Boron Adsorption Kinetics of Microcrystalline Cellulose and Polymer Resin.

Tailoring boron-polysaccharide interactions is an important strategy for developing functional soft materials such as hydrogels, fire retardants, and sorbents for environmental remediation, for example, using lignocellulosic biomass. For such applications to be realized, it is paramount to understand the adsorption kinetics of borate anions on cellulose and their local structures. Here, the kinetic aspects of boron adsorption by microcrystalline cellulose, lignin, and polymeric resin are investigated and compared. Borate anions interact with the vicinal diols in the glucopyranoside moieties of cellulose to yield chemisorbed boron chelate complexes. In contrast to cellulose, technical lignin contains fewer cis-vicinal diols, and it does not have a tendency to form such chelate complexes upon treatment with the aqueous boric acid solution. The formation kinetics and stability of these chelate complexes strongly depend on nanoscale structures, as well as reaction conditions such as pH and concentration of the sorbate and sorbent. Specifically, insights into the distinct boron adsorption sites were obtained by solid-state one-dimensional (1D) 11B magic-angle spinning NMR and the local structures and intermolecular interactions in the vicinities of boron chelate complexes are elucidated by analyzing two-dimensional (2D) 1H-13C and 11B-1H heteronuclear correlation NMR spectra. The total boron adsorption capacity of cellulose is estimated to be in the 1.3-3.0 mg range per gram of sorbent, which is lower than the boron adsorption capacity of a polystyrene-based resin, ∼17.2 mg of boron per gram of Amberlite IRA 743. Our study demonstrates that the local backbone and side chain flexibility as well as the structures of polyol groups play a significant role in determining the kinetic and thermodynamic stability of chelate complexes, yielding to different boron adsorption capabilities of lignocellulosic polymers.

High-Field NMR, Reactivity, and DFT Modeling Reveal the γ-Al2 O3 Surface Hydroxyl Network.

Aluminas are strategic materials used in many major industrial processes, either as catalyst supports or as catalysts in their own right. The transition alumina γ-Al2 O3 is a privileged support, whose reactivity can be tuned by thermal activation. This study provides a qualitative and quantitative assessment of the hydroxyl groups present on the surface of γ-Al2 O3 at three different dehydroxylation temperatures. The principal [AlOH] configurations are identified and described in unprecedented detail at the molecular level. The structures were established by combining information from high-field 1 H and 27 Al solid-state NMR, IR spectroscopy and DFT calculations, as well as selective reactivity studies. Finally, the relationship between the hydroxyl structures and the molecular-level structures of the active sites in catalytic alkane metathesis is discussed.

Rationalization of solid-state NMR multi-pulse decoupling strategies: Coupling of spin I = ½ and half-integer quadrupolar nuclei.

In this paper we undertake a study of the decoupling efficiency of the Multiple-Pulse (MP) scheme, and a rationalization of its parameterization and of the choice of instrumental set up. This decoupling scheme is known to remove the broadening of spin-1/2 spectra I, produced by the heteronuclear scalar interaction with a half-integer quadrupolar nucleus S, without reintroducing heteronuclear dipolar interaction. The resulting resolution enhancement depends on the set-up of the length of the series of pulses and delays of the MP, and some intrinsic material and instrumental parameters. Firstly through a numerical approach, this study investigates the influence of the main intrinsic material parameters (heteronuclear dipolar and J coupling, quadrupolar interaction, spin nature) and instrumental parameters (spinning rate, pulse field strength) on efficiency and resolution enhancement of the scalar decoupling scheme. A guideline is then proposed to obtain quickly and easily the best resolution enhancement via the rationalization of the instrumental and parameter set up. It is then illustrated and tested through experimental data, probing the efficiency of MP-decoupling set up using this guideline. Various spin systems were tested (31P-51V in VOPO4, 31P-93Nb in NbOPO4, 119Sn-17O in Y2Sn2O7), combined with simulations results.

Natural abundance 17O MAS NMR and DFT simulations: New insights into the atomic structure of designed micas.

Combining 17O Magic-Angle Spinning (MAS) NMR at natural abundance with DFT calculations is a promising methodology to shed light on the structure and disorder in tetrahedral sheets of designed micas with enhanced properties. Among brittle micas, synthetic mica is an important alternative to natural ones with a swelling sheet-like structure that results in many applications, by exploiting unique characteristics. Lowenstein's rule is one of the main chemical factor that determines the atomic structure of aluminosilicates and furthermore their properties. In the present article, 17O MAS NMR spectroscopy is used to validate (or not) the agreement of the Lowenstein's rule with the distribution of Si and Al sites in the tetrahedral sheets of synthetic micas. 17O MAS spectra of synthetic high-charged micas exhibit two regions of signals that revealed two distinguishable oxygen environments, namely Si-O-X (with X = Si, Altet, Mg) and Altet-O-Y (Y=Mg or Altet). DFT calculations were also conducted to obtain the 17O chemical shift and other NMR features like the quadrupolar coupling constant, CQ, for all of the oxygen environments encountered in the two model structures, one respecting the Lowenstein's rule and the other involving Altet-O-Altet and Si-O-Si environments. Our DFT calculations support the 17O assignment, by confirming that Altet-O-3Mg and Altet-O-Altet oxygen environments show chemical shifts under 30 ppm and more important, with quadrupolar coupling constants of about 1 MHz, in line with the spectral observation. By quantifying the 17O MAS NMR spectra at natural abundance, we demonstrate that one of the synthetic mica compositions does not meet the Lowenstein's rule.

Grafting of a new bis-silylamido aluminum species on silica: insight from solid-state NMR into interactions with the surface.

The new bisamido aluminum species [AlCl{N(SiMe3)2}2(THF)] (1) was prepared and fully characterized by 27Al and 35Cl solid-state NMR, along with X-ray diffraction studies. 1 was grafted on silica partially dehydroxylated at 700 °C, affording silica-supported Al species. The resulting material (2) was characterized by IR, elemental analysis and 1H, 13C and 27Al solid-state MAS NMR. The 1D and 2D 27Al MAS NMR studies showed the occurrence of two types of species, where the Al center adopts a tetracoordinated coordination sphere, with as an additional coordinated Lewis base, either a THF ligand or a silica-surface siloxane moiety. DFT calculations allowed understanding the grafting mechanism and the spectroscopic properties of the material.

Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on a Mostly Amorphous Alumina: Role of the Local Grafting Environment.

Inorganic oxides play a crucial role in the activation of atomically dispersed metal oxides for catalytic olefin transformations, but the inefficient activation processes remain poorly understood. Activation of methyltrioxorhenium (MTO) for propene metathesis via its deposition on the surface of γ-Al2O3 typically results in <5% active sites, and these sites deactivate rapidly. Simple substitution of the support by a less crystalline (largely amorphous) alumina ( a-Al2O3) results in ca. 4× more activity and at least 10× more productivity. On both types of alumina, metathesis is initiated only at specific sites, whose availability limits the catalytic activity. While the two aluminas have similar total numbers of Lewis acid sites, the less crystalline support activates twice as many grafted MTO sites. Interestingly, a-Al2O3 has nearly double the number of strong Lewis acid sites. However, the number of active sites is ca. 10× lower than the total number of strong Lewis acid sites, and metathesis proceeds even when most are occupied by pyridine. DQSQ and D-HMQC 1H and 27Al solid-state NMR reveal that many Lewis acid sites are co-located with surface hydroxyl groups, which prevent activation and/or cause rapid deactivation. Undercoordinated Al sites on dominant (110) facets, which retain hydroxyl groups under catalyst preparation conditions, are therefore unlikely to lead to stable active sites. In contrast, the minor (100) facets of γ-Al2O3, which are completely dehydroxylated, contain strongly Lewis-acidic five-coordinate Al sites that are necessarily remote from surface hydroxyl groups. Such sites, which are relatively more abundant on less well-crystallized aluminas, are inferred to be responsible for generating stable metathesis sites.

31P MAS NMR spectroscopy with 93Nb decoupling and DFT calculations: A structural characterization of defects in a niobium-phosphate phase.

The present study has investigated the structure of four niobium phosphates compounds using 31P MAS NMR spectroscopy. Niobium 93Nb decoupling, applied during 31P NMR acquisition led to a resolution enhancement by a factor of 2-3, which allowed distinguish phosphorous sites separated by 1 ppm or less. The assignment of 31P spectra has been completed by use of first-principles calculations derived from the original XRD structures. One of the compounds, Na3.04Nb7P4O29, contains a sodium vacancy, clearly revealed in the 31P MAS spectrum obtained with 93Nb decoupling. A series of structural models were proposed to account for the sodium distribution in the structure of Na3.04Nb7P4O29 and the assignment was possible using DFT calculation and a statistical analysis of the NMR parameters.

Well-Defined Molybdenum Oxo Alkyl Complex Supported on Silica by Surface Organometallic Chemistry: A Highly Active Olefin Metathesis Precatalyst.

The well-defined silica-supported molybdenum oxo alkyl species (≡SiO-)MoO(CH2tBu)3 was selectively prepared by grafting of MoO(CH2tBu)3Cl onto partially dehydroxylated silica (silica700) using the surface organometallic chemistry approach. This surface species was fully characterized by elemental analysis and DRIFT, solid-state NMR, and EXAFS spectroscopy. This new material is related to the active species of industrial supported MoO3/SiO2 olefin metathesis catalysts. It displays very high activity in propene self-metathesis at mild (turnover number = 90 000 after 25 h). Remarkably, its catalytic performance outpaces those of the parent imido derivative and its tungsten oxo analogue.

Solvent-Free Ring-Opening Metathesis Polymerization of Norbornene over Silica-Supported Tungsten-Oxo Perhydrocarbyl Catalysts.

Ring opening metathesis polymerization (ROMP) of bicyclo[2.2.1]hept-2-ene (norbornene) is carried out over silica-supported catalysts based on tungsten complexes bearing an oxo ligand (1: [(SiO)W(O)(CH2 SiMe3 )3 , 2: [(SiO)W(O)(CHCMe2 Ph)(dAdPO)], dAdPO  2,6 diadamantyl-4-methylphenoxide, 3: [(SiO)2 W(O)(CH2 SiMe3 )2 ]). The evaluation of the catalytic activities of the aforementioned materials in ROMP indicates that at low reaction time (0.5 min), the highest polymer yield is obtained with catalyst 2. However, for longer reaction time (>2 min), complex 3, a model of the industrial catalyst, exhibits a better monomer conversion. The polymers obtained are characterized. Moreover, these catalysts are shown to be rather preferentially selective to give the cis polynorbornene (>65%), characterized by high melting points (≈300 °C). The experimental values of the average molecular weight (Mn ) of polynorbornenes are found to be close to the theoretical ones for the polymers prepared using catalyst 2 and higher for those originated from catalyst 3.

Ligand Exchange-Mediated Activation and Stabilization of a Re-Based Olefin Metathesis Catalyst by Chlorinated Alumina.

Extensive chlorination of γ-Al2O3 results in the formation of highly Lewis acidic surface domains depleted in surface hydroxyl groups. Adsorption of methyltrioxorhenium (MTO) onto these chlorinated domains serves to activate it as a low temperature, heterogeneous olefin metathesis catalyst and confers both high activity and high stability. Characterization of the catalyst reveals that the immobilized MTO undergoes partial ligand exchange with the surface, whereby some Re sites acquire a chloride ligand from the modified alumina while donating an oxo ligand to the support. More specifically, Re LIII-edge EXAFS and DFT calculations support facile ligand exchange between MTO and Cl-Al2O3 to generate [CH3ReO2Cl+] fragments that interact with a bridging oxygen of the support via a Lewis acid-base interaction. According to IR and solid-state NMR, the methyl group remains intact, and does not evolve spontaneously to a stable methylene tautomer. Nevertheless, the chloride-promoted metathesis catalyst is far more active and productive than MTO/γ-Al2O3, easily achieving a TON of 100 000 for propene metathesis in a flow reactor at 10 °C (compared to TON < 5000 for the nonchlorinated catalyst). Increased activity is a consequence of both a larger fraction of active sites and a higher intrinsic activity for the new sites. Increased stability is tentatively attributed to a stronger interaction between MTO and chlorinated surface regions, as well as extensive depletion of the Brønsted acidic surface hydroxyl population. The reformulated catalyst represents a major advance for Re-based metathesis catalysts, whose widespread use has thus far been severely hampered by their instability.

The design of a bipodal bis(pentafluorophenoxy)aluminate supported on silica as an activator for ethylene polymerization using surface organometallic chemistry.

A new class of well-defined activating supports for olefin polymerization was obtained via the surface organometallic chemistry approach. High activities in slurry polymerization of ethylene along with industrial-grade physical properties of the resulting polyethylene were obtained when these activators were combined with metallocene complexes in the presence of triisobutylaluminium.

Selective Secondary Face Modification of Cyclodextrins by Mechanosynthesis.

α-, ß-, and γ-cyclodextrins (CDs) were modified on their secondary face by mechanosynthesis at room temperature using a laboratory-scale ball-mill. Mono-2-tosylated α-, ß-, and γ-CDs were obtained in good yield from mixtures of native α-, ß-, and γ-CDs, respectively, N-tosylimidazole, and an inorganic base, with each of them being in the solid state. The yields appeared to be dependent upon the nature of the base and the reaction time. A kinetic monitoring by (1)H NMR spectroscopy demonstrated that the highest yields in mono-2-tosyl-CDs were measured using KOH as a base in very short reaction times (up to 65% in 80 s). Mono-(2,3-manno-epoxide) α-, ß-, and γ-CDs were subsequently synthesized by ball-milling a mixture of monotosylated α-, ß-, and γ-CDs, respectively, and KOH. The characterization of the modified CDs was carried out by X-ray diffraction, mass spectrometry, solid-state NMR, and diffuse reflectance UV-vis (DR UV-vis) spectroscopies. Clues to the supramolecular arrangement of the molecules in the solid state provide information on the reaction mechanism.

An investigation of chlorine ligands in transition-metal complexes via ³5Cl solid-state NMR and density functional theory calculations.

Chlorine ligands in a variety of diamagnetic transition-metal (TM) complexes in common structural motifs were studied using (35)Cl solid-state NMR (SSNMR), and insight into the origin of the observed (35)Cl NMR parameters was gained through first-principles density functional theory (DFT) calculations. The WURST-CPMG pulse sequence and the variable-offset cumulative spectrum (VOCS) methods were used to acquire static (35)Cl SSNMR powder patterns at both standard (9.4 T) and ultrahigh (21.1 T) magnetic field strengths, with the latter affording higher signal-to-noise ratios (S/N) and reduced experimental times (i.e., <1 h). Analytical simulations were performed to extract the (35)Cl electric field gradient (EFG) tensor and chemical shift (CS) tensor parameters. It was found that the chlorine ligands in various bonding environments (i.e., bridging, terminal-axial, and terminal-equatorial) have drastically different (35)Cl EFG tensor parameters, suggesting that (35)Cl SSNMR is ideal for characterizing chlorine ligands in TM complexes. A detailed localized molecular orbital (LMO) analysis was completed for NbCl5. It was found that the contributions of individual molecular orbitals must be considered to fully explain the observed EFG parameters, thereby negating simple arguments based on comparison of bond lengths and angles. Finally, we discuss the application of (35)Cl SSNMR for the structural characterization of WCl6 that has been grafted onto a silica support material. The resulting tungsten-chloride surface species is shown to be structurally distinct from the parent compound.

Heteronuclear NMR spectroscopy as a surface-selective technique: a unique look at the hydroxyl groups of γ-alumina.

The surface hydroxyl groups of γ-alumina dehydroxylated at 500 °C were studied by a combination of one- and two-dimensional homo- and heteronuclear (1)H and (27)Al NMR spectroscopy at high magnetic field. In particular, by harnessing (1)H-(27) Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the (1)H magic-angle spinning (MAS) NMR spectrum was demonstrated thanks to (1)H-(27) Al RESPDOR (resonance-echo saturation-pulse double-resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {(1)H}-(27) Al dipolar heteronuclear multiple quantum correlation (D-HMQC), which was used to establish a first coordination map. Then, in combination with (1)H-(1) H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO2 reactivity with alumina.

On the track to silica-supported tungsten oxo metathesis catalysts: input from 17O solid-state NMR.

The grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined precatalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. (17)O labeling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding (17)O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the (17)O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and chemical shift interactions.

A study of transition-metal organometallic complexes combining 35Cl solid-state NMR spectroscopy and 35Cl†NQR spectroscopy and first-principles DFT calculations.

A series of transition-metal organometallic complexes with commonly occurring metal-chlorine bonding motifs were characterized using (35)Cl solid-state NMR (SSNMR) spectroscopy, (35)Cl nuclear quadrupole resonance (NQR) spectroscopy, and first-principles density functional theory (DFT) calculations of NMR interaction tensors. Static (35)Cl ultra-wideline NMR spectra were acquired in a piecewise manner at standard (9.4 T) and high (21.1 T) magnetic field strengths using the WURST-QCPMG pulse sequence. The (35)Cl electric field gradient (EFG) and chemical shielding (CS) tensor parameters were readily extracted from analytical simulations of the spectra; in particular, the quadrupolar parameters are shown to be very sensitive to structural differences, and can easily differentiate between chlorine atoms in bridging and terminal bonding environments. (35)Cl NQR spectra were acquired for many of the complexes, which aided in resolving structurally similar, yet crystallographically distinct and magnetically inequivalent chlorine sites, and with the interpretation and assignment of (35)Cl SSNMR spectra. (35)Cl EFG tensors obtained from first-principles DFT calculations are consistently in good agreement with experiment, highlighting the importance of using a combined approach of theoretical and experimental methods for structural characterization. Finally, a preliminary example of a (35)Cl SSNMR spectrum of a transition-metal species (TiCl4) diluted and supported on non-porous silica is presented. The combination of (35)Cl SSNMR and (35)Cl NQR spectroscopy and DFT calculations is shown to be a promising and simple methodology for the characterization of all manner of chlorine-containing transition-metal complexes, in pure, impure bulk and supported forms.

Extended Czjzek model applied to NMR parameter distributions in sodium metaphosphate glass.

The extended Czjzek model (ECM) is applied to the distribution of NMR parameters of a simple glass model (sodium metaphosphate, NaPO3) obtained by molecular dynamics (MD) simulations. Accurate NMR tensors, electric field gradient (EFG) and chemical shift anisotropy (CSA) are calculated from density functional theory (DFT) within the well-established PAW/GIPAW framework. The theoretical results are compared to experimental high-resolution solid-state NMR data and are used to validate the considered structural model. The distributions of the calculated coupling constant C(Q) is proportional to |V(zz)| and the asymmetry parameter η(Q) that characterize the quadrupolar interaction are discussed in terms of structural considerations with the help of a simple point charge model. Finally, the ECM analysis is shown to be relevant for studying the distribution of CSA tensor parameters and gives new insight into the structural characterization of disordered systems by solid-state NMR.

95Mo solid-state nuclear magnetic resonance spectroscopy and quantum simulations: synergetic tools for the study of molybdenum cluster materials.

The ability of (95)Mo solid-state nuclear magnetic resonance (SSNMR) spectroscopy to probe the atomic and electronic structures of inorganic molybdenum cluster materials has been demonstrated for the first time. Six cluster compounds were studied: MoBr(2), Cs(2)Mo(6)Br(14), (Bu(4)N)(2)Mo(6)Br(14), each containing the octahedral Mo(6)Br(14)(2-) cluster unit, and MoS(2)Cl(3), Mo(3)S(7)Cl(4), and MoSCl that contain metallic dimers, trimers, and tetramers, respectively. To overcome inherent difficulties due to the low sensitivity of (95)Mo SSNMR, both high-magnetic-field spectrometers and the quadrupolar Carr-Purcell Meiboom-Gill sensitivity enhancement pulse sequence under magic-angle-spinning conditions, combined with a hyperbolic-secant pulse were used. Experimental measurements as well as characterization of the (95)Mo electric field gradient and chemical shift tensors have been performed with the help of quantum-chemical calculations under periodic boundary conditions using the projector augmented-wave and the gauge-including projector augmented-wave methods, respectively. A large (95)Mo chemical shift range is measured, ∼3150 ppm, and the isotropic chemical shift of the Mo atoms is clearly correlated to their formal oxidation degree in the various clusters. Furthermore, a direct relation is evidenced between the molybdenum quadrupolar coupling constant and the bond lengths with its surrounding ligands. Our results demonstrate the efficiency of the combined use of quantum-chemical calculations and (95)Mo SSNMR experiments to study inorganic molybdenum cluster compounds.

Fine hierarchy of the V-O bonds by advanced solid state NMR: novel Pb4(VO2)(PO4)3 structure as a textbook case.

We report here a complete structural characterization of a new lead Pb(4)(VO(2))(PO(4))(3) vanadophosphate compound by single crystal X-ray diffraction and (51)V and (31)P solid-state NMR spectroscopy. Although structural data are commonly used for the estimation of bond lengths and further delimitation of the true coordination number (e.g., octahedral: 6 versus 5 + 1 versus 4 + 2), we show here for the first time by solid-state NMR a more accurate appreciation of the V-O bonding scheme in this complex oxide which appears well adapted to the full series of vanado-phosphate materials. The direct characterization of V-O-P bridges through the J-mediated correlation (51)V{(31)P} heteronuclear multiple quantum coherence (J-HMQC) technique allows a contrasted hierarchy of the V-O electronic delocalization and indirectly supports the presence or not of the V-O bond. In the reported lead vanado-phosphate structure, the two vanadium polyhedra that have been assigned to octahedra from a bond length point of view have been finally reclassified as tetra- and penta-coordinated units on the basis of the solid-state NMR results. More generally, we believe that the improved characterization of interatomic bonds in various vanado-phosphate structures by solid-state NMR will contribute to a better understanding of the structure/property relationships in this important class of materials.

17O NMR gives unprecedented insights into the structure of supported catalysts and their interaction with the silica carrier.

Flame silica was surface-labeled with (17)O, through isotopic enrichment of both siloxanes and silanols. After heat treatment at 200 and 700 °C under vacuum, the resulting partially dehydroxylated silica materials were investigated by high-field solid-state (1)H and (17)O NMR. More specifically, MQ MAS and HMQC sequences were used to probe the (17)O local environment. In a further step, these (17)O-tagged supports were used for the preparation of supported catalysts by reaction with perhydrocarbyl transition metal derivatives (zirconium tetraalkyl, tantalum trisalkyl-alkylidene, and tungsten trisalkyl-alkylidyne complexes). Detailed (17)O 1D and 2D MQ and HMQC MAS NMR studies demonstrate that signals in the Si-OH, Si-O-Si, and Si-O-metal regions are highly sensitive to local structural modifications, thanks to (17)O wide chemical shift and quadrupolar constant ranges. Experimental results were supported by DFT calculations. From the selective surface labeling, unprecedented information on interactions between supported catalysts and their inorganic carrier has been extracted.