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Article

Development of 3D-Printed Carbon Capture Adsorbents by Zeolites Derived from Coal Fly Ash

by
Silviya Boycheva
1,*,
Boian Mladenov
1,
Ivan Dimitrov
2 and
Margarita Popova
2
1
Department of Thermal and Nuclear Power Engineering, Technical University of Sofia, 8 Kl. Ohridsky Blvd., 1000 Sofia, Bulgaria
2
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(10), 524; https://doi.org/10.3390/jcs9100524
Submission received: 29 July 2025 / Revised: 9 September 2025 / Accepted: 16 September 2025 / Published: 1 October 2025
(This article belongs to the Special Issue 3D Printing and Additive Manufacturing of Composites)
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Abstract

The present study aims to develop 3D-structured adsorbents for carbon capture with the utilization of coal ash after its conversion into zeolites. For this purpose, printing paste mixtures with a viscosity of 800 Pa·s were developed based on an environmentally friendly and safe polymer binder filled with coal ash zeolite with the addition of bentonite as a filler. The optimal consistency of the printing mixtures for preserving the shape and dimensions of the 3D-printed structures was established. Various model configurations of the macrostructure of 3D adsorbents were developed, and the optimal settings of the extruding system for their printing were established. After calcination, the resulting 3D structures were studied using instrumental analysis techniques, investigating the influence of 3D structuring on the phase composition, surface characteristics, and adsorption capacity for CO2 capture in comparison with the initial powder coal ash zeolite adsorbents. The role of compensating cations in terms of the adsorption ability of powders in 3D-printed adsorbents was investigated. The current study offers an innovative and previously unexplored approach to a more expedient and practically significant utilization of aluminosilicate solid waste and, in particular, coal ash, through their 3D structuring and outlines a new research and technological direction in the development of economically advantageous, technologically feasible, and environmentally friendly 3D adsorbents.

Graphical Abstract

1. Introduction

Noticeable changes in climate and the natural disasters that are intensifying along with them are related to the global warming effect caused by the emission of greenhouse gases into the atmosphere. Carbon dioxide (CO2) is considered the main greenhouse gas due to its emission in huge quantities, mainly from the combustion of fossil fuels in the production of energy [1,2]. Overcoming global warming requires reducing carbon emissions into the atmosphere by limiting the burning of carbon-containing fuels for energy purposes, increasing the share of energy produced from renewable energy sources, producing hydrogen and biofuels, developing nuclear power, and building installations for capturing carbon dioxide at existing thermal power plants [3,4]. Carbon capture and utilization (CCUS) technologies are based on the selective retention of carbon dioxide from exhaust gas streams and the concentration of CO2 into a pure gas, followed by liquefaction through compression to facilitate its transportation and use as a raw material [5,6]. Carbon dioxide has many industrial applications in the synthesis of polymers, steel slag carbonization, the production of concrete and building materials, the intensification of gas and oil drilling, the carbonation of beverages, dry ice, the production of methane and chemicals, etc. [7,8]. Despite exploring various technological options, industrialized carbon capture technologies mainly consist of scrubber processes with solutions of amines, methanol, glycol, or ethers, and they are associated with a number of difficulties due to the environmental incompatibility and toxicity of sorption solutions, their thermal instability, high energy costs for CO2 desorption, corrosion, solvent degradation, toxicity, etc. [9,10,11]. The widespread adoption of carbon capture by solids requires the development of economically viable and environmentally friendly sorbents with a high adsorption capacity and selectivity, as well as advantageous thermal regeneration properties [12,13]. In recent years, solid-phase adsorbents have been intensively developed for carbon capture, with the main disadvantage of capture by solid-phase adsorbents being the limitation of installations to process large gas flowrates due to the high hydraulic resistance of the adsorption layers [14,15,16]. Another critical issue for adsorption efficiency is the synergy of textural and surface properties, i.e., an appropriate pore size and pore distribution, and surface functionalization to create solids with tunable adsorption selectivity [17]. Although effective, most carbon capture tests on solid-phase adsorbents remain at the laboratory level, due to the testing of powdered adsorbents, which do not allow for pilot or industrial tests, and the insufficient development of macrostructuring technologies to achieve the practical implementation of the process [18,19,20]. In our previous studies, a high adsorption capacity of coal fly ash zeolites (CFAZs) was registered with a relatively good selectivity to CO2 in the presence of water vapor and molecular nitrogen, which are the main components of flue gases from fuel combustion [21,22]. Moreover, CFAZs are distinguished by fast adsorption kinetics and favorable thermal regeneration at mild temperatures of 50–60 °C due to their mixed micro-mesoporous structure, facilitating mass transfer processes [23,24]. CFAZs are obtained by the alkaline conversion of coal ash according to three laboratory schemes: a two-step synthesis, including high-temperature alkaline melting (550 °C) of solid-phase mixtures of coal ash and alkaline activator, followed by water dissolution, the homogenization of the reaction mixtures and the subsequent hydrothermal synthesis for several hours at mild temperatures (80–140 °C); hydrothermal synthesis at mild temperatures for several hours of coal ash suspensions in alkaline solution without prior calcination; and the atmospheric crystallization of zeolites at ambient temperatures from alkaline suspensions of coal ash and alkaline activator solution for several months [25]. The last approach allows the production of zeolites from coal ash with zero carbon emissions and no energy costs, and does not require specific equipment, but has a longer reaction time. The resulting zeolite product is a fine powder that is separated from the alkaline solution by filtration. The approach of the alkaline conversion of coal ash into zeolites opens up broad and practically significant opportunities for utilizing the raw material resource of coal ash, which has accumulated in landfills for decades from the production of energy from coal-fired power plants [26,27,28]. Coal ash, in terms of its macro-composition, is an aluminosilicate raw material with a significant content of spinel and non-spinel iron oxides, with a smaller content of alkaline earth and alkali oxides and some transition metal oxides [29,30,31]. The utilization of the aluminosilicate resource of coal ash for the synthesis of zeolites will free up the soil areas occupied by landfills and will preserve natural zeolite deposits. Zeolites are minerals with a large industrial application for the adsorption of pollutants from water and gas streams, and soil restoration; as gas stream dryers, molecular sieves, catalytic carriers, adsorbents, and molecular hosts; etc. [32,33,34]. They are distinguished by a highly developed specific surface area, a microporous structure with a defined pore size, high chemical inertness and chemical stability, health safety, and environmental friendliness [35]. Due to the presence of silanol groups in their structure and a large internal free volume, they allow modification with metal oxides and active groups, which expands the scope of their practical significance [36]. In nature, over 40 zeolite minerals are known, and more than 200 synthetic zeolite phases have been developed, with some of the most industrially significant zeolite phases being obtained from coal ash by varying the conditions of hydrothermal synthesis, such as zeolites X and Y, zeolite A, phillipsite, chabazite, ZSM-5, modernite, etc. [37,38,39]. The application of the ultrasonic homogenization of the reaction mixtures generally accelerates the alkaline conversion process and leads to the production of nanocrystalline zeolites with a high external specific surface area [40]. Zeolites X and Y of the faujasite type, which are of exceptional industrial importance due to their supercellular porous structure, are technologically easy to obtain from coal ash by the relatively short hydrothermal activation at temperatures below 100 °C and even by the atmospheric crystallization of alkaline suspensions of coal ash [41,42]. Faujasite has the highest potential for capturing carbon emissions due to its highly developed specific surface area, which, for the pure synthetic material, exceeds 800 m2/g [43,44]. Zeolite X, obtained from coal ash, has a lower specific surface area of up to 500 m2/g, but with a comparable and even higher adsorption capacity due to its higher surface instauration and the transfer of iron oxides from the raw coal ash into the composition of the zeolite product, which increase the adsorption capacity for CO2 and impart hydrophobicity to the otherwise hydrophilic pure zeolite [23,24]. However, the large pore zeolites of the faujasite type (zeolite X and Y), besides their high adsorption capacities, suffer from lower selectivity as compared to zeolites with small pores, such as zeolite A (LTA), chabazite (CHA), cancrinite (CAN), sodalite (SOD), etc., by which the pore accessibility is a key factor in their exceptional selectivity [45]. Therefore, small-pore zeolites are also the focus of carbon capture investigations; moreover, that could also be obtained from waste aluminosilicates, such as coal fly ash (CFA) [46,47,48].
The established advantages and efficiency of CFAZs in carbon emission adsorption remain undeveloped to practical significance without the macrostructuring of the adsorbents. For this reason, our studies are aimed at exploring the opportunities of 3D additive printing technology for the three-dimensional structuring of carbon dioxide adsorbents developed based on coal ash zeolite X and sodalite. In-depth studies of the synthesis of zeolites from coal ash and their application as carbon emission adsorbents are available in the scientific literature, but there is a lack of research and knowledge of the 3D structuring of coal ash adsorbents and its influence on the textural characteristics and adsorption efficiency of the materials. The 3D additive printing of pastes with the appropriate viscosity attracts increasing research interest for structuring adsorbents and catalysts to increase their adsorption efficiency or catalytic activity [49,50,51]. The better performance of 3D structures is achieved by reducing the pressure drop caused by the resistance of the solid phase to the flow in the bed, improving the fluid dynamics, optimizing the external surface/free volume ratio of the bed, building complex geometric shapes and multilayer configurations, controlling the surface distribution of the adsorption and catalytic centers, etc. [52]. In addition, 3D printing provides economical material consumption and the joint construction of the adsorption columns and adsorbent packing [53].
However, there is still a lack of sufficient research on the development of printing mixture consistencies with the appropriate rheology that would preserve the shape and dimensions of 3D-printed products and provide mechanical strength after calcination, and the influence of macrostructuring on preserving the functionality of the material has not been fully clarified. This study presents our initial successful research on the development of 3D-structured adsorbents for carbon capture based on zeolite X and sodalite obtained by utilizing CFA. The research is aimed at creating 3D models of the adsorbent macrostructure, developing our own printing mixtures with the appropriate rheology, and investigating the textural characteristics and adsorption capacity of the 3D-structured adsorbents for carbon capture in comparison with the powdered zeolites on which they are based. The influence of 3D structuring on the adsorption efficiency of both large-pore zeolite X and small-pore zeolite sodalite will be investigated. Additionally, the effect of 3D structuring on the accessibility of sodium- and calcium-compensating cations in the zeolite framework and those in an extra-framework position will be clarified. For this purpose, zeolite X with sodium compensating ions, but with different contents of calcium ions in its composition, sourced from the raw CFA, was studied.
The study presents a potentially impactful approach by combining CFA utilization for the development of economically beneficial and efficient 3D-printed carbon capture adsorbents, which will expand the opportunities for larger-scale applications of CFAZs as adsorbents and catalysts. The study opens up the prospects of a new field of research for the full-scale use of powdered zeolites from waste aluminosilicates, which have been intensively studied in recent years, by revealing the possibility of their susceptibility to 3D structuring when establishing a suitable consistency of printing inks. The main obstacles to the production of adsorbents and catalysts through 3D additive manufacturing are related to the availability of materials suitable for 3D printing, and this study reveals an affordable opportunity for the industrialization of 3D-structured adsorbents obtained by utilizing waste resources.

2. Materials and Methods

2.1. Synthesis of Zeolite Powders from Coal Fly Ash

CFA employed as raw feedstock for zeolite production was sampled from the electrostatic precipitators of two large Thermal Power Plants in R. Bulgaria, named TPP AES Galabovo (CFAAES) and TPP Maritza 3 Dimitrovgrad (CFADM3), burning domestic lignite coal with low and moderate limestone content, respectively. CFA from both TPPs is class F, containing silica, alumina, and iron oxides more than 70 wt % of its composition. The raw CFA underwent analysis of its chemical and phase composition and the results were provided in detail in Ref. [22]. The calcium-containing components in the composition of CFA, expressed as CaO, were determined to be about 4.5 wt % in CFAAES and 9.4 wt % in CFADM3, respectively, and the iron-containing components, represented as Fe2O3, were about 13 wt % in CFAAES and 4.7 wt % in CFADM3, with the remainder of the composition being aluminosilicates. The chemical composition of the raw materials used is provided in Table 1.
Zeolite X was synthesized from both CFA compositions using sodium hydroxide as alkaline activator by applying double-stage fusion–hydrothermal synthesis with ultrasonic homogenization of the reaction mixtures. The reaction conditions for preparing zeolite X were provided elsewhere [22,40]. Solid mixtures of CFA and NaOH in a weight ratio 1:2 were calcinated at 550 °C for 60 min. Suspensions with alkalinity of 2.5 mol/L NaOH were prepared from the obtained cooled batches with distilled water, which, after ultrasonic homogenization for 15 min, were treated by hydrothermal synthesis at 90 °C for 240 min. The resulting powder product was collected by filtration, washed to neutrality, dried at 105 °C, and examined. Our previous investigations on CFAZs reveal that a significant amount of the calcium content in the raw CFA is transferred during the synthesis to the zeolite products; therefore, zeolite X obtained from the CFAAES with a lower Ca content is hereinafter referred to as NaX, and the one obtained from the CFADM3 with a moderate Ca content is referred to as NaCaX.
Sodalite (SOD) was prepared from CFAAES, applying similar double-stage synthesis procedure—however, at reduced alkalinity of the reaction mixture and adding NaCl between fusion and hydrothermal steps prior to slurry ultrasonic treatment. CFA, NaOH, and NaCl are in a weight ratio 1.0 : 1.6 : 0.4, resulting in a suspension alkalinity of 2 mol/L NaOH. Hydrothermal activation of the reaction slurry was performed at 160 °C for 8 h.

2.2. Preparation of Inks for 3D-Printing of Adsorbents Based on CFAZ

Inks suitable for 3D extrusion were prepared from zeolite powders as an adsorbent, bentonite nanoclay as a binder, and methyl cellulose and polyvinyl alcohol as a plasticizing organic binder in a weight ratio of 85:10:2.5:2.5 wt %. All powder ingredients were mixed using an IKA RW20 mixer (IKA-Werke GmbH & Co., 79219 Staufen, Germany) at 250 rpm. Aqueous paste with suitable viscosity of 800 Pa·s was obtained, adding deionized water gradually upon mixing by THINKY Mixer ARE-250 (Thinky Co., Ltd., Tokyo, Japan). The inks were degassed from air bubbles and foam in a planetary centrifugal mixer, firstly, at 1000 rpm for 2–3 min, and then mixed for 5 min, increasing the speed at 2000 rpm. The resulting homogeneous paste is tempered at room temperature and immediately applied for 3D printing. More details about development of 3D-printing inks by using CFAZ could be found in Ref. [54].

2.3. 3D Printing by Extrusion of Adsorbent Structures

The 3D extrusion of the as-prepared inks based on CFAZ was performed by a Lynxter S600D 3D printer (Lynxter, Bayonne, France) with an open material system for printing from self-developed pastes. The printer device is equipped with Pass 11 printing toolhead, pressurized in order to ensure continuous extrusion. The nozzle diameter used is 1.04 mm, and the printing speed applied is 1 mm/s. The ink extrusion pressure is 450 kPa. The 3D-printed samples were dried for 48 h in a controlled humidity environment of 33 % RH, ensured by saturated salt solution, to keep the shapes without cracking. Thereafter, the 3D-printed samples were calcinated for 4 h at 600 °C, heated by rate of 3 °C/min in air to remove the binding polymer.

2.4. Characterization of Powder and 3D-Printed Adsorbents

Phase analysis was carried out on zeolite powders and powdered 3D-printed samples using X-ray powder diffraction (XRD) on a Bruker D8 Advance diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) equipped with Cu Kα radiation and a LynxEye detector, within 5–80°2θ range at a constant step of 0.02°2θ. Identification of zeolite phases was based on the ICDD PDF2 database, employing the NaX (00-072-2422) and SOD (00-037-0476) reference cards.
The specific surface area and pore volume of powder and 3D-structured zeolites were determined from N2 physisorption isotherms recorded at −196 °C using an AUTOSORB iQ-C-MP-AG-AG analyzer (Quantachrome Instruments, Anton Paar brand, Boynton Beach, FL, USA). Prior to adsorption, samples were degassed under vacuum at 350 °C. Surface characteristics were computed by model studies of the experimental isotherms by multi-point Brunauer–Emmett–Teller (BET), Barrett–Joyner–Halenda (BJH), and t-plot models. The total pore volume was derived at a relative pressure of 0.9, following the Gurvich rule.

2.5. CO2 Adsorption Tests

Static adsorption measurements were conducted on an AUTOSORB iQ-MP-AG analyzer (Quantachrome Instruments, Anton Paar, Boynton Beach, FL, USA) using pure CO2 as the adsorptive gas at 0 and 25 °C. Following evacuation, the sample vessels were charged with CO2 to a defined pressure, and, once equilibrium was achieved, the adsorbed amount of CO2 was determined. Adsorption isotherms were constructed by plotting the equilibrium uptake of CO2 against the relative pressure (p/p0 = 0.001–0.03). The isosteric heats of CO2 adsorption on the investigated material were evaluated from the adsorption isotherms via the Clausius–Clapeyron approach.
CO2 adsorption experiments were performed in dynamic conditions in a flow system with a laboratory adsorption column. The 3D samples were dried at 300 °C for 2 h prior to the experiments; thereafter, the 3D adsorption elements were arranged transversely to the flow direction, and 5 vol% CO2/N2 at a flowrate of 30 mL/min was passed through the structures. The gas was analyzed online by GCNEXIS GC-2030 ATF (Shimadzu, Tokyo, Japan) with 25 m PLOT Q capillary column. The amounts of adsorbed CO2 in the adsorbents were determined and used to calculate the adsorption capacity. The adsorption behavior of the 3D-printed samples was tested in several adsorption–desorption cycles after their regeneration at 100 °C prior to each adsorption measurement.

3. Results and Discussions

3.1. 3D Structuring of Adsorbent Elements

The creation of 3D-printed structures goes through the following steps:

3.1.1. Designing 3D Models

3D models were designed in a CAD software environment using Autodesk FUSION 360, where adsorption elements with a different geometry and ratio of the external surface and free volume of the structures are created. The description of the designed 3D structures of adsorbent elements, and their technical drawings, 3D models, and their geometric dimensions are summarized in Table 2.
Once the 3D model is created, it is exported to a file of stereo lithography type suitable for 3D printing—.STL.

3.1.2. Preparing a 3D Model for Printing

To prepare a 3D model for printing, it must first be converted into G-code. G-code (also known as RS-274) is the most commonly used programming language for computer numerical control and 3D printing (the name derives from ‘geometry’). It is primarily applied in CAM to control automated machine tools and to guide 3D printers in fabricating products. Since G-code exists in multiple variants, a slicer software is required to generate a version compatible with the printer. In this work, SIMPLIFY 3D version 5.1.2 was used. The G-code creation procedure involves the following steps:
Importing the 3D model (in .STL format) into the SIMPLIFY 3D environment.
Configuring the printer parameters, including the nozzle type, filament thickness, printing speed, and layer height.
After defining the layers and print settings, the software computes key information such as the estimated print time, required filament volume, total component volume, XYZ dimensions, and number of layers.
An example of the imported 3D model of adsorbent element, created G-code, and 3D extruded adsorbents is demonstrated in Figure 1.
Adsorbents with several 3D structures have been developed, presented in Table 2, with the expected advantages and disadvantages for each of them. A comparative analysis of the proposed 3D structures of the adsorbent elements is made in Table 3. The largest external surface area and free volume are provided by the cylindrical structure with concentric circles, and the highest mechanical strength is expected for the supported hexagonal and circular structures. However, cylindrical structures with rectangular holes have the advantage of simplicity.

3.2. Phase Characterization of Powder and 3D Printed CFAZ Adsorbents

X-ray diffraction (XRD) analysis was employed to characterize the phase composition of both the starting powders and the 3D-printed zeolite samples (Figure 2). For convenience, the initial powder CFAZ was denoted further as NaX, NaCaX, and Sodalite, while their 3D counterparts were designated as 3D NaX, 3D NaCaX, and 3D Sodalite. The initial zeolite powders—NaX, NaCaX, and Sodalite—exhibited a high phase purity, with XRD patterns indicating that these materials consisted almost entirely of their respective crystalline zeolite structures, with a negligible presence of the secondary phases or amorphous content (Figure 2a).
To fabricate the adsorbents, a direct ink writing (3D printing) technique was applied, enabling precise control over the geometry and porosity of the resulting structures. The printable suspensions were formulated by combining the chosen zeolite phase (NaX, NaCaX, or Sodalite) with bentonite nanoclay, which serves as a rheological modifier and binder. Methyl cellulose was added as a thickening agent to improve the printability and shape retention of the ink, while polyvinyl alcohol (PVA) acted as an additional binder and film-forming agent to enhance the mechanical stability of the adsorbents. The formulated suspensions were extruded layer by layer to produce monolithic adsorbents with the designed porosity and structural integrity. After printing, the samples underwent drying and thermal treatment to remove organic additives and consolidate the printed structure.
An XRD analysis of the 3D-printed samples confirmed the retention of the main zeolite phase after processing, although minor changes in the crystallinity or the presence of secondary phases were observed in some cases, likely due to interactions with the additives or thermal treatment during post-processing (Figure 2b).

3.3. Surface Characterization of Powder CFAZ Adsorbents

Nitrogen physisorption measurements were conducted to investigate the textural properties of powder and 3D-printed CFAZ (Table 4). N2-physisorption isotherms of CFAZ powders of the NaX, NaCaX, and Sodalite types were presented in our previous studies [22,55]. The powder samples reveal type I isotherms typical for porous solids with a steep rising at low relative pressures. CFAZs also possess a mesoporous character with a type H3 hysteresis loop typical of agglomerated nanoparticles with a wide pore size distribution. The adsorption–desorption isotherms for 3D zeolites of the NaX, NaCaX, and Sodalite types exhibit features characteristic of a combination of type I and type II isotherms, according to the IUPAC classification (Figure 3a). The pore-size distribution functions reveal a high micro-pore yield with a size of about 5 nm (Figure 3b).
This hybrid behavior is indicative of materials that possess a hierarchical pore structure, combining a well-developed microporous framework with significant macroporosity. In addition, 3D CFAZ samples display a slight H4-type hysteresis loop, which is typically associated with narrow slit-like pores and is often observed in systems composed of aggregated zeolite crystals. This phenomenon contributes to the formation of a substantial volume of interparticle meso- and macropores, which is reflected in the pore size distribution and total pore volume values reported in Table 4. Between the two powder zeolites X, NaX demonstrates a significantly higher specific surface area and total pore volume compared to NaCaX (Table 4). This enhancement is primarily attributed to its greater micropore content, which likely results from a more uniform and better-organized crystalline framework. The increased microporosity in NaX facilitates a higher nitrogen uptake at low relative pressures, which is consistent with a higher degree of structural regularity and connectivity in the zeolitic network. Despite these differences, both NaX and NaCaX samples exhibit a considerable contribution from meso- and macropores. These larger pores, formed between loosely packed crystals or as a result of structural imperfections, enhance the overall surface area and, more notably, the total pore volume. Such hierarchical porosity is beneficial for applications involving molecular adsorption or catalysis, as it facilitates improved mass transport and accessibility to the internal active sites (Table 4). The relatively low surface area and micropore volume observed in the Sodalite zeolite can be attributed to the occupation of cationic sites by various metal ions originating from the raw CFA. These ions may partially block the pore entrances or internal channels, rendering them inaccessible to nitrogen molecules during physisorption measurements. In a previous study, results from transmission electron microscopy (TEM) demonstrated the preservation of the porous structure in NaX; and NaCaX; after their 3D structuring [56].
In the case of 3D-printed zeolite adsorbents, a more significant deterioration of the surface characteristics as compared to the initial CFAZ powders was found in the 3D NaX, a minor decrease in the specific surface area in the 3D NaCaX, and the preservation of the textural characteristics in the 3D Sodalite. It could be assumed that the reasons for the more significant reduction in the specific surface area and free volume in 3D NaX are due to the higher proportion of micropores in the structure of the powdered zeolite, which are filled and blocked by bentonite particles, which are added as a filler to establish the appropriate rheology of the printing inks.

3.4. CO2 Adsorption Studies at Static Conditions

The CO2 adsorption isotherms of 3D-printed CFAZ adsorbents of the NaX, NaCaX, and SOD zeolite types, recorded at 0 °C and 25 °C, are presented in Figure 4. NaX and NaCaX are of the large-pore-size zeolite type, while the SOD phase is of the small-pore-size zeolite type. It has been observed for powder CFAZ adsorbents that sodium (Na+) and calcium (Ca2+) cations contribute to generating moderate-strength basic sites in the zeolite structures, which play a crucial role for their CO2 adsorption capacity [56]. This study provides additional knowledge about the influence of Na+ and Ca2+ on the interaction strength and adsorption capacity of 3D-structured zeolite towards CO2 for adsorbents containing predominantly Na+ or, simultaneously, Na+ and Ca2+ ions in their structure. The results clearly demonstrate that the type of cations has a significant influence on the CO2 adsorption performance of the 3D CFAZ adsorbents of zeolite type X. The values for the CO2 adsorption capacity of NaX and NaCaX powders fall within the range indicative of a moderate to strong adsorption affinity and suggest the presence of accessible basic sites capable of interacting effectively with CO2 molecules (Table 5). The stronger CO2 adsorption capacity of NaX compared to NaCaX is most probably due to the higher specific surface area developed at this powder sample. However, if the adsorption capacity is referred to per unit surface area of the adsorbent, a higher value is found for NaCaX, 0.21 cm3/m2, compared to NaX, 0.15 cm3/m2. This observation suggests a more favorable interaction between CO2 and the sodium–calcium-exchanged zeolite framework. In our previous study, it was proven by in situ Fourier-transform infrared spectroscopy that Ca2+ ions contribute to an increase in the number of the accessible Na+-adsorption sites, which benefits the adsorption ability of NaCaX zeolites [57]. On the other hand, if there is some presence of CaO species in the NaCaX sample, which may form during synthesis, these CaO domains are less effective in promoting CO2 adsorption, likely due to their lower dispersion and limited surface accessibility within the zeolite matrix. Moreover, CO2 interaction with CaO occurs at high temperature (above 500 °C) and the release of the captured CO2 also needs an even higher temperature (above 800 °C), which is not economically efficient. However, the Ca2+ ions in the zeolite are very efficient adsorption sites for CO2 capture at ambient temperature, and their release proceeds at a relatively low temperature (below 100 °C).
The adsorption isotherms of 3D-printed CFAZ measured at 0 and 25 °C are presented in Figure 4. The obtained data on the adsorption capacity under static conditions at 0 °C and 104 kPa show lower values compared to those of the powdered initial CFAZ of the NaX and NaCaX types (Table 5).
The sodalite adsorbent keeps its surface parameters and CO2 adsorption ability in powder and 3D form. This decrease is more pronounced for the 3D NaX sample by about 37.5 % compared to the powdered NaX adsorbent, while, for 3D NaCaX, the adsorption capacity drops by about 24% compared to NaCaX. And, since, after 3D structuring, the difference in the surface parameters of 3D NaX and 3D NaCaX is not significant (Table 4), while the adsorption capacity of 3D NaX remains higher than that of 3D NaCaX, the formation of calcium particles inaccessible to CO2 adsorption can be expected, most likely due to the agglomeration with the bentonite filler.
In any case, the adsorption capacity of the 3D-structured NaX and NaCaX CFAZ to CO2 remains at values favorable from a practical point of view as compared to other suggested, even amine functionalized, porous silica adsorbents, many of which have a comparable or lower CO2 retention capacity in powder form [12]. The CO2 retention capacity, reported under similar conditions (CO2/N2 gas flow with a pressure of 100 kPa) for different types of zeolite powders, usually varies between 2.0–3.5 mmol/g and, in separate cases, reaches 4 mmol/g for LTA-type zeolites [58]. The results reported for the adsorption capacity of powdered zeolite 13 X, a commercial analogue of zeolite NaX, range from 2.7–6.2 mmol CO2/g, indicating that zeolite X from coal ash exhibits excellent adsorption potential [43,59,60,61]. It has been found that, despite the high capacity of zeolite 13X, its granulation greatly reduces its adsorption capacity to 2 mmol CO2/g at 100 kPa, due to the filling of its micropores with binding molecules [62]. The present study shows an even higher adsorption capacity of 3D-printed coal ash zeolite X adsorbents compared to granular commercial 13X zeolite, which, in powder form, typically has a specific surface area almost twice that of coal ash zeolite X (nearly 800 m2/g for 13X vs. 300–550 m2/g for CFAZ zeolite X). A prerequisite for the better adsorption capacity of 3D CFAZ compared to granular 13X is its mixed micro-mesoporosity, which favors the accessibility of the adsorbate molecules to the internal free volume of the adsorbent.
The adsorption isotherms toward the CO2 of 3D CFAZs were described by fitting to general Langmuir (Equation (1)) and Sips (extended Langmuir) (Equation (2)) models, applying the following equations:
C a d s = C a d s , e q b ( p / p 0 ) ( 1 + b P )
C a d s = C a d s , e q b p / p 0 n 1 + b ( p / p 0 ) n
where Cadsis the amount of adsorbed CO2 per unit mass of adsorbent, mmol/g; Cads,eq is the maximum adsorption capacity (monolayer capacity), mmol/g; b is the Langmuir constant (related to adsorption energy), dimensionless when the relative pressure p/p0 is used as the input parameter; and n is a dimensionless parameter related to surface inhomogeneity and energy dispersion (0 < n ≤ 1).
The classical Langmuir Equation (1) correlates well with the experimental data for monolayer adsorption and a homogeneous adsorbent surface, and in the absence of interactions between adsorbate molecules. Its high correlation (R2 > 0.999) with the experimental CO2 adsorption isotherms at 0 °C was established for powdered CFAZ adsorbents of NaX and NaCaX, synthesized by two-step hydrothermal synthesis with preliminary alkaline melting and ultrasonic homogenization [22]. Model calculations on the experimental adsorption isotherms of the 3D-printed adsorbents show a good correlation with the classical Langmuir model (R2 > 0.99), but are better described by the Sips model (R2 > 0.999). The Sips (extended Langmuir) model describes a heterogeneous surface at n < 1, where the active sites have different adsorption energies. The fitting of the experimental data to the Sips model is presented in Figure 5. The obtained model parameters are presented in Table 5. The comparatively low n values for 3D NaX and 3D NaCaX are indicative of a strong surface inhomogeneity, probably due to some agglomeration between the zeolite and bentonite filler. The adsorption isotherm of 3D Sodalite is described more reliably by the classical Langmuir model at R2 > 0.98, as the Sips model does not show realistic parameters, most likely due to the small range of variation of the amount of adsorbate with p/p0, and the surface heterogeneity of the sample cannot be assessed.
In the present study, the adsorption capacity for CO2 was tested on pieces of 3D-printed structures in order to clarify to what extent the adsorption capacity decreases upon the agglomeration of the powder adsorbent upon the addition of binders and fillers for the development of the printing mixtures and the subsequent calcination of the 3D samples. For this reason, the indicated adsorption capacities are not influenced by the macrogeometry of the 3D adsorbents. The influence of 3D macrogeometry on the adsorption capacity and aerodynamics in the adsorption column will be the subject of further investigation in integrated 3D-printed systems of a laboratory adsorption column and a structured adsorbent bed.
In addition to the adsorption capacity, the heat of CO2 adsorption was estimated for each sample, further confirming the moderate-strength basicity of the zeolitic materials. The results are presented in Figure 6. The registered higher isosteric heat of adsorption for 3D NaX than that calculated for 3D NaCaX and 3D Sodalite materials is due to the stronger interaction between accessible cations and CO2 molecules. The trend in the adsorption capacities and heats of adsorption implies that the sodium form (3D NaX) possesses a more optimal balance of accessible micropores and moderately basic sites, enabling the more effective CO2 capture compared to its 3D calcium-modified counterpart and 3D sodalite zeolite. These findings underscore the importance of the cation type in tuning the basicity and adsorption performance of zeolite-based materials.

3.5. CO2 Adsorption Studies at Dynamic Conditions

The CO2 adsorption capacity of 3D-structured samples under dynamic conditions was investigated in three adsorption–desorption cycles. The breakthrough curves are plotted in Figure 7.
The preservation of the adsorption capacity value in a sequence of adsorption–regeneration cycles is established, which indicates the stability of the 3D structure and adsorption centers. Full adsorbent regeneration was established at a mild temperature of 100 °C, which is a favorable value with respect to the energy demands for practical application. The values of the dynamic adsorption capacity of 3D-structured adsorbents toward CO2 are presented in Table 6.
The CO2 retention capacity values of the 3D adsorbents, measured under dynamic conditions, for all tested samples are higher compared to the equilibrium values for the powdered and 3D-printed zeolites (the tests under equilibrium conditions were conducted on pieces of the 3D structures). This is most likely due to the fact that the adsorption measurements under dynamic conditions were performed on the entire 3D structures, and convincingly confirm the advantages of 3D structuring. The highest adsorption capacity was measured for 3D NaX corresponding to its highest specific surface value. Under dynamic conditions, a higher adsorption capacity was measured for 3D NaX compared to the powdered zeolite and a lower one for 3D NaCaX compared to its powdered counterpart. Since previous studies have shown a mixed physical and chemical adsorption of CO2 on calcium-containing zeolites from coal ash, it is most likely that, in the case of 3D-structured calcium-containing zeolite adsorbents, the binding matrix influences the accessibility of Ca2+ in the adsorption process [22]. Figure 7 shows that the adsorbents 3D NaX and 3D NaCaX have the same time for complete CO2 retention, after which the CO2 concentration in the outlet gas stream increases more smoothly with 3D NaX.

4. Conclusions

Three-dimensionally structured adsorbents based on coal fly ash zeolites of type X and Sodalite have been successfully developed. To study the influence of the type of compensating cations on the ability of zeolite adsorbents to capture CO2, zeolite X was obtained in the form of NaX and NaCaX. Then, 3D printing was performed by extrusion from printing mixtures with the appropriate rheology, preserving the shape and dimensions of the printed product. The printing inks are composed of the corresponding coal ash zeolite, with bentonite as a filler and polymer binders in appropriate ratios. The conducted comparative studies of the phase composition and surface characteristics of powdered and 3D-structured zeolites reveal the preservation of the zeolite phases after the extrusion and calcination of the 3D adsorbents and a more significant reduction in the specific surface area and free volume in 3D NaX with a higher content of micropores in its structure. We found that 3D-structured zeolite X adsorbents reveal a high adsorption capacity toward CO2 molecules in dynamic conditions (above 4 mmol/g for 3D NaX), affordable thermal regeneration, and stable adsorption performance in multiple adsorption/desorption cycles, which makes them suitable for applications in CO2 capture systems. The influence of calcium ions in the zeolite matrix on improving the adsorption capacity of type X zeolites is better pronounced in powder adsorbents, since, in 3D samples, calcium participates in agglomerates with bentonite filler and is inaccessible to CO2 molecules. The fitting of CO2 adsorption isotherms to the Langmuir and Sips mathematical models shows a high correlation with the more pronounced surface inhomogeneity in the 3D-structured zeolites of type X. However, further studies will be aimed at investigating the influence of different fillers on the adsorption capacity of the 3D-printed samples.

Author Contributions

Conceptualization, S.B. and M.P.; methodology, S.B. and M.P.; software, B.M. and I.D.; validation, S.B. and M.P.; formal analysis, S.B., B.M., I.D. and M.P.; investigation, S.B., B.M., I.D. and M.P.; resources, S.B. and M.P.; data curation, B.M. and I.D.; writing—original draft preparation, S.B. and M.P.; writing—review and editing, S.B. and M.P.; visualization, B.M. and I.D.; supervision, S.B. and M.P.; project administration, S.B. and M.P.; funding acquisition, S.B. and M.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by EUROPEAN UNION—NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project № BG-RRP-2.004-0005 and project BG-RRP-2.011-0021-C01.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

S.B. and B.M. are grateful for the financial support of the European Union—NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project № BG-RRP-2.004-0005. M.P. and I.D. are grateful for the financial support of the European Union—NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, and project BG-RRP-2.011-0021-C01.

Conflicts of Interest

The authors declare no conflicts of interest.

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Jcs 09 00524 g001
Figure 1. Software converting of 3D model of adsorbent element to G-code for printing: (a) incorporated 3D model in slicer environment; (b) G-code; and (c) 3D adsorbent structure.
Figure 1. Software converting of 3D model of adsorbent element to G-code for printing: (a) incorporated 3D model in slicer environment; (b) G-code; and (c) 3D adsorbent structure.
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Figure 2. XRD of the powder (a) and 3D-printed (b) adsorbents. Reference XRD of zeolite phases X and SOD are plotted for comparison.
Figure 2. XRD of the powder (a) and 3D-printed (b) adsorbents. Reference XRD of zeolite phases X and SOD are plotted for comparison.
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Figure 3. N2 physisorption isotherms (a) and pore-size distribution functions (b) of 3D CFAZ.
Figure 3. N2 physisorption isotherms (a) and pore-size distribution functions (b) of 3D CFAZ.
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Figure 4. CO2 adsorption–desorption isotherms at 0 and 25 °C for the studied 3D-zeolite-based adsorbents.
Figure 4. CO2 adsorption–desorption isotherms at 0 and 25 °C for the studied 3D-zeolite-based adsorbents.
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Figure 5. Model description of adsorption isotherms of 3D CFAZ: (a) 3D NaX; (b) 3D NaCaX; and (c) 3D Sodalite.
Figure 5. Model description of adsorption isotherms of 3D CFAZ: (a) 3D NaX; (b) 3D NaCaX; and (c) 3D Sodalite.
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Figure 6. Heat of adsorption for the studied 3D zeolite adsorbents.
Figure 6. Heat of adsorption for the studied 3D zeolite adsorbents.
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Figure 7. Breakthrough curves of CO2 adsorption onto 3D-printed zeolite-based adsorbents.
Figure 7. Breakthrough curves of CO2 adsorption onto 3D-printed zeolite-based adsorbents.
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Table 1. Chemical composition of raw coal fly ashes used in the zeolite synthesis.
Table 1. Chemical composition of raw coal fly ashes used in the zeolite synthesis.
Samples Components, wt % Zeolite
Obtained
SiO2Al2O3Fe2O3CaOMgOOthers *LOI
CFAAES50.223.813.04.52.33.82.2NaX; Sodalite
CFADM350.821.34.79.40.84.87.8NaCaX
* Others include total amount of Na2O + K2O + MnO + TiO2 + P2O5 + SO3.
Table 2. The shapes and geometrical dimensions of the model created.
Table 2. The shapes and geometrical dimensions of the model created.
Structure DescriptionTechnical Drawing3D ModelExternal Surface Area, mm2Structure Volume, mm2
The model has a hexagonal shape, composed of concentric hexagons connected by vertical and horizontal support elements. It features an inscribed circle diameter of 30 mm, a circumscribed circle diameter of 25.98 mm, and a height of 5 mm.Jcs 09 00524 i001Jcs 09 00524 i0022133.472041.66
Cylindrical in shape, with symmetrical rectangular holes. The model has a diameter of 27 mm and a height of 3 mm.Jcs 09 00524 i003Jcs 09 00524 i0041615.581393.66
Cylindrical shape, with symmetrical rectangular holes. The model has a diameter of 30 mm, and a height of 3 mm.Jcs 09 00524 i005Jcs 09 00524 i0061795.101548.51
The model has a cylindrical shape characterized by concentric circles, reinforced with vertical and horizontal support elements. Its outer diameter is 42 mm, featuring a central hole of 12 mm diameter and 5 mm height. Additional holes in the model are defined by arcs with a width of 3 mm and a thickness of 5 mm.Jcs 09 00524 i007Jcs 09 00524 i0083707.853782.33
Table 3. Comparative analysis of the developed 3D-printed adsorbent structures.
Table 3. Comparative analysis of the developed 3D-printed adsorbent structures.
3D StructureAdvantagesDisadvantages
Hexagonal (concentric hexagons)
‐
High mechanical stability due to regular support elements;
‐
Good geometric uniformity—predictable flow distribution;
‐
Larger contact surface area compared to simple cylinders.
‐
Smaller open volume, which may reduce adsorption capacity;
‐
More complex printing process, and potential defects at intersections.
Cylindrical Ø 27 mm with rectangular holes
‐
Simple design—easy and fast to print;
‐
Symmetrical holes ensure relatively uniform flow distribution;
‐
Compact geometry.
‐
Reduced external surface;
‐
Lower structural stability compared to reinforced designs.
Cylindrical Ø 30 mm with rectangular holes
‐
Larger diameter compared to Ø 27 mm—slightly higher adsorption surface;
‐
Maintains simple geometry, easy for replication and scaling.
‐
Reduced external surface;
‐
Susceptible to mechanical deformation under higher flow/pressure.
Concentric circular structure (Ø 42 mm, inner Ø 12 mm)
‐
Largest surface area among tested designs;
‐
Combination of vertical and horizontal supports improves mechanical strength;
‐
Open central channel facilitates fluid distribution and reduces pressure drop.
‐
Longer printing time;
‐
Complex geometry;
‐
Higher thickness, which may reduce effective porosity.
Table 4. Textural properties of the CFAZ powders and 3D counterparts.
Table 4. Textural properties of the CFAZ powders and 3D counterparts.
SamplesSBET
(m2/g)		Smicro
(m2/g)		Total Pore Volume (cm3/g)Vmicro
(cm3/g)		Average Pore Diameter
(nm)		DFT Pore Diameter
(nm)
NaX5524330.4080.1712.951.0; 5.1
NaCaX3202240.2560.0903.201.5; 5.1
Sodalite6060.1450.0039.6531.3; 5.3
3D NaX3362330.3110.0923.7050.6; 5.3
3D NaCaX3001890.4950.0766.6000.6; 1.3
3D Sodalite6060.1450.0039.6531.3; 5.3
Table 5. CO2 adsorption at static conditions of the powder and 3D-printed samples: experimental and model calculations.
Table 5. CO2 adsorption at static conditions of the powder and 3D-printed samples: experimental and model calculations.
SampleCads, mmol/g
0 °C, 104 kPa		Cads,eq
mmol/g		bnR2
NaX3.794.09 0.11 *->0.999
NaCaX3.123.40 0.12 *->0.990
Sodalite0.490.478.03->0.980
3D NaX2.374.770.970.34>0.999
3D NaCaX2.103.112.040.45>0.999
3D Sodalite0.490.478.03->0.980
* The values are published in Ref. [22].
Table 6. CO2 adsorption at dynamic conditions of the powder and 3D-printed samples.
Table 6. CO2 adsorption at dynamic conditions of the powder and 3D-printed samples.
SampleCads,flow
mmol/g		Time for Passing of 5 vol% CO2, minTime for Passing of 100 vol% CO2, min
NaX *2.93.514
NaCaX *4.21.214
Sodalite---
3D NaX4.38.420
3D NaCaX3.512.026
3D Sodalite2.114.526
* The values are for coal fly ash zeolite X powders published in Ref. [22].
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Boycheva, S.; Mladenov, B.; Dimitrov, I.; Popova, M. Development of 3D-Printed Carbon Capture Adsorbents by Zeolites Derived from Coal Fly Ash. J. Compos. Sci. 2025, 9, 524. https://doi.org/10.3390/jcs9100524

AMA Style

Boycheva S, Mladenov B, Dimitrov I, Popova M. Development of 3D-Printed Carbon Capture Adsorbents by Zeolites Derived from Coal Fly Ash. Journal of Composites Science. 2025; 9(10):524. https://doi.org/10.3390/jcs9100524

Chicago/Turabian Style

Boycheva, Silviya, Boian Mladenov, Ivan Dimitrov, and Margarita Popova. 2025. "Development of 3D-Printed Carbon Capture Adsorbents by Zeolites Derived from Coal Fly Ash" Journal of Composites Science 9, no. 10: 524. https://doi.org/10.3390/jcs9100524

APA Style

Boycheva, S., Mladenov, B., Dimitrov, I., & Popova, M. (2025). Development of 3D-Printed Carbon Capture Adsorbents by Zeolites Derived from Coal Fly Ash. Journal of Composites Science, 9(10), 524. https://doi.org/10.3390/jcs9100524

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