Ammonia
titration methods were developed to discriminate and quantify
Brønsted acid sites of different strength that compensate aluminum
and boron heteroatoms incorporated within zeolite frameworks. Borosilicate
and boroaluminosilicate MFI zeolites (B-Al-MFI) were synthesized with
different Al contents and crystallite sizes, which are typically correlated
structural properties in aluminosilicates synthesized hydrothermally,
but independently varied here by incorporating boron as a second framework
heteroatom and using ethylenediamine as a structure directing agent.
Temperature-programmed desorption (TPD) of ammonia from B-Al-MFI samples
saturated via liquid-phase NH4NO3 ion exchange
resulted in quantifying the total number of Al and B heteroatoms.
In contrast, TPD performed after NH4-form B-Al-MFI samples
were purged in flowing helium (433 K), or after gas-phase NH3 adsorption (433 K) onto H-form B-Al-MFI samples, quantified only
protons charge-compensating framework Al heteroatoms. Turnover rates
for methanol dehydration to dimethyl ether, when measured in zero-order
kinetic regimes that are sensitive predominantly to Brønsted
acid strength, are dependent only on the number of protons compensating
framework Al atoms in B-Al-MFI zeolites. The NH3 titration
methods developed here are useful in rigorously normalizing turnover
rates of Brønsted acid-catalyzed reactions in boroaluminosilicate
zeolites, which have been recognized previously to be dependent solely
on Al content. The incorporation of B heteroatoms into zeolite frameworks,
which generate protons that are essentially unreactive, provides a
strategy to influence crystallite sizes independently of Al content,
especially relevant in cases where catalytic behavior is influenced
by intracrystalline transport phenomena.
We
report synthetic protocols to independently influence composition
and crystallite sizes of MFI zeolites, properties that contribute
to Thiele moduli, and the proximity of Al heteroatoms at fixed composition
(Si/Al ∼ 50). Crystallite sizes decrease with the addition
of non-catalytic B heteroatoms in B–Al–MFI zeolites.
Using only tetra-n-propylammonium (TPA+) as a structure-directing agent (SDA) leads to occlusion of one
TPA+ per channel intersection and a finite percentage of
framework Al atoms (20–40%) in proximal configurations. In
contrast, using mixtures of ethylenediamine (EDA) and TPA+ (EDA/TPA+ = 15) leads to incorporation of EDA that displaces
some TPA+ and to nearly all (>95%) Al in isolated configurations.
These findings indicate how adding B and EDA to zeolite synthesis
solutions provides a route to crystallizing MFI zeolites of similar
composition but with framework Al distributed at varying relative
proximity, or with similar Al distributions but different diffusion
path lengths.
Alkane dehydrogenation
rates on acidic zeolites measured in the
presence of co-fed H2 during initial contact with reactants
solely reflect protolytic reactions at Brønsted acid sites, while
rates measured without co-fed H2 and at later reaction
times reflect additional contributions from an extrinsic dehydrogenation
function derived from reactants and products. This extrinsic function
consists of unsaturated organic residues that catalyze dehydrogenation
turnovers by accepting H-atoms from alkanes and recombining them as
H2. Such hydrogen transfer routes are inhibited by alkenes
and H2 products and proceed with activation barriers much
lower than for protolytic dehydrogenation at H+ sites,
causing them to become more prevalent at lower temperatures and for
zeolites with lower H+ densities. The number, composition,
and reactivity of these extrinsic carbonaceous active sites depend
on the local concentrations of reactants and products, which vary
with alkane and H2 pressure, bed residence time, and axial
mixing. These extrinsic catalytic moieties form within H2-deficient regions of catalyst beds but can be removed by thermal
treatments in H2, which fully restore zeolite catalysts
to their initial state. Carbonaceous deposits do not catalyze alkane
cracking reactions; thus, cracking rate constants serve as a reporter
of the state of proton sites, and their invariance with product pressure,
residence time, and axial mixing confirms that protons remain unoccupied
and undisturbed as extrinsic organic residues change in number, composition,
and reactivity. The rates of the reverse reaction (alkene hydrogenation)
under H2-rich conditions inhibit the formation and the
reactivity of these organic residues, and taken together with formalisms
based on nonequilibrium thermodynamics, they confirm that alkane dehydrogenation
occurs solely via protolytic routes only at the earliest stages of
reaction in the presence of added H2. These findings provide
a coherent retrospective view of the root causes of the literature
discord about alkane dehydrogenation turnover rates and activation
barriers on acidic zeolites, variously attributed to extraframework
Al or radical active sites and to turnovers limited by alkene desorption
instead of protolytic steps. Importantly, these findings also prescribe
experimental protocols that isolate the kinetic contributions of protolytic
dehydrogenation routes, thus ensuring their replication, while suggesting
strategies to deposit or remove extrinsic organocatalytic functions
that mediate hydrogen transfer reactions.
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