Microwave synthesis of high-quality mordenite membrane by a two-stage varying heating-rate procedure
Graphical abstract
Introduction
Acetic acid (AA), as a raw material or intermediate for producing organic acids, esters, anhydrides, etc., shows many practical applications in chemical industry [[1], [2], [3]]. Naturally, dehydration of water-AA mixtures is an important procedure during AA production process to obtain high pure AA for industrial applications [2,3]. Traditional separation methods (e.g. distillation) for separation of water-AA mixtures are usually energy-extensive and high-cost because of the close volatility of water and AA [3]. Owing to its superior advantages of high-efficiency, energy-saving, environment-friendly and being independent of relative volatilities of components, membrane-based pervaporation (PV) technique has been considered as a high potential alternative technology for separating liquid mixtures especially isomeric, thermo-sensitive, azeotropic and close-boiling, mixtures [4,5].
In the past, zeolite membranes have been extensively explored as industrial potential separators [[6], [7], [8]] especially for liquid separation by pervaporation [[11], [12], [13], [14], [15], [16]] because of their good mechanical strength, chemical-resistance, and high-thermal stability as well as uniform pore size and ordered structure. To date, various zeolite membranes, including NaA [9], T-type [10], CHA [11], DDR [12], mordenite [[13], [14], [15], [16], [17], [18], [19]], ZSM-5 [[20], [21], [22]] and silicalite-1 [23], etc. have been developed for separation and purification of organics. Generally speaking, hydrophilicity of a zeolite membrane decreases while its acid resistance increases with an increase of Si/Al ratio in the zeolite framwork [10]. Mordenite membrane with moderate Si/Al ratio of 3–10 shows high hydrophilicity and good acid resistance [[13], [14], [15]]. In the separation process, preferential water adsorption on a mordenite membrane leads to high water selectivity. Therefore, mordenite membrane is a potential candidate for separating water from AA solutions.
Microwave synthesis technique began to be employed to the synthesis of zeolites in 1990s [24,25] and this field developed quickly, especially after the mid-1990s [26,27]. As a valuable process in Green Chemistry, microwave heating (MH) can make synthesis time shorter, zeolite crystallinity higher, particle size more uniform and energy consumption more economical as compared with conventional heating (CH), which benefits from the fast and homogeneous heating of microwave radiation [28]. These advantages promote researchers to apply MH to the fabrication of zeolite membranes for a shorter synthesis time and thinner membrane layer [29]. Yang's group has made great progress in this field and various zeolite membranes including LTA [[30], [31], [32], [33]], T-type [34,35], and FAU [36] membranes have been reported to be fabricated by MH. SAPO-34 [[37], [38], [39]], MFI [40,41], CHA [42], DDR [43,44] and mordenite [14] zeolite membranes have also been fabricated successfully under microwave assisted heating for different separation applications. It was reported that heating rate has an effect on crystal growth in the microwave synthesis [[45], [46], [47]]. Undoubtedly, heating rate is also an important factor in the preparation of zeolite membrane under microwave assisted heating, whereas research on this is scarce until now. In the present work, we applied the MH technique to the preparation of mordenite membranes on porous α-Al2O3 tubes with a novel two-stage varying heating-rate procedure. The effect of heating rate on fabrication and performance of mordenite membranes was carefully studied for the first time, as well as the influence of holding time. Moreover, we also made an investigation on PV performances for separation of water-AA mixtures and acid resistance of the as-synthesized mordenite membranes.
Section snippets
Chemicals
Colloidal silica (25 wt%, Qingdao Haiyang Chemical), aluminum sulfate octahydrate (99 wt%, Sinopharm), sodium hydroxide (96 wt%, Kermel) and sodium fluoride (98 wt%, Sinopharm) were all used without any treatment. Mordenite crystals were purchased from Shanghai Zhuoyue Chemical Technology.
Macroporous α-Al2O3 tubes (pore size about 2–3 μm, ID 8 mm, OD 12 mm, L 50 mm, porosity about 30–40%, Foshan) were used as support tubes. Before membrane preparation, the outer surface of tube was carefully
Effect of heating rate on preparation and PV performance of mordenite membrane
Firstly, effect of heating rate on mordenite membrane was investigated by a one-stage heating procedure. Membranes M1-M5 were synthesized by increasing temperature from 25 °C to 175 °C in 5, 10, 15, 20 and 25 min respectively, followed by crystallization at 175 °C for 60 min. As shown in Fig. 1a, the surface of M1 which was prepared with the shortest heating time is not fully covered by crystals and the crystal size is not uniform (about 200 nm to 1 μm). As prolonging heating time (e.g., 10 and
Conclusion
In summary, high performances mordenite membranes were successfully prepared from a dilute solution with a molar ratio of SiO2: 0.06 Al2O3: 0.26 Na2O: 0.3 NaF: 125 H2O by a novel two-stage varying heating-rate procedure under microwave assisted heating. Among them, the as-synthesized M14 shows the best performance which was prepared by elevating the temperature from 25 °C to 100 °C in 10 min at the first stage and from 100 °C to 175 °C in 5 min at the second stage respectively and then followed
CRediT authorship contribution statement
Liangqing Li: Conceptualization, Writing - original draft, Resources. Jiajia Li: Data curation, Methodology, Visualization. Linjuan Cheng: Investigation, Validation. Jiaxuan Wang: Investigation, Validation. Jianhua Yang: Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Key Research and Development Plan of Anhui Province (grant number 1804a09020072), the Anhui Provincial Natural Science Foundation (grant number 1808085QB51) and Oversea Visiting Program for Universities and Colleges Youth Talents of Anhui Province (grant number).
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