Zeolite Products Research Articles

Synthesis and characterization of NaX-type zeolites prepared by different silica and alumina sources and their CO2 adsorption properties

Abstract In this work, two pairs of silicon and aluminium sources (sodium silicate solution + Al(OH)3 and sodium silicate powder + sodium aluminate) were used for synthesizing zeolites which were labeled as Z1 and Z2, respectively. The results from XRD, BET and surface analysis indicated that sodium silicate and sodium aluminate sources yielded zeolite with high surface area and pore volume. Cetyltrimethylammonium bromide (CTAB) and heptane were used to modify the CO2 adsorption capacity and selectivity of zeolite NaX. The surface area and pore volume of zeolite products increased due to smaller crystalline size and then resulted in significant enhancement of CO2 uptake of the synthesized zeolite NaX. The role of additives on zeolite formation was to increase the nuclei formation rate and number which gave rise to smaller zeolite crystals. The best adsorbent exhibited high CO2 adsorption capacity (5.08 mmol g−1 at 303 K) and relatively high CO2 selectivity over N2 (116) which indicated the potential of adsorbent to separate CO2 from flue gas application.

Utilization of rice husk and waste aluminum cans for the synthesis of some nanosized zeolite, zeolite/zeolite, and geopolymer/zeolite products for the efficient removal of Co(II), Cu(II), and Zn(II) ions from aqueous media

In this paper, rice husk and waste aluminum cans were exploited as silicon and aluminum sources, respectively for the low-cost synthesis of some nanosized zeolite, zeolite/zeolite, and geopolymer/zeolite products. XRD confirmed that the synthesized geopolymer/zeolite products are geopolymer/zeolite A (has a crystallite size of 58.44 nm & abbreviated as G1) and geopolymer/faujasite (has a crystallite size of 25.58 and 20.26 nm & abbreviated as G2 and G3, respectively). Also, the synthesized zeolite products are sodium aluminum silicate hydrate (has a crystallite size of 27.65 and 41.85 nm & abbreviated as H1 and H2, respectively). Besides, the synthesized zeolite/zeolite product is sodium aluminum silicate hydrate/zeolite A (has a crystallite size of 66.01 nm and abbreviated as H3). Moreover, the synthesized products were characterized using other tools such as HR-TEM, FE-SEM, EDX, and FT-IR. The synthesized products were efficiently applied for removing Co(II), Cu(II), and Zn(II) ions from aqueous media and wastewater which was taken from Abuzaabal- Qalyubiyah-Egypt. The maximum uptake capacity of G3 sample toward Co(II), Cu(II), and Zn(II) ions is 134.24 ± 1.26, 126.26 ± 0.32, and 131.93 ± 0.87 mg/g, respectively. The uptake of the studied metal ions is spontaneous, chemical, exothermic, and fitted well with the Langmuir isotherm and pseudo-2nd-order kinetic model.

CO2 sequestration and simultaneous zeolite production by carbonation of coal fly ash: Impact on the trapping of toxic elements

Coal-fired power plants are main contributors to atmospheric CO2 emissions. They also produce huge amounts of coal fly ash (CFA) waste, which is typically landfilled, posing significant environmental risks due to its high content of potentially toxic elements (PTE). However, CFA is an alkaline aluminosilicate-rich waste, which offers the possibility of CO2 mineral capture and the production of economically-relevant mineral by-products such as zeolites. Yet, the combined carbonation and zeolite production from CFA resulting in PTE trapping has never been explored. Here we show that under mild hydrothermal conditions (150 °C) and depending of various process parameters such as pH and background alkali metal ion in (bi)carbonate solutions, a carbonation efficiency of up to 79 %, with a net CO2 mineral capture of 0.045 g/g CFA can be achieved, even when using a low Ca and Mg (3.72 wt% CaO, 1.74 wt% MgO) Class F fly ash. Moreover, amorphous zeolitic precursors and different crystalline zeolites (yield up to 60 wt%) are simultaneously obtained, and PTE in CFA are effectively trapped into the newly formed calcite, zeolitic precursors, and zeolite phases. This is the first time that a combined study of carbonation, zeolitization and PTE trapping in newly formed phases has been developed. These results have important implications for carbon capture and storage, as well as for the safe reutilization and disposal of CFA waste.

Preparation and evaluation of zeolites for ammonium removal from municipal wastewater through ion exchange process

The application of ion exchange process for ammonium (NH4+-N) removal from wastewater is limited due to the lack of suppliers of engineered zeolites which present high ammonium exchange capacity (AEC) and mechanical strength. This study focuses on the preparation and evaluation of synthetic zeolites (Zeolite1-6) by measuring AEC and resistance to attrition and compression, against natural (clinoptilolite) and engineered zeolite (reference, Zeolite-N). At high NH4+-N concentrations, Zeolite6 and Zeolite2 showed capacities of 4.7 and 4.5 meq NH4+-N/g media, respectively. In secondary effluent wastewater (initial NH4+-N of 0.7 meq NH4+-N/L), Zeolite1, 2 and 6 showed an AEC of 0.05 meq NH4+-N/g media, similar to Zeolite-N (0.06 meq NH4+-N /g media). Among the synthetic zeolites, Zeolite3 and 6 showed higher resistance to attrition (disintegration rate = 2.7, 4.1 NTU/h, respectively) when compared with Zeolite-N (disintegration rate = 13.2 NTU/h). Zeolite4 and 6 showed higher resistance to compression (11 N and 6 N, respectively). Due its properties, Zeolite6 was further tested in an ion exchange demonstration scale plant treating secondary effluent from a municipal wastewater treatment plant. However, Zeolite6 disintegrated after 2 months of operation, whilst Zeolite-N remained stable for 1.5 year. This highlighted the importance of the zeolite’s mechanical strength for successful application. In particular, future work should focus on the optimization of the zeolite production process (temperature, time and dimension of the kiln during calcination) to obtain an engineered zeolite with a spherical shape thus reducing eventual sharp edges which can affect mechanical strength.

Hydrothermal synthesis of zeolites from green container glass

Landfilling and stockpiling unrecycled colored container glass represents a considerable failure in sustainability with respect to the conservation of energy and mineral resources. In this study, the single-step hydrothermal synthesis of low-silica zeolites from a mixture of waste green container glass and aluminum foil (Al:Si = 1) in 4 M NaOH(aq) at 125 °C was followed at 1, 3, 7 and 14 days. The principal phases, sodalite and cancrinite, appeared within 1 day accompanied by minor quantities of hydrogarnet and tobermorite arising from a stoichiometric excess of calcium ions in the parent glass. Products of 63, 67, 71 and 72% crystallinity were obtained at 1, 3, 7 and 14 days, respectively, with partial successive conversion of sodalite to cancrinite over time. Ion-exchange and catalytic applications of sodalite and cancrinite arise from the high anionic charge of the 1:1 ratio of alternating SiO44- and AlO45- units within their aluminosilicate frameworks. In this respect, the uptake capacity of the 14-day zeolitic product for Cu2+ and Cd2+ ions (1.58 meq g-1 and 1.66 meq g-1, respectively) was within the expected range for zeolites and compared favorably with those reported for other inorganic sorbents derived from industrial and municipal wastes. The 14-day product was also found to be an effective basic heterogeneous catalyst for the Knoevenagel condensation reaction.

Formation of EMT/FAU intergrowth and nanosized SOD zeolites from synthesis gel of zeolite NaX containing ethanol

Ethanol can serve as an organic additive in zeolite synthesis due to the ease of availability and simple removal. It can influence the crystallization leading to zeolites with different phases and morphology. This study explores the effect of partial displacement of water in the synthesis gel of zeolite NaX by various amounts of ethanol. With one-pot synthesis, the gels with different ethanol/water molar ratios are crystallized 90 °C for 18 h under a static condition. The products are characterized by several techniques including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning and transmission electron microscopy (SEM and TEM), nitrogen sorption analysis, inductively coupled plasma-optical emission spectrometry (ICP-OES), and thermogravimetric analysis (TGA). The ethanol/water molar ratio of 0.045 produces EMT/FAU intergrowth with a hollow structure and undefined shapes. The ratios of 0.412 and 0.628 give the aggregates of nanocrystalline SOD zeolite. Moreover, the molar ratios of 0.101, 0.174 and 0.273 provide a mixture of the three phases. All zeolite products contain both intrinsic micropores and interparticle mesopores. The higher ethanol/water molar ratio in the gel produces the zeolites with the lower Si/Al ratio due to the higher Al incorporation in the zeolite structure. In summary, we demonstrate alternative template-free approaches to synthesize EMT/FAU intergrowth and nanosized SOD zeolite with short crystallization time and low crystallization temperature. The finding is an example of ethanol influence on the crystallization to control the phase and morphology of zeolite.

Milling Activation for the Solvent‐Free Synthesis of the Zeolite Mordenite

Milling by a planetary mill was applied as a pretreatment of a reagent mixture for the solvent‐free synthesis of the zeolite mordenite. This method was compared with both manual grinding and standard hydrothermal synthesis. The comprehensive characterization of the obtained products was complemented by thorough studies of activated synthesis mixtures before crystallization steps (by Magic Angle Spinning Nuclear Magnetic Resonance and Scanning Electron Microscopy). This experimental approach gave insight into processes behind the synthesis mixture activation by milling. The characteristics of solvent‐free zeolite product were comparable with material obtained by the classic hydrothermal route. The automated milling activation made the crystallization possible due to higher energetic impact in comparison to manual grinding. The solvent‐free process allowed minimization of the contribution of water to a role of the agent catalyzing silica transformation; as a result, less than 5 % water (compared to the classical synthesis) was necessary to perform the crystallization.

Modified seeding method to produce hierarchical nanocrystalline ZSM-5 zeolite

A series of ZSM-5 nano-zeolites with hierarchical porosity was prepared by a modified seed-induced method using a mixture of silicate-1 and commercial Na-ZSM-5 zeolite as a seed to reduce the production cost. The XRD and FTIR results proved that the pure ZSM-5 phase with a high degree of crystallinity was crystallized. Furthermore, the synthesized samples using a mixture of silicate-1 and commercial ZSM-5 as a seed exhibited more dealumination during the hydrothermal aging process, resulting in a higher Smeso/SBET ratio than other samples and the highest ratio of Brönsted/Lewis acid sites among all the samples. The SEM and BET results also confirmed that the final pH after the synthetic process extensively affected the morphology, specific area and pore volume of products. The purposed method by employing a lower amount of the organic template and silica source might lead to a more economical and environmentally friendly approach for the hierarchical ZSM-5 zeolite production to benefit the relevant industries.

Transformation of fly ash based nanosilica extract to BEA zeolite and its durability in hot liquid

Abstract Power generation in South Africa relies heavily on the combustion of coal and during this process, coal fly ash (CFA) is generated as by-product, which raises several environmental issues. The transformation of CFA into a value added zeolite product is a potential beneficial way to manage and reduce the negative environmental impact of the waste. The present study describes suitable formulations of the synthesis of BEA zeolite from South Africa CFA via an indirect hydrothermal process without the addition of an external silica or aluminium source. Herein, the Si/Al ratio of the nanosilica extract significantly increased from 11 to 48, 53 or 61 depending on applied conditions, thus elucidating that the major component in the extract is 92% silica, with a high purity of 94%. A pure phase BEA zeolite was obtained after the hydrothermal crystallisation of the synthesis precursor with Si/Al ratio of 53 or 61 at 140 °C for 24, 48 or 72 h. The BEA zeolites are micron-sized crystals with high thermal framework stability, high surface area and contained mainly framework but some extra framework Al acid sites. Under hot liquid phase treatment, the BEA zeolite framework maintained structural integrity with no phase transformation at elevated treatment duration and temperature.

Lower-temperature preparation of SiC ceramic membrane using zeolite residue as sintering aid for oil-in-water separation

Due to their excellent attributes of high water permeance, good stability and antifouling properties, silicon carbide (SiC) membranes have achieved widespread application in industrial wastewater treatment facilities. However, the high sintering temperature (usually beyond 2000 °C) during the preparation of porous silicon carbide is a challenge for the transfer of the technology from the laboratory to industry. An efficient sintering additive method was proposed herein to address this issue. Inexpensive residues (containing oxides of sodium, aluminum and silicon) from an NaA zeolite production line were used as the sintering additives to make the SiC particles more tightly connected due to the formation of new phases. Thus, a SiC network could be formed during an in-situ reaction at lower sintering temperature (e.g., 1000 °C) in air without inert gas protection. The as-prepared SiC membranes exhibited a high pure water permeance of 3700 L m-2 h-1 bar-1 and open porosity of 46%. The mechanical strength of the membrane reached 45 MPa. After alkali corrosion or thermal shock resistance tests, the bending strength was maintained at least 38 MPa. The membrane with a mean pore size of 0.4 μm exhibited a high rejection rate to oil droplets (˃ 90%) in various oil-in-water emulsions with a high water permeance. Membrane cleaning with an ultrasonic treatment resulted in good stability in long-term filtration tests. The utilization of zeolite residues successfully reduced the operational cost of heating during ceramic membrane fabrication with recycled solid wastes. Hence, the proposed preparation method for SiC membranes has good sustainability and is scalable for oil-water separation applications.

Phosphate recovery from aqueous solution by K-zeolite synthesized from fly ash for subsequent valorisation as slow release fertilizer

The sorption of phosphate by K-zeolites synthesized from fly ash (FA) by hydrothermal conversion is investigated in this study. The aim is the synthesis of Ca bearing K-zeolites to recover phosphate from urban and industrial wastewater effluents. The loaded zeolites are considered as a by-products rich in essential nutrients such K and P (KP1) with a potential use as slow release fertilizer. A number of synthesis conditions (temperature, KOH-solution/FA ratio, KOH concentration, and activation time) were applied on two FA samples (FA-TE and FA-LB) with similar glass content but different content of crystalline phases, to optimize the synthesis of a zeolitic sorbent suitable for the subsequent phosphate uptake. Merlinoite and W rich zeolitic products synthesized from FA-LB and FA-TE were found to have sorption properties for phosphate removal. A maximum phosphate sorption capacity of 250 mgP-PO4/g and 142 mgP-PO4/g for the zeolitic products selected (KP1-LB and KP1-TE, respectively) was achieved. The dominant phosphate sorption mechanism, in the pH range (6–9) of treated wastewater effluents, indicated that sorption proceeds via a diffusion-controlled process involving phosphate ions coupled with calcium supply dissolution from K-zeolitic products and subsequent formation of brushite (CaHPO4 2H2O(s)). The phosphate loaded sorbent containing a relatively soluble phosphate mineral is appropriate for its use as a synthetic slow release fertilizer. The simultaneous valorisation of fly ash waste and the P recovery from treated wastewaters effluents, (a nutrient with scarce natural resources and low supply) by obtaining a product with high potential for land restoration and agriculture will contribute to develop one example of circularity.

Low-toxic zeolite fabricated from municipal solid waste incineration fly ash via microwave-assisted hydrothermal process with fusion pretreatment

Fusion pretreatment is introduced to the microwave-assisted hydrothermal process to improve the zeolite fabrication from municipal solid waste incineration (MSWI) fly ash. The fusion-pretreated microwave-assisted hydrothermal method (FMHM) and microwave-assisted hydrothermal method are conducted and systematically compared. With fusion pretreatment, the quartz is transformed into amorphous form, which is easier to dissolve into a hydrothermal solution and thus accelerates the zeolite fabrication process. The scanning electron microscope, X-ray diffraction, Fourier transform infrared measurement and thermal gravimetric analysis are performed, whose results suggest the formation of zeolite materials in the FMHM product, such as needle-like tobermorite and rose-like sodalite. The thermal gravimetric analysis indicates water adsorption of the FMHM product is improved. The cation exchange capacity of the FMHM product is 1.172 meq g−1, more than twofold larger than that of the MHM product. Additionally, the toxicity test indicates that the leakage of heavy metal ions from the FMHM product is dramatically reduced. The improved safety makes the zeolitic product synthesized from MSWI fly ash promising for further applications. FMHM significantly facilitates the disposal and reuse of the MSWI fly ash.

Oxidative desulfurization of model oil over Ta-Beta zeolite synthesized via structural reconstruction

As to metallosilicate zeolites, ions with larger size such as Ta5+ in the gels greatly retarded their crystallization during the hydrothermal synthesis, affording long-winded synthesis periods, up-limited framework-substituted metal contents, or even frustrated outcome. An efficient hydrothermal synthesis strategy for metallosilicate, in this case of Ta framework-substituted *BEA zeolite, via structural reconstruction was proposed to stride the gap. The Ta content in our developed Ta-Beta-Re-50 zeolite achieved up to 5.48 % (Si/Ta = 52), breaking through the limitation of Ta contents for conventional method (Si/Ta > 100). Additionally, this Ta-Beta-Re zeolite possessed nanosized crystals (20–40 nm) and short crystallization time (8 h), significantly improving space-time yields of practical zeolite production. Through spectroscopic study, it was confirmed that the existence of zeolite structural units intensively facilitated the formation of nucleation and crystal growth. This innovative Ta-Beta zeolite demonstrated high catalytic performances for oxidation desulfurization, far outperforming traditional fluoride-mediated Ta-Beta-F, which was ascribed to its excellent diffusion properties and incredible high isolated Ta contents. Additionally, the catalytic performance of Ta-Beta-Re could be regenerated after simple calcination and the deactivation may be caused by pore blocking of organics. This work provides a new method for rationally design and construction of metallosilicate materials with high activity for catalytic oxidation applications, which can bridge the conceptual and technical gap between periodic trends and zeolite material synthesis.

Excellent adsorption of Zn(II) using NaP zeolite adsorbent synthesized from coal fly ash via stage treatment

Herein, a novel method for the adsorption of metal ions from sewage by NaP zeolite adsorbent synthesized from stage-treated coal fly ash (CFA) was proposed. The pre-treatment of CFA and grading extraction of silicon and aluminum were explored, and the effects of crystallization parameters and sodium salts on the yield and purity of NaP zeolites were investigated. The characteristics of the zeolite product were measured by a series of characterization methods and the adsorption performance was also evaluated. Optimal treatment parameters were calcination at 800 °C for 3 h with m(CFA)/m(Na2CO3) = 1:1 and 3 mol/L HCl with m(CFA)/V(HCl) = 1 g: 10 mL, with high extraction efficiencies (Si: 91.40% and Al: 90.20%) through stage treatment method. NaP zeolite product exhibited higher purity (96.66%) and yield (94.24%) at 120 °C for 8 h with the addition of NaBr (entirely instead of NaOH, the purity was 90.57% and the yield was 87.82%). The effect of organic steric hindrance was connected with its molecular configuration. NaP zeolite with the addition of polyethylene glycol (PEG) at n(Al2O3)/n(PEG) = 1:5 had the highest single-grain dispersion with the smallest grain size (2.13 μm). However, NaP zeolite with the addition of cyclohexanol (CyOH) at n(Al2O3)/n(CyOH) = 1:3 had the largest surface area (80.4 m2/g). During the Zn(II) adsorption, the appropriate pH value was identified at 5, and the adsorption process followed the pseudo second-order kinetics and Langmuir isotherm adsorption equations. The thermodynamic parameters revealed that the adsorption process was spontaneous and endothermic. NaP zeolite adsorbent exhibited excellent adsorption performance with Zn(II) concentration less than 0.2 ppm in the absorption equilibrium. Moreover, NaP zeolite as an effective adsorbent possessed excellent regeneration.

Influence of Alkalinity on the Synthesis of Zeolite A and Hydroxysodalite from Metakaolin

Kaolin is a cheap and abundant source of silica and alumina, which may be used as precursors for the production of zeolites, molecular sieves with pores in the nanometer scale. Brazil, one the largest producers of kaolin, generates tons of kaolin waste in the paper coating process. That waste may be used to synthesize zeolite A and hydroxysodalite, greater added value materials with a wide range of applications. In this work, Zeolite A and hydroxysodalite were synthesized from kaolin waste of processing industries for paper coating. Kaolin was calcined to dehydroxylate kaolinite and obtain metakaolin, an amorphous material with Si/Al ratio equal to 1, being suitable for production of zeolite A and hydroxysodalite. Zeolites were synthesized under static hydrothermal conditions by reacting metakaolin with NaOH solutions of different concentrations. The zeolitic products were characterized by means of X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The results showed that the higher the NaOH concentration in the reaction medium, the higher the proportion of hydroxysodalite in the zeolitic samples.

LTA zeolite synthesis using natural materials and its evaluation by Green Star methodology

The industrial costs of zeolite synthesis can be diminished by the substitution of traditional raw materials by natural sources. In this work, we obtained LTA zeolite from four natural materials, three different diatomites and expanded perlite, and characterize them by X-ray diffraction, X-ray fluorescence and scanning electron microscopy. It was possible to observe the formation of the zeolitic materials through the identification of the Bragg reflections and the cubic morphology characteristic of the aluminosilicate. The Si/Al ratio of the samples resulted next to the unit as expected for this topology. We found small differences between the zeolitic products, remarking the possibility of adapting this process using natural materials to industry. These results were used for the Green Star evaluation, calculating the synthesis degree of greenness, resulting greener due to the complete substitution of the traditional silica source.

Direct Synthesis of 5A Zeolite From Palygorskite: The Influence of Crystallization Directing Agent on the Separation Performance for Hexane Isomers

AbstractThe efficient separation of hexane isomers from the light naphtha fraction is a significant challenge in the petrochemical industry. 5A zeolite adsorbent is used commercially to sieve alkane isomers. In this study, 5A zeolites were synthesized using a low-cost natural clay mineral precursor, i.e. palygorskite (PAL), with the addition of crystallization directing agent (CDA). By varying the mass ratio of CDA/deionized water, 5A zeolites were obtained as CDA-5%, CDA-7.5%, and CDA-10%. All products were submicron particles with an average particle size of 400–800 nm. A sieving test of CDA-induced 5A zeolites was carried out on hexane adsorbates including n-hexane (nHEX), 2-methylpentane (2MP), and 3-methylpentane (3MP). According to vapor-phase batch adsorption experiments, a significant equilibrium amount (0.149 g/g) of nHEX and only 0.0321 g/g 2MP and 0.0416 g/g 3MP were adsorbed on the 5A zeolite product with CDA-5%. The dynamic adsorption performance of 5A zeolite (CDA-5%) was evaluated by breakthrough curves of binary mixtures of nHEX/2MP and nHEX/3MP. Palygorskite 5A (PAL 5A) zeolite achieved maximum dynamic adsorption capacities of nHEX (0.16 g/g in both cases) at 200°C and 1.2 MPa total pressure. This work provided an economic alternative for the synthesis of 5A zeolites using natural clay minerals instead of chemical raw materials.

A CaO/zeolite-based catalyst obtained from waste chicken eggshell and coal fly ash for biodiesel production

The present paper is focused on the development of a new environment-friendly methanolysis catalyst completely based on waste materials: lignite coal fly ash and chicken eggshells. A novel catalyst based on CaO supported on a fly ash-based zeolitic material (CaO/FA-ZM) was obtained from a cancrinite-sodalite group zeolite-like material (vishnevite type) and active CaO by alkali activation in a new miniature autoclave reactor system and hydration-dehydration. Agitation by rotation of the entire reaction mixture led to a more homogeneous zeolitic product and saved both time and energy. The obtained catalyst structure corresponds to gismondine and the crystallographic modification of calcium silicate (α’-dicalcium silicate) with deposited CaO. The characteristics of the synthesized catalyst were determined using ED-XRF, XRD, FT-IR, SEM, Hg-porosimetry, N2-physisorption, LDPSA, and Hammett indicators. The CaO/FA-ZM catalyst exhibited a high activity (97.8% of FAME for only 30 min) and stability (a negligible drop in activity in five consecutive cycles) in the methanolysis reaction under the optimal reaction conditions (temperature of 60 °C, methanol/oil molar ratio of 6:1, and catalyst concentration of 6 wt%). A kinetic study was performed using two different mechanisms: the irreversible pseudo-first-order reaction mechanism in two regimes (heterogeneous and homogeneous) and the changing mechanism combined with the triacylglycerol mass transfer limitation. Both models showed a satisfactory agreement between the experimental and predicted values of conversion degree (R2 > 0.93), confirming their validity for the CaO-based heterogeneously catalyzed methanolysis. The values of the activation energy calculated for both mechanisms were 67.17 and 58.03 kJ mol−1, respectively.

Controlled synthesis of Zeolite adsorbent from low-grade diatomite: A case study of self-assembled sodalite microspheres

Highly efficient and sustainable conversion technologies to generate uniform sodalite (Na8(AlSiO4)6(OH)2) zeolite microspheres with low-grade waste natural diatomite as raw materials via a solution-mediated crystallization route were developed in the present study. The synthesis process can be considered as an in-situ zeolitization of diatomite precursor without involving any mesoscale template and any post-synthetic modification. The mass ratios of diatomite and AlCl3·6H2O have remarkable effect on the morphology, crystal structure and porosity of sodalite zeolite product. The preferred sodalite microspheres with uniform mesoporous of size 3.5–5.5 nm and large surface area of 162.5 m2/g exhibit well removal performance for heavy metal ions (Pb(II), Cd(II), Zn(II), and Cu(II)), with the highest adsorption abilities for Pb(II) ions of 365 mg/g. In addition, the effect of contact time, initial ion concentration, competitive adsorption and solution pH were evaluated. The removal performance results from synergistic effects of dominating cation-exchange and additional surface chemisorption. The study may broadly help unveil chemical control reactions of the zeolitization processes of diatomite, and thus facilitates the development of promising zeolite materials for the use in natural and engineered aquatic environments by recycling waste diatomite resources.

Revisiting the seed-assisted synthesis of zeolites without organic structure-directing agents: insights from the CHA case.

Herein, the crystallization behaviour of the CHA zeolite synthesized via the seed-assisted method and in the absence of an organic structure-directing agent has been revisited. To date, the working hypothesis of the seed-assisted synthesis method is that the parent gel and seed share the common composite building unit (cbu) of the targeted zeolite crystal. In the case of the CHA zeolite, we reveal that the parent gel in the absence of CHA seeds leads to the formation of the MER zeolite, which does not follow the cbu working hypothesis. It appears that smaller, but essential common units, i.e., 4-membered ring (4-MR) aluminosilicate, play a key role, instead of a larger cbu. The parent gel contains 4-MRs, which can grow into MER and/or CHA, depending on several factors, i.e. alkalinity, Si/Al ratio, synergistic effects of Na+ and K+, and the seeds. In this study, the CHA zeolite with an Si/Al ratio of up to 15 was selectively crystallized in the presence of CHA seeds at suitable alkalinity ((Na2O + K2O)/SiO2 < 0.4) with a fixed point at the tie line of Na2O/SiO2 = 0.3 with K2O/SiO2 = 0.1. Subsequently, the ternary phase diagram was drawn as high alkalinity with (Na2O + K2O)/H2O > 0.4 showing the formation of an MER zeolite, a thermodynamically more stable phase than the CHA zeolite, whereas low alkalinity with (Na2O + K2O)/SiO2 < 0.4 showed the less crystalline CHA zeolite or amorphous products with MER as a competing phase. The crystallization of the CHA zeolite was found to be strongly dependent on the synergistic effects of sodium and potassium ions. The former appears to organize the 4-MRs into essential double-six rings (d6rs), while the latter arranges the formed d6rs into cha cages. The seeds are partially dissolved and provide the outer surface for the crystal growth of CHA. We anticipate that these results may provide useful insight for understanding the crystallization of zeolites and stimulate versatile design in the synthesis of zeolites, particularly for the industrially demanding seed-assisted technique.