Catalytic performance of Mo/HZSM5 and Ru-Mo/HZSM5 catalysts prepared by vaporization-deposition of molybdenum trioxide and impregnation with ammonium heptamolybdate was analyzed in terms of catalyst activity and selectivity, nitrogen physisorption analyses, temperature-programmed oxidation of carbonaceous residues, and temperature-programmed reduction. Vaporization-deposition rendered the catalyst more selective to ethylene and coke than the catalyst prepared by impregnation. This result was assigned to lower interaction of molybdenum carbide with the zeolite acidic sites.
Catalytic methane dehydroaromatization is a potential alternative to methane steam reforming to produce hydrogen. It is also an alternative for production of valuable aromatics from oil-associated gases that are usually flared. Since its first report in 1993 [
A number of different preparation methods and incorporation of different promoters have been reported [
The work reported here shows a comparison of the catalytic performance of Mo/HZSM5 and Ru-Mo/HZSM5 catalysts prepared by impregnation and vaporization-deposition and the results are interpreted in terms of catalyst characterization analyses.
Four catalyst samples were prepared and labeled as follows: MV and RMV for Mo/HZSM5 and Ru-Mo/HZSM5 samples prepared by vaporization-deposition of MoO3 and MI and RMI for Mo/HZSM5 and Ru-Mo/HZSM5 samples prepared by impregnation with (NH4)6Mo7O24·4H2O. Materials and reagents used were HZSM5 zeolite (Si/Al = 15, CBV3020E, Zeolyst International, PA), MoO3 (99.95%, Alfa Aesar), (NH4)6Mo7O24·4H2O (99.98%, Sigma-Aldrich), and RuCl3 (99.98%, Sigma-Aldrich).
For the two samples prepared by vaporization-deposition of MoO3, dried MoO3, measured to contain a final Mo load of 4.0 wt%, was added to a small amount of nanopure water in a beaker. For the sample containing Ru (RMV), dried RuCl3, measured to contain a final Ru load of 0.5 wt%, was also added to the beaker. Next, the content of the beaker was sonicated until complete dispersion. After that, dried HZSM5 zeolite was added to the beaker to obtain a paste. This mixture was dried overnight at room temperature and then for 2 hours at 100°C. The solid obtained was crushed and calcined under flowing air (UHP, Norco Inc., ID) in a straight quartz tube reactor placed in a split furnace (Mellen). The temperature was increased from room temperature to 700°C in 2 hours and maintained at 700°C for 30 minutes. Finally, the powder obtained was crushed, pressed, and sieved to obtain particles in the 40–60 mesh range.
For preparing MI and RMI, (NH4)6Mo7O24·4H2O was dissolved (along with dried RuCl3 for the case of RMI) in a minimal amount of water and added to the dried zeolite. Next, the impregnated material was dried, calcined, crushed, and sieved as for the case of MV and RMV.
For comparison, a blank sample was prepared using only zeolite that was mixed with similar amount of water as used for the actual catalyst samples, dried, and calcined at 700°C for 30 minutes.
Reaction studies were performed in a U-tube quartz reactor heated by an electric furnace (Carbolite) as shown in Figure
Outline of the experimental setup (not to scale). (1) Methane, (2) nitrogen, (3) on-off valve, (4) pressure relief valve, (5) mass-flow controller, (6) one-way check valve, (7) switching valve, (8) furnace, (9) reactor, (10) inert material as preheating bed, (11) catalyst, (12) thermocouple, (13) gas chromatograph, and (14) vent to laboratory hood.
After 6 hours of reaction time, the flow of CH4 was stopped. The catalyst bed was maintained at reaction temperature for an additional hour under flowing nitrogen and then allowed to cool down to room temperature. The spent catalyst sample was recovered for further analyses.
Nitrogen physisorption isotherms at liquid nitrogen temperatures were measured on a Quantachrome Autosorb 1-C instrument. Samples were outgassed at 105°C for four hours prior to the analysis. Isotherms points in the P/P0 = 0.05–0.10 range were used to calculate the BET. surface area (SA) [
Temperature-programmed reduction (TPR) analyses were performed on a Pyris/Diamond TG/DTA thermogravimetric analyzer under 100 sccm of flowing mixture of 5% v/v hydrogen in helium (certified, Norco Inc., Idaho). About 12.5 mg of sample was loaded in the thermogravimetric analyzer pan, dried at 100°C for 30 min, and heated at 15°C/min from 100 to 900°C. The derivative of the weight change with time as a function of sample temperature was used to report the TPR profiles. The curves are displayed offset for clarity.
Temperature-programmed oxidation (TPO) of coke deposited on spent catalyst samples was performed in a similar manner as the TPR analyses. The only two differences were the use of flowing air (hydrocarbon free, Norco Inc., Idaho) instead of the hydrogen/helium mixture and heating to 700°C instead of 900°C. Weight differences between 100 and 700°C were measured directly and corrected for the weight increase expected for oxidation of Mo and Ru in the samples.
Reaction testing experiments showed formation of hydrogen, ethylene, propylene, benzene, toluene, and xylenes. A summary of hydrocarbon apparent production rates at 3 h and 6 h TOS is shown in Figure
Summary of hydrocarbon instantaneous apparent production rates. Catalysts, reaction temperatures, and times on stream as indicated.
Nitrogen physisorption analyses are shown in Table
Nitrogen physisorption results.
Samples | Fresh | Spent at 600°C | Spent at 700°C | |||
---|---|---|---|---|---|---|
SA [m2/g] | MPV [cc/g] | SA [m2/g] | MPV [cc/g] | SA [m2/g] | MPV [cc/g] | |
Original zeolite | 403 | 0.12 | ||||
MV | 197 | 0.05 | 203 | 0.05 | 199 | 0.05 |
MI | 327 | 0.10 | 290 | 0.08 | 275 | 0.08 |
RMV | 175 | 0.05 | 194 | 0.05 | 161 | 0.04 |
RMI | 221 | 0.06 | 252 | 0.07 | 227 | 0.06 |
The presence of carbonaceous residues on the spent samples was studied by TPO using a thermogravimetric analyzer. The curves obtained are shown in Figure
Temperature-programmed oxidation of carbonaceous residues on (a) MV 600°C, (b) RMV 600°C, (c) MI 600°C, (d) RMI 600°C, (e) MV 700°C, (f) RMV 700°C, (g) MI 700°C, and (h) RMI 700°C.
From TPO weight differences between 100 and 700°C, the total amount of carbonaceous residues may be estimated. However, TPO conditions were expected to oxidize not only carbonaceous residues but also the carbided molybdenum usually proposed as the active Mo species [
Carbonaceous residues measured by TPO.
Sample | MV | MI | RMV | RMI | ||||
---|---|---|---|---|---|---|---|---|
Spent at (°C) | 600 | 700 | 600 | 700 | 600 | 700 | 600 | 700 |
Carbon deposits [wt%] | 2.5 | 3.3 | 3.7 | 5.5 | 2.7 | 4.1 | 3.8 | 6.0 |
Table
The experimental protocol applied in this work did not allow for continuous measurement of carbonaceous residue deposition. Thus, Figure
Two views of integrated catalyst selectivity to hydrocarbons leaving the catalyst (white) and carbonaceous residues remaining on the catalyst (black).
Temperature-programmed reduction curves are shown in Figure
Weight loss within the 300–900°C range during temperature-programmed reduction.
Sample | MV | RMV | MI | RMI |
---|---|---|---|---|
Weight loss (wt%) | 1.2 | 1.4 | 0.9 | 1.3 |
Temperature-programmed reduction of fresh (a) blank zeolite sample, (b) MV, (c) RMV, (d) MI, and (e) RMI samples.
The reduction of supported MoO3 species has been proposed to occur in two steps, that is, MoO3 → MoO2 and MoO2 → Mo [
Ding and coworkers [
Methane dehydroaromatization catalyst selectivity was studied on Mo/HZSM5 and Ru-Mo/HZSM5 catalyst samples prepared by two different methods: vaporization-deposition of MoO3 and impregnation with ammonium heptamolybdate. Vaporization-deposition rendered the catalyst more selective to ethylene and coke than the catalyst prepared by impregnation. This result is assigned to the formation of larger, more difficult to migrate into the zeolite channels, molybdenum oxide clusters when the vaporization-deposition method was applied. An important fraction of these clusters may have remained on the external surface of the zeolite and away from internal acid sites. Once those clusters were carbided, they became the active sites for methane activation and production of intermediate ethylene-like fragments but were located too far from the internal zeolite acid sites for subsequent reactions. The eventual result was higher selectivity to coke and ethylene and less selectivity to desirable aromatic products. On the other hand, the ammonium heptamolybdate precursor likely produced smaller MoO3 clusters which migrated more easily into the zeolite channels during sample calcination. In this case, the ethylene-like fragments had a higher opportunity to react and produce desirable aromatics. Addition of ruthenium to the catalyst favored molybdenum dispersion and increased total product yields in both cases.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the INL Laboratory-Directed Research and Development Program under DOE/NE Idaho Operations Office Contract DE-AC07-05ID14517. This paper has been authored by Battelle Energy Alliance, LLC, under Contract no. DE-AC07-05ID14517, with the U.S. Department of Energy. The United States Government and the publisher, by accepting the paper for publication, acknowledge that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this paper, or allow others to do so, for United States Government purposes.