J. Phys. Chem. 1993,97, 1420-1425
1420
The Fractal Dimension in Molecular Sieves: Synthetic Faujasite and Related Solids Bogdan Sulikowski Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 1. 30-239 Krakbw, Poland Received: May 14, 1992; In Final Form: October 15, 1992
Synthetic faujasite (Na-Y) and its variously modified forms have been examined by X-ray diffraction, IR spectroscopy, 29Si MAS NMR, and sorption of C I 4 4 alcohols. A surface fractal dimension, Da, of Na-Y, H,Na-Y, and ultrastable, realuminated, and chemically dealuminated zeolite Y has been calculated from the adsorption isotherms of alcohols. The evolution of the fractal dimension of faujasitic solids is discussed in terms of the parent zeolite treatment. We will demonstrate that fractal analysis is a convenient tool and can discern and describe quantitatively the differences between samples prepared from the sodium form of zeolite Y via diverse routes.
Introduction Synthetic counterparts of the rare mineral faujasite belong to the most important zeolites, both from commercial and academic points of view. Synthesized in the 1950s, they were introduced as cracking catalysts 30 years ago and now are manufactured on a large industrial scale. Faujasites prepared by hydrothermal synthesis usually have SIIAI framework ratios between 1.0 and 3.4,’ with much effort often devoted to postsynthesis modification ofthe product. TheframeworkofzeoliteY isamenable tovarious chemical and hydrothermal treatments, thus leading to a practically infinite number of variants. Thermal or hydrothermal treatment of the ammonium-exchanged forms produces either regular “hydrogen” or the so called “ultrastable” material. The latter term was coined because of the improved resistance of such solidstodegradation under steam and acid treatment.2 Numerous dealumination methods have been developed to vary at will the composition of the zeolitic framework. Aluminum can be extracted from the lattice by H4EDTA acid, tartaricacid, COCl2, S02C12, and other volatile compound^.^-^ Aluminum can also be extracted and replaced by external sources of silicon, by using Sic14 in the gas phase5v6or (NH4)2SiF6 in s~lution.~J’ Some of the dealumination processes have been implemented industrially, and catalysts with high activity, selectivity, and thermal stability are now manufactured. Examples are the cracking catalyst LZ210 based on the dealuminated zeolite Y and the Katalistiks Beta series of catalyst^.^ Recently, novel zeolitic solids were prepared by reversing the dealumination procedure and inserting aluminum back into the lattice.I0 Realuminated samples display a very different distribution of silicon and aluminum in the lattice in comparison with the starting material. It seemed therefore of interest to compare some of the physicochemical properties of the parent and modified Na-Y zeolite. The objective of this paper is to determine if there is a correlation between the fractal dimension of the zeolite solid and the method of modification. In particular we would like to quantify the different fractal dimensions of the parent, hydrogen, ultrastable, chemically dealuminated, and realuminated samples. Experimental Section Sample Preparation. ( i ) Ion Exchange. Using zeolite Na-Y (SiIAl = 2.47) as a starting material, the 78% NH4-exchanged form was prepared by 2-fold contact of Na-Y with a 10 wt % aqueous solution of NH4CI at 80 OC followed by washing with distilled water and drying. 78% NH4,Na-Y was used for ultrastabilization (ii). Additional ion exchange of this sample yielded 92% NH4,Na-Y. This material was deammoniated to produce the hydrogen form by in situ heating and evacuation in a Sartorius balance. 0022-365419312097-1420$04.Oo/O
TABLE I: Chemical Composition, Lattice Parameter, .ad Number of Framework Aluminum Atom ( A l l Sample Unit Cell As Determined by XRD, IR,and
sample 1 Na-Y 2 NH4, Na-Y
no.
Si/ AI‘
2.47 2.45 3 us-Y 2.55 2.84 4 real-US-Y 5 NH~,N~-Y’EDTA6.48
uo (A)
NMRb
24.690(2) 24.692(2) 24.51(1) 24.63(2) 24.50(2)
56 f 1.0 55 f 1.0 33 f 1.8 52h 1.1 39 f 1.6
XRDC 55.1 f 0.3 55.3 0.3 34.6 f 1.2 48 & 2.4 34 f 1.8
IRd (f0.8) 61.1 61.1 32.0 54.0 35.1
By wet chemical analysis. Calculated from Gaussiandeconvolution of 29SiMAS NMR spectra. Calculated from AI/(Si AI) = 0.591~ - 14.305 (ref 36). Calculated from AI/(Si + Al) = 4.454- 4.099 X 10-3v, (ref 37). @
+
(ii)Ultrastabilization. A 3-g portion of 78% NHcNa-Y zeolite was placed in a tubular quartz furnace and heated to 550 OC at a rate of 125 OC/1 h. Water was injected into the tube by a peristaltic pump (12 cm’lh), so its partial pressure above the zeolite bed was 1 atm during the ultrastabilization procedure. The presence of steam is essential during the ultrastabilization as it facilitates healing of the aluminum vacancies by silicon. The sample was kept at 550 OC for 18 h and then cooled to ambient temperature. The injection of water was stopped at 100 OC. (iii) Realumination. A portion (2 g) of the ultrastable (US) sample was stirred with 100 mL of 0.25 M KOH solution at 80 ‘C for 25 h. The product was washed with water and dried. The nonframework zeolitic aluminum generated by procedure ii was a source of aluminum. The sample real-US-Y had 52 framework aluminum atoms/uc, while zeolite Na-Y had 56 AlFIuc, as indicated by the 29Si MAS NMR method (Table I). The efficiency of the realumination procedure was therefore 93%. (iv) Chemical Dealumination. Zeolite Na-Y was dealuminated chemically by means of ethylenediaminetetraacetic acid (H4EDTA). Dealumination proceeds stoichiometrically so that the desired aluminum content may be readily obtained by using thecalculated amountsof H4EDTA. Thedegree of dealumination by H4EDTA was chosen so as to match the amount of framework aluminum in the sample prepared via the hydrothermal route (ii). The rate of H4EDTA addition, essential to minimize amorphization of the zeolite, was kept low at 0.01 g of H4EDTA per 1 g of zeolite per 1 h. Dealumination was carried out under stirring at 97 OC. The process was continued for 4 h after all the acid had been added. The dealuminated samples were washed with hot distilled water, dried, and ion exchanged with 10 wt % NH4Cl solution. More details on dealumination with H4EDTA are given in the Appendix. 0 1993 American Chemical Society
The Fractal Dimension in Molecular Sieves
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1421
TABLE Ik Infrared Frequencies Correspoading to Fnmework Vibrations of Zeolitic Samples no. 1
2 3 4 5
asymmetric stretch 1261
1132 1138 1171 1141 1153
1009 1009 1046 1018 1042
A
2
vibration frequencies (cm-I) symmetric stretch D6R T-O bend 790 788 813 788 805
719 721 749 727 732
577 575 587 574 587
506 503 512 503 512
460 460 456 454 456
IR Spectroscopy. The IR absorption spectra in the zeolite framework vibration region (4W1400 cm-1) were obtained with a Nicolet 800 Fourier transform spectrometer equipped with the TGS detector. 128 scans were accumulated per spectrum with 1-cm-I resolution using the KBr pellet technique. 29si MAS NMR. 29SiMAS NMR spectra were acquired on a Bruker MSL-400 spectrometer operating at 79.5 MHz. A homemade NMR probehead with an Andrew-Beams rotor was used. Samples were spun in air at 3-3.5 kHz,and 5-# radiofrequency pulses ( r / 2 pulse angle) were applied with a 25-s recycle delay. X-ray Diffraction. Powder X-ray diffraction patterns were acquired on a Philip PW 1710 automatic diffractometer using Cu Ka radiation selected by a graphite monochromator in the diffracted beam. All the samples were saturated over aqueous NHdNO3 to avoid the change in water content during the measurement. (Water can significantly alter lattice parameters of faujasitic solids)." Silicon powder was used as an internal standard, and lattice parameters were calculated by a refining program. SorptionMeasurements.Adsorption isothermswere taken with the Sartorius 4410 balance at 23 OC. Methanol, ethanol, 2-propanol, and 2-methyl-1-propanol (reagent grade) were purified by distillation and stored over precalcined 5A molecular sieves. Zeoliticsamples weredehydrated at 350 OC under vacuum (ca. 10-5Torr) for several hours before adsorption measurements were carried out. All the alcohol isotherms were taken using the same portion of a given sorbent to minimize possible errors due to the sample handling. A careful desorption step was applied between the measurements,with prolonged evacuation at ambient temperature followed by a slow heating to 350 OC. Parallel adsorption experiments were carried out to check the reproducibility of the sorption experiments. Results and Discussion Conventionalanalytical methods, such as wet chemical analysis, atomic absorption, or X-ray fluorescence, do not distinguish between framework and nonframework A1 species and thus measure overall Si/AI ratios of zeolitic solids. The overall composition of the samples prepared for sorption measurements is given in column 3 of Table I. The starting material (100% crystalline zeolite Y) has a Si/AI ratio of 2.47. Both the ammonium and the ultrastable forms (samples 2 and 3) exhibit ouerall compositions very similar to the parent sample, while the Si/AI ratio of sample 4 is slightly larger. The most siliceous sample is the chemically dealuminated zeolite 5. The number of aluminum atoms in the framework of zeolites X and Y affects their unit cell parameters (Table I), as well as most of the mid-infrared frequencies (Table 11), largely because S i 4 bonds are shorter than A I 4 bonds (1.60 and 1.75 A, respectively). Empirical formulas can therefore be derived that allow estimation of the number of A1 atoms occupying framework sites from XRD and IR measurements. The corresponding data obtained by these indirect methods are listed in Table I. On the other hand, the number of aluminum atoms in frameworkpositions can be calculated in a straightforward manner from the 29SiMAS NMR spectra of the samples. The spectra (Figure 1) consist of five signals correspondingto Si(nAl) groupings,where the number of A1 framework atoms linked via oxygens to silicon is denoted
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