Fibrous intrinsically zeolitic pickering emulsifier

Abstract

The intrinsic amphiphilicity that enables grafting-free zeolite Pickering emulsification has been reported previously by us. However, only one type of anisotropic form, i.e., layer morphology, is reported. In this study, we successfully synthesized another anisotropic form, i.e., fibrous zeolitic TON, with high crystallinity and dispersion. Furthermore, we defined the anisotropic index to evaluate the anisotropy of the zeolite particles. The high anisotropy (anisotropic index = 17.7) and dispersion result in an amphiphilicity that facilitates the stabilization of the emulsion by the bare zeolite. To our surprise, the emulsion exhibits a very stable state in a basic environment, in contrast with the previous emulsion that is stable at low and medium pH (the Brønsted acid site will be quenched under alkaline conditions). In addition to the anisotropy and dispersion, the zeta potential of fibrous particles plays important roles regardless of the counterpart cations present in the zeolite (H⁺, Na⁺, or a mixture). This feature indicates their applicability in catalytic reactions under alkaline conditions. Finally, Pd nanoparticles are further deposited on zeolites, leading to a catalyst for the bromobenzene/water biphasic Suzuki–Miyaura C–C coupling. Pd/TON acts as not only an emulsifier but also a catalyst in the reaction, making Pd/TON a Pickering interfacial catalyst (PIC). The advantages of PICs are: (1) almost 100% product yield is achieved within 20 min; and (2) easy recovery of the product just by filtration or centrifugation, which makes the reaction system environmentally friendly.

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Introduction

An emulsion is a homogeneous system consisting of droplets of the dispersed phase dispersed within the continuous phase.¹ Pickering emulsions represent a class of emulsions formulated with ultrafine solid particles or solid colloidal particles, serving as alternatives to conventional surfactants for emulsification.² These systems exhibit notable advantages, including enhanced stability, ease of control, environmental sustainability, and cost-effectiveness.³ Consequently, they have found extensive applications across diverse sectors such as petroleum engineering,⁴ water treatment processes,⁵ cosmetics formulation,⁶ food technology,⁷ pharmaceuticals,^8,9 and materials science.¹⁰

The primary characteristic of solid particles that facilitate the formation of stable Pickering emulsions lies in their dual wetting properties, unique morphology (including size and shape), and concentration levels.^11,12 Among various types of solid particles, a significant proportion of inorganic solids can effectively function as Pickering emulsifiers. Among these, solid nanoballs, nanorods, and nanoplates are extensively utilized for the stabilization of Pickering emulsions. The self-assembly of these solid particles at the fluid interface enhances the stability of the Pickering emulsion.¹³ However, previous studies have shown that the self-assembly structure of nanoscale spherical solid particles formed at the oil–water interface is mainly controlled by direct interactions between particles, while capillary interactions seem to dominate for non-spherical particles.¹⁴ There is a clear difference between anisotropic particles, including a lowering of the percolation threshold and an expansion of the range of coverage density achieved through random blocking.¹⁵ Highly anisotropic nanoparticles can significantly enhance the viscoelastic properties of the oil–water interface while concurrently diminishing the interfacial tension between these two phases.¹⁶

The catalytic system in which solid particles at the liquid/liquid interface of Pickering emulsions can act as both emulsifiers and catalysts is called the Pickering interfacial catalytic (PIC) system.¹⁷ The solid particles are highly dispersed at the liquid/liquid interface, which makes the Pickering emulsion more stable and allows for a significant increase in catalytic reaction efficiency.^18,19 Therefore, particles with moderate wettability and amphiphilicity are expected to stabilize emulsions. However, most heterogeneous catalysts, including zeolites, are hydrophilic and need grafting modification to improve this weakness.^5,20–23 Although the final materials could emulsify the biphasic system, the amphiphilicity is extrinsic, i.e. not naturally born. Most inorganic solid particles, such as clay minerals, exhibit limited ability (intrinsic amphiphilicity) for stable emulsion formation without any modification or treatment.²⁴ Direct emulsification leads to inadequate emulsion stability or necessitates higher concentrations of solid particles. In many cases, surface grafting of organosilanes or co-emulsification with organic surfactant molecules is required. This conclusion is also true for the zeolite before our previous work. Table S1 shows a comparison between this work and the previous preparation of emulsions with inorganic solid particles.

We previously demonstrated a grafting-free MWW layer zeolite that could emulsify a water/oil system.²⁵ The intrinsic amphiphilicity could be attributed to the ABCD criteria: (A) anisotropy; (B) Brønsted acid; (C) covalency; (D) dispersity. On one side, besides the layer type of anisotropy, will fibrous anisotropy work? On the other side, the genesis of amphiphilicity relies on strong Brønsted acid sites.²⁶ This means the amphiphilicity will be lost at high pH. Can the zeolite stabilize the emulsion in alkaline media? More importantly, even though anisotropy is a well-known concept that describes the particle morphology divided from a cube/sphere, there is no literature providing a quantitative definition to the best of our knowledge. Therefore, all the issues should be addressed in the research work.

In this work, a fibrous zeolite, TON intrinsic Pickering emulsifier, that emulsifies oil–water mixtures without any surface modification is reported. By modifying the synthesis formula in the literature,²⁷ we obtained a TON zeolite with high anisotropy (anisotropic index = 17.7) and dispersion, which is responsible for the intrinsic amphiphilicity. Subsequently, the Pd-loaded TON zeolite successfully stabilized the bromobenzene/water emulsion system and catalyzed the Suzuki–Miyaura reaction in the Pickering emulsion system, exhibiting outstanding catalytic performance (Fig. 1).

Fig. 1 Experimental design of the sample preparation and PIC catalytic reaction.

Results and discussion

A series of samples were prepared with gel compositions: 0.03 Na₂O/0.004 Al₂O₃/1.0 SiO₂/0.45 HDA (1,6-hexamethylenediamine)/50 H₂O/xTEABr, where x = 0, 0.05, 0.10, 0.15, or 0.20, named TON-N or TON-xT. The XRD patterns of the samples are shown in Fig. 2a and SEM images in Fig. 2b–f. Firstly, we attempt the synthesis with HDA only, and TON-N is obtained as a pure phase²⁸ with a hollow sphere interwoven by zeolite needles (diameter ranging from 180–250 nm and an average length of 2 μm). When tetraethylammonium bromide (TEABr) is introduced with x = 0.05 and 0.10, the agglomerated fiber spheres gradually transform into highly dispersed nanofibers, accompanied by a significant increase in single crystal length (Fig. 2c and d). However, as the content of TEABr further increases (x = 0.15, 0.20), the zeolite crystallinity significantly decreases.

Fig. 2 (a) XRD patterns of TON-N and TON-xT (gel compositions: 0.03 Na₂O/0.004 Al₂O₃/1.0 SiO₂/0.45 HDA/50 H₂O/x TEABr). SEM images of (b) TON-N, (c) TON-0.05T, (d) TON-0.10T, (e) TON-0.15T and (f) TON-0.20T.

The average particle size dimensions of the samples are: ā = 152 nm, b̄ = 160 nm, and c̄ = 3390 nm, according to the orientation displayed in Fig. S1b and c. Based on the statistical data and the definition of A (anisotropic index, eqn (2) in Methods), that gives A = 17.7. We posit that an increase in the value of A correlates with enhanced anisotropy of solid particles, which in turn facilitates more favorable conditions for emulsion formation.

The emulsifying properties of TON-N and TON-0.05T were further investigated by evaluating their capacity to form emulsions in an n-tetradecane/water system. As depicted inFig. 3a and b, the dispersion degree of the zeolite significantly influences the formation of an emulsion. TON-N produced unstable emulsions with rapid phase separation; the use of TON-0.05T, which exhibits a higher level of dispersion, results in stable Pickering emulsions that can persist for extended periods with relatively uniform droplet diameters around 75 μm (Fig. 3c and d). To more precisely define the size of the oil–water interface, specifically the contact area of oil–water phases in the emulsion, we suggest the interfacial area calculation formulas (eqn (3) and (4) of the emulsion as seen in the SI). It is evident that the formation of an emulsion system significantly increases the contact area between solid particles/oil/water systems, thereby accelerating and improving the efficiency of the catalytic reaction.

Fig. 3 Photographs and optical microscopy image of tetradecane-in-water emulsions stabilized by 1.0 wt% (a) TON-N and (b and c) TON-0.05T. (d) The size distribution histogram of a Pickering emulsion stabilized by 0.5 wt% TON-0.05T. (e) CLSM image of the tetradecane-in-water emulsion stabilized by Rhodamine B labeled TON-0.05T. (f and g) SEM images of a paraffin-in-water emulsion stabilized by 1.0 wt% TON-0.05T. (h) Change of the three-phase contact angle of TON-0.05T with time. (i) Image of the stable three-phase contact angle of TON-0.05T. (j) Water/tetradecane interfacial tension with and without 0.5 wt% TON-0.05T and zeta potential of TON-0.05T at 25 °C.

To explore the mechanism of Pickering emulsion formation, TON-0.05T was stained with Rhodamine B, and the emulsion was visualized with Confocal Laser Scanning Microscopy (CLSM). A bright red circle around the oil droplets and red fluorescence signals in the continuous phase can be observed. These indicate the presence of TON-0.05T zeolite fibers plays two roles: (1) wrap up the oil droplets; (2) assemble into a three-dimensional structure within the continuous phase to keep distance between droplets (Fig. 3e); both factors stabilize the emulsion.²⁹ Due to the liquid state of tetradecane at room temperature, conventional scanning electron microscopy cannot be used to observe tetradecane/water emulsions. Therefore, 52# solid paraffin was employed as a substitute for tetradecane in the oil phase to prepare a Pickering emulsion. As depicted in Fig. 3f, SEM analysis of the solid paraffin emulsion exhibited consistency with microscopic observations, revealing an average oil drop size of approximately 75 μm. By zooming in, a substantial amount of TON-0.05T zeolite is uniformly distributed on the surface of the oil droplets (Fig. 3g), effectively preventing coalescence and ensuring the stable existence of the emulsion. These findings highlight the favorable impact of the zeolite's high dispersion on emulsion formation and indicate that the zeolite fiber is an excellent support for Pickering interfacial catalysts. We performed an emulsion stabilization mechanism experiment solely using grafting-free TON-0.05T.

To investigate the amphiphilicity of the zeolite, contact angle tests were conducted on TON-0.05T zeolite. The hydrophilicity of TON-0.05T is evident from the results shown in Fig. 3h, the video image capture method was employed for analysis, and the final stable three-phase contact angle is 31.9° (Fig. 3i), indicating the hydrophilicity of TON-0.05T. At the same time, the interfacial tension of tetradecane–water was decreased slightly with TON-0.05T (Fig. 3j), which facilitated emulsion formation. It is generally believed that when the zeta potential value is greater than 30 mV or less than −30 mV, the agglomeration of particles tends to weaken due to the increase of surface charge and the increase of repulsion force between particles. In contrast, when the zeta potential value is in the range of −30 to 30 mV, the solid particles tend to aggregate.³⁰ At 25 °C, the zeta potential of TON-0.05T is −46.5 mV, indicating that zeolite is highly dispersed in water, which is consistent with SEM, which grants it great potential for emulsification.

To gain a more comprehensive understanding of the emulsification ability of TON zeolite, we carried out emulsification experiments of TON-0.05T, H-TON, and Na-TON in different pH environments. As can be seen fromFig. 4a–c, poor emulsification ability is shown in an acidic environment, a large amount of demulsification occurs, oil droplets condense rapidly, and zeolite falls off from the oil–water interface. It can be seen from the microscopic images (Fig. S2) of the emulsion that as the pH value of the system increases, the particle size distribution of the emulsion gradually decreases, and the emulsion tends to be more stable.

Fig. 4 Photographs of Pickering emulsions prepared from (a) TON-0.05T, (b) H-TON and (c) Na-TON at different pH values (the numbers in the boxes indicate the pH values of the systems). (d) Graph of the variation of zeta potential on the TON zeolite with different pH values. (e) Variation diagram of droplet diameters of emulsion and the interfacial contact area between oil and water with pH value. (f) Water/tetradecane interfacial tension with TON-0.05T, H-TON and Na-TON at 25 °C. (g) NH₃-TPD curve profiles of TON-0.05T, H-TON, and Na-TON. All emulsions were observed after 24 h of standing at 25 ± 2 °C. The DFT-optimized adsorption configuration of tetradecane on TON: (h) pH = 7, ΔE_abs = −0.79 eV; (i) pH = 13, ΔE_abs = −1.63 eV.

Concurrently, we also conducted zeta potential tests on TON-0.05T, H-TON, and Na-TON zeolites in different pH environments (Fig. 4d). It was found that in acidic environments, the zeta potential tended to approach the isoelectric point (0 mV), which would make the zeolite particles more prone to agglomeration and thus less conducive to emulsification. In contrast, in neutral or alkaline environments, the zeta potential was always less than −40 mV, making the zeolite particles exhibit a more dispersed state in the dispersed phase. This result also corresponds to the emulsification phenomenon. Based on the formulas in eqn (3) and (4), the oil–water interface contact area of the emulsion stabilized by TON-0.05T at different pH values was calculated (Fig. 4e). It can be seen that the contact area gradually increases with the increase of pH value. At pH = 13, the contact area reached an astonishing 7285.9 cm² cm⁻³. The same situation occurred in Na-TON zeolite and H-TON zeolite. This is probably because the presence of H⁺ in an acidic environment leads to the protonation of the silico-hydroxyl group (Si-OH) of the zeolite, which makes the zeolite more hydrophilic and reduces the emulsification performance. In contrast, in an alkaline environment, the emulsion layer exhibits a very stable state with almost no zeolite loss. At the same time, the interfacial tension of tetradecane–water was decreased slightly with TON-0.05T, H-TON and Na-TON (Fig. 4f), which facilitated emulsion formation. Therefore, this zeolite is particularly well-suited for emulsion preparation and catalysis in alkaline media.

The NH₃-TPD curves are presented in Fig. 4g; the curves of all samples exhibit two NH₃ desorption peaks in the low-temperature region of 150–300 °C (T_dL) and the high-temperature region of 300–500 °C (T_dH), which suggests the presence of weak and strong acid sites, respectively. The coordination environment of Si and Al species was investigated using solid-state NMR. Only an intense peak around 55 ppm was observed in ²⁷Al MAS NMR spectra (Fig. S3a), which can be assigned to the tetrahedral coordinated Al in the framework.³¹ For the ²⁹Si MAS NMR spectra shown in Fig. S3b, five peaks at −114, −113, −111, −110, and −105 ppm were deconvoluted. The first four peaks were associated with Si(0Al) units, while that at −105 ppm corresponded to Si(1Al) units;³² the results are presented in Table S2. The Si/Al atomic ratio of TON-0.05T from the ²⁹Si MAS NMR spectra³³ is 55.8 (calculation procedure as seen in the SI), which is consistent with the result obtained from the ICP elemental analysis in Table S3. A higher Si/Al ratio will make zeolite possess a certain degree of lipophilicity, thereby further enhancing its emulsifying ability.

Therefore, we are trying to rationalize the emulsification mechanism of fibrous morphology compared with the layer one. Firstly, due to its strong anisotropy (same as that of the layer one), TON-0.05T exhibits a fibrous morphology with an anisotropic index as high as A = 17.7. Secondly, high dispersion (same as that of the layer one) avoids mutual interference to properly distribute on the oil-water interface. Thirdly, the high Si/Al ratio endows the zeolite with a certain lipophilic character (higher than that of the layer one, MWW Si/Al ∼10). Finally, the zeta potential (rather than the Brønsted acid) is responsible for the switch of emulsification. Hence, a deeper understanding is necessary to reach a comprehensive theory for both layer and fibrous morphology. Therefore, density functional theory (DFT) calculations were employed to investigate the adsorption mechanism of the TON zeolite toward a model oil (tetradecane) under neutral (pH = 7) and alkaline (pH = 13) conditions. In the neutral system (pH = 7), no explicit hydroxide ions (OH⁻) were introduced (Fig. S4a). In contrast, under the alkaline conditions (pH = 13), three OH⁻ ions were explicitly placed on the surface of the TON zeolite to simulate the high-pH environment (Fig. S4b). Following DFT geometry optimization, these OH⁻ ions were found to form multiple hydrogen bonding interactions with the zeolite framework. Subsequently, the adsorption behavior of tetradecane was compared between the two systems. The calculated adsorption energy of tetradecane on the TON surface at pH = 7 was −0.79 eV (Fig. 4h). In the pH = 13 system, the surface-located OH⁻ ions exhibited a synergistic effect on tetradecane adsorption, effectively stabilizing the hydrocarbon molecule and enhancing the overall adsorption affinity, as evidenced by the significantly increased adsorption energy of −1.63 eV (Fig. 4i). These theoretical findings, in conjunction with previous experimental studies, support the consistency of the observed adsorption trends with fundamental physicochemical principles, thereby providing a rational theoretical basis that complements and reinforces existing experimental results.

Based on the above, a simple surface impregnation method was used to deposit Pd nanoparticles on the surface of TON-0.05T. According to ICP data analysis in Table S3, the content of Pd nanoparticles is 0.91 wt%. Fig. 5a shows the X-ray diffraction (XRD) pattern of raw and Pd-impregnated zeolites. No peaks corresponding to the palladium species can be identified in the XRD pattern. This phenomenon may be due to the small size of Pd nanoparticles and the wide dispersion of Pd species in TON-0.05T zeolite. This result also confirms that the skeleton topology of zeolite can still be preserved intact after Pd impregnation treatment for a long time. Subsequently, TEM characterization was performed on the prepared solid catalyst, as shown in Fig. 5b. The results show that Pd nanoparticles are uniformly deposited on the fibrous surface of TON-0.05T, and the size of Pd nanoparticles on the zeolite surface is about 3.2 nm (Fig. 5c). To evaluate whether the emulsification capability of TON was significantly affected by Pd loading, the contact angle and oil–water interfacial tension of Pd/TON-0.05T were measured using the same characterization methods employed for TON-0.05T. As shown in Fig. S5a, the final contact angle of Pd/TON-0.05T was determined to be 32.5°. Upon the introduction of Pd/TON-0.05T, the interfacial tension between tetradecane and water decreased, reaching a final value of 28.6 mN m⁻¹. Moreover, the zeta potential remained similar to that of the Pd-free TON sample, with a measured value of −45.1 mV (Fig. S5b). These results indicate that the incorporation of Pd does not significantly alter the emulsification performance of TON toward oil and water. Further experimental outcomes consistently support this conclusion.

Fig. 5 (a) XRD patterns of TON-0.05T and Pd/TON-0.05T, (b) TEM image of Pd/TON-0.05T, showing the highly dispersed Pd NPs on the support, (c) Pd NP size distribution of TON-0.05T. (d) Scheme for the Suzuki–Miyaura reaction catalyzed by Pd/TON-0.05T. (e) Optical microscopy and photograph images of the mixture of phenylboronic acid in water and bromobenzene in the presence of Pd/TON-0.05T (successfully emulsified). (f) Photograph of the reaction product. (g) The scheme of the catalytic reaction. (h) TEM image of Pd/TON-0.05T reacted for 5 cycles. (i) Time course change in the conversion of bromobenzene over Pd/TON-0.05T. (j) Durability test of Pd/TON-0.05T in the Suzuki–Miyaura reaction. (k) XRD patterns of fresh and reacted (for 5 cycles) Pd/TON-0.05T.

As shown in Fig. 5d, Pd/TON-0.05T is adsorbed at the oil–water interface to catalyze the Suzuki–Miyaura reaction. In this reaction, bromobenzene and phenylboric acid aqueous solutions were violently stirred under the action of a solid catalyst to obtain a Pickering emulsion reaction system (Fig. 5e). After the reaction at 60 °C for 20 minutes, the solid reaction product was obtained after rapid cooling to room temperature (Fig. 5f). After filtering and separating the reaction products, the solid catalyst can be recovered for the next catalytic experiment. As can be seen from Fig. 5i, the conversion rate of reaction products reached 99% when the reaction time was 30 minutes, and the reaction activity of the solid catalyst was still retained after five cycles of testing (Fig. 5j). Upon calculation, the turnover frequency (TOF) values for Pd/TON-0.05T reached up to 702 h⁻¹ (Table S4), indicating the potent reactivity of the zeolite-supported palladium catalyst. In addition, XRD patterns and TEM images of the solid catalyst showed no significant changes after five cycles (Fig. 5k and h). It can be seen from the above experimental data that Pd/TON-0.05T has good emulsification performance and catalytic capacity, and can catalyze the Suzuki–Miyaura reaction efficiently and can be reused. As shown in Table S4, under similar conditions, our Pd/TON-0.05T catalyst exhibits superior catalytic activity compared with other supported catalysts. This comparison endeavors to highlight the differential catalytic performance, efficiency, and stability of various catalysts under similar reaction conditions.

Conclusions

In summary, the successful preparation of stable Pickering emulsions is attributed to the intrinsic amphiphilicity of TON-0.05T zeolite. This reliable amphiphilicity is obtained through the combined effects of the highly anisotropic morphology of the zeolite, its high dispersion stability, the high Si/Al ratio, and the high absolute value of zeta potential at high pH. Finally, the TON-0.05T zeolite loaded with Pd successfully stabilized the bromobenzene/phenylboric acid aqueous solution emulsion system and demonstrated superior conversion rates for interfacial catalytic reactions.

Using organic reactions in the aqueous phase, we have demonstrated that the catalyst exhibits significant activity at the interface between the reactants and water, and can be easily separated from the reaction mixture. This novel catalyst provides a method to reduce the harmful surfactants and environmentally unfriendly organic solvents generated during processing. These results indicate that zeolite-based Pickering interface catalysts have great potential as high-performance sustainable chemical catalysts.

Author contributions

S. W. and J. J. carried out experiments on zeolite design and characterization, product identification and quantification, and determination of the reaction pathway. X. H. and F. M. helped with experiments on product characterization and catalyst synthesis. W. J., Z. Z., Y. Z. and Y. Y. helped with catalyst synthesis and characterization, and experiments to understand the reaction mechanism. J. J. conceptualized and oversaw the study. S. W. and J. J. co-wrote the paper.

Conflicts of interest

J. X. J., S. J. W. and Z. F. Z. have filed a patent on the preparation and application of PIC catalysts. The remaining authors declare no competing interest.

Data availability

All data are available in the main text and the supplementary information (SI) or available from the authors upon reasonable request. Supplementary information is available. See DOI: https://doi.org/10.1039/d5sc04922a.

Acknowledgements

We acknowledge the financial support by the National Natural Science Foundation of China (No. 22372200), Talent Work Group of the Jiangxi Provincial Committee of China (No. jxsq2023102006), and Guangdong Provincial Department of Science and Technology (No. 2021B1212050005).

Notes and references

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