Experimental Detection of Preferred Lanthanum Siting in Zeolite Y and Its Impact on Catalyst Reactivity - PMC

Experimental Detection of Preferred Lanthanum Siting in Zeolite Y and Its Impact on Catalyst Reactivity - PMC
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. 2025 Apr 23;129(18):8545–8555. doi:
10.1021/acs.jpcc.5c00474
Experimental Detection
of Preferred Lanthanum Siting
in Zeolite Y and Its Impact on Catalyst Reactivity
Anya Zornes
Anya Zornes
School
of Chemical Engineering, Oklahoma State
University, 420 Engineering North, Stillwater, Oklahoma 74078, United
States
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Anya Zornes
Omio Rani Das
Omio Rani Das
School
of Chemical Engineering, Oklahoma State
University, 420 Engineering North, Stillwater, Oklahoma 74078, United
States
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Omio Rani Das
Nabihan B Abdul Rahman
Nabihan B Abdul Rahman
School
of Sustainable Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States
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Nabihan B Abdul Rahman
Jacob Crouch
Jacob Crouch
School
of Sustainable Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States
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Jacob Crouch
Steven Crossley
Steven Crossley
School
of Sustainable Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States
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Steven Crossley
Bin Wang
Bin Wang
School
of Sustainable Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States
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Bin Wang
Walter Alvarez
Walter Alvarez
Phillips
66, Bartlesville Technology Center, Bartlesville, Oklahoma 74004, United States
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Walter Alvarez
Matthew J Wulfers
Matthew J Wulfers
Phillips
66, Bartlesville Technology Center, Bartlesville, Oklahoma 74004, United States
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Matthew J Wulfers
Daniel E Resasco
Daniel E Resasco
School
of Sustainable Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States
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Daniel E Resasco
Jeffery L White
Jeffery L White
School
of Chemical Engineering, Oklahoma State
University, 420 Engineering North, Stillwater, Oklahoma 74078, United
States
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Jeffery L White
†,
School
of Chemical Engineering, Oklahoma State
University, 420 Engineering North, Stillwater, Oklahoma 74078, United
States
School
of Sustainable Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States
Phillips
66, Bartlesville Technology Center, Bartlesville, Oklahoma 74004, United States
Email:
jeff.white@okstate.edu
Received 2024 Oct 9; Accepted 2025 Apr 9; Revised 2025 Apr 4; Collection date 2025 May 8.
© 2025 The Authors. Published by American Chemical Society
Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (
).
PMC Copyright notice
PMCID: PMC12067440  PMID:
40365423
Abstract
Lanthanum and other rare earth cations are routinely
added to commercial
zeolite catalysts to increase stability during hydrothermal regeneration
steps, as well as to modify catalyst reactivity by changing the distribution
of acid sites and operative electric field gradients in working catalysts.
Solution ion-exchange procedures are typically used to introduce La
cations into zeolite Y, primarily as La
3+
or its hydroxylated
moieties, La
3-n
(OH)
, in the formulation
of commercial LaY or ultrastable steam-stabilized Y (USY) catalysts.
Within zeolite Y, multiple possible exchange sites exist for La occupation,
but quantitative measurement of La siting as a function of La loading
in the catalyst is not generally accessible. Specific open questions
involve whether La cations reside in both sodalite and supercage sites,
which sites are preferred for La incorporation, and whether La can
be selectively incorporated into specific site types. In this contribution,
a simple quantitative method based on solid-state NMR coupled with
the preparation of La–HY catalysts without framework defects
reveals that at low La loadings of less than 3 wt %, essentially all
La is incorporated into sodalite cages as La
3+
ions. DFT
calculations support these experimental conclusions. Coincident with
this incorporation, sodalite Brønsted bridging acid sites (BAS)
decrease, but the number of supercage BASs can remain constant depending
on the La concentration. Increased La loadings in the catalyst preferentially
reduce the number of sodalite BASs compared to supercage acid sites,
with both sodalite and supercage BAS amounts, as well as the amount
of newly created La
3-n
(OH)
species,
quantitatively measured using the methods described here. Flow reactor
hexane-cracking experiments, as well as in situ probe reactions, reveal
that catalyst reactivity increases relative to that of HY when La
resides exclusively in sodalite positions.
Introduction
Zeolites have been successfully employed
as industrial cracking,
reforming, hydrogen-transfer, and hydrocarbon synthesis catalysts
for decades.
Requirements for increased selectivity to minimize the production
of deleterious molecules from conventional hydrocarbon feeds, increased
conversion from nonconventional feedstocks like waste and biomass,
and reduced operating costs afforded by longer catalyst lifetimes
necessitate continuous improvements in understanding the structure
and activity relationships of these dynamic and compositionally heterogeneous
silicoaluminate materials.
The faujasite class of zeolites, most notably zeolite
Y, is used in fluid catalytic cracking (FCC) of gas-phase hydrocarbons
at high temperatures typically following postsynthetic metal cation
exchanges into the zeolite interstitial spaces to increase catalyst
performance and lifetime. Zeolite HY, into which relatively low amounts
of rare earth (RE) cations like lanthanum and cerium are exchanged,
is known to exhibit increased hydrothermal stability in the presence
of gas-phase water during steam-regeneration steps.
12
However, with the increased need
to employ some large-pore zeolites like HY to convert oxygenated feedstocks
that evolve water during reactions, often at lower temperatures where
liquid water can be present instead of water vapor,
13
14
coupled with increased demands on overall carbon maintenance in
traditional chemistries, requires a more detailed understanding of
how rare earths like La impact the structure and reactivity of zeolite
HY.
Specific outstanding questions regarding La incorporation
in zeolite
Y that could be addressed by improved experimental methods include:
(1) Does La have a preferred site for incorporation in zeolite Y?;
(2) Where does La reside in the catalyst structure as a function of
the amount of La incorporated, i.e., the La-to-framework Al ratio
(La/Al), and how does this impact the number of Brønsted acid
sites in sodalite vs supercage positions?; (3) What is the state of
the incorporated La cation as a function of La loading and catalyst
preparation method, e.g., La
3+
vs La
3–n
(OH)
?; (4) How do the results of (1)–(3) impact
La-HY reactivity? These questions are not new, and previous computational
and experimental studies have attempted to address them at various
levels. For example, cation locations have been inferred via assessment
of unit cell sizes measured via X-ray diffraction,
15
18
in several spectroscopic studies
primarily involving infrared or NMR methods,
10
11
19
37
and in aberration-corrected microscopy studies.
38
Common to many of these studies is the fact that the standard
aqueous ion-exchange and heating methods used to introduce La cations
cause hydrolysis of the zeolite Y framework prior to complete La incorporation,
further complicating detection and interpretation of preferred La
siting. Recently, it has been demonstrated that even room-temperature
aqueous exchange and water removal methods, which are significantly
milder than those typically employed for La introduction, degrade
the zeolite Y framework, preferentially inducing hydrolysis of sodalite
cage acid sites relative to supercage sites.
39
In this contribution, we demonstrate that La-exchanged HY
with
different La content can be prepared without introducing framework
defects into the catalyst, and introduce a
H MAS standard-addition
NMR experimental strategy that quantitatively and selectively measures
the impact of La introduction in sodalite vs supercage sites. These
quantitative data guide the interpretation of the impacts of La incorporation
on high-temperature flow-reactor hexane cracking data as well as room-temperature
in situ isotopic exchange experiments with toluene-d
and provide a quantitative basis for comparing LaHY to the initial
HY catalysts. Key results of this study demonstrate that: (1) LaHY
catalysts can be prepared with no silanol defects or framework hydrolysis;
(2) La
3+
cations can be selectively incorporated into
only
sodalite cage positions and preserved there without
further hydrolysis to lanthanum hydroxides, with experimental results
supported by computational calculations; (3) catalysts with La
3+
exclusively residing in sodalite cages exhibit the largest
reaction rates per unit La for both high-temperature hexane cracking
and room-temperature H/D exchange, corresponding to ca. one La ion
per 25 Al atoms; and (4) increasing La loading beyond ca. 1 La ion
per 10 Al atoms corresponds to the formation of La
3–n
(OH)
species and concomitant decreases in reaction rates
for the aforementioned reactions. In total, these findings indicate
not only the preferred siting of La in sodalite cage positions of
HY but also the active role of both proton and La sites in the sodalite
cages, even for molecules that are ostensibly too large to access
them. Further, the data provide quantitative guidance for determining
the optimum La amounts to employ in LaHY catalysts.
Experimental Section
Catalyst Samples
Zeolite NH
Y with a nominal
Si:Al ratio of 2.6 was obtained from Zeolyst International. HY was
prepared from NH
Y in a glass reactor by stepwise vacuum
dehydration, with a pressure of less than 1 × 10
–4
Torr maintained throughout the process, heating at 0.5 °C/min
to 100 °C, holding for 2 h, heating at 2 °C/min to 450 °C,
and holding for 12 h. La–Y was prepared from NH
Y by exchanging 500 mg NH
Y in either 20 mL (higher three
loadings) or 10 mL (lower three loadings) of aqueous La(NO
solutions of appropriate concentration for 3 h at 90
°C. Samples were vacuum filtered, rinsed with DI water, and then
dried in a vacuum oven at 80 °C overnight. The samples were then
fully dehydrated by stepwise vacuum dehydration, with pressure maintained
at less than 1 × 10
–4
Torr throughout, heating
at 0.5 °C/min to 350 °C, holding for 10 min, then heating
at 2 °C/min to 550 °C, and holding for 2 h. The low heating
rate for LaY was continued to a higher temperature than when preparing
HY, as both deammoniation and the removal of physically adsorbed water
in La-exchanged zeolite Y occur at 350 °C,
40
and it is undetermined how well La
3+
protects
the framework at these lower temperatures, since the hydration enthalpy
of LaY is more similar to HY than to other cation-exchanged zeolites.
41
The LaY samples were also heated to a higher
final temperature than HY, as the La(OH)
complexes make
it difficult to fully remove water, and the majority of dehydroxylation
does not occur until around 500 °C.
36
39
42
Table
summarizes
the samples and naming convention used throughout the study, including
results from the elemental analysis of each prepared sample. Note
that the elemental La/Al values in the last column of
Table
indicate an upper limit of
incorporated La, irrespective of the synthesis solution concentration.
Table 1. La–Y Naming Conventions, Synthesis
Solution Concentrations, and Elemental La:Al Ratios in the Final Catalysts.
Sample
La(NO
conc.
Solid:liquid ratio
La/Al (soln.)
La wt% (cat.)
La/Al
(cat.)
0.05-LaY
0.01 M
1g:20 mL
0.05
2.82
0.04
0.12-LaY
0.025 M
1g:20 mL
0.12
6.65
0.11
0.24-LaY
0.05 M
1g:20 mL
0.24
9.34
0.16
0.97-LaY
0.1 M
1g:40 mL
0.97
10.0
0.17
2.4-LaY
0.25 M
1g:40 mL
2.4
9.95
0.17
4.8-LaY
0.5 M
1g:40 mL
4.8
--
0.17
Open in a new tab
To further eliminate any contributions from residual
Na ions that
are present in the commercial NH
Y starting material, several
control experiments were done in which three additional ion-exchange
steps with NH
Cl prior to the generation of the HY or LaY
forms, were executed, as discussed in
Figures
and
below.
Figure 1.
Open in a new tab
H MAS NMR spectra of the (a) HY catalyst prepared from
the parent NH
Y via low ramp-rate vacuum dehydration; (b)
same catalyst as in (a) but following a three-time NH
Cl
exchange prior to dehydration; (c) same catalyst as in (b) followed
by exchange with 0.01 M La(NO
prior to dehydration;
(d) same catalyst as in (a) followed by exchange with 0.01 M La(NO
prior to dehydration. The absolute number of
BAS protons in the sodalite cages (signal at 4.9 ppm) and in the supercages
(signal at 3.9 ppm) are reported as
SD
and
SC
on the right of each spectrum as
obtained from a two-component deconvolution following the procedure
in
Figure S1
and as previously published.
39
Note the absence of any SiOH signals. PDMS denotes
the internal standard polydimethylsiloxane.
Figure 6.
Open in a new tab
Kinetic results for room-temperature in situ MAS NMR experiments
monitoring
H/
H exchange between toluene-d
and HY or LaY as denoted.
Some HY and LaY samples were then exposed to ambient
air at 24
°C with a relative humidity of 30–45% in shallow bed conditions
(bed height of less than 1 mm) for 1 week. The samples were then dehydrated
again in the glass reactor at pressures of less than 1 × 10
–4
Torr with the same heating procedure was used to
make HY. It is known that HY will undergo dealumination from exposure
to ambient air and that heating such a sample will further destroy
the framework.
43
45
Table
summarizes the naming conventions for samples before and after exposure
to ambient moisture. Any sample denoted as LaY still contains significant
BASs and is actually a LaHY catalyst but denoted as the former for
convenience.
Table 2. Naming Conventions vs Sample History.
Notation
Sample history
NH
Commercial CBV300 as received
HY
CBV300 calcined (deammoniated
and dehydrated) in vacuum
X-LaY
NH
Y exchanged
with La(NO
then heated to 550 °C
Y-1wk
HY or X-LaY exposed to ambient
air for 1 week, then reheated to 450 °C
Open in a new tab
NMR Hardware and Sample Packing
All NMR experiments
on the solid catalysts were conducted at 9.4 T with a Bruker Avance
II console using a 4 mm double-resonance MAS probe. Samples were packed
in ZrO
rotors, under either ambient air (
27
Al and
29
Si NMR) or in an inert argon gas atmosphere by
using a VAC atmosphere glovebox (
H NMR).
H NMR Experiments
H NMR spectra
were acquired on fully dehydrated catalysts using a single 90°
excitation pulse of 3.67 μs. In all cases, 64 transients were
obtained with a recycle delay of 60 s, which is in excess of 5 times
the longest relaxation time observed at a spinning speed of 10 kHz.
Pulse durations and chemical shifts were calibrated using hexamethylbenzene.
For quantitative spin-counting experiments, measured amounts of
vacuum-dehydrated sample and an inert quantitation standard, poly(dimethylsiloxane)
(PDMS), were packed in the middle third of a ZrO
rotor,
between sulfur (bottom) and a Teflon spacer (top).
27
Al NMR Experiments
27
Al NMR
spectra were acquired on ambient-air hydrated samples with a 15°
pulse of 0.92 μs and a recycle delay of 0.2 s for 4096 transients
at a spinning speed of 10 kHz. Pulse durations and chemical shifts
were calibrated by using a 0.1 M aqueous solution of Al(NO
29
Si NMR Experiments
29
Si NMR
spectra were acquired on ambient-air-hydrated samples with a 90°
pulse of 4.3 μs. All spectra were acquired for 1024 transients
with a recycle delay of 60 s and a spinning speed of 10 kHz. Pulse
durations and chemical shifts were calibrated by using PDMS.
XRD
Powder X-ray diffraction data were acquired at
ambient humidity using a Philips X-ray diffractometer (Phillips PW
3710 MPD, PW2233/20 X-ray tube, copper tube detector, wavelength 1.5418
Å), which operated at 45 kW and 40 mA. Diffractograms were obtained
with 2θ ranging from 2° to 45° and with a diffractometer
difference of 0.02°.
Elemental Analysis
Elemental analysis to determine
the weight percent (wt%) of lanthanum in the exchanged samples was
provided by Galbraith Laboratories using the GLI ME-70 procedure.
Kinetic Measurements
The impact of lanthanum loading
on catalyst activity was measured using two model reactions: hexane
cracking in a continuous-flow microreactor and in situ H/D exchange
solid-state NMR experiments using toluene-d
as the reagent.
For hexane cracking reactions, catalyst samples were pelletized to
a size of 250–425 μm with a mass of 50 mg. Prior to the
reaction, samples were pretreated to remove any moisture present by
heating to 120 °C at 0.5°/min, holding at 120 °C for
2 h, heating to 300 °C at 2°/min, and holding at 300 °C
for 1 h. Following this dehydration pretreatment, catalysts were heated
to the reaction temperature of 425 °C at 2°/min. Both the
reaction and pretreatment were conducted under a 100 mL/min flow rate
of nitrogen carrier gas, to which liquid hexane was injected at a
rate of 0.5 mL/h after reaching the reaction temperature. Each run
was conducted for approximately 6 h, with analysis of reaction products
carried out using an online GC with an FID detector and an HP-Plot
Al
column.
In-situ H/D exchange experiments
were carried out at room temperature using catalyst samples loosely
packed in a ZrO
rotor in an inert argon atmosphere, which
were then evacuated under vacuum. Toluene-d
was adsorbed
onto the catalyst such that, for all samples, a 1:1 molar ratio of
reactant to acid sites and (when applicable) lanthanols was achieved.
The rotor was then sealed using a Teflon spacer and kept in a liquid
nitrogen bath until the sealed rotor was inserted into the NMR probe.
H NMR spectra were then obtained at room temperature over
the course of the reaction, with the integrated area of each peak
correlating directly with the concentration of that species. Rate
constants for the reaction were determined using a least-squares fit
of the growth of the toluene peak area T(t) during the short-time
initial-rate region (≤20 min) of the proton/deuterium exchange
curve using the following equation:
where fully deuterated toluene is assumed
to be in excess under
initial conditions. Corrections for adsorption equilibria or diffusion
limitations were not necessary, as toluene was fully adsorbed prior
to proton exchange.
46
47
Catalyst sample preparation and
reactant adsorption were spatially removed from the location where
the in situ NMR experiments are completed. Immediately following toluene
adsorption, the sample was immersed in liquid nitrogen for transport
and kept cold until placed into the spectrometer a few minutes later.
While every effort was made to preclude any exchange reaction prior
to acquiring the first spectrum, invariably some H/D exchange had
already occurred during the several-minute delay, with more occurring
as the catalyst reactivity increases thus contributing to larger deviations
from the origin for the most active catalysts in
Figure
below (vide infra).
Computational Methods
All density functional theory
(DFT) calculations were performed using the Vienna ab initio simulation
package (VASP)
48
with projector augmented
wave (PAW) potentials
49
and the Perdew–Burke–Ernzerhof
(PBE) exchange and correlation functional.
50
The structural relaxation was carried out with a kinetic cutoff
energy of 400 eV. All atoms were fully relaxed until the atomic net
force was less than 0.02 eV Å
–1
. The DFT-D3
method
51
was used to include van der Waals
interactions. The KPOINTS was set to 3 × 3 × 3 to sample
the Brillouin zone.
A rhombohedral primitive cell was used (with
48 T sites)
52
and the unit cell was relaxed,
allowing it to expand following the addition of La species. To match
the experimentally determined Si/Al ratio of 3, 12 Al atoms were placed
in the unit cell, with the specific distribution determined according
to Loewenstein’s rule
53
and comparison
of calculated
29
Si NMR spectra to experimental data. The
formation energetics of a LaY structure at each of the 7 unique cation
locations was determined by the following equation:
where
·La(OH)
],
[La(OH)
], and
[H
O] are the DFT-calculated total energies of Y zeolite with La species
incorporated, a La(OH)
cluster, and a water molecule.
The value of
equals 1–3 in the calculations
to represent the La monomer, dimer, and trimer. It should be emphasized
that the exact value of this formation energy varies when the references
are changed, but the trend and relative stability remain consistent.
Results and Discussion
Measuring the Impact of La on Sodalite vs Supercage Acid Sites
Dry samples of the initial commercial HY catalyst, as well as the
prepared 0.05-to-4.8 LaY catalysts detailed in
Table
were examined using a quantitative
H spin-counting or standard-addition MAS NMR method employing
known amounts of inert polydimethylsiloxane (PDMS) as an internal
calibrant to yield absolute amounts (mmol/g) of each type of proton
in the catalyst. As demonstrated in several prior studies of zeolite
catalysts, the method is quantitative with respect to different BAS
protons and silanol (SiOH) concentrations, with standard deviations
of 0.05–0.1 mmol/g for typical zeolites and 0.01–0.05
mmol/g for control experiments on known calibrants.
39
54
55
Indeed, recently reported standard deviations
of 0.5–0.7 for commercial HY with Si/Al = 2.6 were much larger
than prior zeolite or silicoaluminophosphate studies, which indicated
a deficiency in the overall stability of HY under standard catalyst
preparation conditions and led to improved preparation protocols.
39
Here, La was introduced to the significantly
more stable NH
Y rather than NaY or HY and then dehydrated
with low ramp-rate heating under vacuum, rather than the standard
practice of dehydrating under ambient air or flowing air. Further,
prior to the acquisition of any spectroscopic data probing La siting,
the La-exchanged catalysts were not exposed to ambient moisture.
For reference and convenience to the reader,
Figure
shows the quantitative
H MAS
NMR results for an important control experiment involving the commercial
HY catalyst used as the parent material for all LaHY samples. In the
HY (1a, 1b) and LaY (1c, 1d) spectra, only three peaks are observed,
including the PDMS spin-counting standard near 0 ppm, the supercage
BAS hydroxyl protons (BAS-sc) at 3.9 ppm, and the sodalite BAS hydroxyl
protons (BAS-sd) at 4.9 ppm. Importantly, there are no peaks observed
in the 1.5–2.5 ppm region, as typically reported for HY catalysts,
22
even for the LaY catalysts in parts 1c and 1d, indicating
that internal silanol groups resulting from framework hydrolysis are
absent. The absence of measurable SiOH or AlOH signals in the 1.5–2.5
ppm region indicates that dry, high-Al-content HY and La–HY
samples can be prepared without introducing framework defects, as
previously demonstrated for HY zeolites.
39
Using the simple deconvolution procedure shown in
Figure S1
, the amount of supercage and sodalite unit acid
sites can be quantified and is reported to the right of each spectrum
in
Figure
The progression of sodalite vs supercage acid site signals in
Figure
a–d, denoted
SD
and H
SC
respectively,
provides clear experimental evidence that La can be selectively incorporated
into sodalite sites while preserving the integrity of the framework,
including the sodalite units themselves.
Figure
a shows the parent HY prepared from the commercial
NH
Y material, with relatively more sodalite acid sites
than the supercage and a total [BAS] = 3.0 mmol/g, well below the
theoretical value of ca. 4.2 mmol/g based on the Si/Al ratio. Following
additional exchange with NH
Cl solutions,
Figure
b shows that the total [BAS]
increases to 3.6 mmol/g, with the majority of that increase taking
place due to the [H
SD
] change from 1.7 to 2.1
mmol/g, which is relatively larger than the increase observed for
[H
SC
]. The additional ammonium-exchange steps
from
Figure
a,b displace
residual Na
cations remaining from the synthesis,
56
which, according to the increase in sodalite
vs supercage acid sites, preferentially remain in sodalite cages.
The HY catalyst in
Figure
b, having a [BAS] much closer to the theoretical limit with
residual Na
removed and without any framework defects
based on the absence of SiOH signals, is now an ideal catalyst to
use for introducing small amounts of La. The spectrum in
Figure
c is for the same
catalyst as used to make the HY shown in
Figure
b, but after exchange with 0.01 M La(NO
solutions prior to dehydration. The catalyst
in
Figure
c is equivalent
in its preparation to the 0.05-LaY listed in
Table
. The red text in the right captions of
Figure
b,c highlights that
upon La incorporation, the
only
signal that is impacted
is that for the H
SD
sites. The [H
SD
] decreases from 2.1 to 1.3 mmol/g upon La incorporation,
while the [H
SC
] remains constant at 1.5 mmol/g,
and again no framework defects are created. Even without doing the
extra ammonium exchange steps on the parent Y catalyst to remove residual
Na
, i.e., starting with the catalyst in
Figure
a, the same La-incorporation
procedure incorporates ca. 80% of all the La cations into the sodalite
units, as can be seen by comparing the data in
Figure
d to
Figure
a. The LaY catalyst in
Figure
d, following exchange from the catalyst in
Figure
a, has slightly more
than half of the initial [H
SD
] while retaining
almost all of the [H
SC
]. The small decrease
in [H
SC
] likely arises from Na being displaced
from the sodalite sites by La and replacing supercage protons.
20
22
57
Figure
shows that
the experimental approach can quantitatively measure the impact of
La incorporation on overall Brønsted acid site density and, more
importantly, selectively quantify the location of La siting and its
differential impact on sodalite vs supercage acid sites. The data
in
Figure
indicate
that La preferentially resides in sodalite cages and, importantly,
suggest that it exists as La
3+
cations due to the absence
of any new signals arising with La introduction that could be assigned
to La
3–n
(OH)
species. The overlay plot
in
Figure
confirms
the findings from
Figure
and further demonstrates that increasing the level of La
introduction preferentially decreases the number of H
SD
sites. The two BAS peaks in
Figure
are normalized to the same intensity for
the H
SC
peak, which clearly shows that almost
all sodalite BASs are eliminated by the time a 0.24 LaY loading is
achieved, but supercage BASs remain. Again, no peaks are observed
for SiOH species, and no peaks are observed that could be assigned
to La
3-n
(OH)
species even at the highest
loading in
Figure
. Signals from La
3–n
(OH)
species are
observed for the 0.97, 2.4, and 4.8 LaY samples listed in
Table
(vide infra)
Figure 2.
Open in a new tab
Overlay plot of
H MAS NMR spectra for samples prepared
from the parent NH
Y via low ramp-rate vacuum dehydration
is described in
Table
, and arranged from top to bottom by the arrow and labeling on the
left. The amount of H
SD
and H
SC
acid sites is given in the table on the right, in units
of
mmol/g
. All spectra are normalized to the same height
for the H
SC
peak at 3.9 ppm. Note the absence
of any SiOH signals. For clarity, the PDMS peak is off-scale in some
spectra.
Preferred La Siting by Experiment and Theory
DFT calculations
probing the expected fate of La-ion siting at low loadings are consistent
with the experimental findings in
Figures
and
. The framework
model had a total Si/Al ratio of 3 to agree with the experimental
29
Si NMR results shown in
Figures S2 and S3
, requiring calculating arrangements of 12 Al atoms per unit
cell. Three different Al distributions were possible following Loewenstein’s
rule: 3 Al in each of the four hexagonal prisms (3333), 3 Al in two
hexagonal prisms with the other two containing 2 and 4 Al atoms (4332),
or all Al atoms in only two hexagonal prisms (6600). Predictably,
the (6600) model was significantly less stable and can be disregarded,
though the other two configurations were energetically similar. While
the experimental
29
Si NMR data were closer to the calculated
spectrum of (4332) than (3333), as seen by comparing
Figures S2 and S3
, there are dissimilarities that indicate
the catalyst is likely comprised of both modeled unit cells. Thus,
the La location was determined for cages connected by a hexagonal
prism with three Al atoms, as shown in
Figure
a. Within the faujasite framework, there
are 7 unique positions in which La is potentially stable, as shown
in the
Figure
c schematic.
DFT calculations show that a single La ion is most stable as La
3+
in site I within the hexagonal prism (
Figure
b). Since acid sites in the hexagonal prism
are included in the H
SD
signal of
H NMR spectra, and a cation in site I is most likely to titrate acid
sites at I and I’ (in the sodalite cage),
58
this corroborates the direct
H NMR experimental
data, which showed a preferential loss of sodalite acid sites. As
La-loading increases, other less stable sites, in which La is more
energetically stable as La(OH)
2+
or La(OH)
are titrated. Even as less favorable sites are titrated,
sodalite sites are still more energetically favorable than supercage
sites, supporting the
H NMR data, which shows that even
as loading increases, sodalite acid sites are preferentially titrated.
We also note that when entropic contributions were included in the
calculations, the thermodynamics for the formation of highly dehydrated
species are more favored due to the release of water molecules upon
La-exchange. Similarly, isolated La species are also more favored
than clustered La at low loadings, and the reader is directed to
Figures S8 and S10
for the quantitative results.
Figure 3.
Open in a new tab
Portions
of the DFT-calculated unit cell of zeolite Y (a) without
La and (b) with La
3+
in the hexagonal prism. (c) Schematic
showing all potential cation locations, and (d) the energy in kJ/mol
of a single La in each framework location as either (red) La
3+
, (orange) La(OH)
2+
, or (gold) La(OH)
The data in
Figures
and
coupled with the calculations
summarized
in
Figure
indicate
that LaY catalysts can be prepared in which La
3+
resides
in sodalite cage sites at the expense of BASs located there, and with
no framework degradation corresponding to the formation of internal
SiOH groups or La
3-n
(OH)
. Further, at
sufficiently high La loadings, almost all H
SD
sites can be titrated, leaving a catalyst that essentially contains
BASs only in the supercage.
Figure
presents
H MAS NMR data for the entire
suite of LaY compositions listed in
Table
, demonstrating that at larger La-ion loadings,
La(OH)
species are observed, as indicated by the broad
signal near 6 ppm for the 0.97-, 2.4-, and 4.8-LaY catalysts. Interestingly,
even though increasing amounts of La(NO)
were used in
the synthesis solutions, the 0.97–4.8 LaY samples all have
the same amount of La in the final catalysts, ca. 10 wt % or a La/Al
ratio of 0.17, as shown in
Table
. LaY catalysts exhibiting the maximum La content that
can be introduced using ion-exchange and heating methods, without
causing any significant framework degradation, are the same catalysts
that contain La(OH)
moieties. For reference,
Figure S4
shows the X-ray diffraction data, which,
along with the
29
Si NMR data previously shown in
Figure S3
, indicates the expected framework crystallinity.
At high La loadings, some amorphous character is indicated by the
broad background of the upper traces in
Figure S4
. Further,
Figure S5
shows examples
of
H MAS NMR spectra for some of the same samples presented
in
Figures
, and
after controlled
framework hydrolysis led to dealumination. In
Figure S5
, intense and clearly resolved signals in the 1.5–2.8
ppm region arising from SiOH and AlOH defect groups are observed,
further supporting the conclusions stated above. With a well-characterized
suite of defect-free variable-loading LaY catalysts, one can now interrogate
the impact of controlled changes in the ratio of [H
SD
]:[H
SC
], the amount of La residing
in the sodalite unit, and the presence of La(OH)
species
on catalyst performance.
Figure 4.
Open in a new tab
H MAS NMR spectra for the entire
suite of LaY catalysts
listed in
Table
showing the appearance of La(OH)
signals at higher La
loadings. Note the absence of any SiOH signals. PDMS denotes the internal
standard polydimethylsiloxane. For clarity, the PDMS peak is off-scale
in some spectra.
Impact of La Distribution on Catalyst Performance
Introducing
lanthanum to zeolite Y is well known to improve the catalyst stability
during hydrothermal regeneration steps, and some reports have also
shown reactivity changes in specific reactions.
26
29
47
57
Given that many prior reports employ catalysts with
varying degrees of framework defects and residual Na cations, a direct
assessment of the impact of La incorporation in sodalite vs supercage
sites, as well as the impacts of La(OH)
on catalyst performance,
is lacking.
Figure
a shows that the addition of only 0.04 La
3+
per
Al causes a significant increase in conversion for the flow-reactor
hexane cracking reaction at 425 °C. The hexane conversion of
the 0.05-LaY catalyst (i.e., 0.04 La per Al) is roughly double that
of an HY sample of equivalent mass. Considering that the incorporation
of La
3+
cations results in a lower concentration of acid
sites, the TOF of the catalyst effectively more than doubles, as shown
in
Figure
b. Given
that at least 80% of the La present in 0.05-LaY is located in the
sodalite cages, this indicates that La
3+
in the sodalite
cages increases hydrocarbon cracking rates under typical gas-phase
reaction conditions. Although an increase in conversion is also seen
in 0.24-LaY compared to 0.05-LaY, it is not proportional to the increase
in La content. The 0.24-LaY has roughly four times the La
3+
content of 0.05-LaY, but its hexane conversion is not even double
that of 0.05-LaY. Interestingly, increasing the lanthanum loading
from 0.24-LaY to 0.97-LaY actually decreases the overall hexane conversion.
A similar loss of activity at high La loadings has been reported in
the literature, though the maximum La content varies significantly
in literature reports, as do differing degrees of framework hydrolysis
and internal defects.
It is not clear
why conversion decreases for the 0.97-LaY, although it is expected
that with increasing La exchange for BAS protons, conversion will
decrease. Indeed,
Figure S7
shows TOF data
as a function of all proton types and versus the sum ([H
SD
] + [H
SC
] + [La(OH)
]), clearly showing a decrease in TOF once lanthanum hydroxides are
detected by the quantitative NMR data in
Figures
and
. From
Figure
, it was observed
that the 0.97-La–Y sample contained La(OH)
species
and no clearly resolved H
SD
sites but fully
resolved H
SC
sites. Indeed, because of the
quantitative nature of the data in
Figure
and fitting the three known species in the
3–6 ppm region to H
SC
, H
SD
, and La(OH)
groups, the total number of
BASs in the 0.97-LaY is 0.25 mmol/g, while the amount of La(OH)
protons is 0.38 mmol/g. In each of the three high-La loading
samples where the broad La(OH)
peak at 6 ppm is observed,
recalling the [H
SD
] and [H
SC
] values from the table on the right of
Figure
, it is clear that these amounts
for the 0.97-LaY are much lower than for HY, 0.05-LaY, and 0.24-LaY,
and thus the conversion decrease is somewhat expected. The clearly
unexpected result is that the HY exhibits much lower conversion than
any
of the LaY’s, and most importantly, that introducing
such a small amount of La at the expense of acidic protons in sodalite
sites only, e.g., the 0.05-LaY, leads to a large increase in conversion.
It is also important to note that, with respect to the data in
Figure
, preparation of
LaY’s requires an additional aqueous exposure step that is
not used in the HY catalyst, as the latter is prepared directly from
deammoniation of the parent NH
Y. If the HY catalyst is
itself exposed to liquid water, as is typical for an aqueous exchange
sequence, then, as previously demonstrated, a significant decrease
in conversion would be expected.
39
Figure 5.
Open in a new tab
(a) Hexane
conversion at 425 °C for HY and LaY catalysts with
La content as indicated. Error bars represent the change in conversion
during the 6 h time on stream in a flow reactor. (b) TOF was normalized
to the sum of ([H
SD
] + [H
SC
]) BAS species. Importantly, additional plots of TOF based on the
number of each type of BAS as well as the sum of all species ([H
SD
] + [H
SC
] + [La(OH)
]) are shown in
Figure S7
, with
the latter demonstrating a decrease in TOF with detectable La(OH)
To further address the unexpected results from
the hexane cracking
experiments and to identify the contributions of La in sodalite sites
to reactivity, in situ isotopic-exchange MAS NMR experiments were
conducted using toluene-d
as the reagent. Rate constants
for the reaction were determined via a least-squares fit of the growth
of the toluene
H NMR peak area, T(t), during the short-time
initial-rate region of the room-temperature
H/
H exchange curve using the equation previously given in the
Experimental section
, assuming first-order kinetics
based on the initial excess of toluene-d
. Isotopic exchange
was observed only for aromatic sites in the toluene and not at the
methyl group, as no signals near 2 ppm were observed, as exemplified
by the typical series of raw data shown in
Figure S5
. Isotopic exchange experiments have been reported for both
toluene and benzene in zeolites, and for HY, it is commonly understood
that these and other typical hydrocarbon reagents cannot access the
sodalite BASs due to steric restrictions
47
59
63
unless specific postsynthetic sodalite-cage-opening reagents are
used.
64
65
The in situ NMR experiments were conducted
at room temperature, approximately 400° below the flow-reactor
hexane cracking experiments, which affords the opportunity to assess
whether La can potentially impact low-temperature processes and expands
the range for identifying potential mechanisms by which both sodalite
BASs and La improve reactivity.
61
64
66
67
Figure
shows a semilogarithmic plot of the growth
of the
toluene peak area vs reaction time in the
H/
H exchange experiment for toluene-d
and the commercial
or parent HY, and the 0.05-LaY samples. In addition, following the
key findings shown in
Figure
quantifying significant increases in [H
SD
] following ammonium exchange of the parent HY, similar data for
the three-times ammonium-exchanged HY (HY–3xNH
and its comparable 0.05-LaY (0.05-LaY–3xNH
) version
are presented. The four catalysts shown in
Figure
are the same as those previously shown in
Figure
, and they are presented
here to separate the impact of La displacing residual Na
from La siting in the sodalite unit itself. The two lower data traces
correspond to the parent HY and its 0.05-LaY variant, while the two
upper data traces are from the HY–3xNH
catalyst
and its corresponding 0.05-LaY–3xNH
. Reaction rate
constants reported in
Figure
generally support findings from the hexane cracking data
in
Figure
, in that
incorporating even the smallest amount of La, with a concomitant decrease
in the number of BASs according to the values for [H
SD
] and [H
SC
] presented in
Figure
increases
the reaction rate constants. For the parent HY, La introduction
leads to a 33% decrease in total [BAS] but increases the rate constant
by 40%.
Figure
shows
that ammonium exchange of the parent HY, yielding the HY–3xNH
catalyst, leads to almost a 3-fold increase in the rate constant.
This increase must be attributed to the significant increase in [H
SD
] (1.7→2.1 mmol/g) resulting from the
removal of residual Na
, since
Figure
showed that [H
SC
remained almost constant after ammonium exchange. Notably, a comparison
of the HY and HY–3xNH
results reveals that increasing
the number of sodalite acid sites can have a large impact on reaction
rates, even for a molecule that is ostensibly too large to access
those sites.
Finally,
Figure
shows that the introduction of La into the HY–3xNH
catalyst, yielding 0.05-LaY–3xNH
increases
the
reaction rate constant by a factor of 2.5 relative to that of its
HY starting material. Again, as shown in the
Figure
data, the 0.05-LaY–3xNH
catalyst has almost 40% less H
SD
sites (1.3
vs 2.1 mmol/g) due to La exclusively displacing sodalite protons,
but the same number of H
SC
sites (1.5 vs 1.5
mmol/g) as its parent HY, and yet it exhibits much higher reactivity.
The high-temperature hexane cracking data and the room-temperature
isotopic exchange results, interpreted with the benefit of the quantitative
information provided by the spin-counting NMR method, reveal that
the exclusive and preferred siting of La cations in sodalite sites
leads to increased catalyst reactivity, even for molecules that are
sterically precluded from such sites. Several postulates have been
published to explain the indirect influence of sodalite acid sites
on reactions,
61
67
and the results shown here, with the additional benefit of quantitative
site specificity and two different reaction types/temperatures, are
commensurate with La cations perturbing electric field gradients within
the catalyst.
62
68
Further,
Figure
shows that increasing amounts of La ultimately
lead to a decrease in hexane-cracking catalyst performance, with a
maximum conversion observed for the 0.24-LaY catalyst. While not shown
in
Figure
, identical
toluene
H/
H exchange experiments were run
for the 0.24- and 0.97-LaY’s, with the 0.24-LaY exhibiting
the largest rate constant for any catalyst prepared from the parent
HY at 3.3 × 10
–4
–1
, in
agreement with the trend shown in
Figure
for hexane cracking. However, the 0.97-LaY
showed an almost 3-fold decrease in rate constant compared to the
0.24-LaY, further demonstrating that as more La is introduced to the
catalyst, the benefit of its incorporation into the preferred sodalite
sites is ultimately negated by the continued decrease in the overall
number of BASs. The La(OH)
species present in the 0.97-LaY
catalysts are not reactive in the toluene reaction, as their peak
area remains constant throughout.
Proposed Explanation of Increased Reactivity in the Presence
of Low La Loadings
Many mechanisms have been proposed to
explain increased reactivity due to cations in the sodalite cages,
and the mechanism proposed here is not unique.
58
63
However, the removal of residual sodium and the lack of framework
defects provide additional support to one proposed mechanism in particular.
The data collected on
H/
H exchanges in HY
and LaY show no indication of exchange taking place on the methyl
group of toluene, since the only
H signal for toluene
at any point in the isotopic exchange arises on the aromatic ring.
As the sodalite BASs are inaccessible to toluene except by the methyl
group and have not been made accessible by framework hydrolysis, this
precludes any direct exchange between toluene and sodalite BASs. Despite
this, the increased reactivity of HY–3xNH
in the
important control experiment described in
Figure
, which gained acid sites primarily in the
sodalite cages relative to the initial HY from which it was prepared,
indicates that proximate H
SD
sites play a significant
role in determining the reaction rate.
The addition of small
amounts of La cations, essentially all of which are present in the
sodalite unit, further increases reactivity. Indeed, La addition increases
reactivity more effectively in the case of HY–3xNH
than in HY. In contrast, higher loadings of La do not necessarily
increase reactivity; for instance, the reaction rate of 0.97-LaY shows
an almost 3-fold decrease relative to 0.24-LaY in the isotopic exchange
studies. It is likely, then, that the mechanism by which La increases
the reaction rates employs proximate H
SD
sites,
and the lack of those sites in samples with excess La inhibits reactivity.
This is consistent with the mechanism proposed for
H/
H exchange with benzene by Almutairi and Hensen, in which
the increased dipole moment of the transition state due to stretching
of the O–H bond is stabilized by the cation in the sodalite
cages—in this case, La
3+
—and its perturbation
of the electric field gradients.
60
62
Other studies
by Hensen, Ryder, and others show that the H
SD
sites participate in hydrogen-transfer reactions by quantum tunneling;
that is, by a proton moving from one oxygen site to another in the
same AlO
tetrahedron.
60
65
68
Thus, while the addition of La
3+
in the sodalite cages
can stabilize the transition state by perturbing the electric field
gradients, it does so most effectively when there are still H
SD
sites present to allow for proton hopping between
sites. Therefore, we can propose that the synergy between proximate
La and H
SD
is greater than that between proximate
SD
sites themselves, which is also consistent
with the hexane cracking TOF data in
Figure
La Impact on HY Stability in the Presence of Ambient Moisture
HY undergoes both irreversible and reversible changes to the framework
in the presence of liquid- and gas-phase water at elevated temperatures,
43
45
and recently it has been shown that internal silanols and aluminols
that are not associated with a BAS, as well as non-framework Al resulting
from dealumination, form under routine room-temperature ion-exchange
conditions.
39
Figure S6
shows
H MAS NMR data for the same series of
catalysts with and without La, before and after exposure to ambient
humidity for 1 week. In all cases, characteristic peaks for SiOH and
AlOH species arising from framework defects are observed following
exposure, as indicated by the large signals in the 1.5–2.8
ppm region. These results are in contrast to what was previously reported
in
Figures
, and
in which defect-free
HY and LaY were measured.
27
Al NMR confirms the presence
of a hexacoordinate Al species (not shown). Quantification of all
species in spectra like those shown in
Figure S6
for HY, 0.05-LaY, 0.24-LaY, 0.97-LaY, and 2.4-LaY reveals
that more than half of the BASs are lost following exposure, indicating
that the presence of La in the catalyst does not prevent water attack
at framework Al sites for the La amounts incorporated here. Indeed,
the 0.24-LaY, which exhibited the best performance in the hexane cracking
and isotopic exchange reactions, loses more than half of its [BAS]
after 1 week of moisture exposure, and the reaction rate constant
for the H/D exchange experiment decreased from the pre-exposure value
of 3.3 × 10
–4
–1
to 0.67
× 10
–4
–1
following exposure.
Conclusions
A combination of spectroscopy and reactor
experiments on HY and
La-exchanged HY catalysts without hydrolysis defects, coupled with
DFT computational results, reveals that La
3+
cations preferentially
reside inside sodalite cages and lead to significant increases in
catalyst reactivity, even though the number of sodalite acid site
protons decreases. Experiments and calculations indicate that lanthanum
exists as La
3+
in sodalite sites and not as hydroxylated
moieties at the low La loadings revealed by the quantitative NMR experiments.
This result is consistent with prior conclusions regarding cation
siting in faujasites based on extensive X-ray analyses.
69
The impact of La incorporation on sodalite vs
supercage acid sites, as well as the introduction of La(OH)
was quantified using spin-counting NMR experiments, which, coupled
with high-temperature flow-reactor data and room-temperature isotopic
exchange reactions, revealed an optimum La loading that, when exceeded,
led to decreases in catalyst performance. At increased La loadings
commensurate with the formation of La(OH)
species, TOFs
decrease in hexane cracking experiments, suggesting that “synergistic”
effects from nonframework lanthanols are not responsible for increased
reactivity. Rather, in total, the experimental and computational results
suggest that proximate La
3+
and H
SD
pairs lead to increased electric field gradients for polarizing
reagents in excess of that afforded by proximate H
SD
pairs. In the absence of La incorporation, quantitative
experiments on defect-free catalyst structures showed that small increases
in the number of sodalite acid sites lead to large increases in catalyst
reactivity, even for reagents that are too large to enter sodalite
cages. In total, these findings indicate not only the preferred siting
of La in sodalite cage positions of HY but also the active role of
both proton and La sites in the sodalite cages, even for molecules
that are ostensibly too large to access them. Further, the data provide
quantitative guidance for determining optimum La amounts to employ
in LaHY catalysts.
Acknowledgments
The authors gratefully acknowledge financial support
provided by Phillips 66 and partial student support from the National
Science Foundation under Grant Nos. CHE-2154398 and CHE-2154399.
Supporting Information Available
The Supporting Information is
available free of charge at
Additional information including XRD, DFT, and NMR data
PDF
The authors
declare no competing financial interest.
Supplementary Material
jp5c00474_si_001.pdf
(714KB, pdf)
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