FCC catalyst testing

One of the major problems in designing improved FCC catalysts is that it is very difficult to scale down the commercial FCC process with its short residence time and rapid deactivation processes. The feedstocks are complex and contain various impurities that can have a major effect on performance, such as Conradsen carbon, metals like Ni and V, oxygenates, and nitrogen- and sulfur-containing molecules. Resid feedstocks require a different operation than VGO, and diesel- or propylene-selective applications again are completely different.

Over the years, various more or less standard methods have been developed for testing FCC catalysts. The first was the “MAT”-test, or Micro Activity Test, according to ASTM D-3907. In this test, a small sample of catalyst is tested in fixed bed. Conversion can be influenced by changing the catalyst-to-oil (CTO) ratio. The test has various drawbacks,  but has nevertheless been very popular over the years. The test contacts the catalyst and feed for prolonged periods, during which deactivation of the FCC catalyst proceeds, and coke- and temperature profiles may develop over the catalyst bed. As a result of the prolonged exposure to feedstock, also the amount of coke deposited on the catalyst material may be unrealistic. The same holds for the observed gas selectivities.

The major drawbacks, concerning contact time and feed vaporization were addressed in various protocols. Kayser developed the so-called ACE (Advanced Cracking Evaluation) units, a catalytic fixed fluid bed system, in which a small catalyst sample (typically about 1 g) is fluidized in a gas stream, and a brief pulse of atomized VGO is passed through the fluidized bed at 538 °C (1000 °F). Another solution capable of handling the heavier feedstock is the Short Contact Time Resid Test, described by Imhof et al. MAT and its refinements (e.g. SCT-MAT and AUTOMAT) and ACE protocols can show ranking differences amongst each other, but also with pilot plant results.

To overcome this, more realistic simulations or even downscaled versions of the riser reactor, like Pilot Riser Units (PRU), have to be applied. The closest approximation on lab scale may be the Micro-riser simulation based on a coiled reactor developed by Dupain et al., and the Micro-downer developed by Corma et al., a moving bed system with short contact time, which also allows testing with heavier feedstocks.

While FCC catalyst testing is already complicated, the protocol will also have to take into account the deactivation of the catalyst during its lifetime of cracking and regeneration cycles. The deactivation of the catalyst is caused by steaming during the regeneration and assisted by the presence of metals like Ni and V (but also Fe, Na and Ca). Deactivated commercial catalysts may contain thousands of ppms of Ni and V, depending on the operation. Mitchell Impregnation (MI) is used to deposit Ni and V on the catalyst particle, usually prior to steaming. The metals are impregnated throughout the catalyst particle, which is maybe (in part) correct for V, but certainly not for Ni. Simple steaming of the catalyst (with or without metals) at increased temperatures mimics the effect of the regenerator in vary crude way.

More realistic procedures mimic the cracking-regeneration cycles, e.g. cyclic propane steaming (CPS), in which the catalysts are exposed to multiple cycles of (propane) cracking, stripping and steaming prior to the actual activity tests. A more elaborate deactivation procedure is the cyclic deactivation (CD) procedure,  in which actual feedstock cracking, depositing metals every cycle, is combined with regeneration for many (up to over 50) cycles to create a more realistic metals profile. Improvements are the two-step CD (2s-CD) and advanced CPS protocols, as described by Psarras et al.

 Zeolite framework stabilization

As mentioned above, the main cracking component in FCC catalysts responsible for the production of gasoline-range molecules is zeolite Y.¹⁹ The structure of zeolite Y, shown in Fig. 10, has a 3-D pore system, in which pores of ∼7.3 Å connect larger (13 Å in diameter) cages, which are known as the supercages of this zeolite.

Fig. 10 The structure of zeolite Y (Faujasite), with the most relevant ion-exchange sites highlighted. The effect of RE-introduction: XRD: comparing RE-stabilized (blue) with non-stabilized Y-zeolite (red), we observe a shift to lower angles (i.e. larger unit cell size, lower SAR, less dealumination), as well as higher crystallinity in the RE-stabilized material; IR: we observe a shift to lower frequency (lower SAR, less dealumination) for the RE-stabilized form; NMR: we observe larger contributions from Si-species with multiple Al-neighbors (i.e., a lower SAR, less dealumination). All spectra are simulated based on literature data from Roelofsen and Scherzer et al.

The addition of solid acids to the catalyst improves both the conversion as well the product selectivity towards gasoline. The original FCC catalyst contained clay, and later amorphous silica-alumina and silica-magnesia. The advent of zeolite-based catalytic cracking was seen shortly after their discovery at Union Carbide,  in the early 1960's. Zeolite Y combines high surface area/pore volume solid acidity (both Brønsted and Lewis) with sufficient room to allow bimolecular (carbenium ion) cracking. The preparation of the zeolite is relatively simple, no organic Structure Directing Agents (SDAs) or even autoclaves are required. However, the as-prepared zeolite is not very stable towards hydrothermal conditions. The stability can be improved by controlled steaming and washing/leaching cycles (to make the so-called ultra stable Y, or US-Y).

A well-known way to improve the effectiveness of the zeolite (i.e. to retain activity longer) is to exchange part of the counter-ions with rare earth (RE) ions. There is a lot of literature on the effect of RE ions on zeolite stability and reaction characteristics. A large body of work in this area was already performed in the 1970's and 1980's.  For example, Rees et al.⁵¹ show that the exothermic peak in differential thermal analysis, which is interpreted as a collapse of the framework, shifts towards higher temperature for RE-exchanged faujasite versus Na-exchanged faujasite. This framework collapse occurs in the range of 800–1000 °C, so outside of the temperature range relevant for FCC. Nevertheless, the effect is an indication for increased lattice thermal stability.  Flanigen et al. provide an assignment of the IR vibrations observed for zeolite Y. Roelofsen et al.⁴⁶ explain that the symmetric stretch vibration at around 790 cm⁻¹ is the most suited to derive the framework silicon-to-aluminum ratio, shortened as SAR, because other IR peaks are more sensitive to the type and amount of cations in the framework, crystallinity, as well as water content. The peak frequency of the IR band at around 790 cm⁻¹ has found to be linearly proportional to the Al/(Al + Si)-ratio.

Rabo et al. describe two IR peaks related to hydroxyl groups in RE-Y. The first peak, at 3640 cm⁻¹, shows strong hydrogen bonding with water, benzene and ammonia, and can thus be interpreted as a Brønsted acid site exposed in the supercage. The other OH-vibration, centered at 3524 cm⁻¹, does not bind with ammonia or benzene, and is thus hidden inside the sodalite cage. Rabo et al. assume these hydroxyls are associated with OH-groups retained between two RE-cations as an electrostatic shield.  Roelofsen et al. investigated the dealumination of zeolite Y with varying loading of RE₂O₃ (mixed rare earths) with IR, XRD, and Si MAS NMR. They find a good correlation between the framework SAR derived from IR and from ²⁹Si MAS NMR. However, the correlation with the SAR derived from the unit cell size using the Breck–Flanigen relation does not hold in this case. The unit cell size is significantly larger than would be expected from the Breck–Flanigen relation. This indicates that the unit cell size is not a good indicator for lattice stabilization.

A variety of authors studied the stability of RE-exchanged zeolite Y in the 1960's and 1970's, mostly based on IR-analyses. Scherzer et al.⁴⁷ conclude that framework vibrations shift to higher frequencies, and the XRD unit cell size decreases, upon increased severity of the thermal treatment. In both cases there is more or less linear dependence of the effect with the RE-loading. In a subsequent paper, Scherzer and Bass⁵⁵ look at the OH-stretching region of the same samples. They conclude that bands at 3600 and 3700 cm⁻¹ indicate that the framework is dealuminated. Bands at 3650 and 3600 cm⁻¹ are shown to be acidic (from interaction with ammonia, pyridine, and sodium hydroxide). They also observed a band at 3540 cm⁻¹, which they ascribe to OH groups attached to lanthanum ions, although there also appears to be a framework band in the same IR region.

Fallabella et al.  study the effects of using different RE-ions in the ion exchange process of zeolite Y. The introduction of RE cations brought about no significant changes in the structural region of the zeolites. However, in the hydroxyl region, a band ranging from 3530 to 3498 cm⁻¹ was observed.

This band, attributed to OH groups interacting with RE-cations (see also Scherzer and Bass), is shifted to higher wavenumbers as the ionic radius of the cations increases. This hydroxyl is not acidic (or at least not active in catalysis), as it resides in the sodalite cages. The authors do note a clear effect of the radius of the RE-ion on the acidity as probed with pyridine and lutidine. Pyridine is capable of detecting both Brønsted and Lewis acid sites, whereas lutidine can only interact Brønsted acid sites due to sterical hindrances generated by the methyl groups. In their study, Dysprosium falls outside the plotted correlation, possibly because remaining chloride ions create extra activity. Van Bokhoven et al. report that high-charge octahedral extra-framework Al in US-Y, as well as La³⁺ ions in the ion exchange positions in La(x)NaY induce local polarization of the Al-atoms in the lattice. In addition, a long-range effect is observed which causes the T–O–T angles to increase (and thus the unit cell size to increase). The authors thus assume that although the type of ion is different, the origin of the enhanced activity in US-Y and RE-Y is identical. Most authors claim that rare earth elements stabilize the zeolites by moving into the hexagonal prisms (site S-I), and retaining the framework Al by some form of electrostatic interaction. Excess rare earth migrates from the hexagonal prism into the supercage (site S-II), and forms strong Brønsted acid sites in connection with framework Al.

Du et al.  claim that the ionic radius of different RE elements has an effect on the stability of the RE-Y zeolite and the framework stability increases with decreasing ionic radius for the set Ho³⁺, Dy³⁺, Nd³⁺, La³⁺. Ce³⁺ does not seem to move into the S-I positions, because under the conditions applied by Du et al. the cerium gets oxidized to Ce⁴⁺, and forms a larger complex that cannot migrate into the sodalite cages.

Schüßler et al. investigated the nature and location of La-species in faujasite with a combination of techniques, including DFT calculations. In order to make full periodic calculations possible, they selected the rhombohedral primitive cell of faujasite. This reduces the number of framework atoms by a factor of 4, from 576 to 144. The authors find small amounts of [La(OH)]²⁺ and [La(OH)₂]⁺-species in the S-II sites, but claim the majority of the La³⁺ is present in the sodalite cages in multinuclear OH-bridged aggregates. The formation of the hydroxylated clusters leads to the formation of Si–OH–Al groups at a distance to the La-clusters. However, the authors claim that isolated La³⁺ species in the S-II site are also able to polarize secondary and tertiary C–H bonds and thus activate alkanes, and point to these species as responsible for the enhanced activity and hydrogen transfer of RE-exchanged zeolites.

Noda et al. performed a combination of temperature programmed desorption (TPD) of NH₃ with DFT cluster calculations. They examined Ba-, Ca-, and La-exchanged zeolite Y and observe an increase in catalytic activity for all ion-exchanged zeolites with the Ba ones producing the lowest activity. They ascribe the formation of stronger acid sites to a removal of OH-sites in the sodalite cages and hexagonal prisms, and strengthening of the supercage-OH sites by a polarization effect induced by the cations. From the above, it is clear that the presence of RE-cations in the structure provide some form of stabilization, to the extent that more aluminum is retained in the lattice as observed with IR and NMR. XRD unit cell size analysis does not correlate with IR and NMR measurements in the normal way for RE-containing zeolite Y.

The effect of the presence of RE in the lattice on performance is dramatic. Plank et al. already in the early 1960's noted an appreciable increase in activity (more than 100 times as active as amorphous silica-alumina's) when using RE-stabilized Y zeolites, although they compared their materials to amorphous SiO₂–Al₂O₃ and Na–Y. Although the activity increase is desirable, the incorporation of RE also increases the rate of hydrogen transfer, which leads to a less desirable drop in research octane number and olefinicity in the LPG range. Fallabella et al. define a hydrogen transfer (HT) index derived from the ratio of different reaction rate constants in the cracking of cyclohexene, which correlates with the atomic ratio of the RE-ion and the acidity. Lemos et al. studied heptane cracking on RE-exchanged Y-zeolites, and observed mainly paraffinic cracking products. The cracking activity seems to correlate with strong protonic acidity, as derived by reactivity comparison.