Synthesis Of Superacid Wox/zro2(pt) Catalysts
SYNTHESIS OF SUPERACID WOx/ZrO2(Pt) CATALYSTS
FOR ISOMERIZATION OF n-HEXANE
V. V. Brei1, M. M. Levchuk2, O. V. Melezhyk1, K. Patryliak2
1Institute of Surface Chemistry, NAS of Ukraine, Kyiv, Ukraine
2Institute of Bioorganic and Petroleum Chemistry, NAS of Ukraine,
Kyiv, Ukraine
At present, WOx/ZrO2 systems are intensively studied as promising superacid catalysts. The familiar method for producing such useful materials involves stages of impregnation of Zr(OH)4 with a solution of ammonium metatungstate, calcination at 870 – 1170 K, and reduction with dihydrogen at 570‑670 K. Mixed Zr-W hydroxides are also prepared by coprecipitation with ammonia from a solution containing ZrOCl2 and ammonium metatungstate. We supposed that ageing of mixed solutions should result in formation of sols or gels with a heteropolynuclear structure. With a view to adjust a degree of hydrolysis, we varied time of boiling and pH of the solution and added an organic base to the solution. In principle, such a refined procedure is more flexible and versatile, and raises new possibilities of exerting control over porous structure and synthesis of modified WO3/ZrO2 samples, for instance, doped with SiO2, Al2O3, and Nb2O5. The structures and acid properties of the samples have been studied by XRD, ultraviolet reflectance spectroscopy, thermogravimetry, and desorption mass-spectrometry techniques. The samples have been tested in reactions of cracking of cumene and isomerization of n-hexane.
As is seen from the Table, the WZrPt samples show a high activity in reactions of isomerization of n-hexane. The products of these reactions are mainly 2- and 3-methylpentanes, and 2,2-dimethylbutane, with some impurities standing out as products of cracking (C1 – C5) and condensation (C6+).
At 230 – 250 °C the yield of isomers is in the 65 – 70 % interval, selectivity for i-C6 amounts to 70‑94%, and degree of hexane conversion is about 80 %. Similar results are produced for H(Pd)-mordenite at 280 – 300 °C and for ZrO2/SO4/Pt at 230–250°C. In the case of the WZrPt catalysts, however, the attained contents of the most valuable product, namely 2,2-dimethylbutane, are higher (2,2-DMB/(i-C6) amounts to 17‑21wt.%).
Modification of WO3/ZrO2 samples with highly dispersed silica and alumina sharply reduces the yield of hexane isomers. The main product is benzene, with other aromatic hydrocarbons being present as impurities. It has been proved that WOx/ZrO2-Al2O3(SiO2)(Pt) samples can primarily catalyze processes of dehydrocyclization of n-hexane. The introduction of Nb2O5 into such mixed catalysts decreases the hexane conversion degree, but the selectivity for i-C6 remains high.
Table. Activity and selectivity of WOx/ZrO2(Pt) catalysts in the reaction of n-hexane isomerization.
Content of Pt is 0,5 wt%. P=3Mpa, H2:C6H14=1:1, LHSV= 1 h-1.
Catalyst
T,oC
Conver-sion of
n-C6H14%
Selec-tivity For
Si-C6, % wt
Content
Of 2,2-DMB*
In Si-C6, % wt
Content of products, % wt
i-pen-
tane
n-pen-
tane
2,2-DMB
2-me-
thyl-pen-tane
2-me-
thyl-pen-tane
WZrPt-1
230
270
81,1
78,8
93,8
87,9
21,5
16,0
2,7
5,2
1,0
2,2
16,3
10,9
39,0
36,1
20,8
21,3
WZrPt-2
230
250
270
79,8
80,8
86,8
96,3
88,4
69,0
18,7
16,8
20,3
1,2
3,7
12,4
0,5
1,4
4,4
14,4
12,0
12,1
40,6
39,0
29,7
21,9
20,5
18,1
WOx/ZrO2-Nb2O5(Pt)
230
270
41,9
65,1
97,1
96,0
3,4
11,3
0
0,9
0,1
0,3
1,4
7,1
25,9
36,6
13,4
18,8
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