Concentration Of Lewis Acid Sites Research Articles

Hierarchical Ga-MFI Catalysts for Propane Dehydrogenation

We report the synthesis, characterization, and enhanced propane dehydrogenation properties of hierarchical Ga-MFI zeolite catalysts synthesized by two different methods: (i) repetitive branching and (ii) utilization of a long chain alkyl SDA. Structural, compositional, and morphological characterizations confirm that the Ga-MFI catalyst materials have hierarchical structures including micropores and mesopores in a single particle and show that Si/Ga ratios comparable to bulk Ga-MFI catalysts can be obtained. Acid site analysis using NH3-TPD and pyridine adsorption followed by in situ IR spectroscopy allowed quantification of the number and types of acid sites present and show that the hierarchical catalysts have considerably higher Lewis acid site concentrations than the bulk catalysts. The hierarchical Ga-MFI catalysts show superior PDH performance (at 600 °C) compared to bulk Ga-MFI catalysts. Propane conversion rates are increased 2–6-fold and propylene selectivities are 10–100% higher. We also examine...

Study of the effect of isomorphic substitutions in the framework of zeolites with a Beta structure on their porosity and sorption characteristics

Zeolites of a Beta structure with different degrees of substitution of aluminum atoms by those of boron in the aluminum–silicon–oxygen framework have been investigated. The characteristics of the porosity-texture of the fabricated zeolites and the surface properties and sorption capacities of samples to a model drug substance (thiamine hydrochloride) have been examined. It has been demonstrated that isomorphic substitutions could affect the properties of the zeolite under study, whereas the largest effect is associated with the simultaneous presence of aluminum and boron atoms in the zeolite framework. Such samples are characterized with a larger specific area and pore diameters, a high concentration of Lewis acidic sites on the surface, and high adsorption capacity.

Relationship between Acid–Base Properties and the Activity of ZrO2‐Based Catalysts for the Cannizzaro Reaction of Pyruvaldehyde to Lactic Acid

AbstractThe Cannizzaro reaction of pyruvaldehyde to lactic acid is investigated in a flow reactor with ZrO2 catalysts with different structures and acid–base properties. The results show that a difference in the crystalline structures of two ZrO2 polymorphs strongly affects the conversion of pyruvaldehyde. The monoclinic phase of zirconia is the most active for this reaction. A good correlation is observed between the reaction rate and the concentration of Lewis acid sites of sufficient strength, which shows that these sites play a major role in the reaction. A reaction mechanism is proposed involving coordinatively unsaturated Zr4+ cations as sites for activating pyruvaldehyde molecules, whereas Zr4+−O2− pairs generate terminal OH groups through water dissociation.

Continuous-flow chemoselective reduction of cyclohexanone in a monolithic silica-supported Zr(OPri)4 multichannel microreactor

Meerwein-Ponndorf-Vereley reduction of cyclohexanone using 2-butanol as hydrogen donor was studied in the multichannel monolithic silica microreactors modified with zirconium species to demonstrate their higher efficacy than of conventional batch systems with powdered catalysts. Zr(OPri)4 species were attached onto the monoliths siliceous surface to obtain a nominal Zr/Si mass ratio in the range of 0.01–0.28. The value of 0.14 gave the largest concentration of Lewis acid sites and hence also the highest conversion (88%) after 20 min reaction/residence time and productivity of 2.22 mmol/g h. The monoliths prepared by the sol-gel process combined with phase separation were characterized using nitrogen adsorption, mercury porosimetry, thermogravimetry, SEM, FTIR and UV–Vis spectroscopy, pyridine adsorption and XRD. They featured very open 3D co-continuous structure of 20 nm textural meso- and flow-through macropores 30–60 μm in diameter (4 cm3/g total pore volume) which was not appreciably changed after their modification with zirconium species. Diffraction peaks characteristic for crystalline zirconia were not detected even in materials with the highest Zr content.

Solid frustrated-Lewis-pair catalysts constructed by regulations on surface defects of porous nanorods of CeO2

Identification on catalytic sites of heterogeneous catalysts at atomic level is important to understand catalytic mechanism. Surface engineering on defects of metal oxides can construct new active sites and regulate catalytic activity and selectivity. Here we outline the strategy by controlling surface defects of nanoceria to create the solid frustrated Lewis pair (FLP) metal oxide for efficient hydrogenation of alkenes and alkynes. Porous nanorods of ceria (PN-CeO2) with a high concentration of surface defects construct new Lewis acidic sites by two adjacent surface Ce3+. The neighbouring surface lattice oxygen as Lewis base and constructed Lewis acid create solid FLP site due to the rigid lattice of ceria, which can easily dissociate H–H bond with low activation energy of 0.17 eV.

Acid clay minerals as eco-friendly and cheap catalysts for the synthesis of β-amino ketones by Mannich reaction

The characterization and catalytic performance of different smectites, one raw saponite and some commercial montmorillonites, exhibiting mesoporous and acid character are herein reported. The catalysts under study are able to catalyze efficiently via Mannich-type reactions the synthesis of β-amino ketones, which are important building blocks for the drugs synthesis and biologically active natural products. Montmorillonite K10 (Mt-K10) was the most efficient catalyst for the Mannich reaction between the pre-formed imine, N-benzylideneaniline, and cyclohexanone resulting in a recyclable catalyst for at least three catalytic cycles without activity loss. Almost quantitative yield to the β-amino carbonyl compound was obtained after 240min when operating under the most favorable conditions. The catalytic performance of the smectites seemed to be influenced by their porosity, but also by both Brönsted and Lewis acid site concentrations. In fact, the concentration of the Lewis acid catalytic sites would be the key in the Michael addition of cyclohexanone to the imine, although the Brönsted acid sites contribution cannot be neglected. The β-amino carbonyl compound was also synthesized in an excellent yield (86%) in only 15min of reaction time through the one-pot three component reaction, by reaction between benzaldehyde, aniline and cyclohexanone.

Structural, Electronic, and Thermodynamic Properties of the T and B Phases of Niobia: First-Principle Calculations.

Different polymorphs of Nb2O5 can be obtained depending on the pressure and temperature of calcination leading to different catalytic properties. Two polymorphs of niobia, T-Nb2O5 and B-Nb2O5, have been investigated by means of density functional/plane waves method. The equation of state predicted that B-Nb2O5 phase is more stable than the T-Nb2O5 at low temperature; however at high pressure both phases are stable. These results are in good agreement with the available experimental data. The calculated cohesive energies of 6.63 and 6.59 eV·atom-1 for the B-Nb2O5 and T-Nb2O5, respectively, also corroborate this conclusion, and it can be compared to the experimental value of 9.56 eV atom-1 estimated for the most thermodynamically stable phase. The topological analyses based on quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) were applied and reveal bonds with large ionic character for both phases. The B-Nb2O5 presented larger stiffness than T-Nb2O5, and the oxygen sites in the T-Nb2O5 are more compressible. The density of states comparison for both structures indicates that B-Nb2O5 has lower concentration of acid sites compared to T-Nb2O5. This result is consistent with the experimental observations that the concentration of Lewis acid sites decreases with the temperature.

Methanol thiolation over Al2O3 and WS2 catalysts modified with cesium

Thiolation of methanol with H2S was studied on Al2O3, WS2/Al2O3, and the Cs-modified counterparts Cs/Al2O3, and Cs-WS2/Al2O3. On Cs-free catalysts, methanol reacts with similar rates to dimethyl ether and to methanethiol. Secondary steps yield dimethyl sulfide and trace amounts of dimethyl disulfide and methane. On Cs-containing catalysts, the dominating reaction is methanol thiolation. This drastic change in selectivity is related to the low strength and concentration of Lewis acid sites in the presence of Cs+. The active sites for condensation and thiolation of methanol are concluded to be Lewis or Brønsted acid-base pairs formed by dissociation of H2S on the Lewis sites. Thiolation and condensation follow Langmuir-Hinshelwood mechanisms with nucleophilic attack of SH or methoxy groups to adsorbed methanol as rate determining step. The formation of dimethyl ether on Cs+ Lewis acid sites is suppressed due to low coverage of methanol, whereas the rate of the nucleophilic attack of SH to methanethiol is enhanced.

Propane dehydrogenation catalyzed by gallosilicate MFI zeolites with perturbed acidity

The propane dehydrogenation (PDH) behavior of gallosilicate MFI catalysts synthesized in the presence of 3-mercaptopropyl-trimethoxysilane (MPS) is investigated. MPS addition to the zeolite synthesis gel results in gallosilicate molecular sieves with reduced Brønsted acidity and enhanced Lewis acidity, as shown by combined NH3 temperature-programmed desorption (TPD), isopropylamine (IPA) TPD and IR analysis of pyridine-loaded catalysts. In particular, the gallosilicate MFI catalysts prepared using MPS provide a significant concentration of strong Lewis acid sites, which are important in controlling the selectivity of PDH. Enhanced PDH performance with higher propane conversion rates and improved propylene selectivity with limited cracking and aromatization products are obtained from the catalysts synthesized with MPS. The gallosilicate MFI materials prepared using MPS are compared with a benchmark chromia-alumina catalyst, and the gallosilicate materials have a lower rate of deactivation (>50% lower) but slightly inferior propylene selectivity (Ga-MFI: 75%; Chromia-Alumina: 85%).

Effect of Fe on the activity of Au/FeO x -TiO2 catalysts for CO oxidation

A thorough characterisation of Au/TiO2 and Au/FeOx-TiO2 catalysts was conducted in order to get a better understanding of the effect of Fe on the Au/TiO2 catalysts and link it to differences observed in their activities for CO oxidation. Several techniques including HRTEM, temperature-programmed reduction (TPR), UV-Vis and temperature-programmed desorption (TPD) of isopropyl amine, CO and CO2 were used to characterise the catalysts. Au/FeOx-TiO2 (300 °C), which was found to be the most active catalyst for CO oxidation, had the highest number of Bronsted acid sites, although its concentration of Lewis acid sites was similar to all other tested catalyst systems. X-ray photoelectron spectroscopy (XPS) revealed that the Fe species on the Au/FeOx-TiO2 catalyst is Fe2+ with a very small amount of Fe3+. Fe2+ is comprised of both FeO and Fe3O4 species, and according to TPR, the ratio (FeO/Fe3O4) between these two species increases with increasing calcination temperature. The presence of Fe on the Au/TiO2 catalyst seems to stabilise the Au nanoparticles from agglomeration. The activation energy of desorption (Ed) of CO from Au/FeOx-TiO2 (300 °C) was 82.7 kJ/mol, whilst on Au/TiO2, the Ed for CO was 108.8 kJ/mol. On both catalysts, the Ed for CO2 was ca. 99 kJ/mol.

Cation–anion double hydrolysis derived mesoporous mixed oxides for reactive adsorption desulfurization

Mesoporous ZnO–Al2O3 mixed oxides (MO) were synthesized by double hydrolysis method at different temperatures and used as the support for the preparation of Ni/MO adsorbents. The reactive adsorption desulfurization (RADS) performance of adsorbents was evaluated in a fixed bed microreactor using thiophene as a model compound. The adsorbents before and after RADS were characterized by X-ray diffraction, N2 adsorption/desorption, thermogravimetric analysis, Fourier transformed infrared spectrometry and transmission electron microscopy techniques. Results show that Ni/MO samples exhibited much higher RADS activity and larger accumulative sulfur capacity than sample NZA-K prepared using the conventional kneading method. The desulfurization activity of Ni/MO adsorbents decreased with increasing the crystallization temperature of MO. As a result, sample Ni/ZnO–Al2O3-60 °C synthesized at 60 °C showed the best desulfurization performance among all Ni/MO adsorbents. Detailed characterization results revealed that the high dispersion of NiO and ZnO, the absence of inactive NiAl2O4 and high concentration of surface Lewis acid sites may account for the superior RADS performance of Ni/MOs samples. Furthermore, based on the experimental results, a mechanism is proposed for the RADS process with Ni/MO adsorbents.

Influence of partial dealumination of BEA zeolites on physicochemical and catalytic properties of AgAlSiBEA in H2-promoted SCR of NO with ethanol

The two-step postsynthesis method allows obtaining AlSiBEA zeolites with different degree of dealumination (Si/Al = 100 and 200). Physicochemical properties (crystallinity, hydroxyl group coverage, acidic sites and nature of silver species) of AlSiBEA and AgAlSiBEA were investigated by XRD, DR UV–vis, XPS, TEM and FT-IR with CO and pyridine as probe molecules. Catalytic properties of the zeolites were studied in the process of selective reduction of NO with ethanol in the presence of hydrogen in the reaction mixture. It was shown that level of H2-promoting effect on the SCR-process depends on the dealumination degree of Ag-containing BEA zeolites and greater effect is observed for the catalysts with higher concentration of Lewis acidic sites.

A novel preparation method of sulfonic functionalized silica: CH3SO3H as the sulfonic source and Al as the bridge

A novel approach is designed to optimize the synthesis of sulfonic-functionalized silica material. Results from 29Si and 27Al NMR suggest that the Al acts as the bridging atom connecting the methanesulfonate and silica matrix. Further pyridine-FTIR spectra followed by catalytic activity tests demonstrate that compared with previous methods, our new approach results in higher Lewis acid site concentration, higher thermal stability and superior catalytic activity. Moreover, the whole catalysis preparation procedure is environmentally friendly. Specifically, the silica matrix is synthesized through hydrolysis of tetraethylorthosilicate employing formic acid as hydro-catalyst, in which no surfactant species or precursors were involved.

Gas-Phase Oxidation of Glycerol to Dihydroxyacetone over Tailored Iron Zeolites

A novel chemocatalytic technology for the valorization of glycerol (GLY) is presented, where the continuous gas-phase oxidation to dihydroxyacetone (DHA) is accomplished over iron-containing zeolites in the presence of molecular oxygen. The catalyst design elucidated the impact of acidic properties and iron speciation on the performance. For this purpose, a series of catalysts displaying a zeolitic or amorphous structure, different Bronsted and Lewis acid site concentrations and strengths, and distinct iron species were synthesized, characterized by a multitechnique approach, and evaluated in a fixed-bed reactor. Fe-silicalite prepared by isomorphous substitution of iron in the all-silica framework followed by steam activation at 873 K exhibits very mild acidity and highly dispersed iron species in the form of isolated cations or small FeOx clusters in extra framework positions, leading to a stable DHA yield of ca. 90%. In contrast, impregnated or hydrothermally prepared and steamed aluminum-containing ca...

Tailoring acidity of HZSM-5 nanoparticles for methyl bromide dehydrobromination by Al and Mg incorporation.

Three kinds of HZSM-5 nanoparticles with different acidity were tailored by impregnating MgO or varying Si/Al ratios. Both the textural and acidic properties of the as-prepared nanoparticles were characterized by nitrogen adsorption-desorption measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), ammonia temperature-programmed desorption (NH3-TPD) and Fourier transform infrared spectroscopy (FTIR or Py-FTIR). It was found that the intensity of Lewis acid sites with weak strength was enhanced by impregnating MgO or reducing Al concentration, and such an enhancement could be explained by the formation of Mg(OH)+ or charge unbalance of the MgO framework on the surface of HZSM-5 support. The effect of HZSM-5 nanoparticles' acidity on methyl bromide dehydrobromination as catalyst was evaluated. As the results, MgHZ-360 catalyst with the highest concentration of Lewis acid sites showed excellent stability, which maintained methyl bromide conversion of up 97% in a period of 400 h on stream. Coke characterization by BET measurements and TGA/DTA and GC/MS analysis revealed that polymethylated naphthalenes species were formed outside the channels of the catalyst with higher acid intensity and higher Brønsted acid concentration during the initial period of reaction, while graphitic carbon formed in the channels of catalyst with lower acid intensity and higher Lewis acid concentration during the stable stage.

Influence of Montmorillonite K10 Modification with Tungstophosphoric Acid on Hybrid Catalyst Activity in Direct Dimethyl Ether Synthesis from Syngas

Dimethyl ether synthesis was performed from syngas over hybrid catalysts comprising a Cu/Zn/Al as metallic function and K10-montorillonite (K10) as acidic function. The acidic properties of K10 were tuned by deposition of one monolayer of tungstophosphoric acid (H3PW12O40·23.5 H2O, TPA). The changes in acidity were determined based on pyridine adsorption with FTIR detection. Due to the fact that TPA does not possess Lewis acid sites, deposition of TPA on the K10 surface (TPA-K10) resulted only in increase in Brønsted acid site concentrations and in increase in their strength in comparison with unmodified K10. Since both modified and unmodified K10 exhibited the same Lewis acid site concentrations and their strength, it was possible to investigate solely the impact of Brønsted acidity differences of acidic functions on the hybrid catalysts activity in syngas-to-DME process (STD). Additionally, the effect of TPA supported on K10 was investigated in methanol dehydration to DME under atmospheric pressure. In order to prepare metallic functions which differed in activity in CO hydrogenation to methanol (first step of STD), two synthesis methods were used: co-precipitation method (metallic function CZA) and decomposition of citrate complexes of metals (metallic function CZAcitric). The hybrid catalysts (CZA/K10, CZA/TPA-K10, CZAcitric/K10, CZAcitric/TPA-K10) for STD were prepared by physical mixing of metallic and acidic function in volume ratio equal to 2:1. The impact of K10 substitution with TPA-10 in hybrid catalysts depended on whether STD process was controlled by methanol synthesis step or dehydration methanol step. When K10 possessed adequate acidity to dehydrate methanol formed on metallic function (i.e. CZAcitric), it substitution with TPA-K10 in hybrid catalyst did not improve DME yield. On the other hand, when metallic function (i.e. CZA) exhibited higher methanol activity and methanol dehydration rate was limited by insufficient acidity of K10 then usage of TPA-K10 of higher acidity in hybrid catalyst was found to increase significantly DME yield without light parrafins formation.

Photoisomerization of n-pentane over nano ZnO/MoO3-ZrO2

A series of nano ZnO/MoO3-ZrO2 catalysts with different ZnO loading (1.0, 2.5, 5.0 wt%) were prepared by impregnation method for n-pentane photoisomerization under hydrogen or nitrogen atmosphere. The properties of the catalysts were characterized with X-ray Diffraction (XRD), Brunauer Emmett Teller (BET), Transmission Electron microscope (TEM) and FTIR. The XRD result showed that the fraction of tetragonal phase of ZnO/MoO3-ZrO2 was about 0.67 for all samples. While, the specific BET surface area was about 24 m2/g. Pyridine adsorbed FTIR results showed that all samples possessed high concentration of strong Lewis acid sites and small concentration of weak Bronsted acid sites. The interaction of hydrogen and surface samples at 298-523 K formed protonic acid sites with the concomitant of the partial elimination of Lewis acid sites. Whereas no changes of the concentration of acid sites were observed in the presence of nitrogen atmosphere. The activity of all samples in the n-pentane photoisomerization was strongly determined by the presence of hydrogen gas. In fact no activity was observed in the absence of hydrogen________________________________________GRAPHICAL ABSTRACT

Hydroisomerization and hydrocracking over platinum loaded ZSM-23 catalysts in the presence of sulfur and nitrogen compounds for the dewaxing of diesel fuel

Hydroisomerization and hydrocracking of n-hexadecane in the presence of sulfur and nitrogen compound were investigated over four different platinum loaded ZSM-23 catalysts (Pt/ZSM-23, Pt–Pd/ZSM-23, Pt–Mg/ZSM-23 and Pt–Pd–Mg/ZSM-23). Sulfur and nitrogen compounds could poison metal and acid sites of bifunctional catalysts, respectively. In this study, effort has been made to improve the sulfur and nitrogen tolerance in the conversion of n-hexadecane by introducing palladium and magnesium as additives. Pt–Pd bimetallic catalysts increased the electron-deficiency of metal sites, leading to the improvement of sulfur tolerance. The addition of magnesium on platinum loaded ZSM-23 increased the concentration of Lewis acid sites and thereby the poisoning of nitrogen compound was discouraged. The applicability of Pt–Pd–Mg/ZSM-23 catalyst for dewaxing process was further examined using diesel feeds with sulfur compound such as dimethyl disulfide in a high pressure pilot plant. Pt–Pd–Mg/ZSM-23 catalyst showed good catalytic performance for dewaxing process in terms of diesel product yield and long-term stability.

Impact of the local environment of Brønsted acid sites in ZSM-5 on the catalytic activity in n-pentane cracking

The impact of the zeolite Brønsted and Lewis acid site concentration on the catalytic cracking of alkanes was explored using n-pentane and H-ZSM-5 as examples. Rates normalized to strong Brønsted acid sites (i.e., the turnover frequencies, TOF) showed that the two samples with the highest Al content had much higher TOF than all other samples. This difference has been unequivocally linked to the presence of extra-lattice alumina. Post-treatment of the zeolites with ammonium hexafluorosilicate and static calcination was used to vary the concentration of extra-lattice alumina. After extraction of extra-lattice alumina from the samples with high TOF, all TOFs were identical. IR spectra of adsorbed pyridine and NH3, coupled with 27Al MAS NMR, showed that the overall enhanced activity is associated with tetrahedrally coordinated extra-lattice alumina in close proximity to strong Brønsted acid sites. The TOF of these sites is approximately 40-times higher than the TOF on normal Brønsted acid sites.

Acid site properties of thermally stable, silica-doped alumina as a function of silica/alumina ratio and calcination temperature

Acid site properties of silica-doped aluminas prepared by a simple solvent deficient hydrolysis of the alkoxides was investigated. The total acid concentration (Brønsted and Lewis sites) of silica-doped aluminas (SDAs) calcined in the range of 700–1200°C with Si/Al ratios of 5, 15, 27wt% was determined using temperature-programmed desorption of ammonia (ammonia-TPD). 27Al solid state MAS NMR (Al SS MAS NMR) was used to measure the intrinsic Lewis acid site concentration, and FTIR was also used as a separate measure of the Brønsted and Lewis acid site concentration. Results indicate that removing hydroxyl groups in the form of water molecules through calcination result in a lower concentration of Brønsted acid sites. Calcination at higher temperature also results in the transformation of unsaturated 5-coordinated aluminum (a strong Lewis acid) to higher concentrations of 6 and 4-coordinated aluminum in 5, 15 and 27% silica-doped alumina samples. Therefore, the total acid site concentration (Brønsted and Lewis sites) decreases by increasing the calcination temperature. In addition, the data show that increasing the silica/alumina ratio increases both the Brønsted and Lewis acid site concentrations. Based on these results, the acid site concentrations can be controlled by altering the Si/Al ratio and calcination temperature while maintaining high surface areas, large pore volumes, and large pore diameters.