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A series of HPW supported on ammonium hexafluorosilicate (AHFs) modified Hβ (30HPW/Hβ‐Si(x)) was successfully prepared. After Hβ zeolite was pretreated with 0.1 mol·L−1 AHF's solution, the Brønsted/Lewis ratio of Hβ‐Si(0.1) increased compared with those of Hβ, and the strong acid density of 30HPW/Hβ‐Si(0.1) decreased compared with those of 30HPW/Hβ respectively. Strong acidic sites and Lewis acid sites are catalytic sites for by‐products diisobutene and ethylene glycol oligomers, which deposited on the catalyst result in the deactivation of catalysts. According to the results of characterization, the internal Si‐OH groups of Hβ (strong acidity sites) were filled with silicon of AHFs. The extraframework aluminum (strong acid sites) was removed by the H+, which is the hydrolysis of AHFs. In addition, the hydrophobicity of 30HPW/Hβ‐Si(0.1) was also improved compared with that of 30HPW/Hβ due to the SiO2/Al2O3 ratio of Hβ increased after treatment by 0.1 mol·L−1 AHFs solution. The 30HPW/Hβ‐Si(0.1) maintained an 80% conversion of methyl tert‐butyl ether and 100% selectivity to ethylene glycol mono‐tert‐butyl ether and DBE ethylene glycol bis‐tert‐butyl ether with 336 h of time on stream. [ABSTRACT FROM AUTHOR]

Data on the thermodynamic analysis of the chemical affinity values of the oxides of the elements that constitute titanium-containing concentrates with respect to fluorinating reagents (HF, NH4F, NH4HF2) and on the thermochemical reaction processes of silicon dioxide, various dispersions, and ammonium hexafluorotitanate upon steam heating are provided. The predominant formation of ammonium hexafluorosilicate, which binds the products of pyrohydrolysis of titanium salts upon formation, sublimates at a temperature of 250–300°C, is captured outside the pyrohydrolysis zone, and reduces the temperature of pyrohydrolysis, is confirmed and recommended for application in the hydrofluoride technology used in the production of titanium dioxide. Data on the morphology of titanium dioxide powders are obtained. [ABSTRACT FROM AUTHOR]

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A spent cracking catalyst (SCC) containing 1 wt % of rare-earth-elements (REEs) oxides is proposed as an alternative raw material to obtain REEs. We have studied the process of removing silicon in the form of ammonium hexafluorosilicate (NH4)2SiF6 involving sintering samples of an fluid cracking catalyst with NH4F, followed by (NH4)2SiF6 sublimation, resulting in the formation of an aluminum-containing concentrate of REEs. The effect of three factors is studied using the central composite design (CCD) of the experiment: the temperature of sublimation (from 350 to 400°C), the sublimation duration (from 40 to 80 min), and the mass of the catalyst fluoride cake (from 5 to 10 g). The effect of these factors on the efficiency of (NH4)2SiF6 sublimation is explored. The results of the experiment are used to develop a second-order model consistent with experimental data. The dynamics of (NH4)2SiF6 recovery using sublimation is determined for a sublimation time of τ = 10, 20, 40, and 80 min at processing temperatures of 350, 375, and 400°C. The degree of (NH4)2SiF6 recovery calculated using the second-order model for τ = 44, 48, 52, 56, 60, 64, 68, 72, and 76 min is consistent with the experimental curves. The spectra of fluorinated catalyst samples are studied before and after sublimation using X-ray diffraction phase analysis (XRD) and IR spectroscopy. IR spectroscopy data does not contradict XRD data, indicating that the sinter containing SCC and NH4F also contains (NH4)2SiF6, (NH4)3AlF6, and unreacted NH4F. Only aluminum compounds NH4AlF4 and AlF3 remain after sublimation. The concentration of REEs is 15% due to the removal of silicon. [ABSTRACT FROM AUTHOR]

Zeolite Y was dealuminated in the solid-state at several degrees (5, 10, 15 and 20%) using (NH 4 ) 2 SiF 6 as dealumination agent. The materials were heated to 80, 100, 120, 140, 160, 180 and 200 °C to investigate the best temperature at which the samples retain their crystallinity and acidity. The solids obtained were characterized by XRD, FTIR, TG/DTG, XRF, 27 Al and 29 Si MAS NMR and N 2 physisorption. They were also subjected to pyridine adsorption and calorimetric experiments. Structural and textural results showed that lower temperatures (80 and 100 °C) and smaller degrees of dealumination (5 and 10%) maintained the zeolite crystallinity and increased its specific surface area, microporosity and mesoporosity. The study of the materials acidity indicated the presence of Brønsted acid sites with different strength, hydrogen bonding sites and Lewis sites originated from acid leaching. The calorimetric measurements showed higher enthalpy values for some of the dealuminated materials than for the parent zeolite. It also evidenced an increase for the number of acid sites between −160 and −140 kJ mol −1 and a reduction for the other sites, between −140 and −80 kJ mol −1 . [ABSTRACT FROM AUTHOR]

To bring zeolite membranes to commercial application, corresponding preparation technology must solve the following issues like low permeance, poor reproducibility, high manufacturing cost and scale-up etc., which is still a grand challenge. The imhomogeneous slurry-like mother liquor required by the preparation of pure silica CHA membrane makes this challenge even tougher. Herein, we realized facile and efficient preparation of pure silica CHA membrane for the first time. The use of an active silica source, ammonium hexafluorosilicate ((NH 4) 2 SiF 6), enabled facile membrane synthesis in a homogeneous mother liquor at mild temperature and ambient pressure. The replacement of conventional imhomogeneous slurry-like mother liquor with homogeneous mother liqour significantly improved the reproducibility of membrane synthesis and favored the formation of thin membrane. The high activity of (NH 4) 2 SiF 6 allowed low temperature synthesis at ambient pressure without sacrificing synthesis time. Moreover, gel-less synthesis significantly reduced material waste at no cost of selectivity and permeance. All these factors combined together resulted into a facile and reproducible preparation of thin pure silica CHA membrane with high material efficiency, energy efficiency and low equipment cost. Calcined pure silica CHA membrane exhibited outstanding CO 2 permence of 2.9 × 10−6mol/(m2⋅s⋅Pa) and high CO 2 –CH 4 selectivity ∼170. This efficient and facile preparation method has great potential for large scale synthesis. [Display omitted] • Active (NH 4) 2 SiF 6 as silica source enabled facile synthesis of all-silica CHA membrane at ambient pressure. • Homogeneous mother liquor facilitated synthesis reproducibility. • Gel-less synthesis significantly improved material efficiency. • Outstanding CO 2 –CH 4 separation performance obtained on much cheaper membrane. [ABSTRACT FROM AUTHOR]

A series of silicon incorporated TiO 2 with anatase crystalline phase were prepared via sol–gel using ammonium hexafluorosilicate (AHFS). The synthesized products were characterized using X-ray diffraction, FTIR, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–vis diffuse reflectance spectrophotometry (DRS), energy dispersive X-ray analysis (EDX) and BET surface area measurement. Presence of Si–O–Ti bonds is revealed based on the XPS results. The products were tested for their activity as photocatalysts for degradation of congo red as a prob molecule. The 20% AHFS incorporation displayed the highest photocatalytic activity. This adequate amount of AHFS was beneficial for transferring photogenerated electron and reducing the recombination of electron–holes. [ABSTRACT FROM AUTHOR]

The dealumination of zeolites has been studied to tune acidity, resulting in enhanced catalytic efficiency. MOR, FER, and ZSM-5 were dealuminated to remove 5, 10, 15, or 20 mol% of Al with (NH 4 ) 2 SiF 6 (solid-state reaction at 80 °C). XRD, SEM, and FT-IR showed structural maintenance. Aluminum removal was confirmed by the total Si/Al ratio (XRF and AAS) and 27 Al MAS NMR. The dealuminated samples were tested in the dehydration of different alcohols and compared to the protonic parent zeolites. Dealuminated MOR samples exhibited a decrease in coke formation (maximum decrease of 18.3%, 18.2%, and 6.1% for methanol, ethanol, and 1-propanol dehydration, respectively), and an increase in conversion values (maximum increase of 29.0%, 13.9%, and 6.2% for methanol, ethanol, and 1-propanol dehydration). H-FER/20% showed an improvement in methanol (19.9%) and ethanol (3.8%) conversions and a reduction of coke (decrease of 8.6% and 5.6%, respectively), corroborating the highest value of acid sites (0.73 mmol g −1 ). All catalysts showed 100% selectivity to each desired product. The mechanisms for all dehydrations were related to the presence of Brønsted acid sites. Methanol dehydration might involve the generation of methoxonion ion, and ethoxy groups might be formed for ethanol conversion. For ZSM-5, Lewis acid sites may also participate in the elimination mechanism. High reaction temperature (300 °C) favored intramolecular dehydration to ethylene, avoiding formation of diethyl ether and oligomers. Structural defects on H-ZSM-5/15% could facilitate the access of 1-propanol to active sites, leading to the best combination of TON (0.67), conversion (100%), and coke formation (0.35%). [ABSTRACT FROM AUTHOR]

• Caries affected dentin when treated with etch and rinse technique without the use of disinfectant displayed the highest SBS. • Utilization of various disinfectants has altered the adhesion capacity or bonding efficacy of caries-affected dentin. The present study intended to evaluate the shear bond strength (SBS) of resin cement bonded to caries affected dentin (CAD) after disinfection with rose Bengal (RB), Ti-Sapphire Laser, Ammonium Hexafluorosilicate (NH 4) 2 [SiF 6 ], and ozonated water (O 3). A total of 100 extracted human mandibular molars were acquired using caries severity code, 6 of the ICDAS criteria. To achieve homogeneity and prevent size-biased distributions, the average cavity preparation of all specimens had a depth of 2 mm and a breadth of 3 mm. Specimens were divided into five groups (n = 20) at random according to type of disinfection. Group 1: control group, Group 2: RB, Group 3: O 3 , Group 4: Ti-sapphire laser, and Group 5: (NH4)2[SiF 6 ]. All specimens were etch and rinsed, bonding agent was applied and restored with resin cement. Estimation of SBS was performed by placing samples (10/group) in universal testing machine. Stereomicroscope under 40 × magnification was employed for failure mode analysis (FMA). Statistical analysis was executed using the ANOVA and the Tukey multiple test (p <0.05). The highest SBS was demonstrated in the control group when CAD bonded to resin cement without disinfection (18.22±1.14 MPa). Likewise, the lowest SBS values were unveiled by CAD disinfection with O 3 (12.44±1.36 MPa). Similarly, CAD when disinfected with RB (16.25±1.01 MPa) and Ti-sapphire laser (16.25±1.22 MPa) bonded to resin cement exhibited comparable bond results (p>0.05). Caries affected dentin when treated with etch and rinse technique without the use of disinfectant displayed the highest SBS. However, it was seen that utilization of various disinfectants altered the adhesion capacity or bonding efficacy of caries-affected dentin. [ABSTRACT FROM AUTHOR]

An integration of different modelling methods including object and mathematical simulation are suggested for sublimation processes of inorganic fluorides to optimally simulate the real technological conditions. The paper presents the results of testing this methodology on ammonium hexafluorosilicate sublimation. [ABSTRACT FROM AUTHOR]

The authors have studied the laws of ammonium hexafluorosilicate desublimation on a pilot production unit. The proposed explanation of desublimation was proved by a set of investigations of physical and mechanical properties of desublimate samples. The paper introduces the construction of the improved desublimator, cyclone type advective desublimator.

The authors have studied thermal properties of refined ammonium hexafluorosilicate obtained by anhydrous fluorine-ammonium scheme. The sublimation stages within the temperature range 20…500 °С were studied by thermogravimetric method. The authors have studied the laws of influence of sample heating rate and evaporator geometry on temperature conditions of heating and sublimation of ammonium hexafluorosilicate. Possible ways of improving the equipment for raising the efficiency of heating and evaporation were proposed.

Highlights: [•] hGFs were exposed to 0.001%, 0.01%, 0.1% and 1% SiF for 1, 5, 10 and 30min. [•] SiF affected cell viability in a dose and time-dependent manner. [•] Its toxicity grade ranged from 0 to 3. [•] Its toxic effect results in the induction of a decrease in MMP and DNA synthesis. [•] SiF also caused cellular GSH depletion. [Copyright &y& Elsevier]

Ammonium hexafluorosilicate (AHF) has been applied to arrest caries without discoloration. The purpose of this study was to observe structural and elemental changes of demineralized and AHF applied primary tooth enamel. Enamel from the labial surface of 20 primary canines was divided into an unground side and ground side at the center of the tooth, and demineralized with 35% phosphoric acid for 6 min. The teeth were divided into 4 groups according to a 3-min application of AHF and 1 week of soaking in artificial saliva, as follows: group A (neither AHF nor saliva), group B (only saliva), group C (only AHF), and group D (AHF and saliva), and then subdi-vided according to whether the enamel was ground or unground. Specimens were analyzed with scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). The data were statisti-cally analyzed using ANOVA and Fisher's PLSD test at α = 0.05. In groups A and B, prism structures were seen, however, in groups C and D, enamel surfaces were covered with spherical particles. Ca/P ratio was significantly higher in groups C and D than in groups A and B. There was no significant difference between ground and unground enamel in the content of any element. The values for F, Na, Mg and Si persents and Ca/P ratio were significantly higher for the enamel surface than for points 10-30 ftm beneath the surface. Results of this study suggest the possibility that AHF treatment arrests caries, although further study will be required to confirm this result. [ABSTRACT FROM AUTHOR]

Silver diamine fluoride (Ag(NH3)2F) arrests caries but stains teeth black. To overcome this drawback, we applied ammonium hexafluoro-silicate (AHF; (NH4)2SiF6) and observed changes in the color and structure of demineralized enamel and dentin of extracted primary teeth. Enamel and dentin were demineralized in 10% EDTA solution for 90 s followed by 35% phosphoric acid gel for 60 s, then soaked in AHF solution for 60 s. Before analysis by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS), enamel and dentin were demineralized in 10% EDTA for 90 s. Teeth were divided into 4 groups according to AHF application and artificial saliva immersion status and then examined. The data were statistically analyzed using one-way ANOVA and Fisher's PLSD test at a significance level of P < 0.05. AHF treatment did not cause visible discoloration. Enamel prisms and dental tubules appeared by demineralization were covered with precipitates by AHF application. A sphere-filled membranous structure was observed in the saliva immersion groups. EDS analysis showed that AHF application had no effect on enamel; however, F% and Ca/P ratio were significantly higher on dentin surfaces after AHF application without artificial saliva immersion. Further study on arresting caries treatment is required. [ABSTRACT FROM AUTHOR]

New information is obtained on the structural changes occurring during fluoridizing of natural crystalline quartz by ammonium hydrogen fluoride. The morphological, structural, and phase particulars of the samples at each stage of fluoride processing of natural crystalline quartz are studied. Scanning electron microscopy is used to study the morphology, microstructure, and particle-size distribution. X-ray phase analysis and IR spectroscopy are used to determine the phase composition of the samples. [ABSTRACT FROM AUTHOR]

Two kinds of fluoroalkyl end-capped vinyltrimethoxysilane oligomer [R-(VM)-R] silica nanocomposites containing biphenylene units were prepared by the sol-gel reactions of the corresponding oligomer with biphenylene-bridged ethoxysilanes or 4,4′-biphenol under alkaline conditions, respectively. One is the fluorinated oligomer/silica nanocomposites containing biphenylene units [R-(VM-SiO)-R/ Ar-SiO], of whose biphenylene units were incorporated into nanocomposite cores through the siloxane bondings, and the other is the fluorinated oligomer/silica nanocomposites containing biphenylene units [R-(VM-SiO)-R/ Biphenol], of whose biphenylene units were directly encapsulated into nanocomposite cores through the sol-gel process. Interestingly, the shape of R-(VM-SiO)-R/ Ar-SiO nanocomposites is morphologically controlled cubic particles; although the shape of R-(VM-SiO)-R/ Biphenol nanocomposites is spherically fine particles. Thermogravimetric analyses H magic-angle spinning nuclear magnetic resonance, Ultraviolet visible, and fluorescent spectra showed that biphenylene units in R-(VM-SiO)-R/ Ar-SiO nanocomposites have a flammable characteristic after calcinations at 800 °C; in contrast, biphenylene units in R-(VM-SiO)-R/ Biphenol nanocomposites have a nonflammable characteristic even after calcination at 800 °C. X-ray photoelectron spectroscopy analyses of these two kinds of fluorinated nanocomposites showed that nonflammable characteristic toward biphenylene units in the silica gel matrices is due to the formation of ammonium hexafluorosilicate during the sol-gel process. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: A method of elaboration of nanometric amorphous silica is proposed using a rational processing of quartz-based ores with the help of a fluorination method. The different steps of the process are described, including the kinetics of the interaction mechanism of ammonium bifluoride with the initial raw materials, the sublimation of ammonium fluorosilicate and the formation of nanometric amorphous silica. Rate constants and activation energy of the chemical reactions are calculated. [Copyright &y& Elsevier]

The composition of volatile products upon heating of a datolite concentrate that was fluorinated with ammonium hydrodifluoride were studied; the temperature parameters of the transition of the boron and silicon compounds to the gaseous phase were determined and it was established that the difference of the volatilities of the boron and silicon fluoroammoium salts allows selective extraction of these products. A closed technical scheme for processing datolite concentrate by ammonium hydrodifluoride to produce commercial products, viz., ammonium fluoborate, boric acid, amorphous silica, and fluorite, was proposed. [ABSTRACT FROM AUTHOR]

Abstract: The aluminum–silicon distribution and mesoporosity of Y zeolites prepared by sequential NaOH desilication and ammonium hexafluorosilicate (AHFS) dealumination are compared with that of Y samples prepared via AHFS dealumination only. AHFS treatment led to severe non-uniform dealumination and substantial surface silicon deposition. Y samples obtained by sequential desilication and dealumination had substantially better dealumination uniformity and aluminum–silicon distribution. The mesopore formation in these zeolites is discussed in detail. The desilication creates defects in the framework of the parent NaY zeolite. These defects improve the intra-crystalline transport and induce mesopore formation during dealumination. The desilication- plus dealumination-treated zeolites showed higher initial activity and lower deactivation tendency in the case of 1,3,5-triisopropylbenzene cracking, and higher conversion rate of cumene than those zeolites modified by AHFS treatment only. These catalytic data indicate that the former could be a viable catalyst in the catalytic cracking of heavy hydrocarbons. [Copyright &y& Elsevier]

Abstract: Ammonium hexafluorosilicate [SiF: (NH4)2SiF6] was prepared in order to overcome the tooth discoloration caused by diamine silver fluoride [AgF: (NH3)2AgF] application. We employed a single concentration of SiF solution in our previous study; therefore, it is still unclear how the concentration of SiF solution affects the occlusion of dentin tubules and composition of the precipitate. Objective: The aim of this study was to evaluate the effects of changing the concentration of SiF on its clinical use as a dentin hypersensitivity treatment. Methods: To simulate dentin tubules subject to dentin hypersensitivity, dentin disks were treated with EDTA for 2min. Then, the disks were treated with several concentrations of SiF solution (from 100 to 19,400ppm) for 3min. The occlusion of dentin tubules was evaluated using scanning electron microscopy (SEM), and the composition of the precipitate formed in the tubules after SiF treatment was assessed using energy dispersive X-ray analysis (EDXA). Results: SEM photographs demonstrated that dentin tubules after treatment with SiF were occluded homogeneously and fully regardless of the concentration of SiF solution. The Ca/P molar ratio of the precipitate formed in dentin tubules after SiF treatment was increased with the concentration of SiF solution. Significance: It was concluded that the capacity to occlude dentin tubules was the same regardless of the concentration of SiF solution. However, the composition of the precipitate formed in the tubules was dependent on the concentration of SiF solution. [Copyright &y& Elsevier]

Abstract: Diamine silver fluoride [AgF: (NH3)2AgF] has been used clinically in Japan, as it reduces dental caries and dentin hypersensitivity. However, AgF stains the teeth black due to silver precipitation. To overcome this drawback, the authors prepared ammonium hexafluorosilicate [SiF: (NH4)2SiF6], which does not stain the teeth, and SiF occluded open dentin tubules completely with silica–calcium phosphate precipitate. Objectives: The aim of this study was to evaluate the duration of dentin tubule occlusion after SiF treatment in a simulated oral environment. Methods: To simulate dentin tubules subject to dentin hypersensitivity, dentin disks were treated with EDTA for 2min. The disks were treated with 0.476mol/L SiF for 3min, and then the disks were immersed in synthetic saliva, which was regularly replenished to maintain its ionic concentration, for up to 7 days. The occluding ability of the dentin tubules was evaluated using scanning electron microscopy (SEM), and the hydraulic conductance was measured following Pashley''s method at regular intervals. Results: SEM photographs demonstrated that dentin tubules were occluded homogeneously and completely with the precipitate at 7 days after treatment with SiF. In addition, newly formed calcium phosphate precipitate was present at the dentin surface. The dentin permeability showed a consistently low value throughout the experimental period. The values immediately after SiF treatment and 7 days after immersion were 11.9±3.7% and 7.9±2.9%, respectively. Significance: Ammonium hexafluorosilicate is useful for the treatment of dentin hypersensitivity, since ammonium hexafluorosilicate induced calcium phosphate precipitation from the saliva; therefore, it has a continuous effect on dentin tubules occlusion under a simulated oral environment. [Copyright &y& Elsevier]

Abstract: Ammonium hexafluorosilicate (AHFS) is an effective dealumination reagent for MCM-22 zeolite, and the Si/Al2 ratio range of MCM-22 was extended largely (SiO2/Al2O3 ∼300) by AHFS treatment. The physicochemical properties of the MCM-22 samples before and after dealumination were characterized by XRD, XRF, N2 adsorption and pyridine-IR measurements. Also, the samples were used as catalysts in C4 alkenes catalytic cracking to propene and the results showed that properly dealuminated MCM-22 (Si/Al2 ratio 58) exhibited higher activity, better stability and similar product distribution relative to the parent MCM-22 zeolite. [Copyright &y& Elsevier]

Ferrierite zeolite has been treated with ammonium hexafluorosilicate (AHFS). The prepared catalysts were characterized by XRD, atomic absorption spectroscopy, ammonia TPD, FT-IR, SEM, BET/pore size distribution and pyridine chemisorption. Their catalytic activity has been evaluated in the skeletal isomerization of n-butene. Ammonium hexafluorosilicate treatment of ferrierite zeolite leads to a little but progressive framework dealumination, although bulk Si/Al ratio determined by chemical analysis only increases slightly, indicating that most of the extracted aluminum atoms remain in the pores. Higher fluorosilicate amounts decrease crystallinity, micropore area, and volume. Ammonium hexafluorosilicate treatment remove mainly ferrierite external acid sites with a marked improvement in the number of medium strength acid sites.The higher dealumination degree the lower isobutene yield, however, the presence of extraframework aluminum species, medium strength acid sites and lower acid site density reduces the conversion of n-butene through consecutive reactions like n-butene dimerization followed by cracking and hydrogen transfer. These reactions are responsible for the formation of by-products and hence dealuminated catalysts show isobutene selectivity values as high as ferrierite zeolite. [Copyright &y& Elsevier]

The objective of this work is to evaluate the dealumination via ammonium hexafluorosilicate treatment as an effective method for enhancing the catalytic performance of H-Y zeolite in chlorinated VOC oxidative destruction. A series of Y zeolites with various Si/Al ratios was prepared from a commercial sample and then tested for the catalytic decomposition of 1,2-dichloroethane as a model reaction. In general, these modified Y zeolites exhibited a higher activity than that of the parent material, the zeolite subjected to 50% dealumination resulting the most active catalyst. This increase in activity was associated with the development of strong acidity due to dealumination. Likewise, 50% dealuminated sample showed an improved catalytic behaviour for the destruction of other typical chlorinated pollutants, namely, dichloromethane (DCM) and trichloroethylene (TCE). The ease of destruction was found to follow this trend: 1,2-dichloroethane > dichloromethane > trichloroethylene. [Copyright &y& Elsevier]

Chemical dealumination by (NH4)2SiF6 treatment has been investigated as an effective method for improving the catalytic behaviour of Y zeolites for chlorinated hydrocarbon combustion. Hence, the deep catalytic oxidation of dichloromethane (DCM), 1,2-dichloroethane (DCE), and trichloroethylene (TCE) was evaluated over two Y zeolites with different Si/Al ratios, at conditions of lean chlorocarbon concentration (around 1000 ppm) in dry and humid (15,000 ppm of water) air between 200 and 550°C. Catalyst-subjected to dealumination exhibited a higher activity in CVOC oxidation compared with that of the untreated one. The activity increase was attributed to the strong Brønsted acidic sites formed during catalyst dealumination. The ease of destruction was found to decrease in the following order: DCE>DCM>TCE. The main oxidation products formed were CO, CO2, HCl, and Cl2. In addition, some other chlorinated by-products/intermediates were detected, namely vinyl chloride, methyl chloride, and tetrachloroethylene in the oxidation of DCE, DCM, and TCE, respectively. The addition of water slightly diminished the zeolite activity and markedly changed the product distribution. Hence, by-product generation was found to be significantly reduced. Water also promoted somewhat the CO2 selectivity and led to complete reaction of chlorine to HCl by the Deacon reaction. [Copyright &y& Elsevier]

The preparation of zeolite membranes usually involves long hydrothermal synthesis at elevated temperature, which leads to issues like lengthy synthesis, high energy consumption, safety concern etc. It is desirable to prepare zeolite membranes at low temperature with short synthesis time. In this study, thin Silicalite-1 zeolite membranes were prepared in 4–6 h by conventional hydrothermal synthesis at ~373K. The use of ammonium hexafluorosilicate ((NH 4) 2 SiF 6), a highly reactive silica source, is the key to the ultrafast crystallization kinetics even at low temperature, which greatly increased productivity of membrane synthesis. The crystal growth rates of Silicalite-1 crystals and membranes were at least 10 times faster than using regular silica source. Hexane permporometry indicated the presence of low defect density in the membrane. High n-butane permeance of 4.3 × 10−7 mol m-2 s-1·Pa-1 and decent n-butane/i-butane selectivity of 17 were obtained on the Silicalite-1 membranes, which is comparable to the leading Silicalite-1 membranes. The fast synthesis of high performance Silicalite-1 membranes at benign condition greatly simplified membrane preparation and lowered membrane cost. This approach might be used for the synthesis of other types of all-silica zeolite membranes. • Fast synthesis of ultrathin Silicalite-1 membranes at low temperature. • Fast crystallization kinetics attributed to ammonium hexafluorosilicate as silica source. • Fast synthesis at low temperature lowered synthesis cost. • High permeance and decent selectivity achieved on the obtained membranes. [ABSTRACT FROM AUTHOR]