Revisiting NH3–catalyst interactions in Cu-SSZ-13 SCR catalysts: an in situ spectro-kinetics study

At low surface coverages, NH3 consumption was found to occur during adsorption at 120 °C over both Cu-SSZ-13, and H-SSZ-13; albeit much faster on Cu-SSZ-13.

A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na2WO4 catalytic active sites for CH4 heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO4 catalytic active sites resulting in surface WO5 sites with retarded kinetics for C-H scission. On the other hand, dimeric Mn2O5 surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH4 than the poisoned surface WO5 sites or the original WO4 sites. However, the surface reaction intermediates formed from CH4 activation over the Mn2O5 surface sites are more stable than those formed over the Na2WO4 surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO x , in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na2WO4 surface sites towards CH4 activation but can yield MnO x surface sites that activate CH4 faster than Na2WO4 surface sites, but unselectively.

New Mechanistic and Reaction Pathway Insights for Oxidative Coupling of Methane (OCM) over Supported Na2 WO4 /SiO2 Catalysts

The complex structure of the catalytic active phase, and surface-gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na₂ WO₄ /SiO₂ catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H₂ -TPR, TAP and steady-state kinetics) experiments, that the long speculated crystalline Na₂ WO₄ active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na-WO_x sites. Kinetic analysis via temporal analysis of products (TAP) and steady-state OCM reaction studies demonstrate that (i) surface Na-WO_x sites are responsible for selectively activating CH₄ to C₂ H_x and over-oxidizing CH_y to CO and (ii) molten Na₂ WO₄ phase is mainly responsible for over-oxidation of CH₄ to CO₂ and also assists in oxidative dehydrogenation of C₂ H₆ to C₂ H₄ . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na₂ WO₄ /SiO₂ catalysts.

Methane activation by ZSM-5-supported transition metal centers

This review focuses on recent fundamental insights about methane dehydroaromatization (MDA) to benzene over ZSM-5-supported transition metal oxide-based catalysts (MO_x/ZSM-5, where M = V, Cr, Mo, W, Re, Fe). Benzene is an important organic intermediate, used for the synthesis of chemicals like ethylbenzene, cumene, cyclohexane, nitrobenzene and alkylbenzene. Current production of benzene is primarily from crude oil processing, but due to the abundant availability of natural gas, there is much recent interest in developing direct processes to convert CH₄ to liquid chemicals. Among the various gas-to-liquid methods, the thermodynamically-limited Methane DehydroAromatization (MDA) to benzene under non-oxidative conditions appears very promising as it circumvents deep oxidation of CH₄ to CO₂ and does not require the use of a co-reactant. The findings from the MDA catalysis literature is critically analyzed with emphasis on in situ and operando spectroscopic characterization to understand the molecular level details regarding the catalytic sites before and during the MDA reaction. Specifically, this review discusses the anchoring sites of the supported MO_x species on the ZSM-5 support, molecular structures of the initial dispersed surface MO_x sites, nature of the active sites during MDA, reaction mechanisms, rate-determining step, kinetics and catalyst activity of the MDA reaction. Finally, suggestions are given regarding future experimental investigations to fill the information gaps currently found in the literature.

Synthesis and molecular structure of model silica-supported tungsten oxide catalysts for oxidative coupling of methane (OCM)

Catalysts with only dispersed phase Na–WO₄ sites where Na/W < 2 are slightly less active but significantly more C₂ selective than the traditional Na₂WO₄/SiO₂ catalysts that contain a crystalline phase where Na/W = 2.

Superstructural diversity in salt-cocrystals: higher-order hydrogen-bonded assemblies formed using U-shaped dications and with assistance of π--π stacking

Salt cocrystals with components that assemble by hydrogen bonds and aromatic anion-molecule stacks (π--π stacks) are reported. U-shaped bipyridines and an isocoumarin carboxylic acid self-assemble to form 5-, 6-, and 10-component aggregates with components in double and quadruple face-to-face stacks. DFT calculations support the π--π stacks to help stabilize the salt cocrystals.

The increasing importance of polymicrobial bacteremia

Polymicrobial bacteremia increased from 6% in 1970 to 13% in 1975 in patients with bloodstream infections. This type of serious infection most commonly complicated genitourinary (27%) and gastrointestinal (26%) conditions, frequently followed invasive procedures (68%), was more common in hospitalized patients (73%), and was often associated with malignancies (25%) or immunosuppressive or cancer chemotherapy (34%). Although polymicrobial endocarditis was more commonly encountered in recent years, this infection accounted for only 4.5% of patients with multiple organism bacteremias. Gram-negative aerobic bacteria were isolated from 62% and anaerobic bacteria in 39% of patients with polymicrobial bacteremia compared with 37% and 14%, respectively, in patients with monomicrobial bacteremia. In the 88 patients with polymicrobial bacteremia, the mortality was 44.5%, compared with 18.0% in patients with monomicrobial bacteremia.

Ceforanide: in vitro and clinical evaluation

Ceforanide, a new cephalosporin antibiotic with a long half-life (3 h), can be administered twice daily. We evaluated its antimicrobial activity, pharmacology, and clinical efficacy. Twenty-seven patients with infections due to susceptible organisms received ceforanide, 0.5, 1, or 2 g, intramuscularly or intravenously every 12 h for 6 to 28 days. In vitro studies with the clinical isolates from 27 patients treated plus 263 additional isolates showed that ceforanide was active against cephalothin-susceptible gram-positive and gram-negative microorganisms. In addition, ceforanide inhibited 65% of cephalothin-resistant Escherichia coli and 65% of Enterobacter spp. at </=12.5 mug/ml. After a single 1-g intramuscular dose, the mean peak plasma concentration at 1 h was 48.9 mug/ml and that at 12 h was 4.7 mug/ml. Plasma accumulation occurred in some patients. The infections included 10 pneumonias, 3 with bacteremia and 1 with empyema; 11 soft tissue infections, 4 with abscesses and 3 with sepsis; and 3 urinary tract infections. One case each of endocarditis, osteomyelitis, and septic thrombophlebitis, all due to Staphylococcus aureus, were treated. Clinical response was satisfactory in all patients; bacteriological response was satisfactory in 26 of 27 patients. Ceforanide was well tolerated. Three patients developed mild increases in liver enzymes, and one developed slight eosinophilia. In another case, the antibiotic was discontinued because of a fivefold rise in serum glutamic-oxalacetic transaminase (aspartate aminotransferase) and serum glutamic-pyruvic transaminase (alanine aminotransferase) and a twofold rise in lactic acid dehydrogenase and alkaline phosphatase.

The compromised host and Legionnaires' disease

Pneumonia caused by Legionnaires' disease bacterium was recognized in eight patients during a 7-month period. The patients were immunosuppressed by their underlying illness, corticosteroid therapy, and other exogenous immunosuppressive agents. Five of the patients had received immunosuppressive therapy for less than 16 days. Clinical presentation was similar to that of other bacterial pneumonias in compromised patients. Legionnaires' disease progressed to necrotizing pneumonia with abscess formation and respiratory failure in two patients. Diagnosis was made by [1] culture of lung tissue and bronchial washings; [2] direct fluorescent antibody staining of lung tissue, sputum, and bronchial washings; and [3] serologic evidence of infection. Therapy with oral erythromycin was ineffective. Intravenous erythromycin was given to six patients, with a good response. However, two patients showed further clinical improvement after rifampin was added. Because this illness may be more severe in compromised hosts, open lung biopsy and special microbiologic tests should be done when Legionnaires' disease is suspected.