Table 1 shows the chemical composition of Ifon kaolin as well as those of the synthesized zeolite-X samples. The main compounds found in both the kaolin are Al2O3 and SiO2 with about 38.2 and 41.53 wt.% of the former respectively. Other compounds found are TiO2, MgO, K2O, Na2O, etc. but they are present in minor amounts. After processing from kaolin to zeolite-X, the amount of Al2O3 on average decreased by about 67%, while the SiO2 increased by about 65% for all zeolite samples. The amount of some hitherto minor constituents increased after the zeolite conversion.
Table 1
Chemical composition analysis of Ifon kaolin and the synthesized zeolite-X
Analyte
|
SiO2 (wt. %)
|
Al2O3 (wt. %)
|
Fe2O3 (wt. %)
|
TiO2 (wt. %)
|
MgO (wt. %)
|
CaO (wt. %)
|
K2O (wt. %)
|
Na2O (wt. %)
|
BaO (wt. %)
|
MnO (wt. %)
|
P2O5 (wt. %)
|
LOI
|
Ifon Kaolin
|
41.53
|
38.2
|
0.36
|
0.53
|
0.094
|
0.01
|
0.47
|
0.31
|
-
|
-
|
-
|
18.5
|
Zeolite-X 7% 700oC
|
69.31
|
13.51
|
2.03
|
0.14
|
1.64
|
3.11
|
3.29
|
1.73
|
|
-
|
0.02
|
5.22
|
Zeolite-X 15% 700oC
|
68.93
|
12.05
|
0.32
|
0.17
|
2
|
4.1
|
3.4
|
1.56
|
0.01
|
0.28
|
0.04
|
7.14
|
Zeolite-X 7% 900oC
|
68.22
|
12.97
|
2.26
|
0.07
|
1.41
|
2.65
|
3.14
|
1.67
|
-
|
-
|
0.01
|
7.6
|
Zeolite-X 15% 900oC
|
66.17
|
10.69
|
0.32
|
0.14
|
1.98
|
3.83
|
3.32
|
1.48
|
0.01
|
0.19
|
0.01
|
11.86
|
The amounts of Fe2O3, MgO, CaO, K2O, and Na2O all increased significantly after the synthesis of zeolite-X. Only the Fe2O3 content of the samples leached with 7% of the diluted acids increased. At a calcination temperature of 900oC, the amounts of Al2O3 and SiO2 slightly decreased compared to that at 700oC. When the leachates were diluted by 15%, a decrease in the amounts of Al2O3 and SiO2 was observed. The values of SiO2/Al2O3 for the zeolite-X samples range from 5.13 to 6.19 which implies that the zeolite-X samples with 7% diluted acid falls within the range of values for zeolite-X as reported by [24]. The samples leached with 15% of diluted acid had slightly higher SiO2/Al2O3 values than 5.6
The FTIR spectra of the zeolite samples leached with 7 and 15% of the diluted acids and calcined at 700oC and 900oC are shown in Fig. 1. The obtained spectra are similar to those of a typical zeolite-X crystal in the literature although with slight changes in the wavenumbers (Otieno et al., 2021; Srilai et al., 2019)[16, 23]. The differences with the standard may be due to the differences in Al2O3 content. The characteristic wavenumbers were observed at 3466cm− 1, 1647cm− 1, 997 cm− 1, 749 cm− 1, 667 cm− 1, 555 cm− 1 and 462 cm− 1 respectively.
The band at 1647 cm− 1 is attributed to H2O deformation due to incomplete dehydration of the zeolite while the band at 3466 cm− 1 is due to OH-stretching of water molecules inside the zeolite [15]. The very intense peak at 997 cm− 1 is due to asymmetric stretching that 749 cm− 1 is attributed to the distortion of both the octahedral and tetrahedral layers. The peak at 667cm− 1 is due to symmetric stretching. The band at 555cm− 1 is due to a double ring. The band at 462 cm− 1 is attributed to T-O bending (where T is either Si or Al) [10]. The zeolite bands from samples calcined at 900oC are slightly different from those at 700oC between wavenumbers at 600–800 cm− 1. The peaks at 900oC are more developed compared to those at 700oC.
The X-ray powder diffraction patterns of the zeolite-X samples are shown in Fig. 2. The obtained patterns closely resemble that of the typical Faujasite NaX zeolite in the literature [25]. There are slight changes in the peak intensities and positions for the samples are observed.
There are slight changes in the peak intensities and positions for the samples are observed. Calcination of the kaolin sample at 700oC and 900oC respectively had some effect on the diffraction patterns of the zeolite-X samples. Some of the diffraction peaks in the sample calcined at 700oC were missing compared to that calcined at 900oC. This implies that the degree of zeolite formation in samples calcined at 900oC was slightly better than those calcined at 700oC. In addition to the diffraction peaks observed in the zeolite-X crystals, other phases were also present in the sample albeit in small quantities. Based on the report by [26] for kaolin transformed to zeolite NaX, a small quantity of quartz (SiO2) and Andalusite (Al2(SiO4)O were observed in addition to the main constituent phase.
The Scanning electron microscope images showing the morphology of the zeolite-X samples are shown in Fig. 3. The zeolite 7%_700oC (Fig. 3a) sample reveals many tiny grains and few large grains. The particle sizes range from 1µm to 5 µm. Most of the grains are quasi-cubic in morphology. For the zeolite 15%_700oC sample (Fig. 3b), there appears to be a gradual change in the shape of the grains to the octahedral shape. The average particle sizes of the grains are also larger than the former zeolite sample.
The micrograph for the 7%_900oC (Fig. 3c) sample shows that most of the grains appear to be octahedral in shape with a few quasi-cubic-like grains. Most of the grains appear to be uniform. For the 15%_900oC (Fig. 3d) sample, most of the grains appear to be octahedral in shape. The grains also appear to be uniform except where there was coagulation. Previous reports in the literature suggest that in the synthesis of zeolite-X especially when the SiO2/Al2O3 ratio is high, there are in some cases traces of zeolite P [27]
The thermal analysis of the zeolite samples using TG/DTA is shown in Fig. 4. The TG of the zeolite samples shows that as the temperature increases from 20oC to 200oC, a steep decline in the weight of the samples was observed such that about 13% of the weight was lost.
The mass loss is due to moisture loss from both the surface of the zeolite crystals and inside the channels. With further increase in temperature, the rate of weight loss decreases gradually to a temperature of 800oC, such that at this temperature, weight losses ranging from 16.3–18.3% for zeolite 15%_900oC sample and 7%_900oC sample respectively were obtained. The zeolite samples can be said to be structurally stable from 200oC to 800oC after the initial loss in weight attributable to the loss of moisture from hydration complexes formed with exchangeable cations. This is an important property for zeolites used in catalysis and as adsorbents [26].
The DTA of the samples shows that as the temperature was increased, a negative heat flow was obtained up to 162oC where an endothermic reaction occurred. The trends in the zeolite samples were similar but values of the heat flow are different and range from − 0.3124 to -0.4318 mW/mg. Further increment in temperature resulted in a positive heat flow and a broad exothermic peak was observed at 300oC. Above this temperature, the zeolites from kaolin samples calcined at 900oC show markedly different characteristics compared to samples calcined at 700oC. While the samples calcined at 900oC maintained a positive heat flow, those calcined at 700oC had a negative heat flow. The difference in behaviour at higher temperatures could be due to the degree of zeolite formed in both categories of zeolites.
The adsorption-desorption isotherms for the zeolite-X samples are shown in Fig. 5.
Adsorption is the sticking of gas (nitrogen) on the zeolite-X particles as well as on all the possible surfaces including those inside the pores. As the pressure of the gas over the zeolite particles increases, at a particular temperature, the adsorption increases. In the same way, the removal of the nitrogen from the surface of the zeolite and all possible surfaces inside the pores is known as desorption. As the pressure of gas decreases, the desorption rate increases. The samples calcined at a temperature of 700oC exhibited the type II isotherm which is associated with unrestricted monolayer-multilayer adsorption. This isotherm is usually found when adsorption occurs on nonporous powders or on powders whose diameters exceed those of micro-pores and the Inflection point occurs near the completion of the first adsorbed monolayer.
The first edge of the adsorption shows the region of complete monolayer coverage which indicates the non-porous nature of the zeolite produced. The samples calcined at a temperature of 900oC exhibit the type I isotherm which is characteristic of microporous solids such that the exposed surface is inside the micro-pores with very little external surface for further adsorption and also concave to the P/Po axis as the plot approaches 1. The sample that was leached with 15% of the diluted acids and calcined at 900oC could not reach full isotherm as the measurement terminated when the p/po is only at 1 x10− 4. The samples calcined at 900oC adsorbed more nitrogen compared to the samples calcined at 700oC.
The BET plot of the zeolite-X samples is shown in Fig. 6 while the parameters obtained from the adsorption isotherms are shown in Table 2. The graphs were obtained using the classical BET expression. The BET surface area of the zeolite 7%-700oC sample is 38.8617 m2/g and is 15.153 m2/g for the zeolite 15%_700oC sample. The BET values for the zeolite 7%_900oC sample is 212.50m2/g while that for zeolite 15%_900oC could not be obtained because the measurement terminated very early. This means that the surface areas of samples calcined at 900oC are about 7 orders of magnitude higher than that calcined at 700oC. The result is also similar to the Langmuir surface area result. The volume of the pores generated in samples calcined at 900oC was also higher than those at 700oC. The pore widths for samples calcined at 900oC were however smaller (21.62 Å) compared to those calcined at 700oC (28 Å). The average particle size of the samples calcined at 900oC is about 6 orders of magnitude (282 Å) lower than that calcined at 700oC (1544 Å).
Table 2
A table showing the parameters from the Nitrogen adsorption isotherm
Parameters Measured
|
IZX 7%_700oC
|
IZX 7%_900oC
|
IZX 15%_700oC
|
IZX 15%_900oC
|
Single point surface Area (m2/g)
|
39.2365
|
215.492
|
15.7006
|
-
|
BET Surface Area (m2/g)
|
38.8617
|
212.5036
|
15.1536
|
-
|
Langmuir Surface Area (m2/g)
|
53.4857
|
305.0091
|
21.6659
|
274.1116
|
Single point total Pore
Volume (cm3/g)
|
0.028147
|
0.114878
|
0.010768
|
-
|
Avg. Pore width (Å)
|
28.9713
|
21.6238
|
28.423
|
-
|
Avg. Particle Size (Å)
|
1543.937
|
282.348
|
3959.459
|
-
|