High-surface thermally stable mesoporous gallium phosphates constituted by nanoparticles as primary building blocks Vasile I. Parvulescu a,⇑ , Viorica Parvulescu b , Dragos Ciuparu c , Christopher Hardacre d , Hermenegildo Garcia e,⇑ a Dept. Chemical Technology and Catalysis, University of Bucharest, B-dul Regina Elisabeta 4-12, Bucharest 030016, Romania b Institute of Physical Chemistry of the Romanian Academy, 202 Splaiul Independentei St., P.O. Box 194, 060021 Bucharest, Romania c Petroleum-Gas University of Ploiesti, Bd. Bucures ßti 39, Ploies ßti, Romania d School of Chemistry and Chemical Engineering, Queen’s University, Stranmillis Road, BT9 5AG Belfast, UK e Instituto de Tecnologia Quimica CSIC-UPV, Universidad Politecnica de Valencia, Av. De los Naranjos s/n, Valencia 46022, Spain article info Article history: Received 22 June 2010 Revised 9 November 2010 Accepted 29 November 2010 Available online 15 January 2011 Keywords: High surface gallophosphate Thermally stable gallophosphate Sol–gel route Toluene oxidation abstract In constant, search for micro/mesoporous materials, gallium phosphates, have attracted continued inter- est due to the large pore size reported for some of these solids in comparison with analogous aluminum phosphates. However up to now, the porosity of gallium phosphates collapsed upon template removal or exposure to the ambient moisture. In the present work, we describe high-surface thermally stable mes- oporous gallium phosphates synthesized from gallium propoxide and PCl 3 and different templating agents such as amines (dipropylamine, piperidine and aminopiperidine) and quaternary ammonium salts (C 16 H 33 (CH 3 )3NBr and C 16 PyCl). These highly reactive precursors have so far not been used as gallium and phosphate sources for the synthesis of gallophosphates. Conceptually, our present synthetic procedure is based on the fast formation of gallium phosphate nanoparticles via the reaction of gallium propoxide with PCl 3 and subsequent construction of the porous material with nanoparticles as building blocks. The organization of the gallophosphate nanoparticles in stable porous structures is effected by the tem- plates. Different experimental procedures varying the molar composition of the sol–gel, pH and the pre- treatment of gallium precursor were assayed, most of them leading to satisfactory materials in terms of thermal stability and porosity. In this way, a series of gallium phosphates with surface are above 200 m 2 g 1 , and narrow pore size from 3 to 6 nm and remarkable thermal stability (up to 550 °C) have been prepared. In some cases, the structure tends to show some periodicity and regularity as determined by XRD. The remarkable stability has allowed us to test the catalytic activity of gallophosphates for the aerobic oxidation of alkylaromatics with notable good results. Our report reopens the interest for gallo- phosphates in heterogeneous catalysis. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction The synthesis of porous phosphates received much interest since Wilson et al. discovered the microporous crystalline alu- minophosphates [1]. Soon after that, Parise [2–5] extended the synthesis of these phosphates to gallium phosphates. Structural analogues of AlPO molecular sieves were synthesized using the structure-directing agent protocol [6]. Over the time, a large family of open-framework phosphate-based structures has been reported [7] of which gallium phosphates were extensively investigated. Since the first structures were synthesized [2], the exploration of gallophosphates via the fluoride method [8] led to the discovery of a large architectural diversity of open-framework structures including the extra-large pore cloverite [9] and DIPYR-GaPO [10] structures, the Mu-n [11–14], ULM-n [15] or cyclam-GaPO [16] types. In these solids, gallium can adopt 4-, 5- or 6-fold coordination. Different routes have been used to prepare gallium phosphates [17] such as the hydrothermal treatment of a gallophosphate mix- ture containing water, an organic structure-directing agent and fluoride. Fluoride was found in the synthesized material as termi- nal Ga–F groups or occluded in small structural units, acting as a stabilizer of the building units [18]. Among the preparative vari- ables considered to improve the stability of these materials were the nature of gallium and phosphorous precursors. In most of the studies, Ga 2 O 3 or GaO(OH) was considered as a gallium source and H 3 PO 4 or P 2 O 5 as a phosphorous source [19]. Ionothermal syn- thesis of microporous gallium phosphates [20,21] has also been re- ported for LTA structures. These syntheses were carried out via a 0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2010.11.021 ⇑ Corresponding authors. Tel./fax: +40 745502052 (V.I. Parvulescu), +34 963877809 (H. Garcia). E-mail addresses: vasile.parvulescu@g.unibuc.ro (V.I. Parvulescu), c.hardacre@- qub.ac.uk (C. Hardacre), hgarcia@qim.upv.es (H. Garcia). Journal of Catalysis 278 (2011) 111–122 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat