In the case of catalytic cracking, the source of the large hydrocarbon molecules is usually the gas oil fraction of crude oil petroleum. Useful "straight run" products, such as gasoline, kerosene, diesel, butane and propane are separated from the crude oil mixture in the Atmospheric Distillation Unit at atmospheric pressure.
Atmospheric residue is typically routed to the Vacuum Distillation unit for separation into gas oil usually termed vacuum gas oil, or VGO for short and vacuum residue; via distillation at reduced pressure. The FCC unit employs a sophisticated powdered catalyst to lower the activation energy of cracking and thereby drive the cracking process. The particle size and density of the powder are set such that the catalyst powder is fluidizable - the powder behaves like a fluid when aerated.
This allows the catalyst to form fluidized beds and be transported between the FCC reactor and regenerator with ease. FCC catalyst are considered to have two catalytically active parts: a the "zeolite", and b what is termed the "matrix".
The zeolite referred to as Zeolite-Y is a rare-earth stabilized synthetic form of a rare naturally occurring aluminosilicate mineral called faujasite. It is also highly crystalline and has a very regular micropore structure with mean pore mouth diameters in the range of diesel molecules - hence it behaves as a molecular sieve.
Acid sites within the crystalline micropores very selectively crack diesel range molecules to gasoline and some light olefins propene and butenes. However, VGO consists of molecules that are bigger than diesel, too large to access the zeolite micropores. The catalyst "matrix" is designed to include a high surface area mesoporous alumina which has acid sites situated in larger pores that are able to pre-crack VGO molecules into the diesel range - in order to feed the zeolite so to speak.
Optimization of FCC yields between gasoline and diesel can be achieved by adjusting the catalyst zeolite-to-matrix ratio. The main role of the FCC unit has until recently been the production of gasoline. This band, attributed to OH groups interacting with RE-cations see also Scherzer and Bass 55 , 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. In their study, Dysprosium falls outside the plotted correlation, possibly because remaining chloride ions create extra activity. Van Bokhoven et al. 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.
Du et al. 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 to The formation of the hydroxylated clusters leads to the formation of Si—OH—Al groups at a distance to the La-clusters. Noda et al. 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. The effect of the presence of RE in the lattice on performance is dramatic.
Plank et al. 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. Lemos et al.
The cracking activity seems to correlate with strong protonic acidity, as derived by reactivity comparison. On the one hand, conventional feedstocks are becoming heavier. Resid cracking in FCC gained popularity in the early 's, and has gained importance since.
Heavier feedstocks imply that larger, more aromatic molecules need to be cracked, which calls for improved accessibility and improved metals tolerance. At the same time, there is a drive to increase activity, but at the same time limit the amount of coke produced to the absolute minimum required for heat balance of the unit. This is a continuous challenge in FCC since the early days, and various improvements have been made over the decades, as illustrated in Fig.
Apart from the conversion of heavier feedstocks, we have recently also seen an increased application of relatively light, paraffinic shale oil as the feedstock to the cracker. At the same time, a similar effect can be observed on the product side. Where aviation gasoline was the desired product for the initial FCC units, we have seen an increased demand for propylene over the last two decades.
Propylene is the raw material for polypropylene, and the FCC unit can be one of the main sources to form propylene the other would be steam cracking of naphtha. Propylene can be produced in the FCC unit as a product mostly of secondary cracking of gasoline range molecule, usually by specific additives containing ZSM-5 zeolite.
The development of specific FCC-propylene capacity follows the demand for olefins. On the other hand, as shown in Fig. The compiled information, based on data from the OPEC World Oil Outlook , 67 shows the ratio between gasoline and diesel demand over the next decades is projected to change in favor of diesel. Historically, the USA had a surplus in diesel, and the EU had a surplus in gasoline, which could be traded.
The two developments combined require a shift from gasoline as the main product to both higher- and lower-boiling products, which is not possible at the same time. In this review paper, we will exclusively focus on ZSMcontaining additives. Even though zeolite ZSM-5 was first described as a synthetic material, a natural mineral form named Mutinaite also exists as it was discovered in Antarctica adjacent to deposits of natural zeolite Beta.
The essentially all-silica form, known as silicalite, has a slightly different structure than the low-SAR material, it has a monoclinic unit cell, whereas the low SAR material crystallizes in an orthorhombic cell.
The framework is exactly the same for both phases. The pores are slightly elliptical and have diameters of 5. The structure has a straight MR pore along the []-direction, and a zig-zag MR pore along the []-direction. The pores intersect, and molecules of the correct dimensions can reach any point in the pore system from any other point. ZSM-5 normally crystallizes in lozenge- or coffin-shaped crystals that are frequently twinned.
The limited room in the pore system of zeolite ZSM-5 compared to the supercages in zeolite Y implies that it is much more difficult to accommodate the larger bimolecular transition states.
As a result, the secondary cracking of gasoline range molecules in ZSM-5 will produce more olefins. This is illustrated in Fig. Just like the primary cracking zeolites, also zeolite ZSM-5 is unstable towards the harsh environments of the FCC process.
Dealumination by repeated contact with steam in the regenerator dislodges the aluminum from its framework position, thus removing the active acid sites, and in the process destroying the zeolite lattice. Although a partial destruction of the zeolite lattice may improve the diffusion characteristics of the zeolite by creating access to the interior through mesopores, this also creates larger pores, and hence the opportunity for bimolecular cracking.
To increase the stability of zeolite ZSM-5, a treatment with phosphorous is often applied. For example, Xue et al. Given the scale of the operation and ease of handling e. Excess phosphorus used during the treatment will deposit on the external surface of the zeolite ZSM-5 crystals as a polyphosphate.
If any aluminum is dislodged during the thermal treatment, it will very likely react with the available phosphate, and form an amorphous aluminophosphate. It should be noted that the ZSMcontaining additive will generally also contain an alumina binder, which will react with excess phosphate to form an aluminophosphate species that may be beneficial for binding the system. The effect of the treatment with phosphate on macroscopically observable parameters is. In view of the mechanistic relations described above, the latter two seem to hint at decreased hydrogen transfer, and the second seems to indicate decreased room in the lattice for the formation of bulky intermediates.
Although generally the catalysts will be exposed to steam after they were stabilized, some authors describe phosphate treatment after initial steaming. This may lead to the formation of extra-framework aluminum EFAL and hydroxyl nests, and dislodged aluminum still partially connected to the lattice.
Various characterization techniques have been used to study phosphated zeolites. Excess of phosphate will remain on the external surface of the zeolite ZSM-5 crystals. Although porosity and accessibility are initially decreased, the bridging hydroxyl groups and thus the acidity appear to remain available at this stage. The zeolite crystals appear to lose some crystallinity after the calcination treatment following the phosphorous impregnation, but this could be due to scattering of the X-rays by P-species in the pores.
Depending on conditions for the calcination, the phosphate species may coordinate to the aluminum, and thus break the Si—OH—Al bridges.
Although this would lower the number of strong acid sites, the Al—O—P OH 3 and Si—OH species formed when this happens may lead to new acid sites, and partially connected Al may form additional Lewis acid sites.
This does not necessarily mean that the Al is dislodged from its framework position. Extra-framework aluminum reacts with the P-sources to form an extra-framework crystalline ALPO phase.
These distorted sites were more or less immune to hydrothermal treatment. Excess phosphate was found on the external surface of the zeolite crystals. Upon heat treatment, van der Bij et al. The exact structure and position of these clusters, as well as mechanisms to form acid sites around these cluster remains as yet unsolved, although it is suggested that the bulky SAPO-species impede the formation of carbenium ions, and thus successfully suppress the bimolecular mechanism, resulting in an improved propylene selectivity for the treated samples.
There are a number of ways to introduce a hierarchical pore structure, in which mesopores and micropores are connected, in zeolites. A review on hierarchical zeolites is presented by Li et al. In the bottom up-approach, extra-crystalline, hard, templates such as carbon black, 3-D ordered mesoporous carbon, or carbon aerogel can be used e. The zeolites form within the structure of the hard template, which is then burnt off to create mesoporosity.
Adaptations of the more standard templates, which introduce mesopore-structure direction in the same molecule, are called soft-templating. Here, different functionalities are combined in one template molecule that direct for micropores and mesopores. For instance, Ryoo et al. Rimer et al. In the top-down approach to achieve hierarchical zeolites, the zeolites are post-treated after synthesis. The easiest way to introduce mesoporosity is by dealumination, which can be achieved by steaming and chemical treatments, such as acid leaching or reaction with EDTA or other chelating agents that remove the resulting extra-framework alumina.
This approach was used in the development of Dow's 3DDM mesoporous mordenite catalyst for the production of cumene, 93 and is also the basis of US-Y zeolites that are used in many applications nowadays. Clearly, dealumination leads to a lower number of acid sites and at least an initial loss of framework integrity. However, these disadvantages are more than offset by the creation of new types of acidic sites and enhanced diffusion properties.
Janssen et al. Another way of producing mesopores is by desilication. Initial work in this field was published by Groen et al. Top-down approaches typically have low yields because they leach away either alumina or silica, and thus give rise to waste-streams.
Li et al. They applied soft and hard templating, as well as a combination of acid leaching and base treatment. Only the combination of acid leaching and base leaching yielded a material with improved accessibility and strong acidity, leading to optimal performance in the isomerization of 2-methylpentene and the alkylation of benzene with benzyl alcohol. Park et al.
When compared to normal ZSM-5 catalysts in the cracking of gas oil, they observe higher overall activity, and higher yield of lower olefins like propylene and butylene. The catalysts contain intracrystalline mesopores. The author assume that pre-cracking of larger molecules inside the mesopores provides the molecules that can be cracked inside the MFI micropores to give the desired products.
Normal ZSM-5 would require conversion of gasoline range molecules to form the desired olefins, whereas the mesoporous catalysts described by the authors have similar or better gasoline yields compared to normal ZSM However, the catalytic performance was tested on pure zeolite samples.
The addition of matrix and binder, as well as the presence of a main Y-zeolite based FCC catalyst in the catalyst system, may cause the observed benefits to change, among others because this would supply a large concentration of gasoline molecules. The conversion and selectivity to propylene observed for the hierarchical ZSM-5 samples described by the authors is not high enough to warrant use by itself see e.
Hansen et al. The authors observe a lower bottoms yield at constant coke, and improved middle distillate over bottoms selectivity in ACE testing. A similar effect is seen for the gasoline over LPG selectivity, since the optimum in the series pathway network is shifted to higher molecular weight.
The starting zeolite in the original process already has a quite high silica-to-alumina ratio of about 30, lower SAR zeolites apparently need an acid pretreatment before they are suitable for post-treatment.
TPD of ammonia shows that the mesostructured material has about the same number of acid sites as normal zeolite US-Y. The zeolites were tested after being introduced in FCC-matrices, and steam-deactivated. At constant conversion, lower bottoms- and coke-make, and higher gasoline and middle distillate yields are observed. They tested the E-cat from the refinery in a FCC test unit before and at the end of the trial, and report lower coke make, higher LCO make and lower bottoms for the catalyst containing hierarchical zeolites.
Early work by Derouane and co-workers explains this effect. The authors describe the role of the curvature of the zeolite pore surface and explain that the interaction between molecules and the zeolite surface is strongest when the radius of the molecule and the surface curvature are similar.
At this exact fit, a number of phenomena are described that have a direct effect on the performance, e. The increased interaction leads to increased concentration of reactants near the acid sites, and expresses itself macroscopically as increased apparent acid strength.
This implies that the 3D structure of the zeolite and its effect on sorption equilibria can play a large role in reaction kinetics; they directly influence the observed rate of reaction, especially when the sorption energetics are magnified by the surface curvature. Zeolite Beta, for instance, has been studied extensively.
Although economics and thermal stability have thus far prevented the application of zeolite Beta in large-scale FCC processes, it is known — that P-stabilized zeolite Beta improves C 4 -yields. Bonetto et al.
The issue of cost and stability returns for many of the new structures proposed for FCC applications. Quite often, complicated organic SDAs, or exotic framework constituents e. Ge and Ga , or fluoride-assisted syntheses are required to even synthesize new zeolite structures. These do not translate well to the scale of operation, catalyst consumption and the severity of the FCC process.
Nevertheless, we will discuss some recent developments in the paragraphs below. When examining the medium pore size zeolite MCM, Corma et al. When using it in an additive similar to zeolite ZSM-5 additives, zeolite MCM produces less gases lower loss in gasoline yield , but with higher olefinicity so higher propylene and butylene selectivity than ZSM ZSM-5 is more active, though.
The specific pore structure induces an increased yield of propylene in VGO cracking. Zeolite ITQ-7 has a pore system similar to zeolite Beta, yet a higher gasoline yield and improved olefin selectivity are observed in FCC cracking, where an ITQ-7 containing additive was used.
Their cracking characteristics are similar to zeolite Y, except for a higher gas LPG and propylene yield but lower gasoline olefinicity in ITQ Zeolite ZSM shows good thermal stability compared to zeolite Y, but this does not directly translate into higher activity.
In their description of the new zeolite IM-5, Corma et al. The structure is described as having MR pores with side pockets, and performance of the material in some cases is close to ZSM-5, possibly with improved thermal stability. Moliner et al. The zeolite performs well in the alkylation of benzene to cumene.
The authors claim the material would be a good additive for FCC since its pore system behaves as an intermediate between zeolites ZSM-5 and Beta. The silica-germanate ITQ, from the same group is another zeolite with a mixed pore system, in this case an intersecting MR—MR system. The authors conclude the material behaves like a MR, i. Economics and stability of the material may impede its widespread application, though.
FCC of biomass-derived oxygenates gives products with higher hydrogen content than the starting biomass-based feedstock by removing oxygen as carbon monoxide as well as carbon dioxide, next to an increased amount of water.
In addition, higher amounts of carbon deposits are found on the FCC catalyst material, which then can be burned off in the regeneration to produce process heat. Another important issue relates to the significant content of water in biomass-derived oxygenates, which may not dissolve into VGO, although some options have been discussed by Corma and co-workers. In this article we focus on the catalytic cracking of biomass-derived feedstocks mixed with petroleum-derived feedstocks making use of real-life FCC catalyst materials.
Other more recent review papers are of the hands of Al-Sabawi and co-workers and Kubicka and Kikhtyanin. Active research groups include those of e.
The ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol gasoline. Hydrocarbons used in petrol gasoline are given an octane rating which relates to how effectively they perform in the engine. A hydrocarbon with a high octane rating burns more smoothly than one with a low octane rating. Molecules with "straight chains" have a tendency to pre-ignition. This double explosion produces knocking in the engine.
Octane ratings are based on a scale on which heptane is given a rating of 0, and 2,2,4-trimethylpentane an isomer of octane a rating of In order to raise the octane rating of the molecules found in petrol gasoline and so make the petrol burn better in modern engines, the oil industry rearranges straight chain molecules into their isomers with branched chains. It is used particularly to change straight chains containing 5 or 6 carbon atoms into their branched isomers.
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