Heteropoly Acid Encapsulated SBA‐15/TiO₂ Nanocomposites and Their Unusual Performance in Acid‐Catalysed Organic Transformations

Abstract

The preparation of SBA-15/TiO(2) nanocomposites with different loadings of Keggin-type 12-tungstophosphoric acid (TPA) nanocrystals in their mesochannels through a simple and effective vacuum impregnation method is reported for the first time. The catalysts have been characterised by various sophisticated techniques, including XRD, HRSEM, and TEM. It has been found that the acidity and the textural parameters of the nanocomposites can be controlled by simply changing the loadings of TPA and TiO(2) or the calcination temperature. TPA and TiO(2) loadings of 15 and 22.4 wt %, respectively, and a calcination temperature of 1123 K have proved to be optimal for obtaining mesoporous nanocomposite materials with the highest acidity. Moreover, the activities of these catalysts in promoting hydroamination as well as Mannich and Claisen rearrangement reactions have been extensively investigated. The results show that the amount of TPA has a great influence on the activity of the nanocomposites in all of the reactions studied. The effects of other reaction parameters, such as temperature and reaction time, on the conversion and product selectivity have also been studied in detail. A kinetic analysis of the formation of the products under various reaction conditions is presented. It has been found that the activity of the nanocomposite composed of 15 wt % TPA deposited on 22.4 wt of TiO(2) on SBA-15 in promoting the studied reaction is remarkably higher than the catalytic activities shown by pure TPA, TiO(2)-loaded SBA-15, or TPA-loaded SBA-15. The results obtained have indicated that the acidity and the structural control of the nanocomposite materials are highly critical for obtaining excellent catalytic activity, and the presented highly acidic nanocomposites are considered to show great potential for use as catalysts in promoting many acid-catalysed organic transformations.

Titania Nanoparticles Stabilized HPA in SBA‐15 for the Intermolecular Hydroamination of Activated Olefins

“…[ 16] However, to get a clear insight into the relation between the textural parameters and the acidity of the catalysts with their performances, some of the results are presented. Low-angle XRD patterns of xT-22.4TO-S-15 with different TPA loadings and calcined at 1123 K reveal that the meso-structure of the materials is intact up to 50 wt % of TPA loading whereas the wide-angle XRD patterns of the samples show that the pure anatase phase of TiO 2 which is critical for the performing organic transformation is changed to rutile phase when the loading of TiO 2 is increased to more than 15 wt % ( Figure S1-see the supporting information).…”

“…[15] As SBA-15 has interesting structural features, we have previously employed this as a support to prepare nanosized titania supported TPA composite catalysts. [16] It has been demonstrated that the SBA-15 has been a good support and the acidity of the catalysts can be tuned by the simple adjustment of the calcination temperature and the loading of the TPA in the mesochannels of the SBA-15. These nanocomposites were later tried for various organic transformations including Mannich, hydroamination (HA), and Claisen rearrangement reactions.…”

“…On the other hand, the partial decomposition of TPA is responsible for the lower activity of the catalyst loaded with the higher amount of TPA wherein the acidity of the catalyst is lower as compared to that of the samples loaded with 15 wt % of TPA. [16] The influence of calcination temperature on the catalytic activity for HA of EA with p-ethylaniline over 1 calcined at different temperatures (923-1273 K) was investigated and the results are shown in Figure 3 B. The 1-923 gave a conversion of only 13.24 %, which is increased to 47.53 % for 1-1123 (Figure 3 B).…”

“…[ 16] As the sample containing 15 wt %TPA, 22.4 wt %TiO 2 on SBA-15 calcined at 1123 K (1-1123) showed the optimum surface area, with a monolayer coverage of TPA and small TiO 2 nanoparticles on the surface and a high total acidity (Figure 1), it was chosen for the HA of activated olefins with a series of aliphatic and aromatic amines (Scheme 1).…”

Preparation, Characterization and Catalytic Properties of SBA-15-SO3H Supported Tungstophosphoric Acid for Esterification Reaction

“…Due to its structural features, TPA has been frequently used as an efficient catalyst for various organic reactions [2][3][4][5][6]. However, TPA has such deficiencies, as extremely low surface area (< 10 m 2 g −1 ), porous structure and thermal stability, which result in the limited usage of it in various acid catalyzed reactions [7]. Therefore, it is being used after impregnation onto various carriers, such as silica, active carbon, molecular sieves, Al 2 O 3 and so on [1].…”