0 Copyright 1995 American Chemical Society
Surfaces and Colloids OCTOBER 1995 VOLUME 11, NUMBER 10
Letters Methane Elimination during Silation of Partially Dehydroxylated y-Alumina Stefan V. Slavov, Karl T. Chuang," and Alan R. Sanger Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received May 1, 1995. In Final Form: August 18, 1995@ Methane, and not HC1, is the gaseous product from the silation of the surface of y-alumina using (CH3)3Sic1 in the temperature range 140-260 "C and during thermolysis of silated y-alumina to 500 "C. A minimum of 1mol of CH4 is produced per mol of (CH3)3SiClreacted, and all chloride remains firmly bound to the silated alumina. Alumina and silica are widely used in analytical and separation techniques and as supports for metallic, organometallic, and metal oxide For each material, surface hydroxyl groups play a n important role in the surface chemistry4-6 and hence affect the catalytic activity of each system. The nature of catalysts supported on either silica or alumina can be modified in a controlled manner by removing or modifymg the surface hydroxyl group^,^-^ thereby rendering the surface less hydrophilic.10-12 Organosilanes are effective agents for the
* Corresponding author. E-mail:
[email protected]. Telephone: (403) 492-4676. Fax: (403) 492-2881. Abstract publishedinAdvunce ACSAbstructs, October 1,1995. (1) Gonzalez, R. D.; Miura, H. Catal. Rev.-Sci. Eng. 1994,36, 145. (2) Zecchina, A.; Otero Arean, C. Catal. Rev.-Sci. Eng. lSSS, 35, 261. (3) Knozinger, H.; Ratnasami, P. Catal. Rev.-Sci. Eng. 1978,17,31. (4) DeCanio, E. C.; Edwards, J. C.; Bruno, J. W. J. Catal. 1994,148, @
76 . -.
( 5 ) Peri, J . B. J. Phys. Chem. 1966, 70, 3168. (6) Ballinger, T. H.; Yates, J. T., Jr. Langmuir 1991, 7, 3041. (7)Kim. S.I.; Woo, S. I. J. Catal. 1992, 133, 124. (8)Spanos, N.;Slavov, S.;Kordulis, Ch.; Lycourghiotis, A. Langmuir . 1994, IO, 3134. (9) DeCollonaue, B.; Garbowski, E.; Primet, M. J. Chem. Soc., Faraday Trans. 1991, 87, 1795. (10)Paul, D. K.; Yates, J. T., Jr. J.Phys. Chem. 1991,95, 1699. (11)Armistead, C. G.; Hockey, J . A. Trans. Faraday SOC.1967,63, 2549. (12)Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (13)Paul, D. K.; Ballinger, T. H.; Yates, J . T., Jr. J. Phys. Chem. 1990, 94, 4617.
0743-746319512411-3607$09.0OlO
modification of surfaces of catalyst supports.13-17 We have found that there exists a striking and previously unrecognized difference between the reactions of (CH3)3SiC1 with the surfaces of silica and alumina and between the gaseous and solid products derived from each support. Modification of the surface of dehydrated y-alumina by silation using (CH3)3SiClhas now been investigated over a wide range of conditions. We have found that, surprisingly, the only derived gaseous product is methane and not hydrogen chloride. The amount of methane produced is dependent upon the duration and temperature of chemisorption of (CH3)3SiCl and subsequent thermal treatment (Table 1). The amount of methane produced a t a given temperature is proportional to the amount of (CH3)3SiC1chemisorbed, which is in turn a function of time up to breakthrough of (CH&SiCl into the effluent stream. The maximum number of moles of methane produced per mole of (CH3)3SiClchemisorbed increases with increasing temperature of the subsequent thermal treatment of the product. This can readily be seen by comparison of the amounts of methane produced in examples 3 and 4 in Table 1. At 250 "C the methane produced per mole of silylating agent was 1.42 mol; at 300 "C it was 1.74 mol. In the experiments reported herein, (14) Kurth, D. G.; Bein, T. J . Phys. Chem. 1992, 96, 6707. (15)Hair, M. L.; Hertl, W. J . Phys. Chem. 1969, 73, 2372. (16)Kang, H.-J.; Blum, F. D. J.Phys. Chem. 1991, 95, 939. (17) Tsutsumi, K.; Takahashi, H. Colloid Polym. Sci. 1985,263,506.
0 1995 American Chemical Society
3608 Langmuir, Vol. 11, No. 10, 1995
Letters
Table 1. Silation of pAlumina: Stoichiometry and Product Analyses" consumed (CH3)3SiClb
no.
pretreatment ofy-alumina T,"C t,h
silation reaction T,"C t , m i n
produced CH4' mmolg-1
a 2 0 3
mmolg-1
produced
lo2
102 mmol m-'
A1203
mmol m-2
HC1' mmol g-'&03
supported sid
supported C1
102
mmol g-1 A1203
mmol m-'
1 2 3 4
500 500 500 500
24 24 24 24
180 220 250 300
60 90 110 120
0.360 0.529 0.581 0.681
0.175 0.257 0.282 0.331
0.382 0.685 0.827 1.188
0.186 0.332 0.401 0.577
0.00 0.00 0.00 0.00
nd nd 0.62 0.66
nd nd 24 32
5 6
500 220
40 72
220 180
55 50
0.320 0.330
0.155 0.160
0.418 0.342
0.203 0.166
0.00 0.00
nd nd
nd nd
102
mmol g-l&03 nd nd 0.60,d 0.56e 0.74d 0.67e nd nd
mmol m-2 nd nd 0.27 0.324 nd nd
a Nitrogen-(CHa)aSiCl: 22 cm3 min-l; (CH3)3SiC17.76 x mg ~ m - nd ~ ;= not determined. Determined gravimetrically. Determined by gas chromatography. Determined by the average of several electron microscopy-energy dispersive X-ray analyses of crushed samples. e Determined by elemental analysis.
up to 2.5 equiv of methane was produced per equiv of silating agent, but this is not necessarily the maximum amount. Samples of granular alumina (La Roche) and fumed alumina Won) were sequentially dried a t 120 "C for 3 h, dehydrated in air a t 500 "C for 6 h, and stored in a n oven before treatment. Controlled gas feed mixtures were prepared using double distilled (CH313SiCl(Aldrich) in a n inert carrier, usually nitrogen (Matheson, UHP). The concentration of (CH3)3SiCl (1.5-18 mg ~ m - was ~ ) confirmed gravimetrically. When helium (Matheson, UHP) was used as carrier, for comparative purposes, no difference was found for the silation reaction or the products. Silation and thermal treatment of alumina were performed dynamically using a U-shaped glass reactor (100 cm3)in a movable electrical furnace. A typical experimental sequence is as follows. A sample of dried alumina (12 g) was weighed in the reactor, then further dehydrated a t a set temperature (150-500 "C) for 24-72 h, and reweighed. The reactor was then reheated to the selected temperature (180-300 "C), and the (CH&SiCl feed mixture was fed (12-60 cm3 min-l; HSV 82-252 h-l) until either (CH&SiCl was detected in the effluent or a minimum of 1h had elapsed. The composition of the effluent gases was continuously monitored, and the gaseous products were collected. The sample was then cooled and reweighed, the (CH3)3SiCl irreversibly adsorbed was determined qavimetrically, and the methane produced was quantified by on-line GC. The sample of alumina with chemisorbed (CH&SiCl was then heated in a programmed manner (12-120 h) in a stream of inert gas, and the gaseous products were analyzed. Following completion of the treatment, the reactor was again cooled and the solid product was weighed and analyzed (C, H, Al, C1, Si) using elemental analysis or electron microscopy coupled with an energy dispersive X-ray analyzer. It has been suggested previously that the reaction of (CH3)3SiClwith surface hydroxyls of alumina gives HC1 as a product, but the fate ofthis HC1was not For the experiments with dehydrated alumina now reported, methane was the only gaseous product and was identified by FTIR, GC, GC-MS, and NMR of a solution ofthe trapped products in CDCl3 (-58 "C)or cyclohexanedlz (6 "C). Eliminated methane captured in a liquid nitrogen trap was quantified by GC and identified spectroscopically. Attempts were made to detect HC1 in the effluent stream, using pH paper, by contacting the effluent gases with a fresh solution of NH40H, and by trapping the product gases using liquid nitrogen. In no case was HC1 detected. The absence of produced HC1 is illustrated by comparison of the FTIR spectra of the gaseous products with those for pure methane and a
i ' IJ
0
--
3d50
I
3010
1
2770
2430 22/90 2dSO WAVENUMBER (cm")
Id10
Ii70
U30
Figure 1. FTIR spectra of (a)HC1-free methane from silation of y-alumina, (b) methane containing 1%HCl by volume, and (c) dry HC1. Bands marked with an asterisk arise from COz or water vapor i n t h e sample beam (positive peaks) or reference beam (negative peaks).
mixture of methane with 1%HC1 (Figure 1). Further, X-ray dispersive and elemental analyses of the solid products show that they contain the chloride, as discussed below. The presence of a minimum of 1equiv of methane and the absence of HC1in the product gas are each unexpected and surprising. By analogy with the reaction of (CH3)3Sic1 with surface hydroxyls of silica (eq 1) it was anticipated that HC1 would be eliminated during silation of alumina, as is assumed in several published reports, and that pendant (CH&SiO groups would be formed to cap the surface sites. For dehydrated alumina there exist residual surface hydroxyls, which are considered to be isolated with respect to each other, coordinatively unsaturated sites, and bridging oxide moieties, several of
Letters
Langmuir, Vol. 11, No. 10, 1995 3609 (CH,),SiCI
HCI
cn
+
t
+
-
SI
-
-
SI I////////////
///ll///l/l/l
-
2. (CH,),SiCI
+ H /
0 I AI
Si(CH,),
-.
H
/
CI I
0 I
-
-
-9
AI I/l/lli/~ll//l////l//ll/
which are ~ t r a i n e d . ~ The ~ ~ absence J ~ - ~ ~of HC1 as a product in the silation and thermal treatment of y-alumina suggests that (CH&SiCl does not immediately attack the isolated hydroxyls but instead first attacks coordinatively unsaturated aluminum cations and oxygen anions. Thus there will exist adjacent or proximate terminal hydroxyl and trimethylsiloxy groups, as shown figuratively in eq
0I AI
0 I At
0 I AI
AI' - - AI - AI / / / / / i l l i / l ~ l l l l / / / / / / / / / / / / l l / / // /i // i l / i / / / / / / / / / / / / l / / / l / / / / / / / / / / / Equation 2 represents the irreversible chemisorption of (CH313SiClon y-alumina. The inductive effect of achloro substituent will increase the acidity of an adjacent hydroxyl group. The elimination of methane on thermolysis of the silated alumin'a can then be understood as the reaction of adjacent hydroxyl and trimethylsiloxy groups, with concomitant formation of a bridging silyl unit (eq 3). The removal of the isolated Al-0-H groups is demonstrated by the considerable reduction or absence of the corresponding bands in the FTIR spectra of the silated alumina compared to the same bands in the spectrum of the original dehydrated alumina. The perpetuation of surface chloride has been demonstrated and quantified using both scanning electron microscopy coupled with a n energy dispersive X-ray (18)Cornelius, E. B.; Milliken, T. H.; Mills, G. A.; Oblad, A. G. J. Phys Chem. 1966,59,809. (19) Hsu. L.-Y.: Shore. S. G.; D'Ornelas, L.; Choplin, A.: Basset, J. M. J. Catal. 1994,149,159. (20)Peri,J. B. J. Phys. Chem. 1966,69,211, 220. (21) Davidov, A. A. IR Spectroscopy Applied to Surface Chemistry of Oxides; Nauka: Novosibirsk, 1984. (22) Knozinger, H. Adu. Catal. 1976,25, 184.
-
0 I AI
0 I
-
AI
-
/llll//il/li/l/l//~llll
analyzer and elemental analysis, and the content as determined by each method (2-3%) is consistent with the degree of silation achieved (Table 1). The amount of methane produced is a function of the temperature and duration of both the treatment with (CH&SiCl and, to a lesser degree, the pretreatment of the y-alumina (Table 1). The methane produced corresponds to up to 2.5 molecules of methane per silyl group chemisorbed, under conditions of slow increase in thermolysis temperature to 500 "C over 5 days. These data show that the number of methyl groups which can be eliminated per silyl group increases with increasing severity of treatment, to sequentially form dimethyl, monomethyl, and finally fully demethylated silicon moieties a t the alumina surface. The above data clearly demonstrate that there exist significant differences in the reactions of (CH&SiCl with silica and alumina surfaces, the gaseous reaction products, and the nature of the surface silyl groups formed. Implications arising from this study include the need for caution in the interpretation of data on products from experiments in which methane is detected if the heated materials used in either the experimental or analytical methods include silated alumina. A detailed parametric study will be presented later. To confirm the proposed mechanism a series of experiments is planned in which silation will be performed using alternative silating agents, including (C2H5)3SiC1, (CH3)2SiC12,and [(CH3)3Si]zNH.
Acknowledgment. The followingdata were obtained a t the University of Alberta: elemental analyses, Microanalytical Service Laboratory, Department of Chemistry; scanning electron microscopy-energy dispersive X-ray analyses, Mr. George D. Braybrook, Department of Geology. LA950338K
