Publications by authors named "Alan L Chaffee"

18 Publications

  • Page 1 of 1

Molecular Clustering in Formaldehyde-Methanol-Water Mixtures Revealed by High-Intensity, High-q Small-Angle Neutron Scattering.

J Phys Chem Lett 2021 Jan 29;12(1):480-486. Epub 2020 Dec 29.

Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia.

Methanol-Water (mw) mixtures, with or without a solute, display a nonideal thermodynamic behavior, typically attributed to the structure of the microphase. However, experimental observation of the microphase structures at the molecular length scale has been a challenge. We report the presence of molecular clusters in mw and formaldehyde-methanol-water (fmw) mixtures using small-angle neutron scattering (SANS) experiments and molecular dynamics (MD) simulations. Hydrophobic clusters of methanol in mw and formaldehyde-methanol in fmw mixtures were observed at low methanol compositions ( ≤ 0.3). A three-dimensional hydrogen-bonded network of water with the solute is observed at = 0.5. Linear chains of methanol surrounding the formaldehyde and water molecules were observed at high methanol compositions ( ≥ 0.7). The calculated size of the molecular clusters ( ≈ 0.5 nm, spherical) from the SANS data and their volume fraction closely matched the MD simulation results.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpclett.0c03515DOI Listing
January 2021

Atomistic Mechanisms of Thermal Transformation in a Zr-Metal Organic Framework, MIL-140C.

J Phys Chem Lett 2021 Jan 15;12(1):177-184. Epub 2020 Dec 15.

Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.

To understand the mechanisms responsible for thermal decomposition of a Zr-MOF (MIL-140C), we perform atomistic-scale molecular dynamics (MD) simulations and discuss the simulation data in comparison with the TEM images obtained for the decomposed Zr-MOF. First, we introduce the ReaxFF parameters suitable for the Zr/C/H/O chemistry and then apply them to investigate the thermal stability and morphological changes in the MIL-140C during heating. Based on the performed simulations we propose an atomic mechanism for the collapse of the MIL-140C and the molecular pathways for carbon monoxide formation, the main product of the MIL-140C thermal degradation. We also determine that the oxidation state of the ZrO clusters, evolved due to the thermal degradation, approximates the tetragonal phase of ZrO. Both simulations and experiments show a distribution of very small ZrO clusters embedded in the disrupted organic sheet that could contribute to the unusual high catalytic activity of the decomposed MIL-140C.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpclett.0c02930DOI Listing
January 2021

Elevated amyloidoses of human IAPP and amyloid beta by lipopolysaccharide and their mitigation by carbon quantum dots.

Nanoscale 2020 Jun;12(23):12317-12328

ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville, VIC 3052, Australia. and Zhongshan Hospital, Fudan University, 111 Yixueyuan Rd, Xuhui District, Shanghai, 200032, China.

Type 2 diabetes (T2D) and Alzheimer's disease (AD) represent two most prevalent amyloid diseases with a significant global burden. Pathologically, T2D and AD are characterized by the presence of amyloid plaques consisting primarily of toxic human islet amyloid polypeptide (IAPP) and amyloid beta (Aβ). It has been recently revealed that the gut microbiome plays key functions in the pathological progression of neurological disorders through the production of bacterial endotoxins, such as lipopolysaccharide (LPS). In this study, we examined the catalytic effects of LPS on IAPP and Aβ amyloidoses, and further demonstrated their mitigation with zero-dimensional carbon quantum dots (CQDs). Whereas LPS displayed preferred binding with the N-terminus of IAPP and the central hydrophobic core and C-terminus of Aβ, CQDs exhibited propensities for the amyloidogenic and C-terminus regions of IAPP and the N-terminus of Aβ, accordingly. The inhibitory effect of CQDs was verified by an embryonic zebrafish model exposed to the peptides and LPS, where impaired embryonic hatching was rescued and production of reactive oxygen species in the organism was suppressed by the nanomaterial. This study revealed a robust synergy between LPS and amyloid peptides in toxicity induction, and implicated CQDs as a potential therapeutic against the pathologies of T2D and AD.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d0nr02710cDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7325865PMC
June 2020

A Multifunctional, Charge-Neutral, Chiral Octahedral M L Cage.

Chemistry 2019 Jun 27;25(36):8489-8493. Epub 2019 May 27.

School of Chemistry, Monash University, Clayton, VIC 3800, Australia.

A chiral, octahedral M L cage, which is charge neutral and contains an internal void of about 2000 Å , is reported. The cage was synthesised as an enantiopure complex by virtue of amino-acid-based dicarboxylate ligands, which assemble around copper paddlewheels at the vertices of the octahedron. The cage persists in solution with retention of the fluorescence properties of the parent acid. The solid-state structure contains large pores both within and between the cages, and displays permanent porosity for the sorption of gases with retention of crystallinity. Initial tests show some enantioselectivity of the cage towards guests in solution.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/chem.201901681DOI Listing
June 2019

Correlations between Oxygen Uptake and Vacancy Concentration in Pr-Doped CeO.

ACS Omega 2017 Jun 7;2(6):2544-2551. Epub 2017 Jun 7.

School of Chemistry, Monash University, 17 Rainforest Walk, Victoria 3800, Australia.

The oxygen uptake of a series of Pr-CeO materials was measured using thermogravimetric analysis at 420 and 600 °C, and at both temperatures, 20% Pr-CeO was found to have the highest uptake. The materials were characterized using X-ray diffraction and scanning transmission electron microscopy. Defects in the materials were identified using Raman spectroscopy, and ultraviolet-visible spectroscopy was used to show the presence of Pr cations in the +3 oxidation state. The existence of these species was attributed to be responsible for the ability of the materials to uptake oxygen. Electron energy loss spectroscopy was used to investigate the effect of Pr addition to CeO; the Ce M/M and O / ratios were calculated to indicate the relative changes in the Ce and oxygen vacancy concentration, respectively. There was no observable increase in the Ce concentration; however, the oxygen vacancy concentration increased with an increase in the Pr content. Thus, Pr increases the defect concentration and the ability of the materials to uptake oxygen.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsomega.7b00550DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6641130PMC
June 2017

Vacancy Generation and Oxygen Uptake in Cu-Doped Pr-CeO Materials using Neutron and in Situ X-ray Diffraction.

Inorg Chem 2016 Dec 2;55(24):12595-12602. Epub 2016 Dec 2.

Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), School of Chemistry, Monash University , Clayton, Victoria 3800, Australia.

The oxygen uptake ability of Pr-CeO-based oxygen carriers, catalysts, and solid oxide fuel cells can be attributed to 3+ cation generation and the presence of vacant oxygen sites. Oxygen occupancies of CeO, Pr-CeO, and 5% Cu-doped Pr-CeO were investigated using neutron diffraction and related to the oxygen uptake as determined using thermogravimetric analysis (TGA). The presence of vacant tetrahedral oxygen sites at room temperature did not correspond to low-temperature oxygen uptake. The materials did not uptake oxygen at 420 °C, but oxygen uptake was observed at 600 °C, which indicated that a minimum temperature needs to be met to generate sufficient vacancies/3+ cations. Variations in the lattice parameter as a function of temperature were revealed using in situ X-ray diffraction (XRD). With increasing temperature the lattice parameter increased linearly due to thermal expansion and was followed by an exponential increase at ∼300-400 °C as cations were reduced. Despite segregation of Cu into CuO at high dopant concentration, at 600 °C a higher O uptake was obtained for CePrCuO (120 μmol g), in comparison to CePrCuO (92 μmol g), and was higher than that for CePrO (55 μmol g). Both Pr and Cu introduce vacancies and promote the O uptake of CeO.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.6b01499DOI Listing
December 2016

Porous Polyrotaxane Coordination Networks Containing Two Distinct Conformers of a Discontinuously Flexible Ligand.

Inorg Chem 2016 Oct 26;55(20):10467-10474. Epub 2016 Sep 26.

School of Chemistry, Monash University , Clayton, Victoria 3800, Australia.

A new divergent homopiperazine-derived ligand N,N'-bis(4-carboxyphenyl)-1,4-diazacycloheptane HL has been prepared, containing a semirigid saturated heterocyclic core which is capable of providing multiple distinct bridging geometries. Reaction of HL with zinc acetate in DMSO gives a two-dimensional parallel interpenetrated polyrotaxane structure 1 in which the loops and rods are formed by the bent cis-(eq,ax) twist boat and trans-(ax,ax) twist chair conformers, respectively. By matching the distances between the solvated metal sites in the structure of 1, a related material 2 can be prepared incorporating the pillaring ligand trans-1,2-bis(4-pyridyl)ethylene bpe. Compound 2 displays a similar polyrotaxane interpenetration mode, permitted by the presence of both cis and trans ligand conformers, but displays a three-dimensional 2.6 topology related to the dia diamondoid network. The guest exchange and gas adsorption properties of both materials were investigated; while compound 1 displays poor stability to guest exchange and negligible gas uptake, the higher connectivity microporous compound 2 shows facile guest exchange and a surprisingly high CO capacity of 12 wt % at 1 bar and 273 K, and a zero-loading enthalpy of adsorption of -32 kJ mol. High-pressure adsorption isotherms also show moderate physisorption of H and CH within the material.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.6b01713DOI Listing
October 2016

Coordination Chemistry and Structural Dynamics of a Long and Flexible Piperazine-Derived Ligand.

Inorg Chem 2016 Jul 21;55(13):6692-702. Epub 2016 Jun 21.

School of Chemistry, Monash University , Clayton, Victoria 3800, Australia.

A long and highly flexible internally functionalized dipyridyl ligand α,α'-p-xylylenebis(1-(4-pyridylmethylene)-piper-4-azine), L, has been employed in the synthesis of a series of coordination polymer materials with Co(II), Cd(II), and Ag(I) ions. In poly-[Cd(L)(TPA)] 1 and poly-[Co(L)(IPA)], 2, (TPA = terephthalate, IPA = isophthalate) the ligand adopts a similar linear conformation to that seen in the structure of the unbound molecule and provides a long (2.6 nm) metal-metal bridging distance. Due to the mismatch of edge lengths with that provided by the carboxylate coligands, geometric distortions from the regular dia and (4,4) network geometries for 1 and 2, respectively, are observed. In poly-[Ag2(CF3SO3)2(L)], 3, the ligand coordinates through both pyridine groups and two of the four piperazine nitrogen donors, forming a high-connectivity 2-dimensional network. The compound poly-[Ag2(L)](BF4)2·2MeCN, 4, a porous 3-dimensional cds network, undergoes a fascinating and rapid single-crystal-to-single-crystal rearrangement on exchange of the acetonitrile guests for water in ambient air, forming a nonporous hydrated network poly-[Ag2(L)](BF4)2·2H2O, 5, in which the well-ordered guest water molecules mediate the rearrangement of the tetrafluoroborate anions and the framework itself through hydrogen bonding. The dynamics of the system are examined in greater detail through the preparation of a kinetic product, the dioxane-solvated species poly-[Ag2(L)](BF4)2·2C4H8O2, 6, which undergoes a slow conversion to 5 over the course of approximately 16 h, a transition which can be monitored in real time. The reverse transformation can also be observed on immersing the hydrate 5 in dioxane. The structural features and physical properties of each of the materials can be rationalized based on the flexible and multifunctional nature of the ligand molecule, as well as the coordination behavior of the chosen metal ions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.6b00933DOI Listing
July 2016

Ewald Summation for Molecular Simulations.

J Chem Theory Comput 2015 Aug;11(8):3684-95

Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), School of Chemistry, Monash University , Melbourne, VIC 3800, Australia.

Ewald summation is an important technique for molecular simulation. In this article, expressions are provided for implementing Ewald summation for any inverse power potential in a range of different simulations. Energies, forces, stresses, and Hessian elements as well as truncation errors are considered. Focus is also given to methods for accelerating Ewald summation in Monte Carlo simulations, particularly in the grand canonical ensemble. Ewald techniques are applied to the simulation of CO2 adsorption and diffusion in the metal-organic framework, MOF-5. These simulations show that optimized Ewald summation can provide increased accuracy at similar computational cost compared to that of pair-based methods.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jctc.5b00093DOI Listing
August 2015

Coordination polymers from a highly flexible alkyldiamine-derived ligand: structure, magnetism and gas adsorption studies.

Dalton Trans 2015 Oct 30;44(40):17494-507. Epub 2015 Jul 30.

School of Chemistry, 19 Rainforest Walk, Monash University, Clayton, VIC 3800, Australia.

The synthesis and structural, magnetic and gas adsorption properties of a series of coordination polymer materials prepared from a new, highly flexible and internally functional tetrakis-carboxybenzyl ligand H4L derived from 1,2-diaminoethane have been examined. The compound poly-[Ni3(HL)2(OH2)4]·2DMF·2H2O 1, a two-dimensional coordination polymer, contains aqua- and carboxylato-bridged trinuclear Ni(II) clusters, the magnetic behaviour of which can be well described through experimental fitting and ab initio modelling to a ferromagnetically coupled trimer with a positive axial zero-field splitting parameter D. Compound poly-[Zn2L]·2DMF·3H2O 2, a three-dimensional coordination polymer displaying frl topology, contains large and well-defined solvent channels, which are shown to collapse on solvent exchange or drying. Compound poly-[Zn2(L)(DMSO)4]·3DMSO·3H2O 3, a highly solvated two-dimensional coordination polymer, displayed poor stability characteristics, however a structurally related material poly-[Zn2(L)(bpe)(DMSO)2]·DMSO·3H2O 4 was prepared under similar synthetic conditions by including the 1,2-bis(4-pyridyl)ethylene bpe co-ligand. Compound 4, containing small one-dimensional solvent channels, shows excellent structural resilience to solvent exchange and evacuation, and the evacuated material displays selective adsorption of CO2 over N2 at 273 K in the pressure range 0-1 atm. Each of the coordination polymers displays subtle differences in the conformation and binding mode of the ligand species, with switching between two distinct conformers (X-shaped and H-shaped), as well as a variable protonation state of the central core, with significant effects on the resulting network structures and physical properties of the materials.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c5dt02323hDOI Listing
October 2015

Metal-organic frameworks as stationary phases for mixed-mode separation applications.

Chem Commun (Camb) 2014 Apr;50(28):3735-7

School of Chemistry, Monash University, VIC 3800, Australia.

Polymorphic metal-organic framework (MOF) materials offer a platform for small-scale separation of complex mixtures of polycyclic aromatic hydrocarbons (PAHs) and polar compounds. Retention factors show dependence on both analyte dimensions and polarity, suggesting mixed-mode separation, allowing complete resolution of some analytes from multi-component mixtures.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c4cc00933aDOI Listing
April 2014

Multidimensional and comprehensive two-dimensional gas chromatography of dichloromethane soluble products from a high sulfur Jordanian oil shale.

Talanta 2014 Mar 2;120:55-63. Epub 2013 Dec 2.

Australian Centre for Research on Separation Science, School of Chemistry, Monash University, Clayton, Victoria 3800, Australia. Electronic address:

A high sulfur Jordanian oil shale was converted into liquid hydrocarbons by reaction at 390 °C under N2, and the dichloromethane soluble fraction of the products was isolated then analyzed by using gas chromatography (GC). Comprehensive two-dimensional GC (GC×GC) and multidimensional GC (MDGC) were applied for component separation on a polar - non-polar column set. Flame-ionization detection (FID) was used with GC×GC for general sample profiling, and mass spectrometry (MS) for component identification in MDGC. Multidimensional GC revealed a range of thiophenes (th), benzothiophenes (bth) and small amounts of dibenzothiophenes (dbth) and benzonaphthothiophenes (bnth). In addition, a range of aliphatic alkanes and cycloalkanes, ethers, polar single ring aromatic compounds and small amounts of polycyclic aromatics were also identified. Some of these compound classes were not uniquely observable by conventional 1D GC, and certainly this is true for many of their minor constituent members. The total number of distinct compounds was very large (ca.>1000). GC×GC was shown to be appropriate for general sample profiling, and MDGC-MS proved to be a powerful technique for the separation and identification of sulfur-containing components and other polar compounds.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.talanta.2013.11.069DOI Listing
March 2014

Evaluation of comprehensive two-dimensional gas chromatography with flame photometric detection: potential application for sulfur speciation in shale oil.

Anal Chim Acta 2013 Nov 15;803:174-80. Epub 2013 Jul 15.

Australian Centre for Research on Separation Science, School of Chemistry, Wellington Road, Monash University, Clayton 3800, Australia.

Flame photometric detection in the sulfur channel has been evaluated for sulfur speciation and quantification in comprehensive two-dimensional gas chromatography [GC × GC-FPD(S)] for S-compound speciation in shale extracts. Signal non-linearity and potential quenching effects were reportedly major limitations of this detector for analysis of sulfur in complex matrices. However, reliable linear relationships with correlation coefficient >0.99 can be obtained if the sum of the square root of each modulation slice in GC × GC is plotted vs. sulfur concentration. Furthermore, the quenching effects are reduced due to essentially complete separation of S-containing components from the hydrocarbon matrix. An increase of S/N of up to 150 times has been recorded for benzothiophene and dibenzothiophene in GC × GC-FPD when compared to GC-FPD due to the modulation process. As a consequence, 10 times lower detection limits were observed in the former mode. The applicability of the method was demonstrated using shale oil sample extracts. Three sulfur classes were completely separated and the target class (thiophenes) was successfully quantified after the rest of the sample was diverted to the second detector by using a heart-cut strategy. Based on the proposed method, 70% of the sulfur in the shale oil was assigned to the thiophenes, 24% to benzothiophenes, and 5% to dibenzothiophene compounds.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.aca.2013.07.021DOI Listing
November 2013

Pyrolysis of phenethyl phenyl ether tethered in mesoporous silica. Effects of confinement and surface spacer molecules on product selectivity.

J Org Chem 2011 Aug 7;76(15):6014-23. Epub 2011 Jul 7.

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6197, USA.

There has been expanding interest in exploring porous metal oxides as a confining environment for organic molecules resulting in altered chemical and physical properties including chemical transformations. In this paper, we examine the pyrolysis behavior of phenethyl phenyl ether (PPE) confined in mesoporous silica by covalent tethers to the pore walls as a function of tether density and the presence of cotethered surface spacer molecules of varying structure (biphenyl, naphthyl, octyl, and hexadecyl). The PPE pyrolysis product selectivity, which is determined by two competitive free-radical pathways cycling through the two aliphatic radical intermediates (PhCH·CH(2)OPh and PhCH(2)CH·OPh), is shown to be significantly different from that measured in the liquid phase as well as for PPE tethered to the exterior surface of nonporous silica nanoparticles. Tailoring the pore surface with spacer molecules further alters the selectivity such that the PPE reaction channel involving a molecular rearrangement (O-C phenyl shift in PhCH(2)CH·OPh), which accounts for 25% of the products in the liquid phase, can be virtually eliminated under pore confinement conditions. The origin of this change in selectivity is discussed in the context of steric constraints on the rearrangement path inside the pores, surface and pore confinement effects, pore surface curvature, and hydrogen bonding of PPE with residual surface silanols supplemented by nitrogen physisorption data and molecular dynamics simulations.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jo200576vDOI Listing
August 2011

Partial exchange of Fe(III) montmorillonite with hexadecyltrimethylammonium cation increases catalytic activity for hydrophobic substrates.

Langmuir 2010 Mar;26(6):4258-65

Centre for Green Chemistry, Monash University, Clayton, Victoria 3800, Australia.

Fe(III) montmorillonite clay that was partially exchanged with hexadecyltrimethylammonium (HDTMA(+)) cations achieved increased catalytic activity for the oxidative coupling of hydrophobic organic substrates. A series of mixed-cation organoclays were produced, where the organic cation content ranged from 6 to 50% relative to the cation-exchange capacity (CEC) of the clay, and were tested for catalytic activity using different Fe(III)-mediated oxidative coupling reactions. Enhanced catalytic activity by Fe(3+)/HDTMA(+) montmorillonite for coupling hydrophobic substrates was observed, with maximum catalytic activity in the oxidative coupling of 2-naphthol observed at 6% HDTMA(+) coverage. However, maximum catalytic activity with a more hydrophobic substrate, anthrone, was achieved with 50% HDTMA(+) coverage, indicating that matching levels of organic modification to substrate hydrophobicity improves catalytic activity. The organization of the organic cations at the clay surfaces proved to be heterogeneous, as determined by scanning transmission X-ray microscopy (STXM) and powder X-ray diffraction. Results from molecular dynamics simulations supported the heterogeneous nature of the catalysts but also pointed toward large regions within the interlayers that may be filled with nonreactive hydrated Fe oxides resulting from the organic cation treatment. The exchangeable Fe content of the organic treated clays, as determined by AAS and ICP measurements, was observed to be higher than expected relative to that of Fe-saturated clay, substantiating this hypothesis. These findings have implications for the development of substrate-specific clay catalysts, where the composition and configuration of exchangeable cations can be matched to a particular substrate or reaction.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/la9033047DOI Listing
March 2010

Gas binding to Au13, Au12Pd, and Au11Pd2 nanoclusters in the context of catalytic oxidation and reduction reactions.

J Chem Phys 2008 Oct;129(16):164712

School of Chemistry, Monash University, Victoria 3800, Australia.

The ability of Au(13), Au(12)Pd, and Au(11)Pd(2) nanoclusters to bind species typically found in the oxidation and reduction of small hydrocarbon has been investigated by means of atom centered density functional theory calculations. Binding energies of CO(2), H(2), CO, O(2), CH(4), H(2)O, *O, *H, *CHO, *CO(2)H, and *OH have been calculated. For pure gold nanoclusters, CO(2), H(2), and CH(4) were found to not bind, and O(2) and H(2)O bound weakly with binding energies less than 15 kcal mol(-1), with the rest binding strongly with binding energies in the range 26-68 kcal mol(-1). Binding additional gas molecules did not greatly reduce the binding energy. Adding palladium to the clusters created binding sites for all of the test gases. Binding to the palladium atom generally increased the binding energy of molecules but decreased the binding energy of radicals.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.2993252DOI Listing
October 2008

Confinement effects on product selectivity in the pyrolysis of phenethyl phenyl ether in mesoporous silica.

Chem Commun (Camb) 2007 Jan 24(1):52-4. Epub 2006 Oct 24.

Chemical Sciences Division, Oak Ridge National Laboratory, Bethel Valley Road, Oak Ridge, Tennessee 37831-6197, USA.

Pyrolysis of phenethyl phenyl ether confined in mesoporous silicas by covalent grafting results in significantly increased product selectivity compared with fluid phases.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/b613757aDOI Listing
January 2007

Pyrolysis of mesoporous silica-immobilized 1,3-diphenylpropane. Impact of pore confinement and size.

J Am Chem Soc 2005 May;127(17):6353-60

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6197, USA.

Mesoporous silicas such as SBA-15 and MCM-41 are being actively investigated for potential applications in catalysis, separations, and synthesis of nanostructured materials. A new method for functionalizing these mesoporous silicas with aromatic phenols is described. The resulting novel hybrid materials possess silyl aryl ether linkages to the silica surface that are thermally stable to ca. 550 degrees C, but can be easily cleaved at room temperature with aqueous base for quantitative recovery of the organic moieties. The materials have been characterized by nitrogen physisorption, FTIR, NMR, and quantitative analysis of surface coverages. The maximum densities of 1,3-diphenylpropane (DPP) molecules that could be grafted to the surface were less than those measured on a nonporous, fumed silica (Cabosil) and were also found to decrease as a function of decreasing pore size (5.6-1.7 nm). This is a consequence of steric congestion in the pores that is magnified at the smaller pore sizes, consistent with parallel studies conducted using a conventional silylating reagent, 1,1,3,3-tetramethyldisilazane. Pyrolysis of the silica-immobilized DPP revealed that pore confinement leads to enhanced rates and altered product selectivity for this free-radical reaction compared with the nonporous silica, and the rates and selectivities also depended on pore size. The influence of confinement is discussed in terms of enhanced encounter frequencies for bimolecular reaction steps and pore surface curvature that alters the accessibility and resultant selectivity for hydrogen transfer steps.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/ja050389wDOI Listing
May 2005