Synthesis and Characterization of Worm-shaped Tubular LanthanumAluminum Composite Mesoporous Materials

SONG Weiming (宋偉明), ZUO Chunling (左春玲) and DENG Qigang (鄧啟剛)

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Synthesis and Characterization of Worm-shaped Tubular LanthanumAluminum Composite Mesoporous Materials

SONG Weiming (宋偉明)*, ZUO Chunling (左春玲) and DENG Qigang (鄧啟剛)

College of Chemistry and Chemical Engineering of Qiqihar University, Qiqihar 161006, China

The lanthanum aluminum mesoporous materials were synthesized using sodium dodecyl sulfate as a template agent by ultrasonic hydrothermal method. The resulting samples were characterized by low angle X-ray diffraction (XRD), N2adsorption-desorption studies, transmission electron microscopy (TEM) and surface morphology analysis (SEM), surface acid (NH3-TPD), reducibility properties (TPR), X-ray energy dispersive spectrometer (EDS) and thermogravimetric analysis (TG/DTG).

Al/La composite oxides, mesoporous material, synthesis, characterization

1 INTRODUCTION

Increasing attention has been given to mesoporous materials because of their great applicability as catalysts, catalyst supports, molecular sieves and host matrices, based on their large internal surface areas [1, 2]. At present, the research was mainly focused on the synthesis of new mesoporous materials with different chemical composition and chemical structures, such as mesoporous metal oxides [3-5], mesoporous carbon, mesoporous metals, such as mesoporous aluminum phosphate. Such materials would be very attractive as adsorbent, support and catalyst in a wide variety of catalytic processes, because the large and uniform pore sizes should lead to fast diffusion to the active sites during catalysis, even for large molecules [6, 7].

Most attempts aimed at synthesizing these structures have led to lamella phases in which the surfactant and metal oxide phases are layered [8]. Alumina has been widely used as a catalyst, such as petro- chemical, reforming and hydrogenation. With the improvement of quality of crude oil production and the innovation for the petro-chemical production process, the performance the alumina catalyst was also need to be improved [9, 10]. It is known that mesoporous alumina has a three-dimensional interconnected pore system in the form of sponge-like structure, which is a great advantage in the catalytic applications [11, 12]. Even though mesoporous alumina has high surface area with narrow pore size distribution its poor thermal stability leads to deficiencies in the application [13]. Lanthanum-doped alumina can not only improve its thermal stability but also to increase their catalytic activity [14]. Although the prepared lanthanum-doped alumina was found to have high thermal stability, other metal catalysts impregnated on alumina inevitably experienced pore blocking by metal species during the metal supporting process. This eventually resulted in metal aggregation and low surface area of active metal species [15, 16].

One possible route to solve this problem is to co-condensate aluminum precursor with metal source in the presence of template molecules, where the interaction between metal ions and template favorably serves for the formation of highly dispersed metal particles. The main purpose of the present study is to prepare double oxide precursor with mesopore networks containing La3+and Al3+cations using the self-combustion method [5]. In this work, we reported the synthesis method of Al/La composite mesoporous material with a high surface area and narrow pore size distribution, medium-strong acid and high thermal stability by ultrasonic hydrothermal method.

2 EXPERIMENTAL

2.1 Synthesis of Al/La composite mesoporous materials

2.2 Characterization

The mesophase architecture of the materials obtained was characterized by X-ray diffraction (XRD), using nickel-filtered Cu Kαradiation (D/Max-IIIC, Ricoh). The electron microscopic images (SEM) of the materials were obtained with a scanning electron microscope (S-4300, Hitachi), using an accelerating voltage of 120 kV. Transmission electron microscopy (TEM) images were recorded with JEOL JEM-2010 and Philips TECNAI-20 high-resolution transmission electron microscopes. The accelerating voltage was 200 kV. X-ray energy dispersive spectrometer (EDS), can be major elements of the qualitative and quantitative analysis for micro-area at 20 kV accelerating voltage (EX-250, Horiba). N2adsorption isotherms were obtained at 77 K with a Quanta Chrome Autosorb-1 (AUTOSORB-1, Quanta Chrome). The surface area and pore size distribution (PSD) were calculated by using multipoint BET (Brunauer Emmett Teller Procedure) analysis and the Barrett-Joyner-Halenda (BJH) method, respectively.

Thermogravimetric analysis (TG/DTG) was carried out in air atmosphere with a Mettler TC-10 thermo balance (Diamond TG/DTA, Perkin Elmer) from room temperature to 1000 °C at a heating rate of 20 °C·min-1. Temperature-programmed desorption (TPD) and temperature-programmed oxidation (TPR) are used to identify the acid amount of the sample and redox properties of Al/La composite mesoporous materials. TPD/TPR experiments were carried out with a chemical adsorption instrument (BEL-Cat, BELCAT-M), Pulse Chem Sorb option, from Ankersmid.

The oven controller unit had a built-in circuit board and signal conditioning device which amplified the thermal conductivity detector (TCD) signal prior to sending it to a PC data acquisition system provided with a lab-developed program. The experiments were conducted on samples of the calcined catalysts weighing approximately 50 mg. NH3-TPD was performed with 10% NH3diluted in He to determine the acidity of the samples. The NH3was adsorbed at 40 °C and desorption was performed in the temperature range of 100-550 °C with a heating rate of 10 °C·min-1. TPR experiments were initially flushed with Ar (50 ml·min-1) as the temperature was increased at a rate of 10 °C·min-1to 100 °C, where it was held for 30 min to remove water. Then, 5.0% H2in Ar (50 ml·min-1) was introduced with a rate of heating of 10 °C·min-1from room temperature to 800 °C and was maintained for 60 min. The TCD determined the amount of hydrogen consumed.

3 RESULTS AND DISCUSSION

3.1 X-ray diffraction analysis

Figure 1 provides the small angle X-ray diffraction (XRD) patterns of the original Al/La composite mesoporous material (A-L) and samples calcined at 650 °C (A-L-650). The patterns of A-L (Fig. 1 a) and A-L-650 (Fig. 1 b) exhibited separately one distinguishable peaks due to the lamellar phase with d-spacing (d100) of 4.89 and 5.85 nm without big difference in the intensity and breadth of the peaks comparing as MA. Hence, we have succeeded in synthesizing calcined Al/La composite mesoporous materials templated with simple anionic surfactant. The mesostructure of metal-surfactant composites is similar to that of surfactant aggregates in aqueous solution because lamellar liquid crystals and rod-like micelles can be formed in aqueous solution. It is well-known that, upon heating, dissolved urea hydrolyzes with the liberation of hydroxyl ions, which can be used to promote the nucleation of metal oxide and at same time, the sample calcination leads easily to the collapse of crystal framework structure of the metal oxide [17]. Therefore, in our case, the precursor with outstanding hydrothermal stability is formed as a result of concurrent urea hydrolysis, anionic surfactant and co-precipitation of metal oxide, accompanied simultaneously by particle growth induced spontaneously in the closed hydrothermal condition at a constant temperature.

Figure 1 XRD pattern of A-L (a) and A-L-650 (b)

3.2 Nitrogen adsorption analysis

Figure 2 shows the nitrogen adsorption-desorption isotherms and pore size distributions of A-L-650. The sample exhibits a typical type IV isotherm with N2hysteresis loop. The sample shows narrow pore size distribution centered atca. 5.6 nm. The specific surface area of the sample is 273.90 m·g-1, with pore volume of 0.235 4 cm3·g-1. It is noticeable that the shape of isotherm of Al/La composite mesoporous materials was slightly different from other mesoporous alumina [18]. The sample shows a capillary condensation at a relative pressure of 0.4–0.6. This is one of the typical characteristics of mesoporous materials, indicating the existence of structural mesoporosity (framework mesoporosity). However, an extra large adsorption at relative pressure above 0.8 is observed only for the sample. This additional adsorption may be attributed to the filling of textural mesoporous, resulting from the intergrowth or aggregation of primary nanoparticles.

(a) Nitrogen adsorption-desorption isotherm

(b) Pore size distribution

Figure 2 Nitrogen adsorption-desorption isotherm (a) and pore size distribution (b) of A-L-650

Figure 3 SEM photographs of uncalcined sample

Figure 4 TEM photographs of sample

Figure 5 X-ray EDS photographs of A-L-650

Table 1 EDS analysis of the samples

Figure 6 TG/DTG spectra of AM and A-L

3.3 SEM and TEM analysis

Figure 3 show SEM photographs of micrometer-range morphologies of the uncalcined sample (A-L) and the calcined sample (A-L-650). The morphology of A-L is similar to that of A-L-650, when the sample was treated at 650 °C, and the particles of the sample seen in the picture are irregular worm-like small crystals which is similar to MA, the size of which is about 35 nm [19].

On the other hand, the TEM image of A-L and A-L-650 samples (Fig. 4) show the characteristic morphology of a wormhole-like mesoporous framework, while the formed A-L-650 sample presents a somewhat lath-like mesoporous framework [Fig. 4 (b)], which is similar to that of mesostructured alumina reported by Bhattacharyya[20]. The distance between layers lined in parallel (black stripes) was 5.6 nm in Fig. 4 (b), which agrees with the XRD measurement.

3.4 X-ray EDS analysis

Qualitative and quantitative elemental analyses for the micro-area of samples were finished by using X-ray energy dispersive spectrometer (EDS). The EDS data from three different surface observations are shown in Table 1.

The La/Al ratio can be about 70︰1 with basically the same molar ratio for Al and La of stuff and La is more evenly distributed in the surface of the samples.

3.5 Thermal analysis

The original A-L precursor was characterized by thermogravimetry analysis (TG-DTG), as shown in Fig. 6. In the TG-DTG curve, A-L exhibits a total mass loss of about 63%. This occurs in three steps, including 5% mass loss at 50-190 °C due to removal of physically absorbed water, 48% mass loss at 190-400 °C due to removal of organic template, crystal water and part of hydroxyl groups in the mixed crystalline Al and La composite oxide, and final 10% mass loss at temperature above 400 °C due to removal of the residual hydroxyl groups. No notable mass loss is observed at temperatures higher than 650 °C, which shows that the synthesis of mesoporous oxides of lanthanum aluminum composite has better thermal stability than MA, which shows still apparent mass loss until 1000 °C. For A-L, because of La3+doped holes occupying the position of alumina, the amount of surface hydroxyl was reduced and that effectively prevents the sintering of A-L [21].

Figure 8 TPR spectra of samples

Table 3 Results of TPR (°C) and reducecontent of the samples

3.6 NH3 temperature-programmed desorption

Temperature programmed desorption (TPD) of NH3on the samples were done to monitor changes in the accessibility of the acidic sites on them. The TPD spectra are shown in Fig. 7 for three different samples with calcined temperature ranging from 550-750 °C. The total ammonia uptake was determined by integrating the area under the TPD curves, and the data are listed in Table 2. Two main peaks are observed in the TPD spectra around 200-220 °C and 600-650 °C. These peaks correspond to weak and strong acid sites respectively, and are consistent with the literature [8]. A-L samples are significantly more acidic than MA because of La3+showed a higher polarization and more H+was adsorbed on its surface [22]. The peak value of ammonia desorption temperature changed with increasing of calcined temperature (Table 2). Ammonia desorption temperature peak (2) increases with the increasing of the calcined temperature, indicating that the sample calcined at high temperature produced a stronger acidic base. The NH3uptake decreased with the increasing of the calcined temperature because of overflow of the framework oxygen as well as narrowing of the pores preventing NH3from accessing internal sites.

Figure 7 NH3-TPD spectra of samples

Table 2 Results of NH3-TPD (°C) andacid content of the samples

3.7 Investigation of reducibility by TPR

TPR experiments were carried out in order to investigate the reducibility of A-L samples. Fig. 8 shows the TPR reduction profiles of Al/La composite mesoporous materials calcined at 550-750 °C. It has also been suggested that strong interactions between reduced metals and main structures are important in preventing sintering of the metal oxide particles [9]. Accordingly, studies of the reducibility of A-L samples are of prime interest. For all samples, only one main peak was observed at 580-700 °C. It has been shown in Table 3 that A-L is reduced in one stage which the peak indicates reduction of La3+to La2+. The sample has a better reducibility by the calcination of 650 °C.

4 CONCLUSIONS

Sodium dodecyl sulfate as a template, hydrothermal synthesis of lanthanum aluminum metal mixed oxides, with more common features of the mesoporous structure, similar to worm-like particles. The specific surface area of the sample is 273.90 m2·g-1, with the average pore size of 5.642 nm, pore volume of 0.235 4 cm3·g-1. The sample which was calcined in 650 °C has a strong acid centers and has got to redox properties.

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* To whom correspondence should be addressed. E-mail: wmsong2000@yahoo.com.cn

2010-04-16,

2010-11-28.