Influence of layer structure preservation on the catalytic properties of the pillared zeolite MCM-36 (original) (raw)
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Catalysis Today
Zeolite MCM-56 is a high alumina, monolayered form of the commercially useful framework MWW but it is a transient product during crystallization, so many factors influence its state and quality. This work examines properties of a series of MCM-56 and MCM-49 samples synthesized for different times using Aerosil, Ultrasil and Ludox as silica sources, hexamethyleneimine as the structure directing agent and additionally aniline as structure promoting agent. It was found that the most important parameter, governing the catalytic activity in the test reaction of Friedel-Crafts alkylation of mesitylene with benzyl alcohol, was availability of the Brønsted acid centers located at the external surfaces of the crystals. The key role in correlating physical characteristic with activity was played by infrared spectroscopy as it enabled the study of many properties of the tested materials, starting from the total concentration of acid centers, and their type (Brønsted or Lewis acids) through the concentration of centers available for the reagent molecules, to investigating the correlation of acidity with the degree of zeolite crystallinity.
Catalysts
MWW type zeolites are characterized by the presence of zeolitic layers of 2.5 nm thickness, containing 10-member ring sinusoidal channels inside and supercavities with 12-member ring openings located on their surfaces. Expansion and pillaring of layered zeolites increase the access to active sites and can enable or facilitate catalytic activity towards larger reactant molecules. This goal is explored in this work reporting the pillaring of layered zeolite MCM-56 with MWW topology by tetraethylorthosilicate (TEOS) treatment with the assistance of isopropanol, aimed at obtaining hierarchical micro-mesoporous systems. MCM-56 (Si/Al = 12) was synthesized with hexamethyleneimine as a structure-directing and aniline as a structure-promoting agent. Hierarchical porous systems were obtained using two different pillaring methods: (1) with TEOS only and (2) with TEOS mixed with isopropanol. The MWW framework was preserved during swelling/pillaring in both methods. Pillared zeolites obtained v...
Delaminated Zeolites: Combining the Benefits of Zeolites and Mesoporous Materials for Catalytic Uses
Journal of Catalysis, 1999
The delamination of the layered precursor of the MCM-22 zeolite (MWW structure) affords monolayers of a crystalline aluminosilicate with more than 700 m 2 g −1 of a well defined external surface formed by cups of 0.7 × 0.7 nm. In this layered structure the circular 10-member-ring microporous system is preserved. The resultant material presents the strong acidity and stability characteristic of the zeolites but, at the same time, offers the high accessibility to large molecules characteristic of the amorphous aluminosilicates. The cracking behavior during the process of small and large molecules has been compared with that of the zeolite MCM-22 and pillared laminar precursor MCM-36.
Journal of Porous Materials
Recently reported groundbreaking discovery of efficient delamination of zeolite MCM-56, producing colloidal suspensions of MWW monolayers dispersed in the liquid phase, created unprecedented possibilities for the synthesis of a zeolite catalyst. Based on this innovation, the concept of using MWW monolayers to prepare silica-supported zeolite nanosheet catalysts suitable for transformations of large organic molecules was explored in this work. A series of silica-MWW preparations was synthesized from colloidal suspensions of the monolayers, using both solid and colloidal silica sources. The synthesized solids were thoroughly characterized with various physicochemical methods and their catalytic performance was tested in alkylation of mesitylene with benzyl alcohol. The obtained results indicate that solids containing MWW layers dispersed on silica show promising catalytic properties. The mixed MWW:silica catalysts synthesized from dispersions of MWW monolayers and liquid silica were f...
Catalysis Today, 2012
Zeolites and related materials (including a wide range of microporous and mesoporous materials with ordered pore structure) have been one of the areas in the field of materials and catalysis with the largest impact on science, technology and industrial processes. We discuss here some recent developments in this field, with particular references how to tailor and design zeolite and related material properties to control/enhance the catalytic performances. Four main topics have been addressed. (i) The recent progress and perspectives in the field of tailored syntheses, with selected examples showing the trend and prospects to develop new structures, control the location of active sites, and the crystal size and morphology, including nanoarchitecture of the final catalysts. (ii) The development and prospects of two-dimensional zeolites presenting an extended view/concept of zeolite structures integrating the classical 3D frameworks and the various lamellar forms. (iii) The progresses in the design and synthesis of hierarchical zeolites, with discussion on the still existing challenges related to the synthesis, characterization and catalytic application. (iv) Novel opportunities and needs in terms of zeolite multifunctional design for catalytic applications, with a discussion of the critical issues related to the use in the field of fine chemicals, organic industrial syntheses and biorefinery, and the prospects for the use in two novel challenging areas of the direct conversion of CO 2 to light olefins and methane to methanol.
Hydrothermal stability and catalytic performance of desilicated highly siliceous zeolites ZSM-5
Journal of Catalysis, 2016
Highly siliceous zeolites, namely MFI type have attracted the great attention due to their higher hydrothermal stability, higher selectivity to organic compounds, and often better catalytic properties in comparison to Al-rich zeolites. The native zeolite (Si/Al = 164) and its desilicated analogues were deeply characterized with regard to their structural and textural properties by X-ray diffraction, low temperature adsorption of nitrogen and solid-state 27 Al MAS NMR. Their acidic properties were evaluated in quantitative IR studies. Finally, the catalytic performance of desilicated zeolites ZSM-5 was evaluated in the cracking of n-decane, 1,3,5-tri-iso-propylbenzene and vacuum gas oil. In this article, it is shown that high silica zeolites prepared by NaOH and NaOH&TBAOH leaching presented good hydrothermal stability with only slightly lower resistance when comparing to native steamed zeolite. The mesoporosity was preserved after the steaming treatment. The influence of the generated mesoporosity on the higher activity was evidenced in both 1,3,5-tri-isopropylbenzene and diesel oil cracking of steamed hierarchical zeolites. In spite of their lowered acidity, the mesopores system benefited the diffusion of the bulky molecule and finally provided higher activity of hierarchical zeolites.
A zeolite with interconnected 8-, 10- and 12-ring pores and its unique catalytic selectivity
Nature Materials, 2003
nature materials | VOL 2 | JULY 2003 | www.nature.com/naturematerials 493 Z eolites are built up by TO 4 tetrahedra (where T represents the central tetrahedrally coordinated atom) forming definite crystalline structures that enclose a large number of small cavities, which may be interconnected by a number of still smaller channels. These cavities and channels are highly uniform in size and shape within a specific zeolite material. Because the dimensions of the channels are such that they adsorb molecules of a certain size, these materials are known as molecular sieves, and are widely used in a variety of ways in industrial catalytic applications. Moreover, tailor-made host systems with designed topologies and adapted surface properties are an important prerequisite for supramolecular inorganic host-guest chemistry 1 . To date, the reported zeolite pore topologies of interest in catalysis were formed by uni-, bi-or tridirectional pore structures with the pores constructed by either 8-(small pores), 10-(medium pores) or 12-(large pores) membered rings (MRs) 2 .Two silica-based zeolites with a unidirectional system of 14MR pore apertures (extra-large pores) have been described (UTD-1 and CIT-5) 3,4 . Among the different structures available,the most successful zeolite catalysts up to now have been those containing either medium or large pores. However, it could be expected that a system containing both 12-and 10MR channels in the same structure could open possibilities for shape selectivity effects. The structure of a naturally occurring zeolite (boggsite), which has 10and 12MR windows bounding a three-dimensional channel system,has been described 5 , showing the possibility for those materials to be synthesized. Indeed, zeolites SSZ-26, SSZ-33 and CIT-1, which form a family of intergrown materials of two polymorphs 6-8 have been synthesized, and are formed by connected 12-and 10MR channels. However, this topology could be better described as a structure containing parallel 12MR channels that are perpendicularly connected by 10MR windows. Therefore, from the catalytic point of view, the diffusion properties of molecules through this zeolite structure will be mostly driven by the 12MR system 9 .Another zeolite,named NU-87,has connected pores with 10-and 12MR apertures, but in this case, only the 10MR pores are opened to the exterior 10,11 .A third zeolite,MCM-22 and its analogues SSZ-25, ERB-1 and PSH-3, have been reported, whose structures are formed by a 10MR system and large cavities delimited by 12MRs,but they are independent and the molecules cannot diffuse from one channel system to the other 12 .
The manifestation of zeolite recrystallization and the formation of amorphous aluminosilicate species during desilication are examined to better understand the properties of alkaline-treated hierarchical zeolites and their catalytic performance. This is achieved using a systematic experimental strategy, starting from treating the filtrate of alkaline-treated silicalite-1 in the presence of various external additives. No recrystallization is evidenced upon addition of tetrapropylammonium (TPA + ) and/or aluminum hydroxide ions [Al-(OH) 4 − ], confirming the low probability of zeolite nucleation and/or growth during desilication. Conversely, ordered mesoporous materials (OMMs) form upon addition of cetyltrimethylammonium (CTA + ) to the filtrate. By using other silicon sources, i.e., tetramethyl orthosilicate or the organosilane dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium, we verify the facile formation of amorphous materials during alkaline treatment of USY zeolites in the presence of hydrophobic micelle-forming alkyl moieties. A systematic characterization by X-ray diffraction, transmission electron microscopy, N 2 and Ar adsorption, inductively coupled plasma optical emission spectroscopy, and Fourier transform infrared spectroscopy of pyridine adsorbed, demonstrates that zeolites exposed to base solutions containing CTA + display weaker zeolitic properties, compared to those prepared using TPA + , and should be considered as hierarchical zeolite/OMM composites. Catalytic tests in the alkylation of toluene with isopropyl alcohol or benzyl alcohol evidence that CTA + -derived composites do not outperform the conventional USY zeolite. Only the hierarchical USY zeolite prepared by alkaline treatment in the presence of TPA + yielded a superior catalytic performance.
2021
Steamed zeolites have improved catalytic properties for hydrocarbon activation (alkane cracking reaction as well as alkane dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was speculated to be related to increased Bronsted acidity during dealumination. We now prove that during steaming aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano)facets of hydroxylated transition-alumina surfaces. Bronsted acidity of zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O5Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al1O5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying reason behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on penta Al(V)1O5 sites of aluminum oxide clusters confined in zeolitic pores. Furthermore, slightly decreased number of adjacent Al framework sites (due to Al dislodgement from the framework) decreases the coking activity, prolonging catalyst lifetime. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites. Furthermore, our findings suggest that similar La2O3 clusters exist for La-containing zeolites and the origin of their cracking activity promotion should be similar. Zeolites are arguably the most important industrial materials employed in petroleum refining and cracking [1-9]. They are crystalline and microporous solids composed of tetrahedral SiO4 and Si-OH-Al units, with ordered arrays of micropores/microcavities: alkane transformations into more