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versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. vol.56 no.3 Concepción 2011
J. Chil. Chem. Soc., 56, N° 3 (2011), págs.: 752-758.
MICROW AVE-ASSISTED SYNTHESIS OF LOW-SILICA/ALUMINA-RATIO ZEOLITES FROM GEOTHERMAL SILICA
BEATRIZ DE LEÓN*, PRÓCORO GAMERO, CECILIA MONTERO-OCAMPO, GREGORIO VARGAS, J. IVÁN ESCALANTE-GARCÍA, ARTURO I. MARTÍNEZ
Centro de Investigación y de Estudios Avanzados del IPN, Unidad Saltillo, Carretera Saltillo-Monterrey, Km 13, 25900, Ramos Arizpe, Coahuila, México. e-mail: firstname.lastname@example.org
Zeolites A, X, hydroxysodalite and Iiydroxycancrinite (ZA, ZX, ZS, and ZC, respectively) were obtained using geothermal silica (GS) as a raw material and microwaves (MW) as an energy source. GS is an industrial byproduct from the Cerro Prieto geothermal plant in Baja California, México. It is a potential raw material for zeolite manufacturing because of its chemical composition and its high reactivity given by its small particle size. First, GS was treated with NaOH to generate zeolite precursors; the resulting products were crystallized by MW using sodium alumínate to adjust the SiO2/Al2O3 ratio in the reaction mixture. An experimental design based on an orthogonal array was employed to study the effects of the main factors involved in the zeolitization process of GS. The obtained yields of zeolites ZC, ZA and ZS were 95, 97, and 76.6% respectively. These zeolites were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray fluorescence (XRF), and their calcium binding capacity (CBC) was determined. The obtained results were similar to those of commercial zeolites.
Keywords: microwaves, geothermal silica, zeolite, Taguchi orthogonal array.
The interaction of geothermal fluids with reservoir rock releases a siliceous material referred to as geothermal silica (GS). GS accumulates during the transit of steam through heat exchangers during the electricity production process in geothermal fields, or it precipitates from geothermal brine before being re-injected into the earth. Purified GS may contain more than 95 % silica1 and may include alkali, transition and rare earth elements 2. Cerro Prieto is one of the most important geothermal fields in Mexico. It is located in Baja California 3, where approximately 50,000 tons of GS 4 is generated annually. GS has been shown to exhibit a pozzolanic behavior in Portland cement 5. Wairakei GS has been used as a raw material in the synthesis of zeolites 6, 7. Because of the expansive use of zeolites in the chemical industry as adsorbents, cationic exchangers, and catalysts, among other important applications 8, the development of low-cost methods for their manufacture is important. Zeolites have traditionally been prepared using long reaction times and high temperatures 9-12. Cancrinite, for example, has been obtained at temperatures and crystallization times of approximately 200°C and 48 h 9. Additionally, chemical additives such as NaHCO3 9 ,Na2CO3 10 or NaNO3 11 are incorporated into the starting gel. Zeolite A has been prepared from chemical reagents by a hydrothermal process at 60-1000C for 3-24 h13. The replacement of chemical reagents by cheaper silica and alumina sources, such as industrial byproducts with no commercial value, has proven to be an attractive alternative for producing zeolites 14-19. Our research group has previously reported the use of other byproducts generated by carbo-electric plants (fly ash) as a raw material for zeolite production 20-22.
Microwaves are electromagnetic energy in the frequency interval between 300 and 300,000 MHz. The energy of microwave photons (0.037 kcal/mol) is low in comparison to the energy required for the breaking of molecular bonds (80-120 kcal/mol). Consequently, microwaves do not affect the structures of molecules 23.
Microwaves interact with materials by two different mechanisms: bipolar rotation and ionic conduction. In the case of bipolar rotation, molecules with permanent or induced dipoles align with the electric field. At 2450 MHz, the field oscillates such that the bipolar molecules do not align themselves completely, and heat is produced as a consequence. In the ionic conduction mechanism, the energy transference is attributed to the movement of ionic species in the solution as they try to follow the oscillating electric-field component of the microwaves. Highly polar compounds tend to warm quickly when irradiated with microwaves, whereas substances with low polarities or highly ordered crystalline structures absorb microwaves poorly. The increased kinetic energy and generated heat promote chemical reactions. Because the energy transferred from the microwaves is directly applied to the reactant species, reactions that take place in days with conventional heating are performed in minutes when irradiated with microwaves 23. Therefore, an additional alternative to reduce the costs of the GS zeolitization process is the replacement of expensive common heating systems with more efficient sources of energy, such as microwave generators.
The first patent to claim the use of microwaves in the synthesis of zeolites was issued in 1988 to Mobil for the synthesis of zeolites NaA and ZSM-5 24. Currently, the microwave syntheses of several zeolites have been disclosed and studied, but only at the laboratory level 25. Studies have shown that microwave energy increases the heating rate of the synthesis mixture and thereby increases the reaction rate 26. Additional investigations aimed at explaining the effects of microwave energy have led to devices that heat the reaction mixture more uniformly 27. Nonetheless, some studies have concluded that hot spots are created within the mixture 28,29. Other studies have shown that microwaves change the association between species within the reaction mixture 30,31 or that microwave energy enhances the dissolution of the precursor gel 32,33.
Microwave heating enhances the reaction rate and improves the selectivity toward desirable products during inorganic synthesis, the modification of zeolites and various catalytic processes 34-36. The dissolution of the starting raw materials in alkalis is enhanced during MW exposure, which is essential to induce the nucleation and further crystallization of zeolites 34, 37, 38. The synthesis of zeolite A has been performed by using a continuous MW process, reagent-grade chemicals 39 and kaolin 40. The dissolution of the starting raw materials in alkalis is essential for inducing the nucleation and further crystallization of zeolites 41-43.
No reports appear in the literature regarding the synthesis of high-added-value zeolites using GS as a starting material and MW as the heating source. Therefore, the present work is aimed at determining the best conditions for the low-cost synthesis of zeolites using GS as a material with no other commercial value and MW as a heating source. An orthogonal array proposed by Taguchi (OAT) was used to study the effects of the SiO2/Al2O3 molar ratio, NaOH concentration, time (tTT) and temperature (TTT) of the initial thermal treatment as well as the time (tc) and temperature of crystallization (Tc). The type and content of zeolite in the product served as the response parameters. The analysis of variance (ANOVA) and the study of the effect of each variable were performed to define the best conditions for the zeolite synthesis. The optimal conditions were calculated statistically and used to synthesize zeolites A, hydroxysodalite, hydroxycancrinite and X. The morphological, mineralogical and chemical properties of these zeolites are discussed.
2.1. Reagents and materials
The GS used in this study was obtained from the Cerro Prieto power plant (Baja California Norte, Mexico). Because of its origin, GS may contain NaCl, KCl, MgCl2 and CaCl2. The initial total chlorides content in the sample of GS was approximately 15%; this sample was cribbed at 88 ìçé and washed with deionized water using a water/GS ratio of 3:1 at 800C under stirring for 20 min to eliminate the salts. This procedure was repeated until the total chlorides content in the GS sample was 0.05%. Sodium aluminate (Na2OAl2O3 3H2O) and sodium hydroxide (NaOH) used were reagent grade (Spectrum) and deionized water was used. Calcium chloride 99% (JT Baker) was used for determining the calcium binding capacity (CBC), Ca concentration was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Commercial zeolites A and X in sodium from Chemical industries of the Ebro (Spain) were used for comparison.
2.2. Equipment and geothermal nanosilica zeolitization process
A mixture of GS and NaOH was homogenized for 15 min in a PM200 planetary mill (Retsch) at 350 rpm using agate vessels and balls. The resultant mixture was thermally treated in a microwave furnace (Phoenix Airwave, model 905400) using an output power of 1400 W at a frequency of 2.45 GHz. The product obtained was placed in a 300 mL Teflon™ reaction vessel. Zeolitization was performed in a microwave oven (CEM, MARSX) operated at 2.45 GHz with an output power ranging from 30 to 1800 W. To determine the zeolitic phases, the crystallization products were analyzed by XRD using a Phillips X' Pert 3040 equipped with Cu-Kal radiation source operated at 45 kV and 40 mA with a scan rate of 2° è min1. The morphology of the products was characterized by scanning electron microscopy (SEM, Phillips XL30) at 20 kV. Particle size was observed by transmission electronic microscopy (TEM, Jeol JEM 201) at an accelerating voltage of 200 kV. Chemical composition was determined by X-ray fluorescence (XRF, Bruker S4 Pionner) by the standardless method with a relative deviation of 0.1%. Loss on ignition (LOI) was determined at 950°C for 1 h using a furnace (Thermolyne F6200-60-80).
The OAT was used to define the main factors involved in the GS zeolitization process. Among the different options for optimizing the time and resources required for different experimental designs, the OAT method has proved to be a useful tool 43 because it allows the collection of reliable information with the smallest number of experiments. OAT is a simplified and standardized fractional factorial experiment design that enables the study of the effects of the main parameters and their contribution to the process. The experimental design was established in accordance with an experimental matrix L18 (21, 37) proposed by Taguchi 44. The method involves 18 experiments (L18), with one of the factors at two levels (21), and seven factors at three levels (37), of which five were investigated. The factor studied at two experimental levels (3 and 4) was the SiO2/Al2O3 molar ratio (S/A). The five additional factors (Tc, tc, tTT, TTT, and NaOH concentration) and their experimental levels are listed in Table 1.
The parameters were ordered according to the OAT. Table 2 shows, in columns 2-7, the experimental conditions of the trials established according to the AOT. These experiments were performed by mixing 12.3 g of GS with the corresponding amount of NaOH, and the mixture was thermally treated at temperatures TTT for times tTT, as shown in Table 2. The thermally treated material was slowly added under stirring to a sodium aluminate solution prepared with 191 mL of water at the corresponding SiO2/Al2O3 molar ratio. The water content of the sodium aluminate reagent was subtracted from the total water for zeolite synthesis. After an aging period of 17 h, the mixture was crystallized at the selected temperature, Tc ± 3°C, for time tc. After cooling, the product was filtered and washed with 1.5 L of deionized water until the effluent exhibited a pH of 10.5. The product was then dried at 105°C for 24 h.
2.3. Calcium binding capacity
The CBC was determined by exchanging the sodium on the zeolite surfaces with calcium ions. Approximately 10 mL of 0.01 M CaCl2 solution was mixed with 0.1 g of zeolite at room temperature. The suspension was stirred for 30 min; it was then separated from the liquid by centrifugation for 30 min at 850 rpm.
3. RESULTS AND DISCUSSION
3.1. Geothermal silica conditioning and characterization
The XRD pattern of GS before washing is shown in Fig. 1(a), and the pattern after washing is shown in Fig. 1(b). As evident in Fig. 1, the reflections of NaCl and KCl in the GS disappeared after the sample was washed (Fig. 1(b)). The particle size was observed using TEM (Fig. 1(c)). The GS was composed of particles with an average size of approximately 20 nm, and the particles exhibited a tendency to agglomerate. After the GS was washed, its chemical composition, as determined by XRF, was 91.07% SiO2, 0.31% Al2O3, 0.01% TiO2, 0.27% Fe2O3, 0.03% SO3, 0.05% Cl, 0.09% MgO, 0.29% K2O, 0.31% Na2O, 0.47% ZnO, and 0.59% CaO; 6.45% of the weight was lost on ignition.
3.2. Mineralogical composition of products
The zeolite content of the product served as the OAT response parameter for optimizing the synthesis. The percentage of the zeolites was estimated semi-quantitatively by computing the area under the reflection peaks of the XRD pattern in the 2è range of 11-40°. After elimination of the background, the total area was assumed to be 100%. The sum of the areas under the diffraction peaks that correspond to each phase was calculated. The amorphous phase was obtained by subtracting the area under the peaks from the total area. Small unidentified peaks were attributed to an amorphous phase. Table 2 shows the experimental conditions and the zeolite content of each trial, as determined from the area under the reflection peaks in the XRD pattern.
Zeolitic phases were obtained for all of the trials. The minimum conversion of GS was 60.3% (trial 7). ZS was obtained at the studied conditions in 16 trials, and ZA was obtained in 11 trials. Therefore, the selected conditions favored the crystallization of both ZS and ZA. In contrast, ZC was obtained in only seven experiments, high percentages were obtained at T = 180°C. ZX mixed with other products was detected at Tc=120°C and a SiO2ZAl2O3 ratio of 4. The optimal conditions to obtain a highest content of ZA, ZS, ZC and ZX were determined. The ANOVA and the study of the main effects of factors in the synthesis of these zeolites are presented in the following section.
3.3. ANOVA for zeolites: A, hydroxysodalite, hydroxycancrinite and X.
The purpose of the ANOVA was to identify the factors that were statistically significant with respect to the zeolitization process. Percentages of the zeolites obtained in the 18 experimental trials (Table 2) were statistically treated. The ANOVA shows the variability of the different factors. The contribution of each factor was measured by eliminating the effects of the other factors. The F values obtained using the ANOVA study helped to confirm the statistical significance. If the F test value is greater than the F theoretical value, then this variance is statistically significant for each of the factors. The F theoretical value was 5.14 for a confidence level of 95%. The residue was obtained by subtracting the degrees of freedom of the factors from the total freedom degrees (Table 3). The percentage of the contribution of each factor and the F values for each response are shown in Table 3.
According to the ANOVA results, the crystallization temperature is statistically significant (F > 5.14) in the synthesis of ZA, ZC and ZX. For ZS, both the crystallization temperature and time were statistically significant (F > 5.14). The contribution percentages of the crystallization temperature for obtaining zeolite were 68.44%, 66.54%, 42.08% and 27.74% for ZC, ZA, ZS, and ZX, respectively. Crystallization time exhibited a contribution of 47.94%, 16.45%, 7.88% and 5.97% for ZS, ZX, ZC and ZA, respectively. The NaOH concentration exhibited a contribution of 2.48%, 3.61%, 12.34% and 6.31% for ZA, ZS, ZC and ZX, respectively. The factors with the least influence on GS transformation were the temperature and time of the initial thermal treatment prior to crystallization. This finding indicates that GS could react with NaOH at a lower temperature than that used in this study. Further studies are required to define the lowest temperature and time needed to perform the GS alkaline pretreatment.
3.4. Effect of the synthesis parameters on zeolite content
The statistical treatment of the experimental data was performed following the procedure of Taguchi 44. The effect of the different parameters was taken into account to define the level of each factor that enabled the highest zeolite content to be obtained. Fig. 2 shows the effect of the main factors on the zeolite content; the plots present the effect of each parameter (x-axis) on the percentages of hydroxycancrinite, zeolite X, zeolite A, and hydroxysodalite (y-axis) according to the zeolite content results for the trials. Fig. 2(a) shows that the percentage of ZC is highest for the reaction conditions Tc= 180°C, tc= 240 min and NaOH = 0.45 eq. Fig. 2(b) indicates that the highest ZX content is obtained under the conditions for trial 1 (Table 2). The conditions for a high content of ZA are shown in Fig. 2(c): Tc = 120°C, SiO2/Al2O3 ratio = 3 and NaOH = 0.35 eq. The content of zeolite ZS is highest when Tc = 150°C and tc = 240 min, as shown in Fig. 2(d). The best conditions for the synthesis of ZC, ZX, ZA and ZS are listed in Table 4.
3.5. Synthesis of zeolites at statistically calculated conditions
ZC was prepared from a starting mixture of GS with NaOH = 0.45 eq and SiO2/Al2O3 ratio = 4. The crystallization process was performed using MW to reach and maintain 180°C for 240 min. After the product was washed and dried, 24.1 g of white powder was obtained. The XRD pattern (Fig. 3(a)) shows ZC as the predominant phase (95%) and the presence of an amorphous material. Cancrinite synthesis has also been reported using a conventional hydrothermal process with the addition of chemical reagents 9-11. When kaolin was used as a silica source for producing cancrinite, higher temperatures (2000C) and longer crystallization times (48 h) were required 9. However, the method developed in this study using an industrial byproduct enabled hydroxycancrinite to be obtained at 180°C after 4 h. ZX was prepared with 0.35 eq of NaOH, SiO2/Al2O3 ratio = 4, Tc = 1200C and tc = 60 min. As a result of GS zeolitization, 20.7 g of a white powder was obtained. The XRD pattern in Fig. 3(b) shows reflections of zeolite ZX (60%), ZA (27%) and an amorphous material (13%). Although ZX was obtained together with zeolite A, the procedure followed here is advantageous, given that the chemical reagents used to produce zeolite X 37 were replaced by GS. Previous reports have shown that zeolite X, when prepared from chemical reagents 45, 46, requires 3 to 6 h of heating, which is longer than the time required in the method described here (1 h). The optimal conditions for ZA synthesis were NaOH = 0.35 eq, SiO2/Al2O3 ratio = 3, Tc = 1200C, tc = 60 min, which yielded 20.3 g of a white powder. The XRD pattern in Fig. 3(c) confirms the presence of ZA as a crystalline phase (97%) along with an amorphous material (3%). The experimental run under the statistically calculated conditions to obtain ZS (NaOH = 0.55 eq, SiO2/Al2O3 ratio = 4, Tc = 1500C, tc = 240 min) yielded 19.8 g of a white powder. The XRD pattern in Fig. 3(d) reveals the presence of ZS (76.7%) as a crystalline phase. The method described here is an alternative to obtaining hydroxysodalite. Table 4 shows the optimal conditions for zeolite synthesis.
3.6. Morphological characteristics of the experimental zeolites
The morphologies and particle sizes of the zeolites synthesized under the conditions established in Table 5 were examined by SEM (Fig. 3 (a')-(d')). As shown in Fig. 3(a'), cancrinite is a homogeneous material composed of quasi-spheres with diameters of approximately 5 ìçé; these particles are constituted by intercrossed laminar needles. The morphology of these needles is attributed to the high heating rate resulting from the use of microwaves. Under these conditions, the temperature increases sharply and different nucleation points are formed. This finding is consistent with earlier reports that have demonstrated that crystal size and morphology depend on the heating rate, although they depend on crystallization temperature, the particle size, purity and solubility in alkaline solutions of the precursor species 47.
The mixture of X and A zeolites consists predominantly of cubic particles with diameters of approximately 1 ìçé (Fig. 3(b')). Many of these particles are heaped plates that probably formed by the high heating rate induced by the accelerated microwave heating process. Fig. 3(c') shows that the experimental zeolite A presents a morphology of small, well-defined cubic particles with an average size of approximately 1 ìôç. The small particle size is attributed to the short induction and crystallization time used in the synthesis. Small particle size observed here agrees with reports of the conventional synthesis of zeolite, in which a small particle size has been associated with accelerated heating and fast reagent dissolution. Under these conditions, the reagents are rapidly consumed, and many nuclei are formed. The process promotes the formation of small crystals in a short period of time 47. Hydroxysodalite is mainly composed of particles with diameters of approximately 2 ìéð. These zeolite particles are formed by many intercrossed discs (Fig.3 (d')). The differences in the zeolite morphologies should be taken into consideration for the development of selective catalysts and adsorbents.
3.7. Chemical composition and calcium binding capacity of the end products
The chemical compositions of the synthesized zeolites, determined by XRF as percentages of oxides, indicate that they are mainly composed of Al2O3, SiO2, Na2O and volatile materials, as shown in Table 5. Furthermore, CaO, K2O, Fe2O3 and ZnO were detected in smaller amounts, and MnO, SO3, TiO2 and ZrO2 were found in all the synthesized zeolites in percentages less than 0.1 weight %. Calcium carbonate was only detected in cancrinite, at a concentration of 0.68%. The concentrations of some elements in GS, such as Ca, K, Fe, Zn, and Cl, were decreased during zeolitization.
The resulting SiO2/Al2O3 molar ratios in the final zeolites were 2.2, 2.7, 2.2 and 2.2 for ZC, ZX, ZA and ZS, respectively. These molar ratios were lower than the SiO2/Al2O3 molar ratio in the starting gel compositions, which were 4, 4, 3, and 4 for ZC, ZX, ZA and ZS, respectively.
The CBC results show that ZX zeolite exhibits a CBC of 5.01 meq/g (87.6% crystalline phase), whereas A zeolite presents a CBC of 5.09 meq/g (97% crystalline phase). As a reference, the CBCs of commercial X and A zeolites were measured, and the zeolite percentages were calculated. The CBC and zeolite percentages were 5.58 meq/g and 99.3% for commercial zeolite X and 5.24 meq/g and 99.8% for zeolite A. The lowest values of CBC were obtained for HS and HC, which exhibited CBC values of 4.55 and 4.17 meq/g, respectively. The ion-exchange capacity is relevant when zeolites are used as adsorbents in water-treatment processes. The CBC depends on the sodium content of the zeolite. Thus, CBC serves as an indicator of the ability of other ions, such as heavy metals in wastewater, to replace the sodium ions. This study has defined the conditions for synthesizing some low-SiO2/Al2O3-ratio zeolites, starting from washed and thermally pretreated GS.
The zeolitization of GS using microwaves as an energy source constitutes an alternative method with economic and environmental advantages for obtaining the zeolites hydroxycancrinite, A, X and hydroxysodalite. The method described here required shorter times and lower temperatures for GS zeolitization compared with the conventional heating method. The Taguchi experimental design provided the optimum conditions for synthesizing these zeolites and for gaining an insight into the effect of the synthesis parameters. The synthesized X and A zeolites exhibited physicochemical properties similar
to and CBC values as high as the corresponding commercial zeolites. These properties indicate that the synthesis of zeolites from GS using microwaves is a good alternative for obtaining low-cost zeolites that are potentially useful as ion exchangers for water-treatment applications.
The research was funded by an institutional Cinvestav-multidisciplinary project and FOMIX-Coahuila 62158 project, Mexico. Thanks are due to Dr. Juan Manuel Cobo Rivera for his important contribution to this work. B. De Leon acknowledges the scholarship provided by the National Council of Science and Technology (CONACYT), Mexico.
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(Received: September 2, 2010 - Accepted: April 8, 2011).