Sol-gel synthesis of mesoporous aluminosilicates with a narrow pore size distribution and catalytic activity thereof in the oligomerization of dec-1-ene

https://doi.org/10.1016/j.micromeso.2016.05.007Get rights and content

Highlights

  • The sol-gel synthesis of mesoporous aluminosilicates without using templates was developed.

  • Mesoporous aluminosilicates having a narrow pore size distribution were synthesized.

  • Catalytic activity of mesoporous aluminosilicates in dec-1-ene oligomerization was studied.

Abstract

Porous aluminosilicates with Si/Al ratio of 80 have been prepared by the sol-gel synthesis using tetraethyl orthosilicate (TEOS) and aluminum nitrate (Al(NO3)3*9H2O). It has been shown that the precipitation pH value and the gelation conditions strongly affected characteristics of the porous structure, the incorporation of aluminum into the silicate framework, and the surface acidity. It has been found that the implementation of the sol-gel synthesis initially in the acidic medium followed by the alkaline one allowed producing the mesoporous aluminosilicate with the specific surface area SBET of 660 m2 g1, the mesopore volume V of 0.74 cm3 g1, and the pore size distribution within the range of 2–5 nm. The high activity and selectivity of synthesized aluminosilicates in the oligomerization of dec-1-ene have been revealed.

The conversion of dec-1-ene reached 92%. The formation selectivity of dec-1-ene oligomers with the oligomerization degree n = 2–3 amounted 87%.

Introduction

Nowadays, the ordered mesoporous aluminosilicates possessing the acid sites formed on their surface and the mesoporous structure that provides the effective diffusion of components in the reaction medium are used in the liquid phase acid-base transformations with ever increasing frequency [1], [2], [3], [4], [5]. The texture of these materials is a regular ordered system of mesopores with a rather narrow pore size distribution within the range of 2–50 nm. This is obtained due to the application of the structure-directing agents (templates). After the templates are removed, the specific surface area can be as high as ∼1000 m2 g−1 and the pore volume of ∼1 cm3 g−1. More often, ordered mesoporous aluminosilicates with amorphous walls are synthesized, Al-MCM-41 and Al-SBA-15 [6], [7], [8], [9] being the most extensively studied ones. At present time, rather intense studies on synthesizing mesoporous aluminosilicates with partially crystallized walls are under consideration [10], [11].

More than 20 years have passed since the synthesis of the catalytically active ordered mesoporous materials was reported for the first time [12], but to date no information is still available with regard to the efficient application thereof for the production of industrial catalysts. In our opinion, this can be explained by the fact that the synthesis of the aluminosilicates in question is complex, multi-stage, and requires the use of expensive templates. In addition, the process of incorporating aluminum into the silicate framework is difficult to control.

The more readily available technique for the preparation of catalytically active mesoporous aluminosilicates could be the sol-gel based on the utilization of hydrolyzable molecular precursors, principally silicon and aluminum alkoxides. The sol-gel synthesis of aluminosilicates for the preparation of high-temperature ceramics, membranes, or drying agents is reported in every detail in the literature [13], [14], [15]. Similar rates of the hydrolysis and the condensation of silicon and aluminum sources are known to be necessary for the formation of Sisingle bondOsingle bondAl bonds [16]. There exist different ways for controlling the rates of hydrolysis and condensation of precursor alkoxides, for example, by means of selecting their activity. Yoldas [17] has studied the hydrolysis of tetraethyl orthosilicate (TEOS) with aqueous solutions of boehmite sol, and in the works [18], [19] – with aluminum formate and nitrate solutions. Colomban and Vendange [20] used precursors with existing Sisingle bondOsingle bondAl bonds, namely, (BusO)2-Al-O-Si(OEt)3. In other citation works, the pre-hydrolysis of the less active component, as a rule, the source of silicon is offered [13], [21]. Another way for the equalization of the hydrolysis rate of silicon and aluminum compounds is to decrease the reactivity of the more active precursor by grafting a chelating ligand; the latter could be either acetyl acetone or acetylacetonate ion [22], [23]. It is also possible to control the hydrolysis rates of the starting silicon and aluminum compounds by changing the conditions of the synthesis. Thus, in the work [23] the hydrolysis of the aluminum tetraethylorthosilicate and aluminum butoxide mixture was performed using merely water vapors present in the air.

Basic issues of the porous structure formation for the mesoporous aluminosilicates obtained by the sol-gel synthesis are discussed in the works [24], [25], [26], [27], [28], [29]. The porous structure characteristics are shown to be affected primarily by the H2O/Si and Si/Al ratio, the pH of hydrolysis, gel ageing conditions, and the temperature of further heat treatment. It should be noted that the authors of the work [29] have succeeded in synthesizing a mesoporous material with Si/Al ratio of 0.85 and the pore size distribution within the range of 3–5 nm from sodium metasilicate and aluminum nitrate without templates. These researchers also have managed to obtain a mesoporous material from TEOS and aluminum nitrate with the pore size distribution within the range of 2–3 nm.

Thus, some literary sources contain the information about controlling the Alsingle bondOsingle bondSi bond formation, whereas others report data on the formation of the mesoporous structure with rather narrow pore size distribution. At the same time, during the synthesis of the catalytically active mesoporous aluminosilicates it is necessary to simultaneously solve two problems, namely, the creation of the surface acid sites resulting from the formation of the Sisingle bondOsingle bondAl bonds and the formation of the mesoporous structure possessing the narrow pore size distribution that provides the efficient component diffusion from the reaction medium [16]. The data to that effect are quite limited.

Recently, we have shown the possibility of producing the catalytically active mesoporous aluminosilicate with the Si/Al ratio of 40 and the narrow pore size distribution within the range of 2–5 nm by the two-stage synthesis using tetraethyl orthosilicate and aluminum nitrate [30]. This study aims at investigating the effect of the synthesis conditions of the high-modular aluminosilicate (Si/Al = 80) on the formation of the mesoporous structure with the narrow pore size distribution and on the incorporation of aluminum into the silicate framework accompanied by the formation of acid sites. It is also intended to consider the relationship between the pore size and the concentration of acid sites with regard to the strength thereof and the effect of the above on the activity of aluminosilicates in the oligomerization of α-olefins.

Section snippets

Materials

Tetraethyl orthosilicate (TEOS, 98%, Acros Organics), Aluminum nitrate (Al(NO3)3*9H2O, 99%, Merck), Ethanol (C2H5OH, 96%, Acros), Aqueous ammonia (30%, Acros), 1,4-Dioxane (C4H8O2, 99%, Acros) and deionized water. All chemicals were used as received without further purification.

Sol-gel synthesis in acidic medium (AS-1, AS-1D)

Microporous aluminosilicates with Si/Al mole ratios equal to 80 were prepared by the sol-gel synthesis. In a typical synthesis, 14 g of tetraethyl orthosilicate and 0.32 g of aluminum nitrate, were added to 12 ml of

The pH of precipitation and its effect on characteristics of the porous structure

It was established that the Si/Al ratios (mol/mol) in the samples of the resulting aluminosilicates were close to those in the reaction mixture (see Table 1). These results can be explained by the fact that only water and alcohol are removed from the reaction mixture during the synthesis.

According to XRD analysis, all obtained samples are amorphous as seen from the characteristic halo between 20 and 30° 2θ (Fig. 1). Analogous diffraction patterns were observed for samples AS-1D and AS-3D.

The pH

Conclusions

Porous aluminosilicates with Si/Al ratio of 80 have been prepared by means of the sol-gel synthesis using tetraethyl orthosilicate and aluminum nitrate: AS-1, AS-1D (micro-mesoporous aluminosilicate obtained in acidic medium); AS-2 (mesoporous aluminosilicate with a broad pore size distribution is obtained in an alkaline medium); AS-3, AS-3D (mesoporous aluminosilicate with a narrow pore size distribution, obtained by a two-step synthesis first in acidic medium and then the alkaline).

It was

Acknowledgments

The authors thank the International charitable scientific fund named by K.I. Zamaraev (the Author) for financial support.

The reported study was funded by RFBR according to the research project №. 16-33-00775 mol_a.

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