This section presents the most relevant results of the activity and catalytic properties of the catalysts used for the production of 1,3-PG, differentiating them according to the metal phase and the type of reactor, both in the liquid and vapor phase.
3.1. Pd, Ru, and Rh Catalysts
The first results with a certain tendency to form 1,3-PG were obtained using homogeneous phase catalysts based on Pd, Ru, and Rh as the active phase.
Table 1 summarizes the results obtained in the liquid phase batch reactors.
One of the first references dates back to a patent developed by Che, who reported that by employing a catalyst composed of Rh and H
2WO
4 in the presence of 1-methyl-2-pyrrolidinone as the solvent, it is possible to achieve a glycerol conversion of 47% with a selectivity to 1,3-PG of 43% at 200 °C and 31.6 MPa of a CO/H
2 (1:2) mixture after 24 h of reaction [
71].
In another patent, Drent et al. employed a Pd-1,2-bis(1,5-cyclooctylenphosphino) ethane (Pd-BCPE) catalyst in a water/sulfolane (10:10) medium at 140 °C and 6 MPa of a CO/H
2 mixture (20:40) and obtained selectivity values to 1,3-PG of about 30% with glycerol conversions of about 7%, leading to low yields of 1,3-PG (~2%) [
72].
Bullock et al. reported the use of homogeneous Ru catalysts in a sulfolane medium for the production of 1,3-PG. Under moderate temperature and pressure conditions, 110 °C and 5.2 MPa H
2, they obtained very low yields to 1,3-PG (~3%) [
73].
The difficulty in separating the homogeneous catalyst and recovering it from the reaction product mixture has led to the consideration of heterogeneous phase catalytic systems.
Given the superior activity of Rh-based homogeneous catalysts, the first supported catalysts used Rh as the active phase.
Table 2 summarizes the main results obtained with these catalysts used in liquid-phase batch reactors.
Chaminand et al. tested Rh/C catalysts in the presence of H
2WO
4 and found very low activity levels for the formation of 1,3-PG. However, their work revealed that, unlike other acids such as HCl, the addition of H
2WO
4 favored the first stage of development of the intermediate in the formation of 1,3-PG [
57]. Later, Kunosoki et al. tested the activity of Ru/C, Pt/C, Pd/C, and Rh/C catalysts in the hydrogenolysis of glycerol to 1,2-PG and 1,3-PG [
56]. With respect to the hydrogenating activity, they found the following order of activity:
Following the work of Chaminand et al., the results by Kunosoki et al. confirmed that Rh is an interesting metal for obtaining 1,3-PG due to the selectivity values obtained with respect to Ru, Pt, and Pd. By also adding H
2WO
4 to the reaction medium in the presence of Ru/C and Rh/C, they obtained a marked increase in the selectivity to 1,3-PG in the presence of Rh/C, suggesting the existence of a Rh–W bond that favors the production of glycol [
56]. Surprisingly, there have been no studies following up on this idea.
Furikado et al. tested Rh/SiO
2 catalysts that were equally selective to 1,3-PG as Rh/C catalysts. In the presence of an Amberlyst resin in the reaction medium, the activity levels improved, but there was no substantial change in selectivity to 1,3-PG [
55]. Other Rh-based systems on heteropolyacids, Cs
2.5H
0.5[PW
12O
40], also showed some tendency to form 1,3-PG [
74]. However, in all cases, the yield to 1,3-PG was very low (<4%).
Higher, but still low yields (12%) were obtained by employing a ReO
x-modified Rh/SiO
2 catalyst. The results indicated that the acidic properties of the ReO
x particles favored the formation of the reaction intermediate, leading to the formation of 1,3-PG following the direct mechanism of hydrogenolysis [
75].
These results, although showing low yields to 1,3-PG, proved that the presence of an active hydrogenating phase and acidic chemical species such as H2WO4 or ReOx particles is promising for the development of improved catalysts for the production of 1,3-PG.
3.2. Ir Catalysts
Ir catalysts and their bimetallic combinations with Ni, Co, Re, and Ru have been reported with some activity for the formation of 1,3-PG. Reports have employed these catalysts in the liquid phase using batch reactors (
Table 3).
Low yields (<1%) were obtained by employing Ir/γ-Al
2O
3 catalysts and their modification with Ni, Ir-Ni/γ-Al
2O
3, even though the presence of the metal produced an increase in glycerol conversion. For these catalysts, the species responsible for the activity were IrO
2 and IrCl
x coming from the decomposition of the precursor H
2IrCl
6.xH
2O [
76]. In contrast, Co–Ir nanorods (Co:Ir = 100) were shown to be more active toward the formation of 1,3-PG, essentially due to the morphology of these structures and the exposure of their {10-10} crystalline planes [
77].
Ir–Re based catalytic systems such as Ir-ReO
x/SiO
2 showed even higher yields due to a promoting effect by Re [
59,
60,
61,
78,
79,
86,
87]. The presence of ReO
x particles results in the formation of 2,3-dihydroxypropoxide from glycerol. Then, gaseous hydrogen, activated on the metal sites of Ir under the hydride form, attacks the 2-position of 2,3-dihydroxypropoxide to form 3-hydroxypropoxide, which finally hydrolyzes to lead to the formation of 1,3-PG (
Figure 8) [
59].
Modifying the Ir/SiO
2 catalyst with other oxides based on W, Mo, V, Cr, Mn, and Ag, Amada et al. found that the selectivity to 1,3-PG varied in the following order:
Their study determined that the cleavage of the C–O bond occurred at the interface between the metal surface of the Ir and the ReO
x particles, where the substrate was adsorbed [
60]. Unlike other promoters, ReO
x allows for the formation of a terminal alkoxide, leading to the formation of 1,3-PG instead of a secondary alkoxide that would favor the formation of 1,2-PG [
61].
In their study, they further proved that by keeping the Re promoter but changing the metal phase to Rh, Ru, and Pt, the order of activity was as follows:
Finally, they concluded that the performance of Ir-ReO
x/SiO
2 was superior to that presented by the Ir/SiO
2 and ReO
x/SiO
2 monometallic catalysts [
60].
To understand a little more about their promoter effect, Gilkey et al. studied the structural properties of ReO
x particles on the Ir-ReO
x/SiO
2 catalysts. During preparation, the catalyst precursor is in the form of IrO
2-ReO
x/SiO
2 with Ir having a +4 oxidation state that is reduced to Ir
0 after reduction. The ReO
x species, on the other hand, are partially reduced and remain with an average oxidation state of +2.6 after reduction [
86]. Amada et al. demonstrated that, once reduced, the surface of such catalysts was covered by metallic Ir particles forming cube-octahedrons covered by ReO
x particles. The presence of Re improved the reducibility of the catalyst due to the interaction between Re and IrO
2 during the calcination process. In addition, the performance of the Ir-ReO
x/SiO
2 catalysts was attributed to the active sites formed by metal bonds between metallic Ir and ReO
x species (
Figure 9) [
87].
In the presence of another metal such as Ru, the Ir-ReO
x/SiO
2 catalysts showed enhanced activity levels toward the formation of 1,3-PG. The presence of Ru at a given concentration dissociates H
2 molecules and transfers H species via a spillover mechanism toward the Ir-ReO
x particles. These species form protons and facilitate the formation of 1,3-PG (
Figure 10) [
80]. Surprisingly, Ru-Re/SiO
2 catalysts were found to be less active and selective than the Ir-ReO
x/SiO
2 catalysts, reinforcing the idea that Ru acts as a promoter in the hydrogenolysis reaction [
88].
In order to explore the reaction mechanism of hydrogenolysis using Ir-ReO
x/SiO
2 catalysts, Varghese et al. carried out an experimental and simulation study in liquid phase batch reactors. The characterization of the catalysts by photoelectron spectroscopy (XPS) revealed the presence of metallic Ir and ReO
x particles with multiple oxidation states (0, +4, +7). A catalyst structural model was developed based on this study, considering that on metallic Ir, nanoparticles are dispersed ReO
x particles with an average oxidation state of +4 for Re. Their results also revealed that once the catalyst was reduced, the ReO
x particles constituted Brønsted acid sites with Ir-H
3Re
3O
6 type clusters [
89].
In addition to SiO
2, other supports were studied to support the Ir-ReO
x couplet. In this regard, Liu et al. investigated Ir-ReO
x catalysts supported on anatase TiO
2, rutile TiO
2, C, ZrO
2, CeO
2, SiO
2, Al
2O
3, and MgO. Of all the materials used as supports, rutile TiO
2 was found to be the most active. The results were attributed to the stabilization and dispersion of the Ir-ReO
x particles on the rutile TiO
2 surface, which allowed for an increase in the number of active sites. In these, the Ir particles interacted with the partially oxidized ReO
x clusters (average Re oxidation state: +3) covering almost the entire rutile TiO
2 surface [
81].
From the zeolite group, Wan et al. prepared IrO
x/H-ZSM-5 catalysts in the absence of ReO
x. Their results showed that the activity is a function of Ir dispersion, Ir (0)/Ir (III) ratio, and Brønsted acid site content. The activity was found to be linear as a function of the content of Brønsted acid sites while the selectivity to 1,3-PG increased with the presence of this type of site, denoted as Ir-O(H)-H-ZSM-5. These sites are generated by the synergistic interaction between IrO
x and H-ZSM-5 species through a hydrogen spillover. The authors proposed a direct mechanism for the formation of 1,3-PG on IrO
x/H-ZSM-5, considering that glycerol adsorption occurs on IrO
x particles and H
2 activation takes place on the Si–Al sites of the H-ZSM-5 zeolite (
Figure 11) [
82].
Recently, Chanklang et al. prepared Ir-ReO
x catalysts supported on H-ZSM-5, which showed high metal dispersion with a decrease in the Ir particle size by the addition of Re. The results indicated that the activity is a function of the Ir metal content and Re/Ir molar ratio. However, even by optimizing the reaction conditions (temperature, reaction time, and catalyst concentration), the catalysts were found to be poorly selective at 1,3-PG (~19%) with very low yields (~2.8%) due to excessive hydrogenolysis of the glycol to form 1-POH. The authors proposed a mechanism similar to the direct mechanism of hydrogenolysis used to explain the results in Ir-ReO
x/SiO
2 (
Figure 12) [
83].
Other Ir–Re systems on mesoporous silica and SiO2–Al2O3 supports were also studied from the structural point of view.
Deng et al. studied the reduction of Ir–Re (Ir:Re = 1) catalysts supported on a mesoporous silica of the KIT-6 type in the range of 400 to 700 °C. Their results showed an improvement in glycerol conversion with an increasing reduction temperature up to 600 °C due to an increase in the number of acid sites that produced a higher Ir–Re interaction. For temperatures between 600 and 700 °C, a decrease in the activity of these catalysts was observed due to a growth in the size of the metal particles [
84]. In another work by these authors, they compared the Ir-Re/KIT-6 catalyst prepared by direct reduction at 500 °C of (Ir-Re/KIT-6-R) with the same precursor, but previously calcined in air at 500 °C (Ir-Re/KIT-6-CR). The results showed that the direct reduction in a H
2 atmosphere allowed for the generation of metallic Re particles that together with Ir formed a much more active Ir–Re alloy than the Ir-ReO
x phase formed during calcination, because the ReO
x particles cannot be easily reduced once the oxide is formed. In the presence of an Amberlyst-15 type resin, it is possible to reach the maximum yield to 1,3-PG [
90].
In Ir–Re catalysts supported on silica-alumina, Ir-Re/SiO
2-Al
2O
3, it has been reported that the presence of Al promotes the dispersion of Re particles and facilitates the formation of large Ir crystals, leading to low Ir–Re interaction and, consequently, low activity levels. When the Al content in the support, SiO
2–Al
2O
3 (–Al), is reduced, the Re particles form an alloy with the Ir particles due to the proximity between them and their interaction, which promotes higher activity levels and selectivity to 1,3-PG (
Figure 13) [
85].
Despite the efforts made with these higher complexity supports, the results show that the highest yields to 1,3-PG (~38%) were obtained with Ir-ReOx/SiO2 catalysts at moderate temperature (120 °C) and pressure (8 MPa) conditions operating in liquid phase batch reactors.
3.3. Pt Catalysts
Pt catalysts were used for the production of 1,3-PG in both the liquid and vapor phases.
Table 4 and
Table 5 summarize the main catalysts tested in the liquid phase using batch and continuous flow reactors, respectively.
Of the set of noble metals, Ru, Pd, Rh, Ir, and Pt, the latter has shown to be the most promising one for obtaining 1,3-PG, since, unlike the other metal phases, Pt acts not only as an active site for hydrogenation, but also facilitates the cleavage of the secondary C–O bond in the glycerol molecule [
91].
Studies have mainly focused on the modification of traditional supports to prepare Pt catalysts, leading to the formation of 1,3-PG. The modification of supports was carried out using W and Re species, but, even so, the yields achieved were less than 70%.
The first studies were based on the modification of Pt catalysts using Re. In this sense, the Pt-Re/C catalysts were found to be more active and selective toward the formation of 1,3-PG than monometallic Pt/C and Re/C catalysts due to the formation of a Pt–Re alloy with an average metal particle size smaller than 2 nm [
92]. The same Pt–Re combination was supported on SiO
2, C, and SiO
2 modified with W. The study revealed that, although Pt was completely reduced in the alloy, Re was oxidized with a distribution of oxidation states, conferring a Brønsted-type acidity to the Pt–Re particles, which influenced the selectivity to 1,3-PG [
93].
With the aim of generating Brønsted acid sites, other authors have prepared Pt catalysts modified with heteropolyacids (HTPA) and tungsten oxides (WOx).
Table 4.
The hydrogenolysis of glycerol to 1,3-PG using Pt catalysts operating in liquid phase batch reactors.
Table 4.
The hydrogenolysis of glycerol to 1,3-PG using Pt catalysts operating in liquid phase batch reactors.
Catalyst | T (°C) | P (MPa) | mgly/mc | t (h) | X (%) | S1,3-PG (%) | Y1,3-PG (%) | Ref. |
---|
Pt/WOx | 180 | 5.5 (H2) | 4.0 | 12 | 18.0 | 39.3 | 7.0 | [63] |
Pt/WOx/AlOOH | 180 | 5.0 (H2) | 0.9 | 12 | 100.0 | 66.0 | 66.0 | [64] |
Pt/WOx/Al2O3 | 200 | 9.0 (H2) | 6.0 | 4 | 62.5 | 61.5 | 38.5 | [66] |
Pt/WOx/SBA-15 | 150 | 4.0 (H2) | 3.0 | 30 | 86.8 | 70.8 | 61.5 | [67] |
Pt/WO3/ZrO2 | 170 | 8.0 (H2) | 2.7 | 18 | 85.8 | 28.2 | 24.2 | [91] |
Pt-Re/C | 170 | 4.0 (H2) | NR | 8 | 45.0 | 29.0 | 13.0 | [92] |
Pt/Al2O3 + H4SiW12O40 | 200 | 4.0 (H2) | 0.5 | 18 | 49.0 | 28.0 | 13.7 | [94] |
Pt/H4SiW12O40-Al2O3 | 240 | 6.0 (H2) | 25.0 | 8 | 45.3 | 10.5 | 4.8 | [95] |
Pt/H4SiW12O40-m-Al2O3 | 200 | 4.0 (H2) | 0.9 | 15 | 60.5 | 33.3 | 20.1 | [96] |
Pt/WO3/TiO2/SiO2 | 180 | 5.5 (H2) | 2.0 | 12 | 15.3 | 50.5 | 7.7 | [97] |
Pt/Al-WOx | 160 | 3.0 (H2) | 2.0 | 30 | 79.0 | 40.6 | 32.1 | [98] |
Pt/WOx | 140 | 1.0 (H2) | 2.0 | 12 | 37.4 | 35.1 | 13.1 | [99] |
Pt-La/WOx | 140 | 1.0 (H2) | 2.0 | 12 | 39.9 | 41.3 | 16.4 | [99] |
Pt-Au/WOx | 140 | 1.0 (H2) | 1.0 | 12 | 81.4 | 51.6 | 42.0 | [100] |
Pt/Au/WO3 | 155 | 5.0 (H2) | 2.0 | 12 | 55.0 | 41.0 | 22.5 | [101] |
Pt/WOx/ZrO2 | 140 | 8.0 (H2) | 6.0 | 24 | 76.2 | 64.8 | 49.4 | [102] |
Pt/WOx/Al2O3 | 140 | 0.5 (H2) | 0.6 | 6 | 35.8 | 29.0 | 10.4 | [103] |
Pt/WOx/Al2O3 | 220 | 6.0 (H2) | 12.0 | 5 | 78.0 | 42.0 | 32.8 | [104] |
Pt/WOx/γ-Al2O3 | 200 | 5.5 (H2) | 3.3 | 4 | 65.0 | 50.0 | 32.5 | [105] |
Pt/WOx/α-Al2O3 | 180 | 5.0 (H2) | 9.0 | 24 | 59.5 | 43.9 | 26.1 | [106] |
Pt/W+Al/SBA-15 | 160 | 6.0 (H2) | 1.5 | 12 | 66.0 | 50.0 | 33.0 | [107] |
Pt/WOx/T-Ta2O5 | 160 | 5.0 (H2) | 0.9 | 24 | 87.0 | 45.6 | 39.7 | [108] |
Pt/WOx/Ta2O5 | 160 | 5.0 (H2) | 9.0 | 24 | 78.0 | 38.0 | 29.6 | [109] |
Pt/sulfated ZrO2 | 170 | 7.3 (H2) | NR | 24 | 66.5 | 83.6 | 55.6 | [110] |
Pt/Mn/WOx/ZrO2 | 180 | 8.0 (H2) | 8.0 | 18 | 56.2 | 42.0 | 23.6 | [111] |
Pt/WOx/SiO2/ZrO2 | 180 | 5.0 (H2) | 5.0 | 12 | 90.1 | 44.5 | 40.1 | [112] |
Pt/WO3/ZrP | 200 | 4.0 (H2) | 1.0 | 12 | 92.3 | 20.9 | 19.3 | [113] |
Pt-Ru/WOx/Al2O3 | 170 | 9.0 (H2) | 3.0 | 36 | 90.4 | 43.0 | 38.9 | [114] |
Pt-Au/WOx/Al2O3 | 180 | 5.0 (H2) | 4.0 | 12 | 57.0 | 41.4 | 23.5 | [115] |
Pt/W-MCFs | 150 | 4.0 (H2) | 3.0 | 24 | 100.0 | 65.0 | 65.0 | [116] |
Pt/WOx/S-TiO2 | 120 | 4.0 (H2) | 1.3 | 8 | 100.0 | 36.0 | 36.6 | [117] |
Pt/WOx/TiO2 | 120 | 4.0 (H2) | 1.3 | 8 | 57.0 | 66.0 | 37.0 | [117] |
Table 5.
The hydrogenolysis of glycerol to 1,3-PG using Pt catalysts operating in liquid phase continuous flow reactors.
Table 5.
The hydrogenolysis of glycerol to 1,3-PG using Pt catalysts operating in liquid phase continuous flow reactors.
Catalyst | T (°C) | P (MPa) | SV (h−1) | X (%) | S1,3-PG (%) | Y1,3-PG (%) | Ref. |
---|
Pt/WO3/ZrO2 | 130 | 4.0 (H2) | 0.25 (LHSV) | 70.2 | 45.6 | 32.0 | [62] |
Pt/H4SiW12O40-SiO2 | 200 | 6.0 (H2) | 0.045(WHSV) | 81.2 | 38.7 | 31.4 | [118] |
Pt-H2Li2SiW12O40/SiO2 | 180 | 5.0 (H2) | 0.09 (WHSV) | 43.5 | 53.6 | 23.3 | [119] |
Pt-H3PMo12O40/SiO2 | 180 | 5.0 (H2) | 0.09 (WHSV) | 27.1 | 7.8 | 2.1 | [120] |
Pt-H3PW12O40/SiO2 | 180 | 5.0 (H2) | 0.09 (WHSV) | 25.5 | 32.9 | 8.4 | [120] |
Pt-H4SiW12O40/SiO2 | 180 | 5.0 (H2) | 0.09 (WHSV) | 24.1 | 48.1 | 11.6 | [120] |
Pt/WOx/Al2O3 | 160 | 5.0 (H2) | 0.09 (WHSV) | 64.2 | 66.1 | 42.4 | [121] |
Pt/WOx/Al2O3 | 160 | 5.0 (H2) | 1.02 (WHSV) | 75.2 | 44.0 | 33.1 | [122] |
Pt/WOx/Al2O3 | 160 | 5.0 (H2) | 1.02 (WHSV) | 80.5 | 47.4 | 38.2 | [123] |
Pt/WOx/SiO2/ZrO2 | 180 | 5.0 (H2) | 1.00 (WHSV) | 54.3 | 52.0 | 28.2 | [124] |
Pt/WOx-ZrO2-TiO2 | 140 | 5.0 (H2) | 0.50 (WHSV) | 75.0 | 33.7 | 25.3 | [125] |
Pt-Li2B4O7/WOx/ZrO2 | 150 | 4.0 (H2) | 0.20 (WHSV) | 90.0 | 47.4 | 42.7 | [126] |
Pt/Mg/WOx/ZrO2 | 150 | 4.0 (H2) | 0.20 (WHSV) | 52.6 | 60.7 | 32.1 | [127] |
Pt/WOx/Al2O3 | 180 | 5.0 (H2) | 1.00 (LHSV) | 80.4 | 35.3 | 28.8 | [128] |
With respect to the use of HTPA, H
4SiW
12O
40 was found to be the most effective in producing 1,3-PG, either by the addition to the reaction medium in the presence of Pt/Al
2O
3 catalysts [
94], or by the modification of supports such as SiO
2 [
118], ZrO
2 [
119,
120], Al
2O
3 [
95], and ordered mesoporous Al
2O
3 (m-Al
2O
3) [
96]. The results indicated that the modification of these supports with the heteropolyacid enhanced the dispersion of Pt and conferred the appropriate acidity for the formation of the intermediate in the production of 1,3-PG. In this respect, it has been verified that there is a linear correlation between the yield to 1,3-PG and the surface concentration of Brønsted acid sites provided by the heteropolyacids. This trend was verified using Pt-H
4SiW
12O
40/ZrO
2 [
119], Pt-H
3PW
12O
40-ZrO
2 [
120], and Pt-H
3PMo
12O
40/ZrO
2 [
120].
With respect to modification with tungsten oxides (WO
x), it has been shown that the presence of these oxides endows the support with Brønsted acid sites for the adsorption of glycerol and the subsequent formation of an alkoxide intermediate in the formation of 1,3-PG, following the direct mechanism of hydrogenolysis [
97]. The tungsten redox cycle, W
+6 ↔ W
+5, further allows in situ regeneration of the surface sites after the formation of 1,3-PG [
129]. In the Pt/WO
x catalysts, the interaction between Pt particles and metallic H
2 produces a hydrogen spillover from Pt to WO
x. The presence of oxygen vacancies in the WO
x facilitates the mechanism of heterolytic cleavage of H
2 into H
+ and H
− ions, which are necessary in hydrogenolysis for the formation of 1,3-PG from the alkoxide intermediate (
Figure 14) [
63].
In situ ATR-IR studies revealed that WO
x plays three fundamental roles in the formation of 1,3-PG: (1) adsorption of the primary –OH group of glycerol; (2) provision of H+; and (3) stabilization of the intermediate carbocation in the formation of 1,3-PG [
66].
Recently, Yang et al. carried out characterization studies by photoelectron spectroscopy and hydrogen chemisorption and determined that the catalytic activity of Pt/WO
x was from the oxygen vacancies found in the WO
x species. By preparing Pt/Al-WO
x catalysts, they determined that by modulating the content of atomically dispersed AlO
x species on the WO
x support, it was possible to generate a higher content of oxygen vacancies and thus promote the formation of 1,3-PG (
Figure 15) [
98].
In order to promote the activity of the Pt/WO
x catalysts, studies on the addition of promoters such as La [
99] and Au [
100] have been reported. Both strategies improved the activity and selectivity toward 1,3-PG due to the extra generation of Brønsted acid sites. In the case of Pt-Au/WO
x, the presence of Au enhanced the dissociative adsorption of H
2 [
100]. Yang et al. characterized Pt/Au/WO
3 catalysts prepared by a surface-modified deposition method and determined that the presence of Au promotes the reduction of Pt and W at low temperatures while making the dispersion of Pt uniform on the WO
3 support. These changes in the electronic properties allow for an improvement in the activity and selectivity of the catalyst [
101].
To increase the dispersion of WO
x particles and improve the catalytic properties, Pt/WO
x catalysts supported on ZrO
2 [
62,
91,
102], Al
2O
3 [
103,
104,
105,
106,
121,
122,
123], AlOOH [
64], SBA-15 [
67,
107], and Ta
2O
5 [
108,
109] were prepared.
With respect to the Pt/WO
x/ZrO
2 catalysts, their activity in the isomerization of alkanes is well-known [
130,
131,
132,
133]. On the surface of these catalysts, H
2 molecules are transformed into protons (H
+) and hydrides (H
−) via heterolytic cleavage [
134,
135]. The hydrogenolysis reaction of glycerol to form 1,3-PG consists of a substitution reaction, in which an H atom replaces the secondary –OH group in the glycerol molecule. For this reason, a possible sequence is the two-step reaction of glycerol with H
+ and H
−, which produces water and the substitution of the secondary –OH group by H [
136,
137]. Following this idea, Pt/WO
x/ZrO
2 catalysts [
62,
91,
102] were used for the hydrogenolysis of glycerol to 1,3-PG.
Compared to other traditional supports, ZrO
2 is one of the most interesting to combine with Pt as the metal phase and to generate Brønsted acid sites. For example, the modification of ZrO
2 with sulfonic groups allows for the preparation of Pt catalysts that showed yields of 55% to 1,3-PG due to the generation of Brønsted acid sites [
110]. It has also been indicated that the tetragonal phase of ZrO
2 is more active than the monoclinic phase to be used as a support, since it generates higher metal dispersion and Brønsted acid sites of higher acidic strength [
102].
In comparison to catalysts such as Pt/WO
3/TiO
2, Pt/WO
3/HY, and Pt/WO
3/AlMCM-41, the Pt/WO
3/ZrO
2 catalyst proved to be the best alternative, reaching the maximum yield to 1,3-PG [
91]. In these catalysts, the formation of 1,3-PG is structurally sensitive to the degree of polymerization of WO
x species. The Pt-(WO
x)
n-H type active sites provide a strong Brønsted acidity that promotes the formation of 1,3-PG [
111].
With the aim of improving the catalytic properties, modifications of Pt/WO
x/ZrO
2 catalysts including the use of SiO
2 [
112,
124], TiO
2 [
125], Li
2B
4O
7 [
126], Mn [
111], Mg [
127], and P [
113] have been reported.
Pt/WO
x/SiO
2/ZrO
2 catalysts resulted in being more active and selective to 1,3-PG due to a promoting effect of SiO
2 that favors not only the dispersion of Pt, but also of the WO
x species [
124]. This effect has been observed in similar Pt/WO
x/TiO
2/SiO
2 catalytic systems [
97]. The presence of an adequate SiO
2 content favors the transformation of crystalline WO
3 species into polytungstate species, which are more active and allow for an increase in the yield to 1,3-PG. When the SiO
2 content exceeds the optimum, the polytungstate species are transformed into monotungstate species that decrease the yield to 1,3-PG (
Figure 16a). This leads to obtaining a maximum yield to 1,3-PG as a function of the SiO
2 content (
Figure 16b) [
124].
Zhao et al. proved that the presence of bibridged (OH
II) and tribridged hydroxyl groups (OH
III) on the SiO
2/ZrO
2 support could react with WO
x species forming Zr–O–W bands and retarding the agglomeration of WO
x species. The high dispersion of WO
x species in conjunction with Zr–O–W bonds benefit the formation of Brønsted acid sites that promote the formation of 1,3-PG [
112].
Xi et al. prepared a series of Pt/WO
x-ZrO
2-ZrO
2-TiO
2 catalysts from an evaporative assembly method using different Ti/Zr molar ratios. The authors detected the presence of ZrTiO
4 and Ti
2ZrO
6 phases, responsible for the activity in the hydrogenolysis of glycerol. For a Ti/Zr = 7:3 molar ratio, the catalyst showed the maximum yield to 1,3-PG (~25.3%) [
125].
The modification of Pt/WO
x/ZrO
2 catalysts with Li
2B
4O
7 in the 0.5–2 wt.% range allowed for a better dispersion of the Pt particles and increased the total number of acid sites, thus increasing the interaction between the metal phase and the support. At 150 °C and 4 MPa H
2, the catalyst with 2 wt.% Li
2B
4O
7 allowed for obtaining the maximum yield to 1,3-PG (~42.7%), being stable for more than 200 h [
126].
Doping Mn into the Pt/WO
x/ZrO
2 catalysts allowed for the generation of a large number of WO
x species with some intermediate degree of polymerization, which increased the number of Pt-(WO
x)
n-H sites promoting the formation of 1,3-PG [
111]. The same effect was found with the incorporation of Mg to the Pt/WO
x/ZrO
2 catalysts. In this case, the formation of small WO
x species with some degree of polymerization was active for the formation of 1,3-PG [
127].
The preparation of Pt/WO
3/ZrP catalysts also led to good yields to 1,3-PG, but lower in comparison to Pt/WO
x/ZrO
2 catalysts. The co-presence of Pt and W in the catalyst composition led to a synergistic enhancement in the catalytic activity and 1,3-PDO selectivity. Highly dispersed Pt nanoparticles in contact with polytungstate-like WO
x species were active centers for the hydrogenolysis reaction [
113].
With respect to the modification of Al
2O
3 with WO
x, it has been reported that the surface density of W determines the type of W species on the support surface and thus the selectivity to 1,3-PG. In the Pt/WO
x/Al
2O
3 catalysts, the presence of monotungstate, polytungstate, and mesoporous WO
3 species generates the Brønsted acid sites necessary for the cleavage of the secondary C–O bond of the glycerol molecule [
121]. In this regard, it has been reported that the transformation of WO
3 to polytungstate is required to achieve the highest yields to 1,3-PG [
124]. The combination of polytungstate species with highly dispersed Pt particles leads to the formation of alkoxide intermediates and their subsequent hydrogenation to form 1,3-PG (
Figure 17) [
65].
It has also been reported that an excessive number of tungsten species can lead to the formation of undesired products such as 1-POH, 2-POH, MeOH, and gases [
138]. Thus, a proper balance between Pt, W species, and support is required, leading to the need for an optimization of the W content in the catalyst. In this regard, Lei et al. studied various compositions of Pt and WO
x species on Al
2O
3. Their results determined that for a fixed Pt content (2 wt.%), an increase in the W concentration up to 7.5 wt.% causes an increase in the selectivity to 1,3-PG; if the W concentration continues to increase, the selectivity to 1,3-PG decreases by the generation of products such as 1-POH. Similarly, maintaining a fixed W content (12.9 wt.%), an increase in the Pt concentration up to 6 wt.% causes an increase in glycerol conversion, while for higher contents of the metal, there is a drop in activity due to a decrease in the metal dispersion. The results of this work indicate that for each W content and optimum Pt content (or vice versa), it is required to achieve the highest levels of glycerol conversion and selectivity to 1,3-PG, which implies maximizing the yield to the glycol of interest, as shown in
Figure 18 [
128].
Zhao et al. developed a strategy to prepare Pt catalysts supported on Al
2O
3 containing highly dispersed WO
x species by employing high-temperature heat treatment. In their work, these authors attributed the catalytic efficiency to the strong interaction between isolated WO
4 species and the Pt metal phase, which would be responsible for generating Brønsted acid sites in situ, according to the results of the IR spectroscopy and NH
3 adsorption–desorption [
106].
In order to improve the properties of the Pt/WO
x/Al
2O
3 catalyst, Wen et al. studied the modification of the metal phase with Ru. The authors found that the presence of Ru favored the redox cycle W
+6 ↔ W
+5, which allowed for an improvement in the adsorption of H
2 and an increase in the number of Brønsted acid sites of medium acidic strength, leading to the attainment of a higher yield to 1,3-PG [
114].
Similar results were obtained with the Pt-Au/WO
x/Al
2O
3 catalysts. In these systems, the addition of Au, which exists on the catalytic surface mostly as single atoms, enhances the hydrogen spillover capacity, decreasing the interaction between the Pt and WO
x species, promoting Pt dispersion (~2 nm) and increasing its exposed area. Since the spillover capacity is directly linked to the number of Brønsted acid sites, the presence of Au favors the production of 1,3-PG [
115].
For catalysts of similar characteristics, but based on bohemite (AlOOH), a reaction mechanism through alkoxide type intermediate species has been proposed, in whose formation Pt, WO
x, and AlOOH type oxides are cooperatively involved. Initially, glycerol is adsorbed on the surface sites of the bohemite through the primary –OH group, forming alkoxide species of the HOCH
2CH(OH)CH
2OAlO
x type. Next, the secondary –OH group of the alkoxide species is protonated on the acid sites of the WO
x and dehydrates, producing a secondary carbocation. This carbocation is attacked by hydride species generated on the WO
x sites due to the H
2 spillover produced by the interaction between the Pt particles and the metallic H
2. Finally, the alkoxide species are hydrolyzed to generate 1,3-PG and the catalytic cycle is completed. On the other hand, the surface of the bohemite plays a fundamental role as it stabilizes the Pt particles and the W oxides [
64]. With this catalyst, it is possible to reach a 66% yield to 1,3-PG, which is the highest yield reported to date.
Recently, yields very close to that value (~65%) have been reported by Cheng et al., who prepared Pt catalysts (Pt/W-MCFs) supported on tungsten-doped siliceous mesocellular foams (W-MCFs). The synergy between the Pt nanoparticles and the tetrahedral sites of W
VO
4 species at the oxide–metal interface were responsible for the catalyst activity as well as the 3-dimensional continuous large pore structure of the W-MCFs, which facilitates the entry of the reactant and the escape of the product [
116].
With respect to the Pt/WO
x/SBA-15 catalysts, the addition of W produces WO
4, which is incorporated into the SBA-15 matrix as a Lewis acid site. The H
2 adsorbed on Pt is transferred by spillover to the W oxide, which is transformed into a Brønsted acid site, allowing for the formation of the intermediate and then of 1,3-PG [
67].
Feng et. al. also prepared Pt/SBA-15 catalysts, but modified with W and Al, which were homogeneously incorporated into the SBA-15 matrix using a one-step synthesis method under an acidic medium. The joint presence of W and Al allowed for an increase in the synergism effect between the Lewis and Brønsted acid sites, which were tightly related to the incorporation of W and Al species on the modified SBA-15. According to these authors, glycerol adsorbs on the catalytic surface at the WO
x sites to form a terminal alkoxide, which then dehydrates and hydrogenates to finally form 1,3-PG. Naturally, the adsorption and activation of H
2 takes place on the Pt metal sites (
Figure 19) [
107].
Recently, Zhou et al. tested Pt/WO
x/TiO
2 catalysts that were found to be active and selective to 1,3-PG, showing 37% yields for 57% glycerol conversions at 120 °C and 4 MPa H
2 after 8 h of reaction. Modification of these catalysts with sulfate ions allowed them to achieve full glycerol conversion while maintaining the overall yield to 1,3-PG under the same reaction conditions. The result was attributed to the increase in Brønsted acid sites as well as a better dispersion of Pt particles and WO
x species [
117].
Other more current systems based on Pt-WO
x have been supported on Ta
2O
5. The calcination of these catalysts at high temperature (900 °C) has allowed for high dispersions of tungsten oxides to be obtained with a low degree of polymerization, increasing the oxygen vacancies on their surface. This generates a higher dispersion of the Pt particles, maximizing the Pt–O–W interfacial bonding, and leads to obtaining higher yields to 1,3-PG [
108,
109].
With respect to obtaining 1,3-PG in the vapor phase,
Table 6 summarizes the main Pt catalysts employed in continuous flow reactors.
The Pt/WO
3/Al
2O
3 catalysts that had demonstrated yields to 1,3-PG close to 40% in the liquid phase showed low yields to 1,3-PG in the vapor phase (14%), probably due to the thermal level in combination with the surface acidity, which promotes the formation of undesired products. In this regard, the activity results indicated that the reaction proceeds via an initial dehydration of glycerol to form acrolein, which is then rehydrated to form 3-HPA, an intermediate of 1,3-PG [
139].
Surprisingly, the modification of SBA-15 with WO
3 allowed for the preparation of Pt catalysts that showed higher yields to 1,3-PG than Pt/WO
3/Al2O
3 (36%), probably due to lower acidity and less severe reaction conditions. In these systems, the activity of the Brønsted acid sites present in WO
3 is combined with the hydrogenation activity of Pt [
140]. Similar yields were obtained with Pt/AlPO
4 catalysts due to the presence of weak Brønsted acid sites in AlPO
4 and the dispersion of Pt particles [
141].
Other Pt [
142] and Pt–Cu [
143] based catalysts on protonated mordenite supports (Pt/H-MOR, Pt-Cu/H-MOR) showed superior performance for the formation of 1,3-PG with yields of 46–52% at 0.1 MPa H
2 and 1 h
−1 (WHSV) in the 225–215 °C range, respectively. In these systems, the dispersion of Pt combined with the concentration of Brønsted acid sites of the support promote the selective formation of 1,3-PG [
142]. In the case of the bimetallic catalyst, the presence of Cu decreases the C–C cleavage side reactions promoted by Pt [
143]. The highest yields to 1,3-PG were obtained with a Pt catalyst on sulfuric acid activated montmorillonite (Pt/S-MMT) (~58%) at 200 °C and 0.1 MPa H
2 [
144].