HZSM-5 zeolite catalysts product distribution

The zeolite structure also plays a large role in RC product distribution. Weitkamp et al.62 conducted experiments with Pt/HZSM-5 catalysts, which have very narrow pore sizes when compared with other zeolites, such as USY or SAPO. They found that c is I trans-1,3-dime thylcyclopentane was formed, while 1,1 and 1,2-DMCP were not. This indicates that the more oval shaped 1,3-DMCP was able to diffuse through the pores, while the more bulky and spherical isomers were not, and thus not seen in the product distribution. In short, when compared with dealkylation to cyclohexane, ring contraction of MCH is a more effective pathway to yield higher ON products. However, in order to further improve the ON, ring-opening of the RC isomers may be necessary, as shown below. 

The title reaction has been studied on HY, HM, HZSM-5 and HBeta zeolites under standard conditions. The reaction pathway involves many parallel and/or successive steps. The reactant can undergo dealkylation, isomerization and disproportionation in various relative ratios, depending on the nature of the catalyst and on the reaction conditions. The influence of pressure was investigated. It was found that, generally, the activity and stability of the catalyst increase with increasing pressure. Products distribution, which strongly depends on the nature of the zeolite, is also affected by pressure. Low conversion (< 10%) runs were also performed at different temperatures to evaluate the activation energy values of the reactions. 

Table 6.19 Product distribution of methylcyclohexane hydroconversion over CLD-modified HZSM-5 zeolite catalyst (400°C)...

Product distribution of the light naphtha conversion (LNl) over the metal incorporated zeolite catalyst is given in Table 8. As can be seen from data presented, the metal modified catalyst is highly active for aromatization, evidenced by increased conversion to 91.5 wt % and aromatic yield 35.2 wt %. This is more obvious when we compare with the results on the parent catalyst (before metal modification) HZSM-5, which showed only 84% conversion and 22.4% aromatic yield. Increase in aromatic yields obtained over the metal incorporated catalyst can be explained by the active participation of metal in the olefin production by dehydrogenation and aromatization steps of the reaction [39-41]. Since aromatization of paraffins is an endothermic reaction, higher reaction temperatures (500 C) were employed for the maximum production of aromatics. 

The alkylation of phenol with propylene over several solid acid catalysts such as HZSM-5 with different silica to alumina ratios, H-Beta, H-USY and Y-AI2O3 has been studied. It has been found that zeolite structure has great influence on product distribution. Apart from shape selectivity taking effect in phenol alkylation with propylene over HZSM-5 zeolites, acidic properties (i.e. acid strength and acid density) also influence product distribution. It has been found that H-ZSM-5 exchanged with different alkali metal ions, such as Na and Cs could apparently enhance the selectivity for para-iso-propylphenol due to the change of acidic properties. The acidic properties of the zeolites were characterized by NH3-TPD. 

In a preliminary screening, the alkylation of 2-methylnaphthalene was studied using a variety of acid zeolites with different pore widths. In principal agreement with the earlier work of Fraenkel et al. [22-25] it was found that the best selectivities for the slim alkylation products, i. e., 2,3-, 2,6- and 2,7-dimethylnaphthalene, are obtained on HZSM-5 and HZSM-11. On these catalysts it was observed that the alkylation is always accompanied by the undesired isomerization into 1-methylnaphthalene. Moreover, a peculiar deactivation behavior was encountered With time on stream, the yield of 1-methylnaphthalene always dropped while the yield of alkylation products remained practically constant or even slightly increased. An example for the conversion and yield curves is given in Fig. 4. The distribution of the dimethylnaphthalene isomers is shown for the same experiment in Fig. 5. Bearing in mind that in equilibrium one would expect roughly 12 mole-% of each of the slim isomers, the... 

In additional experiments, HZSM-5 was precoked by converting methanol alone (into hydrocarbons) at 400 °C. Afterwards the zeolite was exposed to the 2-methylnaphtha-lene/methanol mixture, under the usual reaction conditions. The initial yield of 1-methylnaphthalene was significantly reduced (1.5 % compared to 4 % for the fresh catalyst, cf. Fig. 4). Furthermore, the initial content of 2,6- + 2,7-dimethylnaphthalene in the dimethyl-naphthalene fraction was 84 % instead of 70 % for the fresh catalyst. In another run, HZSM-5 was loaded with 0.5 wt.-% of Pt, and H2 was used as carrier gas instead of N2. Under these conditions, the formation of coke was avoided or at least drastically diminished. In-line with our model, no changes in the product yields and in the distribution of the dimethylnaphthalene isomers were observed with time on stream. 

The aging and combustion kinetics of coke deposited on an HZSM-5 zeolite-based catalyst in the MTG process have been studied. The kinetic study of coke combustion in air was carried out at 500-550°C in a differential scanning calorimeter, by following the evolution of the combustion products with on-line FTIR analysis. The results provide evidence for limitations on coke reactivity that can be attributed to the combined effects of several circumstances (e.g. bad oxygen-coke contact and heterogeneous distribution of coke within the zeolite crystal). The need is demonstrated for a thermal aging treatment which equilibrates reproducibly the coke prior to combustion. The aging of coke is also limited by a peculiar coke deposit in the microporous stmcture of the zeolite. 

There is a great similarity in the catalytic transformation of ethanol and of methanol on a HZSM-5 zeolite, either concerning the reaction mechanism [4], or in the spectra of products [5,6], Nevertheless, the high content of water in the feed in the BTG process markedly influences the distribution of products and catalyst deactivation [5-7], It has been proven that water attenuates deactivation by coke at moderate temperatures [6], but causes irreversible deactivation by dealumination of the zeolite at high temperatures. 

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