3.2 Heterogeneous Reactions for the Production of Methanol …
55
continuous-flow reactor at atmospheric pressure [12]. The major products of the
reaction were formaldehyde, carbon dioxide, and carbon monoxide, and methanol
were also produced as a minor product. The catalytic activity and selectivity showed
a clear dependence on the FePO 4 loading. FePO 4 supported on silica gave the
highest yield of formaldehyde, while alumina-supported FePO 4 exhibited the lowest
selectivity toward formaldehyde. The highest selectivities and space–time yields for
formaldehyde and methanol were observed at lower FePO 4 loadings. Mössbauer
spectra suggested the formation of penta-coordinated Fe species when FePO 4 was
supported on silica. X-ray photoelectron spectroscopy (XPS) results indicated that
the Fe on the surface of the silica support existed in a lower oxidation state, and
the catalysts also displayed a substantial enrichment of phosphorus on the surface,
relative to less selective catalysts. The presence of iron that can undergo a facile
reduction in a high coordination state and its isolation by the phosphate groups was
proposed as the reason for the good performance of the silica-supported FePO 4
catalyst. The stability of the resulting formaldehyde and methanol on the support
was also considered to play an important role in achieving a high yield.
The single crystalline phase CuFe 2 (P 2 O 7 ) 2 catalyzed the conversion of methane
to formaldehyde and methanol in the presence of oxygen molecules as the oxidant
at 673–898 K with high stability [18]. The performance of the catalyst could be
optimized by tuning the atomic ratio of copper to iron in the pyrophosphate matrix and
the calcination conditions [19]. The optimum conversion of methane to formaldehyde
and methanol was achieved at a Cu/Fe atomic ratio close to 1:2. However, its methane
conversion and methanol selectivity were low (<1% and <6%, respectively), while the
selectivity for formaldehyde was ~95%. The methane conversion and the selectivity
toward methanol could be enhanced by using N 2 O as the oxidant. For example,
at 773 K, methane conversion increased from 0.1 to 1%, and methanol selectivity
increased from 5 to 30%. The catalyst was composed predominantly of crystalline
Fe 2 (P 2 O 7 ) 2 with nanodomains containing copper. The interactions between these
phases probably generate a disordered crystalline structure, which may play a role
in the catalytic site. The pulse reaction studies demonstrated that the oxygens in the
lattice react with methane molecules to produce the intermediate that converts to
formaldehyde and methanol, which is assumed to be the CH 3 O species on copper.
The lattice oxygen could be regenerated rapidly by N 2 O, resulting in the production
of the oxygen radical, which did not form when molecular oxygen was used as the
oxidant. The ability to generate oxygen species effectively from N 2 O for mediating
the conversion of methane to methanol is very similar to the process observed in
iron-exchanged ZSM-5 with extra-framework Fe species, which will be discussed
next.
An iron ion-exchanged zeolite was found to convert methane to methanol at room
temperature using nitrous oxide or hydrogen peroxide as the oxidizing agent [20].
The active species of this reaction over iron-exchanged ZSM-5 was a mononuclear
high spin tetravalent iron monoxide species (Fe(IV) = O), i.e., the so-called α-Fe
site [21]. This active species is formed by treating an iron ion (α-Fe(II)) that is
constrained to the “square planar” shape in the zeolite pores with nitrous oxide
(N 2 O) at 523 K. The highly reactive oxygen, called α-O, is derived from N 2 O. α-O
55
continuous-flow reactor at atmospheric pressure [12]. The major products of the
reaction were formaldehyde, carbon dioxide, and carbon monoxide, and methanol
were also produced as a minor product. The catalytic activity and selectivity showed
a clear dependence on the FePO 4 loading. FePO 4 supported on silica gave the
highest yield of formaldehyde, while alumina-supported FePO 4 exhibited the lowest
selectivity toward formaldehyde. The highest selectivities and space–time yields for
formaldehyde and methanol were observed at lower FePO 4 loadings. Mössbauer
spectra suggested the formation of penta-coordinated Fe species when FePO 4 was
supported on silica. X-ray photoelectron spectroscopy (XPS) results indicated that
the Fe on the surface of the silica support existed in a lower oxidation state, and
the catalysts also displayed a substantial enrichment of phosphorus on the surface,
relative to less selective catalysts. The presence of iron that can undergo a facile
reduction in a high coordination state and its isolation by the phosphate groups was
proposed as the reason for the good performance of the silica-supported FePO 4
catalyst. The stability of the resulting formaldehyde and methanol on the support
was also considered to play an important role in achieving a high yield.
The single crystalline phase CuFe 2 (P 2 O 7 ) 2 catalyzed the conversion of methane
to formaldehyde and methanol in the presence of oxygen molecules as the oxidant
at 673–898 K with high stability [18]. The performance of the catalyst could be
optimized by tuning the atomic ratio of copper to iron in the pyrophosphate matrix and
the calcination conditions [19]. The optimum conversion of methane to formaldehyde
and methanol was achieved at a Cu/Fe atomic ratio close to 1:2. However, its methane
conversion and methanol selectivity were low (<1% and <6%, respectively), while the
selectivity for formaldehyde was ~95%. The methane conversion and the selectivity
toward methanol could be enhanced by using N 2 O as the oxidant. For example,
at 773 K, methane conversion increased from 0.1 to 1%, and methanol selectivity
increased from 5 to 30%. The catalyst was composed predominantly of crystalline
Fe 2 (P 2 O 7 ) 2 with nanodomains containing copper. The interactions between these
phases probably generate a disordered crystalline structure, which may play a role
in the catalytic site. The pulse reaction studies demonstrated that the oxygens in the
lattice react with methane molecules to produce the intermediate that converts to
formaldehyde and methanol, which is assumed to be the CH 3 O species on copper.
The lattice oxygen could be regenerated rapidly by N 2 O, resulting in the production
of the oxygen radical, which did not form when molecular oxygen was used as the
oxidant. The ability to generate oxygen species effectively from N 2 O for mediating
the conversion of methane to methanol is very similar to the process observed in
iron-exchanged ZSM-5 with extra-framework Fe species, which will be discussed
next.
An iron ion-exchanged zeolite was found to convert methane to methanol at room
temperature using nitrous oxide or hydrogen peroxide as the oxidizing agent [20].
The active species of this reaction over iron-exchanged ZSM-5 was a mononuclear
high spin tetravalent iron monoxide species (Fe(IV) = O), i.e., the so-called α-Fe
site [21]. This active species is formed by treating an iron ion (α-Fe(II)) that is
constrained to the “square planar” shape in the zeolite pores with nitrous oxide
(N 2 O) at 523 K. The highly reactive oxygen, called α-O, is derived from N 2 O. α-O