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Paramagnetic relaxation enhancement solid-state NMR studies of heterogeneous catalytic reaction over HY zeolite using natural abundance reactant
Solid State Nuclear Magnetic Resonance ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Paramagnetic relaxation enhancement solid-state NMR studies of heterogeneous catalytic reaction over HY zeolite using natural abundance reactant Lei Zhou a, Shenhui Li a,n, Yongchao Su b, Bojie Li a, Feng Deng a,n a National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Key Laboratory of Magnetic Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China b Department of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 21 October 2014 Received in revised form 20 December 2014
Paramagnetic relaxation enhancement solid-state NMR (PRE ssNMR) technique was used to investigate catalytic reaction over zeolite HY. After introducing paramagnetic Cu(II) ions into the zeolite, the enhancement of longitudinal relaxation rates of nearby nuclei, i.e. 29Si of the framework and 13C of the absorbents, was measured. It was demonstrated that the PRE ssNMR technique facilitated the fast acquisition of NMR signals to monitor the heterogeneous catalytic reaction (such as acetone to hydrocarbon) using natural abundance reactants. & 2015 Published by Elsevier Inc.
Keywords: Solid-state NMR Paramagnetic relaxation enhancement (PRE) Heterogeneous catalysis Zeolites
1. Introduction High-resolution solid-state nuclear magnetic resonance (SSNMR) spectroscopy can provide a unique insight into the structure of heterogeneous catalysts and a detailed description of the related catalytic reaction mechanism [1–6]. By utilizing in situ MAS NMR technique, information about the adsorption of reactants and diversity of the nature of surface intermediates and products can thus be clearly clarified [2,4,6]. Thus, the reaction pathway during heterogeneous catalysis process could be elucidated by monitoring the evolution from reactants to intermediates and products accordingly. For in situ MAS NMR approach, 13C isotopic enriched reactants are frequently loaded into the heterogeneous catalysts to gain sufficient NMR sensitivity. However, it is unnecessary to utilize the 13 C isotopic enriched samples for in situ detection of catalytic reaction process by other spectroscopic tools such as infrared and ultraviolet–visible spectroscopy [7,8]. Thus, it is desirable to detect the natural abundance NMR signals of reactants, intermediates and products to save cost by improving the NMR sensitivity. DNP is one of the most promising techniques to detect the trace NMR signals [9–11]. Alternatively, paramagnetic relaxation enhancement (PRE) NMR is a well-established tool for investigating the n
Corresponding authors. Fax: þ 86 27 87199291. E-mail addresses: [email protected] (S. Li), [email protected] (F. Deng).
structure and dynamics of biological proteins and functional materials [12–21]. Introducing paramagnetic centers into biological systems not only can provide lots of structural constraints, but also can efficiently reduce the NMR acquisition time [20,22]. Here, we propose a new technique via in situ paramagnetic relaxation enhancement (PRE) NMR to monitor catalytic reaction processes using natural abundance reactants. In this work, the longitudinal relaxation rates of framework nuclei as well as the reactants loaded onto the catalysts could be significantly enhanced upon introducing paramagnetic centers Cu (II) ions into HY zeolite. The feasibility of the in-situ PRE NMR spectroscopy to monitor the reaction pathway using natural abundance reactants is demonstrated via the catalytic conversion of acetone to hydrocarbons.
2. Materials and methods 2.1. Sample preparation Zeolite Na Y (nSi/nAl ¼ 2.8) was exchanged in 1.0 mol L 1 NH4Cl aqueous solution at 353 K for 4 h. The process was repeated four times. The obtained zeolite NH4Y was washed with distilled water until it became chloride-free. Subsequently, the powder material was dried in air at 383 K for 12 h. An ion-exchange process was used to load Cu(II) metal ions into the HY zeolite. 1.0 g of NH4Y
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zeolite and a certain amount of CuCl2 2H2O were mechanically mixed and added into 30 ml deionized water and was then stirred for 12 h at room temperature. The amount of CuCl2 2H2O was quantitatively calculated according to the molar ratio of Cu(II) to unit cells in HY zeolite. The obtained Cu–NH4Y zeolites were dried overnight at 383 K. To prepare HY and Cu(II)–HY zeolites, the obtained NH4Y and Cu(II)–NH4Y samples were deaminated and dehydrated on a vacuum line. The temperature was raised from room temperature to 383 K at a rate of 1 K/min, kept at 383 K for 2 h, then raised from 383 to 673 K at a rate of 1.5 K/min, and kept at 673 K for ca. 12 h. The prepared Cu(II)–HY samples are denoted hereafter as X-Cu(II)–HY, where X (0.5, 1.0, 1.5, 2.0 and 3.0) represents the number of Cu(II) ions anchored per unit cell. Prior to NMR experiments, 1.0-Cu(II)–HY zeolites were placed in glass tubes and dehydrated at 673 K under a pressure below 10 3 Pa for 12 h on a vacuum line. After the dehydrated samples cooled to room temperature, a known amount (15 molecules/u.c.) of acetone with natural abundance and a small amount of oxygen gas were introduced to the HY and 1.0-Cu(II)–HY zeolites and frozen by liquid N2. Subsequently, the samples were flame-sealed. The reaction was performed in the sealed tube at designated temperatures (298, 323, 353, 383, 473 and 573 K) for 3 h. Thereafter, the sample was transferred into a 4 mm ZrO2 rotor under a dry nitrogen atmosphere in a glove box for NMR measurements. 2.2. Solid-state NMR spectroscopy Solid-state NMR experiments were carried out on a Varian Infinityplus-300 spectrometer equipped with a 4 mm doubleresonance probe. The NMR resonance frequencies for 1H, 13C, 27Al and 29Si channels were set to 299.8, 75.4, 78.1 and 59.6 MHz, respectively. The PRE MAS NMR experiments were carried out under 14 kHz MAS. 1H MAS NMR spectra were acquired with direct polarization. The 13C MAS NMR signals from both direct polarization and cross polarization were collected without hetero-nuclear decoupling as illustrated in Fig. 1. The cross polarization contact time was fixed to 0.5 ms. Typically 70,000–110,000 scans were accumulated to acquire the 13C MAS NMR spectra using a pulse delay of 0.06 s. 27Al and 29Si MAS NMR spectra were acquired using a single pulse scheme. A short π/12 pulse (0.4 μs) was used to irradiate the 27Al NMR signals. The longitudinal relaxation values (T1) of 1H, 13C, and 29Si nuclei were measured utilizing inverse recovery scheme. The π pulse lengths were set to 10 μs for 1H, 13C and 29Si nuclei during T1 measurements. All recycle delays in different NMR experiments were adjusted accordingly and could satisfy the normal measurement condition. 1H, 13C and 29 Si chemical shifts were externally referenced with respect to tetramethylsilane (TMS). 27Al chemical shifts were referenced to 0.1 M aqueous Al(NO3)3 at 0 ppm.
3. Result and discussions Fig. 2 shows 29Si MAS NMR spectra of zeolites HY and Cu(II)–HY with different Cu(II) loadings acquired at designated recycle
Fig. 1. Pulse sequence for collecting 1.0-Cu(II)–HY zeolite.
13
C MAS NMR signals of acetone reaction over
Fig. 2. 29Si MAS NMR spectra of HY zeolite with different Cu(II) loadings: (a) parent HY zeolite, (b) 0.5-Cu(II)–HY, (c) 1.0-Cu(II)–HY, (d) 1.5-Cu(II)–HY, (e) 2.0-Cu(II)–HY, and (f) 3.0-Cu(II)–HY.
Table 1 29 Si T1 values of different Qn sites in HY and Cu(II)–HY zeolites. Sites
Q1 Q2 Q3 Q4
T1 (s) HY
1.0-Cu(II)–HY
2.0-Cu(II)–HY
12.6 22.4 25.1 19.3
5.1 10 2 8.1 10 2 1.1 10 1 1.2 10 1
2.3 10 2 3.6 10 2 4.3 10 2 4.1 10 2
delays, which can satisfy the normal experimental condition (45T1). Four well resolved signals arising from 90, 96, 101, and 106 ppm in Fig. 2 can be unambiguously assigned to Q1, Q2, Q3, and Q4, respectively [23]. All of the longitudinal relaxation rates for these different silicon sites can be quantitatively determined on the basis of T1 measurements. The simulated T1 values for different sites in parent HY and Cu(II)–HY zeolites are summarized in Table 1. As listed in Table 1, 29Si T1 values of different Qn sites in parent HY zeolite were determined to be in the range of 12–25 s, whereas the relaxation rates could be enhanced to 2–3 orders after introducing 1–2 paramagnetic center Cu(II) ions into each unit cell. The small loading level of Cu(II) ions can also be evidenced from the 1H MAS NMR spectra of 1.0-Cu(II)–HY as shown in Fig. 3. The intensity for the peaks at 3.8 and 4.7 ppm due to Brønsted acid sites in 1.0-Cu(II)–HY zeolite is generally comparable with that of HY zeolite (see Fig. 3). Moreover, it is obvious that the enhancement amplitude for all these silicon sites follows the order Q1 4Q2 4Q3 4 Q4, suggesting that the most favorable coordination sites for Cu(II) species are Q1 and Q2 sites in Cu(II)–HY zeolites. In addition, the relative percentages for four Qn sites of zeolites HY and Cu(II)–HY in Fig. 2 are summarized in Table S1. As indicated in Table S1, the relative peak ratios for Q1 and Q2 sites in Cu(II)–HY zeolites compared with parent HY zeolite decreased in the following order: 0.5-Cu(II)–HY 4 1.0-Cu(II)–HY 41.5-Cu(II)– HY 42.0-Cu(II)–HY 43.0-Cu(II)–HY, confirming that Q1 and Q2 sites could be the preferable coordination sites for paramagnetic Cu(II) ions. These results are in good consistency with the DFT theoretical calculation, in which one Cu(II) ion requires at least two framework aluminum to compensate the positive charges
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Fig. 3. 1H MAS NMR spectra of dehydrated HY zeolite (a) and 1.0-Cu(II)–HY zeolite (b).
Table 2 1 H and 13C T1 values of acetone loaded on the dehydrated HY and Cu(II)–HY zeolites. Sites
13
CO CH3 CH3 13
T1 (s) HY
1.0-Cu(II)–HY
2.0-Cu(II)–HY
7.3 10 1 1.5 10 1 1.7 10 1
2.3 10 2 4.6 10 2 2.5 10 2
2.9 10 3 2.5 10 2 5.5 10 3
[24–26]. The extra-framework cationic species would be preferentially located in close proximity to the framework aluminum sites in zeolites. It should be notable that it is impossible to accurately determine the Si/Al ratio for Cu(II)–HY samples from the 29Si MAS NMR spectra. The paramagnetic relaxation enhancement of framework 29Si sites is similar to the recent observation as proposed by Inagaki et al. [27], in which drastic sensitivity enhancement in 29Si MAS NMR can be obtained in mesoporous and microporous silicate zeolites by paramagnetic doping of Cu2 þ . Moreover, our current work reveals the preferable sitting sites of Cu(II) on the basis of T1 values analysis of different Qn sites. It is noteworthy that the line widths for different 29Si and 27Al sites do not broaden significantly upon introducing paramagnetic species into HY zeolites (see Figs. 2, S1 and Table S2). Aldol condensations are industrially important reactions and can be catalyzed by solid-acid catalysts. Acetone constitutes a real alternative source of hydrocarbons since acetone is readily available for aldol condensations [28,29]. According to the previous literatures [30,31], acetone as probe molecule is readily absorbed on the Brønsted acid sites in various zeolites. The 1H and 13C T1 values of adsorbed acetone in HY and Cu(II)–HY zeolites are quantitatively determined and summarized in Table 2. As listed in Table 2, the 13C T1 values of the carbonyl groups are determined to be around 1 s in HY zeolite, whereas they are significantly shortened (by 1–2 orders) in Cu(II)–HY zeolites. In addition, the 1H relaxation rates of the methyl groups in acetone are greatly enhanced in Cu(II)–HY samples in comparison with Cu(II)-free HY zeolite. The extremely short 1H and 13C T1 values allow the fast detection of NMR signals for the reactants loaded on the Cu(II)–HY zeolites. Thus, the presence of paramagnetic relaxation enhancement provides the opportunity to monitor the evolution of the reactants on Cu(II)–HY zeolite using natural abundance reactants. Hereby, in situ PRE MAS NMR technique was employed to determine the reaction pathway of catalytic conversion of acetone to hydrocarbons. The acetone conversion over zeolites is a typical acid-catalyzed reaction, which is known to take place via aldolization
Fig. 4. 13C MAS NMR spectra of reaction of natural abundance acetone on dehydrated zeolite 1.0-Cu(II)–HY at different temperatures: (a) 298 K, (b) 323 K, (c) 353 K, (d) 383 K, (e) 473 K, and (f) 573 K.
and dehydration followed by secondary reactions such as cyclization, aromatization and cracking [29,32]. Fig. 4 shows the 13C MAS NMR spectra of reaction of natural abundance acetone on 1.0-Cu(II)–HY zeolite at temperatures ranging from 298 to 573 K. The 13C MAS NMR signals from both direct polarization and cross polarization were collected simultaneously without hetero-nuclear decoupling as illustrated in Fig. 1. At 298 K, as shown in Fig. 4a, no reaction occurs and the signals at 218 and 31 ppm are due to the carbonyl and methyl groups of the unreacted acetone adsorbed on the zeolite [30]. No obvious alternation can be found for the samples heated at 323 K for 3 h (see Fig. 4b). As the reaction temperature increases to 353 K, apart from the signal at 208 ppm arising from the products of bimolecular aldol reactions of acetone such as mesityl oxide, three additional signals are observable at 140, 128, and 21 ppm, which can be assigned to the aromatization product of acetone such as methylated benzene. The resonances at 140 and 128 ppm are due to substituted and unsubstituted aromatic carbons, respectively [33,34]. The peak at 21 ppm arises from the methyl group attached to the aromatic rings. Further increasing the temperature to 473– 573 K, mesityl oxide is cracked in the presence of water to produce acetic acid (181 ppm) [35]. In order to determine the efficiency of in-situ PRE MAS NMR technique, control experiments with identical acquisition time were carried out using Cu(II)-free HY zeolite. Fig. 5 shows the 13C MAS NMR spectra of acid-catalyzed reaction of natural abundance acetone on HY zeolite at 383 and 473 K. Compared with Fig. 4d and 4e using 1.0-Cu(II)–HY as catalyst, no significant variation can be observed, except for a considerable decrease in the signal intensity, indicating that the low loading level of Cu(II) has negligible influence on the transformation of acetone to hydrocarbons. The 13C CP/MAS NMR spectra of reaction of 2-13C-acetone reacted on dehydrated zeolite HY at various temperatures further confirm that the catalytic effect of copper species is considerably
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Acknowledgments This work was supported by the National Natural Science Foundation of China (21210005, 21221064 and 21373265).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ssnmr.2014.12.008.
References
Fig. 5. 13C direct polarization (DP) or CP/MAS NMR spectra of natural abundance acetone reacted over dehydrated zeolite HY at specified temperatures: (a, b) 383 K and (c, d) 473 K.
negligible in the case of acetone conversion on zeolite H-Y (see Fig. S2) though Cu ions can play essential roles in some zeolite-catalyzed processes, such as carbonylation of methanol to produce methyl acetate over Cu-Η-MOR zeolite [36,37]. With the same amount of reactant loading and identical NMR acquisition time, it is found that for the Cu(II)-free sample, the integrated peak areas of NMR signals could decrease up to 3–19% in comparison with that of 1.0-Cu(II)–HY zeolite (see Tables S3 and S4). It can be concluded that in-situ PRE MAS NMR leads to a 5–30 fold efficiency enhancement for collecting the 13C NMR signals. The whole reaction pathway is similar to the previous observation. Therefore, the catalytic reaction mechanism of the conversion of acetone to hydrocarbons can be elucidated using natural abundance reactants coupled with in situ paramagnetic relaxation enhancement (PRE) NMR techniques. The application of in-situ PRE MAS NMR techniques to elucidate the heterogeneous catalytic reaction pathway using natural abundance reactants not only can save cost for purchasing the 13C isotropic enriched samples, but also is promising to shorten the NMR spectrometer time. Moreover, it is also promising to explore the sitting of paramagnetic metal ions in zeolite framework, and further investigate the host–guest interaction through distance constraints in heterogeneous catalytic systems. This work might open a new way to characterize heterogeneous catalytic reactions using natural abundence reactants in combination with in-situ ssNMR technique. 4. Conclusion In this work, we have demonstrated that in-situ MAS NMR technique in combination with PRE can be employed to monitor the reaction pathway of acetone to hydrocarbon using natural abundance reactant. As Cu2 þ paramagnetic centers are introduced to HY zeolite, the relaxation rates for both the framework 29Si sites and the adsorbed reactants are significantly enhanced, resulting in the fast detection of NMR signals with short recycle delay for signals accumulation. As a result, a 5–30 fold efficiency enhancement for collecting the 13C signals is achieved. It is notable that the line-width of NMR signals is not affected too much upon the doping with paramagnetic centers.
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Please cite this article as: L. Zhou, et al., Solid State Nucl. Magn. Reson. (2015), http://dx.doi.org/10.1016/j.ssnmr.2014.12.008i
Contents lists available at ScienceDirect
Solid State Nuclear Magnetic Resonance journal homepage: www.elsevier.com/locate/ssnmr
Paramagnetic relaxation enhancement solid-state NMR studies of heterogeneous catalytic reaction over HY zeolite using natural abundance reactant Lei Zhou a, Shenhui Li a,n, Yongchao Su b, Bojie Li a, Feng Deng a,n a National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Key Laboratory of Magnetic Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China b Department of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 21 October 2014 Received in revised form 20 December 2014
Paramagnetic relaxation enhancement solid-state NMR (PRE ssNMR) technique was used to investigate catalytic reaction over zeolite HY. After introducing paramagnetic Cu(II) ions into the zeolite, the enhancement of longitudinal relaxation rates of nearby nuclei, i.e. 29Si of the framework and 13C of the absorbents, was measured. It was demonstrated that the PRE ssNMR technique facilitated the fast acquisition of NMR signals to monitor the heterogeneous catalytic reaction (such as acetone to hydrocarbon) using natural abundance reactants. & 2015 Published by Elsevier Inc.
Keywords: Solid-state NMR Paramagnetic relaxation enhancement (PRE) Heterogeneous catalysis Zeolites
1. Introduction High-resolution solid-state nuclear magnetic resonance (SSNMR) spectroscopy can provide a unique insight into the structure of heterogeneous catalysts and a detailed description of the related catalytic reaction mechanism [1–6]. By utilizing in situ MAS NMR technique, information about the adsorption of reactants and diversity of the nature of surface intermediates and products can thus be clearly clarified [2,4,6]. Thus, the reaction pathway during heterogeneous catalysis process could be elucidated by monitoring the evolution from reactants to intermediates and products accordingly. For in situ MAS NMR approach, 13C isotopic enriched reactants are frequently loaded into the heterogeneous catalysts to gain sufficient NMR sensitivity. However, it is unnecessary to utilize the 13 C isotopic enriched samples for in situ detection of catalytic reaction process by other spectroscopic tools such as infrared and ultraviolet–visible spectroscopy [7,8]. Thus, it is desirable to detect the natural abundance NMR signals of reactants, intermediates and products to save cost by improving the NMR sensitivity. DNP is one of the most promising techniques to detect the trace NMR signals [9–11]. Alternatively, paramagnetic relaxation enhancement (PRE) NMR is a well-established tool for investigating the n
Corresponding authors. Fax: þ 86 27 87199291. E-mail addresses: [email protected] (S. Li), [email protected] (F. Deng).
structure and dynamics of biological proteins and functional materials [12–21]. Introducing paramagnetic centers into biological systems not only can provide lots of structural constraints, but also can efficiently reduce the NMR acquisition time [20,22]. Here, we propose a new technique via in situ paramagnetic relaxation enhancement (PRE) NMR to monitor catalytic reaction processes using natural abundance reactants. In this work, the longitudinal relaxation rates of framework nuclei as well as the reactants loaded onto the catalysts could be significantly enhanced upon introducing paramagnetic centers Cu (II) ions into HY zeolite. The feasibility of the in-situ PRE NMR spectroscopy to monitor the reaction pathway using natural abundance reactants is demonstrated via the catalytic conversion of acetone to hydrocarbons.
2. Materials and methods 2.1. Sample preparation Zeolite Na Y (nSi/nAl ¼ 2.8) was exchanged in 1.0 mol L 1 NH4Cl aqueous solution at 353 K for 4 h. The process was repeated four times. The obtained zeolite NH4Y was washed with distilled water until it became chloride-free. Subsequently, the powder material was dried in air at 383 K for 12 h. An ion-exchange process was used to load Cu(II) metal ions into the HY zeolite. 1.0 g of NH4Y
http://dx.doi.org/10.1016/j.ssnmr.2014.12.008 0926-2040/& 2015 Published by Elsevier Inc.
Please cite this article as: L. Zhou, et al., Solid State Nucl. Magn. Reson. (2015), http://dx.doi.org/10.1016/j.ssnmr.2014.12.008i
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zeolite and a certain amount of CuCl2 2H2O were mechanically mixed and added into 30 ml deionized water and was then stirred for 12 h at room temperature. The amount of CuCl2 2H2O was quantitatively calculated according to the molar ratio of Cu(II) to unit cells in HY zeolite. The obtained Cu–NH4Y zeolites were dried overnight at 383 K. To prepare HY and Cu(II)–HY zeolites, the obtained NH4Y and Cu(II)–NH4Y samples were deaminated and dehydrated on a vacuum line. The temperature was raised from room temperature to 383 K at a rate of 1 K/min, kept at 383 K for 2 h, then raised from 383 to 673 K at a rate of 1.5 K/min, and kept at 673 K for ca. 12 h. The prepared Cu(II)–HY samples are denoted hereafter as X-Cu(II)–HY, where X (0.5, 1.0, 1.5, 2.0 and 3.0) represents the number of Cu(II) ions anchored per unit cell. Prior to NMR experiments, 1.0-Cu(II)–HY zeolites were placed in glass tubes and dehydrated at 673 K under a pressure below 10 3 Pa for 12 h on a vacuum line. After the dehydrated samples cooled to room temperature, a known amount (15 molecules/u.c.) of acetone with natural abundance and a small amount of oxygen gas were introduced to the HY and 1.0-Cu(II)–HY zeolites and frozen by liquid N2. Subsequently, the samples were flame-sealed. The reaction was performed in the sealed tube at designated temperatures (298, 323, 353, 383, 473 and 573 K) for 3 h. Thereafter, the sample was transferred into a 4 mm ZrO2 rotor under a dry nitrogen atmosphere in a glove box for NMR measurements. 2.2. Solid-state NMR spectroscopy Solid-state NMR experiments were carried out on a Varian Infinityplus-300 spectrometer equipped with a 4 mm doubleresonance probe. The NMR resonance frequencies for 1H, 13C, 27Al and 29Si channels were set to 299.8, 75.4, 78.1 and 59.6 MHz, respectively. The PRE MAS NMR experiments were carried out under 14 kHz MAS. 1H MAS NMR spectra were acquired with direct polarization. The 13C MAS NMR signals from both direct polarization and cross polarization were collected without hetero-nuclear decoupling as illustrated in Fig. 1. The cross polarization contact time was fixed to 0.5 ms. Typically 70,000–110,000 scans were accumulated to acquire the 13C MAS NMR spectra using a pulse delay of 0.06 s. 27Al and 29Si MAS NMR spectra were acquired using a single pulse scheme. A short π/12 pulse (0.4 μs) was used to irradiate the 27Al NMR signals. The longitudinal relaxation values (T1) of 1H, 13C, and 29Si nuclei were measured utilizing inverse recovery scheme. The π pulse lengths were set to 10 μs for 1H, 13C and 29Si nuclei during T1 measurements. All recycle delays in different NMR experiments were adjusted accordingly and could satisfy the normal measurement condition. 1H, 13C and 29 Si chemical shifts were externally referenced with respect to tetramethylsilane (TMS). 27Al chemical shifts were referenced to 0.1 M aqueous Al(NO3)3 at 0 ppm.
3. Result and discussions Fig. 2 shows 29Si MAS NMR spectra of zeolites HY and Cu(II)–HY with different Cu(II) loadings acquired at designated recycle
Fig. 1. Pulse sequence for collecting 1.0-Cu(II)–HY zeolite.
13
C MAS NMR signals of acetone reaction over
Fig. 2. 29Si MAS NMR spectra of HY zeolite with different Cu(II) loadings: (a) parent HY zeolite, (b) 0.5-Cu(II)–HY, (c) 1.0-Cu(II)–HY, (d) 1.5-Cu(II)–HY, (e) 2.0-Cu(II)–HY, and (f) 3.0-Cu(II)–HY.
Table 1 29 Si T1 values of different Qn sites in HY and Cu(II)–HY zeolites. Sites
Q1 Q2 Q3 Q4
T1 (s) HY
1.0-Cu(II)–HY
2.0-Cu(II)–HY
12.6 22.4 25.1 19.3
5.1 10 2 8.1 10 2 1.1 10 1 1.2 10 1
2.3 10 2 3.6 10 2 4.3 10 2 4.1 10 2
delays, which can satisfy the normal experimental condition (45T1). Four well resolved signals arising from 90, 96, 101, and 106 ppm in Fig. 2 can be unambiguously assigned to Q1, Q2, Q3, and Q4, respectively [23]. All of the longitudinal relaxation rates for these different silicon sites can be quantitatively determined on the basis of T1 measurements. The simulated T1 values for different sites in parent HY and Cu(II)–HY zeolites are summarized in Table 1. As listed in Table 1, 29Si T1 values of different Qn sites in parent HY zeolite were determined to be in the range of 12–25 s, whereas the relaxation rates could be enhanced to 2–3 orders after introducing 1–2 paramagnetic center Cu(II) ions into each unit cell. The small loading level of Cu(II) ions can also be evidenced from the 1H MAS NMR spectra of 1.0-Cu(II)–HY as shown in Fig. 3. The intensity for the peaks at 3.8 and 4.7 ppm due to Brønsted acid sites in 1.0-Cu(II)–HY zeolite is generally comparable with that of HY zeolite (see Fig. 3). Moreover, it is obvious that the enhancement amplitude for all these silicon sites follows the order Q1 4Q2 4Q3 4 Q4, suggesting that the most favorable coordination sites for Cu(II) species are Q1 and Q2 sites in Cu(II)–HY zeolites. In addition, the relative percentages for four Qn sites of zeolites HY and Cu(II)–HY in Fig. 2 are summarized in Table S1. As indicated in Table S1, the relative peak ratios for Q1 and Q2 sites in Cu(II)–HY zeolites compared with parent HY zeolite decreased in the following order: 0.5-Cu(II)–HY 4 1.0-Cu(II)–HY 41.5-Cu(II)– HY 42.0-Cu(II)–HY 43.0-Cu(II)–HY, confirming that Q1 and Q2 sites could be the preferable coordination sites for paramagnetic Cu(II) ions. These results are in good consistency with the DFT theoretical calculation, in which one Cu(II) ion requires at least two framework aluminum to compensate the positive charges
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Fig. 3. 1H MAS NMR spectra of dehydrated HY zeolite (a) and 1.0-Cu(II)–HY zeolite (b).
Table 2 1 H and 13C T1 values of acetone loaded on the dehydrated HY and Cu(II)–HY zeolites. Sites
13
CO CH3 CH3 13
T1 (s) HY
1.0-Cu(II)–HY
2.0-Cu(II)–HY
7.3 10 1 1.5 10 1 1.7 10 1
2.3 10 2 4.6 10 2 2.5 10 2
2.9 10 3 2.5 10 2 5.5 10 3
[24–26]. The extra-framework cationic species would be preferentially located in close proximity to the framework aluminum sites in zeolites. It should be notable that it is impossible to accurately determine the Si/Al ratio for Cu(II)–HY samples from the 29Si MAS NMR spectra. The paramagnetic relaxation enhancement of framework 29Si sites is similar to the recent observation as proposed by Inagaki et al. [27], in which drastic sensitivity enhancement in 29Si MAS NMR can be obtained in mesoporous and microporous silicate zeolites by paramagnetic doping of Cu2 þ . Moreover, our current work reveals the preferable sitting sites of Cu(II) on the basis of T1 values analysis of different Qn sites. It is noteworthy that the line widths for different 29Si and 27Al sites do not broaden significantly upon introducing paramagnetic species into HY zeolites (see Figs. 2, S1 and Table S2). Aldol condensations are industrially important reactions and can be catalyzed by solid-acid catalysts. Acetone constitutes a real alternative source of hydrocarbons since acetone is readily available for aldol condensations [28,29]. According to the previous literatures [30,31], acetone as probe molecule is readily absorbed on the Brønsted acid sites in various zeolites. The 1H and 13C T1 values of adsorbed acetone in HY and Cu(II)–HY zeolites are quantitatively determined and summarized in Table 2. As listed in Table 2, the 13C T1 values of the carbonyl groups are determined to be around 1 s in HY zeolite, whereas they are significantly shortened (by 1–2 orders) in Cu(II)–HY zeolites. In addition, the 1H relaxation rates of the methyl groups in acetone are greatly enhanced in Cu(II)–HY samples in comparison with Cu(II)-free HY zeolite. The extremely short 1H and 13C T1 values allow the fast detection of NMR signals for the reactants loaded on the Cu(II)–HY zeolites. Thus, the presence of paramagnetic relaxation enhancement provides the opportunity to monitor the evolution of the reactants on Cu(II)–HY zeolite using natural abundance reactants. Hereby, in situ PRE MAS NMR technique was employed to determine the reaction pathway of catalytic conversion of acetone to hydrocarbons. The acetone conversion over zeolites is a typical acid-catalyzed reaction, which is known to take place via aldolization
Fig. 4. 13C MAS NMR spectra of reaction of natural abundance acetone on dehydrated zeolite 1.0-Cu(II)–HY at different temperatures: (a) 298 K, (b) 323 K, (c) 353 K, (d) 383 K, (e) 473 K, and (f) 573 K.
and dehydration followed by secondary reactions such as cyclization, aromatization and cracking [29,32]. Fig. 4 shows the 13C MAS NMR spectra of reaction of natural abundance acetone on 1.0-Cu(II)–HY zeolite at temperatures ranging from 298 to 573 K. The 13C MAS NMR signals from both direct polarization and cross polarization were collected simultaneously without hetero-nuclear decoupling as illustrated in Fig. 1. At 298 K, as shown in Fig. 4a, no reaction occurs and the signals at 218 and 31 ppm are due to the carbonyl and methyl groups of the unreacted acetone adsorbed on the zeolite [30]. No obvious alternation can be found for the samples heated at 323 K for 3 h (see Fig. 4b). As the reaction temperature increases to 353 K, apart from the signal at 208 ppm arising from the products of bimolecular aldol reactions of acetone such as mesityl oxide, three additional signals are observable at 140, 128, and 21 ppm, which can be assigned to the aromatization product of acetone such as methylated benzene. The resonances at 140 and 128 ppm are due to substituted and unsubstituted aromatic carbons, respectively [33,34]. The peak at 21 ppm arises from the methyl group attached to the aromatic rings. Further increasing the temperature to 473– 573 K, mesityl oxide is cracked in the presence of water to produce acetic acid (181 ppm) [35]. In order to determine the efficiency of in-situ PRE MAS NMR technique, control experiments with identical acquisition time were carried out using Cu(II)-free HY zeolite. Fig. 5 shows the 13C MAS NMR spectra of acid-catalyzed reaction of natural abundance acetone on HY zeolite at 383 and 473 K. Compared with Fig. 4d and 4e using 1.0-Cu(II)–HY as catalyst, no significant variation can be observed, except for a considerable decrease in the signal intensity, indicating that the low loading level of Cu(II) has negligible influence on the transformation of acetone to hydrocarbons. The 13C CP/MAS NMR spectra of reaction of 2-13C-acetone reacted on dehydrated zeolite HY at various temperatures further confirm that the catalytic effect of copper species is considerably
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Acknowledgments This work was supported by the National Natural Science Foundation of China (21210005, 21221064 and 21373265).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ssnmr.2014.12.008.
References
Fig. 5. 13C direct polarization (DP) or CP/MAS NMR spectra of natural abundance acetone reacted over dehydrated zeolite HY at specified temperatures: (a, b) 383 K and (c, d) 473 K.
negligible in the case of acetone conversion on zeolite H-Y (see Fig. S2) though Cu ions can play essential roles in some zeolite-catalyzed processes, such as carbonylation of methanol to produce methyl acetate over Cu-Η-MOR zeolite [36,37]. With the same amount of reactant loading and identical NMR acquisition time, it is found that for the Cu(II)-free sample, the integrated peak areas of NMR signals could decrease up to 3–19% in comparison with that of 1.0-Cu(II)–HY zeolite (see Tables S3 and S4). It can be concluded that in-situ PRE MAS NMR leads to a 5–30 fold efficiency enhancement for collecting the 13C NMR signals. The whole reaction pathway is similar to the previous observation. Therefore, the catalytic reaction mechanism of the conversion of acetone to hydrocarbons can be elucidated using natural abundance reactants coupled with in situ paramagnetic relaxation enhancement (PRE) NMR techniques. The application of in-situ PRE MAS NMR techniques to elucidate the heterogeneous catalytic reaction pathway using natural abundance reactants not only can save cost for purchasing the 13C isotropic enriched samples, but also is promising to shorten the NMR spectrometer time. Moreover, it is also promising to explore the sitting of paramagnetic metal ions in zeolite framework, and further investigate the host–guest interaction through distance constraints in heterogeneous catalytic systems. This work might open a new way to characterize heterogeneous catalytic reactions using natural abundence reactants in combination with in-situ ssNMR technique. 4. Conclusion In this work, we have demonstrated that in-situ MAS NMR technique in combination with PRE can be employed to monitor the reaction pathway of acetone to hydrocarbon using natural abundance reactant. As Cu2 þ paramagnetic centers are introduced to HY zeolite, the relaxation rates for both the framework 29Si sites and the adsorbed reactants are significantly enhanced, resulting in the fast detection of NMR signals with short recycle delay for signals accumulation. As a result, a 5–30 fold efficiency enhancement for collecting the 13C signals is achieved. It is notable that the line-width of NMR signals is not affected too much upon the doping with paramagnetic centers.
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Please cite this article as: L. Zhou, et al., Solid State Nucl. Magn. Reson. (2015), http://dx.doi.org/10.1016/j.ssnmr.2014.12.008i