Jan-2015

The form of sulphate in pseudo-boehmite and its effect on properties of pseudo-boehmite

γ-Al2O3 is 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.

Zou Sumeng, Yang Qinghe, Zeng Shuangqin and Nie Hong
SINOPEC Research Institute of Petroleum Processing

Article Summary

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 neutralisation reaction between an acid and an aluminum-containing alkali; the neutralisation 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 NaAlO2 solution on Al2 (SO4)3 solution1 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 specific effects of the impurities on the properties of pseudo-boehmite and alumina.

Impurities in alumina prepared by the reaction of NaAlO2 solution on Al2 (SO4)3 solution mainly include Na2O and SO42- radicals. The effect of Na2O on the alumina property has been reported2, but the effect of sulphate radicals on alumina is still not thoroughly known. The sulphate radicals in alumina are mainly introduced by the following ways: (1) through materials like Al2 (SO4)3 solution during the preparation of pseudo-boehmite; (2) through sulphatecontaining extruding additives during the extruding or kneading process of supporters; (3) through the transformation of sulphur element into sulphate radicals introduced by sulphurisation agents in the course of reaction; and (4) through sulphate-containing active ingredient introduced during the impregnation process. Currently, there are a lot of literature reports about the effects of extraneous sulphate radicals on the alumina support. However, in these documents, the sulphate radicals were deliberately introduced by external sources3-6. There are few documents referring to the effects of the sulphate radicals introduced through the materials for preparation of pseudo-boehmite. In this study, four samples were prepared with different sulphate concentrations. The existing form of sulphate radicals in pseudo-boehmite was studied, and their effects on pseudo-boehmite and surface properties of alumina were investigated.

Experimental
Preparation of pseudo-boehmite
Aluminum sulphate was dissolved in water to obtain a solution of Al2 (SO4)3 with an Al2O3 mass concentration of 60 g/L. Aluminum trihydroxide and caustic soda were dissolved in water at 100°C to obtain a NaAlO2 solution with an Al2O3 mass concentration of 230 g/L. The neutralisation reaction of NaAlO2 solution on Al2 (SO4)3 solution was conducted at a specified pH value and temperature. Then the gel obtained was filtered. Deionised water was added to the filter cake and the agitator was used to mixing them evenly. The mass of deionised 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 deionised water. Some different filter cakes of pseudo-boehmite 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 sulphate concentration. Then, the sulphate in filtrate was analysed and the remainder squeezed filter cake was dried at 120°C for 8 h. Another part of filter cake was directly dried at 120°C for 8 h, and the sample was denoted as PB-X

Characterisation
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 2θ was between 5° to 70°.

Specific surface area and pore volume were obtained from nitrogen adsorption-desorption isotherms measured in an Autosorb-6B analyser. The samples of pseudo-boehmite were both annealed at 600°C for 3 h. Sulphate 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 sulphate radicals were obtained by the length and intensity of the characteristic radiation wave. The measurements of sulphate radicals in pseudo-boehmite were carried out by an EMIA-820V type infrared carbon sulphur analyser. 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.

Results and Discussion
The existing form of sulphate radicals in pseudo-boehmite
Some researches⁷ indicated that sulphates in pseudo-boehmite can be divided into two classes, the soluble sulphate and the insoluble sulphate. The soluble sulphate 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 sulphate has the form of aluminium hydroxide sulphate because of incomplete reaction, which exists in the structural framework of the gel.

In the present experiment, the amount of sulphate radicals in the filtrate and filter cake were analysed and calculated and the results are shown in Table 1. As regards the filter cake of these samples, the proportion of soluble sulphate radicals in the total amount of sulphate was low, which was less than 0.8%. Also, some relationships were found among the total sulphate and the sulphate radicals in filtrate and filter cake. With the increase in total sulphate concentration, the amount of sulphate radicals in filtrate and the proportion of insoluble sulphate radicals of filter cake in total amount of sulphate increased. For a better understanding, the following calculation equations are listed:
Sulphate radicals in filtrate = sulphate radicals in filtrate/the density of solution Soluble sulphate radicals in filter cake = difference of filter cake mass before and after drying × sulphate radicals content in filtrate/mass of filter cake after drying Insoluble sulphate radicals in filter cake/total sulphate radical concentration in filter cake = 1-soluble sulphate radicals in filter cake/total sulphate concentration in filter cake
 
Effects of sulphate radicals content on pseudo-boehmite
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-boehmite⁸. 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 crystallinity⁹. The crystal grain size of these samples was determined by calculating the half peak width according to the Scherrer equation¹⁰. 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 reported3, 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 sulphate concentration. The phenomenon was in good agreement with those references^10-11 which considered that the sulphate radicals had hindered the contact between adjacent crystal particles. Therefore, the crystallisation process of pseudo-boehmite was inhibited by the presence of sulphate anions in the gel, and then the sulphate concentration affected the relative crystallinity of pseudo-boehmite. As the crystallisation process was inhibited by more sulphate anions, the grain size became smaller.