Immobilization of Phosphotungstic Acid (PTA) on Imidazole Functionalized Silica: Evidence for the Nature of PTA Binding by Solid State NMR and Reaction Studies

Abstract

Phosphotungstic acid (PTA) immobilized onto imidazole functionalized fumed silica and was used as an efficient catalyst for epoxidation of a variety of olefins using aqueous H 2 O 2 as an oxidant. Negligible leaching of PTA under the reaction conditions employed indicates a strong interaction between PTA and imidazole. The immobilized catalysts could be separated and reused after the catalytic cycle. Evidence for the heterogenization of PTA on the imidazole functionalized fumed silica has been inferred from different spectroscopic techniques like IR, UV-vis, and NMR. Importantly, the nature of binding of PTA on the support has been studied in detail by solid state NMR spectroscopy using 15 N labeled imidazole support. It is clear from the NMR studies that the effective heterogenization of PTA is mainly due to imidazolium ion formation on the support by the acidic protons of PTA and the resultant ion pair.

Temperature‐Dependent Immobilization of a Tungsten Peroxo Complex That Catalyzes the Hydroxymethoxylation of Olefins

“…This change implied that the environment of the nitrogen atoms in the imidazole ring is very different due to the anchoring of HWPy to the imidazole; this observation coincides with previous research on the immobilization of phosphotungstic acid (PTA) onto imidazole-functionalized fumed silica. [32] In addition to the shift of the imidazole ring stretching, another strong piece of evidence that HWPy is anchored on the SiO 2 -IM was the characteristic FTIR peak at 977 cm À1 assigned to the W=O in HWPy, which was shifted to 951 cm À1 in SiO 2 -IM-HWPy. [33] Moreover, as shown in Figure 4 c, the peak at 884 cm À1 assigned to the stretching of the peroxy group (ÀOÀOÀ) was also observed, indicating that the catalytically active HWPy had been anchored on the surface of functional silicon successfully.…”

One-pot synthesis of multisubstituted imidazoles catalyzed by Dendrimer-PWAn nanoparticles under solvent-free conditions and ultrasonic irradiation

“…Aromatic aldehydes having electron-withdrawing groups (Table 3, entries 2, 3, 6-8, 18, 23 and 26) reacted at a faster rate compared with aromatic aldehydes substituted with electron-releasing groups (Table 3,entries 4,5,9,10,(14)(15)(16)(17)24,25,(27)(28)(29). Moreover, the neutral conditions of our methodology make it possible to use acid-sensitive aldehydes without side reactions and/or byproducts.…”

“…In recent years, Keggin-type heteropolyacids (HPAs) have been used as efficient catalysts for various organic reactions because of their superacidic and redox properties, low toxicity, ease of handling, low cost, high proton mobility, high thermal stability and recoverability and reusability [29]. Although these materials are versatile compounds in their acidic form, their main disadvantages are high solubility in polar solvents and low surface area (\10 m 2 /g).…”

“…Therefore, in order to overcome this problem, these materials are dispersed on supports (such as silica, acidic ionexchange resins, active carbon, etc.) which possess a large surface area [29]. Silica is a very important industrial material and is extensively used for a wide range of commercial applications such as catalyst supports, resins, molecular sieves, fillers in polymers, and in biomedical applications [34].…”

Proton conductivity and morphology of new composite membranes based on zirconium phosphates, phosphotungstic acid, and silicic acid for direct hydrocarbon fuel cells applications

“…3. Although PWA is water soluble, but it is well known that it can be immobilized by the addition of silica gel [36,37]. In addition, it was shown in the literature that PWA interacts with silica [38].…”