Chapter 19
Homogeneous Adsorption of Benzene on NaX and NaY Zeolites
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Shang-Bin Liu1, Jin-Fu Wu2,3, Long-Ja Ma1, May-Whei Lin1, and Tun-Li Chen2 1Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 10764, Republic of China 2Department of Chemistry, Tamkang University, Tamsui, Taiwan 25137, Republic of China The adsorption behavior of benzene on dehydrated NaX and NaY zeolites has been investigated directly by 1H and 13C N M R measurements of the adsorbed benzene and indirectly by the combination of 129Xe N M R and isotherm measurements of the co-adsorbed xenon. Powdered zeolite samples of various S i / A l ratios and with varied adsorbate concentrations were investigated. Detailed macroscopic and microscopic adsorption phenomena of benzene in NaX and NaY zeolites, including the loading capacity, mobility, and sites of adsorption are presented in terms of measurements of N M R linewidths and chemical shifts. The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion (1-6) and adsorption (7-9) techniques. Moreover, the adsorbatezeolite interactions and related motion and location of the adsorbate mole cules within the zeolite cavities have been investigated by theoretical calculations (10-15) and by various spectroscopic methods such as U V (16, 11), IR (17-23), neutron (24-27), Raman (28), and N M R (29-39). Adsorption Properties of Benzene i n Faujasite-type Zeolites Despite extensive study of the adsorption of benzene on zeolites, little attention has been devoted to the equilibrium state of adsorption and the
3Current address: Department of Chemistry, National Tsinghua University, Hsinchu, Taiwan, Republic of China
0097-6156/93/0517-0272$06.00/0 © 1993 American Chemical Society
In Selectivity in Catalysis; Davis, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
19. LIU ET AL.
Homogeneous Adsorption of Benzene on Zeolites
distribution of adsorbate within zeolite cavities. Sorption diffusivities, which are characteristic of related transport processes, are commonly inconsistent when obtained from different techniques (40-43). The possible origins of the discrepancies arising from transport resistances (44~4fy and from adsorbate/ adsorbent characteristics (1-4,38,49-53) have been discussed. When benzene molecules are adsorbed on dehydrated faujasite-type zeolites, the common assumption is that the molecules are adsorbed within the zeolite supercage and disperse uniformly. This assumption was first questioned by Pines and co-workers (54,55) who observed a uniform benzene adsorption on NaY zeo lite only after an extensive adsorbate/adsorbent sample thermal pretreatment. Using X e , Ή , and C N M R (56), we confirmed that a prolonged thermal treatment at temperature > 250°C (much greater than the boiling point of bulk benzene) is necessary to ensure an homogeneous benzene dis tribution within cavities of NaX and NaY zeolites. Moreover, this effect depends not only on the nature of the adsorbent samples and related sample-bed configurations but also on the adsorbate concentrations. Following the pioneering works of Ito and Fraissard (57) and Ripmeester (58), X e N M R of xenon adsorbed on zeolite has proven sensitive probe of its local environment due to its chemical inertness and excellent sensitivity (59). In this work, we used Ή and C N M R measurements of the adsorbed benzene in conjunction with X e N M R and adsorption iso therm measurements of the co-adsorbed xenon to study the homogeneous adsorption behavior of benzene on faujasite-type zeolites with various S i / A l ratios. Detailed macroscopic and microscopic adsorption states of the benzene in various NaX and NaY zeolites are discussed in terms of N M R linewidths and chemical shifts and are compared with results obtained from other studies. 129
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Experimental Section Materials and Sample Preparation. Powdered, binderless, sodium-form zeo lite X (Si/Al = 1.23) and zeolite Y (Si/Al = 2.49, 2.70) were obtained from Strem Chemicals. The crystalline framework structure of these mater ials was confirmed by x-ray diffraction (XRD). Their chemical compositions were determined by I C P - A E S and their respective S i / A l ratios were further confirmed by S i M A S N M R measurements. Before adsorption of guest molecule, a known amount (typically ca. 1 g) of hydrated zeolite sample in the sample tube was dehydrated by gradual heating to 400°C in vacuum (< 10" torr) and was then maintained at this temperature for at least 15 hours. The sample tube configuration was designed so that a 10 mm stan dard N M R tube joined to a vacuum valve could be conveniently setup for adsorption or desorption of adsorbate molecules on a vacuum apparatus and for isolation of the sample from the atmosphere during N M R experiments. After dehydration, benzene guest molecules (adsorbates) were introduced into the samples of host zeolites (adsorbents) by vapor transfer at room temperature (22C). Prior to the experiments, samples with various benzene concentrations were subjected to thermal treatment at 250°C for 10 hours to render uniform adsorbate distribution within zeolite cavities. 29
5
Xenon Adsorption Experiments. Gaseous xenon was co-adsorbed onto the samples on a vacuum manifold; the xenon equilibrium pressure was mea sured by an absolute-pressure transducer (MKS Baratron) capable of mea suring pressure with accuracy ± 0.1 torr. The adsorption isotherms of the co-adsorbed xenon in the samples were measured volumetrically at 22 °C.
In Selectivity in Catalysis; Davis, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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SELECTIVITY IN CATALYSIS
N M R Experiments. A l l N M R experiments were done at 22 °C on a spect rometer (Bruker MSL-300) using a broadband N M R probe with proton dec oupling option. The field strength of the wide-bore superconducting magnet was 7.05 tesla, corresponding to resonance frequencies 300.13, 75.47 and 83.01 M H z for nuclei of *H, * C and X e , respectively. For the X e N M R experiments, free induction decays (FID) were recorded at room temperature following a single radio-frequency pulse (ca. 30°) at 0.3 s intervals. We followed this procedure because the adsorbed xenon has a large spin-lattice relaxation time (Ti > 3 s). In order to ensure adequate signal-to-noise (S/N) ratios, we accumulated typically 1,000-240,000 FID depending on the amount of xenon adsorbed on the sample. The reference for the X e chem ical shift values was that of xenon gas extrapolated to zero density foll owing the equation given by Jameson (60). A l l resonance signals of Xe adsorbed on the samples were shifted to higher frequency relative to the reference, which we define to be the positive direction. The *H N M R spec tra were also obtained by the single-pulse sequence using 2 s recycle delay. Typically, 120 accumulated FID were sufficient to generate spectra with adequate S / N ratio. The C N M R spectra, on the other hand, were obtain ed with proton decoupling; typically 6,000 scans were averaged every 0.6 s. The reference for the chemical shifts of both H and C was liquid benzene (adjusted to tetramethylsilane, or T M S , as standard) at the same spectro meter settings. 3
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1 2 9
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Results and Discussion 1 2 9
Xenon Adsorption Isotherms and X e N M R Measurements. Figure 1 dis plays the room temperature (22 °C) xenon adsorption isotherms of the coadsorbed xenon for the three different zeolite samples loaded with various amounts of benzene. A consistent decrease of adsorption with increasing θ was found for each benzene/zeolite system. By comparing the slope at low xenon pressures, i.e. in the Henry's Law region, we obtained for the adsorp tion strength NaX(1.23) > NaY(2.49) > NaY(2.70). Moreover, the saturation benzene concentration in faujasite-type zeolites with different S i / A l ratios follows the relation: NaX(1.23) < NaY(2.49) < NaY(2.70). The dependence on xenon pressure of the measured X e N M R chemi cal shift (δ) and linewidth (Δα;) was also recorded for each adsorbate/adsorbent sample system. In combination with the adsorption data in Figure 1, the dependence of the X e chemical shift on the density of adsorbed xenon is presented in Figure 2. For given 0, the observed 6 increases with increasing xenon pressure due to increasing interactions among xenon atoms. In contrast, for a given xenon loading, the increase of δ with increasing θ characterizes a decrease in the internal void space of the benzene/zeolite samples, hence a decrease in the mean free path of xenon (57). The measur ed xenon chemical shift is described as the sum of three terms: 1 2 9
1 2 9
δ = δ + 4 + a - -pxe 0
xe
xe
;
(1)
here δ = 0 is the reference; d> corresponds to the shift at zero xenon loading which represents in turn the sum of two contributions,
