Molecular engineering of microporous crystals: (III) The influence of water content on the crystallization of microporous aluminophosphate AlPO4-11

The influence of water content on the crystallization of the microporous aluminophosphate AlPO₄-11 was investigated by a combination of in situ UV Raman and ex situ XRD and NMR characterizations. Under high water content conditions (Al₂O₃:H₂O = 1:20-69), an AlPO₄-5 intermediated formed first and later co-existed with AlPO₄-11. In the last stage of crystallization, the crystalline AlPO₄-5 completely disappeared and well-crystallized AlPO₄-11 was obtained. The position of the three signals observed in the ³¹P MAS NMR spectra of the isolated solid samples did not change during the hydrothermal treatment, suggesting that the fragments containing the P site in the junction of 6- and 4-membered rings of both AlPO₄-5 and AlPO₄-11 formed first and that the environments of the P sites were very similar to those in the final structure. A significant increase of Al concentration in liquid media was observed before the appearance of XRD detectable AlPO₄-11. Under low water content conditions (Al₂O₃:H₂O = 1:15), the AlPO₄-11 was directly crystallized from the initial mixture. In situ Raman spectra showed that the 10-membered rings were complete by the time the transformation of octahedral Al to tetrahedral Al was finished and that the conformation of the protonated di-(i-propyl)amine changed little during the crystallization. The present study showed that the composition of the liquid phase is critical to the formation of a specific structure.     

Catalyst effect of metal cations on pyrolysis of hydrocarbon molecules and formation of carbon nanotubes in the channels of AlPO4-5 crystals

Ultra-small single-walled carbon nanotubes (SWNTs) with diameter of 0.4 nm were fabricated in the channels of AlPO₄-5 crystals by pyrolyzing hydrocarbon molecules. In order to improve the structural quality of the SWNTs, we introduced Br?nsted acid sites onto the channel walls by incorporating metal cations (Mn, Mg, Co, and Si) into AlPO₄-5 framework. The Br?nsted acid sites play an important catalytic role in the carbonization of hydrocarbon molecules (tripropylamine) in the AlPO₄-5 channels, and favor the formation of SWNTs, as revealed by the significant decrease in formation energy of the nanotubes. The experimental results agree well with the predictions of first-principles calculations.     

Synthesis of AlPO4 Molecular Sieves with AFI and AEL Structures by Dry-Gel Conversion Method and Catalytic Application of Their SAPO Counterparts on Isopropylation of Biphenyl

AlPO₄-5 and AlPO₄-11 were synthesized by dry-gel conversion (DGC) method. Steam-assisted conversion (SAC) and vapor-phase transport (VPT) techniques were applied for this purpose. The synthesis was successful in presence of a certain minimum amount of external bulk water, without which the crystallization failed. Crystallization by VPT method was slower than corresponding SAC and HTS method. SAPO analogs of the samples, SAPO-5 and SAPO-11 were also synthesized by DGC method. Samples made by DGC methods had higher yield than the conventional hydrothermal synthesis (HTS); otherwise the samples showed similar characteristics as that made by HTS. XRD, SEM and N₂-adsorption results showed high crystallinity and purity of the samples made by DGC, and ²⁷Al MAS NMR spectra indicated the tetrahedral framework nature of Al. SAPO-5 and SAPO-11 were tested for their catalytic activity in isopropylation of biphenyl, and in terms of conversion and selectivity, SAPO-5 was found to be suitable for this application.     

Adsorption of oxygen on the paramagnetic defect centers in aluminophosphate molecular sieves

Temperature-programmed desorption (TPD) and electron spin resonance (ESR) techniques have been employed to investigate the oxygen adsorbed on the paramagnetic defect centers in dehydroxylated AlPO₄-20, AlPO₄-11, AlPO₄-5, and VPI-5 molecular sieves. Two different adsorption sites are observed for the small-pore AlPO₄-20, where oxygen is entrapped within the intracrystalline voids generated during the calcination step, as well as within the β-cages. However, the unidimensional molecular sieves AlPO₄-11, AlPO₄-5, and VPI-5 have only one type of adsorption site within their inner pores. The apparent activation energies of desorption (12.0-21.7 kcal mol^-1) determined from the oxygen TPD results demonstrate that, as the pore size of the AlPO₄ molecular sieve is larger, the interaction between the adsorbed oxygen and the molecular sieve framework becomes weaker. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalyst effect of metal cations on pyrolysis of hydrocarbon molecules and formation of carbon nanotubes in the channels of AlPO4-5 crystals

Ultra-small single-walled carbon nanotubes (SWNTs) with diameter of 0.4 nm were fabricated in the channels of AlPO₄-5 crystals by pyrolyzing hydrocarbon molecules. In order to improve the structural quality of the SWNTs, we introduced Brønsted acid sites onto the channel walls by incorporating metal cations (Mn, Mg, Co, and Si) into AlPO₄-5 framework. The Brønsted acid sites play an important catalytic role in the carbonization of hydrocarbon molecules (tripropylamine) in the AlPO₄-5 channels, and favor the formation of SWNTs, as revealed by the significant decrease in formation energy of the nanotubes. The experimental results agree well with the predictions of first-principles calculations.     

High resolution,quasi-equilibrium sorption studies of molecular sieves

Argon, nitrogen, and neopentane adsorption isotherms from molecular sieves are recorded at 87 K, 77 K, and 273 K, respectively, by a quasi-equilibrium, high resolution gas sorption technique. The molecular sieves used in this study are alkali exchanged zeolite X, AlPO₄-11, AlPO₄-5, VPI-5, KL, CaA, ZSM-5, and ZSM-11. Little relation is observed between the transition pressure for microporous nitrogen adsorption and pore size. Small changes in the effective pore size resulting from variations in cation size are detected in the transition pressure for argon adsorption. Large shifts in the transition pressure for argon adsorption are found for the 10-, 12-, and 18-membered ring pores of AlPO₄-11, AlPO₄-5, and VPI-5, respectively. Argon adsorption combined with neopentane adsorption on microporous materials provides additional information regarding transitions in the isotherm that result from dual pore systems and effects that may be due to adsorbate packing. The step in the nitrogen isotherm atP/P ₀> 0.1 from ZSM-5 is not observed in the nitrogen isotherm from ZSM-11.     

On stability and performance of highly c-oriented columnar AlPO4-5 and CoAPO-5 membranes

Continuous films comprised of highly c-oriented aluminophosphate AlPO₄-5 or cobalt-substituted AlPO₄-5 (CoAPO-5) were grown on porous supports and subjected to heat treatment in order to investigate the potential for membrane applications. A study in the early stages of in-plane crystalline intergrowth revealed a potential mechanism for flake-like crystal formation between the original oriented columnar crystals. Variations in metal substitution (AlPO₄-5, CoAPO-5), support (glass, silicon, porous alumina), and calcination method (conventional, rapid thermal processing) were chosen to examine the conditions by which structural integrity was compromised following secondary (or tertiary) growth, resulting in reduced membrane functionality. Through the use of rapid thermal processing, the structure debilitation could be partially avoided. The membrane quality was inspected through pervaporation measurements consisting of a liquid hydrocarbon feed of n-heptane and 1,3,5-triisopropylbenzene. By investigating the effect of template removal on the oriented, columnar crystalline structure, useful insight is provided into the potential for the membranes to participate in applications such as molecular separations, catalysis, or host-guest assemblies.     

Computational Studies in the AlPO4-34 System

The lattice energies of the as-synthesized fluoride-containing chabazite-like aluminophosphate (AlPO₄-34F) and of the corresponding metal-substituted materials [MeAPO-34F, Me = Mn(II), Co(II), Ni(II)] have been calculated in order to investigate the Al-site preference the transition metal substitution in the AlPO₄-34F. The calculations show that the transition metal ions in MeAPO-34F should preferentially occupy octahedral Al³⁺ sites, and kinetic reasons are suggested as an explanation for the actual preference of tetrahedral sites. The lattice energies have also been calculated for the calcined AlPO₄-34F material (AlPO₄-34) and the rehydrated-calcined product (AlPO₄-34h). The AlPO₄-34 is found to be less stable than either AlPO₄-34F or AlPO₄-34h, which is consistent with the fact that AlPO₄-34 can only be prepared starting from AlPO₄-34F.     

Synthesis and structure of AlPO4-9: An unacquainted member in the family of microporous aluminophosphates

In the family of microporous aluminophosphate, AlPO₄-9 is one of the members reported first in 1982. However, its structure and characteristics have not been known up to now, and this makes it a special member. In the present work, AlPO₄-9 has been synthesized using piperazine as the structure-directing agent and the mixture of H₂O and EG as the solvent. Structure refinement from single crystal X-ray diffraction data shows that AlPO₄-9 is a material with the composition of C₂H₇Al_5.50NO₂₅P₆. It crystallizes in the monoclinic space group C2/c (No: 15), with a = 24.230(5) Å, b = 14.026(5) Å and c = 16.197(3) Å, β = 119.87(4)°, V = 4773(2) Å³, Z = 8. The open-framework of AlPO₄-9 is built of alternating corner-sharing PO₄ tetrahedra and AlO₄ tetrahedra (AlO₆ octahedra) to construct a curved one-dimensional 8-ring channel system running along [1 0 1]. Meanwhile, the strict alternative of PO₄ tetrahedra and AlO₄ tetrahedra (AlO₆ octahedra) results in a negative framework in AlPO₄-9. Thermal stability has been determined on a thermal analyzer and by calcination. It reveals that AlPO₄-9 framework is thermally stable and has a potential to be a catalytic material.     

The retention of copper ions by AlPO4-5/VAPO-5 and their effect on reactant access

The reaction of copper salts with AlPO₄-5 or V^v-VAPO-5 under acidic (CuCl₂, pH adjusted to 2) but especially basic conditions (Cu(NH₃) ₄ ²⁺ , pH adjusted to 9) gives ion incorporations greater than expected by a simple ion exchange mechanism (both AlPO₄-5 and V^v-VAPO-5 could be expected to have no cation exchange capacity). Ion incorporation is proposed to occur initially at defect sites, and examination of the ESR spectrum of a dehydrated, evacuated CuCl₂-exchanged AlPO₄-5 shows that these defect sites give rise to a number of unique environments upon Cu^II incorporation. The CuCl₂-exchanged VAPO-5 retains a significant toluene accessibility to the V^v sites in the VAPO-5. However, the toluene accessibility in the Cu(NH₃) ₄ ²⁺ -exchanged VAPO-5 is significantly reduced and we propose this is due to a combination of the presence of crystalline CuO and structural collapse from reaction with base (NH₄OH). The ability of treatment with base (NH₄OH, pH 13) to restrict access of toluene to the V^v sites of the original VAPO-5 was verified in a separate experiment.     

Synthesis of AlPO4-34 and its large single crystal by partially substituting framework fluorine with oxygen species

AlPO₄-34 is a fluorine containing aluminophosphate with two fluorine atoms located in the bridge of two octahedrally coordinated Al atoms of a 4-ring. It has a unit cell composition (C₄H₁₀NO⁺)₂ (AlPO₄)₆F₂ ^? with the stoichiometric value of Al:P:F being 3:3:1 (F/Al?=?0.33) and to be named as AlPO₄-34F. The presence of fluorine in the reacting gels favors the formation of AlPO₄-34 structure, but hinders the growth of its large crystals. In present work, a seesaw effect between organic template and hydrofluoric acid on the crystallization of AlPO₄-34 has been found and used to synthesize AlPO₄-34 with F/Al in the framework lower than 0.33. Large single crystal of AlPO₄-34 has been prepared from a gel having the molar ratio of HF/Al₂O₃?=?0.125. Single crystals refinement has shown that the unit cell formula of the AlPO₄-34 is (C₄H₁₀NO⁺)₂(AlPO₄)₆F _1.18 ^? (OH) _0.82 ^? (H₂O)_0.5. The molar ratio of Al:P:F:O_b is 3:3:0.59:0.41 with 40% fluoride ions being substituted by oxygen species (O_b: oxygen atoms in the bridges of Al?CO?CAl). Ion chromatography analysis has confirmed the partial substitution of fluorine by oxygen. It was named as AlPO₄-34P. Thermal analyses have shown that oxygen species in the bridge position of two octahedrally coordinated Al atoms are more instable than fluoride ions. Under heating, the AlPO₄-34P will transform to chabazite structure at a temperature lower than AlPO₄-34F.     

Comparison of AlPO4- and Co3(PO4)2-coated LiNi0.8Co0.2O2 cathode materials for Li-ion battery

Electrochemical and thermal properties of Co₃(PO₄)₂- and AlPO₄-coated LiNi_0.8Co_0.2O₂ cathode materials were compared. AlPO₄-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 170.8 mAh g⁻¹ and had a capacity retention (89.1% of its initial capacity) between 4.35 and 3.0 V after 60 cycles at 150 mA g⁻¹. Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 177.6 mAh g⁻¹ and excellent capacity retention (91.8% of its initial capacity), which was attributed to a lithium-reactive Co₃(PO₄)₂ coating. The Co₃(PO₄)₂ coating material could react with LiOH and Li₂CO₃ impurities during annealing to form an olivine Li_xCoPO₄ phase on the bulk surface, which minimized any side reactions with electrolytes and the dissolution of Ni⁴⁺ ions compared to the AlPO₄-coated cathode. Differential scanning calorimetry results showed Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathode material had a much improved onset temperature of the oxygen evolution of about 218 °C, and a much lower amount of exothermic-heat release compared to the AlPO₄-coated sample.     

Effect of Thermal Activation on the Kinetics of Cd2+ Ions Sorption by AlPO4

Abstract

The kinetics of Cd²⁺ uptake on two different samples of AlPO₄ activated at 105°C and 400°C is studied as a function temperature, which shows an increase in the optimum time for the maximum uptake of Cd²⁺ ions with activation. The mechanism of the uptake is observed to change from ion exchange to sorption inside the pores with activation. The rate constants calculated from the first order Lagergren's plots are observed to decrease while activation energy, enthalpy, entropy, and free energy of activation are observed to increase with activation. All the activation parameters are found to be higher for the activated AlPO₄ as compared to the non-activated AlPO₄.     

Hydrodesulfurization and hydrodearomatization activities of catalyst containing ETS-10 and AlPO4-5 on Daqing FCC diesel

A Ni–W loaded ETS-10/AlPO₄-5/Al₂O₃ composite support catalyst was optimized and used in hydrodesulfurization (HDS) and hydrodearomatization (HDA) of Daqing FCC diesel feedstock. The result indicated that ETS-10 and AlPO₄-5 showed positive synergism effect. The effects of operating conditions on its catalytic performance were investigated by using a 100 mL hydrotreating test unit. The catalyst showed a remarkable HDS conversion of 99.9% and a HDA conversion of 73.2%. A clean diesel product with ultra-low sulfur content (<1.0 μg/g) and very low polycyclic aromatic content (<2.0 wt.%) was obtained.     

Characterization and fluorine-free microwave hydrothermal synthesis of AlPO<Subscript>4</Subscript>-5 molecular sieves as adsorbents

With a characteristic of S-shaped water sorption isotherms, AlPO₄-5 molecular sieves have been considered as up-and-coming adsorbents for the utilization in adsorptive cooling and heating systems. In order to avoid toxicity and corrosion of fluoride, this paper introduced a fluorine-free microwave hydrothermal synthesis strategy of pure AlPO₄-5 crystals. The effects of hydrothermal conditions including template agent, crystallization temperature and time on the performances of AlPO₄-5 adsorbents were systematically investigated. Fourier transform infrared spectroscopy, X-ray diffraction (XRD), ²⁷Al and ³¹P solid state magic angle spinning nuclear magnetic resonance (MAS NMR), pore size analyzer and Scanning electron microscopy (SEM) were used to determine the chemical structure, crystalloid phase, framework, pore structure and the morphology. Adsorption and desorption performances were measured by static adsorption, thermogravimetry and temperature programmed desorption. XRD results indicated that both triethylamine and tetraethylammonium hydroxide as templates, AlPO₄-5 crystals could quickly be synthesized within 30–45 min under fluorine-free microwave irradiation. By comparison, the adsorption and desorption performance of the former (recorded as AlPO₄-5A) was superior to that of the latter (recorded as AlPO₄-5H). FT-IR, MAS NMR and SEM results revealed that AlPO₄-5 crystals have the frameworks of alternating AlO₄ and PO₄ units and typical hexagonal rod-like morphologies.     

Hydrothermal synthesis of pure AlPO4-5 without fluoride medium: synthesis, characterization and application as a support

Hydrothermal synthesis of pure AlPO₄-5 crystals using various reaction gels is reported. The influence of synthesis conditions on the purity of the AlPO₄-5 has been studied by changing Al sources and factors affecting the pH of the starting gel. It was observed that the additive acid to adjust the gel pH value has strongly influenced both the crystallinity and purity of the resulting AlPO₄-5 crystals. The crystals were characterized by XRD, FT-IR, TGA, BET, SEM and TPD techniques. The resulting microporous AlPO₄-5 was used as an efficient support for a functional polymer to produce an efficient heterogeneous basic nanocomposite catalyst. The catalytic activity of this novel nanocomposite was tested for Knoevenagel reaction under solvent-free conditions at room temperature. The catalyst showed a considerable degree of reusability besides very good activity.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

Electrochemical performances of lithium ion battery using alkoxides of group 13 as electrolyte solvent

Tris(methoxy polyethylenglycol) borate ester (B-PEG) and aluminum tris(polyethylenglycoxide) (Al-PEG) were used as electrolyte solvent for lithium ion battery, and the electrochemical property of these electrolytes were investigated. These electrolytes, especially B-PEG, showed poor electrochemical stability, leading to insufficient discharge capacity and rapid degradation with cycling. These observations would be ascribed to the decomposition of electrolyte, causing formation of unstable passive layer on the surface of electrode in lithium ion battery at high voltage. However, significant improvement was observed by the addition of aluminum phosphate (AlPO₄) powder into electrolyte solvent. AC impedance technique revealed that the increase of interfacial resistance of electrode/electrolyte during cycling was suppressed by adding AlPO₄, and this suppression could enhance the cell capabilities. We infer that dissolved AlPO₄ components formed electrochemically stable layer on the surface of electrode.     

Towards Rational Synthesis of Microporous Aluminophosphate AlPO4-21 by Hydrothermal Combinatorial Approach

We present a strategy toward the rational synthesis of microporous materials by combination of computational and combinatorial approach. In terms of nonbonding interaction energies of host–guest calculated by the molecular dynamics simulations using Cerius²package, the templating abilities of various organic amines in the formation of microporous aluminophosphate AlPO₄-21 have been evaluated. Through rational selection of the predicted suitable templates, such as ethanolamine, trimethylamine and N, N, N′, N′-tetramethylenediamine, AlPO₄-21 has been successfully synthesized by hydrothermal combinatorial approaches in the reaction system 1.0Al(ⁱOPr)₃-xH₃PO₄-yR-255.0H₂O (R amines). The as-synthesized products are characterized by automated X-ray powder diffraction, ICP, TG, and single-crystal X-ray diffraction analyses. Their framework structures belong to zeotype AWO, which possesses eight-membered ring channels along [001] direction.     

A1PO4-Al2O3 catalysts with low alumina content. III. Surface basicity of catalysts obtained in aqueous ammonia

The amount of basic sites of A1PO₄-Al₂O₃ (APA1-A, 5–15 wt% Al₂O₃) catalysts at two basic strengths was measured by studying the liquid-phase adsorption of two acidic molecules (benzoic acid (BA, pK = 4.2) and phenol (PH, pKa = 9.9) from cyclohexane solutions, through the application of a spectrophotometric method. The data obtained follow the Langmuir adsorption isotherm and the monolayer coverage at equilibrium (at 298 K),X _m, is assumed as the amount of basic sites corresponding to the specific pK of the acid used as titrant. The amount of basic sites of any AlPO₄-Al₂O₃ catalyst is higher than that of AlPO₄, but lower than that of Al₂O₃. Besides, an increase in the Al₂O₃ content from 10 wt% gradually increases the basicity of the APA1-A catalyst. Moreover, calcination at increasing temperatures does not practically affect the surface basicity of APAl-A-5 and APAl-A-10 catalysts. However, for AlPO₄ content higher than 10 wt% we observe a decrease in surface basicity, this decrease depends on alumina content, i.e. it is higher as the amount of alumina increases. The basic sites of APAl-A systems catalyze the Knoevenagel condensation ofp-methoxybenzaldehyde and malononitrile at room temperature and in the absence of solvent.