Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 65
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Am Chem Soc ; 2020 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-32862644

RESUMO

Nonoxidative dehydrogenation is promising for production of light olefins from shale gas, but current technology relies on precious Pt or toxic Cr catalysts and suffers from thermodynamically oriented coke formation. To solve these issues, the earth-abundant iron catalyst is employed, where Fe species are effectively modulated by siliceous zeolite, which is realized by the synthesis of Fe-containing MFI siliceous zeolite in the presence of ethylenediaminetetraacetic sodium (FeS-1-EDTA). Catalytic tests in ethane dehydrogenation show that this catalyst has a superior coke resistance in a 200 h run without any deactivation with extremely high activity and selectivity (e.g., 26.3% conversion and over 97.5% selectivity to ethene in at 873 K, close to the thermodynamic equilibrium limitation). Multiple characterizations demonstrate that the catalyst has uniformly and stably isolated Fe sites, which improves ethane dehydrogenation to facilitate the fast desorption of hydrogen and olefin products in the zeolite micropores and hinders the coke formation, as also identified by density functional calculations.

2.
Chemistry ; 2020 Jun 09.
Artigo em Inglês | MEDLINE | ID: mdl-32515846

RESUMO

Unlike conventional aluminosilicate zeolites synthesized in alkaline media, aluminophosphate molecular sieves (AlPOs) have always been prepared under acidic conditions in the past three decades; this has been regarded as one of essential factors for synthesis, except for the case of silica-substituted analogues (SAPOs). For the first time, we demonstrate herein a simple and generalized route for synthesizing various types of aluminophosphate molecular sieves in alkaline media. A series of aluminophosphate sieves and their analogues have been prepared with different quaternary ammonium cations as structure-directing agents in this manner. The above successes have extended the systematic media from acidic or neutral to alkaline for the preparation of a series of aluminophosphate molecular sieves, which possibly open an alternative route for the synthesis of aluminophosphate molecular sieves.

3.
Artigo em Inglês | MEDLINE | ID: mdl-32453899

RESUMO

There are a large number of zeolites, such as ITH, that cannot be prepared in the aluminosilicate form. Now, the successful synthesis of aluminosilicate ITH zeolite using a simple cationic oligomer as an organic template is presented. Key to the success is that the cationic oligomer has a strong complexation ability with aluminum species combined with a structural directing ability for the ITH structure similar to that of the conventional organic template. The aluminosilicate ITH zeolite has very high crystallinity, nanosheet-like crystal morphology, large surface area, fully four-coordinated Al species, and abundant acidic sites. Methanol-to-propylene (MTP) tests reveal that the Al-ITH zeolite shows much higher selectivity for propylene and longer lifetime than commercial ZSM-5. FCC tests show that Al-ITH zeolite is a good candidate as a shape-selective FCC additive for enhancing propylene and butylene selectivity.

4.
Dalton Trans ; 2020 Apr 20.
Artigo em Inglês | MEDLINE | ID: mdl-32307488

RESUMO

With the merits of the accelerated crystallization process, high volume yields, and reduced waste production, solvent-free syntheses give new opportunities for the fabrication of zeolites. In this Frontier, a background overview is provided, consisting of a brief introduction of challenges in traditional zeolite syntheses and how solvent-free syntheses provide the right combination of properties for achieving a more sustainable route. Next, recent advancements in zeolites synthesized under solvent-free conditions are exemplified along with discussions about the strategies and principles involved. Finally, the remaining challenges and opportunities for this synthetic route are outlined.

5.
Nat Commun ; 11(1): 1722, 2020 Apr 01.
Artigo em Inglês | MEDLINE | ID: mdl-32238812

RESUMO

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

6.
Science ; 367(6474): 193-197, 2020 01 10.
Artigo em Inglês | MEDLINE | ID: mdl-31919221

RESUMO

Selective partial oxidation of methane to methanol suffers from low efficiency. Here, we report a heterogeneous catalyst system for enhanced methanol productivity in methane oxidation by in situ generated hydrogen peroxide at mild temperature (70°C). The catalyst was synthesized by fixation of AuPd alloy nanoparticles within aluminosilicate zeolite crystals, followed by modification of the external surface of the zeolite with organosilanes. The silanes appear to allow diffusion of hydrogen, oxygen, and methane to the catalyst active sites, while confining the generated peroxide there to enhance its reaction probability. At 17.3% conversion of methane, methanol selectivity reached 92%, corresponding to methanol productivity up to 91.6 millimoles per gram of AuPd per hour.

7.
High Alt Med Biol ; 20(4): 421-426, 2019 12.
Artigo em Inglês | MEDLINE | ID: mdl-31618098

RESUMO

Introduction: The relationship between altitude during treatment and common postoperative infections remains to be established. Based on the inverse relationship between oxygen partial pressure and altitude, we hypothesized that hospital elevation would correlate positively with postoperative infectious complication rates, including surgical site infection (SSI), urinary tract infection (UTI), and pneumonia. Methods: We used an event-enriched population of general, urologic, vascular, plastic-reconstructive, orthopedic, and thoracic patients within the 2016 ACS National Surgical Quality Improvement Program (NSQIP) dataset who underwent procedures with high risk of infectious complications. This yielded 82,172, 175,409, and 88,856 patients from 571, 577, and 570 hospitals for the study of 30-day postoperative SSI, UTI, and pneumonia outcomes respectively. Hospital altitudes were determined using Google Maps. Data were analyzed using univariate (altitude) and multivariate logistic regression, with altitude forced into the model, and forward-selection of NSQIP variables, with adjustment for clustering by hospital. Results: When compared in 1000-foot increments above sea level, hospital altitude had no significant effect on SSI or UTI (odds ratio [OR] = 1.0, p > 0.05). The risk of postoperative pneumonia decreased with increased altitude (OR = 0.93, 95% confidence interval: 0.87-0.99, p = 0.03). Conclusions: Patients and providers should be reassured that there is no increased risk of SSI or UTI at higher altitudes. The decreased risk of postoperative pneumonia was surprising and there exist potential explanations warranting future investigation.

8.
J Am Chem Soc ; 141(45): 18318-18324, 2019 Nov 13.
Artigo em Inglês | MEDLINE | ID: mdl-31644275

RESUMO

A large amount of zeolite structures are still not synthetically available or not available in the form of aluminosilicate currently. Despite significant progress in the development of predictive concepts for zeolite synthesis, accessing some of these new materials is still challenging. One example is the IWR structure as well. Despite successful synthesis of Ge-based IWR zeolites, direct synthesis of aluminosilicate IWR zeolite is still not successful. In this report we show how a suitable organic structure directing agent (OSDA), through modeling of an OSDA/zeolite cage interaction, could access directly the aluminum-containing IWR structure (denoted as COE-6), which might allow access to new classes of materials and thus open opportunities in valuable chemical applications. The experimental results reveal that the COE-6 zeolites with a SiO2/Al2O3 ratio as low as 30 could be obtained. Very interestingly, the COE-6 zeolite has much higher hydrothermal and thermal stabilities than those of the conventional Ge-Al-IWR zeolite. In methanol-to-propylene (MTP) reaction, the COE-6 zeolite exhibits excellent selectivity for propylene, offering a potential catalyst for MTP reaction in the future.

9.
Adv Mater ; 31(50): e1901905, 2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-31478282

RESUMO

Supported metal nanoparticles are widely used as catalysts in the industrial production of chemicals, but still suffer from deactivation because of metal leaching and sintering at high temperature. In recent years, serious efforts have been devoted to developing new strategies for stabilizing metal nanoparticles. Recent developments for preparing sinter-resistant metal-nanoparticle catalysts via strong metal-support interactions, encapsulation with oxide or carbon layers and within mesoporous materials, and fixation in zeolite crystals, are briefly summarized. Furthermore, the current challenges and future perspectives for the preparation of highly efficient and extraordinarily stable metal-nanoparticle-based catalysts, and suggestions regarding the mechanisms involved in sinter resistance, are proposed.

10.
Nat Commun ; 10(1): 3059, 2019 07 11.
Artigo em Inglês | MEDLINE | ID: mdl-31296873

RESUMO

Precise control of the outer-sphere environment around the active sites of heterogeneous catalysts to modulate the catalytic outcomes has long been a challenge. Here, we demonstrate how this can be fulfilled by encapsulating catalytic components into supramolecular capsules, used as building blocks for materials synthesis, whereby the microenvironment of each active site is tuned by the assembled wall. Specifically, using a cationic template equipped with a polymerizable functionality, anionic ligands can be encapsulated by ion pair-directed supramolecular assembly, followed by construction into porous frameworks. The hydrophilic ionic wall enables reactions to be achieved in water that usually requires organic solvents and also facilitates the enrichment of the substrate into the hydrophobic pocket, leading to superior catalytic performances as demonstrated by the industrially relevant hydroformylation. Remarkably, the formation of the supramolecular assembly and catalyst encapsulation further engenders reaction selectivity, which reaches an even greater extent after construction of the porous framework.

11.
Angew Chem Int Ed Engl ; 58(35): 12138-12142, 2019 Aug 26.
Artigo em Inglês | MEDLINE | ID: mdl-31283076

RESUMO

Currently, the synthesis of pure silica zeolites always requires the presence of organic structure-directing agents (OSDAs), which direct the assembly pathway and ultimately fill the pore space. A sustainable route is now reported for synthesizing pure silica zeolites in the absence of OSDAs from a combined strategy of zeolite seeding and alcohol filling, where the zeolite seeds direct crystallization of zeolite crystals from amorphous silica, while the alcohol is served as pore filling in the zeolites. Very importantly, the alcohol could be fully washed out from zeolite pores by water at room temperature, which completely avoids calcination at high temperature for removal of OSDAs in the synthesis of pure silica zeolites.

12.
Chem Commun (Camb) ; 55(62): 9180-9183, 2019 Aug 11.
Artigo em Inglês | MEDLINE | ID: mdl-31305813

RESUMO

Herein, we show that by following molecular engineering of the inter-site distance between the two functionalities in porous organic materials, it is possible to enable them to work in a concerted manner. Specifically, the activity can be amplified by the placement of the hydroxyl group in the meta position of the phosphonium salts in the representative cycloaddition of epoxides and CO2.

13.
ACS Appl Mater Interfaces ; 11(26): 23112-23117, 2019 Jul 03.
Artigo em Inglês | MEDLINE | ID: mdl-31252486

RESUMO

For the first time, SSZ-39 zeolite has been directly prepared using conventional colloidal silica and sodium aluminate instead of using FAU zeolite as the raw material in the alkaline media. The adjustment of the Si/Al ratios in the starting materials to the suitable values is a key factor to prepare the aluminosilicate SSZ-39 zeolite. Various characterizations (for instance, X-ray diffraction, scanning electron microscopy, nitrogen sorption, solid 27Al NMR, and NH3-temperature-programmed desorption) display that the aluminosilicate SSZ-39 zeolite owns high crystallinity, uniform cuboid morphology, large surface area, four-coordinated aluminum species, and strong acidic sites. Inductively coupled plasma analysis shows that the SiO2/Al2O3 ratios of the SSZ-39 products are ranged from 12.8 to 16.8. Considering the special framework of the SSZ-39 zeolite, the yield of this synthesis is not higher than 21.3%. Moreover, the catalytic performance of Cu-SSZ-39 catalyst synthesized from this route is excellent in the selective catalytic reduction of NO x with NH3 (NH3-SCR).

14.
J Am Chem Soc ; 141(21): 8482-8488, 2019 May 29.
Artigo em Inglês | MEDLINE | ID: mdl-31063372

RESUMO

Supported rhodium nanoparticles (NPs) are well-known for catalyzing methanation in CO2 hydrogenation. Now we demonstrate that the selectivity in this process can be optimized for CO production by choice of molecular sieve crystals as supports. The NPs are enveloped within the crystals with controlled nanopore environments that allow tuning of the catalytic selectivity to minimize methanation and favor the reverse water-gas shift reaction. Pure silica MFI (S-1)-fixed rhodium NPs exhibited maximized CO selectivity at high CO2 conversions, whereas aluminosilicate MFI zeolite-supported rhodium NPs displayed high methane selectivity under the equivalent conditions. Strong correlations were observed between the nanoporous environment and catalytic selectivity, indicating that S-1 minimizes hydrogen spillover and favors fast desorption of CO to limit deep hydrogenation. Materials in this class appear to offer appealing opportunities for tailoring selective supported catalysts for a variety of reactions.

15.
J Am Chem Soc ; 141(7): 2975-2983, 2019 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-30677301

RESUMO

Classical strong metal-support interactions (SMSI), which play a crucial role in the preparation of supported metal nanoparticle catalysts, is one of the most important concepts in heterogeneous catalysis. The conventional wisdom for construction of classical SMSI involves in redox treatments at high-temperatures by molecular oxygen or hydrogen, sometimes causing sintered metal nanoparticles before SMSI formation. Herein, we report that the aforementioned issue can be effectively avoided by a wet-chemistry methodology. As a typical example, we demonstrate a new concept of wet-chemistry SMSI (wcSMSI) that can be constructed on titania-supported Au nanoparticles (Au/TiO2-wcSMSI), where the key is to employ a redox interaction between Auδ+ and Ti3+ precursors in aqueous solution. The wcSMSI is evidenced by covering Au nanoparticles with the TiO x overlayer, electronic interaction between Au and TiO2, and suppression of CO adsorption on Au nanoparticles. Owing to the wcSMSI, the Au-TiO x interface with an improved redox property is favorable for oxygen activation, accelerating CO oxidation. In addition, the oxide overlayer efficiently stabilizes the Au nanoparticles, achieving sinter-resistant Au/TiO2-wcSMSI catalyst in CO oxidation.

16.
ChemSusChem ; 11(22): 3965-3974, 2018 Nov 23.
Artigo em Inglês | MEDLINE | ID: mdl-30350924

RESUMO

Construction of efficient interfaces to improve the performance of supported metal catalysts is a challenging but effective technique. A newly synthesized catalyst with layered cobalt oxide on the surface of titania (layer-CoOx /TiO2 ) is highly selective towards the aerobic oxidation of C-H bonds in a series of hydrocarbons under sustainable conditions. The layer-CoOx /TiO2 easily outperforms the state-of-the-art noble metal catalysts and homogeneous cobalt salts used in industry. In-depth structural and functional characterization reveal that the layer-CoOx /TiO2 readily reacts with O2 for the adsorption and activation of C-H bonds. The layered structure of CoOx can maximize the interfacial effect of CoOx /TiO2 leading to a good performance for the oxidation of C-H bonds.

17.
ACS Appl Mater Interfaces ; 10(39): 33214-33220, 2018 Oct 03.
Artigo em Inglês | MEDLINE | ID: mdl-30203642

RESUMO

Modern methodologies for synthesizing zeolites typically involve the employment of costly organic structure-directing agents. Herein, we report the design synthesis of aluminosilicate zeolite with ITE structure using an inexpensive nickel-amine complex (nickel-pentaethylenexamine) as a novel structure-directing agent. Characterizations including X-ray diffraction, scanning electron microscopy, N2 sorption isotherms, and 27Al magic-angle spinning NMR techniques show that the ITE zeolite has high crystallinity, perfect crystals, large surface area, and abundant aluminum species in the framework. More importantly, catalytic tests on the hydrogenation of CO2 into methane show that the Ni-ITE zeolite exhibits better catalytic performance than aluminosilicate-supported and silica-supported nickel catalysts. Obviously, the use of nickel-amine complex offers an alternative and facile way to synthesize aluminosilicate zeolites.

18.
Nat Commun ; 9(1): 3236, 2018 08 13.
Artigo em Inglês | MEDLINE | ID: mdl-30104623

RESUMO

Chemical transformations are highly sensitive toward changes in the solvation environment and solvents have long been used to control their outcome. Reactions display unique performance in solvents like ionic liquids or DMSO, however, isolating products from them is cumbersome and energy-consuming. Here, we develop promising alternatives by constructing solvent moieties into porous materials, which in turn serve as platforms for introducing catalytic species. Due to the high density of the solvent moieties, these porous solid solvents (PSSs) retain solvation ability, which greatly influences the performance of incorporated active sites via concerted non-covalent substrate-catalyst interactions. As a proof-of-concept, the -SO3H-incorporated PSSs exhibit high yields of fructose to 5-hydroxymethylfurfural in THF, which exceeds the best results reported using readily separable solvents and even rivals those in ionic liquids or DMSO. Given the wide application, our strategy provides a step forward towards sustainable synthesis by eliminating the concerns with separation unfriendly solvents.

19.
Acc Chem Res ; 51(6): 1396-1403, 2018 06 19.
Artigo em Inglês | MEDLINE | ID: mdl-29738233

RESUMO

Zeolites have been extensively studied for years in different areas of chemical industry, such as shape selective catalysis, ion-exchange, and gas adsorption and separation. Generally, zeolites are prepared from solvothermal synthesis in the presence of a large amounts of solvents such as water and alcohols in sealed autoclaves under autogenous pressure. Water has been regarded as essential to synthesize zeolites for fast mass transfer of reactants, but it occupies a large space in autoclaves, which greatly reduces the yield of zeolite products. Furthermore, polluted wastes and relatively high pressure due to the presence of water solvent in the synthesis also leads to environmental and safety issues. Recently, inspired by great benefits of solvent-free synthesis, including the environmental concerns, energy consumption, safety, and economic cost, researchers continually challenge the rationale of the solvent and reconsider the age-old question "Do we actually need solvents at all in zeolite synthesis?" In this Account, we briefly summarize our efforts to rationally synthesize zeolites via a solvent-free route. Our research demonstrates that a series of silica, aluminosilicate, and aluminophosphate-based zeolites can be successfully prepared by mixing, grinding, and heating starting solid materials under solvent-free conditions. Combining an organotemplate-free synthesis with a solvent-free approach maximizes the advantages resulting in a more sustainable synthetic route, which avoids using toxic and costly organic templates and the formation of harmful gases by calcination of organic templates at high temperature. Furthermore, new insights into the solvent-free crystallization process of zeolites have been provided by modern techniques such as NMR and UV-Raman spectroscopy, which should be helpful in designing new zeolite structures and developing novel routes for synthesis of zeolites. The role of water and the vital intermediates during the crystallization of zeolites have been proposed and verified. In addition to a significant reduction in liquid wastes and a remarkable increase in zeolite yields, the solvent-free synthesis of zeolites exhibits more unprecedented benefits, including (i) the formation of hierarchical micro-, meso-, and macrostructures, which benefit the mass transfer in the reactions, (ii) rapid synthesis at higher temperatures, which greatly improve the space-time yields of zeolites, and (iii) construction of a novel catalytic system for encapsulation of metal nanoparticles and metal oxide particles within zeolite crystals synergistically combining the advantages of catalytic metal nanoparticles and metal oxide particles (high activity) and zeolites (shape selectivity). We believe that the concept of "solvent-free synthesis of zeolites" would open a door for deep understanding of zeolite crystallization and the design of efficient zeolitic catalysts.

20.
Angew Chem Int Ed Engl ; 57(21): 6104-6108, 2018 05 22.
Artigo em Inglês | MEDLINE | ID: mdl-29660228

RESUMO

Methods for the hydrogenation of CO2 into valuable chemicals are in great demand but their development is still challenging. Herein, we report the selective hydrogenation of CO2 into ethanol over non-noble cobalt catalysts (CoAlOx ), presenting a significant advance for the conversion of CO2 into ethanol as the major product. By adjusting the composition of the catalysts through the use of different prereduction temperatures, the efficiency of CO2 to ethanol hydrogenation was optimized; the catalyst reduced at 600 ° gave an ethanol selectivity of 92.1 % at 140 °C with an ethanol time yield of 0.444 mmol g-1 h-1 . Operando FT-IR spectroscopy revealed that the high ethanol selectivity over the CoAlOx catalyst might be due to the formation of acetate from formate by insertion of *CHx , a key intermediate in the production of ethanol by CO2 hydrogenation.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA