Interlayer spacing in pillared and grafted MCM-22 type silicas: density functional theory analysis versus experiment.

Pillaring of synthetic layered crystalline silicates and aluminosilicates provides a strategy to enhance their adsorption and separation performance, and can facilitate the understanding of such behavior in more complex natural clays. We perform the first-principles density functional theory calculations for the pillaring of the pure silica polymorph of an MCM-22 type molecular sieve. Starting with a precursor material MCM-22P with fully hydroxylated layers, a pillaring agent, (EtO)3SiR, can react with hydroxyl groups (-OH) on adjacent internal surfaces, 2(-OH) + (EtO)3SiR + H2O → (-O)2SiOHR + 3EtOH, to form a pillar bridging these surfaces, or with a single hydroxyl, -OH + (EtO)3SiR + 2H2O → (-O)Si(OH)2R + 3EtOH, grafting to one surface. For computational efficiency, we replace the experimental organic ligand, R, by a methyl group. We find that the interlayer spacing in MCM-22 is reduced by 2.66 Å relative to weakly bound layers in the precursor MCM-22P. Including (-O)2SiR bridges for 50% (100%) of the hydroxyl sites in MCM-22P increases the interlayer spacing relative to MCM-22 by 2.52 Å (2.46 Å). For comparison, we also analyze the system where all -OH groups in MCM-22P are replaced by non-bridging grafted (-O)Si(OH)2R which results in a smaller interlayer spacing expansion of 2.17 Å relative to MCM-22. Our results for the interlayer spacing in the pillared materials are compatible with experimental observations for a similar MCM-22 type material with low Al content (Si : Al = 51 : 1) of an expansion relative to MCM-22 of roughly 2.8 Å and 2.5 Å from our x-ray diffraction and scanning transmission electron microscopy analyses, respectively. The latter analysis reveals significant variation in individual layer spacings.

Determining the Three-Dimensional Structures of Silica-Supported Metal Complexes from the Ground Up.

The immobilization of molecularly precise metal complexes to substrates, such as silica, provides an attractive platform for the design of active sites in heterogeneous catalysts. Specific steric and electronic variations of the ligand environment enable the development of structure-activity relationships and the knowledge-driven design of catalysts. At present, however, the three-dimensional environment of the precatalyst, much less the active site, is generally not known for heterogeneous single-site catalysts. We explored the degree to which NMR-based surface-to-complex interatomic distances could be used to solve the three-dimensional structures of three silica-supported metal complexes. The structure solution revealed unexpected features related to the environment around the metal that would be difficult to discern otherwise. This approach appears to be highly robust and, due to its simplicity, is readily applied to most single-site catalysts with little extra effort.

Silica-Supported Organolanthanum Catalysts for C-O Bond Cleavage in Epoxides.

Single-site organolanthanum complexes supported on mesoporous silica nanoparticles, La{C(SiHMe2)3}n@MSNs, catalyze the ring-opening hydroboration reaction of aliphatic and styrenic epoxides with pinacolborane (HBpin). The surface-bound complexes, synthesized by reaction of the homoleptic tris(alkyl)lanthanum La{C(SiHMe2)3}3 and SBA-type MSN treated at 700 °C (MSN700), are mostly monopodal ≡SiO-La{C(SiHMe2)3}2 and contain an average of one bridging La↼H-Si per alkyl ligand. This structure was established through a combination of solid-state NMR (SSNMR) experiments, including J-resolved SiH coupling and quantitative 29Si measurements, diffuse reflectance IR, and elemental analysis. These rigorous analyses also established that grafting reactions in pentane provide a preponderance of ≡SiO-La{C(SiHMe2)3}2 sites and are superior to those in benzene and THF, and that grafting onto MSN treated at 550 °C (MSN550) results in a mixture of surface species. The single-site supported catalysts are more selective and in most cases more active than the homogeneous analogue, allow easy purification of products from the catalyst, are strongly resistant to leaching into solution phase, and may be recycled for reuse at least five times. After reaction of La{C(SiHMe2)3}n@MSN and HBpin, species including ≡SiO-La{C(SiHMe2)3}(H2Bpin) and ≡SiO-La{C(SiHMe2)3}{κ2-pinB-O(CMe2)2OBH3} are identified by detailed 1D and 2D 11B SSNMR experiments.

Control of interfacial pH in mesoporous silica nanoparticles via surface functionalization.

The pH at silica-water interfaces (pHint) was measured by grafting a dual emission fluorescent probe (SNARF) onto the surface of mesoporous silica nanoparticles (MSN). The values of pHint of SNARF-MSN suspended in water were different from the pH of the bulk solution (pHbulk). The addition of acid or base to aqueous suspensions of SNARF-MSN induced much larger changes in pHbulk than pHint, indicating that the interface has buffering capacity. Grafting additional organic functional groups onto the surface of SNARF-MSN controls the pHint of its buffering region. The responses of pHint to variations in pHbulk are consistent with the acid/base properties of the surface groups as determined by their pKa and are affected by electrostatic interactions between charged interfacial species as evidenced by the dependence of ζ-potential on pHbulk. Finally, as a proof of principle, we demonstrate that the hydrolysis rate of an acid-sensitive acetal can be controlled by adjusting pHint via suitable functionalization of the MSN surface. Our findings can lead to the development of nanoreactors that protect sensitive species from adverse conditions and tune their chemical reactivity.

Surface structure of linear nanopores in amorphous silica: Comparison of properties for different pore generation algorithms.

We compare the surface structure of linear nanopores in amorphous silica (a-SiO2) for different versions of "pore drilling" algorithms (where the pores are generated by the removal of atoms from the preformed bulk a-SiO2) and for "cylindrical resist" algorithms (where a-SiO2 is formed around a cylindrical exclusion region). After adding H to non-bridging O, the former often results in a moderate to high density of surface silanol groups, whereas the latter produces a low density. The silanol surface density for pore drilling can be lowered by a final dehydroxylation step, and that for the cylindrical resist approach can be increased by a final hydroxylation step. In this respect, the two classes of algorithms are complementary. We focus on the characterization of the chemical structure of the pore surface, decomposing the total silanol density into components corresponding to isolated and vicinal mono silanols and geminal silanols. The final dehyroxylation and hydroxylation steps can also be tuned to better align some of these populations with the target experimental values.

Probing O-H Bonding through Proton Detected 1H-17O Double Resonance Solid-State NMR Spectroscopy.

The ubiquity of oxygen in organic, inorganic, and biological systems has stimulated the application and development of 17O solid-state NMR spectroscopy as a probe of molecular structure and dynamics. Unfortunately, 17O solid-state NMR experiments are often hindered by a combination of broad NMR signals and low sensitivity. Here, it is demonstrated that fast MAS and proton detection with the D-RINEPT pulse sequence can be generally applied to enhance the sensitivity and resolution of 17O solid-state NMR experiments. Complete 2D 17O → 1H D-RINEPT correlation NMR spectra were typically obtained in less than 10 h from less than 10 mg of material, with low to moderate 17O enrichment (less than 20%). Two-dimensional 1H-17O correlation solid-state NMR spectra allow overlapping oxygen sites to be resolved on the basis of proton chemical shifts or by varying the mixing time used for 1H-17O magnetization transfer. In addition, J-resolved or separated local field (SLF) blocks can be incorporated into the D-RINEPT pulse sequence to allow the direct measurement of one-bond 1H-17O scalar coupling constants (1 JOH) or 1H-17O dipolar couplings ( DOH), respectively, the latter of which can be used to infer 1H-17O bond lengths. 1 JOH and DOH calculated from plane-wave density functional theory (DFT) show very good agreement with experimental values. Therefore, the 2D 1H-17O correlation experiments, 1H-17O scalar and dipolar couplings, and plane-wave DFT calculations provide a method to precisely determine proton positions relative to oxygen atoms. This capability opens new opportunities to probe interactions between oxygen and hydrogen in a variety of chemical systems.

Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles (2): a study by 1H triple-quantum fast-MAS solid-state NMR.

The distribution of organic functional groups attached to the surface of mesoporous silica nanoparticles (MSNs) via co-condensation was scrutinized using 1D and 2D 1H solid-state NMR, including the triple-quantum/single-quantum (TQ/SQ) homonuclear correlation technique. The excellent sensitivity of 1H NMR and high resolution provided by fast magic angle spinning (MAS) allowed us to study surfaces with very low concentrations of aminopropyl functional groups. The sequential process, in which the injection of tetraethyl orthosilicate (TEOS) into the aqueous mother liquor was followed by dropwise addition of the organosilane precursor, resulted in deployment of organic groups on the surface, which were highly clustered even in a sample with a very low loading of ∼0.1 mmol g-1. The underlying mechanism responsible for clustering could involve fast aggregation of the aminopropyltrimethoxysilane (APTMS) precursor within the liquid phase, and/or co-condensation of the silica-bound molecules. Understanding the deposition process and the resulting topology of surface functionalities with atomic-scale resolution, can help to develop novel approaches to the synthesis of complex inorganic-organic hybrid materials.

Pore diameter dependence of catalytic activity: p-nitrobenzaldehyde conversion to an aldol product in amine-functionalized mesoporous silica.

The reaction yield for conversion of p-nitrobenzaldehyde (PNB) to an aldol product in amine-functionalized mesoporous silica nanoparticles (MSN) exhibits a 20-fold enhancement for a modest increase in pore diameter, d. This enhanced catalytic activity is shown to reflect a strong increase in the "passing propensity," P, of reactant and product species inside the pores. We find that P ≈ 0, corresponding to single-file diffusion, applies for the smallest d which still significantly exceeds the linear dimensions of PNB and the aldol product. However, in this regime of narrow pores, these elongated species must align with each other and with the pore axis in order to pass. Thus, P reflects both translational and rotational diffusion. Langevin simulation accounting for these features is used to determine P versus d. The results are also augmented by analytic theory for small and large d where simulation is inefficient. The connection with the catalytic activity and yield is achieved by the incorporation of results for P into a multi-scale modeling framework. Specifically, we apply a spatially coarse-grained (CG) stochastic model for the overall catalytic reaction-diffusion process in MSN. Pores are treated as linear arrays of cells from the ends of which species adsorb and desorb, and between which species hop and exchange, with the exchange rate reflecting P. CG model predictions including yield are assessed by Kinetic Monte Carlo simulation.

Homoleptic Trivalent Tris(alkyl) Rare Earth Compounds.

Homoleptic tris(alkyl) rare earth complexes Ln{C(SiHMe2)3}3 (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) are synthesized in high yield from LnI3THFn and 3 equiv of KC(SiHMe2)3. X-ray diffraction studies reveal 1a-d are isostructural, pseudo-C3-symmetric molecules that contain two secondary Ln↼HSi interactions per alkyl ligand (six total). Spectroscopic assignments are supported by comparison with Ln{C(SiDMe2)3}3 and DFT calculations. The Ln↼HSi and terminal SiH exchange rapidly on the NMR time scale at room temperature, but the two motifs are resolved at low temperature. Variable-temperature NMR studies provide activation parameters for the exchange process in 1a (ΔH⧧ = 8.2(4) kcal·mol-1; ΔS⧧ = -1(2) cal·mol-1K-1) and 1a-d9 (ΔH⧧ = 7.7(3) kcal·mol-1; ΔS⧧ = -4(2) cal·mol-1K-1). Comparisons of lineshapes, rate constants (kH/kD), and slopes of ln(k/T) vs 1/T plots for 1a and 1a-d9 reveal that an inverse isotope effect dominates at low temperature. DFT calculations identify four low-energy intermediates containing five ß-Si-H⇀Ln and one γ-C-H⇀Ln. The calculations also suggest the pathway for Ln↼HSi/SiH exchange involves rotation of a single C(SiHMe2)3 ligand that is coordinated to the Ln center through the Ln-C bond and one secondary interaction. These robust organometallic compounds persist in solution and in the solid state up to 80 °C, providing potential for their use in a range of synthetic applications. For example, reactions of Ln{C(SiHMe2)3}3 and ancillary proligands, such as bis-1,1-(4,4-dimethyl-2-oxazolinyl)ethane (HMeC(OxMe2)2) give {MeC(OxMe2)2}Ln{C(SiHMe2)3}2, and reactions with disilazanes provide solvent-free lanthanoid tris(disilazides).

Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles: a study by conventional and DNP-enhanced 29Si solid-state NMR.

Solid-state NMR spectroscopy, both conventional and dynamic nuclear polarization (DNP)-enhanced, was employed to study the spatial distribution of organic functional groups attached to the surface of mesoporous silica nanoparticles via co-condensation and grafting. The most revealing information was provided by DNP-enhanced two-dimensional 29Si-29Si correlation measurements, which unambiguously showed that post-synthesis grafting leads to a more homogeneous dispersion of propyl and mercaptopropyl functionalities than co-condensation. During the anhydrous grafting process, the organosilane precursors do not self-condense and are unlikely to bond to the silica surface in close proximity (less than 4 Å) due to the limited availability of suitably arranged hydroxyl groups.

Improved strategies for DNP-enhanced 2D 1H-X heteronuclear correlation spectroscopy of surfaces.

We demonstrate that dynamic nuclear polarization (DNP)-enhanced 1H-X heteronuclear correlation (HETCOR) measurements of hydrogen-rich surface species are better accomplished by using proton-free solvents. This approach notably prevents HETCOR spectra from being obfuscated by the solvent-derived signals otherwise present in DNP measurements. Additionally, in the hydrogen-rich materials studied here, which included functionalized mesoporous silica nanoparticles and metal organic frameworks, the use of proton-free solvents afforded higher sensitivity gains than the commonly used solvents containing protons. We also explored the possibility of using a solvent-free sample formulation and the feasibility of indirect detection in DNP-enhanced HETCOR experiments.

Natural Abundance 17 O DNP†NMR Provides Precise O-H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts.

Heterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by 17 O DNP surface-enhanced NMR spectroscopy (SENS). We additionally show that the O-H bond length in these catalysts can be measured with sub-picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O-H bonds.

Mechanistic Insight into Nanoparticle Surface Adsorption by Solution NMR Spectroscopy in an Aqueous Gel.

Engineering nanoparticle (NP) functions at the molecular level requires a detailed understanding of the dynamic processes occurring at the NP surface. Herein we show that a combination of dark-state exchange saturation transfer (DEST) and relaxation dispersion (RD) NMR experiments on gel-stabilized NP samples enables the accurate determination of the kinetics and thermodynamics of adsorption. We used the former approach to describe the interaction of cholic acid (CA) and phenol (PhOH) with ceria NPs with a diameter of approximately 200 nm. Whereas CA formed weak interactions with the NPs, PhOH was tightly bound to the NP surface. Interestingly, we found that the adsorption of PhOH proceeds via an intermediate, weakly bound state in which the small molecule has residual degrees of rotational diffusion. We believe the use of aqueous gels for stabilizing NP samples will increase the applicability of solution NMR methods to the characterization of nanomaterials.

Polarity Control at Interfaces: Quantifying Pseudo-solvent Effects in Nano-confined Systems.

Surface functionalization controls local environments and induces solvent-like effects at liquid-solid interfaces. We explored structure-property relationships between organic groups bound to pore surfaces of mesoporous silica nanoparticles and Stokes shifts of the adsorbed solvatochromic dye Prodan. Correlating shifts of the dye on the surfaces with its shifts in solvents resulted in a local polarity scale for functionalized pores. The scale was validated by studying the effects of pore polarity on quenching of Nile Red fluorescence and on the vibronic band structure of pyrene. Measurements were done in aqueous suspensions of porous particles, proving that the dielectric properties in the pores are different from the bulk solvent. The precise control of pore polarity was used to enhance the catalytic activity of TEMPO in the aerobic oxidation of furfuryl alcohol in water. An inverse relationship was found between pore polarity and activity of TEMPO in the pores, demonstrating that controlling the local polarity around an active site allows modulating the activity of nanoconfined catalysts.

Effects of biradical deuteration on the performance of DNP: towards better performing polarizing agents.

We study the effects of the deuteration of biradical polarizing agents on the efficiency of dynamic nuclear polarization (DNP) via the cross-effect. To this end, we synthesized a series of bTbK and TOTAPol biradicals with systematically increased deuterium substitution. The deuteration increases the radicals' relaxation time, thus contributing to a higher saturation factor and larger DNP enhancement, and reduces the pool of protons within the so-called spin diffusion barrier. Notably, we report that full or partial deuteration leads to improved DNP enhancement factors in standard samples, but also slows down the build-up of hyperpolarization. Improvements in DNP enhancements factors of up to 70% and time savings of up to 38% are obtained upon full deuteration. It is foreseen that this approach may be applied to other DNP polarizing agents thus enabling further sensitivity improvements.

Langevin and fOkker-Planck analyses of inhibited molecular passing processes controlling transport and reactivity in nanoporous materials.

Inhibited passing of reactant and product molecules within the linear pores of nanoporous catalytic materials strongly reduces reactivity. The dependence of the passing propensity P on pore radius R is analyzed utilizing Langevin dynamics to account for solvent effects. We find that P ∼ (R-R(c))(σ), where passing is sterically blocked for R≤R(c), with σ below the transition state theory value. Deeper insight comes from analysis of the corresponding high-dimensional Fokker-Planck equation, which facilitates an effective small-P approximation, and dimensional reduction enabling utilization of conformal mapping ideas. We analyze passing for spherical molecules and also assess the effect of rotational degrees of freedom for elongated molecules.

Analysis of sensitivity enhancement by dynamic nuclear polarization in solid-state NMR: a case study of functionalized mesoporous materials.

We systematically studied the enhancement factor (per scan) and the sensitivity enhancement (per unit time) in (13)C and (29)Si cross-polarization magic angle spinning (CP-MAS) NMR boosted by dynamic nuclear polarization (DNP) of functionalized mesoporous silica nanoparticles (MSNs). Specifically, we separated contributions due to: (i) microwave irradiation, (ii) quenching by paramagnetic effects, (iii) the presence of frozen solvent, (iv) the temperature, as well as changes in (v) relaxation and (vi) cross-polarization behaviour. No line-broadening effects were observed for MSNs when lowering the temperature from 300 to 100 K. Notwithstanding a significant signal reduction due to quenching by TOTAPOL radicals, DNP-CP-MAS at 100 K provided global sensitivity enhancements of 23 and 45 for (13)C and (29)Si, respectively, relative to standard CP-MAS measurements at room temperature. The effects of DNP were also ascertained by comparing with state-of-the-art two-dimensional heteronuclear (1)H{(13)C} and (29)Si{(1)H} correlation spectra, using, respectively, indirect detection or Carr-Purcell-Meiboom-Gill (CPMG) refocusing to boost signal acquisition. This study highlights opportunities for further improvements through the development of high-field DNP, better polarizing agents, and improved capabilities for low-temperature MAS.

Hydrogen spillover and substrate-support hydrogen bonding mediate hydrogenation of phenol catalyzed by palladium on reducible metal oxides.

Substrate-support interactions play an important role in the catalytic hydrogenation of phenolic compounds by ceria-supported palladium (Pd/CeO2). Here, we combine surface contrast solution NMR methods and reaction kinetic assays to investigate the role of substrate-support interactions in phenol (PhOH) hydrogenation catalyzed by titania-supported palladium (Pd/TiO2). We show that PhOH adsorbs on the catalyst via a weak hydrogen-bonding interaction between the -OH group of the substrate and one oxygen atom on the support. Interestingly, we observe that the addition of 20 mM inorganic phosphate results in a ∼2-fold destabilization of the PhOH-support interaction and a corresponding ∼2-fold inhibition of the catalytic reaction, suggesting an active role of the PhOH-TiO2 hydrogen bond in catalysis. A comparison of the data measured here with the results previously reported for a Pd/CeO2 catalyst indicates that the efficiency of the Pd-supported catalysts is correlated to the amount of PhOH hydrogen bonded to the metal oxide support. Since CeO2 and TiO2 have similar ability to uptake activated hydrogen from a noble metal site, these data suggest that hydrogen spillover is the main mechanism by which Pd-activated hydrogens are shuttled to the PhOH adsorbed on the surface of the support. Consistent with this hypothesis, Pd supported on a non-reducible metal oxide (silica) displays negligible hydrogenation activity. Therefore, we conclude that basic and reducible metal oxides are active supports for the efficient hydrogenation of phenolic compounds due to their ability to hydrogen bond to the substrate and mediate the addition of the activated hydrogens to the adsorbed aromatic ring.

Capturing Rare-Earth Elements by Synthetic Aluminosilicate MCM-22: Mechanistic Understanding of Yb(III) Capture.

We studied the mechanism underlying the solid-phase adsorption of a heavy rare-earth element (HREE, Yb) from acidic solutions employing MCM-22 zeolite, serving as both a layered synthetic clay mimic and a new platform for the mechanistic study of HREE adsorption on aluminosilicate materials. Mechanistic studies revealed that the adsorption of Yb(III) at the surface adsorption site occurs primarily through the electrostatic interaction between the site and Yb(III) species. The dependence of Yb adsorption on the pH of the solution indicated the role of surface charge, and the content of framework Al suggested that the Brønsted acid sites (BAS) are involved in the adsorption of Yb(III) ions, which was further scrutinized by spectroscopic analysis and theoretical calculations. Our findings have illuminated the roles of surface sites in the solid-phase adsorption of HREEs from acidic solutions.

Ligand conformation dictates membrane and endosomal trafficking of arginine-glycine-aspartate (RGD)-functionalized mesoporous silica nanoparticles.

Recent breakthrough research on mesoporous silica nanoparticle (MSN) materials has illustrated their significant potential in biological applications due to their excellent drug delivery and endocytotic behavior. We set out to determine if MSN, covalently functionalized with conformation specific bioactive molecules (either linear or cyclic RGD ligands), behave towards mammalian cells in a similar manner as the free ligands. We discovered that RGD immobilized on the MSN surface did not influence the integrity of the porous matrix and improved the endocytosis efficiency of the MSN materials. Through competition experiments with free RGD ligands, we also discovered a conformation specific receptor-integrin association. The interaction between RGD immobilized on the MSN surface and integrins plays an important role in endosome trafficking, specifically dictating the kinetics of endosomal escape. Thus, covalent functionalization of biomolecules on MSN assists in the design of a system for controlling the interface with cancer cells.