Unprecedented activity and stability on zirconium phosphates grafted mesoporous silicas for renewable aromatics production from furans
Graphical abstract
Introduction
Biomass, a CO2-neutral renewable energy-source, has a significant potential for a sustainable production of building block chemicals, and benzene, toluene and xylenes (BTX) are important chemicals for the synthesis of plastics, packaging, clothing, paints, computers, and so on. Among them, xylene isomers are most value-added chemicals, for example, para-xylene (pX) has been widely used in the production of terephthalic acid (TPA), which subsequently transformed to produce polyethylene terephthalate bottles and polyester fibers [1], [2], [3]. The pX has been generally produced from crude oil through catalytic cracking of naphtha, although the method requires an additional energy consuming distillation process to purify pX from other isomers of xylenes as well as benzene and toluene [4], [5]. Different sustainable alternative routes such as aqueous phase reforming of C5 + C6 sugars [6], Gevo process from i-butanol [7], catalytic pyrolysis of biomass [8], selective conversion of ethylene [9] and syngas [10], and dehydrative aromatization of the Diels-Alder product between 2,5-dimethylfuran (DMF) and ethylene [6], [7] have been widely investigated in recent years. The tandem reaction involving Diels-Alder cycloaddition of ethylene (C2H4) to DMF followed by dehydrative aromatization seems to be more advantageous over the other sustainable routes as the tandem-route not only couples the biomass-derived chemicals but also directly gives a very high stereoselectivity (99%) to the para-isomer of xylenes [11], [12], [13]. Besides DMF, the Diels-Alder approach can also be applicable to the other furanic dienes namely 2-methylfuran (MF) and furan (F), which can be readily obtained from many lignocellulosic biomass, to produce toluene (T) and benzene (B), respectively [14]. The results, however, showed a successive decrease in the selectivity of corresponding desired aromatics under the same reaction conditions. For instance, the reaction between DMF and ethylene produces 90% pX selectivity over H-BEA, whereas the reaction of MF and F with ethylene yields only 46% T and 35% B selectivity, respectively [15].
The tandem reaction for the production of benzene, toluene and para-xylene (BTpX) from furans is typically carried out by either a Brönsted (B) or Lewis (L) acid sites [14], [16], [17], [18], [19]. Although dehydration of cycloadduct intermediate and undesired hydrolysis reaction of furans are greatly affected on both acid sites [16], very recent studies suggested that catalysts containing suitable amounts of both B and L acid sites are more active than catalysts comprising predominantly either B or L acid sites [13], [20]. The L acid sites can promote the Diels-Alder cycloaddition reaction of furans and ethylene to the cycloadduct product, which dehydrated to BTpX on both L and B acid sites, but more efficiently catalyzed by B acid sites [13], [14], [20], [21]. Acidic zeolites and trivalent (Ga3+, B3+ and Al3+) or tetravalent (Zr4+, Ti4+ and Sn4+) metals substituted Beta zeolites have been widely used, though undesirable reactions such as isomerization of pX to m- and o-xylenes, hydrolysis of DMF to 2,5-hexanedione and electrophilic reactions of cationic intermediates, which demonstrated to be formed during dehydrative aromatization of the Diels-Alder product, were also reported [11], [15], [17], [18], [22], [23], [24]. Moreover, an increased concentration of DMF from 0.46 to ~3.2 M decreases the targeted pX selectivity significantly from 85% to 45% and inversely increases the undesired product selectivity of dimers and trimers from 15 to 55% over H-BEA catalyst, which are preferentially formed from an inappropriate concentration of both B and L acid sites and their strength [25]. Those competing undesired reactions were minimized in presence of ‘isolated P acid sites’ on phosphorous-containing Beta zeolite [26]. However, the inherent microporous structures of zeolites lead to faster catalyst deactivation, especially during the Diels-Alder cycloaddition reaction of MF and Furan with ethylene, thus decrease the target value-added aromatics by trapping the trimer-like oligomeric compounds over zeolite micropores [24]. Only few researchers have demonstrated that the introduction of mesopores into microporous zeolites [27] as well as some mesoporous WO3/SBA-15 [13], SiO2–SO3H [19] and NbOx–based [20] catalysts has improved the coke-tolerance from deactivation due to a more facile mass transfer through the mesoporous structures than that of the microporous zeolites. Therefore, the synergy effects between the B and L acidic sites, and their strengths along with mesoporosity not only facilitate Diels-Alder reaction of furans and ethylene but also capable of successive conversion of the cycloadduct intermediates to targeted BTpX products [16], [20], [28].
Among many active metal-based heterogeneous catalysts, metal phosphates are promising one due to their hydrothermal stability and cooperation between B and L acidic sites in the sustainable conversion of biomass to fuels and chemicals [29], [30], [31], [32], [33]. Although significant efforts have been made to synthesize the highly active metal phosphates with well-controlled physico-chemical properties, they showed a relatively lower surface area and acid sites concentration, which limits their applications for the production of pX [34]. It is therefore necessary to prepare the metal phosphates with large surface area, tunable pore sizes and reasonable active sites. As reported by Zhang et al., one-pot grafting of metal phosphates onto mesoporous silica may address all the above challenges [35], [36]. As far as we know, only one report is available for the one-pot surface grafting of titanium phosphate [35] and zirconium phosphate [36] on SBA-15 using nonhydrolytic sol-gel method that were mainly used for dehydration of isopropanol and also for cumene cracking reaction. However, their application for biomass conversion reaction is not reported so far.
In continuation of our efforts to develop sustainable ordered mesoporous solid catalysts for the production of hydrocarbon fuels and oxygenates [37], here we prepared a series of highly active and stable ordered mesoporous zirconium phosphate (ZrP) grafted on the various silicas for the production of pX from the relatively higher DMF concentration (2.35 M) at moderate ethylene pressure (2.0 MPa) with smaller catalyst concentration (2 wt%). The higher catalytic activity and stability shown by the ZrP grafted on the SBA-15 (ZrP1.5-SBA, where P/Zr mole ratio of 1.5) as compared to the ZrP grafted either on the KIT-6 (ZrP1.5-KIT) or SiO2 (ZrP1.5-Si) revealed the superior surface properties on the larger specific surface area of the ZrP1.5-SBA with the abundant presence of thermally active ZrP species and its optimal amount of acid sites in the highly ordered mesoporous structures.
Section snippets
Synthesis of silica supports and their modified catalysts
A highly ordered mesoporous KIT-6 and SBA-15 silica supports were prepared using a sol-gel method as described previously [37](a), [38]. Briefly, for the synthesis of KIT-6, Pluronic P123 copolymer (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), Sigma-Aldrich) was dissolved in deionized water and HCl solution under vigorous stirring. After complete dissolution, n-butanol was added to the above solution with continuous stirring for 1 h. Silica source, tetraethoxysilane (TEOS,
ZrP grafted mesoporous silica catalysts
In our earlier work [32], the cascade conversion of glucose to 5-hydroxymethylfurfural (HMF) over ZrP, we found that the conversion of glucose and formation of HMF can be tuned, which were depending upon the concentration of L and B acid sites on the ZrP, respectively. In the present study of the pX production from DMF, similar attempts were applied to use the same zirconium phosphate (i.e., ZrP1) since the present tandem reaction also required both L and B acid sites, however DMF conversion
Conclusions
A series of zirconium phosphates (ZrP) grafted onto the highly ordered mesoporous silica catalysts were prepared and examined for the tandem Diels-Alder cycloaddition and dehydration reactions to produce renewable benzene, toluene and para-xylene using the biomass-derived furan, 2-methylfuran and 2,5-dimethylfuran (DMF) with gaseous ethylene, respectively. The ZrP grafted on the SBA-15 with the optimal P/Zr mole ratio of 1.5 (ZrP1.5-SBA) was found to be a promising catalyst for a higher DMF
Acknowledgements
The authors are grateful to the National Research Foundation (NRF) of South Korea for the financial support (NRF-2018M3D3A1A01018009 and NRF-2020R1A2C2006052).
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Present address: Department of Chemical and Metallurgical Engineering, School of Chemical Engineering, Aalto University, Kemistintie 1, P.O. Box 16100, Espoo, FI-00076, Finland