We
show that cyclic ethers, such 2-methyltetrahydrofuran (2-MTHF),
can undergo dehydration to produce pentadienes over SiO2/Al2O3. The catalyst exhibited reversible deactivation
due to coke deposition, with the yield to pentadienes decreasing from
68% to 52% at 623 K over 58 h time on stream. A reaction network for
2-MTHF dehydration was proposed on the basis of the results of space
time studies. Pentadienes can be produced directly by a concerted
hydride shift and dehydration of carbenium intermediates or indirectly
through dehydration of pentanal and pentenol. Reaction kinetics studies
were performed at temperatures ranging from 573 to 653 K and 2-MTHF
partial pressures from 0.21 to 2.51 kPa. The apparent activation energy
barrier for 2-MTHF conversion to pentadienes and the reaction rate
order for ring opening were determined to be 74 kJ mol–1 and 0.24, respectively, indicating strong interaction between 2-MTHF
and the SiO2/Al2O3 surface. Other
solid acids such as γ-Al2O3, H-ZSM-5,
and Al-Sn-Beta were found to be active for 2-MTHF dehydration to pentadienes.
The rate of ring opening decreased in the order 2,5-dimethyltetrahydrofuran
> 2-MTHF > tetrahydropyran > tetrahydrofuran. Over SiO2/Al2O3, the dehydration of 2,5-dimethyltetrahydrofuran
resulted in 75% yield to hexadiene isomers.
We show that MoO(x)-promoted Au/SiO2 catalysts are active for reverse water-gas shift (RWGS) at 573 K. Results from reactivity measurements, CO FTIR studies, Raman spectroscopy, and X-ray absorption spectroscopy (XAS) indicate that the deposition of Mo onto Au nanoparticles occurs preferentially on under-coordinated Au sites, forming Au/MoO(x) interfacial sites active for reverse water-gas shift (RWGS). Au and AuMo sites are quantified from FTIR spectra of adsorbed CO collected at subambient temperatures (e.g., 150-270 K). Bands at 2111 and 2122 cm(-1) are attributed to CO adsorbed on under-coordinated Au(0) and Au(δ+) species, respectively. Clausius-Clapeyron analysis of FTIR data yields a heat of CO adsorption (ΔH(ads)) of -31 kJ mol(-1) for Au(0) and -64 kJ mol(-1) for Au(δ+) at 33% surface coverage. Correlations of RWGS reactivity with changes in FTIR spectra for samples containing different amounts of Mo indicate that interfacial sites are an order of magnitude more active than Au sites for RWGS. Raman spectra of Mo/SiO2 show a feature at 975 cm(-1), attributed to a dioxo (O═)2Mo(-O-Si)2 species not observed in spectra of AuMo/SiO2 catalysts, indicating preferential deposition of Mo on Au. XAS results indicate that Mo is in a +6 oxidation state, and therefore Au and Mo exist as a metal-metal oxide combination. Catalyst calcination increases the quantity of under-coordinated Au sites, increasing RWGS activity. This strategy for catalyst synthesis and characterization enables quantification of Au active sites and interfacial sites, and this approach may be extended to describe reactivity changes observed in other reactions on supported gold catalysts.
Supported PtMo bimetallic catalysts
were prepared by controlled
surface reactions (CSR) and studied for water gas shift (WGS) at 543
K. Carbon and silica supports were used for the preparation of monometallic
Pt catalysts, and Mo was deposited onto these catalysts by reaction
with cycloheptatriene molybdenum tricarbonyl ((C7H8)Mo(CO)3). Catalysts were characterized by CO chemisorption,
inductively coupled plasma-atomic emission spectroscopy (ICP-AES),
STEM/EDS, and XAS analysis. We report that carbon-supported Pt nanoparticles
are saturated with Mo species at a Mo:Pt atomic ratio of 0.32. Molybdenum
has a strong promotional effect in these catalysts, increasing the
TOF by up to a factor of more than 4000. Silica-supported catalysts
were found to be more active, but the TOF promotional effect of Mo
was smaller than for the carbon-supported catalysts at 15. EDS analyses
and activity studies showed that the formation of bimetallic catalysts
was therefore more efficient using the carbon support. The active
sites for WGS are suggested to be at the interface between Pt atoms
and Mo moieties that are possibly in an oxidized form.
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