Effect of Enhanced Accessibility of Acid Sites in Micromesoporous Mordenite Zeolites on Hydroisomerization of n-Hexane

Abstract

This paper describes a study of the nature and the accessibility of the acid sites in micromesoporous mordenite zeolites obtained by desilication and dealumination and analysis of their activity and selectivity in the hydroisomerization of n-hexane. Alkaline−acid, acid−alkaline−acid, and fluorination−alkaline−acid postsynthesis treatments were employed for the preparation of micromesoporous mordenites. The FTIR spectra of adsorbed d 3 -acetonitrille, 27 Al MAS NMR, HR-TEM, and N 2 adsorption were used for quantitative analysis of the Brønsted and Lewis sites, the coordination of Al atoms, and the textural properties. The alkaline treatment causes desilication, preferably occurring along the crystal defects and resulting in the formation of a secondary mesoporous structure characterized by 5−20 nm cavities and the formation of extraframework (Al Ex ) species and terminal Si−OH groups. The Al Ex species formed by hydrolysis of perturbed or dislodged framework Al easily restrict part of the pseudomonodimensional channel structure of mordenite. The subsequent removal of Al Ex by mild acid leaching or simultaneous removal of Si and Al atoms by desilication of fluorinated zeolite result in a micromesoporous structure with a large number of unrestricted channel openings and lead to a large increase in the accessibility of OH groups for n-hexane. Thus, the sequential leaching treatments enable the formation of active acid sites in an environment of nonrestricted microporous channels with simultaneous enhancement of accessibility of the active sites and molecular transport. It is shown that the micromesoporous structure with high concentration of Brønsted sites of enhanced accessibility directs the hydroisomerization reaction toward high yields of branched isomers and shortening of the main 12-ring channels and that the larger numbers of channel openings result in an increase in selectivity, limiting nonselective subsequent cracking reactions.

Mordenite Nanorods Prepared by an Inexpensive Pyrrolidine‐based Mesoporogen for Alkane Hydroisomerization

“…Taking into account that acid sites located in the 8MR side‐pockets are not accessible to alkanes, the improved catalytic behavior of MOR‐0.25 can be explained by a decreased amount of BAS in the main 12MR channels, where the hydroconversion reaction of alkenes takes place. The lower accessible acidity of Pd/MOR‐0.25 shifts the hydrogenation/acidity balance towards ideal hydrocracking behavior.…”

“…The introduction of a second level of intracrystalline porosity to the MOR pore system would be advantageous to the many acid‐catalyzed hydrocarbon conversion reactions that involve large paraffinic and aromatic molecules. A general “top‐down” approach to prepare mesoporous mordenite zeolites via desilication, dealumination or a combination of these approaches has been extensively examined. Even though these techniques are commercially available and can be scaled up, there is little control over the textural and acidic properties of the final materials.…”

The Physico-Chemical Characterization of Mesoporous Sulfated Zirconia Nanoparticles Prepared under Structure-Directing-Free for <i>n</i>-Hexane Isomerization

“…Isomerization of normal hexane (n-C 6 ) has been recognized to be one of the most economical units to boost octane level especially for the lower boiling gasoline. The process is typically catalyzed by bifunctional Pt/H-mordenite catalysts [2]. Recently, zirconia has been reported as a popular support for various reactions because of its high thermal stability, acid-base properties, and redox surface properties.…”

Hierarchical H-MOR Zeolite Supported Vanadium Oxide for Dimethyl Ether Direct Oxidation

“…Pastvova et al have reported that the controlled alkaline-based treatments of the MOR framework lead to the better preservation of acid sites and the formation of well-developed secondary mesoporosity. The existence of mesopores structure results in high yields of branched isomers and limits nonselective subsequent cracking reactions [22]. Zhang et al found that the formation of mesopores for hierarchical porous MOR used as the supports could be beneficial to the dispersion of Pt species which can improve the contact of the reactants to the active sites.…”