Enhanced catalytic activity of zeolitic imidazolate frameworks (ZIF-8) polyelectrolyte complex composites membranes by laser etching
Published Online: May 08, 2024
Page range: 52 - 61
Received: Jan 05, 2024
Accepted: Mar 28, 2024
DOI: https://doi.org/10.2478/msp-2024-0004
Keywords
© 2024 Ting Yu et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
Polyelectrolyte complexes (PECs) are formed by the complexation of oppositely charged polyelectrolytes in a solution that is primarily driven by electrostatic interactions between water-soluble polyelectrolytes. Hydrogen bonding and hydrophobic interactions can also influence the final structure of PECs [1, 2]. PECs can be found in diverse applications such as drug delivery, biosensing, battery separators, fuel cells, and catalysis [3–7]. PDDA, a cationic polyelectrolyte, and PSS, an anionic polyelectrolyte, are often used to prepare PECs. These two polyelectrolytes can form strong complexes, but the addition of salt during the formation of PECs can swell the PDDA/PSS membranes, increasing their thickness and improving their plasticity [8]. Notably, one significant advantage of PDDA/PSS PECs is their excellent stability against dissolution in most organic solutions [9, 10]. PDDA/PSS PECs can be used as a matrix for the immobilization of various fillers such as conducting particles (carbon black), redox species (MnO2), or even catalytic particles such as metal-organic frameworks [11–13]. The filler, which is usually found inside the membranes, has limited access to the solution, thus reducing the interaction between the filler particle and the solution if needed, for example, for catalytic particles. A possible solution is to use an etching step to erode the surface of the membrane and expose the particle within.
Laser etching is a proven technology used to remove material from a substrate using a high-energy laser beam [14]. Although chemical etching has a lower upfront investment cost and a higher production efficiency, industries such as the glass, plastic, semiconductor, and steel industries [15–18] are increasingly focused on laser etching strategies because of their lower comprehensive cost, precise engraving, and greater environmental friendliness due to zero waste emission [19–22]. While the etching of a polymer surface can be used to induce the formation of chemical structures or erode the surface [23, 24], the etching of polyelectrolyte complexes, although never reported yet, could be of great interest to modify the membrane interface. The need for surface treatment comes from the fact that after compression molding, the membranes are very smooth, and the particles are entrapped within the membranes with limited access to the surface. The particle’s limited access to the surface is the result of the processing method in which heat and pressure to the polymer-filler mixture promote the flow and redistribution of the polymer matrix around the filler particles. As a result, the particles become encapsulated within the matrix rather than being pushed to the surface. This could be interesting, for example, in applications where roughness and high surface area are needed between the electrolyte and the membrane, especially for catalytic surface or solvent and ion transport through the membrane.
To study the effect of laser etching, a composite PEC films were prepared by combining PDDA/PSS as a matrix with oxidized ZIF-8 powder rich in N=C=O bonds that are known to enhance the conversion of triglycerides [25, 26]. Laser etching was employed to erode the PECs and expose the ZIF-8 from the interior of the membrane. The hypothesis in this work was that the laser could etch the PEC surface and, therefore, expose the inner particles. The PECs were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDS). Furthermore, the performance of the catalytic PECs was evaluated in the transesterification of triglycerides found in soybean oil, which is an important reaction to synthesize fatty acid methyl ester (FAME), the main component of biodiesel, as a model experiment.
Poly(diallyldimethylammonium chloride) (PDDA) (Mw=200,000-350,000, 20 wt% in H2O) are purchased from Sigma-Aldrich. Poly(sodium 4-styrenesulfonate) (PSS) (Mw=70,000) is provided by ACROS. Sodium chloride is supplied by Pine Chemical Co., Ltd. Zinc Nitrate Hexahydrate (98%) and 2-Methylimidazole (99%) were obtained from Sigma-Aldrich. Methanol (≥99.9%) was bought from Fisher Chemical. All chemical reagents are analytical grade purity. Soybean oil was purchased by Thai Vegetable Oil Public Company Limited.
ZIF-8 was prepared according to Janosch Cravillon’s report with few modifications [27]. 7.0 mmol Zn(NO3)2 and 56.0 mmol 2-Methy-limidazole (Zinc : 2-Methylimidazole = 1:8) were dissolved in 200 mL methanol. Then, Zn(NO3)2 solution was added into 2-methylimidazole solution quickly. After vigorous stirring of 16 hours at room temperature, the ZIF-8 solid was separated by centrifugation machine (10000 rmp, 30 min) and washed with excess methanol three times. White ZIF-8 solid was dried in a vacuum oven at 60°C overnight to evaporate methanol. The above ZIF-8 was oxidized in the furnace at 300°C for 3 hours in the air.
The ZIF-8 was added to a mixture solution of PDDA/NaCl (PDDA: 0.1 mol/L, NaCl: 1 mol/L). The mixture was sonicated for 15 minutes to disperse ZIF-8 in the PDDA/NaCl solution. The PDDA/NaCl/ZIF-8 solution was combined with an equal volume of PSS/NaCl solution (PSS: 0.1 mol/L, NaCl: 1 mol/L) and stirred slowly for five minutes. The resulting primary solid of PECs was obtained and isolated by vacuum filtration. The primary solid of the PECs was compressed using a compressor at a pressure of 2000 psi and a temperature of 60°C for 10 minutes. The resulting ZIF-8/PDDA/PSS was dried at room temperature until the moisture had evaporated, and it was called xZIF/PEC. Here, x represents the weight fraction of ZIF-8 in the PDDA/PSS membrane.
The untreated xZIF/PEC samples were etched by a 500 mW laser (wavelength: 405 nm). The engraving parameters were a density of 10 lines/mm at a speed of 800 mm/min. These samples were noted as xZIF/PEC-L, with x representing the weight fraction of ZIF-8 in the matrix and L denoting the laser treatment.
A transesterification reaction was used as an example to test the performance of catalytic PECs in the batch reactor. The schematic of the batch reactor and photograph of the batch reactor are provided in Fig. S1 in the supporting materials section. Catalytic PECs, 3 wt% of soybean oil, were added into 72.5 mL soybean oil and 17.4 mL methanol mixture (mole ratio of soybean oil and methanol was 1:6). A sealed batch reactor with a magnetic stirrer was heated in oil bath on 150° C for 4 hours. After the reaction, the PECs were isolated from the product by vacuum filtration. The excess methanol in the product was evaporated off in a rotary evaporator under negative pressure. The product was separated into a FAME mixture and glycerol by gravity using a separatory funnel.
X-ray diffraction (XRD) patterns were acquired using a Rigaku SmartLab instrument (Japan) equipped with Cu-K
The conversion of triglyceride was analyzed by high-performance liquid chromatography (HPLC) using a CTO-10AS VP instrument and an Agilent Eclipse XDB-C18 column, both manufactured by Shimadzu. The FAME mixture was diluted with acetone, and the 20
The yield of fatty acid methyl ester (FAME) was calculated by the following expression:
The initial step in this study was the synthesis of ZIF-8 and oxidized ZIF-8, which were characterized by X-ray diffraction (XRD), and the corresponding data are presented in Figure 1(a). The observed relative peaks, including 011, 002, 112, 022, 013, 222, 114, 233, and 131, are in good agreement with previous reports, confirming the successful synthesis of ZIF-8 with a sodalite structure [28–30]. The crystallinity of the ZIF-8 was significantly improved after oxidation, as indicated by the higher intensity of each peak (XRD spectra are available in the supporting document section). Figure 1(b) shows the field emission scanning electron microscopy (FE-SEM) images of the oxidized ZIF-8, revealing an average particle diameter of approximately 150 nm, which facilitates their dispersion in aqueous solutions.
Fig. 1.
(a) XRD patterns of ZIF-8 and calcined ZIF-8, (b) FE-SEM images of calcined ZIF-8
The formation of ZIF/PECs occurs spontaneously through the complexation process of the PDDA, and PSS, driven by the association between opposite charges [31]. The ZIF-8 were initially dispersed in a PDDA solution due to their anionic character, resulting in the formation of the ZIF/PDDA mixture. Subsequently, the addition of PSS enables electrostatic interactions between PDDA and PSS, leading to the formation of the composite PECs with the ZIF-8. During the formation process, increasing ZIF-8 content tends to increase the stiffness and roughness of the PECs, as shown in the pictures in Figure 2. A smooth and transparent PEC membrane was prepared without the addition of ZIF-8 (0ZIF/PEC), while the incorporation of increasing amount of ZIF-8 led to the formation of a rough and brown xZIF/PEC. When compared to the soft and flexible PDDA/PSS PECs, the increasing content of ZIF-8 filler led to a harder and more brittle composite.
Fig. 2.
Pictures of PECs that (a) 0ZIF/PEC, (b) 10ZIF/PEC, (c) 30ZIF/PEC, (d) 50ZIF/PEC, (e) 70ZIF/PEC and (f) 90ZIF/PEC
As mentioned in the introduction, the main problem with composite materials is that the filler can hardly be accessible to the surface, which is a limiting factor for the utilization of reactive and catalytic particles. Here, to expose more ZIF-8 from the PECs during transesterification, a laser was used to erode the surface of the sample. As shown in Figure 3, engraving marks after laser are noticeable on the surface of the etched samples, while the surface of the PECs before etching was smooth, and shimmy appears rough and dull after etching.
Fig. 3.
Photo of PECs before (left side) and after (right side) laser etching
On the FE-SEM images in Figure 4, the morphology of each (0-90)ZIF/PEC after laser etching is visible. The scratches on the surface of 0ZIF/PEC-L are hardly noticeable because the transparent PEC lets the laser pass through, and almost none of the laser energy is absorbed by the sample. Similarly, only faint scratches can be seen on the surface of 10ZIF/PEC for the same reason. Nevertheless, when more than 30wt% of ZIF-8 was added to the PECs, a clear laser marking was visible on the surface of the samples. From image analysis, it can be calculated that the path of the laser for etching covers around 28% of the surface of the PECs (The calculation method is presented in supporting materials and Fig. S4). The etching lines could not be made in a tighter pattern due to the limitation of the stepper motor used to control the displacement of the laser.
Fig. 4.
FE-SEM images of (a) 0ZIF/PEC-L, (b) 10ZIF/PEC-L, (c) 30ZIF/PEC-L, (d) 50ZIF/PEC-L, (e) 70ZIF/PEC-L, and (f) 90ZIF/PEC-L
The distribution of ZIF-8 at the PECs surface after laser etching was analyzed by SEM with the EDS mapping mode, as presented in Figure 5. The Zinc element was chosen to be detected because only ZIF-8 contains Zn. Results demonstrate that a higher concentration of Zn is detected in the area etched by laser for samples containing more than 50 wt% ZIF-8, particularly 90ZIF/PEC-L, while a lower concentration is found in the area without laser etching. This indicates that during the compression process, ZIF-8 flows within the polymer PECs due to the surface energy and interaction between the polymers and ZIF-8. A laser with high energy can ablate the polymer surface and expose the ZIF-8 inside the PECs to the reactants, resulting in improved catalytic activity. A similar trend is observed in 30ZIF/PEC-L, but to a lesser extent due to the lower initial concentration of ZIF-8. In conclusion, despite being covered by PDDA/PSS during the preparation process of PECs, the surface of PECs can be etched by laser to reveal more ZIF-8 and improve the catalytic activity of the reaction.
Fig. 5.
FE-SEM images of xZIF/PEC etched by laser (left side) and corresponding EDS mapping of Zn distribution (right side): (a) 10ZIF/PEC-L, (b) 30ZIF/PEC-L, (c) 50ZIF/PEC-L, (d) 70ZIF/PEC-L and (e) 90ZIF/PEC-L
The XRD patterns of the samples before and after laser etching are presented in Figure 6. The patterns of 70ZIF/PEC and laser-ablated 70ZIF/PEC illustrate that almost no structural transformation occurred, demonstrating that ZIF-8 could maintain its complete crystal structure under laser etching, thus ensuring its catalytic activity during the reaction.
Fig. 6.
XRD patterns of 70ZIF/PEC before and after laser etching
Figure 7 shows the ATR-FTIR spectra of samples before and after lasing etching. In the spectra of PDDA, peaks observed at 3384 cm−1 and 1640 cm−1 are attributed to –NR3+ stretching vibration and –NH deformation vibration, and peaks around 2924 cm−1 and 1462 cm−1 are associated with C–H bending vibrations of [32]. In addition, peaks of PSS observed at 1040 cm−1 and 1005 cm−1 are designated as symmetric and asymmetric stretching vibrations of the sulfonate groups [32, 33]. For 70ZIF/PEC and 70ZIF/PEC-L samples, the peak at 755 cm−1 is attributed to the C-H bending of ZIF-8, and the peak at 1138 cm−1 is associated with the C–N stretch of ZIF-8 [34, 35]. The intensity of the characteristic peaks of ZIF-8 in 70ZIF/ PEC-L is stronger than that in 70ZIF/PEC, demonstrating that more ZIF-8 powder is exposed from the polymer of PDDA and PSS after laser etching. Noteworthy, peaks of 70ZIF/PEC and 70ZIF/PEC-L remained unchanged, indicating that there is no change in the chemical structure of PECs after laser etching.
Fig. 7.
ATR-FTIR spectra of PDDA, PSS, ZIF-8, 70ZIF/PEC, and 70ZIF/PEC-L
To ensure that the ZIF-8 powder exposed to laser (ZIF-8-L) had similar catalytic efficiency as the original ZIF-8, both powders were used to catalyze triglycerides in soybean oil (Fig. S5). The reaction rate and final conversion of ZIF-8-L were similar to those of ZIF-8, demonstrating that ZIF-8 remained active after being exposed to the laser. This result, along with the XRD pattern (Fig. 6), indicates that the laser had negligible influence on ZIF-8.
The yield of FAME and stability of the catalysts while being used in up to 5 cycles is shown in Fig. 8. Obviously, high yield can be achieved by 90ZIF/PEC-L and 70ZIF/PEC-L, and their activity decreased almost linearly after reusing the catalysts several times. Among all the xZIF/PEC catalysts, 90ZIF/PEC-L presented the best yield, but due to excessive brittleness, the catalyst appeared to break into small parts, which made it harder to collect at the end of the reaction. Considering both activity and mechanical strength, the 70ZIF/PEC-L would be a better choice as the membranes were able to maintain physical integrity while providing sufficient catalytic activity.
Fig. 8.
Triglyceride conversion catalyzed by ZIF/PECL with 10%, 30%, 50%, 70%, and 90% ZIF-8 loadings over five times usages
Therefore, PECs with 70% ZIF-8 were chosen for the comparative analysis of reaction activity before and after laser etching, as shown in Figure 9. The conversion of triglyceride using 70ZIF/PEC-L reached 90%, which is just slightly lower than the ZIF-8 powder but is known to be hard to collect at the end of the reaction, while 70ZIF/PEC reached only 63% conversion of triglyceride. Given that triglycerides undergo catalysis at the active sites on the ZIF-8 surface, augmenting the ZIF-8 content and employing laser etching proves advantageous in exposing more ZIF-8 to reactants, thereby facilitating enhanced mass transfer of molecules from the mixed solution to the active sites. In a similar conversion experiment, pure PDDA/PSS PECs do not have a catalytic function on the reaction, as manifested by a reaction rate similar to the blank experiment.
Fig. 9.
Conversion of triglyceride in soybean oil by powder calcined ZIF-8, PDDA/PSS PEC, 70ZIF/PEC, 70ZIF/PEC-L, and blank experiment
In this research, we have successfully fabricated catalytic PECs by employing oxidized ZIF-8 as the active material. The application of laser etching to expose ZIF-8 particles within the PECs has been demonstrated as an effective strategy to enhance their catalytic activity without compromising their structural integrity. The catalytic performance of these materials was assessed in the transesterification of triglycerides in soybean oil conversion rates. When considering both the catalytic performance and the mechanical stability of the membrane, the 70ZIF/PEC-L sample provided the best conversion while maintaining physical integrity of the membrane during the transesterification reaction. This work presents an innovative approach to catalytic PEC development, with potential applications in biodiesel production and other catalytic processes. The combination of PECs and laser etching opens up new possibilities for designing highly efficient and mechanically robust membranes with superior surface properties.