Catalyst zeolite deposition

Zeolite Y, 2 345t, 5 238-239, 11 678, 679 coke formation on, 5 270 for liquid separation adsorption, 1 674 manufacture, 2 359 structure, 1 675 Zeolite ZSM-5, 11 678 Zeolitic cracking catalysts, 16 835 Zeolitic deposits, 16 813 Zeonex, 10 180 Zeotypes... 

Three different zeolites (USY-zeolite, H-ZSM-5 and H-mordenite) were investigated in a computer controlled experimental equipment under supercritical conditions using the disproportionation of ethylbenzene as test reaction and butane or pentane as an inert gas. Experiments were carried out at a pressure of 50 bar, a flow rate of 450 ml/min (at standard temperature and pressure), a range of temperatures (573 - 673 K) and 0.8 as molar fraction of ethylbenzene (EB) in the feed. The results showed that an extraction of coke deposited on the catalysts strongly depends on the physico-chemical properties of the catalysts. Coke deposited on Lewis centres can be more easily dissolved by supercritical fluid than that on Brnsted centres. 

Now zeolite catalysts have been employed by most FCCUs. Although zeolite catalysts have a much higher initial activity as compared to amorphous catalysts, coke deposit on the catalyst particles rapidly lowers their activity. As the carbon content of zeolite catalysts increases by 0.1 wt%, the activity decreases by 2-3 units. Generally the carbon content of regenerated zeolite catalysts should not be allowed to exceed 0.2 wt. %, or preferably less than 0.1 wt. % in the case of ultrastable Y zeolite (USY). Therefore, how to decrease CRC efficiently for zeolite catalysts in FCCUs has become a significant problem. 

Initial runs using Cu-containing Y zeolites yielded remarkable conversion levels (T = 523-623 K residence time = 1-6 s concentration c = 23 or 37 mg/1, respectively). During these runs, however, we observed deactivation of the catalyst and deposition of crystalline by-products at the exit of the fixed bed reactor. These deposits were identified as congeners of polychlorinated benzenes by means of HRGC/MS. The phenomenon of transchlorination is also known in connection with the thermal decomposition of 1,2-dichlorobenzene [6,7], As we confirmed for catalytic experiments with unchlorinated aromatics carried out in our laboratory [8], no oxygenated products were released from the catalyst surface even in the case of chlorobenzene... 

The rate of coke burning for coke deposited on a zeolite-containing catalyst has been reported to be first order with respect both to coke concentration and oxygen partial pressure (23) ... 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

These metals, when deposited on the E-cat catalyst, increase coke and gas-making tendencies of the catalyst. They cause dehydrogenation reactions, which increase hydrogen production and decrease gasoline yields. Vanadium can also destroy the zeolite activity and thus lead to lower conversion. The deleterious effects of these metals also depend on the regenerator temperature the rate of deactivation of a metal-laden catalyst increases as the regenerator temperature increases. 

Vanadium in the feed poisons the FCC catalyst when it is deposited on the catalyst as coke by vanadyl porphydrine in the feed. During regeneration, this coke is burned off and vanadium is oxidized to a oxidation state. The vanadium oxide (V O ) reacts with water vapor in the regenerator to vanadic acid, HjVO. Vanadic acid is mobile and it destroys zeolite crystal through acid-catalyzed hydrolysis. Vanadic acid formation is related to the steam and oxygen concentration in the regenerator. 

Sulfur is widely distributed as sulfide ores, which include galena, PbS cinnabar, HgS iron pyrite, FeS, and sphalerite, ZnS (Fig. 15.11). Because these ores are so common, sulfur is a by-product of the extraction of a number of metals, especially copper. Sulfur is also found as deposits of the native element (called brimstone), which are formed by bacterial action on H,S. The low melting point of sulfur (115°C) is utilized in the Frasch process, in which superheated water is used to melt solid sulfur underground and compressed air pushes the resulting slurry to the surface. Sulfur is also commonly found in petroleum, and extracting it chemically has been made inexpensive and safe by the use of heterogeneous catalysts, particularly zeolites (see Section 13.14). One method used to remove sulfur in the form of H2S from petroleum and natural gas is the Claus process, in which some of the H2S is first oxidized to sulfur dioxide ... 

Noble metal catalysts such as Pd, Pt, and Ru and transition metals are deposited on or incorporated in the fibers by methods similar to those used in catalyst preparation. Zeolitic (e.g., TS-M, Na-X, ZSM-type) crystals have been grown on the surfaces. 

In the case of a catalytic membrane reactor (CMR), the membrane is (made) intrinsically catalytically active. This can be done by using the intrinsic catalytic properties of the zeolite or by making the membrane catalytically active. When an active phase is deposited on top of a membrane layer, this is also called a CMR because this becomes part of the composite membrane. In addition to the catalytic activity of the membrane, a catalyst bed can be present (PBCMR). The advantages of a CMR are as follows ... 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

For this purpose we studied a temperature-programmed interaction of CH with a-oxygen. Experiments were carried out in a static setup with FeZSM-5 zeolite catalyst containing 0.80 wt % Fe203. The setup was equipped with an on-line mass-spectrometer and a microreactor which can be easily isolated from the rest part of the reaction volume. The sample pretreatment procedure was as follows. After heating in dioxygen at 823 K FeZSM-5 cooled down to 523 K. At this temperature, N2O decomposition was performed at 108 Pa to provide the a-oxygen deposition on the surface. After evacuation, the reactor was cooled down to the room temperature, and CH4 was fed into the reaction volume at 108 Pa. 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

Evaluating the results a clear kinetic picture of the catalysts has been obtained. In the steady state the active sites in Fe- and Cu-ZSM-5 are nearly fully oxidized, while for Co only -50% of the sites are oxidized. The former catalysts oporate in an oxidation reduction cycle, Fe /Fe and CuVCu. Coi in zeolites is hardly oxidized or reduced, but ESR studies on diluted solid solutions of Co in MgO indicate that Co -0 formation is possible, rapidly followed by a migration of the deposited oxygen to lattice oxygen and reduction back to Co [36]. For Fe-ZSM-5 such a migration has been observed, so a similar model can be proposed for the zeolitic systems. Furthermore, it is obvious that application of these catalysts strongly depends on the composition of the gas that has to be treated. 

T5 pically, supported metal catalysts are used in order to hydrogenate or oxidize the educt to the desired compound. Such catalysts often contain a metal (for example, 0.5-5 wt.%), which was deposited on the surface of a support (e.g., Si02, AI2O3, Ti02, zeolites, activated carbon) by means of an appropriate catalyst synthesis procedure (Figure 1). 

Another way of immobilizing catalyst complexes might be to trap them in the pores of solid particles, for instance by synthesizing the complex inside the pores of a zeolite ( ship in a bottle ). Another method could be to trap catalyst complexes in porous materials and deposit a membrane at the outer. surface. These methods of immobilizing a homogeneous catalyst do not involve chemical linkage between the catalyst and the carrier. The fixation is the result of steric hindrance. 

As an example, Figure 3.1.10 illustrates the use of this procedure for elucidating the location of coke deposits on zeolite catalysts [62]. Samples of zeolites H-ZSM-5... 

The basis of the demonstration can be based on already published data on the surface reaction between NOz and adsorbed organic compounds. Yokoyama and Misono have shown that the rates of N02 reduction over zeolite or silica are proportional to the concentration of adsorbed propene [29], whereas Il ichev et al. have demonstrated that N02 reacts with pre-adsorbed ethylene and propylene on H-ZSM-5 and Cu-ZSL-5 to form nitro-compounds [30], Chen et al [2-4] have observed the same nitrogen-containing deposits on MFI-supported iron catalysts. The question on the pairing of nitrogen atoms is not considered here. 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

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