Generation Of Acid Sites Research Articles

Synthesis of Solid Acid Catalysts of MgO–Al2O3 Mixed Oxide Containing Oxyanions by Thermal Decomposition of Anion-Exchanged Hydrotalcites

Solid acid catalysts of MgO–Al2O3 mixed oxides containing B4O72−, HPO42−, Mo7O246−, MoO42−, WO42−, and SO42− were synthesized by anion exchange with Cl− located in the space between anionic layers of hydrotalcite, followed by heat treatment at 773 K. The distance between the hydroxide layers was expanded by the intercalation of oxyanions larger than Cl−. The exchange of oxyanions in the interlayer space was confirmed by IR spectroscopy. Acid sites were generated on the obtained mixed oxides of MgO–Al2O3 by the electron withdrawing effect of the oxyanions. The acid catalyzed ethanol dehydration into ethylene and diethyl ether took place on the obtained catalysts. The effect of exchanged anions in the generation of acid sites was the largest in SO42−.

Effect of Ti4+ and Sn4+ co-incorporation on the catalytic performance of CeO2-MnOx catalyst for low temperature NH3-SCR

A novel Ti4+ and Sn4+ co-incorporated CeO2-MnOx based catalyst was synthesized by an inverse co-precipitation method with the purpose of inhibiting the formation of N2O and enhancing the NOx conversion of CeO2-MnOx catalyst for NH3-SCR at low temperature. The obtained catalysts were characterized by a series of XRD, Raman, N2-physisorption, NH3-TPD, H2-TPR, XPS, and in situ DRIFTS techniques in details. The experiment results suggested that CeO2-MnOx-TiO2-SnO2 (Ce-Mn-Ti-Sn) catalyst displayed the optimal catalytic performance with an enhancement of 30% compared with CeO2-MnOx (Ce-Mn) in the range of 225–275 °C. Moreover, the N2O formation was inhibited remarkably compared to Ce-Mn (152 ppm), and there was no N2O (0 ppm) detected even at 250 °C. This is linked to moderate weakening of redox property, more generation of acid sites, enhancement of acidic strength, as well as large amount of Ce3+, oxygen vacancies, and absorbed oxygen species, which are contributed to improving the chemical property of Ce-Mn-Ti-Sn catalyst, and further enhancing the catalytic performance and inhibiting the generation of N2O.

Porous clay heterostructures intercalated with multicomponent pillars as catalysts for dehydration of alcohols

Abstract Montmorillonites intercalated with silica, silica-alumina, silica-titania and silica-zirconia pillars by surfactant directed method were studied as catalysts for dehydration of methanol and ethanol to dimethyl ether as well as diethyl ether and ethylene, respectively. Moreover, the samples of montmorillonite intercalated with silica pillars were doped with aluminum using template ion-exchange method. In the case of all the samples deposited metal (Al, Ti, Zr) species were present in highly dispersed forms and resulted in generation of acid sites catalytically active in dehydration of methanol and ethanol. The best catalytic results in both studied processes were obtained for the samples doped with aluminum, especially introduced by using template ion-exchange method (first time applied for deposition of metal cations into PCHs), which were more catalytically active in the process of methanol to dimethyl ether conversion than γ-Al2O3 (one of the most important commercial catalysts of this process). High catalytic activity of the aluminum doped samples was related to the nature and strength of acid sites active in the processes of alcohols dehydration.

Generation of acidic sites in Al, Ga, In salts of molybdenum and tungsten Keggin-type heteropolyacids. DFT modeling and catalytic tests

The catalytic properties, both redox and acidic, of 12-tungsto- and 12-molybdo-phosphoric Al3+, Ga3+, In3+ salts were studied. As a model reaction the gas phase ethanol conversion was used. On different sites (redox and acidic) various products are obtained (acetaldehyde and diethyl ether/ethylene respectively). Catalytic activity of MePW and MePMo salts was theoretically predicted, using both periodic and cluster models. Performed experiments confirm the theoretical prediction concerning the influence of addenda atoms (W and Mo) and various counter-cations (Al3+, Ga3+, In3+) on the catalytic properties of studied systems.

Role of Brønsted acidity in NH3 selective catalytic reduction reaction on Cu/SAPO-34 catalysts

In the present work, a series of Cu/SAPO-34 catalysts with varying number of Brønsted acid sites has been prepared by ion exchanging with potassium to investigate the role of Brønsted acidity in the NH3-SCR reaction. The impact of potassium on the structural and acidic properties of Cu/SAPO-34 catalysts has been studied. No significant changes in both the SAPO-34 structure and the copper species were observed in the presence of potassium. With increasing potassium loading, the NH3-SCR activity of the Cu-K/SAPO-34 catalysts decreased in accordance with the decreasing Brønsted acidity. The quantification method of Brønsted acid sites in the Cu-(K)/SAPO-34 was developed by using IR spectroscopy, which was confirmed by acid site generation mechanism and NH3-TPD results. A linear relationship was found between the amounts of Brønsted OH groups determined by DRIFTS and the NH3 desorbed at high temperatures from NH3-TPD.

TiO2 nanoparticles doped SiO2 films with ordered mesopore channels: a catalytic nanoreactor

Titanium dioxide (TiO2) incorporated ordered 2D hexagonal mesoporous silica (SiO2) films on a glass substrate were fabricated for use as a catalytic nanoreactor. Films were prepared using a tetraethyl orthosilicate (TEOS) derived SiO2 sol and a commercially available dispersion of TiO2 nanoparticles (NPs) in the presence of pluronic P123 as the structure directing agent. The effect of TiO2 doping (4-10 mol% with respect to the equivalent SiO2) into the ordered mesoporous SiO2 matrix was thoroughly investigated. The undoped SiO2 film showed a mesostructural transformation after heat-treatment at 350 °C whereas incorporation of TiO2 restricted such a transformation. Among all the TiO2 incorporated films, TEM showed that the 7 equivalent mol% TiO2 doped SiO2 film (ST-7) had an optimal composition which could retain the more organized 2D hexagonal (space group p6mm)-like mesostructures after heat-treatment. The catalytic activities of the TiO2 doped (4-10 mol%) films were investigated for the reduction of toxic KMnO4 in an aqueous medium. ST-7 film showed the maximum catalytic activity, as well as reusability. A TEM study on the resultant solution after KMnO4 reduction revealed the formation of MnO2 nanowires. It was understood that the embedded TiO2 NPs bonded SiO2 matrix increased the surface hydroxyl groups of the composite films resulting in the generation of acidic sites. The catalytic process can be explained by this enhanced surface acidity. The mesoporous channel of the ST-7 films with TiO2 doping can be used as a nanoreactor to form extremely thin MnO2 nanowires.

Glycerol oxidehydration into acrolein and acrylic acid over W–V–Nb–O bronzes with hexagonal structure

This paper deals with an investigation of hexagonal W–Nb–O and W–V–Nb–O bronzes as catalysts for the one-pot oxidehydration of glycerol into acrylic acid. In a previous work, we reported that the best yield to acrylic acid obtained with the W–V–O bronze was 25%; in the current work, the incorporation of Nb in the tri-component bronze structure allowed us to obtain the best acrylic acid yield of 34%. The W–Nb–O bronze was an efficient acid catalyst for the dehydration of glycerol into acrolein – more selective than WO3 at temperatures lower than 300°C –, whereas in the tri-component system the presence of V conferred to the catalyst the redox properties for the partial oxidation of acrolein into acrylic acid. The characterization of the catalysts confirmed the incorporation of Nb5+ in the hexagonal structure, and the generation of acid sites with enhanced strength compared to the bi-component W–V–O system. This also enhanced the stability of catalytic performance during short-term lifetime experiments.

Conversion of Methane into C1 Oxygenates by Deep‐UV Photolysis on Solid Surfaces: Influence of the Nature of the Solid and Optimization of Photolysis Conditions

Deep-UV photolysis (either 165 or 185 nm) of surface hydroxy groups leads to homolytic O-H bond-cleavage with the generation of oxyl radicals that can initiate the room-temperature radical-chain methane activation. Whilst in the absence of oxygen, radical coupling reactions to give low-molecular-weight alkanes are observed in the gas phase, the presence of some oxygen quenches these radicals and increases the selectivity towards C1 oxygenates (methanol, formaldehyde, and formic acid species). The nature of the solid influences the efficiency of the photochemical process and the distribution between products in the gas and solid phases. Using Beta-, delaminated ITQ2 and ITQ6, and medium-pore ZSM5 zeolites, mesoporous MCM41 silicates, and non-porous TiO(2), we observed that confinement and porosity increased the proportion of C1 oxygenates adsorbed onto the solid and reduced the contribution of the gas-phase products. In addition, the presence of aluminum in the zeolite framework, which is responsible for the generation of acid sites, increased overoxidation of methanol and methoxy groups into formaldehyde and formic acids. For a given amount of methane and unchanged photolysis conditions, the conversion increased with the amount of the solid used as photocatalyst. In this way, methane conversions of up to 7% were achieved for the 185 nm photolysis of methane for 1 h with a 76 MJ mol(-1) energy consumption.

Highly Active S-Modified ZnFe2O4 Heterogeneous Catalyst and Its Photo-Fenton Behavior under UV–Visible Irradiation

Ultrafine Zn–Fe composite oxides have been synthesized by the sol–gel method and then modified by S═O. After preparation, these chalybeate compounds were characterized by BET surface area and porosity analyzer, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometry (FT-IR). The influence of introduced S═O on the performance of ZnFe2O4 catalysts was measured, and the reaction mechanism was investigated. To evaluate the activities of different catalysts, a series of photo-Fenton reactions have been conducted to decontaminate the Reactive Black KN-GR simulated wastewater. Results indicated that the prepared catalysts were porous-material with a high specific surface area of 38 m2 g–1. Significantly, the FT-IR spectrum suggested that sulfated ZnFe2O4 calcinated at 773 K (named S773 catalyst) exhibited a strong acid property and XPS analysis proved the presence of S6+(S═O), which led to the generation of acid sites. Under optimum conditions of 9.8 mM H2O2, 0.5 g L–1 catalyst, and pH = 6.0, up to 90% total organic carbon removal and 100% decolorization of a solution containing 100 mg L–1 Reactive Black KN-GR could be achieved after 150 min of treatment. The kinetics of acid catalysis were also discussed, and the stability of the catalyst was estimated on studying the iron leaching and S773 catalyst recycling.

ChemInform Abstract: Solid Acid Catalysts: Roles in Chemical Industries and New Concepts

AbstractReview: 17 refs.

Ammonia IRMS-TPD measurements on Brønsted acidity of proton-formed SAPO-34

By utilizing the advantages of a combined method of IRMS-TPD of ammonia and DFT calculations, the solid acidity of HSAPO-34 was studied. The number, strength and structure of the Brønsted OH were measured experimentally. The quantitative measurements and DFT calculations supported the identification of Brønsted OH to account for the generation model of the Brønsted OH primarily located in the edge of the Si domain (island). The acid strength of SAPO-34 was slightly weaker than that of chabazite, a zeolite with the same structure. Thus, some important insights were obtained to understand the acid site generation of SAPO-34.

Heptane isomerization over molybdenum oxides obtained by H 2 reduction of H xMoO 3 with different hydrogen contents

The effects of the hydrogen content in hydrogen molybdenum bronze (H x MoO 3) on the catalytic properties of its reduction product were studied. H 2 reduction of H x MoO 3 yielded an active and selective catalyst for heptane isomerization. The catalyst with a higher isomerization activity was obtained from H x MoO 3 with a larger content of hydrogen. H 2-reduced MoO 3 was almost inactive for heptane isomerization, and the highest activity was obtained on H 2-reduced Pt/MoO 3. The isomerization activity was well related to the number of acid sites, which was determined from temperature-programmed desorption of NH 3. H 2-reduced H x MoO 3 and Pt/MoO 3 consisted of MoO x H y and Mo metal. In contrast, reduction of MoO 3 yielded a mixture of MoO 2 and Mo metal. We suggest from these results that the generation of acid sites is related to the formation of MoO x H y , and that the acidity of MoO x H y can be enlarged as the hydrogen content in H x MoO 3 is increased.

ChemInform Abstract: Skeletal Isomerization of Hydrocarbons Over Zirconium Oxide Promoted by Platinum and Sulfate Ion.

Abstract The genesis of the high activity of zirconium oxide promoted by platinum and sulfate ion (Pt/SO 4 2− --ZrO 2 ) for skeletal isomerization of butane and pentane in the presence of hydrogen is studied in terms of the interaction of the catalyst with molecular hydrogen. For skeletal isomerization of pentane at 523 K, Pt/SO 4 2− -ZrO 2 showed activity only in the presence of molecular hydrogen, and its activity persisted for a long period. For skeletal isomerization of butane at 523 K, the catalyst showed activity in the absence of hydrogen, but the activity was markedly enhanced in the presence of hydrogen. For pentane and butane skeletal isomerization, the products consisted exclusively of 2-methylbutane and 2-methylpropane, respectively. For a typical acid-catalyzed reaction of cyclopropane ring opening at 373 K, the presence of hydrogen enhanced the activity, but the hydrogen enhancement effect was small. The products consisted exclusively of propene even in the presence of hydrogen: hydrogenation of propene scarcely occurred. Infrared spectroscopic study of adsorbed pyridine showed that by heating the catalyst in the presence of hydrogen in the temperature range 423–623 K, protonic acid sites were formed with concomitant decrease in the number and strength of Lewis acid sites, demonstrating that the protonic acid sites originate from molecular hydrogen. The mechanisms of protonic acid site generation are discussed. It is suggested that molecular hydrogen dissociates on the platinum to hydrogen atoms which undergo spillover on the SO 4 2− -ZrO 2 and convert to an H+ and an e − or H − . The H+ acts as catalytic site for acid-catalyzed reactions.

Highly Stable Platinum Catalysts in Propane Combustion Prepared by Supporting Platinum on Zirconia-Incorporated Silica

Highly active and stable platinum catalysts for propane combustion were prepared by impregnating platinum on zirconia-incorporated silica by combining the high surface area of silica with the exceptional thermal stability of zirconia. The thermal stability of the platinum catalysts was examined after thermal aging at 800 °C for 50 h. The physico-chemical states of the platinum and zirconia were investigated by using various analytical instruments, CO and H2 chemisorption, and temperature-programmed-desorption of ammonia. The platinum catalysts prepared on the zirconia-incorporated silica and treated with hydrogen peroxide showed high catalytic activity and stability in propane combustion. The high dispersion of platinum and the acid site generation achieved due to the incorporation of zirconia on the silica were responsible for the high activity and stability of the catalysts. Highly active and stable platinum catalysts for propane combustion were prepared by impregnating platinum on zirconia-incorporated silica by combining the high surface area of silica with the exceptional thermal stability of zirconia. The platinum catalysts supported on zirconia-incorporated silica maintained their activity after thermal aging at 800 °C for 50 h.]

Solid Acid Catalysts: Roles in Chemical Industries and New Concepts

Extensive studies of the cracking catalyst used in the largest process in chemical industries established the research field of solid acid catalysis. Solid acid catalysts are also utilized in various industrial processes. A new concept for acid site generation, Molecular hydrogen-originated protonic acid site, and its characteristics in alkane isomerization are described.

Influence of the synthesis method on the nanostructure and reactivity of mesoporous Pt/Mn-WO x-ZrO 2 catalysts

The influence of the synthesis method on acid site generation, nanostructure and reactivity of Mn-promoted WO x -ZrO 2 catalysts was investigated. The materials were synthesized by impregnation (IMP), coprecipitation (COP) and reflux (COP-R and IMP-R), using cetyl-trimethylammonium bromide (CTAB) as surfactant in order to promote the textural properties of the catalysts. The Mn-WO x -ZrO 2 samples were calcined at 800 °C and characterized by X-ray diffraction (XRD), nitrogen adsorption–desorption, laser Raman spectroscopy, ultraviolet–visible (UV–vis) spectroscopy, high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). The Mn-WO x -ZrO 2 materials were impregnated with 0.3 wt% of platinum (Pt/Mn-WO x -ZrO 2) in order to enhance H 2 spillover and diminish coke formation and they were evaluated for n-hexane hydroisomerization. The XRD results show that the incorporation of 0.5 wt% Mn cation stabilizes the metastable tetragonal zirconia phase and the Rietveld refinement of the crystalline phases shows that only a fraction of tungsten (about 5 wt%) leads to the formation of the WO 3 segregated phase. Laser Raman and UV–vis spectroscopies verify the presence of different tungsten oxide domains, WO 3 crystallites and a WO x phase not detected by XRD formed by oligomeric and polymeric WO x clusters. The surfactant assisted methods produce mesoporous materials with narrow pore size distributions, allowing access to the WO x clusters of suitable domain size for the hydroisomerization reaction. The simultaneous precipitation of tungsten with the formation of hydrous zirconia and manganese generates a larger number of active sites by the formation of polytungstate species on the zirconia surface with sizes and structures suitable for the isomerization of alkenes. Additionally, the coprecipitation-reflux method (COP-R) results in the formation of WO x species with W O bonds which generates acid sites suitable and accessible to the n-hexane molecule, leading to an increase in the conversion (70%) and selectivity to the high octane biramified products 2,3-dimethyl-butane (2,3-DMB) and 2,2-dimethyl-butane (2,2-DMB). It was found that besides the role of Mn in the stabilization of the tetragonal phase, this cation promotes the catalytic activity of these materials.

Preparation and characterization of transition metal-modified lacunary Keggin 11-tungstophosphates supported on carbon

Transition metal-modified lacunary Keggin heteropolycompounds [PW 11O 39 M] 5−, with M = Ni 2+, Co 2+, Cu 2+, or Zn 2+, were synthesized and characterized. These heteropolycompounds were used to prepare the carbon-supported catalysts. Both the heteropolycompounds and the carbon-supported catalysts showed FT-IR spectra with the characteristic bands of these compounds and provided an indirect measure of the interaction strength between M and the oxygen of the central tetrahedron group of the Keggin structure. The anion decomposition of the modified compounds takes place at equal or lower temperature than that of the bulk and supported parent lacunary [PW 11O 39] 7− anion. The species present on the support surface may be highly dispersed as a non-crystalline form, as a result of the interaction with the support surface groups. The activity of the synthesized catalysts in isopropanol dehydration, reflecting their acidity, increases as the reduction temperature of the decomposition products of the heteropolycompounds decreases. It may be assumed that the generation of acid sites is a result of the interaction of hydrogen, donated by isopropanol during its decomposition, with the M cation, leading to M reduction and protons.

IR spectroscopic investigation of hydrodechlorination on Pt-containing zeolites

Two PtY-FAU zeolite samples, prepared by impregnation and ion exchange methods, and a Pt ion exchanged ZSM-5 zeolite sample reduced with two methods were studied in hydrodechlorination reactions. The generation of acid sites was investigated by IR spectroscopy after pyridine adsorption. Hydrodechlorination of CCl4 resulted CH4 and CHCl3 as main products besides HCl. With increasing temperature the hydrodechlorination of CHCl3 also took place. For CHClF2 no hydrodechlorination was observed at under identical experimental conditions. For trichloroethene (TCE) and tetrachloroethane (TCA) the product distribution corresponded to a hydrodechlorination reaction giving ethane and HCl as the main products. In spite of the hydrogen atmosphere under hydrodechlorination conditions side reactions occurred on Pt-containing zeolites with high Brønsted acidity, small amount of oxygen-containing product, phosgene was formed with the participation of the framework oxygen of the zeolite.

Synthesis of AFI aluminophosphate molecular sieves partly substituted with magnesium by dry-gel conversion method

Aluminophosphate molecular sieves with AFI topology, in which framework Al 3+ is partly substituted with Mg 2+ (MgAPO-5), were synthesized by dry-gel conversion (DGC, including vapor-phase transport [VPT] and steam-assisted conversion [SAC]) method using triethylamine (Et 3N) as a structure-directing agent (SDA). Systematic studies were carried out in DGC method using different compositions of gel (concentration of Mg and SDA), amount of external bulk water and period of synthesis. MgO/Al 2O 3 ratios in the range of 0.025<MgO/Al 2O 3<0.10 gave pure MgAPO-5, and further increase in Mg-content caused contamination of MgAPO-34 with CHA topology. The amount of SDA played an important role, synthesized with a SDA/Al 2O 3 ratio of 0.38–0.76. Very minute amount of external bulk water was necessary for the crystallization. Appropriate crystallization time was 12–18 h. Incorporation of Mg 2+ into the framework was suggested by generation of acid sites and the change of unit cell parameters.

IR spectroscopic reinvestigation of the generation of acid sites in Pt-containing faujasite zeolites

Two Pt/NaY zeolite samples were tested by a combination of instrumental analytical methods such as IR spectroscopy, XRD and TEM. The generation of acid sites and Pt nanoparticles was investigated upon various reductive treatments. The impregnation and the ion-exchange techniques result in different materials with respect to their acidities, particle sizes of platinum nanoclusters and the crystallinity of the zeolite matrices.