The Synthesis And Properties Study Of Polyoxometalates-Based Heterogeneous Catalytic Materials

With the continuous development of society and technological progress, the demands for green chemistry production process are highly increasing in comparison to other processes which negatively affect the environment through different kinds of contamination. Therefore, in order to achieve a harmonious development with respect to economical, social and environmental dynamics, development of new technologies and syntheses of new functional materials is highly required.Consequently, homogenous polyoxometalates (POMs) have high crystallinity and small crystal size,which result to agglomeration, high brittleness and difficulty in recycling. Therefore, the need for appropriate supports by using several methods produce highly dispersed, uniformly dispersed and stably dispersed POMs on the supports which consequently improve the catalytic activity, selectivity and recyclability of the heterogeneous catalytic materials.The advantages of polyoxometalates-based heterogeneous catalytic materials include saving the precious polyoxometalates, increasing its mechanical strength and thermal stability during practical applications. Moreover, in order to protect environmental resources and safeguard human health needs, these POMs-based heterogeneous catalytic materials was used in oil desulfurization and degradation of dye, and therefore investigate the influence of different experimental parameters (i.e. temperature, oxidant dosage, catalyst dosage, etc.) on the reaction results, and further study the corresponding reaction mechanisms.This paper revolves around the excellent performance of polyoxometalates in the field of oxidation catalysis, by preparing different kinds of polyoxometalates-based heterogeneous catalytic materials by various methods such as:1. Immobilization of polyoxometalates onto modified mesoporous silica for deep desulfurization:(1) Na7H2LaW10O36·32H2O (LaW10) has been immobilized onto the amino-modified mesoporous silica through electrostatic interaction, resulting to a polyoxometalate-based heterogeneous catalytic material SiO2/NH3+/LaW10. This catalyst is characterized by ICP-AES, FT-IR, BET, HR-TEM, MAS-NMR, etc. On this heterogeneous catalytic material Na7H2LaW10O36·32H2O is highly dispersed on mesoporous silica, which results to highly efficient deep desulfurization within 35min. Furthermore, this heterogeneous catalytic material has achieved efficient deep desulfurization in model oil with sulfur (S) content as low as 100 ppm and the volume increases to 1000 mL with S content of 1000 ppm. Moreover, this heterogeneous catalytic material is recovered and reused at least 10 times without any loss in catalytic efficiency.(2) The lanthanide-containing polyoxometalate (POM) cluster Na7H2LaW10O36·32H2O is chemically anchored to the dihydroimidazolium-based ionic liquid (IL) modified mesoporous silica, resulting to a new polyoxometalate-based heterogeneous catalytic material LaW10/IL-SiO2. The process of extractive catalytic oxidative desulfurization (ECODS) using this heterogeneous catalytic material is studied without the need for the direct addition of ionic liquid (IL) into the reaction system, because the ionic liquid (IL) was introduced onto the modified mesoporous silica. In comparison to other extractive catalytic oxidative desulfurization (ECODS) systems, LaW10/IL-SiO2 shows many advantages:(1) in mild conditions, this heterogeneous catalytic material can achieve deep desulfurization within 25 min,30 min and 25 min for DBT, BT and 4,6-DMDBT, respectively; (2) when this heterogeneous catalytic material is reused? there is no need for the ionic liquid addition procedure; (3) the reaction kinetics in the DBT model oil deep desulfurization process is in line with the zero-order reaction kinetics; (4) this heterogeneous catalytic material can be easily recovered by filtration and reuse at least 10 times with no catalytic efficiency decrease.2. Polyoxometalates intercalation to Tris modified layered double hydroxides and its application studies:(1) For the first time report using the layered double hydroxides with tris(hydroxymethyl)-aminomethane (Tris) (triangular support structure) modified to prepare [PW]2O40]3- intercalated layered double hydroxides under ambient conditions without necessity of degassing CO2 by ion exchange method resulting in the novel intercalated Tris-LDH-PW12. This synthetic method overcomes the difficulties of restrictions in the charge, geometry and size of polyoxometalates in intercalation to the layered double hydroxides. The LDH layer slight distortion and the energy decrease after Tris modification is proven by the theoretical calculation, indicating the Tris modify LDHs layer as more conducive for [PW12O40]3-intercalation. Under mild conditions, Tris-LDH-PW12 is further applied to sulfides oxygenation, which shows a highly catalytic efficient and selectively catalytic activities. Moreover, the Tris-LDH-PW12 can be recovered easily and reused at least ten times with no catalytic efficiency decrease. This synthetic method opens a completely new door for the preparation of POMs/LDHs advanced nanocomposites.(2) The exploitation of intercalation techniques in the field of two-dimensional layered materials offers unique opportunities for developing nanocomposites with desired functionality. In this paper, a fundamentally novel and facile’one-pot’anion-exchange method was demonstrated for the intercalation of classical polyoxometalate (POM) clusters of Na3[PW12O40]·15H2O (Na3PW,2),K6[P2W18O62]·14H2O (K6P2W18), and Na9LaW10O36·32H2O (Na9LaW10) into tris(hydroxymethyl)amino-methane (Tris) modified layered double hydroxides (LDHs) under ambient conditions without the necessity of degassing CO2 resulting in a series of novel intercalated nanocomposites Tris-LDH-PW12, Tris-LDH-P2W18 and Tris-LDH-LaW10. Investigation of the resultant intercalated materials of Tris-LDH-POMs for the degradation of methylene blue (MB), rhodamine B (RB) and crystal violet (CV) has been carried out, in which Tris-LDH-PW12 shows the best performance in the presence of H2O2. When a mixture of RB, MB and CV is applied, the dyes degradation by Tris-LDH-PW12 follows the order of CV> MB> RB, which is directly related to the designed interlayer space. Tris-LDH-PW12 can be reused for dyes degradation at least ten cycles without measurable decrease of activity and it retains its structural and chemical integrity during recycling process.