J. Phys. Chem. 1994,98, 7436-7439
7436
Infrared Microscopic Study of Sorption and Diffusion of Toluene in ZSM-5 Georg Miiller, Thomas Narbeshuber, Gabriele Mirth, and Johannes A. Lercher' Department of Chemical Technology, University of Twente, Christian Doppler Laboratory for Heterogeneous Catalysis, Postbox 21 7, 7500 AE Enschede, The Netherlands Received: April 14, 1994"
Time-resolved FTIR microscopy was used to investigate in situ the transport and sorption of toluene in individual ZSM-5 crystals with varying degree of perfection. The results indicate that the rate of transport of toluene in the pore system of zeolite ZSM-5 strongly depends upon the degree of crystal intergrowth. The diffusion coefficients observed with single crystals were 3 orders of magnitude higher than the diffusion coefficients measured for a polycrystalline sample. The first results of this new method to study diffusion in microporous materials are compared to the results from other techniques.
Introduction The unique properties of ZSM-5 for adsorption and diffusion of aromatic molecules (in particular, of benzene, toluene, and xylene) were extensively studied during the past decade (e.g., refs 1-6). Various methods are already known and established to measure sorption kinetics such as, e.g., zero-length column chromatography,3s7IR spectroscopy,Q gra~imetry,~.~ single-step frequency response techniques: and NMR spectroscopy.10-12 Although the values of diffusion coefficients reported vary drastically, Garcia et ala2indicated that this is mainly due to the differences in experimental conditions used and is not caused inherently by the different analytical techniques applied. This large amount of information allows, therefore, to use adsorption and diffusion coefficients of aromatic molecules to parametrize new experimental methods and theoretical models for the evaluation of diffusion in molecular sieves. In a previous paper, in situ FTIR microscopy was introduced to study the surface chemistry during the decomposition of template molecules in individual zeolite crystals.13 We adopted this earlier experimental setup to measure the time-dependent uptake of aromatic molecules in single crystals of ZSM-5. The main advantage of using IR microscopy for these measurements is the possible confinementof the analyticalvolume to an individual specimen or a fraction of this specimen. Thus, the transport can be viewed through a specified crystallographic direction of the molecular sieve. The adsorption (and diffusion) of toluene in ZSM-5 single crystals was used to compare in situ FTIR microscopy with previously well-established techniques like gravimetry and conventional transmission absorption IR spectroscopy.
Experimental Section Zeolites. Four ZSM-5 samples with different degrees of intergrowth were used. The Si/AI ratios, crystal sizes, and morphologies of these zeolites are compiled in Table 1. Figure 1 shows scanning electron microscope images of single crystals of the samples and a schematic representation denoting the crystallographic orientations in ZSM-5. IR Microscopy. IR measurements were performed using a Bruker microscope attached to the IFS88 FTIR spectrometer (4-cm-1 resolution). The experiments were carried out in situ in a microreactor placed at the stage of the IR microscope. Figure 2 shows a schematic representation of that microreactor and the general experimental setup. Gas stream 1 contained He with a defined partial pressure of toluene (0.1 mbar, realized by injection Abstract published in Advance ACS Abstracts, July 15, 1994.
0022-3654/94/2098-7436$04.50/0
TABLE 1: Chemical Composition, Morphology, and Size of ZSM-5 Samples Studied sample Si/AI ratio morphology size [pm)] ZSM-5-1000/1 >lo00 singlecrystal, coMin type. 400 X 90 X 90 ZSM-5-1000/2 >lo00 singlecrystal, twinned 100 X 40 X 40 ZSM-5-122 122 single crystal, coffin type. 150 X 40 X 40 ZSM-5-35 35 polycrystalline, spherical 4.2 via a syringe pump), gas stream 2 contained pure He, and gas stream 3 contained synthetic air or pure He. Two dead volume free four-port crossover valves were used to switch between the gas streams. Tubings with a diameter of 1/16 and '18 in. in combination with high gas flow rates were used to provide short transient periods (below 1 s) to reach steady-state concentrations in the reactor after forced changes of the feed composition. All experiments were carried out at 320 K. The radial temperature gradient across the sample holder was found to be less than 10
K. The IR band of toluene at 1495cm-* was used to quantify the amount of toluene adsorbed per weight of the catalyst. Independent gravimetric measurements showed an excellent direct correlation between the intensity of the band at 1495 cm-1 and concentration of adsorbed toluene. Sample Treatment. For the spectroscopic investigations, individualcrystalsof ZSM-5-1000/1,ZSM-5-1000/2, and ZSM5-122 were placed on the CaF2 sample holder. Because of its small particle size, polycrystallinezeolite ZSM-5-35 was pressed into a self-supporting wafer. The samples were activated in synthetic air (flow rate = 150 mL/min at STP) by heating with a rate of 10 K/min up to 820 K, holding this temperature for 10 min. At 820 K, the gas stream was changed to pure He (flow rate = 260 mL/min at STP), and the temperature was lowered to 320 K. The partial pressure of toluene in He was changed in a pressure step from 0 to 0.1 mbar. The uptake of toluene was monitored in situ by collecting IR spectra with a time resolution of 60 s. Gravimetry. A Cahn RG microbalance was used for the gravimetricdeterminationof the time-dependentuptake of toluene on zeolite ZSM-5. Activation of the crystals was carried out in air under static conditions. The measurement was performed in vacuum. A differentially pumped manifold allowed to provide a step function of the partial pressure of toluene from 0 to 0.1 mbar. Calculation of the Diffusion Coefficient. The diffusion coefficients were calculated using the relations of FU = ( 2 4 V ) (Dt/r)O.5 and FU = (m, - mO)/(m- - %).I4 FU,A , V, and D denote the fractional uptake (defined as the ratio of the difference in mass at time t and time t = 0 and the difference in mass at equilibrium and at time t = 0), the surface area, the volume of 0 1994 American Chemical Society
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Twinned ZSM-5- 1000/2
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7437
ZSM-5-122 (shape of ZSM-5- 1000/1 identical)
P Polycrystalline ZSM-5-3 5 Figure 1. REM pictures of the samples.
the sample, and the diffusion coefficient, respectively. Single crystallites were described as cylinders; the individual particles of the polycrystalline sample were assumed to be spheres.
Results and Discussion Activationof the zeolitesandAdsorptionof Toluene. IR spectra of the activated samples are compiled in Figure 3; the corresponding spectra of the zeolites in equilibrium with 0.1 mbar of toluene are shown in Figure 4. The spectrum of the activated zeolite ZSM-5- 1000/ 1 (Figure 4a) showed bands at 2007,1882, 1644, and 1475cm-I, which were assigned to overtones of zeolite latticevibrations.Is IR bands characteristicfor hydroxyl groups were not detected with this sample. The spectrumof theactivated ZSM-5-1000/2 (Figure 4b) showed two bands in the spectral region of the OH stretching vibrations at 3725 and 3500 cm-1, which are characteristicfor SiOH groups of lattice defect sites.16 Inaddition to thesebands,a bandat 361Ocm-I whichisattributed to the OH stretching vibration of the zeolite Si-OH-A1 groups (strong Bransted acid sites) was observed with zeolite ZSM-5122 (Figure 4c).17 The spectrum of the activated sample ZSM5-35 showed a significantly higher intensity of the band at 3610 cm-1 (due to the higher concentration of A1 in the lattice) than ZSM-5- 122as well as an additional band at 3745 cm-1 (assigned to the terminal SiOH groups17). After contacting the zeolites with 0.1 mbar of toluene, new IR bands were observed at 3089,3067,and 3032 cm-* (CH stretching vibrations of the aromatic ring of adsorbed toluene), at 2962, 2923, and 2733 cm-1 (CH stretching vibrations of the methyl group of adsorbed toluene), and at 1606 and 1495 cm-1 (ring vibrations of toluene).18J9 In the spectra of toluene adsorbed on
the samples ZSM-5-122 and ZSM-5-35, the decrease in the intensity of the hydroxyl stretching bands at 3745 and 3610 cm-1 and the appearance of new broad bands at 3600 and 3290 cm-1 were noticed. Both bands are attributed to perturbed OH vibrationscaused by hydrogen bonding of toluene with these OH groups.16J9 The shifts indicate that the strength of interaction of toluene with the SiOH groups is significantly weaker than with the SiOHAl groups.20 Diffusion. The diffusion coefficients (see Table 2) were calculated using the time-dependentuptake of toluene which was monitored spectroscopically (see Figure 5 ) and gravimetrically. As demonstrated in Table 2 for ZSM-5-1000/1, the overall diffusion coefficients determined in the IR microscopic experiments were in very good agreement with the result from gravimetric measurements. The time to reach 50% of the equilibriumuptake(ts)dependsdirectlyon thesizeofthecrystals. Consequently, the slight difference in tN for ZSM-5-1000/1 observed for the two types of measurements is explained by the difference between the individual crystal size of 400 X 90 X 90 pm3 for FTIR microscopy and the somewhat smaller average crystal size in the gravimetricmeasurementsof that sample. The results obtained for the various types of single crystalsagree very well with those of Qureshi and We91 for partially twinned and slightly intergrown single crystals. Also, the direct observations of the intracrystallinetransport of benzene in ZSM-5(100 X 30 X 30 pm3) by xenon pulsed field gradient NMR measurements yielded diffusion coefficients on the order of 1.3 X 1&Iocm2/s at room temperature," which is in good agreementwith thevalues for single crystals reported here. In situ IR studies on wellcrystallized ZSM-5 crystals (8.8 X 5.2 X 3.2 pm3) performed by
7430 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994
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TABLE 2 Diffusion Coefficients of Toluene in ZSM-5 at
,251
320 K and 0.1 mbar of Toluene
3500
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3500
1500
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Figme 4. Spectra of 0.1 mbar of toluene adsorbed on activated samplcs at 320 K.
Niessen and Karge4**yielded diffusion coefficients for benzene and ethylbenzene which range between the values obtained for toluene in perfect single crystals and polycrystalline material in this study.
diffcoeff tu, diffcoeff tu, (IR spec) (IR spec) (microbalance) (microbalance) [SI [cm2/s1 [SI sample [cm2/s] ZSM-5-1000/1 1.1 X IP9 720 1.33 X 10-9 500 ZSM-5-1000/2 3.9 X IO-" 370 ZSM-5-122 6.4 X 10-'' 490 ZSM-5-35 1.5 X l&13 180
A low value of the diffusion coefficient as found for the polycrystalline sample in this study was also reported for polycrystallinematerialby Choudharyet al.u Furthermore,both diffusion coefficient and f atime of this sample (ZSM-5-35) were in excellent agreement with the correspondingresultsdetermined by conventional transmission absorption IR spectroscopy using zeolite wafers.19 Thus, the examplesdemonstrate that excellent agreementbetween the present results and earlier measurements exists, provided that thesame typeof ZSM-5 (crystallinity, degree of intergrowth) is investigated. The influence of strong sorption of the molecules on the rate of transport in the pores of zeolite HZSM-5 was evaluated by comparing the values for the diffusion coefficients for ZSM-51OOO/ 1 (without Branstedacid sites) and ZSM-5-122 (with strong Bransted acid sites). The similarityof the values of the diffusion coefficients with these two samples indicates that strong sorption of molecules did not markedly hinder the rate of transport in the pores of ZSM-5. This finding is in good agreement with the theoretical and experimental work of Wei et aZ.,*3J4indicating that one-component diffusion coefficients are essentially independent of the coverage up to values of 0.75. Comparison of the two samples having the same Si/Al ratio but different degree of crystal intergrowth indicated that the rate of transport in the externallytwinned crystals (see Figure 1) was lower than in the (apparent) single crystals (see Table 2). Externally twinned crystals terminate with a higher fraction of (100) surfaces than monocrystalline samples (see Figure 1). Therefore, a larger fraction of molecules have to enter the twinned crystals through the (100) surface planes,I5which have only pore openings leading into sinusoidal channels. As the transport
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7439
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