Modified HZSM-5 zeolites for intensifying propylene production in the transformation of 1-butene
Graphical abstract
Introduction
Knowledge on the transformation of light olefins (ethylene, propylene and butenes) is essential in the catalytic processes for olefin production from oil fractions by fluid catalytic cracking (FCC) and from other alternative sources and routes, such as methanol by the MTO (methanol to olefins) process and dimethyl ether by the DTO (dimethyl ether to olefins) process. These processes of lower energy requirements and lower CO2 emissions in comparison to steam cracking (major light olefin source) [1] are subject to continuous innovation in order to satisfy the increasing olefin demand [2], [3]. Moreover, new routes have been opened for olefin production by the catalytic transformation of paraffinic streams in refineries [4], [5], lignocellullosic biomass (by bio-oil cracking) [6], bioethanol [7], [8] and pyrolysis/cracking of waste polyolefins [9], [10]. The present demand of individual light olefins is undergoing significant changes conditioned by their interest as raw materials, with a current shortage in propylene production boosting its selective production according to the different processes mentioned above [11]. Furthermore, steam cracking (the main olefin production unit, with ethylene being the major product) will be boosted by feeding ethane (obtained from shale gas extraction) [12]. Consequently, propylene will be produced only in other catalytic processes of low selectivity. Within this scenario, apart from the development of ad hoc processes for the selective production of propylene, light olefin interconversion arouses great interest with a view to adjusting its production to market prices and requirements.
Light olefin formation and their distribution in the product stream in the catalytic processes mentioned above are explained by a complex network based on a carbocationic mechanism for the oligomerization–cracking of these olefins [13]. This mechanism has been widely studied since the implementation of the MOGD (Mobil olefin to gasoline and distillate) process on a HZSM-5 zeolite by Mobil Research and Development Corporation. This process takes place under low temperature conditions in which the oligomerization step prevails, thus obtaining middle distillates suitable for incorporating into the diesel pool [14], [15], [16]. Nevertheless, light olefin interconversion needs temperatures above 300 °C to ensure cracking of oligomers is the main step (higher activation energy than that for their formation). Furthermore, secondary reactions of aromatization and hydrogen transfer are also significant [17], [18], [19].
The HZMS-5 zeolite selectively produces light olefins in the aforementioned processes thanks to its properties involving a three-dimensional porous structure (MFI topology), a medium severity in the shape selectivity (with channels being elipsoidal, straight (0.53 × 0.56 nm) and zig-zag (0.51 × 0.55 nm)) and absence of cages located at micropore intersections. In addition, the moderate acid strength of the acid sites contributes to minimizing secondary reactions of aromatization and hydrogen transfer, as well as the deactivation by coke [20], [21], [22].
The HZSM-5 zeolite has great possibilities for modifying its properties (severity in the shape selectivity and acidity) in order to improve its performance (activity, selectivity and stability) for light olefin production. Accordingly, the steps for light olefin formation must be promoted by attenuating those for gaseous byproducts (paraffins, aromatics, high olefins) and coke formation. Certain treatments pursue the modification of the zeolite porous structure to generate mesopores and so reduce the contact time of products in the zeolite crystal, by carrying out as follows: (i) dealumination [23]; (ii) desilication [7], [24], and (iii) crystal size reduction [25], [26]. In order to decrease the amount and density of acid sites, a higher SiO2/Al2O3 ratio is recommended, which also enhances hydrothermal stability [27], [28]. Al-Dughaiter and de Lasa [29] studied the effect the SiO2/Al2O3 ratio of the HZSM-5 zeolite has on its acid properties, by using adsorption of NH3 and FTIR analysis of adsorbed pyridine. Different strategies have been applied in order to control HZSM-5 acidity, as: (i) controlling calcination temperature [27]; (ii) doping with different components, such as alkali metals and alkaline earth metals [17], [18], [30], P [31], [32], [33], [34], [35], [36], B [37], rare earth metals [38], [39], or W [40]. The HZSM-5 zeolite can also be combined with other compounds by: (i) agglomerating with a binder [4], [9] and (ii) creating hybrid catalysts by physical mixture or successive re-crystallization with other acid catalysts, such as HSAPO-34 [41]. Most of these methods significantly affect the kinetic performance of the HZSM-5 zeolite, and therefore provide high versatility to the zeolite.
This study explores the possibilities of the HZSM-5 for intensifying propylene production, with 1-butene being the raw material. The demand for butene stream is lower than that for ethylene and propylene, especially due to the decrease in the production of methyl tert-buthyl ether (MtBE) as additive for gasoline. The transformation of 1-butene into other more interesting light olefins is an alternative to the oligomerization for the production of middle distillates [16]. The literature studies on butene cracking report the complexity of the reaction scheme and the significant effect of HZSM-5 zeolite properties on product distribution and stability [42], [43], [44], [45].
HZSM-5 zeolites have been prepared based on different strategies suggested in the literature as suitable for propylene intensification, as are: (i) the use of zeolites with different SiO2/Al2O3 ratio [8], [42], [46], [47]; (ii) modification of the zeolite by incorporating different amounts of P [33], [48] and K [49] in order to attenuate the acidity, and (iii) mild in situ steaming treatment for attenuating acidity and generating mesopores [23]. The physical and acid properties of the catalysts have been determined and their catalytic performance has been evaluated, which should strike a suitable balance between conversion and propylene selectivity. Furthermore, special attention is paid to the stability of the catalysts in order to attenuate deactivation by coke deposition.
Section snippets
Catalysts preparation
The zeolites have been supplied by Zeolyst International in ammonium form and with different SiO2/Al2O3 molar ratios: 30, 80 and 280. The acid form of the zeolites (denoted HZ-X, where X indicates SiO2/Al2O3 ratio) was obtained by calcining at 570 °C for 2 h. The incorporation of K or P (nominal content in the 1–5 wt.% range) was carried out by incipient wetness method at 70 °C under vacuum, adding a suitable amount of KOH dissolution (purity 85%, Panreac) or H3PO4 (85%, Merck), respectively, in a
Catalysts properties
Table 1 sets out the chemical and physical properties of the synthesized zeolites (prior to agglomeration). ICP-AES analysis reveals that real K and P metallic content is close to the nominal one, which indicates that impregnation has been carried out correctly. The different modifications (K and P incorporation and mild in situ steaming) entail a drop in the adsorbed volumes of N2 (and consequently in the BET area and pore volume) at a relative pressure of P/P0 = 0.2, with this effect being more
Conclusions
Due to its influence on the density of the acid sites and on their strength, the selection of the SiO2/Al2O3 ratio of HZSM-5 zeolite is a suitable tool to modulate olefin selectivity (and propylene selectivity) in the transformation of 1-butene at 500 °C. An increase in the SiO2/Al2O3 ratio increases propylene selectivity by hindering hydrogen transfer and aromatization reactions (catalyzed by strong acid sites), although this increase also entails a decrease in conversion. Catalyst deactivation
Acknowledgements
This work was carried out with the financial support from the Ministry of Economy and Competitiveness (MINECO) of the Spanish Government (Project CTQ2010-19188), from the Basque Government (Project IT748-13) and from the University of the Basque Country (UFI 11/39). Eva Epelde is grateful for the Ph.D. grant from the Department of Education, University and Research of the Basque Country (BFI08.122) and for the University of the Basque Country. Technical and human support provided by SGIKer
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