Research Article www.acsami.org
PdNP Decoration of Halloysite Lumen via Selective Grafting of Ionic Liquid onto the Aluminol Surfaces and Catalytic Application Gustave K. Dedzo,*,†,‡ Gael̈ le Ngnie,† and Christian Detellier† †
Center for Catalysis Research and Innovation and Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada ‡ Laboratory of Analytical Chemistry, Faculty of Science, University of Yaounde I, B.P. 812, Yaounde, Cameroon S Supporting Information *
ABSTRACT: The synthesis of selectively deposited palladium nanoparticles (PdNPs) inside tubular halloysite lumens is reported. This specific localization was directed by the selective modification of the aluminol surfaces of the clay mineral through stable Al−O−C bonds. An ionic liquid (1-(2-hydroxyethyl)-3-methylimidazolium) was grafted onto halloysite following the guest displacement method (generally used for kaolinite) using halloysite−DMSO preintercalate. The characterization of this clay nanohybrid material (XRD, NMR, TGA) showed characteristics reminiscent of similar materials synthesized from kaolinite. The grafting on halloysite lumens was also effective without using the DMSO preintercalate. The presence of these new functionalities in halloysite directs the synthesis of uniform PdNPs with size ranging between 3 and 6 nm located exclusively in the lumens. This results from the selective adsorption of PdNPs precursors in functionalized lumens through an anion exchange mechanism followed by in situ reduction. In contrast, the unmodified clay mineral displayed nanoparticles both inside and outside the tubes. These catalysts showed significant catalytic activity for the reduction of 4-nitrophenol (4-NP). The most efficient catalysts were recycled up to three times without reducing significantly the catalytic activities. KEYWORDS: palladium nanoparticles, intercalation, grafting, halloysite, nanotubes, catalysis, 4-nitrophenol
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release,21−23 as corrosion inhibitor carrier,24,25 and as nanoreactor for enzymatic reaction.26 Halloysite is a 1:1 dioctahedral phyllosilicate of the kaolin family.27,28 Structurally, it consists of a silicon tetrahedral sheet linked to an aluminum octahedral sheet. This two-dimensional elementary layer is structured in different ways according to natural formation conditions to yield tubular, lamellar or spherical structures. However, the tubular structure is the most commonly encountered.27 These tubes of 0.2−2 mm length have internal and external diameters respectively between 5 and 20 nm and 10−50 nm.29 This mineral has a chemical composition similar to kaolinite, in addition to a maximum of two water molecules per structural unit, located in the interlayer (Al2Si2O5(OH)4·nH2O). Because of the presence of a monolayer of water molecules in the interlayer space, the dspacing ranges from 10 Å for the dihydrated form (n = 2) of halloysite nanotubes (HNTs) to 7 Å for the completely nonhydrated form (n = 0).28,30 The dehydration process is irreversible and occurs even at low temperatures.27,31 The direct synthesis of nanoparticles on halloysite presents some drawbacks including the low amount of immobilized
INTRODUCTION
The importance of catalysis is well established in organic synthesis and in pollution remediation. Metal nanoparticles are of special interest due to their important catalytic properties combined with high stability. In addition, they are produced by relatively simple synthesis strategies.1,2 Among these metals, palladium nanoparticles (PdNPs) are frequently used for various catalytic reactions.3−5 However, their agglomeration during synthesis and recycling difficulties greatly explain why PdNPs are commonly prepared on organic or inorganic support to ensure their dispersions and particles size distribution while facilitating their recovery after use. Clay minerals are used as nanoparticles support mainly because they are inexpensive and show good chemical and thermal stability, enabling catalytic reactions in various experimental conditions without alteration of the inorganic support.6−11 Several published studies report the use of smectites, zeolites, sepiolite, kaolinite and halloysite among other clay minerals, as suitable PdNPs supports.12−15 To improve the affinity between the nanoparticles and clay minerals, prior modification of the support with an organic compound is a widely used strategy.12−14 Because of its tubular structure and robustness, halloysite was used as nanoparticles support,16−20 as nanocontainer for drug © XXXX American Chemical Society
Received: October 29, 2015 Accepted: January 29, 2016
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DOI: 10.1021/acsami.5b10407 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
°C for 4 h. The solid was then washed 4 times with isopropanol (4 × 15 mL) and dried in the oven at 80 °C for 2 h. The solids were redispersed in 100 mL of distilled water and stirred for 1 day. The modified clay (H-IL) was washed 4 times by centrifugation with distilled water (4 × 40 mL) and dried in oven at 80 °C overnight. For the direct grafting of IL on unmodified halloysite (r H-IL), a similar experimental procedure was applied, using halloysite instead of HDMSO. Synthesis of Halloysite-Supported Palladium Nanoparticles. Clay (0.2 g, modified or unmodified) was dispersed in a round-bottom flask containing 100 mL of 0.5 mM K2PdCl4 aqueous solution. The mixture was homogenized by magnetic stirring at room temperature for 30 min, refluxed for 1 h and finally stirred at room temperature for 2 h. The solid was recovered by centrifugation at 4500 rpm. The solid was redispersed in 50 mL of deionized water. To this solution was added 15 mL of aqueous solution of NaBH4 (0.01 M) under stirring for 15 min. The dark gray solid obtained was recovered by centrifugation, washed three times with water (3 × 40 mL). The solid was then redispersed in 40 mL of 0.1 M NaCl aqueous solution to ensure that the counteranions of the grafted ionic liquid were replaced by chlorides. The washing procedure was repeated until the solution was free of chloride ions (silver nitrate test) and the solid dried in the oven. These steps are summarized in Scheme 1. Catalytic Application: 4-NP Reduction. A suspension of either the natural clay mineral or the functionalized clay with PdNPs was prepared by dispersing 4 mg of solid in 1 mL of deionized water. The mixture was stirred in an ultrasonic bath for 2 min to obtain well dispersed particles in water. For the catalytic experiments, 2.5 mL of an aqueous solution of 4-NP 0.1 mM was introduced in a spectrophotometer cuvette. Twenty μL of an aqueous solution of NaBH4 1 M was added in the solution. The mixture took an intense yellow color characteristic of 4-nitrophenolates ions generated by acid−base reaction of 4-NP with alkaline borohydride ions. 50 μL of the suspension of the catalyst was then added in the solution and the cuvette was immediately introduced in the spectrophotometer to monitor the reaction. Characterization. Powder XRD patterns were recorded using a Rigaku Ultima IV diffractometer operating with Cu Kα radiation (λ = 1.54056 Å) using a generator with a voltage of 45 kV and a current of 40 mA. Solid-state 13C NMR CP/MAS spectra were collected on a Bruker AVANCE 200 spectrometer, operating at a spinning rate of 4.5 kHz. 13C NMR spectra in solution were recorded using a Bruker 400 MHz spectrometer. KBr pellets were prepared for the IR analysis and the spectrum recorded on the Thermoscientific Nicolet 6700 FT-IR equipment. Scanning electron microscope (SEM) images were taken on a JEOL JSM-7500F FESEM in low secondary electron imaging (LEI) mode with a 1.5 kV acceleration voltage. Energy Dispersive Xray Spectroscopy (EDS) device coupled to SEM was used to record the EDS spectra of defined area of the samples. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100F electron microscope. The powders were dispersed in ethanol in an ultrasound bath for about 2 min. One drop of the suspension was deposited on copper grid and air-dried before imaging. Thermal gravimetric analyses (TGA) were recorded using a TA Instruments Q5000 under nitrogen flow (25 mL min−1) at a heating rate of 10 °C/ min. Specific surface area was determined by the BET method using nitrogen adsorption−desorption isotherms at 77 K with an ASAP 2010 Micromeritics analyzer. Samples were degassed at 110 °C before the measurements.
nanoparticles and the poor compatibility of the nanoparticles with halloysite.32 To solve these problems, halloysite can be functionalized, for example with an organosilane.17 The result is a better distribution of the particles on the external surfaces of halloysite and the stability of the catalysts obtained.17,32−34 Despite recent reports on the grafting of organic compounds in halloysite interlayer,35,36 there is no report of the use of interlayer grafted halloysite (via Al−O−C bonds) as support for the synthesis of nanoparticles. Such strategy has three major advantages: (i) grafting provides greater density of functionalities as every elemental layer is functionalized. This ensures a better accumulation of the precursors of the nanoparticles at the vicinity of the surface of the clay mineral; (ii) the nanoparticles can be located exclusively on the exposed aluminol functionalized surfaces, allowing the control of the environment of the future catalytic reactions; (iii) the presence of suitable selected modifiers can improve the catalysts efficiency by promoting the accumulation of the reagents on the modified halloysite. In this work, we grafted organic compounds in the interlayer spaces as well as inside the lumen of halloysite following experimental conditions previously used for kaolinite.37−40 These grafted materials are subsequently used as support for the synthesis of PdNPs selectively deposited in the lumen of HNTs using tetrachloropalladates ions as precursors. The grafted compounds employed as catalysts were characterized and their efficiency was tested for the reduction of 4nitrophenol (4-NP) in the presence of borohydride ions. This opens the way for the synthesis of a new class of nanohybrid clay mineral supported catalysts.
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EXPERIMENTAL SECTION
Material and Chemicals. The halloysite sample used was a clay mineral standard (Halloysite No. 13 from Utah, Ward’s natural science establishment). 2-chloroethanol 99%, K2PdCl4 (98%), NaBH4 (≥98.5%), 1-methylimidazole (99%) were purchased from SigmaAldrich. All other chemicals (dimethyl sulfoxide (DMSO), anhydrous diethyl ether and methylene dichloride) were of analytical grade. The ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium (IL) was synthesized according to the procedure previously reported.37,39 Synthesis of Halloysite-Supported PdNPs. Synthesis of Grafted Halloysite. The halloysite-DMSO preintercalate (H-DMSO) was prepared by following previously described procedures depicted in Scheme 1.37,38 The grafting of IL was adapted from procedures previously published.37,39 In practice, 0.6 g of H-DMSO was dispersed in 2 g of IL and stirred vigorously under nitrogen atmosphere at 180
Scheme 1. Halloysite-Ionic Liquid-Supported PdNP Synthesis Pathway
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RESULTS AND DISCUSSION IL Grafting on Halloysite. XRD. The strategy used to graft organic compounds into the interlayer of halloysite was inspired by the method previously used for kaolinite modification37−39 because of the structural similarities between these two clay minerals. To follow the intercalation and grafting throughout this work, we focused mainly on the 001 reflection plan of halloysite because the delamination will affect the c-axis. B
DOI: 10.1021/acsami.5b10407 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces In the XRD pattern of unmodified halloysite (Figure 1), the peak at 7.2 Å is characteristic of the d-spacing of nonhydrated
Figure 1. XRD patterns of unmodified and modified halloysite. 001 reflection of unreacted halloysite is marked by asterisk (*).
Figure 2. 13C CP/MAS solid-state NMR and 13C liquid NMR spectra of intercalated and grafted halloysite and IL. The structure of IL with numbered carbons is presented.
halloysite. The intercalation of DMSO increases the d001 from 7.2 to 11.2 Å, a value very close to those reported previously in the literature.35 DMSO intercalation is promoted by the high polarity of this compound that favors its insertion in the interlayer space and interactions with interlayer’s functionalities. After the grafting of the ionic liquid, further interlayer expansion was observed: d001 reached a value of 13.2 Å (Figure 1). This increase is directly related to the size of the cation of the ionic liquid and to its almost vertical orientation in the interlayer as reported recently during the grafting of the same ionic liquid in kaolinite.39 As expected, the material resulting from the direct grafting of the ionic liquid (IL) on pristine halloysite (r H-IL) presents a pattern superimposable to that of the unmodified clay (Figure 1). This result clearly shows that similarly to kaolinite, grafting of IL in the halloysite interlayer spaces can be performed only through a guest displacement method. NMR. The chemical structures of intercalated or grafted compounds were confirmed by 13C solid state NMR. As indicated in Figure 2, upon intercalation, DMSO which usually shows only one carbon signal due to the identical chemical environment of its two methyl groups presents in this case two distinct signals with equal intensities at 41.6 and 40.6 ppm. This differentiation of the two methyl carbons, also found in kaolinite DMSO intercalates, results from the different orientations of the methyl groups in the interlayer space. According to experimental and computational results obtained for kaolinite DMSO intercalate, one methyl group is oriented so that the S−C bond is parallel to the aluminol surface and the
second one is keyed in the siloxane macroring of the siloxane surface.41,42 The spectra of the modified halloysite show that the structure of the grafted compound is maintained upon grafting. For H-IL (Figure 2), the three aliphatic carbons (C1, C2 and C3) appear as well-defined peaks centered at 34.8, 47.3, and 59.6 ppm respectively while the three imidazolium carbons appear as two peaks centered at 121.1 ppm (C4 and C5) and 134.5 ppm (C6). These peaks appear at the same positions for r H-IL, also close to their positions in the liquid state. The 13C NMR spectrum is identical to the one obtained by grafting of the same IL to the internal surfaces of kaolinite.39 Their lower intensities in r H-IL are due to the smaller amounts of grafted ionic liquid, since only the external inner surfaces of the halloysite tubes are involved in the grafting reaction. There is ample evidence in the literature that the 13C NMR chemical shift of the Al−O−C linkage is very close to the H− O−C one. Consequently 13C NMR chemical shifts are not expected to be strongly affected by the grafting of a hydroxyl group on an aluminol function. For example, this was demonstrated in the case of alkoxyalumoxanes,43 of the transesterification reaction of propanediols on methoxykaolinite44 or on the grafting of ethylene glycol.45,46 In this latter case, a definite and convincing demonstration of the grafting was done by Hirsemann et al. using a 13C NMR REAPDOR pulse sequence to determine the 13C---27Al distance.47 A test for the success of grafting results from the resistance to hydrolysis of the grafted compounds. An intercalated guest is easily removed from the interlayer space C
DOI: 10.1021/acsami.5b10407 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces by water washing while a grafted compound is very resistant.38 This property permitted the use of modified kaolinite in electrochemical aqueous applications37,39,40 or as heavy metal adsorbent in water.48 Both H-IL and r H-IL were submitted to intense water washing during their synthesis procedure, guarantying that the obtained materials are grafted. FTIR. The spectra of H-IL, r H-IL, and H are plotted in Figure S1. H-IL spectrum confirms that the interlayer space OH groups were strongly affected by the presence of IL if one considers the AlO-H stretching region (3800−3600 cm−1). The presence of IL was also confirmed by the aromatic and aliphatic C−H stretching bands (in the range 3200−2800 cm−1). The spectra of H and r H-IL was almost superimposable. This tends to confirm the results of solid state NMR: IL was not intercalated in r H-Il and the amount grafted on lumen surface was not abundant enough for FTIR analysis. TGA. The thermal behavior of halloysite is similar to that of kaolinite. This clay mineral has a characteristic mass loss at 490 °C because of dehydroxylation (Figure 3). There is also a loss
Specific Surface Area (BET). Specific surfaces of natural and modified HNTs were measured by nitrogen adsorption isotherms at 77 K (Figure S2). All the adsorption/desorption isotherms present hysteresis characteristic of mesoporous materials.50 The estimation of the total specific surface area using BET method is summarized in Table 1. Table 1. BET Specific Surface Areas and Average Pore Diameters of Modified and Unmodified Halloysite H H-DMSO H-IL H-IL@Pd r H-IL r H-IL@Pd
S (m 2 g−1)
BET average pore diameter (Å)
44.9 41.4 44.8 37.7 40.2 45.9
133.6 117.3 126.8 125.3 112.6 106.0
The unmodified halloysite has a specific surface area of 44.9 m2 g−1. The intercalation of DMSO results in a slight decrease (41.4 m2 g−1). After the grafting of the ionic liquid, the surface area is identical to that of the unmodified halloysite. This supposes that the voids present in the interlayer after the grafting of the ionic liquid do not contribute to the value of the measured specific surface area. The presence of these voids is confirmed by the presence of water molecules intercalated as a result of the highly hydrophilic nature of the grafted ionic liquid and the accessible functionalities of the halloysite interlayer, as evidenced by TGA (more than 3% by weight of water present in H-IL). Of course, in the operating conditions of the specific surface area measurement, these water molecules were no longer present in the material (after the degassing step at 110 °C). Results obtained with r H-IL (specific surface area of 40.2 m2 g−1) tend to confirm our hypothesis. Indeed, in this sample, only the surface of halloysite lumens was functionalized resulting in the expected decrease of the specific surface area. This result support the previous work reported when kaolinite interlayers were grafted by ionic liquids of different sizes. There were only minor and constant increases in the specific surface area depending on the size of the grafted ionic liquid.39 The average pore diameter of halloysite, determined by the BET method (Table 1) tends to decrease after the modification by organic compounds and PdNPs, in accordance with the expected occlusion of the pores due to the presence of the guest compounds. Nanoparticle Deposition. Upon the deposition of PdNPs on H-IL (Figure S3), the d-spacing decreases from 13.2 to 10.1 Å. This shift suggests a different orientation of the ionic liquid in the interlayer which tilted to adopt an almost parallel orientation with respect to the siloxane surface. This reorientation could be related to a partial hydrolysis of the ionic liquid during the synthesis of nanoparticles. Similar dspacing was obtained by Tonle et al. (2009), during the grafting of the same ionic liquid in kaolinite with stable d-spacing of 10.3 Å after 24 h of water treatment.37 As it could be expected, with r H-IL, the d001 remained characteristic of that of unmodified halloysite (7.2 Å). These results suggest that the PdNPs are not located in the interlayer. The presence of nanoparticles in the interlayer would result in a substantial increase of the d-spacing of halloysite. However, while the gray color of the clay supported palladium indicates the presence of PdNPs, characteristics peaks of PdNPs ((111) (200) and (220) reflections plans)
Figure 3. TGA and DTGA of unmodified and modified halloysite.
around 46 °C due to physically adsorbed water molecules. For the DMSO preintercalate (Figure 3), the loss of DMSO is centered at 170 °C, followed by dehydroxylation at 496 °C. The grafted materials show a different behavior. For H-IL, the high amount of physisorbed water (weight loss below 100 °C, corresponding to 3.5% of the total mass of the sample) is linked to the high hydrophilicity of the grafted ionic liquid.40 On the DTGA, the sharp peak at 370 °C indicates the loss of the ionic liquid and the peak at 460 °C accounts for the dehydroxylation of the clay mineral. The dehydroxylation temperature is shifted to lower temperature (about 30 °C). This is an additional indication of the effective grafting of IL in the interlayer space, as it was shown for kaolinite.49 The r H-IL material is less hydrophilic than H-IL, with only 1.1% of adsorbed water molecules (Figure 3). In addition, the weight loss related to the ionic liquid is less important than that of H-IL and occurs at lower temperature (335 °C). The dehydroxylation of r H-IL takes place almost at the same temperature as pristine halloysite (486 °C). These observations perfectly confirm that the grafting of IL in r H-IL happens only on the external inner surfaces of the nanotubes, and also that for H-IL, physisorbed water molecules are mostly located in the interlayer spaces of the modified halloysite. D
DOI: 10.1021/acsami.5b10407 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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With unmodified halloysite (H@Pd), one obtains a random distribution of particles on the inner and outer surfaces of the tubes, demonstrating that the presence of a directing functionality in the material structure determines the nanoparticles location. For pristine halloysite, these nanoparticles are less abundant and have larger sizes ranging between 5 and 12 nm. The nanoparticles prepared on r H-IL have characteristics identical to those obtained on H-IL: The particles are abundant and selectively confined in the lumens with a uniform distribution of sizes between 3 and 6 nm. This result confirms that the excellent affinity between the grafted ionic liquid and the tetrachloropalladate ions drives the location of the nanoparticles. Indeed, in the case of r H-IL, the ionic liquid was grafted specifically in the lumen which, consequently, was converted to an excellent anion exchanger surface. Only few works in the literature report the selective synthesis of nanoparticles in halloysite lumen. The reported procedures used complexing agents such as citrate or acetate to promote metal ions adsorption in the lumen followed by their reduction.51,52 To the best of our knowledge, it is the first time that the grafting of an organic compound is specifically used in halloysite lumen to direct the in situ synthesis of PdNPs. Application: 4-NP Reduction. The reduction of 4-NP (4nitrophenol) into 4-AP (4-aminophenol) was reported to be a model reaction to evaluate the efficiency of metallic nanoparticles in alkaline medium.53−57 During the reduction of 4NP, the nitro group reduction takes place through a series of intermediates and 4-aminophenol (4-AP) is obtained at the end of the reaction. This reduction process goes through color changes that can be easily monitored by UV−vis spectrophotometry where the decreasing concentration of 4-NP is measured (400 nm) as well as the corresponding increase in concentration of 4-AP (300 nm). Although borohydride ions are powerful reductants, these ions show no effect on 4-NP reduction. Furthermore, with the addition of unmodified or grafted halloysite (H-IL or r H-IL), similar steady state was observed, proving that these materials are not efficient catalysts for the hydrogenation of 4-NP (Figure S5). The use of H@Pd (Figure S6) showed a fast decrease of the absorption peak of 4-NP at 400 nm and the gradual formation of 4-AP absorption peak with a maximum around 300 nm. We observed that 4-NP completely disappears from the solution after about 8 min. This conversion of 4-NP reflects the effectiveness of PdNPs to reduce an organic compound when immobilized on halloysite. With H-IL@Pd (Figure S6), 4-NP reduction is faster, with the complete disappearance of the compound after only 3 min. This result indicates that the presence of grafted organic compounds not only does not reduce the activity of PdNPs, but increases its catalytic performances. The evolution of the absorbance at 400 nm for different catalysts during the first 5 min of reaction is plotted on Figure 5. The curve obtained for H@Pd showed a particular trend with a slow reactivity in the first 30 s. This step reflects the induction period of the catalyst during which the metal atoms reorganized themselves to perform catalysis.54,58 This step was followed by a slow and constant 4-NP reduction even after 5 min of reaction time. For all other materials it was not possible to distinguish the induction time, probably because of their high activity in catalyzing the reduction of 4-NP. Indeed, for all these materials, the reduction of 4-NP was complete within 4
should appear as shown in Figure S3, at 39.4, 45.8, and 66.7° 2θ, respectively. Unfortunately, these palladium characteristic peaks were not clearly observed in the XRD patterns of halloysite supported PdNPs. This could be attributable to the low abundance or the very small sizes of PdNPs which explains why these peaks are embedded in the background of XRD patterns. Therefore, the use of other characterization techniques was required to highlight the presence of PdNPs. TEM. TEM is a powerful characterization method for nanoparticles thanks to its high resolution. Before using this tool, we first tried to have a direct evidence of the presence of PdNPs by using SEM coupled to EDS of H-IL@Pd (Figure S4 in the Supporting Information). Despite the well displayed tubular morphology of halloysite tubes, nanoparticles were not observed due to the poor contrast between the metal and the supported clay surfaces or because nanoparticles were located inside the tubes. However, by recording the EDS spectra of clay particles, we obtained in addition to silicon and aluminum, the signals of palladium. This clearly showed the presence of substantial amount of palladium on clay particles. TEM micrographs of materials were recorded (Figure 4). In the case of the modified materials, the nanoparticles are almost
Figure 4. TEM micrographs of halloysite and halloysite supported Pd nanoparticles.
exclusively localized in the lumens of the tubes, especially in the case of H-IL@Pd. These nanoparticles are abundant with discrete sizes ranging between 3 and 6 nm. The exclusive location of nanoparticles is related to the functionalization of the tubes inner surfaces through the condensation reaction between the aluminol and alcohol function of the modifier to yield Al−O−C bond. These new functionalities act as adsorption sites preferentially occupied by the nanoparticles precursors during their synthesis. Indeed, the grafted ionic liquid will promote the tetrachloropalladate anions adsorption through anion exchange mechanism with the chloride counterion of IL. Hence, the tetrachloropalladate ions are reduced in situ into PdNPs by NaBH4 treatment and deposited at the same location (the inner surface of the lumen of HNTs). The external siloxane surfaces of halloysite tubes, with some silanol functions due to defects17 was not functionalized and consequently showed poor affinity for the nanoparticles precursors. E
DOI: 10.1021/acsami.5b10407 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. Variation of 4-NP maximum absorbance at 400 nm for various catalysts during the catalytic reduction in the presence of NaBH4.
min of reaction time. To quantify the reactivity of different materials, we applied a pseudo-first order kinetic model since borohydride ions were largely in excess relatively to 4-NP. Table 2 includes the values of the apparent rate constants and corresponding correlation coefficients. Table 2. Pseudo-First-Order Apparent Rate Constants and Correlation Coefficients of the Reduction of 4-NP by Halloysite-Supported PdNPs catalysts 2
−1
10 kapp (s ) R2
H-IL@Pd
r H-IL@Pd
H@Pd
1.80 0.99
2.22 0.99
0.72 0.99
It appears from Table 2 that H@Pd is the least efficient catalyst (kapp = 0.72 × 10−2 s−1) because of the small amount of PdNPs present in the material as revealed by TEM images. The small amount of nanoparticles was explained by the low capacity of halloysite to quantitatively accumulate the nanoparticles precursor (PdCl42−). The reduction of 4-NP is more efficient (kapp = 1.80 × 10−2 s−1) with H-IL@Pd, Surprisingly, this performance is further increased in the case of r H-IL@Pd (kapp = 2.22 × 10−2 s−1), showing that the grafting of the ionic liquid on halloysite from H-DMSO preintercalate is not necessary to prepare an effective catalyst. The effectiveness of these catalysts derived from ionic liquid modified halloysite cannot be explained only by the abundance of the nanoparticles. The presence of the ionic liquid can promotes contact between the nanoparticles and the reagents (nitrophenolates and borohydrides) by an anion exchange mechanism. The ability of the catalyst to be used in multiple catalytic cycles without cleaning steps was tested by a simple procedure. After the first reduction experiment was complete, 4-NP (50 μL of a 5 mM aqueous solution) was added in the reaction medium and the reaction monitored by the same UV−vis method. Three sets of reduction experiments have been performed successively by this method using r H-IL@Pd catalyst (Figure 6A). Only minor reductions of the rate constant were obtained (2.22 × 10−2, 2.17 × 10−2, and 2.10 × 10−2 s−1) (inset Figure 6A) which can be related to the gradual reduction of the concentration of borohydride ions in the reaction medium. Indeed, it has been clearly shown that even in
Figure 6. (A) Variation of 4-NP maximum absorbance (400 nm) for r H-IL@Pd during 3 successive catalytic reduction cycles of 4-NP. Inset, Pseudo 1st order apparent rate constants for the 3 successive reactions. (B) Effect of the amount of NaBH4 (1 M) on the catalytic reduction of 4-NP on r H-IL@Pd. Inset, variation of the pseudo 1st order apparent rate constants as a function of the volume of NaBH4.
excess, NaBH4 concentration has an effect on the reaction rate.54 A similar trend was obtained for three successive catalytic reduction of 4-NP using H-IL@Pd in a similar experimental procedure (Figure S7). To test this hypothesis, specifically for these catalysts, series of reductions were performed while keeping constant the concentration of 4-NP and the amount of catalyst and varying the volume of the 1 M solution of NaBH4 added in the reactor. The curves obtained are presented in Figure 6B. A marked increase of the reaction rate as the amount of reductant was increased was observed. The rate constant was plotted as a function of the volume of NaBH4 solutions. At low concentrations, a strong increase of the rate constant was observed, followed gradually by a weaker increase that would reach a plateau at higher volume, giving the concentration at which this reactant could be considered as a constant with no effect on the reaction rate.
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CONCLUSION A simple and reliable method was established for the synthesis of a novel class of halloysite supported PdNPs. The nanoparticles were found to decorate selectively the lumen F
DOI: 10.1021/acsami.5b10407 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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surfaces, favored by the permanent grafting of the imidazolium ionic liquid onto the aluminol functions of halloysite. Two strategies were used: interlayer grafting and lumen surface grafting. This functionality acted as adsorption sites of nanoparticles precursors prior to their synthesis. Application of these materials for the catalytic reduction of 4-NP showed excellent results, particularly for the lumen surface grafted material. The modification strategy developed here could be applied to a broad range of metal nanoparticles by selecting the chemical nature of the modifiers. Thanks to their high thermal stabilities despite the presence of organic functionalities, this novel class of catalysts could be applied for gas pollutants remediation and in organic synthesis taking advantage to the beneficial effect of the location of nanoparticles specifically in the confined lumen of halloysite nanotubes. These materials could also find application in selective catalytic reactions depending on the size of reactants.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10407. N2 adsorption/desorption isotherms of modified and unmodified H, XRD patterns of halloysite@PdNPs, SEM and EDS spectrum of H-IL@Pd, and kinetic of catalytic reduction of 4-NP on halloysite with and without PdNPs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by a Discovery Grant of the Natural Sciences and Engineering Research Council of Canada (NSERC). The Canada Foundation for Innovation and the Ontario Research Fund are gratefully acknowledged for infrastructure grants to the Center for Catalysis Research and Innovation of the University of Ottawa.
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DOI: 10.1021/acsami.5b10407 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.5b10407 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
