Characterization of Chromium Cations in CrAPO-5 Metal Aluminophosphate

Abstract

Chromium-substituted aluminophosphate (CrAPO-5) with the AFI crystal structure was prepared for the first time by using microwave conditions at the stage of gel formation and crystallization. This approach allowed the reduction of the time required for the synthesis of CrAPO-5 from 60–70 h to 6 h. CrAPO-5 metal aluminophosphate prepared by microwave-assisted synthesis is studied by UV-visible and IR spectroscopy and X-ray photoelectron spectroscopy. The material is characterized by the presence of Cr³⁺ and Cr²⁺ ions in the framework with predominating Cr³⁺ ions introduced via isomorphous substitution, as well as some minor amounts of extra-framework Cr³⁺ species, which are present presumably in the state of α-Cr₂O₃. The latter species can be partially reduced to Cr²⁺ species in the presence of CO or H₂. XPS study of CrAPO-5 revealed the presence of Cr³⁺ ions in the framework. A TPR experiment showed that the reduction of chromium starts at about 450–500 °C.

1. Introduction

Metal aluminophosphates (MeAPOs) remain at the forefront of zeolite-like materials that find application in diverse catalytic processes with a special interest being placed on selective oxidation [1,2,3]. These materials provide quite wide opportunities for the isomorphous substitution of both aluminum and phosphorus for various metal ions, as well as silicon. It is the isomorphous substitution that makes it possible to design the materials with controllable acid-base and redox properties. In spite of the abundant literature on MeAPOs, the state of transition-metal ions in such materials as CrAPO, MnAPO, CoAPO, etc., remains disputable. The uncertainty in determining the oxidation and coordination state of such isomorphously substituted metal ions is related to the possibilities of their stabilization in various oxidation states and in different sites of the AlPO framework. Furthermore, the state of the framework metal ions may change in the presence of polar molecules and solvents, like water. Finally, some of the transition-metal ions introduced during the synthesis may form diverse non-framework species that may also be quite active and selective in the redox catalytic processes.

Cr-APOs, including CrAPO-5 (the AFI structure according to International Zeolite Association classification) are among the quite-well-studied MeAPOs [4,5,6,7]. CrAPO-5 materials are studied in a few oxidation-type reactions with either oxygen (air) or organic peroxides serving as oxidizing agents. Among the substrates and reactions chosen for the study of the catalytic properties of CrAPO-5, oxidation of tetraline, which can be easily oxidized with the participation of chromium ions into 1-tetralone; one-step cyclohexane nitrosation to ε-caprolactam; oxidation of oleic acid; or hydrocracking of heavy hydrocarbons [8,9,10,11,12,13,14,15] should be noted. The state of Cr⁺ⁿ species in CrAPO-5 was studied with different sensitive methods, including ESR, UV-visible and IR spectroscopy, and XPS in combination with precise XRD analysis of the samples obtained. Chromium ions in either octahedral or tetrahedral or distorted tetrahedral coordination can be identified by UV-visible spectroscopy. However, the existence of such species, as well as other potentially possible moieties, like Cr⁴⁺ or even Cr⁵⁺ under different conditions (reducing or oxidizing atmospheres, polar solvents, different temperatures of pretreatments) requires a more detailed characterization with a combination of different spectroscopic methods.

In this work, we studied CrAPO-5 with diffuse-reflectance UV-visible and IR spectroscopy in order to reveal the state of different Cr species present in the material and their mutual inter-conversion. Two hypotheses were checked by performing this research: (a) whether well-crystallized CrAPO-5 material could be synthesized by using the microwave technique and (b) what oxidation and coordination states of chromium ions we can expect in the microwave synthesis of CrAPO-5.

2. Materials and Methods

CrAPO-5 was synthesized from a Cr³⁺ precursor under microwave activation of the reaction mixture according to the procedure described in our previous work [16]. Tetraethylammonium hydroxide with added n-dipropylamine was used as an organic structure-directing agent. Aluminum chlorhydrol and ortho-phosphoric acid were used as sources of aluminum and phosphorus, whereas chromium acetate hydroxide (Cr₃(OH)₂(CH₃COO)₇ was used as a source of chromium. The chromium salt (1.4 g) was added to 29 g of water and then 15.4 g of a solution of H₃PO₄ (85 wt% aqueous solution) was added under stirring until complete dissolution of the salt. After that, aluminum chlorhydrol (29.3 g) was added under stirring, followed by the dropwise addition of 12.3 g of a 40% aqueous solution of tetraethylammonium hydroxide with added 13.5 g of n-dipropylamine. The obtained gel was stirred for 1 h and then transferred to a Teflon-lined vessel of 50 mL, which was placed in the chamber of an Anton Paar Multiwave PRO device (Anton Paar GmbH, Graz, Austria). The microwave-assisted hydrothermal synthesis was carried out at 463 K for 6 h. After the synthesis, the CrAPO-5 material was washed five times with distilled water and dried at 333 K.

X-ray diffraction (XRD) patterns were obtained using a DRON-2 diffractometer (Moscow, Russia) (CuKα radiation, λ = 0.1542 nm). The samples were scanned in the 2θ range 5–55° at a rate of 1 deg/min.

XP spectra were measured using a PHI5000VersaProbeII spectrometer (ULVAC-PHI, Inc., Kanagawa, Japan). The source was Al Kα irradiation (hν = 1486.6 eV), and the power was 50 W. The powdered sample was pressed in foil. The spectra in the regions of Al 2p, P 2p were taken at an analyzer transmission energy of 23.5 eV and sweep rate of 0.2 eV/step. The Cr 2p spectrum was taken at an analyzer transmission energy of 29.35 eV and a sweep rate of 0.25 eV/step. The calibration of the E_b scale was carried out according to the lines of Au 4f—83.96 eV and Cu 2p³—932.62 eV.

TPR-H₂ studies were carried out in a half-automatized flow system with a water trap cooled down to −100 °C. The detector (TCD) was calibrated by reduction of CuO (Aldrich-Chemie GmbH, 99%). Prior to the measurements, the samples were pretreated in an Ar flow with a gas flow rate of 30 mL min⁻¹ at 300 °C. Then, the catalysts were heated to 850 °C at the rate of 10 °C/min in a 5% H₂-Ar gas mixture.

Diffuse-reflectance IR spectra were measured using a Perkin Elmer 580B spectrophotometer (Shelton, CT, USA). The UV-visible spectra were registered with a Hitachi M-340 spectrometer (Tokyo, Japan) supplied with an integrating sphere for the measurements of diffuse reflectance spectra. Before the spectroscopic studies, the sample was subject to either a thermal vacuum treatment or treatment in the presence of O₂ or H₂ at different temperatures in special quartz in situ cells that allowed measurements of diffuse-reflectance IR, UV-visible, and ESR spectra, as well as adsorption of measurable amounts of gases.

3. Results and Discussion

It should be noted that the MW-assisted synthesis of CrAPO-5 was not reported so far. The well-known hydrothermal synthesis of CrAPO-5 reported by Flanigen et al. [17] took 68 h. Unlike the conventional thermal synthesis, the MW-assisted synthesis provided a decrease in the crystallization time from 60–70 h to 6 h, i.e., by 10 times.

XRD analysis (Figure 1) shows the presence of a single AFI crystal phase, with microcrystals representing spherical particles with an average diameter of 20 μm. The pore volume determined from the amount of n-butane adsorbed at room temperature was equal to 0.114 cm³/g. The content of chromium in the sample was 1 wt%.

The sample of CrAPO-5 was first studied with TPR and XPS methods. The TPR curve of this sample (Figure 2) shows the reduction of chromium at very high temperatures (over 500 °C), presumably to the Cr²⁺ state with possible further reduction to Cr⁰ at T > 850 °C).

XPS study of CrAPO-5 (Figure 3) revealed the presence of Cr³⁺ ions in the framework. The Al 2p spectrum contains two peaks: at 74.0 eV and 75.5 eV. According to the reference data, the first peak can be attributed to AlPO₄ phosphate, and the second peak is in the region of the binding energies of Al_0.52P_0.48O_2.2: Al 2p—75.5 eV, P 2p—135.0 eV, and O 1s—532.9 eV. The spectrum also contains a doublet of P 2p at 133.8 eV and 134.7 eV. Both doublets are in the region of phosphate binding energies. In the O 1s oxygen spectrum, a low-intensity peak of 530.6 eV is isolated, and the main peak is located at 532.5 eV. The peaks in the spectrum in the region of the Cr 2p doublet are superimposed on the second satellite, the O 1s spectrum, and the intensity is very low. Figure 2 shows the Cr 2p spectrum, which corresponds to Cr³⁺ species in the oxide framework.

The detailed analysis of the CrAPO-5 sample was made by using diffuse-reflectance IR and UV-visible spectroscopy. The as-synthesized dried (100 °C) sample of CrAPO-5 of a pale-green color contains a template, Pr₃N, and water molecules, which is visualized as intense absorption in the region of O-H (3400–3800 cm⁻¹) and C-H (2800–3000 cm⁻¹) vibrations in the IR spectrum (Figure 4a). Prolonged evacuation of the as-synthesized sample at 100 °C results in a significant decrease in the intensities of the bands assigned to physically adsorbed water in the region of O-H stretching vibrations in the IR spectrum of the sample (Figure 4a). An increase in the evacuation temperature to 200 °C and further to 350 and 470 °C leads to a continuous desorption of adsorbed water. The IR spectrum of the sample pre-evacuated at 200 °C is presented in Figure 4b. The sharp lines at 3790 and 3680 cm⁻¹ can be assigned to the stretching vibrations of the O-H bonds in Al-OH and P-OH groups of the aluminophosphate framework, respectively, whereas the bands in the range of 3300–2700 cm⁻¹ belong to the template. A further temperature rise under evacuation until 350 °C causes a complete removal of water from the aluminophosphate. This is also confirmed by the absence of the overtones of OH groups (the region of 7000–7400 cm⁻¹) and combination (O-H stretching + out-of-plane bending) bands (the region of 4300–4700 cm⁻¹), as well as the maximum intensity of the band of P-OH groups at 3680 cm⁻¹ (Figure 4c), which were hydrogen-bonded at lower temperatures of the vacuum pretreatment and were not observed as isolated lines. The further increase in the evacuation temperature does not result in any increase in the intensity of the band of P-OH groups. It follows from Figure 4c that a part of the template molecules is still present in the channels of the aluminophosphate framework after such a rather severe pretreatment. The complete removal of the template from zeolite-like materials occurs, as a rule, at high temperatures of treatment in the presence of oxygen. Figure 4d shows the IR spectrum of the CrAPO-5 sample pretreated at 475 °C in an atmosphere of dry oxygen. It is seen that the bands of C-H (2800–3000 cm⁻¹) and N-H (3200–3400 cm⁻¹) stretching vibrations vanish from the spectrum, which provides evidence for the complete removal of the template.

From the IR spectra, we may assume that chromium cations have a maximum coordination number of six and thus they may exhibit an octahedral coordination and demonstrate characteristic electron transitions in UV-visible spectra. The UV-visible spectra of CrAPO-5 after thermal treatments in O₂ for 2 h in a vacuum at 100 °C, 200 °C, 350 °C, and 475 °C are presented in Figure 5a–d. Analysis of the absorption bands at 22,700 cm⁻¹, 16,130 cm⁻¹, and an unresolved maximum at 38,000 cm⁻¹ in the UV spectrum of CrAPO-5 heated at 100 °C (Figure 5a) allows us to suppose that at least a part of chromium is present in the oxidation state Cr³⁺ (d³) [18], as far as a Cr³⁺ precursor was used in the CrAPO-5 synthesis [17]. Additionally, octahedral complexes of Cr²⁺ cations (d⁴) characterized by only one transition ⁵T_2g–⁵E_2g with the maximum at 14,000 cm⁻¹ [19] may contribute to the absorption around 16,000 cm⁻¹, and their presence in the sample cannot be ruled out.

It is commonly accepted that the isomorphous substitution of Al³⁺ ions for divalent cations in the aluminophosphate matrix requires the presence of an extra proton to compensate the arising negative charge of the framework. Such protons may produce bridged hydroxyl groups Me-(OH)-P possessing Broensted acidic properties. Figure 4d presents the IR spectrum of the CrAPO-5 sample after the high-temperature vacuum pretreatment (470 °C). It should be noted that the spectrum does not change, no matter whether the oxidative or reductive (CO or H₂) treatment was carried out. The spectrum contains the only narrow band at 3680 cm⁻¹ that can be assigned to P-O-H stretching vibrations. No bands with lower frequencies that might be ascribed to bridged Me-(OH)-P groups are observed in the spectrum.

It is noteworthy that the UV-visible spectrum of the sample after evacuation at 100 °C does not change compared to the spectrum of the as-synthesized sample (i.e., evacuated at 20 °C). An increase in the evacuation temperature to 200 °C leads not only to a continuous desorption of adsorbed water (see IR spectra shown in Figure 4b) but also to a partial removal of water molecules from the coordination sphere of the chromium cations. As a result, the symmetry and the strength of the ligand field at the chromium cations decrease, and the UV-visible spectrum of the electron transitions changes (Figure 5b). The position of the maximum of the high-frequency band attributed presumably to the ⁴T_1g–⁴A_2g transition in Cr³⁺ is shifted towards lower frequencies to 21,740 cm⁻¹. The maximum of the low-frequency absorption band that can be ascribed both to the ⁴T_2g–⁴A_2g transition in Cr³⁺ and to the ⁵T_2g–⁵E_2g transition in Cr²⁺ is also shifted to lower frequencies down to 15,300 cm⁻¹. It should be noted that, unlike the UV-visible spectrum of the as-synthesized sample, the spectrum presented in Figure 5b is characterized by a lower intensity of the low-frequency absorption band compared to the intensity of the high-frequency absorption band. This effect can be explained by the much smaller contribution of Cr²⁺ cations in the intensity of the band at 15,300 cm⁻¹ as a result of decreasing coordination number due to the loss of water molecules from the coordination sphere of these ions. However, an absorption band at 9670 cm⁻¹ may be ascribed to electron transitions of Cr²⁺ cations with partially removed ligands in the coordination sphere. Similarly, these Cr²⁺ cations with reduced coordination numbers due to the substitution of water molecules for framework oxygen anions may be responsible for a low-intensity absorption band in the region of charge-transfer bands at around 30,000 cm⁻¹. The absorption band with about the same maximum was reported for Cr²⁺/NaY zeolite [20].

Figure 5c depicts the UV-visible spectrum of the CrAPO-5 sample after the vacuum treatment at 350 °C. The band at 14,500 cm⁻¹ that can be assigned to the tetrahedrally coordinated Cr³⁺ cations predominates in the spectrum. The low-frequency band at 8350 cm⁻¹ is most likely attributed to Cr²⁺ cations in a tetrahedral coordination. This assignment can be performed on the basis of the analysis of the literature data [18,19,20,21,22]. It follows from this assignment that complexes of Cr²⁺ cations with coordination numbers 5 or 4 may quite often exhibit electron transitions in the region of frequencies below 10,000 cm⁻¹, whereas the low-frequency transitions are less probable for Cr³⁺ cations, although they cannot be completely forbidden. The bands in the region of charge-transfer bands with maxima at about 37,000 cm⁻¹ and a shoulder at about 30,000 cm⁻¹ are not characteristic and can be ascribed equally to Cr³⁺ and Cr²⁺ cations. The appearance of absorption in the region of charge-transfer bands results from the decrease in the difference between the positions of the non-bonding orbital of the ligand and the highest loosened orbital of a chromium ion, which participates in the charge transfer. The decrease in this difference may be a result of the weakening of the ligand field due to the reduction in the number of ligands after the vacuum treatment at elevated temperatures or due to the decreasing ionization potential of one of the ligands caused by the substitution of one ligand (H₂O) by another ligand (O²⁻ ions of the framework). Some absorption in the region of 21,000–17,000 cm⁻¹ in the UV-visible spectrum (Figure 5c) is likely attributed to chromium cations interacting with template molecules or products of template decomposition, with Cr ions being in a coordination intermediate between the octahedral and tetrahedral ones.

Figure 5d presents the UV-visible spectrum of the sample evacuated at 470 °C after the preliminary thermal treatment in O₂ at the same temperature. Analysis of this spectrum shows that the complete decomposition of the template results in only slight shifts of the bands previously assigned to tetrahedrally coordinated Cr ions towards lower frequencies, i.e., to 14,400 and 8280 cm⁻¹. The UV-visible spectrum of the sample pretreated in O₂ at 470 °C also exhibits additional absorption bands at about 21,500 and 28,570 cm⁻¹ that are close to the bands observed for α-Cr₂O₃. These data allow us to assume that a part of chromium ions that are not incorporated in the framework of the CrAPO-5 zeolite-like material may form extra-framework oxygen-containing species similar in their structure and composition to Cr₂O₃.

It is known that transition-metal ions that substitute Al or P atoms in the aluminophosphate framework may change their oxidation state. This was clearly demonstrated for cobalt [5]. Therefore, we tried to study in this work the possibility of the transition of chromium cations from one oxidation state to another one under the action of external factors. Figure 6a shows the UV-visible spectrum of CrAPO-5 calcined at 400 °C in O₂ and then pretreated in a vacuum in the presence of CO at 400 °C. Figure 6b presents the spectrum of electron transitions of the same sample that was oxidized again at 400 °C. The comparison of these two spectra demonstrates that the reductive treatment (CO) does not affect the position of the maxima of bands at 14,400 cm⁻¹ and 8260 cm⁻¹ nor the intensity of these absorption bands that belong to tetrahedrally coordinated Cr³⁺ cations. The resistance of these states to redox treatments confirms the earlier made assumption that the isomorphous substitution in the rigid aluminophosphate framework increases the stability of the chromium cations.

Nevertheless, the appearance of the absorption band at 2350 cm⁻¹ in the IR spectrum of the sample treated in CO, which is assigned to CO₂ molecules (the IR spectrum is not presented here), indicates the occurrence of the reduction process. In parallel, the bands of charge-transfer complexes vanish in the UV-visible spectra of the CO-treated material. We may assume that the extra-framework chromium cations lose a part of oxygen atoms in their environment in the course of reduction with CO. The change in the state of a part of the extra-framework chromium cations after reduction is confirmed also by the appearance of a new band in the IR spectrum of adsorbed CO (20 °C) at 2195 cm⁻¹ that can be ascribed to CO complexes with low-coordinated chromium ions.

Similar results were obtained upon reduction of the oxidized CrAPO-5 in a flow of dry H₂ at 400–500 °C with the only difference that, unlike reduction with CO, the treatment in H₂ does not lead to the complete disappearance of the bands at 21,500 and 28,570 cm⁻¹ in the UV-visible spectrum. It should be noted that extra-framework chromium cations in the reduced CrAPO-5 sample can be easily reoxidized with molecular oxygen even at room temperature. The ESR spectrum of such reoxidized material demonstrates a characteristic signal of oxygen, whereas the bands at 21,500 cm⁻¹ and 28,570 cm⁻¹ are completely restored in the UV-visible spectrum.

It follows from the analysis of the spectroscopic data that the only experimental proof of the presence of isomorphously substituted Cr²⁺ cations in the framework of CrAPO-5 together with Cr³⁺ ions is the observation of the absorption band at 8260 cm⁻¹ in the UV-visible spectrum. These data, however, do not exclude the existence of two different coordination states of Cr³⁺ in the framework of CrAPO-5.

It should be noted that difficulties with observation and identification of Cr²⁺ species in heterogeneous catalysts are known from the literature. Diverse approaches have been applied to reveal these cations. For instance, Zecchina et al. [21] reported that the UV-visible spectrum of a reduced Cr/SiO₂ catalyst becomes significantly easier to interpret in the case of ammonia adsorption resulting in the interaction of Cr²⁺ and Cr³⁺ ions existing in different coordination states at the catalyst surface with ammonia with the formation of octahedral complexes visualized as absorption bands at 23,000 and 17,000 cm⁻¹, respectively. Analysis of the positions and intensity ratios of the bands in the observed UV-visible spectrum allowed the authors of [21] to conclude the presence of Cr²⁺ ions in the Cr/SiO₂ catalysts.

Figure 7a presents the UV-visible spectrum of the reduced CrAPO-5 sample after its saturation with ammonia at room temperature. The band maxima at 23,600 cm⁻¹ and 16,700 cm⁻¹ and their intensity ratio in this spectrum are very much similar to the spectrum reported in [21], which makes us come to the same conclusion about the presence of Cr²⁺ ions in CrAPO-5. However, one should keep in mind that the Cr/SiO₂ system is somewhat less complicated compared to the CrAPO-5 material because the latter contains a part of the chromium ions incorporated in the framework during the synthesis via isomorphous substitution, whereas the other part is present as extra-framework CrO_x species, possibly, similar to supported chromium-containing systems. Figure 7b shows the UV-visible spectrum of the reduced CrAPO-5 sample after adsorption of water at room temperature. Similarly to the spectrum presented in Figure 7a, it consists of two absorption bands at 21,800 cm⁻¹ and 16,000 cm⁻¹. The band at 21,800 cm⁻¹ and perhaps a part of the high-frequency tail of the band at 16,000 cm⁻¹ can be ascribed to octahedral complexes of Cr³⁺. Hexaaqua complexes of Cr²⁺ (d⁴) are known to be characterized by only one band around 15,000 cm⁻¹ [22]; therefore, Cr²⁺ cations may also contribute to the absorption with the maximum at 16,000 cm⁻¹ in the spectrum presented in Figure 7b.

Figure 7c depicts the spectrum of the oxidized CrAPO-5 sample after its saturation with water at room temperature. As it has been already noted, extra-framework chromium cations after the high-temperature treatment of the sample in oxygen produce moieties similar to α-Cr₂O₃. Therefore, if the oxidized CrAPO-5 material contains Cr²⁺ ions, they must be present as isomorphously substituted ions in the framework and the band at 8280 cm⁻¹ characteristic of these species should be present in the UV-visible spectrum. Comparison of the spectra presented in Figure 7b,c shows that the maximum of the band corresponding to electron transitions in octahedral complexes of Cr³⁺ in the oxidized sample of CrAPO-5 is shifted towards the value of 22,700 cm⁻¹. This value is closer to the value of 23,000 cm⁻¹ reported in [22] for hexaaquacomplexes than the value of 21,800 cm⁻¹ observed for the reduced CrAPO-5 (Figure 7b). We cannot exclude that the absorption with the maximum around 21,800 cm⁻¹ is also present in the spectrum shown in Figure 7c; however, it is not resolved as a separate band in the spectrum due to its lower intensity and poor resolution. Indeed, the CrAPO-5 sample contains two types of Cr³⁺ cations. Cations of one type are incorporated into the framework via isomorphous substitution and therefore their state does not depend on the type of the pretreatment (oxidative or reductive). These cations form Cr³⁺(O²⁻_framework)₄(H₂O)₂ complexes that are characterized by the absorption band in the region of 23,000–21,700 cm⁻¹ with the constant intensity. The other coordination state and the number of extra-framework Cr³⁺ cations depend substantially on the conditions of the pretreatment. Thus, it is the variation in the states of extra-framework Cr³⁺ cations that results in a considerable dependence of the position of the maximum of the overlapped band corresponding to the octahedral complexes of Cr³⁺ cations. It should be noted that there is a not quite understandable increase in the intensity of the band at 28,570 cm⁻¹ in the spectrum of the oxidized sample after adsorption of H₂O at room temperature (Figure 7c).

Let us consider now the absorption in the region of 16,500–14,500 cm⁻¹, which is characteristic for the transitions of the octahedral complexes of both Cr³⁺ and Cr²⁺ ions. It is noteworthy that both the maximum and the intensity of this band do not change for the oxidized and reduced CrAPO-5 samples (Figure 6 and Figure 7). The following conclusions can be drawn from this observation:

First, octahedral complexes of Cr³⁺ contribute insignificantly to the overall absorption in the region of 16,500–14,500 cm⁻¹, since we do not observe any variation of the intensity of this band when a substantial change in the frequency of the band at 23,000–21,800 cm⁻¹ is observed after different pretreatments. This conclusion is in agreement with the data presented by other authors [18,19,20,21].

Second, octahedral complexes of extra-framework Cr²⁺ ions also do not make any contribution to the absorption in the region of 16,500–14,500 cm⁻¹.

Third, once other contributions have been declined, obviously the absorption in the region of 16,500–14,500 cm⁻¹ should be ascribed to isomorphously substituted Cr²⁺ ions in the framework of the Cr-APO material.

The chromium ions in the framework can flexibly change their coordination state, while remaining (at least up to 450 °C) in the Cr³⁺ oxidation state. This makes them active sites of a number of oxidation reactions. Presumably, when organic peroxides are used as oxidants, the peroxide molecules may enter the coordination sphere of chromium ions with the transformation of the tetrahedra into a penta-coordinated state or octahedra, each of these coordination polyhedra may be distorted. The tetrahedral chromium ions are able to form complexes with strong bases like water and ammonia due to additional coordination, for instance, via transformation of tetrahedra into distorted trigonal bipyramids or octahedra. These data are in agreement with results obtained by Zadorna et al. [23], who showed that CrAPO-5 materials contain predominantly Cr³⁺ ions stably incorporated into framework positions and shall remain in the same oxidation state in a pseudotetrahedral coordination sphere in the framework, whereas two additional water ligands convert this coordination into the favored (pseudo)octahedral one.

The possibility of incorporation of transition metal ions into the frameworks of zeolite-like aluminophosphates via the isomorphous substitution mechanism has been discussed over almost three decades. With iron and titanium, it was demonstrated unambiguously that these ions can substitute Al or Si in the framework. As to other metals, no clear answer was found. The recent data obtained by different structure-sensitive methods demonstrate that, in addition to iron and titanium, other transition metal ions with crystal or ionic radii over 0.55 nm that are larger than the radii of Al³⁺, Si⁴⁺, or P⁵⁺ ions (crystal radii 0.31–0.53 nm, ionic radii 0.17–0.39 nm) can be introduced in the frameworks of pure silica, silica-alumina, or aluminophosphate zeolite-like compositions [24,25,26]. This diversity of the structures and compositions provides a wide range of applications in catalytic reactions, not only of the acid-base type, but also of redox nature. Recently mesoporous metal aluminophosphates (with cobalt ions) were successfully prepared and demonstrated activity in selective oxidation of cyclohexane to cyclohexanol with hydrogen peroxide as an oxidant [27]. Isomorphously substituted AlPOs also found application in hybrid ultrafiltration membranes, where Co-substituted AlPO-5 demonstrated outstanding performance in ultrafiltration of water from biomolecules [28]. MeAPOs can also be used as templates for production of carbon nanotubes and membranes on their basis for hydrogen separation [29].

4. Conclusions

Thus, the CrAPO-5 material is prepared successfully for the first time by using a microwave-assisted method of synthesis, which allows one to significantly decrease the duration of the hydrothermal synthesis from 60–70 h to about 1 h. This zeolite-like material with the AFI structure type is characterized by the presence of Cr³⁺ and Cr²⁺ ions in the framework with predominating Cr³⁺ ions, as well some minor amounts of extra-framework Cr³⁺ species, presumably in the state of α-Cr₂O₃ that can be partially reduced to Cr²⁺ species in the presence of CO or H₂. The framework Cr³⁺ cations are stable and incorporated into the framework via isomorphous substitution and therefore their state does not depend on the type of the pretreatment (oxidative or reductive). These cations form octahedral Cr³⁺(O²⁻_framework)₄(H₂O)₂ complexes upon water adsorption that are characterized by the absorption band in the region of 23,000–21,700 cm⁻¹. The other coordination state and the number of extra-framework Cr³⁺ cations depend substantially on the conditions of the pretreatment. The chromium ions in the framework can flexibly change their coordination state, while remaining (at least up to 450 °C) in the Cr³⁺ oxidation state. This makes them active sites of a number of oxidation reactions. Presumably, when organic peroxides are used as oxidants, the peroxide molecules may enter the coordination sphere of chromium ions with the transformation of the tetrahedra into a penta-coordinated state or octahedra, and each of these coordination polyhedra may be distorted. For CrAPO-5, future potential research directions will probably include new oxidation reactions and CO₂ conversion, in particular the reaction of CO₂-assisted oxidative dehydrogenation of propane into propylene and formation of CO as the only by-product that can be used for downstream carbonylation or hydroformylation reactions. CrAPO-5 can be also of interest in the valorization of platform chemicals produced from biomass via a dual-function mechanism involving a combination of partial oxidation (on Cr centers) and Lewis acid-catalyzed rearrangement and C-C bond cleavage.