Silica-alumina, coke deposits

It has been indicated by several investigators that strong acids rather than weak acids, Lewis acids rather than Bronsted acids favor coke formation, and that the presence of transition metal ions as impurities, e.g., Fe and Ni ions, accelerate the formation of coke. It was reported that coke formation on a nonacidic silica was less than one-twentieth that on acidic silica —alumina. Coke deposition became less serious as the strong acid sites of silica —alumina were weakened by NaOH treatments. Coke formation from hydrocarbons is usually less serious in the case of solid bases. It is reported that the deactivation of MgO and Li/MgO for methane coupling was due to sintering and loss of alkali. ... 

A further important aspect of carbon inertness in catalysis is its much lower coking propensity in comparison with alumina or silica supports. Coke deposition on the surface of the catalyst reduces the life of the catalyst. De Beer et al. (1984) studied this effect and found that the extent of carbon deposition on the blank supports is higher for carbons than for alumina and it increases with increasing surface area. In the absence of a metallic component the cracking appears to be related more to the accessible surface area than to any other particular surface property. However, the addition of metals to the supports causes an increase in the rates and amounts of carbon deposition, but the increase is much higher for the alumina-supported catalysts. 

This interpretation of the experimental data is supported by the differences observed in the deactivation patterns and carbon contents after test, since one notorious effect of Hjp is the capacity to diminish the deactivation caused by coke deposition on the active sites [21,22]. This is supposed to be due to a reaction with the coke precursors, very likely a hydrogenolysis. In pure silica-aluminas, where no source of spillover is present, no special protection against deactivation should be observed. Indeed, the silica-aluminas lose most of their activity (about 80%) before reaching the steady-state and present the highest carbon contents after catalytic test. On the other hand, in the case of the mechanical mixtures, where spillover hydrogen is continuously produced by the CoMo/Si02 phase and can migrate to the silica-alumina surface, the predicted protection effect is noticed. The relative losses of activity are much lower... 

The intracrystalline pore volume of the catalysts was evaluated by n-hexane sorption as shown in Fig. 6. Sorption capacities for samples SI to S3 are comparable to that of the zeolite before Ga impregnation and correspond to the value expected for an unaltered ZSM-5 type material (S10). Sorption capacity decreases for samples S3, S4, S5, and S6, because of intracrystalline volume blockage by coke deposits and possibly also (silica)-alumina debris [6] in the aged catalyt S6. In addition, the sorption rate for S6 is about twice the rate observed for the other samples, suggesting that adsorption occurs mostly at the external surface of the S6 catalyst crystallites. Thus, it appears that coke deposited on S6, probably as polyaromatic species, has almost blocked the channel pore mouths and/or practically occupied the whole intracrystalline pore volume. It explains the poor catalytic performance of S6. 

Nickel and vanadium are contained within the crude oil as their respective porphyrins and napthenates (2). As these large molecules are cracked, the metals are deposited on the catalyst. Nickel which possesses a high intrinsic dehydrogenation and hydrogenolysis activity drastically increases the production of coke and dry gas (particularly H2) at the expense of gasoline. Vanadium on the other hand interacts with the zeolitic component of a cracking catalyst and leads to destruction of its crystallinity. This results in reduced activity as well as an increase in non-selective amorphous silica-alumina type cracking. Supported vanadium also has an intrinsic... 

In the process of catalytic cracking, characteristic reactions such as chain scission, hydrogen transfer and condensation take place under certain temperature and pressure conditions and when an appropriate catalyst is utilized, products with certain range of molecular weights and structures are obtained. Catalysts with surface acid sites and with the ability of hydrogen ion donation such as silica-alumina and molecular sieve catalyst have been already widely utilized. These catalysts can also enhance the isomerization of products and increase the yield of isomeric hydrocarbons. However, large amounts of coke will deposit on the surface of catalysts and consequently lead to their deactivation. Therefore, the recycling of catalysts is difficult to achieve. 

Catalysts tend to be deactivated in the process of plastics pyrolysis because of coke deposition on their surface. The deactivation of HZSM-5, HY, H-zeolite and silica-alumina was compared by Uemichi et al. [86]. In the case of PE pyrolysis and HZSM-5 added as catalyst, no deactivation occurred due to the low coke deposit, and high yields of light hydrocarbons (mainly branched hydrocarbons and aromatics) were achieved. In the case of PS, however, coke production increased dramatically, so HZSM-5 was deactivated very quickly. Silica-alumina catalyst was deactivated gradually and slowly with the increase of cracking gas, while HY- and H-zeolite molecule sieve catalysts were deactivated very quickly. Walendziewski et al. [87] studied the catalytic cracking of waste... 

Correlations of experimental data have shown that relative activity of the catalyst for cracking gas oil also decreases exponentially with time, the exponent in this case being remarkably close to the exponent m — 1 in the formula for rate of coke deposition (73,290). The activity at the shortest times investigated, on the order of 1 to 5 seconds, is roughly 100 times that noted after one hour of use (73). Synthetic silica-alumina shows a slower rate of decline in activity with time than activated clay. Thus, although the initial activities of the two are about equal, the synthetic catalyst is three to four times as active as activated clay after two hours of exposure to oil vapors at identical cracking conditions. 

Gasification Kinetics of Coke Deposited on Silica-Alumina. Within the temperature range 1400 to 1600°F and in the presence of excess steam, the gasification reaction of coke deposited on the silica-alumina cracking catalyst closely followed first-order kinetics with respect to unreacted carbon (Figure 1). First-order rate constants were calculated from the slopes of these plots (Table III), and yielded an activation energy of 55.5 Kcal/mole. 

Figure 1. Kinetics of steam gasification of coke deposited on silica-alumina, for reaction temperatures of 1400°F (O), 1500°F ( Z ), 1550°F (A), and 1600°F (V).

The steam-carbon reaction is known to be catalyzed by metals, particularly transition metals (3,4.). In an effort to improve the rate of gasification, separate samples of the silica-alumina (Durabead) catalyst were impregnated with one of various metals prior to coke deposition, and the results for the subsequent steam-carbon reaction at 1500°F over these materials are shown in Figure 2 and Table IV. The effects of the deposited metal oxides can be summarized as follows ... 

The kinetics of carbon deposition on nickel have been studied in detail previously (17,18,19), and a mechanism which explains most observations has been advanced. Coke precursors proceed through a series of dehydrogenation steps on the nickel surface, resulting eventually in carbonaceous species. These species dissolve in, and precipitate from, the metal phase detaching nickel crystallites from the surface of the bulk nickel or the silica-alumina surface. Further coke deposition incorporates additional nickel into the growing carbon layer, as shown by electron microscopic examination of the deposits (18). 

The activation energies for coke gasification on the three substrates shown in Figure 4 were 33 Kcal/mole for the three alumina-based materials and 54 Kcal/mole, for the silica-alumina catalysts. The increased activity and lower activation energy for the coke deposited on the aluminas (compared to that on the silica-aluminas) cannot be due to a direct catalytic effect of alumina on the gasification reaction, but rather to an indirect effect of the alumina that controls the nature and structure (surface area and structural disorder) of the coke during its deposition. 

Figure 3. Kinetics of steam gasification at 1500°F of coke deposited on mont-morillonite (AJ, silica-alumina (Al, American Cyanamid alumina ( ,), Catapal alumina, Run 58 (9), Catapal alumina, Run 56 (O), and fresh bauxite flj.

Figure 4. Arrhenius plots for steam gasification of coke deposited on American Cyanamid alumina (A), fresh bauxite (O. and silica-alumina (O). Corresponding activation energies are 32.6, 33.8, and 53.1 kcal/mol, respectively.

The BET surface area was determined for both fresh and spent catalysts, during the isobutane alkylation with 1-butene . LaY and Lap zeolites displayed a decrease in the BET area of 45 % approximately due to the coking, while the amorphous silica alumina a decrease of 33 %. This is the case where the pretreatment of the coked catalysts before the BET determination will eliminate some of the carbonaceous deposits, since the reaction temperature is typically below 100°C, and the pretreatment for BET determination with zeolite catalysts, is usually around 250°C. TPO studies clearly demonstrated that this treatment under vacuum eliminates a fraction of the coke, and therefore the real decrease in surface area due to coke deposition is larger than that measured by BET. 

To address the deactivation behavior in more detail, Uemichi et al. recently examined the change in activity of a silica-alumina catalyst with 13 wt% alumina as a function of time on stream. At a reaction temperature of 723 K, the SA catalyst accumulated over 12 wt% coke on the catalyst after 250 min time on stream. The liquid yield increased slightly from 60 wt"/o to approximately 70 wt% as the coke built up on the catalyst. The limited effect of the coke on the reaction was attributed to the inability of coke deposits to block completely the large pores (f/p,ave = 4.4 nm) of the amorphous catalyst. Although SA showed no activity toward cracking of -octane, the reactivity of polyethylene was substantially enhanced in the presence of the catalyst. This was attributed to the facile reaction on the catalyst of olefins which could be formed from thermal degradation of polyethylene at the temperatures used in this study. 

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