Langmuir 1996, 12, 6407-6409
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Using Trichlorosilane as a Probe To Investigate the Role of the Preadsorbed Amine in a Two-Step Amine-Promoted Reaction of Chlorosilanes on Silica C. P. Tripp,* P. Kazmaier, and M. L. Hair Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada Received June 14, 1996X Low frequency infrared data is critical for identifying the chemisorbed species that are formed when HSiCl3 reacts with the surface of silica. That information is now used to probe an important aspect of the two-step amine-promoted room temperature reaction of chlorosilanes on silica. In the reaction of HSiCl3 with a silica whose surface contains preadsorbed NH3, it is shown that the ammonia that is liberated from the surface is consumed exclusively by the HCl that is generated as a byproduct of the surface reaction. The ammonia liberated from the surface is not “free” to react with the adsorbed HSiCl3 species. This has implications in using preadsorbed amines to control the amount and spatial distribution of a chlorosilane on the surface. Scheme 1. Two-Step Amine-Promoted Reaction
Introduction In the previous paper in this issue we have shown that low frequency infrared data is critical to the determination of the surface species that is formed when trichlorosilane (TCS, HSiCl3) reacts with surface silanol groups. The reaction occurs readily at room temperature and thus is in contradistinction to the reaction of the structurally similar organotrichlorosilane that normally reacts only at temperatures above 300 °C.1 Both chlorosilanes give surface adducts that are analogous in structure. i.e.,
(1)
It is well known that the surface attached Si-Cl groups are hydrolyzed by water to give the corresponding silanols, and reaction with other labile molecules such as ammonia2-4 and primary and secondary amine5,6 are known. We have also shown that the attachment of organochlorosilanes to a silica surface can be achieved at room temperature in a two-step amine-promoted reaction.7 The scheme for the amine-promoted reaction using ammonia (NH3) is shown in Scheme 1. The silica surface is first treated with a lone-pair donor such as ammonia, and it is found that the H-bonded adduct reacts readily with organochlorosilanes at room temperature to give a chemisorbed species. In this two-step reaction any excess gaseous ammonia is removed by evacuation at the end of the first step. If a sequential reaction scheme is not used then it is possible that any free or excess NH3 that is not bonded * Corresponding author. Fax (905)822-7022. Phone (905)8237091 x319. E-mail: Carl
[email protected]. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Tripp, C. P.; Hair, M. L. Langmuir 1991, 7, 923. (2) Low, M. J. D.; Severdia, A. G.; Chan, J. J. Catal. 1981, 71, 144. (3) Van Der Voort, P.; Vrancken, K. C.; Vansant, E. F.; Riga, J. J. Chem. Soc. Faraday Trans. 1993, 89, 2509. (4) Van Der Voort, P.; Swerts, J.; Vrancken, K. C.; Vansant, E. F.; Geladi, P.; Grobet, P. J. Chem. Soc. Faraday Trans. 1993, 89, 63. (5) Severdia, A. G.; Low, M. J. D. Langmuir 1988, 4, 1234. (6) Severdia, A. G.; Morterra, C.; Low, M. J. D. J. Colloid Interface Sci. 1984, 99, 208. (7) Tripp, C. P.; Hair, M. L. J. Phys. Chem. 1993, 97, 5693.
S0743-7463(96)00587-2 CCC: $12.00
to the surface could react directly with the incoming chlorosilane to form a pentacoordinate intermediate, and
this pentacoordinate intermediate could then react with a second nucleophile i.e., the surface hydroxyl groups. (The pentacoordinate intermediate is also very reactive with residual water, and this results in uncontrolled polymerization of the chlorosilane.) Tertiary amines such as triethylamine (TEA) and pyridine are preferred for the two-step reaction because primary and secondary amines such as ammonia can directly react with the chlorosilane.
R3SiCl + NH3 f R3SiNH2 + HCl
(3)
It is this aspect of the reaction mechanism that we can probe by studying the reaction of TCS with silica. Although the NH3 is liberated from the surface during the reaction shown in Scheme 1, it does not form a pentacoordinate species with the incoming free chlorosilane or react with the surface species if it is immediately consumed by the HCl generated in the reaction to form a quaternary salt. We can use the reaction of TCS with © 1996 American Chemical Society
6408 Langmuir, Vol. 12, No. 26, 1996
Tripp et al.
Figure 1. (a) Addition of TCS to a silica that has been degassed at 400 °C, evacuated for 5 min, and then (b) exposed to NH3 vapor for 2 s. (c)Addition of NH3 to a silica that has been degassed at 400 °C, evacuated, and exposed to HCl vapor.
Figure 2. Expanded regions of spectra shown in Figure 1. The * denotes salt bands.
silica to probe the selectivity of this deactivation step. Amines such as NH3 are known to react with the adsorbed TCS species and if this occurred in the two-step process it would result in a shift in frequency of the Si-H band. Thus, if we study the two-step reaction using TCS and a primary rather than a tertiary amine, the increased reactivity of the primary amine5 with adsorbed TCS and the sensitivity to direct reaction with the incoming chlorosilane should distinguish the reaction paths. Ammonia is used as the most sensitive probe to test this thesis. Experimental Section The infrared cell, spectrometer and experimental protocols are as described in the preceding article.
Results and Discussion 2
Reaction with Ammonia. Low et al. have provided a detailed account of the reaction of ammonia with the Sis-O-SiHCl2 groups on silica. They followed the reaction using the perturbation of the Si-H band as a probe for the reactions occurring at the Si-Cl bond. (At that time, direct observation of the Si-Cl bond was not experimentally possible.) The reaction produced complex results, and a mixture of species was formed that depended on the reaction protocol. However, in all cases, reaction of NH3 with Sis-O-SiHCl2 was rapid and gave rise to two Si-H bands at 2240 and 2190 cm-1. These were assigned to Sis-O-SiHClX and Sis-O-SiHXX, respectively, where X is NH2‚‚‚NH4Cl. The rapidity of this reaction is demonstrated in Figure 1b where NH3 (0.5 torr) was added for 2-3 s at room temperature. The bands at 3160, 3060, 2925, and 1410 cm-1 are various NH4+ modes.2 (In a separate experiment, these bands (albeit broadened) were reproduced by doping a silica with NH3, evacuating, and then exposing to HCl vapor (see Figure 1c)). The changes that occurred in Si-H stretching, Si-H bending, and SiCl regions are shown in Figure 2. We refer the reader to the preceding article for specific assignment of bands due to chemisorbed TCS species. With exposure to NH3 vapor, three Si-H bands at 2266, 2233, and 2190 cm-1 were formed together with a broad Si-H bending mode located at 875 cm-1. The bands at 590 and 520 cm-1 were eliminated with this quick exposure to NH3 vapor thus proving reaction with the Si-Cl functionality. Evacuation at room temperature for 1 h caused the band at 2266 to increase in intensity at the expense of both 2233 and 2190 cm-1 bands, and this was accompanied by a reduction in the NH4+ bands. The Si-Cl bands do not reappear thus showing that the evacuation step removed NH4Cl from the surface.
Figure 3. (a) Addition of TCS at 25 ˚C to a silica pretreated at 400 °C (i.e., same as Figure 1a. (b) NH3 has been added to a silica that had been degassed at 400 °C, evacuated for 5 min, and then exposed to excess TCS vapor at 25 °C followed by further evacuation. (c) Addition of excess NH3 for 2 s to sample b and then evacuated.
Two-Step Base-Promoted Reaction. The spectrum obtained with addition of TCS to silica predoped with NH3 is shown in Figure 3b. The silica had been degassed at 400 °C, evacuated for 5 min prior to addition of NH3 vapor for 2 s, evacuated for 5 min, and then exposed to excess TCS vapor and further evacuation. Reaction of TCS with the NH3-treated surface is instantaneous. The spectrum (excluding the spectral features attributed to the amine salt) is similar to that obtained for the direct reaction of TCS with silica (Figure 3a). This is more evident in the expanded regions shown in Figure 4 and shows that the species formed during the two-step amine-promoted reaction is the same as that produced by the direct reaction. This result is in full agreement with previous data obtained for the two-step process using alkyltrichlorosilanes.7 Moreover, these results show that the NH3 liberated during the two-step process does not react with the adsorbed TCS species: Any “free” NH3 would be expected to react instantaneously with the adsorbed species (for example, see Figures 1b and 3c) and cause drastic changes in the spectrum. It is noted that if the sample prepared via the two-step amine-promoted reaction (i.e., Figure 3b) is exposed to excess NH3 (0.3 torr for 2-3 s), then the spectrum obtained (Figure 3c) is similar to that shown in Figure 1b. In other words, exposure of the adsorbed species to gaseous NH3 vapor caused the immediate removal of the Si-Cl bands. This reaction with the SiCl groups was not found in the two-step reaction and the Si-Cl bands were of the same relative intensity as those
Amine-Promoted Reaction of Chlorosilanes on Silica
Langmuir, Vol. 12, No. 26, 1996 6409
step amine-promoted reaction of organochlorosilanes with silica in the presence of water: The amine that is consumed by the HCl does not have a chance to react with gaseous or solution chlorosilane. Conclusion
Figure 4. Expanded regions of spectra shown in Figure 3.
bands obtained in the direct reaction with TCS (see Figure 4). Thus, the NH3 liberated in the two-step process must have been consumed immediately by the HCl. It is this immediate consumption and deactivation of the amine by the HCl that explains the lack of polymerization in two-
The extreme reactivity of adsorbed TCS with NH3 has been used to investigate a subtle, yet important aspect of the two-step base-promoted reaction. The NH3 that is liberated from the surface in the two-step reaction does not react with either the adsorbed or free TCS molecules but is instead immediately consumed by the HCl generated in the main reaction. This result leads us to speculate on some possibilities. For example, it should be possible to control the amount of adsorbed chlorosilane on siliceous material by metering the amount of preadsorbed amine on the surface. Furthermore, it may be also possible to control the spatial distribution of the adsorbed chlorosilane by dictating the location of the preadsorbed amines. Work in these areas are currently in progress. LA960587O
