D. Baykan, N.A. Oztas / Materials Research Bulletin 64 (2015) 294–300
297
in Fig. 2. Iron orthophosphate prepared by solution method
appeared to have lost weight in the three steps about 54.94% at
100–550 ꢀC. The first step was due to loss of water molecules
represented by endothermic peaks at 130 ꢀC. The second step in the
temperature range 200–350 ꢀC was an endothermic reaction. The
last step was due to phase transition of iron orthophosphate into
quartz phase about 2% [33]. The solution combustion synthesized
iron orthophosphate had 70.52% total weight lost. The first step
was at 100–200 ꢀC due to loss of water. The second step was at
200–350 ꢀC because of evaluation of large amount of gases during
the combustion reaction which represented by exothermic peak at
212 ꢀC. The last weight loss was phase transition. The results
obtained from TG and DT curves indicated that fuel was decreasing
the synthesis temperature of quartz phase iron orthophosphate.
Fig. 3 represents SEM micrographs of the powders prepared
with different methods. The SEM images of the product obtained
by solution method indicate formation of more ordered structures
like plates compared to the products obtained with solution
combustion method which like foam. Surface areas of iron
phosphate with the solution combustion synthesis and solution
synthesis were found 11.8 m2/g and 5.9 m2/g, respectively. Using
fuel and evaluation of large amount of gases during combustion
were caused the formation of porous material and increased the
surface area. The results obtained from surface measurements also
indicate the effect of synthetic approaches. Iron orthophosphate
prepared by solution combustion method have higher surface area
than the samples prepared by solution method.
orthophosphate. Catalyst amount increased the formation of
hydroxyl radical and occured cyclohexadienyl radical from
benzene [34]. Increasing radical amount was increased the
reaction yield. When the catalyst amount was raised to 0.75 g,
reaction yield was decreased. Because the excess amount of
catalyst was attributed to the accelerated self-decomposition of
hydrogen peroxide and excess amount of hydroxyl radical was
caused to form byproducts like benzoquinone and biphenyl.
Table 1 shows the effects of catalyst in phenol yield and selectivity
of products.
The influence of reaction temperature on the yield of phenol is
shown in Fig. 4b. With increasing reaction temperature between
50 ꢀC and 70 ꢀC, phenol yield raised for both catalysts. Over 70 ꢀC,
the product yield decreased, because the decomposition of
hydrogen peroxide proceeds faster at higher temperature [28].
Therefore, the optimum reaction temperature is found as 70 ꢀC.
Fig. 4c demonstrates the behavior of the phenol yield with
increasing reaction time. The yield of phenol reaches the
maximum value of 24.52% with solution combustion synthesized
iron orthophosphate and the maximum value of 8.90% with
solution synthesized iron orthophosphate at 4 h. With the
increasing reaction time, the phenol yield decreased because of
the oxidation of the phenol to catechol, benzoquinone and
biphenyl [27].
At constant reaction condition (70 ꢀC, 4 h, 0.75 g catalyst) the
influence of oxidant amount was investigated and the effect of
H2O2/benzene mole ratio is shown in Fig. 4d. No phenol was
obtained without hydrogen peroxide. When H2O2/benzene mole
ratio was 1, the phenol yield was maximum. At higher amount of
oxidant, the yield of phenol decreased because of the formation of
byproducts and self-decomposition of hydrogen peroxide to water
and oxygen [35].
After the optimization studies completed, the effect of different
solvents was investigated. Without solvent benzene and hydrogen
peroxide didn’t mixed, solvent was necessary [36]. The activity is
maximum in polar solvents [20]. Fig. 5 shows that both ethanol and
acetone are improper solvents for hydroxylation of benzene to
phenol because of stronger hydrogen bond between solvent and
hydrogen peroxide. In neutral medium, phenol occurred the much
lower than acidic medium [19]. For this purpose, acetic acid was
3.2. Catalytic performance
To examine the catalytic effect of iron orthophosphate
synthesized by different method, liquid phase hydroxylation of
benzene was selected as a model reaction. For optimization of
catalytic reaction, at first the effect of catalyst amount was
investigated.
The effect of iron orthophosphate amount is shown in Fig. 4a.
Without catalyst phenol production wasn’t detected. When the
amount of catalyst increased, the reaction yield increased from
10.42 to 24.52 for solution combustion synthesized iron ortho-
phosphate and from 5.41 to 8.90 for solution synthesis iron
[(Fig._3)TD$FIG]
Fig. 3. SEM images of solution synthesis (a) and solution combustion synthesis (b) of iron orthophosphate.