The Effect of Chemical Activation Using Base Solution With Various Concentrations Towards Sarulla Natural Zeolite

Material characteristics analysis of Sarulla natural zeolite (SNZ) with base activation has been carried out. The base used was NaOH at various concentrations; 0.2 M, 0.5 M, and 2 M. Base activated catalysts were characterized by X-ray Fluorescence (XRF), X-ray diffractometer (XRD), Fourier-transform Infrared (FTIR) Spectroscopy, Scanning Electron Microscopy (SEM) and gas adsorption analysis using BET method. The procedures applied have caused differences in adsorption and desorption rates as well as the crystallinity values of the catalytic materials. Moreover, the ratio of Si/Al contents has also changed at the lowest concentration. On the other hand, the crystallinity of catalysts has decreased at the highest concentration of base applied. The results are reinforced in FTIR characterization which shows the changes of silanol bonds to become silicate and aluminate. Furthermore, morphological analysis of the catalysts shows that homogenous surface was obtained at low concentration, while rough/lumpy surfaces was obtained at higher concentration.


Introduction
Natural Zeolite (NZ) is usually formed as a secondary mineral in several geological activities in the Earth's crust. Indonesia has got many sources of natural zeolite which are widely spread across various regions such as Lampung (Elysabeth et al., 2019), Bayah (Hartono et al., 2019), Tasikmalaya (Dewi et al., 2016), Ende-Flores (Noelaka et al., 2018), Malang (Erlynata et al., 2014), Gunung Kidul (Hernawan et al., 2015) as well as in Sarulla -North Tapanuli (Sihombing et al., 2020). NZ is available in large quantities at low-cost exploitation and until today, it has not been fully utilized.
NZ shows interesting structural properties like a large specific surface, good flexibility, well-defined micropores in molecular dimensions as well as high chemical and physical stability (Zhong et al., 2019). Besides that, its cation exchange capacity selectivity and catalytic activity, especially of Bronsted Acid sites have allowed NZ to be widely used in various applications such as catalysts in oil industries, adsorbents in water treatment, production of cement, separation of gas and applications in biomedical (Gatta and Lee, 2014).
The utilization of NZ as catalysts is inseparable from activation processes, either physically or chemically. Zeolite chemical activation can be done by treatment with acid or base. In this study, SNZ was activated by using base. Lari et al. (2016) reported metal hydroxide of Group 1 in catalyst production by using a base, in which sodium hydroxide was identified to be the most suitable because it produces a stronger catalyst character, forms higher acid sites and minimizes damages in pores. This has led to superior catalytic performance. Rahmayani et al. (2020) has activated zeolite by using NaOH at concentration of 2 N, 4 N and 6 N to bind dissolved iron. The study reported that NaOH-activated zeolite can increase the amount of Fe attached to zeolite. This is shown by the changes of colors and morphology of the catalyst material analyzed by SEM. Sadowska et al. (2013) have done desilicalization of ZSM-5 zeolite by using NaOH and reported that all Al in the activated zeolite was able to form proton sites while some parts of them had gone through dihydroxylation during the thermal process to produce Lewis acid sites. Therefore, in this research, material characteristics of SNZ activated by NaOH at various concentrations of 0.2 M, 0.5 M and 2 M will be examined. The produced material is expected to have better catalytic properties which are going to be observed by using SEM, XRD, BET, FTIR and XRF.

Materials
The materials used in this study were natural zeolites from Sarulla village, Pahae Jae sub-district, North Sumatra, NaOH (p.a), aquadest and aquabidest purchased from CV Bratachem Indonesia, also nitrogen gas from PT Aneka Gas.

Methods
First NZ is crushed and sieved to obtain 100 mesh size. Then it is soaked in aquadest for 24 hours at room temperature. Next zeolite is filtered and the precipitate is dried at 110 o C then calcined until active natural zeolite (ANZ) is obtained. Desilicalization is done next by using 100 ml NaOH 0.2 M, 0.5 M and 2 M is done subsequently with the same working procedure. ANZ is refluxed at 90 o C for 2 hours, and then it is filtered and washed by aquadest until neutral pH is reached. Next, the sample was dried and calcined for 2 hours at 500 o C along with nitrogen flow until the zeolites was activated by base (ZB). ZB 0.2 M, ZB 0.5 M and ZB 2 M were obtained. The catalyst is characterized by X-ray Fluorescence (XRF) to determine the chemical composition and X-ray diffractometer (XRD Shimadzu 6100) to determine the type and crystallinity by using Cu Kα radiation at 40 kV and 30 mA. Functional group analysis was done by using FTIR (FT-IR Shimadzu type 8201-FC), while morphological analysis was done by using SEM type Zeiss EPOMH 10Zss. The surface areas, pore volumes and diameters are analyzed by gas sorption analyzer (NOVA 1200 e) with BET method.

XRD Analysis
NZ obtained from Sarulla village is a mordenite-type zeolite reported by Sihombing et al., (2018). The changes of 2θ degree intensity are able to provide observations on the influence of base (NaOH) activation and calcination towards crystallinity, as shown in Figure 1. The level of NZ crystallinity is known from the high and low changes in peak intensity, by using crystallinity degree calculation based on Kaelble (1967). The result of the calculation is shown in Table 1.  Table 1, catalyst ZB 0.2 M shows higher crystallinity than ANZ, ZB 0.5 M and ZB 2 M. The higher the intensity, the higher the crystallinity is possessed by zeolite. However, the use of NaOH with greater concentrations (0.5 M and 2 M) shows a decrease in the intensity of the main peak of zeolite compared to the use of 0.1 M NaOH and ANZ. The decrease in peak intensity can be seen at 2θ of 21.06 o and 23.65 o . The decrease in peak intensity and the shift of the 2θ angle indicate the changes in catalyst structure. This shows that the higher the use of NaOH base, the more Si can dissolve, causing changes in the zeolite structure. Desilicalization process at light (0.1 M and 0.2 M) conditions do not change the crystallinity and are able to maintain zeolite structures. Meanwhile, a higher concentration than 0.4 M will cause a reduction in crystallinity by 65% (Sadowska et al., 2013).
Desilicalization process causes Al to be extracted in a relatively small amount (4 -5%) with NaOH 0.1 and 0.2 M treatment. When zeolite is treated with 0.5 M concentration, Al loss can be up to 73% (Sadowska et al., 2013). This is consistent with XRD data that shows a decrease in crystallinity of the structure treated with a 0.5 M base.

XRF Analysis
Both chemical and physical activations change the ratio of Si/Al of zeolites as shown in Table 2. From Table 2, desilicalization process is known to increase the Si/Al ratio to be 1.8%. However, at a high concentration, there is a decrease in ratio to be 1.7%. Zeolites which were treated with NaOH contain more aluminium compared to other elements. These results are in accordance with previous studies (Wei et al., 2011;Rownaghi et al., 2011). The desiliconization leads predominantly to the removal of silicon from the zeolite body. Figure 2 shows the asymmetric stretch of TO 4 functional group that appears at 900-1250 cm -1 wavenumber (Rahmayani et al., 2020). Asymmetric stretch in ANZ appears at 1042.35 cm -1 wavenumber. While there are shifts to shorter wavenumber in ZB 0.2 M, ZB 0.5 M and ZB 2 M to 1041.22 cm -1 , 1038.76 cm -1 and 1039.01 cm -1 respectively with decreases in intensity. These results align with (Park andKang., 2008 andPanagiotopoulou et al., 2007) that stated shifts occur due to activation treatment with the base which has reorganized Si-O-Si and Al-O-Si bonds to form silicate and aluminate (Palomo et al., 1999). As a result, zeolite structures become more negative. Base activation is not only caused by silanol structures presented in zeolites, but also by the changes in quartz structures such as silica mineral. This is shown by the appearance of 720-650 cm -1 wavenumber in the catalysts.  Table 3 shows that ZB 0.2 M, ZB 0.5 M and ZB 2 M have got smaller surface areas than ANZ. Catalysts ANZ, ZB 0.2 M, ZB 0.5 M and ZB 2 M give the average pore radii of 1.7221 nm, 1.5989 nm, 1.5997 nm and 1.6038 nm respectively. The data illustrates that at the initial state, ANZ had a bigger pore diameter, but after the activation and calcination processes, changes took place and caused the pores of ZB 0.2 M, ZB 0.5 M and ZB 2 M to be tighter. From the results, the pore sizes of the catalysts are micropores and based on the classification from IUPAC (International Union of Pure and Applied Chemistry) they are categorized into Type 1. The size of micropore materials is less than 2 nm (Kanellopoulus, 2011). Adsorption-desorption isotherm graph of ANZ, ZB 0.2 M, ZB 0.5 M and ZB 2 M can be seen in Figure 3. Based on adsorption-desorption isotherm in Figure 3, the pattern in the diagram is categorized into Type IV with the presence of hysteresis loop and high rises in the graph at P/Po (Sihombing et al., 2020;Sihombing et al., 2018;Hasanuddin and Rahmat 2015;Fathi et al., 2014). Moreover, the graph shows that the rate of adsorption is not the same as the desorption rate.  In figure 3, the adsorption volume of ANZ is greater than ZB 0.2 M, ZB 0.5 M and ZB 2 M. This data correlates to the previous data, that ANZ has a greater surface area and pore diameter than others.

Surface Morphology
SEM micrograph photos of ANZ, ZB 0.2 M, ZB 0.5 M and ZB 2 M are shown in Figure 4. Figure 4(a) shows that the surface morphology of ANZ is heterogeneous with impurities covering the pores. Figure 4(b) presents a more homogenous and equal morphology. While 4(c) and (d) show heterogeneous surfaces with a tendency to clot and form lumps. Pa and Chik (2018) reported that SEM micrograph of zeolites with 2 M NaOH was observed to have the tendency to be lumpy and rough and it follows the SEM photo of mordenite (Yuan et al., 2007). The same thing has been reported by Fathi et al., (2014), stating that the desiliconization process shows a slightly coarser surface morphology, and is in accordance with the adsorption-desorption graph. This has also been reported by Schmidt et al. (2013).