Zou Sumeng; Yang Qinghe; Zeng Shuangqin; Nie Hong

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

The Form of Sulfate in Pseudo-Boehmite and Its Effect on Properties of Pseudo-Boehmite

Zou Sumeng; Yang Qinghe; Zeng Shuangqin; Nie Hong

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

A series of pseudo-boehmite samples with their sulfate radicals(SO42-) concentration ranging from 0.9% to 3.0% were prepared by the reaction of NaAlO2solution on Al2(SO4)3solution. The existing form of sulfate radicals was investigated. Results have shown that sulfates in pseudo-boehmite included two parts, the soluble sulfate radicals and the insoluble sulfate radicals, which accounted for 99% of the total amount of sulfate radicals. XRD, low-temperature N2-adsorption, and TEM were used to characterize the properties of these pseudo-boehmite samples. Results have shown that the relative crystallinity and crystal size of pseudo-boehmite decreased with the increase of sulfate radicals in the support. In the meanwhile, the bound water content in pseudo-boehmite increased. The TEM images of pseudo-boehmite indicated that the pseudoboehmite was prone to become amorphous hydrated alumina. However, the effect of sulfate content on the specific surface area and pore structure of aluminium oxide was insignificant.

NaAlO2-Al2(SO4)3; sulfate; pseudo-boehmite; alumina; surface properties

1 Introduction

γ-Al2O3is widely used as catalyst support in the petroleum refining industry thanks to its high specific surface area, good thermal stability, proper acidity and low cost. Pseudo-boehmite, as the precursor of alumina which is used as the support of hydrogenation catalyst, is generally prepared by three methods, namely: the hydrolysis of aluminum alkoxide; the neutralization reaction between an acid and an aluminum-containing alkali; the neutralization reaction between an aluminum-containing acid and an alkali. Among these routes, the hydrolysis of aluminum alkoxide seems to be an ideal way for preparing the pseudo-boehmite, because the impurities concentration of the product is low and can be suitable for use as the support of catalysts for hydrofining, hydrocracking, hydroisodewaxing and catalytic reforming processes. But the pseudo-boehmite prepared by this method is quite costly. In China, the pseudo-boehmite is mainly prepared by the reaction of NaAlO2solution on Al2(SO4)3solution[1]because of its lower cost compared to the hydrolysis of aluminum alkoxide. However, there are also some disadvantages, such as higher impurities content and lower crystallinity of the product. And the impurities concentration and crystallinity may affect properties of the hydrogenation catalysts caused by changes in the surface property and pore structure of pseudo-boehmite and alumina. Therefore, it is necessary to know the speci fic effects of the impurities on the properties of pseudo-boehmite and alumina.

Impurities in alumina prepared by the reaction of NaAlO2solution on Al2(SO4)3solution mainly include Na2O andradicals. The effect of Na2O on the alumina property has been reported[2], but the effect of sulfate radicals on alumina is still not thoroughly known. The sulfate radicals in alumina are mainly introduced by the following ways: (1) through materials like Al2(SO4)3solution during the preparation of pseudo-boehmite; (2) through sulfatecontaining extruding additives during the extruding or kneading process of supporters; (3) through the transformation of sulfur element into sulfate radicals introduced by sulfurization agents in the course of reaction; and (4) through sulfate-containing active ingredient intro-duced during the impregnation process. Currently, there are a lot of literature reports about the effects of extraneous sulfate radicals on the alumina support. However, in these documents, the sulfate radicals were deliberately introduced by external sources[3-6]. There are few documents referring to the effects of the sulfate radicals introduced through the materials for preparation of pseudoboehmite. In this study, four samples were prepared with different sulfate concentrations. The existing form of sulfate radicals in pseudo-boehmite was studied, and their effects on pseudo-boehmite and surface properties of alumina were investigated.

2 Experimental

2.1 Preparation of pseudo-boehmite

Aluminum sulfate was dissolved in water to obtain a solution of Al2(SO4)3with an Al2O3mass concentration of 60 g/L. Aluminum trihydroxide and caustic soda were dissolved in water at 100 ℃ to obtain a NaAlO2solution with an Al2O3mass concentration of 230 g/L. The neutralization reaction of NaAlO2solution on Al2(SO4)3solution was conducted at a specified pH value and temperature. Then the gel obtained was filtered. Deionized water was added to the filter cake and the agitator was used to mixing them evenly. The mass of deionized water was 15 times that of Al2O3. Then, some aging reagents were added to adjust the pH value required for the ageing process. Meanwhile, parameters like aging temperature and aging time were kept the same in each test. After the aging process, the obtained gel was filtered and washed by deionized water. Some different filter cakes of pseudoboehmite prepared at different ageing pH values were obtained. These filter cakes were divided into two parts. One part of them was squeezed to get the filtrate with less soluble sulfate concentration. Then, the sulfate in filtrate was analyzed and the remainder squeezed filter cake was dried at 120 ℃ for 8 h. Another part of filter cake was directly dried at 120 ℃ for 8 h, and the sample was denoted as PB-X.

2.2 Characterization

XRD analysis of the pseudo-boehmite samples was carried out by an X-ray diffractometer (XPERT, Philips) equipped with a Cu target. The scanning angle 2θwas between 5° to 70°.

Specific surface area and pore volume were obtained from nitrogen adsorption-desorption isotherms measured in an Autosorb-6B analyzer. The samples of pseudo-boehmite were both annealed at 600 ℃ for 3 h.

Sulfate radicals in the filtrate were measured by a Baird PS-4 type inductively coupled plasma atomic emission spectroscopy(ICP-AES). The qualitative and quantitative analysis of the sulfate radicals were obtained by the length and intensity of the characteristic radiation wave. The measurements of sulfate radicals in pseudo-boehmite were carried out by an EMIA-820V type infrared carbonsulfur analyzer. The measurements of sodium oxide in pseudo-boehmite were measured by a Rigaku-3271 type X-ray fluorescence spectrometer (XRF). The line intensity of elements was determined by scintillation counter and proportional counter and the concentration of the elements were determined using the external standard method.

3 Results and Discussion

3.1 The existing form of sulfate radicals in pseudoboehmite

Some researches[7]indicated that sulfates in pseudoboehmite can be divided into two classes, the soluble sulfate and the insoluble sulfate. The soluble sulfate mainly is adsorbed in an anion form on the surface of aluminium hydroxide or exists in the porous channels of gel in the form of solution. In contrast, the insoluble sulfate has the form of aluminium hydroxide sulfate because of incomplete reaction, which exists in the structural framework of the gel.

In the present experiment, the amount of sulfate radicals in the filtrate and filter cake were analyzed and calculated and the results are shown in Table 1. As regards the filter cake of these samples, the proportion of soluble sulfate radicals in the total amount of sulfate was low, which was less than 0.8%. Also, some relationships were found among the total sulfate and the sulfate radicals in filtrateand filter cake. With the increase in total sulfate concentration, the amount of sulfate radicals in filtrate and the proportion of insoluble sulfate radicals of filter cake in total amount of sulfate increased. For a better understanding, the following calculation equations are listed:

Sulfate radicals in filtrate = sulfate radicals in filtrate/the density of solution

Soluble sulfate radicals in filter cake = difference of filter cake mass before and after drying × sulfate radicals content in filtrate/mass of filter cake after drying

Insoluble sulfate radicals in filter cake/total sulfate radical concentration in filter cake = 1-soluble sulfate radicals in filter cake /total sulfate concentration in filter cake

Table 1 The concentration of sulfate radicals in filterate and filter cake

3.2 Effects of sulfate radicals content on pseudo-boehmite

3.2.1 XRD analysis

XRD spectra of four pseudo-boehmite samples are shown in Figure 1. Obviously, X-ray diffraction peaks of these samples were mainly concentrated at around 2θ=14.48°, 28.18°, 38.33°, 49.10°, and 64.90°. By comparing with JCPDS standard diffraction patterns card, these samples were verified as pseudo-boehmite[8]. By picking the peak of 2θ=38.3° and comparing peak areas of these samples with those of commercial SB powder, we could identify the samples’ relative crystallinity[9]. The crystal grain size of these samples was determined by calculating the half peak width according to the Scherrer equation[10]. All the results obtained are shown in Table 2. Meanwhile, the concentration of sodium oxide in samples was measured and test results had shown that the sodium oxide was found in these samples. The effect of sodium oxide on the pseudo-boehmite and alumina has been reported[3], which indicated that when the sodium concentration was very low in pseudo-boehmite, the effect of sodium oxide on pseudo-boehmite could almost be ignored. Table 2 reveals that the relative crystallinity of pseudo-boehmite decreased and the grain size became smaller with an increasing sulfate concentration. The phenomenon was in good agreement with those references[10-11]which considered that the sulfate radicals had hindered the contact between adjacent crystal particles. Therefore, the crystallization process of pseudo-boehmite was inhibited by the presence of sulfate anions in the gel, and then the sulfate concentration affected the relative crystallinity of pseudoboehmite. As the crystallization process was inhibited by more sulfate anions, the grain size became smaller.

Figure 1 XRD spectra of samples

Table 2 The relationship between relative crystallinity and crystal size of pseudo-boehmite samples and sulfate radicals content in samples

3.2.2 Analysis of bound water in pseudo-boehmite

Table 3 shows the bound water of pseudo-boehmite. The pseudo-boehmite was dried at 150 ℃ for 4 h to removethe physically adsorbed water, and then its mass was measured. Afterwards, the sample was put in a calcinator at 900 ℃ for 4 h and the ultimate mass of pseudo-boehmite was measured. During this thermal process, the hydroxyl groups of pseudo-boehmite combined with hydrogen to form water and the pseudo-boehmite was transformed into α-alumina, which was actually a pure Al2O3. Therefore, the bound water of pseudo-boehmite can be obtained by calculating the difference in its mass. The analytical results are shown in Table 3. It can be seen that the concentration of bound water in pseudo-boehmite was between 1.59 and 1.76. Another phenomenon that deserved attention was that the concentration of bound water declined with the decrease in sulfate radicals content. The structural elements in boehmite crystals consist of double chains of AlO6giving double molecules[12]. The paralleled double chains form layers with the external OH groups. Therefore, it indicated that less OH groups existed as the content of sulfate in pseudo-boehmite became less. And we could conclude that the pseudo-boehmite was in better order and the relative crystallinity increased with a decreasing sulfate concentration, which was in agreement with the results of X-Ray powder diffraction analyses.

Table 3 Hydrate water concentration of pseudo-boehmite

3.2.3 N2-adsorption analysis

The BET specific surface area and the pore structure of alumina determined by N2adsorption are summarized in Table 4. The pore size distribution (dV/dlogDvsD) of different alumina samples obtained by desorption isothermal curve are shown in Figure 5.

Table 4 Specific surface area and pore structure of alumina

Figure 2 Pore size distribution of alumina

Table 4 shows that the specific surface area of alumina samples ranged from 253 m2/g to 276 m2/g and their pore volume was approximately in the range of 0.93 mL/g—1.09 mL/g. There is slight difference among these samples in terms of specific surface area and pore volume. These test results indicated that the effect of sulfate radical concentration on the alumina was insignificant, which might be ascribed to the low concentration of sulfate radicals and its existing form. However, the results were contrary to the previous statement in the literature [3]. The statement pointed out that both the pore volume and average pore size were decreased with the increase in specific surface area of alumina. These differences between the literature statement and the test results may be caused by the different synthesis conditions. In the literature description, the way of getting alumina with controlled amounts of sulfate was achieved by blending sulfate salts with SB3 pseudo-boehmite, water, and an extrusion additive. In this way, the pores of pseudo-boehmite could be blocked by the sulfate and a decline in specific surface area and pore volume of alumina would take place.

3.2.4 TEM morphology analysis

Figure 3 shows the TEM images of four pseudo-boehmite samples. As shown in Figure 3, the samples PB-1, PB-2 and PB-3 had a better crystallization degree as compared with the sample PB-4, the sulfate radical concentration of which was about 3.03%. The sample PB-4 obviously displayed a sliced structure, which was inconsistent with that of amorphous hydrated alumina[13]. This phenomenon also indicated that the crystallization process was inhibited when a certain concentration of sulfate radicals existed in the pseudo-boehmite sample.

Figure 3 TEM morphology of pseudo-boehmite samples

4 Conclusions

1) By preparing pseudo-boehmite samples with different concentration of sulfate radicals and measuring the sulfate radicals content in the filtrate and the filter cake, the proportion of sulfate radicals in each part of tested sample was calculated. The sulfate species in the pseudo-boehmite mainly existed in an insoluble form which accounted for 99% of total sulfate content.

2) The sulfate radicals of pseudo-boehmite could inhibit the crystallization process of pseudo-boehmite. When the sulfate radical concentration of pseudo-boehmite increased, the relative crystallinity of pseudo-boehmite and the crystal size decreased, and the crystal particles of pseudo-boehmite showed a tendency to become amorphous with more bound water.

3) Judging from the N2-adsorption results of the samples, the sulfate radicals barely affected the specific surface area and pore structure of alumina.

Acknowledgement:The project is financially supported by the SINOPEC (Contact No. 111010).

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Received date: 2014-04-23; Accepted date: 2014-10-15.

Dr. Yang Qinghe, Telephone: +86-10-82368123; E-mail: yangqh.ripp@sinopec.com.