  *Context The evolution of young massive protoplanetary disks toward planetary
systems is expected to include the formation of gaps and the depletion of dust
and gas. *Aims A special group of flaring disks around Herbig Ae/Be stars do
not show prominent silicate emission features. We focus our attention on four
key Herbig Ae/Be stars to understand the structural properties responsible for
the absence of silicate feature emission. *Methods We investigate Q- and N-band
images taken with Subaru/COMICS, Gemini South/T-ReCS and VLT/VISIR. Our
radiative transfer modeling solutions require a separation of inner- and outer-
disks by a large gap. From this we characterize the radial density structure of
dust and PAHs in the disk. *Results The inner edge of the outer disk has a high
surface brightness and a typical temperature between ~100-150 K and therefore
dominates the emission in the Q-band. We derive radii of the inner edge of the
outer disk of 34, 23, 30 and 63 AU for HD97048, HD169142, HD135344B and Oph IRS
48 respectively. For HD97048 this is the first detection of a disk gap. The
continuum emission in the N-band is not due to emission in the wings of PAHs.
This continuum emission can be due to VSGs or to thermal emission from the
inner disk. We find that PAH emission is not always dominated by PAHs on the
surface of the outer disk. *Conclusions. The absence of silicate emission
features is due to the presence of large gaps in the critical temperature
regime. Many, if not all Herbig disks with Spectral Energy Distribution (SED)
classification `group I' are disks with large gaps and can be characterized as
(pre-) transitional. An evolutionary path from the observed group I to the
observed group II sources seems no longer likely. Instead, both might derive
from a common ancestor.
