Fluid Catalytic Cracking VII:: Materials, Methods and Process Innovations: Materials, Methods and Pr
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The silica freed from the collapsed framework migrates under high temperature steam toward the tetrahedral vacancies of the remaining framework and, by filling them, increases framework stability [30]. Prior sorption studies tend to support this interpretation. Based on sorption studies on USY zeolites, Lohse et al. As shown schematically in Figure 2, the collapse of sodalite units, or even ensembles of them, generates mesoporosity and simultaneously provides the source of Si atoms for healing the framework sites vacated by A1.
Lattice destruction of hydrothermally dealuminated or thermochemically treated FAU materials, leading to the formation of an amorphous silica phase, may be identified in the 29Si MASNMR spectra by a broadened peak or shoulder at about ppm TMS [33]. Upon aging, each of these components generate penta-, tetrahedrally- and octahedrally-coordinated A1 species, and at the same time, Si compounds, which can contribute both to healing and to net silica losses from the FCC particle [34a,b].
Schematic model for mechanisms of dealumination in FAU, showing Si for "healing," and the source of mesoporosity. Nonframework Aluminum- Local Environment Nonframework aluminum NFA is a catchall description for a wide collection of defects that are produced during the formation of USY and during subsequent hydrothermal treatment.
NFA species are themselves composed of several different types, some isolated, some agglomerated, as outlined in Table 1. It has proven to be particularly difficult to characterize the many different NFA species in dealuminated H-Y. Structural studies using X-ray and neutron diffraction have indicated the presence of octahedrally coordihated microcrystalline aluminum species in the supercages [35], and isolated tetrahedral aluminum species in the small sodalite cages [36], but do not give much information about agglomerated noncrystalline species.
The former provides direct information on the composition and Si, A1 distribution of the tetrahedral framework, independently of the presence of non-framework A1 species, while the latter allows distinction between tetrahedral framework A1 ppm and octahedral non-framework A1 -0 ppm. The interpretation of 27A1NMR in terms of A1 species location or state of agglomeration is often ambiguous. Even at low levels of dealumination, the contribution of nonframework species to the tetrahedral resonance cannot be ruled out.
The fact that a wide range of transitional aluminas exhibit a ratio of tetrahedrally coordinated A1 over total A1 content of about 0.
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Extensively dealuminated samples typically show substantial broadening of the tetrahedral resonance [40, 41], as shown in Figure 3, only a small portion of which can be attributed to framework A1 [41, 42]. In addition, as higher magnetic fields and faster sample spinning have become more routine, a new resonance has been observed at 30 ppm Figure 3 , which has been attributed to either an aluminum in a highly distorted tetrahedral environment [] or a penta-coordinated aluminum species [44, 45].
Application of the novel double-rotation DOR spinning technique [46, 47] to the study of 27A1in zeolites [48] has shown two different tetrahedral A1 species for a commercial USY material, one framework and the other nonframework [48]. Nonframework A l u m i n u m and Mesoporosity Previous transmission electron microscope TEM studies of hydrothermal aging of neat USY materials [] and also of USY cracking catalysts [] have shown 5 to 50 nm defect domains, which were attributed to mesopores.
Such features, more pronounced in the presence of vanadium [55, 56], are characteristic of extended hydrothermal treatment. Typical porosity analyses of mildly steamed USY materials show a distribution of mesopore dimensions in the range 5 to 50 nm that is skewed toward the smaller sizes [5 l, 53], supporting the association of the light amorphous zones observed by TEM with the secondary pore system characteristic of USY materials [3 ".
A new understanding of the formation and evolution of mesopores has emerged from combined high resolution electron microscopy HREM and analytical electron microscopy AEM investigations on hydrothermally treated USY materials [42, 59]. It was concluded that the extent of inhomogeneity is driven by non-equilibrium processes represented by accelerated steam-aging treatments in the laboratory.
In regions with high defect concentration, mesopores "coalesce" to form channels and cracks Figures 4, 5a, 6 , which ultimately define the boundaries of fractured crystallite fragments. At these boundaries, a dark band is often observed which is highly enriched in aluminum Figure 5 , while within the mesopore, aluminum appears to be deficient Figure 7 [59]. Such dark bands appear to have been observed in prior studies [53], but their presence was not discussed. The predominant fate of aluminum ejected from lattice sites appears to be closely associated with the dark bands, which often decorate these newly formed fracture Figure 4.
A coalescence of mesopores indicating an evolving fracture is indicated by an arrow [42]. An inhomogeneous distribution of mesopores is seen within individual grains; some grains contain more mesopores than others. In regions with high mesopore concentration, the pores coalesce to form channels indicated by arrows. Many mesopores are formed. Although localized disorder is observed within the pores, the connecting regions remain crystalline [59].
Cracks as indicated by arrows are formed from the evolution of the coalesced mesopores. Dark bands, which were found to be A1 rich, are seen along these cracks [59]. Within a mesopores, A1 is slightly deficient [59]. These features were observed both for the steamed USY cracking catalyst Fig. From this parallel study, it was concluded that the extracrystalline phases represented by the dark bands revealed in the electron microscopy studies contribute the majority of the nonframework aluminum species, tetrahedral, penta-coordinate, and octahedral, that were detected by 27A1NMR [61 ].
Each fractured crystallite is bounded by cracks evolved from coalescence of mesopores. An FCU "young" fraction Figure 9 proved to be similar to the lab-steamed samples except that defect patterns are more homogeneous than for the case of accelerated aging in the lab. An FCU "old" fraction Figure 10 shows more destruction, higher concentrations of mesopores, more A1 enriched bands, and the presence of more highly fractured grains than are found in the laboratory-aged samples Figs.
Extensive crystallite fracture and many prominent dark bands are observed [61]. Background - Some Nonframework Aluminum is Essential The central role of framework A1 content in defining the catalytic properties of dealuminated H-Y was discussed in a classic paper by Pine, et al [62]. These workers concluded that catalyst activity, selectivity, and octane performance are each correlated with unit cell size, which, in turn is directly proportional to the number of aluminum atoms in the framework. The result of this study is genetically summarized in Figure With increasing dealumination and concomitant loss of framework A1, selectivity towards naphtha octane and olefinic content tends to increase, but at the expense of catalyst activity and naphtha yield.
The importance of nonframework aluminum NFA was not revealed in studies such as this.
The more obvious manifestations of NFA species, such as the agglomerated species found in the electron microscopy studies discussed in the previous section, typically lead to decreased activity. Such agglomerated NFA species are thought to contribute to increased coke make 17 and not to desirable products. X-ray photoelectron spectroscopy XPS studies on hydrothermally dealuminated H-Y zeolites have shown that a considerable enrichment of aluminum occurs near the zeolite surface during the process of forming USY, with further accumulation at the surface upon extended hydrothermal treatment [60, 63].
Haag [27] observed that this migration of nonframework A1 to the external crystal surface generally lowers activity, presumably due to NFA species that have migrated to the surface and neutralized some of the BrCnsted acid sites near the crystallite surface [27, 63]. Haag further associated the increase in catalytic activity following ammonium exchange of a steamdealuminated zeolite with the reversal of this surface site neutralization [27].
Prior to the mids, NFA species were generally regarded as undesirable, pore-blocking, amorphous "debris" and were often referred to as "detrital" A1. This picture has been changed by more recent studies which show that high silica H-Y materials, having little or no nonframework A1, exhibit poor catalytic activity for acidity dependent reactions See Ref. It is concluded that the presence of some nonframework A1 is essential for the strong catalytic activity exhibited by fresh and mildly steamed USY materials.
It is now generally accepted 3, 5, 61, 64 that the activity and selectivity of Y zeolites in catalytic cracking are determined by an interplay of framework aluminum and nonframework aluminum species. The implicated NFA species, presumably well dispersed, may exist as isolated, cationic species in the small cages.
Relation between framework composition of USY and unit cell dimension, upon which is superposed a generic representation of the role of unit cell size as a unifying concept in the catalytic properties of USY as discussed by Pine, et al [62]. Direct Measures of Solid Acidity in Zeolites: In a review of studies of solid acidity by adsorption microcalorimetry, Dumesic and co-workers [65] note that, for dealuminated H-Y, the calorimetric results obtained at room temperature do not correlate with the catalytic activity for cumene cracking at K.
Samples with high activity showed essentially the same values of heat of adsorption of ammonia as did samples exhibiting substantially lower activity. Gorte and co-workers [66] carried out microcalorimetry measurements of pyridine and of isopropylamine adsorption, and also measured activities for hexane cracking on a series of steamed and chemically dealuminated H-Y materials.
The calorimetry investigations failed to detect evidence for superacidic sites, and there was no correlation between hexane cracking activities and heats of adsorption for the materials examined. These workers also found no evidence for a very small concentration of strong sites, the presence of which had been previously proposed from calorimetry studies on steam-dealuminated USY catalysts [67].
The results of these studies indicate that stable carbocations in acid catalysis by zeolites may be the exception rather than the rule. In fact, the only such species observed spectroscopically have been exceptionally stable and bulky cations such as the methyl indanyl [84] or trityl [85] cations. This consensus is consistent with the landmark contribution by Kazansky [], who first presented a mechanism for protonation of alkenes that did not involve the formation of stable carbenium ions.
Theoretical studies by the van Santen group [], initially on reactions of H-D exchange, and more recently extended to a wide range of hydrocarbon elementary reactions [76], have demonstrated a closely-related mechanism by which a stable carbocation is avoided. The carbocations represent high-energy activated complexes or transition states [75]. In other words, zeolite catalysts do not stabilize free carbocations, but the transition states associated with reaction pathways do resemble carbocations, consistent with Kazanky's earlier findings [] and with the suggestion by Kramer and McVicker in a review paper [ 14].
In their recent review of the combined NMR and theoretical studies of solid acidity by Haw and co-workers [25, 26, ], Haw, Nicholas and Xu [83] discuss similar conclusions from their studies of zeolites, emphasizing the contrast with comparable studies on true solid superacids. These workers observed that the body of theoretical and complementary experimental studies of zeolite acidity "do not support the long-held interlocked assumptions that zeolites are solid superacids and that free carbenium ions are prolific in zeolites One of the earliest results of our combined theoretical and experimental collaboration was to show that H-D exchange in benzene on zeolites could also proceed without a stable benzenium intermediate.
The quantum chemical calculations indicate that carbocation-like transition state pathways, not stable reaction intermediates are available in zeolites. These are thought to proceed via routes that require stabilization from the lattice, such as through formation of surface alkoxy groups. Synergism between framework and nonframework sites The question remains "What is the nature of the apparent synergism between framework and nonframework aluminum species that gives rise to enhanced catalytic activity? Such a synergism is consistent with the previously proposed concept of superacidity [10, 86] - as distinct from "superacids.
This model appears to be at odds with most direct measures of Br0nsted acidity Sections 3. Such an activity enhancement cannot be correlated with either the number or strength of acid sites. Temperature programmed desorption of NH3 and integral calorimetric adsorption heats of NH3 indicated that acid strength decreased upon fluorine incorporation [87].
What remains unclear is the possible presence and role of a very small concentration of strong acid sites [67]. In contrast, investigations of differential heats of titration with pyridine by Ahsan, Arnett and 21 McVicker [88] revealed a small number of strong sites on both the USY and the 0. At the same time, both the average acid site strength and the total acid site number for the fluorided sample showed a decrease in comparison to USY. The results of these studies raise anew the question: Could a small number of strong sites be key in the strong catalytic acidity exhibited by mildly dealuminated USY catalysts?
The high resolution 27A1NMR studies [89] indicated the presence of two kinds of Lewis sites within the nonframework A1 distribution in dealuminated zeolites - a tetrahedral site and a pentagonal site with isotropic shifts of about 53 ppm and 37 ppm, respectively. The FfIR-CO adsorption studies [90, 91] also revealed the presence of two types of Lewis sites associated with nonframework aluminum.
In the case of dealuminated mordenite, it was further shown that these Lewis sites were highly dispersed. Investigations of the isomerization of n-pentane and of o-xylene over dealuminated mordenites [92] showed initial reaction rates to be proportional to the product of the number of BrCnsted and the number of Lewis sites, suggesting that the the high acidity of dealuminated mordenites derives from a synergistic interaction between BrCnsted framework and highly dispersed Lewis nonframework acid sites [].
They arrived at the quantitative conclusion that a BrCnsted site is an OH bridging an aluminum to a silicon with only one aluminum neighbor. These workers also found that there are fewer BrCnsted sites than framework A1 sites and that the difference between the number of BrOnsted sites and the number of framework A1 sites increased with the amount of nonframework A1 [93]. It was suggested that some of the acidic OH groups have disappeared owing to their neutralization by reaction with NFA species, consistent with the suggestion of Haag [27].
Alternative explanations for enhanced activity in mildly steam-dealuminated zeolites It has recently been suggested that the enhanced catalytic activity exhibited by steamed H-Y zeolites may be an artifact of a diffusion-limited reaction, which is "enhanced" by the formation of structural defects during hydrothermal treatment [94]. In the absence of any direct measurements on micropore diffusion, a model is put forth that incorporates the assumption that most of the BrOnsted sites in HY are inaccessible to the "micropore diffusionlimited bimolecular and oligomeric cracking reactions.
This model fails to explain a number of salient observations in prior literature, including: The results, reproduced in Table 3, show that, over a wide range of sample sizes, total rates and selectivities are little affected. Carbonium ion activity [3, 12, 95] per framework aluminum atom is essentially independent of both sample size and framework aluminum content.
Catalyst deactivation in these studies was minimal, barely detectable in the mass balance [3,12]. Such consistency furnishes compelling evidence against the presence of any diffusion limitations in these studies and strongly supports the contention that carbonium ion activity [95] is directly dependent upon framework aluminum content.
As discussed previously by McVicker et al. Importance of the concentration and type of nonframework species The dependence of catalytic properties of dealuminated H-Y materials on unit cell size, or equivalently, on framework A1 content, can be profoundly altered by the concentration and type of nonframework species. However, "unconventional" ultrastable Y materials, prepared by mild steam treatment of a "clean framework," AHF dealuminated USY were found by Beyerlein, McVicker and co-workers [3] to exhibit enhanced carbonium ion activity [95] for isobutane conversion Figure In these studies, carbonium ion activity, as evidenced by skeletal isomerization and oligomerization and back-cracking of isobutane [12], was found to be directly proportional to framework A1 content.
While such studies demonstrate the critical role of nonframework species in the development of strong acidity, no information is provided on their location. These results provided definitive evidence for the association of isolated cationic species in the small cages with the development of enhanced acidity.
For a given framework A1 content, each La-exchanged material showed substantially increased activity over that of its clean framework parent material and somewhat lower activity than that of dealuminated H-Y Figure 13 [5]. The results of this study, and also those from recent investigations of Lewis acidity in dealuminated zeolites by Fripiat and coworkers [], provide compelling evidence that the critical nonframework A1 species are a highly dispersed, and b quite possibly exist as cationic species in the small cages of dealuminated H-Y, as indicated from earlier structural studies [36].
Carbonium ion rates from studies of isobutane conversion over high silica Y, ultrastable materials: For the materials exhibiting highest activity: It is apparent that Na addition suppresses activity much more rapidly than framework aluminum removal by dealumination. It was concluded from these sodium poisoning studies that only a fraction of the framework A1 atoms are associated with strong acidity. This conclusion is supported by results of studies of isopropylamine desorption from dealuminated H-Y by Gorte and coworkers [66], which demonstrated that framework A1 content is significantly greater than BrCnsted acid site density.
These workers suggested that each alkali ion affects only a single Figure Isobutane conversion studies of Na-poisoning [4, 61 ]. This study showed that the difference between the number of BrCnsted sites and the number of framework A1 sites increased with increasing levels of nonframework A1.
It was further suggested that some of the acidic OH groups have disappeared owing to their neutralization by reaction with NFA species [93]. The consistency of results from many different research teams using a variety of approaches furnishes compelling evidence that only a fraction of the framework A1 represents active sites.
Addition of potassium produces by far the strongest poisoning effect. Similar trends have been reported by P. This group showed that the extent of suppression of hexane cracking activity was directly related to cation size, with the effective degree of poisoning increasing with increasing ionic radius of the alkali metal cations. It was concluded that aluminum sites in ZSM-5 are not isolated, but occur in clusters of two or three depending on aluminum content, consistent with suggestions of A1 pairs by Haag et al. Effect of Acid Site Removal on Selectivity Previous correlations of isobutane conversion activity with framework composition [3] support a direct dependence of carbocation processes on framework aluminum, with a linear dependence of carbonium ion rates on A1F content.
The demonstrated linear dependencies for Na or K addition Figs. Utilizing the demonstrated linear dependencies for Na or K removal, the effect of decreasing acid sites on selectivity by aluminum removal is compared with that of Na addition in Figure 18, and with that of K addition in Figure The measured selectivities Figs. Methane Formation - Tracking of Initiation Step in Carbocation pathways Apparently, the likelihood of the initiation step for carbocation processes is sharply reduced in the vicinity of and below a limiting site density of about 8 A1F per unit cell.
On the other hand, the results for the formation of methane Figs. Comparison of the methane formation results in Figs. These result are consistent with previous discussion of dual mechanisms [12, 14] and also with the results of acidity modification of USY zeolites by fluoride treatment [87]. The enhanced activity per A1F observed upon mild fluoride treatment was suggested to be due to an increased concentration of carbocation intermediates. It was concluded that the sites responsible for increasing the initiation rates, thus increasing the pool of carbocations, are not readily characterized by traditional chemical and physical acidity measurement probes [87, 88].
A combined EPR, NMR and product distribution study of oxidation sites in dealuminated mordenite has shown a strong correlation between the presence of nonframework A1 and the generation of radical cations []. Since Lewis sites in H-mordenite have been shown to be associated with nonframework A1 species [], these results suggest a role of nonframework A1 species as electron acceptor sites for the generation of a radical cations, ultimately leading to the formation of olefins. Such a reaction sequence can serve as an initiation step for carbocation pathways, as proposed earlier [14].
These workers observe that a model involving carbenium ions is kinetically equivalent to a model involving surface alkoxy species, provided that the surface coverages by reactive intermediates are low. Consistent with the results on methane formation discussed here, Dumesic et al.
Methane formation rate vs. Isobutane conversion rate vs. Investigations of differential heats of ammonia adsorption by Yaluris, et al. It has been suggested that the extensive dehydroxylation associated with severe steam treatment leads irreversibly to the formation of strong Lewis sites at the expense of BrCnsted sites [65]. For less severely dealuminated samples, the large scale migration and agglomeration of nonframework species discussed in Section 2 appears to have a retarding effect on activity, as a portion of the acidity and catalytic activity lost by steaming can be recovered by careful acid-leaching [3, 4, , ], which removes both framework and nonframework A1 species.
It has been suggested that the microporosity modifications associated with the generation of NFA species during hydrothermal treatment could change the initial pore diameter and thus the availability of acid sites [ ]. Corma, et al [ ] found that removal of some of the nonframework species from severely steam-dealuminated H-Y led to an enhanced alkylation activity and an extended catalyst life.
In the case of mildly steamed HZSM-5, Lago, Haag and co-workers [7] attributed the generation of acid sites of enhanced activity 45 to 75 times more active than sites in clean framework HZSM-5 to the number of paired NNN A1 sites in the unsteamed parent. HREM investigations of mesopore formation in USY materials subjected to different severities of steam treatment showed an inhomogeneous distribution of mesopores among different USY grains as well as within single grains [42, 59, 61].
Such inhomogeneities were found to be more pronounced for materials that were steam-treated at a higher temperature. These inhomogeneities, which encompass the presence of restricted regions with more severe dealumination and also regions with milder dealumination than the average, were attributed to the non-equilibrium nature of the accelerated steam-aging treatments in the laboratory. The 33 presence of numerous regions, less severely dealuminated than the average, can be expected to give a disproportionate contribution to the measured activity. In consideration of the inhomogeneities in defect concentration found by HREM [42, 59, 61 ], BrCnsted sites can be expected to contribute to measured activity even for severely steamdeactivated zeolitic catalysts where the average crystallinity is low.
During the formation or further dealumination of a USY material by hydrothermal treatment, the collapse of small regions of the crystalline framework generates mesoporosity, simultaneously providing a source of Si atoms for healing the vacancies left by expulsion of A1 atoms from framework T-sites. A new understanding of the formation and evolution of mesopores during hydrothermal treatment has emerged from recent combined high resolution electron microscope HREM and analytical electron microscope AEM investigations on hydrothermally dealuminated USY materials [42, 59, 61].
The nonequilibrium nature of the accelerated steam-aging treatment in the laboratory gives rise to an inhomogeneous distribution of mesopores, which occurs concomitantly with further zeolite dealumination.
In regions with high defect concentration, mesopores "coalesce" to form channels and cracks, which, upon extended hydrothermal treatment, ultimately define the boundaries of fractured crystallite fragments. The predominant fate of aluminum ejected from lattice sites appears to be closely associated with dark bands that often decorate these newly formed fracture boundaries, as observed by H R M. A smaller proportion of ejected aluminum exists as isolated [36] or agglomerated species [35] within the zeolite cages.
Similar defect patterns, although less inhomogeneous, have been observed for age-separated young fractions from a commercial Fluid Cracking Unit [61 ], where the rate of deactivation is slower than for accelerated steam-aging in the lab. High silica H-Y materials, having little or no nonframework A1, exhibit poor catalytic activity for acidity dependent reactions such as paraffin isomerization or cracking. Investigations of mildly dealuminated zeolites indicate that the origin of their high catalytic activity is a synergistic interaction between BrCnsted framework and highly dispersed Lewis nonframework acid sites.
The enhanced cracking, isomerization activity associated with the presence of highly dispersed nonframework AI species is not reflected in direct measures of solid acidity obtained, for example, by calorimetry or by NMR spectroscopy. The question of how nonframework aluminum affects catalytic activity remains an experimental and modeling challenge. The enhanced cracking, isomerization activity of mildly steamed materials almost certainly involves some synergistic interaction between framework BrCnsted sites and highly dispersed nonframework species.
The presence of this enhanced activity is not consistent with a major increase in acid site strength. BrCnsted sites are presumably more dilute than the distribution of framework A1 as only a fraction of the framework A1 appears to represent active sites. The critical nonframework A1 species quite possibly exist as cationic species in the small cages of dealuminated H-Y.
In analogy with conclusions form a recent study reporting similar trends in ZSM-5 [ ], these results are consistent with the presence of next-nearest-neighbor NNN A1 clusters in the FAU framework, such that a single alkali ion can influence more than one acid site. The current body of theoretical and associated experimental studies indicate that zeolites, as a class of solid acids, do not comprise superacids that stabilize free carbocations [].
The quantum chemical calculations show that the transition states associated with reaction pathways resemble carbocations. That is, carbocation-like pathways are available in zeolites, but these proceed via routes that require stabilization from the lattice, such as through formation of surface alkoxy groups. Previous correlations of isobutane conversion activity with framework composition support a direct dependence of carbocation processes on framework aluminum A1F , with a linear dependence of carbonium ion rates on A1F content. The demonstrated linear dependencies for Na or K addition show that the primary effect of poisoning, or of A1F removal, is a decrease in the number of active sites.
Consistent with previous discussion of dual mechanisms, the results for formation of methane, a stable reaction product marker, show that the initiation step and the secondary carbocation processes are intimately linked over the entire range of acid site content, whether manipulated by dealumination or by poisoning. The initiation step appears to be independent of carbocation activity, consistent with the conclusions from independent efforts to describe the reaction kinetics of the catalytic cracking of paraffins []. While these and related studies have provided new understanding of the initiation step for carbocation pathways on zeolite catalysts, significant challenges remain.
The sites responsible for increasing the initiation rates and thus increasing the pool of carbocations are not readily characterized by traditional chemical and physical acidity measurement probes. The nature of the initiation step s for the cracking of paraffins remains a subject of research. The recent advances in understanding of the bifunctional nature of zeolite catalysts reviewed here have led to some notable advances in Fluid Catalytic Cracking technology.
The use of amorphous alumina added to rare earth exchanged Y zeolite to improve FCC activity and conversion to gasoline has been reported by Exxon []. The added alumina is reported to increase the Lewis acidity of the catalyst providing a dehydrogenation function. The olefins produced due to the increased Lewis acidity lead to improved conversion on the BrCnsted acid sites of the zeolite. These findings resulted in the commercialization of a new FCC formulation by Exxon.
Ray, Bill Schuette and Stu Soled. The expert assistance of B. Ziemiak is also gratefully acknowledged.
Carvajal, Po-Jen Chu and J. Prager, Zeolites 9 Ono, Zeolites 8 Sun, Po-Jen Chu and J. Lunsford, Langmuir 7 Guisnet, Zeolites 2 Jansen Elsevier, Amsterdam, p. Liebens, Catalysis Today 37 Heitmann, Catalysis Today 38 State of the Art , Stud.
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H61derich, Elsevier, Amsterdam, Proceedings of 6th International Zeolite Conference, eds. Bisio Butterworths, Surrey, UK, p. Schirmer, Zeolites 6 Elsevier, Amsterdam, p. Occelli, private communication, manuscript submitted to J. Fripiat, Microporous Materials 4, Gobbi, Nature London Fluid Catalytic Cracking 1II: Grobet, et al, Elsevier, Amsterdam, Jacobs, Nature Samosen, Zeolites 13 New Frontiers in Catalysis, Eds.
As a general rule high RegT, high steam or oxygen partial pressures and high vanadium contents increase dealumination and ZSA losses. Initially, there is AI extraction from the lattice with no great destruction of the zeolite framework. The hydrotreating of FCC feed minimizes the dry gas yields. Although the laboratory deactivation protocols currently reported in the literature may emulate the average properties of the catalyst at equilibrium, the effects of aging are more difficult to reproduce. Michoacfin y La Purisima sin Col. Two contributions describe the use of predictive methods to understand FCC aging and deactivation and personal overviews of the development of SOx and combustion promoters technology are presented.
Catalysis by Acids and Bases, Eds. Jaeger, Zeolites 7 Dufresne, Zeolites 7 Characterization of Porous Solids, Eds. Facts, Figures, Future Eds. Ray, Topics in Catalysis, Vol. Dumesic Balzer Science Publishers, Amsterdam, Hall, J-M Dereppe and G. Advances in Catalysis, Vol. Weisz, Academic Press, San Diego, pp. Advanced Zeolite Science and Applications, eds. B Nicholas, Teng Xu, L. Nicholas, Topics in Catalysis, Vol. Nicholas, Teng Xu and J.
Haw, Topics in Catalysis 6 ; b J. Haw and Teng Xu, Advances in Catalysis 42 General This ratio accounts for the major carbonium ion based products arising from isomerization and chain-cracking sequences. Carbonium ion activity was defined as the product of carbonium ion selectivity and total rate See Ref. Patent , May 14, Ferenc L6nyi and J. Facts, Figures, Future, eds. In each case, hydrated zeolite was combined with a properly diluted reagent solution.
Catalysis Letters 10 Madon, Topics in Catalysis 4 Acta [ ] Thiet Dung and P. Mastikhin, Zeolites 9 Relationship Between Structure and Reactivity, eds. Schweizer, EP 0 A3, Studies in Surface Science and Catalysis M. All rights reserved 41 The use of microcalorimetry and solid state nuclear magnetic resonance NMR to study the effects of post-synthesis treatments on the acidity and framework composition of several HY-type zeolites M. Aurou b , M. More than fluid catalytic cracking units are in use worldwide generating the capacity to produce in excess of million gallons of gasoline a day 1.
The lead phaseout, reformulated gasoline, together with ever stringent automotive emission standards, provide the main incentive to continue to study the properties of fluid cracking catalysts FCCs for gasoline production 2. Today, the demand for olefin-rich gasoline is focusing the refiner attention on new FCC compositions in which acidity can be manipulated to generate materials with a low hydrogen transfer index 3,4. To impart the desired catalytic activity to the aluminosilicate framework, several postsynthesis treatments have been proposed to modify the acidic properties of HY type zeolites , the main cracking component of all commercially used FCCs.
However, repeated ammonium ion exchange and calcination cycles can yield HY crystals with properties similar to those of USHY and containing only minor 43 crystals modified by post-synthesis chemical, thermal and hydrothermal treatments. The utility of NMR and adsorption microcalorimetry to characterize zeolites has been reviewed and discussed elsewhere 2. O level to 0. The properties of all the samples under study can be found in Table 1. Prior to analysis, samples weighing from 0. Surface area measurements have been performed using the BET equation Successive doses of gas were sent onto the sample until a final equilibrium pressure of Pa was obtained.
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The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge from Datametrics. Samples were spun in cylindrical 7mm zirconia rotors at spinning speeds 44 near 4 kHz. Chemical shifts were determined relative to tetramethylsilane as an external reference; spectra were deconvoluted into Gaussian lineshape components. Samples were spun in cylindrical 4mm zirconia rotors at a spinning frequency of 12 kHz.
Resonance shifts are reported using liquid samples of 1M aqueous solutions of AI NO3 3 as an external reference standard. Two spectra were obtained from each sample before accepting the results as being representative of the zeolite sample under study.
Cracking VII: Fluid Catalytic Cracking VII: 1st Edition - ISBN: , 1st Edition. Materials, Methods and Process Innovations. Sixty-six years after the introduction of the fluid cracking catalyst Fluid Catalytic Cracking VII:: Materials, Methods and Process Innovations.
The resonance near -I I0 ppm has been attributed to the presence of extraframework silica 2 I. This discrepancy can be safely attributed to the presence of extraffamework Al-species which are clearly visible in the 27AI MAS spectrum shown in Figure 2A. In contrast, HY crystals prepared by reacting NH4Y with ammonium hexafluorosilicate solutions followed by calcination, produce the spectrum shown in Figure 1B, indicating a much stronger retention of framework AI and a completely different distribution of T-sites. As reported elsewhere 11 , the hydrolysis of the fluorosilicate salt forms protons and fluoride anions that cause the dealumination of the 45 Table 1.
The rest of the AI is re-introduced, together with the Si, to occupy vacant defect sites in the dealuminated faujasite framework. As a result, the concentration of Tsites in which Si is coordinated to one or more A1 atoms increases yielding the spectrum 48 shown in Figure lB. Prime Book Box for Kids. Elsevier Science; 1 edition September 7, Language: Related Video Shorts 0 Upload your video. Share your thoughts with other customers. Write a customer review. There was a problem filtering reviews right now. Please try again later. FCC catalyst and catalysis. One person found this helpful.
Amazon Giveaway allows you to run promotional giveaways in order to create buzz, reward your audience, and attract new followers and customers. Learn more about Amazon Giveaway. Set up a giveaway. Pages with related products. See and discover other items: In the s, Vladimir Haensel , [1] a research chemist working for Universal Oil Products UOP , developed a catalytic reforming process using a catalyst containing platinum. Haensel's process was subsequently commercialized by UOP in for producing a high octane gasoline from low octane naphthas and the UOP process become known as the Platforming process.
In the years since then, many other versions of the process have been developed by some of the major oil companies and other organizations. Today, the large majority of gasoline produced worldwide is derived from the catalytic reforming process. Before describing the reaction chemistry of the catalytic reforming process as used in petroleum refineries, the typical naphthas used as catalytic reforming feedstocks will be discussed. A petroleum refinery includes many unit operations and unit processes.
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The first unit operation in a refinery is the continuous distillation of the petroleum crude oil being refined. The overhead liquid distillate is called naphtha and will become a major component of the refinery's gasoline petrol product after it is further processed through a catalytic hydrodesulfurizer to remove sulfur -containing hydrocarbons and a catalytic reformer to reform its hydrocarbon molecules into more complex molecules with a higher octane rating value.
The naphtha is a mixture of very many different hydrocarbon compounds. The naphtha from the crude oil distillation is often further distilled to produce a "light" naphtha containing most but not all of the hydrocarbons with 6 or fewer carbon atoms and a "heavy" naphtha containing most but not all of the hydrocarbons with more than 6 carbon atoms. The naphthas derived from the distillation of crude oils are referred to as "straight-run" naphthas.
It is the straight-run heavy naphtha that is usually processed in a catalytic reformer because the light naphtha has molecules with 6 or fewer carbon atoms which, when reformed, tend to crack into butane and lower molecular weight hydrocarbons which are not useful as high-octane gasoline blending components. Also, the molecules with 6 carbon atoms tend to form aromatics which is undesirable because governmental environmental regulations in a number of countries limit the amount of aromatics most particularly benzene that gasoline may contain.
It should be noted that there are a great many petroleum crude oil sources worldwide and each crude oil has its own unique composition or "assay". Also, not all refineries process the same crude oils and each refinery produces its own straight-run naphthas with their own unique initial and final boiling points. In other words, naphtha is a generic term rather than a specific term. The table just below lists some fairly typical straight-run heavy naphtha feedstocks, available for catalytic reforming, derived from various crude oils.
It can be seen that they differ significantly in their content of paraffins, naphthenes and aromatics:. Some refinery naphthas include olefinic hydrocarbons , such as naphthas derived from the fluid catalytic cracking and coking processes used in many refineries. Some refineries may also desulfurize and catalytically reform those naphthas. However, for the most part, catalytic reforming is mainly used on the straight-run heavy naphthas, such as those in the above table, derived from the distillation of crude oils.
There are many chemical reactions that occur in the catalytic reforming process, all of which occur in the presence of a catalyst and a high partial pressure of hydrogen. Therefore, the naphtha feedstock to a catalytic reformer is always pre-processed in a hydrodesulfurization unit which removes both the sulfur and the nitrogen compounds. Most catalysts require both sulphur and nitrogen content to be lower than 1 ppm. The four major catalytic reforming reactions are: During the reforming reactions, the carbon number of the reactants remains unchanged, except for hydrocracking reactions which break down the hydrocarbon molecule into molecules with fewer carbon atoms.
The isomerization of normal paraffins does not consume or produce hydrogen. However, both the dehydrogenation of naphthenes and the dehydrocyclization of paraffins produce hydrogen. The hydrogen is also necessary in order to hydrogenolyze any polymers that form on the catalyst. In practice, the higher the content of naphtenes in the naphtha feedstock, the better will be the quality of the reformate and the higher the production of hydrogen. Crude oils containing the best naphtha for reforming are typically from Western Africa or the North Sea, such as Bonny light oil or Norwegian Troll.
Owing to too many components in catalytic reforming process feedstock, untraceable reactions and the high temperature range, the design and simulation of catalytic reformer reactors is accompanied by complexities. The lumping technique is used extensively for reducing complexities so that the lumps and reaction pathways that properly describe the reforming system and kinetic rate parameters do not depend on feedstock composition. Rate equations of this type explicitly account for the interaction of chemical species with catalyst and contain denominators in which terms characteristic of the adsorption of reacting species are presented.
The most commonly used type of catalytic reforming unit has three reactors , each with a fixed bed of catalyst, and all of the catalyst is regenerated in situ during routine catalyst regeneration shutdowns which occur approximately once each 6 to 24 months.