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Application of in situ MAS NMR for elucidation of reaction mechanisms in heterogeneous catalysis
Colloids and Surfaces A: Physicochemical and Engineering Aspects 158 (1999) 189 – 200 www.elsevier.nl/locate/colsurfa
Application of in situ MAS NMR for elucidation of reaction mechanisms in heterogeneous catalysis Irina I. Ivanova * Moscow State Uni6ersity, Department of Chemistry, Leninskie Gory, 119899 Moscow, Russia
Abstract This short review deals with the recent applications of in situ MAS NMR techniques for the unravel of the mechanisms of heterogeneous catalytic reactions. In the first part, the different approaches for realization of in situ MAS NMR experiments in static and flow conditions are considered. In the second part, the main application areas are discussed and a cross reference index between the reactions studied, the catalysts used, the mechanistic information obtained and the corresponding literature sources is established. In the third part, the capabilities of the in situ MAS NMR techniques are illustrated with examples taken from our recent results on the mechanistic studies of various catalytic reactions over mono- and bifunctional zeolite and oxide catalysts. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Review; In situ MAS NMR; Adsorbed phase; Mechanisms; Zeolites; Dealumination; Alkylation; Isomerization; Alkane activation
1. Introduction The growing role of heterogeneous catalysis in chemical technology, biotechnology and ecology has been repeatedly emphasized during the last years. However the progress in this field, in particular, in tuning and mastering of the catalytic properties of the existing catalysts and in the development of new methods of catalyst preparation is limited because of the lack of detailed information on the reaction mechanisms and the nature of active sites.
* Fax: + 7-095-9328846. E-mail address: [email protected] (I.I. Ivanova)
Heterogeneous catalytic reactions are usually studied by analysis of the desorbed reactants and products. This method however does not allow to follow the reactant and the catalyst under ‘working’ conditions directly in course of catalytic reaction. This information can be best obtained by means of recently developed in situ spectroscopic techniques (IR, UV–vis, ESR, NMR), among which NMR spectroscopy is considered to be one of the most informative. The first attempts for the investigation of heterogeneous catalytic reactions in situ by means of NMR techniques can be referred to the beginning of 1970s. At that time, the conventional high resolution Fourier transform 13C NMR was applied to studies of reacting adsorbates, looking
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into product formation and reaction kinetics [1– 7]. The application of the technique was limited to the studies of the reactions of simple molecules, which retain sufficient mobility in the adsorbed phase. The main reasons for that were rather low sensitivity and resolution of NMR techniques in adsorbed phase due to chemical shift anisotropy and dipolar interactions between nuclei of strongly adsorbed species in which molecular motion is reduced. At present, not only conventional high-resolution techniques are applied to studies of catalytic reactions, but also more sophisticated methods such as magic-angle-spinning, cross-polarization, J-resolved spectroscopy, correlation spectroscopy, multiple-quantum NMR, spin-echo, spin-echo double resonance, pulse field gradient technique and others [8 – 12]. This contribution is devoted to the applications of MAS NMR techniques for elucidation of reaction mechanisms. It contains a brief review of the experimental approaches used for in situ MAS NMR studies and main application areas of the technique. The emphasis is made on the analysis of the main capabilities of the techniques, which are illustrated with examples taken from our recent works.
2. Experimental approaches At present, there are two general experimental protocols used to carry out in situ MAS NMR
studies. The first one, which is used over the last 10 years models a batch reactor catalytic experiments. This approach makes use of sealed highly symmetrical glass ampoules containing a catalyst and an adsorbate, and fitting precisely into MAS rotors [13–19] or gas tight MAS rotors with a catalyst and an adsorbate [20–22]. In the latter case, CAVERN apparatus were designed for sealing and unsealing MAS rotors directly on a vacuum line [20–22]. The results obtained using these experimental approaches were reviewed and discussed several times [22–27]. The second, recently developed experimental approach, allows to carry out in situ MAS NMR investigations under continuous flow conditions. There are several different designs reported, which allow for in situ flow MAS NMR experiments [28–30]. Fig. 1 shows a drawing of a design proposed by Hunger and Horvath [29], which is the easiest to fabricate. This design is based on a commercial 7-mm Bruker MAS NMR probe. The reactants are injected into the rotor with carrier gas, which passes through the glass tube axially placed in the center of the MAS rotor, then through the catalyst (from the bottom to the top) and leaves the rotor via an annular gap in the rotor cap. It should be mentioned that the reported flow MAS NMR systems do not reproduce completely the conditions in real flow catalytic reactors, however they reveal a significant step forward, especially, for the processes which cannot be modeled in batch reactors.
3. Application areas
Fig. 1. Schematic drawing of the flow system used for in situ MAS NMR investigations in Ref. [29].
The applications of controlled atmosphere MAS NMR in heterogeneous catalysis can be divided into three main areas: characterization of the catalyst structure and active sites; study of the nature of molecular adsorption on catalytic surfaces; and investigation of the transformation of the reactant during the catalytic reactions. The application of MAS NMR for catalyst characterization and investigation of adsorption phenomenon has been reviewed and discussed many times (for recent reviews see Refs. [31–38]). In this chapter, the attention is focused on the cata-
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Table 1 Application of in situ MAS NMR spectroscopy for the investigation of heterogeneous catalytic reactions Reactions
Catalysts
Information obtained
Reference
Conversion of methanol and dimethylether
X, Y, Mor, ZSM-5, SAPO
[13,23,39–59]
Conversion of alcohols and ethers
X, Y, ZSM-5
Conversion of unsaturated hydrocarbons
Y, Mor, ZSM-5, ZnO
Adsorption complexes; surface species; localization of reactants and products on the surface; identification of intermediates and primary products; information on the mechanism of CC bond formation Identification of surface alkoxides; evidences for carbenium ion intermediate formation Identification of surface alkoxides and stable carbenium ions; confirmation of carbenium ion intermediate formation with 13C tracing techniques and adsorption of probe molecules; primary products; mechanisms of oligomerization and catalyst deactivation. Adsorption of hydrocarbons, CO and H2, information on the mechanisms of hydrogenation, dehydrogenation, oligomerization, cracking, isomerization and aromatization on metals; structure of adsorption complexes and intermediates, identification of primary products. Mechanism of polymerization; ionic intermediates; homo- and co-polymerization. Adsorption; active sites; application of 13C label tracing for the investigations of the mechanisms of cracking, dehydrogenation, disproportionation and aromatization; intermediates; diffusion of reactants and products. Adsorption complexes and surface species; nature of alkylating agent; mechanisms of alkylation in the aromatic ring and in the alkyl chain; mechanisms of side reactions; information on the role of alkoxy groups and carbenium ions in alkylation. Application of 13C tracing technique for the investigation of the mechanisms of isomerization, cracking, fragmentation and transalkylation; inter- and intramolecular reactions, identification of intermediates.
Conversion of hydrocarbons on Pt, Os, Ir/Al2O3; Ru, Pt/SiO2, RuY, metals Pt, Pd/Mg(Al)O, Pt/KL
Polymerization
BF3*BH2O, HCl/ZnCl2, HCl/SnCl4
Conversion of alkanes
H-ZSM-5, Ga/H-ZSM-5 X, Y, ZrO2/SO4
Alkylation of aromatics
X, ZSM-5, ZSM-11, Beta
Conversion of alkylaromatics
ZSM-5, ZSM-11, Mor, Y, SAPO-5, Beta
[27,60–69]
[27,65,66,70–75]
[10,11,24,76–82]
[83,84]
[24,85–93]
[24,39,94–100]
[99–103]
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Table 1 (Continued) Reactions
Catalysts
Information obtained
Reference
Reactions of aldehydes and ketones
ZSM-5, Mor, Y, SAPO
[104–107]
Reactions of halogen-containing compounds Reactions of nitrogen-containing compounds Synthesis of methanol
ZSM-5, Mor, Y,
Adsorption complexes; mechanisms of oxidation, condensation and catalyst deactivation. Reactions of methyl halides and acetyl halides, adsorption complexes, surface species. Reactions of amines and cyanides; adsorption complexes and intermediates; primary products. Adsorption of reactants and products, surface species and intermediates
ZSM-5, SiO2
Cu/ZnO/Al2O3
lytic reactions studied by in situ MAS NMR technique. Table 1 establishes a cross reference index between the reactions studied, the catalysts used, the mechanistic information obtained and the corresponding literature sources. It should be mentioned that most of the papers are devoted to the studies of catalytic reactions on oxide and zeolite catalysts. Among them, a great deal of papers is devoted to the methanol conversion [13,23,39– 59]. The nature of the interaction of methanol with zeolite catalysts is of special interest because of the ability of some zeolites to convert methanol to gasoline giving an alternative source of energy and organic chemicals. The considerable efforts have been devoted to the investigation of: (i) the nature of the interaction between MeOH molecules and zeolitic active sites; (ii) identity of the primary reaction products and intermediates; (iii) reasons for an induction period preceding hydrocarbon synthesis; and (iv) the mechanism of the initial CC bond formation. Another topic which received great attention deals with conversion of olefins and alcohols over zeolite catalysts [27,65,66,70 – 75]. The main efforts were focused on the investigation of the intermediate species formed during these reactions. The discussion turned around the question
[44,108–112]
[113–115]
[116,117]
on whether alkoxide or carbenim ion type species are responsible for oligomerization. Among other topics of considerable interest, the conversion of alkanes [24,85–93] and reactions of alkylaromatics [24,39,89,94–103] should be mentioned. A special interest received the problem of the initial stages of light alkanes activation [24,85–89]. Application of in situ 13C tracer techniques was of great help for the elucidation of the mechanisms of these classes of reactions. These reactions will be considered in more details in Section 4.
4. Capabilities of the techniques for the elucidation of reaction mechanisms The potential of the in situ MAS NMR techniques for the elucidation of the mechanisms of organic reactions over various heterogeneous catalysts has been thoroughly investigated during the last decade [23–27]. It has been demonstrated that in situ MAS NMR techniques can be used for the analysis of state and mobility of adsorbed reactants and products; the investigation of their interaction with surface active sites and identification of such sites; the direct observation and the indirect identification of reaction interme-
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diates; the studies of reaction kinetics; the direct identification of primary products; the testing of reaction mechanisms using 13C tracing techniques and various probe molecules (H2, O2, H2O, CO, C6H6 etc.); finally, the direct observation of the shape selectivity, confinement effects in molecularsieve catalysts and catalyst deactivation. In this chapter, the main capabilities of the technique are summarized and illustrated with examples taken from our studies.
4.1. Determination of the surface acti6e sites and their interaction with adsorbed reactants The main advantage of the in situ MAS NMR techniques for the unravel of the reaction mechanisms in heterogeneous catalysis is the possibility to follow both the nuclei of solid catalyst and the nuclei of adsorbed reactant. Active sites of the catalyst can be probed using 1H, 2D (Bronsted acid sites), 27Al, 29Si (Lewis acid sites), 7Li, 23Na, 133 Cs (basic sites), 195Pt (metal sites) and other nuclei exhibiting magnetic moment and associated with active sites. Transformations of adsorbed reactants are usually studied by 13C, 1H and 15N NMR. Fig. 2 shows an example of such complex multinuclear MAS NMR study aimed at the investigation of the surface reaction of carbon tetrachloride with zeolite NaY. The complementary information gained from 13C, 23Na, 27Al and 29 Si MAS NMR spectra allowed to identify the catalyst active sites, to study how they interact with CCl4, to follow the structural changes occurring with zeolite during dealumination procedure and to determine all the consecutive steps of the reaction [118,119]. 13C MAS NMR spectra (Fig. 2(a)) concluded to the formation of weakly bound complex of CCl4 with alkali ions of zeolite (resonance line at 93 ppm) at the initial steps of the reaction, its further transformation into strongly adsorbed surface complex of phosgene (155.5 ppm) exhibiting intermediate behavior and finally to the formation of CO2 (24.4 ppm), the final reaction product. From 23Na MAS NMR results (Fig. 2(b)), it was assumed that CCl4 reacts with Na+ cations sitting in the supercages, since the intensity of the resonance line at − 2 ppm corre-
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sponding to sodium in supercages decreased, while the intensity of the line at − 3.8 ppm attributed to sodium cations in sodalite cages did not change. The new resonance at 35 ppm was attributed to sodium in NaAlCl4, which is usually observed during dealumination. 27Al MAS NMR spectra (Fig. 2c) showed that during the reaction the coordination of a part of aluminum atoms changes from tetrahedral to octahedral and trigonal (broad resonance underlining narrow resonance at 56.5 ppm) and also pointed to the formation of NaALCL4 (resonance at 94 ppm). Finally, 29Si MAS NMR data (not shown) pointed to partial amorphization of zeolite during the dealumination process. On the bases of the above observations, the following reaction mechanism was proposed: (AlO2) − Na+ + CCl4 {(AlO2) − Na+CCl4} {(AlO2) − Na+CCl4} {AlOCl·OCCl2}+ NaCl {AlOCl·OCCl2} AlCl3·NaCl+ CO2 + {…} AlCl3·NaCl NaAlCl4 where {…} denotes the vacancy formed upon aluminum removal.
4.2. Analysis of the state and mobility of adsorbed reactants and products, determination of surface species formed and their reacti6ity NMR spectroscopy exhibits a wide range of experimental techniques for the investigation of the behavior of adsorbed molecules, in particular, cross-polarization techniques, relaxation measurements, variable temperature experiments, pulse field gradient technique, etc. The former techniques are especially useful for the investigation of the surface species and complexes formed during reaction, while the latter can be used for investigation of molecular diffusion [120,121]. Fig. 3 shows an example of the study of surface species formed in reactions of methanol or dimethylether with zeolite H-ZSM-11 and the reactivity of this species in toluene alkylation [39,94]. In both reactions, five strongly bound species were observed upon heating at 433 K. The species were identified as MeOH and DME side-on and end-on adsorp-
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Fig. 2.
13
C (a),
23
Na (b) and
27
Al (c) MAS NMR spectra observed during CCl4 surface reaction on NaY zeolite.
tion complexes and surface methoxides (Fig. 3). The identification was made on the basis of the information on their chemical shifts, sensitivity to cross-polarization conditions, spin-lattice relaxation times, interaction with probe molecules and results of theoretical calculations. End-on adsorbed
complexes (MeOH end-on and DME end-on) and surface methoxides were shown to be reactive in toluene alkylation. However, the reaction of surface methoxides with toluene occurred only to a very small extent due to high stability of surface methoxy species under our reaction conditions.
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Fig. 3.
195
13
C CP MAS NMR spectra observed upon activation of methanol and dimethylether at 433 K on zeolite H-ZSM-11.
4.3. Direct obser6ation and indirect identification of reaction intermediates The direct observation of stable reaction intermediates has been illustrated with examples in the
two previous sections. It should be emphasized however that stable intermediates occur rather rarely in heterogeneous catalysis and in most of the reactions the lifetime of the intermediates is very short on the NMR time schedule. For the
Fig. 4. 13C MAS NMR spectra observed at the initial steps of propane 2-13C activation over Ga/H-ZSM-5 catalyst at 573 K and the proposed reaction intermediate.
Fig. 5. Aliphatic parts of 13C MAS NMR spectra observed during toluene alkylation with methanol over H-ZSM-11 catalyst at 433 K.
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Fig. 6.
13
C MAS NMR spectrum observed after 80 min of n-hexane 1-13C reaction over Pt/Mg(Al)O catalyst at 573 K.
identification of unstable intermediates, various indirect methods such as 13C label tracing techniques or trapping of reactive intermediates with probe molecules can be used. The example given below shows how 13C tracing technique was used for the investigation of the intermediates formed at the initial stages of propane activation over Ga-modified H-ZSM-5 catalyst [85 – 88]. Fig. 4 presents 13C MAS NMR spectra observed at the early stages of propane 2-13C activation. The initial spectrum shows the only resonance at ca. 17 ppm, corresponding to the initially labelled methylene group of propane. After 5 min reaction at 573 K, the line at ca. 16 ppm corresponding to a methyl group in propane appeared. No other significant signals were observed in the spectrum. Scrambling of the 13C label in propane in the absence of any large amount of other reaction products, can only be explained by cyclic intermediate. These data and the analysis of the roles of acid and Ga sites at the early stages of propane activation concluded to the formation of
protonated pseudocyclopropane shown in Fig. 4.
intermediate,
4.4. Obser6ation of real primary products and determination of real kinetic constants The determination of primary products and investigation of reaction kinetics are the central questions for the elucidation of reaction mechanisms. However in many cases the reaction pattern is affected significantly by diffusion, and the products observed at the outlet of conventional continuous flow reactors, do not reflect the real kinetic situation on the catalyst. Information on this situation can be best obtained by means of in situ techniques, which do not suffer from the above limitations. The question of the primary orientation of methyl group in the aromatic ring of toluene during its alkylation with methanol over various zeolite catalysts remained for a long time a source of controversy, as product diffusion limitations
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within zeolite constrains affected the ratio of xylene isomers, preventing the determination of the primary reaction products. The determination of the primary alkylation products over various zeolites using in situ 13C MAS NMR techniques showed that o- and p-xylenes formed in the ratio of two to one are the only primary products (Fig. 5) [94]. The initial o/p ratio observed in NMR experiment confirmed that toluene alkylation with methanol obey general concepts of electrophilic substitution in the aromatic ring [122].
4.5. Probing reaction mechanisms with techniques
13
C tracing
Low natural abundance of isotope 13C is generally considered as one of the main disadvantages of 13C NMR technique because of rather a low sensitivity of this method. This feature however may turn as an advantage if the initial reactant
Fig. 7.
13
197
can be selectively enriched with 13C at specific positions. Tracing the fate of specifically labeled carbons during the time course of the reaction may give a unique information on the reaction mechanism especially in the case of complex reactions, consisting of a large number of parallel and consecutive steps. The advantages of the in situ 13C tracing techniques for the unravel of reaction mechanisms are illustrated in Figs. 6 and 7 on the example of n-hexane isomerization over Pt/Mg(Al)O catalyst [76–78]. n-Hexane 1-13C was used for in situ label tracer experiments. 13C MAS NMR spectra were obtained during the time course of the reaction at 573 K; typical NMR spectrum observed after 80 min of reaction is presented in Fig. 6. The thorough analysis of the set of the NMR spectra allowed to identify the primary and the secondary labeled reaction products and to classify them as the products of cyclic and bond shift isomeriza-
C label displacement during n-hexane 1-13C isomerization over Pt/Mg(Al)O catalyst.
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tion (Fig. 7). Cyclic mechanism involving non-selective bond rupture of methylcyclopentane intermediate accounted for 80% of isomerization products over Pt/Mg(Al)O catalyst. Bond shift isomerization was demonstrated to occur via formation of 1,3- and 2,4-metallocyclobutane intermediates. The behaviour of Pt/Mg(Al)O catalyst was explained on the basis of the specific properties of the metal and support.
5. Conclusions In situ MAS NMR is shown to be a powerful techniques for the investigation of reaction mechanisms in heterogeneous catalysis. It has contributed significantly to elucidation of mechanisms of various catalytic reactions such as methanol conversion to hydrocarbons, transformations of alkenes and alcohols, activation of alkanes, alkylation of aromatics, reactions of aldehydes, ketones, ethers, etc. It allows to follow both the nuclei of the catalyst and the nuclei of the reactant during the time course of the reaction. However the analysis of the literature shows that up to date the main efforts were focused at the investigation of reactant transformations. The future progress in the field will require a combined information on the behavior of the solid catalyst and adsorbed reactant. The breakthrough in this area will depend on the development of the new experimental approaches which will allow to follow simultaneously the nuclei of the solid catalyst and adsorbed reactant and their interaction under working conditions.
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Application of in situ MAS NMR for elucidation of reaction mechanisms in heterogeneous catalysis Irina I. Ivanova * Moscow State Uni6ersity, Department of Chemistry, Leninskie Gory, 119899 Moscow, Russia
Abstract This short review deals with the recent applications of in situ MAS NMR techniques for the unravel of the mechanisms of heterogeneous catalytic reactions. In the first part, the different approaches for realization of in situ MAS NMR experiments in static and flow conditions are considered. In the second part, the main application areas are discussed and a cross reference index between the reactions studied, the catalysts used, the mechanistic information obtained and the corresponding literature sources is established. In the third part, the capabilities of the in situ MAS NMR techniques are illustrated with examples taken from our recent results on the mechanistic studies of various catalytic reactions over mono- and bifunctional zeolite and oxide catalysts. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Review; In situ MAS NMR; Adsorbed phase; Mechanisms; Zeolites; Dealumination; Alkylation; Isomerization; Alkane activation
1. Introduction The growing role of heterogeneous catalysis in chemical technology, biotechnology and ecology has been repeatedly emphasized during the last years. However the progress in this field, in particular, in tuning and mastering of the catalytic properties of the existing catalysts and in the development of new methods of catalyst preparation is limited because of the lack of detailed information on the reaction mechanisms and the nature of active sites.
* Fax: + 7-095-9328846. E-mail address: [email protected] (I.I. Ivanova)
Heterogeneous catalytic reactions are usually studied by analysis of the desorbed reactants and products. This method however does not allow to follow the reactant and the catalyst under ‘working’ conditions directly in course of catalytic reaction. This information can be best obtained by means of recently developed in situ spectroscopic techniques (IR, UV–vis, ESR, NMR), among which NMR spectroscopy is considered to be one of the most informative. The first attempts for the investigation of heterogeneous catalytic reactions in situ by means of NMR techniques can be referred to the beginning of 1970s. At that time, the conventional high resolution Fourier transform 13C NMR was applied to studies of reacting adsorbates, looking
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into product formation and reaction kinetics [1– 7]. The application of the technique was limited to the studies of the reactions of simple molecules, which retain sufficient mobility in the adsorbed phase. The main reasons for that were rather low sensitivity and resolution of NMR techniques in adsorbed phase due to chemical shift anisotropy and dipolar interactions between nuclei of strongly adsorbed species in which molecular motion is reduced. At present, not only conventional high-resolution techniques are applied to studies of catalytic reactions, but also more sophisticated methods such as magic-angle-spinning, cross-polarization, J-resolved spectroscopy, correlation spectroscopy, multiple-quantum NMR, spin-echo, spin-echo double resonance, pulse field gradient technique and others [8 – 12]. This contribution is devoted to the applications of MAS NMR techniques for elucidation of reaction mechanisms. It contains a brief review of the experimental approaches used for in situ MAS NMR studies and main application areas of the technique. The emphasis is made on the analysis of the main capabilities of the techniques, which are illustrated with examples taken from our recent works.
2. Experimental approaches At present, there are two general experimental protocols used to carry out in situ MAS NMR
studies. The first one, which is used over the last 10 years models a batch reactor catalytic experiments. This approach makes use of sealed highly symmetrical glass ampoules containing a catalyst and an adsorbate, and fitting precisely into MAS rotors [13–19] or gas tight MAS rotors with a catalyst and an adsorbate [20–22]. In the latter case, CAVERN apparatus were designed for sealing and unsealing MAS rotors directly on a vacuum line [20–22]. The results obtained using these experimental approaches were reviewed and discussed several times [22–27]. The second, recently developed experimental approach, allows to carry out in situ MAS NMR investigations under continuous flow conditions. There are several different designs reported, which allow for in situ flow MAS NMR experiments [28–30]. Fig. 1 shows a drawing of a design proposed by Hunger and Horvath [29], which is the easiest to fabricate. This design is based on a commercial 7-mm Bruker MAS NMR probe. The reactants are injected into the rotor with carrier gas, which passes through the glass tube axially placed in the center of the MAS rotor, then through the catalyst (from the bottom to the top) and leaves the rotor via an annular gap in the rotor cap. It should be mentioned that the reported flow MAS NMR systems do not reproduce completely the conditions in real flow catalytic reactors, however they reveal a significant step forward, especially, for the processes which cannot be modeled in batch reactors.
3. Application areas
Fig. 1. Schematic drawing of the flow system used for in situ MAS NMR investigations in Ref. [29].
The applications of controlled atmosphere MAS NMR in heterogeneous catalysis can be divided into three main areas: characterization of the catalyst structure and active sites; study of the nature of molecular adsorption on catalytic surfaces; and investigation of the transformation of the reactant during the catalytic reactions. The application of MAS NMR for catalyst characterization and investigation of adsorption phenomenon has been reviewed and discussed many times (for recent reviews see Refs. [31–38]). In this chapter, the attention is focused on the cata-
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Table 1 Application of in situ MAS NMR spectroscopy for the investigation of heterogeneous catalytic reactions Reactions
Catalysts
Information obtained
Reference
Conversion of methanol and dimethylether
X, Y, Mor, ZSM-5, SAPO
[13,23,39–59]
Conversion of alcohols and ethers
X, Y, ZSM-5
Conversion of unsaturated hydrocarbons
Y, Mor, ZSM-5, ZnO
Adsorption complexes; surface species; localization of reactants and products on the surface; identification of intermediates and primary products; information on the mechanism of CC bond formation Identification of surface alkoxides; evidences for carbenium ion intermediate formation Identification of surface alkoxides and stable carbenium ions; confirmation of carbenium ion intermediate formation with 13C tracing techniques and adsorption of probe molecules; primary products; mechanisms of oligomerization and catalyst deactivation. Adsorption of hydrocarbons, CO and H2, information on the mechanisms of hydrogenation, dehydrogenation, oligomerization, cracking, isomerization and aromatization on metals; structure of adsorption complexes and intermediates, identification of primary products. Mechanism of polymerization; ionic intermediates; homo- and co-polymerization. Adsorption; active sites; application of 13C label tracing for the investigations of the mechanisms of cracking, dehydrogenation, disproportionation and aromatization; intermediates; diffusion of reactants and products. Adsorption complexes and surface species; nature of alkylating agent; mechanisms of alkylation in the aromatic ring and in the alkyl chain; mechanisms of side reactions; information on the role of alkoxy groups and carbenium ions in alkylation. Application of 13C tracing technique for the investigation of the mechanisms of isomerization, cracking, fragmentation and transalkylation; inter- and intramolecular reactions, identification of intermediates.
Conversion of hydrocarbons on Pt, Os, Ir/Al2O3; Ru, Pt/SiO2, RuY, metals Pt, Pd/Mg(Al)O, Pt/KL
Polymerization
BF3*BH2O, HCl/ZnCl2, HCl/SnCl4
Conversion of alkanes
H-ZSM-5, Ga/H-ZSM-5 X, Y, ZrO2/SO4
Alkylation of aromatics
X, ZSM-5, ZSM-11, Beta
Conversion of alkylaromatics
ZSM-5, ZSM-11, Mor, Y, SAPO-5, Beta
[27,60–69]
[27,65,66,70–75]
[10,11,24,76–82]
[83,84]
[24,85–93]
[24,39,94–100]
[99–103]
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Table 1 (Continued) Reactions
Catalysts
Information obtained
Reference
Reactions of aldehydes and ketones
ZSM-5, Mor, Y, SAPO
[104–107]
Reactions of halogen-containing compounds Reactions of nitrogen-containing compounds Synthesis of methanol
ZSM-5, Mor, Y,
Adsorption complexes; mechanisms of oxidation, condensation and catalyst deactivation. Reactions of methyl halides and acetyl halides, adsorption complexes, surface species. Reactions of amines and cyanides; adsorption complexes and intermediates; primary products. Adsorption of reactants and products, surface species and intermediates
ZSM-5, SiO2
Cu/ZnO/Al2O3
lytic reactions studied by in situ MAS NMR technique. Table 1 establishes a cross reference index between the reactions studied, the catalysts used, the mechanistic information obtained and the corresponding literature sources. It should be mentioned that most of the papers are devoted to the studies of catalytic reactions on oxide and zeolite catalysts. Among them, a great deal of papers is devoted to the methanol conversion [13,23,39– 59]. The nature of the interaction of methanol with zeolite catalysts is of special interest because of the ability of some zeolites to convert methanol to gasoline giving an alternative source of energy and organic chemicals. The considerable efforts have been devoted to the investigation of: (i) the nature of the interaction between MeOH molecules and zeolitic active sites; (ii) identity of the primary reaction products and intermediates; (iii) reasons for an induction period preceding hydrocarbon synthesis; and (iv) the mechanism of the initial CC bond formation. Another topic which received great attention deals with conversion of olefins and alcohols over zeolite catalysts [27,65,66,70 – 75]. The main efforts were focused on the investigation of the intermediate species formed during these reactions. The discussion turned around the question
[44,108–112]
[113–115]
[116,117]
on whether alkoxide or carbenim ion type species are responsible for oligomerization. Among other topics of considerable interest, the conversion of alkanes [24,85–93] and reactions of alkylaromatics [24,39,89,94–103] should be mentioned. A special interest received the problem of the initial stages of light alkanes activation [24,85–89]. Application of in situ 13C tracer techniques was of great help for the elucidation of the mechanisms of these classes of reactions. These reactions will be considered in more details in Section 4.
4. Capabilities of the techniques for the elucidation of reaction mechanisms The potential of the in situ MAS NMR techniques for the elucidation of the mechanisms of organic reactions over various heterogeneous catalysts has been thoroughly investigated during the last decade [23–27]. It has been demonstrated that in situ MAS NMR techniques can be used for the analysis of state and mobility of adsorbed reactants and products; the investigation of their interaction with surface active sites and identification of such sites; the direct observation and the indirect identification of reaction interme-
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diates; the studies of reaction kinetics; the direct identification of primary products; the testing of reaction mechanisms using 13C tracing techniques and various probe molecules (H2, O2, H2O, CO, C6H6 etc.); finally, the direct observation of the shape selectivity, confinement effects in molecularsieve catalysts and catalyst deactivation. In this chapter, the main capabilities of the technique are summarized and illustrated with examples taken from our studies.
4.1. Determination of the surface acti6e sites and their interaction with adsorbed reactants The main advantage of the in situ MAS NMR techniques for the unravel of the reaction mechanisms in heterogeneous catalysis is the possibility to follow both the nuclei of solid catalyst and the nuclei of adsorbed reactant. Active sites of the catalyst can be probed using 1H, 2D (Bronsted acid sites), 27Al, 29Si (Lewis acid sites), 7Li, 23Na, 133 Cs (basic sites), 195Pt (metal sites) and other nuclei exhibiting magnetic moment and associated with active sites. Transformations of adsorbed reactants are usually studied by 13C, 1H and 15N NMR. Fig. 2 shows an example of such complex multinuclear MAS NMR study aimed at the investigation of the surface reaction of carbon tetrachloride with zeolite NaY. The complementary information gained from 13C, 23Na, 27Al and 29 Si MAS NMR spectra allowed to identify the catalyst active sites, to study how they interact with CCl4, to follow the structural changes occurring with zeolite during dealumination procedure and to determine all the consecutive steps of the reaction [118,119]. 13C MAS NMR spectra (Fig. 2(a)) concluded to the formation of weakly bound complex of CCl4 with alkali ions of zeolite (resonance line at 93 ppm) at the initial steps of the reaction, its further transformation into strongly adsorbed surface complex of phosgene (155.5 ppm) exhibiting intermediate behavior and finally to the formation of CO2 (24.4 ppm), the final reaction product. From 23Na MAS NMR results (Fig. 2(b)), it was assumed that CCl4 reacts with Na+ cations sitting in the supercages, since the intensity of the resonance line at − 2 ppm corre-
193
sponding to sodium in supercages decreased, while the intensity of the line at − 3.8 ppm attributed to sodium cations in sodalite cages did not change. The new resonance at 35 ppm was attributed to sodium in NaAlCl4, which is usually observed during dealumination. 27Al MAS NMR spectra (Fig. 2c) showed that during the reaction the coordination of a part of aluminum atoms changes from tetrahedral to octahedral and trigonal (broad resonance underlining narrow resonance at 56.5 ppm) and also pointed to the formation of NaALCL4 (resonance at 94 ppm). Finally, 29Si MAS NMR data (not shown) pointed to partial amorphization of zeolite during the dealumination process. On the bases of the above observations, the following reaction mechanism was proposed: (AlO2) − Na+ + CCl4 {(AlO2) − Na+CCl4} {(AlO2) − Na+CCl4} {AlOCl·OCCl2}+ NaCl {AlOCl·OCCl2} AlCl3·NaCl+ CO2 + {…} AlCl3·NaCl NaAlCl4 where {…} denotes the vacancy formed upon aluminum removal.
4.2. Analysis of the state and mobility of adsorbed reactants and products, determination of surface species formed and their reacti6ity NMR spectroscopy exhibits a wide range of experimental techniques for the investigation of the behavior of adsorbed molecules, in particular, cross-polarization techniques, relaxation measurements, variable temperature experiments, pulse field gradient technique, etc. The former techniques are especially useful for the investigation of the surface species and complexes formed during reaction, while the latter can be used for investigation of molecular diffusion [120,121]. Fig. 3 shows an example of the study of surface species formed in reactions of methanol or dimethylether with zeolite H-ZSM-11 and the reactivity of this species in toluene alkylation [39,94]. In both reactions, five strongly bound species were observed upon heating at 433 K. The species were identified as MeOH and DME side-on and end-on adsorp-
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Fig. 2.
13
C (a),
23
Na (b) and
27
Al (c) MAS NMR spectra observed during CCl4 surface reaction on NaY zeolite.
tion complexes and surface methoxides (Fig. 3). The identification was made on the basis of the information on their chemical shifts, sensitivity to cross-polarization conditions, spin-lattice relaxation times, interaction with probe molecules and results of theoretical calculations. End-on adsorbed
complexes (MeOH end-on and DME end-on) and surface methoxides were shown to be reactive in toluene alkylation. However, the reaction of surface methoxides with toluene occurred only to a very small extent due to high stability of surface methoxy species under our reaction conditions.
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Fig. 3.
195
13
C CP MAS NMR spectra observed upon activation of methanol and dimethylether at 433 K on zeolite H-ZSM-11.
4.3. Direct obser6ation and indirect identification of reaction intermediates The direct observation of stable reaction intermediates has been illustrated with examples in the
two previous sections. It should be emphasized however that stable intermediates occur rather rarely in heterogeneous catalysis and in most of the reactions the lifetime of the intermediates is very short on the NMR time schedule. For the
Fig. 4. 13C MAS NMR spectra observed at the initial steps of propane 2-13C activation over Ga/H-ZSM-5 catalyst at 573 K and the proposed reaction intermediate.
Fig. 5. Aliphatic parts of 13C MAS NMR spectra observed during toluene alkylation with methanol over H-ZSM-11 catalyst at 433 K.
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196
Fig. 6.
13
C MAS NMR spectrum observed after 80 min of n-hexane 1-13C reaction over Pt/Mg(Al)O catalyst at 573 K.
identification of unstable intermediates, various indirect methods such as 13C label tracing techniques or trapping of reactive intermediates with probe molecules can be used. The example given below shows how 13C tracing technique was used for the investigation of the intermediates formed at the initial stages of propane activation over Ga-modified H-ZSM-5 catalyst [85 – 88]. Fig. 4 presents 13C MAS NMR spectra observed at the early stages of propane 2-13C activation. The initial spectrum shows the only resonance at ca. 17 ppm, corresponding to the initially labelled methylene group of propane. After 5 min reaction at 573 K, the line at ca. 16 ppm corresponding to a methyl group in propane appeared. No other significant signals were observed in the spectrum. Scrambling of the 13C label in propane in the absence of any large amount of other reaction products, can only be explained by cyclic intermediate. These data and the analysis of the roles of acid and Ga sites at the early stages of propane activation concluded to the formation of
protonated pseudocyclopropane shown in Fig. 4.
intermediate,
4.4. Obser6ation of real primary products and determination of real kinetic constants The determination of primary products and investigation of reaction kinetics are the central questions for the elucidation of reaction mechanisms. However in many cases the reaction pattern is affected significantly by diffusion, and the products observed at the outlet of conventional continuous flow reactors, do not reflect the real kinetic situation on the catalyst. Information on this situation can be best obtained by means of in situ techniques, which do not suffer from the above limitations. The question of the primary orientation of methyl group in the aromatic ring of toluene during its alkylation with methanol over various zeolite catalysts remained for a long time a source of controversy, as product diffusion limitations
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within zeolite constrains affected the ratio of xylene isomers, preventing the determination of the primary reaction products. The determination of the primary alkylation products over various zeolites using in situ 13C MAS NMR techniques showed that o- and p-xylenes formed in the ratio of two to one are the only primary products (Fig. 5) [94]. The initial o/p ratio observed in NMR experiment confirmed that toluene alkylation with methanol obey general concepts of electrophilic substitution in the aromatic ring [122].
4.5. Probing reaction mechanisms with techniques
13
C tracing
Low natural abundance of isotope 13C is generally considered as one of the main disadvantages of 13C NMR technique because of rather a low sensitivity of this method. This feature however may turn as an advantage if the initial reactant
Fig. 7.
13
197
can be selectively enriched with 13C at specific positions. Tracing the fate of specifically labeled carbons during the time course of the reaction may give a unique information on the reaction mechanism especially in the case of complex reactions, consisting of a large number of parallel and consecutive steps. The advantages of the in situ 13C tracing techniques for the unravel of reaction mechanisms are illustrated in Figs. 6 and 7 on the example of n-hexane isomerization over Pt/Mg(Al)O catalyst [76–78]. n-Hexane 1-13C was used for in situ label tracer experiments. 13C MAS NMR spectra were obtained during the time course of the reaction at 573 K; typical NMR spectrum observed after 80 min of reaction is presented in Fig. 6. The thorough analysis of the set of the NMR spectra allowed to identify the primary and the secondary labeled reaction products and to classify them as the products of cyclic and bond shift isomeriza-
C label displacement during n-hexane 1-13C isomerization over Pt/Mg(Al)O catalyst.
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tion (Fig. 7). Cyclic mechanism involving non-selective bond rupture of methylcyclopentane intermediate accounted for 80% of isomerization products over Pt/Mg(Al)O catalyst. Bond shift isomerization was demonstrated to occur via formation of 1,3- and 2,4-metallocyclobutane intermediates. The behaviour of Pt/Mg(Al)O catalyst was explained on the basis of the specific properties of the metal and support.
5. Conclusions In situ MAS NMR is shown to be a powerful techniques for the investigation of reaction mechanisms in heterogeneous catalysis. It has contributed significantly to elucidation of mechanisms of various catalytic reactions such as methanol conversion to hydrocarbons, transformations of alkenes and alcohols, activation of alkanes, alkylation of aromatics, reactions of aldehydes, ketones, ethers, etc. It allows to follow both the nuclei of the catalyst and the nuclei of the reactant during the time course of the reaction. However the analysis of the literature shows that up to date the main efforts were focused at the investigation of reactant transformations. The future progress in the field will require a combined information on the behavior of the solid catalyst and adsorbed reactant. The breakthrough in this area will depend on the development of the new experimental approaches which will allow to follow simultaneously the nuclei of the solid catalyst and adsorbed reactant and their interaction under working conditions.
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