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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Toward Catalytic Detoxification of Phosphotriesters: Insights from Kinetics and Docking on Solid Surfaces Lizandra M. Zimmermann, Gizelle I. Almerindo, Michelle Medeiros, Ricardo Ferreira Affeldt, Eduardo H. Wanderlind, Adriana P. Gerola, René A Nome, Milton A. Tumelero, Ricardo Faccio, Andre Avelino Pasa, Haidi D. Fiedler, and Faruk Nome J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09148 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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The Journal of Physical Chemistry

Toward Heterogeneously-Catalyzed Detoxification of Phosphotriesters: Insights from Kinetics and Theoretical Calculations Lizandra M. Zimmermann,† Gizelle I. Almerindo,ψ Michelle Medeiros,† Ricardo F. Affeldt,† Eduardo H. Wanderlind,†,* Adriana P. Gerola,† René A. Nome,,* Milton A. Tumelero,ζ Ricardo Faccio,ϕ André A. Pasa,‡ Haidi D. Fiedler† and Faruk Nome†,# # deceased †

National Institute of Science and Technology of Catalysis in Molecular and Nanostructured

Systems, Department of Chemistry, Federal University of Santa Catarina, Florianópolis, SC, 88040-900, Brazil. ψ

Escola do Mar, Ciência e Tecnologia, Universidade do Vale do Itajaí (UNIVALI), Itajaí, Santa

Catarina, SC, 88302-901, Brazil. 

Institute of Chemistry, State University of Campinas (UNICAMP), Campinas, SP, 13083-970,

Brazil. ζ

Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, 91501-970,

Brazil.

ACS Paragon Plus Environment

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ϕ

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Cryssmat-Lab and Centro NanoMat/CryssMat/Física - DETEMA - Facultad de Química -

Universidad de la República, 11800 Montevideo, Uruguay. ‡

Department of Physics, Federal University of Santa Catarina, Florianópolis, SC, 88040-900,

Brazil.


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ABSTRACT. A mixed Mg2+/Al3+ oxide featuring an incipient spinel phase was employed in the catalytic decomposition of a range of phosphate triesters, including the neurotoxic biocide chlorpyrifos methyl O-analog, with estimated catalytic effects up to 104-fold. The target reaction, namely, the transesterification with 1-propanol, was conveniently chosen as a means to convert toxic organophosphorus substrates into trialkyl phosphates structurally related to a family of flame retardants. Catalytic efficiency depends upon the stereo-electronic properties of the substrate, with both leaving group ability and geometric factors displaying pivotal effects on the rate constants. To evaluate the adsorption of 1-propanol/propoxide and the phosphotriester methyl paraoxon over the MgAl2O4 (100) surface, DFT calculations with periodic boundary conditions were performed, which showed that the most probable reactants` conformation prior to the reaction is such that propoxide is bound to a Mg2+ center with a juxtapositioned methyl paraoxon molecule at a close, neighboring Al3+ site.


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1. INTRODUCTION While phosphate mono- and diesters constitute natural chemical functions that participate in countless biological processes,1–4 phosphate triesters and related organophosphorus compounds find industrial applications, for example, as plasticizers, flame retardants and agrochemicals.5–8 Additionally, their toxicity to the nervous system, primarily due to the anticholinesterase activity, has also motivated the development of nerve gas agents, creating an actual scenario in which harmless stockpiles need to be rendered from herbicides, pesticides and nerve gases.7–11 But because

the

currently

employed

methodologies

for

the

destruction

of

neurotoxic

organophosphorus compounds, which include alkaline hydrolysis and incineration, present disadvantages such as high cost and formation of by-products, the academic community has been continuously focused on the development of novel processes for the safe detoxification of these compounds. In this sense, the use of catalytic systems is of particular interest, generally designed to accelerate a given specific substitution reaction at phosphorus, being either a solvolysis reaction or the reaction with nucleophilic reagents. In this broad scenario, researchers have evaluated the catalytic propensities of a wide range of systems in the decomposition not just of organophosphorus compounds, but also of related phosphate esters, whose structures and reactivities resemble those of some nerve agents, thus giving insights as simulants of the targeted compounds. For example, there has been a number of recent examples of catalytic systems for a variety of nucleophilic

substitution

reactions

in

phosphate

esters

in

homogeneous,12–17

microheterogeneous18–25 and heterogeneous26–29 media. This collection of data available in the literature suggests that the best catalytic effects arise from the combination of several factors including, for example, the presence of at least one metal ion center. This has recently led us to


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investigate the catalytic effects promoted by mixed Mg2+-Al3+ oxides in the decomposition of the organophosphate methyl paraoxon (dimethyl p-nitrophenyl phosphate, DMPNPP) by means of its transesterification with 1-propanol, Scheme 1.27 Among the series of oxides evaluated, the best catalytic activity was observed for the one featuring the incipient spinel phase. The term “incipient spinel phase” defines the solid in which there exists a ternary oxide of general formula AB2O4, where A stands for a divalent metal center and B, a trivalent one, the first constituting a tetrahedral site and the latter, an octahedral one. The incipient MgAl2O4 catalyst promoted a rate constant increase of 2.5×105-fold in the transesterification of DMPNPP with 1-propanol when compared with the substrate spontaneous propanolysis, surpassing the catalytic effect promoted by some of the simple oxides that feature either aluminum or magnesium centers only. Thus, this result is very important because the solid catalyst proved to be highly efficient in the detoxification of DMPNPP, most probably due to the presence of two Lewis acid sites (Mg2+ and Al3+) in the catalyst, in a reaction that results in the formation of a product structurally related to an important family of flame retardants.30 Scheme 1. Propanolysis of methyl paraoxon (DMPNPP) in the presence of the incipient MgAl2O4 spinel.27

Significantly, the MgAl2O4 spinel catalyst also presented high surface area and pore diameter greater than 70.4 Å, which facilitates adsorption and the high activity shown by the active sites of MgAl2O4 spinel on the degradation of DMPNPP. Based on the facts described above, we decided to examine the catalytic activity of the spinel in the propanolysis of a variety of phosphate esters. Although these compounds are somewhat bulky molecules (reaching an


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estimated diameter of about 10.3 Å) they could conveniently fit into the catalyst pores, minimizing pore substrate diffusion limitations.27 Thus, we show that it is possible to extend the use of incipient MgAl2O4 spinel catalyst to effectively catalyze the propanolysis of a wide range of organophosphate triesters. In order to further understand the reaction kinetics, it is important to know how the phosphate triesters interact with the surface of the spinel catalyst by determining site and length of the bonding as well as adsorption energies. To obtain this information, atomistic simulations were performed based on density functional theory (DFT). 2. EXPERIMENTAL SECTION Materials. The incipient MgAl2O4 spinel catalyst was synthesized as described previously.27 The phosphate triesters Chlorpyrifos methyl O-analog (DMCPOA) and triphenyl phosphate (TPhP) were obtained commercially, and the procedures for the synthesis of the other phosphate triesters are described in detail in the Supporting Information. Doubly deionized water with conductance < 5.6 x 10-8 -1 cm-1 and pH 6.0–7.0 from a NANOpure analytical deionization system (type D-4744) was used to prepare the reagents solutions. Catalyst activity and kinetics measurements. In the synthesis procedure, the catalyst was powdered after calcination, being ball milled and sieved to 230 mesh, resulting in a solid with particle size < 63 μm, which was dried at 450 °C for 1.5 h prior to performing the reactions. The kinetic studies were performed as previously described and are briefly presented below. Typically, dried 1-propanol (20 mL) was mixed with 300 mg of catalyst (pre-dried at 450°C for 1.5 h) and the reaction was started by adding an aliquot of a stock solution of the phosphate ester in acetonitrile to give a final concentration in the range of 8.0 x 10-5 to 8.0 x 10-4 mol L-1 in the reaction medium. The reactions were performed at 30 ± 0.2 °C under argon atmosphere. The stirring of the reaction mixture was accomplished using a magnetic stirrer with appropriate


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specifications for the sample volume. A stirring rate of 750 rpm was employed, which showed to be enough to guarantee the appropriate mixing of the reaction medium. At established times, aliquots of 0.5 mL were withdrawn and centrifuged with 0.2 mL of ethanol and 0.2 mL of carbonate buffer at pH 9.0 containing 4.0 mol L-1 of NaCl. Then, 0.7 mL of supernatant solution were diluted with water to a final volume of 2 mL in a quartz cuvette to record the UV-Vis spectrum. First order rate constants for the reactions were obtained by iterative least-squares fitting of the absorbance profiles at the λmax of the aryl oxide product against time. Identification of products. The quantitative determination of the aryl oxide products was carried out by using a UV-Vis spectrophotometer (HP-8453). Additionally, mass spectrometry with electrospray ionization (ESI-MS) in the positive ion mode was employed, using an Applied Biosystems/MDS SCIEX 3200 Q TRAP equipment. Computational methods. The calculations were done with a VASP package,31,32 using plane wave expansion basis and Projector Augmented Waves (PAW)33 method for the treatment of core electrons. For the exchange-correlation, the semilocal GGA functional of Perdew-BurkeErnzerhof (PBE)34 was applied. A supercell method was used for the simulations of the MgAl2O4 surfaces, by adding a vacuum layer of 20 Å. The plane wave basis set was truncated at energy of 300 eV, and the calculations were done using a 2x2x1 mesh of K points, while the convergence was assumed after energy differences of less than 10-6 in relation to previous self-consistent cycle. The studies were divided in three parts, at first the understanding of the spinel surfaces terminations, followed by the analysis of single isolated molecules and after all, the evaluation of the triester properties on top of the spinel surface. Since the focus of the work is to determine the interactions between the triester functional group with the spinel surface, just one organophosphate molecule was selected in order to reduce the computational cost, and the


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selected one was the methyl paraoxon (dimethyl p-nitrophenyl phosphate, DMPNPP).27 The adsorption conformation and the adsorption processes were investigated in this work, while a detailed mechanistic investigation of plausible reaction pathways and transition states will be object of a future publication. 3. RESULTS AND DISCUSSION The incipient MgAl2O4 spinel catalyst was synthesized as described previously.27 After characterization, the MgAl2O4 catalyst was tested in the degradation of a series of organophosphate triesters which include dialkyl aryl phosphates, alkyl diaryl phosphates and triaryl phosphates. Dialkyl aryl phosphates. Inspired by the results obtained in the propanolysis of methyl paraoxon (DMPNPP) we examined the propanolysis of a series of dialkyl aryl phosphates with different substituents (Chart 1) in the presence of the MgAl2O4 catalyst. We initially analyzed the degradation of the Chlorpyrifos methyl O-analog (DMCPOA), which is a compound potentially more toxic than Chlorpyrifos in the inhibition of acetylcholinesterase.5 Figure 1A shows ultraviolet–visible spectra of DMCPOA as a function of time in the presence of the catalyst, showing an increase in absorbance at 325 nm (Figure 1B), which is consistent with the formation of 3,5,6-trichloro-2-pyridynol (TCP).


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Chart 1. Dialkyl aryl phosphate esters.

Figure 1. (A) Representative spectra as a function of time for the propanolysis reaction of DMCPOA in the presence of the MgAl2O4 catalyst, employing 1.7x10-5 mol of substrate per


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gram of catalyst, at 30ºC and 750 rpm, and (B) respective values of absorbance at 325 nm as a function of time. The kinetics of Figure 1B is consistent with the classical Langmuir-Hinshelwood model for heterogeneously catalyzed reactions on solid surfaces, as previously shown for the degradation of methyl paraoxon27 (DMPNPP, Chart 1). According to this model, generalized in Scheme 2 for a dialkyl aryl phosphate, the substrate should bind the catalyst surface according to an equilibrium constant KSubstrate, where the nucleophilic attack of an adsorbed neighboring propoxide on phosphorus leads to the displacement of the aryl oxide leaving group and the formation of the transesterified product. In turn, the products should desorb the solid sequentially, diffusing toward the 1-propanol phase and regenerating the catalyst surface. Thus, on the basis of Scheme 2, eq. 1 can be derived to treat data of Figure 1 quantitatively.


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Scheme 2. Propanolysis of dialkyl aryl phosphates in the presence of the MgAl2O4 catalyst. KPrOH, KArO- and KTriester stand for the adsorption equilibria constants of 1-propanol, the aryl oxide product and the transesterified product, respectively.

d TCP  dt



d  DMCPOA dt

 k1 DMCPOA  DMCPOA  k z DMCPOA

(1)

In eq. 1, DMCPOA stands for the fraction of the surface active sites occupied by DMCPOA, and k1 represents the first order rate constant for the propanolysis reaction on the catalyst surface at low DMCPOA concentration or in the beginning of the reaction where there is not a significant


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amount of the products formed. The rate constant kz corresponds to the zero order reaction observed at higher concentrations of phosphate triester, where usually liberation of TCP or aryl oxides from the surface of the catalyst become rate determining. In order to compare the reactivities of the different phosphate triesters, we choose to work at low concentrations of the organic substrate, where the results are consistent with a simple first order kinetic equation, indicating that the surface is not fully saturated and eq. 1 can be simplified to eq. 2. 

d  DMCPOA dt

 k1 DMCPOA  DMCPOA

(2)

Using the first order dependence of the increase in absorbance as a function of time observed for the degradation of DMCPOA at 325nm allowed us to calculate an observed rate constant of 3.34 x 10-3 s-1, which corresponds to a reaction with half-life of approximately 3.5 minutes. Compared with the value of (6.1 ± 4.8) x 10-7 s-1

35

reported for the spontaneous hydrolysis of

DMCPOA, which can be taken as a reference for the decomposition of this biocide in natural waters, we estimate a catalytic effect in the range of 103–104-fold. Correspondingly, for other dialkyl aryl phosphates shown in Chart 1: diethyl 2,4-dinitrophenyl phosphate (DEDNPP), dimethyl 2,4-dinitrophenyl phosphate (DMDNPP), diethyl 2-pyridyl phosphate (DE2PyP) and dimethyl 2-pyridyl phosphate (DM2PyP), the kinetic behavior for the MgAl2O4-catalyzed propanolysis are also consistent with the Langmuir-Hinshelwood mechanism, and treatment of the data with eq. 2 allow obtaining the first order rate constants given in Table 1, where the previously reported value for the degradation of DMPNPP is included for comparison purposes. The kinetic profiles for the triesters DEDNPP and DM2PyP are presented in Figure S1, and a replicate of DMPNPP propanolysis in the presence of the catalyst is shown in Figure S2. For the triester DE2PyP the rate constant was estimated by the initial rate method. It is important to note that in the case of the triester DMDNPP the reaction is


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so fast that it could not be monitored kinetically and the rate constant given in Table 1 represents a minimum value, considering that a minimum time of 30 seconds was necessary for analysis, and accordingly k1 should be at least ≥ 1.2 x 10-2 s-1. Table 1. Rate constants for the propanolysis of dialkyl aryl phosphates triesters in the presence of the MgAl2O4 spinel catalyst. Substrate

k1 (s-1)

pKa

DM2PyP

1.19 x 10-4

9.09

DM4NPP a

5.25 x 10-4

7.14

DMCPOA

3.34 x 10-3

4.55

DMDNPP

≥ 1.2 x 10-2

4.07

DE2PyP b

3.00 x 10-6

9.09

DEDNPP

3.51 x 10-3

4.07

a

Data from reference 27. b Rate constant obtained by the initial rate method.

The kinetic results of the propanolysis of the dialkyl aryl triesters in presence of the incipient spinel catalyst indicate that the overall reactivity is intrinsically related to the size of the alkyl substituent. For example, note that the half-life time for DE2PyP and DM2PyP are approximately 64 and 1.6 hours, respectively. Furthermore, the rate constant for DEDNPP is 3.51 x 10-3 s-1, while DMDNPP decomposes immediately and the rate constant was estimated to be ≥ 1.2 x 10-2 s-1. Indeed, results reported in the literature for a number of organophosphates with alkyl substituents showed that the increase in the alkyl group leads to a reduction of adsorption capacity of the substrate on the MgO surface by stereo effects.36 Additionally, there is a significant effect of the leaving group and the reactivity for the family of compounds with similar alkyl groups show a marked increase in rate constant with a decrease in the pKa of the leaving group (values included in Table 1).


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Theoretical Calculations. In order to understand in more detail the catalytic effect promoted by the surface of the MgAl2O4 catalyst in the propanolysis of the phosphate triesters, we decided to examine the adsorption of 1-propanol and the representative triester DMPNPP on the surface of the catalyst using theoretical methods. In Figure 2(a) it is shown the MgAl2O4 surface used for the adsorption studies of 1-propanol and DMPNPP. Such slab was chosen for presenting an inversion center along the z axis, meaning that it is symmetrical with respect to the central xy plane of the slab. Another relevance of this surface slab is the stoichiometry, which is the same as that of the bulk cell, avoiding the need for equilibrium conditions. In addition, several configurations for the (100) plane surface termination could be considered. However, most of them do not present the inversion center, while several other possibilities are non-stoichiometric. Since the scope of this work is to understand the adsorption of a phosphate triester molecule in different MgAl2O4 sites, rather than understanding the surface physics of this spinel material, a single slab was selected to simplify the calculation. Due to the high lattice parameter and low electrical polarization (no electrical dipole along z slab direction), a thin slab of about 7.2 Å (a monolayer) is sufficient to calculate the surface free energy (SFE). The calculated SFE for the slab was 1.4 J/m2; it is the smallest one when compared to several other slabs tested, including the non-stoichiometric one reported by Rasmussen et al.37 of 1.53 J/m2. The Al terminated surfaces are rather unstable, with the Al atoms going inward to the Mg layers. Nevertheless, the adsorption in Al sites can be evaluated since Al atoms exist in between the Mg terminations. The low SFE of the selected slab in comparison to other surfaces such as (100), (110) and (111) was also previously reported by Wang et al.38


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Figure 2. Adsorption simulations. In (a) the symmetric and stoichiometric MgAl2O4 (100) surface slab and in (b) the DMPNPP triester considered in the calculation. In (c) the 1-propanol adsorption in the Mg site of the surface slab. In (d) and (e) the DMPNPP adsorption conformation in the (100) slab at Al and Mg sites, respectively. In (f) DMPNPP and 1-propanol in the closest conformation. Once the surface is known, the next step was to calculate the properties of isolated molecules of 1-propanol and DMPNPP, the latter represented in Figure 2(b). As expected, the overall phosphate tetrahedral structure is slightly distorted: the bond length between the phosphorus and the oxygen atoms of the methoxy groups are 1.58 Å, while the corresponding bond lengths involving the oxygen of the 4-nitrophenolate leaving group and the phosphoryl oxygen are 1.62 and 1.47 Å, respectively.


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1-Propanol and DMPNPP molecules were adsorbed separately on top of the surface slab at different sites and the adsorption conformation and adsorption energy were calculated. For the adsorption of 1-propanol on top of Al atoms the bond length between the oxygen and the outermost aluminum atom in the surface is 2.13 Å, and the adsorption energy is about 9.27 kcal/mol. The propanol molecule does not present any major changes in its aliphatic chain and the angle between the chain and the surface normal axis (Al-O-C) is about 117°, indicating that the propanol could lay down on the MgAl2O4 surface. When 1-propanol is adsorbed on a Mg site, as presented in the Figure 2(c), the O…Mg distance is almost the same as that for the O…Al, and the calculated value is 2.12 Å, while the adsorption energy is much higher and of about 19.02 kcal/mol. The higher adsorption energy in Mg site was also calculated for the adsorption of CO2.39 The carbon chain goes through minor reconstruction and the Mg-O-C (last carbon in the chain) angle is about 115°, meaning that the adsorbed 1propanol molecule approaches more to the surface, probably due to the higher electron affinity of neighboring Al atoms. After the deprotonation of the hydroxyl group of 1-propanol the length between the Mg (Al) and O atoms become shorter of about 1.88 Å (1.9 Å). Once deprotonated, the adsorption energy of propanol in Mg site increases significantly to 35.19 kcal/mol, while the adsorption energy on Al site decreases to 6.99 kcal/mol, indicating that the Mg enhances the deprotonation of 1-propanol. The adsorption structure of DMPNPP on an Al site of the (100) surface is presented in Figure 2(d), with a bond length of 1.97 Å between the phosphoryl oxygen and the Al site. The calculated adsorption energy is 10.35 kcal/mol, close to the value found for the adsorption of 1propanol on this site. Two rotations of the benzene group were tested: one with the P atom lying in the benzene plane as in Figure 2(d), and another with the benzene rotated by 90°. No relevant


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differences were found for these two configurations, other than a small increase in the adsorption energy to 11.70 kcal/mol for the latter configuration with the P atom out of the nitrophenol plane and a slight deformation of the phosphate tetrahedron. The distance between P and Al is 3.42 Å and the distance between Al and N is 9.83 Å. The adsorption of DMPNPP on the Mg site of the surface is displayed in Figure 2(e). The distance between Mg and the phosphoryl oxygen is 2.00 Å and the adsorption energy is 22.31 kcal/mol, which is two times greater than the value found for adsorption on Al sites. The chain structure does not present significant changes, but shows a small decrease of the angle between the surface normal and the chain (Mg-P-C angle) in relation to the adsorption in Al sites. That is, the chain becomes closer to the surface, which is also observed from the Mg to N distance of 9.74 Å, smaller than the distance between Al and N for adsorption in Al sites even though the Mg to P distance of 3.48 Å is greater than the Al to P distance in Al sites. The data obtained from the calculations are presented altogether in Table 2. In summary, the results point to the fact that both Mg and Al sites can adsorb the 1-propanol and DMPNPP molecules, with the Mg site being more reactive than the Al site for both reactants. Nevertheless, the distance between two consecutives Mg sites on the surface is about 8.0 Å, a distance which is not conducive for the reaction since the reactants are well separated from each other. On the other hand, the distance between an Al site and a neighboring Mg site at the surface is only 3.04 Å, indicating that in the propanolysis reaction one of the reactants (1-propanol or DMPNPP) will be adsorbed on a Mg site while the other one would be adsorbed on a neighboring Al site in a configuration such that the negatively charged oxygen of 1-propoxide and the electrophilic phosphorus center are positioned at a short, favorable distance for bond formation, as required for a bimolecular reaction.40,41


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Table 2. Adsorption bond lengths, adsorption energy and carbon chain to surface normal angle, for the adsorption of 1-propanol/propoxide and DMPNPP over the MgAl2O4 (100) surface.a Angle System

O–Al or O–Mg Adsorption Bond length (Å) energy (kcal/mol)

C-O(P)-Al or C-O(P)-Mg

1-Propanol On Al

2.13

(after deprotonation of (1.9) the hydroxyl group) On Mg

2.12

(after deprotonation of (1.88) the hydroxyl group)

9.27

117°

(6.99)

(114°)

19.02

115°

(35.19)

(103°)

DMPNPP On Al (0o rotation)

1.97

10.35

146°

On Al (90o rotation)

1.98

11.70

150°

On Mg (0o rotation)

2.0

22.31

143°

a

The atomic positions of the structures used for the adsorption energy calculations (Figure 2) are presented in the Supporting Information. Thus, even though both reactants show preferential adsorption to the Al site, reactivity will result from a hopping process whereby reactants migrate on the catalyst surface until they are eventually optimally positioned, one at each type of Lewis acid site, the distance between the nucleophilic and electrophilic centers being suitable for nucleophilic attack and O–P bond formation. Moreover, provided that the propoxide adsorption energy is greater when coordinated to Mg, compared to the Al site, one could then expect a more straightforward reaction pathway when the reactive 1-propoxide is bound to a Mg site, and the DMPNPP substrate to a neighboring Al center. An illustrative description of this most probable conformation is presented in Figure 2(f). Once the reactants are properly positioned, the reaction takes place, and


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4-nitrophenolate displacement should occur presumably through either a concerted or an addition-elimination mechanism, as expected for a substitution reaction at phosphorus in a neutral triester.42 At any event, the investigation of such plausible mechanisms is beyond the scope of the present work, and a thorough investigation of the mechanism of DMPNPP propanolysis over the surface of metal oxides will be object of a future publication. This work is underway. Alkyl diaryl phosphates. The compounds ethyl bis(4-chlorophenyl) phosphate (EB4ClPP) and ethyl bis(2-pyridyl) phosphate (EB2PyP), Chart 2, were selected because the leaving groups in both compounds (4-chlorophenol and 2-pyridinol) have similar acid dissociation constants. Interestingly, the observed kinetic behavior was completely different for both compounds. Chart 2. Alkyl diaryl phosphate esters.

Propanolysis of EB4ClPP in the presence of MgAl2O4 catalyst is somewhat slow, showing a typical first order behavior, and it releases one equivalent of 4-chlorophenol in about 3000 minutes (kinetic profile is shown in Figure S1). In contrast, the reaction of the triester EB2PyP shows a relatively fast release of 1 equivalent of 2-pyridinol, and the reaction continues up to the release of two leaving group equivalents in about 2000 minutes. The experimental data is typical of reactions proceeding via two irreversible consecutive first order reactions, Figure 3 and Scheme 3. The first order rate constant for the propanolysis of EB4ClPP was calculated by fitting the experimental data to eq. 2, while the rate constants for the propanolysis of B2PyP were obtained by fitting the experimental data to typical equations that describe consecutive


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irreversible first order reactions (equations S1-S3 in the Supporting Information). The rate constants obtained for both substrates are presented in Table 3.

Figure 3. Kinetic profile of the propanolysis of EB2PyP in the presence of the MgAl2O4 spinel catalyst, at 30ºC. Scheme 3. Propanolysis of EB2PyP in the presence of the MgAl2O4 spinel catalyst.

Table 3. Rate constants for the propanolysis of alkyl diaryl phosphate triesters in the presence of MgAl2O4, at 30ºC. Substrate

k1 (s-1)

k2 (s-1)

pKa

EB2PyP

3.73 x 10-4

2.20 x 10-5

9.09

EB4ClPP

3.03 x 10-5

-

9.38


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The release of 2 equivalents of 2-pyridinol in the propanolysis of EB2PyP was confirmed by performing ESI(+)-MS experiments. Figure 4 shows the ESI(+)-MS spectrum of an aliquot taken after approximately 24 hours of propanolysis reaction, and several peaks were observed, which confirm the products of the first and second reaction steps (Scheme 3).

Figure 4. ESI-MS spectrum in the positive ion mode of an aliquot taken after 24 hours of the propanolysis of EB2PyP in the presence of the MgAl2O4 catalyst. Mass spectrometric analysis using ESI-MS in the positive mode monitors the reaction collecting snapshots of its cationic composition. Reagents, intermediates, and products present as cations are expected to be transferred directly from the reaction solution to the gas phase and then detected by ESI-MS. In Figure 4, the peak of m/z 96.0 corresponds to the protonated 2pyridinol and there is a very important fragment of m/z 211.1, which indicates the formation of ethyl bis(n-propyl) phosphate, the final product in Scheme 3. In this spectrum, several key cations were detected and identified including the protonated ethyl n-propyl phosphate of m/z 169.1, which can be formed from the ion of m/z 211.1 by loss of n-C3H7O. Similarly, the ion of m/z 127.0 can be formed from the protonated ethyl n-propyl phosphate (m/z 169.1) by loss of nC3H7O. Also in Figure 4, the ion of m/z 246.1 confirms the formation of the ethyl n-propyl 2-


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pyridyl phosphate, and the ion of m/z 204.1, which corresponds to the protonated ethyl 2-pyridil phosphate, is formed from the ion of m/z 246.1 by loss of n-C3H7O. Triaryl phosphate triesters. The propanolysis of a series of symmetric triaryl phosphates including tris(4-nitrophenyl) phosphate (T4NPP), tris(3-nitrophenyl) phosphate (T3NPP), tris(4chlorophenyl) phosphate (T4ClPP), tris(2-pyridil) phosphate (T2PyP) and triphenyl phosphate (TPhP), Chart 3, were examined in the presence of the MgAl2O4 catalyst. Chart 3. Triaryl phosphate esters.

Representative absorbance versus time profiles for T4ClPP and T4NPP are shown in Figure 5 and other typical absorbance versus time profiles are given in Figure S1. In all cases, the reported absorbance changes correspond to the formation of the products of each reaction at the appropriate maximum absorption wavelength and the solid lines correspond to the theoretical treatment.


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Figure 5. Kinetic profiles of propanolysis reactions of T4ClPP and T4NPP in the presence of the MgAl2O4 catalyst at 30°C. The kinetic behaviors shown in Figure 5 do not correspond to typical first order profiles. Instead, these are typical Burst kinetic profiles, in which there is an initial increase in absorbance according to a first order rate law, turning into a zero order process at longer times due to progressive saturation of the catalyst surface with the formed products. In fact, it is well known that substituted phenolates do present great adsorption affinities toward metal oxides,43 and lowering the substrate concentration should minimize the presence of the zero order step by avoiding progressive overload of the catalyst surface. At any event, since the first order step is well defined in all the profiles, the catalytic activity of the MgAl2O4 spinel can be evaluated with


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confidence, and thus we fitted the kinetic data for the propanolysis of the triaryl phosphates to eq. 1; the values obtained for k1 and kz are presented in Table 4. Table 4. Rate constants for the propanolysis of triaryl phosphate triesters in the presence of the MgAl2O4 catalyst, at 30ºC. Substrate

k1 (s-1)

kz (M s-1)

pKa

T4NPP

9.36 x 10-4

3.39 x 10-10

7.14

T3NPP

8.89 x 10-4

2.83 x 10-9

8.35

T4ClPP

6.17 x 10-5

1.36 x 10-10

9.38

T2PyP

7.20 x 10-4

1.25 x 10-9

9.09

TPhP a

2.38 x 10-7

–

9.99

a

First order rate constant obtained by the initial rate method. It is important to notice that, in the time scale in which the kinetics were followed, it was

observed the release of one equivalent of the aryl oxide moiety from the substrates T3NPP and T4ClPP, leading to the formation of an alkyl diaryl triester. On the other hand, for the T4NPP substrate, the absorbance values in the first order domain increases more than expected for the formation of one equivalent of 4-nitrophenolate, indicating that the initial product n-propyl bis(4nitrophenyl) phosphate is further subjected to propanolysis, giving rise to the second equivalent of 4-nitrophenolate, and then saturation of the catalyst surface is observed. At any event, since the exponentials for the two reactions are not clearly distinguishable in the kinetic profile, the k1 value for this substrate is rather a sum of the two consecutive first order reactions. Similarly, the substrate T2PyP is also doubly transesterified, but in this case the absorbance changes in the first order domain coincide with those calculated for the complete release of one equivalent of 2pyridinol, suggesting that the second propanolysis reaction is most probably observed at longer times only. Finally, the reactivity for the TPhP was far below the other triaryl phosphates, and the first order rate constant was calculated by the initial rate method.


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Even though the reactivity is strongly dependent upon the substrate geometry, the data in Table 4 shows as a general trend that k1 decreases with the leaving group basicity, the only exception being the T2PyP substrate, whose greater reactivity can be a result of the presence of the nitrogen atom in the aromatic rings. This additional nitrogen atom offers additional binding sites to the catalyst surface and, in the leaving group more specifically, it is a means of assistance in the leaving group departure by the catalyst metal centers. In our recent study with a dialkyl aryl phosphate (DMPNPP, k1 shown in Table 1) a slightly larger k1 was obtained when compared with the triaryl T4NPP studied here. This difference can be related to the stereo effect of the three aromatic rings present in the T4NPP. In fact, studies reported in the literature regarding adsorption and decomposition of organophosphorus compounds on the MgO surface warn about factors such as steric hindrance and the basicity of phosphate substituent groups.36 Furthermore, there is a second reaction following formation of one equivalent of 4-nitrophenolate in the case of T4NPP, contributing to the catalyst surface becoming progressively saturated. These results clearly show that the synthesized catalyst is extremely effective in the catalytic cleavage of a large class of phosphate esters. However, it is important to note that computational calculations are required to further a detailed description of the specific interactions of the substrate with the catalytic surface. Also, the structure of the Mg2+/Al3+ mixed oxides is not clearly described in the literature.44,45 In this context, a good model to study the catalytic sites of Mg2+/Al3+ mixed oxides is the Al-doped MgO surface based on the simple MgO.34 More recently, we have focused our attention on reactions with higher substrate concentrations and different reaction variables in the degradation of phosphate esters in the presence of commercial MgO. The results should be published in our next paper.


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4. CONCLUSIONS A MgAl2O4 catalyst featuring the incipient spinel phase was tested in the propanolysis of a series of phosphate triesters at 30ºC, extending studies initiated with the organophosphorus biocide methyl paraoxon (DMPNPP), with catalytic effects estimated up to c.a. 104-fold. Adsorption of 1-propanol and DMPNPP on the surface of the MgAl2O4 (100) surface showed that the Al center is preferred for coordination of the reactants, but the transesterification reaction should probably proceed after migration of the reactants over the surface until an Al3+-adsorbed DMPNPP is found at a neighboring Mg2+-bound propoxide anion, in a configuration wherein the electrophilic and nucleophilic centers are close enough for the P–O bond formation. Particularly, results in this paper show that there is certain specificity to the catalytic effect, which depends largely on the functional groups present in the substrates. In the case of dialkyl aryl triesters, or when experimental conditions allow successive transesterification of the alkyl diaryl and triaryl triesters, reactions result in the formation of products that are structurally related to a family of flame retardants. This result is particularly important because it shows that toxic phosphate triesters can be converted into useful compounds. These results also drive us to new possibilities for the design of highly efficient catalytic systems. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The following file is available free of charge. Experimental procedures for the synthesis of the phosphate triesters and characterization data, kinetic profiles, and coordinates of structures from the DFT calculations. (PDF)


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AUTHOR INFORMATION Corresponding Author *E-mail for E.H.W.: [email protected] *E-mail for R.A.N.: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We dedicate this paper to Prof. Faruk Nome, who deceased on September 24, 2018. The authors thank INCT-Catálise, CNPq, CAPES, FAPESC, PRONEX and TWAS for ﬁnancial support. REFERENCES (1)

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