Facile synthesis of highly thermally stable nanoporous γ-aluminas from aluminum alkoxide precursors

Abstract

The effects of aluminum alkoxide single source precursors on the sol–gel synthesis of highly thermally stable nanostructured alumina by nonionic triblock copolymer P123 were investigated. Different crystalline nanoporous alumina materials with large single or bimodal accessible pores were synthesized via an efficient single-step process starting from alkoxide precursors utilizing the evaporation-induced self-assembly (EISA) method. The effect of different types of organic groups within the molecular aluminum precursor were explored by variation from iso-propoxide (OⁱPr), as an alkoxy group, over phenoxide (OPh), as an aryloxy group, to the presence of ether functions within the alkoxy chains, represented by methoxyethoxide (OCH₂CH₂OCH₃) and methoxyethoxyethoxide (OCH₂CH₂OCH₂CH₂OCH₃) groups. The prepared samples were characterized by small and wide angle X-ray diffraction, N₂ adsorption–desorption, TGA-DTA, and TEM measurements. For the prepared aluminas porosity, surface area, and excellent thermal stability, as well as an adjustable pore size distribution were found which make them suitable for potential application in different processes where single or bimodal accessible pores are necessary.

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1. Introduction

Synthesis of functional inorganic materials such as metal oxides with tunable properties has been considered as a challenging issue for both academic and industrial researchers.^1–6 Among these materials aluminum oxides are one of the most important class of compounds which have found a wide range of applications in industrial catalysts, catalyst supports, adsorbents, optics, electronics, or biomedicine.^7–12 Enhancement of the characteristics of these materials such as surface area, particle size, pore size, pore volume, morphology, and dimensionality continues to attract a considerable amount of interest.^13–19 Since the first report of well-ordered periodically organized mesoporous silica in 1992,²⁰ with outstanding characteristics such as high specific surface area, uniform pore channels, tunable pore size, narrow pore size distribution, and others, the synthesis of different porous materials has become an exciting area of materials chemistry.

Mesoporous aluminas (MAs) have been synthesized by different structural directing agents such as cationic and anionic surfactants,^21,22 and different block copolymers.^23,24 However, thermal stability of these materials and preservation of mesoporous structure upon calcination and processing is of great importance. Since the use of polymeric templates, such as non-ionic triblock copolymers as soft template, large efforts have been devoted to the synthesis of ordered mesoporous aluminas.²⁵ These types of polymeric templates are striking due to their availability, low price and biodegradability and specifically for their ability to induce the formation of uniform and large pores. Different approaches have been reported for the synthesis of ordered mesoporous aluminas. Early results from the usage of block copolymers resulted in amorphous,²⁶ disordered,²³ worm-like²⁷ or disk-shaped mesostructures.²⁸ However, there have been a few drawbacks such as the complicated hydrolysis behavior, the need to maintain strict control of synthetic conditions, in addition to the time consuming procedures. A remarkable step forward in the synthesis of ordered mesoporous γ-aluminas with high thermal stability has been achieved through the solvent evaporation induced self-assembly (EISA) of P123 triblock copolymer ([(EO)₂₀(PO)₇₀(EO)₂₀]) and aluminum precursors (Al(OⁱPr)₃, Al(O^sBu)₃ or Al(NO₃)₃) based on a simple and reproducible sol–gel process in ethanol.²⁹ Thereafter, other metal precursors such as aluminum chloride,³⁰ aluminum nitrate^30,31 and bohemite³² with different additives were used based on this procedure for preparation of MAs. The role of acid concentration in the synthesis medium on MAs has also been explored.³³ The physicochemical properties of alumina are highly dependent on the source of aluminum precursors.³⁴ Aluminum alkoxides are some of the most frequent precursors which have been used for synthesis of aluminas.³⁵ Moreover, the rate of hydrolysis and condensation of aluminum alkoxides strongly depend on the types of alkoxides groups and consequently the coordination environment of the aluminum centers.³⁶

There remains a great interest to explore different parameters in the synthesis of MAs for desired materials. To the best of our knowledge, the effect of utilizing various aluminum alkoxides as precursors in preparation of MAs employing the EISA method in presence of non-ionic polymeric templates has not been explored. Herein, to study the influence of various suitable alkoxides, including OⁱPr, OPh, OEtOMe, and OEtOEtOMe, on the structure, morphology, and physicochemical properties of MAs we prepared a series of highly thermally stable nanoporous alumina with tunable pore size and volume as well as crystallinity.

2. Experimental

2.1. Chemicals

Triblock copolymer Pluronic P-123 (average M_n ∼ 5800, (EO)₂₀(PO)₇₀(EO)₂₀) was purchased from Aldrich Chemical Inc. Ethanol, iso-propanol, phenol, 2-methoxyethanol, and 2-(2-methoxyethoxy)ethanol were obtained from Merck. HNO₃ (65 wt%) was purchased from Carl Roth GmbH. & Co. KG. Aluminum iso-propoxide, and aluminum phenoxide were synthesized from aluminum foil and related alcohol based on the reported procedures.^37,38 Aluminum 2-methoxyehoxide, [Al(OCH₂CH₂OCH₃)₃], and aluminum 2-methoxyethoxyethoxide, [Al(OCH₂CH₂OCH₂CH₂OCH₃)₃] were prepared by alcohol exchange method from aluminum iso-propoxide in toluene.³⁴ Ethanol was dried over sodium and distilled before use under inert atmosphere.

2.2. Synthesis

The Aluminas were synthesized based on the modified procedure reported by Yuan et al.²⁹ The triblock copolymer Pluronic P-123 (2.00 g) was dissolved in dried ethanol (20.0 mL) at ambient temperature and allowed to stir for 4 h. To this clear solution were added the aluminum alkoxide (0.02 mol) followed by dried ethanol (20.0 mL) and nitric acid (3.4 mL). The resulting solution was stirred for 5 h at room temperature. Subsequently the solvent evaporation process was performed at 60 °C in a convection oven for 72 h. Samples 2 and 4 were further dried at 150 °C for 2 h. Calcination was performed at a heating rate of 1 °C min⁻¹ starting from room temperature and kept at 400 or 900 °C for 4 h.

2.3. Characterization

X-ray powder diffraction patterns were collected at room temperature with a Siemens D5000 (Cu-K_α, germanium monochromator). Small angle X-ray scattering (SAXS) measurements were performed on a Bruker AXS Nanostar (Bruker, Karlsruhe, Germany), equipped with a microfocus X-ray source (Incoatec ImSCu E025, Incoatec, Geesthacht, Germany), operating at λ = 1.54 Å. A pinhole setup with 750 mm, 400 mm, and 1000 mm (in the order from source to sample) was used and the sample-to-detector distance was 27 cm. Samples were mounted on a metal rack and fixed using tape. The N₂ adsorption and desorption isotherms were measured using a Quantachrome Autosorb-iQ-MP gas sorption analyzer. Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA) were performed with a NETZSCH STA409PC Luxx apparatus under constant flow of air ranging from room temperature up to 1000 °C with a heating rate of 1 K min⁻¹.

For transmission electron microscopy (TEM), for each sample, 20 mg of aluminum oxide were added to 100 μL of absolute ethanol and sonicated at 37 kHz and 240 W for 20 min. After 10 min of sedimentation a drop of 6 μL supernatant was placed on top of a carbon-coated 400 mesh copper grid (Quantifoil Micro Tools, Jena, Germany). Excess liquid was removed by placing a sheet of filter paper below the grid. The samples were then air dried and analyzed using a CM120 electron microscope (Philips, Eindhoven, Netherlands) operated at 120 kV. Images were acquired using a 2k TemCam F216 CMOS camera (camera and software, TVIPS, Munich, Germany). For scanning electron microscopy (SEM) the 400 mesh copper grids previously prepared for the TEM analyses were sputter coated with platinum (2 nm) in a BAL-TEC SCD005 Sputter Coater (BAL-TEC, Liechtenstein) and imaged in a LEO 1530 Gemini field emission scanning electron microscope (Carl Zeiss, Oberkochen, Germany) at 4 kV acceleration voltage and a working distance of 3 mm using an inlense secondary electron detector.

3. Results and discussion

Four different aluminum alkoxide precursors Al(OⁱPr)₃, Al(OPh)₃, [Al(OCH₂CH₂OCH₃)₃], and [Al(OCH₂CH₂OCH₂CH₂OCH₃)₃] have been utilized as molecular precursors for the synthesis of aluminum oxides by evaporation-induced self-assembly in the presence of P123 triblock copolymer. In order to investigate the role of the polymeric template two additional experiments were performed under similar conditions but in absence of template. The effect of four different functional groups in the aluminum precursors have been explored. This includes the iso-propoxide (OⁱPr) as an alkoxy group, phenoxide (OPh) as an aryloxy group, and the presence of ether functions on the alkoxy chains, such as methoxyethoxide (OCH₂CH₂OCH₃) and methoxyethoxyethoxide (OCH₂CH₂OCH₂CH₂OCH₃), which provide additional donor sites and are therefore expected to lead to decreasing rates of hydrolysis.

The synthetic conditions under which the aluminas were prepared using different molecular precursors are summarized in Table 1. Small angle X-ray scattering (SAXS) measurements were performed to check for the evidence of the formation of mesopores. The SAXS pattern of sample Al-1 calcined at 400 °C is depicted in Fig. 1. It shows a strong reflex (100) at 0.7° and, in addition, a weak reflex at around 1.2°. The SAXS patterns of the other three samples calcined at 400 °C are depicted in Fig. 2. In these samples the main diffraction reflexes (100) are around 0.6°, which is an indication for meso-structure of these samples, but no long range order was observed. For sample Al-1 the highest intensity for peak (100) is observed while for Al-2 the lowest intensity is observed within the series of template assisted materials (Al-1 to Al-4), which indicates that alkyl or alkylether groups have a better effect on the porosity of the final product. In the presence of alkylether and aryl groups the main diffraction peak (100) is shifted towards lower angles, as can be seen from Fig. 1 and 2. This observation could be related to synergetic effects of released organic molecules, such as ether functionalized alcohols or phenol after hydrolysis of the corresponding alkoxides, with the presence of block copolymer template which results in different pores.

Table 1 Synthetic conditions for the preparation of aluminas

Sample	Alkoxides precursor	Template	Acid
Al-1	Al(O^iPr)3	Plutonic P123	HNO3
Al-2	Al(OPh)3	Plutonic P123	HNO3
Al-3	[Al(OCH2CH2OCH3)3]	Plutonic P123	HNO3
Al-4	[Al(OCH2CH2OCH2CH2OCH3)3]	Plutonic P123	HNO3
Al-5	Al(O^iPr)3	—	HNO3
Al-6	[Al(OCH2CH2OCH2CH2OCH3)3]	—	HNO3

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Fig. 1 SAXS pattern of Al-1 calcined at 400 °C.

Fig. 2 SAXS patterns of Al-2, Al-3, and Al-4 calcined at 400 °C.

For most of the reported cases considerable drawbacks are observed, as the prepared MAs possess amorphous walls or involve inconvenient multistep synthetic procedures. Consistently the wide-angle XRD measurements of all samples calcined at 400 °C show the occurrence of amorphous structures. The TGA-DTA data of the samples exhibit an exothermic peak at about 860 °C without any weight loss which is due to the crystallization of amorphous alumina. Therefore, all samples have also been calcined at 900 °C. The conservation of the mesoscopic order of Al-1 after calcination at 800–1000 °C has been proved previously,²⁹ whereas for samples Al-2 to Al-4 the thermal stability of the mesoscopic structure for calcination at 900 °C is indicated by the SAXS measurements depicted in Fig. 3. The wide-angle XRD patterns for the samples Al-1 to Al-4 are depicted in Fig. 4. In all the cases, except for Al-2, calcination at 900 °C results in conversion of amorphous walls to γ-alumina as crystalline phase (JCPDS card no. 10-0425). These results show that in the presence of aliphatic or aliphatic-ether groups, the only observed phase is γ-alumina. However, for Al-2 a different behavior is observed, as in addition to the γ-alumina also sharp and distinct peaks of α-alumina are observed (JCPDS card no. 46-1212). The collective results from SAXS and WAXS (wide angle X-ray scattering) measurements confirm the presence of crystalline mesoporous structures for the samples calcined at 900 °C.

Fig. 3 SAXS patterns of Al-2, Al-3, and Al-4 calcined at 900 °C.

Fig. 4 Wide-angle XRD patterns for samples Al-1 to Al-4 calcined at 900 °C.

Fig. 5 shows nitrogen adsorption–desorption isotherms of Al-1 to Al-4 treated at 400 °C and 900 °C (for Al-5 and Al-6 see Fig. S1 and S2†). According to IUPAC classification six types of sorption isotherms are possible.³⁹ All samples, except Al-2 calcined at 400 °C, exhibit type IV isotherms. In fact, the observed isotherm for Al-2 can be regarded as a combination of type I and IV isotherms. The observed type IV isotherms are typical for mesoporous materials. The most characteristic feature of this type of isotherm is the hysteresis loop, which often accompanies the occurrence of pore condensation. The limiting uptake in a range of high P/P° leads to a plateau in the isotherm, which is indicative of complete pore filling. The first part of this type of isotherms can be attributing to monolayer–multilayer adsorption. It has been widely accepted that there is a correlation between the shape of the hysteresis loop and the texture properties of mesoporous materials such as pore size, pore size distribution, and their connections. Hysteresis loops were first classified by de Boer⁴⁰ and later by the IUPAC.³⁹ After calcination at 900 °C all four samples exhibit hysteresis loops of type H1, which is often associated with porous materials containing a distinct tubular pore network or agglomerates of compacts with nearly identical particles.

Fig. 5 Nitrogen adsorption–desorption isotherms of Al-1 to Al-4 calcined at 400 °C and 900 °C represented with red and blue symbols respectively.

Also noteworthy are the results of N₂ absorption–desorption for the samples calcined at 400 °C. For sample Al-1 a hysteresis loop of type H2 is observed which is usually taken as an indication of the occurrence of channel-like pores of somewhat uniform size with narrow pore size distribution. For sample Al-3 the hysteresis loop is larger than for Al-1 which is due to continued adsorption up to P/P° = 1. This behavior is normally taken as an indication of more complex pore networks involving pores with ill-defined shape and a wide pore size distribution.³⁹ Overall, the pore size distribution is narrow for Al-1 and bimodal pores are observed for Al-3. As mentioned above, the isotherm observed for sample Al-2 can be regarded as a combination of type I and type IV. This means that in addition to the mesopores or agglomerations of identical nanoparticles, a significant amount of micropores are also present. For microporous materials with type I isotherms a high adsorption potential and narrow pore width leads to micropore filling and therefore high uptake (as shown in Fig. 5 Al-2-400 as well) at relatively low pressures. The limiting adsorption is being controlled by the available micropore volume rather than by the interior surface area. As deduced by WAXS, the calcination of sample Al-2 at 900 °C results in crystallization which is accompanied by the change of its isotherm to type IV and a hysteresis loop of H1 type. This is believed to be an indication of structural change with a significant decrease in the micropore component. There were no significant changes observed in the isotherms of Al-4 after calcination at 900 °C which shows that it retains most of its structural characteristics upon crystallization at higher temperature. These results show that the presence of phenolic groups resulted in bimodal porosity and increased micropore volume while the application of ether groups leads to a wider pore size distribution.

In Table 2 the data for surface area (BET), pore volume, and mean diameter of the pores are presented for the four samples prepared in the presence of polymeric template and the two additional samples, Al-5 and Al-6, which were prepared under the same conditions, but in absence of the template. As expected, the results show that calcination at higher temperature (900 °C) leads to a decrease in the surface area and pore volume.

Table 2 Pore structure parameters of the samples obtained from different alkoxides precursors

Compounds	Calcination temperature (°C)	BET surface area (m^2 g^−1)	Total pore volume (cm^3 g^−1)	Average pore width (nm)
Al-1	400	312	0.57	7.3
	900	147	0.33	9.0
Al-2	400	480	0.29	2.4
	900	99	0.22	8.9
Al-3	400	366	0.58	6.4
	900	97	0.40	16.5
Al-4	400	269	0.63	9.3
	900	109	0.47	17.0
Al-5	400	232	0.19	3.2
Al-6	400	168	0.68	16.2

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For the samples calcined at 400 °C the largest surface area is observed in the sample Al-2 but when compared with the others within the same series its pore volume is found to be the lowest. Moreover, for this sample also the smallest average pore diameter within a given calcination temperature is observed, which can be related to its mixed microporous–mesoporous texture. The presence of micropores in this sample reduces the volume and pore size distribution when compared with the other three samples. Besides having the largest observed surface area (480 m² g⁻¹), also the presence of two types of pores (micro- and mesopores) is a significant feature for the sample Al-2, which clearly demonstrates the substantial impact of the aluminum alkoxide precursor.

For calcination at 900 °C two groups can be identified, as the average pore size distribution is concerned, with distinct ranges for Al-1 and Al-2 as well as Al-3 and Al-4 at around 9 and 17 nm, respectively. Within the first group the sample Al-1 possesses mesopores with a narrow pore size distribution, a high surface area, and a corresponding pore volume. A closer inspection of the data for samples Al-3 and Al-4 reveals that the utilization of ether-alkoxides in the aluminum precursor not only affects the surface area, but results in a considerable increase of the average pore diameter and volume. The extension of the alkoxide chain with additional potential donor groups, as for the samples Al-3 and Al-4, leads to a considerably larger pore volume and a slight increase in its average pore size, which is particularly evident for sample Al-4.

To probe the influence of the polymeric template the sample Al-5 was prepared in a manner similar to Al-1 but in absence of template. The results show that the surface area, volume and average pore diameter are significantly decreased and a broader pore size distribution is obtained. For sample Al-6, which was likewise prepared in a manner similar to Al-4 but in the absence of polymeric template, the results show that while the volume and average pore diameter increase, the surface area decreases. The XRD patterns for the samples Al-5 and Al-6 are presented in Fig. 6 and show that their crystallization pattern is similar to the same samples which have been prepared in the presence of template. In Conclusion, these two additional experiments reveal the importance of the presence of polymeric template for the preparation of porous aluminas.

Fig. 6 Wide-angle XRD patterns of samples prepared in absence of any template.

TEM and SEM images of sample Al-1 are depicted in Fig. 7. The images for samples obtained after calcination at 400 °C show the alignment of tubular structures (Fig. 7a and b) and the corresponding presence of a hexagonal arrangement of these tubular structures for both calcination at 400 °C (Fig. 7c) and 900 °C (Fig. 7d). Selected TEM images of the prepared samples are shown in Fig. 8 and 9 which confirm the retention of the porous structure of the samples after heat treatment at 900 °C.

Fig. 7 (a) and (b) TEM and SEM images of Al-1 calcined at 400 °C, view along the 110 direction; (c) TEM image of Al-1 calcined at 400 °C, view along the 001 direction; (d) TEM image of Al-1 calcined at 900 °C, view along the 001 direction. Scale bars 100 nm (a and b) and 50 nm (c and d).

Fig. 8 (a) TEM image of Al-3 calcined at 900 °C, view along the 001 direction; (b) TEM image of Al-3 calcined at 900 °C, view along the 110 direction. Scale bars 100 nm.

Fig. 9 TEM images of Al-4 calcined at 400 °C (a), and calcined at 900 °C. (b). Scale bars 100 nm.

4. Conclusions

The influence of different aluminum alkoxide precursors as one of the most important parameters in the synthesis of nanoporous aluminas has been explored. These results show that all the prepared samples were thermally stable and preserved their porous structure upon elevated heat treatment. SAXS also showed that the alkyl or alkylether groups lead to increased porosity of the final product if compared to the aryl group. All the samples were found to be amorphous after calcination at 400 °C with relatively high surface areas. Additionally, the utilization of the aluminum phenoxide precursor leads to formation of a bimodal microporous–mesoporous structure after calcination at 400 °C and a mixed crystalline phase (γ- and α-alumina) with mesoporous-macroporous structure when calcined at 900 °C, while for the other three samples solely the formation of γ-alumina is observed. The usage of the ether-alkoxides not only affects the surface area, but also results in an increase of the average diameter of the pores and their volumes. The extension of ether-alkoxide chain with additional ether groups leads to a larger pore volume and a slight increase in its average pore size. Thus, depending on the desired catalytic reaction or catalysis support, each of the molecular precursors could be utilized to obtain tailor-made materials for the intended application.

Acknowledgements

We gratefully acknowledge the Iran National Science Foundation (INSF) for supporting this study. Partial support from the Research Affairs Division of Isfahan University of Technology (IUT) is also acknowledged. The financial support of the Deutsche Forschungsgemeinschaft (DFG: PL 155/14) as well as of the Thuringian Ministry for Science, Education, and Culture (TMBWK, grant #B515-11028 SWAXS-JCSM) is gratefully acknowledged. O.A. thanks the “Evangelisches Studienwerk Villigst e.V.” for a scholarship. We thank Mr Reinhardt for the measurement of the thermal analysis data and Mr. Wermann for measuring the powder diffraction data.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05883j

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