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.     

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.     

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.     

Study of alkylation on a Lewis and Brønsted acid hybrid catalyst and its industrial test

A novel catalyst ZnBr₂ modified bentonite is prepared. Pyridine adsorbed FTIR spectra demonstrated the amount of Brønsted and Lewis acid was improved to 2 and 5 times, respectively. Thermal analysis indicated the thermal stability of ZnBr₂-bentonite could reach 350 °C. Mechanism research led to an interesting hypothesis that in the former stage, the active position is Brønsted acid sites and in the later stage, both Lewis acid and Brønsted acid sites act as active positions. Industrial test showed the ability of ZnBr₂-bentonite to remove olefins is 17 times higher than that of bentonite, and the industrial application prospect is good.     

Synthesis and application of a novel strong and stable supported ionic liquid catalyst with both Lewis and Brønsted acid sites

Poly(4-vinylpyridine)-supported ionic liquid with both Lewis and Brønsted acid sites was easily prepared from its starting materials and used as a novel and highly efficient heterogeneous catalytic system for the synthesis of biscoumarins by two-component one-pot domino Knoevenagel-type condensation/Michael reaction between various aliphatic and aromatic aldehydes with 4-hydroxycoumarin. The Lewis and Brønsted acidic sites loading in [P₄VPy-BuSO₃H]Cl-X(AlCl₃) were found to be 2.15 and 0.9 mmol per gram of catalyst, respectively. The effect of the simultaneous presence of Lewis and Brønsted acid sites was evaluated. The catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), elemental analysis, and atomic absorption technique. The catalyst is stable (as a bench top catalyst) and reusable.     

Effect of acid sites on catalytic destruction of trichloroethylene over solid acid catalysts

The catalytic destruction of trichloroethylene (TCE) over several solid acid catalysts (HZSM-5, γ-Al₂O₃ and SBA-15/P) was evaluated under dry conditions. The activity order was found to be: HZSM-5>SBA-15/P>γ-Al₂O₃. It was reported that Brønsted and Lewis acid sites of catalysts both played an important role on TCE catalytic destruction, while the Brønsted acid sites were more decisive. Additionally, the formation of the polychlorinated by-product (tetrachloroethylene, PCE) over HZSM-5 and γ-Al₂O₃ catalysts was observed and attributed to the presence of Lewis acid sites and basic O^2?, and PCE was not detected over SBA-15/P catalyst due to the presence of only Brønsted acid sites. The TCE/O₂-TPSR studies demonstrated that the main oxidation products during TCE catalytic destruction are CO, CO₂ and Cl₂, and the carbon in TCE was firstly converted to CO and then further oxidized into CO₂ by gas phase O₂.     

MAS NMR, ESR and TPD studies of Mo/HZSM‐5 catalysts: evidence for the migration of molybdenum species into the zeolitic channels

NH₃‐TPD, MAS NMR and ESR spectroscopies were employed to investigate Mo‐modified HZSM‐5 catalysts prepared by impregnation. It was found that the modification of Mo ions results in a pronounced decrease in the intensity of ¹H MAS NMR resonance originating from Brønsted acid sites in the zeolites and a distinct splitting of Mo⁵⁺ ESR signals, which is attributed to the interaction of Mo with the Al atom of the zeolite framework. This presents distinct evidence that Mo ions migrate from the external surface of the zeolite into the lattice channels during the impregnation and subsequent treatment. The remaining Brønsted acid sites associated with the migrated Mo ions form the bifunctional catalytic centers that may be responsible for the outstanding catalytic performance in methane aromatization.     

Discrepancy between TPD- and FTIR-based measurements of Brønsted and Lewis acidity for sulfated zirconia

Temperature-programmed desorptions (TPD) of isopropylamine (IPA), NH₃, and pyridine were compared with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine to determine the effect of H₂O on the Brønsted and Lewis acidities of two sulfated zirconia (SZ) catalysts. Although the traditional interpretation of pyridine infrared spectra showed an apparent increase in Brønsted acidity upon treating SZ with H₂O, TPD spectra showed that H₂O displaced IPA from approximately one-fifth of the Lewis sites with no corresponding increase in Bronsted acidity. Water treatment prior to TPD displaced similar amounts of both NH₃ and pyridine. The primary effect of H₂O is displacement of weakly adsorbed basic probe molecules from Lewis sites, rather than the conversion of Lewis sites to Brønsted sites. Finally, different types of analyses (e.g. infrared or TPD) of catalyst acidity yield dramatically different conclusions regarding Brønsted and Lewis acidity.     

Unraveling the spatial distribution of the acidity of HZSM-5 zeolite on the level of crystal grains

Here we developed a new method to quantitatively characterize the spatial distribution of the Brønsted acidity of HZSM-5 zeolite on the crystal grains level indirectly. The Brønsted acid sites of HZSM-5 zeolite completely covered or blocked by previous coke deposition were released gradually from the outer surface to the center of the crystal grains via shrinking core mode isothermal oxidation with high temperature and low oxygen concentration. The spatial distribution of the coke was obtained based on the one-dimensional position coordinate L_rp through building and solving an enhanced shrinking core model. Then the released acidity characterized by n-propylamine temperature-programmed decomposition was correlated to the specific position of L_rp. The results show that the acid density is roughly homogenous while the acid strength is heterogeneous within the crystal grains. From the outer surface to the center of the crystal grains, the strength of the Brønsted acid sites increase gradually.     

Synthesis of Glycerol Triacetate Using a Brønsted–Lewis Acidic Ionic Liquid as the Catalyst

Brønsted–Lewis acidic ionic liquids (IL) were used in the esterification of glycerol and acetic acid to produce glycerol triacetate. The results show that the IL (3–sulfonic acid)–propyltriethylammonium chloroironinate [HO₃S–(CH₂)₃–NEt₃]Cl–[FeCl₃]_x (molar fraction of FeCl₃, x = 0.67) was an efficient catalyst for the esterification reaction. The yield of glycerol triacetate and its content were greater than 98 % when reacted under reflux for 4 h. It was observed that a synergistic effect of Brønsted and Lewis acid sites enhanced the catalytic performance of IL. The reusability of IL was good. After six reaction cycles, the glycerol triacetate yield and concentration were still greater than 98 %. Likewise, the Brønsted–Lewis acidic IL was an efficient catalyst for esterification reactions of high boiling points alcohols with acetic acid.     

On the Environment of the Active Sites in Phosphate Modified Silica–Zirconia Acid Catalysts

The acid sites of a series of phosphated silica–zirconia catalysts have been quantified by the combined use of FTIR and TGA and the data related to the activity of the samples for the isomerisation of but-1-ene. The catalyst series show a linear correlation indicative of constant TOF when the number of Brønsted acid sites is plotted against catalytic activity. However, this relationship only holds when the number of Lewis acid sites exceeds that of Brønsted acid sites suggesting that an optimal arrangement may involve both types of site. The TOF is less when the Lewis/Brønsted ratio falls below 1, a scenario that dominates at high Brønsted acid site densities. When this site arrangement prevails, a higher than expected cis/trans ratio is found in the isomerisation selectivity. These differences in the environment of the active Brønsted acid site may also be deduced from differences in the molar absorption coefficients of adsorbed basic probe molecules.     

Role of acidity of catalysts on methane combustion over Pd/ZSM-5

Palladium-based catalysts were prepared by the impregnation (I) and ion-exchange method (E) with ZSM-5 and γ-Al₂O₃ as support respectively. The high activity of Pd/ZSM-5(I) and Pd-ZSM-5(E) catalysts for methane combustion was observed. The order of activity is consistent with Brønsted acidity of catalysts: Pd/ZSM-5(I) > Pd-ZSM-5(E) > Pd/Al₂O₃. It is shown by FT-IR that methane adsorbs on acidic bridging hydroxyl groups of ZSM-5-supported Pd catalysts. Symmetric v₁ C–H stretching vibrations of methane shift to low frequency due to the interaction between methane molecules and Brønsted acid sites or Pd²⁺, indicating that methane molecules can be activated.     

Alkane isomerization over sulfated zirconia and other solid acids

Some solid acids, including sulfated zirconia and certain industrial isomerization catalysts, catalyze two types of n-butane isomerizations, avoiding primary carbenium ions or carbonium ions: (1) an internal rearrangement of the C atoms in n-butane and (2) skeletal isomerization of n-butane to iso-butane. No superacid sites are required for these reactions. The skeletal isomerization is an intermolecular reaction, involving a C₈ intermediate. Easily accessible Brønsted acid sites and small amounts of olefin are crucial. Spectroscopic examination of the acid sites on sulfated zirconia shows that they are not stronger than the acid sites in zeolites such as HY. The butane isomerization rate is suppressed by CO, even when no CO is adsorbed on Lewis sites; formation of oxocarbenium ions is likely. The decisive role of Brønsted acid sites is demonstrated by results on deuterated catalysts.     

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.     

Unraveling the cooperative effects of acid sites and kinetics for pyrolysis of CHF3 to C2F4 and C3F6 on SO42−/ZrO2-SiO2

The treatment or upgrading of waste trifluoromethane (CHF₃, R23), which has a significant greenhouse effect, is of great importance in industry. Herein, series of SO₄²⁻/ZrO₂-SiO₂ catalysts with different Brønsted and Lewis acid site densities and ratios were prepared for pyrolysis of R23 to tetrafluoroethylene (C₂F₄, TFE) and hexafluoropropylene (C₃F₆, HFP). The effects of impregnation concentration of (NH₄)₂SO₄ on specific surface area, crystal phase, and Brønsted and Lewis acid site densities and ratios were respectively demonstrated. The Brønsted and Lewis acid sites were observed to have cooperative effects on R23 transformation and up to 94.6% selectivity of (TFE + HFP) could be achieved at 750°C. The kinetic studies revealed the decomposition of R23 into CF₂ carbene and HF was the rate-determining step, and a deactivation behavior was found due to the site coverage and pore blockage by the oligomers of TFE and HFP.     

Acid Sites in HZSM‐5 Upon Copper Exchange by FTIR and DSC Using Ammonia

The acidity of the parent HZSM‐5 zeolite and Cu‐HZSM‐5 catalyst has been examined by FTIR and DSC using ammonia desorption. Sites of different strength were found in HZSM‐5, desorbing ammonia at a relative temperature difference of about 100 °C. Upon copper exchange a fraction of Brønsted sites were transformed to Lewis sites, but the acid strength of the remaining Brønsted sites was increased. Aside from Lewis sites originating from copper exchange, there might be some additional sites formed from precipitated copper. This could explain the quantity of adsorbed NH₃ on Cu‐ZSM‐5 which is higher than theoretically expected. While changing their nature, acid sites of higher strength keep their location, which is manifested by some diffusion effect towards ammonia.     

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.     

Direct N2O decomposition over ex-framework FeMFI catalysts. Role of extra-framework species

Extra-framework iron species in FeMFI prepared via an ex-framework route are essential for the formation of reactive oxygen species in direct catalytic decomposition of N₂O, while Lewis or Brønsted acid sites play a minor role in this reaction.     

Production of Furfural from Lignocellulosic Biomass Using Beta Zeolite and Biomass-Derived Solvent

The production of furfural from the C₅ monosaccharides xylose, arabinose and ribose, as well as from real biomass (corn fiber), was studied using H-Beta zeolite as catalyst in a monophasic system with the biomass-derived solvent, gamma-valerolactone. Due to the combination of Brønsted and Lewis acid sites on this catalyst (Brønsted:Lewis ratio = 1.66), H-Beta acts as a bifunctional catalyst, being able to isomerize (Lewis acid) and dehydrate (Brønsted acid) monosaccharides. The combination of Lewis and Brønsted acid functionality of H-Beta was shown to be effective for the isomerization of xylose and arabinose, followed by dehydration. While no advantages were found in the conversion of xylose, higher furfural yields were achieved from arabinose, using H-Beta, 73 %, compared to sulfuric acid (44 %) and Mordenite (49 %). The furfural yields from corn fiber for H-Beta, H-Mordenite and sulfuric acid were 62, 44, and 55 %, respectively, showing that H-Beta is particularly effective for conversion of this biomass feedstock composed of 45 wt% hemicellulose, of which 66 % is xylose and 33 % arabinose.     

Variable temperature FTIR study on the surface acidity of variously treated sulfated zirconias

The acidity of three related sulfated zirconias has been compared by IR spectroscopy using CO and H₂ as probe molecules. The parent material (SZ) was obtained by calcination of a commercial sulfated zirconium hydroxide. The other two samples, SZ-WW and SZ-SO₃, were obtained from SZ by water washing and by sulfation with gaseous SO₃, respectively. The labile sulfate groups responsible for alkane activation and associated with an IR band at 1406 cm⁻¹ are removed by water washing and increased by SO₃-sulfation. No remarkable differences in the strength of either Lewis or Brønsted acid sites have been observed between SZ and SZ-SO₃. Water washing strongly weakens Brønsted acidity but only slightly weakens Lewis acidity.