(211f) The Effects of Metal-Acid Site Proximity on Bifunctional Isomerization of Alkanes and Deoxygenation of Alcohols

Hydroisomerization of alkanes occurs
through kinetic cascades mediated by alkene intermediates that require a
(de)hydrogenation metal function within diffusion distances of the
kinetically-relevant acid function. Hydrodeoxygenation of alkanols similarly
requires a hydrogenating metal function proximal to kinetically relevant acid
sites to scavenge dehydrated product alkenes and prevent catalyst deactivation
with no influence on rates. In practice, the location and distribution of metal
clusters with respect to acid sites—and thus their proximity—is seldom
well-characterized because metal precursors are deposited onto bound solid
acids that also contain inert metal-oxide binders during synthesis protocols.
The accepted intimacy criterion for bifunctional isomerization [1] requires that the metal and acid
functions reside at distances (~100 um for mesoporous solid acids) that prevent
concentration gradients of alkenes—derived from reactant alkanes in
isomerization and formed from oxygenates via dehydration for hydrodeoxygenation—within
acid domains. Indeed, for zeolite crystals with an extracrystalline
metal function present in the form of Pt/SiO₂, distances of ~150 nm
are sufficient to maintain constant alkene reactant concentration within acid
domains, as shown by isomerization rates that are unaffected by the size or intracrystalline site density of zeolite crystals. We find,
however, that as metal-acid distances approach the nanometer scale via
introduction of the metal function as intracrystalline
clusters using exchange methods, isomerization rates are markedly higher. These
remarkable yet previously unexplained effects of site proximity and their
mechanistic underpinnings are addressed in this study. Turnover rates (per H⁺)
(i) for n-heptane isomerization on Pt-exchanged MFI
and on physical mixtures of Pt/SiO₂ with MFI (of varying site
density ([H⁺]) and crystal size) and (ii) for 4-methylcyclohexanol,
2-butanol, and 2-propanol hydrodeoxygenation on physical mixtures of Pt/SiO₂
with FAU and BEA and on physical mixtures of Pt/SiO₂ with
Pt-exchanged FAU and Pt-exchanged BEA. We also report turnover rates for probe
reactions selective to each of the two functions to assess their monofunctional
reactivities as the distances separating them
decrease. Acid sites are characterized by turnover rates for methanol
dehydration [2], a reaction that
sensitively probes acid reactivity and any consequences of changes in acid
strength or confining environment as a result of proximity; the metal function
is assessed using CO oxidation rates [3],
which are able to report on changes in electronic properties of Pt clusters as
a result of their proximity to zeolitic protons.

n-Heptane isomerization turnover rates
(548 K, 60-101 kPa H₂, 5 kPa n-heptane: ~70 molec. (H⁺ks)^-1) on MFI (of different crystallite
size and site density; mixed with Pt/SiO₂) are invariant with the
Thiele modulus, the dimensionless quantity that rigorously describes reactant
concentration gradients, until large values of the Thiele parameter
(corresponding to metal-acid separation of ~250 nm) at which point rates
decrease by about twofold. Thus, smaller metal-acid distances lead to
negligible intracrystalline heptene
concentrations, the required intimacy criterion [1] has been satisfied for these
bifunctional catalysts with metal function present only as extracrystalline
Pt/SiO₂. Alkanol dehydration turnover rates (393 K, 2 MPa H₂, 2.5 kPa
4-methylcyclohexanol: ~25 molec. (H⁺ks)^-1) were measured on BEA and FAU (0.60 and
0.74 nm apertures, respectively, compared to 0.47 nm in MFI [4]). Dehydration turnover rates are
much lower than those for n-heptane isomerization, which, taken together with
the larger molecular diffusivities afforded by large pore frameworks, precludes
the presence of concentration gradients of methylcyclohexene
products. Concentrations of products measured are much lower than those
predicated by equilibrated reactant dehydration, indicating that further
decreases in metal-acid site proximity should be inconsequential. In contrast
to these expectations, we measure unexpectedly significant increases (by
factors of 2-9) in reaction rate constants (Figure 1a for 2-butanol and
4-methylcyclohexanol on BEA, BEA-Pt/SiO₂, and PtBEA-Pt/SiO₂;
Figure 1b for n-heptane isomerization over MFI-Pt/SiO₂ and PtMFI-Pt/SiO₂ for MFI of different site density)
for both isomerization and hydrodeoxygenation when Pt clusters and H⁺
reside within the intracrystalline framework at
distances of ~5-20 nm—as estimated assuming uniform distribution of each of the
two functions during synthesis procedures.

The nanoscale proximity between metal and
acid function could potentially result in cooperative behavior changing the
inherent monofunctional reactivity. Alkene skeletal isomerization and alkanol
dehydration steps limit the respective rates of alkane isomerization and alkanol
hydrodeoxygenation. Any potential changes in acid reactivity, caused by
modification of acid strength and local environment for acid sites as a result
of proximate Pt clusters, were assessed using methanol dehydration turnover
rates (per H⁺; 433 K) were measured on both MFI and on PtMFI (Figure 2a). Turnover rates and rate constants (inset
in Fig. 2a) are the same, consistent with unperturbed reactivity and properties
of the acid sites. CO oxidation turnover rates and rate constants (per Pt_s;
443 K) on Pt/SiO₂ and Pt-MFI are also the same (Figure 2b and
inset), indicating that the reactivity and properties of Pt clusters are
unaffected by their confinement or by their proximity to acid sites within intracrystalline regions. Thus, we conclude that the
monofunctional reactivity of the individual metal and acid functions are
unaffected by their close proximity.

These significant rate enhancements for
isomerization and dehydration reactions must occur via highly reactive
intermediate species. We surmise that these short-lived intermediates are
present only at low concentrations and thus require metal clusters within
nanometer distance from acid sites. Rate enhancements for 2-butanol and
4-methylcyclohexanol but not 2-propanol dehydration (Figure 1a) shed light on
the identity of such reactive intermediates. Additionally, alkanol dehydration
turnover rates are independent of P(H₂) on BEA and on BEA-Pt/SiO₂;
however, turnover rates for 2-butanol and 4-methylcyclohexanol become inversely
dependent on P(H₂) when Pt is present in nanometer proximity from
acid sites (Fig. 1a inset). These kinetic data, together with insights from
density functional theory, implicate unsaturated alkenone/alkenol
analogs of reactants as plausible intermediates for dehydration reactions. Such
a pathway is unavailable for 2-propanol because the analogous alkenol cannot undergo dehydration and consequently does
not impact its measured rates. The identity of highly reactive intermediates
mediating isomerization remains under investigation. Unlike for alkanol
dehydration, isomerization rates show no change in P(H₂) dependence
with encapsulation of Pt clusters, thus excluding diene-derived intermediates.
We continue to apply density functional theory treatments to assess the
plausibility of species as alkene-derived intermediates for isomerization.

We have addressed previously uninterpreted
enhancements in reaction rates as they arise from nanoscale metal-acid site
proximity. Such insights can be applied to optimize industrial catalysts for these
and other analogous reactions.

References

[1]      P.B. Weisz, Adv. Catal. 13 (1962) 137–190.
doi:10.1016/S0360-0564(08)60287-4.

[2]      A.J. Jones, R.T. Carr,
S.I. Zones, E. Iglesia, J. Catal.
312 (2014) 58–68. doi:10.1016/j.jcat.2014.01.007.

[3]      A.D. Allian, et al.,
J. Am. Chem. Soc. 133 (2011) 4498–4517. doi:10.1021/ja110073u.

[4]      C. Baerlocher,
L. McCusker, Database of Zeolite Structures.
http://www.iza-structure.org/databases/.

The authors thank Zhichen
(Jane) Shi for measurement of diffusion timescales of zeolite samples.
Additionally, the authors acknowledge financial support from Chevron Energy
Technology Company and computational resources from the National Science
Foundation’s Extreme Science and Engineering Discovery Environment
(ACI-1053575).