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Article

Synthesis of Hollow Mesoporous Silica Nanospheroids with O/W Emulsion and Al(III) Incorporation and Its Catalytic Activity for the Synthesis of 5-HMF from Carbohydrates

by
Anirban Ghosh
1,
Biswajit Chowdhury
2 and
Asim Bhaumik
1,*
1
School of Materials Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
2
Department of Chemistry, Indian Institute of Technology (ISM), Dhanbad 826004, India
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(2), 354; https://doi.org/10.3390/catal13020354
Submission received: 22 December 2022 / Revised: 25 January 2023 / Accepted: 31 January 2023 / Published: 5 February 2023
(This article belongs to the Special Issue Catalytic Conversion of Biomass to Added Value Chemicals)
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Abstract

:
Controlling the particle size as well as porosity and shape of silica nanoparticles is always a big challenge while tuning their properties. Here, we designed a cost-effective, novel, green synthetic method for the preparation of perforated hollow mesoporous silica nanoparticles (PHMS-1) using a very minute amount of cationic surfactant in o/w-type (castor oil in water) emulsion at room temperature. The grafting of Al(III) through post-synthetic modification onto this silica framework (PHMS-2, Si/Al ~20 atomic percentage) makes this a very efficient solid acid catalyst for the conversion of monosaccharides to 5-HMF. Brunauer–Emmett–Teller (BET) surface area for the pure silica and Al-doped mesoporous silica nanoparticles (MSNs) were found to be 866 and 660 m2g−1, respectively. Powder XRD, BET and TEM images confirm the mesoporosity of these materials. Again, the perforated hollow morphology was investigated using scanning electron microscopic analysis. Al-doped hollow MSNs were tested for acid catalytic-biomass conversion reactions. Our results show that PHMS-2 has much higher catalytic efficiency than contemporary aluminosilicate frameworks (83.7% of 5-HMF yield in 25 min at 160 °C for fructose under microwave irradiation).

Graphical Abstract

1. Introduction

With oxygen and silicon being the two most abundant elements in the Earth’s crust, silica-based materials are extensively used for different practical applications. Owing to the very stable, heat-resistive structure of colloidal silica, the commercial production rate of colloidal silica increased from 2.4 million tons to 2.9 million tons from 2012 to 2018 globally [1]. Various methods have already been developed to synthesize colloidal silica nanoparticles such as, for example, the sol-gel process, pyrogenic route, and microemulsion technique, etc. The sol–gel method for the size-controlled synthesis of silica nanoparticle developed by Stöber and his coworkers in 1968 is a milestone in the history of science [2]. The disadvantage of the microemulsion technique over others is that it lacks particle-size control, monodispersity and uniformity throughout the framework. Porous nanomaterials can be classified into three categories: (i) microporous materials (<2 nm), (ii) mesoporous materials (2–50 nm), and (iii) macroporous materials (>50 nm). Shape-controlled synthesis of mesoporous materials is of considerable interest as it provides a larger pore size to accommodate bigger guest molecules and enables faster mass transport, which is essential for heterogeneous catalysis, chemical sensors, drug delivery and many other surface applications [3,4]. Research in this area has accelerated after the discovery of the M41S family of mesoporous molecular sieves by Mobil researchers in the early 1990s [5,6]. However, the concept of preparing mesoporous materials has always relied on the templating route, where the supramolecular assembly of the surfactants acts as a template or structure directing agent [5,6]. However, it is worth mentioning here that although MCM-41/MCM-48 types of mesoporous silica nanoparticle maintain their mesoporosity, their particle morphology is far away from monodispersive in nature [7]. In contrast to different hydrothermal synthesis methods, there are several procedures, such as the modified Stöber method [8,9], using a double surfactant template [10], or using a microemulsion template [11,12], which were subsequently developed to control particle size as well as tailor the pore size of mesoporous silica nanospheres. Hollow mesoporous silica is another class of porous silica nanoparticle, which has attracted tremendous attention due to its hollow interior void spaces [13]. Hollow mesoporous silica nanoparticles are used in various applications such as drug delivery, the adsorption of organic pollutants, and catalysis [14,15,16,17,18]. Further encapsulation of nanoparticles, creating a quantum dot in a hollow interior, can induce several other properties [19]. Further, several recipes have been developed to synthesize hollow mesoporous silica nanoparticles (HMSN). However, in this work, we choose to achieve our goal using cationic surfactant in o/w microemulsion as a structure directing agent followed by sol-gel synthesis to form mesoporous silica nanospheres at room temperature. An emulsion is made by dispersing one liquid over other, which is otherwise immiscible and generates heterogeneous phases. Emulsions are thermodynamically unstable and try to split from the surface. However, the stability of emulsions can be increased by surface active molecules with the proper control of pH. Two major roles of surface active molecules are: (i) the minimizing of interfacial tension and (ii) allowing the flocculation between the particles [20]. The droplet size of a microemulsion is quite uniform and one can further control the particle size by, typically, changing the surfactant concentration as well as other parameters [21,22]. Several synthesis strategies have already developed to develop silica nanoparticle in both w/o (water in oil)- [23,24,25] and o/w (oil in water)-type microemulsions [26]. Compared with other microemulsion techniques, this strategy provides a green, cost-effective synthesis route which avoids toxic, carcinogenic oil substrates such as benzene, cyclohexane, dodecane, octane or petroleum-derived hydrocarbons etc. [27,28,29]. In this article, we present a room-temperature synthesis procedure to prepare perforated hollow mesoporous silica (PHMS-1) nanospheroids of an average particle size of around 100–150 nm with a pore size of 3.76 nm using o/w-type microemulsion in the presence of a cationic surfactant. Here, we utilized castor oil, a widely accepted healthy vegetable oil, as an oil source to form a microemulsion in a water medium. Thereafter, we successfully grafted Al(III) onto the silica surface (PHMS-2), keeping its morphology the same through post-synthetic modification. A pure silica framework lacks reactivity due to inefficient active sites. However, Al-derivatized silica nanoparticles create acidic active sites, which are largely used for heterogeneous catalysis and sensing etc. [30,31,32]. Typical microporous aluminium-doped silica nanoparticles have limited application due to the diffusional limitations on the guest molecule. Conversely, here, we establish advantages due to increasing an pore size and hollow interior, which overcome the diffusional limitations and favor catalytic applications. Two very common recipes were adopted during the synthesis of Al-doped framework: (i) direct synthesis and (ii) post-synthetic modification. However, as monodispersity and uniform morphology are hard to maintain in the direct-synthesis method, we followed the method of post-synthetic modification starting from PHMS-1. Hollow mesoporous aluminosilicate materials are used in different fields of catalysis reactions such as oxidation, cracking and acid catalysis, etc. [33,34,35]. Here, we explored the catalytic potential of biomass conversion into the platform chemical 5-hydroxymethylfurfural (5-HMF).
Following the Kyoto protocol, the global focus has changed to reducing the dependence on fossil fuels and to identify alternative renewable energy sources for filling this gap. “Biomass” is among the largest feedstocks for alternative renewable energy resources available in nature. Approximately 75% of biomass comprises of carbohydrates, among which hexose (C6-based)- and pentose (C5-based)-based sugars (monosaccharide or polysaccharide) are most abundant; however, a very small amount (3–4%) is consumed by humans [36]. After investigating biomass components, scientists established their widespread potential to produce essential chemicals, which are otherwise derived from directly petroleum or its daughter compounds [37]. In addition, 5-HMF is among the “top 10” essential biomass-derived platform chemicals, as declared by the US Department of Energy [38,39]. Monosaccharides are easily converted to 5-HMF via dehydration in the presence of acid catalysts [40]. Mineral acid shows a great performance in catalyzing the 5-HMF formation reaction from sugar [41]. However, as a homogeneous catalyst is hard to separate from the reaction mixture, a heterogeneous acid catalyst plays an efficient role in catalyzing the reaction. Although numerous porous heterogeneous solid acid catalysts have already detailed in the literature, silica-based catalysts are still superior over others due to their low cost and easy synthesis method [42,43]. Monodispersity while controlling particle size and inherent porosity always improves a catalytic performance for solid catalysis [44]. To date, most of the reports based on biomass conversion to 5-HMF over the porous aluminosilicate family detailed a long duration and high temperature. We showed a very high yield of 5-HMF from the most abundant monosaccharaides of biomass (glucose, fructose, mannose, galactose) over our mesoporous aluminium-doped silica-based nanospheroids under a reasonable temperature and significantly less time.

2. Results

Powder X-ray diffraction (pxrd) of both PHMS-1 and PHMS-2 suggested an amorphous nature for both the material (Figure S1, Supplementary Materials). However, from the low-angle powder X-ray diffraction measurement (Figure 1A), a broad peak around 2θ = 2.45° was observed for both PHMS-1 and PHMS-2, which clearly indicates the presence of mesopores inside both the PHMS-1 and PHMS-2 frameworks. Further, the nature of the small-angle broad PXRD pattern suggests the presence of mesophase in both the materials [45,46]. The corresponding “d” spacing calculated from Bragg’s equation suggested a 3.6 nm spacing between diffraction planes. The permanent porosity and surface area of PHMS-1 and PHMS-2 were determined from N2 adsorption–desorption analysis at 77 K. As shown in Figure 1B, a high nitrogen uptake at low P/P0 followed by slow and steady capillary condensation at higher P/P0 indicates a type-IV isotherm corresponding to mesoporous PHMS-1. Again, the nature of this isotherm at high P/P0 indicates an ink-bottle-type pore in PHMS-1 surfaces [47]. In addition to this, a small hysteresis was observed due to multilayer adsorption followed by capillary condensation. Conversely, the isotherm for PHMS-2 is a type-IV type isotherm with almost no hysteresis. Thus, according to the new report of IUPAC, N2 adsorption–desorption isotherms for PHMS-1 can be more specifically classified as type IVa, with PHMS-2 as type IVb. PHMS-1 and PHMS-2 show Brunauer–Emmett–Teller (BET) specific surface areas of 866 and 660 m2g−1, respectively. The reduction in BET surface area after Al grafting into the PHMS-1 framework is quite noticeable. From the pore-size distribution curve (PSD) of PHMS-1 (shown in Figure 1C), one type of pore with an average size of 3.76 nm was observed. However, the PSD curve of PHMS-2 (Figure 1C) suggest two types of pore, one with a similar size to that of PHMS-1 and the other somewhat small, corresponding to 1.46 nm. This result suggested that in some of the mesopores, Al(III) is grafted to the mesopore surface and, thus, reduces the pore size from that of PHMS-1. Poor hysteresis and deterioration of pore size in PHMS-2 can be predicted due the pore mouthing effect of aluminum inside the pore wall of PHMS-2. Ultra-high resolution transmission electron microscopic (UHR-TEM) analysis is an efficient technique for the visualization of the core shell structure. Figure 2A shows the UHR-TEM images of PHMS-1. Well-dispersed, uniform spheroids of a particle size of around 100–150 nm was observed throughout the specimen. A hollow morphology in the interior surface covering the nanopore channels was clearly visualized in the calcined PHMS-1 sample. The distance between the pore channels was found to be 3.64 nm, which is in close agreement with that of the PSD curve obtained from BET analysis (Figure 1C) and the “d” spacing value obtained from pxrd analysis. Therefore, the mesoporous channels are clearly visualized from the UHR-TEM images of PHMS-1. Further elucidation of the surface topology and overall morphologies was achieved through field emission scanning electron microscopic (FE-SEM) analysis (shown in Figure 2B). A uniform, almost monodispersed perforated hollow silica spheroids/ellipsoids of PHMS-1 with a particle size of around 100–150 nm were observed.
The hollow interior is clearly visualized in the SEM images. The UHR-TEM and FE-SEM images of aluminum containing PHMS-2 material are shown in Figure 3A and Figure 3B, respectively. From these observations, it can be seen that the surface morphology directly replicated that of the PHMS-1 material after post-synthetic modification. Again, elemental mapping (shown in Figure 3C–F) obtained from UHR-TEM shows that aluminum has been significantly incorporated into the outside wall of the hollow interior, which helps acid catalysis. All these results help the understanding of the formation process of the overall architecture, which is discussed in detail in the next part of this article.
In order to determine the chemical environment of the “Si” atom in PHMS-1, a solid-state 29Si NMR study was carried out (shown in Figure 4A). From this spectrum, we clearly see the existence of two types of NMR signal at −108.03 ppm and −99.27 ppm, respectively. 29Si NMR signals are assigned through Qn notation, where “n” indicates the number of other SiO4 tetrahedra connections with a tetrahedral silicate. The predominant peak at −108.03 ppm corresponds to a Q4-type silicate for the presence of a [Si(OSi)4] type network, which suggests a hydrophobic nature for PHMS-1 and the small hump at −99.27 ppm suggests the existence of Q3-type silicate due to the [Si(OSi)3(OH)] in the framework structure [48]. Again, in order to determine the local chemical environment of the 27Al nucleus in PHMS-2, a solid-state 27Al MAS NMR study was performed (shown in Figure 4B). The major two resonance peaks at around 0 ppm and 55.19 ppm are attributed to the octahedral (Oh) and tetrahedral (Td) Al coordination sites, respectively [49,50]. The lower intensity peak at 55.19 ppm is assigned to coordinatively unsaturated AlO4 (Q4-Si4) Td, whereas the higher intensity peak at 0 ppm is assigned to coordinatively saturated AlO6 (Oh) analogues. This seems to be a major disadvantage for catalysis application, as the majority of aluminium active sites are coordinatively saturated (AlOh) in PHMS-2. To determine the surface acidity of PHMS-2, we performed pyridine-adsorbed FT-IR spectroscopic analysis (Figure 1D). Two sharp peaks at 1637 cm−1 and 1543 cm−1 correspond to the pyridinium ion (PyH+) coming from the interaction with the pyridine and Brӧnsted acid sites of the PHMS-2 framework. Brӧnsted acid sites were formed due to the protonation of Si-O-Al moiety. Again, a very weak band at around 1450 cm−1 and 1610 cm−1 corresponds solely to a pyridine-adsorbed bond on the coordinatively unsaturated Lewis acidic Al site of PHMS-2. However, a sharp peak at 1490 cm−1 was present due to the interaction of pyridine with both the Lewis acidic Al site and Brӧnsted acid site of PHMS-2, respectively; however, due to the low intensity of the 1450 cm−1 peak, it is expected that the peak at 1490 cm−1 was primarily due to Brӧnsted acid sites [51]. X-ray photoelectron spectroscopic analysis of PHMS-2 confirms the existence of aluminium in the framework. A sharp peak at 74.4 eV is assigned to the Al 2p spectrum in PHMS-2 [52], which is shown in Figure 4C. Again, intense peaks at 103.1 eV and 532.9 eV correspond to Si 2p and O 1s, respectively (shown in Figure 4D and Figure 4E, respectively). Again, XPS analysis reveals the atomic percentage of Si and Al in PHMS-2, which are 37.77% and 1.86%, respectively. XPS analysis of PHMS-2 reveals a good Al loading (Si/Al ~20 atomic percentage) throughout the framework, which reinforces its application as a solid acid catalyst in biomass conversion reactions.

3. Discussion

3.1. Proposed Mechanism

Following these results, a plausible explanation is proposed in Scheme 1. CTAB containing a cationic head and hydrophobic tail acts as an emulsifier with hydrophobic castor oil. Castor oil serves as a swelling agent by capping with the hydrophobic part of CTAB (Scheme 1B). Again, aliphatic alcohol (Ethanol) helps in emulsification by linking the hydrophobic part (oil) with water and forming microdroplets of a uniform size. Then, TEOS adsorbs on the cationic surface, which is then hydrolyzed over the cationic surface under alkaline conditions (Scheme 1C). The product is then collected through the centrifugation method and dried in vacuum. Now, after the removal of the oil template by calcination, we obtain the hollow morphology (Scheme 1D). The porous channels observed from the UHR-TEM images (shown in Figure 3A) of PHMS-1 come from the CTAB stack in the oil interface, which generates perforation after calcination at 823 K for 5 h. Again, after modification with Al on the silica surface (PHMS-2), it grafts onto the outer part of the wall, as shown in Scheme 1D, with a Si/Al ratio of ~20. The size of the particle varies from 100–150 nm depending on the droplet size of the microemulsion that forms the hollow morphology after calcination. However, the role of oil ingredients is not yet completed. Following the hypothesis of Zhang et al., it is anticipated that the volume ratio and surface tension force at the interface play a major role in determining the spherical or elliptical shape [53].

3.2. Catalysis

Al-doped silica precursors are highly promising catalysts for acid-catalyzed reactions. The HMF conversion reaction is one such acid-catalyzed reaction which has been highly cultivated over the past few decades. However, aluminosilicate-based catalysts are somehow weak in HMF conversion reactions compared to purely Brӧnsted catalysts. Elucidation of catalytic activity from different monosaccharides to 5-HMF was achieved by varying four common substrates, i.e., fructose, glucose, mannose and galactose, in microwave irradiation conditions. HMF yield was very much dependent on four major factors: (i) catalyst loading, (ii) temperature, (iii) reaction time, and (iv) choice of solvent. Taking fructose as the substrate and keeping all other parameters constant, we optimized the temperature by changing the reaction temperature from 140 °C to 170 °C. Figure 5A shows the variation in HMF yield as a function of temperature (°C) for fructose as the substrate. The dependence of reaction time on HMF yield was further confirmed by varying the reaction time (from 10 min to 30 min) at optimized temperatures by taking fructose as the substrate. A suitable kinetic profile was drawn by measuring HMF yield (%) through UV/vis spectrophotometer as a function of reaction time (min) (shown in Figure 5B). At 160 °C, fructose shows the highest yield for the HMF conversion reaction at 25 min. As seen from Table 1, a deterioration of product yield was observed at further higher temperatures and prolonged reaction times. This is because longer exposure and higher reaction temperature often promotes self-polymerization and cross-polymerization of HMF with substrate carbohydrates, to form humins via the dehydration step in the presence of an acid catalyst [38,54]. This is further exemplified by the appearance of a dark colored product in the reaction vial, which corresponds to the humin. HMF yield strongly depends on the catalyst loading under optimized conditions. The catalyst varied from 4.16 wt % to 41.6 wt % and was loaded in a G10 microwave vial containing 2 mL DMSO solvent under optimum time and temperature. The results are tabularized in Table 1. From the results we see that increasing the catalyst amount from 1mg to 3mg increases the product yield but further increases in the catalyst results in a decrease in yield. The reason for this may be attributed to catalyst poisoning or further degradation of HMF to other products. The highest amount of yield, of 83.7%, was found for 3 mg of catalyst. The choice of solvent for fructose conversion to 5-HMF is of utmost importance in the microwave-assisted synthesis method. Solvent polarity is most crucial in microwave technology. The more polar is a solvent, the better the energy transfer will be [55]. Therefore, we chose polar solvents (DMSO, DMF, NMP and H2O) as a variant to optimize the HMF yield. Although polar solvents are necessary for microwave-assisted reactions, they suffer a major drawback for Al-doped catalysts. Polar solvents partially block the Al active sites, which are otherwise accessible for substrates. However, high temperatures and longer reaction times disrupt this solvent effect and activate the active sites as well. Figure 5C shows the variation in HMF yield using fructose as a substrate in different solvents. As seen from Figure 5C, DMSO was superior over other solvents in the fructose conversion reaction. The reason for this can be attributed to the additional effect of DMSO, serving as a Brӧnsted acid in the presence of the aluminosilicate framework [56].
In addition, DMSO shows a higher tangent value over other solvents, which means that in the presence of DMSO, a large amount of heat energy can be converted from microwave radiation at a particular frequency. As seen from the solid-state 29Si NMR study (Figure 4A), the dominance of the Q4-type silicate makes the catalyst highly hydrophobic. That is why a very low yield was found when we used pure water as the reaction medium. However, a slight improvement in HMF yield was found after using a 1:1 mixture of DMSO and H2O as the reaction medium. Variation in different solvents results in a reactivity order corresponding to DMSO > DMSO + H2O > DMF > NMP > H2O for the fructose substrate (depicted in Figure 5C). Table 2 shows a detailed comparison table for the aluminosilicate-based catalyst for the HMF conversion reaction. A considerably higher temperature compared to the normal Brӧnsted acid catalyst is noteworthy. This is due to the weak Brӧnsted acidity of the aluminosilicate framework and activation of the Lewis acidic site. Porosity seems to have a major role during fructose-to-HMF conversion and the reason for this can be attributed to the better mass transport and easier diffusion of reactant molecules.

3.3. HMF Conversion Reaction

Examination of different substrates in the same reaction conditions is shown in Figure 5D. The reactivity order corresponding to fructose > galactose > glucose > mannose was observed using the PHMS-2 catalyst. It is believed that glucose, galactose and mannose isomerize before to fructose and, thereby, it creates 5-HMF through dehydration. A detailed catalytic mechanism over monosaccharides such as glucose, galactose, mannose and fructose to HMF conversion is shown in Scheme 2. A study reported by Marianou et al. suggested that homogeneous aluminum-based catalysts are highly active compared to heterogeneous aluminum-based catalysts for glucose-to-fructose isomerization [57]. For example, NaAlO2 gives almost 25% fructose yield at 80 °C for 30 min, whereas γ-Al2O3 or SiO2/ Al2O3 gives less than 2% fructose yield even at 100 °C for 1 h from glucose in an aqueous medium. The reason for this can be attributed to the fact that glucose isomerization is favored by deprotonation followed by isomerization [58]. However, a deprotonation step is highly reluctant in aluminosilicate-based acid catalysts. Again, following the theorization of Yang et al., it may anticipated that Lewis-acid-based catalysts may proceed through another pathway in the isomerization process by coordinating with the –OH group of the substrate [59]. However, in the case of heterogeneous catalysts, the activity may be questionable as the active sites are hard to access for coordination. Therefore, based on these facts, the slow kinetics of the isomerization process can be explained.
Accumulation of all the results show that our catalyst PHMS-2 serves as an efficient and sustainable acid catalyst compared to other Al-doped silica precursors (shown in Table 2) for the conversion of biomass to 5-HMF. From Table 2, we can easily conclude that our Al-grafted mesoporous silica (PHMS-2) shows a much better result in comparison to other similar Al-doped silica precursors.

3.4. Hot Filtration Test

The heterogeneous nature of HMF conversion over PHMS-2 catalysts was determined by a hot filtration test. In a typical process, 24 mg of fructose and 3 mg of catalyst were placed in a microwave reactor along with 2mL DMSO solvent under optimum conditions. The reaction was then stopped after 15 min and cooled to room temperature. Then, the catalyst was separated from the reaction mixture by centrifugation and an HMF yield of 65.3% was measured by UV/Vis spectrophotometry (mentioned above). Next, the reaction was further continued for up to 15 min under catalyst-free conditions. From Figure S2, it can be seen that the yield was maintained. Therefore, we can conclude that no leaching of aluminum took place during the progress of reaction. Therefore, this reaction was completely heterogeneous under the optimum conditions.

3.5. Catalyst Reusability

The recycling of a catalyst is a major concern in heterogeneous catalysis. To prove this, we chose entry 3 in Table 1 as our model reaction. After the first reaction, the catalyst was separated from the reaction mixture using centrifugation. Then, the catalyst was successively washed with water and methanol and dried at 200 °C for approximately 4 h. The reactivated catalyst was then repeatedly used four times at similar reaction conditions. However, no appreciable amount of change in 5-HMF yield (%) was observed, even after the fourth cycle (shown in Figure S3). Again, after the fourth cycle, the catalyst was separated through centrifugation and the dried catalyst was, then, subjected to powder X-ray diffraction analysis. The result shows that the mesoporosity of the catalyst remains unchanged even after so many cycles (shown in Figure S4). Therefore, all the collective results show that our novel mesoporous PHMS-2 has great potential as a reusable candidate for the conversion of carbohydrate biomass into 5-HMF.
Table
Table 1. Catalytic activity of PHMS-2 material for HMF production under different conditions.
Table 1. Catalytic activity of PHMS-2 material for HMF production under different conditions.
EntrySubstrateSubstrate
Amount (mg)		Catalyst
Amount
(mg)		Solvent ‡Temperature
(°C)		Time
(min)		Yield (%)
1Fructose245DMSO1602569.8
2Fructose244DMSO1602581.8
3Fructose243DMSO1602583.7
4Fructose242DMSO1602578.4
5Fructose241DMSO1602562.8
6Fructose247DMSO1602561
7Fructose2410DMSO1602558.8
8Fructose243DMSO1702575.4
9Fructose243DMSO1502563.6
10Fructose243DMSO1402550.4
11Fructose243DMSO1602074.9
12Fructose243DMSO1601565.3
13Fructose243DMSO1601033.7
14Fructose243DMSO1603078.6
15Fructose243DMSO + H2O1602548.4
16Fructose243DMF1602523.7
17Fructose243NMP1602510.7
18Glucose243DMSO1602526.9
19Mannose243DMSO1602526.1
20Galactose243DMSO1602527.3
‡Solvent used 2 mL.
Table
Table 2. Comparative study of catalytic efficiency in HMF conversion reaction between PHMS-2 and other Al-grafted nanoporous silica materials.
Table 2. Comparative study of catalytic efficiency in HMF conversion reaction between PHMS-2 and other Al-grafted nanoporous silica materials.
CatalystTemp. (°C)SolventTime (min.)SubstrateYield (%)Ref.
Al-MCM-41195H2O/MIBK150Glucose63[60]
Al-KCC-1162DMSO60Fructose92.9[61]
Al-KCC-1170DMSO120Glucose39[61]
Al-KIT-5135DMSO60Fructose79.5[62]
Al-MCM-41170H2O/MIBK75Fructose42.3[63]
Al-SBA15120DMSO120Fructose74[64]
Al-TUD-1170H2O/Toluene360Fructose16[65]
PHMS-2160DMSO25Fructose83.7This work

4. Materials and Methods

4.1. Chemicals

CTAB (cetyltrimethylammonium bromide), TEOS (tetraethyl orthosilicate) and anhydrous AlCl3 were purchased from Sigma Aldrich, Bangalore, India. Castor oil was purchased from Dabur Ltd., Kolkata, India. All solvents were used after purification and double distilled water was used for synthesis and cleaning purposes.

4.2. Instrumentation

Powder X-ray diffraction experiments of both PHMS-1 and PHMS-2 were performed in Bruker D-8 Advanced SWAX, Germany diffractometer using Ni-filtered Cu Kα (λ = 0.15406 nm) radiation. X-ray diffraction patterns in the small-angle region (0.5°–5°) were recorded at a rate of (2θ) = 1° per min. N2 sorption analysis for both the materials was performed at 77 K using surface area analyzer of Isorb-HP-1 gas sorption analyzer of Quantachrome Instruments, USA. For both the materials, pore-size distribution plot was obtained using non-local density functional theory (NLDFT) method. Prior to the sorption analysis, both the samples were dried and degassed at 150 °C for 5 h under high-vacuum conditions. Particle sizes of both PHMS-1 and PHMS-2 frameworks were analyzed by collecting ultra-high-resolution transmission electron microscopic (TEM) images data from UHR-FEG TEM system of JEOL JEM 2100F, Japan instrument operating from 200 kV electron source. Prior to TEM analysis, samples were prepared by drop-casting isopropanolic solution of both the materials in two different carbon-coated copper grids and then drying them under high vacuum. Morphology of self-assembled novel silica as well as Al-doped silica framework were analysed by Zeiss SEM (Germany) system operating with an voltage of 15 kV (FE-SEM) field emission scanning electron microscope (FE-SEM). Atomic percentage and Si:Al ratio of PHMS-2 framework were confirmed from the X-ray photoluminescence spectroscopic (XPS) analysis with the help of 6 Omicron Nanotechnology GmbH XPS machine, Sweden. Solid-state 29Si NMR spectroscopic analysis was performed using 11.74 T Bruker Advanced-II 500 MHz NMR spectrometer, Germany with the equipment of 4 mm MAS probe of spinning rate 11 KHz. 27Al solid-state MAS-NMR experiment was carried out with the help of Bruker AV-300 NMR spectrometer, Germany at a Larmor frequency of 78.172 MHz in 7.01 T magnetic field. UV-vis spectroscopic characterization of 5-HMF was performed using SHIMADZU UV-2401PC (Japan) instrument. Fourier transform infrared (FT-IR) spectra of pyridine adsorbed PHMS-2 material was collected on a Shimadzu FT-IR 8400S (Japan) instrument using KBr pellets. All the catalysis experiments were carried out in an Anton Paar Microwave Synthesis Reactor, USA.

4.3. Synthesis of PHMS-1

A very simple and facile room-temperature synthesis route was optimized to prepare size-controlled monodispersed mesoporous silica nanoparticles named PHMS-1. In the typical synthesis method, 50 mg of cationic surfactant CTAB were taken in a polypropylene bottle containing 200 mL of deionized water. After that, 0.5 mL of castor oil (vegetable oil) immersed in 10 mL ethanol were added into the solution and overall solution was subjected to continuous high-speed stirring with sonication to prepare o/w (oil in water)-type microemulsion. After 2 h of stirring, a milky colloidal solution appeared. Then, 4 mL of TEOS were added in dropwise manner into this milky solution and, again, the solution was kept under stirring condition for another 30 min. Then, slowly and very carefully, NaOH solution (200 mg in 5 mL water) was added into the reaction mixture in a dropwise manner, with pH = 14. After 30 min of continuous stirring, a glimpse of colloidal precipitate appeared. Then, for the flocculation, the overall solution was kept at 5 °C for 8 h. A colloidal precipitate was found and then it was, again, aged at room temperature. The precipitate was collected using high-speed centrifugation, which was further washed with deionized water and rectified spirit, respectively. After drying the precipitate under vacuum, the solid sample was powdered and calcined at 550 °C for 5 h to remove the surfactants and oil ingredients. After cooling to room temperature, we collected template-free novel silica nanoparticle (PHMS-1).

4.4. Synthesis of PHMS-2

An activated form of PHMS-1 prepared by above-mentioned procedure was taken in 10 mL of deionized water. After few minutes of stirring, anhydrous AlCl3 (0.25 mmol with respect to silica) was added very slowly and carefully into the solution while stirring at room temperature. After 6 h of continuous stirring at room temperature, the solid product was collected via centrifugation. Finally, the product was washed several times with deionized water and dried at 200 °C for 24 h. The white solid product was collected and named PHMS-2.

4.5. Pyridine Adsorbed FT-IR

Surface acidity of the acid catalyst (PHMS-2) was investigated through pyridine chemisorption on the acidic sites. Prior to the investigation, the said catalyst was activated at 120 °C temperature for an hour. Thereafter, the activated catalyst was placed in a Teflon lined autoclave in a table-top arrangement with the help of a holder. Pyridine was then carefully and slowly added beneath the holder so that no catalyst was directly in contact with pyridine. The overall system was then sealed carefully and placed in 120 °C oven. During condensation from vapour phase, pyridine adsorbs on the catalyst surface. After 3 h of the treatment, the autoclave was cooled to room temperature, opened and immediately subjected to FT-IR analysis using KBr pellet.

4.6. Catalyst Activation

Prior to each catalysis reaction, the catalyst needs to be activated for the removal of adsorbed solvent molecules which may cause partial hindrance to catalytic activity due to the partial blocking of active sites. Hence, we activated the whole specimen of catalyst (PHMS-2) at 120 °C for 6 h and then placed it in a high vacuum before injection to catalysis chamber. The activated catalyst was then used for all further catalytic experiments.

4.7. 5-HMF Catalysis

Here, 5-HMF was synthesized through the acid-catalyzed dehydration of carbohydrate sources. Briefly, carbohydrate sources (fructose, glucose, mannose and galactose) were placed in a microwave tube and charged with activated catalyst and solvent (2 mL). Then, the tube was properly sealed and placed in a microwave reactor for appropriate time at desired temperature. After that, the reaction mixture was allowed to cool at room temperature and then the catalyst separated from the mixture by centrifugation. Catalysis condition was optimized at 160 °C temperature, which showed 83.7% HMF yield in DMSO medium by 12.5 wt% of activated catalyst within 20 min. The obtained product was extracted with water and DCM. The extracted product was investigated through 1H-NMR spectroscopic analysis in DMSO-d6 medium and the characteristic proton signal of 5-HMF was again confirmed (shown in Figure S5). Then, 5-HMF yield was calculated from brown-color filtered solution using UV/Vis spectrophotometry.

4.8. HMF Yield Determination

In every reaction, 5-HMF yield was calculated quantitatively using UV/Vis spectrophotometry. After completion of every reaction, catalyst was separated through centrifugation. Then, the supernatant solution was further diluted with appropriate amount of solvent and the diluted solution was placed in UV/Vis spectrophotometer. Prior to analysis, an initial calibration was performed using a standard 99% HMF solution. A strong peak at 284 nm confirms the formation of 5-HMF in every reaction (Figure 4F and Figure S6). From the absorbance value and molar extinction coefficient data, we calculated the yield of 5-HMF using Beer’s law [38]. The analysis was repeated and found almost identical data for every reaction. Further, the yield was confirmed with the help of 1H-NMR spectroscopy using mesitylene as reference.

5. Conclusions

In this work, we demonstrated a new synthetic recipe for the preparation of perforated hollow mesoporous silica and aluminosilicate nanospheroids/ellipsoids with the help of the o/w emulsion method. This method is cost-effective and environmentally friendly at the same time, as it includes vegetable oil (castor oil) and biodegradable surfactants in a water medium. This emulsion mediated the synthesis of mesoporous catalysts in an economical way, which is of considerable interest for biomass conversion reactions. Therefore, our work will open up new options in the field of mesoporous silica-based nanomaterials. Although several works have reported the catalytic property of hollow mesoporous aluminosilicate, their catalytic potential in the biofuel (5-HMF) production has not been explored. Thus, in summary, the green synthesis method reported herein could offer a new opportunity for researchers and industries for the sustainable synthesis of biofuel products.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13020354/s1, Figure S1: Wide-angle powder XRD of PHMS-1 and PHMS-2, Figure S2: Hot filtration test, Figure S3: Catalyst recyclability test of the catalyst, Figure S4: Small-angle PXRD of the catalyst after 4th cycle, Figure S5: 1H-NMR spectrum of 5-hydroxymethylfurfural (5-HMF) and Figure S6: UV-Vis spectrum of 5-HMF formation from fructose.

Author Contributions

Experiments, investigation and formal analysis were performed by A.G. B.C. was involved in the formal analysis of the catalysts and products. A.B. provided the resources, investigation and overall supervision of this project. A.G wrote the draft manuscript with the help of A.B. and B.C. All authors have read and agreed to the published version of the manuscript.

Funding

AG wants to thank CSIR, New Delhi (Award No: 09/080(1084)/2019-EMR-I) for a Senior Research Fellowship. A.B. and B.C. would like to acknowledge DST, New Delhi for Indo–Russia collaborative research grant (Project no. DST/INT/RUS/P-25).

Data Availability Statement

Not applicable.

Conflicts of Interest

There are no conflict to declare.

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Figure 1. (A) Small-angle powder XRD pattern of PHMS-1 (red) and PHMS-2 (blue). (B) Nitrogen adsorption/desorption isotherms of PHMS-1 (red) and PHMS-2 (blue). (C) Non-local density functional theory pore-size distributions (PSD) of PHMS-1 (red) and PHMS-2 (blue). (D) Pyridine adsorbed FT-IR of PHMS-2.
Figure 1. (A) Small-angle powder XRD pattern of PHMS-1 (red) and PHMS-2 (blue). (B) Nitrogen adsorption/desorption isotherms of PHMS-1 (red) and PHMS-2 (blue). (C) Non-local density functional theory pore-size distributions (PSD) of PHMS-1 (red) and PHMS-2 (blue). (D) Pyridine adsorbed FT-IR of PHMS-2.
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Figure 2. (A) UHR-TEM images of PHMS-1; (B) FE-SEM micrographs of PHMS-1 at different magnifications (60,000–93,000).
Figure 2. (A) UHR-TEM images of PHMS-1; (B) FE-SEM micrographs of PHMS-1 at different magnifications (60,000–93,000).
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Figure 3. (A) UHR-TEM images of PHMS-2. (B) FE-SEM micrographs of PHMS-2. (C) Elemental analysis specimen. (D) Elemental mapping for Si. (E) Elemental mapping for O. (F) Elemental mapping for Al.
Figure 3. (A) UHR-TEM images of PHMS-2. (B) FE-SEM micrographs of PHMS-2. (C) Elemental analysis specimen. (D) Elemental mapping for Si. (E) Elemental mapping for O. (F) Elemental mapping for Al.
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Figure 4. (A) Solid-state 29Si NMR spectroscopic analysis of PHMS-1. (B) Solid-state 27Al NMR spectroscopic analysis of PHMS-2 and XPS analysis for Al 2p, (C) Si 2p, and (D) O 1s (E) in PHMS-2. (F) Time-dependent UV-Vis spectrum of HMF formation from fructose.
Figure 4. (A) Solid-state 29Si NMR spectroscopic analysis of PHMS-1. (B) Solid-state 27Al NMR spectroscopic analysis of PHMS-2 and XPS analysis for Al 2p, (C) Si 2p, and (D) O 1s (E) in PHMS-2. (F) Time-dependent UV-Vis spectrum of HMF formation from fructose.
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Scheme 1. Illustration of the formation mechanism of PHMS-1 (A-D) and PHMS-2 (D-E) involving the hollow interior by using castor oil and CTAB as templates.
Scheme 1. Illustration of the formation mechanism of PHMS-1 (A-D) and PHMS-2 (D-E) involving the hollow interior by using castor oil and CTAB as templates.
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Figure 5. (A) Temperature dependence of HMF yield (%). (B) Dependence of HMF yield on reaction time (min). (C) Solvent dependence of HMF yield (%). (D) Selectivity of substrate for HMF conversion reaction. The amount of catalyst used was 3 mg, substrate used was 24 mg and solvent used 2 mL under microwave irradiating conditions.
Figure 5. (A) Temperature dependence of HMF yield (%). (B) Dependence of HMF yield on reaction time (min). (C) Solvent dependence of HMF yield (%). (D) Selectivity of substrate for HMF conversion reaction. The amount of catalyst used was 3 mg, substrate used was 24 mg and solvent used 2 mL under microwave irradiating conditions.
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Scheme 2. Reaction pathway for the synthesis of 5-HMF over PHMS-2.
Scheme 2. Reaction pathway for the synthesis of 5-HMF over PHMS-2.
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Ghosh, A.; Chowdhury, B.; Bhaumik, A. Synthesis of Hollow Mesoporous Silica Nanospheroids with O/W Emulsion and Al(III) Incorporation and Its Catalytic Activity for the Synthesis of 5-HMF from Carbohydrates. Catalysts 2023, 13, 354. https://doi.org/10.3390/catal13020354

AMA Style

Ghosh A, Chowdhury B, Bhaumik A. Synthesis of Hollow Mesoporous Silica Nanospheroids with O/W Emulsion and Al(III) Incorporation and Its Catalytic Activity for the Synthesis of 5-HMF from Carbohydrates. Catalysts. 2023; 13(2):354. https://doi.org/10.3390/catal13020354

Chicago/Turabian Style

Ghosh, Anirban, Biswajit Chowdhury, and Asim Bhaumik. 2023. "Synthesis of Hollow Mesoporous Silica Nanospheroids with O/W Emulsion and Al(III) Incorporation and Its Catalytic Activity for the Synthesis of 5-HMF from Carbohydrates" Catalysts 13, no. 2: 354. https://doi.org/10.3390/catal13020354

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