Evaluation
of Zr–Alumina in Production of Polyoxymethylene
Dimethyl Ethers from Methanol
and Formaldehyde: Performance Tests and Kinetic Investigations
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Abstract
Polyoxymethylene dimethyl ethers
(PODE<sub><i>n</i></sub>) converted from methanol and formaldehyde
were investigated with
alumina supported ZrO<sub>2</sub> catalyst in a stainless steel tube
fixed-bed reactor within the temperature range from 333 to 433 K and
the pressure range from 0.1 to 2.5 MPa. First, results from catalytic
performance tests showed that the catalyst γ-Al<sub>2</sub>O<sub>3</sub> containing 4 mol % ZrO<sub>2</sub> exhibited the highest
activity for PODE<sub><i>n</i></sub> synthesis. The stoichiometry
analysis of raw material indicated that ZrO<sub>2</sub> modified catalyst
had the optimum activity at methanol/formaldehyde = 3 (mol/mol). An
increase of the reaction pressure resulted in a significant increase
in methanol conversion from 13.18 to 48.64%, and selectivity to PODE<sub>3–8</sub> was enhanced to 24.82%. The temperature experiment
analysis confirmed the optimization of the reaction temperature to
be 393 K. Second, SEM and TEM showed the relationship between the
catalytic activity and the catalyst-crystal alignment. Incorporation
of ZrO<sub>2</sub> in the framework of γ-Al<sub>2</sub>O<sub>3</sub> was noticed and confirmed by X-ray diffraction (XRD) and
BET analysis. The NH<sub>3</sub>-TPD tests indicated the relationship
between catalytic activity and acidity. Finally, an elimination mechanism
was proposed to explain the PODE<sub><i>n</i></sub> synthesis
reaction rate and kinetic model. The contributions of intraparticle
and external diffusion were eliminated by changing the size of catalyst
particles and space velocity. The proposed model can reasonably predict
the observed behaviors of PODE<sub><i>n</i></sub> synthesis
from methanol and formaldehyde