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1H MAS NMR monitoring of the 13C-labeled carbon scrambling for propane in zeolite H-ZSM-5
Chemical Physics Letters 420 (2006) 574–576 www.elsevier.com/locate/cplett
1
H MAS NMR monitoring of the 13C-labeled carbon scrambling for propane in zeolite H-ZSM-5
Alexander G. Stepanov a
a,*
, Sergei S. Arzumanov a, Horst Ernst b, Dieter Freude
b,*
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia b Abteilung Grenzfla¨chenphysik, Universita¨t Leipzig, Linne´strasse 5, 04103 Leipzig, Germany Received 19 October 2005 Available online 2 February 2006
Abstract It has been demonstrated that 1H MAS NMR spectroscopy can be used as a tool for in situ monitoring the reaction kinetics of 13Clabeled carbon scrambling in alkane molecules adsorbed on zeolite catalysts at the reaction temperature of 540–570 K. The accuracy of the results and the time resolution are improved compared to 13C MAS NMR spectroscopy. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction
The novel technique is applied for the first time to the C-labeled carbon transfer in propane adsorbed on zeolite H-ZSM-5. 13
Solid state NMR spectroscopy has been established for the study of hydrocarbon conversion on solid acid catalysts [1–6]. Valuable information on the reaction mechanisms could be obtained by in situ monitoring the kinetics of the selective 13C-labeled carbon scrambling among the reactants and the products of the reaction [7,8]. However, 13 C MAS NMR experiments are time consuming due to the low sensitivity of 13C nuclei and the long T1 relaxation times. This limits the time resolution in kinetic studies. 1 H NMR experiments exceed significantly the sensitivity and time resolution of kinetic 13C NMR measurements, but it is difficult to detect the scrambling of the 13C-label by 1H NMR. In principle, the J-splitting of the 1H NMR signal of the 1H nuclei connected to the 13C-labeled carbon atom can be utilized. In this Letter, we demonstrate that mechanistic studies of alkane activation and transformation can be performed in situ by 1H MAS NMR observation of the 13C-labeled carbon transfer in alkane adsorbed on solid acid catalyst.
* Corresponding authors. Fax: +7 383 330 80 56 (A.G. Stepanov), +49 341 97 39349 (D. Freude). E-mail addresses: [email protected] (A.G. Stepanov), freude@ uni-leipzig.de (D. Freude).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.12.070
2. Experimental Zeolite H-ZSM-5 was kindly provided by Tricat Zeolites. Its silicon-to-aluminum ratio was equal to 13 [7]. Propane-2–13C (99% 13C) purchased from ICON Inc. was used without further purification. The samples for the NMR measurements were prepared by heating 60 mg of the zeolite sample in the glass tubes of 5.5 mm outer diameter. The temperature was increased under vacuum at the rate of 10 K h1. The samples were maintained at 673 K for 24 h under vacuum (less than 102 Pa). The loading of the zeolite with propane was performed at room temperature with 4 propane molecules per unit cell (ca 600 lmol g1). Then the glass tube (12 mm length) was sealed off. 1 H MAS NMR measurements were performed on a Bruker Avance 400 spectrometer at a Larmor frequency of 400.13 MHz using a Bruker high temperature probe within the temperature range ca. 540–570 K. The rotation frequency of a 7-mm-rotor with the inserted sealed glass tube was 3000 Hz. Free induction decays (FID) were recorded using a Hahn-echo sequence with the p/2-pulse
A.G. Stepanov et al. / Chemical Physics Letters 420 (2006) 574–576
of 4.5 ls duration and 333 ls delay between the two pulses, 4 s recycle delay, and 12 scans for the signal accumulation. The sample temperature was controlled by the Bruker BVT-1000 variable-temperature unit. The calibration of the temperature inside the rotor was performed with an accuracy of ±2 K by using lead nitrate sample as a 207Pb MAS NMR chemical shift thermometer [9]. 3. Results and discussion Propane-2–13C adsorbed on zeolite H-ZSM-5 exhibits in the 1H MAS NMR spectrum a distinct doublet, centered at 1.5 ppm and a singlet at 1.0 ppm, at the temperature above 473 K (see Fig. 1, bottom spectrum). The singlet line belongs to the methyl groups, whereas the doublet line arises from the methylene group of propane. The separation of ca. 125 Hz between the maxima in the doublet corresponds to 1JCH – coupling arising from scalar spin-spin interaction of the 13C nuclei of the methylene group with the attached protons [10]. A reaction of the 13C-labeled carbon scrambling in the adsorbed propane can be described by the scheme kC 1
CH3 13 CH2 CH3
13
kC 2
CH3 CH2 CH3
The apparent rate constant k ¼ k C1 þ k C2 for the C-labeled carbon scrambling from the CH2 group into the CH3 groups of the adsorbed propane-2–13C was determined by the simple exponential fits
13
I t ðsingletÞ ¼ I 1 ðsingletÞ½1 expðktÞ
and
I t ðdoubletÞ ¼ I 0 ðdoubletÞ½expðktÞ þ I 1 ðdoubletÞ½1 expðktÞ;
4.0
3.0
2.0
ð1Þ
1.0
0.0
ppm Fig. 1. Stack plot of the 1H MAS NMR spectra of propane-2–13C adsorbed on H-ZSM-5 obtained at 563 K. The plot shows the signals of the methyl (1.0 ppm) and the methylene (1.5 ppm) groups. The first spectrum (bottom) was recorded 3 min after the temperature was raised to 563 K. The time between subsequent spectra recording was 14 min.
575
where It, I1 and I0 denote the integral intensities of signals from the CH2 group in the 1H MAS NMR spectra at the observation time t, t = 1 (equilibrium), and t = 0, respectively. At t = 0 the temperature was rapidly increased from 503 K, at which the reaction did not yet occur, to the desired temperature of the experiment. A transfer of 13C-labeled carbon from the methylene group into the methyl groups in the course of the reaction should result in decrease of the intensity of the doublet line at 1.5 ppm and appearance and increase of the intensity of the singlet in between of a doublet line. This singlet should correspond to the methylene group with the nonmagnetic isotope 12C, which substitutes the 13C-labeled methylene groups and provides no JCH – splitting with the neighboring protons. So, monitoring of an increase of the intensity of a singlet line at 1.5 ppm and a decrease of the intensity of a doublet line with the same chemical shift as afunction of the reaction time provides the kinetics of 13C-labeled carbon transfer in the adsorbed propane. Fig. 1 shows the time dependent variation of the 1H MAS NMR spectrum of propane-2–13C adsorbed on HZSM-5 at the temperature of 563 K. An increase of the singlet line and a decrease of the doublet at 1.5 ppm with time can be clearly seen. This allows an estimation of the kinetic parameters of 13C-labeled carbon scrambling. The decomposition of the signal at 1.5 ppm as a sum of the singlet line and the doublet line with J-splitting of 125 Hz gives the integral intensities of the corresponding signals. Variation of the intensities of the singlet and the doublet with the reaction time for the four studied temperatures is shown in Fig. 2. Simulations of the kinetic curves yield the apparent rate constants k ¼ k C1 þ k C2 for the 13C-labeled carbon scrambling from the CH2 group into the CH3 groups. The Arrhenius plot offers further the activation energy for the 13C-labeled carbon scrambling of 153 ± 5 kJ mol1 (see Fig. 3). The value k C1 was presented in Fig. 3 for a comparison with Fig. 6 in Ref. [7]. The comparison of the obtained data for the rate constants k and the activation energy of 153 ± 5 kJ mol1 or the 13C-labeled carbon scrambling with similar parameters, e.g. 159 ± 13 kJ mol1, which were obtained earlier by 13C MAS NMR [7], shows a good agreement. The drastically increased signal-to-noise ratio of the 1H MAS NMR spectra compared to the 13C MAS NMR spectra yield more accurate k and k C1 values (cf. Figs. 2 and 3 in this study and Figs. 5 and 6 in Ref. [7]). Perhaps the most important advantage of the novel technique has not yet entirely demonstrated in this study. Indeed, with a 4-s-recycle delay and 12 scans for the signal accumulation, we could measure one spectrum per minute and observe much faster kinetics compared to that we have observed in the present study. Unfortunately the used probe limits our experiments to a maximum temperature of 573 K, at which the rate of the considered reaction is still quite slow.
576
A.G. Stepanov et al. / Chemical Physics Letters 420 (2006) 574–576
T = 550 K
T = 558 K 25
k1C + k2C = 0.0049 min–1
20
Intensity / a.u.
Intensity / a.u.
25
15 10 5 0
0
200
400
k1C + k2C = 0.0081 min–1
20 15 10 5 0
600
0
200
Time / min
T = 563 K
30
Intensity / a.u.
Intensity / a.u.
20 15 10
15 10 5 0
0
150
300
450
k1C + k2C = 0.019 min–1
20
5 0
600
0
100
200
300
400
Time / min
Time / min
Fig. 2. Kinetics of C-labeled carbon scrambling for propane-2– C adsorbed on H-ZSM-5 monitored in situ by 1H MAS NMR. Propane was initially 13 C-labeled in the methylene group, therefore, the intensity of a doublet centered at 1.5 ppm (h) decreases, whereas singlet with the same chemical shift () increases. The solid lines are the fits to the kinetics with the sum k C1 þ k C2 , based on Eq. (1).
Rate constant k 1c / min–1,
13
600
T = 573 K
25
k1C + k2C = 0.0105 min–1
25
400
Time / min
13
by 13C MAS NMR [7]. The accuracy of the kinetic parameters and the time resolution are improved compared to those obtained earlier by 13C MAS NMR spectroscopy.
0.012 0.010 0.008
Acknowledgement 0.006
We are grateful to Professor Dr. D.B. Shah for valuable advice. This work was supported by the Deutsche Forschungsgemeinschaft, project FR 902/15, Russian Foundation for Basic Research (Grant No. 04-03-32372) and INTAS (Grant No. 03-51-5286).
0.004
1.74
1.76
1.78
1.80
1.82
1000 T –1/ K–1 Fig. 3. Arrhenius plot for k C1 for the 13C-labeled carbon scrambling in propane-2–13C on H-ZSM-5, monitored by 1H MAS NMR in situ.
4. Conclusions 1
H MAS NMR spectroscopy can be reliably used as a tool for in situ monitoring the reaction kinetics of 13 C-labeled carbon scrambling in alkane molecules adsorbed on zeolite catalyst at the reaction temperature of 543–573 K. Kinetic parameters for propane adsorbed on zeolite H-ZSM-5 which have been obtained by 1H MAS NMR are in good accordance with those obtained
References [1] [2] [3] [4] [5] [6] [7]
M.W. Anderson, J. Klinowski, Nature (London) 339 (1989) 200. J. Ka¨rger, D. Freude, Stud. Surf. Sci. Catal. 105 (1997) 551. A.G. Stepanov, Russ. Chem. Rev. 68 (1999) 563. I.I. Ivanova, Colloid Surf. A 158 (1999) 189. J.F. Haw, Top. Catal. 8 (1999) 81. M. Hunger, Catal. Today 97 (2004) 3. A.G. Stepanov, S.S. Arzumanov, M.V. Luzgin, H. Ernst, D. Freude, V.N. Parmon, J. Catal. 235 (2005) 221. [8] S.S. Arzumanov, S.I. Reshetnikov, A.G. Stepanov, V.N. Parmon, D. Freude, J. Phys. Chem. B 109 (2005) 19748. [9] T. Mildner, H. Ernst, D. Freude, Solid State Nucl. Magn. Reson. 5 (1995) 169. [10] E. Breitmaier, W. Voelter, 13C NMR Spectroscopy, Methods and Applications in Organic Chemistry, VCH, Weinheim, 1978.
1
H MAS NMR monitoring of the 13C-labeled carbon scrambling for propane in zeolite H-ZSM-5
Alexander G. Stepanov a
a,*
, Sergei S. Arzumanov a, Horst Ernst b, Dieter Freude
b,*
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia b Abteilung Grenzfla¨chenphysik, Universita¨t Leipzig, Linne´strasse 5, 04103 Leipzig, Germany Received 19 October 2005 Available online 2 February 2006
Abstract It has been demonstrated that 1H MAS NMR spectroscopy can be used as a tool for in situ monitoring the reaction kinetics of 13Clabeled carbon scrambling in alkane molecules adsorbed on zeolite catalysts at the reaction temperature of 540–570 K. The accuracy of the results and the time resolution are improved compared to 13C MAS NMR spectroscopy. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction
The novel technique is applied for the first time to the C-labeled carbon transfer in propane adsorbed on zeolite H-ZSM-5. 13
Solid state NMR spectroscopy has been established for the study of hydrocarbon conversion on solid acid catalysts [1–6]. Valuable information on the reaction mechanisms could be obtained by in situ monitoring the kinetics of the selective 13C-labeled carbon scrambling among the reactants and the products of the reaction [7,8]. However, 13 C MAS NMR experiments are time consuming due to the low sensitivity of 13C nuclei and the long T1 relaxation times. This limits the time resolution in kinetic studies. 1 H NMR experiments exceed significantly the sensitivity and time resolution of kinetic 13C NMR measurements, but it is difficult to detect the scrambling of the 13C-label by 1H NMR. In principle, the J-splitting of the 1H NMR signal of the 1H nuclei connected to the 13C-labeled carbon atom can be utilized. In this Letter, we demonstrate that mechanistic studies of alkane activation and transformation can be performed in situ by 1H MAS NMR observation of the 13C-labeled carbon transfer in alkane adsorbed on solid acid catalyst.
* Corresponding authors. Fax: +7 383 330 80 56 (A.G. Stepanov), +49 341 97 39349 (D. Freude). E-mail addresses: [email protected] (A.G. Stepanov), freude@ uni-leipzig.de (D. Freude).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.12.070
2. Experimental Zeolite H-ZSM-5 was kindly provided by Tricat Zeolites. Its silicon-to-aluminum ratio was equal to 13 [7]. Propane-2–13C (99% 13C) purchased from ICON Inc. was used without further purification. The samples for the NMR measurements were prepared by heating 60 mg of the zeolite sample in the glass tubes of 5.5 mm outer diameter. The temperature was increased under vacuum at the rate of 10 K h1. The samples were maintained at 673 K for 24 h under vacuum (less than 102 Pa). The loading of the zeolite with propane was performed at room temperature with 4 propane molecules per unit cell (ca 600 lmol g1). Then the glass tube (12 mm length) was sealed off. 1 H MAS NMR measurements were performed on a Bruker Avance 400 spectrometer at a Larmor frequency of 400.13 MHz using a Bruker high temperature probe within the temperature range ca. 540–570 K. The rotation frequency of a 7-mm-rotor with the inserted sealed glass tube was 3000 Hz. Free induction decays (FID) were recorded using a Hahn-echo sequence with the p/2-pulse
A.G. Stepanov et al. / Chemical Physics Letters 420 (2006) 574–576
of 4.5 ls duration and 333 ls delay between the two pulses, 4 s recycle delay, and 12 scans for the signal accumulation. The sample temperature was controlled by the Bruker BVT-1000 variable-temperature unit. The calibration of the temperature inside the rotor was performed with an accuracy of ±2 K by using lead nitrate sample as a 207Pb MAS NMR chemical shift thermometer [9]. 3. Results and discussion Propane-2–13C adsorbed on zeolite H-ZSM-5 exhibits in the 1H MAS NMR spectrum a distinct doublet, centered at 1.5 ppm and a singlet at 1.0 ppm, at the temperature above 473 K (see Fig. 1, bottom spectrum). The singlet line belongs to the methyl groups, whereas the doublet line arises from the methylene group of propane. The separation of ca. 125 Hz between the maxima in the doublet corresponds to 1JCH – coupling arising from scalar spin-spin interaction of the 13C nuclei of the methylene group with the attached protons [10]. A reaction of the 13C-labeled carbon scrambling in the adsorbed propane can be described by the scheme kC 1
CH3 13 CH2 CH3
13
kC 2
CH3 CH2 CH3
The apparent rate constant k ¼ k C1 þ k C2 for the C-labeled carbon scrambling from the CH2 group into the CH3 groups of the adsorbed propane-2–13C was determined by the simple exponential fits
13
I t ðsingletÞ ¼ I 1 ðsingletÞ½1 expðktÞ
and
I t ðdoubletÞ ¼ I 0 ðdoubletÞ½expðktÞ þ I 1 ðdoubletÞ½1 expðktÞ;
4.0
3.0
2.0
ð1Þ
1.0
0.0
ppm Fig. 1. Stack plot of the 1H MAS NMR spectra of propane-2–13C adsorbed on H-ZSM-5 obtained at 563 K. The plot shows the signals of the methyl (1.0 ppm) and the methylene (1.5 ppm) groups. The first spectrum (bottom) was recorded 3 min after the temperature was raised to 563 K. The time between subsequent spectra recording was 14 min.
575
where It, I1 and I0 denote the integral intensities of signals from the CH2 group in the 1H MAS NMR spectra at the observation time t, t = 1 (equilibrium), and t = 0, respectively. At t = 0 the temperature was rapidly increased from 503 K, at which the reaction did not yet occur, to the desired temperature of the experiment. A transfer of 13C-labeled carbon from the methylene group into the methyl groups in the course of the reaction should result in decrease of the intensity of the doublet line at 1.5 ppm and appearance and increase of the intensity of the singlet in between of a doublet line. This singlet should correspond to the methylene group with the nonmagnetic isotope 12C, which substitutes the 13C-labeled methylene groups and provides no JCH – splitting with the neighboring protons. So, monitoring of an increase of the intensity of a singlet line at 1.5 ppm and a decrease of the intensity of a doublet line with the same chemical shift as afunction of the reaction time provides the kinetics of 13C-labeled carbon transfer in the adsorbed propane. Fig. 1 shows the time dependent variation of the 1H MAS NMR spectrum of propane-2–13C adsorbed on HZSM-5 at the temperature of 563 K. An increase of the singlet line and a decrease of the doublet at 1.5 ppm with time can be clearly seen. This allows an estimation of the kinetic parameters of 13C-labeled carbon scrambling. The decomposition of the signal at 1.5 ppm as a sum of the singlet line and the doublet line with J-splitting of 125 Hz gives the integral intensities of the corresponding signals. Variation of the intensities of the singlet and the doublet with the reaction time for the four studied temperatures is shown in Fig. 2. Simulations of the kinetic curves yield the apparent rate constants k ¼ k C1 þ k C2 for the 13C-labeled carbon scrambling from the CH2 group into the CH3 groups. The Arrhenius plot offers further the activation energy for the 13C-labeled carbon scrambling of 153 ± 5 kJ mol1 (see Fig. 3). The value k C1 was presented in Fig. 3 for a comparison with Fig. 6 in Ref. [7]. The comparison of the obtained data for the rate constants k and the activation energy of 153 ± 5 kJ mol1 or the 13C-labeled carbon scrambling with similar parameters, e.g. 159 ± 13 kJ mol1, which were obtained earlier by 13C MAS NMR [7], shows a good agreement. The drastically increased signal-to-noise ratio of the 1H MAS NMR spectra compared to the 13C MAS NMR spectra yield more accurate k and k C1 values (cf. Figs. 2 and 3 in this study and Figs. 5 and 6 in Ref. [7]). Perhaps the most important advantage of the novel technique has not yet entirely demonstrated in this study. Indeed, with a 4-s-recycle delay and 12 scans for the signal accumulation, we could measure one spectrum per minute and observe much faster kinetics compared to that we have observed in the present study. Unfortunately the used probe limits our experiments to a maximum temperature of 573 K, at which the rate of the considered reaction is still quite slow.
576
A.G. Stepanov et al. / Chemical Physics Letters 420 (2006) 574–576
T = 550 K
T = 558 K 25
k1C + k2C = 0.0049 min–1
20
Intensity / a.u.
Intensity / a.u.
25
15 10 5 0
0
200
400
k1C + k2C = 0.0081 min–1
20 15 10 5 0
600
0
200
Time / min
T = 563 K
30
Intensity / a.u.
Intensity / a.u.
20 15 10
15 10 5 0
0
150
300
450
k1C + k2C = 0.019 min–1
20
5 0
600
0
100
200
300
400
Time / min
Time / min
Fig. 2. Kinetics of C-labeled carbon scrambling for propane-2– C adsorbed on H-ZSM-5 monitored in situ by 1H MAS NMR. Propane was initially 13 C-labeled in the methylene group, therefore, the intensity of a doublet centered at 1.5 ppm (h) decreases, whereas singlet with the same chemical shift () increases. The solid lines are the fits to the kinetics with the sum k C1 þ k C2 , based on Eq. (1).
Rate constant k 1c / min–1,
13
600
T = 573 K
25
k1C + k2C = 0.0105 min–1
25
400
Time / min
13
by 13C MAS NMR [7]. The accuracy of the kinetic parameters and the time resolution are improved compared to those obtained earlier by 13C MAS NMR spectroscopy.
0.012 0.010 0.008
Acknowledgement 0.006
We are grateful to Professor Dr. D.B. Shah for valuable advice. This work was supported by the Deutsche Forschungsgemeinschaft, project FR 902/15, Russian Foundation for Basic Research (Grant No. 04-03-32372) and INTAS (Grant No. 03-51-5286).
0.004
1.74
1.76
1.78
1.80
1.82
1000 T –1/ K–1 Fig. 3. Arrhenius plot for k C1 for the 13C-labeled carbon scrambling in propane-2–13C on H-ZSM-5, monitored by 1H MAS NMR in situ.
4. Conclusions 1
H MAS NMR spectroscopy can be reliably used as a tool for in situ monitoring the reaction kinetics of 13 C-labeled carbon scrambling in alkane molecules adsorbed on zeolite catalyst at the reaction temperature of 543–573 K. Kinetic parameters for propane adsorbed on zeolite H-ZSM-5 which have been obtained by 1H MAS NMR are in good accordance with those obtained
References [1] [2] [3] [4] [5] [6] [7]
M.W. Anderson, J. Klinowski, Nature (London) 339 (1989) 200. J. Ka¨rger, D. Freude, Stud. Surf. Sci. Catal. 105 (1997) 551. A.G. Stepanov, Russ. Chem. Rev. 68 (1999) 563. I.I. Ivanova, Colloid Surf. A 158 (1999) 189. J.F. Haw, Top. Catal. 8 (1999) 81. M. Hunger, Catal. Today 97 (2004) 3. A.G. Stepanov, S.S. Arzumanov, M.V. Luzgin, H. Ernst, D. Freude, V.N. Parmon, J. Catal. 235 (2005) 221. [8] S.S. Arzumanov, S.I. Reshetnikov, A.G. Stepanov, V.N. Parmon, D. Freude, J. Phys. Chem. B 109 (2005) 19748. [9] T. Mildner, H. Ernst, D. Freude, Solid State Nucl. Magn. Reson. 5 (1995) 169. [10] E. Breitmaier, W. Voelter, 13C NMR Spectroscopy, Methods and Applications in Organic Chemistry, VCH, Weinheim, 1978.