Article pubs.acs.org/JPCC
Degradation of Methyl Paraoxon in the Presence of Mg2+-Al3+ Mixed Oxides Lizandra M. Zimmermann,† Gizelle I. Almerindo,† José R. Mora,† Ivan H. Bechtold,‡ Haidi D. Fiedler,† and Faruk Nome*,† †
Department of Chemistry, INCT-Catalysis and ‡Department of Physics, Federal University of Santa Catarina, Florianópolis, Santa Catarina 88040-900, Brazil S Supporting Information *
ABSTRACT: Al3+-Mg2+ mixed oxides were prepared by coprecipitation and characterized with scanning electron microscopy (SEM), energy dispersive Xray fluorescence (EDXRF), temperature programmed desorption of CO2 (CO2-TPD), and N2 adsorption/desorption isotherms (BET and BJH methods). By increasing the MgO concentration up to 31.8% (w/w), X-ray diffraction (XRD) measurements suggested an incipient magnesium aluminate spinel (MgAl2O4) phase. However, the spinel crystalline structure was obtained only after calcination at 950 °C. These materials were tested as catalysts in the propanolysis reaction of methyl paraoxon. This reaction in the presence of the more efficient incipient MgAl2O4 spinel is of the order of 2.5 × 105-fold faster than the spontaneous propanolysis reaction and results in the formation of a product that is structurally related to a family of flame retardants. The different products of propanolysis and hydrolysis were identified by electrospray ionization mass spectrometry (ESI(+)-MS), ESI(+)-MS/MS) and liquid chromatography mass spectrometry (LC-MS/MS).
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INTRODUCTION Phosphate triesters are widely used as pesticides, plasticizers, nerve agents, for heavy metal complexation, and as flame retardants in a variety of polymeric systems.1−3 It is known that these compounds are harmful to the environment and human beings and that the P−O bonds are chemically stable.4−8 In this context, the efficient breakdown and detoxification of phosphate triesters remains a significant challenge1,9,10 and the preparation of a solid catalyst with suitable surface characteristics is desirable. The efficient decomposition of organophosphorus compounds/poisons is a longstanding problem, and the development of solid catalysts that can destroy the large available stockpiles of these compounds is important.1,11−14 To address this issue, the use of heterogeneous catalysis is an interesting approach due to its simultaneous capacity to degrade these compounds and to adsorb the products produced.14−17 Easy separation and recycling of the catalyst from the reaction mixture and, in many cases, the possibility of preparing solid catalysts with different types of active sites (Bronsted, Lewis, or chiral), with a wide range of basic strengths, or even the use of acid−base bifunctional catalysts, are some of the advantages offered by heterogeneous catalysis.18−20 Oxides, such as MgO,21−24 CaO,25 CaO/ FeO3,15 and Al2O3,16,26 have been applied in the degradation of organophosphates, but there is limited knowledge regarding the interaction of organophosphorus compounds with the surfaces of the heterogeneous systems.16,17,27 © 2013 American Chemical Society
The efficiency of homogeneous systems for the degradation of phosphate esters is well-known and a series of in micellar systems involving efficient α-nucleophiles28−30 or metal ions (metallomicelles)31−33 have been examined in detail, showing ingenious exploitation of metallomicellar systems for phosphate hydrolysis,31,32 which help us in our understanding of the role of metal ions in enzymes. Although, homogeneous systems have proven to be very efficient in the hydrolysis procedure, separation and recovery of the catalysts is not always convenient, and we focus our effort in developing a heterogeneous system. In fact, the catalytic efficiency of the Mg2+ covered γ-Al2O3 was tested in aqueous solutions on the hydrolysis of ethyl 2,4dinitrophenyl phosphate (EDNPP), which was chosen since phosphodiester reactions with nucleophiles are well characterized.34 The hydrolysis of EDNPP in the presence of Mg2+ covered γ-Al2O3 proceeds with a rate constant 4-fold faster than hydrolysis of EDNPP in a 1.0 mol L−1 solution of sodium hydroxide. Since a Mg2+ saturated γ-Al2O3 surface efficiently promotes hydrolysis of phosphate diesters, we decided to examine the catalytic efficiency of Mg2+/Al3+ mixed oxides as catalysts for the degradation of organic phosphates. Considering the capacity for the adsorption of organophosphates at the active sites of MgO21−24 and Al2O316,26 and the activity of Mg2+ Received: September 2, 2013 Revised: November 19, 2013 Published: November 20, 2013 26097
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covered γ-Al2O3 in the hydrolysis of EDNPP,34 it seems relevant to investigate solid mixtures of these oxides, which would contain two Lewis acid sites (Mg2+ and Al3+), as well as the high surface area of γ-Al2O3 and the high adsorption capacity of both oxides.19 Intermediate Mg/Al compositions may show changes in coordination (spinel phases), which will affect the density and strength of the acid/basic sites. In this study, we report studies of the degradation of methyl paraoxon (DMPNPP) in the presence of magnesium oxide, aluminum oxide, and magnesium oxide and aluminum oxide solid mixtures, obtained by the coprecipitation method.19,35 For the degradation of DMPNPP, we choose 1-propanol as a solvent because the reaction product, in the presence of catalysts, is dimethyl n-propyl phosphate, a compound structurally related to a family of flame retardants and transforms the pesticide Paraoxon in a useful commercial product. The effect of catalysts on the solvolysis of methyl paraoxon was investigated by evaluating: (i) the influence of the Al/Mg molar ratio and the calcination temperature on the structure of the resultant mixed oxides catalyst, and (ii) the influence of the substrate/catalyst ratio on the rate constants of the solvolysis reaction in the presence of the prepared catalysts.
exceeded. The Mg2+ and Al3+ content (expressed as MgO and Al2O3) of the solid catalysts were quantified using glass discs, and an automatic sampler and the EQUA-OXIDES software application were used for instrument control, data collection, and data analysis (Bruker, Gemany). Before analysis, instrument calibration and a stability check were performed: sample preparation followed described procedures.36 All the materials were dried before use at 300 °C over a period of 3 h. Glass discs were prepared by mixing 0.5 g of powdered sample and 5.0 g of lithium metaborate (34%)/ tetraborate (66%) flux (PANalytical X-ray flux Type 66:34; both sample and flux had been dried at 105 °C, following the procedure described in ASTM D221637 and were fused in a platinum/gold and rhodium crucible (PANanalytical EC2R) in an Eagon 2 automatic fusion machine (equipped with gas burner and muffle furnace) at 1100 °C for 10 min, with periodic swirling. On removal from the furnace the melt was poured automatically into a platinum and gold casting disk (PANanalytical) and cooled down in a 5 min program. The glass discs produced are about 3 mm thick with a 31 mm diameter. X-ray Diffraction (XRD). The XRD measurements were carried out with the X′PERT-PRO (PANalytical) diffractometer using Cu Kα radiation (λ = 1.5418 Å) with an applied power of 1.2 kVA. The scans were performed in continuous mode from 10 to 90° (θ−2θ geometry). The powder was slightly compressed to be flat in the cavity of the sample holder, which was rotated with a rotation time of 2.0 s during diffraction pattern collection. Scanning Electron Microscopy (SEM). SEM micrographs were obtained on JEOL JSM-6390LV equipment, version 1.0, operating at 30 kV. Dried powdered catalysts were dispersed on adhesive tape and covered with gold to prevent surface charging. N2 Adsorption/Desorption Isotherms. All nitrogen adsorption and desorption measurements were performed at −196 °C using a Nova 2200e Quantachrome instrument. In all cases, samples were degassed at 130 °C under vacuum for 3 h. The Brunauer−Emmett−Teller (BET) method was applied in data obtained in the P/Po range between 0.05 and 0.30 to determine specific surface areas Total pore volume and pore size distribution were obtained according to the Barret, Joyner, and Halenda (BJH) method. Temperature Programmed Desorption of CO2 (CO2-TPD). The CO2-TPD measurements were performed using a Quantachrome ChemBET 3000. The samples were treated in situ under a helium atmosphere (100 mL min−1) at 288 °C for 2 h then 500 °C for 1 h before cooling to room temperature. CO2 adsorption was performed by a stream of 100 mL CO2 min−1 at room temperature for 40 min. After the gas phase, physically adsorbed CO2 was purged by means of a helium flow at room temperature for 30 min. 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. Synthesis of DMPNPP. The phosphate triester DMPNPP was prepared as reported in the literature38 and its purity checked using a GC−MS (Shimadzu QP5050A) using a DB− 5MS capillary column (length, 30 m; diameter, 0.25 mm; film thickness, 0.25 μm) and 31P NMR (Supporting Information, Figures S1−S3, Table S1). Product Identification. The quantitative determination of the product 4-nitrophenolate was carried out by using a UV− vis spectrophotometer (HP-8453). The products were
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EXPERIMENTAL SECTION Catalyst Preparation. Al(NO3)3·9H2O (Fluka Analytical P.A) and Mg(NO3)2·6H2O (VETEC P.A) were used as received. Samples of γ-Al2O3, MgO, and the Al3+-Mg2+ mixed-oxide catalyst series were prepared using the coprecipitation method. Milli-Q Nanopure water (resistivity 18 ± 0.2 MΩ cm) was used in all cases. Appropriate amounts of Mg(NO3)2·6H2O and Al(NO3)3· 9H2O were dissolved in water to prepare solutions containing 1.0 mol L−1 of each metal, and the solution was then mixed with a 1.5 mol L−1 ammonium carbonate solution (SigmaAldrich), which was used as precipitating agent, keeping the pH of the solution approximately equal to 8.0. The precipitation was carried out under constant stirring (875 rpm) during 4 h in an oil bath at 95 °C and then, the precipitate was aged for 10− 12 h. The resultant solid was centrifuged, washed with deionized water, and dried at 120 °C during 48 h. The thermal treatment of the amorphous solid involved standard calcination with the sample heated in a muffle furnace with air flow, with the temperature raising at a heating rate of 10 °C min−1 up to a final temperature of 500 °C, which was kept for 4 h. Pure alumina and MgO were prepared following the same procedure. In order to study the temperature effect on the structure, the catalytic material containing 31.8% (w/w) MgO was also calcined at 550, 650, and 950 °C. After heat treatment solids were crushed, ball milled, and sieved to 230 mesh (particle size < 63 μm) by grinding in air atmosphere in a mill where the ball and bowl were ceramic materials. Catalyst Characterization. Analysis Energy Dispersive Xray Fluorescence (EDXRF). Analysis was undertaken in a temperature-controlled room (23 ± 1 °C) using a S2 Ranger (Bruker, Germany). The S2 Ranger measurements were carried out using a Pd X-ray 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 the maximum power of 50 W is not 26098
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identified by ESI(+)-MS, ESI(+)-MS/MS, and LC-MS/MS (Supporting Information, Figure S4). Products were separated using a high performance liquid chromatography (HPLC) system (Agilent Technologies, Waldbronn, Germany) coupled to ESI(+)-MS/MS using a C18 column (length, 30 mm; diameter, 2.0 mm; particle size, 2.2 μm). Catalyst Activity: Kinetics Measurements. Kinetic studies were done using predried catalyst at 130 °C for 3 h and 450 °C for 1.5 h. In all cases, 20 mL of dried 1-propanol (stored with 3 Å molecular sieves) were mixed with 300 mg of the catalyst and the reaction was started adding DMPNPP (from a stock solution in acetonitrile) to give a final concentration of 7.53 × 10−5 mol L−1. The temperature was maintained constant at 30 (±0.2) °C and the system was closed with a rubber septum. It is worth noting that Mg2+-Al3+ mixed oxides were totally insoluble in 1-propanol and the suspension was continuously stirred at 750 rpm for all experiments. Kinetics using predried catalyst at 450 °C were performed under argon atmosphere. Aliquots of 0.5 mL were withdrawn at appropriate times and mixed with 0.2 mL of the ethanol and 0.2 mL of carbonate buffer pH = 9.0, containing 4.0 mol L−1 NaCl. The mixture was stirred in a vortex followed by centrifugation. After that, 0.7 mL of supernatant solution was transferred in to a quartz cuvette and diluted with water to 2 mL before taking the UV−vis spectrum in a Hewlett-Packard (model HP-8453) spectrophotometer and recording the absorbance at 405 nm.
Figure 1. XRD patterns of MgO, γ-Al2O3 and samples 4 and 5 prepared with increasing amounts of MgO. At the bottom are the standard peaks for MgAl2O4 obtained from the crystallographic database.40
CRYSMET Database (see Supporting Information, Figures S5 and S6). The spectrum (c) obtained for catalyst 4 is consistent with the formation of an incipient MgAl2O4 spinel phase,39 where the peaks coincide with the standard peaks for the MgAl2O4 described in American Mineralogist Crystal Structure Database (code amsd 0001398),40 see bottom of Figure 1. Defining MgAl2O4 as a spinel phase, indicates that the sample is a ternary oxide with a chemical formula of AB2O4, where A is a divalent metallic cation at a tetrahedral site and B is a trivalent metallic cation at an octahedral site of the cubic structure.41 Increasing the mole fraction of MgO to 48.8% (w/w) gave the observed peaks in spectrum (b) for sample 5 consistent with a mixture of MgO and the MgAl2O4 spinel. The samples (1−3) with lower contents of MgO showed spectrograms (not shown) similar to that of γ-Al2O3 with increasing amounts of the incipient MgAl2O4 spinel phase. The effects of calcination at different temperatures of sample 4, with 31.8% (w/w) concentration of MgO, which presented the incipient spinel phase, were also examined using powder XRD, and the results are shown in Figure 2. As can be seen, the incipient MgAl2O4 spinel phase still exists for calcination temperatures until 650 °C, spectra (a−c). Conversely, when the sample was treated using a calcination temperature of 950 °C, the peaks remain at exactly the same 2θ angles but become significantly sharper, spectra (d), showing excellent crystallinity, with relative intensity ratios of the most intensive diffraction peaks42 fully consistent with the proposed MgAl2O4 spinel phase. Micrographs of Al3+-Mg2+ mixed oxide powders were performed to compare any change in the surface structure of the mixed oxide catalysts due to changes in composition. Figure 3 presents typical scanning electron microscopic (SEM) images of the prepared samples, which show that in all cases the coprecipitation method led to the formation of large agglomerates with highly irregular surfaces. The samples 3 and 5 showed similar SEM micrographs (not shown). Figure 4A,B shows the N2 adsorption and desorption isotherms for γ-Al2O3 and sample 1, respectively. The N2 adsorption and desorption isotherms obtained for the catalysts prepared were similar and typical of materials with type IV behavior. This type of isotherm indicates the presence of mesoporous materials, and the hysteresis loops observed for
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RESULTS AND DISCUSSION Catalyst Characterization. The compositions of the Al3+Mg2+ mixed oxides were conveniently determined by EDXRF. The individual peaks corresponding to Mg Kα1 and Al Kα1 can be clearly distinguished and identified as arising from their characteristic element. In fact Al3+-Mg2+ mixed oxides with different mass fractions (% w/w) of MgO and Al2O3 were characterized and are showed in Table 1. The labels (1−5) will be used in the rest of the text. Table 1. Results of EDXRF for the Characterization of Al3+Mg2+ Mixed Oxides on Glass Disc Beadsa catalyst
Mg2+ expressed as MgO % w/w
Al3+ expressed as Al2O3 % w/w
Al/ Mg
Al2O3 1
0 3.90 ± 1.1
100 96.1 ± 0.1
19.5
2
5.86 ± 0.9
94.1 ± 0.1
12.7
3
13.6 ± 0.6
86.4 ± 0.1
5.02
4 5
31.8 ± 0.4 48.8 ± 0.3
68.2 ± 0.2 51.2 ± 0.2
1.70 0.83
MgO
100
0
composition expressed as mole % Al2O3 (Al2O3)0.90 + (MgAl2O4)0.10 (Al2O3)0.84 + (MgAl2O4)0.16 (Al2O3)0.60 + (MgAl2O4)0.40 MgAl2O4 (MgO)0.59 + (MgAl2O4)0.41 MgO
a
The certified reference material Natural Moroccan Phosphate rock (BCR 032) was used for calibration, validation, and monitoring of the analysis.
Figure 1 shows the collected diffractograms of the γ-Al2O3 and MgO-composed samples prepared by the procedure described above. The main characteristic peaks related to the pure MgO and the γ-Al2O3 samples were identified in spectra (a) and (d), respectively. The identification was achieved by comparing the diffraction patterns with the standard peaks from 26099
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values of P/Po > 0.4 are in all cases related to capillary condensation and pore size. The curves for the average pore radius distribution, determined according to the Barrett, Joyner, and Halenda (BJH) method, can be seen in the insets of Figure 4A,B for γAl2O3 and catalyst 1, respectively. All of these curves reveal the presence of mesopores. Samples 1, 2, 3, and 5 and MgO showed isotherms similar to the typical examples shown in Figure 4 and all the main textural parameters of the samples are reported in Table 2. The BET Table 2. Texture Parameters Calculated from N2 Adsorption and Desorption Isotherms
Figure 2. XRD diffractograms for sample 4 calcined at different temperatures. At the bottom are the standard peaks for MgAl2O4 obtained from the crystallographic database.40
catalyst
SBET (m2 g−1)a
VBJH (cm3 g−1)b
RP (Å)c
γ-Al2O3 1 2 3 4 5 MgO
180.3 175.7 187.0 129.4 112.0 64.89 10.03
0.37 0.59 0.33 0.26 0.48 0.20 0.026
41.48 66.83 35.23 40.39 85.14 60.42 51.08
a SBET = specific surface area. bVBHJ = pore volume. cRP = pores mean radius.
specific surface areas of Al2O3 and MgO are 180.3 and 10.0 m2 g−1, respectively. It can be noted that the BET surface area of the mixed oxides decreases from 187.0 to 64.9 m2 g−1 with an increase in the MgO content of the material. The pore radius of the catalysts was greater than 35.2 Å (diameter > 70.4 Å, see Table 2), a result which is critical in terms of minimizing the pore diffusion limitations of the bulky methyl paraoxon molecules, which molecular modeling shows have an estimated diameter 10.3 Å. The CO2-TPD data provide a qualitative and quantitative measure of the strength of basic sites in a solid. The desorption curves were deconvoluted and exhibit three peaks (see Supporting Information for catalyst 4, Figure S7) which correspond to three different forms of adsorbed CO2.43,44 According to the temperature range of the CO2 desorption, these peaks were divided into three groups exhibiting weak
Figure 3. SEM micrographs of (A) 1, (B) 2, and (C) 4 catalysts.
Figure 4. N2 adsorption and desorption isotherms for (A) γ-Al2O3 and (B) catalyst 1. The insets in each figure show the average pore radius distribution, based on the BJH method, for γ-Al2O3 and catalyst 1. 26100
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(60−180 °C), medium (180−400 °C) and strong (>400 °C) basicity. As can be seen in Table 3, Al2O3 exhibited lower Table 3. Surface Base Site Properties of Catalysts base site density (μmol CO2/m2 of catalyst)a
a
catalyst
total
weak
medium
strong
γ-Al2O3 1 2 3 4 5 MgO
0.60 0.79 1.07 1.39 1.54 2.32 5.97
0.38 0.50 0.69 1.00 0.95 1.50 2.15
0.13 0.24 0.30 0.36 0.52 0.71 2.95
0.08 0.04 0.08 0.03 0.07 0.11 0.86
Figure 5. ESI-MS spectrum in positive mode of an aliquot taken after 40 min of propanolysis reaction of a sample containing the spinel catalyst 4 and DMPNPP (7.5 × 10−5 mol L−1; 30 °C). The aliquot was diluted with H2O containing 0.1% formic acid.
By TPD of CO2.
basicity compared to MgO, as expected. For the Al3+-Mg2+ mixed oxides, the formation of weak and medium basicity sites was particularly promoted by the addition of MgO and represents more than 85% of the total base site density. The strong basicity sites contribute a little to the total density, with the value of the density of these sites oscillating slightly as the amount of MgO increases. Mass Spectrometric Study of the Propanolysis Reaction. The propanolysis reaction of DMPNPP (Scheme 1) in the presence of the solid mixed oxide catalysts results in the formation of dimethyl n-propyl phosphate, which was identified by mass spectrometric analysis using ESI-MS in the positive mode: this monitors the course of the reaction by 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. Figure 5 shows an ESIMS(+) spectrum recorded after 40 min for the reaction of DMPNPP with catalyst 4, and the spectrum in positive mode was recorded in a sample diluted with an aqueous solution containing 0.1% formic acid. In this spectrum, a series of key cations were detected and identified including (i) the residual protonated reagent DMPNPP of m/z 248; (ii) the protonated dimethyl n-propyl phosphate of m/z 169; (iii) the protonated dimethylphosphate diester of m/z 127.1, produced either by hydrolysis or lost of nC3H7; and (iv) the protonated m/z 110.1, which can be formed from the ion of m/z 169 by loss of n-C3H7O. ESI-MS/MS was then used to characterize the protonated dimethyl n-propyl phosphate of m/z 169 via collision-induced dissociation (CID). The resulting tandem ESI-MS/MS(+) for the alkyl triester (m/ z 169) shows (Figure 6) that it dissociates to dimethylphosphate diester (m/z 127) and confirms the proposed structure. LC-MS/MS shows (Figure 6B) that using a C18 column dimethylphosphate (m/z = 127) is eluted at 2.52 min and npropyl dimethyl phosphate (m/z = 169) appears at 16.7 min. The chromatogram is consistent with a reaction which proceeds largely via propanolysis, and when the catalyst is dried at 450 °C, the reaction proceeds exclusively via
Figure 6. (A) ESI-MS/MS(+) of the protonated n-propyl dimethyl phosphate triester of m/z 169.1. (B) LC-MS/MS for an aliquot of the reaction mixture in a C18 column, at 30 °C; ESI + MS/MS detection.
propanolysis (see Supporting Information, Figure S4). The mass spectrometric analysis and the large increase in absorbance at 405 nm due to the appearance of the 4nitrophenolate leaving group are fully consistent with the
Scheme 1. Propanolysis Reaction of DMPNPP Using the Mixed Oxide Catalysts
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Figure 7. Kinetic of the reaction of catalyst 4 (300 mg) with concentration of DMPNPP (substrate/catalyst) of (A) 5.0 × 10−6 (mol g−1), (B) 1.0 × 10−5 (mol g−1), and (C) 1.1 × 10−4 (mol g−1). Absorbance changes correspond to the formation of p-nitrophenolate at 405 nm and the solid lines correspond to the theoretical treatment considering: (A) a first order reaction and (B, C) a first order reaction followed by zero order behavior.
reaction described in Scheme 1. It is important to notice that in the absence of catalyst, the propanolysis of DMPNPP is very slow and does not interfere with the reaction observed in the presence of catalyst (see below). Kinetics of the Propanolysis Reaction in the Presence of the Heterogeneous Catalysts. As can be seen in Figure 7, when the propanolysis reaction at a constant catalyst concentration is followed by the changes in absorbance in the UV−vis region, the shapes of the kinetic curves depend strongly on the concentration of DMPNPP. The experiments shown in Figure 7 used 300 mg of catalyst 4 and concentrations of DMPNPP in the range of 5.0 × 10−6 (mol g−1) to 1.1 × 10−4 (mol g−1), and in all cases, the reaction was accompanied by the formation of the p-nitrophenolate at 405 nm in UV−vis. Experimental data in Figure 7A are consistent with a typical first order reaction, while Figure 7B and C show reactions that are progressively changing to a profile consistent with a fast pnitrophenolate liberation with typical first order kinetics, followed by a slower zero order reaction. The zero order reaction (Figure 7B,C) observed at higher substrate concentration, indicates that the surface of the catalyst becomes saturated with p-nitrophenolate, which has a very high affinity with the surface of the catalyst, as reported in the literature.45 As a consequence, the Mg2+-Al3+ mixed oxides can preferentially take up p-nitrophenolate, poisoning the active sites, and thus, its dissociation becomes the rate determining step of the process. Based on the results shown in Figure 7, a concentration of 5.0 × 10−6 mol g−1 (substrate/catalyst) was used to measure the reactivity of all the different catalysts with different contents of Mg2+ and Al3+. In all cases, the behavior was similar to that reported in Figure 7A and the first order rate constants obtained following the formation of the p-nitrophenolate anion at 405 nm, for all the different catalysts, are shown in Figure 8 (the values of the rate constants are given in the Supporting Information, Table S2). Comparing the effect of the different heterogeneous catalysts we can clearly see, that catalyst 4, which corresponds to the incipient MgAl2O4 spinel, is clearly more efficient promoting the propanolysis of the dimethyl p-nitrophenyl phosphate triester. In fact, the rate constants for the propanolysis reaction was estimated to be
