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Organic Catalysis by Clay-Supported Reagents
Published in Benny K.G. Theng, Clay Mineral Catalysis of Organic Reactions, 2018
Of the hundred or so heteropoly acids (HPA) that are known to exist, only those with a Keggin structure have featured widely as acid and oxidation (redox) catalysts because of their availability and stability against dehydration at 150°C–200°C (Misono et al. 2000; Timofeeva 2003). In the Keggin HPA, the heteropoly anion has the general formula [XM12O40]n− where X represents P5+, Si4+, and M denotes W6+, Mo6+ while the central XO4 tetrahedron is surrounded by 12 edge- and corner-sharing MO6 octahedra (Kozhevnikov 2009). In terms of clay-supported HPA, dodecatungstophosphoric acid (DTP) or 12-tungstophosphoric acid with the formula H3PW12O40 has received the greatest attention (Yadav 2005; Pacula et al. 2014). With a Hammett acidity function (Ho) of <−8.2 (cf. Figure 2.1), solid DTP may be regarded as a superacid of the Brønsted type (Timofeeva 2003).
Synthesis of highly dispersed phosphotungstic acid encapsulated in MIL-100(Fe) catalyst and its performance in heterogeneous oxidative desulfurization
Published in Chemical Engineering Communications, 2019
Dongxu Zhang, Hua Song, Dandan Yuan
Polyoxometalates (POMs), especially those having a Keggin structure, have excellent catalytic activity in the ODS process (Komintarachat and Trakarnpruk, 2006; Li et al., 2009). Effective ODS methods have been developed that exploit supported POMs, with abundant catalytic active centers, as catalysts. To date, titanium dioxide (Yan et al., 2009), mesoporous graphitic carbon nitride (Zhu et al., 2015b), activated carbon (Xiao et al., 2014), magnetic nanoparticles (Zheng et al., 2011), mesoporous silica (Guo et al., 2000; Li et al., 2011; Zhu et al., 2014), and molecular sieve (Luo et al., 2014; Wu et al., 2014) have been investigated as POM carriers to improve their catalytic activity. However, these supported POM materials have disadvantages such as higher agglomeration of POM particles, low dispersion of active sites, and poor stability in water (Hu et al., 2013). Therefore, improving specific surface area of POM is a key to solve its disadvantages (Zhu et al., 2015; Wu et al., 2016).
Catalytic performance and kinetic modeling of n-hexane isomerization over phosphomolybdic acid (HPMo) combining palladium and platinum supported on metal-organic framework MIL-101(HPW)
Published in Chemical Engineering Communications, 2023
Heteropoly acids are polyoxometalates of Keggin structure containing hydrogen and anionic metal-oxygen clusters (de Oliveira et al. 2019; Zang et al. 2013). They are applicable in catalytic processes due to the preferable characteristics of reduction-oxidation behavior and high acidity (Kim et al. 1985). Compared to zeolites and ordinary mineral acids, they possess stronger Bronsted acid sites and participate in various reactions under homogeneous and heterogeneous conditions. (Zendehdel et al. 2010). They are harmless to the environment, soluble in aqueous solution, and sit on porous solid foundations (de Oliveira et al. 2019; Zang et al. 2013; Zendehdel et al. 2010; Misono 2001). Heteropoly acids contain hetero polyanions XM12O40x-8, where X is the central heteroatom (P5+, Si4+, etc.), M is a metal (Mo6+, W6+, etc.), and x indicates the state of oxidation (Kozhevnikov 1995; Moffat 1989). According to recent research, the bifunctional catalyst comprising platinum and heteropoly acids (especially H3PW12O40) has an acceptable effectiveness performance in alkane isomerization (Alazman et al. 2019). The catalytic activity is attributed to the Keggin structure of heteropoly acids (Kumar et al. 2018). However, heteropoly acids have a low specific surface area (1–10 m2 g−1) and low porosity (Zang et al. 2013; Abd El Rahman et al. 2014; Yan et al. 2019; Pham and Van Doan 2019). Due to the low specific space area of heteropoly acids and the inaccessibility of active sites, they need appropriate supports with a high surface area for dispersion (Yan et al. 2019).
Preparation and photocatalytic kinetic study of ternary composite photocatalyst 12-phosphotungstic acid/PANI/SnO2
Published in Journal of Coordination Chemistry, 2019
The composite catalyst PW12/PANI/SnO2 was synthesized by electrostatic self-assembly. The heteropoly acid PW12 in the composite has Keggin structure. The light influence range after compounding is increased, and the photocatalytic degradation efficiency is enhanced.