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• Methyl orange (MO) is a kind of azobenzene sulfonic acid dyes with complex aromatic ring, which can cause considerable damage to human health originated from carcinogenic and mutagenic effects. In recent years, a wide variety of methods have been employed to remove MO such as chemical oxidation [1], degradation [2, 3], membrane separation [4], ion exchange [5] and adsorption [6, 7]. Among these methods, the adsorption-based process is one of the most favorable approach strategies in terms of versatility, simplicity, promptness, and efficiency. Traditional adsorbent like activated carbon faces poor efficiency and expensive price issues [8]. Currently, novel generation materials are considered as excellent adsorbents, including zeolites [9], functionalized clays [10] and some composite materials [11]. Within these substances, intercalation composites are emerging as promising adsorbent materials due to their potential advantages, including well-ordered layered structure, larger gallery space and controllability of interlayer species [12].

  MoO₃ is a 2D layered structure with covalently bonded oxide layers separated by van der Waals interactions [13]. In some cases, the 2D layered structure of the host lattice can slack as guest species are immobilized, forming intercalation materials with extension of the interlayer separation. In the previous works, small molecules such as acrylamide [14], nicotinamide [15] can be introduced into MoO₃ under room temperature stirring, while larger molecules cannot be inserted diametrically. A method is proposed that MoO₃ can be reduced to [Na(H₂O)₂]_xMoO₃ using for ion exchange based on Na(H₂O)₂⁺ is replaced by macromolecular to achieve intercalation [16-19]. For example, Nazar et al. utilized Al₁₃ to exchange Na(H₂O)₂⁺ further to obtain Al₁₃ pillared microporous MoO₃ hybrid materials, the gallery spacing enlarged to 17.35 Å [20]. However, the combination process of [Na(H₂O)₂]_xMoO₃ has been protected under N₂ and sample deposits in N₂ dry-box [21]. Surprisingly, Pan took a simple and convenient strategy directly to interpolate different lengths alkylamine into MoO₃ layers [22]. Among them, MoO₃ intercalation material (MoO₃-DDA) through introducing dodecylamine (DDA) into MoO₃ improves the interval from 6.9 Å to 30.86 Å, and the larger space area is more conducive to import other guest molecules, which provides an exceptional technology for the preparation of intercalation composites.

  Polyoxometalates (POMs), as one kind of significant metal-oxide clusters with multiple charge have been employed in dyeadsorption [23, 24]. Regrettably, the high solubility in aqueous solution cannot realize selective separation and reuse. A series of outstanding works have been executed to incorporate POMs into layered compounds to prepare POMs-based intercalation composites [25, 26]. For example, Lesbani et al. introduced [α-PW₁₂O₄₀]^3– into Mg/Fe layered double hydroxides (LDH) host, extremely improved Cd²⁺ adsorption [27]. Similarly, Xu et al. reported [P₂W₁₇]^10– and [CoW₁₂]^5– intercalated ZnAlFe LDH, which could enrich with the adsorption function for cationic dye methylene blue [28]. Despite above progress achieved, cationic POMs like AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ (Al₁₃) as guest to construct intercalation composites suffer setbacks, searching for an advisable host encapsulate cationic POMs as an imaginative point.

  Herein, we successfully prepared a series of Al₁₃@MoO₃ composites with different Al₁₃ loading by implanting Al₁₃ guests into extensive airspace of MoO₃-DDA-5 which the molar ratio of DDA to MoO₃ is 5, as shown in Scheme 1. These composites demonstrate a certain adsorption capacity for MO due to the electrostatic interaction effects between cation Al₁₃ and anion MO. Among these, Al₁₃-3.34%@MoO₃ exhibits superior performance with the rate up to 93% in 20 min as well as adsorption capacity to 357.2 mg/g, which is a useful adsorption material to eliminate MO.

  Scheme 1

  

  Scheme 1.  Preparation methods of MoO₃-DDA and Al₁₃@MoO₃.

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  MoO₃-DDA-5 hybrid was synthesized by the literature method [22] and indicated 30.89 Å separation interstice (Fig. S1a in Supporting information). The van der Waals diameter of Al₁₃ cluster is 10.5 Å [20], therefore, it can migrate into the MoO₃-DDA-5 passageway through the capillary phenomenon. Furthermore, replacement of Al₁₃ solution each 24 h results in Al₁₃ embedding easily in MoO₃-DDA-5 thoroughfare derived from concentration disparity between inside and outside of the layer. Subsequently, Al₁₃@MoO₃ composites with diverse Al₁₃ concentration are prepared, denoted as Al₁₃-2.51%@MoO₃, Al₁₃-3.34%@MoO₃, Al₁₃-4.36%@MoO₃, respectively.

  The PXRD patterns indicated that Al₁₃@MoO₃ appeared new characteristic diffraction peaks at 2θ = 3.30° and 2θ = 6.60° compared with MoO₃-DDA-5 (Fig. 1a). Layer spacing can be calculated by 2dsinθ = nλ, the consequent of 26.77 Å is narrower than 30.89 Å of MoO₃-DDA-5. The variability of layer interspace could be associated with introduction of Al₁₃ due to the diameter of Al₁₃ is smaller than DDA molecular size. It is worth noting that Al₁₃@MoO₃ exhibit two spacings of 30.89 Å and 26.77 Å, which corroborated incompletely deintercalated for DDA molecules. In addition, sharp peaks gradually enhance at 3.30° and 6.60° with increasing of immersion time, which can probably be attributed to the improved content of Al₁₃.

  Figure 1

  

  Figure 1.  MoO₃-DDA-5 and Al₁₃@MoO₃ with different Al₁₃ con-centration: (a) PXRD patterns. (b) FT-IR spectra.

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  The FT-IR spectra of Al₁₃@MoO₃ composites were obtained in the range of 4000-400 cm^–1 (Fig. 1b). Al₁₃@MoO₃ composites exhibit a new characteristic peak at 630 cm^–1, which is attributed to the AlO₄ symmetric stretching vibration peak, and the characteristic peak at 3700–3400 cm^–1 correspond to the stretching vibration of the –OH on the Al₁₃ cluster surface [29], further verified that Al₁₃ is successfully encapsulated into MoO₃-DDA.

  To further explore the morphology of composites, scanning electron microscope (SEM) images were observed, as shown in Figs. S4a–e (Supporting information). MoO₃-DDA-5 exhibits a special uniform flake morphology and smooth surface, but the size significantly shrinks than MoO₃. The morphology of Al₁₃@MoO₃ composites gradually changed. Due to immersing time influence of Al₁₃, Al₁₃@MoO₃ appear cracks and holes. To define the content of elements in the Al₁₃@MoO₃ composites, energy dispersive spectroscopy (EDS) experiment was determined (Fig. S5 in Supporting information), suggesting that the weight percentages of Al in the three composites were 2.28 wt%, 2.93 wt% and 3.50 wt%, respectively. It is worth noting that the content of Al₁₃ is proportional with immersion time. The same conclusion can be further confirmed by elemental analyses (Table S1 in Supporting information). The elements of Mo, Al, C, N and O in Al₁₃-3.34%@MoO₃ were corroborated by element X-ray mapping as shown in Fig. S4 (Supporting information), indicating Al and other elements are uniformly distributed on this composite.

  We choose the typical anionic dye MO as the research substance to assess the capacity of these composites. Considering that MoO₃ and MoO₃-DDA-5 also have ordered layer structures could accommodate pollutant, we investigated the adsorption performance for MO. The adsorption curves indicate that MoO₃ shows unobvious adsorption capacity (Fig. S7a in Supporting information), while MoO₃- DDA-m (m represents the molar ratio of DDA to MoO₃, m = 1, 2, 3, 4, 5) composites exhibit a little bit better adsorption behavior (Figs. S7b and f in Supporting information). We found that the amount of adsorbed MO increased as the ratios of DDA raised from 1 up to 5, reaching the adsorption rate value of 82% after 20 min by MoO₃-DDA-5 (Fig. S7f). The possible reason for the different adsorption behaviors is that layer distance with accommodating MO has impact on capability. The above completion leads to MoO₃-DDA-5 is selected as intercalation target for the immersing of Al₁₃.

  Owing to the electrostatic attraction exist between Al₁₃ and anionic MO. We further studied the adsorption properties of Al₁₃@ MoO₃ (Fig. 2a). Expectantly, the results manifest that the adsorption ability enhanced with the amount of Al₁₃ less than 3.34%, the rate value of Al₁₃-3.34%@MoO₃ reach 61% within 1 min, 87.3% after 5 min, attaining 93% after 20 min as adsorption equilibrium already appeared, exhibiting impressive ability toward quickly remove MO. However, with the amount of Al₁₃ above 3.34%, the adsorption rate and adsorption capacity of Al₁₃@MoO₃ composites initiate to decrease unexpectedly (Fig. 2b, Figs. S7c-e in Supporting information). The possible reason for the above phenomenon is believed to be that strong electrostatic interaction [30, 31] between Al₁₃ with highly positive charges and anionic dye MO is propitious to absorb MO, which is incarnated on the raising adsorbed amount of MO as the content of Al₁₃ below 3.34%. With the amount of Al₁₃ further increased, excessive Al₁₃ will occupy the original internal area of Al₁₃@MoO₃ for accommodating MO dye, and the remaining space is inadequate to effectively encase much number of MO molecules, so that the adsorption capacity declined.

  Figure 2

  

  Figure 2.  (a) The concentration changes of MO at different times (Adsorbent: 40 mg, MO initial concentration: 20 mg/L, volume: 40 mL). (b) Adsorption rate of MO with Al₁₃@MoO₃ after 20 min (Al content measured by ICP). (c) Absorption capacity of Al₁₃@MoO₃ and MoO3-DDA-5 (Adsorbent: 10 mg, MO concentration: 100 mg/L, volume: 100 mL). Al₁₃-3.34%@MoO₃ fresh and after five cycles: (d) PXRD patterns, (e) FT-IR spectra, (f) Absorption capacity. (Adsorbent: 10 mg, MO concentration: 100 mg/L, volume: 100 mL).

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  In addition, the adsorption capacity of Al₁₃@MoO₃ composites are investigated by the 100 mg/L concentration of MO dye (Fig. 2c), suggesting that the adsorption capacities of composites are 320.6 mg/g, 357.2 mg/g and 295.1 mg/g after 2 h, respectively. We found that the amount of MO adsorption at the beginning get better with the increase of Al₁₃ loading, then decreases, the consequent is in fair agreement with the previous adsorption behavior. Compared with other adsorbents previously reported in the literature, Al₁₃-3.34%@MoO₃ composites have higher MO adsorption capacity value (Table 1) [32-39].

  Table 1

  

  Table 1.  The adsorption capacities of different adsorbents.

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  We have selected the material with the best adsorption effect to investigate its regeneration ability, PXRD patterns and FT-IR spectra indicate that the recycled adsorbent is consistent with fresh material (Figs. 2d and e), the conclusion can be comprehended in terms of a recyclable material. The experiment result of the adsorption capacity for revived Al₁₃-3.34%@MoO₃ indicates the amount of adsorbed MO decreased slightly (Fig. 2f), which is attributed to the loss of Al₁₃ existing amid interlayers during multiple washing process.

  In summary, we successfully prepared Al₁₃@MoO₃ intercalation composites by encapsulating polycationic Al₁₃ clusters into MoO₃ layers area. These composites are utilized to evaluate the adsorption capacity of MO due to the electrostatic attraction interaction between cation Al₁₃ and anionic MO. The adsorption results show that Al₁₃-3.34%@MoO₃ is provided with the noticeable amounts of MO adsorption capacity as 357.2 mg/g and the rapid rate reaches 93% after 20 min. Recycling experiments illustrate that the structure is pristine during the adsorption process, exhibiting favorable stability to MO removing.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgment

  The authors thank the National Natural Science Foundation of China (Nos. 21872021, 21671033, 22172022 and 22071019).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.09.079.

  
  
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  Scheme 1  Preparation methods of MoO₃-DDA and Al₁₃@MoO₃.

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  Figure 1  MoO₃-DDA-5 and Al₁₃@MoO₃ with different Al₁₃ con-centration: (a) PXRD patterns. (b) FT-IR spectra.

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  Figure 2  (a) The concentration changes of MO at different times (Adsorbent: 40 mg, MO initial concentration: 20 mg/L, volume: 40 mL). (b) Adsorption rate of MO with Al₁₃@MoO₃ after 20 min (Al content measured by ICP). (c) Absorption capacity of Al₁₃@MoO₃ and MoO3-DDA-5 (Adsorbent: 10 mg, MO concentration: 100 mg/L, volume: 100 mL). Al₁₃-3.34%@MoO₃ fresh and after five cycles: (d) PXRD patterns, (e) FT-IR spectra, (f) Absorption capacity. (Adsorbent: 10 mg, MO concentration: 100 mg/L, volume: 100 mL).

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  Table 1.  The adsorption capacities of different adsorbents.

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