ABSTRACT
Small pore zeolites with chabazite structure have been commercialized for selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonium (NH3) from diesel exhaust. However, conventional zeolite synthesis processes detrimental effects on the environment due to the consumption of large amount of water, organic templates. Thus, this study proposed a green synthesis process with addition of minimal amount of water, structure directing agent and shortened steps to prepare nano-sized SSZ-13 (0.12 µm) using trans-crystallization strategy and exhibited enhanced performance for NOx removal after copper ion-exchange. The operation temperature window (NOx conversion >90 %) as well as the SO2 and H2O resistance over the green-route prepared nano-sized SSZ-13 (178-480 °C) outperformed the conventional SSZ-13 (29.8 µm, 211-438 °C) mainly due to the much shorter diffusion path. This clearly implied that the mass transportation was key for NH3-SCR of NOx on such small pore zeolite catalysts, which was further confirmed via an in-depth mass transportation calculation process. These results demonstrate that the Cu-nano-sized SSZ-13 prepared by the environmental benign route has great potential to act as a new generation of deNOx catalyst for diesel exhaust and provided a guideline for researchers to develop new methods to synthesize nano-catalysts for air pollution control.
ABSTRACT
Nitrogen oxides (NOx) are a primary source of air pollutants from combustion of fossil fuels. Though Mn-Ce based catalysts exhibit superior low temperature activities, their water and SO2 tolerance is inferior to other metal oxide catalysts, due to their strong water adsorption and sulfate species formation tendency at low reaction temperatures. Herein, a confinement strategy was adopted to design and synthesize a novel Mn-Ce based catalyst for selective catalytic reduction of NOx with NH3. The confined MnCeOx catalyst was assembled with a simple one pot method, using a mesoporous zeolite (ZSM-5) as the shell and Mn-Ce oxides as the active core (MnCeOx@Z5). Owing to the zeolite shell's shielding effect and the synergy between the alumina-silica zeolite shell's acidic properties and the mixed oxide cores' redox properties, the novel MnCeOx@Z5 catalyst displayed enhanced water and SO2 resistance as compared to the MnCeOx supported on ZSM-5 (MnCeOx/Z5) and its precursor (MnCeOx@Al-SiO2). Evidently, the zeolite sheath hinders sulfate species formation, and this phenomenon was further investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTS). The novel shielding and acid-redox synergy effect/strategy adopted in this work can be applied to design other high performance deNOx catalysts for air pollution control.
ABSTRACT
Hierarchical ZSM-5 zeolite with meso- and micro-pore structures was successfully prepared through a facile one-pot hydrothermal synthesis method using bifunctional template. After copper ion-exchange, it was applied for the selective catalytic reduction of NO with NH3 (NH3-SCR). Compared with conventional Cu-ZSM-5 catalyst containing only micropores, the hierarchical catalyst with ca. 2 wt.% Cu loading displayed significantly improved catalytic performance. Particularly, the hierarchical zeolite catalyst also displayed excellent hydrothermal stability and sulfur resistance that exhibited great potential in practical application. Characterization techniques such as XRD, N2 physisorption, temperature programmed desorption/reduction (TPD/TPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were comprehensively used to reveal the relationship between zeolite structure and catalytic properties. It was concluded that the hierarchically porous structure could not only improve the mass transfer of reactant/product but also provide larger specific surface area, higher surface acidity, larger NO adsorption capacity. And we found that bidentate nitrate species was more active in Cu-ZSM-5-meso than Cu-ZSM-5-C, which were all beneficial to the NH3-SCR reaction. This work can provide a guideline to design other high performance hierarchical zeolites with different crystalline structures (such as CHA, LTA) for efficient catalytic NOx removal processes.
ABSTRACT
BACKGROUND: The 6AL/6VS translocation lines, carrying the wheat powdery mildew resistance gene Pm21, are planted on more than 3.4 million hectares. The NAM-A1 gene, located on chromosome 6AS of hexaploid wheat, has been implicated with increased wheat grain protein content (GPC). However, the NAM-A1 gene was removed from the 6AL/6VS translocation lines after the original chromosome 6AS was replaced by chromosome 6VS of Haynaldia villosa. The present study aimed to clone the NAM homologous gene from chromosome 6VS, to analyze the changes of GPC in the 6AL/6VS translocation lines, and to develop related molecular markers for wheat molecular breeding. RESULTS: A new NAM family gene, NAM-V1, was cloned from 6VS of H. villosa (GenBank ACC. no. KR873101). NAM-V1 contained an intact open reading frame (ORF) and putatively encodes a protein of 407 amino acids. Phylogenetic analysis indicated that NAM-V1 was an orthologous gene of NAM-A1, B1, and D1. The determination of GPC in four Pm21 F2 segregation populations demonstrated that the replacement of NAM-A1 by NAM-V1 confers increased GPC in hexaploid wheat. Multiple sequence alignment of NAM-A1, B1, B2, D1, D2, and V1 showed the single nucleotide polymorphism (SNP) sites for each of the NAM genes, allowing us to develop a molecular marker, CauNAM-V1, for the specific detection of NAM-V1 gene. Our results indicate that CauNAM-V1 can be used as a novel DNA marker for NAM-V1, and can also be used for selecting Pm21 in wheat breeding programs. Further, we developed a marker, CauNAM-ABD, for the amplification and simultaneously distinguish among the NAM-A1, NAM-B1, NAM-B2, NAM-D1, and NAM-D2 genes in a single step. CauNAM-ABD enabled us to develop an efficient "one-marker-for-five-genes" procedure for identifying genes and its copy numbers related with grain protein content. CONCLUSION: Here, we report the isolation of the NAM-V1 gene of H. villosa. This gene contributes to increasing GPC in 6AL/6VS translocation wheat lines. We developed a molecular marker for the specific detection of NAM-V1 and a molecular marker that can be used to simultaneously distinguished among the NAM-A1, NAM-B1, NAM-B2, NAM-D1, and NAM-D2 genes in a single step.
