Preparation and characterization of transitional alumina obtained through ammonia treatment

Article

DOI: 10.1002/bkcs.10582

BULLETIN OF THE

Y. Liu et al.

KOREAN CHEMICAL SOCIETY

Preparation and Characterization of Transitional Alumina Obtained

through Ammonia Treatment

*

Yan Liu, Litao Jia, Bo Hou, and Debao Li

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,

Taiyuan 030001, China. *E-mail: jialitao910@163.com; dbli@sxicc.ac.cn

Received April 18, 2015, Accepted August 15, 2015, Published online November 25, 2015

In this study, different aluminas were obtained by precipitating Al(NO₃)₃ꢀ9H₂O with different concentration

of aqueous solution of ammonium carbonate. The obtained aluminas were treated by 50 (vol/vol) %

ammonia via low temperature immersing and high temperature hydrothermal treatment, respectively, then,

the treated aluminas were calcined again. The crystal types, morphology, textural properties, and pore distri-

bution of the original alumina, ammonia-treated alumina, and the second calcined alumina were studied

by characterizations of X-ray diffraction, thermogravimetric analyses, nuclear magnetic resonance,

Brunauer-Emmett-Teller (BET), transmission electron microscopy, and so on. Several modiﬁed aluminas

with speciﬁc morphology, desired pore structure, and pure crystal types were obtained, which can be used

in some speciﬁc industrial application.

Keywords: Alumina, Pore distribution, Crystal type, Ammonia treatment, Morphology

Introduction

However, traditional alumina exhibited irregular

shape and complicated pore structure, depending on the size

and aggregation of the primary particles, which limit its

practical applications. Hence, the synthesis of alumina

with controllable internal structure and morphology, as

well as adjusting the texture and morphology to modify

alumina performance has attracted attention in recent

years.^8–10

Alumina has been extensively studied as an important indus-

trial material in the ﬁelds of adsorbent, ceramic, membrane,

and catalyst support due to its unique physical and chemical

properties.^1,2

Traditional alumina exhibited different crystal types, and

different aluminas possessed different particle size, pore-size

distribution, and so forth.³Owing to the actual function of the

alumina depending mainly on their textural properties, the

application of alumina in each of these ﬁelds requires a favor-

able combination of morphology and textural properties,

including surface area, pore volume, and pore-size

distribution.^3–7Chaitree et al.⁴prepared χ-Al₂O₃and

employed it as supports for preparation of 20 wt% Co/Al₂O₃

catalysts, which possessed higher cobalt dispersion and CO

hydrogenation activity compared to the use of γ-Al₂O₃with

similar BET surface area. Kim³prepared various aluminum

oxides with different crystalline phases, such as κ-Al₂O₃,

γ-Al₂O₃, ≈-Al₂O₃, δ-Al₂O₃, and θ-Al₂O₃, which possessed

different pore-size distribution, the average pore size increased

in the following order: ≈-Al₂O₃= γ-Al₂O₃< δ-Al₂O₃

< θ-Al₂O₃< κ-Al₂O₃, and the catalytic activity of supported

ruthenium catalysts for the CO oxidation increased in the fol-

Generally, alumina is produced by thermal dehydration

of its precursor, many strategies and techniques in preparing

precursors have been developed to acquire featured alumina.

Shin et al.⁸prepared various precursors of different crystal

structure and chemical composition with varied precipitants,

indicating that the precipitants greatly affect the pores

structure and particles size of the ﬁnal alumina. Bai et al.⁹

synthesized mesoporous γ-Al₂O₃with a bimodal pore-size

distribution through

a

reverse cation–anion double

hydrolysis method (CADH). Li et al.¹⁰prepared meso/macro-

porous γ-Al₂O₃via thermal decomposing nanorods

ammonium aluminum carbonate hydroxide obtained from

hydrothermal synthesis. However, aluminas obtained via

these means are possibly restricted by such as sophisticated

equipment, the long cycle of synthesis, high cost, or the poor

replicability.

lowing

order: θ-Al₂O₃< γ-Al₂O₃< ≈-Al₂O₃< δ-Al₂O₃

In this study, different aluminas were obtained from preci-

pitating Al(NO₃)₃ꢀ9H₂O with different concentration of aque-

ous solution of ammonium carbonate. The as-synthesized

aluminas were treated by 50 (vol/vol)% ammonia via low-

temperature immersing and high-temperature hydrothermal

treatment, respectively, in order to obtain an alumina with

well-distributed particles size and desired pore structure.

The crystal types of the obtained alumina were distinguished

by characterizations of X-ray diffraction (XRD),

<< κ-Al₂O₃. Xiong et al.⁵prepared alumina with different

pore size by calcining the aluminum precursors at different

temperature, which have signiﬁcant effects on the supported

Co₃O₄crystallite diameter, catalyst reducibility, and Fischer–

Tropsch synthesis activity. Pinilla et al.⁶and Giraldo and

Centeno⁷usedbigmesoporousaluminaincrackingandisomer-

ization reactions to upgrade the heavy oil and acquired

special effects.

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thermogravimetric analyses (TGA), and solid-state nuclear

magnetic resonance (SSNMR). The effects of ammonia treat-

ment on the crystal type, morphology, pore distribution of the

original alumina were studied by characterizations of

XRD, TGA, BET, transmission electron microscopy

(TEM), and so on.

Results and Discussion

Phase and Structure of Aluminum Precursors. It was

reported that, during the precipitation reaction, the precipitat-

ingparameterssuchastheprecipitant typeandprecipitant con-

centration affect greatly the composition and structure of the

prepared catalytic materials.^8,11In this work, 1.0, 1.5, and

2.0 mol/L aqueous solutions of ammonium carbonate were

selected as precipitant to precipitate 1.0 mol/L aqueous solu-

tion of Al(NO₃)₃ꢀ9H₂O. Figure 1 shows the XRD patterns of

precursors. As presented in Figure 1, when the concentration

of ammonia carbonate was equal to Al³⁺, boehmite (γ-AlO

(OH), JCPDS Card No. 21-1307, 2θ = 14.4^ꢁ, 28.2^ꢁ, 38.3^ꢁ,

49.2^ꢁ, 64^ꢁ, and71.9^ꢁ¹²)wasobtainedinprecursorP-1.0. How-

ever, the diffraction peak intensity of γ-AlO(OH) was weaker

than that of commercial γ-AlO(OH), indicating the poor crys-

tallization of the produced γ-AlO(OH). It was resulted from

the existence of H⁺in the precipitate, which partly dissolved

the produced γ-AlO(OH) because of the low concentration

of ammonia carbonate, as shown in Eq. (1). γ-AlO

(OH) diffraction peaks were also observed in P-1.5 when

the concentration of ammonia carbonate was increased to

1.5 times of Al³⁺. It has strengthened diffraction peaks com-

pared to γ-AlO(OH) in the P-1.0, indicating its comparatively

well crystallization for lacking of H⁺in the precipitation sys-

tem, as shown in the reaction process of Eq. (2). However,

ammonium aluminum carbonate hydroxide (AACH, NH₄Al

(OH)₂CO₃, JCPDS Card No. 42-0250, 2θ = 15.2^ꢁ, 21.8^ꢁ,

26.9^ꢁ, 30.7^ꢁ, 34.9^ꢁ, 41^ꢁ, 52.8^ꢁ, and 55.3^ꢁ¹³) was obtained in

P-2.0, when the concentration of ammonia carbonate was

twice that of Al³⁺. It was well crystallized due to the strong dif-

fraction peaks, and the reaction process is shown in Eq. (3).

Experimental

Samples Preparation. Aqueous solution (1.0 mol/L) of

Al(NO₃)₃ꢀ9H₂O as aluminum salt and aqueous solution

(1.0, 1.5, and 2.0 mol/L) of ammonium carbonate as alkaline

precipitants were used. Precipitation was conducted in a water

bath at 50 ^ꢁC under a stirring rate of 400 r/min for 1 h by co-

precipitation method. The water bath was then heated to 80 ^ꢁC

and maintained for1h. Finally, the precipitate waswashedand

ﬁltered twice by twofold deionized water of suspension. The

ﬁlter mass was dried at 110 ^ꢁC for 12 h, and the precursor was

obtained and denominated as P-1.0, P-1.5, and P-2.0. The car-

rier aluminas were obtained by calcining the corresponding

precursors at 500 ^ꢁC for 4 h under ambient atmosphere and

denominated as C-1.0, C-1.5, and C-2.0. The aluminas were

added to an excess ammonia water solution (50 vol%) at room

temperature for 6 h, removed and dried, the treated aluminas

were obtained and denominated as LA-1.0, LA-1.5, and LA-

2.0 and calcined again at 500 ^ꢁC for 4 h, the second calcined

aluminas were obtained and denominated as LAC-1.0, LAC-

1.5, andLAC-2.0. Asacontrast, the aluminas wereaddedtoan

excess ammonia water solution (50 vol%) in a hydrothermal

reaction kettle at 150 ^ꢁC for 6 h, removed and dried, the treated

aluminas were obtained and denominated as HA-1.0, HA-1.5,

and HA-2.0 and calcined again at 500 ^ꢁC for 4 h, the second

calcined aluminas were obtained and denominated as HAC-

1.0, HAC-1.5, and HAC-2.0.

Al^{3 +}+ ðNH₄Þ₂CO₃+ H₂O = AlOðOHÞ #

+ 2NH₄⁺+ H⁺+ CO₂"

ð1Þ

Characterization Techniques. The BET surface areas of the

prepared samples were measured with a Micromeritics model

ASAP 2020 (Micromeritics, Atlanta, GA, USA) using nitro-

gen at −196 ^ꢁC. Prior to measurements, all samples were

vacuum pumped at 423 K under 1 × 10⁻⁵Torr residual

pressure. XRD patterns were recorded on a DX-2700 diffrac-

tometer (Dandong Fangyuan, China) using a Cu Ka radiation

(λ = 1.5404 Å). The spectra were scanned at a rate of 8^ꢁ/min in

the range 2θ = 5^ꢁ–80^ꢁ. Scanning electronic micrograph

images were obtained using a LEO 1530VP (Carl Zeiss AG,

Germany). TEM images were recorded using a JEOL-2011

microscope (Akishima, Japan) operated at 200 kV.²⁷Al

SSNMR experiments were carried out at B₀= 9.4 T on a

Bruker Avance III 400 WB spectrometer (Billerica, MA,

USA). The corresponding resonance frequency of ²⁷Al was

104.3 MHz, samples were packed in a 4-mm ZrO₂rotor and

spun at the magic angle of 54.7^ꢁat a rate of 15 kHz. The

²⁷Al chemical shift was externally referenced to a 1.0 M aque-

ous solution of Al(NO₃)₃. TGA were carried out on a TGA-92

under an air ﬂow of 50 mL/min. The temperature increased

from 50 to 1100 ^ꢁCat a rate of 10 ^ꢁC/min.

Figure 1. XRD patterns of alumina precursors.

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2Al^{3 +}+ 3ðNH₄Þ₂CO₃+ H₂O = 2AlOðOHÞ #

temperature. As shown in Figure 2, weight loss peaks at

100–110 ^ꢁC occurred in all the three precursors, which

belonged to the dehydration of the physically adsorbed

water. P-1.5 exhibited weight loss at 190 ^ꢁC, belonging to

the decomposition of γ-AlO(OH). The peak temperature

was lower than that mentioned in literature^14,15because of

the low crystallization of the precursor. It was decomposed

completely at about 400 ^ꢁC, which was in agreement with

the reported value. The maximum rate of water removal

for P-1.0 occurred at 182 ^ꢁC, which was lower than that of

P-1.5 due to the low crystallinity. The complete decomposi-

tionat about 400 ^ꢁC indicates that some integrated crystals still

existed in P-1.0.

+ 6NH₄⁺+ 3CO₂"

ð2Þ

ð3Þ

Al^{3 +}+ 2ðNH₄Þ₂CO₃+ H₂O = NH₄AlðOHÞ₂CO₃#

+ 3NH₄⁺+ CO₂"

Figure 2 illustrates the TGA curves of the prepared precur-

sors. The DTG curves are integrations of the corresponding

TG curves. The Differential Thermal Analysis (DTA) curves

agree well with the TG curves, revealing the same data about

the decomposition of precursors with increasing calcination

Based on the analysis of TG curves (A) in Figure 2

(a) and (b), the total weight loss of P-1.0 and P-1.5 amounted

to ca. 33.5 and ca. 31.0%, respectively, which indicates that

their coefﬁcients were closed to 1.0 according to Eq. (4). P-

2.0 exhibited a weight loss at about 209 ^ꢁC, which was attrib-

uted to the dehydroxylation of AACH. The completed decom-

position temperature was as high as 375 ^ꢁC, which was higher

than the reported value.¹⁶The total weight loss of P-2.0 was

ca. 60% according to curve (A) in Figure 2(c), which is close

to the theoretical value of 63.3% according to Eq. (5). The TG

and DTA curves for the range of high temperature will be ana-

lyzed later in this study.

2AlOðOHÞ ꢀ n H₂O = Al₂O₃+ ð1 + 2nÞH₂Oðn = 0 – 1Þ ð4Þ

2NH₄AlðOHÞ₂CO₃= Al₂O₃+ 2NH₃" + 2CO₂" + 3H₂O

ð5Þ

Phase

and

Structure

of

Original

Alumina.

Figure 3(a) presents the XRD patterns of alumina obtained

from heating three precursors at 500 ^ꢁC for 4 h in air atmos-

phere. The crystal phase of alumina C-1.5 was γ-Al₂O₃, and

the crystal phase of C-2.0 was η-Al₂O₃. It indicated that

γ-Al₂O₃and η-Al₂O₃were obtained by heating γ-AlO

(OH) andAACH, respectively, combining withtheXRDanal-

ysis of precursors. However, precursor P-1.0 was not con-

verted to any crystal because of its low crystallinity and

weak binding force. The diffraction peaks of γ-Al₂O₃

appeared at 2θ = 37.60^ꢁ, 39.49^ꢁ, 45.80^ꢁ, 60.90^ꢁ, and 67.03^ꢁ

(JCPDS Card No. 10-0425), and the diffraction peaks of

η-Al₂O₃appeared at 2θ = 37.44^ꢁ, 39.67^ꢁ, 46.3^ꢁ, 60.90^ꢁ, and

67.76^ꢁ(JCPDS Card No. 04-0875).^17,18

It was difﬁcult to distinguish η-Al₂O₃from γ-Al₂O₃

because of their close diffraction peaks. The ﬁt-liners of

XRD patterns were drawn together to compare the two kinds

of Al₂O₃clearly (Figure 3(B)). According to several

studies,^17,18the η-Al₂O₃and γ-Al₂O₃phases could be distin-

guished based on the relative intensity of diffraction peaks for

(3 1 1), (4 0 0), and (4 4 4) planes. Assuming that the intensity

of the diffraction peak for (4 4 4) plane of γ-Al₂O₃is 100, the

intensity of the diffraction peaks for (3 1 1) and (4 0 0) planes

are 80 and 100, respectively. By contrast, assuming that the

intensity of the diffraction peak for (4 4 4) plane of η-Al₂O₃

Figure 2. TGA curves of precursors: (a) P-1.0; (b) P-1.5; (c) P-2.0;

(A) TG curve; (B) DTG curve; (C) DTA curve.

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Figure 4. ²⁷Al SSNMR spectra of alumina.

The difference between γ-Al₂O₃and η-Al₂O₃could also be

distinguished by DTA characterization. Kul’ko¹⁸claimed that

the transformation sequence of γ-Al₂O₃is γ-Al₂O₃! δ-

Al₂O₃! θ-Al₂O₃! α-Al₂O₃and the transformation

sequence of η-Al₂O₃is η-Al₂O₃! θ-Al₂O₃! α-Al₂O₃with

increasing calcination temperature. As shown in Figure 2(a),

except for the endothermic peaks for the elimination of

adsorbed water, three major endothermic peaks were observed

in curves (C) at 182, 765, and 920 ^ꢁC. The ﬁrst peak was

assigned to the decomposition of γ-AlO(OH) to γ-Al₂O₃,

involving the elimination of OH groups; the second peak

was assigned to the transformation of γ-Al₂O₃to δ-Al₂O₃,

without weight loss detected; the third peak was assigned to

the transformation of δ-Al₂O₃to θ-Al₂O₃, without weight loss

detected; θ-Al₂O₃completely converted to α-Al₂O₃when the

calcination temperature was more than 1100 ^ꢁC. Similarly, as

shown in Figure 2(b), except for the endothermic peaks for the

elimination of adsorbed water, three major endothermic peaks

were observed at 190, 800, and 925 ^ꢁC for precursor P-1.5.

The process of decomposition and transformation was

similar to P-1.0, the transformation of γ-Al₂O₃to δ-Al₂O₃also

terminated at 875 ^ꢁC. Only two major endothermic peaks were

observed at 209 and 910 ^ꢁC for P-2.0 as shown in Figure 2(c),

except for the inconspicuous endothermic peaks for the elim-

ination of adsorbed water at around 100 ^ꢁC. The ﬁrst peak was

assigned to the decomposition of AACH to η-Al₂O₃, invol-

Figure 3. XRD patterns of alumina (a) and their ﬁt-liners (b): (A) C-

1.0; (B) C-1.5; (C) C-2.0.

is 100, the intensity of the diffraction peaks for (3 1 1) and (4 0

0) planes are 60 and 80, respectively. Based on the preceding

analysis, the crystal phase of C-1.5 is γ-Al₂O₃and the crystal

phase of C-2.0 is η-Al₂O₃.

SSNMR experiments were conducted on the Brooker

Avance III to further distinguish the similarity between

γ-Al₂O₃and η-Al₂O₃. Several studies^19–21reported that

γ-Al₂O₃and η-Al₂O₃contain Al³⁺in both octahedral coordi-

nation and tetrahedral coordination. The fraction of octahed-

rally coordinated Al³⁺in γ-Al₂O₃was higher than that in

η-Al₂O₃, and the fraction of tetrahedrally coordinated Al³⁺

in γ-Al₂O₃was lower than that in η-Al₂O₃. As shown in

Figure 4, octahedrally and tetrahedrally coordinated Al³⁺were

observed at ca. 5 and ca. 60 ppm in all the three aluminas,

although C-1.0 was amorphous. The amount of tetrahedrally

coordinated Al³⁺in C-2.0 was more than the amount in C-

1.0 and C-1.5, while the amount of octahedrally coordinated

Al³⁺in C-2.0 was relatively smaller than the other two, which

proved that C-1.5 was γ-Al₂O₃and the alumina C-2.0 was

η-Al₂O₃from another side.

ving elimination of OH⁻, CO₃, and NH₄⁺groups; the second

2−

peak was assigned to the transformation of η-Al₂O₃to

θ-Al₂O₃, without weight loss detected based on the analysis

of TG characterization; it also ﬁnally converted to α-Al₂O₃

when the calcination temperature was 1100 ^ꢁC or more.

In order to verify the previous DTA analysis further, the

XRD patterns of the alumina by calcining the precursors at

800, 925, and 1000 ^ꢁC, respectively, were shown in

Figure 5. As shown in Figure 5(a), diffraction peaks of

δ-Al₂O₃appeared in C-1.0 and C-1.5 when they were calcined

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Figure 6. XRD patterns of ammonia-treated alumina: (a) low-

temperature ammonia treatment and (b) high-temperature ammonia

treatment.

above XRD analysis results were consistent with the TGA

analysis.

Phase and Structure of Ammonia-treated Alumina. The

calcined aluminas were charged into excess 50 (vol/vol)%

aqueous solution of ammonia under room temperature for

6 h, after being removed and dried, then the products were

obtained (low temperature ammonia treatment). By contrast,

the aluminas were added to another excess aqueous solution

of ammonia (50 vol %) and transmitted into a hydrothermal

reaction kettle at150 ^ꢁC for6 hwith thesame continuousproc-

ess (high temperature ammonia treatment). Figure 6 presents

the XRD patterns of the ammonia-treated alumina. As shown

in Figure 6(a), β-Al(OH)₃diffraction peaksappeared inall alu-

mina (JCPDS Card No. 12-0457, 2θ = 18.6^ꢁ, 20.3^ꢁ, 28.0^ꢁ,

40.7^ꢁ, and 53.3^ꢁ). The diffraction peak intensity order of

ammonia-treated alumina was LA-1.0 > LA-1.5 > LA-2.0,

on the contrary, the intensity order of the diffraction peaks

of their untreated alumina was C-1.0 < C-1.5 < C-2.0. Besides,

Al₂O₃diffraction peaks still existed in the patterns. This result

indicated that the original alumina did not completely con-

verted into β-Al(OH)₃, and stronger diffraction peaks of

Figure 5. XRD patterns of precursors calcined at different tempera-

ture: (a) 800 ^ꢁC; (b) 925 ^ꢁC; (c) 1100 ^ꢁC.

at 800 ^ꢁC, while C-1.5 was basically still η-Al₂O₃. As shown

in Figure 5(b), diffraction peaks of θ-Al₂O₃appeared in C-1.0,

C-1.5, andC-2.0whentheywerecalcinedat925 ^ꢁC. Asshown

in Figure 5(c), diffraction peaks of α-Al₂O₃appeared in C-1.0,

C-1.5, and C-2.0 when they were calcined at 1100^ꢁC.^22,23The

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The γ-AlO(OH) diffraction peaks were still observed when

calcined at 900 ^ꢁC, demonstrating that high-temperature

ammonia treatment has greater inﬂuence on the alumina than

low-temperature ammonia treatment.

Figure 8 shows the TGA curves of the ammonia-treated alu-

mina. As shown in Figure 8(a)–(c), all the three low-

temperature ammonia-treated alumina possessed weight loss

peaks at 100–110 ^ꢁC, which belonged to the dehydration of

the physically adsorbed water, the weight loss of water for

LA-2.0waslessthantheothertwo, indicatingthatLA-2.0pos-

sessed less physically adsorbed water. The weight loss peaks

at 250–270 ^ꢁC belonged to the decomposition and dehydra-

tion of β-Al(OH)₃. The decomposition temperature is consist-

ent with the Du’s research result.²⁴Based on the calculated

results of TG curves, the weight losses of LA-1.0, LA-1.5,

and LA-2.0 were 32.4, 33.9, and 33.4%, respectively, which

were close to the theoretical value of 34.6%. As shown in

Figure 8(d)–(f ), the weight loss peaks of the physically

adsorbed water of HA-2.0 at 100–110 ^ꢁC were much weaker

compared to HA-1.0 and HA-1.5. Besides, all the three high

temperature ammonia-treated alumina possessed weight loss

peaks at 200–210 ^ꢁC, which belonged to the decomposition

and dehydration of γ-AlO(OH). The decomposition tempera-

ture were higher than the original precursors P-1.0 and P-1.5,

indicatingthattheproducedγ-AlO(OH)possessedmuchmore

completed crystal and more stable structure than that of P-1.0

and P-1.5.

Phase of the Second Calcined Alumina. Figure 9 provides

the XRD patterns of the second calcined alumina. As shown in

Figure 9(a), when the low-temperature ammonia-treated alu-

mina was calcined again, the obtained alumina was η-Al₂O₃.

Interestingly, strong η-Al₂O₃diffraction peaks appeared in

LAC-1.0, although no diffraction peaks appeared in C-1.0

before ammonia treatment and the second calcination. This

result indicates that ammonia destroyed and restructured the

inner structure ofC-1.0. Thecrystal phaseof η-Al₂O₃emerged

whenitwascalcinedagain. Theγ-Al₂O₃crystal phaseofC-1.5

transformed into η-Al₂O₃after ammonia treatment and second

calcination, and the intensity of the diffraction peaks for

η-Al₂O₃was stronger than the original γ-Al₂O₃. This ﬁnding

reﬂects that ammonia destroyed the intrinsic crystal phase and

rearranged the structure of C-1.5. It also indicates that η-Al₂O₃

was easier to be formed than γ-Al₂O₃owing to the existence of

β-Al(OH)₃in the treated alumina.

Similar to the previous analysis, the crystal type of the

second calcined alumina can also be distinguished by DTA

characterization in Figure 8. For the low-temperature

ammonia-treated alumina, except for the endothermic peaks

for the elimination of adsorbed water, two major endothermic

peaks were observed in all curves (C) of Figure 8(a)–(c) at

about 250 and 845 ^ꢁC. The ﬁrst peaks were assigned to the

decomposition of β-Al(OH)₃to η-Al₂O₃, involving the elim-

ination of OH groups. The second peaks were assigned to the

transformation of η-Al₂O₃to θ-Al₂O₃, without weight loss

detected, and the θ-Al₂O₃was completely converted to

α-Al₂O₃when the calcination temperature was higher than

Figure 7. XRD patterns of ammonia-treated alumina calcined at dif-

ferent temperatures: (a) low-temperature ammonia treatment and(b) -

high-temperature ammonia treatment.

original alumina resulted in weaker diffraction peaks of the

corresponding obtained β-Al(OH)₃. As shown in Figure 6

(b), the appearance of quite strong diffraction peaks of

γ-AlO(OH) (JCPDS 21-1307) in all aluminas and none obser-

vation of Al₂O₃diffraction peaks in the patterns indicated that

the original alumina completely converted into γ-AlO(OH).

Figure 7 provides the XRD patterns of ammonia-treated

alumina calcined at different temperature to investigate their

stability. As shown in Figure 7(a), the β-Al(OH)₃diffraction

peaks for the low temperature ammonia-treated alumina cal-

cined 500 ^ꢁC were obvious, but the intensity became weak

with the increase of calcination temperature and no β-Al

(OH)₃peaks were detected for the 800 ^ꢁC calcined sample.

It suggests that the low-temperature ammonia-treated alumina

was becoming stable with the increase of calcination temper-

ature, and it was completely stable when calcined at 800 ^ꢁC.

As shown in Figure 7(b), when the high-temperature

ammonia-treated aluminas were calcined at different temper-

ature, although the intensity of γ-AlO(OH) diffraction peaks

decreased with the increase of calcination temperature.

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Figure 8. TGA curves of ammonia-treated alumina: (a) LA-1.0; (b) LA-1.5; (c) LA-2.0; (d) HA-1.0; (e) HA-1.5; (f ) HA-2.0; (A) TG curves;

(B) DTG curves; (C) DTA curves.

1100 ^ꢁC. For the high-temperature ammonia-treated alumina,

similar to P-1.0 and P-1.5 in Figure 2, except for the weak

endothermic peaks for the elimination of adsorbed water,

major exothermic peaks were observed in curves (C) of

Figure 8(d)–(f ) at 875 ^ꢁC. The decomposition process was

similar with that of P-1.0 and P-1.5, involving the transforma-

tionofγ-Al₂O₃toδ-Al₂O₃,andδ-Al₂O₃toθ-Al₂O₃. However,

the treated alumina did not convert to α-Al₂O₃when they were

calcined at 1100 ^ꢁC, they became some unknown crystal type

(see Figure 10) at the calcination temperature of 1100 ^ꢁC.

There was no transformation heat detected during the process

of 1000–1100 ^ꢁC as shown in the DTA curves, which indi-

cated that the new produced γ-AlO(OH) were much more sta-

ble than the original ones.

Morphology and Texture Properties of Original Alumina,

Treated Alumina, and the Second Calcined Alumina.

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Figure 10. XRD patterns of high-temperature ammonia-treated alu-

mina calcined again at 1000 ^ꢁC.

as shown in Figure 11(k) and (l). The tiny pores of LAC-

1.0, LAC-1.5, and LAC-2.0 grew to 6–8, 8–10, and 10–12

nm, respectively. When the high-temperature-treated alumina

was calcined again, a part of the crystal faces were destroyed

and new small pores and caves were formed on the original

surface (see Figure 11(m)–(o)).

Figure 12 presents BJH pore-size distribution curves of all

the samples and nitrogen adsorption–desorption isotherm of

the second calcined alumina. As shown in Figure 12(a), the

three original aluminas displayed double-pore distribution,

the small pores were attributed to surface pores, whereas the

big pores were attributed to packed pores formed by the dis-

ordered and agglomerate packed sheets.²⁵The surface pores

were dominated in C-1.5 and C-2.0 due to release of abundant

NH₃, CO₂, and H₂O in the calcination process, while in C-1.0,

the packed pores were dominated for no gas release in the

related process. Figure 12(b) indicates that the amount of

packed pores was decrease, whereas that of surface

pores was increase and the pores became smaller when the alu-

minas were treated by ammonia at low temperature (see

Figure 12(b)). The most possible surface pore sizes of LA-

1.0, LA-1.5, and LA-2.0 were decreased to 3 nm, exhibiting

as single-pore distribution with little parts of packed pores.

On the contrary, as shown in Figure 12(c), when the aluminas

were treated by ammonia at high temperature, the surface

pores amount was decrease, whereas that of packed pores

was increase, the most possible surface pore sizes of HA-

1.0, HA-1.5, and HA-2.0 concentrated at about 40–70 nm,

exhibiting as single-pore distribution with a small parts of sur-

face pores. Moreover, when the low-temperature ammonia-

treated aluminas were calcined again, it displays single-pore

distribution, and the pore sizes were about 6–10 nm (see

Figure 12(d)), smaller than that obtained from the TEM

results. All the three samples exhibit mesoporous structure

(see Figure 12(f )), especially for LAC-2.0, these mesoporous

formed via surface small pores growth during the second

Figure 9. XRD patterns of the second calcined alumina: (a) low-

temperature ammonia treatment and (b) high-temperature ammonia

treatment.

Figure 11 shows the TEM images of original alumina, treated

alumina, and the second calcined alumina. As shown in

Figure 11(a), C-1.0 was bulk although its diffraction

peaks in XRD pattern was diffuse. C-1.5 was fragmentary,

and C-2.0 was cotton-like as can be seen from Figure 11

(b) and (c). When the aluminas were treated by ammonia at

low temperature, the intrinsic structure of the original alumina

was destroyed to some extent, resulting in the porosity of the

treated alumina. As Figure 11(d) shows, LA-1.0 was pierced

with tiny pores, and Figure 11(e) provides that LA-1.5 became

porous compared with C-1.5. LA-2.0 in Figure 11(f ) also

emerged with some distributed pores. When the aluminas

were treated by ammonia at high temperature, the intrinsic

structure of the original alumina was completely destroyed

due to the atoms in alumina deviated from their original posi-

tions. All the treated aluminas were cubic and nearly the same

with the edge-lengths of about 50–150 nm. When the LA-1.0

were calcined again, the original β-Al(OH)₃were completely

decomposed with the small pores gathering and increasing.

The pores of LAC-1.5 and LAC-2.0 were legible and neat

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Figure 11. TEM images of alumina samples: (a) C-1.0; (b) C-1.5; (c) C-2.0; (d) LA-1.0; (e) LA-1.5; (f ) LA-2.0; (g) HA-1.0; (h) HA-1.5; (i) HA-

2.0; (j) LAC-1.0; (k) LAC-1.5; (l) LAC-2.0; (m) HAC-1.0; (n) HAC-1.5; (o) HAC-2.0.

calcinationprocess. It basically displayed a double-pore distri-

bution, when the high-temperature ammonia-treated aluminas

were calcined again, the big pores were the original packed

pores, while the small pores were new superﬁcial pores pro-

duced on the surface of the obtained alumina HAC-1.0,

HAC-1.5, and HAC-2.0 (see Figure 12(e) and Figure 11

(m)–(o)).

The physical properties of the original alumina-treated alu-

mina and the second calcined alumina are listed in Table 1.

Alumina C-2.0 had higher speciﬁc surface area and larger pore

volume than C-1.0 and C-1.5 for release of CO₂, NH₃, and

H₂O during the calcination process. C-1.0 had the smallest

speciﬁc surface area and pore volume for less gas release.

When the aluminas were treated by ammonia at low temper-

ature, the corresponding speciﬁc surface area increased for

the formation of small pores on the surface of LA-1.0, LA-

1.5, and LA-2.0 (see Figure 11(d)). Thus, the average pore

diameter decreased and the pore volume increased. When

the low-temperature ammonia-treated aluminas were

calcined again, the original small pores enlarged and the aver-

age pore diameter of the obtained LAC-1.0, LAC-1.5, and

LAC-2.0 increased to 6.3, 8.2, and 9.9 nm, respectively.

Although the corresponding speciﬁc surface area was

decreased, it was still higher than that of the original alumina.

When the aluminas were treated by ammonia at high temper-

ature, there were block crystals formed, the corresponding

speciﬁc surface area sharply decreased for the crystals recon-

struction (see Figure 11(g)–(i)), the average pore diameter

sharply increased to 26.4, 21.0, and 20.5 nm, and the pore vol-

ume decreased for the same reason. When the high-

temperature ammonia-treated aluminas were calcined again,

the BET surface area of the obtained samples increased due

to the production of new small pores and caves (see

Figure 11(m)–(o)), andthepore volume were stillsmaller than

that of the original alumina although they were increased for

the same reason. The average pore diameter of HAC-1.0,

HAC-1.5, and HAC-2.0 decreased to 11.4, 13.0, and 15.1

nm, respectively.

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Figure 12. BJH pore-size distribution of original alumina (a), ammonia-treated alumina (b, c), the second calcined alumina (d, e) and nitrogen

adsorption–desorption isotherm of the second calcined alumina (f ).

Conclusion

aqueous solution of ammonium carbonate. The obtained alu-

minas were treated by 50 (vol/vol)% ammonia via low-

temperature immersing and high-temperature hydrothermal

treatment, respectively, then, the treated aluminas were

Inthis study, different aluminamaterialswereobtained bypre-

cipitating Al(NO₃)₃ꢀ9H₂O with different concentration of

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Preparation and Characterization of Transitional Alumina

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Table 1. BET data for the original alumina, treated alumina, and the second calcined alumina.

Sample

BET surface area (m²/g)

Average pore diameter (nm)

Pore volume (cm³/g)

C-1.0

C-1.5

C-2.0

148.5

176.6

221.1

228.7

252.3

287.0

34.7

9.7

12.1

13.0

5.6

6.5

7.2

26.4

21.0

20.5

6.3

8.2

9.9

0.43

0.59

0.77

0.54

0.67

0.85

0.16

0.23

0.24

0.55

0.68

0.92

0.20

0.26

0.32

LA-1.0

LA-1.5

LA-2.0

HA-1.0

HA-1.5

HA-2.0

LAC-1.0

LAC-1.5

LAC-2.0

HAC-1.0

HAC-1.5

HAC-2.0

39.5

52.0

210.2

230.3

259.8

100.4

108.3

119.5

11.4

13.0

15.1

10. G. C. Li, Y. Q. Liu, L. L. Guan, X. F. Hu, C. G. Liu, Mater. Res.

Bull. 2012, 47, 1073.

11. K. M. Parida, A. C. Pradhan, J. Das, N. Sahu, Mater. Chem. Phys.

2009, 113, 244.

12. Z. Tang, J. Liang, X. Li, J. Li, H. Guo, Y. Liu, J. Solid State

Chem. 2013, 202, 305.

13. J. Kong, B. Chao, T. Wang, Y. Yan, Powder Technol. 2012,

229, 7.

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38, 3741.

15. H. K. Varma, T. V. Mani, A. D. Damodaran,

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77, 1597.

calcined again. When the original aluminas were low-/high-

temperature treated, the treated alumina and the second cal-

cined alumina had obvious changes, the morphology, crystal

types, and pore distribution also took changes, and the modi-

ﬁed alumina with pure crystal types, desired pore distribution,

and speciﬁc morphology were obtained, which can be used in

some speciﬁc application, such as heterogeneous catalysis,

heavy oil cracking, and so on.

Acknowledgments. The authors would like to acknowledge

the ﬁnancial support from the National Natural Science

Foundation of China (Grant No. 21203232).

16. X. Hu, Y. Liu, Z. Tang, G. Li, R. Zhao, C. Liu, Mater. Res. Bull.

2012, 47, 4271.

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Preparation and characterization of transitional alumina obtained through ammonia treatment

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