Jiang et al. has reported the zirconia supported Kiggin unit 12-
tungstophosphoric acid/zirconia (H3PW12O40/ZrO2), which was
prepared via sol–gel technique, promoted the formation of DMC
from CO2 and methanol effectively under mild condition. The
results shown that with the amount of H3PW12O40 on the catalysts
in the range of 0–50 mg the DMC formation increased almost
linearly. And the mechanistic studies indicated that acid–base
bifunctional catalysis is essential in selective DMC synthesis.
Compared with ZrO2, the H3PW12O40/ZrO2 catalysts has weak
Brønsted acid sites, uniquely, which were more effective than
Lewis acid sites for CH3OH activation [57].
Wu has reported direct synthesis DMC from gaseous methanol
and CO2 over the modified V2O5 catalysts, such as H3PO4/V2O5 and
Cu–Ni/VSO. In the H3PO4/V2O5 catalysts, the direct interaction
between V and P formed weak Brønsted acid sites, which were
more effective for the CH3OH activation. The crystal phase of
H3PO4/V2O5 was influenced by the composition of P/V, significantly,
and with P/V = 0.15–0.50 it was in bicrystal phase
(orthorhombic/tetragonal) showed effective activation of both
CO2 and CH3OH [58].
At our lab, direct synthesis of dimethyl carbonate from CO2 and
methanol was carried out at near supercritical conditions using
nickel acetate as the catalyst. It was demonstrated that DMC could
be produced as the unique product at such low temperature as
305 K and the yield was 12 times higher than that at nonsupercritical
conditions. The synthesis was sensitive to the
reaction pressure and showed a maximum for DMC yield at the
pressure of 9.3 MPa. The concentration of methanol showed an
obvious influence on both the yield and selectivity of DMC. Nickel
acetate appeared to be the precursor of the catalyst. The formation
mechanism of dimethyl carbonate in supercritical phase was
proposed in Scheme 3 [59].