Nature Of Acid Sites Research Articles

Nature of Acid Sites in Silica-Supported Zirconium Oxide: A Combined Experimental and Periodic DFT Study

A combined experimental and DFT study was carried out to investigate the acidity of silica-supported zirconium oxide synthesized via the grafting method. XRD results show the crystal phase of ZrO2 is tetragonal. Sample acidity was characterized via ammonia adsorption microcalorimetry and DRIFTS pyridine desorption. Theoretical results predict that on a vacancy-free surface of t-ZrO2 at low coverage NH3 binds preferentially to Lewis acid sites with an adsorption energy up to 120 kJ/mol and that ammonia adsorption energies below 100 kJ/mol calculated at higher coverage can be well explained by lateral NH3–NH3 interactions. Higher differential heats of adsorption at low coverage (up to 190 kJ/mol) on both the (101) and (201) crystal planes of t-ZrO2 are in agreement with dissociative adsorption of ammonia near an oxygen vacancy site. The theoretical results are in good agreement with our findings in ammonia adsorption microcalorimetry and DRIFTS pyridine desorption on the ZrO2/SiO2 sample.

Role of acid sites and selectivity correlation in solvent free liquid phase dehydration of sorbitol to isosorbide

A number of Brönsted acids (methanesulfonic acid, p-toluene sulfonic acid, triflic acid, sulfamic acid, citric acid, NaHSO4, and boric acid) and Lewis acids (metal sulfate/triflates) were employed in solvent free dehydration of sorbitol and their influence on anhydroalcohols selectivity has been investigated. The outcome indicated that all the acid catalysts produced first mono-dehydrated product sorbitan followed by second dehydration of 1,4-sorbitan to isosorbide. However, the formation and yield isosorbide were found to depend on the nature of acid sites and their acidic strength. The Brönsted acids are more efficient to convert sorbitol to isosorbide than Lewis acids. The Brönsted acids having lower pKa value (i.e. strong acid) exhibited high catalytic activity as well as yield of isosorbide. In the case of Lewis acids, the catalytic activity and selectivity were radically depended on which metal used and their stability during the reaction. The water formed during reaction induced Brönsted acidity on Lewis acid metal site. The Lewis-assisted Brönsted acid site enabled high yield of isosorbide up to 70% at moderate temperature (160°C).

Vapor Phase Dehydration of Glycerol to Acrolein Over Phosphotungstic Acid Catalyst Supported on Niobia

The vapor phase dehydration of glycerol was carried out on niobia-supported phosphotungstic acid (PTA) catalysts. The catalysts were prepared by varying the active component PTA loadings ranging from 10 to 40 wt% on the support. The catalysts were characterized by powder X-ray diffraction, laser Raman spectroscopy, temperature programmed desorption of ammonia, ex-situ FT-IR spectra of adsorbed pyridine, BET surface area and pore size distribution. The XRD results suggest that the active phase of phosphotungstic acid was found to be highly dispersed at lower PTA loadings on the support. The findings of Raman spectra reveals that the catalyst with 30 PTA/Nb-400 clearly exhibit the primary structure of Keggin ion of hetropolyacid on the niobia support. NH3-TPD experiments confirm that the acidity of the catalyst increases with increase of PTA loading and decreases at higher loadings. The nature of acidic sites on the catalyst surface was investigated by the ex-situ pyridine adsorbed FT-IR spectroscopy and the results suggest that the catalyst with 30 PTA/Nb has shown stronger Bronsted acidic sites. Pore size distribution reveals that 30 PTA/Nb-400 exhibited lager average pore diameter than the other loadings. The Nb2O5-supported PTA catalysts have shown remarkable catalytic performance during the dehydration of glycerol to acrolein. The catalytic properties during glycerol dehydration are related to the surface acidic functionality of the catalyst.

ChemInform Abstract: The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity

AbstractReview: 365 refs.

The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity

In this article, we considered all aspects of acidity (nature of acid sites, strength, density, etc.) in solid catalysts and in zeolites in particular. After reminding the definition of acidity in liquid and solid acids, we emphasized acidity characterization by the most used physical techniques, such as Hammett's indicator titration, microcalorimetry of adsorbed probe molecules (ammonia, pyridine or other amines for acidity characterization and CO2 or SO2 for basicity characterization), ammonia or any amine thermodesorption, IR spectroscopy of hydroxyl groups and of several probe molecules adsorbed (ammonia, pyridine, piperidine, amines, CO, H2, etc.), MAS-NMR of 27Al, 29Si, 1H elements and of 1H, 13C, 31P, etc. of adsorbed probe molecules, and model catalytic reactions. Modeling the way the acid features of zeolites influence the catalytic activity of these catalysts toward acid-catalyzed reactions (relation between ammonia desorption activation energy values and catalytic activities, reaction mechanism, and kinetics) completes the general analysis of acidity and zeolite chemistry.

Fine tuning the surface acidity of titanate nanostructures

The effect of protonation on the surface acidic properties of titanate nanowires (TiONWs) was investigated. Nanowires were synthesized by the alkali hydrothermal method which resulted in one dimensional nanostructures of large external surface area and well-defined lamellar interlayer structure. The Na+/H+ ratio in the structure can be tuned by ion-exchange. Our aim was to characterize the morphology of the as-synthesized nanostructures by HRTEM and SEM measurements and assess their surface acidity using in situ infrared spectroscopic measurements and temperature programmed desorption. It was found that the numbers of Lewis and Bronsted acidic sites in the Na-form and the H-form of the TiONWs is different. The ratio and the nature of acidic sites can be tuned by the ion exchange process. The wire-like morphology and the tunable acidity are features of titanate nanowires that may render them a promising material in various heterogeneous catalytic applications.

Glycerol dehydration over calcium phosphate catalysts: Effect of acidic–basic features on catalytic performance

This work focuses on investigation of calcium phosphate compounds with different Ca/P ratios (1.39–1.77). Additionally, hydroxyapatite impregnated with tungsten oxide was also investigated. The structural, textural, and surface properties of these materials have been fully characterized using appropriate techniques (low-temperature adsorption–desorption of nitrogen, X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and temperature-programmed reduction (TPR)). Adsorption microcalorimetry of NH3 or SO2 was used to estimate the population, strength and strength distribution of acid and basic sites. The nature of acidic sites was determined through the adsorption/desorption of pyridine, followed by infrared spectroscopy. Catalytic performance of the catalysts was tested in the gas phase dehydration of glycerol with the intention of finding correlations between catalytic activity and surface acid–base features. Results show that Ca/P ratio, beside the already known influence on acidic/basic features, also influences red-ox properties of these materials. The investigation performed here proved that, in order to get acrolein – with high selectivity – which is formed on acid sites, it is not only necessary to provide acidity, but also to hinder basic sites. Our results also show that reducing of number and strength of acid centers increases the yield of other desired product, acetol.

Effect of preparation parameters of Cu catalysts on their physico-chemical properties and activities for glycerol hydrogenolysis

Cu based catalysts were prepared by co-precipitation, alkali fusion followed by precipitation and direct solid state fusion methods. The changes in the phase formation, morphology, crystallite size, extent of aggregation, strength and nature of acid sites were observed due to variations in precipitating agents and also their order of addition. The catalyst prepared by co-precipitation using Na2CO3 showed the predominant presence of metallic Cu phase with a crystallite size of 5nm, well segregated spherical morphology and highest acidity in the activated sample. These intrinsic properties contributed to achieve the highest glycerol conversion of 62% and 1,2-PDO selectivity of 88% in glycerol hydrogenolysis.

Acidity study of X zeolites modified by nickel and/or chromium cations in the case of binary and ternary exchanges

A series of Ni(x)-, Cr(x) and CrNi(x)-exchanged X zeolites (x being the ion-exchange degree) was prepared and characterised by nitrogen adsorption, XRD, DTA/TGA and pyridine adsorption–desorption followed by IR spectroscopy. The objective is mainly to estimate quantitatively the number, strength and nature of acid sites and their dependence on the type of cation and its exchange level. A comparison between the binary and ternary exchanges was also established. The crystallinity and micropore volume losses were more significant for Cr(x)X and CrNi(x)X. The concentration of Lewis acid sites decreases with increasing the exchange degree for Cr(x)X and Ni(x)X, while it increases up to 39% for CrNi(x)X. An increase of the concentration of Brønsted sites was obtained. The fact that both the Lewis and Brønsted acidities markedly decrease between 423 and 623K, proves that the available acid sites are overall weak. The pyridine desorption from the protonic sites is easier with CrNi(39)X than with Cr(39)X and Ni(40)X at 623K, indicating a decrease in the strength in the case of ternary exchange.

Ni/SiO2 and Ni/ZrO2 catalysts for the steam reforming of ethanol

SiO2 and ZrO2 supported Ni catalysts were prepared for use in the steam reforming of ethanol. The catalytic performances, in terms of both H2 productivity and stability towards coking and sintering, were related to the physico-chemical properties of the catalysts.The samples were prepared either by synthesis of the support by precipitation and subsequent impregnation with the active phase, or by direct synthesis through flame pyrolysis. The latter has been chosen because it leads to nanostructured oxides, often quenched in very disperse or metastable form, characterised by high thermal resistance, important for this high temperature application.Many techniques have been used to assess the physico-chemical properties of the catalysts. The samples showed different textural, structural and morphological properties, as well as different reducibility and thermal resistance, depending on the preparation method and support. Therefore, besides evaluating the effect of catalyst formulation and preparation method on the catalytic performance, the influence of all such properties has been considered. The fundamental parameter governing the final catalyst properties was metal–support interaction. In particular, the stronger the latter parameter, the higher was metal dispersion, leading to small and stable Ni clusters. This influenced both activity and the resistance towards coking. Surface acidity was also taken into account considering the effect of the different nature of acid sites (silanols or Lewis a.s.) of both support and metal phase on catalyst deactivation. The best results were obtained with a 10wt% Ni/SiO2 sample, prepared by FP, when tested at 625°C. H2 productivity of 1.44molH2/minkgcat was reached, corresponding to ca. 80% of the maximum value achievable under the selected conditions. This result was accompanied by the lowest CO/CO2 ratio and 100% carbon balance without by-products in the outflowing gas.

CeO2–Nb2O5 mixed oxide catalysts: Preparation, characterization and catalytic activity in fructose dehydration reaction

In this work, ceria–niobia mixed oxides have been prepared by coprecipitation. The structural, textural, and surface properties of these materials have been fully characterized using appropriate techniques (low-temperature adsorption–desorption of nitrogen, thermogravimetric analysis, X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and temperature-programmed reduction/oxidation (TPR/O)). The acid–base properties were estimated by the adsorption of appropriate probe molecules – NH3 or SO2 were used to estimate the population, strength and strength distribution of acid or basic sites, by means of adsorption microcalorimetry. The nature of acidic sites was determined through the adsorption/desorption of pyridine, studied by infrared spectroscopy. New phases between the oxides were not formed, while there is evidence of interaction between them, as revealed by XRD and Raman spectroscopy. All investigated mixed oxide samples are amphoteric and possess red-ox centers on their surface. Both red-ox and acid–base properties are dependent on the ratio of ceria to niobia in the samples. Both Lewis and Brönsted acid sites are present on the surface of the mixed oxides. In order to check catalytic abilities of these materials, reaction of fructose dehydration has been performed. All the investigated materials are catalytically active in fructose dehydration; conversion of fructose and selectivity to 5-hydroxymethylfurfural (5-HMF) improved with increasing content of niobia in the samples.

Brønsted acidity of amorphous silica–alumina: The molecular rules of proton transfer

The nature of acid sites on amorphous silica–alumina (ASA) is strongly debated, as well as their infrared signature. We report a combined experimental and computational study to unravel this challenging question at the atomic scale, focusing on proton transfer from ASA to lutidine (2,6-dimethylpyridine), an experimentally widely used molecule for probing Brønsted acid sites. The ASA surface model obtained by density functional theory (DFT) calculations is validated by the comparison of infrared frequencies of OH-groups with experimental spectra. The bands observed are assigned to the various OH-groups present, as a function of their hydrogen-bond donor character and of the proximity of silanols toward aluminum atoms. The affinity of lutidine (2,6-dimethylpyridine) for each site of the ASA surface is then evaluated by sampling the DFT model and varying the experimental pretreatment conditions. A general rule is established for Brønsted acidity of ASA, by comparison with calculations on reference silica, alumina, and mordenite models: the driving force for the proton transfer from OH-groups to lutidine is the stabilization of the conjugated base (after deprotonation) of the hydroxyls, more than the intrinsic acidity of the OH-group. Pseudo-bridging silanols (PBS) are thus found to be capable of proton transfer, thanks to the stabilization of silanolate species by the formation of additional O–Al and O–Si bonds. A prominent role of water molecules adsorbed on Al atoms is also shown: they act as a proton reservoir to express intrinsic acidity and to promote the acidity of neighboring silanols. Finally, we suggest that the ν ˜ 8 a and the ν ˜ 8 b modes of lutidinium species are inverted with regards to lutidine, contrary to what was previously thought on the basis of empirical data.

Direct synthesis of carbon-templating mesoporous ZSM-5 using microwave heating

Carbon-templated mesoporous ZSM-5 zeolites were synthesized directly avoiding a drying process. Carbon nanoparticles were simply mixed into synthesis precursor of ZSM-5 and hydrothermally treated by microwave irradiation. The amount of mesopores formed inside the ZSM-5 single crystals was controllable by adjusting the amount of carbon used. For comparison, mesoporous ZSM-5 zeolites have also been synthesized under hydrothermal conditions. The influence of microwave irradiation on mesoporous ZSM-5 materials was thoroughly investigated by using nitrogen adsorption/desorption studies and 27Al MAS NMR. The nature of acid sites both in the micropores (internal) and on the surface of mesopores (external) was investigated by in situ FTIR spectroscopy using pyridine (Py) and 2′,6′-di-tert-butylpyridine (DTBPy) as a probe molecules. Mesoporous ZSM-5 prepared by microwave synthesis showed higher catalytic activity in the bulky molecular reaction of 2′,4′-dimethoxyacetophenone (2′,4′-DMAP) with 4-methoxybenzaldehyde as a model reaction in comparison with the results obtained over hydrothermally prepared ZSM-5. The further catalytic behavior has been studied in condensation reaction and cracking of substituted benzene.

Gas-phase dehydration of glycerol to acrolein catalysed by caesium heteropoly salt

Caesium 12-tungstophosphate, Cs 2.5H 0.5PW 12O 40 (CsPW), possessing strong Brønsted acid sites is an active catalyst for the dehydration of glycerol to acrolein in the gas-phase process at 275 °C and 1 bar pressure. The initial glycerol conversion amounts to 100% at 98% acrolein selectivity, however, decreases significantly with the time on stream (∼40% after 6 h) due to catalyst coking, without impairing acrolein selectivity. Doping CsPW with platinum group metals (PGM) (0.3–0.5%) together with co-feeding hydrogen improve catalyst stability to deactivation, while maintaining high selectivity to acrolein. The enhancing effect of PGM was found to increase in the order: Ru ∼ Pt < Pd. The catalyst 0.5%Pd/CsPW gives 96% acrolein selectivity at 79% glycerol conversion, with a specific rate of acrolein production of 23 mmol h −1 g cat −1 at 275 °C and 5 h time on stream, exceeding that reported previously for supported heteropoly acids (5–11 mmol h −1 g cat −1 per total catalyst mass). Evidence is presented regarding the nature of acid sites required for the dehydration of glycerol to acrolein, supporting the importance of strong Brønsted sites for this reaction.

The state of dispersed niobia species on γ-Al2O3 and their catalytic performance for condensation of isobutylene and isobutyraldehyde

Catalysts of Nb2O5/γ-Al2O3 were prepared by aqueous solution impregnation. The state of niobia species on surface of γ-Al2O3 is characterized by using the technology of X-ray power diffraction (XRD) and analyzed using the “incorporation model”. The acidity and the nature of acid sites of the catalysts were evaluated by means of Fourier transform infrared (FTIR) spectroscopy of adsorbed pyridine. The catalytic activity of Nb2O5/γ-Al2O3 catalysts was evaluated by a condensation reaction from isobutene and isobutyraldehyde to 2,5-dimethyl-2,4-hexadiene. The results of XRD indicate that the dispersion capacity of niobia on γ-Al2O3 is about 0.76 mmol Nb per 100m2 γ-Al2O3, which is almost identical to the theoretical value (0.75 mmol Nb per 100m2 γ-Al2O3) calculated by the “incorporation model”. The results of Py-IR and catalytic activity evaluation indicate that the acidity feature is related to the state of dispersed niobia species as well as the loading of niobia onto the surface of γ-Al2O3 support.

Catalysis of [formula omitted] solid acid for the liquefaction of coal

In order to study the catalysis of SO 4 2 - / ZrO 2 solid acid for the liquefaction of coal, a series of SO 4 2 - / ZrO 2 solid acids were synthesized by the method of precipitation–impregnation. The catalytic behaviours of the SO 4 2 - / ZrO 2 solid acids for the hydro-liquefaction of Shenhua coal and model compounds, such as diphenylmethane, bibenzyl and phenyl ethyl ether, were investigated. In addition, non-catalytic liquefaction and the catalytic liquefaction under N 2 were further compared with the catalytic liquefaction under H 2 in order to understand the catalytic mechanism of SO 4 2 - / ZrO 2 solid acid. The results indicate that hydro-liquefaction conversions of coal and model compounds are related to the strength, amount and nature of acid sites on the surface of SO 4 2 - / ZrO 2 , and the strong acid site responds to their catalytic activities. The SO 4 2 - / ZrO 2 solid acid catalyzes mainly the hydro-cracking, ring-opening and hydrogenation reactions of coal to produce oil and gas during the coal liquefaction. The hydro-cracking reactions in the liquefaction of model compounds and coal catalyzed by SO 4 2 - / ZrO 2 involved via carbenium ion intermediate instead of traditional radicals intermediate.

Highly selective vapor phase nitration of toluene to 4-nitro toluene using modified and unmodified H3PO4/ZSM-5

Abstract The nitration of toluene has been studied in the vapor phase at various temperatures over a series of surface modified and unmodified ZSM-5 (Si/Al = 170) loaded with H 3 PO 4 , differing in the external surface treatment of the zeolites. All samples were characterized by X-ray diffraction, BET and FT-IR. The nature of acid sites of the supports, i.e., H 3 PO 4 /ZSM-5-surf-170 and H 3 PO 4 /ZSM-5-170 was determined using FT-IR-pyridine adsorption study. 2,4,6-Trimethyl pyridine adsorption was used for calculating the external surface acidity of the H 3 PO 4 /ZSM-5-surf-170 and H 3 PO 4 /ZSM-5-170. The superior performance of the externally blocked catalysts (modified) relevant to the H 3 PO 4 /ZSM-5-170 catalyst shows that by blocking the external surface area of the ZSM-5 support, it exhibits a superior catalytic selectivity than the unmodified catalyst. Catalysts containing 0–4.9 wt.% P were prepared using ZSM-5-surf-170 and ZSM-5-170, and their catalytic performance was determined for the vapor phase nitration of toluene to 4-nitrotoluene (4-NT), 2-nitrotoluene (2-NT) and oxidation products. The effect of reaction temperature, H 3 PO 4 content, feed ratio of toluene to HNO 3 , WHSV, TOS and carrier gas flow rate was also investigated.

Phosphotungstic acid on silica with modified acid sites as a solid catalyst for selective cleavage of tert-butyldimethylsilyl ethers

The strength and nature of acid sites of bulk phosphotungstic acid could be modified after dispersing it on silica support which was characterized by XRD, ammonia TPD and 31P MAS NMR techniques. This solid acid catalyst was found to be highly selective for the cleavage of tert-butyldimethylsilyl ethers at room temperature. The catalyst was easily recovered by filtration and could be reused for several times without loss of any activity giving a TON of 9.5 × 105.

Surface acidic properties of supported binary oxides containing CuO coupled with Ga2O3 and SnO2 studied by complementary techniques

Silica–alumina (SA) was used as support for dispersing CuO, Ga2O3, SnO2 oxide phases and the corresponding binary couplings (CuGa/SA and CuSn/SA samples). The acidity of uncovered SA and of SA-supported oxides was studied from a qualitative (nature of acid sites) and quantitative (number, acid strength, and strength distribution of acid sites) points of view by an integrated approach. The adsorption and desorption of basic probes (ammonia and pyridine) onto the acidic surfaces were monitored by the coupled volumetric–calorimetric technique, and by XPS and FT-IR spectroscopy. The results show that the Brönsted acidity of the SA support was converted into predominantly Lewis acidity upon metal oxide deposition. The high dispersion of the copper oxide phase was responsible for this phenomenon. Differences of acid strength distribution were noticed among the samples. Moderate acidity was associated with the Cu sites (100 kJ mol−1 < qdiff < 150 kJ mol−1), whereas the most acidic fraction of the sites (qdiff > 150 kJ mol−1) increased with the presence of Ga and Sn. The experimental results are discussed in the light of the intrinsic acidity of the Cu, Ga, and Sn metal ions derived from electronic parameters.

Acid sites in mesoporous Al-SBA-15 material as revealed by solid-state NMR spectroscopy

The nature of acid sites in Al-SBA-15 materials with various Si/Al ratios was studied by various NMR techniques including 27Al, 29Si, 1H, 31P MAS (magic-angle spinning), and some double-resonance methods such as 1H/ 27Al transfer of population in double resonance (TRAPDOR). XRD and 27Al NMR results indicate that Al has been successfully incorporated into the framework of SBA-15 materials by post-synthesis method. The bridging hydroxyl groups (SiOHAl) associated with Brönsted acid sites are invisible either in 1H MAS or in 1H/ 27Al TRAPDOR spectra of the dehydrated Al-SBA-15 materials. However, after the adsorption of basic probe molecules, such as trimethylphosphine oxide (TMPO), trimethylphosphine (TMP) and acetone, 31P MAS and 13C CP MAS NMR experiments strongly support the presence of Brönsted acid sites. NMR results suggest that Brönsted acid sites in dehydrated Al-SBA-15 materials are in the form of terminal silanol groups in the vicinity of aluminum atoms that can be induced to generate the bridging hydroxyl groups by the adsorbed basic probe molecules. Although the acid strength of Al-SBA-15 materials detected by different probe molecules is different, it is comparable to that of microporous zeolites. It is noteworthy that no Lewis acid site can be found in the Al-SBA-15 materials.