CHAPTER 1 INTRODUCTION
1.4. An idealized model for the orthorhombic structure ofvanadyl pyrophosphate
VPO systems have a complex, yet fascinating structural chemistry. (VO)2P207 exhibits exceptional selectivity in the 14-electron oxidation ofn-butane to maleic anhydride (Centi, G. et al. (1988)). The catalytic performance of this phase was shown to be correlated with crystal morphology and size, and is strongly influenced by the presence of non-stoichiometric P and variations in the bulk oxidation state of V (Cornaglia, L.M. et al. (1991)). In order to fully understand the structure/performance dependance of this system and the mechanistics of site isolation at the active/selective surfaces parallel to the basal (100) plane (Fig. 1.5), a thorough investigation of the crystallography and variation in the structure of vanadyl pyrophosphate has been necessary.
A molecular description of the surface structure and surface chemistry of vanadyl pyrophosphate requires an acceptable crystallographic model of the bulk. Unfortunately, a great deal of confusion has surrounded attempts to determine the structure of this material. For example, crystals and crystallites ofvanadyl pyrophosphate have been observed to have defects.
The nature of these defects can cause severe problems with the refinement of the crystallographic models in single crystal X-ray diffraction studies and this has resulted in lack of confidence in previous structural assignments. Other points of confusion revolve around the fact that vanadyl pyrophosphate catalysts are known to exhibit a structure sensitivity related to the method of preparation (Cavani, F. et al. (1985a)) and that differences in catalytic performance are likely due both to the modification of crystal morphology as well as structure.
The solid-state dehydration reaction, which transforms the vanadyl hydrogen phosphate hemisolvate precursor into the vanadyl pyrophosphate product, has been reported to be topotactic (Bordes, E. et al. (1979)), with an amorphous intermediate phase required to
complete the transformation. Based on symmetry arguments alone, it is clear that this reaction cannot proceed as simple topotaxy of the published crystal structures of VOHP04·'/2H20 and (VO)2P20 7. VOHP04·1/2H20 and (VO)2P207 are representative of the precursor and product, respectively. The point group symmetry around the face-shared vanadyl dimeric unit in the precursor is C2Y, while that of the edge-shared dimer in the vanadyl pyrophosphate product is Cl. It is apparent that there is a considerable reorganization of structure as the catalyst precursors pass through an amorphous intermediate phase during calcination and, after conditioning in the presence of the feed stream in the reactor, yield the catalytically active (VOhP20 7 phase (Thompson, M.R. et al.(1994)).
Large single crystals of vanadyl pyrophosphate vary in colour (either emerald-green or red-brown) and possess subtle structural differences due to variation in the symmetry of the V atom sites within the asymmetric unit (Thompson, M.R. etal. (1994)). No variation in P atom positions are indicated in the single crystals, however, there is evidence of P disorder in catalyst powders.
...
----
"-
Close packed oxide basal plane ... .. ,,..', .... ,. " ..
Close packed oxide basal plane
(a) (b)
Fig. 1.5. (a) The close-packed oxygen basal planes/or the unit cell o/vanadyl pyrophosphate.
(b) The relationship between the coordination spheres o/vanadium (octahedra) and phosphorous (tetrahedra)
The crystal structure of vanadyl pyrophosphate contains two close-packed layers of oxygen atoms that lie parallel to the bc plane at approximately J!4 and % along the a-axis as illustrated in Fig. 1.5.
These layering planes are made up entirely of the basal oxygens of V octahedra and pyrophosphate tetrahedra (Fig. 1.5. (b)). The close-packed pattern for the basal-plane and the relative positions of the V and P sites in the octahedral and tetrahedral interstices are illustrated in Fig. 1.6. The refinement of the crystallographic model indicates a degree of non-planarity and distortion of the oxygen basal plane. These distortions are minor.
Oxide close packing pattern (a)
O.33A.1.. •
Octahedral and lelrahedral sites (b)
. .1..0.38A
Fig. 1.6. (a) Basal oxygen close-packing pattern. (b) Location ofthe octahedral and tetrahedral interstices (Thompson, MR. et al. (1994))
The V octahedra are square-pyramidally distorted. The V atoms lie approximately 0.33
A
out of the basal plane oriented toward the vanadyl oxygen, i.e. V=O. The P atoms lie approximately
0.38 A
out of the basal plane. Fig. 1.7 (a) illustrates the coordination geometry about the V atoms, and Fig. 1.7 (b) the geometry for the P atoms, each idealized from experimental models.o
1.60A\\ lO
P... O-V~O
0 / '----Ot /
~~ O-P
p/ P V-Od-p
, /2.26)\0
g
(a) (b)
Fig. 1.7. (a) The vanadium coordination sphere and (b) the phosphorous atoms in the idealized model ofvanadyl pyrophosphate. Subscripted qxygen atoms represent double-bridged positions
(OciJ and triple-bridged positions (OJ
Four classes of oxygen atoms exist within the structure: double bridging oxygen, triple bridging oxygen, vanadyl oxygen (V=O) and pyrophosphate oxygen (P-O-P). The double- and triple-bridging oxygens lie in the basal plane. The other vanadyl oxygens lie within the unit cell.
It is important to note that the posltions of the vanadyl oxygens are invariant to the direction of the vanadyl bond. The directional sense of the vanadyl column relative to the a-axis is determined by the position of the V ions in that column (Fig. 1.5 (b)). Two positions are possible for each V atom: above or below the basal plane. If the V atoms lie above the basal planes at 114, the direction of the vanadyl column will be aligned with the direction of the a-axis, and ifthey lie below these planes at%, then the direction ofthe column will be anti-parallel to a.
Within every vanadyl column, one V atom will be positioned between any two basal planes of the structure. Similar to the situation for the V atoms, the P atoms can lie above or below the planes at 114 and % on the a-axis. However, both P atoms of an individual pyrophosphate group must lie between two adjacent basal layers. Therefore a column vacancy will occur in every other layer. There are eight pyrophosphate columns within the unit cell, each of which possess two possible orientations. In summary there are 104 atoms contained within the unit cell of
vanadyl pyrophosphate: 48 basal oxygen atoms, 16 vanadyl oxygen atoms, 16 V atoms (8 pairs) and 8 pyrophosphates (24 atoms).
The XRD patterns of vanadyl pyrophosphate catalysts exhibit significant differences when compared with the diffracted intensities from the single crystals. The most probable explanation is that there is a significant amount of variation in the structure of the vanadyl pyrophos'phate in the microcrystalline catalysts (Thompson, M.R. et al. (1994)). The structures of the single crystals are only two of a great number of possible polytypes for vanadyl pyrophosphate. For the observed cell volume, there are 8 columns of vanadyl groups each possessing two possible orientations and 8 columns of pyrophosphates each with two possible orientations, yielding
i
6(65 536) variations.The idealized model of vanadyl pyrophosphate has been presented here primarily to illustrate the point that there are many conceivable variations in the structure of this material. There does not seem to be a simple symmetry preserving mechanistic path between vanadyl hydrogen phosphate and the structures of the emerald-green and red-brown crystal, and therefore, we should not be surprised that an amorphous intermediate phase results during the preparation of the catalyst. Theoretical results point to the fact that the experimental structures may be representative of the most thermodynamically favorable structures for this material (i.e. lowest crystal energy). There is a common misconception that the bulk structure of the vanadyl pyrophosphate is characterized as a compact solid oxide. Consideration of the symmetry of the vanadyl and pyrophosphate building blocks more appropriately leads to a description ofthe bulk as a material with a series of interlayer vacancies or pores. One hypothesis is that the surface topology parallel to the (100) plane in vanadyl pyrophosphate must possess three-dimensional character (Centi, G., Trifiro, F.; (1988)). This stems from the fact that these surfaces cut across bulk vacancies. If vanadyl pyrophosphate can exhibit variations in its bulk structure, then the sizes and the symmetries of these vacancies at the surface termination would likewise be expected to be variable.