PYROLYSIS OF ACETALDEHYDE: A FLEETING GLIMPSE OF VINYLIDENE

Why do this?

The experiments

The products

The mechanism

PYROLYSIS OF ACETALDEHYDE: A FLEETING GLIMPSE OF VINYLIDENE

Senior Participants:

M. Ahmed, J.R. Barker, J.W. Daily, M.R. Nimlos, D.L. Osborn, J.F. Stanton

Postdocs and Students:

•Golan, L. Nguyen, M. Harding, O. Kostko,C. Miller*, B. McKown*, K. Piech, D. Tabor*, A. Vasilou

and……..

*Undergraduates!

The experiments

The products

The mechanism

Lawrence Berkeley LaboratoryUniversity of MichiganUniversity of ColoradoNational Renewable Energy LabCombustion Res. Facility, SandiaUniversity of Texas

Our team leader

Why Biomass?

Plants grow & pull CO2 from the atmosphere.Burning the biomass (combustion) to CO2 + H2O energy Returns CO2 to the atmosphere: Carbon neutral (on

“short” timescale)

• Bio-refining: “grass, sticks” + H2 + CO (syngas) CH3CH2OH Horrifically inefficient !

• Atmospheric Problem? Forest fires (air) + biomass aerosols

D.J. Nowakowski, A.V. Bridgwater, D.C. Elliott, D. Meier, P. de Wild, Lignin fast pyrolysis: Results from an international collaboration J. Analytical Applied Pyrolysis 88 53–72 (2010)

HC

HC CH

CO

CHO

HC

HCC

C

CHC

CHO

OH

OCH3

O

H

HO

H

HO

H

OH

OHH

CH2

H

OH

OO

OH

H2C

O

HH OH

HOH

H

O

H

HO

H

HO

H

OHH

CH2

OH

α-D-glucopyranose

Hcellobiose

Å

Ñ

Å Ñ

O

OHO OH

CH2OH

O

OCH2OH

HOOH O

OHO OH

CH2OH

O

OCH2OH

O

HOOH

Cellulose

β 1,4' glycosididic linkages in cellulose

Å ÑÅ Ñ

Å Ñ

Cellulose is the most abundant compound on the planet. Degree of polymerization is 104 glucopyranose units in wood cellulose and 1.5 x 104 in native cotton cellulose. A linear, covalent chain.

Carbohydrates (cellulose) important biomass component

1.0

0.8

0.6

0.4

0.2

0.010-7 10-6 10-5 10-4 10-3 10-2 10-1 100

Time (sec)

Cellulose 'Active Cellulose' Char Volatiles Stables Tar

TGA analysis

nascent cellulose active cellulose primary vapors

secondary tars

char + H2O char + H2O

secondary gases

kca

kag

kackcc

kvg kvt

kav

Diebold's Cellulose Decomposition Scheme

J. P. Diebold, "A Unified, Global-Model for the Pyrolysis of Cellulose." Biomass & Bioenergy 7 75-85 (1994)

Phenomenological kinetics/rapid heating to 500º C J W Daily

Let’s start with something simple

Aldehydes are involved , and the simplest organic aldehyde is acetaldehyde:

CH3CHO + ∆ products (monitor by PIMS and IR)

What are the bond energies of CH3CHO ?

CH2

C

H

O

H

84.8 ± 0.2

89.4 ± 0.3

94 ± 2

Lowest Path → CH3• X 2A2" + • CHO X 2A'

m/z 15 29

Middle Path → CH3CO• X 2A' + • H 2S m/z 43

Highest Path → H• 2S + • CH2CHO X 2A" m/z 43

~ ~

~

~

Most (all?) studies have focused on the upper pathway; other decomposition routes unknown/unexplored.

Chen nozzle (aka “tubular reactor”)

SiC tube, walls resistively heated to T up to ca. 1500 K

1. PIMS detection at 10.49 eV

2. Infrared detection in Ar matrix

3. PIMS/VUV studies at LBL synchrotron; photon energies from 9 to 15 eV

3 % impurity

CH3CHO + K+OD— CH2DCHO CHD2CHO CD3CHO

CH2=C=O+

CH3+

“new”

Is there anything else?

Limitations of 10.49 eV PIMS:

1.Get only the mass, although isotopic studies can be useful in refining estimates of the carrier.

2.Isomers create headaches (vinyl alcohol, etc.)

3.Not all compounds ionize at this energy (CO, CO2, CH4, HCCH, cold C2H4 …)

More experiments needed: Infrared

Acetylene!See talk by R.W. Field, MSS, 2009.

Not immediately obvious where this comes from.

3635 3630 3625 3620 3615 3610Wavenumber (cm-1)

ν1 CH2 =CHOH

1% CH3 / 1400 , CDO Ar heated to ºC RED1% CH3 / 1050 , CHO Ar heated to ºC BLUE

1400 , Ar heated to ºC GREENν1 CH2=CDOH

Mechanism(s)?

Limitations of matrix IR:

1.Very hard to discriminate some isotopic species.

2.Tremendous amount of spectral congestion complicates search for new compounds (transients). Theorists sometimes needed to estimate line positions.

More experiments needed: ALS synchrotron

6% impurity in CD3CHO

C

O

C

H

D

H

HC:C

H

HHC CH

HOD

HC CD CD

O

CH

HHH2O

(1,1)(1,2)

**

Supported by additional isotopic studies, calculations with semiclassical transitionstate theory (SCTSTa)

See: W.H. Miller JCP 62, 1899 (1975); T.L. Nguyen, JFS and J.R. Barker JCP A 115, 5118 (2011).

Harmonic system

This is the exact Q.M. result!!!!!!!

JFS and C. Miller, unpublished work.

Vinylidene channel

Acetylene channel

Conclusions

Thermal pyrolysis of acetaldehyde is more complicated than commonly believed.

Ketene, acetylene and water are “new” closed-shell (stable) products.

Isomerization to vinyl alcohol is competitive with, and perhaps faster than, decomposition to methyl and CO.

Isotopic evidence shows clearly that a great deal of vinyl alcoholdehydration involves vinylidene as an intermediate.

There is a role for theory in “biomass science”

The end