For class (ii) the name clathrasils has recently been introduced s to indicate that these phases are clathrates with host lattices based on silica frame- works,

The name zeolites stands for the phases of class (iii).

The clathrasils can be subdivided according to their fundamental cages from which the tetrahedral framework cart be constructed. Melanophlogite would then be considered a tetrakaisil since its fundamental cagc is a 14-hcdron Is. In contrast, the dodecasils can be constructed by direct linking of dodecahedra s.

According to this structural classification of frame- work silicates the dodecasils are clathrasils and should not be called zeolites a because they do not show zeolitic properties but rather retain their guest molecules up to ca. 900°C s.

ACKNOWLEDGEMENT

I thank Dr. R. A. Howie for assistance in devising the term pyknolite and Drs. L. S. Dent Glasser and J. A. Gard for critically reviewing the manu- script.

REFERENCES

Short Papers

1 Flanigen, E. M., Bennett, J. M., Grose, R. W., Cohen, J. P,, Patton, R. L., Kirchner, R. M. and Smith, J. V. Nature 1978, 271,512

2 Bibby, D. M., Milestone, N. B. and Aldrige, L. P. Nature 1979, 280,664

3 Wilson, S. T., Lok, B. M,, Messina, C, A., Cannan. T. R. and Flanigen, E. M.J. Am. Chem. Soc. 1982, 104, 1146

4 Schlenker, J. L., Dwyer, F. G.. Jenkins, E. E.. Rohrbaugh, W. J., Kokotailo, G. T. and Meier, W. M. Nature 1981, 294, 340

5 Gies, H., Liebau, F. and Gerke, H. Angew. Chem. Int. Ed. 1982, 21,206

6 Gies, H., Gerke, H. and Liebau, F. Neues Jb. Miner. Mh. Jg. 1982, 1982, 119

7 Wells, A, F. "Structural Inorganic Chemistry', 4th Edn. Oxford University Press, 1975

8 Wells, A. F., I.¢.p. 824 ff 9 Loewenstein, W. Am. Mineral 1954, 39, 92

10 Sahl, K. Z. Kristallogr. 1980, 152, 13 11 Gebert, W. Z. Kristallogr. 1972, 135, 437 12 Gerke, H., Gies, H, and Liebau, F. Bunsentagung 1982 der

Deutschen Bunsen-Gesellsehaft, UIm 1982 13 Kokotailo, G. T., Lawton, S. L., O~son, D. H. and Meier,

W. H. Nature 1978, 272,437 14 Penzhorn, R.-D. and Schuster, P. DBP 2 948 515 (19811 15 Kokotailo, G, T., Chu, P,, Lawton, S. L. and Meier, W. H.

Nature 1978, 275, 119 16 Rees, L. V. C. Nature 1982, 296, 491 17 Jarchow, O, and Klaska, K.-H. -7. Kristallogr. 1981,155, 64 18 Gies, H. and Liebau, F. Acta Crystallogr. 1981, A37, $187

Transition-state selectivity of H-ZSM-5 in isomerization of n-butenes Susumu Tsuchiya and Seiki Ito Department of lndustrial Chemistry, Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan

and Seitaro Namba and Tatsuaki Yashima Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan (Received 10 February 1983)

The isomerization of n-butenes took place over H-ZSM-5 and H-Y zeolite catalysts, The rates of the reaction and the activation energies were measured. Iz was found that H-ZSM-5 acted as a solid 13ri~nsted acid which appeared to suppress the cis-trans isomerization, i t is strongly suggested that this suppression is due to a kind of molecular shape-selectivity, transition-state selectivity.

Keywords; H-ZSM-5 zeolite; isomerization; butene; transition-state selectivity

Much attention has been paid to ZSM-5, synthe- sized by Mobil t' 2, which has recently shown very interesting properties for the conversion of methanol and other compounds to hydrocarbons, especially gasoline with high octane number 3-6. In this paper, we studicd the isomerization of n- butenes, which is the most fundamental and suggestive reaction, to explorc the catalytic properties of H-ZSM--'5.

A closed circulating system having a U-shaped reactor with a vacuum line and a gas-chromato- graph were used to follow the reaction. The ZSM-5 type zeolite was synthesized in a similar manner to the patent 7. The ZSM-5 zeolite pre- pared was idcntified by its X-ray diffraction pattern 8. After the calcination at 773 K for 3 h, the Na cations in the zeolite were removed with 1 mol 1 -l HCI at 333 K for 1 week to give its acid

0144-24491831030193-02503.00 © Butterworth & Co. (Publishersl Ltd. ZEOLITES, 1983, Vo13, July 193

Short Papers

Table 1 The rate constants at 323 K and the activation energies of the isomerization of n-butenes over H-ZSM-5 and H -Y zeolites

Rate constant Activation energy (X 106 min -z m -2) (kJ mol -=)

Reactant Product H-ZSM-5 H-Y H-ZSM-5 H-Y

1-butene trans-2-butene k l t : 92.3 85.6 55.0 51.9 cis-2-butene k lc: 78.9 104.8 57.3 49.4

cis-2-butene 1 -bu tene kcl : 9.52 12.6 63.6 52.5 trans-2-butene kct: 6.99 29.7 82.8 60.6

trans-2-butene cis*2-butene ktc: 2.58 10.96 87.4 66.0 1-bu tene k t l : 4.11 3.82 65.9 60.4

form, H-ZSM-5 (Si/A1 = 49, Na/A1 < 0.005). NH4-Y zeol i te was prepared with SK40 (Na-Y) and 1 mol 1 -I NH4C1 aqueous so lu t ion using a convent iona l ion exchange m e t h o d at 333 K, the degree of ion exchange being 80%. NH4-Y was conver ted to H - Y by calc inat ion at 670 K for 2 h. Before each run, the catalysts were evacuated at 670 K for 2 h. n-Butenes were pur i f ied by repea ted dis t i l la t ion in vacuo and their pur i ty conf i rmed by gas-chromatography to be > 99%.

The double bond migra t ion and the cis-trans i somer iza t ion of n-butenes took place over H-ZSM-5 and H-Y catalysts . The skeletal isomeri- za t ion could no t be de tec ted and no gaseous hydroca rbons , o ther than n-butenes, could be de tec ted in the reac t ion mixtures be low 333 K of the reac t ion tempera ture . At a higher reac t ion tempera ture , e.g. 373 K, however , a variety of hyd roca rbons were formed. The i somer iza t ion react ion of butenes was a lmost f i rs t -order with respect to the pressure of reactants . F rom the rates of reac t ion and the select ivi ty ratios, all six rate constants and the act ivat ion energies of the i somer iza t ion of n-butenes over H-ZSM-5 and H-Y catalysts were de te rmined . The rate cons tants at 323 K and the act ivat ion energies are shown in Table 1.

It was found that the rates and the act ivat ion energies of the double bond migra t ion over H-ZSM-5 catalysts did no t differ great ly from those over H - Y catalyst . The rat io of the rate constants , klt[klc , over H - Z S M - 5 was a lmost un i ty as well as that over H-Y, which was one of the most typical solid acid catalysts with Br6ns ted type acidi ty . These results suggested tha t the ca ta ly t ic act ivi ty of H-ZSM-5 was caused by Br/Snsted Wpe acid sites. The rat ios of rate constants , kca [kct and ktl /ktc over H-ZSM-5 were larger than uni ty . However , those over H-Y were less than uni ty , as r epor ted by L o m b a r d o and Velez 9. The rate constants and the act ivat ion energies in the cis-trans i somer iza t ion over

H-ZSM-5 were much smaller and higher than those over H-Y, respect ively. These facts may be expla ined by the suppression of the cis-trans i somer iza t ion in the nar row space of the channel of H-ZSM-5.

The cri t ical d iameters of 1-, trans-2- and cis-2- butenes are 0.495, 0.495 and 0.558 nm respec- t ively 1°. Chert and Weisz have r epor t ed that the diffusion coeff ic ient of cis-2-butene in Ca -A zeol i te is smaller than tha t of trans-2-butene by a fac tor of more than 20011 . Therefore , the di f fus ion coeff ic ient of cis-2-butene in H-ZSM-5 may be smaller than those of the o ther n-butenes. How- ever, the results over H-ZSM-5 canno t be expla ined by the reac tan t or p roduc t select ivi ty con t ro l l ed by relative diffusivit ies, because the double b o n d migra t ion over H-ZSM-5 was no t suppressed remarkab ly . Consequen t ly , we s t rongly suggest that the suppress ion of cis-trans i somer iza t ion of 2-butenes over H-ZSM-5 is due to the t ransi t ion- state select ivi ty.

R E F E R E N C E S

1 Flanigen, E. M., Bennett, J. M., Grose, R. W., Cohen, J. P., Patton, R. L., Kirchner, R. M. and Smith, J. V. Nature 1978, 271,512

2 Kokotailo, G. T., Lawton, S. L., Olson, D. H. and Meier, W. M. ibid. 1978, 272, 437

3 Chang, C. D., Kuo, J. C. W., Lang, W. H., Jacob, S. M., Wise, J. J. and Silvestri, A. J. Ind. Eng. Chem. Proc. Des. Dev. 1978, 17,255

4 Liedeman, D., Jacob, S. M., Voltz, S. E. and Wise, J. J. ibid. 1978, 17,340

5 Chang, C. D. and Silve'stri, A. J. J. CataL 1977, 47, 249 6 Derouane, E. G., Nagy, J. B., Dejaifve, P., van Hoof-f, J. H. C.,

Spekman, B. P., Vedrine, J. C. and Naccache, C. ibid. 1978, 53, 40

7 Br. Pat. 1 402 981 8 Wu, E. L., Lawton, S. L., Olson, D. H., Rohrman Jr., A. C. and

Kokotailo, G. T. J. Phys. Chem. 1979, 83, 2777 9 Lombardo, E. A. and Velez, J. 'Molecular Sieves Zeolites -- I1'

(Ed. R. F. Gould) Adv. Chem. Set. 1971, 102, 346 10 Eberly Jr., P. E. 'Zeolite Chemistry and Catalysis' (Ed. J. A.

Rabo) ACSMonograph 1976, 171,392 11 Chen, N. Y. and Weisz, P. B. Chem. Eng. Progr. Symp. Set.

(No. 73) 1967, 63, 36

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