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M. Bjørgen et al. / Applied Catalysis A: General 345 (2008) 43–50 49 Fig. 8. (a) Selectivity towards C5+, (b) C4 hydrogen transfer index (HTI), (c) C3/C2 ratio as a function of conversion for the three catalyst samples. The C4 HTI is defined according to the C4 yields as (iso-butane + n-butane)/SC4. All values are carbon- based (C%). sample, the selectivity remains constant and high, at 43–44 C% over the slightly more narrow conversion span investigated. Hence, the selectivity towards gasoline-blendable compounds is on average a factor of 1.7 higher for the most extensively desilicated sample. A related selectivity aspect is borne out in Fig. 8b, in which the C4 hydrogen transfer index (HTI) is displayed. The C4 HTI is defined as the combined yields of iso-butane and n- butane divided by the total yield of C4 alkanes and alkenes and constitutes a convenient measure of the hydrogen transfer activity of the catalyst [38]. Thus, a high C4 HTI implies extensive cyclization and aromatization reactions, which are accompanied by a high alkane production. For all samples, the C4 HTI decreases with loss of activity. This is the expected behavior, which is commonly attributed to the effective reduction in acid site density upon catalyst deactivation [39]. A slightly higher C4 HTI is seen for the 0.05M sample, whereas a distinct increase is found for the 0.20M sample. These increases in the HTIs must, of course, be reflected in the selectivities towards aromatic compounds, as seen from the chromatograms measured at 85% conversion shown in Fig. 9. A fairly stable amount of toluene (not included in Fig. 9), corre- sponding to about 10% of the amount of p/m-xylene, was also detected for all samples. For all catalysts, the combined p- and m- xylene peak is the largest, and for the PARENT sample, the only other significant peak represents modest amounts of 1-methyl-4- ethyl-benzene. For the 0.05M sample, the peaks corresponding to o-xylene and 1,2,4-trimethylbenzene also become clearly visible. The rather minor increase in aromatic compounds seen for this sample, which corresponds well with the moderate increase in the C4 HTI, indicates that a considerable part of the increase in selectivity towards C5+ must be attributed to C5 and C6 aliphatics and cyclic non-aromatic compounds. For the 0.20M sample, however, a profound increase is seen for the aromatic compounds found among the products, both with respect to the yield, and also with respect to the number of different compounds. The peak Fig. 9. Chromatogram detail (GC-FID) of the aromatics region at 85% conversion for the three catalyst samples. representing 1,2,4-trimethylbenzene is now comparable in size to that corresponding to p/m-xylene, and 1,2,4,5-tetramethylbenzene is also a significant component. Clearly, the desilication procedure leads to catalyst modifications which allow larger molecules to diffuse out of the zeolite crystals. It is also interesting to note that o-xylene also increases, even though the p/m-xylene peak is slightly reduced as a result of the treatment, signifying a modification of the para-selectivity of the H-ZSM-5 samples. A final point with respect to product selectivities is illustrated in Fig. 8c. The C3/C2 ratios (mainly alkenes) diverge as a function of deactivation. For all three samples, the ratio is close to 2.5 for a fresh catalyst, but it decreases upon deactivation for the PARENT sample, remains stable throughout the whole range of conversion for the 0.05M sample, and increases to 3.6 for the 0.20M sample. This increased selectivity towards C3 at moderate to low conversion levels might be exploited in a MTP application. The observed increase in the selectivity towards C5+ is the expected outcome of an increased acid site density, an effect which typically favors the cyclizations and dehydrogenations leading to heavier products, but as indicated by NH3-TPD, this is likely to be a minor effect, and other effects may play more significant roles. The DRIFTS spectra presented in Fig. 4 indicates that some Lewis acidity may be present in the treated samples and especially in the 0.20M sample, as is further elaborated in a study in progress [34]. Such sites are frequently suggested to be especially acidic and might therefore favor the reactions leading to formation of heavier products, which require strong acidity [40–42]. It must also be noted that the observed shifts in product selectivities described above are consistent with the notion that the desilication procedure results in mesopore formation and/or increased external surface area as seen with N2 sorption and suggested by the IR measurements, because mesopores will lead to improved product and reactant diffusivity. Further, the IR spectra show a dramatic reduction in the number of internal defects, which may also improve diffusivity. The increase in selectivity towards C5+ and presence of larger aromatic molecules among the products is straightforwardly associated with improved product diffusion and a larger external surface area. However, the increase in the propene/ethene ratio is not as easily explained by improved product diffusivity, as both these molecules are quite small and probably equally (little) influenced by diffusion in the three- dimensional 10 ring ZSM-5 framework, and the underlying reaction mechanism for alkene formation should be considered. Very recently, we have shown that, over H-ZSM-5, ethene is formed predominantly from the xylenes and trimethylbenzenes confined in the zeolite micropores according to a hydrocarbon poolPDF Image | Methanol to gasoline over zeolite H-ZSM-5
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