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M. Bjørgen et al. / Applied Catalysis A: General 345 (2008) 43–50 47 Fig. 3. Scanning electron micrographs of the untreated H-ZSM-5 sample (PARENT) and the sample treated with 0.20 M NaOH (0.20M). The two micrographs are shown at the same magnification. of the various sites have been confirmed in a more extensive spectroscopic study using large probe molecules (2,4,6-trimethyl pyridine) [34]. An H-ZSM-5 free from internal defects will only have a Si–OH contribution from the external surface (giving a peak at 3747 cm1). Fig. 4. DRIFTS spectra of the three H-ZSM-5 samples collected at room temperature. Thus, the remaining Si–OH components located at lower frequencies for the PARENT sample represent internal defects in the framework. Upon NaOH treatment, the vibrational properties of the sample in the n(O–H) region are changed substantially. The 3747 cm1 band increase in intensity, whereas the different Si–OH contributions at the lower frequencies vanish, implying that after treatment the Si– OH sites are more uniform and to a much larger extent unperturbed. The interpretation of this observation is not straightforward, but it seems plausible that the defects are either healed, i.e. Si is inserted into the vacancies that created the Si–OH nests, or the defects become parts of mesopores which effectively may appear like an ordinary external surface with isolated Si–OH sites represented by the band at 3747 cm1. In any case, it is beyond any doubt that the features characterizing Si–OH in structural defects are dramatically reduced. It should also be mentioned that the NaOH treatment inevitably seems to generate partially lattice Al–OH species which give rise to the rather small component at 3664 cm1 clearly seen for the treated samples. These bands have also been associated with the presence of Lewis acidity [35–37]. Based on FTIR and quantum chemical modeling, Brand et al. suggested that the band at 3666 cm1 (close to our observed band at 3664 cm1) can be assigned to terminal and bridging OH groups in dimeric (Al(OH)3)2 species [36]. Also Fleischer et al. found that the 1H NMR chemical shift of the bridging protons in the (Al(OH)3)2 species is in the range assigned to OH groups associated with extra framework Al species [37]. 3.2. Catalytic testing The conversion of methanol into hydrocarbon products (dimethyl ether, DME, is considered to be unconverted reactant and lumped together with methanol in the calculation of conversion and selectivities) at 370 8C and WHSV = 8 g g1 h1 is displayed as a function of time on stream for the three catalyst samples in Fig. 5. The untreated sample displays an initial activity of 86% and deactivation is rather fast, as the activity falls below 20% after 30 h on stream. The initial activity observed for the catalyst treated with 0.05 M NaOH is 89% and slightly higher than for the PARENT sample, and the lifetime of the catalyst is markedly improved, as the conversion remains above 20% for more than 70 h on stream. These effects are even more pronounced for the sample treated with 0.20 M NaOH; the initial activity increases further andPDF Image | Methanol to gasoline over zeolite H-ZSM-5
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