Tribomechanical activation of vanadium phosphates by milling - effects on microstructure and catalytic properties of (VO)₂P₂O₇ in the oxidation of n-butane to maleic anhydride

Since the first publication of the crystal structure of vanadyl pyrophosphate (VPP/(VO)2P2O7) in 1979 [1] the question arose whether the unique catalytic properties of this compound in the oxidation of n-butane to maleic anhydride (MA) are associated with the exposure of a special crystallographic plane of the VPP crystal, i.e. the plane parallel to (100). Compared to the other planes the (100) plane is more exposed in high performance catalysts exhibiting platelet type habit than in VPP catalysts having other habits [2]. Therefore, it has been postulated that the butane oxidation is structure sensitive to the (100) plane [3] and the VPP crystal has been assumed to be catalytically anisotropic [4]. To answer the main question concerning the catalytic anisotropy of VPP vanadyl hydrogenphosphate hemihydrate (VHP/VOHPO4 · 0,5H2O) precursors prepared either by crystallization from aqueous solution (VHPH2O) or from alcoholic medium (VHPROH) and VPP catalysts exhibiting different morphology were tribomechanically treated (TT) in different laboratory mills: mortar (main energy input: shear forces), vibratory disc (shear, pressure), planetary ball (impact, friction). The duration and the strength of the treatment were varied. After shaping VHP granulate (1.25 - 2.5 mm) was transformed into fresh VPP in a tubular reactor under oxidizing conditions. All the VPP samples were conditioned for more than 12 h at reaction conditions to form quasi-equilibrated catalysts. The catalytic tests were carried out in a tubular reactor (1.5 vol% n?butane/air, flow rate: 3 - 18 l/hSTP). VHP and VPP were characterized by X-ray powder diffraction (XRPD) using a STADI P (Stoe, Germany) set-up (transmission, Ge primary monochromator, CuKa1, capillary technique). The orientation factors o1 characterizing the deviation of the habit from cube-shaped (o1=1) to platelet-shaped (o11) were calculated applying the March-Dollase function. The average primary crystallite sizes, internal micro strains, lattice parameters, and the proportions of the X-ray amorphous fraction were determined using appropriate software. The TT of VHPH2O and also of the fresh catalyst VPPH2O led to an improvement of the selectivity and activity in the conversion of n-butane. Hence, the maximum MA yields (temperature range: 430 … 460 °C) were en-hanced by 4 mol % independent of the type of mill used and when milling was carried out (TT of VHP or of fresh VPP). From the differences in the MA selectivities of untreated and milled samples found at low values of n-butane conversion it is to be concluded that milling of VHPH2O enhances both the number of active sites and the ratio of selective to non selective sites of n-butane conversion. In contrast to the changes in selectivity the increase in activity due to the TT of VHPH2O seems to depend on the type of mill used: the higher the sup-posed energy input the higher the catalytic activity. The results for VPPROH differ significantly from those obtained with VPPH2O. Surprisingly, the TT of VHPROH neither influenced markedly the mass related activity nor the MA selectivity at the reaction conditions chosen. However, milling of the fresh VPPROH led to an increase in activity associated with an enhancement in MA yield by 4 mol% due to the lower reaction temperature necessary to achieve the optimum n-butane conversion degree. From XRPD it was concluded that the crystals of both VPPH2O and VPPROH as well as of VHPH2O and VHPROH are distinguished by different orientation factors o1 which are characteristic for a cubic (o1 = 1, VPPH2O and VHPH2O) and a platelet-like habit (o1 = 0.6, VPPROH and VHPROH), resp. These factors behaved differently in the treatment process: for VHPH2O o1 remained constant, i.e. the original cubic habit did not change. In contrast, o1 of the platelet-shaped VHPROH changed drastically from 0.6 to 1.0, i.e. to cubic habit. These results are confirmed by SEM images and speak against a pre-ferred exposure of a special surface plane of the VPP crystals after each kind of milling procedure and, there-fore, against a correlation of catalytic performance and the exposure mentioned. The results of VHPH2O microstructure analysis revealed that TT caused a marked reduction of the unit cell volumes and a relatively isotropic lattice contraction. Furthermore, TT led to drastically changed crystallite sizes and internal micro strains whereas both the selectivity and the activity of all the milled catalytic materials increased as described above. Analogous microstructure changes were obtained in the case of VHPROH/VPPROH. These results of the microstructure analysis and of the catalytic tests show that VPP is not catalytically anisotropic: the improved catalytic properties of the tribome-chanically treated VPP samples cannot be due to the exposure of a special surface plane since the orienta-tion factors of all materials became 1 after milling. In the case of VHPH2O, the enhanced catalytic performance could be associated with an increase in disorder in VPPH2O crystals expressed by the observed changes in micro strains, crystallite sizes, and unit cell volumes. On the other hand, such properties like the number of step sites of the comminuted particles should be considered, too. In this sense a structure sensitivity of the n-butane oxidation on VPPH2O cannot be excluded. However, the unique catalytic properties of VPP in the oxidation of n-butane to MA are not associated with an exclusive exposure of the active sites on surface planes parallel to (100