Boron Phosphate and Aluminum Phosphate Aerogels

Journal of the Arkansas Academy of Science Volume 48 Article 21 1994 Boron Phosphate and Aluminum Phosphate Aerogels David A. Lindquist University of Arkansas at Little Rock Steven M. Poindexter University of Arkansas at Little Rock Sterling S. Rooke University of Arkansas at Little Rock D. Ritchie Stockdale University of Arkansas at Little Rock Kirk B. Babb University of Arkansas at Little Rock See next page for additional authors Follow this and additional works at: http://scholarworks.uark.edu/jaas Part of the Organic Chemistry Commons Recommended Citation Lindquist, David A.; Poindexter, Steven M.; Rooke, Sterling S.; Stockdale, D. Ritchie; Babb, Kirk B.; Smoot, Alison L.; and Young, William E. (1994) "Boron Phosphate and Aluminum Phosphate Aerogels," Journal of the Arkansas Academy of Science: Vol. 48 , Article 21. Available at: http://scholarworks.uark.edu/jaas/vol48/iss1/21 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact , . Boron Phosphate and Aluminum Phosphate Aerogels Authors David A. Lindquist, Steven M. Poindexter, Sterling S. Rooke, D. Ritchie Stockdale, Kirk B. Babb, Alison L. Smoot, and William E. Young This article is available in Journal of the Arkansas Academy of Science: http://scholarworks.uark.edu/jaas/vol48/iss1/21 Journal of the Arkansas Academy of Science, Vol. 48 [1994], Art. 21 Boron Phosphate and Aluminum Phosphate Aerogels David A. Lindquist*, Steven M.Poindexter, Sterling S. Rooke, D. Ritchie Stockdale, KirkB. Babb, Alison L.Smoot and William E. Young The University of Arkansas at Little Rock Department of Chemistry Little Rock, AR 72204 *Author to whom correspondence should be addressed Abstract Anhydrous sol-gel condensation of triethyl phosphate [(CH3CH2O)3PO] with boron trichloride (BCL3) or triethyl aluminum [(CH3CH2)3A1] in organic solvents, led to formation of metallophosphate gels. The pore fluid of the gels was removed under supercritical conditions in a pressurized vessel to form aerogels. The aerogels were then calcined at progressively higher temperatures to produce high surface area phosphates. Since the initial gel reagent mixtures contained several NMR active nuclei, the condensation chemistry prior to the gel point was monitored by solution nB NMR. The surface areas, distribution of pore sizes, and total pore volumes of the aerogel products were determined using nitrogen gas physisorption methods. Introduction Materials and Methods The orthophosphate (MPO 4) compounds of boron, aluminum, and iron(III) may be described as covalent network solids of oxygen bridging alternating PO 4 and MO4 tetrahedra (Van Wazer, 1958). These phosphates are consequently structurally isomorphous with one or more of the various forms of silica (SiO 2) and also share similar chemical and physical properties with silica. The formation of silica by sol-gel routes has been intensively studied for various applications such as coatings and formation of high surface area materials (Brinker and Scherer, 1990), but comparatively little has been written on the sol-gel preparation of covalent phosphates (Gerrard and Griffey, 1961; Kearby, 1967; Glenz et al., 1991; Rebenstorf et al., Gel Syntheses and Aerogel Processing. Allsolvents were dried under a dry nitrogen atmosphere by distillation from P2O5 in the case of chlorobenzene, potassium carbonate for acetone, and sodium benzophenone ketyl for pentane. The triethyl phosphate was also freshly distilled and all gel syntheses conducted under a dry nitrogen atmosphere using Schlenk techniques. The boron phosphate gels in this work were synthesized using the method of Gerrard and Griffey (1959) by reaction of triethyl phosphate with boron trichloride to yield boron phosphate and ethyl chloride as a byproduct. To prepare the boron phosphate gels, 8 mL (46 mmol) of triethyl phosphate (CH3CH2O)3PO was dissolved in 18 mL of chlorobenzene. The solution was cooled in an ice bath and a flask containing 4 mL (46 mmol) of boron trichloride (BCL3) was mated to the triethyl phosphate solution flask to allow condensation of the BCL3 vapor into the stirring phosphate solution over a period of about three hours. During this time, the flasks were closed off from nitrogen purge so that no BCL3 was lost from the system. The resulting solution of adduct was then aged at 60 °C for 8 hours. During this time, the flask was periodically vented to allow for escape of the ethyl chloride byproduct. The gel point occurred, on average, 1 hour after beginning heating at 60 °C. The gel was then allowed to age in a sealed flask at room temperature for two days. The A1PO 4 gel was prepared by a novel method using triethyl aluminum instead of the chloride compound due to the low solubility of aluminum chloride in organic solvents. In a flask equipped with a water condenser, a neat 1991). Phosphate gels of aluminum and boron were pre(FePO 4) willbe described in future studies. Since phosphates of acidic metal cations have useful solid acid catalytic properties, it was desirable to prepare them with a high surface area. Aerogels have high surface areas since the liquid in the jjels is removed under supercritical conditions, and colapse of the pore structure, which can be problematic for evaporatively dried gels, is greatly reduced. We chose to )repare gels under nonaqueous conditions because most organic solvents have a considerably lower critical temperature than water. Asecond rationale for anhydrous condiions is the difficulty in making stoichiometric phosphate ompositions from aqueous solutions. In water solutions one may obtain some metal oxide phase in addition to lie desired phosphate due to competing hydrolysis reacions. jared in this work; iron phosphate — Proceedings Arkansas Academy of Science, Vol. 48, 1994 Published by Arkansas Academy of Science, 1994 100 100 Journal the Arkansas Academy of Science, Vol. 48 D.Ritchie [1994], Art. 21Stockdale, KirkB. Babb, M.ofPoindexter, Sterling David A Lindquist, Steven S. Rooke, Alison L.Smoot and Willliam E. Young mixture of 10 mL (73 mmol) of (CH3CH2)3ALand 12.4 mL of (CH3CH2O)3PO (73 mmol) were heated to 175°C in an oil bath on a hot plate stirrer overnight. This formed an oligomeric oil. The oligomer (2 mL) was then dissolved in 24 mL of acetone and the solution cooled in a salt ice bath (-40 °C). Anhydrous NH3 was then bubbled vigorously through the solution with a syringe needle. After a few minutes the mixture gelled with a (...truncated)