Reactions of Dibutylmagnesium with Modified Silica Gel Surfaces

Feb 13, 1998 - Dibutylmagnesium reacts with two hydrogen-bonded silanols to give bonds to the surface with two surface silicons. Both butyl groups are...
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Langmuir 1998, 14, 1122-1129

Reactions of Dibutylmagnesium with Modified Silica Gel Surfaces Jonathan P. Blitz,*,† Craig C. Meverden,‡ and Richard E. Diebel, III† Department of Chemistry, Eastern Illinois University, Charleston, Illinois 61920, and Equistar Chemicals, L.P., Allen Research Center, 11530 Northlake Drive, Cincinnati, Ohio 45249 Received April 16, 1997. In Final Form: January 9, 1998 The reaction products of dibutylmagnesium with modified silica gel surfaces have been studied as relevant olefin polymerization catalyst precursors. Reactions were carried out on thermally and/or chemically pretreated silica gel. The pretreatments effect changes in silica surface silanol type and concentration. A combination of methods including infrared spectroscopy, NMR spectroscopy, and elemental analysis were used to characterize the chemically modified surfaces. Dibutylmagnesium reacts with non-hydrogenbonded silanols to give a bond to the surface with one surface silicon. One butyl group is retained on magnesium. Dibutylmagnesium reacts with two hydrogen-bonded silanols to give bonds to the surface with two surface silicons. Both butyl groups are lost in the reaction with non-hydrogen-bonded silanols. Dibutylmagnesium also reacts with surface siloxanes to give a singly bonded magnesium, analogous to the reaction with non-hydrogen-bonded silanols. The adjacent silicon in the siloxane is believed to react with the second butyl group to form a silicon-butyl surface species. A majority of surface-reacted magnesium on the thermally pretreated silica gels studied is a result of siloxane bond breaking.

Introduction Silica-supported heterogeneous catalysts are widely used for the manufacture of many industrial chemicals. The silica support is generally thought of as a carrier for the catalytically active phase. The support serves to maximize the active-phase surface area and allows the catalyst to be used in industrially useful reactor processes. Silica gel supported catalysts are currently used to manufacture polyolefins on a multimillion ton scale. The support serves technically relevant functions which are often not well-understood. One type of silica-supported polyolefin catalyst is the thermally activated chromium catalyst. It is generally recognized that for the chromium catalyst surface chemical reactions are involved. Silica surface hydroxyl (silanol) groups react to stabilize hexavalent chromium as silyl chromates. These silyl chromates then react with ethylene to form active sites. Silica gel surface chemistry has thus played a large role in the study and development of these catalysts.1 A second type of heterogeneous polyolefin catalyst is the Ziegler-Natta catalysts. In their original simplest form, Ziegler-Natta catalysts consist of the reaction product between a transition-metal halide and a metal alkyl.2 Ziegler-Natta catalysts are now often supported on silica gel. In contrast to chromium catalysts, the role of silica surface chemistry in the function of Ziegler-Natta catalysts is often overlooked. This report represents a continuing effort to investigate the role of silica gel surface chemistry as it relates to Ziegler-Natta olefin polymerization catalysis. * To whom correspondence should be addressed. † Eastern Illinois University. ‡ Equistar Chemicals, L.P. (1) See, for example: (a) Myers, D. L.; Lunsford, J. H. J. Catal. 1985, 92, 260. (b) McDaniel, M. P. Ind. Eng. Chem. Res. 1988, 27, 1559. (c) Jozwiak, W. K.; Dalla Lana, I. G.; Fiederow, R. J. Catal. 1990, 121, 183. (d) Kim, C. S.; Woo, S. I. J. Mol. Catal. 1992, 73, 249. (e) Augustine, S. M.; Blitz, J. P. J. Catal. 1996, 161, 641. (2) Boor, J., Jr. Ziegler-Natta Catalysts and Polymerizations; Academic Press: New York, 1979.

One of the most widely used classes of Ziegler-Natta catalysts is based on a mixture of Ti (such as TiCl4), Mg (such as dibutylmagnesium), and Al (such as triethylaluminum) compounds. These species are often supported on silica gel. Numerous reports have considered the reactions of TiCl4 with silica.3 The reactions of TiCl4 with silica gel can be controlled by appropriate pretreatments. These pretreatments effect changes in the relative silanol types existing on the surface.3c An unmodified silica surface, pretreated at 250 °C in N2 to desorb molecular water, contains both hydrogen-bonded and non-hydrogenbonded silanols.4,5 Titanium tetrachloride reacts with nonhydrogen-bonded silica surface silanols (I) to give a single surface bond:

Si(s)-OH + TiCl4 f Si(s)-O-Ti-Cl3 + HCl (I)

(1)

and with hydrogen-bonded silanols (II) to give a “bridged” surface bound species: Si(s)–OH

Si(s)–O

+

O Si(s)–OH

TiCl4

O

TiCl2 + 2HCl

(2)

Si(s)–O

(II)

When TiCl4 reacts with unmodified silica gel, a mixture of the above two surface species results. A 600 °C thermal pretreatment of the silica gel primarily condenses hydrogen-bonded silanols. TiCl4 reaction with this thermally pretreated silica gel results in predominantly singly (3) See, for example: (a) Kinney, J. B.; Staley, R. H. J. Phys. Chem. 1983, 87, 3735. (b) Morrow, B. A.; Tripp, C. P.; McFarlane, R. A. J. Chem. Soc., Chem. Commun. 1984, 1282. (c) Blitz, J. P. Colloids Surf. 1992, 63, 11. (d) Haukka, S.; Lakomaa, E. L.; Root, A. J. Phys. Chem. 1993, 97, 5085. (4) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (5) It is generally believed that geminal or disilanols exist on the surface of silica gel; however, their relative proportion with respect to the other silanols is probably less than 20% in all cases.

S0743-7463(97)00394-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/13/1998

Reactions of DBM with Silica Gel Surfaces

bonded surface groups (eq 1). Chemical pretreatment of the silica gel surface with hexamethyldisilazane (HMDS) results in a reaction with predominantly non-hydrogenbonded silanols. A titanium tetrachloride reaction with this chemically pretreated silica results in predominantly bridged-bonded species (eq 2). Finally, 600 °C thermal pretreatment of the silica gel followed by silylation with HMDS leaves very few surface silanols. Titanium tetrachloride was unreactive with this surface under the conditions studied. Relating this work to Ziegler-Natta catalysis, the singly bonded Ti-catalyst precursor was found to be more active toward ethylene polymerization than the bridged-bonded precursor.6 The presence of magnesium alkyls in Ziegler-Natta catalysts exert a tremendous beneficial effect on catalyst activity.6,7 The first step in the synthesis of many titanium-based Ziegler-Natta catalysts involves the reaction of a magnesium alkyl with the silica surface. This reaction product is then treated with a transition-metal compound such as TiCl4. Since the first catalyst synthesis step is the reaction of a magnesium alkyl with silica gel, a systematic study of this reaction is important. After reasonable surface structures of magnesium alkyl reactions with silica are proposed, further studies can be carried out to correlate structure with catalytic properties. In this report, reactions of dibutylmagnesium (DBM) with silica gel surfaces are investigated. Dibutylmagnesium was chosen for this study for a variety of reasons. The reactions of DBM with silica gel should be fairly representative of other dialkylmagnesium compounds. DBM is soluble in heptane in the absence of aluminum alkyls which are often added to improve solubility. This simplifies data interpretation. Also, since both alkyl groups on DBM are essentially the same, it is possible to obtain at least semiquantitative infrared data of the amount of hydrocarbon on the surface from a DBM reaction. The reactions of magnesium alkyls with silica for the synthesis of supported Ziegler-Natta catalysts are of unquestionable importance. Early work consisted of the reactions of silica with alkylmagnesium halides dissolved in ether as catalyst precursors (largely in patents) and as silica surface probes.8 Recently, hydrocarbon solutions of dialkylmagnesium compounds became commercially available. These hydrocarbon solutions are widely used for Ziegler-Natta catalyst synthesis. Whereas silica surface reactions of these dialkylmagnesium compounds has received some attention in the patent literature,9 little has been published in the scientific literature.10 The study of DBM reactions with the pretreated silicas provides new information concerning these important catalyst precursors. If alkylmagnesium reactions with the surface can be varied, analogous to the previous TiCl4 reacted silicas,3c,6 additional studies to correlate structure with catalytic properties of this important class of catalysts can be performed. (6) Blitz, J. P. In New Advances in Polyolefins; Chung, T. C., Ed.; Plenum Press: New York, 1993. (7) Hsieh, H. L.; McDaniel, M. P.; Martin, J. L.; Smith, P. D.; Fahey, D. R. In Advances in Polyolefins; Seymour, R. B., Cheng, T., Eds.; Plenum Press: New York, 1987. (8) (a) Fripiat, J. J.; Uytterhoeven, J. J. Phys. Chem. 1962, 66, 800. (b) Pullukat, T. J.; Hoff, R. E.; Shida, M. J. J. Polym. Sci., Polym. Chem. 1980, 18, 2857. (c) McDaniel, M. P.; Welch, M. B. J. Catal. 1983, 82, 98. (9) See, for example: (a) Shida, M. J.; Pullukat, T. J.; Hoff, R. E. U.S. Patent 4, 263, 753, 1981. (b) Nowlin, T. E.; Wagner, K. P. U.S. Patent 4, 481, 301, 1984. (c) Mink, R. J.; Nowlin, T. E. European Patent 0 518 604 A2, 1992. (10) Nowlin, T. E.; Mink, R. I.; Lo, F. Y.; Kumar, T. J. Polym. Sci., Polym. Chem. 1991, 29, 1167.

Langmuir, Vol. 14, No. 5, 1998 1123

Materials and Methods Materials. Davison 948 silica gel (W.R. Grace) was pretreated in four ways: (1) fluidized in N2 at 250 °C for 6 h (called unmodified silica); (2) fluidized in N2 at 600 °C for 6 h (called 600 °C silica); (3) reacted with HMDS as previously described,11 followed by fluidization in N2 for 6 h at 250 °C (called HMDS silica); (4) reacted 600 °C silica with HMDS (10 mmol/g) in heptane for 2 h at room temperature, followed by evacuation and heating under flowing N2 at 100 °C for an additional 2 h (called 600 °C/HMDS silica). Dibutylmagnesium (FMC Lithco) was used as received. There is approximately a 1:1 ratio of n-butyl/sec-butyl groups in the DBM. Given the likelihood of ligand exchange, it is undoubtedly a mixture of (n-Bu)2Mg, (sBu)2Mg, and (n-Bu)(s-Bu)Mg. The concentration of DBM (14 wt % solution) was provided by the manufacturer in an analysis sheet. Independent experiments support the concentration provided by the manufacturer. Heptane was rigorously dried and deoxygenated prior to use by passing through columns containing 4A molecular sieves and R-311 catalysts (BASF). Anhydrous THF (Aldrich) and methylmagnesium iodide (3.0 M solution in diethyl ether; Aldrich) were used as received. Synthesis. The magnesium-treated silicas were prepared under dry nitrogen by reacting 2-5 g of silica with DBM in 100 mL of heptane at room temperature. After 1 h, the solid product was allowed to settle and the supernatant liquid siphoned off. The solids were resuspended in 100 mL of heptane and stirred for several minutes, and the washing procedure was repeated five times. After the last wash, the solids were dried by stirring under a flow of dry nitrogen. When THF was used to wash the products, the same procedure was used for a total of three washes. All materials before and after synthesis were stored in an inert atmosphere drybox. Characterization. Infrared spectra were obtained using the diffuse reflectance sampling technique. A Nicolet 7199 FT-IR spectrometer purged with dry air and equipped with a narrow band mercury-cadmium-telluride detector was used. Spectra were acquired at 4-cm-1 nominal resolution by coaddition of 128 scans. Samples for analysis were prepared in an inert atmosphere drybox by making a 10% (w/w) dispersion of the sample in predried, ground KCl. Spectra were obtained using the Harrick “praying mantis” diffuse reflectance accessory (DRA-2CN) and a controlled atmosphere cell (HVC-DRP). Diffuse reflectance spectra were analyzed and plotted in Kubelka-Munk units. The cell was modified for precise control of sample position for optimal S/N ratio.12 The solid-state CP-MAS 13C NMR spectra were obtained with a Varian 300 MHz spectrometer under crosspolarization using a 2.5-ms contact time and a delay time of 5 s. The 13C 90° pulse width was 6.3 ms and during decoupling the 1H field strength was approximately 75 kHz. The samples were packed into silicon nitride rotors in an inert atmosphere drybox. The rotational frequency of the samples in these experiments was approximately 2500 Hz. Typically, 4000-5000 transients were signalaveraged to obtain an adequate signal-to-noise ratio. The FID was processed using 75 Hz of line broadening. The chemical shifts were referenced using hexamethylbenzene as an external standard. (11) Pullukat, T. J.; Hoff, R. E. U.S. Patent 4, 530, 912, 1985. (12) (a) Murthy, R. S. S.; Leyden, D. E. Anal. Chem. 1986, 58, 1228. (b) Murthy, R. S. S.; Blitz, J. P.; Leyden, D. E. Anal. Chem. 1986, 58, 3167.

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Figure 1. Infrared spectra of unmodified silica gel before (A) and after (B) reaction with dibutylmagnesium.

The concentration of surface silanol groups was estimated by reaction of the silica gels with a 3.0 M ether solution of methylmagnesium iodide.8 Although numerous methods exist to measure surface silanol content,13 this method was chosen because it should give the most relevant measure of the number of reactive silanols with dibutylmagnesium. The reactions were carried out in a sealed vial connected to a manifold fitted with a pressure transducer. Measurement of silanol groups on all of the silicas was reproducible, except values on HMDS silica were not reliable. An indirect method for obtaining silanol values on this silica was used. Knowing the silanol value of unmodified silica provides a baseline. Carbon analysis of HMDS silica provides the data necessary to determine the number of silanols reacted by HMDS. Subtraction of this latter value from the silanol content of unmodified silica gives an estimate of the silanol content of HMDS silica. Elemental analysis of the magnesium-treated silicas was obtained by fusing the products with a 4:1 sodium carbonate:lithium tetraborate flux. The fused product was dissolved in nitric acid and analyzed for Mg and Si using a Leeman Labs PS-1000 inductively coupled plasma emission spectrometer. The wavelengths used for Mg and Si were 279.553 and 251.611 nm, respectively. Results Infrared spectra of the four silica gels (unmodified, 600 °C silica, HMDS silica, and 600 °C/HMDS silica) before and after reaction with dibutylmagnesium are shown in Figures 1-4. The infrared spectrum of unmodified silica gel in Figure 1A exhibits a sharp peak at ∼3745 cm-1 from predominantly non-hydrogen-bonded silanols. The broad absorbance from 3700 to 3200 cm-1 is from hydrogen-bonded silanols in various hydrogen-bonding environments. The pair of peaks at 1800 and 1600 cm-1 are Si-O-Si combination and overtone bands, respec(13) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Elsevier Science: Amsterdam, 1995.

tively. After a dibutylmagnesium reaction (Figure 1B), the non-hydrogen-bonded silanol band disappears, the shape of the hydrogen-bonded silanol bands are slightly altered, intense CH stretching peaks are detected between 3000 and 2800 cm-1, and CH bending modes from surfacereacted dibutylmagnesium are detected below 1400 cm-1. No differences in the Si-O-Si combination and overtone bands are detected after reaction. On 600 °C silica gel prior to a dibutylmagnesium reaction (Figure 2A), the peak(s) due to hydrogenbonded silanols are not detected. Thermal treatment results in the condensation of the hydrogen-bonded silanols, so only non-hydrogen-bonded silanols are present. After a dibutylmagnesium reaction the non-hydrogenbonded silanol band disappears, while CH stretching and bending vibrations are detected (Figure 2B). The data in Figures 1 and 2 indicate that dibutylmagnesium reacts with non-hydrogen-bonded silanols. Alkyl groups from dibutylmagnesium remain on the surface after the DBM reaction. The infrared spectrum of HMDS-modified silica gel (Figure 3A) exhibits no non-hydrogen-bonded silanol peaks. This is a result of trimethylsilylation of nonhydrogen-bonded silanol groups by HMDS. Hydrogenbonded silanols remain largely unreacted. Two intense CH stretching peaks due to Si-(CH3)3 groups are observed at 2960 and 2906 cm-1. After reaction with dibutylmagnesium, small peaks in the CH stretching and bending regions, separate from peaks due to HMDS, are barely detectable. Aside from these small peaks arising from DBM, there is little difference between the infrared spectra in Figure 3. Infrared spectra of silica gel pretreated by first being heated to 600 °C to condense hydrogen-bonded silanols and then reacted with HMDS to silylate the remaining non-hydrogen-bonded silanols (600 °C/HMDS silica), before and after reaction with dibutylmagnesium, are shown in Figure 4. The absence of surface silanols before reaction with dibutylmagnesium is evidenced by the lack of appreciable absorbance in the O-H stretching region (3800-3200 cm-1) in Figure 4A. Peaks from Si-(CH3)3

Reactions of DBM with Silica Gel Surfaces

Langmuir, Vol. 14, No. 5, 1998 1125

Figure 2. Infrared spectra of 600 °C treated silica gel before (A) and after (B) reaction with dibutylmagnesium.

Figure 3. Infrared spectra of HMDS-modified silica gel before (A) and after (B) reaction with dibutylmagnesium.

groups are observed at 2960 and 2906 cm-1 as a result of HMDS reaction. After a dibutylmagnesium reaction, as seen with the 600 °C pretreated silica, a weak broad band centering around 3500 cm-1 is evident. This may arise from magnesium hydroxyl groups, a small amount of water, and/or CH combination and overtone bands from the dibutylmagnesium. A CH stretching band at 2920 cm-1 is clearly detected from surface-reacted dibutylmagnesium. CH bending mode vibrations below 1400 cm-1 are also seen in Figure 4B. Even in the absence of silanols on this surface, strong infrared peaks arise from butyl groups presumably derived from dibutylmagnesium. Additional information was obtained by measurement of the silica gel silanol content before reaction with DBM

and the magnesium content after DBM reaction. Data are shown in Table 1. Dibutylmagnesium is most reactive with unmodified silica gel. This is not surprising since the maximum number of silanol groups exist on this surface. When an excess of DBM (with respect to surface silanols) is reacted with unmodified, 600 °C, and 600 °C/ HMDS silicas, the amount of magnesium which remains on the surface is significantly greater than the measured silanol content. Previous workers have found that alkylmagnesium compounds react in excess of the measured silanol content.9,10 Approximately the same amount of magnesium is measured on both HMDS silica and 600 °C/HMDS silica, even though the former contains a significant amount of silanols whereas the latter has very few (Table 1). Dibutylmagnesium must be bonded dif-

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Figure 4. Infrared spectra of 600 °C/HMDS-modified silica gel before (A) and after (B) reaction with dibutylmagnesium. Table 1. Dibutylmagnesium Reactions with Various Silica Gel Surfaces silica 600 °C HMDS

silanol conc. (mmol/g)

DBM added (mmol/g)

Mg conc. on silica (mmol/g)

0.6

7.0 1.7 0.85 7.0 2.9 1.7 0.85

1.56 1.60 0.79 0.75 0.73 0.70 0.72

1.7 0.85 10

0.78 0.74 2.17

0.8

600 °C/HMDS

0

unmodified

1.7

a

Mg conc. after THF wash (mmol/g)

CH/Mg ratioa 0.11 ( 0.01

1.62 0.75

0.017 ( 0.002

0.77 0.71 0.086 ( 0.009

0.74 0.74

0.068 ( 0.007

Explained in text, normalized ratio.

ferently on these two silica gels. Very little CH is detected from dibutylmagnesium on HMDS silica gel (Figure 3A), but considerable CH is detected from dibutylmagnesium on 600 °C/HMDS silica gel (Figure 4A). Table 1 contains FT-IR data which semiquantitatively compares the amount of CH present on the surface as a result of dibutylmagnesium reaction. Inspection of the CH stretching region from 3000 to 2800 cm-1 reveals that there are no bands from dibutylmagnesium resolved from HMDS trimethylsilyl groups. A CH bending band was therefore used for the following analysis. The ratio of an integrated CH bending band (1390-1370 cm-1) to an SiO-Si combination band centered at approximately 1860 cm-1, used as an internal standard, was obtained. The resulting unitless number is a quantitative measure of the amount of CH present on each of the four silica gels from dibutylmagnesium. This approach is quantitatively valid for determining the extent of silylation reaction on silica gel surfaces using diffuse reflectance infrared spectroscopy.12 To obtain the data shown in Table 1, the unitless band area ratio was normalized to the amount of magnesium determined by inductively coupled plasma atomic emission spectroscopy in mmol of Mg/g of silica gel. This value is designated as the “CH/Mg ratio”. This ratio decreases in the order 600 °C silica g 600 °C/HMDS silica g unmodified silica . HMDS silica. Dibutylmagnesium on 600 °C and 600 °C/HMDS silica has lost

relatively few alkyl groups upon reaction; dibutylmagnesium on HMDS silica has lost the most alkyl groups upon reaction. In percentage terms, a large excess of magnesium with respect to the number of silanols measured is detected on DBM-reacted 600 °C and 600 °C/HMDS silica gel. Similar results have been seen by previous workers,9,10 and two hypotheses have been proposed to explain these results. The first hypothesis is that the metal atom coordinates to the oxygen atom in the siloxane group:10 Si O: +

Si (C4H9)2Mg

Si

O:MgR2

(3)

Si

The second hypothesis9a is that not only are the surface silanol groups reactive, but some siloxanes may also be. No specific reactions were hypothesized for the magnesium alkyl case. A reaction path which has been hypothesized by many researchers for aluminum alkyl reactions3a,14 with silica is as follows: Si O: + Si

Si–OAlMe2 Al(Me)3

+ Si–CH3

(4)

Reactions of DBM with Silica Gel Surfaces

Langmuir, Vol. 14, No. 5, 1998 1127

Figure 5. Solid state 13C CP-MAS NMR spectra of dibutylmagnesium-reacted (A) HMDS-modified silica gel, (B) 600 °C/HMDSmodified silica gel, and (C) 600 °C silica gel.

It may be that DBM reacts analogously: Si O: + Si

Si–OMgC4H9 (C4H9)2Mg

+

(5)

Si–C4H9

The product in eq 3 contains only butyl groups attached to magnesium. The product of eq 5 possesses butyl groups attached to both magnesium and silicon. We sought to differentiate these two groups by 13C CP-MAS NMR spectroscopy. Solid-state CP-MAS 13C NMR spectra of HMDS silica, 600 °C/HMDS silica, and 600 °C silica are shown in Figure 5. The spectrum of HMDS silica gel reacted with dibutylmagnesium in Figure 5A contains a single resonance at 0 ppm from trimethylsilyl groups; no resonances from butyl groups is distinguishable from the noise. The 13C NMR spectra of 600 °C/HMDS and 600 °C silica gels reacted with dibutylmagnesium (Figure 5B,C) show a series of resonances in the 10-35 ppm range, which can be attributed to butyl carbons. The spectrum of 600 °C/ HMDS silica gel also shows the trimethylsilyl peak at 0 ppm. While the R-carbon resonances of Si-C4H9 and MgC4H9 groups should be readily distinguishable (15-20 ppm15 for Si-CH2- and 30-35 ppm16 for Mg-CH2-), the (14) (a) Yates, D. J. C.; Dembinski, W.; Kroll, W. R.; Elliott, J. J. J. Phys. Chem. 1969, 73 (3), 911. (b) Peglar, R. J.; Hambleton, F. H.; Hockey, J. A. J. Catal. 1971, 20, 309. (c) Low, M. J. D.; Severdia, A. G.; Chan, J. J. Catal. 1981, 69, 384. (d) Bartram, M. E.; Michalske, T. A.; Rogers, J. W., Jr. J. Phys. Chem. 1991, 95, 4453. (e) Kratochvila, J.; Kadle, Z.; Kazda, A.; Salajka, Z. J. Non. Cryst. Solids 1992, 143, 14. (f) Molotvshchikova, M. B.; Dodonov, V. A.; Lysenko, G. N.; Ignatov, S. K.; Razuvaev, A. G. Russ. Chem. Bull. 1995, 44, 1827. (15) (a) Ziegler, R. C.; Maciel, G. E. In Chemically Modified Surfaces in Science and Industry; Leyden, D. E.; Collins, W. T., Eds.; Gordon and Breach: New York, 1988. (b) Albert, K.; Pfleiderer, B.; Bayer, E. In Chemically Modified Surfaces in Science and Industry; Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach: New York, 1988. (16) Mann, B. E.; Taylor, B. F. 13C NMR Data for Organometallic Compounds; Academic Press: London, 1981.

remaining butyl carbon peaks overlap significantly with the Si-CH2- peak.17 The Si-C4H9 groups are therefore not directly observable. However, the presence of SiC4H9 groups can be observed in the reaction product with TiCl4. Magnesium alkyls reduce TiCl4 and form Mg-Cl bonds:18

-Mg-R + TiCl4 f -Mg-Cl + TiCl3 + olefins, alkanes (6) Solid state 13C NMR spectroscopy of DBM-reacted 600 °C and 600 °C/HMDS silicas, after TiCl4, reaction shows that the peak at 30-34 ppm has disappeared (Figure 6). The remaining peaks centered at 14 and 26 ppm are due to surface-bound Si-C4H9 groups. Since Si-C4H9 resonances remain after TiCl4 reaction, this suggests that siloxane bond breakage is occurring. To further differentiate the reaction pathways depicted by eqs 3 and 5, an extraction experiment was performed. Results are discussed for 600 °C/HMDS silica gel. Similar results for 600 °C and HMDS silicas are provided in Table 1. Since 600 °C/HMDS silica possesses no detectable silanols, dibutylmagnesium on this silica must be adsorbed in a way that does not include reaction with silanols. Dibutylmagnesium-reacted 600 °C/HMDS silica gel was synthesized as described in the Experimental Section. Elemental analysis indicated a magnesium concentration of 0.78 mmol of Mg/g of silica. This sample was washed three more times with THF. THF has a higher dielectric constant (7.6) than either silica (4.3) or organosiloxanes (2.2-2.7). THF is thus expected to solvate and desorb most of the dibutylmagnesium that is either complexed or associated with surface species. A magnesium con(17) Expected chemical shifts: [Si-CH2-CaH2-CbH2-CH3] and [SiCH-(CH3)-CcH2-CH3] Ca, Cb, Cc, CH ) 20-25 ppm, CH3 ) 13-15 ppm; [Mg-CH2-CaH2-CbH2-CH3] and [Mg-CH-(CH3)-CcH2-CH3] all carbons 30-37 ppm.16 (18) (a) Haward, R. N.; Roper, A. N.; Fletcher, K. L. Polymer 1973, 14, 365. (b) Lichelli, J. A.; Haward, R. N.; Parsons, I. W.; Caunt, I. D. Polymer 1981, 22, 667.

1128 Langmuir, Vol. 14, No. 5, 1998

Figure 6. Solid state reaction.

13C

Blitz et al.

CP-MAS NMR spectra of DBM-reacted 600 °C silica gel (A) before TiCl4 reaction and (B) after TiCl4

centration of 0.74 mmol of Mg/g of silica was measured after washing with THF. It is unlikely that the excess magnesium exists as a siloxane complex or is associated with bonded magnesium groups. These results suggest surface-bond formation, presumably by reaction with siloxanes as shown in eq 5. Discussion HMDS-Modified Silica. The DBM reaction with HMDS-modified silica can now be interpreted. Data in Table 1 show that 0.7-0.8 mmol of Mg/g of silica bonds to the surface of HMDS silica regardless of whether one reacts with a stoichiometric DBM/SiOH ratio or a 10-fold excess. Both FT-IR and NMR spectra show little evidence of retained butyl groups on the surface. The reaction paths depicted in eqs 3 and 5 are thus not dominant. Since HMDS-modified silica contains only hydrogen-bonded silanols (Figure 3A), these silanols may be in close enough proximity to react with DBM to give a bridged type structure as shown in eq 7. This reaction is analogous to the TiCl4 reaction with HMDS-modified silica shown in eq 2.3c Si(s)–OH

Si(s)–O

+ (C4H9)2Mg

O Si(s)–OH

O

Mg + 2C4H10

between the two ligands. Data in Table 1 indicate the same result regardless of reaction stoichiometry. With the lowest amount of DBM added (0.85 mmol/g), 0.72 mmol/g react. If half the groups in DBM are n-butyl and half are sec-butyl, then both ligands must react. This does not rule out the possibility of a difference in reactivity, but both ligands exhibit equivalent reactivity under the conditions in this study. 600 °C Silica Gel. DBM reaction with 600 °C modified silica results in a significant level of Mg bonded to the surface beyond the silanol content (Table 1). When 0.85 mmol of DBM/g of silica is added, a stoichiometric excess to the number of silanols measured, 0.79 mmol of DBM reacts. When larger amounts of DBM are added, about 1.5-1.6 mmol of Mg/g of silica gel reacts. This is wellabove the 0.6 mmol/g of silanol content measured. Data in Table 1 also show that the excess magnesium is not extracted with THF. Both the NMR (Figure 5C) and infrared (Figure 2B) spectra indicate that a significant quantity of butyl groups from DBM remain after reaction. The IR spectra of 600 °C modified silica indicates that isolated silanol groups are present almost exclusively (Figure 2) and that these silanols are consumed by a DBM reaction. Isolated silanols react with DBM to bond Mg in a singly attached structure (eq 6).

(7)

Si(s)–O

(II)

The proposed structure is largely consistent with the results. A significant quantity of magnesium is bonded to the surface of HMDS silica, and very few butyl groups are detected by infrared or NMR spectroscopy. In the above reaction, both butyl groups are lost as butane. The small amount of hydrocarbon detected by infrared spectroscopy is probably the result of DBM bonded singly to the surface through a siloxane (eq 5) or through a silanol. Since DBM consists of a mixture of n-butyl and secbutyl groups, there may be a difference in reactivity

Si(s)-OH + (C4H9)2Mg f (I) Si(s)-O-Mg-C4H9 + C4H10 (8) Both NMR (Figure 6) and elemental analysis (Table 1) data suggest that excess dibutylmagnesium reacts with siloxanes as shown in eq 5.19 If ∼1.5 mmol of Mg/g of silica is reacted and 0.6 mmol/g react with non-hydrogen(19) Another reaction which has been proposed for aluminum alkyls is also possible. This reaction is a continuation of that shown in eq 6, where the singly bonded metal alkyl reacts with an adjacent siloxane to give a bridged bonded species and a silicon-alkyl analogue. The extent to which this reaction occurs is unknown.

Reactions of DBM with Silica Gel Surfaces

bonded silanols, then ∼0.9 mmol of Mg/g of silica reacts with siloxanes. More than 50% of the magnesium bonded results from reaction with siloxanes. 600 °C/HMDS Silica Gel. DBM reaction with completely dehydroxylated silica (600 °C/HMDS-modified) also gives a product having a significant level of bonded Mg (0.7-0.8 mmol/g), even though the surface contains almost no silanol groups with which to react (Figure 3a, Table 1). THF washing does not remove any Mg. The dominant reaction path for dibutylmagnesium reaction with 600 °C/HMDS silica gel is proposed to be that shown in eq 5. It is interesting to note that the extent of reaction on 600 °C/HMDS silica closely matches the extent of reaction with siloxanes in 600 °C silica gel (0.7-0.8 vs ∼0.9 mmol/ g). Unmodified Silica Gel. Unmodified silica gel possesses both non-hydrogen-bonded and hydrogen-bonded silanols. One might predict a mixture of bridged-bonded and singly bonded structures resulting from reaction of both of these groups (eqs 7 and 8, respectively). Since a significantly higher level of magnesium is measured after the dibutylmagnesium reaction with unmodified silica gel (Table 1), both types of silanols are probably reacting. The CH/Mg ratios in Table 1 support this conclusion. CH/Mg Ratio Comparisons. The observed trend of the CH/Mg ratio, 600 °C g 600 °C/HMDS g unmodified . HMDS silica, is largely consistent with the discussion thus far. For 600 °C silica, a silanol content of 0.6 mmol/g was obtained, nearly all of which are non-hydrogenbonded. Assuming that 0.6 mmol/g of dibutylmagnesium reacts with these silanols, each losing one butyl group, and the remaining dibutylmagnesium (∼0.9 mmol/g [1.5 mmol of Mg/g-0.6 mmol of SiOH/g]) reacts with siloxanes, losing no butyl groups, then on average one should detect a 1.75 butyl/Mg ratio. Reaction of dibutylmagnesium with 600 °C/HMDS silica gel theoretically results in no loss of butyl groups, giving a butyl/Mg ratio of 2. The values presented in Table 1 for these two materials are equal within experimental error. Unmodified silica gel, which probably contains a mixture of singly bonded DBM through non-hydrogen-bonded silanols (loss of one butyl group), singly bonded DBM through siloxanes (loss of no butyl groups), and bridged-bonded DBM through hydrogenbonded silanols (loss of two butyl groups), is expected to have a lower CH/Mg ratio than either 600 °C or 600 °C/

Langmuir, Vol. 14, No. 5, 1998 1129

HMDS silicas. Although this ratio is not statistically different from 600 °C/HMDS silica, a paired t-test indicates a difference at the 95% confidence level with 600 °C silica. HMDS silica has the lowest CH/Mg ratio, supporting the conclusion that the major reaction pathway results in loss of both butyl groups. Conclusions Dibutylmagnesium reacts with silica gel in different ways, depending on the silica surface pretreatment. On HMDS silica gel, DBM reacts to form predominantly bridged-bonded surface species (eq 7), reaction with siloxanes being a minor reaction path. On 600 °C/HMDS silica, DBM reacts primarily with siloxanes to give predominantly singly bonded surface species (eq 5). On 600 °C silica DBM reacts with both non-hydrogen-bonded silanols and surface siloxanes, both giving singly bonded surface species (eqs 5 and 8). DBM apparently reacts with siloxanes to a greater extent on 600 °C and 600 °C/ HMDS silica gels compared to the nonthermally treated silicas. This, at least in part, explains why larger amounts of DBM react with 600 °C silica compared to HMDS silica. Reactions of DBM with unmodified silica gel give a mixture of singly bonded and bridged-bonded groups as a result of three reactions (eqs 5, 7, and 8). This work shows how dibutylmagnesium reacts with the silica surface and how various reaction paths can be altered by silica surface pretreatment. The reaction of dibutylmagnesium with surface siloxanes was found to be an important pathway under the reaction conditions studied. The extent of siloxane bond breaking on thermally pretreated silicas was surprising. Limiting the extent of this reaction may help in preparing Ziegler-Natta catalysts of known and variable structures. Future work will focus on the correlation of catalyst performance with the structures proposed in this report. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. We also acknowledge Millenium Petrochemical Corporation for partial support of this work. The authors thank Mr. John Rogers for help in synthesis and Dr. Doug McFaddin for obtaining NMR data. LA970394G