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The production of olefins via on-purpose dehydrogenation of alkanes allows for a more efficient, selective and lower cost alternative to processes such as steam cracking. Silica-supported pincer-iridium complexes of the form [(≡SiO-R4POCOP)Ir(CO)] (R4POCOP = κ3-C6H3-2,6-(OPR2)2) are effective for acceptorless alkane dehydrogenation, and have been shown stable up to 300 °C. However, while solution-phase analogues of such species have demonstrated high regioselectivity for terminal olefin production under transfer dehydrogenation conditions at or below 240 °C, in open systems at 300 °C, regioselectivity under acceptorless dehydrogenation conditions is consistently low. In this work, complexes [(≡SiO-tBu4POCOP)Ir(CO)] (1) and [(≡SiO-iPr4PCP)Ir(CO)] (2) were synthesized via immobilization of molecular precursors. These complexes were used for gas-phase butane transfer dehydrogenation using increasingly sterically demanding olefins, resulting in observed selectivities of up to 77%. The results indicate that the active site is conserved upon immobilization.

Pincer-ligated iridium complexes of the form [(R4PCP)IrL] (R4PCP = κ3-C6H3-2,6-(XPR2)2; X = CH2, O; R = tBu, iPr) have previously been shown competent for acceptorless alkane dehydrogenation when s...

While several metal phosphides have attracted significant attention in the last several years due to their potential use as photocatalytic and hydrotreating catalysts, iridium phosphide has remained largely unexplored. In this work, silica-supported pincer-iridium species are thermolyzed, resulting in deconstruction of the tridentate ligand precursor and formation of a sub-nanometer iridium phosphide phase characterized by 31P magic angle spinning nuclear magnetic resonance (31P-MAS-NMR), X-ray absorption spectroscopy (XAS), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The support material was found to play an active role in determining the product of the surface thermolysis, with the silica supported material generating phosphorus rich iridium phosphide nanoparticles. The resulting silica-supported iridium phosphide phase is explored as a thermocatalyst for non-oxidative butane dehydrogenation, achieving high initial reaction rates up to 900 molbutenes molcatalyst-1 hr-1 and a terminal olefin selectivity of up to 70 %.

Supported MOx (M= Mo, V) on mixed CeO2-TiO2 were investigated for the oxidative dehydrogenation of ethane (ODHE) using CO2 as a mild oxidant. Raman spectroscopic characterization of the synthesized catalysts under dehydrated conditions suggest that surface MoOx species prefer to anchor on the crystalline domains of TiO2. Upon increasing the amount of CeO2 in the mixed oxide support, an intense and broad band at ~930cm-1 underscored that the prevalent species tend to be polymeric (MoOx)n domains. On the other hand, in the case of VOx catalysts, a gradual shift in the symmetric stretching of the vanadyl (V=O) Raman band with increasing CeO2 content was observed that points at the gradual anchoring of the surface vanadia species on both TiO2 and CeO2 thus highlighting the possible existence of the amorphous VOx to be located at the interface of the two mixed oxides. The catalytic behavior of Mo and V were distinct. As the ceria content in the support increased, MoOx catalysts promoted the ODHE via Mars van Krevelen mechanism while VOx catalysts appeared to favor ethane direct dehydrogenation. Investigation of structure-function relationships via in-situ Raman spectroscopic efforts revealed that adding ceria not only changed the redox properties of the support but also improved those of the deposited metal oxide. We also show that upon incorporation of ceria into the support, CO2 directly participates in the reoxidation of the dispersed MoOx species during catalysis. This effect was distinct from the reaction of CO2 in the reverse water gas shift reaction. Operando Raman spectra revealed that the presence of CO2 enhances the stability of the bridging Mo–O–Mo bond of polymeric molybdena domains, which is proposed to affect the relative contribution of oxidative versus non-oxidative pathways in ethane dehydrogenation.

Commercial anatase phase titanium dioxide was annealed under various gases (hydrogen, nitrogen, argon, and air) to induce the formation of defects. While annealing in the absence of oxygen there was a notable increase in the concentration of paramagnetic defects as measured by Electron Paramagnetic Resonance (EPR) and X-ray Photoelectron Spectroscopy (XPS). The presence of these defects increased the metal-free photocatalytic activity of the samples towards hydrogen evolution from photocatalytic methane steam reforming (MSR) under UV illumination. Catalyst activity was stable for over 42 h while illuminated owing to the regeneration of Ti3+ defects by UV photoexcitation, but rapidly decayed in the dark. The high concentration of unique Ti3+ defect sites generated during annealing catalyze hydrogen evolution, avoiding the need for precious metal cocatalysts, while anatase lacking these defects is inactive. This work shows that the implementation of defect-rich anatase TiO2 provides new catalytic pathways for hydrogen generation from photocatalytic methane steam reforming.

The methane steam reforming (MSR) mechanism on the anatase TiO2 (101) surface has been investigated using periodic density functional theory. Reaction energies for several pathways were calculated, and pathways involving high energy intermediates were eliminated from consideration. The remaining pathways all involved formaldehyde and formyl as intermediates. Activation energy barriers were calculated for these, revealing that C–H activation in methane and elementary dehydrogenation reactions in general involved very high energy transition states when calculated on clean surfaces. When water was coadsorbed on reaction surfaces as chemisorbed OH and H, hydrogen transfer reactions were facilitated by the proximity between hydrogen donor and acceptor and the strong affinity of hydroxyl for hydrogen. As the product of such hydrogen transfer reactions was physisorbed water, water activation completes the catalytic cycle, and the role of water can be termed cocatalytic. Cocatalytic water lowered the most difficu...

The water gas shift reaction (WGS) mechanism on the anatase TiO2 (101) surface has been investigated using periodic density functional theory. Reaction energies and activation energy barriers for direct CO oxidation and associative pathways with carboxyl and formate intermediates were calculated and compared. Unassisted by water, the formation of formate was infeasible due to the difficulty in transferring hydrogen from the anatase surface to hydrogenate CO, while the carboxyl mechanism had the lowest apparent activation energy barrier among the three. Coadsorbed water species were found to be effective hydrogen transfer cocatalysts, with molecularly physisorbed water acting as a hydrogen donor and dissociatively chemisorbed water acting as a hydrogen acceptor. Using water as the hydrogen donor, CO hydrogenation became competitive with the carboxyl mechanism, indicating that the reactivity of water could change the relative competitiveness of different WGS pathways. These results suggest that adsorbed wat...

Periodic, self-consistent generalized gradient approximation (GGA PW-91) density functional theory (DFT) was applied to study the effect of tin coverage in platinum–tin alloys in the dehydrogenation of ethane. Light alkane dehydrogenation to olefins can add significant value to hydrocarbon processes that generate ethane and propane by converting low value commodity fuels to high-value chemical and polymer precursors. Supported Pt catalysts are known to be active but show significant coke formation and deactivation, which can be reduced by alloying with Sn. Model surfaces of substitutional surface alloys of Pt and Sn were constructed with 1/4 and 1/2 of a monolayer of Pt atoms in the surface of fcc Pt(111) replaced by Sn atoms. Potential energy surfaces for all C1 and C2 derivatives from C–H and C–C bond cleavage from ethane were computed with kinetic reaction barriers obtained using the climbing-image nudged elastic band method. While the presence of Sn weakened the binding energies of all species, the ef...

The reactions of 1-pentene over acid zeolites were investigated in the liquid phase at 473 K. The primary reactions were isomerization, dimerization, and subsequent cracking of dimers. Zeolites consisting of only 10-membered (MFI) or 12-membered rings (FAU, BEA) behaved similarly, with dimerization and subsequent cracking products observed. Zeolites possessing 8-membered rings (MOR, FER) showed very different selectivity from each other and from other zeolites. MOR showed almost complete conversion of C 10 olefins, such that hexene and butene from cracking were the dominant products. FER showed high activity and selectivity for dimerization, with very small amounts of cracking products observed.

The oligomerization of propene was investigated over H-MFI zeolites with varying Si/Al ratios. For a constant space time per active site, the conversion of propene as well as the selectivity to products of different carbon number was affected by the density of the sites within the zeolite. In particular, as the Si/Al ratio decreased, corresponding to an increase in site proximity, the rate of oligomerization per site decreased but the selectivity to dimers relative to cracking products increased. These effects were shown to arise from the effects of molecular crowding on the rate coefficient for propene trimer formation and were confirmed by quantum chemical analysis of the energetics of propene oligomerization. It was found that the activation for propene dimerization is unaffected by the presence of oligomers on nearby sites, but the activation energy for propene trimerization relative to desorption of hexene increases by 19 kcal mol−1 when two next nearest neighbor sites are occupied by oligomers. In situ IR spectroscopy observations showed the buildup of aromatic species with time-on-stream. The accumulation of these species increases with decreasing Si/Al ratio, suggesting that increasing proximity of Bronsted-acid sites enhances the formation of aromatic species.

Heterogeneous rhodium catalysts supported on SiO2 were modified with PPh3 for the gas-phase hydroformylation of propene to produce n- and isobutanal. High selectivity to aldehydes was achieved, with no propane or alcohols formed. Investigation of the effects of reaction temperature, reactant partial pressures, total pressure, and PPh3/Rh ratio suggested that the supported catalyst behaved similarly to the homogeneous catalyst. In particular, the supported catalyst showed similar activation energies and partial and total pressure dependences of the reaction rates to those observed in homogeneous, liquid-phase reactions. The first order dependence of the hydroformylation rate on the partial pressures of propene, CO, and H2 individually led to a cubic dependence of butanal formation on total pressure for equimolar reactant mixtures. High regioselectivity with a typical n/i ratio of 14 was achieved.

In situ IR spectroscopy was used to observe the intermediates formed on zeolites FAU and MFI during the synthesis of methyl methoxyacetate (MMAc) via carbonylation of dimethoxymethane (DMM) and the disproportionation of DMM to dimethyl ether (DME) and methyl formate (MF). Both reactions are initiated by the reaction of DMM with the Bronsted acid protons of the zeolite to form methanol and methoxymethoxy groups (MMZ). The latter species then undergoes one of two processes – carbonylation to form methoxyacetyl species, the precursors to MMAc, or reaction with DMM, resulting in DMM disproportionation. Surface intermediates for both DMM carbonylation and disproportionation respond to changes in reaction conditions in a manner consistent with observed steady-state kinetics. DMM carbonylation occurred more rapidly in the presence than absence of physisorbed DMM, a phenomenon attributed to solvation of the carbocationic transition state involved in the addition of CO to MMZ predicted by DFT calculations. The surface concentration of the methoxyacetyl species at steady state was 10 times smaller on FAU than on MFI, consistent with the higher rate of DMM carbonylation on FAU. Rate expressions for the formation of each product, based on the proposed mechanisms, in combination with a suitable set of rate coefficients, give a good description of the experimentally observed dependences of the rates of product formation on temperature and the feed partial pressures of CO and DMM.

This work reports on the effects of zeolite framework type and Si/Al ratio on the carbonylation of dimethoxymethane (DMM) to produce methyl methoxyacetate (MMAc). Faujasite (FAU), ZSM-5 (MFI), Mordenite (MOR) and Beta (BEA) showed very similar activity for DMM carbonylation. However, FAU had a very high selectivity to MMAc compared to MFI, MOR and BEA because of very low rates of dimethyl ether (DME) and methyl formate (MF) formation, by-products of the disproportionation of DMM. The high rate of DMM disproportionation observed for MFI, MOR and BEA is ascribed to the small pores of these zeolites, which facilitate a critical initial step in the formation of DME and MF. FER showed very low activity for both carbonylation and disproportionation. Increasing the Si/Al ratio for both FAU and MFI led to an increase in the turnover frequency for DMM carbonylation. It is proposed that the low rate of MMAc formation found at low Si/Al ratios is due to repulsive interactions occurring between adsorbed species located within the same supercage (FAU) or channel intersection (MFI).

Because the adrenal glands are common locations for metastases, pheochromocytoma is frequently misdiagnosed as adrenal metastasis in patients with a history of cancer. An incidental adrenal mass was detected during an abdominal computed tomography (CT) scan performed to stage the nasopharyngeal carcinoma in a 35-year-old male patient. The features of an adrenal mass on the CT, magnetic resonance imaging (MRI), and fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) were thought to show adrenal metastasis. However, the patient did not complain about flushing, palpitation, headache or excessive sweating. His blood pressure was 132/74 mmHg, and his pulse rate was 82 bpm. A pheochromocytoma was found during a biochemical diagnosis that evaluated the catecholamine in urine collected over a 24-hour period. The urine had elevated urinary adrenaline, metanephrine, and vanillylmandelic. An I123 MIBG scan showed avid tracer uptake in the right adrenal mass with no evidence of abnormal uptake elsewhere. A right adrenalectomy operation was performed and a diagnosis of pheochromocytoma was confirmed histopathologically. Incidental adrenal masses detected in the presence history of cancer should always be subjected to hormonal evaluation. Although patients may be asymptomatic, the probability of incidental pheochromocytoma should not be ignored.

Carbonylation gets a phase lift: The usual liquid-phase, high-pressure processes for carbonylating formaldehydes are avoided in a novel vapor-phase reaction. Using an acid zeolite (Faujasite) at near-atmospheric pressure dimethoxymethane (DMM; the dimethyl acetal of formaldehyde; see scheme) is carbonylated to produce methyl methoxyacetate (MMAc). This approach provides a new route to ethylene glycol under mild conditions.