Clean synthesis of hierarchically structured boehmite and γ-alumina with a flower-like morphology

Accepted Manuscript Clean Synthesis of Hierarchically Structured Boehmite and γ-Alumina with A FlowerLike Morphology Zongbo Shi, Wenqian Jiao, Li Chen, Peng Wu, Yimeng Wang, Mingyuan He PII:

S1387-1811(15)00729-5

DOI:

10.1016/j.micromeso.2015.11.064

Reference:

MICMAT 7501

To appear in:

Microporous and Mesoporous Materials

Received Date: 19 June 2015 Revised Date:

25 November 2015

Accepted Date: 29 November 2015

Please cite this article as: Z. Shi, W. Jiao, L. Chen, P. Wu, Y. Wang, M. He,, Clean Synthesis of Hierarchically Structured Boehmite and γ-Alumina with A Flower-Like Morphology, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2015.11.064. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Template-free Phase transfer

FFlloow weerr--lliikkee A AllO OO OH H

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A All((O OH H))333 PPrreeccuurrssoorr

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Graphical Abstract

ACCEPTED MANUSCRIPT Clean Synthesis of Hierarchically Structured Boehmite and γ-Alumina with A Flower-Like Morphology

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Zongbo Shi, Wenqian Jiao, Li Chen, Peng Wu,* Yimeng Wang, Mingyuan He*

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Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, North

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Zhongshan Rd. 3663, Shanghai 200062, P. R. China

E-mail: [email protected]; [email protected]

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Tel/Fax: 86-21-62232292

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ACCEPTED MANUSCRIPT Abstract Flower-like boehmite, composed of the nanosheets with 1‒5 nm thickness, was synthesized from a bayerite precursor via phase transformation in a clean way. This

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template-free method is featured with the advantages that the mother solution containing ethanol, water and ammonium sulfate could be recovered and reused. The transformation procedure of bayerite to boehmite was clarified, in which bayerite

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gradually dehydrated and evolved to boehmite from the crystal outside to the inside,

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with the primary crystallite size of boehmite and the coordination state of aluminum remaining almost unchanged. The corresponding flower-like γ-Al2O3 derived by calcination of flower-like boehmite exhibited not only a large specific surface area (281 m2 g‒1) but also bimodal porosities with the diameter distributions centered at 5.2

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nm and 18.4 nm, respectively. Possessing large pore size and open pore structures, the flower-like γ-alumina showed an improved catalytic performance in the cracking of

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1,3,5-triisopropylbenzene in comparison to the commercial alumina catalysts.

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Keywords: boehmite, γ-alumina, hierarchical structure, morphology control

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ACCEPTED MANUSCRIPT 1. Introduction Transition aluminas have been large used as catalysts and catalyst carriers or supports [1]. Among others, γ-alumina (γ-Al2O3) is of greatest interest since it has a

most

catalytic

reactions

[2].

Hierarchical

porous

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high surface area and is relatively stable over the temperature range of interest for (HP)

γ-aluminas

with

open meso/macro pore structures show significant enhancements in pore accessibility the

potential

to

further

extend

its

industrial applications in a

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and have

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variety of liquid-phase and gas-phase reactions [3, 4]. Comparing with the mesostructured aluminas obtained using various structure-directing agents, the HP aluminas have larger pore diameter always at the sacrifice of surface area [5, 6]. In order to increase the surface area, one approach was to synthesize HP aluminas

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with different morphologies by using various organic templates [3, 7-10]. Organic templates used in the synthesis process directed the growth of boehmite (precursor of

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γ-alumina) particles and tuned their morphologies. A number of macropore/mesopore hierarchically structured aluminas with the surface area of >400 m2/g have been

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reported, which the alumina particles had parallel arrays of macropores interconnected with mesopores [5, 11, 12]. Nevertheless, this method might be not suitable for large-scale preparation of alumina due to the high cost of the organic templates and the related post-treatment problems. Another way was to synthesize HP aluminas in the absence of organic templates. Generally, a decrease in the size of the primary alumina particles would result in an increased surface area [13]. Several strategies have been made to synthesize boehmite of discrete nano-structured, 3

ACCEPTED MANUSCRIPT including nanofibres [14-17], nanowires [18, 19], nanorods [20-22] nanoribbons [23, 24] and nanosheets [25], with the diameter or thickness ranging from 1‒4 nm to 20‒30 nm, however, these HP alumina precursors were always hard to be separated and

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recovered. To gather those nano-structured boehmite, vaporization of the mother solvent or high-speed centrifugation were usually inevitable, which leaded to a high energy-consumption. Otherwise, alternative separation methods like spray-drying

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process were employed to recover the nano-structured boehmite [26]. Therefore,

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researchers have been devoting their efforts to the synthesis of HP boehmite with assembled nano-structured clusters [27-33]. For instance, Tang and co-workers once fabricated bunches of aligned boehmite nanowires with an average diameter of 20 nm, using AlCl3 as aluminum source toxic Na2B4O7 as mineralizers [33]. Zhang et al.

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prepared flower-like alumina which consisted by several alumina nanosheets of 2‒5 nm in thickness, through solvo-thermal reaction of AlCl3 or Al(NO3)3 with a large amount of toxic toluene as solvent [34, 35]. Su’s group developed an one-pot self-formation

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procedure to synthesize HP alumina which random assembled by fibrous nanoparticles

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of 5-6 nm in size, but the chemical reagent of Al(CH3)3 was costly [36, 37]. As mentioned above, the soluble aluminum compounds (AlCl3 or Al(NO3)3,

NaAlO2, aluminum alcohol, etc.) and liquid aluminum compounds (Al(CH3)3, etc.) were more favor used for control of the particle morphology of HP aluminas. Bayerite (Al(OH)3), as an kind of insoluble solid aluminum compound, is one of the initial product of Bayer or Sinter processes for purifying bauxite [38], where the products of these processes are an intermediate stage for those soluble or liquid aluminum 4

ACCEPTED MANUSCRIPT compounds [1]. In general, the bayerite results in the η-alumina, while the boehmite leads to γ-alumina after the calcination [38]. Direct transformation of bayerite to HP boehmite would be an effective way to obtain HP γ-alumina. Wang et al. [39] once

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converted sodium-free bayerite to boehmite which only owned surface areas of 35‒59 m2 g‒1. Jiao et al. [13] have successfully transformed of HP bayerite to clusters of boehmite nanorods with the diameter of 10 nm. However, an excessive amount of

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acetic acid used easily led to the formation of aluminum carboxylate.

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Herein, a template-free method is presented to synthesize pure HP boehmite and γ-alumina with flower-like morphology using bayerite as a starting material, where the thickness of primary alumina nanosheets is only 1‒5 nm. The resultant boehmite is easy separated by traditional filtration, and the mother solvent can be recovered and

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reused for further synthesis of boehmite. The catalytic performance of γ-alumina is tested in the cracking of 1,3,5-triisopropylbenzene.

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2. Experimental section 2.1 Chemicals

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NaAlO2 (Al2O3, 54.09 wt. %; Na2O, 40.44 wt. %) and commercial Al2O3-2 were

purchased from J&K Chemical Ltd. Sodium bicarbonate, ammonium sulphate, ethanol (EtOH), commercial Al2O3-1, all in AR grade, were supplied by Sinopharm Chemical Reagent Co. Ltd. The chemicals were used directly without further purification. 2.2 Preparation of bayerite and flower-like boehmite and alumina The starting bayerite sample was synthesized according to the reference [40]. 20.1 g NaAlO2 was dissolved into 134.0 g water at 90 °C under stirring. After cooling to 5

ACCEPTED MANUSCRIPT the room temperature, 190 mL 1 M NaHCO3 solution was added drop-wise into this solution, resulting in a white suspension. After agitated for about 4 h, the white solid product formed (bayerite) was collected by filtration, washed thoroughly with hot

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water and dried at 80 °C overnight. Typical synthesis of aluminum hydroxide from bayerite was as follows. A mixture of 0.8 g bayerite, 0.6 g (NH4)2SO4, 2.7 g H2O and 2.3 g EtOH were sealed into a 25

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mL Teflon-lined stainless autoclave and heated at 175 °C for a different period of time

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and then cooled down naturally. The obtained white solid was recovered by filtration, washed with hot water and dried at 80 °C overnight. The product is denoted as FL-AHO-t (t = 2h, 4h, 6h, 12h) where t represents the hydrothermal synthesis time. The η-Al2O3 and FL-Al2O3-t samples were obtained from bayerite and FL-AHO-t,

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respectively, by calcination at 550 °C for 6 h with a heating rate of 2 °C min‒1. 2.3 Characterization methods

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Powder X-ray diffraction (XRD) patterns were collected on a Rigaku-Ultima diffract meter using a Cu Kα radiation source (λ = 0.15432 nm) in the 2θ range from

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5o to 80o. Scanning electron microscope (SEM images) was performed on a Hitachi S-4800 microscope. Transmission electron microscopy (TEM images) was conducted on TECNAI G2 F30 operating at 300 kV after the specimens were dispersed in ethanol and deposited on holey copper grids. N2 adsorption-desorption isotherms were measured at ‒196 °C on a Quanta chrome Autosorb-3B volumetric adsorption analyzer. Before the measurements, the aluminum hydroxide samples were out-gassed in the degas port of the adsorption apparatus at 150 °C for 6 h, while the alumina 6

ACCEPTED MANUSCRIPT samples were out-gassed at 300 °C for 6 h. The BET specific surface area was calculated using adsorption data acquired at a relative pressure (P/P0) range of 0.05‒0.30 and the total pore volume determined from the amount adsorbed at P/P0 of

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about 0.99. The pore size distribution (PSD) curves were calculated from the analysis of adsorption isotherm branches using the Barrett-Joyner-Halenda (BJH) algorithm. The bulk Al and Na contents were determined by inductively coupled plasma

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Al MAS NMR spectra were measured on a VARIAN VNMRS

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spectrometer. The

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emission spectrometry (ICP) on a Thermo IRIS Intrepid II XSP atomic emission

400WB NMR spectrometer. Temperature-programmed desorption of NH3 (NH3-TPD) testing was performed using a TP-5080 chemisorption instrument (Xianquan Co., Ltd, Tianjin, China) with a thermal conductivity detector (TCD). After pretreatment at

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550 °C under flowing helium (25 mL/min) for 2 h, each sample (100 mg) was cooled to 50 °C, and then adsorbed to saturation by ammonia for 30 minutes. Ammonia physically adsorbed on the catalyst was removed by flushing the sample with helium

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(25 mL/min) for 2 h at the adsorption temperature. Thermal desorption of ammonia

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was carried out in the temperature range of 100–550 °C increasing at a rate of 10 °C/min.

2.4 Catalytic cracking of triisopropyl benzene A schematic diagram of the experimental setup used for measuring the cracking

activity of the catalysts is presented in Fig. 1. A stainless steel tube microreactor with an inner diameter of 10 mm was used for the cracking experiments. The alumina samples were all ion-exchanged with ammonium chloride before catalytic test. 1.0 g of

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ACCEPTED MANUSCRIPT the catalyst particles (40‒60 mesh size) was loaded in the reactor followed by activated at 500 °C for 2 h under a 40 sccm flow of N2. Thereafter, N2 was switched off, and 1.506 g triisopropyl benzene (TIPB) was injected into the carburetor with the flow rate

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of 1.29 g min‒1 at the same temperature. TIPB was immediately vaporized and flowed to the reactor. The reaction products was cooled with an ice-water bath. After the reaction, the N2 was switched on for purging the residual products, with a flow rate of

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40 sccm for 10 min. The weight of liquid products and the volume of gas products were

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recorded. The liquid products were analyzed by an offline automatic injection gas chromatograph (GC) equipped with a flame ionization detector (FID) and a HP-Plot Q column (Ø 0.32 mm and 50 m length). The gas products were analyzed by an online GC equipped with a thermal conductivity detector (TCD), a FID and a DM-Alumina

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column (Ø 0.53 mm‒50 m length). Almost no TIPB thermal cracking was observed at 500 °C in the absence of catalyst.

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3. Results and discussion

3.1 Phase transformation

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Firstly, bayerite was produced by the neutralization of NaAlO2 solution with NaHCO3 at room temperature as shown in equation (1). NaAlO2 + NaHCO3 + H2O → Al(OH)3 (bayerite) + Na2CO3 (1) Equation (1) can be integrated into the sintering process in the industrial production of alumina [1], where the aqueous Na2CO3 solution in the products is partially recycled to the procedure for sintering bauxite ore, followed by the leaching of sintered bauxite to produce a supersaturated sodium aluminate solution. Meanwhile, 8

ACCEPTED MANUSCRIPT the remaining Na2CO3 solution can directly react with CO2 to regenerate the sodium bicarbonate reagent solution, NaHCO3. Fig. 2a shows the wide-angle XRD patterns of bayerite. The bayerite sample

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obtained by reacting NaAlO2 with NaHCO3 well matched with typical diffraction peaks of bayerite (JCPDS card number 20-11). Although bayerite can be converted into boehmite via thermal treatment, it is still quite difficult to obtain a pure boehmite phase

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by calcination due to the complication of the dehydration process [5]. For example,

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thermal decomposition of large bayerite particles can lead to the formation of boehmite, thanks to the inter-granular hydrothermal conditions resulting from the buildup of steam pressure in the coarse particles, while very fine bayerite particles showed less boehmite formation [41]. Compare to the thermal synthesis approach, hydrothermal

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approach is more favorable for the formation of boehmite, where amorphous precipitates, pseudo-boehmite, and aluminum hydroxides including bayerite are

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usually used as precursors.

Secondly, the bayerite structure could be transformed to boehmite gradually under

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hydrothermal conditions (Fig. 2b-e). The pattern of the as-synthesized FL-AHO-12h sample is identical to JCPDS card number 21-1307, characteristic of a pure crystalline structure of boehmite. As calculated according to Scherrer-Debye equation, the primary crystallite size of FL-AHO-12h is about 7.8 nm (Table 1). The primary crystallite size mentioned here may not be accurate value [42], and can only be used for qualitative comparation. After increasing synthetic time from 2 h to 12 h, the diffraction peaks corresponding to crystalline phase of bayerite gradually diminish, 9

ACCEPTED MANUSCRIPT while those corresponding to the boehmite phase persistently increase. In spite of the phase transformation, the full width at the half maximum height (FWHM) of the diffraction peaks remains almost unchanged (Table 1), indicating that boehmite

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inherits the primary crystallite size of bayerite. Under the hydrothermal conditions with the assistance of ammonium sulfate and EtOH, the transformation of bayerite to boehmite takes place in the way as shown in

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equation (2):

(2)

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Al(OH)3 (bayerite) → AlOOH (boehmite) + H2O

In equation (2), no other products are generated except H2O and boehmite, and thus the filtrate can be recovered and reused to synthesize boehmite in the next runs. The XRD pattern and SEM images of the boehmite sample (see supporting

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information Fig. S1 and S2) obtained by re-using the filtrate are very similar to those of FL-AHO-12h prepared using fresh solution.

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It is worth mentioning that when the bayerite sample was subject to the thermal

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treatment in air at 175 °C for 1 d, it maintained the crystalline phase of bayerite. This suggests that aqueous solution involving ethanol and ammonium sulfate decreases the dehydration temperature of bayerite. The pH value has a great influence on the morphology of boehmite, as the acidic condition favors the formation of boehmite with small particles size [13, 43]. In the present synthesis, ammonium sulfate provides a stable acidic condition suitable for synthesizing boehmite. Unlike acetic acid that may chemically modify bayerite as excessive amount of acetic acid used would easily

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ACCEPTED MANUSCRIPT result in the formation of aluminum carboxylate [13], ammonium sulfate gives rise to pure phase of crystalline bayerite even when the reaction time is prolonged to 48 h or the content of ammonium sulfate is doubled.

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Thirdly, the calcination causes a further phase transformation from boehmite to alumina (Fig. 2f). After the calcination in air atmosphere at 550 °C for 6 h, FL-AHO-12h loses the crystalline structure of boehmite and changes to a phase

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showing typical diffraction peaks of γ-alumina (JCPDS card number 04-0880) [38].

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The progressive transformation of bayerite to boehmite was also verified by FT-IR spectra. Fig. 3 shows the spectra of as-synthesized bayerite and FL-AHO-t samples. The bayerite and FL-AHO-12h samples demonstrate the typical spectra of bayerite and boehmite, respectively. The intensive bands at 3658, 3549 and 3465 cm−1

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are attributed to the Al–OH stretching vibrations of bayerite, while those at 3303 and 3085 cm−1 are assigned to the Al–OH stretching vibrations of boehmite [13, 35, 44].

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The bands at 1027 and 980 cm−1 are attributed to the Al–OH bending vibrations of bayerite, while those at 1161 and 1068 cm−1 are due to those of boehmite. The bands

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at 770 and 532 cm−1 and the bands at 735, 611 and 479 cm−1 belong to the Al–O stretching vibrations of bayerite and boehmite, respectively. With prolonging the synthetic time from 2 h to 6 h, the bands belonging to bayerite gradually reduce in intensity, while those of boehmite become more intensive. These results are indicative of a phase transformation from bayerite to boehmite. Except for the characteristic IR bands of bayerite and boehmite, there are no obvious other bands can be observed, for example the band at 1432 cm‒1 ascribing to 11

ACCEPTED MANUSCRIPT CO32‒ appears or the band at 835 cm‒1 assigning to the internal mode of HCO32‒ [45, 46], which means the residual of CO32‒ or HCO32‒ debris in the samples is trace. When NaAlO2 reacts with NaHCO3, the product of bayerite is purified with water, and the

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residual content of Na2CO3 or NaHCO3 is very low. The sodium content measured by ICP is only 0.17 wt. % for bayerite. Furthermore, after the hydrothermal treatment of bayerite with ammonium sulfate, the sodium decrease to 0.03 wt. % for the

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FL-AHO-12h sample.

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3.2 Physicochemical properties

Fig. 4 exhibits the 27Al NMR spectra of as-synthesized bayerite, FL-AHO-t and FL-Al2O3-12h samples. The bayerite and FL-AHO-12h samples show typical resonance at ~10 ppm, which is due to the octahedrally coordinated aluminum in well

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crystallized bayerite and boehmite. With synthetic time varying from 2 to 6 h, the FL-AHO-t samples show only the resonance at ~10 ppm, but no signals at 30‒40 ppm

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or 60‒70 ppm. The latter is often observed for amorphous aluminum hydroxide sample [23]. There is an additional peak at ~68 ppm in the spectrum of FL-Al2O3-12h sample

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(Fig. 4f), indicating the generation of tetrahedral aluminum after the calcination of boehmite.

Bayerite and boehmite both have a layer lattice with the O anions locating in close

packed face-centred-cubic (f.c.c) oxygen layers [47], only the dehydration is required for the transition from bayerite to boehmite. Meanwhile, the octahedral coordinated aluminum remains constant. Though the gibbsite, the other initial Al(OH)3 product from the Bayer or Sinter process, has the similar structure to bayerite, composed of 12

ACCEPTED MANUSCRIPT the same basic layers of Al‒OH octahedral. The c-plane layer of bayerite is arranged in AB-AB sequence, and that of gibbsite is in AA-BB-AA-BB sequence [38]. The phase transformation to boehmite from gibbsite usually needs higher dehydrated

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temperatures than that from bayerite [48]. As transferred from boehmite, γ-alumina also possesses the packed f.c.c oxygen layers, while the dehydration and diffusion of Al3+ cations are required [47]. The phase transformation from boehmite to γ-alumina

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causes the tetrahedrally coordinated aluminum formed.

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The SEM image indicates that the as-synthesized bayerite sample has an irregular morphology and consists of relatively large particles in size of 100‒500 nm (Fig. 5a). After the hydrothermal treatment for 2 h, aluminum hydroxide fibers begin to appear (Fig. 5b). By prolonging the synthetic time, aluminum hydroxide fibers become longer

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and wider (Fig. 5c) and eventually turn into sheets (Fig. 5d, 5e). The FL-AHO-12h sample has a pure crystalline structure of boehmite with flower-like morphology,

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consisting of sheets with 50‒200 nm in length, 30‒50 nm in width and 1‒5 nm in thickness (Fig. 5g). The thickness does not equal to the crystallite size calculated by the

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Scherrer-Debye equation, as the latter analysis is always based on assuming the crystallite in spherical shape [49, 50]. As the primary nanosheets are highly aggregated into secondary flower-like spheres, the resultant boehmite product could be easily separated and recovered by conventional filtration. After the calcination at 550 °C, the FL-AHO-12h sample turns into γ-alumina. Importantly, this γ-alumina sample preserves the morphology and sheet thickness of the corresponding boehmite precursor, as evidenced by SEM and TEM images (Fig. 5f and 5h). The nano-particles of the 13

ACCEPTED MANUSCRIPT formed γ-alumina (1‒5 nm) are thinner than the ones of the assembled nano-structure aluminas reported previously [27-32, 34, 35, 51, 52]. The small particle size of flower-like boehmite and alumina may lead to large external surface area.

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In the small angle XRD pattern (Fig. 2, insert), no peaks appear at the starting bayerite sample, but the single broad peaks at 2θ = 0.8–1.5° for FL-AHO-t and FL-Al2O3-12h samples are observed, which attributes to the relatively uniform pore

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structure. The detailed variation trend of pore structure in the FL-AHO-t and

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FL-Al2O3-t samples is further investigated by N2 adsorption/desorption measurements. Fig. 6 displays the N2 adsorption/desorption isotherms and pore size distribution curves of the as-synthesized bayerite and FL-AHO-t samples. The starting bayerite exhibits very a low adsorption amount of N2 even at high relative pressure (Fig. 6a), giving a

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small surface area of 28 m2 g‒1 (Table 2, No.1). The FL-AHO-t samples show characteristics of type IV isotherms along with the presence of hysteresis loop, where

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the adsorbed amount continuously increases with the increase of P/P0. With the synthetic time increasing from 2 h to 12 h, the hysteresis loop of FL-AHO-t samples

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becomes more obvious (Fig. 5b-f). The textural properties of all the samples are listed in Table 2. The surface area of the boehmite sample (FL-AHO-12h) is as high as 221 m2 g‒1 and the total pore volume exceed 0.50 cm3 g‒1. Fig. 7 displays the N2 adsorption/desorption isotherms and pore size distribution curves of the as-synthesized η-alumina and FL-Al2O3-t samples. Although the bayerite exhibits very low surface area, the surface area of η-alumina (calcination of bayerite) runs up to 255 m2 g‒1. Bayerite is built of the basic layers of Al-OH octahedra, and the 14

ACCEPTED MANUSCRIPT dehydration is happened within the two Al-OH. When bayerite dehydrates to η-alumina by calcination, some layers of the bayerite separate which creates inter-nanoparticle pores [38]. Because the boehmite contains much less Al-OH than

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bayerite, the surface area of the FL-Al2O3-12h reaches only to 281 m2 g‒1 (Table 2, No. 12), but is still higher than those of the η-alumina and the assembled nano-structure γ-aluminas reported before [27-32, 34, 35, 51, 52].

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The pore size distributions shown in Fig. 7 (insert) indicate that η-Al2O3 displays

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only a single pore system, but the FL-Al2O3-12h samples possess bimodal pore systems with the pore distribution centered at ~ 5.2 nm and ~ 18.4 nm, respectively, which is similar to the bunches-like aligned boehmite nanowires [33]. Fig. 8 gives the schematic representation for the transformation of bayerite to

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boehmite. As mentioned above, bayerite and boehmite phases co-exist during the hydrothermal treatment. The proportion of bayerite phase gradually reduces and that

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of the boehmite phase increases with the increasing of the synthetic time. As the primary crystal size of bayerite and boehmite measured from XRD patterns remains

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almost the same (Table 1), bayerite may directly transfer to an intermediate structure with a particular morphology (such as flower-like) firstly, which has the similar primary crystal size to that of starting bayerite. Exposing to the mother liquid, the external surface of the bayerite crystals may loss water rapidly, while the inner part undergoes a much slower and more delayed dehydration. The dehydrated external surface grows into primary petals of boehmite and constructs more cavities, making the inner parts of bayerite accessible to the mother liquid. During the phase transformation 15

ACCEPTED MANUSCRIPT process, the dehydration and the growth of the flower petals carry on from exterior to interior. The flower-like pure boehmite is formed after the phase transformation is

3.3 Catalytic performance of flower-like γ-alumina

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complete.

Flower like aluminas with high surface area and open pores has potential

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applications as the matrix of fluid catalytic cracking (FCC) catalyst, the key material for converting heavy oil to transport fuels with small molecular size. FAU zeolite,

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with high micropore surface area and high acidity, serves as the active component of FCC catalyst, but it provides poor mass transfer of bulky molecules [4]. As the hydrocarbons in residue and heavy oil are larger in size than the opening of the micropores of FAU zeolite, they need to pre-crack on the external surface of the

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zeolite and/or matrix. Here, cracking of 1,3,5-triisopropylbenzene (TIPB) with a kinetic diameter of 9.4 Å is used as a probe reaction to test the catalytic property of

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flower-like γ-alumina [53-56].

The activity of FL-Al2O3-12h is compared to that of commercial γ-Al2O3 materials

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and the results are listed in Table 3. All of the alumina samples contain < 0.05 wt. % Na2O, and they have the same crystalline structure and comparable surface areas. NH3 temperature-programmed desorption (NH3-TPD) technique is used to investigate the acidity of alumina [57]. The three samples display similar acid strength as shown in Fig. 9. The total acid sites, determined by NH3-TPD measurement, are in the order of commercial Al2O3-1< FL-Al2O3-12h< commercial Al2O3-2. However, the conversion of TIPB on FL-Al2O3-12h is 2.1 times as high as that on commercial Al2O3-1 and is 1.6 16

ACCEPTED MANUSCRIPT times

as

high

as

that

on

commercial

Al2O3-2

(Table

3,

No.

1-3).

Diisopropylbenzene (DIPB) isomers are the dominant products for the three catalysts. Generally, the higher external surface area and/or higher external pore volume may

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lead to higher conversion for the catalytic reactions involving bulky molecules. For instance, the dealuminated Y zeolite with larger mesopore volume gives a higher conversion in the vacuum gas oil cracking reaction [58]. However, the importance of

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the mesopore accessibility should not be despised in the catalytic applications. Pirez et

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al. [59] found the turnover frequency (TOF) towards bulky acid esterification was greatly enhanced on the 3D interconnected mesopore architectures in comparison to 2D ones. Xie and coworkers [60] reported the bimodal aluminosilica showed a higher 1,3,5-TIPB conversion than a mesoporous counterpart with comparable acidity. Here,

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the increase of pore size and introduce of bimodal pore system for FL-Al2O3-12h sample leads to an improvement in pore accessibility, on the other hand suppresses the internal diffusion limitation of the TIPB, thus makes the acid sites more reachable for

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TIPB molecules, giving rise to higher conversion of TIPB.

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We also compared the activities of TIPB cracking between FL-Al2O3-12h and η-Al2O3 catalysts, latter of which was prepared by the calcination of bayerite. η-Al2O3 is reported to contain more Lewis acid sites than γ-Al2O3 with the same surface area [61]. FL-Al2O3-12h shows a higher conversion of TIPB cracking than η-Al2O3 does, which should also be ascribed to the better pore accessibility of FL-Al2O3-12h. Thus, the phase transformation from bayerite to flower-like boehmite is beneficial to

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ACCEPTED MANUSCRIPT increasing the surface area of alumina, improving the acid sites accessibility, and then enhancing the cracking activity for bulky molecules. 4. Conclusions

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Bayerite is transformed to boehmite and γ-Al2O3 both with flower-like morphology through sequentially hydrothermal and calcination treatments. The resultant boehmite and alumina have small particle size and large surface area. Only the dehydration is

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required for the phase transformation from bayerite to boehmite, during which the

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primary crystal size and the coordination state of aluminum remain unaltered. The flower-like aluminum hydroxide preferentially grows from the exterior of bayerite to the interior. Ammonium sulfate in the mother solution may decrease the phase transformation temperature and provide a suitable acidic condition for the synthesis of

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the flower-like boehmite. The mother liquid could be recovered and re-used in the whole synthesis procedure. The γ-alumina derived from boehmite via calcination can

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inherit the flower-like morphology and retain the high surface area and bimodal mesopore structure. Thanks to its enhanced acid site accessibility, flower-like γ-Al2O3

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exhibits superior activity in the cracking of bulky TIPB than η-Al2O3 and commercial γ-Al2O3 do.

Acknowledgements

The authors gratefully acknowledge the financial supports from NSFC of China (21373089, 21533002), Ministry of Science and Technology (2012BAE05B02), and Programs Foundation of Ministry of Education (2012007613000). 18

ACCEPTED MANUSCRIPT References [1] C. Misra, Industrial Alumina Chemicals, Washington, DC,, 1986. [2] C.N. Satterfield, Heterogeneous Catalysis in Industrial Practice, McGraw Hill, 1991. [3] W. Deng, B.H. Shanks, Chem. Mater., 17 (2005) 3092-3100. [4] C.M.A. Parlett, K. Wilson, A.F. Lee, Chem. Soc. Rev., 42 (2013) 3876-3893. [5] C. Márquez‐Alvarez, N. Žilková, J. Pérez‐Pariente, J. Čejka, Catalysis Reviews, 50 (2008)

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[61] K. Sohlberg, S.T. Pantelides, S.J. Pennycook, J. Am. Chem. Soc., 123 (2001) 26-29.

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ACCEPTED MANUSCRIPT Figure capations

Fig. 1 Schematic representation of the experimental setup used for the cracking tests.

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Fig. 2 Wide-angle XRD patterns of as-synthesized bayerite (a), FL-AHO-2h (b), FL-AHO-4h (c), FL-AHO-6h (d), FL-AHO-12h (e),and FL-Al2O3-12h (f).

Fig. 3 FT-IR spectra of as-synthesized bayerite (a), FL-AHO-2h (b), FL-AHO-4h (c),

27

Al NMR spectra of as-synthesized bayerite (a), FL-AHO-2h (b),

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Fig. 4

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FL-AHO-6h (d), and FL-AHO-12h (e).

FL-AHO-4h (c), FL-AHO-6h (d), FL-AHO-12h (e), and FL-Al2O3-12h (f).

Fig. 5 SEM images (a-g) and TEM (g, h) of as-synthesized bayerite (a), FL-AHO-2h

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(b), FL-AHO-4h (c), FL-AHO-6h (d), FL-AHO-12h (e, g), and FL-Al2O3-12h (f, h).

Fig. 6 N2 adsorption/desorption isotherms (left) and pore size distribution curves

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(right) of as-synthesized bayerite (a), FL-AHO-2h (b), FL-AHO-4h (c), FL-AHO-6h (d), and FL-AHO-12h (e) samples. The isotherms of b, c, d, e and f are offset by 10,

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50, 90, and 160 cm3 g-1 (STP) for clarity, respectively

Fig. 7 N2 adsorption/desorption isotherms and pore size distribution curves (insert) of as-synthesized bayerite (a), FL-Al2O3-2h (b), FL-Al2O3-4h (c), FL-Al2O3-6h (d), and FL-Al2O3-12h (e) samples. The isotherms of b, c, d and e are offset by 70, 140, 200 and 290 cm3 g-1 (STP) for clarity, respectively.

22

ACCEPTED MANUSCRIPT Fig. 8 Schematic representation for the phase transformation of bayerite to flower-like boehmite.

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Commerical-Al2O3-1 (c), and Commercial Al2O3-2 (d).

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Fig. 9 NH3-TPD profiles of alumina samples: η-Al2O3 (a), FL-Al2O3-12h (b),

23

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Table 1. Primary crystallite sizes of the aluminium hydroxide samples.

No.

Boehmite phase PCS b

Sample 2θ (°)

hkl

FWHM

a

2θ (°) (nm)

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Bayerite phase

18.76

001

0.249

35.4

-

2

FL-AHO-2h

18.76

001

0.242

36.6

14.06

3

FL-AHO-4h

18.70

001

0.246

35.9

4

FL-AHO-6h

-

-

-

-

5

FL-AHO-12h

-

-

-

-

-

020

0.991

8.1

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Bayerite

14.20

020

0.968

8.3

14.16

020

1.026

7.8

020

0.974

8.2

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FWHM

PCS b (nm)

1

-

hkl

a

14.24

a

Full width at the half maximum height of the specific diffraction peak for bayerite or boehmite.

b

Primary crystallite size (PCS), calculated using Scherrer-Debye equation: D = Kλ/B cosθ, where

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K = 0.89, λ = 0.154056 nm and B is the FWHM.

24

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Table 2. Textual properties of the aluminium hydroxide and alumina samples.

Sample

SBET (m2 g-1)

Vtotal (cm3 g-1)

1

Bayerite

28

0.15

2

FL-AHO-2h

109

0.32

3

FL-AHO-4h

141

0.59

4

FL-AHO-6h

238

0.46

5

FL-AHO-12h

221

6

η-Al2O3

255

7

FL-Al2O3-2h

291

8

FL-Al2O3-4h

285

9

FL-Al2O3-6h

10

FL-Al2O3-12h

-

2.7/15.2

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3.2/11.6/22.1 3.3/15.4 3.4/17.3

0.44

4.1

0.45

3.2

0.66

3.1/18.1

284

0.68

5.2/17.4

281

0.60

5.2/18.4

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0.52

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The pore size is derived from the adsorption isotherm by BJH method.

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a

dBJH a (nm)

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No.

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Table 3. Comparison of catalytic properties of various alumina catalysts in the TIPB cracking .a

SBET 2

1 2

3 4

FL-Al2O3-12h Commercial Al2O3-1 Commercial Al2O3-2

η-Al2O3

Pore

(mmol

(m

sizeb

g-1)

(nm)

281

5.2/18.4

0.77

257

5.1

331 255

conv.

C3 =

benzene.

24.6

5.8

0.2

0.69

11.7

2.6

0.1

5.8

0.85

15.1

3.5

4.1

0.75

16

3.4

-1

g )

DIPB

0.9

17.7

97.2

0.3

8.6

97.5

0.1

0.4

11.1

95.9

0.1

0.4

12

97.3

a

Reaction conditions: temp., 773 K; cat., 1.0 g; TIPB, 1.5 g; feed rate, 1.29 g/min.

b

Determined by N2 adsorption at 77 K.

c

Determined by NH3-TPD measurement.

d

Recovery

cumene

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Catalyst

Product yield

sitesc

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No.

b

TIPB cracking (wt. %) d Acid

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Textural properties

Conv. is defined as the sum of propylene (C3=), benzene, cumene and DIPB. DIPB is the sum of

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1,2-DIPB, 1,3-DIPB and 1,4-DIPB. Recovery indicates the carbon balance.

26

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Fig. 1 Schematic representation of the experimental setup used for the cracking tests.

27

2

3

4

a b c d e f

20

30

40

50

2 Theta (Degree)

60

70

80

SC

10

1

a b c d e f 5

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Intensity (a.u.)

Intensity (a.u.)

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Fig. 2 Wide-angle XRD patterns of as-synthesized bayerite (a), FL-AHO-2h (b), FL-AHO-4h (c), FL-AHO-6h (d), FL-AHO-12h (e),and FL-Al2O3-12h (f).

28

ACCEPTED MANUSCRIPT

3658 3549

b 3465

1027 980 770

c d

532

e 1068 3303

3085

1161

735 611 479

3600 3400 3200 3000 1400 1200 1000 800 600 400 -1

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Wavenumbers (cm )

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Transmittance (%)

a

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Fig. 3 FT-IR spectra of as-synthesized bayerite (a), FL-AHO-2h (b), FL-AHO-4h (c), FL-AHO-6h (d), and FL-AHO-12h (e).

29

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Al

a b

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c d e

IV

Al

80 60 40 20

0 -20

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δ (ppm)

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f

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Fig. 4 27Al NMR spectra of as-synthesized bayerite (a), FL-AHO-2h (b), FL-AHO-4h (c), FL-AHO-6h (d), FL-AHO-12h (e), and FL-Al2O3-12h (f).

30

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a

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b

c

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d

f

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e

Fig. 5 SEM images (a-g) and TEM (g, h) of as-synthesized bayerite (a), FL-AHO-2h (b), FL-AHO-4h (c), FL-AHO-6h (d), FL-AHO-12h (e, g), and FL-Al2O3-12h (f, h). 31

e d dV/dlogd

100 0 0.0

b a 1

300 200

c

10

100

Diameter (nm)

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400

3

-1

500

e d c b a 0.2

0.4

0.6

0.8

P/Po

1.0

SC

N2 Volume adsorbed (cm g , STP)

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Fig. 6 N2 adsorption/desorption isotherms and pore size distribution curves (insert) of as-synthesized bayerite (a), FL-AHO-2h (b), FL-AHO-4h (c), FL-AHO-6h (d), and FL-AHO-12h (e) samples. The isotherms of b, c, d and e are offset by 10, 50, 90 and

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160 cm3 g-1 (STP) for clarity, respectively.

32

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1

400

e

300

d c b a

200 100 0 0.0

10

100

Diameter (nm)

0.2

0.4

P/Po

0.6

0.8

SC

500

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dV/dlogd

600

3

-1

700

e d c b a

1.0

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N2 Volume adsorbed (cm g , STP)

800

Fig. 7 N2 adsorption/desorption isotherms and pore size distribution curves (insert) of as-synthesized bayerite (a), FL-Al2O3-2h (b), FL-Al2O3-4h (c), FL-Al2O3-6h (d), and FL-Al2O3-12h (e) samples. The isotherms of b, c, d and e are offset by 70, 140, 200

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and 290 cm3 g-1 (STP) for clarity, respectively.

33

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4h

20

0h

20

40

60

80

40

60

Bayerite + boehmite

Bayerite + boehmite 40

60

80

20

40

Bayerite

60

80

boehmite

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20

80

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12h

2h

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Fig. 8 Schematic representation for the phase transformation of bayerite to flower-like boehmite.

34

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14

d

10 8

c

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TCD signal (a. u.)

12

6

b

4 2

a 200

300

400

Temperature (°C)

500

600

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100

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0

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Fig. 9 NH3-TPD profiles of alumina samples: η-Al2O3 (a), FL-Al2O3-12h (b), Commerical-Al2O3-1 (c), and Commercial Al2O3-2 (d).

35

ACCEPTED MANUSCRIPT Highlights


Flower-like boehmite was template-free synthesized from a bayerite precursor via phase transformation. The transformation procedure of bayerite to boehmite was clarified.




Flower-like boehmite consists the nano-sheets of 1 - 5 nm thickness.




Flower-like alumina showed an improved catalytic cracking performance in

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comparison to commercial alumina.

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