Synthesis and Characterizations of Titanium Tungstosilicate and Tungstophosphate Mesoporous Materials

Abstract

The work reports a development approach for the synthesis of novel multi-components mesoporous materials of titanium tungstate (meso-TiW) titanium tungstosilicate (meso-TiWSi) and tungstophosphate (meso-TiWP) mixed oxides that have high surface area and ordered mesoporous structures at nanometer length scale. Using the solvent evaporation-induced self-assembly (EISA) new oxides of bi- and tri-component of meso-TiW, meso-TiWSi and meso-TiWP oxides with different compositions and porosity were achieved. The physicochemical properties of the mesoporous oxides were characterized by X-ray diffraction, BET surface area analyzer, scanning, and transmission electron microscopes. Subject to the oxide composition, the obtained meso-TiW, meso-TiWSi and meso-TiWP exhibits high surface area, ordered 2D hexagonal mesostructured with order channels extended over a large area. The produced meso-TiW, meso-TiWSi, and meso-TiWP adsorbents exhibit good adsorption efficiency for the removal of Pb(II), Cd(II) and Hg(II) ions from water solution due to the presence of high surface area and accessibility of surface active sites. The adsorption efficiency of these mesoporous oxide reaches up to 95% and is found to be dependent contact time and adsorbents dose. The synthesis strategy is particularly advantageous for the production of new complex (multi-component) inorganic mesoporous materials that might have an application in the field of environmental, catalysis or energy storage and production.

1. Introduction

The approaches used for the synthesis of nanostructured materials show a critical role in synthesis the required materials and determining its final properties. To achieve mesoporous inorganic oxide powders with high surface area, uniformity and even mesoporous structures, there are many protocols have been implemented [1, 2] and the most important ones are: mesoporous silica materials as a host [3, 4], sol-gel precipitation, solution co-precipitation, thermal solid state, and surfactant self-assembled templates [5, 6, 7, 8]. Generally, the sol-gel precipitation, co-precipitation, and surfactant self-assembled templates were the most widely used methods for preparation of mesoporous ion exchange materials.

The synthesis of nonsiliceous nanostructured mesoporous materials with high surface area using surfactant self-assembled templates has been recognized as a method for controlling the phase structure, ordering and pore size of these functional materials [9, 10, 11, 12]. A wide range of mesoporous oxides, metals and semiconductors materials with regular geometries have been successfully prepared by direct templating from liquid crystalline phases [13]. Nevertheless the synthesis of nonsiliceous mesostructured still lags behind the synthesis of mesoporous silica materials, particularly for complex and multi-components inorganic materials. The reasons for this come from the complexity, reactivity, chemical stability and interaction of the surfactant with the inorganic precursor. Establishment of synthetic routes for multi-component metal oxide mesostructured those are effective, applicable, reproducible and suitable for large-scale production still remains a challenge.

Inorganic ion exchange materials are widely used for water treatment, in the semiconductor industry, and in nuclear technology. Multi-component ion-exchange materials, such as titanium tungstosilicate (TiWSi) and tungstophosphate (TiWP) mixed oxides have better chemical and thermal stability and exchange capacity compared to Ti (IV) tungstate, Ti (IV) silicate and Ti (IV) phosphate. The crystalline phases of these materials have been utilized for separating heavy metals in aqueous media. The adsorption, separation and exchange properties of these systems could potentially be enhanced significantly if the materials were obtained with mesoporous structures.

In this work, the solvent evaporation-induced self-assembly (EISA) template method is adapted to produce a selection of mesoporous ion exchange materials such as titanium tungstate (meso-TiW) titanium tungstosilicate (meso-TiWSi) and tungstophosphate (meso-TiWP) mixed oxides with the high surface area and controlled porous structures that similar to mesoporous silica. Using this method mesoporous ionic exchange materials can be synthesized with homogeneous and uniform structures in the form of powders, amenable to mass production, or as membranes depending on the conditions employed during the synthesis. The characterization and adsorption capacity of the resulting nanostructured mesoporous materials is investigated for water demineralization and the removal of heavy metal ions from water solution.

Using the template strategy to synthesize nanostructured mesoporous materials with a different shape, size and morphology was firstly introduced by Mobil scientists [14] and Martin et al. [15, 16]. In this template method, the desired materials are chemically formed using self-assembled surfactant templates (sol-gel) or electrochemically deposited inside the pores of a nanoporous membrane that subsequently dissolves away leaving a replica of nanomaterials structures. The templates are divided into two different types: hard and soft templates. The commonly used hard templates include anodic aluminum oxide [16], silica [17], and polystyrene spheres [18] and mesoporous carbon [19]. The soft templates are formed from surfactant molecules that are self-assembled into organized mesophases (soft matter) that are intermediate in order between crystalline solids and isotropic liquids.

In recent years, the sol-gel process is widely utilized concurrency with organic-directed assembly to synthesize a variety of non-siliceous mesostructured materials such as transition metals, metal oxides, semiconductors, carbons, and polymers. Such mesostructured materials have attracted significant attention because of the application of these materials in nanoscale devices, sensors, microelectronic devices, catalysts, energy storage media, and adsorbent materials. Mesoporous silica-based materials, for example, have been widely attracted an attention for heavy metal ions adsorption from wastewater [20, 21, 22, 23]. This because they exceptionally have the large specific surface area, regular pore structure, and tunable pore sizes. Consequently, the mesoporous materials are expected to exhibit better heavy metal ions adsorption due to improved accessibility to the adsorption sites and/or speeding up the diffusion process within the mesopores. Toward this end and very recently, mesoporous silica materials were prepared by microwave assistant heating using silica fume as silica source and evaluated as adsorbents for the removal of Cu²⁺, Pb²⁺, and Cd²⁺ from aqueous solutions [24]. The results showed that the produced mesoporous silica could efficiently confiscate the heavy metal ions from aqueous solution within pH range of 5–7 and the maximum removal capacities of the mesoporous silica for Cu²⁺, Pb²⁺, and Cd²⁺ were 36.3, 58.5, and 32.3 mg/g, respectively.

In addition, mesoporous silica modified with suitable organic functional groups have shown enhanced heavy metals adsorbents capacity due to combining the open porous structure of mesoporous silica with suitable organic compounds that exhibit high binding affinity toward the target metal ions. As an example, Lee et al. [22] reported the adsorption of Pb(II) and Cu(II) metal ions on mesoporous silica modified with organic amine and thiol functional groups and results reveal that the thiol-functionalized mesoporous silica adsorbents exhibit enhanced adsorption affinity for Pb²⁺ than the amino-mesoporous silica. Moreover, other studies reported an excellent Hg removal efficiency on thiolated mesoporous silica compared to the aminated ones [23, 25].

Sol-gel process using polyethylene oxide (PEO) as well as low molecular weight surfactants have been adapted for the preparation of thermally stable, ordered and large pore size mesoporous non-silica materials of metal oxides including MnO₂, TiO₂, ZrO₂, Nb₂O₅, Ta₂O₅, Al₂O₃, SnO₂, WO₃, HfO₂, and mixed oxides of Al₂TiO_y, ZrTiO_y, ZrW₂O_y [26, 27, 28]. The pore topology and pore size of the mesoporous oxides can be controlled by varying the surfactant or by the addition of hydrophobic swelling agents to the template mixture. However, the nature of the inorganic precursors of transition metals plays a critical for this process and it has to be compatible with organic co-assembly [29]. Therefore the evaporation-induced self-assembly (EISA) was introduced for the synthesis of non-silica mesostructured materials and has shown to be the very useful approach for both controlling macroscopic form (powder, thin films, and membranes) and mesostructured [30, 31]. In this EISA process, the mixture of volatile solvent, (commonly ethanol) surfactant and inorganic precursors are exposed to evaporation which drives the co-assembly of surfactant molecules and inorganic precursors to form the ordered mesophases. Subsequently using aging, chemical or heat treatments induce the formation of the desired mesostructures. Recently mesostructures of titanium tungstate with the high surface area, large pore volume, uniform pore size, and tunable W/Ti ratios were successfully fabricated by using (EISA) strategy [32]. Also, mesoporous zirconium phosphotungstate with large specific surface area ~170 m²/g, large pore volume, uniform pore size distribution, and tunable W/Zr ratios were prepared by adopting this EISA method [33]. In another template strategy, mesoporous TiO₂/WO₃ hollow fibers and interconnected nanotubes are prepared by hollow fibers as a framework, in addition, to block copolymer as a template. The mesostructures showed a surface area of ∼130 m²/g with improved photodegradation activity [34].

Earlier studies on the synthesis of inorganic ion-exchangers are well documented by Amphlet [35] and Clearfield [36]. A comprehensive review of inorganic ion-exchangers with layered structures [37] has been published which suggests that group IV phosphates, molybdates, tungstates, etc. possess interesting cation-exchange properties. Silicates of Ti(IV) have been shown to have porous properties and are being used as molecular sieves [38]. Two new crystalline phases of α-titanium phosphates with layered structures have been reported [39]. Siddiqui et al. [40] have synthesized crystalline phases of titanium tungstosilicate (TiWSi) and titanium tungstophosphate (TiWP) with better ion exchange capacity and better chemical and thermal properties as compared to titanium phosphate, titanium tungstate, and titanium silicate. Furthermore, ion-exchange materials with the amorphous state, crystalline state, and hybrid organic-inorganic states have been recently synthesized and studied for environmental applications [41, 42].

2. Experimental

2.1 Chemicals and materials

Triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly-(ethylene oxide) copolymer Pluronic^® P123 (EO₂₀PO₇₀EO₂₀, Mw = 5800, Sigma-Aldrich) was used as the structure-directing agent (SDA) and titanium tetraisobutoxide (Ti(OBu)₄, 98%, Aldrich), tungsten hexachloride WCl₆, phosphorus pentachloride PCl₅ (reagent grade, 95%, Sigma-Aldrich) and tetraethyl orthosilicate (TEOS, 98%, AnalaR) were used as Ti, W, P and Si source respectively. n-Propanol (n-C₂H₈O, BDH AnalaR) was used for dissolve the surfactant and hydrochloric acid HCl (37 wt. %, Analar) were all supplied by Shanghai Chemical Corp. Copper(II) nitrate hydrate, lead(II) nitrate (ACS reagent, ≥99.0%) and mercury(II) nitrate monohydrate (ACS reagent, ≥98.0%) are purchased from Sigma-Aldrich. All other chemicals were used as received without further purification and all aqueous solutions were prepared using Milli-Q (Millipore, Inc.) high quality deionized (DI) water (resistance 18.2 MΩ cm).

2.2 One-pot synthesis of bi-components mesoporous titanium tungstate (meso-TiW-x) using EISA approach

Mesoporous bi-components titanium tungstate (meso-TiW-x) oxides were synthesized with different composition through a modified evaporation-induced self-assembly (EISA) method. Typically, with a total Ti-W molar content of 18 mmol, Ti(OBu)₄ and WCl₆ were first dissolved in a mixture containing 20 g of n-propanol and 2.0 g of Pluronic 123 (EO₂₀PO₇₀EO₂₀). Then, the mixture was allowed for stirring for 2.0 hours at room temperature in a sealed vial. The obtained sol was cast into Petri dishes and kept under ambient conditions for 1 hour as shown in Scheme 1. Subsequently, the as-made gel was cured at 40°C for 2 days and then calcined in flowing air at 400°C for 4 hours with a ramping rate of 1°C min⁻¹. The obtained samples of meso-TiW were denoted as meso-TiW-x, where X equals 20, 30, and 40 wt. % the percentage of W in the original template mixture.

Scheme 1.

Schematic diagram for the synthesis of mesoporous materials of titanium tungstate (meso-TiW) titanium tungestophosphate (meso-TiWP) and titanium tungstosilicate (meso-TiWSi) by EISA.

Specifically, for the synthesis of meso-TiW-20, meso-TiW-30 and meso-TiW-40, a 4.90, 1.43, 4.28 g of Ti(OBu)₄ were added to 2.14, 3.68, 2.86 g of WCl₆, respectively. In order to further moderate the hydrolysis rate, a 1.20 g of hydrochloric acid (37 wt. %) was added to the solution for synthesizing meso-TiW-20 oxides. A control experiment was performed to prepared pure meso-TiO₂ using EISA approach for comparison purpose.

2.3 Synthesis of mesoporous titanium tungstosilicate (meso-TiWSi) and titanium tungstophosphate mixed oxides (meso-TiWP)

The mesoporous titanium tungstosilicate oxides (meso-TiWSi-x, where x is equal 5, 10, 15, 20, 40, 60 wt. % the percentage of Si in the original template mixture) and titanium tungstophosphate mixed oxides (meso-TiWP-x, where x is the wt. % of P in template mixture equal 5, 10, 15, 20 and 30 wt. %) were synthesized through adopted (EISA) method as shown above in Scheme 1. In a typical experiment, 2.86 g of WCl₆ was dissolved in 10 g of n-propanol, this mixture was mixed by stirring until getting the clear solution then 3.68 g of Ti(OBu)₄ was added to this solution to obtain a solution (A). Then 2.0 g of nonionic surfactant (triblock copolymer templates (P^®123), was dissolved in 10 g n-propanol and stirred for 10 min and different amounts of TEOS (5, 10, 15, 20, 40, 60 wt. %) were added to this solution and stirring for another 5.0 min to obtain a solution (B). In case of titanium tungstophosphate 5, 10, 15, 20 and 30 wt. % of PCl₅ was added to this solution and stirring for 5 min to obtain solution B. The solution (A) was added dropwise to (B) then the mixture was stirred for 2.0 hours at room temperature in a sealed vial. In order to further moderate the hydrolysis rate, 1.20 g of hydrochloric acid (37 wt%) was added to the solution for all prepared mixtures of mesoporous meso-TiWSi oxides. The obtained dark-blue colored homogeneous solution (shown in the inset in Scheme 1) was cast into Petri dishes and kept under ambient condition for 1.0 hour. Subsequently, the as-made gel was cured at 40°C for 2 days to completely remove solvents. Finally, the blue deposit was scraped and crushed into powder and then calcined in flowing air at 400°C for 4.0 hours with a ramping rate of 1.0°C/min.

2.4 Materials characterizations

X-ray powder diffraction (XRD) and Small-angle X-ray (SAXS) measurements were recorded respectively on a Bruker D4 powder X-ray diffractometer (Germany) and Nanostar U small-angle scattering system (Bruker, Germany) by using Cu K_α radiation (40 kV, 40 mA). The BET surface area, pore volume, and the pore size distributions were measured by nitrogen adsorption/desorption using a Micromeritics Tristar 3020 analyzer (USA). Prior to the measurements, the samples were degassed at 180°C for at least 6.0 hours. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area. By using the Barrett-Joyner-Halenda (BJH) model, the pore volumes and pore size distributions were derived from the adsorption branches of isotherms. Transmission electron microscopy (TEM) analysis was conducted on a JEOL 2011 microscope (Japan) operated at 200 kV and equipped with an energy dispersive X-ray (EDX) detector. Field-emission scanning electron microscopy (FESEM) images were collected on the Hitachi Model S-4800 field-emission scanning electron microscope. The heavy metals analysis was performed using an inductively coupled plasma optical emission spectroscopy (ICP-OES) system (Thermo Scientific; UK, Model ICAP 6000).

2.5 Procedure for heavy metal ions adsorption from water by mesoporous materials

The standard procedure for the removal of heavy metal ions from water using the produced mesoporous materials was performed by mixing a weighted amount of the mesoporous absorbent with 3.0 mL solutions containing 0.2 mg/L Cu(II), Cd(II) or Pb(II) nitrate salt in polypropylene tubes. The samples were placed in a shaking water bath at 25°C for 2.0 hours and then the solution was separated by centrifuge at rpm 7000 for 10 min in each run. Each solution was then diluted by taking 100 μl and transferred into 10 mL conical polypropylene flask for analysis by ICP. The amount of metal ion adsorbed at the mesoporous adsorbents was calculated by the difference of the initial concentration (C_o) and the equilibrium concentration (C_e). The removal efficiency and equilibrium adsorption capacity of Cd(II), Pb(II), and Hg(II) ions from the solution was calculated using the following equations:

$$\left(\text{E \%}\right)=\left({\text{C}}_{\text{o}}-{\text{C}}_{\text{e}}{\text{/C}}_{\text{o}}\right)\times 100$$E1

$$\left({\text{Q}}_{\text{e}}\text{\%}\right)=\left({\text{C}}_{\text{o}}-{\text{C}}_{\text{e}}\right)\text{V/m}$$E2

Where, E, Q_e, C_o, C_e, V, and m are the removal efficiency, equilibrium adsorption capacity (mg/g), initial concentration (mg/L), equilibrium concentration (mg/L), solution volume (L) and adsorbent mass (g) respectively. To investigate metal ion loss during the adsorption due to the glassware or any of the experimental equipment a parallel experiment was performed in the absence of the mesoporous adsorbents and it was regarded as a standard solution.

3. Results and discussion

3.1 Synthesis and characterizations of bi-components meso-TiW prepared by EISA approach

In this section, we present the result for the synthesis of bi-components meso-TiW with various ratios using modified solvent evaporation-induced self-assembly (EISA). Firstly the synthesis of bi-components of mesoporous titanium tungstate (meso-TiW) with various compositions was executed. In EISA process the mixture of volatile solvent, (commonly ethanol), copolymer surfactant and inorganic precursors are mixed together and exposed to evaporation which drives the co-assembly of surfactant molecules and inorganic precursors to form the ordered mesostructures. The W content was varied between 20 and 40 moles and the meso-TiW powder after annealed at 400°C for 4.0 hours shows the light yellow color crystal as such shown in the inset of Scheme 1 which indicating on the precipitation of TiW oxide while pure meso-TiO₂ exhibits white color.

Figure 1 shows the wide-angle XRD patterns for meso-TiW-20 and for pure meso-TiO₂ prepared using EISA approach for comparison purpose. The XRD pattern of pure meso-TiO₂ displayed a well-crystallized anatase phase (JCPDS Card No. 01-083-2243), while for meso-TiW-20 the XRD peaks intensity is slightly decreased in addition to the disappearance of some other peaks. This can attribute to the addition of 20 wt. % of W precursor hinders the crystallization of anatase phase and instead an amorphous composite of Ti▬W oxides were formed.

Figure 1.

X-ray diffraction patterns for meso-TiW-20 and pure meso-TiO₂ prepared by EISA approach and annealed at 400°C, 4.0 hours.

The nitrogen adsorption-desorption isotherms and the plot for pore size distribution of the mesoporous TiW-x oxides are shown in Figure 2. From the isotherms we can see that the meso-TiW oxide showed typical type IV isotherms with sharp capillary condensation steps at a relative pressure (P/P^o) of 0.35–0.85 that characteristic for mesoporous materials.

Figure 2.

(a) Nitrogen adsorption-desorption isotherms, (b) the corresponding pore size distribution curves of the meso-TiW oxide.

The average pore size, surface area (BET), pore volume for the meso-TiW oxide sample in comparing with pure mesoporous TiO₂ (prepared using EISA approach but without adding WCl₆) are shown in Table 1. Obviously, the meso-TiW-30 oxide shows the highest surface area; lower pore volume and a narrow pore size distribution which demonstrates the formation of highly ordered and uniform mesoporous structure. On the other hand, the pure mesoporous TiO₂ prepared in absence of WCl₆ shows lower surface area (81 m²/g) and higher pore volume with bimodal pore diameters. This can be attributed to the incorporation of W precursor effectively improve the order of mesoporous structure and because of the tungsten oxide has a high density (7.16 g/cm³) the incorporation of W leads to slightly reduce the pore volume of meso-TiW oxide.

Samples	SBET (m^2/g)	Pore volume (cm^3/g)	Pore size (nm)
meso-TiW-20	189	0.27	6.1
meso-TiW-30	206	0.21	3.9
meso-TiW-40	188	0.37	9.5
meso-TiO2	81	0.33	6/12.5

Table 1.

The textural properties of the samples of mesoporous TiW oxides.

Figure 3 shows the TEM image of the mesoporous meso-TiW samples. Clearly, the fine structure of the meso-TiW oxides shows typical morphologies of the highly ordered 2D hexagonal mesostructures with highly orderly channels extended over large domains which are consistent with high surface area obtained by N₂ adsorption results and confirms the successful production of bi-components mesoporous TiW oxides by EISA approach.

Figure 3.

TEM images of the samples (a) meso-TW-20, (b) meso-TW-30, and (c) meso-TW-40.

3.2 Synthesis and characterization of tri-component mesoporous titanium tungstosilicate (meso-TiWSi) materials

Here we investigate the possibility of synthesis more complicated mesoporous materials of tri-components mesoporous oxides of Ti, W, and Si. Because the bi-components meso-TiW-30 showed the highest surface area (206 m²/g) and ordered pore structure we used its composition of mole ratio (Ti₇₀W₃₀) as start material for the synthesis of tri-component mesoporous titanium tungstosilicate meso-TiWSi-x and tungstophosphate (meso-TiWP-x) materials with various compositions of silica (x = 10, 15, 20, 40 and 60 wt. %) via the facile EISA method employing P^®123 surfactant, Ti(OBu)₄, WCl₆ and TEOS as described in Section 2 above and shown in Scheme 1. Optical images of the produced meso-TiWSi materials with Si ratio of 10, 15, 20, 40 and 60 wt. % after annealing at 400°C in the air are shown in the inset of Scheme 1.

Figure 4 shows the low-angle and wide-angle X-ray diffraction patterns of mesoporous meso-TiWSi-x calcined at 400°C in the air. As shown in Figure 4a, there’s a distinct peak at 2θ = 0.53°can be observed which is indexed as (100) reflections characterize of the 2D hexagonal mesostructure [32, 43]. Furthermore, another one broad and weak diffraction peak can clearly observe at 2θ = 1–1.5° indicates the long-range order of the mesopores structure. An increase in the Si ratio up to 60 wt. % result in one broad peak can be seen in the SXRD diffraction pattern, suggested that an excessive increase in Si incorporation decreased the ordering of the mesoporous structure. For the TiWSi-10, the first scattering peak shifts from a 0.53 nm⁻¹ to a larger q value of a 0.56 nm⁻¹, corresponding to the framework shrinkage during the removal of the template. The wide angle X-ray diffraction (WXRD) patterns of these samples are shown in Figure 4b. All the patterns kept consistent with each other and only exhibited two broad peaks in the 2θ range of 20–40° and 40–60°, indicating the existence of an amorphous or semicrystalline mesoporous TiWSi wall structure. No individual crystalline peak of titanium tungstate or titanium was observed.

Figure 4.

(a) Low-angle and (b) wide-angle X-ray diffraction patterns of mesoporous meso-TiWSi-x calcined at 400°C in the air for 4.0 hours.

Meanwhile, there were no peaks of tungsten oxide in the patterns, illustrating the high dispersion of tungsten and silicon oxide among the mesoporous framework even in meso-TiWSi-0, which had silica ratio of 0 wt. %. This might be accounted to the silica atoms in the materials were segregated by titanium and tungsten atoms due to the advantage of using the one-pot EISA method [44]. We presumed that the presence of W and Si species somehow retarded the crystallization of TiO₂ during the calcination and hence benefited the preservation of the ordered mesostructured which is consistent with Figure 4b and the previous studies [45, 46].

In order to further confirm the presence of mesopores in meso-TiWSi-x materials, the SEM and TEM characterizations were performed and Figure 5 shows the SEM images of the meso-TiWSi-x samples with different Si ratios after the annealing process. For the meso-TiWSi-10 (Figure 5a) and meso-TiWSi-20 (Figure 5b), the SEM images clearly show the highly ordered mesopores uniformly exist through the TiWSi framework over a wide micrometer area. However for meso-TiWSi-40 and meso-TiWSi-60, the SEM images in Figure 5c and d show wormlike mesoporous structure indicating on less order pore arrangement when Si ratio is increased to 60 wt. %.

Figure 5.

SEM images of (a) meso-TiWSi-10 (b) meso-TiWSi-20 (c) meso-TiWSi-40 (d) meso-TiWSi-60.

Figure 6 shows the TEM images for different composition of meso-TiWSi after calcination and obviously, the TiWSi framework exhibits a 2D hexagonal mesostructured with ordered mesopores of about 10 nm in diameter. It also clearly observes the existence of ordered pores along the [110] and [001] directions in Figure 6b of meso-TiWSi-20. Nevertheless by further increasing the Si ratios to 60 wt. % (Figure 6d), the structure showed wormlike mesoporous channels rather than the long-range ordered hexagonal pores. The results of SEM and TEM agreed quite well with the results of the SXRD characterization, which predicted the presence of ordered mesoporous structure for samples with Si ratio from 0 to 20% and wormlike mesopores structure for meso-TiWSi-60.

Figure 6.

TEM images of (a) meso-TiWSi-10 (b) meso-TiWSi-20 (c) meso-TiWSi-40 (d) meso-TiWSi-60.

For the elemental composition analysis, the backscattering SEM image and energy-dispersive X-ray (EDX) analysis of the elemental composition of the meso-TiWSi-20 materials are shown in Figure 7. The backscattering SEM image (Figure 7a) obviously shows very and highly order channels running through the meso-TiWSi framework confirming the formation of long-range and ordered mesopores. The EDX elemental analysis in Figure 7b shows evidently the existence of Ti, W, O and Si elements approving that all the elements were successfully introduced into the framework of the mesoporous structure. The Si wt. % derived from the EDX analysis were very close to the target composition set up in the template mixture as shown in Table 2.

Figure 7.

(a) Backscatter SEM image of meso-TiWSi-20 (b) EDX elemental profile of meso-TiWSi-20.

Samples	Si wt. %
meso-TiWSi-10	7.4
meso-TiWSi-20	18.4
meso-TiWSi-60	54.3

Table 2.

EDX analysis of the Si wt. % in the meso-TiWSi samples.

The textural properties and the surface area of the ordered meso-TiWSi-x with different Si ratios were characterized by the N₂-physisorption technique. Nitrogen adsorption and desorption isotherms and the corresponding pore size distributions of the meso-TiWSi materials are shown in Figure 8. For all the materials, the isotherms exhibited typical type IV isotherms and well-defined H₂ shaped hysteresis loops with sharp capillary condensation steps at a relative pressure (P/P^o) of 0.4–0.8, that characteristic of mesoporous materials [45, 47]. Besides, all the capillary condensation steps of the hysteresis loops for all the samples were very steep, pointing to the presence of a uniform and ordered pore size in the framework of meso-TiWSi. The average pore size of these samples was narrowly distributed at around 4–10 nm, implying the existence of the uniform mesoporous channels within the obtained materials. The details textual parameters including specific surface area, pore size and volume of meso-TiWSi-x obtained from the N₂ adsorption-desorption isotherms, are summarized in Table 3. Clearly, all the samples had excellent textural properties with large specific surface area (131–188 m² g⁻¹), uniform pore size (4–10 nm) and pore volume (0.17–0.37 cm³ g⁻¹).

Figure 8.

(a) Nitrogen adsorption isotherms and (b) the corresponding pore size distribution curves of the meso-TWSi materials.

Samples	Specific surface area ± 5 (m^2 g^−1)	Pore size ±0.5 (nm)	Pore volume (cm^3 g^−1)
meso-TiWSi-0	188	9.5	0.37
meso-TiWSi-5	131	7.0	0.20
meso-TiWSi-10	164	5.5	0.19
meso-TiWSi-15	138	7.0	0.20
meso-TiWSi-20	142	7.5	0.22
meso-TiWSi-40	151	6.0	0.17
meso-TiWSi-60	156	5.0	0.19

Table 3.

Textural properties of the meso-TWSi-x materials derived from nitrogen adsorption and desorption analysis.

3.3 Synthesis and characterization of tri-component mesoporous titanium tungstophosphates (meso-TiWP-x) materials

Tri-components mesoporous titanium tungstophosphates meso-TiWP-x materials were fabricated via the facile EISA method employing P^®123 surfactants as a template and Ti(OBu)₄, WCl₆ and PCl₅ as precursor for Ti, W and P respectively. Starting with Ti: W mole ration of 70:30, various compositions of P (5, 10, 15, 20 and 30 wt. %) were incorporated into the meso-TiW-30 and an optical image of the produced meso-TiWP-x samples is shown in the inset of Scheme 1.

Figure 9 shows the small and wide angle X-ray diffraction pattern of meso-TiWP-x samples after annealing at 400°C for 4 hours. As indicates in Figure 9a there’s a broad scattering peak in the 2θ range 0.5–1° which indicates on the formation of less ordered mesoporous TiWP structures. However, the meso-TiWP-10 shows rather well-defined peak among the other samples; presumably, due to better order pores have been formed. The wide-angle X-ray diffraction patterns of the TiWP samples shown in Figure 9b are consistent with each other and only exhibited two broad peaks in the 2θ range of 20–30° and 40–60° which is evidence for the existence of an amorphous or semi-crystalline framework of mesoporous TiWP. Based on the above results it can be concluded that the produce meso-TiWP-x samples have amorphous and less order porous structures than in the case of TiWSi.

Figure 9.

X-ray diffraction patterns for meso-TiWP-x materials, (a) low angle and (b) high angle.

Figure 10 reveals the surface morphology of the meso-TiWP-x samples and by examining the SEM images, the formation of the pores are evidently observed within the TiWP framework and generally, the pores order seems to be irregular. However, for the meso-TiWP-10 sample, it shows some degree of ordered pores domains is formed. In addition the X-ray mapping and EDX composition analysis (not shown here) demonstrate the presence of Ti, W, P and oxygen elements which are uniformly distributed through the mesoporous TiWP materials.

Figure 10.

SEM images of meso-TiWP-X: (a) meso-TiWP-5 (b) meso-TiWP-10 (c) meso-TiWP-15 (d) meso-TiWP-20.

The transmission electron microscope characterization of the meso-TiWP-x materials is illustrated in Figure 11. The TEM images of the mesoporous TiWP-x materials display large domains of the porous structure obtained after direct calcination at 400°C in the air due to the removal of templates. Clearly, the pores arrangement is rather random presumably due to the source of phosphor (PCl₅) affected the formation of the ordered mesostructured during the self-assembly process. However the TEM image for meso-TiWP-10 with phosphor content of 10 wt. % shows the pores are arranged in the highly ordered structure. It implies that adding phosphorus with 10 wt. % retains the pore order of the titanium tungstophosphates.

Figure 11.

TEM images of (a) meso-TiWP-5, (b) meso-TiWP-10, (c) meso-TiWP-15, (d) meso-TiWP-20 after annealing at 400°C in air for 4 hours.

The N₂ adsorption/desorption isotherms of meso-TiWP-x materials after annealing at 400°C in the air are shown in Figure 12a. Evidently, the N₂ adsorption/desorption isotherms for meso-TiWP-x materials show typical type IV curves with a sharp capillary condensation step at P/P^o = 0.8–0.9 and an H₂-type hysteresis loop, which is typical of large-pore mesoporous materials with cylindrical channels. The pore size derived from the adsorption branches of the isotherms by using the Barrett-Joyner-Halenda method is shown in Figure 12b. Generally, the pore diameter for the meso-TiWP-x materials shows a narrow distribution and a gradual decrease as the phosphorus content increases, with a pore diameter of 9.6 nm for meso-TiWP-0 and 4.3 nm for meso-TiWP-30 as reported in Table 4. The Brunauer-Emmett-Teller surface area and pore volume of the meso-TiWP-x materials are calculated from the N₂ adsorption/desorption isotherms and reported in Table 4. The BET surface area of the meso-TiWP-x materials is varied between 188 m²/g for no phosphorus and a 128 m²/g for meso-TiWP-15 which presumably due to the formation of the more crystalline mesoporous wall as the phosphorus is added to titanium tungstate. However, it can conclude that mesostructured of titanium tungstate can be retained during the addition of phosphorus and annealing treatment which confirms the versatility of EISA soft template approach to producing multi-components mesoporous oxides with various compositions.

Figure 12.

(a) Nitrogen adsorption/desorption isotherms of meso-TiWP-x and (b) the corresponding pore size distribution curves.

Material	Pore diameter/nm	Pore volume/cm^3 g^−1	Specific surface area/m^2 g^−1
meso-TiWP-0	9.5	0.37	188
meso-TiWP-5	7.5	0.232	140
meso-TiWP-10	7.6	0.220	129
meso-TiWP-15	5.6	0.200	128
meso-TiWP-20	4.5	0.150	153
meso-TiWP-30	4.3	0.120	154

Table 4.

Textural properties of the meso-TiWP-x materials derived from nitrogen adsorption/desorption analysis.

3.4 Removal of heavy metal ions from water by mesoporous titanium tungstosilicate and tungstophosphate materials

This section reports the preliminary evaluation of the produced mesoporous materials for heavy metal ions removal from synthetic water solution. The ability of mesoporous titanium tungstosilicate to enrich the metals ions absorption from water was tested by adding aliquots of metal ions solution to a known weight of meso-TiWSi-x absorbent. Figure 13 shows the effect of the meso-TiWSi-x loading on the removal efficiency of Cd(II), Hg(II) and Pb(II) ions at pH equals 7 and 25°C. It is observed that the Cd(II) ion uptakes are independent on absorbent loading of meso-TiWSi-10 (Figure 13a) and the removal efficiency stabilized at around 92%. On the other hand, for Hg(II) ion uptakes gradually increase as the meso-TiWSi-10 absorbent load increases reaching about 92% at 200 mg meso-TiWSi-10 loading. However, for Pb(II) ion adsorption at meso-TiWSi-10 is inconsistent and unexpectedly shows a decrease in absorption efficiency as the absorbent loading increased. Figure 13b shows with the addition of meso-TiWSi-20 adsorbent in the range of 50–200 mg, no significant variation in the metal ions uptakes were observed for Hg(II), whereas slight decrease in Cd(II) ions removals was maintained. In contrast, the Pb(II) ions uptakes show a remarkable increase in a number of meso-TiWSi-20 adsorbent increases and 80% removal efficiency was achieved at 200 mg adsorbent. Effect of meso-TiWSi-40 adsorbent dose on the adsorption of Cd(II), Pb(II) and Cd(II) are shown in Figure 13c. The removal efficiency of Hg(II) and Pb(II) gradually increases with increasing the adsorbent dose and record about 90 and 85% respectively. This can attribute to the greater availability of the exchangeable sites or surface area at a higher concentration of the adsorbent. While increasing the meso-TiWSi-40 adsorbent dose gave rise to a remarkable decrease in adsorption of Cd(II).

Figure 13.

Effect of the adsorbent dose of (a) meso-TiWSi-10, (b) meso-TiWSi-20, (c) meso-TiWSi-40 on the adsorption of Pb(II), Cd(II) and Hg(II) from aqueous solution at pH of 7, a contact time of 2 hours and a metal ion concentration of 20 ppm, (d) effect of contact time on the adsorption of Pb(II), Cd(II) and Hg(II) from aqueous solution by m-TiWSi-20 at pH of 5, 0.15 g m-TiWSi-20/3 mL solution and a metal ion concentration of 0.2 mg/L.

Figure 13d shows the effects of agitation time on the removal efficiency of Hg(II), Cd(II) and Pb(II) using 150 mg of meso-TiWSi-20 adsorbent at pH = 6. It was noted that the adsorption of Cd(II) and Pb(II) gradually increases with an increase in agitation time and attains an equilibrium adsorption of 95 and 89% respectively within 40 min. Then the removal efficiency gradually decreases at longer agitation time. This result is important as the equilibrium time is a key parameter for economical wastewater treatment applications. On the other hand, the adsorption equilibrium time for Hg(II) takes about 2 hours to reach adsorption efficiency of 90%.

The mesoporous titanium tungstophosphate (meso-TiWP) adsorbent was also evaluated for the adsorption of heavy metal ions from water. Figure 14a shows the effect of the meso-TiWP-15 and meso-TiWP-20 adsorbent dose of on the uptakes of Hg(II) ions from water at pH 6. At a fixed Hg(II) ion concentration, it can be observed that the removal % of Hg(II) ion slightly increases with increasing weight of the adsorbents. This is attributed to the high surface area and greater availability of adsorption active sites of meso-TiWP-15 and meso-TiWP-20 adsorbents. For 95% removal of Hg(II) ion a maximum of 100 mg is required from meso-TiWP-15 or meso-TiWP-20 adsorbent. Figure 14b shows the effect of adsorption time on the removal efficiency of Hg(II), Cd(II) and Pb(II) ions using 150 mg of the meso-TiWP-15 absorbent. It is seen that for meso-TiWP-15 absorbent exhibited a rapid adsorption for Cd(II) and Pb(II) ions with a removal efficiency reaching about 90% within the first 10 min while it takes about 1.0 hour to achieve 80% for Hg(II) adsorption.

Figure 14.

(a) Effect of the meso-TiWP-15 and a meso-TiWP-20 dose of on the adsorption of Hg(II) from aqueous solution, experimental conditions included a solution pH of 6, a contact time of 2 hours and a metal ion concentration of 20 mg/L, (b) effect of contact time on the adsorption of Pb(II), Cd(II) and Hg(II) from aqueous solution by m-TiWP-15. Experimental conditions included a solution pH of 5, 0.15 g m-TiWP-15/3 mL solution and a metal ion concentration of 0.2 mg/L.

Finally, Table 5 shows a comparison of the heavy metal ions removal efficiency of the best performance of our mesoporous titanium tungstophosphate and tungstosilicate materials with some of the modified mesoporous silica-based material widely reported in the literature at similar experimental conditions. Mesoporous meso-TiWP-20 showed a good performance for removing Cu²⁺, Pb²⁺, and Cd²⁺ in the pH region of 5.0–7.0 which is comparable to the performance, for example, mesoporous silica modified with amine functional group and magnetic iron oxide. This metal ion adsorption enhancement can be apparently attributed to the higher surface area and active sites available to metal ion binding at the surface of meso-TiWP and meso-TiWSi adsorbents [7].

Adsorbent	Cu	Pb	Cd	Hg	Ref.
SBA-15-3-aminopropyl trimethoxy-silane	98.0	99.0	90.0	—	[48]
KIT-6-ethylenediaminetetraacetic acid (EDTA)	99.5	98.0	99.0	—	[49]
SBA-15-ethylenediaminetetraacetic acid (EDTA)	99.0	98	99.5	—	[49]
SBA-15-aminopropyl (SBA/NH2)	—	96	50	—	[50]
Magnetic MCM-48 mesoporous silica amine (▬NH2)	91	96	90	—	[51]
meso-TiWP-20	95.9	95.5	95.8	—	This work
meso-TiW-20	50	76	—	—	This work
meso-TiWSi-10	—	82	91	92	This work

Table 5.

A comparison of the heavy metal ions removal efficiency of the best performance meso-TWSi-x and meso-TiWP-x materials with some of the modified mesoporous silica-based material reported in the literature.

4. Conclusion

In this work we investigate the synthesis of new bi- and tri-component of mesoporous oxides of titanium tungstate (TiW), titanium tungstosilicate (TiWSi) and titanium tungstophosphate (TiWP) oxides. For the mesoporous titanium tungstosilicate, the silica content was varied between 10, 15, 20, 40 and 60 wt. % using TEOS precursor, while for mesoporous titanium tungstophosphates the ratio of phosphorus was varied as 5, 10, 15, 20 and 30 wt. % by adding PCl₅ into the EISA template mixture of P^®123 surfactants, Ti(OBu)₄ and WCl₆ precursors. Subject to silica and phosphorus content, the obtained meso-TiWSi-x and meso-TiWP-x exhibits high surface area, ordered 2D hexagonal mesostructure with orderly channels extended over a large domain. The EISA approach demonstrates a successful synthesis approach for the synthesis of new complex inorganic nanostructured and mesoporous materials for potential application in environmental and catalysis. The produced meso-TiW, meso-TiWSi, and meso-TiWP adsorbents exhibit good adsorbent efficiency for the removal of Pb(II), Cd(II) and Hg(II) ions from water due to the presence of high surface area and accessibility of surface active sites. The adsorption of these metal ions is found to be dependent on contact time and adsorbents dose.

Acknowledgments

This work was funded by The National Plan for Science, Technology, and Innovation (MAARIFAH) King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award number AT34-203. The authors appreciate Dr. Yuhui Li and Prof. Dongyuan Zhao from Fudan University for their cooperation in the SEM and TEM analysis.

Conflict of interest

There is no any conflict of interest can be declared.