Pierluigi Villa

Catalyst preparation methods

Solid catalysts are highly sophisticated products derived from chemicals by means of several different procedures. The catalytic properties of heterogeneous catalysts are strongly affected by every step of the preparation together with the quality of the raw materials. The choice of a laboratory method for preparing a given catalyst depends on the physical and chemical characteristics desired in the final composition. It is easily understood that the preparation methods are also dependent on the choice of the base materials and experience shows that several ways of preparation can be considered, even for a given selection of the base material. This variety of possibilities might seem to confirm that making a catalyst is a ‘black art’

Characterization of the surface properties of polycrystalune WO3

Abstract Tungsten trioxide was precipitated from an ammonium paratungstate aqueous solution at acid pH. The precipitated material consists of a mixture of hexagonal and pseudocubic WO3. After calcination at about 700 K, it transforms into monophasic pseudocubic WO3 (15m2 g−2). FTIR investigation of adsorbed water, ammonia, pyridine, pivalonitrile and CO2 indicates an acid nature of the surface. Two types of strong Lewis acid sites, as well as Bronsted acid sites, have been detected. Models are proposed for the surface sites of WO3, and the catalytic activity of WO3-containing catalysts is briefly discussed in relation to their surface chemistry.

The nature of active components in Fe2O3MoO3 System: IV. Bi-doped Fe(III)-molybdate: Characterization and activity

Abstract An investigation on the FeMoBi oxides system in the Fe-rich region was carried out by adding various amounts of Bi to Fe(III)-molybdate. The principal catalytic modifications which already occurred at very low Bi levels (0.5% Bi 2 O 3 by wt), were: (i) strong decreasing in isomerizing power at low temperature; (ii) strong increase in the selectivity to and yield of butadiene; iii) decrease in 1-butene conversion; (iv) strong decrease in CO formation. The characterization of these catalysts with Mossbauer, ir, ATR, diffuse reflectance, X-rays seems to indicate that no new phase is present at these concentrations of Bi. Only at much higher levels (3.5% Bi 2 O 3 by wt) where the catalytic activity is already constant, does a new phase form in increasing amounts. This new phase is Bi-molybdate 2:1 modified probably by Fe 3+ . It is proposed that the active and selective phase of these ternary systems is Fe(III)-molybdate with Bi as dopant in interstitial or substitutional sites, which might give rise to Bi(V)Mo(VI) continuous groups. Other particular features of these catalysts seem to be: 1. i. A higher activity than that of the typical values of Bi-molybdates. 2. ii. An activity constant also in very reducing conditions where Bi-molybdates become inactive. 3. iii. A strong influence of oxygen partial pressure: at very low oxygen level the selectivity increases considerably.

Synthesis of alcohols from carbon oxides and hydrogen. V. Catalyticbehaviour of pure Cr, Zn, Mn oxides towards CO/H2

Abstract Pure oxides of Mn, Zn and Cr have been prepared and characterized. Their catalytic behaviour in the direct synthesis of higher alcohols from CO and H2 has been studied in terms of the elementary reactions: formation of methanol, chain growth to higher alcohols, direct water-gas shift reaction, methanation, formation of heavier hydrocarbons. The results provide useful references when analyzing catalysts with more complex formulations.

Oxidation of 1-butene to maleic anhydride and carbon oxides on molybdate-based catalysts

Abstract The catalytic activity of MnMoO4, β-FeMoO4, a-CoMoO4 and CdMoO4 in the oxidation of butenes has been investigated in a wide range of experimental conditions both in a stirred-tank gradientless reactor and in a pulse microreactor. Some of the catalysts investigated showed good selectivity to maleic anhydride; the other products were mainly CO and CO2. The oxidation runs carried out in a pulse microreactor showed that the catalytic behaviour of the catalysts investigated strongly depended on whether oxygen was or was not present in the gas phase. At a low oxygen percentage, butadiene is the main product for all the catalysts. Therefore the partial pressure of oxygen is a key to modify the selectivity. In the absence of oxygen, CdMoO4 was not active, whereas CO and CO2 were the main products. It was therefore proposed that gaseous oxygen must take a direct part in the formation of oxidised products. As the second metal (Mn(II), Fe(II), Co(II), Cd(II)) of the catalysts investigated is unable to oxidise the reduced form of molybdenum, a direct interaction of reduced molybdenum with oxygen must be hypothesised. In addition, the tetrahedral coordination of Mo (a common structural feature of all the catalysts investigated) can play an important role in giving a reactive form of adsorbed molecular oxygen which is responsible for the formation of maleic anhydride, CO and CO2. Bismuth molybdates, which present an octahedral coordination of molybdenum and a second metal (Bi(III)) that can oxidise Mo(V), show a catalytic behaviour quite different from that of the investigated catalysts.

Preparation and activity of bismuth tungstates in oxidation and ammoxidation of olefins

Investigations have been carried out on the catalytic behavior of Bi tungstates at the different ratios BiW: 23, 11, 1.81, 21, 61 in the oxidation and ammoxidation of propylene and 1-butene. Bi2WO6 is the only active and selective compound. WO3 is active but the main products of oxidation of both 1-butene and propylene are CO and CO2. At low temperatures WO3 shows a high isomerizing power towards 1-butene while Bi2WO6 is inactive. Ammonia present in the reagent mixture modifies the catalytic behavior of WO3, keeping the distribution of products of ammoxidation of olefines nearer to the distribution given by Bi2WO6. The surface acidic sites and the formation of CO and CO2 were attributed to the pressence of bonds of covalent type. These types of bonds are not present in Bi2WO6 which has a rather polar structure.

YBa2Cu3O7 preparation by the amorphous citrate method

Abstract A modification of the amorphous citrate method to obtain high-Tc superconducting oxides is reported. The method starts from organic salts of yttrium, barium and copper, such as acetates, and avoids the sudden and uncontrollable decomposition of the organic fraction which occurs if nitrates are used as starting materials. The aqueous solution obtained with citric acid in an alkaline medium is concentrated under vacuum. The solid so prepared is decomposed in an oxygen-nitrogen atmosphere at about 300 °C, thus giving an oxide precursor containing well-interspersed yttrium, barium and copper. Preliminary characterization of the properties of the YBa2Cu3O7 system obtained after the high-temperature treatment is reported.

Propylene oxidation to acrolein on FeSbTiO catalysts

Abstract Coprecipitated FeSbTi mixed oxides with an (Fe + Sb) (Fe + Sb + Ti) atomic ratio in the 0–1 range and with an Sb Fe atomic ratio of 2.0 were characterized by means of X-ray diffraction, Mossbauer, and infrared spectroscopies and by attrition resistance tests. Catalysts were tested in the oxidation of propene to acrolein. Catalysts with high and similar selectivities (80–90%) are obtained in the 0.3–1.0 (Fe + Sb) (Fe + Sb + Ti) atomic ratio range where X-ray diffraction analysis shows the presence of either only a single crystalline rutile phase or a single rutile phase together with α- and β-Sb 2 O 4 in variable amounts. No correlation was found between selectivity to acrolein and the presence of crystalline Sb 2 O 4 . On the contrary, selectivity to acrolein seems to be associated with Sb ions stabilized at the surface of the catalyst. The results of physicochemical characterization and interpretation thereof are consistent with the crystalline rutile-like single phase being constituted by a solid solution originating from either FeSbO 4 , FeSbO 4 doped with Ti, and TiO 2 doped with Fe and Sb phases, or a lattice of largely uniform periodicity, but with localized variations in composition.

Deactivation of Silica Supported Fe2 03 -MoO3 Catalyst for the Oxidation of Methanol

The deactivation of Fe 2 O 3 -MoO 3 supported on silica (1.7% b.w.) in the oxidation of methanol to formaldehyde is studied, and fresh and deactivated catalysts are characterized by chemical analysis of Fe and Mo contents, Raman and Mdssbauer spectroscopy. Partial destruction of Fe 2 (MoO 4 ) 3 , a corresponding increase of Fe ions within silica, and loss of Mo occur during reaction. The fundamental processes involved in the catalyst deactivation are discussed and a model is given for deactivation-reaction kinetics, which provides a good fitting for experimental data.

A Study of the Chemisorption of Chromium(VI), Molybdenum(VI) and Tungsten (VI) Onto γ-Alumina

In the pulse and step addition of ammonium dichromate, ammonium paramolybdate and ammonium paratungstate solutions to γ-alumina a large increase of the pH was observed in the liquid phase which suggests a common mechanism of interaction with the basic sites of γ-alumina. This mechanism involves the decondensation of the isopolyanion in the presence of alumina followed by the chemisorption of monomeric species. The most relevant difference between the three metal ions is the limiting amount of the chemisorbed oxospecies on γ-alumina: CrO 3 3. 28; MoO 3 7. 72; WO 3 19. 2 wt. %. The electronic and Raman spectra of the chemisorbed molybdenum(VI) and tungsten(VI) at low coverage can be interpreted as due to oxospecies in tetrahedral co-ordination. The relevance of the adsorption phenomena in the preparation of the commercial catalyst is finally pointed out.