Facile Synthesis of Monodisperse Magnesium Oxide Microspheres via Seed-Induced Precipitation and Their Applications in High- Performance Liquid Chromatography

DOI: 10.1002/adfm.200600906

Facile Synthesis of Monodisperse Magnesium Oxide Microspheresvia Seed-Induced Precipitation and Their Applications inHigh-Performance Liquid Chromatography**

By Zhiping Zhang, Yajun Zheng, Jiping Chen,* Qing Zhang, Yuwen Ni, and Xinmiao Liang*

1. Introduction

The fascination with the so-called monodisperse colloids canbe traced back to more than a century ago when Faraday de-scribed his gold sols, the brilliant colors of which were found todepend on the particle size.[1] Many decades later, significantprogress has been made in the preparation of a large numberof inorganic colloids consisting of particles of different chemi-cal composition, shape, and modal size. The importance ofwell-defined fine particles has been recognized in numerousapplications, such as in ceramics, catalysis, pigments, recordingmaterials, medical diagnostics, and many others.[2] From thepoint of chromatographers’ view, monodisperse spherical col-loids have been extensively exploited as the packing materialsfor high-performance liquid chromatography (HPLC) due totheir particular packing density, dispersion property, and fluid-ity.[3] Most work in this area, however, has been limited to silica

beads because of its high mechanical strength, small particlesize, narrow particle size distribution, high specific surface,variation of pore size and well documented chemistry of sur-face modification.[4] In terms of the hydrolytic stability of bothacidic and alkaline eluents, inorganic oxides should providesome immediate advantages over silica and thus seem promis-ing candidates for applications in HPLC.[5] Although a numberof inorganic oxides (e.g., Al2O3, ZrO2, TiO2) recently havebeen synthesized as monodisperse spheres and demonstratedsuperior efficiency in the separation of basic, neutral and acidiccompounds,[5] the diversity of materials still needs to be greatlyexpanded in order to fully capitalize the advantages of the ma-terials in the aspect of separation. Here we would like to addanother inorganic material to the list: magnesium oxide(MgO).

As an exceptionally important material, MgO has been ex-tensively used in catalysis, toxic waste remediation, or as addi-tives in refractory, paint and superconductor products, and isalso used in steel manufacturing because of its highly corro-sion-resistant.[6] In addition, MgO is a typical wide bandgap in-sulator, and its electronic and optical properties have been in-vestigated.[7] MgO is unique in its basicity with an isoelectricpoint of about 12, and it forms a class apart from other sup-ports and sorbents such as alumina, zeolites, active carbon, sili-ca, etc., which are acidic, neutral, or amphoteric.[8] For this rea-son or others, many analytical scientists have also used MgO asa packing material for liquid chromatography in the early re-search. For example, Strain[9] noted that the relative adsorptionof the carotenes on MgO was determined largely by the num-ber of olefinic double bonds, with polar groups playing a lessimportant role. Sabacky and co-workers[10] reported that ethyl

Adv. Funct. Mater. 2007, 17, 2447–2454 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2447

–[*] Prof. J. Chen, Prof. X. Liang, Z. Zhang, Dr. Q. Zhang, Dr. Y. Ni

Dalian Institute of Chemical Physics, Chinese Academy of SciencesDalian 116023 (P.R. China)E-mail: [email protected]; [email protected]. ZhengInstitute of Chemistry for Functionalized MaterialsDepartment of Chemistry, Liaoning Normal UniversityDalian 116029 (P.R. China)

[**] The work was financially supported by the National Natural ScienceFoundation of China (Grant No. 20577050) and the National BasicResearch Program of China (Grant No. 2003CB415001). We wouldlike to thank Prof. Lefeng Zhang for his many helpful suggestionsand discussion to the manuscript. Supporting Information is avail-able online from Wiley InterScience or from the authors.

Low-cost, uniform, and monodisperse spherical particles are desirable for a variety of applications. We report the realization ofuniform spherical magnesium oxide with a diameter of 10.5 lm and specific surface area of 140.9 m2 g–1 resulting from a facileseed-induced precipitation in the presence of a trace amount of phosphate species. By optimizing the experimental parameters,the results demonstrate that the morphologies of magnesium oxide precursors are very sensitive to the amount and the se-quence of addition of Mg5(CO3)4(OH)2 · 4H2O seeds, as well as the amount and type of phosphate species. These particles havealso been used as a packing material for high performance liquid chromatography. In comparison with the commercial sphericalsilica, as-synthesized spherical magnesium oxide exhibits excellent efficiency in the separation of polycyclic aromatic hydrocar-bons, and has a larger retention to planar compounds in contrast to non-planar ones. It is believed that this method will providea simple and versatile approach to large-scale production of spherical magnesium oxide via a facile seed-induced mechanism.The spherical magnesium oxide may find widespread use as a packing material in the group separation of polycylic aromatic hy-drocarbons from target samples.

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ether and acetone had a weaker adsorption on MgO than ben-zene. Snyder[11] found that olefinic bonds and aromatic groupswere preferentially held on MgO, relative to other groups. It iswell known that persistent organic pollutants (POPs), espe-cially polycyclic aromatic hydrocarbons (PAHs) and their de-rivatives, have gained considerable attention due to their toxic-ity. Nevertheless, these pollutants exist in the environment as acomplex mixture, and the concentrations of some compoundsare usually very low (ppb ∼ ppt level), which make a serious ob-stacle for analyzing their levels in environmental samples withthe conventional methods. Hence, MgO may be an ideal pack-ing material for the separation and enrichment of differentPAHs from target samples. Unfortunately, MgO from differentsources are highly variable, and most of commercial chromato-graphic MgO are fine powders,[11] which to a large extent re-stricts the application of MgO as a packing material.

Up to now, the preparation of spherical oxides mainly in-cludes methods of spray drying,[12] microemulsion,[13] sol-gel,[14]

and polymerization-induced colloid aggregation.[15] For thesemethods, however, particular attentions should be paid to theemulsification conditions for generating droplet size, or to as-certain whether particle classification is demanded after syn-thesis, or to prepare and handle the costly and hazardous metalorganic precursors. To our knowledge, the direct precipitationmethod in which synthesis and self-assembly are coupled in situto produce highly ordered structures have potential applica-tions in this regard, due to the fact that the particle classifica-tion and the introduction of organic reagents can be effectivelyavoided. At the same time, precipitation has been proved to bestraightforward and versatile due to its various advantages suchas a single-step process and facile large-scale production.

In a recent study,[16] we discovered thatMg5(CO3)4(OH)2 · 4H2O with variousmorphologies can be synthesized by care-fully changing the pH value and reactiontemperature of the initial reaction solu-tion during the precipitation of magne-sium nitrate (Mg(NO3)2) and potassiumcarbonate (K2CO3). Although sphericalMgO can also be obtained by the decom-position of spherical Mg5(CO3)4(OH)2 ·4H2O, there are some exploding particlesamong them,[17] which to a certain extentreduces their corresponding mechanicalstrength during the packing with slurrymethod. Herein we demonstrate thatuniform MgO miscrospheres can bewell obtained by the calcination ofMg5(CO3)4(OH)2 · 4H2O, and the spheri-cal Mg5(CO3)4(OH)2 · 4H2O is preparedvia a facile seed-induced precipitation inthe presence of a trace amount of phos-phate species (sodium phosphate, sodiumtripolyphosphate, or sodium polypho-sphate). On the basis of our results fromscanning electron microscope (SEM) ob-servation, we believe that phosphate ions

play a crucial role in changing the crystal orientation ofMg5(CO3)4(OH)2 · 4H2O. In addition, the chromatographicproperty of as-synthesized MgO demonstrates that thesemicrospheres may be a promising packing material for the sep-aration and enrichment of different PAHs from our targetsamples.

2. Results and Discussion

The uniform spherical MgO was synthesized by using thecommercial Mg5(CO3)4(OH)2 · 4H2O as the seeds, phosphatespecies (sodium phosphate, sodium tripolyphosphate, or so-dium polyphosphate) as the regulator of Mg5(CO3)4(OH)2 ·4H2O morphology, and Mg(NO3)2 and K2CO3 as the inorganicsources. The synthesis mixture was stirred for 1 min and thenallowed to react under static conditions. Figure 1 shows theSEM images and X-ray diffraction (XRD) of a typical ex-tracted spherical MgO by the calcination of their precursors,which were synthesized by adding 2 drops (D) of 0.4 mg mL–1

Mg5(CO3)4(OH)2 · 4H2O seed solution after pouring K2CO3

into Mg(NO3)2 solution in the presence of 0.15 g sodium tripo-lyphosphate. Figure 1a presents the lower-magnification SEMimage of as-synthesized product. One can see that the panora-mic morphology of the sample consists of spherical particlesthat are usually 9–11 lm in diameter and with an average di-ameter of about 10.5 lm. Figure 1b shows a magnified view ofan individual sphere, revealing that all these microspheres arerather round. The magnified SEM image as shown in Figure 1cdemonstrates that the microspheres are mainly composed ofnanosheet structure with thicknesses of about 40–60 nm, and

2448 www.afm-journal.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 2447–2454

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Figure 1. SEM images of spherical MgO by the calcination of its precursor obtained by adding 2 Dof Mg5(CO3)4(OH)2 · 4H2O seed solution (0.4 mg mL–1) after pouring K2CO3 into Mg(NO3)2 solu-tion in the presence of 0.15 g sodium tripolyphosphate: a) panoramic morphologies, b) individualsphere, c) enlarged section on an individual sphere, and d) a typical XRD pattern of the obtainedMgO products.

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Z. Zhang et al./Facile Synthesis of Monodisperse Magnesium Oxide Microspheres

has a very similar structure as Mg5(CO3)4(OH)2 · 4H2O givenin the previous reports,[8,16–18] indicating that the crystal nucleitend to assemble into a sheetlike structure in MgO-CO2-H2Osystem. The phase of as-synthesized products was examined byXRD as shown in Figure 1d. All of the reflections in this figurecan be readily indexed to a cubic phase of MgO with latticeconstant of a = 4.213, in agreement with the values reported inthe literature (JCPDS Card: 4-829). No other crystalline impu-rities were detected by XRD, suggesting that the presence ofsodium tripolyphosphate has little influence on the phase ofthe final products.

By examining the influence of reaction conditions on themorphologies of MgO precursors, the results demonstrate thatthe shapes and surface structures of the products have a greatdiversity with the variation of experimental parameters. Fig-ure 2 shows the SEM images of MgO precursors synthesizedby adding various drops of Mg5(CO3)4(OH)2 · 4H2O seed solu-tion after pouring K2CO3 into Mg(NO3)2 solution in the pres-ence of 0.15 g sodium tripolyphosphate. When there was noMg5(CO3)4(OH)2 · 4H2O seeds in the reaction system (Fig. 2a),the aggregates of microspheres with diameters of ca. 36 lmwere retained, and there were some gel-like particles on thesurface of these mircrospheres. However, the addition of 1D ofMg5(CO3)4(OH)2 · 4H2O seed solution (0.4 mg mL–1) into thereaction solution resulted in the uniform particles 12–14 lm indiameters (Fig. 2b). If further increasing the amount ofMg5(CO3)4(OH)2 · 4H2O seed solution ranged from 4D to 12D,the particle size decreased from 10 to 7 lm as shown in Fig-ure 2c–e. More interesting is that there was a tunnel in the mid-dle of these particles, and the tunnel became more obviouswith the decrease of the particle size. To our knowledge, itshould be attributed to the function of sodium tripolypho-sphate, which will be discussed as follows. From these results, itcan be learned that Mg5(CO3)4(OH)2 · 4H2O seeds played animportant role on the formation of MgO precursors. It is wellknown that the function of seeds is to afford the nuclei and cut

off the nucleation period because of the relatively lower nucle-ation energy, thus reduce the crystal size. As reported in our re-cent study,[17] the initial products favored to form(MgCO3)0.8((Mg(OH)2)0.2 · 1.3H2O after the “burst” mixtureof Mg2+ and CO3

2–, but this composition is not stable from thepoint of thermodynamic view. It will transfer into the morestable Mg5(CO3)4(OH)2 · 4H2O via the intermediateMgCO3 · xH2O. When Mg5(CO3)4(OH)2 · 4H2O seed solutionwas introduced into the present reaction system, the transfor-mation energy among (MgCO3)0.8((Mg(OH)2)0.2 · 1.3H2O,MgCO3 · xH2O, and Mg5(CO3)4(OH)2 · 4H2O might be greatlyreduced. Therefore, the formation rate of Mg5(CO3)4(OH)2 ·4H2O was promoted, which can be clearly seen with the in-crease of the amount of Mg5(CO3)4(OH)2 · 4H2O seed solutionduring the aging period. With the increase of the formationrate, the particle sizes of the products also continue to decreaseas discussed above. On the other hand, these sheetlikeMg5(CO3)4(OH)2 · 4H2O seeds (Fig. S-1) may also act as anepitaxial “substrate” or guide the directional aggregation pro-cess for the formation of spherical MgO precursors. As can beseen in Figure 1c, the MgO microspheres are self-assembledby sheetlike structures, which displays that with thetransformation from (MgCO3)0.8((Mg(OH)2)0.2 · 1.3H2O toMgCO3 · xH2O, then to Mg5(CO3)4(OH)2 · 4H2O,[17] the initialparticles may gradually grow into sheetlike structure in the ori-ented effect of the seeds, and then the sheetlike structure willfurther self-assemble into spherical MgO precursors.

The sequence of addition of Mg5(CO3)4(OH)2 · 4H2O seedsis another important parameter for the kinetically controlledprocess, thus one would expect it could affect the shape evolu-tion of MgO precursors. Figure 3 shows the typical SEM im-ages of MgO precursors obtained by adding 2D ofMg5(CO3)4(OH)2 · 4H2O seed solution at various times in thepresence of 0.15 g sodium tripolyphosphate. If theMg5(CO3)4(OH)2 · 4H2O seeds were added into K2CO3 solu-tion before mixing with Mg(NO3)2 solution, the products dis-

Adv. Funct. Mater. 2007, 17, 2447–2454 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.afm-journal.de 2449

20 µm20 µm

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Figure 2. SEM images of MgO precursors synthesized by adding various drops of Mg5(CO3)4(OH)2.4H2O seed solution after pouring K2CO3 into

Mg(NO3)2 solution in the presence of 0.15 g sodium tripolyphosphate: (a) 0 D, (b) 1 D, (c) 4 D, (d) 8 D, and (e) 12 D, and the inserted parts in (b)-(e)are the enlarged section of a typical particle.

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played a aggregate of spheres (Fig. 3a). However, the uniformmicrospheres, as shown in Figure 3b–d, were obtained by add-ing 2D of Mg5(CO3)4(OH)2 · 4H2O seed solution after pouringK2CO3 into Mg(NO3)2 solution in the presence of 0.15 g of so-dium tripolyphosphate. Simultaneously, the particle size of theproducts also increased with the prolonged time of addition ofMg5(CO3)4(OH)2 · 4H2O seed solution into the reaction mix-ture, and their corresponding diameters ranged from 13 to20 lm when the time of introduction of Mg5(CO3)4(OH)2 ·4H2O seed solution into the reaction mixture was varied from0 to 40 s. To the best of our knowledge, the addition ofMg5(CO3)4(OH)2 · 4H2O seeds before the nucleation of Mg2+

and CO32– ions might lead to the adsorption of CO3

2– ions onthe surface of Mg5(CO3)4(OH)2 · 4H2O seeds. During the“burst” mixture of Mg2+ and CO3

2– ions, the nucleation be-

tween the CO32– ions adsorbed on the Mg5(CO3)4(OH)2 · 4H2O

seeds and Mg2+ ions will proceed. As a result, the initial gel willfill on the surface of Mg5(CO3)4(OH)2 · 4H2O seeds, which to alarge extent restricts the function of Mg5(CO3)4(OH)2 · 4H2Oseeds, and the products display a similar morphology as thoseobtained without introduction of Mg5(CO3)4(OH)2 · 4H2Oseeds after pouring K2CO3 into Mg(NO3)2 solution in the pres-ence of 0.15 g of sodium tripolyphosphate (Fig. 2a).

On the contrary, the Mg5(CO3)4(OH)2 · 4H2O seeds willreduce the transformation energy among (MgCO3)0.8-((Mg(OH)2)0.2 · 1.3H2O, MgCO3 · xH2O, and Mg5(CO3)4-(OH)2 · 4H2O, and uniform particles are produced. With theprolonged time of addition of Mg5(CO3)4(OH)2 · 4H2O intothe reaction mixture, however, the growth speed of the initialnuclei will gradually precede nucleation speed, and the largernuclei are produced. With the addition of Mg5(CO3)4(OH)2 ·4H2O seeds, the larger nuclei will aggregate or assemble intolarger particles. Therefore, it should be carefully chosen the in-troduction time of Mg5(CO3)4(OH)2 · 4H2O seeds according tothe practical requirements.

Figure 4 shows the typical SEM images of MgO precursorssynthesized by adding 2D of Mg5(CO3)4(OH)2 · 4H2O seed so-lution after pouring K2CO3 into Mg(NO3)2 solution in thepresence of various amounts of sodium tripolyphosphate(STPP). When there is no STPP in the reaction system, it is in-teresting that the product looks like self-assembly by bothhemispheres, and there is an obvious suture in the middle ofthe particle (Fig. 4a). With the increase of the amount of STPP,the suture in the surface of the particle gradually disappears,which can be clearly seen from Figure 4b–d. However, increas-ing the amount of STPP up to 0.20–0.25 g results in the parti-cles with coarse surface structure, and it also leaves behindsome gel-like particles on their corresponding surfaces asshown in Figure S-2, which can be attributed to the slow nucle-ation speed during aging period. As reported in the litera-ture,[19] the addition of small amounts of phosphate or hypo-phosphite ions has little effect on the chemical composition ofthe precipitate formed on aging of ferric salt solutions, butthese anions can greatly influence the particle morphology.


20 µm 20 µm

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(a) (b)

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Figure 3. SEM images of MgO precursors synthesized by adding 2 D ofMg5(CO3)4(OH)2 · 4H2O seed solution in different times: a) adding theseeds into K2CO3 solution before mixing with Mg(NO3)2 solution, b) add-ing the seeds 0 s, c) 20 s, and d) 40 s after mixing of K2CO3 andMg(NO3)2 solutions in the presence of 0.15 g sodium tripolyphosphate.

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(a) (b) (c) (d)

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Figure 4. SEM images of MgO precursors synthesized by adding 2 D of Mg5(CO3)4(OH)2 · 4H2O seed solution after pouring K2CO3 into Mg(NO3)2 solu-tion in the presence of various amounts of sodium tripolyphosphate: a) 0 g, b) 0.05 g, c) 0.10 g, d) 0.15 g, e) 0.20 g, f) 0.25 g, and g) 0.30 g.

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The authors attributed the phenomenon to the fact that phos-phate ions can be readily adsorbed on the iron (hydrous) ox-ides in general and on hematite in particular,[20] and then suchadsorption was responsible for the influence of the subsequentgrowth mechanism, resulting in anisometric a-Fe2O3 particles.In our case, the phenomena also can be ascribed to the readyadsorption of phosphate ion on the surface of crystals. Becauseof the strong complexing ability between phosphate and Mg2+,the Mg2+ in the structure of hydrate magnesium carbonate maycomplex with the phosphate ions. As stated in many previousreports,[21] the characteristic feature of the formed hydratemagnesium phosphate is the presence of large tunnels occupiedby alkali metal ions, which will have a great influence on thestructure of the nuclei of Mg5(CO3)4(OH)2 · 4H2O, and then af-fect the morphology of the final products. The characteristictunnel of hydrate magnesium phosphate in the structure of as-synthesized products could be well confirmed by the particlesas shown in Figure 2. From this figure, it is clear that when theaddition amount of Mg5(CO3)4(OH)2 · 4H2O seed solution ishigher than 8 D, the tunnel becomes more obvious. However,it is difficult to observe the tunnel in the MgO precursor ob-tained from a smaller amount of Mg5(CO3)4(OH)2 · 4H2Oseeds (1–4 D), which should be attributed to the slower nuclea-tion speed. For this reason, the crystal growth dominates, andthese microspheres with more smooth surfaces tend to be ob-tained although the tunnel in the initial product can also be ob-served by optical microscope during the aging period. In addi-tion, by determining the percentages of tripolyphosphate ionsin the particles obtained by adding various drops ofMg5(CO3)4(OH)2 · 4H2O seed solution, the results (Table S-1)illustrate the tripolyphosphate ions play an comparative roleduring the growth of the particles.

On the other hand, Li and Lü[22] found that the complexingability between the metal ions (including Ca2+, Mg2+, and Fe3+)and phosphates increased with the increase of polymerization

of phosphate. In the present study, the influence of variousphosphate species (e.g., sodium phosphate and sodium poly-phosphate) on the morphology of the products had a similarchange pattern (Figs. S-3 and S-4), which further confirmedthat the effect of phosphate ions on the morphology was viathe complexing interactions between phosphate and Mg2+ ions.These results suggest that the influence of hydrate magnesiumphosphate on the morphology of Mg5(CO3)4(OH)2 · 4H2O in-creased with the increase of the nucleation speed, the amountof phosphate ions and the polymerization of phosphates. Atthe same time, it should be noted that with the increase of theamount of phosphate ions in the reaction mixture, the com-plexing ability between Mg2+ and phosphate ions also greatlyinfluenced the nucleation speed of hydrate magnesium carbon-ate, because the introduction of phosphate ion into K2CO3 so-lution had a little effect on other parameters such as pH value(Fig. S-5). This phenomenon could be confirmed by the gel-like particles on the surface of the particles obtained in a largeamount of phosphate ions (Fig. 4g and Fig. S-2). At the sametime, although these parameters had a significant effect on themorphology of MgO precursors, their corresponding composi-tions had little been affected (Fig. S-6), which was similar tothe report of Ozaki et al.[19]

To better evaluate the properties of as-synthesized MgO as apacking material, a reference column was packed with a com-mercially available SiO2, and both columns were subjected tothe same chromatographic separation tests in the normal-phasesystem. According to the supplier, the chosen reference materi-al, SiO2, had a particle size of 5 lm, which were different fromthose of as-synthesized MgO (ca. 10.5 lm) as discussed above.The two materials were first compared on the basis of nitrogenphysisorption measurements as shown in Figure 5. In nitrogenphysisorption measurements, SiO2 displayed a type-IV iso-therm with H1 hysteresis loop (Fig. 5a), which is characteristicof mesoporous materials.[23] In contrast to the commercial


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Figure 5. a) N2 adsorption-desorption isotherms of (a) the commercial SiO2 and b) spherical MgO obtained by calcination of Mg5(OH)2(CO3)4 · 4H2Oobtained by adding 2 D of Mg5(CO3)4(OH)2 · 4H2O seed solution after pouring K2CO3 into Mg(NO3)2 solution in the presence of 0.15 g sodium tripoly-phosphate. The inserted parts show the corresponding BJH pore size distribution curves.

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SiO2, the isotherms of MgO exhibited type-IV char-acteristics with a significant H3 loop (Fig. 5b). Thetype H3 loop, which did not clearly exhibit any ad-sorption plateau at relative pressures close to unity,was observed with aggregates of platelike particlesgiving rise to slit-shaped pores.[23,24] It is also indica-tive of a pore size distribution extending into themacropore range, a fact that is also clear from thepore size distribution in the inserted part of Fig-ure 5b. These measurements revealed that the porediameter of SiO2 was 10.2 nm (Fig. 5a), whereas itwas 17.7 nm for MgO (Fig. 5b). Besides them, theother textural properties of SiO2 and MgO are listedin Table 1.

The investigated mixture contained benzene, naph-thalene, biphenyl, fluorene, anthracene and p-terphe-nyl (Table 2), which were also used in many reportsof chromatographic applications of spherical silicaand metal oxide particles. All peaks in the chromato-grams of the mixtures were identified on the basis ofmeasurement of the individual component. Figure 6demonstrates the separation of various polycyclicaromatic hydrocarbons (PAHs) with 95:5 (v:v) hex-ane/dichloromethane at a flow rate of 0.8 mL min–1

for MgO and 0.6 mL. min–1 for SiO2. From this chro-matogram, it is obvious that these solutes could bewell separated on MgO column despite the similarstructures of biphenyl and naphthalene, p-terphenyland anthracene, and the separation factors (a) forthem were 1.32 and 1.40, respectively. Despite thelarger specific surface area and smaller particle sizeof the commercial SiO2 relative to MgO, these PAHshad the relatively short retention times on SiO2 col-umn, and the retention time for the last solute is onlyabout 5.5 min (the value for MgO is as large as170 min). Simultaneously, it is not possible to achieve the base-line separation for biphenyl, naphthalene, fluorine and anthra-cene. To the best of our knowledge, the different retentions of

these PAHs between the commercial SiO2 and as-synthesizedMgO can be attributed to the various oxygen vacancies at thesurface or sub-surface region of MgO.[25] Different from othercovalent oxides, the removal of oxygen atoms in the structureof MgO does not result in the formation of new bonds butrather cavities, which may be filled with electrons trapped bythe strong electrostatic potential of ionic crystal.[25d] Becausethe solutes in this study are all aromatic compounds with richelectrons, the oxygen cavities on the surface of MgO willstrongly interact with the electrons of these aromatic com-pounds. Consequently, the solutes with more electrons havelarger retention behavior on MgO column relative to SiO2 col-umn.[26] At the same time, it is well known that reversed-phase


Table 1. Texture properties of as-synthesized MgO and the commercial SiO2.

Oxides Specific surface

area [m2 g–1] [a]

Average pore

diameter [nm] [b]

Average particle

size [lm]

Pore volume

[cm3 g–1]

MgO 140.9 17.7 10.5 0.62

SiO2 332.1 10.2 5.0 0.87

[a] Using the standard Brunauer–Emmett–Teller (BET) method. [b] Usingthe Barret–Joyner–Halenda (BJH) method.

Table 2. Structural formulae and planarity for the mixture of polycyclic aromatic hydrocarbons. Note +, planar; -, non planar; (+) containing hydrogenatoms existing out of the plane of the aromatic ring.

benzene biphenyl naphthalene fluorene p-terphenyl anthracene

Structural Formula

Planarity + – + (+) – +

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Figure 6. Chromatograms of the separation of polycyclic aromatic hydrocarbons on A)as-synthesized MgO and B) a commercial SiO2. Detection wavelength: 254 nm; Mo-bile phase: 5 % (v/v) dichloromethane/hexane; Flow rate: 0.80 mL min–1 for MgO col-umn and 0.60 mL. min–1 for SiO2 column; Column: 150 mm × 4.6 i.d. mm (MgO) and100 mm × 4.6 i.d. mm (SiO2); Solutes: 1 = benzene, 2 = biphenyl, 3 = naphthalene,4 = fluorene, 5 = p-terphenyl, 6 = anthracene.

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liquid chromatography (LC) with various chemically bondedoctadecylsilane (C18) stationary phases, such as polymericphases, intermediate phases and monomeric phases,[27] has be-come the most popular LC mode for the separation of PAHs,but few materials[28] can be used as the normal-phase stationaryphases for the separation of PAHs. On the basis of the advan-tages of as-synthesized material, such as its unique selectivityfor PAHs, facile preparation and needless surface modification,MgO microspheres may be a potential stationary phase for theseparation of PAHs, and thus normal-phase LC can become analternative method for the analysis of PAHs.

In addition, by careful observation of this chromatogram, itis very interesting that in contrast to the commercial SiO2 sta-tionary phase, the planar solutes had a larger retention onMgO relative to non-planar ones, namely, the k′ (retention fac-tor) value of naphthalene > k′ (biphenyl) and k′ (anthrace-ne) > k′ (p-terphenyl). In our opinion, this observed behaviorof PAHs on as-synthesized MgO may be related to an effect ofspecific donor-acceptor interactions between solutes and sta-tionary phase.[26] Due to the complex factors that influenceshape selective separation of PAHs,[29] the detailed retentionmechanism of PAHs on MgO should be further investigated.

3. Conclusions

In summary, the uniform MgO microspheres with a diameterof about 10.5 lm have been successfully synthesized by a facialseed-induced precipitation, and the spherical MgO was ob-tained by the calcination of Mg5(CO3)4(OH)2 · 4H2O preparedvia the precipitation of Mg(NO3)2 and K2CO3 solution. Thepresence of both Mg5(CO3)4(OH)2 · 4H2O seeds and phos-phate species in the reaction solution was found to be criticalto the control of the shape and surface structure of MgO pre-cursors. In contrast to the commercial normal-phase SiO2, theMgO microspheres were able to well separate all tested PAHmixture, despite the larger particle size and smaller surfacearea. By comparison with the conventional preparation ap-proaches of metal oxides and SiO2, the synthetic route was sim-ple, mild, and controllable, and provided a convenient route tosynthesize MgO microspheres based on seed-induced precipi-tation mechanism, demonstrating that it is possible to easilyrealize large-scale production. In addition, the uniform MgOmicrospheres and their unique basicity (with an isoelectricpoint of about 12) can open up a wide range of interesting ap-plications in the separation of PAHs and other compounds(Fig. S-7) from target samples. By the way, it should be notedthat MgO similar to SiO2 and Al2O3 as a stationary phase fornormal-phase LC, is hydrophilic, which is the common disad-vantage of normal-phase LC, so the moisture percentage in theeluant should be strictly controlled.

4. Experimental

Magnesium nitrate (Mg(NO3)2), potassium carbonate (K2CO3),phosphate species, and Mg5(CO3)4(OH)2 · 4H2O seeds were all of ana-

lytical grade and used without further purification. In a typical synthe-sis, 10.26 g of Mg(NO3)2 · 6H2O was dissolved in 50 mL of double-de-ionized water. Then, the Mg(NO3)2 solution was transferred to a250 mL three-necked flask, and heated to 70 °C. Subsequently, 0.15 gof sodium tripolyphosphate was added into 100 mL of 0.4 M K2CO3 so-lution, and the mixture was also heated to 70 °C. Under the vigorousstirring (800 ± 10 rpm), the K2CO3 mixture was poured into theMg(NO3)2 solution in 4–5 s. After that, 2 D of 0.4 mg mL–1

Mg5(CO3)4(OH)2 · 4H2O seed solution, which had been dispersed inwater under ultrasonic treatment for minutes, was rapidly added intothe reaction mixture, and continued to stir for 1 min. Then the mixturewas maintained at the temperature of 70 °C for 2 h under static condi-tions followed by maintaining at 100 °C for 2 h. Finally, the resultingproduct was collected, filtered off, and washed with double-deionizedwater and ethanol for several times. The MgO sample was prepared bycalcination of the products in air from room temperature to 300 °C witha rate of 2 °C min–1, then kept for 2 h. After that, the product was cal-cined from room temperature to 550 °C in muffle furnace, and thenmaintained at that temperature for 7 h. The products were character-ized by SEM, XRD, N2 physisorption, FT-IR spectra and chromato-graphic techniques. The SEM images were taken by using a JSM-6360LV scanning electron microscope operated at an acceleratingvoltage of 10–20 kV. The crystal structures of these products were char-acterized by XRD on a D8 Advance diffractometer using Cu Ka radia-tion, and the operation voltage was 40 kV and the current was 40 mA.The Brunauer-Emmett-Teller (BET) surface area (SBET) and averagepore size were determined by using a NOVA 4000 nitrogen adsorptionapparatus (Quantachrome, USA) at 77 K. Prior to the adsorption mea-surements, these samples were degassed at 200 °C for 5 h. The BETsurface area was determined by the multipoint BET method using theadsorption data, and the pore-size distribution, from the desorptionbranch of the isotherm, was calculated by the Barrett-Joyner-Halen-da(BJH) method. The pore volume was estimated from the adsorbedamount of N2 at P/P0 = 0.99, where P/P0 denotes the relative pressure.The Fourier transform infrared (FT-IR) spectra of the obtained sam-ples were recorded with a Perkin-Elmer GS-II FT-IR spectrometer inthe range of 4000–400 cm–1, and the resolution was 4 cm–1 and 8 scanswere signal-averaged in each interferogram. The chromatographic con-ditions were as follows: the commercial SiO2 and as-synthesized MgOwere packed into the stainless-steel tubes by slurry packing method.The eluent used was 5 % (v/v) dichloromethane/hexane, and the flowrate was 0.6 mL min–1 for SiO2 column and 0.8 mL min–1 for MgO col-umn. The elution solvents, n-hexane (HPLC grade), isopropyl alcohol(HPLC grade) and dichloromethane (Analytical grade), were pur-chased from Shandong Yu Wang Chemical Reagent Ltd., and the so-lutes used were of analytical grade or better.

Received: September 30, 2006Revised: November 25, 2006

Published online: August 7, 2007

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