Dimitris E. Petrakis

Synthesis and acidic catalytic properties of ordered mesoporous alumina–tungstophosphoric acid composites

A series of well-ordered mesoporous alumina–tungstophosphoric (HPW) acid composite frameworks has been prepared by a sol–gel copolymerization route in the presence of non-ionic surfactants. The resulting materials feature a high loading of HPW acids (up to ∼53 wt%) in composite framework and possess hexagonal p6mm pore structure with uniform large pores. The mesoscopic order of these structures was evidenced by SAXS analysis, TEM images and N2 physisorption measurements. The composite materials exhibited BET surface areas in the range of 54–71 m2 g−1, total pore volumes in the range of 0.11–0.14 cm3 g−1 and quite narrow pore size distributions with peak maxima in the 7.1–8.3 nm range. The Keggin clusters were incorporated in mesoporous alumina walls by strong chemical bonds according to the FT-IR and UV/vis spectroscopy analysis. This chemical linkage of HPW to the alumina matrix is responsible for the outstanding stability of these materials against water-leaching. The mesoporous surfaces exhibited exceptional acidity that arises from the unique alumina–HPW composite structure. As the loading of HPW increases, the surface acidic character of the composites enhanced, and this is reflected in the higher catalytic activity towards isopropanol conversion.

Influence of the structure of alumina–aluminium phosphate (AAP) catalysts containing Fe3+ and Cr3+ on their catalytic activity for isopropyl alcohol decomposition

Phosphate catalysts of the general formula Al100PxM20, where M = Al, Cr, Fe and x= 0, 4.5, 9, 18, 36, 72 and 144, have been prepared by coprecipitation of the corresponding nitrate salts and phosphoric acid with ammonia at pH 9.5. The resulting solids, after characterization by surface area and XRD measurements, were checked for their total surface acidity (Bronsted and Lewis) by pyridine titration and also tested for their catalytic activity towards the isopropanol dehydration. It was found that for the Al100PxAl20 solids the surface density of the acid sites increases on addition of phosphorus in a fairly regular manner. Substitution of some Al by Cr or Fe blurrs the picture of increasing acidity described above. The catalytic activity for isopropyl alcohol dehydration, calculated as the number of molecules decomposed per acid site per second, shows a minimum at x= 18–36 for Al100PxAl20 and Al100PxFe20, while for Al100PxCr20 a continuous increment with x is observed. The activity minima of the first two sets of solids coincide with the total absence of crystallinity as detected by XRD measurements. On the contrary, the continuous increment of activity for the Cr-containing catalysts is related to the presence of crystalline α-Cr2O3. This behaviour is attributed to competition between the diffusional and chemical steps controlling the reactions process. On the amorphous solids, where high values of activation energy and decreased activity are observed, the chemical reaction seems to be the slow and rate-determining step, while diffusion is fast because of the large porosity of the solids. The other samples show activation energies that are half those of the amorphous solids and larger activities. The controlling step in these cases seems to be diffusion, a fact related either to smaller pores or larger rate constants.

Influence of Cr3+ and Fe3+ cations on the structure of alumina–aluminium phosphate solids and their catalytic activity for N2O decomposition

Phosphate salts of the metals Al, Fe and Cr supported on alumina were prepared by co-precipitation with NH3 from solutions containing calculated amounts of the metal nitrates and phosphoric acid. The atomic ratio of the elements participating in the solids was Al : P : M = 100 : x : 20 where M = Al, Fe, Cr and x= 0, 4.5, 9, 18, 36, 72 and 144. The parent compounds Al100PxAl20 are formed by an endothermic decomposition of NH4NO3, which exists in the dried precipitant, to NH3 and HNO3. The resulting solids for x= 0 contain just γ-Al2O3, for x= 144 AlPO4 plus an amorphous phase, while for intermediate values of x a totally amorphous phase is formed. Substitution of 20 % Al by Fe alters slightly the route of decomposition, the endo-effect noticed above is followed by a small exothermic one, meaning probably the decomposition of part of the resulting nitric acid through anion breakdown to N2, H2O and O2. The structure of the resulting solids corresponds to α-M2O3 for x= 0 and MPO4 for x= 144, while for intermediate values amorphous materials are formed. An addition of Cr instead of Fe alters completely the route of decomposition of NH4NO3 to N2, H2O and O2 for reasons similar to those noticed for Fe. For x= 0 the resulting solids contain α-M2O3 oxides alone, for x= 144 they are totally amorphous and for intermediate values of x an amorphous phase plus crystalline Cr2O3 is formed. The behaviour of these systems for a simple redox reaction, namely N2O decomposition, shows a continuous drop of activity with phosphorous content. An analysis of the results according to the kinetic theory of poisoning indicates an order of deactivation between one and two, depending on the catalyst and the temperature. A mechanism which may explain this result is proposed. Thiele modulus and effectiveness factors which have been calculated show that for AlPAl an AlPCr species internal diffusion is the rate-determining step and the reaction occurs mainly on the external surface. For the AlPFe catalysts the rate of diffusion is almost equal to that of reaction.

Synthesis and properties of some mesoporous aluminophosphates with acidic surface sites

Mesoporous solids of the general formula Al100PxFe5-y[x= 0, 15, 60, and y(the final firing temperature in °C)= 400 or 600] have been prepared and characterised by surface area and porosity measurements, X-ray diffraction (XRD), Mossbauer spectroscopy and solid-state NMR (27Al and 31P). The surface acidities of the solids were also determined by ammonia adsorption, and the ζ-potential of the particles was measured. The gradual incorporation of phosphate groups into the alumina results in an increase of the surface area. The materials possess a pore-size distribution approximated by a mixed Gaussian and Lorentzian component which varies in a controllable manner according to the elemental composition. The material Al100P15Fe5 corresponds to that with the maximum surface area and pore volume. The XRD analysis showed that the materials without phosphorus possess the γ-Al2O3 structure, but they become amorphous upon addition of 15% P. A further increase in the amount of phosphorous to 60% initiates the transformation of the solids to AlPO4. The Mossbauer spectra suggest that, in the samples without phosphorus, the iron species are segregated, but the addition of phosphorus results in a dispersion of the iron into the aluminium phosphate matrix. The 27Al MAS NMR spectra indicate that the aluminium atoms are predominately in octahedral environments in the samples Al100P0Fe5-600 and Al100P15Fe5-600 but they become equally distributed between octahedral and tetrahedral sites in Al100P60Fe5-600. The ζ-potential of the particles in aquatic suspension indicates that this parameter is sensitive to the temperature of thermal treatment. The surfaces of the solids showed increased acidity which is linearly related to the phosphorus content.

Effect of the first row transition metal cations on the mode of decomposition of ammonium nitrate supported on alumina-aluminum phosphate and the final products obtained

Abstract The mode of decomposition of ammonium nitrate in mixtures with 80% Al 2 O 3 and 20% of first row transition metal oxides MOx (M = Mn, Cr, Fe, Co, Ni, Cu, Zn) and containing different amounts of phosphorus (Al:M:P = 100:20:18 and 100:20:144) has been examined under non-isothermal conditions in a thermogravimetric balance. Pure ammonium nitrate is decomposed endothermically while addition of Al 2 O 3 shows a stabilizing action of this endo effect. Substitution of 20% of aluminum by a first row transition metal (M = Mn, Cr, Fe, Co, Ni, Cu and Zn) shows that zinc does not alter the route of decomposition while the rest show a tendency to decompose ammonium nitrate exothermically. The catalytic action of the cations toward such an exothermic route is Fe = Ni 4 NO 3 are primarily related to electrochemical Gibbs free energy Δ G = − nFE of the process M oxidized /M reduced . On top of this the crystal field stabilization effect for each particular cation should be taken into account. The extent of exothermic decomposition of the ammonium nitrate seems to influence the specific surface area of the final product in a positive manner in the absence of phosphorus but in a negative manner in its presence. The reasons underlying this effect should be attributed to the stabilizing action of phosphorus on the decomposition route.

De-sintering action of phosphorus in alumina–aluminium phosphates (AAP) containing transition-metal ions (M = Cr, Mn, Fe, Co, Ni, Cu, Zn)

The de-sintering action of phosphorus added in different amounts to alumina containing 20% of a second cation M (M = Cr, Mn, Fe, Co, Ni, Cu, Zn) has been quantified from measurements of the surface areas of the solids. The solids were prepared by co-precipitation with NH3 from solutions containing calculated amounts of metal nitrates and phosphoric acid and by heating at 600 °C. The atomic ratio of the elements in the obtained solids was Al : P : M = 100 : x : 20 where x= 0, 4.5, 9, 18, 36, 72 and 144. The surface areas of the solids were measured by the B.E.T. method. The results showed that the maximum de-sintering action of phosphorus, as estimated from the change of the surface area of the solids, appeared at x= 9. Thereafter the surface area decreased, at a different pace for each cation, and at x= 36 it returned to the original value at x= 0. Further addition of phosphorus decreased the surface area even further and at x= 144 it fell almost to zero, depending on the second cation. The strongest sintering ability was exhibited by Cu and Mn and the smallest by Cr, Fe and Ni, with the rest occupying intermediate positions. An explanation of the results is attempted by taking into account the enthalpies of formation of the corresponding metal phosphates and aluminates formed in each case.

Microporosity, pore anisotropy and surface properties of organized mesoporous silicates (OMSi) containing cobalt and cerium

Nanostructured, organized mesoporous silicate materials, containing cerium plus cobalt, were synthesized from a system based on polyacrylic acid (Pac), C16TAB and TEOS. The synthesis took place along a titration path from low pH (≈2) to high pH (≈10). Along this path the C16TAB species are attached to the Pac, and on the formed micelles, hydrolysis of TEOS (as well as of cobalt and cerium species) takes place. The initial atomic ratios of Ce(IV) : Co(II/III) were 1 : 9, 2.5 : 7.5 and 5 : 5. In each of those three cases, three different samples were isolated at pH = 5.5, 7.5 and 9.5 which, after drying and firing at 600 °C, were characterized by X-ray diffraction (XRD), nitrogen adsorption porosimetry, diffuse reflectance spectrospopy (DRS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Cerium is already quantitatively incorporated into the solid at pH = 5.5. At this low pH value, only ≈20% of the cobalt is incorporated into the solid, but it is fully incorporated at the high pH of 9.5 The silicate species containing a low content of Ce(IV) and Co(II/III) (x : z = 1 : 9) show a self-organized morphology as previously observed in samples free of cerium. This self-organization is destroyed when the content of cerium is increased (x : z = 2.5 : 7.5 and 5 : 5). The samples with a low content of Ce(IV) and Co(II/III) possess an MCM-41 structure, which is gradually degenerated as the amount of the metals increases, and tiny crystallites of CeO2 and Co3O4 become apparent with XRD. The surface area of the solids was in the range 1230–770 m2 g−1, and it drops almost linearly as the content of the metals in the solids increases with the increase of pH. The I-point methodology was successfully applied to detect and distinguish distinct domains of microporosity and mesoporosity in the solids, as well as for the estimation of the percentage of micropore volumes (%Vmic), found in the range 15–30%, using the variation of the C parameter of the BET equation. The pore anisotropy, b, was also determined in the range 10 < b < 2300. A correlation of the form log b = log b0 − (%Vmic) was observed for the materials of the present study, in line with previous findings.

Solid-state NMR study of mesoporous phosphoro-vanado-aluminas

The structures of mesoporous solids of abbreviated formula Al100PxVy where x, y = 0, 5, 10 have been characterised by multinuclear magnetic resonance techniques. 31P MAS spectra indicate that two phosphorus environments (threefold or fourfold O–Al coordination) are incorporated within the aluminium network. 51V magic-angle spinning (MAS) spectra suggest that vanadium species are not (or only weakly) incorporated into the alumina network, forming V2O5-like and tetrahedral species at the surface of the pores. 27Al MAS spectra show that amorphous AlPO4 species are formed upon addition of phosphorus, already occurring for the lower phosphorus input. However, 27Al triple-quantum (3Q)-MAS cannot detect this amorphous AlPO4 phase, therefore indicating distorted environments close to the surface. For the Al100P10V10 sample, crystalline AlPO4 is detected. 31P–{27Al} and 31P–{51V} TRAPDOR experiments confirm that all the phosphorus atoms are surrounded by at least three aluminium atoms in the second coordination sphere, and suggest the heterogeneous location of vanadium species at the surface of the mesopores.

Controlled modulation of mesoporosities in metal (M Al, Fe) phosphate solids

Abstract Mesoporous solids which possess average pore diameters between 7 and 20 nm, depending on the composition, have been prepared. The solids have the general formula Al100PχM20 where M  Al or Fe, and χ = 0, 4.5, 9, 18, 36, 72 or 144. The initial addition of phosphorus as phosphate transforms the originally crystalline oxide/oxides into amorphous solids. These amorphous materials possess a narrow pore size distribution: 80–90% of the pores lie within 1–2 nm of the average pore diameter. Subsequent incremental amounts of phosphorus transform the material into a crystalline solid whilst the pore size distribution becomes much wider and the maximum moves towards larger pore diameters. Substitution of 20% of the aluminium by iron results, at a low phosphorus content, in pores with smaller pore volumes and smaller surface areas. The data in the dVp/dDp = > Dp) graphs, where Vp is the incremental pore volume and Dp is the average pore diameter, can be approximated using an admixture of Gaussian and Lorentzian curves. For low phosphorus contents the dVp/dDp = (Dp) curves have a mainly Gaussian profile but the gradual addition of phosphorus transforms them to Lorentzian-type curves. An attempt to approximate the histograms dVp =(Dp) with the minimum number of distribution curves made up of the corresponding Gaussian and Lorentzian components indicates that each successive addition of phosphorus creates a dominant new pore component at a larger pore diameter. At the same time, the components at smaller pore diameters are diminished and eventually disappear as more phosphorus is added.

Influence of phosphorus and vanadium additives in the development of surface acid catalytic properties of mesoporous alumina

Alumino-phosphoro-vanadate catalysts of the general formula Al100PxVy, where x,y = 0, 5, 10 and 20, have been prepared by co-precipitation of calculated amounts of Al(NO3)3·9H2O, H3PO4 and V2O5 dissolved in NH4OH, with ammonia solution at pH = 9.5. The resulting solids, after characterization by N2-porosimetry and XRD measurements, were checked for their surface acidity by ammonia–TPD and were also tested for their catalytic activity towards isopropanol decomposition. It was found that the addition of phosphorus increases the acid sites per square metre of the Al100PxVy solids and the same effect is also observed by the addition of vanadium. The catalytic activity of the solids with y < 20, expressed as molecules decomposed per surface acid site per second, shows maxima for catalysts with higher cumulative addition of phosphorus and vanadium, at those P plus V concentrations that result in amorphous solids while the onset of crystallization results in reducing the activity. The opposite behaviour is observed for catalysts with 20% V where the crystal phase of V2O5 is apparent in XRD right from the beginning but becomes gradually amorphous at high phosphorus addition at x = 20.