Catechol, hydroquinone, and phenol are known to be environmental pollutants due to their ability to generate environmentally free radicals, which cause millions of deaths worldwide. Recently, efforts have been done to precisely identify the origin and the nature of those free radicals employing EPR-LTMI technique. All the three precursors generate cyclopentadienyl radical as major pyrolysis products and phenoxyl radical as both pyrolysis and photolysis products which were obtained from phenol; ortho-semiquinone and para-semiquinone were seen, respectively, from the pyrolysis of catechol and hydroquinone. However, it has been suspected that the solely use of the EPR-LTMI did not allow the isolation of the more labile radicals that is supposedly terminated by radical-radical or radical-surface interaction. The present study reports the gas chromatography mass analysis of the pyrolysis products from catechol, hydroquinone, and phenol. Naphthalene , indene, and hydroxyindene were observed as the pyrolysis products of hydroquinone, while fluorene, 1H-indenol and its isomer 1H-inden-1-one 2,3 dihydro, acenaphthylene, benzofuran-7-methyl, and benzofuran-2-methyl were observed as pyrolysis products of catechol. Dibenzo dioxin and dibenzo furan were observed from pyrolysis of catechol and hydroquinone. Those products result from the combination of radicals such as cyclopentadienyl, para-semiquinone, ortho-semiquinone, hydroxyl-cyclohexadienyl, phenoxyl, and most importantly Hydroxycyclopentadienyl which was not identified by EPR-LTMI.
Combustion-generated Particulate Matter (PM) has been extensively reported in the literature [
Persistent Free Radicals (PFRs) are chemical compounds with one or more unpaired electrons, sufficiently stable towards decomposition, and resistant to further reaction and can exist for long period of time in the atmosphere. These free radicals that potentially include semiquinone-type and phenoxyl-type radicals are highly resonance stabilized and are supposed to form in combustion systems or thermal processes such as burning of cigarette, biomass fuels, fossil fuels, coal, and hazardous materials [
From radical stand point, during these past ten years, extensive efforts have been made in identifying PFRs that can be associated with PM [
For the pyrolysis of catechol, hydroquinone, and phenol, the same experimental procedure is almost followed, except for some minor differences due to their vapor pressures. Catechol (CAT) and hydroquinone (HQ) have very low vapor pressures. Thus they needed to be vaporized prior to their pyrolysis. Experiments were carried out by loading HQ, CAT, or phenol (>99.5% pure from Aldrich) into a Pyrex container vaporizer in a constant temperature oven held at 50–75°C for the vaporization of the sample of CAT and HQ and at 15°C for phenol. To avoid condensation of the vaporized CAT and HQ samples on the transfer line to the cold finger of the Dewar, the transfer line was insulated and held at temperature of 80–90°C employing heating tapes and blankets. All samples were at a total pressure of carrier gas of ~0.1–0.3 Torr for the low pressure pyrolysis of the precursors and 760 Torr for the atmospheric pyrolysis measured by the online pressure gauges. The experimental setup for the low pressure pyrolysis is described elsewhere (my references). The atmospheric conditions pyrolysis is depicted in Figure
Assembly for atmospheric pyrolysis experiments.
The flow meters in the system allow monitoring the time of residence of the vaporized sample in the pyrolysing reactor held at the experimental temperature by a thermoelectric furnace. The radical accumulation starts when the Dewar is filled with liquid nitrogen. A rotary pump is used to pump the pyrolyzed products through the liquid nitrogen cooled Dewar. Frozen radicals onto the cooled finger of the Dewar can be observed by the dropping of the total reagent pressure (from 0.1 Torr to 10−3 Torr) and most of the time by the slight change of cold finger color. The accumulation lasts generally 10 to 12 minutes during which EPR spectra of frozen radicals are acquired every minute. The end of the accumulation of radicals is marked by the nonincreasing of EPR signal intensity of the radicals. At the end of the accumulation, the carrier gas flow was stopped, and the sample vaporizer was closed. More EPR spectra of total radical intensity were then registered in the absence of thermal noise generated by the interaction of the hot gas flow with the cooled finger of the Dewar. This is the time in the experiment where the power dependence of radical is achieved. After acquisition of the radical spectra, the Dewar was removed from the EPR cavity under liquid nitrogen temperature.
The pyrolysis products are thereafter dissolved in approximately 5 cm3 ethanol under hood and dried up to 1 cm3. one
The radical intensity is calculated employing the method of double integration of the first derivative signal and compared with a standard sample of 2,2-di(4-tertoctylphenyl)-1-picrylhydrazyl (DPPH or 1,1-diphenyl-2-picrylhydrazyl (DPPH).
Products were analyzed using an Agilent Technologies 6890N GC system coupled with a 5973 Masse Selective Detector. Products separation was completed employing a 30 m, 0.25 mm i.d., and 0.25
For each sample and for a given pyrolysis temperature, pyrolysis products are accumulated for 10 to 12 minutes and EPR spectra acquired every minute. The radical intensity is thereafter calculated by transferring a given EPR spectrum to SimFonia software (
Beside its natural occurrence [
Its pyrolysis, as it can be seen from Figure
Time dependence of radicals intensity from the pyrolysis of phenol from 750 to 950°C. A linear trend with excellent correlation intensity versus time was observed in the entire temperature domain.
Unlike CAT and HQ, accumulation of the pyrolysis products of phenol shows no saturation which means accumulation is possible beyond 16 minutes with a fine tune of the EPR resonator, while for CAT and HQ saturation is reached after 10 to 12 minutes of their pyrolysis product accumulation. This phenomenon can be due to several reasons. The phenol high vapour pressure in comparison to those of CAT and HQ favours a steady feed of material to the reactor throughout the experiment, while CAT and HQ low vapour pressure leads to a quick saturation. Phenol molecular structure compared to those of CAT and HQ renders its thermal degradation easier than that of the others [
The thermal degradation of HQ scheme shows, on the left hand side, that phenol will decompose to yield phenyl, phenoxy, and CPD. Phenol thermal degradation leads to phenoxy radical by elimination of the hydrogen atom of the hydroxyl group of phenol. Further reaction of phenoxy proceeds by CO elimination to form CPD [
Unlike the pyrolysis of phenol, the time dependence of radical intensity from the pyrolysis of HQ and CAT showed saturation towards the end of the accumulation as depicted by Figure
A comparative total radical intensity yields from 0 to 12 min accumulation time of radicals from the pyrolysis of CAT(blue), HQ ( red), and phenol (yellow). HQ shows the lowest yield. While total radical intensity from phenol keeps an increasingly linear trend, saturation is reached at approximately 10 min of total radical accumulation from CAT and HQ.
As it was seen, the pyrolysis of phenol from 400 to 1000°C shows a linear time dependence of radical signal intensity (Figure
Temperature dependence of total radical yield. CAT values to the scale, and HQ values have been multiplied by 2 and phenol’s value divided by 10. CAT and HQ have the same trends with maximum at 800°C and 850°C while phenol’s values have a sudden increase by 800°C.
The temperature dependence of HQ pyrolysis showed an increase in signal intensity from 300 to 850°C, followed by a decrease above 850°C (Figure
It has been demonstrated that the pyrolysis of CAT and HQ promotes formation of methyl (CH3), ethyl (CH3CH2) [
Gas chromatogram of pyrolysis products of catechol.
The GC-MS analysis of the pyrolysis products of CAT revealed the formation of fluorene (Scheme
From hydroquinone, a computer-generated hydroxycyclopentadienyl (HO-CPD) radical EPR spectrum, a five-line spectrum with intensity distribution 1 : 4 : 6 : 4 : 1 was compared with the EPR spectra from the pyrolysis of HQ in all temperature regions (400–1000°C) [
The formation of indene and hydroxyindene during the pyrolysis of HQ is the evidence of OHCPD gas-phase formation. Therefore OHCPD is formed but was not persistent enough to acquire its EPR spectrum with the various techniques we used in the previous studies [
Fluorene is second to naphthalene in the family of poly-aromatic hydrocarbons (PAHs).
Fluorene detection as the pyrolysis products of CAT is a confirmation that CPD radicals are formed during the pyrolysis/photolysis of CAT as demonstrated in previous studies [ The detection of 1H-indenol gives evidence of the formation of labile radicals such as Hydroxycyclopentadienyl radical while that of fluorene confirms not only the formation of CPD radical but also of aliphatic radicals such as ethenyl and acetylene radicals [
Proposed mechanism of the formation of napthalene diol-1,8 and 1H-inden-7-ol.
Formation of dibenzodioxin (DD) results from the condensation of one molecule of phenol and one phenoxyl radical followed by successive H-radical abstraction and cycling as depicted in Scheme
Scheme
The present study, by combining LTMI-EPR and GC-MS, has shined light on, and thus completing, CAT, HQ, and phenol thermal decomposition understanding. The mass analysis permits to identify molecules such as naphthalene. The experimental identification of those compounds gives the proof of how they are formed. Radical-radical, radical-neutral molecule and even radical-wall interactions are plausible explanation of the molecular gas phase formation. Thus, naphthalene would result from the condensation of two molecules of CPD radical, 1-hydroxynaphthalene from the condensation of one HO-CPD and one CPD, followed by elimination of CO to yield indene (Scheme
Proposed mechanism of the formation of 1-hydroxynaphthalene from the condensation of one molecule of CPD and one molecule of HydroxyCPD followed by the formation of 1H-indene by elimination of CO from the formed hydroxynaphthalene.
Formation of fluorene. It starts with H-radical abstraction from indene followed by its condensation with a cyclopentadienyl radical followed by the cycling with elimination of a second H-radical.
Formation DD from radical-radical condensation reactions [
Formation of DF from radical-radical recombination reactions [
The authors greatly acknowledge the partial support to this research by the National Science Foundation (NSF) Grant CTS-0317094, Philip Morris USA, the British American Tobacco, and Patrick F. Boyd chair held by Dr. Barry Dellinger, the Director of the Interdepartmental Colloquium on Environment. The authors are deeply indebted to Dr. Dellinger and Dr. Lavrent Khachatryan their mentors, both from the Chemistry Department of Louisiana State University (USA) for providing them with a very nice working environment and support.