Catalytic Selective Oxidation - ACS Publications - American Chemical

Chapter 16

V—Ρ—Ο Catalysts in n-Butane Oxidation to Maleic Anhydride Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on November 12, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch016

Study Using an In Situ Raman Cell 1

1

1

2

J. C. Volta , K. Bere , Y. J. Zhang , and R. Olier 1

Institut de Recherches sur la Catalyse, Centre National de la Recherche Scientifique, 2 avenue Albert Einstein, 69626 Villeurbanne Cédex, France Laboratoire de Physicochimie des Interfaces, Centre National de la Recherche Scientifique, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, B.P. 163, 69131 Ecully Cédex, France 2

The evolution of the structure of four vanadyl phosphate hemihydrates has been studied and then used as catalysts for n-butane oxidation using an in-situ Raman 31

cell and P MAS-NMR. The catalytic performance for maleic anhydride formation can be explained by their transformation into α and δVOPO4on the II

(VO) P O 2

2

7

matrix as evidenced by Raman spectroscopy during the course of 5+

the reaction. The best catalytic results correspond to a limited number of V

sites forming small domains situated on and strongly interacting with the (100) (VO) P O 2

2

7

crystalline face. This is therefore another example of a Structure

Sensitive Reaction.

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO) P O 2

2

7

is the central phase of the Vanadium Phosphate system for butane

oxidation to maleic anhydride (1 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430°C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO) P O , V (IV) is mainly detected by X-Ray Diffraction. 2

2

7

0097-6156/93/0523-0217$06.00/0 © 1993 American Chemical Society Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

218

CATALYTIC SELECTIVE OXIDATION

However, the use of techniques which analyze both the short and the long range orders of the VPO materials, like RED of X-Rays (2 ), 31p and

5 1

V MAS-NMR (3,4 ),

showed the possible participation of some V (V) structures to the reaction. Previously, there has been some ambiguity insofar as these structures should be a consequence of the reoxidation of the starting VOHPO4, 0.5 H2O precursor or of the basic

Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on November 12, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch016

(VO)2P2C>7 matrix and hence do not intervene directly in the reaction mechanism of butane oxidation to maleic anhydride. The possible role of a limited amount of V(V) sites to control the selectivity to maleic anhydride was previously postulated (5 ). With the aim to study the characteristics of VPO catalysts in the course of butane oxidation to maleic anhydride together with a simultaneous evaluation of the catalytic performance, we have used Raman spectroscopy which is a very sensitive probe for determining the presence of VOP04-like entities together with (VO)2P207- An in situ Laser Raman Spectroscopy (LRS) cell was built in our laboratory (6). In the corresponding publication (6 ), the preparation and the characterization by XRD, 31p and V NMR of the different VPO phases, (VO2P2O7, α , β, γ and δ VOPO4 has 5 1

π

been described. The LRS spectra were registered up to 430°C in butane/air atmosphere. In this communication, we compare VPO catalysts which differ by their conditions of preparation, considering both their LRS spectra registered under reaction conditions and the corresponding catalytic results. LRS data are discussed in relation with results for η-butane oxidation to maleic anhydride.

Experimental A schematic diagram of the in situ cell, built at the Institut de Recherches sur la Catalyse and used for the Laser Raman study of the materials has been described elsewhere (6 ). It is presented in Figure 1. It is made of three parts of stainless steel with different functions. The solid to be examined under reaction conditions was placed in the lower part on a glass sintered disc. For the present study, 1.5 g of VPO catalyst was used for each run. The temperature was controlled by a thermocouple placed at the centre of the catalyst powder. The reaction gases flowed through the powder in the middle of this lower part which was heated by three thermoregulated fingers. The upper part held a glass window transparent to the laser beam. The gaseous effluent from the cell was analyzed by gas-chromatography. The tightness of the cell was ensured by two goldrings.The composition and flow rate of the reacting gases (2.4% butane/air) were controlled by two flow meters. Experiments were done with a flow

Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


VOLTA ET AL.

V-P-O Catalysts in η-Butane Oxidation

219

Y Figure 1 : A/Chamber upper part B/Chamber lower part C/Glass window D/ Isolating shell E/Thermal screen F/Anticaloric filter 1. Catalyst 2. Sintered glass 3 and 4. Tightening rings 5. Temperature controller 6. Heating finger lodging 7. Chamber heating controller 8. Thermal security 9. Mounting bolts 10. Rotating lens.

(Reproduced with permission from ref. 6. Copyright 1992 Academic Press.)


220


rate of 3.61. h' . Detection of evolved gases was done by a FID detector. CO and CO2 1

amounts were determined by conversion to CH4 on a Pt/yAi203 catalyst working at 300°C. It was thus possible to analyze all the gases with the only FID detector. Two columns were used in parallel: a lm 1/4 in. Porapak Q column to separate CO and CO2 which were further transformed into CH4, whereas butane, acetic and acrylic Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on November 12, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch016

acids and maleic anhydride were separated on a 3m 1/8 in. Lac 2R (13%)/H3P04 (2.5%) on Gas ChromQ column. The two columns were heated at 140°C. The tube connecting the cell to the chromatograph was heated at 120°C in order to avoid any condensation of the reaction products. Helium was the carrier gas. Raman spectra were recorded on a DILOR OMARS 89 spectrophotometer equipped with an intensified photodiode array detector. The emission line at 514.5 nm from A r

+

ion laser (SPECTRA PHYSICS, Model 164) was used for excitation. The power of incident beam on the sample was 36 mW. Time of acquisition was adjusted according to the intensity of the Raman scattering. LRS spectra were recorded during the activation of the different VOHPO4,0.5 H2O precursors up to 440°C in the butane/air atmosphere, with a simultaneous measurement of the butane conversion and selectivity to maleic anhydride. Temperature was maintained at 420°C for 16 hours. 1000 spectra were accumulated for these three periods in order to improve signal to noise ratio. The wavenumber values obtained from the spectra were accurate to within about 2 cm~l. In order to reduce both thermal and photodegradation of samples, the laser beam was scanned on the sample surface by means of a rotating lens in the same way as described in ref. (7 ) . The scattered light was collected in the back scattering geometry. The presence of the different VPO structures in catalytic conditions was determined from the LRS spectra of the pure VPO reference phases recorded at the same temperature and published elsewhere (6 ). The 31p spectra were recorded on a Brucker MSL-300 spectrometer operating at 121.4 MHz s. The 31Ρ NMR spectra were obtained under MAS conditions by use of a double bearing probehead. A single pulse sequence was used in all cases and the delays were chosen allowing the obtention of quantitative spectra (typically the pulse width was 2 ms (10°) and the delay was 10 to 100 s. The number of scans was 10 to 100. Spectra were refered to external H3PO4 (85%). Four different VOHPO4,0.5 H2O precursors were studied. They were prepared in organic medium according to the EXXON method with isobutanol as reducing reagent (8 ). They differed by the starting vanadium material (Guilhaume N. and Volta J.C.,


16. VOLTA ET AL.

V-P-0

221

Catalysts in η-Butane Oxidation

unpublished results). After the LRS cell examination, the sample powder was stored under dry argon atmosphere to avoid any hydration, prior to a XRD and 31p NMR examination.


Results and Discussion Figure 2 shows the LRS spectra (1000 accumulations) of the four VPO catalysts recorded at 420°C under reaction conditions with the corresponding catalytic results at stationary state obtained in the LRS cell. It is noteworthy that both butane conversion and MA selectivity are lower than those obtained on the same catalysts in our laboratory reactor for butane oxidation. Note that the thermal characteristics of the LRS cell are highly different from those of a classical tubular reactor for which the catalytic bed is made of cylindrical pellets and is settled in a salt bed. It is likely that there should be a large thermal gradient in the perpendicular direction of the cell from the thermoregulated fingers up to the glass window, so that, in spite of the thinness of the catalytic bed, the sample powder may work at temperature lower than 420°C, as measured. These reasons may explain the poor results obtained in the LRS cell. It appears that the four VPO catalysts differ in the LRS spectra by the relative distribution of the three (νθ)2Ρ2θγ, ajj and δ V O P O 4 phases. Raman spectroscopy is highly sensitive to the detection of the 1

VOPO4

structures which are unambiguously

1

observed at 994 cm" and 1015 cm" for ajj and δ V O P O 4 respectively (6 ). Their presence is confirmed by the XRD spectra (Figure 3) and the 31p NMR spectra (Figure 4) of the catalysts recorded at room temperature after the LRS cell run. The signal observed at - 20.3 ppm is characteristic of ajj, while the signal at - 13.3 ppm in the absence of β V O P O 4 (Figure 3) is characteristic of δ V O P O 4 (6 ). Note the absence of any broad signal which is indicative of a low v5+/v4+ interaction between the large VOPO4

domains and the (VO)2?207 structure. For the presence of (Xjj and δ

VOPO4,

there is a very good agreement between the LRS informations obtained at

420°C and the XRD and 31p NMR informations obtained at room temperature. The coexistence of the two ajj and δ V O P O 4 phases at 420°C is logical insofar as it was previously observed that δ V O P O 4 is partially transformed into CCJJ

VOPO4

conditions (6 ).


in catalytic



222


V-P-0 Catalysts in η-Butane Oxidation


16. VOLTA ET AL.


223



224


16. VOLTA ET AL.

225

V-P-O Catalysts in η-Butane Oxidation

From Figure 2, it appears that higher selectivity for MA at 420°C is observed for catalysts 1 and 2 for which (VO)2P2C>7 is principally observed with a small extent of (XJJ

VOPO4. Catalysts 3 and 4 which highly develop OCJJ and δ VOPO4 phases together

with (VO2P2O7 are less selective but more active. The lower relative intensity of the


(200) line (Figure 3) is associated with a higher development of the (XJJ and δ lines, so that we can postulate that nucleation of these two crystalline phases is not associated with the basal (100) (VO)2P2Û7 face but is more in relation with the lateral faces of the corresponding crystals. In order to study the influence of the atmosphere of activation both on the catalytic M

results and on the physicochemical characteristics of the VPO catalyst, a (VO)2P207" catalyst (600 mg) was prepared by dehydration of a classical VOHPO4, 0.5 H2O precursor under an oxygen-free argon atmosphere at 440°C during 26 hours until the stationary state was reached. This catalyst was then tested in a classical reactor with 2.4% butane/air from 300°C up to 450°C for two consecutive runs. Catalytic performances are given in Figure 5. It is clear that both butane conversion and MA selectivity are improved in the second run when compared to the first run. After the second run, catalytic performances approach those of the VPO catalyst prepared from the same VOHPO4, 0.5 H2O precursor treated directly in the 2.4% butane/air atmosphere. Figure 6 shows the evolution of the 31p NMR spectra before catalysis (Figure 6a), after the first run (Figure 6b) and after the second run (Figure 6c). The first run implies both an increase of the number of the V^+ ions interacting with V^+ and an increase of the size of their domains (decrease of the band width) (Figure 6b), while the second run implies a redispersion of the V^+ ions and a decrease of their number (Figure 6c). This evolution is correlated with the modification of the X-Rays spectra of the catalysts examined before and after the two consecutive runs (Zhang, Y.J., Sneeden, R. and Volta, J.C., Catalysis Today, in press). The main feature is a changing of the profile of the (200) (VO)2P27

l i n e

a t

around 23° 2Θ, the width of

which diminishes from spectra of "(VO)2P207" to spectra of "(VO)2P207"(lst run) and spectra of "(VO)2P207" (2nd run). It is difficult to give a clear explanation to this evolution but we consider that it could be the result of an increase of the organization


226



100

• ConvB .(l) ut

2

ο ConvBut.( )

250

300

350

400

450

•

S A M ( D

•

scod)

•

S

C

0 2 ( D

"

S M(2)

*

S

*

S 02(2)

A

C

0

( 2 )

C

500

T(°C) Figure 5 : Evolution of the performances of the "(VO)2P207" catalyst activated under argon atmosphere after two succesive catalytic runs (300-450°C).




16. VOLTA ET AL.


227

228


of the (VO)2P2C>7 structure (a progressive disappearance of the (201) (VO)2P2C>7 line is simultaneously observed) which could be connected with the modification of the dispersion of the V^+ ions observed by 31p NMR. The fact that only the profile of the (200) ( ν θ ) 2 Ρ 2 ^ 7

l i n e

i s

perturbated may be

explained by suggesting that V^+ species principally affect the corresponding (100)


basal plane of (V0)2P2O7- The improvement of the catalytic performances may be thus associated with a subsequent variation of the v5+/v4+ local interaction : the number of the V5+ sites interacting locally with

sites changes depending on conditions of

activation so that there should be a specific ratio for the best catalyst corresponding to small V^+ domains on (VO)2P2Û7 with low interaction.

Conclusions This study has resulted in interesting informations concerning the active sites of the VPO catalysts for η-butane oxidation to maleic anhydride being obtained. The study of VPO catalysts in the course of η-butane oxidation by an in-situ Raman cell has shown that catalytic performances can be explained by the presence of the ctjj and δ VOPO4 phases on the ( ν θ ) 2 ? 2 θ γ matrix. However, if ( ν θ ) 2 ? 2 θ γ is the basic phase for this reaction, the present study confirms the participation of V^+ entities detected by 31p 4

5

NMR. Superficial V +/V + distribution is determined by the atmosphere of treatment of the catalysts. This was evidenced both by the catalytic and the physicochemical evolution of a (VO)2P2Û7 catalyst from the activation of the VOHPO4, 0.5 H2O precursor under an oxygen-free argon atmosphere of calcination to the butane/air atmosphere of catalysis. Best catalytic results correspond to a limited number of V^+ sites forming small domains with a strong interaction with the ( ν θ ) 2 ? 2 θ γ matrix. From the evolution of the X-Ray diffraction spectra, it can be postulated that these domains affect principally the basal (100) crystal face. We previously observed that the γ 4 + / γ 5 + distribution depended also on the morphology of the VOHPO4, 0.5 H2O precursor which could be determined by the conditions of its preparation (9 ). The oxidation of butane to maleic anhydride appears as another example of a Structure Sensitive Reaction (10 ). The formation of maleic anhydride (MA) could occur on the basal (100) ( ν θ ) 2 Ρ 2 θ γ face as it was previously proposed (77 ), but with a participation of a suitable number of V^+ entities. The local superficial v5+/v4+


16.

VOLTA E T AL.


229

distribution in this face should control the catalytic results. Side faces of (VO)2P2C>7 located in the direction which appear to be faces for nucleation and growing of (XJJ and δ VOPO4 phases, should be less selective for M A . The presence of ajj VOPO4 is partially a consequence of the transformation of δ VOPO4 under the reaction


conditions (6 ). Depending on these ones, a redispersion of the VOPO4 structures should occur from the side faces to the basal (100) (VCO2P2O7 face. A model of the catalyst is proposed in Figure 7.

dispersed V sites or small V O P O 4 domains Vistrong interaction with (ΥΌ)2Ρ2θ7) 5 +

side faces ( direction)

large V O P O 4 domains, precursors of the V O P O 4 structures δ-* απ; (low interaction with (νθ)2Ρ2^7) Figure 7 : Model for the transformation of η-butane into maleic anhydride on the "(VO)2P207 catalyst M

Acknowledgments Authors are indebted to Dr. F. Lefebvre for 31p NMR experiments and for the interpretation of the corresponding results. They thank Dr. J.C. Védrine for fruitfull discussions.


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Literature cited

(1) Centi, G., Trifiro, F., Ebner, J.R. and Franchetti, V.M., Chem. Rev., 88, 55, (1988). (2) Bergeret, G., Broyer, J.P., Gallezot, P., Hecquet, G. and Volta, J.C. J. Chem. Soc., Chem. Comm., 825, (1986). (3) David, M., Lefebvre, F. and Volta, J.C., 11th Iberoamericano Symposium on Catalysis, Guanajuato, 1988, IMP Edit, p. 365. (4) Harrouch-Batis, N., Batis, H., Ghorbel, Α., Védrine, J.C. and Volta, J.C., J. Catal, 128, 248, (1991). (5) Cavani, F., Centi, G., Trifiro, F., Grasselli, R.K., Preprints ACS Symposium of Division of Petroleum Chemistry, "Hydrocarbon Oxidation", New Orleans Meeting, Sept. 1987. (6) Ben Abdelouahab, F., Olier, R., Guilhaume, N., Lefebvre, F. and Volta, J.C., J. Catal., 134, 151, (1992). (7) Zimmerer, N., Kiefer, W., Appl. Spectrosc., 28, 279, (1974). (8) Yang, T.C., Rao, K.K., Der Huang, I., Exxon Res. Eng. Co., US. Patent, 4.392.986, (1987). (9) Guilhaume, N., Roullet, M., Pajonk, G., Grzybowska, B. and Volta, J.C., " New Developments in Selective Oxidation by Heterogeneous Catalysis, Stud. in Surf. Sci. and Catal., P. Ruiz and B. Delmon Ed., Elsevier, Amsterdam, Vol. 72, p.255, (1992). (10) Volta, J.C. and Moraweck, B., J. Chem. Soc., Chem. Commun., 338, (1980). (11) Ziolkowski, J., Bordes, Ε. and Courtine, P., J. Catal., 122, 126, (1990). RECEIVED November 6,

1992


Catalytic Selective Oxidation - ACS Publications - American Chemical

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