This manuscript demonstrates the synthesis of selective Lewis-acid sites in a metal–organic framework (MOF) for glucose transformation to 5-hydroxymethylfurfural (HMF). These sites are synthesized via partial phosphate modification of zirconia-cluster nodes in MOF NU-1000, which titrates strong Lewis-acid sites that would lead to undesired side reactions. Our mechanistic study using isotope tracer analysis and kinetic isotope effect measurements reveals that an isomerization–dehydration mechanism mainly occurs on the MOF catalyst, where fructose is an intermediate. This mechanism suggests that dilute concentrations are favorable in order to suppress undesired intermolecular condensation of glucose/fructose/HMF and maximize HMF yield. We demonstrate both high yield and selectivity of HMF formation of 64% with the MOF catalyst, at an initial glucose concentration of 1 mM in water/2-propanol. In stark contrast, similar partial phosphate modification of a bulk zirconia yields a catalyst that exhibits poor HMF selectivity, while possessing nearly identical Brønsted acidity to the selective NU-1000-based catalyst.

The low-energy isomers of Irx(CO)y(NHC)z (x = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD(T) results compared with the other four functionals in the benchmark calculations. The NHC’s behave as stronger σ donors compared with CO’s and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the CO’s and the decrease in the LDEs of the NHC’s as more NHC’s are substituted for CO’s are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a μ3-CO resulting from the high electron density of the metal centers in these complexes, as the bridging CO’s and the μ3-CO’s can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (C3v derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters.

The stabilization of isolated grafted Fe3+ sites on siliceous supports is investigated by a comparative study of crystalline versus amorphous materials. Our synthetic approach treats crystalline delaminated zeolite DZ-1 and amorphous silica (SiO2) with an aqueous NaFeEDTA cation precursor complex, to result in grafting of isolated Fe3+ sites via covalent attachment to support hydroxyl groups. Thermogravimetric analysis and UV–visible spectroscopy demonstrate the complete detachment of chelating EDTA ligand upon Fe3+ grafting on both supports. Before calcination treatment, both Fe/DZ-1 and Fe/SiO2 have similar UV–visible spectral features, with absorption bands at 208–225 and 257 nm, characteristic of framework Fe3+ sites in zeolites. Calcination does not affect the UV–visible spectroscopic characteristics of Fe/DZ-1 but changes the spectrum of Fe/SiO2 to a single absorption band at 260 nm, indicating better thermal stability of Fe3+ sites in Fe/DZ-1 as compared to Fe/SiO2. This stability persists for Fe/DZ-1 even during alkane oxidation catalysis in the presence of hydrogen peroxide, which causes aggregation of Fe3+ into oxide oligomers for Fe/SiO2. 57Fe Mössbauer spectroscopy of calcined materials indicates a more uniform distribution of sites in Fe/DZ-1 relative to Fe/SiO2. We thus attribute the greater robustness and site uniformity of Fe/DZ-1 to the chelation of Fe3+ by the rigid crystalline silicate DZ-1 framework, engendered by the spatial preorganization of grafting hydroxyls groups within its uniform defect sites, which are templated by framework B3+ removal during delamination. Such preorganization enables cooperativity between neighboring hydroxyl groups. This contrasts with more randomly distributed hydroxyl groups on SiO2, which lack such preorganization, leading to decreased hydrothermal stability and an Fe3+ grafting density that is ∼7-fold lower for Fe/SiO2 relative to Fe/DZ-1. These observations reveal how the silicate surface onto which a cation is grafted can act as a relevant ligand, capable of controlling material synthesis and functionality akin to ligands in homogeneous metal complexes, and demonstrate the advantages of support crystallinity in having this ligand be hydrothermally stable and tunable via templating.

The synthesis of high surface-area colloidal assemblies of calixarene-phosphine-capped nanoporous gold is reported under reductive electrochemical conditions. These materials uniquely exhibit a remarkably thin wall thickness down to 10 nm, while possessing pore sizes on the order of up to hundreds of nanometers, which can be controlled via choice of organic ligand.

Organic–inorganic materials consisting of organophosphonic-acid-supported-on-silica materials C3/SiO2 and C4/SiO2 are described, where C3 is propane-1,2,3-triphosphonic acid and C4 is butane-1,2,3,4-tetraphosphonic acid. Solid-state structures of both of these phosphonic acids are analyzed using single-crystal X-ray diffraction, and these data reveal extensive intermolecular hydrogen bonding and no intramolecular hydrogen bonds. Thermogravimetric analysis/mass spectroscopy (TGA/MS) data show a lack of combustion for these materials in air at temperatures below 400 °C, and only release of water corresponding to reversible organophosphonic acid condensation below 150 °C. A comparative series of silica-supported materials were synthesized, consisting of organophosphonic acid CX8, which represents a calixarene macrocycle that is decorated with a high density of organophosphonic-acid substituents on both the lower and upper rim, as well as polyvinylphosphoric acid (PVPA). Material CX8/SiO2 possesses a significantly lower thermal stability and lower combustion temperature of 300 °C in air, whereas PVPA demonstrates comparable thermal stability as observed with C3 and C4. TGA coupled with base-probe titration was used to determine the Brønsted acid site density of all silica-supported phosphonic acids at various coverages and temperatures. Material C4/SiO2-37% (corresponding to 37% (by mass) loading and half-monolayer coverage on silica) exhibited the highest Brønsted acid-site density of all materials, corresponding to 0.84 mmol/g at 150 °C, and 0.62 mmol/g at 300 °C. All supported phosphonic acids treated with pyridine at room temperature were strong enough acids to protonate pyridine at room temperature as exhibited by a distinct pyridinium cation band in the infrared spectrum; however, in contrast to much stronger acid sites in silica-supported phosphoric acid materials, almost all adsorbed pyridine was lost by 150 °C. Use of a stronger base for acid-site titration consisting of diisopropylamine (DIPA) demonstrates acid sites in all materials up to 300 °C, at which temperature the acid site was too weak to adsorb DIPA. Thus, these oxidatively stable materials are deemed to be useful in applications requiring weak Brønsted acid sites, while exhibiting high-temperature oxidative stability up to 400 °C.

This manuscript develops a surface polymerization and cross-linking approach for the stabilization of single-site catalysts on solid surfaces, which is demonstrated here for grafted Ti(IV)-calixarene Lewis acids on silica. Our approach relies on cationic polymerization that is initiated by an adsorbed B(C6F5)3 and uses styrene as the monomer and diisopropenylbenzene as the cross-linking agent. The mildness of this polymerization method is demonstrated by its lack of blocking micropores and only slight consumption of mesopore internal surface area on the basis of N2 physisorption data at 77 K, both of which are in contrast to previously reported surface-polymerization approaches. Catalysis of samples before and after polymerization and cross-linking was investigated with a probe reaction consisting of the epoxidation of 1-octene with tert-butyl hydroperoxide as oxidant, which is known to be catalyzed by Lewis-acid sites, and a comparison of catalyst hydrolytic stability was performed. Added water in the latter was used as a a trigger to induce site aggregation, as a stress test to determine the effectiveness of site protection by our polymerization approach. Consistent with the N2 physisorption data, catalysis data demonstrate that surface polymerization does not block small-molecule reactant and product access to Lewis-acid sites on the surface, since the conversion remains essentially unchanged before and after surface polymerization and cross-linking. DR UV–vis, TGA, and catalysis data reveal that the grafted Ti(IV)-calixarene sites on silica maintain their catalytic activity even after being treated with corrosive protic stress-test solution. In sharp contrast, grafted sites without the polymer layer leach nearly all of their calixarene and Ti contents during similar stress testing, resulting in the near complete loss of catalytic activity. We hypothesize that the surface polymer acts as a nanoreactor gatekeeper, which prevents the large Ti(IV)-calixarene site from leaching and keeps surface complexes as single sites grafted on the silica surface, by blocking access for the migration of sites from the surface to bulk solution.Keywords: leaching; Lewis acid catalyst; nanoreactor gatekeeper; olefin epoxidation reaction; surface polymerization

Metal–organic framework (MOF) material NU-1000 adsorbs dimers cellobiose and lactose from aqueous solution, in amounts exceeding 1250 mg gNU-1000−1 while completely excluding the adsorption of the monomer glucose, even in a competitive mode with cellobiose. The MOF also discriminates between dimers consisting of α and β linkages, showing no adsorption of maltose. Electronic structure calculations demonstrate that key to this selective molecular recognition is the number of favorable CH–π interactions made by the sugar with pyrene units of the MOF.

The effect of calixarene ligand symmetry, as dictated by lower-rim substitution pattern, on the coordination to a Ti(IV) cation is assessed in solution and when grafted on SiO2, and its effect on epoxidation catalysis by Ti(IV)-calixarene grafted on SiO2 is investigated. C2v symmetric Ti-tert-butylcalix[4]arene complexes that are 1,3-alkyl disubstituted at the lower rim (di-R-Ti) are compared to previously reported grafted Cs symmetric complexes, which are singly substituted at the lower rim (mono-R-Ti). 13C MAS NMR spectra of complexes isotopically enriched at the lower-rim alkyl position indicate that di-R-Ti predominantly grafts onto silica as the conformation found in solution, exhibiting a deshielded alkyl resonance compared to the grafted mono-R-Ti complexes, which is consistent with stronger alkyl ether→Ti dative interactions that are hypothesized to result in higher electron density at the Ti center. Moreover, 13C MAS NMR spectroscopy detects an additional contribution from an “endo” conformer for grafted di-R-Ti sites, which is not observed in solution. Based on prior molecular modeling studies and on 13C MAS NMR spectroscopy chemical shifts, this “endo” conformer is proposed to have similar Ti–(alkyl ether) distances at the lower-rim and electron density at the Ti center relative to grafted mono-R-Ti complexes. Differences between grafted mono-R-Ti and di-R-Ti sites can be observed by ligand-to-metal charge transfer edge-energies, calculated from diffuse-reflectance UV–visible spectroscopy at 2.24 ± 0.02 and 2.16 ± 0.02 eV, respectively. However, rates of tert-butyl hydroperoxide consumption in the epoxidation of 1-octene are found to be largely unchanged when compared to those of the grafted mono-R-Ti complexes, with average rate constants of ~1.5 M−2 s−1 and initial TOF of ~4 ks−1 at 323 K. This suggests that an “endo” conformation of grafted di-R-Ti may prevail during catalysis. Despite this, grafted di-C1-Ti complexes can be more selective than mono-C1-Ti complexes (45 vs. 34 % at a 50 % conversion at 338 and 353 K), illustrating the importance of the Ti coordination environment on epoxidation catalysis.

This manuscript quantitatively investigates the effect of weak acid site surface density on adsorption and catalytic hydrolysis of long-chain β-glucans, with post-synthetically modified zeolite-templated carbon (ZTC) catalysts. Our approach requires ZTC-surface modification and overcomes previous limitations of pore collapse in accomplishing this, which has previously necessitated electrochemical methods. We demonstrate that mild ZTC treatment in hydrogen peroxide preserves the 1.1 nm micropores of ZTC, which were previously shown to be ideal for β-glucan adsorption, while synthesizing surface-modified ZTC catalysts that hydrolyze adsorbed β-glucans to glucose in up to 87% yield. Our results demonstrate a direct increase in catalytic hydrolysis activity and glucose yield upon increasing acid site density via surface functionalization. Upon investigating the mechanism of catalytic hydrolysis under buffered conditions, we rule out the synthesis of acid sites with stronger acidity as a result of possible greater anion delocalization as well as the possibility of a cooperative acid–base bifunctional mechanism. Our data instead argue for the importance of a high density of surface carboxylic acid functionality as promoting the likelihood of pairing a surface acid site with a glycosidic oxygen of an adsorbed glucan on a length scale that is commensurate with that required for general acid catalysis. From this perspective, our ZTC catalysts function much like zeolites - wherein both achieve high rates with weak-acid sites by coupling adsorption of reactant into a confined domain containing the acid site, and general-acid catalyzed reaction.Keywords: Adsorption; Catalytic hydrolysis; Cellulosic biomass; Micropores; Post-synthetic modification; Weak acid site catalysis; Zeolite-templated carbon;

Polyanion dispersants stabilize aqueous dispersions of hydrophilic (native) inorganic oxide particles, including pigments currently used in paints, which are used at an annual scale of 3 million metric tons. While obtaining stable aqueous dispersions of hydrophobically modified particles has been desired for the promise of improved film performance and water barrier properties, it has until now required either prohibitively complex polyanions, which represent a departure from conventional dispersants, or multistep syntheses based on hybrid-material constructs. Here, we demonstrate the aqueous dispersion of alkylsilane-capped inorganic oxide pigments with conventional polycarboxylate dispersants, such as carboxymethylcellulose (CMC) and polyacrylate, as well as a commercial anionic copolymer. Contact-angle measurements demonstrate that the hydrophobically modified pigments retain significant hydrophobic character even after adsorbing polyanion dispersants. CMC adsorption isotherms demonstrate 92% greater polyanion loading on trimethylsilyl modified hydrophobic particles relative to native oxide at pH 8. However, consistent with prior literature, hydrophobically modified silica particles adsorb polyanions very weakly under these conditions. These data suggest that Lewis acidic heteroatoms such as Al3+ sites on the pigment surface are necessary for polyanion adsorption. The adsorbed polyanions increase the dispersion stability and zeta potential of the particles. Based on particle sedimentation under centrifugal force, the hydrophobically modified pigments possess greater dispersion stability with polyanions than the corresponding native hydroxylated particles. The polyanions also assist in the aqueous wetting of the hydrophobic particles, facilitating the transition from a dry powder into an aqueous dispersion of primary particles using less agitation than the native hydroxylated pigment. The application of aqueous dispersions of hydrophobically modified oxide particles to waterborne coatings leads to films that display lower water uptake at high relative humidities and greater hydrophilic stain resistances. This improved film performance with hydrophobically modified pigments is the result of better association between latex polymer and pigment in the dry film.

This manuscript represents a comparative study of Lewis acid catalysis using heteroatom-substituted delaminated zeolites, which are synthesized using an approach that obviates the need for surfactants and sonication during exfoliation. The comparison involves heteroatom substitution into silanol nests of delaminated zeolites consisting of DZ-1 and deboronated UCB-4. Diffuse reflectance ultraviolet (DR-UV) spectroscopy demonstrates framework heteroatom sites, and the Lewis acidity of these sites is confirmed using infrared spectroscopy of adsorbed pyridine. The enhanced catalytic accessibility of these Lewis acid sites is confirmed when performing Baeyer–Villiger oxidation of substituted 2-adamantanones with hydrogen peroxide as the oxidant. Comparison of delaminated Sn-DZ-1 with three-dimensional Sn-Beta for this reaction shows that the delaminated zeolite is more active for bulkier ketone substrates. The role of the two-dimensional crystalline framework of the delaminated zeolite on catalysis is highlighted by comparing delaminated zeolites Sn-DZ-1 with Sn-UCB-4. The former exhibits a significantly higher activity for Baeyer–Villiger oxidation, yet when comparing Ti-DZ-1 with Ti-UCB-4, it is the latter that exhibits a significantly higher activity for olefin epoxidation with organic hydrogen peroxide, whereas both delaminated zeolites are more robust and selective in epoxidation catalysis compared with amorphous Ti/SiO2.Keywords: Baeyer−Villiger oxidation; delaminated zeolites; epoxidation; heteroatom substitution; Lewis acid catalysis; silanol nest

We demonstrate adsorption and depolymerization of long-chain β-glu strands derived from cellulose, within the microporous confines of a zeolite-templated carbon (ZTC) material. The ZTC adsorbs β-glu strands that have a radius of gyration several-fold larger than the ZTC micropore diameter and do so rapidly (less than 2 min) and in adsorbed β-glu coverages of up to 80% of the ZTC mass. Principles of supramolecular chemistry predict that such adsorption occurs inside of the ZTC based on its micropore size as host being nearly ideal for glucan guest. A comparative study of partially etched materials and nitrogen physisorption at −196 °C indeed demonstrates β-glu adsorption to occur within internal ZTC micropores rather than on the external surface. Such adsorption under micropore confinement is expected to place significant mechanical strain on the β-glu strand, and this strain can be in principle relieved by depolymerization via hydrolysis. This hypothesis motivated us to investigate depolymerization of adsorbed β-glu strands in ZTC, where the ZTC serves as a catalyst for adsorbed β-glu hydrolysis. After a 3 h treatment in water at 180 °C, adsorbed β-glu was converted to soluble glucose in 73% yield. This represents the highest glucose yield observed to date for a carbon catalyst without postsynthetic surface functionalization and speaks to the effectiveness of weak-acid sites for β-glu hydrolysis within a constrained micropore environment.Keywords: adsorption; catalytic hydrolysis; cellulosic biomass; confinement; weak-acid sites; zeolite-templated carbon

Tetrameric Ti(IV)–calix[4]arene complexes were synthesized and characterized as well as grafted on a hydroxylated SiO2 inorganic support as an example of patterned Lewis-acid sites on an inorganic oxide surface. These complexes consist of a novel calixarene organic ligand with varying lengths of tethers to a central aromatic core, and with the calixarene coordinating four titanium(IV)-cations via tetrahedral recognition on the lower rim. Catalysis of these materials was investigated with a probe reaction consisting of the epoxidation 1-octene with tert-butylhydroperoxide as the oxidant, and a comparison of their hydrolytic stability was performed. The oligomeric calix[4]arenes showed similar behavior in catalysis to the monomeric control, with the exception of the material with the shortest tether length, which shows a 1.3-fold higher activity that may be due to a modest cooperativity effect. In hydrolytic leaching tests, the oligomeric complexes showed higher stability compared to the monomeric complex, and this stability appeared to be more thermodynamic rather than kinetic in nature. We hypothesize that encapsulatation of the tetrameric active site within a silica mesopore of the support contributes to this stability.

Delaminated zeolite UCB-3 exhibits 2.4-fold greater catalytic activity relative to its three-dimensional (3D) zeolite counterpart, Al-SSZ-70, and 2.0-fold greater activity (per catalyst mass) when compared with industrial catalyst MCM-22, for the alkylation of toluene with propylene at 523 K. The former increase is nearly equal to the observed relative increase in external surface area and acid sites upon delamination. However, at 423 K for the same reaction, UCB-3 exhibits a 3.5-fold greater catalytic activity relative to 3D Al-SSZ-70. The higher relative rate enhancement for the delaminated material at lower temperature can be elucidated on the basis of increased contributions from internal acid sites. Evidence of possible contributions from such acid sites is obtained by performing catalysis after silanation treatment, which demonstrates that although virtually all catalysis in MCM-22 occurs on the external surface, catalysis also occurs on internal sites for 3D Al-SSZ-70. The additional observed enhancement at low temperatures can therefore be rationalized by greater access to internal active sites as a result of sheet breakage during delamination. Such breakage leads to shorter characteristic internal diffusion paths and was visualized using TEM comparisons of UCB-3 and 3D Al-SSZ-70.Keywords: aromatic alkylation; cumene; cymene; delamination; exfoliation; layered zeolite precursor; MCM-22; SSZ-70

Chiral AlIII-calixarene complexes were investigated as catalysts for the asymmetric Meerwein–Ponndorf–Verley (MPV) reduction reaction when using chiral and achiral secondary alcohols as reductants. The most enantioselective catalyst consisted of a new axially chiral vaulted-hemispherical calix[4]arene phosphite ligand, which attained an enantioselective excess of 99%. This ligand consists of two lower-rim hydroxyl groups, with the remaining two lower-rim oxygens directly connected to the phosphorus of the phosphite, which is derived from a chiral diol. The results emphasize the importance of the rigid calix[4]arene lower-rim substituents and point to a possible role of a lower-rim chiral pocket and Lewis-basic phosphorus lone pairs in enhancing asymmetric hydride transfer.Keywords: asymmetric hydride transfer; calixarene complexes; chiral; Lewis-acid catalysis; MPV reduction; phosphite ligand

Layered borosilicate zeolite precursor ERB-1P (Si/B = 11) is delaminated via simultaneous deboronation and SDA removal, to yield material DZ-1 consisting of silanol nests, using a simple aqueous Zn(NO3)2 treatment. Characterization of this synthesis process by PXRD shows loss of long-range order, and transmission electron microscopy (TEM) demonstrates transformation of rectilinear layers in the layered zeolite precursor to single and curved layers in the delaminated material. N2 physisorption confirms the expected decrease of micropore volume and increase in external surface area for delaminated materials relative to their calcined 3D zeolite counterpart. Elemental analysis shows loss of B and absence of Zn in the delaminated material. Resonances corresponding to silanol nests are evident via29Si solid-state NMR spectroscopy in DZ-1, which should be located within 12-MR pockets near the external surface. We have successfully utilized these nests as tetrahedral recognition sites for incorporation of Ti within an isolated framework coordination environment in material Ti-DZ-1. Diffuse-reflectance ultraviolet (DR-UV) spectroscopy of Ti-DZ-1 confirms isolated framework Ti sites, which are assigned to bands in the range of 210 nm–230 nm. Infrared spectra of Ti-DZ-1 consist of a distinct absorption band at 960 cm−1, which is absent in DZ-1 prior to Ti incorporation and has been previously correlated with the presence of framework Ti species. Infrared spectra after pyridine adsorption demonstrate bands consistent with Lewis-acid sites in the resulting Ti-substituted delaminated zeolite. The accessibility of these Lewis-acid sites is confirmed when using Ti-DZ-1 as a catalyst for cyclohexene epoxidation using tert-butyl hydroperoxide as the organic oxidant – a reaction for which both DZ-1 and TS-1 are inactive.

The first example of selective synthesis 5-methyl-2,3-dihydrofuran (MDHF) via dehydration of tetrahydro-2-furanylmethanol (THFM) is described. This synthesis is accomplished in a single-step reaction and uses readily available catalysts consisting of sodium-exchanged faujasite zeolites (Na-X and Na-USY). The mechanism is hypothesized to involve a hydride shift in reactant THFM, which leads to MDHF. This is fundamentally different from the known ring-expansion pathway for hydrolysis, which synthesizes 3,4-dihydro-2H-pyran (DHP). The activity and selectivity of the catalysts to MDHF was observed to increase with time on stream, and decrease upon increasing reaction temperature. Upon increasing either the initial water content of the catalyst or water partial pressure during reaction, an increase in the MDHF/DHP ratio was observed. This last observation helps correlate the above-mentioned trends in the catalyst activity and selectivity with increased time on stream, since the dehydration reaction synthesizes water.

Layered borosilicate zeolite precursor ERB-1P (Si/B = 11) is delaminated via isomorphous substitution of Al for B using a simple aqueous Al(NO3)3 treatment. Characterization by PXRD shows loss of long-range order, and TEM demonstrates transformation of rectilinear layers in the precursor to single and curved layers in the delaminated material. N2 physisorption and base titration confirm the expected decrease of micropore volume and increase in external surface area for delaminated materials relative to their calcined 3D zeolite counterpart, whereas FTIR and multinuclear NMR spectroscopies demonstrate synthesis of Brønsted acid sites upon delamination. Comparative synthetic studies demonstrate that this new delamination method requires (i) a borosilicate layered zeolite precursor, in which boron atoms can be isomorphously substituted by aluminum, (ii) neutral amine pore fillers instead of rigid and large quaternary amine SDAs, and (iii) careful temperature control, with the preferred temperature window being around 135 °C for ERB-1P delamination. Acylation of 2-methoxynaphthalene was used as a model reaction to investigate the catalytic benefits of delamination. A partially dealuminated delaminated material displays a 2.3-fold enhancement in its initial rate of catalysis relative to the 3D calcined material, which is nearly equal to its 2.5-fold measured increase in external surface area. This simple, surfactant- and sonication-free, mild delamination method is expected to find broad implementation for the synthesis of delaminated zeolite catalysts.

The synthesis of the first delaminated borosilicate layered zeolite precursor is described, along with its aluminosilicate analogue, which consists of Al-containing UCB-3 and B-containing UCB-4 from as-made SSZ-70. In addition, the delamination of PREFER (which is the precursor to ferrierite zeolite) under similar conditions yields delaminated layered zeolite precursors consisting of Al-containing UCB-5 and Ti-containing UCB-6. Multinuclear solid-state NMR spectroscopy (11B and 27Al), diffuse-reflectance UV-vis spectroscopy, and heteroatom/Si ratios measured via elemental analysis are consistent with a lack of heteroatom leaching from the framework following delamination. Such mild delamination conditions are achieved by swelling the zeolite precursor in a fluoride/chloride surfactant mixture in DMF solvent, followed by sonication. Powder X-ray diffraction, argon gas physisorption, and chemisorption of bulky base probes strongly suggest delamination, and demonstrate a 1.5-fold increase in the number density of external acid sites and surface area of calcined UCB-3, relative to calcined Al-SSZ-70. The synthesis of microporous pockets in materials UCB-3–UCB-5 suggests the possibility of interlayer porosity in SSZ-70, which is a layered zeolite precursor material whose structure remains currently unknown. The mildness of the delamination method presented here, as well as the lack of need for acidification in the synthesis procedure, enables the delamination of heteroatom-containing zeolites while preserving the framework integrity of labile heteroatoms, which could otherwise be leached under harsher conditions.Keywords: delamination; exfoliation; ferrierite zeolite; layered zeolite precursor; MCM-22(P); SSZ-70;

The homogeneous versus heterogeneous nature of the active site of gold catalysis of the 4-nitrophenol reduction to 4-aminophenol is investigated using poisoning experiments that employ various organic ligands and 4 nm gold nanoparticles as catalysts. DDT (dodecanethiol)-bound gold nanoparticles are unable to catalyze this reaction, whereas nanoparticles capped with calixarene ligands consisting of calix[6]arene phosphine C6P and calix[4]arene thiol MBC are active. Poisoning of residual terrace sites upon addition of 2-naphthalenethiol (2-NT) in these latter two catalysts results in a gold nanoparticle that consists solely of ostensibly similar pinhole defect sites. However, the reaction rate for the catalyst consisting of C6P + 2-NT is 6.5-fold higher relative to the rate for catalyst consisting of MBC + 2-NT. This observation along with lack of activity for the DDT-bound catalyst suggests that pinhole defect sites and the gold nanoparticle surface cannot be the active site for catalysis. Instead, the active site is suggested to be a leached gold species that is present in exceedingly small concentrations (cannot be detected by disappearance of gold nanoparticles from solution during catalysis). Such a supposition is supported by observations of induction time and the interplay between observed induction time and kinetics. It is observed that the composition of the organic ligands in the system controls the kinetics and the induction time. Additionally, there is an absence of an induction time in a solution containing used catalyst, to which reactants are added. In the initial catalysis, it is observed that as the thiol ligand concentration on the surface increases, the induction time increases and the reaction rate decreases. The leached species were unable to be detected via changes in the surface-plasmon resonance absorption of the gold nanoparticles in solution before and after catalysis, as well as electron microscopy studies of used nanoparticle catalysts. This suggests that the leached species concentration is low, and their catalytic activity in turn must be quite high.

We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open “coordinatively unsaturated” active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(I)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH–π interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(I) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core.

An approach for the control and understanding of supported molecular catalysts is demonstrated with the design and synthesis of open and closed variants of a grafted Lewis acid active site, consisting of Al(III)-calix[4]arene complexes on the surface of silica. The calixarene acts as a molecular template that enforces open and closed resting-state coordination geometries surrounding the metal active sites, due to its lower-rim substituents as well as site isolation by virtue of its steric bulk. These sites are characterized and used to elucidate mechanistic details and connectivity requirements for reactions involving hydride and oxo transfer. The consequence of controlling open versus closed configurations of the grafted Lewis acid site is demonstrated by the complete lack of observed activity of the closed site for Meerwein-Ponndorf-Verley (MPV) reduction; whereas, the open variant of this catalyst has an MPV reduction activity that is virtually identical to previously reported soluble molecular Al(III)-calix[4]arene catalysts. In contrast, for olefin epoxidation using tert-butyl-hydroperoxide as oxidant, the open and closed catalysts exhibit similar activity. This observation suggests that for olefin epoxidation catalysis using Lewis acids as catalyst and organic hydroperoxide as oxidant, covalent binding of the hydroperoxide is not required, and instead dative coordination to the Lewis acid center is sufficient for catalytic oxo transfer. This latter result is supported by density functional theory calculations of the transition state for olefin epoxidation catalysis, using molecular analogs of the open and closed catalysts.

The synthesis and characterization of new cluster compounds represented by the series Ir4(CO)12−xLx (L = tert-butyl-calix[4]-arene(OPr)3(OCH2PPh2); x = 2 and 3) is reported using ESI mass spectrometry, NMR spectroscopy, IR spectroscopy and single-crystal X-ray diffraction. Thermally driven decarbonylation of the cluster compound series represented by x = 1–3 according to the formula above is followed viaFTIR and NMR spectroscopies, and dynamic light scattering in toluene solution. The propensity of these clusters to decarbonylate in solution is shown to be directly correlated with number density of adsorbed calixarene phosphine ligands and controlled via Pauli repulsion between metal d and CO 5σ orbitals. The tendency for cluster aggregation unintuitively follows a trend that is exactly opposite to the cluster's propensity to decarbonylate. No cluster aggregation is observed for clusters consisting of x = 3, even after extensive decarbonylationvia loss of all bridging CO ligands and coordinative unsaturation. Some of the CO lost during thermal treatment viadecarbonylation can be rebound to the coordinatively unsaturated cluster consisting of x = 3. In contrast, the clusters consisting of x = 1 and x = 2 both aggregate into large nanoparticles when treated under identical conditions. Clusters in which the calixarene phosphine ligand is replaced with a sterically less demanding PPh2Me ligand 6 lead to significantly less coordinative unsaturation upon thermal treatment. Altogether, these data support a mechanical model of accessibility in coordinatively unsaturated metal clusters in solution, which hinges on having at least three sterically bulky organic ligands per Ir4 core.

Dissociative chemisorption of ethanol on partially dehydroxylated silica is investigated by (i) exposing silica to gas-phase ethanol at various temperatures (ranging between 373 and 773 K) and (ii) analyzing the material using temperature-programmed desorption and in situ infrared spectroscopy. This chemisorption leads to formation of isolated surface ethoxide species via dehydration of ethanol at reaction temperatures above 573 K, and, at lower temperatures, it favors the synthesis of silanol–ethoxide functionality via a pathway involving opening of siloxane Si–O–Si bridges. The activation barrier for ethene desorption from the isolated surface ethoxide species is considerably higher relative to that for ethanol desorption from the hydrogen-bound silanol–ethoxide pairs. These single-turnover experiments allow predicting the product distribution of ethanol chemisorption on silica depending on the treatment conditions, e.g. temperature of interaction between ethanol and silica, and suggest why, in general, dehydration catalysis on silica requires high temperatures, in order to avoid non-productive chemisorption via opening of siloxane bridges.

The adsorption of cellulose-derived long-chain (longer than ten glucose repeat units on size) glucans onto carbon-based acid catalysts for hydrolysis has long been hypothesized; however, to date, there is no information on whether such adsorption can occur and how glucan chain length influences adsorption. Herein, in this manuscript, we first describe how glucan chain length influences adsorption energetics, and use this to understand the adsorption of long-chain glucans onto mesoporous carbon nanoparticles (MCN) from a concentrated acid solution, and the effect of mesoporosity on this process. Our results conclusively demonstrate that mesoporous carbon nanoparticle (MCN) materials adsorb long-chain glucans from concentrated acid hydrolyzate in amounts of up to 30% by mass (303 mg/g of MCN), in a manner that causes preferential adsorption of longer-chain glucans of up to 40 glucose repeat units and, quite unexpectedly, fast adsorption equilibration times of less than 4 min. In contrast, graphite-type carbon nanopowders (CNP) that lack internal mesoporosity adsorb glucans in amounts less than 1% by mass (7.7 mg/g of CNP), under similar conditions. This inefficiency of glucan adsorption on CNP might be attributed to the lack of internal mesoporosity, since the CNP actually possesses greater external surface area relative to MCN. A systematic study of adsorption of glucans in the series glucose to cellotetraose on MCN shows a monotonically decreasing free energy of adsorption upon increasing the glucan chain length. The free energy of adsorption decreases by at least 0.4 kcal/mol with each additional glucose unit in this series, and these energetics are consistent with CH−π interactions providing a significant energetic contribution for adsorption, similar to previous observationsin glycoproteins. HPLC of hydrolyzed fragments in solution, 13C Bloch decay NMR spectroscopy, and GPC provide material balance closure of adsorbed glucan coverages on MCN materials. The latter and MALDI-TOF-MS provide direct evidence for adsorption of long-chain glucans on the MCN surface, which have a radius of gyration larger than the pore radius of the MCN material.

New material UCB-1 is synthesized via the delamination of zeolite precursor MCM-22 (P) at pH 9 using an aqueous solution of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride at 353 K. Characterization by powder X-ray diffraction, transmission electron microscopy, and nitrogen physisorption at 77 K indicates the same degree of delamination in UCB-1 as previously reported for delaminated zeolite precursors, which require a pH of greater than 13.5 and sonication in order to achieve exfoliation. UCB-1 consists of a high degree of structural integrity via 29Si MAS NMR and Fourier transform infrared spectroscopies, and no detectable formation of amorphous silica phase via transmission electron microscopy. Porosimetry measurements demonstrate a lack of hysteresis in the N2 adsorption/desorption isotherms and macroporosity in UCB-1. The new method is generalizable to a variety of Si:Al ratios and leads to delaminated zeolite precursor materials lacking amorphization.

The delamination of layered zeolite precursor PREFER is demonstrated under mild nonaqueous conditions using a mixture of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride in N,N-dimethylformamide (DMF) as solvent. The delamination proceeds through a swollen material intermediate which is characterized using powder X-ray diffraction (PXRD). Subsequent addition of concentrated HCl at room temperature leads to synthesis of UCB-2 via delamination of the swollen PREFER material and is characterized using PXRD, transmission electron microscopy (TEM), and argon gas physisorption, which shows lack of microporosity in UCB-2. 29Si magic angle spinning (MAS) NMR spectroscopy indicates lack of amorphization during delamination, as indicated by the entire absence of Q2 resonances, and 27Al MAS NMR spectroscopy shows exclusively tetrahedral aluminum in the framework following delamination. The delamination process requires both chloride and fluoride anions and is sensitive to solvent, working well in DMF. Experiments aimed at synthesizing UCB-2 using aqueous conditions previously used for UCB-1 synthesis leads to partial swelling and lack of delamination upon acidification. A similar lack of delamination is observed upon attempting synthesis of UCB-1 under conditions used for UCB-2 synthesis. The delamination of PREFER is reversible between delaminated and swollen states in the following manner. Treatment of as-made UCB-2 with the same reagents as used here for the swelling of PREFER causes the delaminated UCB-2 material to revert back to swollen PREFER. This causes the delaminated UCB-2 material to revert back to swollen PREFER. Altogether, these results highlight delamination as the reverse of zeolite synthesis and demonstrate the crucial role of noncovalent self-assembly involving the zeolitic framework and cations/anions/structure-directing agent and solvent during the delamination process.Keywords: delamination; exfoliation; layered zeolite precursor; PREFER;

The design of new materials for gaseous NOx (NO and NO2) removal at ambient temperature using organic active sites is reported. The materials consist of unfunctionalized silica and silica modified by immobilized aminoxyls and function via sequential processes consisting of (i) NO oxidation to NO2 and (ii) NO2 storage. NOx removal by physical mixtures of immobilized PTIO (2-phenyl-4,4,5,5-tetramethyl-imidazoline-3-oxide-1-oxyl) sites on silica as the NO oxidant and hydrated silica as the NO2 trap occurs with significant degradation of the PTIO oxidant via undesired side reactions with NO2 when NO2 adsorption sites are fewer than required for its complete removal along the packed bed. The use of packed beds with sufficient NO2 adsorption sites requires a large excess of unfunctionalized silica, because of its low surface density of geminal silanols, which are shown to be the relevant sites for NO2 storage on silica at ambient temperature based on density functional theory calculations. This PTIO degradation is circumvented by the design of NOx traps consisting of immobilized PTIO on silica as the NO oxidant and immobilized TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) on silica as an adsorbent with a high density of strong NO2 binding sites. Packed beds consisting of a 2:1 molar mixture of PTIO and TEMPO sites consume NOx as predicted by stoichiometry without detectable PTIO degradation and also without a contribution from geminal silanols as NO2 storage sites. This result requires that PTIO and TEMPO sites on silica render geminal silanols as inactive towards NO2 storage presumably because of the titration of these silanols by immobilized aminoxyls.

The design, synthesis and characterization of materials consisting of grafted poly(1 → 4-β-glucan) strands on silica is reported. The silanol-rich environment provided in these materials activates the glycosidic bond for hydrolysis under mild conditions.

Metal nanoparticles bound with polymeric and oligomeric ligands are commonly employed in areas where accessibility to the underlying metal is critical, such as in catalysis. Developing a fundamental understanding of molecular-scale factors that control accessibility to the metal surface in these systems enables approaches for ligand design. Here, we implement a comparative synthetic approach to investigate the role of an induced-fit binding mechanism on accessibility, which reduces to elucidating the correlation between ligand flexibility and accessibility. Four nm gold nanoparticles are bound with a calix[8]arene phosphine ligand in the comparative series 3a−c, and ligand molecular footprints of ∼230 Å2/calix[8]arene are measured on the gold surface using UV−vis spectroscopy of the surface plasmon resonance absorption band. Ligand flexibility in CDCl3 solution is characterized using variable-temperature 1H NMR spectroscopy, and results demonstrate 3c to be the most rigid of all three ligands. This is further reinforced by conformational analysis of 3a−c using molecular modeling calculations of the free ligand in the gas phase. Additional conformational analysis of bound 3a−c is modeled by using a constrained variant of the corresponding phosphine oxide 2a−c, in which the P═O bonds of the phosphine oxide are aligned, as they would be when bound to a surface underneath. Interestingly, the most rigid ligand in the bound state is 3a, a result that is in stark contrast to experiments in solution and simulations in the gas phase described above. The amount of accessible surface is measured using steady-state fluorescence of 2-naphthalenethiol (2NT) as a chemisorption probe. The most rigid ligand in the bound state (3a) synthesizes no accessible surface whereas the more flexible ligands in the bound state (both 3b and 3c) synthesize ∼0.5 2NT worth of accessible metal surface per calix[8]arene ligand, corresponding to 5−7% of the gold surface of the nanoparticle being accessible. These results demonstrate a correlation between accessibility and ligand flexibility in the bound state, and suggest an induced-fit binding mechanism in which a flexible bound ligand subtly changes shape in order to accommodate adsorption of an incoming molecule on the metal surface.

Kinetic poisoning experiments employing organic ligands were conducted using a gold nanoparticle–catalyzed reaction consisting of the reduction of resazurin to resorufin. The kinetic contributions of three distinct types of sites along with the number density of each of these site types during reaction were determined. The calculated number densities of each of the three types of sites, hypothesized to be corners, edges, and terraces, correlates well with atomic-resolution micrographs of the supported gold nanoparticles, obtained using aberration-corrected transmission electron microscopy and with predictions based on geometric models of idealized gold nanoparticles. The most active sites comprising 1% of the surface atoms exhibit at least 30% of the total activity of the catalyst for resazurin reduction. The selective mechanical blocking of surface sites on nanoparticles, particularly undercoordinated sites, paves the way for novel approaches utilizing organic ligands to quantify the activity of different active sites and control catalysis on metal surfaces.Graphical abstractSupported gold nanoparticle catalysts were synthesized for the reduction of resazurin to resorufin as a model reaction, on a trimethylsilyl-capped silica support. These were used in kinetic poisoning experiments using organic ligands. Results show the distinct catalytic contributions of a corner, edge, and terrace sites. Download high-res image (105KB)Download full-size imageHighlights► A silica-supported Au nanoparticle catalyst is used for resazurin reduction. ► One percentage of the surface atoms accounts for 30% of the catalyst’s activity. ► Kinetic poisoning experiments reveal two different types of active sites. ► The catalytically active sites are suggested to be corner and edge atoms. ► More than an order of magnitude difference in activity is shown between sites.

Catalytic Meerwein–Ponndorf–Verley (MPV) reduction using AlIII–calix[4]arene complexes is investigated as a model system that requires the bringing together of two different chemical species, ketone and alkoxide, within a six-membered transition state. Two-point versus one-point ketone binding is demonstrated to be the most salient feature that controls MPV catalysis rate. A 7.7-fold increase in rate is observed when comparing reactants consisting of a bidentate Cl-containing ketone and sterically and electronically similar but looser-binding ketones, which are substituted with H and F. The one-point and two-point nature of ketone binding for the various ketones investigated is independently assessed using a combination of structural data derived from single-crystal X-ray diffraction and DFT-based molecular modeling. Using MPV catalysis with inherently chiral calix[4]arenes, the effect of multiple point reactant binding on enantioselectivity is elucidated. A higher denticity of ketone binding appears to increase the sensitivity of the interplay between chiral active site structure and MPV reduction enantioselectivity.Graphical abstractAl(III)–calix[4]arene complexes act as site-isolated Lewis acid catalysts for homogeneous MPV reduction. Two-point versus one-point binding of ketone reactant is a crucial feature that controls the catalytic rate and enantioselectivity.Download high-res image (91KB)Download full-size imageHighlights► Al(III)–calix[4]arene complexes are active site-isolated catalysts for MPV reduction. ► Two-point binding of ketone reactant accelerates rate of catalysis versus one-point binding. ► Electronic and steric requirements at the active site are investigated. ► Interplay between calixarene chirality and MPV reduction enantioselectivity is controlled by rigidity of ketone binding.

Metal–organic framework NU-1000 selectively adsorbs furanics, while completely excluding the adsorption of monomeric sugars from the same aqueous mixture. The highly refined degree of molecular recognition exhibited by NU-1000 is exemplified with it selectively adsorbing 5-hydroxymethylfurfural, even in the presence of up to a 300-fold excess of glucose in solution.

The design, synthesis and characterization of materials consisting of grafted poly(1 → 4-β-glucan) strands on silica is reported. The silanol-rich environment provided in these materials activates the glycosidic bond for hydrolysis under mild conditions.

The synthesis and characterization of new cluster compounds represented by the series Ir4(CO)12−xLx (L = tert-butyl-calix[4]-arene(OPr)3(OCH2PPh2); x = 2 and 3) is reported using ESI mass spectrometry, NMR spectroscopy, IR spectroscopy and single-crystal X-ray diffraction. Thermally driven decarbonylation of the cluster compound series represented by x = 1–3 according to the formula above is followed viaFTIR and NMR spectroscopies, and dynamic light scattering in toluene solution. The propensity of these clusters to decarbonylate in solution is shown to be directly correlated with number density of adsorbed calixarene phosphine ligands and controlled via Pauli repulsion between metal d and CO 5σ orbitals. The tendency for cluster aggregation unintuitively follows a trend that is exactly opposite to the cluster's propensity to decarbonylate. No cluster aggregation is observed for clusters consisting of x = 3, even after extensive decarbonylationvia loss of all bridging CO ligands and coordinative unsaturation. Some of the CO lost during thermal treatment viadecarbonylation can be rebound to the coordinatively unsaturated cluster consisting of x = 3. In contrast, the clusters consisting of x = 1 and x = 2 both aggregate into large nanoparticles when treated under identical conditions. Clusters in which the calixarene phosphine ligand is replaced with a sterically less demanding PPh2Me ligand 6 lead to significantly less coordinative unsaturation upon thermal treatment. Altogether, these data support a mechanical model of accessibility in coordinatively unsaturated metal clusters in solution, which hinges on having at least three sterically bulky organic ligands per Ir4 core.

The design of new materials for gaseous NOx (NO and NO2) removal at ambient temperature using organic active sites is reported. The materials consist of unfunctionalized silica and silica modified by immobilized aminoxyls and function via sequential processes consisting of (i) NO oxidation to NO2 and (ii) NO2 storage. NOx removal by physical mixtures of immobilized PTIO (2-phenyl-4,4,5,5-tetramethyl-imidazoline-3-oxide-1-oxyl) sites on silica as the NO oxidant and hydrated silica as the NO2 trap occurs with significant degradation of the PTIO oxidant via undesired side reactions with NO2 when NO2 adsorption sites are fewer than required for its complete removal along the packed bed. The use of packed beds with sufficient NO2 adsorption sites requires a large excess of unfunctionalized silica, because of its low surface density of geminal silanols, which are shown to be the relevant sites for NO2 storage on silica at ambient temperature based on density functional theory calculations. This PTIO degradation is circumvented by the design of NOx traps consisting of immobilized PTIO on silica as the NO oxidant and immobilized TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) on silica as an adsorbent with a high density of strong NO2 binding sites. Packed beds consisting of a 2:1 molar mixture of PTIO and TEMPO sites consume NOx as predicted by stoichiometry without detectable PTIO degradation and also without a contribution from geminal silanols as NO2 storage sites. This result requires that PTIO and TEMPO sites on silica render geminal silanols as inactive towards NO2 storage presumably because of the titration of these silanols by immobilized aminoxyls.

We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open “coordinatively unsaturated” active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(I)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH–π interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(I) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core.

Layered borosilicate zeolite precursor ERB-1P (Si/B = 11) is delaminated via simultaneous deboronation and SDA removal, to yield material DZ-1 consisting of silanol nests, using a simple aqueous Zn(NO3)2 treatment. Characterization of this synthesis process by PXRD shows loss of long-range order, and transmission electron microscopy (TEM) demonstrates transformation of rectilinear layers in the layered zeolite precursor to single and curved layers in the delaminated material. N2 physisorption confirms the expected decrease of micropore volume and increase in external surface area for delaminated materials relative to their calcined 3D zeolite counterpart. Elemental analysis shows loss of B and absence of Zn in the delaminated material. Resonances corresponding to silanol nests are evident via29Si solid-state NMR spectroscopy in DZ-1, which should be located within 12-MR pockets near the external surface. We have successfully utilized these nests as tetrahedral recognition sites for incorporation of Ti within an isolated framework coordination environment in material Ti-DZ-1. Diffuse-reflectance ultraviolet (DR-UV) spectroscopy of Ti-DZ-1 confirms isolated framework Ti sites, which are assigned to bands in the range of 210 nm–230 nm. Infrared spectra of Ti-DZ-1 consist of a distinct absorption band at 960 cm−1, which is absent in DZ-1 prior to Ti incorporation and has been previously correlated with the presence of framework Ti species. Infrared spectra after pyridine adsorption demonstrate bands consistent with Lewis-acid sites in the resulting Ti-substituted delaminated zeolite. The accessibility of these Lewis-acid sites is confirmed when using Ti-DZ-1 as a catalyst for cyclohexene epoxidation using tert-butyl hydroperoxide as the organic oxidant – a reaction for which both DZ-1 and TS-1 are inactive.

Metal–organic framework (MOF) material NU-1000 adsorbs dimers cellobiose and lactose from aqueous solution, in amounts exceeding 1250 mg gNU-1000−1 while completely excluding the adsorption of the monomer glucose, even in a competitive mode with cellobiose. The MOF also discriminates between dimers consisting of α and β linkages, showing no adsorption of maltose. Electronic structure calculations demonstrate that key to this selective molecular recognition is the number of favorable CH–π interactions made by the sugar with pyrene units of the MOF.

The homogeneous versus heterogeneous nature of the active site of gold catalysis of the 4-nitrophenol reduction to 4-aminophenol is investigated using poisoning experiments that employ various organic ligands and 4 nm gold nanoparticles as catalysts. DDT (dodecanethiol)-bound gold nanoparticles are unable to catalyze this reaction, whereas nanoparticles capped with calixarene ligands consisting of calix[6]arene phosphine C6P and calix[4]arene thiol MBC are active. Poisoning of residual terrace sites upon addition of 2-naphthalenethiol (2-NT) in these latter two catalysts results in a gold nanoparticle that consists solely of ostensibly similar pinhole defect sites. However, the reaction rate for the catalyst consisting of C6P + 2-NT is 6.5-fold higher relative to the rate for catalyst consisting of MBC + 2-NT. This observation along with lack of activity for the DDT-bound catalyst suggests that pinhole defect sites and the gold nanoparticle surface cannot be the active site for catalysis. Instead, the active site is suggested to be a leached gold species that is present in exceedingly small concentrations (cannot be detected by disappearance of gold nanoparticles from solution during catalysis). Such a supposition is supported by observations of induction time and the interplay between observed induction time and kinetics. It is observed that the composition of the organic ligands in the system controls the kinetics and the induction time. Additionally, there is an absence of an induction time in a solution containing used catalyst, to which reactants are added. In the initial catalysis, it is observed that as the thiol ligand concentration on the surface increases, the induction time increases and the reaction rate decreases. The leached species were unable to be detected via changes in the surface-plasmon resonance absorption of the gold nanoparticles in solution before and after catalysis, as well as electron microscopy studies of used nanoparticle catalysts. This suggests that the leached species concentration is low, and their catalytic activity in turn must be quite high.