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2020 Volume 36 Issue 4
2020, 36(4): 190201
doi: 10.3866/PKU.WHXB201902018
Abstract:
Phosphate glass is widely used in optical applications; however, its generally low chemical stability and poor thermal mechanical properties hinder the application of phosphate glass to the rapidly evolving laser industry. The addition of a small amount of silicon can form a six-coordinate Si (Si(6)) network and improve the above-mentioned poor properties of phosphate glass. Therefore, it is important to characterize and understand the structural details of phosphosilicate glasses. It is difficult to investigate the glass structure because of its complicated and disordered characteristics. However, solid-state nuclear magnetic resonance (NMR) spectroscopy can provide detailed local structural information, regardless of the presence of its long-range order. To study the effect of alkaline earth metals on Si(6) species formation, we prepared phosphosilicate glasses (2MO-3P2O5)(1−x)·(SiO2)x (M = Ca, Sr, Ba) by conventional melt-quenching, and the glass structure was investigated by solid-state NMR and Raman spectroscopy. The 31P and 29Si NMR spectra indicated that the glass networks consisted of P(2) and P(3) tetrahedrons linked via four- and six-fold coordinated silicon units (Si(4) and Si(6)). The fraction of six-coordinated silicon Si(6) decreased with increasing SiO2 content. Similarly, the Raman spectra showed that the vibration band of the P＝O stretching mode in P(3) linked with Si(6) neighbors reduced as the silica content increased. The connectivities between various phosphorus species were probed by 31P one- and two-dimensional refocused INADEQUATE experiments. This experimental technique is based on homonuclear J-coupling and yields correlation peaks between nuclei engaged in P―O―P linkages (P(2) and P(3) units). The signals from isolated 31P nuclei are suppressed because of the absence of J-coupling, which precludes the formation of double quantum coherences. The results indicated the segregation of P(2) and P(3) units in the prepared glass, which were also compared with those in the previously reported Na2O-P2O5-SiO2 glasses. They differed from alkali phosphosilicate glasses, where each P(3) unit exhibited a maximum average of one Si(6)―O―P(3) linkage, and in the alkaline earth phosphosilicate glasses, the average was approximately 0.4–0.7. When the content of Si(6) units reached its maximum, further increase in the SiO2 content did not increase the Si(6) content, and the surplus Si were present as Si(4). Alkaline earth metal ions exhibit weaker stabilizing effects for Si(6) species. Based on the results presented herein, we constructed sketches to illustrate the local structural organization of the glass. The relationships between the compositions and structures are important for glass composition and property design. It is important to improve the performance of phosphate glass by changing its composition, particularly for large laser device applications. ![]()
Phosphate glass is widely used in optical applications; however, its generally low chemical stability and poor thermal mechanical properties hinder the application of phosphate glass to the rapidly evolving laser industry. The addition of a small amount of silicon can form a six-coordinate Si (Si(6)) network and improve the above-mentioned poor properties of phosphate glass. Therefore, it is important to characterize and understand the structural details of phosphosilicate glasses. It is difficult to investigate the glass structure because of its complicated and disordered characteristics. However, solid-state nuclear magnetic resonance (NMR) spectroscopy can provide detailed local structural information, regardless of the presence of its long-range order. To study the effect of alkaline earth metals on Si(6) species formation, we prepared phosphosilicate glasses (2MO-3P2O5)(1−x)·(SiO2)x (M = Ca, Sr, Ba) by conventional melt-quenching, and the glass structure was investigated by solid-state NMR and Raman spectroscopy. The 31P and 29Si NMR spectra indicated that the glass networks consisted of P(2) and P(3) tetrahedrons linked via four- and six-fold coordinated silicon units (Si(4) and Si(6)). The fraction of six-coordinated silicon Si(6) decreased with increasing SiO2 content. Similarly, the Raman spectra showed that the vibration band of the P＝O stretching mode in P(3) linked with Si(6) neighbors reduced as the silica content increased. The connectivities between various phosphorus species were probed by 31P one- and two-dimensional refocused INADEQUATE experiments. This experimental technique is based on homonuclear J-coupling and yields correlation peaks between nuclei engaged in P―O―P linkages (P(2) and P(3) units). The signals from isolated 31P nuclei are suppressed because of the absence of J-coupling, which precludes the formation of double quantum coherences. The results indicated the segregation of P(2) and P(3) units in the prepared glass, which were also compared with those in the previously reported Na2O-P2O5-SiO2 glasses. They differed from alkali phosphosilicate glasses, where each P(3) unit exhibited a maximum average of one Si(6)―O―P(3) linkage, and in the alkaline earth phosphosilicate glasses, the average was approximately 0.4–0.7. When the content of Si(6) units reached its maximum, further increase in the SiO2 content did not increase the Si(6) content, and the surplus Si were present as Si(4). Alkaline earth metal ions exhibit weaker stabilizing effects for Si(6) species. Based on the results presented herein, we constructed sketches to illustrate the local structural organization of the glass. The relationships between the compositions and structures are important for glass composition and property design. It is important to improve the performance of phosphate glass by changing its composition, particularly for large laser device applications.
2020, 36(4): 190502
doi: 10.3866/PKU.WHXB201905021
Abstract:
Partially reduced TiO2 nanomaterials have attracted significant interest because of their visible-light activity for catalysis and photodegradation. Herein, we prepared a partially reduced anatase TiO2 (Re-A-TiO2) nanoparticle material using a fast combustion method, demonstrating good activity toward decomposing methyl orange under visible light irradiation. The surface structure of the prepared material, after being surface-selectively 17O-labeled with H217O (17O-enriched water), was studied via 17O and 1H solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy, and the obtained results were compared to those of non-reduced anatase TiO2 (A-TiO2). The EPR results showed that the concentrations of paramagnetic species (i.e., oxygen vacancies (OV) and Ti3+) in Re-A-TiO2 were much higher than that in A-TiO2, while the former was associated with a higher OV/Ti3+ ratio. The intensities of the EPR signals were significantly affected by the adsorbed water, and this phenomenon was explored in combination with 1H NMR spectroscopy. The 1H species on Re-A-TiO2 appeared at larger chemical shifts, denoting the increased acidity of the sample, and these 1H species on Re-A-TiO2 were more difficult to remove than those on A-TiO2. On the other hand, different features were observed for the signals arising from the two-coordinated oxygen atoms (μ2-O) in 17O NMR, suggesting a typical anatase TiO2(101) surface on A-TiO2, but a more complex surface environment for Re-A-TiO2. Furthermore, a larger amount of hydroxyl groups (OH) were observed on Re-A-TiO2 compared to that on A-TiO2, indicating a larger proportion of exposed (001) facets on Re-A-TiO2. However, the μ2-O signals broadened and became similar when the drying temperature was increased to 100 ℃, indicating a non-faceted anatase TiO2 surface in such conditions. Based on the EPR and NMR results, a significant fraction of the OH species is believed to be formed from the reaction of the paramagnetic centers and adsorbed water molecules. The 1H→17O cross polarization (CP) MAS and two-dimensional heteronuclear correlation (2D HETCOR) NMR spectra were used to verify the spatial proximity of the hydrogen and oxygen species, confirming the spectral assignments of a strongly adsorbed water and one type of surface OH species. In particular, the 1H NMR signals at approximately 11 ppm were ascribed to the hydrogen species in the intramolecular hydrogen bond. In summary, this study investigated the paramagnetic species and surface structure of anatase TiO2 materials by combining EPR along with 1H and 17O solid-state NMR spectroscopy. The differences in the surface structures of Re-A-TiO2 and A-TiO2 should be closely related to their different properties toward the photodegradation of methyl orange. ![]()
Partially reduced TiO2 nanomaterials have attracted significant interest because of their visible-light activity for catalysis and photodegradation. Herein, we prepared a partially reduced anatase TiO2 (Re-A-TiO2) nanoparticle material using a fast combustion method, demonstrating good activity toward decomposing methyl orange under visible light irradiation. The surface structure of the prepared material, after being surface-selectively 17O-labeled with H217O (17O-enriched water), was studied via 17O and 1H solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy, and the obtained results were compared to those of non-reduced anatase TiO2 (A-TiO2). The EPR results showed that the concentrations of paramagnetic species (i.e., oxygen vacancies (OV) and Ti3+) in Re-A-TiO2 were much higher than that in A-TiO2, while the former was associated with a higher OV/Ti3+ ratio. The intensities of the EPR signals were significantly affected by the adsorbed water, and this phenomenon was explored in combination with 1H NMR spectroscopy. The 1H species on Re-A-TiO2 appeared at larger chemical shifts, denoting the increased acidity of the sample, and these 1H species on Re-A-TiO2 were more difficult to remove than those on A-TiO2. On the other hand, different features were observed for the signals arising from the two-coordinated oxygen atoms (μ2-O) in 17O NMR, suggesting a typical anatase TiO2(101) surface on A-TiO2, but a more complex surface environment for Re-A-TiO2. Furthermore, a larger amount of hydroxyl groups (OH) were observed on Re-A-TiO2 compared to that on A-TiO2, indicating a larger proportion of exposed (001) facets on Re-A-TiO2. However, the μ2-O signals broadened and became similar when the drying temperature was increased to 100 ℃, indicating a non-faceted anatase TiO2 surface in such conditions. Based on the EPR and NMR results, a significant fraction of the OH species is believed to be formed from the reaction of the paramagnetic centers and adsorbed water molecules. The 1H→17O cross polarization (CP) MAS and two-dimensional heteronuclear correlation (2D HETCOR) NMR spectra were used to verify the spatial proximity of the hydrogen and oxygen species, confirming the spectral assignments of a strongly adsorbed water and one type of surface OH species. In particular, the 1H NMR signals at approximately 11 ppm were ascribed to the hydrogen species in the intramolecular hydrogen bond. In summary, this study investigated the paramagnetic species and surface structure of anatase TiO2 materials by combining EPR along with 1H and 17O solid-state NMR spectroscopy. The differences in the surface structures of Re-A-TiO2 and A-TiO2 should be closely related to their different properties toward the photodegradation of methyl orange.
2020, 36(4): 190502
doi: 10.3866/PKU.WHXB201905029
Abstract:
The considerable demand of robust solid-state nuclear magnetic resonance (NMR) sequences has been met by the development in solid-state NMR hardware and probe design, particularly for fast magic angle spinning (MAS). Fast MAS enhances spectral resolution, however, it makes many conventional methods unusable because of the need of significantly high radiofrequency (RF) field strength and the intrinsic inefficiencies under such condition. Dipolar-based homonuclear recoupling sequences are widely used for structural analysis, and radio-frequency driven recoupling (RFDR) is one of the most popular zero-quantum (ZQ) homonuclear recoupling sequence. Previous studies demonstrated that RFDR efficiency strongly depends on factors such as MAS frequency, resonance offset, RF field inhomogeneity, and chemical shift anisotropy (CSA). To alleviate these dependencies, different RFDR phase cycles have been proposed. To completely understand the principle of ZQ recoupling sequences and achieve uniform broadband homonuclear recoupling under fast MAS conditions, we herein utilize the theory of symmetry sequences and propose a series of RNN1 (N ≥ 4, N is even) sequences with various phase cycles under both moderate and fast MAS conditions. We simulated the influence of MAS rate, resonance offset, RF field strength, RF mismatch, and heteronuclear decoupling on ZQ homonuclear polarization transfer efficiency. We verified the ZQ dipolar recoupling efficiencies of various RN symmetry sequences using U-13C, 15N-labeled L-histidine and microcrystalline U-13C, 15N-labeled dynein light chain (LC8) protein. The basic R4 sequence showed the worst broadband ZQ polarization transfer performance theoretically and experimentally, while the basic R6 sequence could efficiently achieve ZQ dipolar recoupling within moderate bandwidth. Under low to moderate MAS conditions, high-power 1H decoupling could considerably enhance the polarization transfer efficiency, while homonuclear recoupling sans heteronuclear decoupling is recommended under fast MAS conditions. Super phase cycling enhanced ZQ polarization transfer efficiency and bandwidth and resulted in significantly reduced sensitivity to RF mismatch. RNixy3 and RNixy4 sequences with 6*N and 8*N phase cycling steps, respectively, were preferred. The R4ixy3 sequence with fewer phase cycling steps showed comparable, or even slightly better, performance to the R4ixy4 sequence. As shown in the simulations, by choosing proper RF field strengths, 1.5*ωr < ω1 < 3*ωr, uniform broadband ZQ recoupling with R4ixy3 or R4ixy4 sequences could be achieved under fast MAS conditions, which would be significant for the accurate determination of spatial proximities and internuclear distances. By prolonging the mixing time, the RN ZQ scheme could provide more cross peaks, where medium- to long-range spatial correlations could be included; these correlations are essential for structural determination in complex systems.
The considerable demand of robust solid-state nuclear magnetic resonance (NMR) sequences has been met by the development in solid-state NMR hardware and probe design, particularly for fast magic angle spinning (MAS). Fast MAS enhances spectral resolution, however, it makes many conventional methods unusable because of the need of significantly high radiofrequency (RF) field strength and the intrinsic inefficiencies under such condition. Dipolar-based homonuclear recoupling sequences are widely used for structural analysis, and radio-frequency driven recoupling (RFDR) is one of the most popular zero-quantum (ZQ) homonuclear recoupling sequence. Previous studies demonstrated that RFDR efficiency strongly depends on factors such as MAS frequency, resonance offset, RF field inhomogeneity, and chemical shift anisotropy (CSA). To alleviate these dependencies, different RFDR phase cycles have been proposed. To completely understand the principle of ZQ recoupling sequences and achieve uniform broadband homonuclear recoupling under fast MAS conditions, we herein utilize the theory of symmetry sequences and propose a series of RNN
2020, 36(4): 190503
doi: 10.3866/PKU.WHXB201905035
Abstract:
The flexible chemical composition of the frameworks with tunable pore size and geometry of molecular dimensions makes zeolites widely used in chemical and petrochemical industry fields. The understanding of crystallization mechanism is important for a rational design of new zeolite with target structure and property, which however is still a big challenge in the field of material science. In this work, the specific spatial correlations/interactions between the SiO-···HO―Si hydrogen bonds within the charged framework of silicalite-1 (MFI topology) zeolite and the alkyl chains of tetrapropylammonium ion (TPA+) organic structure direction agents (OSDAs) were studied by one-dimensional (1D) and two-dimensional (2D) solid state-NMR spectroscopy in combination with other techniques, with the aim to shed light into the crystallization mechanism of silicalite-1. The "solvent-free" route was used to study the crystallization process. Silicalite-1 crystals were also prepared following the hydrothermal synthesis route. The structural properties of as-synthesized TPA-silicalite-1 samples during the crystallization were characterized by XRD and scanning electron microscopy (SEM) images, which showed the evolution of long-range periodic structure and cyrtal growth. The 1H-29Si CP/MAS NMR experiments showed that the reorganization of the silica or silicates occurred in the crystallization process. The lH-13C CP/MAS NMR experiments performed on the samples synthesized with different time indicated that the TPA+ ions in the amorphous samples experienced a constrained environment, forming the inorganic-organic composites. The splitting of the methyl carbon signal from TPA+ ions was observed in the 13C NMR spectra, which is the direct reflection of the interactions between the methyl groups and the silicate framework in the straight and zig-zag channels of silicalite-1. Two types of SiO-···H―OSi hydrogen bonds (SiO-···H―OSi hydrogen bond in-cage and SiO-···H―OSi hydrogen bond between lamellae) have been identified by 2D 1H double quantum (DQ)-single quantum (SQ) MAS NMR and 2H MAS NMR during the crystallization of silicalite-1. The SiO-···H―OSi hydrogen bonds between lamellae are formed and gradually transformed into the in-cage ones during the crystallization process. Their functions have been revealed in the formation of silicalite-1: the SiO-···H―OSi hydrogen bond in-cage provides the stereoscopic counterbalance for the positive charges from TPA+ ions and this stereoscopic electrostatic interaction is the key factor to transform inorganic-organic composites with the MFI structure property, even though the long-range periodic MFI structures have not been established yet; the SiO-···H―OSi hydrogen bond between lamellae acts as a connector to assemble the silicate species together to generate the zeolite framework. 2H MAS NMR spectra show that the SiOH nests exist in the zeolite framework even though the long-range periodic structures have been fully established. ![]()
The flexible chemical composition of the frameworks with tunable pore size and geometry of molecular dimensions makes zeolites widely used in chemical and petrochemical industry fields. The understanding of crystallization mechanism is important for a rational design of new zeolite with target structure and property, which however is still a big challenge in the field of material science. In this work, the specific spatial correlations/interactions between the SiO-···HO―Si hydrogen bonds within the charged framework of silicalite-1 (MFI topology) zeolite and the alkyl chains of tetrapropylammonium ion (TPA+) organic structure direction agents (OSDAs) were studied by one-dimensional (1D) and two-dimensional (2D) solid state-NMR spectroscopy in combination with other techniques, with the aim to shed light into the crystallization mechanism of silicalite-1. The "solvent-free" route was used to study the crystallization process. Silicalite-1 crystals were also prepared following the hydrothermal synthesis route. The structural properties of as-synthesized TPA-silicalite-1 samples during the crystallization were characterized by XRD and scanning electron microscopy (SEM) images, which showed the evolution of long-range periodic structure and cyrtal growth. The 1H-29Si CP/MAS NMR experiments showed that the reorganization of the silica or silicates occurred in the crystallization process. The lH-13C CP/MAS NMR experiments performed on the samples synthesized with different time indicated that the TPA+ ions in the amorphous samples experienced a constrained environment, forming the inorganic-organic composites. The splitting of the methyl carbon signal from TPA+ ions was observed in the 13C NMR spectra, which is the direct reflection of the interactions between the methyl groups and the silicate framework in the straight and zig-zag channels of silicalite-1. Two types of SiO-···H―OSi hydrogen bonds (SiO-···H―OSi hydrogen bond in-cage and SiO-···H―OSi hydrogen bond between lamellae) have been identified by 2D 1H double quantum (DQ)-single quantum (SQ) MAS NMR and 2H MAS NMR during the crystallization of silicalite-1. The SiO-···H―OSi hydrogen bonds between lamellae are formed and gradually transformed into the in-cage ones during the crystallization process. Their functions have been revealed in the formation of silicalite-1: the SiO-···H―OSi hydrogen bond in-cage provides the stereoscopic counterbalance for the positive charges from TPA+ ions and this stereoscopic electrostatic interaction is the key factor to transform inorganic-organic composites with the MFI structure property, even though the long-range periodic MFI structures have not been established yet; the SiO-···H―OSi hydrogen bond between lamellae acts as a connector to assemble the silicate species together to generate the zeolite framework. 2H MAS NMR spectra show that the SiOH nests exist in the zeolite framework even though the long-range periodic structures have been fully established.
2020, 36(4): 191201
doi: 10.3866/PKU.WHXB201912016
Abstract:
Intermolecular interactions are the key to control the final structure and properties of polymers; however, molecular-level detection of complex interactions remains a challenge. In this study, a series of poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO) solid films were prepared from aqueous solutions at different pHs. Multinuclear solid-state NMR (SSNMR) experiments, including one- and two-dimensional (1D and 2D) 1H CRAMPS (Combined Rotation And Multiple Pulse NMR Spectroscopy) based on the continuous phase modulation technique, high-resolution 13C CPMAS (Cross-Polarization and Magic-Angle Spining), and 23Na MQMAS (Multiple-Quantum MAS) experiments, were used to this in situ investigation of the structure and dynamics of these polymer blends. The 1H CRAMPS experiments revealed different types of protons in the blends from the mutually hydrogen-bonded COOH groups, from the free COOH groups, the COOH groups bounded with water that undergo fast chemical exchange mutually, and the COOH groups interacting with PEO and from main chain groups. With increasing pH, most of these peaks decreased except for the main chain protons owing to the decrease in the hydrogen bonding interaction among PAA and PEO as well as water. These CRAMPS NMR techniques were also used to elucidate the molecular mobility of the different groups. Furthermore, 2D 1H-1H spin-exchange NMR experiments provided more detailed information about the interpolymer and water–polymer interactions. 1H spin-diffusion experiments indicated the presence of phase separation in these blends, and the determined domain size of the mobile phase was approximately 17 nm. Two types of 23Na sites were revealed by MQMAS experiment; in particular, the Na+ ionic location and interaction between individual polymers was revealed by 1H detected 23Na-1H CP experiments, which showed that 23Na is in the proximity of PAA instead of PEO. These SSNMR experimental results provide detailed information about the influence of hydrogen bonding interactions on the microcosmic structure and dynamics of PAA/PEO blends at the molecular level. The influence of different pH levels on the hydrogen bonding interactions, miscibility between PAA and PEO, microstructure, water–polymer interactions, and molecule mobility of individual compositions was clarified. Based on the above-mentioned NMR studies, we proposed a novel structural model of these PAA/PEO blends. This model successfully revealed the influence of pH on the microstructure and dynamics of PAA/PEO blends at the molecular level for the first time. Our results indicate that solid-state NMR is a powerful tool that can be used to study the complex interactions of multiphase polymer materials. Our research is of great significant to both the development of new methods to probe the weak interactions in polymers and the development of new polymer materials based on hydrogen bonding interactions.
Intermolecular interactions are the key to control the final structure and properties of polymers; however, molecular-level detection of complex interactions remains a challenge. In this study, a series of poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO) solid films were prepared from aqueous solutions at different pHs. Multinuclear solid-state NMR (SSNMR) experiments, including one- and two-dimensional (1D and 2D) 1H CRAMPS (Combined Rotation And Multiple Pulse NMR Spectroscopy) based on the continuous phase modulation technique, high-resolution 13C CPMAS (Cross-Polarization and Magic-Angle Spining), and 23Na MQMAS (Multiple-Quantum MAS) experiments, were used to this in situ investigation of the structure and dynamics of these polymer blends. The 1H CRAMPS experiments revealed different types of protons in the blends from the mutually hydrogen-bonded COOH groups, from the free COOH groups, the COOH groups bounded with water that undergo fast chemical exchange mutually, and the COOH groups interacting with PEO and from main chain groups. With increasing pH, most of these peaks decreased except for the main chain protons owing to the decrease in the hydrogen bonding interaction among PAA and PEO as well as water. These CRAMPS NMR techniques were also used to elucidate the molecular mobility of the different groups. Furthermore, 2D 1H-1H spin-exchange NMR experiments provided more detailed information about the interpolymer and water–polymer interactions. 1H spin-diffusion experiments indicated the presence of phase separation in these blends, and the determined domain size of the mobile phase was approximately 17 nm. Two types of 23Na sites were revealed by MQMAS experiment; in particular, the Na+ ionic location and interaction between individual polymers was revealed by 1H detected 23Na-1H CP experiments, which showed that 23Na is in the proximity of PAA instead of PEO. These SSNMR experimental results provide detailed information about the influence of hydrogen bonding interactions on the microcosmic structure and dynamics of PAA/PEO blends at the molecular level. The influence of different pH levels on the hydrogen bonding interactions, miscibility between PAA and PEO, microstructure, water–polymer interactions, and molecule mobility of individual compositions was clarified. Based on the above-mentioned NMR studies, we proposed a novel structural model of these PAA/PEO blends. This model successfully revealed the influence of pH on the microstructure and dynamics of PAA/PEO blends at the molecular level for the first time. Our results indicate that solid-state NMR is a powerful tool that can be used to study the complex interactions of multiphase polymer materials. Our research is of great significant to both the development of new methods to probe the weak interactions in polymers and the development of new polymer materials based on hydrogen bonding interactions.
2020, 36(4): 190201
doi: 10.3866/PKU.WHXB201902019
Abstract:
The rapid development of batteries, especially lithium-ion batteries, has dramatically changed our daily lives. From portable electronics to electric vehicles and smart grids, batteries are extensively used in many fields and are difficult to be replaced in terms of their excellent energy and power densities. The advancement of battery technology requires the thorough understanding of electrochemical reaction mechanisms, which strongly depends on the collaboration of researchers from different fields. Magnetic resonance spectroscopy includes the important techniques of nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), and the former is suitable for studying light elements commonly found in batteries including Li, Na, P and O, while the latter is suitable for studying heavier transition metals such as Co, Mn, Fe and V. In addition, NMR and EPR are capable of quantitatively analysis in a nondestructive manner regardless of sample crystallinity. Hence, NMR and EPR spectroscopies have allowed for significant research progress and have become increasingly important for battery research over the past three decades. Herein, we will provide our perspective of magnetic resonance methods and first summarize the main interactions and the Hamiltonian forms of solid-state NMR and EPR (dipole-dipole interaction, electric quadrupole interaction, chemical shift, and hyperfine interaction). Subsequently, we summarize the important and frequently-used methods of solid-state NMR and EPR spectroscopies and introduce their representative applications in metal ion battery research (mainly lithium- and sodium-ion batteries). Specifically, we introduce the basic principles and representative applications of (ⅰ) MQMAS (multiple-quantum magic angle spinning), (ⅱ) pjMATPASS (MAT = magic-angle turning, PASS = phase-adjusted sideband separation, and pj = projection), (ⅲ) WURST-CPMG (WURST = wide band uniform rate smooth truncation, CPMG = Carr-Purcell Meiboom-Gil), (ⅳ) 2D homonuclear correlation and exchange (2D EXSY), (ⅴ) 2D homonuclear correlation based on dipole coupling (i.e. RFDR), (ⅵ) perpendicular mode EPR, (ⅶ) parallel mode EPR, (ⅷ) in situ NMR, and (ⅸ) in situ EPR. In addition, we briefly introduce representative applications of 2D heteronuclear correlation (i.e. CP-HETCOR), pulsed field gradient NMR, spin-lattice relaxation (SLR), spin alignment echo (SAE), DFT calculations, and dynamic nuclear polarization (DNP). Previous reviews regarding the application of magnetic resonance technology in battery research are almost all reported in terms of the classification of battery materials. In other words, they are written from the perspective of applications in cathode, anode, and electrolyte research. Herein, we summarize from the perspective of solid-state NMR and EPR methods, which may be beneficial for the readers to fully understand the value of these important technologies. We believe this review can serve as a guide to solve challenges related to using solid-state NMR and EPR spectroscopies in battery research. ![]()
The rapid development of batteries, especially lithium-ion batteries, has dramatically changed our daily lives. From portable electronics to electric vehicles and smart grids, batteries are extensively used in many fields and are difficult to be replaced in terms of their excellent energy and power densities. The advancement of battery technology requires the thorough understanding of electrochemical reaction mechanisms, which strongly depends on the collaboration of researchers from different fields. Magnetic resonance spectroscopy includes the important techniques of nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), and the former is suitable for studying light elements commonly found in batteries including Li, Na, P and O, while the latter is suitable for studying heavier transition metals such as Co, Mn, Fe and V. In addition, NMR and EPR are capable of quantitatively analysis in a nondestructive manner regardless of sample crystallinity. Hence, NMR and EPR spectroscopies have allowed for significant research progress and have become increasingly important for battery research over the past three decades. Herein, we will provide our perspective of magnetic resonance methods and first summarize the main interactions and the Hamiltonian forms of solid-state NMR and EPR (dipole-dipole interaction, electric quadrupole interaction, chemical shift, and hyperfine interaction). Subsequently, we summarize the important and frequently-used methods of solid-state NMR and EPR spectroscopies and introduce their representative applications in metal ion battery research (mainly lithium- and sodium-ion batteries). Specifically, we introduce the basic principles and representative applications of (ⅰ) MQMAS (multiple-quantum magic angle spinning), (ⅱ) pjMATPASS (MAT = magic-angle turning, PASS = phase-adjusted sideband separation, and pj = projection), (ⅲ) WURST-CPMG (WURST = wide band uniform rate smooth truncation, CPMG = Carr-Purcell Meiboom-Gil), (ⅳ) 2D homonuclear correlation and exchange (2D EXSY), (ⅴ) 2D homonuclear correlation based on dipole coupling (i.e. RFDR), (ⅵ) perpendicular mode EPR, (ⅶ) parallel mode EPR, (ⅷ) in situ NMR, and (ⅸ) in situ EPR. In addition, we briefly introduce representative applications of 2D heteronuclear correlation (i.e. CP-HETCOR), pulsed field gradient NMR, spin-lattice relaxation (SLR), spin alignment echo (SAE), DFT calculations, and dynamic nuclear polarization (DNP). Previous reviews regarding the application of magnetic resonance technology in battery research are almost all reported in terms of the classification of battery materials. In other words, they are written from the perspective of applications in cathode, anode, and electrolyte research. Herein, we summarize from the perspective of solid-state NMR and EPR methods, which may be beneficial for the readers to fully understand the value of these important technologies. We believe this review can serve as a guide to solve challenges related to using solid-state NMR and EPR spectroscopies in battery research.
2020, 36(4): 190500
doi: 10.3866/PKU.WHXB201905004
Abstract:
The fast-growing demand for safe energy storages with high power and energy density drives the continuous improvement of rechargeable Li-ion batteries (LIBs). In situ characterization is a potential way to understand the mechanism (metaphases, diffusion, kinetics, inhomogeneity etc.) of battery under operation conditions. Solid-state nuclear magnetic resonance (SS-NMR) is very sensitive to the local environment of 1H, 6, 7Li, 11B, 13C, 17O, 19F, 23Na, and 31P isotopes, which are widely used in battery materials, regardless of their ordering degree. In addition to providing well-resolved spectra obtained under fast magic angle spinning (MAS), NMR can effectively serve as a non-invasive tool to capture the evolution of electrodes/electrolyte upon charge/discharge electrochemical cycling. Subsequently, in situ NMR and imaging (MRI) have been developed for extending toward temporal and spatial dimensions in working batteries. Complementarily, highly sensitive electron paramagnetic resonance (EPR) and imaging (EPRI) have been employed to track and map the redox of transition metals and oxygen species (O2n−) within electrodes. The insights gained from in situ NMR/EPR and their imaging can serve as a guide for the structural design of energy storage materials and the fabrication of batteries with optimized performance. As such, this review summarizes the applications of both NMR and EPR in the field of battery community. In particular, we first introduce the combination of fast magic angle spinning and phase-adjusted sideband separation (pjMATPASS) to obtain highly resolved spectra for extreme broad signal mediated by unpaired electrons, which is usually found in battery materials, as well as isotope-oriented NMR to determine the Li pathway in the composite electrolyte by the aid of 6Li replacing 7Li in their transport pathway. Secondly, we introduce the combination of NMR/MRI measurement while battery under electrochemical cycling by (1) briefly summarizing the advantages and disadvantages of home-made cells (coin cell, bag cell, and cylindrical cell) developed for in situ NMR study; (2) using different isotopes for conducting in situ NMR on batteries: 7Li, 23Na, and 31P spectra; and (3) performing in situ MRI on electrolytes and electrodes with and without chemical shift information (CSI, S-ISIS, and stray-field MRI). Furthermore, in situ EPR determines and quantifies the evolution of active Li microstructure, transition metals, and oxygen species together with in situ EPRI mapping of the concentration of the paramagnetic center within a functioning battery. Finally, we point out the limitations and perspective of in situ NMR and EPR for cycling batteries in real-time. This review will provide illuminating insights on the magnetic technologies in the battery community and pave a way for carrying out NMR/EPR on functional materials. ![]()
The fast-growing demand for safe energy storages with high power and energy density drives the continuous improvement of rechargeable Li-ion batteries (LIBs). In situ characterization is a potential way to understand the mechanism (metaphases, diffusion, kinetics, inhomogeneity etc.) of battery under operation conditions. Solid-state nuclear magnetic resonance (SS-NMR) is very sensitive to the local environment of 1H, 6, 7Li, 11B, 13C, 17O, 19F, 23Na, and 31P isotopes, which are widely used in battery materials, regardless of their ordering degree. In addition to providing well-resolved spectra obtained under fast magic angle spinning (MAS), NMR can effectively serve as a non-invasive tool to capture the evolution of electrodes/electrolyte upon charge/discharge electrochemical cycling. Subsequently, in situ NMR and imaging (MRI) have been developed for extending toward temporal and spatial dimensions in working batteries. Complementarily, highly sensitive electron paramagnetic resonance (EPR) and imaging (EPRI) have been employed to track and map the redox of transition metals and oxygen species (O2n−) within electrodes. The insights gained from in situ NMR/EPR and their imaging can serve as a guide for the structural design of energy storage materials and the fabrication of batteries with optimized performance. As such, this review summarizes the applications of both NMR and EPR in the field of battery community. In particular, we first introduce the combination of fast magic angle spinning and phase-adjusted sideband separation (pjMATPASS) to obtain highly resolved spectra for extreme broad signal mediated by unpaired electrons, which is usually found in battery materials, as well as isotope-oriented NMR to determine the Li pathway in the composite electrolyte by the aid of 6Li replacing 7Li in their transport pathway. Secondly, we introduce the combination of NMR/MRI measurement while battery under electrochemical cycling by (1) briefly summarizing the advantages and disadvantages of home-made cells (coin cell, bag cell, and cylindrical cell) developed for in situ NMR study; (2) using different isotopes for conducting in situ NMR on batteries: 7Li, 23Na, and 31P spectra; and (3) performing in situ MRI on electrolytes and electrodes with and without chemical shift information (CSI, S-ISIS, and stray-field MRI). Furthermore, in situ EPR determines and quantifies the evolution of active Li microstructure, transition metals, and oxygen species together with in situ EPRI mapping of the concentration of the paramagnetic center within a functioning battery. Finally, we point out the limitations and perspective of in situ NMR and EPR for cycling batteries in real-time. This review will provide illuminating insights on the magnetic technologies in the battery community and pave a way for carrying out NMR/EPR on functional materials.
2020, 36(4): 190501
doi: 10.3866/PKU.WHXB201905019
Abstract:
Biological assemblies with specific function or pathogenicity are widespread within organisms; however, their insolubility, amorphous properties, and large size are the major obstacles for structure determination via solution NMR spectroscopy and X-ray crystallography. In contrast, solid-state NMR (ssNMR) spectroscopy is not limited by the solubility or crystallinity of the sample and is a potent method to determine the structure of protein assemblies at atomic resolution. High magnetic field, fast magic-angle spinning (MAS), isotope labeling schemes, and improved methodology in ssNMR have enabled resonance assignment and restraints in structure determination among protein assemblies. This review first discusses methods of obtaining structural restraints by ssNMR. Optimization of sample preparation is an effective approach to increase homogeneity in the conformation, thus also improving the resolution of ssNMR spectra. Furthermore, the resolution of 13C spectra can be further improved by using 13C sparse labeling strategies with selective labeling of carbon sources during protein expression. Structure characterization by ssNMR is based on structural restraints via multidimensional experiments correlating resonance between 13C and 15N. Protein secondary structure can be ascertained through chemical shifts involving 13Cα, 13Cβ, 13C', and 15N. The backbone torsion angle can be predicted using TALOS+ based on these chemical shifts. Site-specific structural restraints are accessible from 2D experiments such as 13C-13C, e.g., proton-driven spin diffusion (PDSD), dipolar-assisted rotational resonance (DARR), proton-assisted recoupling (PAR) and 13C-15N, e.g., transferred-echo double-resonance (TEDOR), rotational-echo double-resonance (REDOR). An additional issue is to distinguish inter-molecular and intra-molecular restraints. Preparations of mixed labeled samples (e.g., 50% 13C uniformly labeled subunits and 50% uniformly 15N labeled subunits) have yielded abundant structural restraints from ssNMR data, facilitating high-resolution structural analysis. Further, hybrid approaches based on ssNMR are discussed. Electron microscopy (EM) is a suitable method to investigate structural features including the diameter of the protein assemblies, which is "invisible" through ssNMR analysis. Scanning transmission electron microscopy (STEM) can help determine the mass-per-length parameters (MPL) of unbranched fibrils, thus confirming the number of subunits in a layer of fibrils. Cryo-EM is a powerful technique to describe the molecular envelope of protein assemblies. Cryo-EM potentially yields the density map and long-range symmetry parameters, while ssNMR provides atomic-level structural details; hence, Cryo-EM and ssNMR are highly complementary methods. X-ray diffraction can help determine the distance (4.5–4.7 Å, 1 Å = 0.1 nm) along the fibril axis between adjacent polypeptide chains in β-strand conformation, generally referred to as the "cross-β" structure. Rosetta has simulated the protein structure in accordance with structural data obtained from protein data bank (PDB) with the same peptide sequence. On combining ssNMR with those methods, more abundant structural information may be obtained, thus shortening the structural calculation cycle. Finally, a detailed description of the ssNMR structural data on amyloid-β (Aβ) fibrils and T3SS needles are provided as examples. Various structural characteristics of Aβ40/Aβ42 were reported by several groups, including the trimeric or dimeric conformations, parallel or antiparallel, in-register or out-of-register arrangements of the β-strands, demonstrating the structural polymorphism of Aβ40/Aβ42. Atomic-resolution structures of T3SS needles were analyzed on the basis of high-resolution spectra, using 13C sparse-labeled and ssNMR-Cryo-EM-Rosetta hybrid approaches, indicating that hybrid approaches based on ssNMR are a powerful tool to determine the high-resolution structure of protein assemblies. ![]()
Biological assemblies with specific function or pathogenicity are widespread within organisms; however, their insolubility, amorphous properties, and large size are the major obstacles for structure determination via solution NMR spectroscopy and X-ray crystallography. In contrast, solid-state NMR (ssNMR) spectroscopy is not limited by the solubility or crystallinity of the sample and is a potent method to determine the structure of protein assemblies at atomic resolution. High magnetic field, fast magic-angle spinning (MAS), isotope labeling schemes, and improved methodology in ssNMR have enabled resonance assignment and restraints in structure determination among protein assemblies. This review first discusses methods of obtaining structural restraints by ssNMR. Optimization of sample preparation is an effective approach to increase homogeneity in the conformation, thus also improving the resolution of ssNMR spectra. Furthermore, the resolution of 13C spectra can be further improved by using 13C sparse labeling strategies with selective labeling of carbon sources during protein expression. Structure characterization by ssNMR is based on structural restraints via multidimensional experiments correlating resonance between 13C and 15N. Protein secondary structure can be ascertained through chemical shifts involving 13Cα, 13Cβ, 13C', and 15N. The backbone torsion angle can be predicted using TALOS+ based on these chemical shifts. Site-specific structural restraints are accessible from 2D experiments such as 13C-13C, e.g., proton-driven spin diffusion (PDSD), dipolar-assisted rotational resonance (DARR), proton-assisted recoupling (PAR) and 13C-15N, e.g., transferred-echo double-resonance (TEDOR), rotational-echo double-resonance (REDOR). An additional issue is to distinguish inter-molecular and intra-molecular restraints. Preparations of mixed labeled samples (e.g., 50% 13C uniformly labeled subunits and 50% uniformly 15N labeled subunits) have yielded abundant structural restraints from ssNMR data, facilitating high-resolution structural analysis. Further, hybrid approaches based on ssNMR are discussed. Electron microscopy (EM) is a suitable method to investigate structural features including the diameter of the protein assemblies, which is "invisible" through ssNMR analysis. Scanning transmission electron microscopy (STEM) can help determine the mass-per-length parameters (MPL) of unbranched fibrils, thus confirming the number of subunits in a layer of fibrils. Cryo-EM is a powerful technique to describe the molecular envelope of protein assemblies. Cryo-EM potentially yields the density map and long-range symmetry parameters, while ssNMR provides atomic-level structural details; hence, Cryo-EM and ssNMR are highly complementary methods. X-ray diffraction can help determine the distance (4.5–4.7 Å, 1 Å = 0.1 nm) along the fibril axis between adjacent polypeptide chains in β-strand conformation, generally referred to as the "cross-β" structure. Rosetta has simulated the protein structure in accordance with structural data obtained from protein data bank (PDB) with the same peptide sequence. On combining ssNMR with those methods, more abundant structural information may be obtained, thus shortening the structural calculation cycle. Finally, a detailed description of the ssNMR structural data on amyloid-β (Aβ) fibrils and T3SS needles are provided as examples. Various structural characteristics of Aβ40/Aβ42 were reported by several groups, including the trimeric or dimeric conformations, parallel or antiparallel, in-register or out-of-register arrangements of the β-strands, demonstrating the structural polymorphism of Aβ40/Aβ42. Atomic-resolution structures of T3SS needles were analyzed on the basis of high-resolution spectra, using 13C sparse-labeled and ssNMR-Cryo-EM-Rosetta hybrid approaches, indicating that hybrid approaches based on ssNMR are a powerful tool to determine the high-resolution structure of protein assemblies.
2020, 36(4): 190601
doi: 10.3866/PKU.WHXB201906018
Abstract:
Nuclear magnetic resonance (NMR) is an effective and widely adapted technique that can be used for medical diagnosis and chemical analysis. However, its application has been limited by low sensitivity originating from the extremely low polarization of nuclear spins that follow a typical Boltzmann distribution. In principal, it is possible to break this Boltzmann distribution using different physical or chemical mechanisms to generate hyperpolarization and increase NMR sensitivity by several orders of magnitude. The crucial point of hyperpolarization is to transfer the polarization from highly polarized systems to nuclear spins. For example, the unpaired electrons in organic radicals or other systems exhibit much higher polarization than that of nuclear spins (~660 times higher than 1H) under the same magnetic field. The high polarization of electrons at thermal equilibrium can be transferred to nuclear spins via microwave irradiation and hyperfine coupling. This hyperpolarization method is called dynamic nuclear polarization (DNP) and has been successfully and widely applied for the evaluation of the protein structure and the examination of nanomaterial surface chemistry. Electron spins can also be hyperpolarized using circularly polarized light (CPL) or nonpolarized light in some systems, and this polarization can be transferred to nuclear spins as well. These hyperpolarization methods are referred to as optical pumping (OP) and optical nuclear polarization (ONP), respectively. A common application of OP is the production of hyperpolarized noble gases, including hyperpolarized xenon-129, which can be used in magnetic resonance imaging of lungs or evaluation of porous structures. For ONP, the nitrogen-vacancy center in diamond is the most promising system that has demonstrated the ability to track the precession of a single spin. In addition, electrons can be polarized by certain chemical reactions as used in chemically induced dynamic nuclear polarization (CIDNP). CIDNP can be used to study the active sites of proteins and identify low-concentration intermediates that are formed during chemical processes. In addition to electrons, hydrogen molecules with unique spin state, i.e., parahydrogen, can be converted to hyperpolarized NMR signals via hydrogen addition reactions, which is known as parahydrogen induced polarization (PHIP). PHIP was originally used to understand the mechanisms of hydrogenation processes, but has recently become a promising hyperpolarization technique via the protocols of signal amplification by reversible exchange (SABRE). Herein, the basic mechanisms and potential applications of DNP, OP, CIDNP, and PHIP techniques are reviewed. These emerging hyperpolarization techniques have the potential to push the limits of NMR beyond current conceptions. ![]()
Nuclear magnetic resonance (NMR) is an effective and widely adapted technique that can be used for medical diagnosis and chemical analysis. However, its application has been limited by low sensitivity originating from the extremely low polarization of nuclear spins that follow a typical Boltzmann distribution. In principal, it is possible to break this Boltzmann distribution using different physical or chemical mechanisms to generate hyperpolarization and increase NMR sensitivity by several orders of magnitude. The crucial point of hyperpolarization is to transfer the polarization from highly polarized systems to nuclear spins. For example, the unpaired electrons in organic radicals or other systems exhibit much higher polarization than that of nuclear spins (~660 times higher than 1H) under the same magnetic field. The high polarization of electrons at thermal equilibrium can be transferred to nuclear spins via microwave irradiation and hyperfine coupling. This hyperpolarization method is called dynamic nuclear polarization (DNP) and has been successfully and widely applied for the evaluation of the protein structure and the examination of nanomaterial surface chemistry. Electron spins can also be hyperpolarized using circularly polarized light (CPL) or nonpolarized light in some systems, and this polarization can be transferred to nuclear spins as well. These hyperpolarization methods are referred to as optical pumping (OP) and optical nuclear polarization (ONP), respectively. A common application of OP is the production of hyperpolarized noble gases, including hyperpolarized xenon-129, which can be used in magnetic resonance imaging of lungs or evaluation of porous structures. For ONP, the nitrogen-vacancy center in diamond is the most promising system that has demonstrated the ability to track the precession of a single spin. In addition, electrons can be polarized by certain chemical reactions as used in chemically induced dynamic nuclear polarization (CIDNP). CIDNP can be used to study the active sites of proteins and identify low-concentration intermediates that are formed during chemical processes. In addition to electrons, hydrogen molecules with unique spin state, i.e., parahydrogen, can be converted to hyperpolarized NMR signals via hydrogen addition reactions, which is known as parahydrogen induced polarization (PHIP). PHIP was originally used to understand the mechanisms of hydrogenation processes, but has recently become a promising hyperpolarization technique via the protocols of signal amplification by reversible exchange (SABRE). Herein, the basic mechanisms and potential applications of DNP, OP, CIDNP, and PHIP techniques are reviewed. These emerging hyperpolarization techniques have the potential to push the limits of NMR beyond current conceptions.
2020, 36(4): 190302
doi: 10.3866/PKU.WHXB201903021
Abstract:
SSZ-13 zeolite with a chabazite (CHA) topology structure has important applications for methanol to olefin (MTO) conversion and selective catalytic reduction of nitrogen oxides (NOx) by ammonia (NH3-SCR) to reduce diesel engine exhaust emissions. It has been reported that the Al-rich SSZ-13 zeolite can be used to tune the selectivity of olefins in the MTO reaction, and significantly enhance NO conversions at lower temperatures in NH3-SCR. Thus, the aluminum content and distribution as well as the corresponding acidity in SSZ-13 zeolite determine the catalytic performance of the zeolite for different catalytic reactions. Herein, quantum chemical computing using density functional theory (DFT) combined with multinuclear solid-state nuclear- magnetic-resonance (NMR) experiments were performed to investigate the correlation of Al location and Brønsted acidity of H-SSZ-13 zeolite with the Si/Al ratio varying from 5.8 to 25. The most favorable acid site in the 1Al model is O(1)―H in which a proton is bonded with the O(1) atom near the isolated Al atom of the zeolite framework. Nevertheless, energy differences were rather small when comparing the substitution energies of an Al atom replacing a Si atom in the zeolite framework with a proton located in different O sites. As the Si/Al ratio decreased, the Al-rich SSZ-13 zeolite contained more Al substitutions in its framework. This system exhibited the lowest substitution energy when two Al atoms were located at the diagonal of the same six-membered ring for the Al-Si-Si-Al (NNNN) sequence in the framework of the Al-rich SSZ-13 zeolite. However, for the Al-Si-Al (NNN) sequence, the most favorable distribution involved two Al atoms located in different six-membered rings of the double six-membered ring units (D6R). The proton affinities (PA), NH3 desorption energies, and 1H NMR chemical shifts after d3-acetonitrile adsorption were calculated in the most stable models to characterize the Brønsted acid strength of the SSZ-13 zeolite with different Si/Al ratios. All computing results suggested that the Al-rich SSZ-13 zeolite exhibited weaker Brønsted acid strength than that of the Si-rich counterpart due to the presence of Si(2Al) groupings with the NNN sequence in the framework. Quantitative 29Si magic-angle spinning (MAS) NMR measurements after deconvolution demonstrated that the content of Si(2Al) groupings in the Al-rich SSZ-13 was > 43%. The 1H MAS NMR experiments after d3-acetonitrile adsorption showed that the chemical shift of the bridging hydroxyls in the Al-rich SSZ-13 moved to the lower field, further confirming that it had a weaker Brønsted acid strength than the Si-rich counterpart. ![]()
SSZ-13 zeolite with a chabazite (CHA) topology structure has important applications for methanol to olefin (MTO) conversion and selective catalytic reduction of nitrogen oxides (NOx) by ammonia (NH3-SCR) to reduce diesel engine exhaust emissions. It has been reported that the Al-rich SSZ-13 zeolite can be used to tune the selectivity of olefins in the MTO reaction, and significantly enhance NO conversions at lower temperatures in NH3-SCR. Thus, the aluminum content and distribution as well as the corresponding acidity in SSZ-13 zeolite determine the catalytic performance of the zeolite for different catalytic reactions. Herein, quantum chemical computing using density functional theory (DFT) combined with multinuclear solid-state nuclear- magnetic-resonance (NMR) experiments were performed to investigate the correlation of Al location and Brønsted acidity of H-SSZ-13 zeolite with the Si/Al ratio varying from 5.8 to 25. The most favorable acid site in the 1Al model is O(1)―H in which a proton is bonded with the O(1) atom near the isolated Al atom of the zeolite framework. Nevertheless, energy differences were rather small when comparing the substitution energies of an Al atom replacing a Si atom in the zeolite framework with a proton located in different O sites. As the Si/Al ratio decreased, the Al-rich SSZ-13 zeolite contained more Al substitutions in its framework. This system exhibited the lowest substitution energy when two Al atoms were located at the diagonal of the same six-membered ring for the Al-Si-Si-Al (NNNN) sequence in the framework of the Al-rich SSZ-13 zeolite. However, for the Al-Si-Al (NNN) sequence, the most favorable distribution involved two Al atoms located in different six-membered rings of the double six-membered ring units (D6R). The proton affinities (PA), NH3 desorption energies, and 1H NMR chemical shifts after d3-acetonitrile adsorption were calculated in the most stable models to characterize the Brønsted acid strength of the SSZ-13 zeolite with different Si/Al ratios. All computing results suggested that the Al-rich SSZ-13 zeolite exhibited weaker Brønsted acid strength than that of the Si-rich counterpart due to the presence of Si(2Al) groupings with the NNN sequence in the framework. Quantitative 29Si magic-angle spinning (MAS) NMR measurements after deconvolution demonstrated that the content of Si(2Al) groupings in the Al-rich SSZ-13 was > 43%. The 1H MAS NMR experiments after d3-acetonitrile adsorption showed that the chemical shift of the bridging hydroxyls in the Al-rich SSZ-13 moved to the lower field, further confirming that it had a weaker Brønsted acid strength than the Si-rich counterpart.
2020, 36(4): 190701
doi: 10.3866/PKU.WHXB201907014
Abstract:
2020, 36(4): 191203
doi: 10.3866/PKU.WHXB201912038
Abstract:
2020, 36(4): 191204
doi: 10.3866/PKU.WHXB201912040
Abstract:
2020, 36(4): 191101
doi: 10.3866/PKU.WHXB201911014
Abstract:
2020, 36(4): 191207
doi: 10.3866/PKU.WHXB201912074
Abstract:
