BIPHASIC SILICATE MATERIALS BASED ON POROUS GLASSES PREPARATION AND PROPERTIES

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1 BIPHASIC SILICATE MATERIALS BASED ON POROUS GLASSES PREPARATION AND PROPERTIES W. SCHWIEGER*, M. RAUSCHER*, F. SCHEFFLER*, D. FREUDE, U. PINGEt, F. JANOWSKI* >institut für Technische und Makromolekulare Chemie, Martin-Luther-Universität Halle Wittenberg, Halle /Saale, Germany >Fakultät für Chemie und Geographie, Universität Leipzig, Linnestraße 3, Leipzig ABSTRACT Biphasic silicates with a bimodal pore system were prepared by hydrothermal treatment of porous glass in a reaction mixture. Tue products were characterized by chemical analysis, XRD, SEM and MAS-NMR-spectroscopy. INTRODUCTION Porous silicates such as porous glasses. silica gels, and zeolites play an important role as adsorbents. catalysts and catalyst supports. New applications of porous silicates currently under development include hydrogen storage, optical information storage. media for chemical reactions. and formation ofthin crystallized membranes Tue development is accelerated by a more detailed knowledge of the silicate formation conditions, and by the combination of different preparation techniques /3,4/. Tue knowledge gathered here allows to "tailor" a porous material for specific purposes. Tue information from these investigations, including the preparation and characterization of porous bisilicate materials, broadens the potential application fields of porous glasses as catalysts and adsorbents. Tue porous silicates prepared in this work cxhibit a combination of the properties of the amorphous Si0 2 of porous glasses and those of high silica zcolites. Furthermore, the proposed routc of preparation of the bisilicatic materials could be looked at as a supported crystallization process. Such materials could be useful as catalysts, adsorbents and membranes th International Zeollte Conference Materlllls Research Soc1ety

2 EXPERIMENTAL Biphasic silicates with a bimodal pore system can be obtained by using granular Si0 2 - materials like porous glasses or xerogel pellets as the silica source and matrix in which the crystallization takes place. The synthesis procedures are based on those given in ref. /SI for a template and a template-free synthesis. The batch composition expressed in mole ratios of the oxides was as follows: 8 Na 2 0 / R / 1Al / x Si0 2 / 3877 H 2 0 (x: 30 up to 200) with propylamine (PA) or tetrapropylammonium iodide (TP A) as the template (R) and 3-5 Nap I Si0 2 I 1Al I H 2 0 in the template-free case. Granular porous glasses (based on glass melts of the Schuler AG) and pelletized xerogels (Chemiewerk Köstritz) of different particle size were used as the silica source, sodium aluminate or aluminium sulphate as Al-source. NaOH as alkali source and propylamine or tetraalkylammonium salts as template. The synthesis was carried out in stainless steel autoclaves (50 ml) at 448 K. The autoclaves were removed from the crystallization oven after present times. After cooling down the mixture the grained fraction of the crystallization products were separated from a powdered fraction through a sieving process. Both fractions were dried at slightly elevated temperature (about 50 C), rehydrated in air and then characterized by chemical analysis (ICP: Plasma 4000, Perkin Eimer) and powder-xrd (URD 63, Seifert) in a range between 4 and 40 (20). Selected samples were investigated through thermal methods (DTNDTG - SDT 2960, TA instruments), electronmicroscopy (SEM ) and 27 Al- and 11 8-MAS-NMR-spectroscopy (MSL 400, Bruker). RESULTS The preparation route of the biphasic porous materials based on the raw material sodium borate glass PG 960. Zeolite containing pellets can be prepared by a two-step process including (i) the formation of the pores in the glass melt by a thermal treatment followed by a leaching process and (ii) the transformation of the amorphous glass phase in a crystalline one by a hydrothermal crystallization. Synthesis rcsults are summerized in Table I to 3. Table 1 shows the effect of different Si0 2 -sources on the direction of the

3 crystallization process, in comparison to the raw materials Waterglass (WG 30) and the colloidal silica (K.S 30), usually used as a silica source for such crystallizations. Whereas the reaction with Waterglass or colloidal silica leads always to well-crystalline ZSM-5 material, the pelletized silica gels (B, S and E 36) and the porous glass (PG 960; AJOO, Al ) act differently under otherwise identical crystallization conditions. After a crystallization time of24 hat a crystallization temperature of 448K ZSM-35 or ZSM-5 (or both) has/have been obtained as the crystalline phase in the pelletized biphasic products. Raw material Phase composition Description of the raw material tk=24 h silica gel B ( 1 ) ZSM35 macroporous silica gel silica gel S (2) ZSM5 silica gel, macro- and mesoporous E36 ZSM5 silica gel, acidic post treatment of sample ( 1) PG960 ZSM 35/5 boron silicate glass. thennal treated. Ieached A 100 ZSM 35/5 boron silicate glass. only quenched A 100 (200) ZSM5 A 100, thennal treated KS30 ZSM5 silica sol (-30 wt% Si0 2 ): Chemiewerk Köstritz WG (30) ZSM5 sodium silicate solution (-30 wt% Si0 2 ) Table 1: The influence of the Si0 2 -raw material on the crystallization. Composition ofthe reaction mixture (RM): 8 Na 2 0 I 36.7 PA! 93.5 Si0 2 I 3877 H 2 0! 1 Al ; Crystallization temperature: T.-= 448 K. (PA: propylamine) Table 2: Dependence of the product composition on the initial SiO/ Al 2 0~ molar ratio of the reaction mixture (conditions see Table 1, a: amorphous fraction) Si0 1 source Si0 1 / Al ratio Phase composition of thc pellet porous glass (PG 960) silica gel S a>>zsm-5 a>zsm-35. ZSM-5 a>zsm-35 a a>zsm-5 a>zsm-5 a>>zsm-5 From Table 2 it can be concluded that the crystallization process can be optimized in respect to a desired crystalline phase by varying the crystallization conditions. The different Si0/Al ratios (modules) in the reaction mixture affect the direction of the crystaiiization as weil. If a common Si0 2 -source and as template propylamine or TP A - ~. cations have been used only ZSM-5 is formed independent fonn starting Si0 2 /Al ratios. up to ratios of about 250. Such common crystallization process result always in fine ' 1851

4 powdered products [5]. Different from the crystalfü.ation of the pelletized silica gel of the type S leading also only to ZSM-5 products, the crystallization direction changes in the case of porous glasses with an increasing starting SiO/ Al 2 0rratio, form ZSM-5 to ZSM- 35 if propylamine is used as the template. However, at this stage such an unexpected crystallization behaviour of the pelletized porous glasses can not be explained satisfactoril y. In general, depending on the crystallization time, the composition of the reaction mixture and the crystallization conditions, the crystallization can be directed in such a way so as to only synthesize zeolite-containing pellets without an additional powdered phase. The crystallization can be carried out as weil in a template containing reaction mixture. The pellets consist of the typical matrix of the starting silica source. e.g. the porous glasses, and the zeolitic crystallites with porosity and acidity typical for high silica zeolites. such as ZSM-5. Figure 1 shows the XRD pattems of a series of template-free crystallized products. As seen in this figure all pattems contain the typical lines of ZSM-5-material with increasing intensities for larger crystallization times. The relative crystallinity 161 increased from 0.13 to 0.85 when the crystallization time was incresed from 24 to 96 h. One can conclude from these results that thc zeolite content in thc ZSM-5-containing pellets, prepared from the porous glasses. can be varied in a!arge range by optimizing the crystallization conditions. Figure 2 shows 11 8-MAS-NMR spectra of samples obtained in different preparation stages during the preparation of the bisilicatic products. These measurements show that boron from the glass phase is introduced into the zeolitic framework. The boron coordination changes from a threefold to a tetrahedral zeolite-like coordination indicating that boron plays an active rote in the crystallization process ofthe zeolite. Table 3: Product comparison Characterization of a crystallized porous glass and a porous glass used as starting material (PG 960) Pore volume (cmj/g) Pore size, maxima of the destribution (nm) Porous glass (PG 960) Crystallized porous glass (98 %) 61.0 ( 11.5) 14.0 \

5 Figure 1: XRD-Pattems of partially crystallized porous gjasses. The samples were taken after crystallization times: 24 h (QAI = 0,1) 48 h (QAI = 0,53) 72 h (QAl = 0,61) 96 h (QAl = 0,95) : ~ i o..l---~ ~ /gn:I -2.6 tctrahcdral (zcolitic) llm -cfold -_. cnurd inatcd lhrccfold.. coorditfatc tlnwfold counlin11tcd tctrnhcdral ( amorrhous) Figure 2: 11 B-MAS-NMR spectra of samples obtained of different preparation stages during the preparation of the bi-silicatic products ppm 1853

6 Tue adsorption data (Table 3) and the SEM-images (Figure 3) of the PG 960-sample show that the crystallization can take place inside of the porous system as weil as on the. outer shell of the porous glass pellets. Table 3 shows that the total pore volume of the porous glass increases drastically after the hydrothermal crystallization. lt also indicates that the pores already present in the porous glasses dilate remarkably caused by the consumption of the glass phase during the crystallization process. In addition to these meso- and macropores, micropores have been also detected by the nitrogen adsorption isotherms if a zeolitic fraction has been determined in the granules. SEM images (Figure 3) indicate clearly that the crystallization takes place into the inner pore system and on the outer surface of the particle. A large amount of ZSM-5 crystals has been obtained in the so-called leaching cavities. Figures 3a and 3c show ZSM-5 (and mordenite) crystals at the entree of a pore and in a ground plane of the pellet. Beside this, the whole outer surfäce is covered with very small ZSM-5 crystals as weil. This crystallization behaviour may explain the very complex changes in the pore size distribution at the different pore size ranges._considering the relation between the crystalline fraction in the composite and the increasing surface area and micropore volume one has to conclude that the forrned micropores of the zeolitic fraction in the composite are accessible to the test molecules at every crystallization stage. This proves that the crystallization process inside of the porous glass matrix starts from the inner surface area into the matrix volume of the porous glasses. Catalyst QAi'l Micropore volume S, m 2 /g (name) cm 3 /g (BET)b1 H-PG/175/ , H-PG/175/156 0,60 0, H-PG/ 175/160 0,95 0, H-ZSM-5 0,98 0, Table 4: Crystallinity QAI textural properties and the n-hexan cracking rate k,,, for different template free crystallized ZSM-5 containing catalysts (H-PG/175/XX: on base of porous glasses PG 960; pelletized, l mm H-ZSM5: pure template-free ZSM-5, Si0;/Al :30 powdered product, about 5 µm) a) QA = l % ZSM-5 content/6/, b) calculated from nitrogen adsorption isotherms, c) rate constant for the n-hexan cracking at 400 C 1854

7 c) 1---i 5 µm 0.1 µm 1---i 1---i 0.1 µm 0.1 µm Figure 3: SEM images ofbisilicatic samples prepared ofporous glass PG 960. a) Leaching cavity with MFI crystallites (1 :3 000) b) MFI crystallites on the edge of pore ( 1: ) c) MFI crystallites in the glass material (ground section, 1 : ) d) Glass surface with MFI crystallites (1: ) 1855