Pyrolysis of Two- and Three-Carbon Monosaccharides to Understand Hemicellulose and Cellulose Pyrolysis

Phillip R. Westmoreland and Patrick J. Fahey [Presently post-doc at U.S. Naval Academy, Chemistry Dept]

Dept of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh NC

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 Sugars, cellulose, & hemicelluloses have many OHs.

dehydrogenation and dehydration:  Studied extensively in the aqueous phase and over metal-oxide and organic catalysts;  However, less study of the gas-phase and melt-phase reactions.

 Experiments and calculations are used to probe the fates of the OH groups, including their influence on each other:  Pyrolyses of simple with detailed GCxGC-TOFMS analysis.  Observed products correspond to dehydrogenation, dehydration, and cyclic Grob fragmentation.  Mechanistic steps are proposed, building on our prior work on cellulose and glucose pyrolysis.

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 2 Use mono-, di-, tri-ols as biomass model compounds.

 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 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 by Seshadri and Westmoreland; Assary and Curtis, Mayes, Broadbelt, Vinu, et al.; and Auerbach et al.).

V. Seshadri, P. R. Westmoreland, “Concerted reactions and mechanism of glucose pyrolysis and implications for cellulose kinetics,” J. Phys. Chem. A 116:49 (2012) 11997-20013. V. Seshadri, P. R. Westmoreland, “Roles of Hydroxyls in the Noncatalytic and Catalyzed Formation of Levoglucosan from Glucose," Catalysis Today 269 (2016) 110-121. Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 3 Cellulose: Polymer of β-glucose (β-D-glucopyranose).

β-D-glucopyranose unit

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 Hemicellulose is less ordered; here, xyloglucan.

The heptamer block is shown (glucan 4-xylose 3). In blue, backbone β-D-glucans; in red, α-D-xylose; in black, α-D-galactose and in brown, α-L-fucose residues. M. Ochoa-Villarreal et al., “Ch. 4. Plant Cell Wall Polymers” in Polymerization (A. De Souza Gomes ed.), 2012.

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 5 Our earlier finding: Cellulose forms levoglucosan end.

 Our flash pyrolysis with liquid chromatography / MS revealed cello-n-san’s early formation.  Hypothesis: The polymer scission step is breaking the glycosidic bond and forming bicyclic LGA group. Y.-C. Lin, J. Cho, G. Tompsett, P. R. Westmoreland, G. Huber, J. Phys. Chem. C 113:46 (2009) 20097–20107. Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 6 How? Pericyclic breaking of glycosidic C1-O bond.

6

1

Found and unimolecular bimolecular 6 transition 6 transition state 1 state (~235 kJ/mol (~180 kJ/mol 1 or or 56 kcal/mol) 43 kcal/mol)

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 7 Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 8 Pericyclic TSs for the other glucose reactions, too…

Note how many poly-ols are present; How do they decompose? What are the multi-ol effects? Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 9 Pyrolyses with GCxGC-TOFMS give details.

TGA/DSC Flash pyrolyzer Pulse-Injected Deactivated-SS or Quartz flow reactors

TA Instruments, CDS Analytical, SDT Q600 Pyroprobe 5200

GCxGC/TOFMS LECO Corp., Pegasus 4D

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 10 Serial columns and TOF MS resolve close elutions.

 Here, analyzing t-butyl alcohol revealed butan-2-ol as an impurity in the feed.  Pyrolysis at 400°C in the Pulse-Injected Deactivated-SS reactor implies:  t-Butyl alcohol mainly dehydrates to isobutene (2-methylpropene).  Butan-2-ol impurity mainly dehydrogenates to methylethyl ketone but also dehydrates to but-2-ene.

Secondary: StabilWax®-DA with Carbowax-20M polyethylene glycol stationary phase (Restek Corp.)

Primary: 30 m, 0.25 mm ID, Rtx-200 fused-silica capillary column with polytrifluoropropylmethylsiloxane stationary phase (Restek Corp.)

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 11  Mono-alcohols:

propanol 2-propanol (& d8 version)

butan-2-ol t-butyl alcohol 5-HDMF  Diols:

ethan-1,2-diol propan-1,2-diol propan-1,3-diol

 Triol: propan-1,2,3-triol

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 12 All but one of the saturated 1° and 2° mono- alcohols mainly underwent 1,2-dehydrogenation.

 Forms a carbonyl group in place of the alcohol’s C-OH.  1° alcohol aldehyde.  (Methanol formed methanal, ethanol formed ethanol, propanol formed propanal).  2° alcohols ketone with no alteration of alkyl chains.  (Propan-2-ol formed dimethylketone, butan-2-ol formed methyl ethyl ketone).  Not radical chemistry:  Mixture of propan-2-ol and propan-2-ol-d8 was pyrolyzed.  No products of any H-D exchange.  Any other products like created much smaller chromatographic peaks.

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 13 The main exception was neopentyl alcohol.

 Dehydrogenated to 2,2-dimethylpropanal

(CH3)3C-CH=O and dehydrated to 2- methylpropene (CH3)2C=CH2.

 Two other mono-alcohols were not 1° or 2° alkyl alcohols.  Saturated tertiary mono-alcohol 2- methylpropan-2-ol (t-butyl alcohol) only dehydrated, forming 2-methylpropene,

(CH3)2C=CH2.  Phenol did not form any detectable reaction products at 400°C.

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 14 Pericyclic decompositions will dominate at 400°C.

 Methanol provides illustrative examples, depending on [ROH]:

“meta”

“para”

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 15 Compare mono-ols: meta vs. para dehydrogenation.

 Meta molecular catalysis has lowest activation energies (kcal/mol).

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 16 Multiple types of reactions occur for diols (and triol).  Symmetrical ethan-1,2-diol:

 Dehydrogenation product HO-CH2CH(=O)

 But also fragmentation to CH3CH=O and CH2=O.

 Compare to propandiols…

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 17 Multiple types of reactions for diols (2).  Propan-1,2-diol:  Dehydrogenated to single carbonyl at the

internal C-OH position, HOCH2CH(=O)CH3, and to two carbonyls, O=CHCH(=O)CH3.  Dehydrated at internal C-OH position to form

CH3CH2CH=O .

 Fragmentation to CH3CH=O and CH2=O.  Symmetrical propan-1,3-diol:  Some 1,2-dehydration product

CH2=CHCH2OH, but more CH2=CHCH=O .

 CH3CH2CH2OH and CH3CH2CH=O, implying either re-hydrogenation of the carbon-carbon double bond or displacement of the OH.

 Fragmentation to CH3CH=O and CH2=O.

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 18 Multiple types of reactions for triol.  Propan-1,2,3-triol:  Forms products suggesting dehydration, dehydrogenation, and displacement or re-

hydrogenation: CH2=CH-CH=O, CH3CH(=O)CH=O, CH3C(=O)CH2OH .

 Fragmentation to CH3CH=O and CH2=O.

13 13  Paine et al. pyrolyzed C1- and C2-labelled 1,2,3-triol; suggested cyclic 1,3-dehydration (naming it “cyclic Grob fragmentation”) to explain the labelling patterns in methanal and ethanal products.

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 19 Extend with molecular-beam mass spectrometry.

TOF-MS detection

Electron-ionization MBMS system at NCSU

VUV-Photoionization at Lawrence Berkeley Natl Lab Advanced Light Source

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 Thick-walled reactor has a slip-on, formed heater.

section with ¼” thick sheath, 3.3 in.

0.5 1.0 in. in.

total length to be heated, 6 in.

http://www.thermocoax.com 21 Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 22 Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 To conclude:

 Experiments and calculations are used to probe the fates of the OH groups, including their influence on each other.  Detailed speciation is provided by GCxGC-TOFMS and labelling.  The results indicate the importance of pericyclic reactions in pyrolysis of OH groups in sugars and cellulosics.  Continuing work computes rate coefficients for the steps and probes further with molecular-beam mass spectrometry.  Results point to possible routes for radical influences – pertinent for lignin-hemicellulose interactions.

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 23 Grateful acknowledgements:

 Vikram Seshadri, Jordan Keith.  US Department of Energy NABC program through a subaward from RTI International.  San Diego Supercomputing Center’s comet supercomputer under XSEDE grant TG-CHE100121.  N.C. State University’s henry2 supercomputing cluster.

 More details of the work are now available as: P. Westmoreland, P. Fahey, “Dehydration and dehydrogenation kinetics of OH groups in biomass pyrolysis,” Chem. Eng. Transactions 50 (2016) 73-78 (DOI: 10.3303/CET1650013)

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 24 25 Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016  Mono-alcohols:

methanol ethanol propanol 2-propanol (&d8)

butan-2-ol t-butyl alcohol neopentyl alcohol 5-HDMF  Diols:

ethan-1,2-diol propan-1,2-diol propan-1,3-diol

 Triol: propan-1,2,3-triol

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 Cellulose: Polymer of β-glucose (β-D-glucopyranose).

Carbon 1 β-D-glucopyranose unit

1,4-glycosidic β: OH position bond

relative to CH2OH

Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016 Use glucose as a model compound.

 Is that okay? It gives mix of α-glucose LGA/AGF and furan-type LGA AGF species ( ).

 From their data, Sanders et β-glucose

al. (2003) proposed a 5-HDMF network of reactions:  α- and β interconvertible; can form linear D-glucose.

 α- and β could form the D-glucose anhydrosugars levoglucosan and AGF.  D-glucose would mainly form furans and furfurals.

28 Symposium on Thermal and Catalytic Sciences for Biofuels and Biobased Products, November 1-3, 2016