One-pot synthesis of UiO-66@SiO₂ shell–core microspheres as stationary phase for high performance liquid chromatography

Abstract

The unusual properties of ultrahigh surface area, adsorption affinity and shape selectivity make metal–organic frameworks (MOFs) a promising candidate as the stationary phase for high performance liquid chromatography (HPLC). However, the problems of high column backpressure and low column efficiency resulting from the direct packing of irregular MOF particles still remain in the HPLC separation. Herein, a facile one-pot synthesis method for the fabrication of MOFs@SiO₂ shell–core microspheres was developed with aminosilica as the supporting substrate to grow the MOF shell. The density and particle size of the MOF shell could be easily controlled by adjusting the concentration of reactants, reaction temperature and time. UiO-66 (UiO for University of Oslo) was chosen as a model MOF because of its excellent chemical stability and unique reverse shape selectivity. The selected compounds including xylenes and ethylbenzene were effectively separated on the prepared UiO-66@SiO₂ packed column with high resolution, good reproducibility and low column backpressure. The UiO-66@SiO₂ packed column showed both reverse shape selectivity and a molecular sieving effect, making it attractive for the separation of structural isomers. Besides, the retention of analytes was also ascribed to the synergic effect of the hydrogen bonding between analytes and the amino groups of aminosilica, the hydrophobic effect and the π–π interaction between analytes and the aromatic rings of the UiO-66 shell.

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1. Introduction

Metal–organic frameworks (MOFs) have received much attention due to their outstanding properties, such as ultrahigh surface area, excellent thermal stability, diverse structures and pore topologies, and tuneable surface properties.^1–3 These unique features make MOFs attractive as highly efficient adsorbents and novel separation media.^4–10 Recently, a series of MOFs, for instance, MIL-53,^11–14 MIL-47,^14–16 MIL-101,^17,18 MOF-5,¹⁹ HKUST-1,^19–21 MIL-100,²² ZIF-8 (ref. 23) and UiO-66,²⁴ have been utilized as stationary phases for high performance liquid chromatography (HPLC). However, in comparison with the extensive studies carried out on gas chromatography (GC) separation,^25–28 the attempts at HPLC separation using MOFs as stationary phase are largely lagging behind. The problems of high column backpressure, low column efficiency and undesirable peak shape still remain in the HPLC application, which is caused by the direct packing of MOFs particles with irregular morphology, sub-micrometer size and wide particle size distribution.

To avoid these problems, an effective strategy is to fabricate uniform spherical MOFs composites for HPLC, which combines the excellent separation ability of MOFs with the perfect packing property of spherical substrates. It is known that silica microspheres are standard packing materials for HPLC columns, thus in the previous reports MOFs were synthesized in the mesopores of silica²¹ or mixed with silica microspheres.²⁹ However, the fabrication of MOFs@SiO₂ shell–core microspheres is a more valuable strategy to improve chromatographic performance. Yan et al. reported the preparation of monodisperse ZIF-8@SiO₂ shell–core microspheres for HPLC separation of endocrine-disruption chemicals and pesticides.²³ Zhang et al. prepared HKUST-1 nanocrystals on sphere-on-sphere silica microspheres for the fast separation of toluene/ethylbenzene/styrene and toluene/o-xylene/thiophene.²⁰

Although a variety of MOFs shell–core composites have been prepared by heterogeneous nucleation and growth of MOFs on the surface of carboxylate or hydroxylate-modified substrates, including silica,^30,31 polystyrene,³² Fe₃O₄,^33,34 and Al₂O₃ (ref. 35) spheres, few composites were explored as HPLC stationary phases up to now. Moreover, in most studies, the density of MOFs was controlled through altering the number of growth cycles, which would be complicated and hardly controlled.^23,31–33 Herein, in this work, a facile one-pot synthesis method for the preparation of MOFs@SiO₂ shell–core microspheres was developed. The loading amount of MOFs in shell–core structures could be easily regulated by adjusting the concentration of reactants, reaction temperature and time. We also made our initial efforts to assemble UiO-66 (UiO for University of Oslo) onto amino-terminated silica microspheres to prepare a novel HPLC stationary phase.

UiO-66 is a zirconium based MOF built up from hexamers of eight coordinated ZrO₆(OH)₂ polyhedra and terephthalate ligands.³⁶ UiO-66 possesses a cubic rigid 3D porous structure consisting of octahedral cavities with a diameter of 1.1 nm and tetrahedral cavities with a diameter of 0.8 nm, which are connected with narrow triangle windows of a diameter of 0.5–0.7 nm. UiO-66 was used as an example of MOFs here because of its excellent stability and unique reverse shape selectivity. Except for good thermal stability, UiO-66 also exhibits a suitable mechanical stability as well as an excellent chemical resistance toward solvents such as water, benzene, dimethylformamide (DMF) and acetone.³⁶ In addition, it is worth noting that UiO-66 is the first MOF that is observed to have reverse shape selectivity.³⁷ Multicomponent equimolar breakthrough experiments showed that the adsorption hierarchy of structural isomers in UiO-66 is opposite to the one observed in conventional adsorbents.³⁷ The UiO-66 shaped as powder, agglomerates and tablets also displayed an interesting o-xylene preference in the liquid phase, presenting reverse shape selectivity for the xylene isomers.³⁸ UiO-66 particles were furthermore coated onto capillary column as the stationary phase for GC and performed reverse shape selectivity for the preferential retention of branched alkane isomers over their linear isomer.³⁹ Recently, silica–UiO-66 composite was synthesized as HPLC stationary phase.²⁴ However, the prepared composite was merely a mixture of silica spheres and UiO-66 nanoparticles, with few UiO-66 attached on silica surface, which may result in the problems of instability, wide particle size distribution and the leakage of UiO-66 nanoparticles through the chromatographic column filter.

In this work, the fabrication of UiO-66@SiO₂ shell–core microspheres as a novel HPLC stationary phase was reported for the separation of small organic molecules including xylene isomers and ethylbenzene. The slurry-packed UiO-66@SiO₂ column showed high resolution, short analysis time and good reproducibility for the separations of neutral, basic and acidic analytes, exhibiting both reverse shape selectivity and molecular sieving effect, which makes it promising for the chromatographic separation of positional isomers.

2. Experimental

2.1 Chemicals and reagents

All of the organic reagents were of analytical grade and used without further purification unless otherwise indicated. Aminosilica (5 μm in diameter) was obtained from Tianjin Borui Jianhe Chromatographic Technology Co., Ltd. (China). Zirconium chloride (ZrCl₄) was from Strem Chemicals (USA). Terephthalic acid (H₂BDC) was purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Acetic acid and phenol were from Tianjin Fuchen Chemical Reagent Plant (China). Xylenes, ethylbenzene, dichloromethane, chlorobenzene, 1,3,5-trimethylbenzene (TMB), p-chloroaniline, aniline, N,N-dimethylaniline and p-phenylenediamine were all obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Benzene, toluene, nitrobenzene and DMF were purchased from Beijing Chemical Works (China). 2-Chlorophenol and 2,4-dichlorophenol were from AccuStandard Inc. (USA). Acetonitrile (ACN, HPLC grade) was purchased from Amethyst Chemicals (China).

2.2 Synthesis of UiO-66@SiO₂ shell–core composites

The synthesis of UiO-66@SiO₂ shell–core composites was carried out by immobilization of Zr⁴⁺ ions on aminosilica particles followed by solvothermal synthesis in the presence of H₂BDC solution. Briefly, ZrCl₄ (0.08 g; 0.16 g; 0.32 g; 0.64 g; 1.28 g; 2.56 g) was dissolved with 40 mL of DMF in a 250 mL round-bottom flask. 0.5 g of aminosilica was added and stirred at room temperature for 1 h to anchor the Zr⁴⁺ ions onto the surface of silica microspheres by the coupling of Zr⁴⁺ and amino groups. Afterwards, a corresponding amount of H₂BDC and acetic acid in 40 mL of DMF were added to the mixture. The reaction was then carried out at 120 °C for 24 h under stirring. After cooling, the prepared composites were collected by centrifugation at 750 rpm for 1 min and washed several times with DMF to remove the unreacted precursors. The individual UiO-66 nanoparticles in the products were also removed readily from the microspheres due to their density difference during the centrifugation process. The obtained UiO-66@SiO₂ composites were then immersed in a dichloromethane solution for 3 days, and the dichloromethane was changed once every day. The white solids were subsequently collected by centrifugation and dried at 60 °C under vacuum. Finally, the composites were activated under vacuum at 190 °C overnight to remove the solvent molecules in the micropores. The obtained products prepared with different concentration of precursors were referred as UiO-66@SiO₂-0.08/0.16/0.32/0.64/1.28/2.56 according to the dosage of ZrCl₄. The detailed experimental conditions were supplied in Table S1, ESI.† Furthermore, to prepare UiO-66@SiO₂ particles for HPLC assessment, a scaled-up synthesis method was used.

Except for varying the concentration of precursor solution, the loading amount of UiO-66 on the shell–core microspheres was also controlled by altering the reaction temperature (85 °C, 100 °C, 110 °C, 120 °C) and time (3 h, 6 h, 12 h, 24 h), under otherwise identical reaction conditions to those of UiO-66@SiO₂-0.64, the detailed experimental conditions were listed in Table S1, ESI.†

UiO-66 nanocrystals were also prepared for comparison. Typically, 0.64 g of ZrCl₄ in 40 mL of DMF, 0.456 g of H₂BDC in 40 mL of DMF and 4.0 mL of acetic acid were mixed in a 250 mL round-bottom flask, and then the mixture was stirred at 120 °C for 24 h. After cooling down to room temperature, the white solid was collected by centrifugation and washed thoroughly with DMF. The obtained UiO-66 was then immersed in a dichloromethane solution for 3 days, dried at 60 °C under vacuum, and finally activated under vacuum at 190 °C overnight.

2.3 Chromatographic separation

The as-prepared UiO-66@SiO₂ composites and aminosilica microspheres (about 1.7 g) were slurry-packed into stainless steel tubes (150 mm × 4.6 mm i.d.) using a mixture of 10 mL of chloroform and 8 mL of cyclohexanol as homogenate solvent. Packing pressure was 250 bar and a mixture of methanol and isopropanol (50/50, v/v) was used as displacement liquid. All chromatographic separations were performed on an Agilent 1100 HPLC system equipped with a quaternary pump, Rheodyne 7725i injector with 20 μL sample loop and UV-Vis detector. The separation performance of UiO-66@SiO₂ packed column was compared with the commercial C18 column (LabTech C18, 150 mm × 4.6 mm i.d., 5 μm).

2.4 Characterizations

The surface morphologies of particles were observed on a Sirion 200 field emission scanning electron microscopy (SEM, FEI, Holland). Brunauer–Emmett–Teller (BET) specific surface areas were obtained by nitrogen adsorption on Micromeritics ASAP 2010C (Micromeritics, USA). Fourier transformed infrared spectra (FTIR, Perkin Elmer, USA) were recorded using a spectrometer in the frequency range of 600–4000 cm⁻¹ with a resolution of 4 cm⁻¹. Thermogravimetric analyses (TG, Mettler Toledo, Switzerland) for the samples were carried out in the range of 30–900 °C at a ramp rate of 10 °C per minute with oxygen as the purge gas. A D8 Advance X-ray diffractometer (XRD, Bruker, Germany) was used to measure the X-ray diffraction patterns using CuKα radiation (λ = 1.5406 Å).

3. Results and discussion

3.1 Characterization of UiO-66@SiO₂ shell–core microspheres

The silica microspheres terminated with amino groups were used as nucleation seeds and meanwhile the scaffold for the growth of UiO-66 nanocrystals. The amino groups could bind Zr⁴⁺ ions to form the Zr⁴⁺@SiO₂ precursor. Subsequently, UiO-66 nanoparticles were grown from the Zr⁴⁺@SiO₂ precursor by solvothermal synthesis in the presence of H₂BDC solution. The overall synthetic procedure of the UiO-66@SiO₂ shell–core microspheres is shown in Fig. 1.

Fig. 1 Synthetic procedure of UiO-66@SiO₂ shell–core microspheres.

The surface morphologies of aminosilica and UiO-66@SiO₂ shell–core microspheres prepared with different concentration of precursor solutions are shown in Fig. 2 and S1, ESI.† It can be seen that the density and particle size of UiO-66 crystals were easily controlled by adjusting the concentrations of reactants. Few UiO-66 particles were formed on the surface of aminosilica under a low precursor concentration (Fig. S1A, ESI†), and meanwhile a portion of silica microspheres were crushed. While as shown in Fig. 2C–H, the synthesized UiO-66 nanocrystals were uniformly and closely bound to the silica spheres, and the size of UiO-66 particles increased from about 60 nm to 250 nm with the precursor concentration increased. The SEM images also reveal the monodispersity and narrow particle size distribution of the prepared composites. However, with further increment of precursor concentration, an excess of individual UiO-66 nanoparticles existed in the product which were difficult to completely remove from the composites by centrifugation (Fig. S1B, ESI†). A higher precursor concentration gave rise to the formation of the irregular lamellar UiO-66, as can be observed from Fig. S1C, ESI.† The above SEM results indicate the successful fabrication of UiO-66 shell on silica core with controllable density and particle size.

Fig. 2 SEM images of (A and B) NH₂–SiO₂; (C and D) UiO-66@SiO₂-0.16; (E, F) UiO-66@SiO₂-0.32; (G, H) UiO-66@SiO₂-0.64.

FTIR spectra (Fig. 3A) were further recorded to evaluate the fabrication of UiO-66@SiO₂ composites. In the FTIR spectra of UiO-66@SiO₂ composites, the appearance of the characteristic peaks of UiO-66 at around 1664, 1585, 1506, 1395 and 745 cm⁻¹ confirms the successful incorporation of UiO-66 onto the aminosilica. Moreover, the intensity of these characteristic peaks increases gradually with the concentration of precursor solutions, indicating a progressively increasing loading amount of UiO-66 nanocrystals. Besides, the peaks at 1102 and 798 cm⁻¹ are assigned as the stretching vibration of Si–O–Si and bending vibration of SiO–H, respectively. The XRD results could also proof the formation of UiO-66@SiO₂ composites, as shown in Fig. 3B. Both the characteristic peaks of UiO-66 and the characteristic signal of aminosilica at 20–25° are observed on the XRD patterns of UiO-66@SiO₂ composites. Additionally, the signal intensity of the characteristic peaks of UiO-66 increases with the concentration of precursor solutions, which is consistent with the FTIR results.

Fig. 3 (A) FTIR spectra; (B) XRD patterns; (C) N₂ adsorption–desorption isotherms (empty symbols: adsorption, filled symbols: desorption); (D) TG curves of aminosilica, UiO-66 and UiO-66@SiO₂ composites.

From Fig. 3C, the isotherm curve of UiO-66 by N₂ sorption exhibits a type I plot, indicating its typical microporous structure. However, the isotherm curves of the UiO-66@SiO₂ composites show a type IV plot due to the mesoporous structure of silica microspheres. The increased BET specific surface areas of the UiO-66 coated composites (276.0 m² g⁻¹ for UiO-66@SiO₂-0.16, 308.9 m² g⁻¹ for UiO-66@SiO₂-0.32 and 371.1 m² g⁻¹ for UiO-66@SiO₂-0.64) as compared to the bare aminosilica (269.9 m² g⁻¹) also imply the effective loading of UiO-66 (930.6 m² g⁻¹) on the silica surface. For the three kinds of.

UiO-66@SiO₂ composites, the ratio of micropore area to total surface area rises from 10.1% to 45.9% with the increment of UiO-66 loading amount. Thermogravimetric analysis results (Fig. 3D) show that the composites contain four parts in weight loss, which correspond to the surface water evaporation (<403 K), the loss of solvent molecules trapped inside the micropores (473–573 K), the decomposition of the organic linkers of UiO-66 (753–793 K) and the decomposition of amino groups on aminosilica (473–773 K). The residue was identified to be SiO₂ and ZrO₂. Based on the TG weigh losses, the weight percentages of UiO-66 in the composites were calculated to be 12.7% for UiO-66@SiO₂-0.16, 18.6% for UiO-66@SiO₂-0.32 and 26.2% for UiO-66@SiO₂-0.64.

The density and particle size of UiO-66 nanocrystals could also be tuned through altering the reaction temperature and time, since they are the key factors that influence the crystal growth.

Comparing the SEM images in Fig. 4A–C and 2H as well as the FTIR spectra in Fig. 4D, it can be seen that the sample prepared at 85 °C did not exhibit obvious formation of UiO-66 particles owing to the low crystallinity of UiO-66 under this condition. However, with the increased reaction temperature, a gradually denser UiO-66 shell was observed, and the characteristic signals of the UiO-66 structure became more distinct in the FTIR spectra. This is due to the improved growth of UiO-66 crystals at higher temperature. Similarly, extending the reaction time could also lead to the increase in UiO-66 loading amount and particle diameter, as shown in Fig. 5 and 2H. The XRD patterns of the UiO-66@SiO₂ composites synthesized under different reaction temperature and time are listed in Fig. S3 and S4, ESI.†

Fig. 4 SEM images of UiO-66@SiO₂ composites synthesized at (A) 85 °C; (B) 100 °C; (C) 110 °C for 24 h; (D) FTIR spectra of UiO-66@SiO₂ composites synthesized at 85 °C, 100 °C, 110 °C and 120 °C for 24 h.

Fig. 5 SEM images of UiO-66@SiO₂ composites synthesized at 120 °C for (A) 3 h; (B) 6 h; (C) 12 h; (D) FTIR spectra of UiO-66@SiO₂ composites synthesized at 120 °C for 3 h, 6 h, 12 h and 24 h.

The stability of UiO-66 shell plays an important role in the application of UiO-66@SiO₂ microspheres as HPLC stationary phase. Hence, to investigate their stability, the composites were treated under dramatic ultrasonic in a dichloromethane solution for 10 minutes, and then collected by centrifugation. From the SEM images exhibited in Fig. S5, ESI,† it is noticed that the UiO-66 nanocrystals were bound firmly to the silica microspheres without obvious detachment by the applied treatment, thus implying the good stability of the shell–core composites.

3.2 HPLC evaluation of UiO-66@SiO₂ packed column

Fast separation with low backpressure and high efficiency is still a challenge for HPLC application. While in most previous studies, MOFs particles were packed directly into the columns, thus resulting in the increased column backpressure and decreased efficiency. The UiO-66@SiO₂ microspheres represent a mesoporous core–microporous shell structure, where the mesoporous silica core provides perfect packing property, low column backpressure and decreased mass transfer resistance, and the microporous UiO-66 shell gives a high retention of analytes. The chromatographic performance of UiO-66@SiO₂ packed columns was evaluated in a reverse phase mode using ACN and H₂O as mobile phase.

Fig. 6 shows the separation of four groups of analytes on UiO-66@SiO₂-0.16 packed column. The relative standard deviations (RSDs) of the retention time for six replicate runs were in the range of 0.10–0.46% for xylenes and ethylbenzene, 0.06–0.24% for alkyl benzenes, 0.07–0.38% for anilines and 0.09–0.35% for substituted benzenes, respectively, showing the good reproducibility. Furthermore, the UiO-66@SiO₂ packed column was retained for at least 400 runs over one month without observable change in the chromatographic performance. These results indicate the high stability and reproducibility of the prepared UiO-66@SiO₂ packed column. The UiO-66@SiO₂ packed column also exhibits good permeability. With the flow rate of the mobile phase (100% ACN) increases from 1.0 to 5.0 mL min⁻¹, the column backpressure increases linearly from 13.05 to 67.12 bar (Fig. S6, ESI†). Compared with the high column backpressure caused by the direct packing of MOFs particles, the UiO-66@SiO₂ packed column gives a very low backpressure, which is attractive for its application in HPLC separation. The role of the mobile phase composition in HPLC separation was also studied in Fig. S7, ESI.† Fast and high-resolution separation of analytes could be achieved on UiO-66@SiO₂ packed column through controlling the composition of mobile phase.

Fig. 6 Chromatograms on the UiO-66@SiO₂-0.16 packed column for the separation of four groups of analytes: (A) xylenes and ethylbenzene; mobile phase, 30% ACN; UV detection, 215 nm; flow rate, 1.0 mL min⁻¹. (B) Alkyl benzenes; mobile phase, 35% ACN; UV detection, 215 nm; flow rate, 1.0 mL min⁻¹. (C) Anilines; mobile phase, 15% ACN; UV detection, 280 nm; flow rate, 1.0 mL min⁻¹. (D) Substituted benzenes; mobile phase, 25% ACN; UV detection, 254 nm; flow rate, 1.0 mL min⁻¹.

The separation performance of C18 column was also investigated for comparison. As shown in Fig. S8, ESI,† although a much higher percentage of ACN was applied as mobile phase, longer analysis time was needed for complete separation of the three groups of analytes (alkyl benzenes, anilines and substituted benzenes) on C18 column, thus indicating the fast separation of UiO-66@SiO₂ packed column. Besides, ethylbenzene and xylenes were hardly separated on C18 column because of their similar hydrophobicity.

To study the effect of UiO-66 shell on the chromatographic separation, these analytes were also separated on the aminosilica packed column with different composition of the mobile phase for comparison (Fig. 7). However, the aminosilica packed column provided a poor resolution for the tested analytes. Adjustment in the mobile phase composition brought about no obvious improvement of the separation. The abundant amino groups on the surface of silica could lead to the formation of hydrogen bond with analytes, and this is the main separating power of the aminosilica packed column. Therefore, p-phenylenediamine with two amino groups obtained the longest retention time in the separation of aniline compounds (Fig. 7C), and similarly aniline exhibited the strongest retention in the separation of substituted benzenes (Fig. 7D). The result indicates that the UiO-66 shell played a significant role in the effective separation of the four groups of analytes on UiO-66@SiO₂ packed column.

Fig. 7 Chromatograms of the separation of (A) xylenes and ethylbenzene, (B) alkyl benzenes, (C) anilines and (D) substituted benzenes on aminosilica packed column with different ratio of ACN/H₂O as mobile phase. Other separation conditions are identical to Fig. 6.

The loading amount of UiO-66 shell also made an obvious influence on the chromatographic performance. Three composites (UiO-66@SiO₂-0.16, UiO-66@SiO₂-0.32 and UiO-66@SiO₂-0.64) were investigated as HPLC stationary phases. As exhibited in Fig. 8, aniline compounds (p-phenylenediamine, aniline, N,N-dimethylaniline and p-chloroaniline) were selected as an example to study the role of UiO-66 loading amount in the HPLC separation. The retention on the UiO-66@SiO₂ packed columns depends on the synergic effect of the hydrogen bonding between the compounds and the amino groups on the surface of aminosilica, and the hydrophobic effect as well as π–π stacking interaction between the compounds and the organic linkers of UiO-66 shell. As the UiO-66 loading amount increased, the hydrogen bonding between anilines and amino groups decreased, whereas the hydrophobic interaction and π–π stacking interaction between anilines and UiO-66 shell increased. Hence, as can be seen from Table S2, ESI,† the retention factors of p-phenylenediamine, aniline and N,N-dimethylaniline firstly decrease as the UiO-66 load raises from 12.7% (UiO-66@SiO₂-0.16) to18.6% (UiO-66@SiO₂-0.32), and afterwards increase with the UiO-66 load increases to 26.2% (UiO-66@SiO₂-0.64). While unlike other aniline compounds, the retention factor of p-chloroaniline increases gradually with the increment of UiO-66 coating, since the p–π conjugate effect existing between the lone pair electrons of chlorine atom and the aromatic rings of UiO-66 contributed to the strong retention of p-chloroaniline. The above results prove that the UiO-66 loading amount has an important effect on the retention and resolution of analytes.

Fig. 8 Chromatograms of the separation of anilines on (A) UiO-66@SiO₂-0.16 packed column, (B) UiO-66@SiO₂-0.32 packed column, (C) UiO-66@SiO₂-0.64 packed column using 17.5% ACN as the mobile phase at a flow rate of 1.0 mL min⁻¹. The signals were monitored with a UV detector at 280 nm.

3.3 Retention behavior of analytes on UiO-66@SiO₂ packed column

Xylenes are generally produced as a mixture containing ethylbenzene and the three xylene isomers (o-, m-, and p-xylene). The separation of ethylbenzene and xylene isomers is an important process because they are essential constituents of raw chemicals in chemical industries. Nevertheless, their coherent dimensions, boiling points and hydrophobicity make the separation challenging. In the previous studies, UiO-66 powders or shaped as agglomerates and tablets, and UiO-66 coated GC capillary column were evaluated for the selective adsorption and separation of xylene isomers, showing a unique o-xylene preference and moreover presenting an overall reverse shape selectivity. Here, we also explored the feasibility of HPLC separation of ethylbenzene and xylenes on UiO-66@SiO₂ packed column. As shown in Fig. 6A, the four analytes were baseline separated with short analysis time, high resolution and good precision. The UiO-66@SiO₂ packed column provided o-xylene stronger retention in comparison with m-xylene, ethylbenzene and p-xylene, exhibiting the o-xylene preference. According to the classical view on adsorption, at low coverage, an adsorbed molecule is attracted by van der Waals forces towards the pore walls; the closer the molecule approaches the pore walls, the higher the intensity of the interactions.³⁷ Therefore, as the critical diameter of ethylbenzene, m-xylene, p-xylene and o-xylene is 0.67, 0.71, 0.67 and 0.74 nm, respectively, the largest critical diameter of o-xylene led to the closest distance between the molecules and the pore walls, and thus the strongest retention of o-xylene on the UiO-66@SiO₂ packed column. However, the retention sequence according to reverse shape selectivity should be ethylbenzene/p-xylene < m-xylene < o-xylene, which is not in accordance with the retention order in Fig. 6A. It is known that m-xylene possesses a much higher basicity relative to other three compounds,⁴⁰ and meanwhile the amino groups on the surface of silica are also basic, hence resulting in the weakest retention of m-xylene. Moreover, the stronger hydrophobicity of p-xylene led to its longer retention time in comparison with ethylbenzene. Despite the exceptional retention of m-xylene, the UiO-66@SiO₂ packed column exhibited an overall reverse shape selectivity for ethylbenzene and xylene isomers, showing its potential in the separation of structural isomers.

Effective separation of neutral alkyl benzenes was also realized on the UiO-66@SiO₂ packed column (Fig. 6B). The retention order of benzene < toluene < p-xylene indicates that the retention behavior corresponds to the hydrophobic interaction between the compounds and the aromatic walls of UiO-66. However, TMB, with the strongest hydrophobicity, shows the weakest retention on the UiO-66@SiO₂ packed column. The bulky TMB molecules with a critical diameter of 0.86 nm, which is much larger than the pore window size (0.5–0.7 nm) and tetrahedral cavities diameter (0.8 nm) of UiO-66, were excluded from the micropores of UiO-66 and could only be adsorbed through surface interaction, thus leading to its shortest retention time. The experimental result reveals the existence of molecular sieving effect on the UiO-66@SiO₂ packed column. Additionally, the retention time of polycyclic aromatic hydrocarbons (PAHs) followed an increasing order of acenaphthene < phenanthrene < naphthalene (Fig. S9, ESI†), which is not in accordance with both the hydrophobic interaction and π–π interaction between the PAHs and the UiO-66 stationary phase. The strongest retention of naphthalene is attributed to its smaller molecule size relative to acenaphthene and phenanthrene, further supporting the presence of molecular sieving effect.

Four kinds of basic aniline compounds were baseline separated on the UiO-66@SiO₂ packed column with high resolution and good reproducibility in 6 minutes (Fig. 6C). The hydrophobicity strength of anilines follows the order of p-phenylenediamine < aniline < p-chloroaniline < N,N-dimethylaniline, while N,N-dimethylaniline was eluted earlier than p-chloroaniline on the UiO-66@SiO₂ packed column. The longer retention time of p-chloroaniline is ascribed to the p–π conjugate effect existing between the lone pair electrons of chlorine atom and the benzene rings of organic linkers in UiO-66.

The UiO-66@SiO₂ packed column also showed satisfactory separation performance for the substituted benzenes (Fig. 6D). The elution order of aniline, nitrobenzene and toluene is in accordance with the hydrophobic interaction between the solutes and the aromatic walls of UiO-66 shell. However, the p–π interaction between the chlorine atoms and the aromatic rings of UiO-66 gave rise to the longest retention time of chlorobenzene.

The HPLC columns packed with UiO-66@SiO₂ shell–core composites and aminosilica microspheres were subsequently used for the separation of phenol, 2-chlorophenol and 2,4-dichlorophenol. From the chromatograms shown in Fig. S10, ESI,† it can be seen that the three acidic phenolic compounds were effectively separated on the two columns under their optimal separation conditions. However, the intense interaction between chlorine substituent groups and the UiO-66 shell results in the obvious peak broadening and tailing of 2-chlorophenol and 2,4-dichlorophenol on the UiO-66@SiO₂ packed column, which is consistent with the separation performance of p-chloroaniline and chlorobenzene.

4. Conclusions

A facile one-pot method for the synthesis of UiO-66@SiO₂ shell–core microspheres as a novel HPLC stationary phase was reported. The density and particle size of UiO-66 shell could be simply adjusted by altering the precursor concentration, reaction temperature and time. The prepared UiO-66@SiO₂ packed column provided effective separations for the tested neutral, basic and acidic compounds with high resolution, good reproducibility and low column backpressure, displaying both reverse shape selectivity and molecular sieving effect, making it attractive for the separation of structural isomers. Besides, the retention of analytes also depends on the synergic effect of the hydrogen bonding between analytes and the amino groups of aminosilica, the hydrophobic interaction and π–π stacking interaction between analytes and the aromatic rings of UiO-66 shell. The excellent separation performance and multiple retention mechanism make the UiO-66@SiO₂ shell–core microspheres a valuable candidate as HPLC stationary phase. The developed method could supply a general path for the controllable preparation of MOFs@SiO₂ shell–core composites that can be utilized as HPLC stationary phase or in other applications.

Acknowledgements

This project was supported by the National Nature Science Foundation of China (no. 21075074).

Notes and references

1. O. M. Yaghi, M. O'Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Nature, 2003, 423, 705–714.
2. S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334–2375.
3. G. Ferey, Chem. Soc. Rev., 2008, 37, 191–214.
4. P. Llewellyn, P. Horcajada, G. Maurin, T. Devic, N. Rosenbach, S. Bourrelly, C. Serre, D. Vincent, S. Loera-Serna and Y. Filinchuk, J. Am. Chem. Soc., 2009, 131, 13002–13008.
5. S. Han, Y. Wei, C. Valente, I. Lagzi, J. J. Gassensmith, A. Coskun, J. F. Stoddart and B. A. Grzybowski, J. Am. Chem. Soc., 2010, 132, 16358–16361.
6. M. Maes, L. Alaerts, F. Vermoortele, R. Ameloot, S. Couck, V. Finsy, J. F. Denayer and D. E. De Vos, J. Am. Chem. Soc., 2010, 132, 2284–2292.
7. J. R. Li, J. Sculley and H. C. Zhou, Chem. Rev., 2012, 112, 869–932.
8. L. Li, X. L. Liu, H. Y. Geng, B. Hu, G. W. Song and Z. S. Xu, J. Mater. Chem. A, 2013, 1, 10292–10299.
9. B. Van de Voorde, B. Bueken, J. Denayer and D. De Vos, Chem. Soc. Rev., 2014, 43, 5766–5788.
10. Y. Yu, Y. Ren, W. Shen, H. Deng and Z. Gao, TrAC, Trends Anal. Chem., 2013, 50, 33–41.
11. L. Alaerts, M. Maes, L. Giebeler, P. A. Jacobs, J. A. Martens, J. F. Denayer, C. E. Kirschhock and D. E. De Vos, J. Am. Chem. Soc., 2008, 130, 14170–14178.
12. C.-X. Yang, S.-S. Liu, H.-F. Wang, S.-W. Wang and X.-P. Yan, Analyst, 2012, 137, 133–139.
13. S. S. Liu, C. X. Yang, S. W. Wang and X. P. Yan, Analyst, 2012, 137, 816–818.
14. M. Maes, F. Vermoortele, M. Boulhout, T. Boudewijns, C. Kirschhock, R. Ameloot, I. Beurroies, R. Denoyel and D. E. De Vos, Microporous Mesoporous Mater., 2012, 157, 82–88.
15. L. Alaerts, C. E. Kirschhock, M. Maes, M. A. van der Veen, V. Finsy, A. Depla, J. A. Martens, G. V. Baron, P. A. Jacobs, J. F. Denayer and D. E. De Vos, Angew. Chem., Int. Ed., 2007, 46, 4293–4297.
16. L. Alaerts, M. Maes, P. A. Jacobs, J. F. Denayer and D. E. De Vos, Phys. Chem. Chem. Phys., 2008, 10, 2979–2985.
17. C. X. Yang and X. P. Yan, Anal. Chem., 2011, 83, 7144–7150.
18. C. X. Yang, Y. J. Chen, H. F. Wang and X. P. Yan, Chem.–Eur. J., 2011, 17, 11734–11737.
19. R. Ahmad, A. G. Wong-Foy and A. J. Matzger, Langmuir, 2009, 25, 11977–11979.
20. A. Ahmed, M. Forster, R. Clowes, D. Bradshaw, P. Myers and H. Zhang, J. Mater. Chem. A, 2013, 1, 3276–3286.
21. R. Ameloot, A. Liekens, L. Alaerts, M. Maes, A. Galarneau, B. Coq, G. Desmet, B. F. Sels, J. F. M. Denayer and D. E. De Vos, Eur. J. Inorg. Chem., 2010, 2010, 3735–3738.
22. Y.-Y. Fu, C.-X. Yang and X.-P. Yan, J. Chromatogr. A, 2013, 1274, 137–144.
23. Y. Y. Fu, C. X. Yang and X. P. Yan, Chem.–Eur. J., 2013, 19, 13484–13491.
24. Z. Yan, J. Zheng, J. Chen, P. Tong, M. Lu, Z. Lin and L. Zhang, J. Chromatogr. A, 2014, 1366, 45–53.
25. B. Chen, C. Liang, J. Yang, D. S. Contreras, Y. L. Clancy, E. B. Lobkovsky, O. M. Yaghi and S. Dai, Angew. Chem., Int. Ed., 2006, 45, 1390–1393.
26. V. Finsy, H. Verelst, L. Alaerts, D. De Vos, P. A. Jacobs, G. V. Baron and J. F. Denayer, J. Am. Chem. Soc., 2008, 130, 7110–7118.
27. Z. Y. Gu and X. P. Yan, Angew. Chem., Int. Ed., 2010, 49, 1477–1480.
28. A. S. Münch and F. O. R. L. Mertens, J. Mater. Chem., 2012, 22, 10228–11234.
29. K. Tanaka, T. Muraoka, D. Hirayama and A. Ohnish, Chem. Commun., 2012, 48, 8577–8579.
30. C. Jo, H. J. Lee and M. Oh, Adv. Mater., 2011, 23, 1716–1719.
31. S. Sorribas, B. Zornoza, C. Tellez and J. Coronas, Chem. Commun., 2012, 48, 9388–9390.
32. H. J. Lee, W. Cho and M. Oh, Chem. Commun., 2012, 48, 221–223.
33. F. Ke, L.-G. Qiu, Y.-P. Yuan, X. Jiang and J.-F. Zhu, J. Mater. Chem., 2012, 22, 9497–9500.
34. J. Zheng, C. Cheng, W.-J. Fang, C. Chen, R.-W. Yan, H.-X. Huai and C.-C. Wang, CrystEngComm, 2014, 16, 3960–3964.
35. S. Aguado, J. Canivet and D. Farrusseng, J. Mater. Chem., 2011, 21, 7582–7588.
36. J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga and K. P. Lillerud, J. Am. Chem. Soc., 2008, 130, 13850–13851.
37. P. S. Bárcia, D. Guimarães, P. A. P. Mendes, J. A. C. Silva, V. Guillerm, H. Chevreau, C. Serre and A. E. Rodrigues, Microporous Mesoporous Mater., 2011, 139, 67–73.
38. M. A. Moreira, J. C. Santos, A. F. Ferreira, J. M. Loureiro, F. Ragon, P. Horcajada, K. E. Shim, Y. K. Hwang, U. H. Lee, J. S. Chang, C. Serre and A. E. Rodrigues, Langmuir, 2012, 28, 5715–5723.
39. N. Chang and X. P. Yan, J. Chromatogr. A, 2012, 1257, 116–124.
40. D. A. McCaulay and A. P. Lien, J. Am. Chem. Soc., 1951, 73, 2013–2017.

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthesis conditions, additional tables, SEM images, XRD patterns and HPLC results. See DOI: 10.1039/c4ra12263a

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