I. INTRODUCTION
Molybdic acid mixed with aniline and its derivatives in acidic solutions forms polymeric trimolybdates, layered penta- and octamolybdates which can form isolated clusters. The compounds we synthesized have promising catalytic properties and can be applied in numerous industrial processes (Bożek et al., Reference Bożek, Neves, Valente and Łasocha2018). The investigated molybdates are frequently applied in catalysis, medicine and chemistry of new materials. Like many layered or polymeric compounds, molybdates can also be used as ion exchangers. Moreover, they can be applied in the production of valuable catalysts for oxidation processes in organic chemistry and petroleum industry.
Starting from the same reactants, changing the time of the synthesis, or using various agents to obtain an acidic environment, the chemical synthesis can result in new compounds with unexpected compositions and promising properties. The compounds we synthesized and investigated are new, obtained for the first time, and there is a lack of information about them in the chemical and crystallographic databases (neither crystallographic data nor analytical and spectroscopic data are reported).
II. EXPERIMENTAL
A. Sample preparation
1. Synthesis of tetrakis(3-ethylanilinium) octamolybdate Mo8O26(C8H12N)4
Molybdic acid (1.8 g) was dissolved in boiling water (150 ml) under reflux. To this solution, the mixture of 3-ethylaniline (1.3 ml) with acetic acid (50 ml) was added and heated for 2 h, then filtered off and left for crystallization. After 1 day, the obtained crystals (white, very small, not suitable for single-crystal study) were separated, dried in air and investigated by an X-ray powder diffraction (XRPD) technique. Chemical analysis: C 23.15% (calc. 22.98%), H 2.938% (calc. 2.89%) and N 3.36% (calc. 3.35%). The elemental (C, N and O) analysis was performed using a Euro Vector EA 300 for all investigated samples.
2. Synthesis of tetrakis(3-ethylanilinium) octamolybdate tetrahydrate Mo8O26(C8H12N)4·(H2O)4
Molybdic acid (1.8 g) was dissolved in boiling water (150 ml) under reflux. To this solution, the mixture of 3-ethylaniline (1.3 ml) with hydrochloric acid (10 ml of 1 m) was added and heated for 2 h. The obtained precipitate (white, very fine powder) was filtered off, dried in air and investigated by an XRPD technique. Chemical analysis: C 22.13% (calc. 22.03%), H 3.274% (calc. 3.24%) and N 3.24% (calc. 3.21%).
3. Synthesis of bis(3-ethylanilinium) pentamolybdate Mo5O16(C8H12N)2
Molybdic acid (1.8 g) was dissolved in boiling water (150 ml) under reflux. To this solution, the mixture of 3-ethylaniline (1.3 ml) with hydrochloric acid (10 ml of 1 m) was added and heated for 16 h. The obtained precipitate (white, very fine powder) was filtered off, dried in air and investigated by an XRPD technique. Chemical analysis: C 19.56% (calc. 19.61%), H 2.51% (calc. 2.47%) and N 2.84% (calc. 2.86%).
B. XRPD measurements and crystallographic studies
Before the measurements, each sample was thoroughly powdered (using agate mortar and pestle; particle size after grinding ~1 µm) and back-loaded into a sample holder to avoid preferred orientation. The XPRD measurements were performed at the Faculty of Chemistry Jagiellonian University using an X'Pert PRO MPD diffractometer, a diffracted-beam graphite monochromator, and with a PIXcel 1D detector and CuKα radiation (generator setting: 40 kV and 30 mA) at 298 K. The diffraction data were collected over the angular range from 3° to 70° 2θ with a step size of 0.02° (time of diffraction measurement ~1 h). The divergence of the incident X-ray beam was 0.25°.
III. RESULTS AND DISCUSSION
The obtained powder diffraction data were analyzed using the Data Viewer and HighScore – X'Pert PRO diffractometer software (peak search, detection of α 2 lines and phase analysis). To test the purity of the sample (looking for impurity phases, similar compounds, etc.) PDF-4+ (Gates-Rector and Blanton, Reference Gates-Rector and Blanton2019), database was used. Then, the PROSZKI package was used to index the patterns (Lasocha and Lewinski, Reference Lasocha and Lewinski1994). Experimental powder diffraction patterns are depicted in Figure 1. XRPD data of the investigated compounds are shown in Tables I–III. The intensity of the diffraction lines (heights) and their positions were determined using the program of Sonneveld and Visser (Reference Sonneveld and Visser1975). For lattice parameters refinement, the least-squares program by Appleman et al. (Reference Appleman, Evans and Handwerker1966) was used. The crystallographic characteristic and indexing figures of merit (de Wolff, Reference de Wolff1968; Smith and Snyder, Reference Smith and Snyder1979) for all three compounds are reported in Table IV. Figure 2 presents the molecular diagrams of the investigated compounds.
Figure 1. Experimental powder diffraction patterns of the investigated molybdates: tetrakis(3-ethylanilinium) octamolybdate (a), tetrakis(3-ethylanilinium) octamolybdate tetrahydrate (b) and bis(3-ethylanilinium) pentamolybdate (c). Axes in the figure: x – 2θ (°) and y – square root of the number of counts.
Figure 2. Molecular diagram of the investigated compounds.
TABLE I. XPRD data of tetrakis(3-ethylanilinium) ctamolybdate Mo8O26(C8H12N)4.
a I obs is in the range of 1–1000, FWHM for a single, well-resolved line is in the range of 0.11–0.19 (°) 2θ.
TABLE II. XPRD data of tetrakis(3-ethylanilinium) octamolybdate tetrahydrate Mo8O26(C8H12N)4·(H2O)4.
a I obs is in the range of 1–1000, FWHM for a single, well-resolved line is in the range of 0.13–0.22 (°) 2θ.
TABLE III. XPRD data of bis(3-ethylanilinium) pentamolybdate Mo5O16(C8H12N)2.
a I obs is in the range of 1–1000, FWHM for a single, well-resolved line is in the range of 0.12–0.21 (°) 2θ.
TABLE IV. X-ray crystal structure data for the investigated compounds.
Diffraction patterns of polyoxometalates usually contain one (or few) very strong low-angle diffraction lines, while the other lines are weak or very weak which can hinder the phase analysis process, indexing and space group determination. Such patterns are very common in the case of low symmetry, complicated organic or hybrid inorganic–organic materials. The preferred orientation effect (strong in the case of layered compounds measured applying Bragg–Brentano geometry) may additionally increase the problems with the analysis of such compounds by powder diffraction techniques.
IV. CONCLUSION
Molybdic acid reacting with 3-ethylaniline forms different compounds depending on the time of reaction and the acid used to obtain an acidic environment. Interestingly, using 3-ethylaniline, we managed to obtain two different types of polyoxometalates: octamolybdate of the type of an isolated cluster and a layered pentamolybdate. The resulting pentamolybdate extends the group of known layered pentamolybdates of aromatic amines.
Practical importance of the investigated compounds is connected with the fact that the catalysts based on Mo, W or V derivatives are produced in amounts of hundred tons every year. The investigated polyoxometalates can be applied to crystal engineering of complex hybrid inorganic–organic materials with tailored properties. As we reported earlier (Bożek et al., Reference Bożek, Neves, Valente and Łasocha2018, Szymańska et al., Reference Szymańska, Nitek, Oszajca, Pamin, Połtowicz and Łasocha2016), pentamolybdates are very promising catalysts in reactions of oxidation of cyclic hydrocarbons and their epoxidation. The investigated pentamolybdates are thermally stable and also stable under the conditions of chemical reactions. Moreover, their synthesis is efficient, relatively simple and inexpensive.
V. DEPOSITED DATA
CIF and/or RAW data files were deposited with ICDD. You may request this data from ICDD at info@icdd.com.
ACKNOWLEDGEMENT
This study was partially supported by ICDD Grant-in-Aid No. 97-04, 2018/2019.
