J . Phys. Chem. 1989, 93, 350-356
350
Hydrothermal Isomorphous Substitution of Aluminum in Faujasitic Frameworks: Second-Generation Zeolite Catalysts Halimaton Hamdan, Bogdan Sulikowski,t and Jacek Klinowski* Physical Chemistry Laboratories, University of Cambridge, Lensfield Road, Cambridge CB2 1 EP, England (Received: March 9, 1988)
Magic-angle-spinning NMR, infrared spectroscopy, and X-ray diffraction measurements all indicate that aluminum atoms eliminated from the framework of zeolite Y by hydrothermal treatment (ultrastabilization) can be subsequently reinserted into the framework by treatment with aqueous solution of KOH at elevated temperatures. Sample crystallinity is largely retained in the process and depends upon the concentration of the base. The products have a very different Si,AI distribution from as-prepared zeolite Y with the same AI content. The realumination-dealuminationp r m s can be performed in a cyclic fashion yielding a wide variety of second-generationfaujasitic catalysts. Aqueous NaOH is less effective than KOH in achieving reinsertion of Al.
Introduction
TABLE I: Conditions of Preparation of Samdes'
Properties of zeolites are intimately related to the type of occupancy of the tetrahedral sites. Modification of the compasition of the framework, particularly important in the case of faujasitic catalysts (zeolites X and Y) and pentasil zeolites, is involved with thermal stability (more siliceous structures being generally more stable) and with catalytic activity for a variety of reactions' (by altering the number and strength of acidic sites). It also affects the extent of coke deposition. It is thus desirable to be able to alter the Si/Al ratio of the framework, and it would be particularly convenient to achieve this by Ysecondary synthesis", i.e., isomorphous substitution of Si or A1 on the tetrahedral sites after the completion of the original zeolite crystallization. It is also of interest to introduce other elements, such as Ga, Ge, P, and B, into the tetrahedral sites. While the aluminum content of zeolites can be decreased relatively easily by a variety of methods,2 ways of increasing it have been devised only recently. A1 can be isomorphously substituted for Si in the framework of highly siliceous zeolite ZSM-5 by treatment with AICIJ vapor3 or alumina4 at elevated temperatures, but these processes are inconvenient and limited in extent. A preliminary account has been givenS of experiments in which the process of ultrastabilization of zeolite Y is reversed by a simple hydrothermal treatment at moderate temperatures. We have also shown6 that highly siliceaps silicalite can be hydrothermally converted into zeolite ZSM-5 with a substantial A1 content. We now present a full study of the preparation and characterization of "second-generation" faujasites. We shall demonstrate that (i) virtually all the extraframework aluminum in dealuminated zeolite Y can be resubstituted into the framework by treatment with an aqueous solution of KOH; (ii) the samples remain crystalline upon treatment, and their degree of crystallinity depends upon the concentration of the base, the temperature, and the number of dealumination-realumination cycles to which the sample was subjected; (iii) the distribution of Si and A1 among the tetrahedral sites in the treated material is very different from that in as-prepared zeolite Y with the same %/A1 ratio; and (iv) the process of dealumination-realumination can be repeated in a cyclic fashion yielding a great variety of novel faujasitic solids. Experimental Section
Sample Preparation. ( i ) Ultrastabilization. The starting material (sample 1) was 78% NH4-exchanged zeolite Y (Si/Al = 2.50) prepared by 2-fold contact of Na-Y with a saturated aqueous solution of NH4N03at 80 OC followed by washing with water and drying. Twenty-gram portions of the zeolite were heated in a tubular quartz furnace (see Figure 1) with water being injected at a rate of 12 mL/h into the tube by a peristaltic pump 'On leave from the Krakdw Technical University, Poland.
0022-3654/89/2093-0350$01.50/0
sample no. 2 3 4 5 6
I 8 9 10 11
prepared from treatment" sample no. 1 (parent) ( S ) 550 OC, 18 h 2 (W) 0.25 M KOH, 80 O C , 24 h 1 (S) 725 OC,2 h 4 (W) 0.25 M KOH, 80 OC,24 h 1 ( S ) 550 OC,3 h; 3-fold exchange with ",NO,; (S) 800 OC, 3 h 6 (W) 0.25 M KOH, 80 OC,24 h 1 (S) 550 "C, 18 h 8 (W) 0.25 M KOH, 80 "C, 25 h 9 (S) 350 'C, 14.5 h 10 (W) 0.10 M KOH, 80 OC, 16.5 h
" (S) denotes hydrothermal treatment (steaming), and (W) denotes washing with KOH solution. so that the partial pressure of H 2 0 above the sample was 1 atm. Times and temperatures of treatment are given in Table I. (ii) Realumination. Portions of ultrastabilized samples were stirred with aqueous solution of KOH for 24 h. We normally used 50 mL of 0.25 M solution per gram of zeolite at 80 OC, but other reaction conditions were also considered. Potassium (rather than sodium) hydroxide was used because faujasites cannot recrystallize from potassium-bearing solutions,' but we have also carried some exploratory experiments using NaOH and other bases. The products were washed with water and dried in air at 80 OC. Magic-Angle-Spinning NMR (MAS NMR). 29SiMAS NMR spectra were measured at 79.5 MHz with a Bruker MSL-400 multinuclear spectrometer. An Andrew-Beams and a doublebearing MAS probehead were used with rotors spinning in air at 3-4 kHz. Radio-frequency pulses (45O, of 2.25-fis duration) were applied with a 20-s recycle delay. 29Sichemical shifts are (1) Lago, R. M.; Haag, W. 0.; Mikovsky, R. J.; Olson, D. H.; Hellring, S. D.; Schmitt, K. D.; Kcrr, G . T. In New Developments in Zeolite Science and Technology,Proceeding 7th International Zeolite Conference;Murakami, Y . , Iijima, A., Ward, J. W., Eds.; Elsevier: Amsterdam, 1986; p 677. (2) Schemer, J. In Catalytic Materials: Relationship between Structure and Reactiuity; Whyte, Jr., T. E., Dalla Betta, R A., Derouane, E. G., Baker, R. T. K., Eds.; American Chemical Society: Washington, DC, 1984; ACS Symp. Ser. No. 284, p 157. (3) Anderson, M. W.; Klinowski, J.; Liu, X. J . Chem. SOC.,Chem. Commun. 1984, 1596. (4) Chang, C. D.; Hellring, S. D.; Miale, J. N.; Schmitt, K.D.; Brigandi, P. W.; Wu, E. L. J . Chem. Soc., Faraday Trans. 2 1985, 81, 2215. (5) Liu, X.; Klinowski, J.; Thomas, J. M. J. Chem. Soc., Chem. Commun.
1986, 582. (6) Sulikowski, B.; Rakoczy, J.; Hamdan, H.; Klinowski, J. J. Chem. Soc., Chem. Commun. 1987, 1542. (7) Barrer, R. M. In Hydrothermal Chemistry of Zeolites; Academic: London, 1982.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 1 , 1989 351
Second-Generation Zeolite Catalysts peristaltic pump
TABLE II: Cubic Unit Cell Parameters, ah and Sorption Capacities for Nitrogen at 77 K and p / p a = 0.5 before and after Hydrothermal Treatment
,
tube f u r n a c e
-
sample no. 1 2 3 4 5 6 I 8 9 10 11
.......
prepared from sample no. I
2 1 4 1 6 1 8 9 10
7% ao.
A
24.66 24.51 24.67 24.58 24.68 24.39 24.69 24.51 24.62 24.51 24.58
change
0.65 0.41 1.23 0.45 0.29
uptake of N21 Big 0.281 0.175 0.219 0.180 0.206 0.137 0.137 0.175 0.131 0.144 0.119
beam. The samples were hydrated over saturated aqueous N H 4 N 0 3 before measurement. Infrared Spectra. IR absorption spectra in the zeolitic framework vibration region (1400-400 cm-’) were recorded by using a Nicolet MX-1 Fourier transform spectrometer and the conventional KBr disk technique. Nitrogen Adsorption. In order to obtain information about pore size and void volume, sorption measurements of N 2 at 77 K were carried out. The volumetric method was used as follows. About 0.2 g of zeolite powder was weighed into a sample tube attached to a gas-handling line and, prior to measurement, dehydrated overnight at 450 OC under vacuum. Dehydrated sample weights were calculated by subtracting the water content known from thermogravimetric analysis; sorption uptakes were computed from the measured gas pressure drop in the sample tube of known volume. Sorption capacities for Nz are given in Table I1 for p/po = 0.5.
zocm
v
-A4 Figure 1. Apparatus for the hydrothermal dealumination of zeolites.
quoted in ppm from external tetramethylsilane (TMS). 27Al MAS N M R spectra were measured a t 104.2 MHz by using very short (0.6-ps duration) pulses with a 0.2-s recycle delay. Chemical shifts are quoted in ppm from external A1(Hzo),3+. X-ray Diffruction. Powder X-ray diffraction (XRD) patterns were acquired on a Philips vertical goniometer using Cu Ka radiation selected by a graphite monochromator in the diffracted
Results and Discussion 29Si MAS N M R spectra of dealuminated samples before treatment with KOH (see Figure 2) are in complete agreement with earlier work&” and consist of up to five signals corresponding
2 1
-80
-90
