Pt/ZSM-5 catalysts

TABLE 4. Shape-selective hydrogenation of alkene mixtures in the presence of 1% Pt-ZSM-5 catalyst reduced in a mixture of alkenes and H2... 

At our knowledge, this is the first time that the catalytic behavior of Cu-Al-MCM-41 for the HC-SCR of NO is reported. HC-SCR of NO was previously reported on the Pt-MCM-41, Rh-MCM-41 and Co-MCM-41 catalysts [8]. Pt-MCM-41 resulted the most active catalyst, but no comparison was made with the activity of Pt-ZSM-5 catalysts measured under the same experimental conditions. 

It should be noted that whereas the isomerization activity of Pd ZSM-5 is about 1/3 of the Pt-ZSM-5 catalyst, the Pd-containing hybrid catalyst (Pd/Si02-i- H-ZSM-5) shows comparable activity and selectivity to those of the Pt containing hybrid catalyst. In this case, the supported Pd on H-ZSM-5 seems to poison the active site on H-ZS.M-5, to some extent. 

Watanabe, M., Uchida, H., Igarashi, H., and Suzuki, M. Development of Pt/ZSM-5 catalyst with high CO selectivity for preferential oxidation of carbon monoxide in a reformed gas. Chemistry Letters, 1995, 24, 21. 

Figure 2. Conversion of toluene with reaction temperature using the indicated Pt/ZSM-5 catalysts. (Pt i.—=5000ppm. Po,=12vol%, W/F=0.06g s/ml).

It was found that the amount of N2 (and also N2O) formation decreases with increasing offset time At from 0 to 1 second for the lean mixture B. The amoxmt and peak shape of the N2 formed during the NO pulse at At = 1 second are identical to those observed when NO is pulsed over a preoxidised surface. This indicates that one second after the propene/Oa pulse no residual carbonaceous species are present on the surface which can reduce NO. When the next propene/02 pulse enters the catalyst, two seconds after the NO pulse, still some N containing adspecies are on the surface as some N2 (and also N2O) formation is visible. At At = 0 and 0.01 seconds more oxygen leaves the catalyst after the propene/02 pulse then at larger offset times. Also more N2 is formed during the NO pulse at At = 0 and 0.01 seconds. This effect can be caused by a competition between O2 and NO for a direct reaction with propene or reaction products of propene. Another possibility is that O2 and NO compete for reduced adsorption sites. Rottlander et al. [14] recently reported similar results with TAP experiments on a Pt/ZSM-5 catalyst. They proposed that carbon containing surface species, formed from propene, are mainly responsible for the NOx reduction at T < 600 K. 

Figure lb. Concentration traces over a Pt/ZSM-5 catalyst in a heating ramp. 

The Pt SiC catalyst (Fig. Ic) shows light-off around 220°C and NO reduction in the same temperature range as for the other catalysts. The NjO formation maximises around 235°C and is of similar magnitude as for the Pt/ZSM-5 catalyst. The NOj formation rate increases rapidly around this temperature and shows a maximum at 320°C. Adsorption of neither hydrocarbons nor NOx on the Pt SiC sample is obvious from Fig Ic. In Table 2 the NOx reduction efficiency and the selectivity towards N2 and N2O formation are summarised for the flow reactor experiments. Both the NOx reduction activity and the N2 selectivity of the Pt/SiC and Pt/ZSM-5 system appear to be similar, while Pt/Al203 shows a higher peak reduction value for the heating ramp experiments. 

For the Pt/ZSM-5 catalyst (Fig. lb) there is light-off at somewhat lower temperature (210°C) and a significant over-shoot in the COj formation just above light-off. This behaviour is probably connected with combustion of hydrocarbons adsorbed at lower temperatures. The NOx reduction window occurs around the same temperatures as for Pt/Al203 but is less pronounced. The maximum in N2O formation is somewhat higher in magnitude than for Pt/AljOj and occurs at a lower temperature (230°C). The NO2 formation starts at about this temperature and has a maximum aroimd 340°C. There is no desorption of adsorbed NOx below light-off. 

The i5N2-formation is described by the oxidation of reduced platinum sites by 15NO (reaction (3)). Obviously, the limitation of vacant sites also results in an increasing yield of N20 (reaction (4)). This is in a good agreement with the results described by Rottlander et al. for a Pt-ZSM-5 catalyst, who observed an increase in N20-yields after deposition of oxygen on the catalyst. 

Besides the position of the Pt clusters, the zeolite particle size is another important parameter for selective hydrogenation of the trans fatty acid methyl ester isomer. Larger crystals have a higher ratio of internal to external surface and consequently a lower ratio of external to internal Pt clusters, which induce an improving hydrogenation selectivity. Also the Si/Al ratio influences the catalytic system. A Pt/ZSM-5 catalyst with an Si/Al ratio up to 80-150 results in further improvement of the reaction selectivity based on shape selectivity of diffusional origin. 

The shape-selective Pt/ZSM-5 catalyst was also tested in the hydrogenation of model triacylglycerols. In agreement with the results obtained with the fatty acid methyl esters, a... 

Figure 18.15 Selectivity to C4—Cg isoparaffins at 40% total n-paraffin conversion over microporous/mesoporous Pt/ZSM-5 catalysts (reaction conditions P = 30 bar ...

Heracleous, E., Iliopoulou, E.F., Lappas, A.A., 2013. Microporous/mesoporous Pt/ZSM-5 catalysts for hydroisomerization of BTL-naphtha. Industrial and Engineering Chemistry Research 52, 14567-14573. 

Na-ZSM-5(a molar SiOz/AlaOa ratio=23.8) provided by Tosoh Corp. was used. ln(4wt%)/H-ZSM-5 and lr(1wt%)/H-ZSM-5 catalysts were prepared by the ion exchange method using NH4-ZSM-5 derived from the Na-ZSM-5 with aqueous solutions of ln(NOs)3 at 368 K for 8 h and lrCI(NH3)sCl2 at room temperature for 24 h, respectively. Addition of precious metals, 1wt% platinum and iridium to ln/H-ZSM-5 was carried out by impregnating the ln/NH4-ZSM-5 in aqueous solutions of Pt(NH3)4Cl2 and lrCI(NH3)5Cl2, respectively. The catalysts were calcined at 813 K for 3 h. 

Further research has been performed and is continued to be reported, mostly with zeolites unloaded or loaded with Pt, and Ga- and Zn-promoted H-ZSM-5 or H-[Al]ZSM-5 catalysts to clarify the details of the complex transformations taking place and make further improvements. In addition, new catalysts were studied and reported. Reference should also be made to work addressing the problems of the modification of catalyst features of ZSM-5404 and the development of a new light naphtha aromatization process using a conventional fixed-bed unit.405 406... 

Besides Ga, other metals such as Zn (11, 12) and Pt (13) have also been used in combination with ZSM-5 zeolite for C2-C4 aromatization. However, besides aromatization, Pt also catalyzes other undesired reactions, such as hydrogenolysis, hydrogenation and dealkylation that leads to excessive formation of methane and ethane, and limits the selectivity to aromatics. Therefore, Ga- and Zn-ZSM-5 catalysts are preferred over Pt-ZSM-5 except, perhaps, in the case of the more refractory ethane, in where a higher dehydrogenating function is needed to activate the reactant. The catalytic performance of Ga and Zn/ZSM-5 for propane aromatization is compared in Table 2.2. The results obtained on the purely acidic H-ZSM-5 are also included in the table. As observed, a higher conversion and yield of aromatics is obtained for the Ga/ZSM-5 catalyst. 

When only 0.1 wt% of platinum was introduced into catalyst, the selectivity increased drastically in both of the Pt-ZSM-5 system and Pt-hybrid system. It can be said that the existence of platinum is necessary for the selective formation of iso-pentane. 

Figure 4. Effect of time-on-stream on the catalytic activities of n-heptane over un- and presulfided Pt-Re/Al203 and Pt-Re/Al203-t-ZSM-5 catalysts.

For the composite Pt-Re/Al203-ZSM-5 catalyst, the increased activity and aromatic selectivity can be ascribed to the suppression of hydrogenolysis by sulfur adsorption on the rhenium surface reducing the ensemble size of platinum [3]. As a result, the... 

Differences in adsorption behaviour are observed for the investigated systems. No NO or NO2 desorption peaks are observed for Pt/SiC or Pt/ZSM-5, while a clear desorption of NO, with a maximum at 158°C, can be observed for Pt/Al203. The Pt/SiC system is also inert towards hydrocarbon adsorption, while Pt/ZSM-5 adsorbs a substantial amount of hydrocarbons. It can be observed that for all tested catalysts, the CO2 formation and the NOx reduction are closely correlated the maximum in NOx reduction is observed at almost complete hydrocarbon oxidation. 

Catalyst Pt-Si02- Al203 Pt-HY Pt-ZSM-5, 80 Si02/Al203 Pt-ZSM-5, 650 Si02/Al203 Pt-Na- BETA Pt- SAPO- 11... 

Finally, we shall discuss two examples that demonstrate the shape selectivity of bifunctional zeolite catalysts. Thus the difiusivity of trans-2-hutene in zeolite CaA is 200 times higher than that of cis-2-butene. Doping with Pt allows selective hydrogenation of fmns-2-butene to be carried out [T35]. Also of interest is shape selective hydrogenation on [Pt]ZSM-5, which is compared to hydrogenation on a conventional supported Pt catalyst in Table 7-8. With the zeolite catalyst, hydrogenation of the unbranched alkene is favored. 

Alkene Reaction temperature rq Conversion [%] Catalyst [Pt]ZSM-5 Pt/AljOj... 

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