Chemical industries heavily rely on the use of heterogeneous catalysts. The development
of more sustainable chemical processes requires, however, better catalyst formulations
and ultimately tailoring of these catalytic materials for a specific application.
A showcase example is fluid catalytic cracking (FCC), which is industrially applied
to convert heavy oil fractions into more valuable chemicals, such as gasoline and
olefins.1 The detrimental effect of metals originating from crude oil, especially
Ni and V, on FCC catalysts is widely recognized.2 The metals damage the active zeolite
phase, being either ultrastable zeolite Y (USY) or zeolite ZSM-5. As a result, pore
accessibility and acidity are decreased, while dehydrogenation–hydrogenation reactions
are favored, leading to increased coke formation. In the case of Ni, the main detrimental
effect is coke formation,3 while V poisoning is associated with permanent zeolite
damage in the presence of steam at high temperatures.2a
Several research groups have attempted to understand the mechanism of metal poisoning
by observing the distribution of metals across the 50–150 μm FCC catalyst particle
and between the different catalyst components; that is, zeolite, matrix (for example,
alumina and clay), and additives, which comprise the FCC catalyst particle. Such characterization
studies have been mostly conducted with invasive characterization methods.4 These
investigations revealed that V is much more mobile than Ni and proceeds more quickly
towards the interior of the FCC catalyst particle.4d
The mayor drawback with these previous studies is that they require an invasive preparation
step, where the FCC catalyst particle is cut along the desired plane of analysis.
This bisection is far from trivial and in most of the cases an alteration in the distribution
of the distinct FCC components is observed. Therefore, a non-invasive approach can
be expected to deliver more truthful information about the location of metal poisons.
Furthermore, the relationship between metal poisoning and zeolite deactivation is
not well understood, and to date there are no studies providing detailed information
about the effect of deactivation on the crystalline zeolite structure within an individual
FCC catalyst particle.
Herein we report for the first time the detrimental effect of metal poisons on the
zeolitic material after deactivation in a commercial FCC unit at the level of a single
catalyst particle. Using synchrotron-based hard X-ray radiation, the presence of Ni,
V, as well as the crystalline phases can be determined with micrometre resolution
in 2D or 3D. Furthermore, the non-invasive nature of the experimental approach avoids
the pre-bisection of the FCC particle, avoiding damage and contamination to the catalyst
material. Our findings lead to a better understanding of the deactivation processes
taking place in real-life FCC catalysis and open the possibility to apply this approach
for the study of other important catalytic materials, comprising both metals and crystalline
phases.
Recent developments in synchrotron radiation now make it possible to image catalyst
materials with high spatial resolution in a non-invasive fashion.5 Microfocus X-ray
fluorescence (μ-XRF) microscopy is a powerful imaging technique widely applied in
several disciplines.6 Furthermore, when combined with a scanning monochromator, it
is possible to obtain microfocus X-ray absorption near-edge structure (μ-XANES) data
so as to be able to obtain information on the oxidation and coordination state of
the elements of interest. However, these methods fall short on giving information
about the crystalline phases of the catalytic material; for example, the zeolite and
clay phase. This can be done by using micro X-ray diffraction (μ-XRD), more in particular,
XRD computed tomography (XRD-CT), which has been demonstrated recently in the field
of heterogeneous catalysis to gain spatiotemporal insight into the active phase of
a nickel supported γ-alumina catalyst body.7 Therefore, to accurately understand how
metal poisons deactivate FCC catalysts at the single particle level and assess their
effect on the local structure of the zeolite material embedded in the FCC matrix,
the application of an integrated method comprising μ-XRF, μ-XANES, and μ-XRD is required
in one set-up. This unique multipronged approach is illustrated in Figure 1; more
details about the experimental approach can be found in the Supporting Information.
Figure 1
Investigating the deactivation of an individual catalyst particle mounted on a holder
attached to a goniometer. μ-XRF and μ-XANES are collected by a polycapillary with
confocal arrangement, which is placed on one side of the sample, while μ-XRD is measured
in transmission mode by a 4000×2500 pixel CCD camera and subsequently radially integrated
and transformed into 1D XRD patterns. The corresponding individual phases can be selected
from the 1D XRD patterns and sinograms can be constructed. Finally, the sinograms
can be back-projected, which provide the 2D maps of the corresponding crystalline
phases present. Scale bars: 20 μm.
In a first step, 2D μ-XRF images of Ni and V were recorded on the FCC catalysts (Figure
2 a–f); 3D μ-XRF images were also acquired (Supporting Information, Movies S1 and
S2). As Ni and V are not included in the catalyst formulation, no traces of these
metals were observed in the fresh FCC sample (Figure 2 a and d). The same approach
was applied for an equilibrium FCC catalyst (further denoted as Ecat), which is the
fresh catalyst material after catalytic testing in an industrial cracking unit during
real operation. In this sample, Ni and V were present with an uneven 2D distribution
(Figure 2 b,c and e,f). A pronounced egg-shell profile was found for Ni with a shell
thickness of 10–15 μm. Interestingly, V X-ray fluorescence was seen to span more evenly
across the FCC particle, although its signal still strongly decreases in intensity
with increasing probing depth within the particle. These results are in good agreement
with previous invasive studies on FCC particles, which were bisected and metal profiles
monitor with SIMS
[4c] and SEM-EDX.8 It should be noted that the Ni and V X-ray fluorescence on the
right side of the FCC particle starts to fade, which is due to X-ray fluorescence
attenuation. To ensure that such fluorescence attenuation does not affect the distribution
of the metal poisons, an additional μ-XRF study was carried out. Identical μ-XRF measurements
were performed on FCC catalysts particles impregnated with Ni and V, namely a Mitchell
FCC catalyst particle. This lab-based deactivation method leads to an even Ni and
V distribution across the FCC particle (Figure 2 c and f) and therefore the influence
of X-ray attenuation can be accurately determined. More details on this reference
experiment can be found in the Supporting information. 1D XRF intensity profiles (Figure
2 h and i) corroborate these findings.
Figure 2
μ-XRF 2D chemical maps of Ni (a–c) and V (d–f) for a fresh (a,d), Ecat (b,e), and
Mitchell (c,f) FCC catalyst particle. The orange lines illustrate the position where
the intensity profiles were taken. Scale bars: 20 μm. g–i) 1D X-ray fluorescence intensity
profiles as a function of the position inside the FCC catalyst particle derived from
the 2D images for a g) fresh, h) Ecat, and i) Mitchell FCC catalyst particle. j) Ni
and k) V μ-XANES spectra of the spots highlighted with the orange circle for the Ecat
particle. The spatial resolution of μ-XRF maps and μ-XANES spectra was 5×5 μm2.
Ni and V K-edge XANES spectra of the Ecat sample were then collected and the results
are given in Figure 2 j and k, respectively. The results derived from the first-derivative
data reveal that the Ni K-edge is at 8343 eV, which is indicative for Ni2+.9 The small
pre-edge at 8330.5 eV is attributed to the 1s–3d transition and suggests the presence
of an oxide-type phase, most likely NiAl2O4 and/or NiO species in which the Ni2+ is
six-coordinate.10 In the case of V, the XANES spectrum shows a K-edge at 5480.4 eV
and a pre-edge peak at 5469.4 eV. The position of both features is indicative for
a mixture of V4+ and V5+. Furthermore, the intensity of the pre-edge peak implies
that most likely there is a mixture of octahedral V2O4 and square-pyramidal V2O5 species.11
To validate our findings, an additional XANES study was performed where spectra from
V2O4 and V2O5 reference compounds and bulk measurements of the Ecat sample were measured
(Supporting Information, Figure S1).
μ-XRD-CT provides complementary information regarding the crystalline phases. Representative
XRD patterns of both samples (Figure 3) display the sum of the diffraction patterns
recorded for the acquired projections of the middle plane of fresh and an Ecat FCC
catalyst particles. This represents an average of 2700 collected μ-XRD patterns per
sample and allows the detection of all of the crystalline phases present. A number
of reflections can be identified for the fresh FCC catalyst particle, which together
represents all the FCC components with the exception of silica (amorphous). More specifically,
the diffraction patterns of zeolite Y, kaolinite, and boehmite can be identified in
the fresh FCC particle. An additional phase, anatase, is also detected, which most
probably originates from TiO2 impurities in the kaolinite clay fraction. Interestingly,
the summed XRD pattern of the Ecat sample is very different to that of the fresh sample.
More specifically, the reflections corresponding to zeolite Y are less intense, which
can be related to a decrease in the overall crystallinity of the zeolite material.
Kaolinite, which has a layered structure, undergoes also a series of phase transformations
resulting in the formation of a mullite phase. Boehmite dehydrates and subsequently
forms a γ-alumina phase. Finally, no phases from the Ni and V could be detected, which
suggest that they are below the detection limit or distributed as small oxide and
aluminate nanoparticles.
Figure 3
Summed XRD patterns of an individual fresh and Ecat FCC catalyst particle. The colored
squares denote the characteristic reflections of the crystalline phases present; asterisks
refer to the reflections selected for the 2D reconstruction of the crystalline phases.
These identified XRD-active phases can be also transformed into 2D distribution maps
by using a reconstruction algorithm. More details about this mathematical procedure
can be found in the Supporting information as well as in recent literature.12 Figure
4 a–f illustrate how this approach can discriminate between the different phases present
and also spatially resolve them. Furthermore, the transformation of these crystalline
components upon poisoning/deactivation can be assessed by using a specific diffraction
peak per phase, indicated in the summed diffraction patterns by asterisks in Figure
3.
Figure 4
2D maps of a) zeolite Y, b) clay, and c) boehmite for a fresh FCC particle, and 2D
maps of d) zeolite Y, e) mullite, and f) γ-alumina for the Ecat particle. g) Magnification
of the (111) diffraction peak for the fresh (lower) and Ecat sample (upper) to evaluate
the peak shift experienced after deactivation. 2D reconstructions of the zeolite Si/Al
ratio for the h) fresh and i) Ecat samples; thermal scale bar shows the position of
the diffraction peak for the fresh and the calculated zeolite Si/Al ratio for the
Ecat FCC catalyst particle. Scale bars: 20 μm.
We now focus our attention on the physicochemical changes taking place in zeolite
Y upon deactivation/poisoning. By scanning the whole FCC particle 2D zeolite Y maps
can be generated from the most intense diffraction peak at 2θ≍3.3°; that is, the (111)
reflection. Figure 4 a reveals a random distribution of the zeolite component throughout
the fresh FCC particle, which is in agreement with previous studies.13 Interestingly,
after deactivation, the zeolite undergoes significant changes in crystallinity and
spatial distribution, leading to a less ordered material, with a preferential destruction
of the zeolite at the periphery of the particle, resulting in an egg-yolk distribution
of the zeolitic material (Figure 4 d). Our previous results using confocal fluorescence
microscopy and a fluorescent probe reaction that is very sensitive to the acidity
of the zeolite were unable to monitor such a zeolitic distribution in the interior
of the FCC catalyst bodies.13a This is most probably due to the attenuation suffered
by the fluorescent light in the interior of the catalyst particle, which allows investigation
of only the first 10–15 μm of the material in an accurate manner. More importantly,
the egg-yolk distribution of the zeolite phase mirrors with the egg-shell distribution
of the poisons in the Ecat sample, providing direct evidence for the negative effect
of Ni and V on the crystallinity of the zeolite phase.
In a next step, we aimed for a better understanding of this 2D egg-yolk zeolite phase
degradation by studying in more detail the obtained XRD patterns. Inspection of the
(111) reflection (peak at 2θ≍3.3°, as depicted in Figure 4 g) and the (220) reflection
(peak at 2θ≍5.4°; Supporting Figure S2a) reveals significant peak shifts at different
locations in the FCC catalyst particle. To corroborate that these peak shifts are
not just artefacts that are due for example to changes in the experimental setup geometry
by a longer distance between the sample and the CCD camera detector, bulk XRD analysis
with a standard powder X-ray diffractometer were performed (Supporting Information,
Figure S3). However, this analysis rules out this hypothesis and confirms that the
peak shifts are real. These peak shifts to higher 2θ values are well-described in
the literature and indicate a shift towards a smaller d-spacing and reduced unit cell
parameters of the zeolite, as a result of partial zeolite dealumination.14 As a consequence
of this dealumination process, the zeolite experiences a loss in Brønsted acidity,
which is one of the main causes of catalyst deactivation.15 Therefore, our approach
allows an estimation of the extent of dealumination, which can be represented in the
form of a 2D map of Si/Al ratios across an Ecat FCC catalyst particle (Figure 4 i).
Although a narrow range of peak positions is evenly distributed for the fresh FCC
catalyst particle (Figure 4 h), the Ecat FCC particle shows a shift in 2θ positions
to higher 2θ values, more specifically in the periphery, which can be translated into
a higher degree of zeolite dealumination in the outer rim of the particle. Translating
the peak shifts into Si/Al ratios is possible by making use of a proper calibration
curve (Supporting Information, Figure S4). A unit cell analysis of bulk powder XRD
patterns was performed for zeolite Y samples with known Si/Al ratios. Furthermore,
the same was carried out for fresh catalyst particles, which were deactivated under
distinct steaming conditions. The corresponding d-values from the samples were then
used to estimate a particular Si/Al ratio for the Ecat samples under study. The fresh
sample shows a range of Si/Al ratio, which is in line with the values that the catalyst
manufacturer employs for the synthesis of the catalyst particles. The unequal spatial
zeolite destruction and related dealumination of a single FCC catalyst particle (Figure
4 i) is very significant as the Si/Al ratios range from about 60 at the periphery
to circa 35 in the interior of the Ecat FCC catalyst particle. Furthermore, it has
to our best knowledge never been reported and can be explained in two ways. A first
plausible explanation is that Ni and V, in combination with high boiling point hydrocarbons,
are unable to access the center of the particle and therefore results in significantly
higher coke formation at the external part of the particle. During regeneration, coke
combustion will create hot spots and therefore this localized severe hydrothermal
condition will have a higher impact on the zeolite destruction and related dealumination.
A second reasoning relates to the egg-shell distribution of V, which promotes zeolite
destruction by mechanisms described in the literature.2a
In conclusion, the developed combined μ-XRF/μ-XANES/μ-XRD approach is capable of revealing
2D and 3D chemical information in commercially used FCC catalyst materials at the
individual particle level. μ-XRF revealed in an Ecat particle egg-shell distributions
for V and Ni, while μ-XRD indicated for the same catalyst particle egg-yolk distributions
for zeolite crystallinity and the Si/Al ratio, directly linking the detrimental effect
of metal poisoning with zeolite destruction and dealumination. It is clear that this
multipronged X-ray microscopy method offers great potential for the 2D and 3D spatiotemporal
characterization of other catalyst materials containing metals and crystalline structures.