Kinetics and adsorption calculations: insights into the MgO-catalyzed detoxification of simulants of organophosphorus biocides

Article abstract of DOI:10.1039/c9ta14028j

We report the targeted decomposition of the organophosphate methyl paraoxon by means of its transesterification with 1-propanol catalyzed by magnesium oxide. Catalyst characterization by energy dispersive X-ray fluorescence (EDXRF), nitrogen adsorption/desorption measurements (BET and BJH methods), and temperature programmed desorption of CO₂(CO₂-TPD) showed that the employed MgO presents properties favorable for the methyl paraoxon adsorption and transesterification to occur. A thorough kinetic investigation showed that rate enhancements up to 3 × 10⁶-fold can be achieved in comparison with the spontaneous propanolysis of the substrate, and that the material can be used in additional cycles without loss of catalytic activity, with the catalyst recovery achieved through a simple washing procedure. Energies for adsorption of 1-propanol and methyl paraoxon onto a MgO model surface were obtained by density functional theory calculations, which showed that the latter displays a stronger affinity for the catalyst surface, and that the reaction should proceed with methyl paraoxon and 1-propanol molecules juxtapositioned at adjacent Mg²⁺sites, with nucleophilic and electrophilic centersca.2.4 ? away from each other. Additionally, MgO also promoted rate enhancements up to 5 × 10⁴-fold in the propanolysis of a further range of representative phosphate triesters, and in most of the cases the final transesterified products are trialkyl phosphates structurally related to a family of flame-retardants. The results thus provide insights into the development of novel systems for the targeted conversion of organophosphorus compounds into value-added products by employing simple, highly efficient, and low-cost metal oxide catalysts.

Full text of DOI:10.1039/c9ta14028j

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Buratto, E. H. Wanderlind, L. M. Nicolazi, P. Sangaletti, M. Medeiros, F. S. S. Schneider, G. F. Caramori, R. L.
T. Parreira, G. A. Micke and H. D. Fiedler, J. Mater. Chem. A, 2020, DOI: 10.1039/C9TA14028J.
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DOI: 10.1039/C9TA14028J
ARTICLE
Kinetics and Adsorption Calculations: Insights into the MgO-
Catalyzed Detoxification of Simulants of Organophosphorus
Biocides
Gizelle I. Almerindo,a,b,* Suelen C. Buratto,c Eduardo H. Wanderlind,c,d,* Lucas M. Nicolazi,c Patrícia
Sangaletti,c Michelle Medeiros,c Felipe S. S. Schneider,c Giovanni F. Caramori,c Renato L. T. Parreira,e
Gustavo A. Micke,c Haidi D. Fiedlerc,* and Faruk Nomec,†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report the targeted decomposition of the organophosphate methyl paraoxon by means of its transesterification with 1-
propanol catalyzed by magnesium oxide. Catalyst characterization by energy dispersive X-ray fluorescence (EDXRF), nitrogen
adsorption/desorption measurements (BET and BJH methods), and temperature programmed desorption of CO2 (CO2-TPD)
showed that the employed MgO presents properties favorable for the methyl paraoxon adsorption and for the
transesterification to occur. A thorough kinetic investigation showed that rate enhancements up to 3 x 106-fold can be
achieved in comparison with the spontaneous propanolysis of the substrate, and that the material can be used in additional
cycles without loss of catalytic activity, being the catalyst recovery attained through a simple washing procedure. Adsorption
energies for 1-propanol and methyl paraoxon onto a MgO model surface were obtained by density functional theory
calculations, which showed that the latter displays a stronger affinity with the catalyst surface, and that the reaction should
proceed with methyl paraoxon and 1-propanol molecules juxtapositioned at adjacent Mg2+ sites, with nucleophilic and
electrophilic centers c.a. 2.4 Å distant from each other. Additionally, MgO also promoted rate enhancements up to 5 x 104-
fold in the propanolysis of a further range of representative phosphate triesters, and in most of the cases the final
transesterified products are trialkyl phosphates structurally related to a family of flame-retardants. The results thus provide
insights into the development of novel systems for the targeted conversion of organophosphorus compounds into value-
added products by employing simple, highly efficient, and low cost metal oxide catalysts.
nerve agents, which include incineration and alkaline
hydrolysis, are not convenient to effectively detoxify a large
body of substrates, presenting in many instances disadvantages
1. Introduction
Organophosphorus compounds are amongst the most toxic
classes of compounds to the human being, primarily because of
the inhibition of acetylcholinesterase activity in the nervous
system.1–3 This mechanism of action has led to the development
in the past of highly potent nerve agents for military
applications, requiring the investigation of methodologies for
the safe and effective decomposition of stockpiles of such
compounds.1,4–6 This is because most of the preferred
methodologies currently employed for the destruction of these
such as the formation of toxic by-products.1,4,6 In the continuous
effort for the development of methodologies for the
detoxification of organophosphorus compounds, researches
have focused on the development of catalytic systems as
effective means to accelerate a given targeted reaction.1,4
Indeed, there is a number of recent examples in the
literature highlighting a variety of catalytic systems designed for
the cleavage of organophosphorus compounds as well as their
simulants, frequently used in worldwide laboratories as a
precautionary measure due to the high toxicity of the
homogeneous7–12
and
a. Faculdade de Engenharia Química, Escola do Mar, Ciência e Tecnologia,
Universidade do Vale do Itajaí (UNIVALI), 88302-901, Itajaí, Santa Catarina (SC),
Brazil. E-mail: gizelle.almerindo@univali.br (G.I.A)
substrates.
These
include
microheterogeneous13–18 systems, displaying different modes
of catalysis in a diverse range of systems including, for example,
metal complexes, polymers, metallic nanoparticles, ionic
liquids, etc. Significantly, developing mechanistic understanding
of enzymes that exhibit phosphotriesterase activity promises
improved medical treatments for cases of intoxication by
organophosphorus compounds.19–21
b. Programa de Pós-Graduação em Ciências Farmacêuticas, Universidade do Vale do
Itajaí (UNIVALI), 88302-901, Itajaí, Santa Catarina (SC), Brazil.
c. Departamento de Química, Universidade Federal de Santa Catarina (UFSC),
88040-900, Florianópolis, Santa Catarina (SC), Brazil. E-mail: haidi.fiedler@ufsc.br
(H.D.F)
d. Institute of Chemistry, University of Campinas (UNICAMP), 13083-970, Campinas,
São Paulo, Brazil. Emails: ewanderlind@gmail.com or ehw@unicamp.br (E.H.W)
e. Núcleo de Pesquisas em Ciências Exatas e Tecnológicas, Universidade de Franca,
Franca, SP, 14404-600, Brazil.
Additionally, a great number of heterogeneous systems is
also reported.22–29 Heterogeneous catalysis has proved to be of
great value in the detoxification of organophosphorus
compounds, since it provides decisive practical features
† F.N. is deceased since Sept. 24, 2018.
Electronic Supplementary Information (ESI) available: Data of characterization of
solids, kinetic measurements, identification of products, and Cartesian coordinates,
as well as energies, of structures studied by DFT. See DOI: 10.1039/x0xx00000x
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compared with homogeneous media, such as convenient
separation of the catalyst and its recycling.30–32 More
specifically, metal oxides attract particular attention because
they are simple, robust catalysts known to be highly active while
being stable in a wide range of temperatures.1,33 Several reports
highlight the catalytic activity of simple metal oxides toward the
solvolysis of organophosphorus compounds, hydrolysis being
the most studied reaction,34–38 with a lower number of
examples of solvolysis by alcohools.39–41 Besides the acid-base
properties of the metal oxides, their geometric and textural
properties, as well as the nature of the metal, are intrinsically
related to the catalytic effect. Thus, researchers have directed
efforts into the use of solids that present proper geometric and
textural arrangements for a targeted mechanism of catalysis.
In this sense, we investigated recently the propanolysis
reaction of methyl paraoxon (dimethyl 4-nitrophenyl
phosphate) catalyzed by a series of mixed Mg2+/Al3+ oxides
(Scheme 1).42 The solid featuring an incipient spinel phase
showed to be the most active one, leading to rate
enhancements of ca. 2.5 x 105-fold in comparison to the
spontaneous propanolysis of the substrate.42 Significantly, the
transesterification of methyl paraoxon leads to a product
structurally related to a family of flame-retardants of particular
interest, since the use of trialkyl organophosphate esters has
increased considerably as substitutes for environmentally
persistent flame-retardants currently employed by the
industry.43 Also, the incipient MgAl2O4 spinel showed to be
highly efficient in the propanolysis of a wide range of phosphate
triesters, including dialkyl aryl, alkyl diaryl and triaryl substrates,
evidencing that the catalytic effect depends on the nature of the
triester substituents.44
Additionally, we also focused in the evaluation of the effect
of lanthanide metal centers in solid catalysts: a novel aluminum-
titanate-supported erbium oxide promoted catalytic effects of
at least 7 x 105-fold in the propanolysis of methyl paraoxon.45
And more recently, we described the greatest catalytic effect
reported so far for this target reaction: a 107-fold effect,
achieved with a catalyst of strontium alkaline compounds
featuring strontium oxide as main component, being the
superior catalytic effect well correlated to the strong base site
density of the material surface.46
Envisioning the application of metal oxides in the
decomposition of organophosphorus compounds, we report
herein an investigation of the model magnesium oxide (MgO) as
catalyst in the propanolysis of methyl paraoxon and a further
range of phosphate triesters. Additionally, the experimental
results are complemented with density functional theory
calculations employed to obtain adsorption energies and gain
insights into the reaction mechanism.
2. Experimental
View Article Online
DOI: 10.1039/C9TA14028J
Materials
Magnesium oxide (Vetec; lot 021953) and γ-alumina (Merck; lot
TA1328576-542) were purchased and used as received. The
phosphate triesters used in this study, methyl paraoxon
(dimethyl 4-nitrophenyl phosphate), dimethyl 2,4-dinitrophenyl
phosphate (DMDNPP), diethyl 2,4-dinitrophenyl phosphate
(DEDNPP), tris-2-pyridyl phosphate (T2PyP), and tris-4-
nitrophenyl phosphate (T4NPP), were all synthesized as
described previously (experimental procedures and
characterization data are presented in the ESI), except for
chlorpyrifos methyl O-analog (CPO), which was purchased
(Sigma Aldrich) and used as received. Doubly deionized water
with conductance < 5.6 x 10-8 -1 cm-1 and pH 6.0–7.0 used to
prepare reagent solutions was from a NANOpure analytical
deionization system (type D-4744).
C
atalyst Characterization
Thermogravimetric analysis (TGA). The thermogravimetric
analyses were carried out using a Shimadzu TGA-50 analyzer,
under an airflow of 20 mL min-1, with heating from 25 to 800 °C
under oxidizing atmosphere and heating rate of 10 °C min−1.
Energy Dispersive X-Ray Fluorescence (EDXRF). Analyses of
EDXRF were performed in a Bruker S2 Ranger instrument and
the software EQUA-OXIDES was used for instrument control,
and data collection and analysis. Samples were analyzed as
pressed pellets following the procedures of ASTM D2216.47,48
Nitrogen Adsorption/Desorption Isotherms. Measurements of
nitrogen adsorption and desorption were performed at -196 °C
using a Quantachrome Nova 2200e instrument. The samples
were degassed at 300 °C under vacuum for 1 h. Specific surface
areas were calculated according to the Brunauer–Emmett–
Teller (BET) method, and the total pore volume and pore size
distributions were obtained according to the Barret–Joyner–
Halenda (BJH) method.49
Temperature Programmed Desorption of CO2 (CO2-TPD). The
CO2-TPD analyses were performed using a Quantachrome
ChemBET 3000 instrument. The samples were treated in situ
under a helium atmosphere (100 mL min−1) at 500 °C for 1 h
before cooling to room temperature, and then the samples
were saturated with 100 mL CO2 min−1 for 40 min. After,
physically adsorbed CO2 was purged by a helium flow at room
temperature for 30 min. CO2-TPD were carried out in a stream
of helium (100 mL min−1) with a heating rate of 10 ºC min−1
reaching a final temperature of 800 °C.
Catalyst Activity: Kinetic Measurements
Typically, the reactions were performed as follows: 20 mL of
dried 1-propanol (stored with 3Å molecular sieves) were mixed
with 300 mg of pre-dried catalyst (450 °C/45 min), and the
reaction was started by adding an aliquot of a stock solution of
the substrate (in acetonitrile, stocked in freezer) under
continuous magnetic stirring and argon atmosphere, the system
being sealed with a rubber septum. All the reactions were
performed employing previously optimized stirring velocity and
reaction temperature of 640 rpm and 80 ºC, respectively (for
Scheme 1 Propanolysis of methyl paraoxon catalyzed by mixed MgAl2O4
oxides.
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details, see the ESI). In order to monitor the kinetics of the 3-atom layer fixed but relaxing the rest. The same box
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DOI: 10.1039/C9TA14028J
reactions, appropriate aliquots of the reaction media were dimensions were employed in all calculations. 1-Propanol, 1-
withdrawn periodically and added to a solvent mixture of 0.2 propoxide, and the phosphate educt were fully optimized on
mL of ethanol and 0.2 mL of tris buffer at pH 9.0 containing NaCl this surface. The semi-relaxed surface, as well as gas-phase
4.0 mol L-1. The mixture was stirred in a vortex and sequentially optimizations of the adsorbents, were used in the calculation of
centrifuged. Then, 0.7 mL of supernatant solution were adsorption energies. For 1-propoxide, a proton was added
transferred to a quartz cuvette and diluted with a mixture of 0.2 bound to the oxygen closest to the adsorbent in order to create
mL of ethanol and 1.5 mL of water, and absorbance changes the most stable ion pair possible.
were followed at the specific wavelength of the phenolic
products by recording UV-Vis spectra with a Hewlett-Packard
HP-8453 UV-Vis spectrophotometer. For methyl paraoxon
3. Results and discussion
specifically, a set of experiments was performed with different
ratios of moles of substrate per mass unit of catalyst from 5 x
10-6 to 5 x 10-3 mol g-1, by adjusting both the concentration of
methyl paraoxon in the reaction media and the mass of the
catalyst to achieve the desired final ratio. The propanolysis
reactions of a series of representative phosphate triesters
catalyzed by MgO were performed as described in the general
Catalyst Characterization
Since the catalytic activity of solid materials is intrinsically
related to textural and acid-base properties, the commercial
MgO was initially subjected to a series of characterization
techniques. Firstly, TGA analysis shows a mass reduction of 15%
with increasing temperature up to 400 ºC, above which residual
materials, probably water and CO2, are fully eliminated (TGA
profile is shown in Figure S1 in the ESI). Thus, standard thermic
activation involved degassing the MgO at 450 ºC before
performing the reactions for catalytic activity evaluation.
Additionally, the composition of MgO was verified by semi-
quantitative EDXRF analysis: the results indicate that 97.7% of
the degassed solid is MgO, the remaining material composed of
small amounts of chloride and oxides of calcium, sulfur, silicon
and iron (complete EDXRF data is given in Table S1 in the ESI).
Nitrogen adsorption and desorption isotherms were
obtained to elucidate textural properties (graphs are presented
in Figure S2 in the ESI). Altogether, the isotherms show typical
procedure above employing 4.6
(substrate/catalyst).
x
10-5 mol g-1
Characterization of products
The phenolic products were determined quantitatively by UV-
Vis spectrophotometry using a Hewlett-Packard HP-8453
instrument. The transesterified products were characterized
using high performance liquid chromatography coupled to mass
spectrometry. An Allcrom SynergiTM 4 µm Polar RP column (150
mm length; 2.0 mm diameter) was employed with a flux of 200
µL/min at 25 ºC using an Applied Biosystems/MDS SCIEX 3200
Q TRAP® LC/MS/MS System.
type IV behavior, characteristic of mesoporous materials.53
A
specific surface area of 176.0 m2 g-1 was calculated by the
Brunauer–Emmett–Teller (BET) method, and the Barret–
Joyner–Halenda (BJH) method was used to calculate the total
pore volume of 0.397 cm3 g-1 and mean pore radius of 45.2 Å. In
particular, it is important to note that the mean pore radius is
significantly greater than the estimated kinetic radius of methyl
paraoxon of 5.1 Å,42 facilitating the diffusion of bulk paraoxon
toward the interior of the MgO pores. Additionally, the profile
of pore radius distribution obtained by the BJH method indicate
a small contribution of macropores (i.e., pores with radius
greater than 250 Å).
The strength of the basic sites of MgO was evaluated by CO2-
TPD. Deconvolution of the profile of total CO2 desorption as a
function of temperature originates three CO2 desorption peaks,
corresponding to weak (20 - 160 ºC), medium (160 - 400 ºC) and
strong (> 400 ºC) basic sites.54 Table 1 presents the total density
of MgO basic sites and the values calculated for each
deconvoluted peak. Note that the major contribution to the
Catalyst recycling
At a ratio of 4.6 x 10-5 mol g-1, propanolysis of methyl paraoxon
was performed three times consecutively using the same
starting MgO. The reactions and kinetic data treatment were
performed as described above. Between reactions the catalyst
was subjected to a reactivation procedure as follows. MgO was
washed with deionized water five times being centrifuged after
each wash, and this procedure was performed sequentially with
a solution of NaCl 4 mol L-1. Then, MgO was washed once with
deionized water, vacuum filtrated and dried at 120 ºC/5h. Post
characterization of MgO was performed by TGA, EDXRF and
nitrogen adsorption/desorption measurements.
Computational methods
In order to investigate the adsorption process of reactants,
quantum chemical calculations were employed, which were
done using slab calculations (revPBE-D3BJ/DZP50) as
implemented in ADF51 (version 2019.103). Bulk, single unit cell,
periclase (MgO) calculations were carried out with an initial
experimental52 cell constant of 4.210 Å, which was allowed to
relax completely (final cell constant of 4.212 Å). A single surface
unit cell, 6-atom thick periodic slab model of the (100) surface
of the bulk structure, had half its outermost layers relaxed, the
Table 1 Base site density of MgO (µmol CO2/g catalyst).
Total
Weak
Medium
Strong
785.3
238.3
197.1
349.9
other atoms being held fixed. A 14.884 x 14.884 Ų (Mg150O150
)
clear cut was further optimized and employed as a supercell
slab for representing the surface, always holding the innermost
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total basicity of MgO is from strong basic sites, whose density
corresponds to ca. 45% of the total, followed by weak and
medium basic sites, with densities in the range 25–30% relative
to the total value. It is worth to note that the total base density
of MgO is some 3.5-fold greater than that of commercial γ-
alumina (γ-Al2O3), a known weakly basic support (data shown in
Table S5 in the ESI), which highlights the basic character of MgO.
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DOI: 10.1039/C9TA14028J
Kinetic Measurements and Catalytic Activity
In order to evaluate the catalytic activity of MgO in the
propanolysis of methyl paraoxon and understand the overall
process in detail, we performed a thorough kinetic investigation
by executing a series of reactions at different ratios of moles of
substrate per mass unit of catalyst, lying between 5 x 10-6 and 5
x 10-3 mol g-1 (substrate/catalyst) at 80 ºC. Aliquots of the
reaction media were withdrawn at appropriate time intervals
and diluted in order to quantify spectrophotometrically the
formation of the product p-nitrophenol as a function of time,
and the data of fraction conversion against time were treated
mathematically. Importantly, as shown later, experiments of LC-
ESI-MS confirm that there is no side reaction, and thus the
obtained kinetic profiles can be directly attributed to the
substitution of p-nitrophenolate by 1-propanol.
When employing a constant amount of 300 mg of catalyst
and varying the initial concentration of the substrate in the
range of 0.0734 to 1.08 mM, first-order kinetics were observed,
as indicated by the typical plot of fraction conversion against
time in Figure 1A, obtained with 0.942 mM of the substrate (the
complete set of kinetic data is shown in Figure S4 in the ESI).
Thus, data of Figures 1A and S4 were fitted to eq. 1, in which
pNP stands for the product p-nitrophenol, and k1 is the
observed first-order rate constant of the propanolysis reaction
over the MgO surface.
Figure 1 Kinetic data of the MgO-catalyzed propanolysis of methyl paraoxon,
at 80ºC. (A) Typical kinetic profile obtained when employing 300 mg of the
catalyst and 0.942 mM of the substrate. Solid line represents fit to eq. 1. (B)
Kinetic profiles obtained employing 3 mg of the catalyst and different
amounts of the substrate, as indicated in the figure. Solid lines represent fit
to eq. 2. (C) Data of kobs as a function of the ratio of mmols of substrate per
gram of catalyst.
(d[pNP]/dt) = – (d[methyl paraoxon]/dt) =
= k1[methyl paraoxon]
(1)
Differently from that observed in Figure 1A, results show
that enlarging significantly the concentration of the substrate
induces a change in the overall kinetic behavior. Figure 1B
presents two related examples of kinetic profiles of reactions
performed with 3 mg of MgO and concentrations of methyl
paraoxon of 0.379 and 0.688 mM. At initial time intervals, when
the MgO surface is exposed with active sites fully available for
substrate adsorption, formation of the product p-nitrophenol
obeys an exponential first order law, and then there is a change
to a zero order process at longer times. Thus, such profiles were
fitted to eq. 2, which is similar to eq. 1 and accounts for the zero
order processes at longer times by the introduction of the kZ
term, a zero order rate constant. For the reaction performed at
0.379 mM of methyl paraoxon, a kZ value of 5.43 x 10-10 M s-1 is
calculated and, consistently, a slightly lower value of 3.83 x 10-
consequently, lowering reaction rates and catalytic effects. In
fact, it is possible to notice in Figure 1B that reaction goes to
completion when employing 0.379 mM of methyl paraoxon in
spite of the catalyst surface saturation. However, the same does
not apply for the reaction with greater amount of the substrate:
in the kinetics with 0.688 mM of methyl paraoxon, saturation of
the catalyst surface occurs and allows conversion of up to ~75%
of the substrate, as evidenced by the deviation of the three last
experimental points at longer times from the expected course
of the zero order process (dashed line). Most probably,
saturation of the MgO catalyst surface is related to p-
nitrophenol formation and its adsorption as the reaction
proceeds, as also reported when employing mixed MgAl2O4
oxides and aluminum-titanate-supported erbium oxide as
catalysts for methyl paraoxon propanolysis.42,44,45 Indeed,
nitrophenols are known to present strong adsorption
capabilities onto metal oxides in heterogeneous systems.55
10
M s-1 is obtained when using 0.688 mM of substrate. This
scenario strongly suggests poisoning of the catalyst active sites
by the substrate itself and/or the products dimethyl n-propyl
phosphate and p-nitrophenol: the strong adsorption of the
different species influences the progress of the reaction by
affecting the catalyst surface, hindering the adsorption of
further methyl paraoxon molecules onto the MgO surface, and
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(d[pNP]/dt) = – (d[methyl paraoxon]/dt)
= k1[methyl paraoxon] + kZ
the propanolysis of methyl paraoxon with ESI(+)-MS detection,
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(2)
and it is observed the elution of the tranDseOsI:te10r.i1f0ie3d9/Cpr9oTAd1u4c0t28aJt
Taking the first order rate constants for the kinetics as a 2.58 minutes, whose ESI(+)-MS/MS characterization is shown in
function of the ratio of moles of substrate per gram of catalyst Figure 2(B).
in the full range from 5 x 10-6 to 5 x 10-3 mol g-1
In the ESI(+)-MS/MS of Figure 2(B) it is possible to identify
(substrate/catalyst), the plot of Figure 1C is obtained (for the the protonated dimethyl n-propyl phosphate of m/z 169 and
kinetics that show surface saturation, the first order rate the protonated species of dimethyl hydrogen phosphate of m/z
constant that describe the kinetics at initial time intervals were 127, formed by the loss of CH2=CH2CH3 from the species of m/z
considered). In Figure 1C, it is observed an exponential decrease 169. The LC-MS results confirm the formation of dimethyl n-
of the rate constants up to ca. 0.25 mmol g-1 propyl phosphate and are in agreement with
(substrate/catalyst), followed by a plateau for larges ratios, a spectrophotometric observations, sustaining the reaction
typical behavior expected for heterogeneously catalyzed pathway outlined.
reactions.30,31 Considering that the first order rate constant for
the spontaneous propanolysis of methyl paraoxon is estimated
Adsorption calculations and mechanistic insights
to be < 1.50 x 10-9 s-1,42 rate enhancements of up to 3 x 106-fold In order to gain insight into the mechanism of methyl paraoxon
can be obtained in the transesterification reaction by the proper transesterification with 1-propanol over the surface of the MgO
choice of the experimental conditions: favoring lower substrate catalyst, we performed DFT calculations and determined
concentrations avoids active site overload by the substrate itself adsorption energies for the reactants. Provided that the
and/or adsorption of products and leads to greater catalytic reaction is heterogeneously catalyzed, adsorption of the
activities.
reactants onto the oxide surface is an important, initial step of
the overall catalytic cycle, thus evaluating their adsorption
energies and detailed geometric parameters are indeed
Experiments of LC-ESI-MS
As described above, the MgO-mediated propanolysis of methyl insightful, as well as the relative positioning of the reactants
paraoxon was followed spectrophotometrically and prior to the reaction, as discussed later. In our calculations, we
characteristic UV-Vis spectra of p-nitrophenolate were used a slab model of the (100) surface of the bulk structure of
observed, suggesting that the reaction over the MgO surface periclase (MgO, Figure 3), in which a typical distance between
proceeds through the nucleophilic attack of the solvent on the adjacent surface oxygen atoms is 2.977 Å, while the typical
phosphorus of the substrate with p-nitrophenolate surface cell O-O diagonal distance is 4.210 Å and similarly for
displacement. In order to confirm this reaction pathway, we Mg-Mg distances (Figure 3). The MgO (100) surface has been
performed LC-MS experiments to characterize in detail the widely employed as a model in adsorption DFT calculations
reaction products, and the results show the formation of the because of the simple theoretical description for such ionic,
phosphate triester dimethyl n-propyl phosphate as the non-polar surface.56–58 Figure 3 also shows the geometries
transesterified product from the propanolyis of methyl obtained for non-adsorbed methyl paraoxon and 1-propanol,
paraoxon. Figure 2(A) shows a chromatogram obtained after while Figure 4 shows the final adsorbed structures obtained for
the reactants. In all calculations, the MgO slab was composed
of six atomic layers with the top three layers allowed to relax,
while the bottom three were fixed, and we employed the
Figure 3 Phosphate educt (top right) and 1-propanol (bottom right) structures
from gas phase optimization and (100) MgO surface model used (left).
Selected geometry parameters are highlighted (Å): (i) typical d(Mg-Mg) = d(O-
O) = 2.977, (j) typical d(Mg-Mg) = 4.182, d(O-O) = 4.235, (k) d(P=O) = 1.482, (l)
d(O-H) = 0.980, (m) d(C-O) = 1.448.
Figure 2 (A) Total ion current chromatogram of product ion of m/z 169, from
an aliquot of the reaction medium of the MgO-catalyzed methyl paraoxon
propanolysis at 4.61 x 10-5 mol g-1 (substrate/catalyst) after the reaction
finished, and (B) respective ESI(+)-MS/MS of the ion of m/z 169 eluted at 2.58
min.
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Figure 5 Distance map for adsorbed reactants, in which the orange dot
represents the phosphate educt and the blue dots represent different
adsorption sites for 1-propanol. Numbers represent the smallest d(O···P)
distances between reactive groups. Dot sizes are proportional to the square
of the distances.
Figure 4 Top (top) and side view (bottom) of adsorbed 1-propanol (left) and
phosphate educt (right). Selected geometry parameters are highlighted (Å):
(i) d(O···Mg) = 2.159, (j) d(OH···O) = 1.731, (k) d(P=O···Mg) = 2.250, (l) d(P=O)
= 1.492, (m) d(NO···Mg) = 2.675.
significant insights into a better comprehension of the overall
catalytic process from a mechanistic point of view. In this sense,
the reactants adsorb on the catalyst surface, where the
transesterification takes place and leads to the formation of
dimethyl n-propyl phosphate and 4-nitrophenol, and then the
products should desorb from the MgO surface in order to
regenerate the catalyst surface. In particular, the DFT
calculations enlighten that the reaction most probably occurs
when the reactants are found at adjacent Mg2+ sites, where the
distance between the nucleophilic and electrophilic centers is
ca. 2.4 Å. On the basis of the general scheme for phosphoryl
transfer reactions from phosphate triesters to oxygen
nucleophiles,62–64 and considering the basic character of the
MgO surface, the reaction should thus be favored by base
catalysis promoted by the catalyst surface, assisting
nucleophilic attack of 1-propanol on a juxtapositioned methyl
paraoxon molecule. This argument is in agreement with the
optimized structure for the proton dissociation from 1-propanol
over the MgO surface, which readily converged back to the 1-
propanol structure (a structure comparison is shown in Figure
S13). Thus, the probable adsorbed structure is 1-propanol and
not 1-propoxide, allowing the consideration that the latter
should only be formed as an incipient species along the reaction
coordinate, leading to either a short-lived phosphorane
intermediate or directly to the products through a single
transition state.
Grimme’s D3 dispersion correction, as these have been shown
to be adequate for calculations of adsorption energies when
interactions between lone-pairs of adsorbates and surface
Lewis acid sites play an important role.59–61
1-Propanol adsorbs through its O-H group by interacting
with a Mg-O group on the surface, in a structure that suggests
dipole-dipole interactions together with hydrogen donation to
the surface (Figure 4). The O···Mg contact obtained was 2.159
Å, while OH···O amounts to 1.731 Å. Furthermore, while the O-
H bond of 1-propanol elongates upon adsorption (d(O-H)
increases from 0.980 to 1.010 Å), its C-O bond shortens (d(C-O)
decreases from 1.448 to 1.431 Å).
The phosphate educt adsorption was modeled through a
P=O···Mg contact, which amounted to 2.250 Å (Figure 4). A
NO···Mg contact, from the nitro group of the ring to the surface,
was also observed (d(NO···Mg) = 2.675 Å). We also observe a
small elongation of the P=O distance upon adsorption (from
1.482 to 1.492 Å), as well as an elongation of the Ph-O bond of
the leaving group (from 1.387 to 1.403 Å).
Adsorption energies for 1-propanol and the phosphate
educt were found to be -18.4 and -57.9 kcal/mol, respectively,
which suggests that 1-propanol binds less strongly to the
surface than methyl paraoxon. Furthermore, the phosphate
product showed a smaller adsorption energy (-36.9 kcal/mol)
than the educt (Figure S12). In order to estimate the
relationship of reactive sites of the surface, Figure 5 shows an
approximate distance map of the surface, created by simple
translation of the optimized 1-propanol structure around the
surface, while keeping the phosphate educt static at the origin,
where each dot represents a Mg adsorption site. Since the
adsorption of both reactants on the same Mg2+ site is precluded
by steric reasons (in this case, d(O···P) = 1.419 Å), the shortest
possible distance between the two educts is around 2.4 Å, when
the reactants are adsorbed on adjacent Mg sites (Figure 5).
Altogether, the experimental and theoretical results provide
Catalyst recycling
Focusing on one of the greatest advantages of heterogeneous
catalysis, the reuse of the catalyst, we evaluated the activity of
the MgO employed in the propanolysis of methyl paraoxon at a
ratio of 4.6 x 10-5 mol g-1 (substrate/catalyst) in two further
sequential reactions. Provided that the products may adsorb
onto the catalyst surface, p-nitrophenol in particular, the
utilized MgO was subjected to a washing reactivation treatment
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between the reactions looking for the full recovery of the MgO paraoxon propanolysis, allowing the conduction of the
View Article Online
DOI: 10.1039/C9TA14028J
active sites (for details, see Experimental Section). First order adsorptive transesterification.
rate constants were calculated from the kinetic data of the
MgO-mediated propanolysis of dialkyl aryl and triaryl phosphate
triesters
three sequential reactions, and we obtained 2.83 x 10-3, 2.09 x
10-3 and 1.95 x 10-3 s-1, and within experimental error, these
values show that there was no loss of catalytic activity of the Since MgO showed to be highly active towards the
MgO in the propanolysis of methyl paraoxon.
transesterification of methyl paraoxon, we evaluated the
Importantly, we performed post-characterization catalytic activity of MgO towards the propanolysis of a range of
experiments to evaluate possible interference of the representative dialkyl aryl and triaryl phosphate triesters,
reactivation process in the structure and properties of MgO. whose structures are shown in Chart 1. The reactions were
TGA analysis after the first cleaning procedure of MgO show a performed under the optimized experimental conditions for the
mass reduction of 5% with increasing temperature up to 300 ºC propanolysis of methyl paraoxon, and the kinetics were
(TGA profile shown in Figure S6 in the ESI). Differently from that similarly followed spectrophotometrically in order to quantify
observed in the semi-quantitative EDXRF analysis before the the formation of the phenolic product as a function of time.
first use, the MgO reactivated after the second use shows the
For the propanolysis of the dialkyl aryl phosphate triesters,
presence of 0.5% of Na2O in the degassed material, a result that typical first order kinetic profiles were obtained, similar to the
indicates trace amounts of NaCl as a residue from the scenario described for methyl paraoxon, and thus the data for
reactivation procedure without promoting significant such substrates were fitted to an equation of the type of eq. 1.
modifications in the original structure of MgO (complete EDXRF First order rate constants obtained are presented in Table 2,
data is shown in Table S3 in the ESI). Though the ionic radius of along the rate enhancements, which highlight that MgO
sodium is greater than that of magnesium, which hinders the effectively promotes the transesterification of the three
incorporation of sodium into the MgO structure,65 the presence substrates. LC-MS analyses of aliquots of the reaction media
of sodium could eventually obstruct the catalyst pores, resulting after the reactions have finished show that a trialkyl phosphate
in lower surface area.66–69
ester is obtained, either dimethyl n-propyl phosphate or diethyl
Textural properties of the reactivated MgO after the third n-propyl phosphate (representative LC-MS data is shown in
use show a reduction of c.a. 35% of the specific surface area Figure S7 in the ESI).
(SBET = 114.3 m2 g-1), compared to that of MgO before the first
Interestingly, the absorbance changes in the kinetic profiles
use, which did not influence significantly the catalytic activity, of the propanolysis of the triaryl phosphates are as much as two
as shown by the above comparison of the first order rate or three times greater than that expected for the substitution
constants. The decrease of the specific surface area could be an of only one of the aromatic substituents for the T2PyP and
effect of the successive procedures for reactivating the catalyst
and for its thermic activation prior to the reactions, the latter
possibly resulting in the generation of active phases and
sintering processes,70 for example. Additionally, the recycled
MgO mean pore radius was determined to be 65.3 Å, a value
that is c.a. 45% greater compared to that of the original MgO,
indicating that the diffusion of bulky methyl paraoxon onto the
catalyst active sites is still allowed. Lastly, volume pore of
recycled MgO was 0.373 cm3 g-1, a value c.a. 6% lower than that
of the original catalyst.
Finally, evaluation of MgO basicity after its third use was
performed by CO2-TPD, and the results show a total base
density of 364.1 µmol CO2/g of catalyst, from which 226.5, 79.4
Chart 1 Phosphate triesters subjected to propanolysis over MgO.
and 58.2 µmol CO2/g of catalyst are discriminated as weak,
medium and strong base site density, respectively. Though a
Table 2 Rate constants, transesterified products and rate enhancements for
the propanolysis of dialkyl aryl phosphate esters catalyzed by MgO at 4.61 x
10-5 mol g-1 (substrate/catalyst), at 80 ºC.
reduction of c.a. 46% of the total base density of MgO is
observed in comparison to the scenario prior to the first use, it
seems that the basicity is still enough for the reaction without
loss of catalytic activity. Actually, MgO total base density after
third use is still satisfactory, being superior to the weakly basic
γ-Al2O3 support, for instance (see Table S5 in the ESI).
Furthermore, it is important to notice that the catalytic activity
of a solid material is rarely explained by means of one single
aspect, and generally several properties contribute
simultaneously to the catalytic efficiency. Thus, the MgO
properties after the third use seem to be sufficient altogether
to maintain the initial catalytic activity observed in the methyl
a
Substrate
DMDNPP
k1, s-1
Transesterified product
dimethyl n-propyl
phosphate
diethyl n-propyl
phosphate
dimethyl n-propyl
phosphate
k1/k0
5.13 x 10-3
3.56 x 104
3.88 x 102
3.43 x 103
DEDNPP
CPO
3.10 x 10-3
2.09 x 10-3
a k0 refers to the spontaneous solvolysis first order rate constants of the substrates
as benchmarks: propanolysis, in the case of DMDNPP, estimated by the initial
velocity method, and hydrolysis, for DEDNPP and CPO, obtained from
literature.44,71
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Table 3 Rate constants, transesterified products and rate enVhieawncAermticelenOtsnlfinoer
T4NPP substrates, respectively. Thus the kinetic profiles can not
be appropriately fitted to a monoexponential first order
equation, as shown in Figure 6. These observations indicate that
two or three aromatic moieties of the substrate are substituted
by 1-propanol in two or three consecutive first order
transesterification reactions, as depicted in Scheme 2, with rate
constants significantly different. Thus, the kinetic profiles were
fitted to equations in which the absorbance at a given time is
related to the sum of the concentrations of phenolic product
arising from the first, second, and third step, when applicable
(equations are given in the ESI).
Table 3 presents the first order rate constants k1, k2 and k3,
which refer to the first, second and third steps, respectively, in
the propanolysis of the triaryl substrates. Table 3 shows that the
rate constant for the substitution reaction of one aromatic
substituent (first step, k1) is up to c.a. 5x104-fold greater than
those of the spontaneous propanolysis of the substrates.
Interestingly, it is worth noting that the time required for the
substitution of two aromatic substituents in the T2PyP by 1-
propanol is c.a. 750-fold lower than that required for the
substitution of solely the first equivalent in the spontaneous
the propanolysis of triaryl phosphate esters catalyzed by MgO at 4.61 x 10-5
DOI: 10.1039/C9TA14028J
mol g-1 (substrate/catalyst), at 80 ºC.
c
Rate constants (s-1)
Transesterified products
T2PyP a
k1/k0
k1 = 2.79 x 10-3
k2 = 3.40 x 10-4
bis-2-pyridyl n-propyl phosphate
2-pyridyl bis-n-propyl phosphate
T4NPP
6.17 x 103
–
k1 ≥ 7.15 x 10-2 b
k2 = 9.36 x 10-4
k3 = 1.55 x 10-4
bis-4-nitrophenyl n-propyl
phosphate
4-nitrophenyl bis-n-propyl
phosphate
5.14 x 104
–
–
tris-n-propyl phosphate
a LC-MS analyses show that the third step does not occur to a significant extent for
the T2PyP substrate in the time domain at which the reaction was followed. b Must
be rather considered as a minimum value, since the first reaction was too fast to
c
collect enough experimental points in its time domain. k0 refers to the
spontaneous propanolysis first order rate constants of the substrates, estimated
by the initial velocity method.
solvolysis of the substrate, evidencing the high activity of MgO
toward
successive
transesterification
reactions
of
organophosphate esters. In the case of the T4NPP substrate,
the time required for substitution of the three aromatic
aromatic moieties is some 100-fold lower compared to that for
the spontaneous substitution of just the first substituent.
Experiments of LC-ESI-MS in the positive ion mode are in
agreement with the proposed pathways of Scheme 2. For
example, from a chromatographic separation of the reactants,
intermediate and products in the MgO-catalyzed propanolysis
of the T2PyP at approximately 40 min, it is possible to extract
the chromatograms of Figure 7(A) that show the elution of the
species with m/z 317 and 282, consistent with the sodium
adducts of n-propyl bis-2-pyridyl phosphate and bis-n-propyl 2-
pyridyl phosphate, respectively. Additionally, the identification
of the above mentioned phosphates is confirmed by the ESI(+)-
MS spectra of the products. In the spectrum of the product
eluted at 3.24 min, Figure 7(B), besides the sodium adduct of
the n-propyl bis-2-pyridyl phosphate, it is observed the ion of
m/z 295, consistent with a protonated species of the triester,
and the ion of m/z 253, formed by the loss of CH3CH2=CH2 from
the ion of m/z 295. Similarly, the ESI(+)-MS spectrum of the
product eluted at 3.45 min, Figure 7(C), is fully consistent with
the formation of the bis-n-propyl 2-pyridyl phosphate, the
species with m/z 282 and 260 being consistent with the sodium
adduct and the protonated species of the triester, respectively.
In Figure 7(C), it is also possible to identify the species of m/z
218 and 176, formed by the loss of one and two CH3CH2=CH2
groups, respectively, from the m/z 260. This attribution is
consistent by the ESI(+)-MS/MS spectrum obtained for the ion
of m/z 260 (Figure S8 in the ESI).
Figure 6 Kinetics of the propanolysis reactions of T2PyP and T4NPP catalyzed
by MgO, both at 4.6 x 10-5 mol g-1 (substrate/catalyst), at 80 ºC. The
concentration of the aromatic product was expressed in the y-axis as the
number of equivalents relative to the substrate.
4. Conclusions
Herein, we reported the propanolysis of methyl paraoxon
catalyzed by a commercial magnesium oxide at 80 ºC. A series
of kinetic experiments with varying ratios of moles of substrate
Scheme 2 Successive propanolysis reactions observed with triaryl substrates.
OAr represents an aromatic substituent of the phosphate triester.
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called contact distances, which are usually shorter than solvent
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DOI: 10.1039/C9TA14028J
diameter, with the observed reactivity ascribed to the
persistent interaction between the functionalities (provided
they are found at appropriate geometries) overcoming the time
required for reactants to diffuse and collide effectively in the
corresponding bimolecular counterpart. Thus, in our system,
besides the fact that the acid Mg2+ sites could facilitate the
reaction by enhancing the substrate electrophilicity and/or
stabilizing partial negative charges along the reaction
coordinate,64 an important part of the catalytic efficiency
should be consequence of the ability of the MgO surface to offer
adjacent Mg2+ sites: this allows for the juxtapositioning of the
reactants for persistent interaction at adequate distances, as
supported by the distance map obtained by DFT calculations.
Additionally, the basicity is clearly important to explain the
MgO catalytic efficiency contributing with a significant increase
in reactivity of the nucleophile. To test this hypothesis, we
performed a set of kinetics with the weakly basic γ-Al2O3 to
evaluate its catalytic activity towards the propanolysis of methyl
paraoxon (complete results of commercial γ-Al2O3
characterization and kinetic data are shown in the ESI). The
largest first order rate constant obtained for the reaction in the
presence of γ-Al2O3 was 1.20 x 10-4 s-1 at a ratio of 4.6 x 10-5 mol
g-1 (substrate/catalyst), a rate constant which is some 24-fold
lower than the calculated first order rate constant of 2.83 x 10-
3
s-1 for the reaction over the MgO surface at same ratio of
substrate moles/catalyst mass. Provided that γ-Al2O3 and MgO
present similar textural properties, but MgO base site density is
c.a. 3.5-fold greater, this result is a good indication of the
importance of basicity as a factor to explain the greater catalytic
activity of MgO toward methyl paraoxon propanolysis. More
basic surfaces facilitate proton abstraction and nucleophilic
attack upon the phosphorus atom.
Since MgO is shown to be significantly active in the
propanolysis of a range of phosphate triesters besides methyl
paraoxon, the results are encouraging for the design of novel
efficient solid catalysts for targeted transesterification
processes. In addition to convenient textural properties,
basicity seems to play an important role in the efficiency of
metal oxides in catalyzing the propanolysis of phosphate
triesters, highlighting the importance of the exploration of the
solids’ properties in the continuing design of simple metal oxide
catalysts.
Figure 7 (A) Extracted ion chromatograms of the species of m/z 317 and 282
from an aliquot of the reaction medium of the MgO-catalyzed propanolysis of
T2PyP, obtained by LC-ESI(+)-MS, and complete ESI(+)-MS spectra of the
products eluted at: (B) 3.24 min, and (C) 3.45 min.
per mass unit of catalyst was performed, in the range 5 x 10-6 to
5 x 10-3 mol g-1 (substrate/catalyst), and the results showed that
rate enhancements up to 3 x 106-fold can be obtained
depending upon the experimental conditions, in comparison
with the spontaneous propanolysis of the substrate. The MgO-
catalyzed propanolysis of methyl paraoxon leads to the
formation of dimethyl n-propyl phosphate in a process that
converts a toxic organophosphate into a phosphate triester
structurally analogous to
a family of flame-retardant
compounds. Additionally, MgO could be reactivated through a Conflicts of interest
simple washing procedure, maintaining its catalytic activity
There are no conflicts to declare.
when employed in two additional sequential batches.
Furthermore, MgO presented high catalytic activity toward the
transesterification of a series of phosphate triesters, and in the
Acknowledgements
particular
case
of
triaryl
substrates,
successive
We dedicate this paper to Prof. Faruk Nome, deceased since
September 24, 2018, who dedicated more than five decades to
teaching and research. With his enthusiasm for science, he
became a paragon of commitment to research among his
colleagues and collaborators, leaving a legacy that will certainly
be transmitted to generations of students to come. The authors
transesterification reactions were observed.
Although our system is heterogeneous, the catalytic effects
promoted by MgO in the propanolysis of methyl paraoxon can
be nicely viewed in light of the spatiotemporal theory for
intramolecular reactions of organic molecules in solution.72–76 In
this approach, reactive groups are considered to be caged at so-
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24 S.-W. Zhan, W.-B. Tseng and W.-L. Tseng, Environmental
thank the Brazilian funding agencies CNPq, Capes and FAPESC,
and the National Institute of Science and Technology in
Molecular and Nanostructured Systems (INCT-Catálise). E.H.W
thanks São Paulo Research Foundation (FAPESP, grant
#2019/07899-5). G.F.C. thanks CNPq (grant 311963/2017-0) for
the research fellowship, and F. S. S. S. thanks CNPq for a PhD
scholarship (grant 140485/2017-1).
View Article Online
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