A Novel Process for the Preparation of Binder‐Free Zeolite Beads (Lta‐5a) via Photopolymerization Applied in Co2 Adsorption Field

The increasing CO2 concentration in the atmosphere has a severe influence on both the environment and human health, resulting in global warming. According to the high adsorption capacity for CO2 removal, zeolites play an important role for CO2 capture and sequestration. However, its powdery form limits their use in adsorption applications. When binding agents are used, the zeolite porosity is partially occluded that compromises the CO2 adsorption capacity severely. How to prepare binder‐free zeolite beads with a certain structure strength and how to keep the high surface area and porosity are a great challenge. In this work, LTA‐5A beads composites (80 wt.%) are prepared via cationic photopolymerization and then calcination is performed to remove organic part to obtain LTA‐5A beads with different sizes (<1.0 mm, 1.0–2.0 mm, and 2.0–3.0 mm). These beads (0.1 g) are characterized by good mechanical strength without any binder (partial bead fragmentation under 200 g‐load) with a preserved zeolite porosity. This method allows to exploit the zeolite beads reaching a high specific surface area (586 688 m2 g−1) and good CO2 adsorption capacity (4.5‐4.9 mmol g−1). These values are higher than those of commercial LTA‐5A beads (515 m2 g−1 and 3.8 mmol g−1, respectively), which is in agreement with their binder‐free structure.


Introduction
During the past 150 years, the carbon dioxide (CO 2 ) concentration in the atmosphere has increased from 250 to 418 ppm, which is mainly from anthropic activities, recognized one of DOI: 10.1002/admt.202300699 the main causes of global warming problem. [1]Regulating the CO 2 emissions and capturing CO 2 from the environment are the main effective methods to ameliorate this problem.11] As a class of microporous crystalline aluminosilicate materials, zeolites are widely used in gas adsorption or separation field, such as the sorption of CO 2 and H 2 O, and the separation of CO 2 from CO 2 /CH 4 /N 2 mixtures. [12,13]They also are widely employed for the adsorption of toxic matters from water (metal ions, dyes, phenolic components). [14,15]hey consist of crystalline particles having size in the range of several nanometers to few micrometers, which is unsuitable for column filling and their use in continuous flow. [14]In order to be used as industrial adsorbents, zeolites in powder form need to be transformed into zeolitic objects (most common shape: beads and pellets) as a bulk material to avoid a high-pressure drop in cyclic adsorption processes. [16]The conventional ways to prepare zeolitic objects are to mix zeolite powder and inorganic binders together, such as kaolin, [17,18] and to extrude pellets of desired sizes [19] or to mix powder and binder in a kneader-mixer where beads of different size are formed. [19]However, zeolitic objects normally have a binder content of ≈20%, which decreases the adsorption capacity or catalytic performance of the material. [20]Hence, how to develop the synthesis of binder-free zeolite beads is an interesting researching area. [12,21]In order to overcome this issue, many types of binder-free zeolite beads/pellets have been developed, and the binder is itself transformed into zeolite matter during hydrothermal conversion. [22]][25] At the same time, aluminosilicate beads also can be prepared by ion exchange of aluminosilicate species into ion exchange resins followed by combustion of the organic resin, which also can be seen as binder-free beads. [26]In addition, the other method is to obtain structured objects with amorphous aluminosilicate extrudates through hydrothermal synthesis and calcination treatment. [27]he use of organic binder is the other efficient approach to prepare zeolitic objects, which can help to avoid reducing the volumetric efficiency as inorganic binders causing clogging of microporous particle surface and therefore lower the mass transfer rate. [28]Normally, this way to prepare zeolitic objects is to mix zeolite powder and organic binder (polymer, such as chitosan, [16,29] alginate, [30] methyl cellulose, [19] polyvinyl alcohol, [31] etc.) homogenously to obtain formulations and to prepare 3D composite based on the cross-link between zeolite and organic part firstly.Then, depending on whether the presence of organic part affects the porosity of zeolite, the calcination step is considered or not to remove the organic part.[33] Luzzi et al. reported their work about zeolite 13×/chitosan aerogel beads for CO 2 capture. [16]Their prepared beads composites containing 90 wt.% 13× zeolite by the phase inversion process.These beads had a high specific surface area (561 m 2 g −1 ) and good CO 2 capture (4.23 mmol g −1 at 1000 mbar).Isawi successfully prepared nanocomposite beads containing natural zeolite/polyvinyl alcohol/sodium alginate for removal of some heavy metals from wastewater. [31]The natural zeolite consists of 1% silicon oxide, 2.6% zeolite SSZ-73, 81.1% clinoptilolite and 15.3% donpeacorite.The beads were able to eliminate 60-99.8% of heavy ions from the natural wastewater samples collected from 10th Ramadan City, Cairo, Egypt, which contain Al 3+ , Fe 3+ , Cr 3+ , Co 2+ , Cd 2+ , Zn 2+ , Mn 2+ , Ni 2+ , Cu 2+ , Li 2+ , Sr 2+ , Si 2+ , V 2+ , and Pb 2+ .
However, during the preparation of zeolite beads or other shapes, organic binders, zeolite, or any other co-binders should be mixed in different types of solvent (such as an acid/alkaline-based solvent (acetic acid, NaOH), [16,33,34] saltbased solvent (CaCl 2 ), [30,33] or water together.Then, sample drying processes must be performed on these beads, including by freeze drying or by heating in oven. [16,33]This indicates that the process of preparing these samples can be complex and time consuming.Hence, it is of significance to propose a simple and direct method for preparing binder-free zeolite beads.
One efficient way to implement this idea is the use of photopolymerization for zeolitic objects preparation, according on the unique advantages of photopolymerization, such as excellent spatial and temporal control, high production rate, mild reaction conditions (absence of organic solvents as volatile compounds (VOCs), and low-cost and safe irradiation source).In our previous work, we have successfully prepared LTA-5A zeolite based composites (zeolite content at 80-95 wt.%) with polyethylene glycol diacrylate (PEGDA; Mw ≈ 610 g mol −1 ) via radical photopolymerization. [13] Then calcination (600 and 700 °C) was performed on these composites to prepare zeolitic objects.The calcinated zeolite materials kept porosity and have good CO 2 adsorption capacity.However, this work was carried out with the presence of solvent toluene to prepare zeolite composites, which is not environmentally friendly.Therefore, proposing a new way to elaborate zeolitic objects without solvent and binder after calcination is a very interesting challenge for zeolitic objects manufacturing.
In this study, we proposed a new approach to prepare zeolite beads for CO 2 adsorption application.Millimeter-sized composite beads with different diameters were obtained through highspeed mixing combined with epoxy cationic photopolymerization processes in a mild condition (room temperature, LED@405nm, and absence of solvent).After calcination treatment at 600 °C for 30 min, white beads were obtained.These beads properties were characterized in detail by different techniques: i) scanning electron microscope (SEM) was used to observe the surface of the calcinated beads and their core, ii) N 2 adsorption-desorption to determine the textural properties of beads and iii) CO 2 adsorption tests were performed to estimate the adsorption capacity of calcinated beads.In order to see the structure strength of calcinated beads, compression tests with different weights were also performed.

Results and Discussion
The sample preparation process is schematized in Scheme 1 and the details are reported in Experimental Section.As shown in Scheme 1a, two types of photopolymerization mechanism (i.e., free radical polymerization for acrylate and cationic polymerization for EPOX) are presented respectively.As shown in Scheme 1b, during the high-speed mixing process, the liquidpowder two phases (monomer-zeolite powder) are in full contact to form lots of small beads quickly, which can grow into large beads layer-by-layer.Monomer acts as organic binder to bind zeolite powder layer by layer to form beads.After photopolymerization and calcination processes, the organic binder (polymer generated through polymerization) was removed and these beads retained the spherical shape.Compared to our previous works, [13,35] the method in this work can successfully prepare composite beads without any solvent, which is more environmentally friendly, and then binder-free beads can be obtained directly after calcination process and used for CO 2 adsorption.

LTA-5A Beads Composite Preparation
EPOX monomer was used as organic binder first to prepare bead composites with different mixing rate from 500 to 2000 rpm.As shown in Figure 1a, when the mixing rate was set at 500 rpm, few beads were obtained after three batches.As shown in Figure 1c, when the mixing rate was increased to 1000 rpm, the number of beads increased after three batches (first, second, and third).As the mixing rate was set at 1500 rpm, the number of beads was less than that prepared at 1000 rpm, but these beads have more uniform size and their shape is spherical (see Figure 1d).However, when the mixing rate was 2000 rpm, no beads were successfully prepared, as shown in Figure 1b.Therefore, mixing rates have significant influences on beads preparation, because high-speed mixing rate (1000 or 1500 rpm) can make monomer and zeolite dispersed homogenously under the effect of strong centrifugal force and beads were formed rapidly.However, when the mixing rate is too high, such as 2000 rpm, monomer was too quickly dispersed compared to zeolite powder which prevented the beads formation.PEGDA has also been used as organic binder.Few beads were observed at 1500 rpm (see Figure 1e).When the mixing rate was 1000 rpm (see Figure 1f), more beads could be prepared, but their size were not yet uniform.This could be explained by the low viscosity of PEGDA, which makes it negative to be used as binder to crosslink zeolite powder to obtain beads like EPOX.
Based on above discussion, EPOX based formulation at 1500 rpm is more efficient for beads preparation, according the uniform beads size; cationic polymerization is also found more adapted for bead formation.The weight yield in beads formation (all sizes) is 85%.It's detailed according to the beads size in Table 1.In our previous work, when a formulation containing 80 wt.% zeolite was selected, the structured objects could not be obtained with the absence of organic solvent toluene, and after these objects should be placed at 50 °C oven overnight for solvent evaporation.Here, bead composites can be obtained directly after photopolymerization, without solvent, and solvent evaporation process also can be omitted.

Thermal Property of LTA-5A Beads Composites
In order to ascertain the LTA-5A content (ZC) of composite beads, TGA experiments were performed on composite beads from three batches.As shown in Figure 2, TGA were carried out on LTA-5A powder, pure EPOX based polymer and composite beads obtained at 1500 rpm.The weight loss observed for LTA-5A at T < 350 °C (WL1, weight loss from 30 to 350 °C) is related to water desorption (physisorbed water).EPOX polymer degrades in two steps, at 30-350 °C (WL1) and 350-600 °C (WL2, weight loss from 350 to 600 °C).After TGA process, the LTA-5A powder and EPOX polymer lost ≈18.1 wt.% and 98.6%, respectively.
Composite beads (mixture of 1st, 2nd, and 3rd batch) are logically associated to weight losses from zeolite and EPOX polymer.They have a similar residual weight (RW, 67.3, 68.1, and 65.6 wt.%, respectively) that no longer changes after 500 °C.As reported in Table 2 taking into account the hydration rate of zeolite, ZC in each batch was ≈80 wt.%.It means that after  high-speed mixing and photopolymerization processes, beads composition is similar whatever the size.However, the first and second weight losses values depend on the batch: the first decreases while the second increases with the batch number.It can be related to the beads size of batch 1st, 2nd, and 3rd that correspond to 2.5-3.0 mm, 1.0-2.5 mm, and <1 mm (0-1 mm), respectively.Because of the high heating rate (10 °C min −1 ), water desorption takes more time in bigger beads because of diffusion.Thus, water desorption has not finished when the polymer begins to decompose that induces increasing of the second weight loss.
In addition, two T max values corresponding to polymer decomposition process are observed: they are all associated to temperature range of ≈230-260 and 400-410 °C (Figure 2b).Therefore, the method proposed in this work can help to prepare composites beads with different sizes and LTA-5A content also can be controlled at 80 wt.%.

Load Bearing Experiment
After calcination, in order to evaluate the structure strength of these beads, load bearing experiments were performed on calcined beads (0.1 g) by using weights (100 and 200 g).All calcined beads can keep the geometry shape and structure when a 100 g weight was placed on the beads (Figure S1, Supporting Information).But when the test was continuously performed with a 200 g weight, different results were observed.As shown in Figure 3a, Calcined Beads@2.0-3.0mm can stand the 200 g weight without breaks, which means these beads have stable structure.When a 200 g weights was placed on Calcined Beads@1.0-2.0mm(Figure 3b), some beads broke, as highlighted in red circle.When the same test was performed on Calcined Beads@0-1.0mm, it can be observed that many of these beads were broken as highlighted in red circle in Figure 3c.It is obvious to observe that more and more beads can be broken with the decreased bead size, which indicates that the structure strength could be affected by the bead sizes.It could be related to the layerby-layer process for bead formation, because when the more layers are formed, the more stable the structure would be.But it is interesting to note that a small proportion of beads was fragmented, which means that these binder-free beads able to withstand a certain mechanical strength.The same experiment was also performed on commercial LTA-5A beads, and they could keep the structure without breaks, which means commercial beads have a good loading bearing ability.

X-Ray Diffraction and X-Ray Fluorescence
After calcination, these calcined zeolite beads with different sizes were characterized by XRD to control the stability of the zeolite structure.As shown in Figure S2 (Supporting Information), the XRD patterns of these calcined zeolite beads were similar to that of LTA-5A powder, in agreement with the literature (ICDD 04-017-3644).This indicates that the calcination process did not have a significant effect on zeolite structure.X-ray fluorescence was performed on LTA-5A powder and Calcined Beads@2.0-3.0mm,Calcined Beads@1.0-2.0mm(see Table S2, Supporting Information).Compared to LTA-5A powder, the change of the Si/Al ratio and Na/Al ratio for Calcined Beads@2.0-3.0mm,Calcined Beads@1.0-2.0mm is not significant, which means photopolymerization and calcination treatment do not have obvious influence on zeolite.

N 2 sorption and CO 2 adsorption
N 2 sorption measurements were performed to evaluate the textural properties of calcined zeolite beads.As shown in Figure 4a, isotherms of LTA-5A powder, calcined LTA-5A powder calcined beads are type I characteristic of microporous solids.Compared to LTA-5A powder, a similar microporous surface area (SMicro) of calcined LTA-5A powder, which means the calcination process does not have an obvious influence on the zeolite textural properties (see Figure 4a and Table 3).At the same time, the calcined beads have lower BET surface area (SBET) and microporous volume (VMicro) than LTA-5A powder (especially Calcined Beads@1.0-2.0mm and Calcined Beads@2.0-3.0mm), and the micropore size at 0-1 nm also have an obvious decline tendency (see Figure 4c), which could be explained by the low residual carbon formed during calcination partially blocking the micropores, and crystal agglomerates.Because when the beads with larger size are prepared, the crystal aggregation could be more obvious, which results in the decreasing textural properties.Interestingly, as shown in Figure 4d, mesopores (diameter 3.4 nm) from interparticle porosity can be observed on Calcined Beads@1.0-2.0mm and Calcined Beads@2.0-3.0mm, and compared to our calcined beads, LTA-5A commercial beads have lower BET surface area and microporous volume and bigger mesopores (23 nm).In addition, no mesopores can be observed for calcined LTA-5A powder.The ratio of Si/Al of LTA-5A commercial beads is 1.3, which is higher than that of LTA-5A powder (Si/Al = 0.9), which indicates inorganic binder was used for LTA-5A commercial beads preparation, resulting in the presence of mesopores and the lower SBET.Therefore, these results indicate that the calcined zeolite beads could be used as adsorbents, depending on their interesting microporous volumes.
The values for the calcined beads are 5-8% lower than that of LTA-5A powder, but 14-24% higher than that of commercial beads.This could be explained by the presence of low residual carbon that would partially block access to micropores in calcined beads, and by the presence of inorganic binder in the commercial product.In addition, compared to the CO 2 adsorption capacity values of zeolite beads from other references listed in Table 4, zeolite beads in this work have a comparable CO 2 adsorption capacity.Therefore, the calcined beads in this work can be used as adsorbent for CO 2 adsorption.

Scanning Electron Microscope
Scanning Electron Microscope (SEM) analyses were performed to characterize beads surface and core (see Figure 5).As shown in Figure 5c, a spheroidal shape can be observed from calcinated beads, and the surface is flat without roughness.This demonstrates the process of high-rate mixing is favorable to form spheroidal beads and calcination process did not have an obvious damage on surface structure.On the cross-section view (Figure 5a,b,d), significant layers can be observed, which in-dicates a layer-by-layer process to manufacture beads was performed during high-rate mixing process.As shown in Figure 5e, LTA-5A crystals kept easily recognizable well-shaped cubic morphology after photopolymerization and calcination processes.

Conclusion
Zeolite LTA-5A powder was successfully transformed into zeolite beads composites (zeolite content 80 wt.%) by consecutive high-speed mixing and photopolymerization steps, and then zeolite beads were obtained via calcination process.According to the thermal stability of LTA-5A zeolite, calcination process did not have significant influence on zeolite crystal structure and calcined beads had a good structure strength (partial bead fragmentation (0.1 g beads) under 200 g-load).Besides providing mechanical strength, calcined beads also had good CO 2 adsorption capacity (4.5-4.9 mmol g −1 ), higher than commercial LTA-5A beads (3.8 mmol g −1 ).Therefore, this work can help to provide a new way to prepare binder-free zeolite beads for many applications.
Preparation of Zeolite Beads: First, ITX/Iod/EPOX1500 or BDMK/PEGDA were added into a non-transparent bottle and were mixed homogenously in Speedmixer (DAC 150.1FVZ-K) with a speed at 1000-1500 rpm.Second, zeolite powder was added into the bottle first and the monomer liquid mixture was dropped on the surface of zeolite powder after.Zeolite powder and monomer liquid mixture were mixed together to form composite beads with speed at 500-2000 rpm for 5 min.During this step, many intact uncured beads were formed, and the surplus mixture deposited on the bottom of the bottle formed a block.After being weighed, the surplus part was mixed again to form more beads.This step should be carried out for three times to prepare three batches of beads with different sizes, and these beads were defined as composite beads-1st, composite beads-2nd, and composite beads-3rd, respectively.In a LED-curing box, these uncured beads were placed vertically under the near-UV light (LED@405nm, 4500 mA for 1 min) to obtain cured composite beads.
After photopolymerization, all cured composite beads (1st, 2nd, and 3rd) were mixed together and then calcination was carried out on the mixed cured composite beads at 600 °C under air for 30 min in the Muffle Furnace (heating rate at 1 °C min −1 , under air) to remove organic compounds to obtain calcined beads.According to their different sizes, calcined beads were sieved to obtain three categories: 0-1.0 mm, 1.0-2.0mm, and 2.0-3.0 mm and referenced as Calcinated Beads@0-1.0mm,Calcinated Beads@1.0-2.0mm, and Calcinated Beads@2.0-3.0mm,respectively.
Material Characterization: Thermogravimetric analyses (TGA) were carried out using a thermal analyzer (METTLER-TOLEDO TGA/DSC 3+) by heating from 30 to 600 °C under nitrogen atmosphere (N 2 flow rate: 100 mL min −1 , heating rate: 10 °C min −1 ).Zeolite crystalline structures were identified by X-ray diffraction (XRD) measurements using a PANalytical MPD X'Pert Pro diffractometer operating with Cu K radiation (K = 0.15418 nm) equipped with an PIXcel real-time multiple strip detector (active length = 3.347°(2)).The powder patterns were collected at 295 K in the range 3 < 2 < 70, step = 0.013°(2), time/step = 220 s.Elemental analyses were performed by X-Ray fluorescence spectrometry (XRF) on a PANalytical Zetium (4 kW) spectrometer.Samples were mixed with boric acid and pressed into pellets of 13 mm diameter with 5 tons of pressure before the analysis.Zeolite crystal size and morphology, and their corresponding were investigated by scanning electron microscope (SEM) with a JEOL JSM-7900F microscope.
N 2 Adsorption Measurements: Nitrogen (N 2 ) adsorption-desorption isotherms were performed at −196 °C with a Micromeritics ASAP 2420 Instrument on calcined beads.Before each measurement, the samples were outgassed to a residual pressure of <0.8 Pa at 90 °C for 1 h and 300 °C for 15 h.Specific surface areas were calculated according to the Brunauer-Emmett-Teller (BET) method (0.01 < p/p 0 < 0.03).Microporous volume (V micro ) and microporous surface area (S micro ) were calculated using the tplot method.The external surface (S ext ) was obtained by subtracting S micro from the total surface.Micropore size distributions and Mesopore size distributions were calculated by the DFT method and the BJH (Barrett-Joyner-Halenda) method on the desorption branch of the isotherm respectively.
CO 2 Adsorption Measurements: The CO 2 adsorption capacity of the composites after thermal treatments was achieved at 0 °C with a Micromeritics ASAP 2420 Instrument under 105 KPa.Before each measurement, the samples were outgassed to a residual pressure of <0.8 Pa at 90 °C for 1 h and 300 °C for 15 h.

Scheme 1 .
Scheme 1. Schematic representation for a) polymerization reactions and b) zeolite beads preparation by high-speed mixing/photopolymerization/calcination steps.

Table 1 .
The yield of beads with different diameters.

Table 2 .
Weight loss values highlighted by TGA for LTA-5A, EPOX polymer, and LTA-5A filled composite beads.

Table 3 .
Textural properties of LTA-5A powder, calcined beads with different diameter and LTA-5A commercial beads after N 2 adsorption-desorption and CO 2 adsorption.

Table 4 .
CO 2 adsorption capacity of different zeolite beads.
a) CO 2 adsorption was carried out on commercial binder-free 4A zeolite beads on Fixed bed.