Effect of postsynthesis preparation procedure on the state of copper in CuBEA zeolites and its catalytic properties in SCR of NO with NH3
Graphical abstract
Introduction
Copper containing zeolites have been extensively tested in selective catalytic reduction of NO with ammonia. Especially high activity and selectivity to N2 were reported for Cu-ZSM-5 and Cu-BEA zeolites [1], [2], [3], [4]. However, recently, zeolites with smaller pores size, such as SSZ-13 and SAPO-34, have been indicated as the most promising materials for application in diesel systems SCR unit due to high chemical and thermal stability [5], [6], [7], [8]. Nevertheless, further development in catalytic NOx abatement technologies is necessary due to severe environmental regulation related to nitrogen oxides emission.
The main obstacles leading to deactivation of copper containing zeolites are hydrothermal dealumination and poisoning of active species as well as migration and sintering of copper species over SCR process. A possible solution of those issues is the improvements in catalysts preparation methods. An optimal catalyst preparation procedure should allow controlling the speciation of active sites and to obtain isolated species well fixed to support. Currently used conventional methods, such as ion-exchange and impregnation, leads to introduction of active species in various forms i.e. isolated framework species, isolated cations in exchange positions and extra-framework nanoclusters.
Some of scientists involved in DeNOx processes proposed a direct synthesis of Cu containing zeolites by the addition of organo-copper complexes in the synthesis gel as an alternative method to conventional ion exchange and impregnation [9]. However, this kind of method resulted in an undesired mixture of copper in framework and extra-framework positions.
On the other hand, Dzwigaj et al. [10], [11], [12] have proposed a new method for postsynthesis modification of beta zeolite which consists of two steps. In the first step, vacant T-atom sites are created by treatment of parent zeolite with nitric acid solution. In the second step, the metal ions react with silanol groups of vacant T-atom sites forming framework metal species with well-defined environment.
In this work it was studied the influence of two different catalyst preparation procedures, conventional wet impregnation and two-step postsynthesis method, on the nature and environmental of copper introduced in Cu2.0HAlBEA and Cu2.0SiBEA, respectively. Moreover, both copper containing catalysts were applied for selective catalytic reduction of NO with ammonia and their performances were compared.
Section snippets
Materials
Copper-containing BEA zeolites (with 2 Cu wt%) were prepared from parent TEABEA zeolite by a two-step postsynthesis method (Cu2.0SiBEA) and a conventional wet impregnation (Cu2.0HAlBEA) using SiBEA and HAlBEA as the supports whose preparation was described in our earlier work [13].
Cu2.0SiBEA and Cu2.0HAlBEA were prepared by impregnation of 2 g of SiBEA and HAlBEA, respectively, with Cu(NO3)2·6H2O solutions with appropriate concentration of copper. Firstly, the suspensions were stirred for 24 h at
X-ray diffraction
The experiments of chemical analysis of zeolite samples allowed determine the Si/Al ratio and Cu content and the results are shown in Table 1.
Both Cu2.0HAlBEA and Cu2.0SiBEA zeolites are characterized by similar XRD patterns as that of HAlBEA and SiBEA supports (Fig. 1). It indicates that introduction of copper into both type of BEA supports did not affect crystal structure of these materials. The observations of the zeolites XRD patterns before and after introduction of copper (Fig. 1) showed
Conclusions
Modifications of zeolite beta by two different methods led to obtaining two types of catalysts with considerable different nature and environment of copper species.
Copper species in Cu2.0HAlBEA zeolite, prepared with conventional wet impregnation method, were predominantly present as extra-framework octahedral Cu(II) species, whereas copper species in Cu2.0SiBEA zeolite, obtained by two-step postsynthesis method, occurred mostly as framework mononuclear Cu(II) species.
Both Cu2.0SiBEA and Cu2.0
Acknowledgements
This project was funded by the National Science Center “PRELUDIUM” UMO-2012/07/N/ST5/00171 (R.B., S.D.). Special thanks to DSc Ireneusz Kocemba for help with carrying out TPR experiments and for Laetitia Valentin for acidity measurement on Cu2.0HAlBEA by FTIR of pyridine adsorption.
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