1 Introduction

Porous materials are relevant owing to large surface area, uniform pore size, well defined pores and the potential for functionalization with several chemical. International Union of Pure and Applied Chemistry classifies porous materials in three groups: microporous (pore size < 2 nm), mesoporous (2–50 nm) and macroporous (> 50 nm) [1,2,3,4]. Since 1990, mesoporous silica has become a significant class of ordered materials widely studied. Two types of mesoporous silica are described in the literature: MCM-41 and SBA-15 materials. In 1992, Mobil Corporation researchers discovered the family M41S with several members of which MCM-41 is recognized by properties such as: high surface area (> 800 m2 g−1), well-defined pores, uniform pore size distribution, pore diameter from 2 to 10 nm and adjustable pore diameter [5,6,7,8]. However, SBA-15 mesoporous silica is favorite because possesses surface area (600–1000 m2 g−1), large pore volume (up to 2.5 cm3 g−1), pore diameter ranging from 2 to 30 nm and thermal and hydrothermal stability higher than MCM-41 [9,10,11]. Moreover, it has great potential as support for the active phase of heterogeneous catalysts and easy diffusion due to minimizing obstruction by pore diameter adjustment [5, 9, 10].

Zhao and coworkers synthesized SBA-15 (Santa Barbara Amorphous of number 15), in 1998, at the University of California in the United States. It is a mesoporous sieve, silica-based, with unidirectional and hexagonally organized pore structure. They synthesized SBA-15 using as structure-directing agent, the amphiphilic triblock copolymer (Pluronic P123, (poly (ethylene oxide)–block–poly (propylene oxide)–block–poly (ethylene oxide), EO20PO70EO20) under acidic conditions [11, 12]. SBA-15 possesses a defined pore framework containing cylindrical and parallel mesopores accompanied or not by disordered micropores or mesopores located in the silica wall [12,13,14,15].

Synthesis of SBA-15 in general involves dissolving of amphiphilic triblock copolymer in acidic solution, addition of the silica source and hydrothermal treatment. Dissolution of the amphiphilic triblock copolymer occurs at mild temperatures (~ 40 °C) in acidic medium under agitation; after 2 h the silica source is added by keeping the systems under the same conditions (40 °C and under stirring) for 24 h. Subsequently, the hydrothermal treatment is realized at 100 °C for 48 h. The formed product is cooled to room temperature, filtered, washed with water, dried and calcined at temperatures ranging from 500 to 550 °C for 5 h [11].

As for the surfactant, the pluronic triblock copolymer P123 is used to synthesize the SBA-15, because it has a low ratio EO/PO, providing the formation of micelles that generate the desired structure of the SBA-15, besides having commercial availability, low cost and biodegradability [16]. As a source of silica, tetraethyl orthosilicate (TEOS) is used because it produces a high purity material and facilitates doping with any material due to low temperatures involved [15,16,17].

SBA-15 has classified as inert support, although this classification it is not absolute, it serves only as a support to disperse the metallic oxides [14]. This support with remarkable properties, such as well-defined pores, good thermal and hydrothermal stability, uniform pore size distribution, pore diameter from 2 to 10 nm and high surface area aroused much interest to anchor nanoparticles or metallic oxides [12, 14, 15, 18]. Functionalized SBA-15 has several applications, such as: catalysis reaction [19,20,21], wastewater treatment [22,23,24], biorefinery production [25], drug deliver [26, 27], CO2 adsorption [28], photodegradation [36]. The multi-functionization modified SBA-15 has been widely used in adsorption, separation and catalysis [10]. Many studies have focused on the properties and applications of SBA-15 and SBA-15 functionalized as it is presented on Table 1.

Table 1 Functionalization of SBA-15 with various metals to potentialize its application in catalysis
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Table 1 illustrates the great interest of the scientific community to functionalize SBA-15. It is also highlights the active phase diversity used for functionalization, as well as highlights the applications in catalysis. The synthesis methodology reported in Table 1 indicates that the direct synthesis and post-synthesis methods are the most common, since the sol–gel, pH-adjusting and hydrothermal methods are only adaptations of the direct synthesis method and wet impregnation, incipient wetness impregnation and two-solvents are post-synthesis adaptations. So, in this review we don’t describe specifically about the methods sol–gel, pH-adjusting, Hydrothermal and two-solvents.

Despite of the interest of the scientific community and researches accomplished, the functionalization of SBA-15 is challenging task because the heteroatoms can be introduced into the silica wall by replacing Si4+, within the mesopores or on the outer surface and modify the original properties of the support [15]. Literature report several methods of SBA-15 functionalization such as: direct synthesis or “one pot” procedure [25, 38,39,40,41], wetness impregnation [40, 42,43,44], deposition–precipitation (DP) [45,46,47], graft hybrid (GH) [48, 49], colloid immobilization (CI) [50,51,52,53], nanoparticles encapsulation (NE) [54, 55]. We briefly reviewed already mentioned methods of SBA-15 functionalization, because it is are the most reported in the literature, to verify if such methods are satisfactory or if we need of new functionalization method/strategies.

2 Main SBA-15 functionalization methods

2.1 Direct synthesis method

Direct synthesis or “one-pot” method consists in the insertion of the precursor source of the metal in the gel synthesis support, as the co-condensation of the transition metal precursors and silicon occurs during the formation of the silica mesostructure. Generally, a synthetic gel is formed following a molar composition: a TEOS/b precursor metal/c P123/d HCl/e H2O (a, b, c, d and e are mole numbers of each species). This method involves the following steps: dissolution of the species under acidic conditions, agitation and mild temperatures; ageing at the same dissolution conditions (mild temperatures and controlled stirring); hydrothermal treatment (generally at 100 °C), filtration, washing, drying and calcination as reported in literature [38, 40, 56, 57]. Scheme 1 shows the general steps of the functionalization of SBA-15 by the direct synthesis and impregnation method.

Scheme 1
scheme 1

Main steps of SBA-15 functionalization by synthesis direct and post-synthesis methods, respectively [5, 57]

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This method has the advantage of promoting a strong interaction between the embedded metal and SBA-15 support. However, this interaction alters the properties of the SBA-15 mesostructure during the synthesis process, reducing the surface area and diameter of the pores, in addition it presents low rate of incorporation of metal due to the high solubility of their precursors. [57].

2.2 Post-synthesis or post-grafting method

This method involves two routes: incipient wetness impregnation and wet impregnation. The difference between mentioned routes is the volume of the solvent used to moisten the support and the evaporation step. Incipient wetness impregnation uses the minimum volume to moisten the support (solvent volume equals the volume of the carrier pores) not requiring the evaporation step; while in wet impregnation, evaporation is necessary to eliminate the excess volume of the solvent containing the active sites [58].

This method is one of the most commonly used when the species to be inserted into the SBA-15 remain in cationic forms under acidic conditions, similar to those used in the synthesis of SBA-15, for example Ga and Al at acidic pH (~ 1) do not condense [42]. Another advantage in comparison to direct synthesis is the high percentage of impregnation of the components in the active sites, the values approach or even exceed the estimated theoretical percentage without destroying the support mesostructure [59,60,61]. Despite the mentioned advantages, the post synthesis method presents some drawbacks, such as: production of non-uniform materials due to evaporation process that leads to the migration of the material from the pores to the surface [58]; reduction of surface area, pore volume, pore diameter [25, 59,60,61]; dependence on the nature of the solvent to ensure adequate distribution and dispersion of the particles in the carrier [43]; and oxidation of transition metal oxides giving low heteroatom dispersion [62]. It is difficult to obtain an adequate interaction between the precursor species of the heteroatom and the surface silane groups of the support [46, 63, 64]. Table 2 shows the effects of SBA-15 functionalization on textural properties by the direct synthesis and post-synthesis methods.

Table 2 Effects of SBA-15 functionalization on textural properties direct synthesis and post-synthesis methods
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Table 2 shows that both methods affect the textural properties of SBA-15 support. The Si/M ratio is lower in the post-synthesis method, showing that this route promotes a greater insertion of the active phase in the support than the direct synthesis method. The decrease in texture properties (surface area, pore volume and pore diameter) is attributed to the presence of metal oxide within the mesoporous structure of the support [65]. The increase of the surface area as observed in the sample LaSBA-15 (synthesis II) can occur owing the larger surface area of mesopore ascribed to the enhancement long-range arrangement of structure of support. Addition of the precursor source of sites actives might to favor the mentioned the long-range arrangement of structure of support [65].

The incorporation of metal may also alter the lattice parameter in a unit cell, when occurs substitution of the short Si–O bond by a longer M–O bond, and expansion occurs in the unit cell evidenced by the displacement of the peaks at low angles in the XRD [65, 66].

2.3 Deposition–precipitation (DP) method

This technique was developed to gain catalysts with well-dispersed metal or metal oxide on the surface of the support. It is based on the solubility of the cations intended for deposition in the carrier, generally if the cation chosen for deposition in the carrier is precipitable, the respective catalyst may be manufactured by the deposition–precipitation method [14, 67,68,69,70,71].

This method involves dissolution of the precursor source of the metal by adjusting the pH to ensure complete precipitation of the metal generally in hydroxide form which is deposited on the surface of the support (SBA-15) followed by filtration, washing, drying and calcination which decomposes the precipitated hydroxide into oxides, water and undesirable residues. The pH control of the solution is a crucial factor because the precipitation of the metal depends on the pH. The deposition of nanoparticles involves three steps: supersaturation, nucleation and particle growth of the precipitate on the surface of the support. Nucleation can be controlled by the slow and homogeneous introduction of the precipitating agent. The hydrolysis of urea at 90 °C provides a slow and homogeneous increase in pH [14, 72,73,74,75,76]. Scheme 2 shows the main steps deposition–precipitation (DP) and graft hybrid (GH) methods, respectively.

Scheme 2
scheme 2

Main steps of SBA-15 functionalization by deposition–precipitation (DP) and graft hybrid (GH) methods, respectively [14]

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The main advantage of this route of synthesis is the obtaining of supported catalysts with high deposition rate of the active phase formed by small particles highly dispersed. The filtration step removes the chloride ions from the precursor source of the metal, avoiding interference in the catalytic reaction [14]. However, this methodology presents the difficulty of obtaining particles of specific uniform size to a particular application, because a wide distribution of particles occurs due to low control of their growth [76, 77]. In addition, some reported procedures require the excessive use of reducing agent, such as NaBH4 (sodium borohydride) and H2, to ensure the formation of nanoparticle of the metal in the support, needing to be removed after the reaction [76]. The methodology is complex, requiring the control of some parameters that interfere in the formation of the precipitate: pH, time and temperature [14].

Chirieac et al. [12] synthesized SBA-15 with copper and nickel by methods: incipient wetness impregnation followed by mild drying and partially extracted (IWI-MD/EC), Precipitation with Na2CO3 (sodium carbonate) (P) and Precipitation with urea (DP). IWI-MD/EC method promoted greater interaction of the support with the active phase than the others. Method (DP) caused a partial collapse in the ordered mesoporous structure, evidenced by the absence in the low angle diffractograms of the reflections (210) and (300), reflection reduction (100) and shifted to lower values 2θ. Similar result was reported by other authors and attributed to partial dissolution of the silica wall, in harsh conditions such as, pH and temperature [12, 78].

2.4 Graft hybrid (GH) method

The principle of this method is based on electrostatic interaction. The general steps involve: the functionalization of the previously prepared support (for example SBA-15) in the pores/surface by the insertion of organic groups; solubilization of the precursor source of the metal; mixing the functionalized support with the solution containing the precursor source of the metal to promote the interaction of the opposites between the functionalized groups and the metal cation followed by washing, drying, calcination and reduction [14, 48].

The literature reports the preparation of Ag/SBA-15 [51], Pt/SBA-15 [14, 48] and Au/SBA-15 [79] by the grafting method. In the case of Pt/SBA-15, SBA-15 support was pretreated in water for 1 h under reflux, followed by heating at 150 °C under vacuum. Pre-treated SBA-15 was suspended in toluene and N-trimethoxysilypropyl-N, N-trimethylammonium chloride (TPTAC, 50 wt% in methanol) was added. The mixture was stirred at 80 °C for 6 h. The obtained sample was washed with toluene and ethanol and dried. SBA-15 functionalized with TPTAC (TPTAC-SBA-15) was blended with ethanol solution containing hexachloroplatinic(IV) acid hexahydrate (H2PtCl6). After 10 min the solid was washed with ethanol, dried at room temperature and finally reduced in hydrogen flow at 100–300 °C to get Pt/SBA-15 GH.

Graft Hybrid (GH) method has the advantage of adjusting the amount of functionalized organic groups, pore structures and amount of charged metal. However, the textural properties: surface area, volume and pore size decrease significantly suggesting that the pores of support are blocked and/or nanoparticles do metal are attached on the channels of support [14, 48, 80].

2.5 Colloid immobilization method

This method consists in the use of a dendrimer to encapsulate the metal nanoparticles prior to be deposited on the carrier. In brief, the method involves the following steps: choosing a suitable dendrimer to interact with the solution of the precursor source of the metal; mixing the solutions of the precursor source of the metal and source of the dendrimer; reduction of the metal by the NaBH4 reducing agent; immobilization of the nanoparticles in the support by ultrasound; separation of nanoparticles followed by drying. The separation step is by centrifugation or evaporation system [14, 50,51,52,53]. Scheme 3 shows the main steps colloid immobilization (CI) and nanoparticles encapsulation (NE) methods, respectively.

Scheme 3
scheme 3

Main steps of SBA-15 functionalization by colloid immobilization (CI) and nanoparticles encapsulation (NE) methods, respectively [14, 55]

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The reduction step is very important, so it is necessary to select the most suitable reducing agent in minimizing the particle size, the literature points out NaBH4 as more efficient than H2, alcohol, glycol and ethylene glycol. [81,82,83,84,85]. Advantage is in controlling the size of the nanoparticles and avoiding the pre-dispersion in the support [85,86,87]. However, this method has some disadvantages: the challenge of obtaining nanoparticles from the active phase to be introduced in the support; low yield of nanoparticles due to incomplete reduction of precursor ions [88]; long calcination time to remove the stabilizing agent from the nanoparticles [55, 89].

2.6 Nanoparticle encapsulation method

Also called the inclusion method, the previously synthesized nanoparticles are included in the synthesis gel during the formation of the support. This method includes the following steps: synthesis of the nanoparticles to be deposited on the support; addition of the nanoparticles in the SBA-15 synthesis gel; continued synthesis of SBA-15, namely, aging of the synthesis gel, hydrothermal treatment, lapping, drying and calcinations [54, 55].

The literature reports the synthesis of catalysts supported in SBA-15 using this method [54, 55] which show that the presence of the nanoparticles does not interfere in the formation of SBA-15. In this method, nanoparticles are stabilized in the support and present sizes similar to the initial one and allows the deposition on the support without the necessity of synthesizing it separately. However, there are some challenges, such as: difficulty into get nanoparticles from the active phase introduced in the SBA-15 and keeping them stable in the acid medium (pH ~ 1) of the SBA-15 synthesis gel and at high calcination temperatures used to remove the triblock copolymer P123.

The summary of the discussion of the conventional SBA-15 functionalization methods described in this review is presented in Table 3, which summarizes the main characteristics of the methods described in items 2.2–2.6, facilitating the comparison in the synthesis steps, advantages and disadvantages of the methods described in this review.

Table 3 Summaries of characteristics the conventional methods of functionalization of SBA-15 described in this review
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2.7 Alternative routes of functionalization of SBA-15

The methods described (items 2.1–2.6) provide SBA-15 catalysts functionalized with various metal oxides, but have some disadvantages. Thus, some changes in the synthesis methods have been made to overcome the drawbacks. TiO2/SBA-15 was obtained by altering the route of the direct synthesis method by introducing the TiO2 nanoparticles on the SBA-15 synthesis gel before the silica source, this method called an anchor in situ (ISA) by the authors [61, 90]. Li et al. [10] reported a synthesis route with two templates for Al–Ce–SBA-15 to overcome the problem of metal agglomeration. This methodology provided SBA-15 functionalized with Al and Ce highly dispersed on the surface, narrow pore distribution with surface area and volume of pores respectively 773 m2 g−1 and 0.87 cm3 g−1).

Another change in the route of the direct synthesis method is the adjustment of pH, because under very acidic conditions such as the synthesis of SBA-15 the metals are very soluble. This methodology is important to incorporate metal oxides that are strongly affected by the pH of the SBA-15 synthesis gel. The literature reports SBA-15 functionalized by this strategy, such as: Co–SBA-15 [15], La–SBA-15 [66, 91], Fe–SBA-15]33] and Ga/SBA-15 [20].

Chirieac et al. [12] modified the post-synthesis method to obtain SBA-15 functionalized with copper and nickel. The modification consisted of using SBA-15 with partially extracted surfactant, by solvent extraction, use of SBA-15 occluded with P123 was shown to be a good strategy to obtain high dispersion of the Ni and Cu nanoparticles on the internal surface of SBA-15 maintaining the textural properties of mesoporous materials. Chi et al. [92] report novel method to fabricated Ag/SBA-15 with well dispersed silver nanoparticles inside the mesoporous channels silica, using incipient wetness impregnation method, toluene as fill agent, Ag(NH3)2NO3 solution as source of silver and glucose how mild reducing agent. Prashar et al. [93] obtained PtSBA-15 with nanoparticles of Pt inside mesoporous channels of SBA-15 using novel in situ template method which was similar to the direct synthesis method adding CTAB surfactant on the synthesis gel.

Functionalization of SBA-15 can potentiate the applications in several fields: catalysis, adsorption, separation. The conventional methods of direct synthesis, post-synthesis, deposition–precipitation, graft hybrid, colloid immobilization and nanoparticle encapsulation, as well as new functionalization routes have provided SBA-15 functionalized with several active phases (metal, metal oxides, organic groups). Despite the advances in the fabrication of functionalized SBA-15, the challenge of obtaining SBA-15 functionalized while maintaining the properties of the support (SBA-15) and the active phase still remains. The current challenges are to functionalize the SBA-15 with unstable inorganic materials at the pH of obtaining the SBA-15 (pH < 1), inserting active sites phases in the pores of the SBA-15. Thus, strategies such as: obtaining SBA-15 at less acidic pH (pH ~ 5), increasing the pore diameters of SBA-15, as well as reducing the particle size of the active phase to be inserted into the support should be investigated and to provide more advances in the area of ​​catalysis and materials.

3 Choosing the best method for functionalization SBA-15

The selection of the most suitable method depends on the application of the catalyst and the type of metal or metal oxide to be introduced into the SBA-15. The application in catalysis, such as: hydrogenation, oxidation of volatile organic compounds depend on the surface area of the support, dispersion of active sites and particle size. The literature indicates that high surface area, good dispersion of active sites and small particles are responsible for high catalytic activity [94, 95]. So if application will be in these reactions should select the method that favors these properties.

The type of metal or metal oxide to be inserted influences the choice since some metal cations such as Al and Ga are very soluble in acidic medium synthesis of SBA-15 (pH ~ 1), limiting the insertion of metal cations. In this case, the direct synthesis method is not indicated.

Some articles influence the synthesis methods on the performance of the catalysts: Chirieac et al. [12] functionalized SBA-15 with Cu and Ni by wet incipient impregnation methods followed by mild conditions; Precipitation with Na2CO3 and precipitation with urea. Catalyst obtained by the wet incipient impregnation method has high activity (100% in conversion in 120 min) for the hydrogenation of cinnamaldehyde. VOx/SBA-15 obtained by the post-synthesis method (impregnation) showed good selectivity and conversion to methane oxidation [9]. Zu et al. [95] Synthesized Pt/SBA-15 by the Nanoparticle (NE) encapsulation method and applied them to CO oxidation, obtaining a satisfactory result suggesting that Pt/SBA-15 by the NE method is a good catalyst for CO oxidation with good thermal stability.

4 Conclusion

Functionalization of SBA-15 support is of great interest of the scientific community, the main methods described in this brief review are most commonly used for this purpose. There are advantages and challenges in all methods described here. Direct synthesis (DS) method promotes strong interaction between the support and the inserted active phase. The post-synthesis method (PS) has high efficiency in the incorporation of the species to be introduced in the support and avoids the problem of solubility of metal cations in acidic medium (pH ~ 1) present in the direct synthesis method. The direct synthesis and post-synthesis methods are the most used. Deposition–precipitation method (DP) provides high deposition rate of the well-dispersed active phase formed by small particles. Graft Hybrid (GH) method has the advantage of adjusting the amount of functionalized organic groups, pore structures and amount of charged metal. Colloid Immobilization (CI) method controls the size of the nanoparticles and prevents pre-dispersion in the carrier. Nanoparticle Encapsulation (NE) method allows stabilization of the particles in the carrier maintaining the initial size and eliminates a step of synthesis (impregnation). Despite the benefits already mentioned, these methods alter the textural properties of the SBA-15 support, reducing the surface area, average pore diameter and pore volume. They do not provide SBA-15 functionalized with all desirable features at the same time, such as: textural properties very similar to those of SBA-15, efficient active phase insertion, particle size control and particle distribution. The alternative to overcome these difficulties in the described methods has been the change in the routes of the mentioned methods. The choice of method must be based on the application of the catalyst and the properties of the active phase to be inserted in the support.