165
Aqueous-Phase Reforming and BioForming Process
catalysts were used in APR of polyols, and superior performance was shown in
terms of carbon conversion, hydrogen yield, selectivity, and hydrogen production
rate compared to Pt catalysts supported on activated carbon or two-dimensional
CMK-3 (ordered mesoporous carbon synthesized by silica hard template).
The study by Davda et al. [2,16,41,58,59] also indicated that the best support for
Pt was Al 2 O 3 for hydrogen production and the effect of support on reforming activity
and selectivity is greater than that of metal dispersion. They also analyzed bimetallic
catalysts and concluded that Ni–Sn catalysts show potential for APR. The selectivity
for hydrogen and alkanes for different oxygenates at 225°C and 265°C using Pt/Al 2 O 3
catalyst is illustrated in Figure 6.3 [16].
6.5.3 eFFeCTS oF PromoTerS And ACidiTy oF liquid And SolidS
The addition of a promoter can also have some effect on the catalyst performance.
Re was found to be an effective promoter for Pt/C catalyst. The selectivity of Pt–Re/C
was found to be different from that of Pt/C. Hydrogen selectivity with promoter was
lower, although hydrogen productivity was higher. Following reduction, Pt–Re/C catalyst was significantly more active for APR of glycerol than Pt/C catalyst. The presence
of Re created surface acidity that favored a pathway of C–O bond breaking (dehydration), resulting in lower hydrogen and CO(CO 2 ) selectivity and higher alkanes
selectivity [1–6,16]. The literature [1–6,16] also showed that an addition of KOH (base)
affected APR selectivity of glycerol for 3%Pt3%Re/C catalyst.
The effects of liquid and solid acidities on carbon selectivity for sorbitol at
538 K and 57.6 bar with Pt/Al catalysts were also examined by Dumesic et al.
[1–6,16]. The results indicated that lower pH of both liquids and solids produce
higher carbon number alkanes. In general, an increase in acidity by either the use
of acid catalyst support (i.e., SiO 2 /Al 2 O 3 ) or the addition of the mineral acid such as
HCl to increased the feed alkanes selectivity due to the increased rate of dehydration and hydrogenation pathways compared to hydrogenolysis and reforming reactions. The nickel supported on SiO 2 or AI 2 O 3 was found to have low selectivity for
hydrogen and favored the formation of alkanes. However, an addition of an Sn promoter to Raney R–Ni-based catalysts enhanced the production of hydrogen from
sorbitol, glycerol, and ethylene glycol [1–6,16]. While the promoters and acidity
can be used to produce alkanes, some C–C bond needs to be broken to produce
hydrogen needed for the production of alkanes. For example, the hydrogenation
and complete deoxygenation of sorbitol results in the following set of reactions
[1–6,16]:
C H O + 6 H → C H + 6 H O
(6.5)
6 14 6
2
6 14
2
However, complete deoxygenation occurs as [1–6,16]
(6.6)
These reactions indicate the need for hydrogen for the production of alkanes.
C H O
C H
CO
H O
6 14 6
6 14
2
2
13 19
36 19
42 19
→
+
+
/
/
/