Ind. Eng. Chem. Res. 1993,32, 751-752
75 1
Synthesis of NaY Zeolite on Preformed Kaolinite Spheres. Evolution of Zeolite Content and Textural Properties with the Reaction Time Elena I. Basaldella,'*+Rita Bonetto, and Juan C. Tara Centro de Inuestigacibn y Desarrollo en Procesos Catahticos (CINDECA),calle 47 No. 257, 1900 La Plata, Argentina
The synthesis of NaY zeolite was carried out on fired kaolinite microspheres. Changes in porosity, chemical composition, and crystallinity of the solid show zeolite growth on both internal and external microsphere surfaces. It was also observed that, as a consequence of the alkaline treatment, the sioz/&o3 ratio in the solid diminishes prior to the appearance of the zeolite, but increases when the zeolite begins to crystallize.
Introduction One of the main routes for the cracking catalysts preparation is the so-called "in situ" process (see, for example, Haden et al. (19721,Breck (19741,and Brown et al. (1985)). In this approach, the starting solid phase in the hydrothermal synthesis of NaY zeolite is constituted by fired kaolinite microspheres with adequate size and shape for subsequent use as cracking catalyst in a fluidized bed. This paper deals with a technique specificallydeveloped to produce microspheres containing a high percent of Y zeolite (Brown, 1985;Altomare, 1986). At present, this process has only briefly been described in the patent literature. Howden (1978,1983),has studied the in situ synthesis proposed by Haden et al. (1972, 1976) for obtaining catalysts with low zeolite content. Therefore, on the basis of the work of Howden, we investigate the way in which high zeolite content could be obtained. This was done by studying the evolution of the porosity, chemical composition, and crystallinity of the solid phase over the course of the reaction.
Experimental Section Materials. Kaolinite-type clay from deposits in Argentina was used. NaOH (CarloErba analytical reagent), commercialsolublesilicate(SiOdNazO= 3.18w/w, density 1.36 g/mL), and distilled water were used for the alkaline treatment. For the preparation of seed solution, commercial gibbsite was used as an alumina source. Raw Material Characterization. The starting clay was characterized by chemical analyses,X-ray diffraction (XRD), and differential thermal analysis (DTA). X-ray powder diffraction patterns were taken using a Philips PW 1732/10diffractometer with Cu K a radiation. In the DTA study, calcination was from 20 to 1100 "C, in air, at a heating rate of 10 OC/min. Hydrothermal Synthesis and Product Characterization. The in situ synthesis of NaY zeolite was performed according to variations of the procedure proposed by Brown (1985). The solid consistsof microspheres composed of metakaolinite and kaolin clay that has been calcined through its characteristic exotherm. A solution of sodium hydroxide, commercial solublesilicate solution, and distilled water were added to this solid. This mixture was hydrothermally treated at 98 "C in a crystallization reactor provided with reflux and a stirring device. A portion, 4.8%, of the total alumina present in the reactor was incorporated by a zeolite seed solution which + Profesional CIC-CINDECA-UNLP.
had the following molar ratio: Na,O/SiO, = 1.09; SiO,/Al,O, = 16.45; H,O/Na,O = 20.30 The global molar ratios at the beginning of the reaction were Na,O/SiO, = 0.38; SiO,/Al,O, = 10.64; H,O/Na,O = 32.3 During the development of the reaction, samples of the product were taken, separating the liquid from the solid. The solids so obtained were washed with water, dried in an oven at 110 'C, and characterized by XRD, mercury intrusion, nitrogen adsorption, scanning electron microscopy (SEMI,and energy dispersiveX-ray analysis (EDX). Qualitative diffraction analyses of the synthesized zeolites were carried out by comparingthe diffractograms with the characteristic peaks mentioned in the bibliography (Breck, 1974). The zeolite percentage and its unit-cell size were determined according to the ASTM procedures D-3906/ 80 and D-3942/80, using a standard Nay-550 C (Davison) as reference. Pore size distributions were measured by mercury intrusion using an Aminco penetrometer. The specific surface area was measured by nitrogen adsorption using a Micromeritics Accusorb 2100 E. The obtained microspheres with different zeolite contents were broken for analysis of its exterior and interior surfaces. For this purpose, a Philips SEM 505 microscope with a magnification of lOOOOX was used. The zeolite crystals formed in the interior surface of the microspheres cannot be differentiated by the zeolite's morphology. Therefore a scanning of the broken microspheres for the distribution of the exchangeablecationswould showwhere the zeolite is situated (Howden, 1978). As the detection of Na+ with SEM is very poor, some Na+ cations present in the zeolite were exchanged by Mn2+cations, to obtain Mn/NaY zeolite. This treatment was also performed on originalinitial microspheres. Mn2+cations were detected by EDX. The chemical analyses to estimate the Si02/Al203ratio were carriedout with the same equipment. An accelerating potential of 25 kW was used with an incident angle of 90° with respect to the sample, and a 24" take-off angle. The counting time for each analysis was 150 s (live time). The oxide concentrations for the various areas scanned were determined by a conventionalmatrix correctiontechnique with the SW 9100 program.
Results and Discussion Study of Raw Materials. The chemical composition of the starting clay is given in Table I. The XRD analysis
0888-5885/93/2632-0751$04.00/00 1993 American Chemical Society
752 Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 Table 1. Chemical Composition of the Clay (in wt W ) Si02 49.7
AkOa 34.9
Fez03 1.2
TiOa 0.5
lostwtat950°C 13.3
Table 11. Crystallinity. Porosity, Chemical Composition, and BET A- of the Solid Product as a Function of Reaction Time reaction time (h)
9% zeolite (DRX)
0
0 0
2 4 6 8
10 15 21 24
0 0 0 6
25 42 55
SiOdAhOs (moVmol) 2.41 2.19 2.18 2.19 2.19 2.43 2.48 3.61 4.10
pore vola
BET area
(cm3/g)
0.369 0.360 0.340 0.322 0.320 0.313 0.290 0.113
(mz/g) 15 15 11 12 30 40 150 371
0.098
360
‘Memured for porous radius in the range 0.01-1 rrm.
determines that kaolinite is the main component (quartz 13%; kaolinite 87%). The DTA study shows an endothermal peak at 550 OC, charaderistic of the transformation into metakaolinite, and an exothermalpeak at 970 O C corresponding to spinel transformation. Hydrothermal Synthesis. The reaction medium consists of a solid phase (clay calcined microspheres) in contact with an alkaline mother liquor. The analysis methods employed show the evolution of the porosity, chemical composition, and BET area of the solid over the course of the hydrothermal synthesis. The results are summarized in Table 11. It can be seen that at the beginning of the synthesis, for a reaction time t , = 0, the solid phaseconsistsofan macroporous amorphousmaterial with a SiOdA1203 = 2.14. For reaction times previous to zeolite crystallization, an expecteddissolutionprocessoccurs. The alkaline solution leaches silica and alumina from the solid (Howden, 1978). More silica than alumina is extracted at this stage,causing a decrease of the mentioned SiOz/A1203 ratio. The leaching does not generate additional porosity in the porous range 0.01-1 pm, since the pore volume decreases slightly with the reaction time. This fact can beexplainedassumingthat themesoporesmay he partially filled by amorphous material and seeds which will grow, forming zeolite crystals. A t longer reaction times, when zeolite crystals are detected by X-ray diffraction (XRD),the SiOdAlzO3ratio of the solid increases. Transport of silica and alumina from the mother liquor to the solid produces this increase by formation of zeolite NaY with a SiOz/Al203molar ratio of 5.2 on an amorphous matrix with a SiOdA1203 ratio of 2.19. A t the same time, an appreciable decrease of mesopore volume is observed, giving evidence that the zeolite is growing within the microspheres pore channels. Furthermore, zeolite crystals in the range 0.2-0.7 pm can be clearly seen over the external surface of the microspheres (Figure 1). The EDX analysis shows an uniform distri-
I
I
W0f-l Eyre 1. Microsphere external surface at the end of the synthesis.
bution of Mn2+ in the interior and exterior surfaces, confirmingthepresence of zeolite not only on the external surface but also in all the complete volume. On the other hand, the values of BET areas ratify the formation of zeolite for reaction times longer than 10 h. Conclusions
A t the beginning of the synthesis, a dissolution process takes place and a decrease of the si02/&03 ratio of the
solid iscaused hytransferenceof silicaand aluminatoward the liquid phase. After a few hours treatment, zeolite crystallization can be detected within the solid phase. Changes in the SiOdAl~03ratio,BETarea, and mesopores volume of the solid indicate Nay formation not only on the external surface but also on the mesopores, with uniform distribution of the active material inside the microspheres. Acknowledgment The authors gratefully thank S. Garcia for chemical analysis of the samples. Literature Cited Altomare, C. A. Eur. Patent APPL 86/0 204 455 A2,1986. Breck, D. W. Zeolite Molecular Sieues;Wiley: New York, 1971; pp 315.614. Brown, S. M.; Durante, V. A,; Reagan,W. J.; Speronello,B. K. US. Patent 4,493,902, 1985. Haden, W. L.; Dzienanowski, F. J. U.S.Patent 3,667,154, 1972. Haden, W. L.; Dzienanowski, F. J. U.S. Patent 3,932,268,1976. Howden, M. G. “Control of the concentration of Nay zeolite 8yntheskdinfmed kaolinitemicrospheres”;CSIFtReport:CENG 262; CSIR Pretoria, South Africa, 1978. Howden, M. G. The in situ synthesis of zeolite Y in fired kaolinite matrices. Spec. Publ. Geol. Sac. S. Afr. 1983, 7, 369-373. Received for review March 24, 1992 Revised mnnuscript receiued September 1.1992 Accepted December 18,1992
