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The selective catalytic reduction of N2O over steam-activated FeZSM-5 zeolite has been investigated at 473-823 K using C1-C3 hydrocarbons (methane, ethane, ethene, ethyne, propane, and propyne). The assessment of the hydrocarbons for nitrous oxide abatement is discussed according to different criteria: (i) de-N2O activity and operation temperature, (ii) hydrocarbon utilization, that is, the selectivity to react with N2O in O2 excess, (iii) emission of CO and CO2. (iv) sensitivity to NO and NH3, and (v) cost Alkanes are generally more effective reducing agents than unsaturated hydrocarbons. In particular, alkynes require higher temperature to activate N2O and are unselective because of their proneness to react with O2. Ethane is an optimal reductant, featuring a high de-N2O activity, high selectivity, and compared to methane, a lower degree of inhibition by NO. [ABSTRACT FROM AUTHOR]

Halogen chemistry plays a central role in the industrial manufacture of various important chemicals, pharmaceuticals, and polymers. It involves the reaction of halogens or halides with hydrocarbons, leading to intermediate compounds which are readily converted to valuable commodities. These transformations, predominantly mediated by heterogeneous catalysts, have long been successfully applied in the production of polymers. Recent discoveries of abundant conventional and unconventional natural gas reserves have revitalized strong interest in these processes as the most cost-effective gas-to-liquid technologies. This review provides an in-depth analysis of the fundamental understanding and applied relevance of halogen chemistry in polymer industries (polyvinyl chloride, polyurethanes, and polycarbonates) and in the activation of light hydrocarbons. The reactions of particular interest include halogenation and oxyhalogenation of alkanes and alkenes, dehydrogenation of alkanes, conversion of alkyl halides, and oxidation of hydrogen halides, with emphasis on the catalyst, reactor, and process design. Perspectives on the challenges and directions for future development in this exciting field are provided. [ABSTRACT FROM AUTHOR]

AlPO-5, SAPO-5, SAPO-11, and SAPO-34silicoaluminophosphates (SAPOs)are exposed to various acid and base treatments aimed at mesoporeformation and investigation of associated physicochemical modifications.SAPOs amorphize strongly in aqueous NaOH, requiring the use of organicbases (e.g., tetrapropylammonium hydroxide or diethylamine) to preservethe crystallinity during base treatment. In acid media (HCl, H4EDTA, and Na2H2EDTA), SAPO-11 remainsfully crystalline, while SAPO-34 strongly amorphize. No clear influenceof the framework topology is established. The high resistance in alkalinemedia and low stability in acid media of SAPO-34 is attributed toits relatively high silicon content. Base treatment of SAPOs leadspredominately to the formation of intercrystalline porosity. Still,an up to 4-fold increase in external surface and pore volume in SAPO-11are achieved. Besides the formation of secondary porosity, base treatmentof SAPOs induces a variety of (correlated) physicochemical changes.For example, the silicon distribution clearly influences the dissolutionbehavior of SAPO-11 in alkaline media, as zeolitic-like Si-domainsare more resistant than AlPO domains. As a result, base leaching isselective to phosphorus and leads, depending on the silicon distribution,to either aluminum or silicon enrichment. The resulting changes inbulk composition can be directly related to the secondary porosity,as the Si and Al enrichment takes place predominately on the externalsurface. Acidity characterization (TPD of ammonia and IR spectroscopyof pyridine or 2,6-di-tert-butylpyridine adsorbed)shows that base treatment of SAPO-11 slightly reduces the concentrationof Brønsted sites, while the number of Lewis sites is substantiallyincreased. Moreover, the amount of Brønsted acid sites associatedwith the external surface is largely enhanced. The behavior of zeotypes,that is, AlPOs, SAPOs, and zeolites, in acid and basic aqueous solutionsis generalized, highlighting the role of charge balancing cations.Catalytic evaluation of SAPO-11 shows the potential of base-treatedsamples in the alkylation of benzyl alcohol with toluene. [ABSTRACT FROM AUTHOR]

Thegas-phase oxidation of hydrogen halides to the correspondinghalogens has gained industrial relevance following the recent discoveryof active and stable heterogeneous catalysts based on RuO2and CeO2. These developments originated from the needof polyurethane and polycarbonate producers to recycle the large amountsof byproduct HCl to Cl2for further use in the phosgenationstep. More recently, the catalyzed HBr oxidation to Br2has been regarded as an enabling technology for bromine-mediatedalkane functionalization processes. The highly exothermic and corrosivecharacter of these reactions, especially HBr oxidation, requires particularprecautions regarding temperature control to guarantee stable andsafe operation in connection with the catalyst, the reactor, and ultimatelythe plant. Adopting experimental data available in the literatureand considering model simplifications, we rationalize effective strategiesto recover bromine, which involve simulations of staged RuO2/TiO2and CeO2/ZrO2catalyst bedsof variable activity in fixed-bed reactors. A cooled tubular and anadiabatic cascade configuration are considered. Options to optimizeprocess economics by minimizing catalyst cost and number of unit operationsare discussed. [ABSTRACT FROM AUTHOR]

The porosity of layered hydroxides can be greatly enhanced by thermal activation. Despite intense interest in the associated structural transformations, porosity changes have been almost exclusively described in terms of primitive bulk descriptors (surface area, pore volume, and average pore size), providing limited insight into the pore architecture (i.e., the geometry, location, and connectivity of the pores). This article maps porosity upon thermal activation of well-crystallized Mg(OH)2 (brucite) and isostructural layered double hydroxide {LDH, [M2+1-xM3+x(OH)2]x+(An-x/n)•mH2O} materials through novel characterization approaches such as transmission electron microscopy (TEM) coupled with in situ heating, differential hysteresis scanning (DHS) by high-resolution argon sorption, and positron annihilation lifetime spectroscopy (PALS). Despite exhibiting comparable surface area enhancements, arising from the formation of micro- and mesoporosity upon activation up to 873 K, the findings highlight striking differences in the structure of the developed pore networks. As visualized by TEM, the substitution of trivalent metals like aluminum into the brucite-like sheets restricts the growth and improves the thermal stability of the pores, which form abundantly throughout the sample. Quantitative analysis by DHS reveals that the geometry of mesopores in the most highly porous samples is significantly constricted. This is supported by low-temperature measurements (55 K), which permits precise assessment of the small mesopores (approximately 2-3 nm) in the LDH. Complementary insights are obtained by PALS, which evidences the asynchronous development of micro- and mesopores and readily distinguishes differences in the transport properties. The complexity of the pore evolution demonstrated by this approach illustrates the scope for improving the mechanistic understanding of the design of porous materials derived from layered hydroxides. [ABSTRACT FROM AUTHOR]

A generalizedapproach to determine the complete distribution ofmacropores, mesopores, and micropores from argon adsorption and mercuryporosimetry is developed and validated for advanced zeolite catalystswith hierarchically structured pore systems in powder and shaped forms.Rather than using a fragmented approach of simple overlays from individualtechniques, a unified approach that utilizes a kernel constructedfrom model isotherms and model intrusion curves is used to calculatethe complete pore size distribution and the total pore volume of thematerial. An added benefit of a single full-range pore size distributionis that the cumulative pore area and the area distribution are alsoobtained without the need for additional modeling. The resulting completepore size distribution and the kernel accurately model both the adsorptionisotherm and the mercury porosimetry. By bridging the data analysisof two primary characterization tools, this methodology fills an existinggap in the library of familiar methods for porosity assessment inthe design of materials with multilevel porosity for novel technologicalapplications. [ABSTRACT FROM AUTHOR]

Cuprous delafossites exhibit exceptionalelectrical, magnetic,optical, and catalytic properties. Through the application of a batteryof in situand ex situcharacterizationmethods complemented by density functional theory (DFT) calculations,we gathered an in-depth understanding of the synthesis of CuMO2(M = Al, Cr, Fe, Ga, Mn) by the solid-state reaction of Cu2O and M2O3and of their stability againstoxidative disproportionation to CuM2O4and CuO.TGA-DTA and XRD studies of the synthesis revealed that the natureof the M3+cation strongly impacts (i)the formation temperature of the delafossite phase, which occurredat a much lower temperature for CuCrO2than for the othermetals (1073 versus 1273–1423 K), (ii) themechanism of formation of the CuMO2in different atmospheres,which was found to comprise up to four steps in air and a single stepin N2, and (iii) the kinetics of the process,which could be significantly accelerated upon mechanochemical activationof the precursors by ball milling. The identification of unstableintermediate phases and, thus, a proper description of the synthesismechanism was only possible by the application of in situXRD. Electron microscopy, nitrogen sorption, and mercury porosimetryanalyses of the precursor oxide mixtures at different stages of thesynthesis in air revealed that particle agglomeration took place priorto the solid-state reactions forming the intermediate spinel phaseand the delafossite, respectively, and that these led to a substantialdrop in porosity and specific surface area. On the basis of XRD andHe pycnometry, the resulting CuMO2samples exhibit puredelafossite phase with rhombohedral structure (R3̅m), except for CuMnO2which features a monoclinicstructure (C2/m). Upon heating inair, CuCrO2retained its structure up to 1373 K, whileall other delafossites decomposed, CuAlO2at 1073 K, CuGaO2at 873 K, CuFeO2at 773 K, and CuMnO2at 673 K. The DFT-calculated surface phase diagram of CuCrO2and CuAlO2indicated that, at elevated oxygenpressures, the terminations with 1/2 and 0 ML of Cu are the most stablefor the (0001) facet. The formation enthalpy for interstitial oxygenspecies in the bulk is endothermic for both delafossites, while thatfor oxygen insertion in subsurface layers of these terminations isstill endothermic for CuCrO2but slightly exothermic forCuAlO2. These results provide an improved understandingof the chemistry of these mixed oxides, enabling their optimizationfor specific applications. [ABSTRACT FROM AUTHOR]

The manifestation of zeolite recrystallizationand the formationof amorphous aluminosilicate species during desilication are examinedto better understand the properties of alkaline-treated hierarchicalzeolites and their catalytic performance. This is achieved using asystematic experimental strategy, starting from treating the filtrateof alkaline-treated silicalite-1 in the presence of various externaladditives. No recrystallization is evidenced upon addition of tetrapropylammonium(TPA+) and/or aluminum hydroxide ions [Al(OH)4–], confirming the low probability of zeolite nucleationand/or growth during desilication. Conversely, ordered mesoporousmaterials (OMMs) form upon addition of cetyltrimethylammonium (CTA+) to the filtrate. By using other silicon sources, i.e., tetramethylorthosilicate or the organosilane dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium,we verify the facile formation of amorphous materials during alkalinetreatment of USY zeolites in the presence of hydrophobic micelle-formingalkyl moieties. A systematic characterization by X-ray diffraction,transmission electron microscopy, N2and Ar adsorption,inductively coupled plasma optical emission spectroscopy, and Fouriertransform infrared spectroscopy of pyridine adsorbed, demonstratesthat zeolites exposed to base solutions containing CTA+display weaker zeolitic properties, compared to those prepared usingTPA+, and should be considered as hierarchical zeolite/OMMcomposites. Catalytic tests in the alkylation of toluene with isopropylalcohol or benzyl alcohol evidence that CTA+-derived compositesdo not outperform the conventional USY zeolite. Only the hierarchicalUSY zeolite prepared by alkaline treatment in the presence of TPA+yielded a superior catalytic performance. [ABSTRACT FROM AUTHOR]

Hierarchical clinoptilolite and Lzeolites are prepared using optimizedtandem dealumination–desilication treatments. The main challengein the postsynthetic modification of these zeolites is the high Alcontent, requiring a tailored dealumination prior to the desilicationstep. For natural clinoptilolite, sequential acid treatments usingaqueous HCl solutions were applied, while for L a controlled dealuminationusing ammonium hexafluorosilicate is required. Subsequent desilicationby NaOH treatment yields mesopore surfaces of up to 4-fold (clinoptilolite,64 m2g–1; L, 135 m2g–1) relative to the parent zeolite (clinoptilolite,15 m2g–1; L, 45 m2g–1). A thorough characterization sheds light on thecomposition, crystallinity, porosity, morphology, coordination, andacidity of the modified clinoptilolite and L zeolites. It is elaboratedthat, besides the degree of dealumination, the resulting Al distributionis a critical precondition for the following mesopore formation bydesilication. Adsorption experiments of Cu2+and methyleneblue from aqueous solutions and the catalytic evaluation in alkylationsand Knoevenagel condensation evidence the superiority of the hierarchicalzeolites, as compared to their purely microporous counterparts. Finally,the postsynthetic routes for clinoptilolite and L are generalizedwith other recently reported modification strategies, and presentedin a comprehensive overview. [ABSTRACT FROM AUTHOR]