Fluid catalytic cracking: recent developments on the grand old lady of zeolite catalysis

Simulation Study on the Reaction-Diffusion Coupling in Simple Pore Structures

Most porous media (just like catalyst pellets) have complicated pore structures, and understanding the coupling of the diffusion and reaction processes in these pores is very important for improving their performance. In this work, a diffusion factor (D) and a reaction factor (R) are proposed to quantitatively describe the diffusion and reaction performance in these pores respectively at molecular level. The yield in unit time is used to quantify their productivity and is expressed as the product of D and R. Molecular dynamic simulations with the hard-sphere algorithm are carried out to study the reaction-diffusion coupling in several simple pore structures with the same volume, such as straight, T-shaped, and cross-shaped pores. The reaction formula based on activation energy is given for a simple irreversible reaction process from A to B. In terms of the proposed factors, D and R, analysis on the simulation results shows clearly that the overall productivity of these pore structures depends on the competition of D and R, which are both determined by the size and shape of the pore structures. The results demonstrate the effectiveness of the simulation approach used for evaluating the performance of the simple pore structures for simple reactions and the potential of its application in more complicated and practical cases. It also suggests the effectiveness of the proposed factors, D and R, for charactering the diffusion and reaction processes at molecular level.

A three-dimensional view of structural changes caused by deactivation of fluid catalytic cracking catalysts

Since its commercial introduction three-quarters of a century ago, fluid catalytic cracking has been one of the most important conversion processes in the petroleum industry. In this process, porous composites composed of zeolite and clay crack the heavy fractions in crude oil into transportation fuel and petrochemical feedstocks. Yet, over time the catalytic activity of these composite particles decreases. Here, we report on ptychographic tomography, diffraction, and fluorescence tomography, as well as electron microscopy measurements, which elucidate the structural changes that lead to catalyst deactivation. In combination, these measurements reveal zeolite amorphization and distinct structural changes on the particle exterior as the driving forces behind catalyst deactivation. Amorphization of zeolites, in particular, close to the particle exterior, results in a reduction of catalytic capacity. A concretion of the outermost particle layer into a dense amorphous silica-alumina shell further reduces the mass transport to the active sites within the composite.Catalyst deactivation in fluid catalytic cracking processes is unavoidably associated with structural changes. Here, the authors visualize the deactivation of zeolite catalysts by ptychography and other imaging techniques, showing pronounced amorphization of the outer layer of the catalyst particles.

Localization and Speciation of Iron Impurities within a Fluid Catalytic Cracking Catalyst

Fluid catalytic cracking is a chemical conversion process of industrial scale. This process, utilizing porous catalysts composed of clay and zeolite, converts heavy crude-oil fractions into transportation fuel and petrochemical feedstocks. Among other factors iron-rich reactor and feedstream impurities cause these catalyst particles to permanently deactivate. Herein, we report tomographic X-ray absorption spectroscopy measurements that reveal the presence of dissimilar iron impurities of specific localization within a single deactivated particle. Whereas the iron natural to clay in the composite seems to be unaffected by operation, exterior-facing and feedstream-introduced iron was found in two forms. Those being minute quantities of ferrous oxide, located near regions of increased porosity, and impurities rich in Fe³⁺ , preferentially located in the outer dense part of the particle and suggested to contribute to the formation of an isolating amorphous silica alumina envelope.

Single-Molecule Fluorescence Microscopy Reveals Local Diffusion Coefficients in the Pore Network of an Individual Catalyst Particle

We used single-molecule fluorescence microscopy to study self-diffusion of a feedstock-like probe molecule with nanometer accuracy in the macropores of a micrometer-sized, real-life fluid catalytic cracking (FCC) particle. Movies of single fluorescent molecules allowed their movement through the pore network to be reconstructed. The observed tracks were classified into three different states by machine learning and all found to be distributed homogeneously over the particle. Most probe molecules (88%) were immobile, with the molecule most likely being physisorbed or trapped; the remainder was either mobile (8%), with the molecule moving inside the macropores, or showed hybrid behavior (4%). Mobile tracks had an average diffusion coefficient of D = 8 × 10-14 ± 1 × 10-13 m2 s-1, with the standard deviation thought to be related to the large range of pore sizes found in FCC particles. The developed methodology can be used to evaluate, quantify and map heterogeneities in diffusional properties within complex hierarchically porous materials.

Probing Gas Adsorption in Zeolites by Variable-Temperature IR Spectroscopy: An Overview of Current Research

The current state of the art in the application of variable-temperature IR (VTIR) spectroscopy to the study of (i) adsorption sites in zeolites, including dual cation sites; (ii) the structure of adsorption complexes and (iii) gas-solid interaction energy is reviewed. The main focus is placed on the potential use of zeolites for gas separation, purification and transport, but possible extension to the field of heterogeneous catalysis is also envisaged. A critical comparison with classical IR spectroscopy and adsorption calorimetry shows that the main merits of VTIR spectroscopy are (i) its ability to provide simultaneously the spectroscopic signature of the adsorption complex and the standard enthalpy change involved in the adsorption process; and (ii) the enhanced potential of VTIR to be site specific in favorable cases.

Ruthenium(II)-Catalyzed C-C Arylations and Alkylations: Decarbamoylative C-C Functionalizations

Ruthenium(II)biscarboxylate catalysis enabled selective C-C functionalizations by means of decarbamoylative C-C arylations. The versatility of the ruthenium(II) catalysis was reflected by widely applicable C-C arylations and C-C alkylations of aryl amides, as well as acids with modifiable pyrazoles, through facile organometallic C-C activation.

A DNP-supported solid-state NMR study of carbon species in fluid catalytic cracking catalysts

A combination of solid-state NMR techniques supported by EPR and SEM-EDX experiments was used to localize different carbon species (coke) in commercial fluid catalytic cracking catalysts. Aliphatic coke species formed during the catalytic process and aromatic coke species deposited directly from the feedstock respond differently to dynamic nuclear polarization signal enhancement in integral and crushed FCC particles, indicating that aromatic species are mostly concentrated on the outside of the catalyst particles, whereas aliphatic species are also located on the inside of the FCC particles. The comparison of solid-state NMR data with and without the DNP radical at low and ambient temperature suggests the proximity between aromatic carbon deposits and metals (mostly iron) on the catalyst surface. These findings potentially indicate that coke and iron deposit together, or that iron has a role in the formation of aromatic coke.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

In the future, (electro-)chemical catalysts will have to be more tolerant towards a varying supply of energy and raw materials. This is mainly due to the fluctuating nature of renewable energies. For example, power-to-chemical processes require a shift from steady-state operation towards operation under dynamic reaction conditions. This brings along a number of demands for the design of both catalysts and reactors, because it is well-known that the structure of catalysts is very dynamic. However, in-depth studies of catalysts and catalytic reactors under such transient conditions have only started recently. This requires studies and advances in the fields of 1) operando spectroscopy including time-resolved methods, 2) theory with predictive quality, 3) kinetic modelling, 4) design of catalysts by appropriate preparation concepts, and 5) novel/modular reactor designs. An intensive exchange between these scientific disciplines will enable a substantial gain of fundamental knowledge which is urgently required. This concept article highlights recent developments, challenges, and future directions for understanding catalysts under dynamic reaction conditions.

Sulfonic acid functionalized silica nanoparticles as catalysts for the esterification of linoleic acid

Sulfonic acid functionalized SiO₂-nanoparticles showed excellent performance in linoleic acid esterification: 100% conversion, 2 h reaction time, TOF = 53–498 h⁻¹.

Neutron spectroscopy as a tool in catalytic science

The unique power of neutron spectroscopy to probe molecular behaviour in catalytic systems is illustrated. Vibrational spectroscopy and quasielastic scattering techniques are introduced, along with their use in probing methanol-to-hydrocarbons and methane reforming catalysis, and also hydrocarbon behaviour in microporous catalysts.

Surface modification of a polyvinyl alcohol sponge with functionalized boronic acids to develop porous materials for multicolor emission, chemical sensing and 3D cell culture

Surface modification of a polyvinyl alcohol sponge with functionalized boronic acids led to the formation of porous materials applicable for multicolor emission, chemical sensing and 3D cell culture.

Crude oil to chemicals: light olefins from crude oil

The possibility to fulfill the increasing market demand and producers' needs in processing straightforwardly crude oil, a cheap and universally available feedstock, to produce petrochemicals appears to be a very attractive strategy.

Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal

Visualizing proton-transfer processes at the nanoscale is essential for understanding the reactivity of zeolite-based catalyst materials. In this work, the Brønsted-acid-catalyzed oligomerization of styrene derivatives was used for the first time as a single molecule probe reaction to study the reactivity of individual zeolite H-ZSM-5 crystals in different zeolite framework, reactant and solvent environments. This was accomplished via the formation of distinct dimeric and trimeric fluorescent carbocations, characterized by their different photostability, as detected by single molecule fluorescence microscopy. The oligomerization kinetics turned out to be very sensitive to the reaction conditions and the presence of the local structural defects in zeolite H-ZSM-5 crystals. The remarkably photostable trimeric carbocations were found to be formed predominantly near defect-rich crystalline regions. This spectroscopic marker offers clear prospects for nanoscale quality control of zeolite-based materials. Interestingly, replacing n-heptane with 1-butanol as a solvent led to a reactivity decrease of several orders and shorter survival times of fluorescent products due to the strong chemisorption of 1-butanol onto the Brønsted acid sites. A similar effect was achieved by changing the electrophilic character of the para-substituent of the styrene moiety. Based on the measured turnover rates we have established a quantitative, single turnover approach to evaluate substituent and solvent effects on the reactivity of individual zeolite H-ZSM-5 crystals.

Visualizing Dealumination of a Single Zeolite Domain in a Real-Life Catalytic Cracking Particle

Fluid catalytic cracking (FCC) catalysts play a central role in the chemical conversion of crude oil fractions. Using scanning transmission X-ray microscopy (STXM) we investigate the chemistry of one fresh and two industrially deactivated (ECAT) FCC catalysts at the single zeolite domain level. Spectro-microscopic data at the Fe L3 , La M5 , and Al K X-ray absorption edges reveal differing levels of deposited Fe on the ECAT catalysts corresponding with an overall loss in tetrahedral Al within the zeolite domains. Using La as a localization marker, we have developed a novel methodology to map the changing Al distribution of single zeolite domains within real-life FCC catalysts. It was found that significant changes in the zeolite domain size distributions as well as the loss of Al from the zeolite framework occur. Furthermore, inter- and intraparticle heterogeneities in the dealumination process were observed, revealing the complex interplay between metal-mediated pore accessibility loss and zeolite dealumination.

Relating structure and composition with accessibility of a single catalyst particle using correlative 3-dimensional micro-spectroscopy

To understand how hierarchically structured functional materials operate, analytical tools are needed that can reveal small structural and chemical details in large sample volumes. Often, a single method alone is not sufficient to get a complete picture of processes happening at multiple length scales. Here we present a correlative approach combining three-dimensional X-ray imaging techniques at different length scales for the analysis of metal poisoning of an individual catalyst particle. The correlative nature of the data allowed establishing a macro-pore network model that interprets metal accumulations as a resistance to mass transport and can, by tuning the effect of metal deposition, simulate the response of the network to a virtual ageing of the catalyst particle. The developed approach is generally applicable and provides an unprecedented view on dynamic changes in a material's pore space, which is an essential factor in the rational design of functional porous materials.

FIB-SEM Tomography Probes the Mesoscale Pore Space of an Individual Catalytic Cracking Particle

The overall performance of a catalyst particle strongly depends on the ability of mass transport through its pore space. Characterizing the three-dimensional structure of the macro- and mesopore space of a catalyst particle and establishing a correlation with transport efficiency is an essential step toward designing highly effective catalyst particles. In this work, a generally applicable workflow is presented to characterize the transport efficiency of individual catalyst particles. The developed workflow involves a multiscale characterization approach making use of a focused ion beam-scanning electron microscope (FIB-SEM). SEM imaging is performed on cross sections of 10.000 μm2, visualizing a set of catalyst particles, while FIB-SEM tomography visualized the pore space of a large number of 8 μm3 cubes (subvolumes) of individual catalyst particles. Geometrical parameters (porosity, pore connectivity, and heterogeneity) of the material were used to generate large numbers of virtual 3D volumes resembling the sample's pore space characteristics, while being suitable for computationally demanding transport simulations. The transport ability, defined as the ratio of unhindered flow over hindered flow, is then determined via transport simulations through the virtual volumes. The simulation results are used as input for an upscaling routine based on an analogy with electrical networks, taking into account the spatial heterogeneity of the pore space over greater length scales. This novel approach is demonstrated for two distinct types of industrially manufactured fluid catalytic cracking (FCC) particles with zeolite Y as the active cracking component. Differences in physicochemical and catalytic properties were found to relate to differences in heterogeneities in the spatial porosity distribution. In addition to the characterization of existing FCC particles, our method of correlating pore space with transport efficiency does also allow for an up-front evaluation of the transport efficiency of new designs of FCC catalyst particles.

Nanoscale Chemical Imaging of an Individual Catalyst Particle with Soft X-ray Ptychography

Understanding Fe deposition in fluid catalytic cracking (FCC) catalysis is critical for the mitigation of catalyst degradation. Here we employ soft X-ray ptychography to determine at the nanoscale the distribution and chemical state of Fe in an aged FCC catalyst particle. We show that both particle swelling due to colloidal Fe deposition and Fe penetration into the matrix as a result of precracking of large organic molecules occur. The application of ptychography allowed us to provide direct visual evidence for these two distinct Fe-based deactivation mechanisms, which have so far been proposed only on the basis of indirect evidence.

Organosilane with Gemini-Type Structure as the Mesoporogen for the Synthesis of the Hierarchical Porous ZSM-5 Zeolite

A new kind of organosilane (1,6-bis(diethyl(3-trimethoxysilylpropyl)ammonium) hexane bromide) with a gemini-type structure was prepared and used as a mesoporogen for the synthesis of hierarchical porous ZSM-5 zeolite. There are two quaternary ammonium centers along with double-hydrolyzable -RSi(OMe)3 fragments in the organosilane, which results in a strong interaction between this mesoporogen and silica-alumina gel. The organosilane can be easily incorporated into the ZSM-5 zeolite structure during the crystallization process, and it was finally removed by calcination, leading to secondary pores in ZSM-5. The synthesized ZSM-5 has been systematically studied by XRD, nitrogen adsorption, SEM, TEM, TG, and solid-state one-dimensional (1D) and two-dimensional (2D) NMR, which reveal information on its detailed structure. It has a hierarchical porosity system, which combines the intrinsic micropores coming from the crystalline structure and irregular mesopores created by the organosilane template. Moreover, the mesoposity including pore size and volume within ZSM-5 can be systematically tuned by changing the organosilane/TEOS ratio, which confirms that this organosilane has high flexibility of use as a template for the synthesis of hierarchical porous zeolite.

Upcycling spent petroleum cracking catalyst: pulsed laser deposition of single-wall carbon nanotubes and silica nanowires

From waste to single-wall carbon nanotubes and silica nanowires: the first high-tech outlet for FC3R.

Photo-spectroscopy of mixtures of catalyst particles reveals their age and type

Within a fluid catalytic cracking (FCC) unit, a mixture of catalyst particles that consist of either zeolite Y (FCC-Y) or ZSM-5 (FCC-ZSM-5) is used in order to boost the propylene yield when processing crude oil fractions. Mixtures of differently aged FCC-Y and FCC-ZSM-5 particles circulating in the FCC unit, the so-called equilibrium catalyst (Ecat), are routinely studied to monitor the overall efficiency of the FCC process. In this study, the age of individual catalyst particles is evaluated based upon photographs after selective staining with substituted styrene molecules. The observed color changes are linked to physical properties, such as the micropore volume and catalytic cracking activity data. Furthermore, it has been possible to determine the relative amount of FCC-Y and FCC-ZSM-5 in an artificial series of physical mixtures as well as in an Ecat sample with unknown composition. As a result, a new practical tool is introduced in the field of zeolite catalysis to evaluate FCC catalyst performances on the basis of photo-spectroscopic measurements with an off-the-shelf digital single lens reflex (DSLR) photo-camera with a macro lens. The results also demonstrate that there is an interesting time and cost trade-off between single catalyst particle studies, as performed with e.g. UV-vis, synchrotron-based IR and fluorescence micro-spectroscopy, and many catalyst particle photo-spectroscopy studies, making use of a relatively simple DSLR photo-camera. The latter approach offers clear prospects for the quality control of e.g. FCC catalyst manufacturing plants.

The curious case of zeolite–clay/binder interactions and their consequences for catalyst preparation

Zeolite-based catalyst bodies are commonly employed in a range of important industrial processes. Depending on the binder and shaping method chosen, vast differences in the reactivity, selectivity and stability are obtained. Here, three highly complementary micro-spectroscopic techniques were employed to study zeolite ZSM-5–binder interactions in SiO₂-, Al₂O₃-, SiO₂ : Al₂O₃- (2 : 1 mix) and kaolinite-bound catalyst pellets. We establish how their preparation influences the zeolite–clay/binder interactions. Using thiophene as an acid-catalyzed staining reaction, light absorbing oligomers produced in each sample were followed. To our surprise, kaolinite decreased the overall reactivity of the sample due to the phase change of the binder, creating a hard impenetrable outer layer. Aluminum migration to the zeolite was observed when Al₂O₃ was selected as a binder, creating additional Brønsted acid sites, which favored the formation of ring-opened thiophene oligomers compared to the larger oligomer species produced when SiO₂ was used as a binder. In the latter case, the interaction of the Si–OH groups in the binder with thiophene was revealed to have a large impact in creating such large oligomer species. Furthermore, the combination of a SiO₂ : Al₂O₃ mix as a binder enhanced the reactivity, possibly due to the creation of additional Brønsted acid sites between the two binder components during pellet preparation. It is evident that, independent of the shaping method, the intimate contact between the zeolite and binder heavily impacts the reactivity and product selectivity, with the type of binder playing a vital role.

Methanol diffusion in zeolite HY: a combined quasielastic neutron scattering and molecular dynamics simulation study

The diffusion of methanol in zeolite HY is studied using tandem quasielastic neutron scattering (QENS) experiments and molecular dynamics (MD) simulations at 300–400 K.