On the NiO, MoO3 mixed oxide correlation between preparative procedures thermal activation and catalytic properties

SOLID STATE IONICS

Solid State Ionics 63-65 (1993) 731-735 North-Holland

On the NiO, MoO3 mixed oxide correlation between preparative procedures thermal activation and catalytic properties C. M a z z o c c h i a , A. K a d d o u r i , R. A n o u c h i n s k y Politecnico-Dipartimento di Chimica lndustriale, Piazza Leonardo da Vinci 22, Milano, Italy

M. Sautel a n d G. T h o m a s C.R.E.S.A., Ecole Nationale Supbrieure des Mines, 158 cours Fauriel, 42100 Saint Etienne, France

In order to achieve a better control of the final characteristics of nickel molybdate catalysts employed in the oxidative dehydrogenation of propane, a study of the dependence of the catalytic activity upon the preparation methods has been undertaken. A new oxalic precursor is compared to those precipitated from an ammonia solution and is particularly studied because it leads to more selective phase I~stabilized at room temperature.

1. Introduction Nickel molybdate is a well-known active catalyst for oxidation reactions and particularly for the oxidative dehydrogenation of propane to propene; at atmospheric pressure two phases can be observed, the low temperature phase ct and the high temperature phase [L Furthermore recent studies [1,2] have shown that the high temperature phase [3 gives the best yield in the oxidative dehydrogenation of propane [2]. This phase is formed, accordingly to the H T X R D study, after heating the low temperature ct phase up to 700°C for 15 min. [1 ]. In a successive paper [3], it has been pointed out that the phase crystallizing after the thermal treatment of the precursor at a temperature around 500°C is the fl phase, and catalytic tests have been run activating "in situ" the precursor directly in the reactor employed for the reaction. In this case a technical problem arises, as the [3 phase undergoes a phase transition on cooling at around 300°C (13--,a) [4]. To maintain the catalyst in the more selective phase for a longer period, it requires the continuous heating o f the reactor, which cannot be cooled at room temperature. That is the reason why the aim o f the research and the object o f the present paper is developing a new NiMoO4 precursor that could, after thermal activa-

tion, lead to a room temperature-stabilized l] phase [51.

2. Preparation of precursors A Mettler RC 1 reactor calorimeter is employed for the preparation of the a m m o n i a precursors with a constant stirring speed of 100 rpm. Thus a careful control o f the conditions (pH, T) particularly during the precipitation is obtained. Experiments have been carried out with several techniques: T G - D T A , high temperature X R D and mass spectroscopy. Surface area measurements were also performed. 2. I. Yellow powder (A)

The starting compounds are nickel nitrate 6H20 and molybdic acid ( F L U K A ) . A m m o n i a is added as 30% volume solution. Molybdic acid is dissolved with a m m o n i a in 800 ml distilled water (0.25 M) and heated at 90°C. The p H is adjusted to 5.4. An equimolar nickel nitrate solution maintained at constant temperature is added at this point to the molybdic solution, and a yellow precipitate is immediately formed. Filtered after 30 min in thermo-

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732

C. Mazzocchia et al. / NiO, MoO3 mixed oxide correlation

regulated conditions, the precursor is a b u n d a n t l y washed with hot distilled water and heated at 120 °C during 15 h. 2.2. Green powder (B) The starting c o m p o u n d s are nickel nitrate 6 H 2 0 and a m m o n i u m h e p t a m o l y b d a t e 12H20 ( F L U K A ) . A m m o n i a is a d d e d as a 30% volume solution. A m m o n i u m h e p t a m o l y b d a t e after being dissolved in 800 ml distilled water (0.25 M ) is heated at 90°C, and the p H a d j u s t e d with a m m o n i a in order to attain the required [ N H 3 ] / 2 [ M o ] ratio (see table 1). A concentrated nickel nitrate solution, containing a stoichiometric a m o u n t o f Ni with respect to Mo and kept at the same t e m p e r a t u r e ( 9 0 ° C ) is then a d d e d to the molybdic solution with a d r o p p i n g rate o f 7 ml/min. A green precipitate is i m m e d i a t e l y formed and the powder filtered after 30 rain in thermoregulated conditions. After washing the filtrate with hot distilled water, the precursor is heated at 120°C during 15 h. In table 1 are s u m m a r i z e d the precipitation conditions and the formula for each precursor. 2.3. Oxalic precursor (C) A m m o n i u m h e p t a m o l y b d a t e ( F L U K A ) is a d d e d to a 250 ml solution containing a large excess o f oxalic acid, and after dissolution a quantity o f nickel nitrate ( F L U K A ) corresponding to an atomic N i / Table I Formula and precipitation conditions of NiMoO4 ammonia precursors. pH

< 5.4 5.6 6 7.5

Temperature

Precursor obtained

precipitation (°c)

filtration (°c)

25 65 85 90

25 85 90

A a) E b~ S c) V d)

a) A (blue) = (NH4)4H6NiM06024, m H20, n NH3 (4). b) E=S+A (S=stoichiometric). c) S (yellow) =NiMoO4, m HzO, nNH3 (m= 1/3; n=5/3). ,1) V (green)=S+Ni(OH)2 (m=3/4; n=3/4).

Mo ratio equal to one is introduced at ambient temperature. The solution (0.14 M in Ni or M o ) is then slowly heated under v a c u u m until a t e m p e r a t u r e o f 40 °C is attained. The precipitation starts i m m e d i a t e l y and it increases while water is being evacuated. After total elimination o f water, the precursor is heated at 120°C during 15 h.

3. Thermal activation of the different precursors 3. 1. Anal.vtical techniques" The X-ray diffraction patterns o f the sample are p e r f o r m e d on a D5000 Siemens diffractometer with a filtered Cu K a radiation and a count time o f one second in the interval 10 ° < 2 0 < 6 0 °. F o r the experiments performed at high temperature, an Anton Paar camera is used. The heating rate is 1 ° C / s with a count time o f 3 s and the temperature stabilized during 60 s before the diffractogram acquisition. A 1700 Perkin-Elmer and a Mettler TA 2000C thermal analyzer have been used for differential thermal and thermogravimetric analysis. Experiments have been carried out under two different gases (nitrogen N 50 or oxygen N 48 ), with a heating rate o f 2 0 ° C / r a i n and a cooling rate o f 1 5 ° C / m i n . In order to identify the desorbed gases by a Balzers QMG-111-A mass spectrometer, some milligrams are heated in a vertical furnace under vacuum, with a heating rate o f 10°C/rain. The spectrometer source ionized molecules in ionic fragments and the measured m / e ratio between mass and charge is within the range 1-200. 3.2. Characterization o f precursors 3.2.1. Yellow and green powders The X-ray diffraction pattern observed for the yellow a m m o n i a precursor is the same as the one indicated by Corbet [6], who identified the comp o u n d as an hydrated nickel m o l y b d a t e whose formula would be NiMoO4, m HzO, n NH3, while the X-ray spectra o f the green powder have been reported by Astier [7] for a large range o f N i / M o ratios ( 0 . 5 - 2 ) .

C. Mazzocchia et al. / NiO, MoOa mixed oxide correlation

The detailed study of the thermal evolution of both the ammonia precursors has been described elsewhere [ 3,8]. It may be reminded that for these two precursors the polymorphic form obtained at 450 °C is the high temperature 13phase (figs. 1 and 2), and that after cooling to room temperature the tt phase is present almost pure in the case of the yellow precursor, while in the case of the green powder the return transition takes place only after 30 min, and there is probably a partial stabilization of the 13phase at ambient temperature due to a small nickel excess [4].

into account. The oxalic precursor is a mixture of the following solids: (NiC204, 2H20> and (MOO, C204, 4H20>. 3.2.3. Evolution on heating (02)

(MoOC204, 4H20>

~

i

75

/

MoO~ 3 --'-~-~--.45~"/

_--~IZ7~% 15

20

. 25

.

30

.

. 35

40

NiMoO$~

~O/~recursor

ANGLE 2 THETA

Fig. 1. HTXRD of the yellow precursor.

oO4~ - ~ - -15- ~ - ~20

25

~~ 30~

w ~ 0 2 ~ .......

35

ANGLE 2 THETA

Fig. 2. H T X R D of the green precursor.

1°°°c' ( M 0 0 C 2 0 4 >

+ 4 H20 (NiC204, 2H20)

(MoOC204 > + 3.2. 2. Oxalic precursor The behavior of the oxalic precursor during thermal activation, and consequently the phase composition of the final catalyst, can be better understood if the different decomposition patterns of the simple nickel and molybdenum oxalates are taken

733

10 2

210°c> ( N i C 2 0 4 ) + 2 280°C'

H20

+ 2 CO2

+ ½02 >2sooc (MOO3 > +~O2 3s°°c' ( N i O > + 2 CO 2 + >45°°c' 13. In the case of molybdenum, the great excess of oxalic acid reduces the Mo (VI) ions leading to an oxalic salt containing Mo (IV), as evidenced by the XRD spectra obtained after thermal activation under nitrogen, which showed only M002 diffraction lines. The endothermic effects recorded under 02 are related to the weight loss of water first (peaks at 180 and 250°C), and then to the decomposition of the oxalate (peak at 310°C). The strong exothermic effect recorded at around 370 °C is rather independent from the weight loss and attributable to the oxidation and crystallization of MO (IV) into M003. Under nitrogen the same endothermic effects are observed, but a smaller exothermic peak appears at a higher temperature (420 oC) and could be attributed to the crystallization of MOO2. Comparing the results of thermal activation of the precursor under different gaseous conditions, the only remarkable difference is that no exothermic effect is observed under nitrogen while a strong one is seen with oxygen. Mass spectra recorded during the decomposition of respectively Mo and Ni oxalates show that only water and CO2 are desorbed. The mixed oxalate precursor XRD spectrum is a superposition of its Ni and Mo oxalic components and it decomposes accordingly, showing all the thermal effects described before; a final exothermie peak

734

c. Mazzocchia et al. I NiO, MoOj mixed oxide correlation

corresponds to the crystallization o f NiMoO4. In fig. 3 the mass spectra and weight loss behavior versus temperature for this c o m p o u n d are shown. D e p e n d ing on the thermal treatment, and particularly considering heating rate and p e r m a n e n c e time, the final phase composition could change significantly (table 2). To obtain a crystallized NiMoO4 containing a limited a m o u n t o f MoO3 and N i O it is necessary to d e c o m p o s e the oxalates as soon as possible (direct introduction in the preheated oven at 550°C). The catalyst obtained in this way will show a remarkable stabilization o f the 13 phase at room temperature. NiMoO4 can also crystallize, containing different amounts ofct and I~ phase other than MoO3 and N i O

~4eight loss < MOOC204,4H20

-H20 A

O% m/e = 18

< NiC204,2H20

~,/-H20

>

>

A B

A+B

20%

10-7

60~

10-8

i00

200

300

400

500

°C

Fig. 3. TPD spectra of mixed nickel and molybdenum oxalates. Table 2 Thermal treatments of the oxalic precursor and phase composition ") Temp. ( ° C)

Heating procedure

Phase composition

350

oven b) oven c) HTXRD oven b) oven c) oven b) oven c)

MoO3 NiO'e[NiMoO4]a+13 MoO3 NiO MoO3 NiO NiMoO4 MoO3 NiO NiMoO4 ct+13 MoO3 NiMoO4 a+13 I~-NiMoO4+ e [NiMoO4 ] a NiMoO4 a + 13

450 550

a) Except for the HTXRD (high temperature XRD) assay, all diffractograms are recorded at room temperature. b) Thermal treatments performed from room temperature on the same sample. c) Sample directly introduced in the preheated oven.

in varying proportions, depending on the operating t e m p e r a t u r e and the treatment time. 3.3. Catalytic properties

0.5 g o f catalyst is mixed with a large quantity o f silicon carbide granules o f the same size in order to avoid severe t e m p e r a t u r e gradients within the catalytic bed. The temperature inside the catalyst layer is measured by a type K thermocouple placed in a thin quartz tube ( d i a m e t e r 2.5 m m ) . The gas feed consists o f 18% 02, 15% C3H8 and 67% nitrogen, with a total flow rate o f 15 £/h. Before each experiment, the catalyst was heated progressively from a m b i e n t until the desired temperature, under a mixture o f oxygen and nitrogen (ratio 0 2 / N 2 = 0.7). Propene was introduced after 30 min once the required temperature was obtained. The reactor effluent (COx, C2H4, C3H6, C2H40 and C3H40) was analyzed by gas chromatography: 02, N2 and CO after separation by a 5 A molecular sieve column connected to a thermal conductivity detector; ethylene, propylene, propane and oxidation products after separation on a porapak QS column were analyzed with both a flame ionization and T C D detectors. In table 3 data are presented comparing catalytic runs performed with NiMoO4 catalysts prepared from the three precursors studied. It may be observed that there is a remarkable difference between A and B catalysts; the second one despite a small a m o u n t o f 13 phase stabilized at r o o m temperature and a high surface area (47 mZ/g) does not present a remarkable catalytic activity. M o r e o v e r it becomes less active with time due to surface properties modification that could be interesting to understand. The results obtained starting from the oxalic precursor show how the different a / ~ ratio o f this catalysts directly influences its catalytic activity. This could be due to the presence o f the stabilized 13phase already at room temperature. Surface area measurements gave similar results (32 and 34 m 2 / g ) for the two active catalysts, respectively A and C.

4. Conclusions The research for organic precursors o f active catalytic systems based on N i / M o / O presents the ad-

735

C. Mazzocchia et al. / NiO, MoO3 mixed oxide correlation

Table 3 Comparison among different NiMoO4 catalysts. Catalytic conditions: Flow rate: 15 ~/h, catalyst weight: 0.5 g. 15% propane, 17.85% oxygen in N2 as diluent. T (°C)

System A a) (tt phase) System B b) (xo/x~ = 7 ) * System C c) (xdx~=0.4) ( x d x a = 2.1 ) d)

Conversion (%)

Selectivity (%)

CaH s

C3H 6

CO

CO 2

C2H40

C3H40

500

14.7

50.6

25.1

22.3

0.3

1.7

500

7.0

53.0

16.0

25.0

0.7

3.7

450 490 500 510 520

7.3 12.5 16.7 18.5 26.5

70.0 65.2 56.1 55.3 42.2

16.0 21.1 26.0 27.6 35.3

10.7 9.2 13.8 12.0 17.9

0.3

2.4 4.1 3.7 4.0 3.4

0.4 0.3 0.3

") Yellow NiMoO4 (obtained after two hours at 550 ° C ). b) Green NiMoO4 (obtained after two hours at 550°C). c) N iMoO4 from oxalic precursor (obtained after 15 h at 450 ° C ). d) NiMoO4 from oxalic precursor (obtained after two hours at 550 °C). * x~ is the molar fraction of the ct phase in the mixture, equal to F'/I'~ (ratio between the intensity of the principal line of the ct phase in the mixture and the intensity of the principal line of pure ct NiMoO4). xl~is calculated similarly ( = I ~/I~o).

v a n t a g e o f a l o w e r t e m p e r a t u r e o f c r y s t a l l i z a t i o n . By using an oxalic precursor, this product decomposes at a t e m p e r a t u r e l o w e r t h a n i n t h e case o f t h e a m m o n i a o n e s , a n d d i f f e r e n t t h e r m a l t r e a t m e n t s will determine the final phase composition of the catalysts d e p e n d i n g o n t h e a d v a n c e m e n t o f t h e crystall i z a t i o n . P a r t i c u l a r l y , a p a r t i a l s t a b i l i z a t i o n at r o o m temperature of the more selective high temperature p h a s e [3 h a s b e e n o b s e r v e d . T h i s c o u l d h a v e a n ind u s t r i a l i n t e r e s t b e c a u s e it a v o i d s t h e c o n t i n u o u s h e a t i n g o f t h e r e a c t o r t o m a i n t a i n t h e c a t a l y s t in its [3 c o n f i g u r a t i o n , t o p r e v e n t t h e r e t u r n t r a n s i t i o n i n t o t h e m o r e a c t i v e b u t less s e l e c t i v e ct p h a s e .

Acknowledgement This work was performed within the frame of a

research contract commissioned by ELF-ATOCHEM.

References [ 1 ] C. Mazzocchia, C. Aboumrad, C. Diagne, E. Tempesti, J.M. Hermann and G. Thomas, Catal. Lett. 10 ( 1991 ) 181. [2] C. Mazzocchia, C. Aboumrad and E. Tempesti, French Patent 89-00522, 18 Jan. 1989 to Norsolor. [ 3 ] C. Mazzocchia, R. Anouchinsky, A. Kaddouri, M. Sautel and G. Thomas, Proc. 10th ICTA Congr., Hatfield U.K. (1992), to be published. [4] C. Mazzocchia, F. Di Renzo, C. Aboumrad and G. Thomas, Solid State Ionics 32/33 (1989) 228. [5] C. Mazzocchia, F. Di Renzo, P. Centola, R. Del Rosso, in: Chemistry and Uses of Molybdenum, eds. H.F. Barry and P.C.H. Mitchell (Climax Molybdenum Co., Ann Arbor, MI, 1982) p. 406. [6] F. Corbet, Thesis (Lyon, 1960). [7 ] M.P. Astier, G. Dti and S.J. Techner, Ann. Chim. Sci. Mater. 12 (1987) 337. [ 8 ] F. Di Renzo, C. Mazzocchia and R. Anouchinsky, Therm. Acta 133 (1988) 163.