Rational direct synthesis methodology of very active and hydrothermally stable Cu-SAPO-34 molecular sieves for the SCR of NO_x

Abstract

A one-pot direct synthesis of Cu-SAPO-34 has been achieved that allows more than 90% yield in the material synthesis. By this method it is easy to control the Cu-loading in the Cu-SAPO-34. It is presented that a maximum in hydrothermal stability with very high activity for NO_x SCR with NH₃ is obtained for an optimum Cu loading.

Introduction

Nitrogen oxides (NO_x), involving nitric oxide (NO), nitrogen dioxide (NO₂) and nitrous oxide (N₂O) are major air pollutants [1]. NO_x is generated primary by the combustion of fossil fuels, both in transportation and industrial processes. However, transports, and especially those working with diesel engines, are the main sources of NO_x emissions [2]. Selective catalytic reduction (SCR) of NO_x by ammonia has become the most used emission control [3], and catalysts based on vanadia are in commercial use since 2005 for diesel vehicles [4]. However, vanadia catalysts suffer some serious drawbacks, as oxidation of SO₂ to SO₃ (which would react with H₂O resulting in H₂SO₄), and low activity and selectivity at high temperatures due to the sintering of vanadia species [5].

The discovery of copper-exchanged zeolites as active and stable catalysts for the SCR of NO_x by Iwamoto et al. introduced an attractive alternative to vanadia-based materials [6]. Several metal-exchanged molecular sieves, primary medium and large pore zeolites, have been reported in the last two decades for the SCR of NO_x [7]. Unfortunately, those materials present low hydrothermal stability when reacting under harsh conditions (presence of steam at high temperatures). Recently, BASF researchers have described that copper-containing small-pore chabazite zeolite shows much better hydrothermal stability than large pore zeolites (Cu-Beta or Cu-Y) [8]. This description has revived the industrial interest in metal-containing molecular sieves for NO_x abatement [9]. In this sense, Fickel and Lobo [10] have reported that the higher hydrothermal stability and better catalytic behavior of Cu-CHA for SCR of NO_x is due to the localization of copper atoms coordinated to the double six-membered rings units (D6-MR) present in the large cavities of chabazite structure.

Conventionally, those metal-containing molecular sieves are obtained by post-synthesis ion-exchange procedures. Indeed, several steps, such as hydrothermal synthesis of the molecular sieves, calcination, metal ion exchange, and calcination are required to get the final metal-containing molecular sieves.

Recently, Xiao et al. [11] have nicely reported the direct preparation of Cu-SSZ-13 zeolite (SSZ-13 is the silicoaluminate form of the CHA structure) by using a low-cost copper–amine complex (Cu²⁺ with tetraethylenepentamine, Cu-TEPA) as an efficient template. This methodology allowed the direct introduction of extra-framework copper species in the CHA cages, revealing promising results in SCR of NO_x reaction. Despite the remarkable impact that this discovery could have on the NO_x field, the materials reported in this manuscript suffer from important drawbacks. SSZ-13 syntheses with several Si/Al ratios (5, 7.5, 12.5, and 17.5) were attempted, but the final Si/Al ratios into the solids were much lower (4.1, 4.3, 5.3, and 7.5, respectively). These values clearly indicate that large part of the initially introduced silicon species remained in solution when the Si/Al ratio was increased, notoriously affecting to the SSZ-13 yield. Moreover, the desired industrial catalysts for the SCR of NO_x must show high hydrothermal stability due to the reaction conditions, i.e. high temperature and presence of steam. It is well known that zeolites with low Si/Al ratio (less than 10) suffer severe dealumination processes in presence of steam at high temperature [12]. In fact, the SCR of NO_x experiment described by Xiao was performed on the sample with Si/Al ratio of 4.1 under mild conditions (low space velocity), and the stability of the synthesized samples under hydrothermal treatments was not studied. Furthermore, the different Cu-SSZ-13 examples reported by Xiao et al. show similar Cu loadings (Cu/Si = 0.09–0.10), indicating that the amount of Cu on SSZ-13 samples cannot be easily varied in a controlled manner.

In addition to SSZ-13, the chabazite molecular sieve can be synthesized as silicoaluminophosphate form, SAPO-34 [13]. Cu-exchanged SAPO-34 has also been shown as a very stable and active material for SCR of NO_x [14]. In the last years, some groups have attempted the direct preparation of Cu-SAPO-34 in order to achieve an inexpensive and more efficient synthetic route for this metal-substituted material [15]. In those cases morfoline and copper oxide were used as organic structure directing agent (OSDA) and copper source, respectively. Those crystalline Cu-SAPO-34 materials show very low Cu contents [Cu/(Al + P) = 0.02], and samples with higher Cu loadings direct into amorphous materials, low solid yields (lower than 70% of initial sources), and mixture of metal in framework positions and extra-framework cationic positions [15c]. During the preparation of the present manuscript, a very interesting paper studying the effect of the synthesis approach (cation exchange, chemical vapor deposition and direct synthesis) on the nature of the Cu active site, and therefore, on the SCR of NO_x activity, has been published [16]. In that work, the authors showed the direct synthesis preparation of Cu-SAPO-34 material by using a combination of templates, such as the Cu²⁺-triethylenetetramine complex and tetraethylammonium cations, obtaining interesting catalytic results for the SCR of NO_x. Those experiments were performed in absence of steam, and moreover, the catalyst stability tests against severe ageing treatments were not carried out.

Herein, we will show a very detailed and rationalized “one-pot” Cu-SAPO-34 preparation, from the use of a Cu-complex (Cu²⁺ with tetraethylenepentamine, Cu-TEPA) as the unique template, to the cooperative and fundamental role of a small organic molecule (as diethylamine, DEA) acting as co-template. Following this rationalized direct synthesis methodology, the Cu-loading in extra-framework positions into the material can be easily controlled, and the final solid yields obtained are very high (larger than 90% of expected solids from initial precursors). Those Cu-SAPO-34 samples are extremely active and hydrothermally stable when tested in the SCR of NO_x reaction under very severe reaction conditions (very high space velocity, and presence of steam at very high temperatures). The combination of the direct synthesis of Cu-SAPO-34 together with hydrothermal stability and excellent catalytic results obtained for the SCR of NO_x introduces new opportunities for the industrial use of this type of material.