Lewis Surface Acid Research Articles

The ideal treatment for heart failer

The acid-base characteristics of the catalyst material are frequently linked to catalytic activity. Metal-organic frameworks (MOFs) have been tested as catalysts for a variety of processes. Based on the observed performance in diverse catalytic processes, Cu-BTC (with a well-defined crystalline structure) and Fe-BTC (with a gel-like porous structure) have both shown the presence of Lewis surface acid sites. In this study, researchers used Inverse Gas Chromatography (IGC) and n-butylamine thermal desorption analysis to assess the acid-base characteristics of both materials. Using acidic, basic, and amphoteric molecules, IGC experiments can access the acid-base characteristics of the material surface. The n-butylamine thermal desorption measurements, on the other hand, allow quantification of those acid and basic sites from the n-butylamine desorption curves of a sample previously saturated with that probe molecule. The Lewis acid properties obtained by both methodologies showed to be more pronounced for the Fe-BTC MOF sample. The IGC measurements exhibited values of acidity constants (KA) of 0.81 and 0.60 for Fe-BTC and Cu-BTC, respectively. Furthermore the n-butylamine thermo desorption experiments indicate total acidities of 2.06 and 0.49 mmol g−1 for Fe-BTC and Cu-BTC, respectively.

Acid matrix production from treated natural vermiculite-hydrobiotite

Given the potential of acid-activated clays in adsorptive and catalytic processes, several studies focusing on acid matrices have grown over the years, making it possible to study vermiculite, a mineral composed of a type 2:1 unit cell with tetrahedral and octahedral sheets. Therefore, this work aimed to study vermiculite treated with acids for the production of matrices. In this work, the treatment of natural vermiculite-hydrobiotite, with nitric acid in concentrations of 0.25, 0.50, 0.75, and 1 mol.L-1, followed by the dealumination process with sulfuric acid 25% v/v, aiming the production of acid matrices for applications as support for biocatalysts, chemical adsorbents, among others. The produced materials were characterized using different techniques, including X-ray fluorescence spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and isotherm adsorption using the BET method. The results showed that after acid leaching the vermiculite showed a high SiO2 content, high Brønsted and Lewis surface acidity, good adsorption/desorption performance, and significant growth of the specific surface area, qualifying the vermiculite as a promising material for future applications in catalytic and biocatalytic processes. The acid concentration had a considerable influence on the compositional and textural modification of vermiculite-hydrobiotite.

Surface acid-base properties of Cu-BTC and Fe-BTC MOFs. An inverse gas chromatography and n-butylamine thermo desorption study

Catalytic activity is usually related to the acid-base properties of the catalyst material. Metal-organic frameworks (MOFs) have been evaluated as potential catalysts for several reactions. Cu-BTC (with a well-defined crystalline structure) and Fe-BTC (with a gel-like porous structure) have both shown the presence of Lewis surface acid sites based on the observed performance in different catalytic reactions. In this work, experiments of Inverse Gas Chromatography (IGC) and n-butylamine thermo desorption analysis were carried out in order to determine the acid-base properties of both materials. The IGC measurements allow to access the acid-base properties of the material surface by using acidic, basic and amphoteric molecules. Otherwise the n-butylamine thermo desorption measurements allow the quantification of those acid and basic sites from the n-butylamine desorption curves of a sample previously saturated with that probe molecule. The Lewis acid properties obtained by both methodologies showed to be more pronounced for the Fe-BTC MOF sample. The IGC measurements exhibited values of acidity constants (KA) of 0.81 and 0.60 for Fe-BTC and Cu-BTC, respectively. Furthermore the n-butylamine thermo desorption experiments indicate total acidities of 2.06 and 0.49 mmol g−1 for Fe-BTC and Cu-BTC, respectively.

Determination of the Surface Thermodynamic Characteristics of Phthalate Plasticizers by Inverse Gas Chromatography

The dispersive component of surface energy (γsd) and Lewis surface acid–base parameters (Ka, Kb) for phthalate plasticizers, including dimethyl phthalate, di-n-butyl phthalate, di-2-ethylhexyl phth...

Tailored Brønsted and Lewis surface acid sites of the phosphotungstic Wells Dawson heteropoly-acid

The synthesis of transition metals Wells Dawson heteropoly salts along with their elemental composition, water content, structure and specific surface area were thoroughly investigated. Additionally, the use of a molecular probe such as pyridine allowed the quantification of Lewis and Brønsted surface acid sites. The transition metals counter-cations provide both Lewis and Brønsted acid sites. The concentration of Brønsted acid sites is influenced by the crystallization water associated with the heteropoly anion.Additionally, the dispersion of the phosphotungstic Wells Dawson heteropolyacid over transition metal oxides evidenced the presence of both Brønsted and Lewis surface acid sites. The concentration of these sites is related to the surface dispersion of the crystals of the HPA over the oxide supports and the degree of hydration of the HPA.

A new approach for gasoline upgrading: Coupling octane enhancement and desulfurization of heavy straight-run naphtha over Ni/HZSM-5 catalyst

The effect of incorporating nickel in HZSM-5 zeolite on its performance of desulfurization alongside octane enhancement processes of heavy straight-run naphtha was investigated for the first time. Experimental results indicated that the synergy between Brönsted and Lewis surface acid sites of Ni/HZSM-5 at a low SiO2/Al2O3 ratio and consequently of high acidity made a great impact on the coupling aromatization and desulfurization reactions of heavy naphtha as a proper feed. After using a SiO2/Al2O3 ratio of 60, the amount (wt%) of total sulfur decreased by 88%. In addition, the research octane number increased from 52 to 94 at the SiO2/Al2O3 ratio of 40.

Molybdenum-containing systems based on natural kaolinite as catalysts for selective oxidation of aromatic sulfides

A natural kaolinite from Argentinean deposits, converted into metakaolin by thermal treatment at 823K and chemically modified, was used as a support for catalysts containing iso- and heteropolyanions (POMs and HPOMs) such as ammonium heptamolybdate (AHM) and ammonium Anderson phases [XMo6O24H6]3− [X(III)=Al, Co]. The catalytic performances were analyzed in the clean selective oxidation of diphenyl sulfide (DPS) with H2O2. The chemical modification included treatment in acid solution and functionalization with 3-(aminopropyl)trimethoxysilane (F). The characterization was carried out by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), FTIR and Raman spectroscopies. Textural properties were determined by N2 adsorption-desorption isotherms (BET method); surface acid properties (Brønsted and Lewis surface acid sites) were measured by FTIR of adsorbed pyridine.A noticeable increase of surface areas and chemical composition change of the support occurred by acid treatment. Functionalization with amino-siliceous agents yielded an appropriate surface for the adsorption of POMs and HPOMs. The system containing Mo presented conversions of diphenyl sulfide of around 90% and high selectivity to diphenyl sulfone near 95%.

Co-aromatization of olefin and methane over Ag-Ga/ZSM-5 catalyst at low temperature

The massive exploitation of shale gas in the past decade has boosted the production of natural gas and reduced its price dramatically. The methane activation and following conversion into more valuable fuels and chemicals have thus become more and more attractive, while the introduction of hydrocarbons to enhance the methane activation at mild conditions represents a promising approach. In the present work, the co-aromatization of methane with propylene has been studied at 400°C. The presence of methane would increase the toluene to benzene ratio as well as the average carbon number of the formed liquid aromatic products compared to its propylene alone counterpart. Among the gas products, the formations of C3H8, C4H8 and C4H10 also get promoted when methane is present. The incorporation of methane into the product molecules is also directly evidenced by the 1H, 2D and 13C NMR spectroscopy of the liquid products obtained from the reaction between propylene (or styrene) and isotope labelled methane. Hydrogen from methane would contribute a large portion of the hydrogen in the product molecules, while the benzylic and aromatic hydrogen sites are favored compared with those on the alkyl side chains. The activation of methane is also observed in the DRIFT spectra when deuterium enriched methane is engaged as the methane source and evidenced by the escalated exothermic feature when olefin aromatization takes place under methane environment. The excellent catalytic performance of Ag-Ga/ZSM-5 might be because of the better dispersion of Ag and Ga on the ZSM-5 surface and moderate amount of strong Brösted and Lewis surface acid sites. All the observations suggest that methane might be activated nonoxidatively and converted into aromatics if suitable catalyst is charged under the assistance of co-existing olefin. The reported synergetic effect could potentially lead to the more economic utilization of abundant natural gas and petrochemical intermediates.

Tailored visible-light driven anatase TiO2 photocatalysts based on controllable metal ion doping and ordered mesoporous structure

Visible-light driven ordered mesoporous metal ion doped anatase TiO2 photocatalysts were synthesized via a hard-template chemical route. The effects of metal ion doping on the band energy level, surface area, pore volume, pore diameter, and photocatalytic properties of the ordered mesoporous anatase TiO2 were systematically investigated. X-Ray photoelectron spectra and UV-vis absorption spectra indicate that the W and Mn ions enter into the lattice of anatase TiO2 in the presence of W6+ and Mn3+/Mn4+, resulting in a band gap transition with a red shift from the ultraviolet to the visible range, preventing the recombination rate of electron-hole pairs, therefore the photocatalytic activity can be greatly enhanced in the visible region. There exists an optimal doping level of 5 mol% W and 0.5 mol% Mn, respectively, with 95% MB and 82% MB being photocatalytically decomposed within 70 min under the visible light for 5% W and 0.5% Mn doped mesoporous anatase TiO2, respectively. Importantly, W ion doped TiO2 displays much higher photocatalytic efficiency than Mn ion doped TiO2, which can be attributed to the presence of much higher Lewis surface acidity of W6+ doped TiO2 surface with a higher affinity for chemical species having unpaired electrons than Mn doped TiO2. In addition, the W doping induced grain refinement, highly crystalline state, large surface area and large pore size additionally contribute to the improved photocatalytic activity of the mesoporous W-doped TiO2 samples. The enhanced ability to absorb visible light makes the ordered mesoporous metallic ion doped TiO2 an effective photocatalyst for solar-driven applications.

Photocatalytic Behavior of WO3/TiO2in Decomposing Volatile Aldehydes

Removal of volatile organic compounds (VOCs) by photocatalytic treatment has drawn extensive interest as an environmentally-benign technique over the last few decades. Low molecular weight aldehydes are one of the major VOCs contaminating indoor air. They are usually generated from new architectural ornaments, furniture, merchandize, and plastics, as well as from paint, glue, or other chemicals, and known to be carcinogenic, mutagenic or teratogenic. Thus far, photocatalytic decomposition of aldehydes with TiO2 has been reported by several researchers, but the decomposition characteristics of each aldehyde has not been fully investigated. Previously, we reported that the monolayer coverage of WO3 on the surface of TiO2 particle enhances the photocatalytic activity by 3-4 times in decomposing gaseous 2propanol or benzene. It was proposed that the major advantage of the WO3-modified TiO2 originates from much higher adsorption of organic compound on its surface because of the high Lewis surface acidity of WO3 covering TiO2 surface. In the present work, we studied the decomposition trends of the three volatile aldehydes, formaldehyde, acetaldehyde, and propionaldehyde, with the WO3modified TiO2 photocatalyst. Unexpectedly, increase of photocatalytic activity was not observed in decomposing formaldehyde, differently from other aldehydes. We analyzed the surface of formaldehyde-adsorbed photocatalysts to investigate the unusual decomposition behavior of formaldehyde. The obtained result will also provide a new insight to understand the photocatalytic enhancement in the WO3modified TiO2. The commercial TiO2, Degussa P25, with a surface area of 50 m/g, was chosen as a standard. The 3 mol% WO3/97 mol% TiO2 (denoted to WO3/TiO2; the TiO2-based composite whose surface is covered with monolayer of WO3) was prepared by an incipient wetness method. That is, 1.00 g of P25 TiO2 was suspended in 40 mL of 14.0 M aqueous ammonia solution containing 3.87 × 10−4 mol of the tungstic acid (99%, Aldrich), and dried in a water bath at 70 C while stirring. The dried sample was then heat treated at 200 C for 2 hr in a flowing oxygen. The TEM images in Figure 1 show the pure TiO2 (Degussa p25) and WO3/TiO2 nanoparticles. The uniform lattice fringes observed over an entire particle with the size of about 25 nm indicate that the individual TiO2 nanoparticle consists of a single grain. The WO3/TiO2 nanoparticles also showed the lattice fringes similar to those observed in pure TiO2, and no WO3 cluster was found around the TiO2. This suggests that WO3 is uniformly dispersed on the surface of TiO2. The prepared WO3/TiO2 and pure TiO2 samples were used for the photocatalytic decomposition of each aldehyde in gas phase. For the measurements, an aqueous colloidal suspension containing 2.0 mg of WO3/TiO2 or TiO2 was spread on a 2.5 × 2.5 cm Pyrex glass, and subsequently dried at room temperature. Then it was located in the center of a 200 mLsized gas-tight reactor, and the whole area of the sample was irradiated by a 300 W Xe lamp. After evacuation of the reactor, 26 μmol of aldehyde and 170 μmol of water were added to obtain the partial pressures of 2 and 16 Torr, respectively. The total pressure of the reactor was then controlled to 700 Torr by addition of oxygen gas. The gas mixtures in the reactor were magnetically convected during the irradiation. The remained aldehyde and evolved CO2 during the photocatalytic reaction were monitored by a gas chromatography. The detailed description for the measurement of photocatalytic activity is given elsewhere. Figure 2 shows the photocatalytic removal of the three alkylaldehydes with the WO3/TiO2 and pure TiO2 under a UV light irradiation. The WO3/TiO2 was much more effici-

The effect of copper loading on the acidity of Cu/HZSM-5 catalysts: IR of ammonia and methanol for methylamines synthesis

The Cu/HZSM-5 catalysts (1.2, 3.7 and 6.0%, p/p, in Cu) were tested in methanol amination showing that the 3.7% copper sample was fairly more selective to MMA than the other metallic catalysts. The results of textural and superficial analyses obtained in this work showed that the increase in copper content is associated with the procedure adopted in the samples preparation and accounts for the variations in Lewis surface acidity and metallic areas. The infrared spectra of 3.7% Cu/HZSM-5 and HZSM-5 after methanol and ammonia uptake in successive steps at 623 K presented mono-, di- and tri-methylamines that were influenced upon copper addition. The metal should facilitate amine desorption, probably, due to the decreasing Brönsted acidity.

제주 Scoria에 코팅된 WO3/TiO2광촉매를 이용한 Humic Acid의 광분해 특성

This study aimed at improving the <TEX>$TiO_2$</TEX> photocatalytic degradation of HA. In this study, the Degradation of Humic Acid using Jeju Scoria Coated with <TEX>$WO_3/TiO_2$</TEX> in the presence of UV irradiation was investigated as a function of different experimental condition : photocatalyst dosage, <TEX>$Ca^{2+}\;and\;HCO_{3}_{-}$</TEX> addition and pH of the solution. Photodegradation efficiency increased with increasing photocatalyst dosage, the optimum catalyst dosage is 2.5 g/L and Photodegradation efficiency is maximized to <TEX>$WO_3/TiO_2=3/7$</TEX>. This indicates that <TEX>$WO_3$</TEX> retains a much higher Lewis surface acidity than <TEX>$TiO_2,\;and\;WO_3$</TEX> has a higher affinity for chemical species having unpaired electrons. The addtion of cation(<TEX>$Ca^{2+}$</TEX>) in water increased the photodegradaion efficiency. But the addtion of <TEX>$HCO_{3}^{-}$</TEX> ion in water decreased a photodegradation efficiency. Photodegradation efficiency increased with decreasing pH < pzc, the electrostatic repulsion between the HA and the surface of <TEX>$TiO_2$</TEX> decreased.

Infrared spectroscopic study of surface species and of CO adsorption: a probe for the surface characterization of sulfated zirconia catalysts

IR spectra of surface species and of CO adsorption at ∼77 and ∼300 K on pure ZrO 2, sulfated ZrO 2, sulfated/calcined ZrO 2 have been obtained and related to the catalytic activity of these systems in the isomerization of n-butane. Spectral data indicated that: (i) the catalytic activity, when present, requires the presence of both Brønsted and Lewis surface acidity; (ii) a catalyst calcination step is necessary for the elimination of sulfates from crystal defects, on which strong Lewis acid sites must form; (iii) strong Lewis acid sites actually involved in the catalytic process represent a small yet strongest fraction of all Lewis sites formed on the catalyst upon thermal activation.

Physico-chemical properties of WO3/TiO2 systems employed for 4-nitrophenol photodegradation in aqueous medium

A series of polycrystalline WOx/TiO2 samples were prepared by means of a conventional impregnation method. The samples were characterized by means of X-ray diffraction, Vis-UV diffuse reflectance and Raman spectroscopies, nitrogen adsorption at 77 K for determining specific surface areas and surface texture, scanning electron microscopy, and FT-IR monitoring of pyridine adsorption for measuring the surface acidity. Catalytic activity of the samples has been assessed by carrying out as a ``probe'' reaction the photodegradation of 4- nitrophenol in aqueous medium. The results obtained indicate that incorporation of tungsten on titania leads to formation of different surface species, depending on the tungsten loading. Tungsta microcrystals were detected by X-ray diffraction when the nominal molar W/Ti ratio reached a value of 8.0%. FT-IR investigation indicated that the presence of tungsten induces formation of Bronsted and Lewis surface acid sites. The photoactivity results confirm the beneficial effect of tungsten in TiO2 for 4-nitrophenol photodegradation in aqueous medium. The reaction rates are higher than those reported in literature for another set of samples and maximum photoactivity was achieved for a sample containing 1.96 moles of W per 100 moles of Ti.

The roles of Br�nsted and Lewis surface acid sites in acid-treated montmorillonite supported ZnCl2 alkylation catalysts

The catalyst known as “clayzic” (ZnCl2 supported on acid-treated montmorillonite) exhibits both BrOnsted and Lewis surface acidities. The relative catalytic activities of these two types of acid site have been determined in a range of reactions, by comparing the activities of clayzic with those of related catalysts which exhibit only one type of surface acidity. Acid-treated montmorillonite has been used as the BrOnsted acid catalyst in this comparison, and ZnCl2 supported on a mesoporous silica has been used as the Lewis acid catalyst. The results confirm that the relative activities of the two types of acid site depend on both the nature of the reaction and the polarity of the reaction medium. They will be of value in optimising the choice of support and the ZnCl2 loading in clayzic-type catalysts for specific reactions.

FT-IR Assessment Through Pyridine Adsorption of the Surface Acidity of Alkali-Doped MoO3/TiO2

Adsorption of pyridine on MoO3/TiO2 doped with alkali metal cations (Li, Na, K, and Rb) has been studied by FT-IR spectroscopy in order to assess the effect of the nature and concentration of alkali cations on the surface acidity. It has been found that incorporation of the alkali cations leads to a decrease in the surface acidity and of the ratio between the concentration of Brønsted and Lewis surface acid sites. These decreases are due to (i) formation of weak Lewis acid sites (alkali cations) with lower electron affinity than Mo6+ and Ti4+ and (ii) cancellation of Brønsted sites (Mo-OH) because of formation of alkali molybdates. These effects are more pronounced as the alkali content increases and/or the polarizing character of the alkali cation is increased.

Surface characterization of modified aluminas I. The Lewis acidity of Sm-Doped Al 2O 3

The surface behavior of an alumina catalyst (support) containing ≈3 wt% Sm 2O 3 and fired at 773, 1273, and 1473 K has been investigated by IR spectroscopy and compared with the surface behavior of the corresponding undoped aluminas. It was observed that the addition of Sm, which at high temperatures stabilizes the transition (spinel) alumina phases, does not modify appreciably the spectral features of the surface hydrated layer (free surface OH groups), but modifies the Lewis surface acidity of the material. The ambient temperature adsorption of CO, pyridine, and CO 2 was investigated in some detail, and it was revealed that coordinatively unsaturated Sm centers progressively substitute, with increasing firing temperature, (all) the tetrahedrally coordinated Al centers in the surface layer. Spectral data indicate that the Lewis acidity of surface Sm 3+ sites is intermediate between that of tetrahedrally and octahedrally coordinated surface Al sites. Samples activated under nonoxidizing conditions tend to present at the surface (relatively stable) reduced Sm centers.