Multiple Acid Sites Research Articles

Efficient absorption of low pressure NH3 by non-ionic phenol-based deep eutectic solvents with multiple acidic sites

In chemical industry, efficient absorption of ammonia (NH3) from industrial exhaust remains a challenge and significant issue for resource utilization and pollution control. Present study proposed a new strategy to design non-ionic deep eutectic solvents (DESs) using phenolic analogues. Several phenol-based DESs with multiple acidic sites were constructed by paring catechol (Cat) or resorcinol (Res) with phenol (PhOH) and investigated as feasible NH3 absorbents from physical property, NH3 absorption performance, and mechanism analysis. Among the as-obtained DESs, Res/PhOH (1:3) demonstrated attractive NH3 absorption capacity at 303.15 K (11.862 mol/kg at 101.3 kPa, and 6.391 mol/kg at 5.0 kPa), surpassing most DESs and ionic liquids (ILs) reported in the literatures. Meanwhile, it displayed good reversibility, satisfactory thermal stability, and high NH3/CO2 selectivity. The mechanism of NH3 absorption was further elucidated using FTIR and NMR spectra, and Quantum chemistry calculation, and inferred as the strong and multiple interaction between NH3 and weakly acidic sites of DESs.

Phenomenological description of the acidity of the citric acid and its deprotonated species: informational-theoretical study.

In spite of the fact that molecular acidity is a fundamental physicochemical property of molecular systems, the vast majority of theoretical studies have focused attention on monoprotic acids and on the prediction of pKa's. Polyprotic acids, represent a challenge for electronic structure calculations since the multiple acidic sites result in a vast group of species with different conformations and reactivities. In this work, Information-theoretic (IT) concepts of localizability, order and uniformity are applied to the Citric Acid and its deprotonated species through the one-electron density functionals: Shannon entropy (S), Fisher information (I) and Disequilibrium (D), respectively. We pursue the goal of characterizing the acidity of the aforementioned species with the aim to associate the IT concepts to chemical features such as the polarizability of the protonated/deprotonated species, the liability of the acidic sites, atomic electrostatic potentials, covalent bonding. IT analyses looks very promising for future studies on the acidity of specific deprotonation-sites of polyprotic acids. Density functional theory (DFT) calculations were performed with Gaussian 09 program. A sensitivity analysis of the IT-measures was performed for the citric acid and the citrate using B3LYP, B3PW91, BPW91, M05-2X, M06-2X and PBEPBE functionals with the 6-311++g(3df,2p), 6-311++g(d,p), 6.311+g(d,p) and aug-cc-pVDZ basis sets. The rest of the analysis was performed with the M05-2X/6-311+G(d,p) level of theory. Additionally, aqueous media was considered by use of the SMD solvent model. The IT-measures were calculated using a suite of programs developed in our laboratory jointly with the DGRID software package.

Clay-supported acidic ionic liquid as an efficient catalyst for conversion of carbohydrates to 5-hydroxymethylfurfural

Halloysite nanoclay has been used as a natural support for the immobilization of an acidic ionic liquid with multiple acidic sites. To graft the ionic liquid on the halloysite, amino-functionalized halloysite was reacted with 2,4,6-trichloro-1,3,5-triazine and 2-aminopyrimidine successively to form heterocyclic ligand on halloysite, which was then converted to acidic ionic liquids through reaction with chlorosulfuric acid. The resultant catalyst was then characterized via SEM, EDS, and elemental mapping analysis, FTIR, XRD and TGA. Noteworthy, acid-base titration method confirmed that introduction of acidic ionic liquid significantly improved the Brønsted acidity of halloysite. The catalytic activity of the catalyst was investigated for conversion of various carbohydrates to 5-hydroxymethylfurfural. Gratifyingly, the results showed that the catalyst could promote the reaction of fructose, sucrose, glucose and galactose under mild reaction conditions (catalyst (0.03 g), at 85 °C in 70 min) to give the product in high to excellent yields. Computational DFT studies unveiled the reaction mechanism, pointing out the limiting steps kinetically, and in addition how important are the non-covalent interactions. Furthermore, the recycling tests confirmed high recyclability of the catalyst up to six runs.

Efficient synthesis of 5-hydroxymethylfurfural by MCM-41 modified with multiple acid sites

Catalytic conversion of biomass derivative glucose to high-purity HMF over modified MCM-41 with multiple acid sites in biphasic system.

Distribution of multiple Al substitution in HY zeolite and Brønsted acid strength - A periodic DFT study

In petrochemical industry, zeolites as solid acid catalysts play an important role and Y zeolites with high surface area and large pore openings are mainly used for catalytic cracking. Aiming at the problem that the specific positions of Al atoms in zeolites cannot be accurately distinguished experimentally and the acid strength can only be studied qualitatively, we used computational model and method to study the position of Al substitution and the resulted acid strength of Y zeolite at atomic and molecular level. The position of Al substitution with gradually decreasing Si/Al ratio (Si/Al = 47–2.4) and the corresponding Brønsted acid strength of the resulting multiple acid sites of HY zeolites are systematically investigated based on periodic DFT calculation including dispersion correction (GGA-PBE-D3). The thermodynamically more stable structures prefer Al distribution having the next-nearest neighbor of aluminum atom pairs (Al–O–Si–O–Al), and based on these stable configurations, the adsorption of ammonia (NH3) and pyridine (C5H5N) has been computed to estimate the Brønsted acid strength. It is found that the adsorption of NH3 and C5H5N prefers the formation of ammonium (NH4+) and pyridinium (C5H5NH+) ions at each acid sites and the adsorption enthalpy does not correlate with the Bader charge of the acid proton. Most importantly, the adsorption enthalpy decreases with the decrease of Si/Al ratio, indicating that the more the Al substitution the weaker the Brønsted acid strength. It is noted that the adsorption enthalpy of all individual acid sites at a given Si/Al ratio can differ strongly, despite the formation of NH4+ and C5H5NH + ions, indicating their mutual interaction and local confinement effect. The calculated adsorption enthalpies agree fully with the experimentally measured results (Si/Al = 8.6–2.4). Hyroxyl nest, defect sites formed from dealumination, has much weaker acid strength than the Brønsted acid sites. Our results provide a rational basis for further studies on the enhanced catalytic activity of HY zeolites with different Si/Al ratios.

Construction of framework confined ordered mesoporous Pt/TixAlOy catalysts and applied for the catalytic oxidation of propane

The framework confined Pt/TixAlOy catalysts were prepared by a one-step evaporation-induced self-assembly (EISA) technique, and their activities for the catalytic oxidation of propane were investigated. The Pt/TixAlOy catalysts exhibited high specific surface area due to the ordered mesoporous structure. With the increase of TiO2 contents, the specific surface area and acidic sites of Pt/TixAlOy catalysts increased. The specific surface area and acidic sites could improve the exposure of the active sites and electron transfer of Pt and TiO2, which was beneficial to catalytic oxidation of propane. However, too high acidic sites of catalysts could lead to coke deposition and clogging of the pores, and the addition of TiO2 had the optimal value. The Pt/Ti0.1AlOy catalyst exhibited superior catalytic activity, which could be attributed to the higher specific surface area, higher the percentage of Pt2+ and lattice oxygen, abundant acid sites, and higher redox properties. In addition, the Pt/Ti0.1AlOy catalyst also exhibited superior sulfur resistance due to the multiple acidic sites of catalysts.

Direct production of ethyl levulinate from carbohydrates and biomass waste catalyzed by modified porous silica with multiple acid sites

Three modified porous silica solid acid catalysts with multiple acid sites (SO3H-Al/Silica, SO3H-Al-Zr/Silica, and SO3H-Zr/Silica) were prepared and characterized. The Pyridine-infrared spectra showed that both Brønsted (B) and Lewis (L) acid sites were present in all three catalysts. Catalytic acid sites with different acid strengths were also found, including strong sulfonic acids and several weak acids. The prepared solid acid catalysts were used to catalyze the ethanolysis of glucose, glucosyl-based carbohydrates, and starchy food waste to produce ethyl levulinate (EL). The formation of EL was promoted by the synergistic effect between multiple acid sites on the catalysts. A higher B/L ratio and –SO3H amount promoted the production of EL. The catalytic activity was positively correlated to the specific surface area of the catalysts. Promising results were obtained for SO3H-Al/Silica: the maximum EL yield from glucose was 47.9%, and a small amount of 5-ethoxymethylfurfural was detected as a byproduct. Cellobiose, cellulose, expired wheat flour, and kitchen waste were also used as raw materials to produce EL, affording EL yields in the range of 1.6–38.9%. Finally, the stability of the prepared catalysts was explored after reuse and hot filtration.

Computational Prediction of All Lanthanide Aqua Ion Acidity Constants.

The protonation state of lanthanide-ligand complexes, or lanthanide-containing porous materials, with many Brønsted acid sites can change due to proton loss/gain reactions with water or other heteroatom-containing compounds. Consequently, variations in the protonation state of lanthanide-containing species affect their molecular structure and desired properties. Lanthanide(III) aqua ions undergo hydrolysis and form hydroxides; they are the best characterized lanthanide-containing species with multiple Brønsted acid sites. We employed constrained ab initio molecular dynamics simulations and electronic structure calculations to determine all acidity constants of the lanthanide(III) aqua ions solely from computation. The first, second, and third acidity constants of lanthanide(III) aqua ions were predicted, on average, within 1.2, 2.5, and 4.7 absolute pKa units from experiment, respectively. A table includes our predicted pKa values alongside most experimentally measured pKa values known to date. The approach presented is particularly suitable to determine the Brønsted acidity of lanthanide-containing systems with multiple acidic sites, including those whose measured acidity constants cannot be linked to specific acid sites.

Selective deoxygenation of biomass volatiles into light oxygenates catalysed by S-doped, nanosized zinc-rich scrap tyre char with in-situ formed multiple acidic sites

In this paper, we report the unique properties and catalytic performance of scrap tyre char, an otherwise low-value waste derivative on the selective deoxygenation of biomass volatiles (derived from flash pyrolysis of lignocellulosic biomass at 500 °C) into value-added light oxygenates including furfural and phenol. Due to the inclusion of sulphur (S) and zinc oxide (ZnO) as additives during the prior tyre manufacturing process, the pyrolysis-derived tyre char is rich in both organically bound S and nano-sized zinc sulphide (ZnS) that are highly dispersed within the carbonaceous matrix. Detailed characterisations of the fresh and spent catalysts have been conducted to elaborate the unique mechanisms, upon the use of XPS, TEM-SAED, XAS, NEXAFS, Pyridine-FTIR, and NH3-TPD. As has been confirmed, multiple acidic sites are present in tyre char, including organically bound S serving as a weak Brønsted acid and nano-sized ZnS being a strong Lewis acid. The former acid is active for dehydration, whilst the latter one mainly catalyses the decarboxylation and decarbonylation reactions. Upon the interaction with adsorbed oxygen-bearing species and water molecules, the ZnS-centered active acid site can in-situ transform into a ZnSOx-centered super strong acid site, which is a Brønsted acid that is able to enhance the deoxygenation extent remarkably. More interestingly, the organic S enables an in-situ sulphidation of the less active ZnO on the catalyst surface, thereby enabling a continuous exposure of bio-oil vapour to the highly active ZnS and its sulphate derivative. This in turn enlarges the lifetime of the catalyst and its strong stability upon cyclic tests.

Redox & acidity optimizing of LDHs-based CoMnAl mixed oxides for enhancing NH3-SCR performance

A series of Co4−xMnxAl1 mixed metal oxides were obtained via calcining the CoMnAl layered double hydroxides (LDHs) and employed as catalysts in low-temperature NH3-SCR. The results proposed that Co3Mn1Al1-500 catalyst possessed broader temperature window than all other control catalysts Co4Al1-500, Mn4Al1-500 due to its synergistic effect of Co and Mn species. Furthermore, the catalytic behavior of the obtained CoMnAl mixed oxides catalysts were sensitive to calcination temperature and less affected by Co/Mn molar ratios, where Co3Mn1Al1-500 exhibited excellent DeNOx activity (NOx yield above 90%), N2 selectivity and sulfur tolerance in the temperature range of 120–280 °C. Multiple characterization manifested that the outstanding catalytic behavior of Co3Mn1Al1-500 could be attributed to its more favourable ratios of Mn4+ to Mn3+, Co3+ to Co2+ and plentiful surface adsorbed oxygen species which consequently provided stronger redox property and multiple acid sites. Furthermore, kinetics analysis verified that the calculated activation energy significantly decreased over Co3Mn1Al1-500 in low-temperature NH3-SCR reaction.

Performance Effect of Pt/MO<sub><em>x</em></sub>-SiO<sub>2</sub> (M=Ce, Zr, Al) Catalysts for CO and C<sub>3</sub>H<sub>8</sub> Oxidation

MOx-SiO2(M=Ce,Zr,Al) mixed oxides with a MOx:SiO2 mass ratio of 1:1 were prepared by co-precipitation.Pt-only diesel oxidation catalysts supported on these mixed oxides were obtained by the incipient wetness method.The catalytic activities in simplified diesel exhaust gas before and after SO2 treatment were analyzed.The catalysts were characterized by X-ray diffraction,N2 adsorption-desorption,NH3/O2/CO2 temperature programmed desorption(NH3/O2/CO2-TPD) and X-ray photoelectron spectroscopy(XPS).The results of NH3-TPD suggested that the surface of the catalysts had multiple acidic sites,and the number of medium-strength acidic sites increased following treatment with SO2.The results of O2-TPD revealed that there were α and β oxygen species in the catalysts,and the amount of O2 desorption decreased for the SO2-treated catalysts.The Pt/Al2O3-SiO2 catalyst exhibited the lowest surface acidity and the largest amount of oxygen desorption.XPS indicated that the binding energy of Pt 4f5/2 decreased when the catalysts were treated with SO2.All the catalysts showed excellent activity for CO and C3H8,and the Pt/ZrO2-SiO2 catalyst exhibited the best SO2 poisoning resistance,showing the potential for these catalysts to be applied in diesel oxidation.

Gas-Phase Thermochemical Properties of Pyrimidine Nucleobases

The gas-phase acidity and proton affinity of thymine, cytosine, and 1-methyl cytosine have been examined using both theoretical (B3LYP/6-31+G*) and experimental (bracketing, Cooks kinetic) methods. This paper represents a comprehensive examination of multiple acidic sites of thymine and cytosine and of the acidity and proton affinity of thymine, cytosine, and 1-methyl cytosine. Thymine exists as the most stable "canonical" tautomer in the gas phase, with a DeltaH(acid) of 335 +/- 4 kcal mol(-1) (DeltaG(acid) = 328 +/- 4 kcal mol(-1)) for the more acidic N1-H. The acidity of the less acidic N3-H site has not, heretofore, been measured; we bracket a DeltaH(acid) value of 346 +/- 3 kcal mol(-1) (DeltaG(acid) = 339 +/- 3 kcal mol(-1)). The proton affinity (PA = DeltaH) of thymine is measured to be 211 +/- 3 kcal mol(-1) (GB = DeltaG = 203 +/- 3 kcal mol(-1)). Cytosine is known to have several stable tautomers in the gas phase in contrast to in solution, where the canonical tautomer predominates. Using bracketing methods in an FTMS, we measure a DeltaH(acid) for the more acidic site of 342 +/- 3 kcal mol(-1) (DeltaG(acid) = 335 +/- 3 kcal mol(-1)). The DeltaH(acid) of the less acidic site, previously unknown, is 352 +/- 4 kcal mol(-1) (345 +/- 4 kcal mol(-1)). The proton affinity is 228 +/- 3 kcal mol(-1) (GB = 220 +/- 3 kcal mol(-1)). Comparison of these values to calculations indicates that we most likely have a mixture of the canonical tautomer and two enol tautomers and possibly an imine tautomer under our conditions in the gas phase. We also measure the acidity and proton affinity of cytosine using the extended Cooks kinetic method. We form the proton-bound dimers via electrospray of an aqueous solution, which favors cytosine in the canonical form. The acidity of cytosine using this method is DeltaH(acid) = 343 +/- 3 kcal mol(-1), PA = 227 +/- 3 kcal mol(-1). We also examined 1-methyl cytosine, which has fewer accessible tautomers than cytosine. We measure a DeltaH(acid) of 349 +/- 3 kcal mol(-1) (DeltaG(acid) = 342 +/- 3 kcal mol(-1)) and a PA of 230 +/- 3 kcal mol(-1) (GB = 223 +/- 3 kcal mol(-1)). Our ultimate goal is to understand the intrinsic reactivity of nucleobases; gas-phase acidic and basic properties are of interest for chemical reasons and also possibly for biological purposes because biological media can be quite nonpolar.

The acidity of uracil and uracil analogs in the gas phase: four surprisingly acidic sites and biological implications

The gas phase acidities of a series of uracil derivatives (1-methyluracil, 3-methyluracil, 6-methyluracil, 5,6-dimethyluracil, and 1,3-dimethyluracil) have been bracketed to provide an understanding of the intrinsic reactivity of uracil. The experiments indicate that in the gas phase, uracil has four sites more acidic than water. Among the uracil analogs, the N1-H sites have ΔH acid values of 331–333 kcal mol −1; the acidity of the N3 sites fall between 347–352 kcal mol −1. The vinylic C6 in 1-methyluracil and 3-methyluracil brackets to 363 kcal mol −1, and 369 kcal mol −1 in 1,3-dimethyluracil; the C5 of 1,3-dimethyluracil brackets to 384 kcal mol −1. Calculations conducted at B3LYP/6-31+G* are in agreement with the experimental values. The bracketing of several of these sites involved utilization of an FTMS protocol to measure the less acidic site in a molecule that has more than one acidic site, establishing the generality of this method. In molecules with multiple acidic sites, only the two most acidic sites were bracketable, which is attributable to a kinetic effect. The measured acidities are in direct contrast to in solution, where the two most acidic sites of uracil (N1 and N3) are indifferentiable. The vinylic C6 site is also particularly acidic, compared to acrolein and pyridine. The biological implications of these results, particularly with respect to enzymes for which uracil is a substrate, are discussed.

A thermodynamic analysis of the Cal–Ad method with respect to gas–solid calorimetry

A recent article claims that the Cal–Ad method cannot discriminate multiple acid sites in the analysis of the interaction of pyridine (dilute pyridine in n-hexane) with HZSM-5. The Cal–Ad technique measures the enthalpy of a base-displacement reaction, while gas–solid calorimetry is a measure of an adduct formation between pyridine and the solid. A Born–Haber cycle with the appropriate values can demonstrate that the enthalpies derived from gas–solid calorimetry are an average of the two enthalpies of the interaction of pyridine with the two acid sites observed with the Cal–Ad method.

A Solid Acidity Scale Based on the31P MAS-NMR Shift of Chemisorbed Triethylphosphine Oxide

The change in chemical shift of a chemisorbed probe molecule is an informative tool for the characterization of solid acids. This work demonstrates that the phosphorus-31 isotropic chemical shift of the probe triethylphosphine oxide (TEPO) can be used to identify the acidity of multiple acid sites on a surface. The difference in the phosphorus-31 chemical shift of chemisorbed TEPO with respect to physisorbed TEPO, Δδ, establishes a scale of solid acidity that can be validated by comparison to enthalpies of calorimetric methods. A linear correspondence is observed for the strong solid acid (silica-gel)nSbVCl3with four acid sites and with enthalpies of −66, −46, −33, and −21 Kcal/mol for interaction of the solid acid with pyridine in a cyclohexane slurry that correlate with Δδ values of 66, 48, 36, and 22 ppm, respectively.