(25e) Biomass Conversion: Novel Analytical Methods and Techniques

Biomass derived fuels are becoming increasingly attractive in
the light of growing demand for oil as well as political instability and
economic tension. The wide abundance of biomass material, in particular lignin,
makes it a promising feedstock for diesel-range fuel production. The conversion
of lignin to liquid energy carriers requires novel catalytic approaches in
order to obtain a uniform pool of hydrocarbons.[1]
While lignin, a highly branched phenol-based polymer, contains much less oxygen
than cellulose, the high oxygen content in the lignin-derived molecules still
requires substantial amounts of hydrogen for complete reduction to hydrocarbons.[2] The
hydrogenation of lignin-derived bio-oils on metals, such as Pd
or Ni, leads to a range of C6-C9 cycloalkanols. The
alcohols must be dehydrated to olefins and then further alkylated and
hydrogenated to obtain diesel-range alkanes. While a direct, one pot,
conversion of lignin would be most advantageous, the dehydration step of the
reaction cascade still poses significant complications and is rate-limiting for
the reaction sequence.

In the past hydrodeoxygenation
(HDO) has been successfully performed on dual functional metal/acid catalysts even
in liquid water, which is the solvent of choice due to its ubiquitous presence.[3]
Also, it has been shown that phenol hydroalkylation
on solid acids, such as HBEA, can take place in aqueous environment thereby
providing a way to increase the bio-oil hydrodeoxygenation
product carbon numbers.[4]
This can be tentatively attributed to the unique environment of the zeolitic pores (often referred to as confinement and nest
effects[5])
leading to better stabilization of the sorbed and
transition states.[6]A fundamental study of
the active centers, reaction intermediates and reaction dynamics/kinetics is
critically needed to facilitate the development of catalysts for the practical
conversion of biomass to energy-carrying alkanes. Therefore, we introduce the
application of a combination of nuclear magnetic resonance (NMR) and extended
X-ray absorption fine structure (EXAFS) spectroscopies that enable us to monitor
the state of the reacting molecules and determine the catalyst active site structure.

We have recently reported an in-situ study of a cyclohexanol reaction (a
common cycloalkanol from phenol HDO) on HBEA in
liquid water at elevated temperatures using high-resolution magic angle
spinning (MAS) NMR spectroscopy.[7] In
particular, this method not only enabled us to discriminate between different mechanisms
of water elimination, but also to follow the distribution and catalyst
adsorption of products during the reaction. It was shown that the initial rates
of functional group migration in the reactants/products during cyclohexanol dehydration on HBEA in liquid water are better
explained by the E1 mechanism. This new insight can lead to improved reaction
control and process design for cycloalkanol dehydration
in the future. The developed technique is also suitable for study of other industrially
relevant reactions, e.g. alkylation and isomerization reactions.

In a separate effort the structure of the catalysts, HBEA150
and HBEA25, was analyzed in unprecedented detail. As a note in passing, Al³⁺
substitutes Si in the tetrahedral (T) positions of the zeolite framework
forming the catalytically active sites. HBEA has nine different T-sites. Previously,
it has been difficult to determine experimentally the distribution of Al³⁺
amongst the different T-sites in the zeolite.[8] We
report a quantitative analysis of the Al K-edge EXAFS that allowed us to
determine the Al³⁺ lattice distribution. The experimental EXAFS
spectra were interpreted by invoking a linear combination fit of the molecular
dynamics (MD) EXAFS derived for each T-site from the DFT modeled structure. It
has been determined that most of the Al³⁺ populating T-sites that
are part of one or more 4-member framework rings. In contrast, dramatically
different Al³⁺ distributions are found for the T-sites occupying only
5- and 6-member rings.

Using the method described above, we studied the integrity of
HBEA framework under hot liquid water conditions that are typical for HDO.
Commonly there are two mechanisms describing zeolite lattice degradation: acid
catalyzed dealumination, removal of Al³⁺ from
the T-sites and base-catalyzed hydrolysis of siloxane
bonds. It was determined that dealumination does not occur under high-temperature water conditions
up to the point where the zeolite undergoes complete lattice degradation. It was
also found that the Al³⁺ site retains its coordination and local structure, remaining identical
to that of the initial HBEA T-site. For the same samples, however, we observed
severe loss of crystallinity. These results suggest
that the desilication mechanism is primarily
responsible for the increasing number of framework defect sites, which form as
a function of time. We believe future work in the field of catalyst design in
particular in respect to framework Al³⁺ distribution will benefit greatly
from this capability.

In summary, a combination of
two novel approaches for both reaction and catalyst studies is presented and
their application is demonstrated. The new understanding of the dehydration
reaction step as part of the overall HDO cascade is an important advance toward
future industrial implementation of the bio-oil conversion to diesel-range
fuels. The quantitative analysis of Al³⁺ T-site distribution and the
zeolite framework stability in liquid water are highly relevant to refining. In
particular, the optimization of zeolite performance and increase of catalyst
life cycle will greatly reduce both environmental impact and refinery operation
costs to the industry.