Acido-basic Properties Research Articles

Synthesis and FT–IR study of the acido–basic properties of the V2O5 catalysts supported on zirconia

In this paper we describe the synthesis and characterization of the acido–basic properties of catalysts containing varied amounts of vanadium supported on ZrO2. The preparation of the zirconia was carried out using a hydrolysis method and the vanadium was introduced by impregnation with a porous volume in several stages, followed by calcinations under air at a temperature of 723K. The obtained samples are characterized by adsorption–desorption of nitrogen and infrared spectral analysis of different species formed by acidic and basic probes. This adsorption on the surface of these compounds has been studied in order, in the hand to investigate information on their surface acidity and in the other hand to know particularly the nature and strength of acidic and basic sites. Among the molecular probes, we used carbon monoxide, carbon dioxide, pyridine and 2,6-dimethylpyridine. The adsorption of CO has shown that contrary to pure zirconia and oxidized V2O5/ ZrO2, the reduced V2O5/ ZrO2 samples favour the formation of CO co-ordinated on Lewis acidic sites of reduced V2O5 species (CO on V4+ or V3+). We also observe the creation of Brønsted acidic sites by means of the incorporation of vanadium.

Effet des cations extra-réseau de la zéolithe Y sur l’activité catalytique du palladium dans la réaction de combustion du méthane

Alkali and alkaline earth metal cations were introduced into zeolite to explore their influence on the activity of supported palladium in the total combustion of methane and to study how the acido-basic properties of the support affected the structure and the oxidation state of Pd. The results show that all samples exhibit a slight activation with time. Moreover, at temperatures below 430 °C, PdLiNaY seems to be the most active catalyst, while PdHY is the most active sample at higher temperatures. Except for PdHY, all Pd catalysts lead to an activation energy (Ea) value in the range of 79–97 kJ mol–1.

Vapor phase alkylation of indole with methanol and dimethylcarbonate over Ni–Mn based ferrospinels

Abstract Vapor-phase methylation of indole with methanol and dimethylcarbonate (DMC) was carried out over Ni 1− x Mn x Fe 2 O 4 ( x = 0, 0.25, 0.5, 0.75, 1.0) type ferrospinel in a fixed-bed reactor. 3-Methylindole was found to be the major product. It was observed that systems possessing higher ‘ x ’ values were highly selective for 3-methylindole. A maximum yield of 54.6% with selectivity 88.3% of 3-methylindole was obtained when methanol was used as alkylating agent, while yield of 48.6% with selectivity 80.1% was obtained using DMC as alkylating agent over MnFe 2 O 4 under optimized conditions. Ni 2+ /Mn 2+ ionic distribution in the spinel lattice influences the acido-basic properties. Effect of alkylating agent and different reaction parameters are discussed.

N-doped carbon nanotubes for liquid-phase C [dbnd]C bond hydrogenation

The introduction of foreign elements inside the channel of carbon nanotubes could lead to a significant modification of the intrinsic properties of these nanomaterials. Nitrogen atoms entering in the graphene sheets as substitute of carbon could modify in a large extend the acido-basic properties and also adsorption of the nanotube itself. Depending on the synthesis conditions, i.e. nature of the N-source, temperature and C-to-N atomic ratio, various N-doped carbon nanotubes can be synthesized with different surface properties. The aim of the present work is to report the synthesis of N-doped CNTs using a common nitrogen source precursor namely ammonia (NH 3) with C 2H 6 as carbon source. The as-synthesized N-CNTs were subsequently employed as catalyst support in the liquid-phase hydrogenation of cinnamaldehyde using palladium as an active phase.

FT-IR Evidence of Two Distinct Protonic Sites in BEA Zeolite: Consequences on Cationic Exchange and on Acido-Basic Properties in the Presence of Cesium

Two acidic H−BEA zeolites having similar Al content but different structural and textural properties were used to prepare two series of BEA samples containing increasing Cs amounts, added by exchange in solution and then impregnation. Whatever the zeolite, the introduction of Cs up to a Cs/Al ratio close to 1 leads to solely exchanged Cs+ ions, indicating that almost all Al are framework atoms giving cationic exchange capacity. At higher Cs contents, Cs-overloading takes place, giving Cs-oxide-like species with strong basicity on which carbonates are formed upon adsorption of CO2, as followed by Fourier transform infrared (FT-IR) spectroscopy. In spite of such comparable behaviors, the two parent BEAs act in a strongly different manner toward cationic exchange in solution. This is related to the presence, in the most defective sample, of a distinct type of protons able to protonate NH3 but significantly less acidic than protons on regular tetrahedrally coordinated framework sites, as is shown by FT-IR spe...

Redox and selective oxidation properties of Mn complexes grafted on SBA-15

Manganese complexes of 5,10,15,20-tetraphenylporphyrin (Mn(TPP)Cl) were grafted on SBA-15 functionalized with propyl-amine, -thiol and -sulfonic acid groups. The acido-basic properties of the support influenced the oxidation state, redox behavior and selective oxidation properties of the Mn complexes. Acidic supports (SBA-15- pr-SH and SBA-15- pr-SO 3H) promoted the reduction of Mn ions in Mn-TPP complexes from 3+ to 2+ oxidation state. A similar behavior was found also when Mn(Salen)Cl complexes were immobilized on those supports. However, the extent of such facile reduction was more in the case of Mn-TPP than in Mn-Salen complexes. The former exhibited superior catalytic activity (turnover frequency) than the latter-type Mn complexes in regio-, stereo- and chemoselective oxidation of R-(+)-limonene with molecular oxygen (1 atm) at ambient temperatures. The catalytic oxidation activity correlates with the redox behavior of the grafted Mn complexes.

Ac-Electrogravimetry study of ionic and solvent motion in polypyrrole films doped with an heteropolyanion, SiMo 12O 404−

One way to immobilize heteropolyanions on an electrode is to entrap them into a film of conducting polymer. In the present work, a polypyrrole film doped with a Keggin-type heteropolyanion, SiMo 12O 40 4−, obtained by electrochemical way was studied. We focus on the charge compensation process, which occurs when the heteropolyanions entrapped into the polymer are reduced or re-oxidized. First, electrochemical quartz crystal microbalance analyses were performed, then, to determine if solvent motion takes place, ac-electrogravimetry measurements were carried out. In fact, in concentrated acidic solution, only protons are involved in the charge compensation. In low acidic solution (pH 3) having a high potassium concentration (0.5 M) both proton and potassium ions are involved whereas at pH 5.2 only potassium motion takes place. These findings are in good agreement with the acido-basic properties of SiMo 12O 40 6−. In all cases, no solvent motion was pointed out.

Lanthanum oxides for the selective synthesis of phytosterol esters: Correlation between catalytic and acid–base properties

The selective synthesis of phytosterol esters from natural sterols and methyl esters was investigated on new grounds by a chemical process using basic solids that are reusable, active, and more selective than the currently used homogeneous catalysts (e.g., alkali hydroxides, carbonates). Various lanthanum oxides with different synthesis procedures and varying specific surface areas were tested. The catalytic performance of the new solids and, more specifically, the ability of La 2O 3 oxides in increasing phytosterol ester selectivity, was correlated with their acido-basic properties, which were fully characterized by infrared spectroscopy. All La 2O 3 oxides involved surface carbonate species with different structural states and strengths, as revealed by propyne adsorption. The selectivity to phytosterol ester varied from 90 to 96%, confirming that the side reaction of dehydration of β-sitosterol was strongly inhibited. The phytosterol ester yield could reach 89% and was related to the basic strength of the carbonate species. The lower the carbonate basicity, the higher the phytosterol ester yield. Moreover, IR spectra of catalysts diluted in KBr powder showed that the higher the intensity of unidentate carbonate bands, the higher the yield. It was concluded that the synthesis of phytosterol esters from fatty methyl esters and sitosterols in the presence of La 2O 3 catalysts requires ionic carbonate species of medium basic strength.

Development and validation of HPLC method for determination of clotrimazole and its two degradation products in spray formulation

A novel simple isocratic HPLC method with UV detection for the determination of three compounds in spray solution (active component clotrimazole and two degradation products imidazole and (2-chlorophenyl)diphenylmethanol) using ibuprofen as an internal standard was developed and validated. The complications with different acido-basic properties of the analysed compounds in HPLC separation – while clotrimazole has p K a 4.7, imidazole has p K a 6.9 compared to relatively more acidic (2-chlorophenyl)diphenylmethanol – were finally overcome using a 3.5 μm Zorbax ® SB-Phenyl column (75 mm × 4.6 mm i.d., Agilent Technologies). The optimal mobile phase for separation of clotrimazole, degradation products imidazole and (2-chlorophenyl)diphenylmethanol and ibuprofen as internal standard consists of a mixture of acetonitrile and water (65:35, v/v) with pH* conditioned by phosphoric acid to 3.5. At a flow rate of 0.5 ml min −1 and detection at 210 nm, the total time of analysis was less than 6 min. The method was applied for routine analysis (batch analysis and stability tests) in commercial spray solution.

Effect of additives on properties of V 2O 5/SiO 2 and V 2O 5/MgO catalysts : II. Structure and physicochemical properties of the catalysts and their correlations with oxidative dehydrogenation of propane and ethane

Effect of additives (A) of main group elements (P and K) and of transition metal ions (Ni, Cr, Nb and Mo) introduced to VO x /SiO 2 (VSiA) and VO x /MgO (VMgA) catalysts on their structure and physicochemical properties, has been studied with the use of XRD, XPS, Raman spectroscopy, H 2-TPR and TPD of adsorbed propane. Acido-basic properties were determined from IR spectra of adsorbed pyridine and from isopropanol decomposition. The results of the characterization have been correlated with catalytic data for oxidative dehydrogenation (ODH) of propane and ethane reported in Part I. The additives do not affect markedly the catalysts’ structure, but modify their acido-basic properties and reducibility. For both VSi and VMg catalysts number of acidic sites increases with the electronegativity of the additive. For VSiA catalysts, in which V 2O 5 is the main phase and acidic properties dominate over dehydrogenating, the selectivity to propene increases with the decrease in acidity and with the increase in the M O bond energy. For VMgA catalysts, containing magnesium orthovanadate, in which dehydrogenating properties dominate over acidic ones, the selectivity to propene does not depend clearly on the acido-basic properties, but appears to be influenced by the properties of the catalysts’ oxygen. Selectivity to ethene in ODH of ethane does not depend on acidic properties for both series of the catalysts.

1H MAS NMR study of the coordination of hydroxyl groups generated upon adsorption of H 2O and CD 3OH on clean MgO surfaces

The coordination of OH groups on MgO has been characterized by 1H MAS NMR after appropriate pre-treatment conditions to obtain surfaces free from carbonates and with a controlled degree of hydroxylation. On MgO-sol–gel sample treated at 1023 K under flowing nitrogen, hydroxylated with water and evacuated at 673 K for 2 h, at least six lines have been identified at 1.2, 0.7, 0.0, −0.4, −0.7 and −1.8 ppm. Upon dehydroxylation at higher temperature, the signals become sharper and a relative decrease of intensity of the lines at δ ≥ −0.4 and at −1.8 ppm is observed. After adsorption of CD 3OH and further desorption at 473 K, an OH signal with two main lines at 0.4 and 0.0 ppm and a broad contribution around 1.0 ppm are observed. Thus deuterated methanol dissociation generates only OH groups at high chemical shift. These observations are related to the coordination of the oxygen atom of the OH groups. It is thus suggested that, when water dissociates on the surface, the OH groups coming from protonation of O 2− ions and being the more coordinated, give a signal at a higher chemical shift than the OH groups formed by coordination of a hydroxide ion to a Mg 2+. The link with the acido-basic properties is then discussed on the basis of literature data.

Hydrodynamic flow and electroosmotic flow in zirconia‐packed capillaries

Fused-silica capillaries were packed with Zirchrom-PBD stationary phase for application in CEC, nanoLC and pseudoelectrochromatography (PEC). Acido-basic properties of zirconia can be used to control the EOF even if the zirconia particles were coated by polybutadiene. As for native zirconia, the EOF is pH-dependent and the pI is close to pH 5. The mixed-mode pressure-voltage technique induced a modulation of the mobile-phase velocity as well as an electrophoretic migration of the solutes in order to improve the resolution of the separation. A significant increase of the flow appeared when both hydrodynamic and EOFs were in the same direction. But an important reduction of the electroosmotic velocity was observed when the hydrodynamic flow and EOF were opposed in Zirchrom-PBD columns. This behaviour has been observed at high or low pH on several columns. Separations of neutral and charged compounds have been performed with these columns in PEC mode.

Redox properties of alkaline and acidic L zeolites as studied by EPR spectroscopy using TCNE and perylene as probe molecules

The oxidizing and reducing character of various exchanged L zeolites has been studied by EPR spectroscopy via the formation of perylene and tetracyanoethylene ion radicals, respectively, and correlated with the acido-basic properties of zeolites. In the series of alkaline L forms the reducing character increases from LiL to CsL in parallel with the calculated framework average negative charge on oxygen atoms (oxygen basicity). An opposite variation is obtained for the oxidizing properties. The exchange of K + ions by H +, enhancing the acidity of zeolites, decreases the reducing and increases the oxidizing properties. The present work reveals the links which exist between the acidity of zeolites with oxidizing centers on one hand and the basicity with reducing character on the other hand. This observed parallelism between the redox properties and the acido-basicity of zeolites could be of great importance to explain reliably the catalytic behaviour of L zeolites as support. The first attempt has been made concerning the correlations between the changes in the redox properties and the selectivity for the aromatization of n-hexane on Pt alkaline L zeolite and the sulphur resistance of Pt/acidic or basic zeolites.

Interaction of Porphyrins with a Dendrimer Template: Self-Aggregation Controlled by pH

The interaction between self-aggregated porphyrins such as 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and 5,10,15,20-tetrakis(4-phosphonatophenyl)porphyrin (TPPP), and a generation 5 (G5) PAMAM dendrimer template is governed by minute differences of porphyrin acido-basic properties. While at neutral pH both monomeric TPPS and TPPP form complexes with G5, decreasing pH did not lead to porphyrin ring protonation (pK(a) approximately 5) but rather to the preferential formation of H-aggregates (probably H-dimers), most likely due to protonation of the G5. Upon further acidification of the solution, this face-to-face orientation of the porphyrin units is being converted to edge-to-edge aligned J-aggregates with a tightly defined structure. This process starts by protonation of the porphyrin ring at pH below 2.3 and 2.8 for TPPS and TPPP, respectively. The AFM imaging of porphyrin/G5 nanostructures obtained at pH 0.7 shows the formation of long nanorods of TPPS with partially aggregated G5 and small aggregates of TPPP connected to individual G5 molecules.

Acido-Basic Properties of Selective Oxidation Catalysts — An Overview

For Abstract see ChemInform Abstract in Full Text.

Sorption of selenium anionic species on apatites and iron oxides from aqueous solutions

The sorption of selenite and selenate ions from aqueous solutions was investigated on hydroxyapatite, fluorapatite, goethite and hematite, in order to simulate the behavior of radioactive selenium in natural or artificial sorbing media. Correlation studies with acido-basic properties and solubility of the solids were also performed. The sorption is pH dependant, but these solids are very efficient for retaining selenite at pH values generally encountered in natural waters, with however higher K d values for oxides than apatites. Selenate ions are much less sorbed than selenite. Several methods such as electron microscopy and spectroscopic techniques were used to identify the sorption mechanisms. In the case of hydroxyapatite, sorption proceeds by substitution of phosphate groups in the lattice of the apatite crystal in the superficial layers of the solid. In the case of goethite and hematite, sorption can be interpreted and modeled by a surface complexation process, but there is a discrepancy between sorption site densities for selenite and for protons.

Direct catalytic conversion of chloromethane to higher hydrocarbons over a series of ZSM-5 zeolites exchanged with alkali cations

The direct catalytic conversion of chloromethane to higher hydrocarbons at various temperatures over a series of zeolites ZSM-5 exchanged with alkali cations has been studied by means of in situ FTIR and catalytic testing. Hydrocarbons such as aromatics, alkanes and alkenes are formed. A correlation between the acido-basic properties of ZSM-5 zeolites exchanged with alkali cations (Li, Na, K and Cs) and their activity in the direct catalytic conversion of chloromethane has been established. The reaction mechanism has been discussed. The IR observations indicate a production of HCl by elimination from CH3Cl during the first step of the conversion. A co-operative role of counter ions and the framework oxygen atoms is responsible for the conversion of CH3Cl in cationic zeolites. Coke formation during the reaction has been investigated by thermal analysis. Significant differences in activity and selectivity can be observed between Li- and NaZSM-5 on one side and K- and CsZSM-5 on the other side. The conversion of chloromethane is quite higher on Li- and NaZSM-5. On the Li and Na forms, the selectivities in C5–C6 are higher. While on K- and CsZSM-5 zeolites the C4 unsaturated products are favoured.

Preparation of Zr-Containing Layered Double Hydroxides and Characterization of the Acido-Basic Properties of Their Mixed Oxides

Multicomponent M(II)/Al/Zr layered double hydroxides (LDHs) with M(II) = Mg, Ni, and Zn, exhibiting single hydrotalcite-like phases by XRD, were obtained by coprecipitation at constant pH = 10. The...

Acidobasic equilibria and spectral characteristics of acetyl and tosyl derivatives of 4,4′-diaminostilbene-2,2′-disulfonic acid

Acetyl and tosyl derivatives of 4,4′-diaminostilbene-2,2′-disulfonic acid and their N, N′-dimethyl analogues were prepared. The course of syntheses was checked by HPLC and products were identified with NMR spectroscopy. Chemical stability was tested in phosphate buffers. Acidobasic properties of prepared substances were studied with potentiometric titration and also with absorption and fluorescence spectrophotometry. The fluorescence quantum yields of pure acidobasic forms were measured. The tosylamino derivative was found to behave as N-acid and in aqueous alkali medium exhibits a strong fluorescence. The p K A values were estimated.

Effect of doping of TiO 2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia–titania catalysts

Vanadia phase (one monolayer) was deposited on TiO 2 anatase doped with Ca 2+, Al 3+, Fe 3+ and W 6+ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO 2 support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO 2. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi>VWTi>VTi>VAlTi>VCaTi (3), whereas the selectivity to propene follows the sequence: VWTi<VTi<VFeTi<VAlTi<VCaTi (4). No clear correlation has been observed between the activity of the catalysts and the acido–basic properties. On the other hand, the rate of the isopropanol dehydration (the acidity) decreases in the same order as the selectivity increases (4). The increase in the selectivity with the decrease in the acidity is ascribed to the easier desorption of propene from the less acidic surface, preventing the consecutive total combustion of propene. The selectivity to propene can be also correlated with the work function values which decrease in the series: VWTi>VTi>VFeTi>VAlTi>VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O 2+4e→2O 2− is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface.