For catalytic processes, not only HPCs in their initial state are important, but also the so-called "blues". Blues are formed during the partial reduction of a HPC and contain oxides and hydroxides of a metal whose valency is less (VI) [31, 32]. Despite the fact that HPC blues are actively studied [20, 33, 34] the nature of their bonds remains not fully understood, therefore, in this work, molybdenum blues were also studied. Molybdenum blues were obtained naturally in the open air for five days. The initial PMA crystals are bright yellow; after five days they acquire a dark blue, which is characteristic of the reduced form of the acid. Figure 1 shows the mass spectra of the original and reduced forms of PMA in the mass range up to 1600 Da.
According to Fig. 1, the mass spectra of the original PMA are characterized by a large set of registered peaks. To compare the obtained data, the main results were summarized in Tables 1 and 2.
Table 1
Results of a mass spectrometric study of crystalline phosphomolybdic acid in the negative ion detection mode.
m/z, Da | Average peak intensity, a. u. | The gross formula of the proposed particle |
Original form | Reduced form |
82 | 420 | 270 | Н3РО3 |
224 | 280 | 340 | HPO3∙MoO3 |
288 | 240 | 820 | (MoO3)2 |
319 | - | 510 | P(MoO3)2 |
368 | 250 | 500 | HPO3∙(MoO3)2 |
432 | 830 | 3100 | (MoO3)3 |
512 | 2370 | 1840 | HPO3∙(MoO3)3 |
576 | 715 | 1430 | (MoO3)4 |
656 | 1600 | 535 | HPO3∙(MoO3)4 |
720 | 550 | 250 | (MoO3)5 |
800 | 1010 | - | HPO3∙(MoO3)5 |
864 | 580 | - | (MoO3)6 |
944 | 335 | - | HPO3∙(MoO3)6 |
1009 | 340 | - | (MoO3)7 |
Table 2
Results of a mass spectrometric study of crystalline phosphomolybdic acid in the positive ion detection mode.
m/z, Da | Average peak intensity, a. u. | The gross formula of the proposed particle |
Original form | Reduced form |
352 | 4290 | - | (MoO3)2∙HPO2 |
432 | 3500 | - | (MoO3)3 |
496 | 4190 | 5430 | (MoO3)3∙HPO2 |
548 | - | 5800 | H3PMo3O13·H2O or HPO3∙(MoO3)3∙2Н2O |
560 | - | 5750 | (MoO3)2∙Mo2O5 |
640 | 4910 | 4260 | (MoO3)4∙HPO2 |
704 | - | 3360 | (MoO3)3∙Mo2O5 |
784 | 3870 | 2590 | HPO3∙(MoO3)3∙Mo2O5 |
848 | 1140 | - | (MoO3)4∙Mo2O5 |
929 | 1450 | - | (MoO3)6∙HPO2 |
1201 | 390 | - | (MoO3)6∙Mo2O5∙HPO2 |
An analysis of the data in Table 1 showed that when registering negative ions of crystalline PMA, both in the original and in the reduced form, peaks appear in the mass spectra that are characteristic of clusters of two types: (MoO3)n and HPO3∙(MoO3)n.
Registration of positive PMA ions (Table 2) made it possible to record peaks characteristic of particles containing molybdenum compounds with mixed valence (V) and (VI). Compounds of this type are highly reactive and are widely used in various processes [32, 35, 36].
In addition, particles with mixed valence of the form HPO2·(MoO3)n were registered. Particles of this type are more common in the parent molecule than in blues. It can be assumed that these particles arise when one of the O–Mo bonds is completely broken. At the same time, part of the P-Mo bonds is preserved. The first representative of compounds of the HPO2·(MoO3)n type contains two molybdenum atoms, both in the + 5 oxidation state. Subsequent representatives of this type of compounds contain molybdenum in mixed oxidation states: +5 and + 6.
Comparison of the obtained mass spectra (Fig. 1) showed that the initial acid is more prone to the formation of particles with relatively large m/z values (more than 800 Da). It should be noted that the intensity of the peaks that appeared in the mass spectra of the reduced acid, in most cases, exceeds that of the original acid. If the intensity is considered as a quantity proportional to the concentration, then it can be concluded that the reduced form of PMA forms a smaller number of particles of different composition, but with a higher concentration, compared to the initial acid.
The tendency of PMA to form oxide clusters of various compositions is also traced for molybdenum (VI) oxide crystals (Fig. 2). It should be noted that molybdenum oxide is also capable of forming particles of mixed valence (m/z = 494 Da).
In a detailed study of the PMC mass spectra in the range from 1500 Da, peaks with masses greater than those of the molecular ion were recorded (Fig. 3). In the negative ion detection mode, peaks appeared with m/z = 2196, 2310, and 2419 Da (Fig. 3a). Taking into account a small error, it can be assumed that the following particles correspond to these peaks: H3PMo12O40·(MoO3)2·4H2O (2198 Da), H3PMo12O40·(MoO3)3·2H2O (2308 Da) and H3PMo12O40·(MoO3)4 (2418 Da).
The peaks that appeared in the positive ion detection mode can be attributed to particles containing the initial molecule: H3PMo12O40·(MoO3)3·H2O (2290 Da), H3PMo12O40·HPO3·(MoO3)3·3H2O (2358 Da), and H3PMo12O40·HPO3·(MoO3)4 (2498 Da). However, particles of another type can also correspond to such m/z values: (MoO3)15·6H2O (2282 Da), (MoO3)14·Mo2O5·5H2O (2394 Da), and (MoO3)15·Mo2O5·3H2O (2503 Da). To understand which of the assumptions is correct, the isotopic distributions of the theoretically assumed particles and the experimentally obtained peak were considered (Fig. 4).
Analysis of Fig. 4 showed that the proposed particles do not contain the PMA molecule or its molecular ion, but are hydrated molybdenum oxides, the number of which is more than 15.
Thus, the mass spectrometric study of phosphomolybdic acid in the form of crystals made it possible to detect particles that may contain the molecular ion of the parent compound, but the intensity of the corresponding peaks is relatively low.
Since most catalytic processes take place in solution or at the solution-solid catalyst interface, it is important to understand the state of the catalyst under these conditions. In addition, HPCs in solutions undergo a multiple process of formation and destruction of final particles. Dissolution and subsequent crystallization can lead to the formation of new compounds. Therefore, the next stage of the work was the study of aqueous solutions of PMA.
To study PMA solutions, a certain amount of the starting compound was dissolved in water. Then a drop of the resulting solution was placed on the inert surface of a standard plate and allowed to dry completely. The results of the study of solutions of phosphorusmolybdic acid of various concentrations in the mode of registration of negative ions are summarized in Table 3.
Table 3
Results of mass spectrometric study of aqueous solutions of phosphomolybdic acid in the negative ion detection mode.
m/z, Da | Peak intensity, a.u. | The gross formula of the proposed particle |
Solution concentration, % |
0.01 | 0.1 | 1 | 2 | 5 |
79 | 1760 | 3170 | 5250 | 5470 | 8900 | PO3− |
144 | 1300 | 300 | 160 | 190 | 80 | MoO3 |
162 | 570 | 140 | 145 | 2750 | 120 | MoO3∙H2O |
180 | 550 | - | - | - | - | MoO3∙(H2O)2 |
207 | 450 | 775 | 630 | 610 | 250 | MoO3∙PO2− |
224 | - | 300 | 515 | 740 | 900 | HPO3∙MoO3 |
288 | 1830 | 1770 | 1140 | 800 | 360 | (MoO3)2 |
432 | 1890 | 1650 | 1710 | 2060 | 540 | (MoO3)3 |
512 | - | - | 1175 | 2145 | 1820 | HPO3∙(MoO3)3 |
576 | 1395 | 580 | 1460 | 1775 | 430 | (MoO3)4 |
656 | - | - | 690 | 1280 | 930 | HPO3∙(MoO3)4 |
720 | 1000 | - | 530 | 840 | 200 | (MoO3)5 |
864 | 790 | - | - | - | - | (MoO3)6 |
1009 | 730 | - | - | - | - | (MoO3)7 |
Analysis of the data in Table 3 showed that, at low concentrations of an aqueous solution of PMA, only peaks characteristic of particles of the (MoO3)n type are observed in the mass spectra. When moving to more concentrated solutions, mass spectra can reveal more complex particles, which include the central phosphorus atom and molybdenum (VI) atoms. It should be noted that almost all mass spectra of the studied solutions are characterized by the presence of highly reactive PO3− and MoO3∙PO2− particles. It can be assumed that, for this reason, catalysts containing HPC have shown good results in oxidation reactions [11, 12].
The results of the mass spectrometric study of aqueous solutions of PMA in the positive ion detection mode are summarized in Table 4.
Table 4
Results of mass-spectrometric study of aqueous solutions of phosphomolybdic acid in the positive ion detection mode.
m/z, Да | Peak intensity, a.u. | The gross formula of the proposed particle |
Solution concentration, % |
0.1 | 1 | 2 | 5 |
81 | 5910 | 4140 | 9290 | 4570 | HPO3∙H+ |
99 | 5190 | - | - | - | H3PO4∙H+ |
311 | 1780 | - | - | - | ? |
546 | 1160 | 4410 | 5470 | 3900 | MoO3∙Mo2O7∙H3PO4 |
560 | - | 4060 | 4760 | 3420 | (MoO3)2∙Mo2O5 |
640 | - | 3565 | 4290 | 8115 | (MoO3)2∙Mo2O10 |
784 | - | 2740 | 2380 | 6900 | (MoO3)3∙Mo2O10 |
929 | - | - | - | 1430 | (MoO3)6∙HPO2 |
According to the results obtained, at low concentrations of aqueous solutions of PMA, the mass spectra do not show peaks that could be attributed to the particles of the compound under study. In addition, it was not possible to obtain informative mass spectra for a 0.01% solution. When moving to a more concentrated solution, the mass spectra reveal particles, most of which contain molybdenum atoms in mixed oxidation states: +5 and + 6. Thus, particles containing both Mo (V) and Mo (VI) are characteristic of PMA not only in the crystalline state, but also in aqueous solution.
To obtain the mass spectrum of the HPS solution, in which it would be possible to fix the intense peak of the particle containing the PMA molecular ion, a mass spectrometric study of a 1% aqueous solution of the acid was carried out together with a matrix of 2,5-dihydroxybenzoic acid (DHB) at various experiment parameters. The laser power (LP) and the number of shots at a time (Nshots) were chosen as variable parameters. The influence of these parameters was taken into account both in the reflex and in the linear modes when registering negative ions. Since the purpose of this stage of the study was to obtain a peak or peaks in which the molecular ion of phosphomolybdic acid (M = 1825) could be identified, studies were carried out in the range of 1500–5000 Da. The main results obtained in the negative ion detection mode are shown in Figs. 5 and 6.
According to the results obtained, for any values of the main parameters of the mass spectrometric experiment, the study by the linear method leads to much more intense spectral peaks. It should be noted that the analysis of the PMA solution with a matrix can provide more informative mass spectra with m/z values exceeding the mass of the HPC molecular ion. In this case, the study in the reflex or linear mode does not matter.
In almost all cases, the following regularities are characteristic: 1) for the linear method, in contrast to the reflex method, the intensity of the peak depends ambiguously on the parameters LP and Nshot; 2) in the general case, the dependencies I – LP and I – Nshot are symbatic; 3) peak intensity depends more on the laser energy than on the number of shots.
Practically under all conditions of mass spectrometric analysis, the spectra contained five peaks, with the peaks at 1772, 1916, and 2162 Da recorded both by the reflex and by the linear method. Table 5 illustrates the particles that can be correlated with the obtained peaks.
Table 5
Presumed particles recorded in the mass spectrometric study of a 1% solution of phosphomolybdic acid in the negative ion detection mode.
Method | m/z, Da | Presumed particle |
linear | 2046 | (MoO3)14·H2O |
2272 | H3PMo12O40·(MoO3)3 |
reflex | 2052 | H3PMo12O40·MoO3·4H2O |
2283 | (MoO3)15·6H2O |
linear, reflex | 1772 | (MoO3)12·2H2O |
1916 | (MoO3)13·H2O |
2162 | H3PMo12O40·(MoO3)2·2H2O |
To confirm the correctness of the assumptions about fixed particles, we compared the theoretical and experimental isotopic distributions of the considered peaks (Fig. 7).
Taking into account the error of the mass spectrometric experiment, the obtained peaks with m/z 2052 and 2272 Da and those assumed theoretically have good convergence. A detailed examination of the 2162 Da peak for both the linear and reflex modes shows a slight deviation from the theoretical one. However, in both cases, with a small experimental error, it can be argued that the peak with a value of 2162 Da corresponds to a particle of the composition H3PMo12O40(MoO3)2·2H2O.