Zeolite crystallization on mullite fibers

Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.

ZEOLITE CRYSTALLIZATION

ON MULLITE

527

FIBERS

V. Valtchev ~, S. Mintova a, B. Schoeman b, L. Spasov~ and L. Konstantinov ~ a Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, 92 Rakovski St., 1000 Sofia, Bulgaria b Chemical Technology, Lulea University of Technology, 971 87, Lulea, Sweden Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences, G. Bonchev St., 1113 Sofia, Bulgaria

In situ crystallization of zeolite A, Y and silicalite-1 on crystalline and amorphous mullite fibers is studied. It was found that the fiber degree of coverage is determined by the chemical composition of the initial gel and by the physicochemical features of fibers. The role of the fiber pretreatment is also discussed.

INTRODUCTION Microporous materials are of interest as sorbents and gas separators due to their large internal surface and well-defined pore system [1]. In industry zeolites have been usually used in the form either of granules with adhesive additives or as pallets and blocks of bulk particles. Neither the use of zeolites as bulk material nor as a part of a composite has been fully utilized so far. This is due to the obvious disadvantage that in the bulk usage a large part of the zeolite consists of material which is not available in the interior, but in other instances, when the zeolite is embedded within the matrix material, the effective surface~substantially decreases. A combined usage of the whole zeolite surface can be achieved through deposition of the zeolite on a supporting substrate. During the past few years a large number of articles and patents have been published on the preparation of composite materials containing continuous zeolite films [2-7]. Suitable substrates for this purpose are fibrous materials due to their flexibility and the ability to use them for forming matrices of various shape and size [8,9]. Therefore, the synthesis of zeolite-containing fibers of high thermal and acid stability is of a considerable technological importance.

528 In the present communication we discuss the in-situ synthesis of zeolites on mullite fibers. In addition, the effect of the chemical pre-treatment of the fibers on their reaction ability is studied.

EXPERIMENTAL Two types of muUite fibers were used by us with A1203 to SiO2ratios equal to 80/20 and 72/28. The mullite fibers were prepared by the sol-gel method with a centrifuge equipped with ejection nozzles.The spinning composition was prepared from basic aluminium chloride (A1/C1 atomic ratio of 1.85), silicasol, and a rheology modifier (polyvinyl alcohol of molecular weight 70 000). Further, the amorphous precursor obtained (the so called Green fibre) was calcined at a given temperature to form an oxide fibre. The 80/20 muUite was amorphous, while the 72/28 muUite crystalline. These two types of fiber were chemically treated before synthesis in 10% H3PO4for 30 mill under ultrasound action. The hydrothermal synthesis of zeolite A, Y and silicalite-1 was performed on fibers either treated or treated. The chemical composition of the gels used for the synthesis were: zeolite A: 4,1 Na20: A1203: 2,4SIO2:155H20 zeolite Y: 8Na20: A1203: 20SIO2:400H20 silicalite-l: 25Na20: 10TPA: 26SIO2:1528H20 After preparing the initial gels the muUite fibers were directly immersed into the autoclaves. After the hydrothermal synthesis the fibers were filtered, washed and dried at 110~ The zeolite coated muUite fibers were investigated by thermogravimetric analysis (Stanton Redcroft 781). The type of synthesized zeolites was specified by X-ray powder diffraction using a DRON-3M diffractometer with Cu K~ radiation, while the distribution of the zeolite particles by scanning electron microscopy (Philips 515).

RESULTS and DISCUSSION

The main results on the synthesis of zeolite A, Y and silicalite-1 on muUite fibers are systemized in Table 1. SEM measurements revealed that the crystallization of the three types of zeolite on untreated fibers had proceeded in different way. The most effective zeolite formation was observed for zeolite A on both type of fibers. Although the fiber coverage in both cases was not dense,

529 Table 1. Zeolites A, Y and silicalite-1 grown on different mullite fibers MuUite fibers Degree of coating "A12(~3/SiO2 80/20* 80/20** 72/28* 72/28** * untreated fibers

zeolite A medium hi$1a medium medium ** treated fibers

zeolite Y low medium none low

silicalite-1 none low none low

one can see that the content of zeolite A is higher on amorphous than on crystalline fibers (see Figure la). Zeolites Y and silicalite-1 crystallize much worse and only isolated crystals were observed on the fibers. It is worth noting that among all zeolites stidied the amount of siliealite-1 grown on the fibers is the smallest. The results obtained point that the composition of the initial gel plays an important role for the zeolite growth on mullite fibers. The highest is the reaction capacity of the gel yielding zeolite A, which is of the highest content of hydroxyl groups. The zeolite crystallization on the mullite fibre surface was confirmed by X-ray phase analysis and thermogravimetric analysis. X-ray diffraction method indicated the presence of crystalline 72/28 mullite and zeolite A. In the case of zeolite Y and siliealite-l,the X-ray phase analysis revealed a very low content of zeolite phase as only the most intense 1• were detected. The thermal analysis indicated the formation of a composite of zeolite A and mullite. Figure 2 shows DTA curves of 72/28 mullite fibers (curve a), zeolite A (curve b) and the composite (curve c). The DTA curve of mullite fibers shows a low-temperature endothermal effect caused by the surface desorption of water at 77~ The DTA curve of zeolite A is typical of zeolites of stable three-dimensional structures, which indicates a continuous water desorption in the temperature range 100-200~ The DTA curve of muUite fibers covered by zeolite A indicates relatively high-temperature exothermal effects at 265,350 and 420~ which are not characteristic both of zeolite A and mullite. The presence of these effects points to the formation of a combined zeolite-mullite structure which decomposes after heating and speaks in favour of a strong bond between the zeolite and mullite. On the other hand, we performed a series of experiments with samples deposited on pre-treated fibers. It was found that after the pre-treatment the fiber coverage has generally improved, the effect being more pronounced on amorphous fibers. For example, as Figure l b shows, zeolite A forms a homogeneous dense coating on treated amorphous fibers, while does not on untreated ones. In addition, one can see that the size of crystals decreases from

530

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Figure 1. SEM photographs of coatings of zeolite A on" untreated (a) and treated (b) 72/28 mullite fibers. M=I 0 ~tm

a

b

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I

I

100 300

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500~

Figure 2. DTA curves of: 72/28 mullite fibers (a), zeolite A (b) and zeolite A crystallized on mullite fibers (c).

531

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0

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~ 0

~o

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0

,~0 o

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2 Thefo Figure 3. X-ray diffraction pattern of zeolite A(o) on 72/28 treated mullite fibers (v)

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Figure 4. SEM photographs of zeolite Y (a) and silicalite-1 mullite fibers. M=100 ~tm

(b) on treated 72/28

3 ~tm for untreated to 1 ~tm for treated fibers. The high-quality powder diffraction pattern of zeolite A on 72/28 mullite indicates the high content of the zeolite on treated fibers (Figure 3). Figure 4 shows that the content of zeolite Y and silicalite-1 also increases after treatment of the mullite fiber. These two

532 zeolites do not cover densely the fibers, but the crystals are homogeneously distributed on the surface of pre-treated fibers. In contrast, on untreated fibers the zeolite crystals are distributed inhomogeneously. The preliminary treatment of fibers leads to an increase of their surface and to intensification of their reaction ability. These effects are more pronounced in the case of amorphous fibers, on which the combined action of ultrasound and acid treatment leads to a disorientation of the structural dements and to the formation of active centres serving as sites for zeolite formation. The great number of seeds in the latter case leads to a reduction in the size of the zeolite crystals. The effect of the pre-treatment is weaker for crystalline mullite fibers, where the reaction ability depends on the crystalline-to-amorphous ratio. In this case the treatment by acid influences only the amorphous areas of the fibers which results in the observed weaker effect of the fiber chemical treatment on the zeolite deposition.

CONCLUSION The crystallization of zeolite A, Y and silicalite-1 on amorphous and crystalline muUite fibers is studied. The composition of the initial gel is the main parameter controlling the zeolite crystallization on the surface. Another important parameter is the fiber physicochemical state. The reaction ability of amorphous muUite fibers has been found higher than that of crystalline ones. The fiber ability capacity can be improved by chemical treatment which increases the amount of seeds for zeolite formation on the surface. REFERENCES

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