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Spectroscopic evidence for the formation of carbenium ions on H-ZSM-5 zeolites
MATERIALS CHEM;6TJ&isND ELSEVIER
Materials
Spectroscopic
Chemistry
and Physics
39 (1994)
evidence for the formation H-ZSM-5 zeolites A.V. Demidov”,
13-20
of carbenium
ions on
A.A. Davydov
Boreskov Institute of Catalysis, Novosibirsk
630090, Russian Federation
Received 27 October 1993; accepted 30 May 1994
Abstract The formation of alkenyl and alkylaromatic cations from methanol, olefin, diene and compounds with a single aromatic ring on H-ZSM-5 zeolites has been detected using IR and UV-Vis spectroscopy. The formation of alkenyl cations can be characterized reliably by the appearance of vas ,--c bands at 1490-1530 cm-’ in IR spectra and bands at 290-310 (allyl), 350-380 (dienyl) and 430-450 (trienyl cations) nm in UV-Vis spectra. The formation of alkylaromatic cations can be characterized by the appearance of stretching vibrations of the aromatic ring at 1500 and 1600 cm-’ and bands at 320-330 and 4OW40 nm, taking into account the thermal stability of surface complexes. Keywords:
Zeolites; Carbenium ions; Adsorption
1. Introduction
The hypothesis concerning the carbenium ion mechanism of hydrocarbon transformation on strong, acidic, heterogeneous catalysts at moderate temperatures is widely discussed in the literature [1,2]. However, direct data on the nature of charged surface compounds that can be obtained only with the use of physical methods are limited and contradictory. Thus, the adsorption of olefins on zeolites is followed by the formation of oligomer-saturated structures (recorded by IR spectroscopy) at room temperature; their cationic nature is discussed in the literature [3-91. It is supposed, on the basis of UV-Vis data [lO-141, that the interaction of olefins with H-ZSM-5, HY and HM zeolites at 298-473 K leads to the formation of ally1 cations characterized by an absorption band at 300 nm. According to the IR spectra of strong, acidic, homogeneous systems, the formation of ally1 carbenium ions should be followed by appearance of an absorption band at 1530 cm-’ [15]. In fact, when the temperature increases during the interaction of olefins with H-ZSM5 and HY zeolites, an absorption band at 1500 cm-’ appears [7,X-19]. However, various authors suggest different interpretations of this absorption band. Some of them associate it with the formation of aromatic structures [7,16,17], *Corresponding
author.
0254-0584/94/$07.00 0 1994 Elsevier Science SSDI 0254-0584(94)01384-S
S.A. All rights
reserved
COO- groups [18], and complexes with the transformation of proton to olefin ‘nonclassic carbenium ion’ [19]. On investigating the interaction of propene with H-ZSM5 zeolites, the authors of Refs. [20-221 (in comparing data obtained by the coupling of IR and UV-Vis methods) suggested that the absorption band at 1500-1540 cm-l belongs to ally1 and polyenyl carbenium ions - r&c - and observed the formation of alkenyl carbenium ions by UV-Vis. Similar results were obtained for IR and UV-Vis studies of the interaction of propene and cyclopropane with HM zeolites [13,14]. In earlier works devoted to the interaction of complex hydrocarbons, such as diphenylethene, on aluminum silicates, the formation of alkylaromatic carbenium ions has been ascertained with UV-Vis spectra [2]. The spectral manifestation of carbenium ions observed is close to that for corresponding homogeneous media. The formation of aromatic carbenium ions such as protonated benzene is expected in the case of olefin and methanol adsorption on H-ZSMJ zeolite, as the 370 nm absorption band has been observed in UV-Vis spectra [ll]. At the same time, the appearance of the corresponding band for H-ZSM5 zeolite similarly treated is attributed to the formation of nonaromatic dienyl carbenium ions on the basis of UV-Vis and IR spectroscopy data [20-221.
14
A.K Demidov, AA. Davydov i Materiab ~~ern~t~ and Physics 39 (1994) 13-20
preliminary purified reagents are given in the figure captions (with dimethylnaphthalene and triphenylcarbinol, the treatment was performed under pressure of saturated vapors). All spectra are given with a subtracted background absorption of the sample.
Thus, an analysis of the literature data shows that the use of physical methods, especially IR spectroscopy together with UV-Vis, allows one to solve the problem of carbenium ion identi~cation on zeolites. At the same time; it is obvious that a more profound investigation of the spectral manifestations of such compounds is necessary (including using molecules with isotope substitution, molecules with different chemical natures, investigating the nature of surface sites involved in the fo~ation of surface impounds, etc.).
3.1. Alkenyl &arb~~i~ ions on H-ZSM-5 zeolites
2. Experimental
3.1.1. Adsorption of propene The adsorption of propene
Zeolites with a SiO,/Al,O, (modulus M) ratio of 30 and 90 were synthesized at Irkutsk State Universi~. According to XPS and IR spectroscopy, these zeolites are similar to H-ZSM5 zeolite. The parameters of the unit cell are as follows: a = 20.08 and 20.05, b = 19.90 and 19.81, and c = 13.38 and 13.37 A for M=30 and 90, respectively. Fig. 1 presents the IR spectrum in the region of fundamental lattice vibrations. The residual content of Na after exchange on NH,+ was not more than 0.12%. The H-ZSM-5 zeolites were treated in 0, and in vacuum for 2-4 h at 773-823 IS. H-ZSM5 zeolite with M=60 has a low degree of crystallization, which is evidenced by the low intensi~ [23] of the absorption band at 545-550 cm-’ (Fig. 1, spectrum 3). Exposure to temperatures above 773 K led to the complete removal of acidic hydroxide groups (with respect to pyridine), characterized by a 3600 cm-’ absorption band. Zeolite samples with 2ft40 mg cm-’ were treated with reagents in a quartz cell [2] with NaCl and optical quartz windows. Deuterated zeolites were prepared through H-D exchange by means of multiple DzO treatment at 573 K. IR spectra were recorded with a Karl Zeiss UR-20 and UV-Vis with a Shimadzu UV300 inst~ment, using a standard reflection attachment at room temperature. Conditions for treatment with
I 500
Fig. 1. IR spectra lattice vibrations:
600
700
800
4,cd
of H-ZSMJ zeolites in the region of fundamental (1) M= 30; (2) iw=90; (3) M=60.
3. Results and discussion
on H-ZSM-5 zeolite at 298 Kwith subsequent outgassing leads to the formation of surface compounds that are characterized in IR spectra by a number of absorption bands in the 6 and 2, regions of saturated C-H bonds (Fig. 2, spectrum 1). In this case strong changes in the absorption of acidic hydroxide groups occurs, i.e., a sharp decrease of the 3610 cm-l absorption band intensity and the appearance of a low-intensi~ band at 3450-3550 cm-’ take place. The appearance of bands in the absorption region of C-H bonds indicates the formation of saturated hydrocarbon surface structures, namely, oligomers [3-81. The heating of oligomer-containing H-ZSM5 zeolite gives rise to a 1510 cm-’ absorption band. Moreover, the intensity of the absorption band at 1510 cm-l tends to increase (Fig. 2, spectra 2 and 3). Similarly, the appearance (at 298 K) and increase in the absorption intensity occur in the UV-Vis spectra in the 300 nm region (Fig. 2, spectra la-3a). Consequently, oligomer heating is followed by the fo~ation of compounds of a new type; a simultaneous increase in the 1510 cm-’ and 290-310 nm absorption band intensities allows them to be attributed to a single surface compound.
ba
Fig. 2. IR and (a) UV-Vis spectra for H(D)-ZSM-5 zeolite with M=30 treated with propene: H-ZSMJ zeolite treated with CsH6 at 298 K, 13.33 kN m-‘, and outgassed at 298 K (spectra 1 and la), at 373 K (2 and 2a) and at 473 K (3 and 3a); H-ZSM-5 (D-ZSM5) zeolite treated with C,H, (spectra 4 and 4a) (or with C,D, (5)) at 473 K 26.66 kN m-* (for C,D, 8 kN m-r). for 2 h and outgassed at 473 K; samples 4 and 4a were outgassed at 573 K (6 and 6a).
The temperature rise to 573 K during sample outgassing is followed by practically complete removal of the compound observed and restoration of the intensity of the 3610 cm-’ absorption band (Fig. 2, spectra 6 and 6a). The formation of the compound characterized by 1510 cm-’ and 290 nm bands can be observed in larger amounts for H-ZSM5 zeolite treated with propene at 473 K (Fig. 2, spectra 4 and 4a). With the use of deuterated reagents (C,D, and DZSM), the 1510 cm-l absorption band shifts to 1480 cm-’ (Fig. 2, spectrum 5), i.e., the isotope shift upon H-D exchange is 30 cm-‘. Both the location and the isotope shift are not typical for C-H bond vibrations [24]. The appearance of the 1510 cm-’ absorption band cannot be attributed to aromatic complexes, since the corresponding band at 1600 cm-’ is not observed (along with the 1500 cm-’ band, this band is typical of the majority of aromatic compounds [24,X]). The appearance of an absorption band at 1550-1600 cm-’ does not occur if deuterated reagents are used. UV spectroscopy has not revealed the presence of aromatic hydrocarbons in the decomposition products of surface compounds. In the 180-240 nm region, the bands typical of olefin and diene hydrocarbons are observed. One can hardly attribute the 1510 cm-’ absorption band to the COO- group vibrations, since the thermostability of these groups (obtained during the study of HCOOH, CH,COOH and CO2 adsorption on the H-ZSM-5 zeolites) is very high and absorption bands are absent above 240 nm in UV-Vis spectra. To attribute the absorption band at 1510-1520 cm-’ observed after treatment of H-ZSM5 zeolites with olefins at 373473 K to the complex with the proton transfer to the olefin C=C bond (‘nonclassic ion’) 1191 seems not to be reasonable. The activation process of the formation of such a complex with relatively high stability in vacuum (up to 573 K) and its manifestation in the 300 nm region in the UV-Vis spectra are not yet clear. Deno [15] has registered a 1530-1540 cm-’ band in IR spectra and attributed it to ally1 cations in strong acidic homogeneous media. The appearance of the 1500 cm-’ absorption band during the investigation of olefin-HM zeolite interaction has also been attributed to the absorption of ally1 ~arbenium ions ($&) 1141. The abode-mentioned literature data and analysis of the 1510 cm-’ band origin may be taken as support for attributing it to ally1 carbenium ions (z$&). The low-frequency location of the ally1 structure (Y&-) with respect to the location of this band in spectra for olefins may be explained by charge delocalization between three hydrocarbon atoms, i.e., by the hybrid structure formation [2]. This attribution of the observed 1510 cm-’ observed to ally1 carbenium ions is in conformity with the data obtained by UV-Vis. It has been established that an absorption band in the 300 nm region in olefin-strong
acid systems corresponds to ally1 carbenium ion formation [15,26]. The analogous conclusion has been made during the investigation of olefin interaction with HY, H-ZSM-5 and HM zeolites [lO-13,20-221 and with other strong acidic oxide systems [2]. The alternative variant of attributing the 300 nm band to the polyene hydrocarbons (triene, tetraene 1261) is not confirmed by IR spectral data, since an absorption band v,,, at 1600 cm- ’ should be observed for such compounds
F% Thus,on
the basis of the data obtained, we can state that the appearance of 1510 cm-’ and 290 nm absorption bands is direct evidence for the formation of ally1 carbenium ions resulting from heating of oligomers on H-ZSM5. It should be noted that the formation of a saturated structure as well as ally1 carbenium ions is caused by the presence of acidic (with respect to pyridine) hydroxyl groups. The formation of the above surface structures does not occur on a partly dehydroxylated sample of H-ZSM-5 zeolite with a low degree of c~stallization, which has nonacidic hydroxyl groups, characterized by 3750 and 3680 cm-’ absorption bands. We suppose that the revealing of oligomer structures as precursors of ally1 carbenium ions indicates the ionic character of the oligomer reaction with the surface (formation of an alkyl ‘carbenium ion’ is assumed). By calling the oligomer surface structure an ‘alkyl carbenium ion’ we do not mean that it is bound via the pure ionic bond; there is obviously some contribution from a covalent component. We only emphasize its difference from pure covalent compounds, namely surface alkoxides (e.g., on -y-Al,O,), for which olefin insertion into a R-C-O-M”’ bond does not occur
WIThe following scheme of alkenyl cation formation from propene has been adopted in the literature [11,13]: + CH,--CR--CR, H+ + CH,=CH-CH,
(1, (R+)
CHa-YH-CH&H-CH,
+c~H~ b (2)
3
CR, +R+
CH,-CH-CH=CH-CH,
_RH(4) 8
CR, CR, CH,-_C---GH-CIJ-CH, +
+
etc.
According to this scheme, the further course of the complex of reactions is expected to be followed by polyenyl (di-, tri-, tetraenyl) carbenium ion formation [13]. Indeed, treatment of H-ZSM-5 zeolite with pro-
16
A.V. Debit,
AA.
Da~dov
I ~afe~a~
pene at 473 K is followed by the appearance of 350-370 and 430-440 nm bands in the UV-Vis spectrum, along with ally1 carbenium ion formation (Fig. 2, spectrum 4a). The location of the bands observed is typical for dienyl and trienyl carbenium ion absorption, respectively (mainly for cyclic carbenium ions); these carbenium ions are obtained in acidic homogeneous media [15,26]. In the IR spectra of D-ZSM5 zeolites treated with GD, at 473 K, a 1470 cm-’ band is observed, as a shoulder near a 1480 cm-’ band (Fig. 2, spectrum 5). We may suppose that along with ally1 cations, similar nonaromatic compounds containing C-C bonds are also formed. The 1470 cm-l band may be attributed to polyenyl cations or, as expected for HM zeolites, to dienyl cations [14]. Thus, the appearance of 350-370 and 430-440 mn absorption bands indicates the formation of dienyl and trienyl carbenium ions, and the corresponding absorbance (found for deuterated reagents) is observed at 1470 cm-l in IR spectra. The thermostability of polyenyl cations, like that of ally1 cations, does not exceed 573 K in vacuum. Such thermostability may be explained by conjugation of rough C=C bonds in alkenyl carhenium ions as well as by their cyclic structure. These factors lead to an increase in the number of negatively charged 02- ions in a solvating cation shell and thus to stabil~ation of cations on II-ZSM-5. The schemes proposed in Refs. Ill] and f133 can be tested - for example, adsorption of diene hydrocarbons (as well as their oligomerization) on zeolites should be followed immediately (at 298 K) by formation of ally1 and polyenyl cations.
3.1.2. Adsorption of butadiene Butadiene adsorption on H-ZSMJ zeolite at 298 K is followed by the appearance of bands at 310,350-380, 440 and 520-570 nm (Fig. 3, spectra la-3a). The distribution of absorption maxima over these wavelengths is close to that reported by Sorensen for alkenyl carbenium ions in acidic homogeneous media [26]. Similar results have been obtained for alkene adsorption on HM zeolites [13]. By monitoring the butadiene dosage it is possible to detect the predominant fo~ation of ally1 carbenium ions using UV-Vis spectroscopy (Fig. 3, spectrum la). An increase in the amount of dienyl and trienyl carbenium ions occurs upon increasing the amount of adsorbed butadiene (Fig. 3, spectra 2a and 3a). Using IR spectroscopy, we can see (more evidently for deuterated reagents) the appearance of 1500 and I480 cm-’ absorption bands (the isotope shift upon H-D exchange is 30 cm-‘) (Fig. 3, spectra 2-4). By analogy with the data obtained for propene and the results of the HM zeolite investigation [14], the bands observed may be attributed to ally1 and polyenyl carbenium ions.
C.kem&y
and Physics 39 (1994) 13-20
IJV-Vis
I
wo
MOD &lfQca
sbo
700
’x,nm
Fig. 3. IR and WV-b% spectra for H(D)-ZSM-5 zeolite (M=30f treated with butadiene and methanol: D-ZSM-5 zeolite treated with C,D, at 298 K, 1.33 N m-* (spectrum la), 133.3 N mW2 (2 and 2a) and 13.33 N me2 (3 and 3a); H-ZSMJ zeohte treated with C,H, at 0.66 kN m-* and outgassed at 473 K (4 and 4a); H-ZSM-5 zeolite treated with CH,OH at 495 K, 6.66 kN m-‘, for 4 h and outgassed at 473 K (5 and 5a); D-ZSM-5 zeolite (M=90) treated with CD,OD at 573 K, 5.33 kN m-‘, for 2 h and outgassed at 473 K (6). Thus, the study of butadiene and H-ZSM-5 zeolite interaction supports the above conclusion about the formation of alkenyl carbenium ions from propene. The the~ostab~i~ of alkenyl cations formed from butadiene and propene is the same. The formation of alkenyl carbenium ions is probably followed by their cyclization, as observed for homogeneous systems [15,26].
3.1.3. Adsorption of methanol It is known that olefins (ethene, propene) are intermediates in methanol conversion on H-ZSM-5 zeolites [28]. Evidently, the interaction of methanol with H-ZSM-5 zeolite should also be followed by formation of alkenyl carbenium ions. Indeed, at temperatures above 473 K, the H-ZSM-5 zeolite treated with methanol exhibits absorption bands at 300, 350-370 and 440 nm in the UV-Vis spectrum (Fig. 3, spectrum 5a). Such a high temperature is necessary because hydrocarbon formation from methanol is limited by formation of C-C (C--C) bonds [11,28]. The position of the above absorption bands is typical for allyl, dienyl and trienyl cations observed for adsorption of propene and butadiene. In addition, in the IR spectra one can observe the appearance of the strong 1510 cm-’ band, the position of which is typical of ally1 carbenium ion vz& (Fig. 3, spectrum 5). When studying the interaction of CD,OD + D-ZSM-5 zeolite with a modulus of 90, we succeeded in detecting only alkenyl carbenium ions. In this case the appearance of a 1480 cm- ’ band with a 1470 cm-’ shoulder was observed; these are typical of ally1 and polyenyl ion absorption, respectively (Fig. 3, spectrum 6). Thus, conversion of methanol, propene and butadiene is followed by formation of alkenyl cations on H-ZSM-5 zeolites.
A.V. Demidov,
3.2. Akylaromatic
A.A. Davydov
/ Materials
carbenium ions on H-ZSM-5 zeolites
3.2.1. Adsorption of benzene, toluene and isopropylbenzene
Adsorption of benzene, toluene and isopropylbenzene on H-ZSM-5 zeolite at 298 K leads to the formation of surface compounds with spectral manifestations in IR and UV-Vis spectra that are close to those of individual hydrocarbons. Spectra of adsorbed toluene are presented in Fig. 4 (spectra 1 and la). The compounds formed are weakly bound to the surface (decomposition in vacuum occurs at 373-473 K) and are probably the r-complexes of aromatic hydrocarbons
PIIt is known [30-321 that for the aromatic hydrocarbon-strong acid homogeneous system, the formation of benzene and benzyl (the names are given according to the classification of Koptyug [30]) carbenium ions is observed, which are sufficiently characterized by absorbance in the 400 nm region. However, in the above-mentioned cases, the formation of carbenium ions does not occur, since their absorbance above 330 nm is not observed. 3.2.2. Adsorption of 1,3_dimethylnaphthalene Adsorption of 1,3_dimethylnaphthalene at 298 K gives rise to a 290 nm absorption band in the UV-Vis spectrum and a low-intensity band at 325 nm (Fig. 4, spectrum
Chemistry and Physics 39 (1994) 13-20
17
3a). The band observed is typical of a&l-substituted naphthalenes. Thus the adsorption of aromatic compounds with two condensed rings also does not lead to the formation of stable carbenium ions. According to the IR spectra, the concentration of adsorbed dimethylnaphthalene is very low, and considering the molecule size, it is more probable that the adsorption occurs on the outer surface of a crystalline H-ZSM-5 zeolite support. This conclusion comes from the absence of disturbance of acidic hydroxyl groups, most of which are located in channels of these zeolites [33]. 3.2.3. Adsorption of triphenylcarbinol
Adsorption of triphenylcarbinol on H-ZSM-5 zeolites is characterized by the appearance of an absorption band with a 400-440 nm maximum, and the increase in absorbance at 260 and below 240 nm (Fig. 4, spectrum 4a). The sample color is yellow. The appearance of a 400-440 nm absorption band may indicate triphenylmethyl cation formation [2,34]. Indeed, the corresponding spectrum of the cations, obtained in acidic homogeneous media, has two bands of similar intensity at 404 and 431-434 nm in the visible region [31,32]. Obviously, the reaction on H-ZSM-5 zeolites occurs as follows:
+H
c
B -H,O
The increase in absorbance at 1635 cm-’ and hydroxyl group disturbance can indicate the appearance of water during this reaction. The presence of adsorbed aromatics is not observed by IR spectroscopy. Evidently, the formation of carbenium ion from such a large molecule takes place only on the outer surface of crystallites. Decomposition of the carbenium ions formed occurs at 473 K under vacuum. 3.2.4. Adsorption of styrene The adsorption of styrene
Fig. 4. IR and UV-Vis spectra for H-ZSMJ zeolite (M=30) treated with the following: toluene and then propene; 1,3-dimethylnaphthalene; triphenylcarbinol; propene and then methanol; propene with water. H-ZSM-5 zeolite treated with C,Hx at 298 K (spectra 1 and la) and then with C,H, at 473 K, 13.33 kN m-*, for 1 h and outgassed at 473 K (2 and 2a); zeolite treated with CIZH12 at 298 K (3a); zeolite treated with CIJ-IISOH at 298 K (4a); zeolite treated with C,H, at 473 K, 26.6 kN m-*, for 2 h and then outgassed at 473 K with further treatment with CH,OH at 473 K, 5.33 kN m-*, for 2 h and outgassing at 473 K (5 and 5a); zeolite treated with &H, (1.33 kN m-‘)+HZO (0.4 kN m-‘) at 473 K for 2 h and outgassed at 473 K (6 and 6a).
on H-ZSM-5 zeolites is followed by the appearance of bands at 320 and 400 nm and an increase in their intensity (Fig. 5, spectra la-3a) with a gradual rise in the amount of adsorbed molecules (doses under 133 N m-*). In the IR spectra of these samples the appearance and intensity increase of the 1415, 1455, 1495 and 1615 cm-’ bands and absorption bands of ~o_~ in the 2800-3100 cm-’ region are observed (Fig. 5, spectra l-3). A simultaneous increase in the intensity of the 320 and 400 nm bands in the UV-Vis spectra and of the more intense 1495 and 1615 cm-’ bands of the aromatic ring vC_e in the IR spectra suggest that these bands can be attributed to the same type of aromatic surface compounds. The appearance of a maximum at long wavelength in the UV-Vis spectrum (320 and 400 nm) is not due
18
A. K Demidov, A.A. Daydov
I Materials Chemistry and Physics 39 (1994) 13-20
cyclic noncharged
Fig. 5. iR and UV-Vis spectra for H-ZSM-5 zeolite (M=30) treated with styrene. Successive treatment with CsH, at 298 K under a pressure of 133 N m-’ (spectra 1 and la)+ 133 N m-* (2 and 2a) + 133 N m-’ (3 and 3a) and outgassed at 473 K (4 and 4a) and 573 K (5 and 5a).
to formation of condensed (with conjugated rings) compounds, since their fo~ation is hardly probable at room temperature. The styrene molecule has the largest-wavelength maximum at 280-290 nm. The 320 and 400 nm absorption bands can be attributed to al~laromatic carbenium ions, since in homogeneous systems the formation of similar structures corresponds to 310-340 and 380-450 nm bands [31,32]. IR spectroscopy data indicate the disappearance of unsaturated C=C bonds conjugated with the ring during styrene adsorption, since in the 1600 cm-” region the doublet typical of styrene molecules is not observed [25] and absorption bands and vibrations of C-H saturated bonds appear at 1455 and 28~3~ cm-‘, respectively. The C=C bond disappearance may be associated with its protonation and formation of carbenium ions. Indeed, according to the UV-Vis data, the absorption bands observed in the IR spectra may be attributed to the alkylaromatic carbenium ions formed. It should be noted that no significant differences are observed in the IR spectra at 1600 and 1500 cm-’ for charged and neutral aromatic rings 1341; this fact complicates the identification of alkylaromatic carbenium ions from IR spectra. The identification of the 320 and 400 nm bands and v,--~ bands at 1495 and 1615 cm-’ as arising from the same type of surface compounds, alkylaromatic carbenium ions, is also confirmed by the simultaneous disappearance of the above bands after outgassing at 573 K (a strong decrease in intensity is observed in the UV-Vis spectra) (Fig. 5, spectra 4, 5, 4a and 5a). From the data obtained it follows that styrene adsorption (this is especially true for large amounts of adsorbents) is accompanied by secondary processes, which is evidenced by the intensity increase of the 1615 cm-’ band and the appearance of new absorption bands at 1480 cm-” (Fig. 5, spectrum 3). The nature of these processes is still unclear; however, the formation of
structures
should not be excluded
]351* Thus, the formation of aromatic cations on H-ZSM5 zeolite does not occur, in the case of adsorption of aromatic hydrocarbons and condensed hydrocarbons (dimethylnaphthalene). Alkylaromatic carbenium ions form either during protonation of an unsaturated substituent of the aromatic ring (for styrene) or during triphenylcarbinol dehydro~lation. It is evident that the presence of carbenium ions on H-ZSM-5 zeolite is due to conjugation of the phenyl ring(s) with a carbenium ion being formed via the reactions mentioned above. The lower stability of triphenylmethyl carbenium ions, in comparison with alkylaromatic carbenium ions formed from styrene, is due to the arrangement of the latter in the zeolite pore volume, which causes a threedimensional arrangement of solvating elements (oxygen ions).
3.2.5. Propene adsorption on the preadsorbed toluene As mentioned above, toluene adsorption at 298-473 K results in the fo~ation of complexes weakly bound to the surface. At 298-473 K, propene adsorption is followed by alkenyl carbenium ion formation, for which absorbances at 1510 cm-’ and 290,350-370 and 430-440 nm are typical (see Section 3.1). Successive feeding of propene on the toluene-pretreated H-ZSM-5 at 298 K and sample heating at 473 K are followed by a considerable increase in intensity of the 1615 cm-’ absorption band (Fig. 4, spectrum 2). In this case the UV-Vis spectrum shows the 290 nm absorption band typical of ally1 carbenium ions; 320 and 400 nm bands are also observed (Fig. 4, spectrum 2a). The rather high stability of the surface compounds observed in vacuum (up to 573 K) and the appearance of a maximum at large wavelengths in the UV-Vis spectrum do not suggest the presence of products of toluene alkylation by propene (alkylbenzenes) after outgassing at 473 K. As in the case of alkylaromatic carbenium ion formation from styrene on H-ZSM-5 zeohte, spectral manifestations (320 and 400 nm bands and a 1615 cm-’ absorption band) and the thermal stability of the surface compounds obtained from a propene + toluene mixture indicate alkylaromatic carbenium ion formation. The presence of an absorption band at 1505 cm - ’ is due to +_e bands overlapping phenyl rings and v~ecc (ally1 carbenium ions). By analogy with the mechanism suggested for aromatic hydrocarbon alkylation by propene [35], one may suppose that alkylaromatic carbenium ions are formed (ally1 carbenium ions as an alkylation agent) via the following reaction:
A.V. Demidov,
AA.
Davydov
/ Materiab
+ R-Cl--CH-CH-CH: +
+ CH7
<’ o-
-
H,C-C-CH2-CH,-R
-0
rd. \ \ ..’
L
CH3
In this instance the appearance of minor amounts of alkenyl carbenium ions (weak absorbance at 290 nm, as compared with that of pure propene) may also be explained by their interaction with toluene molecules. 3.2.6. Adsorption of butadiene At 298 K, the adsorption of butadiene on H-ZSM5 zeolites leads to the formation of alkenyl carbenium ions (see Section 3.1), which are characterized by absorption bands at 1500-1530 cm-’ in the IR spectra and at 310, 350-380 and 440 nm in the UV-Vis. The increasing amount of adsorbed butadiene results in the appearance of a 1615 cm-’ absorption band (Fig. 3, spectrum 4). For deuterated reagents (C.,D6 and DZSM5 zeolites), the corresponding absorption band is observed at 1590 cm-‘, i.e., the isotope shift is ca. 25 cm-‘. Removal of the compounds formed occurs after outgassing at 573 K. The appearance of the 1615 cm-’ absorption band and the same constant thermal stability may be taken as evidence for the formation of alkylaromatic carbenium ions during adsorption of butadiene, as in the case of styrene adsorption and propene-toluene interaction at 473 K. The isotope shift revealed is typical of aromatic hydrocarbons [24]. We have not succeeded in detecting the corresponding UV-Vis absorption bands at 320-330 and 400 nm owing to the presence of intense absorption bands ascribed to alkenyl carbenium ions. The formation of alkylaromatic carbenium ions appears to be obvious, since the cyclization of alkenyl carbenium ions proceeds easily enough [15,31], and a cyclic system with a large number of r-electrons (6 or more, as follows from UV-Vis data) would be aromatic, taking the Hiickel rule into consideration. 3.2.7. Adsorption of methanol Starting at 47-93 K, the methanol treatment produces absorption bands corresponding to alkenyl cations (see Section 3.1) and a 1615 cm-’ absorption band (Fig. 3, spectrum 5). On the basis of the data mentioned in previous sections, a 1600 cm-’ absorption band indicates the formation of alkylaromatic carbenium ions. Only at 573 K does the outgassing lead to the decomposition of such a structure, which is additional support for the assignment of the 1615 cm-’ band to alkylaromatic carbenium ions. As in the case of butadiene adsorption on H-ZSM-5 zeolites, we were not able to register a 400 nm absorption band, typical of alkylaromatic carbenium ions. Alkylaromatic compounds characterized by a 1615-1620 cm- * absorption band are evident when a H-ZSM5 zeolite is treated with methanol at 423 K
Chemistry and Physics 39 (1994) 13-20
19
(H-ZSM-5 zeolite consists of alkenyl carbenium ions, synthesized from propene at 473 K) (Fig. 4, spectra 5 and 5a). Consequently, the data obtained may confirm the assumption of alkenyl carbenium ion transformation to alkylaromatic ions. 3.2.8. Coadsorption of propene with water The interaction of propene with H-ZSM-5 zeolite at 373-473 K gives no absorption bands at 1600 cm-’ or 4ON30 nm (see Section 3.1), which indicates that alkylaromatic carbenium ions are not formed. However, new bands at 1615-1620 cm-’ and 405 nm (Fig. 4, spectra 6 and 6a) appear upon addition (preadsorption) of water molecules to propene at 473 K. The position of the above bands and the thermal stability of the complexes observed give evidence of the formation of alkylaromatic carbenium ions. Thus, water molecules positively affect the rate of alkenyl carbenium ion conversion to alkylaromatic cations.
4. Conclusions (1) Adsorption of propene at 373-473 K, of butadiene at 298 K and of methanol at a473 K on H-ZSM-5 zeolites is accompanied by formation of alkenyl carbenium ions, which are stable up to 523-573 K. (2) Alkenyl carbenium ions have a characteristic v?& band at 1490-1530 cm-’ in the IR spectra, an isotope shift of ca. 30 cm-’ upon D substitution for H, and a series of bands, namely at 290-310 nm (allyl), 350-380 nm (dienyl) and 430-450 nm (trienyl carbenium ions), in the UV-Vis spectra. (3) The formation of alkylaromatic carbenium ions on H-ZSM5 zeolites has been registered upon adsorption of styrene (298 K) and of triphenylcarbinol (298 K); coadsorption of toluene with propene (473 K) and of propene with water (473 K); adsorption of butadienc (298 K) and methanol (2473 K); as well as upon methanol interaction with alkenyl carbenium ions previously obtained from propenc. Decomposition of carbenium ions under vacuum occurs at 523-573 K, except for triphcnylcarbenium ions obtained from triphcnylcarbinol, whose decomposition occurs at 473 K. For adsorption of benzene, toluene, isopropylbenzene and 1,3-dimethylnaphthalene, alkylaromatic carbenium ions have not been detected; however, formation of more weakly bound complexes has been observed. (4) Alkylaromatic carbenium ions are characterized by stretching vibrations of the aromatic ring at 1500 and 1600 cm-’ in the IR spectra and by 320-330 and 4OO~l40 nm bands in the UV-Vis spectra. (5) Using the data obtained, one can follow the route of carbenium ion formation. For propene the reactions steps are:
20
A.V. Demidov,
A.A.
Davydov
I Materials Chemistry
N.K. Deno, in S.G. Cohen, A. Strcihviser, Jr., and R.W. Taft (eds.), Progress in Physical Organic Chemistry, Vol. 1, Wiley, New York, 1963, p. 393. WI J. Novakova, L. Kubelkova, Z. Dolejsek and P. Jiru, Collect. Czech. Chem. Commun., 44 (1979) 3341. Proc. Jnt. Symp. P71 J. Haber, J. Komorek and T. Romotowski, Zeolite Catalyst, Siofok, Hungary, 1985, p. 671. M. Gorska, J. Eysymantt and A. Salek, WI B. Kontnik-Mattechka, J. Mol. Strucr., 80 ( 1982) 199. and V. Nenova, J. 1191 H. Lechert, C. Dimitrov, C. Bezuhanova Catal., 80 (1983) 457. A.V. Demidov, A.A. Davydov and L.N. Kurina, Proc. 4th AIIPI Union Conf Mechanisms of Catalytic Reactions, Moscow, 1986, Part 1, p. 295. A.V. Demidov, A.A. Davydov and L.N. Kurina, IN. Akad. Nat& PI SSSR, Ser. Khim., 6 (1989) 1229. A.V. Demidov, A.A. Davydov and L.N. Kurina, IN. Akad. Nauk PI SSSR Ser. Khim., 7 (1989) 1486. P.A. Jacobs, E.G. Derouane and J. Weitkamp, J. Chem. Sot., PI Chem. Commun., 12 (1981) 591. [241 L.M. Sverdlov, M.A. Kovner and Ye.P. Krajnov, Vibration Spectra of Multiatomic Molecules, Nauka, Moscow, 1970. LI. Bellamy, Infra-Red Spectra of Compler Molecules, Wiley, PI New York, 1958. T.S. Sorensen, 3. Am. Chem. Sot., 87 (1965) 5075. PI 1271 A.A. Davydov, IR Spectroscopy in Chemistty of Oxide Sutfaces, Nauka, Novosibirsk, 1984. E.G. Derouane, J.B. Nagy, P. Dejaifre, J.H.C. Van Hooff, B.P. PI Spekman, J.C. Vedrine and C. Naccache, /. Caral., 53 (1978) 40. and T. Romotowski, Pal. J. Gem., 1291 J. Haber, J. Komorek-Hodzik 53 (1979) 2589. [301 V.A. Koptyug, Arenonium Ions: Structure and Reactivity, Nauka, Novosibirsk, 1983. 1311 G.A. Olah, C.U. Pittman, R. Waack and M. Doran, /. Am. Chem. Sot., 88 (1966) 1488. 1321 H.H. Perkampus, in V. Gold (ed.), Advances in Physical Organic Chetitty, Vol. 4, Academic Press, New York, 1966. [331 N.-Y. Topsoe, K. Pedersen and E.G. Derouane, J. Catal., 70 (1981) 41. H.G. Karge, Surf: Sci., 40 (1973) 157. J.E. Gcrmain, Catalytic Conversion of Hydrocarbons, Academic ;z; Press, New York, 1969.
WI
oligomer -
ally1 carb. ions --+
polyenyl carb. ions -
alkylaromatic carb. ions
References 111 P.A. PI I31 I41 151 [61 I71
181 I91 WI (111
PI u31
1141
Jacobs, Carboniogenic Activity of Zeolites, Elsevier, Amsterdam, 1977. H.P. Leftin and M.C. Hobson, in D.D. Eley, H. Pines and P.B. W&z (eds.), Advances in Catalysis, Vol. 14, Academic Press, New York, 1963, p. 115. B.V. Liengue and W.K. Hall, Trans. Faraday Sot., 62 (1966) 3229. P.E. Ebcrli, J. Phys. Chem., 71 (1967) 1717. T.J. Week, C.L. Angel], I.R. Iadd and A.P. Bolton, J. Carol, 33 (1974) 256. L.I. Lafer, B.A. Nabiev and V.U. Yakerson, IN. Akad. Nauk SSSR, Ser. Khim., 9 (1975) 1942. V. Boiis, J.C. Vedrine, J.P. Van de Berg, J.P. Wohhuizen and E.G. Derouane, /. Chem. Sot., Faraday Trans. I, 76 (1980) 1606. S. Dzwigai, J. Haber and T. Romotowski, Zeolitq 4 (1984) 147. V.B. Kazanskii, Kinet. Katal., 28 (1987) 47. E.D. Garbowski and H. Praliand, /. Chim. Phys. Phys-Chins. Biol., 76 (1979) 687. J.C. Vedrine, P. Dcjaifve, E.D. Garbowski and E.G. Derouane, in B. Imelic et al. (eds.), Pmt. Int. Symp. Cata&sis by Zeolites, Elsevier, Amsterdam, 1980, p. 29. G. Coudurier, T. Dekamp and H. Praliand, J. Chem. Sot., Faraday Tmns. 1, 78 (1982) 2661. P. Fejes, H. FBrster, I. Kiricsi and J. Seebode, in P.A. Jacobs et al. (eds.), Proc. Inf. Con5 Structure and Reactivity of Modified Zeoiites, Elsevicr, Amsterdam, 1984, p. 91. H. Forster and J. Seebode, Proc. Inl. Symp. Zeolite Cataiyst, Siofok, Hungary, 1985, p. 413.
and Physics 39 (1994) 13-20
Materials
Spectroscopic
Chemistry
and Physics
39 (1994)
evidence for the formation H-ZSM-5 zeolites A.V. Demidov”,
13-20
of carbenium
ions on
A.A. Davydov
Boreskov Institute of Catalysis, Novosibirsk
630090, Russian Federation
Received 27 October 1993; accepted 30 May 1994
Abstract The formation of alkenyl and alkylaromatic cations from methanol, olefin, diene and compounds with a single aromatic ring on H-ZSM-5 zeolites has been detected using IR and UV-Vis spectroscopy. The formation of alkenyl cations can be characterized reliably by the appearance of vas ,--c bands at 1490-1530 cm-’ in IR spectra and bands at 290-310 (allyl), 350-380 (dienyl) and 430-450 (trienyl cations) nm in UV-Vis spectra. The formation of alkylaromatic cations can be characterized by the appearance of stretching vibrations of the aromatic ring at 1500 and 1600 cm-’ and bands at 320-330 and 4OW40 nm, taking into account the thermal stability of surface complexes. Keywords:
Zeolites; Carbenium ions; Adsorption
1. Introduction
The hypothesis concerning the carbenium ion mechanism of hydrocarbon transformation on strong, acidic, heterogeneous catalysts at moderate temperatures is widely discussed in the literature [1,2]. However, direct data on the nature of charged surface compounds that can be obtained only with the use of physical methods are limited and contradictory. Thus, the adsorption of olefins on zeolites is followed by the formation of oligomer-saturated structures (recorded by IR spectroscopy) at room temperature; their cationic nature is discussed in the literature [3-91. It is supposed, on the basis of UV-Vis data [lO-141, that the interaction of olefins with H-ZSM-5, HY and HM zeolites at 298-473 K leads to the formation of ally1 cations characterized by an absorption band at 300 nm. According to the IR spectra of strong, acidic, homogeneous systems, the formation of ally1 carbenium ions should be followed by appearance of an absorption band at 1530 cm-’ [15]. In fact, when the temperature increases during the interaction of olefins with H-ZSM5 and HY zeolites, an absorption band at 1500 cm-’ appears [7,X-19]. However, various authors suggest different interpretations of this absorption band. Some of them associate it with the formation of aromatic structures [7,16,17], *Corresponding
author.
0254-0584/94/$07.00 0 1994 Elsevier Science SSDI 0254-0584(94)01384-S
S.A. All rights
reserved
COO- groups [18], and complexes with the transformation of proton to olefin ‘nonclassic carbenium ion’ [19]. On investigating the interaction of propene with H-ZSM5 zeolites, the authors of Refs. [20-221 (in comparing data obtained by the coupling of IR and UV-Vis methods) suggested that the absorption band at 1500-1540 cm-l belongs to ally1 and polyenyl carbenium ions - r&c - and observed the formation of alkenyl carbenium ions by UV-Vis. Similar results were obtained for IR and UV-Vis studies of the interaction of propene and cyclopropane with HM zeolites [13,14]. In earlier works devoted to the interaction of complex hydrocarbons, such as diphenylethene, on aluminum silicates, the formation of alkylaromatic carbenium ions has been ascertained with UV-Vis spectra [2]. The spectral manifestation of carbenium ions observed is close to that for corresponding homogeneous media. The formation of aromatic carbenium ions such as protonated benzene is expected in the case of olefin and methanol adsorption on H-ZSMJ zeolite, as the 370 nm absorption band has been observed in UV-Vis spectra [ll]. At the same time, the appearance of the corresponding band for H-ZSM5 zeolite similarly treated is attributed to the formation of nonaromatic dienyl carbenium ions on the basis of UV-Vis and IR spectroscopy data [20-221.
14
A.K Demidov, AA. Davydov i Materiab ~~ern~t~ and Physics 39 (1994) 13-20
preliminary purified reagents are given in the figure captions (with dimethylnaphthalene and triphenylcarbinol, the treatment was performed under pressure of saturated vapors). All spectra are given with a subtracted background absorption of the sample.
Thus, an analysis of the literature data shows that the use of physical methods, especially IR spectroscopy together with UV-Vis, allows one to solve the problem of carbenium ion identi~cation on zeolites. At the same time; it is obvious that a more profound investigation of the spectral manifestations of such compounds is necessary (including using molecules with isotope substitution, molecules with different chemical natures, investigating the nature of surface sites involved in the fo~ation of surface impounds, etc.).
3.1. Alkenyl &arb~~i~ ions on H-ZSM-5 zeolites
2. Experimental
3.1.1. Adsorption of propene The adsorption of propene
Zeolites with a SiO,/Al,O, (modulus M) ratio of 30 and 90 were synthesized at Irkutsk State Universi~. According to XPS and IR spectroscopy, these zeolites are similar to H-ZSM5 zeolite. The parameters of the unit cell are as follows: a = 20.08 and 20.05, b = 19.90 and 19.81, and c = 13.38 and 13.37 A for M=30 and 90, respectively. Fig. 1 presents the IR spectrum in the region of fundamental lattice vibrations. The residual content of Na after exchange on NH,+ was not more than 0.12%. The H-ZSM-5 zeolites were treated in 0, and in vacuum for 2-4 h at 773-823 IS. H-ZSM5 zeolite with M=60 has a low degree of crystallization, which is evidenced by the low intensi~ [23] of the absorption band at 545-550 cm-’ (Fig. 1, spectrum 3). Exposure to temperatures above 773 K led to the complete removal of acidic hydroxide groups (with respect to pyridine), characterized by a 3600 cm-’ absorption band. Zeolite samples with 2ft40 mg cm-’ were treated with reagents in a quartz cell [2] with NaCl and optical quartz windows. Deuterated zeolites were prepared through H-D exchange by means of multiple DzO treatment at 573 K. IR spectra were recorded with a Karl Zeiss UR-20 and UV-Vis with a Shimadzu UV300 inst~ment, using a standard reflection attachment at room temperature. Conditions for treatment with
I 500
Fig. 1. IR spectra lattice vibrations:
600
700
800
4,cd
of H-ZSMJ zeolites in the region of fundamental (1) M= 30; (2) iw=90; (3) M=60.
3. Results and discussion
on H-ZSM-5 zeolite at 298 Kwith subsequent outgassing leads to the formation of surface compounds that are characterized in IR spectra by a number of absorption bands in the 6 and 2, regions of saturated C-H bonds (Fig. 2, spectrum 1). In this case strong changes in the absorption of acidic hydroxide groups occurs, i.e., a sharp decrease of the 3610 cm-l absorption band intensity and the appearance of a low-intensi~ band at 3450-3550 cm-’ take place. The appearance of bands in the absorption region of C-H bonds indicates the formation of saturated hydrocarbon surface structures, namely, oligomers [3-81. The heating of oligomer-containing H-ZSM5 zeolite gives rise to a 1510 cm-’ absorption band. Moreover, the intensity of the absorption band at 1510 cm-l tends to increase (Fig. 2, spectra 2 and 3). Similarly, the appearance (at 298 K) and increase in the absorption intensity occur in the UV-Vis spectra in the 300 nm region (Fig. 2, spectra la-3a). Consequently, oligomer heating is followed by the fo~ation of compounds of a new type; a simultaneous increase in the 1510 cm-’ and 290-310 nm absorption band intensities allows them to be attributed to a single surface compound.
ba
Fig. 2. IR and (a) UV-Vis spectra for H(D)-ZSM-5 zeolite with M=30 treated with propene: H-ZSMJ zeolite treated with CsH6 at 298 K, 13.33 kN m-‘, and outgassed at 298 K (spectra 1 and la), at 373 K (2 and 2a) and at 473 K (3 and 3a); H-ZSM-5 (D-ZSM5) zeolite treated with C,H, (spectra 4 and 4a) (or with C,D, (5)) at 473 K 26.66 kN m-* (for C,D, 8 kN m-r). for 2 h and outgassed at 473 K; samples 4 and 4a were outgassed at 573 K (6 and 6a).
The temperature rise to 573 K during sample outgassing is followed by practically complete removal of the compound observed and restoration of the intensity of the 3610 cm-’ absorption band (Fig. 2, spectra 6 and 6a). The formation of the compound characterized by 1510 cm-’ and 290 nm bands can be observed in larger amounts for H-ZSM5 zeolite treated with propene at 473 K (Fig. 2, spectra 4 and 4a). With the use of deuterated reagents (C,D, and DZSM), the 1510 cm-l absorption band shifts to 1480 cm-’ (Fig. 2, spectrum 5), i.e., the isotope shift upon H-D exchange is 30 cm-‘. Both the location and the isotope shift are not typical for C-H bond vibrations [24]. The appearance of the 1510 cm-’ absorption band cannot be attributed to aromatic complexes, since the corresponding band at 1600 cm-’ is not observed (along with the 1500 cm-’ band, this band is typical of the majority of aromatic compounds [24,X]). The appearance of an absorption band at 1550-1600 cm-’ does not occur if deuterated reagents are used. UV spectroscopy has not revealed the presence of aromatic hydrocarbons in the decomposition products of surface compounds. In the 180-240 nm region, the bands typical of olefin and diene hydrocarbons are observed. One can hardly attribute the 1510 cm-’ absorption band to the COO- group vibrations, since the thermostability of these groups (obtained during the study of HCOOH, CH,COOH and CO2 adsorption on the H-ZSM-5 zeolites) is very high and absorption bands are absent above 240 nm in UV-Vis spectra. To attribute the absorption band at 1510-1520 cm-’ observed after treatment of H-ZSM5 zeolites with olefins at 373473 K to the complex with the proton transfer to the olefin C=C bond (‘nonclassic ion’) 1191 seems not to be reasonable. The activation process of the formation of such a complex with relatively high stability in vacuum (up to 573 K) and its manifestation in the 300 nm region in the UV-Vis spectra are not yet clear. Deno [15] has registered a 1530-1540 cm-’ band in IR spectra and attributed it to ally1 cations in strong acidic homogeneous media. The appearance of the 1500 cm-’ absorption band during the investigation of olefin-HM zeolite interaction has also been attributed to the absorption of ally1 ~arbenium ions ($&) 1141. The abode-mentioned literature data and analysis of the 1510 cm-’ band origin may be taken as support for attributing it to ally1 carbenium ions (z$&). The low-frequency location of the ally1 structure (Y&-) with respect to the location of this band in spectra for olefins may be explained by charge delocalization between three hydrocarbon atoms, i.e., by the hybrid structure formation [2]. This attribution of the observed 1510 cm-’ observed to ally1 carbenium ions is in conformity with the data obtained by UV-Vis. It has been established that an absorption band in the 300 nm region in olefin-strong
acid systems corresponds to ally1 carbenium ion formation [15,26]. The analogous conclusion has been made during the investigation of olefin interaction with HY, H-ZSM-5 and HM zeolites [lO-13,20-221 and with other strong acidic oxide systems [2]. The alternative variant of attributing the 300 nm band to the polyene hydrocarbons (triene, tetraene 1261) is not confirmed by IR spectral data, since an absorption band v,,, at 1600 cm- ’ should be observed for such compounds
F% Thus,on
the basis of the data obtained, we can state that the appearance of 1510 cm-’ and 290 nm absorption bands is direct evidence for the formation of ally1 carbenium ions resulting from heating of oligomers on H-ZSM5. It should be noted that the formation of a saturated structure as well as ally1 carbenium ions is caused by the presence of acidic (with respect to pyridine) hydroxyl groups. The formation of the above surface structures does not occur on a partly dehydroxylated sample of H-ZSM-5 zeolite with a low degree of c~stallization, which has nonacidic hydroxyl groups, characterized by 3750 and 3680 cm-’ absorption bands. We suppose that the revealing of oligomer structures as precursors of ally1 carbenium ions indicates the ionic character of the oligomer reaction with the surface (formation of an alkyl ‘carbenium ion’ is assumed). By calling the oligomer surface structure an ‘alkyl carbenium ion’ we do not mean that it is bound via the pure ionic bond; there is obviously some contribution from a covalent component. We only emphasize its difference from pure covalent compounds, namely surface alkoxides (e.g., on -y-Al,O,), for which olefin insertion into a R-C-O-M”’ bond does not occur
WIThe following scheme of alkenyl cation formation from propene has been adopted in the literature [11,13]: + CH,--CR--CR, H+ + CH,=CH-CH,
(1, (R+)
CHa-YH-CH&H-CH,
+c~H~ b (2)
3
CR, +R+
CH,-CH-CH=CH-CH,
_RH(4) 8
CR, CR, CH,-_C---GH-CIJ-CH, +
+
etc.
According to this scheme, the further course of the complex of reactions is expected to be followed by polyenyl (di-, tri-, tetraenyl) carbenium ion formation [13]. Indeed, treatment of H-ZSM-5 zeolite with pro-
16
A.V. Debit,
AA.
Da~dov
I ~afe~a~
pene at 473 K is followed by the appearance of 350-370 and 430-440 nm bands in the UV-Vis spectrum, along with ally1 carbenium ion formation (Fig. 2, spectrum 4a). The location of the bands observed is typical for dienyl and trienyl carbenium ion absorption, respectively (mainly for cyclic carbenium ions); these carbenium ions are obtained in acidic homogeneous media [15,26]. In the IR spectra of D-ZSM5 zeolites treated with GD, at 473 K, a 1470 cm-’ band is observed, as a shoulder near a 1480 cm-’ band (Fig. 2, spectrum 5). We may suppose that along with ally1 cations, similar nonaromatic compounds containing C-C bonds are also formed. The 1470 cm-l band may be attributed to polyenyl cations or, as expected for HM zeolites, to dienyl cations [14]. Thus, the appearance of 350-370 and 430-440 mn absorption bands indicates the formation of dienyl and trienyl carbenium ions, and the corresponding absorbance (found for deuterated reagents) is observed at 1470 cm-l in IR spectra. The thermostability of polyenyl cations, like that of ally1 cations, does not exceed 573 K in vacuum. Such thermostability may be explained by conjugation of rough C=C bonds in alkenyl carhenium ions as well as by their cyclic structure. These factors lead to an increase in the number of negatively charged 02- ions in a solvating cation shell and thus to stabil~ation of cations on II-ZSM-5. The schemes proposed in Refs. Ill] and f133 can be tested - for example, adsorption of diene hydrocarbons (as well as their oligomerization) on zeolites should be followed immediately (at 298 K) by formation of ally1 and polyenyl cations.
3.1.2. Adsorption of butadiene Butadiene adsorption on H-ZSMJ zeolite at 298 K is followed by the appearance of bands at 310,350-380, 440 and 520-570 nm (Fig. 3, spectra la-3a). The distribution of absorption maxima over these wavelengths is close to that reported by Sorensen for alkenyl carbenium ions in acidic homogeneous media [26]. Similar results have been obtained for alkene adsorption on HM zeolites [13]. By monitoring the butadiene dosage it is possible to detect the predominant fo~ation of ally1 carbenium ions using UV-Vis spectroscopy (Fig. 3, spectrum la). An increase in the amount of dienyl and trienyl carbenium ions occurs upon increasing the amount of adsorbed butadiene (Fig. 3, spectra 2a and 3a). Using IR spectroscopy, we can see (more evidently for deuterated reagents) the appearance of 1500 and I480 cm-’ absorption bands (the isotope shift upon H-D exchange is 30 cm-‘) (Fig. 3, spectra 2-4). By analogy with the data obtained for propene and the results of the HM zeolite investigation [14], the bands observed may be attributed to ally1 and polyenyl carbenium ions.
C.kem&y
and Physics 39 (1994) 13-20
IJV-Vis
I
wo
MOD &lfQca
sbo
700
’x,nm
Fig. 3. IR and WV-b% spectra for H(D)-ZSM-5 zeolite (M=30f treated with butadiene and methanol: D-ZSM-5 zeolite treated with C,D, at 298 K, 1.33 N m-* (spectrum la), 133.3 N mW2 (2 and 2a) and 13.33 N me2 (3 and 3a); H-ZSMJ zeohte treated with C,H, at 0.66 kN m-* and outgassed at 473 K (4 and 4a); H-ZSM-5 zeolite treated with CH,OH at 495 K, 6.66 kN m-‘, for 4 h and outgassed at 473 K (5 and 5a); D-ZSM-5 zeolite (M=90) treated with CD,OD at 573 K, 5.33 kN m-‘, for 2 h and outgassed at 473 K (6). Thus, the study of butadiene and H-ZSM-5 zeolite interaction supports the above conclusion about the formation of alkenyl carbenium ions from propene. The the~ostab~i~ of alkenyl cations formed from butadiene and propene is the same. The formation of alkenyl carbenium ions is probably followed by their cyclization, as observed for homogeneous systems [15,26].
3.1.3. Adsorption of methanol It is known that olefins (ethene, propene) are intermediates in methanol conversion on H-ZSM-5 zeolites [28]. Evidently, the interaction of methanol with H-ZSM-5 zeolite should also be followed by formation of alkenyl carbenium ions. Indeed, at temperatures above 473 K, the H-ZSM-5 zeolite treated with methanol exhibits absorption bands at 300, 350-370 and 440 nm in the UV-Vis spectrum (Fig. 3, spectrum 5a). Such a high temperature is necessary because hydrocarbon formation from methanol is limited by formation of C-C (C--C) bonds [11,28]. The position of the above absorption bands is typical for allyl, dienyl and trienyl cations observed for adsorption of propene and butadiene. In addition, in the IR spectra one can observe the appearance of the strong 1510 cm-’ band, the position of which is typical of ally1 carbenium ion vz& (Fig. 3, spectrum 5). When studying the interaction of CD,OD + D-ZSM-5 zeolite with a modulus of 90, we succeeded in detecting only alkenyl carbenium ions. In this case the appearance of a 1480 cm- ’ band with a 1470 cm-’ shoulder was observed; these are typical of ally1 and polyenyl ion absorption, respectively (Fig. 3, spectrum 6). Thus, conversion of methanol, propene and butadiene is followed by formation of alkenyl cations on H-ZSM-5 zeolites.
A.V. Demidov,
3.2. Akylaromatic
A.A. Davydov
/ Materials
carbenium ions on H-ZSM-5 zeolites
3.2.1. Adsorption of benzene, toluene and isopropylbenzene
Adsorption of benzene, toluene and isopropylbenzene on H-ZSM-5 zeolite at 298 K leads to the formation of surface compounds with spectral manifestations in IR and UV-Vis spectra that are close to those of individual hydrocarbons. Spectra of adsorbed toluene are presented in Fig. 4 (spectra 1 and la). The compounds formed are weakly bound to the surface (decomposition in vacuum occurs at 373-473 K) and are probably the r-complexes of aromatic hydrocarbons
PIIt is known [30-321 that for the aromatic hydrocarbon-strong acid homogeneous system, the formation of benzene and benzyl (the names are given according to the classification of Koptyug [30]) carbenium ions is observed, which are sufficiently characterized by absorbance in the 400 nm region. However, in the above-mentioned cases, the formation of carbenium ions does not occur, since their absorbance above 330 nm is not observed. 3.2.2. Adsorption of 1,3_dimethylnaphthalene Adsorption of 1,3_dimethylnaphthalene at 298 K gives rise to a 290 nm absorption band in the UV-Vis spectrum and a low-intensity band at 325 nm (Fig. 4, spectrum
Chemistry and Physics 39 (1994) 13-20
17
3a). The band observed is typical of a&l-substituted naphthalenes. Thus the adsorption of aromatic compounds with two condensed rings also does not lead to the formation of stable carbenium ions. According to the IR spectra, the concentration of adsorbed dimethylnaphthalene is very low, and considering the molecule size, it is more probable that the adsorption occurs on the outer surface of a crystalline H-ZSM-5 zeolite support. This conclusion comes from the absence of disturbance of acidic hydroxyl groups, most of which are located in channels of these zeolites [33]. 3.2.3. Adsorption of triphenylcarbinol
Adsorption of triphenylcarbinol on H-ZSM-5 zeolites is characterized by the appearance of an absorption band with a 400-440 nm maximum, and the increase in absorbance at 260 and below 240 nm (Fig. 4, spectrum 4a). The sample color is yellow. The appearance of a 400-440 nm absorption band may indicate triphenylmethyl cation formation [2,34]. Indeed, the corresponding spectrum of the cations, obtained in acidic homogeneous media, has two bands of similar intensity at 404 and 431-434 nm in the visible region [31,32]. Obviously, the reaction on H-ZSM-5 zeolites occurs as follows:
+H
c
B -H,O
The increase in absorbance at 1635 cm-’ and hydroxyl group disturbance can indicate the appearance of water during this reaction. The presence of adsorbed aromatics is not observed by IR spectroscopy. Evidently, the formation of carbenium ion from such a large molecule takes place only on the outer surface of crystallites. Decomposition of the carbenium ions formed occurs at 473 K under vacuum. 3.2.4. Adsorption of styrene The adsorption of styrene
Fig. 4. IR and UV-Vis spectra for H-ZSMJ zeolite (M=30) treated with the following: toluene and then propene; 1,3-dimethylnaphthalene; triphenylcarbinol; propene and then methanol; propene with water. H-ZSM-5 zeolite treated with C,Hx at 298 K (spectra 1 and la) and then with C,H, at 473 K, 13.33 kN m-*, for 1 h and outgassed at 473 K (2 and 2a); zeolite treated with CIZH12 at 298 K (3a); zeolite treated with CIJ-IISOH at 298 K (4a); zeolite treated with C,H, at 473 K, 26.6 kN m-*, for 2 h and then outgassed at 473 K with further treatment with CH,OH at 473 K, 5.33 kN m-*, for 2 h and outgassing at 473 K (5 and 5a); zeolite treated with &H, (1.33 kN m-‘)+HZO (0.4 kN m-‘) at 473 K for 2 h and outgassed at 473 K (6 and 6a).
on H-ZSM-5 zeolites is followed by the appearance of bands at 320 and 400 nm and an increase in their intensity (Fig. 5, spectra la-3a) with a gradual rise in the amount of adsorbed molecules (doses under 133 N m-*). In the IR spectra of these samples the appearance and intensity increase of the 1415, 1455, 1495 and 1615 cm-’ bands and absorption bands of ~o_~ in the 2800-3100 cm-’ region are observed (Fig. 5, spectra l-3). A simultaneous increase in the intensity of the 320 and 400 nm bands in the UV-Vis spectra and of the more intense 1495 and 1615 cm-’ bands of the aromatic ring vC_e in the IR spectra suggest that these bands can be attributed to the same type of aromatic surface compounds. The appearance of a maximum at long wavelength in the UV-Vis spectrum (320 and 400 nm) is not due
18
A. K Demidov, A.A. Daydov
I Materials Chemistry and Physics 39 (1994) 13-20
cyclic noncharged
Fig. 5. iR and UV-Vis spectra for H-ZSM-5 zeolite (M=30) treated with styrene. Successive treatment with CsH, at 298 K under a pressure of 133 N m-’ (spectra 1 and la)+ 133 N m-* (2 and 2a) + 133 N m-’ (3 and 3a) and outgassed at 473 K (4 and 4a) and 573 K (5 and 5a).
to formation of condensed (with conjugated rings) compounds, since their fo~ation is hardly probable at room temperature. The styrene molecule has the largest-wavelength maximum at 280-290 nm. The 320 and 400 nm absorption bands can be attributed to al~laromatic carbenium ions, since in homogeneous systems the formation of similar structures corresponds to 310-340 and 380-450 nm bands [31,32]. IR spectroscopy data indicate the disappearance of unsaturated C=C bonds conjugated with the ring during styrene adsorption, since in the 1600 cm-” region the doublet typical of styrene molecules is not observed [25] and absorption bands and vibrations of C-H saturated bonds appear at 1455 and 28~3~ cm-‘, respectively. The C=C bond disappearance may be associated with its protonation and formation of carbenium ions. Indeed, according to the UV-Vis data, the absorption bands observed in the IR spectra may be attributed to the alkylaromatic carbenium ions formed. It should be noted that no significant differences are observed in the IR spectra at 1600 and 1500 cm-’ for charged and neutral aromatic rings 1341; this fact complicates the identification of alkylaromatic carbenium ions from IR spectra. The identification of the 320 and 400 nm bands and v,--~ bands at 1495 and 1615 cm-’ as arising from the same type of surface compounds, alkylaromatic carbenium ions, is also confirmed by the simultaneous disappearance of the above bands after outgassing at 573 K (a strong decrease in intensity is observed in the UV-Vis spectra) (Fig. 5, spectra 4, 5, 4a and 5a). From the data obtained it follows that styrene adsorption (this is especially true for large amounts of adsorbents) is accompanied by secondary processes, which is evidenced by the intensity increase of the 1615 cm-’ band and the appearance of new absorption bands at 1480 cm-” (Fig. 5, spectrum 3). The nature of these processes is still unclear; however, the formation of
structures
should not be excluded
]351* Thus, the formation of aromatic cations on H-ZSM5 zeolite does not occur, in the case of adsorption of aromatic hydrocarbons and condensed hydrocarbons (dimethylnaphthalene). Alkylaromatic carbenium ions form either during protonation of an unsaturated substituent of the aromatic ring (for styrene) or during triphenylcarbinol dehydro~lation. It is evident that the presence of carbenium ions on H-ZSM-5 zeolite is due to conjugation of the phenyl ring(s) with a carbenium ion being formed via the reactions mentioned above. The lower stability of triphenylmethyl carbenium ions, in comparison with alkylaromatic carbenium ions formed from styrene, is due to the arrangement of the latter in the zeolite pore volume, which causes a threedimensional arrangement of solvating elements (oxygen ions).
3.2.5. Propene adsorption on the preadsorbed toluene As mentioned above, toluene adsorption at 298-473 K results in the fo~ation of complexes weakly bound to the surface. At 298-473 K, propene adsorption is followed by alkenyl carbenium ion formation, for which absorbances at 1510 cm-’ and 290,350-370 and 430-440 nm are typical (see Section 3.1). Successive feeding of propene on the toluene-pretreated H-ZSM-5 at 298 K and sample heating at 473 K are followed by a considerable increase in intensity of the 1615 cm-’ absorption band (Fig. 4, spectrum 2). In this case the UV-Vis spectrum shows the 290 nm absorption band typical of ally1 carbenium ions; 320 and 400 nm bands are also observed (Fig. 4, spectrum 2a). The rather high stability of the surface compounds observed in vacuum (up to 573 K) and the appearance of a maximum at large wavelengths in the UV-Vis spectrum do not suggest the presence of products of toluene alkylation by propene (alkylbenzenes) after outgassing at 473 K. As in the case of alkylaromatic carbenium ion formation from styrene on H-ZSM-5 zeohte, spectral manifestations (320 and 400 nm bands and a 1615 cm-’ absorption band) and the thermal stability of the surface compounds obtained from a propene + toluene mixture indicate alkylaromatic carbenium ion formation. The presence of an absorption band at 1505 cm - ’ is due to +_e bands overlapping phenyl rings and v~ecc (ally1 carbenium ions). By analogy with the mechanism suggested for aromatic hydrocarbon alkylation by propene [35], one may suppose that alkylaromatic carbenium ions are formed (ally1 carbenium ions as an alkylation agent) via the following reaction:
A.V. Demidov,
AA.
Davydov
/ Materiab
+ R-Cl--CH-CH-CH: +
+ CH7
<’ o-
-
H,C-C-CH2-CH,-R
-0
rd. \ \ ..’
L
CH3
In this instance the appearance of minor amounts of alkenyl carbenium ions (weak absorbance at 290 nm, as compared with that of pure propene) may also be explained by their interaction with toluene molecules. 3.2.6. Adsorption of butadiene At 298 K, the adsorption of butadiene on H-ZSM5 zeolites leads to the formation of alkenyl carbenium ions (see Section 3.1), which are characterized by absorption bands at 1500-1530 cm-’ in the IR spectra and at 310, 350-380 and 440 nm in the UV-Vis. The increasing amount of adsorbed butadiene results in the appearance of a 1615 cm-’ absorption band (Fig. 3, spectrum 4). For deuterated reagents (C.,D6 and DZSM5 zeolites), the corresponding absorption band is observed at 1590 cm-‘, i.e., the isotope shift is ca. 25 cm-‘. Removal of the compounds formed occurs after outgassing at 573 K. The appearance of the 1615 cm-’ absorption band and the same constant thermal stability may be taken as evidence for the formation of alkylaromatic carbenium ions during adsorption of butadiene, as in the case of styrene adsorption and propene-toluene interaction at 473 K. The isotope shift revealed is typical of aromatic hydrocarbons [24]. We have not succeeded in detecting the corresponding UV-Vis absorption bands at 320-330 and 400 nm owing to the presence of intense absorption bands ascribed to alkenyl carbenium ions. The formation of alkylaromatic carbenium ions appears to be obvious, since the cyclization of alkenyl carbenium ions proceeds easily enough [15,31], and a cyclic system with a large number of r-electrons (6 or more, as follows from UV-Vis data) would be aromatic, taking the Hiickel rule into consideration. 3.2.7. Adsorption of methanol Starting at 47-93 K, the methanol treatment produces absorption bands corresponding to alkenyl cations (see Section 3.1) and a 1615 cm-’ absorption band (Fig. 3, spectrum 5). On the basis of the data mentioned in previous sections, a 1600 cm-’ absorption band indicates the formation of alkylaromatic carbenium ions. Only at 573 K does the outgassing lead to the decomposition of such a structure, which is additional support for the assignment of the 1615 cm-’ band to alkylaromatic carbenium ions. As in the case of butadiene adsorption on H-ZSM-5 zeolites, we were not able to register a 400 nm absorption band, typical of alkylaromatic carbenium ions. Alkylaromatic compounds characterized by a 1615-1620 cm- * absorption band are evident when a H-ZSM5 zeolite is treated with methanol at 423 K
Chemistry and Physics 39 (1994) 13-20
19
(H-ZSM-5 zeolite consists of alkenyl carbenium ions, synthesized from propene at 473 K) (Fig. 4, spectra 5 and 5a). Consequently, the data obtained may confirm the assumption of alkenyl carbenium ion transformation to alkylaromatic ions. 3.2.8. Coadsorption of propene with water The interaction of propene with H-ZSM-5 zeolite at 373-473 K gives no absorption bands at 1600 cm-’ or 4ON30 nm (see Section 3.1), which indicates that alkylaromatic carbenium ions are not formed. However, new bands at 1615-1620 cm-’ and 405 nm (Fig. 4, spectra 6 and 6a) appear upon addition (preadsorption) of water molecules to propene at 473 K. The position of the above bands and the thermal stability of the complexes observed give evidence of the formation of alkylaromatic carbenium ions. Thus, water molecules positively affect the rate of alkenyl carbenium ion conversion to alkylaromatic cations.
4. Conclusions (1) Adsorption of propene at 373-473 K, of butadiene at 298 K and of methanol at a473 K on H-ZSM-5 zeolites is accompanied by formation of alkenyl carbenium ions, which are stable up to 523-573 K. (2) Alkenyl carbenium ions have a characteristic v?& band at 1490-1530 cm-’ in the IR spectra, an isotope shift of ca. 30 cm-’ upon D substitution for H, and a series of bands, namely at 290-310 nm (allyl), 350-380 nm (dienyl) and 430-450 nm (trienyl carbenium ions), in the UV-Vis spectra. (3) The formation of alkylaromatic carbenium ions on H-ZSM5 zeolites has been registered upon adsorption of styrene (298 K) and of triphenylcarbinol (298 K); coadsorption of toluene with propene (473 K) and of propene with water (473 K); adsorption of butadienc (298 K) and methanol (2473 K); as well as upon methanol interaction with alkenyl carbenium ions previously obtained from propenc. Decomposition of carbenium ions under vacuum occurs at 523-573 K, except for triphcnylcarbenium ions obtained from triphcnylcarbinol, whose decomposition occurs at 473 K. For adsorption of benzene, toluene, isopropylbenzene and 1,3-dimethylnaphthalene, alkylaromatic carbenium ions have not been detected; however, formation of more weakly bound complexes has been observed. (4) Alkylaromatic carbenium ions are characterized by stretching vibrations of the aromatic ring at 1500 and 1600 cm-’ in the IR spectra and by 320-330 and 4OO~l40 nm bands in the UV-Vis spectra. (5) Using the data obtained, one can follow the route of carbenium ion formation. For propene the reactions steps are:
20
A.V. Demidov,
A.A.
Davydov
I Materials Chemistry
N.K. Deno, in S.G. Cohen, A. Strcihviser, Jr., and R.W. Taft (eds.), Progress in Physical Organic Chemistry, Vol. 1, Wiley, New York, 1963, p. 393. WI J. Novakova, L. Kubelkova, Z. Dolejsek and P. Jiru, Collect. Czech. Chem. Commun., 44 (1979) 3341. Proc. Jnt. Symp. P71 J. Haber, J. Komorek and T. Romotowski, Zeolite Catalyst, Siofok, Hungary, 1985, p. 671. M. Gorska, J. Eysymantt and A. Salek, WI B. Kontnik-Mattechka, J. Mol. Strucr., 80 ( 1982) 199. and V. Nenova, J. 1191 H. Lechert, C. Dimitrov, C. Bezuhanova Catal., 80 (1983) 457. A.V. Demidov, A.A. Davydov and L.N. Kurina, Proc. 4th AIIPI Union Conf Mechanisms of Catalytic Reactions, Moscow, 1986, Part 1, p. 295. A.V. Demidov, A.A. Davydov and L.N. Kurina, IN. Akad. Nat& PI SSSR, Ser. Khim., 6 (1989) 1229. A.V. Demidov, A.A. Davydov and L.N. Kurina, IN. Akad. Nauk PI SSSR Ser. Khim., 7 (1989) 1486. P.A. Jacobs, E.G. Derouane and J. Weitkamp, J. Chem. Sot., PI Chem. Commun., 12 (1981) 591. [241 L.M. Sverdlov, M.A. Kovner and Ye.P. Krajnov, Vibration Spectra of Multiatomic Molecules, Nauka, Moscow, 1970. LI. Bellamy, Infra-Red Spectra of Compler Molecules, Wiley, PI New York, 1958. T.S. Sorensen, 3. Am. Chem. Sot., 87 (1965) 5075. PI 1271 A.A. Davydov, IR Spectroscopy in Chemistty of Oxide Sutfaces, Nauka, Novosibirsk, 1984. E.G. Derouane, J.B. Nagy, P. Dejaifre, J.H.C. Van Hooff, B.P. PI Spekman, J.C. Vedrine and C. Naccache, /. Caral., 53 (1978) 40. and T. Romotowski, Pal. J. Gem., 1291 J. Haber, J. Komorek-Hodzik 53 (1979) 2589. [301 V.A. Koptyug, Arenonium Ions: Structure and Reactivity, Nauka, Novosibirsk, 1983. 1311 G.A. Olah, C.U. Pittman, R. Waack and M. Doran, /. Am. Chem. Sot., 88 (1966) 1488. 1321 H.H. Perkampus, in V. Gold (ed.), Advances in Physical Organic Chetitty, Vol. 4, Academic Press, New York, 1966. [331 N.-Y. Topsoe, K. Pedersen and E.G. Derouane, J. Catal., 70 (1981) 41. H.G. Karge, Surf: Sci., 40 (1973) 157. J.E. Gcrmain, Catalytic Conversion of Hydrocarbons, Academic ;z; Press, New York, 1969.
WI
oligomer -
ally1 carb. ions --+
polyenyl carb. ions -
alkylaromatic carb. ions
References 111 P.A. PI I31 I41 151 [61 I71
181 I91 WI (111
PI u31
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and Physics 39 (1994) 13-20