JournaZ of Molecuh CataZysis, 3 (1977178) 453 - 457 0 Ekevier Sequoia S.A., Lausanne - Printed in the Netherlands

453

Preliminary Nok

13GNMR study of the dehydration of methanol on a synthetic near-faujasite germanïc molecular sieve

E. G. DEROUANE*, P. DEJAIFVE and J. B. NAGY

FacuZtés Universitaires N_D_ de la Pa& Laboratoire de CataZyse. 61, rue de BruxeZZes, B-5000-Namur (Belgium)

(Receiver? September 30,1977; in revised form November 9,1977)

13C-NMR is used to study the conversion of less than one monolayer of methanol on a synthetic, germanic near-faujasite zeolite. Formation of dimethyl ether and partial methoxylation of the surface occurs at 300 “C Surface methoxy-groups are back-hydrolyzed to methanol in the presence of water at 25 “C. Small additional amounts of dimethyl ether and surface formate are also observed after the latter treatment.

Our recent 13C-NMR investigation [l] of the conversion of methanol into hydrocarbons on a new type of zeolite [2] has prompted US to use the same technique for studying the dehydration of methanol on a synthetic, near-faujasite germanic molecular sieve, which we wïll refer to as the NaGeX zeolite (Na indicates that the counterions are sodium, Ge that germanium substitutes for Si, and X refers to the zeolite type). Untïl recently, most of the investigations on the reaction of alcohols wïth oxidic surfaces have dealt with Alz03, SiOz, and Mg0 [4] : an extensive review has been proposed some time ago by Knözinger [5] _ Only a few studies were devoted to the reactivity of alcohols on zeolite surfaces [l - 3,6].

By contrast wïth the formation of surface methoxide, formate, and carbonate, which is observed in the case of Mg0 [4], methanol is usuahy dehydrated to dimethyl ether, at least in the initial reaction stages, when zeolites are used. The aim of this note is to provide a better understandïng of the nature of the methanol-zeolite interaction

The NaGeX zeolite was slowly dehydrated and activated at 300 “C over 1 h, down to a final pressure of about 2 X lom6 Torr, using the same experimen tal set-up as described previously [7] _ CH30H (92.2% enriched in 13C, fiom Prochem) was diluted to 30% v/v by normal freshly-vacuum-distilled methanol- About 0-8 monolayer of methanol was adsorbed on the zeolite referred to as NaGeX-300.

13C-NMR spectra were recorded on a WP-60 Bruker Fourier Transform NMR spectrometer, usïng broad band proton decouphng and with extemal

*To whom querïes conceming this paper should be sent.

454

lock of the magnetic field, The chemical shifts are referenced indirectly to TMS via the transmitter frequency, as described elsewhere [7] _

The chemical shift for pure liquid methanol is 49.5 ppm. Its small varïation (- 0.6 ppm) when it is adsorbed, indicates the presence of some (but weak) interaction with the support. At 300 “C (spectra 2 and 3, Fig- 1 and Table 1), the dehydration of methanol to dïmethyl ether occurs readily, as seen from the appearance of a peak at about 60 ppm, of which the inten- sity is increasing. The dehydration reaction must, however, be reversible: indeed, spectrum 4, obtained after heating at 300 “C and cooling of the system back to room temperature, shows less dimethyl ether and more methanol than spectrum 3.

TABLE 1

13C-NMR results for the dehydration of methanol on NaCeX-300 zeolite

Spectrum Reaction conditions** number*

Temp- Time (“C)

Chemical shift Lïnewïdth IdentïfïcationfIntensityft (ppm from TMS) (Hz)*** (%)

1 250 5min 48-9 61 A 100 2 300 30 min 49.7 52 A 86.5

59.4 - B 13.5 3 300 1-5 h 49.4 61 A 68.2

60.6 70 B 31.8 4 300 1.5 h 49.0 61 A 82.9

25 2.0 h 58.7 - B 17.1 5 300 6.0 h 49.1 85 A 74.3

57-0 - B, B 25-7 6 300 16.5 h 47.7 70 A 36.9

56.7 10 B, B 63.1

*Spectra characteristicsr 1 - 4: 100 stans; 5: 400 scans; 6: 2 000 seans. Instrumental line broadeningr 5 Hz_

**Successive treatments of the same sample- ***Linewidth measured at half-height.

TA. Methanol, B. Dimethyl ether, B’_ Surface methoxy-groups. #TotaI spectrum intensity normalized to 100%; relative intensities of the 13C-NMR peaks as calcuIated by triangulation- Values are only indicative, the nuclear Overhauser effect (NOE) differing from one 13C nucleus to anothër.

Brolonged heating at 300 “C (spectra 5 and 6) results in the conversion of about 60% of the methanol, the latter value being accurate to 5 - 10%. only because of possible differences in polarization (NOE) and relaxation effects for the various types of 13C nuclei.

By contrast with the results obtained using the synthetic H-ZSM-5 zeolite [ 1,2 J , for which alkanes, alkenes, higher ethers, and aromatics were observed, onIy dimethyl ether is formed in the present case.

An interesting feature sterns, however, from the careful analysis of spectra 4,5, and 6 (see Fig- 1 and Table 1) A slight and progressive decrease

455

CH,OH 4

T0:3000C-Sh

CHLIO”

ccn,>,o

TY300°c-

E!

h 6

Fig. 1. Typical l3 C-NMR spectra observed dwing the static dehydration of 13CH30H (30% V[V) on NaGeX-300 zeolite- The spectra are numbered from 1 to 6: see Table 1 for specfxal details.

456

in the chemieal shifts is observed as weII as a strong variation in spectraI intensities as a fimction of heating time at 300 “C. As approximately calcu- Iated from the number of stans needed to get closely equivalent spectra, relative ïntensities are 4.5 (1.5 h at 300 “C), 2.3 (6 h at 300 “C), and l(16.5 h at 300 “C). These observations can be interpreted in the light of surface methoxylation (formation of Si-O-CHB groups) as recently reported on an HY zeoIite by SaIvador and KIadnig [S] _ The brpad resonance Ene, und&rIying the methanol and dimethyl ether resonances, in spectra 5 and 6, is probably attributable to these methoxy groups of which the chemieal shift shouId not dïffer mucb f&om these of CH30H and (CH&O_

FinaIIy, one can compare the behavïors of the NaGeX, tbe NaY, and the NaX zeolites. NO reaction of methanol was observed on the NaY zeolite, even at 410 “C ES], whïIe higb actïvity and selectïvity (98%) in dimethyl ether production were reported at 340 “C on NaX zeoIite [9] _ These differ- ences cannot, hence, be ïnterpreted simply in terms of varïations in Brönsted acïdity: structuraI features of the zeolites could then also play a role.

ITI view of the former results, it seemed interesting to pay more atten- tion to the possible roIe played by surface methoxylation as a possible inter- mediate stage in the dehydration of methanol. Therefore, the sample giving spectrum 6 was desorbed, The desorbed products (methanol and dimethyl ether, no other compounds observed by mass spectrometry) were quantita- tively estimated and a comparison with the NMR data gïves, as a tentatïve quantïtative analysis of the species giving rise to spectrum 6, an approximate composition of methanol: ether: surface methoxy-groups of 36.9: 45.4: 17.7.

The residual surface methoxy-groups are also observed by 13C-NMR as a broad peak (216 Hz) at about 61.6 ppm (from TMS). Adsorption of water at 20 “C on these (Hz0 in excess by about an order of magnitude) Ieads to their complete hydrolysis as shown in TabIe 2. SmaU amounts of dimethyl ether and surface formate are aIso formed, as identïfied by the position of the NMR peaks-

The observation that surface methoxy-groups are readily hydrolyzed back to methanol (as a major peoduct) at 20 “C explains the decrease in the dimethyl ether to methanol ratio, as noticed from spectra 3 and 4, when the sample is kept for some time at room temperature- On the other hand, the simultaneous formation of ether (about 10% of the amount of methanol) could support the proposal that the initial step in the dimethy1 ether forma- tion is the methoxylation of the surface [S] _ Dimethyl ether wouId then be formed by reaction between a surface methoxy-group and a nearby, adsorbed methanol molecuIe.

The mechanism by which a surface formate is formed is not known. The existente of such species is, however, weII ascertained by their characteristic resonance and it should be noted that theïr formation has aIso been reported previously on Mg0 [4] and alumina [lol.

Further work is in progress in order to gather more quantitative infor- macion on the reactivity of surface methoxide-

457

TABLE 2

Presence and hydrolysïs of surface methoxy-groups

SampIe and pretreatment* Compound Intensity** Remarks (70)

Sample pretreated as to CH30H 36.9 give spectrum 6 of tcH3)20 45-4 Fig. 1 and Table 1 Surface methoxy- 17-7

Relative intensities obtained by comparing methanol/ dimethyl ether ratios from NMR and mass spectro-

Evacuation to lO@ Torr at 100 “C

Adsorption of Ha0 at 20 “C; 48 h at 20 “C

Surface methoxy-100-0

CH30H 84.0

(CH3)20 8-0 Surface formate 8-0

metry data_ 61.6 ppm from TMS_ Linewidthr 216 Hz. Only resonance observed. Surface methoxy-groups are completely hydrolyzed; fonnate resonance at 177-1 ppm from TMS

*Successive treatments- **Intensity (%) expressed relative to overall spectral intensity. Values are on1 indicative (within about lO%), the Nuclear Overhauser Effect (NOE) varying from one ‘C-nucleus to another.

Acknowledgements

b. D. thanks the Facultés Universitaires de Namur for a Research Fellowship. The authors thank G. Poncelet (Université Catholique de Lou- vain) for having supplied them with the synthetic germanium zeolite. They also acknowledge valuable comments from the referee-

References

1 E. G. Derouane, P. Dejaifve, J. B. Nagy, J_ H. C. van Hooff, B_ P_ Spekman, C_ Nacca- che and J_ C. Védrine, C R_ Acad. Sci., Ser. C, 284 (1977) 945.

2 C. D. Chang and A. J_ Silvestri, J_ Catal., 47 (1977) 249. 3 P. Salvador and W. Kladnig, J. Chem. Sec., Faraday Trans. I,73 (1977) 1 153. 4 D_ C. Foyt and J. M. White, J. Catal., 47 (1977) 260_ 5 H_ Knözinger, Angew- Chem. Int. Ed. Engl., 7 (1968) 791_ 6 P. B_ Venuto and P_ S_ Landis, in D. D. Eley, H. Pines and P_ B. Weisz (eds.), Advances

in CataIysis, Vol_ 18, Academie Press, New York, 1968, p_ 259_ 7 J_ B_ Nagy, M_ Gigot, A. Gourgue and E_ Derouane, J_ Mol_ Catal., 2 (1977) 265_ 8 G- Poncelet, M. Dubru and P_ A_ Jacobs, in J_ R_ Katzer (ed_). Molecular Sieves. Am_

Chem. Sec. Symp_ Scr. NO 40, Vol. 11, American Chemical Society, 1977, p_ 606_ 9 W_ J_ Mattox, U. S_ Patent 3-036-134 (1962)_

10 R. G. Greenler, J. Chem. Phys., 37 (1962) 2 094_