A Concise Description of the Bulk Structure of Vanadyl Pyrophosphate and Implications for n-Butane Oxidation

V. CortCs Corbersn and S. Vic Bellon (Editors), New Developments in Selective Oxidation 11 0 1994 Elscvier Science B.V. All rights rescrved.

167

A Concise Description of the Bulk Structure of Vanadyl Pyrophosphate and Implications for n-Butane Oxidation Michael R. Thompson*a, A.C. Hessa, J.B. Nicholasa, J.C. Whitea, J. Anchella, J.R. Ebnerb aMolecular Sciences Research Center, Pacific Northwest Laboratory', Box 999, Richland, Washington, USA, 99352; bMonsanto Corporate Research Laboratories, 800 N. Lindbergh Avenue, St. Louis, Missouri, USA, 63303 ABSTRACT The evolution of the structure of vanadyl pyrophosphate from its vanadyl hydrogen phosphate precursors occurs with a change in point group symmetry and a transition through an amorphous phase. Based on the crystal structures of these materials, there are no simple topotactic pathways between the precursor and product. An idealized model of the solid-state structure of vanadyl pyrophosphate is introduced and the notion of polytypism discussed with respect to the preparation of vanadiumphosphorus-oxide (VPO) based catalysts. Periodic ub initio Hartree-Fock calculations have been used to compute energy differences between various polytypical vanadyl pyrophosphate crystal structures. These calculations indicate that the experimentallydetermined structures for emerald-green and red-brown crystals

of vanadyl pyrophosphate are expected to be among the most stable for this material. Implications to catalysis relate to the method of synthesis and equilibration of VPO catalysts, and to variation in the expected surface structuresfor vanadyl pyrophosphate. 1. INTRODUCTION

The family of vanadium-phosphorus-oxidespossess a fascinating and complex structural chemistry [2]. Relative to catalysis, the primary focus has been on the vanadyl pyrophosphate phase, (VO)2P2O7,

which exhibits exceptional selectivity in the 14electron oxidation of n-butane to maleic anhydride [3]. The catalytic performance of this phase has been shown to be correlated with crystal morphology and size, and is also strongly influenced by the presence of non-stoichiometric phosphorus and variations in the bulk oxidation state of vanadium [4]. In order to fully understand the structure/performance dependence in this system and the mechanistics of site isolation 1.51 at the activdselective surfaces parallel to (1.0.0) [6], a thorough investigation of the crystallography and variation in the structure of vanadyl pyrophosphate has been necessary. A molecular-level 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

* Author to whom correspondence should be addressed

has surrounded attempts to determine the solid-state structure of this material [7]. For example, crystals and crystallites of vanadyl pyrophosphate have been observed to be defected [8]. The nature of these defects cause severe problems with the refinement of the crystallographic model in single crystal X-ray diffraction studies and this has resulted in a lack of confidence in the previous structural assignment. 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 [9] 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 [8,10], with an amorphous intermediate phase required to complete the transformation. Based on symmetry arguments alone, it is clear that this reaction cannot proceed as a simple topotaxy if the published crystal structures of VOHP04 . 0.5 HzO I l l ] and (vo)2Pzo7 [7b] are representative of the precursor and product, respectively. The point group symmetry around the face-shared vanadyl dimeric unit in the precursor is C ~ while V that of the edge-shared dimer in the vanadyl pyrophosphate product is C1. It is apparent that there is a considerable reorganization of structure as the catalyst precursors pass through the amorphous intermediate. Experimentally, we have determined that vanadyl pyrophosphate exists in at least two polytypical forms and it is probable that this phase exhibits a broad range of structure. The intent of this paper is to introduce an idealized model of the solid-state structure of vanadyl pyrophosphate which is consistent with experimental studies, to use this model to illustrate the concept of polytypism. and to briefly outline preliminary results from theoretical studies of the bulk structure.

2. RESULTS AND DISCUSSION

2.1 An Idealized Model for the Orthorhombic Structure of Vanadyl Pyrophosphate Large single crystals of vanadyl pyrophosphatevary in color (either emerald-green or red-brown) and possess subtle structural differences relative to variation in the symmetry of the vanadium atom sites within the asymmetric unit [12]. No variation in phosphorus atom positions are indicated in the single crystals, however, there is evidence of phosphorus disorder in catalyst powders [13]. We have developed an idealized model for the bulk structure of vanadyl pyrophosphate based on these experimental observations. For the sake of simplifying the crystallographic description, minor adjustments have been made to the coordinates of the experimental model in order to maximize the apparent symmetry and remove minor variations in bond lengths and bond angles. This idealized model possesses atom connectivity consistent with the experimental structures, and all equivalent vanadium, phosphorus, and oxygen atoms possess identical bonding environments. The coordinates are tabulated below and a general description of the structure and the structural variables are included. The crystal structure of vanadyl pyrophosphate contains two close-packed layers of oxygen atoms which lie parallel to the bc-plane at approximately 114 and 3/4 along the a-axis (‘Fig. la). These layering planes are made up entirely of the basal oxygens of vanadium octahedra and pyrophosphate tetrahedra (Fig. lb). Figure 2 illustrates the close-packed pattern for the basal-plane and the relative positions of the vanadium and phosphorus sites in the octahedral and tetrahedral interstices. The refinement of the crystallographicmodel indicates a degree of non-planarity and distortion of the oxygen basal plane. These

169

+c

b

Figure 1. (a) The close-packed oxygen basal planes for the unit cell of vanadyl pyrophosphate. (b) The relationship between the coordination spheres of vanadium (octahedra) and phosphorus (tetrahedra) . distortions are minor and idealizing the basal layer by forcing the oxygen atoms to lie precisely in the planes at x a . 2 5 and 0.75, simplifies the description and produces a set of coordinates which possess maximum symmetry. Coordinates for all basal oxygen atoms lying within the unit cell are listed in Table I. The vanadium octahedra are square-pyramidally distorted. The vanadium atoms lie approximately 0.33A out of the basal plane oriented toward the vanadyl oxygen (formally V=O). Figure 3a illustrates the coordination geometry about the vanadium atoms, and Fig. 3b the geometry for the phosphorus atoms, each idealized from the experimental model. Four classes of oxygen atoms exist within the structure: double-bridging oxygen (V-0-P), triple-bridging oxygen (P-0-Vz), vanadyl oxygen (V=O), and pyrophosphate oxygen (P-0-P). The double- and triple-bridging oxygens all lie in the basal plane and are listed above in Table I. The coordinates for all of the vanadyl oxygens which lie within the unit cell are listed in Table 11. It is important to realize that the positions 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 vanadium a t o m in that column. Two positions are possible for each vanadium atom: above or below the basal plane. If the vanadium atoms lie above the basal planes at 1/4 and 3/4, the direction of the vanadyl column will be aligned with the direction of the a-axis, and if they

Oxide Close Packmg Pattern

Octahedral and Tetrahedral Siles

Figure 2. (a) Basal oxygen close-packing pattern (numbers are in accordance with the labeling scheme in Table I). (b) Location of the octahedral and tetrahedral interstices.

170

Table I. Idealized Fractional Coordinates for Basal Oxygen Plane for Vanadyl Pyrophosphate. Idealized Lattice Constants: a= 7.710A,b=9.650& c=16.650A, a=k=90.00 ~~~

~

~

x a . 2 5 , x'd.75

Name 01 02 03

04 05 06

v 0.1600 0.1375 0.1600 0.1600 0.1375 0.1600

Name

z 0.0850 0.2500 0.4150 0.5850 0.7500 0.9150

09

010 011 012 07 08

v 0.3450 0.3625 0,3450 0.3450 0.3625 0.3450

v

z

0.6375 0.6550 0.6550 0.6375 0.6575 0.6575

0.1675 0.3325 0.5000 0.6675 0.8325

Name

z

0.3325 0.5000 0.6675 0.8325 0.W 0.1675

013 014 015 016 017 018

Name

0.W

019

020 021 022 023 024

v

2

0.8400 0.8625 0.8400 0.8400 0.8625 0.8400

0.0850 0.2500 0.4150 0.5850 0.7500 0.9150

1.62A

o 1.6oA

0

(4

@)

Figure 3. Bond lengths for (a) the vanadium coordination sphere, and (b) the phosphorus atoms in the idealized model of vanadyl pyrophosphate. Subscripted oxygen atoms represent double-bridged (V-0-P) positions (Od) and triple-bridged (P-0-V2) positions (03. lie below these planes, then the direction of the column will be anti-parallel to a. Table III lists all possible positions for the vanadium atoms within the unit cell. Unprimed atoms are located below the basal plane, primed atoms above. Note that only one of these two related positions will be occupied, and that the two occupied vanadium sites within a column must be either both primed or both unprimed to construct two chemically reasonable vanadyl moieties (e.g., V1 and V2, or V3' and V4').

Table 11. Idealized Coordinates for the Vanadyl Oxygens Name

y

z

025 026 027

0.W

0.1500 0.3500 0.6500

0.W

0.W

Name 028 029 030

x=O.00. x'=0.50 y z

O.oo00

0.5000 0.5000

0.8500 0.1000 0.4000

Name

y

z

031 032

0.5000 0.5000

0.6000 0.9000

Within every vanadyl column, one vanadium atom will be positioned between any two basal planes of the structure. Similar to the situation for the vanadium atoms, the phosphorus atoms can lie above or below the planes at 1/4 and 3/4 on the a-axis. However, both phosphorus atoms of an individual pyrophosphate group must lie between two adjacent basal layers. Therefore, a column vacancy will necessarily occur in every other layer. There are eight pyrophosphate columns within the unit cell, each of which possess two possible orientations, and these are listed in Table IV. The atom labels in Table IV are unprimed and primed, denoting whether the pyrophosphate group lies below or above the basal plane at x=1/4, respectively.

171

In summary, there are 104 atoms contained within the unit cell of vanadyl pyrophosphate: 48 basal oxygen atoms listed in Table I, 16 vanadyl oxygen atom from Table II, 16 vanadium atoms (8 pairs) from Table III, and 8 pyrophosphates (24 atoms) from Table IV.As an example, the crystal structure reported

Table 111. Idealized Coordinates for the Vanadium Atoms in Vanadyl Pyrophosphate Name

V1 V1' V2 V2' V3 V3' V4 V4'

x

y

0.2075 O.oo00 0.2925 O.oo00 0.7075 O.oo00 0.7925 O.oo00 0.2075 O.oo00 0.2925 O.oo00 0.7075 O.oo00 0.7925 O.oo00

z

Name

0.1500 0.1500 0.1500 0.1500 0.3500 0.3500 0.3500 0.3500

V5 V5' V6 V6' V7 V7' V8 V8'

x

0.2075 0.2925 0.7075 0.7925 0.2075 0.2925 0.7075 0.7925

y O.oo00 O.oo00 O.oo00 O.oo00 O.oo00 O.oo00 O.oo00 O.oo00

Name

z

0.6500 0.6500 0.6500 0.6500 0.8500 0.8500 0.8500 0.8500

x

V9 0.2075 V9' 0.2925 V10 0.7075 V10 0.7925 V11 0.2075 V11'0.2925 V12 0.7075 V12'0.7925

y

Name

z

0.5000 0.1000 0.5000 0.1000 0.5000 0.1000 0.5000 0.1000 0.5000 0.4000 0.5000 0.4000 0.5000 0.4000 0.5000 0.4000

x

y

z

V13 0.2075 0.5000 0.6000 V13'0.2925 0.5000 0.6OoO V14 0.7075 0.5000 0.6000 V14'0.7925 0.5000 0.6OoO V15 0.2075 0.5000 0.9000 V15' 0.2925 0.5000 0.9oOo V16 0.7075 0.5000 0.9000 V16'0.7925 0.5000 0.9oOo

by Linde et al. can be constructed by using all 64 atoms in Tables I and II, the following pairs of vanadium: Vl/V2, V3'/V4', V5'/V6', V7/V8, V9'/VlO, Vll'/V12', V13/V14, V15/V16, and the pyrophosphate groups associated with Pl', P3', PS, P7, P9,P11, P13', P15'.

Table IV. Idealized Coordinates for Phosphorus and Pyrophosphate Oxygen Atoms Name

x

y

z

Name

x

y

z

Name

x

y

z

Name

x

y

z

P1 0.2000 0.2000 O.oo00 P5 0.2000 0.2000 0.5000 P9 0.2000 0.8000 O.oo00 P13 0.2000 0.8000 0.5000 033 O.oo00 0.1850 O.oo00 035 O.oo00 0.1850 0.5000 037 O.oo00 0.8150 O.oo00 039 O.oo00 0.8150 0.5000 P2 0.8000 0.2000 O.oo00 P6 0.8000 0.2000 0.5000 P10 0.8000 0.8000 O.oo00 P14 0.8000 0.8000 0.5000 P1' 0.3000 0.2000 O.oo00 P5' 0.3000 0.2000 0.5000 P9' 0.3000 0.8000 O.oo00 P13' 0.3000 0.8000 0.5000 033' 0.5000 0.1850 O.oo00 035'0.5000 0.1850 0.5000 037'0.5000 0.8150 O.oo00 039 0.5000 0.8150 0.5000 P2' 0.7000 0.uXx) O.oo00 P6' 0.7000 0.2000 0.5000 P10 0.7000 0.8000 O.oo00 P14' 0.7000 0.8000 0.5000 P3 0.2000 0.3000 0.2500 P7 0.2000 0.3000 0.7500 P11 0.2000 0.7000 0.2500 P15 0.2000 0.7000 0.7500 034 O.oo00 0.3150 0.2500 036 O.oo00 0.3150 0.7500 038 O.oo00 0.6850 0.2500 040 O.oo00 0.6850 0.7500 P4 0.8000 0.3000 0.2500 P8 0.8000 0.3000 0.7500 P12 0.8000 0.7000 0.2500 P16 0.8000 0.7000 0.7500 P3' 0.3000 0.3000 0.2500 P7' 0.3000 0.3000 0.7500 P11' 0.3000 0.7000 0.2500 P15' 0.3000 0.7000 0.7500 034' 0.5000 0.3150 0.2500 036'0.5000 0.3150 0.7500 038'0.5000 0.6850 0.2500 040 0.5000 0.6850 0.7500 P 4 0.7000 0.3000 0.2500 P8' 0.7000 0.3000 0.7500 P12 0.7000 0.7000 0.2500 P16' 0.7000 0.7000 0.7500

2.2 The Structures of Emerald-Green and Red-Brown Crystals of (VO)2PzO7

As we have reported [121, the crystallographic models which result from the single crystal X-ray studies of emerald-green and red-brown crystals of vanadyl pyrophosphate are not grossly different from that published by Linde et al., with the exception that the previous authors neglected to account for vanadium atom disorder in the lattice. The refinement of the crystallographicmodel neglecting vanadium disorder results in a woefully inadequate fit to the data. The description of the crystallographicrefinement

172 utilizing a disorded model will appear shortly in the literature. A discussion of the disorder and crystal defects warrant some extra discussion here. Diffraction Streaks,. It is possible to assign the cause of the diffraction streak effects by considering the coordinates tabulated above for each of the building blocks of vanadyl pyrophosphate, and the manner in which each of these contribute to the structure factors for reflections in the two affected parity groups. The structure factor for a reflection (h,k,I) has the form: Fm= fj [COS2n ( hxj + k ~+jlzj ) + i sin 2x ( hxj +

ky, + lz, )], where f, is the scattering factor for the j-th atom type; (xj,y,,zj) are the fractional coordinates for the j-th atom; and the sum runs over all j atoms in the unit cell. For each of the oxygen atoms which lie in the basal plane (Table I) with coordinates (x,y,z), there exist identical atoms with coordinates (1/2+x,y,lm). It is simple to show that for the calculated structure factor the sine and cosine terns for the oxygen at (x,y,z) will have the same magnitude but opposite sign to the sine and cosine terms for the

oxygen at (x,y,l/Bz), for any reflection in the even-even-odd (ew) parity group. Precisely the same situation exists for any even-odd-odd (eoo) reflection for the basal oxygen atoms at (x,y,z) and (1/2kx,y,z). Therefore, the contribution of the basal oxygen atoms to a computed structure factor for any reflection in either of these two parity groups will be zero. In addition, it can be shown that the phosphorus, bridging pyrophosphate oxygen, and vanadyl oxygen atoms do not contribute to reflections in the two affected parity groups due to similar relationships within the cell. The only atoms which contribute to the eeo and eoo reflections are those of vanadium, and the magnitude of the structure factor is quite sensitive to the site-occupancy-factorswhich relate the relative disorder of the vanadium atoms between the equivalent sites above or below the basal plane. The vanadium disorder occurs in a manner in which the vector between the two related V-sites lies parallel to the a-axis, and the effects of the disorder (line broadening) are evident only in a select subset of reflections. There are other examples of this type of disorder and diffraction streaking [14]. Pattern of Vanadium Disorder: Enantiomomhism. The above discussion provides a basis of understanding the diffraction streaks, but the pattern of disorder for the four independent vanadium atoms is not random. Consider that the space group P c u 1 is non-enantiomorphic. The implication of this is that the two enantiomorphic structures (mirror images) represented by the coordinate sets (x,y,z) and (-x,y,z) cannot be supported together in an entirely ordered lattice. In other words, an ordered lattice with this continuous structure and this space group infers an enantiomorphicallypure crystal. Our observation for the crystal structure of the emerald-green specimens of vanadyl pyrophosphate is that half of the vanadium sites in the structure disorder and half do not. As shown in Fig. 4, the vanadium atoms which lie along a vector parallel to the c-axis with y= ln disorder with site-occupancy-factors reflecting approximately 3:l disorder (but variable from 4:l to 2:l for various crystals), while those along the edge of the cell with y= 0.0 are fully occupied. The explanation of the disorder can be found by generating models for the two enantiomorphs equivalent to the structure reported by Linde et al. If these models are superimposed, all of the oxygen and phosphorus atoms within the structure superimpose, as well do half of the vanadium atoms. Those vanadium atoms with y = l n are not super-impsable. The interpretation of this disorder is that the emerald-green crystals are composed of the two enantiomorphic isomers equivalent to the structure reported by Linde et al. The red-brown crystals of vanadyl pyrophosphate exhibit a pattem of disorder distinctly different from their emerald-green counterparts. All vanadium atoms disorder, however, those which lie along the unit

173 cell edge with y= 0.0 consistently disorder with site-occupancy-factors of 0.5W.03 for the sites above and below the basal plane. Those vanadium atoms with y= 112 disorder in a manner consistent with the emerald-green materials (variable ranging from 4:l to 2:l). While the interpretationof this is not entirely straightforward, we believe that the statistical disorder along the cell edge is caused by a change in symmetry of adjacent vanadium-centereddimers with y= 0.0 (Fig. 5). To construct the crystal structure of

Figure 4. A plot of the crystal structure of vanadyl pyrophosphate projected on the bc-plane. the red-brown material, the following atomic coordinates are used: all 64 oxygen atoms in Tables I and 11; vanadium atom pairs Vl/V2, V3'/V4', V5/V6, V7'/VS', V!Y/VlO', Vll'/V12', V13/V14, V15/V16, and the pyrophosphate groups associated with Pl', P3', P5, P7, P9, P11, P13', P15'. Superpositioned enantiomorphs of this structure disorder all vanadium sites.

Figure 5. Proposed cell edge columnar orientation for (a) emerald-green and (b) red-brown crystals. While both the emerald-green and red-brown crystal structures possess the same space group, P c a ~ ~ , the symmetry of the vanadium atoms which lie inside the asymmetric unit of the cell is different in each case. The mechanism of the transformation which generates the vanadyl pyrophosphate structure from its precursors must provide more than one path to the product, and in this case, it results in subtly different columnar orientations of the vanadyl moieties. The terminology appropriate for this type of structural variation refers to the structures of emerald-green and red-brown crystals as polytvpes [15]. 2.3 Differences in Non-Bonded Contacts, Theoretical Calculations The XRD patterns of vanadyl pyrophosphatecatalysts exhibit significant differences when compared to the diffracted intensities from the single crystals. The most obvious differences are associated with the extreme broadening or extinction of the 000 parity group in the microcrystallinematerials [13]. The most probable explanation is that there is a significant amount of variation in the structure of vanadyl

174 pyrophosphate in the microcrystalline catalysts. 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 216 (65,536) variations. - i h r 1 s. A property of any vanadyl pyrophosphate structure constructed from the coordinates listed above is that irrespective of which vanadium and pyrophosphate coordinates are chosen, the bonding shells of all vanadium, phosphorus and oxygen atoms will be identical with any other coordinate set. However, the symmetries of these structures will be variable, and more importantly, consecutive next-neighbor (non-bonding) shells will change as a function of vanadium and pyrophosphate positions. Next-neighbor distance relationships which vary from sbucture to structure involve: V-V,

V-P, P-P, V--Opop. and P--Opop interactions. Given any two models, differences in interatomic distances are quite small for the first near-neighbor shell, however, these can become very significant for subsequent shells. As an example (Fig. 6). for models with the vanadyl moieties oriented either cis- or trans- across the edge-shared dimer, the first near-neighbor V-V distances differ by less than 0.07A (3.33A vs 3.40A). the distances to the second near-neighbor vanadium atoms are identical in either case ( 3 . 8 6 h however the third near-neighbor V--V shells differ by 0.50A ( 5 . l l A vs 4.62A) for cis- and trans-vanadyl structures. Similar arguments can be made for the variation in non-bonded shells for the phosphorus atoms.

0

0

(4 Figure 6. Illustration of V-V

0

0

(b)

interactions for (a) cis-, and (b) trans-vanadyl dimers.

The importance of these non-bonded interactions in determining the relative energetics of a crystal structure is difficult to assess without resorting to theoretical methods. If the energy differences are small and interconversion possible, many differing structures might be accessible under butane oxidation conditions. Clearly, a thorough quantum chemistry study of the energetics of all variations of vanadyl pyrophosphate is a ridiculous task. However, it is possible to learn something about the relative dependence of the crystal energy on variations of the vanadyl and pyrophosphate networks for a select number of structures. Ab Initio Ouantum Chemistrv of Vanadvl PvroDhosDhate. Wavefunctions for various polytypical vanadyl pyrophosphates have been computed using a WriodiC nb initio Hartree-Fock formalism known as Crystal [16]. These techniques are capable of computing the solutions to the Hartree-Fcck-Roothan equations subject to periodic boundary conditions for a broad variety of crystalline systems, taking full advantage of the space group symmetry. We can use these methods to compute the ground state energy, G, of crystalline vanadyl pyrophosphates evaluated as a function of the nuclear coordinates. Calculations performed on several carefully chosen polytypes can be used to understand the effects on the electronic

175

structure coincident with changes in near-neighbor environments. Since the V - 0 and P-0bond distances and bond angles do not change for any of these structures, the expected energy differences will be due principally to changes in the Coulomb energy. There are a small set of polytypical structures of vanadyl pyrophosphate which possess coordinates for which the c-axis can be halved (c = 8.325A) to create a unit cell containing half the number of atoms (52). Use of these models for theoretical calculations greatly reduces the computational requirements. Within this set of "half-cell" polytypes, structures exist which place the pyrophosphates in various "networked" or "layered" symmetries, as depicted in Fig. 7. For each of the emerald-green and red-brown vanadyl pyrophosphate crystals, a networked structure exists where half of the pyrophosphate groups surrounding a given edge-shared vanadyl dimer are oriented above the basal plane, and half are oriented below the plane, similar to Figure 7a. As for the vanadyl moieties, half-cell structures exist for both cis- and transvanadyl symmetries.

Figure 7. (a) networked and (b) layered structures of the pyrophosphate groups. Wavefunctions have been computed for four polytypcial vanadyl pyrophosphate structures. All calculations were performed with basis sets optimized for the solid 8441+(3-1)d for V; 8-31 for P; and 8-41 for 0. Two polytypes were chosen which possessed identical pyrophosphate networks and differing cis- and trans-orientations orientations of the vanadyl groups. In the same manner, two polytypes with identical vanadyl structures and differing pyrophosphate networks were chosen to evaluate the energy difference due to changes in pyrophosphate structure. The results from these quantum mechanics calculations have been somewhat surprising. For polytypes with identical vanadyl structures (either all cisor all trans-vanadyl groups), only small energy differences exist between layered pyrophosphates and those with networked structures: AEHF 15-20kcals/cell. However, very significant differences in energy exist between structures with variations in vanadyl symmerry: AEm > 250 kcals/cell.

-

Several consistent features appear in these calculations. Consider first the non-bonded near-neighbor distance variations for the phosphorus atoms in networked and layered structures. The closest interaction between two pyrophosphate groups surrounding an individual vanadyl dimer occur at either end of dimeric moiety (Fig. lb). The distance between the phosphorus atoms when they are positioned on the same side of the basal plane is 3.86A. and approximately 0.lOA greater in length when positioned on opposite sides of the plane. While this is a minor difference in the interatomic separation, it should be noted that a van der Waal contact distance for two phosphorus atoms is on the order of 3.8A. Considerable changes in distance

176 are apparent for the O-.O interactions between the bridging pyrophosphate oxygens for these two differing orientations. Elongation of critical contacts by shifting the closely positioned pyrophosphate groups to opposite sides of the basal plane are predicted to result in a stabilization of approximately 3-5 kcals/pyrophosphate,based on the electronic structure calculations. The more dramatic results come as a consequence of positioning the vanadyl groups in environments where they are oriented either cis- or transacross the edge-shared dimer. The vanadium and vanadyl oxygen atoms, in particular, are quite sensitive to changes in near-neighbor environments. For each of the structures with cis-vanadyl groups within the dimer, the effective charges (Muliken populations) on the vanadium and vanadyl oxygen atoms are equivalent throughout the structure, as illustrated in Fig. 8a. However, for structures with trans-vanadyl groups, a segregation of approximately one unit of charge occurs between vanadyl groups within the dimer, and a complementary shift of charge occurs within the column (Fig. 8b). The driving force for -0.44

0

0 -09J

-0.94

6

0 .nod

Figure 8. Effective charges on the V and vanadyl oxygen atoms for columns of (a) cis- and (b) trans-. the charge segregation stems from a significant lowering of the Coulomb energy by minimizing the charge on closely placed vanadium atoms and increasing the charge on the next-near-neighbor shells. Under the assumption that the crystal energies for structures of vanadyl pyrophosphate are driven primarily by the Coulomb energy, we have made an attempt to simplistically rationalize the energetics of the 2'6 possible variations in the structure of this material. Given the effective charges for the vanadium, phosphorus, and four classes of oxygen atoms computed from the various models above, Ewald sums (an evaluation of the Coulomb energy for the solid) have been calculated for wide variations in structure, and this is schematically illustrated in Fig. 9. If we define a quantity equivalent to the mole fraction of vanadyl pairs within the structure which are positioned trans- across the dimeric unit relative to the total number of pairs in the cell, we can evaluate the Coulomb energy as a function of the variation in vanadium atom order, while holding the pyrophosphate structure constant, i.e. E (v;p). Likewise, the mole fraction of pyrophosphate groups positioned between x= 0 and x= 114 relative to the number positioned between x= 0 and x= 1/2 can be used to compute, E (p;v). Figure 9 illustrates the trends computed for a total of 1024 hypothetical polytypical structures for vanadyl pyrophosphate. While the results yield only a semiquantitative description of the energetics of the crystal system, the results are very satisfying. The structure with the minimum Coulomb sum it that of the emerald-greencrystals of vanadyl pyrophosphate, equivalent to that reported by Linde et al. Its Coulomb energy is approximately 120 kcalslcell lower than the polytype with all cis-vanadyl groups and a completely layered pyrophosphate structure. The second lowest

177

Coulomb energy is computed for the polytype noted above for the structure of the red-brown crystals and it lies approximately 2 kcal/cell above the minimum.

I Green

amax Figure 9. A schematic of the trends in Coulomb energy for polytypes of vanadyl pyrophosphate. 3.

CoNcLusrorvs

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 in the preparation of the catalyst. The crystallography of this system is very complex and riddled with high pseudosymmetry. We note, as we have in the past, that the powder diffraction patterns of the microcrystallinecatalysts, while easily indexed on the crystal structure of vanadyl pyrophosphate, indicate a great deal of structural variation. Intevretation of experimental observables of the bulk material, particularly powder diffraction, should be made with great care. Our preliminary theoretical results point to the fact that the experimental structures may be representative of the most thermodynamicallyfavorable structures for this material (i.e., lowest crystal energy). As to models of the active site and the expected surface topology. we would likewise expect variation. There is a common misconception that the bulk structure of 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 of the bulk as a material with a series of interlayer vacancies or pores. The size and location of the interlayer pore is determined by the symmetry of the pyrophosphate network in the solid. Our hypothesis that the surface topology parallel to (1 ,O,O) in vanadyl pyrophosphate must possess three-dimensional character [3] 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 symmetriesof these vacancies at surface terminationwould likewise be expected to be variable.

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4. REFERENCES 1 The Pacific Northwest Laboratory is operated for the United States Department of Energy by the Battelle Memorial Institute under contract DE-AC06-76RLO-1830. This research is supported by the Office of Conservation and Renewable Energy, Advanced Industrial Concepts Division.

2 (a) Bordes, E., Catalysis Today, 1 (1987),499;(b) Hodnett, B.K., Catal. Rev.- Sci. Eng., 27,373, (1985);(c) Centi, G., Trifiro, F., Chim. I d . (Milan), 68 (1986),74;(d) Hodnett, B.K., Catalysis Today, 1 (1987), 477. 3 (a) “Forum on Vanadyl Pyrophosphate Catalysts”, Centi, G. (Ed.), Catalysis Today, 16 (1993). 1-147; Trifiro, F., Centi, G., Ebner, J.R., Franchetti, V.M., Chem. Rev., 88, (1988),55.

4. (a) Comaglia, L.M., Caspani, C., Lombardo, E., Appl. Catal., 74,(1991).15; (b) Yamazoe, N., Morishige, H., Teraoka, Y., Stud. Sug. Sci. Catal., 44,(1989), 15; (c) Garbassi. F., Bart, J.C., Tassinari, R., Vlaic, G., Largarde, P., J. Catal., 98, (1986),317;(d) Hodnett, B.K.. Pemanne, P., Delmon, B.,Appl. Caul.,6,(1983).231; (e) Hodnett, B.K., Delmon, B., ibid., 23,(19841,465.

5. Grasselli, R.K., in: ‘‘ Surface Properties and Catalysis by Non-MetaLs”,Nonnelle, J., and Derouane, E. (Eds.), Elsevier, Amsterdam, (1983), 273. 6.(a) Bordes, E., Catalysis Today, 16 (1993).27;(b) Okuhara, T., Inumaru, K., Misono, M., ACS Symposium Series, American Chemical Society, Washington, D.C., August, 1992,523,157; Busca, G.,Cavani, F., Centi, G., Trifiro, F., J. Catal., 99, (1986).400. (a) Middlemiss, N.E., Doctoral Dissertation, Department of Chemistry, McMaster University, Hamilton, Ontario, Canada, 1978;(b) Linde, S.A., Gorbunova, E., Dolk. Akad. Nauk, SSSR (English Trans.), 245, (1979),584. 8. Bordes, E., Courtine, P., Johnson, J., J. Solid State Chem., 55,(1984).270. (a) Cavani, F., Centi, G., Trifiro, F., J. Chem. Commun., (1985). 492;(b) Centi, G.,Trifiro, F., Busca, G., Ebner, J.R., Gleaves, J.,Faraday Discuss. Chem. Soc., 87,(1989).215;(c) Horowitz, H. Blackstone, C., Sleight, A.W., Tenfer, G., Appl. Catal., 38, (1988), 193.

10. Bordes, E.,Courtine, P., J. Catal., 57,(1979),236. 11. Torardi, C.C., Calabrese, J.C., Inorg. Chem., 23,(1984),1308.

12.Thompson, M.R., Ebner, J.R., ‘Studies in Surface Science and Catalysis”, Ruiz, P, and Delmon, B., (Eds.), Elsevier, Amsterdam, 72, (1992),353. 13.Diffraction peaks in XRD pattern of microcrystallinecatalysts belonging to the odd-odddd parity group are generally broadened by an order of magnitude relative to any other class of reflections. It is possible to show that these peaks are particularily sensitive to phosphorus atom order in the lattice. 14. (a) Miller, K.M., Strouse, C.E., Inorg. Chem., 23, (1984). 2395;(b) Gryder, J.W.; (b) Donnay,

G., Ondik, H.M., Acta. Crystallogr., 11, (1958)38.

15.Wells, A.F., “Structural Inorganic Chemistry”,5th Edition (1987),Oxford University Press, Oxford, p. 10,987. 16. Pisani, C., Dovesi, R., Roetti, C., “Hanree-FockAb Initio Treatment of Crystalline Solids”,

Springer-Verlag,Berlin, 1988.

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DISCUSSION CONTRIBUTIONS F. Trifiro (Department of Industrial Chemistry and Materials, University of Bologna, Bologna, Italy): What are the analogies or correlation's between the defect nature of the calcined catalysts (broadened x-ray lines) and the defects and structural polytypism you propose to exist in well defined crystalline materials?

Michael R. Thompson (Molecular Sciences Research Center, Pacific Northwest Laboratory, Richland, WA, USA): The defects in the single crystals are caused by the co-crystallizationof both enantiomorphsof vanadyl pyrophosphate within each crystal. A good analogy for this situation can be found in a paper by Charles Strouse and Kathy Miller (reference 14a). I would not expect this to be observable in the typical powder XRD pattern for a catalyst. The reason for this is that you would have to effect the relative ratio of the amount of each enantiomorph generated in the preparation. I do not believe we have much control over this. However, these defects are relevant to the catalysts. As Elizabeth Bordes found, microcrystallites in catalyst powders indicate diffraction spot streaking. These effects are completely analogous to the siluation in our single crystals. There probably exist domains of each enantiomorph within every mature microcrystalliteof the catalyst. The broader question you ask relates to the breadth effects of diffraction lines in catalyst XRD patterns. I think that the most illustrative peak broadening effect in XRD patterns is actually that for the (1.1,l) reflection at approximately 15.8' 2 8 ~ " .For the very mature catalysts which I am familiar, this peak is generally an order of magnitude broader than any other peak of it size in the pattern. Few, in any, other odd-odd-odd reflections are observed. Using arguments similar to those in text of our paper, it can be shown that this parity group (odd-odd-odd) is highly dependent on the phosphorus atom positions in the lattice. This is brought about by the symmetry of the structure. While I realize that the theoretical arguments made in the paper are somewhat obtuse in this brief format, the conclusions are important here. We observe that changing the placement of the phosphorus atoms within the structure of vanadyl pyrophosphate from one symmetry to another costs little in the way of energy to the system (15-20 Kcals). The conclusion that follows is that I might expect a great deal of variability in the phosphorus atom network as a consequence. Variability in phosphorus positions means greater breadth in these peaks. While these effects will be dramatic to the odd-odd-odd reflections, other peaks will be affected for other reasons. Anyone who has built a scale model of vanadyl pyrophosphate can tell you that there exists a great deal of rotational freedom within the basal plane. Would I expect two structures with different pyrophosphate symmetries to be able to pack their basal planes with exactly the same periodicity? The answer to this question is no. The dimensions of the cell (lattice constants) will not be identical and the placement of the peaks will change slightly, adding to breadth. Variability in structure which occurs in this manner will effectively reduce intensity in an exponential manner and the patterns will appear dramatically simple relative to what would be expected for a pattern for a discrete phase. With crystallite dimensionallity, catalyst maturity, and structural variation all playing rolls in broadening peaks, I would not put a great deal of effort into over interpreting these patterns.

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E. Bordes (Departement de Genie Chimique, Universite de Technologie de Compiegne, Compiegne Cedex, France): The question of knowing if the transformation of the hemihydrate is a true topotactic reaction or not could be secondary if this idea would not allow control in the manufacture of the catalyst instead of using a vague recipe. We showed that in a poster at Europa-Cat 1. Now I have two questions. How did you get these crystals and did you observe any morphological differences between them? What would be the selective force determining the formation of MAA: the cis- or trans(vanadyl)? Michael R. Thompson (Molecular Sciences Research Center, Pacific Northwest Laboratory, Richland, WA, USA): First, I think that the concept of topotaxy in this case is very important. The initial steps of the dehydration likely occur topotactically. and in effect, this begins to “zip” the structure together and places limits on the possible outcomes. But clearly, since the reaction proceeds through an amorphous intermediate with a change in the point group symmetry of the basal layer, a less proscriptive mechanism seems in order. The crystals were generated by carefully controlling the atmosphere and temperature of a mature microcrystalline commercial catalyst placed in a Lindberg oven for periods of up to six or seven days. The temperature program involved heating to approximately 1250’K and slow cooling of approximately 1‘ per hour. We believe that the most important factor was the maturity of the material used: these catalysts had been taken from reactors after more than 5000 hours in the butane oxidation reaction. As to the crystal morphology, crystals which we studied were varied. Green crystals were found which had plate-like dimensions, and others were cube or block shaped. Almost all crystals of the red-brown materials were block shaped. With respect to differences in reaction selectivity that might be exhibited by any of these polytypical vanadyl pyrophosphates, there is no way of knowing. We have hypothesized that the pyrophosphate termination of the crystal can effectively generate isolated active sites and we can also extrapolate that these sites possess different reactive centers depending upon the vanadium site occupancy. However, at this point in time the practical theoretical tools do not exist which would allow us to look at reactions at these surfaces. G. Centi (Department of Indusrrial Chemistry and Materials, University of Bologna, Bologna, Italy): One conclusion that can be derived from your data is that there exists the possibility of a surface reorganization of vanadyl pyrophosphate during the catalytic reaction, because the energetics of transformation between the various possible surface structures corresponding to the different degrees of disorder in the structure are evidently low. This suggests that during the catalytic reaction there is some degree of flexibility of the surface and therefore the actual surface seen by the molecules may change in the course of the reaction. What is your opinion about this question which suggests also a greater role of the conditions of reaction on the in-situ reorganization of the catalyst surface during the catalytic reaction?

Michael R. Thompson (Molecular Sciences Research Center, Pacific Northwest Laboratory, Richland, WA, USA): The theoretical calculations were performed to assess the differences which

181 wouId be expected for the crystal energies of materials as a consequence of changing the nearneighbor environments in vanadyl pyrophosphate. If framed properly. this might tell us something about the phase diagram for vanadyl pyrophosphate, but it does not give us any understanding about the kinetics nor the barriers for transformation.

J.J. Lerou (DuPont CR&D, Experimental Station, Wilmington, DE): There are two parts of the problem of selective oxidation on VPO - the structure of the solid phase is one, but there is also the interaction of the molecules at the surface. When do you expect to be able to model dynamically these interactions? Michael R. Thompson (Molecular Sciences Research Center, Pacific Northwest Laboratory, Richland, WA, USA): Quantum dynamics is a current hot topic in theoretical chemistry, There are people working both in the molecular and molecular cluster regime, and there are those working the methods to approach periodic materials (surfaces). For simple surfaces like MgO with simple reactions (c.f., chemisorption of water), methods will be generally available probably in two years or less depending on how it’s done. Surfaces and reactions as complex as n-butane to MAA on (1.0.0) vanadyl pyrophosphate, far longer if ever. We can look at the properties of static surfaces of (1,0,0) vanadyl pyrophosphate currently. These require hundreds of hours of supercomputer time to converge a single wavefunction. Consider that the consequences of reacting a substrate on a surface generally requires the lowering of the surface symmetry, and hence a requirement to explicitly treat a greater number of atoms. For this system, the computational requirements of the program Crystal scale approximately as n2.5. Hundreds of Cray-hours per energy point quickly escalate to thousands. More efficient means are needed.