Use of 129Xe NMR spectroscopy for the study of gaseous reactant diffusion in a fixed bed of zeolite: application to benzene diffusion in HZSM-5

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

2939

Use of ~29Xe NMR spectroscopy for the study of gaseous reactant diffusion in a fixed bed of zeolite: application to benzene diffusion in HZSM-5 P. N'Gokoli-Kekele, M.-A. Springuel-Huet, J.-L. Bonardet and J. Fraissard Laboratoire de Chimie des Surfaces-SIEN, CNRS-ESA 7069, Universit6 P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05 France The diffusion of benzene, during its adsorption on HZSM-5 zeolite under various conditions (powder or pellet samples with different lengths, mixture of the zeolite with a binder expressing), has been studied by the 129Xe NMR of adsorbed xenon. The two differential equations expressing the mass balance of the adsorbate in the macropores and in the micropores have been solved by taking into account the boundary conditions of the adsorbate-adsorbent system. The solution gives the adsorbate concentration profiles in both the macropores and the micropores which are used to simulate 129Xe NMR spectra. Comparison of simulated and experimental spectra leads to the intracrystalline diffusion coefficient of benzene (3x 1014 m 2 sl). For most of the samples, the time necessary to obtain the adsorption equilibrium is imposed by the intercrystalline diffusion, except with shallow bed or small pellets. The presence of a binder increases the influence of the macropores but an apparent adsorption equilibrium constant must be considered in order to take into account the adsorption of benzene by the binder. 1. INTRODUCTION

129Xe~ resonance of adsorbed xenon has proved to be very sensitive to the internal free volume of zeolites and to interactions with other chemical species; it depends therefore on the nature and the concentration of co-adsorbed molecules [ 1-3]. The concentration change of co-adsorbed molecules during their diffusion within zeolite crystallites leads to a variation of the 129XeNMR parameters (chemical shitt, linewidth, signal intensity). The evolution with time of spectra makes it possible to describe the diffusion process, to determine concentration profiles and to estimate diffusion coefficients in systems out of adsorption equilibrium [4]. In this paper we report the study of benzene diffusion in a bed of HZSM-5 zeolite, during the benzene adsorption process, by 129Xe NMR. The influence of various effects such as the compression, the size and the form (powder or pellet) of the sample or the dilution of the zeolite with a binder (silica-alumina), have been investigated. 2. E X P E R I M E N T A L Samples were outgassed at 673 K overnight under vacuum (10 -2 Pa) before xenon was preadsorbed at a pressure (~2.7x104 Pa) low enough to assume that it has a negligible influence on the benzene diffusion. The diffusion coefficient of xenon in HZSM-5 is several orders of magnitude higher than that of benzene [5]. We can therefore assume that xenon is always at adsorption equilibrium.

2940 HZSM-5 zeolite was used in powder form (samples A, C, D was differentiated by the length of the beds) or compressed into one pellet (sample B) which was then broken in small pieces (sample E), see Table 1. To be closer to industrial catalytic conditions where zeolites are used with a binder, HZSM-5 zeolite were also mixed with silica-alumina (specific surface area, 740 m 2 g-l; pore volume, 0.69 +0.06 cm 3 g-l) in powder form (sample F) or compressed into one pellet (sample G). A reservoir, containing benzene Gas phase and xenon, the latter at a partial pressure of 2.7x 104 Pa, macropore is attached to the NMR tube and separated by a (~, Dinter, 1;inter) ; micropore stopcock.At time t = 0 the stopcock is opened, the tube is (R, Dintra, 1;intra) introduced in the NMR probe ~>>R,Dinter >> Dintra of a 400 MHz Bruker x I I spectrometer and spectra (90 ~ -R R pulse, repetition time 500 ms, about 1000 scans) are Fig. 1. Schematic representation of the different types of recorded as a function of time diffusion. Horizontal line represents the size of until adsorption equilibrium is crystallites. reached.

Preliminary experiments where benzene is adsorbed at different equilibrium pressures give the correlation of the 129Xe NMR parameters (chemical shift, linewidth, intensity) with the benzene concentration. There are essentially two types of diffusion in the zeolite bed which occur in parallel: diffusion in macropores (intercrystalline volume) and diffusion in micropores (intracrystalline volume). Each type of diffusion has a specific characteristic time, Xinter or Xintra, which depends on the intercrystalline (Dinter) or intracrystalline (Dintra) diffusion coefficient and the distance to cover: ~, the sample length, or R, the crystallite radius (Fig. 1). We use a model, which we call the "non-uniform model", where we consider a gradient of adsorbate concentration in both intercrystalline macropores and intracrystalline micropores [6]. 3. THEORY Writing the mass balance in the macro- and micropores gives two differential equations:

0C 02C ),Dintra/0Q / 8inter-'~-= Dinter~:inter---~-0-Sinter" 0Z R ~" X=l aQ 0t

c32Q

= Dintra ~0X 2

(1) (2)

2941 where ~inter is the ratio of maeropore volume over the bed volume, Z = z/s and X=x/R (see Figure 1). C (= e/e~) and Q (= q/q~) are the relative adsorbate concentrations in the macropores and mieropores, respectively. If the diffusion rate is limited by the intererystallite diffusion, one should solve the Eq. (1). The adsorbate concentration is homogeneous in each crystallite and depends on the location of the crystallites in the bed. Therefore the NMR spectum corresponds to the superposition of different uniform intracrystalline concentrations of crystallites located at different heights in the bed. On the contrary, if the overall diffusion is limited by the intracrystalline diffusion, Eq. (2) must be solved. The diffusion rate does not depend on the crystallite size. The gradient of adsorbate concentration in the macropores can be neglected. At time t, the concentration profiles inside the micropores are independent of the position of a crystallite in the bed. In this case, the spectrum corresponds to the concentration profile inside all the crystallites. If the diffusion rate is controlled by both micro- and macropores, the system of the two equations must be solved and the spectrtma corresponds to the superposition of different intracrystalline concentration profiles of crystallites situated at different heights in the bed. We chose this last hypothesis to simulate the NMR spectra. Taking into acount the boundary conditions in the adsorbate-adsorbent system, these two equations can be solved either numerically using "orthogonal collocation" techniques [7] or even analytically using Laplace transformation (this work). The solution gives the analytical expressions of the adsorbate concentration in the macropores, C(Z,t), and in the micropores, Q(Z,t), as a function of time and of parameters depending on Dinter, Dintra, 8inter, ~, R, and the adsorption equilibrium constant K [8]. These concentration profiles (Fig. 2) are calculated for different values of the ratio t/1;intra.They allow the simulation of the 129Xe NMR spectra assuming Lorentzian lineshape and using the NMR parameters determined from the preliminary benzene adsorption experiments (see above). The contribution to the NMR line, at time t, of a crystallite located at a distance Z in the bed is given by: I z ( f , t ) = ~Izx (5,Q, t)p(Q, t)dQ

(3)

where Izx(5,Q,t) is the contribution of a sub-crystalline region situated between X and X+dX and p(Q,t) is the relative weight of this region compared to the whole crystallite. Izx(~5,Q) is given by: w2 I z x ( 5 , Q) = I (4) w 2 + 4(6'-5) 2 with I, w, 5' being the intensity, the linewidth and the chemical shifcs respectively. The comparison of simulated and experimental spectra leads to the diffusion coefficients. For the simulation, ~inter is determined using the expression:

Xinter =

s (1 - Cinter)K a In P 3Dinter~inte r a l n q

(5)

2942 where K is the adsorption equilibrium constant and Dinter m 8interDgas, Dgas being the diffusion alnP coefficient of the gas phase. The thermodynamic factor ~ is equal to 1/(1-0) for the linear alnq part and to 1/(1-0) 2 for the non-linear part of a Langrnuir isotherm. 1.0

1.0 t = 2 0 1;intra 0.8

t=201;lntra 0.8

t=15 1;intra

- ~ - 1 0 ~|ntra

t=5 ~intra t=10 1;intra

~

0.6

0.6

o II t~ 0.4

~Y

t=2 "Uintra 0.4

t=5 "Cintra 0.2

0.2 t=--2 "[intra

~intra

0.0 0.0

0.0 0.2

0.4

0.6

Z

0.8

1.0

0.0

,

I 0.2

,

I

9

0.4

, 0.6

9

,

9

0.8

X

Fig. 2. Concentration profiles along the bed, C(Z, t) (a) and inside the crystallites situated at a height Z = 0.8 in the bed, Q(Z = 0.8, X, t) (b)

4. RESULTS AND DISCUSSION The spectra have two signals: a narrow line corresponding to Xe not interacting with benzene and a broad one, more shifted, corresponding to Xe interacting with benzene (Fig. 3). Using this method we determined the intracrystalline diffusion coefficient of benzene during its adsorption under constant pressure (equals to saturation pressure), Dintra, and found 3x 10-14 m 2 sl with sample A. The increase in the equilibrium time for sample B reflects the decrease of the size of macropores due to compression. The experimental equilibrium time, too, is even longer that the calculated Tinter. The influence of macropores on the overall diffusion is clear with samples A and C: when the length of the bed increases from 15 to 20 mm, too increases and is equal to Xinter, which shows that the diffusion in macropores is the dominant step. On the contrary, for a shallow

1.0

2943 bed (5 ram), t~ is much longer that 1:inter showing that the mieropores now control the overall process. When we have small pellets (sample E), each of them being an independent sub-system in a gas phase of uniform concentration, the micropore again imposes the equilibrium time but the evolution of the spectra suggests that the influence of the maeropores is important.

time (h) 12.5 9.00 7.50 6.00 5.00 3.00

1.00

I

180

~

I

l

I

160

"

I

l

I

l

140

I

~

I

120

'

I

~

I

100

(ppm) Fig. 3. Experimental 129Xe NMR spectra as a function of time during adsorption of benzene on sample A.

When a binder is added to the HZSM-5 zeolite, one may consider three simultaneous steps: diffusion in the macropores, in the mesopores of the binder and in the micropores of the zeolite. Actually, the benzene diffusion rate is of the same order of magnitude in both the macro- and the mesopores; therefore, the zeolite/binder system may be reduced to a system with two types of pores. Nevertheless, the equilibrium constant, K, must be replaced by another coefficient, K', since the binder adsorbs benzene (there is a corresponding weak NMR signal at about 100 ppm). It is evident that the presence of the binder increases the influence of the macro-mesopores on the overall diffusion since, for a given mass of zeolite, it increases the sample length. "[interhas been calculated by replacing K by K' which depends on K, Einter, EB, the porosity of the binder and the ratio qa/q (qB, the amount adsorbed by the binder) [8].

2944 The role of the binder is seen by comparing samples B and G; 1;interis equal to 23 and 17 h, respectively. For a given length, the presence of the binder increases the intercrystalline diffusion rate through the apparent equilibrium constant K'. Table 1. Sample A B C D E F G

s (10 3 m) 15 (powder) 7x10 (1 pellet) 20 (powder) 5 (powder) lx2x3 (n pellets) 7 (powder) 7x 11 (1 pellet)

Oequilibrium

I~intra

I~inter

I~B

Xinter (h)

0.58 0.22 0.58 0.58 0.52 0.58 0.34

0.18 0.32 0.19 0.19 0.32 0.09 0.14

0.69 0.38 0.73 0.72 0.38 0.64 0.38

0 0 0 0 0 0.18 0.27

12.3 19.0 17.1 2.1 2.0 7.2 16.1

too (h) 13 23 17 8 10 8 17

Oequilibrium is equal to the ratio of the adsorbed quantity at too over the quantity adsorbed at saturation of adsorption (Qoo/Qsat); ~intra is the (micro)pore volume ratio of the sample and eB is the mesopore volume of the silica-alumina which has been mixed with the HZSM-5 zeolite in samples F and G. The pellets were obtained by compressing the powder at a pressure of 3 tons cm -2. REFERENCES

1. J.-L. Bonardet, J. Fraissard, A. G6d6on and M.-A. Springuel-Huet, Catal.Rev.-Sci. Eng., 41-42 (1999) 115. 2. A. G6d6on, T. Ito and J. Fraissard, Zeolites, 8 (1988) 376. 3. M.C. Barrage, J.-L. Bonardet and J. Fraissard, Catal. Lea., 5 (1990) 143. 4. J. K~ger, H. Pfeifer, T. Wutscherk, S. Ernst, J. Weitkamp and J. Fraissard, J. Phys. Chem., 96 (1992) 5059. 5. J. K~ger, H. Pfeifer, F. Stallmach and H. Spindler, Zeolites, 10 (1990) 288. 6. F.D. Magalh~es, R.L. Laurence, W.C. Conner, M.-A. Springuel-Huet, A. Nosov and J. Fraissard, J. Phys. Chem. B, 101 (1997) 2277. 7. J.V. Villadsen and M.L. Michelsen, Solution of Differential Equation Models by Polynomial Approximation, Prentice Hall, New Jersey, 1978. 8. P. N'Gokoli-Kekele, M.-A. Springuel-Huet, J.-L. Bonardet and J. Fraissard, to be published