International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 3.7, p. 310

Section 3.7.2.4.2. Vanadium phosphate butane-oxidation catalyst

J. A. Kaduka,b,c*

aDepartment of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, IL 60616, USA,bDepartment of Physics, North Central College, 131 South Loomis Street, Naperville, IL 60540, USA, and cPoly Crystallography Inc., 423 East Chicago Avenue, Naperville, IL 60540, USA
Correspondence e-mail: kaduk@polycrystallography.com

3.7.2.4.2. Vanadium phosphate butane-oxidation catalyst

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Vanadyl pyrophosphate [(VO)2P2O7] catalysts are used commercially for the selective oxidation of butane to maleic anhydride. Modern third-generation search/match programs [using the background-subtracted, Kα2-stripped data (files goed80.gsas, GOED80.raw and d8v3.prm); Fig. 3.7.4[link]] had no trouble in identifying the desired major phase (VO)2P2O7, but had difficulty with the minor phases that were clearly present. Unless the display of duplicate entries is turned off, most programs will yield several duplicate hits at the top of the list. Both 00-050-0380 and 04-009-2740 are Star quality, but only the Linus Pauling File (LPF) entry 04-009-2740 contains atom coordinates for a Rietveld refinement. Entry 01-070-8726 has the lower-quality B mark.

[Figure 3.7.4]

Figure 3.7.4 | top | pdf |

The results of applying a commercial search/match program (Jade 9.5; Materials Data, 2012[link]) to the (background-subtracted, Kα2-stripped) powder pattern of a butane-oxidation catalyst. The first three patterns in the hit list had equivalent figures of merit. The PDF entries 00-050-0380 and 04-009-2740 had Star quality marks and 04-009-2740 contained the atomic coordinates necessary for a Rietveld refinement. Additional peaks are apparent. The phases that give rise to them were identified using the native capabilities of the Powder Diffraction File.

The native capabilities of the PDF proved helpful in identifying the minor phases. The lowest-angle peak not accounted for by the major phase is at a d-spacing of 7.2107 Å. A search for phases containing just the elements V, P, O and H (known from the synthesis procedure) and having one of their three strongest peaks in the range 7.21 ± 0.05 Å (an estimated range) yielded only the single hit 00-047-0967: H4V3P3O16.5(H2O)2. This is a low-precision (O quality mark) pattern from a US Patent (Harju & Pasek, 1983[link]), and the pattern contains only four lines. The comments in the PDF entry indicate that this hydrated phase was formed by exposing a catalyst to ambient conditions, so it seems chemically reasonable but poorly defined.

To see whether this phase had been better characterized by a crystal structure, the four peaks were entered into SIeve+ and a Hanawalt search using a wider than default tolerance of 0.3° on the peak positions and the `just' chemistry filter V, P, O and H was carried out. As expected, PDF entry 00-047-0967 was at the top of the hit list, but close to the top was entry 04-017-1008 (Shpeizer et al., 2001[link]): [H0.6(VO)3(PO4)3(H2O)3](H2O)4. The article by Shpeizer et al. (2001[link]) indicates that this phase was formed from an anhydrous precursor by exposing it to ambient conditions. The single-crystal structure was obtained at 173 K. The similarity of the two PDF entries (Fig. 3.7.5[link]) and the difference in data-collection temperatures makes it clear that these correspond to the same phase, and that the structure of [H0.6(VO)3(PO4)3(H2O)3](H2O)4 could be used in a Rietveld refinement.

[Figure 3.7.5]

Figure 3.7.5 | top | pdf |

Comparison of the low-quality experimental PDF entry 00-047-0967 with the high-quality calculated pattern 01-074-2749 located by searching the experimental pattern against the rest of the PDF. The similarity in patterns and chemistry demonstrated that the two phases were the same and that the coordinates used to calculate entry 01-074-2749 could be used in a Rietveld refinement of a butane-oxidation catalyst.

There were still unaccounted-for peaks at 3.5823 and 3.0760 Å. Under the assumption that these came from a single phase, two separate searches for phases containing just V, P, O and H and with one of their three strongest lines in the ranges 3.58 ± 0.03 and 3.08 ± 0.03 Å were carried out and then combined (using the History option) with a Boolean `and' operation. All five of the entries on the hit list corresponded to α-VOPO4. This yellow V5+ compound was consistent with the altered colour of the V4+-based catalyst, and is a common impurity.

Close examination of the Rietveld difference plot from a refinement including these three phases indicated that there was a weak shoulder at a d-spacing of 3.985 Å. A search for phases containing just V, P, O and H and having a strong peak near this d-spacing yielded β-(VO)(PO3)2, another common catalyst impurity (Fig. 3.7.6[link]). Including this compound as a fourth phase yielded a satisfactory Rietveld refinement and a quantitative analysis of 84.8 (1) wt% (VO)2P2O7, 5.9 (1) wt% [H0.6(VO)3­(PO4)3(H2O)3]­(H2O)4, 5.6 (1) wt% α-VOPO4 and 3.7 (1) wt% β-VO(PO3)2.

[Figure 3.7.6]

Figure 3.7.6 | top | pdf |

The four crystalline phases identified in a butane-oxidation catalyst.

References

Harju, P. & Pasek, P. (1983). Vanadium–hydrogen–phosphorus–oxygen catalytic material. US Patent 4374756; PDF entry 00-047-0967.Google Scholar
Shpeizer, B., Ouyang, X., Heising, J. M. & Clearfield, A. (2001). Synthesis and crystal structure of a new vanadyl phosphate [H0.6(VO)3(PO4)3(H2O)3].4H2O and its conversion to porous products. Chem. Mater. 13, 2288–2296.Google Scholar








































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