Microwave-assisted method for the direct synthesis of nano-sized Cu-zeolite

: Recently, the direct synthesis methods for zeolite modification have received much attention from scientists to improve zeolites as catalysts in a variety of applications. Unlike conventional methods, well advanced physicochemical properties could be introduced to zeolites through direct synthesis. During this work, Cu-loaded zeolite was directly synthesized using a microwave-assisted method, and the resulted samples were characterized by Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and Raman microscopy. The PXRD and SEM results revealed that the Cu-loaded zeolite has LTA (Linde Type A) crystal structure with high crystallinity and particle size obtained within the 300 - 600 nm range. The FTIR and Raman microscopic data showed the bond formation of Cu within the zeolite framework. Unlike other methods, modification of zeolite was done at a low temperature of 110 ℃. During this microwave-assisted method, nano-sized, LTA type Cu-zeolite was successfully obtained.


INTRODUCTION
Zeolites are mesoporous materials that can be found in nature, and due to their high demand in various applications, synthetic zeolites are also available widely. Most of the zeolite types are naturally occurred due to volcanic activity (Moshoeshoe et al., 2017). Even though zeolites are utilized in many industries and scientific research during the early 1960s, they have been known for 250 years (Kannangara et al., 2020;Kustov, 2020). Artificial zeolite synthesis was begun during the 19 th century, and it was inspired by R.M. Milton and his co-worker, D.W. Breck. As a result, several types of zeolites, such as zeolite A, X and Y, were synthesized (Breck, 1964;Flanigen, 2001;Lutz, 2014;Zimmermann and Haranczyk, 2016).
Zeolites are crystalline aluminosilicates of the alkaline and alkaline earth metal elements. The chemical composition of zeolites is represented by the empirical formula: M 2/n O•Al 2 O 3 •ySiO 2 •wH 2 O (Flanigen, 2001;Dogan, 2003;Hashimoto, 2003;Valdés et al., 2006). Usually, zeolites are composed of a three-dimensional framework of oxygen sharing AlO 4 and SiO 4 tetrahedral building units. In their structure, some Si 4+ ions are substituted by Al 3+ ions, originating an anionic framework that arises from the difference of valency between (SiO 4 ) 4and (AlO 4 ) 5tetrahedra. These resulted negative sites are balanced by counterions, such as alkaline and alkaline earth metal cations. Hence, it reveals the ion exchange property of zeolites (Armbruster and Gunter, 2001;Hashimoto, 2003;Valdés et al., 2006;Tago et al., 2012;Moshoeshoe et al., 2017).
Nowadays, zeolites are synthesized commercially for various applications due to their specific tunable properties, such as porosity, large surface area, thermal stability and ion exchange ability. Due to the cation exchange capability of zeolites, they are widely used in water purification treatments (Kalló, 2001;Kosobucki et al., 2008;Cardoso et al., 2015). On the other hand, zeolites gain much more attention in the industrial sector due to their molecular sieving property. Another significant characteristic property of zeolites is their catalytic activity because of their strong acidic nature. By applying this property of zeolites, they are used in catalytic cracking of long hydrocarbons, production of fine chemical products (Heinemann, 1981), and reduction of harmful gases from vehicle exhaust (Fritz and Pitchon, 1997). Furthermore, with the help of previously mentioned properties, zeolites are utilized in medicinal applications, such as cancer treatment (Moore et al., 1998;Pavelić et al., 2001;Moshoeshoe et al., 2017).
The high interest in nano-sized porous materials is originated with a new era in nanotechnology. The synthesis of nano-sized crystalline zeolites has acquired more consideration because unlike micro-scale zeolites, they have more advanced properties, such as adjustable surface characteristics, large surface area, fine-tuning bulk properties and high stability in suspensions (Zaarour et al., 2013). The capability of these materials to offer sustainable solutions to prevailing global problems is attracted by researchers (Valtchev and Tosheva, 2013). Despite the fact that there are multiple strategies available to synthesize nano-sized zeolites, microwave-assisted technique is more effective than other techniques (Katsuki, 2001). The synthesis of zeolites using microwave heating method has been shown to have many advantages over conventional heating methods, such as rapid product formation at low temperature within minimum time period, improved selectivity and improved product uniformity (Panzarella, et al., 2007). Moreover, nowadays, site directing agents are getting more attention because of their ability to control particle size during crystallization (Tago et al., 2012;Abdel et al., 2015;Kannangara et al., 2020).
Recently, strong attention has been directed towards the synthesis of modified zeolite materials using transition metals, such as Cu, Fe, Zn, Pt, Pd, Mn, Cr and Ni (Karge and Beyer, 1991;Jiang et al., 2020). There are various applications of modified zeolites, such as the utilization of TiO 2 encapsulated zeolite Y in photovoltaic solar cells (Atienzar et al., 2007), usage of silver containing zeolites as antimicrobial agents (Nagy et al., 2011), usage of copperdoped zeolites as electrochemical sensors (Walcarius, 1999), selective catalytic reduction of NO x from vehicle exhaust by Cu-zeolite (Moliner et al., 2012;Giordanini et al., 2013;Mihai et al., 2013) and adsorptive desulfurization by Cu(I)-Y(III)-Y zeolite (Jiang et al., 2020).
The general procedure for zeolite modification is the solid state ion exchange technique (Karge and Beyer, 1991). Other than that, wet ion exchange and chemical vapor deposition ion exchange methods are also mostly used in research fields and industries (Deka et al., 2012). All the above mentioned post-synthetic modification methods are carried out through dealumination, calcination, acid leaching, or chemical treatment steps. The synthesis of modified zeolite with sufficient physicochemical properties for their application in an industrial chemical process has to be improved with the consideration of the overall economic cost of its manufacture. In this manner, the direct synthesis of modified zeolites has received more attention over the multi-step synthesis of zeolite modification (Moliner, 2012). There are various disadvantages that can be found in conventional methods that are used for zeolite modification. Generally, the organic molecules are used as site directing agents which has increased the overall cost of the synthetic procedure. On the other hand, all other multistep methods require high temperature for the calcination process (Sultana, et al., 2011;Deka et al., 2012). The main advantage of inserting transition metals by direct synthesis is correlated to the capability of accomplishing a high dispersion of the metal in the zeolite structure. However, the inclusion of a metal in framework positions is complicated and will depend on the synthesis conditions, such as pH, temperature and source of metal used (Moliner, 2012). In this study, copper loaded zeolite A was synthesized using a microwave heating method and resulted samples were characterized using several techniques.

Synthesis of zeolite A
The synthesis of zeolite A via microwave-assisted method was performed as follows: for the microwave-assisted method, MARS 6 microwave digestion system was used. First, a sample of 0.534 g of NaOH pellets was dissolved in 5.00 mL of de-ionized water. Then, a sample of 0.242 g of NaAlO 2 was slowly added into the alkaline solution while shaking. Then, 0.343 g sample of Na 2 SiO 3 .5H 2 O was added and the solution was kept at 60 ℃ for 48 h for aging. Thereafter, the microwave heating was applied to the aged sample with 900 W at 110 ℃ for 3 h. Finally, the synthesized sample was taken out and washed with de-ionized water until washings reached pH 8 and dried at 80 ℃ (Kannangara et al., 2020).

Synthesis of Cu-zeolite
First, the seed solution was prepared as previously mentioned, and it was kept at 60 ℃ for 48 h for aging. Thereafter, 0.5 mL of Cu(NO 3 ) 2 was added to the aged sample and a sample series was prepared by varying the Cu 2+ concentration from 0.01 -1.0 mol L -1 . Then, the microwave heating was applied to the sample with previously mentioned parameters. Finally, the synthesized sample was taken out and washed with de-ionized water until washings reached pH 8 and dried at 80 ℃.

Characterization techniques
Bond association of the synthesized crystal structure was determined using the FTIR data performed on Thermo Fisher Scientific Nicolet iS 50 FT-IR spectrometer and Raman microscopy measurements were performed using RENISHAW in Via Raman microscope operating with 514 nm edge LASER state, 2400 l/mm spectrometer state and 10.00 s of exposure time. Crystallinity and purity of the synthesized products were identified by powder X-ray diffraction (PXRD) technique using a powder X-ray diffractometer (Rigaku-Ultima IV) operating with Cu-K radiation (λ=1.54 Å). Scanning electron microscopic (SEM) imaging was conducted for all the samples on EEVO/LS 15 ZEISS microscope .

Fourier-transform infrared spectroscopy studies
The FTIR data were collected to determine the bond association within the crystal structure of synthesized samples. The FTIR spectra of synthesized products obtained after microwave-assisted synthesis treatment are shown in Figure 1. As depicted in Figure 1, the absorption bands of the synthesized products show the symmetric and asymmetric vibration bands characteristic of LTA zeolite in the region 400-1200 cm -1 (Barnes et al., 1999). The fingerprint region of the LTA zeolite is considered to be at 400 -800 cm -1 . The presence of the double rings (D4R and D6R) in the framework zeolite structure is revealed from the bands that appear in the region 650 -500 cm -1 . The bands in the region 500 -420 cm -1 appeared due to the internal tetrahedron vibrations of Si−O and Al−O of the zeolite structure (Reyes et al., 2013). The region below 300 cm -1 is related to T−O−T bending mode and the region 400 -550 cm -1 is related to the bending mode of O−T−O (T = Si/Al). The asymmetric stretching mode of T−O−T is shown in the region 900 -1200 cm -1 while the symmetric stretching mode of T−O−T is shown in the region 550 -850 cm -1 (Henderson and Taylor, 1977). The peaks at 461, 557, 665 and 1006 cm -1 which are shown in Figure 1 are related to the LTA zeolite structure. Investigation of the structural changes concerning Cu and Al-related acid sites can be done by comparison between the band intensities of the individual samples before and after introducing Cu into zeolite (Mohamed et al., 2017). The sharpness of the band appearing at 665 cm −1 is increased while loading Cu, and it may due to the internal deformation vibration modes of T−O−T bridges. The band at 1006 cm −1 is due to the asymmetrical vibrations related to (Si, Al)O 4 tetrahedral of zeolite A. However, the sharpness of that peak reduces while increasing the Cu loading and it may due to the formation of Cu−O bonds, and deformation of Si−O bonds. This band occurs may be due to O−Cu−O bridge at the covalently bonded Cu centers. The band at 1647 cm −1 can be assigned to the bending mode of adsorbed water. The presence of a broad band at 3444 cm −1 in Cu-zeolite samples can be assigned to the O-H stretching mode and Cu−O vibrations. The band intensity slightly reduced while increasing the copper loading to the zeolite A sample. This might be due to decrease in O-H bond strength (Fanta et al., 2019).

Powder X-ray diffraction studies
To determine the crystal structure and phase transformation of the synthesized zeolite A and Cu-loaded zeolites, PXRD measurements were taken. The PXRD patterns of zeolite A and synthesized Cu-zeolites via microwaveassisted synthesis are shown in Figure 2. According to the PXRD pattern, all the samples were found to have higher crystalline nature. The similarity of the PXRD patterns of the Cu-loaded zeolites to that of zeolite A indicates that the original zeolite structure remains unchanged during modification. Though a slight reduction in the peak intensity after the Cu loading process was also found as a result of the introduction of metal ions, it may due to the high absorption coefficient of Cu compounds in Cu-zeolite for the X-ray radiation. As depicted in Figure 2, clear peaks are found at 7.2°, 10.1°, 12.5°, 16.1°, 24.0° and 30.3° which are characteristic peaks for LTA zeolite (Chudasama et al., 2005). Two additional peaks are observed in the PXRD spectra of Cu-Zeolites at 35.5° and 38.6° which are characteristic peaks for CuO which cannot be observed in the PXRD pattern of synthesized zeolite A (Černík and Padil, 2013). PXRD patterns give minor peaks for CuO which is not sufficient to identify its phase. The overall results from PXRD pattern suggest the incorporation of copper into the zeolite A framework.

Scanning electron microscopic studies
To study the particle morphology and the crystal growth process of Cu-loaded zeolites, synthesized samples were characterized using SEM images as shown in Figure 3. Typically, well-developed LTA-type zeolites show cubic morphology in their crystal structure (Reyes et al., 2013). As depicted in SEM images, Cu-loaded zeolites have cubic crystal structure with sharp planer surfaces and well-defined edges. Therefore, it can be suggested that the morphology of zeolite A remain unchanged after the incorporation of copper. Since microwave heating was applied at low temperature, particle size is not much reduced while crystallization and crystals have a particle size distribution of 300 -600 nm.

Raman microscopic studies
The Raman microscopic data were collected to determine the bond association within the crystal structure of synthesized samples. Since Raman spectroscopy is complementary to FTIR, it is limited to probing molecules that are IR active. This is because if a bond is strongly polarized, vibrations will have only a small additional effect on polarization, thus exhibiting a weak response on Raman scattering (Juneau et al., 2019). Raman peaks in the spectral region of 380 -530 cm −1 reveal the vibration modes of T−O−T bonding, where T stands for Si or Al (Tsai et al., 2021). The bands between 900 -1100 cm −1 are corresponded to the T−O asymmetric stretching vibrations within the TO 4 tetrahedra (Yang et al., 2013). The three bands in the region 900 -1100 cm −1 are highly depended on the Si/Al ratio and when increasing the Si/Al ratio, band broadening and peak intensities are changed. As depicted in Figure 4, zeolite A shows a band at 1041 cm −1 along with weak shoulders at 975 and 1102 cm −1 assigned to T−O asymmetric stretching vibrations indicating that the Si/Al ratio equal to 1 (Dutta and Del Barco, 1988). Dominant bands in the region 250 -550 cm −1 originate from the T−O−T bending vibrations. The bands below 250 cm −1 are frequently associated with the rotational and translational modes of TO 4 tetrahedra, as well as the lattice vibrational modes (Yang et al., 2012). It has been reputed that the wavenumbers of Raman bands attributed to the T−O−T bending modes are inversely correlated to the tetrahedral ring size, as well as the Si/Al ratio is (Wopenka et al., 1998;Yu et al., 2001;Yang et al., 2013). Yu et al. (2001) proposed that the Raman bands at 470 -530, 370 -430, 290 -410 and 220 -280 cm −1 can be related to the bending modes of 4-, 5-, 6-, and 8-membered rings of aluminosilicate zeolites, respectively. As depicted in Figure 4, bands at 280, 330 and 490 cm −1 are assigned to be bending modes of 8-, 6-and 4-membered rings of aluminosilicate zeolites, respectively. The peak at 700 cm −1 is characteristic of the double ring present in the zeolite A framework (Dutta et al., 1992). That peak does not appear in Raman spectra of Cu-loaded zeolites and it can be due to distortion of double rings by copper ions. As shown in Figure 4, with the increase of Cu loading, peaks in the range 900 -1100 cm −1 disappear while a new peak appears at 619 cm −1 . This may be due to the weakening of the T−O bonds and the formation of Si−O−Cu bonds in the zeolite framework.

CONCLUSION
The Cu-zeolite was successfully synthesized using a novel microwave-assisted direct synthesis method. The SEM and PXRD results reveal that all the Cu-zeolite samples have formed LTA-type zeolite structures with a high degree of crystallinity. Moreover, according to the results obtained from FTIR spectroscopy and Raman microscopy, the formation of Si−O−Cu bonds and weakening of Si−O and Al−O bonds in the crystal structure are confirmed. Unlike conventional methods, modification of zeolite was done during this method at a low temperature of 110 ℃, and nano-sized, LTA type Cu-zeolite was successfully obtained. Hence, this method could be improved as a cost-effective direct synthesis method in industrial applications.