Outer Surface Of Zeolite Crystallites Research Articles

Dehydration of 1-Octadecanol over H-BEA: A Combined Experimental and Computational Study

Liquid-phase dehydration of 1-octadecanol, which is intermediately formed during the hydrodeoxygenation of microalgae oil on zeolite H-BEA, has been studied, combining experiment and theory. Both the OH group and the alkyl chain of 1-octadecanol interact with zeolite Bronsted acid sites, inducing inefficient utilization in the presence of high acid-site concentrations. The parallel intramolecular and intermolecular dehydration pathways, leading to octadecene and dioctadecyl ether, have different activation energies and pass through different reaction intermediates. The formation of surface alkoxides is the rate-limiting step in the intramolecular dehydration, whereas the intermolecular dehydration proceeds via a bulky dimer intermediate, occurring preferentially at the pore mouth or outer surface of zeolite crystallites. Despite the main contribution of Bronsted acid sites toward both dehydration pathways, Lewis acid sites are also active to form dioctadecyl ether.

Cycloisomerization of 5-(o-Tolyl)-Pentene over Modified Zeolite BEA

Liquid-phase cycloisomerization of 5-(o-tolyl)-pentene (OTP) was conducted using a commercial nanocrystalline zeolite BEA. Zeolite BEA was a good catalyst for selective synthesis of 1,5-dimethyltetralin (1,5-DMT). However, over certain zeolite BEA, isomerization of 1,5-DMT to other isomers occurred after cyclization of OTP was almost complete at high temperatures. Textual properties of nanocrystalline BEA did not influence the cyclization activity, but the degree of postsynthetic treatments such as surface poisoning, passivation, and dealumination had great effects. Reactions over BEA modified by triphenylphosphine poisoning and Si layer coating of the outer surface of zeolite crystallites revealed that strong acid sites residing only on the external surface of zeolite BEA were responsible for isomerization activity. The 27Al magic-angle spinning nuclear magnetic resonance study for dealuminated zeolites together with the IR study of adsorbed pyridine clearly demonstrated that the concentration of Brønsted acid sites due to the framework aluminum was directly related to cyclization activity. On the other hand, octahedral aluminum did not participate in either OTP cyclization or DMT isomerization.

133Cs and 23Na MAS NMR studies of zeolite X containing cesium

The local structures around cesium and sodium in the zeolites X containing cesium in excess of the ion-exchange capacity (cesium-added zeolite X ) were examined by 133Cs and 23Na MAS NMR to elucidate the location of the added cesium and the formation of the basic sites caused by addition of the excess cesium. The cesium species located in the intracrystalline cavities of zeolite and those on the outer surface of zeolite crystallites were distinguished by a combination of 133Cs and 23Na MAS NMR data. The excess cesium species are located in the supercages for the samples containing an excess of 1.1 and 2.9 cesium atoms per supercage, whereas for the samples containing 5.1 and 9.8 cesium atoms per supercage, parts of the cesium species are on the outer surface of the zeolite crystallites. The cesium-added zeolites which contain cesium species in the supercages and possess a high surface area exhibit high activities for 1-butene isomerization.

Effect of High Temperature Water Steaming on the Active Centers of ZnHZSM-5

The effects of high temperature water steaming (HTWS) on the structure of HZSM-5 and ZnHZSM-5 catalysts have been investigated using XRD, XPS and IR spectrum of adsorbed pyridine, and the aromatization of propane has been tested as a probe reaction to show the change of catalytic performance. The result indicate that the treatment of HTWS nearly has no significant effect on the crystallinity of zeolite catalysts, but it lead to the dealumination of zeolite framework and the migration of zinc ions departing from the cation positions and results in the enrichment of aluminum and zinc content on the outer surface of zeolite crystallites and the decrease of catalyst acidity, and comparing the variation of Si/Al with Zn/Si and Zn/Al, it is shown that the framework aluminum species with zinc counterions are more difficult to detach from the framework than those with proton counterions. The variation of the reaction activity and selectivity of propane aromatization also verifies that the active centers of B and Zn-L acids are diminished differently depending on the treatment of HTWS processing.