Impact of Controlling the Site Distribution of Al Atoms on Catalytic Properties in Ferrierite-Type Zeolites

Zeolites with the ferrierite (FER) topology are synthesized using a combination of tetramethylammonium (TMA) cations with differently sized cyclic amines (pyrrolidine (Pyr), hexamethyleneimine (HMI), and 1,4- diazabicyclo[2.2.2]octane (DAB)). Using these organic structure-directing agents (SDAs), low Si/Al ratios and concentrated synthesis mixtures favor the crystallization of FER materials. Increasing the size of the cyclic amine or decreasing the aluminum content leads to the crystallization of other phases or the creation of excessive amounts of connectivity defects. TMA cations play a decisive role in the synthesis of the FER materials, and their presence allows the use of HMI to synthesize FER. Proton MAS NMR is used to quantify the accessibility of pyridine to acid sites in these FER samples, where it is found that the FER + HMI + TMA sample contains only 27% acid sites in the 8-MR channels, whereas FER + Pyr and FER + Pyr + TMA contain 89% and 84%, respectively. The constraint index (CI) test and the carbonylation of dimethyl ether (DME) with carbon monoxide are used as probe reactions to evaluate how changes in the aluminum distribution in these FER samples affect their catalytic behavior. Results show that the use of Pyr as an SDA results in the selective population of acid sites in the 8-MR channels, whereas the use of HMI generates FER zeolites with an increased concentration of acid sites in the 10-MR channels

Decisive role of transport rate of products for zeolite para-selectivity: effect of coke deposition and external surface silylation on activity and selectivity of HZSM-5 in alkylation of toluene

Toluene alkylation with methanol and ethylene over HZSM-5 zeolites has been investigated in order to understand the effects of deposited ¿coke¿ and postsynthesis surface silylation on the toluene conversion and product selectivity. Long time-on-stream kinetic runs, performed under conditions in which total toluene conversion did not decrease but concentration of coke continuously increased were used to follow the changes in the individual xylene selectivity, the activity of the external surface, and the transport rates of individual xylene isomers. A very small increase in ortho- and para-xylene selectivity, no changes in xylene diffusivities, but a substantial decrease in the activity of the external surface were observed with increasing coke deposition. Under severe reaction conditions when coking caused a substantial decrease in toluene conversion, nearly no increase in para-xylene or para-ethyltoluene selectivity was found. The silylation of ZSM-5 zeolite with tetraethylorthosilicate blocked active sites on the external zeolite surface and decreased considerably the diffusivities of xylenes and ethyltoluenes. This modification also increased substantially the para-selectivity at high toluene conversions. Comparing the effect of coke deposition and surface silylation on the para-selectivity it is concluded that a decrease in the transport rates of xylenes is decisive for an enhancement of the para-selectivity, whereas secondary isomerization of para-isomers on the external surface plays a minor role

Analysis of the state and size of silver on alumina in effective removal of NOx from oxygen rich exhaust gas

Ag/alumina catalysts with different silver contents for octane-SCR were prepared by impregnation and incipient wetness methods. Activity tests revealed that the decisive factor for high activity is not only a high dispersion of silver, but also the ability of the system to redisperse clustered silver. Determination of dispersion by TEM/HAADF and O-2-chemisorption experiments resulted in values close to each other even if the results were not directly comparable. This is suggested to be due to not complete silver reduction below 700 degrees C and the samples being very heterogeneous in terms of particle size, e.g., having a bimodal size distribution. Small charged Ag-n(delta+) clusters containing 2-8 silver atoms highly prevailed in the samples containing 400 degrees C), large metallic particles are stabilized, resulting in poor conversion of NOx to N-2