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Journal of Natural Gas Chemistry 14(2005)181–188

Influence of Successive Washing on Porous

Structure of Pseudoboehmite

Yuefeng Yan, Jianping Zhi∗, Gaoyong ZhangState Engineering Center of Surfactant, China Research Institute of Daily Chemical Industry, Taiyuan 030001, China

[Manuscript received May 26, 2005; revised August 15, 2005]

Abstract: The effect of successive washing instead of traditional intermittent washing on the porousstructure of pseudoboehmite was investigated by mercury porosimetry, N2 adsorption and thermal analysis,while the stabilities of dif ferent types of crystals were investigated by X-ray diffractometer. Experimentalresults show that successive washing is a continuation of the aging process of intermittent washing. After asuccessive washing, the pore types showed no difference with the intermittent washing. During successivewashing, the characteristics of the pores in the range of 2–15 nm changed only very little. However, thedistributions of the pore radius for pores of 20–50 and 300–1000 nm were obviously influenced. It wasshown that the volume of larger pores decreased only to a smaller extent after the successive washing, ascompared with that of the intermittent washing, and the pore size was affected by the condition of thesuccessive washing. The roles of physisorbed water, intermicellar liquid, weakly bonded water, as well asthe role of stirring, have been discussed.

Key words: successive washing, intermittent washing, pseudoboehmite, γ -Al2O3, porous structure

1. Introduction

As we know, the structure and performance of

γ -Al2O3 are determined by the pore structure of its

precursor, namely, the pseudoboehmite. Some factors

influencing the pore structure of the pseudoboehmite

have been reported, and among them the washing

process is one of the important factors because: (1)

the adsorbing on the surface of the pseudoboehmite

acts as a stabilizing factor for amorphous particles [1],

and washing can remove the Cl− anions, thus enhanc-

ing the transformation of the colloidal amorphous par-

ticles into the pseudoboehmite; (2) the residual amor-phous particles have become smaller after aging, and

their solubility in the medium is higher, thus they can

be easily removed by washing; (3) the removal of the

impurity anions is beneficial for enhancing the stabil-

ity of the sol formed in the acidification course and

hence improve the mechanical strength of the dried

granules [2].

Many researchers have emphasized the effect of

the washing medium [3] and the additives [4] during

the washing stage in the preparation of γ -Al2O3. In-

termittent washing is employed in almost all of the γ -

Al2O3 preparations both in manufacturing and labo-

ratory scales. Hou et al. [5] have studied the improve-

ments attained with various washing modes. They

introduced countercurrent washing to replace the re-

peated dispersion mode for the sake of saving water.

However, their approach still belonged to the inter-

mittent washing mode. The disadvantages of inter-

mittent washing are as follows: (1) unfavorable yield,

(2) larger water consumption, (3) too long manufac-turing cycle, (4) low production efficiency, (5) high

labor intensity, and (6) large amount of liquid waste.

It is possible to solve the above problems if the inter-

mittent washing mode is replaced by the successive

washing mode. However, when utilizing the succes-

sive washing, it is important to first understand what

changes will be brought about to the structure and the

∗ Corresponding author. Tel(0351)4132049; E-mail: [email protected].



182 Yuefeng Yan et al./ Journal of Natural Gas Chemistry Vol. 14 No. 3 2005

performance during the successive washing. There-

fore, we have investigated the ef fects of successive

washing on the pore structure of the pseudoboehmite,

and the results are reported here.

2. Experimental

2.1. Successive washing technologypar

The equipment of washing and the two different

washing modes are shown in Figure 1.

Figure 1. Flow sheet of washing

1—BMJ 0.5/310 open delivery washability plate-and-frame

type filter press (closed delivery unwashability type), 2—LZBF-

15flowrator, 3—Pressure gange, 4—QFK-100enamel antirot re-

actor, 5—I-1Bscrew pump, 6—Stainless steel store tank, 7—

Temperature recorder and controller

The procedure of the intermittent washing in-

cludes the transportation of the aged stuff, filtering,discharging of the filter cakes and their dispersion

in water, and repeating of the procedure for three

times (Figure 2). During the intermittent washing,

pH value, temperature and stirring are important pa-

rameters for controlling the solubility equilibrium of

the Cl− anions.

Figure 2. Flow sheet of intermittent washing

The successive washing process is different from

the intermittent one. As shown in Figure 3, the proce-

dure only includes one filtration and one discharging

of the solid stuff. After transporting of the aged stuff

by opening valve 1 and filtering, the successive wash-

ing is started. By opening water valve and closingstuff valve, water will be pumped with an appropri-

ate flux into the pipeline and enter the space between

the plate and the filtering cloth, as indicated by the

solid line in Figure 3. Then, the exit valve of filter

plate 2 (see Figure 3) should be closed. Water will

then be pressed through the filtering cloth, the filter

cake, and the filtering cloth on the other side, and

finally flows out of the filtering equipment (washing

plates 1 and 3 in Figure 3). In order to make com-

parison with the results of intermittent washing, the

washing water was stored in an anti-corrosion enamelreactor, and was divided into three successive por-

tions to pass through the washing equipment during

successive washing.

Figure 3. Flow sheet of successive washing

2.2. Orthogonal design

Preferable technical parameters were obtained by

the orthogonal experiment. Levels of factors are

shown in Table 1. We used the Table L27(133 ) [6] to

conduct the experiment. A series of samples (n–m)

were obtained, n=1–27, denoting the batch number

of successive experiment, and m=0–3, denoting three

washing stages. For n=0, the samples were taken from

the intermittent washing mode.



Journal of Natural Gas Chemistry Vol. 14 No. 3 2005 183

Table 1. Levels of factors in orthogonal experiment

Levels of First washing stage Second washing stage Third washing stage

factors θ1/ pH1 V 1/(L/h) t1/min θ2/ pH2 V 2/(L/h) t2/min θ3/ pH3 V 3/(L/h) t3/min

1 40 9.3 50 5 40 9.8 50 5 40 10.0 50 5

2 60 10.3 100 10 60 10.6 100 10 60 10.6 100 10

3 80 11.3 150 15 80 11.0 150 15 80 11.4 150 15

θ: temperature; V : velocity of flow; t: time of air flow.

2.3. Preparation of samples

Parameters of successive washing are shown in Ta-

ble 2, which are chosen through variance analysis for

orthogonal experiment, using mass fraction of Cl− in

the filter cake as index. After slurrying, the aged stuff

were pumped into an open delivery plate-and-frame

type f ilter press. Then the temperature and pH of

deionized water were adjusted according to Table 2

for conducting the three stage washing, and finally

the f ilter cake was discharged. During this course,

concentrations of Cl− in the filter liquor and the cake

were exact determined.

Table 2. Parameters of successive washing process

First washing stage Second washing stage Third washing stagen

θ1/ pH1 V 1/(L/h) t1/min θ2/ pH2 V 2/(L/h) t2/min θ3/ pH3 V 3/(L/h) t3/min

7 40 11.3 150 15 40 9.8 50 15 80 11.3 100 10

9 40 11.3 150 15 80 11.0 150 10 60 10.6 50 5

25 80 11.3 100 5 40 11.3 100 15 60 10.0 100 5

27 80 11.3 100 5 80 10.6 50 10 40 11.3 50 15

n: Batch number of the successive washing experiment

2.4. Characterization of samples

2.4.1. X-ray diffraction determination

X-ray diffraction determinations of the pretreated

samples were conducted by using a Japanese Rigaku

D/max-RB X-ray diffractometer with a K α operatedat 40 kV, 100 mA, DS=SS=1o, RS=0.3 mm−1, and

scan rate 0.25o/min (RC=1s).

2.4.2. N2 adsorption

Surface areas, pore volumes, and pore radius of

narrow pores were measured by an ASAP2010 instru-

ment made in Japan. Samples were dried at 120 to

get rid of adsorbates on the surface before the mea-

surement. BET areas were calculated according to

the Brunauer-Emmett Teller equation, and the total

pore volumes were obtained with p/ p0=0.99.

2.4.3. Mercury porosimetry measurement

Distribution of macropore radius was determined

by a Thermoquest Pascal-240 Mecury porosimetry of

America manufacture after drying at 120 .

2.4.4. Thermal analysis-TG(DTG) and DTA

DTA and TG data were obtained by a Setaram

Tga 92 thermal analysis instrument made in France,

with Pt/Pt-10%Rh, and α-Al2O3 as the reference.

Heated in argon gas at increments of 10 /min, the

weight of the samples decreased gradually. If the tem-

perature was above 600, the types of the crystals of

the samples would be changed, so 650 was chosen

as the ultimate temperature in thermal analysis.

3. Results and Discussion

3.1. Change of pore types after successive

washing

The isotherm plots are shown in Figures 4 and

5. The hysteresis loop shows the presence of medium

pores in the samples obtained either by intermittent

or successive washing. These medium pores were orig-inated mainly from the interspaces of crystalline par-

ticles or their aggregates.

The samples 0-0 and 0-1 prepared by intermit-

tent washing consisted of mainly ink-bottle type pores

(H2 type or E type), and part of them was cylindrical

pores. Similar conclusion can also be obtained from

the increasing trend of the t-ns curve. Results of the

t-equation after correction are shown in Table 3. The




Figure 4. Isotherm plots of intermittent washing

ink-bottle type pores cannot be easily explained, but

different mechanisms in capillary shrinkage and evap-

oration between the narrow neck and the wide bodyare possible explanations, that is to say, the pore type

Figure 5. Isotherm plots of successive washing

is influenced by the texture of the material. Whatever

the reason for forming of the ink-bottle type pores, the

adsorption branch should be used for the analysis of the pore distribution.

Table 3. orrected results of t-equation for pore types of the samples

SamplesPhysical-chemical parameters of the samples

0-0 7-3

Specific surface area (m2/g) 117.1 264.6

Pore volume ( p/ p0=0.99) (ml/g) 0.20 0.47

t-Area (m2/g) 90.39 233.7

Micropore volume (ml/g) 0.00004 0.014

Pore volume (ml/g) cylindrical pores (0.12) 0.48

slit-shaped pores — (0.46)

ink-ottle pores 0.20 —

Pore diameter (nm) cylindrical pores adsorption (3.32) 7.1

slit-shaped pores desorption — (2.13)

ink-ottle pores adsorption 6.67 —

Specific surface area (m2/g) cylindrical pores (90.1) 257.6

slit-shaped pores — (243.0)

ink-ottle pores 117.3 —

From Figure 5, we can conclude that for the sam-

ples of 0-2, 0-3 and 7-3, 25-3, 27-3 etc, the pore types

mainly consisted of cylindrical pores (H1 type or A

type), and part of them was slit-shaped pores (H3

type or B type). The values of specific surface area

and the t-ns curve could confirm these results as well,

since the value of 257.6 m2/g is approximated to the

BET value of 264.6 m2/g. Besides, the isotherm plot

shows that the pores also have the characteristics of

the slit-shaped pores, because adsorption is unlim-

ited when p/ p0 is high enough. The size of the mi-

cropore radius is 1.13 nm calculated using the slit-

shaped pores mode, which is typical of micropores,

and this kind of pores constitute the layer structure

of the pseudoboehmite.With the increment in the extent of washing, the

pore type changed from the ink-bottle shape into

the cylindrical shape, because the neck of the pores

was shortened and widened. Thus, we can conclude

that there is no difference in pore types between the

pseudoboehmites prepared by successive washing and

intermittent washing.




3.2. Effect of successive washing on pore ra-

dius distribution

3.2.1. Results of thermal analysis

DTG curves of the samples 0-0, 0-3, 7-3 and 27-3

with different peaks are shown in Figure 6. It has

been reported [6] that the peak at the higher temper-ature is associated with intermicellar water bonded

with the crystal lattice via the —OH group, and the

lower temperature peak is originated from the loss

of water in macropores of the pseudoboehmite, while

the peak appearing at the mid-temperature range is

resulted from the loss of the water weakly bonded to

the crystallites. As shown in Figure 6, there are two

peaks in the DTG curve of the samples 7-3 and 27-3,

Figure 6. DTG curves

Figure 7. DTA curves

where the peak at the higher temperature represents

the existing of less structural liquor as well as larger

crystallite particles obtained by successive washing.

Consequently, we can infer that the radius of the nar-

row pores yielded after successive washing are larger

than those formed after intermittent washing, and the

pore volume of the former is smaller than the latter.

This conclusion can also be obtained from Table 6.

In Figure 7, the DTA plots have the same trend as

that of the DTG patterns, due to the fact that there

was no transformation of the crystal type. The degree

of crystallization follows the order of: (27-3)>(27-3)>(7-3)>(25-3)>(3-0).

3.2.2. Effect of successive washing on total

pore volume

The total pore volumes (mm3/g) obtained by

mercury porosimetry are shown in Table 4, and the

increment in volume relative to the intermittent wash-

ing samples are also given. It can be seen that the two

washing modes showed the same trend of alteration,

that is, they both increase at first, and then decrease.

But the values of the pore volumes are different fromeach other. Combining with the technical parameters

in Table 2, we can derive the following conclusion by

making lateral and perpendicular comparisons: lower

temperatures and higher pH values are beneficial to

the forming of larger pore volumes. The difference in

pore volumes is mainly determined by the amount of

ammonia present in the wash water, and this is consis-

tent with the results of Ref. [8], which reported that

ammonia is an effective pore-enlargement reagent. By

compared with the results of intermittent washing, it

can be found that the technical parameters of sample

7 are the optimal ones.

Table 4. Total volume of the samples

Washing Total volume of the samples

stage (n) (mm3/g)

(m) 0 7 25 27

0 668.6 668.6 668.6 668.6

1 1042.9 1151.1 942.5 948.4

2 1041.5 1201.8 1014.3 358.3

3 979.5 925.8 761.9 528.9

3.2.3. Data of pore structure

Distributions of the pore radius of different sam-

ples are shown in Figure 8, including the distribution

of narrow pores (Figure 8) obtained by N2 adsorp-

tion measurement, and the distribution of wide pores

(Figure 9) obtained by mercury porosimetry.




Figure 8. Distribution of pore with narrow radius

For the sake of comparison, pores are generally

divided into three ranges: medium pores of 3–10 nm,

medium pores of 10–100 nm and wide pores of 100–

1000 nm. The volumes of these three sorts of pores

are shown in Table 5. It can be noted that sample 7-2

shows data quite different from the others. General

parameters for the porous structure of the samples are

shown in Table 6 to serve as references for studying

the effect of successive washing on narrow pores and

wide pores.

Table 5. Distributions of three sorts of pores

Volume of Percentage of Volume of Percentage of Volume of Percentage of

Sample 3–10 nm pores 3–10 nm pores 10–200 nm pores 10–200 nm pores 200–1000 nm 200–1000 nm

(cm3/g) in all pores (%) (cm3/g) in all pores (%) pores (cm3/g) in all pores (%)

0-0 0.09 13.5 0.16 24.0 0.41 61.4

0-1 0.20 19.9 0.20 19.9 0.65 60.2

0-2 0.20 19.2 0.17 16.3 0.67 64.4

0-3 0.17 17.3 0.22 20.4 0.61 62.2

7-1 0.20 18.0 0.24 21.6 0.71 64.0

7-2 0.19 15.7 0.51 42.1* 0.51 42.1*

7-3 0.17 18.1 0.28 29.8 0.49 52.1

25-1 0.15 15.7 0.19 20.0 0.61 64.2

25-2 0.20 19.6 0.20 19.6 0.62 60.8

25-3 0.11 14.2 0.17 22.1 0.49 63.6

* Sample 7-2 have a great change, as shown in the Table.

Table 6. Parameters of the porous structure of the samples

Sample R1/nm V 1/(mm3/g) S P1/(m2/g) R2/nm V 2/(mm3/g) S P2/(m2/g) R3/nm V 3/(mm3/g) S P3/(m2/g)

0-0 3.34 200 117 786.6 578.4 0.0008

0-1 3.08 470 297 969.9 896.2 0.0650

0-2 4.00 540 259 889.1 843.4 0.0810

0-3 3.98 510 251 868.5 814.2 0.0750

7-1 4.10 500 243 803.6 948.7 0.0960

7-2 3.95 500 255 27.0 10.2 1.0 821.4 1017.0 0.0660

7-3 3.55 470 265 889.3 914.2 0.0750

25-1 4.00 500 252 820.9 926.0 0.0750

25-2 4.10 500 244 830.5 1023.2 0.1500

25-3 4.20 480 229 770.3 919.8 0.0730

27-1 3.95 490 248 888.9 923.5 0.0440

27-2 3.95 490 248 830.1 987.1 0.0320

27-3 3.85 480 249 911.1 908.2 0.4400

R1, R2, R3: the main pore radius of 3–10 nm, 10–100 nm and 100–1000 nm pores respectively

V 1, V 2, V 3: total volume of pores of 3–10 nm, 10–100 nm and 100–1000 nm pores respectively

S P1, S P2, S P3: specific area of pores of 3–10 nm, 10–100 nm and 100–1000 nm pores respectively




3.2.4. Effect of successive washing on narrow

pores

3.2.4.1 Micropores

When calculating according to the t-plot equa-

tion, only a small amount of micropores (0.012–0.017

mm3

/g ) are present in the pseudoboehmite after suc-cessive washing. The same conclusion was obtained

for intermittent washing. Micropores are controlled

by the self-evaporation process in a general sense, so

the distribution of micropores would not change very

much by the change in the washing mode.

3.2.4.2 Medium pores (3–10 nm)

As has been reported in some references [7], fac-

tors influencing the radius of narrow pores include

technical conditions of the washing process, such as

neutralization, aging, acidification and so on. Trans-formation of crystal types is the key factor affecting

the particle radius and the pore distribution during

the aging and washing process.

X-ray patterns shown in Figure 10 indicate that

there is no difference among the crystal types of

the samples, but the degree of crystallization in-

creased more remarkably after the successive washing

than the intermittent washing, which resulted in the

difference of the distribution of pore radius in Figures

8 and 9. But all in all, the changes are not obvious.

3.2.4.3 Medium pores (10–50 nm)

In Figure 9, there is a wide peak at 27 nm for the

sample 7-2, but it is a reverse of sample 25-2 which

was washed at a different pH. This was resulted from

the changes in the assembling state of the samples.

During continuous mass transfer, the net structure of

the gel has come into being. The stablizing factor

of the gel is electrostatic force. When pH is higher

in the first washing stage, the NH+4 ions surrounding

the colloid surface will lead to the formation of bridges

in the gel, so that the change in assembling state isnot obvious. But when the pH is decreased suddenly,

hydrophobicity of the colloid surface is weakened as

wel, so that the space between the colloid particles

or the micelles are shortened, resulting in a drastic

contraction and the collapse of the original structure.

Consequently, medium pores of 10–50 nm could be

formed under the above conditions.

Figure 9. Distribution of pore with wide radius

Figure 10. X-ray pattens of samples

3.2.5. Effect of successive washing on wide

pores

Wide pores mainly distribute in the range of 300–

1000 nm, as shown in Figure 9. It is obvious from

Table 5 that different washing conditions could lead

to different pore radius and volumes, because of the

different assembling behaviors of secondary particles

and their aggregates.




In Table 5, we can see that successive washing

and intermittent washing have different effects on the

pore volumes of wide pores. The volumes of the wide

pores basiscally remained constant after intermitant

washing for three times, but for the successive wash-

ing the situation was the opposite. This difference

may be due to the stirring in the first stage of the

intermitant washing. Repeated stirring in a washingmedium might break the interface between the sec-

ondary particles and their relative displacement might

change, so that the interspace between the secondary

particles was increased and the pore volume was en-

larged. Meanwhile, the process of stirring did not

exist in the successive washing, and the mutual trans-

ferring ratio of the elements in secondary crystals was

decreased because of the addition of superfluous am-

monia. Therefore, the surface of the pseudoboehmite

was covered by the particles, causing a compacting of

the structure and the decrease of the wide pore vol-

ume.

It is interesting to note that the higher the pH,

the larger the radius of the wide pores, as shown in

Figure 9. This might be resulted from the breaking

function of ammonia to the structure of the secondary

particles. More ammonia can suppress the mixing of

the aggregates, consequently decrease the percent of

the water content. Thus, the space and pores radius

of pseudoboehmite can be enlarged.

Both mixing and additives such as ammonia play

the roles of changing the amount of water between

the particles and their assemblage by changing theassembling states. Water is one of the simplest and

the most important additives, in some sense, it is wa-

ter that created the space of the molecules. If the

amount of water is increased, the porosity of the sam-

ples would increase, too. On the other hand, it is

water that supports the force between the particles

and the aggregates. By bonding with the particles

and the assemblages by the —OH groups, water can

act as bridges for stabilizing the assembling state of

the particles. The more the —OH groups, the higher

the stability fo the particle assemblages, and the more

difficult for the breaking of the wide pore structure.

4. Conclusions

The optimum technical conditions for successive

washing have been found to be θ1=40 , pH1=11.3,

V 1=150 L/h , t1= 15 min , θ2=40, pH2=9.8, V 2=50

L/h, t2=15 min. The advantages of successive wash-

ing are as follows: (1) alumina yield can be increased

to a large extent; (2) production cycle can be short-

ened greatly; (3) deionized water consumption can be

reduced; (4) labor intensity can be lowered.

Effects of technical parameters on the pore struc-

ture during successive washing have been evaluated,

offering information for the preparation of γ -Al2O3

supports with diversified pores structures.

References

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1996, 134(2): 331

[2] Martens W N, Kloprogge J T, Frost R L et al. J

Colloid Interface Sci, 2002, 247: 132

[3] Xiang D H, Weng Y P, Li Q Sh. Solid catalyst. Bei-

jing: Chemical Industry Press, 1983. 255

[4] Tsukacha T. JP 08268715 1996

[5] Hou CH L, Liu J, WR Q et al. Transaction ZhongnanIndustrial University , 1997, 28(3): 238

[6] Decleer J G M. Bull Soc Chim Belg , 1992, 101(2): 89

[7] Trimm D L, Stanislaus A. Appl Catal , 1986, 21: 215

[8] Fedorov B M, Danyushevskii Y Y, Fialko V M. Kinet

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