Zeolites are hydrated aluminosilicates having a structure formed by SiO4 and AlO4 tetrahedra linked between them by an oxygen atom. Thus, they form three dimensional frameworks of structural formula as follow: Mex/n (AlO2) x(SiO2)y) w H2O, where n is the valence of exchanged cation Me, w is the number of water molecules and (x + y) is the total number of tetrahedra per unit cell (Smith, 1963; Baerlocher et al., 2001). Many of the interesting properties of zeolites are based on the fact that the framework is anionic and the balancing cations exchangeable. Pure silica (Si O2) framework is neutral, but if some of the tetravalent Si is replaced by trivalent Al to produce an aluminosilicate, the ramework becomes negative and counter ions such as Na+ are needed to balance its charge (Dyer, 1988; Barrer, 1978).
A very large number of high silica zeolites (Si/Al = 10-100) prepared by direct synthesis have now been reported, including beta, ZSM-5, -11, -12, -21, -34, NU-1 and FU-1 and ferrisilicate and borosilicate analogous of the aluminosilicate structures (Nibou, 1999; Breck, 1974). The intermediate zeolites (Si/Al of 2-5) consist of the natural zeolites erionite, chabazite, clinoptilolite and mordenite and the synthetic zeolites Y, omega and L.
But according to literature, few works were referring to the morphology and form of obtained ZSM-5 and mordenite crystals and the influence of the concentration of SiO2. Initially, the aim of the present work will be to synthesis and characterization ZSM-5 and mordenite aluminosilcate structures according the hydrothermal crystallization method. In the second place, we will show that the crystal growth environment during nucleation and crystallization was occurred at the liquid-gel interface in the dispersed gel-solution mixtures.
MATERIALS AND METHODS
Elaboration process: Samples of zeolites were prepared by hydrothermal crystallisation (Table 1) at different temperatures, pH and heating times from starting gels of molar composition Al2O3. (x) SiO2.(y) Na2O. (z) R. (w) H2O.
As shown in Table 1, zeolites products could only be obtained
in the case of samples 2 and 4. As synthesis example, sample 2 was carried out
following molar reactants 1, x, y, z and w with the pH of the mixtures between
7.5 and 8.
|| Synthesis conditions and obtained phases
The reaction mixture was heated at 170°C and autoclaved for 48 h. After
crystallization, the product was washed with distillate water and dried at 100°C.
The used reactants are NaOH (Merck), SiO2 (Degussa, 41%), Al2O3
(Catapal, 77%), tetrapropylammonium bromide (Merck, 98%).
Characterization: The obtained phases were characterized by powder X ray diffractometer (Philips PW 1800, using Cu Kα radiation), scanning microscope (Philips XL 30) and infrared spectroscope (Philips PU 9800). The differential thermal analysis was carried out using a M2 BDLSETARAM microanalyser.
RESULTS AND DISCUSSION
Table 1 shows the synthesis conditions of zeolite samples. For samples 2 and 4, pure ZSM-5 and Mordenite (structure code MOR) were obtained. The XRD patterns of as-elaborated samples are given in Fig. 1 and 2. They are in good agreement with that reported by Breck (1974) and the simulated pattern published in the collection of simulated powder patterns for zeolites (Treacy and Higgins, 2001).
Scanning electron micrographs of samples 4 are shown in Fig. 3. Individual hexagonal prismatic crystals (as needle) of about 10-16 μm in length are observed in Fig. 3. The morphology and/or form of zeolite crystals appear generally to be influenced by the concentration of SiO2, guest molecule type cation and crystal growth inhibitors (Jansen, 2001). A frequently observed crystal form of mordenite is the needle form (with pore channel system parallel to needle direction). According to the synthesis conditions used (Table 1), the favour crystal form seems to be the needle.
Generally, the typical form and morphology of zeolite crystal reveals not only
information on the type of the zeolite formed but also on the crystal growth
|| XRD pattern of as-elaborated ZSM-8 (sample 2)
|| XRD pattern of as-elaborated Mordenite (sample 4)
There are four cases of crystal growth environment during nucleation and crystallization:
clear solution, dispersed low density gel, separated high density gel and solid
phase (Jansen, 2001).
According to the obtained form of crystallization product shown in Fig. 4 (sample 2), it seems that the nucleation process is heterogeneous and occurs at the liquid-gel interface in the dispersed gel-solution mixtures. These results are in good agreement with those obtained by Kuperman et al. (1993).
The anhydrous framework composition found for samples 2 and 4 by chemical analysis
is Na0.0497Si0.8962Al0.054O2 and
These results show that Al is essentially substituted for Si only according
to the mechanism (sample 4): 2 Al3+ + Na+ + (C3H7)3NH2+
= 2 Si4+.
||Scanning electron micrographs of samples 4 (a) Individual
hexagonal prismatic crystals (as needle). (b) Typical needle aggregates
Thus, each aluminium atom incorporated into the framework will generate a
Brönsted acid site after removal of the template by calcinations. In the
other hand, the cation Na+ of mordenite (sample 2) compensates the
negative charge of the framework according to the following mechanism: Al3+
+ Na+ = Si4+.
Moreover, the investigation by infrared spectroscopy technique in the mid-infrared region of the zeolite spectrum is useful in this regard since it contains the fundamental vibrations of the framework Al, Si-O4 or (TO4) tetrahedral. A different vibration bands are observed in the spectra of samples 2 and 4. The first class of vibrations found at 1223-1230, 680-700 and 438-454 cm-1 are assigned to the internal tetrahedral. The second group of frequencies observed at 547-546, 446, 792-789 and 1084-1102 cm-1 are assigned to the linkages between tetrahedral and the topology of the units of structure of samples.
Figure 5 shows the DTA curve of ZSM-5 (sample 2). Four transformations
during the heating are observed at 100-150, 350, 750 and 1200°C.
|| Scanning electron micrograph of samples 2
|| Thermal decomposition curve (DTA) of sample 2
|| Thermal decomposition curve (DTA) of sample 4
They may be attributed to desorption of water, the oxidation of template, the
beginning of framework decomposition and the total decomposition, respectively.
Thus, it seems that ZSM-5 is stable to calcinations to at least 800°C. In the other hand, the mordenite zeolite (sample 4) appears very stable as shown in Fig. 6.
Aside from the broad endotherm which is characteristic of the dehydration of sample 4 at lower temperatures, mordenite exhibits no thermal reactions up to a temperature of about 1150°C. However at 1200°C, a glass transformation was observed.
The hydrothermal elaboration route leads to well prismatic crystals as needle of mordenite which are very stable to calcinations to at least 1100°C. Scanning electron observations of ZSM-5 showed that heterogeneous nucleation is occurred at the liquid-gel interface in the dispersed gel-solution mixtures. The infrared spectroscopy technique in the mid-infrared region allowed characterizing the fundamental vibrations of the framework Al, Si-O4 or (TO4) tetrahedral.