Mini review

Catalysis Science & Technology

contaminants—notably acidic CO₂—which generally proceeds by heating.² Furthermore, thermal processing provides the energy to create new bonds between the magnesia and silica phases forming new crystalline magnesium silicates.⁵,²⁵¹ Consequently, calcination conditions can influence the activity of MgO–SiO₂ catalysts by modifying their acid–base and structural properties. Zhu *et al.* studied the relationship between the calcination temperature under air of MgO–SiO₂ and its chemical properties;⁵ their results are summarized in Fig. 31. As illustrated, the ratio between the number of Lewis acid sites and basic sites was significantly affected by the calcination conditions. Calcination between 773 and 873 K generated a balanced number of Lewis acid and basic sites, resulting in higher butadiene selectivity. However, higher temperature generated too many basic sites, favoring undesirable side reactions. Weckhuysen *et al.* also found the thermal treatment conditions to influence the properties of CuO/MgO–SiO₂, including the nature of the atmosphere, noting that stagnant air was preferable to a N₂ atmosphere.⁵¹

Over bare MgO–SiO₂ catalysts, temperature-programmed surface reactions, catalytic tests, DFT calculations and poisoning studies suggest that the conversion of ethanol to acetaldehyde limits the reaction.¹⁰⁰,¹¹¹,¹²⁶,¹³⁸,¹⁶⁰,²⁵⁹

<figure><img src="image_76.png" /><figcaption>**Fig. 31** Effect of calcination temperature on the acid and basic properties of MgO–SiO₂ catalysts versus butadiene selectivity in the Ostromislensky process.⁵ Acid sites quantified with pyridine chemisorption; basic sites quantified with CO₂ chemisorption.</figcaption></figure>

Consequently, several dehydrogenation promoters have successfully been used to improve catalytic performances. Taifan *et al.*¹⁸¹ and Weckhuysen *et al.*⁵¹,²⁶⁸ studied the promoting role of copper. It was found to increase the butadiene yield due to its dehydrogenation abilities improving the acetaldehyde formation rate, but also by poisoning acid sites, ostensibly suppressing undesired side-reactions. However, contrary to bare MgO–SiO₂, Cu-modified samples deactivated faster. Both the sintering of metallic copper particles and blockage by coke have been proposed as deactivation mechanisms. Ag, Au and Pd have also been used to promote the activity of MgO–SiO₂.⁹³,¹⁵⁸,¹⁶⁰ Transition and noble metal nanoparticles, well established for promoting alcohol dehydrogenation reactions, are believed to provide redox properties to the catalytic system. Shylesh *et al.* investigated the potential of all three elements and copper: the catalytic test results are listed in Table 5.⁹³

A clear hierarchy can be seen in terms of selectivity at iso-conversion: Au > Ag > Cu ≫ Pd, but its origin has yet to be determined. Several parameters can influence the nonoxidative dehydrogenation ability of noble and transition metals, such as particle size, pretreatment conditions and metal-support interactions.

Sels *et al.* used Ag to enhance the activity of dry-milled MgO–SiO₂ after conducting a screening of promoters which included Zn, Cu, Ni, Co, Fe, Mn and Cr.⁹²,¹⁶⁰ The introduction of Ag by aqueous impregnation improved acetaldehyde formation and generated new Lewis acid sites attributable to silver cations. Additionally, water itself increases the mixing between the oxide by dissolving Mg cations. Tripathi *et al.* reported a Cu–Ag/MgO–SiO₂ catalyst with high activity and stability compared to catalysts with monometallic promoters, a phenomenon which was attributed to a synergism between Cu and Ag.¹⁵⁸

Zinc oxide and zirconium oxide dispersed on MgO–SiO₂ were also used to enhance its catalytic performances. Zinc oxide contributed to the dehydrogenation reactivity of MgO–SiO₂. Zn-Modified MgO–SiO₂ was not only found to yield more butadiene and acetaldehyde, but also to be more stable than a Cu-modified equivalent, possibly due to the lower reducibility of Zn(n).¹⁸¹ Both Zn and Zr oxides introduced new Lewis acid sites believed not only to promote the aldol condensation reaction (see above), but also to increase the rate of ethanol dehydration.²³⁰,²⁶⁶ As a result, another dimension is added to catalyst design: the ZnO–ZrO₂ must

<table><caption>**Table 5** Effect of metallic promoters on the selectivity of MgO–SiO₂ at 25% conversion of ethanol studied by Shylesh *et al.*⁹³</caption><thead><tr><th rowspan="2">Catalyst</th><th colspan="6">Selectivity at 25% conversion, %</th></tr><tr><th>Butadiene</th><th>Acetaldehyde</th><th>Ethylene</th><th>Butenes</th><th>Butanol</th><th>Methane</th></tr></thead><tbody><tr><td>Au/MgO–SiO<sub>2</sub></td><td>61</td><td>30</td><td>5</td><td>3</td><td>1</td><td>—</td></tr><tr><td>Ag/MgO–SiO<sub>2</sub></td><td>54</td><td>43</td><td>2</td><td>1</td><td>—</td><td>—</td></tr><tr><td>Cu/MgO–SiO<sub>2</sub></td><td>37</td><td>55</td><td>2</td><td>1</td><td>5</td><td>—</td></tr><tr><td>Pd/MgO–SiO<sub>2</sub></td><td>2</td><td>12</td><td>1</td><td>—</td><td>—</td><td>85</td></tr></tbody></table>

Conditions: T = 523 K, WHSV<sub>EtOH</sub> = 1.1 h⁻¹.

<figure><img src="image_77.png" /></figure>
also be balanced to maximize ethanol conversion to butadiene.¹⁹¹ One may consider that the contribution of zinc and zirconium goes beyond that of a promoter. In fact, Kyriienko *et al.* found ZnO–ZrO₂/SiO₂ to be more active than ZnO–ZrO₂/MgO–SiO₂.²⁶⁶

Examples of MgO–SiO₂ catalysts and performance data

Men *et al.* reported the most productive MgO–SiO₂ catalyst in the literature: at a high WHSV<sub>EtOH</sub> of 4.1 h<sup>-1</sup> and a temperature of 723 K, it exhibited an initial conversion of 95% with a butadiene selectivity of 77%, resulting in a productivity of 1.76 g<sub>BD</sub> g<sub>cat</sub><sup>-1</sup> h<sup>-1</sup>.<sup>267,273</sup> These performances were attributed to the morphological and chemical properties of the catalyst resulting from the synthesis method used. The authors prepared MgO using an ethylene glycol mediated self-assembly method reported by Cui *et al.*<sup>274</sup> Magnesium acetate and polyvinylpyrrolidone—a structure-directing polymer—are dissolved in ethylene glycol and hydrothermally-treated in a Teflon-lined autoclave before calcination of the MgO precursors at 773 K. This procedure afforded flower-like hierarchical oxide with a larger surface area and pore size than commercial MgO. It was combined with SiO₂ by a wet-kneading process.

The acidic and basic properties were optimized by tuning the Mg-to-Si ratio between 1 and 19, which also affected the textural properties of the mixed oxides; butadiene selectivity ranged between 50 and 80%. Ultimately, the authors obtained an optimal ratio of 1.87, arguing that it provided the material with the right balance of acid and basic sites, notably by limiting the number of strong acid sites. They also attributed the impressive catalytic performances to the high surface area and better access to catalytic sites provided by the hierarchical morphology. A stability test indicated that the catalyst deactivated slowly over a period of 42 hours: ethanol conversion and butadiene selectivity decreased from 95% and 77% to 51% and 49%, respectively (Fig. 32). The authors attributed the loss of activity to gradual covering of active sites by coke deposition. This catalyst was further put to test by comparing it with materials possessing different morphologies.<sup>273</sup> Flower-like MgO–SiO₂ outperformed catalysts with nanodisk and nanosheet morphologies. The authors identified the greater presence of Mg–O–Si linkages and a better balance of acid–base properties in the former catalyst to be the cause of its superiority.

As previously mentioned, Shylesh *et al.* used metallic dopants to enhance the catalytic activity of MgO–SiO₂.⁹³ Their work illustrates the beneficial effects of promoters. Prepared by incipient wetness impregnation of commercial silica using a magnesium nitrate solution, the bare initial catalyst with a Mg-to-Si ratio of 2.6 performed poorly. With an ethanol conversion below 10% at 573 K and a WHSV<sub>EtOH</sub> of 4.1, it mainly yielded ethylene. Au was added using the deposition-precipitation method with urea, well-known for introducing highly dispersed nanoparticles. As illustrated in Fig. 33, ethanol conversion underwent a seven-fold increase;

<figure><img src="image_78.png" /><figcaption>**Fig. 32** Stability test of a highly active hierarchical MgO–SiO₂ catalyst prepared using magnesium acetate and polyvinylpyrrolidone. *T* = 723 K, WHSV<sub>EtOH</sub> = 4.1 h<sup>-1</sup>. Reproduced from ref. 267 with permission from The Royal Society of Chemistry.</figcaption></figure>

the addition of Au further suppressed the ethylene selectivity, increasing the selectivity towards butadiene to near 60%. The authors attributed the performance enhancement observed to the redox properties provided by Au which participates in the dehydrogenation of ethanol. Characterization further indicated a close, but limited mixing of the magnesia and silica phases, a property that Weckhuysen *et al.* had associated with superior activity. The contribution of the deposition-precipitation method to the catalytic activity was not explored. It may be possible that a basic pH combined with the mixing required replicated conditions akin to those of wet-kneading, further enhancing the textural and chemical properties of the catalyst. The work of Shylesh *et al.*

<figure><img src="image_79.png" /><figcaption>**Fig. 33** Effect of 3 wt% Au on the performance of MgO–SiO₂.⁹³ *T* = 573 K, WHSV<sub>EtOH</sub> = 1.1 h<sup>-1</sup>.</figcaption></figure>

<figure><img src="image_80.png" /></figure>
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illustrates how post-synthesis modifications can drastically improve the catalytic performances of MgO-SiO$_2$.

While Weckhuysen et al. correlated butadiene yield with the relative quantity of layered hydrous magnesium silicates,²⁵⁵ Baba et al. studied the catalytic activity of talc, a layered hydrous magnesium silicate, in the ethanol-to-butadiene reaction.¹³⁹,¹⁴²,²⁷¹ Pure synthetic talc produced very little butadiene from ethanol but showed >75% selectivity towards dehydration products at 38.8% conversion. The addition of zinc oxide to the hydrothermal synthesis process produced talc catalysts with Zn(n) substituted within the layered structure and highly selective towards butadiene. By tuning the Zn concentration, it reached 51.8% at a WHSV$_{EtOH}$ of 8.4 h⁻¹ and a temperature of 673 K, amounting to a productivity of 1.1 g$_{BD}$ g$_{cat}$h⁻¹—one of the highest in the literature. Catalytic tests with acetaldehyde alone demonstrated that Zn not only suppressed ethanol dehydration, but also crotonaldehyde formation, indicating that pure talc was more active in the aldol condensation. Experimental and computational studies were conducted to explain the promoter effect of Zn. XPS revealed that the growing incorporation of Zn increased the binding energy of the O 1s level, indicating a progressive lowering of the basic character of talc. These results not only support the theory that limiting the amount of strong basic sites is beneficial to butadiene yield, but also indicate that strong basic sites are more active in the aldol condensation. The promoter effect of Zn was attributed to its contribution to ethanol dehydrogenation. Based on DFT calculations and the hard-soft acid-base theory, the authors argued that Zn cations, acting as softer Lewis acid-base pairs, favored dehydrogenation compared to purely Mg-containing catalysts. Zinc also increased the lifetime of the catalyst. Pure talc deactivated after 1 hour due to severe coking, whereas ethanol decreased by around half over a period of 15 hours with Zn-talc. However, the selectivity towards acetaldehyde progressively increased at the expense of butadiene,

indicating that the stronger basic sites responsible for aldol condensation were progressively poisoned.

MgO-SiO$_2$ has proved to be one of the best catalytic systems for the Lebedev process. Table 6 provides the performance details of many catalysts directly or indirectly addressed in this section. Again, accurately comparing the different materials reported in the literature is hindered by the different reaction conditions employed. Nonetheless, from an industrial standpoint, it is worth highlighting that high butadiene selectivity, low selectivity towards dehydration products and high butadiene productivity were achieved, despite the challenges associated with MgO-SiO$_2$ catalyst design. Part of the issue can be attributed to unknowns regarding the identity of active sites. For instance, the extent to which SiO$_2$ is necessary to the formation of the active phase has yet to be identified.¹ Nevertheless, it is understood that a balance between acidic and basic properties is required to maximize catalytic activity. The prevalent theory on catalyst design suggests that weak and medium basic sites found on the MgO phase are required to catalyze the ethanol dehydrogenation and aldol condensation, respectively, with strong basic sites being detrimental to activity. However, the role of acid sites remains under debate: although moderate numbers of Lewis acid sites have been correlated with superior activity, there is no consensus on their identity. Identifying their ideal acid strength and the influence of neighboring Si atoms requires further investigation. Finally, the role of different magnesium silicates formed by the mixing of MgO and SiO$_2$ must be clarified.²⁵⁵

Even with the knowledge of the active phase, achieving the ideal balance of acidic and basic properties would be challenging. No systematic understanding of the numerous synthesis parameters susceptible to affect the acid-base character of MgO-SiO$_2$ (Mg-to-Si ratio, calcination temperature, mixing of the oxide phases, solvents, etc.) has been formulated. An overview of the best performing catalysts suggests that a limited mixing at the interface of

<table><caption>**Table 6** Performances in the Lebedev process of reviewed MgO-SiO$_2$ catalysts and others found in the literature</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>Mg: Si</th><th>WHSV h<sup>-1</sup></th><th>TOS h</th><th>T K</th><th>X %</th><th>BD S. %</th><th>AcOH S. %</th><th>C<sub>2</sub>= S. %</th><th>BD Y. %</th><th>P<sub>BD</sub></th><th>Ref</th></tr></thead><tbody><tr><td>1</td><td>MgO-SiO<sub>2</sub><sup>a</sup></td><td>1.86</td><td>4.1</td><td>—</td><td>723</td><td>95</td><td>77</td><td>2</td><td>13</td><td>73</td><td>1.76</td><td>267</td></tr><tr><td>2</td><td>3%Au/MgO-SiO<sub>2</sub></td><td>2.6</td><td>1.1</td><td>3.3</td><td>573</td><td>45</td><td>60</td><td>28</td><td>7</td><td>27</td><td>0.14</td><td>93</td></tr><tr><td>3</td><td>Zn-talc</td><td>—</td><td>8.4</td><td>7</td><td>673</td><td>42</td><td>52</td><td>22</td><td>8</td><td>22</td><td>1.06</td><td>139</td></tr><tr><td>4</td><td>MgO-SiO<sub>2</sub><sup>b</sup></td><td>1</td><td>0.275</td><td>4</td><td>748</td><td>93</td><td>41</td><td>—</td><td>—</td><td>38</td><td>0.06</td><td>275</td></tr><tr><td>5</td><td>MgO-SiO<sub>2</sub><sup>c</sup></td><td>1</td><td>1</td><td>—</td><td>673</td><td>41</td><td>57</td><td>5</td><td>34</td><td>24</td><td>0.14</td><td>266</td></tr><tr><td>6</td><td>1%Ag/MgO-SiO<sub>2</sub></td><td>2</td><td>1.2</td><td>3.3</td><td>753</td><td>84</td><td>50</td><td>6</td><td>10</td><td>42</td><td>0.29</td><td>160</td></tr><tr><td>7</td><td>1%CuO/MgO-SiO<sub>2</sub></td><td>1</td><td>1.1</td><td>0.5</td><td>698</td><td>74</td><td>48</td><td>7</td><td>5</td><td>38</td><td>0.25</td><td>51</td></tr><tr><td>8</td><td>6.3%Zr-1.4%Zn/MgO-SiO<sub>2</sub></td><td>3</td><td>0.62</td><td>3</td><td>648</td><td>40</td><td>36</td><td>8</td><td>32</td><td>30</td><td>0.13</td><td>191</td></tr><tr><td>9</td><td>4%ZnO/MgO-SiO<sub>2</sub></td><td>1</td><td>1</td><td>3</td><td>648</td><td>56</td><td>62</td><td>22</td><td>10</td><td>35</td><td>0.20</td><td>230</td></tr><tr><td>10</td><td>2.5%Cu-2.5%Ag/MgO-SiO<sub>2</sub></td><td>2</td><td>—</td><td>—</td><td>573</td><td>64</td><td>72</td><td>15</td><td>9</td><td>46</td><td>—</td><td>158</td></tr><tr><td>11</td><td>0.5%Ag/MgO-SiO<sub>2</sub><sup>d</sup></td><td>1</td><td>0.2</td><td>6</td><td>598</td><td>85</td><td>76</td><td>6</td><td>3</td><td>64</td><td>0.02</td><td>276</td></tr><tr><td>12</td><td>MgO-SiO<sub>2</sub></td><td>0.63</td><td>0.03</td><td>11</td><td>623</td><td>53</td><td>30</td><td>10</td><td>53</td><td>16</td><td>0.003</td><td>1</td></tr></tbody></table>

a Wet-kneading with hierarchical MgO. b Wet-kneading with nano-sized MgO. c Mechano-chemical synthesis. d Hydrogen was co-fed. WHSV: weighted hourly space velocity of ethanol. X: ethanol conversion. BD, AcH, C$_{2}$= are butadiene, acetaldehyde and ethylene, respectively. S., Y. and P. are selectivity, yield and productivity, respectively. Productivity is expressed in terms of g$_{BD}$ g$_{cat}$ h⁻¹. Active phases are expressed in terms of wt%.

<figure><img src="image_81.png" /></figure>
<figure><img src="image_82.png" /></figure>

MgO and SiO₂ phases, resulting in layered hydrous amorphous silicate, is preferable. Ostensibly, the remaining MgO phases provides the weak and medium basic sites required, while the new magnesium silicate phase introduces new Lewis acid sites required for catalytic activity. Excessive mixing between the two oxides leads to amorphous magnesium silicates responsible for ethanol dehydration.

The most successful preparation method appears to be wet-kneading MgO and SiO₂ precursors with morphological properties (nanosized, hierarchical) intended to maximize the limited interaction between them. The optimal Mg-to-Si ratio has varied depending on the synthesis method used. Contrarily, the ideal calcination temperature was found to vary between 773 K and 873 K.⁵

Because the rate limiting step of the ethanol-to-butadiene conversion over MgO-SiO₂ is believed to be the formation of acetaldehyde, the catalytic activity can be further improved by introducing transition and noble metals active in the dehydrogenation of ethanol. According to the results of Shylesh et al. Au is the most promising promoter for MgO-SiO₂.⁹³

As illustrated in Fig. 32, MgO-SiO₂ is susceptible to deactivation, ostensibly by coke deposition believed to be formed by strong basic sites. Yet, few studies have addressed the improvement of catalytic stability, or even the regeneration of the spent catalyst.²⁷² It may be worth investigating the deactivation mechanism of MgO-SiO₂.

## Rare earth elements

Rare earth metal oxides have found numerous uses in heterogeneous catalysis.²⁷⁷,²⁷⁸ Recently, some authors have investigated the potential application of rare earth elements as catalysts for the conversion of ethanol to butadiene. Li et al. studied the activity of various rare earth (Y, La, Ce, Pr, Nd) oxide catalysts supported on dealuminated zeolite. Although zeolite-supported monometallic catalysts primarily dehydrated ethanol to ethylene and diethyl ether, Y, La and Ce were found to be capable of converting acetaldehyde to crotonaldehyde, indicating their condensation ability. By combining these elements with metal oxides capable of ethanol dehydrogenation, catalysts highly active in the ethanol-to-butadiene reaction were recently reported in the literature. However, due to their novelty, these catalytic systems have not been investigated as other catalytic systems detailed above. Consequently, the understanding of their activity is limited.

Owing to the work of Li et al., yttrium-containing catalysts have been the most studied of all rare earth-based ones for the ethanol-to-butadiene reaction.¹¹⁹,¹⁸⁰ Supported on dealuminated β zeolite, Y lead primarily to ethanol dehydration products. Promotion with copper marginally increased the butadiene yield, yet diethyl ether and other carbonaceous species remained the principal products. Significant activity was obtained by combining Zn and Y. Li et al. tuned the Zn-to-Y ratio, ultimately obtaining 2%–

Zn8%Y/SiBEA as the most active catalyst. Butadiene selectivity as high as 81% was reported. This catalyst achieved an exceptional productivity of 2.33 g<sub>BD</sub> g<sub>cat</sub><sup>-1</sup> h<sup>-1</sup> at 673 K and a WHSV<sub>EtOH</sub> of 7.9 h<sup>-1</sup>, making it one of the best for the Lebedev process found in the literature. The authors attributed these remarkable performances to a synergic effect between both Zn(II) and Y(III), which was enhanced by their proximity within the zeolite pores. Owing to the confinement effect of zeolites, Zn-Y clusters were suggested to form within the zeolite cages, as evidenced by the fact that MCM-41-supported Zn-Y did not perform as well. Furthermore, XPS analysis found a stronger interaction between Zn(II) and Y(III) unique to the zeolite supported catalyst. The deactivation of 2%Zn8%Y/BEA was discussed previously. Li et al. determined that pore blockage did not play a significant role in the loss of activity.¹³¹ Rather, cyclic unsaturated aldehydes and ketones gradually cover the Zn and Y sites of the catalyst. Under conditions enabling high productivity, the butadiene yield dropped by half in the first 10 hours of catalyst testing (Fig. 34), which is more severe than other highly productive catalysts recently reported, such as hierarchical MgO-SiO₂ and Zn-Ta-TUD-1.¹⁰³,²⁶⁷ Nevertheless, ZnY/SiBEA recovered its high productivity after calcination under air for 4 hours.

Li et al. have proposed a model of activity and deactivation of their 2%Zn-8%Y/SiBEA catalyst.¹⁶³ Since Zn/SiBEA produced not only acetaldehyde from ethanol, but also some butadiene, the authors argued that Zn(II) primarily provided the catalyst with redox properties active in the dehydrogenation reaction. Since each component taken as a stand-alone catalytic system was found by MAS-NMR coupled with NH₃ and acetone-2-¹³C adsorption experiments to introduce Lewis acidity to SiBEA, the authors argued that both metals contributed to the condensation activity, although Y(III) as found to be more active. Li et al. concluded

<figure><img src="image_83.png" /><figcaption>**Fig. 34** Stability and regeneration test of 2%Zn-8%Y/SiBEA. Regeneration under air flow at 673 K for 4 hours. T = 673 K, WHSV<sub>EtOH</sub> = 7.9 h<sup>-1</sup>. Reproduced from ref. 180. Copyright 2017, American Chemical Society.</figcaption></figure>
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that not only the synergistic effect between Zn(II) and Y(III), but also the proximity of the two phases enables the high selectivity observed, as it increased the chance of the intermediates to react with each other on these sites during the aldol condensation.

Zhu *et al.* focused on understanding the structure-activity relationship of Y/SiO₂ catalysts in the two-step process, providing insight on the coupling activity of yttrium-containing catalysts.²⁷⁹ Y/SiO₂ catalysts with varying Y:Si ratios were prepared *via* wet impregnation of fumed silica. Characterization revealed that the amount and coordination of Y–O–Si linkages depended on the elementary composition of the catalyst. This parameter had repercussions on the acid–base properties of the materials and, in turn, on their catalytic activity. The authors used chemical probes to quantify and characterize the acidic and basic sites of their Y/SiO₂ samples. In an approach reminiscent of works on MgO–SiO₂,²⁵⁵,²⁶⁷ they were able to correlate the chemical properties with the product distribution. Accordingly, materials possessing a high concentration of strong Lewis acid sites and intermediate basicity displayed higher selectivity towards butadiene; such properties were obtained with moderate Y loading. High loadings resulted in bulk Y₂O₃ formation on the surface of fumed silica. Such samples displayed stronger basic sites monitored with CO₂-TPD, which favored butanol formation. Therefore, as with MgO–SiO₂, Y-containing catalysts are subject to a careful balance between acidic and basic properties.

Kyriienko *et al.* mixed lanthanum oxide, which the literature identifies as silica-supported La₂O₃, a multifunctional catalyst,²⁸⁰ with silica, zirconia and zinc oxide to prepare a catalyst highly active in the Lebedev process.²³⁰ The preparation process combined incipient wetness impregnation and wet-kneading; several samples with different compositions were synthesized to study the role of each component in catalytic tests at 748 K and a WHSV<sub>ETOH</sub> of 1 h⁻¹. La₂O₃–SiO₂ proved poorly active, yielding primarily dehydration products. The addition of zinc oxide successfully suppressed ethanol dehydration—a phenomenon observed in many catalytic systems for the ethanol-to-butadiene reaction.¹²⁷,¹⁹⁷,²⁴⁷ Still, the butadiene yield remained below 20%. Only with the addition of ZrO₂ could a yield of 52.5% be achieved, marked by a sharp consumption of acetaldehyde compared to other samples. Ultimately, the best catalyst was 2%ZnO–7%La₂O₃–1%ZrO₂–SiO₂, which reached a butadiene yield of 60.2% at 743 K. The authors

suggested that a synergy between each component explained the catalytic performances of the mixed catalyst. ZnO promoted ethanol dehydrogenation, whereas both ZrO₂ and La₂O₃ catalyzed the aldol condensation and MPVO reactions. In fact, La₂O₃ been shown to be active in aldol condensation¹⁸⁰,²⁸¹ and MPVO reactions.²⁸² The condensation activity of La₂O₃ can be attributed to its basic properties, confirmed by Kyriienko *et al.* with pyrrole-FTIR, but also well-known in the literature on La₂O₃.²²⁹ Potential active sites are La–O acid–base pairs and isolated O²⁻.

Zhao *et al.* reported a Zn–Ce catalyst encapsulated within mesoporous SBA-15 active in the Lebedev process.²⁸³ 10%Zn–5%Ce/SBA-15 catalysts were prepared using the solid-state grinding method to mix SBA-15 samples with metal precursors. By mixing the as-prepared SBA-15 sample containing the organic templates, a higher degree of active phase dispersion was obtained. This dispersion increased the concentration of Lewis acid sites compared to calcined SBA-15, which resulted in higher ethanol conversion and butadiene selectivity in the Lebedev process. At 648 K and a WHSV<sub>ETOH</sub> of 1.62 h⁻¹, the butadiene yield reached 36% after 5 hours on stream. These performances were ostensibly attributed to a better condensation activity provided by the greater number of Lewis acid sites.

Development of rare earth catalysts for the ethanol-to-butadiene reaction is a recent subject. Characterization studies together with the literature available for other organic reactions suggest that parallels may be drawn with other catalytic systems, as Y, La and Ce oxides possess chemical properties comparable to those of the previously discussed systems: basic sites and Lewis acid-base pairs. Correlating these properties with specific catalytic activity remains to be fully elucidated before proceeding to rational design. As demonstrated by Li *et al.*¹⁶³ rare earth metal oxides are active in the aldol condensation and MPVO reaction, implying that dehydrogenation promoters must be introduced to make them active in the Lebedev process. So far, Zn has been the preferred promoter, which some authors have suggested to be in synergy with Y.¹⁸⁰ The performances of the most active catalysts reviewed are listed in Table 7.

Alkali & alkaline earth dopants

The balance of acidic and basic properties influences the activity of catalysts in the ethanol-to-butadiene reaction. A

<table><caption>**Table 7** Reviewed rare earth catalysts active in the Lebedev process</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>WHSV<br>h<sup>-1</sup></th><th>TOS<br>h</th><th>T<br>K</th><th>X<br>%</th><th>BD S.<br>%</th><th>AcOH S.<br>%</th><th>C<sub>2</sub>–S.<br>%</th><th>BD Y.<br>%</th><th>P<sub>BD</sub></th><th>Ref.</th></tr></thead><tbody><tr><td>1</td><td>2%Zn–8%Y/SiBEA</td><td>7.9</td><td>1</td><td>673</td><td>82</td><td>63</td><td>7</td><td>2</td><td>52</td><td>2.33</td><td>180</td></tr><tr><td>2</td><td>2%ZnO–7%La<sub>2</sub>O<sub>3</sub>–1%ZrO<sub>2</sub>–SiO<sub>2</sub></td><td>2</td><td>—</td><td>673</td><td>100</td><td>60</td><td>—</td><td>—</td><td>60</td><td>0.71</td><td>229</td></tr><tr><td>3</td><td>10%Zn–5%Ce/SBA-15</td><td>1.62</td><td>5</td><td>648</td><td>79</td><td>45</td><td>22</td><td>22</td><td>36</td><td>0.339</td><td>283</td></tr></tbody></table>

WHSV: weighted hourly space velocity of ethanol. X: ethanol conversion. BD, AcH, C₂= are butadiene, acetaldehyde and ethylene, respectively. S., Y. and P. are selectivity, yield and productivity, respectively. Productivity is expressed in terms of g<sub>BD</sub> g<sub>cat</sub><sup>-1</sup> h<sup>-1</sup>. Active phases are expressed in terms of wt%.

<figure><img src="image_84.png" /></figure>
<figure><img src="image_85.png" /></figure>

simple, but efficient strategy has been used to modulate them with alkali and alkaline-earth dopants,³,⁸,⁹,¹¹⁸,¹⁷²,¹⁹¹,²⁸⁴ an approach employed when dealing with solid acids.²⁸⁵,²⁸⁶ The main benefit is the suppression of ethanol dehydration, sometimes at the expense of the overall activity.¹⁶,¹⁹⁸ Although successfully put in practice, the fundamental aspects of this approach are not fully understood and no established method for using these dopants has been formulated.

The suppression of side reactions is attributed to the modulation of acid sites resulting from their interaction with alkali and alkaline-earth cations, which can be introduced into catalysts *via* impregnation.³,⁸,⁹,¹¹⁸,¹⁷²,¹⁹¹,²⁸⁴ In theory, these cations undergo proton-exchange with Brønsted acid sites, thereby deactivating them. The suppression of ethylene and diethyl ether formation are attributed to this poisoning effect, as Brønsted acid sites are known dehydration active sites. In practice, alkali and alkaline-earth cations have been shown to not only weaken or poison acid sites, but also to act as Lewis acid sites themselves, owing to their positive charge.²⁸⁷ The influence of alkali and alkaline-earth dopants depends on both their nature and the amount added to the parent material. Baylon *et al.* demonstrated that the principal effect of Na doping was to weaken the acid strength of a ZnO–ZrO₂ catalyst without significantly lowering the total number of sites (Fig. 35).⁸ As a result, acetaldehyde and butadiene formation became favored over ethanol dehydration (Table 6, entries 1 and 2). However, the enhancement of butadiene yield plateaued when the acid strength decreased. Similarly, Da Ros *et al.* progressively increasing the amount of Na from 0 to 2 wt% on ZnO–ZrO₂/MgO–SiO₂ which lowered the butadiene yield (Table 9, entry 3).¹⁹¹ These results suggest that, although these dopants can alleviate unwanted side-reactions by tuning the acid properties of a catalyst through poisoning, a minimum number of acid sites with moderate strength is required to enable to conversion of ethanol to butadiene.

<figure><img src="image_86.png" /><figcaption>**Fig. 35** Influence of alkali and alkaline-earth dopants on the acid (NH₃-TPD)/base (CO₂-TPD) site concentration of the reference 1% ZnO-5%ZrO₂/SiO₂ *versus* butadiene yield.¹⁷²</figcaption></figure>

Beyond their role as dehydration suppressors, Patil *et al.*¹⁷² have proposed that alkali and alkaline-earth oxides could also contribute to butadiene production since they are known to possess basic properties.²³⁰ After all, the basic alkaline-earth metal oxide MgO is active in the ethanol-to-butadiene reaction. In comparing the performance enhancement of various promoters (Li, Na, K, Cs), the authors found that—although every dopant increased the number of basic sites and suppressed ethylene selectivity—only Cs increased the butadiene yield compared to the ZnO–ZrO₂ reference material (Fig. 36). Notably, this promoter effect was verified at equimolar amounts of alkali and alkaline-earth dopants. The authors argued that Cs alone possessed the basic strength necessary to promote the aldol condensation step, thereby increasing the butadiene yield (Table 6, entry 3).

However, Klein *et al.* possibly contradicted this conclusion. Catalytic tests of alkali and alkaline-earth (Cs, K, Ca, Mg) doped β zeolites during the Ostromislensky process indicated that Cs/BEA did not catalyze the aldol condensation, reaching only 5% conversion; magnesium-doped samples alone showed significant activity. This suggests that magnesium oxide is better suited for providing the condensation activity required in the ethanol-to-butadiene reaction.¹³⁹,¹⁴²

The only instance of an alkaline-earth oxide other than MgO being used as the catalyst component providing condensation activity was reported by La-Salvia *et al.*²⁸⁸ The authors used barium oxide to promote the condensation of acetaldehyde on the acidic mesoporous aluminosilicate Al-MCM-41, with chromium oxide being used as a dopant for ethanol dehydrogenation. With barium oxide and the aluminosilicate alone, ethylene was the predominant species, indicating that the alkaline-earth oxide did not perform adequately in the dehydrogenation of ethanol. Only by adding chromium oxide and barium oxide could selectivity

<figure><img src="image_87.png" /><figcaption>**Fig. 36** Product yield for the Lebedev process and the concentration of weak, medium and strong acid sites on ZnZrOₓ (Zr : Zn = 10) *versus* the concentration of the Na dopant.⁸</figcaption></figure>
<table><caption>**Table 8** Effect of morphological properties and the type of alkali and alkaline-earth metal on the performances of Zn–Hf catalysts on MFI zeolites.³ T = 593 K, WHSV = 0.47 h⁻¹, TOS = 3 h</caption><thead><tr><th rowspan="2">Catalyst</th><th rowspan="2">Conversion</th><th colspan="3">Selectivity</th></tr><tr><th>Butadiene</th><th>Acetaldehyde</th><th>Ethylene</th></tr></thead><tbody><tr><td>1.5%Zn–8.9%Hf/MFI (M)</td><td>63.6</td><td>43.0</td><td>1.1</td><td>11.1</td></tr><tr><td>1.5%Zn8.9%Hf/MFI (NS)</td><td>76.2</td><td>53.3</td><td>0</td><td>14.6</td></tr><tr><td>1.7%Li–1.5%–Zn–8.9%Hf/MFI (M)</td><td>21.3</td><td>36.4</td><td>10.3</td><td>3.8</td></tr><tr><td>1.7%Li–1.5%–Zn8.9%Hf/MFI (NS)</td><td>64.6</td><td>73.0</td><td>0</td><td>5.3</td></tr><tr><td>1.7%Na–1.5%Zn–8.9%Hf/(NS)</td><td>54.2</td><td>45.2</td><td>3.2</td><td>2.3</td></tr><tr><td>1.7%K–1.5%Zn–8.9%Hf/MFI (NS)</td><td>35.6</td><td>32.1</td><td>5.6</td><td>1.2</td></tr></tbody><tfoot><tr><td colspan="5">NS = nanosheets. M = microporous.</td></tr></tfoot></table>

towards butadiene attain a maximum of 28% (Table 8, entry 5). Compared to other doped catalysts reviewed in this section, the ethylene selectivity remained very high, a phenomenon ostensibly due to Brønsted acids formed by the presence of Al cations within the silicate framework.

Wang *et al.* recently demonstrated how the morphology of a catalyst plays a significant role in the promoter effect of alkali and alkaline-earth cations.³ Two zinc–hafnium oxide catalysts were compared: one supported on microporous MFI zeolite, another on nanosheets of the same molecular sieve. As listed in Table 8, the latter performed better, ostensibly due to the hierarchical morphology of the carrier. Interestingly, introducing 1.5 wt% Li led to very different changes in activity depending on the support. In the microporous material, the alkali cation reduced ethanol conversion by two-thirds, also suppressing the butadiene selectivity. Contrarily, adding Li to the nanosheet-supported catalyst significantly enhanced the butadiene selectivity while lowering ethylene formation at minimal loss in ethanol conversion. The reason for the discrepancy is unclear, but it demonstrated that the properties of the catalyst support influenced the dopant effect of alkali and alkaline-earth dopants. Furthermore, the authors compared the promoter effect of other cations (Na, K), concluding that Li was a better dopant (Table 8).

Post-synthesis introduction of alkali and alkaline-earth cations in various catalytic systems—mixed oxides, supported metal oxides, zeolites—suppresses side-reactions, most notably ethanol dehydration. Generally, this is accompanied with a decrease in ethanol conversion. Alkali and alkaline-earth modifications also influence acetaldehyde and

butadiene selectivity. The extent to which the catalytic performances are modified depends on the type and amount of dopant, as well as the nature of the catalytic system. Various studies indicated that these oxides lower the strength and/or number of acid sites.

Generally, an excess of dopant results in a clear decrease in butadiene yield, to the benefit of acetaldehyde yield, a phenomenon attributed to the poisoning of acid sites required to form butadiene. It is still unclear whether alkali and alkali-earth oxides also promote condensation activity *via* their basic properties, as discrepant results have been published.⁹,¹⁷² Table 9 lists the performances of notable instances in which alkali and alkaline-earth dopants were used to enhance the catalytic activity of a parent material. Judging from the studies reviewed, it is difficult to conclude whether one dopant is inherently superior in their ability to enhance catalytic performances, a problem exacerbated by the different conditions used. For instance, Ohishi *et al.* reported one of the best catalytic performances by adding Na₂O to MgO–SiO₂ (Table 9, entry 7). However, many authors have pointed out that product distribution was recorded at a very short time on stream, thereby ignoring any contribution from deactivation.¹¹,¹² Nevertheless, alkali and alkaline-earth doping offer a facile approach to modify the crucial acid–base properties of a given catalytic system. And, as Da Ros *et al.* pointed out, the loss of butadiene yield sometimes observed can be compensated for by the reduction of undesired byproducts considering that acetaldehyde can be recycled to take part in a two-step process.¹⁹¹

<table><caption>**Table 9** Notable catalytic systems using alkali and alkaline-earth dopants to enhance the activity in the Lebedev process</caption><thead><tr><th rowspan="2">Entry</th><th rowspan="2">Catalyst</th><th>WHSV</th><th>TOS</th><th>T</th><th>X</th><th>BD S.</th><th>AchOH S.</th><th>C<sub>2</sub>= S.</th><th>BD Y.</th><th rowspan="2">PBD</th><th rowspan="2">Ref.</th></tr><tr><th>h<sup>-1</sup></th><th>h</th><th>K</th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th></tr></thead><tbody><tr><td>1</td><td>2000 ppm Na/ZnZrOx<sup>a</sup></td><td>6.2</td><td>—</td><td>623</td><td>54</td><td>26</td><td>37</td><td>28</td><td>14</td><td>0.49</td><td>8</td></tr><tr><td>2</td><td>2000 ppm Na/ZnZrOx<sup>a</sup></td><td>0.2</td><td>—</td><td>623</td><td>97</td><td>47</td><td>11</td><td>47</td><td>46</td><td>0.06</td><td>8</td></tr><tr><td>3</td><td>1.2%K–1.5%Zr–0.5%Zn/MgO–SiO<sub>2</sub></td><td>0.62</td><td>3</td><td>648</td><td>26</td><td>55</td><td>17</td><td>12</td><td>27</td><td>0.12</td><td>190</td></tr><tr><td>4</td><td>0.5%Cs<sub>2</sub>O–17%ZnO–5%ZrO<sub>2</sub>/SiO<sub>2</sub></td><td>1</td><td>—</td><td>673</td><td>98</td><td>56</td><td>16</td><td>15</td><td>55</td><td>0.32</td><td>172</td></tr><tr><td>5</td><td>1.4%Cr–16%Ba/Al-MCM-41</td><td>0.07</td><td>10</td><td>723</td><td>90</td><td>28</td><td>—</td><td>—</td><td>25</td><td>0.01</td><td>288</td></tr><tr><td>6</td><td>1.7%Li–1.5%Zn8.9%Hf/MFI (NS)</td><td>0.47</td><td>3</td><td>593</td><td>65</td><td>73</td><td>0</td><td>5</td><td>47</td><td>0.13</td><td>3</td></tr><tr><td>7</td><td>0.1%Na<sub>2</sub>O/MgO–SiO<sub>2</sub></td><td>0.18</td><td>0.17</td><td>623</td><td>100</td><td>87</td><td>—</td><td>—</td><td>87</td><td>0.09</td><td>284</td></tr></tbody></table>

<figure><img src="image_88.png" /></figure>

<footnote>a Zr: Zn ratio = 10.</footnote>
Conclusions

After decades of being considered an obsolete technology, ethanol conversion to butadiene has begun to gather attention again. The primary cause of interest is the necessity of finding alternatives to the steam cracking of naphtha, which suffers from sustainability and supply issues. Under the right circumstances, notably the geographical location and the type of biomass feedstock, butadiene from bio-ethanol is more sustainable than fossil-based routes. Financially, ethanol-to-butadiene processes cannot yet compete with current production methods, in part due to low petroleum cost, as well as due to insufficient catalyst performances.

In the last 8 years, significant progress has been made in gaining insight into several aspects of the ethanol-to-butadiene reaction, namely its mechanism and the design of materials exhibiting optimal catalytic activity. The mechanism of the ethanol-to-butadiene reaction is known to be complicated, partly because several of the intermediate species believed to be involved are seldom detected. Nevertheless, recent development owing to the use of techniques such as infrared spectroscopy, isotope tracing experiments and DFT studies has provided new evidence. The general pathway formulated by various authors more than fifty years ago has now come to be recognized as valid: ethanol first dehydrogenates to acetaldehyde, which condensates to crotonaldehyde; the latter reacts with another ethanol molecule to form crotyl alcohol and acetaldehyde via an MPVO reaction; crotyl alcohol dehydrates to butadiene.

At the molecular level, there remains differences amongst the conclusions of the various teams, particularly concerning the aldol condensation mechanism, which may consist of an Eley-Rideal or a Langmuir-Hinshelwood-type mechanism. These discrepancies are not inconsequential, as kinetic modelling greatly benefits from our understanding of the reaction. Incidentally, the ethanol-to-butadiene reaction lacks adequate kinetic studies that can be used in process simulations. Consequently, studies on the viability of this technology often rely on industrial data—only for the Ostromislensky process—obtained after the Second World War with catalysts that have now be outclassed by recent findings. Another topic important to the implementation of ethanol-to-butadiene processes is the deactivation mechanism. The major cause of deactivation on almost all catalytic systems has been identified as the deposition of coke, with calcination under air being a viable method of catalyst regeneration. However, some studies have found alternative sources of deactivation, such as the poisoning of active sites by bulky oxygenated species that are not coke, the reduction of metal oxide active phases and particle sintering.

A wide variety of catalysts have been reported throughout the years. Two primary categories of catalytic systems have been reviewed: group 4 and 5 transition metals, and MgO-SiO₂. In the first case, the catalytic activity has been attributed to the Lewis acid characteristics of these metals

when in their oxide form, generally supported over silicates. Lewis acid sites are known to catalyze the aldol condensation, making Ta, Hf and Zr catalysts some of the most active found in the literature. The condensation ability of these catalysts is maximized by achieving monoatomic dispersion, preferably in the form of tetrahedral “open” sites within the framework of silicate carriers. To this end, several metal dispersion methods and catalyst carriers have been investigated, with exceptional performances obtained using mesoporous silica supports. The Lebedev process requires the addition of dehydrogenation promoters, with Ag, Cu and Zn being the most successful. Recent research has found rare-earth metal oxides to be also active in the ethanol-to-butadiene reaction. Notably, zeolite-supported yttrium oxide catalysts have shown remarkable performances when combined with zinc oxide. Ostensibly, the Lewis acid properties of Y play a role similar to Ta, Hf, Zr and Nb catalysts, which is to say to provide the material with condensation abilities.

MgO-SiO₂ catalysts are inherently active in the Lebedev process and have also shown remarkable performances. The main issue regarding this catalytic system concerns the uncertainty regarding the nature of the active sites. Weak and medium-strength basic sites on the MgO phase have been shown to enable the dehydrogenation of ethanol and the aldol condensation. However, the role and character of the acid sites formed by the mixing between MgO and SiO₂ remain under debate. To maximize the activity of MgO-SiO₂ catalysts, a subtle balance between the acid and base properties must be attained. However, the effect that many synthesis parameters have on this balance makes the design of catalysts challenging. The activity can be further improved by the use of dehydrogenation promoters, as the formation of acetaldehyde has been identified as the rate-limiting step on MgO-SiO₂.

Modifying catalysts with alkali and alkaline-earth ions was shown to be a practical strategy to tune the acid-base properties of catalysts. These can poison stronger acid sites associated with undesired side-reactions, enhancing the selectivity towards butadiene. To some extent, alkali and alkaline-earth ions can also introduce basic sites, although the remains no clear evidence that these new sites significantly participate in the reaction.

Although much insight has been acquired in recent years, there remains room for improving our understanding of the ethanol-to-butadiene reaction. A full elucidation of the molecular-level mechanism has been proposed, but its validity on different catalysts remains to be confirmed. Ostensibly, the subtle differences between the conclusions reached by various research groups could be explained by the different properties of each active phase. With such elucidation, kinetic modelling and accurate process simulation will be facilitated. Such modelling should include the effect of co-feeding water and deactivation. Improving the reactivity of catalysts—all systems included—will not only require further investigation of the structure-activity

<figure><img src="image_89.png" /></figure>
Mini review

relationship, but also the development of synthesis methods better suited to generate the most desirable active sites.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the support from the French National Research Agency (ANR-15-CE07-0018-01). Chevreul Institute (FR 2638), Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation, Région Hauts-de-France and FEDER are acknowledged for support and partial funding of this work. We thank Dr. Lee and Dr. Cheong for providing us with high-resolution TEM images.

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