Acidity characterization of a titanium and sulfate modified vermiculite
Introduction
In the past few years, a wide range of solids with strong acidic properties have been developed and applied in catalytic reactions. Particularly, the promotion of oxides such as ZrO2, TiO2 and Fe2O3 with sulfate species has led to the production of materials whose catalytic properties have come to be considered as super acidic solids [1], [2]. On the other hand, the synthesis of pillared clays by intercalation of polycationic species in smectites has allowed us to obtain materials which combine acidic properties with a variably porosity and relatively high thermal stability. All these factors depend on the chemical nature of the mixed species, the type of clay mineral and the synthesis procedure used [3], [4], [5]. As a result of the combination of knowledge that has been acquired in the synthesis procedures of these two types of materials, pillared sulfated clays (of the smectite type) have been suggested as new materials characterized by a higher acidic strength than non-sulfated clays. Their physical–chemical properties may be modulated in function of the type of oxide that makes up the pillar, the method of sulfating and the sulfate/metal ratio, among other variables [6], [7], [8], [9], [10]. For instance, it has been reported that the pillarization of smectites by intercalation of titanium species results in an increase in the acidity of the sample, generating both Brönsted and Lewis acid sites [11]. This acidity induced by titanium is higher than the one obtained if any other metal intercalating species is used. In addition, the acidic strength of the system is enhanced when sulfate anions, mainly those associated with the pillar, are incorporated. This effect is due to the ability of SO in sulfate complexes to accommodate electrons from basic molecule, this effect being a strong function of the environment of sulfate ions [12]. Besides this, the typology and properties of the acidic sites found in pillared clays depend on the nature of the parent clay. In the case of vermiculite, this mineral may be considered as swelling trioctahedral micas with Al for Si substitutions in the tetrahedral layers, and Fe and Al for Mg substitutions in the octahedral layers. They are intermediate minerals in the natural weathering sequence of micas to smectites, with a negative layer charge density between that of micas and smectites, and where the charge neutralizing K ions of the parent mica have been replaced by exchangeable hydrated cations, most often Mg2+ and Ca2+, providing partial swelling properties [13].
Unfortunately, the high density of the interlaminate charge present in the vermiculite hinders the intercalation process of the metallic species. Recently, efforts have been applied to diminish the charge density and, consequently, to make intercalation easier [13], [14].
This work explores for the first time, the modification of a vermiculite with an intercalating titanium solution and the application of two methods of sulfating in order to study the changes in the acidic properties of the material produced by the presence of titanium and the oxo-anion sulfate. The methods of sulfating evaluated were: (i) intercalation (the sulfate comes into contact with the intercalated solution) and (ii) impregnation over the modified and calcined solids. The type and strength of the acid centers present in the solids obtained were investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption (TPD) experiments, employing ammonia as a probe molecule. On the other hand, the catalytic potential of the acidic sites generated in the solids was evaluated in the hydroconversion of heptane, considering that this reaction is catalyzed by materials of acid nature, including pillared clays [14], [15], [16] and whose selectivity towards isomerization or hydrocracking products, is strongly affected by the strength of the acid centers, and the accessibility towards it of the adsorbed species.
Section snippets
Starting materials
The initial clay mineral is a natural vermiculite of high interlaminar charge coming from Santa Marta (Colombia). The high purity and crystallinity of the material, as well as its high interlaminate Mg2+ cationic content has been recently reported [14], [17], [18]. The natural vermiculite was crushed and sifted, and the fractions smaller than 150 μm was subjected to hydrothermal treatment (THT, 2.0 L h−1 of 50% water vapour pressure in nitrogen, for 6 h at 400 °C) in order to diminish the
Results and discussion
Fig. 1 shows the XRD diagrams of the considered solids. The d0 0 1 plane of the vermiculite THT is observed at 1180 pm. Besides this, the THT treatment induces a little collapse of the layers, as deduced from the little signal observed at 930 pm. The success of the modification of the solid with the titanium and titanium sulfate intercalation solution is confirmed by the shift of the d0 0 1 plane towards 1380 pm. This shift is low and suggests a low size of the species incorporated, probably due to
Conclusions
Through the acidity study carried out, by means of DRIFTS and TPD-NH3 techniques and heptane hydroconversion, it was possible to establish the existence, on the solids prepared, of both types of acid centers, Brönsted and Lewis type, at a quantity above that present in the parent clay. In addition, it was found that on the materials of the VT series, Brönsted acidity prevails over Lewis acidity, being more stable facing the thermal treatments applied.
Likewise, the beneficial effect generated by
Acknowledgements
Our gratitude to joint project CSIC-COLCIENCIAS-2006CO0015 (Spain-Colombia) and the financial support of VRI-DIB at Universidad Nacional de Colombia (QUIPU 20201007579) and Junta de Andalucia (PAI, TEP106). WYH wishes to thank Universidad Nacional de Colombia for the financial support through the “Outstanding Postgraduate Students” scholarship.
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