Ultrastable and Highly Acidic, Zeolite-Coated Mesoporous Aluminosilicates

Abstract

The acidity and hydrothermal stability of mesoporous aluminosilicates (MesoAS) are relatively low compared to those of zeolites, which limits their potential applications as catalysts in petroleum refining and fine chemicals synthesis. [1±7] Much effort has therefore been undertaken to synthesize a new type of materials, which combines the advantages of both zeolites and mesoporous molecular sieves. One might expect to improve both the stability and acidity of these materials if zeolite-like order could be introduced into the mesopore walls. Recent advances showed that amorphous walls of the mesostructure could indeed be converted to a partially zeolitic product. [8±13] For example, MCM-41/MFI composites containing interconnected mesopores and micropores were prepared by using dual templates and exhibited enhanced acidity and steam stability compared to the corresponding MCM-41. [8,9] Furthermore, the use of zeolite seeds as precursors for the assembly of mesoporous aluminosilicates with high hydrothermal stability and acidity was reported by Pinnavaia et al. [10] A similar zeolite beta type of materials was also prepared in the presence of co-templates of tetraethylammonium hydroxide and cetyltrimethylammonium bromide. [11] Recent results from our group showed the preparation of a new type of materials with semicrystalline zeolitic mesopore walls based on a templated solid-state secondary crystallization of zeolites starting from the amorphous walls of SBA-15. Bright-and dark-field transmission electron microscope (TEM) images recorded on the same area of the sample indicated that nanocrystals were embedded in the continuous amorphous inorganic matrix to form semicrystalline wall structures while preserving the mesoporous structure. The resulting mesoporous materials had much stronger acidity than those of the corresponding amorphous alumosilicates and a much improved hydrothermal stability. [12] Herein, we describe a new approach for the production of unusual zeolite-coated mesoporous aluminosilicates (ZCMeso-AS) using diluted clear solutions containing primary zeolite units. Hydrothermally ultrastable and highly acidic ZCMeso-AS result owing to the nanocrystalline zeolitic nature of their pore wall surface. These features open new possibilities for using this type of materials as acid catalysts.It is of special concern that due to the size of primary ZSM-5 units templated by tetrapropylammonium ions (2.8 nm in diameter), [13±14] the pore diameter of the mesoporous precursor molecular sieves should be higher than 30 ä. The methods in references [2] and [3] are therefore useful in this context for the preparation of mesoporous precursors, such as SBA-15 and siliceous mesostructured cellular foams (MCFs).The N 2 adsorption/desorption isotherms obtained from the calcined sample before and after coating are shown in Figure 1 A. The parent and ZSM-5-coated samples display the typical behavior of a mesoporous molecular sieve with a mesopore volume saturation capacity of about 1.56 and 0.78 cm 3 g À1 , respectively...

Overview and Industrial Assessment of Synthesis Strategies towards Zeolites with Mesopores

“…Thus, as soon as crystallization of the mesopore walls occurs, the constraints often involve distortion and even collapse of the mesostructure. To avoid this phenomenon, several measures can be taken including using more stable mesoporous materials, that is, with thicker walls, such as SBA‐15,102, 115 using milder conditions like lower temperatures or other solvents, such as glycerol, to limit silica dissolution and moderate zeolite growth 103. 107, 116 An alternative is to fill the mesopores with an organic compound which, upon pyrolysis under nitrogen, will give a CMK‐type carbon material that maintains the mesostructure during the crystallization 117.…”

“…Another approach is based on the use of an amorphous mesoporous matrix as a support for zeolite precursors or nanocrystals. After impregnation of preformed mesoporous materials with zeolite seeds and treatment at high temperature, semicrystalline zeolitic mesopore walls are obtained 103. 105, 108, 109…”

Structure and Properties of Al−MSU−S Mesoporous Catalysts: Structure Modification with Increasing Al Content

“…While these methods perhaps show some derivation from early reports of the conversion of zeolites into mesostructures, or indeed vice versa, those early routes tended to result only in intimate mixtures of separate domains of the zeolite and the mesostructure. , Indeed, a more recent report suggested that pore-wall crystallization of mesoporous aluminosilicates can be achieved by using the mesostructure as the silicate source in a modified zeolite synthesis . Analysis of that data, however, strongly suggested that the mesostructure was simply being dissolved and subsequently recrystallized in situ, once again producing separate domains of mesostructure and zeolite.…”

Hierarchical ZSM-5 based MCM-41 aluminosilicates: Ostwald ripening effect of 8 years old aged samples

“…An increase in surface area of about 56% was observed along with pore volume (30%) (figure 4). Ultra-stable mesostructured aluminosilicate with high acidic property using such primary zeolitic units of seed solution was reported to exert high stability attributed mainly to the presence of nanolayered zeolitic units at the mesopore walls of aluminosilicates [29]. Similar technique was used to synthesize different structured mesoporous molecular sieves involving MCM-48 with zeolite nanolayers (building units) [30].…”