energies 2020, 13, x FOR PEER REVIEW 11 of 17
This pathway leads to the stereoselective formation of cis enol from EG (reaction E-c in Figure 10). Therefore, the influences of these stereoisomers during the keto–enol tautomerization were studied further (Figure 11). The cis enol (Ea 74.6 kcal mol−1) was more stable than the corresponding trans isomer (Ea 58.6 kcal mol−1) by assuming the unimolecular mechanisms. In the gas phase, -O–H would associate with the double bond π electron to accomplish the proton-transfer required for tautomerization to the aldehyde isomer. The hydrogen bond formed between the two OH groups in the cis enol make this transition difficult. Consequently, pathway E-f would compete with the hydrogen abstraction pathway E-g reading to the formation of GO. The intramolecular hydrogen bonding lowers the BDE of the -O–H bond in the cis isomer to 70.6 kcal mol−1. The trans enol, which is not formed from the radical reactions of EG, may be transformed to GA through the unimolecular keto–enol tautomerization, because of the low Ea of 58.6 kcal mol−1. EnergiesBased2020, 13 on, 3726 the above discussion, both heterolysis and radical chain pathways are suggested11 to of be 18 involved in the gas-phase pyrolytic conversion of EG into gasifying intermediates.
Cis isomer Trans isomer
BDE 70.6 kcal mol-1 H H O OOH
OH -1 -1 Ea 74.6 kcal mol Ea 58.6 kcal mol
Figure 11. Influences of the stereochemistry of cis/trans enol on activation energy (Ea, kcal mol−1) 1of Figure 11. Influences of the stereochemistry of cis/trans enol on activation energy (Ea, kcal mol− ) ofketo–enol keto–enol tautomerization tautomerization by by assuming assuming the the unim unimolecularolecular mechanisms, mechanisms, as as calculated at thethe MP4(SDQ)MP4(SDQ)//DFT(M06-2X)//DFT(M06-2X) level. level. BDE:BDE: bondbond dissociationdissociation energy.energy.
3.2.2.Based Propylene on the Glycol above (PG) discussion, both heterolysis and radical chain pathways are suggested to be involved in the gas-phase pyrolytic conversion of EG into gasifying intermediates. Energies 2020Pald, 13 represents, x FOR PEER 54.8% REVIEW of the condensable products from PG, followed by AllyOH, AA, FA,12 and of 17 3.2.2.other Propyleneminor products Glycol (ACE, (PG) MeGO, and ACR) at 600 °C. Heterolysis and radical pathways, which formationswere considered of other for minor the pyrolysis products of are PG, also are explainedsummarized with in Figures the pathways 12 and 13,P-c, respectively. P-d, P-g, P-j, and P-l for HAPald Forand unimolecular representsP-c for MeGO. 54.8% heterolysis ACR of the may reactions, condensable form througPald products canh formoxidation fromfrom PG,threeof AllyOH. followed pathways, Pald, by P-iv, AllyOH, ACE P-v, and and AA, AllyOH P-vii, FA, wouldandwith other alsothe values minorform ofthrough products Ea of 73.6, P-a, (ACE, 68.3, P-f, and MeGO, and 76.6 P-h,and kcal ACR)respmol−ectively.1, atrespectively. 600 ◦C.Therefore, Heterolysis Pathway radi P-ii andcal ( radicalEpathwaysa, 66.7 pathways,kcal aremol −also1) expectedwhichand pathways were to contribute considered P-i (E toa, for76.1the the degradationkcal pyrolysis mol−1) ofand of PG, PG. P-vi are (71.7 summarized kcal mol in−1) Figures can produce 12 and AllyOH13, respectively. and ACE, respectively. The lower values of Ea of 68.3 (P-v) and 66.7 kcal mol−1 (P-ii) are reasonable to explain whyDirect Pald dehydration and AllyOH are produced as the major products. ThePinacol reactions rearrangement forming ACE as a minor
a −1 productHO (1.7%,H C-based) have the greater values of E of 76.1 (P-i) HOand 71.7H kcal mol (P-vi). P-i P-v BimolecularOH mechanisms lower the values of Ea of direct dehydration reactions P-i, P-ii, andO P- O 76.1 (uni) 68.3 iv. As a result, the Ea of reaction P-iiOH forming AllyOHO as the second greatest product decreased to the 68.2 (bi) −1 H Pald level of 57.4 kcal mol . Furthermore, the isomerACEization from AllyOH to enol, which exhibited H extremely high Ea (93.8 kcal mol−1) via a unimolecular mechanism (reaction P-iii), was reduced to 60.9 H O H O H O kcal mol−1 with the P-iibimolecular mechanism.P-iii The ∆G‡, ∆H‡, and T∆S‡ valuesP-vi calculated for the O OH OH OH unimolecular and bimolecular66.7 (uni) AllyOH isomerizations93.8 (uni) of AllyOH at 25, 400, and 600 71.7°C are summarized in Table 4. The ∆G‡ at 60057.4 (bi)°C decreased from60.9 95.7 (bi) (unimolecular) to H63.2 kcal mol−1 (bimolecular)ACE due to the stabilizationH HO of the TS with a six-membered ring. Consequently, the preferential formation of Pald P-iv O H O and AllyOH fromH PG is explainable with the pathways P-ii + P-iii followedP-vii by the keto–enol OH tautomerization,OH by73.6 assuming (uni) the bimolecular mechanisms. When the product76.6 water formed from 68.0 (bi) H3C Pald reaction P-ii can be used for the bimolecularOH reactionPald O P-iii before leaving the product AllyOH, the biomolecular reaction would proceed smoothly. FigureFigure 12. 12. PossiblePossible heterolytic heterolytic dehydration pathways pathways for for propylene propylene glycol glycol (PG). (PG). The valuesThe values next tonext the to As discussed in the case of EG, the formations of1 AA, FA, and other minor products (HA, MeGO thearrows arrows represent represent the the activation activation energies energies (kcal (kcal mol −mol), as−1), calculated as calculated at the at MP4(SDQ) the MP4(SDQ)//DFT(M06-//DFT(M06-2X) and ACR) cannot be explained with the heterolysis pathways. These products would produce 2X)level. level. Pald, Pald, propionaldehyde; propionaldehyde; AllyOH, AllyOH, allyl allyl alcohol; alcohol; ACE, ACE, acetone; acetone; uni, unimolecularuni, unimolecular mechanism; mechanism; and through radical pathways (Figure 13). AA and FA would form via the pathways P-i and P-k. The andbi, bi, bimolecular bimolecular mechanism. mechanism.
For unimolecular heterolysisC-centered radical reactions, Pald can form from three pathways, P-iv, P-v, and P-vii, OH O with the values of E of 73.6, 68.3, and 76.6 kcal mol 1, respectively. Pathway P-ii (E , 66.7 kcal mol 1) a − Pald a − P-a1 OH OH O 1 and pathways P-i (Ea, 76.1 kcal mol− ) and P-vi (71.7 kcal mol− ) can produce AllyOH and ACE, CH3 + OH O + O OH P-b 1 respectively. The lower valuesb a of Ea of 68.3 (P-v)cis and 66.7GA kcal mol GO− (P-ii) are reasonable to explain O d OH c H O O why Pald and AllyOH are producedH asP-c the majorOH products. The reactions forming ACE as a minor OH + O P-d cis 1 product (1.7%, C-based) have the greater values of Ea of 76.1 (P-i)HA andMeGO 71.7 kcal mol− (P-vi). OH O
O OH HA O
P-e OH e HA H OH H O O g P-f OH f OH ACE O P-g OH OH HA OH P-h OH AllyOH h OH O i OH P-i AA + O OH H2C FA
O-centered radical O OH OH O P-j HA OH OH k O H O j P-k H H + O O H2C FA AA
O
OH O P-l m HA OH l O H P-m OH CH3 + GA Figure 13. Possible radical chain pathways from the C- and O-centered radicals of propylene glycol (PG). The bond with a bold line, which is located at the β-position to the radical, is expected to be cleaved through a β-scission reaction. Pald, propionaldehyde; AllyOH, allyl alcohol; MeGO,
Energies 2020, 13, x FOR PEER REVIEW 12 of 17 formations of other minor products are also explained with the pathways P-c, P-d, P-g, P-j, and P-l for HA and P-c for MeGO. ACR may form through oxidation of AllyOH. Pald, ACE and AllyOH would also form through P-a, P-f, and P-h, respectively. Therefore, radical pathways are also expected to contribute to the degradation of PG.
Direct dehydration Pinacol rearrangement
HO H HO H P-i P-v OH O O 76.1 (uni) 68.3 OH O 68.2 (bi) H Pald ACE H H O H O H O P-ii P-iii P-vi O OH OH OH 66.7 (uni) AllyOH 93.8 (uni) 71.7 57.4 (bi) 60.9 (bi) H ACE
H HO O H O H P-iv P-vii OH OH 73.6 (uni) 76.6 68.0 (bi) H3C Pald OH Pald O Figure 12. Possible heterolytic dehydration pathways for propylene glycol (PG). The values next to the arrows represent the activation energies (kcal mol−1), as calculated at the MP4(SDQ)//DFT(M06- 2X) level. Pald, propionaldehyde; AllyOH, allyl alcohol; ACE, acetone; uni, unimolecular mechanism; and bi, bimolecularEnergies mechanism.2020, 13, 3726 12 of 18
C-centered radical OH O
Pald P-a OH OH O
CH3 + OH O + O OH P-b b a cis GA GO O d OH c H O O H P-c OH OH + O P-d cis HA MeGO OH O
O OH HA O
P-e OH e HA H OH H O O g P-f OH f OH ACE O P-g OH OH HA OH P-h OH AllyOH h OH O i OH P-i AA + O OH H2C FA
O-centered radical O OH OH O P-j HA OH OH k O H O j P-k H H + O O H2C FA AA
O
OH O P-l m HA OH l O H P-m OH CH3 + GA Figure 13. Possible radical chain pathways from the C- and O-centered radicals of propylene glycol Figure 13. Possible(PG). radical The bond chain with a bold pathways line, which is locatedfrom at the the β -positionC- and to theO-centered radical, is expected radicals to be cleaved of propylene glycol (PG). The bond withthrough a bold a β-scission line, reaction. which Pald, is propionaldehyde; located at AllyOH,the β allyl-position alcohol; MeGO, to the methylglyoxal; radical, is expected to be HA, hydroxyacetone; ACE, acetone; GA, glycolaldehyde; GO, glyoxal; AA, acetaldehyde; and cleaved through aFA, β formaldehyde.-scission reaction. Pald, propionaldehyde; AllyOH, allyl alcohol; MeGO,
Bimolecular mechanisms lower the values of E of direct dehydration reactions P-i, P-ii, and P-iv. a As a result, the Ea of reaction P-ii forming AllyOH as the second greatest product decreased to the level Energies 2020, 13, 3726 13 of 18
1 of 57.4 kcal mol− . Furthermore, the isomerization from AllyOH to enol, which exhibited extremely 1 1 high Ea (93.8 kcal mol− ) via a unimolecular mechanism (reaction P-iii), was reduced to 60.9 kcal mol− with the bimolecular mechanism. The ∆G‡, ∆H‡, and T∆S‡ values calculated for the unimolecular and bimolecular isomerizations of AllyOH at 25, 400, and 600 ◦C are summarized in Table4. The ∆G‡ at 1 600 ◦C decreased from 95.7 (unimolecular) to 63.2 kcal mol− (bimolecular) due to the stabilization Energiesof 2020 the, TS13, x with FOR PEER a six-membered REVIEW ring. Consequently, the preferential formation of Pald and13 of AllyOH 17 from PG is explainable with the pathways P-ii + P-iii followed by the keto–enol tautomerization, by assumingmethylglyoxal; the bimolecularHA, hydroxyacetone; mechanisms. ACE, When acetone; the productGA, glycolaldehyde; water formed GO, from glyoxal; reaction AA, P-ii can be usedacetaldehyde; for the bimolecularand FA, formaldehyde. reaction P-iii before leaving the product AllyOH, the biomolecular reaction would proceed smoothly. Table 4. The ∆G‡, ∆H‡, and T∆S‡ values (kcal mol−1) calculated for isomerization of allyl alcohol 1 (AllyOH)Table to 4. enolThe by∆ Gassuming‡, ∆H‡, and the unimolecularT∆S‡ values (kcal(uni) moland− bimolecular) calculated (bi) for mechanisms isomerization at 25 of and allyl 600 alcohol °C from(AllyOH) the minimum to enol by energies assuming of the the transition unimolecular states, (uni) as andcalculated bimolecular at the (bi) MP4(SDQ)//DFT(M06-2X) mechanisms at 25 and 600 ◦C level.from the minimum energies of the transition states, as calculated at the MP4(SDQ)//DFT(M06-2X) level.
kcal mol1-1 kcal mol− ΔG‡ ΔH‡ TΔS‡ ∆G‡ ∆H‡ T∆S‡ AllyOH (uni) 25°C 94.2 93.5 -0.7 25 C 94.2 93.5 0.7 AllyOH (uni) 600°C◦ 95.7 93.2 − -2.5 600 ◦C 95.7 93.2 2.5 AllyOH (bi) 25°C 61.6 60.6 − -1.0 25 C 61.6 60.6 1.0 AllyOH (uni) AllyOH (bi) AllyOH (bi) 600°C◦ 63.2 61.2 − -2.0 600 C 63.2 61.2 2.0 ◦ − 3.2.3. Glycerol (Gly) As discussed in the case of EG, the formations of AA, FA, and other minor products (HA, MeGO andGly ACR)formed cannot HA, beACR, explained AA, GA, with and the heterolysisFA, along pathways.with other These minor products products would at 600 produce °C. The through postulatedradical heterolysis pathways (Figure pathways 13). AAas summarized and FA would in formFigure via 14 the explain pathways the P-i formation and P-k. Theof the formations major of componentsother minor except products for GA. are The also pathway explained with with the the lowest pathways Ea (66.7 P-c, P-d,kcal P-g,mol P-j,−1) with and P-lunimolecular for HA and P-c mechanismfor MeGO. is the ACR pinacol may rearrangement form through oxidation(G-v) giving of AllyOH.FA and AA Pald, via ACE the retro-aldol and AllyOH fragmentation would also form (G-vi,through Ea 41.7 P-a,kcal P-f,mol and−1). Cyclic P-h, respectively. Grob fragmentation Therefore, (G-i, radical Ea 68.8 pathways kcal mol are−1), also which expected is a characteristic to contribute to reactionthe degradationof Gly, also forms of PG. AA and FA. By assuming the bimolecular reactions, direct dehydration reactions G-ii (66.6 kcal mol−1) and G-iii 3.2.3.(63.4 Glycerolkcal mol− (Gly)1) become more reactive to explain the formation of HA and ACR, respectively, more reasonably as the other major products. Gly formed HA, ACR, AA, GA, and FA, along with other minor products at 600 ◦C. The postulated heterolysisAll of the dehydration pathways as summarizedproducts listed in Figurein Figure 14 explain 14 can the be formationproduced ofthrough the major radical components reactions except illustrated in Figure 15. Many of the radical reactions explain1 the formation of GA, a major product for GA. The pathway with the lowest Ea (66.7 kcal mol− ) with unimolecular mechanism is the pinacol which is not expected to form by heterolysis reactions. Other minor condensable intermediates 1 rearrangement (G-v) giving FA and AA via the retro-aldol fragmentation (G-vi, Ea 41.7 kcal mol− ). (MeGO and GO) can also be produced by the radical 1pathways. Cyclic Grob fragmentation (G-i, Ea 68.8 kcal mol− ), which is a characteristic reaction of Gly, also forms
AA and FA. Cyclic-Grob fragmentation
H HO 1 By assuming the bimolecularO reactions, direct dehydrationO reactions G-ii (66.6 kcal mol ) and G-i − 1 H 68.7 AA G-iii (63.4 kcal mol− ) become more reactive+ to explain the formation of HA and ACR, respectively, O O H2C more reasonably as the other majorH products.O FA Direct dehydration All of the dehydration products listed inOH Figure 14 canHO be produced through radical reactions
illustrated in Figure 15. ManyO ofH the radicalG-ii reactions explain the formation of GA, a major product OH 76.7 (uni) HO OH O HA which is not expected to formH by heterolysis66.6 (bi) reactions. Other minor condensable intermediates (MeGO H O and GO) can also be producedH by the radical pathways. H G-iii G-iv O O 71.5 (uni) 30.9 O O H OH ACR H 63.4 (bi)
Pinacol rearrangement O CH FA H 2 H O O H G-v G-vi O HO + HO O H 66.7 41.7 H O AA H O O H HO O G-vii O 70.7 H H HA
H H O H O H O HO O G-viii O O + 71.4 CH2 FA AA Figure 14. Possible heterolytic dehydration pathways for glycerol (Gly). The values next to the arrows represent the activation energies (kcal mol−1), as calculated at the MP4(SDQ)//DFT(M06-2X) level. ACR, acrolein; HA, hydroxyacetone; AA, acetaldehyde; FA, formaldehyde; uni, unimolecular mechanism; and bi, bimolecular mechanism.
Energies 2020, 13, x FOR PEER REVIEW 13 of 17 methylglyoxal; HA, hydroxyacetone; ACE, acetone; GA, glycolaldehyde; GO, glyoxal; AA, acetaldehyde; and FA, formaldehyde.
Table 4. The ∆G‡, ∆H‡, and T∆S‡ values (kcal mol−1) calculated for isomerization of allyl alcohol (AllyOH) to enol by assuming the unimolecular (uni) and bimolecular (bi) mechanisms at 25 and 600 °C from the minimum energies of the transition states, as calculated at the MP4(SDQ)//DFT(M06-2X) level.
kcal mol-1
ΔG‡ ΔH‡ TΔS‡
AllyOH (uni) 25°C 94.2 93.5 -0.7 600°C 95.7 93.2 -2.5
AllyOH (bi) 25°C 61.6 60.6 -1.0 600°C 63.2 61.2 -2.0 AllyOH (uni) AllyOH (bi)
3.2.3. Glycerol (Gly) Gly formed HA, ACR, AA, GA, and FA, along with other minor products at 600 °C. The postulated heterolysis pathways as summarized in Figure 14 explain the formation of the major components except for GA. The pathway with the lowest Ea (66.7 kcal mol−1) with unimolecular mechanism is the pinacol rearrangement (G-v) giving FA and AA via the retro-aldol fragmentation (G-vi, Ea 41.7 kcal mol−1). Cyclic Grob fragmentation (G-i, Ea 68.8 kcal mol−1), which is a characteristic reaction of Gly, also forms AA and FA. By assuming the bimolecular reactions, direct dehydration reactions G-ii (66.6 kcal mol−1) and G-iii (63.4 kcal mol−1) become more reactive to explain the formation of HA and ACR, respectively, more reasonably as the other major products. All of the dehydration products listed in Figure 14 can be produced through radical reactions illustrated in Figure 15. Many of the radical reactions explain the formation of GA, a major product which is not expected to form by heterolysis reactions. Other minor condensable intermediates (MeGO and GO) canEnergies also2020 be, 13, 3726 produced by the radical pathways. 14 of 18
Cyclic-Grob fragmentation
H HO O O G-i H 68.7 AA + O O H2C H O FA Direct dehydration OH HO
O H G-ii
OH 76.7 (uni) HO OH O HA H 66.6 (bi) H O H H G-iii G-iv O O 71.5 (uni) 30.9 O O H OH ACR H 63.4 (bi)
Pinacol rearrangement O CH FA H 2 H O O H G-v G-vi O HO + HO O H 66.7 41.7 H O AA H O O H HO O G-vii O 70.7 H H HA
H H O H O H O HO O G-viii O O + 71.4 CH2 FA AA Figure 14. Possible heterolytic dehydration pathways for glycerol (Gly). The values next to the arrows Figure 14. Possible heterolytic dehydration pathways1 for glycerol (Gly). The values next to the arrows represent the activation energies (kcal mol− ), as calculated at the MP4(SDQ)//DFT(M06-2X) level. ACR, represent the activationacrolein; energies HA, hydroxyacetone; (kcal AA, mol acetaldehyde;−1), as FA,calculated formaldehyde; at uni, the unimolecular MP4(SDQ)//DFT(M06-2X) mechanism; level. and bi, bimolecular mechanism. ACR, acrolein; HA, hydroxyacetone; AA, acetaldehyde; FA, formaldehyde; uni, unimolecular 3.3. Discussion for Syngas and Hydrocarbon Gas Production from Polyalcohol mechanism; and bi, bimolecular mechanism. Gas-forming pathways suggested from the results of the present investigation are summarized in Figure 16. Several heterolysis pathways are suggested to explain the formation of dehydration products from polyalcohols along with the radical chain pathways, where C–OH bonds are cleaved by the β-scission type reactions occurring for the C-centered radicals. On the other hand, many of the radical chain pathways particularly from the O-centered radicals give aldehydes/ketones that bear the hydroxymethyl and hydroxymethyne groups such as glyceraldehyde and glycolaldehyde, leading to the syngas production. Accordingly, relative efficiency of hydrogen abstractions from C–H and O–H bonds may affect the relative composition of syngas and hydrocarbon gas in addition to the occurrence of heterolytic dehydration reactions. The higher concentration of syngas in the gaseous products from polyalcohol gasification would be due to the greater contribution of the radical chain fragmentations via the O-centered radicals, although further systematic study is needed to conclude it. Energies 2020, 13, x FOR PEEREnergies 2020REVIEW, 13, 3726 15 of 18 14 of 17
C-centered radical
O O H O ACR H OH O G-a O OH + O + O CH OH HO OH 2 G-b cis GA GO b a O d c H HO OH HO O H G-c OH O cis G-d OH HO OH O O + H C O 2 GA FA Gald O O O O + + HO O GO MeGO e G-e H OH HO O DHA OH HO OH HO O f G-f
HA
O-centered radical OH O G-k AA m H OH O HO O G-l HO OH k + G-g O O HO OH cis H l + GA GO g O O OH G-m H C h 2 O H G-h FA GA
OH O HO OH O + H C O 2 GA FA Galde O O O + + O
HO O MeGO GO
G-i OH HO O j DHA OH i OH HO O H G-j + + CH3OH O CH2 GA FA Figure 15. Possible radical chain pathways from the C- and O-centered radicals of glycerol (Gly). Figure 15. Possible radicalThe bond withchain a bold pathways line, which is located from at thetheβ-position C- and to the O-centered radical, is expected radicals to be cleaved of glycerol (Gly). The bond with a bold line,through whichβ-scission is reaction. located ACR, at acrolein; the MeGO,β-position methylglyoxal; to Gald,the glyceraldehyde;radical, is DHA,expected to be cleaved 1,3-dihydroxyacetone; HA, hydroxyacetone; GA, glycolaldehyde; GO, glyoxal; AA, acetaldehyde; through β-scission reaction.FA, formaldehyde. ACR, acrolein; MeGO, methylglyoxal; Gald, glyceraldehyde; DHA, 1,3- dihydroxyacetone; HA, hydroxyacetone; GA, glycolaldehyde; GO, glyoxal; AA, acetaldehyde; FA, formaldehyde.
3.3. Discussion for Syngas and Hydrocarbon Gas Production from Polyalcohol Gas-forming pathways suggested from the results of the present investigation are summarized in Figure 16. Several heterolysis pathways are suggested to explain the formation of dehydration products from polyalcohols along with the radical chain pathways, where C–OH bonds are cleaved by the β-scission type reactions occurring for the C-centered radicals. On the other hand, many of the radical chain pathways particularly from the O-centered radicals give aldehydes/ketones that bear the hydroxymethyl and hydroxymethyne groups such as glyceraldehyde and glycolaldehyde, leading to the syngas production. Accordingly, relative efficiency of hydrogen abstractions from C– H and O–H bonds may affect the relative composition of syngas and hydrocarbon gas in addition to the occurrence of heterolytic dehydration reactions. The higher concentration of syngas in the gaseous products from polyalcohol gasification would be due to the greater contribution of the radical chain fragmentations via the O-centered radicals, although further systematic study is needed to conclude it.
Dehydration O Oxidation OH Heterolysis O (Radical chain) Radical chain
H H C H2 OH OH
CO + H2 Hydrocarbon gas CO + H2 Figure 16. Syngas and hydrocarbon gas formation pathways from polyalcohol.
Energies 2020, 13, x FOR PEER REVIEW 14 of 17
C-centered radical
O O H O ACR H OH O G-a O OH + O + O CH OH HO OH 2 G-b cis GA GO b a O d c H HO OH HO O H G-c OH O cis G-d OH HO OH O O + H C O 2 GA FA Gald O O O O + + HO O GO MeGO e G-e H OH HO O DHA OH HO OH HO O f G-f
HA
O-centered radical OH O G-k AA m H OH O HO O G-l HO OH k + G-g O O HO OH cis H l + GA GO g O O OH G-m H C h 2 O H G-h FA GA
OH O HO OH O + H C O 2 GA FA Galde O O O + + O
HO O MeGO GO
G-i OH HO O j DHA OH i OH HO O H G-j + + CH3OH O CH2 GA FA Figure 15. Possible radical chain pathways from the C- and O-centered radicals of glycerol (Gly). The bond with a bold line, which is located at the β-position to the radical, is expected to be cleaved through β-scission reaction. ACR, acrolein; MeGO, methylglyoxal; Gald, glyceraldehyde; DHA, 1,3- dihydroxyacetone; HA, hydroxyacetone; GA, glycolaldehyde; GO, glyoxal; AA, acetaldehyde; FA, formaldehyde.
3.3. Discussion for Syngas and Hydrocarbon Gas Production from Polyalcohol Gas-forming pathways suggested from the results of the present investigation are summarized in Figure 16. Several heterolysis pathways are suggested to explain the formation of dehydration products from polyalcohols along with the radical chain pathways, where C–OH bonds are cleaved by the β-scission type reactions occurring for the C-centered radicals. On the other hand, many of the radical chain pathways particularly from the O-centered radicals give aldehydes/ketones that bear the hydroxymethyl and hydroxymethyne groups such as glyceraldehyde and glycolaldehyde, leading to the syngas production. Accordingly, relative efficiency of hydrogen abstractions from C– H and O–H bonds may affect the relative composition of syngas and hydrocarbon gas in addition to the occurrence of heterolytic dehydration reactions. The higher concentration of syngas in the gaseous products from polyalcohol gasification would be due to the greater contribution of the radicalEnergies 2020chain, 13 fragmentations, 3726 via the O-centered radicals, although further systematic study is needed16 of 18 to conclude it.
Dehydration O Oxidation OH Heterolysis O (Radical chain) Radical chain
H H C H2 OH OH
CO + H2 Hydrocarbon gas CO + H2 FigureFigure 16. 16. SyngasSyngas and and hydrocarbon hydrocarbon gas gas format formationion pathways pathways from from polyalcohol. polyalcohol.
4. Conclusions Thermal degradation of gas-phase simple polyalcohols (glycerol, propylene glycol, and ethylene glycol) was studied experimentally and theoretically, focusing on the formation and roles of condensable gas-forming intermediates at 400, 600, and 800 ◦C under nitrogen flow (residence times: 0.9–1.4 s), and the results were compared with those of glyceraldehyde. The following conclusions are obtained:
1. All polyalcohols become reactive at 600 ◦C and produce condensable products in 15.7–24.7% yields (C-based), corresponding to 33.9–38.4% based on the amounts of reacted polyalcohols. Because the polyalcohols are completely converted into non-condensable gases at 800 ◦C, the condensable products observed at 600 ◦C act as gas-forming intermediates. 2. Most of the condensable products are aldehydes, which can be formed from by dehydration and the following keto–enol tautomerization. Loss of some oxygen atoms as water is also suggested by the elemental compositions of gas and gas-forming intermediates.
3. Yields of gas-forming intermediates (600 ◦C, mostly aldehydes) bearing alkyl groups are proportionally correlated to the yields of hydrocarbon gases (800 ◦C), suggesting the alkyl groups being converted into hydrocarbon gases. 4. Fragmentation pathways of aldehydes via acyl radicals can explain the formation of characteristic hydrocarbons from polyalcohols (methane from ethylene glycol, ethylene and acetylene from glycerol and propylene glycol).
5. Glyceraldehyde directly fragments into CO and H2 with a smaller contribution of dehydration reaction to form methyl glyoxal leading to the methane production, which explains why glyceraldehyde produces syngas selectively. 6. Theoretical calculations indicate heterolysis mechanisms for polyalcohol dehydration along with the contribution of radical chain mechanisms. 7. By assuming the bimolecular mechanisms for heterolytic dehydration reactions, the formations of gas-forming intermediates are explainable more reasonably. 8. These findings provide insight into upgrading gasification processes and controlling the gas compositions between hydrocarbons and syngas for sustainable utilization of biomass in biofuel and biochemical applications.
Author Contributions: A.F. conducted the gasification experiments and theoretical calculations under the supervision of H.K. Draft was prepared by A.F. and edited by H.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by Grants-in-Aid for Scientific Research (B) (no. 24380095, 2012.4-2016.3) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Acknowledgments: The author would like to express great appreciation for the kind assistance of Takashi Hosoya of Kyoto Prefectural University, Kyoto, with regard to theoretical calculations and the use of associated software. This research was partly conducted using the supercomputer system of the Academic Center for Computing and Media Studies, Kyoto University. Conflicts of Interest: The authors declare no conflict of interest. Energies 2020, 13, 3726 17 of 18
Abbreviations
Gly glycerol Gald glyceraldehyde HC hydrocarbon GA glycolaldehyde MeGO methylglyoxal HA hydroxyacetone BDE bond dissociation energy PG propylene glycol DHA 1,3-dihydroxyacetone FA formaldehyde Pald propionaldehyde ACR acrolein AllyOH allyl alcohol Uni unimolecular EG ethylene glycol OX oxygenated AA acetaldehyde GO glyoxal ACE acetone Ea activation energy bi bimolecular
References
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