Effect Of Fluoride And Preparation Methods
Effect of fluoride and preparation methods
on n-butylamine and n-butylmercaptane chemisorption on CoMo/Al2O3 catalysts
Z. Sarbak, A. Pogorzelska
Faculty of Chemistry, Adam Mickiewicz University,
Grunwaldzka st. 6, 60-780 Poznan, Poland
Cobalt-molybdenum catalysts are mainly applied in hydrofining reactions such as hydrodesulfurisation and hydrodenitrogenation. In order to increase the activity of hydrogenolysis of the C-N and/or C-S bonds, the catalysts are subjected to appropriate modification, e.g. by introduction of fluorine ions. These ions can be either introduced simultaneously with cobalt and molybdenum or in a two-stage process before or after those of cobalt and molybdenum. Depending on the mode of the fluorine ions introduction the catalysts obtained show different activity. The paper reports results of a study of n-butylamine and n-butylmercaptan chemisorption on the above-described samples and for comparison also catalysts containing sodium ions were used.
The catalysts CoMo/Al2O3 (denoted as CoMoAl) were prepared by the method of impregnation of g-Al2O3 (180 m2/g) with solutions of Co(NO3)2, (NH4)6Mo7O24 and NH4F or NaNO3. After each stage of impregnation the samples were dried for 4 hours at 120° C and calcined at 500° C for the night. The composition of the samples and the sequence of the ion introduction are presented in Table 1.
Table 1. The composition and preparation modes of the catalysts
Samples
CoO
(% wag.)
MoO3
(% wag.)
Atomic ratio of introduced ions to 20 Al
Procedure
1
2
3
4
5
CoMoAl F (0)
3
15
0
One-stage
CoMoAl F (1)
3
15
1
One-stage
CoMoAl F (2)
3
15
2
One-stage
CoMoAl F (4)
3
15
4
One-stage
1
2
3
4
5
CoMo(I) F (II)
3
15
2
Two-stage, CoMo followed by F
CoMo(II) F (I)
3
15
2
Two-stage, F followed by CoMo
CoMoAl Na (2)
3
15
2
One-stage
CoMo (I) Na (II)
3
15
2
Two-stage, CoMo followed by Na
CoMo(II) Na (I)
3
15
2
Two-stage, Na followed by CoMo
Chemisorption of n-BA on the catalysts CoMo/Al2O3 was studied by gas-chromatography. Adsorption was conducted in a column adsorber of 10cm in length and 1 cm in diameter, equipped with a steel net. The net was covered with a layer of quartz cotton wool then with a layer of the catalyst in the amount of 200 ± 0,2 mg, which was covered with 1 cm thick layer of quartz wool. The adsorber was connected to a gas chromatograph W. Giede GCHF 18 through which a stream of nitrogen, used as a carrier gas, was passed at a rate of 30 ml/min. The catalyst was activated at 400° C, maintained for 1 hour. On the catalyst 10 ml of n-BA was introduced by injection with a Zimmermann syringe. The injections were repeated 5 times at 15 minutes intervals. The experiment was repeated three times. The degree of adsorption was found from the formula: X = B-A/B × 100 %, where A is the mean area under the peak after the injection, while B is the mean area under the peak after the injection directly onto the chromatographic column. The chemisorption of n-BM was studied in an analogous way.
The results presented in Fig. 1. have shown that the chemisorption of the adsorbed compounds depends on the amount of the fluorine ions introduced.
Fig. 1. Effect atomic ratio F/20Al. On chemisorption of n-butylamine (D) and n-butylmercaptane (O)
The chemisorption of n-butylamine increases with increasing concentration of fluorine ions, which can be explained by generation of Bronsted acidic centres and enhancement of the already present Lewis acidic centres.
F O –– H+
Al Al Al
O O O O
The chemisorption of n-butylmercaptane does not show a strong dependence on the concentration of fluorine ions. The obtained results do not suggest that the sequence of the ion introduction during the catalyst preparation significantly affects the chemisorption of n-BA and n-BM.
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