27Al MAS NMR Research Articles

Tailoring selectivity in the liquid-phase isomerization of α-pinene on dealuminated ferrierite-type zeolites

Abstract Ferrierite-type zeolite (Zeolyst Intern.) with Si/Al = 10 was dealuminated sequentially using solutions of hydrochloric acid of different concentrations. The course of dealumination was followed by XRD, XPS, sorption of nitrogen, scanning electron microscopy, FTIR, and solid-state 29Si, 27Al and 1H MAS NMR. The dealuminated samples retained high crystallinity and revealed good sorption properties. The tetrahedrally coordinated aluminium in ferrierite was distributed between TB sites located in the main channels and TA ones siting in hidden positions. It was shown that dealumination removed aluminium from ferrierite in a non-random manner; up to 60% of aluminium was removed from TB and up to 19-% from TA sites. The amount and strength of Bronsted acid sites decreased with dealumination. The samples were tested in the liquid-phase isomerization α-pinene at 348–363 K; camphene and limonene were the main products. The initial reaction rates and turnover frequencies have been estimated and compared to other zeolite catalysts. The selectivity to camphene and limonene was in the range of 77–98.9 %.

Synthesis of aluminophosphate xerogels by non-hydrolytic sol–gel condensation of EtAlCl2 with trialkylphosphates

We have investigated the preparation of high-surface-area mesoporous aluminophosphates by non-hydrolytic sol–gel method based on reactions of EtAlCl2 with trialkylesters of phosphoric acid (OP(OR)3, R = Me, Et, iPr, nBu, in dry organic solvents. The condensations proceed by alkylchloride elimination. Various reaction and calcination conditions were examined. Porosity is obtained after calcination by removal of organic residual groups. This thermal processing at 300 °C of as-synthesized precursor gels leads to amorphous aluminophosphate xerogels with surface areas of 400–500 m2 g–1 provided by small mesopores (2–8 nm). Changes in the coordination environment of aluminium from six- to four-coordinate are evidenced by shift of 27Al MAS NMR resonances.

Formic Acid Production Via Methane Peroxide Oxidation Over Oxalic Acid Activated Fe-MFI Catalysts

The process of synthesis of formic acid via partial peroxide oxidation of methane over Fe-MFI zeolites, as well as the influence of the catalyst activation by oxalic acid on the process parameters (conversion and selectivity) was studied. XRD, ICP-OES, SEM, UV–Vis DR, ESR, NH3-TPD, 27Al MAS NMR, N2 adsorption techniques were used for the catalyst characterization. The observed increase in TOF, selectivity to formic acid and efficiency of H2O2 utilization upon the catalyst activation with oxalic acid was accounted for by the formation of oligomeric oxo-clusters of Fe-ions and by an increase in the total acidity of the catalysts. The process of oxidation of main intermediates of the reaction, such as methanol and formic acid, was studied in the presence and in the absence of methane that allowed the mechanism of methane oxidation to formic acid to be suggested.

Regeneration of deactivated H-ZSM-5 for aromatization by dielectric barrier discharge plasma

A dielectric barrier discharge (DBD) plasma regeneration reaction was conducted at atmospheric pressure and temperature. This method was found to be very effective in removing carbon deposits after a non-oxidative aromatization from acetylene. When a fresh H-ZSM-5 catalyst was used for the aromatization, coke was by-produced and it could be ascribed to the primary reason for deactivation. By applying the DBD plasma to the deactivated catalyst bed at ambient temperature and pressure, the removal of carbon deposition with microdischarge and aeration was very effective; therefore, it is believed that additional thermal activation might not be necessary. More amount of coke was removed by supplying higher power to the catalyst bed. According to the results of XRD, physisorption, 27Al MAS NMR, and NH3-TPD analyses, it was confirmed that the structure and the acid site distribution of H-ZSM-5 were successfully retained even after the plasma treatment. Aromatization over the plasma-regenerated catalyst led to mild deactivation at the early stage and afforded comparable BTX selectivity to fresh H-ZSM-5 catalyst. Compared with the results obtained using the thermally regenerated catalyst, the structural stability was found to be higher and the BTX selectivity was kept higher during most of the reaction time for the plasma-regenerated catalyst.

Rapid green synthesis of ZSM-5 zeolite from leached illite clay

Abstract A seed-induced rapid synthesis of submicron ZSM-5 zeolites from acid-leached illite clay was achieved via solid-like state conversion. The optimal synthesis conditions and crystallization mechanism were investigated. The synthesized ZSM-5 zeolites were characterized using X-ray diffraction, scanning electron microscopy, N2 adsorption–desorption, X-ray fluorescence, NH3 temperature-programmed desorption, and 27Al MAS NMR. The results clearly indicated that well-dispersed submicron-size ZSM-5 zeolites were synthesized in the relatively short period of 6 h. In the solid-like state conversion synthesis system induced by crystal seeds, the sample gradually transformed from amorphous to well-dispersed submicron ZSM-5 as the crystallization time increased. Aluminum entered the ZSM-5 zeolites framework to provide acidic sites. The obtained ZSM-5 zeolites showed slightly better catalytic performance in toluene disproportionation than ZSM-5 seeds.

27Al MAS NMR investigations on Al23Te77 glass: Observation of 5- coordinated Al and its influence on electrical switching

27Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopic studies show that Al resides in 4-, 5- and 6- fold coordination in bulk Al23Te77 glass. Also, Al23Te77 glass is found to exhibit threshold type switching. Earlier studies report memory type switching in Al-Te glasses wherein Al has been found only in [4]Al and [6]Al environments. X-Ray Photo Electron Spectroscopic (XPS) measurements show the presence of Al2O3 in Al23Te77 glass network. The structural network of Al-Te glass has been cross linked by the presence of higher coordinated Al atoms. The structural reorganization becomes difficult due to the increased network connectivity. Thus, the memory switching (which requires an easy structural reorganization) is retarded, resulting in the observation of threshold switching.

Nano-sized plate-like alumina synthesis via solution combustion

Nano-sized plate-like alumina is an important inorganic material in catalytic, ceramic and electronic applications due to its unique two-dimensional structure and properties, while its facile synthesis is still under investigation. In this work, nano-sized alumina plate was synthesized by solution combustion method with glycine and polyvinyl alcohol as composite fuels for the first time. The calcination temperature during solution combustion process was optimized as confirmed by XRD, FT-IR, TG-DSC, Raman, 27Al MAS NMR, SEM, TEM, and N2 physisorbtion. Obtained alumina displayed special nano-sized plate morphology and porous structure. It was further adopted as support for cobalt acetate and used as catalyst for selective oxidation of cyclohexane. This work develops a facile approach to synthesize two-dimensional plate-like alumina for catalytic application.

Mineralogical investigations of Jamaican hematite-rich and goethite-rich bauxites using XRD and solid state 27Al and 31P MAS NMR spectroscopy

Bauxite is the ore that is the world's primary source of aluminum. It is a significant natural resource for the Caribbean island of Jamaica. Large portions of Jamaica's future bauxite resources are goethitic and have significantly higher phosphorus concentrations than currently-processed hematite-rich ores. Depending on mineralogy, phosphate minerals may dissolve during alumina extraction and cause various process problems, consequently identifying them is critical. This study investigates phosphorus and alumina minerals in current and future Jamaican bauxites and extraction residues using powder X-ray diffraction (XRD) and solid state 27Al and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies. The use of powder XRD conclusively identified crandallite as the major P mineral in the goethitic bauxites but its presence in the hematite-rich ore was uncertain. Bauxite analysis using solid state 27Al MAS NMR, gave central peaks (δ = 6.60–8.60 ppm) and associated spinning sidebands that were consistent with the octahedral coordination of Al3+ in gibbsite however one high phosphorus ore had an additional peak at 50 ppm attributable to the tetrahedral coordination of Al3+ in an unidentified aluminosilicate mineral. Solid state 31P MAS NMR studies identified crandallite in both the goethitic and hematite-rich bauxites while peaks corresponding to chlorapatite, wavellite and an unspecified aluminophosphate mineral were observed in high P bauxite samples; phosphorus existed as apatite in the red mud residue. These results indicate that solid state 31P MAS NMR can give meaningful information on the P mineralogy of bauxite resources. The technique complements the use of powder XRD for investigating the P mineralogy of both hematite-rich and goethitic bauxites and can therefore assist in managing their exploration and predicting their processability.

Quantifying the impact of dispersion, acidity and porosity of Mo/HZSM-5 on the performance in methane dehydroaromatization

The catalytic performance of the bifunctional catalyst Mo/HZSM-5 for methane dehydroaromatization (MDA) depends on the Mo dispersion and on zeolite acidity. Here we separately quantify the effect of dispersion and the effect of acidity on aromatic yields and coke selectivity. Also, the effect of porosity on the same is quantitatively assessed. For that, a suite of 17 samples with varying Mo dispersion were synthesized by means of several methods, including chemical vapor deposition with MoCl5, MoO2Cl2 and Mo(CO)6 as precursors and the conventional methods, incipient wetness impregnation and solid ion exchange. These catalysts were characterized with pyridine IR-spectroscopy, XPS, UV–vis spectroscopy, N2 adsorption, XRD, TGA and 27Al MAS NMR. The combined results yielded a measure of how much Mo is anchored to the zeolite as well-defined cationic species and how much is present as bigger clusters on the outer surface of the zeolite. Through relating these characterization results to the catalytic behavior of the catalysts, it was found that the maximum instantaneous benzene and naphthalene yields as well as the integral selectivities during methane dehydroaromatization linearly increase with the amount of Mo present as mono- or dimeric species. At the same time, the selectivity to coke increases with the amount of Mo present as bigger clusters or nanoparticles on the outer surface of the zeolite. The number of Mo cationic sites is the most important factor determining the activity of Mo/HZSM-5 for low loadings of Mo. But at higher loadings, the high rate of aromatics formation requires an easily accessible pore structure as well.

Magnesium substitution in calcium and strontium fluoro-phospho-aluminosilicate glasses by multinuclear 19F, 31P, 27Al, and 29Si MAS-NMR spectroscopy

Abstract Two series of glasses were produced by systematically substituting magnesium for calcium and strontium in the glass compositions, 4.5SiO2. 3Al2O3. 1.5P2O5. (3-x)CaO/SrO. xMgO. (2-y)CaF2/SrF2. yMgF2, 0 ≥ x ≥ 3, 0 ≥ y ≥ 2 and these glasses were characterised by 19F, 31P, 27Al, and 29Si MAS-NMR spectroscopy. The substitution of magnesium for calcium and strontium resulted in the gradual formation of F-Mg(n) species over the F-Ca(n) and F-Sr(n) with additional loss of Al F species. The 31P and 27Al MAS-NMR spectra showed the presence of Al-O-PO33− species charge balanced by a mixture of Ca2+ and Mg2+ or Sr2+ and Mg2+. In addition to that, the 31P MAS-NMR spectrum of the fully magnesium substituted glass showed the presence of amorphous magnesium pyrophosphate, Mg2P2O7. Al(IV) was the major Al species with small amount of Al(V) and Al(VI), but the amount of Al(V) increased with increase in magnesium substitution. Moreover, the fully magnesium substituted fluoro-phospho-aluminosilicate glass resulted in the more polymerised silicate network with increased number of the bridging oxygen on SiO4 tetrahedra.

Layered double hydroxides for phosphorus recovery from acidified and non-acidified dewatered sludge

Phosphate, which contains the essential element phosphorous (P), is a necessary fertilizer for agriculture, but the current phosphate deposits are running out and alternative sources are needed. Sludge obtained from wastewater treatment plants contains high concentrations of phosphorus and represents an alternative, sustainable source. In this study, sludge obtained from a wastewater treatment plant with biological and chemical phosphorus removal was acidified (pH = 3, 4, 5 and 6) to release orthophosphate followed by sequestration of the orthophosphate by a zinc aluminum layered double hydroxide (Zn2Al-LDH). Sulfuric acid (H2SO4), nitric acid (HNO3), and hydrochloric acid (HCl) was tested, which showed that only sulfate anions compete with phosphate and results in reduced phosphate recovery (25–35%). The orthophosphate concentration in the liquid phase increased from 20% (raw sludge) to 75% of the total phosphorus concentration at a pH of 3, which enhanced the phosphate uptake by the ZnAl-LDH from 1.7 ± 0.2% to 60.3 ± 0.6%. During acidification, the competing anion carbonate is degassed as CO2, which further improved the phosphate uptake. PXRD showed the intercalation of carbonate in the LDH in the raw sludge at pH = 8, whereas orthophosphate was intercalated at lower pH values. 27Al MAS NMR spectroscopy and powder X-ray diffraction (PXRD) proved preservation of the LDH at all pH values. Furthermore, about a fourth of the Al is present as an amorphous aluminum phosphate (AlPO4) upon exposure to phosphate at low pH (pH = 3 and 5) based on 27Al MAS NMR spectroscopy. At a pH of 6 about a third of the P is present as brushite (CaHPO4·2H2O).

One-Pot Catalytic Conversion of Cellobiose to Sorbitol over Nickel Phosphides Supported on MCM-41 and Al-MCM-41

MCM-41- and Al-MCM-41-supported nickel phosphide nanomaterials were synthesized at two different initial molar ratios of Ni/P: 10:2 and 10:3 and were tested as heterogeneous catalysts for the one-pot conversion of cellobiose to sorbitol. The catalysts were characterized by X-ray diffractometer (XRD), N2 adsorption-desorption, scanning electron microscope (SEM), transmission electron microscope (TEM), 27Al-magnetic angle spinning-nuclear magnetic resonance spectrometer (27Al MAS-NMR), temperature programmed desorption of ammonia (NH3-TPD), temperature-programmed reduction (H2-TPR), and inductively coupled plasma optical emission spectrophotometer (ICP-OES). The characterization indicated that nickel phosphide nanoparticles were successfully incorporated into both supports without destroying their hexagonal framework structures, that the catalysts contained some or all of the following Ni-containing phases: Ni0, Ni3P, and Ni12P5, and that the types and relative amounts of Ni-containing phases present in each catalyst were largely determined by the initial molar ratio of Ni/P as well as the type of support used. For cellobiose conversion at 150 °C for 3 h under 4 MPa of H2, all catalysts showed similarly high conversion of cellobiose (89.5–95.0%). Nevertheless, sorbitol yield was highly correlated to the relative amount of phases with higher content of phosphorus present in the catalysts, giving the following order of catalytic performance of the Ni-containing phases: Ni12P5 > Ni3P > Ni. Increasing the reaction temperature from 150 °C to 180 °C also led to an improvement in sorbitol yield (from 43.5% to 87.8%).

Vanadium contamination on the stability of zeolite USY and efficient passivation by La2O3 for cracking of residue oil

It is of great significance to identify the destructive role of high deposited vanadium on the USY-based cracking catalysts and find a facile approach to vanadium passivation for processing residue oil. Herein, the destruction degree of the steam-treated vanadium-contaminated USY zeolites, which also contained a certain amount of Na ions, was investigated by N2 physical adsorption, NH3-TPD, HRTEM, 27Al MAS NMR and catalytic cracking evaluation. The results reveal that high deposited vanadium causes severe damage to the structure of USY, resulting in a sharp deterioration in cracking performance. Furthermore, CeO2 and La2O3 were studied as vanadium passivating agents. The introduction of La2O3 using physical mixing could preserve the zeolite structure to a large extent without altering the properties of USY. The superior passivation performance might be mainly attributed to the fact that La2O3 introduced by simple physical mixing prefers to interact with vanadium species, rather than migrate into the channels and cavities of zeolites USY.

ZnO Nanoclusters Supported on Dealuminated Zeolite β as a Novel Catalyst for Direct Dehydrogenation of Propane to Propylene

AbstractSmall ZnO nanoclusters supported on dealuminated β zeolite were prepared and evaluated for catalyzing direct dehydrogenation of propane to propylene (PDH), exhibiting high catalytic performance. N2 sorption, XRD, TEM, 27Al and 28Si MAS NMR, IR, XRF, DR UV‐vis, XPS, and NH3‐TPD techniques were employed to characterize the physicochemical properties of this novel catalyst system. It is found that the Zn species can be accommodated in the vacant T‐atom sites of dealuminated β zeolite due to the reaction of aqueous zinc acetate solution with silanol groups, and thus, producing massive small ZnO nanoclusters as active phases in PDH. Additionally, dealuminated β zeolite can greatly depress side reactions attributable to the absence of strong acid sites, thereby guaranteeing high catalytic activity, propylene selectivity and stability. As a result, the optimal catalyst of 10 wt% Zn loaded on dealuminated β zeolite exhibits a high initial propane conversion of around 53 % and a superior propylene selectivity of about 93 % at a space velocity of 4000 cm3 gcat−1 h−1, together with the high stability and satisfactory reusability. This study may open a new way to design and synthesize highly active PDH catalysts with high selectivity and stability.

Synthesis and characterization of Al@MOF materials

Abstract The potential impregnation of Al(0) nanoparticles in the pores of three different metal-organic frameworks (MOFs), MIL-53-Al, HKUST-1, and UiO-67, was investigated through the suspension of the MOFs in AlH3·NMe2Et (1), followed by filtration, toluene wash, and heating to 150 °C under vacuum. Calculations based on the ratios of molecular and pore volumes provided idealized, benchmark impregnation capacities. Three successive impregnation cycles were performed to provide maximum incorporation of Al in the pores, and the materials were characterized after each impregnation cycle by ICP elemental analysis, BET surface area, and pore volume measurements. For MIL-53-Al, about half of the calculated amount of Al was incorporated into the MIL-53 pore structure, and PXRD data indicated a loss of crystallinity after the third incorporation cycle. Little Al incorporation was observed with HKUST-1, and the large decrease in surface area and pore volume, without significant change in the PXRD pattern, is attributed to pore blockage. Reaction of a large excess of 1 with UiO-67 was highly exothermic and evolved gas, likely from reaction with the μ3-OH groups in the UiO-67 structure. The resulting material was amorphous apart from metallic Al(0) crystals approximately 30 nm in size and larger than the UiO-67 pores, as determined by PXRD and 27Al MAS NMR spectroscopy. This material exhibited no apparent reaction with air or water and exposure to air gave little change in the 27Al MAS NMR spectrum. The Al(0) crystals thus appear to be protected from oxidation, presumably by the remaining UiO-67 framework.

On the preparation and NMR spectroscopic characterization of potassium aluminium tetrahydride KAlH4.

Potassium aluminium tetrahydride KAlH4 of high phase purity (space group Pnma (62)) was synthesized via a mechanochemical route. The thus obtained material was studied by 27Al and 39K MAS NMR spectroscopy. For both nuclei precise data for the isotropic chemical shift and the quadrupole coupling at T = 295 K were derived (27Al: δiso = (107.6 ± 0.2) ppm, CQ = (1.29 ± 0.02) MHz and η = 0.64 ± 0.02; 39K: δiso = (6.1 ± 0.2) ppm, CQ = (0.562 ± 0.005) MHz and η = 0.74 ± 0.02). The straightforward NMR spectroscopic approach applied here should also work for other complex aluminium hydrides and for many other materials containing half-integer nuclei experiencing small to medium-sized quadrupole couplings.

The partial dehydrogenation of aluminium dihydrides.

The reactions of a series of β-diketiminate stabilised aluminium dihydrides with ruthenium bis(phosphine), palladium bis(phosphine) and palladium cyclopentadienyl complexes is reported. In the case of ruthenium, alane coordination occurs with no evidence for hydrogen loss resulting in the formation of ruthenium complexes with a pseudo-octahedral geometry and cis-relation of phosphine ligands. These new ruthenium complexes have been characterised by multinuclear and variable temperature NMR spectroscopy, IR spectroscopy and single crystal X-ray diffraction. In the case of palladium, a series of structural snapshots of alane dehydrogenation have been isolated and crystallographically characterised. Variation of the palladium precursor and ligand on aluminium allows kinetic control over reactivity and isolation of intermetallic complexes that contain new Pd-Al and Pd-Pd interactions. These complexes differ by the ratio of H : Al (2 : 1, 1.5 : 1 and 1 : 1) with lower hydride content species forming with dihydrogen loss. A combination of X-ray and neutron diffraction studies have been used to interrogate the structures and provide confidence in the assignment of the number and position of hydride ligands. 27Al MAS NMR spectroscopy and calculations (DFT, QTAIM) have been used to gain an understanding of the dehydrogenation processes. The latter provide evidence for dehydrogenation being accompanied by metal-metal bond formation and an increased negative charge on Al due to the covalency of the new metal-metal bonds. To the best of our knowledge, we present the first structural information for intermediate species in alane dehydrogenation including a rare neutron diffraction study of a palladium-aluminium hydride complex. Furthermore, as part of these studies we have obtained the first SS 27Al NMR data on an aluminium(i) complex. Our findings are relevant to hydrogen storage, materials chemistry and catalysis.

Modification of mordenite by carbon black template and utilization as supports for iron catalyst in phenol hydroxylation

Abstract The aim of this work was to prepare mordenite zeolite in sodium form by using silica-alumina source and carbon as the second template (C-NaMOR) then transformed to proton form (C-HMOR). Both C-NaMOR and C-HMOR were characterized by XRD and N2 adsorption-desorption to confirm their structure, morphology and textural properties including surface area, micro- and mesopores volume, respectively. Results from 27Al MAS NMR provided the information of structural defect corresponding to Bronsted and Lewis acid sites. NH3-TPD suggested the amount of acid increasing with structural defects. Both C-NaMOR and C-HMOR were impregnated with Fe(NO3)3⋅9H2O. The form of Fe in 5Fe/C-NaMOR and 5Fe/C-HMOR catalysts which analyzed by XANES was mainly Fe2O3 and the oxidation state was +3. Catalytic testing of Fe supported on C-xMOR zeolites showed improvement the conversions (68%) and selectivities (catechol/hydroquinone ratio = 1).

Insight into the active site nature of zeolite H-BEA for liquid phase etherification of isobutylene with ethanol.

The nature of active acid sites of zeolite H-BEA with different Si/Al ratios (15–407) in liquid phase etherification of isobutylene with ethanol in a continuous flow reactor in the temperature range 80–180 °C has been explored. We describe and discuss data concerning the strength and concentration of acid sites of H-BEA obtained by techniques of stepwise (quasi-equilibrium) thermal desorption of ammonia, X-ray diffraction, low-temperature adsorption of nitrogen, FTIR spectroscopy of adsorbed pyridine and solid-state 27Al MAS NMR. The average values of the adsorption energy of NH3 on H-BEA were experimentally determined as 63.7; 91.3 and 121.9 mmol g−1 (weak, medium, and strong, respectively). In agreement with this, a correlation between the rate of ethyl-tert-butyl ether synthesis and the concentration of weak acid sites (ENH3 = 61.6–68.9 kJ mol−1) has been observed. It was concluded that the active sites of H-BEA for this reaction are Brønsted hydroxyls representing internal silanol groups associated with octahedrally coordinated aluminum in the second coordination sphere.

Nickel aluminate spinel-derived catalysts for the aqueous phase reforming of glycerol: Effect of reduction temperature

Bulk nickel aluminate (NiAl2O4) was synthesised by co-precipitation at a Ni/Al mole ratio of 1:2 (stoichiometric ratio). The prepared sample was reduced at different temperatures, in the 300 to 850 °C range, and obtained assays were analysed by a wide range of analytical techniques (XFR, XRD, H2-chemisoprtion, H2-TPR, DRS UV–vis NIR, FTIR, 27Al MAS NMR, NH3-TPD, CO2-TPD, TPO) and tested for the APR of glycerol. The spinel precursor allowed the formation of small and stable Ni particles (<14 nm) upon reduction with good performance in the APR of glycerol (NiAl-850 93% conversion, 57% conversion to gas, at 250 °C/45 bar and WHSV 24.5 h−1). Hydrogen was the main gaseous product and the activation temperature did not substantially alter selectivity to gaseous products; however, selectivity to intermediate oxygenated liquid compounds was substantially modified. Overall, glycerol dehydrogenation route was dominant at high reduction temperature. The good stability of the spinel led to stable H2 yield in the long-term runs (50 h) and proved potential to be used in the APR of glycerol.