t1-Noise Eliminated Dipolar Heteronuclear Multiple-Quantum Coherence Solid-State NMR Spectroscopy

Heteronuclear correlation (HETCOR) spectroscopy is one of the key tools in the arsenal of the solid-state NMR spectroscopist to probe spatial proximity between two different nuclei and enhance spectral resolution. Dipolar heteronuclear multiple-quantum coherence (D-HMQC) is a powerful technique that can be potentially utilized to obtain 1H detected 2D HETCOR solid-state NMR spectra of any NMR active nucleus. A long-standing problem in 1H detected D-HMQC solid-state NMR experiments is the presence of t1-noise which reduces sensitivity and impedes spectral interpretation. In this contribution, we describe novel pulse sequences, termed t1-noise eliminated (TONE) D-HMQC, that suppress t1-noise and can provide higher sensitivity and resolution than conventional D-HMQC. Monte-Carlo and numerical simulations confirm that t1-noise in conventional D-HMQC primarily occurs because random MAS frequency fluctuations cause variations in the NMR signal amplitude from scan to scan, leading to imperfect cancellation of uncorrelated signals by phase cycling. The TONE D-HMQC sequence uses 1H p-pulses to refocus the evolution of 1H CSA across each recoupling block, improving the stability of the pulse sequence to random MAS frequency fluctuations. The 1H refocusing pulses also restore the orthogonality of in-phase and anti-phase magnetization for all crystallite orientations, enabling the use of 90° flip-back or LG spin-lock trim pulses to reduce the intensity of uncorrelated signals. We demonstrate the application of these methods to acquire detected 2D 1H-35Cl and 1H-13C HETCOR spectra of histidine•HCl•H2O with reduced t1-noise. To show generality, we also apply these methods to obtain 2D 1H-17O spectra of 20%-17O fmoc-alanine and for the first time at natural abundance, 2D 1H-25Mg HETCOR spectra of magnesium hydroxide. The TONE D-HMQC sequences are also used to probe 1H-25Mg and 1H-27Al proximities in Mg-Al layered double hydroxides and confirm the even mixing of Mg and Al in these materials

Metal–Organic‐Framework‐Derived Carbons: Applications as Solid‐Base Catalyst and Support for Pd Nanoparticles in Tandem Catalysis

The facile pyrolysis of a bipyridyl metal‐organic framework, MOF‐253, produces N‐doped porous carbons (Cz‐MOF‐253), which exhibit excellent catalytic activity in the Knoevenagel condensation reaction and outperform other nitrogen‐containing MOF‐derived carbons. More importantly, by virtue of their high Lewis basicity and porous nature, Cz‐MOF‐253‐supported Pd nanoparticles (Pd/Cz‐MOF‐253‐800) show excellent performance in a one‐pot sequential Knoevenagel condensation‐hydrogenation reaction

Facile Fabrication of Hierarchical MOF–Metal Nanoparticle Tandem Catalysts for the Synthesis of Bioactive Molecules

Multifunctional metal–organic frameworks (MOFs) that possess permanent porosity are promising catalysts in organic transformation. Herein, we report the construction of a hierarchical MOF functionalized with basic aliphatic amine groups and polyvinylpyrrolidone-capped platinum nanoparticles (Pt NPs). The postsynthetic covalent modification of organic ligands increases basic site density in the MOF and simultaneously introduces mesopores to create a hierarchically porous structure. The multifunctional MOF is capable of catalyzing a sequential Knoevenagel condensation–hydrogenation–intramolecular cyclization reaction. The unique selective reduction of the nitro group to intermediate hydroxylamine by Pt NPs supported on MOF followed by intramolecular cyclization with a cyano group affords an excellent yield (up to 92%) to the uncommon quinoline N-oxides over quinolines. The hierarchical MOF and polyvinylpyrrolidone capping agent on Pt NPs synergistically facilitate the enrichment of substrates and thus lead to high activity in the reduction–intramolecular cyclization reaction. The bioactivity assay indicates that the synthesized quinoline N-oxides evidently inhibit the proliferation of lung cancer cells. Our findings demonstrate the feasibility of MOF-catalyzed direct synthesis of bioactive molecules from readily available compounds under mild conditions

Layered double hydroxide-derived nanomaterials for Suzuki−Miyaura cross-coupling reaction

Layered double hydroxide (LDH) is an inorganic solid with a brucite-like structure that is similar to hydrotalcite, which contains two main metal plates and an anion interlayer in between them. The main metal plates contain both trivalent and divalent metal cations that result in positively charged metal plates. The anion interlayer can compensate for the positive charge on metal plates and result in a natural LDH solid. Their high tunability of chemical composition ordered and uniform metal cation dispersion, and well-distributed layered architecture allows them to become a potential source of intermetallic compound’s (IMC) precursor. Layered double hydroxide can be converted to the mixed metal oxide film (layered double oxide, LDO) after high-temperature calcination, which has a large specific surface area and is more thermally stable. The layered double oxide materials that have specific stoichiometry and homogeneity of composition have been applied directly as catalysts or as catalyst supports. Intermetallic compounds are well-known for their particular electronic structures and geometries, which show high activity and selectivity towards various types of heterogeneous catalysis reactions as compared with alloys. Non-precious metal IMCs derived from LDH precursors have attracted research attention because they are low-cost, readily available, and could exhibit meritorious catalytic performance. The synthesizing process of LDH precursors does not involve any toxic organic reagent. Therefore, it is more environmentally friendly than colloidal chemical synthesis methods. We use LDH as a precursor for two research projects. For the first project, we synthesized the single-atomic Pd which immobilized on the Fe3GaMg7 layered double oxide. Single-atom catalysts have attracted significant research interest because of their extremely excellent catalytic efficiency and superior selectivity. The Pd/ Fe3GaMg7 catalyst demonstrates promising activity for the Suzuki coupling reaction of bromobenzene and phenylboronic acid at room temperature （Turn over Frequency = 2312h-1). This catalyst has been recycled five times without a significant loss of catalytic activity. It also indicates high tolerance towards a broad scope of the functional groups on the aryl halogen compounds. For the second project, we synthesized Ni-Ga, Co-In, and Fe-Ga LDH precursors with various metal compositions and generated the corresponding IMCs via the high-temperature reduction in the gas phase.</p

Layered double hydroxide-derived nanomaterials for Suzuki−Miyaura cross-coupling reaction

Layered double hydroxide (LDH) is an inorganic solid with a brucite-like structure that is similar to hydrotalcite, which contains two main metal plates and an anion interlayer in between them. The main metal plates contain both trivalent and divalent metal cations that result in positively charged metal plates. The anion interlayer can compensate for the positive charge on metal plates and result in a natural LDH solid. Their high tunability of chemical composition ordered and uniform metal cation dispersion, and well-distributed layered architecture allows them to become a potential source of intermetallic compound’s (IMC) precursor. Layered double hydroxide can be converted to the mixed metal oxide film (layered double oxide, LDO) after high-temperature calcination, which has a large specific surface area and is more thermally stable. The layered double oxide materials that have specific stoichiometry and homogeneity of composition have been applied directly as catalysts or as catalyst supports. Intermetallic compounds are well-known for their particular electronic structures and geometries, which show high activity and selectivity towards various types of heterogeneous catalysis reactions as compared with alloys. Non-precious metal IMCs derived from LDH precursors have attracted research attention because they are low-cost, readily available, and could exhibit meritorious catalytic performance. The synthesizing process of LDH precursors does not involve any toxic organic reagent. Therefore, it is more environmentally friendly than colloidal chemical synthesis methods. We use LDH as a precursor for two research projects. For the first project, we synthesized the single-atomic Pd which immobilized on the Fe3GaMg7 layered double oxide. Single-atom catalysts have attracted significant research interest because of their extremely excellent catalytic efficiency and superior selectivity. The Pd/ Fe3GaMg7 catalyst demonstrates promising activity for the Suzuki coupling reaction of bromobenzene and phenylboronic acid at room temperature （Turn over Frequency = 2312h-1). This catalyst has been recycled five times without a significant loss of catalytic activity. It also indicates high tolerance towards a broad scope of the functional groups on the aryl halogen compounds. For the second project, we synthesized Ni-Ga, Co-In, and Fe-Ga LDH precursors with various metal compositions and generated the corresponding IMCs via the high-temperature reduction in the gas phase

t1-Noise Eliminated Dipolar Heteronuclear Multiple-Quantum Coherence Solid-State NMR Spectroscopy

Heteronuclear correlation (HETCOR) spectroscopy is one of the key tools in the arsenal of the solid-state NMR spectroscopist to probe spatial proximity between two different nuclei and enhance spectral resolution. Dipolar heteronuclear multiple-quantum coherence (D-HMQC) is a powerful technique that can be potentially utilized to obtain 1H detected 2D HETCOR solid-state NMR spectra of any NMR active nucleus. A long-standing problem in 1H detected D-HMQC solid-state NMR experiments is the presence of t1-noise which reduces sensitivity and impedes spectral interpretation. In this contribution, we describe novel pulse sequences, termed t1-noise eliminated (TONE) D-HMQC, that suppress t1-noise and can provide higher sensitivity and resolution than conventional D-HMQC. Monte-Carlo and numerical simulations confirm that t1-noise in conventional D-HMQC primarily occurs because random MAS frequency fluctuations cause variations in the NMR signal amplitude from scan to scan, leading to imperfect cancellation of uncorrelated signals by phase cycling. The TONE D-HMQC sequence uses 1H p-pulses to refocus the evolution of 1H CSA across each recoupling block, improving the stability of the pulse sequence to random MAS frequency fluctuations. The 1H refocusing pulses also restore the orthogonality of in-phase and anti-phase magnetization for all crystallite orientations, enabling the use of 90° flip-back or LG spin-lock trim pulses to reduce the intensity of uncorrelated signals. We demonstrate the application of these methods to acquire detected 2D 1H-35Cl and 1H-13C HETCOR spectra of histidine•HCl•H2O with reduced t1-noise. To show generality, we also apply these methods to obtain 2D 1H-17O spectra of 20%-17O fmoc-alanine and for the first time at natural abundance, 2D 1H-25Mg HETCOR spectra of magnesium hydroxide. The TONE D-HMQC sequences are also used to probe 1H-25Mg and 1H-27Al proximities in Mg-Al layered double hydroxides and confirm the even mixing of Mg and Al in these materials.</p

t1-Noise Eliminated Dipolar Heteronuclear Multiple-Quantum Coherence Solid-State NMR Spectroscopy

Heteronuclear correlation (HETCOR) spectroscopy is one of the key tools in the arsenal of the solid-state NMR spectroscopist to probe spatial proximity between two different nuclei and enhance spectral resolution. Dipolar heteronuclear multiple-quantum coherence (D-HMQC) is a powerful technique that can be potentially utilized to obtain 1H detected 2D HETCOR solid-state NMR spectra of any NMR active nucleus. A long-standing problem in 1H detected D-HMQC solid-state NMR experiments is the presence of t1-noise which reduces sensitivity and impedes spectral interpretation. In this contribution, we describe novel pulse sequences, termed t1-noise eliminated (TONE) D-HMQC, that suppress t1-noise and can provide higher sensitivity and resolution than conventional D-HMQC. Monte-Carlo and numerical simulations confirm that t1-noise in conventional D-HMQC primarily occurs because random MAS frequency fluctuations cause variations in the NMR signal amplitude from scan to scan, leading to imperfect cancellation of uncorrelated signals by phase cycling. The TONE D-HMQC sequence uses 1H p-pulses to refocus the evolution of 1H CSA across each recoupling block, improving the stability of the pulse sequence to random MAS frequency fluctuations. The 1H refocusing pulses also restore the orthogonality of in-phase and anti-phase magnetization for all crystallite orientations, enabling the use of 90° flip-back or LG spin-lock trim pulses to reduce the intensity of uncorrelated signals. We demonstrate the application of these methods to acquire detected 2D 1H-35Cl and 1H-13C HETCOR spectra of histidine•HCl•H2O with reduced t1-noise. To show generality, we also apply these methods to obtain 2D 1H-17O spectra of 20%-17O fmoc-alanine and for the first time at natural abundance, 2D 1H-25Mg HETCOR spectra of magnesium hydroxide. The TONE D-HMQC sequences are also used to probe 1H-25Mg and 1H-27Al proximities in Mg-Al layered double hydroxides and confirm the even mixing of Mg and Al in these materials.</p

t1-Noise Eliminated Dipolar Heteronuclear Multiple-Quantum Coherence Solid-State NMR Spectroscopy

Heteronuclear correlation (HETCOR) spectroscopy is one of the key tools in the arsenal of the solid-state NMR spectroscopist to probe spatial proximity between two different nuclei and enhance spectral resolution. Dipolar heteronuclear multiple-quantum coherence (D-HMQC) is a powerful technique that can be potentially utilized to obtain 1H detected 2D HETCOR solid-state NMR spectra of any NMR active nucleus. A long-standing problem in 1H detected D-HMQC solid-state NMR experiments is the presence of t1-noise which reduces sensitivity and impedes spectral interpretation. In this contribution, we describe novel pulse sequences, termed t1-noise eliminated (TONE) D-HMQC, that suppress t1-noise and can provide higher sensitivity and resolution than conventional D-HMQC. Monte-Carlo and numerical simulations confirm that t1-noise in conventional D-HMQC primarily occurs because random MAS frequency fluctuations cause variations in the NMR signal amplitude from scan to scan, leading to imperfect cancellation of uncorrelated signals by phase cycling. The TONE D-HMQC sequence uses 1H p-pulses to refocus the evolution of 1H CSA across each recoupling block, improving the stability of the pulse sequence to random MAS frequency fluctuations. The 1H refocusing pulses also restore the orthogonality of in-phase and anti-phase magnetization for all crystallite orientations, enabling the use of 90° flip-back or LG spin-lock trim pulses to reduce the intensity of uncorrelated signals. We demonstrate the application of these methods to acquire detected 2D 1H-35Cl and 1H-13C HETCOR spectra of histidine•HCl•H2O with reduced t1-noise. To show generality, we also apply these methods to obtain 2D 1H-17O spectra of 20%-17O fmoc-alanine and for the first time at natural abundance, 2D 1H-25Mg HETCOR spectra of magnesium hydroxide. The TONE D-HMQC sequences are also used to probe 1H-25Mg and 1H-27Al proximities in Mg-Al layered double hydroxides and confirm the even mixing of Mg and Al in these materials.This article is published as Venkatesh, Amrit, Xuechen Luan, Ivan Hung, Frédéric A. Perras, Wenyu Huang, and Aaron J. Rossini. "t1-Noise Eliminated Dipolar Heteronuclear Multiple-Quantum Coherence Solid-State NMR Spectroscopy." Physical Chemistry Chemical Physics 22 (2020): 20815-20828. DOI: 10.1039/D0CP03511D. Posted with permission.</p

Investigation of the Flow Intensity in an Inverted Seven-Point Well Pattern and Its Influence on the EOR Efficiency of S/P Flooding

Polymer and surfactant (S/P) binary flooding is a widely used chemical flooding technology for enhanced oil recovery (EOR). However, it is mostly used in the five-spot well pattern, and there is little research on the effect of well patterns on its flow law and EOR efficiency in the reservoir. In this paper, the flow intensity of S/P flooding in an inverted seven-spot well unit and its EOR efficiency are investigated. Based on the theoretical derivation and simulation, the flow distribution at different positions in the inverted seven-spot well pattern unit was calculated. The oil displacement efficiency was evaluated by simulating different flow intensities with various flow velocity. The microscopic residual oil of the core at the end of displacement was scanned and recognized. The 2D model was used to simulate the well pattern to clarify the EOR of S/P flooding. The results show that the swept area in the well unit can be divided into the strong swept region (>0.2 MPa); medium swept region (0.1–0.2 MPa); weak swept region (0.03–0.1 MPa); and invalid swept region (<0.03 MPa), according to the pressure gradient distribution. Compared to the five-spot well pattern, the inverted seven-spot well pattern featured a weak swept intensity, but a large swept area and lower water cut rise rate. Increasing the flow intensity can improve oil displacement efficiency, and disperse and displace continuous cluster remaining oil. The 2D model experiments show that the incremental oil recoveries by SP flooding after water flooding in the five-spot well pattern and inverted seven-spot well pattern are 25.73% and 17.05%, respectively. However, the ultimate oil recoveries of two well patterns are similar by considering the previous water flooding

Facile Fabrication of Hierarchical MOF–Metal Nanoparticle Tandem Catalysts for the Synthesis of Bioactive Molecules

Multifunctional metal–organic frameworks (MOFs) that possess permanent porosity are promising catalysts in organic transformation. Herein, we report the construction of a hierarchical MOF functionalized with basic aliphatic amine groups and polyvinylpyrrolidone-capped platinum nanoparticles (Pt NPs). The postsynthetic covalent modification of organic ligands increases basic site density in the MOF and simultaneously introduces mesopores to create a hierarchically porous structure. The multifunctional MOF is capable of catalyzing a sequential Knoevenagel condensation–hydrogenation–intramolecular cyclization reaction. The unique selective reduction of the nitro group to intermediate hydroxylamine by Pt NPs supported on MOF followed by intramolecular cyclization with a cyano group affords an excellent yield (up to 92%) to the uncommon quinoline N-oxides over quinolines. The hierarchical MOF and polyvinylpyrrolidone capping agent on Pt NPs synergistically facilitate the enrichment of substrates and thus lead to high activity in the reduction–intramolecular cyclization reaction. The bioactivity assay indicates that the synthesized quinoline N-oxides evidently inhibit the proliferation of lung cancer cells. Our findings demonstrate the feasibility of MOF-catalyzed direct synthesis of bioactive molecules from readily available compounds under mild conditions.</p