Crystallization of MFI-type structure zeotype materials with a trivalent metal present in tetrahedral (T) positions has had a tremendous impact in synthesis of new shape selective industrial catalysts having tunable acidic strength.1 Thus, the isomorphous substitution of Si by other tetrahedrally coordinated heteroatoms such as B(III) provides new materials showing structural modifications and specific catalytic properties, namely cracking of olefins with 95% of propylene as product and conversion of heavy hydrocarbons into gasoline, BTX and other important aromatic products.2-5 However, crystallization of a borosilicate zeotype with MFI/MEL structure, having a three dimensional medium pore system of a 10-membered ring, confers its unique characteristics.
The most common method for preparing zeolites is the conventional hydrothermal synthesis (HTS). The new synthetic method for zeolites and zeotypes, in which an aluminosilicate gel dried in advance is crystallized into a zeolitic phase in a gas environment, has been developed and named dry gel conversion (DGC).6-9
In 1990, Xu et al.17 introduced a new technique, in which they converted an amorphous aluminosilicate in contact with steam and vapors of volatile amines into ZSM-5 zeolite. This technique has been referred to as the vapor-phase transport method (VPT). If non-volatile quaternary ammonium ions are used as templates, then only water vapor is supplied via the gas phase; therefore the method is rather named steam-assisted conversion (SAC).10,18 The SAC method enables us to achieve rapid and full crystallization of an amorphous dry gel, which contains tetraalkylammonium cations, into a crystalline zeotype phase.15
Synthesis of high silica borosilicate zeotype by steam assisted conversion method was first reported by Banyopadhyay et al.6 In his study, synthesis and characterization of [B]-MFI,-BEA and -MTW phases were investigated using tetraethylammonium cations. Clearly, due to the different conformations of TEA+ entrapped in divergent host lattices, the three phases have been obtained as it was shown by Curtis et al.12 It is obvious that organic templates are a powerful tool in synthesis of high silica zeolites having high stability, however, their role has not been completely understood yet since we are still not able to predict what structures will produce a particular organo-cation template in different synthesis conditions, and this is due mainly to the very complex guest/host relationships.17
Moreover, Song-Jong Hang13 has successfully synthesized new high silica zeolite structure SSZ-47 using a mixture of quaternary ammonium cations, so it has been showed that they still have high potential and possibility to produce novel structures. Even though they were used by Barrer et al.14 in the early 1960s, they result in the discovery of numerous new zeolitic structures yet.
In the present study, we have explored, via the Xray investigation based on powder diffraction data and other techniques, the phase nature of the materials obtained when using the TBA+ cations as structure directing agent under the new synthesis conditions of the SAC method.
Synthesis of materials
The following reagents have been used for the synthesis of zeoborosiles (borosilicate zeolites): sodium hydroxide NaOH (Aragonesas 99.99%), boric acid H3BO3 (Labosi 99.5%), triethylborate (C2H5O)3B (Aldrich 99%), borate sodium Na2B4O7.10H2O (PANREAC 99.5%), colloidal silica SiO2 (Ludox-HS-40 Dupont, 40 wt.%), and tetra-n-butylammonium hydroxide C16H37NO (MERCK 20%).
The synthesis of borosilicate zeotypes was performed following the general dry gel conversion technique described elsewhere.6,8,16 For this method amorphous solid oxide powders were prepared as follows, an appropriate amount of tetralkylammonium hydroxide was mixed with a silica source, and the mixture has been stirred for 10 min. Boron source was dissolved in deionised water and added to the above mixture and the final mixture was further stirred for 2 hours. Then, the gel was dried at 80-90 ℃ over an oil bath with continuous stirring allowing evaporation of water. When the gel became thick and viscous, it was homogenized by hand using a teflon rod until it dried, so a white solid was formed. It was ground into a fine powder and was poured into a small teflon cup. This cup was placed in a teflon-lined autoclave (65 ml) with water which was the source of steam. The dry gel never came into the direct contact with water. The autoclaves were heated in forced convection ovens for prescribed times at 175 ℃ in autogeneous pressure. After the crystallization period was over, the autoclaves were quenched with cold water and the zeoborosile samples were taken out from the cups, filtered and washed thoroughly with distilled water, and dried overnight at 100 ℃.
The removal of organic template occluded inside the borosilicate zeolite pores was carried out by a heat treatment. The as-synthesized samples were placed in a muffle furnace and heated stepwise in a flow of nitrogen. The temperature was raised to 550 ℃ over a period of 4 hours and kept at the same temperature for 4 hours. The temperature was then raised again to 600 ℃ over a period of 4 hours and kept again at this temperature for another 4 hours and finally the sample was cooled to room temperature.
X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1830 diffractometer (CuKα: λ=1.5406 Å, 40 kV, 20 mA). KBr pellet technique was used to perform FT-IR spectroscopy of the samples using a Nicolet 460 FT-IR spectrometer; the samples were ground with KBr and pressed into thin wafers. Nitrogen adsorption isotherms were collected at 77 K on Micrometrics Gemini II 2370 area analyser. The degassing and activation pre-treatment was carried out at 673 K for 4 h prior to the analysis.
RESULTS AND DISCUSSION
Optimal chemical composition of the reaction gel
Crystallization of borosilicate zeotypes from borosilicate gels containing sodium and TBA+ cations by steam-assisted crystallization method has not been reported yet, however, there are only some recipes for preparation of zeoborosiles using other quaternary cations as templates in the literature. Comparative data on these recipes in terms of molar ratios of components in gel are listed below (see Table 1). In this study, the optimal molar composition of gel is as follows:
SiO2 : 0.042 Na2O : 0.0125 B2O3 : 0.098 TBA2O
Table 1.Molar compositions of some zeoborosiles in the literature
X-ray powder diffraction
The X-ray diffraction patterns of the borosilicate powder (dry gel) and as- synthesized borosilicate materials are shown in Fig. 1. The XRD pattern of the calcined sample is shown in Fig. 2. The data presented in both figures shows that both of the materials calcined and as-synthesized samples obtained using tetrabutylammonium cations (TBA+) as template were highly crystalline: The XRD patterns in this work were identified to be one of the MFIMEL intermediates family (see Fig. 3).
Fig. 1.XRD patterns of (a) amorphous borosilicate (b) crystalline borosilicate.
Fig. 2.XRD Pattern of [B]-MFI/MEL (calcined sample).
Fig. 3.Simulated XRD patterns for MFI-MEL series .
The reticular distances dhkl of our materials are higher than those of pure MFI zeolites as shown in Table 2, and this leads us to suggest that our product has a different and specific structure. The apparent topology is that of MEL since there exist two peaks between 2θ=8-9˚ and two others at 2θ=23-24˚ much more intense (see Fig. 1). However, the appearance of two weak peaks at 2θ=45˚ usually characterizing the MFI structure has led us to proceed to a higher resolution (0.007). We tried to identify and localize all the peaks characterizing the MFI structure and we have found some peaks are dramatically diffused according to the absence conditions, in particular the reflections of planes (151) and (133) and others are found to be superimposed, namely reflections of planes (501) and (051), (see Table 2). By comparison with Fig. 3, the structure of the product is supposed to be an intergrowth of two topological structures MFI and MEL in the ratio (90% MFI and 10% MEL).21
Table 2.Principal reticular distances of prepared zeoborosiles and simulated zeolite
Midinfrared spectroscopic studies of the zeolite framework vibrations in the framework region have been widely used to characterize and to differentiate various zeolite structures. Fig. 4 revealed the FT-IR spectra of framework vibrations in the 1400-400 cm-1 range of the three as-synthesized zeosilicate versions, namely zeosile, zeolite and zeoborosile with (Si2O/B2O3 or Si2O/Al2O3=80) as well as the calcined borosilicate version. Several IR bands are observed at 1224, 792 and 550 cm-1. Two Large bands at 1080 and 440 cm-1 and shoulder at 580 cm-1 are also observed. As to as-synthesized borosilicate sample, it should be noted that there are one additional IR band at 900.75 cm-1 and two shoulders at 781 and 536 cm-1.
Fig. 4.Midinfrared spectra of samples with MFI/MEL structure of (a) as-syn. silicalite, (b) as-syn. borosilicate (c) calcined borosilicate and (d) as-syn. aluminosilicate (zeolite).
The band at 550 cm-1 is characteristic of ZSM-5 (MFI), while absorption band at ~ 450 cm-1 is common to pentasil zeotypes and amorphous silicates. Zeolites that contain five-membered rings (pentasil structure) usually present IR bands at 1220 and 560 cm-1.40
The intense IR band at 1220 cm-1 was assigned to the extensive asymmetric stretching vibrations of the framework (four chains of five-membered rings for the pentasile family), while the IR band at 560 cm-1 was attributed to the double five-membered ring blocks and it is sensitive to the topology and building units of the zeolite frameworks. This band was observed at 580 cm-1 in mordenite (MOR) which is a large-pore zeolite. However, the additional absorption band at 900.75 cm-1 is observed, but Bandyopadhyay et al.6 observed the absorption bands at 930 cm-1 and 1397 cm-1 which were attributed to the symmetric and asymmetric stretching vibrations of B-O-Si group.41 In our spectra, the absorption band at 1397 cm-1 is not observed It should be noted that the presence of the band at 900.75 cm-1 only in the borosilicate version Fig. 4(b) confirms the incorporation of boron atoms and it is due to framework vibrations of tetrahedral entities B(OSi)-46 and it could be attributed to the stretching of the Si-O-B bond.44 On the other hand, the presence of a shoulder at 586 cm-1 may be attributed to the double five-membered ring blocks vibrations sensitive to the particular structure of the intergrowth of both MFI and MEL pore systems namely , the two types of pore system bi- and three dimensional channel systems.37 The IR spectra reported in Fig. 4 are in agreement with the pentasil structure type confirmed by the XRD data. We notice that the large absorption band of the zeolitic sample is moved towards smaller wavenumbers compared to the zeosile and the zeoborosile samples.
Nitrogen adsorption measures at 77 K
Fig. 5 illustrates the nitrogen adsorption isotherms from the starting oxide powder and the solids (MFI and MFI/MEL) obtained from the SAC method after calcination in nitrogen flow at 550 ℃ and then 600 ℃ to remove tetrabutylammonium cations.
Fig. 5.Nitrogen adsorption on (a) amorphous borosilicate (b) [B]-MFI and (c) [B]-MFI/MEL.
Nitrogen adsorption at 77 K allowed us to discuss the texture of the amorphous sample Fig. 5(a) and it indicates that the amorphous solid adsorbs nitrogen at low P/Po values indicating the presence of pores in the same range as the crystalline samples Fig. 5(b) and (c). However, the porous structure of the sample is dramatically different and shows an isotherm of type II and this means the adsorption took place on non-porous powders or powders with pores diameters higher than micropores (pores of 20A diameter or less).36,42 The inflexion point of the isotherm usually comes near the complete filling of the first adsorbed monolayer and along with the increase of relative pressure; the following layers are filled till the number of layers be infinite. Moreover, there is some resemblances to isotherm of type IV where there exist pores with radii 15-1000 A, which confirms the presence of micropores in the borosilicate dry gel.43 The adsorption isotherms in Fig. 5(b) and (c) of the samples [B]-MFI/MEL and [B]-MFI show the typical isotherm of type I characterizing the microporous structures. The adsorption capacity of the product [B]-MFI/MEL presumably larger than that of the product [B]-MFI because of the increased crystallinity and porosity. The BET and Langmuir surface areas of the samples with SiO2/B2O3=80 measured at liquid nitrogen temperature are listed above in Table 3.
Table 3.at-plot method bspecific area measured by Brunauer-Emmet-Teller (BET) method cspecific area measured by Langmuir Method
The textural data draw attention to the fact that the sample [B]-MFI/MEL shows higher microporosity (0.160 cc/g) than that of the pure phase [B]-MFI (0.119 cc/g), which is probably attributed to the larger cages of MEL structure.45 Also, it should be noted that the external surfaces are different and consequently the crystal size and the form of crystals are likely to be different too.
It should be pointed out that from a thermodynamic viewpoint the majority of known zeolites constitute metastable phases. Under the influence of various synthesis parameters, they may undergo further changes such as recrystallization into phases with slightly lower free energy. This explains the strong dependence of the hydrothermal synthesis of zeolites upon numerous kinetic parameters.19 In spite of the fact that the size and the shape of the guest molecules often correlate well with the void dimensions within the host, there are still aspects of phase selectivity in zeolite synthesis that are kinetically controlled, so that the same organo-cation guest may be capable of crystallizing more than one zeolitic phase.27 For this reason, following the crystallization process is of most importance. Thus, the time evolution of the sample with SiO2/B2O3=80 has been investigated. The experiment was performed at 175 ℃ and autogeneous pressure. Figs. 6 and 7 show the XRD patterns obtained from the solid phases collected at various time intervals (40 h, 72 h, 120, and 168 h.). Initially, the dry gel is essentially amorphous. After only 40 hours of heating the sample is mainly highly-crystalline zeolite and at first it shows a strong similarity to that of zeolites ZSM-11 (MEL).21-29 As mentioned above our product consists of an intergrowth of two pentasil phases, namely 10% MEL and 90% MFI. It should be noted that after a crystallization period of 100 hours, the XRD patterns start to show the appearance of peaks characterizing the phase of quartz. Henceforth, the prolongation of time of crystallization has led to the total conversion of the zeophases into quartz phase, which is the most stable phase.
Fig. 6.XRD patterns of products using TBA as template after different periods of crystallization (0-72 hours).
Fig. 7.XRD patterns showing partial conversion of the zeophase (MFI/MEL) into quartz after 100 hours.
The present steam-assisted crystallization technique is an effective method for the synthesis of an intermediate pentasile zeoborosile with an intergrowth type of (10% MEL/ 90%MFI). This type of intergrowth is thought to be a very stable phase when varying the space parameters; however, it is an unstable kinetic phase. This material show higher cristallinity and higher microporosity compared to pure MFI-type zeoborosile owing to the presence of larger cages of MEL structure. On the other hand, the presence of band absorption at ~900 cm-1 indicates the incorporation of boron atoms in the silicate matrix of the products compared to other versions.