Effect of the amount and time of addition of a dye template on the adsorption and photocatalytic performance of molecularly imprinted silica

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Highlights

  • A molecularly imprinted (Rhodamine B as the template) photocatalysts were prepared.

  • The effect of the amount and time of addition template were evaluated.

  • Higher photocatalytic degradation (37.82%) was achieved by addition of template at t = 0.

  • There is a threshold of template addition in which degradation is improved.

  • A higher template content may be more difficult to remove from the silica network.

Abstract

A molecularly imprinted (MI) photocatalyst was prepared via an acid-catalyzed sol-gel route using Rhodamine B (RhB) as the template. The aim of this study was to evaluate the effect of the amount and time of addition of RhB on the photocatalytic degradation. The adsorption kinetics of MI silica xerogels were also investigated. Several methods of extraction were employed, and the use of ultrasound (in methanol) exhibited the best results with respect to increasing the surface area, template removal and economy of spent solvent volume. The samples were characterized in terms of their pore diameter, surface area and aggregation of primary particles using small-angle X-ray scattering. For systems containing a fixed mass of template (150 mg), the textural evidence suggests that the large pores and large surface areas may explain the improved results that were achieved by systems where the template was added at t = 0. In addition, the effect of the added template amount at a fixed time indicated that there is a threshold at which degradation was improved. The results suggest that using a higher template content in the synthesis of the photocatalysts may hinder removal from the silica network and increase the aggregation of molecules, affecting the photocatalytic performance.

Introduction

Synthetic dyes have become an integral component for a variety applications. The majority of these dyes (ca. 70%) belong to the azo group [1]. Approximately 7 × 105 metric tons of dye are produced annually [2] for various applications, such as the plastic, textile, printing and cosmetic. Some estimation suggests that for 1 kg of textile materials the textile finishing industries consume 100 L of water [4]. Such consume is partially a consequence of textile industry, from which it is estimated that each year ca. 2.8 × 105 tons of dyes are discharged [5]

These discharged possibly affect the aquatic life by hindering sunlight penetration in water. Therefore, removing these dyes from wastewater remains a difficult challenge, especially the removal of dyes at a low concentration [6].

Chemical treatment, filtration, oxidation, adsorption, biological treatment and advanced oxidation processing, are among the most commons methods available for remediation of dye wastewaters [3], [7]. However, a condition that is necessary for complete removal is the mineralization of the contaminant [8], which can be achieved by using advanced oxidation processes (AOPs).

Considering emerging contaminants, the heterogeneous photocatalysis is one of more promising strategies among AOPs. The use of TiO2 as catalyst encompasses several advantages, such as low cost and low toxicity [9]. However, the low selectivity to most hazardous compounds is a serious disadvantage of heterogeneous photocatalytic oxidation [10], [11].

To address this drawback, some studies have suggested combining molecularly imprinted (MI) technology with a photocatalytic process. One of its main feature is that MI technology demonstrates high affinity and selectivity for the molecules employed as templates [12], [13]. In this approach, the cavities generated by MI technology could enhance the molecular recognition ability. MI materials have been used in many different applications, such as drug delivery [14], catalysis [15], adsorption [16], [17], [18], chemical sensing [19], preconcentration [20], biological antibodies, and receptor systems [21].

The production of MI as sorbent material is majority based on organic matrix. However, the adsorption sites generated are usually heterogeneous and also possess non-specific interactions between the template and the imprinted sites that compromise the adsorption phenomenon [22]. In order to overcoming these inconveniences, a promising alternative is the use of inorganic matrices, such as silica-based material.

Silica-based imprinted materials can be produced by sol-gel process. Briefly, a percursor (e.g., tetraethylorthosilicate (TEOS)) interacts with a chosen template producing a silica network. After the extraction of the template, specific cavities are created and the produced material can be used as an adsorbent. As shown in the literature [23], [24], the main advantage of silica prepared by sol-gel reactions is that material can be tolerant to a several chemical environments [25], which include exposure to very strong base, very strong acid, and oxidizer species. Silica materials as adsorbents in solid-phase for drug extraction are among the majority of the studies discussing the preparation of silica material containing molecularly imprinting technology [26], [27], [28].

Imprinted photocatalysts based on inorganic matrix have not yet been extensively explored in the literature [29], [30], [31], [32], [33]. In this context, the aim of the current study was to evaluate the effect of the amount and time of addition of a template (RhB) on the characteristics and catalyst activity of a photocatalyst that was prepared via an acid-catalyzed sol-gel route. Different extraction methods were studied and, after selecting the best extraction method, the adsorption kinetics of the MI xerogels (without calcination) were investigated.

Section snippets

Materials and methods

Tetraethoxysilane (Si(OCH2CH3)4, TEOS, Merck,>98%), Rhodamine B (Vetec), and titanium tetrachloride (TiCl4, Merck,>99%) were used as received. Hydrochloric acid (HCl, Nuclear, 38%) was employed as the catalyst.

Preparation of sol-gel materials

The photocatalysts were prepared using a sol-gel process using HCl (0.2 M) as the catalyst. TEOS (45 mmol) was used as the raw material at a 1:2 (HCl:TEOS) volume ratio followed by the addition of TiCl4 (10.4 mmol). The effect of the amount of template was investigated by adding 150, 300 and

Xerogel and Photocatalysts Characterization

Table 1 shows the C and N content, the SBET values and the solvent volume used for the different extraction methods for the 15A0 system (non-calcined). Although a decrease in the C content was observed compared to that in encapsulated silica, the carbon content can be misleading due to the potential presence of residual ethoxide groups in silica that are produced by the sol-gel method [37].

According to Table 1, the highest removal of N was obtained using thermal extraction. However, no increase

Conclusions

In the development of a supported photocatalyst prepared using an acid-catalyzed sol-gel process, the amount of employed template (RhB) and the time of its addition to the reaction medium affect the characteristics and catalyst activity of the resulting photocatalyst. The adsorption kinetic data were well represented by pseudo-second-order kinetics, and the values of kinetic constants are comparable to those of typical silica-based adsorbents. For the systems with a fixed template mass, the

Acknowledgements

This project was partially funded by the CNPq. C. Escobar is grateful for the grant provided by the CAPES. The authors wish to thank the LNLS (Project D11A-SAXS1-8691) for the SAXS beamline measurements.

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