Aqueous-Phase Reforming and BioForming Process
163
6.5 KinetiCs and Catalysis OF aPr PrOCess
As shown in Figure 6.1, the APR process can be tailored toward the four distinct kinetic
steps depending on the desired product [1–6,11–33] (Tanksale et al., 2008, pers. comm.).
APR can produce hydrogen, syngas, alkanes, or monofunctional groups depending on
the catalyst and support system, promoters, and other operating conditions.
The original purpose of APR was to generate either hydrogen or alkanes by an
APR of sugar, other oxygenated compounds, and polyols (with a ratio of 1:1). The
kinetics of APR depends on the temperature, the pressure, the nature of the catalyst
and its support, the presence of promoters, the pH of the slurry, the acidity of catalyst
active sites, and the nature of the feedstock. Here, we briefly examine the effects
of these operating variables on the APR process. While the literature has shown
numerous ways to generate hydrogen from biomass under high-temperature conditions, APR is unique in that it is the only process that can be carried out in liquid
water [5,34–57]. While APR can only be used for selective feedstock without their
pretreatments, it is faster than anaerobic digestion process for generating hydrogen
from cellulosic waste [5,34–57].
6.5.1 eFFeCTS oF TemPerATure, CArBon numBer, And PreSSure
As shown in Figure 6.1, APR provides multiple options of reaction paths depending
on the operating conditions. Figure 6.3 illustrates that hydrogen selectivity decreases
with an increase in carbon number of oxygenated compounds and an increase in
temperature. The temperature effect shown in this figure is valid for all oxygenated
feedstock. The literature has shown that compounds such as furanone and acetic
acid are not amenable to the production of hydrogen by APR [1–7,16]. The hydrogen
selectivity depends on the nature of the bond breaking in oxygenated compounds;
the breakage of C–C bond favors the hydrogen formation and the breakage of C–O
bond favors the formation of alkanes. Following the preferred pathway is the key to
the hydrogen formation. Dumesic et al. [1–6,16] also showed that an increase in pressure reduced the hydrogen selectivity. For example, for the reaction of 5 wt% sorbitol
over Pt–SiAl at 498 K, the hydrogen selectivity at 25.8 atm pressure was 21, whereas
the same selectivity at a pressure between 33.1 and 52.1 atm was <2 [1–6,16].
6.5.2 eFFeCTS oF CATAlySTS And SuPPorTS
As shown in Table 6.1, the major primary feedstock for APR are glucose (and sorbitol), alcohols, ethylene glycol, and glycerol. For all of these feedstock, Dumesic et al.
[1–6,16], among others [5,15,17–26,27–57] (Tanksale et al., 2008, pers. comm.), have
clearly shown that the nature of metal and support has an important influence on the
reaction paths and the rates of reactions in the APR process. The product selectivity
can be tuned depending on the metal and support. For example, Pt-black and Pt supported on Al 2 O 3 , TiO 2 , and ZrO 2 have been demonstrated to be active and selective
for the APR of methanol and ethylene glycol to produce hydrogen. Catalysts based
on Pd have shown similar activity compared to Pt analogs. Ru, Rh, and Ni, however,
showed lower activity for hydrogen.
163
6.5 KinetiCs and Catalysis OF aPr PrOCess
As shown in Figure 6.1, the APR process can be tailored toward the four distinct kinetic
steps depending on the desired product [1–6,11–33] (Tanksale et al., 2008, pers. comm.).
APR can produce hydrogen, syngas, alkanes, or monofunctional groups depending on
the catalyst and support system, promoters, and other operating conditions.
The original purpose of APR was to generate either hydrogen or alkanes by an
APR of sugar, other oxygenated compounds, and polyols (with a ratio of 1:1). The
kinetics of APR depends on the temperature, the pressure, the nature of the catalyst
and its support, the presence of promoters, the pH of the slurry, the acidity of catalyst
active sites, and the nature of the feedstock. Here, we briefly examine the effects
of these operating variables on the APR process. While the literature has shown
numerous ways to generate hydrogen from biomass under high-temperature conditions, APR is unique in that it is the only process that can be carried out in liquid
water [5,34–57]. While APR can only be used for selective feedstock without their
pretreatments, it is faster than anaerobic digestion process for generating hydrogen
from cellulosic waste [5,34–57].
6.5.1 eFFeCTS oF TemPerATure, CArBon numBer, And PreSSure
As shown in Figure 6.1, APR provides multiple options of reaction paths depending
on the operating conditions. Figure 6.3 illustrates that hydrogen selectivity decreases
with an increase in carbon number of oxygenated compounds and an increase in
temperature. The temperature effect shown in this figure is valid for all oxygenated
feedstock. The literature has shown that compounds such as furanone and acetic
acid are not amenable to the production of hydrogen by APR [1–7,16]. The hydrogen
selectivity depends on the nature of the bond breaking in oxygenated compounds;
the breakage of C–C bond favors the hydrogen formation and the breakage of C–O
bond favors the formation of alkanes. Following the preferred pathway is the key to
the hydrogen formation. Dumesic et al. [1–6,16] also showed that an increase in pressure reduced the hydrogen selectivity. For example, for the reaction of 5 wt% sorbitol
over Pt–SiAl at 498 K, the hydrogen selectivity at 25.8 atm pressure was 21, whereas
the same selectivity at a pressure between 33.1 and 52.1 atm was <2 [1–6,16].
6.5.2 eFFeCTS oF CATAlySTS And SuPPorTS
As shown in Table 6.1, the major primary feedstock for APR are glucose (and sorbitol), alcohols, ethylene glycol, and glycerol. For all of these feedstock, Dumesic et al.
[1–6,16], among others [5,15,17–26,27–57] (Tanksale et al., 2008, pers. comm.), have
clearly shown that the nature of metal and support has an important influence on the
reaction paths and the rates of reactions in the APR process. The product selectivity
can be tuned depending on the metal and support. For example, Pt-black and Pt supported on Al 2 O 3 , TiO 2 , and ZrO 2 have been demonstrated to be active and selective
for the APR of methanol and ethylene glycol to produce hydrogen. Catalysts based
on Pd have shown similar activity compared to Pt analogs. Ru, Rh, and Ni, however,
showed lower activity for hydrogen.