Article pubs.acs.org/JPCC
Propanolysis of Methyl Paraoxon in the Presence of AluminumTitanate-Supported Erbium Oxide Gizelle I. Almerindo,† Priscila Bueno,† Lucas M. Nicolazi,† Eduardo H. Wanderlind,† Patrícia Sangaletti,† Sandra M. Landi,‡ Lidia A. Sena,‡ Braulio S. Archanjo,‡ Carlos A. Achete,‡ Haidi D. Fiedler,*,† and Faruk Nome*,† †
Instituto Nacional de Ciência e Tecnologia de Catálise em Sistemas Moleculares e Nanoestruturados (INCT-Catálise), Departamento de Química, Universidade Federal de Santa Catarina (UFSC), CEP 88040-900 Florianópolis, Santa Catarina, Brazil ‡ Divisão de Metrologia de Materiais (Dimat), Diretoria de Metrologia Científica e Industrial (Dimci), Instituto Nacional de Metrologia, Qualidade e Tecnologia (Inmetro), CEP 252502-020 Xerém, Duque de Caxias, Rio de Janeiro, Brazil S Supporting Information *
ABSTRACT: Er2O3/Al2TiO5 and Al2TiO5 were evaluated as catalysts for the propanolysis of the organophosphate pesticide dimethyl 4-nitrophenyl phosphate (methyl paraoxon). The solid catalysts were characterized by energy-dispersive X-ray fluorescence (EDXRF), N2 physisorption (BET and BJH methods), and scanning and transmission electron microscopy (SEM and TEM). Physical−chemical characterizations revealed that the erbium content is 11.7% w/w in the novel solid and results in improved catalyst performance when compared with Al2TiO5, with textural properties favorable for methyl paraoxon diffusion in agglomerates composed of polydisperse spherical nanoparticles. Kinetic measurements at 80 °C show that the Er2O3/Al2TiO5 catalyst promotes a catalytic effect of at least 7 × 105-fold when compared with the first-order rate constant of the spontaneous propanolysis of methyl paraoxon.
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INTRODUCTION
A variety of complexes of lanthanide ions have been shown to be potentially useful for the detoxification of pesticides and warfare nerve agents.6−9 And the catalytic efficiency of the parent lanthanide ions (Ln3+ = La, Sm, Tb, Eu, and Er) in homogeneous systems was tested in hydrolysis reactions of phosphate esters.9,10 Although homogeneous catalysts were found to be efficient in the hydrolysis of phosphate esters, the separation and recovery of the catalysts is generally difficult and expensive. In this context, the development of heterogeneous systems is desirable and has been object of study.3,11,12 The use of Er3+ in catalytic systems shows numerous advantages, such as the low toxicity of its salts,13−15 low prices,16 ability to act as a dopant to increase stability of the catalytic supports,17 and small radius that contributes to its high oxyphilicity and Lewis acidity.18 Indeed, erbium catalysts have shown to be efficient in a large number of systems, which include the protection and deprotection of alcohols and carbonyl compounds,19−21 the conversion of cellulose to lactic acid,22 and the formation and cleavage of O-tert-butoxy carbonates.21 On the basis of this collection of interesting properties, we decided to examine the ability of Er3+ in heterogeneous catalytic systems to promote the cleavage of phosphate esters. We report a detailed study of the
The properties of phosphate triesters are significantly different from those of the more familiar mono- and diesters. Because they are polar but uncharged molecules, only the lower trialkyl derivatives are soluble in water, and triesters have been introduced into the biosphere primarily as agrochemicals. As a consequence, their chemistry becomes of intense interest because of the need to render harmless stockpiles of some organophosphorus biocides, which include nerve gases as well as herbicides and pesticides.1−6 The objective is to engineer an efficient phosphate transfer to a nucleophile (water or a shortchain alcohol) to convert the active triester agent to an unreactive product (Scheme 1).4,7−10 The literature shows many studies of the destruction, decomposition, and decontamination of these compounds by means of incineration, base-catalyzed hydrolysis, and the use of alkoxides and peroxides and of reactive alpha-nucleophiles.1 Despite these continuing efforts, the development of better cost-effective cleanup technologies is still needed.2 Recently, we reported a heterogeneous system efficient for the degradation of dimethyl 4-nitrophenyl phosphate (methyl paraoxon), whereby an incipient magnesium aluminate spinel (MgAl2O4) catalyzed the propanolysis of methyl paraoxon about 2.5 × 105 times faster than the spontaneous propanolysis reaction to give dimethyl n-propyl phosphate, a product that is structurally related to a family of flame retardants. © 2016 American Chemical Society
Received: June 16, 2016 Revised: August 23, 2016 Published: September 12, 2016 22323
DOI: 10.1021/acs.jpcc.6b06085 J. Phys. Chem. C 2016, 120, 22323−22329
Article
The Journal of Physical Chemistry C Scheme 1. General Scheme for the Reaction of a Phosphate Triester with an Alcohola
a
The reaction can be catalyzed by better nucleophiles and/or general bases.
propanolysis of methyl paraoxon in the presence of aluminum titanate (Al2TiO5) and of Er3+ supported on Al2TiO5, which shows that the effect of Er2O3/Al2TiO5 on the degradation of methyl paraoxon is a large increase in the rate of propanolysis with the formation of a trialkyl phosphate, opening up the use of novel solid catalysts featuring different lanthanide sites for the targeted transesterification of toxic organophosphates into value-added compounds.
microscopy (SEM) using a Magellan 400 (FEI Company) equipped with a field-emission gun (FEG), operating at an accelerating voltage of 15 kV and 100 pA current. For SEM analyses, samples were dispersed in isopropyl alcohol at ca. 0.1 mg/mL and sonicated for 20 min; then, 100 μL aliquots were dripped on cleaved silicon substrates (10 × 10 mm2) and let dry. Transmission Electron Microscopy/Scanning Transmission Electron Microscopy (STEM/TEM). Transmission electron microscopy/scanning transmission electron microscopy (STEM/TEM) analysis was performed in a Cs probe-corrected Titan 80-300 (FEI Company) transmission electron microscope operating at 300 kV accelerating voltage. EDS (energydispersive X-ray microanalysis) in STEM mode was used to investigate the presence of erbium in the nanostructures. In STEM analyses, a high-angle annular dark filed (HAADF) detector was used. For STEM/TEM analyses, samples were dispersed in isopropyl alcohol at ca. 0.1 mg/mL and sonicated for 20 min; then, 10 μL aliquots were dripped on holey carbon copper grid (from Electron Microscopy Sciences) and let dry. N2 Adsorption/Desorption Isotherms. The textural properties were determined by N2 physisorption using a Nova 2200e Quantachrome instrument. The samples were degassed at 300 °C under vacuum for 1 h prior to N2 physisorption. Brunauer− Emmett−Teller (BET) and Barret−Joyner−Halenda (BJH) methods were used to determine the specific surface areas and pore size distributions, respectively. Temperature-Programmed Desorption of CO2 (CO2-TPD). The CO2-TPD analyses were performed using a Quantachrome ChemBET 3000 instrument. The catalysts were treated in situ under a helium atmosphere (100 mL min−1) at 300 °C for 50 min; then, the samples were saturated with 100 mL CO2 min−1 for 40 min at room temperature. 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 °C min−1 reaching a final temperature of 800 °C. Catalyst Activity and Kinetics Measurements. The general procedure for the kinetic studies was performed as described in the literature.3 Predried catalyst (150 mg) at 300 °C for 20 min was mixed with 10 mL of dried 1-propanol (stored with 3 Å molecular sieves). An aliquot of the stock solution of methyl paraoxon in acetonitrile was added to a mixture of catalyst and 1-propanol (to achieve final concentrations of 6.9 × 10−5 and 6.9 × 10 −4 mol L−1) under a continuous magnetic stirring of 640 rpm and 80 °C. The ratios of moles of substrate per mass unit of catalyst used were 4.6 × 10−6 and 4.6 × 10−5 mol g−1. The quantification of one of the products (p-nitrophenol) was carried out by visible spectroscopy at 405 nm using an HP-8453 spectrophotometer. First-order rate constants for the reactions were obtained by
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EXPERIMENTAL SECTION Materials. Aluminum titanate (Al2TiO5) and metallic erbium were purchased from Sigma-Aldrich. Dimethyl 4nitrophenyl phosphate (methyl paraoxon) was prepared by classical methods described in the literature for the synthesis of this compound.3,23,24 Catalyst Preparation. Initially, Al2TiO5 was alkalinized in NaOH 1 M. Then, a known amount of the Er0 (15% by wt) was dissolved in HCl 0.1 M suprapur and mixed slowly with the Al2TiO5 mixture under stirring. The resultant solid was centrifuged, washed with deionized water to a neutral pH, and dried at 105 °C during 10 h. Er2O3/Al2TiO5 sample was obtained by calcining the dried samples at 450 °C in airflow for 4 h. A sample of Al2TiO5 was also subjected to the same treatment in the absence of erbium and used as a blank to test the effect of the preparation of the catalyst in the catalytic properties of the original Al2TiO5 sample. Catalyst Characterization. Thermogravimetric Analysis (TGA). TGA analysis was performed with a Shimadzu TGA-50 analyzer using 10 mg of sample with a heating rate of 10 °C min−1 and airflow of 20 mL min−1. Analysis of Energy-Dispersive X-ray Fluorescence (EDXRF). Analysis was undertaken in a temperature-controlled room (23 ± 1 °C) using a S2 Ranger spectrometer (Bruker, Germany). The S2 Ranger measurements were carried out using a Pd Xray tube, operated with Cu filter and 50 kV and 250 μm. The acquisition time was 250 s (measurement time per region in the presence of air), and the X-rays used to excite the sample were produced using a 50 W, 50 kV/2 mA Xray VF50 tube. The tube and generator are capable of operating at voltages ranging from 10 to 50 kV and currents from 1 to 2000 μA, providing that a maximum power of 50 W is not exceeded. The Er, Ti, and Al content (expressed as Er2O3, TiO2, and Al2O3) of the catalyst was quantified using glass discs (pearls). An automatic sampler and the EQUA-OXIDES software application were used for instrument control, data collection, and data analysis (Bruker, Germany). The certified reference material Natural Moroccan Phosphate rock (BCR 032), Lead Glass (BCR-126A), and Road Dust (BCR723) were used for calibration, validation, and monitoring of the analysis. Scanning Electron Microscopy Analysis. The morphology of the samples was investigated in a scanning electron 22324
DOI: 10.1021/acs.jpcc.6b06085 J. Phys. Chem. C 2016, 120, 22323−22329
Article
The Journal of Physical Chemistry C iterative least-squares fitting of the profiles of absorbance against time. Characterization of Products. In addition to the quantitative determination of the product 4-nitrophenol by UV−vis spectrophotometry, analyses of high-performance liquid chromatography coupled to mass spectrometry with electrospray ionization were carried out using an Applied Biosystems/MDS SCIEX 3200 Q TRAP LC−MS/MS System. Chromatographic separation was performed using a Phenomenex Synergi Polar-RP 80 Å column (150 mm length; 2.0 mm i.d.; 4 μm particle size), at 20 °C, with a mobile phase flux of 200 μL min−1. Mobile phase was constituted of (A) methanol/ water (95:5, v/v) and (B) aqueous solution of formic acid 0.1%, and a gradient elution was performed as follows: 0−2 min, 50% of each solvent; 2−5 min, 50% to 95% of solvent A; 5−15 min, 95% of solvent A.
with a significant change in pore volume and pore radius, indicating that the presence of erbium and the preparation method caused such changes. It is interesting to note that the pore radius of the erbium-doped samples is much greater than that of the estimated radius of the methyl paraoxon molecule (5.1 Å), and accordingly, pore diffusion should not be a limitation to the catalytic activity of the Er2O3/Al2TiO5 samples.3 Figure 1 presents TEM images of Al2TiO5 (Figure 1a,b) and Er2O3/Al2TiO5 (Figure 1c,d), which, in general, are quite
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RESULTS AND DISCUSSION Catalyst Characterization. The support (Al2TiO5) chosen to be used in this work presents interesting features, such as low thermal expansion coefficient, high thermal shock resistance, low thermal conductivity, and high melting point.25 Consistent with the properties described above, the thermogravimetric analysis for the Er2O3/Al2TiO5, presented in Figure S1 (see Supporting Information), shows only ca. 2% weight loss between 30 and 300 °C. Despite this small weight loss value it was decided to thermally activate (300 °C) the catalyst before the catalytic tests and subject the sample to N2 physisorption analysis, to ensure complete availability of active sites and a quantitatively characterized specific surface area. The composition of Er2O3/Al2TiO5 catalyst was determined by quantitative EDXRF analysis. The results shown in Table 1
Figure 1. TEM images of samples: (a,b) Al2TiO5 and (c,d) Er2O3/ Al2TiO5. In panel c, the arrows point to Er-rich and Er-free regions. In panel d, a higher resolution TEM image of the erbium-rich regions is shown: the white arrows might indicate the presence of sub-10 nm size erbium oxide nanoparticle supported in an Al2TiO5 nanospheroid.
similar, showing the presence of agglomerates made of polydisperse nanospheroids with diameters below 100 nm. SEM images in lower magnification can be seen in Figure S2 in the Supporting Information. TEM images of Al2TiO5 at low magnification show smooth changes in the contrast along the nanospheroids: Conversely, the catalyst with erbium was observed to present morphologically different regions. The two different regions are indicated by black arrows in Figure 1c. The region having a higher contrast variation inside the nanospheroids was identified via nanoscale chemical analyses (EDS) to have high erbium concentration (see also Figure 2). Looking at higher magnification (Figure 1b,d), both Al2TiO5 and erbium-rich Er2O3/Al2TiO5 regions can show similar contrast variations, although we can recognize defined edges in erbium-rich Er2O3/Al2TiO5 nanospheroids, as highlighted by the white arrows in Figure 1d. High-resolution TEM (HRTEM) images of samples with and without erbium show that the nanoparticles are mostly crystalline (Figures S3−S5 in the Supporting Information). To further understand how the erbium oxide is supported on aluminum titanate, we used STEM-HAADF and EDS nanoscale chemical analyses. Figure 2a shows a low-magnification STEM image where the square in the center was scanned, and its chemical maps are shown in Figure 2b. The Er2O3/Al2TiO5 sample clearly shows an uneven distribution of erbium on the surface of the support (Al2TiO5). A typical EDS spectrum from the erbium-rich areas is shown in the inset. A single Al2TiO5 nanospheroid from an erbium-rich area can be seen in Figure 2c. This image clearly shows brighter regions, which have high concentrations of erbium and oxygen (confirmed by EDS). Also, the brighter
Table 1. Results of EDXRF for the Characterization of Er2O3/Al2TiO5 on Glass Disc Beads composition
concentration, w/w (%)
Al2TiO5 Er2O3 mineral part LOI (n = 2) 450 °Ca total
85.7 ± 0.2 11.7 ± 0.1 97.4 2.6 ± 0.2 100.0 ± 0.5
a
n = number of individual determinations. LOI = Fire assay (mainly H2O + CO2).
indicate that 11.7% by weight of the sample is constituted by Er2O3 and 85.7% by Al2TiO5. However, it is important to note that when the analysis is expressed as mole %, the sample contains 93.9% of Al2TiO5 and only 6.1% of Er2O3. As can be seen in Table 2, the catalyst containing erbium (Er2O3/Al2TiO5) showed a small decrease in surface area when compared with commercial support (Al2TiO5). However, there are significant differences in the values of textural properties Table 2. Textural Properties Calculated from N2 Adsorption and Desorption Isotherms catalyst
SBET (m2 g−1)a
VPBJH (cm3 g−1)b
RP (Å)c
Al2TiO5 Er2O3/Al2TiO5
40.0 36.0
0.06 0.22
35.3 119.8
a
SBET: specific surface area. bVPBJH: pore volume. cRP: pores mean radius. 22325
DOI: 10.1021/acs.jpcc.6b06085 J. Phys. Chem. C 2016, 120, 22323−22329
Article
The Journal of Physical Chemistry C
Er2O3/Al2TiO5 was performed at 80 °C by adding a small aliquot of a methyl paraoxon stock solution to a mixture of catalyst and 1-propanol using ratios of moles of substrate per mass unit of catalyst of 4.6 × 10−6 and 4.6 × 10−5 mol g−1. The kinetics were followed using UV−visible spectroscopy to quantify the 4-nitrophenolate ion. Using a ratio of 4.6 × 10−6 moles of methyl paraoxon per gram of Er2O3/Al2TiO5, a typical first-order profile is obtained, with a rate constant of (1.17 ± 0.15) × 10−3 s−1 (Figure 3A), and the first-order rate constant is consistent with a catalytic effect of at least 7 × 105-fold in the propanolysis of methyl paraoxon, in comparison with the spontaneous propanolysis rate constant, which has been estimated to be
