Efficient synthesis of the Cu-SSZ-39 catalyst for DeNOx applications

Abstract

The use of small pore zeolites with large cavities has been reported for relevant applications in catalysis or gas separation.[1] One of the most important applications of these materials as catalysts is the selective catalytic reduction (SCR) of nitrogen oxides (NO x ), where copper-containing zeolites with small pore sizes show excellent activities to remove these hazardous compounds produced during combustion in diesel engines. [1,2] Interestingly, these small-pore zeolites show higher hydrothermal stability compared to other zeolitic materials presenting larger pore sizes, such as Cu-ZSM-5 or Cu-Beta. [3] The particular stabilization of the Cu 2+ cations within the double 6-rings (D6Rs) present in some structures of these small pore zeolites, i.e. chabazite (CHA structure), has been claimed as a plausible reason for their higher hydrothermal stability.[4]Another zeolite with related structural properties to chabazite is SSZ-39 (AEI structure), which is an aluminosilicate with large cages connected by a three-directional small 8-ring (8-R) pore system, and also with D6R as secondary building units in its structure.[5]Recently, we have reported that Cu-exchanged SSZ-39 zeolite is an active and hydrothermally stable catalyst for the SCR of NO x with ammonia, showing even better catalytic performance than Cu-exchanged CHA.[6]The preferred synthesis procedure of SSZ-39 requires the use of simple alkylsubstituted cyclic quaternary ammonium cations as organic structure directing agents (OSDAs) in alkaline conditions. [7] These organic cations could be easily prepared from commercially-available pyridine precursors, [8] making attractive the use of these OSDAs for the synthesis of the SSZ-39 from an economic point of view. Unfortunately, this methodology can afford the preparation of the SSZ-39 in low solid yields (lower than 50%), since the final crystalline solids show much lower Si/Al ratios than the Si/Al ratios initially introduced in the synthesis gels, [6,7,8] indicating that most of the Si species remain in solution after the crystallization procedure. This fact limits the potential use of SSZ-39 in commercial applications.Recently, the synthesis of the high silica AEI zeolite with high solid yields (above 80%) has been reported using USY zeolite as silicon source and tetraethylphosphonium (TEP)cations as OSDA. [9] Although this achievement is highly relevant in terms of the optimization of the SSZ-39 synthesis, the use of P-based OSDAs still presents some 3 important drawbacks. On one hand, phosphine-derived organic molecules show important environmental and health hazards, and, on the other hand, the complete removal of the phosphorous-species entrapped within the zeolitic cavities is very difficult, especially within small pore zeolites, and their calcination process requires high temperatures and hydrogen atmospheres for the complete decomposition/elimination of these compounds.[9]It is worth mentioning that in the last years the use of pre-formed crystalline zeolitic precursors to synthesize different...

Iron‐Containing SSZ‐39 (AEI) Zeolite: An Active and Stable High‐Temperature NH3‐SCR Catalyst

“…Besides zeolites with the CHA framework structure, we have shown that Cu‐SSZ‐39 zeolite (AEI structure) is an excellent NH 3 ‐SCR catalyst, which in some cases even outperform Cu‐CHA in terms of activity and stability. Thus, it would be expected that Fe‐SSZ‐39 would perform comparatively well as a high‐temperature NH 3 ‐SCR catalyst.…”

“…However, the incorporation of iron within the AEI structure has not yet been described, which is probably due to the fact that this material was only achieved under very limited synthesis conditions and with low solid yields (below 40 %) . Recently, we reported the synthesis of high‐silica SSZ‐39 with improved yields (above 85 %) by means of a zeolite‐to‐zeolite transformation procedure …”

Building Zeolites from Precrystallized Units: Nanoscale Architecture

“…[13] In general, well-formed zeolite precursors are described to decompose into small fragments containing characteristic SBUsi nt he synthesis gel when treated hydrothermally,t hus favoring the rearrangement and crystallization of other structures sharing similar SBUs. [15] These zeolite-to-zeolite transformations allow control over the particle sizes,h eteroatom substitutions,o rs olid yields of different zeolites that could not be easily achieved when using other amorphous bulk silica sources. [15] These zeolite-to-zeolite transformations allow control over the particle sizes,h eteroatom substitutions,o rs olid yields of different zeolites that could not be easily achieved when using other amorphous bulk silica sources.…”

Improving the catalytic performance of SAPO-18 for the methanol-to-olefins (MTO) reaction by controlling the Si distribution and crystal size

“…[20,30] This material has been tested for the MTO reaction at 350ºC and WHSV=0.8 h -1 , obtaining an excellent catalytic activity and catalyst lifetime (X 50 =860 min, see SSZ-39_50 nm in Figure 8b). …”