Beta zeolite deactivation

Deactivation is also a function of the catalyst used. Under similar reaction conditions. Beta zeolite deactivated more rapidly than did H-ZSM5. The Beta zeolite was found to contain more adsorbed molecules as well as larger amounts of coke. On the other hand, sulfated zirconia deactivated more rapidly than H-ZSM5 but formed much less coke. It has previously been... 

When Ga203 is incorporated on a large pore hi silica-alumina zeolite, such as Beta, propane is also converted into aromatics, being in this case the selectivity much higher for the formation of naphthalene and methyl naphthalene than on Ga-ZSM-5 zeolite. Meanwhile, the Beta zeolite deactivates much faster (164). 

Figure 3-a shows the propylene conversion obtained on ITQ-21 and beta zeolites at space velocity of 18 h"1 and different temperatures. It can be seen that both zeolites, ITQ-21 and BETA, are initially highly active, but whereas ITQ-21 maintains full propylene conversion along the 8 hours of reaction at 125°C, zeolite BETA is completely deactivated at TOS=100 min. Increasing contact time (WHSV=12 h 1) results in a slight improvement for BETA (Figure 3-b), as conversion at 50 min TOS is in this case above 90%, but results are still far from those obtained with ITQ-21. 

ITQ-21 presents excellent catalytic properties for the production of cumene, being more active and stable towards deactivation and presenting lower selectivity to NPB than a comparable beta zeolite. The benefits of ITQ-21 can be directly related to its open three-dimensional crystalline structure that favors diffusion of the products and minimizes undesired consecutive reactions. 

Moreover, the catalyst deactivation must also be considered in order to use these solid materials in industrial processes. Figure 13.8 shows the variation of catal54ic activity (2-butene conversion) with the time on stream obtained under the same reaction conditions on different solid-acid catalysts. It can be seen how all the solid-acids catalysts studied generally suffer a relatively rapid catalyst deactivation, although both beta zeolite and nafion-sihca presented the lower catalyst decays. Since the regeneration of beta zeolite is more easy than of nafion, beta zeolite was considered to be an interesting alternative. ... 

Zeolite Beta has also been studied for isobutane/butene alkylation (65, 66), but it was less selective to the desired TMP than USY, suggesting some diffusional limitations for these highly branched products at the relatively low reaction temperatures used. In fact, an increase of activity was observed when decreasing the crystal size of the Beta zeolite (66). As for USY zeolites, the activity, selectivity and deactivation rate of Beta zeolite were influenced by the presence of EFAL species (67). Medium pore zeolites, such as ZSM-5 and ZSM-11 were also found active for alkylation, but at temperatures above 100°C (68, 69). Moreover, the product obtained on ZSM-5 and ZSM-11 contained more light compounds (C5-C7), and the Os fraction was almost free of trimethylpentanes, indicating serious pore restrictions for the formation of the desired alkylation products. 

It is interesting to note that the destruction of the structure of beta zeolite by treatment with strong acids or high temperature leads to a complete deactivation of the catalyst for limonene alkoxylation. By using a higher reaction temperature only isomerisation and polymerisation products have been obtained. 1-methyl-4-[alpha-methoxy-isopropyl]-1 -cyclohexene or other addition products cannot be found. 

H-beta zeolite proved to be an active and selective catalyst for alkylation of benzene with propene. In situ spectroscopic methods were applied to follow the formation and the evolution of surface intermediates and products,. It was found that when benzene is taken alone on the zeolite surface, its adsorption is reversible up to 473 K. On the contrary propene undergoes to several transformations even at 295 K. Isopropylbenzene behaves as propene, giving the same intermediates and products by decomposition at higher temperatures. Isopropyl cations formed upon chemisorption of propene on Broensted acid sites are the key intermediates for the alkylation reaction and are responsible for the faster deactivation via unsaturated caibenium ions formation. 

A similar shape selective effect was observed in the liquid phase. Those catalysts with the smaller pore and channel openings were more selective for para-nitrotoluene. However, in the liquid phase, no induction period was observed. Rather, all catalysts exhibited significant deactivation throughout time on stream and after 5 hrs. little of the original activity remained. As shown in Fig. 3, the para selectivity was found to decrease with time on stream. This would indicate that deactivation occurs within the pore channels effectively reducing the preferential capacity of the catalyst to generate the para isomer. The decrease in para selectivity was not evident on Beta zeolite, which has larger pores and may allow for a more uniform production of... 

Fig. 6 shows the normalized rate profiles for reaction at 94 °C in the presence of different catalysts. This plot was constructed from the concentration profiles of these catalysts similarly to the ones caleulated above for H-ZSM5. It can be observed that H-ZSM5 and H-Mordenite exhibit a very similar deactivation pattern while sulfated zirconia is a little faster and beta zeolite significantly faster. Beta zeolite showed the highest initial activity but a much faster deaetivation. At the same time, beta zeolite exhibited a poor selectivity to the para isomer. 

Use of beta zeolite catalyst does not require the benzene feed to be clay treated prior to use in alkylation service. Some of the unsaturated material in the benzene can lead to the formation in the alkylation reactors of polycyclic-aromatic material which will get preferentially trapped in some zeolites having relatively small-sized pores. This can lead to increased deactivation rates in such small-pore zeolites. Beta zeolite s large pore structure makes it possible to more easily handle this polycyclic-aromatic material and as a result does not require further treatment of the benzene feed to remove unsaturated material. In addition, alpha-methylstyrene (AMS) is produced by alkylation of benzene with methylacetylene or propadiene. Some of the AMS alkylates with benzene, forming diphenyl-propane, a heavy aromatic that leaves the unit with the DIPB column bottoms. 

Recently, Diaz-Mendoza et al. (1998) have studied the catalytic behaviour and the catalyst decay of Beta, RY and USY zeolites. They propose that Lewis acid sites promote the formation of unsaturated compounds (favoring coke formation). However, Bronsted acid sites with intermediate acid strength appear to be appropiate sites for maintaining good alkylation catalytic performance. They observe that the best catalytic performance and the slowest deactivation were achieved with Beta zeolite, followed by RY and USY with low sodium content. However, only butene isomerization was observed over USY zeolite with high sodium content. 

When H-Y and H-beta zeolites, which have three-dimensional channel systems, are used, intraporous catalysis is dominant. The reaction selectivity is apparently closely related to the sorption selectivity. In the case of zeolite beta this means that the observed formation of the large indole isomer 4 can only be effected by acid sites located on the outer surface of the crystals. When intraporous formation of 4 would occur, it would lead to rapid deactivation of the catalyst because the molecule would neither desorb nor be converted (Although in zeolite beta the intraporous formation of 4 is largely suppressed, it is probably not completely inhibited some deactivation was actually observed when 1(X) mg of zeolite beta was used in stead of 3 g). The fact that removal of outer-surface aluminium leads to a substantial selectivity improvement is consistent with this interpretation. 

FTIR spectra of (a) fresh and (b) deactivated Beta zeolite, at 75°C. From Ref 56... 

P-06 - Study of coke and deactivation over H-Beta zeolite... 

The catalytic behavior of an Al-ITQ-7 zeolite, with a three-dimensional system of large pore channels, has been evaluated for the liquid phase alkylation of isobutane with 2-butene, and compared to that of a Beta zeolite. In absence of deactivation (TOS=l min), zeolite ITQ-7 gives a higher proportion of C5-C7/C5+, obtained by cracking of Cs and specially of the bulky C9+. However, the main differences are observed in the distribution of the trimethylpentane (TMP) isomers. Although zeolite ITQ-7 is more selective to TMP in the C8 fraction than Beta, the most abundant isomers are 2,3,3- and 2,3,4-TMP instead of the primary 2,2,3-TMP or the thermodynamically favored 2,2,4-TMP. This is a clear shape selectivity effect, due to the smaller pore size of ITQ-7 as compared to Beta, and the fact that 2,3,3- and 2,3,4-TMP are the isomers with less restricted transition states and smaller diffusion problems. 

It has been shown in the literature that isobutane (Pc = 36.5 bar, = 408 K)/butene (Pc= 40.2 bar, Tcf= 420 K) alkylation on solid acid catalysts at supercritical temperatures suffers from increased butene oligomerization and cracking reactions at these temperatures, increasing the catalyst deactivation potential [16-18]. Lower temperatures tend to favor the alkylation reaction. Supercritical operation at 95°C can be facilitated by diluting the isopar-affin/olefin feed with suitable amounts of a low inert solvent such as CO2 (Pc = 73.8 bar. Pc = 304 K), and has been shown to give rise to steady alkylation activity on USY and beta zeolites [19]. However, the alkylate yields are very low (< 10%) on these catalysts, attributed to severe pore diffusion limitations on these catalysts. 

The conversion of sucrose to LA in water was assessed with Sn-Beta zeolite, but only low yields of LA of less than 30% were encountered [107]. Besides LA, HMF and levulinic acid were analyzed in the product mixture. One reason of the significant change in product spectrum was related to the auto-catalytic effect of LA. Its acidity lowers the pH of the medium, thereby enhancing the dehydration rate with an increase of HMF and the likes in the product spectrum as a result [107]. Pronounced carbonaceous deposits on the catalysts confirmed its deactivation by LA, similar to the triose reaction in water. 

At a hydrothermal aging temperature of 670 °C, minor deactivation in NOx reduction with NH3 was observed for the leading Cu/zeolite and Fe/zeolite catalysts in 2006. For these early generations using beta zeolites, the never-to-exceed temperature for Cu was established to be only 775 °C while Fe was determined to be as high as 925 °C. Under standard SCR reaction conditions (NOx — NO),... 

D.Y. (2006) Isomerization of n-butane to isobutane over Pt-modified beta and ZSM-5 zeolite catalyst catalyst deactivation and regeneration. Chem. Eng. ]., 120, 83-89. 

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