Introduction

Zeolites are hydrated aluminosilicates which have a crystalline structure formed by the combination of silica (SiO4)4− and alumina (AlO4)5− tetrahedra joined by oxygen atoms (Klunk et al. 2019a; Lu et al. 2019; Auerbach 2003). For each aluminium atom contained in the structure, an excess negative charge is generated, producing an electrical imbalance (Cherkasov et al. 2016; Matsunaga et al. 2012). Alkali and alkaline Earth act as stabilizers, a striking feature of clay minerals (Klunk et al. 2019b; Elgamouz and Tijani 2018).

The mordenite zeolite (MOR) has 12-ring pores about 6.5 × 7.0 Å in the crystallographic direction of [001] (Ma et al. 2016; Zang et al. 2008; Wahono et al. 2019). They are connected by small pores 8-ring along the direction [010]. MOR belongs to the orthorhombic crystalline system consisting of three mutually perpendicular crystallographic axes with different lengths (a = 18.1, b = 20.5, and c = 7.5 Å). This is an important feature, because the diffusion process is slow compared to the other two- or three-dimensional molecular sieves (13 × and 4A) (Fischer et al. 2018; Garshasbi et al. 2017; Panda and Kumar 2017; Zhou et al. 2015). This behaviour is intensified when the adsorbate molecules are approximately the same size as the pore diameter (Klunk et al. 2019c; 2020). As a result, a small fraction of specific molecules can be adsorbed into the cavities of MOR, giving it selectivity, a fundamental feature of molecular sieves (Ma et al. 2016; Nishiyama et al. 1996).

Zeolites can be used for carbon dioxide (CO2) adsorption to mitigate the environmental impacts caused by greenhouse gas emissions (Santos et al. 2018; Klunk et al. 2019d; Cataluña et al. 2017, 2018; Caetano et al. 2015a). The wide application of zeolites for CO2 adsorption is enabled due to larger pore cavities compared to the molecular diameter of carbon dioxide (Klunk et al. 2019e; Caetano et al. 2015b, c, 2018). Gas separation in zeolite structure depends on three factors: crystal lattice structure and composition, cationic forms, and zeolite purity (Singh et al. 2018; Hernández-Huesca et al. 1999).

To achieve this purpose, this work aims to synthesize MOR in high purity and crystallinity, in the absence of an organic template. To obtain MOR, rice husk ash (RHA) (silicon source) was used, and for the aluminium source, Metakaolin (MK) (Al2O3.2SiO2) was used (Jun Zhang et al. 2018; Zhang et al. 2007). In general, standard synthesis of zeolitic material involves a mineralizing agent (sodium hydroxide) and an organic template (tetrapropylammonium hydroxide) (Hamidzadeh et al. 2018; He et al. 2013; De la Iglesia et al. 2006; Casado 2003). The high cost of manufacturing the zeolitic material is associated with the organic template as they are high-value reagents (Klunk et al. 2019c). Given all these problems, many researchers have sought to develop a methodology for synthesizing MOR in the absence organic template (Radman et al. 2019; Singh et al. 2018; Idris et al. 2018; Zhang et al. 2015; Aly et al. 2012; Huang et al. 2012; Cheng 2008; Kalipçiliar 2007).

To develop efficient adsorbents, MOR was synthesized with different Si/Al ratios (SAR) of 5, 10, 15, and 20. Thus, this work sought to deepen the understanding of the adsorption process and which parameters influence the synthesis of zeolitic material.

Materials and methods

The materials and methodology applied in this work consisted of characterizing the raw materials (RHA and MK) as well as the zeolitic material formed.

Synthesis of mordenite

Mordenite synthesis in absence organic template was performed according to Costa and Araujo (2010) with some modifications. For this purpose, RHA (silicon source) from a thermoelectric power plant located in the state of Rio Grande do Sul-Brazil was used. This company uses biomass for power generation under the method “Moving Grade Reactor” (Fernandes et al. 2016, 2017a, 2018; Moraes et al. 2014). Therefore, MK (Al2O3.2SiO2) was used as an aluminium source. This feedstock is related to the passage from the hydrated state of kaolin (Al2O3.2SiO2.2H2O) to dehydrate through dehydroxylation of the kaolinite molecule (Al2Si2O5(OH)4) through calcination at controlled temperatures (Gardolinski et al. 2003).

The contribution of metakaolin plays an important role in the adjustment of the effective charges in the crystal lattice of the zeolitic material. According to Table 1, we can identify that the major compounds are SiO2 and Al2O3. Unlike RHA, which has 90% SiO2, metakaolin enters the mordenite synthesis not to aggregate SiO2 to the structure, but to compensate for the alumina (AlO4)5− deficit in the molecular sieve tetrahedral lamellae. Another important point of MK is the contribution of SiO2 in zeolite formation, even if less significant compared to Al2O3.

Table 1 Chemical analysis of the compounds present in the RHA (Fernandes et al. 2016) and MK (wt%) by XRF
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The characteristics of the two raw materials, precursors of the zeolitic material, are shown in Table 1. Potential applications of RHA and MK were defined by the chemical composition, which was determined by X-ray fluorescence. The present compounds are derived from the inorganic fraction present in RHA and MK, so the results are presented as more stable binary oxides (Fernandes et al. 2017b).

The synthesis methodology consists of reactions under alkaline conditions (NaOH—3.5 mol/L). The reaction system is divided into two equal parts of the same volume (500 mL of NaOH). The first part is used to dissolve the RHA and the second part to dissolve the MK according to the mass in grams of each raw material to obtain the Si/Al ratios (Table 2). The two solutions were then carefully mixed and left at room temperature to allow the transformation of the sol–gel process. The gel was then oven-dried at 90 °C for 24 h. The dried solid was ground and subsequently calcined at 550 °C for 6 h.

Table 2 Quantities of raw material and molar composition used for the synthesis of mordenite in absence organic template for the different Si/Al ratios
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Characterization of materials

The characterization of the zeolitic material is fundamental for understanding the adsorption process. Thus, the techniques used in this work are: (1) specific surface area (BET), (2) chemical composition (XRF), (3) mineral composition (XRD), (4) identification of specific functional groups (FTIR), (5) SEM/EDS was utilized in the present work to complement the other characterization techniques, (6) thermal analysis in which the variation of sample mass (loss or gain) is determined as a function of temperature and/or time (TGA), and (7) cation-exchange capacity (CEC). To evaluate CEC, the methodology of Shinzato et al. 2008 with some modifications was applied.

The cation-exchange capacity (CEC) was performed as follows:

  • (1) 1.0 g of sample with 100 mL of one molar sodium acetate solution (CH3COONa.3H2O).

  • (2) Stirring at room temperature for 24 h at 120 rpm.

  • (3) Filter the sample, and the analyte retained on the filter, should be washed with 1.0 L of deionized water, and oven-dried at 80 °C for 2 h.

  • (4) In the dry sample, stir at room temperature for 24 h at 120 rpm with 100 mL of one molar ammonium acetate solution (CH3COONH4).

  • (5) Filter the suspension and the Na+ ions and concentration determined by the inductively coupled plasma optical emission spectrometry (ICP-OES) technique. The amount of sodium ion was expressed in mg/L (ppm) and converted to the usual milli-equivalent of CEC units (meq/g).

Results and discussion

Characterization of mordenite

Table 3 shows the amounts in grams of unreacted material after alkaline treatment as well as the yield of mordenite synthesis.

Table 3 Gel quantity and leftover grams of Si and Al forming impurities as well as the yield of the reactions
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We can observe that for all samples, the reaction system yield is above 70%. The residues found in the reactions come from the sol–gel system filtration step. The remaining (unreacted) precursor materials composed of SiO2 (RHA) and Al2O3.2SiO2.2H2O (MK) obstruct the filtration system making it difficult to separate sol–gel.

Measurements of the surface area (BET), volume, and average pore diameter of the zeolitic material are listed in Table 4. For low and intermediate silicon ratios, between 5 and 10, the BET ranged from 314 to 328 m2/g, with an average pore diameter of 5.9 Å. Therefore, for high Si/Al ratios, between 15 and 20, the surface area is in the range of 339–347 and an average pore diameter between 6.0 and 6.2 Å. The BET results found in this study are in agreement with the works of Zhang et al. (2011), Mignoni et al. (2007), Dimitrova et al. (2006), and Hincapie et al. (2004) that use the synthesis of mordenite by the traditional method.

Table 4 Textural properties of mordenite zeolites in different SARs
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Synthesized mordenite has, in all Si/Al ratios, pore volume identified in the micro-range (less than 2 nm) according to Meier and Olson (1992).

Increased SAR results in an 11% increase in surface area (Table 4). This increase is related to the formation of a crystal lattice made up of Si and Al tetrahedra. The results indicated that as Si and Al are incorporated into the crystal lattice, the micropore volume of mordenite increases, which leads to a larger pore volume. Another factor contributing to the increase in surface area is the fact that adsorption of nitrogen molecules does not occur uniformly in a monolayer, but in the form of irregular layers, where the monolayer is often not fully filled until another layer begins to form, which, consequently, changes the value of the specific surface area.

After obtaining the zeolitic material, the micropore volume has increased from 0.198 to 0.279 cm3/g. The size of the adsorbed molecules plays an important role in their adsorption on zeolite. Zeolite cavities are a selective factor for adsorbed molecules. Mordenite can adsorb molecules that have a maximum diameter of 0.4 nm, indicating that CO2 adsorption (diameter = 0.33 nm) is not limited by this steric factor (Bonenfant et al. 2008; Aguilar-Armenta et al. 2001; Hernández-Huesca et al. 1999). In addition, the time required to reach the middle CO2 adsorption capacity is proportional to the diameter of the adsorbed molecules (Dunne et al. 1996; Hayhurst 1980). This behaviour is similar when other forms of adsorbents such as non-mineralizable coals are used (Klunk et al. 2018). Figure 1 shows the nitrogen adsorption–desorption isotherms for mordenite zeolite samples in the different SARs. The results indicate a slight increase in relative pressure (p/p0) in the ranges of 0.01 and 0.97.

Fig. 1
figure 1

N2 adsorption and desorption isotherms of mordenite

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For low relative pressure, this behaviour can be interpreted by the fact that zeolites are filling the micropores of their structure, which means the presence of microporous cavities. Therefore, for high pressures, rapid absorption occurs indicating the existence of empty cavities resulting from the crystalline structure of the zeolitic material. In addition, the formed zeolites show a hysteresis loop at relative pressures (p/p0) between 0.42 and 0.96 caused by mesoporous channels. The obtained isotherms demonstrated that all samples exhibit the typical type IV adsorption isotherm with the H3 hysteresis loop, as identified by the IUPAC.

According to Table 5, there was a wide variation between the contents of the main components present in zeolites formed with different SARs.

Table 5 Chemical analysis of the compounds present in the MOR (% by mass)
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For 5-MOR zeolite, X-ray fluorescence reveals that SAR is 4.64. In sample 10-MOR, the Si/Al ratio is 10.32. Thus, in zeolitic materials 15-MOR and 20-MOR, the SAR are 15.47 and 21.96, respectively. Due to experimental errors, reagent impurity, and instrumental inaccuracy, the exact SARs of 5, 10, 15, and 20 could not be obtained by synthesis. The zeolites formed presented, in general, high contents of silica, alumina, ferric oxide, and sodium oxide that favour the formation of mordenite. Calcium oxide, titanium, magnesium, sulfur, potassium, and other compounds were also found in amounts of less than 5%.

The low sulfur content and high amounts of calcium enhance the use of these materials. Sodium content is related to its incorporation into the zeolitic material by hydrothermal synthesis, where metakaolin was used as an external aluminium source and activating agent, respectively. Exposure of the material to solutions with higher NaOH concentrations during synthesis increased the amount of sodium in the zeolite structure (Scott et al. 2001).

To identify the formation of the crystal structure of MOR in the absence of organic template, they were submitted to XRD analysis. The diffractograms obtained are shown in Fig. 2. The results of XRD were based on the International Union Database Database’s Joint Committee on Powder Diffraction Standards (JCPDS) (Scapin 2003; Atkins and Jones 2001). The peaks between 2θ of 5° and 30° correspond to mordenite in all SARs. Thus, mordenite is favoured by the addition of metakaolin due to the high amounts of calcium and potassium elements that are part of its crystal lattice. The chemical formulas of the crystalline phases of the zeolitic material are shown in Table 6.

Fig. 2
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Diffractogram of mordenite zeolites with different Si/Al ratios: a 5, b 10, c 15, and d 20

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Table 6 Crystalline structures and chemical formula of zeolitic compounds
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The diffractograms reveal the formation of kaolinite, mullite, and quartz between 2θ of 31° and 50° (peaks with low intensity). These results are in accordance with research conducted by Mignoni et al. (2007) and Shao et al. (2001) who synthesized mordenite with external sources of aluminium. The occurrence of kaolinite and mullite is related to the reactions that occur between SiO2 (RHA) and Al2O3.2SiO2 (metakaolin).

In general, quartz cannot be dissolved by hydrothermal processes and remain in the zeolitic material. These compounds were found in varying amounts in all samples. Aluminium is an element that constitutes the crystalline network of zeolites, and therefore, its quantities directly influence the formation of adsorbents. It is noteworthy that the identification of the crystalline phases in the samples with the addition of metakaolin served to obtain the various Si/Al ratios.

The results, compared with literature data, prove that the molar ratios used and the synthesis conditions favour the formation of the zeolitic material (Hisham et al. 2012; Xianfeng et al. 2009; Zhang et a. 2002).

Fourier transform infrared spectroscopy (FTIR) of the 5-MOR, 10-MOR, 15-MOR, and 20-MOR samples are in Fig. 3. All samples exhibit the same profile in the FTIR spectra. In this way, the samples reveal typical mordenite vibrational bands (Ivanova and Knyazeva 2013).

Fig. 3
figure 3

Infrared spectrum for mordenite zeolites without different Si/Al ratios and without the use of organic guidance: a 5-MOR, b 10-MOR, c 15-MOR, and d 20-MOR

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The mordenite FTIR spectra exhibit two vibrational peaks of 3744 cm−1 and 3605 cm−1 associated with the silanol end-groups (Si–O–H and Si–OH–Al). In addition, the band observed around 3655 cm−1 corresponds to aluminium vibrations (AlOH–). The H–OH flexion bands were observed at 1636 cm−1 and 1230 cm−1, and are attributed to the –OH vibration. Vibrational stretches were found at 1040 cm−1 related to the Si–O group.

The wavelengths of 800 cm−1, 550 cm−1, and 468 cm−1 are related to the vibrations of Al–O–Al, Si–O–Si, and internal asymmetric stretching of Al–O and Si–O (Bevilacqua and Busca 2002).

Fig. 4a and b shows the SEM/EDS demonstrations from mordenite zeolite (sample 20-MOR). This sample has a typical appearance characterized by the presence of irregular-shaped angular particles (RHA burning takes place in the pulverized form) of varying sizes (Fig. 4a). The combustion conditions of RHAs in the thermoelectric power plants determine the morphology of the particles.

Fig. 4
figure 4

Scanning electron micrographs (a) and energy-dispersive spectroscopy (b) of mordenite zeolites formed from RHA and MK

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According to the EDS technique, the presence of the chemical elements Ca, Na, K, Al, Si, and O2 acts as evidence for the formation of zeolite called mordenite.

Thermogravimetric tests were performed to identify the mass loss regions for the zeolitic material formed. Thermogravimetric curves are shown in Fig. 5. These curves show a region of marked mass loss from 100 °C to approximately 300 °C. This loss comprises the desorption of water not bound to the structure and because they do not have organic material. The samples obtained in the absence of organic template had two less mass loss steps when compared to the studies by Frantz (2015), Costa (2013), and Caldeira (2011) that used organic template. Mass loss is higher for samples with lower Si/Al ratio. However, all samples stabilize mass losses around 400 °C.

Fig. 5
figure 5

TGA plots of mordenite in SAR 5, 10, 15, and 20 in absence of organic template

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The results of the cation-exchange capacity measurements using ICP-OES of the zeolitic material are shown in Table 7.

Table 7 CEC for mordenite zeolites according to Si/Al ratio
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Mean CEC values ranged from 1.10 to 1.78 meq/g. There was a 62% increase in CEC of 20-MOR zeolite compared to 5-MOR in the zeolitization process. This was due to the high surface area (about 11% greater than 5-MOR compared to 20-MOR) associated with the micropore volume which increased 41%. 20-MOR zeolites are easier to exchange NH4+ when compared to 5-MOR. The CEC values found for all MOR zeolites, synthesized without an organic director, indicate that these materials have a high potential for use as ion exchangers.

Conclusions

Synthesized mordenite from external sources of Si (RHA) and Al (MK) in absence organic template resulted in a high crystallinity zeolitic material, proven by XRD with 70% of the yield. Zeolite characterization revealed an 11% increase in surface area when compared to 5-MOR with 20-MOR. As a consequence, the N2 adsorption/desorption isotherms obtained for mordenite presented type IV adsorption isotherm with the H3 hysteresis loop of microporous materials. The hysteresis curves revealed that they can be reused without loss in adsorption capacity. The chemical composition of Si/Al ratios in the formed zeolites 5-MOR (4.64), 10-MOR (10.32), 15-MOR (15.47), and 20-MOR (21.96) proved the success of the synthesis. These transformations were due to the efficient alkaline treatment that provided the formation of zeolitic material. Fourier transform infrared spectroscopy reinforces the success of mordenite synthesis by the vibration bands corresponding to Al–O–Al and Si–O–Si stretches, and coupling vibration Al–O and Si–O typical of zeolitic material. The SEM demonstrations in the presence of irregular-shaped angular particles from RHA burning takes place in the pulverized form), and according to EDS technique, the presence of the chemical elements Ca, Na, K, Al, Si, and O2 acts as evidence for the formation of zeolite called mordenite. The results of thermogravimetric tests reveal water desorption and absence of organic director with high mass loss in the range of 100–300 °C, acquiring stability after 400 °C. Thus, it can be concluded that the higher the Si/Al ratio in the samples, the higher the thermal stability. The 20-MOR cation-exchange capacity was 62% higher compared to the low Si/Al ratio. This result is satisfactory and promising for the use of MOR as adsorbent material.