Carmen I. Cabello

Anderson type heteropolyoxomolybdates in catalysis:: 1. (NH4)3[CoMo6O24H6]·7H2O/γ-Al2O3 as alternative of Co-Mo/γ-Al2O3 hydrotreating catalysts

A new preparation method of Co-Mo/γ-Al2O3 HDS catalyst has been developed by using the (NH4)3[CoMo6O24H6]·7H2O Anderson type heteropolyoxomolybdate supported on γ-Al2O3. The system can be considered as an interesting and simplified alternative to the traditional bimetallic catalyst. Diffuse reflectance spectroscopy measurements have been employed to confirm that the planar-arrangement of the heteropolyanion is not substantially altered by the adsorption process. The corresponding adsorption isotherm has been registered at room temperature due to the good solubility of the phase. In comparison with the traditional HDS catalysts, this new system shows the following relevant aspects: (a) the activity for the HDS of thiophene is similar in both cases, although the Co content is considerably lower than that of the impregnation of the ammonium heptamolybdate and cobalt nitrate hexahydrate solutions ([Co]/([Co]+[Mo])=0.14 in relation to 0.25–0.40 in the conventional system); (b) the phase adsorbs well onto the support; (c) the activity of molbydenum seems to be promoted by the presence of cobalt in the ordered distribution of the heteropolyanion on the surface of γ-Al2O3 (synergy-like effect). The primary structure of supported heteropolyoxomolybdate is clearly preserved; (d) whereas the conventional catalysts can be prepared through a series of steps (two impregnation operations, one for drying and another for the calcination at 550°C), the proposed method only requires one impregnation step and a drying step at 80°C, suppressing the calcination at 550°C.

(NH4)6[TeMo6O24] . 7H2O Anderson phase as precursor of the TeMo5O16 catalytic phase : thermal and spectroscopic studies

Abstract The thermal behaviour of the (NH 4 ) 6 [TeMo 6 O 24 ] · 7H 2 O Anderson phase has been analysed in N 2 and H 2 /N 2 atmospheres b differential thermal analysis and thermogravimetry (DTA-TG) and temperature-programmed reduction (TPR) techniques. X-ray diffraction, IR and Raman spectroscopies as well as scanning electron microscopy (including electron microprobe) have also been employed to characterize the intermediate and final products of the pyrolysis. Some studies on ammonium heptamolybdate, telluric acid, TeO 2 paratellurite and (NH 4 ) 3 [AlMo 6 O 24 H 6 ] · 7H 2 O Anderson phase have been carried out for comparative purposes. MoO 3 and TeO 2 are the solid products of the therm treatment in N 2 . The mild reducing effect of NH 3 decreases the temperature of the Te(VI) reduction. On the other hand, the TPR study permits the formation of the TeMo 5 O 16 , which is an interesting catalytic phase, to be established. Its stability range, between 400 and 500 °C, can be supported by the Mo(IV)-Te(IV) and Mo(VI)-Te 0 redox incompatibility. Mo 4 O 11 is the thermal product of the TeMo 5 O 16 decomposition, together with TeO 2 and MoO 3 according to the stoichiometry. However, some traces of MoO 2 induce the cyclic redox process, which makes it difficult to detect TeO 2 and MoO 3 . Likewise, Mo4O 11 shows a large range of stability (between 550 and 650 °C), unlike the thermal behaviour of other related molybdic phases.

Anderson-type heteropolyoxomolybdates in catalysis: 2. EXAFS study on γ-Al2O3-supported Mo, Co and Ni sulfided phases as HDS catalysts

Abstract The nature of the Co (or Ni) and Mo sulfided species obtained starting from γ-Al 2 O 3 -supported Anderson heteropolyoxomolybdates (CoMo 6 and NiMo 6 ) has been analyzed by means of the extended X-ray absorption fine structure (EXAFS) technique with the aid of temperature-programmed reduction (TPR) and spectroscopic measurements. Results are discussed in relation to the adsorption isotherms and to the catalytic activity for the thiophene hydrodesulfurization (HDS) reaction. A comparison with traditional catalysts has been made.

Reducibility and thermal behaviour of some Anderson phases

Abstract The thermal behaviour of the (NH 4 ) 3 [M(III)Mo 6 O 24 H 6 ] · 7H 2 O, Anderson phases, where M is Al, Co, Fe has been analysed in N 2 , N 2 /O 2 and N 2 /H 2 atmospheres using TG-DTA, TPR, and XRD techniques. Additional measurements were made by IR, EPR and Mossbauer spectroscopies, and also by scanning electron microscopy. Similar studies were carried out on ammonium heptamolybdate for comparative purposes. Molybdenum oxide and metallic molybdate are the solid products of the thermal treatment in N 2 and N 2 /O 2 atmospheres. While M 2 (MoO 4 ) 3 is observed when M is Al and Fe, the β-CoMoO 4 binary oxide is stabilized under both experimental conditions. The intermediate [(NH 4 ) 2 O] 0.8 MoO 3 (H 2 O) 0.3 phase is only observed when working in static air atmosphere at a very low heating rate. The temperature-programmed reduction (TPR) study permits the analysis of the different reduction steps of the Mo(VI) and M(III) is Fe, Co components. Mo 4 O 11 , MoO 2 , Mo and MMoO 4 (M is Co, Fe) are also observed. The formation of an Al-Mo reduced oxidic phase (related to MoO 2 ) can be inferred for the Al compound.

Γ-Al2O3-Supported XMo6 Anderson Heteropolyoxomolybdates: Adsorption Studies for X = TeVI, AlIII, CoIII, CrIII and NiII by DR Spectroscopy and TPR Analysis

The support–overlayer adsorptive interactions, generated when solutions of Anderson heteropolyoxomolybdates were supported as a monolayer on γ-Al2O3 by means of the equilibrium adsorption method, were studied by XRD analysis, DR spectroscopy, electron microscopy (SEM–EDAX) and TPR methods. The adsorption isotherms and corresponding adsorption parameters (number of active sites and adsorption constants) of the γ-Al2O3-supported ammonium heptamolybdate were obtained and discussed. The following sequence of adsorption strength was suggested: CoMo6 > CrMo6 > TeMo6 ~ AlMo6 > NiMo6. The effectiveness of the TPR technique as a tool for elucidating interaction effects was clearly demonstrated. The formation of a γ-Al2O3-supported Te–Mo mixed valence oxide appears to provide new and interesting catalytic possibilities.

Reductibility and spectroscopic behaviour of the (NH4)4[H6CuMo6O24] · 4H2O anderson phase

Abstract The vibrational spectrum of (NH 4 ) 4 [H 6 CuMo 6 O 24 ] · 4H 2 O was analyzed and is discussed on the basis of the different Mo-O bond lengths in the lattice. The temperature-programmed reduction (TPR) process was also studied by means of several techniques, such as X-ray diffraction, scanning electron microscopy, and infrared, Raman and electron spin resonance (ESR) spectroscopies. The results revealed that the MoO 2 oxide, together with Cu metal, is formed at ≈ 385 °C, while the final reduction to Mo 0 occurs at 783 °C. These reduction steps for molybdenum are at ≈ 350 and ≈ 70 °C, respectively, below those observed for ammonium heptamolybdate. The presence of Mo(V) and Cu(I) species was also suggested in the course of the thermal reduction by ESR spectroscopy. The TPR data of some other Anderson phases have been used for purposes of comparison. The effect of the redox potential of the heteroatom on the stability of these phases has been analyzed. The presence of metallic copper seems to increase the capability of the reducing agent to form Mo 0 .

Thermal behaviour and properties of the (NH4)6MnMo9O32·8H2O Waugh phase

Abstract The thermal degradation of (NH4)6MnMo9O32·8H2O has been investigated by TG, DTA and TPR methods. The decomposition process in a stream of N2 takes place in several steps. Only a reversible dehydration process occurs up to 200°C. The subsequent steps show the formation of [(NH4)O]0.083MoO3(H2O)0.25 and MoO3 phases. The reduction of Mn(IV) to Mn(III) can be considered to take place in this inert atmosphere. Likewise, the TPR process clearly reveals the successive formation of Mo4O11, MoO2 and metallic molybdenum. In this treatment the manganese seems to be incorporated in the Mo oxide lattices. Hence, the expansion of the MoO2 cell volume, observed by XRD with the aid of DRS spectroscopy, presumably leads to the formation of the MoIV0.9MnII0.1O1.9 solid solution. This last phase also affects the final process, increasing the temperature of molybdenum formation by 111°C, compared to the temperature of molybdenum formation from MoO2. In this step the manganese segregates as MnO. The (NH4)6MnMo9O32·8H2O vibrational spectrum was analysed and discussed on the basis of the different MoO bonds in the lattice. The magnetic measurements, together with other spectroscopical information, allow an estimation of the degree of covalence of the MnO bond to be made.

Vibrational and electronic spectra of the PV14O9-42 polyanion

Abstract The polyanion PV 14 O 9- 42 represents a new structural-type, constituted by the well known α-Keggin skeleton with two additional trigonal-bipyramidal VO 5 caps. The vibrational and electronic (reflectance) spectra of different salts of this polyanion have been investigated. They were assigned on the basis of the structural data and by comparison with the spectroscopic behaviour of related species.

Bulk and surface characterization of some heteropolymolybdates and the products of their reduction and sulfidation

[CoMo6O24H6](NH4)3·7H2O Anderson-type hexamolybdate (as prepared, supported on γ-alumina and after thermal treatment in hydrogen or in a thiophene/hydrogen mixture) has been characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), thermal analysis and X-ray photoelectron spectroscopy (XPS). The Anderson phase also contains Co3+ and Mo6+ species at the surface. After thermal treatment at 773 K in hydrogen and exposure to air at room temperature, MoO3 and CoMoO4 are formed. The XP spectrum of the Anderson phase supported on γ-alumina showed the presence of two Mo6+ species, probably due to the coexistence of the molybdenum contained in the structural framework of the Anderson phase and a species derived from the interaction of the hexamolybdate with γ-alumina. The XP spectrum of the supported Anderson phase after sulfidation revealed the presence of two molybdenum species which could be due to the coexistence of sulfide and oxisulfide phases; in addition, the Mo: Al atomic ratio was increased by sulfidation, suggesting a good dispersion of the MoS2-like layered structure with small crystallite sizes. [AIMo6O24H6](NH4)3· 7H2O treated in hydrogen at 1073 K was also characterized and its XRD pattern showed the presence of metallic molybdenum.

Thermal behaviour of some phosphorus/chromium polyoxoanions

Abstract The thermal degradation of K 2 HPCr 2 O 10 , Na 3 PCr 3 O 13 · 3H 2 O, K 3 PCr 4 O 16 and (NH 4 ) 3 PCr 4 O 16 has been investigated by TG and DTA methods. The main decomposition process for the three firstly mentioned compounds occurs at around 400°C and is associated with the rupture of P-O-Cr bonds. The ammonium salt shows a different behaviour and decomposes explosively at 230°C. In the case of K 2 HPCr 2 O 10 a new polyoxoanion of formula K 4 P 2 Cr 4 O 19 is generated as an intermediate during the thermal degradation. Detailed stoichiometries, sustained by the TG/DTA results as well as by the chemical and infrared spectroscopic analyses of the solid residues could be given. It is shown that, in all cases, CrPO 4 is one of the decomposition products.