10
4
Route II
8
3
6
2
4
Route I
2
Route I
1
0
Conventional USY
Conventional USY
Route II All Brønsted acidity
0
-2
Mesoporosity
Figure 4 Middle distillate yield (MD, left) and delta of temperature of 60% conversion (right) as a function of common descriptors such as the mesoporosity (left) and overall Brønsted acidity (right)
the application of suitable descriptors to characterise the mesoporous USY zeolite is worthwhile. This becomes par- ticularly relevant in cases where conventional descriptors fail. For example, take two seemingly similar mesoporous USY zeolites obtained via two different routes: ‘Route I’ and ‘Route II’. Both materials feature a largely retained micro- pore volume and a roughly doubled mesopore volume (see Figure 4, left ). Additionally, both samples have an overall acidity of about 80% as compared to the conventional USY zeolite (see Figure 4, right ). Taking the overall Brønsted acidity into account, one would expect both mesoporous zeolites to be substan- tially less active than the conventional USY. Moreover, using mesoporosity as a descriptor for selectivity, both mesoporous zeolites should yield a similar benefit in mid - dle distillates make. The above hypotheses are obviously not in line with the remarkably different catalytic results in Figure 4. A broader evaluation of the samples within this study confirms the sub-optimal correlation of established descriptors with the performance of mesoporous zeolites. Combining accessibility with acid strength By closely evaluating the top-performing optimal samples,
descriptors have been derived based on porosity and acid- ity, which correlate the set of materials to the hydrocracking selectivity and activity in a superior fashion. A key ingredient in these descriptors proved the acid strength, expressed as the number of strong acid sites (probed at high temperatures) relative to the overall num- ber of acid sites (probed at lower temperatures). Whereas Route I yielded a relative strength of about 40%, Route II yielded a relative strong acidity of 60%, similar to a con- ventional USY zeolite. The acidity of the mesoporous Route II sample represents a breakthrough: never before was a mesoporous USY zeolite with this level of mesoporosity prepared with the same acid strength as a conventional zeolite. The acid strength was, therefore, included in advanced descriptors for selectivity and activity, giving rise to a selec- tivity and an activity number. The selectivity number refers to the relative number of strong acid sites compared to the total. The activity number is formed by correcting the strong Brønsted sites for undesired strong Lewis sites, both as measured with FT-IR spectroscopy of pyridine-probed zeolites. It was hypothesised that the acid strength may account
4
10
Route II
8
3
6
2
4
Route I
2
Route I
1
0
Conventional USY
Conventional USY
Route II
0
-2
Relative strong acidity x mesoporosity
Activity number
Figure 5 Middle distillate yield (MD, left) and delta of temperature of 60% conversion (right) as a function of advanced descriptors such as the mesoporosity multiplied with the relative strong acidity (left) and strong Brønsted acidity corrected with strong Lewis acidity (activity number, right).
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