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Local Structure of Supported Keggin and Wells-Dawson Heteropolyacids and Its Influence on the Catalytic Activity Elisa I. Garcí

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Local Structure of Supported Keggin and Wells-Dawson Heteropolyacids and Its Influence on the Catalytic Activity Elisa I. Garcí Page 1 of 23 The Journal of Physical Chemistry 1 2 3 Local Structure of Supported Keggin and Wells-Dawson Heteropolyacids and its Influence 4 on the Catalytic Activity 5 6 7 Elisa I. García-López1, Giuseppe Marcì2,*, Igor Krivstov3,4, 8 Jorge Casado Espina5, Leonarda F. Liotta6, Aida Serrano7,* 9 10 1 11 Department of Biological, Chemical and Pharmaceutical Sciences and Technologies 12 (STEBICEF), Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy. 13 2“Schiavello-Grillone” Photocatalysis Group. Dipartimento di Ingegneria (DI), Università di Palermo, 14 Viale delle Scienze, 90128 Palermo, Italy. 15 3 Department of Organic and Inorganic Chemistry, University of Oviedo, 33006 Oviedo, Spain. 16 4 17 Department of Environmental and Chemical Technology, South Ural State University, Chelyabinsk, Russia. 18 5 Scientific Technical Services, University of Oviedo, 33006, Oviedo, Spain. 19 6 Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)-CNR, via Ugo La Malfa, 153, 90146 Palermo, 20 Italy. 21 7Spanish CRG BM25 SpLine Beamline at the ESRF, 71 Avenue de Martyrs, F-38043 Grenoble, France. 22 23 * 24 E-mails: giuseppe.marcì@unipa.it, [email protected]. 25 26 27 Abstract 28 29 3- 6- 30 Keggin [PW12O40] and Wells-Dawson [P2W18O62] heteropolyanions are nanosized transition metal-oxygen 31 clusters belonging to the heteropolyacids (HPAs) family. They are widely used as catalysts, due to their 32 strong Brönsted acidity, and their dispersion on solid supports favours the accessibility to their acid sites 33 generally increasing the catalytic activity. A series of binary materials composed of Keggin or Wells- 34 35 Dawson HPAs and SiO2, TiO2 and ZrO2 has been prepared by impregnation or solvothermal methods. 36 Remarkable differences have been found in the catalytic activities among the unsupported and supported 37 HPAs. These differences have been correlated in the past to the structural changes of the HPAs due to the 38 cluster-support interaction, which is different depending on preparation methodologies of the binary 39 material. In the present work, the modes of interaction between the two types of HPA, Keggin and Wells- 40 41 Dawson, and various supports have been studied by X-ray absorption spectroscopy. The obtained data have 42 been compared with the characterization of the same materials reported before by using different bulk and 43 surface physicochemical techniques. The characterization results were then used to correlate the interaction 44 modes between the HPAs and the supports with the catalytic performances reported for 2-propanol 45 46 dehydration to propene and for propene hydration to 2-propanol. The results reveal that the deposition of 47 HPA by impregnation or solvothermal treatment may cause distortions in the H3PW12O40 cluster structure 48 depending on stronger (TiO2 and ZrO2) or weaker (SiO2) basic sites presence in the support, respectively. 49 Moreover, the type of preparation method affects the structure and acidic properties of the supported HPAs. 50 In particular, during the preparation of TiO and ZrO with HPA by in-situ solvothermal method, the reaction 51 2 2 52 of the HPA with the products of metal alkoxides hydrolysis occurs with consequent destruction of the 53 Keggin structure. Therefore, the catalytic activity of such materials is poor. These modifications, in addition 54 to the bulk and surface features of the supported HPAs, affected the catalytic 2-propanol dehydration to a 55 significant extent. On the contrary, the propylene hydration was less influenced, probably, due to the propene 56 non-polar nature. 57 58 59 1. Introduction 60 Metal oxides can be subdivided into classical solid oxides and nanosized transition metal oxygen clusters, 1 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 2 of 23 1 2 3 also called polyoxometalates (POMs) that offer a fascinating range of structures and properties.1 The most 4 explored POM materials are the heteropolyacids (HPAs) which are strong inorganic acids possessing defined 5 6 molecular structures. In order to classify the HPAs, one can consider the symmetrical ‘parent’ polyanion, 7 among them the Keggin or Wells-Dawson structures. The use of HPAs in catalysis has attracted great 8 interest because of their strong Brönsted acidity and redox abilities, so they are extensively used as 9 bifunctional catalysts.2,3 These materials are also economically and environmentally interesting because the 10 majority of POMs are inexpensive and not toxic.4 In addition, HPAs show photocatalytic activities both in 11 5 6 12 homogeneous and heterogeneous systems. 13 Wells-Dawson and Keggin heteropolytungstates are formed by polymeric WO6 units. The Keggin structure, 3- 14 in the polyanion [PW12O40] , is composed of a globe-like cluster of corner and edge sharing WO6 units that 15 enclose a central tetrahedron PO4. The tungsten WO6 units possess one short W=O terminal bond, four W-O- 16 17 W bonds, and one long W-O-P bond to the central PO4 tetrahedron. The four peripheral [W3O13] blocks 18 complete twelve octahedra in total, whose centers are occupied by tungsten atoms, which are the so-called 7,8 19 “addenda” atoms. This cluster has a diameter of ca. 1 nm and, in the acidic form H3PW12O40 will be 20 labelled in the following as PW12. The lack of one (monovacant) or more addenda atoms results in the 21 formation of the ‘lacunary’ HPA, in contrast with the ‘plenary’ one. The phosphotungstic Wells-Dawson 22 6- 23 heteropolytungstate possesses the formula [P2W18O62] , and it may be considered being derived from two 8,9 24 Keggin units. In the following H6P2W18O62 will be labelled as P2W18. Figure 1 shows a geometrical 25 representation of both Keggin (a) and Wells-Dawson (b) clusters. The orange central atom represents P, 26 whereas the blue octahedral positions of W are surrounded by oxygens, as red dots. 27 28 ≈1 nm 29 ≈1 nm 30 31 32 33 ≈1 nm ≈1.8 nm 34 35 36 37 38 (a) (b) 39 3- FIGURE 1. Polyhedral representations of the structures of (a) plenary Keggin-type [PW12O40] and (b) 40 Wells-Dawson [P W O ]6- heteropolyanions. 41 2 18 62 42 43 The acidity of both Keggin and Wells-Dawson HPAs is higher than that of mineral acids possessing pure 44 Brönsted acidity. Their protons play a major role in catalytic active sites. Misono et al. determined the 45 presence of hierarchical structures in HPAs i.e., primary, secondary and tertiary structures3,7 that result of 46 paramount importance to understand their role as catalysts. The primary structure corresponds to those in 47 48 Figure 1, i.e. the heteropolyanion itself; the secondary structure is assigned to a three-dimensional 49 arrangement including counter cations, in our case H+ and water molecules.3,7-10 In addition, the arrangement 50 of the particles, that is dependent on their morphology and size, generates the tertiary structure that is 51 responsible for the surface area of HPAs and for the accessibility of substrates to the HPAs acid sites. The 52 + 53 active sites in the solid bulk, i.e. the protons, represented by H5O2 species on the secondary structure, taking 54 part in the catalytic reaction are the active Brönsted acid sites on the HPAs secondary structure. Polar 55 reactants can be absorbed into and therein react in the pseudo-liquid phase reaction. Hence, according to 56 Misono et al., the catalytic behaviour of HPAs is strongly influenced by the modifications that may occur in 57 the secondary structure3,7,10 which would be altered by the number of water molecules involved. In this 58 59 context, the catalytic behaviour of these materials is strongly influenced by the structural modifications that 60 may occur in their primary, secondary and tertiary structures and it depends on their hydration state which determines the number and strength of surface acid sites along with the accessibility of the substrates to these 2 ACS Paragon Plus Environment Page 3 of 23 The Journal of Physical Chemistry 1 2 3 acid sites.11,12 The pseudo-liquid behaviour observed in catalytic reactions with HPAs is attributed to the 4 flexibility of the HPA lattice, which allows that reagent molecules can be absorbed and react on the solid 5 6 bulk. These properties provide to the HPA catalysts exclusive selectivity and high activity. However, it is 7 important to highlight that the extent of the pseudo-liquid behaviour observed in catalytic reactions depends 8 on the pre-catalytic treatment of HPAs13 and, anyway, it concerns only polar reagents. Among the HPAs, 9 those having Keggin-type structure are currently used in industrial catalytic processes14 while the Wells- 10 Dawson HPAs have been used for the important selective catalytic oxidative dehydrogenation of isobutane 11 15 16 12 to 2-methyl-propene (isobutylene) and for methyl tert-butyl ether (MTBE) production. 13 HPAs clusters have also been used as photocatalysts because they absorb light by the ground electronic state 14 producing a charge transfer-excited state (HPA*) which can easily become HPAn-, the so-called 15 “heteropolyblue” species, by means of one (or more) electron transfer from another molecule. 16 n- 5 17 Heteropolyblues (HPA ) species are relatively stable and are readily re-oxidized. The enhanced degradation 18 of organic compounds in the UV/TiO2 process in the presence of supported Keggin-type HPAs has been 19 reported.6,17,18 20 Dispersing HPAs onto solid supports with high specific surface area is generally considered beneficial to 21 increase their very low (ca. 5 m2 g-1) area which could hamper its use as heterogeneous catalysts. However, 22 23 contrasting results are reported, depending upon the support and its degree of interaction with the HPA. 24 Gaigneaux et al.
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