Ammonium Dichromate Research Articles

Synthesis, characterization and catalytic application of Cr–SBA-1 mesoporous molecular sieves

A series of Cr–SBA-1 ( Pm3 n) mesoporous molecular sieves were directly synthesized under strongly acidic conditions using ammonium dichromate as chromium source and characterized by various techniques. X-ray diffraction (XRD) showed that the synthesized mesoporous materials had a well-ordered cubic structure. N 2 adsorption/desorption measurements confirmed that the resultant samples had large mesopores (>23 Å) and highly surface areas (>1000 m 2 g −1). FT-IR and UV–vis results revealed that both monochromate and polychromate coexisted on these mesoporous materials. All these samples prepared by direct synthesis have been used as catalysts for dehydrogenation of ethane with carbon dioxide. Among these catalysts, Cr–SBA-1(III) exhibited the highest catalytic performance, giving 21.2% of ethylene yield with a selectivity of 86.9% at 650 °C, whereas, its catalytic activity is lower than that of Cr–SBA-1 prepared by conventional impregnation.

Nanocrystalline pirochromite spinel through solution combustion synthesis

The production of magnesium–chromium oxides by solution combustion synthesis was investigated using glycine and urea for the first time. Ammonium dichromate, urea/glycine and ammonium nitrate aqueous solutions were used as the precursors of the oxides. The effect of different reaction parameters, such as fuel richness, stoichiometry and fuel leanness was evaluated; such parameters were modified by changing the reagents and the fuel/oxidant ratio. The results suggest that glycine is an interesting complexing/combustible agent for ammonium dichromate to produce chromite spinel. Addition of extra ammonium nitrate to stoichiometric compositions improved the specific surface area and reduced the crystallite size. The highest specific surface area (153.40 m 2/g) was obtained for the stoichiometric fuel/oxidant mixtures containing glycine as combustible in combination with ammonium nitrate; however, the smallest crystallite size (approximately 9 nm) of Pirochromite (MgCr 2O 4) was synthesized using urea as combustible.

Thermal stability and degradation of starch derivatives

A complex precursor of composition Cu/Cr=1 has been synthesized, dissolving copper oxalate and ammonium dichromate in aqueous ammonia followed by evaporation and drying. The thermal decomposition of the precursor has been studied in different environments, air/argon. In air, overlapping decomposition of the complex and simultaneous oxidation of the products of decomposition (oxidative decomposition) occur in four steps and the overall process is highly exothermic, whereas the process completes in two stages only in argon preventing oxidation of the products and so resulting negligible heat of decomposition. The elemental analysis and thermogravimetric measurements suggest the stoichiometric formula of the precursor to be CuNH3C2O4NH4CrO4. The precursor is termed as ammoniacal copper oxalate chromate (ACOC).

Synthesis of La[sub 0.7]Ca[sub 0.3]CrO[sub 3] SOFC Interconnect Using a Chromium Source

A chromium source, ammonium dichromate, has been used to synthesize ultrafine La 0.7 Ca 0.3 CrC 3 interconnect powder for solid oxide fuel cells (SOFCs)by combustion synthesis technique. Ammonium dichromate is the replacement of conventional chromium nitrate which is hygroscopic and relatively expensive. Besides being a chromium source, ammonium dichromate acts as a fuel in the auto combustion reaction. The La 0.7 Ca 0.3 CrO 3 powders as derived from dichromate source can be sintered to near theoretical density at a temperature as low as 1400°C and the properties of the samples are at par with those prepared using the conventional chromium nitrate under identical processing conditions.

Adsorption of Cr(VI) on Cross‐Linked Chitosan Beads

A possibility of Cr(VI) removal by the adsorption method is discussed in the paper. An adsorbent were hydrogel chitosan beads are produced by the phase inversion method (by changing pH). The possibility of removing Cr(VI) ions by both pure chitosan hydrogel and its chelate compounds (chitosan cross‐linked with Cu(II) and Ag(I) ions) was investigated. The adsorption proceeded from the solutions of potassium dichromate and ammonium dichromate (NH4)2Cr2O7 and K2Cr2O7. The process rates and adsorption isotherms were determined and described by relevant equations. The process rate was described by the pseudo‐ and second‐order equations, and adsorption equilibria by the Langmuir equations. A slight advantageous change in adsorption properties of chitosan beads was revealed after cross‐linking (for chromium concentration up to 10 g/dm3). A maximum adsorption was 1.1 gCr/g chitosan. Results of the studies show that chitosan hydrogel proves useful in the removal of Cr(VI) ions, additionally, cross‐linking with Cu(II) and Ag(I) ions has an advantageous effect in the case of low‐concentrated solutions.

New combined polymer/chromium approach for investigating the phototransformations involved in hologram formation in dichromated poly(vinyl alcohol)

AbstractThe photochemical behavior of dichromated poly(vinyl alcohol) films has been analyzed upon exposure at 365 nm in relationship with the performances of holograms recorded in such photosensitive materials. The evolution of both involved species, poly(vinyl alcohol) and chromium, has been quantified by the implementation of an innovative approach. This approach combines the monitoring of the structural modification of the polymeric matrix and the fate of the various chromium species. For the first time, it has been established that chromium(V) not only is an indicator of the hologram quality, as previously assessed, but also appears to be responsible for the crosslinking implied in the hologram formation. Actually, the crosslinking has been proved to proceed through coordination bonds around chromium(V), which acts as a bridge between the polymeric chains. The role played by the ammonium cations in the hologram quality has also been elucidated. The improvement brought by ammonium dichromate with respect to potassium dichromate involves amide groups resulting from poly(vinyl alcohol) phototransformation. These groups provide additional chelating sites for chromium(V), leading to an increase in the matrix crosslinking. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1317–1325, 2006

Synthesis of mesoporous chromium phosphate through an unconventional sol–gel route

Mesoporous chromium (III) phosphate has been synthesized first time using an unconventional sol–gel technique using inexpensive chromium source ammonium dichromate. In situ synthesized chromium (III), prepared through reduction of ammonium dichromate using ethanol and nitric acid, forms gel in presence of ammonium dihydrogen phosphate and urea at elevated temperature where tetradecyltrimethylammonium bromide (TTBr) is used as structure directing agent. High specific surface area of 384 m 2/g with narrow pore size distribution has been achieved after extracting the surfactants with ethanolic sodium acetate solution. EDS analysis shows that the Cr/P ratio is 1.05. The material was characterized by powder low angle X-ray diffraction (XRD), wide angle XRD, N 2 adsorption–desorption, HRTEM, FTIR, UV–visible and ESR spectroscopy. Wide-angle XRD patterns show the amorphous nature of the compound. UV–visible and ESR spectroscopic data shows the presence of octahedrally coordinated Cr 3+ solely.

Selective Oxidative Deprotection of Trimethylsilyl Ethers in Solution and under a Solvent Free Condition

Direct oxidation of trimethylsilyl ethers to their corresponding carbonyl compounds has drawn considerable attention in recent years. However, some of the reported methods suffer from disadvantages such as long reaction time, low yields of the products and tedious work-up procedure. Therefore, introduction of new methods for such functional group transformation is still in demand. Recently, we have reported that (NH4)2Cr2O7 in the presence of Al(HSO4)3 and wet SiO2 can be used as an efficient reagent for the oxidation of alcohols to their corresponding carbonyl compounds. In continuation of this study, we were interested in extending the applicability of this reagent system to the oxidation of the other functional groups. In this paper, we report a new, efficient and selective method for the oxidative deprotection of trimethylsilyl ethers to their corresponding carbonyl compounds using the above mentioned reagent system in solution and under a solvent free condition. Oxidative deprotection of different types of trimethylsilyl ethers was investigated in the absence of solvent with ammonium dichromate in the presence of Al(HSO4)3 and wet SiO2 at room temperature (Scheme 1). Yields and reaction times are given in Table 1. Benzylic trimethylsilyl ethers, including electron donating and withdrawing groups are converted to their corresponding carbonyl compounds in high yields (Table 1). Trimethylsilyl ethers containing alkyl ethereal groups are converted to their corresponding aldehydes and ketones without cleavage of carbon-oxygen bond (Table 1, entries 9, 10). This method is also very effective for the oxidation of non-benzylic trimethylsilyl ethers (Table 1, entries 13-16). Over-oxidation of the products was not observed by this method. In order to compare the results obtained under a solvent free condition with those obtained in solution, we studied the oxidation reaction in n-hexane. As shown in Table 1, in most cases, there are appreciable differences between the results obtained in solution and those under a solvent free condition. By omitting the solvent, the reaction time was reduced in addition to ease of the work-up procedure. It should be noted that the oxidation did not proceed using any of Al(HSO4)3, ammonium dichromate or wet SiO2 alone, which presumably suggested the in situ generation of H2CrO4 in low concentration at the surface of wet SiO2 by the solid inorganic acid salts Al(HSO4)3 and (NH4)2Cr2O7. Tetrahydropyranyl ethers do not undergo oxidative deprotection by this method. Therefore, in order to show the selectivity of the described method, we have also performed competitive oxidative deprotection reactions. The experimental results show that trimethylsilyl ethers are oxidized selectively in the presence of tetrahydropyranyl ethers (Scheme 2). These selectivities are useful achievements in organic synthesis. In conclusion, we have introduced a mild, efficient and selective method for the selective oxidative deprotection of trimethylsilyl ethers using ammonium dichromate in the presence of Al(HSO4)3 and wet SiO2 in solution and under a solvent free condition.

Synthesis and characterisation of MgCr2O4 spinel precursor

MgCr 2 O 4 spinel was synthesized from Mg-Cr based hydrogels precursors prepared by coprecipitation technique. Ammonium hydroxide, ammonium carbonate and hexamethylene tetramine (HMT) used as different basic media for coprecipitation. Magnesium nitrate and ammonium dichromate were used as starting materials for the preparation of mixed solution containing Mg 2 + and Cr 3 + in molar ratio for MgO : Cr 2 O 3 as 1 : 1. It had been noticed that spinel formation was influenced by the mixed hydroxides precursors derived by using different basic precipitating agents. DTA, TGA, XRD, IR spectroscopy and SEM technique were utilized to characterize the coprecipitates as well as the final products generated by heat treatment of the coprecipitates at different temperatures. Thermal treatment of the coprecipitates led to the formation of MgCr 2 O 4 spinel at low temperature (500°C). Mixed hydroxides derived by using ammonium hydroxide medium was the most effective one towards the formation of MgCr 2 O 4 spinel at relatively low temperature.

Voltammetry of chromium(VI) at the liquid|liquid interface

The voltammetry of hexavalent chromium (ammonium dichromate) at the interface between two immiscible electrolyte solutions is reported. Detection of Cr(VI) by ion transfer voltammetry is possible by use of an organic phase ionophore, which facilitates the transfer of Cr(VI) from the aqueous into the organic phase. The ionophore was the penta protonated form of polyamine macrocycle 2,5,8,11,14-pentaaza[15]-16,29-phenanthrolinophane (NeoTT). Cyclic voltammetry showed an increase of the peak current on increasing the concentration of Cr(VI). Square wave voltammetry with background subtraction was employed for low level concentration detection. The lowest concentration detected was 0.25 parts per million of Cr(VI).

Nanocrystalline Cr 2O 3 and amorphous CrO 3 produced by solution combustion synthesis

The synthesis of chromium oxides by solution combustion synthesis was investigated. Ammonium dichromate, glycine, urea and ammonium nitrate dissolved in aqueous solution were used as the precursors of the oxides. The effect of different reaction parameters, such as fuel richness, stoichiometry and fuel leanness was evaluated; such parameters were modified by changing the reagents and the fuel/oxidant ratio. Amorphous CrO 3 and crystalline Cr 2O 3 were synthesized. The results suggest that glycine is a better complexing/combustible agent for ammonium dichromate than urea. Addition of extra ammonium nitrate to stoichiometric compositions improved the specific surface area and reduced the crystallite size. The smallest crystallite size (≈20 nm) of Cr 2O 3 was obtained with glycine as fuel/complexant agent in fuel-lean mixtures. The highest specific surface area (63 m 2/g) was observed with urea in fuel-rich mixtures, forming amorphous CrO 3.

Rosin (colophony) holograms sensitized with ammonium dichromate®

Abstract We report a photosensitive emulsion by mixing ammonium dichromate with rosin resin diluted in isopropyl alcohol. This material can be easily elaborated. A phase grating in this material was recorded using an argon-ion laser at λ = 457 nm, shows a moderate diffraction efficiency. This material is capable of a high resolution when we record some diffraction gratings, observing the order of 2000 l/mm (grooves). A characteristic of the developed process is its simplicity that it takes approximately 25 s. It describes a hypothesis with respect to some mechanisms of photosensitivity in emulsions.

Relief hologram replication using a dental composite as an embossing tool

A simplified method for holographic embossing tool production is presented. Surface relief diffraction gratings are holographically recorded in pullulan sensitized with ammonium dichromate (DCP). The surface structure is copied into dental photopolymer composite by direct contact and subsequent photo-polymerization. It was found that arbitrary surface micropattern can be replicated. Due to its excellent mechanical and thermal properties, micro-patterned dental composite can be further used as an embossing tool for mass production of holograms.

Supercritical water oxidation of wastewater from LCD manufacturing process: kinetic and formation of chromium oxide nanoparticles

Supercritical water oxidation (SCWO) has been known as one remarkably effective process for the treatment and disposal wastewater. In this work, SCWO of the wastewater from a liquid crystal display (LCD) manufacturing plant was carried out in an isothermal, isobaric tubular flow reactor with H 2O 2 as an oxidant. All experiments were performed at pressures from 25 to 29 MPa and at temperatures from 396 to 615 °C. At the reactor entrance, the initial chemical oxygen demand (COD) concentrations of wastewater were between 126 and 19,428 mg/l; the oxidant concentrations were between 4.66 × 10 −3 and 1.34 × 10 −1 mol/l. During the SCWO process, in situ formation of chromium oxide nanoparticles (α-HCrO 2 and Cr 2O 3) was found by decomposition of ammonium dichromate, which is contained in wastewater. Based on scanning electron microscope and electrophoretic light scattering analyses for particle size, non-faceted crystalline chromium particles are obtained in the range 200–400 nm. The COD conversion rate was increasing with higher wastewater and oxidant feed concentration. An assumed first-order global power-law rate expression was determined with activation energy of 21.71 ± 1.67 kJ/mol and a pre-exponential factor of 46.84 ± 1.52 s −1 to a 95% confidence level. By taking into account the dependence of the reaction rate on wastewater and oxidant concentration, a global power-law rate expression was regressed from the complete set of data. The resulting activation energy were 47.79 (±1.52) kJ/mol; the pre-exponential factor was 2.78 (±0.71) × l0 2 l 1.075 mmol −0.075 s −1; and the reaction orders for wastewater (based on COD) and oxidant were 1.01 ± 0.01 and 0.065 ± 0.01, respectively.

Oxidative Coupling of Thiols in Solution and under Solvent‐Free Conditions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Oxidation of Alcohols Using (NH4)2Cr2O4 in the Presence of Al(HSO4)3 and Wet SiO2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Full Color Screen by EPD Combined with Photolithography for Flat Panel Displays

Electrophoretic deposition (EPD) was employed for applying a phosphor coating for use in the fabrication of a full color screen consisting of red (R), green (G), and blue (B) phosphors. Patterned phosphor layers with a high packing density and uniformity can be fabricated by EPD combined with photolithography. For this process, R, G, and B phosphors were deposited on a patterned indium tin oxide glass by means of a commercial positive photoresist based on novolac resin and naphthoquinone diazide. The phosphor can be deposited on the patterned conductive substrate, but not on the surface of the photoresist film, because the photoresist has insulating properties. Conventional photolithographic conditions could not be used for this deposition because the commercial photoresist was soluble in the alcoholic solvent, which was used during the phosphor deposition. Instead, an insoluble photoresist was obtained by optimizing the baking temperature. Patterned color screens were studied using scanning electron microscopy and photoluminescence spectroscopy. The adhesion strength of the patterned color screens was substantially improved by the use of a coating of aqueous solution including polyvinyl alcohol and ammonium dichromate, followed by curing under UV light. © 2004 The Electrochemical Society. All rights reserved.

Oxidative coupling of thiols in solution and under solvent-free conditions

A mild and efficient method for the oxidative coupling of thiols by ammonium dichromate in the presence of silica chloride and wet SiO2 in solution and under solvent free conditions is reported.

Solvent‐Free Oxidation of Thiols by (NH4)2Cr2O7 in the Presence of Mg(HSO4)2 and Wet SiO2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The New Criteria Of Separation Of Oxidises And Ammonium Salt In SeaTransport

The research was aimed at finding objective magnitudes which could make it possible to determine the levels of separation of oxidisers and ammonium salts, required for sea transport. The differential thermal analysis can be used to search. Self-ignition condition and explosive processes were determined. In this paper the results separation of some oxidisers from ammonium nitrate(V), ammonium dichromate(V1) and ammonium chloride are presented. The determination the temperature change rate during reactions is the basis of oxidants and incompatible materials separation. It should be stated, that the compounds will require the 2* separation level if the temperature change rate during thermal reaction of oxidises and ammonium salts exceed 1,8 [Cls], the 1'' separation level can be applied if the temperature change rate values are between 0,8 and 1,8 ['Cls]. The thermal decomposition of oxidisers needs higher temperatures than for oxidisers and ammonium salts in mixtures.