Redistributing Cu species in Cu-SSZ-13 zeolite as NH3-SCR catalyst via a simple ion-exchange

The nature and distribution of Cu species in Cu-SSZ-13 play a vital role in selective catalytic reduction of NO by NH3 (NH3-SCR),but existing methods for adjusting the Cu distribution are complex and difficult to control.Herein,we report a simple and effective ion-exchange approach to regulate the Cu distribution in the one-pot synthesized Cu-SSZ-13 that possesses sufficient initial Cu species and thus provides a "nat-ural environment" for adjusting Cu distribution precisely.By using this proposed strategy,a series of Cu-SSZ-13x zeolites with different Cu contents and distributions were obtained.It is shown that the dealu-mination of the as-synthesized Cu-SSZ-13 during the ion-exchange generates abundant vacant sites in the double six-membered-rings of the SSZ-13 zeolite for relocating Cu2+ species and thus allows the redistribution of the Cu species.The catalytic results showed that the ion-exchanged Cu-SSZ-13 zeolites exhibit quite different catalytic performance in NH3-SCR reaction but superior to the parent counterpart.The structure-activity relationship analysis indicates that the redistribution of Cu species rather than other factors (e.g.,crystallinity,chemical composition,and porous structure) is responsible for the improved NH3-SCR performance and SO2 and H2O resistance.Our work offers an effective method to pre-cisely adjust the Cu distribution in preparing the industrial SCR catalysts.     

Porous washcoat structure in CeO2 modified Cu-SSZ-13 monolith catalyst for NH3-SCR with improved catalytic performance

A series of CeO₂ modified Cu-SSZ-13 monolith catalysts were prepared by embedding CeO₂ into the washcoat of Cu-SSZ-13 monolith catalyst through solvent combustion method. These CexCu-SSZ-13 catalysts were studied in the selective catalytic reduction (SCR) of NO with NH₃, among which the Ce2Cu-SSZ-13 catalyst exhibited the best low-temperature activity, hydrothermal stability, and sulfur resistance. The physicochemical properties of the catalysts were characterized using multiple methods. Results showed that the acidity, redox capacity, and ammonia adsorption capacity significantly enhanced after CeO₂ modification, thus leading to the high performance of Ce2Cu-SSZ-13 catalyst. Furthermore, the introduction of CeO₂ induced the fast SCR reaction by promoting the oxidation of NO to NO₂. Analog calculation suggested that the porous structure generated via solvent combustion in the washcoat effectively increased the diffusion rate of reaction. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT) analysis showed that Brønsted acid sites were the main active center and the reaction followed Eley–Rideal mechanism.     

过渡金属/分子筛催化剂上选择性催化还原氮氧化物的研究进展

综述了近十年来过渡金属/分子筛催化剂上氨和碳氢化合物选择性催化还原NOx方面的研究进展。在NH3-SCR体系,着重介绍了铜基和铁基分子筛催化剂的研究状况,探讨了分子筛催化剂在该体系中的失活原因;在HC-SCR体系,总结了不同过渡金属、分子筛类型、还原剂、H2O和SO2等对催化剂活性的影响,探讨了目前比较公认的碳氢化合物选择性催化还原NOx的反应机理。最后展望了分子筛催化剂在选择性催化还原NOx领域今后的研究方向。     

焙烧程序对一步合成Cu-SSZ-13催化剂NH3-SCR性能的影响

焙烧程序影响一步合成法制备Cu-SSZ-13催化剂中Cu物种的种类及分布,是影响催化剂在NH₃选择性催化还原（NH₃-SCR）氮氧化物反应中催化性能的重要因素。为研究焙烧程序对该方法制备Cu-SSZ-13催化剂性能的影响,采用不同的焙烧温度及升温速率制备Cu-SSZ-13催化剂,并考察各催化剂的催化活性、水热稳定性及活性物种形态。结果表明焙烧温度不改变催化剂的晶型结构,但影响催化剂的活性物种形态及稳定性。当焙烧温度为600℃时,催化剂中Cu物种全部为孤立的Cu²⁺,并具有极高的稳定性,催化剂具有最佳的活性及水热稳定性。固定焙烧温度为600℃,随升温速率的提高,催化剂活性及水热稳定性表现出下降趋势,考虑经济成本,最佳的升温速率应为1℃/min。因此,以1℃/min的升温速率升至600℃焙烧6h是一步合成法制备Cu-SSZ-13催化剂的最佳焙烧程序,所得催化剂具备优异的NH₃-SCR活性和水热稳定性。     

Reactivity of titanosilicates-supported catalysts for the selective catalytic reduction of NO with NH3

The zeolites with MEL structure were synthesized via the hydrothermal method and the zeolites-supported catalysts, such as Cu²⁺, Ga³⁺, Co³⁺, Ce²⁺ and VO²⁺/zeolites, were prepared by the incipient wetness impregnation. The structures of the synthesized zeolites were characterized by techniques of XRD, FT-IR, SIMS, ²⁹Si and ²⁷Al MAS NMR. The selective catalytic reduction (SCR) of NO by ammonia was carried out with a glass reactor under a downstream flow. The synthesized TS-2 showed no significant DeNO_x activity, instead of catalyzing the ammonia oxidation at a high temperature. Furthermore, the catalytic activity of TS2 zeolite can be effectively modified and tuned up through incorporating second metal ion such as Fe³⁺, Co³⁺, and Al³⁺ into the framework (i.e., [Fe,Ti]Z11, [Co,Ti]Z11, and [Al,Ti]Z11). Among the synthesized bimetallosilicates, the [Fe,Ti]Z11 zeolite is the most active catalyst for the SCR DeNO_x with ammonia; the NO conversion and the N₂ yield reach around 80%. In addition, impregnating the metal ions on TS2 or bimetallosilicates is also a very effective way to improve the SCR DeNO_x activity. Ga³⁺/[Fe40,Ti40]Z11 and Co³⁺/[Fe40,Ti40]Z11 are the most active catalysts and show a potential for the practical applications.     

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

An overview is given of the selective catalytic reduction of NO_x by ammonia (NH₃‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH₃‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH₃‐SCR reaction over Fe‐ZSM‐5, with isolated species being the most active.     

The Effect of Copper Loading on the Selective Catalytic Reduction of Nitric Oxide by Ammonia Over Cu-SSZ-13

Abstract  

The effect of Cu loading on the selective catalytic reduction of NO_x by NH₃ was examined over a series of Cu ion-exchanged (20–80%) SSZ-13 zeolite catalysts. High NO reduction efficiencies (80–95%) were obtained over all catalyst samples between 250 and 500 °C, and at the gas hourly space velocity of 200,000 h⁻¹. Both NO reduction and NH₃ oxidation activities under these conditions were found to increase slightly with increasing Cu loading at low temperatures. However, NO reduction activity was suppressed with increasing Cu loadings at high temperatures (>500 °C) due to excess NH₃ oxidation. The optimum Cu ion exchange level appears to be ~40–60% since higher than 80% NO reduction efficiency was obtained over 50% Cu ion-exchanged SSZ-13 up to 600 °C. The NO oxidation activity of Cu-SSZ-13 was found to be low regardless of Cu loading, although it was somewhat improved with increasing Cu ion exchange level at high temperatures. During the “fast” SCR (i.e., NO/NO₂ = 1), only a slight improvement in NO_x reduction activity was obtained for Cu-SSZ-13. Regardless of Cu loading, near 100% selectivity to N₂ was observed; only a very small amount of N₂O was produced even in the presence of NO₂. The apparent activation energies for NO oxidation and NO SCR were estimated to be ~58 and ~41 kJ/mol, respectively.     

Screening of Fe–Cu-Zeolites Prepared by Different Methodology for Application in NSR–SCR Combined DeNOx Systems

The NO_x NH₃-SCR performance of several Cu and Fe catalysts supported on BETA and ZSM-5 zeolites has been studied in single SCR and double NSR–SCR configuration, and the activity related to the nature and reducibility of metal species on the catalyst surface. Intermediate ammonia formed in NSR improved greatly NO_x conversion at the exit of the double NSR–SCR configuration, which was practically totally converted to N₂.     

铜系低温选择性催化还原脱硝催化剂的研究进展

选择性催化还原（SCR）技术已广泛应用在燃煤电站烟气脱硝技术中,开发低温高活性、高抗中毒性能的催化剂体系已经成为国内外学者的研究重点。Cu系催化剂由于具有良好的脱硝性能及水热稳定性,得到了广泛的研究和关注。本文综述了近年来活性组分Cu负载在TiO₂、Al₂O₃、碳基材料和分子筛等材料上的研究进展;重点分析了Cu系催化剂低温SCR反应机理,主要包括Eley-Rideal （E-R）机理和Langmuir-Hinshelwood （L-H）机理,同时分析了SCR反应的两个必然过程：吸附（NH₃吸附和NO_x吸附）和反应;简要地介绍了Cu系催化剂的抗水抗硫中毒性能研究现状以及反应机理,同时介绍了碱金属中毒、飞灰和催化剂烧结对催化剂失活的影响,结合生命周期分析SCR脱硝系统还原剂氨和尿素对NO排放的影响。在此基础上展望了未来铜系催化剂的研究方向：采用新型方式对催化剂进行改性、进一步采用表征和模拟技术研究催化体系的反应机理、优化锅炉和催化剂设计减轻催化剂失活以及研究适用于其他还原剂条件的高选择性催化剂。     

不同沸石催化剂上苯与1-十二烯烷基化

利用NH3 TPD测定了 4种沸石分子筛的酸性 ,并考察了 4种分子筛在苯与 1-十二烯烷基化反应中的催化性能 ,揭示了沸石分子筛的酸性、孔结构对其催化活性和产物选择性的作用规律 ,发现沸石分子筛经水热处理和酸处理 ,调变了孔结构和酸性 ,进而提高其催化活性     

Analysis of the Coupling of HC–SCR by Ethanol and NH3–SCR on Real Engine Emissions

Selective catalytic reduction by ethanol on silver-based catalysts was proved to be very effective to abate the nitrogen oxides emitted at the exhaust of an automotive engine. Moreover, the selectivity to ammonia of this reaction may be exploited to further enhance the NOx reduction using a dedicated transition metal exchanged zeolite catalyst. This coupling between HC– and NH₃–SCR is called Dual SCR. In order to control the silver-based catalyst efficiency via ethanol injection, a NOx sensor is located downstream of it, as usually done for urea–SCR on series vehicles. Furthermore, based on the cross-sensitivity of this NOx sensor, large amounts of ammonia were estimated that would help to reduce the remaining NOx on the zeolite based catalyst. However, when measured by FTIR technique, the concentrations of ammonia produced by the HC–SCR catalyst were surprisingly not as high as expected, while large amounts of acetaldehyde were detected and, in a lesser extent, formaldehyde and hydrogen cyanide. NOx were partly reduced over the iron-exchanged zeolite catalyst, improving the overall deNOx efficiency by up to 15 points, while acetaldehyde to formaldehyde ratio reversed and ammonia concentration remains unchanged. The cross-sensitivity of the NOx sensor was further investigated on synthetic gas bench. If its partial dependence on the ammonia concentration is rather well known, the influence of aldehydes and hydrogen cyanide in presence of ammonia had not yet been investigated. The NOx sensor’s signal remains unchanged whatever the aldehydes concentration and a strong sensitivity to the hydrogen cyanide was highlighted.     

选择性催化还原NOx催化剂的研究进展

综述了选择性催化还原法脱除NOx的各类催化剂,包括贵金属、分子筛、金属氧化物等。传统催化剂由于不能同时兼备优良的催化活性、热稳定性、抗中毒能力,因此难以真正实现商业化。由不同类型催化剂优化组合而成的复合型催化剂和NOx的存储一还原型催化剂克服了传统催化剂的上述缺点,在脱除NOx领域具有广阔的应用前景。     

基于NH3-SCR反应铜基小孔分子筛催化剂Na中毒对比研究

以浸渍法模拟碱金属中毒NH₃-SCR催化剂过程，制备不同Na含量（质量分数）的铜基小孔分子筛Cu/SSZ-13和Cu/SAPO-34，对比研究了二者的碱金属中毒机理。结果表明，外引Na离子均可严重影响两种催化剂的NH₃-SCR催化活性，造成催化剂的晶相结构坍塌，酸性量减少，活性物种减少。不同的是，Na引入量较低（<1.82%）时，Cu/SAPO-34比Cu/SSZ-13具有更强的Na离子耐受性，而当Na含量高于3.48%时，Cu/SAPO-34几乎完全丧失NH₃-SCR催化活性。通过催化剂的结构表征（BET、XRD和SEM）和酸性位表征（DRIFTS、NH₃-TPD和H₂-TPR），研究表明随着Na中毒程度的加深，Cu/SSZ-13的结构破坏是渐变式的，而Cu/SAPO-34的结构破坏是突变式的；Na中毒的机理研究表明，酸性位的减少是Cu/SSZ-13的SCR活性下降的主导原因，结构坍塌是Cu/SAPO-34的SCR活性下降的主导原因。     

Comparison study of Na poisoning effect on copper-based chabazite micropore catalysts for NH3-SCR reaction

Using wet impregnation method to mimic alkali metal poison of ammonia-selective catalytic reduction (NH₃-SCR) catalysts, Cu-based micropore zeolites Cu/SSZ-13 and Cu/SAPO-34 with different Na contents(mass fraction) were prepared, and the Na poisoning mechanism on them was studied. The results show that the externally introduced Na ions can severely affect the NH₃-SCR catalytic activity of the two catalysts, resulting in the collapse of the crystal structure of the catalyst, the decrease of acidity and the reduction of active species. In detail, when Na content was less than 1.82%, Cu/SAPO-34 has higher resistance of Na ions than Cu/SZZ-13; while when Na> 3.48%, Cu/SAPO-34 catalysts almost deactivated thoroughly. By structural characterization (BET, XRD and SEM) and acidity characterization (DRIFTS, NH₃-TPD and H₂-TPR), it was found that with Na poisoning deeper and deeper, Cu/SSZ-13 took a gradual style of the structural destruction, but Cu/SAPO-34 adopted a sudden way. Studies on the mechanism of Na poisoning show that the decrease of acid sites is the main reason for the decrease of SCR activity of Cu/SSZ-13, and the structural collapse is the main reason for the decrease of SCR activity of Cu/SAPO-34.     

High Temperature SCR Over Cu-SSZ-13 and Cu-SSZ-13 + Fe-SSZ-13: Activity of Cu2+ and [CuOH]1+ Sites and the Apparent Promoting Effect of Adding Fe into Cu-SSZ-13 Catalyst

Cu-SSZ-13 catalysts were synthesized with Si: Al?=?4.5 and 25, to obtain materials with isolated Cu²⁺ and [CuOH]¹⁺ sites, respectively. The catalysts were tested for the selective catalytic reduction of NOx (SCR), NO oxidation and NH₃ oxidation. Cu²⁺ sites presented the highest NO rates and lowest NH₃ rates, as the temperature was increased from 300 °C to 650 °C, during SCR and NH₃ oxidation, respectively. None of the Cu-SSZ-13 catalysts presented activity for NO oxidation, consistent with the absence of copper oxide clusters. In addition, catalysts composed by mechanical mixtures of Cu-SSZ-13?+?Fe-SSZ-13 with Si: Al?=?4.5 and 25 were tested for SCR, NO oxidation and NH₃ oxidation, to study the effect of the presence of iron together with Cu-SSZ-13 for improving its SCR working temperature range. Higher reaction rates for NO oxidation and NH₃ oxidation over Cu-SSZ-13?+?Fe-SSZ-13 showed a more relevancy of side reactions that makes a combined effect of Fe-SSZ-13 and Cu-SSZ-13 not a real improvement in high temperature SCR.

Graphic Abstract

     

Selective Catalytic Reduction of NO by Ammonia over Raney‐Ni Supported Cu‐ZSM–5 I. Catalyst Activity and Stability

Composite materials containing Raney Ni and Cu‐ZSM‐5 are highly active catalysts for the selective catalytic reduction (SCR) of NO by NH₃. Their catalytic properties were studied with particular attention to the influence of moisture and SO₂ in the feed, and to effects of catalyst shaping operations. Composite materials (16–20 wt‐% zeolite) were prepared by mixing the components, with different degree of segregation in the resulting pressed particles, or by growing ZSM‐5 crystallites on the surface of leached Raney Ni, which were then exchanged with Cu ions. Catalytic tests were performed with 1000 ppm NO, 1000 ppm NH₃, 2 % O₂ in He, at 3–6.5 · 10⁵ h^–1 (related to zeolite component). With physical mixtures, the catalytic behaviour strongly depended on the mixing strategy, particles containing both Ni and zeolite being inferior to mixed Ni‐only and zeolite‐only particles. The SCR activity was promoted by 2 % H₂O in the feed, SO₂ (200 ppm) was a moderate poison at low temperatures, but indifferent or slightly promoting at high temperatures. A catalyst prepared from ZSM‐5 grown on Raney Ni, which was ranked intermediate in dry feed, was promoted to excellent performance in H₂O and SO₂ containing feed at T > 700 K and was stable for 38 h at 845 K. The results suggest that SCR catalysts containing highly active zeolites should be produced avoiding shaping operations e.g. by use of zeolite crystallites grown on wire packings.     

The acidity and catalytic behavior of Mg-ZSM-5 prepared via a solid-state reaction

A series of Mg-ZSM-5 zeolite catalysts with different content of magnesium were prepared via a solid-state reaction of HZSM-5 with magnesium chloride at 327°C. X-ray determinations demonstrated that the structure and crystallinity of the catalysts were not changed as compared with HZSM-5 zeolite. The acidity of the catalysts was characterized by temperature programmed desorption of ammonia and infrared spectra of pyridine adsorption. The studies indicated that their Brønsted acid sites decreased and Lewis acid sites slightly increased with increasing magnesium amount in the zeolites. The catalytic properties of the catalysts have been examined by choosing the alkylation of toluene with methanol as probe reaction. The results showed that the modified zeolite catalysts significantly improved the initial activity and lifetime. Para-selectivity also increases to a level of 80–90%. These results indicate that the solid-state reaction is an effective and convenient route for modification of the zeolite.     

生物基乳酸（酯）制丙烯酸的工艺研究

以乳酸（酯）脱水反应为模型反应,分别考察了X、Y、ZSM-5和SBA等分子筛、硫酸盐、磷酸盐以及复配催化剂的催化性能。结果表明,适当调节分子筛催化剂的酸碱度,实现多种催化剂的复配应用,催化剂性能较好,丙烯酸及其酯的总选择性达80.3%。     

多级孔分子筛在重油加氢裂化催化剂的应用进展

分子筛是加氢裂化催化剂关键组分,其性质影响着加氢裂化反应效率和产品分布。微孔分子筛的孔结构降低了大分子反应物的扩散效率和酸中心的可及性,不宜直接用作加氢裂化催化剂的载体。本文从分子筛的孔结构和酸中心可及性的角度出发,介绍了具有多级孔体系的分子筛和具有核壳结构分子筛的加氢裂化性能。与相应的参比剂比较,分子筛的多级孔结构能大幅提高反应物种的扩散效率和酸中心的可及性,呈现出更好的催化活性、稳定性以及目标产物选择性。此外,金属加氢活性中心与分子筛裂化活性中心的合理调配,也是多级孔分子筛在重油加氢裂化应用中面临的挑战。     

Selective catalytic reduction of NO on copper-exchanged zeolites: the role of the structure of the zeolite in the nature of copper-active sites

Copper-exchanged zeolites with different structures (CuMFI, CuMOR and CuY) used as catalysts on the selective catalytic reduction (SCR) of NO by propene have been studied. Different types of Cu species were identified (Cu²⁺, Cu⁺, and CuO) by H₂-TPR and NO TPD. The structure of each zeolite determines the nature and concentration of those species and the catalytic behavior for SCR of NO by propene in the presence of oxygen. A correlation was observed between the catalytic activity, and the presence of isolated Cu²⁺ species, which is enhanced by MFI structure.