171
Aqueous-Phase Reforming and BioForming Process
6.5.6 SummAry
The above-described discussion on thermodynamics, kinetics, and catalysis of the
APR process gives the following conclusions about the APR process [1–6,16]:
1. The basis for the APR process is that while alkanes reforming is only
favorable at high temperatures, the reforming of oxygenated carbon (with
a C/O ratio of 1:1) and the water–gas shift reaction are possible at low
t emperatures. This allows APR to be carried out in the liquid phase.
2. The activation energy required to break up the C–C bond in oxygenated
compounds is lower than that required in alkanes. Thus, H 2 and CO 2 from
oxygenated compounds can be obtained in a single reactor. This can be
accomplished in liquid phase only for high boiling compounds such as glucose and sorbitol, whereas for low boiling compounds such as glycerol,
ethylene glycol, and methanol, the reactions can occur in both the gas and
liquid phases.
3. The choice of a catalyst can affect the products. Pt, Pd, and Ni–Sn alloys
show high selectivity for hydrogen, whereas Ni catalysts tend to make
more alkanes. Ru and Rh catalysts also make alkanes with very little
hydrogen. More acidic support favors alkanes production, whereas more
basic/neutral support such as alumina favors hydrogen production. The
acidic aqueous solution similarly promotes alkanes production due to
acid-catalyzed dehydrogenation reactions (followed by the hydrogenation
on the metal). The basic aqueous solution favors hydrogen production.
The promoters such as Re add acidity to the catalyst, thereby reducing
hydrogen formation.
4. The type of feed and its concentration affect the product distribution.
Sorbitol gives higher selectivity for hydrogen than glucose. Within polyols,
hydrogen selectivity decreases with an increase in carbon number of the
feed and an increase in feed concentration due to an increase in side reactions. APR of platform chemical glycerol has been very widely studied.
APR can also be applied to the secondary feedstock as long as they are
properly pretreated by hydrolysis (either acid or enzymatic) and/or hydrogenation depending on the nature of the feedstock.
5. Davda et al. [2,16,41,58,59] outlined a number of different pathways that
can occur in the APR reactor depending on the nature of catalyst, its acidity and acidity level of aqueous solution, the temperature, and the pressure to obtain the desired product distributions. Generally, higher carbon
number in the feed and more acidity on the catalyst or aqueous solution
favor C–O scission and more alkanes production. The reverse conditions
promote C–C bond cleavages to form hydrogen and CO 2 . The latter compounds can, however, undergo undesirable methanation and FT reactions
to produce more alkanes. Some metals such as Ru and Rh favor C–O
scission and form more alkanes. Pt and Pd, however, favor C–C scission.
More bifunctional catalysis can occur by the combination of metal, support, and solutions. In general, high hydrogen selectivity requires high
Aqueous-Phase Reforming and BioForming Process
6.5.6 SummAry
The above-described discussion on thermodynamics, kinetics, and catalysis of the
APR process gives the following conclusions about the APR process [1–6,16]:
1. The basis for the APR process is that while alkanes reforming is only
favorable at high temperatures, the reforming of oxygenated carbon (with
a C/O ratio of 1:1) and the water–gas shift reaction are possible at low
t emperatures. This allows APR to be carried out in the liquid phase.
2. The activation energy required to break up the C–C bond in oxygenated
compounds is lower than that required in alkanes. Thus, H 2 and CO 2 from
oxygenated compounds can be obtained in a single reactor. This can be
accomplished in liquid phase only for high boiling compounds such as glucose and sorbitol, whereas for low boiling compounds such as glycerol,
ethylene glycol, and methanol, the reactions can occur in both the gas and
liquid phases.
3. The choice of a catalyst can affect the products. Pt, Pd, and Ni–Sn alloys
show high selectivity for hydrogen, whereas Ni catalysts tend to make
more alkanes. Ru and Rh catalysts also make alkanes with very little
hydrogen. More acidic support favors alkanes production, whereas more
basic/neutral support such as alumina favors hydrogen production. The
acidic aqueous solution similarly promotes alkanes production due to
acid-catalyzed dehydrogenation reactions (followed by the hydrogenation
on the metal). The basic aqueous solution favors hydrogen production.
The promoters such as Re add acidity to the catalyst, thereby reducing
hydrogen formation.
4. The type of feed and its concentration affect the product distribution.
Sorbitol gives higher selectivity for hydrogen than glucose. Within polyols,
hydrogen selectivity decreases with an increase in carbon number of the
feed and an increase in feed concentration due to an increase in side reactions. APR of platform chemical glycerol has been very widely studied.
APR can also be applied to the secondary feedstock as long as they are
properly pretreated by hydrolysis (either acid or enzymatic) and/or hydrogenation depending on the nature of the feedstock.
5. Davda et al. [2,16,41,58,59] outlined a number of different pathways that
can occur in the APR reactor depending on the nature of catalyst, its acidity and acidity level of aqueous solution, the temperature, and the pressure to obtain the desired product distributions. Generally, higher carbon
number in the feed and more acidity on the catalyst or aqueous solution
favor C–O scission and more alkanes production. The reverse conditions
promote C–C bond cleavages to form hydrogen and CO 2 . The latter compounds can, however, undergo undesirable methanation and FT reactions
to produce more alkanes. Some metals such as Ru and Rh favor C–O
scission and form more alkanes. Pt and Pd, however, favor C–C scission.
More bifunctional catalysis can occur by the combination of metal, support, and solutions. In general, high hydrogen selectivity requires high