(544ee) Kinetic Assessments of the Location and Proximity of Brønsted Acid Sites in MFI Zeolites Containing Boron and Aluminum Heteroatoms

Kinetic assessments
of the location and proximity of Brønsted acid sites in MFI zeolites containing
boron and aluminum heteroatoms

Philip M. Kester, Elizabeth E. Bickel,
Jeffrey T. Miller, and Rajamani Gounder^*

Charles D. Davidson School of Chemical Engineering,
Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA

*rgounder@purdue.edu

Crystallite
sizes and framework Al contents influence the rates, selectivity, and
deactivation of MFI zeolites in hydrocarbon upgrading processes (e.g. olefin
oligomerization[1], methanol-to-olefins[2]) because
longer diffusion paths and higher Al contents preferentially increase
intracrystalline residence times of larger products. The combined effects of
crystallite size and Al content are reflected in a characteristic diffusion
parameter in Thiele modulus expressions, proportional to the square of the
characteristic diffusion length and the volumetric active site density. While these
properties are typically correlated in MFI zeolites synthesized hydrothermally,
they are decoupled here through addition of B and Al heteroatoms into synthesis
solutions containing tetra-n-propylammonium and ethylenediamine as structure
directing agents, resulting in independent variations in crystallite size (0.3
– 10 μm) and Si/Al ratios (74 – 153), and diffusion
parameters that vary by three orders of magnitude. Protons at framework B are
weaker in acid strength than those at Al [3], requiring methods to discriminate
and quantify protons originating from Al prior to interpreting catalytic
phenomena on boroaluminosilicates.

Here,
ammonia titration methods developed previously to differentiate between Lewis
and Brønsted acid sitesin MFI zeolites [4] were adapted to do so
for protons of different Brønsted acid strength that charge compensate Al and B
heteroatoms. Temperature programmed desorption (TPD) of ammonia of B-Al-MFI after
liquid-phase NH₄⁺-exchange quantified all framework B and
Al heteroatoms. In contrast, TPD performed after NH₄-form B-Al-MFI
were purged in flowing He (433 K), or after gas-phase NH₃ adsorption
(433 K) onto H-form B-Al-MFI, quantified only protons at framework Al atoms (H⁺_Al). Turnover rates of methanol
dehydration to dimethyl ether (415 K, per H⁺_Al)
measured in zero-order kinetic regimes, in which rate constants are primarily
sensitive to acid strength, provide quantitative validation that these
titration protocols measure protons at framework Al in boroaluminosilicates. These
methods should be adaptable to other boroaluminosilicates, and are especially
useful for small-pore and Al-dense frameworks for which using larger base titrants
may be challenging.

Methanol
dehydration rates (415 K, per H⁺_Al)
measured in first-order kinetic regimes, in which rate constants reflect
differences in both acid strength and confinement, are lower on B-Al-MFI
zeolites than those previously reported for Al-MFI [5], suggesting that Al is
preferentially sited within MFI intersections (~0.7 nm) in boroaluminosilicate
compositions. We will also discuss how this suite of materials enables
decoupling the influence of Al density and proximity, and crystallite size, on alkene
oligomerization rates and selectivities (473-523 K) in MFI zeolites.

References:

[1] M. L. Sarazen,
E. Doskocil, and E. Iglesia. ACS Catal.
6 (2016) 7059.

[2] R. Khare, D. Millar, and A. Bhan. J.
Catal. 321 (2015) 23.

[3] A. J. Jones,
and E. Iglesia. ACS Catal. 5 (2015) 5741.

[4] G. L. Woolery,
G. H. Kuehl, H. C. Timken, A. W. Chester, and J. C. Vartuli.
Zeolites. 19 (1997) 288.

[5] J. R. Di Iorio, C. T. Nimlos, R. Gounder.
ACS Catal. 7 (2017) 6663.