5836
Langmuir 1999, 15, 5836-5840
Hydrocarbon Diffusion Measurements and Coke Distribution in Zeolite Pellets: A Study by 1H NMR Imaging and 129Xe NMR Spectroscopy† J.-L. Bonardet,* T. Domeniconi, P. N’Gokoli-Ke´ke´le´, M.-A. Springuel-Huet, and J. Fraissard Laboratoire de Chimie des Surfaces, S.I.E.N., CNRS-ESA 7069, Universite´ P. et M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France Received September 22, 1998. In Final Form: March 4, 1999
Diffusion of benzene in HZSM-5 zeolite in powder or pellet form and of 2,3-dimethylpentane (DMP) in HY zeolite pellets, more or less deactivated by formation of coke during n-heptane cracking, was studied by 1H NMR imaging and 129Xe NMR of adsorbed xenon used as a probe. The shape and the evolution of the signal intensity of the 1D profile, representing the proton distribution along the axis of the cylindrical tube containing the catalyst, make it possible to determine the intracrystalline molecular diffusion coefficient of benzene when this is adsorbed (≈10-14 m2 s-1). Comparison of the 129Xe NMR spectra, recorded during benzene adsorption, with those simulated assuming intracrystalline diffusion gives a similar value of the diffusion coefficient (≈10-15 m2 s-1). For DMP in HY coked pellets, the NMR profile shows a maximum, which moves during DMP adsorption. This was interpreted as the superposition of two competing processes: intercrystalline diffusion and intracrystalline diffusion of DMP from uncoked to highly coked zones of the crystallites. 1D and 2D images at equilibrium adsorption of the probe clearly confirm the heterogeneous distribution of the coke shown by 129Xe NMR spectroscopy.
Introduction
Experimental Section
The properties and the efficiency of solid catalysts are strongly influenced by the rate of molecular transport. Therefore, diffusion of reactants and products plays an essential role in reactions catalyzed by microporous solids. The methods of studying molecular diffusion in porous solids are numerous and varied. Among them, NMR techniques are particularly relevant. Beside pulsed field gradient (PFG) NMR1,2 which is a classical technique to measure molecular displacements over a scale from 10 nm to 100 µm, NMR imaging has been successfully used to study solvent penetration into polymers and coals3,4 and permeation resistance of cement5 as well as drying kinetics of alumina or titania pellets.6 This technique can also be used to determine diffusion coefficients in zeolites. We present here a comparative study of benzene and 2,3dimethylpentane (DMP) diffusion by 129Xe NMR of adsorbed xenon used as a probe7 and by 1H NMR imaging. These two techniques have also been used to show the heterogeneous coke distribution in more or less deactivated zeolite pellets.
Materials. HSM5 (Sud Chemie AG, Si/Al ) 130, mean crystallite size about 40 µm) and HY zeolite (LZ54, UOP, Si/Al ) 2.3, mean crystallite size about 1 µm) were used either in powder or in pellet form. In that latter case, the samples were compressed at a pressure of 103 bar into cylindrical pellets 12 mm long and 6-7 mm in diameter. Before adsorption of hydrocarbons or xenon, samples were outgassed overnight under a vacuum of 10-8 bar at 673 K. Deactivation of the catalysts was performed by cracking nheptane at 720 K for periods between 2 and 10 h to obtain various coke contents (2.5, 7.5, 10% w/w). Xenon adsorption measurements were performed using a homemade volumetric apparatus at 300 K in the pressure scale 0-2 bar. Benzene and DMP (Aldrich Chem. Co., purity >98%) were chosen because their kinetic diameters (0.55 and 0.6 nm, respectively) are quite close to the diameter of the openings of the zeolite channels (0.55 nm for HZSM5) or of the cavities (0.74 nm for HY). For the diffusion studies by 1H NMR imaging, we used liquid hydrocarbons in equilibrium with their vapor so that diffusion occurred at constant pressure. Nuclear Magnetic Resonance. 129Xe NMR spectra were recorded on a Bruker MSL 400 spectrometer operating at a frequency of 110.67 MHz and using a simple 90° pulse sequence with a repetition time of 1 s. 1H NMR imaging experiments were performed on a Bruker MSL 300 apparatus operating at 300.13 MHz. Images were obtained by superimposition, during a time, τ′, of a pulsed field gradient (a few G/cm) in one (1D imaging) or two (2D imaging) directions on a composite Hahn echo sequence used for the signal detection (Figure 1). In these conditions, if the field gradient is applied along the z direction, which is the symmetry axis of the pellet, the signal can be expressed by
* Corresponding author. Fax: 33 144 27 55 36. E-mail: bonardet@ ccr.jussieu.fr. † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998. (1) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288. (2) Ka¨rger, J.; Caro, J. J. Chem. Soc., Faraday Trans. 1 1977, 73, 1363. (3) Blackband, S.; Mansfield, P. J. Phys. C: Solid State Phys. 1986, 19, L49. (4) Cody, G. D.; Botto, R. E. Macromolecules 1994, 27, 2607. (5) Papavassiliou, G.; Milia, F.; Fardis, M.; Rumm, R.; Laganas, E.; Jarh, O.; Sepe, A.; Blinc, R.; Pintar, M. M. J. Am. Ceram. Soc. 1993, 76, 2109. (6) Koptyug, I. V.; Fenelonov, V. B.; Khitrina, L. Y.; Sagdeev, R. Z.; Parmon, V. N. J. Phys. Chem. B 1998, 102, 3090. (7) Springuel-Huet, M.-A.; Bonardet, J.-L.; Fraissard, J. Appl. Magn. Reson. 1995, 8, 427.
i)n
S(2τ) )
[ ]
∑ F (z) exp[2iγg z τ] exp i
i)1
0 i
-2τ T2
(1)
where Fi(z) is the spin density at zi, g0 is the field gradient, γ is the gyromagnetic ratio of the probe nucleus, and T2 is its
10.1021/la981308v CCC: $18.00 © 1999 American Chemical Society Published on Web 05/08/1999
Hydrocarbon Diffusion Measurements
Langmuir, Vol. 15, No. 18, 1999 5837
Figure 1. Schematic representation of a 1H NMR imaging experiment.
Figure 2. Time dependence of benzene profiles during the adsorption of C6H6 on zeolite HZSM5 (loose powder). Similar profiles are obtained with compressed sample. transverse relaxation time. The Fourier transform of S(2τ) leads to the 1D spectrum which is the envelope of individual lines with amplitude Fi appearing at zi for an echo detected at t ) 2τ, τ being the time interval between the 90° and the 180° pulses of the Hahn echo sequence.
Results and Discussion Diffusion of Benzene in Fresh HZSM5 Zeolite. 1H Imaging. The 1D NMR profiles of benzene adsorbed in loose or compressed zeolite have the same form (Figure 2). The signal intensity increases with adsorption time. The rectangular shape of the profiles at any time proves that the benzene concentration is the same in any part of the sample and that diffusion of the hydrocarbon is controlled by micropore diffusion as discussed by Heink et al.8 The adsorption equilibrium is reached after 8 h, whatever the form of the sample (loose or compressed). Nevertheless, we observe a difference between loose and compressed samples when we plot the ratio C(t)/C(∞) against the time and the square root of time (Figure 3). If we assume that the benzene concentration in the gas phase is negligible and the T2 of protons of the probe is constant, C(t), the concentration of benzene in the sample at time t, is directly proportional to the integral of NMR profiles; C(∞) represents the benzene concentration at adsorption equilibrium. The sigmoidal shape of the curves C(t)/C(∞) ) f(xt) at the beginning of the adsorption can be attributed to the external surface barrier of the crystallites. Moreover, the curves do not exactly coincide; this can be explained by the difference in the porosity of the sample (ratio of the macro-mesopore volume by the total sample volume) which is lower for the compressed sample compared to the loose powder. This decrease (8) Heink, W.; Ka¨rger, J.; Pfeifer, H. Chem. Eng. Sci. 1978, 33, 1019.
Figure 3. Variation of the relative concentration C(t)/C(∞) of benzene adsorbed on HZSM5: (a) versus t and (b) versus xt.
in should lead to a decrease of the benzene mobility inducing a decrease in the transfer rate from the gas phase to the external surface of the crystallite. To obtain quantitative information (diffusion coefficient and transfer rate), we have simulated the kinetic curves. Assuming that the benzene concentration in the gas phase is constant and equal to that of its saturation vapor pressure (≈5.35 × 10-6 mol cm-3 at 300 K), C(t) is obtained by solving the Fick equation, expressed in spherical coordinates
(
)
∂C ∂2C 2 ∂C )D 2 + ∂t r ∂r ∂r
(2)
where D is the diffusion coefficient and r the radial coordinate, respectively. The boundary conditions are
t ) 0, C ) 0; t > 0, C ) C* for r ) R; dC/dr ) 0 for r ) 0 where R is the mean radius of the crystallites assumed to be spherical, and C* is the adsorbate concentration at the external surface of the crystallites, given by
C*(t) ) C(∞ )[1 - exp(-βt)] where β is the transfer coefficient of molecules from the gas phase to the surface of crystallites. Under these
5838 Langmuir, Vol. 15, No. 18, 1999
Bonardet et al.
Figure 4. Evolution with time of 129Xe NMR spectra of adsorbed xenon during the adsorption of benzene in HZSM5: (a) experimental, (b) simulated.
conditions, C(t) is given by the relation of Crank,9
C(t) C(∞)
)1-
3 λ
( )
exp -
λt
6λ π2
θ
(1 - xλ cot xλ) +
∞
∑
n)1
1 n2(n2π2 - λ)
exp
( ) -n2π2t θ
(3)
where λ ) βR2/D and θ ) R2/D. Starting from an approximate value of θ, estimated as the time constant of the equilibrium adsorption and then using λ and θ as adjustable parameters, we simulated the experimental curves and obtained an identical benzene diffusivity, D, of about 1 × 10-14 m2 s-1 for compressed and loose powders, which is in good agreement with the literature.10 As expected, the values of β are different, 1.18 × 10-3 s-1 for the powder and 0.78 × 10-3 s-1 for the pellet, confirming that compression has reduced the transfer rate of molecules to the surface. 129Xe NMR. Since the beginning of the 1980s, 129Xe NMR has proved to be a powerful tool to investigate porosity of a lot of materials (zeolites, clays, coals, polymers, ...);11 we used xenon coadsorbed with hydrocarbon as a “spy” to study diffusion of benzene in HZSM5 pellet. Xenon was preadsorbed at a pressure low enough (
