Surface Lewis Acid Sites Research Articles

Upgrading oil sand bitumen under superheated steam over ceria-based nanocomposite catalysts

Upgrading of oil sand bitumen by catalytic cracking of its heavy oil fraction via ceria-based catalysts was investigated in a fixed-bed flow-type reactor, in the presence of superheated steam and addition of water. The reaction was carried out over CeZr, FeCoCeZr1 and FeCoCeZr2 catalysts at 470 °C, Wcat/FFeed of 0.4 h and FH2O/Ffeed = 2. The oxygen species in the crystal lattice of the catalysts and the surface Lewis acid sites are responsible for the oxidative decomposition and catalytic cracking of the heavy oil, respectively. Higher light oil yield of approximately 60 mol%-C (gas oil and vacuum gas oil) and lowest coke yield (20.45 mol%-C) was obtained over CeZr catalyst. FeCoCeZr1 and FeCoCeZr2 gave lower residue and higher gas yield, with higher H2 and lower CO2 composition when compared to CeZr. The spent catalysts showed structural stability which is supported by the X-ray diffraction analysis, and thermal stability which agrees to the minimal weight loss from thermogravimetric analysis. The catalysts also exhibited good potential for reusability based on the analysis of the spent samples.

Silica deposition as an approach for improving the hydrothermal stability of an alumina support during glycerol aqueous phase reforming

Silica deposition on the benchmark aqueous phase reforming (APR) catalyst Pt/γ-Al2O3 is studied to prevent or limit hydrolytic attack of the support under hydrothermal APR conditions, for which boehmite formation by support hydration is a known cause for catalyst deactivation. Tetraethyl orthosilicate (TEOS) is employed as a silicon source in a straightforward liquid-phase, silylation process followed by catalyst calcination and reduction. Characterization by X-ray diffraction, temperature-programmed desorption of NH3, infrared, 27Al nuclear magnetic resonance and X-ray photoelectron spectroscopy of the fresh catalysts suggests that silica addition occurs preferentially on the support surface, resulting in weak Brønsted acid sites as well as in the formation of Si-O-Al linkages at the expense of specific surface Lewis acid sites. Silylation and calcination of Pt/γ-Al2O3 causes partial blockage of the metal surface area (12% loss), whereas γ-Al2O3 surface silica modification prior to Pt deposition makes controlled metal deposition difficult. Catalytic performance tests show the overcoated samples to be active in the APR of 5 wt% glycerol, albeit with lower H2 production rates compared to the benchmark catalyst. Characterization of spent APR catalysts clearly demonstrates that silylation/calcination treatments effectively slows down the transformation of the γ-Al2O3 support due to the formation of a Si-O-Al interface. Overall, the lifetime of the catalyst is increased three-fold as a result of the surface overcoating treatment, with repetitive recycling ultimately leading to loss of the protective silica layer.

Molecular Level Investigation of CH4 and CO2 Adsorption in Hydrated Calcium–Montmorillonite

We have studied the mechanism of intercalation and methane adsorption from a H2O/CH4/CO2 mixture on a prototypical swelling shale component, Ca–montmorillonite. We employed ab initio molecular dynamics simulations at 323 K and 90 bar to obtain molecular level information on adsorption energetics, speciation, and structural and thermodynamic properties. Interaction of CH4 with surface Lewis acidic sites (Ca2+, surface OH) results in large induced dipoles (∼1 D) that lead to relatively strong adsorption energies compared to interactions of the normally apolar CH4 that level off once a CH4 layer is formed. Intercalated CH4, also exhibits large induced dipoles at lower hydration levels, when the interaction with Ca2+ cations are less hindered. CO2 displaces CH4 in the coordination sphere of the cations (in the interlayer) or on the surface, thereby driving CH4 extraction. Our simulations indicate that there is an optimal pressure range (∼70–90 bar) where scCO2-facilitated CH4 extraction will be maximized.

An insight on the influence of surface Lewis acid sites for regioselective C[sbnd]H bond C3-cyanation of indole using NH4I and DMF as combined cyanide source over Cu/SBA-15 catalyst

The Cu dispersed on mesoporous SBA-15 has demonstrated the regioselective CH bond C3-cyanation of indoles by in-situ generation of CN using DMF and NH4I in presence of O2. Pyridine adsorbed diffuse reflectance infrared spectroscopy (DRIFTS) results revealed that surface Cu2+ species are acting as Lewis acid sites in the in-situ generation of cyano- group for the synthesis of indole nitriles. A direct correlation between Cu metal surface area and the indole nitrile yields are established and the dual role of copper is substantiated by N2O titration and XPS techniques. The 10wt%Cu/SBA-15 demonstrated superior performance when compared to Pd, Ru supported on SBA-15. The 10wt%Cu/SBA-15 catalyst showed consistent activity and selectivity after the 4th cycle.

Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts

Ammonia (NH3) is an essential chemical in modern society, currently manufactured via the Haber–Bosch process with H2 and N2 under extremely high pressure (>200 bar) and high-temperature conditions (>673 K). Toxic nitrate anion (NO3–) contained in wastewater is one potential nitrogen source. Selective NO3–-to-NH3 transformation via eight-electron reduction, if promoted at atmospheric pressure and room temperature, may become a powerful recycling process for NH3 production. Several photocatalytic systems have been proposed, but many of them produce nitrogen gas (N2) via five-electron reduction of NO3–. Here, we report that unmodified TiO2, when photoexcited by ultraviolet (UV) light (λ > 300 nm) with formic acid (HCOOH) as an electron donor, promotes selective NO3–-to-NH3 reduction with 97% selectivity. Surface defects and Lewis acid sites of TiO2 behave as reduction sites for NO3–. The surface defect selectively promotes eight-electron reduction (NH3 formation), while the Lewis acid site promotes nonselect...

Selective response of pyrylium-functionalized nanozeolites in the visible spectrum towards volatile organic compounds

An optical detection of volatile organic compounds (VOCs) using nanosized zeolites functionalized with pyrylium salts is presented. Nanosized zeolite crystals firstly are functionalized with pyrylium salt (para-dimethylamino-2,4,6-triphenylthiopyrylium tetrafluoroborate, PNS). Then the optical response of the pyrylium-functionalized nanozeolites towards volatile organic compounds is investigated. Two possible modes of zeolite functionalization are proposed based on: (1) ion exchange between the pyrylium and sodium cations (Na+) in the zeolite (Si-O−-Al active sites), and (2) adsorption via electrostatic interaction between the electron lone-pair of amine of the PNS and the orbital vacancy of Al on the zeolite surface (Lewis acid sites). The experimental results verify that the great number of Lewis acid sites of the nanozeolites led to a significant adsorption of the PNS (mode 2) causing the hypsochromic (band shift to lower wavelength) shift in the visible light absorbance. Pyrylium-functionalized nanozeolites demonstrate a good sensitivity and reversibility toward volatile organic compounds investigated in this work.

Catalytic ozonation of clofibric acid over copper-based catalysts: In situ ATR-IR studies

The current study describes the catalytic ozonation of clofibric acid (CFA) under ambient conditions using copper oxide catalysts synthesized by different methods. The objective of this study is to provide novel catalysts and reaction mechanism for the degradation of emerging pharmaceutical compounds in aqueous solution. Among the various Cu catalysts screened in this study, the Cu1–Al1 oxide catalyst showed an excellent activity and stability in the degradation and mineralization of CFA. In situ attenuated total reflection IR (ATR-IR) spectroscopy was used to examine the interaction of ozone with the active sites of the catalyst in presence of water and to investigate the possible catalytic mechanism. The presence of Lewis acid sites in the Cu1–Al1 catalyst increased the amount of chemisorbed water enhancing stronger interaction of ozone to form surface activated species, resulting in higher catalytic activity. The results obtained from in -situ ATR-IR study indicate that surface hydroxyl groups and Lewis acid sites are responsible for promoting the generation of hydroxyl radicals (OH) from aqueous ozone.

The role of Lewis acid–base pair sites in ZnO–ZnCr2O4 catalysts for cyclization via dehydrogenative condensation of crude glycerol and 1,2-propanediamine for the synthesis of 2,6-dimethylpyrazine

DRIFT spectroscopic data revealed the role of surface Lewis acid sites on the synthesis of 2,6-dimethylpyrazine.

Efficient NH3-SCR removal of NOx with highly ordered mesoporous WO3(χ)-CeO2 at low temperatures

To eliminate nitrogen oxides (NOx), a series of highly ordered mesoporous WO3(χ)-CeO2 nanomaterials (χ represents the mole ratio of W/Ce) were synthesized by using KIT-6 as a hard template, which was used for selective catalytic reduction (SCR) to remove NOx with NH3 at low temperatures. Moreover, the nanomaterials were characterized by TEM, XRD, Raman, XPS, BET, H2-TPR, NH3-TPD and in situ DRIFTS. It can be found that all of the prepared mesoporous WO3(χ)-CeO2 (χ=0, 0.5, 0.75, 1 and 1.25) showed highly ordered mesoporous channels. Furthermore, mesoporous WO3(1)-CeO2 exhibited the best removal efficiency of NOx, and its NOx conversion ratio could reach 100% from 225°C to 350°C with a gas hourly space velocity of 30 000h−1, which was due to higher Ce3+ concentrations, abundant active surface oxygen species and Lewis acid sites based on XPS, H2-TPR, NH3-TPD and in situ DRIFTS. In addition, several key performance parameters of mesoporous WO3(1)-CeO2, such as superior water resistance, better alkali metal resistance, higher thermal stability and N2 selectivity, were systematically studied, indicating that the synthesized mesoporous WO3(1)-CeO2 has great potential for industrial applications.

Highly Efficient Vapor‐Phase Hydrogenation of Biomass‐Derived Levulinic Acid Over Structured Nanowall‐Like Nickel‐Based Catalyst

AbstractWith ever increasing demand of sustainable energy, biomass has been regarded as an ideal alternative to fossil resources. Herein, hierarchical three‐dimensional nickel‐based nanowalls on a nickel foam substrate were fabricated by a Ni–Zr–Al layered double hydroxide (NiZrAl–LDH) precursor route, which involved in situ growth of the Zr‐containing precursor on the Ni foam strut through surface activation without an external nickel source. After calcination–reduction treatment, the nanowalls were applied as a structured catalyst for selectively hydrogenating biomass‐derived levulinic acid to γ‐valerolactone. Systematic characterization revealed that highly dispersed Ni nanoparticles could be generated on the platelet‐like Zr‐containing oxide matrix derived from hierarchical 3 D NiZrAl–LDH nanowalls. Under vapor‐phase, solvent‐free hydrogenation conditions (250 °C, ambient pressure), the yield and productivity for as‐fabricated Ni‐based structured catalyst could reach as high as 97.7 % and 5.747 kgGVL kgcat−1 h−1, respectively. Such high catalytic efficiency was reasonably attributable to highly dispersed Ni nanoparticles and abundant surface Lewis acid sites, as well as favorable heat‐transfer nature of present catalytic system. Moreover, the newly developed structured nanowall‐like Ni‐based catalyst possesses high structural and chemical stability, which makes the vapor‐phase hydrogenation process promising in terms of green sustainable chemistry.

Highly Ordered Hierarchical Macroporous-Mesoporous Alumina with Crystalline Walls

Highly ordered hierarchical macroporous-mesoporous alumina with crystalline walls are synthesized by facial route. The general synthesis strategy is based on a sol–gel process associated with block copolymers as soft templates and polystyrene colloidal crystals as hard template to produce macropore and mesopore structures, respectively. Small-angle XRD, TEM and Nitrogen adsorption and desorption results show that hierarchical macroporous-mesoporous alumina possess highly ordered two-dimensional hexagonal mesostructured and highly ordered face centered cubic macropore arrays structure. The present work reveals that the hierarchical macroporous-mesoporous structure can endure high temperature up to 900 °C. FTIR pyridine adsorption measurements show that the amount of Lewis acid sites provided by hierarchical macroporous-mesoporous alumina is nearly two times more than that for mesoporous alumina, indicating that the open macroporous structure may be in favor of the reactant transfer, and the consequently superior activity. Large surface areas, high thermal stability, uniform pore structures and large amounts of surface Lewis acid sites illustrate that these materials are expected to find wide applications in catalysis realm. The highly ordered hierarchical macroporous-mesoporous alumina with crystalline walls are synthesized by a sol–gel process associated with block copolymers as soft templates and polystyrene colloidal crystals as hard template to produce macropore and mesopore structures, respectively.

Physico-Chemical and Catalytic Properties of Mesoporous CuO-ZrO2 Catalysts

Mesoporous CuO-ZrO2 catalysts were prepared and calcined at 500 °C. The performance of the synthesized catalysts for benzylation of benzene using benzyl chloride was studied. The bare support (macroporous ZrO2) offered 45% benzyl chloride conversion after reaction time of 10 h at 75 °C. Significant increase in benzyl chloride conversion (98%) was observed after CuO loading (10 wt. %) on porous ZrO2 support. The conversion was decreased to 80% with increase of CuO loading to 20 wt. %. Different characterization techniques (XRD, Raman, diffuse reflectance UV-vis, N2-physisorption, H2-TPR, XPS and acidity measurements) were used to evaluate physico-chemical properties of CuO-ZrO2 catalysts; the results showed that the surface and structural characteristics of the ZrO2 phase as well as the interaction between CuO-ZrO2 species depend strongly on the CuO content. The results also indicated that ZrO2 support was comprised of monoclinic and tetragonal phases with macropores. An increase of the volume of monoclinic ZrO2 phase was observed after impregnation of 10 wt. % of CuO; however, stabilization of tetragonal ZrO2 phase was noticed after loading of 20 wt. % CuO. The presence of low-angle XRD peaks indicates that mesoscopic order is preserved in the calcined CuO-ZrO2 catalysts. XRD reflections due to CuO phase were not observed in case of 10 wt. % CuO supported ZrO2 sample; in contrast, the presence of crystalline CuO phase was observed in 20 wt. % CuO supported ZrO2 sample. The mesoporous 10 wt. % CuO supported ZrO2 catalyst showed stable catalytic activity for several reaction cycles. The observed high catalytic activity of this catalyst could be attributed to the presence of a higher number of dispersed interactive CuO (Cu2+-O-Zr4+) species, easy reducibility, and greater degree of accessible surface Lewis acid sites.

Lewis Acid-Promoted Heterogeneous Platinum Catalysts for Hydrogenation of Amides to Amines

For the hydrogenation of a tertiary amide (N-acetylpiperidine), Lewis acidic support materials (Nb2O5 and MoOx/TiO2) increase the catalytic activity of Pt metal nanoparticles by a factor of >100. In situ infrared (IR) study shows that coordination of the C=O group of amide to surface Lewis acid sites weakens the C=O bond, and the activated amide undergoes hydrogen transfer from H species on Pt sites. Among 19 types of heterogeneous catalysts tested, Pt/Nb2O5 shows the highest catalytic activity. Pt/Nb2O5 is generally effective for the selective hydrogenation of various tertiary amides and ɛ-caprolactam to the corresponding amines under additive-free and solventless conditions and shows an order of higher turnover number (TON) than previously reported catalysts. Pt-MoOx/TiO2 is the secondary most effective and is reusable 10 times after the reaction.

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.

Mesoporous zinc aluminate (ZnAl2O4) nanocrystal: Synthesis, structural characterization and catalytic performance towards phenol hydroxylation

Mesoporous zinc aluminate (ZnAl2O4) nanocrystal with rhombus-shaped morphology was for the first time synthesized by a vacuum-promoted self-assembly and alkaline hydrothermal approach. The obtained mesoporous ZnAl2O4 (Meso-ZA) was characterized by X-ray diffraction (XRD), nitrogen sorption, transmission electron micrograph (TEM), pyridine adsorbed infrared spectra (Pyridine-FTIR) and temperature-programmed desorption of ammonia (NH3-TPD). The characterization results indicate that the final Meso-ZA possesses rhombus-shaped morphology, worm-like mesoporous structure, crystalline framework, narrow pore size distribution, large surface area (up to 216 m2/g) and rich surface Lewis acid sites. The catalytic test on phenol hydroxylation shows that the synthesized Meso-ZA exhibits a superior catalytic performances to titanium silicalite (TS-1) and bulk zinc aluminate.

Surface Lewis acid-promoted copper-based nanocatalysts for highly efficient and chemoselective hydrogenation of citral to unsaturated allylic alcohols

Chemoselective hydrogenation of α,β-unsaturated aldehydes or ketones to unsaturated alcohols (UAs) is one of the key processes for the production of various important intermediate chemicals. In the present work, well-dispersed ZnO-promoted supported copper nanocatalysts were generated from Cu–Zn–Al layered double hydroxide (CuZnAl-LDH) precursors for liquid-phase chemoselective hydrogenation of citral to allylic alcohols (geraniol and nerol isomers). A series of characterizations including XRD, TEM, STEM, XPS, H2-TPR, and Py-IR demonstrated that the microstructure and catalytic performance of as-formed Cu-based nanocatalysts were significantly affected by the incorporation of Zn into catalyst precursors. It was found that the addition of more ZnO to catalysts could result in better metal dispersion and an increase in the surface Cu+/(Cu+ + Cu0) ratio and surface Lewis acid sites. In liquid-phase chemoselective hydrogenation of citral, a high selectivity toward allylic alcohols (>75%) at complete citral conversion was achieved successfully on as-formed non-noble-metal Cu-based nanocatalysts with a Cu/Zn molar ratio of 2 : 1 under mild reaction conditions (e.g. 80 °C, 1.0 MPa). The high efficiency of the catalysts was attributed mainly to both the synergism between Cu0 and Cu+ species and the promotion of surface Lewis acid sites, thereby improving the dissociation of hydrogen and facilitating the adsorption of the citral molecule and the following activation of the carbonyl group during the citral hydrogenation.

Rich surface Co(iii) ions-enhanced Co nanocatalyst benzene/toluene oxidation performance derived from Co(II)Co(III) layered double hydroxide.

A hierarchical CoCo layered double hydroxide (LDH) nanostructure was constructed through a facile topochemical transformation route under a dynamic oxygen atmosphere. Self-assembled coral-like CoAl LDH nanostructures via the homogeneous precipitation method were also inspected under different ammonia-releasing reagents and solvents. Benzene and toluene were chosen as probe molecules to evaluate their catalytic performance over the metal oxide CoCoO and CoAlO calcined from their corresponding LDH precursors. Nanocatalyst of trivalent Co ions replaced Al(3+) ions in the bruited-like layer had a higher catalytic activity (T99(benzene) = 210 °C and T99(toluene) = 220 °C at a space velocity = 60 000 mL g(-1) h(-1)). Raman spectroscopy, XPS and H2-TPR demonstrated the existence of abundant high valence Co ions that serve as active sites. TPD verified the types of active oxygen species and surface acid properties. It was concluded that the high valence Co ions induced excellent low-temperature reducibility. Surface Lewis acid sites and the surface Oads/Olatt molar ratio (0.61) played relevant roles in determining its catalytic oxidation performance. Our design in this work provides a promising approach for the development of nanocatalysts with exposed desirable defects.

Synthesis of mesoporous silica-zirconia supported phosphotungstic acid and its catalytic performance for oxidative desulfurization of fuel

Mesoporous silica-zirconia supported phosphotungstic acid was synthesized by evaporation induced self-assembly method and used as oxidative desulfurization catalysts. The structural properties of as-prepared catalysts were characterized using various analytical techniques including X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and nitrogen adsorption desorption. The experimental results showed that HPW was highly dispersed on mesoporous framework. The surface acidity of catalysts was analyzed by FTIR measurement of adsorbed pyridine.The surface Lewis acidity was improved with increasing the content of zirconium in the samples. The mesoporous composites were used as catalysts with H2O2 as oxidant for oxidative desulfurization of model fuel. The catalytic activity results showed that the surface Lewis acid sites acted as selective adsorption active sites for dibenzothiophene, which facilitated the sulfur removal from model fuel in the presence of arene. A slight decrease in activity of the recovered catalyst used in the proceeding rounds indicated the reusability of the catalyst.

Mechanism of catalytic ozonation in Fe ₂O₃/Al ₂O₃@SBA-15 aqueous suspension for destruction of ibuprofen.

Fe2O3 and/or Al2O3 were supported on mesoporous SBA-15 by wet impregnation and calcinations with AlCl3 and FeCl3 as the metal precursor and were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectra (FTIR) of adsorbed pyridine. Fe2O3/Al2O3@SBA-15 was found to be highly effective for the mineralization of ibuprofen aqueous solution with ozone. The characterization studies showed that Al-O-Si was formed by the substitution of Al(3+) for the hydrogen of surface Si-OH groups, not only resulting in high dispersion of Al2O3 and Fe2O3 on SBA-15, but also inducing the greatest amount of surface Lewis acid sites. By studies of in situ attenuated total reflection FTIR (ATR-FTIR), in situ Raman, and electron spin resonance (ESR) spectra, the chemisorbed ozone was decomposed into surface atomic oxygen species at the Lewis acid sites of Al(3+) while it was converted into surface adsorbed (•)OHads and O2(•-) radicals at the Lewis acid sites of Fe(3+). The combination of both Lewis acid sites of iron and aluminum onto Fe2O3/Al2O3@SBA-15 enhanced the formation of (•)OHads and O2(•-) radicals, leading to highest reactivity. Mechanisms of catalytic ozonation were proposed for the tested catalysts on the basis of all the experimental information.

Facile preparation of ordered mesoporous MnCo2O4 for low-temperature selective catalytic reduction of NO with NH3.

Ordered mesoporous MnCo2O4 nanomaterials were successfully prepared through the nanocasting route using SBA-15 and KIT-6 as hard templates. These mesoporous nanomaterials were characterized using XRD, BET, TEM, NH3-TPD, H2-TPR, NO-TPD, XPS and DRIFT. The low temperature selective catalytic reduction (SCR) activity of NO with NH3 was investigated, which revealed that 3D-MnCo2O4 using KIT-6 as a template can totally clean all NO over a wide temperature range of 100-250 °C with a gas hourly space velocity (GHSV) of 32,000 h(-1), while 2D-MnCo2O4 with SBA-15 as a template had 95% conversion rate at the same condition. 3D-MnCo2O4 showed the best performance to clean NO due to its typical three-dimensional porous structure, large specific surface area, abundant active surface oxygen species and Lewis acid sites. All the results indicate that a novel, cheap catalyst for catalytic removal of NO can be designed by controlling the morphology at the nanoscale.