mesoporous zeolites preparation characterization and applications pdf

Mesoporous Zeolites Preparation Characterization And Applications Pdf

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Johnson Matthey Technol. The book describes mesoporous materials, their synthesis, characterisation and applications. In addition to mesoporous materials, several chapters of the book review nanoporous materials.

Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: Garcia-martinez and K.

Mesoporous zeolites as efficient catalysts for oil refining and natural gas conversion

The article reviews different strategies towards obtaining mesoporous zeolites Y: desilication; surfactant templating and assembly of zeolite crystals. The impact of those methods on physicochemical properties is covered, with a special focus on the acidity of the samples measured with infrared IR spectroscopy. The methods of characterization of acidity are presented.

Quaternary ammonium cations used for desilication lead to obtaining crystalline; mesoporous and highly acidic zeolites. Si-OH-Al groups of extremely high acidity can be produced by calcination in a humid atmosphere.

When the conditions are optimized, post-synthetic surfactant templating allows crystalline mesoporous zeolite to be obtained with no loss of material.

All mesoporous zeolites Y proved to be active catalysts in liquid phase isomerization, catalytic cracking, and other reactions.

These unique properties are responsible for their application as catalysts in many major petro chemical processes, e. The fact that the active sites are situated inside the micropores is advantageous for the stabilization of carbocations, by negative charges in the framework, and shape selectivity in the case of some zeolite frameworks.

However, diffusional limitations are frequently observed when large molecules are processed in large-crystal zeolites with narrow pores. Moreover, such mass transfer limitations may cause a fast catalyst deactivation [ 4 ]. To improve the catalytic effectiveness in chemical reactions, several approaches were proposed. One of them was the synthesis of extra-large pore zeolites [ 5 , 6 ]. Another one was the preparation of zeolite nanoparticles [ 7 , 8 , 9 ] or assembly of zeolitic micrograins producing micro-meso-macroporous structures [ 10 , 11 ].

A very effective way was the synthesis of hierarchical zeolites containing more than one level of pores. Carbon particles [ 12 , 13 , 14 , 15 , 16 ] or template micelles [ 17 , 18 , 19 , 20 , 21 , 22 ] were incorporated into zeolite crystals, during the synthesis, and these were next burned off producing the additional mesopores.

Apart from some examples of organic surfactants that produce crystalline material like silane-terminated quaternary amines [ 17 , 18 , 19 , 22 ] , this approach often results in phase separation and amorphous material [ 23 ]. Mesoporous aluminosilicates can be produced as well by micelles-templating of zeolite seeds [ 24 ].

Dealumination is, in general, a less efficient method to create mesopores, while the desilication of zeolites in alkaline solutions turns out to be the most effective way to produce mesoporous zeolites.

Most desilication studies have been applied to MFI zeolites [ 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 ], among many others and zeolites possessing good porosity, acidity, and catalytic activity were obtained.

Many data on the synthesis of mesoporous zeolites, their properties and catalytic applications were presented in the monograph of J.

Garcia-Martinez and Kunhao Li [ 49 ]. Desilication of zeolite Y was much less frequently studied [ 35 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 ]. Most of the studies concerning hierarchical zeolites Y, including those that obtained them by incorporation of template micelles and by desilication, presented the results of structural and textural studies, as well as of catalytic tests.

Even though most of the reactions catalyzed by hierarchical zeolites are catalyzed by acid sites, their acid properties of these zeolites were less frequently investigated.

The present paper deals with the acidity of hierarchical zeolites Y, both these obtained by desilication and by template method. Assembly of zeolitic grains has also been briefly covered. Many reactions in the industry are catalysed by acid sites, therefore characterization of acidity is a very important field of study in the chemistry of zeolites.

The examination of acidity is performed using spectroscopic methods, such as infrared IR and nuclear magnetic resonance NMR spectroscopy, microcalorimetry, and thermoprogrammed desorption TPD. Solid-state NMR has been used for the characterization of acidity in zeolites and other materials for decades [ 67 , 68 , 69 , 70 ].

Different nuclei can be used for the characterization of SiOHAl groups, i. In the 1 H spectrum, signals from silanol groups and acidic Si-OH-Al groups can be easily discerned and provide quantitative information [ 71 ].

In principle, the oxygen nucleus 17 O could also be used to examine acidity, but the experiments with 17 O are not prevalent due to low natural abundance and its high quadrupole moment [ 73 ]. The interactions of acid sites with probe molecules are also studied using various nuclei. A growing interest is observed in using phosphorous-containing probe molecules like trimethylphosphine TMP or trimethylphosphine oxide TMPO [ 74 , 75 ].

The details of NMR characterization of acid sites in zeolites are presented in recent reviews [ 82 , 83 ]. Temperature-programmed desorption TPD is prevalently used method to study acidic properties of catalytic materials [ 84 , 85 ]. It allows for the measuring of the amount of base molecules that can be desorbed from acid sites during thermal desorption, which indicates the concentration of acidic OH groups [ 84 ].

The most widely used probe molecule for these studies is ammonia, but other more reactive amines ethylamine, n-propylamine, isopropylamine, tertbutylamine have also been studied [ 86 ]. Microcalorimetry is another tool for measuring acidity, namely heats of adsorption of probe molecules. It has been widely used by Auroux et al. The measurement results in the plot of enthalpy of adsorption as the function of the coverage [ 93 ], from which the number of acid sites, their strength and heterogeneity can be deduced.

IR is by far the most widely used comprehensive method to study the acidity of zeolites. The concentration of these acid sites can be determined if the extinction coefficients the diagnostic bands are known [ 94 ].

The best method is to analyse the spectrum at low temperatures, possibly coupled with OH to OD exchange [ 95 ]. Pyridine and ammonia are the most important probe molecules for acidity studies. Pyridine is the most popular probe molecule, as it exhibits narrow diagnostic bands and is suitable for quantitative studies. It is also quite a bulky molecule, therefore it does not penetrate voids and channels with a diameter smaller than 0.

It has to be mentioned that quantitative studies in IR spectroscopy can be made only when the extinction coefficient is known [ 98 ]. Additionally, the catalytic behaviour can be predicted, if the reactant or similar molecules are sorbed and measured in IR experiment. Lercher [ 99 ] and Knozinger [ ] presented the features that probe molecules should exhibit to provide a reliable examination of acidity.

The shift is an indirect measurement of acid strength. Desilication is the most effective and economical method of production of hierarchical zeolites. As mentioned, most of the desilication research was done with zeolites of the MFI type.

Therefore such a zeolite should be dealuminated before desilication. Such zeolites were nearly destroyed even in diluted NaOH, the micropore volume diminished dramatically and an amorphous material of relatively good mesoporosity but very low acidity was obtained [ 50 , 51 ].

Similarly, even very diluted ammonia solution [ 59 , 66 ], which practically extracted neither Si nor Al, caused framework destruction, and material of reasonable mesoporosity was produced. The reason why tetraalkylammonium cations stabilize zeolite structure is not well elucidated. Li and Shanz [ ] revealed strong bonding of tetraalkylammonium ions with silica entities protecting zeolitic Si against an alkali attack.

If tetraalkylammonium cations known as a pore directing agent are added to NaOH they act as an efficient pore-growth moderator during OH- assisted Si extraction [ 61 ]. It is possible that strong bonding of such cations with silica surfaces, shown by Li and Shanz [ ], protects framework Si atoms against OH- attack. The data were adapted with the permission from [ 51 ]. The zeolite which was the subject of desilication studies described in [ 51 , 52 , 53 , 54 , 66 ] was commercial CBV Zeolyst , and was steamed and acid-treated.

The acid properties of this zeolite were subject of detailed studies [ 51 ]. The concentration of acid sites was determined by quantitative IR studies of pyridine sorption and are presented in Table 1.

Moreover, some AlO 4 - tetrahedra may be neutralized by positively charged extraframework Al species. Very important information concerns Si-O 1 H-Al groups. They were found to be very strongly acidic. It may be supposed that this very high acidity may be related to the specific geometry of the environment of Si-O 1 H-Al bridge in the faujasite type structure.

It should be noted that recent quantum-chemical density functional theory DFT simulations within an ab initio molecular dynamic MD approach applied to a periodic model of faujasite structure in actual experimental temperature evidenced also very high acidity of these hydroxyls. Another very important property of Si-O 1 H-Al group is their homogeneity, all of them have the same acid strength.

This statement is supported by three facts. The second argument is the fact, that the OH band which restores upon adsorption of pyridine and desorption step-by-step at increasing temperatures restores at the same frequency Figure 1 B. In the case of heterogeneous hydroxyls more weak hydroxyls of higher stretching frequency restore at lower temperatures than more acidic ones of lower acid strength.

Therefore, the band of heterogeneous hydroxyls restoring at increasing temperatures shifts to lower frequency with the increase of desorption temperature. On the other hand, no such shift is observed for homogeneous hydroxyls. Spectra are normalized to the same band intensity. According to the results presented in [ ], homogeneous hydroxyls are present in zeolites in which all of them have the same number of Al near the bridge and all have the same bridge geometry the same bridge angle.

All Si-O 1 H-Al have also the same bridge angle. Generally, in most of the zeolites, Si-OH-Al are heterogeneous because there are hydroxyls of various number of Al near the bridge or of various bridge geometries , there are only a few zeolites with homogeneous OH groups: zeolites NaHA [ ] and NaHX [ , ], these zeolites have homogeneous and very weakly acidic hydroxyls.

Dealuminated zeolite Y is the only zeolite with homogeneous, but very strongly acidic hydroxyls. As most of the catalytic reactions catalysed by hierarchical zeolites are catalysed by acid sites the characterization of acidity of zeolites is essential for catalytical applications.

Li et al. The catalytic activity in triisopropylobenzene increased, indicating, that for this bulky molecule the improvement of transport inside mesopores is more important than the distinct loss of acid sites. On the contrary, Oruji et al. Desilication was done with NaOH without and with ultrasonication.

While desilication with ultrasonication practically changed neither mesoporosity nor acidity, it preserved the microporosity which was substantially decreased if zeolite was desilicated without ultrasonication. As a consequence, the cracking activity increased in ultrasonicated samples. Very interesting results were obtained [ 58 ] if zeolite desilicated with NaOH was previously steamed and next treated with NH 4 F solution.

NaOH treatment produced mesopores with only a small decrease of microporosity and caused an important increase of protonic acidity. Rac et al. The authors discussed the distribution of the acid strength of sites obtained from microcalorimetry, and the concentration of acid sites from IR experiments. Desilication of zeolite ZSM-5 changed neither the concentration nor the acid strength of the acid sites.

The catalytic activity of fructose dehydration increased. Even though all these bases show similar OH - concentration, their reaction with the zeolite framework is different. Figure adapted with the permission from [ 51 ]. The results of porosimetric and the acidity studies are compiled in Table 1. It diminished the concentration of protonic sites and produced a large amount of Lewis sites.

It seems that the different acidity of our zeolites and those studied by Rac et al.

Mesoporous Zeolites: Preparation, Characterization and Applications

Zeolites have been regarded as one of the most important catalysts in petrochemical industry due to their excellent catalytic performance. However, the sole micropores in zeolites severely limit their applications in oil refining and natural gas conversion. To solve the problem, mesoporous zeolites have been prepared by introducing mesopores into the zeolite crystals in recent years, and thus have the advantages of both mesostructured materials fast diffusion and accessible for bulky molecules and microporous zeolite crystals strong acidity and high hydrothermal stability. In this review, after giving a brief introduction to preparation, structure, and characterization of mesoporous zeolites, we systematically summarize catalytic applications of these mesoporous zeolites as efficient catalysts in oil refining and natural gas conversion including catalytic cracking of heavy oil, alkylation, isomerization, hydrogenation, hydrodesulfurization, methane dehydroaromatization, methanol dehydration to dimethyl ether, methanol to olefins, and methanol to hydrocarbons. Download to read the full article text. Corma A.

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Mesoporous Zeolitic Materials (MZMs) Derived From Zeolite Y Using a Microwave Method for Catalysis

Mesostructured zeolitic materials MZMs with relatively high acidity in comparison with the mesoporous siliceous MCM were prepared via an efficient, mild, and simple post-synthetic treatment of Y zeolite facilitated by microwave irradiation, i. MZMs possess a total acidity of about 0. Additionally, a comprehensive comparison of the state-of-the-art mesoporous materials concerning their sustainable aspects was made, showing that MZMs are promising mesoporous materials for further development and functionalization for catalysis. Mesoporous materials are an important class of porous materials with a wide range of proposed applications in catalysis particularly as the additives for petrochemical conversions, especially large hydrocarbons , environmental remediation e. However, although the well-structured mesoporosity is valuable to improve the accessibility and molecular diffusion within the framework, the amorphous nature of mesoporous silica frameworks makes them, generally, hydrothermally less stable compared to the crystalline microporous zeolites.

Metrics details. A novel soft-template ST is fabricated and successfully employed as mesoporogen to synthesis hierarchical ZSM-5 zeolites with outstanding mesoporosity and high hierarchy factors. The as-produced soft-template can connect steadily with the MFI frameworks by covalent bonds of —Si—O—Si— during the high-temperature hydrothermal crystallization process.

Metal and Semiconductor Nanocrystals View all 10 Articles. Metal nanoparticles NPs exhibit desired activities in various catalytic reactions. However, the aggregation and sintering of metal NPs usually cause the loss of catalytic performance in practical reaction processes. Microporous zeolites, owing to their rigid frameworks and porous structural features, have been considered as one of ideal inorganic supports.

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The article reviews different strategies towards obtaining mesoporous zeolites Y: desilication; surfactant templating and assembly of zeolite crystals. The impact of those methods on physicochemical properties is covered, with a special focus on the acidity of the samples measured with infrared IR spectroscopy. The methods of characterization of acidity are presented.

“Mesoporous Zeolites: Preparation, Characterization and Applications”

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Mesoporous Zeolites: Preparation, Characterization and Applications

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Mesoporous Zeolites Preparation Characterization And Applications

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