Ionothermal Synthesis of Aluminophosphate Molecular Sieve Membranes through Substrate Surface Conversion

Abstract

Applications of zeolite membranes in separation, chemical sensors, and catalytic membrane reactors have led to extensive studies on their fabrication. [1,2] To date, mainly silicatebased zeolite membranes of the structure types MFI, [3] DDR, [4] LTA, [5] and FAU [6] have been synthesized on different substrates and evaluated for single-or multiple-gas permeation. Routes for the synthesis of zeolite membranes can be broadly classified into two categories: [7] in situ and secondary growth synthesis, both of which are derived from the strategy of hydrothermal zeolite synthesis. However, the high autogenous pressure [8] associated with hydrothermal procedures is inconvenient for preparation of zeolite membranes, especially for large-scale production, due to safety concerns and the risk of molecular sieve collapse. On the other hand, although aluminophosphate [9,10] and silicoaluminophosphate [11,12] membranes for gas separation have been prepared, there are few reports on the preparation of aluminophosphate molecular sieve membranes, [9][10][11][12] which suggests that hydrothermal preparation of such membranes may be difficult.Ionothermal synthesis, [13] which uses ionic liquids as reaction media instead of water or organic solvents, allowed highly efficient preparation of aluminophosphate molecular sieves under ambient pressure. [13][14][15] Ionic liquids (ILs) are commonly defined as salts that consist of organic cations and inorganic anions with melting temperatures below 100 8C. [16] ILs are well known as environmentally benign and designable solvents, [17] which endow ionothermal synthesis with interesting features and many potential advantages over the hydrothermal method. As a result of the negligible vapor pressure of ILs, ionothermal synthesis can take place at ambient pressure, which eliminates safety concerns. The organic cation of the IL can act as a structure-directing agent (SDA), [13c] or play a cooperative structure-directing role together with introduced amines and quaternary ammonium compounds. [18][19][20] AEL, AFI, CHA, LTA, and ÀCLO types of aluminophosphate molecular sieves have been successfully synthesized by ionothermal methods.Ionothermal synthesis is a promising method for the preparation of aluminophosphate molecular sieve membranes under ambient pressure. Yan and co-workers prepared AlPO-11 and SAPO-11 films on aluminum alloys as anticorrosion coatings by in situ ionothermal synthesis. [21] To the best of our knowledge, no permeable membrane has been previously prepared on a porous substrate by the ionothermal method. Here we report an ionothermal method for the synthesis of permeable aluminophosphate molecular sieve membranes on porous alumina disks by substrate-surface conversion.Molecular sieve membranes of types AEL, AFI, CHA, and LTA were prepared by placing homemade d-alumina substrates (2.0 mm thickness, 20 mm diameter, and 10 nm average pore size; see Supporting Information for details) in a solution of IL, phosphoric acid, hydrofluoric acid, and, if required, organic amines w...

In Situ Electrochemical Synthesis of Oriented and Defect‐Free AEL Molecular‐Sieve Films Using Ionic Liquids

“…These results indicated that the amount of Al dissolved by acid at ambient pressure was not sufficient to form ac ontinuous AlPO 4 -11 film without an additional Al source in the synthesis solution. In accordance with our observation, Tian and co-workers [21] also reported that aluminophosphate molecular-sieve membranes can only be ionothermally synthesized on d-Al 2 O 3 through substrate surface conversion in as ealed vessel. Electrochemical oxidation can effectively generate asuitable amount of Al ions precisely at the surface of the Al substrate without adding an Al source directly to the synthesis solution.…”

Ionothermal synthesis of a CHA-type aluminophosphate molecular sieve membrane and its formation mechanism

“…The single gas permeation experiments were conducted at room temperature in an apparatus that was fabricated in-house [19]. The gas pressure p on the feed side of the membrane was maintained at 0.1 MPa, whereas the permeation side was at ambient pressure.…”

“…The CHA-type molecular sieve membranes were prepared according to the method reported elsewhere [19]. The typical synthesis procedure of the "substrate surface conversion" method can be described as follows: a d-aluminia disc was placed into a 1-ethyl-3-methyl imidazolium bromide ([EMIm]Br) solution containing reagent quantities of phosphoric acid (H 3 PO 4 , 85 wt% in water, AR), hydrofluoric acid (HF, 40 wt% in water, AR) and N-methylimidazole (N-MIm).…”

“…Yan and coworkers have reported ionothermal synthesis of AlPO 4 -11 and SAPO-11 films on aluminum alloys by microwave heating. Recently, we reported an ionothermal approach for the synthesis of permeable aluminophosphate molecular sieve membranes on porous alumina discs through a so-called "substrate surface conversion" method [19,20]. During the synthesis, the substrate not only acted as a support for the membrane but also as an Al source.…”

Ionothermal Synthesis of AEL-Type Aluminophosphate Molecular Sieve Membrane and Its Formation Mechanism

“…在之前的工作中, 我们探讨了离子热基底自转晶法 合成磷酸铝分子筛膜的机理 [14] : 离子热基底自转晶过 程中, 氧化铝基底表面与溶液中的磷酸和氢氟酸发生反 应, 可能在其表面形成前驱物, 前驱物进而成核、晶化 成分子筛膜.…”