Synthesis and CO₂ Capture Properties of Co- and Nd-Modified ZIF-8 Materials Loaded onto Electrospun Polyacrylonitrile Fibers

Abstract

Zeolitic imidazolate framework (ZIF)-8 materials exhibiting zinc metal centers partially replaced by cobalt or neodymium were successfully synthesized via a convenient coprecipitation method. The resulting materials were structurally and microstructurally characterized by SEM, XRD, FT-IR, and TGA, among other techniques. Subsequently, ZIF-8 nanoparticles were integrated into polyacrylonitrile fibers (PAN) via the electrospinning technique, followed by a secondary growth step to increase the ZIF-8 loading on the fiber’s surface. Furthermore, the characterization and evaluation of the materials’ CO₂ adsorption properties at low pressures revealed their volumetric CO₂ uptake capacities. The samples containing ZIF-8 powders modified with Co cations exhibited the best CO₂ capture performances of 26.48 and 8.08 cm³·g⁻¹ (at STP) for the unsupported and PAN-anchored materials, respectively. The strategy of seeding followed by secondary growth to anchor ZIF-8 onto PAN fibers is proposed as a novel and practical approach for adsorbent processing.

1. Introduction

The excessive release of CO₂ into the atmosphere has drawn the attention of the scientific community, which seeks to mitigate the negative impact of such emissions on the global environment [1]. It is estimated that the amount of CO₂ emitted today represents 150% of its value compared to the amount emitted in 1750, at the beginning of the Industrial Era [2,3]. Due to the varied emission sources, which differ in volume, composition, type, and industry, a comprehensive strategy is necessary that encompasses the advancement of a wide array of carbon capture and storage technologies, new materials, and processes [4]. Cryogenic distillation, membrane separation, absorption, and adsorption are examples of promising approaches to reach net-zero carbon emissions [4,5].

Adsorption-based processes are among the most efficient technologies for effective carbon capture. In fact, adsorbents such as porous materials have been widely explored and proven suitable for widespread use [6,7]. Metal–organic frameworks (MOFs) have been positioned as promising emerging materials for designing CO₂ adsorbents due to their outstanding properties, including large surface area, high porosity, and selectivity, which promote their excellent adsorption performance. The subcategories of MOFs include zeolitic imidazolate frameworks, commonly referred to as ZIF materials. ZIF-8s stand out primarily due to their high microporosity and strong affinity for CO₂, making them suitable for adsorption applications [8,9,10,11,12]. However, notwithstanding their degree of development, and similar to other absorbents, these systems still exhibit low capture efficiencies, regeneration capacities, and selectivities that may limit their usage in industrial-scale CO₂ capture.

Surface modification approaches have been mainly explored to enhance the CO₂ capture properties of ZIF-like materials [13]. For example, the effects of functionalization with amino compounds such as tetraethylenepentamine, ethanolamine, propylamine, and hexadecylamine [14] as well as methylamine, ethylenediamine, and N,N-dimethyl-ethylenediamine have been studied [15]. In all these cases, the results showed a significant enhancement in the material’s absorption performance, ranging from 18% to 110% compared to pristine ZIF-8, due to improvements in the interactions between the adsorbent and the adsorbate. In this case, stronger interactions were observed between the acidic CO₂ molecule and the Lewis basic amine groups. Structural modifications were also proposed to optimize CO₂ capture. These approaches may involve designing the specific surface area, pore size, and pore volumes, primarily for physisorption-based processes. For example, the solvent-assisted ligand exchange method was used to obtain mixed-linker ZIF-8s exhibiting 2-methyl-imidazole ligands replaced by 2-nitroimidazole and some halogenated (-Cl, -CF₃, and -Br) imidazolate linkers, resulting in an improvement of the observed CO₂ uptake at low pressure from 11 up to 155% in the best case [16]. This was attributed not only to the observed changes in the microstructural features of the absorbents but also, at the structural level, to the electron-withdrawing groups of the introduced linkers, which increased the electrostatic/Van der Waals interactions between the modified ZIFs and the CO₂ molecules. In the same sense, a different approach of doping or partial replacement of the zinc metal center in ZIF-8 materials with varying ions of metal, for example, nickel, cobalt, copper and iron, has been successfully applied to modify their pore structure, morphology, and acid-base properties, which introduces possibilities for their application not only in absorption processes but also in the sensing and catalysis fields [17,18,19,20]. The use of rare earth elements for the doping of ZIF-8 materials is barely documented; however, some efforts to prepare bimetallic Ce/Zn and Ce/Co-containing ZIF-8 materials have yielded interesting results for achieving stable and enhanced carbon dioxide capture [21]. The structural and morphological features of ZIF-8 materials doped with trivalent Eu³⁺, Y³⁺, and La³⁺ lanthanides have also been reported, particularly for luminescent and sensing applications [22,23].

Finally, the handling of particulate solid adsorbents presents specific challenges, including their limited use in continuous processes due to the potential for pipe clogging, complications related to their reuse, and other specific considerations that must be addressed [24,25]. Therefore, hybrid systems have been developed to address the above limitation, wherein MOFs are packaged into macroscopic discrete shapes, as well as pellets, extrudates, granules, or continuous macroscopic supports, such as bulky monoliths, fibers, and membranes [26,27,28,29,30,31,32]. For example, polymer fibers obtained by electrospinning can be used to load several MOFs. Electrospinning is a straightforward technique for obtaining polymer fibers with suitable properties for absorbents, such as high porosity and a nonwoven interconnected mesh. The introduction of MOF loading via electrospinning into polymer fibers has resulted in better adsorption behavior compared to other MOF packaging techniques, such as flat sheet membranes and coatings on ceramics. In the former type of experimental arrangements, the MOF particles remain available for adsorption [33,34,35,36].

Several studies have focused on elucidating and developing practical strategies to homogeneously anchor and distribute MOFs into hybrid systems [36,37,38]. However, there are still challenges to increasing MOF loading and thereby improving the adsorption capacity of these materials [36,39].

This work reports the use of the practical electrospinning method to seed ZIF-8 particles (pure and structurally modified with Co and Nd cations) within PAN microfibers, followed by a secondary growth stage to improve the anchorage as well as to increase the amount of the porous sorbent on the polymeric support, thereby promoting the CO₂ uptake properties of the materials.

2. Materials and Methods

2.1. Synthesis of Co- and Nd-Doped ZIF-8 Powders

In a general procedure, 1.02 g of zinc nitrate hexahydrate, Zn(NO₃)₂·6H₂O (99%, Sigma-Aldrich, St. Louis, MO, USA), and the corresponding stoichiometric amounts of neodymium (III) nitrate hexahydrate, Nd(NO₃)₃·6H₂O (99.9%, Sigma-Aldrich, St. Louis, MO, USA), or cobalt (II) nitrate hexahydrate, Co(NO₃)₂·6H₂O (99%, Meyer, Mexico City, Mexico), were used for obtaining 5 mol % doped ZIF-8 materials. Salt powders were dissolved in 50 mL of methanol (CH₃OH, 99.9%, Fermont, Monterrey City, Mexico) and sonicated for 20 min. Simultaneously, 1.13 g of 2-methylimidazole (C₄H₆N₂, 99.0%, Sigma-Aldrich, St. Louis, MO, USA) was dissolved in 50 mL of methanol and stirred for 20 min. Then, these two solutions were mixed and subsequently stirred for one hour at room temperature, resulting in a turbid mixture. The suspended solid phase was separated by centrifugation at 8000 rpm. The precipitates were washed three times with methanol and then dried overnight at 90 °C. For comparison, conventional pure ZIF-8 was also synthesized without doping. Samples were labeled as ZIF8, ZIF8Co, and ZIF8Nd to distinguish between undoped and Co- or Nd-containing samples, respectively.

2.2. Polyacrylonitrile Preparation

The synthesis was performed using emulsion polymerization techniques in a semicontinuous process. The synthesis system consists of an addition tank connected to a 1 L glass-jacketed reactor equipped with a condenser, operating under a constant flow of nitrogen (Figure 1). The glass reactor contained an initial mixture of 0.6 g of ABEX EP 110 (SOLVEY, Brussels, Belgium) surfactant solution with 0.5 wt.% concentration and 1.2 g of initiator solution (i.e., sodium persulfate reagent grade (Na₂S₂O₈, 98% Sigma-Aldrich, St. Louis, MO, USA), 2.0 wt.% concentration, and 32 g of deionized water) which was stirred mechanically (HELDOLPH, Schwabach, Germany) at 300 rpm. An addition tank was connected to the reactor, containing a pre-emulsion (

Table 1. Polyacrylonitrile synthesis reagent.

Reagent	Glass Reactor (g)	Tank (g)
Surfactant solution A (0.5 wt%)	0.6	--
Surfactant solution B (3.73 wt%)	--	20
Acrylonitrile monomer	--	40
Initiator solution (2.0 wt%)	1.2	6.5
Deionized water	32	--

), which was continually fed, using a dosing pump (ISMATEC, Wertheim, Germany) at a rate of 0.5 g/min. The temperature was maintained at 75 °C and controlled by a thermal bath (PolyScience, Niles, IL, USA). At the final stage of the reaction, the product was collected and dried for 24 h at 60 °C [40].

2.3. Synthesis of ZIF-8/PAN Microfibers

For the preparation of composite fibers, 0.63 g of PAN was added to 4.265 g of anhydrous N,N-dimethylformamide (99.8%, Sigma-Aldrich) and stirred overnight until a homogeneous solution was formed. Simultaneously, 0.105 g of the previously prepared ZIF-8 powder was added to 2.0 g of N,N-dimethylformamide solvent. The prepared solutions were vigorously stirred for two hours. The solutions were then mixed and stirred overnight at room temperature. The electrospinning equipment consisted of a syringe and pumping system, a high-voltage power supply, and a grounded rotating roller (Figure 2). In brief, the prepared solution was loaded into a syringe and electrospun by applying a 17 kV voltage at a working distance of 17 cm, using a feed rate of 1.5 mL·h⁻¹ to form the ZIF-8/PAN fibers.

2.4. In Situ Secondary Growth of ZIF-8 on ZIF-8/PAN Microfibers

Generally, 60 mg of as-synthesized ZIF-8/PAN fibers was soaked for 24 h in a ZIF-8 precursor solution previously prepared with 50 mL of methanol, which was sonicated for 3 h at room temperature before the addition of the fibers (as described in Section 2.1). After the secondary growth stage, which resulted in the coating of composite fibers with small ZIF-8 crystals, the obtained modified fibers were dried at 60 °C to remove the remaining methanol and moisture. This procedure was conducted for loading the fibers with ZIF8, ZIF8Co, and ZIF8Nd powders. Samples were labeled as ZIF8PAN-f, ZIF8CoPAN-f, and ZIF8NdPAN-f, respectively.

2.5. Materials Characterization

The X-ray diffraction (XRD) patterns of the ZIF-8 synthesized materials were recorded using a powder diffractometer (Bruker AXS model D8 Advance (Bruker, Billerica, MA, USA). The equipment was operated using Kα radiation from a Cu source (λ = 1.54056 Å). The XRD patterns were obtained by operating the diffractometer over a 2θ range from 5 to 70 degrees.

The samples were analyzed by Fourier Transform Infrared (FT-IR) spectroscopy using a PerkinElmer Frontier spectrometer (PerkinElmer, Waltham, MA, USA) in the 4000–500 cm⁻¹ range. The surface area and pore volume of the samples were analyzed based on the N₂ adsorption/desorption isotherms recorded using a Quantachrome Autosorb iQ apparatus (Quantachrome, Boynton Beach, FL, USA). Prior to measurement, the powders were degassed at 120 °C for 10 h on a vacuum line. The specific surface area values were calculated using the BET model, while the pore diameter was determined using the desorption data via the DFT method.

The microstructural features of the synthesized particles and fibers were characterized using a JEOL scanning electron microscope model JSM-6701F (JEOL, Tokio, Japan). The secondary electron imaging mode was used to determine the ZIF-8 morphology, and the compositional mode was useful for obtaining additional microstructural information regarding the presence of ZIF-8 particles formed on the fiber surface through secondary growth. Energy-dispersive spectroscopy (EDS) analysis was performed to elucidate the presence of rare elements and their distribution within the ZIF-8 particles. Analysis of the thermogravimetric behavior of the materials was performed using a Simultaneous Thermal Analyzer model STA 6000 (PerkinElmer, Waltham, MA, USA). The samples were heated from 50 to 700 °C at a ramping rate of 10 °C/min under an air atmosphere.

2.6. Gas Adsorption Measurements

CO₂ adsorption isotherms of the samples were measured at 298 K within a pressure range of 0–1 bar using a volumetric apparatus model BELSORP-Max (BEL, Toyonaka, Japan). Prior to each adsorption measurement of N₂ and CO₂, samples were degassed at 120 °C under vacuum for 12 h.

3. Results

Figure 3a shows the XRD results of the ZIF-8 synthesized samples. The diffraction peaks at 2θ = 7.3°, 10.4°, 12.7°, 14.7°, 16.4°, and 18.2° corresponding to (1 1 0), (2 0 0), (2 1 1), (2 2 0), (3 1 0), (2 2 2) diffraction planes, respectively, indicate the obtained well-crystallized structure of ZIF-8 materials with SOD-type topology [41]. Similar diffraction profiles were obtained for the Co- and Nd-containing materials. The FT-IR spectra (Figure 3b) also show good agreement with previous work, thereby supporting the successful synthesis of the ZIF-8-like materials [10,14,20,31]. The bands at 3136 and 2930 cm⁻¹ are related to the C–H stretching vibration of the aromatic rings and the aliphatic chains of the imidazole linkers. The bands observed at 1583 and 1146 cm⁻¹ were attributed to C=N stretching vibrations, while the intense signals between 1310 and 1456 cm⁻¹ are due to the stretching vibration mode of the whole imidazole ring. The bands at 995, 760, and 694 cm⁻¹ are associated with the bending mode of C–N vibrations of the ring. Additionally, small bands observed at 431 and about 460 cm⁻¹ were assigned to the vibration of the Zn–N and M–O bonds, respectively, wherein M=Zn, Co, or Nd [42,43].

Rietveld analysis of the XRD data was conducted to elucidate the structural modifications resulting from the exchange of Zn²⁺ by Co²⁺ or Nd³⁺ cations in the crystal structure of ZIF-8. Figure 4 illustrates an example of the Rietveld refinement of the ZIF8Co sample, demonstrating that the observed data points (XRD profile) were well-fitted with the calculated data points (modeled profile). Blue lines represent the Bragg reflections. The R_WP (weighted residual error) parameter value of approximately 8% for the three samples demonstrated the fitting quality of the obtained results. This quality parameter is defined as ${100\left[{\displaystyle \frac{\sum _{\mathrm{i}}^{\mathrm{M}}{\mathrm{w}}_{\mathrm{i}}{({\mathrm{y}}_{\mathrm{i}}-{\mathrm{y}}_{\mathrm{i}\mathrm{c}})}^2}{\sum _{\mathrm{i}}^{\mathrm{M}}{\mathrm{w}}_{\mathrm{i}}{\mathrm{y}}_{\mathrm{i}}^2}}\right]}^{{\displaystyle \frac{1}{2}}}$, where M is the number of data points, y_i is the measured intensity at pattern data point I, y_ic is the computed intensity at pattern data point I, w_i = T_i/y_i is the weight at pattern data point i, and T_i is the counting time as preselected for pattern data point i. The goodness-of-fit indicator (χ²) values were approximately 4.4, which falls within an acceptable range. It is defined as ${\left({\displaystyle \frac{{\mathrm{R}}_{\mathrm{w}\mathrm{p}}}{{\mathrm{R}}_{\mathrm{e}\mathrm{x}\mathrm{p}}}}\right)}^2$,where the expected residual error ${\mathrm{R}}_{\mathrm{e}\mathrm{x}\mathrm{p}}={100\left[{\displaystyle \frac{\mathrm{n}-\mathrm{p}}{\sum _{\mathrm{i}}^{\mathrm{M}}{\mathrm{w}}_{\mathrm{i}}{\mathrm{y}}_{\mathrm{i}}^2}}\right]}^{{\displaystyle \frac{1}{2}}}$, and (n − p) is the number of degrees of freedom.

Table 2. XRD data: cell parameters and atomic positions.

Sample	Cell Parameter a (nm)	Atomic Position (Fracc)
		C1	C2	C3	N
ZIF-8 (Reference [44])	1.7033	x = 0.36910	x = 0.3760	x = 0.40520	x = 0.4100
		y = 0.10140	y = 0.99350	y = 0.91430	y = 0.03182
		z = 0.68660	z = 0.6230	z = 0.59470	z = 0.68241
ZIF8	1.7031 (31)	x = 0.3673(33)	x = 0.3309(18)	x = 0.4212(15)	x = 0.3720(21)
		y = 0.1044(17)	y = 1.0213(16)	y = 0.9414(30)	y = 0.0592(35)
		z = 0.6833(26)	z = 0.5930(17)	z = 0.6199(14)	z = 0.6955(16)
ZIF8Co	1.7026 (32)	x = 0.3529(38)	x = 0.3351(27)	x = 0.4122(38)	x = 0.3901(22)
		y = 0.0939(39)	y = 1.0231(10)	y = 0.9221(21)	y = 0.0733(26)
		z = 0.6851(36)	z = 0.5894(20)	z = 0.6131(40)	z = 0.6921(15)
ZIF8Nd	1.7033 (34)	x = 0.3775(21)	x = 0.3427(24)	x = 0.3862(24)	x = 0.3994(32)
		y = 0.0965(29)	y = 1.0067(24)	y = 0.8988(22)	y = 0.0092(33)
		z = 0.6835(25)	z = 0.6006(23)	z = 0.5862(20)	z = 0.7025(34)

and

Table 3. XRD data: cell volume and crystallite size along different directions.

Sample	Cell Volume (nm3)	Crystallite Size (nm)
		(1 1 0)	(2 0 0)	(2 22)
ZIF8	4.940	102.8 ± 0.8	82.8 ± 1.7	111.8 ± 1.5
ZIF8Co	4.936	87.3 ± 0.6	72.5 ± 1.3	93.8 ± 1.1
ZIF8Nd	4.941	92.5 ± 0.7	75.5 ± 1.4	100.0 ± 1.2

present the estimated structural and microstructural features of the samples. Table 2 presents the atomic positions of carbon (1, 2, and 3) and nitrogen, along with the corresponding values from the reference crystalline structure. Values in parentheses represent the random error of the refinement. Additionally, in Table 3, an anisotropic crystal size model using spherical harmonics was employed for each sample to model the profile broadening. A detailed analysis of the diffraction peaks for the Co-doped and Nd-doped ZIF-8 materials revealed slight shifts toward higher and lower diffraction angles, respectively, when compared to the pristine ZIF-8 structure (Figure 3a). These shifts are consistent with the changes in unit cell parameters listed in Table 2; they are attributed to differences in both ionic radius and charge among the dopants (Zn²⁺ = 74 pm, Co²⁺ = 72 pm, Nd³⁺ = 98 pm). In addition, it has been previously reported that doping Zn-MOFs with lanthanide ions can introduce structural defects and/or partially disrupt the framework, leading to the accommodation of the trivalent lanthanide ion in new stable coordination sites. This incorporation provides a mechanism for charge compensation within the crystalline lattice [22,23].

Anisotropic crystallite sizes were calculated using the Scherrer equation (Table 3). The variation in crystallite size across different crystallographic planes is attributed to differences in peak broadening within the same sample. In contrast, differences in crystallite size between samples for the same plane are likely due to variations in surface interactions. Doping in ZIF materials can have a considerable impact on crystallite size, resulting in a decrease in its size by hindering growth. This is a well-known phenomenon caused by the dopant atoms disturbing the regular crystal lattice, making it more difficult for coherent domains to grow to greater sizes. Additionally, crystallite growth is a complex process involving mass diffusion at the crystal interface. A detailed analysis of these phenomena, however, is beyond the scope of this work.

The morphology and particle size of the samples were observed using scanning electron microscopy. Moreover, the histograms of particle size (Figure 5) were constructed based on particle sizes estimated from SEM images using ImageJ software (Version 1.54p). The images revealed that the synthesized particles were on the nanometric scale. All materials exhibited normal and unimodal particle size distributions. Even though there is no clear trend for the differences in particle sizes observed among the samples, it can be stated that certain slight changes were observed as a result of the type of cations incorporated; ZIF8 showed particle sizes in the range between 80 and 240 nm and a mean particle size of 160 nm, ZIF8Co exhibited particles between 100 and 280 nm and a mean particle size of 174 nm, and ZIF8Nd showed particle sizes between 80 and 240 nm and a mean particle size of 163 nm. Furthermore, the insets of the SEM images provide detailed views of the crystal morphology, revealing rhombic dodecahedra with truncated corners, which is considered the most stable equilibrium morphology of ZIF-8 materials compared to the cubic [39].

The energy-dispersive X-ray spectroscopy elemental mapping spectrum is shown in Figure 6. As expected, C and Zn elements were found in all the synthesized materials. On the other hand, Figure 6b,c show the presence of Co and Nd in samples ZIF8Co and ZIF8Nd, respectively. Once again, the mapping area proves the homogenous composition of the materials; therefore, no segregation of metal cations was observed.

Thermogravimetric analysis was performed to investigate the potential effect of composition on the thermal degradation behavior of the material; the results are presented in Figure 7. All the samples exhibit a very similar degradation process, showing only negligible differences in their resulting thermograms. ZIF samples presented three stages of weight loss; the first two were presented at a temperature below 460 °C. First, all materials exhibited a weight loss of less than 2 wt% until reaching 200 °C; thereafter, between 200 and 460 °C, a constant weight loss of approximately 10 wt% was observed. This phenomenon is related to the release of guest molecules, such as methanol and humidity, that are superficially adsorbed; the latter are followed by those guest molecules in the materials’ voids which require higher temperatures to be removed [45]. Finally, at temperatures above 460 °C, a marked loss in weight is attributed to the complete decomposition of ZIF-8, resulting from the decomposition and elimination of organic linkers [46]. The total weight loss at 700 °C was approximately 35 wt% in all synthesized materials. Figure 7b shows the FT-IR spectra of a pristine ZIF8 sample previously calcined at 150, 300, 450, and 600 °C for 10 min in air; this test was performed to support the TGA results. It was observed that even the sample calcined at 450 °C shows all the characteristic bands of the ZIF-8 structure, and as expected, at higher temperatures, the ZIF-8 structure decomposes; in the latter case, the corresponding spectra show bands at 1046, 890, 794, 722 and 546 cm⁻¹ that were assigned to ZnO as the main calcination product [47,48,49]. As mentioned, ZIF8Co and ZIF8Nd present a slight change in thermal behavior compared to the pristine sample; however, results suggest that neither the partial incorporation of Co nor Nd on the ZIF-8 structure involves a significant detriment to the well-known outstanding thermal stability of this material, at least for the proposed 5 mol% doping. It is well worth mentioning this fact as the incorporation of metal cations in higher amounts has been reported; therefore, certain bimetallic systems are obtained, for example, 30% Cu and 70% Zn [39] or 20–70% Co and 80–30% Zn [20], which results in a significant decrease in the thermal stability of the ZIF-8 structure.

Figure 8 shows the N₂ adsorption–desorption isotherms of the synthesized ZIF-8 materials, which exhibit type I adsorption isotherms and narrow hysteresis loops. The presence of microporosity can explain these results. Furthermore, the calculated surface area, pore size, and volume values are presented in

Table 4. Textural properties of pristine and metal cation-modified ZIF-8 samples.

Sample	Specific Surface Area a SBET (m2·g−1)	Average Pore Size b Pore Diameter (Å)	Pore Volume Vtotal (cm3·g−1)
ZIF8	699.6	11.72	0.38
ZIF8Co	1178.2	11.77	0.63
ZIF8Nd	1474.7	11.81	1.05

^a Measured using N₂ adsorption isotherm and applying Brunauer–Emmett–Teller (BET) method. ^b Pore size in diameter was calculated from the desorption data using the DFT method.

. ZIF8Nd exhibited a larger surface area of 1474.74 m²·g⁻¹. Interestingly, the pore sizes slightly increase in both cation-modified samples. The observed pore size values around 11.7 Å (Table 4) agree with the values reported for the pore diameter defined as the largest sphere that will fit into the ZIF’s cage without contacting the framework atoms [50]. On the other hand, the pore volume significantly increased from 0.38 to 0.63 and further increased to 1.05 cm³·g⁻¹ for the corresponding ZIF8, ZIF8Co, and ZIF8Nd samples, respectively. Therefore, from this perspective, cation doping improves the textural properties of the proposed ZIF materials. Regarding pore size distribution, it was observed that although the main pore size did not show remarkable differences as a result of doping, the samples ZIF8Co and ZIF8Nd exhibited a broadened distribution of pore sizes. In fact, sample ZIF8Nd depicted a bimodal distribution in the micropore range (Figure 8b). This fact was attributed to the presence of interparticle pores exhibited by non-rigid aggregates; indeed, sample ZIF8Nd showed the highest degree of hysteresis, which is a result of these microstructural features.

CO₂ adsorption capacities were evaluated from adsorption–desorption isotherms measured at 298 K in the range of 0–1 bar. The adsorption test revealed that ZIF8Co has the highest CO₂ adsorption capability among the studied materials, exhibiting 26.48 (cm³·g⁻¹ (STP)) at 1 bar. On the contrary, the lowest adsorption behavior was observed for the pristine ZIF8 sample, which presented 15.78 (cm³·g⁻¹ (STP)). Meanwhile, ZIF8Nd showed an intermediate adsorption capacity value of 19.44 (cm³·g⁻¹ (STP)) of CO₂ (Figure 9). In general, these results indicate an increase in CO₂ adsorption on the modified materials compared to pure ZIF8. Moreover, ZIF8Co exhibited a desorption isotherm with a certain hysteresis, suggesting that the adsorbent could not completely release the previously adsorbed CO₂ without exerting thermal energy. This is likely due to the stronger interaction between the CO₂ adsorbate and the ZIF8Co adsorbent compared to the other samples [51]. Considering that the ZIF8Co sample did not show the highest surface area or pore volume, the observed adsorption capacity must be related not only to its textural properties but also to the sample chemistry, i.e., the Co metal centers. Data in

Table 5. Summary of some characteristics and CO₂ adsorption properties of ZIF-8 samples. Experimental conditions of 298 K and a pressure of 1 bar.

	Specific Surface Area (BET Model) (m2/g)	CO2 Uptake (mmol/g)	CO2 Uptake (cm3/g)	Ref.
FJU-88a (FJU-88-derived adsorbent)	3.68	0.49	10.98	[52]
ZIF-8	1645	1.39	31.1	[14]
MIL-101 (Cr)	2166	1.17	26.22	[53]
UPC-110	1384	1.08	24.21	[54]
MIP-202	279	0.55	12.33	[55]
ZIF-67	2189	1.19	26.6	[56]
MOF-801	839	1.35	30.3	[57]
MOF-508b (Cu)	364	0.58	13	[58]
SNU-70	5290	0.80	17.93	[59]
SNU-71	1770	1.05	23.53	[59]
JUC-199	821	1.78	39.90	[60]
NU-1000-BzTz	1530	2.00	44.83	[61]
ZIF8	699.6	0.71	15.78	This work
ZIF8Nd	1474.7	0.87	19.44	This work
ZIF8Co	1178.2	1.18	26.48	This work

show the CO₂ uptake values of different MOF-like adsorbents for comparison purposes with the literature regarding powder materials.

The recycling stability of the adsorbent is of critical importance. The results of the adsorption–desorption behavior for three cycles are shown in Figure 10. It could be observed that there was a negligible change in the CO₂ capture properties of the materials after the third consecutive test.

Scanning electron microscopy was used to analyze the morphologies of the synthesized polyacrylonitrile fibers, which were prepared by incorporating previously characterized ZIF8 samples. In Figure 11, the new composite materials are labeled as ZIF8PAN-f, ZIF8CoPAN-f, and ZIF8NdPAN-f, respectively, based on the use of ZIF8, ZIF8Co, or ZIF8Nd powders as fillers. A relatively homogeneous distribution of ZIF-8 filler is observed within the different fibers that acted as a matrix. Synthesized fibers show a circular cross-section and present good continuity throughout the material. This fact demonstrates that the 1.5 wt% filler allows fiber processing using electrospinning. However, it is possible to observe the presence of ZIF-8 agglomerates. Therefore, future efforts must be made to improve the dispersion of the filler into the PAN matrix; for example, by including additional dispersing stages based on colloidal processing strategies for inorganic fillers before their incorporation into the polymeric precursor. Composite fibers have diameters ranging from 1.04 to about 1.95 µm. The resulting diameters and particle sizes were estimated from the SEM images using ImageJ software. At higher magnifications, it was possible to observe the presence of ZIF-8 particles inside the fibers. Therefore, it is argued that the electrospinning method can be proposed as a convenient strategy for seeding the previously synthesized ZIF-8 fine particles.

Figure 12 shows the SEM images obtained after the secondary growth stage was performed on the previously electrospun composite fibers. The images reveal the obtained high number of ZIF-8 particles and their homogeneous distribution on the fiber surface. Furthermore, it is feasible to observe in Figure 12a,b some larger ZIF-8 agglomerates; as mentioned, these agglomerates were previously correlated with the ZIF-8 particles inside the fibers. During the secondary growth stage, they served as nucleation sites to guide the growth and anchor the new ZIF-8 crystals on the fiber surface.

Data in

Table 6. ZIF-8 loading on PAN fibers due to secondary growth.

Sample	ZIF-8 Loading After Secondary Growth (wt%)
	Seeded Fibers	Non-Seeded Fibers
ZIF8PAN-f	27.56	21.38
ZIF8CoPAN-f	25.12	20.63
ZIF8NdPAN-f	24.23	19.59

provide a quantitative comparison of the loading of ZIF-8 on seeded and non-seeded fibers after secondary growth; therefore, the data values represent the weight gained, excluding the initial 2.5 wt% of seeds. Results show the effect of the seed stage, which increases the amount (wt%) of ZIF-8 particles in the formed composite. Moreover, qualitatively, it was observed that seeded fibers promoted better anchorage of the grown ZIF-8 particles on the fiber’s surface; this fact was evidenced by the easier detachment of particles grown on fibers that were not previously seeded when these were handled.

CO₂ adsorption capacities were also evaluated for materials after fiber coating using a secondary growth approach. Results (Figure 13) revealed the same trend observed for the unsupported ZIF-8 materials; therefore, the CO₂ adsorption capability at 1 bar was ZIF8CoPAN-f > ZIF8NdPAN-f > ZIF8PAN-f. These results indicate a slight increase in CO₂ adsorption for fibers containing modified fillers compared to ZIF8. The performance of pure PAN fibers was also evaluated to demonstrate that the PAN matrix also contributes to the sorption properties.

The data in

Table 7. Summary of some characteristics and CO₂ adsorption properties of ZIF-8/polymer samples. Experimental conditions of 273–298 K and a pressure of 1–1.2 bar.

Material	MOF (Filler)	Polymer Matrix	Total Filler Amount (wt%)	CO2 Capture Capacity (mmol/g)	Ref.
TEPA-MIL-101MA	MIL-101(Cr)	Tetraethylenepentamine (TEPA)	—	0.3	[62]
BCZ-0.5	ZIF-8-NH2	Bacterial cellulose (BC)	—	0.44	[63]
ZIF8-PEI-10%	ZIF-8	Polyethanolamine (PEI)	90	0.46	[64]
ZIF8-PEI-30%	ZIF-8	Polyethanolamine (PEI)	70	1.40	[64]
PANI@MIL-101(Cr)	MIL-101(Cr)	Polyaniline (PANI)	70	1.62	[65]
PEI-MIL-101-50	MIL-101	Polyethanolamine (PEA)	50	4.0	[66]
PEI-MIL-101-75	MIL-101	Polyethanolamine (PEA)	75	4.64	[66]
ZIF8PAN-f	ZIF-8	Polyacrylonitrile (PAN)	30.06	0.32	This work
ZIF8CoPAN-f	ZIF-8 (doped Co)	Polyacrylonitrile (PAN)	27.62	0.36	This work
ZIF8NdPAN-f	ZIF-8 (doped Nd)	Polyacrylonitrile (PAN)	26.73	0.34	This work

show the CO₂ uptake values of different MOF-like adsorbents for comparison purposes with the literature regarding composite materials. Despite the CO₂ capture capacity exhibited by the obtained composite material being relatively low compared to other polymer ceramic systems, it is worth noting that the amount of filler in the synthesized materials is also significantly lower. Therefore, further efforts must be focused on increasing and optimizing the ZIF-8 loading in the fabricated fibers.

4. Conclusions

Different ZIF-8 samples were synthesized by the conventional precipitation method, with the successful incorporation of various metal centers, including Zn, Co, and Nd. The synthesis of bimetallic systems (samples ZIF8Co and ZIF8Nd) showed structural, microstructural, and textural changes compared with pristine ZIF-8. Despite the small 5% mol of the substitutional cation, it results in changes in the cell parameters, crystal size, specific surface area, and pore volume. Moreover, the ZIF-8 structure, crystal morphology, and thermal stability are preserved. The partial substitution of zinc cations enhances the adsorption properties, where the ZIF8Co materials exhibit the highest CO₂ adsorption capacity in both cases: the powders and PAN-supported samples. The electrospinning approach, combined with the secondary growth strategy, enables the formation of a homogeneous, well-distributed, and well-attached ZIF-8 crystal on the PAN fibers used as the support. Moreover, it was observed that PAN fibers, although modestly, also contribute to CO₂ adsorption; therefore, the use of PAN as a matrix shows advantages from the perspective of processability and sorption capability.

In this study, the most significant improvements in CO₂ capture properties were observed in bare modified ZIF-8 particles, where CO₂ uptake increased from 0.71 to 0.87 and up to 1.18 mmol/g for the pristine, Nd- and Co-doped samples, respectively. In contrast, only a modest improvement was seen in the composite series, with the ZIF8CoPAN-f sample reaching a CO₂ uptake of up to 0.36 mmol/g. These results could be further enhanced by optimizing the loading of ZIF-8 particles in the composite absorbents. This area could be explored in future research.