Chitosan Hybrid Microspheres

Article pubs.acs.org/IECR

Synthesis of Monodisperse Zeolite A/Chitosan Hybrid Microspheres and Binderless Zeolite A Microspheres Liang Yu,† Jie Gong,† Changfeng Zeng,‡ and Lixiong Zhang*,† †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, and ‡College of Mechanic and Power Engineering Nanjing University of Technology, Nanjing 210009, PR China ABSTRACT: Zeolite A/chitosan hybrid microspheres with uniform particle size of ∼700 μm and high crushing strength were prepared by bioorganic molecule-assisted impregnation-gelation-hydrothermal synthesis of silica/chitosan hybrid microspheres in a sodium aluminate alkaline solution. Binderless zeolite A microspheres with similar particle size and medium crashing strength were subsequently obtained after calcination of the zeolite A/chitosan hybrid microspheres. The silica/chitosan hybrid microspheres were prepared by solidification of chitosan microdroplets, which were formed in a microfluidic device from a silica sol−chitosan aqueous solution, in the sodium aluminate alkaline solution. X-ray diffraction, FT-IR, scanning and transmission electron microscopy, thermogravimetric analysis, and adsorption were applied to examine the obtained products. The zeolite A/ chitosan hybrid microspheres are composed of zeolite A crystals with particle sizes of 200−400 nm and chitosan, with zeolite A crystals covered with and linked by chitosan. Removal of chitosan in the zeolite A/chitosan hybrid microspheres by calcination has no influence on the structure of the microspheres. Furthermore, the resulting binderless zeolite A microspheres exhibit a high surface area of 30 m2 g−1, high static water adsorption amount of 290 mg g−1, and quite different adsorption capacities for CH4 from H2, with a high ideal separation factor for CH4/H2 of 12. The crushing strength of binderless zeolite A microspheres can be greatly enhanced after a secondary growth. In addition, the formation process of the microspheres was analyzed.

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INTRODUCTION Bioorganic molecule-assisted synthesis of various organic,1 inorganic,2 macromolecular,3 and various hybrid materials4−6 has attracted much attention in the past few years. Among these materials, inorganic/organic hybrid materials exhibit improved physical and chemical performance by containing rigid and rugged inorganic materials, thus multifunctionizing their organic materials.7−10 Furthermore, the hybrid materials extend the application of both their inorganic and organic materials because of their improved properties, such as better mechanical strength and biocompatibility.11 Inorganic/organic hybrid materials are usually prepared by using supramolecular organic materials, such as cholesterolbased gelators and polysaccharide-type biopolymers.12,13 Chitosan, as an inexpensive, abundant, biocompatible, nontoxic polysaccharide, is commonly used to synthesize bioinspired hybrid materials.14,15 Ayers and Hunt16 prepared chitosan− silica aerogel by using an acidic solution of chitosan to catalyze the hydrolysis and condensation of tetraethylorthosilicate, which exhibits adjustable physical properties, high surface area, and very high value of hemolysis in hemolysis screening using rabbit blood. Yeh et al.17 prepared chitosan/SiO2 hybrid materials that are made of tetraethylorthosilicate/vinyltriethoxysilane and chitosan in different weight ratios via a sol−gel process. The hybrid materials exhibited better thermostability and mechanical performances than pure chitosan. Molvinger et al.18 synthesized porous chitosan/silica hybrid microspheres with changeable structures and catalytic properties for monoglyceride synthesis. Zeolites are aluminosilicate crystals composed of an intrinsic ordered three-dimensional microporous system. They are commonly used in traditional fields, such as adsorption, © 2012 American Chemical Society

separation, ion-exchange, and catalysis as well as in new areas, such as separation membranes,19 sensors,20 low-k materials,21 and corrosion-resistant22 and microbiocidal coatings,23 etc. A union of zeolites with chitosan may produce hybrid materials exhibiting improved physical and chemical properties and broaden application of both zeolites and chitosan. Zeolites are conventionally synthesized by an autogenous hydrothermal process of synthesis solutions prepared from alumina, silica, and alkali sources in powder form with water. This process is more complex than hydrolysis of tetraethoxysilane in the above-mentioned preparation of chitosan/silica hybrid materials. Furthermore, zeolite powders have to be shaped to desired geometries using undesired binders for practical application.24 Thus, binderless zeolite monoliths have been an object of efforts since the 1960s.24 On the other hand, chitosan is easily solidified in an alkali solution, indicating that we cannot prepare zeolite/chitosan hybrid materials by simply mixing a zeolite synthesis solution with chitosan or a chitosan solution. Thus, we have developed an impregnation−gelation− hydrothermal synthesis process for this purpose. It has been reported that unique zeolite particles or films can be synthesized in the presence of chitosan. For example, cubes of zeolite A consisting of a thin crystalline shell and an amorphous core were grown within non-cross-linked chitosan hydrogels.25 Oriented titanium silicalite-1 (TS-1) zeolite films on chitosan film-precoated porous α-Al2O3 substrates were Received: Revised: Accepted: Published: 2299

October 1, 2011 January 17, 2012 January 18, 2012 January 18, 2012 dx.doi.org/10.1021/ie202242e | Ind. Eng. Chem. Res. 2012, 51, 2299−2308

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic illustrating the process to fabricate zeolite A/chitosan hybrid microspheres.

microspheres were collected by toppling the liquid from the beaker. Zeolite A/Chitosan Hybrid Microspheres and Binderless Zeolite A Microspheres. The obtained silica/chitosan hybrid microspheres prepared with 18 mL water phase were first impregnated in a 19.5 mL sodium aluminate solution contained in a tightly capped 100 mL polypropylene bottle at room temperature for 24 h. The sodium aluminate solution was prepared by dissolving 3.90 g NaOH and 1.64 g NaAlO2 (Sinopharm Chemical Reagent Co., Ltd.) in 15.3 mL of deionized water under vigorous stirring until it became clear. The molar composition of the mixture including the silica/ chitosan microspheres was 5.87Na2O/1.0Al2O3/1.35SiO2/ 185H2O. Then the bottle was put into an oven that was preheated to 80 °C for 3 h. The zeolite A/chitosan hybrid microspheres were finally obtained after filtration, washing, and drying. Binderless zeolite A microspheres were prepared by simply calcining the zeolite A/chitosan hybrid microspheres in air at 550 °C for 4 h. To further enhance the crushing strength of binderless zeolite A microspheres, they experienced a secondary growth in the synthesis solution prepared with a molar composition of 2Na2O/Al2O3/2SiO2/120H2O at 100 °C for 3.5 h. The synthesis solution was prepared by mixing 5.68 g of sodium metasilicate nonahydrate (Sinopharm Chemical Reagent Co., Ltd.), 1.64 g of NaAlO2, and 18.36 g of deionized water and stirring at room temperature for 30 min. The mass ratio of binderless zeolite A microspheres to the synthesis solution was 1:10. The process is illustrated in Figure 1. Instruments and Characterization. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance powder diffractometer using a Ni-filtered Cu Kα radiation source at 40 kV and 20 mA. The samples were prepared by grinding the microspheres by hand using a mortar and pestle. The Fourier transform infrared (FT-IR) spectra were obtained on a Nexus 870 FT-IR spectrometer. Samples were mixed and ground with KBr (in a mass ratio of 1:10) for FT-IR measurement in the wavenumber range of 4000−400 cm−1. Optical microscopy (Pentax), a scanning electron microscope (SEM, Hitachi S-4800 and Quanta 200), and a transmission electron microscope (TEM, JEL-200CX) were used to investigate the particle size, morphology, and microstructure of the microspheres.

synthesized by using direct in situ crystallization, in which the chitosan film was used as an orientation-directing matrix.26 In this paper, we report preparation of zeolite A/chitosan hybrid microspheres that preserve the porosity of zeolite A and biocompatibility of chitosan and exhibit quite strong mechanical properties, by impregnation−gelation−hydrothermal synthesis of silica/chitosan hybrid microspheres and confining the synthesis of zeolite A in chitosan microspheres. Furthermore, binderless zeolite A microspheres, which possess moderate mechanical strength and quite high specific surface area, can be readily obtained after removal of the chitosan by calcination.

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EXPERIMENTAL SECTION Silica/Chitosan Hybrid Microspheres. Silica/chitosan hybrid microspheres with diameters of ∼1 mm were prepared in a microfluidic device assembled by inserting a syringe needle (0.3 mm i.d.) perpendicularly into a poly(vinyl chloride) tube (1.2 mm i.d., 1.6 mm o.d.) (Figure 1).35 Before preparation of the microspheres, a water phase containing silica sol and chitosan was prepared. Typically, silica sol (40 wt %, particle size 30−40 nm, Nanjing Erbang Chemical Industry Co., Ltd.; 2.07 g) was added to deionized water (15 g), followed by addition of a certain amount of chitosan (90% deacetylated, Sinopharm Chemical Reagent Co., Ltd.). Afterward, an acetic acid aqueous solution (36 wt %, Shanghai Lingfeng Chemical Reagent Co., Ltd.; 0.65 mL) was added dropwise under stirring until the chitosan was completely dissolved. The mixture was stirred overnight at room temperature. The silica and chitosan contents in the final mixture were 4.5 and 3.2 wt %, respectively. For preparation of the microspheres in the microfluidic device, the water phase and an oil phase (pure fatty acid methyl ester, lab-made with cotton seed oil27) were fed into the microfluidic device at flow rates of 3 and 20 mL h−1 , respectively, by two syringe pumps (LSP02−1B, LongerPump, China). Aqueous solution microdroplets were thus formed at the end of the needle by shearing force from the oil phase. The microdroplets were dropped into a beaker halffilled with a 2 wt % NaOH (Sinopharm Chemical Reagent Co., Ltd.) aqueous solution. The oil phase would be floating at the surface, and the microdroplets would settle to the bottom and immediately be solidified, leading to formation of silica/ chitosan hybrid microspheres. The silica/chitosan hybrid 2300

dx.doi.org/10.1021/ie202242e | Ind. Eng. Chem. Res. 2012, 51, 2299−2308


Article

Figure 2. Optical micrographs of the silica/chitosan hybrid microspheres prepared at different NaOH and chitosan concentrations. (a) Chitosan concentration, 2.3 wt %; NaOH concentration, 5 wt %; (b) chitosan concentration, 2.3 wt %; NaOH concentration:, 2 wt %; (c) chitosan concentration, 3.2 wt %; NaOH concentration, 5 wt %; (d) chitosan concentration, 3.2 wt %; NaOH concentration, 2 wt %.

Thermogravimetric (TG) analysis was carried out on zeolite A/chitosan hybrid microspheres and zeolite A microspheres in air up to 800 °C using a Netzsch STA 409 instrument with a heating rate of 10 °C min−1. N2 adsorption/desorption measurements were performed at 77 K on a Micromeritics ASAP 2020 instrument. The samples were all outgassed at 200 °C for 3 h before measurement. The Al and Si contents in the inner core of the silica/ chitosan hybrid microspheres after impregnation in a sodium aluminate solution for 24 h were analyzed by an energy dispersive X-ray analyzer (EDX, Sigma) attached to the SEM (Quanta 200). For analysis, the microsphere was cut in half using a sharp blade. The Al and Si contents in the solution after hydrothermal treatment were analyzed by inductively coupled plasma (ICP) emission spectroscopy using an Optima 2000 DV system. The static water adsorption amount of the zeolite A microspheres was determined using the Chinese national standard method (GB6287-86). Briefly, the sample was first calcined in air at 550 °C for 1 h and then cooled in a vacuum desiccator at a pressure below 1.0 × 103 Pa to room temperature. After the sample was weighed, it was put back into the desiccator in which a bottle of saturated NaCl solution was also loaded. The desiccator was put into an oven preheated to 35 °C (±1 °C), and it was allowed to remain for 24 h. Afterward, the weight increase of the sample was measured. The static water adsorption amount was thus calculated as the ratio of the weight increase to the weight of the sample. H2 and CH4 adsorption measurements were performed at 298 K on a Micromeritics ASAP 2020 instrument. The samples were all outgassed at 300 °C for 10 h before measurement. The crushing strength of the hybrid microspheres was determined using the microspheres with a diameter of ∼1.2 mm. First, the maximum load (N) of the hybrid microspheres and binderless zeolite microspheres at 25 °C was determined using a smart

particle strength tester (ZQJ-II, Dalian, China) with a 500 N load cell operating at a cross-head speed of 4 N s−1. Then the maximum crushing strength (σm) was calculated from the maximum applied load and the cross-sectional area of the microspheres as described in the following equation: σm = 0.4Pm/πr2, where Pm is the maximum load at failure (N) and r is the radius of the microspheres (m).28

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RESULTS AND DISCUSSION Silica/Chitosan Hybrid Microspheres. The silica/chitosan hybrid microspheres were prepared in a simple two-phase microfluidic device by solidification of chitosan in a NaOH aqueous solution (Figure 1). We found that both the NaOH concentration and the chitosan concentration have a strong influence on the sphericity of the microspheres. Figure 2 shows the optical micrographs of the silica/chitosan hybrid microspheres prepared at different chitosan and NaOH concentrations. At a chitosan concentration of 2.3 wt % and a NaOH concentration of 5 wt %, the resulting microspheres are of irregular morphology and agglomerated (Figure 2a). A decrease in the NaOH concentration to 2 wt % leads to nonagglomeration but also irregular morphology of the composite microspheres (Figure 2b). This may result from the poor mechanical strength of the microspheres obtained at low chitosan concentrations. Thus, we increased the chitosan concentration to 3.2 wt %. The resulting microspheres show mostly sphericity but some ellipse at a NaOH concentration of 5 wt % (Figure 2c) and nice spherical morphology and uniform particle size at a NaOH concentration of 2 wt % (Figure 2d). Therefore, the silica/chitosan hybrid microspheres with perfect sphericity and narrow size distribution were prepared with a chitosan concentration of 3.2 wt % and a NaOH concentration of 2 wt %. Zeolite A/Chitosan Hybrid Microspheres. Synthesis of zeolite A/chitosan microspheres was carried out by first 2301



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Figure 3. EDX analysis of the inner core (a) and FT-IR spectrum (b-1) of the silica/chitosan hybrid microspheres after impregnation in a sodium aluminate solution for 24 h and FT-IR spectra of pure chitosan powders (b-2), the zeolite A/chitosan hybrid microspheres (b-3), and binderless zeolite A microspheres (b-4).

Figure 4. SEM images of silica-contained chitosan spheres after hydrothermal synthesis (a); the surface of the obtained microspheres (b); the inner core of the microspheres (c); a high magnification of the inner portion showing the linkage among cubic particles (d); and a TEM picture of the microspheres (e).

impregnating the silica/chitosan hybrid microspheres in a sodium aluminate solution for 24 h, followed by hydrothermal synthesis at 80 °C for 3 h. After impregnation in the sodium aluminate solution, sodium aluminate diffuses into the silica/ chitosan microspheres, as verified by the EDX analysis and FTIR spectrum of the microspheres after impregnation (Figure 3), resulting in formation of aluminasilicate gels.

The products after hydrothermal synthesis and drying are nice spheres, with a diameter of about 700 μm (Figure 4a), smaller than silica/chitosan hybrid microspheres resulting from the shrinkage during drying. Closer observation of the surface of the obtained microspheres (Figure 4b) reveals that the spheres are composed of cubic particles with particle sizes ranging from 200 to 400 nm. The particle sizes are smaller than those of zeolite A synthesized with the same recipe under the 2302



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The zeolite A/chitosan hybrid microspheres inherit the microporous property of zeolite A and possess a certain amount of macroporosity, as indicated from their N2 adsorption− desorption isotherm (Figure 7a). The BET surface area was 8.8

same hydrothermal condition without addition of chitosan (about 500−600 nm), possibly because of the space-confining effect of chitosan hydrogels.29 After breaking the microspheres with a strong force, we observe that the inner core of the microspheres is also composed of cubic particles covered with and interconnected by, quite possibly, chitosan (Figure 4c). The linkage between two cubic particles could be more clearly observed by a SEM picture at high magnification (Figure 4d) and a TEM picture (Figure 4e). The XRD pattern of the as-prepared microspheres (Figure 5a) indicates that the microspheres are composed of zeolite A

Figure 7. Nitrogen adsorption−desorption isotherms of zeolite A/ chitosan hybrid microspheres (a), zeolite A powders (b), and binderless zeolite A microspheres (c).

m2 g−1. They also exhibit a quite strong mechanical property because it is quite difficult to cut the sphere with a sharp blade. The measured crushing strength is 11.3 MPa, higher than those of some cancellous bones32 and cellulose-based calcium carbonate pellets.28 Binderless Zeolite A Microspheres. By calcining the hybrid microspheres in air at 550 °C for 4 h, we obtain binderless zeolite A microspheres, which reserve the welldefined spherical structure of the hybrid microspheres (Figure

Figure 5. XRD patterns of silica/chitosan hybrid spheres after impregnation in the sodium aluminate solution for 24 h and hydrothermal synthesis at 80 °C for 3 h (a) and the obtained sample calcined at 550 °C for 4 h (b).

with high crystallinity. By comparing the FT-IR spectra of chitosan powder, the microspheres, and zeolite A microspheres (Figure 3b), we observe well-defined absorption bands at 1007, 553, and 466 cm−1 assigned to the stretching vibration of (Si, Al)O4 tetrahedron units of zeolite A,30 and absorption bands at 2921 and 2879 cm−1, assigned to the stretching vibration of CH3− and −CH2− of chitosan in the microspheres.31 These results indicate that the microspheres are composed of zeolite A and chitosan, with zeolite A crystals covered with and linked by chitosan. To further elucidate the structure of the hybrid microspheres, we first carbonized them at 550 °C, followed by dissolving the zeolite crystals using 1 M hydrochloric acid. From the SEM picture of the carbonized sample (Figure 6a), we could observe

Figure 8. SEM images of binderless zeolite A microspheres produced by calcining zeolite A/chitosan hybrid microspheres at 550 °C for 4 h: (a) overall morphology and (b) inner core.

8a). The content of the chitosan in the hybrid microspheres is ∼20 wt %, as measured by TG analysis (Figure 9a). Complete removal of chitosan and intergrowth of zeolite A crystals could be clearly observed from a SEM picture of the inner core of the microspheres (Figure 8b) and TG analysis (Figure 9b). The crystal morphology and habit of zeolite A do not change after calcination, as indicated from the XRD pattern (Figure 5b) and FT-IR spectrum (Figure 3b) of the calcined microspheres. Their crushing strength is 0.7 MPa, comparable with that of binderless zeolite A bars prepared from amorphous aluminosilicate extrudates (0.4 MPa)33 and much higher than that of self-supporting silicalite-1 monoliths (0.05 MPa).34 The binderless zeolite A microspheres exhibit a higher N2 adsorption capacity (Figure 7c) than the hybrid microspheres and zeolite A powders obtained by grinding the zeolite A microspheres using a mortar and pestle for about 30 min

Figure 6. FESEM images of zeolite A/chitosan hybrid microspheres after being carbonized at 550 °C for 4 h (a) and dissolving the zeolite crystals using 1 M hydrochloric acid (b).

that the zeolite A crystals are linked together. After removal of the zeolite crystals, the leaving carbon material exhibits many cavities with shapes and sizes similar to those of zeolite crystals (Figure 6b). The above results further confirm that the zeolite A/chitosan hybrid microspheres are composed of zeolite A crystals covered with and linked by chitosan. 2303



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crystals after removal of chitosan, resulting in high surface area. This is verified by the N2 adsorption−desorption isotherm of the binderless zeolite A microspheres as well as the reduced surface area of the zeolite A powders obtained by grinding the binderless microspheres (12 m2 g−1). The high surface area provides more adsorption sites for water and CH4, leading to a high static water adsorption amount and CH4 adsorption capacity. Since the adsorption of CH4 on zeolite A results from a high polarizability of CH4 molecules while the polarizability of H2 is relative low, the adsorption capacity of CH4 should be higher than that of H2 on the adsorbent with a higher surface area.37,42,43 To further enhance the crushing strength of binderless zeolite A microspheres, they underwent a secondary hydrothermal treatment at 100 °C for 3.5 h. The resulting sample still keeps the well-defined spherical structure of the microspheres, with a diameter of 700 μm (Figure 11a), similar to the binderless zeolite A microspheres before the secondary growth. A layer of well-intergrown zeolite A film is formed on the surface of the parent binderless zeolite A microspheres, as shown in the SEM picture of the microsphere’s surface after secondary growth (Figure 11b). The thickness of the film is about 4 μm, as can be seen from the cross-sectional view of a broken sphere (Figure 11c). On the other hand, the internal structure of the microspheres is almost not affected (Figure 11d). Although the secondary growth on the binderless zeolite A microspheres results in a quite dense zeolite film on the microspheres (Figure 11b), the process leads to hardly any change in the surface area and the static water adsorption amount, with their values of 29 m2 g−1 and 293 mg g−1, respectively. This is ascribed to existence of intercrystalline pathways in the dense zeolite film, resulting in the fully accessible surfaces of the zeolite A crystals in the microspheres, because defects quite commonly exist in zeolite membranes on various substrates.44,45 However, the crushing strength is increased from 0.7 to 1.79 MPa, making them more favorable for practical applications.46 Formation Process of the Zeolite A/CS Microspheres. To examine the formation process of the zeolite A/CS microspheres, we first analyzed the Al and Si contents in the synthesis solution as a function of the hydrothermal treatment time of the silica/chitosan hybrid microspheres impregnated in the sodium aluminate solution for 24 h by ICP (Figure 12a). Obviously, a small amount of silica diffuses from the silica/ chitosan hybrid microspheres to the synthesis solution during the 30 min impregnation and hydrothermal synthesis process. As the hydrothermal treatment time is lengthened, the Si content in the synthesis solution decreases sharply and then stays constant after 60 min. On the other hand, the Al content in the synthesis solution decreases sharply from a synthesis time of 0−60 min and stays almost constant afterward. These results indicate that most of the Si and Al species are in the chitosan microspheres and are finally converted to zeolite A crystals. In fact, the calculated yield of zeolite A is ∼95%. Figures 13 and 14 show the internal structures and XRD patterns of the silica/chitosan hybrid microspheres after impregnation in the sodium aluminate solution for 24 h and being hydrothermally synthesized at 0, 10, 20, 30, 60, and 90 min duration. The SEM picture (Figure 13a) illustrates that the microspheres after impregnation and before the synthesis are composed of fibrous materials combined with many small particles of about 40 nm as the scaffold and some round voids with diameters of 100−450 nm. After hydrothermal synthesis,

Figure 9. TG curves of the samples: (a) zeolite A/chitosan hybrid microspheres and (b) binderless zeolite A microspheres.

(Figure 7b). Their BET surface area is 30 m2 g−1, much higher than those of even ultrafine zeolite A powders29,35 and commercial zeolite adsorbent.36 Their static water adsorption amount is 290 mg g−1, higher than those of commercial zeolite desiccants (200−265 mg g−1). Furthermore, the binderless zeolite A microspheres exhibit adsorption capacities for CH4 that are quite different from H2, with an ideal separation factor for CH4/H2 of 12 at 100 kPa (Figure 10), higher than that of commercial zeolite 5A as well as Sr2+ ion-exchanged submicrometer hierarchical zeolite A.37

Figure 10. Adsorption isotherms (298 K) of H2 and CH4 on binderless zeolite A microspheres.

The higher surface area, static water adsorption amount, and CH4 adsorption capacity of the binderless zeolite A microspheres may be attributed to their more porous structure. Chitosan is known to act as a template for formation of micropores, mesopores, and macropores when it is used to prepare inorganic porous materials.38−41 Molvinger et al.18 synthesized porous chitosan/silica hybrid microspheres, exhibiting a micropore volume and BET surface area of 0.029 cm3/g and 300 m2/g, respectively. After removal of chitosan by calcination, the micropore volume and BET surface area increased to 0.061 cm3/g and 500 m2/g. Pedroni et al.38 obtained monolithic siliceous mesoporous− macroporous materials using chitosan as a template, which show a multimodal pore size distribution between 3 and 10 nm. Furthermore, macropores (>100 nm) were formed in significant numbers in mesoporous silica−aluminosilicate composites synthesized via a sol−gel process under acidic (pH 6.5) and basic (pH 11.5) conditions in the presence of cetyltrimethylammonium bromide (CTAB) and chitosan.40 Figures 4 and 6 reveal that zeolite crystals are covered with chitosan in the zeolite A/chitosan hybrid microspheres, while Figure 8 shows that zeolite A crystals are intergrown together in the binderless zeolite A microspheres. Thus, it is reasonable to postulate that pores are formed among the intergrown zeolite A 2304



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Figure 11. SEM images of NaA microspheres after secondary growth, (a) overall morphology (b) outside surface, (c) cross-section, (d) internal.

Figure 12. The ICP results of the Al and Si variation tendency of the silica/chitosan hybrid microspheres after their impregnation in the sodium aluminate alkaline solution for 24 h and hydrothermal treatment in the same solution at 80 °C for different times (a) and the calculated Si/Al ratio in the silica/chitosan microspheres during the synthesis (b).

small particles start to grow in size after 10 min, and several particles 150 nm in size can be observed after 20 min (Figure 13b and c). The XRD patterns of corresponding samples (Figure 14a, b and c) show no peaks, indicating amorphous structure of the particles. More and more cubic particles 200− 300 nm in size are formed after 30−90 min of synthesis (Figure 13d−f). The XRD patterns of the corresponding samples (Figure 14d−f) exhibit typical diffraction peaks ascribed to zeolite A, with their peak intensity increasing with increasing synthesis time. Note that obvious XRD peaks can be observed after a hydrothermal time of 30 min, in which a weak XRD peak at 2θ of 6.19° (Figure 14) ascribed to a FAU-type zeolite appears. These indicate that formation of a large amount of zeolite A crystals starts at a hydrothermal time of 30 min in addition to formation of a trace amount of FAU-type zeolite resulting quite possibility from the Si/Al ratio higher than 1 at the early stage of the hydrothermal treatment, as indicated from the calculated Si/Al ratio in the microspheres during the synthesis (Figure 12b). XRD peaks ascribed to FAU-type zeolite disappear on samples synthesized after a hydrothermal time of 60 min, suggesting that zeolite A is the main phase of the product at

long synthesis times because of diffusion of much more Al into the silica/chitosan hybrid microspheres.

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CONCLUSIONS Zeolite A/chitosan hybrid microspheres with strong mechanical strength and microporous properties could be readily prepared by bioorganic molecule-assisted impregnation−gelation−hydrothermal synthesis procedures. The improved properties of the hybrid microspheres give them potential biomedical applications, such as bone tissue engineering, because chitosan-based hybrid materials exhibit quite good biocompatibility47−50 and zeolite A induces the proliferation and differentiation of cells of the osteoblast lineage.51 Furthermore, binderless zeolite A microspheres are thus obtained by simply burning off chitosan in zeolite A/chitosan hybrid microspheres. They exhibit moderate mechanical strength, quite a large surface area, a high static water adsorption capacity, and adsorption capacity for CH4 that is different from H2, suggesting potential application in adsorption and separation. After secondary growth, the crushing strength of the binderless zeolite A microspheres is obviously enhanced without a change in the adsorption properties. Zeolite A/chitosan hybrid microspheres are believed to be formed following the process 2305



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Figure 13. SEM images of the internal structures of the silica/chitosan hybrid microspheres impregnated in the sodium aluminate solution for 24 h and after hydrothermal treatment at 80 °C for 0 (a), 10 (b), 20 (c), 30 (d), 60 (e), and 90 min (f).

spheres may be quite useful in biomedicine, adsorption, and separation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (20676059, 20801027, 21076107), Specialized Research Fund for the Doctoral Program of Higher Education (20093221110002), the Natural Sciences Foundation of Education Department of Jiangsu Province (09KJB150002), and PAPD of Jiangsu Higher Education Institutions.

Figure 14. XRD patterns of the silica/chitosan hybrid microspheres impregnated in the sodium aluminate solution for 24 h and after hydrothermal treatment at 80 °C for 0 (a), 10 (b), 20 (c), 30 (d), 60 (e), and 90 min (f).

as diffusion of the aluminum source during the impregnation and at the early stage of crystallization and growth of zeolite A crystals in the chitosan framework microspheres afterward. The zeolite A crystal sizes in the microspheres are in the range of 200−400 nm, smaller than those prepared in a batch synthesis with the same receipt and condition, resulting from the confinement effect of the chitosan framework. The zeolite/ chitosan hybrid microspheres and binderless zeolite micro-

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REFERENCES

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