Abundant Acid Sites Research Articles

Selective catalytic reduction of NOx with NH3 over a novel MOF- derived MnOx catalyst

A novel MnOx catalyst (Mn-MP) derived from MOF-Mn3(BTC)2(H2O)6 was fabricated and investigated for the selective catalytic reduction of NOx by NH3 at low temperatures. Mn-MP catalyst showed a superior activity and noticeably improved tolerance to SO2. The MOF precursor and doping PVP led to large surface area, small particle size and more crystalline defects of Mn-MP catalyst. The formation of abundant acid sites, Mn4+ and surface chemical oxygen would promote the NH3-SCR reaction. In situ DRIFTs demonstrated the high SO2 tolerance over the Mn-MP catalyst not only results from the reduced irreversible sulfurization rate, but also the promoting adsorption of active bidentate nitrates and NH4+ in the coexistence of sulfates. Our work shed light on the design of porous MOF-derived MnOx catalysts with high SO2 resistance at low temperatures.

Effects of the preparation method of Cu⁃SAPO⁃34 catalysts in de-NOx SCR by propylene

Cu-SAPO-34 catalysts prepared by incipient wet impregnation (IWI), liquid ion-exchange (LIE) and one-pot (OP) hydrothermal methods were applied in selective catalytic reduction of NO with propylene (C3H6-SCR). Under diesel engine exhaust conditions containing 10% O2 and 5% H2O, Cu-SAPO-34(OP) obtained ~95% NOx conversion and ~95% N2 yield at 250–300 °C. Compared the IWI and LIE samples, the Cu-SAPO-34(OP) sample had abundant Lewis acidic sites, the largest specific surface area, and the most isolated Cu2+ species, thus it exhibited the best de-NOx performance. In contrast, more inactive CuO species formed on the surface of the LIE and IWI samples. The isolated Cu2+ ions in the 8MR rings have high reducibility, and are supposed to be the main active sites of Cu-SAPO-34 catalysts in the C3H6-SCR reaction. In situ DRIFTS suggests that isolated copper ions are favourable for the adsorption and activation of NOx and C3H6, thus enhancing the formation of isocyanate (-NCO) and -CN species, which act as key intermediates and further produce N2. This study can be helpful to provide some references for the design of HC-SCR catalyst and application of diesel NOx abatement.

Delaminated MWW-type zeolite and its catalytic performance in alkylation of benzene with cyclohexene

A type of delaminated MWW zeolite is synthesized in the presence of hexamethyleneimine and dicyclohexylamine via a hydrothermal crystallization method. The appropriate molar ratio range of HMI/DCHA is found to correlate with the laminar morphology and crystallinity of the prepared zeolite samples. The delaminated MWW zeolite possesses thinner layered structure, larger external surface area and more accessible active sites than the conventional MCM-22 sample, and its protonated form can be used as a superb catalyst in benzene alkylation with cyclohexene. Under the optimized reaction conditions, the conversion of cyclohexene reaches 99.9 % while the selectivity to the target products, cyclohexylbenzene and dicyclohexylbenzene is 95.4 % in 1000 h lifetime experiment. Because of better diffusion properties and more abundant accessible acid sites on external surface, the delaminated MWW is superior to the conventional MCM-22 in benzene alkylation with cyclohexene, despite of the similar microporous structure and bulk acid properties of these two samples.

Enhanced Catalytic Oxidation of Toluene over Hierarchical Pt/Y Zeolite

The development of efficient Pt-supported zeolite catalysts with tunable micro/mesopore structures for the removal of volatile organic compounds (VOCs) presents a major challenge. Herein, hierarchical Pt/Y zeolites with tunable mesopores are fabricated by varying the etching time before the surfactant-templated crystal rearrangement method and used as catalyst supports for VOC oxidation. The hierarchical Pt/Y zeolites provided an excellent environment for Pt nanoparticle loading with abundant accessible acidic sites. The catalytic performance of the obtained hierarchical Pt/Y zeolites is analyzed using toluene oxidation, with the modified zeolites exhibiting improved catalytic activities. The hierarchical Pt/Y zeolites exhibited higher catalytic toluene oxidation activities than non-hierarchical Pt/Y zeolites. Pt/Y-6h demonstrated the highest catalytic toluene oxidation activity of the prepared catalysts, with a T90 of 149 °C, reaction rate of 1.15 × 10−7 mol gcat−1 s−1, turnover frequency of 1.20 × 10−2 s−1, and an apparent activation energy of 66.5 kJ mol−1 at 60,000 mL g−1 h−1 at a toluene concentration of 1000 ppm. This study will facilitate the fine-tuning of hierarchically porous materials to improve material properties and achieve higher catalytic performance toward VOC oxidation.

Assembling core-shell SiO2@NiaCobOx nanotube decorated by hierarchical NiCo-Phyllisilicate ultrathin nanosheets for highly efficient catalytic combustion of VOCs

Configuring Ni-based catalyst with superior activity and stability is one efficient strategy in achieving its application for VOCs catalytic combustion. Here, we report the rational design and synthesis of core-shell SiO2@NiaCobOx nanotube derived from three-layered C@SiO2@NiCo-Phyllosilicate nanofiber. Through a typical hydrothermal strategy, the hierarchical NiCo-Phyllosilicate ultrathin nanosheets are fitly grown on the surface of order mesoporous SiO2 nanotube, which is obtained through thermal treatment of core-shell C@SiO2 nanofiber in air. Whereafter, a core-shell SiO2@Ni2Co1Ox nanotube is obtained to achieve the highly efficient conversion of VOCs in the presence of 5vol%H2O. We observe this special structural design can own superior redox ability, provide active oxygen species (Oads), and generate the abundant acid sites. Ideally, the low-temperature catalytic combustion of toluene over the core-shell SiO2@Ni2Co1Ox nanotube is realized through the combined action of Marse-van Krevelen (MvK) and Langmuir-Hinshelwood (L-H) mechanism. Simultaneously, a good thermal stability and water resistance is also achieved, which benefits from the guided growth of order mesoporous SiO2 nanotube, the presence of strong interactions (Ni-O-Si band) and the contribution of surface -OH groups in the special “fibrous” structure of phyllosilicate.

Progress in Metal-Organic Framework Catalysts for Selective Catalytic Reduction of NOx: A Mini-Review

Nitrogen oxides released from the combustion of fossil fuels are one of the main air pollutants. Selective catalytic reduction technology is the most widely used nitrogen oxide removal technology in the industry. With the development of nanomaterials science, more and more novel nanomaterials are being used as catalysts for the selective reduction of nitrogen oxides. In recent years, metal-organic frameworks (MOFs), with large specific surface areas and abundant acid and metal sites, have been extensively studied in the selective catalytic reduction of nitrogen oxides. This review summarizes recent progress in monometallic MOFs, bimetallic MOFs, and MOF-derived catalysts for the selective catalytic reduction of nitrogen oxides and compares the reaction mechanisms of different catalysts. This article also suggests the advantages and disadvantages of MOF-based catalysts compared with traditional catalysts and points out promising research directions in this field.

Oxygen vacancies enriched Nb-WOX supported Ru for polychlorinated aromatics oxidation: Reaction mechanism exploration

The current catalysts for polychlorinated aromatics (PCAs) oxidation showed either poor low-temperature catalytic activity, low mineralization ability or bad stability, and developing oxygen vacancies enriched catalysts with abundant acid sites is a potential solution for the above drawbacks. Herein, oxygen vacancies enriched Nb-WOX were developed for o-DCB (o-dichlorobenzene, a PCAs model compound) oxidation, and they showed notably better low-temperature catalytic activity with T50% <350 °C than did commercial WO3 (T50% >400 °C) under the WHSV of 36000 mL g−1h−1. Nb-WOX supported Ru exhibited even lower T50% (173 °C) and T90% (238 °C), as well as high COX yield with >99% ratio of CO2 to CO and good stability (>99% conversion for 500 mins) when with 1.5 wt% Ru loading at 300 °C. The above phenomenon was attributed to the abundant oxygen vacancies and acid sites in Nb-WOX that contributed to the better redox property, stronger interaction with Ru species and the removal of accumulated chloride. The o-DCB oxidation mechanism revealed that the higher CO2 ratio to CO on Ru/Nb-WOX arose from the deep oxidation of CO byproducts. The work described here will instruct us to degrade PCAs effectively through catalysts design.

Tuning the acidity and pore structures of NiP based bifunctional catalyst for efficient n-hexane isomerization

Isomerization of alkane is one of the most important catalytic processes for producing environmentally friendly gasoline fraction with high octane number. However, the development of non-noble metal based catalysts with high conversion and selectivity towards multi-branched isomerized products is still challenging. Herein, we report that a NiP-Hβ/modified Kaolin catalyst was successfully prepared by ball-milling of amorphous NiP nanoparticles with Hβ/modified Kaolin composites and applied for n-hexane isomerization. The introduction of chemically modified Kaolin with abundant weak-medium acid sites and mesopores not only alleviates the cracking of isomerized products on the strong acid sites on Hβ, but also provides more active sites for the formation of di-branched isomerized products with high octane number. As a result, the NiP-Hβ/modified Kaolin catalyst shows an excellent catalytic performance for n-hexane isomerization in terms of good conversion (78.1%), high selectivity towards mono-branched (75.4%) and di-branched (21.4%) i-C6 products, and high liquid yield (99.9%). This work paves the way to the development of high performance non-noble metal based catalysts for large scale production of clean gasoline.

Fabricating Bifunctional Co−Al2O3@USY Catalyst via In‐Situ Growth Method for Mild Hydrodeoxygenation of Lignin to Naphthenes

AbstractTo enhance the catalytic activity and stability of metal catalysts in the hydrodeoxygenation of lignin derivatives into naphthenes, a bifunctional Co−Al2O3@USY catalyst was fabricated by the reduction of CoAl layered double hydroxide in‐situ grown on the USY zeolite. In the hydrodeoxygenation of guaiacol, a 100.0 % conversion with cyclohexane yield up to 93.6 % was achieved at 180 °C, 3 MPa for 4 h, which should be the hitherto lowest reaction temperature that has been reported over Co‐based metal catalysts. This catalyst was also relatively stable with 5 runs and exhibited excellent catalytic performance in the hydrodeoxygenation of other lignin model compounds and even real lignin feedstock into naphthenes. The high‐efficiency of Co−Al2O3@USY was attributed to the synergistic effect between well‐dispersed small Co nanoparticles and abundant acidic sites on the USY surface, while the outstanding stability was attributed to the anchoring effect of Al2O3 matrix to Co nanoparticles which avoided the leaching of Co species and particle agglomeration. This work provides a potential strategy for the design of an efficient and stable catalyst for lignin utilization.

Low-Temperature NH3-SCR on Cex-Mn-Tiy Mixed Oxide Catalysts: Improved Performance by the Mutual Effect between Ce and Ti

A series of Cex-Mn-Tiy catalysts were synthesized using the coprecipitation method, and sodium carbonate solution was used as a precipitant. The various catalysts were assessed by selective catalytic reduction of NOx with NH3, and characterized by X-ray diffraction, Raman spectroscopy, H2 temperature-programmed reduction, NH3 temperature-programmed desorption, and X-ray photoelectron spectroscopy to investigate the physicochemical properties, surface acidity, and redox abilities of the Cex-Mn-Tiy catalysts. The Ce0.1-Mn-Ti0.1 catalyst exhibited the best catalytic performance (more than 90% NOx conversion in the range of 75 to 225 °C), as a result of proper redox ability, abundant acid sites, high content of Mn4+ and Ce3+, and surface-adsorbed oxygen (OS). The results of in situ DRIFT spectroscopy showed that the NH3-SCR reaction followed both the E-R and L-H paths over the Ce0.1-Mn-Ti0.1 catalyst, and it occurred faster and more sharply when it mainly abided by the E-R mechanism.

Selective adsorption of trace gaseous ammonia from air by a sulfonic acid-modified silica xerogel: Preparation, characterization and performance

The toxic and corrosive gaseous ammonia (NH3) poses serious environmental and health risks. However, removal of low concentration NH3 from air remains challenging. In this work, a SO3H-modified silica xerogel (MPTS-X) with good porous structure stability and abundant Bronsted acid sites was fabricated, and its physicochemical properties and adsorption performance for NH3 were systematically investigated. Static adsorption experiments showed the MPTS-1.0 had high NH3 adsorption capacity of 4.02 mmol⋅g−1 at 0.1 bar and 7.00 mmol⋅g−1 at 1.0 bar, which was much higher than that of MPTS-0 (3.19 and 1.95 mmol⋅g−1). Langmuir-Freundlich isotherm model fitted the experimental data of NH3 adsorption well, implying that the MPTS-1.0 surface is not homogeneous and there are different adsorption sites, which was further confirmed by temperature programmed desorption spectrum analysis. According to IAST calculation, the adsorption selectivity of MPTS-1.0 for NH3/N2 (1: 9) and NH3/CO2 (1: 1) was 6175 and 165 at 1.0 bar, respectively, indicating the MPTS-1.0 has excellent adsorption selectivity. Dynamic adsorption experiments and field test demonstrated the MPTS-1.0 could quickly capture trace NH3, even if the initial concentration is as low as 2 ppm. Successive adsorption–desorption experiments demonstrated the MPTS-1.0 had good regenerability. In-situ infrared spectroscopy analyses indicated NH3 adsorption on the MPTS-1.0 was mainly due to hydrogen bonding interaction and proton transfer to form Si-OH…NH3 and -SO3-NH4+, respectively. NH3 molecules bound firmly to acidic sites on the xerogel surface, which reduced re-release risk. This work may provide a feasible way to develop promising adsorbents for the removal of gaseous ammonia from air.

Realizing Fast Synthesis of High‐Silica Zeolite Y with Remarkable Catalytic Performance

AbstractHigh‐silica zeolite Y (FAU) plays a vital role in (petro)chemical industries. However, the slow nucleation and growth kinetics of the high‐silica FAU framework limit its direct synthesis and the improvement of framework SiO2/Al2O3 ratio (SAR). Here, a facile strategy is developed to realize the fast crystallization of high‐silica zeolite Y, which involves the combination of high crystallization temperature, ultra‐stable Y (USY) seeds and efficient organic‐structure directing agent (OSDA). The synthesis can be finished in 5–16 h at 160 °C and with tunable SAR up to 18.2, and the key factors affecting crystallization kinetics and phase purity are elucidated. Moreover, the crystallization process was monitored to reveal the fast crystal growth mechanism. The high‐silica products possess high (hydro)thermal stability and abundant strong acid sites, which endow them excellent catalytic cracking performance, obviously superior to commercial USY.

Realizing Fast Synthesis of High-Silica Zeolite Y with Remarkable Catalytic Performance.

High-silica zeolite Y (FAU) plays a vital role in (petro)chemical industries. However, the slow nucleation and growth kinetics of the high-silica FAU framework limit its direct synthesis and the improvement of framework SiO2 /Al2 O3 ratio (SAR). Here, a facile strategy is developed to realize the fast crystallization of high-silica zeolite Y, which involves the combination of high crystallization temperature, ultra-stable Y (USY) seeds and efficient organic-structure directing agent (OSDA). The synthesis can be finished in 5-16 h at 160 °C and with tunable SAR up to 18.2, and the key factors affecting crystallization kinetics and phase purity are elucidated. Moreover, the crystallization process was monitored to reveal the fast crystal growth mechanism. The high-silica products possess high (hydro)thermal stability and abundant strong acid sites, which endow them excellent catalytic cracking performance, obviously superior to commercial USY.

Synergistic effect in bimetal Cr-Pt/ZSM-5 catalysts with high activity for CVOCs low-temperature removal

A series of Cr-Pt/ZSM-5 catalysts (0.5 wt% Pt) with various doping amounts of Cr species were prepared to investigate the synergy effect of MOx and ZSM-5 zeolites with micro-mesoporous structure on the oxidation performance of CVOCs low-temperature removal. The results indicate that the strong interaction between Pt and Cr species can be promoted by the doping of appropriate amount of Cr species, which is beneficial for the formation of more Cr6+ species and acid sites (especially for the strong acid sites). The synergistic effect of strong oxidizing sites (Cr6+/Pt species) and abundant acid sites (ZSM-5) is in favor of the dechlorination of DCE as well as the deep oxidation of intermediate products and by-products. Among these catalysts, the 10%Cr-Pt/ZSM-5 catalyst possesses the greatest oxidation activity for 1,2-dichloroethane (DCE) degradation with the T90% value as low as 255 °C, and also possesses excellent durability and catalytic activity for other non-chlorinated VOCs degradation. However, the structure of ZSM-5 zeolites is partly destroyed by the introduction of excess Cr species and the serious agglomeration of Cr species also occurs, which would weaken the interaction between Cr/Pt species and ZSM-5, resulting in a significant decline in its Cr6+ species and strong acid sites.

ZnO as a simple and facile catalyst for acid-base coordination transformation of biomass-based monosaccharides into lactic acid

Catalytic production of lactic acid (LA) from biomass has attracted increasing attention due to the wide applications of LA. However, the industrial-scale production of LA is seriously limited by the complex synthetic process of efficient catalyst. In this work, we applied commercially available ZnO as a simple and facile heterogeneous catalyst to catalyze the conversion of glucose into LA under hydrothermal conditions. The ZnO possesses abundant strong Lewis acid and base sites and acts as bifunctional catalyst to promote the cascade reactions from glucose to LA, leading to a high yield of 41.5%. The robustness of catalytic process and the wide scope of applicable substrates like monosaccharides and disaccharides further extend the application of hydrothermal biomass conversion using facile catalysts.

Mechanistic insights into the role of acidity to activity and anti-poisoning over Nb based catalysts for CVOCs combustion

A series of Nb-based catalysts with different acid types of carriers (TiO2, Beta molecular sieve, and CeO2) were synthesized. In the catalytic oxidation of dichloromethane (DCM), Nb/CeO2 exhibited the best catalytic activity, resistance to chlorine poisoning/carbon deposits, and stability, due to the abundant acid sites and appropriate ratio of Lewis to Bronsted acid sites on the surface. Both Lewis and Bronsted acid sites could participate in the breakdown of C-Cl bonds, whereas the former exerted a greater influence. The Bronsted acid sites participated in the desorption of Cl species, and were prone to the generation of carbon deposits. The chlorine species easily accumulated at the Lewis acid sites of Nb/TiO2(containing predominantly Lewis acid sites), formed metal chlorides, and caused chlorine poisoning. Carbon deposits made it difficult for Nb/Beta (containing predominantly Bronsted acid sites) to maintain long-term activity. Additionally, the acid sites generated by the active component were more important for DCM degradation than those generated by the carrier.

Under-coordinated AlF3 clusters confined in carbon matrix with robust sintering resistance for dehydrofluorination of hydrofluorocarbons

The development of catalysts for the resource utilization of potent greenhouse gases, hydrofluorocarbons (HFCs) by catalytic pyrolysis is of great significance to alleviate the greenhouse effect. Fabricating small catalyst particles with high specific surface area leading to more Lewis acid sites is the effective strategies to achieve highly functional catalysts. However, catalysts tend to sinter at elevated temperatures. Herein, we synthesized carbon matrix confined Al2O3 clusters via the calcination of MIL-53-Al under N2 atmosphere. Then the Al2O3 clusters were in situ fluorinated and turned into AlF3 clusters accordingly. The derived carbon confined AlF3 clusters exhibited outstanding sintering resistance even following thermal treatment at 600 °C. Moreover, the carbon confined AlF3 clusters presented superior catalytic performance for 1,1-difluoroethane (HFC-152a) conversion with near 40% higher than that of AlF3 prepared by traditional precipitation. The excellent catalytic activity and stability were mainly ascribed to the abundant under-coordinated Al species with strong interaction with carbon matrix and high specific surface area. Thus, in this paper, we developed new practicable method to synthesize highly efficient Lewis acid catalysts with strong sintering resistance and abundant acid sites.

Confinement and synergy effect of bimetallic Pt-Mn nanoparticles encapsulated in ZSM-5 zeolite with superior performance for acetone catalytic oxidation

Catalytic oxidation is one of the most promising methods to remove VOCs. Herein, a zeolite-confined PtMn bimetal catalyst (PtMn0.2 @ZSM5) was synthesized using a ligand-assisted hydrothermal method, which showed extraordinary catalytic activity with a T95 of 165 ℃ for the acetone oxidation in the presence of 5 vol% water. Thanks to the zeolite confinement and bimetallic synergy effect, the PtMn0.2 @ZSM5 exhibited small nanoparticle sizes, abundant acid sites, higher active Pt0 content, and sufficient active oxygen species. These excellent properties promoted the adsorption of VOC, the deep oxidation of VOC, and the desorption of CO2. In-situ DRIFTS proposed an L-H mechanism for acetone oxidation over PtMn0.2 @ZSM5. Significantly, the PtMn0.2 @ZSM5 presented good durability and water resistance under high GHSV conditions. Also, it was confirmed effective for various VOCs oxidation, such as toluene, ethyl acetate, propane, and dichloromethane, showing a promising industrial prospect for eliminating VOCs.

Correlations of Lewis acidic sites of nickel catalysts with the properties of the coke formed in steam reforming of acetic acid

Coking is a major challenge in the reactions of steam reforming. In addition to the nature of metal species, the properties of carrier also affect coking behaviors of a reforming catalyst. In this work, we prepared nickel carried on Al2O3, SiO2 or the molecular sieves (25-H, 38-H) for assessed in steam reforming of acetic acid. The Ni/Al2O3 catalyst contained abundant Lewis acidic sites while the Ni/SiO2, Ni/38-H and Ni/25-H catalysts did not. The Lewis acidic sites in Ni/Al2O3 induced the polymerisation reactions to form the polymeric coke, which wrapped metallic nickel species and was the primary reason for deactivation of catalyst, even though the amount of coke was small. The substantial amount of coke formed over the other three catalysts did not lead to rapid catalyst deactivation. With the form of carbon nanotube or other forms was mainly the type of catalytic coke, depending on characteristic of the carrier. Comparing with the polymeric coke in the catalyst of Ni/Al2O3, the catalytic coke had a high C/H ratio, a much higher thermal stability and was more aromatic rich.

Development and characterisation research on SnO2‐Al2O3‐NiO/SO42− catalysed epoxidation of soybean oil under hydraulic cavitation

AbstractA new hydraulic cavitation‐assisted epoxidation scheme was developed by using a cost‐effective, environment‐friendly, and recyclable solid‐acid catalyst. SnO2‐Al2O3‐NiO/SO42− solid‐acid catalysts were prepared using co‐precipitation and impregnation methods, which were characterised by several techniques and analysed using the Hammett acidity function. The prepared catalysts were found to have a large specific surface area (53.95 m2/g) and abundant acid sites (94 μmol/g). Exhibits higher stability and more effective catalytic activity compared with H2SO4. Under the optimal reaction conditions, the relative conversion to oxirane of 86.97% was obtained in only 1.5 h of reaction time. Moreover, the catalyst could be easily separated and showed satisfactory catalytic activity after five reuse cycles. The current work has clearly demonstrated that the application of hydraulic cavitation technology to the solid‐acid‐catalysed epoxidation reaction of soybean oil can shorten the reaction time, increase the relative conversion to oxirane, and be a viable method with promising applications.