Amorphous Aluminophosphate Research Articles

Through-bond and through-space connectivities of amorphous aluminophosphate by two-dimensional 27Al– 31P heteronuclear NMR

The connectivities between Al and P through chemical bond and internuclear distance have been studied for an amorphous aluminophosphate (a-AlPO 4) using two-dimensional (2D) solid-state 27Al– 31P correlation NMR (MAS J-HMQC and CP HETCOR). Whereas the conventional 31P MAS spectrum provides less informative results because of poor resolution caused by large distributions of the nucleus surroundings, the 2D HETCOR shows much better resolution and at least four non-equivalent P sites in the a-AlPO 4. These P sites are found to be correlated with one [4]Al, two [5]Al and one [6]Al species, and have different chemical shifts. This result might indicate that the mean P–O– [ n] Al ( n = 4, 5, 6) bond angles are different each other, and they are estimated using the relationship with the 31P chemical shifts in crystalline AlPO 4 previously reported.

Structure and catalytic activity correlation of CoAlPO 4 in the synthesis of N,N-biphenyl urea via alkylation of aniline using dimethyl carbonate

Two sets of cobalt aluminophosphate materials, one at different pH (4.5, 7, 10, and 11.5) and the other containing different percentage of cobalt (0.5–20) were prepared by co-precipitation method. All the materials were characterized by XRD, SEM, 27Al MASS NMR, TGA and DSC, FTIR, for their textural and structural properties. The surface area of the samples was determined by BET method and total surface acidity by n-butylamine titration method. The CoAlPO 4 obtained under acidic pH were found to be crystalline AlPO 4 with nonmicroporous tridymit structure associated with cobalt phosphate, whereas the one obtained under alkaline pH were amorphous aluminophosphate and also associated with cobalt phosphate. Tridymit-AlPO 4 crystals exhibited Al atom in a tetrahedral environment of phosphate groups. In the amorphous material the Al atoms were in a mixed tetrahedral and octahedral environment. The concentration of acid sites were found to be higher in the sample obtained under alkaline pH. This has been attributed due to the partial coordination of hydroxyl groups in the octahedral aluminum sites. The amorphous materials obtained under alkaline pH were found to be thermally stable up to 900 °C. The catalytic activity of all the materials was investigated in aniline alkylation using dimethyl/ethyl carbonates (DMC/DEC) under refluxing conditions. N-methyl/ethyl aniline (NMA/NEA) and biphenyl urea (BPU) were formed as the major products with the catalysts prepared at acidic and alkaline pH, respectively. A good correlation has been observed between the catalytic activity of CoAlPO 4 materials towards their selectivity for NMA/NEA and/or BPU and their textural and structural properties. Reaction conditions such as the amount of the catalyst, duration of the reaction, reusability and efficiency of the catalyst have also been evaluated. CoAlPO 4 prepared under alkaline pH with 5% of cobalt has been found to be the best catalyst for BPU synthesis from aniline and DMC/DEC under refluxing conditions.

AlPO4-18 synthesized from colloidal precursors and its use for the preparation of thin films

Nanosized AlPO4-18 is synthesized from clear precursor solutions; a complete transformation from amorphous to crystalline colloidal particles is observed after 60, 20 and 18h at 100, 150, and 170°C, respectively. The evaluation of the long-range ordering of nanocrystals as a function of crystallization time was followed by X-ray diffraction and dynamic light scattering. The time-dependent 27Al NMR measurements in the liquid phase before and after purifications with different crystallization times show that the tetrahedrally coordinated Al is formed at RT prior to hydrothermal treatment indicating the formation of crystalline phase. The mean hydrodynamic radius of the amorphous aluminophosphate entities and the crystalline AlPO4-18 particles does not change during the entire process of crystal growth. Further these AlPO4-18 nanocrystals were employed for the preparation of thin films on silicon wafers via spin coating of stable AlPO4-18/EtOH solutions, and additional crystal growth of the seeded layers at elevated temperature was carried out. The structural evolution of the thin films was studied with grazing incidence diffraction using synchrotron X-ray radiation. The orientation of growing crystals is governed by the preferred orientation of the seeded particles deposited by spin coating approach.

Investigation of the Evolution of Intermediate Phases of AlPO4-18 Molecular Sieve Synthesis

In the present work, we examined the intermediate phases formed during the hydrothermal synthesis of microporous material, AlPO4-18. The evolution of the long-range ordering of the gel samples as a function of crystallization time was followed by powder X-ray diffraction. The development of the local environments of P and Al atoms was monitored by 31P and 27Al magic angle spinning (MAS) NMR. Several representative intermediate phases were further characterized by 27Al multiple quantum MAS and 27Al → 31P cross-polarization experiments. Our results show that the formation of AlPO4-18 undergoes several stages. Mixing phosphorus and aluminum sources together with a template at room temperature yielded a mixture containing an ordered aluminophosphate (AlPO) phase and a phosphate material. These materials were converted to an amorphous AlPO phase under hydrothermal conditions. Continuous heating resulted in the transformation of the amorphous phase into a crystalline microporous material, AlPO4-5. A further inc...

Aldol Condensations on Solid Catalysts: A Cooperative Effect between Weak Acid and Base Sites

An amorphous aluminophosphate (ALPO) catalyst containing weak acid and base centers can carry out the aldol condensation of heptanal with benzaldehyde at much higher rates and selectivities than conventional solid acid (amorphous or crystalline aluminosilicates) or base catalysts (MgO, hydrotalcites, KF/Al2O3, nitrurated ALPO). With the weak acid-base catalyst, the reaction occurs through a bifunctional acid-base mechanism that involves the activation of benzaldehyde, by protonation-polarization of the carbonyl group on the acid sites, and the attack of the enolate heptanal intermediate generated on the basic sites. With this type of bifunctional acid-base catalyst, compounds with weak basicities are already able to undergo the reaction with a much higher selectivity than those obtained on stronger acid or base catalysts.

Synthesis of methylpseudoionones by activated hydrotalcites as solid base catalysts

Preparation of methylpseudoionones (precursors of methylionones) has been carried out by aldol condensation between citral and methyl ethyl ketone (Mek) using an amorphous aluminophosphate (ALPO), KF–Alumina and differently activated Al-Mg hydrotalcites as solid base catalysts. Activated hydrotalcites (calcined or calcined–rehydrated hydrotalcites) exhibited the best activity and selectivity to obtain methylpseudoionones. The effect of the chemical composition of the calcined hydrotalcites as well as the effect of the water content of the rehydrated samples have been studied. The main process variables such as reaction temperature and Mek∶citral molar ratio have been studied on these different activated catalysts, and in general it has been found that an increase of reaction temperature increases in rate of formation of methylpseudoionones, whereas the influence of the reaction temperature on the selectivity to this compound strongly depends on the type of activated hydrotalcite used. Owing to basicity/adsorption differences between calcined and rehydrated hydrotalcite, a different behaviour of reaction rate and selectivity to the desired product versus Mek∶citral ratio is observed, which should be attributed to the different nature and distribution of basic sites existing for both samples.

Synthesis, structure and surface properties of some mesoporous cero-phosphoro-aluminates

Mesoporous cero-phosphoro-aluminates (Al100PXCeY, X, Y=0, 5, 10, 20) have been prepared and characterized by XRD analysis, surface area and porosity measurements, XPS analysis and surface acidity determination. The preparation of the Al100PXCeY solids took place in an aqueous environment with NH3 as precipitating agent and heating at 600°C. The addition of phosphorus leads to an increase in the specific surface area (SSA) of the γ-Al2O3 parent solid from 230 to 390 m2 g−1 and the formation of amorphous aluminophosphates. The addition of cerium does not result in its incorporation into the alumina or the aluminophosphate solids but a separate phase of CeO2 is formed at X,Y<20 while at X,Y=20 CePO4 becomes apparent. At the same time the specific surface area drops from 230–390 m2 g−1 to 170–180 m2 g−1 depending on the sample. All the Al100PXCeY samples are truly mesoporous with mean pore diameters around 6–10 nm and full width at half maximum (FWHM) of the pore size distribution at 3–7 nm. The size of the crystallites of CeO2 (Sherrer relationship) is around 4–8 nm depending on the sample. The surface acid density, determined by NH3 adsorption, increases linearly with the addition of phosphates from 5×1017 sites m−2 for X=0 to 13×1017 sites m−2 for X=20, while it is practically invariant on addition of cerium. The XPS analysis showed that the surface of those porous materials is highly enriched in cerium as compared to the nominal solid composition. The enrichment is more substantial in pure alumina, reaching relative values of 100–150%, and decreases with the addition of phosphorus, where the relative enrichment is 30–60%. The surface of the solids without or with the addition of a small amount of cerium (Y=0,5,10) is moderately enriched in phosphorus (10–30%) while for high cerium content (Y=20) the surface is moderately depleted of phosphorus to 14–23%. The surface enrichment, or depletion, in phosphorus and/or cerium is corroborated by linear relationships between the surface composition (XPS) and the acidity (TPD NH3) in the case of phosphorus or the intensity of the main reflection in XRD of CeO2 in the case of cerium.

Designing the adequate base solid catalyst with Lewis or Bronsted basic sites or with acid–base pairs

A series of heterogeneous catalysts with Lewis and Bronsted basic sites, and acid–base bifunctional pairs has been used in order to perform organic reactions. By changing the chemical composition and activation conditions it is possible to have predominantly Lewis or Bronsted base catalysts within a large range of well defined basicities. This allows to select the most appropriate catalyst for a specific reaction. Thus, MgO, calcined hydrotalcites, rehydrated hydrotalcites and grafted quaternary organic ammonium hydroxides on MCM-41, have been used as catalysts in Knoevenagel condensation, aldolization and Michael additions. Catalysts containing mild acid–base pairs such as those existing in amorphous aluminophosphates (ALPOs) allow to achieve high selectivities with still very reasonable activities.

Synthesis of Pseudoionones by Acid and Base Solid Catalysts

The preparation of pseudoionones by aldol condensation reaction between citral and acetone have been carried out in the presence of acid (HY and beta zeolites), an acid–base (amorphous aluminophosphate) and basic catalysts such as an aluminophosphate oxynitride, MgO and different activated hydrotalcites. The results showed that acid or acid–base catalysts were not successful for performing in one pot the preparation of ionones. MgO and calcined hydrotalcites showed excellent activity and selectivity to pseudoionones, with calcined hydrotalcite more selective than MgO. Moreover, the rate of reaction can be improved by activating the hydrotalcite through rehydration. This activation can be successfully done by simply adding the optimum amount of water to the calcined hydrotalcite before reaction. The inhibiting effect of the concentration citral on the catalytic activity of rehydrated hydrotalcites that has been reported to occur at 273 K can be avoided by working at a reaction temperature of 333 K.

Acid–Base Bifunctional Catalysts for the Preparation of Fine Chemicals: Synthesis of Jasminaldehyde

Jasminaldehyde was prepared by condensation between benzaldehyde and heptanal. Large-pore acid zeolites (HY and Beta), mesoporous aluminosilicate (Al MCM-41), and amorphous aluminophospates (ALPO) were used as catalysts. The results indicated that zeolites showed lower activity and selectivity than mesoporous aluminosilicate (Al MCM-41). These results were attributed to the confinement effects of the reactants and products inside of the voids of the microporous materials, which lead to the preferential formation of the heptanal self-condensation product, as well as to a fast deactivation of the catalyst. However, the amorphous aluminophosphate (ALPO) which possesses weaker acid sites than zeolites and MCM-41, but combines acidic and basic sites, showed the maximum activity and selectivity to jasminaldehyde. This finding was explained on the basis of the acid–base bifunctional character of the ALPO catalyst. The role of the weak acid sites is to activate benzaldehyde by protonation of the carbonyl group, favoring then the attack of the enolate heptanal intermediate generated on the relatively weak basic sites of ALPO.

Surface Stability of Amorphous Aluminophosphate Oxynitrides (AIPON)

Surface Stability of Amorphous Aluminophosphate Oxynitrides (AIPON)

The Short-Range Structure of Aluminophosphate Oxynitride Catalysts. An ab Initio and Experimental Study

In this study we combine spectroscopic data (diffuse reflectance infrared Fourier transform spectroscopy, DRIFTS, and X-ray photoelectron spectroscopy, XPS) with ab initio SCF calculations on model systems in order to get insight into the short-range order of amorphous aluminophosphate oxynitride (AlPON) catalysts. The use of fixed P/Al = 1/1 atomic ratio and varying N/O atomic ratio in the models, allowed us to consider stoichiometries with nitrogen contents in the range 0−20 wt %, similar to those found experimentally. The observed trends in computed Koopman ionization potentials (IPs), vibrational frequencies, and thermodynamic stability are compared with experimental XPS and DRIFTS data and available information of aluminophosphate and related systems. They are used to propose a structural model for the short-range structure of amorphous aluminophosphate oxynitrides. From the XPS data it can be concluded that first nitrogen enters the phosphorus coordination sphere; this results in the formation step ...

Characterisation, surface hydrolysis and nitrogen stability in aluminophosphate oxynitride (AlPON) catalysts

Solid amorphous aluminophosphate oxynitride (AlPON) catalysts with a nitrogen content ranging from 0% up to 20% N (w/w) have been obtained by reaction between AlPO 4 and NH 3 at 800°C. Samples are analysed by X-ray photoelectron (XPS) and diffuse reflectance Fourier transform infrared (DRIFTS) spectroscopies, mass spectrometry (MS) and thermogravimetry (TG). Similar to that of AlPO 4, AlPON local structure is pictured as a network of PO 4 and AlO 4 tetrahedra in which nitrogen preferentially replaces oxygen bonded to phosphorous, although some contribution from Al–N bonds in an AlON phase is also proposed. Nitridation stages yielding [PO 3N] and [PO 2N 2] building units are characterised below 7.2% N and above 11% N (w/w), respectively. Also terminal –PNH 2 groups are detected from early nitridation stages. Freshly prepared AlPON catalysts readily undergo hydration (10–15% by weight) and hydrolysis at atmospheric conditions. Hydration is more intense on low nitrogen containing samples and sharply decreases above 7.2% N (w/w). Ammonia is the product of both surface hydrolysis and bulk decomposition. Hydrolysis is almost independent on nitrogen content while bulk decomposition grows parallel to nitrogen percentage. Bulk decomposition below 500°C is more intense than hydrolysis and always involves terminal –PNH 2 reaction either as a condensation around 230–240°C or through a reaction with water around 320–340°C. Condensation depends on –PNH 2 coverage and is the preferred decomposition mechanism in high nitrogen containing AlPON.

Magic angle spinning NMR investigations on amorphous aluminophosphate oxynitrides

Download options Please wait... Article information DOI https://doi.org/10.1039/A904244J Article type Paper Download Citation Phys. Chem. Chem. Phys., 1999,1, 4493-4499 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Magic angle spinning NMR investigations on amorphous aluminophosphate oxynitrides T. Blasco, A. Corma, L. Fernández, V. Forne′s and R. Guil-López, Phys. Chem. Chem. Phys., 1999, 1, 4493 DOI: 10.1039/A904244J To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Teresa Blasco Avelino Corma Lorenzo Fernández Vicente Forne′s Rut Guil-López

Diffuse reflectance infrared spectra and their relation to the thermal stability of aluminophosphate oxynitrides as a function of nitrogen content

Amorphous aluminophosphate oxynitrides (AlPON) solids with a variable nitrogen content (2.8, 3.6, 7.2, 11, 17.5 and 20%w/w) have been prepared by nitridation of AlPO 4 using gaseous NH 3. Their thermal stability below 500°C is investigated using diffuse reflectance infrared spectroscopy and mass spectrometry. Water and ammonia, coming from AlPON surface hydrolysis upon exposure to air, are desorbed around 150°C from sites involving Al–OH. At a higher temperature (350°C) terminal –PNH 2 groups are attacked by gaseous water to produce NH 3 and additional P–OH in tetrahedral co-ordination. There is a simultaneous surface reaction between –PNH 2 groups and –PO generating P–OH (tet) but no NH 3. Besides, highly nitrided AlPON samples suffers –PNH 2 condensation around 250°C to yield gaseous NH 3 in a process thermodynamically favoured but controlled by the coverage of –PNH 2 groups.

Study of aluminophosphate oxynitride (AlPON) materials by X-ray photoelectron (XPS) and diffuse reflectance Fourier transform IR spectroscopy (DRIFTS)

A series of amorphous aluminophosphate oxynitrides (AlPON) have been obtained by nitridation of AlPO4 in a tubular furnace at 750 °C under a dry NH3 stream. Bulk nitrogen content is controlled by the reaction time and quantified by alkaline digestion with molten KOH. Samples have been analysed by X-ray photoelectron spectroscopy (XPS) and diffuse reflectance IR spectroscopy (DRIFTS). XPS results indicate that nitrogen selectively replaces oxygen atoms bonded to phosphorus in the [PO4 ] tetrahedra of AlPO4 leading to a mixture of [POxN4–x] units in AlPON. Simultaneously, an Al2O3-type phase develops. Compositions obtained by XPS are in good agreement with those expected from bulk formulation. Nitridation reaction starts at the surface of AlPO4 by breaking P–O bonds and generating terminal P–NH2 . Once surface P–NH2 saturation is achieved (above 3.6 mass% N), bulk nitridation occurs. In nitrogen saturated AlPON samples (ca. 20 mass% N) the presence of a surface P3N5 phase is proposed to be responsible for the slight surface nitrogen enrichment observed. The presence of structural (–NH–) groups, isoelectronic to (–O–), and their growing concentration with nitridation time have been detected by DRIFTS. Diffuse reflectance IR spectroscopy also suggests the presence of cyclic (P–N–P) structures.

Geometric and Electronic Structure of Amorphous Aluminophosphates. Ab Initio and Experimental Studies

A combination of experimental (diffuse reflectance infrared Fourier transform spectroscopy, DRIFTS, and X-ray photoelectron spectroscopy, XPS) and ab initio studies of model clusters is used to understand the geometric and electronic structure of aluminophosphate (AlPO) systems used as catalytic materials. The presence of an intense IR band in the DRIFTS spectra, around 1300 cm-1, together with literature data suggested to represent the system using cluster models based on metaphosphate-like structures. By varying the P/Al atomic ratio the basic features of AlPO4−Al2O3 catalysts are modeled. The calculated geometrical parameters (bond distances and associated stretch force constants) are discussed in relation to the structure of the catalyst and, despite the inherent approximations in modeling solids with cluster models, fits quite well-the experimental X-ray data for aluminum metaphosphate. The computed ionization potentials (IP), OKVV transitions, and IR spectra are in reasonable agreement with experime...

Aluminophosphates Oxynitrides as Base Catalysts: Nature of the Base Sites and Their Catalytic Implications

By means of IR spectroscopy it has been found that Al–NH–P and P–NH2groups are formed during the high temperature NH3treatment of amorphous aluminophosphates (ALPONs). The amount of nitrogen incorporated increases with time of exposure to NH3, while also increasing the ratio of the more basic –NH– with respect to the less basic –NH2groups. This is reflected in the catalytic activities of these materials for the Knoevenagel condensation reaction using the activated methylenic groups of different pKavalues as reactants. ALPON materials have stronger basic sites than alkaline exchanged Y zeolites and weaker basic sites than MgO and Hydrotalcites. However, in the case of Knoevenagel condensation reactions the ALPONs present a very adequate basicity, especially when the competing Michael addition can also occur. In such cases the selectivities of the oxynitrides are better that those showed by MgO and hydrotalcites.

Surface basicity of a new family of catalysts: aluminophosphate oxynitride (ALPON)

A new family of basic catalysts, amorphous aluminophosphate oxynitrides (AIPONs) with variable nitrogen content, have been synthesized by nitridation of amorphous AlPO4(AIPO). The samples were analysed by X-ray photoelectron spectroscopy (XPS) and diffuse reflectance IR spectroscopy (DRIFTS). XPS indicates that nitrogen is preferentially bonded to phosphorus in the AlPON framework. Residual surface NH4+ ions coming from the nitridation reaction were used to evaluate the Brønsted acidity of these new catalysts. The decrease in the Brønsted acidity along the series is parallel to a shift of the P—O stretching to lower wavenumbers. Malonitrile–benzaldehyde condensation was employed as a test reaction to estimate the basicity of AIPON samples. The conversion trend observed also correlates with the decrease in the frequency of the P—O stretching. Consequently, the position of the P—O stretching reflects both the reduction of the Brønsted acidity and the enhancement of the basicity of the AlPON specimen.

Structure–property relationships of some amorphous and crystalline aluminophosphates

Precipitation of aluminophosphates was carried out over a wide range of alkalinity (pH 4.5–8.5) and P : Al ratios (0.17–1.02). X-Ray diffraction (XRD) showed that for P : Al < 1 the materials were completely amorphous whereas at P : Al = 1.02 they were crystalline, although the crystallinity decreased markedly with increasing pH. A large increase in porosity occurred as P : Al increased above 0.17 accompanied by major changes in 27Al and 31P magic-angle spinning (MAS) NMR spectra. A strong correlation exists between the 31P chemical shift and the pore-size distribution. Catalytic activity was probed via, dehydrogenation and dehydration of n-butanol. Dehydrogenation selectivity is maximal at P : Al = 1.02 whereas the overall catalytic activity correlates with total pore volume and is maximal at P : Al = 0.66. NMR, XRD and studies of catalytic selectivity confirm the absence of separate alumina and aluminium phosphate phases in these aluminophosphates.