Generation of Brønsted Acid Over Alumina-Supported Niobia Calcined at High Temperatures

Nb₂O₅/Al₂O₃ catalysts calcined at high temperatures exhibited the Brønsted acid property. A monolayer of niobic acid-like compound, which has distorted octahedral symmetry, was stabilized over 16 wt% Nb₂O₅/Al₂O₃ catalyst calcined at 1,173 K. The two-dimensional Nb–O–Nb network of stabilized niobic acid-like compound probably accounts for the generation of Brønsted acid.     

Effect of Treatment with NaAlO2 Solution on the Surface Acid Properties of Zeolite β

The influence of treatment with an NaAlO₂ aqueous solution of NaAlO₂ on the surface acid properties of zeolite β was studied as a method to increase the number of both Brønsted and Lewis acid sites. The Hβ samples, prepared by treating with an aqueous solution of NaAlO₂, were characterized by XRD, NH₃-TPD, pyridine-IR, ²⁹Si MAS NMR, and ²⁷Al MAS NMR. The surface properties of the Hβ zeolite changed after treatment with the aqueous NaAlO₂ solution: the number of strong and weak acid sites, the total amount of acid, and the Brønsted and Lewis acid sites increased. The treatment of zeolite β with an aqueous solution of NaAlO₂ may realuminate an Si(0Al)_A site by the isomorphous substitution of Al(OH)₄⁻ anions.     

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.     

Surface acid property and its relation to SCR activity of phosphorus added to commercial V2O5(WO3)/TiO2 catalyst

To examine the influence of phosphorus on the commercial V₂O₅(WO₃)/TiO₂ SCR catalyst, measurements were carried out by means of infrared and Raman spectroscopy, XPS, and NO reduction measurement as a function of phosphorus loading. Phosphorus added to the catalyst was found to disperse well over the catalyst without a significant agglomeration up to 5 wt% P₂O₅ addition. The number of the hydroxyl groups bonded to the vanadium and titanium species decreased readily with increasing amount of phosphorus. Correspondingly, the hydroxyl groups bonded to the phosphorus species were formed. NH₃ adsorbed on both hydroxyl groups bonded to vanadium and phosphorus as ammonium ions, implying that the P–OH groups formed are also responsible for the Brønsted acidity. The NO reduction activity was found to be decreased with increasing amount of phosphorus; however, the influence of phosphorus was relatively small irrespective of the large amount of phosphorus addition. The deactivation might be caused by the change in the nature of the surface hydroxyl groups as Brønsted acid sites. Phosphorus species might partially wrap the surface V=O and W=O groups, which might also contribute to the deactivation.     

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.     

A density functional theory study of the effects of metal cations on the Brøsted acidity of H-ZSM-5

Density functional theory calculations have been carried out to establish the influence of mono- and polyvalent cations on the Brønsted acidity of H-ZSM-5. The zeolite was modeled as a cluster containing 41-45 atoms, in the center of which is an Al⁽¹⁾(OH)SiOAl⁽²⁾(OM)unit, where M⁺ = H⁺, Li⁺, Na⁺, K⁺, Ca(OH)⁺, AlO⁺, Al(OH)⁺ ₂. The local geometry of the Brønsted acid site is affected by the nature of M⁺ and this in turn causes a change in the value of the proton affinity (PA) for the site. The highest value of PA is 330 kcal/mol for M⁺ = H⁺ and the lowest value of PA is 305 kcal/mol for M⁺ = AlO⁺. No correlation was found between the value of PA and the Mulliken charge on Al⁽¹⁾. With the exception of the case where M⁺ = AlO⁺ the binding energy of CO with the Brøsted acid proton is approximately 8.8 kcal/mol, independent of the nature of M⁺. When M⁺ = AlO⁺, the binding energy for CO is 11.1 kcal/mol. The present calculations suggest that different factors affect proton affinity and the binding energy for CO adsorption.     

Direct synthesis of hydrogen peroxide over Pd nanoparticles embedded between HZSM-5 nanosheets layers

Direct synthesis of hydrogen peroxide (DSHP) was studied over Pd loaded on HZSM-5 nanosheets (Pd/ZN). Pd nanoparticles with average size of ca. 4.3 nm were introduced into the adjacent nanosheet layers (thickness of ca. 2.9 nm) by impregnation method. Pd/ZN with theoretical Si/Al molar ratio of 25 showed the highest selectivity for H₂O₂ among the prepared catalysts, together with highest formation rate of H₂O₂ (38.0 mmol·(g cat)⁻¹·h⁻¹), 1.9 times than that of Pd supported on conventional HZSM-5 zeolite (Pd/CZ-50). Better catalytic performance of nanosheet catalysts was attributed to the promoted Pd dispersion which promoted H₂ dissociation, more Brønsted acid sites and stronger metal-support interaction which inhibited the dissociation of OO bond in H₂O₂. The embedded structure sufficiently protected the Pd nanoparticles by space confinement which restrained the Pd leaching, leading to a better catalytic stability with 90% activity retained after 3 cycles, which was almost 3 times than that of Pd/CZ-50 (30.4% activity retained).     

Brønsted acidity of amorphous silica–aluminas studied by 1H NMR

¹H broad-line (4 K) and MAS (room temperature) NMR have been used to study the acid strength of two amorphous silica–aluminas interacting or not with adsorbed water. The study is more difficult than for zeolites, because the acidic SiO(H)Al bridges are reversibly destroyed by dehydration. However, an acidity coefficient value (H₃O⁺ concentration per Brønsted acid site when one water molecule interacts with each Brønsted site) of 0.34±10%; has been determined. This value is equal to that obtained for H-faujasite and H-mordenite samples with Si/Al ratios high enough for maximum acid strength.     

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.     

The dynamics of hydrogen bonds and proton transfer in zeolites – joint vistas from solid-state NMR and quantum chemistry

The different aspects of zeolite Brønsted acid sites are reviewed from the perspective of solid-state NMR spectroscopy and quantum-chemical calculations. The strength of the combined use of these two methods is demonstrated. Special emphasis is dedicated to the structure and dynamics of hydrogen-bonded complexes of zeolites with H₂O and CH₃OH.     

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.     

Quantitative Determination of Brönsted Acid Sites on Zeolites: A New Approach Towards the Chemical Composition of Zeolites

An original quantitative method based on H/D exchange between H₂O/D₂O molecules and the OH groups of different zeolites has been developed for the titration of the Brönsted acid sites present on the solid surface. The measured Brönsted acid sites density appears to be in good agreement with the theoretical amount estimated by the Si/Al ratio. In contrary to classical methods, this non-destructive anhydride method titrates the whole quantity of Brönsted acid sites of zeolites.     

One‐Pot Hydroxy Group Activation/Carbon‐Carbon Bond Forming Sequence Using a Brønsted Base/Brønsted Acid System

A new sequential two‐step multicatalytic strategy is presented consisting in the efficient DBU‐catalysed trichloroacetimidation of an alcohol followed by a ditriflylamine (Tf₂NH)‐catalysed intermolecular alkylation by silicon‐based nucleophiles and C H nucleophiles. The distinct feature of the trichloroacetimidate group allows use of weaker acid catalysts such as 1,1′‐bi‐2‐naphthol (BINOL)‐derived phosphoric acid, pointing out the possible development of an enantioselective variant. This unprecedented sequential one‐pot Brønsted base‐Brønsted acid catalysis further expands the synthetic scope of the trichloroacetimidate group.     

A quantitative description of the active sites in the dehydrated acid catalyst HSAPO-34 for the conversion of methanol to olefins

A neutron diffraction study of the deuterated form of a microcrystalline specimen of the H⁺ SAPO-34 catalyst for converting methanol to light olefins reveals two distinct bridging hydroxyl Brønsted sites. These sites are also identified by IR spectroscopy which distinguishes their acid strength from the shift in O-H frequencies following adsorption of CO at 77 K. Protons attached to the O(4) bridging oxygen (OH distance 0.95 Å) exhibit greater acidity than those attached to O(2) (OH distance 0.91 Å).     

Quantitative characterization of the solid acidity of montmorillonite using combined FTIR and TPD based on the NH3 adsorption system

The complete parameters of montmorillonite solid acidity, namely amount, strength, and types of acidity, were determined and the properties of the acid sites after heating were proposed by combining the temperature-programmed desorption (TPD) and Fourier transform infrared spectroscopy (FTIR) based on the NH₃ adsorption system. The total amount of montmorillonite acid sites was 1.15 mmol/g, which was higher than the value obtained by the Hammett indicator method because of the detection of solid acid sites in the montmorillonite interlayer space. These acid sites were composed of 1.00 mmol/g Brønsted and 0.15 mmol/g Lewis acid sites. The acidity of montmorillonite was primarily derived from the interlayer polarized water, Si–OH, H₃O⁺ adsorbed by negatively charged tetrahedral AlO₄, and unsaturated Al^3 + ions, all of which were attributed to the Brønsted acid sites with the exception of the unsaturated Al^3 + ions (Lewis acid sites). Heating led to an increase in the acid strength and the acid amount and altered the type of the partial acid sites. The interlayer polarized water provided more protons after heating at 120 °C and exhibited higher acid strength than that of raw montmorillonite. After heating at 400 °C, the interlayer polarized water acted as very strong acid sites. The H₃O⁺ adsorbed by tetrahedral AlO₄ was attributed to weak-strength acid sites and transformed into Si–O(H)–Al after dehydration, while displaying strong-strength acidity. The unsaturated Al^3 + ions showed medium-strength Lewis acidity, although a portion of these ions adsorbed water molecules and exhibited weak Brønsted acidity. After dehydroxylation at 600 °C, an abundance of unsaturated Al^3 + ions appeared and the amount of Lewis acid sites increased.     

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.     

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.     

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.     

Acid centers in sulfated, phosphated and/or aluminated zirconias

On sulfated ZrO₂, the comparison of the effects of adsorbing water or ammonia on the infrared bands between 1400 and 1000 cm^?1 suggests that besides structural Lewis sites on the surface of ZrO₂, strong Lewis sites are made from chemisorbed SO₃. Upon adsorption of water, SO₃ is converted, partially, into a surface sulfated species which may act as strong Brønsted sites. At moderate surface hydration, both types of sites may coexist. The catalytic activity in the isomerization of isobutane is a function of the overall nominal surface density in SO₄. The acid sites on the surface of phosphated mesoporous zirconia are attributable to surface P–OH groups working as weak Brønsted sites. On both sulfated and phosphated zirconia, surface coating of alumina stabilizes the porosity, but it does not modify the nature of their acid centers.