Concentration Of Lewis Acid Sites Research Articles

Effect of γ-Al2O3 hydrothermal treatment on the formation and properties of platinum sites in Pt/γ-Al2O3 catalysts

The effect of hydrothermal treatment (HTT) of γ-alumina on the state of its surface functional cover, anchoring of hexachloroplatinate, and properties of platinum sites was investigated. Hydrothermal treatment of γ-Al2O3 was found to increase the fraction of surface bridging OH groups and concentration of Lewis acid sites (LAS) and alter the metal complex–support interaction. Adsorption of hexachloroplatinate on the support surface is accompanied mainly by the formation of outer-sphere complexes, which are characterized by a lower reduction temperature. As a result, dispersion and electronic state of supported platinum are changed. Influence of γ-Al2O3 hydrothermal treatment on the catalytic properties of Pt/Al2O3 in n-hexane transformations and propane dehydrogenation was demonstrated. The revealed effect of γ-Al2O3 hydrothermal treatment on the formation of platinum sites in Pt/Al2O3 catalysts can be of fundamental importance for the steps of supported system synthesis where solid phase is contacting with water and aqueous solutions (impregnation and drying).

Tailoring silica–alumina-supported Pt–Pd as poison-tolerant catalyst for aromatics hydrogenation

The tailoring of the physicochemical and catalytic properties of mono- and bimetallic Pt–Pd catalysts supported on amorphous silica–alumina was studied. Electron-energy-loss spectroscopy and extended X-ray absorption fine structure analyses indicated that bimetallic Pt–Pd and relatively large monometallic Pd particles were formed, whereas the X-ray absorption near edge structure provided direct evidence for the electronic deficiency of the Pt atoms. The heterogeneous distribution of metal particles was also shown by high-resolution transmission electron microscopy. The average structure of the bimetallic particles (Pt-rich core and Pd-rich shell) and the presence of Pd particles led to surface Pd enrichment, which was independently shown by IR spectra of adsorbed CO. The specific metal distribution, average size, and surface composition of the Pt–Pd particles depend to a large extent on the metal precursors. In the presence of NH3 ligands, Pt–Pd particles with a fairly homogeneous bulk and surface metal distribution were formed. Also, high Lewis acid site concentration of the carrier leads to more homogeneous bimetallic particles. All catalysts were active for the hydrogenation of tetralin in the absence and presence of quinoline and dibenzothiophene (DBT). Monometallic Pt catalysts had the highest hydrogenation activity in poison-free and quinoline-containing feed. When DBT was present, bimetallic Pt–Pd catalysts with the most homogenous metal distribution showed the highest activity. The higher resistance of bimetallic catalysts toward sulfur poisoning compared to their monometallic Pt counterparts results from the weakened metal–sulfur bond on the electron-deficient Pt atoms. Thus, increasing the fraction of electron-deficient Pt on the surface of the bimetallic clusters increases the efficiency of the catalyst in the presence of sulfur-containing compounds.

Seeds-induced fluoride media synthesis of nanosized zeolite Beta crystals

This study reports on the seeded synthesis of zeolite Beta in fluoride media. Crystal growth kinetics of the material as a function of seed content, crystallization temperature and Si/Al ratio of the initial gel is studied. After the optimization of crystallization conditions a highly crystalline material with crystal size 100–300nm is synthesized. A counterpart zeolite Beta sample with similar characteristics is obtained in hydroxyl media. The physicochemical properties of fluoride and hydroxyl media syntheiszed zeolites are compared as particular attention is paid to the nature, number and distribution of their active sites. Zeolite Beta sample synthesized in fluoride media exhibits larger number of Broensted acid sites, while the concentration of Lewis acid sites is higher in the zeolite synthesized in hydroxyl media. The two materials are tested in m-xylene transformation, where zeolite Beta synthesized in fluoride medium shows superior performance. The set of experimental data unambiguously proves that seeded synthesis of zeolites in fluoride medium is a promising alternative of the conventional zeolite synthesis in hydroxyl medium.

Superior Performance of Metal–Organic Frameworks over Zeolites as Solid Acid Catalysts in the Prins Reaction: Green Synthesis of Nopol

The catalytic performance of a set of metal-organic frameworks [CuBTC, FeBTC, MIL-100(Fe), MIL-100(Cr), ZIF-8, MIL-53(Al)] was investigated in the Prins condensation of β-pinene with formaldehyde and compared with the catalytic behavior of conventional aluminosilicate zeolites BEA and FAU and titanosilicate zeolite MFI (TS-1). The activity of the investigated metal-organic frameworks (MOFs) increased with the increasing concentration of accessible Lewis acid sites in the order ZIF-8<MIL-53(Al)<FeBTC<MIL-100(Cr)<MIL-100(Fe). Unwanted β-pinene-like isomerization takes place on the strong Brønsted acid sites of zeolites BEA and FAU, which showed significantly lower selectivity to the target nopol than the MOFs. Its high activity, the preservation of its structure and active sites, and the possibility to use it in at least three catalytic cycles without loss of activity make MIL-100 (Fe) the best performing catalyst of the series for the Prins condensation of β-pinene and paraformaldehyde. Our report exemplifies the advantages of MOFs over zeolites as solid catalysts in liquid-phase reactions for the production of fine chemicals.

Catalytic performance of Metal-Organic-Frameworks vs. extra-large pore zeolite UTL in condensation reactions.

Catalytic behavior of isomorphously substituted B-, Al-, Ga-, and Fe-containing extra-large pore UTL zeolites was investigated in Knoevenagel condensation involving aldehydes, Pechmann condensation of 1-naphthol with ethylacetoacetate, and Prins reaction of β-pinene with formaldehyde and compared with large-pore aluminosilicate zeolite beta and representative Metal-Organic-Frameworks Cu3(BTC)2 and Fe(BTC). The yield of the target product over the investigated catalysts in Knoevenagel condensation increases in the following sequence: (Al)beta < (Al)UTL < (Ga)UTL < (Fe)UTL < Fe(BTC) < (B)UTL < Cu3(BTC)2 being mainly related to the improving selectivity with decreasing strength of active sites of the individual catalysts. The catalytic performance of Fe(BTC), containing the highest concentration of Lewis acid sites of the appropriate strength is superior over large-pore zeolite (Al)beta and B-, Al-, Ga-, Fe-substituted extra-large pore zeolites UTL in Prins reaction of β-pinene with formaldehyde and Pechmann condensation of 1-naphthol with ethylacetoacetate.

The effect of MFI zeolite lamellar and related mesostructures on toluene disproportionation and alkylation

The effect of MFI zeolite nanosheet morphology and related mesostructures on the accessibility of acid sites evaluated from FTIR spectra of different adsorbed probe molecules and conversions and selectivities in toluene disproportionation and alkylation with isopropyl alcohol was investigated. Two-dimensional MFI lamellar zeolite (1.5 nm thickness of layers) and related mesoporous sponge morphologies, most probably with 10-ring channel system not yet structurally fully determined, were synthesized and characterized by XRD, SEM, TEM, and adsorption measurements. FTIR spectroscopy using pyridine, deuterated acetonitrile and 2,6-di-tert-butyl pyridine evidenced high concentrations of accessible Lewis acid sites at the expense of Bronsted acid sites. Ordered mesoporous zeolites exhibited slightly higher toluene conversion in its alkylation with isopropyl alcohol than lamellar MFI or conventional MFI due to easy access to active sites. The lamellar zeolite consisting of less than a unit cell layer was slightly more active than MFI but still exhibited high selectivity to p-cymene. It evidences that alkylation of toluene with isopropyl alcohol proceeds mainly in the 10-ring channel system of MFI zeolite, although the effect of large external surface cannot be neglected. Increased para-selectivity compared with thermodynamic value is achieved even in the case of extremely thin zeolite nanosheets.

Hydrogenolysis of glycerol to 1,3-propanediol over bifunctional catalysts containing Pt and heteropolyacids

Hydrogenolysis of glycerol to 1,3-propanediol was conducted over zirconia supported bifunctional catalysts containing Pt and heteropolyacids using H4SiW12O40 (HSiW), H3PW12O40 (HPW) and H3PMo12O40 (HPMo) as active compounds. Pt/ZrO2 was also examined for comparison. The as-prepared catalysts were characterized by BET, CO chemisorption, XRD, Raman spectra, NH3-TPD and FTIR of adsorbed pyridine. Compared with Pt/ZrO2, heteropolyacids modified Pt/ZrO2 catalysts showed higher acidity and better catalytic performance of glycerol hydrogenolysis to 1,3-propanediol. Among them, Pt-HSiW/ZrO2 exhibited superior performance due to the high Brønsted acid sites and good thermal stability. Independent of the heteropolyacid type, the concentration of Brønsted acid sites appeared as a key to the selective formation of 1,3-propanediol from glycerol hydrogenolysis, whereas the concentration of Lewis acid sites was related to the formation of 1,2-propanediol. We also investigated the reaction network and proposed a possible reaction pathway.

Influence of the temperature of calcination of bayerite-containing aluminum hydroxide pellets on the water vapor adsorption capacity and acid-base properties of alumina

The paper considers the influence of the temperature of calcination (350–550°C) of alumina-based water adsorbents prepared from bayerite-containing aluminum hydroxide on the static water sorption capacity at high and low relative air humidities and on the acid-base properties of the surface. The acid-base properties of aluminas have been determined by means of IR spectroscopy of adsorbed CO probe molecule (Lewis sites) and by the spin probe method, which was used to determine the concentrations of electron acceptor and electron donor sites. It has been demonstrated that, at a high relative air humidity (60%), there is no correlation between the static capacity of the drier and its acid-base properties, while at low relative humidities (1–1.5%), an increase in the concentration of electron donor sites and a decrease in the concentration of electron donor and Lewis acid sites enhance the water sorption capacity of alumina.

Aluminum oxide and systems based on it: Properties and applications

The metastable forms of aluminum oxide that exist in the range of 300–800°C are characterized; differences in the microstructures of homogeneous γ-, η-, and χ-Al2O3 are demonstrated; and the acid-base properties of the above modifications are compared. The catalytic properties of aluminum oxide in ethanol dehydration and propionitrile ammonolysis were studied. It was found that an increased surface concentration of Lewis acid sites, including strong acid sites (ν(CO) = 2237 cm−1), is required for preparing an effective catalyst for the dehydration of ethanol, whereas the rate of propionitrile conversion increased proportionally to the surface concentration of Bronsted acid sites. γ-Aluminum oxide was used to prepare catalysts for carbon monoxide oxidation. It was found that the supporting of Pd on γ-Al2O3 did not change the support structure. Palladium on the surface of γ-Al2O3-550 (Tcalcin = 550°C, SBET = 300 m2/g) occurred as single particles (2–3 nm) and aggregates (∼100 nm). The single particles were almost completely covered with a layer of aluminum oxide to form core-shell structures. According to XPS data, they were in atypical states (BE(Pd 3d5/2) = 336.0 and 338.0 eV), which were not reduced by hydrogen in the range of 15–450°C and were resistant to the action of the reaction mixture. Palladium on the surface of γ-Al2O3-800 (SBET = 160 m2/g) was in the states Pd0 and PdO, which are typical of Pd/Al2O3, and the proportions of these states can change under the action of the reaction mixture. An increase in the Tcalcin of the Pd/Al2O3(800)-450 catalyst from 450 to 800 → 1000 → 1200°C led to the agglomeration of palladium particles and to an increase in the temperature of 50% CO conversion from 145 to 152 → 169 → 189°C, respectively. α-Aluminum oxide was used in the preparation of an effective Mn-Bi-O/α-Al2O3 supported catalyst for the synthesis of nitrous oxide by the oxidation of ammonia with oxygen: the NH3 conversion was 95–97% at 84.4% N2O selectivity.

Bimetallic Pt–Pd/silica–alumina hydrotreating catalysts – Part I: Physicochemical characterization

Supported Pt–Pd catalysts were synthesized using the respective tetrammonium complexes as precursor salts. The materials used as carriers were silica and amorphous silica–alumina (ASA) with Al2O3/SiO2 wt.% ratio of 55/45, 20/80, and 5/95. The concentration of Lewis acid sites increased with the alumina content of support, whereas the concentration of Brønsted acid sites reached a maximum at 20wt.% of alumina. According to the chemical analysis, the content of Pt and Pd was close to 0.3 and 0.5wt.%, respectively, keeping the Pd/Pt molar ratio of 3. The average particle size slightly varied from 1.4 to 1.8nm (TEM). The analysis of the EXAFS and CO adsorption IR characterization revealed that the general morphology of the bimetallic clusters was Pt-rich core and Pd-enriched surface. The Pt atoms in the bimetallic clusters were electron deficient due to electron transfer to Pd (XANES). Furthermore, monometallic Pd clusters coexist with those Pt–Pd particles. The proportion of surface Pt increased (from 13.9% to 16.9%) with increasing alumina content in the support. The catalyst supported on ASA(55/45) exhibited the highest coverage of Pt. The varying surface metal composition is attributed to different interaction strengths between the metal precursors and the supports during the preparation steps.

Effect of Al content on hydrocracking of n-paraffin over Pt/SiO2–Al2O3

Various SiO2–Al2O3 supports with different Al contents were prepared using a co-precipitation method and employed with Pt catalysts in the hydrocracking of n-paraffin to middle distillate (C10–C20). The concentration of Lewis acid sites increased with the Al content of the support. The fraction of Brønsted acid sites depended on the Al content of the support and affected the conversion of n-paraffin and the yield of the middle distillate.

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.

Effect of the nature of the additives of metal cations (Sr, Ba, and La) on the properties of Co-Mo hydrodesulfurization catalysts

The effect of the nature of the support modified with the ions of alkaline earth and rare earth elements (Sr, Ba, and La) on the properties of Co-Mo catalysts for the hydrodesulfurization of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied using a set of physicochemical and catalytic techniques. It was found that the introduction of modifying additives decreased the surface concentration of Lewis acid sites (LASs) and increased the concentration of basic sites (BSs) in aluminum-containing supports; these changes were more significant upon modification with lanthanum and strontium. The modification affected the distribution and degree of dispersion of Co-Mo-S sulfide packets. It was found that the rate constants of DBT and 4-MDBT conversion increased with decreasing total surface concentration of both LASs and BSs on the support. The highest rate constant of 4,6-DMDBT conversion was reached at an optimum concentration of weak Lewis sites on Co-Mo catalysts, whose support was modified with strontium or barium.

Hydroisomerization of benzene-containing gasoline fractions on a Pt/SO 4 2− -ZrO2-Al2O3 catalyst: II. Effect of chemical composition on acidic and hydrogenating and the occurrence of model isomerization reactions

The acidic and hydrogenating of Pt/SO 4 2− -ZrO2-Al2O3 samples containing from 18.8 to 67.8 wt % Al2O3 as a support constituent were studied by the IR spectroscopy of adsorbed CO and pyridine, and the model reactions of n-heptane and cyclohexane isomerization on these catalysts were examined. The total catalyst activity in the conversion of n-heptane decreased with the concentration of Al2O3; this manifested itself in an increase in the temperature of 50% n-heptane conversion from 112 to 266°C and in an increase in the selectivity of isomerization to 94.2%. In this case, the maximum yield of isoheptanes was 47.1 wt %, which was reached on a sample whose support contained 67.8 wt % Al2O3. A maximum yield (69.6 wt %) and selectivity (93.7%) for methylcyclopentane formation from cyclohexane were also reached on the above catalyst sample. This can be explained by lower concentrations of Lewis and Bronsted acid sites in the Pt/SO 4 2− -ZrO2-Al2O3 system, as compared with those in Pt/SO 4 2− -ZrO2. The experimental results allowed us to make a preliminary conclusion that the Pt/SO 4 2− -ZrO2-Al2O3 catalyst whose support contains 67.8 wt % Al2O3 is promising for use in the selective hydroisomerization of benzene-containing gasoline fractions in the thermodynamically favorable process temperature range of 250–300°C.

Modifying the functional cover of the γ-Al2O3 surface using organic salts of aluminum

A method is suggested for modifying the surface properties of alumina without changing its chemical composition. The sorption of aluminum complexes with organic acid ligands on the γ-Al2O3 surface is reported. The thermal decomposition of the adsorbed oxalate complexes yields supported aluminum oxide compounds on the surface of the initial support. This modifies the functional cover of the γ-Al2O3 surface, altering the proportions of different types of surface hydroxyl groups, reducing their total number, and lowering the concentration of weak Lewis acid sites.

Design of solid acid catalysts for aqueous-phase dehydration of carbohydrates: The role of Lewis and Brønsted acid sites

We have prepared a series of well-characterized acid catalysts, including Zr–P, SiO2–Al2O3, WOX/ZrO2, γ-Al2O3, and HY zeolite and tested them for aqueous-phase dehydration of xylose. We have characterized the concentration of both Brønsted and Lewis acid sites in these catalysts with TPD and FT-IR spectroscopy using gas-phase NH3 and compared the catalytic activity and selectivity with that of homogeneous catalysts for the dehydration of aqueous solutions of xylose. The catalyst selectivity is a function of the Brønsted to Lewis acid site ratio for both the heterogeneous and homogeneous reactions. Lewis acid sites decrease furfural selectivity by catalyzing a side reaction between xylose and furfural to form humins (insoluble degradation products). At 20% xylose conversion, catalysts with high Brønsted to Lewis acid ratios, such as Zr–P, exhibit furfural selectivities as much as 30 times higher than catalysts with higher Lewis acid site concentrations. Dehydration reactions using ion-exchange polymer resins with high Brønsted acid site concentrations showed similar selectivities to Zr–P and HCl. Using HY zeolite revealed a low furfural selectivity due to strong irreversible adsorption of the furfural in the pores, causing an increase in the rate of humin formation. Thus, to design more efficient aqueous-phase dehydration catalysts, it is desirable to have a high ratio of Brønsted to Lewis acid sites. Furthermore, gas-phase characterization of acid sites can be used to predict catalytic activity in the aqueous phase.

Zeolites Promoting Quinoline Synthesis via Friedländer Reaction

Catalytic performance of zeolites H-BEA, H-MFI, H-FAU and H-MOR, exhibiting textural differences and with different Bronsted and Lewis acid sites concentration, have been studied in the synthesis of quinolines via Friedlander reaction. H-BEA and H-FAU efficiently promoted the condensation of 2-aminoaryl ketones 3 with ethyl acetoacetate (4a) or acetylacetone (4b) under mild reaction conditions, those being the first examples of zeolites as catalysts for this transformation. While H-FAU showed similar catalytic behaviour than that reported for (Al)SBA-15 affording mixtures of quinolines 5 and quinolones 6, H-BEA mainly led to quinolines 5 in almost total selectivity and good yields. However, H-MFI and H-MOR zeolites afforded quinolones 6 as the major reaction product. Methodology reported here was found to be useful for the synthesis of biologically active compounds with excellent yields avoiding unnecessary purifications protocols and tedious work-up procedures.

Effect of thermal treatment conditions on the acid properties of zeolite Beta

The acid properties of zeolite Beta were studied by IR spectroscopy with the use of adsorbed NH3 and CO probe molecules. It was found that the strength of the Bronsted acid sites (BASs) of zeolite Beta was the same as that of zeolite ZSM-5. Approximately a third of the total amount of BASs participated in the formation of hydrogen bonds at interdomain boundaries; thus, they were inaccessible to molecules larger than the ammonia molecule. The qualitative and quantitative compositions of acid sites in zeolite Beta can be regulated by changing the conditions of calcination of the initial zeolite form (temperature and/or gas atmosphere). Calcination under vacuum conditions afforded the highest acidity of the zeolite with respect to all types of acid sites. Calcination in a flow of air resulted in the lowest acidity of the sample, especially, with respect to the concentration of strong Lewis acid sites, because of the formation of an alumina phase. Calcination in an atmosphere of helium resulted in a decrease (as compared with a vacuum) in the rate of removal of the organic template decomposition products of the initial zeolite form from the channel volume. The resulting ethylene was mainly adsorbed at strong Lewis sites and converted into undesorbed condensation products.

Effects of sulfate species on V2O5/TiO2 SCR catalysts in coal and biomass-fired systems

Sulfation occurs when commercial vanadia SCR catalysts are exposed to SO2-laden coal combustion flue gases. Effects of sulfation on the surface chemistry of vanadia/titania catalysts and SCR activity have not been adequately addressed in previously published work. In this work, in situ FTIR spectroscopy and post situ XPS investigations were performed during vanadia/titania catalyst sulfation under simulated coal combustion flue gas conditions. In situ FTIR spectroscopy combined with XPS analyses on fresh and sulfated TiO2, 2% and 5% V2O5/TiO2 indicate that sulfate does not form on vanadia sites but rather on titania sites. FTIR spectroscopy data show that sulfation inhibits NO adsorption in the presence of oxygen, but greatly enhances NH3 adsorption by generating additional Brønsted acid sites while reducing the concentration of Lewis acid sites. Observed increases in the intrinsic NO reduction activity of sulfated vanadia/titania catalysts (relative to fresh catalysts) are consistent with spectroscopy data showing that the sulfation enhances NO reduction activity by increasing the number of active sites without changing the activation energy or site acid strength.

Alkaline Modification of MCM-22 to a 3D Interconnected Pore System and its Application in Toluene Disproportionation and Alkylation

Modification of HMCM-22 zeolite by alkaline treatment was investigated by various characterization techniques and in toluene disproportionation and alkylation with isopropyl alcohol. This ‘desilication’ process led for mild alkaline concentrations (~0.10–0.20 M NaOH at 323 K for 45 min) to the partial destruction of the zeolite framework, but also to the formation of additional mesoporosity. Furthermore, the accessibility/availability of Lewis acid sites, investigated by d 3-acetonitrile and pyridine adsorption using FTIR spectroscopy, increased for these mild alkaline treatments, while the Bronsted acidity decreased. Higher alkaline concentrations (up to 0.50 M NaOH) led to a too severe framework and pore destruction and a decrease of both the Lewis and Bronsted acid site concentration. Decomposition and deconvolution of 29Si MAS-NMR spectra confirmed the Si extraction and partial framework destruction, since more Q3 SiOH groups were formed at the expense of the Q4 T-atoms in the framework. Furthermore, the T6 and T7 Si-atoms were preferentially extracted, which would indicate that an interconnection between the intralayer and the interlayer and/or outer surface is formed. The toluene conversion in its disproportionation reaction increased for the mildly treated sample, while the selectivity to xylene isomers (and cymene and n-propyltoluene isomers in the alkylation reaction with isopropyl alcohol) was similar to the thermodynamic equilibrium, suggesting that the reaction primarily occurs at outer surface cups of the HMCM-22 zeolite.