(600ak) Effects of Sodium Substituted Solid Base ZrO2 On the Hydrogenolysis of Phenethyl Phenyl Ether (PPE) in near-Critical Water

Lignin
is uneatable biomass and large amounts of lignin are produced by the lignocellulosics biomass-to-ethanol process and by pulp
& paper refineries process as waste. Most of it is burned as low-grade fuel.
However, the complex chemical structure of aromatic groups in lignin suggests
that it may be a good source of valuable aromatic chemicals if it could be
broken into smaller molecular units. Phenethyl phenyl
ether (PPE), model substance of the b-O-4 linkage prevalent in lignin, was hydrolyzed in near-critical water. The influence of Na-ZrO₂
base catalyst on these reactions was tested at 400° for 0.5~2 h. The prepared
Na-ZrO₂ catalysts were characterized by several methods (BET, XRD, ICP
and CO₂-TPD) to investigate the relationship between its
physicochemical structure and the activity in PPE hydrogenolysis. The valuable
chemicals such as phenol, styrene and ethyl benzene were largely produced by
the hydrogenolysis reaction of PPE.

Na-ZrO₂
catalysts were prepared by precipitation method. 3.0 g of amphiphilic
block copolymers (PEO₂₀PPO₇₀PEO₂₀, Pluronic P 123) was dissolved in a 110.6 g of absolute
ethanol and 13.4 g of zirconium (IV) n-propoxide (70 wt.%
solution in 1-propanol) were separately dissolved with 1.5 g of acetylacetone
under stirring. Then above two solutions were mixed under vigorous stirring.
Upon stirring at room temperature for 1 h, 5.4 g of deionized water was added
dropwise. The mixture was gelated in a closed vessel at 60° for 48 h and the
obtained transparent resin hybrid was partly refluxed in the vessel with 0.5 mol/L NaOH aqueous solution for
another 48 h. Then the suspension was washed thoroughly with deionized water
and drying at 100° for 24 h. Finally, the samples were calcined at 700° for 5
h (ramping rate: 1°/min)

Reaction tests
were performed in a stainless-steel-tube (SUS 316) bomb reactor with a volume
of 5 mL. The reactor was loaded with 4 ml of
deionized water containing 2.5 wt.% PPE and 0.1 g of
prepared catalysts. And then pressurized to 20 bar with gaseous hydrogen (ultra high purity, >99.999%). The reactor was then
placed into an electric furnace and quickly heated to the reaction temperature.
After the reaction, the reactor was removed from the electric furnace and
quickly quenched to room temperature. The mixture of reactants and products was
diluted with THF to a volume of 12 mL. The reaction
products were analyzed by gas chromatography–mass spectroscopy (GC-MS, Aglient 6890 GC coupled with 5973N mass selective detector,
column: Restek, Rxi-5Sil MS, 30 m•0.25 mm•0.25 mm).

The Na-ZrO₂
catalyst showed the higher basicity than pure ZrO₂ (from CO₂-TPD
results, abbreviated) and the PPE conversion and the yields of the valuable
products increased sharply. As the reaction time increased, the conversion of
PPE and the product yields increased but above 2 h reaction, the yields of
phenol and ethyl benzene decreased sharply while the conversion of PPE was 95%.
This result was due to the consecutive reaction to form the repolymerization
product (char) was occurred between reactive intermediate like phenolic
radicals.

Figure 1. CO₂–TPD
profiles of pure zirconia and the as-made Na–ZrO₂

Figure 2. . PPE
conversion and product yields at different reaction times (4 mL of aqueous solvent;
0.1g of PPE; non-catalyst; reaction temperature: 400°; initial pressure: 20
bar H₂; reaction time: 0.5~2 h).

Figure 3. PPE conversion and product yields at different reaction times (4
mL of aqueous solvent; 0.1g of PPE; 0.1 g of Na-ZrO₂; reaction temperature:
400°; initial pressure: 20 bar H₂; reaction time: 0.5~2 h).