Concentration Of Lewis Acid Sites Research Articles

Isomerization of α-pinene with a hierarchical mordenite molecular sieve prepared by the microwave assisted alkaline treatment

Abstract To develop effective catalysts for α-pinene isomerization reaction is important to get high valuable products. A hierarchical mordenite molecular sieve is prepared using alkaline treatment assisted by the microwave radiation. After the alkaline treatment, the relative crystalline decreases by ~20% and mesopores are introduced in the mordenite molecular sieve. Compared to conventional heating method, microwave assisted alkaline treatment exhibits a selective etching of zeolitic silicon atoms. Aluminum atoms adjacent to silicon atoms are extracted to form Al–OH hydroxyl groups. These extracted aluminum atoms are reconnected to zeolitic silicon atoms. Mordenite after microwave assisted alkaline treatment has a good dispersion of mesopores, higher retention of BrOnsted acid sites in 12-membered rings, and lower concentration of Lewis acid sites. The isomerization reaction of α-pinene suggests that alkaline treatment increases the accessibility of acid sites for α-pinene molecules. The hierarchical porosity increases the conversion of α-pinene from 46.1 to 94.7%. The yields for camphene and limonene increase by 17.0 and 5.4%, respectively. Disproportionation of limonene is inhibited when using microwave assisted alkaline treated mordenite due to a low concentration of Lewis acid sites. These results are important to propose a facile post modification method to realize a selective extraction of zeolitic framework atoms.

Spectroscopic studies of MFI and USY zeolite layers over stainless steel 316L wire gauze meshes.

The objective of our study was to develop and optimize the in situ synthesis of zeolitic thin coatings with USY (ultrastabilised form of faujasite) and MFI (Model Five) type structure on metallic structured catalysts supports using the hydrothermal method. Thus, obtained zeolitic materials were studied in terms of their prospective activity in selective catalytic reduction of nitrogen oxides (SCR of NOx) with ammonia. Optimization of the preparation method consisted of several steps including: the pretreatment of steel carrier to obtain an adhesive surface, hydrothermal synthesis of zeolites at different conditions and adjustment of the zeolite structure type (MFI vs. USY). As a result, uniform zeolitic layers were deposited on steel supports. Prepared structured supports were ion-exchanged with copper or cobalt precursors to obtain active catalysts and then characterised by various physicochemical methods with a particular reference to the in situ Fourier-Transform Infrared Spectroscopy (FTIR), Ultraviolet-Visible Diffusion Reflectance Spectroscopy (DRS-UV/VIS) and Raman spectroscopy. For CuUSY sample, slightly better catalytic properties are related to higher copper content. In the case of Co-samples, worse catalytic properties in comparison with Cu counterparts might imply from higher concentration of Brønsted acid sites, lower cobalt loading (thus concentration of Lewis acid sites) and the presence of cobalt cation significantly in oxide form (evidenced by Raman, DRS-UV/VIS spectroscopy and by in situ FT-IR sorption studies).

New synthesis routes and catalytic applications of ferrierite crystals. Part 2: The effect of OSDA type on zeolite properties and catalysis

Tailoring of physicochemical properties of synthetic zeolites is of paramount importance when these materials are applied as heterogenous catalysts. In this work, the effect of the synthesis system and in particular of the organic structure directing agent (OSDA) on acidity, crystal morphology, and catalytic behaviour of FER-type materials is studied. FER-type zeolites with similar Si/Al ratio and crystal size range 0.3–15 μm are synthesised by changing OSDA type or by adding surfactant. A crystallization kinetic analysis shows that the stability of the zeolitic phase strongly depends on the OSDA type. In particular, the crystallization time strongly affects the purity of the FER phase when tetrahydrofuran is used as a template. Furthermore, it is found that long amines as 1,8-diaminooctane can play a role of OSDA obtaining highly crystalline FER-type crystals and with a high Lewis acid sites concentration. FT-IR and 27Al-NMR analyses show that all of the synthesised samples possess different Lewis acid sites concentration associated with extra-framework Al–OH species that are not detected by NMR analysis. Catalytic tests in methanol-to-dimethyl ether reaction reveal that synthesised samples exhibit different catalytic behaviour. In particular, large (10–15 μm) FER-type catalyst synthesised with tetrahydrofuran suffer deactivation caused by a higher carbon deposit level. On the contrary, nano-sized FER-type crystals, prepared by using pyrrolidine as OSDA and a surfactant, exhibits high selectivity and considerable resistance to coke deposition.

Characterization of Acid-Modified Alumina as a Support for Reforming Catalysts

The textural and acid–base characteristics of initial alumina and alumina modified with acetic acid at different concentrations (from 0.83 to 17.4 mol/L) were studied. It was found that treatment with acetic acid did not influence on the texture characteristics and phase composition of the supports. According to the IR-spectroscopic data on adsorbed CO, an increase in the concentration of medium-strength Lewis acid sites and an increase in the total acidity of the modified supports were observed, as compared with those of the initial alumina. Among the Pt‒Re systems, a catalyst based on the support treated with acetic acid at a concentration of 1.67 mol/L exhibited the highest activity in n-heptane reforming due to an optimal composition of the acid sites of the support.

Quantification of Lewis acid sites in 3D and 2D TS-1 zeolites: FTIR spectroscopic study

Abstract Titanium silicalite 1 (TS-1) zeolite is an important selective oxidation catalyst. Recently prepared layered and pillared forms expanded the application of TS-1 to the catalytic oxidation of bulky molecules. Despite progress in designing and application of titanosilicate zeolites in catalysis, only qualitative information of their acidity is available. Herein, we report thorough characterization of acid sites in TS-1 zeolites of different morphologies (3D TS-1, layered 2D TS-1, 2D TS-1 pillared either with silica (TS-1-PISi) or silica-titania (TS-1-PITi)) using FTIR spectroscopy and probe molecules. FTIR of adsorbed pyridine was used for quantification of Ti-associated Lewis acid sites based on the integral intensity of υ8a absorption band ca. 1608 cm−1 and measured integrated molar extinction coefficient (e1608(Ti-LAS) = 0.71 ± 0.01 cm μmol–1). Thermodesorption of pyridine monitored with FTIR showed that distribution of strength of Lewis acid sites is, to some extent, dependent on the way of Ti incorporation in the samples. TS-1-PITi, containing large fraction of external surface Ti(OH)(OSi)3 species introduced post-synthesis, showed increased concentration of stronger Lewis acid centres. FTIR spectroscopy of TS-1 with pre-adsorbed quinoline and d3-acetonitrile probe confirmed enhanced relative concentration of external Lewis acid sites in all layered TS-1 materials (28–38%) vs. 3D TS-1 (2%).

Prins cyclization of (-)-isopulegol with benzaldehyde for production of chromenols over organosulfonic clays

Prins cyclization of (-)-isopulegol with benzaldehyde was investigated with sulphur containing halloysite nanotubes, K10 clays and Cloisite clays. The catalysts were characterized by TEM, SEM, XRD, XPS, pyridine adsorption desorption by FTIR, nitrogen adsorption. The most active catalyst was K10 modified with chlorosulphonic acid giving 95% selectivity to the desired chromenol at 90% conversion and 30 °C. Selectivity to chromenol was increasing with increasing Lewis acid site concentration while no linear correlation of chromenol selectivity with the Brønsted acid sites concentration was observed. The diastereoisomer R/S ratio of chromenols was the highest, 11.5, at 70 °C and 90% conversion over sulphur modified halloysite nanotubes exhibiting rather low acidity. A higher stability of the R-isomer confirmed by quantum mechanical calculations can partially explain preferential formation of the R diastereomer.Successful catalyst reuse was demonstrated with the best performing catalyst, organosilylated sulphur containing K10.

Monolithic microreactors of different structure as an effective tool for in flow MPV reaction

Silica monolithic microreactors of the same topology of the skeleton but different pore sizes were functionalized with zirconium isopropoxide (Zr/Si = 0.14), and they were studied in Meerwein-Ponndorf-Verley (MPV) reduction of cyclohexanone using n-butanol as a hydrogen donor. It was shown that the concentration of Lewis acid sites was proportional to the value of the specific surface area, and this directly translated into the higher reaction rate, larger TOF-value and productivity. The monolith with the largest surface area, 625 m2/g was characterized by the small flow-through pores (2 µm), and thus very high flow resistance. Catalytic studies confirmed the first order kinetics of MPV reduction, and that the overall/observed reaction rate depends on the rate of the intrinsic chemical reaction and internal diffusion. Moreover, the higher efficiency of zirconium functionalized mesoporous catalysts over homogeneous zirconium isopropoxide has been demonstrated in batch process.

Ethanol-to-ethylene dehydration on acid-modified ring-shaped alumina catalyst in a tubular reactor

New results of endothermic dehydration of undiluted ethanol on alumina catalyst in pilot wall-heated tubular reactor are presented. Activity and physicochemical characteristics of the proprietary acid-modified alumina (AMA) prepared by gibbsite flash calcination in TSEFLAR™ reactor were compared to the properties of commercially available alumina samples prepared by precipitation (CS1) or flash calcination of gibbsite in a flue gas (CS2). The samples differed in phase composition, which corresponds to γ-Al2O3 in CS1, mixed γ- and χ-Al2O3 in AMA and CS2; sodium content and concentration of Lewis acid sites (LAS) were different as well. Acid modification resulted in AMA catalyst with improved acidic properties and catalytic activity higher than that of CS2, despite the lower sodium content in the latter. Ethanol-to-ethylene dehydration in the wall-heated tubular reactor proceeded at the catalyst bed temperatures mainly below 385 °C, where AMA catalyst is the most active due to the highest content of strong LAS. The heat-agent temperature favors yield of ethylene rather than that of by-products. Higher linear velocity and porosity contribute to a flatter axial temperature profile in the bed; a higher average integral temperature gives a quantitative estimate. This ensures higher ethylene yield with a lower ethanol consumption even at a reduced heat-agent temperature. On the ring-shaped AMA catalyst, the greater ethylene yield, catalyst productivity and sharply reduced hydraulic resistance are reached compared to CS2 cylinders. For the first time we showed that use of AMA ring-shaped catalyst in wall-heated tubular reactor in contrast to the SynDol alumina based catalyst in adiabatic reactor can improve the catalyst productivity by 5–8 times and WHSV by 5–12 times.

The ZSM-5-Catalyzed Oxidation of Benzene to Phenol with N2O: Effect of Lewis Acid Sites

The oxidation of benzene to phenol (BTOP) with N2O as the oxidant has been studied with a variety of Fe/ZSM-5 catalysts. The literature has conclusively proven that Fe2+ sites are the active sites. However, some studies have suggested that the Lewis acidic sites (LAS) are responsible for the generation of the active chemisorbed oxygen. Nevertheless, there is no clear relationship between the LAS and the N2O selectivity to phenol. In an effort to elucidate the effects of LAS on BTOP with various ZSM-5 catalysts, we investigated the initial N2O selectivity to phenol. Here we show that the initial N2O selectivity to phenol is negative with the amount of LAS over a certain range. The catalyst H-ZSM-5-ST (H-ZSM-5 treated with water vapor) showed a remarkable initial N2O selectivity to phenol as high as 95.9% with a 0.021 mmol g−1 LAS concentration on the surface of the catalyst, while the Fe/ZSM-5 catalyst demonstrated the lowest initial N2O selectivity to phenol (11.7%) with the highest LAS concentration (0.137 mmol g−1). Another remarkable feature is that steaming was more effective than Fe ion exchange and high temperature calcining. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), N2-adsorption-desorption, UV-vis, NH3-TPD and pyridine Fourier transform infrared (FT-IR) techniques. Our results demonstrate how the concentration of LAS is likely to affect the initial N2O selectivity to phenol within a certain range (0.021–0.137 mmol g−1). This research has demonstrated the synergy between the active Fe2+ sites and LAS.

Acid properties of M-SBA-15 and M-SBA-15-SO3H (M = Al, Ti) materials and their role on esterification of oleic acid

Abstract

Effect of acidity and porosity of hierarchical HBEA zeolite on catalytic stability of α-methylnaphthalene isomerization

Hierarchical HBEA catalysts were fabricated through alkaline treatment and evaluated in a fixed-bed reactor. The relative crystallinity, pore property, Si/Al molar ratio, coke formation and structure of catalysts were investigated by the methods including X-ray diffraction, N2 physisorption, X-ray fluorescence, thermogravimetric analysis and transmission electron microscope, respectively. The acidity of catalysts was studied by Fourier transform infrared spectrometry using pyridine as the probe molecule. These results demonstrated that pure NaOH treatment improved the mesoporosity of catalyst, while decreased the β-methylnaphthalene selectivity and catalyst stability due to the partially destroyed framework and an increase in the concentration of Lewis acid sites. The Lewis acid sites are responsible for the hydride transfer and subsequent reaction that form coke. In contrast, the presence of tetra-propyl-ammonium hydroxide in the mixed-alkali treatment resulted in regular mesopores development, preserved intrinsic zeolite properties and Lewis sites proportion, leading to the maintained β-MN selectivity and coke deposition rate. The increased mesoporosity and strong Bronsted acid sites amount per number of coke were responsible for the improved coke tolerance of mixed-alkali treated HBEA, and thus its deactivation rate was 41.7% lower than that of HBEA catalyst.

Effect of support on the acidity of NiMo/Al2O3-MgO and NiMo/TiO2-Al2O3 catalysts and on the resulting competitive hydrodesulfurization/hydrodenitrogenation reactions

The influence of Al2O3, MgO-Al2O3, and TiO2-Al2O3 supports on the activity of NiMoS catalysts was evaluated for the competitive hydrodesulfurization/hydrodenitrogenation (HDS/HDN) reactions using dibenzothiophene and quinoline as model compounds. Nitrogen physisorption results show that the Al2O3-MgO and TiO2-Al2O3 supports present specific surface areas of 362 m2/g and 170 m2/g, respectively. Also, Al2O3-MgO and TiO2-Al2O3 supports have net surface pH values respectively of 9.0 and 4.9. The supports were impregnated with Ni/Mo aqueous precursor solutions and characterized at their oxide state using UV–vis diffuse reflectance and Raman spectroscopies after drying and calcination steps. Results show that the NiMo/MgO-Al2O3 catalyst presents mainly Ni2+Oh/MoO42− species while Ni2+Oh/Mo7O246−/Mo8O264− species are found on NiMo/TiO2-Al2O3 catalyst. NiMo catalysts supported on Al2O3-MgO and TiO2-Al2O3 were characterized at the sulfide state mainly by infrared spectroscopy using pyridine as a probe. In this respect, the NiMo catalyst supported on Al2O3 shows the highest concentration of Lewis acid sites of all catalysts series. Finally, the sulfide catalysts were evaluated in the HDS/HDN competitive reaction using dibenzothiophene and quinoline compounds. The highest DBT HDS activity for the NiMo/Al2O3 catalyst was associated with Lewis acid properties. However, the HDS inhibition caused by quinoline addition depends on the nature of the support. The NiMo/MgO-Al2O3 catalyst exhibits lower inhibition than NiMo/Al2O3 and NiMo/TiO2-Al2O3 catalysts. This behavior was related to the fact that the pyridine adsorption enthalpy depends on the support nature. The NiMo catalyst supported on MgO-Al2O3 requires more energy to adsorb pyridine than the catalysts supported on Al2O3 or TiO2-Al2O3.

Hierarchical Zr‐MTW Zeolites Doped with Copper as Catalysts of Ethanol Conversion into 1,3‐Butadiene

AbstractThe hierarchical Zr‐MTW zeolites are prepared using NH4F and HF as mineralizing agents and structure‐directed agent like Gemini‐type surfactant and studied by X‐ray diffraction (XRD), 29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR), Fourier transform infrared (FTIR) spectroscopy with pyridine, 2,6‐di‐tert‐butylpyridine and CD3CN as probe molecules. Structural characteristics and acid‐base properties of the zeolites depend on concentration of fluorine precursor in the reaction medium (a ratio of Si/F). The hierarchical Zr‐MTW zeolites (included doped with copper) have been tested in ethanol conversion into 1,3‐butadiene by Lebedev method. The symbatical dependence of 1,3‐butadiene productivity with the concentration of Lewis acidic sites determined by FTIR spectroscopy of adsorbed CD3CN is observed. 1.3‐Butadiene selectivity of 68 % with ethanol conversion of 81 % are achieved in the presence of Cu/Zr‐MTW catalyst.

Selective catalytic reduction of NOx by NH3 at low temperature over manganese oxide catalysts supported on titanate nanotubes

We report the NO conversion with NH3 at low temperature over MnOx-supported on titanate nanotubes. The effect of SO2 and water on catalytic activity was also analyzed. For the catalytic activity tests, three catalysts with 1, 3, and 5 wt.% of MnOx were synthesized. A high NO conversion (92%) at 180 °C is reported for the 1Mn/NTiO2 catalyst. The further addition of MnOx to the support improves the catalytic activity but NO conversion (95%) was shifted to a higher temperature (from 180 to 220 °C). However, the presence of SO2 (50 ppm) and water (5 vol.%) diminishes the NO conversion up to 60%, which remains after 300 min. We found that addition of MnOx increases the Lewis acid sites concentration of the nanotubes. As the Lewis acid sites increase (from 1.86 to 3.56 µmol m−2), the Mn4+/Mn3+ ratio on the surface of the nanotubes decreases (from 4 to 1.6), which indicates that the surface of the catalysts is deficient in electrons. We concluded that a high Lewis acid sites concentration and a low Mn4+/Mn3+ ratio, hence a surface deficient in electrons, improves the catalytic activity to remove NO on the Mn/NTiO2 catalysts.

Effect of the SiO2 support on the catalytic performance of Ag/ZrO2/SiO2 catalysts for the single-bed production of butadiene from ethanol

A ternary Ag/ZrO2/SiO2 catalyst system was studied for single-step conversion of ethanol to butadiene by varying the catalyst composition (Ag, Ir, or Pt metal component, Ag/ZrO2 loading, and choice of SiO2 support) and operating conditions (space velocity and feed gas composition). Exceptional catalytic performance was achieved over a 1%Ag/4%ZrO2/SiO2-SBA-16 catalyst leading to 99% conversion and 71% butadiene selectivity while operating under mild conditions (325 °C, 1 atm, and 0.23 h−1). Several classes of silica—silica gels, fumed silicas, mesoporous silicas)—were evaluated as catalyst supports, and SBA-16 was found to be the most promising choice. The SiO2 support was found to significantly influence both conversion and selectivity. A higher SiO2 catalyst surface area facilitates increased Ag dispersion which leads to greater conversion due to the accelerated initial ethanol dehydrogenation reaction step. By independently varying Ag and ZrO2 loading, Ag was found to be the main component that affects ethanol conversion. ZrO2 loading and thus Lewis acid sites concentration was found to have little impact on the ethanol conversion. Butadiene selectivity depends on the concentration of Lewis acid site, which in turn differs depending on the choice of SiO2 support material. We observed a direct relationship between butadiene selectivity and concentration of Lewis acid sites. Butadiene selectivity decreases as the concentration of Lewis acid sites increases, which corresponds to an increase in ethanol dehydration to ethylene and diethyl ether. Additionally, adding H2 to the feed had little effect on conversion while improving catalytic stability; however, selectivity to butadiene decreased. Finally, catalyst regenerability was successfully demonstrated for several cycles.

Concretes Using Modified Mineral Extenders

The possibility of modification of mineral extenders with a specific surface of 350 m2/kg with ultraviolet irradiation is considered. Infrared spectroscopy confirmed that under the influence of ultraviolet irradiation, the bonds of OH-groups with SiO2 tetrahedra are weakened, that contributes to the formation of extenders of the acid centers of Lewis and Bronsted on the surface. The optimum time of ultraviolet exposure is determined, at which the concentration of active Lewis and Bronsted acid sites on the extenders surface increases by more than 100%. It was found that replacing 10% of cement mass with a modified extenders makes it possible to produce concretes with strength values 20-30% higher than for composites without extenders. Obtaining equal strength concrete is possible with replacing 20% of cement with modified extenders.

Synthesis of 1,3-Diols from Isobutene and HCHO via Prins Condensation-Hydrolysis Using CeO2 Catalysts: Effects of Crystal Plane and Oxygen Vacancy

We herein report the synthesis of 3-methyl-1,3-butanediol from isobutene and HCHO in water via a Prins condensation-hydrolysis reaction over CeO2, which is a water-tolerant Lewis acid catalyst. The CeO2 exhibits significant catalytic activity for the reaction, giving 95% HCHO conversion and 84% 3-methyl-1,3-butanediol selectivity at 150 °C for 4 h. The crystal planes of CeO2 have a significant effect on the catalytic activity for the Prins reaction. The (110) plane shows the highest catalytic activity among the crystal planes investigated (the (100), (110), and (111) planes), due to its higher concentration of Lewis acid sites, which is in line with the concentration of oxygen vacancies. Detailed characterizations, including NH3-TPD, pyridine-adsorbed FT-IR spectroscopy, and Raman spectroscopy, revealed that the concentration of Lewis acid sites is proportional to the concentration of oxygen vacancies. This study indicates that the Lewis acidity induced by oxygen vacancy can be modulated by selective synthesis of CeO2 with different morphologies, and that the Lewis acidity and oxygen vacancy play an important role in Prins condensation and hydrolysis reaction.

Enhancement of the Sorption Ability of Aluminum Oxide Desiccants by Alkaline Modification

The influence of impregnation of aluminum oxide desiccants prepared by centrifugal thermal activation of hydrargillite with alkali (KОН and NaOH) and carbonate (Na2CO3 and K2СО3) solutions on the physicochemical properties of the products was studied. Impregnation with alkali solutions increases the dynamic capacity of the desiccants by a factor of 2 and more, whereas impregnation with carbonate solutions decreases the sorption characteristics of the desiccants at similar texture characteristics. Introduction of alkaline modifiers leads to a considerable decrease in the concentration of Lewis acid sites on the surface and to an increase in the concentration of strong base sites. Linear correlation was revealed between the concentration of strong base sites on the surface of the desiccants and their dynamic capacity in drying of humid air. The desiccants modified by impregnation exhibit not only high static and dynamic capacity, allowing improvement of the drying efficiency, but also considerably enhanced mechanical strength.

Cadmium Modified HZSM-5: A Highly Efficient Catalyst for Selective Transformation of Methanol to Aromatics

Cadmium modified HZSM-5 catalyst (Cd/ZSM-5) prepared by incipient wetness impregnation shows markedly high selectivity of aromatics and low selectivity of light alkanes and C5–C9 nonaromatics in the methanol-to-aromatics (MTA) transformation. The majority of the introduced Cd cations can exchange with protons of the bridged hydroxyl groups of HZSM-5, leading to a reduced number of Bronsted acid sites while generating new Lewis acid sites on the catalysts. The selectivity of aromatics with the Cd/ZSM-5 is found to be linearly correlated with the concentration of the strong Lewis acid sites. Compared to Zn/ZSM-5, the most extensively investigated catalyst thus far for the MTA, Cd/ZSM-5 is found to be more effective in promoting the aromatics formation. The different catalytic behavior observed for Cd/ZSM-5 and Zn/ZSM-5 catalysts is most likely due to the difference in ionic radius of Cd and Zn.

Acidic and catalytic co-functionalization for tuning the sensitivity of sulfated tin oxide modified by ruthenium oxide to ammonia gas

This work is aimed at elaborating a route for co-modification of acidic and catalytic sites on the surface of nanocrystalline tin oxide with the aim to enhance the sensitivity to ammonia gas. Sulfated tin oxide with controlled amount 0–10 wt.% of SO42− as the acidic modifier was synthesized by wet chemical deposition. It was modified by the catalytic additive of ruthenium oxide (0–5 wt.% Ru). Surface acidity was evaluated by temperature-programmed desorption and in situ infrared spectroscopy of adsorbed ammonia. The concentration of Lewis and Broensted acid sites was strongly dependent on sulfate content. Increasing amount of ruthenium oxide depressed surface acidity of catalytically-modified materials. Sensor response to 2–100 ppm NH3 in air was measured depending on the materials composition. At Ru-loading lower 0.5 wt.% the sensor signal was detected at temperature 200–300 °C and its value was monotonously dependent on SO42− content. The highest ammonia sensitivity was detected at lower temperature of 140–150 °C for co-functionalized tin oxide with comparable contents of Ru and SO42− (1–3 wt.%). From the sensing tests and in situ infrared spectroscopy data it was deduced that the improved sensing of ammonia gas was provided by equivalent impacts from the processes of target molecules adsorption on sulfate-promoted acid sites and NH3 oxidation on the catalytic clusters of ruthenium oxide.