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.     

Catalytic effect of metal cations on the formation of carbon nanotubes inside the channels of AlPO4-5 crystal

Single-walled 0.4 nm carbon nanotubes (SWNTs) were fabricated using various metal cation substituted AlPO₄-5 (MeAPO-5) molecular sieves as the template. The catalytic behavior varies with different metal incorporation. The incorporation of metal cations (Mn, Mg, Co) and Si gives rise to the formation of negatively charged frameworks and Brønsted acid sites. These frameworks thus play an important catalytic role in the pyrolysis and conversion of the organic molecules to SWNTs within the crystal channels. It is shown that the MeAPO-5 single crystals have a higher density of SWNTs than those crystals without the metal cations. The experimental results agree with the predictions of first-principles calculations, which show the metal-incorporated frameworks to be favorable for the SWNTs formation.     

Optical Properties and van der Waals–London Dispersion Interactions in Berlinite Aluminum Phosphate from Vacuum Ultraviolet Spectroscopy

The interband optical properties of single‐crystal berlinite AlPO₄ have been investigated in the vacuum ultraviolet (VUV) range using VUV spectroscopy and spectroscopic ellipsometry. The complex optical properties were directly determined from 0.8 to 45 eV. Band gap energies, index of refraction and complex dielectric functions, oscillator index sum rule, and energy loss functions were calculated through Kramers–Kronig transformation. Direct and indirect band gap energies of AlPO₄ over the absorption coefficient range of 33–11 000 cm^?1 are 8.06 and 7.89 eV, respectively. The index of refraction at 2 eV, n_vis, is 1.51. The interband transition features at 10.4, 11.4, 14.2, 16.2, 17.3, 21, 22.5, 24.5, and 31 eV were indexed and correlated with the electronic structure of AlPO₄. Strong similarities were observed between AlPO₄ and its structural isomorph SiO₂ in the exciton and interband transitions, resulting from the similarity of their constituent tetrahedra and the strong electron localization therein. The London dispersion spectrum for AlPO₄ was calculated, and the Hamaker coefficients for AlPO₄ with various interlayers were calculated using the Lifshitz method. These results elucidate the role of phosphate complex anions on the electronic structure and van der Waals forces in important organic and inorganic systems.     

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.     

Characterization of AlPO4-based molecular sieves and methanol conversion to light olefins

Of the AlPO₄-based molecular sieves, A1PO₄, SAPOs, and MeAPOs of different pore sizes were prepared at 100-200°C by a hydrothermal crystallization method. This study was purposed to maximize the yield of light olefins through methanol conversion. Crystal structure was confirmed by means of XRD and SEM, and acidity was examined by TPD and IR of adsorbed ammonia on the catalysts. It was found that SAPO-34 exhibited more than 90% selectivity for light olefins such as ethylene, propylene, and butylene due to shape selectivity through small pores, although it had a strong acidity. MeAPO-34 exhibited slightly lower selectivity for light olefins than SAPO-34 and different product distribution, depending on the electronegativity of the metal in its framework. SAPO17 and SAPO-44, which have the same pore size with SAPO-34 but different pore structure from SAPO-34, showed less selectivity for light olefins than SAPO-34.     

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.     

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.     

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 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.     

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.     

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 High Temperature Stable Carbon Coated Metal Nanoparticles in AlPO4 Based Matrix In Situ in Oxidative Atmosphere

In this work, we have reported a universal method for the synthesis of metal nanoparticles coated with graphite layer in AlPO₄ based matrix. As an example, graphitized carbon coated Ag, Pt, Cu and Ni nanoparticles were synthesized in the amorphous AlPO₄ based matrix. The metal nanoparticles were protected from oxidation up to very high temperatures due to the low oxygen diffusivity in AlPO₄ based matrix and carbon coating over the metal nanoparticles. The oxidation states of the Ag and Ni nanoparticles were detected with the help of X‐ray photoelectron spectroscopy. The synthesis technique followed very simple methodology. The entire processing including heat treatments at higher temperatures were carried out in oxidative atmosphere. The mechanism for the formation of metal particle in AlPO₄ based matrix has also been addressed. This approach can be a universal approach to achieve metal nanoparticles in AlPO₄ based matrix. Finally, catalyzing activities of the AlPO₄‐Cu nanocomposites in the oxidation of cyclohexane, AlPO₄‐Ni and AlPO₄‐Pt nanocomposites in the reduction in 4‐nitrophenol were successfully investigated.     

Conductometric studies of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) tetrafluoroborates in N,N—dimethylacetamide solutions

Results are reported for the molar conductivities at 25°C of N,N—dimethylacetamide (DMA) solutions of Bu₄NBF₄ and Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) tetrafluoroborates. The limiting molar conductivities of [M(DMA)₆]²⁺ (M  MN, Co, Ni, Cu, Zn) and BF^?₄, as well as association constants for Co(BF₄)₂ in DMA solutions have been calculated. The slight differences between conductometric curves of different metal ions are discussed.     

Fluoride treatment of AlPO4-Al2O3 catalysts. II. Poisoning experiments by bases for cyclohexene conversion and cumene cracking

Brønsted acid sites on fluoride-modified AlPO₄-Al₂O₃ (2.5 wt% F; APAl-P-2.5F) catalyst are poisoned by the presence of 2,6-dimethylpyridine (DMPY) and hexamethyldisilazane (HMDS), thus decreasing the catalytic activity for cyclohexene and cumene reaction processes, while the effect of pyridine (PY) was scarce. Besides, the drop in activity for cyclohexene conversion was accompanied by a change in reaction selectivity so that hydrogen transfer sites are much more sensitive to base poisoning (getting greater as the poisoning effect increased) than isomerization sites. Moreover, surface trimethylsilyl (TMS) complexes (formed by covalent reaction of HMDS with surface hydroxyls) decomposed and thus, the activity progressively increased at increasing time intervals, thus reaching greater values (at ca. 4 h) than the unpoisoned APAl-P-2.5F catalyst. So, DMPY was more suitable than PY and HMDS for the poisoning of Brønsted acid sites on APAl-P-F catalyst.     

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.     

Removal of toxic aqueous ions using amorphous AlPO4

Aluminium phosphate (AlPO₄) was characterized using X-ray diffractometry (XRD), Fourier Transform infrared (FTIR), point of zero charge (PZC) and dissolution studies. XRD showed the sample to be amorphous, FTIR confirmed the presence of OH groups on the surface and PZC was determined at pH 3.45. The dissolution study illustrated a decrease in dissolution with an increase in the pH. Potentiometric titration data were fit to the Gaines–Thomas equation, which showed that AlPO₄ is a weakly acidic ion exchanger. Sorption studies were carried out at pH 4–6 and temperatures 293–323 K. The uptake of metal ions was observed to increase with an increase in the pH and temperature. The surface selectivity towards metal ions was found in the order Pb²⁺> Cu²⁺> Cd²⁺. Sorption data were fit to the new equation derived from the proposed mechanism for metal ion uptake. Various parameters such as stoichiometry of the surface H⁺ ion release, equilibrium constant, standard enthalpy, entropy and free energy changes were evaluated from the plots. The values of all these parameters were found to be closely related to the values reported in the literature.     

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.     

Unique vapor phase synthesis of 1,2,3,5,6,7-hexahydrodicyclopenta[b,e]pyridine selectively over Co–Al-MCM-41

Cyclization of cyclopentanone, formaldehyde and ammonia in vapor phase gives 1,2,3,5,6,7-hexahydrodicyclopenta[b,e]pyridine (HHDCP) and spiro[cyclopentane-1,8′-(1′,2′,3′,5′,6′,7′,8′,8′a) octahydrodicyclopenta[b,e]]pyridine (SCOHDCP) over zeolites HY, HZSM-5, Hβ and mesoporous Al-MCM-41 molecular sieves. The preliminary screening of catalysts clearly shows that Al-MCM-41 is more suitable for the vapor phase synthesis of HHDCP. As the NH₃-TPD profiles of Al-MCM-41 show wide range distribution of acid sites in the temperature range of 200–600 °C (weak–medium–strong), Al-MCM-41 is further modified with transition metal ions like V(V), Mn(II), Fe(III), Co(III), Cu(II), La(III) and Ce(III) to fine tune the acid sites. Correlation of activity and selectivity of transition metal modified Al-MCM-41 with the NH₃-TPD profiles show that though the conversions are high, selectivity of either HHDCP or SCOHDCP is a preference of acid site strength formed on metal ion modification. Interestingly Co²⁺ ion modification of Al-MCM-41 resulted distinctly into two sets of acid sites with T_max around 218 °C (weak–medium) and 673 °C (strong). The reaction is studied on Co–Al-MCM-41 by adsorbing pyridine at 300 °C. The typical acidity available on pyridine adsorbed Co–Al-MCM-41 around 300 °C is showing cyclization activity forming only HHDCP indicating that weak–medium acid sites are responsible for the formation of HHDCP. Based on the product distribution plausible reaction mechanism is proposed.     

Electric field gradient tensor in positions of rare-earth metals in RBa2Cu3O7 lattices

The parameters of the electric field gradient tensor (EFG) created in rare-earth metal (REM) sites of RBa₂Cu₃O₇ lattices (R = Nd, Sm, Gd, Dy, Y, Ho, Er, Tm) by crystal lattice ions have been determined by the method of emission Mössbauer spectroscopy on ⁶⁷Ga(⁶⁷Zn) and ¹⁵⁵Eu(¹⁵⁵Gd) isotopes. The EFG tensor has been calculated for REM sites in the approximation of the point charge model. It was shown that the agreement between the experimental and calculated parameters of the EFG tensor can be achieved if the holes are preferably localized in the chain oxygen sublattice.     

Hydrothermal synthesis of AlPO4-5 type zeolitic materials by using aluminum dross as a raw material

The hydrothermal synthesis of AIPO₄-5, which is one of the large pore size aluminum phosphate condensates like zeolitic materials, was carried out by using aluminum dross as a raw material. Triethylamine (TEA) was used as a structure directing agent (SDA) for AIPO₄-5 synthesis. Various physical properties such as crystal structure, surface texture and specific surface area were investigated for the obtained reaction product.AlPO₄-5 can be synthesized from aluminum dross under hydrothermal conditions at 453–473 K for 3 h. AlPO₄ salt as a by-product, which is a non-porous material, is formed at the same time. The crystal of the obtained AlPO₄-5 is hexagonal as observed by SEM photographs. It is desirable to heat-treat the reaction product at the temperature around 823 K to remove TEA. The crystal structure of AlPO₄-5 changes to non-porous aluminum phosphate in case of a heat treatment exceeding 973 K. The specific surface areas of the reaction product before and after heat treatment at 823 K are 18 and 360 m²/g, respectively.