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Blocking of Acidic Centers upon Hydrothermal Dealumination of ZSM-5 Zeolites.

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operations were carried out under inert gas. Yield: 0.32 g P,Se, 3 (5.62% of the
Received: October 1, 1990 [Z 4220 IE]
German version: Angew. Chem. 103 (1991) 620
Y. C. Leung, J. Waser, S. van Houten, A. Vos. G. A. Wiegers, E. H.
Wiebenga, Actu Crystullogr. 10 (1957) 574.
E. Keulen, A. Vos, Acta Crystaltogr. 12 (1959) 323.
G. W. Hunt, A. W Cordes, Inorg. Chem. 10 (1971) 1935.
G. J. Penney, G. M. Sheldrick, Acra Crystullogr. Sect. B26 (1970) 2092.
R. Blachnik, G. Kurz, U. Wickel, Z . Nuturforsch. B 3 9 (1984) 778.
B. W. Tattershall, J. Chem. Soc. Dalton Trans. 1987, 1515.
R. Blachnik, W. Buchmeier, C. Schneider, U. Wickel, Z . Nuturforsch. B 42
(1987) 47.
3: IR(Csl): v[cm-'] = 500 m, 389 s.358 vs, 322 w, 310 s, 245 w, 220 w.
NMR (121.5 MHz, 85% H,PO,, CS,, 25°C): 5 = 104.95; "Se NMR
(57.3 MHz, sat. H,SeO,, CS,, 25°C): b = - 408.24 (Se(a)), - 388.28
(Se(c));. 'J(P(A)Se(a)) = - 243.49(6) Hz, 'J(P(C)Se(c)) = - 283.6(3) Hz,
2J(P(A)Se(c)) = 14.7(7) Hz, 'J(P(A)P(C)) = 181.0(2) Hz; secondary isotope shifts of the 77Se-containingisotopomers, relative to the I I P main
spectrum: 'dP(A)(Se(a)) = 0.0046, 'dP(C)(Se(c)) = + 0.0044 and
'dP(A)(Se(c)) = 0.0013 ppm. MS (El, 70eV): mjz 458 (P,SeF, 13%),
300 (P,Sey, 24), 222 (P'Sef, 3, 191 (PSey, 9), 160 (Sef, 20), 142 (P,Se@,
13) 111 (PSee, 100).
Programm NUMARIT, S.E.R.C. NMR. Program Library, Daresbury.
B. W. Tattershall, R. Blachnik, H. P. Baldus, J. Chem. Sac. Dalton Trans.
1989. 977.
J. J. Berzelius in Gmelins Hundbuch der anorganischen Chemie, System
No. 9-12. Verlag Chemie, Leipzig 1942, p. 244.
M. V. Kudchadker, R. A. Zingaro, K. J. Irgolic, Can. J. Chem. 46 (1968)
Y Monteil, H. Vincent, Z. Anorg. Allg. Chem. 428 (1977) 259.
D. L. Price, M. Misawa, S. Susman, T. I. Morrison, G. K. Shenoy, M.
Grimsditch, J. Non-Cryst. Solids 66 (1983) 443.
(smaller amount of p-xylene), i.e. to a diminished shape sel e c t i ~ i t y61. ~This
~ ~ is explained in terms of a partial extraction
of Ale, and the associated exposure of blocked centers on the
inner and outer surfaces of the zeolite.[61Since the reaction
of m-xylene on ZSM-5 zeolites is disguised by internal mass
transport effects,r51which vary according to the concentration of AI,,, it cannot be decided on the basis of activity and
selectivity data whether the blocking of acidic centers by Ale,
or the exposure of such centers is responsible for the effects
observed during the extraction.
The present communication demonstrates for the ZSM-5
zeolites discussed in Refs. [3 -61, by checking their catalytic
activity, that the extraction of Ale, does not expose acidic
centers, and therefore the prior hydrothermal dealumination
has not blocked acidic centers. The isomerization of mxylene and the conversion of ethylbenzene served as test
reactions. Due to the smaller kinetic diameter of the ethylbenzene the latter reaction is not disguised by internal mass
transport effects.
Starting from the NH,-form of the template-free synthesized ZSM-5 zeolite HS 30, samples with various degrees of
dealumination by hydrothermal treatment were prepared
and the catalytic activity of the dealuminated samples and
of the untreated zeolite were determined, both without as
well as after extraction with 2 M HNO, (385 K, 2 h). Further details of the experimental determination of the
Si/Al(lattice) ratio, of the acidity characterization, and of the
measurements of the catalytic activity are given in the References [4-6, 81. The samples investigated here are listed in
Table 1.
Table 1. Zeolite samples investigated [a]
Blocking of Acidic Centers upon
Hydrothermal Dealumination of ZSM-5 Zeolites
By Manfred Richter *
Treatment of the ZSM-5 zeolites with water vapor at high
temperatures removes aluminum from lattice positions and
thus reduces the concentration of acidic Brsnsted centers.
The question whether the extra-lattice aluminum species
(Alex)block acidic centers of the lattice during the dealumination is controversial. Vogt et aI.,['l e.g., reported an increase in the concentration of acidic centers after dealumination with HC1 in the case of the zeolite ZSM-5. Apparently,
the acid treatment leads to the extraction of Ale, and thus to
exposure of lattice Brsnstedt centers. Muvrodinovu et al. report analogous findings in the case of the zeolite US-Y: on
the basis of the NH, thermodesorption, after extraction of
Ale, both an increase of the total concentration of acidic
centers as well as a modification of the distribution of acidity
is observed. The authors concluded that Ale, species partially
neutralize lattice Brsnsted centers. In the case of the ZSM-5
zeolites T3 and HS 30 (Chemie AG, Bitterfeld), which were
synthesized without organic templates, IR spectroscopic investigations and NH,-thermodesorptionr3~41 show that neither a further dealumination occurs nor does the number of
acidic centers increase upon extraction of hydrothermally
dealuminated samples with 1 M HNO,. However, the
isomerization of m-xylene on samples treated this way leads
to higher conversions and to a modified isomeric ratio
Dr. M. Richter
Institut fur Physikalische Chemie, Bereich Katalyse
Rudower Chaussee 5 , 0-1 199 Berhn-Adlershof (FRG)
Angew. Chem. Ini. Ed. Engl. 30 (1991) No. S
2 h, 775 K
shallow bed
hydrothermal treatment at 775 K
1 h[b]
Si/Al (lattice)
Ale, (total)[c]
Al,, (extr.)[d]
[a] Starting material was the ZSM-5 zeolite H530 in the NH,-form, Si/Al (lattice) = 20. [b] Additional calcination 2 h at 775 K under shallow bed conditions
(ca. 2 cm layer height). [c] Concentration of Ale, after dealumination, data in
mmol Al g-', total AI content 0.871 mmol g-I. [d] Amount of the soluble Al,,
in mmolg-' after extraction with 2 M HNO, (385 K, 2 h).
The isomerization of m-xylene on the zeolite samples
shows (Fig. 1 a) that at Si/AI (lattice) ratios > 30 the conversions decrease, the extraction of about 20% of Ale, (cf.
Table 1) however increase the conversions by maximally
20 YO.Since a loss of activity on continuing dealumination is
unavoidable (the concentration of acidic centers is reduced
by the transformation of A1 of the zeolite lattice into Alex),
the influence of an additional blocking of acidic centers by
Ale, cannot be estimated. The higher activity of the extracted
samples would be reconcilable with the concept that the extraction of Ale, exposes acidic centers. The lower shape selectivity after the extraction (lower p-lo-xylene ratio (Fig. 2)
would come about by the exposure of blocked centers on the
outer zeolite surface, which is not shape-selective. This interpretation is not tenable however if one takes into consideration the results of the ethylbenzene conversion under the
same conditions (Fig. 1 b). In the case of the ethylbenzene
conversion an extraction of acid-soluble Ale, species has no
recognizable influence on the activity of the samples, i.e.
acidic centers are apparently not exposed by the extraction.
Verlugsgesellschufi mbH, W-6940 Weinheim, 1991
0570-0833[91j0S0S-060?S 3.50
+ .25/0 607
Fig. 1. Dependence of the conversion X of m-xylene (a) and ethylbenzene (b)
on the Si/AI (lattice) ratio of the ZSM-S zeolites. o samples dealuminated, not
extracted, 0 samples dealuminated. extracted. Reaction conditions: normal
pressure flow reactor, carrier gas N, (10 L h-l, 1 vol % C8), 1 g zeolite (without binder), reaction temperature 623 K. ---thermodynamic equilibrium conversion of the m-xylene at 623 K [12].
A blocking of these centers by the Ale, species formed in the
hydrothermal dealumination can therefore be ruled out (under the plausible assumption that both test reactions take
place, in principle, at the same acidic centers). The activity
maximum, which is observed for all samples in dependence
on the Si/AI ratio, could, as in the case of n-hexane conversion on the H form of ZSM-5J9. be attributed to a special
Fig. 2. Ratio R of the p-lo-xylene formation as a function of the m-xylene
conversion for Sample 4. o sample dealuminated, not extracted, 0 sample
dealuminated, extracted. Reaction temperature 573 K. Conversion was
changed by variation of residence time. --- thermodynamic equilibrium value
at 573 K [12].
0 VCH Verlagsgesellschafi mbH, W-6940 Weinheim. 1991
interaction of the substrate with Brernsted centers and Ale,
species. Since the dependence of the activity on the Si/AI
(lattice) ratio of the zeolite does not change, even after extraction of the soluble Ale, species, it is conceivable, as in the
case of the substrate n-hexane, that insoluble Ale, species are
decisive for the modified interaction of the substrate with the
active centers. Hence, the following conclusions can be
1. The modification of activity/selectivity in the isomerization of m-xylene is only apparently determined by a
blocking or exposure of acidic centers on the internal/external surfaces of the ZSM-5 zeolites.
2. The results of the ethylbenzene conversion confirm the
IR s p e c t r o s ~ o p i cand
~ ~ . the
~ ~ NH,-desorption data:r41 an
extraction of Ale, neither exposes acidic centers nor leads to
the formation of new acidic sites.
3. The modification of activity/selectivity in the isomerization of rn-xylene is obviously a result of the mass transport
influenced by Alex.This not only concerns the diffusion of
the isomers but also the movement of rn-xylene in the pore
system. The reaction takes place in the range of internal
diffusion as is suggested by the activation energy for the
m-xylene onv version.^^] Since p-/-o-xylene values above the
thermodynamic equilibrium ratio (Fig. 2) are also observed
on the extracted catalysts, the mass transport barriers are
still not completely removed with the removal of the acidsoluble Ale, species.
Received: December 20, 1990 [Z 4348 IE]
German version: Angew. Chem. 103 (1991) 606
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(1990) 167-117.
[7] Starting from the NH,-form of the zeolite, the hydrothermal treatment is
accomplished by passage of water vapor (ca. 1.5 mol h-') at 775 K for the
times quoted in Table 1. In the case of shorter hydrothermal treatment an
additional calcination was necessary for complete conversion of the NH,form into the H-form. The H-form of the non-dealuminated sample was
prepared by thermal decomposition of the NH,-fonn.
[S] W. Schwieger, K.-H. Bergk, E. Alsdorf, U. Lohse, B. Parlitz, Z . Phys.
Chem. (Leipzig) 271 (1990) 243-257.
[9] R. M. Lago, E. 0. Haag, R. J. Mikovsky, D. H. Olson, S. D. Hellring,
K. D. Schmidt, G. I. Kerr in L. Murakami, A. Iijima, J. P. Ward (Hrsg.):
New Developments in Zeolite Science and Technology (Proc. 7th Inr. Zeoliie
Conf. Tokyo 1986) Elsevier, Amsterdam 1986, S. 677-687.
[lo] V. L. Zholobenko, L. M. Kustov, V. B. Kazansky, E. Loefler, U. Lohse,
C. Peuker, G. ohlmann, Zeolites 10 (1990) 304-306.
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[12] W. W. Kaeding, C. Chu, L. B. Young, B. Weinstein, S. A. Butter, 1 Caial.
67 (1981) 159-174.
0570-0833/91/0505-06083 3.50+ .25/0
Angew. Chem. tnr. Ed. Engl. 30 (1991) No. 5
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upon, zeolites, hydrothermal, acidic, zsm, blocking, center, dealumination
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