Catalytic transformation of ethylene to propylene and butene over an acidic Ca-incorporated composite nanocatalyst
Graphical abstract
Introduction
Light olefins (ethylene, propylene, and butene) are the key building blocks in the petrochemical industries [1]. These platform chemicals are produced industrially mainly through steam cracking [[1], [2], [3], [4], [5], [6]], catalytic cracking [[7], [8], [9]], and dehydrogenation [10,11]. To these pathways should be added the selective dimerization of ethylene to 1-butene [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21]]. Hence, a lot of research is still being generated on the related catalysts [10,[22], [23], [24], [25], [26]].
The growing demand for propylene (the average of 5.7% per annum) along with the reduction in the availability of conventional sources for three to four carbon olefins have lead to the emergence of relevant on-purpose technologies to fulfill the need for the ample derivatives of propylene [[3], [4], [5], [6],22,[27], [28], [29]]. The frequent discoveries of rich gas recourses and the new technologies presented for natural gas processing in areas such as the Middle East, the Mid-Atlantic States, Russia, and the South West of the USA have led to surplus in light alkanes such as ethane which is regarded as a low-cost and available feedstock for the steam cracker units. This potential provides the largest cost advantage for the petrochemical industry [30]. Perhaps, the sole disadvantage of ethane as a feedstock is its limited product distribution (ethylene and some fuel) [30,31]. Having all of these in mind, an integrated scheme of ethane to ethylene followed by ethylene to propylene could be quite attractive by coining a new pathway that applies ethane as its starting material and ends at more demanded products such as propylene [22,32]. In this sense, the direct conversion of ethylene to propylene (ETP) or a combination of valuable chemicals including butenes and higher oligomers is emerged as one of the on-purpose technologies with the capability of responding to the displacement of the balance between supply and demand of light olefins in a flexible manner. A comprehensive overview of heterogeneous catalysts for the relevant gas-phase conversion pathways was presented in two recent review papers [22,33] that addressed all aspects of the relevant systems including the catalytic performances, productivities, life times, chemistry of active sites, etc. At variance with the relatively well-established technologies that include butene as a co-feedstock, the direct process with ethylene as the sole feedstock normally produces butene as a valuable by-product. Notably, the ETP process is viable through different reaction scenarios, such as, metathesis and oligomerization [22,34]. The former is carried out over transition metal catalysts such as Re, Mo, W, Ni and Ru [28,[34], [35], [36], [37]]. However, the process suffers from several drawbacks including the reaction intricacies and relatively quick catalyst poisoning due to the sensitivity to different types of impurities such as sulfur, water, and other sorts of hydrocarbons [22]. Taking into account these complexities, the latter pathway becomes more preferred, with the control of the reactions being relatively facile. Either way, the ETP process is known to be highly favorable thermodynamically in such a manner that, below 900 K, the standard Gibbs free energy of the oligomerization-cracking reactions is negative and below 600 K, the reaction is at complete equilibrium [38,39]. The oligomerization pathway is efficiently performed over acidic micro- and meso-porous materials such as HZSM-5, HMOR, HY, HSAPO-34, HMCM-41 and so forth [28,40]. Among those, HZSM-5 and HSAPO-34 are the most frequently applied catalysts for producing light olefins due to their high selectivity toward light olefins, fine pores which prohibit the formation of higher olefins, long lifetime particularly after metal-modification, thermal stability, shape selectivity, and also adjustable acidity [10,[41], [42], [43]]. By far, several catalysts have been tested for this purpose and improvements have been made in the relevant catalyst systems. Examples include the incorporation of various metals such as Cr, Se, and Ni into the catalyst structure, application of composite catalysts such as ZSM-5/SAPO-34 with improved characteristics to the MTO reaction [44], and the composite catalyst of HAlZSM-5 added to silica-alumina matrix for ethene and propene oligomerization [45]. Nevertheless, more investigations are required to develop highly efficient catalysts for this process in terms of propylene selectivity, productivity, and catalyst regenerability at improved conditions, e.g., the appropriate dispersion and strength of the acidic sites on the surface of the catalyst [8,11,28,36,39,46].
Having these in mind and to further advance the field, the focus of the present study was the direct conversion of ethylene to propylene (and butene) through an oligomerization-cracking pathway over a recently developed proprietary composite catalyst called GNM-1, which is characterized by the means of XRD, FTIR, FESEM, EDS, N2 physisorption, and NH3-TPD techniques. Both the reaction temperature and partial pressure of ethylene were altered to probe the catalytic performance of GNM-1 in the ETP reaction.
Section snippets
Catalyst preparation
The HZSM-5 catalyst was prepared following the well-established recipes [47,48]. A high-silica ZSM-5 sample (close to silicalite-1) was also prepared according to the same procedure except that the aluminum source was almost negligible. Typically, a clear solution of sodium aluminate and the same amount of sodium hydroxide in water was added to tetrapropylammonium hydroxide (TPAOH, 40 wt%). Then, tetraethylorthosilicate (TEOS) was slowly added and the mixture was left with stirring for 7200 s
X-ray powder diffraction
Fig. 2 displays the X-ray diffraction patterns of the three catalysts. The diffractograms of HZSM-5 and HSAPO-34 were typical patterns expected from the reference books [52,53]. For the GNM-1 sample, the multiple indicator peaks in the XRD diffractogram were attributable to a combination of different phases including pentasil (MEL, MFI) aluminum silicates, calcite, aluminum phosphate, and Ca-CHA zeolite structure with a descending order of quantity, which clearly represented the composite
Conclusion
The activity of the new acidic composite nanomaterial (GNM-1) was compared against reference HZSM-5 and HSAPO-34 samples for the ETP reaction. The XRD patterns revealed that the new composite catalyst comprised of pentasil (MEL, MFI) aluminum silicates, calcite, Ca-CHA zeolite, and aluminum phosphate, with a descending order of quantity. The NH3-TPD analyses uncovered the presence of acidic sites of medium strength on GNM-1 compared to those of the other closely relevant benchmark catalysts
Acknowledgments
The authors gratefully acknowledge the assistance from Mr. Vahid Farzaneh and the support received from IPPI under grant 53791101.
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