Catalysts with Time on Stream

Fig. 2 shows the development of the activities (reported as TOF for the formation of propene) of the catalysts with time on stream at 700 K. Initial activities and selectivities are listed in... 

Still deactivation of the catalyst with time on stream (TOS) can be observed. This deactivation is mainly due to the adsorption of dodecene and heavy products on the catalyst which block its active sites for further reaction. Elementary analysis of the catalyst after the reaction has been carried out, and a significant increase of the carbon content was found. This confirms that the deactivation of the catalyst is due to strong adsorption of organic materials on the surface of the catalyst. 

Industrial application of the Au catalysts would be feasible if 10% conversion of propene could be achieved. However, there still remains a number of serious problems low PO 4elds (<2%), low selectivities at high reaction temperature, deactivation with time-on-stream and low H2 efficiency (<30%). The first two problems could be somewhat improved when titania-modified silica and titanosilicates were used as supports for Au nanoparticles. The initial conversion of propene was increased to ca. 5% from 2% or below, when the reaction temperature was raised to 423 K from 353 K or below without considerable loss in PO selectivity [40,402]. To date, only small improvements in the deactivation of Au catalysts with time-on-stream have been reported,... 

Reversible deactivation by coke formation led to selective deactivation of the Zn/ZSM-5 catalyst with time on stream. The metal sites were more affected than the acid sites and access to the zeolite was constrained, which increased the diffusion resistance for larger molecules. This resulted in a selectivity shift to lighter products and a decrease in the aromatics selectivity, paraffin to olefin ratio and conversion. 

Iron catalysts present special problems for the determination of the adsorbed species. The iron may be carburized during the reaction and this leads to a change in the bulk composition of iron catalyst with time on stream. Bianchi et al. determined that iron was partly converted into a mixture of e -Fe2 2C and x Fe2 5C during the reaction using Mossbauer spectroscopy. The adsorbed carbon surface species were determined to be present in three forms small amounts of reactive CH species which produced the bulk of the hydrocarbon products during reaction, a carbidic species with some associated hydrogen, and inactive graphitic carbon species. 

The catalytic pyrolysis of R22 over metal fluoride catalysts was studied at 923K. The catalytic activities over the prepared catalysts were compared with those of a non-catalytic reaction and the changes of product distribution with time-on-stream (TOS) were investigated. The physical mixture catalysts showed the highest selectivity and yield for TFE. It was found that the specific patterns of selectivity with TOS are probably due to the modification of catalyst surface. Product profiles suggest that the secondary reaction of intermediate CF2 with HF leads to the formation of R23. 

Fig. 1(b) represents the selectivity to styrene as a ftmcfion of time fijr the above catal ts. It is observed that the selectivity to styrene is more than 95% over carbon nauofiber supported iron oxide catalyst compared with about 90% for the oxidized carbon nanofiber. It can be observed that there is an increase in selectivity to styrene and a decrease in selectivity to benzene with time on stream until 40 min. In particrdar, when the carbon nanofiber which has been treated in 4M HCl solution for three days is directly us as support to deposit the iron-precursor, the resulting catalyst shows a significantly lows selectivity to styrene, about 70%, in contrast to more than 95% on the similar catalyst using oxidized carbon nanofiber. The doping of the alkali or alkali metal on Fe/CNF did not improve the steady-state selectivity to styrene, but shortened the time to reach the steady-state selectivity. 

Let us now use the sequence of elementary steps to explain the activity loss for some of the catalysts The combination of hydrogen chemisorption and catalytic measurements indicate that blocking of Pt by coke rather than sintering causes the severe deactivation observed in the case of Pt/y-AljOj The loss in hydrogen chemisorption capacity of the catalysts after use (Table 2) is attributed mainly to carbon formed by methane decomposition on Pt and impeding further access. Since this coke on Pt is a reactive intermediate, Pt/Zr02 continues to maintain its stable activity with time on stream. 

As shown in Table 3, after a pretreatment performed at 333 K, the activity of the K3P sample increased with time on stream (TOS), giving rise to a high production of dimethylhexanes (DMH) and of olefins (Cg" ). After a dehydratation performed at 423 K, the conversion of C4= and the selectivities towards TMP were initially high. As generally observed in the aliphatic alkylation reaction with solid acids, the decrease of the catalyst activity was accompanied by a concomitant decrease of the selectivity in TMP and an increase of the selectivities in DMH and olefins (C4 dimerization) indicating... 

Since it is reported in the literature that small amounts of water may improve the catalyst life with time-on-stream, reducing the formation of tar [26], a small percentage of water was added in the feed (Table 39.5), producing a very negative effect on the selectivity in MDB. Furthermore the formation of a new by-product, 2-methoxyphenol (2-MP), was observed. On the contrary, no increase was found in the C-balance values. 

Therefore, we attempted to regenerate the catalyst by calcination at 773 K for 6 h the calcined catalyst showed a slight decrease in the surface area (450 m g versus 530 m g of the fresh catalyst) and recovered its activity significantly, although proving less stable with time-on-stream than the fresh one. 

Figure 39.4. Activity with time-on-stream at 623 K for TS-1 catalyst in the synthesis of MDB by reaction of pyrocatechol (PYC) and diethoxymethane (DEM) ( Yield and selectivity referring to PYC).

Haneda et al. [134,135] studied the formation and reaction of adsorbed species in NO reduction by propene over Ga203-Al203. IR transient reaction technique was employed to examine the reactivity and dynamic behaviour of surface species. The catalyst was first exposed to either C3H6/02/Ar or NO/Oz/Ar at 623 K for a long time to form and accumulate the surface species. The catalyst was further purged with pure Ar and the reaction gas then switched to various gas mixtures. Changes in the intensity of IR bands were measured with time on stream. The main surface species detected by IR during... 

Figure 2 also includes a comparative experiment, where the solid acid catalyst is a sample of non-fluorided (but calcined), acidic mordenite. Here we see a) a significant loss of alkylation activity with time on stream and b) a measurably lower... 

For the Pd-silk catalyst,2 PdCl2 was deposited on silk and reduced to Pd° moderate enantioselectivities were obtained for the hydrogenation of a C=C bond (66% enantiomeric excess, ee, which is the difference between enantiomers divided by the sum of enantiomers), but the silk support presented two problems it tended to deteriorate with time on stream and it varied from source to source, so enantioselectivities were not reproducible (Scheme 3.2). On the other hand, deterioration was not a problem with the metal-quartz catalysts. 

There are several factors that may be invoked to explain the discrepancy between predicted and measured results, but the discrepancy highlights the necessity for good pilot plant scale data to properly design these types of reactors. Obviously, the reaction does not involve simple first-order kinetics or equimolal counterdiffusion. The fact that the catalyst activity varies significantly with time on-stream and some carbon deposition is observed indicates that perhaps the coke residues within the catalyst may have effects like those to be discussed in Section 12.3.3. Consult the original article for further discussion of the nonisothermal catalyst pellet problem. 

The two limiting cases for the distribution of deactivated catalyst sites are representative of some of the situations that can be encountered in industrial practice. The formation of coke deposits on some relatively inactive cracking catalysts would be expected to occur uniformly throughout the catalyst pore structure. In other situations the coke may deposit as a peripheral shell that thickens with time on-stream. Poisoning by trace constituents of the feed stream often falls in the pore-mouth category. 

Coke builds up on the catalyst since the start up of operation. In the first weeks of operation, an amount between 5% and 8% of coke accumulates on the catalyst. The rate of deposition decreases with time on stream, a careful monitoring of temperature and of feed/H2 ratio is the basis for controlling deposition. Coke deposition primarily affects the hydrogenation reactions (and so denitrogenation), but the deposition rate determines the catalyst life. As mentioned above, deactivation is compensated by an increase in temperature (and some times in pressure, when denitrogenation has to be adjusted, as well). However, increasing severity, increases coke deposition and shorten catalyst life. 

Initial activity (TOF) was measured on fresh catalysts and EB conversion was also followed with time on stream (table 1). ZSM-5 zeolite catalyst is respectively 40, 25 and 2 times initially more active for the EB conversion than the Ferrierite, ZSM-22 and EU-1 catalysts (table 1). Except for ZSM-5, high deactivation occurs on zeolite catalysts as shown by EB conversion drop at different contact time (table 1). 

The activity of a cracking catalyst declines with time on stream. A case of gas oil cracking has the rate equation, dC 8.5C ... 

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