Large Number Of Acid Sites Research Articles

Low-Temperature Reduction of NOx by NH3 with Unity Conversion on Nanofilament MnO2/Activated Semi-Coke Catalyst.

Selective catalytic reduction of nitrogen oxides with NH3 at low temperatures remains a crucial goal for industrial applications. However, effective catalysts operating at 70-90 °C are rarely reported, limiting SCR scenarios to high-temperature conditions. Herein, we report a unique MnO2 nanofilament catalyst grown on activated semi-coke synthesized via a one-step in situ hydrothermal approach, which exhibits a stable and marked 100 % conversion rate of NO to N2 with 100 % selectivity at 90 °C, superior to the other prepared structures (nanowires, nanorods, and nanotubes). Temperature-programmed desorption shows a large number of acid sites on MnO2(NFs)/ASC, benefiting the formation of NH4 + ions. Meanwhile, diffuse reflectance infrared Fourier transform spectroscopy reveals the activation of NO with O2 to form bidentate nitrate/bridging nitrate NO2 intermediates via bidentate nitrate species, triggering the Fast SCR with NH3 at low temperatures. Such an effective, easy-to-prepare, and low-cost catalyst paves a new pathway for low-temperature SCR for a wide range of application scenarios.

Acid-activated α-MnO2 for photothermal co-catalytic oxidative degradation of propane: Activity and reaction mechanism

In order to further reduce the energy consumption of the conventional thermal catalytic oxidation system and improve the degradation efficiency of pollutants, photothermal synergistic catalytic oxidation (PTSCO) system was constructed in this paper with propane as simulated pollutant representing VOCs, and then the modified α-MnO2 catalysts were prepared by using the acid activation method, which were used for the catalytic oxidation of propane in PTSCO. The α-MnO2 with appropriate acid concentration possessed excellent low-temperature reducibility, abundant active oxygen species, fast oxygen migration rate and a large number of acid sites. The optimal catalyst, H0.05-MnO2, had a T90 of 204 °C in the PTSCO system, which reduced by more than 30 °C relative to the α-MnO2 (T90 of 235 °C). Moreover, H0.05-MnO2 demonstrated excellent water resistance and long-term stability (T = 45 h). It was shown that the combination of photocatalysis and thermocatalysis can improve propane degradation by examining the kinetics of propane degradation in the PTSCO system and the conformational relationship of propane degradation by catalysts. Furthermore, a multi-pathway synergistic mechanism between photocatalysis and thermocatalysis in the PTSCO system was proposed. This work provided a theoretical basis for the preparation of high-performance catalysts and the catalytic degradation of propane.

Spatial isolation effect improves the acidity and redox capacity of ZSM-5 encapsulating Pt catalyst to achieve efficient toluene oxidation

This paper introduces a novel approach of encapsulating Pt nanoparticles within the pores of a molecular sieve. By comparing the dispersion, reducibility and surface acid sites, the effect of the combination of molecular sieves and Pt particles on the catalytic performance of toluene was studied. The findings demonstrate that Pt nanoparticles were encapsulated within the molecular sieve framework of ZSM-5 molecular sieves using an in-situ synthesis method, exhibiting remarkable catalytic activity towards toluene oxidation (T90=172 ℃). Mainly due to the spatial confinement of the ZSM-5 molecular sieve framework reduces the size of Pt nanoparticles, enhances their dispersion of the Pt@ZSM-5 catalysts. In addition, the space separation effect of ZSM-5 on Pt metal enhances their interaction and promotes the valence state conversion of Pt species. It leads to a large number of acid sites and oxygen vacancies in the catalyst, which improves the adsorption and migration of toluene, thereby improving the catalytic activity. This study will provide valuable insights into the preparation of new nanocatalysts and the catalytic treatment of toluene.

New insights into electronic structure effect of the SnMnOx nanorod catalysts for low-temperature catalytic combustion of o-dichlorobenzene.

For the catalytic combustion reaction of chlorinated volatile organic compounds (CVOCs), the electronic structure of the catalyst surface are important factors in determining the activity, selectivity, and chlorine-resistance stability. Herein, a series of SnMnOx catalysts for the catalytic combustion of CVOCs were prepared by the changing of Sn-doping way to regulate the electron valance state of Mn element, including reflux (R-SnMnOx), co-precipitation (C-SnMnOx) and impregnation (I-SnMnOx). It was discovered that the R-SnMnOx catalyst had better activity and chlorine resistance than the R-MnOx, C-SnMnOx and I-SnMnOx catalyst, and the doping ways of Sn in MnOx catalyst could regulate greatly the surface acidity, active oxygen species, the chemical state of Mn&+ species, and redox ability. Especially, the R-SnMnOx catalysts exhibit excellent water resistance, and the reasons were related to the strong interaction of Sn&+ and Mn&+, which could promote obviously the dispersion of active Mn species, form a large number of acid sites, provide the abundant lattice oxygen species, and own the excellent redox ability, which accelerate the rate of charge transfer between Sn&+ and Mn&+ (Sn4++Mn2+→Sn2++Mn4+) to produce the abundant active species and accelerate the rapid conversion of benzene and intermediates conversion.

Promotion effect of SO42−/Fe2O3 modified MnOx catalysts for simultaneous control of NO and CVOCs

To improve the simultaneous removal of NOx and CVOCs catalytic activity for MnOx catalysts, SO42−/Fe2O3 modified catalysts were prepared by the coprecipitation method. Compared with pure MnOx, the activity of the SO42–0.10Fe–MnOx catalysts was significantly improved, with >90% NO conversion and CB oxidation. The synergistic effect between SO42−/Fe2O3 and Mn can lead to higher BET, Mn4+/Mn3+ and oxygen vacancy contents, which can provide large number of acid sites. In addition, the characterization results showed that SO42−/Fe2O3 modification led to the generation of more Fe2+/Fe3+ species and chemisorbed oxygen species, and the reduction performance of the catalyst can be influenced by the redox cycle: Fe2+ + Mn4+ ↔ Fe3+ + Mn3+. Therefore, the enhancement of redox performance and abundant surface acidic sites are the main reasons for the improvement of synergistic catalytic activity over SO42–0.10Fe–MnOx catalysts.

Glycerol acetalization over highly ordered mesoporous molybdenum dioxide: Excellent catalytic performance, recyclability and water-tolerance

Highly ordered mesoporous molybdenum dioxide (meso-MoO2) was successfully synthesized by a nano-replication method using a mesoporous silica template with 3D cubic Ia3d mesostructure. Structural analyses using X-ray diffraction, N2 sorption, and electron microscopy indicated that the meso-MoO2 material exhibited high surface area (106 m2/g), large pore volume (0.66 cm3/g), and well-defined mesopores. Under the optimized reaction conditions, the meso-MoO2 catalyst showed excellent catalytic performances for the acetalization of glycerol with acetone (95.8% of glycerol conversion, and 97.8% of solketal selectivity) at 20 °C, due to its high surface area with large number of acid sites. During recycling, the meso-MoO2 catalyst exhibited a slight decrease in activity; and after a simple regeneration at 350 °C, the catalytic performances could be completely recovered. More importantly, a partial hydrophobic treatment of the meso-MoO2 catalyst dramatically enhanced the water-tolerance during the catalytic reaction.

Alkali and Phosphorus Resistant Zeolite-like Catalysts for NOx Reduction by NH3.

At present, the deactivation of selective catalytic reduction (SCR) catalysts caused by the coexistence of alkali metal and phosphorus (P) remains an urgent problem and lacks corresponding strategies against catalyst poisoning. Herein, a novel zeolite-like Ce-Si5Al2Ox catalyst derived from an ultrasmall nanozeolite EMT precursor was synthesized without organic templates at ambient temperature. This catalyst was able to maintain above 95% NOx conversion in the 270-540 °C temperature range. Moreover, 1 wt % potassium (K) and 5 wt % P loading had no influence on the SCR performance of the Ce-Si5Al2Ox catalyst at 300-480 °C. It was demonstrated that cerium (Ce) was highly dispersed in the amorphous aluminum (Al) silicate derived from EMT zeolites and expressed high catalytic performance. Besides, a large number of acid sites were reserved to absorb ammonia allowing effective participation in the SCR reaction and capturing alkali metals, thus improving the SCR performance and K resistance. Additionally, the strong interaction between Ce and aluminosilicate decreased cerium phosphate production, preventing deactivation of the catalysts. Thus, this novel low-cost zeolite-like Ce-Si5Al2Ox catalyst with a highly active ion-exchanged metal phase and abundant surface acid sites paves a way for designing new efficient and poisoning-resistant SCR catalysts for practical applications.

Catalytic Dehydration of Glycerol to Acrolein over Aluminum Phosphate Catalysts

Gas phase conversion of glycerol to acrolein over a variety of aluminum phosphate (AlP) catalysts synthesized by a simple replacement reaction method with a variation in calcination temperature (300-700 oC, AlP-300, AlP-400, AlP-500, AlP-600, and AlP-700, respectively) has been investigated. The textural properties, acidities and coke contents of the samples were also determined. The catalysts were presented in an amorphous state when the AlP sample was calcined below 500 oC. Further increasing the calcination temperature promoted the formation of orthorhombic α-AlPO4 crystal. The weak acid sites increased when the calcination temperature was raised from 300 to 500 oC. However, the weak acid sites decreased when the AlP was calcined above 500 oC. The acidity of the catalyst played a crucial role in the glycerol dehydration reaction. The maximum acrolein selectivity of 66% at 98% glycerol conversion was obtained over AlP-500 catalyst, due to the largest number of acid sites and appropriate textural properties. AlP-700 exhibited the lowest glycerol conversion, owing to the formation of orthorhombic α-AlPO4 crystalline phase and the lowest amount of acid sites under high calcination temperature. The significant reduction in the acidity of the used sample led to a decrease of glycerol conversion.

Effects of MOx (M=Mn, Cu, Sb, La) on V–Mo–Ce/Ti selective catalytic reduction catalysts

A series of MOx–V2O5–MoO3–CeO2/TiO2 (M = Mn, Cu, Sb, and La) catalysts were prepared via an impregnation method. The physico-chemical properties of the catalysts were characterized and their NH3-SCR of NO performance was compared. The Mn-loaded catalyst (Mn5V1Mo3Ce7/Ti) exhibits a large number of acid sites of varying strength, and together with good reducibility of the catalyst, contributes to the optimal SCR performance. The sulphate species formed in the presence of SO2 significantly enhance the H2O and SO2 tolerance of Mn5V1Mo3Ce7/Ti. The Cu-loaded catalyst (Cu5V1Mo3Ce7/Ti) demonstrates potential in flue gas applications in the absence of SO2 at low temperatures because of the excellent redox ability observed and the high degree of weak acid sites. The Sb and La loaded catalysts (Sb5V1Mo3Ce7/Ti and La5V1Mo3Ce7/Ti), especially La5V1Mo3Ce7/Ti, exhibit the largest number of acid sites and the lowest reducibility, and therefore, may be suitable for use in high temperature denitrification applications.

Pd/UiO-66(Hf): A highly efficient heterogeneous catalyst for the hydrogenation of 2,3,5-trimethylbenzoquinone

The conversion of 2,3,5-trimethylbenzoquinone (TMBQ) to 2,3,5-trimethylhydroquinone is regarded as one of the most important reactions for the production of vitamin E. Here, we report on a Hf-based metal–organic framework (MOF), UiO-66(Hf), with supported palladium nanoparticles as a highly active, selective, and recyclable catalyst for the direct hydrogenation of TMBQ with H2 under mild reaction conditions. The developed Pd/UiO-66(Hf) catalyst exhibited a marked increase in activity compared with its Pd/UiO-66(Zr) counterpart and conventional Pd/C catalysts. Its excellent catalytic performance is ascribed to the large number of acid sites in the UiO-66(Hf) framework, which promote hydrogenation.

Glucose conversion to methyl levulinate catalyzed by metal ion-exchanged montmorillonites

Various metal ion-exchanged montmorillonite catalysts were prepared, characterized, and evaluated in the conversion of glucose to methyl levulinate in methanol. The bulk and surface features of these catalysts were investigated using X-ray fluorescence spectroscopy, inductively coupled plasma optical emission spectroscopy, powder X-ray diffraction, Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis, scanning electron microscopy, N2 adsorption–desorption, temperature-programmed NH3 desorption, and pyridine adsorption FT-IR spectroscopy. Al3+-exchanged montmorillonite had the best activity and maximum methyl levulinate selectivity because of presence of a large number of acid sites and a good balance of Brønsted and Lewis acid sites on the catalyst. The montmorillonite catalyst can be easily recovered from the reaction mixture by filtration, and reused at least five times without any loss of activity and selectivity after treatment with H2O2 solution to remove carbon species deposited on the catalyst surface.

2‐Butanol Dehydration over Highly Dispersed Molybdenum Oxide/MCM‐41 Catalysts

Highly dispersed MoO3 on SiMCM‐41 was applied to the dehydration of 2‐butanol. MoO3 /MCM‐41 catalysts were prepared using a modified atomic layer deposition method. The structural characteristics of MoO3 supported on SiMCM‐41 were examined by inductively coupled plasma spectrometry, nitrogen adsorption, X‐ray photoelectron spectroscopy, and X‐ray diffraction. The textural characteristics of SiMCM‐41 were retained after loading with MoO3 . Ammonia temperature‐programmed desorption and Fourier transformation infrared spectroscopy of adsorbed pyridine showed that Lewis acid sites were formed on the MoO3 /MCM‐41 catalysts. Among the MoO3 /MCM‐41 catalysts, the MoO3 (16.4 wt %)/MCM‐41 catalyst showed the highest catalytic activity for butene synthesis from 2‐butanol dehydration due to the largest number of acid sites.

Efficient production of propylene in the catalytic conversion of glycerol

Vapor-phase catalytic conversion of glycerol into propylene was performed over Cu/Al2O3 and acid-loaded Cu/Al2O3 catalysts in an H2 flow at ambient pressure. Acidic substances such as WO3, MoO3, V2O5 and H3PO4 were loaded on a commercial Cu/Al2O3. The addition of WO3 was found to be effective for promoting the formation of 1-propanol and propylene from glycerol, and 9.3wt% WO3-loaded Cu/Al2O3 (WO3/Cu/Al2O3) catalyst showed the best catalytic performance. WO3/Cu/Al2O3 calcined at a low temperature of 320°C had the largest number of acid sites and gave a 1-proponol selectivity of 38.2% and a propylene selectivity of 47.4% in an H2 flow at 250°C. The catalytic reaction of glycerol was performed over double-bed catalysts, in which WO3/Cu/Al2O3 was charged in the upper bed and a commercial silica–alumina was charged in the lower bed, to promote the conversion of 1-propanol into propylene. A high propylene selectivity of 84.8% was obtained over the double-bed catalysts at 100% glycerol conversion. An efficient catalytic process for the production of propylene from glycerol was proposed.

Designed SO42−/Fe2O3-SiO2 solid acids for polyoxymethylene dimethyl ethers synthesis: The acid sites control and reaction pathways

The designed model SO42−/Fe2O3-SiO2 catalysts with varied acidic strength and Brönsted acid sites were successfully prepared. The properties of acid sites can be regulated by preparation method and silica incorporation. The S/Fe sample without silica had stronger acid strength, more Brönsted acid sites and higher acid density, which showed the best catalytic performance. NH3-TPD, pyridine-IR and methanol/dimethoxymethane adsorption–desorption techniques were employed to reveal the relationship between acid properties of the catalysts and catalytic activity for the reaction of trioxane with methanol. Further, this reaction is determined by the strength of acid sites on catalyst surface and does not affect by the inner pore diffusion. These findings suggest that the catalytic activity and selectivity can be monitored mainly by acid strength and Brönsted acid sites on the sulfated oxides, while trioxane decomposition process seems to be a vital step and polyoxymethylene hemiacetals are supposed to be the key intermediates. Meanwhile, a large number of acid sites and a higher surface area of the catalysts can also improve the catalytic performance. These results should provide some specific guidance for understanding of the reaction mechanism and the catalyst design.

Hydrothermal treatment on ZSM-5 extrudates catalyst for methanol to propylene reaction: Finely tuning the acidic property

ZSM-5 powder and extrudates are hydrothermally treated by varying temperature and time. Before and after treatment, the materials are characterized and evaluated in the methanol to propylene (MTP) process. The changes in textural properties are compared and particular attention is paid to the nature, number, and ratio of Brønsted (B) and Lewis (L) acid sites. The extrudates catalyst contains substantially a large number of acid sites, especially the L acid sites and hierarchical porous structure due to the addition of the alumina binder. The number of acid sites and the B/L ratio could be finely adjusted by proper hydrothermal treatment. And the hydrothermal treatment parameter on the extrudates plays an important effect on its MTP catalytic performance and product distribution. After steaming at 500°C for 12h, both the zeolite and extrudates catalyst show a high initial activity, achieving ca. 12 percentage point rise in the propylene selectivity during the MTP reaction. However, with time on stream, the treated extrudates perform the steady propylene selectivity (around 43%) and in particular the longest was 89h of catalytic lifetime, compared to the non-treated extrudates (45h) and treated zeolite (57h). The enhanced catalytic performance could be contributed by the appropriate B/L ratio and number of acid sites, in combination with the mesoporous structure, which are crucial for decreasing the hydrogen transfer reaction and inhibiting the coke deposition.

In Situ Delamination of Ferrierite Zeolite and its Performance in the Catalytic Cracking of C4 Hydrocarbons

Ferrierite (FER) zeolites were synthesized by solid transformation at different alkalinities (OH−/Al2O3 molar ratios). The in situ delamination of FER zeolites were achieved and their catalytic performances in the catalytic cracking of C4 hydrocarbons were examined. The relationships among the OH−/Al2O3 molar ratio, FER structure, composition, surface acidity and catalytic performance in C4 hydrocarbon cracking were investigated. The results of X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, inductively coupled plasma atomic emission spectroscopy, N2 adsorption, NH3 temperature-programmed desorption and catalytic cracking showed that with increasing OH−/Al2O3 molar ratio in the synthesis gel, the SiO2/Al2O3 molar ratio of the as-synthesized FER zeolite decreased, the amount of acid sites in the corresponding H-FER increased, and the acid strength weakened. Additionally, the FER zeolite was delaminated at the mesoscale. H-FER5 synthesized at the highest alkalinity had the largest number of acid sites and exhibited the highest catalytic activity in C4 hydrocarbon catalytic cracking among three of the prepared catalysts. H-FER3 synthesized at the second-highest alkalinity showed that the highest yield of benzene and toluene because of the secondary pores resulted from the gaps between the layers, which were beneficial to the diffusion and formation of large molecules.

Catalytic conversion of waste particle board and polypropylene over H-beta and HY zeolites

The catalytic copyrolysis of waste particle board (WPB) and polypropylene (PP) was investigated for the first time over HY (5.1), HY (30), H-Beta and Ga/H-Beta catalysts. The catalysts were characterized by BET and NH3-TPD analyses. The catalytic pyrolysis of the WPB increased the production of gas products (CO, CO2, C1–C4) compared to non-catalytic pyrolysis. Acids and levoglucosan, which are the main components of bio-oil produced from non-catalytic pyrolysis, were converted to more valuable aromatics, phenolics, and furans through dehydration, deoxygenation and aromatization. The most abundant products from the copyrolysis of WPB and PP were large-molecular-mass hydrocarbons (≥C10). However, catalytic copyrolysis increased the yields of small-molecular-mass hydrocarbons in the gasoline range, aromatics and phenolics. The water content in bio-oil was reduced significantly by copyrolysis with PP, contributing to the improvement in oil quality. HY (5.1) with the largest number of acid sites showed higher catalytic activity than HY (30) and H-Beta because the decomposition and reforming reactions during catalytic copyrolysis occurred on the acid sites of the catalysts. Ga/H-Beta showed even higher selectivity toward the aromatics than H-Beta despite the smaller quantity of acid sites, suggesting that Ga promoted the dehydrocyclization of the reaction intermediates.

Chemoselective hydrogenation of aromatic aldehydes over SiO2 modified Co/γ-Al2O3

With benzaldehyde as a model compound, the hydrogenation of aromatic aldehydes to corresponding benzyl alcohols was investigated in a fixed-bed reactor. Co/γ-Al2O3 doped with a amount of SiO2 displayed excellent catalytic performance for this reaction, Co/γ-Al2O3 is modified by SiO2 in two ways, the strong interaction between metal oxide and support is obviously reduced, and also a large number of acid sites are diminished on the catalyst. The first aspect apparently facilitated the reduction of metal oxide, and improved the hydrogenation activity of the catalyst; while the second aspect greatly inhibited the hydrogenolysis of the CO bond, and thus increased the selectivity toward benzyl alcohol.

Synthesis of porous clay heterostructures from high charge mica-type aluminosilicates

High charge micas are ideal materials to be used as porous solid acids because of their extraordinarily high content of framework aluminium atoms and their thermal stability. However, all efforts to create porosity in these materials have led to a disordered porous structure, since the full process is compromised by the highly layer attractive forces and hence by the incorporation stage of the porous wall precursors between the clay layers. Thermally stable and ordered mesoporous materials were synthesized for the first time from those swelling brittle micas through a surfactant templating approach. The interlayer space was firstly pre-expanded using alkylammonium cations, giving rise to a homogeneous organic–inorganic hybrid structure, which directs the polymerization step of the silica source. A regular porous structure in the supermicropore to the small mesopore range (15–25 A) together with an exclusive acidity was found after sample calcination. The extraordinary content of framework aluminium is responsible for the large number of acid sites, both in the rare medium and strongly acidic regions. The thermal stability of the new synthesised materials was also tested.

Controlling the selectivity to chemicals from lignin via catalytic fast pyrolysis

The catalytic fast pyrolysis of alkaline lignin to useful chemicals was investigated using zeolite catalysts with different acidity and pore size. The catalyst played dual roles in this process. In its acid form, it catalytically converted the depolymerized intermediates into desirable and more stable products. This and their surface prevented repolymerization and coke formation. The yield of liquid and the selectivity to desired products can be controlled by tuning of the acidity and pore size of the catalyst. Using no catalyst yielded 40wt.% of liquid, which mainly consisted of 6wt.% (carbon yield) of phenols and 19wt.% (carbon yield) of phenol alkoxy species. The highest yield of phenol alkoxy species was obtained over H-ZSM5 of extremely low number of acid sites; liquid yield of 51wt.% and carbon yield of 24wt.%. The highest yield of liquid (75wt.%) was obtained over H-USY, which had the largest pore size and lowest Si/Al ratio, thus the largest number of acid sites among all the catalyst tested; the carbon yield of aromatic hydrocarbons was around 40wt.% at 650°C. Depolymerized lignin products undergo consecutive reaction to form phenol alkoxy, phenols, and eventually aromatic hydrocarbons.