In situ investigations of the bulk structural evolution of vanadium-containing heteropolyoxomolybdate catalysts during thermal activation

Abstract

The bulk structural evolution of a vanadium containing heteropolyoxomolybdate (HPOM), H 4 [PVMo 11 O 40 ] * 13 H 2 O, with vanadium substituting for Mo in the Keggin ion was studied under reducing (propene) and partial oxidation reaction conditions (propene and oxygen) by in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) combined with mass spectrometry. During treatment in propene, the loss of crystal water in the temperature range from 373 K to 573 K is followed by a partial decomposition, reduction of the average Mo valence, and formation of a characteristic cubic HPOM at 573 K. This behavior is similar to the structural evolution of ] to an oxidized state under propene oxidation reaction conditions.

Role of Vanadium and Phosphorus in Substituted Keggin‐Type Heteropolyoxo Molybdates Supported on Silica SBA‐15 in Selective Propene Oxidation

“…HPOM were prepared as follows [5] :M oO 3 (16.89 g; Sigma-Aldrich) and V 2 O 5 (2.13 g; Sigma-Aldrich) were dissolved in water (675 mL) and heated to reflux. Phosphoric acid (97.8 mL, 0.12 m)w as added dropwise to the reaction mixture.…”

“…Given the pore diameter and lattice constant, the wall thickness of the SBA-15m aterialu sed amounted to approximately 2nm. In contrast to the wide-angle XRD patterns of amechanical mixture of bulk H 5 Figure 2b. [6,19] )i nt he range between 2.5 and 3.8 w as previously reported for PVMo 11 and PVMo 11 -SBA-15.…”

“…Thep reedge peak heights of in situ Va nd Mo K-edge XANES spectra at 298 K( Figure 4) were attributed to the distorted MO 6 (M = V, Mo) building units of the Keggin ion with the metal centers in their highest oxidation states. [5,21] The XANESs pectra remained unchanged in the temperature range 298-473K.H ence, the Keggin structure appeared to be stable up to 473 K. Between 473 and 648 K, the pre-edge peak heighti ncreased with temperature. This indicated structural changes from octahedral to tetrahedral [MO x ]( M = V, Mo) units during thermal treatment under catalytic conditions.…”

“…Heteropolyoxom olybdates (HPOMs) with aK eggin structure exhibit ab road compositional range while maintaining their characteristic structural motifs. [4][5][6] Substituting Mo atoms with addenda atoms (i.e.,V ,W ,o rN b) make Keggin-type HPOM as uitable model systemt os tudy structure-activity relationships. Moreover,P -a nd V-containing catalysts are active and selective in oxidative dehydrogenation (ODH) reactions, which possibly exhibit similar functionalities as catalysts for selective propene oxidation.…”

“…The incorporated V centers result in ap ronounced destabilization and accelerated decomposition of the Keggin ion at elevated temperatures. [5,35] The structural characteristics of model systemss uch as MoO x -SBA-15 and VO x -SBA-15 depend on their hydration states. [22,36] Ac omparable effect could be responsible for the structural evolution of HPOMs supported on SBA-15.…”