Research Article pubs.acs.org/journal/ascecg
Mesoporous Microcapsules through D‑Glucose Promoted Hydrothermal Self-Assembly of Colloidal Silica: Reusable Catalytic Containers for Palladium Catalyzed Hydrogenation Reactions Basuvaraj Suresh Kumar,† Arlin Jose Amali,*,†,‡ and Kasi Pitchumani*,†,‡ ‡
Centre for Green Chemistry Processes, †Department of Natural Products Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai−625021, Tamilnadu India
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S Supporting Information *
ABSTRACT: A facile methodology is reported to fabricate hierarchically ordered silica nanoassembled microcapsules (SiO2 NACs) with tailored mesopores by combining polymerization of D-glucose with self-assembly of colloidal silica nanoparticles (SiO2 NPs). This controlled self-assembly of SiO2 NPs during a hydrothermal process enables the formation of core−shell (organic/inorganic) hybrid microspheres of carbon and SiO2 NPs. After removal of carbon, spherical hollow SiO2 NACs are formed having mesopores and their surface area was observed as 248 m2/g. The synthesized mesoporous SiO2 NACs can be effectively used to encapsulate palladium nanoparticles (Pd NPs) to act as a heterogeneous catalyst in hydrogenation reactions. The position of Pd NPs in SiO2 NACs (either inside the nanopores or throughout the wall of the capsules) can be dictated by the method of encapsulation which can impart selectivity in hydrogenation of various nitroaromatic compounds, alkyne, and alkenes. The advantages of our catalytic system are greener synthesis of catalyst, that lower Pd content (0.3 mol %) was utilized for the catalytic hydrogenation reaction, heterogeneous nature and reusability. KEYWORDS: Microcapsules, Mesoporous, Colloidal silica, Self-assembly, Hydrogenation
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INTRODUCTION One particular challenge in science and technology of artificial materials is the construction of self-assembled structural hierarchies from the molecular level over mesoscopic length scales (nm and μm) to macroscopic dimensions with highest possible precision and with lesser efforts and cost.1 Selfassembly is realized by spontaneous organization of molecular units into structural hierarchies as a result of local and weak interactions (e.g., van der Waals, electrostatic, π−π, hydrogen bonding, and capillary), in order to maximize their thermodynamic stability.2−4 Several approaches have been developed for self-assembled hierarchies of polymers,5,6 dendritic crowns,7 supramolecular dendrimers,8 amphiphilic trisamides,9 organic10−12 and inorganic materials.13−15 In particular, nanoparticulate self-assembled structural hierarchies are superior to those of the individual building blocks and their well-defined superstructures play important roles for advanced device applications in the areas of photonics, plasmonics, electronics, information storage, catalysis, cancer diagnostics, and biological sensors.16−21 Silica nanoparticles (SiO2 NPs) assembled hollow silica nanoassembled capsules (SiO2 NACs) are a prominent class of hybrid capsules that have low densities, high surface area, mechanical stability, ease of functionalization, low toxicity, and good biocompatibility.22 Owing to these favorable properties (also accessibility, stability, recyclability, variability, and generally low price), they are among the most promising © 2016 American Chemical Society
materials for applications as solid supports. Many methods for hybrid capsule synthesis have been reported involving layer by layer assembly of polymers and nanoparticles by Caruso,23 block copolypeptides assisted nanoparticle assembly by Stucky,24,25 polyamine−salt assembly induced assembly of nanoparticles by Wong,26−28 and supramolecular host−guest assembly upon microdroplet platform by Scherman and Abell.29 Despite their efficiency in encapsulating cargos, majority of the reported methods are validated and implemented only on small scales partially due to high cost or complicated synthetic procedures. Consequently, research for more convenient, economical, and scalable novel methods and approaches is pursued to obtain hybrid capsules with controllable hierarchical structures, compositions, porosity, and novel properties.30 Herein, we identify a reaction condition under which colloidal SiO2 NPs effectively self-assemble into hierarchically ordered hollow microcapsule structures in the presence of a naturally occurring most abundant saccharide, namely Dglucose. We demonstrate that in a hydrothermal process, Dglucose dehydrates, polymerizes, and carbonizes into carbon microspheres and negatively charged SiO2 NPs deposit around the microspheres to form simultaneously a multilayer thick Received: August 22, 2016 Revised: October 17, 2016 Published: October 26, 2016 667
DOI: 10.1021/acssuschemeng.6b02025 ACS Sustainable Chem. Eng. 2017, 5, 667−674
ACS Sustainable Chemistry & Engineering
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RESULT AND DISCUSSION Our methodology to fabricate SiO2 NACs and to entrap the catalytically active Pd NPs into SiO2 NACs is illustrated in Scheme 1. A preformed stable dispersion of glucose and
shell. The resulting organic/inorganic hybrid microsphere contains carbon in the core interior and SiO2 NPs as the shell exterior. Removal of carbon core by simple calcination results in SiO2 NACs with porosity. The synthesized mesoporous SiO2 NACs (formed by the self-assembly of SiO2 NPs) can be effectively used to encapsulate Pd NPs. Their controlled size and large surface area with mesoporous behavior, prevent agglomeration to ensure stability and accessibility of the Pd NPs. The position of Pd NPs in SiO2 NACs can be dictated by the method of encapsulation which can impart selectivity in hydrogenation of various nitroaromatic compounds, alkynes, and alkenes.
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Research Article
Scheme 1. Synthetic Route for the Preparation of Pd NPs Incorporated Mesoporous, Hierarchically Ordered Silica Nanoparticle Assembled Capsules (Pd@SiO2 NACs) by the Self-Assembly of Silica Nanoparticles (SiO2 NPs) under Hydrothermal Conditions Using D-Glucose
EXPERIMENTAL SECTION
Synthesis of SiO2 NPs Assembled Capsules (SiO2 NACs). Colloidal SiO2 NPs (5 mL) were purified by membrane dialysis (molecular weight cut off 14 kDa) against ultrapure water for 48 h and mixed well with the solution of D-glucose (1.6 g, 5 mL). The resultant solution was transferred into a Teflon lined stainless steel autoclave, sealed, and heated to 160 °C for 10 h. After completion of reaction time, the dark brown precipitate, containing the core−shell microspheres (carbon core and SiO2 NPs assembled shell), was collected by centrifugation (3000 rpm, 15 min). Finally, the core−shell microspheres were washed with water and dried in vacuum followed by calcination at 650 °C for 5 h to give colorless SiO2 NACs in pure form. Incorporation of Pd NPs on SiO2 NACs [Pd@SiO2 NACs (SSM)] by a Single Solvent Method (SSM). Typically, SiO2 NACs (100 mg) were well dispersed in water (8 mL) under magnetic stirring. To this, a solution of Na2PdCl4 (13 mg, 2 mL) was added dropwise under vigorous stirring and the stirring was continued for another 1 h at room temperature. After completion of the reaction time, the reaction mixture was concentrated using rotary evaporator and dried under vacuum. The Pd (II) incorporated SiO2 NACs was redispersed in water (8 mL) and added dropwise with a solution of NaBH4 (10 mg, 3 mL) under constant stirring for 3 h. The color of the solution turns to black from brown indicating the reduction of Pd(II) to Pd(0). The synthesized Pd NPs@SiO2 NACs (SSM) were collected by centrifugation (3000 rpm, 20 min), washed with water, and dried under vacuum for 24 h. Incorporation of Pd NPs on SiO2 NACs [Pd@SiO2 NACs (DSM)] by a Double Solvent Method (DSM). Typically, SiO2 NACs (100 mg) was well dispersed in n-hexane (10 mL) by ultrasonication for 15 min. To this, a solution of Na2PdCl4 (13 mg, 0.100 mL) was added dropwise under stirring and the stirring was continued for another 1 h at room temperature to obtain an emulsion. The above emulsion was stirred at room temperature for 1 h. After completion of the reaction time, the reaction mixture was concentrated using rotary evaporator and dried under vacuum. The Pd(II) incorporated SiO2 NACs was redispersed in water (8 mL) and added dropwise with a solution of NaBH4 (10 mg, 3 mL) under constant stirring for 3 h. The color of the solution turns to black from brown indicating the reduction of Pd(II) to Pd(0). The synthesized Pd NPs@SiO2 NACs (DSM) was collected by centrifugation (3000 rpm, 20 min), washed with water, and dried under vacuum for 24 h. Catalytic Hydrogenation of Nitroaromatic Compounds Using Pd@SiO2 NACs as Catalyst. Pd@SiO2 NACs (0.30 mol %, 10 mg), nitroaromatic compounds (0.5 mmol), hydrazine hydrate (3 equiv), and EtOH (3 mL) are taken in a Schlenk tube with a Teflon stopcock, sealed and treated in RT for a given time with constant stirring. After completion of the reaction, the Pd@SiO2 NACs were separated by centrifugation (3000 rpm, 10 min). After removal of Pd@ SiO2 NACs, the solution was concentrated and the residue is subjected to GC analysis followed by column chromatography for further purification. The purified compounds were further characterized by 1H NMR spectroscopy with CDCl3 as solvent and TMS an internal standard (Supporting Information, Figure S6a−m).
colloidal SiO2 NPs (0.9 M of glucose and 15 wt % of colloidal SiO2 NPs in 10 mL H2O) under hydrothermal conditions (160 °C for 10 h) afforded core−shell structure of carbon and SiO2 NPs (C@SiO2 NACs). Dehydration, polymerization and carbonization of glucose followed by the assembly of SiO2 NPs formed C@SiO2 NACs. Calcination (650 °C, 6 h) of C@ SiO2 NACs furnished mesoporous, hollow structured SiO2 NACs. To encapsulate Pd(II) ions in SiO2 NACs (Pd@SiO2 NACs), a single solvent method, using water as solvent, or double solvent method, using a hexane/water system, was adopted for the in situ reduction of Pd(II) ions using NaBH4. In the double-solvent method, the SiO2 NACs are dispersed first in a hydrophobic solvent (n-hexane) and kept for constant stirring. To this solution, a minimum amount (matching the pore volume of SiO2 NACs) of a hydrophilic solvent (H2O) containing 13 mg (5 wt %) of Pd precursor (Na2PdCl4) was added slowly. This mode of NPs synthesis was adopted, since, Pd ions can easily be introduced inside the pores of SiO2 NACs (mesoporous nature, average pore size is 4 nm) by capillary effect (replacing n-hexane) and hence ultrafine Pd NPs are immobilized predominantly inside the nanopores of silica shell during the reduction of Pd ions. Thus, the fabricated Pd@SiO2 NACs (SSM) possessed Pd NPs throughout the SiO2 NACs and in Pd@SiO2 NACs (DSM), the Pd NPs were present inside the hollow interior to impart selectivity while using as a catalyst in hydrogenation reactions. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images revealed the spherical morphology with sizes of 0.5 to 1.5 μm and the assembly of SiO2 NPs in as-formed C@SiO2 NACs (Figure 1a−c). The hollow SiO2 NACs obtained by the removal of carbon-core matrix by calcination still possess spherical morphology as revealed by SEM and TEM micrographs (Figure 1d−f). Calcination does not cause any morphological change in SiO2 NACs. TEM images clearly reveal the hollow nature of the robust SiO2 NACs, with a shell thickness of 150−200 nm and the SiO2 NACs are of same size (0.5−1.5 μm) as that of C@ SiO2 NACs. The shell wall is assembled of SiO2 NPs of ∼15 nm size (inset, Figure 1e). Nitrogen adsorption−desorption isotherms for C@SiO2 NACs display its nonporous nature and the surface area is found to be 4 m2/g (Supporting Information, Figure S1). 668
DOI: 10.1021/acssuschemeng.6b02025 ACS Sustainable Chem. Eng. 2017, 5, 667−674
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. SEM images of (a) carbon with silica nanoassembled capsules (C@SiO2 NACs) and (d) silica nanoassembled capsules (SiO2 NACs). TEM images of (b, c) C@SiO2 NACs, (e, f) SiO2 NACs, (g) carbon spheres, and (h,i) rattle type of C@SiO2 NACs.
Interestingly, SiO2 NACs show increased surface area of 248 m2/g (Figure 2a) with the average pore distribution of 4 nm
Figure 2. (a) N2 adsorption−desorption isotherms at 77 K and (b) the corresponding pore size distributions for SiO2 NACs.
(Figure 2b) and have a Type 4 shape, indicating that the materials are mesoporous.31 The porosity in SiO2 NACs developed by the assembly of SiO2 NPs and by the removal of carbon in the SiO2 NACs is evident from the HRTEM image (Figure 1f). The summary of nitrogen adsorption desorption values are tabulated in Supporting Information Table S1. The total carbon content in as formed C@SiO2 NACs is about 80 wt %, as estimated by gravimetric analysis. The organic/inorganic hybrid nature of C@SiO2 NACs is confirmed by Fourier transform-infrared (FT-IR) spectroscopic studies (Figure 3). The bands at 1714, 1612, 2970, and 1365 cm−1 correspond to CO, CC, C−H stretching, and C−H bending vibrations, respectively, confirming the presence of
Figure 3. FT-IR spectral analysis of C nanospheres (a), C@SiO2 NACs (b), and SiO2 NACs (c).
functionalized carbon (Table S2), formed from aromatization/ carbonization sequence in D-glucose during hydrothermal treatment. The bands at 1117 and 795 cm−1 are attributed to Si−O−Si asymmetric and symmetric stretching vibrations of inorganic SiO2 NPs. After the removal of carbon by calcination, strong bands at 1077 and 800 cm−1 for Si−O−Si asymmetric and symmetric stretching vibrations of SiO2 NPs remain in the SiO2 NACs (Figure 3c). 669
DOI: 10.1021/acssuschemeng.6b02025 ACS Sustainable Chem. Eng. 2017, 5, 667−674
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ACS Sustainable Chemistry & Engineering
Scheme 2. Schematic Representation for the Formation of C@SiO2 NACs under Hydrothermal Conditions from Glucose and Colloidal Silica NPs
1117 and 795 cm−1 due to the presence of Si−O−Si asymmetric and symmetric stretching vibrations. Also, the C− O stretching vibration at 1291 cm−1 is shifted to 1206 cm−1 due to the assembly of SiO2 on the hydrophilic surface of carbon spheres. The absence of the hydroxyl stretching frequency in C@SiO2 NACs confirms the interaction of silanol groups of SiO2 NPs with hydroxyl groups of carbon core to form new O− Si−O bonds. After the removal of carbon core by calcination, the Si−O−Si asymmetric and symmetric stretching vibrations are shifted to 1077 and 800 cm−1. This variation in the stretching frequency Si−O−Si in C@SiO2 NACs further confirms the interaction of silanol groups with the carboxylic groups of the carbon surface. The driving force for the uniform assembly of SiO2 NPs on carbon core is to form covalent linkages on the carbon surface. When the microcapsules used in this study are compared with other solid silica supports (silica spheres, SBA-15, FSM-16, KIT-6 and MCM-41) with the same composition and size, the density is much lower. Also, in many instances, the other solid silica supports lack a regular shape and have disordered structures and hence lack uniform pore size throughout. Mesoporous SiO2 NACs with hollow interiors could confine the active species (metal/metal oxide nanoparticles or the molecular catalysts) to generate a nanoreactor for catalytic reactions, and the shell could protect the active species under harsher conditions. The porous wall structure of the silica shell provides channels that may allow efficient penetration and absorption of organic species and this offers a microenvironment with a high concentration of organic substrates inside the shell.37−39 To introduce catalytic components, SiO2 NACs were loaded with Pd NPs by DSM and SSM. The DSM used in this work for facilitating Pd NP incorporation on the nanopores of SiO2 NACs is based on use of a hydrophilic solvent (water) and a hydrophobic solvent (hexane), the former containing the metal precursor (Na2PdCl4) with a volume set less than the pore volume of the adsorbent (SiO2 NACs), which can be absorbed within the hydrophilic pores and the latter, in a large amount, playing an important role to suspend the adsorbent and facilitate the impregnation process. The small amount of aqueous Na2PdCl4 could go inside the hydrophilic pores by capillary force, which greatly minimizes the deposition of Pd(II) on the outer surface. In contrast, in the conventional SSM, a large amount of solvent containing the metal precursor is used, which will be deposited throughout SiO2 NACs. The entrapped Pd ions were reduced by NaBH4 to obtain Pd NPs entrapped SiO2 NACs, Pd@SiO2 NACs (SSM) and Pd@SiO2
The morphology of C@SiO2 NACs as core−shell structure can be confirmed by either removing carbon or etching SiO2 (Figure 1). To etch SiO2 NPs, C@SiO2 NACs were treated with NaOH (3 N). The resultant material was characterized by TEM and FTIR and confirmed as pure carbon nanospheres of 600 nm size (Figure 1g). Calcination of C@SiO2 NACs for reduced time (650 °C, 3 h) furnished rattle-type SiO2 NACs containing carbon spheres inside hollow SiO2 NACs, providing the possibility of offering space between core and shell (Figure 1h,i). An increase in calcination time (650 °C, 5 h) of C@SiO2 NACs provided hollow structured SiO2 NACs (Figure 1f). These results clearly proved the core−shell nature of C@SiO2 NACs with a carbon core and SiO2 NPs assembled shell, formed from the hydrothermal reaction of D-glucose and SiO2 NPs. Change in concentration of D-glucose (0.45, 0.22, and 0.12 M) failed to assemble the SiO2 NPs around the carbon nanospheres (Supporting Information, Figure S2). Similarly, reducing the amount of SiO2 NPs to 7.5 wt % was not sufficient to assemble C@SiO2 NACs completely on the carbon nanosphere (Supporting Information, Figure S3). Formation of SiO2 NACs was also attempted with various saccharides. Under optimized reaction conditions, monosaccharides, such as galactose and fructose, afforded C@SiO2 NACs, as that of glucose (Supporting Information, Figure S4c−f). On the other hand a disaccharide (maltose) and polysaccharide (starch) formed only agglomerated and interconnected nanospheres of carbon and SiO 2 NPs (Supporting Information, Figure S4g−j). Based on these observed results and by analogy with previously reported systems of carbon spheres from saccharides32 using hydrothermal method, the observed results can be rationalized as follows. During the hydrothermal carbonization of D-glucose, carbon nanospheres consisting largely of aromatic nucleus (hydrophobic) and a hydrophilic shell containing high concentration of hydroxyl, phenolic, and carboxylic groups are obtained via dehydration, condensation or polymerization and aromatization sequence (Scheme 2). This is also confirmed from FTIR spectrum (Supporting Information, Table S2).32−36 The bands at 1699, 1612, 2950, and 1364 cm−1 correspond to CO, CC, C−H stretching, and C−H bending vibrations for carbon spheres. The bands at 3397, 1291, and 1022 cm−1 are due to O−H, C−O, and C−O− C stretching vibrations, implying the existence of large numbers of residual hydroxyl groups to impart the hydrophilicity on the surface of carbon spheres and make avenue for the interaction with SiO2 NPs. The C@SiO2 NACs show two new bands at 670
DOI: 10.1021/acssuschemeng.6b02025 ACS Sustainable Chem. Eng. 2017, 5, 667−674
Research Article
ACS Sustainable Chemistry & Engineering
Figure 4. HRTEM images of (a, b) Pd incorporated silica nanoassembled capsules (Pd@SiO2 NACs) from SSM and (d,e) Pd@SiO2 NACs from DSM. SAED pattern of (c) Pd@SiO2 NACs (SSM) and (f) Pd@SiO2 NACs (DSM), (g) uniform distribution of Pd NPs on SiO2 NACs, and (h) particle size distribution histogram for figure g.
NACs (DSM) (Scheme 1).40,41 Advantages of DSM of impregnation are Pd NPs are now localized predominantly inside the nanopores of silica shell, and this method is beneficial to impart selectivity in catalysis. Since the catalytically active Pd NPs are present inside the nanopores, passing the silica wall to access the Pd NPs will not be easy for substrates containing hydroxyl functional groups, which due to the interaction with silica shell, remain largely unaffected. HR-TEM images reveal few darker spots, representing Pd NPs of 1−2 nm diameter (Figure 4b). A closer look into Pd@ SiO2 NACs (SSM) suggests that Pd NPs are present throughout the capsules, mainly on the surface (Figure 4a,b) with uniform Pd NPs distribution. The distribution and the particle size histogram (Figure 4g,h) show that more number of Pd NPs are present in the range of 1−2 nm. In the case of Pd@ SiO2 NACs (DSM), most of Pd NPs are present in the inner wall of the surface and fewer Pd NPs are present on the surface of the capsules (Figure 4d,e). The formation of well-dispersed, smaller-sized Pd NPs may be the result of their stabilization in a constrained medium, created by the assembly of SiO2 NPs. The porosity developed due to the assembly of SiO2 NPs in the SiO2 NACs is evident in the TEM image and BET studies. The existence of Pd in a metallic state is a primary requirement for Pd catalyzed hydrogenation reactions. In the PXRD patterns of Pd@SiO2 NACs (SSM) and Pd@SiO2 NACs (DSM), appearance of a diffraction peak at 39.86° is
due to Pd in metallic state which can be indexed as the {111} diffraction of crystalline Pd(0) (Figure 5a,b). Using Scherrer’s equation, the average crystallite size is estimated as 1 nm. The crystallite size matches with the average particle size (1−2 nm) seen in the TEM analysis. To further confirm the oxidation state of the Pd, X-ray photoelectron spectroscopic (XPS) studies were carried out. Pd@SiO2 NACs (SSM) and Pd@SiO2 NACs (DSM) show Pd 3d5/2 binding energy (BE) at 336.4 and 336.6 eV respectively. The literature value for pure metallic Pd is 335.1 eV (Pd 3d5/2) (Figure 5c,d).42 It is known that increase in BE up to 1.6 eV is ascribable to a decrease in size to smaller particles of
