Immobilization Of 12-phosphotungstic Acid (Salt) On Modified Palygorskite And Its Application

Liquid mineral acids catalysts, such as H2SO4, HCl, HF, H3PO4 and ClSO2OH, are usually applied in esterification reactions, which cause a series of problems as equipment corrosion, byproducts and environmental pollution. Recently, solid acid catalysts have received considerable attention for their enviromental friendly. Based on the status of the heteropolyacid systematic analysis of the application and development, modified palygorskite was selected as supports for the immobilization of heteropolyacid or its salts as acidic catalyst. An extensive study was performed on the mineral characteristics, modification and immobilization mechanism of palygorskite in terms of chemical composition (FTIR spectroscopy), crystalline structure (XRD), thermal properties (TGA/SDTA), N2 adsorption-desorption, the surface composition chemical states (XPS) and morphology (FESEM and TEM) etc.. The catalytic activity of immobilized heteropolyacid (salt) was tested by esterification models.The nitrogen content in silylated palygorskite withγ-aminopropyltriethoxysilane (KH550) increased with the increasing of activity-degree of acidic palygoeskite. Furthermore, there were lots of waters and hydroxy groups in the palygorskite, with which the ethoxysilane group of KH550 directly reacted with the surface hydroxyl group of palygorskite in refluxing dry tolune. KH550 modified surface presents more stable and homogeneous surface properties, which facilitates the immobilization of heteropolyacid.Palygorskite was modified with different silanes KH792 and KH550. Silylated palygorskite was denoted by KH792-Pa and KH550-Pa, respectively. The results showed that the amount of HPW adsorbed was relayed on the content of monolayer amino group in the supports. KH792-Pa contained larger numbers of adsorption sites (-NH- and -NH2) on its surface and higher nitrogen content (3.02 wt%) than that of KH550-Pa (1.89 wt%), which could trap more amount of HPW. For KH792-Pa and KH550-Pa, the equilibrium adsorption amount of HPW (mg·g-1) was 189 mg·g-1 and 175 mg·g-1, respectively. The adsorption kinetics results demonstrated that the adsorption process of HPW onto silylated-Pa closely followed a pseudo-second-order kinetic model. Equilibrium data were fitted well to the Langmuir adsorption isotherm models at all studied temperatures. The FTIR and XPS characteristic results of HPW/KH792-Pa and HPW/KH550-Pa indicated that heteropolyanion was reacted with NH2 group by chemical reaction;the result of XRD revealed that HPW was finely and effectively distributed on the silylated-palygorskite, which was constent with Langmuir equation. The characteristic of DRIFT and NH3-TPD showed that the Bronsted and total acidity were existed. The different loading HPW/KH792-Pa catalysts were prepared by incipient impregnation (abbreviate as HPW/KH792-Pa). The effect of aging temperature on the catalytic activation was studied and 160℃was proved as suitable aging temperature. The reaction of n-butanol with acetic acid was selected a model reaction, the butanol conversion increased with the HPW loading increasing, 20%HPW/KH792-Pa catalyst showed the best stability during reuseablity and around 60% butanol conversion. The reactions following the Eley-Rideal mechanism takes place between n-butanol chemisorbed on the active sites of the catalyst surface. The rate of the esterification reaction was fitted with a first-order nature. Furthermore, 20%HPW/KH792-Pa showed 90% oleic acid conversion in optimum conditions during the esterification of oleic acid with methanol. From the above, the silylated modification for palygorskite was contributed to the stabilization of heteropolyacid.Acidic Cs salt of 12-tungstophosphoric acid (Cs2.5H0.5PW12O40, CsHPW) was supported on acid-activated palygorskite, which partly overcome the filtration problems in liquid phase reaction because of the tendency of CsHPW to form milky colloid in polar solutions. CsHPW inside CsHPW/Pa was retained intact Keggin anion and highly and uniformly supported on the fibre of Pa, which performed stable catalytic activity for esterification of oleic acid with methanol. Under optimum conditions the conversion of oleic acid reached 73%.Therefore, silylated-Pa should be a suitable candidate for the immobilization of HPW and provide enough encouragement for future exploration.