PYROLYSIS OF GLYCERIN AT DIFFERENT TEMPERATURES AND HEATING RATES

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Serban C. Moldoveanu, Ching-En Chou

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Background• Glycerin is one of the main components of the liquids (e-liquids) used in

electronic nicotine delivery systems (ENDS). • During the use of ENDS the e-liquid is exposed to increased temperatures

and glycerin may suffer thermal decomposition (or pyrolysis).• Information about glycerin pyrolysis is available in the literature (e.g. R.R.

Baker, L.J. Bishop, “The pyrolysis of tobacco ingredients”, J. Anal. Appl. Pyrol., 71 (2004) 223-311; S.C. Moldoveanu, Pyrolysis of Organic Molecules with Applications to Health and Environmental Issues, Elsevier, Amsterdam, 210, pp. 284-286).

• However, this information covers only a limited temperature range, typically higher than the temperatures attained in ENDS.

• For this reason, a more detailed study on pyrolysis of glycerin at different heating conditions was considered necessary.

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General description of the study• Glycerin (containing about 10% water) was subject to pyrolysis at five different

temperatures in the range 350 oC to 750 oC, at three different heating rates 20 oC/ms, 5 oC/ms and 1 oC/ms.

• In addition, a mixture 50/50 w/w of glycerin and silica gel (Selecto Scientific, GA, USA) 100-200 mesh was subject to pyrolysis at heating rate of 20 oC/ms at the same range of temperatures.

• The dependence on temperature of the generation of compounds such as 1-propene, formaldehyde, acetaldehyde, 2-propenal, acetic anhydride, 2-propen-1-ol, lactic acid, acetol, and glycidol, was evaluated.

• The transfer of un-decomposed glycerin was also measured.• The study was performed on a pyrolyzer Pyroprobe 5250 with autosampler (CDS

Analytical, CDS, Oxford PA, USA), on-line with a GC/MS system 6890 GC, 5973 MS (Agilent, 5301 Santa Clara CA, USA).

• The levels of various decomposition products described in the study DO NOT represent the levels generated in an electronic cigarette. The results show only the relative tendency of level increase of various decomposition compounds as a function of temperature.

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The schematics of the pyrolyzer, and a picture of the instrument

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Conditions utilized for the pyrolysis of glycerin

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Temperature Teq Heating rate β Heating rate β Heating rate β

750 oC 20 oC/ms 5 oC/ms 1 oC/ms

650 oC 20 oC/ms 5 oC/ms 1 oC/ms

550 oC 20 oC/ms 5 oC/ms -

450 oC 20 oC/ms 5 oC/ms -

350 oC 20 oC/ms 5 oC/ms -

Conditions utilized for the pyrolysis of glycerin on silica gelTemperature Teq Heating rate β Heating rate β

750 oC 20 oC/ms 1 oC/ms

650 oC 20 oC/ms 1 oC/ms

550 oC 20 oC/ms -

450 oC 20 oC/ms -

350 oC 20 oC/ms -

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Parameters for the GC/MS on-line analysis

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Parameter Description Parameter Description

Initial oven temperature 37 °C Flow rate 1.1 mL/min

Initial time 4.0 min Nominal initial pressure 17.5 psi

Oven ramp rate 2 °C/mm Split ratio 70:1

Oven final first ramp 60 °C Split flow 76.0 mL/min

Final time first ramp 0 min GC outlet MSD

Oven ramp rate 5 oC/mm Outlet pressure Vacuum

Oven final temperature 280 °C MSD transfer line temp. 280 °C

Final time 20 min Ion source temperature 230 °C

Total run time 75.5 min Quadrupole temp. 150 °C

Inlet temperature 280 °C MSD EM offset 250 V

Inlet mode Split MSD solvent delay 2.0 min

Carrier gas Helium MSD acquisition mode TIC

Flow mode Constant flow Mass range 29-550 a.u.

The separation of the pyrolyzates was performed on a DB-1701 chromatographic column, 60 m long, 0.25 mm i.d. with 1.0 μm film (from Agilent)

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Compounds generated from glycerin upon heating at Teq = 750 oC, β = 20 oC/ms, and their relative mole ratio

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Compound Ret. time Cas# Rel. Mole %

carbon dioxide 4.06 124-38-9 0.571-propene 4.28 115-07-1 0.18formaldehyde 4.48 50-00-0 3.75acetaldehyde 5.52 75-07-0 5.512-propenal (acrolein) 8.21 107-02-8 10.32pyruvaldehyde 9.31 78-98-8 0.112-propen-1-ol 13.88 107-18-6 0.37lactic acid 19.78 50-21-5 0.18acetol 22.26 116-09-6 4.60glycidol 26.66 556-52-5 1.71glycerin 40.10 56-81-5 70.491,4-dioxane-2,6-dimethanol 44.18 54120-69-3 0.28other small peaks - - 1.92 *

* Note: Difference from 100

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Variation of formaldehyde relative mole ratio with temperature

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00.5

11.5

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%

Temperature oC

Formaldehyde β = 20 oC/ms

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Rel

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%Temperature oC

Formaldehyde β = 5 oC/ms

Note: The levels of formaldehyde (and of other decomposition products) DO NOT represent the levels generated in an electronic cigarette. The graphs show only the relative tendency of level increase as a function of temperature.

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Variation of acetaldehyde relative mole ratio with temperature

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Temperature oC

Acetaldehyde β = 20 oC/ms

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Temperature oC

Acetaldehyde β = 5 oC/ms

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Variation of acrolein relative mole ratio with temperature

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Temperature oC

Acrolein β = 20 oC/ms

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Acrolein β = 5 oC/ms

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Variation of acetol relative mole ratio with temperature

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Temperature oC

Acetol β = 20 oC/ms

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Acetol β = 5 oC/ms

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Variation of glycidol relative mole ratio with temperature

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Glycidol β = 20 oC/ms

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Glycidol β = 5 oC/ms

The decrease in glycidol formation at Teq = 750 oC and β = 20 oC/ms may be its decomposition at higher temperature and heating rate.

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Variation of glycerin relative mole ratio with temperature

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Temperature oC

Glycerin β = 20 oC/ms

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Temperature oC

Glycerin β = 5 oC/ms

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Results and discussion for glycerin pyrolysis

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• Several compounds that are formed at 750 oC (and listed at the previous table) are not formed at all at temperatures equal or lower than 550 oC.

• Formaldehyde, acetaldehyde, acetol, and glycidol are formed at lower temperatures than 550 oC although at low or (for acetol and glicydol) at very low levels.

• The change of the heating rate to β = 5 oC/ms does not indicate much differences from the results obtained for β = 20 oC/ms.

• Some decomposition compounds were generated at slightly larger levels for β = 5 oC/ms in particular for Teq = 650 oC and Teq = 750 oC.

• An explanation for this effect can be the slower transfer of glycerin to the GC/MS at β = 5 oC/ms, which led to the apparent increase in the level of other compounds that are much more volatile.

• Pyrolysis at a heating rate β = 1 oC/ms did not provide additional information regarding the influence of heating rate on glycerin decomposition.

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Compounds generated from glycerin on silica gel upon heating at Teq = 750 oC, β = 20 oC/ms, and their relative mole ratio

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Compound Ret. time Cas# Rel. Mole %carbon dioxide 4.20 124-38-9 1.021-propene 4.42 115-07-1 0.65formaldehyde 4.62 50-00-0 2.022-butene 4.93 107-01-7 0.13cyclobutene 5.13 822-35-5 0.55acetaldehyde 5.63 75-07-0 5.211,3-pentadiene 7.05 504-60-9 0.05furan 7.39 110-00-9 0.051,3-pentadiene 7.68 504-60-9 0.081,3-cyclopentadiene 8.20 542-92-7 0.162-propenal (acrolein) 8.28 107-02-8 16.202-propanone 8.68 67-64-1 0.30pyruvaldehyde 9.39 78-98-8 0.232-propen-1-ol 13.59 107-18-6 0.081,3-cyclohexadiene 15.05 592-57-4 0.03benzene 15.79 71-43-2 0.24lactic acid 19.43 50-21-5 0.08acetol 21.86 116-09-6 3.89toluene 22.48 108-88-3 0.13glycidol 25.58 556-52-5 2.24glycerin 40.44 56-81-5 65.741,4-dioxane-2,6-dimethanol 44.18 54120-69-3 0.40other small peaks - - 0.52*

* Note: Difference from 100

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Variation of formaldehyde relative mole ratio with temperature (β = 20 oC/ms)

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0

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Temperature oC

Formaldehyde from glycerin on silica gel

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44.5

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Mol

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Temperature oC

Formaldehyde from glycerin only

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Variation of acetaldehyde relative mole ratio with temperature (β = 20 oC/ms)

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Acetaldehyde from glycerin on silica gel

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Acetaldehyde from glycerin only

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Variation of acrolein relative mole ratio with temperature (β = 20 oC/ms)

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Temperature oC

Acrolein from glycerin on silica gel

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Acrolein from glycerin only

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Variation of acetol relative mole ratio with temperature (β = 20 oC/ms)

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Temperature oC

Acetol from glycerin on silica gel

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Acetol from glycerin only

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Variation of glycidol relative mole ratio with temperature (β = 20 oC/ms)

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Temperature oC

Glycidol from glycerin on silica gel

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Glycidol from glycerin only

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Variation of glycerin relative mole ratio with temperature (β = 20 oC/ms)

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0

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Temperature oC

Glycerin on silica gel

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Glycerin only

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Results and discussion for glycerin on silica gel pyrolysis

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• The presence of silica gel modifies to a certain extent the decomposition of glycerin, by favoring the decomposition process.

• The decomposition is affected mainly at higher temperatures (as listed in the previous table).

• Similar to the pyrolysis of glycerin only, most decomposition compounds are not formed at all at temperatures equal or lower than 550 oC.

• For identical temperatures, the levels of formaldehyde, acetaldehyde, propenal(acrolein), acetol, and glycidol are formed on silica gel at levels only slightly higher than those generated from glycerin only.

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Conclusions

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• Pyrolysis of glycerin was studied for the temperature range between 350 oCand 750 oC.

• The results indicated that most decomposition compounds start forming at temperatures above 500 oC to 550 oC.

• Heating rate β of the sample evaluated at 20 oC/ms, 5 oC/ms and 1 oC/msdoes not affect significantly the result of pyrolysis.

• Formaldehyde, acetaldehyde, acetol, and glycidol were formed at lower temperatures, but their level is low or very low below 500 oC.

• The presence of a solid support with potential catalytic activity like silica gel increases the decomposition of glycerin, similar levels of specific analytesbeing produced at slightly lower temperature when silica is present.

• The effect of silica gel is considerably more pronounced at higher temperatures (650 oC to 750 oC) than it is at lower temperatures.

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