A characterization of the two-step reaction mechanism of phenol decomposition by a Fenton reaction

Article abstract of DOI:10.1016/j.cplett.2015.10.005

Phenol is one of the worst contaminants at date, and its degradation has been a crucial task over years. Here, the decomposition process of phenol, in a Fenton reaction, is described. Using scavengers, it was observed that decomposition of phenol was mainly influenced by production of hydroxyl radicals. Experimental and theoretical activation energies (E_a) for phenol oxidation intermediates were calculated. According to these E_a, phenol decomposition is a two-step reaction mechanism mediated predominantly by hydroxyl radicals, producing a decomposition yield order given as hydroquinone > catechol > resorcinol. Furthermore, traces of reaction derived acids were detected by HPLC and GS-MS.

Full text of DOI:10.1016/j.cplett.2015.10.005

Accepted Manuscript
Title: A Characterization of the Two-step Reaction
Mechanism of Phenol Decomposition by a Fenton Reaction
´
Author: Cristian Valdes Jans Alzate-Morales Edison Osorio
˜
Jorge Villasenor Carlos Navarro-Retamal
PII:
S0009-2614(15)00749-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cplett.2015.10.005
CPLETT 33330
To appear in:
Received date:
Revised date:
Accepted date:
2-7-2015
8-9-2015
4-10-2015
´
˜
Please cite this article as: C. Valdes, J. Alzate-Morales, E. Osorio, J. Villasenor,
C. Navarro-Retamal, A Characterization of the Two-step Reaction Mechanism
of Phenol Decomposition by a Fenton Reaction, Chem. Phys. Lett. (2015),
http://dx.doi.org/10.1016/j.cplett.2015.10.005
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1
2
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4
5
6
7
Highlights
Oxidation of phenol (using Fenton) decrease in the presence of OH° scavengers.
The theoretical Ea of liquids systems are lower than gas phases.
Good agreement is found between DFT studies and experimental results.
8
9
Theoretical and experimental order shows the trend:
hydroquinone>catechol>resorcinol.
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11
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A Characterization of the Two-step Reaction
Mechanism of Phenol Decomposition by a
Fenton Reaction
14 Cristian Valdés2, Jans Alzate-Morales1, Edison Osorio3, Jorge Villaseñor2, * and Carlos
15
Navarro-Retamal1, *
16
17
1 Centro de Bioinformática y Simulación Molecular, Facultad de Ingeniería,
Universidad de Talca, 2 Norte 685, Casilla 721, Talca, Chile
18 2 Laboratorio de Fisicoquímica, Instituto de Química y Recursos Naturales, Universidad
19
de Talca, 2 Norte 685, Casilla 721, Talca, Chile
20
21
3 Departamento de Ciencias Básicas, Fundación Universitaria Luis Amigó, Medellin,
Colombia
22 AUTHOR EMAIL ADDRESS: cnavarro@utalca.cl and jvillase@utalca.cl
23 * CORRESPONDING AUTHORS FOOTNOTE:
24 C. Navarro-Retamal, Phone number: 56-71-201798, Fax number: 56-71-201561,
25 Additional Email Address: carlos.navarro87@gmail.com
26
J. Villaseñor, Phone number: 56-71-2200272
27
28
29
30 ABSTRACT
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31 Phenol is one of the worst contaminants at date, and its degradation has been a crucial
32 task over years. Here, the decomposition process of phenol, in a Fenton reaction, is
33 described. Using scavengers, it was observed that decomposition of phenol was mainly
34 influenced by production of hydroxyl radicals. Experimental and theoretical activation
35 energies (E ) for phenol oxidation intermediates were calculated. According to these
a
36 E , phenol decomposition is a two-step reaction mechanism mediated predominantly by
a
37 hydroxyl radicals, producing a decomposition yield order given as hydroquinone >
38 catechol > resorcinol. Furthermore, traces of reaction derived acids were detected by
39 HPLC and GS-MS.
40
41
42 INTRODUCTION
43 Phenol is one of the most common contaminants ever known. This compound may
44 come from the pharmaceutical industry, oil, pesticides and others. Lower amounts of
45 phenol (in concentrations of ppm) are enough to induce high levels of toxicity [1].
46 Phenol is considered by the United States Protection Agency (USEPA) as one of the
47 most pollutant compounds, causing considerable damage to human health and to the
48 ecosystems [2]. The hydroxylation of phenol with oxidizing agents, like ozone and
49 hydrogen peroxide, has a great impact on different fields such as environmental
50 remediation and the photography industry (by using different phenol derivatives as
51 photographic developers) [3].
52 The catalytic oxidation of phenol has proven to be really important along the last years,
53 where different derivatives of this reaction, such as catechol (CT) and hydroquinone
54 (HQ), can serve as raw materials for the generation of photographic supplies, pesticides,
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55 inks and additives for medicines [4]. In order to improve the oxidation of this kind of
56 molecules, many researches have used on the last decades a specific kind of oxidation
57 process named advanced oxidation process (AOP), which usually operates at or near
58 normal temperature and pressure conditions. According to Glaze et al., AOP is defined
59 as a process that “involves the generation of hydroxyl (OH) radicals in sufficient
60 quantity to affect water purification” [5]. OH radicals are extraordinarily reactive
61 species. They are able to degrade the most part of organic compounds, with constant
62 rates in the order of the 106 – 109 M-1s-1[6]. OH radicals are effective in destroying
63 organic chemicals because they are reactive electrophiles that react rapidly and non-
64 selectively with nearly all electron-rich organic compounds. It has been reported that
65 one of the best source of OH radicals corresponds to the hydrogen peroxide (H2O2),
66 because it has low cost, and more importantly, because is not harmful to the
67 environment [7, 8]. On the other hand, the Fenton mechanism is based on the
68 continuous production of radicals using the decomposition of H2O2 mediated by the ion
69 Fe (II) as catalyst at pH 3 in a homogeneous system [9]. It is also known that phenol can
70 be efficiently degraded by Fenton type processes [10]. Since Fenton processes are
–
71 characterized by the generation of several radicals (i.e. •OH, •O2 and •O2H) [11, 12], the
72 radical scavengers Chloroform (CF) and Isopropyl alcohol (IPA) were used in order to
73 study its effect on the oxidation of phenol, considering their highly selectivity by
74 superoxide (k=3,0 x 1010 M-1 S-1) and hydroxyl radical (k=9,0 x 109 M-1 S-1) respectively
75 [9].
76 According to literature, three different intermediates are produced during the oxidation
77 of phenol: catechol (CT), resorcinol (RC) and hydroquinone (HQ). Among them, HQ
78 and CT are the most abundant, followed by RC [13]. It has been reported that other
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79 radical species, such as superoxide radical, are also able to oxidase different aromatic
80 compounds, including phenol [3, 14].
81 Some reaction mechanisms have been proposed to explain the oxidation of phenol
82 mediated by H2O2 and OH radicals [15, 16]. For instance, the oxidation of phenol by
83 OH radicals can be achieved by 3 different reaction mechanisms (Scheme 1). Each one
84 consists in a two-step reaction, where the first step corresponds to the addition of an OH
85 radical onto one of the carbons at the ortho, metha or para positions of phenol, leaving
86 a delocalized electron on aromatic ring and thereby generating an adduct intermediate.
87 The second step corresponds to the hydrogen abstraction (H-abst) at the same position
88 of the previous addition, leading to the formation of a water molecule and the
89 corresponding derivative. Depending on the carbon atom at which the second OH
90 radical is added; three different intermediates can be produced. If both reactions occur
91 at the o-carbon, the product will be CT (Scheme 1C); if instead both reactions occur at
92 the m-carbon, the reactions will produce RC (Scheme 1B) and if the reactions occur at
93 the p-carbon, the final product will be HQ (Scheme 1A). Furthermore were detected by
94 HPLC and GS- MS: traces of maloneic, malic, pyruvic and acetic acids among others
95 (data not shown).
96
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97
98
Scheme 1. Reaction mechanism describing the different phenol intermediates based on the
interaction of phenol with two hydroxyl radicals. A) Generation of HQ by the interaction of the
hydroxyl radicals onto the p-carbon of phenol. B) Generation of RC by the interaction of the
hydroxyl radicals onto the m-carbon of phenol. C) Generation of CT by the interaction of the
hydroxyl radicals onto the o-carbon of phenol.
99
100
101
102
103 Until date, the addition step has been scarcely described at a theoretical level. For
104 instance, Wu et al. described the addition step of the OH radical on gas phase in 4
105 possible positions at phenol ring: ipso, ortho, meta and para of phenol. They found that
106 addition at the o-carbon was thermodynamically favorable, resulting in an overall
107 production of 96.7-99.8% of the total adduct products in the reaction [17]. In a similar
108 way, Morales-Roque et al. were able to study the same step, but now including the
109 solvent effects onto the overall system. They found that adducts at o- and p-carbons
110 were the chemical entities that showed the lower energy barriers, thereby producing a
111 bigger amount of o-adducts and p-adducts [8]. At date, there is not theoretical evidence
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112 related to the second step of the reaction mechanism, neither in gas phase or water as a
113 solvent.
114 In this research work, the main goal is twofold: first, to experimentally elucidate the
115 importance of the OH radical during phenol decomposition; and second, to characterize
116 the two-step decomposition reaction mechanism pathway of phenol by means of
117 theoretical approaches. The research involves the theoretical study of the solvent effect
118 into this kind of reaction.
119
120 METHODOLOGY
121 Quantification of Phenol and its intermediates by HPLC
122 The concentration of phenol was analyzed using a HPLC (Perkin Elmer series 200)
123 equipment with a LiChrospher® column RP-18 with UV absorbance detector. The
124 following conditions were used: λ=280 nm, flow = 1 mL / min, mobile phase methanol
125 (34%) / water (64%) / acetic acid (2%), column at temperature 35° C with an injection
126 volume of 10 μL.
127 The concentration of phenol intermediates was analyzed using a HPLC (Perkin Elmer
128 series 200) equipment with a LiChrospher® column 60 RP Select B coupled to a
129 Shimadzu Prominence RF-20-A / 20AXS fluorescence detector. The following
130 conditions were used: λ emission = 325, λ excitation = 280, flow of 1 mL / min, mobile
131 phase 0.007 M H3PO4 (90%), acetonitrile (10%), column temperature at 45 °C, with an
132 injection volume of 10 μL.
133 Phenol and its intermediates concentrations were determined using calibration curves
134 obtained from solutions of known concentration for each case.
135
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136 The solutions were prepared in triplicate and transferred to the autosampler microvials
137 and samples of 200 μL were injected into the column. Calibration curves were
138 constructed by plotting the peak area against the nominal concentration of each
139 compound.
140 Five samples of each compound were prepared at the quality control in four different
141 concentrations, in three different days, in order to determine intra- and inter-day
142 precision and accuracy.
143 The acceptance criteria for the limit of quantification was that the accuracy for three
144 analyzed samples presented less than 2 % variability and the limit of detection was
145 measured taking into account a signal-to-noise ratio of three.
146
147
148 Radical mediated phenol decomposition in the presence of isopropyl alcohol (IPA) and
149 chloroform (CF) as scavengers
150 Radical mediated decomposition of phenol was achieved using an Advanced Oxidation
151 Process (AOP) methodology, which is based in the application of Fenton reaction
152 mechanism. In order to verify the effect of radicals, some scavenger molecules like
153 isopropyl alcohol (IPA) and chloroform (CF) were used.
154 The system components, Fenton-Phenol-scavenger, were mixed by a continuous stirring
155 in the next concentrations: phenol 50 ppm (Fluka), H2O2 (Merck), 1 mM, Fe(SO4) •
156 7H2O (as source of Fe (II) ions) (Merck), 0,5 g/L and 250 mM of IPA (Sigma Aldrich)
157 or 2 mM of CF (Merck), depending on each case. All the reaction assays were
158 performed at 25ºC, pH 3 in a total volume of 50 mL of water. Fenton reaction was
159 stopped at basic pH using a phosphate buffer at pH 8 [18].
160
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161 Experimental determination of Activation energy
162 In order to measure the activation energy for phenol decomposition into the different
163 intermediates HQ, CT and RC, the components Fenton-Phenol were mixed by a
164 continuous stirring using phenol 50 ppm (Fluka), H2O2 (Merck) 1 mM and Fe(SO4) •
165 7H2O (Merck) 0,5 g/L. All the assays were performed at pH 3 in a total volume of 100
166 mL of water, the reactions were stopped after one minute using a phosphate buffer at
167 pH 8 and then they were analyzed by HPLC. The experiment was performed at different
168 temperatures (5, 15, 25, 35 and 45 °C).
169
170 In order to estimate the activation energies, the initial velocities method was used,
171 where Ea was measured by the Arrhenius equation as follows:
172
Ea
173
174
Log V0  Log A -
(Equation 1)
RT
175 Where R is the ideal gas constant, T is the absolute experimental temperature and Ea is
176 the activation energy, A is a constant and V0 is the initial velocity of the reaction [19].
177
178 Computational Studies on Phenol and its byproducts
179 Phenol local reactivity
180 In order to determine the reactivity of phenol, local theoretical descriptors in the
181 framework of density functional theory (DFT) were used [20, 21]. The Fukui function
182 (adjusted to radicals) was used according to the following equation[22]:
183
184
(Equation 2)
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185
186 Where
corresponds to the electron density of the N-electron species when the
187 system gains an electron (anion), and
when it loses an electron (cation). In
188 order to obtain the electron density for a given region at the molecule, it is necessary to
189 divide the molecular space with subsequent integration of the Fukui function on those
190 regions. To do so, Tiznado et al. proposed a methodology to condense the Fukui
191 function to regions using the following expression [23, 24]:
192
193
(Equation 3)
194 Where the ωk region is obtained from the topological analysis of the Fukui function, and
fk0 is the condensed value of the Fukui function in that specific region [25].
195
196 All calculations were performed with the B3LYP [26, 27] method using the 6-311+(d,
197 p) basis set [28] as available in Gaussian 09 electronic structure software [28-30].
198
199 Characterization of Transition States for Phenol Decomposition Reactions
200
201 Transition state search was performed at an unrestricted DFT level of theory [31] using
202 the LST (Linear Synchronous Transit) method. All geometries (reactants, transition
203 states and products structures) were fully optimized at the B3LYP/6-31G (d) [26, 27]
204 level of theory. By performing frequencies analysis, the character of each stationary
205 point was evaluated, which means that geometries with only positive frequencies
206 (eigenvalues) correspond to a minimum and geometries with only one imaginary
207 frequency should correspond to a transition state geometry. The motion of the main
208 vibrational mode corresponded to the desire reaction pathway. The solvents effects were
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209 introduced by a polarizable continuum model (PCM) based on both, the solute and its
210 electron density and the solvent as a continuum dielectric medium [32].
211 To check if the transition states were properly located, an intrinsic reaction coordinate
212 algorithm (IRC) was used [33] that is defined as the steepest descent (SD) curves
213 connecting a first-order saddle point (the transition structure) with a minimum, which is
214 characteristic of a stationary point (corresponding to either reactants or products). IRC
215 parameters used were: MaxPoints = 80, maxcycle = 300 and stepsize = 2.
216
217
218
219 RESULTS AND DISCUSSION
220 Decomposition of phenol mediated by OH radicals
221 With the aim to show that decomposition of phenol by AOP process, using H2O2 and Fe
222 (II) ions in a Fenton reaction, is dependent on the OH radical concentration; the phenol
223 decomposition was measured in presence of two scavengers: Isopropyl alcohol (IPA)
224 and chloroform (CF). IPA is specific for hydroxyl radicals (OH•) while CF is specific
–•
225 for superoxide radical (O2 ).
226 Phenol decomposition was measured in three different times (10, 20 and 30 minutes) at
227 25 ºC and using 6 different experimental conditions: a) Only H2O2 (unmediated by
228 radicals), b) H2O2 + IPA, c) H2O2 + CF, d) Fenton, e) Fenton + IPA and f) Fenton + CF
229 (see Figure 1). Overall, it was found that phenol decomposition by H2O2 barely reached
230 the 12% at 30’. When scavengers were added, there was only a 2% decrease in phenol
231 decomposition at 30’ using IPA and less of 1% using CF, suggesting that H2O2 is unable
232 to spontaneously decompose on radical species, which may explain the overall low
233 decomposition of phenol under those initial experimental conditions. By using a Fenton
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234 system, it was found that overall decomposition of phenol increases significantly at
235 different measuring times, reaching about 30% in the first 10 minutes. Since it is well
236 known that catalysts based in transition metals are able to accelerate the decomposition
237 of H2O2 into radical species [9, 34], this last result confirmed that radicals are the
238 mainly species involved in decomposition process of phenol. When IPA and CF
239 scavengers were added into experiment d) (H2O2 + Fe (II), Fenton), two phenomena
240 could be observed: By adding CF, the overall phenol decomposition process did not
241 show a marked decrease in the percentage of inhibition (barely 2%), which roughly
242 means that the superoxide radical could not be responsible of the phenol decomposition
243 process. On the other hand by adding IPA as scavenger, and comparing experiments d
244 and e, it is observed that the yield of decomposition is decreased about 3 times at all
245 measured times, which could indicate that the OH radical is the key chemical specie
246 that drives the phenol decomposition process. The effect of CF in phenol decomposition
247 (experiment f) was lower only about 6% when compared with experiment d, which
248 could indicate that the effect of superoxide radical in these conditions is not relevant
249 compared to the hydroxyl radical.
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250
251
252
Figure 1. Bars representation of phenol decomposition percentages measured at different times
and using several experimental conditions.
253
254
255
256 Phenol decomposition and characterization of its intermediates
257 The concentrations (in ppm) of three different intermediates (CT, RC and HQ), from
258 H2O2 induced phenol decomposition, were measured during time at two different
259 conditions: in presence and absence of Fenton reactant, now on called F and wF,
260 respectively. The experiments were carried out at 25º C and pH 3 (Figure 2). It was
261 found that HQ is the main intermediate produced in the experimental degradation
262 assays, and moreover its production was enhanced by the presence of Fenton reactant
263 (more than two times with respect to the non catalytic process). The other main
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264 products determined in phenol degradation reactions were CT and RC, which is in
265 agreement with previous literature [35]. It is important to note that the amount of HQ
266 produced is about 5 times greater than RC, both in presence and absence of the Fenton
267 system. The amount of CT is produced 3 to 4 times greater than RC in the presence of
268 Fenton system.
269
270
271
272
273
274
Figure 2. Concentration (in ppm) of intermediates CT, RC and HQ during induced
decomposition of phenol. The experiments were carried out in presence (F) and absence (wF) of
Fenton process
275 Experimental determination of Activation energies
276 In order to explain the differences in the amount of each intermediate produced during
277 the decomposition of phenol, the activation energy (E ) for each reaction was measured.
a
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278 Using different residence times and temperatures, E was estimated for the three
a
279 reaction mechanisms (Scheme 1) based on the slopes calculated from plots reported in
280 Supplementary Material (Figure 1S, Figure 2S and Figure 3S). It was found that the
281 production of RC presented the highest E with a value of 2,534 kcal/mol. On the other
a
282 hand, the production of CT and HQ exhibited lower activation energies (1,174 and
283 0,855 kcal/mol, respectively).
284 These E differences could be explained by the electron density properties of phenol,
a
285 which indicates that its m-carbon presents a lower electron density while the o-carbon
286 and p-carbon present a higher electron density. It has been reported the appearance of
287 HQ and CT in electrochemical oxidation processes of phenol, and moreover these two
288 intermediates were detected in phenol oxidation processes in the presence of hydrogen
289 peroxide and acetone [13, 36].
290
291 DFT Reactivity Indexes Analysis
292
293 As a previous step, before the theoretical characterization of the reaction mechanisms of
294 phenol decomposition, the electron density of phenol was studied by measuring its
295 Fukui local indexes for a radical reaction.
296 According to the radical Fukui index (FI) analysis (Figure 3 and Table 1), it was found
297 that o- and p-carbons are more susceptible to a radical attack, by the first hydroxyl
298 radical, than the m-carbon (FI values: ortho = 0.0387-0.0387 meta = 0.0251-0.0249 and
299 para = 0.1241). It is worth mentioning that this type of method is useful to find
300 regioselectivity of molecules against radicals, but it does not discriminate the type of
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301 radical. We found that, according to FI, the most favored intermediaries are
302 hydroquinone (para) and catechol (meta).
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
Figure 3. Ball and sticks representation of phenol molecule that represents its Fukui local
indexes at each numbered carbon atom (according to Table 1). In yellow, red and grey are
represented the carbons, oxygen and hydrogen atoms, respectively. The electron density
obtained by the Fukui analysis is represented on green.
Table 1. Carbon atoms in phenol molecule and their radical Fukui index values.
Carbon atom Fukui index (FI)
1
2
0,0848
0.0387
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3
4
5
6
0.0251
0.1241
0.0249
0.0387
323
324 Characterization of transition states geometries and phenol decomposition reaction
325 paths.
326 A single imaginary frequency (lower than -90 cm-1) was found for each of the transition
327 state species calculated both in gas or solvent phase (see Table S1).
328
329 OH radical addition as first reaction step
330
331 The oxidation of phenol molecules is mediated by a two-step reaction mechanism,
332 which comprises the attack of electrophilic radicals (OH radicals) at three different
333 positions in the phenolic ring. While the first step has been described both on gas and
334 solvent [8, 17], to the best of our knowledge, there are not theoretical studies related to
335 the hydrogen abstraction step neither in gas or solvent phase.
336 The reactants, TSs and products structures, as well as their corresponding activation
337 energies, for the first reaction step in each possible phenol decomposition reaction are
338 presented in Figures 4 and 5, for gas and solvent phase, respectively. The oxygen atom,
339 belonging to the hydroxyl radical, was located around 2.02 – 2.05 Å with respect to the
340 corresponding carbon atom (position ortho, meta and para) in each TS structure (Table
341 S2 and S3). The addition of the first OH radical (on the 3 different systems) showed an
342 energy barrier from 0,504 to 1,573 kcal/mol. According to previous reports, some
343 reactions between phenols and electrophilic compounds are in the order of the 4
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344 kcal/mol [8, 17, 37]. Based on those results, the first reaction step that showed the
345 lowest E in decomposition of phenol is favored on the p-carbon, which in turn is in
a
346 agreement with our experimental data that reported more production of the HQ
347 intermediate. The E values for CT and RC decomposition products are higher in
a
348 energy and, thereby, their production would be disfavored.
349
350
351
352
Figure 4: Reactants, TSs and products structures for step 1 in the gas phase, with their relative
energies, are shown. Atom types are: Carbon (yellow), oxygen (red) and hydrogen (cyan).
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353
354
355
Figure 5: Reactants, TSs and products structures for step 1 in solvent phase, with their relative
energies, are shown. Atom types are: Carbon (yellow), oxygen (red) and hydrogen (cyan).
356
357 Hydrogen Abstraction, mediated by OH radicals, as the second reaction step
358
359 With respect to the second reaction step, the transition state geometries showed that a
360 second OH radical was located at 2.37 – 2.41 Å with respect to the carbon atom at the
361 corresponding position in phenol ring. The oxygen atom from OH radical was located at
362 a distance between 1.30 – 1.34 Å with respect to the leaving hydrogen atom (see Table
363 S1 in Supplementary Information). According to IRC energy profiles for second
364 reaction step in gas phase, that means the leaving of H atom mediated by a second OH
365 radical (Figures 6 and 7), it was found that phenol decomposition reaction that showed
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366 the highest E values corresponds to the H-abst onto the m-carbon that produces RC
a
367 (25.893 kcal/mol), followed by the reaction occurring onto the p-carbon (22.137
368 kcal/mol) that produces HQ and finally by the reaction occurring onto the o-carbon
369 (19.131 kcal/mol) that produces CT. Those calculated E values are not in fully
a
370 agreement with our experiments, however they could explain the fact that the
371 intermediate RC showed the lowest concentration during the phenol decomposition
372 reaction studied through our experiments. In order to fully understand the correct
373 reactivity trends in the production of RC, CT and HQ intermediates, subsequent
374 estimation of IRCs including the solvation effect were performed.
375
376 Since this second reaction step occurs on water, the E values were also estimated in
a
377 presence of a continuum dielectric field that represents an implicit water solvent. With
378 respect to the OH addition, the final transition state structures showed distances,
379 between the OH radical oxygen atom and the carbon atom in phenol ring, ranging from
380 2.01 to 2.12 Å (Table S1). When comparing the E values for this first reaction step
a
381 calculated in implicit solvent, against those obtained in gas phase, it can be seen that
382 they decreased for all three reactions in the presence of water as solvent. This
383 phenomenon could be explained by the fact that in organic reactions the electrostatic
384 component of the solvent acts as a catalyst reducing the energetic barrier of the reaction
385 [38] even in the case of reactions that involves free-radicals [39]. It has been reported
386 that E is affected by the degree of polarity of the solvent in reactions of polymerization
a
387 [40] and polar organic solvents tend to decrease E in enzymatic systems [41].
a
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388
389
390
Figure 6: IRC profile with relative energies representing the second step intermediates, in the
oxidation of phenol, calculated in gas phase. R (Reactant), TS (Transition State), P (Product).
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391
392
393
394
Figure 7: IRC profile with relative energies representing the second step intermediates, in the
oxidation of phenol, calculated in solvent phase. R (Reactant), TS (Transition State), P
(Product).
395
396 Regarding to the H-abst, the transition state structures showed distances, between the
397 OH radical oxygen and the carbon atom, that were between 2.06 – 2.63 Å. Likewise, the
398 distances between the OH radical oxygen and the leaving hydrogen atom showed values
399 between 1.55 – 1.74 Å (Table S3).
400 The calculated E values for H-abst in solvent, when compared with their counterpart
a
401 values in gas phase systems, showed a marked decrease in all cases (for instance a
402 marked decrease of ~11 kcal/mol could be seen in the case of the H-abst onto the p-
403 carbon), This seems reasonable due to the fact that all these kind of reactions occurs,
404 and are favored, on water [25, 26]. Then, the determined solvent E values for the
a
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405 reaction giving RC as product confirmed us that its low production could be due to the
406 high-energy barrier of the process. With respect to the other phenol derivatives, it was
407 observed that in presence of the electrostatic component of the solvent, the E values for
a
408 production of HQ were lower than the ones for CT (11.42 and 12.99 kcal/mol,
409 respectively) indicating now the correct trend found in our experimental results, that is:
410 during phenol oxidation HQ is the most abundant phenol derivative, followed by CT
411 and finally RC (that is barely produced in vitro).
412
413 CONCLUSIONS
414
415 The decomposition of phenol, by using the oxidizing agent hydrogen peroxide, was
416 studied using HPLC and DFT approaches. It was found that decomposition increased
417 about 5 times in the presence of Fe (II) in a homogeneous Fenton system. Thus,
418 decomposition of phenol in these conditions seems to be highly dependent on hydroxyl
419 radicals, because in the presence of scavenger IPA the performance of reaction
420 decreases to the half when using catalyst. Regarding the use of CF as scavenger, the
421 effect in lowering the performance of decomposition reaction is subtle. So, we suggest
422 that primarily hydroxyl radicals would carry out the decomposition of phenol in the
423 studied conditions. Generating a high amount of hydroxyl radicals permits efficient
424 decomposition of a variety of harmful substances because it is one of the strongest
425 oxidizing described [42, 43]. Further analysis are required to validate our hypothesis (i.e
426 spin radical experiments), but we believe that our protocol suggest an implication of
427 hydroxil radicals on phenol decomposition.
428
429
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430 On the other hand, the two-step phenol decomposition reaction mediated by OH
431 radicals was successfully described by both theoretical and experimental studies.
432 According to our experimental approach, HQ was the derivate produced in larger
433 quantities, followed by CT and finally RC. From the in silico approach, it was found
434 that the activation energy profile for each reaction, estimated on a continuum solvation
435 model, showed the same trend. Finally, from experimental and theoretical approaches, it
436 was possible to determine that during this phenol decomposition reaction mechanism,
437 the intermediates HQ and CT were the main decomposition products, which is in
438 agreement with previous reported studies. The HPLC method proved highly efficient
439 for quantifying phenol and related compounds, in addition to the use of a fluorescence
440 detector that had a high sensitivity for products generated even in concentrations of
441 parts per billion.
442 To the best of our knowledge, there are no previous reports of a theoretical
443 characterization, through IRC method, of intermediates participating in the second step
444 of phenol oxidation mechanism mediated by hydroxyl radicals.
445
446 ACKNOWLEDGMENT
447 The authors thank to “Centro de Bioinformática y Simulación Molecular” at
448 Universidad de Talca for providing the computational time and resources to
449 successfully accomplish all computational simulations reported in this work. C.N-R
450 thanks Government of Chile for a doctoral fellowship awarded through CONICYT Nº
451 21120691. C.V. thanks Universidad de Talca and Pacific Chemical for a doctoral
452 fellowship through the Applied Sciences Ph.D. program.
453
454
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