If necessary, additives such as shaping aids e. Such mainly organic compounds are burnt out in subsequent thermal treatment steps. In order to create a defined secondary pore system see below thermally or chemically, removable spacers e. Scheme of the principle manufacturing procedures for binder-containing zeolite molecular sieve shapes bulk material.
After the mainly dry mixing of all aforementioned materials in the pre-mixture usually a certain amount of water is added in order to adjust the viscosity and plasticity of the mixture for the related shaping process that can be agglomeration granulation [ 59 ], spray granulation [ 61 ], or extrusion [ 62 ]. After the shaping process, the molecular sieve has to be dried and activated in a thermal step, for example, in a rotary kiln [ 63 ] or belt calciner [ 64 ] to remove the water and other adsorbed compounds.
The applied temperature is often higher than required for the zeolite activation, since the binder system needs such higher temperatures for setting. Due to the limited hydrothermal stability [ 67 — 69 , 95 ] of hydrophilic zeolites, the activation step has to be carried out carefully.
To avoid partial zeolite destruction during the activation step, the released water needs to be removed from the shape as fast as possible in order to avoid the appearance of hydrothermal conditions. The higher the temperature and higher the moisture concentration directly at the zeolite centers the higher the probability for partial zeolite destruction [ 70 ].
Depending on the particular conditions, a residual water content of less than one percent can be reached. During the cooling step as well as during the storage of the activated material, the presence of humidity or other adsorptive components should be avoided to prevent undesired adsorption on the material.
Therefore, activated zeolite molecular sieves are usually packed in hermetically sealed packaging units, as, for example, sealed steel drums or big bags equipped with sealed inliners. The mentioned use of a binder system opens a large variety of zeolite molecular sieves with different properties.
Depending on the final application, the properties of the single shapes or the bulk material have to be optimized. For instance, with regard to the particle size, two limits should be considered: The bigger the single shape, the higher the probability for a possible limitation of the mass transfer within the shape. But the smaller the single shapes, the larger the pressure drop over the fixed bed and the higher the risk of fluidization.
Therefore, the adjustment of the size of the shapes is a critical parameter in the design of an adsorption process. Because dust may cause problems in a running adsorption process e. To receive smooth surfaces on single shapes of bulk material, the shaping process has to be run accurately.
Besides dusting, the mechanical stability crush resistance of the shapes has to be considered. The higher the bulk bed in the adsorber the bigger the weight and force that affects the lowest single shapes in the bed, which could finally result in a destruction of the shapes, and hence, generation of fines and an increased pressure drop. The binding mechanism of activated binder-containing zeolite molecular sieves is based on the generation of a network of binder material, wherein the zeolite crystals are embedded.
Figure 2 shows SEM pictures of the raw materials. The zeolite crystal agglomerates top left and the binder top right form a physically strong bound shape, which is demonstrated in Figure 2 downright. As mentioned earlier, the use of adsorption inert binder material reduces the adsorption capacity zeolite molecular sieves by approximately the percentage of binder in the shape see Table 1.
Adsorption capacities for zeolite NaX pure zeolite NaX powder and binder-containing bulk material with Hg intrusion curves of binder-containing zeolite NaX bulk material with In addition to the mechanical properties, the type and amount of binder and the shaping process applied have an essential influence on the structure of the secondary pore system of the shape—the part of the shape through which the transport of the molecules to and from the zeolites crystals within the shape is realized [ 66 , 73 ].
Said secondary or transport pore system strongly influences the kinetics of the adsorbent, and hence, the adsorption process. Using a lower amount of binder material, the adsorption capacity increases and the transport pores become slightly wider; see Figure 3. A faster diffusion is possible. But using a lower amount of binder material leads to an increased attrition value 0. To avoid the above-mentioned disadvantages of binder-containing zeolite molecular sieve bulk materials such as adsorption capacity reduction by adsorption inert binders or influence of the secondary pore structure by the binder material, the so-called binderless zeolite molecular sieves were developed.
There are different manufacturing procedures for binderless molecular sieves described in the open or patent literature. In most of those processes, the same shaping principles as mentioned earlier for the manufacturing of binder-containing zeolite molecular sieve are applied. They differ in the raw materials, respectively, the composition—especially the type and amount of the so-called temporary binder—of the starting pre-mixture, or the conditions for the conversion of the temporary binder into zeolite matter.
Important for the generation of binderless zeolite molecular sieve is that the temporary binder contains only those elements, which are present in the target zeolite matter; that means in most of the cases silicon, aluminum, and sodium, for example, for binderless molecular sieves of A, NaX, and NaY type or mixtures thereof.
Only for the preparation of binderless zeolite LSX-type molecular sieves, potassium-containing systems are used [ 74 ]. Thus, it is possible to generate binderless zeolite shapes using temporary binder material such as kaolin as starting material [ 75 ]. A mixture of temporary binder material such as kaolin, metakaolin or silica, and zeolite powder or zeolite filter cake is also mentioned [ 76 — 83 ]. The conversion step can be a wet chemical [ 75 , 79 ] or an at least partially autogenic thermal reaction [ 84 ].
Scheme of the principle manufacturing procedure of binderless zeolite molecular sieve shapes bulk material. Figure 4 shows the principle manufacturing procedure starting with shaping a pre-mixture using the above-mentioned technologies agglomeration granulation, spray granulation, or extrusion.
Taggart [ 75 ] reported about shaping a mixture of kaolin and sodium hydroxide followed by drying of the shapes.
It should be considered that kaolin has good binding properties but nearly no chemical reactivity. Usually, the shapes are aged and further processed in that reaction solution to convert the temporary binder into the desired zeolite matter.
Finally, the shapes are washed, dried, and activated. Taggart mentioned that, due to the limited accessibility of the interior of the shapes by the mentioned reaction solution blocked or too tight secondary pore system , the degree of the conversion of the temporary binder into zeolite matter, and hence, the adsorption capacity of the resulting material is limited, and can be enhanced if zeolite powder is used in the starting mixture.
However, the mechanical stability of the resulting shapes is lower. Therefore, Howell et al. Goytisolo et al. The zeolite powder in the mixture has obviously crystallization triggering properties and supports the generation of an open, for the reaction solution accessible pore system. It has to be considered that metakaolin has practically no binding properties. Thus, one has to make sure, that the process is carried out in a way, that the shapes, which are coming out of the shaping process, are mechanically stable enough until the conversion of the non-zeolitic compounds into zeolite matter in the reaction solution is completed.
Said conversion is the basis for the mechanical stability of binderless molecular sieve shapes see below. If it is possible to put all necessary compounds into the starting mixture for the shaping the reaction solution can be water [ 77 , 78 ], otherwise all missing components for the desired zeolite formation have to be present in the reaction solution [ 75 , 79 , 80 ].
Another way to manufacture binderless zeolite molecular sieves is the use of silica as synthetic temporary binder. The shapes obtained are aged to achieve a certain water stability [ 81 ] followed by the conversion of the temporary binder in a solution consisting of aluminum and sodium components [ 82 , 83 ].
The wet chemical reaction for the conversion of the temporary binder is preferably carried out by recirculating the reaction solution over the bulk material at suitable temperatures without moving the single shapes in order to avoid attrition between the still relatively soft shapes. In the course of the chemical conversion of the non-zeolitic components into zeolite matter, the binding mechanism changes.
Said intergrowths are formed during the conversion of the metakaolin into a polycrystalline zeolite matter [ 86 ]. Such unusual crystallization behavior can be explained by considering the available space for crystallization in a shape. On the outer surface of a single shape, there is enough space for a conventional and epitaxial crystal growth see Figure 4 down left for zeolite NaX , whereas in the interior, the space is obviously limited in a way that typical zeolite crystals such as octahedrons or cubes with rounded corners cannot be formed, but polycrystalline structures consisting of zeolite in untypical shape only see Figure 4 bottom right for zeolite NaX [ 86 ].
After the chemical conversion, the bulk material is separated from the mother liquor which can reused for further reactions and washed until a desired pH or conductivity of the effluent is achieved. Optionally, an ion exchange applying a suitable ion exchange solution can be affiliated. Finally, the material is dried and thermally activated in a suitable device e. The as-synthesized zeolite Y was denoted as NaY- -. The sodium form of zeolite was converted to the hydrogen form by ammonium ion exchange method.
The above cycle was repeated three times to get complete exchange of sodium. The zeolite cracking activity was determined with cumene as the probe molecules, which used a flow-type apparatus equipped with a fixed-bed reactor. Nitrogen was used as carrier gas at flows of 3. The catalysts were pressed binder-free and crushed to a particle size of 60—80 meshes, and the catalyst amount was 0.
Reaction products were analyzed by an online gas chromatograph with flame ionization detector. According to the demonstrations, the alkalinity of the synthesis system directly affects the crystallization time. In order to calculate relative crystallinity of the samples, a modified procedure described in [ 24 ] was used. Figure 2 reflects the relations between the alkalinity of the synthesis system and the crystallization times.
In this study, the sulfuric acid works as an additive to reduce the basicity of the synthesis system and the solubility of the aluminosilicate, which led to the decreased crystallization rate. Sample a showed relatively uniform spherical particles in the size range of 0.
Under the lower alkalinity of the synthesis system, the particles size of sample c increased to 1. Figure 4 shows the N 2 adsorption-desorption isotherms of the samples obtained at different alkalinity of gel, and the detailed data are in Table 2. All the samples exhibited a sharp uptake of N 2 at very low relative pressures , implying the presence of micropores.
On the other hand, samples A and B exhibited another uptake appeared at high relative pressure , which originated from intraparticle macropores [ 23 ] because of the uneven grain sizes of sample C Figure 3 e ; it could not form intraparticle macropores. As shown in Table 2 , the micropore surface area was decreased from Following ion exchange and calcinations, the XRD features of sample A weaken obviously except for peak, but the positions of , , and peaks change, indicating the part of the HY structure collapsed, whereas samples B , C , D , and E show only a diminution of the original XRD intensity, but the peaks positions do not change clearly.
Because the temperature of structural collapse was lower than the ammonia decomposition, NH 4 X and the low silica NH 4 Y zeolite could not be transformed to H-form zeolite by calcination method. The catalytic activity of the samples is shown in Figure 6. The sample HY had the lowest activity, the conversion of decomposition was only It should be caused by the low content of zeolite; the sample HY was mainly composed of amorphous silica-alumina material. However, samples HY In addition, we found the activity reduces with the increasing reaction time.
This is mainly caused by the coke deposit on catalysts; the coke deposition can block the zeolite pores and prevent active sites contact with cumene. We have demonstrated that high silica Y zeolite can be synthesized using direct synthesis method without adding any organic additive or SDA.
The as-synthesized zeolite Y nanocrystals show high N 2 adsorption and BET surface area and micropore volume are determined to be In addition, compared with lower silica Y zeolite, the as-synthesized high silica Y zeolite shows excellent cracking performance. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors. Read the winning articles. Journal overview. Special Issues. Academic Editor: Sanjay Behura. Received 24 Feb Revised 17 May Accepted 09 Jun Published 07 Aug Abstract A series of high silicon zeolites Y were prepared through direct synthetic method by using silica sol as the silicon source and sodium aluminate as the aluminum source.
Introduction Zeolites are an important class of crystalline aluminosilicates materials with open-framework structures, and they have been widely used as separations, ion exchange, and acidic catalysts for size and shape selective catalytic reactions by molecular-sized microporosity [ 1 — 3 ]. Experimental Details 2.
Chemicals All the chemicals were directly used as received with no further purification. Preparation of Zeolite Y Sodium aluminate, sodium hydroxide, and deionized water were placed in a beaker to ensure good mixing on a magnetic stirrer at room temperature.
Preparation of H-Type Zeolite Y The sodium form of zeolite was converted to the hydrogen form by ammonium ion exchange method. H-Type Zeolite Y Activity Evaluation The zeolite cracking activity was determined with cumene as the probe molecules, which used a flow-type apparatus equipped with a fixed-bed reactor. Results and Discussion 3. Figure 1. Figure 2. Table 1. A defining feature of zeolites is that their frameworks are made up of 4-coordinated atoms forming tetrahedra.
These tetrahedra are linked together by their corners and make a rich variety of beautiful structures. The framework structure may contain linked cages, cavities or channels, which are big enough to allow small molecules to enter. The system of large voids explains the consistent low specific density of these compounds. In zeolites used for various applications, the voids are interconnected and form long wide channels of various sizes depending on the compound. These channels allow the easy drift of the resident ions and molecules into and out of the structure.
The aluminosilicate framework is negatively charged and attracts the positive cations that reside in cages to compensate negative charge of the framework. Unlike most other tectosilicates [3], zeolites have largeer cages in their structures. Zeolite-like materials have structures similar to zeolites but elements other than Si, Al and O can be present in them.
Considerable success have been done recently on making tetrahedral frameworks with the congeners of Al and Si in the next row of the periodic table, namely, Ga and Ge [] , and only recently the first chiral germanosilicate ITQ has the been discovered with a zeolite-like framework consisting entirely of tetrahedrally coordinated positions occupied by germanium and silicon [15]. Although hundreds of laboratories are trying to synthesize new materials with novel zeolite framework structures, only zeolite framework types have been approved by the International Zeolite Association IZA Structure Commission IZA-SC.
The Atlas of Zeolite Structure Types published by the IZA Structure Commision assigns a three letter code to be used for a known framework topology irrespective of composition.
The codes are normally derived from the name of the zeolite or "type material", e. The more detailed information on topology of zeolites and related germanates compound is included in the Reticular Chemistry Structure Resource [16 ] and the Database of Periodic Porous Structures [17]. The naturally occurring zeolites are an important group of minerals for industrial and other purposes [18]. The discovery in of largedeposits of relatively high purity zeolite minerals in volcanic tuffs in the western United States and in a number of other countries represents the beginning of the commercial natural zeolite era.
Prior to that time there was no recognized indication that zeolite minerals with properties useful as molecular sieve materials occurred in large deposits.
Commercialization of the natural zeolites chabazite, erionite, and mordenite as molecular sieve zeolites commenced in with their introduction as new adsorbent materials with improved stability characteristics. The applications of clinoptiolite in radioactive waste recovery and in waste water treatment during the same period of the 60's were based not only on superior stability characteristics but also high cation exchange selectivity for cesium, strontium, and for ammonium ion.
Zeolites A Fig. Milton at the Union Carbide Corporation Laboratories represent a fortunate optimum in composition, pore volume, and channel structure. As a consequence they contain the maximum number of cation exchange sites balancing the framework aluminum, and thus the highest cation contents and exchange capacities. These compositional characteristics combined give them the most highly heterogeneous surface known among porous materials, due to exposed cationic charges nested in an aluminosilicate framework which results in high field gradients.
Their surface is highly selective for water, polar and polarizable molecules which serves as the basis for many applications particularly in drying and purification. Therefore, zeolites with higher content of silicon were needed, primarily to improve stability characteristics, both thermal and to acids. Breck [18]. Besides improvement in stability over the more aluminous X, the differences in composition and structures had a striking, unpredicted effect on properties making zeolites Y based catalysts valuable in many important catalytic applications involving hydrocarbon conversion since their initial commercial introduction in [19].
Figure 1. A representation of the zeolite A structure LTA as an assembly of framework's cages tiles. Center of a tile is the center of a void in the framework. Voids are connected with adjacend ones through the large "windows" which are faces of tiles.
The next commercially successful synthetic zeolite introduced in the early 's was a large pore mordenite Fig. The improvement in thermal, hydrothermal, and acid stability coupled with its specific structural and compositional characteristics resulted in application of mordenite as an adsorbent and hydrocarbon conversion catalyst [19].
Type L zeolites Fig. Breck and N. They were adapted as commercial catalysts in selective hydrocarbon conversion reactions. Figure 2. The most recent stages in the quest for more siliceous molecular sieve compositions was achieved in the late 's and the early 's with the synthesis at the Mobil Research and Development Laboratories of the "high silica zeolites" [18]. First in that row was zeolite beta Fig. Wadlinger, G. Kerr and E. Rosinski, and later ZSM-5 Fig. Argauer and G. In contrast to the "low" and "intermediate" silica zeolites, representing heterogeneous hydrophilic surfaces within a porous crystal, the surface of the high silica zeolites is more homogeneous with an organophilic-hydrophobic selectivity [19].
They adsorb stronger the less polar organic molecules and only weakly interact with water and other polar molecules. Figure 3. Tiling representation of the structure of the zeolite L LTL. Blue tiles are channels in the structure running along direction of the crystallographic c axis. In addition to this novel surface selectivity, the high silica zeolite compositions still contain a small concentration of aluminum in the framework and the accompanying stoichiometric cation exchange sites.
Thus, their cation exchange properties allow the introduction of acidic OH- groups via the well known zeolite ion exchange reactions, essential to the development of acid hydrocarbon catalysis properties.
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