Steam-assisted Conversion Method Research Articles

Vapor-phase methods for synthesizing metal-organic framework thin films

<p>Due to their unique structures and exceptional physical and chemical properties, metal-organic framework (MOF) materials have garnered extensive attention in various fields, including catalysis, separations, sensing, and optics. Compared with powders or bulk MOF materials, MOF thin films exhibit large vertical and horizontal dimensions, higher specific surface areas, and abundant active sites and undergo facile combination with other functional centers for adsorption/separation, catalysis, and photoelectronic device applications. Among the methods used in preparing MOF thin films, the vapor phase approach enables more effective growth of MOF films with controllable thicknesses, uniformity, and compatibility; thus, it has attracted significant interest. This extensive review presents four vapor-phase approaches for preparing MOF thin films: the steam-assisted conversion method, vapor-phase transformations of metal oxide templates, vapor-phase linker exchange, and the atomic layer deposition/molecular layer deposition method. We summarize the advantages and disadvantages of these different vapor-phase-based methods for thin-film preparation, aiming to promote their use in precise and controllable surface syntheses.</p>

One-pot conversion of α-pinene into biomass high energy density fuel over a bifunctional zeolite catalyst

A shape-selective bifunctional catalyst (Rux/SOT-Hβ) was furnished by loading Ru onto the Hβ zeolite prepared via seed organic template (SOT) method using the seeds obtained via steam-assisted conversion (SAC) method. The Rux/SOT-Hβ catalyst exhibited both favorable acid-catalytic performance and hydrogenation performance in the two-stage one-pot conversion of α-pinene, enabling an energy-efficient process for producing biomass blended high energy density fuels (HEDF). The effect of preparation conditions of Hβ zeolite carrier, metal type, loading amount, and loading method on the catalytic performance of the bifunctional catalyst was investigated. After the optimization of the catalytic process over Ru1.2/SOT-Hβ using pinane as the reaction medium, the resulting liquid reaction mixture without undergoing further distillation and other operations was found to possess good biomass HEDF properties. Feasible recyclability of Ru1.2/SOT-Hβ catalyst in α-pinene one-pot conversion was also demonstrated by a 3-run recycling test.

Secondary cracking suppression through Pt/H-BEA: n-Hexadecane hydrocracking

This paper reports secondary cracking suppression through Pt/H-BEA in n-C16 hydrocracking. H-BEA (C) and H-BEA (H) of SiO2/Al2O3 ratios: 17, 23, and 25 were prepared by hydrothermal method and steam-assisted conversion method, respectively, and platinum loading was performed using incipient wetness impregnation. Pt/H-BEA (H) catalysts exhibited predominantly mesoporous structure by nitrogen physisorption and lower acid site density by NH3-TPD and Pt/H-BEA (C) exhibited unusually high mesoporosity based on our synthesis gel chemistry. Both catalysts had high secondary cracking reactions suppression but Pt/H-BEA23 (H) exhibited superior and perfect suppression with C6/C10 ∼ 1 and symmetrical cracking products carbon number molar distribution at several n-C16 conversions. Pt/H-BEA (H), consequently, had highest jet fuel hydrocarbons (C9+) selectivity with 81% achieved by Pt/H-BEA23 (H) at 50% conversion. It, additionally, had lower coke deposition which decreased with increased SiO2/Al2O3 ratio as a result of lower diffusion limitations and secondary cracking reactions. Pt/H-BEA, in conclusion, exhibits excellent ability to suppress secondary cracking reactions for high jet fuels hydrocarbons selectivity and diesel selectivity in Fischer-Tropsch wax hydrocracking considering n-C16 as a feed model and H-BEA (H) greatly improves the performances.

Synthesis and CO2 photoreduction of two 3d–4f heterometal–organic frameworks

Two 3d–4f heterometal–organic frameworks with similar structures were synthesized by a steam-assisted conversion method and exhibited high activity and selectivity for the photoreduction of CO2.

An Effective Approach to Synthesize High-Performance SSZ-13 Membranes Using the Steam-Assisted Conversion Method for N2/CH4 Separation

Thin and high-quality SSZ-13 membranes were synthesized by the steam-assisted conversion (SAC) method for the first time. More than 95% gel/solution was saved for membrane growth compared with conv...

Fabrication of pure-silica *BEA-type zeolite membranes on tubular silica supports coated with dilute synthesis gel via steam-assisted conversion

A hydrophobic pure-silica *BEA-type zeolite membrane was prepared using the steam-assisted conversion (SAC) method on a seeded tubular silica support coated with dilute synthetic gel. This study systematically investigates the effects of the synthetic conditions on the membrane formation process and separation performance. Under the optimized conditions, when using a dilute synthetic gel with a H2O/SiO2 ratio of 500, a dense and continuous membrane layer was formed in 24 h. This was attributed to dissolution of the surface of the silica support during the steaming process, which functions as a silica source for the growth of a seed crystal layer. An increase in silica the concentration on the support surface locally enabled the formation of a membrane under the dilute synthetic gel composition. The obtained membrane exhibited selectivity toward n-butanol, in addition to a high flux with a separation factor of 132 and a flux of 1.00 kg m−2h−1 for a 1 wt% n-butanol/water mixture at 318 K. This novel simple membrane formation method combines a dilute synthesis gel with SAC, takes advantage of the dissolution of a silica support, and reduces the consumption and cost of chemical raw materials used for membrane synthesis.

Fast synthesis of hierarchical Al-free Ti-BEA plate-like nanocrystals from low-templated dry gel via a steam-assisted conversion method

A hierarchical nanosized Ti-BEA zeolite prepared via a green steam-assisted conversion (SAC) method.

Fast synthesis of hierarchical nanosized pure Si-Beta zeolite via a steam-assisted conversion method

Abstract A hierarchical nanosized pure Si-Beta zeolite was firstly prepared via a micro-zone synchronous crystallization method by steam-assisted conversion taking inorganic silicon sources as raw material. Neither zeolite seeds nor fluoride mineralizing agent was required, only a bit of organic template agent (TEAOH/SiO2 = 0.05) was sufficient for constructing a self-sustaining plate-like nanosized aggregation with high surface area (631.09 m2/g) and pore volume (0.7 cm3/g) after 24 h period, much less than reported (TEAOH/SiO2≥0.54, 7–21 d). The pure Si-Beta zeolite also had a superior hydrophobicity than that of hydrothermal synthesis due to the advantage of micro-zone mass transfer. All the properties of resulting samples were investigated by comprehensive characterizations, including X-ray diffraction, scanning and transmission electron microscopy, nitrogen adsorption-desorption, water adsorption, Flourier transform infrared and solid state 29Si nuclear magnetic resonance. Importantly, a “micro-zone synchronous crystallization” mechanism was proposed to explain the formation of mesoporous structure and intrinsic micropores, so as to providing the theoretical guidance for one-step synthesis of hierarchical Beta and other zeolites.

Effects of seed crystal type on the growth and microstructures of silicalite-1 membranes on tubular silica supports via gel-free steam-assisted conversion

In this study, silicalite-1 membranes were prepared on seeded tubular silica supports using a gel-free steam-assisted conversion (SAC) method. The effects of seed crystal type on the growth and microstructure of the membranes were investigated. The morphologies of the membranes obtained by this preparation method reflected the orientation of the seed crystal layer covering the tubular support; when spherical seed crystals were used, the membrane surface was composed of randomly oriented crystals; whereas when coffin-type seed crystals were used, the membrane surface was composed of crystals arranged in a relatively b-axis orientation. This suggests that membrane layers can be grown with the same orientation as the seed crystals on the tubular support. That is, the gel-free SAC method is able to suppress changes in the orientation of zeolite crystals during membrane formation, which is a problem in conventional hydrothermal syntheses. This is a very interesting phenomenon that occurs only under gel-free SAC conditions, and accounts for the successful growth of the b-oriented silicalite-1 membrane layer on the tubular support. Our findings successfully demonstrate approaches for the formation of membrane layers that retain the orientation of zeolite seed crystals on tubular supports, and for the synthesis of oriented zeolite membranes with high permeability.

Effects of Silica-Particle Coating on a Silica Support for the Fabrication of High-Performance Silicalite-1 Membranes by Gel-Free Steam-Assisted Conversion.

Silicalite-1 membranes with high pervaporation performance were prepared successfully on a silica-particle-coated tubular silica support using a gel-free steam-assisted conversion (SAC) method. The effects of the silica-particle layer formed on the top surface of the silica support and the physical properties of the silica particles themselves on the membrane-formation process were investigated. The silica particles coated served as the additional silica source for growing the silicalite-1 seed crystal layer into the silicalite-1 membrane. As a result, it was possible to form a dense and continuous membrane even under gel-free conditions. Furthermore, it was found that the properties of the silica particles, such as their primary particle diameter, had a determining effect on their solubility during the steam treatment, that is, on the supply rate of the silica source. The silicalite-1 membrane obtained using the spherical-silica-particle-coated support had an approximately 9-μm-thick separation layer and showed very high pervaporation performance, exhibiting a separation factor of 105 and a flux of 3.72 kg m−2 h−1 for a 10 wt % ethanol/water mixture at 323 K. Thus, the gel-free SAC method can be used with a silica support coated with silica particles to readily prepare high-performance membranes without producing any chemical waste.

Hierarchical Hβ zeolite as a highly efficient solid acid catalyst for alkenylation of p-xylene with phenylacetylene

In this work, using phenylacetylene as alkenylating agent, the alkenylation of p-xylene on a series of hierarchical Hβ (Hier-Hβ) zeolites prepared by a modified steam-assisted conversion (SAC) method under different preparation conditions was firstly investigated. Various characterization techniques including X-ray diffraction (XRD), X-ray fluorescence (XRF), argon adsorption-desorption, ammonia temperature-programmed desorption (NH3-TPD), pyridine-IR (Py-IR), and thermogravimetric Analysis (TGA) were adopted to discover the relationship of structure-performance in alkenylation. The results demonstrate that the optimized Hier-Hβ zeolite exhibits an outstanding alkenylation catalytic performance, which is much better than the traditional microporous Hβ zeolite. It can be found that the alkenylation catalytic properties of the as-prepared Hier-Hβ zeolites are dramatically affected by their properties of acidic sites and textural properties, which are significantly dependent on the preparation process parameters for SAC process. Moreover, the spent Hier-Hβ zeolite catalyst can be completely refreshed by a simple calcination method. In a word, the optimized Hier-Hβ catalyst prepared by a modified SAC approach demonstrates great potential to promote alkenylation of aromatics with alkynes to their corresponding alkylaromatics.

Sphere-like Hierarchical Y Zeolite Fabricated by Steam-assisted Conversion Method

Sphere-like Hierarchical Y Zeolite Fabricated by Steam-assisted Conversion Method

Hierarchical Beta Zeolite Prepared by Steam-assisted Conversion Method

Hierarchical Beta Zeolite Prepared by Steam-assisted Conversion Method

A simple secondary growth method for the preparation of silicalite-1 membrane on a tubular silica support via gel-free steam-assisted conversion

Silicalite-1 membranes with high pervaporation (PV) performance were prepared on a seeded tubular silica support using modified steam-assisted conversion (SAC) under gel-free conditions, called the gel-free SAC method, for the first time. The seeded support was first dipped in a TPAOH aqueous solution and dried. Subsequently, the TPAOH-coated silicalite-1 seed layer was converted into a silicalite-1 membrane layer, followed by heating in an autoclave with a small amount of water at the bottom of the autoclave. In this method, silica support was not only used as the support but also as the silica source to grow the zeolite seed layer into a continuous and dense zeolite membrane layer. In other words, no additional supply of silica source such as synthesis gel to grow the seed layer was required. The silicalite-1 membrane prepared in this study had a separation layer with a thickness of approximately 7µm and exhibited high PV performance with a separation factor of 66 and high flux of 4.47kgm−2h−1 for 10wt% ethanol/water mixtures at 323K. This membrane preparation method is quite simple and uses minimum necessary reagents. Therefore, it is economical and suitable for the preparation of membranes on an industrial scale.

A noncovalently connected metal–organic framework assembled from a Ni(iii)-supported polyoxometalate–imidazolate hybrid

The first mixed-valence nickel-supported polyoxometalate-based metal–organic framework, (H2mim)[NiIII(Hmim)4][SiW11NiIIO37(OH)2(Hmim)]·3H2O (1) (Hmim = 2-methylimidazole), has been synthesized using a steam-assisted conversion method, and is constructed from noncovalent interactions (hydrogen bonds and π⋯π stacking interactions). 1 has a repeating honeycomb-shaped internal structure with one-dimensional channels along the direction of the c axis, where the diameter of the aperture is as large as 18.83 A. Furthermore, the stability and the optical band gaps are investigated.

A crystalline AlPO4-5 intermediate: designed synthesis, structure, and phase transformation.

AlPO4-5 is the most extensively studied material in exploring the crystallization mechanism of aluminophosphate molecular sieves, and it can be synthesized using 4-dimethylaminopyridine (DMAP) as an organic structure-directing agent (OSDA). Achieving the AlPO4-5 intermediate in the crystalline form would be of great importance and significance for this purpose. In this study, we proposed a concept that by carefully regulating the synthesis conditions, crystalline alumimophosphate intermediates could be rationally prepared. A crystalline AlPO4-5 intermediate, (C7H11N2)2(H3O)(Al3P4O16)·(H2O)2.5 (1), was, therefore, synthesized by significantly increasing the amounts of orthophosphoric acid and DMAP. The intermediate 1 consists of alternating organic DMAP and inorganic [Al3P4O16]3- layers with abundant hydrogen bonds and can transform to the AlPO4-5 framework upon heating in air or through a steam-assisted conversion (SAC) method. To unravel the role of DMAP in the transformation, pure AlPO4-5 (2) was also synthesized by altering the synthesis conditions. The supramolecular assembly templating (SAT) effects of DMAP in 1 and 2 were verified via fluorescence spectra. Thus, the π-π stacking interactions between the DMAP molecules and the abundant hydrogen bonds between the adjacent layers are considered to be important for the phase transformation of 1. This crystalline intermediate would promote the understanding of the crystallization mechanism in the system of aluminophosphate molecular sieves.

An unprecedented 3D architecture based on a Nb/W mixed-addendum polyoxometalate and lanthanide–organic complex: synthesis, crystal structure and properties

For the first time, a 3D framework based on lanthanide-containing Nb/W mixed-addendum polyoxometalate has been obtained by utilizing the steam-assisted conversion method.

Synthesis of hierarchical zeolite Beta with low organic template content via the steam-assisted conversion method

We report the synthesis of hierarchical zeolite Beta with low organic template (tetraethylammonium hydroxide, TEAOH) content by the steam-assisted conversion (SAC) method. Neither zeolite seeds nor the secondary mesoporogen were required. Only a bit amount of organic template (denoted as TEAOH/SiO2 ratio=0.06) was used to obtain highly crystalline zeolite Beta, which was remarkably lower than the reported lowest value (0.10). Physicochemical properties of the resulting samples were investigated by comprehensive characterizations including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, solid state 27Al MAS NMR spectra, nitrogen adsorption–desorption, X-ray fluorescence, temperature-programmed desorption of ammonia and infrared spectra of pyridine adsorption. These characterization results showed that the SAC method led to hierarchical crystalline zeolite Beta aggregates assembled by 20–40nm nanosized crystals with controllable SiO2/Al2O3 ratios among 29–47 by varying the compositions of synthetic gels with the Na2O/SiO2 ratio of 0.12 and the H2O/dry gelratios at 0.20–0.30. These zeolite Beta aggregates possessed large external surface area (158–180m2/g) and mesoporous volume (0.27–0.30cm3/g) resulted from the formation of intercrystalline mesopores and of the small zeolite crystal sizes. The obtained hierarchical zeolite Beta showed 41.2% conversion in the esterification reaction of levulinic acid with ethanol and four catalytic runs without obvious decrease in conversion, which were better than those on the zeolite Beta samples from the conventional hydrothermal synthesis. This was due to the promotion of the accessibility of reactants to the acid sites distributed on the external surface of zeolite nanocrystals and of the prominent diffusion in intercrystalline mesopores.

Dry gel conversion as a suitable method for increasing the lifetime of SAPO-18 in MTO process

The crystalline and pure SAPO-18 was synthesized by dry-gel conversion (DGC) methods (including steam-assisted conversion (SAC) and vapor phase transport (VPT)) and by conventional hydrothermal (CHT) method. The physicochemical properties of the catalysts were analyzed by XRD, FESEM, EDXS, and TPD techniques. It was found that the DGC methods have no significant effect on the crystallization time of SAPO-18. The catalysts with smaller crystallite size and in a higher solid yield were achieved by DGC methods. The results also revealed that DGC method changes significantly the acidic properties of SAPO-18 by affecting the amount and mechanism of silicon incorporation into the framework. The catalytic behavior of the samples was investigated in methanol-to-olefins (MTO) reaction. Although no considerable change was observed in light olefin selectivity over the samples, SAC method showed an increase of about 20% in useful lifetime of SAPO-18. It can be due to the smaller crystallite size and desirable acidic characteristic. Finally, the results were compared with those of DGC-synthesized SAPO-34 reported in the literature. It suggests that SAPO-18 synthesized by SAC method can be an acceptable alternative to SAPO-34 by exhibiting a comparable activity and longer lifetime in MTO process.

Steam-Assisted Synthesis of an Extra-Stable Polyoxometalate-Encapsulating Metal Azolate Framework: Applications in Reagent Purification and Proton Conduction.

Different from the conventional synthesis approaches, such as hydrothermal or solvothermal synthesis, a porous metal azolate framework encapsulating Keggin-type [SiW12 O40 ](4-) anions was prepared by an environmentally friendly, low-cost, and highly efficient steam-assisted conversion method for the first time. The nanosized polyoxometalates as a template were encapsulated by a zeotype 6(4) 4(8) cage constructed by 28 nuclear zinc atoms connected through 32 Trz ligands. The obtained composite exhibits excellent thermal and chemical stability; meanwhile, its special ability to selectively absorb water from alcohols makes it efficiently separate water from analytically pure ethanol, with the result that water content decreases from 0.23 to 0.05 wt %, which is superior to the standard of chromatographic grade ethanol (<0.1 wt %). Besides, alternating current (ac) impedance experiments also reveal that the hybrid is a kind of proton conductive material.