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Electrochemical Oxidation of Phenolic Compounds at Borondoped Diamond Anodes: Structure-Reactivity Relationships Yi Jiang, Xiuping Zhu, and Xuan Xing J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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The Journal of Physical Chemistry

Electrochemical Oxidation of Phenolic Compounds at Boron-doped Diamond Anodes: Structure-Reactivity Relationships

Yi Jiang1,4*, Xiuping Zhu2,4, Xuan Xing3,4 1

Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, United States

2

Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803, United States 3

College of Life and Environmental Sciences, Minzu University of China, Beijing 100081, China

4

Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China

Submitted to Journal of Physical Chemistry A May, 2017

To whom correspondence should be addressed: Dr. Yi Jiang, E-mail: [email protected], Phone: +1-314-562-1830 1 ACS Paragon Plus Environment


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Abstract Electrochemical oxidation of phenolic compounds using boron-doped diamond (BDD) anodes has been shown as an effective approach to remove these contaminants from water. However, the understanding of the reaction mechanisms of substituted phenolic compounds at the BDD anode remains incomplete. In the present work, we investigated the electrochemical oxidation of 12 representative phenolic compounds (with varied substitution groups (e.g., -CH3, -OCH3, -NH2, Cl, -OH, -COOH, -NO2, -CHO) and positions (-ortho, -meta, and -para)) at the BDD anode. Our analysis show that unlike previous studies, the two parameters, the Hammett constants of the substituents and the highest atomic charge on the aromatic ring, fail to adequately describe the reaction rate change when the chemical structures become complicated (i.e., with increased steric effects). Instead, a quantitative structure-property relationship (QSPR) was established with 26 molecular descriptors and using a partial least squares regression approach. The QSPR analysis shows that the energy gap between the lowest unoccupied molecular orbital and the highest occupied molecular orbital, ELUMO-EHOMO, which reflects the chemical stability of a molecule, is the predominant molecular descriptor determining the reaction rate constant. Furthermore, the predicated rate constants agree well with the observed ones. The findings are consistent with previous studies of SnO2 anodes, suggesting chemical structural parameters such as the molecular orbital energies are critical to consider when elucidating and predicating the electrochemical reactivity of phenolic compounds at these non-active anodes.

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Introduction Phenolic compounds are produced in various industrial processes, including dyes, textiles, coking, pharmaceuticals, and pesticides, among others.1-5 While many phenolic compounds are listed in the Toxic Pollutant List by the US EPA, they cannot be effectively removed from wastewaters through conventional biological processes due to their biological recalcitrance. To address this challenge, electrochemical advanced oxidation technology has been developed and demonstrated.6-10 With this technology, various radicals, especially hydroxyl radicals that have strong oxidizing power, are produced to react with pollutants. The critical component in this electrochemical system is the electrode, with some of the most popular ones being Pt, RuO2, IrO2, SnO2, PbO2, and boron-doped diamond (BDD). In particular, electrochemical oxidation at the BDD anode, due to its remarkable mineralization ability, wide potential window, strong anticorrosion stability, and low surface adsorption, has attracted significant attention and been extensively studied.11-14 BDD films are grown by chemical vapor deposition from a variety of carbon-containing precursors, whereby doping boron (with a B/C ratio of about 10-5 to 10-3) creates a p-type, more conductive semi-conducting character to diamond.15 This feature renders BDD both indirect and direct oxidation reaction regimes under different applied potentials.16 The direct oxidation occurs in the potential region before oxygen evolution (i.e., water stability), and the indirect oxidation takes place in the potential region of oxygen evolution (i.e., water decomposition), which mainly involves reactions of electrogenerated hydroxyl radicals.16-17 The indirect oxidation pathways have been proven very effective to degrade organics into CO2 and H2O, including phenolic ones.6, 13, 16, 18-20



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One area of research focus has been understanding the reaction mechanisms of substituted phenolic compounds at the BDD anode. This has been partially achieved through comparing the reactions of structurally similar compounds at the anode, regarding reaction rates and pathways. Flox et al. revealed that o-, m- and p-cresol have a similar minerization rate at the BDD anode, suggesting the negligible role of the position of the methyl substitution.21 We however found that for mono-substituted nitrophenols, the degradation rate followed: o-nitrophenol > p-nitrophenol > m-nitrophenol > phenol.19 Further, Cañizares et al. showed the release of chloro- or nitro- group was the first step in the degradation of nitro- or chloro- substituted phenols.18 We further confirmed that the release of p-substituted groups from the aromatic ring was the rate-limiting step.16 Overall, these studies have shown that the substituents and positions would have a major impact on the electrochemical reactivity and subsequent reaction pathways, and electrophilic attack of hydroxyl radicals is the main reaction mechanism at the BDD anode. A quantitative understanding has been attempted by correlating the reaction rate constants (or reaction rates) with the Hammett constants (σ) of the substituents.16, 19-20 The Hammett constants, representing the electron-donating or withdrawing character of the substituents, were shown to have an overall linear relationship with the reaction rates of simple phenolic structures, such as the p-substituted phenols16 and nitro-substituted phenols.19 Further, to elucidate the reaction pathways, we identified the carbon atom(s) on the aromatic ring with high atomic charge (electron-rich) as the active site(s) for the electrophilic attack of hydroxyl radicals produced at the BDD anode.19-20 This hypothesis has been confirmed by analyzing the reaction intermediates.19-20 These studies were among the first ones to explain reaction rates and pathways based on available/calculated structural parameters at the BDD anode. However, this knowledge has been very limited due to relatively simple chemical structures (mostly mono-substituted


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phenols). For phenolic compounds with more complicated chemical structures, where the steric effect plays a significant role, such information remains incomplete. In this work, we investigated the electrochemical oxidation of 12 substituted phenolic compounds at the BDD anode. Their degradation kinetics were fitted by pseudo first-order kinetics and correlated with two structural parameters, the Hammett constants of the substituents and the highest atomic charge on the aromatic ring. Followed by that, we calculated the quantum chemical molecular descriptors using a semi-empirical approach (PM 3), and performed a quantitative structure-property relationship analysis. The study shows that either the Hammett constants or the carbon atomic charge cannot adequately correlate with the electrochemical reactivity, due to their fragmentary role in describing the whole chemical structure. The QSPR analysis, on the other hand, identifies the energy gap ELUMO-EHOMO to be the most important molecular descriptor that determines the reaction rate constant. The predicated rate constants agree well with the observed ones, suggesting the applicability of the structure-reactivity model in explaining and predicting the rate constants. This study deepens our understanding on how chemical structures affect the electrochemical reactivity of phenolic compounds at non-active anode such as the BDD anode.

Materials and Method Chemicals. 12 phenolic compounds, including phenol (Ph), o-nitrophenol (o-NO2), mnitrophenol (m-NO2), p-nitrophenol (p-NO2), resorcinol (m-OH), p-hydroxybezonic acid (pCOOH), p-hydroxybezaldehyde (p-CHO), p-cresol (p-CH3), p-methoxyphenol (p-OCH3), paminophenol (p-NH2), p-chlororesorcinol (p-Cl+m-OH), and nitrocatechol (p-NO2+o-OH) were purchased from Beijing Chemical and used as received. All chemicals were analytical grade and 5 ACS Paragon Plus Environment


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used without further purification. Solutions were prepared using deionized Milli-Q water (Millipore). The BDD electrode was purchased from CONDIAS GmbH, German. Bulk Electrolysis. For bulk electrolysis, a BDD electrode with a working geometric area of 4 cm2 was used as the anode, while a stainless steel plate with the same area was used as the cathode. The distance between the two electrodes was set to be 10 mm. The experimental conditions followed those in our previous work.19 The bulk electrolysis was performed in a beaker reactor under galvanostatic conditions, with a current density of 20 mA cm-2 and at ambient temperature (25 oC). The initial concentrations of the phenol solutions were 1 mM, with 0.2 M Na2SO4 as the supporting electrolyte. The pH was adjusted to be 11. The 250 mL solution was stirred by a magnetic stirring bar during electrolysis to promote mass transfer. Samples were collected at each time interval (40 min) and stored in 4 °C if immediate analysis was not performed. All the experiments were carried out in duplicate. Before experiments started, BDD electrode was subjected to ultrasound for 5 min to remove contaminants and then washed with deionized water; stainless cathode was polished and then washed with deionized water. Chemical Analysis. The concentration of these phenols was measured by high performance liquid chromatography (HPLC) with a ZORBAX SB-C18 column and a DAD detector (Agilent HP1100).19 The mobile phase was methanol/water (50:50), and the flow rate was 1.0 mL min-1. The wavelength for UV detection was 314 nm for p-nitrophenol, 210 nm for p-nitrocatechol, and 280 nm for other phenols. Molecular Descriptors Calculation. Unlike the ab initio molecular orbital calculations, semiempirical methods start with the general form of ab initio Hartree-Fock calculations, but make approximations for the various Integrals, thus being more convenient yet with satisfactory results. Here we used the PM3 semi-empirical method (Parameterized Model number 3), by which 6 ACS Paragon Plus Environment

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electrons are included explicitly and many of the detailed integrals are replaced by empirical parameters.22 The quantum chemical calculation was performed with the software WinMopac (Ver. 7.20, Fujitsu Co. Ltd), with the following keywords: PM3, EF, ESP, POLAR, DIPOLE, BONDS, ENPART, PRECISE, NOINTER. In total, 26 descriptors were calculated as shown in Table 1. The detailed calculation results were provided in the Supporting Information. Partial Least Squares (PLS) Regression Analysis. Partial least squares (PLS) regression is a widely used technique that generalizes and combines features from principal component analysis and multiple regression. PLS regression is particularly useful when the matrix of independent variables has larger sizes than observations. This technique has been used in a number of previous studies to construct a quantitative structure-property relationship between rate constants (observed Y variables) and molecular descriptors of organic compounds.23-25 The general underlying model of multivariate PLS can be described as follows: X = TPT + E

(Eqn. 1)

Y = UQT + F

(Eqn. 2)

Where X is an n × m matrix of predicators, Y is an n × p matrix of responses, T and U are n × l matrices that are projections of X and Y respectively (factor matrix), P and Q are m × l and p × l orthogonal loading matrices, and matrices E and F are the error terms. The decompositions of X and Y are made so as to maximize the covariance between T and U. The PLS regression analysis was performed using software OriginPro 2016, with an in-built leave-one-out cross validation method (OriginLab Corporation).

Results and Discussion



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Bulk Electrolysis. Figure 2a shows that the concentrations of the 12 phenols decreased with the electrolysis time, but to different degrees. Among all phenols, p-NO2+o-OH (p-nitrocatechol) degraded at a fastest rate while p-COOH at the slowest rate. At the end of the 4 h electrolysis, oNO2 and p-NO2+o-OH were completely removed, while p-COOH has the most residue. The remaining quantity of the phenols followed this order: p-COOH (0.49 mM) > p-CH3 (0.45 mM) > p-OCH3 (0.44 mM) > Ph (0.42 mM) > p-CHO = m-OH (0.35 mM) > p-NH2 (0.31 mM) > pCl+m-OH (0.13 mM) > m-NO2 (0.11 mM) > p-NO2 (0.08 mM) > o-NO2 (0.02 mM) > p-NO2+oOH (0 mM). The degradation of all phenols was fitted to pseudo first-order kinetics (Figure 2b) and the apparent reaction rates were obtained (Table 2). The degradation meets pseudo firstorder kinetics (R2 = 0.950-0.999), revealing a mass transfer-controlled process. The pseudo firstorder kinetics are also consistent with previous observations of electrochemical degradation of nitrogen-heterocyclic compounds at the BDD anode20 and phenols at the SnO2 anode.24 The calculated reaction rate constants show that the fastest rate constant (p-NO2+o-OH, k = 0.0200 min-1) can be almost one order of magnitude higher than the slowest (p-COOH, k = 0.0029 min1

). Take phenol as a benchmark, almost all have higher reaction rate constants, except p-COOH,

p-CH3, and p-OCH3. Correlations between Reaction Constants and Hammett Constants. The Hammett constant was firstly developed from linear free energy relationships (LFERs) as a quantitative measurement of the effect of structural moieties on the electronic character of a given aromatic system.26 A positive value of the Hammett constant indicates an electron-withdrawing group, while a negative value indicates an electron-donating group. However, due to its nature in describing the electronic effects, the Hammett constants usually can only be applied to para- and meta- substitution positions where the steric effects are minimal. Overall, the phenols with


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electron withdrawing substituent (e.g., -NO2 and -Cl) have higher reaction rates than those with electron-donating substituents (e.g., -CH3 and -OCH3 groups). We here correlated the reaction rate constants and Hammett constants of the para- and/or meta- substituted phenols through the Hammett equation:26

log ( ) = (∑ )

(Eqn. 3)

Where k and kH are the reaction rate constants of un-substituted and substituted compounds respectively, ρ is the susceptibility factor, and ∑σ is the sum of the Hammett constants. For all meta- and para- substituted phenols, there exists a poor correlation between the rate constants and the Hammett constants (R2 = 0.40, Figure 3a). However, after excluding two outlier data points for p-COOH and p-NH2, there appears to be a linear relationship between the rate constants and the Hammett constants (R2 = 0.92, Figure 3b). The rate constant for p-COOH appeared lower and the rate constant of p-NH2 was higher than what was expected from the Hammett constants. For the degradation of p-COOH, 1, 4-hydroquinone, 1, 4-benzoquinone, and 3, 4-dihydroxybenzoic acid were observed as intermediates, when the hydroxyl radical was the predominant oxidizing species present.27 The slow reaction rate for p-COOH was likely due to the production of polymeric materials from those intermediates that rapidly decreased electrode activity (electrode fouling).28-29 On the other hand, for the electrochemical oxidation of p-NH2, a concurrent hydrolysis reaction producing hydroquinone was observed before,30 which resulted in a fast disappearance rate for p-NH2 in this case. The limitation of using Hammett constants is the difficulty to describe the orthosubstitution, where the steric effect comes into play. As a result, o-NO2 and p-NO2+o-OH were not able to be taken into analysis. Further, the Hammett constants lack the consideration for



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intramolecular interaction (e.g., the formation of hydrogen bond bridges, tautomerism, and saline bridge),24 thus further limiting its applicability and accuracy. Correlations between Reaction Rate Constants and Carbon Atom Charges. The electrophilic attack of hydroxyl radicals was revealed to be the predominant reaction mechanism at the BDD anode.12, 16, 19 Accordingly, the electron density distribution is expected to have an impact on the reaction rate and the production of intermediates. We have previously revealed that through the calculation of electron density distribution, the reaction active sites were identified as those with abundant carbon atom charge, which was confirmed by subsequent intermediate analysis using HPLC and GC-MS.19-20 Through theoretical calculation by WinMOPAC, the highest carbon atom charges on the aromatic ring were obtained (Figure 1). Correlations between the reaction constants and highest carbon atom charge on the aromatic ring were presented in Figure 4. From Figure 4a, for the seven p-substituted phenols except p-NH2 and p-COOH, the reaction rate constant has a linear relationship with the highest (most negative) carbon atom charge (R2 = 0.860). Further, for meta- and para- substituted phenols, the linear relationship remained (R2 = 0.765), although there appears an appreciable deviation for m-NO2. When considering all the 12 phenols, the correlation coefficient continued to decrease to 0.673. Overall, the trend of decreasing correlation coefficients indicates that the highest atom charge does play a significant role in determining the electrochemical degradation rate of substituted phenols (R2 = 0.673 – 0.860), but the steric effect becomes significant in more complicated structures (e.g., multiple substituted phenols). As a result, when complicated by both significant electronic and steric effects, it is difficult to use one single parameter, highest carbon atom charge on the aromatic ring, to explain the differences of electrochemical activities of different substituted phenols at the BDD anode. More sophisticated methods, such as establishing a


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quantitative structure-property relationship that considers parameters derived from whole molecules, are needed to offer a better explanation of the electrochemical reactivity. Quantitative Structure-Property Relationship Analysis. The PLS analysis has shown two principle components lead to the minimum root mean of the prediction error sum of squares (PRESS, min = 0.6372). The two principle components contribute to 61.9% and 87.3% of cumulative X and Y variance, respectively. The VIP (variable importance in the projection) values were used to reduce the number of variables in the model (Table S2). Usually, a descriptor (or X variable) with the VIP value smaller than 1 can be considered as unimportant and thus be excluded from the further QSRP modeling.24, 31 From Table S2, molecular descriptors with VIP >1, namely, Mw, u, EE, TE, CCR, q+, q-, ELUMO, (ELUMO-EHOMO), (ELUMOEHOMO)2, (ELUMO+EHOMO), qx, qc, and C were more important than others, and were used to construct the model. We obtained the relationship between ln(k) and the structural descriptors: ln(k) = -2.2263 + 0.0025Mw + 0.0435u + 2.4686 × 10-5EE+ 1.3175 × 10-4TE + 3.0262× 10-5CCR + 0.1021q+ - 0.2675q--0.0639ELUMO - 0.3000(ELUMO-EHOMO) - 0.0166(ELUMO-EHOMO)2 - 0.0197 (ELUMO+EHOMO) + 0.0733qx - 0.2769qc - 0.0224C

(Eqn. 4)

From the standardized coefficients in Table 3, the energy gap between the lowest unoccupied orbital and the highest occupied orbital, ELUMO-EHOMO is the most predominant molecular descriptor in determining the electrochemical reactivity, followed by the dipole moment u and the coulombic interaction energy of the two-center term for the carbon-substituent atom bond (C). According to the molecular orbital theory, EHOMO reflects the electron donating ability of one molecule when interacts with others, and electrons will be more easily shared with higher EHOMO. On the contrary, ELUMO denotes the electron accepting ability. Taken together, the energy



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gap ELUMO-EHOMO shows the energy needed for one electron to migrate from the highest occupied orbital to the lowest unoccupied orbital. As a result, ELUMO-EHOMO reflects the chemical stability of a molecule, with higher value showing more stability. The ELUMO-EHOMO is also defined as twice the chemical hardness, with hard molecules resisting electron transfer or rearrangement, thus being less reactive.32 The term therefore has a negative impact on the reaction rate constant as shown in the equation. In our previous study, we have been able to identify the role of the delocalization energy of a molecule on the degradation rate of nitrogenheterocyclic compounds, where the delocalization energy also reflects the chemical stability.33 Further, this crucial role of ELUMO-EHOMO has also been discovered in the photolysis of chlorinated biphenyls, in which the radical reaction with homolysis of the C-Cl bond was the main reaction mechanism.25 The dipole moment u is due to non-uniform distributions of positive and negative charges on the various atoms. As an indicator of polarity, the dipole moment reflects the electronic forces that each substituent is applying on the phenyl ring, to generate a particular electronic architecture that is suitable for electrochemical oxidation.24 The higher the dipole moment, the more uneven the electron distribution is. As a result, the molecules are more prone to electrophilic or nucleophilic attacks. The coulombic interaction energy of the two-center term for the carbon-substituent atom bond (C = EE2+EN2+NN2) includes three terms: the electronelectron repulsion energy of the two-center term for the carbon-substituent atom bond (EE2), the electron-nuclear repulsion energy of the two-center term for the carbon-substituent atom bond (EN2), the nuclear-nuclear repulsion energy of the two-center term of the carbon-substituent atom bond (NN2). The higher the interaction energy, the lower the reaction rate will be. The inclusion of the C term in the model indicates the degradation of substituted phenols in


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electrochemical process is associated with the destruction of carbon-substituent atom (N or Cl) bond.23 This was consistent with our previous experimental observations that the release of the substituent groups is likely the first step in degradation of p-substituted phenols.16 Overall, our QSPR model has been able to identify a similar set of molecular descriptors as a previous study did,24 which investigated the electrochemical degradation of phenols at the SnO2 anode. The set of molecular descriptors include ELUMO, EHOMO, and dipole moment, among others. This similarity can be attributed to similar reaction mechanisms of BDD and SnO2 anodes, where hydroxyl radicals are the dominant reaction species.12 This implicates that our models may be applicable to the electrochemical reactions at other non-active electrodes where hydroxyl radicals are predominant. Further, Figure 5 shows the predicated rate constants (calculated from Eqn. 4) agree well with the observed ones, suggesting the applicability of the structure-reactivity model in explaining and predicting the rate constants.

Conclusions Our results show that both the single use of Hammett constants and the highest carbon atom charge on the aromatic ring cannot accurately predicate the electrochemical reaction rate constants of phenolic compounds with complicated chemical structures at the BDD anode. They fail because only a fragment contribution was used to account for the observed response. A quantitative structure-property relationship was established between quantum chemical descriptors and the reaction rate constants. The generation of molecular descriptors from quantum molecular mechanics calculations was made considering the whole molecule, thus having considerably improved the accuracy of the model. Our model shows that the energy gap between the lowest unoccupied molecular orbital and the highest occupied molecular orbital,



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ELUMO-EHOMO, is the most important molecular descriptor affecting the electrochemical reactivity of phenols at the BDD anode. The results further confirm that electrophilic attack by hydroxyl radicals is the main reaction mechanism at the BDD anode. Further, as discussed in our earlier work,16, 19 the removal kinetics of the phenolic substrates were found to be different to that of the chemical oxygen demand (COD), symbolizing different production rates of intermediates. For example, the degradation kinetics of phenols with electron-withdrawing groups such as –NO2 (e.g., p-NO2, p-CHO, m-NO2, o-NO2), are usually faster than that of phenol (Ph), but they accumulate more intermediates than phenol during the electrolysis.16, 19 Efforts will be needed to further correlate such structure-reactivity models with the production of intermediates and final products, and thus to identify/prevent potential risks associated with the intermediates and final products.

Supporting Information Detailed values of the molecular structure parameters, variable importance in the projection (VIP) and regression coefficients from the PLS analysis were provided in the supporting information. This material is available free of charge at http://pubs.acs.org.

Acknowledgement This work was supported by National Natural Science Foundation of China (Grant NO. 20877001 and Grant NO. 51409285).


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33. Xing, X.; Zhu, X.; Li, H.; Jiang, Y.; Ni, J., Electrochemical oxidation of nitrogenheterocyclic compounds at boron-doped diamond electrode. Chemosphere 2011, 86 (4), 368-375.



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Figure 1. Chemical structures of studied phenols with carbon atom charge (marked in purple) calculated by the PM3 method


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Figure 2. (a) Evolution of phenols concentration with time during electrochemical oxidation at the BDD anode; (b) linear relationship between Ln(C0/ C) and electrolysis time.



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Figure 3. Correlations between reaction rate constants and Hammett constants of meta- and para- substituted phenols: (a) includes all nine phenols and (b) includes all phenols but p-NH2 and p-COOH.


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Figure 4. Correlations between reaction constants and highest carbon atom charge on the aromatic ring (a) p-substituted phenol; (b) meta- and para- substituted phenol; (c) all 12 substituted phenols



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Figure 5. Comparison between observed and predicated values of reaction rate constants



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Table 1. Molecular descriptors calculated by the PM3 method. The detailed calculation results were provided in the Supporting Information. Nomenclature Mw u ɑ HOF EE TE CCR q+ qELUMO EHOMO ELUMO - EHOMO (ELUMO - EHOMO)2 ELUMO + EHOMO BO qx qc EE1 EN1 J K EE2 EN2 NN2 C TE2

Molecular Descriptors molecular weight dipole moment average molecular polarizability heat of formation electron energy total energy core-core repulsion energy most positive net atom charge most negative atom charge energy of the lowest unoccupied molecular orbital energy of the highest occupied molecular orbital absolute hardness electron negativity bond order; for phenol, the weakest C-H was selected; for phenols with more than two substituents, the weakest C-X was selected (X is the substituent atom) net atomic charges on the substituent atom atomic charge of the carbon atoms connected with the substituent atom electron-electron repulsion energy of one-center term for the substituent atom electron-nuclear attraction energy of the one-center term for the substituent atom resonance energy of the two-center term for the carbon-substituent atom bond exchange energy of the two-center term for the carbon-substituent atom bond electron-electron repulsion energy of the two-center term for the carbonsubstituent atom bond electron-nuclear repulsion energy of the two-center term for the carbonsubstituent atom bond nuclear-nuclear repulsion energy of the two-center term of the carbonsubstituent atom bond Coulombic interaction energy of the two-center term for the carbonsubstituent atom bond, C = EE2 + EN2 + NN2 total of electron and nuclear energies of the two-center term for the carbon-substituent atom bond


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Table 2 Pseudo first-order kinetics of electrochemical degradation of phenols at BDD anode Phenols

k (min-1) Pseudo first order equation t1/2 (min)

R2

Ph

0.0036

ln(C0/Ct)=0.0036t

193

0.997

o-NO2

0.0146

ln(C0/Ct)=0.0146t

47

0.985

m-NO2

0.0102

ln(C0/Ct)=0.0102t

68

0.950

p-NO2

0.0093

ln(C0/Ct)=0.0093t

75

0.957

p-NO2+o-OH

0.0200

ln(C0/Ct)=0.0200t

35

0.991

m-OH

0.0046

ln(C0/Ct)=0.0046t

151

0.979

p-COOH

0.0029

ln(C0/Ct)=0.0029t

239

0.999

p-CHO

0.0042

ln(C0/Ct)=0.0042t

165

0.989

p-CH3

0.0034

ln(C0/Ct)=0.0034t

204

0.994

p-OCH3

0.0034

ln(C0/Ct)=0.0034t

204

0.998

p-NH2

0.0045

ln(C0/Ct)=0.0045t

154

0.972

p-Cl+m-OH

0.0087

ln(C0/Ct)=0.0087t

80

0.990



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Table 3. The variable importance in the projection (VIP) and regression coefficients from the PLS analysis (with VIP > 1 shown) Descriptor

VIP

Coefficient

Mw u EE TE CCR q+ qELUMO (ELUMO-EHOMO) (ELUMO-EHOMO)2 (ELUMO+EHOMO) qx qc C = EE2+EN2+NN2

1.1573 1.3586 1.1788 1.1917 1.1752 1.3224 1.2938 1.2777 1.2625 1.2558 1.1558 1.2895 1.2466 1.0596

0.0025 0.0435 0.0000 0.0001 0.0000 0.1021 -0.2675 -0.0639 -0.3000 -0.0166 -0.0197 0.0733 -0.2769 -0.0224


Standardized Coefficient 0.07042 0.10498 0.05627 0.06222 0.05491 0.08446 -0.07231 -0.06288 -0.13435 -0.13313 -0.03532 0.07288 -0.08771 -0.10408

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Table of Content Graphic


Electrochemical Oxidation of Phenolic Compounds ... - ACS Publications

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