Facile synthesis of pure phase γ-AlOOH and γ-Al₂O₃ nanofibers in a recoverable ionic liquid via a low temperature route

Abstract

Mesoporous γ-AlOOH and γ-Al₂O₃ with fibrous morphology were successfully synthesized by an ionic liquid ([OMim]Br) assisted low temperature precipitation method using aluminum nitrate and ammonia as aluminum source and precipitant, respectively. The samples were characterized by XRD, TG, FT-IR, TEM, and N₂ adsorption–desorption technique. The results showed that the concentration of [OMim]Br had a significant effect on the phase, porous structure, and morphology of γ-Al₂O₃ due to the strong interactions between [OMim]Br and γ-AlOOH. High purity γ-AlOOH was synthesized at 0.2 M [OMim]Br and γ-Al₂O₃ with a specific surface area of 243.53 m² g⁻¹, pore volume of 0.68 cm³ g⁻¹, and average pore size of 11.10 nm was obtained after calcination. Furthermore, the recyclability of [OMim]Br was investigated. The recovery rate of [OMim]Br could reach 95.0 wt%, and the recycled [OMim]Br still had effects on the synthesis of γ-AlOOH and γ-Al₂O₃.

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1. Introduction

Al₂O₃ is a vital industrial material which has been used in an extensive range of fields, such as catalysis, catalyst supports, purification, fillers, abrasives, paints and so on.^1–3 Al₂O₃ exists in many forms, α, χ, η, δ, κ, θ, γ, ρ, etc. Among them, γ-Al₂O₃ is the most important one due to its widespread applications owing to its specific physicochemical properties, such as textual properties, acid/base characteristics, morphology and strength.⁴ Therefore, the preparation of Al₂O₃ with unique characteristics has received increased attention. The precipitation method is widely used to synthesize γ-Al₂O₃, in which low temperature precipitation, characterized by lower energy expenditure, is more mild and economical in the synthesis process. Aluminum nitrate salt is one of the most common sources of aluminium for preparing precursors of γ-Al₂O₃. However, using low temperature precipitation to prepare precursors of γ-Al₂O₃ from aluminum nitrate salt is prone to obtain the mixed precursors of gibbsite and boehmite (γ-AlOOH).^5,6 γ-AlOOH is a desirable precursor of γ-Al₂O₃ due to the energy saving with morphology and textual structure preserved.^7,8 Thus, it is important to investigate the appropriate technology or suitable additives in low temperature precipitation to control the synthesis of γ-AlOOH.

In previous research, the appropriate surfactant has been proven to be an effective approach to synthesis γ-AlOOH and improve its properties.⁹ Ionic liquids (ILs), with the properties of wide liquid range, negligible vapor pressure, good thermal stability, and adjustable physical and chemical properties, have been investigated as novel and green media for synthesis materials.^10–14 Particularly, the property of forming extended hydrogen bond systems in the liquid state and reusability favor their application performance.^15,16 Up to now, various inorganic materials have been prepared using ILs with different approaches. For instance, nanosized metal fluorides have been fabricated in BMIMBF₄ ionic liquids solvent has been reported.¹⁷ Titania,^18,19 iron oxide and Al₂O₃/titania composite have been prepared in ionic liquid.^20,21 γ-Al₂O₃ nanostructures with various morphologies has been controllably synthesized via ionic-liquids-assisted hydrothermal routes and the effect models of ILs has been found by Zheng' group.^22–24 However, most approaches are hydrothermal or ionothermal routes, which require high temperature, pressure and a large amount of ionic liquids. Hence, it is a challenge to fabricate pure γ-AlOOH and γ-Al₂O₃ and adjust the textual structure with ionic liquids using inorganic aluminum source by low temperature precipitation. In addition, it requires great effort to recycle ionic liquids from the perspective of green chemistry.

In this paper, we report a facile and environmentally friendly route to synthesize fibrous γ-AlOOH and γ-Al₂O₃ with high purity via 1-butyl-3-methylimidazole bromide ([OMim]Br) assisted low temperature precipitation method using inorganic Al(NO₃)₃·9H₂O and NH₃·H₂O as aluminum source and precipitant, respectively. The effects of [OMim]Br concentration on the phase, textual structure, and morphology of γ-AlOOH and γ-Al₂O₃ products are elucidated. In addition, the recyclability of [OMim]Br is highlighted in this paper.

2. Experimental section

2.1. Materials and preparation

Materials. Aluminum nitrate (A.R.), ammonia (A.R.), absolute ethanol (A.R.), and ethyl acetate (A.R.) were obtained from Sinopharm Chemical Reagent Co., Ltd. N-Methylimidazole (A.R.), and 1-bromooctane (A.R.) were purchased from Aladdin. All of the reagents were used without further purification. Ionic liquid 1-octyl-3-methylimidazole bromide ([OMim]Br) was synthesized according to the ref. 25.

The synthesis of fibrous γ-Al₂O₃ with [OMim]Br : 0.01 mol Al(NO₃)₃·9H₂O and different amounts of [OMim]Br (C_[OMim]Br = 0, 0.05, 0.1, 0.2, 0.4 M) were added into 100 mL of deionized water under vigorous stirring to form a homogeneous solution. Then, γ-AlOOH precipitate was formed after ammonia was added into the solution with a feeding rate of 500 μL min⁻¹. The pH of the solution was adjusted to pH 8.5, which was monitored by a PHS-2F pH meter. After aging at 55 °C for 24 h, the mixture was processed with centrifugation and divided into two layers. The upper layer was the mixture of ionic liquid [OMim]Br and H₂O and the lower layer was the precipitate of precursor γ-Al₂O₃ (γ-AlOOH), which was washed with deionized water and ethanol and dried under vacuum at 60 °C for 12 h. The upper layer and the washed residual liquids were processed with vacuum distillation and then ionic liquid [OMim]Br was recovered. γ-Al₂O₃ was obtained after γ-AlOOH was calcinated at 600 °C for 6 h.

2.2. Characterization

The XRD was performed on a Philips X'pert MPD Pro diffractometer equipped with Ni-filtered Cu Kα radiation (λ = 0.15418 nm). FT-IR spectra were collected on a Thermo Nicolet 870 spectrophotometer with a resolution of 5 cm⁻¹ using anhydrous KBr as dispersing agent. The thermogravimetric (TG) and differential scanning calorimetry (DSC) were conducted with use of a Netzsch STA 449F3 simultaneous DSC-TG thermogravimetric analyzer. N₂ adsorption–desorption measurements were carried out on a Micromeritics ASAP 2020 system model instrument at 77 K. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) algorithm. The mesoporous pore size distribution was obtained using the Barrett–Joyner–Halenda (BJH) theory. TEM images were obtained with a JEOL (model 794) instrument employing an acceleration voltage of 120 kV.

3. Results and discussion

3.1. Characterization of γ-AlOOH and γ-Al₂O₃

The phase and purity of samples are characterized by XRD. The precursors synthesized without/with [OMim]Br are shown in Fig. 1(A). The broad diffraction peaks of the as-synthesized samples reveal their nanoscale nature. In the absence of [OMim]Br, the precursor can be mainly indexed as γ-AlOOH (JCPDS 21-1307) and the weak diffraction peaks observed at 20°, 40° and 53° correspond to the Al(OH)₃ impurity phase (JCPDS 77-0114). The intensities of Al(OH)₃ diffraction peaks firstly become stronger then weaker by increasing the concentration of [OMim]Br from 0 to 0.05 M then to 0.1 M. However, the diffraction peaks of Al(OH)₃ disappear for the sample synthesized at C_[OMim]Br = 0.2, 0.4 M, indicating the complete phase transformation from Al(OH)₃ to AlOOH. Fig. 1(B) presents XRD patterns of all alumina after calcination at 600 °C. The alumina is mainly amorphous without [OMim]Br. While all alumina, synthesized in the presence of [OMim]Br, can be assigned to γ-Al₂O₃ (JCPDS 10-0425). The weak intensity and broad bands indicate the low crystallinity of γ-Al₂O₃.

Fig. 1 XRD patterns of the as-synthesized precursors (A) and corresponding γ-Al₂O₃ (B) at varying C_[OMim]Br = 0 (a), 0.05 (b), 0.1 (c), 0.2 (d), 0.4 M (e); FT-IR spectra (C) of as-synthesized precursors at varying C_[OMim]Br = 0 (a), 0.05 (b), 0.2 (c) and Al₂O₃ synthesized at C_[OMim]Br = 0.2 M (d).

To further confirm the chemical compositions of as-synthesized precursor and alumina, FT-IR measurement is carried out in the region of 400–4000 cm⁻¹. In the FT-IR spectra (Fig. 1(C)) of as-synthesized samples, two broad bands at 3420 and 3096 cm⁻¹ belong to the ν_as(Al)–OH and ν_s(Al)O–H stretching vibrations of γ-AlOOH. In the spectra of γ-AlOOH synthesized at C_[OMim]Br = 0.2 M, the weak bands at 2949 and 2866 cm⁻¹ due to the asymmetric and symmetric CH₂ stretch respectively, are detectable. This result demonstrates trace amount of [OMim]Br is still in γ-AlOOH though the sample has been washed for several times. These phenomena may be related to the hydrogen bond between [OMim]Br and –OH in γ-AlOOH. Another sharp peak at 1069 cm⁻¹ and the weak shoulder at 1147 cm⁻¹ are assigned to the δ_sAl–O–H and δ_asAl–O–H bending vibrations of γ-AlOOH, respectively. The three bands at 739, 629 and 480 cm⁻¹ represent the vibration mode of [AlO₆], while the 853 cm⁻¹ structure results from the stretching of [AlO₄] atomic group. The above bands are typical features of γ-AlOOH,^13,26 which are consistent with XRD results. The weak band at 1640 cm⁻¹ is attributed to the bending vibration band of adsorbed water. After calcination, the bands at 543 cm⁻¹ and 798 cm⁻¹ were assigned to [AlO₄] and [AlO₆] sites in γ-Al₂O₃.^27,28 The broad bands around 3450 and 1640 cm⁻¹ show the existence of –OH.

Thermal stabilities of as-synthesized precursors without/with [OMim]Br are studied by thermogravimetry-differential scanning calorimetry (TG-DSC). Typical TG-DSC curves of as-synthesized samples synthesized at C_[OMim]Br = 0, 0.2 M are shown in Fig. 2. The initial weight loss from 25 °C to 150 °C, accompanied with an endothermal peak around 75 °C in the DSC curves, can be assigned to the removal of physical adsorbed water. The second weight loss from 150 °C to 450 °C, with an endothermic peak around 250 °C, is due to the first stage of dehydration by condensation of Al–OH groups. The last region of weight loss in the region of 450–800 °C is associated with a second stage of dehydration by the condensation of hydroxyl groups.²⁹ However, compared with the DSC curves of sample synthesized at C_[OMim]Br = 0, there is an additional exothermic peak around 350 °C in the DSC curves synthesized at C_[OMim]Br = 0.2 M, which can be attributed to the decomposition of [OMim]Br. The weight loss at 150 °C and 450 °C is 15.1 wt% and 16.7 wt%, respectively. However, the theoretical weight loss of the phase transition from γ-AlOOH to Al₂O₃ is 15 wt%, which means the actual weight loss of samples synthesized at C_[OMim]Br = 0.2 M is higher than the theoretical value. The extra mass loss (1.7 wt%) can be ascribed to the decomposition of [OMim]Br.

Fig. 2 TG-DSC curves of as-synthesized precursors synthesized at C_[OMim]Br = 0 (a), 0.2 M (b).

The effect of adding the [OMim]Br in the formation of γ-Al₂O₃ is analyzed by N₂ adsorption–desorption. For N₂ adsorption–desorption isotherms in Fig. 3(A), all isotherms are classified as type IV, according to the International Union of Pure and Applied Chemistry (IUPAC) classification,³⁰ verifying they are mesoporous materials. When the relative pressure is between 0.6 and 1, the isotherms of γ-Al₂O₃ synthesized at C_[OMim]Br = 0, 0.05, 0.1, 0.2 M show H1-type hysteresis loops, as shown in Fig. 3(A). However, the isotherm of γ-Al₂O₃ synthesized at C_[OMim]Br = 0.4 M exhibits H2-type hysteresis loops, indicating interconnected pore network. Fig. 3(B) shows the pore size distributions of all γ-Al₂O₃, calculated from the desorption data of isotherms using the Barrett–Joyner–Halenda (BJH) method. All γ-Al₂O₃ are featured with bimodal mesoporous distribution in the range of 5–20 nm. The texture properties of γ-Al₂O₃ products, the BET specific surface area, pore volume and the average pore size, are summarized in Table 1. The γ-Al₂O₃ synthesized at C_[OMim]Br = 0.1 M presents the highest BET surface area (258.02 m² g⁻¹) and the highest pore volume (0.78 cm³ g⁻¹) with an average pore diameter of 12.13 nm. The γ-Al₂O₃ with specific surface area of 243.53 m² g⁻¹, pore volume of 0.68 cm³ g⁻¹, and average pore size of 11.10 nm was obtained after the calcination of high purity γ-AlOOH, which was synthesized at 0.2 M [OMim]Br. It is interesting that the influence of [OMim]Br on the texture properties of γ-Al₂O₃ products shows an evident trend. As shown in Fig. 3(C–E), the BET specific surface area, pore volume and the average pore size of these γ-Al₂O₃ products exhibit a linear trend which firstly increases and then decreases with increasing [OMim]Br. Therefore, we can handily control the textual structure of Al₂O₃ by adjusting the concentration of [OMim]Br.

Fig. 3 (A) N₂ adsorption–desorption isotherms, (B) BJH pore-size distribution and (C) BET specific surface area, (D) pore volume and (E) average pore size changes of γ-Al₂O₃ with varying C_[OMim]Br.

Table 1 The comparison of morphology of γ-Al₂O₃ with different templates

	Reaction conditions				Morphology	Specific surface area/(m^2 g^−1)	Ref.
	Temperature/°C	Time/h	Solution	Template
1	430–450	192	DI water	SiO2	Nanosheets	21.40	32
2	100	24	DI water	P123	Amorphous	182.00	5
3	120	24	DI water	TX-100	Hollow urchin	146.91	9
4	100	48	DI water	PEO	Nanofibers	347.40	33
5	25–30	24	Ethanol	[Bmim]PF6	Rod	85.00	34
6	170	36	DI water	[OMim]Cl	Flower	164.60	35
7	55	24	DI water	[OMim]Br	Nanofibers	258.02	This work

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The morphology of γ-Al₂O₃ synthesized at C_[OMim]Br = 0 and 0.2 M is observed by transmission electron microscopy. The sample synthesized without [OMim]Br contains some fibers and particles with irregular morphology. However, the γ-Al₂O₃ synthesized at C_[OMim]Br = 0.2 M consists mainly of fibers, which are relatively more uniform than that at C_[OMim]Br = 0. Furthermore, the sample is composed of flocculence-like mesostructures as previous report.³¹ As shown in Fig. 4(b), the fiber, with the length of 20–30 nm, width 2–3 nm, is composed of many small crystalline nanosized particles with interconnected pores. The selected area electron diffraction (SAED) pattern (inset in Fig. 4(a)) of the nanofibers confirms the crystal structure with γ-Al₂O₃ phase, which is consistent with the results of XRD.

Fig. 4 TEM images of γ-Al₂O₃ synthesized at C_[OMim]Br = 0 (a), 0.2 M (b) and 0.4 M (c) and corresponding SAED pattern of γ-Al₂O₃ synthesized at C_[OMim]Br = 0.2 M (d).

Results of the morphologies of γ-Al₂O₃ with different templates were investigated, which were listed in Table 1. The morphology of γ-Al₂O₃ is greatly affected by the templates, and varies from nanosheets, hollow urchin, rod to flower under different templates. As shown in Table 1, the specific surface area of nanofibers γ-Al₂O₃ prepared by ionic liquid ([OMim]Br) assisted low temperature precipitation in this paper is higher than that synthesized via other ionic liquids assisted method in previous reports, and is similar with that prepared by conventional templates.

3.2. The effect of [OMim]Br

According to the above results, [OMim]Br leads to changes of the phase, morphology and pore structure. A possible formation mechanism of γ-AlOOH nanofibers is proposed as shown in Scheme 1. [OMim]Br is a solvent and template, widely used in the synthesis of inorganic materials. When it is in aqueous solution, it can mix with water in all compositions and form micelles.^36,37 In this system, [OMim]Br can easily adsorb onto the surface of AlOOH particles to form aligned hydrogen bonds and additional π–π interactions.¹² Thus, the surface energy of aluminum oxyhydroxide can be reduced. The aluminum oxyhydroxide is more stable with [OMim]Br than that without [OMim]Br, inducing well-grown nanofibers. At the same time, due to the long side-chain on the imidazole ring, the bulky molecules prevent the aggregation effectively. With increasing [OMim]Br, more [OMim]Br molecules adsorb on the surface of particles, the growth predominance of nanofibers increases.

Scheme 1 The schematic demonstration of the formation process of γ-AlOOH.

In addition, the increase by the addition of [OMim]Br gives rise to the viscosity of the system. The viscosity of the system and the aggregation of [OMim]Br micelles increases quickly with increasing [OMim]Br, which will suppress the access of [OMim]Br to aluminum oxyhydroxide causing the decrease in the textual parameter of alumina, namely the “viscosity-controlled” mechanism.³⁸ However, with the concentration of [OMim]Br increases and ultimately reaches 0.2 M, the number of the [OMim]Br molecules is increasing. The distribution of [OMim]Br is not uniform, some [OMim]Br may not adsorb on the surface of aluminum oxyhydroxide and the crystallites of aluminum oxyhydroxide aggregate together and form particles.

3.3. The recyclability of [OMim]Br

Though ILs do not release harmful gases into the atmosphere because of the low volatility, they may become waste liquor causing unpredictable water and/or land contamination for their inaccessible biodegradability. In addition, the price of ILs is relatively high in comparison with other general templates employed in synthesis of inorganic materials. Therefore, the recycling of ILs plays an important role in the green chemistry and economics theory of inorganic materials synthesis.

To investigate the recyclability of [OMim]Br, the [OMim]Br was used for 5 runs and recycled by available vacuum distillation. Moreover, it is noteworthy that the recovery rate of [OMim]Br could reach as high as 95.0 wt%, the remaining 5.0 wt% loss might include the loss in the reaction process and precursor. The recycled [OMim]Br is further applied into resynthesizing the AlOOH and Al₂O₃. And the result of γ-Al₂O₃ with C_[OMim]Br = 0.2 M after washed by water and ethanol, dried and calcined is shown in Fig. 5. From Fig. 5, it can be seen that using recycled [OMim]Br we can also obtain γ-AlOOH (JCPDS 21-1307) and γ-Al₂O₃ (JCPDS 10-0425). The obtained γ-Al₂O₃ has mesoporous structure with type IV N₂ adsorption–desorption isotherm further characterized by N₂ adsorption–desorption. The textual parameters of obtained γ-Al₂O₃ are respectively BET surface area 278.96 m² g⁻¹, pore volume 0.97 cm³ g⁻¹ and average pore diameter of 13.6 nm. In addition, as shown in the TEM image, the morphology of obtained γ-Al₂O₃ is also fibrous. In summary, the similar fibrous γ-Al₂O₃ can be produced with mesoporous structure using recycled [Omim]Br by simple recovery process. Effects of the recycled [OMim]Br on the synthesis of γ-AlOOH were almost the same as those of the fresh [OMim]Br.

Fig. 5 (A) XRD patterns: as-synthesized AlOOH (a) and γ-Al₂O₃ (b); (B) N₂ adsorption–desorption isotherm and (C) TEM images of γ-Al₂O₃ synthesized at C_[OMim]Br = 0.2 M with recycled [OMim]Br for 5 times.

4. Conclusions

In summary, we have successfully synthesized γ-AlOOH and γ-Al₂O₃ with fiber morphology by [OMim]Br assisted low temperature precipitation method. Based on results of characterizations, the concentration of [OMim]Br affects the phase of γ-AlOOH and the optional concentration of [OMim]Br is 0.2 M at which γ-AlOOH with high purity can be obtained. After calcination, γ-Al₂O₃ is obtained with the specific surface area of 243.53 m² g⁻¹, pore volume of 0.68 cm³ g⁻¹ and average pore size of 11.10 nm. All of them firstly increase then decrease with the increasing concentration of [OMim]Br with 0.1 M as the critical point, which makes it possible to precisely control the synthesis of γ-Al₂O₃ with specific textual properties. Furthermore, the recyclability of [OMim]Br has been investigated. The recovery rate of [OMim]Br could reach 95.0 wt% and the recycled [OMim]Br had effects on the γ-Al₂O₃. The recyclability of [OMim]Br may have a promising potential in the field of inorganic materials synthesis.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21176121, no. 21306082), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (12KJB530004), and the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University (ZK201305).

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