5.1. Preliminary Studies
Pamin et al.
prepared 12-tungstophosphoric acid on Y zeolite by impregnation in water [35
]. Before encapsulation, the zeolite was first dealuminated by treatment with EDTAH4
. The resulting Si/Al ratio of the material was 4.24. This treatment led to the formation of a secondary pore system with pores ranging from 12 to 50 Å and a mean value of 15 Å. The polyoxometalate loading varied between 1 and 10 wt %. The samples were characterized by various physicochemical methods including XRD, thermogravimetry, infrared spectroscopy and solid-state MAS NMR spectroscopy. The main conclusions were that two types of Keggin units were present on the surface: those strongly interacting with the hydroxyl groups of the support (probably via their protonation [36
]), prevailing at low coverage, and those interacting weakly with the zeolite crystals and corresponding to bulk heteropolyacid, prevailing at high coverage. These solids were tested in the transformation of m
-xylene by isomerization, disproportionation and dealkylation. Deposition of the 12-tungstophosphoric acid dramatically increased the selectivity for disproportionation. The results were comparable to those obtained by the same group with the encapsulated polyoxometalate [27
These solids were also used for catalysis in liquid phase for the oxidation of various substrates by hydrogen peroxide [37
]. Only activated molecules (containing, for example, hydroxyl groups) could be oxidized but unfortunately no studies were made on leaching and recycling.
Sulikowski also prepared samples by impregnation of the same dealuminated zeolite with HPW12
in diethyl ether [38
]. The mixture was stirred in a closed vessel at room temperature for 1.5 h and then the solid was isolated by filtration and washed with diethyl ether. PW12
loadings ranging from 2 to 38 wt % were prepared by this protocol. Similar conclusions to Parmin et al
. were drawn about the presence of two types of Keggin units, even if in the present case the bulk heteropolyacid was also present at low coverage. These solids were tested in the disproportionation of toluene and its transalkylation with 1,2,4-trimethylbenzene. All hybrid catalysts exhibited enhanced catalytic activity in both processes, but the Keggin units located in the mesopores were found to be the most active species.
Haber et al.
] performed a detailed study on HPW12
supported on dealuminated zeolite as a function of the HPW12
loading (between 0.00016 and 46.8 wt %). The samples were prepared by impregnation in water as in the first paper [35
]. These catalysts were fully characterized and studied in the vapor-phase dehydration of ethanol. It was concluded that the catalytic behavior was determined by few very active sites, composed of Keggin anions and lacunary Keggin anions located in the supercages opened at the surface, which has a honeycomb structure. This structure was formed during the zeolite surface pretreatment and dealumination, resulting in a disintegration of the zeolite crystals and a breaking of the supercages forming “bowls” which could perfectly fit the Keggin anions (Figure 4
5.2. Applications in Refining (Hydroisomerization of Alkanes, Hydrodesulfurization)
There are some reports on the use of catalysts based on PW12 and platinum on zeolites in the hydroisomerization of alkanes. These catalysts are quite similar to those prepared on more classical supports such as silica. Their main interest is to combine three different functions: the acidity of the polyacid, that of the zeolite and the noble metal.
The hydroisomerization of n
-heptane was studied on PW12
/DUS-Y (dealuminated ultra-stable Y zeolite) [40
]. The catalysts were prepared by impregnation of the US-Y or DUS-Y zeolite with 12-tungstophosphoric acid, followed by wet impregnation with a platinum salt. Although Pt/zeolite catalysts were active for this reaction, an additional synergic effect was observed in the case of the DUS-Y sample in the presence of platinum and PW12
. Unfortunately, the selectivity decreased when the temperature was increased, irrespective of sample composition, and around 300 °C the formation of toluene was also observed. This promotion effect was not observed on US-Y or silica. It was assumed that the polyoxometalate was not located in the supercages of faujasite but in the mesopores of the solid, their mean pore diameter being 1.8 nm for US-Y and 2.3 nm for DUS-Y. In the case of US-Y, the low catalytic activity was explained by assuming that the protons of PW12
had been neutralized by extra framework aluminum, resulting in a poorly active system.
Wei et al.
] studied the hydroisomerization of n
-heptane on HPW12
supported on Hβ zeolite in presence of platinum. As this zeolite can only accommodate spheres of 0.67 nm in diameter and possesses channels ca
. 0.6 nm in diameter [11
], the polyoxometalate cannot fill them and is deposited on the external surface of the crystallites. The zeolite was used as received or dealuminated by a hydrothermal treatment followed by HCl washing. The samples were prepared by impregnation as in [41
] with 0.4 wt % Pt and 5 to 15 wt % HPW12
. The catalytic experiments were made at 250 °C (0.5 g catalyst, weight hourly space velocity 2 h−1
-C7 molar ratio 7.9, pretreatment under hydrogen at 300 °C). Only the XRD patterns were presented and did not show any peak for Pt or HPW12
. Unfortunately, no further characterization of the solids was made. The cracking products were only propane and butane. The catalytic results are given below (Table 2
) where Hβ-m
) is a Hβ zeolite treated hydrothermally at m
°C during n
These results are relatively difficult to explain as they depend on many parameters, with some of them being not known. For example, the amount of acidic sites, the Pt dispersion or the porosity of the samples. Some features can still be deduced from the data, such as the ideal PW12 loading for Hβ is 10 wt %. As the polyoxometalate cannot fill the channels of the zeolite, this value is probably related to the optimal amount of isolated Keggin units. A puzzling feature of the data is the large deviation of the conversion observed for zeolite pretreated at 650 °C for 5 h, as compared to zeolite pretreated at 650 °C for 4 or 6 h. There is probably another unobserved variable responsible for the anomalous result for zeolite pretreated at 650 °C for 5 h, with all the other pretreatments yielding conversions comparable to zeolite without hydrothermal treatment. If one accepts this data point, the results show conversion close to 20%, with a selectivity between 90% and 95%, showing that a pretreatment at 300 °C is sufficient to roughly halve the conversion.
Later, the same group studied the effect of the incorporation of another metal, namely chromium or lanthanum, on the catalytic activity [43
]. The catalysts were characterized by X-ray diffraction (absence of peaks arising from crystalline polyoxometalate), infrared spectroscopy (presence of the bands of the polyoxometalate), BET surface area analysis and H2
adsorption (increase of Pt dispersion when the Cr amount increases). The addition of chromium leads to an increase of both the conversion and the selectivity to isomerization. A study was made as a function of both the Cr and PW12
loading and the reaction conditions (temperature and ratio alkane/catalyst). The conversion could reach 75.3% with 91.3% selectivity to isomerization. This effect was attributed to the improvement of the polyoxometalate, the higher dispersion of platinum and the creation of a “desorption-transfer promoting site”, due to the introduction of chromium in the catalyst. Introduction of lanthanum instead of chromium also led to an improvement of the catalytic activity but the effect was less pronounced.
This study was also extended to the introduction of Cerium in the Pt-PW12
/DUS-Y system with quite the same conclusions, the presence of cerium increasing the dispersion of platinum [44
]. As above, the solids were characterized by X-ray diffraction, infrared spectroscopy, BET surface area analysis and hydrogen chemisorption.
Finally, the same authors reported the synthesis of cesium salts of PW12
supported on dealuminated US-Y and their use in the hydroisomerization of n
]. For that purpose, the desired amount of cesium was first deposited by impregnation of Cs2
on the zeolite, followed by drying and calcination at 550 °C. The desired amount of PW12
and platinum were then introduced by the same methods as above. The solids obtained were characterized by the same techniques as listed above, and additionally NH3
TPD analysis. The key result was the confirmation of the presence of the cesium salt of PW12 by X-ray diffraction. These solids were also active in hydroisomerization of n
-heptane with high activity and selectivity (for example, at 310 °C 76.2% conversion was observed with a selectivity to isomerization of 92.2%).
Kostova et al.
prepared Mo-containing β-zeolite by use of PMo12
and a mechanochemical approach [46
]. The solid was characterized by infrared spectroscopy and Temperature Programmed Reduction (TPR) analysis. It was then used in the hydrodesulfurization of thiophene. The activity was 50% higher than that of a sample prepared by classical impregnation.
5.3. Applications in Organic Chemistry
While the above section was devoted to the transformation of apolar molecules (if one accepts thiophene), where the heteropolyacid is not soluble, the following applications involve polar molecules where it is highly soluble. Some examples which are not from purely organic chemistry but involving polar substrates will also be included here.
Patel et al.
studied the esterification of oleic acid with methanol catalyzed by HPW12
supported on Hβ zeolite [47
]. Heterogeneous acid catalysts comprised of 12-tungstophosphoric acid (10%–40%) and zeolite Hβ were synthesized by classical impregnation from an aqueous solution of HPW12
and subsequent drying at 100 °C. The 30 wt % loaded catalyst was characterized by various physicochemical techniques. The use of this catalyst was explored for biodiesel production by esterification of a free fatty acid, oleic acid, with methanol (Scheme 5
). The effect of various reaction parameters such as catalyst concentration, acid/alcohol molar ratio, and temperature were studied to optimize the conditions for maximum conversion. The catalyst showed high activity in terms of high conversion (84%) and high turnover number, 1048. A kinetic study, as well as a Koros−Nowak test were carried out, and it was found that esterification of oleic acid followed first order kinetics with a calculated activation energy Ea
of 45.2 kJ·mol−1
and a pre-exponential factor A of 5.4 × 104
. As the reaction occurred in methanol, where the heteropolyacid is highly soluble, the question of recycling was very important. The catalyst showed potential for being used as a recyclable catalytic material, with simple regeneration and no significant loss in conversion after recycling observed. As an application, preliminary studies were carried out for biodiesel production from waste cooking oil and using jatropha oil as feedstock.
Later, the study was extended to HSiW12
supported on the same Hβ zeolite [48
]. The catalyst was also used for the trans
-esterification of soybean oil with methanol. Recycling studies were performed and did not show any deactivation of the catalyst.
Srinivas et al.
prepared catalysts by impregnation of 12-tungstophosphoric acid on Y zeolite [49
]. Unfortunately, the Si/Al ratio of the zeolite used for these experiments was not given. The PW12
loading varied from 10 to 25 wt %. The solids were characterized by BET analysis, X-ray diffraction, infrared spectroscopy and NH3
TP. They were used for the etherification of glycerol with t
-butanol. The highest conversion (84%) and highest selectivity to the mono-ether were achieved on the sample containing 20 wt % PW12
, which had also been shown to be the most acidic by TPD of ammonia. The reusability was studied by recovering the catalyst at the end of the reaction by filtration, washing it with methanol and drying at 100 °C for 2 h. No appreciable variation of activity was detected after four cycles. Another important point is that the filtrate of the reaction mixture did not contain tungsten, proving that the polyoxometalate was well anchored on the surface. However, as the Si/Al ratio of the starting zeolite was not given, one cannot exclude a partial decomposition of the zeolite by the polyoxometalate, resulting, for example, in an insoluble aluminium salt of PW12
Zhang et al.
prepared a series of catalysts by immobilization of 12-tungstophosphoric acid and its cesium salt on ultra-stable Y zeolite and its dealuminated form (DUS-Y) and silica materials. The acid was deposited by impregnation while the cesium salts were obtained by a two-step protocol: the desired amount of cesium was first introduced by impregnation of cesium carbonate followed by calcination, after which the polyacid was then introduced by a second impregnation. The different materials were characterized by various physicochemical methods including X-ray powder diffraction, BET surface area analysis, 31
P and 29
Si MAS NMR spectroscopy and SEM. The early work reported that they were tested in the liquid-phase esterification of acetic acid with butanol [50
]. They were then used in the synthesis of fructone by acetalization of ethylacetoacetate with ethylene glycol [51
] (Scheme 6
Various parameters were studied (amount of PW12, of Cs, reaction conditions, etc.). The best systems were those based on the Cs salts of PW12 supported on dealuminated US-Y zeolite and a conversion as high as 98.7% with a selectivity of 97% to fructone could be achieved over 30% Cs2.5H0.5PW12/DUS-Y. A study of leaching by treatment with water showed that the systems based on the pure heteropolyacid were not stable, with most of the Keggin units being released in water. In contrast, those based on the cesium salt did not show a significant leaching.
The same authors also studied the synthesis of fructone-B by acetalization of ethylacetoacetate with 1,2-propanediol and observed similar results, the Cs salts being active with minimal leaching [54
]. In another paper [55
], they used these catalysts for the liquid-phase esterification of acetic acid with n
-butanol. The conversion was higher than that observed when using the zeolite or the polyoxometalate alone. Again, the supported cesium salts exhibited only minimal leaching, leading to only a small decrease of the catalytic activity upon recycling.
Moosavifar reported the use of heteropolyacids supported on Y zeolite for the synthesis of dihydropyrimidinones by the Biginelli reaction [56
] (Scheme 7
were deposited on a Y zeolite dealuminated by treatment with HClO4
. Good yields were obtained for all three systems within a few hours and the catalysts could be recycled after washing with hot water and ethanol. Such behavior is surprising, as the heteropolyacids are highly soluble in these polar solvents. However, the catalytic reaction involves urea, which is probably protonated by the polyacid, leading to the formation of an insoluble salt. The synthesis of such a species has been reported recently [57
]. Due to the great size of the molecules, polyacids encapsulated in Y zeolite such as those described in [20
] were inactive for this reaction.
More recently, Narkhede and Patel studied this reaction over HPW12
supported on Hβ zeolite [58
]. The catalysts were those reported previously [47
] and led to higher yields in smaller times (for example a 98% yield in 15 min instead of a 97% yield in 360 min). The reaction was then extended to other substrates (Table 3
Nandiwale et al.
prepared a catalyst based on HPW12
supported on desilicated H-ZSM-5 and studied it in the esterification of levulinic acid by ethanol, in view of the synthesis of ethyl levulinate, which can be used as diesel miscible biofuel [59
]. The desilification was performed by treatment with NaOH at various concentrations (from 0.2 to 1.5 M) during 30 min at 65 °C. The resulting solids possessed an increased surface area with a high number of mesopores where HPW12
could be deposited by the incipient wetness technique. These solids were used in the esterification of levulinic acid and various parameters were studied (relative amounts of reactants, of catalyst, temperature, speed of agitation, etc
.) including the recycling of the catalyst (reused directly after filtration). Unfortunately, no study of leaching was made and even if a kinetic law was proposed, the high conversions (more than 80%) prevented some conclusions to be made.