4.2 Results and discussion
4.2.1 Characterization of FAU and MFI zeolite powder .1 XRD analysis
MFI-type zeolites (as-synthesized and calcined) were characterized to verify its purity and structure through XRD profile as illustrated in Figure 4.4. The powder XRD pattern of MFI zeolite shows the high crystallinity and the obtained profile is good agreement with patterns of MFI zeolites described elsewhere (Wegner et al. 1999). The distinctive peaks are obtained in 2θ range around 7.5 and 23.5 with some other bearing peaks in both samples signifying the occurrence of pure phase of the zeolite.
Figure 4.4: XRD pattern of MFI zeolite
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0 50 100 150
200 As synthesized
2 (Degrees)
Intensity (A.U.)
0 50 100 150 200 250 300
Calcined
4.2.1.2 FTIR analysis
The FTIR spectrum of the FAU zeolite is illustrated in Figure 4.5. The spectrum exhibits a strong and broad band at the region of 3200-3600 cm-1 due to the presence of the silanol OH group (Si- OH). The dominant absorbance peak appeared at 979 cm-1 is attributed to Si-O-Al groups. The bands observed between 456 and 740 cm-1 correspond to Al-O, Si-O, Si-Al, and Si-O-Si. The absorption band appeared at 668 cm-1 is due to Si-O-Na groups (Sadeghi et al. 2014).
Figure 4.5: FTIR spectrum of FAU zeolite
In Figure 4.6, FTIR spectrum of MFI zeolite confirms that the creation of zeolite phase in both samples presenting well defined bands around 450 cm-1(T-O bending), 540 cm-1 (double ring vibration), 790 cm-1 (external symmetric stretch) and 1080 cm-1 (internal asymmetric stretch) (Szostak 1998). The external asymmetric stretching vibration near 1225 cm-1 occurred in the
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50 60 70 80 90 100
Transmittance (%)
Wave number (cm-1)
pattern of MFI structures is allocated to four chains of 5-member rings formed around a two- fold screw axis. The band appeared at 1622 cm-1 belongs to the bending vibration of adsorbed water. For the as-synthesized sample, the sharp intense bands occurred near 2,900 and 2,850 cm−1 correspond to the presence of C-H stretching of the structure directing agent (TPA).
Moreover, the spectrum verifies the decrease of silanol group after calcination of zeolite at 400ºC. The bands representing 3200-3700 cm-1 (OH groups), also including water, as well as the band allied with silanol nests (950 cm-1) evidently show a reduced intensity after calcination due to the removal of the structure directing agent (TPA) (Kuhn et al. 2007).
Figure 4.6: FTIR spectra of MFI zeolite
4.2.1.3 TGA and DTG analysis
The thermogravimetric (TGA) analysis curve of the FAU zeolite is displayed in Figure 4.7. It can be seen from TGA data that the majority of weight loss occurs at the temperature ranging
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0 20 40 60 80 100
Wavenumber
(
cm-1)
Transmittance (%)
As synthesized Calcined
between 80 and 330 oC and total weight loss is found to be 18%. The weight loss below 110 °C is attributed to the loss of water molecules present in the framework of the zeolite. The first stage of weight loss occurred above 130 °C is most likely due to intrazeolite water desorption (Figueiredo et al. 2006). The second stage of weight loss at 150-330 °C is due to the liberation of crystal water from the sample and no further significant weight loss is noticed after 330 °C.
Figure 4.7: TG analysis of FAU
The TGA and derivative thermogravimetric (DTG) curves of as-synthesized MFI zeolite material are presented in Figure 4.8. The weight loss below 150 °C corresponds to the removal of the physically adsorbed water present in the sample and the loss at 500 °C is due to condensation of silanol groups. The sample exhibits a derivative peak in the range of 350-500
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20 40 60 80 100
Weight loss (%)
Temperature (oC)
°C, which belongs to the release of structure directing agent (template) present inside the zeolite channels. The total weight loss of the zeolite is found to be 23.22%, which is mainly due to the structure directing agent loosely occluded inside the zeolite channels, resultant in a mass loss (Li et al. 2014).
Figure 4.8: TGA and DTG curves of MFI zeolite
4.2.1.4 Zeta potential measurements
Figure 4.9 displays the variation of zeta potential of FAU and MFI zeolite powder at various pH. The surface charge of the membrane will be positive or negative, depending upon the pH of the contact solution. In order to find out the surface charge of the zeolite composite
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30 40 50 60 70 80 90 100
Weight (%)
Temperature C)
TGA DTG
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
Derivate weight (% / C)
membranes, the zeta potential of FAU and MFI zeolite powder particles in solution is measured by electrostatic light scattering method. The surface potential of zeolite powders is altered by the addition of NaOH to make higher pH and HCl solution is added to make lower pH. The surface charge of zeolites becomes positive due to larger amount of H+ ions at lower pH and at higher pH, negative hydroxyl (OH-) ions become more. Therefore, the zeta potential value is 4 mV for FAU zeolite and 11.5 mV for MFI zeolite at pH 2 while it is approximately -45 mV for FAU zeolite and -62.42 mV for MFI zeolite at pH 12, respectively. Hence, electrostatic interactions play a major role in the surface activity of FAU zeolite. The iso-electric point (IEP) of FAU and MFI zeolite powder is obtained to be 3.8 and 4.0, respectively. When pH is < IEP, the zeolite membrane is positively charged and if pH > IEP, it is negatively charged.
Figure 4.9: Zeta potential measurement of FAU zeolite
2 4 6 8 10 12
-80 -70 -60 -50 -40 -30 -20 -10 0 10 20
Zeta potential (mv)
pH
FAU zeolite MFI zeolite
4.2.1.5 Particle size distribution
Particle size and shape of the particle is important parameter that controls the porosity and pore size of membrane. Smaller particles (less than the membrane pore size) can easily penetrate in the membrane pores and reduce the pore size. On the other hand, larger particles are stick on the membrane surface and also reduce the pore size. Figure 4.10 shows the particle size distribution of the as synthesized FAU zeolite particles. The average particle size of the FAU zeolite is calculated to be 157 nm.
Figure 4.10: PSD of FAU zeolite
Figure 4.11 illustrates the particle size distribution of the MFI zeolite particles before and after calcination. It is to be noted that the average particle size is 972.4 nm and 1,005 nm for
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-2 0 2 4 6 8 10 12 14 16
Diameter (nm)
Differential intensity (%)
before and after calcination, respectively. This indicates that the average particle size is slightly increased after calcination.
Figure 4.11: PSD of MFI zeolite