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In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni 2+ and Cr 3+ . Here we report that silica-supported, single site catalysts containing immobilized, main group Zn 2+ and Ga 3+ ion sites catalyze ethylene and propylene oligomerization to an equilibrium distribution of linear olefins with rates similar to that of Ni 2+ . The molecular weight distribution of products formed on Zn 2+ is similar to Ni 2+ , while Ga 3+ forms higher molecular weight olefins. In situ spectroscopic and computational studies suggest that oligomerization unexpectedly occurs by the Cossee-Arlman mechanism via metal hydride and metal alkyl intermediates formed during olefin insertion and β-hydride elimination elementary steps. Initiation of the catalytic cycle is proposed to occur by heterolytic C-H dissociation of ethylene, which occurs at about 250 °C where oligomerization is catalytically relevant. This work illuminates new chemistry for main group metal catalysts with potential for development of new oligomerization processes. Silica-supported, single site, main group Zn(II) and Ga(III) ions catalyze ethylene and propylene oligomerization. Here, experimental and theoretical evidence suggests a Cossee-Arlman reaction mechanism similar to that for transition metal catalysts.

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NOₓ) with ammonia (NH₃), but the low-temperature rate dependence on copper (Cu) volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O₂)–mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

Cu zeolites catalyse low-temperature (<523 K) selective catalytic reduction (SCR) of nitrogen oxides (NO x ) via a redox cycle involving dynamic interconversion between NH 3 -solvated mononuclear Cu I and binuclear Cu II complexes. Cu I oxidation requires the pairing of two mobilized Cu I (NH 3 ) 2 complexes to form binuclear intermediates, implying that Cu I oxidation kinetics should depend on framework Al density, given that Cu ions are ionically tethered to anionic charges at Al sites in zeolite lattices. Here we combine statistical simulations, steady-state kinetics and operando X-ray absorption spectroscopy to interrogate Cu–chabazite (Cu–CHA) zeolites of varying framework Al density (0.2–1.7 Al centres per cha cage). Increasing the Al density leads to systematic increases in both the fraction of Cu I ions that are SCR active (that is, O 2 oxidizable) and Cu I oxidation rate constants (per Cu), revealing insights into how anionic Al centres in zeolite frameworks regulate the mobility of ionically tethered Cu cations and their dynamic reactivity during low-temperature NO x SCR. The dynamic transformation of Cu ions during the selective catalytic reduction of NO x on Cu zeolites is well documented, although the function of the zeolite framework has not been fully understood. Here the authors unravel the role of anionic Al sites in the zeolite framework in regulating the mobility and reactivity of Cu cations during catalysis.

Copper ions in zeolites help remove noxious nitrogen oxides from diesel exhaust by catalyzing their reaction with ammonia and oxygen. Paolucci et al. found that these copper ions may move about during the reaction (see the Perspective by Janssens and Vennestrom). Zeolite catalysts generally fix metals in place while the reacting partners flow in and out of their cagelike structures. In this case, though, x-ray absorption spectroscopy suggested that the ammonia was mobilizing the copper ions to pair up as they activated oxygen during the catalytic cycle. Science , this issue p. 898 ; see also p. 866 Copper ions can move about and pair up in a zeolite framework as they catalyze nitric oxide removal from diesel exhaust. Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NO x ) with ammonia (NH 3 ), but the low-temperature rate dependence on copper (Cu) volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O 2 )–mediated Cu I →Cu II redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni and Cr . Here we report that silica-supported, single site catalysts containing immobilized, main group Zn and Ga ion sites catalyze ethylene and propylene oligomerization to an equilibrium distribution of linear olefins with rates similar to that of Ni . The molecular weight distribution of products formed on Zn is similar to Ni , while Ga forms higher molecular weight olefins. In situ spectroscopic and computational studies suggest that oligomerization unexpectedly occurs by the Cossee-Arlman mechanism via metal hydride and metal alkyl intermediates formed during olefin insertion and β-hydride elimination elementary steps. Initiation of the catalytic cycle is proposed to occur by heterolytic C-H dissociation of ethylene, which occurs at about 250 °C where oligomerization is catalytically relevant. This work illuminates new chemistry for main group metal catalysts with potential for development of new oligomerization processes.

While organic structure directing agents (OSDAs) are well known to have a directional influence on the topology of a crystallizing zeolite, the relationship between OSDA charge and siting of aliovalent ions on a primarily siliceous framework is unclear. Here, we explore the relationship between OSDA orientation, Al3+ siting, and lattice energy, taking as a model system CHA zeolite occluded with N,N,N-trimethyl-1-adamantyl ammonium (TMAda+) at an Si/Al ratio of 11/1. We use density functional theory calculations to parametrize a fixed-charge classical model describing van der Waals and electrostatic interactions between framework and OSDA. We enumerate and explore all possible combinations of OSDA orientation and Al location (attending to Lowenstein's rule) within a 36 T-site supercell. We find that interaction energies vary over 60 kJ/double-six-ring-unit (d6r). Further, analysis of configurations reveals that energies are sensitive to Al-Al proximity, such that low energies are associated with Al3+ pairs in 8-membered rings and higher energies associated with Al3+ pairs in smaller 6- and 4-membered rings. Comparisons with Al siting inferred from CHA zeolite crystallized with TMAda+ suggests that these computed interaction energies are useful reporters of observed Al siting in CHA synthesized with TMAda+.

The catalytic consequences of confinement within zeolite voids were examined for several elimination (alkane cracking and dehydrogenation, alkene cracking, alkanol dehydration) and addition (alkene hydrogenation, alkylation and oligomerization) reactions catalyzed by Brønsted solid acids. These reactions are mediated by cationic transition states that are confined within voids of molecular dimensions (0.4-1.3 nm) and proceed at rates that reflect the Gibbs free energies of late ion-pairs at transition states relative to those for the relevant reactants. Ion-pair stabilities depend on electrostatic interactions between organic cations and catalyst conjugate anions and on dispersion interactions between these cations and framework oxygen atoms. The former interactions are essentially unaffected by confinement, which influences weakly Brønsted acid strength, while the latter depend strongly on the sizes and shapes of voids and the species confined within them. The catalytic effects of confinement in stabilizing ion-pairs are prevalent when transition states are measured relative to gaseous reactants, but are attenuated and in some cases become irrelevant when measured with respect to confined reactants that are similar in composition and size. Zeolite voids solvate confined species by van der Waals forces and mediate compromises in their enthalpic and entropic stabilities. Confinement is generally preferred within locations that benefit enthalpic stability over entropic freedom at low temperatures, in which free energies depend more strongly on enthalpic than entropic factors. For example, the carbonylation of dimethyl ether (400-500 K) occurs with high specificity within eight-membered (8-MR) zeolite voids, but at undetectable rates within larger voids. This specificity reflects the more effective van der Waals stabilization of carbonylation transition states within the former voids. In contrast, entropic consequences of confinement become preeminent in high temperature reactions. Alkane activation turnovers (700-800 K) are much faster on 8-MR than 12-MR protons of mordenite zeolites because the relevant ion-pairs are confined only partially within shallow 8-MR side pockets and to lesser extents than within 12-MR channels. The site requirements and confinement effects found initially for elimination reactions were also pertinent for addition reactions mediated by ion-pair transition states of similar size and structure. Ratios of rate constants for elimination and addition steps involved in the same mechanistic sequence (e.g., alkane dehydrogenation and alkene hydrogenation) reflected solely the thermodynamic equilibrium constant for the stoichiometric gas-phase reaction. These relations are consistent with the De Donder non-equilibrium thermodynamic treatments of chemical reaction rates, in spite of the different reactant pressures used to measure rates in forward and reverse directions. The De Donder relations remained relevant at these different reaction conditions because the same elementary step limited rates and surfaces remained predominantly unoccupied in both directions. Rate constants for elementary steps catalyzed by zeolitic Brønsted acids reflect the combined effects of acid strength and solvation. Their individual catalytic consequences can be extricated using Born-Haber thermochemical cycles, which dissect activation energies and entropies into terms that depend on specific catalyst and reactant properties. This approach was used to show that thermal, chemical and cation-exchange treatments, which essentially change the sizes of faujasite supercage voids by addition or removal of extraframework aluminum species, influence solvation properties strongly but acid strength only weakly. These findings have clarified controversial interpretations that have persisted for decades regarding the origins of chemical reactivity and acid strength on faujasite zeolites. Born-Haber thermochemical relations, together with Marcus theory treatments of charge transfer reaction coordinates, provide a general framework to examine the effects of reactant and catalyst structure on ion-pair transition state enthalpy and entropy. The resulting structure-function relations lead to predictive insights that advance our understanding of confinement effects in zeolite acid catalysis beyond the largely phenomenological descriptions of shape selectivity and size exclusion. These findings also open new opportunities for the design and selection of microporous materials with active sites placed within desired void structures for reasons of catalytic rate or selectivity. The ability of zeolite voids to mimic biological catalysts in their selective stabilization of certain transition states by dispersion forces imparts catalytic diversity, all the more remarkable in light of the similar acid strengths among known aluminosilicates. This offers significant promise to expand the ranges of materials used and of reactions they catalyze.

Wastewater discharge from world tannery sector is about 600 million m3/annum. The tanneries in Asia discharge more than 350 million m3 of wastewater per annum from the process of 8 to 10 million tons of hides and skins. The ground and surface water resources in many locations in and around tannery cluster contain high Total Dissolved Solids (TDS) and not fit for domestic and industrial use. The conventional treatment systems implemented all over the world reduce Biochemical Oxygen Demand, Chemical Oxygen Demand, Suspended Solids, Heavy Metals etc. and not TDS and salinity which are mainly contributed by chlorides, hardness and sulphates. The treatment plants are unable to meet the standards in terms of TDS, chlorides and salinity which are being enforced in India and many other countries. The pollution control authorities also insist on water recovery integrated with Zero Liquid Discharge (ZLD) system. Naval treatment systems such as special Micro Filter, Ultra Filtration, Membrane Bio-Reactor, Nano Filtration, Reverse Osmosis, etc. have been developed for recovery of water from domestic and tannery wastewater. The achievement of ZLD concept has got many technical challenges. Management of the concentrated saline stream treatment by adopting energy intensive evaporation system is one of the major sustainable issues. The innovative treatment technologies developed and adopted for water recovery, saline stream management, etc. are dealt in this paper.

Rheumatoid arthritis is a chronic disease that is mainly characterized by asymmetric erosive synovitis, particularly affecting the peripheral joints. Radiation synovectomy or radiosynovectomy, also known as radiosynoviorthesis was first described in 1950's as a adjuvant treatment for rheumatoid arthritis. Radiosynovectomy is based on the irradiation of the joint synovium by the intra-articular administration of various β-emitting radiopharmaceuticals. Lu-177 has presence of gamma photons of imagable energy with low abundance which provides the additional benefit of carrying out simultaneous scintigraphy. We describe the first case report of use of Lu-177 hydroxylapatite particulates in a 35-year-old female patient who was presented with elbow joint synovitis due to rheumatoid arthritis.