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J. Phys. Chem. 1981, 85,532-537
Surface Halides of Silica. 1. Chloride M. P. McDanlel Philllps Research Center, Bartlesvllle, Oklahoma 76004 (Received: Aprll23, 1980: In Final Form: October 24, 1980)
The reactivities of surface silanol and siloxane groups on silica with chlorinating agents have been examined in a simple flow system. At 400 "C, CC14reacted with silanol but not siloxane, whereas at 600-800 OC both groups were quickly chlorinated. SOClzwas more reactive with siloxane, especially on precalcined samples where reaction occurred even at 200 "C. Both reagents yielded maximum surface coverages of 3.5 Cl/nm2, 02,or HzO. Sic14 produced higher coverages, up to 4.6 Cl/nm2, but which could be partly removed by Hz, annealing at 800-900 "C lowered this back the previous maximum. The number of paired hydroxyls, where two OH groups react with one SiC14,was large at 200 "C but fell to zero near 600 "C, whereas the number of singles remained more constant. Extensive reaction between SiC14and siloxane was not found.
Introduction That surface hydroxyl groups on silica play an important role in many catalyst systems, by interacting with the active component or by influencing adsorption characteristics, is generally accepted. One can sometimes profoundly alter a catalyst's activity or selectivity by replacing these hydroxyl groups with another species such as an &oxide, alkyl, or even halide. Two surface halides of silica have been reported-the fluoride, a hydrophobic, rather unreactive support, and the chloride which, due to its higher reactivity, is sometimes used as a starting material for introducing alkyls or =SiH. This report concentrates on the chloride, its formation and reactivity. Although several studies of silica chloride have been reported, there has been little agreement about the extent of surface coverage possible. Earliest attempts at chlorination mainly used thionyl chloride at low temperatures. Boehm and Schneiderl found the reaction with surface hydroxyl groups to be quantitative, while others2-sreported that only a fraction of the available surface hydroxyls was replaced by chloride. The accessibility of smallest pores was raised as a possible complication.6 However, this should also have affected determination of hydroxyl concentration by reaction with CH3MgI or CH3Li. Peri et al.'p8 used infrared spectroscopy to study the reaction of silica aerogel with CC14, C12, HC1, and other agents not containing chlorine. Although exact chloride populations were not reported, all hydroxyl groups could be removed by treatment with CC4at 350-600 "C or with Clz at 700-950 "C. Surface chloride groups were very reactive with NH,and water. On the other hand, HC1 was not adsorbed by silica and it did not chlorinate or even exchange very rapidly with deuterated hydroxyls. All hydroxyls reacted with SiC14below 600 OC, sometimes in a 1:l stoichiometry but mainly in a 2:l ratio, indicating considerable pairing of sites, suggesting that some hydroxyls are very close even at 800 "C. However, the possibility that strained siloxane bridges might also be reactive was not mentioned. These findings inspired a new model ~~~
of the silica surface based on the 100 face of P-cristobalite. Hockey et al."13 using infrared spectroscopy and chloride analysis further examined the reaction of silica with methylchlorosilanes and metal chlorides like TiC14,AICI,, and BC13. Again evidence for paired and single hydroxyls was observed. In addition to the usual silanols, siloxane bridges were also found to react provided the silica had first been dried at 600-800 OC. In fact, 15-20% of the trimethylchlorosilane reacting with a silica dried at 700 OC was estimated to arise from siloxane. Hair and HertP4J6examined the direct chlorination of silica with CC14,COCl2,and Clz,and also its reaction with metal chlorides up to 400 "C. Chlorination by CC14at 400 "C removed all hydroxyl groups and the reaction occurred much more readily than when C0Clz or C12 (the possible decomposition products of Cc4) were used. However, the low chloride coverage, 1.6 Cl/nm2, equaled the initial OH population reported, indicating no chlorination of siloxane. Direct chlorination of silanol by metal chlorides was proposed in which chloride but not metal was deposited on the surface. This paper briefly examines several methods of preparing the surface chloride of silica and in each case the extent of coverage by chloride has been determined and compared to the measured change in hydroxyl population. In this way, the reactivity of siloxane can sometimes be distinguished from that of silanol.
Experimental Section Silica. To minimize the possible effects of reagents diffusing into and out of pores, a commercial wide-pore silica was chosen for use in most of the experiments described here. Davison Grade 952 microspheroidal silica has a pore volume of about 1.6 cm3/g and a surface area in the vicinity of 300 m2/g. Analysis of X-ray fluorescence indicated that this material contained 0.012% Na,O, 0.06% CaO, and 0.057% AZO3.Silica A was prepared free of other oxides by hydrolysis of Stauffer pure grade ethyl silicate (99%), double distilled under argon in acid-washed
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(1) H. P. Boehm and M. Schneider, 2.Anorg. Allg. Chem., 301,326 (1959). (2)J. Uytterhoeven and H. Naveau, Bull. SOC. Chem. Fr., 27 (1962). (3)R.Schwarz and H. W. Hennicke, Z. Anorg. Allg. Chem., 283,345 (1956). (4)M.Folman, Trans. Faraday SOC.,57,2000 (1961). (5)K.Unger, W.Thomas, and P. Adrian, Kolloid. Z. Z. Polymn., 251, 45 (1975). (6)H. P. Boehm, Angew. Chem., Int. Edit. Engl., 5,No.6,533(1966). (7)J. B. Peri, J. Phys. Chem., 70,2937 (1966). (8)J. B. Peri and A. L. Hensley, Jr., J.Phys. Chem., 72,2926 (1968). 0022-3654/81/2085-0532$01.25/0
(9)J. Kenawicz, P.Jones, and J. A. Hockey, Trans. Faraday SOC., 67, 848 (1971). (10)R. J. Peglar, F. H. Hambleton, and J. A. Hockey, J. Catal., 20, 309 (1971). (11)G.'C. Armistead, A. J. Tyler, F. H. Hambleton, S. A. Mitchell,and J. A. Hockey, J.Phys. Chem., 73,3947(1969). (12)J. A. Hockev. J. Phvs. Chem.. 74. 2570 (1970). (13)C.G.ArmisGad andJ. A. Hockey,'Trans: Faraday SOC.,63,2549 (1967). (14)M.L. Hair and W. Hertl, J.Phys. Chem., 77, 2070 (1973). (15)M.L. Hair and W. Hertl, J.Phys. Chem., 73, 2372 (1969).
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 5, 1981 533
Surface Halides of Silica OH OR Cl/nrn 2 4.0 \
,
I 2.0
1.O
200 300 400 500 600 700 800 900 1OOOC Flgure 1. (A) OH density on silica calcined in air for 2 h. (B) CI density on silica raised to temperature in CCI, vapor and held for 2 h.
glassware. It had a BET surface area of 774 m2/g and pore volume of 0.5 cm3/g. Chlorination. In typical chlorination experiments, 10 g of silica was supported on a porous sintered glass plate inside a vertical 46-mm 0.d. quartz tube surrounded by an electrical furnace. Gas flowed up through the sample at 35 L (STP)/h producing a fluidized bed about 6 cm deep. The gas, usually air or Nz, was bubbled through the chlorinating agent before passing through the silica bed, sometimes by use of a bubbler system, other times by injecting the chlorine compound into a glass wool plug set in the gas stream. All samples were afterward flushed with Nz at the same temperature for at least 5 min to prevent any excess reagent from being physisorbed as the silica cooled. N2 ( =Si-Br > =Si-Cl. The iodide could not be completely removed by O2 even at 800 "C, but rather formed a heat-stable, yellow-orange species having a strong ESR signal. This species was a powerful oxidant and could even be regenerated by 0,after reduction in CO. However, reduction in H2 or hydrocarbons was permanent. Moisture also destroyed the species, but left most of the oxidizing capacity. These fads and others suggest an unusual oxyanion of iodine(V1) stabilized by the silica surface.
Introduction Although there are a few references in the literature to the surface chloride of silica, practically nothing has been devoted to the lower halides of silica. Perhaps this is because they are more difficult to prepare, owing to the weaker oxidizing strength of bromine or iodine. In this paper the bromide and iodide of silica were prepared, the surface coverages measured, and their reactivities briefly examined. Experimental section Sample Preparation. The gas purification and flow system used here has already been described in part 1of this series. Liquids, such as Br, or SOBr,, were injected into the gas stream ahead of the fluidizing silica. Iodine, when used, was packed (25 g) on a quartz wool plug in the gas stream below the silica bed and just outside of the furnace. At the desired time, it was slowly raised into the furnace over 1h so that it evaporated and passed through the silica. All portions of the sample tube were eventually heat treated to prevent any contamination by unreacted halogen. Halide Analysis. The analysis of bromide and iodide on silica was identical with that used for chloride in part 1. Controls were made, again in 1 N NaOH, to contain iodide, iodate, and various combinations of the two species, which can react with each other in neutral or acidic solution. Neutralization of the NaOH occurred only in the presence of mecuric thiocyanate. The method was found to be an accurate measure of iodide concentration without interference from iodate, to which it was not sensitive. Iodometric titrations were performed under N,. Silica samples were added to acidic 1 N KI solution, protected from air by Schlenkware. 1, was liberated immediately and then stopped. This solution was then titrated against 0.04 N sodium thiosulfate to the starch endpoint. Instrumentation. X-ray photoelectron spectra were taken on a Varian IEE spectrometer with Mg Ka source (1253.6 eV) and retarding grid mode at 100-eV pass energy. Electron spin resonance samples were sealed by flame 0022-365418112085-0537$01.25/0
under N2 inside quartz tubes. Spectra were obtained on a Varian V-4502 double-cavity spectrometer modulated at 100-kHz and 400 Hz for the reference side. Results Bromination of Silica. Figure 1 plots the bromide concentration on the surface of Davison 952 wide-pore silica after being treated with SOBrz. In Figure 1A each sample was calcined and then treated with SOBr2at the same temperature. Bromide adsorption peaked at 500 "C and then declined at higher temperatures as the OH population (Figure 1C) also declined. This suggests less reactivity with siloxane than was found in part 1 when SOCl, was used. In Figure lB, all samples were first dried at 800 "C, leaving 0.9 OH/nm2 on the surface. Bromination with SOBr2then followed a t the temperature indicated. The low bromide density, even at 800 "C, again indicates little reactivity with siloxane. The bromination of siloxane was accomplished more completely in the presence of a strong reducing agent like carbon monoxide and a large excess of bromine. Si-0-Si CO Br, 2Si-Br + COz Figure 2 indicates the surface bromide density after each sample was dried at the temperature shown and then treated with a large excess of Br2vapor in carbon monoxide at the same temperature. At 750 "C, all hydroxyls were gone and at 900 "C a maximum bromide density of 3.5 Br/nm2 was found, which was also the highest chloride level found by any direct treatment in part 1. No carbon deposits were found on any of these CO-treated samples, nor was surface area or porosity affected. Exposing silica to bromine vapor at 750 "C in a Nzatmosphere, instead of CO, left only 0.9 Br/nm2 on the surface. When dry air was used as the carrier, little or no bromide was found on the sample, but the hydroxyl population decreased 10%. Iodination of Silica. Figure 2B shows the iodide density found when silicas were treated with Iz in carbon monoxide. Since iodine is a weaker iodizing agent than bromine or chlorine, and makes a larger anion, it is perhaps
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0 1981 American Chemical Society
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