Solid acid catalysts from clays: Preparation of mesoporous catalysts by chemical activation of metakaolin under acid conditions

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Abstract

Natural kaolin was treated at 850 or 950 °C in air flow to give respectively the metakaolin samples MK8 and MK9. The obtained materials were successively treated at 90 °C with a 1 M solution of H2SO4, for various time lengths. The acid treatment of MK8 was found to give a high surface area microporous material with good catalytic properties related to the high density of acid sites, while MK9 gave an ordered mesoporous material with a low density of acid sites. The materials were characterized by several techniques, X-ray powder diffraction, thermogravimetric analysis, N2 physisorption, scanning electron microscopy, and temperature-programmed desorption of ammonia. The 1-butene isomerization was used as test reaction to evaluate the acidity of the samples.

Graphical abstract

Natural kaolin was treated at 850 or 950 °C to give respectively the metakaolin samples MK8 and MK9. The obtained materials were successively treated at 90 °C with a 1 M solution of H2SO4, for various time lengths. The acid treatment of MK8 was found to give a high surface area microporous material with good catalytic properties related to the high density of acid sites, while MK9 gave an ordered mesoporous material with a low density of acid sites.

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Introduction

Solid acids find a wide range of catalytic applications in oil and chemical conversion processes [1].

The use of clay-type aluminosilicates in heterogeneous catalysis as solid acid catalysts is almost as old as the catalysis concept itself. Among the earliest heterogeneous catalysts there were, in fact, acid-activated bentonites and kaolinites [2]. Despite the actual dominant use of microporous zeolite acid catalysts in modern petrochemical processes, there is still a wide interest in the use of acid-modified clays, not only because of their low cost but also because the structure and dimension of their pores appear more suitable, compared to zeolites, for the conversion of larger molecules [3], [4], [5], [6], [7]. The so-called acid activation, used as a chemical treatment to improve the surface area and catalytic properties of clays [8], [9], [10], [11], [12], consists of the leaching of the natural clays with inorganic acid, for various time lengths at various temperatures, and causes a disaggregation of the clay sheets, elimination of the impurities, and dissolution of the external layers, thus altering their chemical composition and structure. The result is an increase of the surface area, of the porosity, and of the number of the acid sites with respect to the parent clays, depending on the intensity of the treatment. The acid-activation procedure of natural bentonites is known to involve the treatment of the uncalcined clay with mineral acids at 90 °C for several hours, leading to aluminum leaching (and to a lesser extent silicon leaching) from the TOT layers [8], [13], [14], [15], [16].

The high octahedral aluminum content makes kaolin resistant to acid leaching; therefore, metakaolin (produced by heating kaolin in air at temperatures between 550 and 950 °C, that causes the dehydroxylation of the crystalline clay) is generally used because of its higher susceptibility to acid activation [9], [17]. The acid treatment on metakaolin leads to the leaching of aluminum, magnesium, and iron cations from the octahedral layer. While kaolin, as major raw material for the fabrication of conventional ceramics, may be considered to be well studied in the past decades, there is, in the last years, a renewed interest in the study of its conversion to metakaolin and mullite, for the purpose of further understanding the preparation of very strong and thermally stable technical ceramics [18], [19].

Furthermore acid activation of metakaolin can give porous materials with peculiar acid base properties.

The phase transformations that occur when kaolinite is calcined up to very high temperatures has been deeply studied [9], [20], [21], [22], [23]:Al2O32SiO2H2O(kaolinite)400–500 °CAl2O32SiO2(metakaolinite)+2H2O,2(Al2O32SiO2)(metakaolinite)950 °CSi3Al4O12(Al-Si-spinel)+SiO2(amorphous),3Si3Al4O12(Al-Si-spinel)1050 °C2(3Al2O32SiO2)(mullite)+5SiO2(amorphous). The temperatures of the phase transitions depend on the chemical–physical properties of the raw kaolin.

When kaolinite is calcined above 400–500 °C, dehydroxylation occurs (reactions (1), (2), (3)) to give metakaolinite. This transformation involves the loss of structural water with a reorganization of the structure; only part of the AlO6 octahedra is maintained while most is transformed into much more reactive tetra and pentacoordinated units [9].

The conditions of the calcination strongly influence the reactivity of the obtained material and various authors [16], [17], [18], [19] have discussed the best conditions to use in order to obtain a very reactive material.

Acid leaching of metakaolin usually gives materials with acidity comparable to activated montmorillonites [8]. Nevertheless the high purity of kaolin, compared to the other natural clays, and the consequent compositional reproducibility of the obtained products, makes them very interesting for the catalyst producers.

In this paper we describe the acid leaching, with an H2SO4 1 M solution, of some metakaolin samples, obtained by thermal treatment of natural kaolin at 850 and 950 °C, and the textural and physico-chemical characterization of the prepared materials.

Section snippets

Materials

Kaolin and all the other products used in this paper are Aldrich products and no further purification was carried out.

Preparation of the acid-treated metakaolins MK8 and MK9

MK8 and MK9 samples are kaolin samples calcined at 850 and 950 °C, respectively (10 °C/min, keeping constant the temperature for 2 h).

H2SO4 1 M (12 mmol/g) was used to leach the metakaolins MK8 and MK9, leading to the formation of the samples designated as MK8SA or MK9SA, respectively.

The temperature was maintained at 90 °C, while the time of the acid treatment was 4 or 20 h

Results and discussion

The TG/DSC profile, recorded during the heating process from 20 to 1200 °C, of the kaolin sample, previously pretreated at 120 °C to eliminate the adsorbed nonstructural water, is reported in Fig. 1. The thermogravimetric curve shows only a one-step weight loss correlated to the endothermic peak centered at 510 °C due to the dehydration of the kaolin to metakaolin.

No weight loss was registered between 750 and 1050 °C. The DSC curve exhibits two main peaks, the former, endothermic, centered at

Conclusions

Aluminosilicate samples, with a suitable pore system and a tuned acidity, were prepared by calcination of natural kaolin at 850 or 950 °C, followed by treatment with H2SO4 1 M solution. Acid treatment was found to give in the former case a high surface area microporous material with good catalytic properties attributable to the high density of acid sites, in the latter an ordered mesoporous material with a low density of acid sites. The different behavior was related to the different thermal

Acknowledgements

The authors thank Dr. Davide Cristofori for the SEM micrographs. The MIUR is gratefully acknowledged for financial support.

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