[ACS Symposium Series] Inorganic and Organometallic Polymers II Volume 572 (Advanced Materials and Intermediates) || Reaction of Boehmite with Carboxylic Acids

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<ul><li><p>Chapter 13 </p><p>Reaction of Boehmite with Carboxylic Acids New Synthetic Route to Alumoxanes </p><p>Christopher C. Landry, Nina Papp, Mark R. Mason, Allen W. Apblett, and Andrew R. Barron </p><p>Department of Chemistry, Harvard University, Cambridge, MA 02138 </p><p>Reaction of pseudo-boehmite, [Al(O)(OH)]n, with carboxylic acids (RCO2H) results in the formation of the carboxylate-alumoxanes, [Al(O)x(OH)y(O2CR)z]n where 2x + y + z = 3 and R = C1 - C13. The physical properties of the alumoxanes are highly dependent on the identity of the alkyl substituents. The alumoxanes have been characterized by scanning electron microscopy, IR and NMR spectroscopy, and thermogravimetric analysis. A model structure of the alumoxanes is proposed, consisting of a boehmite-like core with the carboxylic acid substituents bound in a bridging mode. All of the alumoxanes decompose under mild thermolysis to yield -alumina. </p><p>The facile formation of ceramic materials from molecules has undoubtedly been one of the significant contributions made by chemistry to materials science (7). However, it is desirable not only to produce the ceramic per se but also to do so in a specific form, for example a fiber. Therefore, one of the key requirements for any ceramic precursor should be its processability. For this reason, there has been continued research effort aimed at the design of precursors with physical properties suitable for processing prior to pyrolysis. Two examples with significant commercial application are polyacrylonitrile and polyorganosilanes, both of which may be spun into fibers, and upon pyrolysis allow for the manufacture of carbon-graphite (2) and silicon carbide (3) fibers, respectively. Despite much effort, the extension of this polymer-type precursor strategy to other ceramic systems has only met with limited success. </p><p>In the case of alumina fibers a common synthetic route has involved the use of alumina gels, which are formed by the neutralization of a concentrated aluminum salt solution (4). However, the strong interactions of the freshly precipitated alumina gels with ions from the precursors' solutions makes it difficult to prepare the gels in pure form (5). Furthermore, the yield of alumina fibers from the gel is low because of the low processability of the precursor during spinning. To avoid these difficulties, alumina fibers have been prepared from alumoxanes (6). Alumoxane is the generic term given to aluminum oxide "polymers" (7) formed by the hydrolysis of aluminum compounds, AIX3 (Eq. 1) where X = alkyl, alkoxide, etc. (8). </p><p>0097-6156/94/0572-0149$08.00/0 1994 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>OK</p><p>LA</p><p>HO</p><p>MA</p><p> on </p><p>Aug</p><p>ust 2</p><p>6, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: N</p><p>ovem</p><p>ber </p><p>18, 1</p><p>994 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1994</p><p>-057</p><p>2.ch</p><p>013</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>150 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>AIX3 2 0 , </p><p>* [Al(0)(X)]n (1) - 2 H X </p><p>While early examples of alumoxane precursors for alumina fiber synthesis had processing characteristics superior to alumina gels, they were found to be unstable and decomposed during spinning. In addition their structures were a complete mystery, making further developments difficult </p><p>The structure of alumoxanes have (despite contradicting spectroscopic data) traditionally been proposed to consist of linear or cyclic chains (I) (9). </p><p>However, recent work from this laboratory (70) has redefined the structural view of alumoxanes and shown that, while the core structure is dependent on the identity of the organic substituent, alumoxanes are not chains or rings but three-dimensional cage compounds. Thus, alkyl-alumoxanes, (RA10)n, adopt cage structures analogous to those observed for gallium sulfides (77) and iminoalanes (72), while the structure of the hydrolytically stable siloxy-alumoxanes, [Al(0)(OH)x(OSiR3)i-x]n, consists of an aluminum-oxygen core structure () analogous to that found in die mineral boehmite, [Al(0)(OH)]n, with a siloxide-substituted periphery () (10a). </p><p>We have reported that the physical properties of these siloxy-substituted alumoxanes are highly dependent on the relative organic content (10c). Low molecular weight clusters (M 2400 g mol"1) with high siloxide content (Si:Al 1.4) are soluble in hydrocarbon solvents. However, the alumoxanes formed from the equilibrium hydrolysis of aluminum compounds have a low siloxide content (Si:Al 0.14), are insoluble in all solvents, and infusible; a similar trend had previously been observed for carboxylate-alumoxanes (13). Without the advantage of hindsight, Kimura proposed that the instability of the alumoxanes used for fiber formation was due to the coordinative unsaturation at aluminum, and suggested that the use of carboxylate ligands in an appropriate ratio with aluminum would allow for the latter to be "properly coordinated". Kimura and co-workers subsequently demonstrated that carboxylate-alumoxanes were excellent precursors to alumina ceramic fibers with properties superior to those formed from alumina gels (14). These preceramic carboxylate-alumoxanes were prepared via a multi-step synthesis requiring accurate control over the reaction conditions (Eq. 2). </p><p>(I) </p><p>Et3SiO QSiEt3 </p><p>() () </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>OK</p><p>LA</p><p>HO</p><p>MA</p><p> on </p><p>Aug</p><p>ust 2</p><p>6, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: N</p><p>ovem</p><p>ber </p><p>18, 1</p><p>994 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1994</p><p>-057</p><p>2.ch</p><p>013</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>13. LANDRY ET AL. Reaction of Boehmite with Carboxylic Acids 151 </p><p>+ HO2CR +H 2 0 AlEt 3 &gt; AlEt 2(0 2CR) &gt; </p><p>- EtH - 2 EtH </p><p>+ H0 2 CR A1(0)(Q2C)- &gt; A1(0)(Q2CR)(HQ2CR) (2) </p><p>Furthermore, while some of the carboxylate-alumoxanes formed gels in THF only those with long chain substituents (e.g., dodecanoic acid) and hence low ceramic yield were melt-processable. It would thus be desirable not only to prepare alumoxane preceramics in a one-pot bench-top synthesis from readily available starting materials, but also to determine if lower hydrocarbon substituents could yield better processability. </p><p>Synthetic Strategy </p><p>If we assume that all hydrolytically stable alumoxanes have the boehmite-like core structure () it would seem logical that instead of synthesizing alumoxanes from a low molecular weight precursor they could be prepared directly from the mineral boehmite. In the siloxy-alumoxanes we have shown the "organic" unit surrounding the boehmite core itself contains aluminum (). Thus, in order to prepare the siloxy-alumoxane similar to those we have previously reported (10a) the anionic moiety the "ligand" [Al(OH)2(OSiR3)2]~ would be required as a bridging group; adding this unit would clearly present a significant synthetic challenge. However, the carboxylate-alumoxanes represent a more realistic synthetic target since the carboxylate anion, [RC0 2]-, is an isoelectronic and structural analog of the organic periphery found in our siloxy-alumoxanes, cf., and IV. </p><p>R </p><p>I </p><p>I . A I JAII JAIL </p><p>SIXI (IV) In previous studies carboxylate-alumoxanes were prepared via a multi-step </p><p>synthesis involving the reaction of a carboxylic acid with an alkoxy-alumoxane (75), Eq. 3. </p><p>[Al(0)(OR)]n +RCO2H &gt; [Al(0)(02CR)]n + HOR (3) </p><p>In order to simplify discussion, the alkoxy-alumoxanes were represented as having the general formula [(RO)A10]n. However, data from our laboratory has demonstrated that the alkoxide-alumoxanes have a high hydroxide content, i.e., [ A ^ O ^ O H ^ O R ) ^ (16). Based on spectroscopic characterization, the carboxylate-alumoxanes resulting from the synthesis have a lower hydroxide content; therefore, the </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>OK</p><p>LA</p><p>HO</p><p>MA</p><p> on </p><p>Aug</p><p>ust 2</p><p>6, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: N</p><p>ovem</p><p>ber </p><p>18, 1</p><p>994 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1994</p><p>-057</p><p>2.ch</p><p>013</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>152 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>carboxylic acid must also react with the hydroxide groups as shown in Eq. 4. It is entirely reasonable therefore that the reaction of boehmite, [Al(0)(OH)]n, with a carboxylic acid, RCO2H should yield the appropriate carboxylate-alumoxane (Eq. 5). </p><p>OH 0 2 CR I r </p><p>A l + RC0 2 H &gt; A l + H 2 0 / \ / \ (4) </p><p>[Al(0)(OH)]n +RCO2H &gt; [Al(0)(02CR)]n + H 2 0 (5) </p><p>Despite the fact that boehmite is a naturally occurring mineral, the majority of commercial samples are man-made, often by the hydrolysis/thermolysis of aluminum salts. In addition, they commonly contain significant quantities of gibbsite, Al(OH)3, and are of variable porosity. Although we have shown die reactions discussed below to be applicable for samples from different commercial sources, to be self consistent we have chosen to discuss results obtained using a single source of low gibbsite (&gt; 99 % boehmite) obtained from American Cyanamid. </p><p>Synthesis and Characterization of Carboxylate-alumoxanes </p><p>Refluxing powdered pseudo-boehmite, in air, with an excess of a carboxylic acid, RCO2H, either neat (e.g., R = CH3, acetic acid) or as a xylene solution (e.g., R = C5H11, hexanoic acid), results in the formation of the corresponding carboxylate-alumoxane (see Table 1). The alumoxanes are isolated by either filtration of the cooled reaction mixture or removal of all volatiles under vacuum followed by washing with Et 2 0 to remove traces of free acid. During the course of the reaction in xylene, gel formation is observed for most of the carboxylates; however, no gelation is observed if boehmite is refluxed in xylene in absence of an acid. Despite gel formation during synthesis only the hexanoate and octanoate alumoxanes form gels in organic solvents once isolated. A list of the alumoxanes synthesized and their appropriate synthetic routes is given in Table 1. </p><p>The as-received boehmite is a free flowing (fluid) white powder with no aggregation. The physical appearance and solubilities of the materials resulting from reaction of the boehmite with carboxylic acids are highly dependent on the identity of the carboxylate substituent. Thus, for R = C n H 2 n + i (n = 1 - 3 and 13) and CH2C1 the alumoxanes are white microcrystalline powders, insoluble in common organic solvents, whereas for R = C5H11 and C7H15 the products are white solids, which readily form homogeneous gels in aromatic and other solvents. A summary of the physical appearance and solubility of each alumoxane is given in Table 1. </p><p>A sample of the boehmite examined by scanning electron microscopy (SEM), prior to reaction with the carboxylic acid, was found to consist of spherical particles varying in size from 10 -100 pm in diameter (average 50 pm), see Figure 1. At higher magnifications the spheres may be seen to actually consist of small crystallites, packed together. From the SEI micrograph it is difficult to estimate a crystallite size; however, the largest distinct feature is ca. 0.1 pm in diameter, suggesting a crystallite size of &lt; 0.1 pm. The average crystallite size as determined by XRD is in fact ca. 64 (020 plane) (77). </p><p>In contrast to the near-perfect spheres observed by SEM for the boehmite starting material, the carboxylate-alumoxanes exist as large "fluffy" conglomerates, 50 - 200 pm in size (e.g., Figure 2a), with a particle size estimated from SEM to be less than 0.1 pm in diameter. At higher magnification these constituent particles of the conglomerates can be more readily seen. Figure 2b shows some of the individual needle-like particles of the hexanoate alumoxane. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>OK</p><p>LA</p><p>HO</p><p>MA</p><p> on </p><p>Aug</p><p>ust 2</p><p>6, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: N</p><p>ovem</p><p>ber </p><p>18, 1</p><p>994 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1994</p><p>-057</p><p>2.ch</p><p>013</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>Tabl</p><p>e 1.</p><p> Syn</p><p>thet</p><p>ic R</p><p>oute</p><p>s an</p><p>d Se</p><p>lected</p><p> Phy</p><p>sical</p><p> and </p><p>Spec</p><p>trosc</p><p>opic</p><p> Dat</p><p>a fo</p><p>r Car</p><p>boxy</p><p>late</p><p> Sub</p><p>stitu</p><p>ted A</p><p>lum</p><p>oxan</p><p>es </p><p>Carb</p><p>oxyl</p><p>ic A</p><p>cid </p><p>Synt</p><p>hesi</p><p>sb So</p><p>lubi</p><p>lity </p><p>Cera</p><p>mic</p><p> yie</p><p>ldd </p><p>IR </p><p>(</p><p>2</p><p>, cm</p><p>-1 13</p><p>C N</p><p>MR </p><p>2? A</p><p>l NM</p><p>R </p><p>(RC</p><p>0 2H</p><p>), R</p><p> %</p><p> (C)</p><p> an</p><p>tisym</p><p> sy</p><p>m </p><p>(ppm</p><p>) (p</p><p>pm) </p><p>acet</p><p>ic, C</p><p>H3 </p><p>neat</p><p> no</p><p>ne </p><p>30 (3</p><p>90) </p><p>1586</p><p> 14</p><p>66 </p><p>179.</p><p>3 1.</p><p>0 </p><p>chlo</p><p>roac</p><p>etic</p><p>, CH</p><p>2C</p><p>I xy</p><p>lene </p><p>none</p><p> 27</p><p> (410</p><p>) 16</p><p>19 </p><p>1464</p><p> 17</p><p>5.4 </p><p>-4.7</p><p>prop</p><p>ioni</p><p>c, C</p><p>2H5 </p><p>neat</p><p> no</p><p>ne </p><p>28 (3</p><p>50) </p><p>1589</p><p> 14</p><p>73 </p><p>180.</p><p>0 e </p><p>buta</p><p>noic</p><p>, C3H</p><p>7 ne</p><p>at </p><p>none</p><p> 23</p><p> (347</p><p>) 15</p><p>86 </p><p>1466</p><p> 18</p><p>1.7 </p><p>-0.5</p><p>piva</p><p>lic, C</p><p>(CH</p><p>3)3 </p><p>xyle</p><p>ne </p><p>none</p><p> 25</p><p> (351</p><p>) 15</p><p>86 </p><p>1466</p><p> 18</p><p>3.7 </p><p>e </p><p>hexa</p><p>noic</p><p>, C5H</p><p>11 </p><p>xylen</p><p>e TH</p><p>F, p</p><p>y ar</p><p>omat</p><p>ics0 </p><p>30 (3</p><p>52) </p><p>1587</p><p> 14</p><p>65 </p><p>180.</p><p>0 1.</p><p>5' </p><p>octa</p><p>noic</p><p>, C7H</p><p>i5a </p><p>xylen</p><p>e ar</p><p>omat</p><p>icsc </p><p>30 (3</p><p>32) </p><p>1586</p><p> 14</p><p>66 </p><p>180.</p><p>8 2</p><p>.4 </p><p>dode</p><p>cano</p><p>ic, C</p><p>11H</p><p>23</p><p> xy</p><p>lene </p><p>pyrid</p><p>ine,</p><p> DM</p><p>F 60</p><p> (465</p><p>) 15</p><p>87 </p><p>1466</p><p> 18</p><p>1.1 </p><p>a wax</p><p>y so</p><p>lid,b </p><p>see </p><p>Expe</p><p>rimen</p><p>tal S</p><p>ectio</p><p>n,c g</p><p>el fo</p><p>rmati</p><p>on in</p><p> aro</p><p>mat</p><p>ic h</p><p>ydro</p><p>carb</p><p>ons,d</p><p> fro</p><p>m T</p><p>GA</p><p>,e no</p><p>t obt</p><p>aine</p><p>d,f s</p><p>olut</p><p>ion </p><p>spec</p><p>trum</p><p>. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>OK</p><p>LA</p><p>HO</p><p>MA</p><p> on </p><p>Aug</p><p>ust 2</p><p>6, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: N</p><p>ovem</p><p>ber </p><p>18, 1</p><p>994 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1994</p><p>-057</p><p>2.ch</p><p>013</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>154 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>Figure 1. SEI micrographs of unreacted boehmite particles. Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>OK</p><p>LA</p><p>HO</p><p>MA</p><p> on </p><p>Aug</p><p>ust 2</p><p>6, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: N</p><p>ovem</p><p>ber </p><p>18, 1</p><p>994 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1994</p><p>-057</p><p>2.ch</p><p>013</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>LANDRY ET AL. Reaction of Boehmite with Carboxylic Acids </p><p>Figure 2. SEI micrographs of hexanoate-alumoxane particles. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>OK</p><p>LA</p><p>HO</p><p>MA</p><p> on </p><p>Aug</p><p>ust 2</p><p>6, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: N</p><p>ovem</p><p>ber </p><p>18, 1</p><p>994 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1994</p><p>-057</p><p>2.ch</p><p>013</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>156 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>Despite the particulate nature of the alumoxanes, homogenous continuous films and bodies may readily be prepared. Films of the hexanoate-alumoxane can be formed by dissolution of the alumoxane in either CH2CI2 or THF followed by spin coating. For example, evaporation of a CH2CI2 solution of the hexanoate-alumoxane on a glass slide yields a thin film, the SEM of which is shown in Figure 3. The homogenous nature of these films implies that they consist of an interpenetrating organic/inorganic matrix. While films of the alumoxanes are contiguous, many show significant shrinkage upon drying, resulting in cracking of the surface. </p><p>The surface area of the carboxylate-al...</p></li></ul>