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.     

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.     

Structural study of the 0.4-nm single-walled carbon nanotubes aligned in channels of AlPO4-5 crystal

A detailed synthesis process of ultra-small single-walled carbon nanotubes (SWNTs) aligned in the channels of AlPO₄-5 zeolite single crystals is reported. The structure of such ultra-small SWNTs was analyzed by means of transmission electron microscopy (TEM) and polarized Raman scattering. TEM images showed that the diameter of the SWNTs is as small as 0.4 nm, which is the size of three possible structures: the zigzag (5,0), armchair (3,3) and chiral (4,2). The polarization dependence of the Raman lines indicates that the structures of the nanotubes are dominated by the zigzag (5,0) form. Line-shape analysis of the tangential A_1g Raman mode shows that the SWNTs are metallic with a finite electronic density-of-state near the Fermi energy level, which is in agreement with the band structural calculation using local-density function approximation (LDA) as well as the electric transport measurement.     

Encapsulation of pyrrole into AlPO4 molecular sieves with chromium ions

Encapsulation of pyrrole in one-dimensional channels of AlPO₄-5 and AlPO₄-11 with chromium ions was attempted by a gas diffusion process and investigated by means of thermoanalytic, spectroscopic and conductivity methods. From the results, it was found that conducting polypyrrole was synthesized in channels of AlPO₄-5 with chromium ions which acted as oxidizing agents. Polypyrrole, however, could not be occluded in channels of AlPO₄-11 with chromium ions.     

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.     

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.     

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.     

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.     

Effects of water vapor, CO2 and SO2 on the NO reduction by NH3 over sulfated CaO

Gas effects on NO reduction by NH₃ over sulfated CaO have been investigated in the presence of O₂ at 700–850 °C. CO₂ and SO₂ have reversible negative effects on the catalytic activity of sulfated CaO. Although H₂O alone has no obvious effect, it can depress the negative effects of CO₂ and SO₂. In the flue gas with CO₂, SO₂ and H₂O co-existing, the sulfated CaO still catalyzed the NO reduction by NH₃. The in situ DRTFTS of H₂O adsorption over sulfated CaO indicated that H₂O generated Br?nsted acid sites at high temperature, suggesting that CO₂ and SO₂ competed for only the molecularly adsorbed NH₃ over Lewis acid sites with NO, without influencing the ammonia ions adsorbed over Br?nsted acid sites. Lewis acid sites shifting to Br?nsted acid sites by H₂O adsorption at high temperature may explain the depression of the negative effect on NO reduction by CO₂ and SO₂.     

Synthesis, characterization and catalytic application of mesoporous molecular sieves SBA-15 functionalized with phosphoric acid

The phosphoric acid groups has been successfully grafted on the surface of mesoporous molecular sieve SBA-15 to form a solid acid catalyst P-SBA-15 by post-synthesis method. The samples were characterized by means of XRD, FT-IR, SEM, TEM, ³¹P MAS NMR, NH₃-TPD, N₂ adsorption/desorption and the esterification reaction. The XRD and N₂ adsorption/desorption results indicate that P-SBA-15 catalyst keeps the 2D hexagonal mesoporous structure and opens channels with the lower surface area of 554.35?m²/g compared with that of SBA-15. The SEM and TEM results show that the figuration of P-SBA-15 behaves ??rod-like?? and phosphoric acid groups is scattered differently on the surface of P-SBA-15. The FT-IR and ³¹P MAS NMR results display that phosphorus species graft onto the surface of SBA-15, which provide the Br?nsted acid sites on the surface of P-SBA-15. The acidity of the generated Br?nsted acid sites on P-SBA-15 significantly is verified by NH₃-TPD. The esterification reaction results shows that catalytic activity of P-SBA-15 is enhanced significantly compared with that of SBA-15, and its stability is good.     

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.     

Characterization of New Bimetallic Oxycarbide (MoWC0.5O0.6) for Bifunctional Isomerization of n-Heptane

A new bimetallic oxycarbide was synthesized and characterized by XRD, TEM, EDS, XPS and adsorption–desorption of probe molecules. All the molybdenum was reduced and 35% of tungsten was present as WO_x. The number of metallic sites, Lewis and Br?nsted acid sites were estimated. A turnover rate of 0.1 s⁻¹ was measured at 300 °C for the first order n-heptane isomerization.     

The catalytic and acidic properties of an i‐Pr2NH‐templated SAPO‐11 and dealuminated derivatives

Crystalline silicoaluminophosphate (SAPO‐11) samples were synthesized with i‐Pr₂NH as a novel template, then dealuminated in H₄EDTA solution. These samples were characterized by catalytic skeletal isomerization of linear butylenes, IR analysis of adsorbed pyridine and ³¹P, ²⁹Si MAS‐NMR. Strong acid sites were concerned with Si domains, that were formed by either increasing the Si content of SAPO‐11 via a synthesis process or dealuminating treatment of as‐synthesized SAPO‐11. Acid sites of medium strength were located around isolated Si together with adjacent Al and the second nearest element P. Deep dealumination led to amorphous AlPO₄, and reduced crystallinity degree. The maximum yield of isobutylene in catalytic skeletal isomerization of linear butylenes were achieved around 763 K. It was confirmed that these reactions were mainly performed on Brønsted acid sites of medium strength.     

Interaction between ammonium heptamolybdate and NH4ZSM-5 zeolite: the location of Mo species and the acidity of Mo/HZSM-5

Mo/HZSM-5 catalysts show high reactivity and selectivity in the activation of methane without using oxidants. Mo/HZSM-5 catalysts with Mo loading ranging from 0 to 10% were prepared by impregnation with an aqueous solution of ammonium heptamolybdate (AHM). The samples were dried at 393 K, and then calcined at different temperatures for 4 h. The interaction between Mo species and NH₄ZSM-5 zeolite was characterized by FT-IR spectroscopy, differential thermal analysis (DTA) and temperature programmed decomposition (TPDE) and NH₃-TPD at different stages of catalyst preparation. The results showed that if Mo/HZSM-5 catalysts were calcined at a proper temperature, the Mo species will interact with acid sites (mainly with BrØnsted acid sites) and part of the Mo species will move into the channel. The Mo species in the form of small MoO₃ crystallites residing on the external surface and/or in the channel, and interacting with BrØnsted acid sites may be responsible for the methane activation. Strong interaction between Mo species and the skeleton of HZSM-5 will occur if the catalyst is calcined at 973 K. This may lead to the formation of MoO ₄ ^2– species, which is detrimental to methane activation.     

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.     

Cobalt-exchanged mordenites: preparation,characterization and catalytic activity for the abatement of NO with CH<Subscript>4</Subscript> in the presence of excess O<Subscript>2</Subscript>

The abatement of NO with methane in the presence of oxygen was studied on various commercial MOR in the Na-form (Na-MOR) and H-form (H-MOR), or exchanged to various extents with cobalt (Co-MOR). The sodium and cobalt contents were determined by atomic absorption. Samples were characterized by FTIR and volumetric measurements of CO adsorption. Chemical analysis indicated that one cobalt species replaced two Brønsted acid sites in H-MOR and two Na⁺ ions in Na-MOR. The IR analysis of the OH stretching region, evidencing an unexpected presence of Brønsted acid sites (band at 3610 cm^?1) in Co-MOR, indicated that the exchange process had a more complex stoichiometry. The adsorption of CO at RT on Co-MOR, in addition to the bands of the corresponding H-MOR and Na-MOR matrices, yielded two types of Co^II-carbonyls, the first type occupied the?mordenite main channels, and the second one the mordenite smaller channels. Brønsted acid sites in mordenites were active for the selective catalytic reduction of NO with CH₄. Co-MOR samples were far more active than Na-MOR and H-MOR samples, showing that acid protons play a negligible role when Co is present. Co-MOR catalysts showing the highest activity had the largest amount of Co^II-carbonyls in the main channels. This result strongly suggests that Co^II in the main channels of MOR are the active sites for the CH₄ + NO + O₂ reaction.     

High growth of SWNTs and MWNTs from C2H2 decomposition over Co-Mo/MgO catalysts

A series of MgO supported catalysts having Co and Mo metals 5-40 wt.% in a ratio of 1:1 was prepared by impregnation method. Carbon nanotubes (CNTs) were grown over the catalysts by decomposition of C₂H₂ at 800 °C for 30 min. It was found that 5 and 10 wt.% Co-Mo/MgO catalysts produced single-wall nanotubes (SWNTs), whereas 20, 30 and 40 wt.% Co-Mo/MgO catalysts produced multi-wall nanotubes (MWNTs). The catalyst Mo/MgO was inactive in growing CNTs. In Co-Mo/MgO catalysts, however Mo generated a favorable environment to grow SWNTs. The growth of SWNTs was strongly dependent on the formation of small clusters of cobalt, which may generate from the decomposition of CoMoO₄ species during the nanotube growth. MWNTs were produced over comparatively larger cobalt clusters generated from Co₃O₄ phase during the nanotube growth stage. The yields of SWNTs were about 6% and 27% over 5 and 10 wt.% Co-Mo/MgO catalysts, respectively. MWNTs yield (576%) was observed over 40 wt.% Co-Mo/MgO catalyst. Carbon yield (%) highly varied with acetylene concentration.     

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.     

Mechanisms and Energetics for Br?nsted Acid-Catalyzed Glucose Condensation, Dehydration and Isomerization Reactions

The mechanisms and energetics for Br?nsted acid-catalyzed glucose condensation, dehydration and isomerization reactions were discussed based on our earlier CPMD?CMTD simulation results. Glucose condensation reaction is initiated by the protonation of C1?COH, whereas both dehydration and isomerization reactions are initiated by the protonation of C2?COH to form a common 5-member ring intermediate. Glucose dehydration to form HMF occurs via the direct cyclic mechanism, rather than via the open chain mechanism converting glucose to fructose then to HMF. Fructose is formed via a 1,2 hydride shift process following the formation of 5-member ring intermediate. The barriers for Br?nsted acid-catalyzed glucose reactions are largely solvent induced due to the competition for proton from the solvent molecules.     

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.