  We study the origin of the cold molecular clumps in quasar outflows, recently
detected in CO and HCN emission. We first describe the physical properties of
such radiation-driven outflows and show that a transition from a momentum- to
an energy-driven flow must occur at a radial distance of R ~ 0.25 kpc. During
this transition, the shell of swept up material fragments due to
Rayleigh-Taylor instabilities, but these clumps contain little mass and are
likely to be rapidly ablated by the hot gas in which they are immersed. We then
explore an alternative scenario in which clumps form from thermal instabilities
at R >~ 1 kpc, possibly containing enough dust to catalyze molecule formation.
We investigate this processes with 3D two-fluid (gas+dust) numerical
simulations of a kpc^3 patch of the outflow, including atomic and dust cooling,
thermal conduction, dust sputtering, and photoionization from the QSO radiation
field. In all cases, dust grains are rapidly destroyed in ~10,000 years; and
while some cold clumps form at later times, they are present only as transient
features, which disappear as cooling becomes more widespread. In fact, we only
find a stable two-phase medium with dense clumps if we artificially enhance the
QSO radiation field by a factor 100. This result, together with the complete
destruction of dust grains, renders the interpretation of molecular outflows a
very challenging problem.
