Vapor-phase Transformation 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>

Synthesis of hierarchical LTA zeolite membranes by vapor phase transformation

Weakening the mass transfer resistance inside the selective layer of membrane can effectively increase the membrane separation performance. In this study, we introduce macropores into the conventional LTA zeolite membrane layer, resulting in an unprecedented hierarchical membrane structure. Specifically, the hierarchical LTA zeolite membranes composed of a macroporous zeolite layer and a thin dense top layer (600 nm), are fabricated on rough α-Al2O3 tubular supports by vapor phase transformation method using mesoporous silica spheres (MSPs) as the porogen in the precursor layer. The H2 (N2) permeance of the hierarchical LTA zeolite membrane is near 10 times that of the conventional LTA zeolite membrane, indicating that the hierarchical pore structure can effectively weaken the mass transfer resistance inside the selective layer of membrane. And it has good pervaporation performance for 90 wt% ethanol aqueous solution at 348 K, i.e., the flux can reach 4.54 kg m−2 h−1 with a separation factor of >10,000, which is much higher than that of the conventional LTA zeolite membrane. The preparation of LTA zeolite membranes with hierarchical pore structure using MSPs as a porogen is expected to open up a new way for the construction of zeolite membranes.

Decomposition of hydrogen solution in the neighborhood of the coolant saturation temperature as a possible cause of increased oxidation of VVER fuel element

The analysis carried out in this work has shown that a sharp local decrease in the hydrogen concentration and a corresponding increase in the radiolytic oxygen content is caused by two phase transitions occurring in the reactor coolant at successive temperatures. Decomposition of hydrogen solution in the coolant in the neighborhood of the saturation temperature is followed by the liquid–vapor phase transformation. In this case, the coolant may in some zones deviate from the strict water-chemistry regime aimed to suppress radiolysis. It has been shown that the hydrogen concentration in the coolant may decrease by orders of magnitude in a short time. In this case the efficiency of radiolytic oxygen suppression reduces drastically, and concentration of the radiolytic oxygen in the coolant may increase considerably, thus promoting local oxidation of VVER fuel element claddings in E110.

Fabrication of 2D bimetallic metal-organic framework ultrathin membranes by vapor phase transformation of hydroxy double salts

At present, the construction of a bimetallic and oriented MOF nanosheet membrane that integrates the advantages of both bimetallic MOF and oriented membranes is highly desirable but also challenging. Herein, we report a vapor phase transformation (VPT) strategy for directly growing a highly oriented bimetallic (Zn/Co) 2 (bIm) 4 (bIm = benzimidazole) nanosheet membrane, based on the conversion of hydroxy double salts (HDS) under bIm ligand vapor treatment. The fabricated strategy includes first the growth of a layer of bimetallic Zn/Co-HDS on a porous tube and then the conversion of the HDS layer into a highly oriented bimetallic nanosheet membrane via the VPT strategy. The formation mechanism of the membrane is discussed. The resulting ultrathin nanosheet membrane with the thickness of about 80 nm exhibits excellent gas separation performance with H 2 permeance of 1.68 × 10 -7 mol m -2 s -1 ·Pa -1 and H 2 /CO 2 ideal separation selectivity of 54.1. This approach shows great potential in large-scale fabrication of nanosheet membranes, especially the bimetallic MOF nanosheet membranes. • A VPT strategy is developed for growing bimetallic MOF nanosheet membrane based on the conversion of HDS. • The achieved bimetallic MOF nanosheet membrane combines the advantages of nanosheets and bimetallic MOFs. • Both HDS layer and VPT strategy cooperatively facilitate the generation of bimetallic MOF nanosheet membrane. • The membrane exhibited excellent molecular sieving performance for H 2 /CO 2 .

Thermal properties of indium tris-dipivaloylmethanate as a volatile precursor for indium oxide materials

Thermophysical and thermochemical data including condensed and vapor phase transformations and temperature stability ranges, melting, sublimation and vaporization processes for a volatile metal-organic precursor are of current importance to optimize the preparation of functional nano-materials through chemical vapor deposition, atomic layer deposition and their modifications. Here, we present the results of a comprehensive thermal characterization of indium tris-dipivaloylmethanate, In(thd)3 or In(dpm)3, which is a convenient air-stable precursor for widely demanded indium oxide ceramics. Thermogravimetry was applied to evaluate the condensed phase stability both in inert and oxidizing atmosphere. The melting temperature and enthalpy were measured by differential scanning calorimetry. The pressure of saturated and unsaturated In(thd)3 vapors was measured using a static method with glass membrane-gauge manometer. The p(T) equations of saturated vapors over the solid and liquid complex were obtained and the thermodynamic characteristics of the processes under study were calculated. The thermal decomposition of In(thd)3 vapors on a heated surface in vacuum and in oxygen presence was investigated by in situ mass spectrometry. The temperature range of this process and its intramolecular character were revealed. A critical review of the literature data on the thermal properties of In(thd)3 was also performed.

Boosting alkaline hydrogen evolution and Zn–H2O cell induced by interfacial electron transfer

Directly use of metal-organic frameworks (MOFs) as electrocatalysts by taking the intrinsic advantages of MOFs architecture for water splitting have attracted tremendous interest. Herein, we reported a vapor-phase transformation method to construct well-aligned nickel based MOFs (Ni-MOF) nanoarrays grown on electrochemical exfoliated graphite (EG) foil and further phosphorization treatment to derive a Ni-MOF/Ni2P@EG. In such a hybrid structure, Ni2P with a particle size of ~9 nm was encapsulated into Ni-MOF nanorod arrays on EG foil. Profiting from super hydrophilic nature and nanoarray feature, the Ni-MOF/Ni2P@EG hybrid displayed an excellent activity of hydrogen evolution reaction (HER) with low overpotentials of 132 and 233 mV at 10 and 150 mA cm−2, respectively, as well as a small Tafel slope of 59 mV dec−1 in alkaline media, which are among the best values in all previously reported MOFs-based hybrid HER electrocatalysts and even outperform commercial Pt/C at high current density (250 mV at 150 mA cm−2). The strong coupling effect of the constructed interfaces in this hybrid resulted in an efficient electron transfer from Ni2P to Ni-MOF, which promotes the adsorption of water molecules for accelerating water dissociation step, thus boosting hydrogen generation. Further, this Ni-MOF/Ni2P@EG could be used as cathode in alkaline Zn–H2O cell, reaching a power density of 4.1 mW cm−2 with high stability to integrate hydrogen production with electricity generation.

Sublimation‐Vapor Phase Pseudomorphic Transformation of Template‐Directed MOFs for Efficient Oxygen Evolution Reaction

AbstractUsing vapor phase transformation to synthesize template‐directed metal–organic frameworks (MOFs) shows great promise as an approach to avoid the shortcomings of solution‐based strategies. However, among current research, either the products are confined to zeolitic imidazolate frameworks or the conversion technologies are limited to complex processes such as chemical vapor deposition. Here, a well‐designed sublimation‐vapor phase pseudomorphic transformation method is reported to fabricate vertically aligned nanosheet arrays of NiFe‐based MOFs with a uniform and controlled thickness, derived from NiFe‐layered double hydroxides. Benefiting from the optimized morphology and the high intrinsic activity originating from the synergistic coupling effect of NiFe metal clusters, the as‐prepared MOF electrocatalyst displays a superior oxygen evolution reaction performance, requiring an overpotential of 318 mV at 50 mA cm−2 with a Tafel slope of only 47 mV dec−1. Furthermore, a string of metal oxide‐MOFs are obtained, demonstrating the universality of this strategy. By observing the different stages of transformation, the transformation and growth mechanism of MOF crystals is unveiled for the first time. These findings may inspire the exploration and preparation of more species of MOFs, further broadening their application areas.

Liquid–Vapor Phase Transitions and Critical Properties of the C3H7OH–C6H14 System

Proceeding from the experimental (p,T,x) and (p,ρ,T,x) dependences for mixtures of 1-propanol and n-hexane (mole fractions 0.2, 0.5, 0.8, and 0.9) in the two-phase (liquid–vapor), single-phase (liquid and vapor), near-critical, and supercritical regions, the parameters of the points of liquid–vapor phase transformations have been determined by the method of isochore kinks p = f(T)ρ,x and the parameters of the critical points have been found by the semigraphical method with allowance for the scaling behavior. The dependences of the pressure on temperature, density, and composition along the phase-coexistence curve are described by a three-parameter polynomial equation of state: expansion of the compressibility factor Z = p/RTρ in powers of the reduced density, reduced temperature, and composition. The mean relative error of deviation of the calculated pressures from experimental values does not exceed 1%. The temperature dependence of the system density along the liquid–vapor phase-coexistence curve is described by two power-law functions at critical exponent β0 = 0.338 ± 0.002: far from the critical point and in the symmetric part of the equilibrium curve. The mean relative error is 1.47%.

Solvent-Free Secondary Growth of Highly b-Oriented MFI Zeolite Films from Anhydrous Synthetic Powder.

Up to date, zeolite films have been mainly fabricated by in situ crystallization, secondary growth in a solution/hydrogel, or occasionally by vapor phase transformation of dry gel. Here we demonstrate for the first time a solvent-free secondary growth method for b-oriented silica MFI zeolite films using the synthetic powder from ground anhydrous raw solids in the presence of NH4F. Typically, precisely b-oriented MFI zeolite films are synthesized from seed layers of highly b-oriented MFI zeolite crystals in the synthetic powder of 1SiO2:0.035TPABr:0.05NH4F at 175 °C for 6 h. If needed, b-oriented MFI zeolite multilayer films can be acquired by changing the synthesis time or the amount of NH4F in the synthetic powder. Compared with the traditional hydrothermal synthesis, the approach developed here may provide a new avenue for fabricating high quality zeolite films/membranes.

Bottom-up fabrication of two-dimensional Co-based zeolitic imidazolate framework tubular membranes consisting of nanosheets by vapor phase transformation of Co-based gel for H2/CO2 separation

Two-dimensional (2D) metal-organic framework (MOF) nanosheet membranes demonstrate superhigh separation properties and offer great potential in practical separation applications, but the preparation of such membranes, especially tubular ones, still remains a huge challenge. Here, a vapor phase transformation strategy is presented for the direct synthesis of a 2D Co-based nanosheet MOF membrane on a porous tubular substrate via ligand vapor phase transformation of the Co-based gel layer coated on the substrate. The gel layer acts as a multifunctional role such as providing active metal source, guiding the oriented growth of the nanosheets and controlling the membrane thickness in the formation of the nanosheet membrane. The achieved Co-based nanosheet membrane with a thickness of around 57 nm exhibits excellent gas separation performance with H2/CO2 ideal selectivity of as high as 58.7. This method shows great promise for the scalable fabrication of MOF nanosheet membranes.

Vapor Phase Decarbonylation of Furfural to Furan over Nickel Supported on SBA-15 Silica Catalysts

Vapor-phase transformations of furfural over SBA-15 silica supported Ni catalysts under H2 in a continuous-flow reactor at atmospheric pressure and 170°C and 230°C were investigated. Two different samples having Ni loadings of 5 and 20 wt% (denoted here by SBA-5Ni and SBA-20Ni, respectively) were prepared by impregnation and characterized by atomic absorption spectroscopy, N2 sorption analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Furan and furfuryl alcohol were two primary products resulting, respectively, from decarbonylation and hydrogenation of furfural. Under the conditions of the study, both reactions exhibited structure sensitivity evidenced by changes in product selectivities with variable Ni loadings. Compared with SBA-20Ni, the 5 wt% Ni catalyst showed better catalytic activity, reaching a furfural conversion of 100 mol% and a selectivity to furan of 98 mol%, after 5 h of time-on-stream at 230°C.

Synthesis of Monolith Hierachical ZSM-5 Zeolite Composed of Nanocrystals by Vapor-phase Transformation Method

A monolith ZSM-5 composed of nano-sized MFI crystals was prepared from a dry gel by a vapor-phase transformation method, in which the dry gel was obtained from a mixture of sodium carboxymethylcellulose and a precursor yielding ZSM-5 zeolite crystals. The as-synthesized samples were characterized by X-ray diffraction(XRD), scanning electron microscope(SEM), N2 adsorption-desorption, in-situ IR spectra of pyridine and FT-IR. Organic Sior/and Al-species replace the classic inorganic Si- or/and Al-species to take part in the construction of MFI zeolite frameworks. And the long chain organic groups, which connect with the Si- and Al-species, disturb the normal growth of the zeolites crystals in the manner of the so called "bond block". The evenly nano-sized zeolite crystals with the sizes of 100–150 nm are therefore formed and then subsequently self-assembly form a monolith ZSM-5. A mesoporous system which centers around 2–20 nm is therefore introduced into the as-synthesized hierachical zeolites samples. As compared with the references catalysts, the monolith ZSM-5 zeolite composed of nano-sized crystals displays an enhanced conversion of isopropylbenzene because of the dramatically increased external surface areas and mesopores volume as tested by the catalytic cracking of isopropylbenzene.

Preparation and Permeation Properties of NaA Zeolite Membrane by Vapor Phase Transformation Method

The NaA zeolite membranes with high permeance were synthesized by seeding and secondary growth both using vapor phase transformation method (VPT). The SEM image of seed layer on substrate indicated that the size of seeds was about 60nm. The XRD patterns showed that the formation of NaA zeolite membrane was after vapor phase transformation of 12h, and the pure NaA zeolite crystals formed on substrate. The SEM images showed that the NaA zeolite membrane consisted of compact polycrystalline crystals with an average size of ca. 2µm. The permeation results showed that the H2 permeance s of NaA membranes attained at the level of 10-6 mol/Pa·m2·s, and the maximum permselectivities of the H2/N2, H2/CO and H2/C3H8 were 4.15,4.78 and 10.1, respectively, which were higher than those of the corresponding Knudsen diffusion selectivities. However, the permeation of C3H8 suggested that there existed unwanted intercrystalline pores or defects in the membranes.

Microwave-Assisted Secondary Synthesis of NaA Zeolite Membranes Grown on ZrO<sub>2</sub> Substrate

The highly intergrown NaA zeolite membranes on seeded ZrO2substrate were synthesized by microwave heating method. The preparation of seeds with the size of ca.60nm employed the vapor phase transformation method (VPT). The XRD patterns indicated that the pure NaA zeolite membranes formed on the seeded ZrO2substrate for varied synthesis times. However, the peak intensity of NaA zeolite membrane after 50min obviously decreased, suggesting that the NaA membrane could take place the dissolution at that time. The SEM images indicated that the NaA zeolite membranes for 15-30min had a good integrity and consisted of highly intergrown zeolite crystals, but the NaA membrane after 50min appeared some large defects, further verifying the result of XRD pattern. The gas permeability showed that the H2/N2and H2/C3H8permselectivities on zeolite membrane attained 4.01 and 8.13, respectively, higher than those of the corresponding Knudsen diffusion. These results suggested that the diffusion of gases, at least in part, are affected by the pore size of zeolite and the function of molecular sieving can be embodied on the synthesized membrane.

Microwave-assisted Synthesis of NaA Zeolite Membrane with High Separating Performance by Seeding Using VPT Method

Abstract A novel synthesis process was developed for synthesis of NaA zeolite membrane with high quality. The method involved two steps, i.e., the seeding with crystal size of ca. 60 nm formed by vapor-phase transformation and the secondary growth by using microwave heating.

Transformation of ZnO Nanobelts into Single-Crystalline Mn3O4 Nanowires

Single-crystalline Mn(3)O(4) nanowires were prepared using the vapor-phase transformation of ZnO nanobelts. Mn(3)O(4)-decorated ZnO nanobelts and ZnO-ZnMn(2)O(4) core-shell nanocables (NCs) were also obtained as reaction intermediates. Heteroepitaxial growth of tetragonal spinel Mn(3)O(4) (or ZnMn(2)O(4)) on wurtzite ZnO is a possible reason for the growth of single-crystalline Mn(3)O(4) nanowires. Growth interfaces are possibly formed between the wurtzite (101[overline]0)/(21[overline]1[overline]0) and spinel (1[overline]01)/(4[overline]11) planes. Various one-dimensional homonanostructures and heteronanostructures consisting of n-ZnO, p-Mn(3)O(4), and p-ZnMn(2)O(4) can be used to design high-performance gas sensors.

High Structure Sensitivity of Vapor-Phase Furfural Decarbonylation/Hydrogenation Reaction Network as a Function of Size and Shape of Pt Nanoparticles

Vapor-phase transformations of furfural in H(2) over a series of Pt nanoparticles (NPs) with various particle sizes (1.5-7.1 nm size range) and shapes (rounded, cubes, octahedra) encapsulated in poly(vinylpyrrolidone) (PVP) and dispersed on MCF-17 mesoporous silica were investigated at ambient pressure in the 443-513 K temperature range. Furan and furfuryl alcohol (FFA) were two primary products as a result of furfural decarbonylation and hydrogenation reactions, respectively. Under conditions of the study both reactions exhibited structure sensitivity evidenced by changes in product selectivities, turnover rates (TORs), and apparent activation energies (E(A)'s) with Pt particle size and shape. For instance, upon an increase in Pt particle size from 1.5 to 7.1 nm, the selectivity toward FFA increases from 1% to 66%, the TOR of FFA production increases from 1 × 10(-3) s(-1) to 7.6 × 10(-2) s(-1), and E(A) decreases from 104 kJ mol(-1) to 15 kJ mol(-1) (9.3 kPa furfural, 93 kPa H(2), 473 K). Conversely, under the same experimental conditions the decarbonylation reaction path is enhanced over smaller nanoparticles. The smallest NPs (1.5 nm) produced the highest selectivity (96%) and highest TOR values (8.8 × 10(-2) s(-1)) toward furan formation. The E(A) values for decarbonylation (∼62 kJ mol(-1)) was Pt particle size independent. Furan was further converted to propylene via a decarbonylation reaction, but also to dihydrofuran, tetrahydrofuran, and n-butanol in secondary reactions. Furfuryl alcohol was converted to mostly to 2-methylfuran.

Submicrometer Zeolite A Crystals Formation: Low-Temperature Crystallization Versus Vapor Phase Gel Transformation

In the present study, low-temperature hydrothermal crystallization and vapor phase gel transformation have been employed to synthesis LTA-type zeolite crystals of submicrometer size. The crystal growth curves under hydrothermal conditions at 35, 50, and 65 °C were obtained. From these curves, the induction periods and the times for reaching maximum crystallinity for respective temperatures were determined. A set of characterization methods, including XRD, FTIR, SEM, TEM, and DLS, was employed to obtain complementary information. The inspection of the crystalline solids obtained under hydrothermal conditions showed that the size of the crystals is a function of the crystallization temperature. The largest crystals (300 × 300 nm) were obtained at 65 °C, while much smaller particles were synthesized at lower temperatures. Second approach involved vapor-phase transformation of the solid part of initial gel. This approach has also yielded submicrometer-sized zeolite crystallites. The characteristics of crystal...

Preparation of Binder-free Utrafine ZSM-5 Zeolite Monoliths by Vapor-phase Transformation Method

Using vapor-phase transformation method,binder-flee monoliths of ultrafine ZSM-5 zeolite crystals were prepared by conversion of the preformed bodies of diatomite and silica sol,which mixed with some zeolite seeding directors.The tiny zeolite crystals and units in the seeding directors inducted the conversion of the alumino-silicalite raw materials into intergrown ZSM-5 zeolites,which resulted in binder-free zeolite monoliths.The prepared materials possess enriched porosity,high surface area,good mechanical strength and tunable acidity which can be verified by the characterization.The products have promising applications in catalyst and adsorbent.

Zeolite membrane with orientation-controlled hexagonal crystals from a mesostructured gel supported on γ-Al 2O 3

The crystals well oriented zeolite membranes were synthesized by the vapor-phase transformation (VPT) of the mesostructured gel coated and spinning on γ-Al 2O 3 substrate and characterized by a series of techniques such as XRD, SEM, TEM and elemental analysis. It is shown that the orientation of crystals can be controlled by variation of synthesis atmosphere and the compactness and the thickness of the membrane can be tailored by variation of the coating method and the number of coated layers. The gas separation performances of synthesized membranes have been evaluated. The selectivity of C 3H 8/O 2 of membrane obtained by dip coating is much higher than that by spin coating due to the difference in compactness of these membranes.