(265c) Production of Hydrogen From Dimethyl Ether Over Promoted Bifunctional Catalysts

Hydrogen
fuel cells generating electricity from hydrogen and oxygen show higher energy
efficiency than Carnot cycle, which the byproduct is only water. Its main fuel
is hydrogen or hydrogen-rich gas. Dimethyl ether (DME) is an ideal liquid fuel
as hydrogen carrier due to its high H/C ratio, high energy density and ease
storage and transportation. Furthermore, DME is inert, non-carcinogenic,
non-mutagenic, non-corrosive, and virtually non-toxic. It can also be
conveniently stored and handled because its physical properties are very
similar to those of LPG. DME will, therefore, play an important role in the
future's energy supply system.

Steam reforming (SR) of DME is a
distinguished process to produce hydrogen, and is an attractive route to
provide hydrogen for fuel cells on a small/medium scale. In this work we investigate the hydrogen
production from the DME SR over catalyst Z417 and the self-made catalysts Cu/¦Ã-Al₂O₃
prepared by the impregnation coupling intermittent microwave heating
method (imp-IMH) or the sole impregnation method (imp). Hydrogen production
from DME SR on the commercially catalyst Z417 and
self-made catalysts Cu/¦Ã-Al₂O₃ was investigated under
different conditions in a bench-scale fixed-bed reactor. Catalyst Z417 was
chosen for steam reforming of DME at high-temperature of >
350 ºC and catalysts
Cu/¦Ã-Al₂O₃ were used for low-temperature of <350 ºC.
The hydrogen yield and the DME conversion over catalyst Z417 increased greatly
with increasing temperature, which reached the maximum value of 66.4% and
91.8%, respectively at 500 ºC. The catalysts Cu/¦Ã-Al₂O₃
made by the impregnation coupling with imp-IMH and the
imp were performed on the SR of DME at 350 ºC. The hydrogen yield
was just 33.7% over 12wt% Cu/¦Ã-Al₂O₃ (imp-IMH) with 8.6%
higher than that over 12wt% Cu/¦Ã-Al₂O₃ (imp) after the
catalyst keeps stable, and the DME conversion was just 45.1% with 4.6 % higher
than that over 12wt% Cu/¦Ã-Al₂O₃ (imp). The selectivity of CO is lower and the selectivity of CO₂ over 12wt% Cu/¦Ã-Al₂O₃
(imp-IMH) is higher than that over 12wt%
Cu/¦Ã-Al₂O₃ (imp). This
phenomenon might be explained that imp-IMH can control heating method to
adjust crystalline structure and grain size of catalyst, which may be easily
led to carry water gas shift reaction.

According to the SEM
images, the copper particles in the Cu/¦Ã-Al₂O₃ prepared
with impregnation coupling with intermittent microwave heating method were finer
and more uniform distributed than that in the Cu/¦Ã-Al₂O₃ prepared
with impregnation method. Cu/¦Ã-Al₂O₃ (imp) consisted of
large particles (about 5 µm and above) with a platelike morphology. In
addition, it also could be found that the surface of carriers ¦Ã-Al₂O₃
prepared by the imp-IMH method was less smooth, which may be attributed
to that the frequency of microwave radiation ranging from 0.3 to 300 GHz could
make the ions migrate and the dipolar molecules rotate. Therefore, microwave
heating is more interesting method to prepare catalyst as compared to
conventional heating. The catalysts made by imp-IMH had the higher hydrogen
yield and DME conversion, and the better stability.

Fig.1 DME steam reforming
over Cu/¦Ã-Al₂O₃ (a) the hydrogen
yield and DME conversion, and

(b) C_i selectivity (Steam to DME ratio
of 5:1, 350 ºC, total gas flow rate of 9900 ml g^-1h^-1).

Fig.2 SEM images of the catalysts 12wt% Cu/¦Ã-Al₂O₃
prepared with (a) impregnation method, and (b) impregnation coupling with
intermittent microwave heating method.