HZSM-5 zeolites

Fig. 2. Isomerization of 1-methylnaphthalene on zeolite HZSM-5 at 300 °C and 400 C. There are no side reactions, hence Xi.m.np = Y2.M.NP ...

Trapped by a suitable compound, a transient intermediate can be converted into a more stable species for unequivocal identification. Stepanov and Luzgin (82) investigated the reaction of acetonitrile with 1-octene or tert-butyl alcohol on acidic zeolite HZSM-5 ( 2si/ Ai = 49) at 296 K by in situ MAS NMR spectroscopy under batch reaction conditions. Upon coadsorption of acetonitrile and 1-octene, a C MAS NMR signal at 108 ppm was observed, indicative of TV-alkylnitrilium ions 2 in Scheme 3. As depicted in Scheme 3a, the formation of these cations was explained by trapping the chemically unstable alkylcarbenium ions (formed from the adsorbed... 

Carboxylic acids can also be formed by a reaction of small alkanes, carbon monoxide, and water on solid acid catalysts (93,94). By in situ C MAS NMR spectroscopy (93), the activation of propane and isobutane on acidic zeolite HZSM-5 was investigated in the presence of carbon monoxide and water. Propane was converted to isobutyric acid at 373 73 K, while isobutane was transformed into pivalic acid with a simultaneous production of hydrogen. On SZA, methyl isopropyl ketone was observed as evidence for the carbonylation of isobutane with carbon monoxide after the sample was held at 343 K for 1 h (94). When the reaction of isobutane and carbon monoxide was carried out in the presence of water, pivalic acid was identified as the main reaction product (94). These observations are rationalized by the existence of a small number of sites capable of generating carbenium ions, which can be further trapped by carbon monoxide (93). 

Isotopic labeling is a powerful tool being used to understand the nature of transition states as well as the mechanisms of surface-catalyzed reactions. By in situ MAS NMR spectroscopy, the scrambling of labels at room temperature was investigated upon adsorption of p C-1]- 1-octene on calcined zeolite HZSM-5 (wsi/... 

Figure 19 shows the C CP/MAS NMR spectra recorded lOmin to 53 h after adsorption of [ C-l]-l-octene on zeolite HZSM-5. The spectra indicate that the C-labeled terminal olefinic =Cll2 groups evident in the spectra at ca. 113 ppm are transformed into terminal C-labeled paraffinic CH3 groups, evident at 14.3 ppm. Furthermore, the selective label penetrates into internal -CH2- groups of the octene hydrocarbon skeleton, characterized by resonances occurring at 33.0 ppm. 

B. Keto-Enol Isomerization on Acidic Zeolite HZSM-5 Evidenced by H/D Exchange... 

Figure 20 shows the H MAS NMR spectra of zeolite HZSM-5 (ksi/wai — 21.5) before (a) and after (b) adsorption of acetone-tfg at room temperature. The ll MAS NMR signals at 4.0 and 1.8 ppm are attributed to bridging OH groups (SiOHAl) and silanol groups (SiOH), respectively (Fig. 20a). Upon adsorption of 0.33 mmol...

The H/D exchange between the methyl groups of adsorbed acetone molecules and the Bronsted acid sites of zeolite HZSM-5 was also observed upon adsorption of C-2-acetone on a deuterated catalyst (D,HZSM-5, nsi/ Ai = 21.5) at room temperature (Figs 20c and d). The " C MAS NMR spectrum of C-2-acetone adsorbed on zeolite D,HZSM-5 (Fig. 20e) consists of the carbonyl signal at 223 ppm with a featured sideband pattern and a methyl signal at 29 ppm. No significant... 

In contrast, an extremely low activity was observed for the gallium-modified silicalite-1. scrambling started first at 723 K, which clearly indicates that Bronsted acid sites are necessary to activate propane adsorbed on zeolites Ga/ HZSM-5 179,181. A low activity was also observed for C-2-propane adsorbed on zeolite HZSM-5 in the absence of gallium. On this catalyst, C scrambling was observed after heating at 573 K for 20 min, and the theoretical 2 1 ratio of the signal intensities of methyl and methylene groups was reached after 80 min at 573 K. 

In a recent work (207), in situ ll and Al MAS NMR spectroscopy was used to investigate the Bronsted acid sites of zeolite HZSM-5 during the conversion of methanol under CF conditions. As a consequence of the application of MAS instead of the spin-echo technique, only framework Al atoms involved in quadrupolar interactions according to Cqcc < 5 MHz could be observed. Upon injection of methanol into the MAS NMR rotor reactor filled with calcined zeolite HZSM-5, a single Al MAS NMR signal arose at 54 ppm at reaction temperatures up to... 

Among the early investigations of methanol adsorption and conversion on acidic zeolites, most of the H and C MAS NMR experiments were performed under batch reaction conditions with glass inserts in which the catalyst samples were fused. Zeolites HZSM-5 76a,204,206,264-272), HY 71,72), H-EMT 273), HZSM-12 274), HZSM-23 275), H-erionite 275), H-mordenite 271,272), and H-offretite 275,276), silicoaluminophosphates H-SAPO-5 271,274), H-SAPO-11 274), and H-SAPO-34 76,277,278), as well as montemorillonite 279) and saponite 279) were investigated as catalysts. 

To shed more light on this issue, the steady state of methanol conversion on zeolites HZSM-5, H-SAPO-34, and H-SAPO-18 was characterized by CF MAS NMR spectroscopy under CF reaction conditions (49,261). 

Fig. 36. C and H CF MAS NMR spectra recorded during the alternating conversion of C-enriched and non-enriched methanol (IV/F= 25gh/mol) on calcined zeolite HZSM-5 (nsi/wAi = 22) at reaction temperatures of 548 and 573 K. The relative intensities given below the spectra were determined by integration in the absolute intensity mode by using the spectra obtained during the conversion of C-enriched methanol as intensity standard. Reproduced with permission from (300). Copyright 2003 Kluwer Academic.

Recently, a novel CF MAS NMR-UV/Vis technique (Fig. 17, Section III.B) was applied to characterize the formation of hydrocarbons by the conversion of methanol on a weakly dealuminated zeolite HZSM-5 6S). The C MAS NMR spectrum recorded at 413 K during the continuous conversion of C-enriched methanol (Fig. 37a, left) consists of signals at 51 and 61 ppm attributed to methanol and DME, respectively. The very weak signal at ca. 23 ppm is probably an indication of alkanes or alkylated cyclic compounds. The appearance of the signals at 23 and 61 ppm indicates that the conversion of methanol on weakly dealuminated zeolites HZSM-5 starts even at 413 K. The simultaneously recorded UV/Vis spectrum (Fig. 37a, right) consists of bands at 275, 315, and 375 nm. The band at 275 nm indicates the formation of neutral aromatic compounds 301,302), and those at 315 and 375 nm may be assigned to mono- and dienylic carbenium ions (301,302), respectively. Because the UV/Vis spectrum of the non-dealuminated zeolite HZSM-5, that... 

Fig. 37. C CF MAS NMR (left) and UV/Vis (right) spectra of a dealuminated zeolite HZSM-5 recorded during conversion of ( -enriched methanol (W/F = 25gh/mol) at 413 K for 2h (a), during a subsequent conversion of T l U = T l U (W/F = 10 g h/mol) at 413 K for 1 h (b), and during conversion of V H2 = V H2 (W/F= 10 g h/mol) at 413 K on a fresh catalyst for 2 h (c). Asterisks denote spinning sidebands. The narrow peaks at ca. 500 nm in UV spectra were caused by the equipment. Reproduced with permission from 168). Copyright 2004 The Royal Society of Chemistry.

The weakly dealuminated zeolite HZSM-5 used to convert methanol was subsequently applied to investigate the conversion of ethylene ( C-isotopes in natural abundance) (Fig. 37b). MAS NMR signals, appearing at 14, 23, and 32 ppm during conversion of ethylene at 413 K for 1 h (Fig. 37b, left), are assigned to alkyl groups of small amounts of alkylated cyclic compounds, such as cyclopentene, cyclohexene, cyclohexadiene, and/or benzene. The simultaneously recorded UV/Vis spectrum (Fig. 37b, right) shows bands at 300 and 375 nm, which characterize the formation of neutral cyclic compoimds and dienylic carbenium ions, respectively (301). 

The conversion of ethylene on a fresh zeolite HZSM-5 catalyst, which had not been used beforehand for methanol conversion, led to the spectra shown in Fig. 37c. The MAS NMR spectrum consists of signals at 14, 24, and 34 ppm caused by alkyl groups of cyclic compounds. Furthermore, a broad signal in the chemical shift range of alkenic and aromatic compounds appeared at ca. 120 ppm. The UV/Vis spectrum consists of bands similar to those shown in Fig. 37b and an additional weak band at ca. 450 nm. The latter may be attributed to condensed aromatics or trienylic carbenium ions (301). A weak shoulder observed at ca. 400 nm is an indication for the formation of hexamethylbenzenium ions (302). 

The simultaneous investigation of the methanol conversion on weakly dealuminated zeolite HZSM-5 by C CF MAS NMR and UV/Vis spectroscopy has shown that the first cyclic compounds and carbenium ions are formed even at 413 K. This result is in agreement with UV/Vis investigations of the methanol conversion on dealuminated zeolite HZSM-5 performed by Karge et al (303). It is probably that extra-framework aluminum species acting as Lewis acid sites are responsible for the formation of hydrocarbons and carbenium ions at low reaction temperatures. NMR spectroscopy allows the identification of alkyl signals in more detail, and UV/Vis spectroscopy gives hints to the formation of low amounts of cyclic compounds and carbenium ions. 

Fig. 1. 300-MHz H MAS spectra of zeolite HZSM-5 over a range of temperatures. In addition to the well-known Brpnsted sites (4.3 ppm) and external silanols (2.0 ppm), a broad shoulder at 296 K sharpened to a third peak at 6.9 ppm when the sample temperature was reduced to 123 K The spinning speed was 3.5 kHz. (Reprinted with permission from Beck et al. (24). Copyright 1995 American Chemical Society.)...

Fig. 12. 75.4-MHz 13C CP/MAS spectra showing the formation of cyclopentenyl cations 8 from cycIopentene-13Ci (random) on zeolite HZSM-5 and 4 from propene-2-,3C on zeolite UY.

Fig. 14. 90.4-MHz 13C MAS spectra of styrene- -l3C reacting on zeolite HZSM-5. The methylindanyl cation 12 (251 ppm), formed through the cracking of the cyclic dimer (cf. Fig. 6) followed by intramolecular hydride transfer, was converted to naphthalene at 523 K. This is the clearest example of a free carbenium ion as a reaction intermediate on a zeolite.

Our study of propene-J-13C, -2-13C, and -5-13C reacting on zeolite HZSM-5 clearly shows that the isopropyl cation is not formed in measurable concentration as a persistent species (45). Furthermore, there is no label scrambling of the 2 position, although 1,3-label scrambling is facile on the zeolite. This strongly argues against a free isopropyl cation—even as a transient intermediate At low temperature, the equilibrium structure of propene is a 77 complex 22 with the Brpnsted site. This mode of coordination... 

The analogous cations 29 and 30 readily formed in HZSM-5, HY, and even the much weaker acid HX by the reaction of methanol with dimethyl sulfide or dimethyl selenide (145). Formation of the trimethylselenonium ion in zeolite HZSM-5 was established unambiguously by the observation the 77Se resonance (22 ppm downfield of dimethyl selenide). Extensive evaluation of the reactivity of methanol in the presence of trimethylchalco-genonium ions found no evidence of a rate enhancement in MTG chemistry, and this mechanism (Scheme 2) was rejected. 

Grey and Vega (161) have demonstrated the use of the TRAPDOOR experiment for measuring the 27Al quadrupole coupling constant in zeolites and applied it to studies of trimethylamine in zeolites. In other spectroscopic work, Fripiat and co-workers (162) used REDOR and various other NMR methods to characterize acid sites in zeolites treated with ammonia. Ernst and Pfeiffer (163) have reported a 13C MAS NMR study of the reactions of methanol and ammonia to make methylamines in zeolite HZSM-5. We reported NMR evidence of the synthesis of cyclopentylamine from cyclo-pentanol and ammonia on zeolite CsX (102). 

Bosacek and co-workers (146) reported spectroscopic evidence that acetone reacts with ammonia on zeolite HZSM-5 (MFI) to form the corre-... 

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