(211b) Characterization of the Active Site and Mechanism for 1,6-Hexanediol Production from Tetrahydrofuran-Dimethanol over Pt-Based Catalysts

Characterization of
the Active Site and Mechanism for 1,6-Hexanediol Production from
Tetrahydrofuran-Dimethanol over Pt-Based Catalysts

Terminal diols such as 1,4-butanediol
(1,4-BDO) and 1,6-hexanediol (1,6-HDO) are important building-block chemicals and
find application in the synthesis of specialty chemicals and a variety of
polymers, primarily in polyesters and polyurethanes, and polyamides such as
nylon-6,6.^1,2 Currently, 1,6-HDO is produced industrially from cylohexanone/cyclohexanol (KA
oil) by oxidation with nitric acid to form adipic
acid, followed by hydrogenation of the dimethoxy ester to yield 1,6-HDO. The
attractiveness of the overall process is tempered by low conversions (4-8% for
the oxidation of KA oil), difficult separations, the use of non-renewable
fossil feedstocks, and emission of greenhouse gases, namely N₂O.³
Various attempts to synthesize 1,6-HDO, as well as other alpha,omega-diols, from biomass resources have been reported.
Using supported catalysts that contain precious metals with a reducible metal
oxide such as Rh-ReO_x,^4-9 and Ir-ReO_x,^10,11
tetrahydropyran-2-methanol (THP2M), a biomass-derived feedstock,¹²
can be converted into 1,6-HDO with high selectivity. However, the high cost of
the catalyst, in combination with a low productivity, limits the industrial applicability
of those systems.¹³ More recently, it has been shown that 1,6-HDO
can be produced from THP2M without the use of precious metals. However, 1,6-HDO
selectivity only reached ~40%.¹⁴ Therefore, this route is not likely
to be industrially applicable soon either. 

            In an effort to make the route to 1,6-HDO from biomass more
industrially viable, our group has developed a complete reaction network
producing 1,6-HDO from woody biomass.¹⁵ The early steps in this
process convert biomass to tetrahydrofuran-dimethanol
(THFDM) with high selectivity, and will likely be discussed elsewhere at this
conference. In this study, we focus on the final steps to 1,6-HDO from THFDM,
shown in Scheme 1.

Scheme 1.
Reaction sequence to 1,6-hexanediol (1,6-HDO) from tetrahydrofuran-dimethanol (THFDM)

We have developed a Pt-based
catalyst that performs each step of the above reaction sequence with a total
1,6-HDO selectivity >90%. In comparison to the state-of-the-art, a
previously described Rh-ReO_x/C catalyst is
not active for this reaction. Moreover, while Rh-ReO_x/C
is active for 1,6-HDO production from THP2M with selectivity >90%, the
reaction rate is an order of magnitude lower than the total production rate of
1,6-HDO from THFDM over our Pt-based catalysts. Our catalyst, therefore,
produces 1,6-HDO from a further upstream product, and at a much higher rate
than the state-of-the-art Rh-ReO_x/C
system.

            In situ characterization of this
catalyst has revealed important information with regards to the active site and
mechanism for 1,6-HDO production over our Pt-based catalysts. Our primary
findings are that the oxidation state and acidity of the oxophilic
additive (Mo, V, W, Re, for example) are crucial for this reaction. All
catalysts are reduced with H₂ prior to reaction, so the same
conditions were used for characterization. We studied these catalysts with
pyridine FTIR spectroscopy to obtain the Bronsted/Lewis
acid ratio in each of our Pt-based catalysts, and compare that to reactivity
data we had already compiled for the reaction of interest. As we observed that Bronsted acidity is correlated to the turnover frequency.
Moreover, we found that this ratio increases in all catalysts during
pre-reduction, a necessary step to achieve any reactivity. This data seems to
indicate that the Bronsted acid site necessary for
this reaction to take place, which are also confirmed by XPS and Raman characterizations.
Furthermore, we are able to alter and characterize the distance between Pt
nanoparticles and the oxophilic additive by changing
the catalyst synthesis technique. Based on these studies we proposed a different
mechanism to those which have been proposed previously, which will be discussed
in more detail during the conference.

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