PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 1995 Elsevier Science B.V.

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Systematic Control of C r y s t a l Morphology during Preparation of Selective Vanadyl Pyrophosphate.

E. Kesteman a, M. Merzoukia, B. Taouk a, E. Bordes a and R. Contractorb,

a D6partement de G6nie Chimique, Universit6 de Technologic de Compi~gne, B.P. 649, 60206 Compi~gne C6dex, France.

b E. I. du Pont de Nemours, Experimental Station Bldg 262, P.O. Box 80262, Wilmington, DE 19880-0262, USA.

The crystal morphology of (VO)2P207 can be determined by control of the pseudomorphic precursor VOHPO4.0.5H20. The standard method of preparation and calcination shows that the primary factor is the nature of alcohol used for reflux, which influences crystallinity and shape of secondary particles of precursor and catalyst. Surface area depends mainly on thickness and width of plates, the later being related to selectivity in oxidation of butane to maleic anhydride.

1. INTRODUCTION

When pseudomorphic relations exist between a solid catalyst and its precursor, one can expect that a control of its properties can be exerted through the control of those of precursor. In the case of vanadyl pyrophosphate used for the selective oxidation of n-butane to maleic anhydride, it has been noticed since a long time that the best performance of (VO)2P207 is related to the presence of (100) planes [1], which is characteristic of samples obtained by calcination of VOHPO4.0.5H20 precursor. To account for {100} specificity we have used the Crystallochemical Model of Active Sites (C.M.A.S.), and shown that this face displays the right cluster of selective sites allowing the formation of one maleic anhydride and four water molecules, while perpendicular faces would lead to oxidized by-products [2, 3]. These findings have been recently confirmed by Inumaru et al., [4] who have shown that poisoning {100} faces by orthosilicate results in making more CO2, which is produced therefore on perpendicular planes.

According to the equilibrium shape of (VO)2P207 crystals, {100} faces would be only 32 % of the external surface area of the crystal [3], the prismatic shape of which is like that for samples prepared by reduction of IB-VOPO4 at 760~ in N2 [5]. To optimize the area of selective {100} comparatively to other faces, the best way is to prepare VOHPO4.0.5H20 precursor because (001) VOHPO4.0.5H20 is structurally

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related to (100) (VO)2P2OT, so that pseudomorphic crystals of the catalyst exhibiting {100} faces are obtained (Fig. 1) [6, 7].

If the numerous parameters involved during prepara t ion and calcination- activation and their effect on the catalytic properties of (VO)2P207 have exten- sively been studied [1, 8-10, 15], relatively little work [4, 9, 12, 13] is devoted to the effect of preparation on crystal morphology of the catalyst although it finally rules selectivity. We have decided to focus on the effect of the alcoholic medium from which the precursor is precipitated after reflux, since this parameter decides on the temperature, on the amount of water and on the reduction power, that is on the final state of vanadium phosphorus compound. Al though it is known that performance is greater when P /V is close to 1.1-1.2:1, we have deliberately chosen the ratio 1:1 in order to favor the crystallization of the precursor. The influence of some other parameters, like addit ion of an acid or of tetraethylorthosil icate (TEOS) [11], dilution of H3PO4, duration of reflux, etc., has also been examined. The calcination of the precursor has always been performed in the same way, that is in a flow of inert gas supposed to induce the least changes in vanad ium valence. To summarize the question to be answered in the present paper: is controlling the crystal morphology of precursor controls the crystal morphology of the catalyst?

2. EXPERIMENTAL

V205 (0.26 mol.1 "1) and H3PO4 (85%) (P/V = 1:1) were mixed and refluxed (6 hrs) in various primary and secondary C2-C8 alcohols (Table 1), other parameters being kept constant. In the case of 2-Ethyl,l-Hexanol, VOPO4.2H20 was first prepared [6] and refluxed. The precipitate was filtered, washed three times with 80 ml of the same alcohol and dried at 120~ (12 hrs) in an oven. All samples of VOHPO4. 0.5H20 (P) were calcined in N2 at 500~ during 6 hrs (heating rate 30~ Other parameters were made to vary: TEOS was added to raw materials (@ 0.1 mol.1-1) in isobutanol and benzyl alcohol mixture (IBA/BA = 91/9) before reflux. Oxalic acid (0.26 mol.1-1) was added to raw materials in 2-butanol. H3PO4 was used at 99% and 50%. A longer time of reflux (24 hrs) was also experimented with most of alcohols.

Several methods (XRD, FTIRS, SEM, TEM, BET) were used to characterize the morphology of crystallites of P and resulting catalyst (C). XRD data were obtained with the same amount and same display of the powder on the sample holder. The mean thickness of {001}, {100} and "length" {220}, {042} of plates of P and C respectively (Fig. 1) was calculated using Scherrer formula. The ratios Rp = (220)/(001) and RC = (042)/(200) between the Full Width at Height Medium (FWHM) of (hkl) planes for P and C respectively were computed to have an evaluation of the relative dimensions of crystal plates. The angles between planes perpendicular to (001)P and (100)C were measured on TEM and SEM micrographs and compared to theoretical values computed from cell parameters [14]. In the case of C, the best fit was obtained for planes (043) and (092) (Fig. la) al though these values are not in accordance with the principle of low index faces. This could be due to a lack of precision of parameters (structure solved at Rw - 9 %) [14].

709

11o

I <oo,> VOHP04.0.5 H z 0 (Precursor)

04

iT<,oo, (VO) 2 P zO 7 (Catalyst)

Figure 1. Pseudomorphism between VOHPO4.0 .5H20 precursor (left) and (VO)2P207 catalyst (right): a) crystal shapes (angles and indexation of faces), b) TEM showing growing dislocations on EK14 (2-ethyl-l,hexanol), and catalyst.

3. RESULTS AND DISCUSSION

The powder pattern and FTIR spectrum of all precursors and catalysts prepared in alcoholic medium are consistent with those found in literature for VOHPO4. 0.5H20 (P) and (VO)2P207 (C) respectively, except in the cases of ethanol (presence of remaining V205, even after 24 hrs of reflux) and of H3PO4 50% (presence of VOPO4.2H20 and other lines corresponding to VOHPO4.nH20, n > 2) [13].

3.1. Inf luence of alcohol. Data on the temperature of reflux (6 hrs) and on surface area (SA), ratio R and

thickness of crystals of P and C are reported Table 1 and Figs. 2-4. The measured temperature of reflux can be very different from the theoretical point of ebul- lition, as for C9 and C62 (120 and 140 instead of 185 and 205~ for pure alcohols). When surface areas of P and C are plotted against temperature of reflux the high-

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Table 1 Effects of solvent on some characteristics of precursors (reflux 6 hrs) and catalysts

Precursor Catalyst

So l v en t T S.A. Rp* Thickness # S.A. RC* Thickness # (~ (m2/~) (001) (220) (m2/g) (200) (042)

C8 2-But.ol 100 5 1.0 545 560 6 2.3 185 425 C61 Isobut.ol 105 10 1.2 445 530 15 5.2 130 510 C2 I B A / B A 110 16 1.4 365 540 23 2.1 185 375 C41w ibid. 110 27 2.1 240 530 32 3.0 130 385 C14 ibid+TEOS 110 31 2.8 170 515 31 3.4 125 425 C9 @ Eth.Hex.ol 120 2 1.0 545 580 5 2.2 175 390 C62 Benz. alc. 140 23 2.0 250 530 28 1.0 350 510

*Rpand Rc = Ratio of FWHM of XRD lines; VOHPO4.0.5H20: Rp = (220)/(001); 20(001) = 15.5 ~ 20(220) = 30.5~ (VO)2P207 " RC = (042)/(200); 20(200)= 22.8 ~ 20(042) = 28.5 ~ (CuK00; #Plate thickness (~) by means of Scherrer formula: T (.~) = 0.89 Z/FWHM.cos0. IBA/BA = 91/9; Eth.Hex.ol = 2-Ethyl-l-Hexanol; TEOS = TetraEthylOrthoSilicate. w IBA/BA = 91/9 and P/V = 1.2:1; @ Prepared by reduction of VOPO4.2H20.

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Figure 2. Effect of reflux temperature of alcohol on surface area of precursor and catalyst (6 hrs) (symbols: open for precursor, black for catalyst)

est values are observed with i sobutanol /benzy l alcohol (C2) and pure benzyl alcohol (C62) (Fig. 2). They increase when P / V is increased up to 1.2:1 in IBA/BA (C41) and also when TEOS is added [11]. SA of catalysts are remarkably parallel to those of precursors and always greater. This is probably due to the high heating rate and use of nitrogen during calcination of the powdered precursor. Not only

711

water molecules escape as the result of dehydration, but also some molecules of alcohol, which were trapped inside the particles, escape.

Thicknesses and values of R characterize the morphology: the greater the values of Rp and RC the thinner the plates. T(220) and T(042) are only indicative of the mean "length" between {220}P and {042}C faces, while T(001) and T(200) are more representative of the actual thickness. Owing to the physical principle of X- Ray diffraction these values are valid for coherent crystals, so that thicknesses measured from SEM pictures are always greater because of stacking of plates (vide infra). When plotted against reflux temperature, the thickness of P plates follows the inverse trend of SA (Fig. 3): the thinner the plates, the higher the surface area. One could think that the shape of primary particles (agglomeration of plates, eventually as rosettes) would rule surface area, but it seems that the primary factor is the thickness of secondary particles. The low value of SA found with 2- ethyl,l-hexanol is probably related to the high crystallinity of the raw material (plates of VOPO4.2H20) which yields in turn large plates of VOHPO4.0.5H20. Catalyst plates are always (except C62) thinner than those of the corresponding precursor (mean value 180 instead of 365 ~), but there is no direct relation between thickness of {100}C or {001}P and surface area of C or P (Fig. 4). If it seems normal to consider that dehydration of P yields thinner plates of C because crystals are delaminated, the size of alcohol molecules trapped can influence the final thickness, as for C62 (benzyl alcohol). Data in Table 1 and Figs. 2-4 show that increasing P /V to 1.2:1 or adding TEOS are two methods to obtain the thinnest plates of P and C. Since TEOS is a nucleating agent the effect is better seen on P than on C, as {001}P makes only 170 ~ compared to the mean thickness 365 ~. In the case of P /V = 1.2:1 the excess of phosphoric acid adsorbed on plates could play the role of trapped alcohol, so that thickness of C is decreased.

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Figure 3. Influence of temperature and time of reflux on surface area (large symbols) and thickness (small symbols) of precursor; a) 6hrs; b) 24 hrs .

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Figure 4. Influence of surface area on thickness of plates for precursor and catalyst (large symbols: 6 hrs; small symbols" 24 hrs).

Plates 1, 2 present TEM and SEM pictures of precursors and catalysts. The characteristic shape of VOHPO4.0.5H20 is retained after calcination in (VO)2P207 and faces exhibited are {001}P and {100}C [6, 7, 13]. Dislocations are evidenced on some crystals of precursor as they grow spirally, but also on the catalyst (Fig. lb). Plates are generally well-shaped and more or less wide (mean size 2-8 x 1-2 x 0.1- 0.3 I~m). In the case of benzyl alcohol (Plate ld) primary particles have a shape rounder than generally observed (0 4 I~m).

SEM pictures of primary and secondary particles of P with TEOS and resulting C are very different from others as already observed by Horowitz et al., [11]. Very thin petals (1-3 I~m) with thickness = 50 nm are displayed as well developed rosettes of same size for P and C (Plates If and 2a, b).

3.2. I n f l u e n c e of other parameters . The concentration of H3PO4 as a raw material has some influence on the

characteristics of the solids (Table 2). Using butan-2,ol (C4, C8, C5) the highest surface area is obtained with H3PO4 85%. There is no effect on the thickness of P plates but catalyst plates are thicker when H3PO4 99% is used. Dilution at 50% yields hydrates other than VOHPO4.0.5H20, as evidenced on Plate 2e exhibiting square crystals characteristic of VOPO4.2H20 [14]. XRD pattern shows that a mix- ture of (VO)2P207 and other phases is obtained by calcination. With the same alcohol, addition of oxalic acid which is also a reductor leads to comparable surfa- ce area and slightly thinner plates (C7), but primary particles look like thick rosettes (O 4 I~m) (Plate 2f). With IBA/BA the trend is similar to that in butan- 2,ol, that is higher surface area and thinner plates with use of H3PO4 85% (C2-C3, Table 2). Addition of TEOS again reinforces this trend (C14-C13).

An increased time of reflux (24 hrs) corresponds to a lower surface area and thicker plates for all alcohols (0.2-0.6 l~m) (Tables 1, 3, Figs. 3b, 4). E2 is an exception

713

Plate 1. Examples of crystal morphology (SEM). a) 2-Propanol, x5K; b) 2-Butanol, x5K (24hrs); c) Isobutanol, x5K; d) Benzyl alcohol, xl0K; e) 2-Ethyl-lHexanol, x5K (24 hrs); f) TEOS in IBA/BA, xl0K (catalyst).

714

w : i: = ~iiiiil :~2:1~;=:i ::::ili!i!!!ii~,~,~!!~ii::iiii~!!~!'~'!~:::::ii~'!~,::,,, .......... :iiii:~::~ ::~::~

Plate 2. Addition of TEOS: a) Precursor, xl0K et b) Catalyst, xl0K. Dilution of phosphoric acid in 2-Butanol: c) 99%, x5K; d) 85%, x5K; e) 50%, x5K. Addition of oxalic acid: f) x5K.

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(benzyl alcohol) because the catalyst particles are thinner and smaller after 24 hrs than after 6 hrs.

Table 2 Dilution of phosphoric acid (Ph.A.) and effect of oxalic acid or TEOS

Precursor Catalyst

Solvent P h . A . S . A . Rp Thickness S.A. RC Thickness % (m2/~) (001) (220) (m2/g) (200) (042)

C4 2-But.ol 99 2 C8 ibid. 85 5 C5 ibid. 5O 2 C7 ibid.+Ox. Ac. 85 6 C2 IBA/BA 85 16 C3 ibid. 99 10 C14 IBA/BA+TEOS 85 31 C13 ibid. 99 26

1.0 547 560 5 2.0 217 1.0 545 560 6 2.3 185 higher hydrates* (VO)2P207 + X

1.1 510 560 12 2.3 165 1.4 365 540 23 2.1 125 1.3 385 525 10 1.2 320 2.8 170 515 31 3.4 125 1.4 360 510 22 2.5 130

438 425

385 425 395 425 340

* higher hydrates: VOPO4.2H20 + VOHPO4.nH20 (n > 2); X: extra lines (XRD).

Table 3 Influence of reflux time (24 hrs).

Precursor Catalyst

Solvent S.A. Rp Thickness S.A. RC Thickness (m2/[~) (001) (220) (m2/~) (200) (042)

E5 2-Propanol 7 0.9 525 570 8 2.2 195 435 E7 2-But.ol 3.5 1.0 540 560 6 2.3 200 450 E3 Isobutanol 13 1.2 465 560 20 2.9 160 400 E14 Eth.Hex.ol 6 1.0 565 580 15 2.0 210 425 E2 Benzyl alc. 8 0.9 465 560 11 2.5 180 450

4. CONCLUSION

By using standard conditions of preparation and calcination while keeping several parameters constant we have shown that the nature of alcohol with its properties (boiling point modified by H3PO4, acidity, reducing power, molecule size) influences surface areas, crystal shapes, thickness and areas of the exposed faces, that is crystal morphology. The pr imary factor on surface area is the thickness of plates and not the way plates are displayed. Increasing P /V or adding

716

TEOS results in higher surface areas, thinner plates being obtained with TEOS, while adding oxalic acid has little effect. It is seen that the conditions claimed to get the best performing catalyst (H3PO4 85%, P /V = 1.1-1.2, isobutanol/benzyl alcohol and TEOS) [11] are indeed optimum for morphology. Our study shows that, when topotaxy and pseudomorphism have been evidenced, controlling the morphology of precursor is indeed controlling the morphology of catalyst, in the sense that wide (and thin) particles of {001}VOHPO4.0.5H20 yield wide (and thin) particles of {100} (VO)2P207. Consequently the faces known to be selective are large enough while unselective perpendicular faces have a minimum area. A high heating rate seems to be a means to get higher surface area for the catalyst. However, as we performed calcination in nitrogen, the presence of V5+ on the surface, which seems to be necessary for better activity [5], cannot be favored. Atmosphere and temperature of calcination-activation are very impor tan t parameters and will have to be studied.

REFERENCES

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