Intrinsic Acidity Research Articles

Selective production of propylene from methanol over high-silica mesoporous ZSM-5 zeolites treated with NaOH and NaOH/tetrapropylammonium hydroxide

The influence of alkaline treatments with NaOH and NaOH/TPAOH mixtures on the physicochemical properties and catalytic performance of high-silica ZSM-5 zeolites (Si/Al = 175) during the methanol-to-propylene (MTP) reaction have been investigated. It was found that alkaline treatment in an NaOH/TPAOH solution with TPAOH/(NaOH + TPAOH) = 0.4 ensures the formation of narrow and uniform intracrystalline mesoporosity without severely damaging the crystal structure and the intrinsic acidity of the zeolite, leading to the best catalytic performance, including the highest propylene selectivity (47.2) and P/E ratio (4.97) as well as the longest catalyst lifetime (80 h).

Solution and gas-phase acidities of all-trans (all-E) retinoic acid: an experimental and computational study.

Retinoic acid is of fundamental biological importance. Its acidity was determined in the gas phase and in acetonitrile solution by means of mass spectrometry and UV/Vis spectrophotometry, respectively. The intrinsic acidity is slightly higher than that of benzoic acid. In solution, the situation is opposite. The experimental systems were described theoretically applying quantum chemical methods (wave function theory and density functional theory). This allowed the determination of the molecular structure of the acid and its conjugate base, both in vacuo and in solution, and for computational estimates of its acidity in both phases.

Effects of synthesis conditions on the yields and properties of HZSM‐5

In the SiO2‐Al2O3‐TPAOH‐NH4OH system through a two‐step procedure, the effects of synthesis parameters on the yields and properties of HZSM‐5 were studied. As the feeding n(SiO2)/n(Al2O3) ratio increased, the yields of synthesized samples were between 11% and 14% while the crystallinity tended to decrease at the ratio > 300. At the ratio of 150, the yield was 13.7% and the ratio of actual yield to theoretical yield was 93%. The sample shaped ellipse‐like particles of 3 × 10−6 m. At the n(NH3)/n(SiO2) ratio of 1.5, the yield was 13.1% and the sample showed regular ellipse‐like particles of 2 × 10−6 m. Too high dosage of NH4OH lowered the yield and the crystallinity. The addition of seed crystal favored the HZSM‐5 synthesis in high crystallinity. At the addition amount of 4%, the yield reached 13.9% and the sample shaped in spherical particles of 0.5 × 10−6 m. Synthesized HZSM‐5 with a surface area of 455 m2·g−1 and intrinsic acidity showed highly catalytic activity toward the methanol conversion to olefins reaction.

Exploitment of niobium oxide effective acidity for xylose dehydration to furfural

Xylose together with other pentose and hexose sugars can be dehydrated to produce interesting platform chemical compounds, like 5-hydroxyfurfural (HMF) and furfural. This study continues our investigation on the niobium oxide based catalysts in connection with the research of adequate solvents systems to minimize catalyst deactivation and increase catalyst stability and durability during the dehydration of sugars. Silica-zirconia supported niobia samples (10wt.% of Nb) prepared by impregnation or sol–gel in comparison with pure niobic acid are here presented for xylose dehydration. The reactions have been studied at different temperatures (130–180°C) in batch or fixed bed continuous catalytic reactors in various solvents. Green solvents soluble in the aqueous solution of xylose or biphasic systems have been taken into account: water, water–isopropanol mixtures, water-γ-valerolactone, and water-cyclopentylmethyl ether. The surface acidities of the catalysts have been measured in cyclohexane (intrinsic acidity) and also in water to determine the effective catalyst acidity. The continuous tests and batch-recycling tests showed that the supported Nb-catalysts, even if initially less active, are more stable than niobic acid. The presence of isopropanol in water improves both the activity and stability of the catalysts in comparison with water and the use of cyclopentylmethyl ether gave the most interesting selectivity to furfural preserving the catalyst stability.

Protic Anions [H(B12X12)]− (X=F, Cl, Br, I) That Act as Brønsted Acids in the Gas Phase

The acidity of protic cations and neutral molecules has been studied extensively in the gas phase, and the gas-phase acidity has been established previously as a very useful measure of the intrinsic acidity of neutral and cationic compounds. However, no data for any anionic acids were available prior to this study. The protic anions [H(B12X12)](-) (X = F, Cl, Br, I) are expected to be the most acidic anions known to date. Therefore, they were investigated in this study with respect to their ability to protonate neutral molecules in the gas phase by using a combination of mass spectrometry and quantum-chemical calculations. For the first time it was shown that in the gas phase protic anions are also able to protonate neutral molecules and thus act as Brønsted acids. According to theoretical calculations, [H(B12I12)](-) is the most acidic gas-phase anion, whereas in actual protonation experiments [H(B12Cl12)](-) is the most potent gas-phase acidic anion for the protonation of neutral molecules. This discrepancy is explained by ion pairing and kinetic effects.

Are Benzoic Acids Always More Acidic Than Phenols? The Case ofortho‐,meta‐, andpara‐Hydroxybenzoic Acids

AbstractTo address the title question, the relative intrinsic acidities of phenol and benzoic acid as well as the isomeric family ofortho‐,meta‐, andpara‐hydroxybenzoic acids were compared. Dissociation of the [PhCO2···H···OPh]–proton‐bound dimer showed slightly greater acidity for benzoic acid. Using traveling‐wave ion mobility mass spectrometry (TWIM‐MS) with CO2as the drift gas and post‐TWIM collision‐induced dissociation, the gaseous deprotonated molecules of the isomeric hydroxybenzoic acids were properly separated and characterized. For theparaisomer, an intrinsic gas‐phase acidity order inverse to that in solution was found, as before, that is, the phenol site ofpara‐hydroxybenzoic acid was found to be considerably more acidic than its benzoic acid site, whereas the opposite was observed in solution, regardless of the solvent. However, for theorthoandmetaisomers, the inversion in acidity order upon going from solution to the gas phase was not observed, and gaseous carboxylate anions were still found to predominate over phenoxide anions. Actually, for theorthoisomer, an even greater acidity as estimated from proton affinity values for the benzoic acid site relative to that of the phenol site was predicted, and accordingly, only a single isomer was sampled by TWIM‐MS. Rationales for these contrasting trends based on interfering inductive effects, charge delocalization by resonance, and intraionic H‐bonds that govern the acidity of benzoic acid relative to that of phenols are presented.

Superacidity of closo-dodecaborate-based Brønsted acids: a DFT study.

The structures and intrinsic gas-phase acidities (GA) of some dodecaborane acids, the derivatives of YB12H11H (Y = PF3, NH3, NF3, NMe3), B12H12H2, and B12H12H(-) (HA, H2A, and HA(-), respectively) have been computationally explored with DFT B3LYP method at the 6-311+G** level of theory as new possible directions of creating superstrong Brønsted acids. Depending on the nature and number of the substituents different protonation geometries were investigated. In general, the GA values of the neutral systems varied according to the substituents in the following order: CF3 < F < Cl and in case of anionic acids: CF3 < Cl < F. The dodecatrifluoromethyl derivative of H2A, B12(CF3)12H1H2, emerges as the strongest among the considered acids and is expected to be in the gas phase at least as strong as the undecatrifluoromethyl carborane, CB11(CF3)11H1H. The GA values of the respective monoanionic forms of the considered acids all, but the (CF3)11 derivative, remained higher than the widely used threshold of superacidity. The HA derivatives' (Y = PF3, NF3) GA's were approximately in the same range as the H2A acids'. In the case Y = NH3 or NMe3 the GA values were significantly higher. Also, the pKa values of B12H12H2, CB11H12H, and their perfluorinated derivatives in 1,2-dichloroethane (DCE) were estimated with SMD and cluster-continuum model calculations. The obtained estimates of pKa values of the perfluorinated derivatives are by around 30 units lower than that of trifluoromethylsulfonylimide, making these acids the strongest ever predicted in solution. The derivatives of B12H12H2 are as a rule not significantly weaker acids than the respective derivatives of CB11H12H. This is important for expanding practical applicability of this type of acids and their anions, as they are synthetically much more accessible than the corresponding CB11H12(-) derivatives.

Acidity enhancement of unsaturated bases of group 15 by association with borane and beryllium dihydride. Unexpected boron and beryllium Brønsted acids.

The intrinsic acidity of CH2[double bond, length as m-dash]CHXH2, HC[triple bond, length as m-dash]CXH2 (X = N, P, As, Sb) derivatives and of their complexes with BeH2 and BH3 has been investigated by means of high-level density functional theory and molecular orbital ab initio calculations, using as a reference the ethyl saturated analogues. The acidity of the free systems steadily increases down the group for the three series of derivatives, ethyl, vinyl and ethynyl. The association with both beryllium dihydride and borane leads to a very significant acidity enhancement, being larger for BeH2 than for BH3 complexes. This acidity enhancement, for the unsaturated compounds, is accompanied by a change in the acidity trends down the group, which do not steadily decrease but present a minimum value for both the vinyl- and the ethynyl-phosphine. When the molecule acting as the Lewis acid is beryllium dihydride, the π-type complexes in which the BeH2 molecules interact with the double or triple bond are found, in some cases, to be more stable, in terms of free energies, than the conventional complexes in which the attachment takes place at the heteroatom, X. The most important finding, however, is that P, As, and Sb ethynyl complexes with BeH2 do not behave as P, As, or Sb Brønsted acids, but unexpectedly as Be acids.

Enhanced CuCl dispersion by regulating acidity of MCM-41 for catalytic oxycarbonylation of ethanol to diethyl carbonate

CuCl supported on molecular sieves has attracted increasing attention in catalyzing oxidative carbonylation of ethanol to diethyl carbonate. Mesoporous MCM-41 has been widely used as catalyst support due to its large surface area and well defined mesoporous structure. Considering its intrinsic weak acidity, MCM-41 was modified by a simple impregnation method to incorporate Al. The incorporation of Al components resulted in the high dispersion of Cu species and the increase of acid sites without changing the mesoporous structure of MCM-41, and thus enhanceed the catalytic activity of CuCl/MCM-41for diethyl carbonate synthesis.

Adsorption of hydrocarbons from industrial wastewater onto a silica mesoporous material: Structural and thermal study

In this paper, the use of a mesoporous silica alumina (MSA) for the removal of hydrocarbons from water is outlined. In particular, we focused our attention on the structural modifications that occur during the adsorption process using toluene and benzene as target contaminants and two different material shapes: particle and extruded. Different characterization techniques such as SEM, powder XRD, TGA and IR absorption are used to gain a deep investigation of MSA characteristics. Indeed, morphological features of MSA, the thermal behavior of the loaded and unloaded material including its regeneration and host–guest interactions were largely examined. SEM images and EDX analysis of the two materials have confirmed their amorphous nature and their elemental composition. Powder XRD patterns of as-synthesized (unloaded) and loaded MSA displayed the appearance of some reflection peaks after the adsorption of benzene and toluene. A Le Bail refinement indicated a monoclinic structure for these molecules when adsorbed onto the MSA. FT-IR measurement on loaded and unloaded MSA highlighted the interaction of water with the free silanol groups while the aromatic hydrocarbons probably adsorbed onto different active sites (Lewis acid sites). Indeed, intrinsic acidity measurements by pyridine adsorption showed the presence of a fairly good number of these acid sites. Finally, the thermal behavior of loaded MSA samples with different contaminants has shown the strong influence of water on the desorption temperature of pollutants from MSA.

Cs-substituted tungstophosphate-supported ruthenium nanoparticles as efficient and robust bifunctional catalysts for the conversion of inulin and cellulose into hexitols in water in the presence of H2

Cellulose and inulin, two important biomasses, can be transformed to polyols using bifunctional catalysts combining acid sites for hydrolysis and metal nanoparticles for hydrogenation. Here, we report that Ru nanoparticles loaded on a Keggin-type polyoxometalate, i.e., Cs3PW12O40, without intrinsic Bronsted acidity exhibit superior catalytic performances for the transformation of inulin and cellulose into hexitols in water in the presence of H2. We demonstrated that new Bronsted acid sites were generated from H2 on the Ru/Cs3PW12O40 catalyst. The H2-originated reversible Bronsted acid sites were robust during the transformation of biomass under hydrothermal conditions. We further found that the mean size of Ru nanoparticles determined the product selectivity in the conversion of inulin under H2. The catalyst with larger Ru nanoparticles favoured the formation of fructose, the hydrolysis product, while the major products were hexitols over the catalyst with a smaller Ru particle size. We clarified that, as compared to that of inulin hydrolysis, the rate of fructose hydrogenation increased more rapidly upon decreasing the Ru particle size.

Transformation of non-edible vegetable oils into biodiesel fuels catalyzed by unconventional sulfonic acid-functionalized SBA-15

A real understanding of intrinsic acidity and catalytic property of unconventional and sulfonic acid-functionalized SBA-15 platelet catalysts synthesized with the aid of highly solvated Zr4+ salts (shortly termed SA-SBA-15-p) for transesterification of non-edible vegetable oils into biodiesel fuel (BDF) was reported, in comparison with conventional SBA-15 catalyst (shortly termed SBA-15-conv), unconventional SBA-15 platelet catalyst (shortly termed SBA-15-p) and commercial sulfonic resins of Amberlyst-15 and SAC-13. Thorough characterizations and catalytic study revealed that the content of fatty acid methyl ester (FAME) as the main component of BDF was increased in the order of SBA-15-conv<SAC-13<SBA-15-p<Amberlyst-15<5-25SA-SBA-15-p, which correlated closely with the loading, distribution, strength of acidic sites. Among them, the unconventional 15SA-SBA-15-p catalyst with proper loadings of propylsulfonic groups (1.15mmolS−1g−1) and superficial Zr4+ clusters (0.66mmolg−1) as a mixed solid Bronsted and Lewis acid, which had differential heat of NH3 adsorption in the range of 90–125kJmol−1, gave highest FAME content with low concentrations of byproducts, such as glycerides and free fatty acid (FFA). In addition to high water and FFA tolerance levels, the used 15SA-SBA-15-p catalyst could be easily regenerated by ethanol extraction and its high activity was maintain. By contrast, the SBA-15-conv catalyst without any Lewis and Bronsted acids had almost no activity in BDF synthesis. The unconventional SBA-15-p catalyst with superficial Zr4+ clusters as a pure solid Lewis acid gave a moderate FAME content accompanying with a high concentration of unreactive glycerides. Commercial sulfonic resins of Amberlyst-15 and SAC-13 with strongly phenylsulfonic and Nafion sulfonic groups as pure solid Bronsted acids gave moderate FAME contents accompanying with high concentrations of unreactive glycerides and unwanted FFA byproducts. Furthermore, they were significantly deactivated by water, FFA and regeneration treatment due to the poison and leaching of sulfonic groups and the decomposition of thermally unstable framework.

Surface acidity of 2:1-type dioctahedral clay minerals from first principles molecular dynamics simulations

We report a first principles molecular dynamics (FPMD) study of the acid chemistry of 2:1-type dioctahedral phyllosilicates. Using the FPMD based vertical energy gap method, we computed intrinsic acidity constants of phyllosilicate edge sites. The investigated models include both neutral and charged frameworks (i.e., Mg for Al replacement in octahedral sheets and Al for Si replacement in tetrahedral sheets) and the common edge surface types (i.e., (010) and (110)). The result of the neutral framework agrees with the experiment of pyrophyllite. For charged frameworks, it is found that Al(OH) sites in T-sheets (i.e., Al-sub) and Mg(OH2) sites (i.e., Mg-sub) have extremely high pKas and thus they all keep protonated. Both types of substitutions increase pKas of the apical oxygen sites on (110) surfaces and Mg substitution also increases the pKas of neighboring silanol sites. With the calculated pKas, we explore the mechanism of heavy metal cations complexation on edge surfaces. As a quantitative basis, the results in this work can be used in future modeling and experimental studies for understanding acid reactivity of phyllosilicates.

Protonation and deprotonation enthalpies of alloxan and implications for the structure and energy of its complexes with water: a computational study

The optimized geometries, harmonic vibrational frequencies, and energies of the structures of monohydrated alloxan were computed at the DFT/ωB97X-D and B3LYP/6–311++G** level of theory. Results confirm that the monohydrate exists as a dipolar alloxan–water complex which represents a global minimum on the potential energy surface (PES). Trajectory dynamics simulations show that attempt to reorient this monohydrate, to a more favorable orientation for H-bonding, is opposed by an energy barrier of 25.07 kJ/mol. Alloxan seems to prefer acting as proton donor than proton acceptor. A marked stabilization due to the formation of N–H–OH2 bond is observed. The concerted proton donor–acceptor interaction of alloxan with one H2O molecule does not increase the stability of the alloxan–water complex. The proton affinity of the O and N atoms and the deprotonation enthalpy of the NH bond of alloxan are computed at the same level of theory. Results are compared with recent data on uracil, thymine, and cytosine. The intrinsic acidities and basicities of the four pyrimidines were discussed. Results of the present study reveal that alloxan is capable of forming stronger H-bonds and more stable cyclic complex with water; yet it is of much lower basicity than other pyrimidines.

Intrinsic Acidity of Surface Sites in Calcium Silicate Hydrates and Its Implication to Their Electrokinetic Properties

Calcium Silicate Hydrates (C–S–H) are the major hydration products of portland cement paste. The accurate description of acid–base reactions at the surface of C–S–H particles is essential for both understanding the ion sorption equilibrium in cement and prediction of mechanical properties of the hardened cement paste. Ab initio molecular dynamics simulations at the density functional level of theory were applied to calculate intrinsic acidity constants (pKa’s) of the relevant ≡SiOH and ≡CaOH2 groups on the C–S–H surfaces using a thermodynamic integration technique. Ion sorption equilibrium in C–S–H was modeled applying ab initio calculated pKa’s in titrating Grand Canonical Monte Carlo simulations using a coarse-grained model for C–S–H/solution interface in the framework of the Primitive Model for electrolytes. The modeling results were compared with available data from electrophoretic measurements. The model predictions were found to satisfactorily reproduce available experimental data.

Using unnatural amino acids to probe the energetics of oxyanion hole hydrogen bonds in the ketosteroid isomerase active site.

Hydrogen bonds are ubiquitous in enzyme active sites, providing binding interactions and stabilizing charge rearrangements on substrate groups over the course of a reaction. But understanding the origin and magnitude of their catalytic contributions relative to hydrogen bonds made in aqueous solution remains difficult, in part because of complexities encountered in energetic interpretation of traditional site-directed mutagenesis experiments. It has been proposed for ketosteroid isomerase and other enzymes that active site hydrogen bonding groups provide energetic stabilization via “short, strong” or “low-barrier” hydrogen bonds that are formed due to matching of their pKa or proton affinity to that of the transition state. It has also been proposed that the ketosteroid isomerase and other enzyme active sites provide electrostatic environments that result in larger energetic responses (i.e., greater “sensitivity”) to ground-state to transition-state charge rearrangement, relative to aqueous solution, thereby providing catalysis relative to the corresponding reaction in water. To test these models, we substituted tyrosine with fluorotyrosines (F-Tyr’s) in the ketosteroid isomerase (KSI) oxyanion hole to systematically vary the proton affinity of an active site hydrogen bond donor while minimizing steric or structural effects. We found that a 40-fold increase in intrinsic F-Tyr acidity caused no significant change in activity for reactions with three different substrates. F-Tyr substitution did not change the solvent or primary kinetic isotope effect for proton abstraction, consistent with no change in mechanism arising from these substitutions. The observed shallow dependence of activity on the pKa of the substituted Tyr residues suggests that the KSI oxyanion hole does not provide catalysis by forming an energetically exceptional pKa-matched hydrogen bond. In addition, the shallow dependence provides no indication of an active site electrostatic environment that greatly enhances the energetic response to charge accumulation, consistent with prior experimental results.

Complexation of Ca2+ with selenocysteine and effects on its intrinsic acidity

The interactions of Selenocysteine (Sec) with Ca have been investigated through the use of B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) calculations. The global minimum of the [SecCa] potential energy surface is a charge-solvated (CS) species in which the metal dication is bound to the SeH, to the amino groups and to the carbonyl oxygen. Within a gap of 5.0 kJ mol in terms of free energies there are four more complexes three of which are salt-bridged (SB) structures. The interactions between Ca and the amino acid are essentially electrostatic, in contrast with what happens when Ca is replaced by Cu, where the charge transfer from Sec to the metal cation is so large that in the [Sec-Cu] global minimum the amino acid moiety is oxidized. The significant electron density redistribution undergone by the amino acid when interacting with both doubly-charged metal ions is reflected in a significant enhancement of its intrinsic acidity, although this effect is much larger for Cu than for Ca. One of the direct consequences of the oxidation undergone by the amino acid when attached to Cu is that the complexes formed are thermodynamically unstable with respect to a proton loss since the process Sec + Cu → [(Sec-H)Cu] + H is strongly exothermic. Conversely, a similar process when Cu is replaced by Ca is endothermic and therefore the [Sec-Ca] complexes should be stable with respect to a proton loss.

Intrinsic (gas-phase) acidity and basicity of paracetamol

Intrinsic (gas-phase) acidity and basicity of paracetamol

Phosphate‐Doped Carbon Black as Pt Catalyst Support: Co‐catalytic Functionality for Dimethyl Ether and Methanol Electro‐oxidation

AbstractNiobium‐phosphate‐doped (NbP‐doped) carbon blacks were prepared as the composite catalyst support for Pt nanoparticles. Functionalities of the composite include intrinsic proton conductivity, surface acidity, and interfacial synergistic interactions with methanol and dimethyl ether (DME). The supported Pt catalysts show significant improvement in catalytic activity towards the direct oxidation of methanol and DME, attributable to the enhanced adsorption and dehydrogenation of methanol and DME, as well as the presence of activated OH species in the catalysts. The latter is demonstrated to facilitate the removal of CO intermediates formed during the oxidation reactions.

Can Conventional Bases and Unsaturated Hydrocarbons Be Converted into Gas‐Phase Superacids That Are Stronger than Most of the Known Oxyacids? The Role of Beryllium Bonds

The association of BeX2 (X: H, F, Cl) derivatives with azoles leads to a dramatic increase of their intrinsic acidity. Hence, whereas 1H-tetrazole can be considered as a typical N base in the gas phase, the complex 1H-tetrazole-BeCl2 is predicted to be, through the use of high-level G4 ab initio calculations, a nitrogen acid stronger than perchloric acid. This acidity enhancement is due to a more favorable stabilization of the deprotonated species after the beryllium bond is formed, because the deprotonated anion is a much better electron donor than the neutral species. Consequently, this is a general phenomenon that should be observed for any Lewis base, including those in which the basic site is a hydroxy group, an amino group, a carbonyl group, an aromatic N atom, a second-row atom, or the π system of unsaturated hydrocarbons. The consequence is that typical bases like aniline or formamide lead to BeX2 complexes that are stronger acids than phosphoric or chloric acids. Similarly, water, methanol, and SH2 become stronger acids than sulfuric acid, pyridine becomes a C acid almost as strong as acetic acid, and unsaturated hydrocarbons such as ethylene and acetylene become acids as strong as nitric and sulfuric acids, respectively.