Hydrothermal Stability Research Articles

Toward the microporous zeolite family with tunable large-medium cage and pore opening

Modulation of zeolite porosity, including the size and type of channel and cage, is essential for catalysis and separation. Although zeolites with a variety of porous systems have been synthesized by hydrothermal or post-synthetic routes, there is still a lack of rational control of zeolite porosity in the range of large to medium pore/cage. Herein, based on the rational structure building, the structure similarity between IWV topology with large-pore opening and supercage and NES topology with medium-pore opening and medium cage is discovered. Based on the guidance of structure building, two IWV-derived daughter zeolites with different framework compositions, (Si,Ge)-ECNU-31 and (Si,Ge,Al)-ECNU-31, are hydrothermally prepared with built-in structural weakness due to the presence of a large amount of framework Ge atoms, which are utilized to prepare ECNU-32 zeolite with NES topology through subsequent post-synthetic treatment under controlled condition. It is demonstrated that the parent IWV zeolite with only Ge and Si as framework atoms is benefit in post-treatment to obtain a highly crystalline NES zeolite. In contrast, the co-existence of framework Al atoms in (Si,Ge,Al)-ECNU-31 zeolite enhances its hydrothermal stability in water. However, the treatment with acid and amine solutions causes the partial collapse of zeolite structure. Our results demonstrate that rational selection of the framework composition and post-synthetic parameters are crucial for the transformation of large-pore zeolite to medium-pore zeolite.

Catalytic performance of Pd catalyst supported on CeO2 or ZrO2 modified beta zeolite for methane oxidation

Two kinds of oxide-zeolite composite support, Ce-beta and Zr-beta were prepared by a simple wet impregnation method and adopted for the preparation of palladium-based catalysts for catalytic oxidation of methane. The Pd/6.8Zr-beta catalyst showed superior methane oxidation performance, achieving T50 and T90 of 417 °C and 451 °C, respectively, together with robust hydrothermal stability. Kinetic analysis has shown that incorporating Zr into the catalyst significantly enhanced its efficiency, nearly tripling the turnover frequency (TOF) for methane combustion compared to the Pd/beta catalyst. This enhanced performance was attributed to the dispersion of Zr on the zeolite surface, which not only promoted the formation of active PdO sites but also helped maintain the high Pd2+ content via facilitating the oxygen migration during the reaction, thus improving both the catalyst's activity and stability. In the Pd/8.6Ce-beta catalyst, doped CeO2 tended to aggregate in the zeolite's pores, adversely affecting the catalyst's efficiency. This aggregation promoted the formation of inactive Pd4+ species, a result of the enhanced metal-support interaction. This finding is critical for understanding the implications of dopant selection in the design of high-activity methane oxidation catalysts.

Stability and durability assessment of alum-tawed skin by light transmission analysis

Alum-tawed skin has been widely used in the past for the production of bookbindings elements because of its physical–mechanical properties and production process, easier and faster than that required for leather. Nevertheless, unlike for other skin-based materials employed in books production, such as parchment and leather, for alum-tawed skin the manufacturing process appears to be less standardized and rather few data are available in the literature on its durability and sensitivity to deterioration.In this work, a number of commercial samples of alum-tawed skin, from different manufactures and belonging to different animal species, were analysed by Light Transmission Analysis (LTA) to characterize their collagen hydrothermal stability, so as to gather useful information regarding their possible use for restoration works. The samples were then subjected to an artificial ageing process to assess their possible long term stability compared with that of some historical samples coming from 14th, 15th and 16th century bindings.The obtained results show that, as expected, the treatments the investigated alum-tawed skin samples undergo during the manufacture processes does not induce a level of stability of the collagen molecules higher than that of the vegetable and chrome tanning used for the leather treatment, although such treatment, in some cases, makes the alum-tawed skin slightly more stable than parchment. Moreover, accelerated ageing analysis showed that whatever the initial stability of the analysed samples, it decreases with ageing more rapidly and markedly than for parchment and leather, revealing a general lower durability of the alum-tawed skin to hydrothermal deterioration.

Compositing of Co3O4 with boron nitride to promote the catalytic performance for methane oxidation

Engineering a suitable catalyst support holds great importance to ensure a high catalytic performance in heterogeneous catalysis. Herein, an advanced boron nitride nanosheet (BNNS) supported Co3O4 composite (Co3O4/BNNS) was tailor-orchestrated to serve as an active and stable catalyst for methane oxidation. BNNS helps dispersion and binding small nano Co3O4 catalyst with augmented defective structures e.g., oxygen vacancies, lattice distortion etc. This facilitates the creation of more active oxygen species on Co3O4. Meanwhile, an interfacial electronic interaction was built to maintain Co3O4 in an electron-rich state. An excellent methane oxidation performance was guaranteed for Co3O4/BNNS with methane conversion rate of 58.3 mol·gCo3O4-1·h−1, 1.9 times that of Co3O4 catalyst at reaction temperature of 366 °C and weight hourly space velocity of 40,000 mL·g−1·h−1. Moreover, a strong metal and support interaction (SMSI) was in-situ formed, resulting a BOx over-layer confined Co3O4, which promises a robust hydrothermal stability for Co3O4/BNNS catalyst. This work exemplifies the pivotal role of BNNS as an excellent support to enhance the methane oxidation performance of Co3O4 catalyst.

Understanding Na functions on distribution of Cu species within Cu-SSZ-13 for the selective reduction of NOx with NH3

Cu-SSZ-13 catalysts have been extensively investigated for NH3-SCR over numerous years. However, whether sodium (Na) promoted or inhibited the NH3-SCR performance of Cu-SSZ-13 catalysts was still controversial and needed to be further elucidated. In this work, two Cu-SSZ-13 catalysts, one containing Na and the other without Na, were prepared through a stepwise ion-exchange technique. The objective was to explore the promoting influence of Na on both catalytic activity and hydrothermal stability of Cu-SSZ-13. Analysis via H2-TPR and NH3-DRIFTS revealed a notable increase in the formation of [Cu(OH)]+-Z species due to the presence of Na. DFT calculations stimulated the ion-exchange process, and confirmed the migration of Cu species from 6MR to 8MR sites in the presence of Na. This study provides direct experimental evidence regarding the influence of Na on Cu distribution, thereby deepening the understanding of the interaction between Cu sites and co-cations.

Optimization of isolated copper species on the NH3-SCR performance over Cu/SSZ-39 modified by ammonia water

To further improve the activity of Selective Catalytic Reduction of nitrogen oxide (NOx) with Ammonia (NH3-SCR) and the hydrothermal stability at 900 °C on Cu-based small-pore zeolite catalyst, the interaction between ammonia water and Cu2+ is used to regulate the interaction between Cu2+ and Si-O-Al structures of the zeolite, and affect the migration of Cu2+ in the channels of zeolite. The characterization results show that the microchemical environment of isolated Cu2+ species is optimized, and more isolated Cu2+ species can stably exist over the surface of Cu/SSZ-39, exhibiting a strong redox property. Besides, the ability of acid sites to adsorb and activate NH3 is also enhanced, and the amount of H2O adsorbed in the pores of zeolite is reduced due to the stable surface isolated Cu2+ species, which is conducive to the stability of the catalyst after hydrothermal aging at 900 °C. When the mole ratio of ammonia and copper is 3, the NH3-SCR performance and the hydrothermal stability at 900 °C are both optimized on the modified catalyst. The temperature window that the NOx conversion is higher than 90% (ΔT90) on the unmodified catalyst is 200–475 °C, but the ΔT90 on the modified catalyst is broadened to 178–544 °C. More importantly, after hydrothermal aging at 900 °C, the ΔT90 on the modified catalyst is shifted to the higher temperature by 48 °C, with the maximum NOx conversion of 96%. This work would provide a new strategy for further optimizing Cu/SSZ-39 catalyst for deNOx emitted from diesel engines to meet higher emission standards in future.

Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries

Covalent organic polymers (COPs), as emerging porous materials with well-defined architectures and high hydrothermal stability, have attracted extensive attention in the field of electrocatalysis. Herein, we report a rational design method for preparing oxygen reduction reaction electrocatalysts with the assistance of a predesigned macrocyclic COP model molecular. With the predesigned nitrogen position and structural features in macrocyclic chain-like COP-based materials, the obtained COPMCT-Co-900 catalyst provided excellent oxygen reduction performance, where the half-wave potential (E1/2) reaches 0.85 V (vs . RHE), comparable to commercial Pt/C. We also extended the strategy to similar macrocycle COPs and Fe-based and Ni-based metal sources and studied the oxygen reduction reaction performance of corresponding catalysts, proving the universality of the method. Interestingly, we assemble COPMCT-Co-900 catalyst as air electrode catalyst of the self-made rechargeable zinc-air flow batteries, which exhibit outstanding power density (155.6 mW·cm-2) and long cycle life (90 h, 270 cycles at 10 mA·cm-2). Our studies provide a new method for the development of high-performance oxygen electrodes applied in zinc-air flow battery devices.

Efficient hydrogen production by methanol aqueous phase reforming over KMnO4 modified PtMnK/AC catalyst: Regulating the hydrophilicity of carbon support

The production of hydrogen through the aqueous phase reforming of biomass-derived oxygenates has broad application prospects in distributed hydrogen energy systems. Activated carbon is commonly employed as a catalyst support in catalytic aqueous phase reforming due to its high surface area and hydrothermal stability. Nevertheless, its hydrophobicity hinders the efficient adsorption of water molecules, which affects the activity of the aqueous phase reforming reaction. In this study, catalysts using carbon as support with varying hydrophilicity were synthesized using nitric acid and potassium permanganate as modifiers. The efficiency of these catalysts was then examined in the aqueous phase reforming reaction of methanol and the water gas shift reaction, respectively. The impact of the hydrophilicity of the support on the reaction performances was examined with diverse characterization techniques. The results demonstrated that the PtMnK/AC catalyst exhibited 1.2 times the hydrogen production yield compared to the conventional Pt/AC catalyst, indicating that enhancing the hydrophilicity of the support promoted the water gas shift reaction while limiting methanation side reactions in methanol aqueous phase reforming. The effective interaction between hydrophilic surfaces and oxygen vacancies enhances the catalyst's capability to dissociate H2O and react with CO* intermediates to generate more hydrogen, which is regarded as the rate determining step of methanol aqueous phase reforming.

Cellulose conversion to 5-hydroxymethylfurfural via a simple and efficient phosphate-doped hafnium oxide catalyst

The efficient catalytic conversion of cellulose to 5-hydroxymethylfurfural (HMF) can provide a valuable reference for the high-value utilization of biomass. In this work, a series of simple phosphate-doped hafnium oxide catalysts (x-P-HfO2) were prepared, and their physicochemical properties were analyzed by ICP-OES, XRD, XPS, FTIR, SEM-EDX, N2 adsorption-desorption isotherms, NH3-TPD and Py-FTIR characterizes. Detailed evaluation of the reaction conditions demonstrated the ideal catalytic performance of the x-P-HfO2 catalysts, and the HMF yield from cellulose reached a satisfactory 61% in the water(NaCl)/tetrahydrofuran biphasic reaction system. Based on the captured intermediates and detected by-products, possible catalytic pathways are summarized. Meanwhile, the hydrothermal stability of the catalyst was also investigated, and the reason for its performance degradation after five cycles was determined to be the loss of useful Brønsted active sites. In addition, the effects of the phosphating and sulfating modification approaches on the catalyst activity were also compared, verifying that the introduction of phosphate is more conducive to the catalyst expressing high activity for the formation of HMF from cellulose.

Catalytic dehydrogenation cracking characteristics of vacuum residue over solid basic catalyst

The basic calcium aluminate with appropriate catalytic cracking activity, excellent anti-coking performance, and hydrothermal stability is of great potential for the utilization of heavy oil. However, there is a lack of systematic research on the catalytic dehydrogenation cracking characteristics of vacuum residue (VR) over solid basic catalyst. Thus, by combining the TG and Py-GC/MS methods, the dehydrogenation cracking behavior of S-VR was investigated. The results indicated that S-VR cracking on calcium aluminate leads to less coking and higher olefin content in liquid products compared with FCC catalyst and quartz sand. Then, the activation energy Ea is computed from the catalytic dehydrogenation cracking of S-VR on calcium aluminate. The results show that the Ea values from Friedman and OFW methods range from 68 to 143 kJ∙mol−1 and 114 to 192 kJ∙mol−1, respectively. After that, the molecular-level carbon number distributions of the products obtained from cracking S-VR on different reaction conditions are investigated, and the optimal reaction temperature and the heating rate are 650 °C and 1000 °C/s, respectively. Finally, the free radical regulation theory was constructed to explain the catalytic dehydrogenation cracking mechanism of basic calcium aluminate catalyst.

SSZ-39 zeolite-based Ru catalysts for selective hydrogenation of levulinic acid to γ-valerolactone: Influence of synthesis method and zeolite acidity

Novel SSZ-39 zeolite-based Ru catalysts were synthesized by impregnation on as-synthesized metallic and protonated SSZ-39 and a one-pot synthesis strategy. The study focused on the influence of the SSZ-39 zeolite characteristics and catalyst preparation method type toward levulinic acid (LA) selective conversion to γ-valerolactone (GVL) under relatively mild reaction conditions (150 °C, 4 h and 2 MPa of H2 with H2O and THF as solvents). Catalysts exhibited excellent catalytic efficiency (100 % LA conversion and 98.3 % GVL selectivity) and remarkable hydrothermal stability. The results revealed a correlation between excellent catalytic activity and the structural features of the synthesized catalysts, predominantly dependent on their appropriate surface acidity (strong Lewis acid sites) and enhanced electronic interaction between Ru species and SSZ-39. The one-pot method promoted the formation of mesopores besides the zeolitic micropores present in SSZ-39; therefore, the catalyst prepared by the one-pot method (Ru/SSZ-39) showed purer SSZ-39 crystallites and a slightly larger catalytic surface area of 439 m2/g than the original zeolite surface area of 424 m2/g. In addition, the original zeolite crystals retained their morphology as spherical aggregates and remained unaffected by Ru impregnation on the metallic or protonated support during the synthesis of Ru/Na-SSZ-39 and Ru/H-SSZ-39 catalysts, respectively. Notably, it has been demonstrated that SSZ-39 zeolite effectively acts as a support for the preparation of Ru catalyst in biomass conversion reactions. Moreover, the reused catalyst exhibited consistent activity over six cycles, maintaining its structural framework without significant loss in activity.

Tuning a green carboxymethyl cellulose-based pre-tanning agent via peroxide oxidation for high chrome exhaustion in leather industry

The low chrome uptake by collagen in the conventional tanning process leads to the pollution of the wastewater. Due to environmental concerns, leather scientists are already searching for innovative ways to produce pre-tanning agents as a high exhaustion chrome tanning auxiliary. Herein, a novel kind of pre-tanning agent is engineered by converting carboxymethyl cellulose (CMC) to oxidized carboxymethyl cellulose (OCMC) via the hydrogen peroxide process. FT-IR and carboxyl content analysis demonstrated the increase in carboxyl content after oxidation. After that, the obtained OCMC was utilized as a pre-tanning agent, resulting in a high exhaustion of chrome (92.76 %) which is 27.76 % more than conventional chrome tanning (65 %), and the amount of chrome in wastewater reduced to 7.24 %. The hydrothermal stability of wet-blue increased by increasing the uptake of chrome (Ts = 118 °C). The obtained crust leather represented excellent mechanical properties (Tensile strength: 305.68 kg/cm2; tear strength: 50 kg/cm) and desirable organoleptic properties. The environmental analysis signifies a significant step towards a cleaner and sustainable tanning process (COD = 1600, BOD5 = 560 mg/L) compared to the conventional chrome tanning process. Consequently, the obtained results offer a green pre-tanning agent to meet the requirements of the sustainable development of the leather industry.

Synergistic bimetallic nanozymes of Ni/ZIF-8 and Cu/ZIF-8 as carbonic anhydrase mimics

The remarkable diversity of metal nodes and ligands, coupled with the extensive array of connection modes, enables the precise design and assembly of metal-organic frameworks (MOFs), which have garnered significant attention in the field of nanozyme. In this study, the synthesis of zeolite imidazole framework materials (ZIFs) with varying Zn2+ and Ni2+ or Cu2+ concentrations were investigated and regulated by methanol, aiming to mimic the CO2 hydration activity of carbonic anhydrase (CA). The materials were comprehensively characterized using TEM, XRD, FTIR, UV-Vis DRS, XPS, and subsequently the catalytic performance of the materials was assessed. The results demonstrated that the structural characteristics of bimetallic Ni/ZIF-8 and Cu/ZIF-8 were influenced by varying concentrations of metal ions. Notably, ZIF-8 doped with lower metal content exhibited enhanced stability, while a precursor containing nickel and copper ions exceeding 50% hindered the formation of the desired ZIFs, resulting in an unstable cluster structure. However, methanol can counteract the inhibitory effect of metal ions on the structure. The bimetallic synergies of doped metals in ZIFs CA mimics can simultaneously expedite both two rate-limiting steps in the CO2 hydration process. The esterase activity of ZIF-8 doped with a small quantity of Ni2+ or Cu2+ can be achieved at a level comparable to that of human carbonic anhydrase (hCA II) at 80 °C, along with remarkable catalytic performance and hydrothermal stability. Specifically, Cu66%/ZIF-8 exhibited an activity of 0.31 U·mg−1 at 25 °C, which was 2.31 times higher than that of ZIF-8, moreover, the esterase activity at 80 °C was approximately 2.21 times greater than that of ZIF-8 under the same condition. While the relative activity of Ni50%/ZIF-8 and Cu50%/ZIF-8 still exceeding 50% even after five repeated utilization cycles. The bimetallic Ni/ZIF-8 and Cu/ZIF-8 can be easily synthesized using a simple method, resulting in enhanced catalytic activity. Therefore, it holds great potential for future applications in CO2 capture.

Hydrolysis of Cellulose using boronic-acid multifunctional carbocatalysts

Background Developingeffective and sustainable catalysts for cellulose hydrolysis is crucial for transforming this abundant biopolymer into renewable chemicals and biofuel. Carbonaceous solid acids are promising catalysts for this reaction due to their low cost, chemical and hydrothermal stability, tunable surface, and high affinity for cellulose. MethodsWe developed multifunctional carbocatalysts for cellulose hydrolysis by introducing boronic groups and weak acid sites along with high-density sulfonic acid through borylation of mesophase pitch carbon (MPC) followed by sulfonation. The functionalized MPC was characterized using various techniques, including FT-IR, XPS, SS-NMR, Raman spectroscopy, and TGA. Significant findingsThe results of surface characterization revealed the successful introduction of multifunctional groups on the peripheral surface of MPC. This carbocatalyst can effectively hydrolyze pretreated cellulose into glucose, resulting in a maximum yield of 64 % in water at 180 °C for 3 h. Boronic groups and weak acid sites could mediate the interaction of cellulose molecules with catalyst surface to enhance the catalytic hydrolysis. The apparent activation energy for breaking the glycosidic bonds using the functionalized MPC is 99 kJ/mol, which is lower than the one derived from homogeneous acids. Furthermore, the carbocatalyst is recyclable at least four cycles prior to the significant loss for reactivity.

One-Pot Synthesis of CHA/ERI-Type Zeolite Intergrowth from a Single Multiselective Organic Structure-Directing Agent.

We report the one-pot synthesis of a chabazite (CHA)/erionite (ERI)-type zeolite intergrowth structure characterized by adjustable extents of intergrowth enrichment and Si/Al molar ratios. This method utilizes readily synthesizable 6-azaspiro[5.6]dodecan-6-ium as the exclusive organic structure-directing agent (OSDA) within a potassium-dominant environment. High-throughput simulations were used to accurately determine the templating energy and molecular shape, facilitating the selection of an optimally biselective OSDA from among thousands of prospective candidates. The coexistence of the crystal phases, forming a distinct structure comprising disk-like CHA regions bridged by ERI-rich pillars, was corroborated via rigorous powder X-ray diffraction and integrated differential-phase contrast scanning transmission electron microscopy (iDPC S/TEM) analyses. iDPC S/TEM imaging further revealed the presence of single offretite layers dispersed within the ERI phase. The ratio of crystal phases between CHA and ERI in this type of intergrowth could be varied systematically by changing both the OSDA/Si and K/Si ratios. Two intergrown zeolite samples with different Si/Al molar ratios were tested for the selective catalytic reduction (SCR) of NOx with NH3, showing competitive catalytic performance and hydrothermal stability compared to that of the industry-standard commercial NH3-SCR catalyst, Cu-SSZ-13, prevalent in automotive applications. Collectively, this work underscores the potential of our approach for the synthesis and optimization of adjustable intergrown zeolite structures, offering competitive alternatives for key industrial processes.

Mechanistic insights into the deactivation of Pd/BEA methane combustion catalysts by hydrothermal aging

Three Pd/Beta zeolite catalysts with low, intermediate and high support Si/Al2 (SAR) ratios were synthesized for methane combustion in both fresh and hydrothermally aged forms. SAR was found to dramatically influence catalyst performance and hydrothermal stability. Via an array of structural and textural characterizations, it is discovered that (1) at low SAR, catalyst deactivation is due primarily to the transformation of catalytically active PdO to inactive Pd2+-ions via protonolysis; (2) at intermediate SAR, catalyst deactivation during aging is due partly to the protonolysis chemistry, and partly to Al-decoration of the remaining PdO particles; (3) at high SAR, both of these deactivation mechanisms are greatly mitigated, leading to much improved catalyst hydrothermal stability. Our work here demonstrates that PdO particles stabilized by silanol nests, sites that are highly active for methane combustion, can only survive harsh hydrothermal aging at very high support SAR.

Cu-OFF/ERI Zeolite: Intergrowth Structure Synergistically Boosting Selective Catalytic Reduction of NOx with NH3.

Cu-SSZ-13 has been commercialized for selective catalytic reduction with ammonia (NH3-SCR) to remove NOx from diesel exhaust. As its synthesis usually requires toxic and costly organic templates, the discovery of alternative Cu-based zeolite catalysts with organotemplate-free synthesis and comparable or even superior NH3-SCR activity to that of Cu-SSZ-13 is of great academic and industrial significance. Herein, we demonstrated that Cu-T with an intergrowth structure of offretite (OFF) and erionite (ERI) synthesized by an organotemplate-free method showed better catalytic performance than Cu-ERI and Cu-OFF as well as Cu-SSZ-13. Structure characterizations and density functional theory calculations indicated that the intergrowth structure promoted more isolated Cu2+ located at the 6MR of the intergrowth interface, resulting in a better hydrothermal stability of Cu-T than Cu-ERI and Cu-OFF. Strikingly, the low-temperature activity of Cu-T significantly increased after hydrothermal aging, while that of Cu-ERI and Cu-OFF substantially decreased. Based on in situ diffuse reflectance infrared Fourier transform spectra analysis and density functional theory calculations, the reason can be attributed to the fact that NH4NO3 formed on the CuxOy species within ERI polymorph of Cu-T underwent a fast SCR reaction pathway with the assistance of Brønsted acid sites at the intergrowth interfaces under standard SCR reaction conditions. Significantly, Cu-T exhibited a wider temperature window at a catalytic activity of over 90% than Cu-SSZ-13 (175-550 vs 175-500 °C for fresh and 225-500 vs 250-400 °C for hydrothermal treatment). This work provides a new direction for the design of high-performance NH3-SCR catalysts in terms of the interplay of the intergrowth structure of zeolites.

Recent Advances in Hydrodeoxygenation of Lignin‐Derived Phenolics over Metal‐Zeolite Bifunctional Catalysts

AbstractThe hydrodeoxygenation (HDO) reaction provides a promising catalytic strategy to remove oxygen in biomass‐derived bio‐oil to produce renewable transportation fuels and value‐added chemicals. The development of highly efficient and stable HDO catalysts plays an essential role in biomass valorization. Metal‐zeolite bifunctional catalysts have been well‐developed as the effective HDO catalysts in upgrading lignin‐derived phenolics due to their excellent activity, selectivity, and thermal and hydrothermal stability. However, clarifying the roles of the active sites and their synergistic effect, and establishing effective structure‐performance relationships in the HDO process still face challenges. In this review, we first survey the conventional catalysts applied in the HDO of bio‐oil, followed by thoroughly discussing the roles of metal centers, acid sites, supports, and their impacts on the HDO process of phenolic model compounds or bio‐oil. Finally, a discussion on the stability and deactivation of metal‐zeolite catalysts, especially in the aqueous‐phase HDO reaction, is provided. This critical review offers new insights into the development of state‐of‐the‐art metal‐zeolite bifunctional catalysts with well‐defined porosity and metal‐acid properties for viable biomass valorization.

Al-rich Cu-SSZ-13 zeolite synthesized by interconversion from Y: Crystallization process resolution and application in the selective catalytic reduction of NO with NH3

Cu-SSZ-13 zeolite is a promising catalyst in the selective catalytic reduction (SCR) of NOx with NH3 and the interzeolite conversion is considered as a convenient method with relatively low cost to get the Al-rich SSZ-13 zeolite with great hydrothermal stability. However, the crystallization process of interzeolite conversion and its relation to the catalytic performance of Cu-SSZ-13 in the NH3-SCR of NOx are still poorly understood. Herein, the interzeolite conversion process of Y to SSZ-13 was well resolved and the effect of major synthesis factors on the hydrothermal stability and catalytic performance of Cu-SSZ-13 in the NH3-SCR of NOx was investigated. The results indicate that highly crystalline SSZ-13 can be synthesized by the interzeolite conversion from Y, using N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as the structure-directing agent (SDA) and with a SDA/Si molar ratio of 0.06–0.2. The Y zeolite of FAU framework is first degraded and decomposed into the oligomeric aluminosilicate fragments quickly in the alkaline circumstance; thereafter, the oligomeric fragments develop into the crystal nuclei that arouse the rapid crystallization of the SSZ-13 zeolite of CHA framework with the help of SDA and solid Al(OH)3, during which the Al(OH)3 source is dissolved and restructured to tetrahedral Al species and incorporated into the CHA framework. The synthesized Cu-SSZ-13 zeolite exhibits excellent catalytic performance in the NH3-SCR of NO, achieving an NO conversion of nearly 90% at a rather low temperature of 150 °C and a gaseous hourly space velocity (GHSV) of 100,000 h−1. Moreover, the Cu-SSZ-13 catalyst displays great hydrothermal stability and remains highly active even after aging at 800 °C. These results help to clarify the mechanism of interzeolite conversion and provide an effective method to synthesize highly crystalline Al-rich SSZ-13 zeolite with great potential for application in the NH3-SCR of NOx.

Praseodymium effects on great improving low-temperature hydrothermal stability of Cu/SAPO-34 NH3-SCR catalysts

Based on the previous findings that the presence of hydroxyl groups on the outer surface is crucial for maintaining skeletal stability, we propose a strategy modified Cu/SAPO-34 using Pr ions in this study. Therefore, we conducted several measurements to investigate the effect of Pr ions on the low-temperature hydrothermal stability of Cu/SAPO-34. We find that Pr exists only on the surface of Cu/SAPO-34 as ions and oxides, with Pr3+ ions playing a protective role in occupying surface acidic sites. The addition of small amounts of Pr leads to the re-dispersion of Cu, resulting in improved low-temperature selective catalytic reduction (SCR) activity in the as-synthesized samples. Furthermore, it enhances the resistance to decomposition of the Si–(OH)–Al framework during low-temperature hydrothermal aging, thereby preserving the framework structure and allowing detached active Cu species to return to exchangeable positions, ultimately restoring SCR activity. However, as the Pr content increases, the enhanced acidity causes some structural damage, gradually weakening the protective effect. Our work demonstrates that Pr modification is a simple and effective solution to the issue of poor low-temperature hydrothermal stability in Cu/SAPO-34, providing a promising way for the application of light rare earth elements.