Thermally Stable Amorphous Mesoporous Aluminophosphates with Controllable P/Al Ratio:  Synthesis, Characterization, and Catalytic Performance for Selective O-Methylation of Catechol

Abstract

Amorphous mesoporous aluminophosphates (AlPO) with P/Al molar ratio in the range 0.8-1.15 are synthesized by using the citric acid (CA) route and are systematically characterized using N(2)-adsorption, XRD, SEM, solid-state CP-MAS NMR, FT-IR, TG-DTA, CO(2)-TPD, and NH(3)-TPD. The characterization studies show that the change in P/Al ratio could affect the structure, texture, thermal stability, and surface acid-base properties of AlPO. Samples with a relatively low P/Al ratio (< or =1.0) exhibit uniform amorphous mesoporous character and high thermal stability (up to 1173 K). Partial crystallization of the AlPO framework easily occurred on the sample with higher P/Al ratio (> or =1.1), thus leading to significant decrease of surface area and formation of particle pile mesopores. Both weak acid and weak base sites are observed over AlPO materials, and the amounts of acid-base sites can be effectively controlled by adjusting the P/Al ratio. The presence of suitable interaction between citric acid and AlPO framework is critical for the formation of mesoporous structures. Both CA and PO(4) units are considered to be ligands to coordinate with aluminum ions, forming relative uniform complexes (such as CA-Al-PO(4)) in the as-synthesized AlPO materials. The mesoporous structure of AlPO materials is obtained after the rapid decomposition of citric acid. Vapor phase selective O-methylation of catechol with methanol reaction is carried out to investigate the catalytic performances of AlPO materials with different P/Al ratios. Among them, AlP(1.1)O shows the highest activity (88.4% conversion of catechol) and the highest yield of guaiacol (74.3%). The presence of suitable weak acid-base pairs may play an important role on the title reaction.

Aldol condensation of acetone over Mg–Al mixed oxides catalyst prepared by a citric acid route

“…2B). It is known that the hysteresis loop in the partial pressure region from 0.4 to 0.8 is associated with framework mesoporosity, and the hysteresis loop at high partial pressure (P/P 0 [ 0.8) is indicative of textural mesoporosity and/or macroporosity, whereas the sharp increase in nitrogen uptake at P/P 0 [ 0.9 is indicative of a significant amount of interparticle mesoporosity [19]. These results suggest that the porosity of Mg x AlO and MgO should mainly arise from voids of small particles.…”

“…Previously, such approach has shown some advantages on the preparation of a variety of porous materials, including silica, alumina, aluminophosphates, carbon, etc. [17][18][19][20]. Herein, the catalytic properties of the resultant Mg x AlO materials are studied for the gas-phase aldol condensation of acetone.…”

Recent Advances in the Preparation and Application of Mesoporous Aluminophosphate-Based Materials

“…The use of a solid acid catalyst offers many advantages over the conventional route, including convenient separation of catalyst and product, catalyst reusability, reduction of waste, etc. Indeed mesoporous aluminophosphate solids have been reported to possess appropriate weak acid-base pairs to facilitate this reaction [57]. The vapour-phase reaction was carried out at 280 • C and the primary product was guaiacol.…”

Preformed precursor of microporous aluminophosphate coating on mesoporous SBA-15: Synthesis, characterization, and catalytic property for selective O-methylation of catechol

“…By this strategy, some ordered mesoporous AlPOs with relatively high thermal stability have been obtained [4][5][6][7][8]. Additionally, the sol-gel route has also been developed to synthesize mesoporous AlPOs in the presence of organic additives [9][10][11][12]. The mesoporous materials obtained by this route, although lack of long-range order due to their amorphous characteristics, exhibit very high thermal stabilities and high specific surface areas [10][11][12].…”

“…Different kinds of materials have been investigated as catalysts for this reaction, including metal oxides [24,25], zeolites [26] and supported heteropoly acids [27]. Recently, we have found that the aluminophosphatebased materials, prepared by conventional precipitation method [28][29][30] and by citric acid route [11], have shown the catalytic potential in this vapor-phase reaction due to their suitable weak acid-base properties, and that the catalytic performance of the aluminophosphate-based catalysts is greatly dependent on the P/Al ratio and the synthetic method, which could affect the structures, specific surface areas and surface acid-base properties of these catalysts [11,30].…”