SBA-15 as a Support for Rh Catalysts
An ordered mesoporous silica preparation of the type SBA-15 with larger pore diameter, pore volume and higher hydrothermal stability has been used as a catalyst support for Rh catalyst as well. In this experiment Fe was used as a promoter for SBA-15-supported Mo catalyst [30]. The highest ethanol selectivity of 20.6% with 19.5% conversion of CO was reported with 5wt% Rh-2.5wt% Fe/SBA-15 catalyst [30].
Multiwalled Carbon Nanotubes (MWCNTs) as a Support for Rh Catalysts
Multiwalled carbon nanotubes (MWCNTs) have attracted interest as a catalyst support due to factors like high surface area and chemical and thermal stability. In this example, MWCNTs were compared with activated carbon as supports for the Co (4.5 and 6wt%) promoted K (9wt%) modified Rh-Mo catalysts (1.5wt% Rh and 15wt% Mo) [33]. These catalysts were extensively characterized in both oxide and sulfide phases, and a drastic fall in surface area over the activated carbon-supported catalysts was observed after impregnating
|
Catalysts3 |
Particle size d(nm)b |
T (K) |
P (MPa) |
GHSV flv1) |
CO Con% |
Selectivity |
|||||
|
C2+oxyc |
EtOH |
ch4 |
CHd |
co2 |
MeOH |
||||||
|
RhMnFe/OMC |
2.4 |
573 |
5.0 |
12,000 |
3.2 |
38.0 |
24.0 |
41.8 |
47.6 |
4.5 |
9.9 |
|
RhFe/MnOMC |
2.6 |
573 |
5.0 |
12,000 |
16.2 |
44.6 |
28.7 |
29.4 |
34.9 |
11.9 |
8.6 |
|
RhMn/FeOMC |
2.8 |
573 |
5.0 |
12,000 |
15.5 |
41.9 |
28.8 |
27.7 |
34.7 |
16.4 |
7.0 |
|
Rh/MnFeOMC |
2.5 |
573 |
5.0 |
12,000 |
25.5 |
46.3 |
34.5 |
30.7 |
38.5 |
11.5 |
3.7 |
|
Table 13.5 Comparison of some ethanol selectivities of RhMnFe/OMC catalysts prepared with different particle sizes [32]. |
|
a Fe loading on RhFe/MnOMC, Mn loading on RhMn/FeOMC and Mn, Fe loadings on ordered mesoporous carbons (OMCs) have been optimized |
b Assuming CO/Rh =1
o surface
cC2+oxy denotes oxygenates containing two and more carbon atoms such as ethanol, acetaldehyde, and acetic acid d CFI denotes all the hydrocarbons
with metal species. Diffraction peaks were observed in the X-ray diffraction (XRD) patterns of the sulfided alkali-modified trimetallic catalysts, due to the characteristic reflections of the K-Mo-S mixed phase. Furthermore, Surisetty et al. reported that activated carbon — supported trimetallic catalysts are less active and show relatively poor selectivity compared to the MWCNTs-supported catalyst, and metal dispersions were higher on the MWCNTs-supported catalysts. The MWCNTs-supported, alkali-promoted trimetallic catalyst with 4.5wt% Co showed the highest total alcohols yield of 0.244 g/ (g cat h), ethanol selectivity of 20.1%, and higher alcohols selectivity of 31.4% at 320°C and 8.28 MPa using a gas hourly space velocity (GHSV) of 3.6 m3 (STP)/(kg catalyst h). A maximum total alcohol yield of 0.261g/(g cat h) and a selectivity of 42.9% were obtained on the 4.5wt% Co-Rh-Mo-K/MWCNTs catalyst at a temperature of 330°C. Additionally, the total alcohol yield increased from 0.163 to
0. 256g/(g cat h) with increased pressure from 5.52 MPa to 9.65 MPa over the 4.5wt% Co-Rh-Mo-K/MWCNTs catalyst. Important results of this Surisetty and coworkers study on using MWCNTs supports for Rh-based catalysts are summarized in Table 13.6.
Rh Nanoparticles — MnO2 Mesoporous Silica Nanoparticle (MSN)
To date the highest C2 oxygenate selectivity has been achieved by the use of rhodium nanoparticles on a mesoporous silica nanoparticle (MSN)-type catalyst [42]. According to this 2012 report in ChemCatChem, well-defined and monodispersed rhodium nanoparticles as small as approximately 2 nm were encapsulated in situ and stabilized in a mesoporous silica nanoparticle (MSN) framework during the synthesis of the mesoporous catalyst material. As Huang and coworkers reported, both the activity and selectivity of MSN-encapsulated rhodium nanoparticles in CO hydrogenation could be improved by the addition of manganese oxide; the carbon selectivity for C2 oxygenates (including ethanol and acetaldehyde) was unprecedentedly high at 74.5% with a very small amount of methanol produced [42].