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48 M. Bjørgen et al. / Applied Catalysis A: General 345 (2008) 43–50 Fig. 5. Conversion of methanol/DME into hydrocarbons as a function of time on stream at 370 8C and WHSV = 8 g g1 h1. is now 92% and the lifetime is even longer than for the 0.05M sample. These observed increased catalyst lifetimes confirm the early assumptions published by Lietz et al. [26] mentioned introductorily. The fact that only slight differences in initial activities are found among the samples again indicates that the acid site densities are fairly similar for the three samples, in agreement with the results from NH3-TPD, and that the Al content determined with ICP-AES also includes a significant fraction of Al not giving rise to acidity. Moreover, an improved accessibility to and from the acidic sites caused by the desilication procedure, as indicated by the increase in the external surface areas and mesopore volumes (Table 1), might play a role. If the diffusion pathways are shortened for the treated samples, this will also be reflected in the product distribution (vide infra). In order to obtain a more quantitative and precise measure of the improvement in catalyst performance, the loosely defined concept of catalyst lifetime will be supplemented by the more meaningful total conversion capacities. Briefly, this is done by plotting the methanol conversion against the grams of methanol converted per gram catalyst and extrapolating to zero conversion, thus obtaining a value for the total capacity of the catalyst for converting methanol into hydrocarbons until complete loss of activity. This is shown in Fig. 6. For the untreated sample, a conversion capacity of 165 g(methanol) (g(catalyst))1 is found, for the 0.05M sample the conversion capacity is 400 g g1, whereas for the 0.20M sample, it is 550 g g1. Thus, the conversion capacity increases by a factor of 3.3 as a result of the most severe desilication procedure. Industrially, Fig. 6. Conversion plotted against the cumulative amount of methanol that has been converted to hydrocarbons. Extrapolation to zero conversion gives the total conversion capacity of the three samples. catalytic reactions are seldom allowed to run until complete deactivation, as the catalyst is regenerated at appreciable conversion levels. Hence, the above comparison of conversion capacities, which is indeed based on completely deactivated samples, is not necessarily the best way of evaluating the catalyst performance. However, if the conversion capacities are compared at considerable conversion much improvement is still seen. When the conversion has dropped to e.g. 80%, the PARENT sample has converted 55 g g1, whereas the 0.05M and the 0.20M samples have converted 148 and 183 g g1, respectively. Importantly, this is the same relative improvement, by a factor of 3.3 for the 0.20M sample, as found when considering the entire lifetime of the catalysts. This implies that similar improvements should be expected in experiments conducted at conditions more similar to those employed industrially (lower WHSV giving full conversion). At this point, it should be emphasized that the improvement in conversion capacity seen even for the 0.05M sample is much too large to be caused by the minor increase in Si/Al ratio compared to the PARENT sample (Table 1), even if we assume, in spite of the NH3-TPD results, that all Al gives rise to acidity. Similarly, also for the 0.20M sample is the increase in conversion capacity (by a factor 3.3) larger than the increase in overall Al content (by a factor 1.7). The initial product selectivities obtained over fresh catalysts (10 min on stream) presumably relatively free from coke, which may alter the diffusion properties, are given in Fig. 7. For the PARENT sample, the most abundant product is C3 (32 C%, mainly propene), followed by various C6+ hydrocarbons (20 C%), C2 (17 C%, mainly ethene), and the butenes (15 C%). In addition to the products displayed in Fig. 7, minor amounts of methane were always detected, reaching a maximum selectivity of 1.1 C% at the most severely deactivated measurements. It seems that the desilication procedure leads to decreased selectivities towards C2 and C3 (predominantly alkenes). A non-systematic behavior is seen for the C4 alkenes, whereas the C4 alkanes increase as a consequence of the treatment. The selectivities towards C5 increase slightly, whereas C6+ increases substantially. More information may be extracted by evaluating the effluent composition over the full range of conversions, as shown in Fig. 8a– c. Fig. 8a displays the selectivity towards C5+, which may loosely be defined as the gasoline cut, as a function of conversion, i.e. during deactivation, as shown in Fig. 5. Clearly, there is a remarkable effect of the desilication on the C5+ selectivity in the entire conversion range, as considerable increases are seen for the treated samples. For the PARENT sample, the selectivity to C5+ starts at 30 C% and drops off to 20 C% as the activity declines. The 0.05M sample starts at 36 C% C5+ and this value declines to 30 C%, whereas for the 0.20M Fig. 7. Product selectivities (C%) at initial conversion at 3708C and WHSV = 8 g g1 h1 for the three catalyst samples.PDF Image | Methanol to gasoline over zeolite H-ZSM-5
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