Chemistry Reference
In-Depth Information
Fig. 14 C
3
(left panel) and C
5 þ
aliphatic (right panel) yield as a function of conversion
over the various MTH catalysts at 350
1
C and P(methanol)
=
13 kPa.
topologies (MFI, MEL, TNU, IMF), which form an ensemble of similar C
3
selectivities, and subsequently the 12-ring topologies (BEA, MOR, AFI),
which form another ensemble with lower C
3
selectivities. Interestingly,
the 1D 10-ring topology, TON, gives similar C
3
product selectivity as the
12-ring materials. The product yield versus conversion curves are
generally linear in the 0-80% conversion range, and then change slope in
the upwards or downwards direction. The slope change mainly corres-
ponds to an increased net conversion of light alkenes to aromatic mol-
ecules and alkanes (See Fig. 16) with the exhaustion of methanol. This
observation is in agreement with C
2
-C
4
methylation studies performed
over MFI, which showed that alkene interconversion reactions were
suppressed in the presence of methanol, possibly due to competitive
adsorption at the Brønsted acidic sites (See Section 2.3).
125
As mentioned
in Section 3.2, CHA is the only structure among those presented here for
which a change in selectivity has been observed with increasing
deactivation. As such, the upwards slope of the C
3
yield versus methanol
conversion curve for CHA is in agreement with a higher diffusion re-
sistance for a coked catalyst, thereby favoring propene (over ethene)
production only at high conversion. No straightforward explanation
has so far been found for the upwards curvature observed for TON and
MOR at high conversions. TON has previously been tested at different
residence times, leading to the conclusion that its product selectivity is
independent of deactivation.
160
Turning to the C
5
þ
aliphatics product group (Fig. 15 right), 1D 10-ring
TON zeolite gives dramatically higher C
5
þ
aliphatics selectivity than any
of the other catalysts in the entire methanol conversion range. In fact,
when summing the yields of C
5
þ
aliphatics and aromatics (Fig. 16 right),
then the TON zeolite gives the highest overall C
5
þ
hydrocarbons yield of
all tested catalysts. The 3D 10-ring zeolites form an ensemble of inter-
mediate selectivities to C
5
þ
aliphatics, while the 12-ring zeolites give
scattered C
5
þ
aliphatics selectivities, although at the lower range of the
