Insertion of Vanadium or Molybdenum as Oxides in ... - Pascal-Man

1037

J. CHEM. SOC. FARADAY TRANS., 1995, 91(6), 1037-1043

Insertion of Vanadium or Molybdenum as Oxides in LaNaY Zeolite: Comparison with Nay Jean Thoret," Pascal P. Man and Jacques Fraissard Laboratoire de Chimie des Surfaces, URA 1428 CNRS, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252Paris Cedex 05,France

When mixtures of NaY or LaNaY zeolites with V,O, or MOO, are calcined in air at atmospheric pressure within given temperature and composition ranges, the oxides can migrate into the pores of the zeolite lattice. The extent of insertion is higher for LaNaY than for Nay; this is attributable to the space in the supercages left vacant by the migration of the La3+ ions into the sodalite cages or hexagonal prisms. Beyond certain temperature and oxide concentrations the NaY or LaNaY lattice is destroyed and new crystalline phases are formed. These and 29Simagicsolid-state interactions and reactions have been followed by X-ray diffraction (XRD) and by angle spinning (MAS) NMR.

For about 20 years zeolite modifications have been intensively studied with a view to obtaining interesting catalytic properties. The first modifications were based on the isomorphous substitution of framework silicon or aluminium by another element'-6 and by these means new zeolites with remarkable catalytic properties were obtained. Following these first framework substitutions came non-framework substitution, either in solution or in the solid state, which led to zeolites modified by replacement of the compensating cation. Beyer and c o - w o r k e r ~exchanged ~~~ ions by solid-solid interaction between the alkali-metal chlorides and H-ZSM-5 or NH,-ZSM-5, as well as with various oxides (MnO, Cu20, CuO and NiO)9 and the chlorides of the same metals. Kucherov and Slinkin extended these investigations in the solid state to H-ZSM-5 and to H-mordenite with the following oxides: CuO, Cu,O, C r 2 0 3 , MOO, and V 2 0 5 . These modified zeolites have very specific catalytic properties, amongst which some are of particular industrial importance : the oxidation of benzene to phenol by nitric acid on Fe-ZSM-5;16 the synthesis of ammonia on Y zeolite exchanged with R u ; ' ~ the conversion of methanol to dimethyl ether, then to alkenes and aromatics on Ga-ZSM5;18 the conversion of acrylonitrile to acrylamide in the aqueous phase with a yield of 90% on Cu-exchanged Y;I9 the aromatisation of ethene on Cu-ZSM-5 and Zn-ZSM-5;20 the cracking of n-decane on La-exchanged Y catalysts in the presence of small amounts of water;21 the alkylation of toluene by B-exchanged ZSM-5;22 the conversion of light gaseous alkanes into aromatics by 211-2sM-5~~ etc. Rabo2, went further, with Y zeolite, by performing not only exchange reactions in the solid state but also more or less reversible insertions of anions (halide, nitrate) into the sodalite cages. In the same way Jirka et aL2, have shown by ESCA that Cu2+ can be inserted into the cavities of NH,Y by interaction with Cu,O above 670 K. Recently, other solid-state interactions between a zeolite and an oxide have been performed, but in the presence of a gas stream. Petras and Wichterlowa26 investigated the interaction of H-ZSM-5 and V 2 0 5 at high temperature in the presence of nitrogen; Kanazirev et aL2' doped H-ZSM-5 with G a in the form of Ga,O, in the solid state in the presence of hydrogen; Huang et aE.28introduced vanadium in the form of (V0,)3+ into the zeolite cavities by heating HY with V,O, at 700 K in a stream of air containing water vapour. Solid-gas reactions between NaY and MoCl, at 673 K have been attempted by Dai and Lunsford2' who introduced Mo4+ into Y but with loss of crystallinity; Mo ions have also been introduced by Johns and Howe3' into mordenites by adsorption of MoCl, in the vapour phase.

These insertions of oxide or ions into the cavities of zeolites also lead to particularly important physicochemical and catalytic properties. For example, Y zeolites loaded with ytterbium or europium in the form of cations can be used for the isomerisation of but- 1-ene and the hydrogenation of ethene;,' octanol can be aminated in the gas phase by use of Y zeolite impregnated with (U0,)2+.32 In our previous work on the interaction of Y zeolites with oxides we suggested that oxide insertion was related to the physical properties of the o ~ i d e s . ~We ~ -now ~ ~ extend this work to LaNaY to compare the degree of crystallinity and the extent of oxide insertion with the results of Nay. The choice of LaNaY was imposed by its exceptionally high thermal stability, attributed to the presence of polynuclear cations in the sodalite cages containing an oxygen and by the fact that the volume of the supercages increases by partial migration of the compensating cations towards the sodalites when it is calcined.,, The interest of studies on the insertion of oxides (particularly V 2 0 5 and MOO,) into Y zeolite lies in the catalytic properties which this association confers (possible modification of the form selectivity of the zeolite by adding the physicochemical and catalytic properties of the oxide). The large amount of published work testifies to the growing industrial interest in oxide-loaded zeolites, particularly molybdenum oxide for catalytic cracking, hydrocracking, hydrorefining of oil residues etc. Examples of catalytic hydrocracking involving Y zeolite and molybdenum oxide include : (i) High-octane petrol fractions are obtained from hydrocarbons treated with hydrogen on a hydrocracking catalyst (Y) containing one or several hydrogenating metal compounds, such as M o . ~(ii)~Y zeolite containing several oxides: COO, NiO and specially MOO,, as well as boehmite, activated at 704°C (16 h) and then at 815 "C (1 h), has been used to transform gas oil into petrol with a yield of 45%;,, MOO, helps to reduce coke formation.,, (iii) Heavy oil fractions are cracked with catalysts consisting of combinations of boehmite, amorphous aluminosilicate and Y zeolite (ammonium exchanged) with Al,O, containing (iv) A matrix consisting of MOO, in which Y zeolite is dispersed, covered with a layer of mixed A1 and Ni oxides, is an excellent catalyst for obtaining diesel fuel from oil fractions.,' Catalysts based on zeolites (particularly Y) modified by MOO, also play an essential role in the synthesis of alkenes from light the aromatisation of ethene at 550 0C,49 and the synthesis of acrolein by vapour-phase condensation of formaldehyde and acetaldehyde.,' Note also that Mocontaining Y zeolites display the best catalytic properties for

J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

1038

diethyl sulfide hydrogenolysis at considerably low M o content (5 wt.%) and at comparatively low temperature^.^'

Experimental The starting materials used are: V 2 0 5 , MOO, (Prolabo), La(NO,), 6 H 2 0 (Fluka) and NH4Y zeolite (LZY-64) corresponding to the composition (NH,),,Na,, (A102)56(Si02)l * 260H20. LaNaY was obtained from LZY-64. La was exchanged by adding 10 g of LZY-64 to 0.5 dm3 of a 0.1 mol dm-, solution of La(NO,), * 6 H 2 0 at room temperature and stirring for 24 h. The sample was then filtered off, washed with distilled water and dried for 12 h. This exchange was repeated twice under the same conditions. Chemical analysis gives the following chemical composition per unit cell: (NH4)7NaloLa,,(A102),,(Si02),,,260H20 with an (Si : Al)NMR ratio of 2.44 &- 0.10. The sample was then calcined for 3 h at 593 K so that the hydrated La3+ ions lost their hydration sphere and migrated into the sodalite cages and the hexagonal prisms; this is confirmed by the fact that the 139LaN M R spectrum shows two signals, one narrow line (La in the supercage) and another overlapping one, which is broad and weak and corresponds to La in the small cages. These results are consistent with those given in previous publ i c a t i o n ~ . ~Before ~ - ~ ~mixing V 2 0 5 or MOO, with LaNaY, the zeolite was placed in a desiccator containing supersaturated NH,NO, for 48 h to saturate it with water. All the mixtures are defined by the ratio R ; number of V or Mo atoms : number of (A1 + Si) atoms in the LaNaY lattice, denoted as R , and R M o .We prepared 2 g of each LaNaYoxide mixture, with different values of R < 0.4. The ratios R , and R,, in the mixtures were varied systematically from 0 to 0.15 with a step of 0.01, except near the highest insertion level where the step is 0.005. After homogenisation and magnetic crushing for 30 min, the mixtures were deposited in rectangular refractory boats in the form of a 1 mm thick bed, 15 cm2 *

*

in area. They were then heated slowly in air to a temperature, T , between 450 and 1050 K in steps of 40 K and maintained for 16 h at the chosen temperature. After they had been heated, the samples were cooled to room temperature in the heating element, then transferred to the desiccator until completely rehydrated, i.e. when the samples reached constant weight. At this stage, the variation of the sample mass caused by the heat treatment can be explained either by obstruction of the zeolite pores by the oxide or one of its derivatives or by amorphisation of the zeolite, which inhibits rehydration to a certain extent. This mass loss is due solely to the quantity of water which has not returned into the lattice rather than loss to the additives, because samples of additive show no mass loss over the given temperature range. The same is true for Y zeolite which, when heated alone and rehydrated with NH4N0,, weighs the same as before the treatment. Temperatures of the heating element are given to + l o K. After this treatment the samples were studied by XRD, 29Si and 27Al MAS NMR. The X-ray diffractograms of the watersaturated samples were obtained with a Philips PW 1025/30 diffractometer with a Cu-Kcr anti-cathode. For each value of R, an X-ray diffraction diagram was established, from which the unit-cell parameters of NaY and LaNaY were calculated. a, was determined to &-5 x lo-, A and refined at 294 K with a least-squares refinement based upon 25 reflections between 3" < 8 6 30". Silicon was used as the internal standard. The increase of a, with R (Aa,) depends on the temperature. The percentage increase, A', in a. from one composition to another (listed in Tables 1 and 2) is given by A' = 100(Aa,/a,). The very small decrease in a, for NaY and LaNaY zeolites alone ( R = 0) treated at 850 and 930 K under self-steaming conditions is a consequence of their slight dealumination (Tables l and 2). In what follows, the maximum degree of insertion will correspond to the greatest increase in a, of the zeolite, which should be pure (monophase system by XRD) with crystallinity higher than 80%. The sample crystallinity was deter-

Table 1 Variation of unit cell parameters and crystallinity of NaY and LaNaY at different contents and temperature treatments NaY

T/K

Rv

a0/A

730

0 0.050 0.075 0.100 0.150 0.200 0.400

24.659 24.675 24.673 24.674 24.674

0 0.050 0.075 0.100 0.200 0.400

24.657 24.666 24.659 24.659

0 0.015 0.050 0.100 0.200 0.400

24.654 24.663

850

930

LaNaY A'

0.06 0.06 0.06 0.06

cryst. (YO)

99 91 87 69 61

a C

-0.01 0.03

98 86 69 51

e

B

-0.02 0.01

88 68 47

I

k m

Rv

a0iA

A

cryst. (Yo)

0 0.05 0.065 0.100 0.150 0.200 0.400

24.672 24.688 24.696 24.691 24.686

0.07 0.10 0.08 0.06

100 91 84 76 65

0 0.050 0.075 0.100 0.200 0.400

24.671 24.692 24.684 24.685

0 0.015 0.050 0.100 0.200 0.400

24.671 24.675

b d

0.08 0.05 0.05

97 85 74 56

f h

0.01

98 77 55

J 1 n

Amorphisation + NaY (cryst. 56%)+ V 2 0 , . Slight amorphisation + LaNaY (cryst. 62%) + V,O, . Marked amorphisation + V,O, + NaV,O,, . Amorphisation + LaNaY + V,O, . Amorphisation + NaY + V,O, + NaV,O,, . Marked amorphisation + LaNaY + V,O, + LaVO, + SiO, (cristobalite). Amorphisation + E V , ~+, Na,V,,O,, + NaV,O,,. Amorphisation + NaV,O,, + LaVO, + unidentified phase@). Amorphisation + NaY + V,O, + NaV,O,, . Amorphisation + ELaNaY + V,O, + LaVO,.+ NaV6Ol5. Marked amorphisation + ENaY + V,O, + NaV,O,, . ' Amorphisation + LaVO, + NaV,O,, + SiO, (cristobalite)+ unidentified phase@). Marked amorphisation + NaV,O,, + Na,V,,O,, + Si0, (cristobalite)+ unidentified phase(s). " LaVO, + NaV,O,, + Si0, (crktobalite) + unidentified phase@).(E indicates small amounts.)

a

f

@

j

Ir


1039

Table 2 Variation of unit-cell parameters and crystallinity of NaY and LaNaY at different Mo contents and temperature treatments NaY TK

730

850

RMO

4

0 0.05 0.075 0.100 0.125 0.200 0.400

24.659 24.670 24.683 24.681 24.676

0 0.050 0.065 0.100 0.150 0.250

24.657 24.669 24.666 24.66 1

0 0.015 0.050 0.100 0.200 0.400

A 0.06 0.10 0.09 0.07

RMO

99 96 87 82 68

0 0.050 0.100 0.105 0.150 0.250 0.400

24.672 24.687 24.703 24.706 24.696

0 0.050 0.065 0.125 0.150 0.250 0.400

24.67 1 24.682 24.688 24.68 1

0 0.015 0.050 0.100 0.200 0.400

24.669 24.676

C

-0.01 0.04 0.03 0.01

95 86 79 62 41

e #

24.654 24.664 24.657

-0.02 0.02

%/A

cryst. (%)

a

0.400

930

LaNaY

85 65 29

I

k k

A 0.06 0.13 0.14 0.10

cryst. (YO) 99 93 84 85 67

b d

0.04 0.07 0.04

99 88 77 43 37

I h

- 0.01 0.02

81 68 37

j

I m

Amorphisation + NaY (cryst. 58%) + MOO,. Amorphisation + LaNaY (cryst. 56%) + &MOO,. ‘ Amorphisation + ENaY + MOO,. Amorphisation MOO,. Marked amorphisation + ENaY + &MOO,+ unidentified phase@). Amorphisation + AI,(MoO,), + &MOO,. Marked amorphisation + A12(MOO,), + NaAl(MoO,), . Strong amorphisation + &MOO,+ AI,(MoO,), + unidentified phase(+ Amorphisation + ENaY + unidentified phase(s). Amorphisation + ELaNaY + NaLa(MoO,), . Marked amorphisation + Al,(MoO,), + NaAl(MoO,), + unidentified crystalline phase@). Amorphisation + Al,(MoO,), + NaLa(MoO,), . Marked amorphisation Al,(MoO,), + NaLa(MoO,), + unidentified crystalline phase(s).( E indicates small amounts.)

a

+

+

mined by comparing the sum of the peaks corresponding to the 331, 333,440, 533, 642, 660, 555 and 664 Miller indices of the treated samples with those of LaNaY or NaY taken as reference (1W% crystallinity at room temperat~re).’~ A Bruker MSL-400 multinuclear spectrometer was used for ,’Si (79.5 MHz) and 27Al (104.2 MHz) MAS NMR spectra. Chemical shifts are reported in ppm from external tetramethylsilane (TMS) for ,’% and from Al(H,0),3+ in a nitric acid solution of Al(NO,), for 27Al. The sign convention of high-frequency (low-field, paramagnetic, deshielded) shifts being positive is used. For ,’Si MAS NMR spectra the standard Bruker double bearing probe and a 7 mm od ZrO, rotor were used. The acquisition parameters are as follows: a 71/4 pulse of 2.5 ps was applied with a 5 s recycle delay and a rotor spinning rate of 4 kHz. The spectra were simulated with the program WINFIT using Gaussian lineshapes. 27Al MAS NMR spectra were acquired with the same probe, a single 71/20 pulse was used with 1 s recycle delay. The Si : A1 ratios of the lattice were calculated by the usual method,’, after spectrum simulation.

Results X-Ray Difiiaction at Ambient Temperature LaNaY-V,O, In the LaNaY-V,O, system, when T < 530 K, the two initial phases coexist regardless of the composition and the length of the thermal treatment. After heating followed by water saturation at room temperature, there is no change in the mass of the system, compared to its original state. This proves that rehydration of the LaNaY zeolite is not hindered by possible obstruction of the pores; the unit-cell parameter is not changed. Above 530 K, whatever the composition, the XRD diagrams indicate an attenuation of the intensity of the lines due to V,O, , until they disappear completely when R, = 0.065 at 650 K. In this temperature range (530-650 K) the vanadium

oxide penetrates partially and progressively into the LaNaY pores, up to the value of R, given above. This temperature range is fundamental; it corresponds to the gradual loss of water from the zeolite which is then able to dissolve partially the V , 0 5 . Thanks to this wetting phenomenon, the oxide coats the zeolite cavities. These deductions are supported by the continuous increase in the unit-cell parameter which is 24.672 A at zero insertion and 24.696 A at maximum insertion (R, = 0.065) at 530 K and 650 K, respectively, and also by the loss of mass after thermal treatment: zero at 530 K and reaches a ceiling at 650 K (this mass loss corresponds to maximum hindrance to rehydration of the zeolite). From 650 to 770 K the results depend on R, (Table 1, 730 K): (i) At R, < 0.065 the diffraction diagram shows only the lines characteristic of LaNaY, a, varying from 24.672 A for LaNaY (R, =0) to 24.696 A (R, = 0.065), and the crystallinity being close to 84% for T = 730 K [Fig. l(a)]. This increase in a, (A’ = 0.10%) can be explained by the introduction of the oxide or of a derivative, whose form remains to be discovered, into the LaNaY cavities. This insertion is confirmed by the reduction in the mass of the rehydrated zeolite, which is almost proportional to R, in this composition range. (ii) When 0.065 < R , < 0.15 the diffraction lines of LaNaY weaken with increasing R,; the crystallinity falls from 84% when R, = 0.065 to 76% for R, = 0.10, and a, = 24.691 A for T = 730 K. The amount of oxide inserted in the zeolite pores decreases slightly in this composition range, despite a higher value of R, and a slight amorphisation of the lattice, particularly when R, z 0.10, is observed. The mass variation (drop after thermal treatment) does not increase in this range, contrasting with what was observed at R , d 0.065, where progressive oxide insertion hindered total rehydration of the zeolite. When R, 2 0.10 the phase V,05 coexists. (iii) When 0.15 < R, d 0.2 the diffraction diagrams reveal two phases, V,O, and LaNaY, with an amorphisation of the latter in proportion to the increase of R , . Fig. l(b) shows the XRD pattern for R, = 0.2. (iv) At R, > 0.2,


1040

destroy the initial phases and give rise to the following new crystalline phases: NaV,O, , LaVO, and SiO, (cristobalite) as well as unidentified phases [Fig. l(f), T = 930 KJ. The first two crystallise in the monoclinic system, as indicated previously, while cristobalite crystallises in the cubic system Fd3m with a, = 7.13 8, (JCPDS no. 27-605). When R , = 0.4 and T = 930 K [Fig. l(g)], the above phases corresponding to R , = 0.2 persist, accompanied, as previously, by the same unidentified diffraction lines. When T = 1010 K and R , < 0.4 the phases of the previous temperature range remain, but are degraded for R , = 0.2 [Fig. l(h)].

,

26/degrees

2ejdegrees

LaNaY is degraded and the V,Os phase predominates for R, = 0.4 [Fig. l(c)]. When T is between 770-890 K the concentration range where pure LaNaY is detected is much smaller than in the previous temperature range (Table 1) and the lattice is damaged at lower R , values. Thus when: (i) R, < 0.05 there is only the LaNaY phase, very slightly amorphous. The crystallinity is 85% and a, is 24.692 A for R, = 0.05 at 850 K. (ii) 0.05 < Rv d 0.15, a, of LaNaY decreases as R, increases; the crystallinity is only 43% for R, = 0.15 at 850 K. (iii) Rv > 0.15 the diffraction diagram shows considerable amorphisation, accompanied by LaNaY (degraded), V,O, , LaVO, and SiO, (cristobalite) lines for R, = 0.2 [Fig. l(d)]. (iv) For R , = 0.4, the initial phases no longer exist but are replaced by LaVO, and NaV,O,, phase^'^-'^ [Fig. l(e)J which crystallise in the monoclinic system with a, = 7.070 A, b, = 7.291 A, c, = 6.771 A and B = 105",space group P2,/m for LaVO, (JCPDS no. 25-427); a, = 10.081 A, b, = 3.615 A, c, = 15.383 A and B = 109.45", space group A2/m for NaV,O,, (JCPDS no. 24-1 155). Despite the relatively low treatment temperature (850 K), there are already solid-solid reactions, explained mainly by the acidity of vanadium anhydride (V,O,). On the other hand, the diagram comprises unidentified diffraction lines. When the treatment temperature is between 890 and 970 K, the pure LaNaY phase can be detected only when R, < 0.015; for example, for R , = 0.050 the crystallinity of LaNaY is 55% at 930 K, accompanied of V,O,. For Rv = 0.10, the system is largely amorphous and multiphase (traces of LaNaY with NaV,O,, , V20, and LaVO,). In the same temperature range for R , = 0.2, solid-solid reactions completely

LaNaY-MOO, As in the previous system, below 530 K the two initial phases, LaNaY and MOO,, coexist whatever the composition and the length of the heat treatment. The unit-cell parameter of the zeolite and the mass of the system after heat treatment are unchanged which proves that no molybdenum anhydride (MOO,) has been inserted. In the 530-650 K range, as in the case of LaNaY-V20,, MOO, gradually enters the LaNaY cavities up to a maximum insertion level of 0.105 at 650 K (progressive decrease of the MOO, lines until they disappear completely at 650 K for R,, = 0.105). This is a temperature range where the zeolite steadily eliminates its water molecules and is therefore able to dissolve the MOO, partially. We observe that the system, after heat treatment, regularly loses weight as the temperature increases and that a, increases continuously up to R , = 0.105 with a, = 24.706 A. For 650 < T/K d 770 the results depend on R , (Table 2). When (i) R,, d 0.105, the XRD pattern reveals a large monophase region, corresponding to pure LaNaY, albeit slightly amorphised in the range 0.09 < R , < 0.105. The unit-cell parameter increases from 24.672 A for LaNaY (RMo= 0) to 24.706 A for R,, = 0.105 at 730 K (A' = 0.14%),corresponding to the greatest increase in this work, with 85% crystallinity [Fig. 2(a)]. (ii) 0.105 < R , d 0.250, the system is biphasic, containing LaNaY, which diminishes as R , increases, and MOO, phases. The crystallinity of LaNaY decreases from 85% for R,, = 0.105 down to 56% for R,, = 0.25 at 730 K. (iii) 0.25 < R , < 0.4, the LaNaY is degraded (strong amorphisation) and mainly the diffraction lines of the molybdenum anhydride are detected for RM0= 0.4 at 730 K [Fig. 2(b)]. From 770 to 890 K molybdenum anhydride progressively enters LaNaY, but only when R,, < 0.065 at 850 K (Table 2) [Fig. 2(c)]. The zeolite is almost destroyed when R , is greater than 0.15. When RMolies between 0.25 and 0.4 there are two crystalline phases: A12(MOO4)3, of orthorhombic symmetry with a, = 12.552 8, b, = 8.935 A, co = 9.044 A, space group Pbcn (JCPDS no. 23-0764), and NaLa(Mo0,) , of quadratic symmetry with a, = 5.343 A and c, = 11.743 space group 14,/u (JCPDS no. 24-1103) [Fig. 2(4, R,, = 0.4 at 850 K]. From 890 to 970 K, depending on RMo, the following results are obtained (Table 2): (i) Molybdenum anhydride enters LaNaY when R,, < 0.015, and the zeolite is 68%crystalline for R,, = 0.015 at 930 K. (ii) When 0.015 < R , < 0.1 the LaNaY is progressively destroyed and a new crystalline phase NaLa(MoO,), is seen for R,, = 0.1 at 930 K. (iii) When 0.1 < RMo< 0.2 there is slight amorphisation and two crystalline phases, NaLa(MoO,), predominating with the onset of Al,(MoO,), for R,, = 0.2 [Fig. 2(e)]. (iv) For R,, > 0.2, the system is multiphasic; for R , = 0.4 it corresponds to NaLa(MoO,), , Al,(M00,)3 increasing and unidentified phase@)[Fig. 2(f)]. At this point solid-solid reactions occur which give rise to new crystalline phases at the expense of the original ones.

ft,

1041


...

200 28/degrees 28/deg rees Fig. 2 XRD patterns of LaNaY-MOO, samples containing (D) LaNaY, (*) M o o , , (4A1,(Mo04),, (0) NaWMoO,),, (0) Na(Si,Al),, unidentified phase(s). (a) 730 K, RM, = 0.105 (m), (b) 730 K, RM, = 0.4 (*), (c) 850 K, RM, = 0.065 (m), (d)850 K, RM, = 0.4 (A,,(4 930 K, RM, = 0.2 (A,0, O), (f)930 K, RM, = 0.4 (4 0, 01, (g) 1010 K,RM, = 0.2 (0, 0, 0) and (h) 1010 K, RM, = 0.4 (A,0,

(a)

17). At even higher temperatures (970-1050 K) the XRD patterns indicate new crystalline phases which depend on R,, . (i) At low R,, (0.1 or less) there is evidence for marked amorphisation of the LaNaY. Two compounds have been identified for R,, = 0.2 at T = 1010 K: disordered albite, Na(Si3A1)08 (JCPDS no. 10-0393) with triclinic structure and a, = 8.165 A, bo = 12.872 A, c, = 7.111 A, a = 93" 45', = 116" 43' and y = 90" 28', space group Pi and NaLa(MoO,),, accompanied by unidentified phases, [Fig. 2(g)]. (ii) When R,, is greater (0.2-0.4) XRD reveals Al,(MoO,), and NaLa(MoO,), ,along with one or several unidentified phases for R,, = 0.4 [Fig. 2(h)]. In the last three temperature ranges that we have discussed (770-1050 K) the zeolite is destroyed by small amounts of oxides, and structural rearrangement gives rise to new crystalline phases, specially for R,, > 0.10 and T > 890 K.

0

100

-1 00

6 MAS NMR of (a) La-exchanged NH,NaY zeolite Fig. 3 calcinated at 593 K and treated at 730 K to give (b)R , = 0.075 and (c) RM, = 0.075

much, contrary to the 45 ppm framework A1 shoulder. These two facts show that the oxides did not dealuminate the zeolite framework, in agreement with the XRD results [Fig. l(a) and 2(a)]. The ,'Si MAS NMR spectrum of NH,Y [Fig. 4(a)] corresponds to a Si : A1 ratio of 2.44 0.15 and consists of four lines at -90.4 [Si(3Al)], -95.1 [Si(2Al)], - 100.4 [Si(lAl)] and -105.7 ppm [Si(OAl)]. Fig. 5 displays the simulation of the spectrum of LaNaY treated at 593 K shown in Fig. 4(b), which consists of five lines at -90.9 [Si(3AI)], -93.9 [Si(3Al)], -97.3 [Si(2Al)], - 101.6 [Si(lAl)] and - 106.0 ppm [Si(OAl)]. The Si : A1 ratio given by this simulated spectrum is 2.65. Compared with the spectrum of Fig. 4(a), that of Fig. 5 shows a split signal (-90.9 and -93.9 ppm) corresponding to the signal at -90.4 ppm, the other lines are shifted and that corresponding to Si(OA1) broadened. The broadening, the shift and the splitting of the signals are due to the migration of La3f ions into the small cages. Fig. 4(c)-(f) give the 29Si MAS NMR spectra of four samples treated as follows: Fig.

27A1and 29SiMAS NMR The 27Al MAS NMR spectrum of La exchanged NH,Y calcined at 573 K shows three lines: an intense tetrahedrally coordinated framework A1 peak located at 61 ppm; a weak framework A1 shoulder located at ca. 45 ppm, assigned to migration of lanthanum ions into the sodalite cagess8 and a weak extra-framework Al, at about 0 ppm due to dealumination during cation exchange in the liquid state [Fig. 3(a)]. The shoulder at 45 ppm cannot be attributed to tetracoordinated extra-framework aluminium59 insofar as the XRD patterns (not shown) indicate no amorphisation. Fig. 3(b) and 3(c) show 27Al MAS NMR spectra of previous samples treated at 730 K with R, = 0.075 and R,, = 0.075, respectively. The extra-framework A1 peak did not increase

1 1 1 1 1 -80 -100 -120

L -80

I

I

-100

u -120

L--L-_II.-.J -100

-80

-120

6 Fig. 4 29Si MAS NMR of (a) NH,NaY zeolite and (b) Laexchanged zeolite calcined at 593 K and treated at 730 K with oxides to give (c) R , = 0.075, (6)R , = 0.10, (e) RM, = 0.075 and (f)RM, = 0.15


1042

-80.00

-1 04.00 -1 12.00 6 Fig. 5 Simulated 29SiMAS NMR spectrum of Fig. 4(b) -88.00

-96.00

4(c) and 4(d) correspond to the introduction of V,O, at 730

K for R, = 0.075 and 0.10, respectively; Fig. 4(e) and 4(f) correspond to the introduction of MOO, at 730 K for RM, = 0.075 and 0.15, respectively. These four spectra show a superposition of broadened signals, dificult to simulate. For this reason, we have not attempted to determine the Si : A1 ratio. Line broadening increases with the amount of oxide. With V,O, at 753 K and for R, = 0.10 [Fig. 4(d)] we obtain a spectrum with lines broader than for R , = 0.075 [Fig. 4(c)]. Analogous results are obtained for MOO,. These results are in full agreement with those of XRD. The broadening of all these signals is probably due to a wider distribution of the T-0-T bond angles (T = Al, Si), resulting from incomplete rehydration (insertion of oxides), rather than to framework dealumination, given that the *?A1 spectra (Fig. 3) show no increase in the signal at 0 ppm following oxide insertion.

Discussion and Conclusions XRD, '?A1 and "Si MAS NMR indicate that there are solidsolid interactions and reactions between V,O, or MOO, and LaNaY in the 530-1050 K temperature range. The degree of oxide insertion is greatest in the range 650-770 K for the compositions R , = 0.065 and RM, = 0.105, corresponding to a variation in the unit cell parameters of 0.10 and 0.14%, respectively, and crystallinities of 84 and 85% for LaNaY. These oxides enter the zeolite in forms which are undetermined: probably V,O, or ions such as (VO,)' or VO(0H) for vanadium, and MOO, or trimers such as Mo,O, or ions such as - for molybdenum. These different species begin to insert in the 530-650 K temperature range in which the zeolite loses its water. Prolonging the heat treatment beyond 12 h has no effect on the degree of insertion, which, therefore, probably occurs at the beginning of the treatment. It can be seen from Tables 1 and 2 that the greatest level of insertion in and LaNaY, both with V,O, and with MOO,, occurs in an optimal temperature range (650-770 K). Above this temperature range, the unit-cell parameter and the degree of crystallinity of the NaY or LaNaY zeolite decrease. The decrease in crystallinity can be partly explained by the slightly acidic character of V,O, or MOO, which attack the zeolite structure and progressively disorganise it; this depends on the temperature and the amount of oxide in the mixture. This disorganisation not only prevents further absorption of oxide but probably rejects part of what was already inserted into the cavities. The amount of insertion is certainly related to the physical properties of the oxides (melting point, solubility in hot water), as we saw in our previous work.,' These insertion levels in LaNaY are greater than that in Nay, which can be attributed to the space in the

supercages left vacant by the migration of the La3+ ions into the sodalite cages and the hexagonal prisms during heat treatment. Consequently, the structural properties of the host, are also of fundamental importance in the insertion of elements, regardless of their form. To date the insertion mechanism has not really been elucidated.60Although exchanges or insertions are usually performed in the presence of water vapour, which is the case with LaNaY and Nay, such reactions can also occur in a dry atmosphere or with compounds which are insoluble in water, such as AgC1.60 Above 770 K the insertion maximum decreases as the temperature increases and becomes zero above 930 K when R > 0.015. New crystalline phases appeared: NaV60,, , LaVO, and SiO, (cristobalite) for V,O, , and NaLa(MoO,), , Al,(MoO,), and Na(Si,Al)O, for MOO,, while the LaNaY progressively disappears. At this point real solid-solid reactions occur with the appearance of the new crystalline phases. We are grateful to Mrs E. Copin (Laboratoire de Reactivite des Surfaces, Universite Pierre et Marie Curie) and Dr N. Lequeux (Laboratoire des Ceramiques, ESPCI) for XRD experiments.

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Paper 4/046121; Received 27th July, 1994