Citronellal
cyclization was carried out in a continuous mode over
H-beta-25 zeolite-based extrudates in a trickle-bed reactor at 35
°C and 10 bar of Ar. The physicochemical properties of zeolitic
catalysts in the form of extrudates with a different diameter (1.4–3
mm) containing 30 wt % of a binder were correlated with the catalytic
results. Alumina, aluminosilicate clay, and colloidal silica were
used as binders. For extrudates with 1.4 mm in diameter, a significant
decrease in the surface area, pore volume, and the total number of
acid sites was observed in comparison to the values expected from
a simple mechanical mixture of constituents. The specific surface
area was similar, while the pore volume and mechanical strength decreased
with increasing extrudate diameter. For all catalysts, conversion
of citronellal and the yield of the desired pulegols decreased because
of lower acidity and more prominent mass transfer with a size increase,
while the selectivity ratio for different pulegols was similar. Overall,
the effect of mass transfer on catalytic results in citronellal cyclization
was larger than the effect of acidity. The yield of pulegols and the
ratio of isopulegol ethers were correlated with strong Brønsted
and strong Lewis acid sites, respectively. A proper selection of the
binder and diameter of the extrudates plays a crucial role in the
cyclization of citronellal.