In this laboratory you will prepare the synthetic zeolite ZSM-5 and show that it is a shape selective catalyst for the isomerization of xylenes to p-xylene. ZSM-5 is a strong acid with a characteristic structure containing channels, which allow molecular diffusion if the size of the molecule is appropriate. This is known as shape selectivity. The preparation of this zeolite involves hydrothermal crystallization, a reaction carried out in aqueous solutions above the normal boiling point of water. Such reactions take a few days heating at about 160°C.
Zeolites are usually synthesized under hydrothermal conditions, from solutions of sodium aluminate, sodium silicate, or sodium hydroxide . Such conditions are typical of those found in the earth's crust where some zeolites are found naturally. The precise zeolite formed is determined by the reactants used and the particular synthesis conditions used, such as temperature, time, and pH; particularly critical is the templating ion, in this case the propylammonium cation. The templating ion is usually an organic cation around which the aluminosilicate lattice is formed, so that the tunnel size is determined by the templating cation. The main zeolite formula is M2/nO.Al2O3.xSiO2.yH2O, with M defining the compensating cation with valence n . The structural component is Mx/n[(AlO2)x(SiO2)y].zH2O, with the general structure as arrangements of tetrahedra in building units from ring structures to polyhedra .
In the experimental section the formation of ZSM-5 is formed by a careful procedure of heating and purification of the product. Periodically, X-ray analyses are taken to confirm the formation of the correct product. Finally, the zeolite is utilized as a shape-selective catalyst for the o-xylene to p-xylene reaction.
Today, zeolites are desired catalysts because of their high concentration of active acid sites, their high thermal/hydrothermal stability, and high size selectivity. ZSM-5 is used commercially as a catalyst in fluid cat-cracking (FCC) units in oil refineries to increase the motor octane of gasoline, increase the total LPG and increase the olefin content of the fraction . This is known as the secondary "cracking" of gasoil .
The properties of the ZSM-5 and its use in the conversion of xylenes to the preferred isomer, p-xylene, is described in the discussion section.
Preparation of Zeolite ZSM-5
You will need the reagents listed in the table below. Calculate and fill in the blanks in the table before continuing; hydrothermal reactions can be rather sensitive to impurities and so it is important to be able to trace the batch of chemical used. Make note of any particular observations you make whilst weighing out the components.
|Lot Number of Reagent||Note your Observations here|
Weigh out about 0.510 g of sodium hydroxide pellets and finely grind with a mortar and pestle. Place this mixture in a 250 ml beaker, and add 2.01 g of silicic acid and 1.01 g of tetrapropylammonium bromide. Mix with 5.0 ml distilled water, then add 1.0 ml of n-propylamine and mix the solution again.
Place 1.0 ml of a prepared 1 molar solution of aluminum sulfate along with about 0.05 ml of the concentrated sulfuric acid in a separate 50 ml beaker. Then add the first solution to this beaker as well. Add enough distilled water to raise the volume to about 25 ml, and mix the solution (26 ml total volume) on a stir plate for ten minutes. Then transfer the solution to the Parr "Bomb" and seal it. Gently shake it, before placing in the oven. Heat the sample to 160°C and hold there for 44 hours (note how long it takes to reach 160°C). Do not worry if you cannot remove every last trace of the solid out of the beaker.
After 2 days heating, turn off the oven and remove the Parr reactor and let it cool to room temperature. Remove a small sample for x-ray analysis. If the x-ray pattern matches the expected one, see figure 2, then filter the rest of the reactant in a Buchner funnel with fine filter paper (541 grade). Wash it three times with copious amounts of water and then dry for 20 minutes on the filter paper.
You are now going to calcine the sample to remove the organic cation, so set-up the tube furnace assembly (Figure 2). Place the zeolite inside, on top of the frit in the middle of the tube, and spread out to maximize the surface area. Fit a ground glass elbow at each end, one attached to a nitrogen cylinder and the other immersed in a beaker of water, to regulate the flow of nitrogen gas. Slowly heat the tube to 500°C in increments of 50 to 100°C, when water vapor will be released, then in increments of 100 to 500°C. Heat for two hours after reaching temperature, where the tetrapropylammonium bromide will decompose to tripropylamine, propylene, and water. Cool and weigh the materials (note the weight in g). Remove another sample for x-ray analysis; how does it compare to the previous x-ray?.
Any sodium ions remaining in the zeolite will now be ion exchanged for protons to fully convert the zeolite to the acid form. The sodium product from above is placed in a 100 ml beaker, and stirred with 12.60 ml of 1 M aqueous ammonium sulfate for 15 minutes. Collect the zeolite by Buchner filtration (repeat for a total of 3 washings). Wash the product with small amounts of acetone. Then wash several times with distilled water to remove all sulfate ions. Test the washings for this anion by adding drops of an aqueous solution of BaCl2; the formation of a BaSO4 precipitate indicates the presence of the ion.
When the solution no longer produces a precipitate, wash the zeolite once more with acetone to dry it, and then dry in the oven at 120°C for 20 minutes. The acid hydrogen form of the compound is prepared by transferring the oven-dried compound to a tube furnace. Heat the ammonium zeolite for 3 hours to ensure the thermal decomposition of the NH4+ ions. Over the course of this process, the zeolite should turn from a white to brown/black to an off-white color. Cool the material and store in a desiccator to preserve the acid hydrogen form.
Xylene isomerization is done using a simplified fixed-bed catalytic reactor (Figure 5). For the set-up, first place glass wool in the glass tube provided, then one of two thin-pressed zeolite pellets, then glass wool, the other pellet, and finally glass wool. Place this glass tube in the tube furnace, and heat to 425°C. Meanwhile connect one end to the flask filled with o-xylene boiling at 140°C, and connect the ground glass elbow at the other end of the glass tube to a receiving flask, with an attached condenser. Start flowing nitrogen gas through the tube when the furnace reaches 425°C. To ensure condensation of the xylenes place the receiver in an ice bath, and sufficient quantity of product should begin to form within 10 minutes. Take an NMR spectrum of both the o-xylene starting material and of the product to determine how much p-xylene products was formed.
ZSM-5 Properties/Formation of p-Xylene
ZSM-5 is a zeolite with orthorhombic symmetry, determined by model building, and single crystal or powder x-ray data . One of its important features is the ratio of aluminum to silicon. For syntheses of pH less than 10, the amount of incorporated Al increases and the Si/Al ratio is between 5 and 20  . This is due to the stabilization of the structure by interactions between positive and negative charges. Likewise, as the Si:Al ratio increases, cation density and electrostatic field strengths decrease affinity of the zeolite surface for non-polar sorbates increases, as silica-rich zeolites "prefer" hydrocarbons to water .
In this way, the ability of this zeolite to function as a catalyst is determined by several properties. These are the presence of active sites (acidic, basic, cationic, etc.), the spatial arrangement and size of the channels and pores, and the presence of extraneous compounds within channels of outer parts . The fact that the compound is shape-selective is due to the fact that most active sites are present in intracrystalline pores or cavities which are of molecular dimensions . For ZSM-5 specifically, the channels are linked by intersections, and access to the interior of the molecule occurs by 8, 10, or 12 peripheral oxygen atoms .
These properties allow ZSM-5 to act as an effective "molecular sieve," with characteristic absorption properties, and illustrates its use in the production of p-xylene from o-xylene . In these procedures, Friedel and Crafts catalysts causes isomerization to occur accompanied by a transalkylation reaction, giving toluene and trimethylbenzene. The high isomerization to disproportion ratio observed in ZSM-5 zeolites is due to the fact that its pore size matches the diameter of xylene molecules . The mechanism occurs only through a unimolecular 1,2 methyl-shift mechanism . In this way, ZSM-5 is an important catalyst for the conversion of p- to o-xylene.
X-Ray Diffraction Analysis
The purpose of XRD patterns is to be able to determine the unit cell parameters and thus unit cell volume when the zeolitic structure is known, then one can determine if an element has been introduced into the lattice framework position . It is also a measure of the purity of a compound, compared with reference spectra. If there is no evidence of crystalline or amorphous contaminants present, then one must compare the intensity of the reference with the authentic sample to check for the same composition and crystal size.
However, it is known that zeolite is either orthorhombic or, to a lesser extent, monoclinic . In Figure 11, three ways of depicting octahedra framework are illustrated . According to reference materials, small changes in the XRD pattern can indicate the change from a monoclinic to orthorhombic structure, with the loss of the characteristic "doublet" at 24.4 (2 theta) . Meanwhile, peaks at 23.3 and 23.8o also show structural changes of the sorbate, while the crystalline structure remains intact .
From Figures 2 & 3, the two x-ray diffraction experimental patterns are presented. In comparison to reference peaks of Figure 4, the patterns appear to be almost the same. In addition, the peaks correspond to the appropriate reference peaks listed in Rollmann et al's report, also included in Figure 4 . Clearly, we produced a pure compound of ZSM-5.
For the conversion of o- to p-xylene, an NMR analysis was also included in the procedure. The results of the lab in Figures 6 & 7 indicated that the o-xylene was present at 1.51627 (Figure 9), while the chloroform standard occurred at 7.25009. Also, in the p-xylene NMR spectra, the chloroform was still present at about 7.25, while at around 6.7 were the aromatic peaks, and the 2 small, symmetrical peaks around the 2.275 peak were most likely 1% C . However, in reference to Figures 8 & 10, the p-xylene chemical shift is at 2.275 ppm. The relative peak area could be used to determine the extent of conversion of o- to p-xylene. In this spectra, the extremely small peak at 1.51445 (o-xylene) is almost negligible compared to the tremendous peak at 2.275. By analysis, the multiplicities of the lines reflect differences in local geometries of different silicon atoms in the unit cell and therefore they are related to diffraction-determined structures . Thus, NMR reveals the presence of the pure ZSM-5 product, and is useful as a determinant of the success of the experiment.
To be completed
By following the above procedures, you have synthesized a very important material, the size and shape selective catalyst ZSM-5. You have learnt how to ion-exchange a solid, how to characterize a solid using x-ray powder diffraction analysis, and how to determine its purity. You then performed a small scale experiment of a major industrial process, the selective isomerization of o-xylene to p-xylene. Finally you used two key organic analytical tools, NMR and GC, to measure the success of the isomerization reaction.
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