Concentration Of Lewis Acid Sites Research Articles

Boosting the Formation of Brønsted Acids on Flame-made WOx/ZrO2 for Glucose Conversion.

WOx/ZrO2with ahigher concentrationof Brønsted acid sites (BAS) and a bigger ratio of Brønsted to Lewis acid sites (B/L) than achievable by conventional impregnation (IM)were synthesized usingflame spray pyrolysis (FSP). The rapid quenching and short residence time inherent to FSP prevent the accumulation of W atoms on the ZrO2 support and thus provide an excellent surface dispersion of WOx species. As a result, FSP-made WOx/ZrO2 (FSP-WOx/ZrO2) has a much higher surface concentration of three-dimensional Zr-WOx clusters than corresponding materials prepared by conventional impregnation (IM-WOx/ZrO2). The coordination of W-OH to the unsaturated Zr4+ sites in these clusters results in a remarkable decrease of the concentration of Lewis acid sites (LAS) on the surface of ZrO2 and promotes the formation of bridging W-O(H)-Zr hydroxyl groups acting as BAS. FSP-WOx/ZrO2 possesses ~80% of BAS and a B/L ratio of around 4, while IM-WOx/ZrO2 exhibits ~50% BAS and a B/L ratio of around 1. These catalysts were evaluated in the dehydration of glucose to HMF. The catalytic study demonstrated that B/L ratio plays a crucial role in glucose conversion. The best catalyst, FSP-WOx/ZrO2 with a W/Zr ratio of 1/10 affords nearly 100% glucose conversion and an HMF selectivity of 56-69%.

STUDY OF CATALYTIC BEHAVIOURS OF THE SPINEL STRUCTURE NdxMg1- xAl2O4 COMPOSITION WITH ZSM-5 ZEOLITE IN ETHYLBENZENE DISPROPORTIONATION

Nanosized (10–15 nm) powders of spinel structures NdxMg1–xAl2O4 were obtained by low-temperature combustion of aluminium, magnesium, neodymium nitrate solutions, diethylmalonate, and hydrazine monohydrate with further calcination of nanopowders at 1000 °C, which were used in the preparation of catalytic compositions with ZSM-5 zeolite. The obtained catalytic compositions were characterized by X-ray diffraction, pyridine adsorption, and low-temperature nitrogen adsorption. The acidic properties and porosity of the catalysts were altered by controlling the concentration of nanopowder (1.0–7.0 wt.%) in the catalytic composition. The interaction of the modifier with HZSM-5 zeolite leads to a decrease in specific surface area and pore volume, as well as reduse of strong Bronsted acid sites (B) and an increase in the concentration of medium Lewis acid sites (L), resulting in a change in the B/L ratio from 3.61 to 0.23. The ongoing variations contributed mainly to an enhancement of the para-selectivity of the catalytic composition in the ethylbenzene disproportionation reaction. The catalytic composition of 5% Nd0.1Mg0.9Al2O4 with a pore volume of 0.15 cm3/g and an acid site B/L ratio of 0.27 demonstrates a maximum selectivity to p-diethylbenzene of 75.6% with a conversion of ethylbenzene of 23.7%.

Cu/ZnO/Al2O3 sorbent promoted by Zr for the ultra-deep removal of thiophene in simulated coke oven gas

The ultra-deep removal of thiophene from coke oven gas (COG) is critical for the conversion of COG to methane. Currently, Cu/ZnO/Al2O3 is considered as a promising sorbent for thiophene removal; however, its desulfurization performance is currently unable to meet the requirements for industrial applications. Here, a zirconium modification strategy is proposed to overcome this problem, allowing zirconium loaded copper-based sorbents (Cu/ZnO/Al2O3/ZrO2) with significantly improved thiophene removal performance and high activity to be obtained. Characterization results showed that zirconium improved both the dispersion of copper and the concentration of Lewis acid sites on the sorbent surface. A ZrO2 concentration of 2 % resulted in stronger Cu-Zn interaction, and the dispersion of Cu on the surface of the sorbent reached its maximum value (49.20 %). Additionally, the Lewis acid strength was also maximized, which greatly improved the desulfurization activity and resulted in a breakthrough adsorption capacity of thiophene (Qb-thiophene) of 36.24 mg/g, which was about 17 % higher than that of the unmodified Cu/ZnO/Al2O3 sorbent. These results demonstrate that the sulfur capacity of the sorbent can be increased by increasing the number of acidic sites on its surface, paving the way for ultra-deep thiophene removal in industrial applications.

Boosting the low-temperature NH3-SCR performance via metals co-doping with inequality Mn/Ce ratios in the carbon-based catalyst prepared by Cr-containing leather waste

The resource utilization of Cr-containing leather waste is still a great challenge. In this work, waste from leather production as raw material was attempted to be used to synthesize activated carbon, a series of MnxCe1-xZr0.5/AC composite catalysts were prepared via loading three metal elements onto activated carbon by impregnation method, the NH3-SCR performance was measured in a low temperature range of 50 ∼ 200 °C. Among them, the catalyst Mn0.7Ce0.3Zr0.5/AC displays the optimal low-temperature SCR activity, excellent resistance to GHSV, H2O and SO2. The characterization results indicated that increasing the Mn/Ce inequality ratios within a certain range would promote the electron transfer between the active metal components, thereby enhancing the synergistic effect and the redox capacity of the catalysts, and it also could significantly improve the concentration of Lewis acid sites to accelerate the adsorption and activation of NH3, resulting in different catalytic activities. The in situ DRIFTS results prove that the Lewis acid site plays a vital role in the “fast SCR” reaction, and the whole reaction process conforms to the Langmuir-Hinshelwood (L-H) mechanism.

Characterization of Bambu Clay as A Precursor for The Synthesis of Zeolite for Catalytic Application

The clay in Bambu town, north-central Nigeria, has been used as a ceramic formulation for several generations without understanding its properties. In this paper, the mineralogical study of the Bambu clay sample was investigated in order to deduce its properties and its potential for technological applications. Samples of the clay were first pre-treated and subjected to elemental composition using powder diffraction, morphology, Fourier transform infrared, and area-volume-pore size analyses. The results indicated that Bambu clay can be classified as kaolinite with a Si/Al ratio of approximately 1:85. From the SEM, the particles show the presence of pseudo-hexagonal structures within the clay structure. The average crystallite size was deduced from the XRD analysis to be 43.5nm. The BET analysis reveals that the surface area of the clay was 429.70 m2g- 1, pore volume was 0.042 cm3g- 1 and the pore radius was 2.14 nm. The clay showed that the sample possessed a concentration of Bronsted acidic sites of 31.57 μmol/g and Lewis acidic sites of 34.40 μmol/g. To ascertain the clay’s viability for industrial application, the results of the analyses were compared with other clays from the literature. It can be deduced from the analyses that the Bambu clay is kaolinite, mesoporous, has a crystalline structure, a low Si/Al ratio, a higher surface area than zeolite Y, and a high concentration of Lewis acidic sites and a low concentration of Bronsted acidic sites. The clay could be a good precursor material for the synthesis of zeolite Y for catalytic applications.

Surface modification of PtSn/Al2O3 catalyst by organic acid chelation and its effect on propane dehydrogenation performance

A series of Pt-based catalysts has been prepared by a chelating-agent-assisted impregnation method, and characterized by N2 adsorption/desorption, XRD, XPS, H2-TPR, NH3-TPD, pyridine-FTIR, CO-DRIFT-IR, Raman, and TG techniques to investigate the influence of organic acids on physicochemical properties. It was found that, except in the case of tartaric acid, organic acid-assisted preparation not only led to higher dispersion of Pt particles in comparison with organic acid-free catalyst PtSn/Al2O3, but also changed the concentration of Lewis acid sites and modified the interaction between the metallic Pt sites and the support. Experiments on the catalytic dehydrogenation of propane have revealed that the optimal dehydrogenation performance, with a propane conversion of 35% and a propylene selectivity of 93%, was obtained over the citric acid-assisted catalyst (PtSn/Al2O3-CA). Lewis acid sites are mainly responsible for activating C–H bonds during the tandem activation–cleavage process, whereas coke deposition and the aggregation of Pt sites are mainly responsible for deactivation of the catalyst.

Comparative study of interzeolite transformed metallosilicates for the chemisorption of benzene, toluene, and xylenes

Zeolitic metallosilicates with Lewis acidity are effective in the chemisorption of benzene, toluene, and xylenes (BTX) because of the strong π-interactions between the Lewis acid sites and BTX adsorbates. In this study, we prepared boro-, nickel-, and zincosilicate delMWW (two-dimensional delaminated MWW) and beta zeolite (*BEA) adsorbents by using the interzeolite transformation method and compared them with their mother MWWs for BTX chemisorption. Different structural properties, framework-atom species, and hydroxyl and Lewis acid site concentrations were systematically characterized by using various analytical methods. Among the studied adsorbents, nickel silicate beta zeolite (Ni-BEA) exhibited the highest concentration of Lewis acid sites and a hydrophobic character because of the stable framework-bound Ni species. In addition, its advantageous textural properties, large pore size, high surface area, and nano-sized crystals, induced the highest chemisorption content and a strong chemisorption ability of BTX at high-temperature (120 °C) adsorption under humid conditions.

One-Pot Transformation of Citronellal to Menthol Over H-Beta Zeolite Supported Ni Catalyst: Effect of Catalyst Support Acidity and Ni Loading

Citronellal was converted to menthol in a one-pot approach using H-Beta zeolite-based Ni catalyst in a batch reactor at 80 °C, under 20 bar of total pressure. The effects of H-Beta acidity (H-Beta-25 with the molar ratio SiO2/Al2O3 = 25 and H-Beta-300 with SiO2/Al2O3 = 300) and Ni loading (5, 10 and 15 wt %) on the catalytic performance were investigated. Ni was impregnated on H-Beta support using the evaporation-impregnation method. The physico-chemical properties of the catalysts were characterized by XRD, SEM, TEM, ICP-OES, N2 physisorption, TPR, and pyridine adsorption–desorption FTIR techniques. Activity and selectivity of catalysts were strongly affected by the Brønsted and Lewis acid sites concentration and strength, Ni loading, its particle size and dispersion. A synergetic effect of appropriate acidity and suitable Ni loading in 15 wt.% Ni/H-Beta-25 catalyst led to the best performance giving 36% yield of menthols and 77% stereoselectivity to ( ±)-menthol isomer at 93% citronellal conversion. Moreover, the catalyst was successfully regenerated and reused giving similar activity, selectivity and stereoselectivity to the desired ( ±)-menthol isomer as the fresh one.Graphical

Micro-mesoporous catalysts based on ZSM-5 zeolite synthesized from natural clay nanotubes: Preparation and application in the isomerization of C-8 aromatic fraction

Micro-mesoporous zeolite with ZSM-5 morphology was successfully synthesized by soft template and template-free methods using natural halloysite aluminosilicate nanotubes as a hard template and a source of aluminum and silicon. Formation of ZSM-5 crystal structure was confirmed by XRD. Textural properties and structural characteristics of functional materials, supports, and Pt-containing catalysts were examined by N2 physisorption, NH3-TPD, Py-FTIR, TEM, SEM, XRF techniques. Specific BET surface area of ZSM-5 zeolite prepared using TPABr as an organic template (ZSM-5(t):HNT) is 583 m2/g, whereas for the template-free counterpart (ZSM-5(tf):HNT) it reaches 225 m2/g. According to the Py-FTIR data, concentrations of Brønsted acid sites for ZSM-5(t):HNT and ZSM-5(tf):HNT are 295 and 93 μmol/g, whereas concentrations of Lewis acid sites are 79 and 41 μmol/g, respectively. Activity and selectivity of Pt catalysts based on the micro-mesoporous ZSM-5 zeolite-derived supports were evaluated in isomerization of C-8 aromatic fraction depending on the process conditions (temperature, feed space velocity). Because of the hierarchical micro-mesoporous structure, both catalysts provide xylenes transformation through the two isomerization routes (intra- and intermolecular). However, for the catalyst based on support synthesized through the template-free methods, the monomolecular reaction route is more preferable, and therefore the number of side reactions (disproportionation, transalkylation, and dealkylation) decreases.

Adsorptive desulfurization using period 4 transition metals oxide: A study of Lewis acid strength derived from the adsorbent ionic-covalent parameter

Pearson’s Hard and Soft Acids and Bases (HSAB) principle has been widely utilized in describing the adsorption trend of sulfur compounds when using transition metal/metal oxides as adsorbents. This study is the first to report on the use of the ionic covalent parameter (ICP) for explaining the adsorption trend of sulfur compounds. Period 4 transition metal oxides (Zn-oxide, Cr-oxide, Mn-oxide, Co-oxide, Fe-oxide, Cu-oxide, Ni-oxide), which act as intermediate Lewis acids, were incorporated into activated carbon (AC), and used in the adsorptive desulfurization (ADS) of model and commercial diesel samples. The study findings show that desulfurization activity in model diesel increased in proportion to the concentration of adsorbent Lewis acid sites. With commercial diesel, the loading of metal oxides significantly enhanced the sulfur adsorption capacity of AC and increased its selectivity for steric sulfur compounds. The results showed a poor correlation between sulfur removal and the Pearson hardness ηth of the intermediate metal oxide cations. However, a strong inverse linear relationship was observed: sulfur removal decreased as the ICPth of the adsorbent metal oxide increased. The study findings suggest a novel concept, i.e. that the ICP is a more accurate measure of the Lewis acidic strength of transition metal oxides with varying crystal structures and metal geometries, and their subsequent desulfurization performance. Kinetic modelling showed that chemisorption was the controlling mechanism. The fixed-bed ADS of commercial diesel when using unmodified AC is best described by the Yoon-Nelson and Thomas models. Adsorption when using the Ni-oxide/AC adsorbent was correlated to the filtration advection–dispersion equation.

Abundant Lewis acidic sites of peculiar fibrous silica zeolite X enhanced toluene conversion in side chain toluene methylation

Side chain toluene methylation reaction (SCTM) is a reliable reaction to produce styrene and ethylbenzene from toluene. Nevertheless, due to the lack of efficient catalyst caused by less Lewis acid sites and mesoporosity properties, the challenges in this SCTM reaction are still ongoing. Hence, this work is accomplished to gain a better understanding in designing a robust catalyst for side chain toluene methylation. Herein, this work emphasizes the uniqueness of top-notch metal-free fibrous silica zeolite X (FZX) by microemulsion method. This FZX showed superior catalytic activity towards side chain methylation with 15.9% yield of styrene and ethylbenzene, which was 16-fold higher compared to the commercial zeolite X (CZX) catalyst. This was achieved by the excellent properties of dendrimeric silica fibers of FZX possessing high concentration of basic sites and Lewis acid sites as well as high mesoporosity, as proved by the FTIR-Pyrrole, FTR-Pyridine and nitrogen physisorption analyses respectively. The presence of intermediates was confirmed by in-situ FTIR toluene-methanol. 27Al MAS NMR analysis proved that the presence of excessive extraframework alumina (EFAl) species could act as additional Lewis acid sites in FZX to enhance toluene adsorption on the FZX catalyst, thus improving the toluene conversion and desired styrene and ethylbenzene products. Silica MCM41 exhibited lower yield of styrene and ethylbenzene by the fact imbalance of acid base property of catalyst. The kinetic study proved that the side chain toluene methylation occurred via dissimilar dual site mechanism of Langmuir-Hinshelwood. This rationally designed metal-free FZX demonstrates its high potential in polymer and/or petrochemical as well as other industrial applications.

Key Factors During the Milling Stage of the Seed‐assisted and Solvent‐free Synthesis of MFI and Catalytic Behavior in the Alkylation of Phenol with Tert‐butyl Alcohol

AbstractThe mechanochemical synthesis of zeolites reduces the generation of liquid residues and gaseous pollutants. Herein, MFI zeolites were synthesized by a seed‐assisted mechanochemical pathway. The effect of different synthesis variables, the Na2O/SiO2 and H2O/SiO2 molar ratios and the milling time and speed, and their interactions on physicochemical properties and catalytic performance on alkylation of phenol with tert‐butyl alcohol were studied. Results showed that the Na2O/SiO2 molar ratio may direct the synthesis to the preferential production of MOR instead of MFI. Also, the milling time and speed and their interaction, i. e., (milling time)×(milling speed), impacted the texture of MFI. Meanwhile, the triple interaction, (milling time)×(milling speed)×(H2O/SiO2 molar ratio), changed the concentration of Lewis acid sites. These effects were rationalized by considering that sodium can act as a structure directing agent during the mechanochemical synthesis of MFI while also promoting the incorporation of aluminum to its structure. Meanwhile, milling time and speed were non‐linearly correlated to the milling energy required for forming the aluminosilicate precursor that crystallizes during the hydrothermal stage of the process. Overall, all the mechanochemically synthesized zeolites were less crystalline than both the seed and a sol‐gel synthesized MFI. The catalytic behavior of the mechanochemical MFI was positively correlated with both the density of acid sites and with the concentration of Brønsted sites.

Study of catalyst performance of two inorganic/organic and inorganic/inorganic hybrid catalysts on the CO2 cycloaddition to propylene oxide: kinetics and thermodynamics

The behavior of two hybrid catalysts: inorganic/organic (KI/chitin) and inorganic/inorganic (KI/amorphous silica-alumina) on the CO2 cycloaddition to Propylene oxide has been compared. Conversions of KI/chitin and KI/Si-Al were 99 and 97% respectively. KI/Si-Al shows a type IV N2 adsorption isotherm at P0/P of 0.6-0.9 of mesoporous nature with a significant concentration of Lewis acid sites, its FT-IR analysis shows that CO2 can be adsorbed as organic “bridging” carbonate onto the support’s surface, which has an important role in the reaction mechanism. Kinetic analysis shows that the organic co-catalyst is up to 1.5 times faster for the CO2 cycloaddition than the inorganic co-catalyst at 160 °C bu both show very similar rate at 120 °C. Both supported catalysts show a significantly lower Ea than that calculated for free KI, although both show comparable activation energies. This insight gives interesting findings on how hybrid catalyst systems behave in an utmost crucial CO2 fixation reaction, being the foundations of future investigation on scale-up applications.

Enhancement effects of Er modification on comprehensive performance of FeMn/TiO2 catalysts for selective reduction of NO with NH3 at low temperature

Erbium (Er) has been used as a novel dopant to modify FeMn/TiO2 catalyst for the selective catalytic reduction of NO with NH3. The optimized ErxFeMn/TiO2 catalyst exhibited excellent catalytic performance at low temperature under a high space velocity of 30,000 h−1. The temperature window for NO conversion efficiency higher than 80% ranged from 100° to 240°C. The catalysts were characterized using XRD, BET, H2-TPR, NH3-TPD, XPS, TGA and In-situ DRIFT. The results showed that Er modification on FeMn/TiO2 catalysts increased the specific surface area, dispersion, reducibility, Mn4+ concentration, and especially the concentration of Lewis acid sites and surface chemisorbed labile oxygen, resulting in an excellent catalytic performance at low temperature. Additionally, Er modification also restrained the formation of sulfate and/or bisulfate salts, which improved the resistance to SO2 and H2O of the catalyst.

Substoichiometric titanium oxide Ti2O3 exhibits greater efficiency in enhancing hydrolysis of 1,1,2,2-tetrachloroethane than TiO2 nanomaterials

Oxygen-deficient substoichiometric titanium oxides, or “titanium suboxides,” are produced incidentally from coal combustion and are environmentally abundant. Additionally, titanium suboxide nanomaterials are promising new materials with likely future environmental release. How these materials may affect contaminant fate differently than stoichiometric TiO2 (nano)materials is largely unknown. Here, we show that Ti2O3 (selected as a model titanium suboxide) exhibits significantly greater efficiency in enhancing the hydrolysis of 1,1,2,2-tetrachloroethane (TeCA), a common groundwater contaminant, than the stoichiometric anatase and rutile TiO2. At environmentally relevant pH (6.5–7.5), the surface area-normalized pseudo-first-order hydrolysis rate constant in the presence of Ti2O3 is approximately an order of magnitude higher than those associated with TiO2. The superior catalytic efficiency of Ti2O3 can be attributed to both its higher surface hydrophobicity, which renders higher adsorption affinity for TeCA, and its higher concentration of Lewis acid sites (mainly the Ti3+ and the five-coordinated Ti4+). Particularly, the deprotonated hydroxyl groups attached to Ti3+ (a weaker Lewis acid than Ti4+) exhibit higher basicity and thus, are more effective in catalyzing the base-promoted hydrolysis reaction. The findings call for further understanding of the environmental implications of titanium suboxide (nano)materials, which may not be readily predictable based on the knowledge acquired for TiO2.

The effect of Si/Al ratio of zeolite Y in NiW catalyst for second stage hydrocracking

The effect of Si/Al ratio of zeolite Y on the performance of NiW catalysts for second stage hydrocracking was studied. The zeolites were characterized by IR spectroscopy of adsorbed pyridine, low-temperature N2 adsorption, and temperature-programmed desorption of ammonia. The observed decrease of Brønsted and Lewis acid sites concentration relates to the increase of Si/Al ratio in zeolites. According to HRTEM and XPS, sulfide component of catalysts differs in morphology, NiWS phase content and tungsten sulfidation degree. The catalysts were tested in hydrocracking of model (squalane) and real feedstock (unconverted oil (UCO) from two-stage industrial hydrocracker) at different temperatures. The increase of Si/Al ratio in zeolites was found to result in the decrease of catalytic activity and the increase of selectivity to middle distillates. However, similar selectivity of 52 and 53% at the 50% of UCO conversion was observed for catalysts containing zeolites with Si/Al ratio 24 and 37 respectively. This can be attributed mainly to the differences in the textural properties of the zeolites. The catalyst based on amorphous silica-alumina (without zeolites) was found to have a higher selectivity to middle distillates and lower activity as compared to zeolite-based catalysts.

Acid-Base Properties of Cobalt Ferrite Surface Examined by Different Physicoсhemical Methods

The acid-base properties of cobalt ferrite synthesized by the mechanochemical method with different ratios of the main components Fe2O3 : Co3O4 have been investigated using the temperature-programmed desorption of ammonia, IR spectroscopy using adsorbed pyridine with ammonia, and the Hammett indicator method. A comparison of the results for the total content of Lewis and Bronsted acid sites by three methods has shown that all three samples were quite similar, with a probability of 0.5%. The introduction of cobalt oxide into the composition of cobalt ferrite leads to an increase in the concentration of Bronsted and Lewis centers with a simultaneous increase in the number of strong basic centers determined by the temperature-programmed desorption of ammonia. The indicator method is the most informative method for a quantitative study of the acid-base properties of the cobalt ferrite surface, since it allows one to assess the concentration and strength of Lewis and Bronsted acid sites and basic sites.

Characterization and cracking performance of zirconium-modified Y zeolite

Zirconium-modified Y zeolites (ZrUSY) were prepared by impregnation of the zeolite with anhydrous-ethanol and aqueous ion exchange methods. The ZrUSY samples were characterized by powder X-ray diffraction, nitrogen physisorption, and Fourier transform infrared spectroscopy. The results indicated that ZrUSY prepared by the impregnation with anhydrous-ethanol method showed higher crystallinity, specific surface area and hydrothermal stability. The incorporation of Zr into other Y zeolites leads to an increase of the Brønsted acid site concentration and a decrease of the Lewis acid site concentration compared to the ultra-stable Y zeolite (USY). The ZrUSY was much more favorable towards heavy oil FCC conversion with decreasing coke yield and improving olefins selectivity.

Effect of alumina polymorph on the dehydrogenation activity of supported chromia/alumina catalysts

In this work, the effect of surface properties of different alumina supports for CrOx/Al2O3-catalysts on Cr-speciation and dehydrogenation activity was studied. For this purpose, a series of CrOx/Al2O3-catalysts with a monolayer coverage of CrOx (ca. 4.4 at. Cr/nm2) was prepared with γ-, δ-, (δ + θ)-, η-, θ-Al2O3 as supports. Systematic investigation of fresh catalysts with a set of techniques (chemical analysis, N2 adsorption, XRD, Raman and ESR spectroscopy) showed that (i) chemical state distribution of Cr (Cr6+, Cr5+, Cr3+) was the same for all of the catalysts; (ii) CrOx-species were in X-ray amorphous state; (iii) surface chromate species were virtually of the same oligomerization degree. However, aggregation of Cr3+ ions both in calcined and reduced catalysts is support-dependent. ESR data and infrared spectroscopic results of adsorbed CO showed that increase of Lewis acid sites (LAS) surface density on bare aluminas promotes the growth of the relative amount of Cr3+ ions in a relatively large Cr2O3-like clusters in the fresh catalysts. Catalytic testing in a cycling dehydrogenation-regeneration mode leads to catalyst deactivation. Investigation of spent catalysts revealed that deactivation is accompanied with sintering of Cr3+Ox-species with partial migration of Cr3+ ions inside alumina. It was observed that increase of LAS surface density on aluminas promotes the growth of both the initial dehydrogenation activity and stability upon cycling. On the basis of experimental results, it was concluded that (i) Cr3+ ions on the surface of relatively large Cr2O3-like clusters are more active than isolated and poorly agglomerated Cr3+ ions; (ii) increase in LAS concentration on alumina induces growth of surface coverage by Cr2O3-like clusters with concomitant modification of intrinsic activity of active sites through their interaction with support.

Role of pore structure on the activity and stability of sulfide catalyst

Effect of pore structure on the activity and stability of sulfide phase in hydrodesulfurization (HDS) of diesel were systematically investigated. A series of alumina with different pore structure were prepared by calcining pseudo boehmite at different temperatures. Various characterizations suggested that the surface concentration of OH groups and Lewis acid sites generally decreased with calcination temperature. The properties of sulfide phase in fresh and spent catalysts were analysis by X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). The results showed that the concentration of CoMoS species was greatly decreased after the reaction while the average length of MoS2 slabs was substantially increased. The concentration of preserved CoMoS species was proportional to the average pore size of support indicating that the strip-off of Co from CoMoS phase can be reduced when the average pore size is larger. Furthermore, the activity evaluation showed that larger pore structure of support is beneficial for the intrinsic activity and stability of CoMoS phase in the HDS of diesel. The reason was partially attributed to that the pore with larger diameter can reduce the coke deposition on the catalyst so that increase the intrinsic activity and stability of CoMoS sites.