Synthesis and characterization of acidic properties of Al-SBA-15 materials with varying Si/Al ratios

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Abstract

Al-SBA-15 of varying Si/Al ratios in the range 11.4–78.4 was synthesized using tri-block copolymer P123. The calcined materials were examined by XRD, pore size distribution, surface area, 27Al NMR spectroscopy. The acidity and acid strength distribution were studied using microcalorimetric adsorption of NH3. The acidic properties were also examined by cumene cracking reaction as a function of Si/Al ratios. Systematic variation of acidity and activity was observed as a function of Si/Al ratio. The initial heats of NH3 adsorption correlated well with activity indicate that acid sites with ΔH > 100 kJ/mole is responsible for cumene cracking activity. Linear correlations were obtained with total acidity and cumene cracking activities. The tetrahedral aluminum was found to be responsible for the observed acidities and catalytic activities.

Introduction

Acidic solids like zeolites are some of the most important materials that catalyze organic transformations of industrial interest [1], [2], [3], [4]. These materials are constrained by pore size to process bigger molecules. In an attempt to prepare large pore inorganic structures researchers at Mobil discovered novel MCM-41 constituent of M41S family [5], [6]. Discovery of MCM-41 is followed by synthesis of a series of mesoporous structures like HMS [7], SBA-15 [8], [9], KIT [10], FSM-16 [11] etc., type of materials and their metal substituted analogues [12], [13]. Among these materials Al-MCM-41 is most extensively studied [14], [15]. In the recent past HMS, Al-HMS, Ti-HMS, Al-SBA-15 and other metal substituted structures attracted increasing attention. Among large number of possible metal ions that can be isomorphously substituted; Al is the most favored one from the point of view of generating acidity. It is well documented that Al substitution in silicate structures generate Bronsted acidity and these acid sites participate in many industrially important reactions [3], [4], [16].

In an attempt to prepare improved acidic materials a large number of mesoporous materials containing Al have been examined [17]. Al-MCM-41 was extensively studied with respect to acidic properties by microcalorimetry [17], [18], Temperature programmed desorption (TPD) of probe molecules [19] and IR methods [20]. Al-HMS and SBA-15 type of materials received comparatively less attention. There are limited number of studies on Al-HMS materials with respect to acidic properties and acid catalyzed reactions. Mokaya and Jones [7], [21] used IR pyridine adsorption method to elucidate Bronsted and Lewis acid sites. They have concluded that Al-HMS materials exhibit Bronsted acidity that is comparable with HY zeolites. Yue et al. [22] examined acidic properties of Al-HMS of various Si/Al ratios using NH3-TPD, which showed that these materials are comparable to HY zeolites in amount of acid sites as well as its strength. Acidity of Al-SBA-15 materials has been briefly examined by few authors but a detailed systematic study is lacking. Ying Li et al. [23] examined Al-SBA-15 materials and concluded that Al-SBA-15 contains Bronsted as well as Lewis acid sites and these sites increase with increasing Si/Al ratio. However, Luan and Fournier from IR pyridine adsorption studies suggested that the Bronsted acidity is invariant with Si/Al ratio [24]. These investigations gave valuable information on the nature of acidic sites; however, a complete description of acidic properties requires the amount of acid sites and also acid strength distribution as a function of Si/Al ratio. It is also well known that reactions like cracking, isomerization, and dehydration; isomerization requires different strength of acid sites. A complete description of total acidity and acid strength distribution is, therefore an essential requirement for the determination of acid properties of solid acid catalyst. Microcalorimetry is ideally suited method to extract such information. With a view to further understand the acidity and acid strength distribution of the materials, in this investigation, we have taken up Al-SBA-15 of various Si/Al ratios and characterized using micro calorimetric adsorption of NH3. The acidic functionality was also examined by using cumene cracking test reaction. Physico-chemical characterization of Al-SBA-15 materials, evaluation of acidic properties by micro calorimetry and its relation with catalytic activities forms the contents of this publication.

Section snippets

Synthesis

The SBA-15, and Al-SBA-15 (X) with various Si/Al ratios were synthesized following published procedures [8], [25], [26]. The incorporation of Al into SBA-15 mesostructure was carried out by direct synthesis method using Al-isopropoxide as Al source [25]. In a typical synthesis, 9 ml of tetra ethyl ortho silicate (TEOS) and a calculated amount of aluminum isopropoxide in order to obtain a Si/Al ratios equal to 10, 20, 30, 40 are added to 10 ml of aqueous HCl solution at pH 1.5. This solution was

X-ray diffraction

Fig. 1 shows the low angle powder X-ray diffraction pattern of siliceous SBA-15 sample. It can be noticed that a well-resolved diffraction peaks that can be indexed to (1 0 0), (1 1 0), and (2 0 0) are present. These diffraction lines are associated with long-range 2D hexagonal ordering in the P6mm space group. The unit cell parameter calculated from a0 = 2d100/√3 is 119.8 Å. These results are in agreement with the data reported by Zhao et al. for SBA-15 [8]. In the same figure, the XRD patterns for

Conclusions

SBA-15 and Al-SBA-15 materials with varying Si/Al ratios in the range 11.4-78.4 were synthesized using tri-block copolymer Pluronic P123. The calcined compounds were examined by XRD, pore size distribution, surface area, and 27Al NMR methods. These investigations indicate that a hexagonal mesoporous structure is obtained for all Si/Al ratios. 27Al NMR analysis indicates that Al is incorporated in tetrahedral positions. The microcalorimetric NH3 adsorption indicates that these materials contain

Acknowledgements

The authors are grateful to Dr. M.O. Garg, Director, Indian Institute of Petroleum, Dehradun for his encouragement and Muthu Kumaran, Shelu Garg, Kapil Soni thanks CSIR, New Delhi, India for providing Research Fellowships.

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