Direct Detection of Multiple Acidic Proton Sites in Zeolite HZSM-5

Abstract

Direct observation of multiple reactive sites in the zeolite HZSM-5, a member of the MFI family of zeolite structures, contradicts the traditional view of only one type of active protonic species in industrially important zeolites. In addition to the well-known Brönsted acid site proton, two other protonic species undergo room-temperature hydrogen-deuterium exchange with an alkane hydrocarbon reagent, including one zeolite moiety characterized by a broad H chemical shift at ca. 12-15 ppm that is reported here for the first time. Although the ca. 13 ppm chemical shift value is consistent with computational predictions from the literature for a surface-stabilized hydroxonium ion in a zeolite, data suggest that the signal does not arise from hydroxonium species but rather from hydroxyls on extra-lattice aluminol species proximate to Brönsted lattice sites, i.e., a small population of highly deshielded acid sites. Double-resonance experiments show that this species is proximate to Al atoms, similar to the Brönsted acid site proton. These sites can be removed by appropriate postsynthesis chemical treatment, yielding a catalyst with reduced activity for isotopic H/D exchange reactions. Additionally, other extra-lattice aluminum hydroxyl groups previously discussed in the literature but whose protons were considered unreactive are also shown for the first time to react with hydrocarbon probe molecules. Two-dimensional exchange NMR reveals direct proton exchange between the Brönsted site and these two types of extra-lattice Al-OH species, and it also reveals unexpected proton exchange between extra-lattice Al-OH species and an alkane reagent.

The acidic nature of “NMR-invisible” tri-coordinated framework aluminum species in zeolites

“…Therefore, understanding the structures and properties of the acidic sites in zeolites is essential to exploring the reaction mechanism and to the optimization of their catalytic performance. [5][6][7] It is well-known that the BAS is associated with the tetrahedral framework aluminum (FAL) in the form of a bridging hydroxyl (Si-OH-Al) in zeolites. Furthermore, the LAS is generally recognized as an extra-framework or framework aluminum species, formed upon calcination or steaming of zeolites.…”

“…8,9 The properties of BAS associated with the tetrahedral framework Al and LAS associated with various extra-framework Al (EFAL) species have been extensively studied, from their structural characteristics [10][11][12][13][14] to their functions in catalytic reactions. [15][16][17][18] In particular, the LAS in zeolites has been proved to play an important role in the catalytic transformations of hydrocarbons such as the hydrogen transfer process, 7,19,20 and also generates the Brønsted/Lewis acid synergy owing to the vicinity of the corresponding Al species in the local architecture of dealuminated zeolites. 11,[21][22][23] Although the tri-coordinated FAL is commonly considered as a LAS in zeolites, only a few characterizations [24][25][26] have been attempted to conrm the existence of the "moisture sensitive" tri-coordinated Al species in the zeolite framework.…”

Recyclable Nanoporous Materials with Ordered Tunnels Self‐Assembled from α‐ and γ‐Cyclodextrins

“…Porous materials, such as covalent organic frameworks (COF), [1,2] Metal-organic frameworks (MOF), [3,4] zeolites, [5,6] mesoporous silica, [7,8] aerogels, [9][10][11] and sponges, [12,13] have received considerable attentions in recent years due to their crucial applications in gas storage, absorption, separation, catalysis, sensing, energy, thermal insulation, and biomedicine etc. The physical properties and applications of the porous materials mostly rely on their pore parameters, [14,15] such as pore size, pore volume, porosity, regularity, and dispersity, [16][17][18] and can be tuned by different synthesis methods.…”

Developing two-dimensional solid superacids with enhanced mass transport, extremely high acid strength and superior catalytic performance

“…The acidity of the synthesized 2D solid acids was investigated by using 1 H and 31 P MAS NMR, a reliable and state-of-the-art technique for determination of the acidity of solid acid catalysts. 6,9,24,38,39 In particular, 31 P trialkylphosphine (TMP) NMR has been used to distinguish Brønsted and/or Lewis acid types. When TMP molecules are bound to Lewis acid sites, they give rise to 31 P resonance ranging from ca.…”