Oxidehydrogenation of isobutyric acid to methacrylic acid
on iron phosphate catalysts: an experimental
and theoretical study (*)
J. C. VE´DRINE, J. M. M. MILLETand S. BORSHCH
Institut de Recherches sur la Catalyse, CNRS, associé a l’Université Claude-Bernard, Lyon I 2 avenue A. Einstein, F-69626 Villeurbanne Cedex, France
(ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)
Summary. — Iron phosphates and hydroxyphosphates of different compositions and therefore of different structural organizations have been used for isobutyric acid oxidehydrogenation to methacrylic acid in the presence of excess water. XRD and Mössbauer studies of the solids allowed us to characterize the crystalline structure of the samples before and after catalytic reaction. The best catalysts were shown to exhibit trimers of FeO6octahedra sharing oxygen ion faces and isolated one from the other by a cationic vacancy. Theoretical calculation using a simple extended Hückel theory allowed us to show that for such iron oxide trimers the excess electron of Fe2 1 with respect to the two Fe3 1cations was localized over the central and a lateral Fe cation. For dimers or larger clusters than trimers either the excess electron was shown to be localized on one Fe cation or completely delocalized along the Fe cations, respectively. The optimum catalytic property was assigned to the limited delocal-ization of the excess electron to a nearby cation while no electron delocaldelocal-ization or a too large electron delocalization led to a less performing catalyst.
PACS 82.65.Jv – Heterogeneous catalysis at surfaces. PACS 01.30.Cc – Conference proceedings.
1. – Introduction
Iron phosphates and hydroxyphosphates are known to be very efficient catalysts [1, 2] for the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) in the presence of excess water (IBA : H2O=12 : 1). The
reaction was shown to proceed via a Mars and van Krevelen mechanism [3] with the redox process being provided by the Fe3 1
OFe2 1cationic couple. It was also shown that the majority of the phases taken in the FeO-Fe2O3-H2O-P2O5phase diagram are more
(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.
or less active and selective for the reaction [4, 5]. The role of the excess of water was shown to maintain a sufficient hydroxylation extent of the active sites either on the surface or even in the bulk of the compound [6].
The aim of this presentation is to try to describe the optimum active-site for the reaction and to use a simple theoretical calculation based on semi-empirical molecular orbital theory to propose an explanation for such an optimization of the active-site structure. This work enters in a more general approach which aims at describing active sites in partial oxidation and oxidative dehydrogenation reactions as surface ensembles of atoms of a limited size isolated from the whole surface atoms and able to fulfil all the requirements necessary for the reaction to occur.
2. – Experimental
A series of iron phosphates and hydroxyphosphates has been synthesized in the laboratory and their properties compared to those of an industrial catalyst. The latter was supplied by Elf Atochem and was shown to be a mixture of the phases: CsFeP2O7,
aFe3(P2O7)2, FePO4and Fe2P2O7. It has been shown that CsFeP2O7, Fe2P2O7and FePO4
were not very active phases [7] and that the catalytic activity of the industrial catalyst was mainly due to the phase aFe3(P2O7)2[8]. The preparation of bFe3(P2O7)2by
solid-state reaction [9] and that of the hydroxyphosphates by hydrothermal synthesis [4] are presented elsewhere. Catalytic reaction was carried out in a flow microreactor w i t h 5 0 t o 6 0 m g c a t a l y s t a n d a t a fl o w r a t e e q u a l t o 1 c m3P s21 a n d a ga s f e e d o f
IBA : H2O : O2: N24 5.86 : 72.0 : 4.26 : 19.2 kPa under atmospheric pressure. The
tem-perature range was 633 to 693 K and the analysis of the products by gas chromatography on line. Methacrylic acid (MAA), acetone (ACE), propene (PRO) and CO2are the products formed.
Mössbauer spectroscopy was used to characterize both the oxidation state and the local environment of iron cations. The technique appeared to be very complementary to XRD analysis.
Theoretical modeling was performed using a simple model of the extended Hückel type of molecular orbitals to describe the electron distribution patterns in mixed-valence clusters composed of adjacent octahedral FeO6entities.
3. – Results and discussion
Iron hydroxyphosphates of different compositions as described in table I have been synthesized and characterized by XRD and Mössbauer spectroscopy before and after catalytic runs for 48 hours. The hydroxyphosphates compositions changed and after catalytic testing reached compositions which correspond all to the same solid solution of the type Fe3 1
4 2xFe2 13 x(PO4)3(OH)3 23xO3 x. The structure of this solid solution is a
framework built up from chains of face sharing Fe-O octahedra aligned along two perpendicular directions (100) and (010) in bFe2(PO4)O and along two nearly
perpen-dicular (857) directions (1, 2 3, 2) and (1, 3, 2) in Fe4(PO4)3(OH)3 (fig. 1), bFe2(PO4)O
and Fe4(PO4)3(OH)3 corresponding to the two end member compounds of the solid
solution.
These chains are connected one to the other by octahedra corners and PO4
TABLE I. – Catalysts composition before and after catalytic run at 658 K as determined from
XRD and Mössbauer analyses; B: barbosalite, L: lipscombite.
Composition of the catalysts
before catalytic test after catalytic test
Fe31 4 (PO4)3(OH)3 Fe31 4.25(PO4)3(OH)2.33O0.77 Fe31 2 Fe21(PO4)2(OH)2 B Fe31 2 Fe21(PO4)2(OH)2 L Fe31 5 (PO4)3O3 Fe31Fe21(PO 4)O Fe31 4 (PO4)3(OH)3 Fe31 3.87Fe210.38(PO4)3(OH)2.66O0.38 Fe31 4.25Fe210.25(PO4)3(OH)2.25O0.75 Fe31 4.25Fe210.25(PO4)3(OH)2.25O0.75 Fe31 3.50Fe211.50(PO4)3(OH)1.50O1.50 Fe31Fe21(PO 4)O
octahedra regularly separated by vacancies to a full occupation of the sites in the chains. This evolution was followed by Mössbauer spectroscopy which showed that when large clusters of face-sharing iron octahedra were present, an electronic charge transfer with a frequency superior to 108Hz did not allow observation of doublets characteristic of the ferrous and ferric cations but only one doublet corresponding to mixed valent (2.5) cations as previously observed [10]. The results showed that the mixed-valent-cation content of the catalysts were increasing up to 100% when the composition was closer to that of bFe2(PO4)O and that the barbosalite was containing
only small clusters of iron octahedra, presumably trimeric clusters, compared to lipscombite which had approximately the same composition after catalytic run. This has been attributed to the fact that the former phase was presenting in its structure before catalytic test only trimeric iron octahedra clusters. The structure of aFe3(P2O7)2 has
not been totally solved but it has been shown that it also contains trimeric clusters of face-sharing octahedra (Fe3O12)162[8]. The structure of bFe3(P2O7)2 is very particular
compared to other iron phosphates [11]. It is built up like aFe3(P2O7)2from (Fe3O12)162
Fig. 1. – Schematic representation of the structure of the solid solution Fe31
TABLEII. – Comparison of the catalytic properties of the hydroxyphosphates, bFe3(P2O7)2and an
industrial catalyst at 658 K in the oxidehydrogenation of isobutyric acid; PRO: propene, ACE: acetone, MAA: methacrylic acid. B: barbosalite, L: lipscombite, RMAA: rate of formation of MAA.
Phases Selectivity (%) RMAA
( 1028mol Q s21m22)
CO2 PRO ACE MAA
Fe31 4 (PO4)3(OH)3 Fe31 4.24(PO4)3(OH)2.38O0.72 Fe31 2 Fe21(PO4)2(OH)2B Fe31 2 Fe21(PO4)2(OH)2L Fe31 5 (PO4)3O3 bFe31Fe21(PO 4)O Ind. cat.(aFe3(P2O7)2)
bFe3( P2O7)2 7 9 6 9 11 22 2 69 6 11 7 6 8 j 9 2 41 25 21 31 31 46 19 26 46 55 66 54 50 32 70 8 55 113 223 114 87 82 222 10
TABLEIII. – Catalytic properties of the hydroxyphosphates at 658 K as a function of their mixed
valent iron cations content as determined from Mössbauer spectroscopic data. RIBA: rate of
transformation of IBA.
Compound Fe2.51
(%)
Selectivity (%) RIBA
( 1028mol Q s21m22)
MAA ACE CO2 PRO
Fe3(PO4)2(OH)2B Fe4.25(PO4)3(OH)2.33O0.77 Fe3(PO4)2(OH)2L Fe5(PO4)3O3 bFe2( PO4) O 0 6 15 33 100 66 55 54 50 39 21 25 31 31 33 6 9 9 8 23 7 11 6 11 5 340 206 210 174 210
clusters connected to each other via (P2O7) groups but with clusters composed of two
octahedra sharing faces with a central trigonal prism rather than three octhaedra sharing faces.
The catalytic properties of the synthesized hydroxyphosphates are summarized in table II and compared to those of bFe3( P2O7)2 and an industrial catalyst whose main
active phase was aFe3( P2O7)2. These properties have previously been
discussed [4, 5, 9]. Fe31
4 (PO4)3(OH)3 was the least efficient, being less active and less
selective than the other catalysts, while barbosalite was the most efficient catalyst with catalytic activity comparable to that of the industrial catalyst. Lipscombite, Fe31
5 (PO4)3O3 and Fe4.2431(PO4)3(OH)2.28O0.72 presented intermediate catalytic properties.
bFe3(P2O7)2was very poorly active and selective.
The hydroxyphosphates catalysts have obviously the same redox couple as aFe3(P2O7)2, the active phase of the industrial catalyst, but different basic sites since
O2 2 have been substituted for (PO
barbosalite (2):
Fe3(PO4)x(PO3OH)42x1y IBAKFe3(PO4)x2y(PO3OH)42x1y1y MAA ,
(1)
Fe3(PO4)2(OH)1.4O0.61y IBA Fe3(PO4)2(OH)1.412yO0.622y1y MAA .
(2)
The catalytic results clearly showed that the best catalysts are containing small clusters of iron octahedra and preferentially clusters of three octahedra [5]. Furthermore, when the catalytic properties of the samples with different electron delocalization as evidenced by Mössbauer spectroscopy were compared (table III) the selectivity to methacrylic acid decreased while that of total oxidation increased.
Finally, it can be noted that the configuration of the clusters may be an important parameter since it was observed that bFe3(P2O7)2 was poorly active. The trigonal
prismatic coordination of the Fe2 1seems to inhibit it from undergoing any oxidation as
is the case when Fe2 1 occupies an octahedron sharing faces with one or two other
octahedra [9].
As the experimental studies unambiguously showed that the catalytic activity of iron phosphates was linked to the intracluster electron transfer properties, the aim of theoretical studies was to follow the changes in these properties for clusters of different structures. Semi-empirical quantum-chemical calculations based on the extended Hückel theory were carried out as described in ref. [12, 13], for clusters with different numbers and arrangments of FeO6 octahedra. The intramolecular electron
delocalization in mixed-valence clusters is controlled by the interaction of the electronic states with the non-totally symmetric cluster distortions. We studied the stability of all clusters against different types of asymmetric distortions. Special attention was paid to the distortions containing displacements of bridging atoms (one atom in corner-bridged structures and three atoms in face-sharing clusters). The total energy was approximated as a sum of one-electron energies and the potential surfaces were built in the spaces of one or two considered active distortions. The resulting electronic distributions were then determined by minimizing the potential surfaces.
The most important features from such calculations can be summarized as below: i) for dimer at realistic iron-iron distances the excess electron (Fe2 1 with
respect to Fe3 1) is localized on one of the two cations;
ii) for trimer at similar metal-metal distances there is a delocalization (fast exchange) of the excess electron between the central atom and one of its two neighbors; iii) for larger clusters the excess electron is delocalized over several iron cations. By comparing the experimental results and the theoretical data, it can be proposed that the best conditions for catalytic reaction is when there is electron delocalization over two nearest cations. If delocalization does not occur as in dimeric clusters or if it is too extended as for clusters larger than trimers, the catalytic performances are not optimum. The same conclusion can be drawn up for bFe3(P2O7)2
where the intracluster electron transfer is hindered by the central FeO6prismatic unit.
4. – Conclusion
Some conclusions may be drawn from our experimental and theoretical studies. Iron hydroxyphosphates constitute an interesting family of active phases for oxidative dehydrogenation of isobutyric acid to methacrylic acid. For such materials a redox
mechanism only occurred without large structural changes. The best catalysts
corresponded to surface and bulk structures where a trimeric cluster of FeO6
octahedra sharing common oxygen ion face was present. Theoretical calculation of iron oxide clusters of different sizes showed that there is no electron delocalization for dimers while a strong electron delocalization over several Fe ions occurred on clusters with three octahedra and more.
In conclusion, the reaction deals with a Mars and van Krevelen redox mechanism and electron transfer. Moreover, Mössbauer spectroscopy data and theoretical calculations led us to propose that for high catalytic performances such an electron transfer should be limited to two nearby Fe cations. It follows that the best active sites should be composed of a limited amount of atoms (presently trimers of FeO6octahedra
sharing a face with three O atoms) isolated from the other by cation vacancies or PO4
groups, limiting electron transfer capability. This constitutes an example of a more general phenomenon and corresponds to a general description of active sites in partial oxidation reactions on metallic oxides.
R E F E R E N C E S
[1] DANIELC., assigned to Ashland Oil Inc., US patent 4 410 727 (1983).
[2] MILLETJ. M. M., VE´DRINEJ. C. and HECQUETG., New Developments in Selective Oxidation, in Study in Surface Science and Catalysis, edited by G. CENTI and F. TRIFIRO, Vol. 55 (Elsevier, Amsterdam) 1990, pp. 833-840.
[3] VIRELYC., FORISSIERM., MILLETJ. M. M. and VE´DRINEJ. C., J. Mol. Catal., 71 (1992) 199. [4] ROUZIESD., MILLETJ. M. M., SIEWHEWSAMD. and VE´DRINEJ. C., Appl. Catal., 124 (1995)
189.
[5] MILLETJ. M. M., ROUZIESD. and VE´DRINEJ. C., Appl. Catal., 124 (1995) 205.
[6] MILLETJ. M. M., FORISSIERM., ROUZIESD., BONNETP. and VE´DRINEJ. C., Proceedings of
the XI International Congress on Catalysis, in Stud. Surf. Sci. and Catal., edited by J.
HIGHTOWERet al., Vol. 101 (Elsevier, Amsterdam) 1996, pp. 1011-1020.
[7] MILLETJ. M. M. and VE´DRINEJ. C., Appl. Catal., 76 (1991) 209. [8] MILLETJ. M. M., PhD Thesis, Lyon (1990), no. 259-90.
[9] BONNETP. and MILLETJ. M. M., J. Catal., 161 (1996) 198.
[10] AMTHAUERG. and ROSSMANG. R., Phys. Chem. Min., 11 (1984) 37.
[11] IJJAALIM., VENTURINIG., GERARDINR., MALAMANB. and GLEITZERC., Eur. J. Solid State
Inorg. Chem., 28 (1991) 983.
[12] ROBERTV., BORSHCHS. A. and BIGOTB., Chem. Phys. Lett., 236 (1995) 491. [13] ROBERTV., BORSHCHS. A. and BIGOTB., J. Phys. Chem., 100 (1996) 580.
