9 research outputs found
Mechanisms of the Deactivation of SAPO-34 Materials with Different Crystal Sizes Applied as MTO Catalysts
SAPO-34 materials with comparable
Brønsted acid site density
but different crystal sizes were applied as methanol-to-olefin (MTO)
catalysts to elucidate the effect of the crystal size on their deactivation
behaviors. <sup>13</sup>C HPDEC MAS NMR, FTIR, and UV/vis spectroscopy
were employed to monitor the formation and nature of organic deposits,
and the densities of accessible Brønsted acid sites and active
hydrocarbon-pool species were studied as a function of time-on-stream
(TOS) by <sup>1</sup>H MAS NMR spectroscopy. The above-mentioned spectroscopic
methods gave a very complex picture of the deactivation mechanism
consisting of a number of different steps. The most important of these
steps is the formation of alkyl aromatics with large alkyl chains
improving at first the olefin selectivity, but hindering the reactant
diffusion after longer TOS. The hindered reactant diffusion leads
to a surplus of retarded olefinic reaction products in the SAPO-34
pores accompanied by their oligomerization and the formation of polycyclic
aromatics. Finally, these polycyclic aromatics are responsible for
a total blocking of the SAPO-34 pores, making all catalytically active
sites inside the pores nonaccessible for further reactants
Improved Postsynthesis Strategy to Sn-Beta Zeolites as Lewis Acid Catalysts for the Ring-Opening Hydration of Epoxides
Nanocrystalline
Sn-Beta zeolites have been successfully prepared
via an improved two-step postsynthesis strategy, which consists of
creating vacant T sites with associated silanol groups by dealumination
of parent H-Beta and subsequent dry impregnation of the resulting
Si-Beta with organometallic dimethyltin dichloride. Characterization
results from UV–vis, XPS, Raman, and <sup>119</sup>Sn solid-state
MAS NMR reveal that most Sn species have been successfully incorporated
into the framework of Beta zeolite through the postsynthesis process
and exist as isolated tetrahedral SnÂ(IV) in open arrangement. The
creation of strong Lewis acid sites upon Sn incorporation is confirmed
by FTIR spectroscopy with pyridine adsorption. The Sn-Beta Lewis acid
catalysts are applied in the ring-opening hydration of epoxides to
the corresponding 1,2-diols under near ambient and solvent-free conditions,
and remarkable activity can be obtained. The impacts of Lewis acidity,
preparation parameters, and reaction conditions on the catalytic performance
of Sn-Beta zeolites are discussed in detail
Investigation of Selective Catalytic Reduction of N<sub>2</sub>O by NH<sub>3</sub> over an Fe–Mordenite Catalyst: Reaction Mechanism and O<sub>2</sub> Effect
We systematically investigated the reaction mechanism
and effect
of O<sub>2</sub> on N<sub>2</sub>O reduction by NH<sub>3</sub> over
an Fe–Mordenite (MOR) catalyst. O<sub>2</sub> has no inhibitory
effect on N<sub>2</sub>O reduction, and NH<sub>3</sub> selective catalytic
reduction (SCR) of N<sub>2</sub>O is superior to NH<sub>3</sub> oxidation
by O<sub>2</sub>. We found that the mechanism of NH<sub>3</sub> SCR
of N<sub>2</sub>O involves the redox cycle of FeÂ(III)–OH sites,
with FeÂ(III)–OH reduction by NH<sub>3</sub> as the first and
rate-determining step. Then N<sub>2</sub>O is activated at the reduced
FeÂ(II)–OH sites into NO/N or N<sub>2</sub>/O, reoxidizing the
FeÂ(II)–OH into FeÂ(III)–OH sites. Next, the NO formed
in situ reacts with adsorbed NH<sub>2</sub> to form NH<sub>2</sub>NO, which further decomposes to N<sub>2</sub> and water. In addition,
some NO may join with O to form NO<sub>2</sub>, which reacts with
NH<sub>4</sub><sup>+</sup> to produce NH<sub>4</sub>NO<sub>2</sub> and further decomposes to N<sub>2</sub> and water. It is possible
that under the steady state, N–NO breaking accounts for two-thirds
of N<sub>2</sub>O splitting. The formation of NO intermediates plays
a crucial role in this reaction. The structural arrangement of MOR
zeolites and the high content of Fe ions provides two proximal Fe
ions, that is, FeÂ(III)···FeÂ(III) pairs, as the active
sites for this N–NO breaking, resulting in the high activity
of Fe–MOR
Mechanistic Insights into One-Step Catalytic Conversion of Ethanol to Butadiene over Bifunctional Zn–Y/Beta Zeolite
Bifunctional Zn–Y/Beta
catalyst was applied in the reaction
mechanism study of the ethanol to butadiene conversion to clarify
the roles of Zn and Y functional sites in each individual reaction
step. According to the results of several complementary methods, i.e.,
ethanol temperature-programmed desorption (TPD), temperature-programmed
surface reaction (TPSR), and in situ diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS), the reaction network consisting
of several key steps, i.e., ethanol dehydrogenation, acetaldehyde
aldol condensation, and crotonaldehyde reduction, was elucidated.
An enolization mechanism was verified to involve in the coupling step.
During this reaction, the Lewis acidic Zn and Y species in [Si]ÂBeta
zeolite were both active in the ethanol dehydrogenation, aldol condensation,
and Meerwein–Ponndorf–Verley reduction. In this cycle,
Zn species exhibited the higher dehydrogenation activity but lower
coupling activity than that of Y species. Through the combination
of the two species in one catalyst, i.e., Zn–Y/Beta, the synergistic
effect of the bifunctional sites could be achieved. Our study provides
mechanistic insights into the cascade transformation of ethanol to
butadiene and the fundamental guidelines for the rational design of
eligible catalysts for the reaction
Zeolite Structural Confinement Effects Enhance One-Pot Catalytic Conversion of Ethanol to Butadiene
The one-pot conversion
of ethanol to butadiene is a promising route
for butadiene production; however, simultaneous attainment of high
butadiene productivity and high butadiene selectivity is challenging.
Here, zeolite-confined bicomponent Zn–Y clusters were constructed
and applied as robust catalysts for ethanol-to-butadiene conversion
with a state-of-the-art butadiene productivity of 2.33 g<sub>BD</sub>/g<sub>cat</sub>/h and butadiene selectivity of ∼63%. Structural
confinement effects are responsible for the enhanced butadiene production
efficiency via a multiple-step cascade reaction
Selective Catalytic Hydrogenolysis of Carbon–Carbon σ Bonds in Primary Aliphatic Alcohols over Supported Metals
The
selective scission of chemical bonds is always of great significance
in organic chemistry. The cleavage of strong carbon–carbon
σ bonds in the unstrained systems remains challenging. Here,
we report the selective hydrogenolysis of carbon–carbon σ
bonds in primary aliphatic alcohols catalyzed by supported metals
under relatively mild conditions. In the case of 1-hexadecanol hydrogenolysis
over Ru/TiO<sub>2</sub> as a model reaction system, the selective
scission of carbon–carbon bonds over carbon–oxygen bonds
is observed, resulting in <i>n</i>-pentadecane as the dominant
product with a small quantity of <i>n</i>-hexadecane. Theoretical
calculations reveal that the 1-hexadecanol hydrogenolysis on flat
Ru (0001) undergoes two parallel pathways: i.e. carbon–carbon
bond scission to produce <i>n</i>-pentadecane and carbon–oxygen
bond scission to produce <i>n</i>-hexadecane. The removal
of adsorbed CO on a flat Ru (0001) surface is a crucial step for the
1-hexadecanol hydrogenolysis. It contributes to the largest energy
barrier in <i>n</i>-pentadecane production and also retards
the rate for <i>n</i>-hexadecane production by covering
the active Ru (0001) surface. The knowledge presented in this work
has significance not just for a fundamental understanding of strong
carbon–carbon σ bond scission but also for practical
biomass conversion to fuels and chemical feedstocks
Understanding the Early Stages of the Methanol-to-Olefin Conversion on H‑SAPO-34
Little
is known on the early stages of the methanol-to-olefin (MTO)
conversion over H-SAPO-34, before the steady-state with highly active
polymethylÂbenzenium cations as most important intermediates
is reached. In this work, the formation and evolution of carbenium
ions during the early stages of the MTO conversion on a H-SAPO-34
model catalyst were clarified via <sup>1</sup>H MAS NMR and <sup>13</sup>C MAS NMR. Several initial species (i.e., three-ring compounds, dienes,
polymethylÂcyclopentenyl, and polymethylÂcyclohexenyl cations)
were, for the first time, directly verified during the MTO conversion.
Their detailed evolution network was established from theoretical
calculations. On the basis of these results, an olefin-based catalytic
cycle is proposed to be the primary reaction pathway during the early
stages of the MTO reaction over H-SAPO-34. After that, an aromatic-based
cycle may be involved in the MTO conversion for long times on stream
Lewis Acid Catalysis Confined in Zeolite Cages as a Strategy for Sustainable Heterogeneous Hydration of Epoxides
We
report a heterogeneous catalysis strategy to the sustainable
hydration of epoxides by designing robust Lewis acid catalysts confined
in zeolite cages as natural shape-selective nanoreactors. In the case
of ethylene oxide hydration, Sn-H-SSZ-13 zeolite exhibits remarkable
catalytic performance, with an ethylene oxide conversion above 99%
and a monoethylene glycol selectivity above 99%, at approaching stoichiometric
water/ethylene oxide ratios and near-ambient reaction temperatures.
It is revealed by theoretical studies that partially hydroxylated
Sn species are the preferred Lewis acid sites for the hydration of
ethylene oxide. The concept of Lewis acid catalysis confined in zeolite
cages may be applied in the future in the chemical industry to develop
energy-saving and environmentally benign processes
Effect of <i>n</i>‑Butanol Cofeeding on the Methanol to Aromatics Conversion over Ga-Modified Nano H‑ZSM‑5 and Its Mechanistic Interpretation
Ga-modified
nano H-ZSM-5 zeolites with different Ga contents were
prepared and applied as methanol-to-aromatics (MTA) catalysts. The
Ga introduction can strongly increase the selectivity to aromatics
but also decrease the catalyst lifetime simultaneously. Upon the cofeeding
of <i>n</i>-butanol with methanol, a significant prolongation
of the catalyst lifetime from 18 to ca. 50 h can be achieved. According
to several spectroscopic results, e.g., TGA, GC–MS, in situ
UV/vis, and solid-state MAS NMR spectroscopy, the addition
of <i>n</i>-butanol during the MTA conversion shows no impact
on the deactivation mechanism but can influence the dual-cycle mechanism.
Namely, <i>n</i>-butanol preferentially adsorbs on Brønsted
acid sites over methanol, followed by dehydration into <i>n</i>-butene. The formed <i>n</i>-butene can directly participate
in the olefin-based cycle and, therefore, significantly alter the
proportions of the dual-cycle mechanism. These results provide mechanistic
insights into the roles of <i>n</i>-butanol cofeeding in
the MTA conversion and exemplify a simple but efficient strategy to
prolonged the catalyst lifetime, which is crucial to the industrial
application