Synthesis and characterization of (3-aminopropyl) triethoxysilane (APTES) functionalized zeolite AlPO-18

Over the years, functionalization of zeolite is gaining popularity among researchers to further modify the properties of the zeolite for wide applications. The procedure of functionalization is crucial to ensure that the framework and structure of the zeolite would not be destroyed by the functionalization process. In this work, zeolite AlPO-18 was synthesized via hydrothermal synthesis method and functionalized by (3-Aminopropyl) triethoxysilane (APTES). The effect of the APTES functionalization on zeolite AlPO-18 was investigated in this work. Both unfunctionalized and silane-functionalized zeolite AlPO-18 were characterized using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and Thermogravimetric analysis (TGA) for their properties. The morphology and the composition of the elements present in zeolite AlPO-18 and zeolite NH2-AlPO-18 were examined using Field Emission Scanning Electron Microscopy (FESEM) and Energy-Dispersive spectroscopy (EDX) respectively. The XRD pattern of NH2-AlPO-18 was similar to that of zeolite AlPO-18, however, the intensity of the peaks was lower compared to zeolite AlPO-18. Based on the FTIR spectra, the presence of N-H stretching and bending vibration band of aminosilane were observed in the NH2-AlPO-18 sample. According to FESEM images, the morphology of NH2-AlPO-18 was comparable to that of zeolite AlPO-18 even after functionalization, proving that functionalization of aminosilane on zeolite does not affect on the zeolite structure. Besides that, EDX proves the presence of 3.02 % of element N in the NH2-AlPO-18 sample which is absent in the zeolite AlPO-18 sample. All of the characterizations evinced the presence of aminosilane, APTES in the NH2-AlPO-18 sample.


Introduction
Zeolites are crystalline aluminosilicates with a well-defined pore framework made from interlinked tetrahedra of SiO4 and AlO4 [1]. Currently, there are more than 200 types of zeolites have been documented, such as zeolite T, SAPO-34, DDR, SSZ-13, Si-CHA, and AlPO-18 [2]. Zeolites can be differentiated based on their crystal structure and framework. It exhibits a well-defined and uniform pore size with a large surface area and high porosity. Zeolites can be categorized based on the pore size IOP Publishing doi: 10.1088/1757-899X/1195/1/012047 2 such as small pore (6-, 8-and 9-membered ring), medium pore (10-membered ring), and large pore (12-membered ring) [3]. Zeolites have been used in various industrial applications such as adsorption, catalysis, and gas separations due to their intrinsic characteristics such as high chemical, thermal and mechanical stabilities [3].
Zeolite AlPO-18 is a type of zeolite that is gaining favor among researchers because of its small pore structure. Zeolite AlPO-18 is a type of zeolite that is made up of aluminum, phosphorus, and oxygen and consists of 3D structured pores possessing 8-membered rings with a pore size of 3.8 Å. The framework density of AlPO-18 of 15.1 T/nm 3 is the lowest among the aluminophosphate family. Zeolite AlPO-18 also has a low hydrophilicity nature. The low framework density, as well as the low hydrophilicity properties, are the predominant feature of AlPO [4]. It can be synthesized at a moderate synthesis temperature and a short synthesis duration. Zeolite AlPO-18 has been researched for applications including gas separation. It is remained an interest among researchers to explore approaches in modifying or improving the properties of zeolite AlPO-18 to be suitably applied at different applications.
In the present work, APTES has been chosen to be used as the silane coupling agent as it has been extensively used for a wide range of applications and it could also be considered as one of the less expensive silane coupling agents compared to the others [20][21][22][23][24][25][26][27][28]. APTES consists of an amine group (NH2) and three ethoxy groups that can be adhered to the zeolite surface [22,29]. Figure 1 depicts the schematic illustration for zeolite AlPO-18 functionalization using an APTES silane coupling agent. The novelty of the work is the functionalization of zeolite AlPO-18 using APTES and investigating the effect of the functionalization on the zeolite. Both unfunctionalized and silane-functionalized zeolite AlPO-18 were characterized using XRD, FT-IR, TGA, and FESEM. To date, there are no research works reported on the functionalization of zeolite AlPO-18 by using APTES.

Synthesis of zeolite AlPO-18
Zeolite AlPO-18 was synthesized via hydrothermal synthesis method by following the literature reported [30,31]. A precursor solution with a molar composition of 1.0 Al2O3: 3.16 P2O5: 6.32 TEAOH: 186 H2O was prepared. Aluminium isopropoxide, TEAOH, and DI water were mixed and stirred for an hour at room temperature to form a homogeneous mixture. Phosphoric acid was added dropwise into the stirring solution. The resulting precursor solution was stirred again for 2 hours before heated hydrothermally at 150°C for 20 hours. The synthesized particles were centrifuged at 6000 rpm for 10 minutes to collect the seeds and washed with DI water. The resulting crystal was dried at 50 °C for overnight.

Functionalization of zeolite AlPO-18
The synthesized zeolite AlPO-18 powder was dried overnight at 50 °C before functionalization. For functionalization, 2 g of zeolite AlPO-18 powder was uniformly dispersed into 50 mL of toluene. Then, 4 mL of APTES was added dropwise to the resultant mixture and refluxed at 110 °C for 4 hours. After the reflux process, the mixture was filtered and rinsed with toluene and ethanol absolute in order to remove the unreacted APTES. The washed particles were dried at 50 °C overnight. AlPO-18 that was functionalized by the silane group was denoted as NH2-AlPO-18 in the current project.

Characterization of zeolite AlPO-18 and NH2-AlPO-18
XRD (X'Pert 3 Powder & Empyrean, PANalytical) was used to study the crystallinity of zeolite AlPO-18 and NH2-AlPO-18. The analysis was carried out at an accelerating voltage of 40 kV and current of 40 mA and by using Cu Kα radiation at 2θ in the range of 5 o -45 o with a step size of 0.05 o . Fourier Transform Infrared-Attenuated Total Reflection, FTIR-ATR (Perkin Elmer, Frontier) was used to identify the functional groups and chemical bondings presence in the zeolite AlPO-18 and NH2-AlPO-18. The analysis was carried out with a wavelength of 4000 cm -1 to 400 cm -1 . Thermogravimetric Analysis, TGA (Perkin Elmer, STA 6000) was used to study the thermal stability of zeolite AlPO-18 and NH2-AlPO-18 based on the weight loss of the sample due to change in temperature over time. The samples were heated from 30 °C to 800 °C at a constant heating rate of 10 o C/min under N2 atmosphere. Field Emission Scanning Electron Microscopy, FESEM (Zeiss Supra 55VP) was used to study the morphology of the zeolite AlPO-18 and NH2-AlPO-18. Energy-Dispersive spectroscopy (EDX) was used to identify the elemental compositions of zeolite AlPO-18 and NH2-AlPO-18.

Crystallinity analysis of zeolite AlPO-18 and NH2-AlPO-18
The XRD pattern of the calcined zeolite AlPO-18 and NH2-AlPO-18 are illustrated in figure 2. From figure 2(a), it can be observed that the XRD pattern display the peaks at two theta of 9.6°, 12.8°, 16.8°, 21°, 23.6°, 26.2°, and 32.0°. All the XRD peaks of the synthesized zeolite AlPO-18 were similar to the XRD patterns reported in the literature previously [32][33][34][35]. The results obtained prove that zeolite AlPO-18 was successfully synthesized. According to Carreon et al. [35], the XRD pattern exhibit broad and less intense peaks due to the presence of amorphous regions and/or there is a high degree of structural disorder in the synthesized zeolite AlPO-18 framework.
From figure 2(b), it can be seen that the XRD pattern of NH2-AlPO-18 was almost similar to the zeolite AlPO-18 peaks. This implies the zeolite structure was not affected by the functionalization of aminosilane on the zeolite surface. Despite that, the intensity of NH2-AlPO-18 peak was lower compared to the zeolite AlPO-18 patterns. This could be due to the pore filling effect, where the pore surface of zeolite AlPO-18 is probably covered by APTES groups. Thus, it affects the crystallinity of the sample by causing a slight decrement in the peak crystallinity [36,37].  Figure 3 and figure 4 shows the FTIR spectra of calcined zeolite AlPO-18 and NH2-AlPO-18 in the region of 4000-400 cm -1 . FTIR was used to identify the functional group and type of bonding present in zeolite AlPO-18 and zeolite NH2-AlPO-18. From figure 3(a), it can be seen that the FTIR spectra exhibits the bands around 3500 cm -1 , 1640 cm -1 , 1100 cm -1 , 600 cm -1 and 500 cm -1 . All these bands are the typical zeolite AlPO-18 peaks that are in good agreement with the literature reported previously [34,38,39].
From figure 3(b) and figure 4(b), it can be seen that the FTIR spectra of NH2-AlPO-18 was almost similar to the zeolite AlPO-18 spectra with a few additional bands around 1500 ~ 1300 cm -1 and 1560 cm -1 . Compared to zeolite AlPO-18, the zeolite NH2 -AlPO-18 showed a broader band at the frequency of 3800-3200 cm -1 , where the band assigned to N-H stretching of a primary amine overlaps with the O-H stretching of hydroxyl group, indicating the presence of aminosilane in the sample [15,49]. Besides that, Si-CH2 and Si-CH3 stretching vibrations of the aminosilane were also apparent around 1500-1300 cm -1 in NH2-AlPO-18 [12,40,50]. Moreover, the presence of an absorption peak at 1560 cm -1 , which indicates N-H bending of the amine, was also absent in zeolite AlPO-18 [15].
The type of bonding represented by each of the peak presence in zeolite AlPO-18 and NH2-AlPO-18 spectra were summarised in table 1.    Figure 5 illustrates the TGA diagram of zeolite AlPO-18 and NH2-AlPO-18. TGA profile shows the percentage of weight loss of the zeolite AlPO-18 and NH2-AlPO-18 as a function of time.

Thermal analysis of zeolite AlPO-18 and NH2-AlPO-18
From the figure 5, it can be observed that zeolite AlPO-18 exhibits a single-stage decomposition process in the temperature range of 30-200 o C. Whereas, NH2-AlPO-18 shows a two-stage decomposition at the temperature ranged from 30-200 o C and 200-800 o C. In figure 5(a), the TGA profile of zeolite AlPO-18 shows a significant weight loss of 24.72% below 200 o C, which was almost similar to the TGA profile reported in the literature [51,52]. This weight loss was ascribed to the removal of physiosorbed water molecules within the zeolite pores [51,53,54]. After 200 o C, there is no obvious mass variation of zeolite AlPO-18 up to 800 o C, which clarifies that zeolite AlPO-18 has good thermal stability.
The TGA profile of NH2-AlPO-18 in figure 5(b) illustrates the first-stage decomposition with a weight loss of 19.52% at the temperature range of 30-200 o C. The thermal decomposition rate of NH2-AlPO-18 was lower compared to zeolite AlPO-18. This justifies the attachment of APTES silane groups on the hydroxyl groups (-OH) of zeolite AlPO-18 during the grafting process [55][56][57]. Thus, it can be concluded that the small weight loss in NH2-AlPO-18 is because of less -OH group presence in the NH2-AlPO-18 sample. The second-stage decomposition of NH2-AlPO-18 with a weight loss of 3.94 % occurs at the temperature range 200-800 o C. This weight loss was attributed to gradual organic volatilizationdecomposition of propyl chain in APTES molecule [22,[58][59][60][61]. The thermal properties of zeolite AlPO-18 and NH2-AlPO-18 are summarized in table 2.   Figure 6(a) and (b) shows the FESEM images of zeolite AlPO-18 and NH2-AlPO-18 at a magnification of 20kX respectively. FESEM was used to examine the morphology of the zeolite AlPO-18 and NH2-AlPO-18.

Morphological analysis of zeolite AlPO-18 and NH2-AlPO-18
The FESEM images in figure 6(a) show a thin elongated plate-like structure of zeolite AlPO-18. The morphology obtained is the typical zeolite AlPO-18 structure that looks similar to the literature reported previously [30,51]. However, the morphology of the samples displays particle aggregates. Besides that, from figure 6(b), it can be observed that the morphology of NH2-AlPO-18 was similar to that of zeolite AlPO-18 even after functionalization on the surface of zeolite AlPO-18. This shows that the functionalization of aminosilane on the zeolite surface had no effect on the zeolite structure.
Moreover, EDX spectroscopy was used to identify the composition of the element presence in zeolite AlPO-18 and NH2-AlPO-18. To ensure the consistency of the composition, five trials of the analysis were carried out on the samples. The elemental composition of the samples is summarized in

Conclusion
In the current project, the effect of aminosilane functionalization on zeolite AlPO-18 was investigated. The XRD pattern of NH2-AlPO-18 was comparable to that of zeolite AlPO-18, although the peak intensity was lower than that of zeolite AlPO-18. The existence of N-H stretching and bending vibration bands of aminosilane was observed in the FTIR spectra of NH2-AlPO-18 sample. According to FESEM analysis, even after functionalization, the morphology of NH2-AlPO-18 was similar to that of zeolite AlPO-18, demonstrating that aminosilane functionalization has no effect on zeolite structure. Furthermore, EDX confirms the existence of 3.02 percent element N in the NH2-AlPO-18 sample. All of the characterizations revealed the presence of APTES in the NH2-AlPO-18 sample. The obtained NH2-AlPO-18 can be used widely in industrial applications such as molecular separation, adsorption, and catalysis. For future research, the NH2-AlPO-18 can be potentially used as filler for CO2/CH4 gas separation. Besides that, the effect of various types of silane coupling agents such as (3-aminopropyl) dimethylethoxysilane (APDMES) and (3-aminopropyl) methyldiethoxysilane (APMDES) on the zeolite properties can also be investigated.