(544ak) Thermodynamic Complexity of Sulfated Zirconia Catalyst

Thermodynamic Complexity
of Sulfated Zirconia Catalyst

Naiwang
Liu ^†*_,Li Shi ^†_,Di
Wu ^¡Î, and
Alexandra Navrotsky ^‡

^† State Key Laboratory of Chemical Engineering, East
China University of Science and Technology, Shanghai 200237, China

^¡Î The Gene and Linda Voiland School of Chemical
Engineering and Bioengineering, Washington State University, Pullman,
Washington 99163, United States

^‡ Peter A. Rock Thermochemistry Laboratory and NEAT
ORU, University of California, Davis, One Shields Avenue, Davis, California
95616, United States

ABSTRACT

Heterogeneous
catalysts play a critical role in modern chemical industry, sulfated zirconia
(SZ) has been applied extensively in the chemical industry as an acid catalyst,
due to its strong surface acidity. Many works has been done to study the
surface structure of SZ, but most of them based on theoretical simulation, lack
of experimental support.

In
this paper, thermodynamic properties of SZ are analyzed using immersion and
high-temperature oxide melt drop solution calorimetry. Sulfated zirconia
catalysts was synthesized by immersion of amorphous zirconium hydroxide in
sulfuric acid of various concentrations (1 to 5 N). Experimental results shows
that the enthalpies of the complex interactions between sulfur species and the
zirconia surface (DH_sz)
for the sulfated zirconia precursor (SZP), ranging from -109.46 +- 7.33 (1 N)
to -42.50 +- 0.89 (4 N) kJ/mol S. DH_sz
appears to be a roughly exponential function of sulfuric acid concentration (C_s). The enthalpy of SZ formation (DH_f),
becomes more exothermic linearly as sulfur surface coverage increases, from
-147.90 +- 4.16 (2.29 nm^-2) to -317.03 +- 4.20 (2.14 nm^-2)
kJ/mol S.

These
thermochemical insights are tightly correlated to the surface coverage and
configuration of sulfur species. In a broader context, knowing such energetic
insights is critical to understand catalyst synthesis and active compounds-catalyst
support interactions for heterogeneous catalysts. The thermodynamic insights support
basic experimental data for the structure theoretical
simulation and helpful
for optimization
of design and synthesis of new catalytic materials with fine-tuned stability
and activity, in particular, dispersed super-acids and
supported metal clusters on solids.

Table
1. Thermodynamic
cycles
for sulfate moiety-zirconia surface bonding energies.

^a  g, ads and sol denotes gas,
adsorption and in sodium molybdate solution, respectively.

^b Ref. Majzlan et al. [1]
and Drouet et al. [2]

^c DH_ds and DH₁ (DH₁  = 27.06 +- 0.71 kJ/mol ZrO₂ [3]) are drop solution enthalpies of sulfated
zirconia (SZ) and nanophase tetragonal zirconia obtained using high temperature
oxide melt solution calorimetry, respectively. DH_ds values are listed in Table 3.

^d DH₂ is enthalpy of H₂O
adsorption on sulfated zirconia surface.

^e DH₃ = 25.65 kJ/mol H₂O [3]

^f DH₄ is drop solution enthalpy for SO₃, it cannot be determined
directly experimentally but has been 
calculated from reaction the formation enthalpies of SO₂ and
O₂. According to previous work, DH₄
= -217.02 +- 4.17 kJ/mol [1].

Table 2. Post immersion
calorimetry SZP sample compositions and enthalpies of sulfur species-zirconia
interactions (DH_SZ, with
different units). These enthalpies data come directly from immersion
calorimetry.

Table 3. Sample
characterization and thermodynamic data for high temperature oxide melt
solution calorimetry, including enthalies of dissolution directly measured by
calorimetry, DH_ds and enthalpies
of SZ formation DH_f,
calculated using thermodynamic cycle listed in Table 1.

References

[1] J. Majzlan, A. Navrotsky, J. Neil, Energetics of
anhydrite, barite, celestine, and anglesite: a high-temperature and
differential scanning calorimetry study, Geochimica et Cosmochimica Acta, 66
(2002) 1839-1850.

[2] C. Drouet, A. Navrotsky, Synthesis, characterization, and
thermochemistry of K-Na-H₃O jarosites, Geochimica et Cosmochimica
Acta, 67 (2003) 2063-2076.

[3] M.W. Pitcher, S.V. Ushakov, A. Navrotsky, B.F. Woodfield,
G. Li, J. Boerio-Goates,
B.M. Tissue, Energy crossovers in nanocrystalline zirconia, Journal of the
American Ceramic Society, 88 (2005) 160-167.