(617m) Oxidative Coupling of Methane over Zeolite Supported Na-Mn-W

Oxidative Coupling of Methane over Zeolite Supported
Na-Mn-W

Naseem S. Hayek and Oz M. Gazit*

Chemical Engineering, Israel Institute of
Technology-Technion, Haifa, Israel  ozg@technion.ac.il

The proven reserves of natural gas
are rapidly increasing.¹ Currently,
it is a highly underutilized hydrocarbon source for chemicals and liquid fuels
synthesis, except for steam reforming.² One
possible route is the direct conversion of methane, the main component of
natural gas, to Ethane (C₂H₆) and Ethylene (C₂H₄)
via oxidative coupling (Eq. 1).

            Oxidative coupling of
methane (OCM) has received much attention since the fundamental
work of Hinsen et al.³
and Keller et al.⁴ This
is because the prospect of direct conversion of methane to C₂
molecules is highly attractive, being a vital building block in the industry
with an expected increase demand. Moreover, OCM is an exothermic reaction and
therefore not limited by thermodynamics.

            Since 1981, hundreds of
materials have been tested as catalysts.^5,6 However, until now only
a few catalysts have shown the potential for being economically viable for
incorporation in a large scale process. The main challenge for the catalyst is the
need to be highly selective for C₂ products, overcoming total
oxidation of CH_4, which is by far the thermodynamically favored
reaction route under these conditions (Eq. 2).

(1)                                 

(2)                                                

             Mn-Na₂WO₄/SiO₂
catalyst has been found to have remarkable stability at the required reaction
conditions.⁷
Moreover, this catalyst obtained performance of ~33% CH₄ conversion
at 80% C₂ selectivity giving a C₂ yield of ~25%.⁸
However, the performance needs to be improved in order to achieve above 30%
yield, the indicated threshold for practical applications.⁹

            Many
variations in the composition of the metals based on this catalyst platform
have been tested in an effort to understand structure-function relationship.^10-14
What is known suggests a strong synergetic effect for all three components (Na,
Mn, W) on silica, where the absence of either one of them leads to inferior
catalytic performance.⁸
For example, sodium induces the transition of amorphous silica to
α-cristobalite. This is considered to be crucial for obtaining above
mentioned catalytic performance, despite the dramatic 2 orders of magnitude loss
in surface area. This phase transition is thought to improve the dispersion of
the active site, however, the dispersion of which of the components or both remains
an open question.

            In this study we
investigate the fundamental aspects of active site dispersion for the Mn/Na/W
based catalyst, while avoiding the phase transition of silica. This is done
using the post-synthetic modification of zeolites (i.e. dealumination) to form
open T sites, in which the active metals can be atomically dispersed regardless
of phase transition.^15-18 Furthermore,
the use of zeolites supports provides high surface area, a crystalline
structure and high thermal stability. As control, we use amorphous silica-alumina
to parallel the various Si/Al ratios used as zeolites.

            Preliminary results
indicate that contrary to native zeolite, dealuminated zeolite undergoes phase
transition to 𝛼-cristoballite, upon impregnation
with Na₂WO₄ and calcination at 850ᵒC,
analogous to silica supported catalyst. This is accompanied by a dramatic loss
in surface area from ~680 m²/g to ~2.5 m²/g. Alumina is
found to preserve its surface area and crystalline structure after the same
impregnation and calcination progress. Amorphous silica-alumina is found to keeps
its amorphous structure at low Si/Al ratios, while it transforms to 𝛼-cristoballite
at high Si/Al ratios.  Manganese atoms have been post-synthetically dispersed
in the open T sites following the dealumination process. The Mn-incorporated
zeolite was characterized by FTIR, XRD, XPS, UV-vis, Chemisorption and
Physisorption. Dispersion of tungsten and catalytic evaluation of the different
catalysts in a tubular fixed-bed reactor are in progress.

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