Understanding the Role of Defect Sites in Glucan Hydrolysis on Surfaces

Though unfunctionalized mesoporous carbon consisting of weakly Brønsted acidic OH-defect sites depolymerizes cellulose under mild conditions, the nature of the active site and how this affects hydrolysis kineticsthe rate-limiting step of this processhas remained a puzzle. Here, in this manuscript, we quantify the effect of surface OH-defect site density during hydrolysis catalysis on the rate of reaction. Our comparative approach relies on synthesis and characterization of grafted poly­(1→4-β-glucan) (β-glu) strands on alumina. Grafted β-glu strands on alumina have a 9-fold higher hydrolysis rate per glucan relative to the highest rate measured for β-glu strands on silica. This amounts to a hydrolysis rate per grafted center on alumina that is 2.7-fold more active than on silica. These data are supported by the lower measured activation energy for hydrolysis of grafted β-glu strands on alumina being 70 kJ/mol relative to 87 kJ/mol on silica. The observed linear increase of hydrolysis rate with increasing OH-defect site density during catalysis suggests that the formation of hydrogen bonds between weakly Brønsted acidic OH-defect sites and constrained glycosidic oxygens (i.e., those juxtaposed adjacent to the surface) activates the latter for hydrolysis catalysis. Altogether, these data elucidate crucial structural requirements for glucan hydrolysis on surfaces and, when coupled with our recent demonstration of long-chain glucan binding to mesoporous carbon, present a unified picture, for the first time, of adsorbed glucan hydrolysis on OH-defect site-containing surfaces, such as unfunctionalized mesoporous carbon

Grafted Poly(1→4-β-glucan) Strands on Silica: A Comparative Study of Surface Reactivity as a Function of Grafting Density

Grafted poly(β-glucan) (β-glu) strands on the surface of silica are synthesized with varying degrees of grafting density, and display an amorphous-like environment via ¹³C CP/MAS NMR spectroscopy. Thermal gravimetric analysis of these materials under oxidative conditions shows increased β-glu thermal stability with higher degrees of grafting density. The range of temperature stability between the most and least hydrogen-bound grafted β-glu strands spans 321 to 260 °C. This range is bound by the combustion temperature previously measured for crystalline and amorphous cellulose, with the former having greater oxidative stability, and is likely controlled by the extent of hydrogen bonding of a grafted β-glu strand with the underlying silica surface. When using these materials as reactants for glycosidic bond hydrolysis, the total number of reducing ends formed during reaction is quantified using a BCA colorimetric assay. Results demonstrate that the material with greatest interaction with silica surface silanols undergoes hydrolysis at an initial rate that is 6-fold higher than the material with the lowest degree of such interaction. The role of the surface as a reactive interface that can endow oxidative stability and promote hydrolysis activity has broad implications for surface-catalyzed processes dealing with biomass-derived polymers

Al^III–Calix[4]arene Catalysts for Asymmetric Meerwein–Ponndorf–Verley Reduction

Chiral Al^III-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

Catalytic Hydrolysis of Cellulose to Glucose Using Weak-Acid Surface Sites on Postsynthetically Modified Carbon

We demonstrate depolymerization of adsorbed (1 → 4)- β-d-glucans (<i>β-</i>glu) derived from crystalline cellulose (<i>Avicel</i>), using weak-acid sites of postsynthetically surface-functionalized mesoporous carbon nanoparticle (MCN) catalysts HT₅-HSO₃-MCN and COOH-MCN and investigate the role of acid-site density and <i>β-</i>glu molecular weight on this depolymerization. Both HT₅-HSO₃-MCN and COOH-MCN hydrolyze adsorbed <i>β-</i>glu strands and afford glucose yields of 73% and 90%, respectively, at a buffered pH of 2.0 after 3 h treatment at 180 °C. These yields are significantly higher than the 16% yield of an unfunctionalized MCN-control catalyst under otherwise identical conditions, demonstrating the importance of postsynthetic surface functionalization for achieving weak-acid catalytic hydrolysis. Highlighting the important role of confinement in this catalysis, all yields are generally depressed when using a lower rather than higher molecular weight of adsorbed <i>β-</i>glu strands on the same catalyst. The catalytic hydrolysis rate also generally increases upon decreasing buffer pHparticularly so for the more acidic carboxylic acid-functionalized catalyst COOH-MCN. This is interpreted on the basis of a higher local density of surface weak-acid sites upon protonation of surface conjugate-base functionality, as demonstrated by a comparison of zeta potential measurements of catalysts COOH-MCN and MCN

Silica-Supported Phosphonic Acids as Thermally and Oxidatively Stable Organic Acid Sites

Organic–inorganic materials consisting of organophosphonic-acid-supported-on-silica materials <b>C3</b>/<b>SiO₂</b> and <b>C4/SiO₂</b> are described, where <b>C3</b> is propane-1,2,3-triphosphonic acid and <b>C4</b> 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 <b>CX8</b>, 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 (<b>PVPA</b>). Material <b>CX8</b>/<b>SiO₂</b> possesses a significantly lower thermal stability and lower combustion temperature of 300 °C in air, whereas <b>PVPA</b> demonstrates comparable thermal stability as observed with <b>C3</b> and <b>C4</b>. 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 <b>C4/SiO</b>_<b>2</b><b>-37%</b> (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

Glucan Adsorption on Mesoporous Carbon Nanoparticles: Effect of Chain Length and Internal Surface

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

Heteroatom-Substituted Delaminated Zeolites as Solid Lewis Acid Catalysts

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

Role of N‑Heterocyclic Carbenes as Ligands in Iridium Carbonyl Clusters

The low-energy isomers of Ir_<i>x</i>(CO)_<i>y</i>(NHC)_<i>z</i> (<i>x</i> = 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 (<i>C</i>_3<i>v</i> derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters

Hydrolysis Catalysis of <i>Miscanthus</i> Xylan to Xylose Using Weak-Acid Surface Sites

Adsorption and hydrolysis of xylan polysaccharides extracted from <i>Miscanthus</i> biomass are demonstrated, using surface-functionalized MCN (mesoporous carbon nanoparticle) materials that comprise weak-acid sites, at a pH corresponding to biomass extract. Extracted xylan polysaccharides consist of a peak molecular weight of 2008 g/mol according to GPC (gel-permeation chromatography), corresponding to approximately 15 xylose repeat units, and, a calculated length of 7 nm and radius of gyration of 2.0 nm based on molecular dynamics simulations. A highly active material for the adsorption and depolymerization of xylan is a hydrothermally treated sulfonated MCN material, which consists of 90% weak-acid sites. In spite of the large polysaccharide size relative to its 1.6 nm pore radius, this material adsorbs up to 76% of xylan strands from extract solution, at a weight loading of 29% relative to MCN. Starting with a 9.7% xylose yield in <i>Miscanthus</i> extract, this material hydrolyzes extracted xylan to xylose, and achieves a 74.1% xylose yield, compared with 24.1% yield for the background reaction in acetate buffer, at 150 °C for 4 h. Catalytic comparisons with other MCN-based materials highlight the role of confinement and weak-acid surface sites, and provide some correlation between activity and phenolic OH acid-site density. However, the lack of a directly proportional correlation between weak-acid site density and catalyzed hydrolysis rate signifies that only a fraction of weak-acid surface sites are catalytically active, and this is likely to be the sites that are present in a high local concentration on the surface, which would be consistent with previously observed trends in the hydrolysis catalysis of chemisorbed glucans on inorganic-oxide surfaces

Selective Metal–Organic Framework Catalysis of Glucose to 5‑Hydroxymethylfurfural Using Phosphate-Modified NU-1000

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