Efficient and selective conversion of methanol to para-xylene over stable H[Zn,Al]ZSM-5/SiO2 composite catalyst
Graphical abstract
Introduction
Benzene, toluene, xylene (BTX) are fundamental raw materials in the chemical industry [[1], [2], [3], [4], [5], [6]]. Especially, para-xylene (p-X) becomes the most valuable component among xylene isomers (p-, m-, o-xylene), owing to the wide use of terephthalic acids in the polyethylene terephthalate industry [7,8]. So far, commercial p-X productions mostly depend on petroleum-based processes, such as toluene alkylation, toluene disproportionation, and separation of xylene isomers [[9], [10], [11], [12]]. However, given the oil exhaustion and energy-consuming separation, alternative p-X production ways are urgently needed. One such alternative way is the methanol to aromatics (MTA) reaction over zeolite catalysts, as methanol can be conveniently obtained from various raw materials, such as coal, natural gas and biomass [[13], [14], [15]].
HZSM-5, owing to its hydrothermal stability, adjustable acidic properties, and regular crystal structure, has become the most promising catalytic component for MTA reaction [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. In this complicated reaction, several key steps including methanol dehydration, olefin oligomerization, cyclization and aromatization are important processes for aromatics formation [[27], [28], [29]]. Furthermore, side reactions (such as xylene alkylation, xylene dealkylation, xylene isomerization and hydrogen transfer reaction) coinstantaneously take place in MTA due to the acid strength, acid amount and acid distribution in HZSM-5 [7,30]. As a result, the p-X yield in MTA catalyzed by HZSM-5 is very low, which severely decreases the economic efficiency of this protocol and further limits the development of the polyethylene terephthalate industry [14,[31], [32], [33]].
Several studies have been done recently to improve the p-X selectivity in xylene [[34], [35], [36], [37]]. For example, Miyake and coworkers reported a HZSM-5/silicalite-1 core-shell composite catalyst constructed of a HZSM-5 core with acid sites and a silicalite-1 shell (Al free MFI zeolite) without acid sites, and the p-X selectivity in xylene largely increased from 23% to 94% after coating [30]. This was because the silicalite-1 coating suppressed the side reactions (such as alkylation and isomerization) on the external surface of HZSM-5 through the elimination of external acid sites. Additionally, given the effective dehydrogenation interaction between metal species and acid sites on zeolite, Zn, Ag, Ga species was introduced into HZSM-5 to improve the aromatics yield in MTA reaction [[38], [39], [40], [41], [42]]. Miyake and coworkers prepared a Zn/ZSM-5/silicalite-1 catalyst by Zn ion exchange and subsequent coating of silicalite-1 on HZSM-5, and largely improved the p-X yield from 7.6% to 40.7% and p-X selectivity from 24% to 99% [13]. Similarly, Zhang and coworkers designed a Zn/P/Si/ZSM-5 catalyst from the introduction of Zn and P species by impregnation and subsequent coating of SiO2 on the HZSM-5 external surface by chemical liquid deposition [7]. This catalyst exhibited a high total aromatics yield (61.7%) and p-X selectivity (89.6%) under reaction conditions of 475 ℃, 0.1 MPa, and 0.79 h−1 WHSVmethanol in MTA reaction due to the strong acid of Zn/P/ZSM-5 core and the weak acid of SiO2 shell. However, the lifetime of this catalyst was only 1 h, which would limit its commercial application due to the frequent catalyst regeneration, low production efficiency and high cost. Accordingly, Li and coworkers reported that Mg modification of Zn-Si-HZSM-5 core-shell catalyst selectively decreased the density of strong acid sites in the HZSM-5 channels, which enhanced the catalytic stability by suppressing the coke formation at the channel intersections of zeolite [14]. The lifetime of Mg-Zn-Si-HZSM-5 at 460 ℃ and 1.0 h−1 WHSVmethanol under N2 flow (30 ml min−1) in MTA was largely prolonged from 5 h to 12 h after Mg modification. However, this lifetime was not long enough for commercial production, and the Mg introduction complicated the catalyst preparation. Moreover, the state of the introduced metal species and its interaction with zeolite were not elaborated.
Isomorphous substitution is an alternative way to prepare metal-doped zeolite catalysts [[43], [44], [45]]. Su and coworkers reported the incorporation of Ga into the structural framework of HZSM-5 by isomorphous substitution, which enhanced the aromatization performance [41]. Ni and coworkers prepared nano-sized H[Zn,Al]ZSM-5 by isomorphous substitution, and the incorporation of Zn into zeolite framework improved both BTX yield and catalytic lifetime [2]. Niu and coworkers prepared Zn(IM)/ZSM-5 and Zn(DS)/ZSM-5 by impregnation and isomorphous substitution, respectively, and investigated the influence of preparation method on the performances of Zn-modified catalysts in MTA reaction [46]. Zn(DS)/ZSM-5 possessed an almost double catalytic lifetime than Zn(IM)/ZSM-5 due to the presence of more ZnOH+ species and larger mesopore volume, which enhanced the coke deposition resistance in the reaction.
Therefore, we designed and prepared a H[Zn,Al]ZSM-5/SiO2 composite consisting of a H[Zn,Al]ZSM-5 core with strong acid sites and a SiO2 shell with weak acid sites, and studied its aromatization performance in MTA reaction. This new catalyst simultaneously obtained high BTX yield, long catalytic lifetime and high p-X selectivity in xylene. The enhancements in catalytic activity (BTX yield) and catalytic stability (catalytic lifetime) of the H[Zn,Al]ZSM-5 core and in the p-X selectivity of the SiO2 shell were studied. In addition, a controlled Zn/HZSM-5/SiO2 catalyst was synthesized in a similar way as H[Zn,Al]ZSM-5/SiO2, except that in the Zn introduction step for Zn/HZSM-5 core preparation, an impregnation method was adopted to study the influence of Zn introduction method on MTA performance. The deactivation of MTA reaction over the two composite catalysts were also elaborated to further elucidate the excellent catalytic stability of H[Zn,Al]ZSM-5/SiO2.
Section snippets
Catalyst preparation
HZSM-5 zeolites were synthesized using tetraethyl orthosilicate (TEOS, 28 wt.% SiO2) and aluminum isopropoxide (AIP, 25 wt.% Al2O3) as Si and Al sources, respectively. Tetrapropylammonium hydroxide (TPAOH, 25 wt.% aqueous solution) was used as structure-directing agent (SDA). Other reagents, such as zinc nitrate hexahydrate (Zn(NO3)2·6H2O), ammonium hydroxide (NH4OH 25–28 wt.%) and hexane were used as received without further purification.
The synthesis procedure of H[Zn,Al]ZSM-5 zeolites were
Structural properties of catalysts
Fig. 1 showed the XRD patterns of the as-synthesized zeolites. All zeolites exhibited the diffractions assigned to the MFI-type zeolite [41]. The characteristic peaks of ZnO at 31.8° and 36.3° were not observed in H[Zn,Al]ZSM-5, Zn/HZSM-5, H[Zn,Al]ZSM-5/SiO2 and Zn/HZSM-5/SiO2, indicating the Zn species were highly dispersed after modification [46]. However, the characteristic peaks at 2θ of 22.0–25.0° have slightly shifted to the low angle in H[Zn,Al]ZSM-5 and Zn/HZSM-5 (Fig. 1b), suggesting
Conclusions
A highly shape-selective H[Zn,Al]ZSM-5/SiO2 composite catalyst for MTA was prepared by direct incorporation of Zn species via isomorphous substitution into the zeolite framework and subsequent SiO2 deposition on zeolite surface. A control Zn/HZSM-5/SiO2 catalyst was also prepared in a similar way as H[Zn,Al]ZSM-5/SiO2 except that an impregnation method was adopted in the Zn introduction step. The Zn introduction method had obvious influences on the textural properties, acidic properties and the
Acknowledgments
This wok was supported by the financial support from National Natural Science Foundation of China (Nos. 21276050, 21406034, and 21676054), Natural Science foundation of Jiangsu (No. BK20161415), Fundamental Research for the Central Universities.
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