“…Reaction of TMA on Silica. The results qualitatively agree with those of previous studies. − Both ligand exchange reaction with hydroxyl groups and dissociation in siloxane bridges took place; the former was confirmed by the consumption of OH groups in the reaction, and the latter was shown by the presence of methyl groups bonded to silicon. Multiple methyl groups bonded to the same silicon atom, as identified by Lakomaa et al, were also observed in this study.…”
Section: Discussionsupporting
confidence: 91%
“…On silica, TMA reacts with isolated and hydrogen-bonded hydroxyl groups (OH) through a ligand exchange reaction, and with siloxane bridges (Si−O−Si) through dissociation, − as schematically depicted in Figure . Methane is released in the reaction with OH groups, as TMA abstracts a hydrogen atom from the group.…”
The saturating reaction of ammonia was studied on trimethylaluminum (TMA)-modified porous silica. This reaction step completes a reaction cycle of TMA and ammonia in aluminum nitride growth by atomic layer chemical vapor deposition (ALCVD), a technique based on well-separated saturating gas-solid reactions. The reaction was studied from 423 to 823 K. In addition, the separate reactions of TMA at 423 K and ammonia at 823 K were studied on silica dehydroxylated at 1023 K. The reaction products on the surface were identified by IR and 29 Si, 13 C, and 1 H NMR spectroscopy, and they were quantified by element determinations and 1 H NMR. In the reaction of TMA on silica, methyl groups were attached to the surface indirectly through aluminum and through direct bonding to silicon. In the subsequent ammonia reaction, ligand exchange of ammonia with the methyl groups occurred at all reaction temperatures, resulting in primary amino groups and the release of methane. Also, secondary amino groups were found on the surface, and quantitative determinations indicated the presence of tertiary amino groups, especially at high reaction temperatures. In addition, especially at low reaction temperatures, ammonia chemisorbed associatively on the TMA-modified silica. All of the methyl groups bonded to aluminum were removed with ammonia at 573-623 K, and about 80% of the methyl groups bonded to silicon were removed at 823 K; amino groups bonded to both aluminum and silicon were left behind. The higher the reaction temperature, the smaller was the average number of hydrogen atoms (x) in the amino groups (NH x ).
“…Reaction of TMA on Silica. The results qualitatively agree with those of previous studies. − Both ligand exchange reaction with hydroxyl groups and dissociation in siloxane bridges took place; the former was confirmed by the consumption of OH groups in the reaction, and the latter was shown by the presence of methyl groups bonded to silicon. Multiple methyl groups bonded to the same silicon atom, as identified by Lakomaa et al, were also observed in this study.…”
Section: Discussionsupporting
confidence: 91%
“…On silica, TMA reacts with isolated and hydrogen-bonded hydroxyl groups (OH) through a ligand exchange reaction, and with siloxane bridges (Si−O−Si) through dissociation, − as schematically depicted in Figure . Methane is released in the reaction with OH groups, as TMA abstracts a hydrogen atom from the group.…”
The saturating reaction of ammonia was studied on trimethylaluminum (TMA)-modified porous silica. This reaction step completes a reaction cycle of TMA and ammonia in aluminum nitride growth by atomic layer chemical vapor deposition (ALCVD), a technique based on well-separated saturating gas-solid reactions. The reaction was studied from 423 to 823 K. In addition, the separate reactions of TMA at 423 K and ammonia at 823 K were studied on silica dehydroxylated at 1023 K. The reaction products on the surface were identified by IR and 29 Si, 13 C, and 1 H NMR spectroscopy, and they were quantified by element determinations and 1 H NMR. In the reaction of TMA on silica, methyl groups were attached to the surface indirectly through aluminum and through direct bonding to silicon. In the subsequent ammonia reaction, ligand exchange of ammonia with the methyl groups occurred at all reaction temperatures, resulting in primary amino groups and the release of methane. Also, secondary amino groups were found on the surface, and quantitative determinations indicated the presence of tertiary amino groups, especially at high reaction temperatures. In addition, especially at low reaction temperatures, ammonia chemisorbed associatively on the TMA-modified silica. All of the methyl groups bonded to aluminum were removed with ammonia at 573-623 K, and about 80% of the methyl groups bonded to silicon were removed at 823 K; amino groups bonded to both aluminum and silicon were left behind. The higher the reaction temperature, the smaller was the average number of hydrogen atoms (x) in the amino groups (NH x ).
“…Grafting reactions analogous to eqs 1 and 2 have long been proposed for AlMe 3 , whose deposition on silica can be used to create Lewis acid sites that activate metallocene catalysts for olefin polymerization . These grafted sites have been described in many studies as three-coordinate ⋮SiOAlMe 2 and (⋮SiO) 2 AlMe. − However, we recently concluded that AlMe 3 reacts with the surface of a fumed silica to give four-coordinate dialuminum surface sites, on the basis of the reaction stoichiometry …”
The room-temperature, gas−solid reaction of volatile GaMe3 with a nonporous silica was studied by
elemental and gas-phase analysis, in situ IR and 1H, 13C, and 29Si solid-state NMR, and extended X-ray
absorption fine structure (EXAFS) spectroscopy. Most of the grafting (∼85%) occurred on Q3 sites,
O3SiOH, but a small amount (∼15%) of siloxane (O3SiO−SiO3) bond cleavage was also observed. The
major, if not the only, gallium product has the empirical formula ⋮SiOGaMe2, but it is not an “isolated”
site. The Ga K-edge EXAFS of GaMe3-modified silica, recorded at 10 K, reveals that each Ga has a Ga
neighbor at 2.97−2.99 Å. The sites are best described as [GaMe2(μ-OSi⋮)]2. To strengthen this assignment,
a molecular analogue, [GaMe2(μ-OSiPh3)]2, was characterized by both single-crystal X-ray diffraction
and EXAFS. The Ga2O2 rings in the molecular complex and the silica-supported gallium dimer have
very similar dimensions. The gallium dimer is formed on the silica surface regardless of the extent of
partial dehydroxylation (varied by pretreatment in vacuo at 100 and 500 °C). This result is interpreted in
terms of a vicinal disposition for the majority of Q3 grafting sites.
“…Regardless of the application, a fundamental knowledge of the nature of these reactions remains important for future development. Using trimethylaluminum (TMA) as the most studied example, − there is general agreement that TMA reacts with a surface silanol, releasing methane (eq 1). …”
Reactions of trimethylaluminum, triethylaluminum, and diethylaluminum chloride and ethylaluminum dichloride with silica gel have been studied experimentally by infrared spectroscopy and elemental analysis. The silica gel was subjected to different pretreatments to alter surface functionalities prior to reaction. In all cases the extent of surface modification reaction follows the trend unmodified > 600 degrees C pretreated > hexamethyldisilazane (HMDZ) pretreated > 600 degrees C/HMDZ pretreated. All of the aluminum compounds studied completely react non-hydrogen-bonded silanols, while also reacting with hydrogen-bonded silanols and siloxanes. Primarily monomeric surface species result from the surface modification reaction. Ethylaluminum chlorides preferentially react with silanols through cleavage of the Al-C bond rather than the Al-Cl bond. Singly bonded Si(s)-O-AlCl(2) surface species are readily synthesized by reaction of ethylaluminum dichloride with HMDZ-pretreated silica gel. Bridged bonded (Si(s)-O)(2)-AlCl surface species are readily synthesized by reaction of diethylaluminum chloride with HMDZ-pretreated silica gel. Computational ab initio studies of the cluster Si(4)O(6)(OH)(4) as a model to study the reaction of monomeric and dimeric methylaluminum dichloride with a silica silanol are also described. Comparison of the potential energy surface (PES) of monomer and dimer indicates that the energetics favor monomer reaction, consistent with experimental results. The energy cost in the dimer reaction is primarily from cleavage of a bridged Al-Cl bond upon adsorption. This does not occur when the monomer adsorbs. A comparison of the PES for the two reaction pathways resulting from cleavage of either an Al-Cl or Al-C bond indicates that while the former reaction is slightly kinetically favored (E(a) = 23.1 kJ/mol for Al-Cl bond cleavage versus E(a) = 31.1 kJ/mol for Al-C bond cleavage), the latter is strongly thermodynamically favored with an overall free energy difference between the two reaction pathways of 135 kJ/mol favorable to Al-C bond cleavage. These reactions are thermodynamically controlled.
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