Organic hydroperoxide formation in the acid-catalyzed heterogeneous oxidation of aliphatic alcohols with hydrogen peroxide

Article abstract of DOI:10.1039/c4ra02486a

Organic hydroperoxides (ROOH) are reactive species which play significant roles in atmospheric processes, such as acid precipitation, hydroxyl radical cycling and secondary organic aerosol formation. Despite their observation in the atmosphere, our understanding of their formation mechanism is still incomplete. In the present work, ROOH formation was found in the acid-catalyzed heterogeneous oxidation of aliphatic alcohols with hydrogen peroxide. The kinetics and mechanism of acid-catalyzed heterogeneous oxidation of three aliphatic alcohols (2-methyl-2-butanol, 3-buten-2-ol and 2-butanol) with hydrogen peroxide were investigated. Based on the experimental results, tertiary or allyl alcohols may contribute to ROOH formation through this route while secondary alcohols may not. The kinetic experiments were conducted in a rotated wetted-wall reactor coupled to a mass spectrometer at room temperature (298 K) with 40-70 wt% H₂SO₄ solution. The reactive uptake coefficients were acquired for the first time. The generation and degradation mechanisms of ROOH in acidic media were proposed according to the product information. Once formed, ROOH are found to undergo two degradation pathways: the acid-catalyzed rearrangement reaction and the organic hydrogen peroxysulfate formation pathway. The newly found acid-catalyzed process may occur under certain conditions and influence particle growth in the atmosphere.

Full text of DOI:10.1039/c4ra02486a

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PAPER
Organic hydroperoxide formation in the acid-
catalyzed heterogeneous oxidation of aliphatic
alcohols with hydrogen peroxide†
Cite this: RSC Adv., 2014, 4, 19716
*
*
Qifan Liu, Weigang Wang, Ze Liu, Tianhe Wang, Lingyan Wu and Maofa Ge
Organic hydroperoxides (ROOH) are reactive species which play significant roles in atmospheric processes,
such as acid precipitation, hydroxyl radical cycling and secondary organic aerosol formation. Despite their
observation in the atmosphere, our understanding of their formation mechanism is still incomplete. In the
present work, ROOH formation was found in the acid-catalyzed heterogeneous oxidation of aliphatic
alcohols with hydrogen peroxide. The kinetics and mechanism of acid-catalyzed heterogeneous
oxidation of three aliphatic alcohols (2-methyl-2-butanol, 3-buten-2-ol and 2-butanol) with hydrogen
peroxide were investigated. Based on the experimental results, tertiary or allyl alcohols may contribute to
ROOH formation through this route while secondary alcohols may not. The kinetic experiments were
conducted in a rotated wetted-wall reactor coupled to a mass spectrometer at room temperature (298
K) with 40–70 wt% H2SO4 solution. The reactive uptake coefficients were acquired for the first time. The
generation and degradation mechanisms of ROOH in acidic media were proposed according to the
product information. Once formed, ROOH are found to undergo two degradation pathways: the acid-
catalyzed rearrangement reaction and the organic hydrogen peroxysulfate formation pathway. The
newly found acid-catalyzed process may occur under certain conditions and influence particle growth in
the atmosphere.
Received 21st March 2014
Accepted 7th April 2014
DOI: 10.1039/c4ra02486a
www.rsc.org/advances
Aliphatic alcohols (ROH), an important class of volatile
organic compounds, are emitted into the atmosphere by
different natural and anthropogenic sources.12,13 It could be
Introduction
Organic hydroperoxides (ROOH) play signicant roles in the
atmosphere due to their multiple roles as oxidants and reser-
voirs of radicals.1,2 Moreover, they are considered to be impor-
tant species of secondary organic aerosol (SOA) and have
negative impacts on vegetation.3–5 A series of ROOH, including
methyl hydroperoxide (MHP), hydroxymethyl hydroperoxide
(HMHP), and ethyl hydroperoxide (EHP) have been measured in
the atmosphere.6–8 Nowadays, it is believed that three reaction
pathways may contribute to the formation of ROOH. The
bimolecular reaction between organic peroxy radicals (RO2) and
hydroperoxyl radicals (HO2) is the rst route.9 The second one is
the ozonolysis reaction of alkenes.9,10 The third pathway
involves reversible addition of hydrogen peroxide (H2O2) to
aldehydes.11 Here, we propose a new route through oxidation
with H2O2 under acidic conditions that may result in ROOH
formation from aliphatic alcohols.
highly abundant in certain regions, for example, high rates of
emission of 2-methyl-3-buten-2-ol (MBO) were measured from
pine species in western United States.14–16 It is well recognized
that H2O2 plays a vital role in both aqueous-phase and gas-
phase oxidation.17 H2O2 was observed in cloud water with the
concentration ranging from 37.8 mM to 283.2 mM,18 and was
expected to be present in ne particles at a concentration of 0.1–
1mM.19 On the basis of lter extracts of ambient aerosols and
model calculations, Arellanes et al.20 suspected that H2O2
concentration in aerosol liquid water might be up to 70 mM.
Previous eld observations and experimental studies21–23 have
revealed that acid-catalyzed particle-phase reactions of biogenic
volatile organic compounds (for example, isoprene and
terpenes) or their reactive oxidation products (for example,
epoxides) provide a potential source for SOA. Laboratory
experiments have also suggested that acid-catalyzed heteroge-
neous oxidation of isoprene with H2O2 makes a contribution to
SOA formation.24,25 Given the cloud water samples collected
from eld measurements were oen acidic26,27 and the esti-
mated pH of liquid aerosol droplets was on the order of ꢀ0.8 to
1.18 in northeastern United States,28 heterogeneous reactions of
ROH and H2O2 in acidic media may take place under certain
atmospheric conditions. Previous studies of ROH (for example,
Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for
Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, 100190, P. R. China. E-mail: wangwg@iccas.ac.cn;
gemaofa@iccas.ac.cn; Tel: +86-10-62558682; +86-10-62554518
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02486a
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methanol, butanol, decanol and MBO) mainly focus on their upstream to expose the solution to the reactant gas and the
heterogeneous interactions with sulfuric acid (H2SO4) because signal dropped down simultaneously. Reactive uptake was
of their high activities to form organosulfates,29–32 these reac- indicated by a constant offset between the original signal S0 and
tions may contribute to aerosol growth. However, the hetero- the reactive uptake signal with time, S. The observed rst-order
geneous chemistry between ROH and H2O2 in acidic media rate constant for removal of the reactant gas from gas-phase,
remains largely uncertain. To the best of our knowledge, little kobs (sꢀ1) can be calculated from eqn (1):
ꢀ
ꢁ
attention is paid on the heterogeneous oxidation of ROH with
H2O2 in the presence of H2SO4, especially a systematic study of
heterogeneous interactions between different structures of
ROH and H2O2 in acidic media is still lacking. Hence, the acid-
catalyzed heterogeneous reactions of three different structures
of ROH (2-methyl-2-butanol, 3-buten-2-ol and 2-butanol), rep-
resenting tertiary, allyl and secondary alcohols, respectively,
were investigated in this study. The purpose of this research is
to gain more knowledge about the uptake kinetics and corre-
sponding chemical mechanisms of diverse structures of ROH
into H2SO4–H2O2 mixed solution. ROOH, organic peroxides
(ROOR) and organosulfates were found to be produced by the
heterogeneous process.
S
L
ln
¼ ꢀkobs
(1)
S0
vave
where vave (cm sꢀ1) is the average gas ow velocity of the reac-
tant gas, and L (cm) is the contact distance of the solution and
reactant gas. kobs can be determined more accurately by placing
the injector at various positions in the reactor to change the
contact distance. Fig. S1a† depicts the loss of 2-butanol signal
as a function of injector position. The rate constant for removal
of the reactant gas, kgas–liquid (sꢀ1), can be determined by cor-
recting kobs for diffusion:34,35
ꢀ
ꢁ
1
1
1
3:66Di
r2
¼
ꢀ
kdiff
¼
(2)
kgasꢀliquid kobs kdiff
where Di (cm2 sꢀ1) is the diffusion coefficient which can be
calculated from the Huller–Schettler–Gidding method,36 r (cm)
is the inner radius of the rotating cylinder, and kdiff is the
diffusion-limited rate (sꢀ1). Finally, the g can be acquired from
eqn (3):
Experimental section
Uptake measurements
The uptake measurements were conducted in a rotated wetted-
wall (RWW) ow tube reactor coupled to a signal-photon ioni-
zation time of ight mass spectrometer (SPI-TOFMS), similar to
our previous study.33 Briey, it is a reactor consisted of a Pyrex
tube with a glass jacket for thermostatic control. A rotating
cylinder (length L ¼ 30 cm, inner radius R ¼ 1.25 cm, rotating
rate r ¼ 10–15 rpm) was put into the Pyrex tube, holding small
volume of solutions (about 3.5 mL) to form liquid lm (about
0.15 mm thickness) evenly on the inner wall. A glass stirring bar
was placed on the bottom of the cylinder to ensure that the
solution could be mixed and spread sufficiently. To avoid the
change of solution composition during one experimental
period of time, a mixture of helium (He) and water vapor in
equilibrium with the solution was used as carrier gas. Reactant
gas was introduced into the reactor at a small ow rate (ten
percent of the carrier gas) through a movable glass injector (D ¼
6 mm) which was centered in the rotating cylinder. This glass
injector allowed for the variation of the contact time between
the solution and reactant gas. Typically, the total pressure was
in the range of 5.9 to 24.0 Torr and the reactant concentration in
the reactor was on the order of 4.8 ꢁ 1014 to 1.9 ꢁ 1015 mole-
cules per cm3. All of the experiments were carried out at the
room temperature (298 K). The Reynolds number calculated
was smaller than 2000 under our experimental condition. In
this situation, the measurements were operated under the
approximate laminar ow condition. Details of SPI-TOFMS are
described in the ESI.†
4kgasꢀliquid
uA
V
g ¼
(3)
where V (cm3) is the volume of the reaction zone, A (cm2) is the
geometric area of the exposed solution, and u (m sꢀ1) is the
mean molecular speed of reactant alcohol. More calculation
details are shown in the ESI.†
Gas-phase products identication
To further survey the gas-phase products, off-line FTIR spec-
trometer experiments were performed. The gas-phase species
were collected in a U-shape collector located in a liquid nitrogen
bath and then analyzed by FTIR spectrometer (Nicolet 6700,
Thermo Scientic). The IR spectra provide information about
the groups of the products molecules in the spectral range from
650 to 4000 cmꢀ1
.
Aqueous–phase reactions
Aqueous-phase reactions of ROH (2-butanol, 2-methyl-2-
butanol, tert-butyl alcohol and 3-buten-2-ol) and H2SO4–H2O2
mixed solution were performed to further study the mecha-
nism. Mixture of 0.1 mL ROH and 5 mL H2SO4 (or H2SO4–H2O2
mixed solution) were shaken thoroughly at 298 K for 2 h before
analysis. Then the mixture was extracted by 2 mL dichloro-
methane (CH2Cl2). To further investigate the formation mech-
anism of ROOH, 200 mg 2 wt% H218O2 was added into tert-amyl
sulfate (TAS) solution, which was prepared by the reaction of 40
Reactive uptake coefficient (g)
As an uptake experiment just began, the movable injector was mg 2-methyl-2-butanol and 200 mg H2SO4 (0.2 M). Another
placed at the zero position and the solution was unexposed. In reaction between 40 mg 2-methyl-2-butanol and 400 mg H2SO4
this situation, the unperturbed mass signal of reactant gas can (pH ¼ 1)–H218O2 (1 wt%) mixed solution was also conducted.
be recorded as baseline S0. Then the injector was pulled The mixture was extracted by CH2Cl2 aer 2 h reaction. Only the
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organic-phase aer extraction was analyzed by GC-MS and ESI-
MS because a large amount of H2SO4 remained in the water-
phase, hindering the detection of products signals. The pH of
the reaction mixture was around 1. This acidity is relevant to the
measured range of acidity in atmospheric aerosols.28 As for
H2O2, we assume 70 mM to be its upper limit in the atmosphere
according to previous work.20 0.1 wt% H2O2 (about 30 mM)
solutions were used in the uptake measurements and higher
concentration (1 wt%) of H2O2 solutions were used to investi-
gate the mechanism readily. As for the aqueous-phase reac-
tions, considering a certain loss of products in the water-phase,
the limit of detection for GC-MS and ESI-MS, high concentra-
tion of H2O2 (10 mM and 300 mM) solutions were used during
the experiments. GC-MS analysis, ESI-MS analysis and the
chemicals used in these experiments are described in the ESI.†
Results and discussion
Uptake behaviors and kinetics
Uptake measurements were performed by exposing the gaseous
alcohols to a certain length of the H2SO4 solution and moni-
tored the MS-signal change meanwhile. Fig. 1 depicts the
temporal proles of 2-butanol signals during the uptake
measurements. As the H2SO4 concentration increased, the
uptake behavior of 2-butanol changed from reversible to irre-
versible. For 50 wt% H2SO4 solution, the signal dropped
instantly upon exposure and returned to its original level as the
solution was saturated. Pushing the injector back to its initial
position produced an opposite change in signal, corresponding
to the release of 2-butanol back to the gas-phase. The similarity
in shape and total area of the adsorption and desorption curves
implied the occurrence of reversible uptake. For 70 wt% H2SO4
solution, the uptake displayed a steady-state feature and
exhibited no saturation on the time scale of the experiment,
indicating that irreversible reactions dominated the uptake
process. In a number of experiments, partially irreversible
uptake were observed in the variation of the signal (Fig. 1b): 2-
butanol was found to be taken up and released at a later time,
but additionally a constant signal offset was observed. The
similar cases were also reported in previous publications.30,31
Absorption and desorption likely explain the reversible uptake,
the formation of organosulfates and dehydration process could
account for the irreversible uptake. For the partially irreversible
uptake, a part of gaseous alcohol molecules are physically
absorbed while the others could undergo irreversible reactions.
The uptake behavior of 3-buten-2-ol was similar with that of 2-
butanol. For 2-methyl-2-butanol, reversible uptake was
observed for the solution with H2SO4 concentration below 40 wt
%. The partially irreversible uptake occurred for the 50–60 wt%
H2SO4 and the uptake on 70 wt% H2SO4 was totally irreversible.
When adding H2O2 into H2SO4 solution, the steady-state uptake
of 2-methyl-2-butanol and 3-buten-2-ol occurred at a lower
acidity while the uptake behavior of 2-butanol stayed
unchanged.
Fig. 1 Typical experimental profiles of 2-butanol. (a) reversible uptake
into 50 wt% H2SO4 solution; (b) partially irreversible uptake into 60 wt%
H2SO4 solution; (c) irreversible uptake into 70 wt% H2SO4 solution.
2-methyl-2-butanol and 3-buten-2-ol, but has little impact on
the uptake of 2-butanol. As for reactive gas uptake, Davidovits
et al.37 have suggested that the chemical reactions mainly
contribute to the uptake process in many cases. Different
chemical mechanisms could be used to explain the diverse
uptake behaviors of these compounds. The reactions between
ROH and H2SO4 at the surface or in the bulk liquid involve two
process: dehydration pathway and the formation of organo-
sulfates. When exposing gaseous ROH to H2SO4–H2O2 mixed
solution, a different route that produces ROOH is found except
for the reaction of 2-butanol. These reactions are described in
detail in the following section. Increasing acidity could result in
faster reactions for all three alcohols, thus leading to the
enhancement of g. More concentrated H2O2 also accelerates the
Table 1 summarizes the g of these three compounds and the
corresponding experimental conditions are listed in Table S1.†
It seems that H2O2 plays a role in the enhancement of g for
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Table
1 Summarization of reactive uptake coefficients for three
performed to further survey the products. Fig. S3a† shows the
ESI-MS spectrum (in the negative mode) of extracted organic-
phase from aqueous-phase reactions between 2-butanol and
H2SO4 (pH ¼ 1). The peak at m/z ¼ 153 (C4H9SO4ꢀ) represents
the signal of 1-methylpropyl sulfate. Organosulfates were
common products formed through this pathway, which were
also observed in previous studies.29–32 These results indicate
that heterogeneous reactions of 2-butanol and H2SO4 could
result in (E)-2-butene and organosulfates formation.
aliphatic alcoholsa
Gas reactant
2-Butanol
H2SO4 (wt%)
H2O2 (wt%)
g† (ꢁ 10ꢀ4
)
60
60
70
70
40
40
50
50
50
50
60
60
60
60
70
70
70
70
50
50
60
60
60
60
70
70
70
70
0
1
0
1
0
1
0
0.1
0.5
1
0.98 ꢂ 0.02
0.82 ꢂ 0.01
1.98 ꢂ 0.05
2.01 ꢂ 0.04
2-Methyl-2-butanol
—
0.46 ꢂ 0.02
0.27 ꢂ 0.02
0.51 ꢂ 0.16
2.21 ꢂ 0.15
5.89 ꢂ 0.12
1.99 ꢂ 0.04
2.93 ꢂ 0.02
5.65 ꢂ 0.15
18.43 ꢂ 1.09
12.52 ꢂ 0.68
16.00 ꢂ 0.03
29.63 ꢂ 0.62
66.49 ꢂ 0.64
—
1.61 ꢂ 0.13
1.10 ꢂ 0.07
1.28 ꢂ 0.04
1.57 ꢂ 0.04
7.60 ꢂ 0.37
2.21 ꢂ 0.44
2.57 ꢂ 0.07
3.93 ꢂ 0.16
17.13 ꢂ 0.32
A new peak at m/z ¼ 70 appeared in the mass spectrum
(Fig. 2a) aer gaseous 2-methyl-2-butanol was exposed to 70 wt%
H2SO4 solution. An alkene likely accounted for this peak as we
0
0.1
0.5
1
0
0.1
0.5
1
0
1
3-Buten-2-ol
0
0.1
0.5
1
0
0.1
0.5
1
a
— represents no obvious uptake. † Each value is the average of at least
three measurements, and the error corresponds to one standard
deviation (s).
reaction rates except for that of 2-butanol. The g of these three
alcohols under the same experimental conditions (same H2SO4
and H2O2 concentration) are found to follow the sequence: 2-
methyl-2-butanol > 3-buten-2-ol > 2-butanol. The generation of
carbocation could be a key step either in the dehydration
process or the formation of organosulfates and ROOH.38 The
stability of carbocations formed during the reactions likely
determines the reaction rates which follow the sequence:
tertiary > allyl > secondary carbocation. Hence, if the carboca-
tion formation is the central process, it is reasonable that the
value of g for 2-methyl-2-butanol is the biggest among these
three alcohols under the same conditions.
Products identication
For the uptake of 2-butanol into 70 wt% H2SO4 solution
(Fig. S1b†), reactant signals (m/z ¼ 44, 45, 59 and 74) dropped
down as soon as gaseous 2-butanol was exposed to H2SO4
solution, meanwhile a new peak at m/z ¼ 56 was detected from Fig. 2 Typical uptake experimental profiles of 2-methyl-2-butanol. (a)
The real-time mass spectrum after gaseous 2-methyl-2-butanol was
exposed to 70 wt% H2SO4 solution; (b) the profiles of monitoring ion
peaks in real-time during the uptake of 2-methyl-2-butanol into 70 wt
% H2SO4–1 wt% H2O2 mixed solution; (c) real-time mass spectrum at
the mass spectrum. This peak is ascribed to (E)-2-butene formed
through the dehydration of 2-butanol, which is conrmed by
the gas-phase products analysis using FTIR spectroscopy
(Fig. S2b†). This result coincides with the Zaitsev's rule (see the
the time marked by dash line in (b). The peak at m/z ¼ 87 is not shown
ESI† for more details). Aqueous-phase reactions were in (b) because the SPI-TOFMS can only monitor five peaks at one time.
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found evident resemblance between the gas-phase product IR 523 K (see Fig. S6a† for detailed mechanism).40 Considering the
(Fig. S2d†) and reference 2-methyl-2-butene IR. Based on the inlet temperature of GC-MS was set at 523 K, acetone and
aqueous-phase products analysis, the peak at m/z ¼ 167 butane might be formed through this pathway. TAHP and DTAP
(C5H11SO4ꢀ) in Fig. S3b† is due to the generation of organo- were also generated in H2SO4 (pH ¼ 1)–H2O2 (10 mM) mixed
sulfates. These results suggest that organosulfates and alkene solution (see Fig. S5b†).
are created by the heterogeneous interactions.
Compared to the aqueous-phase reactions between 2-methyl-
ꢀ
In contrast to the uptake of 2-methyl-2-butanol on H2SO4, 2-butanol and H2SO4, a new peak at m/z ¼ 183 (C5H11SO5
)
obvious changes appeared when adding H2O2 into H2SO4 appeared in the ESI-MS spectrum of extracted organic-phase
solution. As shown in Fig. 2b and c, 2-methyl-2-butanol signals from the reaction between 2-methyl-2-butanol and H2SO4–H2O2
(m/z ¼ 59 and 73) dropped down meanwhile four new peaks at mixed solution (Fig. S3c†). This peak was inferred to be related
m/z ¼ 58, 71, 87 and 104 appeared aer the gaseous reactant to tert-amyl hydrogen peroxysulfate (TAPS), which was produced
was exposed to 70 wt% H2SO4–1 wt% H2O2 mixed solution. by the reaction of TAHP and H2SO4. In order to conrm our
Evident changes also appear in the IR spectrum of gas-phase hypothesis, both the uptake of gaseous TAHP on H2SO4 solution
product (Fig. S4b†), the band at 1740 cmꢀ1 is deduced to belong and the aqueous-phase reactions between TAHP and H2SO4
to a C]O stretch. In the light of these results, the peak at m/z ¼ were conducted. The same peak at m/z ¼ 183 obtained from the
58 detected in the mass spectrum is attributed to the molecule ESI-MS (Fig. S3d†) was also found during the aqueous-phase
ion of acetone. The peak at m/z ¼ 71 (C5H11+) is deduced to be a reactions of TAHP and H2SO4. Fig. 3a depicts the mass spec-
fragment of tert-amyl hydroperoxide (TAHP) according to trum of TAHP and ve peaks (m/z ¼ 58, 59, 71, 87 and 104) are
following experimental evidences. A similar peak was also observed. These peaks have a good agreement with the peaks
detected in our previous study on the uptake of MBO into shown in Fig. 2c. Exposing gaseous THAP to H2SO4 (Fig. 3b) led
H2SO4–H2O2 mixed solution:39 in that paper, a new peak at m/z to an increment of the signal intensity for the peak at m/z ¼ 58
¼ 69 was observed through heterogeneous reactions and was while other peaks dropped to a certain extent at the same time.
ascribed to an online-product which needed to be further Although the peak at m/z ¼ 58 is one of fragments of TAHP, we
identied. The peaks at m/z ¼ 87 and 104 likely stand for TAHP suppose acetone makes a main contribution to peak at m/z ¼ 58
fragment and TAHP molecular ion, respectively. To further in Fig. 2c for three reasons: (i) the peak at m/z ¼ 58 in Fig. 2c is
investigate this reaction pathway, a series of experiments of the strongest fragment, which can hardly caused only by the
aqueous-phase reactions were performed. Fig. S5a† shows the fragment of TAHP according to the relative peak intensity
gas chromatogram of the extracted organic-phase from between the molecular ion peak at m/z ¼ 104 and fragment peak
aqueous-phase reactions of 2-methyl-2-butanol and H2SO4 (pH at m/z ¼ 58 (shown in Fig. 3a). (ii) Fig. S4d† shows the IR
¼ 1)–H2O2 (300 mM) mixed solution. Peaks 1–4 in the chro- spectrum of products formed in the heterogeneous reaction of
matogram correspond to butane, acetone, solvent dichloro- TAHP and H2SO4, the band around 1740 cmꢀ1 allows a
methane and reactant 2-methyl-2-butanol, respectively, all of straightforward detection of C]O stretching modes. This result
these mass spectra match excellently with those from the MS gives a strong support to the formation of carbonyl compounds.
library. Due to the lack of mass spectra of possible products (iii) Acetone may be formed through the protolytic cleavage-
(TAHP and di-tert-amyl peroxide (DTAP)) in the MS library, rearrangement reactions of TAHP in acidic media according to
liquid TAHP and DTAP were injected into GC-MS to obtain the previous work.41 Based on all these experimental evidences,
corresponding mass spectra. Additionally, aqueous-phase TAHP, DTAP, TAPS and acetone are considered to be generated
reactions of tert-butyl alcohol and H2SO4–H2O2 mixed solution by the heterogeneous reactions of 2-methyl-2-butanol and
were also performed in the same experimental conditions H2SO4–H2O2 mixed solution. Detailed reaction mechanism is
because of the similar structure of these two compounds, the discussed in the subsequent section.
products were identied using GC-MS. Therefore, we infer that
As for the uptake of 3-buten-2-ol in H2SO4 solution (Fig. 3c),
peak 5 and 6 in Fig. S5a† belongs to TAHP and DTAP signals the reactant signals (m/z ¼ 43, 57 and 72) decreased quickly
respectively for several reasons: (i) mass spectrum of peak 5 in aer gaseous 3-buten-2-ol was exposed to H2SO4, simulta-
Fig. S5a† is nearly the same as that of TAHP, obvious resem- neously a new peak at m/z ¼ 54 appeared. Similar to the uptake
blance is also found between the mass spectrum of peak 6 and of former two alcohols, this peak is caused by an alkene because
that of DTAP. (ii) According to the GC-MS results, tert-butyl the product IR (Fig. S2f†) is nearly the same as the reference 1,3-
hydroperoxide and di-tert-butyl peroxide are found to be butadiene IR. In the light of ESI-MS result (Fig. S3e†), the peak
produced during the aqueous-phase reactions of tert-butyl at m/z ¼ 151 (C4H7SO4ꢀ) is inferred to be 1-methylallyl sulfate
alcohol and H2SO4–H2O2 mixed solution. Considering the signal, just like the reactions of former two alcohols and H2SO4.
similar structure of 2-methyl-2-butanol and tert-butyl alcohol,
Fig. 3e shows the temporal proles of all ion peaks during the
ROOH maybe also formed from the reaction of 2-methyl-2- uptake of 3-buten-2-ol into 70 wt% H2SO4–1 wt% H2O2 mixed
butanol and H2SO4–H2O2 mixed solution. (iii) In the realm of solution, and Fig. 3f is the real-time mass spectrum aer expo-
organic chemistry, TAHP and DTAP could be synthesized by the sure. The 3-buten-2-ol signals dropped down as soon as the
reaction of tert-amyl alcohol and H2SO4–H2O2 mixed solution, reaction started and two new peaks at m/z ¼ 44 and 55 appeared.
which was under very strict control.40 (iv) Acetone and butane The peak at m/z ¼ 55 (C4H7+) is 1 amu larger than the dehydra-
provide an indirect evidence for the formation of DTAP because tion product 1,3-butadiene. The similar cases also occurred in
they are suggested to be the main pyrolysis products of DTAP at the uptake experiments of MBO and 2-methyl-2-butanol.39
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Fig. 3 Typical experimental profiles of TAHP and 3-buten-2-ol. (a) The vacuum UV mass spectra of TAHP. (b) The profiles of monitoring all ion
peaks in real-time during the uptake of TAHP into 70 wt% H2SO4 solution. (c) The profiles of monitoring all ion peaks in real-time during the
uptake of 3-buten-2-ol into 70 wt% H2SO4 solution. (d) Real-time mass spectrum at the time marked by dash line in (c). (e) The profiles of
monitoring all ion peaks in real-time during the uptake of 3-buten-2-ol into 70 wt% H2SO4–1 wt% H2O2 mixed solution. (f) Real-time mass
spectrum at the time marked by dash line in (e).
Consequently, we infer methylallyl hydroperoxide (MAHP) is of MAHP is limited and the aqueous-phase products were only
generated by this heterogeneous reaction and the peak at m/z ¼ analyzed by ESI-MS. As mentioned above, organic hydrogen
55 is one of its fragments. As shown in Fig. S4f,† a strong band peroxysulfate (ROOSO3H) is created by the reaction of ROOH
around 1745 cmꢀ1 is observed in gas-phase products, which and H2SO4. Therefore, if MAHP was formed in the aqueous-
should be caused by carbonyl compounds. On the basis of these phase reactions, ROOSO3H should be detected by ESI-MS. The
results, the peak at m/z ¼ 44 in Fig. 3f is inferred to belong to the peak at m/z ¼ 167 (C4H7SO5ꢀ) (Fig. S3f†) is deduced to be 1-
molecular ion of acetaldehyde. The reaction pathway is dis- methylallyl hydrogen peroxysulfate signal. These results reveal
cussed in detail in the following section. Due to the lack of that ROOH are also formed through the acid-catalyzed oxidation
standard compound, the information about the mass spectrum of 3-buten2-ol with H2O2.
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chemical mechanism, two experiments are designed: the rst
one is adding H218O2 into TAS, the other one is adding 2-methyl-
2-butanol into mixed H2SO4–H218O2 solution. As shown in
Fig. 4, if HO2ꢀ pathway is the main route, TAHP formed by this
way will further react with H2SO4, thus a peak at m/z ¼ 185
(C5H11S16O418Oꢀ) should be detected by ESI-MS. Conversely, if
OHꢀ pathway controls the reaction, a peak at m/z ¼ 183
(C5H11S16O5ꢀ) should be found. Referring to Fig. S3g and h,†
the peak at m/z ¼ 167 is attributed to TAS and the peak at m/z ¼
185 represents the ROOSO3H formed though HO2ꢀ route. Weak
peaks at m/z ¼ 183 were also detected, this might be due to the
OHꢀ pathway or the inuence of H218O2 (contain 10 atom%
16O). Considering the strong intensity of the peak at m/z ¼ 185,
Reaction mechanism
As stated above, TAHP, DTAP, TAPS and acetone are considered
to be produced by the heterogeneous reactions of 2-methyl-2-
butanol and H2SO4–H2O2 mixed solution. Combining the
results obtained from SPI-TOFMS, FTIR spectrometer, GC-MS,
ESI-MS and previous research,40 the proposed chemical mech-
anism for the formation and degradation of TAHP is shown in
Fig. 4. The initial step involves the addition of a proton to the
hydroxyl of 2-methyl-2-butanol followed by the elimination of
H2O and generation of tertiary carbocations. Based on the
previous studies, ROOH may be prepared by nucleophilic attack
of concentrated H2O2 on carbonium ions and the reaction iꢀs
proposed to follow an SN1 pathway.38,41 Considering HO2
(caused by heterolysis of H2O2) is a strong nucleophile, the
ꢀ
we conclude that HO2 route is the main pathway.
Once generated in H2SO4 solution, the degradation of TAHP
occurs simultaneously. On one hand, cleavage of oxygen to
oxygen (O–O) bond in TAHP produces alkoxyl radicals (RO),
which then react further with H2SO4 to produce TAPS or
combine with other RO to form DTAP. On the other hand, TAHP
undergoes an acid-catalyzed rearrangement which leads to the
formation of acetone. Fig. 5 depicts the rearrangement mech-
anism. The cleavage of O–O bond of conjugate acid, caused by
the protonation at oxygen atom, leads to the formation of a
highly energetic oxenium ion. Then the oxenium ion rearranges
to the alkylated ketone immediately, which reacts further to
produce acetone and ethanol. The relative migratory ability of
different groups follows the trend ethyl > methyl during the
rearrangement. Although magic acid (not H2SO4) was used in
the study of rearrangement of ROOH in previous research,41 we
thought this process could also take place in the presence of
H2SO4 because it was reported that tert-butyl hydroperoxide
underwent a similar rearrangement reaction in H2SO4.42
Ethanol is a missing product of the rearrangement process in
our study for two reasons: (i) ethanol tends to stay in the
aqueous-phase for its great solubility in water and reacts further
with H2SO4 to form ethyl sulfate, the similar route is also
reported in former paper.42 (ii) Even though gaseous ethanol is
produced, the SPI-TOFMS can hardly detect it because our
instrument can only detect species whose ionization energy is
below 10.5 eV. Fortunately, according to the products analysis of
aqueous-phase reactions between TAHP and H2SO4, the peak at
m/z ¼ 125 (C2H5SO4ꢀ) (Fig. S3d†) which is ascribed to ethyl
sulfate, gives an indirect evidence for the generation of ethanol.
ꢀ
possible ROOH formation mechanism is HO2 attack the car-
bocations. Thus, HO2 or hydrogen sulfate ion (HSO4ꢀ) could
ꢀ
attack carbocations in the following step, giving TAHP and
organosulfates, respectively. In the realm of organic chemistry,
TAHP was synthesized based on the method introduced by
Milas and Surgenor:40 adding concentrated H2O2 into tert-amyl
sulfate (TAS) solution (prepared by reaction of 2-methyl-2-
butanol and H2SO4) could lead to the generation of TAHP (main
product) and DTAP (byproduct). Under this condition, HO2ꢀ or
OHꢀ may attack different positions of TAS, thus producing
TAHP. To the best of our knowledge, in previous literature little
attention was paid to the question that which route might be
the main pathway. To get a better understanding of the
Fig. 4 Proposed chemical mechanism for the formation of TAHP
during the heterogeneous acid-catalyzed oxidation of 2-methyl-2- Fig. 5 Proposed chemical mechanism for the acid-catalyzed rear-
butanol with H2O2.
rangement of TAHP.
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Moreover, when extending the formation mechanism of ROOH environment in the past two decades.6–8 According to the review
to our previous work,39 it is logical to infer that 1,1-dimethylallyl summarized by Reeves and Penkett,45 high performance liquid-
hydroperoxide is generated by the uptake of MBO into H2SO4– phase chromatography with post-column enzyme derivatization
H2O2 mixed solution. If ROOH formed from MBO undergoes is a useful method to detect individual ROOH. Horseradish
the same rearrangement mechanism just as TAHP does, peroxidase (HRP) was used to catalyze the reduction of ROOH in
acetone and acetaldehyde should be the products (see Fig. S6b† previous eld measurements.6–8,46,47 It should be pointed out
for detailed mechanism). Previous results have shown a good that HRP can only effectively catalyze the reduction of H2O2 and
agreement with our conjecture. Considering all this, it is short-chain (C # 2) ROOH because of its specicity for the
reasonable to conclude that TAHP undergoes a rearrangement hydrogen receptor.48 Furthermore, many standards of long-
reaction in H2SO4.
chain ROOH (for example, 1,1-dimethylallyl hydroperoxide) are
Asforthechemicalmechanismduringthereactionof3-buten- not available. These two factors may inhibit the measurement of
2-ol and H2SO4–H2O2 mixed solution, ROOH is also formed and ROOH formed through the acid-catalyzed pathway. Based on the
follows the same rule as 2-methyl-2-butanol undergoes (see experimental evidence (2-methyltetrols was found to be
Fig. S6c† for detailed information). Experimental evidence is produced by the reaction of isoprene and H2SO4–H2O2 mixed
limited to validate the generation of ROOR, nevertheless, it is a solution) and eld measurements, Claeys et al. suggested that
possible route for the formation of methylallyl peroxide during multiphase acid-catalyzed oxidation of isoprene with H2O2 may
the heterogeneous interaction as DTAP is produced under the contribute to SOA formation.24 Similarly, on the basis of the
similar condition. It is very interesting that H2O2 changes the aqueous-phase reaction results shown in the present work, it
chemical mechanism of 2-methyl-2-butanol and 3-buten-2-ol, seems logical to assume that the acid-catalyzed reactions of ROH
while it has no obvious effect on 2-butanol, leading to a higher g may also occur under certain conditions and contribute to
for 2-methyl-2-butanol and 3-buten-2-ol. The different reactivity ROOH formation. Our results imply that this acid-catalyzed
may be caused by the stability of carbocations formed during the route is a potential source for ROOH formation and more
reactions: more stable carbocations may contribute to the research is needed to elucidate the role of this pathway.
generation of ROOH while the less stable carbocations may not.
The heterogeneous reactions may inuence particle growth,
especially in the upper troposphere where sulfate aerosols are
more concentrated. Organosulfates were deduced to be
produced during the acid-catalyzed reactions. They were repor-
ted to undergo a slow hydrolysis reaction and likely to be stable
during the lifetime of most ambient SOA.49,50 The uptake of ROH
into existing acidic particles could lead to the formation of low-
Our results are consistent with previous ndings.43
Conclusion and atmospheric
implications
In this work, the g of three different structures of ROH into volatility organosulfates, which tend to stay stable in the
H2SO4–H2O2 mixed solution were calculated and the corre- particle-phase and contribute to the particles growth. In addi-
sponding chemical pathways were deduced according to the tion, aldehydes and ketones are found to be generated by the
products information. For 2-methyl-2-butanol and 3-buten-2-ol, acid-catalyzed rearrangement reaction of ROOH. These carbonyl
ROOH, ROOR, ROOSO3H and organosulfates were found to be compounds may undergo aldol condensation and polymeriza-
created by the heterogeneous interactions. The newly found tion in the acidic media,3,51 thus leading to the formation of
acid-catalyzed pathway may provide a potential route for ROOH higher-molecule weight compounds which may also make a
formation and inuence particle growth.
contribution to the particle growth for their low volatility.
It has been suggested that the majority of the ROOH in the
gas-phase are formed via the recombination reaction of HO2 and
RO2 during the daytime.9 Other mechanisms for the formation
Acknowledgements
of ROOH in the absence of light include the ozonolysis reaction This research was nancially supported by the Strategic Priority
of alkenes9,10 and aqueous-phase reaction between H2O2 and Research Program (B) of the Chinese Academy of Sciences
aldehydes.11 Here, we introduce another possible pathway for (XDB05010400) and National Natural Science Foundation of
the formation of ROOH during the acid-catalyzed oxidation of China (Major Program: 21190052 & Contract no. 41173112,
ROH (limited to tertiary or allyl alcohols) with H2O2. Tertiary and 21077109,40925016).
allyl aliphatic alcohols are emitted into the atmosphere by
different natural and anthropogenic sources. For example, a
series of tertiary and allyl alcohols are emitted to the atmosphere
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they may contribute to ROOH formation through the acid-cata-
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