2.1. Catalyst Characterization
The mesoporous phosphate heterostructure material (PPH) employed as support, is the result of the formation of silica galleries within the interlayer space of the CTMA expanded lamellar zirconium phosphate, MCM-50 type. After removing the surfactant by calcination, the structure was preserved and a porous material formed, which was reflected in the X-ray diffraction (XRD) pattern as a single peak at low angle corresponding to the d001
diffraction at ca. 40 Å (Figure A1
). No diffraction lines were observed in the high angle region, indicating no co-precipitation of silica or zirconium phosphate during the process of synthesis of the support.
depicts the XRD patterns of the supported catalysts. In all cases a broad band at 2θ between 20° and 30°, which is characteristic of amorphous materials, is found. In the case of PdMo/PPH fresh and spent catalysts, no well-defined diffraction lines are observed in the high angle region, probably due to the high dispersion of the active phases, although it could be also explained by the low Pd and Mo loading, perhaps below the detection limit of the technique. As regards the PtMo/PPH catalyst, only two weak diffraction signals at 2θ = 39.7°, and 46.3°, assigned to metallic platinum (PDF 00-001-1190), can be observed, indicating the formation of metallic particles during the process of calcination; these signals remain unchanged after the catalytic HDO test. These results suggest a better dispersion of the metallic phase in the case of the Pd based catalyst. The diffraction peak at 2θ = 35.6° (PDF 01-074-2307) in both spent catalysts is due to the presence of silicon carbide (SiC) which was used as diluent in the preparation of the samples for the catalytic test.
Transmission electron microscopy (TEM) study of the catalysts gives an idea of the degree of dispersion of the metallic phase on both catalysts, the corresponding micrographs are shown in Figure 2
. For PdMo/PPH (Figure 2
A,B), the images show the presence of highly dispersed small particles homogeneously distributed on the material support. Although some agglomerates are also observed, the majority of particles are in the range 1–2 nm (see Figure 3
A). The same can be said about PtMo/PPH catalyst (Figure 2
C,D), although the average particle size of the metallic fraction seems to be slightly higher (~2–3 nm) and the particle size distribution is wider than that observed for PdMo/PPH, as shown in Figure 3
B. Furthermore, a greater number of agglomerates of large particles were observed in this material. As previously observed from XRD, better active phase dispersion is achieved in the case of Pd catalyst.
Moreover, additional micrographs in STEM mode were done in order to obtain mapping results of both samples as well as a quantification of Pt(Pd)-Mo in these samples (Figure 4
Quantification results indicated that the compositions were similar to the nominal one, with Pd/Mo mean ratios of 1.05 and Pt/Mo ones of 0.98.
The textural properties of the support and catalysts were obtained from N2
adsorption-desorption isotherms at −196 °C, and are depicted in Figure 5
. In addition, Table 1
lists the corresponding textural properties derived from them.
All the isotherms are of type IV and exhibit a hysteresis loop, which is characteristic of mesoporous materials. The isotherm of PtMo/PPH catalyst is similar to that of the bare support [30
]; however, the catalyst PdMo/PPH showed a decrease of the N2
adsorbed over all the range of relative pressures studied. It is possible that a certain number of mesopores had been blocked by the entrance of Pd0
particles, which, as it has been discussed, showed a very narrow distribution of sizes; furthermore, these particle sizes were always smaller than the average pore diameter of the bare material (Table 1
In fact, by adding the active phase, the Pt-containing catalyst suffers only a slight loss in BET surface area value relative to the surface area presented by the pristine support, while this loss is much more remarkable in the case of the Pd-based catalyst. Instead, pore volume decreases in the latter, probably due to the main location of the small particles present in this material in smaller pores, mainly in the inner part of the silica galleries. The greater mean pore diameter of the catalysts compared to the bare support could be due to the formation of additional pores by interaction of nanoparticles of bigger size that cannot enter into the channels and are located outside between the packets’ layers (see Picture 1 and Figure 5) [32
]. This effect is more important for PtMo/PPH catalyst where the presence of larger particles is higher. In fact, a sharp increase is clearly observed in the adsorbed volume at high relative pressures.
The concentration of acid centers and strength were determined by NH3
-TPD. Table 2
shows the amount of desorbed NH3
) based catalysts supported of PdMo and PtMo and the fresh PPH support.
From these data, the moderate high acidity of these catalysts is due to the acidic nature of PPH and the incorporation of the active phases. Furthermore, the most important desorption occurs at low temperature, indicating that the acidity is mainly of a weak nature. Additionally, both catalysts show considerable amounts of NH3 desorbed in the range 300–500 °C, indicating that they also possess average acidity. Finally, the amount of desorbed ammonia was normalized by the surface area of the sample. It can be seen that the amount of ammonia desorbed per m2 of sample is higher for the catalysts, indicating that the addition of Pt, Pd, and Mo increased the acidity.
The surface chemical composition of the fresh catalysts and used catalysis was evaluated by XPS. Considering the Mo signal (Figure 6
), in both cases two contributions due to the spin-orbit doublet of Mo 3d
were observed, the binding energy of the Mo 3d5/2
component located at 232.3 eV for the Pt/Mo sample and 232.8 eV for the Pd/Mo sample. These values are slightly higher (between 0.3 and 0.9 eV), than that reported for MoO3
], probably due to the greater interaction of the active phase with the carrier, being more acidic, as discussed below. The Mo 3d
signal does not show significant changes after the catalytic process in the case of the Pt/Mo sample, and was not detected in the case of the Pd/Mo one.
The Pt 4f
core level spectrum the PtMo/PPH catalyst shows two doublets, as can be clearly seen in Figure 7
. The Pt 4f7/2
component located at 71.1 eV corresponds to metal Pt and that located at 72.5 eV is characteristic of the Pt2+
ion corresponding to PtO [34
]. After catalysis, both of the two contributions due to Pt0
and PtO are also observed, the contribution of the metallic phase being the most important one. The presence of PtO in the used catalyst is associated with the oxidation of Pt0
by water, a byproduct of the reaction.
also shows the Pd 3d
core level spectra. The spectrum for the catalyst PdMo/PPH fresh presents a contribution at 334.0 eV corresponding to the Zr 3p
signal coming from the carrier. Furthermore, the presence of PdO is also observed with a Pd 3d5/2
contribution at 337.1 eV, as well as its corresponding doublet [35
The surface atomic concentration of the elements is also included in Table 3
. The catalyst containing Pt possesses larger Pt/Sup and Mo/Sup ratios than the Pd based one. On the other hand, the atomic noble metal/Mo ratio, in both cases is lower than the nominal composition, 1, which points to a surface enrichment of Mo species, especially in the PdMo/PPH catalyst, having the lowest value.
2.2. Catalytic Results
As it was stated previously, the synthesized catalysts were evaluated in the HDO reaction of dibenzofuran. The reaction was carried out at 275 °C and at two different pressures, 15 and 30 bar, to evaluate conversion as a function of reaction time. Figure 8
shows the comparison between both samples at both reaction pressures studied.
PtMo/PPH catalyst showed total and HDO conversions very similar, at around 80%. This catalyst shows a substantially constant conversion with the reaction time and the activity is similar at different pressures. Very different is the behavior of the catalyst PdMo/PPH; that is, the total conversion is greater than the HDO conversion in both cases, showing a practically constant difference of 15% less compared to the total conversion in the reaction at 30 bar and practically 40% lower in the reaction at 15 bar. The conversion decreases with the reaction time and the conversion is higher at 30 than at 15 bar (see Figure 8
). Moreover, HDO conversion is much more affected by the reaction pressure. These data indicate the important role of the active phase in these reactions. Thus, the presence of Pt improves the hydrogenolysis capability, being much more selective to O-free products. Instead, Pd shows greater hydrogenation ability and therefore is much more sensitive to reaction pressure conditions. It is of note that in the case of PtMo based catalyst, the H2
pressure had no influence on the conversion, being practically equal to 80% at both pressures (Figure 8
). Nevertheless, in the case of PdMo, the effect of hydrogen pressure is significant. After six hours of reaction (Figure 9
) it is clearly observable.
In terms of catalyst stability, PtMo/PPH seems to achieve steady state conditions at both reaction pressures evaluated. PdMo/PPH suffers a decrease in both total and HDO conversion but the observed tendency is that after six hours on stream, it achieves a plateau. However, by studying the process of HDO of DBF, the main products obtained as non-oxygenated compounds were: bycyclohexane (BCH) and cyclohexane (CH) and in a lesser extent cyclohexylbenzene (CHB), cyclopentilcyclohexane (C-PE-CH), and cyclopentylmethylcyclohexane (C-PE-ME-CH). As intermediate oxygenates compounds 2,3,4,9 tetrahydrodibenzofuran (THDBF), 2,3,4,4a,9,9a hexahydrodibenzofuran (HHDBF), 2-cyclohexylphenol (2-CHP): 1-1-bi(cyclohexane)-3-en-2-ol isomers (BCH-3-en-2-ol), and bicyclohexane-2-ol (BCHol) were detected. Figure 10
and Figure 11
compile the main reaction products obtained at 15 and 30 bar for PtMo/PPH and PdMo/PPH, respectively (Scheme 1
For PtMo/PPH catalyst, the selectivity is similar at 15 and 30 bar. It can be observed that the major product obtained is BCH with selectivity almost constant between 60% and 70%. CH is the second major non-oxygenated product obtained with a proportion of about 20% during the whole time of the reaction. Other non-oxygenated products such as CBH, C-PE-ME-OH, and C-PE-CH, were obtained in a proportion of less than 5% for both reaction pressures. In both reactions, oxygenated products (THDBF, HHDBF, 2-CHP, BCH-3-en-2-ol) were detected in a proportion of less than 3%.
In the case of PdMo/PPH catalyst (Figure 11
), it can be seen that the main deoxygenated products at 15 bar were CH and BCH; however, as the reaction proceeds, the major product is CH, while the BCH selectivity decreases. Conversely, at 30 bar, the main product is BCH, but BCH selectivity decreases with reaction time similar to that observed at 15 bar. These data indicate that the reaction pressure has an important role in the reaction products for the Pd catalyst. So, at 15 bar, the BCH molecule may break and cause CH, while this break appears inhibited at 30 bar. Furthermore, BCH selectivity decreases slightly while increasing the selectivity to 2-CHP. This points to the blockage of the sites where the CO bond rupture takes place.
Regarding the selectivity towards oxygenated intermediates, at 15 bar, the main compound is 2-CHP. At 30 bar, where the selectivity to these products is lower, the major product is initially HHDBF and after 6 h of reaction, the selectivity to 2-CHP is greater.
Selectivity data after six hours of reaction (Figure 12
) indicate that the catalyst with better selectivity is that based on Pt. Furthermore, it is noted that this catalyst is the most selective toward deoxygenated compounds as BCH and CHB, and CH to a lesser extent. As oxygenate compound, a minimum amount of 2-CHP is formed. The amount of other species formed is not significant. It is also of note that the selectivity towards deoxygenated compounds is slightly greater at 30 bar than that at 15 bar, which coincides with the yields obtained for this catalyst. The catalyst based on Pd showed a CH selectivity close to 2-CHP at both pressures. Only when the pressure is higher, the selectivity towards non oxygenated products is higher than 60%. Another oxygenated compound, HHDBF, is also formed in relative high quantity, at both pressures. The rest of the non-oxygenated products were obtained in a proportion less than 2%.
Therefore, catalysts based on Pt gave better results than Pd catalysts. The pressure had a significant effect on the selectivity distribution of deoxygenated product, mainly BCH. CH was obtained at both pressures with a selectivity of around 20%–30%, and depending on the reaction pressure. Both catalysts were able to hydrogenate and deoxygenate the DBF to BCH, and also were able to dissociate the molecule to form CH. Moreover, the Pt based sample did not show deactivation during the catalytic reaction of HDO, while PdMo suffered a decrease in its activity. Furthermore, it was found that, while catalysts based on PtMo had no significant change of activity with variations in pressure, those based on PdMo improved operation by increasing the reaction pressure.
If the results are compared (see Table 4
) with those obtained with a non-acidic and commercial support such as silica (fumed silica from Aldrich, St. Louis, MO, USA), it can be seen that the Pt based catalyst showed a lower conversion when supported on SiO2
was more selective to BCH and CH. Moreover, the total percentage of deoxygenated products was higher with the PtMo/PPH sample. In the case of PdMo/SiO2
, it can be seen that the HDO conversion and selectivity towards deoxygenated compounds is higher than for PdMo/PPH. However, the latter is much more selective to CH.
These results highlight the importance of both, noble metal and support in this reaction. PPH material can be considered a good candidate as catalyst support, since high conversion and selectivity values are attained in both cases and the results are comparable or even better than those achieved with a commercial support.