8 research outputs found
Role of the Filters in the Formation and Stabilization of Semiquinone Radicals Collected from Cigarette Smoke
The fractional pyrolysis of Bright
tobacco was performed in a nitrogen
atmosphere over the temperature range 240–510 °C in a
specially constructed, high temperature flow reactor system. Electron
paramagnetic resonance (EPR) spectroscopy was used to analyze the
free radicals in the initially produced total particular matter (TPM)
and in TPM after exposure to ambient air (aging). Different filters
have been used to collect TPM from tobacco smoke: cellulosic, cellulose
nitrate, cellulose acetate, nylon, Teflon, and Cambridge. The collection
of the primary radicals (measured immediately after collection of
TPM on filters) and the formation and stabilization of the secondary
radicals (defined as radicals formed during aging of TPM samples on
the filters) depend significantly on the material of the filter. A
mechanistic explanation about different binding capabilities of the
filters decreasing in the order cellulosic > cellulose nitrate
> cellulose
acetate > nylon ∼ Teflon is presented. Different properties
were observed for the Cambridge filter. Specific care must be taken
using the filters for identification of radicals from tobacco smoke
to avoid artifacts in each case
Hydroxyl Radical Generation from Environmentally Persistent Free Radicals (EPFRs) in PM<sub>2.5</sub>
Hydroxyl
radicals were generated from an aqueous suspension of
ambient PM<sub>2.5</sub> and detected utilizing 5,5-dimethyl-1-pyrroline-<i>N</i>-oxide (DMPO) as a spin trap coupled with electron paramagnetic
resonance (EPR) spectroscopy. Results from this study suggested the
importance of environmentally persistent free radicals (EPFRs) in
PM<sub>2.5</sub> to generate significant levels of ·OH without
the addition of H<sub>2</sub>O<sub>2</sub>. Particles for which the
EPFRs were allowed to decay over time induced less hydroxyl radical.
Additionally, higher particle concentrations produced more hydroxyl
radical. Some samples did not alter hydroxyl radical generation when
the solution was purged by air. This is ascribed to internal, rather
than external surface associated EPFRs
Molecular Products from the Thermal Degradation of Glutamic Acid
The
thermal behavior of glutamic acid was investigated in N<sub>2</sub> and 4% O<sub>2</sub> in N<sub>2</sub> under flow reactor
conditions at a constant residence time of 0.2 s, within a total pyrolysis
time of 3 min at 1 atm. The identification of the main pyrolysis products
has been reported. Accordingly, the principal products for pyrolysis
in order of decreasing abundance were succinimide, pyrrole, acetonitrile,
and 2-pyrrolidone. For oxidative pyrolysis, the main products were
succinimide, propiolactone, ethanol, and hydrogen cyanide. Whereas
benzene, toluene, and a few low molecular weight hydrocarbons (propene,
propane, 1-butene, and 2-butene) were detected during pyrolysis, no
polycyclic aromatic hydrocarbons (PAHs) were detected. Oxidative pyrolysis
yielded low molecular weight hydrocarbon products in trace amounts.
The mechanistic channels describing the formation of the major product
succinimide have been explored. The detection of succinimide (major
product) and maleimide (minor product) from the thermal decomposition
of glutamic acid has been reported for the first time in this study.
Toxicological implications of some reaction products (HCN, acetonitrile,
and acyrolnitrile), which are believed to form during heat treatment
of food, tobacco burning, and drug processing, have been discussed
in relation to the thermal degradation of glutamic acid
Kinetic Modeling of Cellulose Fractional Pyrolysis
The kinetics of cellulose
fractional pyrolysis was studied for
the first time in the temperature range of 200–900 °C,
with 25 °C increment under nitrogen atmosphere. A detailed analysis
of the major and minor pyrolysis products was performed using a System
for Thermal Diagnostic Studies (STDS) and FTIR techniques. A semiglobal
kinetic model was proposed, with products grouped into kinetic lumps,
based on their formation profile similarity. Kinetic parameters (pre-exponential
factor <i>A</i> and activation energy <i>E</i><sub>a</sub>) for formation of major products grouped into heavy
volatiles 1 lump (levoglucosan and anhydrosugars) and light volatiles
2 lump (furans and carbonyls) were obtained based on the performed
experimental studies. The final model accurately predicts not only
the weight loss, the temperature-distribution of major lumped products,
and the total yields of tar and gases from the fractional pyrolysis
of cellulose but also shows a good performance toward literature data
for experimental studies of others
Peculiarities of Pyrolysis of Hydrolytic Lignin in Dispersed Gas Phase and in Solid State
The unique decomposition pathways
of hydrolytic lignin (HL) dissolved
in an acetone/water mixture (9:1) and dispersed by a droplet evaporation
technique under nitrogen gas flow has been investigated in a conventional
reactor at atmospheric condition, a temperature region of 400–550
°C, and a residence time of 0.12 s. The results validate the
fact that dispersion of the lignin into the gas phase by decreasing
the sample size (as well as “minimizing the char area to avoid
catalytic contact” of molecular products/radicals with the
surface) may open new perspectives in understanding the chemistry
of the depolymerization of lignin. Using Laser Desorption Ionization-Time
of Flight-Mass Spectrometry (LDI-TOF-MS) the intrinsic ion <i>m</i>/<i>z</i> = 550, as the major MS peak from fresh
HL dissolved in an acetone/water mixture before pyrolysis, was detected.
Surprisingly, the expected phenolic compounds after pyrolysis were
in trace amounts at less than 15% conversion of lignin. Instead, oligomeric
intermediate substances with low (<550 Da) and high molecular weight
(>550 Da) containing lignin-substructures (trapped on quartz wool
located at the end of the reactor at ∼300 °C) were detected
as major products using LDI-TOF-MS. The hypothesis about a largely
disputed key question on lignin pyrolysis as to whether the phenolic
compounds or oligomers (dimers, trimers, etc.) are the primary products
is discussed. Additionally, a focus on the free-radical mechanism
of depolymerization of solid lignin by formation of free intermediate
radicals from initial lignin macromolecules as well as from inherent,
low molecular weight oligomer molecules is developed based on the
Low Temperature Matrix Isolation (LTMI) EPR technique
Environmentally Persistent Free Radicals (EPFRs). 3. Free versus Bound Hydroxyl Radicals in EPFR Aqueous Solutions
Additional experimental evidence
is presented for <i>in vitro</i> generation of hydroxyl
radicals because of redox cycling of environmentally
persistent free radicals (EPFRs) produced after adsorption of 2-monochlorophenol
at 230 °C (2-MCP-230) on copper oxide supported by silica, 5%
Cu(II)O/silica (3.9% Cu). A chemical spin trapping agent, 5,5-dimethyl-1-pyrroline-<i>N</i>-oxide (DMPO), in conjunction with electron paramagnetic
resonance (EPR) spectroscopy was employed. Experiments in spiked O<sup>17</sup> water have shown that ∼15% of hydroxyl radicals formed
as a result of redox cycling. This amount of hydroxyl radicals arises
from an exogenous Fenton reaction and may stay either partially trapped
on the surface of particulate matter (physisorbed or chemisorbed)
or transferred into solution as free OH. Computational work confirms
the highly stable nature of the DMPO–OH adduct, as an intermediate
produced by interaction of DMPO with physisorbed/chemisorbed OH (at
the interface of solid catalyst/solution). All reaction pathways have
been supported by <i>ab initio</i> calculations
Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound <i>p</i>‑Coumaryl Alcohol
The fractional pyrolysis
of lignin model compound para-coumaryl
alcohol (<i>p</i>-CMA) containing a propanoid side chain
and a phenolic OH group was studied using the System for Thermal Diagnostic
Studies at temperatures from 200 to 900 °C, in order to gain
mechanistic insight into the role of large substituents in high-lignin
feedstocks pyrolysis. Phenol and its simple derivatives <i>p-</i>cresol, ethyl-, propenyl-, and propyl-phenols were found to be the
major products predominantly formed at low pyrolysis temperatures
(<500 °C). A cryogenic trapping technique was employed combined
with EPR spectroscopy to identify the open-shell intermediates registered
at pyrolysis temperatures above 500 °C. These were characterized
as radical mixtures primarily consisting of oxygen-linked conjugated
radicals. A comprehensive potential energy surface analysis of <i>p-</i>CMA and <i>p-</i>CMA + H atom systems was performed
using various DFT protocols to examine the possible role of concerted
molecular eliminations and free-radical mechanisms in the formation
of major products. Other significant unimolecular concerted reactions
along with formation and decomposition of primary radicals are also
described and evaluated. The calculations suggest that a set of the
chemically activated secondary radical channels is relevant to the
low temperature product formation under fractional pyrolysis conditions
New Features of Laboratory-Generated EPFRs from 1,2-Dichlorobenzene (DCB) and 2‑Monochlorophenol (MCP)
The present research is primarily focused on investigating
the
characteristics of environmentally persistent free radicals (EPFRs)
generated from commonly recognized aromatic precursors, namely, 1,2-dichlorobenzene
(DCB) and 2-monochlorophenol (MCP), within controlled laboratory conditions
at a temperature of 230 °C, termed as DCB230 and MCP230 EPFRs,
respectively. An intriguing observation has emerged during the creation
of EPFRs from MCP and DCB utilizing a catalyst 5% CuO/SiO2, which was prepared through various methods. A previously proposed
mechanism, advanced by Dellinger and colleagues (a conventional model),
postulated a positive correlation between the degree of hydroxylation
on the catalyst’s surface (higher hydroxylated, HH and less
hydroxylated, LH) and the anticipated EPFR yields. In the present
study, this correlation was specifically confirmed for the DCB precursor.
Particularly, it was observed that increasing the degree of hydroxylation
at the catalyst’s surface resulted in a greater yield of EPFRs
for DCB230. The unexpected finding was the indifferent behavior of
MCP230 EPFRs to the surface morphology of the catalyst, i.e., no matter
whether copper oxide nanoparticles are distributed densely, sparsely,
or completely agglomerated. The yields of MCP230 EPFRs remained consistent
regardless of the catalyst type or preparation protocol. Although
current experimental results confirm the early model for the generation
of DCB EPFRs (i.e., the higher the hydroxylation is, the higher the
yield of EPFRs), it is of utmost importance to closely explore the
heterogeneous alternative mechanism(s) responsible for generating
MCP230 EPFRs, which may run parallel to the conventional model. In
this study, detailed spectral analysis was conducted using the EPR
technique to examine the nature of DCB230 EPFRs and the aging phenomenon
of DCB230 EPFRs while they exist as surface-bound o-semiquinone radicals (o-SQ) on copper sites. Various
aspects concerning bound radicals were explored, including the hydrogen-bonding
tendencies of o-semiquinone (o-SQ)
radicals, the potential reversibility of hydroxylation processes occurring
on the catalyst’s surface, and the analysis of selected EPR
spectra using EasySpin MATLAB. Furthermore, alternative routes for
EPFR generation were thoroughly discussed and compared with the conventional
model