Hydrothermal Crystallization Process Research Articles

Tuning the size and spatial distribution of Pt in bifunctional Pt-zeolite catalysts for direct coupling of ethane and benzene

Controlling the dispersion and position of metal active sites in bifunctional metal–acid zeolite catalysts is essential for achieving synergy between the metal and acid sites in tandem catalytic processes, but still challenging nowadays. Herein, we propose an approach to modulate the dispersion and position of Pt particles in ZSM-5 zeolite by employing various types of organic structure-directing agents (OSDAs) in the one-pot synthesis procedure. Systematic characterizations show that the OSDAs mainly influence the encapsulation of the Pt precursor into the ZSM-5 zeolite crystallites during the hydrothermal crystallization process, resulting in discrepancies in the dispersion and spatial distribution of Pt sites in the final Pt-zeolite catalysts. Comparative studies on different Pt-zeolite catalysts show that the formation of subnanometer Pt clusters and the close spatial proximity of Pt and Brønsted acid sites in the zeolite efficiently promote the tandem dehydrogenation-alkylation process and enhance the production rate of ethylbenzene in the direct ethane-benzene coupling reaction. This study offers insights into the design and synthesis of bifunctional metal-zeolite catalysts for tandem catalysis.

Preparation and growth mechanism of YFeO3 magneto optic crystals

YFeO3 is one of important magneto optic crystals with perovskite structure. The anisotropy of the crystal leads to different physical and chemical properties of the crystal in different directions. In this work, YFeO3 crystals were grown by hydrothermal method and the effects of different mineralizers on the crystal growth were investigated. The results showed that the mineralizer could affect the growth direction of crystal plane during the hydrothermal crystallization process. YFeO3 crystals with various morphologies were grown by adjusting the concentration of the mineralizer under hydrothermal conditions. The transformations between the crystal planes during crystallization were studied by modeling, and the stabilities of {002} and {110} crystal planes under the action of KOH and NaOH were determined. The effects of urea on YFeO3 crystal planes and NH4+ on {002}, {020} and small crystal planes were investigated. Furthermore, the exposure ratio of YFeO3 crystal plane has great influence on its magnetization and coercive field. Finally, the formation and change mechanism of YFeO3 crystals by the hydrothermal method were proposed, providing evidence for further study of the dependence of the shape of YFeO3 on ferromagnetism.

Effect of Metal Complexing on Mn–Fe/TS-1 Catalysts for Selective Catalytic Reduction of NO with NH3

TS-1 zeolite with desirable pore structure, an abundance of acidic sites, and good thermal stability promising as a support for the selective catalytic reduction of NO with NH3 (NH3-SCR). Herein, a series of Mn-Fe/TS-1 catalysts have been synthesized, adopting tetraethylenepentamine (TEPA) as a metal complexing agent using the one-pot hydrothermal method. The introduced TEPA can not only increase the loading of active components but also prompts the formation of a hierarchical structure through decreasing the size of TS-1 nanocrystals to produce intercrystalline mesopores during the hydrothermal crystallization process. The optimized Mn-Fe/TS-1(R-2) catalyst shows remarkable NH3-SCR performance. Moreover, it exhibits excellent resistance to H2O and SO2 at low temperatures. The characterization results indicate that Mn-Fe/TS-1(R-2) possesses abundant surface Mn4+ and Fe2+ and chemisorbed oxygen, strong reducibility, and a high Brønsted acid amount. For comparison, Mn-Fe/TiO2 displays a narrower active temperature window due to its poor thermostability.

Directly preparing well-dispersed ultra-hydrophilic NiFeP nanoparticle/Mxene complexes from spent electroless Ni plating solution as efficient hydrogen evolution catalysts

A novel approach is proposed to prepare well-dispersed ultra-hydrophilic NiFeP nanoparticle/Mxene complex by recovering Ni, P, and organic acid from spent electroless Ni-plating solution. The mechanism of NiP-based nanoparticle formation and size regulation during the hydrothermal process is analyzed. Ni2+ chelated with sodium citrate in the electroless Ni plating effluent anchors on the Ti3C2 surface with -OH functional groups by chemical reaction and charge adsorption owing to Mxene surface negativity. The as-prepared ultrafine NiFeP nanodots are connected with alkali-induced Ti3C2 Mxene nanosheets, which possess ultra-hydrophilic performance and rapid H2 bubble desorption behavior. The introduction of alkali-induced Ti3C2 MXene as a substrate controls particle agglomeration in the hydrothermal crystallization process and achieves perfect dispersed distribution of NiFeP nanoparticles to further increase the specific surface area of catalyst. Consequently, the NiFeP/Ti3C2 nanohybrid catalyst exhibits outstanding hydrogen evolution reaction (HER) electrocatalyst performance as a result, with an overpotential of 122 mV at 10 mA cm−2, quick reaction kinetics of 68 mV dec−1, and strong long-term stability over 24 h in 1 M KOH. This research develops a strategy for directly preparing high-value catalyst materialization from spent electroless Ni-plating solution.

Promotion effects of FeCu co-doping on Co3O4-based catalysts for linear α-olefins to alcohols

For the preparation of alcohols from linear α-olefins, Cu or/and Fe doped Co3O4 catalysts were synthesized using two processes and their structural and catalytic properties were studied. Using microwave-assisted and hydrothermal (HT) crystallization processes, doped Co3O4 with the rod-like morphology and the lamellar morphology were obtained, respectively, and the two morphology both showed the dominantly exposed Co3O4 (111) and (220) planes which were related to the successive hydroformylation and hydrogenation steps of olefins. With microwave-assisted process, Cu doped Co3O4 dramatically improved the hydroformylation reactivity of 1-octene. FeCu or NiCu co-doped Co3O4 raised nonanol selectivity and the promotion of FeCu co-doping was better. At 150 °C and 7 MPa for 4 h reaction, 2Fe2Cu-Co3O4(nFe:nCu:nCo = 2:2:100) exhibited 1-octene conversion of 99.82% and nonanol selectivity of 50.80%. With HT crystallization process, 2Fe2Cu-Co3O4-HT displayed 1-octene conversion of 99.69% and nonanol selectivity of 67.87% under 150 °C, 7 MPa for 5 h reaction. The effects of reaction temperature, pressure, time and catalyst dosage on the activity were discussed. At the set reaction conditions, 2Fe2Cu-Co3O4-HT exhibited high activity for C6 ∼ C12 olefins converting to C7 ∼ C13 alcohols. The roles of Cu or/and Fe doping and the effects of the exposed crystal planes of Co3O4 were revealed by characterizations.

Synthesis of Pt-based TS-1 catalysts for selective hydrogenation to produce C15–C18 alkanes from the FAME: Effect of rare-earth metal additives

Green biodiesel is a clean, renewable, and environmental-friendly biofuel, which is commonly produced through catalytic conversion of lipids. The properties of Pt-based catalyst carriers and additives are important factors determining the production of green biodiesel. Herein, titanium silicalite-1 (TS-1) with a large specific surface area was synthesized by a hydrothermal crystallization process, and the rare-earth metal additives including LaOx, CeOx, PrOx, and YOx were used to modify Pt/TS-1 on the fatty acid methyl ester (FAME) hydrotreatment to enhance the selectivity of C15–C18 alkanes. It is shown that the addition of rare-earth metals decreases the particle size of Pt, increases the content of Pt0 species, and changes the major reaction pathway from hydrodecarbonylation (DCO)/hydrodecarboxylation (DCO2) to hydrodeoxygenation (HDO). The Pt–Ce/TS-1 showed the best catalytic performance on the FAME hydrotreatment due to its smaller active metal particle size, more amounts of weak acid, abundant oxygen vacancies, and more content of low chemical states of Pt. The liquid yield, efficiency of deoxygenation, and selectivity of C15–C18 alkanes were 75.7%, 100%, and 97.3% over Pt–Ce/TS-1. The heating value of upgraded liquid products from Pt–Ce/TS-1 reached 49.1 MJ/kg. This work provides a new idea for Pt-based TS-1 catalysts in the preparation of green hydrocarbon biodiesel.

In situ fabrication of layered double hydroxide film immobilizing gold nanoparticles in capillary microreactor for efficient catalytic carbonylation of glycerol

• Layered double hydroxides crystallite were in situ fabricated on inner wall of capillary. • Thickness and morphology of LDH film on inner-wall of capillary was finely controlled. • Au NPs were immobilized on layered double hydroxides film. • Au/LDH film showed good catalytic activity in carbonylation of glycerol with urea. • The productivity of glycerol carbonate reached 3.78 g•h −1 •g −1 on Au/LDH catalytic film. Microreactor is capable of intensifying catalytic reaction processes. The fabricating of catalytic film inside microchannels for catalytic reaction remains a challenge. Here we report a facile method for in situ fabrication of layered double hydroxide (LDH) film immobilizing gold nanoparticles in capillary microreactor. Through adjusting the reaction condition of in situ hydrothermal crystallization process and flow deposition process inside microchannel, film thickness, film morphology and Au loading of Au/LDH coating can be finely controlled. Such Au/LDH film inside microchannel exhibited good performance in catalytic carbonylation of glycerol with urea. The yield of glycerol carbonate reached 31.9% at flow rate of 10 µl/min, residence time of 6.62 min and reaction temperature of 413 K under atmospheric pressure. The maximum productivity of 3.78 g∙h −1 ⋅g −1 was obtained at flow rate of 40 µl/min and residence time of 1.65 min under same reaction temperature and pressure. Continuously running the microreactor for 30 h proved the high stability of this catalytic film inside microchannels. This approach could be extended to fabricate other diatomic and triatomic metal LDH films immobilizing metal nanoparticles inside microchannels for heterogeneous catalysis.

Controlling the Morphology and Titanium Coordination States of TS-1 Zeolites by Crystal Growth Modifier.

Developing an effective strategy to synthesize perfect titanosilicate TS-1 zeolite crystals with desirable morphologies, enriched isolated framework Ti species, and thus enhanced catalytic oxidation properties is a pervasive challenge in zeolite crystal engineering. We here used an amino acid l-carnitine as a crystal growth modifier and ethanol as a cosolvent to regulate the morphologies and the Ti coordination states of TS-1 zeolites. During the hydrothermal crystallization process, the introduced l-carnitine can not only tailor the anisotropic growth rates of zeolite crystals but also induce the formation of uniformly distributed framework Ti species through building a suitable chemical interaction with the Ti precursor species. Condition optimizations could afford the generation of perfect hexagonal plate TS-1 crystals and elongated platelet TS-1 crystals enriched in tetrahedral framework Ti sites (TiO4) or mononuclear octahedrally coordinated Ti species (TiO6). Both samples showed significant improvement in catalytic activity for the H2O2-mediated epoxidation of alkenes. In particular, the elongated platelet TS-1 enriched in "TiO6" species afforded the highest activity in 1-hexene epoxidation, with a turnover frequency (TOF) of up to 131 h-1, which is approximately twice as high as that of the conventional TS-1 zeolite (TOF: 65 h-1) and even higher than those of the literature-reported TiO6-containting TS-1 catalysts derived from the hydrothermal post-treatment of TS-1 zeolites. This work demonstrates that the morphologies and the titanium coordination states of TS-1 zeolites can be effectively tuned by directly introducing suitable crystal growth modifiers, thus providing new opportunities for developing highly efficient titanosilicate zeolite catalysts for important catalytic applications.

In-situ passivation reaction for synthesis of a uniform ZrO2-coated ZrB2 powder in alkaline hydrothermal solution

A convenient coating process was developed to prepare zirconia-coated zirconium diboride (ZrB2@ZrO2) powder by in-situ passivation reaction in the mixed hydrothermal solutions of sodium hydroxide (NaOH) and hydrogen peroxide (H2O2). Amounts of techniques consisted of field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) were used to study the effects of coating parameters (NaOH concentration, H2O2 concentration, reaction temperature and reaction time) on the microstructure and composition. The results show that the coated powder exhibits a flat surface similar to the raw powder. The coating has a full coverage and its content is controllable by adjusting reaction time. It has a double layered structure. The inner layer of the coating mainly derived from the in-situ passivation reaction is the mixture of tetragonal zirconia (t-ZrO2) and amorphous zirconia (a-ZrO2). The outer layer produced by the hydrothermal crystallization process is monoclinic zirconia (m-ZrO2). The coated powder with 30 wt% ZrO2 has excellent sintering property and contributes to reforming a dense coated structure of the sintered composite. Thus, its oxidation resistance is significantly improved at 1400 °C.

One-Step in Situ Synthesis of Reduced Graphene Oxide/Zn-Al Layered Double Hydroxide Film for Enhanced Corrosion Protection of Magnesium Alloys.

As an effective and environmentally friendly material for corrosion prevention, layered double hydroxide (LDH) films have usually been degraded due to their inherent microporous structure. In this study, graphene derivatives were employed to enhance the corrosion resistance of LDH films. After ultrasonic treatment of a reaction solution mixture containing graphene oxide (GO) powder, a reduced graphene oxide/zinc-aluminum LDH (RGO/Zn-Al LDH) film was in situ synthesized on a magnesium alloy substrate by a one-step facile hydrothermal crystallization process. The characterization results demonstrated that the LDH nanosheets grew on both the GO surface and the magnesium substrate, and thus the agglomeration of graphene was effectively prevented. Furthermore, the GO plates were simultaneously reduced into RGO, which has better corrosion resistance. The as-prepared samples were individually assessed by potentiodynamic polarization measurements, and the RGO/Zn-Al LDH film showed good corrosion resistance with a lower corrosion current density (0.546 μA/cm2) than that of the bare substrate (33.2 μA/cm2) and Zn-Al LDH film (4.33 μA/cm2). The penetration resistance of the Zn-Al LDH film to a corrosive environment was significantly improved through the organic combination with graphene oxide, and this method provides a simple and facile approach to effectively enhance the corrosion protection performance of LDH materials.

Synthesis of an excellent MTP catalyst: hierarchical ZSM-5 zeolites with great mesoporosity.

A unique organosiloxane-polyether amine (OPA) was produced and used as mesoporogen to efficiently synthesize hierarchical ZSM-5 zeolites with great mesoporosity. We have employed silica sol and tetraethylorthosilicate, respectively, to investigate the influence of different silicon sources on hierarchical zeolites in the presence of OPA. The mesopores of synthesized samples focused on 6–15 nm, and the external surface area varied from 185 to 463 m2 g−1 where the micropore surface area was maintained at 245–334 m2 g−1. Benefiting from the superior structure properties, these samples were used as catalysts in the reaction of methanol to propylene, and the optimal one catalysed for 180 h with methanol conversion above 95%. The as-produced OPA could connect steadily with zeolite frameworks through covalent bonds (–Si–O–Si–) during the hydrothermal crystallization process. This type of connection mode could effectively avoid the formation of amorphous phase and the special molecular structure of OPA could efficiently introduce abundant mesopores with few micropores being consumed. The samples synthesized with silicon sol were made up of quasi-circular particles of about 800 nm in size and further consisted of nanocrystals of 40 nm, and the samples produced with TEOS have a particle size of about 1–2 µm aggregated with nanocrystals of 300 nm.

Synthesis of hierarchically porous silicate-1 and ZSM-5 by hydrothermal transformation of SiO2 colloid crystal/carbon composites

Hierarchically porous silicate-1 and ZSM-5 zeolites were successfully prepared by a hydrothermal crystallization process in the presence of SiO2 colloid crystal/carbon composite. The inter-crystalline mesopores and macropores were successfully created inside these zeolite materials. The effect of synthesis conditions including hydrothermal reaction temperature, reaction time, silica source and aluminum source on the morphology and porous structure were studied in detailed. All the samples were characterized by XRD, SEM, N2 adsorption-desorption analysis and NH3-TPD. The advantage of hierarchically porous structure was evaluated by the catalytic application. The hierarchically porous ZSM-5 as a catalyst displayed higher catalytic activity than the conventional ZSM-5 when using the acetalization reaction of cyclohexanone as a model reaction.

Continuous synthesis of mesoporous Y zeolites from normal inorganic aluminosilicates and their high adsorption capacity for dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT)

Mesoporous Y composites which exhibit both crystallized micropores and large mesopores with narrow mesopore diameter distributions and controlled pore diameters were successfully synthesized. The composite is synthesized through an elaborate design of the SiO2@cationic surfactant cetyltrimethylammonium bromide (SiO2@CTAB) micelle swelled by an appropriate amount of 1,3,5-trimenthylbenzene (TMB); it is subsequently fabricated from the condensing of the Y zeolite precursors, which contain Y zeolite seeds, on the mesoscale micelle followed by a hydrothermal crystallization process. The effect of synthesis conditions such as the amounts of CTAB, TMB and TEOS added to the synthesis system on the physicochemical properties of the obtained products was examined. The as-synthesized materials were carefully characterized by employing XRD, N2 adsorption–desorption, SEM, TEM and pyridine-adsorbed FTIR techniques. The mesoporous Y zeolite exhibited both the typical diffraction characteristics of Y zeolites in the wide-angle XRD pattern, and the typical diffraction characteristics of a narrowly dispersed mesopore structure in the small-angle XRD pattern is successfully synthesized for the first time. Finally, the adsorption capacity for DBT and 4,6-DMDBT by samples with different mesopore diameters were assessed by a batch method. Sample MY-0.5C-0.35T-0.9B showed the highest capacity for adsorbing DBT in model diesel, and sample MY-0.5C-0.35T-1.5B showed the highest capacity for adsorbing 4,6-DMDBT in model diesel. Most importantly, sample MY-0.5C-0.35T-1.5B showed the highest adsorptive rate for both DBT and 4,6-DMDBT, which is at least 10 times faster than that of sample MY-0C-0.35T-0B because of the elimination of the diffusion limitation.

Adsorption of lead ion on amino-functionalized fly-ash-based SBA-15 mesoporous molecular sieves prepared via two-step hydrothermal method

Using an alkali two-step hydrothermal method, SBA-15 mesoporous molecular sieves were prepared from fly ash used in a thermal power plant in Inner Mongolia (China). The effect of alkali solution (i.e., NaOH), added in different amounts at 95 °C, on the desilication rate of the ash was determined. During the 48-h hydrothermal crystallization process at 110 °C, sieves were obtained using Na2SiO3 solution as the silicon source, hydrochloric acid to adjust the pH value, and P123 (PEO-PPO-PEO) as a template. Amino-functionalized molecular sieves were prepared at 120 °C, using 3-aminopropyltriethoxysilane as a modifier and toluene as the solvent. The prepared SBA-15 and NH2-SBA-15 molecular sieves were characterized via X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and N2 adsorption. The effects of adsorption time, temperature, pH value, and the initial concentration of lead ions (Pb2+) on the efficiency of Pb2+ removal were investigated using NH2-SBA-15 as an adsorbent. The results revealed that, like its SBA-15 counterpart, NH2-SBA-15 has a typical two-dimensional hexagonal pore structure, with lower specific surface area, average pore diameter, and pore volume compared with its SBA-15 counterpart. Furthermore, the Pb2+-removal efficiency increased gradually with increasing adsorption time, adsorption temperature, and initial concentration of Pb2+. However, when the pH value was increased, the efficiency increased initially and decreased thereafter. The maximum removal rate (>98%) after 60 min at 30 °C was obtained at a pH value of 5 and a Pb2+ concentration of 100 mg/L. The adsorption process of Pb2+ and the corresponding adsorption isotherm were well described by the second-order kinetics model and the Langmuir isotherm model, respectively, and an equilibrium adsorption amount of 131 mg/g was realized. The fly-ash-based SBA-15 mesoporous molecular sieve adsorbent, developed and subsequently modified in the present study, exhibited improved selectivity and adsorption capacity, and was less expensive than existing adsorbents. This sieve can provide effective technical support for control of the industrial heavy-metal-contaminated wastewater, and provides a new approach for the efficient utilization of fly ash.

Study of the hydrothermal crystallization process of barium titanate by means of X-ray mass attenuation coefficient measurements at an energy of 59.54 keV

In this work, the X-ray mass attenuation coefficients of hydrothermally synthesized barium titanate (BaTiO3) samples were calculated with the purpose of determining the crystallization sequence of BaTiO3. Hydrothermally synthesized samples prepared at 100°C and 200°C, and reacted for varying reaction times between 15min up to 120h were studied. Attenuation coefficient measurements were done with a coaxial HPGe gamma detector (Ortec, GEM55P4-95) with a working range in the X-ray energy region. The samples were made into pellets and were exposed to 241Am radioisotopes at an energy of 59.54keV for 300s. Additionally, FT-Raman and XRD measurements were done to support the X-ray mass attenuation measurements. It was found that secondary barium titanate (BT) phases (BaTi2O5 and Ba2TiO4) were formed from the precursor material at the early stages of the hydrothermal reaction and that phase pure BaTiO3 was formed at longer reaction times. The sequence of barium titanate crystallization was determined as follows: BaTi2O5; BaTi2O5 and BaTiO3; BaTi2O5, Ba2TiO4 and BaTiO3: and phase pure BaTiO3.

Revealing the hydrothermal crystallization mechanism of ilmenite-type sodium niobate microplates: the roles of potassium ions

The roles of K ions in the hydrothermal crystallization process of plate-like NaNbO3 microcrystals were revealed.

Prepared MnO2 with different crystal forms as electrode materials for supercapacitors: experimental research from hydrothermal crystallization process to electrochemical performances

The aim of this study was to prepare manganese dioxide with different crystal forms through hydrothermal treatment of MnSO4·H2O–KMnO4 precursors at various precursor ratios, temperatures, time periods, and pH values.

Systematic study of the crystallization process of CrAPO-5 using in situ high resolution X-ray diffraction

Systematic study of the hydrothermal crystallization process of CrAPO-5 by in situ high resolution X-ray powder diffraction (HRXRD).

In situ impregnation−gelation−hydrothermal crystallization synthesis of hollow fiber zeolite NaA membrane

Chitosan-assisted in situ impregnation−gelation−hydrothermal (IGH) crystallization process has been developed for the preparation of hollow fiber zeolite NaA membranes. Firstly, chitosan-zeolite NaA composite hollow fibers were prepared successfully by assistance of a simple homemade tube-in-orifice spinneret. The composite hollow fibers were initially prepared by in situ impregnation–gelation–hydrothermal transformation of chitosan-silica hollow fibers in aluminate solution. Zeolite NaA membranes can be subsequently obtained on the outer surface of chitosan-zeolite NaA composite hollow fibers by in situ microwave hydrothermal treatment. The zeolite crystals in the composite hollow fibers serve as seeds for the growth of zeolite membrane. Moreover, the chitosan-silica hollow fibers were prepared by solidification of chitosan hollow fibers, which were formed in the tube-in-orifice spinneret from a chitosan-silica sol viscous aqueous solution, in the sodium hydroxide solution. Pervaporation for separation of 90 wt% ethanol aqueous solution was employed to examine the obtained membranes. The hollow fiber membranes showed high permeation flux and high stability.

Time resolved in situ X-ray diffraction study of crystallisation processes of large pore nanoporous aluminophosphate materials.

Time resolved high-resolution X-ray powder diffraction was utilized to obtain detailed changes in the crystal structure parameters during the hydrothermal crystallization process of the nanoporous aluminophosphate AlPO-5 (AFI) structure. This in situ study offered not only the influence of metal ions on the onset of crystallization and estimation of the activation energy of the process, but also allowed us to determine in detail the changes in lattice parameters during this process. More importantly the time-resolved study clearly showed the lattice expansion in the divalent metal ions substituted system right from the on-set of crystallization process, compared to the one without any dopant ions, which suggest that an amorphous or poorly crystalline network is formed prior to crystallization that contains the large divalent ions (compared to Al(iii), the substituting element), which is in agreement with the combined XAS/XRD study reported earlier. A mechanism based on this and the earlier study is suggested.