  We present Spitzer IRS low resolution, mid-IR spectra of a sample of 25 high
luminosity QSOs at 2<z<3.5. When combined with archival IRS observations of
local, low luminosity type-I AGNs, the sample spans five orders of magnitude in
luminosity. We find that the continuum dust thermal emission at
lambda(rest)=6.7um is correlated with the optical luminosity, following the
non-linear relation L(6.7um) propto L(5100A)^0.82. We also find an anti
correlation between the ratio L(6.7um)/L(5100A) and the [OIII]5007A line
luminosity. These effects are interpreted as a decreasing covering factor of
the circumnuclear dust as a function of luminosity. Such a result is in
agreement with the decreasing fraction of absorbed AGNs as a function of
luminosity recently found in various surveys. We clearly detect the silicate
emission feature in the average spectrum, but also in four individual objects.
These are the Silicate emission in the most luminous objects obtained so far.
When combined with the silicate emission observed in local, low luminosity
type-I AGNs, we find that the silicate emission strength is correlated with
luminosity.
  The silicate strength of all type-I AGNs also follows a positive correlation
with the black hole mass and with the accretion rate. The Polycyclic Aromatic
Hydrocarbon (PAH) emission features, expected from starburst activity, are not
detected in the average spectrum of luminous, high-z QSOs. The upper limit
inferred from the average spectrum points to a ratio between PAH luminosity and
QSO optical luminosity significantly lower than observed in lower luminosity
AGNs, implying that the correlation between star formation rate and AGN power
saturates at high luminosities.
