Preparation and Characterization of Mesoporous Zirconia Made by Using a Poly (methyl methacrylate) Template
© to the authors 2008
Received: 29 November 2007
Accepted: 14 February 2008
Published: 28 February 2008
Superfine powders of poly (methyl methacrylate) (PMMA) have been prepared by means of an emulsion polymerization method. These have been used as templates in the synthesis of tetragonal phase mesoporous zirconia by the sol–gel method, using zirconium oxychloride and oxalic acid as raw materials. The products have been characterized by infrared spectroscopy, X-ray diffraction analysis, transmission electron microscopy, N2adsorption-desorption isotherms, and pore size distribution. The results indicate that the average pore size was found to be 3.7 nm.
Due to their large surface areas, controllable pore size, and easy functionalization , mesoporous materials have opened many new possibilities for applications in catalysis, as catalyst supports, in separation science, and in drug delivery, as well as for protein encapsulation. Over the past few decades, preparation techniques for mesoporous structures have attracted the attention of many researchers [2–4]. Most research in this field has been focused on silica as a framework structure [5, 6]. However, mesoporous materials derived from transition metal oxides rather than silica frameworks are expected to be quite useful for several applications [7, 8]. At present, stabilized t-ZrO2 at room temperature is considered to be an important structural ceramic because of its excellent mechanical properties, such as fracture toughness, high strength, and hardness [9, 10], and there have been several reports concerning the synthesis of mesoporous structures . However, synthetic mesoporous zirconia has poor structural stability . One difficulty stems from a facile crystallization of zirconia, which is accompanied by structural collapse, during formation of the mesoporous phase and removal of the template [13–15]. A further difficulty is the phase transformation of zirconia from tetragonal phase to monoclinic phase when the products are cooled down from high temperature to ambient conditions [16, 17]. The resulting defects will affect the potential applications of the mesoporous zirconia.
In this work, we have used the inexpensive inorganic material zirconium oxychloride and oxalic acid as raw materials to prepare mesoporous zirconia by a sol–gel method  with the aid of PMMA as a template. Importantly, the space occupied by the PMMA particles shrinks from the micron or submicron size range to the nanosized range as a result of the structural collapse that accompanies the crystallization of zirconia.
Methyl methacrylate (MMA) and sodium dodecyl sulfate (SDS), both of chemical reagent grade, were purchased from Shanghai Lingfeng Chemistry Co. Ltd., China; zirconium oxychloride (ZrOCl2 · 8H2O) and ammonium peroxydisulfate (APS), both of analytical reagent grade, were purchased from the China National Medicine Group Shanghai Chemical Reagent Company; oxalic acid dihydrate (C2H2O4 · 2H2O) and absolute ethanol, both of analytical reagent grade, were purchased from Nanjing Chemical Reagent No. 1 Factory, China. Prior to use, MMA was extracted with brine to remove the polymerization inhibitor.
First of all, superfine powders of poly (methyl methacrylate) (PMMA) were prepared according to a previous literature report . SDS (0.5 g) was dissolved in distilled water (50 mL) with magnetic stirring, the solution was heated to 80 °C, and then APS (0.25 g) was added. The mixture was stirred to form a homogeneous solution, and then MMA (10 mL) was added dropwise over a period of 1.5–2.0 h, ensuring that the reaction proceeded to completion. Subsequently, the appropriate amount of saturated sodium chloride solution was added to the reaction mixture to obtain the products by destroying the emulsion. The precipitate was centrifuged, washed three times with distilled water, and dried at 60 °C in a vacuum oven. The loose, superfine powders of PMMA were obtained by further grinding of these precipitates.
Superfine PMMA powder (1.0 g) was added to a 0.25 M solution of oxalic acid in ethanol (40 mL) with continuous dispersal under ultrasonic conditions until a white suspension was obtained. This suspension was then slowly added to a 0.5 M solution of zirconium oxychloride in water (20 mL) under magnetic stirring to form a white sol . The sol was left to stand for 24 h so as to form a gel. Subsequently, this gel was dried at 40 °C and then ground to obtain the white precursor, which was calcined at 500 °C for 1 h to form the mesoporous zirconia. All other experiments were performed at ambient temperature.
Specific surface area, pore volume, and pore size distribution of the nanosized mesoporous zirconia were determined from N2adsorption–desorption isotherms at 77 K (Micrometrics ASAP 2010). Surface area was calculated using the BET equation; pore volume and pore size distribution were calculated by the BJH method. Prior to the adsorption measurement, samples were degassed under vacuum at 100 °C for 1 h to eliminate any physisorbed moisture. The surface groups were studied by Fourier-transform infrared (FT-IR) microwave spectrophotometry (Bruker Vector22); spectra were recorded in the range 400–4,000 cm−1 from samples in KBr pellets. The crystalline phase and crystallinity of the samples were measured by X-ray powder diffraction (XRD) analysis (Bruker D8 Advance) using Cu–K αradiation (λ = 0.15406 nm); the samples were scanned over a 2θ range of 0–70° at a scanning rate of 1 degree per minute. The morphology was determined by transmission electron microscopy (TEM) using a JEOL JEM-2100 apparatus.
Results and Discussion
Mechanism of Preparation
Superfine PMMA powders have been prepared using an emulsion polymerization method. Using the PMMA particles thus obtained as templates, and zirconium oxychloride and oxalic acid as raw materials, tetragonal phase mesoporous zirconia has been synthesized using a sol–gel method. FT-IR, XRD, TEM, and N2 adsorption–desorption isotherms have been used to characterize the products. The results have indicated that the average pore size of mesoporous zirconia was found to be 3.7 nm, and the pore size distribution was favorably narrow.
This work was supported by the National Science Foundation of Jiangsu Province, China (Grant No. BK2003097). The authors are grateful for the grant.
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