J. Pkys. Ckem. 1993,97, 7321-7327
7321
An in Situ Solid-state NMR Study of the Formation and Reactivity of Trialkylonium Ions in Zeolites Eric J. Munson, AK A. Kheir, and James F. Haw* Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received: February 5, 1993; In Final Form: April 12, 1993
In situ 13C and 77Semagic angle spinning (MAS) N M R was used to investigate the formation and reactivity of trialkylonium species on zeolite catalysts HZSM-5 and HY. Trimethyloxonium, trimethylsulfonium, and trimethylselenonium ions were all formed on HZSM-5 by adsorption of the corresponding dimethyl chalcogenide. The identity of these species was confirmed by comparison to solution-state chemical shifts as well as to an authentic sample of trimethylsulfonium-ZSM-5.Trimethyloxonium formed only on the strongly acidic zeolite HZSM-5 whereas trimethylsulfonium formed on both HZSM-5 and the less acidic HY. This result suggests that onium ions may be useful for measuring the strengths of Bronsted acid sites in catalysts. Unlike trialkyloxonium ions, trialkylsulfonium ions were stable at high temperatures and in the presence of alcohols. The adsorption of more than 1 equiv per acid site of dimethyl sulfide poisoned the catalyst by titration of the Bronsted acid sites. With less than 1 equiv per acid site of dimethyl sulfide, the remaining sites were active for MTG chemistry, but the trimethylsulfonium was not consumed in the formation of hydrocarbons. These results are interpreted in terms of onium-ylide mechanisms proposed for methanol-to-gasoline chemistry on HZSM-5. In situ 13C MAS N M R studies of dimethyl ether at various loadings reconcile these studies with a previous in situ FTIR investigation by Forester and Howe ( J . Am. Chem. SOC.1987, 109, 5076). Trimethyloxonium ion formation on HZSM-5is strongly loading dependent. At low loadings of dimethyl ether (e.g., 0.2 equiv per acid site), no trimethyloxonium formed, and protonation shifts were observed as the sample was heated. Trimethyloxonium formed readily at higher dimethyl ether loadings (e.g., 2 equiv per acid site).
SCHEME I
Introduction In a recent communication,’ we reported 13C magic angle spinning (MAS) results that showed that, upon adsorption of high loadings of dimethyl ether on zeolite HZSM-5, trimethyloxonium formed to a significant extent (Scheme I) in thevicinity of room temperature. The 13Cchemical shift of trimethyloxonium was identicalto values reported previously for superacid solutionsZ as well as a measurement by Chang and co-workers on a sample prepared by direct ion exchange of trimethyloxonium into the zeolite.’ The disproportionationof dimethyl ether also produced a stoichiometric amount of methanol. Trimethyloxonium did not form on the less strongly acidic zeolite HY, nor did it form on NaZSM-5, which has no strong Bronsted acidity.l The formation of trimethyloxonium in the zeolite was a significant observation, because onium ions are proposed intermediates in some of the most often cited mechanisms4J proposed for the conversion of methanol to gasoline (MTG p r o c e ~ s )on ~.~ zeolite HZSM-5 or analogous reactions of other heteroatomsubstituted methane derivatives on a variety of acidic and bifunctional catalysts (Scheme II).4 These mechanisms proposed that trimethyloxonium(or trimethylsulfoniumfor the conversion of methanethiol and dimethyl sulfide) is deprotonated by a basic site to form the corresponding onium-ylide. Evidence for this mechanism has come from trapping studies on bifunctional catalysts, for which the existence of a basic site was not open to question.4 The existence of a suitable basic site on HZSM-5 is more problematic, and this has been a frequent criticism of oxonium-ylide mechanisms for MTG chemistry.697 In one of the mechanisms, methylation of the ylide in the critical step of carboncarbon bond formation forms the dimethylethylonium.4 The latter ion was then thought to eliminate ethylene under reaction conditions on the bifunctional catalyst. In an alternate pathway, the oxonium-ylide undergoes a Stevens-type rearrangement to form ethyl methyl ether.5
* To whom correspondence should be addressed.
H 2CH3OCH3
H3C
I
+
-c
(?.
CH3
‘0’. I
SI AI
+
CH3OH
CH3
/
6 ‘.
si Al SCHEME II H I
CHnXCH], CHJXH
x =
0 / ‘\ Si AI
H3C
CH3
\X4 CHI
basic &
site
H E
CH3
\x4 CHI
-
0,s
CHz=CHz
+ CH3XCH3
H E -H* c-
CHI
CH3CHaXCH3
‘x/+ eHaCH3
Although the observation of trimethyloxonium formation on HZSM-5 from a known intermediate (dimethyl ether) in MTG chemistry satisfied one of the predictions in Scheme 11, other evidence in the initial’ and subsequent investigations8is difficult to reconcile with onium-ylide mechanisms. Trimethyloxonium did not form at observable levels in the presence of excess methanol, and it invariably decomposed at higher temperatures, but well below theonset of hydrocarbon synthesis! Furthermore,noNMR evidence of the oxonium-ylide or dimethylethyloxonium was obtained in studies of dimethyl ether or methanol on HZSM-5.8 Another objection to onium-ylide mechanisms is that trimethyloxonium was not observed during in situ FTIR studies of methanol or dimethyl ether on HZSM-5. Forester and Howe reported no infrared evidence of trimethyloxonium formation when very low levels of dimethyl ether were flowed over a freestanding catalyst wafer.9 Instead, they observed that dimethyl
0022-3654/93/2097-7321$04.00/0 0 1993 American Chemical Society
1322 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993
ether was hydrogen bonded in the zeolite at room temperature and that protonation occurred as the temperature was raised to 373 K or above. The present contribution addresses three objectives. We report observations that generalize the formation of onium species in acidic zeolites to include trialkylsulfonium and trialkylselenonium ions. In the latter case onium ion formation in HZSM-5 is established unambiguously by Y7Se MAS NMR as well as 13C MAS NMR. The formation of the coproduct CH3SeH was verified by the observation of the expected oxidative coupling to CH3SeSeCH3 upon intentional exposure to atmosphere. Trimethylsulfonium was easily ion exchanged into ZSM-5 from solution, and the 13C NMR properties of the authentic sample were identical to those in which the trimethylsulfonium was generated in situ by disproportionation of dimethyl sulfide or reaction with methanol. Sulfonium and selenonium ions are significantly more stable than their oxonium analogues. The ready formationof trimethylsulfoniumbut not trimeth yloxonium in zeolite HY suggests that onium ion formation could be used as an experimentalscaleof solid Bronsted acid strength analogous to the Hammet indicator dye scale,l0 which is not easily applied to zeolite acids. The second objective was to use the more thermally stable trialkylsulfonium and selenonium ions to test whether the presence of these species in the zeolite during methanol conversion has a positive effect on the kinetics of MTG chemistry. Trialkylsulfonium ions were stable on the zeolite even at 523 K, and they exerted no catalytic effect. Indeed, high loadings of dimethyl sulfide deactivated the catalyst by titration of the Bronsted sites with trimethylsulfonium. Furthermore, when dimethylethylsulfonium was formed from dimethyl sulfide and ethanol, it did not eliminate ethylene under MTG conditions at 523 K, in contrast to the prediction in Scheme 11. The third objective was to reconcile the in situ MAS NMR observationthat trimethyloxoniumforms upon the adsorption of high loadings of dimethyl ether with Forester and Howe's in situ FTIR evidence that this ion does not form when very small concentrations of ether are pulsed onto the catalyst under flow conditions. We report that the formation of trimethyloxonium is strongly dependent on dimethyl ether loading, probably as a result of both mass action and solvation. Although the formation of trimethyloxonium was easily reproduced using high loadings of dimethyl ether (ca. 2 equiv), none formed at loadings of 0.2 equiv or less. Furthermore, we report evidence of protonation shifts when low loadings of dimethyl ether were heated in the zeolite. The infrared and NMR studies are thus substantially reconciled. Experimental Section Materials. Zeolite HZSM-5 was obtained from UOP Corp. The stated Si/Al ratio of the catalyst was 38, but Z9Si and Z7Al NMR as well as elemental analysis of the batch used in this study indicated that the Si/Al ratio of the framework was 20. This correspondsto 0.80 mmol/g acid sites, and all adsorbate loadings in this contribution are reported as equivalents relative to this value. Several experimentswere also performed using HZSM-5 samplesfrom alternate sources, and similar results were obtained, but all of the results reported here were obtained using the material described above. Zeolite HY (Si/Al = 2.5) was also obtained from UOP Corp., and y-alumina was obtained from Laroche chemicals. All materials were activated using a multistep procedure described elsewhere.' The catalysts were transferred to a glovebox containing a dry nitrogen atmosphere, packed into a zirconia rotor, and placed in a glass version of the CAVERN apparatus.11J2 The CAVERN was attached to a vacuum line and evacuated to ca. 5 X Torr. In some cases, an alternate CAVERN apparatus designed for shallow bed (< 2-mm bed depth) adsorption was used to facilitate a uniform distribution of adsorbates without resort to heating.
Munson et al. Dependingon the experiment, one or more of dimethyl sulfide (Aldrich), dimethyl selenide(Strem), methanol-13C(Cambridge Isotopes), dimethyl ether-Wl (MSD Isotopes), ethyl methyl ether-1-13C (MSD Isotopes), and ethanol-l-IT (Cambridge Isotopes) were adsorbed onto the catalyst. Each sample was subsequently capped and taken to the NMR probe. To prepare an authentic sample of trimethylsulfonium-ZSM5,2 g of trimethylsulfoniumiodide (Aldrich) was ion exchanged with 1 g of HZSM-5 while slurrying in 40 mL of deionized water at 333 K for 12 h. The catalyst was then filtered and washed three times with deionized water and allowed to air dry for 1 h before drying in vacuo (1 Torr) at 313 K for 1 h. NaZSM-5 was prepared by ion exchangeof 10g of HZSM-5 twice while slurrying in 1 M NaNO3, washing with distilled water, and activating via the multistep activation procedure described in ref 11. NMR Spectroscopy. All 13C spectra were acquired on a modified Chemagnetics CMC-200 spectrometer operating at 50.06 MHz. Hexamethylbenzene (17.4 ppm) was used as an external chemical shift standard, and all chemical shifts are reported relative to TMS. In most cases, chemical shifts were also referenced internally to methanol or other adsorbates. Except in a few cases of especially broad lines, all l3C chemical shifts are believed to be accurate within 1 ppm. Cross-polarization can increase l3C spectral intensities by up to a factor of 4, and larger discrepanciescan result if very mobile species (which may not cross-polarize efficiently)and rigid species are commingled. To avoid any possibility of intensity errors, all of the spectra reported in this contribution were obtained using single-pulse excitation. We note however that cross-polarization spectra were also obtained and that these were in all important respects consistent with the single-pulse spectra. 13C T1measurements showed that adsorbed species generally had 13C Tl values of < 100 ms; therefore, repetition delays of 1 4 s were used to ensure that all spectra were quantitative. The spectral intensity observed was in every case consistent with the amount of label adsorbed. There was no indication of spectral intensity being lost to coke or other species which might be difficult to observe. Typically, 400 scans were obtained for W-enriched samplesand 12 000 scans for samples without added label. All spectra were measured using a 400 ppm spectral width, but a narrower region is shown for clarity. No signals were observed outside of the region shown. 77Se spectra were acquired on a Chemagnetics CMX-360 spectrometer operating at 68.63 MHz. Ammonium selenate (1040 ppm) was used as an external chemical shift standard, and all chemical shifts are reported relative to dimethyl selenide (0 ppm). A single-pulse (repetition delay 3 s, 12 000 transients) experiment with proton decoupling was used to obtain the W e spectrum shown. All NMR experimentswere carried out in zirconia rotors spun at the magic angle at between 3.5 and 4 kHz in Chemagnetics pencil module probes equipped with variable-temperature accessories. A typical in situ protocol involved adsorption at room temperature followed by spectral acquisition as the sample temperature was raised in 40 K steps to a final temperature of 523 K. About 20-60 spectra of various types were taken in each study. In some cases, entire studieswere reproduced several times; all results reported here are representative. Results 13Cchemical shift assignments for all of the relevant species observed in zeolite HZSM-5 are summarized in Table I. Many of these assignments were further verified by direct adsorption or ion exchange of the species into the zeolite. All of the shifts in Table I are in close agreement with literature values from solution studies.13-20 Figure 1 reports a survey of some of the NMR evidence for the formation of the various trimethylonium ions from the correspondingdimethyl chalcogenides on HZSM5. Figure l a shows the 13C MAS spectrum after the adsorption
Reactivity of Trialkylonium Ions in Zeolites
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7323
d
exchange. As shown in Figure IC, the 13C chemical shift is identical to that observed from the disproportionationof dimethyl sulfide. An analogous experiment was performed by Chang and co-workers to prepare and characterize trimethyloxonium-ZSM5;3 their I3Cchemical shift was identical to what we observe when trimethyloxonium forms in situ. The I3C assignments in Figure Id, trimethylselenonium (22 ppm), dimethyl selenide ( 5 ppm), and methaneselenol (-5 ppm), correspond very closely to their solution-statevalues. Nevertheless, two experimentswere carried out toverify these assignments. When thesample used for Figure Id was intentionallyexposed to theatmosphere, the I3C resonance at -5 ppm was replaced by a new peak of equal intensity at 10 ppm (not shown). Oxidative coupling of alkyl selenols to dialkyl diselenols is a very facile reaction, and the new peak at 10 ppm corresponds to the solution-state chemical shift of dimethyl diselenide.21 The oxidativecoupling of methanethiol to dimethyl disulfide was also observed when a sample like that shown in Figure 1b was intentionally exposed to the atmosphere (spectrum not shown). Figure l e shows the 77SeMAS spectrum of a sample similar to that used for Figure Id. The resonance at 245 ppm is due to trimethylselenonium,that at 0 ppm is due to dimethyl selenide, and the peak at -130 ppm is methaneselenol. The 77Se NMR results provide unambiguous evidence for the formation of onium ions in zeolite HZSM-5. The relative stabilities of the trimethylonium ions follow the order oxonium < sulfonium 4 selenonium, and this is reflected in the extent to which they form on different catalysts. Although trimethyloxonium forms readily on HZSM-5 followingadsorption of a high loading of dimethyl ether-I-13C (Figure 2a), this ion does not form when an analogous experiment is carried out on the less strongly acidic zeolite HY (Figure 2b). On the latter zeolite, unreacted dimethyl ether is observed,and there is a small shoulder at 63 ppm. Bronnimann and Maciel observed an analogous feature in a 13Cstudy of methanol in HY and assigned that shoulder to methanol adsorbed into the sodalite cagesSz2 Drawing from that work, we assign the shoulder in Figure 2a to dimethyl ether in the sodalite cages and attribute the larger peak to dimethyl ether in the supercages. Trimethyloxoniumdoes not form or persist in HZSM-5 in the presence of an excess of methanol. Trimethyloxoniumis a fairly strong methylating agent, and in an excess of alcohol it reverts to ethers. In contrast, we find that trialkylsulfoniums and selenoniumsare stable in the presence of large excessea of alcohols. Indeed, this permits a convenient preparation of labeled onium ions in a zeolite from a labeled alcohol and an unlabeled dialkyl sulfide or selenide. As demonstrated in Figure 2c,d, W-labeled trimethylsulfonium is formed from '3CH3OH and dimethylsulfide in either HZSM-5 or HY. Excess methanol is also seen in those spectra as well as some enriched dimethyl sulfide formed by label exchange. A similar experiment was also performed on yalumina, whichdoesnot havestrong Bronsted acid sites. As expected, no trimethylsulfonium formed without Bronsted acidity (Figure 2e). The labeled alcohol procedure has been used to prepare a variety of sulfonium ion species in addition to the simple ones reported in this contribution. In every case, there is close agreement between the chemical shift observed in the zeolite and the correspondingvalue in solution. At 298 K,the formation of trimethylsulfonium from dimethyl sulfide and methanol on H Z S M - 5 was ca. 0.8 equiv. At 523 K, the reaction went further to completion, forming a total of 1 equiv of trimethylsulfonium. These results suggest that trimethylsulfonium may be used as a probe molecule to quantitatively titrate strong Bronsted acid sites in acidic catalysts. Other experiments using various loadings of dimethylsulfide (spectra not shown) support quantitativetitration, although further experiments with other materials are necessary in order to establish the generality of this procedure as a means to quantify strong Bronsted sites. Also, the minimum acid strength necessary for formation of these ions must be quantified.
I I ? 150
l
-50
h
e
400
0
50
100
l
l
300
l
A l
200
l
l
100
l
l
0
l
l
l
i
-100
-200 PPm Figure 1. 13C and "Se MAS NMR spectra showing the formation of trialkylonium ions from the disproportionationof dimethyl chalcogenides on zeolite HZSM-5: (a) I3C spectrum of dimethyl ether adsorbed on HZSM-5, (b) 13C spectrum of dimethyl sulfide on HZSM-5, (c) 13C spectrum of an authenticsample of trimethylsulfonium-ZSM-5,(d) I3C spectrum of dimethyl selenide on HZSM-5, (e) 77Sespectrumof dimethyl selenide and methanol on HZSM-5. TABLE I: 1jC Chemical Shift Assignments for the Oxygen, Sulfur, and Selenium Containing Species Observed in This Study (All Values in Dpm from TMS) obsd lit. species chemical shift chemical shift ref -5 13 -5 methaneselenol 5 5 14 dimethyl selenide 5 7 15 methanethiol 10 13 10 dimethyl diselenide 14,67 14,65 16 diethyl ether 17.63 17,57 16 ethanol 19 17 18 dimethyl sulfide 18 22 22 trimethylselenonium 28 17 28 trimethylsulfonium 28 19 28 ethyl methyl sulfide 38 17 38 dimethylethylsulfonium (CH2) 38 17 40 diethylmethylsulfonium (CH2) 20 50 50 methanol 59 2 60 dimethyl ether 79 2 80 trimethyloxonium of 1.7equivofdimethylether-l-13C. As wasreportedpreviously,' some of the dimethyl ether (40.3 equiv) disproportionated to trimethyloxonium (80 ppm) and a stoichiometric amount of methanol (see Scheme I). The same reaction stoichiometries were observed upon adsorption of natural abundance dimethyl sulfide (Figure lb) or dimethyl selenide (Figure Id), but the positions of equilibriumfavored the onium ions to a greater extent than for dimethyl ether. One-bond scalar couplings between 77Se(natural abundance 7.6%) and 13Care typically ca. 50 Hz;l* these are not resolved in Figure Id due to the line widths of the 13C resonances near the base line. Further evidence for the assignment of the 28 ppm resonance in Figure 1b to trimethylsulfoniumwas obtained by preparing an authentic sample of trimethylsulfonium-ZSM-5 by direct ion
Munson et al.
7324 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993
A
298 K
I
b
I
10 min at 393 K
d
10 min at 473 K
,
I
70 min at 523 K
1
150
1
1
100
1
1
50
1
1
0
1
1
-50
PPm Figure 2. 13C MAS NMR spectra showing the formation of trimethyloxonium and trimethylsulfonium on various catalysts: (a) dimethyl ether-l-Won HZSM-5, (b) dimethyl ether-l-13Con HY, (c) methanolI3Cand dimethyl sulfide on HZSM-5, (d) methano1J3Cand dimethyl sulfide on HY, (e) methanol-IT and dimethyl sulfide on y-alumina. Trimcthyloxonium (80 ppm) formed on zeolite HZSM-5 but not on zeolite HY. Trimethylsulfonium (28 ppm) formed on zeolites HZSM-5 and HY but not on the less acidic y-alumina.
When dimethyl ether is heated on HZSM-5, the trimethyloxonium disappears at temperatures well below the onset of hydrocarbon synthesis.* The inability to observe the oxonium ions while hydrocarbon formation is in progress precludes some of the clearest tests that one might imagine for the mechanisms in Scheme 11. Such tests are, however, possible with the more stable sulfonium ions. Several in situ MAS NMR experiments were performed using various loadings of labeled methanol and unlabeled dimethyl sulfide such that the total loadingwas always 2.9 equiv (1.2 mmol/g). Figures 3 and 4 show representative results. When the loading of dimethyl sulfide was greater than 1 equiv (e.g., 1.2 equiv, Figure 3), most of the methanol was consumed in the formation of trimethylsulfonium. Label scrambling also produced labeled dimethyl sulfide upon heating. A small amount of dimethyl ether also formed, but no hydrocarbons were produced even after prolonged heating at 523 K. Analogous in situ experiments without dimethyl sulfide readily formed hydrocarbonsunder these conditions. When the dimethyl sulfide loading was below 1 equiv (e.g., 0.3 equiv, Figure 4), the catalyst remained active for MTG chemistry. All of the dimethyl sulfide was converted to trimethylsulfonium (28 ppm), but the excess methanol formed hydrocarbons upon prolonged heating at 523 K. The trimethylsulfonium, however, was not consumed in the MTG chemistry, and the rate of the reaction in Figure 4 was, if anything, slower than analogous in situ experiments without dimethyl sulfide.* An analogousexperiment was performed with dimethyl selenide (1.4 equiv) and 13CH3OH (0.7 equiv). Once again, onium ion formation titrated the acid sites and poisoned the catalyst for MTG chemistry. Trimethylselenoniumdid not react to any extent after 50 min at 523 K (not shown). One of the routes in Scheme I1 proposed a dimethylethylonium ion as the first species with a carbon4arbon bond.' That ion is proposed to eliminate ethylene under reaction conditions on a bifunctional catalyst to initiate hydrocarbon synthesis. This last step in the onium-ylide pathway for MTG chemistry was tested by the following experiment: 0.6 equiv of ethanol-Z-13Cand 2.3
hlL I
150
I
I
I
50
100
I
0
-50
PPm Figure 3. 13CMAS NMR spectra showing the reactions of 1.2 equiv of unlabeled dimethyl sulfide and 1.7 equiv of methanol-l3C on HZSM-5. The formation of trimethylsulfonium poisons the catalyst for MTG chemistry (see text). 298 K
I
10 min at 523 K
60 min at 523 K
~
70 min at 523 K
-2LL.L I
80 min at 523 K
1
150
1
1
100
1
1
50
1
1
0
1
1
-50
PPm F m e 4 . L3CMASNMRspactraofthereactionsof0.3equivofunlabeled dimethyl sulfide and 2.6 equiv of methan01-~3 e 61
400 450 500 5 io Temperature (K) Figure 7. 13Cchemical shift change of high (2.2 equiv/acid site) and low (0.2 equiv/acid site) loadings of dimethyl ether on HZSM-5 and a low loading of dimethyl ether on NaZSM-5 as the samples were heated from 393 to 513 K. Protonation shifts were observed at higher temperatures for low loadings of dimethyl ether on HZSM-5. 6%0
300
350
was raised. Rather, one would expect a single resonance at the population-weighted average shift for protonated and nonprotonated dimethyl ether. Thus, on the basis of infrared studies of Forester and Howe? one would expect the I3C chemical shift of dimethyl ether in HZSM-5 to depend on loading as well as temperature. This is in fact the case as shown in Figure 7. The *3Cchemical shift of 0.2 equiv of dimethyl ether on HZSM-5 shifted downfield over 6 ppm upon heating, whereas a 2.2-equiv sample shifted at most 1 ppm. (The line widths at high temperature were too broad to distinguish protonated dimethyl ether from framework-bound methoxyls, a second high-temperature process proposed by Forester and Howe.9) The chemical shift of dimethyl ether on NaZSM-5 showed no temperature dependence, as it must if the observed shifts on HZSM-5 are to be assigned to the extent of protonation.
Munson et al.
7326 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 SCHEME IV
SCHEME I11 CH~XCHJ
+
CHjOH
+
H I /O,,
Si X = S, Se
H3C ___+
‘X/+ I CH3
Al
H3C
CH3
+
H10
-
/OS.
Si
CH3
\o/+ I
CH3
CHjOCHj
+
CH3 I
/O*.
Si
Al
0
AI
The experiments depicted in Figures 6 and 7 thus reconcile to a significant extent theNMRand FTIR evidence for thechemistry of dimethyl ether on zeolite HZSM-5.
Discussion The formation of onium ions in acidic zeolites has been generalized to include sulfonium and selenonium ions. These species readily form by disproportionation of the corresponding dialkyl chalcogenides by analogy to Scheme I. The I3Cchemical shifts of these ions are identical to their values either in solution or for authentic samples of onium zeolite prepared by direct ion exchange. The ”Se MAS spectrum of dimethyl selenide on HZSM-5 provides unambiguous evidence for trialkylonium ion formation under these conditions. Oxonium ions are considerably less stable than their sulfur or selenium analogues. The formation of trimethyloxonium in HZSM-5 requires a high loading of dimethyl ether, which probably stabilizes the ion through solvation. The formation of trimethylsulfonium and trimet hylselenonium from methanol and dimethyl sulfide or dimethyl selenide (Scheme 111) and their relative stabilityverses trimethyloxonium in various zeolites can be understood in terms of hard and soft acid-base t h e o r ~ . ~The ~ , ~high $ electronegativity and low polarizability of oxygen make methanol and dimethyl ether hard bases, whereas the d electrons of sulfur and selenium are easily polarized, and dimethyl sulfide and dimethyl selenide are classified as soft bases. Since H+ is a harder acid than CH3+,hard-soft acid-base theory predicts that dimethyl ether (a hard base) would tend to be protonated than alkylated and that dimethyl sulfide or dimethyl selenide (soft bases) would tend to be alkylated rather than protonated. The same argument applies for the formation of trimethylsulfonium and selenonium upon addition of methanol. We assume that protonated methanol methylates dimethyl sulfide or dimethyl selenide to form water and trimethylsulfonium or trimethylselenonium, respectively. Protonated methanol is not a strong enough alkylating agent to methylate dimethyl ether, and dimethyl ether does not form trimethyloxonium in the presence of an excess of methanol. The greater stability of sulfonium and selenonium ions verses oxonium ions is also consistent with observations of synthetic chemists. Trimethylsulfonium and trimethylselenonium can be isolated as halide salts, while the conjugate base of a much stronger acid (e.g., BF4-, SbFa-) is required to isolate trimethyloxonium salts.24 The formation of trimethyloxonium on HZSM-5 but not HY and the formation of trimethylsulfonium on both zeolites but not on y-alumina can be understood by comparing the strength of the Bronsted acid sites for each catalyst. HZSM-5 and HY possess strong Bronsted acid sites, but the sites are stronger on the former than the latter. The surface hydroxyl groups on y-alumina are only weakly acidic. These observations suggest the application of onium ions in an NMR protocol for measuring the strengths of solid acids. Tanabe has reviewed the use of indicator dyes as probes of acid site strength of amorphous solid acid catalysts.1° Two problems generally preclude the application of such dyes to zeolites and other microporous materials. Many of the standard dyes are too large to be sorbed into small channels, and measurement of UV-visible absorbance changes may be precluded in a zeolite. The simple trialkylonium ions discussed in this contribution are small enough to be formed in most microporous materials of interest in acid catalysis, and in the absence of particles with undesirable magnetic properties, NMR
/ ’\ Si Al
does not suffer from matrix problems with solid acids. A number of molecules that show N M R chemical shift changes upon protonation or interaction with Lewis sites have been proposed as measures of acidity in liquid or solid acid catalysts.2”28 A possible advantage of onium ions in such a strategy is that a variety of ions of similar size but different stabilities could be used to bracket the acidity within narrow limits. The identification of trimethylsulfonium explains a recent 13C N M R study of the reactions of methanol and hydrogen sulfide on HZSM-5.29 In that study, Nosov and co-workers found that when a sample containing methanol coadsorbed with hydrogen sulfide was heated outside the spectrometer to 633 K, the roomtemperature spectrum showed three peaks at 29, 18, and 6 ppm. They identified the latter species as dimethyl sulfide and methanethiol, respectively. They characterized the peak at 29 ppm as due to a species which does not desorb from the catalyst and should contain sulfur, but they were unable to assign a structure. The present study makes clear that their resonance at 29 ppm was due to trimethylsulfonium formed in the catalyst and strongly suggests that zeolitedeactivation by H2S andalcohols is a result of titration of the acid sites in the formation of the sulfonium ion. A number of workers have studied the reactions of trimethyloxonium in attempts to prove or disprove the oxonium-ylide mechanism. Sommer and co-workers studied the reactions of trimethyloxonium hexachloroantimonate with 2,2,6,6-tetramethylpiperidyllithium and found that methyl ethyl ether was formed.30 Van Hoof and co-workers heated trimethyloxonium tetrafluoroborate either neat or in nitromethane and found olefins and methyl ethyl ether as products.31 Olah and co-workers32 and Sommer and co-workers,” however, showed that the products formed in van Hoof‘s experiments were due to dimethylethyloxonium impurity and that, in the absence of base, trimethyloxonium tetrafluoroborate does not generate C-C bond containing species. Hunter and Hutchings have reported that lithium aluminum tetraisopropoxide, which they used to model the conjugate base of a zeolite, did not deprotonate trimethyloxonium but was instead methylated,34 This result is reminiscent of the experiment by Chang and co-workers in which trimethyloxonium was directly ion exchanged into the zeolite.) After several days a t 293 K, rather than forming theoxonium ylide or hydrocarbons, the oxonium ion apparently methylated the conjugate base site to form a framework-bound methoxyl (Scheme IV). The following observations were made based on our studies of onium ions in zeolites. Trimethyloxonium is not observed at high temperatures, a t low dimethyl ether concentrations, or in the presence of excess methanol. It does not form a t all in zeolite HY, although it is possible to synthesize hydrocarbons from methanol or dimethyl ether on that catalyst. We have never observed NMR evidence for the oxonium ylide or the conversion of trimethyloxonium to dimethylethyloxonium. In this investigation, the previously proposed analogy between oxonium and sulfonium routes has been used in direct tests of the effects of onium species on MTG reaction conditions. Trimethylsulfonium does not catalyze the conversion of methanol to hydrocarbons on HZSM-5 at 523 K. On the contrary, the conversion of dimethyl sulfide to trimethylsulfonium poisons the catalyst by titration of the acid sites. No evidence was observed for the formation of dimethylethylsulfonium from trimethylsulfonium and methanol. There was no evidence of any species which could be assigned to the sulfonium ylide or to ethyl methyl sulfide. When dimeth-
Reactivity of Trialkylonium Ions in Zeolites
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7327
ylethylsulfoniumwas generated in the zeolite by other means, it failed to elminate ethylene under MTG reaction conditions at 523 K. From these results we can conclude that oxonium ions and ylides, if they do form at high temperatures, are too unstable to be observed using in situ NMR. Not surprisingly, the properties of the sulfonium ions in ZSM-5, including their stabilities, are quite different from the correspondingoxonium ions. This calls into question the extent to which the former ions can model the reactions of the latter. However, there is no hydrocarbon formation from the sulfonium ions studied at 523 K or higher35 or evidence of a catalytic effect on methanol conversion. It is thus concluded that, with the exception of the formation of trimethyloxoniumfrom dimethyl ether, the results reported here do not support the onium-ylide mechanisms proposed for MTG chemistry on zeolite HZSM-5. It is often said that reaction mechanisms can never be proven, only disproven. The results of this study add to the preponderance of evidence from several research groups that the onium-ylide mechanismsdo not explain methanol-to-gasoline chemistry on HZSM-5. The reconciliationof the in situ NMR and in situ FTIR results on trimethyloxonium formation and dimethyl ether protonation is a positive development. An understanding of the chemistry of heterogeneous catalysis will only be possible with a multiple technique approach, and it is satisfying that infrared and NMR experiments arrive at a consistent picture of the chemistry of dimethyl ether on zeolite HZSM-5.
Acknowledgment. This work was supported by the National Science Foundation(Grant CHE-8918741). Funds for upgrading the spectrometer were provided by the National Science Foundation and the Texas Advanced Technology Program. E.J.M. is an ACS Division of Analytical Chemistry Fellow sponsored by Eastman Chemical. A.A.K. is a Robert A. Welch Predoctoral Fellow. References and Notes (1) Munson, E. J.; Haw, J. F. J. Am. Chem. Soc. 1991, 113, 6303. (2) Olah, G. A.; Doggweiler, H.; Felbcrg, J. D.; Frohlich, S.J. Org. Chem. 1985,50, 4847. (3) Hellring, S.D.; Schmitt, K.D.; Chang, C. D. J. Chem. Soc., Chem. Commun. 1987,- 1320. (4) Olah, G. A.; Doggweiler, H.; Feldbcrg, J. D.; Frohlich, S.;Grdina,
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