Charge distribution and conformational stability effects of organic structure-directing agents on zeolite synthesis

While the use of the so-called organic structure-directing agents (OSDAs) is no stranger to zeolite synthesis at all, the effects of their charge distribution and conformational stability on the phase selectivity of the crystallization remain elusive. To systematically investigate these effects, we have prepared a series of imidazolium and pyrazolium derivatives with the same geometric shape but different positions of nitrogen atoms in the five-membered heterocyclic aromatic ring and used them as OSDAs for the synthesis of zeolites with pure-silica composition. Meaningful differences in the zeolite product were observed.11Ysciescopu

Particulate matter exposure during pregnancy is associated with birth weight, but not gestational age, 1962-1992: a cohort study

<p>Abstract</p> <p>Background</p> <p>Exposure to air pollutants is suggested to adversely affect fetal growth, but the evidence remains inconsistent in relation to specific outcomes and exposure windows.</p> <p>Methods</p> <p>Using birth records from the two major maternity hospitals in Newcastle upon Tyne in northern England between 1961 and 1992, we constructed a database of all births to mothers resident within the city. Weekly black smoke exposure levels from routine data recorded at 20 air pollution monitoring stations were obtained and individual exposures were estimated via a two-stage modeling strategy, incorporating temporally and spatially varying covariates. Regression analyses, including 88,679 births, assessed potential associations between exposure to black smoke and birth weight, gestational age and birth weight standardized for gestational age and sex.</p> <p>Results</p> <p>Significant associations were seen between black smoke and both standardized and unstandardized birth weight, but not for gestational age when adjusted for potential confounders. Not all associations were linear. For an increase in whole pregnancy black smoke exposure, from the 1^st(7.4 μg/m³) to the 25^th(17.2 μg/m³), 50^th(33.8 μg/m³), 75^th(108.3 μg/m³), and 90^th(180.8 μg/m³) percentiles, the adjusted estimated decreases in birth weight were 33 g (SE 1.05), 62 g (1.63), 98 g (2.26) and 109 g (2.44) respectively. A significant interaction was observed between socio-economic deprivation and black smoke on both standardized and unstandardized birth weight with increasing effects of black smoke in reducing birth weight seen with increasing socio-economic disadvantage.</p> <p>Conclusions</p> <p>The findings of this study progress the hypothesis that the association between black smoke and birth weight may be mediated through intrauterine growth restriction. The associations between black smoke and birth weight were of the same order of magnitude as those reported for passive smoking. These findings add to the growing evidence of the harmful effects of air pollution on birth outcomes.</p

Fluoride-free synthesis of high-silica, medium-pore zeolites PST-22 and PST-30

We recently reported the synthesis of three high-silica (Si/Al similar to 10) zeolites with new framework structures (i.e., PST-21, PST-22, and PST-30) via an excess fluoride approach which uses synthesis mixtures with HF/OSDA(n+) = 2/n (n = 1 or 2). However, the use of toxic and corrosive fluoride ions should be avoided for the green manufacturing of zeolites. Here we examine the structure-directing abilities of eight diazolium-based mono- and dications in hydroxide media which could crystallize PST-21, PST-22, or PST-30 under excess fluoride conditions. Among them, 1,1 &apos;-(1,4-butanediyl)bis(2,4-dimethyl-1H-pyrazol-2-ium) (14DMP-C-4(2+)) and 1,1 &apos;-(1,4-butanediyl)bis(2,5-dimethyl-1H-pyrazol-2-ium) ions were found to direct the synthesis of PST-22 and PST-30 in the presence of Na+ ions, respectively. Powder X-ray diffraction and Rietveld analyses of as-made PST-22 reveal that while the pyrazolium moieties in 14DMP-C-4(2+) are located near the center of t-pww cages, the tetramethylene chain runs through a 10-ring window connecting two t-pww cages. The proton forms of PST-22 and PST-30 synthesized here exhibit better performance in 1-butene skeletal isomerization than H-ferrierite, one of the most selective catalysts for this reaction.11Nscopu

Mechanisms for the Reverse Skeletal Isomerization of n-Butenes to Isobutene over Zeolite Catalysts

The use of 13C-enriched isobutene in the reverse skeletal isomerization of 1-butene over zeolite catalysts allowed us to corroborate that the mechanisms for the reverse reaction were essentially the same as those for the forward reaction, regardless of the structure type of the catalyst employed.111sciescopu

Effect of framework Si/Al ratio on the adsorption mechanism of CO2 on small-pore zeolites: II. Merlinoite

Potassium merlinoite with a framework Si/Al ratio of 2.9 (K-MER-2.9) is synthesized using K-MER with a higher Si/Al ratio of 3.8 (K-MER-3.8) as seed crystals. The CO11Nscopu

Palladium-exchanged small-pore zeolites with different cage systems as methane combustion catalysts

The catalytic properties of four palladium-exchanged small-pore zeolites with different cage systems (i.e., Pd-SSZ-13, Pd-LTA, Pd-PST-7, and Pd-RTH) have been evaluated for methane (CH4) combustion and compared with those obtained from Pd-ZSM-5 and Pd-Al2O3 in an attempt to investigate the effects of zeolite support structure and acidity on the oxidation activity of supported Pd catalysts for this greenhouse gas. When Pd loading level was fixed to be ca. 1.0 wt%, Pd-SSZ-13 was found to show the best CH4 oxidation activity and durability under wet feed conditions among the catalysts studied. The overall characterization results of this work demonstrate that the strong acid strength of SSZ-13 may render strong interactions between the intrazeolitic PdO species and the zeolite framework so as to effectively suppress not only PdO sintering but also dealumination during CH4 combustion.116sciescopu

The Origin of Selective Adsorption of CO2 on Merlinoite Zeolites

Herein, we describe the CO2 adsorption behavior at 25‐75 °C and 0−1.0 bar of various alkali cation‐exchanged forms of merlinoite (framework type MER) zeolites with Si/Al = 2.3 and 3.8. The adsorption isotherms at 25 °C on the Na+ , K+ , Rb+ , and Cs+ forms of MER zeolite with Si/Al = 2.3 are characterized by a clear step, the CO2 pressure of which differs notably according to the type of their extraframework cations. Structural analysis shows that CO2 adsorption on the former three zeolites includes the relocation of gating cations with high site occupancy and the remarkable concomitant structural breathing. We define this unusual adsorption phenomenon as a cooperative cation gating‐breathing mechanism. The overall results suggest that the actual mechanism of selective CO2 adsorption on intermediate‐silica small‐pore zeolites can change from cation gating to cooperative cation gating‐breathing to breathing, depending on a combination of their topological and compositional (framework Al content and extraframework cation type and concentration) flexibilities.11Nsciescopu

A Zeolite Family Nonjointly Built from the 1,3-Stellated Cubic Building Unit

From a technological point of view, the synthesis of new high-silica zeolites is of prime importance owing to their high potential as industrial catalysts and catalyst supports. Two such materials have been synthesized which are made up of the 1,3-stellated cubic unit (hexahedral ([4254]) bre unit) as a secondary building unit, with the aid of existing imidazolium-based structure-directing agents under “excess fluoride” conditions. One of them, denoted PST-21, is the first aluminosilicate zeolite consisting of 9-ring apertures solely; it displays exceptional activity towards steering the skeletal isomerization of 1-butene to isobutene and bridges the gap between small- and medium-pore structures. A series of hypothetical structures are also described that are nonjointly built from the bre unit; all of these structures are chemically feasible and will thus be helpful in designing the synthesis of novel zeolites containing 9-ring and/or 10-ring channels. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim11Nsciescopu

Synthesis of High-Silica LTA and UFI Zeolites and NH3-SCR Performance of Their Copper-Exchanged Form

A series of LTA zeolites with Si/Al = 8.3-infinity and a UFI zeolite with Si/Al = 11 are synthesized using benzylimidazolium-based cations as organic structure-directing agents in fluoride media, if required, together with the tetramethylammonium ion. Two LTA zeolites with Si/Al = 11 and 16 showed an enhanced operating temperature window with excellent thermal durability for selective catalytic reduction of NOx with NH3, compared to Cu-SSZ-13 commercially being used for automotive applications.112813sciescopu

Economical synthesis of high-silica LTA zeolites: A step forward in developing a new commercial NH3-SCR catalyst

Here we report an economical synthesis method for high-silica (Si/A1 similar to 17) LTA zeolites using colloidal silica and ammonium fluoride as Si and F sources, respectively. When both 1,2-dimethy1-3-(4-methylbenzyl)imidazolium and tetramethylammonium ions are used as organic structure-directing agents, the LTA crystallization was found to strongly depend on the particle size of the colloidal silica employed. However, the type of Al and F sources had little effect on the formation of this small-pore zeolite. The catalytic properties of three high-silica Cu-LTA catalysts synthesized using the same Si source (Ludox HS-40 colloidal silica) but different Al sources (aluminum hydroxide, aluminum metal, and pseudoboehmite) for the selective catalytic reduction of NOx with NH3 (NH3-SCR) are also presented. Despite the presence of a large amount of extraframework Al, all of the catalysts exhibited considerably higher activity maintenance than the current commercial Cu-SSZ-13 catalyst even after hydrothermal aging at 900 degrees C.11Nsciescopu