A.K. Vasudevan, et al.
AppliedCatalysisA,General601(2020)117604
present in a sample was calculated using the calibration curve.
The detection limit for the GC–MS system used is specified as 1 pg/
μl for octafluoronaphthalene for the signal to noise ratio of 60.
Assuming this detection limit to remain approximately similar for dif-
ferent compounds, for a 1-ml sample, 1 ng of the product of interest
could be detected. The catalysts without recognizable peaks for MNT
were thus assumed to generate less than 1 ng of MNT or have less than
0.000027 % yield.
195.7, and 87 GPa, respectively. Greater bulk moduli thus correlate
with the slower surface area increase during milling. Note that the
observed surface increase for MoO3 is less significant if the reactant
NaNO3 is added (in the amount shown in Table 2) before pre-milling.
Surface areas for the recovered solids were also measured after the
mechanochemical nitration of toluene. The results for all surface area
measurements, before and after the reaction, are summarized in
Table 3. For pre-milled oxides, a relatively minor change in the specific
surface area after nitration runs is observed. In most cases, the surface
area is somewhat decreased. Conversely, samples with pre-milled MoO3
and NaNO3 show a noticeable increase in the solids’ surface area after
the reaction.
2.5. Sources of errors and uncertainties
The inconsistencies in the reported results could be caused both by
lack of reproducibility in the milling experiments and by measurement
techniques used to characterize the nitration products. The reproduci-
bility of the milling experiments could be affected by catalyst caked in
selected runs, presence of iron impurities from the milling media, and
slight differences between individual milling vials. These issues can be
addressed by repeating multiple experiments. Indeed, some of our tests
were repeated; in such cases, error bars showing standard deviations of
results are shown. Generally, such error bars are smaller than the
meaningful changes in the measured quantities reported and discussed
here. Further reduction of the error bars could be achieved by addi-
tional experiments, which were outside the scope of this effort.
The errors in the concentration measurements are first associated
with the efficiency of extraction of the reaction products. This efficiency
is hard to access; however, the procedure used here was refined to
achieve reproducible results in multiple repeated experiments per-
formed in this and previous similar studies [35,36]. The errors in GS-MS
analysis can be assessed based on the relative standard deviation for
selected repeated experiments as not exceeding 5% of the measured
value for the signal to noise ratio of 60.
Initial experiments were carried out to test the effect of pre-milling
sodium nitrate with all three oxides, MoO3, WO3 and V2O5 on the ni-
tration rate. Results of these experiments shown in the supplement,
Table S1 suggest that the MNT yield was higher for MoO3 than for the
other two oxides. Respectively, further experiments involving pre-
milled sodium nitrate and catalyst focused on using MoO3.
XRD patterns for several MoO3- based powders used as catalysts are
shown in Fig. 2. The bottom pattern represents as-received MoO3. The
second and third patterns from the bottom show powders pre-milled for
15 and 30 min. Finally, the top three patterns show MoO3 and NaNO3
powders pre-milled for different times. With the increase in milling
time, the MoO3 peaks broaden suggesting a reduction in the crystallite
sizes. No other changes in the XRD are noted, suggesting that any re-
action occurring during pre-milling is negligible.
Using Williamson-Hall method, crystallite sizes for some of the
powders were evaluated as shown in Fig. 3. The reduction in the
crystallite size is noted for MoO3 milled with and without NaNO3. Ex-
tended milling times, in excess of ca. 100 min do not lead to further
reduction in the crystallite sizes.
3.2. MNT production
3. Results
Results for a set of nitration experiments with 30-min reaction time
employing the as-received and pre-milled catalysts V2O5, WO3, and
MoO3 are shown in Fig. 4. Yields are shown in Fig. 4a in terms of
conversion of the initially used toluene. The yield is higher for MoO3
while the yields for V2O5 and WO3 are comparable to each other. For
both, MoO3 and V2O5, pre-milling results in increased yield. However,
this trend is not as clearly observed for WO3. The effect of acidity
(quantified using the relative scale from Ref. [38], see Table 1) is not
clearly observed either, at least comparing WO3 and V2O5.
In initial experiments, all catalysts listed in Table 1 were tested.
Detectable yields of MNT were obtained only for MoO3, WO3, and V2O5.
Accordingly, these catalysts were used for further experiments. The
complete summary of results for all performed experiments is given in
the supplement, Tables S1 – S3. Most important results are discussed
here with more details provided.
3.1. Surface area and structure of catalysts
The same results are shown in Fig. 4b in terms of the absolute
amount of MNT produced per unit of surface area of the catalyst. For
this presentation, the surface areas were measured for solids recovered
after the nitration runs (Table 3). Fig. 4b shows a clear correlation of
amount of MNT produced and the catalyst’s relative acidity.
For MoO3, showing the highest yield of MNT, the effect of pre-
milling times longer than 30 min was further investigated. In addition,
results compared yields of MNT for MoO3 pre-milled with NaNO3 for
different times. These comparisons are shown in Fig. 5; as before, the
reaction time was fixed at 30 min. Longer pre-milling does not result in
significant increase in the surface area of MoO3 (cf. Table 3). The MNT
yield is lower at 120 min pre-milling despite nearly the same surface
areas observed for the solids after reaction for pre-milling times of 30
and 120 min. Conversely, for MoO3 pre-milled with NaNO3, the yield
increases with an increase in the pre-milling time (as does the surface
area, cf. Table 2). The amount of MNT produced per unit of the surface
area also increases, nearly linearly.
The specific surface area of the different catalysts as a function of
pre-milling time is shown in Fig. 1. Longer pre-milling results in a
greater surface area for all three oxides. The greatest increase in surface
area was observed for pre-milled V2O5. The change in the surface area
caused by milling correlates with mechanical properties of oxides. For
example, bulk modulus values calculated using density functional
theory (BLYP method) for WO3, MoO3 [39], and V2O5 [40] were 257,
3.3. Effect of the reaction time on MNT yield
The effect of reaction time on MNT yield is shown in Fig. 6; the
complete set of results is given in the supplement, Table S2. The error
bars representing the relative standard deviations are shown for
Fig. 1. Specific surface area of different powders as a function of the pre-milling
time.
3