HZSM-5.57 zeolite-based catalyst

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

The aging and combustion kinetics of coke deposited on an HZSM-5 zeolite-based catalyst in the MTG process have been studied. The kinetic study of coke combustion in air was carried out at 500-550°C in a differential scanning calorimeter, by following the evolution of the combustion products with on-line FTIR analysis. The results provide evidence for limitations on coke reactivity that can be attributed to the combined effects of several circumstances (e.g. bad oxygen-coke contact and heterogeneous distribution of coke within the zeolite crystal). The need is demonstrated for a thermal aging treatment which equilibrates reproducibly the coke prior to combustion. The aging of coke is also limited by a peculiar coke deposit in the microporous stmcture of the zeolite. 

The effect of the Si/Al ratio of HZSM-5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption calorimetry of ammonia and tert-butylamine. The observed increase in light olefin selectivity and decrease in methanol conversion with increasing Si/Al ratio can be explained by a decrease in total acidity [141]. 

Sophisticated catalysts, such as ZSM-5 or HZSM-5 [22] and other zeolites are also suggested in numerous papers, e.g. KEY [23], HY and H-mordenite [24], Re-zeolite-based Engelhardt FCC commercial catalyst [25], and steamed commercial zeolite catalyst [26]. These investigations are mainly devoted to fundamental studies and the correlation between feed composition, catalyst properties, process parameters and efficiency connected with prodnct distribution. Iron supported on silica-alumina, mesoporous silica and active carbons serves as the next example of materials applied in the waste plastics cracking [27, 28]. On the other hand, according to some results [29] application of cracking catalysts such as Zn-13X, Fe-5A and CoMo-HY are ineffective in waste plastics cracking. 

In recent years, a lot of research effort has been directed towards dehydroaromatisation of methane in which methane is converted to aromatic products such as benzene and naphthalene in addition to hydrogen. Perhaps the most well studied system has been that employing Mo/ZSM-5 based catalysts, where the bifunctional interaction between the zeolite Bronsted acidity and molybdenum species is well recognised. Under reaction conditions, the active molybdenum species are known to be in the form of carbides or oxycarbides, and recently it has been proposed that the a-MoCi-x phase is the most active form. Deactivation, primarily due to coke formation, is well precedented in this reaction and represents a major obstacle to be overcome in the successful application of these catalysts. In this respect, it is interesting to note that Ichikawa and co-workers have published studies indicating that the inclusion of low levels of CO or CO2 in the feed can promote the reaction via the suppression of coke formation in the case of both Mo/HZSM-5 and Re/HZSM-5 catalysts. Other approaches adopted towards this aim have been the inclusion of second metal components and a reduction of the acid strength of the HZSM-5 support. ... 

A study of the product selectivites of variously supported Co catalysts (kieselguhr, silica, alumina, bentonite, Y-zeolite, mordenite, and ZSM-5) was carried out by Bessel (37). AAdiereas the lower acidity supports such as silica and alumina produced mainly linear hydrocarbons, the acidic supports produced more branched products. At higher temperatures, the latter produced aromatics as well. The isomerization and aromatization are secondary, acid-promoted reactions of the FT olefins. This is then equivalent to a combination of the FT and the Mobil olefins to gasoline process. (With iron-based catalysts, this approach is unlikely to be successful because alkali promotion is essential and the alkali would neutralize the required acid sites on the zeolite support.) Calleja and coworkers (38) studied the FT performance of Co/HZSM-5 prepared by incipient wetness impregnation. Promotion with thorium, being basic, decreased the acidity of the zeolite and so less aromatics were formed and consequently more of the heavier hydrocarbons emerged from the reactor because of the depressed level of secondary reactions. 

Improvements in chemical processes are very often based on the discovery or development of new catalysts or adsorbents. One particularly exciting example in the field of zeolite catalysis is the replacement of the formerly used amorphous silica-aliunina catalysts in fluid catalytic cracking (FCC) of vacuiun gas oil by rare earth exchanged X-type zeoUtes [1]. This resulted in considerably improved yields of the desired gasoUne and, hence, a much more efficient utilization of the crude oil. Fiuther examples are the introduction of zeolite HZSM-5 as catalyst in the synthesis of ethylbenzene from benzene and ethylene [2], for xylene isomerization [3] and for the conversion of methanol to high-... 

Ternary composites have also been used comprising a Fischer-Tropsch catalyst, a methanol synthesis catalyst, and a zeolite [100]. Two Fe-based catalysts (ie, one promoted by K and the other by Ru), two HY zeolites with different acidities, a commercial HZSM-5, and Cu/ZnO/AljOj (methanol synthesis catalyst) were tested in these composites. Dimethyl ether (DME), methanol, and hydrocarbons were formed. Addition of the Cu/ZnO/Al Oj catalyst to a binary mixture of a Fischer-Tropsch catalyst and HZSM-5 results in the increase of the CO conversion by more than 20 times. The DME selectivity decreases as the conversion increases. Y zeolites and the Fischer-Tropsch synthesis catalyst promoted by Ru generated the most active composites. The role of zeolites in the ternary composite is assumed with the DME synthesis. First, methanol is synthesized from syngas on Cu/ZnO/Al Oj then it is dehydrated by an acid catalyst to produce DME and finally, DME initiates FT synthesis, which is then propagated by CO. 

The capsule catalyst Co/SiO -Z with zeolite HZSM-5 as the shell and the Co/SiO -Z-Pd capsule catalysts were used for isoparaffin synthesis from syngas at H2/CO=2/l, 1.0 MPa and 533 K [107]. The isoparaffin and olefin selectivity increased for the HZSM-5-based capsule catalysts compared to the conventional Co/SiO catalyst. Supporting Pd by sputtering caused the selectivity to isoparaffins to increase, and the selectivity to olefins to decrease compared to the samples prepared by incipient wetness, because metallic Pd supported by sputtering on the zeolite shell could hydrogenate olefins efficiently. The CO conversion, CH and CO selectivity on the catalyst Co/SiO -Z-Pd prepared by sputtering increased with a temperature rise from 513 to 553 K. At elevated temperatures, the isoparaffin selectivity also increased, whereas the olefin selectivity diminished. 

An alternative process based on two sequential catalytic cracking stages aimed at obtaining gasoline and diesel from waste plastics or heavy oil/waste plastics mixtures is shown in Figure 3.16 [99]. The catalyst employed in the first step is made up of powder alumina, waterglass and HZSM-5 zeolite and is mixed up directly with the waste plastics in a screw reactor preferably at 600-700°C. The second catalytic step consists in a fixed... 

Influence of Catalyst Preparation. Pebrine reported on the influence of the synthesis conditions of HZSM-5 on the selectivity toward light olefins. Synthesizing ZSM-5 in the presence of a tetra-urea-cobalt(II) complex resulted in an ethylene yield of 24.3 wt% of the hydrocarbon fraction at 43.7% methanol conversion, whereas the conventionally prepared ZSM-5 yielded only 18 wt% ethylene at the same conditions and conversion. Heering et al. mentioned that the conversion of dimethylether on ZSM-5 catalysts crystallized from a sodium-free gel with 1,6-dicunino-hexane as organic base was more selective toward both ethylene and propylene than on a conventionally prepared zeolite in the sodium form from a gel containing tetrapropylammonium. 

Although the catalyst (prepared based on a HZSM-5 zeolite) performs well despite deactivation and the operating conditions are suitably chosen in view of this, the deactivation by coke in the MTG (methanol to gasoline) process is somewhat rapid [1]. (Consequently, the economy of the MTG process requires periodic regeneration of the catalyst by coke combustion with air within the reactor itself [2-4]. In addition to the rapid deactivation, an additional aspect that conditions the design of the MTG process is the fact that the reaction is highly exothermic. 

A kinetic model for the deactivation by coke deposition of a catalyst (based on a HZSM-5 zeolite) used in the transformation of aqueous ethanol into hydrocarbons has been proposed. The experiments have been carried out in an isothermal fixed bed reactor by feeding the reactor with ethene, ethanol/water with different mass ratios and diethyl ether. The kinetic model quantifies the effect on coke deposition of the concentration of the organic components in the reaction medium, and takes into account the attenuating effect of water on coke deposition. 

Due to the similarity of the catalyst and of the characteristics of the deactivation in the transformation of methanol and of ethanol, eq. (5) has been taken as a basis for the establishment of possible deactivation equations for the transformation of ethanol into hydrocarbons. In eq. (5), X is the mass fraction (based on the organic components in the reaction medium) of the lumps of the kinetic scheme that can be considered coke precursors. This is the way in which the composition of the reaction medium is commonly expressed in the literature for the kinetic study of the processes of transformation of methanol on a HZSM-5 zeolite [8,12-14,16] and on a SAPO-34 [9]. In eq. (5), activity, a, is defined as the ratio between reaction rate at t time and at zero time ... 

The most widely used catalysts for acid-catalyzed aldol condensations are the molecular sieve zeolites, for example, crystalline aluminosilicates of group I and II elements, in which the latter have been replaced by protons. The surface protons confer Br0nsted acidity. Among the acidic zeolites we can mention HZSM-5 (pentasil zeolite), HY (faujasite), or HM (mordenite). Recently, polystyrene-supported sulfonic acids such as those of the macroreticular strongly acidic cation-exchange resins (59) and acid-base functionalized mesoporous materials such as amine and sulfonic acid-containing SBA-15 (60) have been shown to promote the acid-catalyzed aldol condensation of aldehydes with ketones at low temperatures. 

A kinetic analysis for the ring-opening (using a high hydrogen concentration in the reaction medium) of methylcyclohexane over a catalyst based on HZSM-5 zeolite [47] was... 

Daramola M O, Burger A J and Giroir-Fendler A (2011), Modelling and sensitivity analysis of a nanocomposite MFI-alumina based extractor-type zeolite catalytic membrane reactor for m-Xylene isomerization over Pt-HZSM-5 catalyst , Chem Eng J, 171,618-627. 

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