As-synthesized Form Research Articles

Structural changes in SAPO-34 due to hydrothermal treatment. A NMR, XRD, and DRIFTS study

When SAPO-34 is used as an industrial MTO catalyst, structural transformations leading to permanent deactivation are inevitable. The performance loss is linked principally to a redistribution of Si in the material, leading to the formation of Si-islands/aggregates with a concomitant loss of Brønsted acidic sites and catalytic activity. In this work we have studied transformations taking place in a SAPO-34 sample after hydrothermal treatment by studying two samples with different levels of Si; 7 atomic % and 13 atomic % Si T-atoms, corresponding to about one and two Si per CHA cage respectively. The 13% Si sample contains significant amount of silicon islands in its as-synthesized form, while the 7% Si sample does not. The 7% Si sample was steamed for a week at 700 °C and a partial pressure of steam of 0.7 atm. The changes were analysed in the context of Si-island formation, and compared with the 13% Si sample. The results clearly illustrated existence of two distinct types of local aggregation of silicon: Silicon islands produced during synthesis and aggregate silicon reminiscent of silicon islands induced by hydrothermal treatment. The materials were synthesized with full 29Si isotopic enrichment and allowed us, for the first time, to characterise the multiplicity of silicon species in great detail by 29Si solid state NMR.

Structure analysis of a BEC-type germanosilicate zeolite including the location of the flexible organic cations in the channels

A BEC-type zeolite (polymorph C of zeolite beta) with low germanium content (Si : Ge = 5.1) has been synthesised using the flexible linear diquaternary cationic form of pentamethyldiethylenetriamine (2,2′-(methylazanediyl)bis(N,N,N-trimethylethanammonium) as the organic structure-directing agent (SDA). The distribution of germanium within the framework and the location of the SDA within the pores have been determined by analysing synchrotron X-ray powder diffraction data collected on the as-synthesised form. The findings, which are corroborated by 13C and 19F NMR data, indicate that the structure of the SDA is retained during the synthesis and that the Ge atoms occupy only sites in the double 4-ring. Although the BEC framework structure has a three-dimensional 12-ring channel system, the SDA molecules are found to orient themselves in two dimensions along the channels running parallel to the a and b axes.

Quest for lead free relaxors in YIn(1-x)Fe(x)O₃ (0.0 ≤ x ≤ 1.0) system: role of synthesis and structure.

The B-site tailored YIn(1-x)Fe(x)O3 (0.0≤ x≤ 1.0) series was synthesized by glycine-aided gel-combustion technique and subjected to extensive structural and electrical investigations. The temperature had tremendous bearing on the phase evolution exhibited by the system. The entire system crystallized as C-type metastable polymorph in the as-synthesized form. Hexagonal polymorphs of Fe(3+)-rich compositions could be isolated by controlled heat treatment at 750 °C. Raman spectroscopic investigations showed that, while there is a general shrinkage of the lattice due to substitution of a smaller ion at In(3+)-site, there is an apparent dilation of the Y-O bond, and this anomaly reflects in the electrical behavior exhibited by the system. The single-phasic hexagonal nominal compositions, YIn(1-x)Fe(x)O3 (0.0 ≤ x ≤ 0.3), were also studied by impedance spectroscopy. The dielectric constant was found to drastically increase from 10 for YInO3 to 1000 for YIn(0.7)Fe(0.3)O3 at room temperature stressing the role of B-site tailoring on electrical behavior. More interestingly, careful substitution of Fe into YInO3 could tune the electrical behavior from a dielectric to relaxor ferroelectric in the temperature range studied. The nominal composition YIn(0.7)Fe(0.3)O3 showed a classical relaxor ferroelectric like behavior which is an important observation in context of the search for new lead free relaxor materials.

Atomically Precise Doping of Monomanganese Ion into Coreless Supertetrahedral Chalcogenide Nanocluster Inducing Unusual Red Shift in Mn2+ Emission

We report a simple and yet effective method to introduce Mn(2+) ions into semiconducting nanoclusters with atomically precise control. Our method utilizes one type of micrometer-sized crystals, composed of well-defined isolated supertetrahedral chalcogenide nanoclusters (∼2 nm, [Cd6In28S52(SH)4]) whose core metal site is unoccupied in as-synthesized pristine form. This unique model structure with vacant core site makes it possible to achieve ordered distribution of Mn(2+) dopants, and at the same time effectively preclude the formation of Mn(2+) clusters in the host matrix. A two-step synthesis strategy is applied to realize an atomically precise doping of Mn(2+) ion into the core site of the nanoclusters, and to achieve uniform distribution of Mn(2+) dopants in the crystal lattice. The PL, X-ray photoelectron (XPS), as well as the electron paramagnetic resonance (EPR) spectra reveal the successful incorporation of Mn(2+) ion into the core site of the nanocluster. Different from the pristine host material with weak green emission (∼490 nm), the Mn(2+)-doped material shows a strong red emission (630 nm at room temperature and 654 nm at 30 K), which is significantly red-shifted relative to the orange emission (∼585 nm) observed in traditional Mn(2+)-doped II-VI semiconductors. Various experiments including extensive synthetic variations and PL dynamics have been performed to probe the mechanistic aspects of synthesis process and resultant unusual structural and PL properties. The quaternary semiconductor material reported here extends the emission window of Mn(2+)-doped II-VI semiconductor from yellow-orange to red, opening up new opportunities in applications involving photonic devices and bioimaging.

Interlayer Expansion of the Hydrous Layer Silicate RUB-36 to a Functionalized, Microporous Framework Silicate: Crystal Structure Analysis and Physical and Chemical Characterization

The hydrous layer silicate RUB-36, (C6H16N)4 [H4Si36O76], has been used for an interlayer expansion reaction with dichlorodimethylsilane to interconnect neighboring ferrierite-type layers to a three-dimensional framework silicate. The linker group (−O–Si(CH3)2–O−) still has the two methyl groups in the as-synthesized form (material name COE-3 [Si20O38(CH3)4] for the silicate framework) rendering hydrophobic properties. The interlayer expanded zeolite, IEZ, is thermally stable and can be calcined at 550 °C to yield a hydrophilic material COE-4 [Si20O38(OH)4]. 29Si solid state MAS NMR experiments confirm the insertion of the linker group and the methyl and hydroxyl substitution in the as-made and calcined form, respectively. The BET surface area is 238 m2/g for COE-3 and 350 m2/g for COE-4. COE-3 and COE-4 crystallize in space group Pm with a = 12.2503(3) Å, b = 13.9752(2) Å, c = 7.3850(1) Å, and β = 107.33(1)° and a = 12.16985(4) Å, b = 13.95066(3) Å, c = 7.37058(2) Å, and β = 107.30(1)°, respectively. Rietveld crystal structure refinement of the PXRD pattern of COE-3 and COE-4 reveal the expanded, two-dimensional 10-ring pore system including the linker group as homogeneous structural property of the materials.

Study of Fe sites in the framework of large pore FeAlPO-5 synthesized from different templates

Abstract This paper reported results of in situ XANES/EXAFS studies of iron substituted AlPO-5. Fe-AlPO-5s were synthesized with different templates. These materials were also studied using XRD, SEM, EDX and in situ XRD/EXAFS. Here the aim is to investigate the influence of template on the state of iron in the as-synthesized and calcined form. It was found that, depending on the nature of template, the coordination of Fe(III) ions changes from octahedral to tetrahedral at different temperatures.

Morphological Control of Porous SiC Templated by As-Synthesized Form of Mesoporous Silica

Porous silicon carbide with fiber like morphology was successfully prepared via a hard templating approach using as-synthesized form of mesoporous silica SBA-15 as template and sucrose as carbon precursor. The obtained materials have been characterized by various sophisticated techniques such as XRD, nitrogen adsorption, HRTEM, HRSEM, FT-IR and TGA. The XRD and the nitrogen adsorption results revealed that the materials are highly crystalline and exhibit porous structure with excellent textural characteristics. It has been demonstrated that the morphology of the materials can be finely controlled from rod shaped morphology to fiber like nanostructures by the simple adjustment of the reaction time from 5 to 12 h at the reaction temperature of 1300 degrees C. The reaction time of 12 h at 1300 degrees C was found to be the best condition to obtain highly crystalline nanofiber like morphology. The obtained materials display beautiful nanofiber morphology which has the diameter of 20 to 60 nm and a length of 7 to 10 microm which is much longer than the previously reported SiC nanofibers.

High Sorptive Removal of Borate from Aqueous Solution Using Calcined ZnAl Layered Double Hydroxides

Borate uptake studies were carried out over both as-synthesized and calcined ZnAl layered double hydroxides (LDHs) containing carbonate or chloride as interlayer anions, with different Zn/Al atomic ratios (2, 3, and 4). Chloride-containing materials in both as-synthesized and calcined forms showed higher borate uptake than carbonate-containing forms. A maximum borate uptake of 31 mgB/g was achieved for the 400 °C heat-treated Zn2Al–Cl LDH material (referred here as Zn2Al–Cl–CLDH). The mechanism of borate uptake for heat-treated materials involves reconstruction as elucidated from PXRD, FT-IR, and SEM measurements. Interestingly, solid state transformation of tetraborate to monoborate anion occurred in the interlayer on drying for longer time. At a ratio of 4 g Zn2Al–Cl–CLDH per liter of 100 mgB/L solution, the uptake of borate reached 97% further, and at 5 mgB/L solution the borate concentration reduced below the value of 0.5 mgB/L recommended by the World Health Organization (WHO), using 1 g/L ratio of s...

Influence of co-bivalent ions in Cu-containing LDHs and solvent on hydroxylation of benzene to phenol

Selective hydroxylation of benzene to phenol using H 2O 2 as the oxidant was studied over copper-containing layered double hydroxides (LDHs) and their calcined oxides containing different co-bivalent ions (Mg, Zn, Ni or Co). In all the catalysts studied, the Cu/M(II) ratio was 10/90, the trivalent metal ion was aluminum and an atomic ratio of the bivalent to the trivalent metal ion was about 3.0. Catalysts were synthesized using co-precipitation under conditions of low supersaturation and were characterized by various physicochemical methods. Samples with zinc as the co-bivalent ion showed high activity whereas cobalt showed the least activity in both the as-synthesized and calcined forms. The high catalytic activity of the zinc-containing compound was attributed to the facile formation of Cu +, the likely active species for this reaction, as confirmed through X-ray photoelectron spectroscopy (XPS) measurements. Solvent variation studies revealed that water was the best solvent and that addition of a small amount of sulfolane as the co-solvent increased the selectivity for phenol. The calcined CuZnAl oxide catalyst can be reused up to three cycles without any significant change in the yield of phenol. The retention of the dispersion of copper verified through energy dispersive X-ray analysis (EDAX) measurements was likely to be responsible for the recyclability.

ChemInform Abstract: Production of Fine, High-Purity Beta Silicon Carbide Powders.

Submicrometer SiC (β-form) powders were synthesized by reacting silica and carbon black at temperatures between 1450° and 1800°C. Simultaneous application of vacuum and mixing provides the condition for full conversion of silica to SiC. It was shown that two different reaction mechanisms are possible, depending on the reaction temperature and the partial pressure of CO. At lower temperatures (below approximately 1400°C), the dominant mechanism for silicon carbide formation involves the solid-state reaction of silica and carbon. At higher temperatures (above approximately 1400°C), the dominant mechanism is the reaction between gaseous SiO and C. Above 1400°C, the rate of SiC formation is controlled by the rate of SiO formation. In as-synthesized form, the SiC powders typically contain < 0.2 wt% of unreacted silica and free carbon in the range between 6 and 15 wt%. Precise control of partial pressure of CO in the reaction chamber and continuous mixing of the reactants provide the conditions under which the rate of silicon carbide formation may be increased by one order of magnitude. The process is suitable for large-scale commercial production of SiC, requiring no postfabrication acid leaching or major milling.

Phenol Hydroxylation over Cu-Containing LDHs and Their Calcined Forms: Profound Cobivalent Metal Influence

Copper containing layered double hydroxides (LDHs) with less than 1 atom % Cu along with different cobivalent ions such as magnesium, zinc, nickel or cobalt at (Cu+M(II))/Al atomic ratio 3.0 were synthesized. Calcination of these materials offered mixed metal oxides (refereed here as CLDHs) without any discrete copper oxide phase as confirmed by PXRD and STEM analysis. Hydroxylation of phenol to catechol (CAT) and hydroquinone (HQ) was carried out over both as-synthesized and calcined materials. Among the cobivalent ions, zinc containing samples both in the as-synthesized and calcined forms showed very high activity while cobalt showed the minimum. Variation of copper content for CuZnAl ternary system (0.009 atom % to 0.056 atom %) showed a maximum activity for the sample with 0.038 atom % (CuZnAl-5-CLDH). High activity was attributed to better dispersion of isolated copper species as inferred through TPR measurements. Highly active CuZnAl-5-CLDH can be reused up to three cycles without any significant change in the activity. No significant variation in the conversion was noted when the reaction was scaled up to 200 times over CuZnAl-5-CLDH renders the promise of exploring it to the next level.

AlF·1,4-benzenedicarboxylate: synthesis and absorption properties

AlF(bdc), bdc = 1,4-benzenedicarboxylate, a new member of the family of MX(bdc) compounds has been synthesized under solvothermal conditions. It is formed without H(2)bdc in the channels and substantial absorption capacity is observed in its as-synthesized form. AlF(bdc) does not absorb water at ambient temperatures and shows absorption properties similar to VO(bdc).

IM-19: a new flexible microporous gallium based-MOF framework with pressure- and temperature-dependent openings

Five metal-organic frameworks (MOFs) based on the same three-dimensional gallium terephthalate network (IM-19) are described, and an incommensurate structure (for the as-synthesized form) as well as two remarkable guest-free polymorphs (open and closed) are highlighted.

Self-Assembled, Nanostructured Aniline Oxidation Products: A Structural Investigation

The structure and oxidation state of self-assembled nanostructures formed by oxidation of aniline with ammonium persulfate in the presence of alanine have been investigated by solid-state 13C and 15N NMR, FTIR, GPC, elemental, UV−vis, and SEM methods. These techniques have been applied to samples obtained 1 and 20 h after the beginning of the reaction in their doped as-synthesized form or after reduction with hydrazine or dedoping with NH4OH or LiOH. Peaks at 66 and 72 ppm and a peak at 241 ppm in the 15N NMR spectra are shifted downfield by about 1−6 ppm and upfield by about 80 ppm, respectively, relative to the positions expected for the amine and imine N atoms in polyaniline. This indicates a difference in molecular structure between the nanotube material and standard polyaniline. This may be attributed to the presence of either strong H-bonding or a fundamentally different bonding connectivity in the nanotube material. Despite the significant difference in morphology of the samples after 1 and 20 h as...

Structure and NMR assignment in calcined and as-synthesized forms of AlPO-14: a combined study by first-principles calculations and high-resolution 27Al–31P MAS NMR correlation

The high-resolution 27Al and 31P NMR spectra of two as-synthesized forms of the microporous aluminophosphate AlPO-14 and the corresponding calcined-dehydrated form were assigned using both "first-principles" calculations of NMR parameters (GIPAW, as implemented in NMR-CASTEP) and a 27Al-31P heteronuclear correlation NMR experiment (MQ-J-HETCOR) that exploits 27Al multiple-quantum coherences and J couplings to identify Al-O-P linkages. NMR parameters calculated from published AlPO-14 crystal structures, which are derived from powder X-ray diffraction (XRD) data, are in poor agreement with experiment and it was necessary to optimize the structure geometry using energy minimization before satisfactory agreement was obtained. Comparison of simulated powder XRD patterns from the experimental and the energy-minimized structures shows that the changes in relative atomic positions in the optimized structure are relatively small and yield only minor adjustments in the Bragg peak intensities. These results indicate that a combination of NMR spectroscopy and first-principles calculation of NMR parameters may soon be considered a generally useful step in the refinement of the structures of microporous materials derived from powder diffraction data.

Surface and subsurface characterization of epoxy-mesoporous silica composites to clarify tribological properties

As-synthesized and calcined DAM-1 (Dallas Amorphous Material-1) mesoporous silica were used as particulate reinforcements in an epoxy matrix. The mesocomposites showed enhanced tribological properties over the base epoxy. The particles were introduced as both as-synthesized and calcined forms to evaluate the effect of porosity on tribological behavior. Epoxy-DAM1 as-synthesized (DSE) mesocomposites exhibited the longest duration low coefficient of friction (COF) values during the run-in regime as well as the lowest COF values in the higher friction regime. These results were attributed to the low cross-linking density envelope around the as-synthesized particles and their plate-like morphology which work together to form friction-lowering transfer films. Subsurface HRSEM imaging using focused ion beam (FIB) cross-sectioning inside wear tracks showed that shear band formation is the mode of deformation in DSE, while for Epoxy-DAM1 calcined (DCE) mesocomposites and pure epoxy, it was determined to be surface fatigue resulting from brittle cracking. This study shows that the as-synthesized particles increase the toughness of the epoxy by altering the mode of deformation and wear debris formation.

The catalytic activity of space versus terrestrial synthesized zeolite Beta catalysts in the Meerwein Ponndorf Verley Reactions: Support for PFAL as the Lewis active site for cis-alcohol selectivity

The Lewis activity of the Meerwein-Ponndorf-Verley (MPV) reactions is hypothesized to be due to partial framework aluminum (PFAl) that is either octahedrally or tri-coordinated. Crystals grown in the free-fall environment of low earth orbit (LEO) are more uniform; that is, have fewer lattice “defects” compared to those grown in a gravity field (i.e., on earth). Therefore, crystals grown in orbit should be less catalytically active relative to their earth grown counterparts. The catalytic activity towards the MPV reaction, and the associated IR and XPS spectrum for zeolite Beta that was synthesized on earth (1g) and aboard the International Space Station (10−3–10−5g) were compared in their as-synthesized forms, and after applying heat treatment protocols designed to stress the crystal structure to generate Lewis acid sites (i.e., tri and octahedrally coordinated PFAl). The activity of the MPV reaction and cis-alcohol selectivity over the heat-treated flight samples was observed to be lower than the identically heat-treated terrestrial zeolite Beta samples. Higher MPV activity as well as cis-alcohol selectivity is related to both a higher number of partial framework Al atoms (PFAl), and a constrained pore volume. As PFAl are created by the destruction of the framework upon heat treatment, flight samples were shown to be thermally more stable with fewer lattice defects and less associated stress in zeolite Beta crystals. The changes observed in the IR spectra, as well as the XPS Al Auger and 2p peaks, of the terrestrial samples support this conclusion. Additionally, the flight samples showed higher tr-alcohol selectivity, which implies more pore volume and less channel blockage. This is consistent with the fact that crystals grown in space have less stress, fewer lattice defects, and thus there are fewer channel obstructions.

Structural evolution of H 4PVMo 11O 40⋅ xH 2O during calcination and isobutane oxidation: New insights into vanadium sites by a comprehensive in situ approach

H 4PVMo 11O 40⋅8H 2O was studied during thermal activation as well as during isobutane oxidation by simultaneous in situ-EPR/UV–vis/Raman spectroscopy, in situ-FTIR spectroscopy, and quasi-in situ- 1H and - 51V-MAS-NMR. In as-synthesized form, most V sites are pentavalent, octahedrally coordinated, and located within the intact Keggin anions. Stepwise dehydration in N 2 up to 350 °C leads to partial reduction and disintegration of the V sites from the Keggin units, followed by their condensation on the outer surface of the latter in square-pyramidal form. In water-free H 4PVMo 11O 40, only V 5+ (not V 4+) is stable inside the Keggin unit. Thus, disintegration of V from the latter is favored by its reduction to the tetravalent state and thus depends on the redox properties of the atmosphere. Active sites in isobutane oxidation are most likely composed of single O 4V 4+/5+ O species connected to Mo 6+ via oxygen bridges. Partial deactivation occurs by formation of carbon-containing deposits.

Catalytic applications of hydrotalcite-like materials and their derived forms

Hydrotalcite-like (HT-like) materials have been studied extensively in the last three decades. Although these materials initially perceived as potential anion exchangers, unrelenting efforts have also been attempted to use them as catalytic materials or as catalytic supports. There are varied ways by which one can use them for catalysis viz.; 1. Use them in as-synthesized form 2. Use them after calcining where the structural integrity of layered lattice is lost 3. Use the interlayer as functional spaces with moieties of catalytic interest 4. Use them after destruction or delamination and restructuring and 5. Use them as support or matrix. In this review, we are reporting the catalytic applications of HT-like materials and their derived forms carried out in our research group for the last 15 years in these various ways, covering reactions such as nitrous oxide decomposition, selective oxidation of organic compounds of industrial relevance and base-catalyzed isomerization of chemicals of perfumery interest. Although this review primarily focuses on our work, wherever applicable, related or relevant works from other groups are also presented to bestow comprehensiveness.

Dynamics on the Microsecond Timescale in Microporous Aluminophosphate AlPO-14 as Evidenced by 27Al MQMAS and STMAS NMR Spectroscopy

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an 27Al (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AlPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the 31P and 13C (both I = 1/2) MAS NMR spectra are also presented. A comparison of the 27Al MQMAS and STMAS NMR results show that, although calcined AlPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four Al sites in each material. Variable-temperature 27Al STMAS NMR studies of the two as-synthesized AlPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.