Identifying zeolite catalysts that
can simultaneously optimize p-xylene selectivity
and feed utilization is critical to
toluene alkylation with methanol (TAM). Here, we show that zeolite
MCM-22 (MWW) has an exceptional catalyst lifetime in the TAM reaction
at high operating pressure, conversion, and selectivity. We systematically
probe the catalytic behavior of active sites in distinct topological
features of MCM-22, revealing that high p-xylene
yield and catalyst stability are predominantly attributed to sinusoidal
channels and supercages, respectively. Using a combination of catalyst
design and testing, density functional theory, and molecular dynamics
simulations, we propose a spatiotemporal coke coupling phenomenon
to explain a multistage p-xylene selectivity profile
wherein the formation of light coke in supercages initiates the deactivation
of unselective external surface sites. Our findings indicate that
the specific nature of coke is critical to catalyst performance. Moreover,
they provide unprecedented insight into the synchronous roles of distinct
topological features giving rise to the exceptional stability and
selectivity of MCM-22 in the TAM reaction.
Crystallization in media comprised of amorphous precursors is becoming a more common phenomenon for numerous synthetic, biological, and natural materials that grow by a combination of classical and nonclassical pathways....
The synthesis of zeolites with nano‐sized dimensions is often limited to a narrow design space that conventionally relies upon the design of organics to direct hierarchical materials. Here, it is demonstrated that the addition of an inorganic modifier, germanium oxide (GeO2), to a zeolite growth mixture directs the formation of crystals with ultrasmall dimensions. This effect is observed for zeolites ZSM‐11 and ZSM‐5 over a range of synthesis conditions wherein the role of GeO2 in zeolite crystallization deviates from its typical function as a heteroatom. Notably, the final products contain trace amounts of Ge, which indicates the inorganic modifier does not compete for sites in the zeolite framework based on its formation of a discrete phase that enables GeO2 recovery. Catalytic tests using the methanol‐to‐hydrocarbons reaction reveal significant enhancement in the performance of zeolite catalysts prepared with GeO2 compared to reported examples of nano‐sized zeolites. These findings highlight a potentially generalizable and commercially viable synthesis method to reduce mass‐transport limitations in zeolites for diverse applications.
The hydrothermal stability and catalytic activity of
zeolite Y
(faujasite, FAU) is highly dependent on its composition. High silicon
content is often desirable for catalytic applications; however, direct
synthesis of faujasite with high silicon content (Si/Al > 2.5)
is
nontrivial. Here, we present an organic-free synthesis of FAU-type
zeolite with Si/Al = 3.4 using zinc oxide as a modifier. A combination
of spectroscopy and microscopy techniques confirms that ZnO is well-distributed
within zeolite pores as extra-framework species, and the nature of
these species differs from bulk ZnO and framework zinc in Zn-FAU crystals.
We demonstrate that the increased Si/Al ratio leads to improved hydrothermal
stability, while catalytic cracking of 1-hexene and cumene show that
ZnO-FAU exhibits a significantly longer lifetime compared to in-house
and commercial zeolite Y. Collectively, this study presents a facile
and efficient method to prepare more siliceous FAU with enhanced catalytic
performance.
There are few studies in zeolite catalysis literature that have developed structure-performance relationships for a particular reaction by considering a broad class of zeolite framework structures possessing identical pore size...
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