Poisoning of acid sites

Poisoning experiments. Poisoning of acidic sites by adsorption of bases is another technique that has been applied to identify the centers of catalytic activity for various reactions. The results of many of the earlier studies were interpreted in terms of interaction of the adsorbed bases with... 

Sodium deactivates FCC catalysts by three separate mechanisms poisoning of acid sites, enhancing the sintering of matrix, and accelerating the destruction of... 

Evidence of poisoning of acidic sites was also obtained by IR spectroscopy. The number of protons on the ferrierite surface was estimated by the intensity of the OH band at 3600 cm". After a few hours on stream, the absorbance of this OH vibration was reduced by a factor of 2.4 in contrast, the acidity of the sohd probed by ammonia temperature-programmed desorption (TPD) decreased by a larger factor (53,58). These results were considered as evidence of the poisoning of a large number of acidic sites and a partial blocking of pores, rendering some acidic sites inaccessible to the reactant. 

Poisoning of acid sites is straightforward. Basic constituents are required to neutralize acidity. This is found in alkali and alkaline earth compounds and in basic organic molecules. Process poisoning by alkaline and alkaline earths is rare. These materials are added as deliberate promoters to remove acidity but are not normally encountered in process streams in the basic form, One exception is Na ions, encountered in steam used for stripping cracking catalysts and other purposes. 

A series of catalytic tests was performed in the presence of benzoic acid (poison of basic sites) and tripropylamine (poison of acidic sites). The poison weight is equal to 15 % of the catalyst weight. Those poisons were chosen for practical reasons GC retention time, boiling temperature and chemical inertness of the poisons. 

Ko El, Oumeslc JA Ctrarecterization and selective poisoning of acid sites on sulfated zirconia. Catal Lett 1995, 32 241-251. 

Nevertheless, sooner or later the alkylation activity of the zeolite will inevitably decline due to the accumulation of carbonaceous deposits on its surface. A thorough understanding of the nature of the carbonaceous deposits formed and the possible deactivation mechanisms is thus crucial for designing zeolite-based alkylation catalysts with enhanced lifetime. In this respect, poisoning of acid sites or pore blockage (or both) caused by the accumulation carbonaceous deposits on the zeolite surface have been proposed as the most likely deactivation mechanisms (143-148). After the initial stable alkylation period, olefins appear in the reaction medium (conversions below 100%) and the rate of olefin addition to an adsorbed carbenium ion to form heavy polyalkylated intermediates increases with respect to that of hydrogen transfer. Such bulky intermediates remain attached to the active site and thus reduce the number of available Bronsted acid sites. Concomitantly, a decrease in the micropore volume does also take place. In the latter stage of the reaction, where deactivation is accelerated, polymers are formed at the outer zeolite surface and may eventually lead to pore mouth... 

The inhibition of the oxidation process between -0.3 V and 0.4 V must be due to the poisoning of active sites, since there are no obvious adsorption products, and so the poison must be present in very low amounts. Similar inhibition was observed by the authors in acid solution. In order to increase the sensitivity of their system, the authors carried out a PDIR experiment and observed adsorbed CO which, they concluded, was the main poison in acid and base. At potentials > 0.4 V, the oxidation recommences as a result of the freeing up of active sites as the poison is oxidised. 

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

In this work the reactions of ammonia with chlorobenzene and benz-aldehyde over a series of metal ion-exchanged zeolites were investigated by the microreactor method, and attempts were made to relate the catalytic activity of the zeolites to properties of metal cations. Ammonia was a reactant and a poison for acidic sites. 

An apparently straightforward method for the determination of number and strength of acid sites consists of the determination of the amount of base required to poison catalytic activity for a model reaction. By means of plots of activity versus amount of added base, the number of acid sites is obtained from the threshold amount of base required to remove catalytic activity acid strength is gauged from the slope of the titration curve. This method can therefore be called a catalytic titration. 

For the first type of poisons, we can either try to apply the same approach as with vanadium meaning specific catchers or offer a sufficient excess of acid sites sacrificial sites in order to reduce the relative impact of the poisoning. For instance, in the case of a nitrogen resistant catalyst, usually the catalyst design will involve a relatively high rare-earth and high zeolite content catalyst with the option of additional sacrificial sites in the form of active alumina (12). 

As expected for silica-alumina as a mixed oxide (see also Section IV.B.5), the PyH+ and PyL species are observed simultaneously (160, 205,206,221-223). Two distinct types of Lewis acid sites could be detected (19b mode at 1456 and 1462 cm-1, respectively) on a specially prepared aluminum-on-silica catalyst (160). On water addition, the Lewis sites can be converted into Br nsted sites (160, 205, 221), The effect of Na+ ions on the acidity of silica-aluminas has been studied by Parry (205) and by Bourne et al. (160). It can be concluded from Parry s results that Na+ ions affect both types of acid sites, so that alkali poisoning does not seem to eliminate the Br nsted sites selectively. For quantitative determination of the surface density of Lewis and Br nsted acid sites by pyridine chemisorption, one requires the knowledge of at least the ratio of the extinction coefficients for characteristic infrared absorption bands of the PyH+ and PyL species. Attempts have been made to evaluate this ratio for the 19b mode, which occurs near 1450 cm-1 for the PyL species and near 1545 cm-1 for the PyH+ species (160,198,206,221,224,225). The most reliable value as calculated from the data given by Hughes and White (198) seems to be... 

Carbon dioxide fulfills some of the relevant criteria and contradicts others. Evidently, although C02 exhibits acidic properties, the adsorbed amounts cannot be taken as a measure of surface basicity strong chemisorption of C02 occurs through interaction with acid-base pair sites preferentially. Thus, specific poisoning of basic sites by C02 chemisorption is not possible. Furthermore, a... 

Poisoning occurs by strong adsorption of stable molecules on the active sites of catalysts. Basic heterocyclic nitrogen compounds can usually poison the acidic sites. Adsorption of... 

Sodium on fluid cracking catalyst, FCC, comes from the raw materials used in the catalyst manufacturing process as well as salt contamination in the feedstock. Sodium can deactivate cracking catalysts by poisoning the acid sites on the matrix and zeolite and by promoting sintering of silica-alumina (1). Sodium can act synergistically with vanadium to accelerate the destruction of zeolite (2). 

This evidence suggests that not all Na species are mobile. Some Na species must in fact have reacted irreversibly with components on the catalyst, leaving it unavailable to poison the acid sites. It is likely that these reactions occur during the early stages of hydrothermal deactivation. The exact mechanism is unclear, but may involve reactions with extraffamework alumina. As the zeolite dealuminates from 24.55 to 24.25A unit cell size, approximately 65% of the initial framework alumina (about 15 wt% of the zeolite) comes out of the zeolite structure. Sodium, which also must leave the exchange sites as the zeolite dealuminates may react with this very reactive form of alumina. The other possibility is that as kaolin undergoes its transition to metakaolin at 800K... 

Additionally, these authors also found that the presence of lube oil decreased the activity of the catalysts due to the presence of both sulphur- and nitrogen-containing compounds (4000 ppm of sulphur and 85 ppm of nitrogen) that poisoned the acid sites. Consequently, higher temperatures (450-500°C) should be employed to obtain complete conversion in the catalytic cracking of the LDPE-lubricating oil mixture over Al-MCM-41 and nanocrystalline HZSM-5 catalysts. 

Based on the above results, it can be mentioned that the catalyst having both hydrogenation and acidic functions can successfully convert heavy oil derived from plastic wastes (relatively clean) into environmentally acceptable transport fuels. However, for the heavy oils containing impurities, the dual functional hydrocracking catalysts still need to be improved. In the hydrocracking process over the acidic catalyst, nitrogen content in feed is limited because basic nitrogen compounds poison the acidic sites of the catalyst. 

Xu et al. (57) reported for PER that the number of acidic sites decreased with TOS (Table VI). The decrease in the number of detected acidic sites could be due to poisoning and/or to pore blocking. Table VI indicates that the larger the probe molecule used to detect acidic sites, the fewer the sites detected evidently, the accessibility of acidic sites in fcrricritc supported on alumina depends on the size of the adsorbate. 

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