Sulfonated Metal-Oxide Surfaces: What Makes them So Acidic?
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539 Mat. Res. Soc. Symp. Proc. Vol. 318. ©1994 Materials Research Society
RESULTS AND DISCUSSION Two of the more common synthetic methods for sulfonating metal-oxide surfaces are treatment of the hydrolyzed metal halide (or alkoxide) with ammonium sulfate ((NH 4 )2 S0 4 ) or dilute sulfuric acid, followed by calcination steps. A simplified synthetic path is illustrated in Figure 1 showing some of the possible reaction products. For both cases, it is likely that the initial reaction involves a condensation step, eliminating water in the process. Both Bronsted and Lewis acidity can be associated with the reaction products of each step, with the exception of the illustrated symmetric products in the second condensation reaction. The chemical bonding characteristics of the metal determine the coordination number of the metal site. OH, OH2 HO -M 120
-OH
+
H2 SO4
OH
Step 1 Condensation
dehydration via heating OH ,OH2
HO-M-0SO3
H120
+ H20
OH more heating
Step 2 Condensation
-
H2 0
HO
o
MO07 Ho
\
0
Ho
0
•
HO0
/
single site interaction, symmetric 6 coordinate
\\O
H O /O\/O Hor M O/S
single site interaction symmetric 4 coordinate
HO
H
single site interaction, asymmetric 6 coordinate Figure 1 - Schematic synthesis of model sulfonated metal-oxide surface through reaction of hydrolyzed metal-oxide and sulfuric acid. Thermodynamic and StructuralAnalysis The electronic structure calculations of the model complexes for the surface sulfate species reveals several interesting trends both for the nature of the metal-oxide species, but also about its local coordination. Estimates of thermodynamic quantities for the first condensation step would indicate that the first water would be lost exothermically. In Tables 1 and 2, we present the estimated heats of reaction for model silicate and titanate complexes with sulfuric acid, and the proton affinities for the resulting complexes.
540
Considering solution acid/base equilibria effects, the first condensation step could occur between a neutral species and a singly ionized one. Since the proton affinity for sulfuric acid is about 55 kcal/mole lower than the acidic hydrogens of the model M(OH)x complexes (Table 2), it would be more likely that the anionic form of sulfuric acid would react with a neutral surface species. The relative stabilization energies for reaction of anionic form of sulfuric acid ([HSO4]1") with the silicate surface model is approximately twice that for the neutral form, although this trend did not extend to the titanate example (Table 1). In Figure 2, we present the molecular structures of the reaction product resulting from the first condensation reaction between the hydrolyzed metal-oxide precursor and sulfuric acid. The molecular geometries for both the neutral and anionic complexes indicate there would be strong interactions between the sulfate group and hydroxyls bound to the metal site. Notably, the oxygen atoms derived from the sulfuric acid group are within hydrogen bonding distances of the metal-bound hydroxide, with the prot
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