React. Kinet. Catal. Lett., Vol. 22, Nos 1-2, 23-27 (1983)

BORON MIGRATION AND T H E R M A L STABILITY OF POROUS GLASS CATALYSTS

F. Janowski, W. Heyer and F. Wolf

Martin-Luther-Universit~it, Sektion Chemie, Wissenschaftsbereich Technische Chemic, DDR-4020 Halle, Schloi~berg 2

Received September 7, 1982 Accepted October 18, 1982

Surface boron concent ra t ion o f porous glasses f rom sodium borosilicate glass was deter- mined in dependence on heat ing be tween 5 0 0 - 7 0 0 ~ The inf luence o f boron migra- t ion to surface on dispersity and activity for dehydrocycl iza t ion o f n-hexane on porous glass-plat inum catalysts was investigated.

]~bma onpeRes~ena KOrlKerrrpatil4~ 6opa Ha noBepXHOCTH nopncrbIX er6"Kon n3 HaTpHe- ~oro 6opoKpeMnea6Mrloro ereKna B 3aBrlCaMOC'rri OT cTenenrl HarpeBann~ nocne~mero B TeMneparypnoM nHTepBane Meg;ay 5 0 0 - 7 0 0 ~ C. Bl, ino nccne~toBaao Bnrlamfe ~rar- paunH 6opa K IIoBepxHoCTH na ~cnepcnocT~, n aKTrmnoerb n o p n c r o r o cTeKno-naarn- HOBOFO KaTa~naaTopa, IlpejJjta31taqenHoro ~ Jlern/Ipot~KnH3at~ari n-reKcaHa.

The use of porous glasses as adsorbents and catalyst supports requires information concerning the thermal stability and surface chemistry as well. Porous glasses result- ing from extracted (phase-separated) sodium borosilicate glasses still contain residual boron oxide and also sodium oxide in the form of unextracted separation ranges inaccessible for extraction media/1/ . It is supposed that a migration of boron takes place from unextracted separation ranges to the surface of porous glasses. According to Zhdanov/2/, boron migrates from bulk phase to surface during heating at tem- peratures about 600 ~ due to a change of the coordination number of boron from four to three. Other investigations /3/ have revealed the connection between thermal stability of porous glasses and the composition of the initial glass and especially the ratio of network-SiO2 and finely dispersed SiO2 /1/ deposited in the pore system, too. It was also affirmed that the utility of microporous glasses as catalyst supports is limited by their unsatisfactory thermal stability /4/.

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JANOWSKI et al.: BORON MIGRATION

For this reason we have investigated the dehydrocyclization (DHC) of n-hexane over porous glass supported paltinum catalysts in dependence on heating of the porous glass supports from 500 ~ up to 700 ~ for 25-150 h. The surface con- centration of boron was measured by subsequent acid treatment for resolving the surface boron of the heated porous glasse~ Specific surfaces of porous glasses as well as the platinum dispersities of the corresponding metal catalysts were also deter-

mined.

EXPERIMENTAL

Porous glass. An initial sodium borosilicate glass (70 wt. % SiO2, 23 wL % B2 03 and 7 wt. % Na2 O) was used with an average particle size of 0.3-0.5 mm for making porous glasses. The preparation is given in more detail in Ref. /1, 5/.

Platinum catalysts. Porous glasses were impregnated with a solution of H2 PtC16 to a content of 0.5 wt. % platinum. Dispersity of platinum on different porous

glasses was measured by means of CO adsorption. Surface boron. Specific surfaces of porous glasses were determined by BET

based on N2 adsorption. 1"he determination of boron oxide concentration was performed by chemical analysis from the acidic extraction solution of heated porous

glasses. Catalytic measurements. Dehydrocyclization of n-hexane (500 ~ 0.5 LHSV) was

carried out in an isothermally operated 5 ml flow reactor at normal pressure. Hyd- rogen as carrier gas was used with a hydrogen to hydrocarbon molar ratio of 6. Before runs catalysts were activated in flowing hydrogen at 500 ~ The analysis of the reactor effluent samples was accomplished by GC.

RESULTS AND DISCUSSION

Figure 1 shows the DHC activity (expressed as benzene yield after the 1 h and 25 h run) of the different catalysts based on porous glasses which underwent heat- ing at temperatures between 500 ~ and 700 ~ from 25 h up to 150 h. For com- parison the surface areas are given as well as the surface boron concentrations of porous glasses and the platinum dispersities of the corresponding catalysts. From Fig. 1 it follows that the DHC activity (initial activity) depends on the dispersity of Pt catalysts. There is an unimportant decrease of the surfaces after heat,_'ng porous glasses at 500 ~ In the 500 ~ interval of heating the activity is maintained although there is a decrease of dispersity during the 25 h run. Therefore, it is suppo~d that

24

JANOWSKI et al.: BORON MIGRATION

I0:I

20:1

30:1 wt .%

50 L0 30 2O 10

m2/g 120

80

L0

H/Pf 0.4

0.3

0.2

0.1

~ , , , . x: I

~203/Na20 ,,

, i

I '

3 enzene ~x- - -x-,,,: }

1

Surface

[

t

~ 'N I j , \ i �9 . [

Disperslfy I x '~-x-- : I I I ] I I I ~"

2550 100 150 50 100 150 Time (hrs)

S-

\

L 25 150

t -~-- 5 0 0 ~ 6 0 0 -'---~ 7 0 0 oC

Fig. 1. Influence o f the heating o f porous glasses on the properties o f the corresponding Pt ca- talysts (surface boron, benzene yield, specific surface and Pt dispersity) - - after 1 h run; . . . . after 25 h run

dispersity of platinum on porous glasses is not alone the deciding criterion for the DHC activity of porous glass catalysts.

In the range of heating interval at 600 ~ the decrease of surface is not dramatic but there is a significant difference in dispersity and activity of the corresponding catalysts after the 1 h and 25 runs.

From the concentration of Surface boron it can be seen that at 600 ~ (150 h) a maximum is reached at the ratio B203/Na20 of 35 : 1. The reducing effect of boron on the dispersity of platinum is well known /6/.

After heating the porous glasses at 700 ~ the SiO2 skeleton begins to sinter. The surface is diminished in dependence on heating time and alkali content. The ratio B2 O3/Na20 then corresponds to that of the initial glass without boron mig-

25

JANOWSKI et al.: BORON MIGRATION

Table 1

Influence of surface boron on the catalytic activity of porous glasses heated at 600 ~

Porous glass

Specific surface (m 2/g) Average pore radius (nm) Surface boron (rag Ba O~/m 2 )

n-Hexane conversion (DHC)

Benzene after 1 h (wt. %) Benzene after 25 h

Heating at 600 ~

25 h 50 h 50 h a

95 92 97 1.7 1.7 1.7 0.010 0.047 0.005

0.5 wt. % Pt/Porous glass

44 47 42 47 10 40

aAfter heating, the porous glass has been extracted with 3 N HC1 for removing surface boron oxide

ration caused by heating. In Table 1 is given a comparison of three different cata-

lysts based on porous glasses heated at 600 ~ the latter sample has been acid

treated in order to remove the surface boron oxide which results from migration

during heating. Whereas the dispersity and surface of the samples are nearly con-

stant, the difference is given by the surface boron concentration. It is obvious that surface boron diminishes the dispersity of platinum on porous glasses. It is also reasonable to suppose a lower life time level for platinum catalysts supported on

porous glasses with higher surface boron concentrations. Removing the boron oxide from the surface leads to more active catalysts having/DHC activities (25 h) com- parable with those of porous glasses with low boron concentrations on the surface.

CONCLUSION

Microporous glasses as catalyst supports are thermally stable at temperatures up

to 700 ~ Evidence is given for an actual migration of boron oxide from the bulk phase to the surface of porous glasses during heating at about 600 ~ A stabi- lizing effect on catalyst activity by removing the boron oxide by acid treatment of

heated porous glasses used as catalyst supports has been shown.

26

JANOWSKI et al.: BORON MIGRATION

REFERENCES

1. F. Janowski, W. Heyer: Por6se Gl~iser. VEB Deutscher Verlag l'fir Grundstoffindustrie, Leipzig 1982.

2. S. P. Zhdanov: Dokl. Akad. Nauk. SSSR, 21 7, 581 (1974). 3. W. Heyer: Chem. Techn. Leipzig, 32, 86 (1980). 4. US 3,923,688. 5. F. Wolf, F. Janowski, W. Heyer: Chem. Techn. Leipzig, 28, 491 (1976). 6. H. Klotsche, C. Szkibik: Freiberger Forschungshefte,A 474, 7 (1969).

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