Impact of the Local Concentration of Hydronium Ions at Tungstate Surfaces for Acid-Catalyzed Alcohol Dehydration.

Tungstate domains supported on ZrO2, Al2O3, TiO2, and activated carbon drastically influence the hydronium-ion-catalyzed aqueous-phase dehydration of alcohols. For all catalysts, the rate of cyclohexanol dehydration normalized to the concentration of Brønsted acid sites (turnover frequencies, TOFs) was lower for monotungstates than for polytungstates and larger crystallites of WO3. TOFs were constant when reaching or exceeding the monolayer coverage of tungstate, irrespective of the specific nature of surface structures that continuously evolve with the surface W loading. However, the TOFs with polytungstates and large WO3 crystallites depend strongly on the underlying support (e.g., WOx/C catalysts are 10-50-fold more active than WOx/Al2O3 catalysts). The electrical double layer (EDL) surrounding the negatively charged WOx domains contains hydrated hydronium ions, whose local concentrations change with the support. This varying concentration of interfacial hydronium ions ("local ionic strength") impacts the excess chemical potential of the reacting alcohols and induces the marked differences in the TOFs. Primary H/D kinetic isotope effects (∼3), together with the substantially positive entropy of activation (111-195 J mol-1 K-1), indicate that C-H(D) bond cleavage is involved in the kinetically relevant step of an E1-type mechanistic sequence, regardless of the support identity. The remarkable support dependence of the catalytic activity observed here for the aqueous-phase dehydration of cycloalkanols likely applies to a broad set of hydronium-ion-catalyzed organic reactions sensitive to ionic strength.

In Situ Identification of Reaction Intermediates and Mechanistic Understandings of Methane Oxidation over Hematite: A Combined Experimental and Theoretical Study.

Effective methane utilization for either clean power generation or value-added chemical production has been a subject of growing attention worldwide for decades, yet challenges persist mostly in relation to methane activation under mild conditions. Here, we report hematite, an earth-abundant material, to be highly effective and thermally stable to catalyze methane combustion at low temperatures (<500 °C) with a low light-off temperature of 230 °C and 100% selectivity to CO2. The reported performance is impressive and comparable to those of precious-metal-based catalysts, with a low apparent activation energy of 17.60 kcal·mol-1. Our theoretical analysis shows that the excellent performance stems from a tetra-iron center with an antiferromagnetically coupled iron dimer on the hematite (110) surface, analogous to that of the methanotroph enzyme methane monooxygenase that activates methane at ambient conditions in nature. Isotopic oxygen tracer experiments support a Mars van Krevelen redox mechanism where CH4 is activated by reaction with a hematite surface oxygen first, followed by a catalytic cycle through a molecular-dioxygen-assisted pathway. Surface studies with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations reveal the evolution of reaction intermediates from a methoxy CH3-O-Fe, to a bridging bidentate formate b-HCOO-Fe, to a monodentate formate m-HCOO-Fe, before CO2 is eventually formed via a combination of thermal hydrogen-atom transfer (HAT) and proton-coupled electron transfer (PCET) processes. The elucidation of the reaction mechanism and the intermediate evolutionary profile may allow future development of catalytic syntheses of oxygenated products from CH4 in gas-phase heterogeneous catalysis.

Importance of Methane Chemical Potential for Its Conversion to Methanol on Cu-Exchanged Mordenite.

Copper-oxo clusters exchanged in zeolite mordenite are active in the stoichiometric conversion of methane to methanol at low temperatures. Here, we show an unprecedented methanol yield per Cu of 0.6, with a 90-95 % selectivity, on a MOR solely containing [Cu3 (µ-O)3 ]2+ active sites. DFT calculations, spectroscopic characterization and kinetic analysis show that increasing the chemical potential of methane enables the utilization of two µ-oxo bridge oxygen out of the three available in the tricopper-oxo cluster structure. Methanol and methoxy groups are stabilized in parallel, leading to methanol desorption in the presence of water.

Importance of Methane Chemical Potential for Its Conversion to Methanol on Cu-exchanged Mordenite.

Invited for the cover of this issue is the collaborative team of researchers from TU Munich, PNNL and TU Delft. Read the full text of the article at 10.1002/chem.202000772.

H-Transfer reactions of internal alkenes with tertiary amines as H-donors on carbon supported noble metals.

A hydride transfer reaction with tertiary amines was observed in the presence of noble metals on a carbon support. Hydride transfer had been documented previously in terms of activated allyl-type carbon-carbon double bonds containing carbonyl derivatives in the presence of triethyl amine (conjugate reduction). The proposed mechanism is a hydride transfer reaction in which the metal serves as the reaction partner of the hydrido-metal iminium adduct formation. The saturation of a non-activated internal double bond containing compound, such as methyl oleate and trans-5-decene as substrates, was observed for the first time in this work. The pre-reduced catalyst samples showed high activity; in the presence of Pd/C, Pt/C and Rh/C partial to complete conversion was detected at 140 °C in a p-xylene solvent without molecular hydrogen. Higher molecular weight byproducts of the amines were formed, while in the case of the substrates negligible amounts of unreacted but double bond migrated species were present. There is a possibility of usage of alkyl amines other than triethylamine; thus use of tributyl-, tripentyl-, trihexylamine and N,N-diisopropylethylamine, as well as cyclic 1-ethylpyrrolidine and 1-ethylpiperidine, was investigated. Cyclic amines and diisopropyl derivatives as H sources produced the highest conversion, while amines with longer alkyl chains showed minor activity. As a clear indication of H-donation, the formation of unsaturated amine species such as 1-ethyl-pyrrole and pyridine was observed.

Role of Spatial Constraints of Brønsted Acid Sites for Adsorption and Surface Reactions of Linear Pentenes.

The Brønsted acid sites of H-ZSM-5 and ferrierite reversibly adsborb linear pentenes via hydrogen bonding, rapidly isomerizing the double bond. On H-ZSM-5, dimerization of adsorbed pentenes occurs at a slower rate and leads to pentyl ester covalently bound to the surface. Pentene adsorbed on zeolites with narrower pores, such as ferrierite, remained stable in a hydrogen-bonded state even up to 423 K. Comparing the differential heat of adsorption of 2-pentene on silicalite and ferrierite allowed for the determination of the enthalpy difference between physically adsorbed pentene in ZSM-5 and the localized hydrogen-bonded π-complex at Brønsted acid sites, -36 kJ/mol. The activation energy (35 kJ/mol) for dimerization is almost identical to this enthalpy difference, suggesting that the rate-determining step is associated either with the mobilization of π-bonded 2-pentene or with the equally large activation barrier to form an alkoxy group via a carbenium-ion transition state. In a closed system, the dimerization rate is first order in the concentration of the π-complex that is both in equilibrium with the mobile pentene phase and in production of the carbenium ion that reacts with the mobile pentene. Overall, the alkoxy group is -41 ± 7 kJ/mol more stable than physisorbed pentene, establishing a series of energetically well-separated groups of reactive surface species.

Fundamental Role of Oxygen Stoichiometry in Controlling the Band Gap and Reactivity of Cupric Oxide Nanosheets.

CuO is a nonhazardous, earth-abundant material that has exciting potential for use in solar cells, photocatalysis, and other optoelectronic applications. While progress has been made on the characterization of properties and reactivity of CuO, there remains significant controversy on how to control the precise band gap by tuning conditions of synthetic methods. Here, we combine experimental and theoretical methods to address the origin of the wide distribution of reported band gaps for CuO nanosheets. We establish reaction conditions to control the band gap and reactivity via a high-temperature treatment in an oxygen-rich environment. SEM, TEM, XRD, and BET physisorption reveals little to no change in nanostructure, crystal structure, or surface area. In contrast, UV-vis spectroscopy shows a modulation in the material band gap over a range of 330 meV. A similar trend is found in H2 temperature-programmed reduction where peak H2 consumption temperature decreases with treatment. Calculations of the density of states show that increasing the oxygen to copper coverage ratio of the surface accounts for most of the observed changes in the band gap. An oxygen exchange mechanism, supported by (18)O2 temperature-programmed oxidation, is proposed to be responsible for changes in the CuO nanosheet oxygen to copper stoichiometry. The changes induced by oxygen depletion/deposition serve to explain discrepancies in the band gap of CuO, as reported in the literature, as well as dramatic differences in catalytic performance.

Adsorption of multimeric T cell antigens on carbon nanotubes: effect on protein structure and antigen-specific T cell stimulation.

Antigen-specific activation of cytotoxic T cells can be enhanced up to three-fold more than soluble controls when using functionalized bundled carbon nanotube substrates ((b) CNTs). To overcome the denaturing effects of direct adsorption on (b) CNTs, a simple but robust method is demonstrated to stabilize the T cell stimulus on carbon nanotube substrates through non-covalent attachment of the linker neutravidin.

Controlling the particle size of ZrO2 nanoparticles in hydrothermally stable ZrO2/MWCNT composites.

The composite of multiwalled carbon nanotubes (MWCNTs) decorated with ZrO(2) nanoparticles, synthesized by a grafting method followed by high-temperature annealing, was studied. The oxygen functionalized MWCNT surface uniformly disperses and stabilizes the oxide nanoparticles to an extent that is controlled by the metal oxide loading and thermal annealing temperature. This ZrO(2)/MWCNT also withstands decomposition in a hydrothermal environment providing potential applications in the catalysis of biomass conversion (e.g., aqueous phase reforming). The ZrO(2)/MWCNT have been characterized by (scanning) transmission electron microscopy ((S)TEM), X-ray diffraction (XRD), in situ small-angle X-ray scattering (SAXS), in situ wide-angle X-ray scattering (WAXS), and near edge X-ray fine structure (NEXAFS) for the purpose of a comprehensive analysis of the ZrO(2) particle size and particle size stability.

Role of the ionic environment in enhancing the activity of reacting molecules in zeolite pores.

Tailoring the molecular environment around catalytically active sites allows for the enhancement of catalytic reactivity through a hitherto unexplored pathway. In zeolites, the presence of water creates an ionic environment via the formation of hydrated hydronium ions and the negatively charged framework aluminum tetrahedra. The high density of cation-anion pairs determined by the aluminum concentration of a zeolite induces a high local ionic strength that increases the excess chemical potential of sorbed and uncharged organic reactants. Charged transition states (carbocations for example) are stabilized, which reduces the energy barrier and leads to higher reaction rates. Using the intramolecular dehydration of cyclohexanol on H-MFI zeolites in water, we quantitatively show an enhancement of the reaction rate by the presence of high ionic strength as well as show potential limitations of this strategy.

Selective synthesis of subnanometer diameter semiconducting single-walled carbon nanotubes.

Subnanometer single-walled carbon nanotubes (sub-nm SWNTs) were synthesized at different temperatures (600, 700, and 800 degrees C) using CoMn bimetallic catalysts supported on MCM-41 silica templates. The state of the catalyst was investigated using X-ray absorption, and the (n,m) indices of the sub-nm SWNTs were determined from Raman spectroscopy and photoluminescence measurements. We find that the size of the metallic particles that seed the growth of sub-nm SWNTs (diameter approximately 0.5-1.0 nm) is highly sensitive to the reaction temperature. Low reaction temperature (600 degrees C) favors the growth of semiconducting tubes whose diameters range from 0.5 to 0.7 nm. These results were also confirmed by electrical transport measurements. Interestingly, dominant intermediate frequency modes on the same intensity scale as the Raman breathing modes were observed. An unusual "S-like" dispersion of the G-band was present in the Raman spectra of sub-nm SWNTs with diameters <0.7 nm.

Clustering of stimuli on single-walled carbon nanotube bundles enhances cellular activation.

Functionalized single-walled carbon nanotube bundles (f-bSWNT) adsorbed with T-cell-stimulating antibodies are shown to enhance both the kinetics and magnitude of T cell stimulation compared to the same concentration of free antibodies in solution. This enhancement is unique to f-bSWNT compared to other artificial substrates with high surface area and similar chemistry. We explored the origins of this enhanced activity with FRET microscopy and found the preferential formation of large antibody stimuli clusters (5 to 6 microm) on the surface of functionalized versus untreated nanotubes. This highlights the important aspect that antigen clusters can be formed on f-bSWNT, impacting the potency of the T cell stimulus. Clustering of T cell antigens on artificial substrates impacts the avidity of interaction with cells facilitating rapid stimulation dynamics and an overall greater magnitude of response. These findings support the use of chemically treated nanotube bundles as an efficient substrate for the presentation of antigens and point to their potential in clinical applications involving artificial antigen-presentation for ex vivo T cell expansion in adoptive immunotherapy.

Controlling of physicochemical properties of nickel-substituted MCM-41 by adjustment of the synthesis solution pH and tetramethylammonium silicate concentration.

The effect of initial synthesis solution pH and tetramethylammonium silicate concentration in the synthesis solution on the physical and chemical properties of MCM-41 was systematically investigated using N(2) physisorption, X-ray diffraction, temperature-programmed reduction, in situ Fourier transform IR, UV-vis, and X-ray absorption spectroscopies. pH and tetramethylammonium (TMA) fraction affect the porosity of MCM-41 and the reducibility of incorporated Ni cations; higher pH and TMA concentration produced more porosity with higher stability against reduction, which is attributed to more metal ions locating in the interior of the silica walls. The control of the pore diameter of mesoporous MCM-41 at the sub-nanometer scale may be accomplished by adjusting the pH and TMA fraction. pH may be used to control the surface free silanol group density and nickel reduction degree as well, and this is useful in the design of a specific catalyst for particular reactions, such as CO methanation, which requires highly dispersed, stable metallic clusters with controllable size.

Pore curvature effect on the stability of Co-MCM-41 and the formation of size-controllable subnanometer Co clusters.

Samples of Co-MCM-41 with different pore diameters have been synthesized using organic templates with different alkyl chain lengths. The reducibility of cobalt in these highly stable samples was investigated by TPR and X-ray absorption spectroscopy. We have found that the reducibility correlates strongly with the pore diameter of the MCM-41, with the cobalt incorporated in the smaller pore MCM-41 being more resistant to complete reduction. It is proposed that the distribution of cobalt ions in the pore wall is affected by both the preparation procedure and the pore diameter. The size of the metallic Co clusters formed after different reducing treatments correlates linearly with the pore size, giving direct evidence for the effect of the radius of curvature on reducibility. Complete cobalt reduction after TPR causes an inverse variation of the cluster size with the pore size, resulting from differences in the density of Co clusters and from differences in the rate of Co migration and aggregation outside the pores of MCM-41 with different pore sizes.

Synthesis, characterization, and stability of Fe-MCM-41 for production of carbon nanotubes by acetylene pyrolysis.

Fe-substituted MCM-41 molecular sieves with ca. 1, 2, and 3 wt % Fe were synthesized hydrothermally using different sources of colloidal silica (HiSil and Cab-O-Sil) and characterized by ICP, XRD, N2 physisorption, UV-vis, EPR, TPR, and X-ray absorption. Catalysts synthesized from Cab-O-Sil showed higher structural order and stability than those from HiSil. The local environment of Fe in the mesoporous material as studied by UV-vis reveals the dominance of framework Fe in all the as-synthesized Fe-MCM-41 samples. Dislodgement of some Fe species to extraframework location occurs upon calcination, and this effect is more severe for Fe-MCM-41 (2 wt %) and Fe-MCM-41 (3 wt %), as confirmed by EPR and X-ray absorption. These materials have been used as catalytic templates for the production of carbon nanotubes (CNTs) by acetylene pyrolysis at atmospheric pressure. A relationship between the Fe loading in MCM-41 and the carbon species produced during this reaction has been established. Using our optimized conditions for this system, Fe-MCM-41 with ca. 2 wt % Fe showed the best results with particularly high selectivity for single-wall carbon nanotube (SWNT) production. This catalyst was selective for carbon nanotubes with a low amount of amorphous carbon for a narrow range of temperatures from 1073 to 1123 K. To account for the different selectivity of these catalysts for CNTs production, the local environment and chemical state of Fe in the used catalyst was further probed by X-band EPR.

Synthesis and characterization of highly ordered Ni-MCM-41 mesoporous molecular sieves.

Highly ordered Ni-MCM-41 samples with nearly atomically dispersed nickel ions were prepared reproducibly and characterized. Similar to the Co-MCM-41 samples, the pore diameter and porosity can be precisely controlled by changing the synthesis surfactant chain length. Nickel was incorporated by isomorphous substitution of silicon in the MCM-41 silica framework, which makes the Ni-MCM-41 a physically stable catalyst in harsh reaction conditions such as CO disproportionation to single wall carbon nanotubes or CO2 methanation. X-ray absorption spectroscopy results indicate that the overall local environment of nickel in Ni-MCM-41 was a tetrahedral or distorted tetrahedral coordination with surrounding oxygen anions. Hydrogen TPR revealed that our Ni-MCM-41 samples have high stability against reduction; however, compared to Co-MCM-41, the Ni-MCM-41 has a lower reduction temperature, and both the H2-TPR and in situ XANES TPR reveal that the reducibility of nickel is not clearly correlated with the pore radius of curvature, as in the case of Co-MCM-41. This is probably a result of nickel being thermodynamically more easily reduced than cobalt. The stability of the structural order of Ni-MCM-41 has been investigated under SWNT synthesis and CO2 methanation reaction conditions as both require catalyst exposure to reducing environments leading to formation of metallic Ni clusters. Nitrogen physisorption and XRD results show that structural order was maintained under both SWNT synthesis and CO2 methanation reaction conditions. EXAFS results demonstrate that the nickel particle size can be controlled by different prereduction temperatures but not by the pore radius of curvature as in the case of Co-MCM-41.

X-ray absorption spectroscopic investigation of partially reduced cobalt species in Co-MCM-41 catalysts during synthesis of single-wall carbon nanotubes.

Chemometric tools were employed to analyze the in-situ dynamic X-ray absorption spectroscopy data to probe the state of Co-MCM-41 catalysts during reduction in pure hydrogen and under single-wall carbon nanotube synthesis reaction conditions. The use of the progressive correlation analysis established the sequence in which changes in the spectral features near the Co K edge occurred, and the evolving factor analysis provided evidence for the formation of an intermediate Co(1+) ionic species during reduction of the Co-MCM-41 catalyst in pure hydrogen up to 720 degrees C. This intermediate species preserves the tetrahedral environment in the silica framework and is resistant to complete reduction to the metal in H(2). While the Co(2+) species is resistant to reduction in pure CO, the intermediate Co(1+) species is more reactive in CO most likely forming cobalt carbonyl-like compounds with high mobility in the MCM-41. These mobile species are the precursors of the metallic clusters growing carbon nanotubes. Controlling the rates of each step of this two-stage reduction process is key to controlling the size of the metallic Co clusters formed in Co-MCM-41 catalysts.

New approach to avoid erroneous interpretation of results derived from generalized two-dimensional correlation analysis for applications in catalysis.

Surface characterization and catalysis can significantly benefit from the application of generalized two-dimensional (2D) correlation analysis. This two-dimensional approach allows a better resolution of overlapping peaks, can reveal new features not readily observable in the raw spectra, gives clear evidence for spectral intensities that change as an effect of a perturbation applied to the system, and allows the establishment of time sequences for the changes occurring in different spectral features of interest for determining reaction intermediates and/or mechanisms. The interpretation of the synchronous and asynchronous plots was observed to lead to erroneous time sequences when spectral features change in a non-monotonic way, such as a biphasic or oscillatory behavior, under the influence of a perturbation. We propose a new approach to the 2D correlation analysis to avoid misinterpretation of the results calculated in the asynchronous plot. Progressive correlation analysis (ProCorA) calculates the synchronous plot from the first two spectra of the data matrix and one spectrum is added at every step of the analysis. The sequence of changes can be set up from the progressive evolution of peaks in both the synchronous and asynchronous plots.

A carbon nanotube-polymer composite for T-cell therapy.

Clinical translation of cell therapies requires strategies that can manufacture cells efficiently and economically. One promising way to reproducibly expand T cells for cancer therapy is by attaching the stimuli for T cells onto artificial substrates with high surface area. Here, we show that a carbon nanotube-polymer composite can act as an artificial antigen-presenting cell to efficiently expand the number of T cells isolated from mice. We attach antigens onto bundled carbon nanotubes and combined this complex with polymer nanoparticles containing magnetite and the T-cell growth factor interleukin-2 (IL-2). The number of T cells obtained was comparable to clinical standards using a thousand-fold less soluble IL-2. T cells obtained from this expansion were able to delay tumour growth in a murine model for melanoma. Our results show that this composite is a useful platform for generating large numbers of cytotoxic T cells for cancer immunotherapy.

Chiral-selective CoSO4/SiO2 catalyst for (9,8) single-walled carbon nanotube growth.

Electronic and optical properties of single-walled carbon nanotubes (SWCNTs) correlate with their chiral structures. Many applications need chirally pure SWCNTs that current synthesis methods cannot produce. Here, we show a sulfate-promoted CoSO(4)/SiO(2) catalyst, which selectively grows large-diameter (9,8) nanotubes at 1.17 nm with 51.7% abundance among semiconducting tubes and 33.5% over all tube species. After reduction in H(2) at 540 °C, the catalyst containing 1 wt % Co has a carbon yield of 3.8 wt %, in which more than 90% is SWCNT. As compared to other Co catalysts used for SWCNT growth, the CoSO(4)/SiO(2) catalyst is unique with a narrow Co reduction window under H(2) centered at 470 °C, which can be attributed to the reduction of highly dispersed CoSO(4). X-ray absorption spectroscopy (XAS) results suggested the formation of Co particles with an average size of 1.23 nm, which matches the diameter of (9,8) tubes. Density functional theory study indicated that the diameter of structurally stable pure Co particles is scattered, matching the most abundant chiral tubes, such as (6,5) and (9,8). Moreover, the formation of such large Co particles on the CoSO(4)/SiO(2) catalyst depends on sulfur in the catalyst. XAS results showed that sulfur content in the catalyst changes after catalyst reduction at different conditions, which correlates with the change in (n,m) selectivity observed. We proposed that the potential roles of sulfur could be limiting the aggregation of Co atoms and/or forming Co-S compounds, which enables the chiral selectivity toward (9,8) tubes. This work demonstrates that catalysts promoted with sulfur compounds have potentials to be further developed for chiral-selective growth of SWCNTs.