Catalysts for ethanol-to-butadiene process and method of making the catalysts

Abstract

A core-shell structured catalyst (502) for an ethanol to butadiene process includes a core (506) made of MgO particles, a shell (504) made of Si particles, and an interface (508) between the core (506) and the shell (504), the interface being made of magnesium silicate Mg-O-Si. A molar ratio of the Si to Mg is equal to or larger than 0.08.

Description

CATALYSTS FOR ETHANOL-TO-BUTADIENE PROCESS ANDMETHOD OF MAKING THE CATALYSTSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 528,997, filed on July 26, 2023, entitled “PREPARATION METHODS OF METAL LOADED CATALYSTS FOR ETHANOL-TO-BUTADIENE,” the disclosure of which is incorporated herein by reference in its entirety.BACKGROUND OF THE INVENTIONTECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate to catalysts and methods for making the catalysts, and more particularly, to a more efficient catalyst for single step, ethanol-to-butadiene process.DISCUSSION OF THE BACKGROUND

[0003] The ethanol-to-butadiene (ETB) process is an alternative to current technology (naphtha steam cracking) for butadiene production. 1 ,3-Butadiene (butadiene) finds its various applications in polymer industry such as styrenebutadiene rubber (SBR), acrylonitrile-butadiene-styrene rubber (ABS), and poly- butadiene-styrene rubber (PBR). Currently, over 95 % of butadiene is produced as a byproduct from ethylene production units, that is, in naphtha steam crackers. The recent unstable oil prices and the issues related to the environment and climatechanges have incentivized the industry to find an alternative process for butadiene production. Taken together with the recent availability of bioethanol, the ETB process is considered as a sustainable alternative. Compared to a two-step ethanol-to- butadiene process (Ostromislensky process), where ethanol is first partially converted to acetaldehyde by dehydrogenation and then the mixture of ethanol and acetaldehyde is reacted to form butadiene, the one-step Lebedev process has its advantages of simplicity and economic vitality [1].

[0004] Although the reaction mechanism of the Lebedev process is still under discussion, the most plausible mechanism involves ethanol dehydrogenation to acetaldehyde as a first step. Then, multiple reactions occur, such as aldol condensation between two acetaldehydes to form 3-hydroxybutanal (acetaldol), subsequent dehydration and hydrogenation steps to crotonaldehyde and crotyl alcohol, respectively, and the intramolecular dehydration, to yield butadiene. Owing to these multiple steps, multi-functional catalysts are typically required for ETB process, especially having both acidic and basic sites [2],

[0005] In particular, silica-magnesia (SiO2:MgO) has been a benchmark catalyst for over 70 years, considering its originality in the industrial process and its stable catalytic performance [3]. The catalytic performance of the silica-magnesia catalyst is largely influenced by its preparation procedure. The traditional preparation procedure is the wet-kneading method, which can generate the most active silica- magnesia catalyst [4], Wet-kneading typically utilizes solid precursors such as Stober silica 102 and magnesium hydroxide 104, which are placed in water 106 under continuous mixing, as schematically illustrated in FIG. 1 [5]. As the pristine MgO andSiO2, and their physical mixtures show limited catalytic performances, the magnesium silicates (Mg-O-Si) 110 formed during wet-kneading (after separation and drying calcination) have active sites for the one-step Lebedev process.

[0006] In recent years, different magnesium silicates have been formed, which originated by the dissolution and cross-deposition of Si and Mg subunits during the wet-kneading [5]. More specifically, two distinct phases (Si on MgO 112 and Mg on SiC>2 114) can be separated (see FIG. 1 ) via modified wet-kneading, which can lead to the preferential formation of butadiene and ethylene, respectively [6].

[0007] Therefore, the traditional methods for obtaining magnesium silicates based catalysts dissolve and redisperse silica on MgO (Si on MgO) and the remaining SiO2 particles are used for the formation of detrimental acid sites (Mg on SiO2), which is undesirable. Thus, there is a need to fully utilize Si to form active sites selective to butadiene (Si subunits on the surface of MgO), with minimum or no Mg on SiO2.SUMMARY OF THE INVENTION

[0008] According to an embodiment, there is a core-shell structured catalyst for an ethanol to butadiene process, and the catalyst includes a core made of MgO particles, a shell made of Si particles, and an interface between the core and the shell, the interface being made of magnesium silicate Mg-O-Si. A molar ratio of the Si to Mg is equal to or larger than 0.08.

[0009] According to another embodiment, there is a method for making a core-shell structured catalyst for an ethanol to butadiene process, and the method includes providing Mg(OH)2 platelets, providing tetraethyl orthosilicate (TEOS) as a source of Si particles, adding cetyltrimethylammonium bromide (CTAB) to the Mg(OH)2 platelets, mixing the TEOS, CTAB, and the Mg(OH)2 platelets to form a mixture, and calcinating particles extracted from the mixture to obtain the core-shell structured catalyst.

[0010] According to still another embodiment, there is a method of transforming ethanol into butadiene using a core-shell structured catalyst, and the method includes placing the core-shell structured catalyst (502 / 1110 / 1120) into a reactor to form a bed and passing ethanol through the core-shell structured catalyst bed to generate butadiene with a Lebedev process. The core-shell structured catalyst includes a core made of MgO, an interface of Mg-O-Si, and a shell made of Si, and Cu particles are distributed either within the interface or within the shell.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0012] FIG. 1 is a schematic view of a process for making a catalyst for a one- step ethanol-to-butadiene process;

[0013] FIG. 2 is a schematic view of a process for selectively making a targeted catalyst for a one-step ethanol-to-butadiene process;

[0014] FIGs. 3A and 3B illustrate catalytic activity testing results of Si / MgO and Mg / SiO2 catalysts compared to MgO and wet-kneaded silica-magnesia (WK- 72h-calc) catalysts;

[0015] FIG. 4 illustrates a scheme of acid-base proximity for a Si / MgO catalyst;

[0016] FIG. 5 is a schematic view of a process of making a core-shell structured catalyst for a one-step ethanol-to-butadiene process;

[0017] FIG. 6 is a flowchart of a method for making the core-shell structured catalyst;

[0018] FIG. 7 illustrates results of thermogravimetric analysis coupled with mass spectrometry (TGA-MS) for dried core-shell structured Mg(OH)2-SiO2 samples;

[0019] FIG. 8 illustrates the measured SiO2 shell thickness over about 50 particles, from high resolution microscopy images;

[0020] FIG. 9 is a simulated structure of the core-shell structured Mg(OH)2- SiC>2 catalyst;

[0021] FIG. 10 illustrates the catalytic performance of pristine MgO and SiO2, the conventional wet-kneaded silica-magnesia (WK-4h), and the core-shell structured cs-Si / Mg-0.08 catalyst at 400 °C;

[0022] FIG. 11 illustrates the formation of Cu functionalized catalysts for the ethanol-to-butadiene process;

[0023] FIGs. 12 A and 12B illustrate the addition of the Cu particles to the core and the shell, respectively, for forming the ethanol-to-butadiene catalysts; and

[0024] FIG. 13 is a flowchart of a method for transforming ethanol into butadiene with one of the catalysts discussed in the figures.DETAILED DESCRIPTION OF THE INVENTION

[0025] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

[0026] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0027] According to an embodiment, magnesium silicates formation occurs via dissolution of Si and Mg subunits from SiC>2 and Mg(OH)2 precursors, initiated by the alkaline pH of the aqueous wet-kneading medium, followed by cross-deposition of the dissolved species on the precursor surfaces. Two individual model systems (Mg / SiC>2 and Si / MgO) were synthesized, representative of the constituents of the wet-kneaded silica-magnesia catalyst, by selective dissolution / deposition induced by pH alteration of the aqueous wet-kneading medium. Based on these model catalysts, the inventors found that the location of the magnesium silicates ( / .e., Mg on SiC>2 or Si on MgO) governs not only their chemical nature but also the ethanol adsorptionconfiguration, which ultimately cause the catalyst material to be selective mainly for ethylene or butadiene, respectively. The inventors found that if the acid and basic sites are in close proximity (around or less than 5 A) at the particle level, they better promote butadiene formation than the traditional wet-kneading method. Thus, the new structure-performance relationships that correlate catalytic activity with types and nature of magnesium silicates offer new possibilities for the development of next generation Lebedev catalysts.

[0028] FIG. 2 schematically illustrates a “modified wet-kneaded process” 200 for forming silica-magnesia catalysts that predominantly include Si on MgO 112. The modified wet-kneaded silica-magnesia catalysts were prepared using spherical Stober SiO2 and platelet-shaped Mg(OH)2 at the nominal molar Si / Mg ratio of 1 .0. More specifically, method 200 used the Stober SiC>2, which was prepared by hydrolysis of tetraethyl orthosilicate in a mixture of ethanol and ammonium hydroxide solution. After 15 h of aging, solid SiC>2 nanoparticles were obtained using a rotary evaporator at reduced pressure. The Mg(OH)2 was synthesized by adding 0.4 M NaOH aqueous solution dropwise to 0.2 M Mg(NOa)2 aqueous solution until the pH reached a value of 12. The precipitated Mg(OH)2 particles were separated by centrifugation and washed multiple times with deionized water.

[0029] The as-made wet-kneading precursors were dried overnight in a convection oven at 120°C. The dried precursors were then wet-kneaded in deionized water (the nominal molar ratio of Si and Mg was 1 .0 and the mass ratio of liquid to solid was 95.7 g / g) at room temperature while varying wet-kneading time (10 min to 72 h). After wet-kneading, the samples were obtained by centrifugation and driedovernight at 120 °C (named WK-t / me-dried, where time = 10 min to 72 h). Samples obtained after the drying were finally calcined at 500 °C for 5 h with a heating rate of 5 °C min-1(denoted as WK-t / me-calc). For comparison, a physical mixture of SiC>2 and Mg(OH)2 was prepared using mortar for 10 min. For the two model catalysts (Mg / SiO2 and Si / MgO), modified wet-kneading was performed for 72 h in 3 M ammonium nitrate and 7.3 M ammonium hydroxide aqueous solutions (to control the pH of the mixture, as discussed later), respectively. The model catalysts were obtained by centrifugation, dried and calcined as described above. The loading of Mg and Si on SiC>2 and MgO were 2.6 and 9.8 wt.%, respectively.

[0030] The pH of the wet-kneading solution was monitored in situ over 72 h (not shown). The sharp, initial increase in pH to 10.4 ( / .e., in first 2 min) is owing to the dissolution of brucite (Mg(OH)2 Mg2++ 2OH~), which initiated the wetkneading process. The pH of the wet-kneading solution then gradually decreased to 9.2 and stabilizes after 10 h of wet-kneading. Elemental analysis by inductively coupled plasma optical emission spectrometry (ICP-OES) showed the molar ratio of Si and Mg in solution to follow the same trend as the pH (exponential decay over wet-kneading time), while the ratio in solid to remain consistent at various synthesis times. This suggests that the increased amount of hydroxide ions at the initial stage of the wet-kneading process triggers the dissolution of surface Si species of SiC>2, which subsequently redeposit and form magnesium silicate composites.

[0031] Even though the rapid pH variation occurs only once, at the onset of the process, the inventors have observed that wet-kneading is a dynamic process and the reaction still proceeds when the pH has plateaued. Specifically, not only Sibut also Mg species continuously dissolve out from Mg(OH)2, as confirmed by elemental analysis (not shown). The dissolved Si and Mg subunits can precipitate on the surfaces of Mg(OH)2 and SiC>2, respectively, as a result of the opposite surface charges in this pH range (cross-deposition). The effect of longer-term wet-kneading ( / .e., how dynamic the wet-kneading process is) can be inferred from studies on cement and concrete where changes in similar systems (hydrated magnesiumsilicate materials) have been investigated on timescales of a year. For example, some researchers conducted the long-term synthesis of MgO-SiO2-H2O system for one year and obtained materials containing poorly crystalline magnesium silicates without pristine phases of SiC>2 and MgO. This result suggests that the dissolution / deposition of Si and Mg occurred continuously throughout the experiments.

[0032] To investigate the deposited Si and Mg species at nanoscale, high- angular annular dark-field scanning transmission electron microscopy imaging (HAADF-STEM) with energy dispersive X-ray spectrometry (EDX) analyses were used to characterize a silica-magnesia catalyst isolated after 10 min of wet-kneading (WK-10min-dried, not shown). The precursor SiC>2 and Mg(OH)2 particles were found to be well mixed and in close contact, which was reported to be beneficial for the Lebedev process owing to intimate interactions of the two components. The high magnification STEM-EDX images (not shown) of WK-10min-dried indicate that isolated Si clusters are deposited on the surface of Mg(OH)2. While not as evident as the Si deposition on Mg(OH)2, Mg species are also found on the SiC>2 domains (not shown), indicating deposition of dissolved Mg subunits on the SiC>2. The decreasedcrystallinity of Mg(0H)2 observed after wet-kneading by PXRD also suggests that the dissolution of Mg subunits from brucite layers with the occurrence of crossdeposition of Si subunits (not shown).

[0033] The catalytic activity of pristine materials (SiC>2 and MgO) and wet- kneaded silica-magnesia catalysts obtained after different wet-kneading times was investigated for a 40h timespan. The wet-kneaded silica-magnesia samples do not show any significant deactivation. Pristine SiC>2 and MgO and their physical mixture catalysts exhibited (not showed) limited catalytic performance not only in ethanol conversion rate, but also in selectivity toward butadiene. For example, pristine Stober SiO2 showed no catalytic activity in the Lebedev process, whereas MgO can produce butadiene at very low rates (3.1 pmolbutadiene gcat-1min-1). This indicates that the multiple reaction steps toward butadiene can occur to a limited extent on the intrinsic acidic / basic sites on the pristine MgO surface. Meanwhile, compared to the physical mixture catalyst, a silica-magnesia catalyst results in 4-fold increase in ethanol conversion rates, after 10 min of wet-kneading, and an increase in butadiene selectivity by a factor of 7. This suggests the cross-deposition by wet-kneading allows for the interplay of acidic and basic sites needed for the multiple reaction steps toward butadiene from ethanol.

[0034] As seen above, the wet-kneading method generates active sites by providing an environment for simultaneous dissolution / cross-deposition of Si and Mg, to ultimately generate a mixture of two particles with contrasting catalyst surfaces (e.g., Mg on SiC>2 114 and Si on MgO 112 as illustrated in FIG. 2). These heterogeneous features, inherent in the traditional wet-kneaded silica-magnesiacatalysts, hamper the establishment of direct structure-performance relationships for the Lebedev process. To disentangle these contributions, in this embodiment, the inventors modified the traditional wet-kneading method and prepared two model catalysts where either SiC>2 or MgO are selectively loaded with Mg and Si (Mg / SiC>2 and Si / MgO). The materials were prepared either in ammonium nitrate or ammonium hydroxide solutions, which provide wet-kneading medium pH values of 8.3 or 1 1 .4, respectively. At a pH of 8.3 or below, the dissolution of SiC>2 is limited, whereas the solubility of magnesium species is greatly enhanced by >1014with respect to pure Mg(OH)2 owing to the formation of magnesium nitrate (Ksp,Mg(NO3)2 and Ksp,Mg(OH)2 = 2.4 x 103and 5.6 x 10-12, respectively). In contrast, SiC>2 nanoparticles can be completely dissolved at a pH of 1 1 .4 or more (~10 times solubility increase compared to a pH of 7), while Mg(OH)2 retains its morphological structure. Thus, by controlling the pH 202 to be about 1 1 .4 or more, the modified wet-kneading method primarily generates Si / MgO catalyst and substantially no Mg / SiC>2 catalyst.

[0035] HAADF-STEM with EDX analysis confirmed the disentanglement of two constituents: neither SiC>2 nor MgO phases observed on the Si / MgO and Mg / SiO2 model systems, respectively. PXRD results (not shown) support the above observation that pristine SiO2 and MgO are not present in the corresponding model systems of Si / MgO and Mg / SiO2, respectively, whereas the wet-kneaded catalyst (WK-72h-calc) showed contributions from both pristine SiO2 and MgO. The crystallite sizes of MgO in WK-72h-calc and Si / MgO were similar (8.3 and 8.4 nm, respectively), indicating that the modified wet-kneading only alters the surface of MgO by selective Si dissolution / deposition.

[0036] FIGs. 3A and 3B highlight the catalytic performances of the two model catalysts (Mg / SiO2 1 14 and Si / MgO 1 12) with pristine MgO and WK-72h-calc catalysts for comparison. The two model catalysts 1 12 and 1 14 displayed contrasting catalytic activity, showing about 3 times higher ethanol conversion activity for Si / MgO compared to Mg / SiO2. In addition, the highest selectivity towards ethylene and butadiene were observed for Mg / SiO2 and Si / MgO catalysts, respectively, among all tested catalysts (LHSV 1 .0 h~1and 425 °C). The model Si / MgO catalyst has 36% higher butadiene production rates compared with the conventional wet-kneaded silica-magnesia catalysts (490 vs. avg. 358 pmolbutadiene gCat“1min-1), while minimizing unwanted ethylene production rate (272 vs. avg. 278 pmolethyiene gcat-1min-1).

[0037] The chemical structure of the species on Mg / SiO2 and Si / MgO catalysts was investigated using solid-state29Si and25Mg magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The results show that a linear combination of the spectra from the two model catalysts closely resembles the spectral features of the traditional wet-kneaded silica-magnesia catalysts. For example, the Mg / SiO2 catalyst showed29Si resonances from bulk SiO2 (Q3and Q4at -100 and -110 ppm, respectively) with two additional peaks at -84 and -92 ppm, which are attributed to amorphous magnesium silicates (Q2(1 Mg,1 OH,2Si) and Q3(1 Mg,3Si), respectively. The PXRD patterns (not shown) suggest that the Mg species on SiO2 surface do not form crystalline-layered magnesium silicates owing to the lack of crystalline octahedral Mg2+layers. The Mg / SiO2 showed a distinct25Mg resonance with a large quadrupolar constant (Cq= 2.6 MHz at 6iso 15 ppm),indicating that the bonding geometries of the surface Mg species are considerably different from the Mg species in pristine MgO and those in the Si / MgO catalyst. The acid / base properties of the Mg / SiC>2 catalyst show a remarkable increase in the number of acidic sites compared with the original SiC>2 materials, suggesting isolated Mg species on SiC>2 surface retain an acidic nature toward ethylene formation.

[0038] Meanwhile, for the Si / MgO catalyst, four29Si resonances were observed at -77, -85, -93 and -97 ppm with characteristic1H correlations (0.5 < 51H < 2 ppm), attributed to structural hydroxyl groups (Si-OH-Mg) (not shown). The NH3-TPD data shows that for this catalyst, additional acidic sites are created, showing about 20- and 3-times higher numbers of acidic sites than pristine SiC>2 and MgO, respectively. These results, together with the electron microscopy measurements, indicate that new acidic sites are created in close proximity to a matrix of basic MgO sites by the modified wet-kneading method 200 of FIG. 2.

[0039] To provide deeper and more accurate information about the proximity between acid and basic sites, a novel spectroscopic titration method was developed including the co-adsorption of NH3 and CHCI3 on MgO and Si / MgO catalysts followed by 2D1H double-quantum single-quantum (DQ-SQ) MAS NMR spectroscopy. This 2D correlation NMR technique is able to study the dipolar coupling interactions of a certain proton with the other protons in the proximities of ca. 5 A range, which are the distances between the protons of the chemisorbed NH3 and CHCI3 on acid and basic sites, respectively. For the Si / MgO catalyst, the1H chemical shifts of the adsorbed NH3 on Bronsted acid and Lewis acid sites show self-correlations at 5.0 and -0.1 ppm, respectively. This indicates the acid sitedensity of Si / MgO is higher than MgO, in line with NH3-TPD results. After adsorption of CHCI3 on the pristine MgO and Si / MgO, additional protons are observed at ca. 3.5 and 0.9 ppm, respectively, indicating the proton in CHCI3 can interact with the basic sites in various configurations. Notably, only for Si / MgO catalyst, the acid sites showed correlations to basic sites (5I H,NH3 at 5.0, -0.1 ppm and 5I H,CHCI3 at 3.5, 0.9 ppm, respectively). This suggests that the Si modification on MgO (Si / MgO) creates additional acid sites and these sites are close to basic sites in the 5 A range (see FIG. 4), which is not observed for the pristine MgO and PM catalysts.

[0040] To further investigate the effect of acid / base proximity, the catalytic performance of WK-10min-calc, WK-30min-calc wet-kneaded samples, and the Si / MgO sample, showing similar acid / base ratios but different acid-base proximities, were compared. The Si / MgO sample shows about 50% higher butadiene rates compared to the wet-kneaded samples, which contain a fraction of their acid and basic sites physically separated. Catalytic performance measurements were performed by physically mixing two model catalysts (Si / MgO and Mg / SiO2) and the results show that the physical mixtures are not able to achieve higher butadiene formation rates. All these results highlight the importance of having acid and basic sites in close proximity (about 5 A or less) for the Lebedev reaction, as is the case for the 112 catalyst.

[0041] The modified wet-kneading method 200 (i.e., controlling the pH environment to prevent the formation of the catalyst 114) is rather a non- conventional catalyst synthesis method, but essential for the preparation of active silica-magnesia catalysts for the one-step ethanol-to-butadiene process. Themodified method 200 induced chemical changes in pristine SiC>2 and Mg(OH)2 precursors, in particular the nature of the magnesium silicates thought to be the active components. The modified method demonstrated that under wet-kneading conditions, the cross-deposition of Si and Mg species on Mg(OH)2 and SiC>2, respectively. Two model catalysts prepared by modified wet-kneading, i.e., Si / MgO and Mg / SiC>2, serve different catalytic roles in the Lebedev process. More specifically, it has been found that the magnesium silicates on Si / MgO can be held responsible for butadiene formation, while the magnesium silicates on the Mg / SiO2 primarily produce ethylene. The close proximity (about 5 A or less) of acidic and basic sites in Si-OH-Mg species in Si / MgO is believed to be key to efficient conversion, by lowering the activation energy of multiple steps toward butadiene. This close proximity may be controlled by selecting the pH during the modified method to be about 11 or more for mainly forming catalyst 1 12 and not the catalyst 1 14 or catalyst mixture 112 / 114.

[0042] According to another embodiment, to fully utilize the Si source during a process of making the catalyst for ETB, and to form active sites selective of butadiene (Si subunits on the surface of MgO), a series of core-shell structured MgO-SiC>2 catalysts were made. FIG. 5 schematically illustrates various products of a method 500 for making core-shell MgO-SiC>2 catalyst 502 (called herein “core-shell MgO-SiC>2”) and FIG. 6 is a flowchart corresponding to method 500. Compared to the conventional wet-kneaded silica-magnesia catalyst system illustrated in FIG. 1 , these catalysts substantially reduce the amount of Si source by about 40 times, while showing enhanced catalytic activity towards butadiene.

[0043] The magnesium hydroxide (Mg(OH)2), which is the core material, was obtained via precipitation, adding a 0.4 M sodium hydroxide solution dropwise to a 0.2 M aqueous magnesium nitrate solution. After the pH 12 is reached, the solution was aged for 1 h with constant stirring. The precipitated Mg(OH)2 platelets were separated by centrifugation and washed thoroughly with deionized water and dried overnight in a convection oven at 120 °C. The core-shell structured MgO-SiC>2 catalysts were prepared from the Mg(OH)2, which was provided in step 602 and tetraethyl orthosilicate (TEOS), which was provided in step 604. The TEOS is the only source of Si in this embodiment. The Mg(OH)2 was modified using cetyltrimethylammonium bromide (CTAB) [7], relative to the previous methods. The Mg(OH)2 containing solution was prepared by adding Mg(OH)2 to a solution of anhydrous ethanol, deionized water (18.2 MQ), and an ammonium hydroxide solution (25%). After adding CTAB to the solution in step 606, the mixture was stirred for 30 min to obtain clear and homogeneous solution. For homogeneous core-shell structure, the desired amount of TEOS (to obtain a molar Si / Mg ratio of 0.08, 0.15, 0.23, 0.45, 0.78, and 1 .37) was diluted in ethanol solution and added dropwise in step 608 to the Mg(OH)2 containing solution with continuous stirring. After 24 h, solid particles were separated in step 610 by centrifugation and washed multiple times with deionized water. The as-made solid samples were dried in step 612 overnight in a convection oven at 120 °C. The samples were calcined in step 614 at 500 °C for 5 h with a heating ramp of 5 °C min-1. The calcined catalysts 502 are denoted with molar Si / Mg ratio as cs-Si / Mg- ratio. FIG. 5 shows sample catalysts 502 having a molar Si / Mg ratio of 0.08, 0.15, 0.23, 0.45, 0.78, and 1.37. Note that the molar ratiois selected for the solution used to make the catalyst 502 but it also valid for the final composition of the catalyst 502. It is noted that for a molar Si / Mg ratio smaller than about 0.24, the Si units do not fully cover the MgO particles, while for a Si / Mg ratio equal to or larger than about 0.24, the Si forms a continuous shell 504, i.e., it fully encloses the MgO core 506. An interface 508 is formed between the core 506 and the shell 504 and includes Mg-O-Si. The pristine MgO was obtained by calcination of Mg(OH)2 and the pristine SiO2 was precipitated based on the above procedure without Mg(OH)2 as the core-material. For comparison, the conventional wet- kneaded SiO2-MgO catalyst was prepared with 4 h of wet-kneading time.

[0044] Core-shell structured MgO-SiO2 catalysts 502 were prepared by using platelet-shaped Mg(OH)2 as a core material, varying the thickness of SiO2 shell (by choosing in step 608 a molar Si / Mg ratio between 0.08-1 .37, with a ratio of about 0.24 or more for forming a continuous Si shell, and a ratio smaller than 0.24 for avoiding the Si shell, only depositing Si units on the surface of the core MgO) as schematically illustrated in FIG. 5. To achieve the core-shell structure 502, the inventors promoted the preferential deposition of Si subunits on the surface of Mg(OH)2. By using the method illustrated in FIGs. 5 and 6, CTAB and aqueous ammonia forms micelles in ethanol in step 606, and then the hydrolysis of TEOS in step 608 takes place after diffusion through the micelle walls, having silicate monomers and oligomers (SiO42-species) in the micelles. Then, the deposition of the silicate micelles occurs on the surface of Mg(OH)2 by electrostatic interaction, which results in the formation of the active sites for ethanol-to-butadiene process, that is, magnesium silicates (Mg-O-Si) on the MgO surface. Meanwhile, for thesamples containing a higher amount of Si, the TEOS hydrolysis continues at the magnesium silicates surface, which can lead to the gradual growth of SiC>2 shells.

[0045] FIG. 5 shows Si subunits being sparsely being attached to the MgO platelets when a molar Si / Mg ratio is up to 0.23. However, it is believed that as the molar Si / Mg ratio increases over 0.24, the Si subunits form a shell around the Mg core. It is also believed that the shell is “continuous” around the core. A “continuous” shell can be defined, in one embodiment, as forming a “sieve” around the Mg core, and the sieve has a pore size given by the characteristics of the SiC>2 material, for example, pore size in the range of 1 .6 to 4.4 nm, with possible intermediary values of 3.06 and 3.88 nm. In one application, the sieve has plural pore sizes, for example, 1 .6, 3.06, 3.88, and 4.37 nm, distributed around the core. One or more of these pore sizes are expected to be present all around the Mg core, i.e. , no third-party atom or molecule, having a size larger than the pore size (or the largest pore size if multiple pore sizes are present), is expected to physically penetrate the shell and reach the core. For this definition, it is assumed that the electromagnetic forces between the various ions are ignored, and the sieve concept is a purely mechanical concept. One skilled in the art would understand that this definition works for most of the third-party atoms or molecules, but always there are exceptions, i.e., there are atoms or molecules that have a size larger than the sieve’s pore (or average pore) and they are able to penetrate the shell. However, these isolated cases are the exception and it is expected to not happen in more than 10% of the cases.

[0046] FIG. 7 shows the results of thermogravimetric analysis coupled with mass spectrometry (TGA-MS) for the dried core-shell structured Mg(OH)2-SiO2samples. During the analysis, four stages of the sample mass loss were observed ( / - iv). The first stage (30-150 °C, stage / ) is related to the removal of the physisorbed water from the sample surface. The second mass loss between 200 and 350 °C (stage / / ) is associated with the decomposition of CTAB, which was evidenced by the online MS identification of fragments from CTAB (CH3CH2CH2 and CO2, m / z 43 and 44). With the increased amount of Si content of the samples, a higher mass loss was observed, which resulted in a maximum of -4% of the total mass loss. The decomposition temperature of CTAB shifted to higher temperature for the core-shell structured samples, which is related to the thickness of the porous shell structure.

[0047] During the third step of mass loss (stage (iii)), the dehydroxylation of brucite occurs to form a periclase phase, which is responsible for about 25% of total mass loss. The mass loss at the stage (iii) corresponds to the relative amount of Mg(OH)2 in the samples. Compared to the pristine Mg(OH)2, the core-shell structured samples 502 exhibited a dehydroxylation temperature shifted to higher temperatures (-46 °C), due to the formation of surface magnesium silicates (see inset of FIG. 7). For example, in the cs-Si / Mg-0.08 sample, the dehydroxylation occurred at 376 °C (Tdehydroxyitation of pristine Mg(OH)2 = 327 °C). As the dehydroxylation of brucite starts from its surface and then inwards, it is expected that the incorporation of Si subunits on the surface of Mg(OH)2 retards the dehydroxylation of surface magnesium species, shifting dehydroxylation to higher temperatures. After brucite dehydroxylation, the fourth step (stage iv) at -500 °C resulted in the final mass loss of less than 3%, which is related to further surface dehydroxylation.

[0048] The calcined core-shell structured MgO-SiC catalysts 502 were imaged by scanning electron microscopy (SEM) and transmission electron microscopy combined with electron energy loss spectroscopy (TEM-EELS) (not shown). The pristine MgO has platelet-shape with the average size of ~50 and ~20 nm for the apothem and the thickness of the platelets, respectively. The prepared SiC>2 without core-material showed agglomerated structures composed of few nanometer-sized particles (<3 nm). The SEM and TEM-EELS images of the coreshell MgO-SiC>2 structures 502 showed that SiC>2 is selectively deposited on the surface of MgO. This suggests that the electrostatic interaction between Si subunits and the surface of Mg(OH)2 promoted the surface deposition of Si on Mg(OH)2, while the formation of individual SiO2 particles is suppressed.

[0049] For example, the low Si content samples (cs-Si / Mg-0.08, 0.15 and 0.23) showed that Si subunits homogenously attached to MgO surfaces without forming a complete shell. This is in line with the TGA-MS results that the dehydroxylation of the brucite phase was retarded by Si subunits, due to the formation of magnesium silicates (Mg-O-Si). It should be noted that the formation of magnesium silicates on the surface of MgO created few nanometer sized patterns, which were also observed for the silica-magnesia catalyst prepared by modified wetkneading.

[0050] For the samples 502 with the higher Si amount (larger than 0.24 molar ratio), the MgO core-particles 506 were fully coated by a SiO2 shell 504 with different thicknesses. For the cs-Si / Mg-0.45 sample, each MgO particle 506 is covered by a ~10 nm SiO2 shell 504. The further growth of SiO2 on the surface led to the formationof SiO2 shell 504 as a continuous domain, as observed for cs-Si / Mg-0.78 and 1 .37 catalysts. Meanwhile, the dispersion of core MgO materials were still preserved, as confirmed by the cross-sectional image obtained from focused ion beam scanning electron microscopy (FIB-SEM) and TEM-EELS results (not shown). FIG. 8 illustrates the average thickness of the SiC>2 shell 504 for the core-shell structured catalyst 502 measured by high-resolution bright-field TEM images, as a function of the sample Si content. It is noted that a thickness of the shell is 7 nm or higher when the molar ratio of Si / Mg is 0.24 or higher, respectively.

[0051] Because the textural properties of the core-shell structured catalysts 502 are largely influenced by the shell thickness, nitrogen (N2) physisorption was performed for all the core-shell structured MgO-SiC catalysts (not shown) to explore its properties. The pristine MgO samples exhibited type IV isotherm with the H1 hysteresis, related to the interparticle voids. The SiO2 presented type I isotherms with a sharp N2 adsorption at the beginning (p / po ~ 0.04), indicating the presence of micropores. Meanwhile, the core-shell structures of the catalysts showed hysteresis loops at around p / po 0.5, attributed to capillary condensations in mesopores. The BET surface area of the prepared catalysts increases linearly with the amount of Si.

[0052] X-ray diffraction (XRD) patterns were obtained (not shown) for the prepared core-shell structured MgO-SiO2 catalysts. For the dried samples, hexagonal brucite phase (trigonal symmetry, P-3m1) was observed. After calcination at 500 °C, crystalline periclase phases (cubic symmetry, Fm-3m) were observed, in line with TGA-MS results. The intensities of Mg(OH)2 (101) and MgO (200) planes decreased for the higher amount of Si in the samples, as expected from their Mgcontents in the sample. Notably, the crystallite size of periclase phase decreased with the surface decoration of Si subunits, which has been also observed for the conventional wet-kneaded SiO2-MgO catalysts. This suggests that, during the dehydroxylation of brucite to periclase phases, the surface Si subunits can regulate the growth of Mg(OH)2 crystallites, forming magnesium silicates (Mg-O-Si).

[0053] 20 shifts of the amorphous silica region (20 15-25°) were observed for the calcined catalysts, which were not observed for dried samples (not shown). This suggests that the calcination step results in these shifts, that is, the surface dehydroxylation and related formation of magnesium silicates, as observed by TGA- MS analysis. The local coordination environment and the structure of the core-shell structured MgO-SiC samples were further refined using pair distribution function (PDF) analysis (not shown). For the prepared pristine SiC>2 sample, the bond lengths of 1 .62 and 3.06 A were observed, corresponding to the Si-0 and Si-Si, respectively. The less intense coherence lengths observed at higher distances (>10 A) represents the nano-sized and amorphous characteristics of the pristine SiO2. For cs-Si / Mg-0.08 and 0.45 samples, both the structures of MgO and forsterite (Mg2SiO4) with a sphere refinement model were used for the fitting. Compared to the pristine SiO2 sample, cs-Si / Mg-0.08 did not exhibit the Si-0 bond length around 1 .6 A and most of the interactions are attributed to the Mg-0 (2.14 A) and Mg-Mg (3.04, 5.19, and 6.30 A). This indicates that the Si atoms are homogeneously dispersed around the surface of MgO nanoplatelets.

[0054] For cs-Si / Mg-0.08 and 0.45 samples, two additional peaks were observed at r = 3.65 and 4.71 A, suggesting the Mg-Si distances are similar to thegeometry of forsterite (Mg2SiC>4). For the cs-Si / Mg-0.45 sample, it was observed the corresponding peaks related to the core-MgO structure (around 2.1 and 3.0 A) and to the interface magnesium silicates (around 3.2 and 4.4 A). This indicates that the Si subunits strongly interact with the MgO surface at the interface of the core-shell structure forming magnesium silicates. Meanwhile, albeit its decreased intensity compared to the pristine SiC>2, there is a peak observed related to Mg-Si (3.62 A), which is also observed at the Mg dispersed on SiC>2 domain (Mg on SiC ). This phase was identified for ethylene formation since the surface Mg can act as a Lewis acid site, which is responsible for ethylene production.

[0055] Moreover, the Si-0 interaction was observed for cs-Si / Mg-0.45 sample (r = 1 .66 A), which indicates the SiO2-shell 504 formation at the core-shell structure 502, as schematically illustrated in FIG. 9. Based on the PDF analysis, it is believed that the observed 20 shifts in PXRD is related to the local coordination of the Si subunits. For example, with the increased amount of Si, the formation of amorphous SiO2-shell (Si-Si, ~3.0 A) can lead the Si-0 bond length longer, compared to the magnesium silicates on MgO (Mg-Si, ~3.5 A). FIG. 9 illustrates the atomic structure of the core 506, interface 508, and shell 504 of the catalyst 502.

[0056] The surface chemical composition and chemical nature of the calcined core-shell structured MgO-SiO2 catalysts 502 were also investigated. X-ray photoelectron spectroscopy (XPS) was performed (not shown). The characteristic peaks corresponding to Mg 2p (49 eV), Mg 2s (88 eV), Si 2p (103 eV), Si 2s (154 eV), Mg KLL auger (306 and 350 eV), O 1 s (532 eV), and O KLL auger (980 eV) are present in the wide range XPS spectra of the prepared catalysts. With the increasedSi content, the Si / Mg ratio of the surface composition abruptly increased compared to the bulk Si / Mg ratio, showing the core-MgO particles are coated with the different SiC>2 shell thickness.

[0057] Mg 2p XPS spectra showed that the pristine MgO exhibited a band centered at 49.1 eV, while the shifts to higher binding energy were observed for the core-shell structured catalysts (ABE, max = 1 .3 eV). On the contrary, compared to the pristine SiO2 (103.2 eV), the shifts of Si 2p spectra to lower binding energy were observed for the core-shell catalysts ( BE, max = 1.1 eV), due to the incorporation of Si into the MgO domain. The above binding energy shifts indicate the formation of new type of Mg and Si species for the core-shell structured catalysts 502, attributed to the formation of Mg-O-Si linkages. O 1 s XPS spectra of SiO2 presented a peak centered at 532.6 attributed to Si-0 bond. In the case of MgO, two types of oxygen species were observed at 531 .3 and 529.3 eV, attributed to the oxygen in Mg(OH)2 and / or MgCOa, and the one in MgO, respectively. Shifts to lower binding energy in the O 1 s spectra were also observed, which evidences the formation Mg-O-Si. The electronegativity of Si is higher than Mg (1 .8 and 1 .2, respectively) and the surface distribution of Si subunits on MgO can lead to the increase in binding energy of O 1 s, as result of the decreased outer electron density of oxygen.

[0058] Solid-state NMR spectroscopy has been used to characterize the magnesium silicates formed during wet-kneading. The1H-29Si cross-polarization (CP) MAS NMR spectra of the prepared catalysts was measured (not shown). For the core-shell structured catalysts 502, three29Si resonances were found at -77, -83, and -93 ppm, attributed to the magnesium silicates formed on MgO, which canbe assigned as Q1(3Mg.1 Si), Q2(2Mg,2Si) and lizardite-type Q3(1 Mg,3Si). As observed by PDF analysis (not shown), the chemical environment of Mg-O-Si of core-shell MgO-SiO2 samples is structurally similar to forsterite and enstatite (intermediate species between forsterite and enstatite). With the higher Si content, additional29Si resonances were observed at -91 , -100, and -110 ppm (Q2, Q3and Q4Si species, respectively), indicating the formation of SiC -shell 504.

[0059] Next, the inventors investigated the catalytic performance of the coreshell structured MgO-SiO2 catalysts 502. The pristine MgO, SiC>2, and their physical mixture catalysts have been reported for their insignificant catalytic performances for one-step ETB process. The measurements performed by the inventors confirm that the MgO and SiO2 are not active for butadiene production. This suggests that the pristine catalysts cannot provide the appropriate catalytic sites for the multiple steps required for ETB process, that is, the cooperation of acidic and basic sites. As a comparison, a conventional silica-magnesia catalyst was prepared after 4 h of wetkneading (WK-4h, Si / Mg = 1 .0), which showed the highest butadiene yield in the series of catalysts of the different Si / Mg ratio. During the conventional wet-kneading, the active sites (magnesium silicates) for ETB process can be formed via dissolution and cross-deposition of Si and Mg subunits on the surfaces of MgO and SiO2, respectively. The TEM-EDX analysis on WK-4h showed that MgO and SiO2 are in well-mixed phase and Si species were found on MgO particles, that is, the active magnesium silicates for Lebedev process. The conventional WK-4h catalyst showed ~25 times higher butadiene yield compared to the pristine MgO (YBD = 0.16 and3.8% for MgO and WK-4h at 400 °C, respectively), showing the importance of the magnesium silicates on the butadiene formation from ethanol.

[0060] Compared to the conventional WK-4h catalyst, albeit its low Si content, the core-shell structured MgO-SiC>2 catalyst (cs-Si / Mg-0.08) 502 exhibited about 3 times higher ethanol conversion with the drastically enhanced acetaldehyde formation, as illustrated in FIG. 10. The dehydrogenation of ethanol to acetaldehyde is considered to be catalyzed via heterolytic elimination of hydrogen from ethanol, which can be enhanced by the synergy between acid and basic sites of the SiO2— MgO based catalysts. For example, the dehydrogenation of ethanol is known to be influenced by the strength of basic sites of the catalysts. It was reported in the literature that, over the strong basic sites of the SiO2-MgO catalyst, the dehydration of ethanol is preferred rather than the dehydrogenation of ethanol. For the core-shell structured MgO-SiO2 catalysts 502, the inventors observed the complete changes in the chemistry of basic sites, that is, the formation of new weak and medium strength basic sites with the disappearance of the strong basic sites related to the pristine MgO. Meanwhile, it was observed that the number of acid sites increased by 15 times after the distribution of the Si subunits on the surface of MgO (0.002 and 0.030 mmolNH3 givigo-1for the pristine MgO and cs-Si / Mg-0.08, respectively). Accordingly, it is expected that the formation of the surface magnesium silicates on MgO can result in the complete alteration of acidic / basic characteristics of the catalysts, such as not only the proper balance, but also the close proximity between the acidic and basic sites.

[0061] The inventors then tested the catalytic activity of the prepared coreshell structured MgO-SiC>2 catalysts 502 with different Si / Mg ratios for the Lebedev process, varying the reaction temperatures from 325 to 500 °C. It was observed that the increase of the reaction temperature can lead to not only the improved ethanol conversion, but also the higher butadiene selectivity. The inventors also observed that the experimental butadiene selectivity is much higher than secondary products such as butenes, although the equilibrium concentration of butadiene is much less than butenes (at 500 °C, 56% and 9% for butenes and butadiene, respectively). Moreover, the experimental butene selectivity increased at higher reaction temperatures, whereas the butenes become thermodynamically unfavorable. Therefore, it is believed that the practical Lebedev process is not governed by thermodynamics and that the ethanol conversion, and the product selectivity are mainly dependent on the catalytic activity of the catalysts.

[0062] For the low Si / Mg catalysts (Si / Mg = 0.08-0.23), most of ethanol was converted to acetaldehyde at low reaction temperatures (<400 °C), while the butadiene formation is more favorable with the consumption of acetaldehyde at higher reaction temperatures (>425 °C). It was also observed the formation of heavy oxygenate compounds (C5+) at high temperatures, especially over low Si / Mg ratio catalysts. These compounds can be formed on the basic sites by aldol condensation, which also indicates the favored aldol condensation at high reaction temperature above 400 °C.

[0063] The increase of Si / Mg ratio of the core-shell structured catalysts 502 resulted in the decrease of the ethanol conversion. This suggests that, as observedfor the pristine SiO2, the excessive SiC>2 shell of the core-shell structured catalysts is not active for the ethanol conversion. It is noted that the butadiene selectivity for the high Si / Mg catalysts (Si / Mg = 0.45-1 .37) is comparable to the low Si / Mg catalysts. This result reveals that the formed magnesium silicates (Mg-O-Si), which are located at the interface 508 between core-MgO 506 and shell-SiO2504, provided the catalytic active sites required for the ethanol-to-butadiene pathway. Meanwhile, the ethylene selectivity increased with the higher Si / Mg ratio of the core-shell structured MgO-SiC>2 catalysts. For cs-Si / Mg-1 .37, the ethylene selectivity is higher than 50% with the unfavorable formation of acetaldehyde, indicating that the competitive reaction between dehydration and dehydrogenation of ethanol to ethylene and acetaldehyde, respectively.

[0064] As discussed above, recent studies showed that wet-kneading provides an environment for the concomitant dissolution and cross-deposition of silicon and magnesium subunits, which can lead to the formations of not only the beneficial sites (Si on MgO), but also the detrimental ones (Mg on SiC>2) for the Lebedev process. The use of conventional silica sources to formulate the silica— magnesia catalysts is in part responsible for this negative outcome, as they are eventually dissolved and redispersed on MgO and the remaining SiO2 particles are used for the formation of detrimental sites. In this regard, the method discussed with regard to FIGs. 5 and 6 demonstrated core-shell structured MgO-SiO2 catalysts 502 as a second generation of Lebedev catalyst, where the Si subunits were selectively deposited on the MgO nanoparticles as surface magnesium silicates. The catalytic activity of core-shell structured MgO-SiO2 catalysts were largely influenced by theamount of Si on MgO surface. With increased Si amount, the acidity of catalyst increased due to the formation of different types of magnesium silicates (Mg-O-Si type 1 and type 2). For the optimized Si amount of the catalyst, the acidity and basicity of catalysts were also fine-tuned to promote not only ethanol dehydrogenation to acetaldehyde, but also the remaining reaction steps in Lebedev process, such as aldol condensation, Meerwein-Ponndorf-Verley-Oppenauer, and dehydration reactions, boosting butadiene selectivity while reducing ethylene selectivity. For these catalysts, the inventors reduced the amount of Si source (i.e. , TEOS) by about 40 times with the enhanced catalytic performance towards butadiene, compared to the conventional wet-kneaded silica-magnesia catalyst system.

[0065] A different approach for generating an ETB catalyst is now discussed. Nano-sized reactor catalysts, having different locations of metal functionality, either at the surface of the catalyst or at the interface at Mg-O-Si, between the core MgO and the shell Si, are now discussed. Adding metal functionality is believed to improve the catalytic activity of the catalyst in the Lebedev process. Due to the metal functionality, the dehydrogenation of ethanol to acetaldehyde, which is considered to be the rate-determining step, has been greatly improved. Transition metals can enhance (de)hydrogenation functionality of the silica-magnesia catalyst, reducing the activation energy of acetaldehyde formation, which has been reported as a ratedetermining step. It was found that the location of the metal site is important for the efficiency of the catalyst.

[0066] To control the location of the metal sites, the inventors prepared two different metal loaded core-shell structured MgO-SiO2 catalysts (see catalyst 1 1 10 and catalyst 1 120 in FIG. 11 ). The effects of the proximity of the metal sites to the surface of the catalyst on the butadiene formation from ethanol was investigated for these catalysts. Typically, core and shell materials show independent activity in catalysis. For example, (i) the shell is used to cover core materials, making the ‘inert’ shell to keep the core as itself, or (ii) the core is used for structure directing material to make ‘shell-crust’ and empty core, or (iii) the core and shell act independently but boost the overall reaction, e.g., core for CO2 hydrogenation and shell for MTH owing to its acidic properties after hydrogenation of CO2. The inventors found MgO-SiC core-shell catalysts 1110 and 1120 prove their activity, showing enhanced activity towards BD as ETB catalyst.

[0067] The Cu-promoted MgO-SiC core-shell structured catalysts 1110 and 1 120 (having the Cu located at different locations) were prepared using the coreshell structured MgO-SiC>2 catalyst discussed above with regard to FIGs. 5 and 6, at the nominal molar Si / Mg ratio of 0.78, to secure a thick SiO2 shell over the MgO core particles. Other Si / Mg ratios may be used. The most stable copper has been chosen to be added at the ‘interface Mg-O-Si’ to boost butadiene pathway for catalyst 11 10. For the catalyst 1 120, the Cu particles have been distributed within or on the exterior surface of the shell 504.

[0068] The catalysts 1110 / 1120 are made from magnesium hydroxide (Mg(OH)2) as core material, which was obtained via precipitation, adding a 0.4 M sodium hydroxide solution dropwise to a 0.2 M aqueous magnesium nitrate solution.After the pH 12 is reached, the solution was aged for 1 h with constant stirring. The precipitated Mg(OH)2 platelets were separated by centrifugation and washed thoroughly with deionized water and dried overnight in a convection oven at 120 °C. The core-shell structured MgO-SiC>2 catalysts were prepared from the Mg(OH)2 and TEOS, after modification using CTAB, as discussed with regard to FIGs 5 and 6. The Mg(OH)2 containing solution was prepared by adding Mg(OH)2 to a solution of anhydrous ethanol, deionized water (18.2 MQ), and an ammonium hydroxide solution (25%). After adding CTAB to the solution, the mixture was stirred for 30 min to obtain clear and homogeneous solution. For homogeneous core-shell structure, the desired amount of TEOS was diluted in ethanol solution and added dropwise to the Mg(OH)2 containing solution with continuous stirring. After 24 h, solid particles were separated by centrifugation and washed multiple times with deionized water. The as-made solid samples were dried overnight in a convection oven at 120 °C. The samples were finally calcined at 500 °C for 5 h with a heating ramp of 5 °C min-1. The calcined catalysts are denoted with Si / Mg ratio as cs-Si / Mg- ratio. The pristine MgO was obtained by calcination of Mg(OH)2 and the pristine SiC>2 was precipitated based on the above procedure without Mg(OH)2 as core-material. For comparison, the conventional wet-kneaded SiO2-MgO catalyst was prepared with 4 h of wetkneading time.

[0069] The Cu-functionalized core-shell structured MgO-SiC catalysts 1 110 / 1 120 were obtained by placing the Cu sites either directly on the MgO core (catalyst 1 110) or within the SiO2 shell (catalyst 1 120). These catalysts were prepared by altering the sequence of Cu impregnation as indicated in FIGs. 12A and12B. For example, the Cu-Core catalyst 1 110 was prepared using Cu-loaded Mg(OH)2 (i.e., step 602 in FIG. 6 provided the Cu-loaded Mg(OH)2) and then the SiO2-shell covered the core and the Cu particles. In this case, it is expected that the Cu species are located selectively at the interface 508 between the MgO-core 506 and the SiC -shell 504 (Mg-O-Cu-O-Si). The Cu-shell catalyst 1 120 was prepared using impregnation of Cu on the already prepared core-shell structured MgO-SiC 502, in step 610 or 612, which can yield CuO particles preferentially on or within the SiO2-shell 504.

[0070] It is noted that the pristine MgO and SiC>2 showed negligible catalytic performances for the ETB process. Because the SiC -shell is inactive for the ETB process (the pristine SiC>2 showed less than 1 .5% ethanol conversion), it is expected that the ethanol diffuses through the pores of SiO2-shell and react at the interface Mg-O-Cu-O-Si 508 of the catalyst 11 10, as illustrated in FIG. 1 12A. Because the Cu species 1 102 are selectively formed at the interface 508, the Cu-Core catalyst 1 110 can have the proximity of dehydrogenation and acid / base sites at atomic distances (Mg-O-Cu-O-Si), closely located to the acid-base active sites of magnesium silicates (Mg-O-Si).

[0071] Meanwhile, for the Cu-Shell catalyst 1120, the CuO species 1 102 located at the particle surface can efficiently convert ethanol into acetaldehyde as illustrated in FIG. 12B, which is considered to be the key intermediate of the ETB process. Then acetaldehyde can diffuse to the interface of Mg-Si composite (Mg-O- Si) to be converted to butadiene via aldol condensation. The un-reacted, remaining ethanol should also go through the SiO2-shell 504 to reach the Mg-O-Si interface508, for the Meerwein-Ponndorf-Verley (MPV) reduction to convert crotonaldehyde to crotyl alcohol. In both cases, the final dehydration by Mg-O-Si interface 508 results in the dehydration of crotylalcohol, to form the final product, 1 ,3-butadiene.

[0072] Based on various measurements performed by the inventors, for the Cu-Core catalyst 1110, the particle size of copper species is 2 nm or less (but not zero), suggesting that the copper on the core MgO is in the status of solid-solution. As expected, the SiC>2 shell 504 is covering the core of CuO / MgO domain. For the Cu-Shell catalyst 1120, it was found that the core-shell structure of MgO-SiC has been maintained but the surface of the shell 504 is partially covered by CuO particles of about 5-15 nm in particle size.

[0073] The Cu functionalized core-shell MgO-SiC>2 catalysts 1 110 and 1 120 were investigated to characterize their properties. First, PXRD was used to characterize the catalysts at different Cu locations. For the comparison, the inventors also loaded Cu on the pristine MgO and SiO2 (Cu / MgO and Cu / SiO2). No differences between MgO and CuO / MgO were observed, due to the formation of solid-solution between CuO and MgO. For CuO / SiO2, the formation of CuO on SiO2 was noted, indicating the distinctive formation of CuO particles on SiO2 support. The observed planes of monoclinic CuO (space group C2 / c) are (1 10), (002), (1 11 ), and (20-2) planes at 20 32.6, 35.7, 49.0, and 53.6°, respectively. The particle size is roughly ~10 nm. For the Cu-Core 1 110 and Cu-shell 1120, they resemble the CuO / MgO and CuO / SiO2, respectively.

[0074] The crystallographic features of the prepared catalysts 1 110 and 1 120 were further characterized by PDF analysis. The features of Si-Si, and Mg-Mg wereobserved. The formation of Mg-Si was observed at 3.4 A. The formation of Cu-Cu, Cu-O, Cu-Si, and Cu-Mg were observed for the Cu-Core sample. For Cu-Shell catalyst, the Cu-Mg was not observed. The active sites for Cu-Core sample are Mg- O-Cu-OH-Si. For the shell, the ETB occurs on CuO / SiC>2, and the remaining reaction pathways to butadiene occurs on Mg-O-Si.

[0075] To investigate the coordination environment of Cu atoms, X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of catalysts at the Cu K edge were collected (not shown). The XANES region provides information about oxidation state and local geometry, while EXAFS yields structural details including interatomic distances, coordination numbers and so on. The XANES edge profile of all catalysts was located close to that of its oxide Cu(ll)O suggesting that Cu atoms are in bivalent states. A characteristic feature for Cu2+ions was seen at around 8985 eV, which corresponds to 1 s-4p transition. This feature correlates to the Cu valence state and local symmetry on the Cu sites. Notably, this feature is shifted to higher energy which is more significant in CuO / MgO and Cu-Core catalysts.

[0076] The wavelet plot of the Cu shell have similar coordination structure of Cu( I l)O, indicating a majority of Cu atom exist as a copper oxide particles. The CuO / SiC>2 catalyst shows some characteristic features of Cu(OH)2. To determine the atomic content, the spectrum has been fitted to the combination of Cu(ll)O, Cu(OH)2 and CU2O using Linear Combination Fitting (LCF) method. The LCF fit results indicate considerably higher amount of Cu(ll)O contribute to CuO / SiC>2 spectrum, while Cu-shell has surface hydroxylated copper species (Cu(OH)2). The result isconsistent with the wavelet transform (WT) analysis. Since the EXAFS signals of CuO / SiO2 catalysts have quite similar characteristic features of CuO, the scattering paths at 1 .98 A, 2.91 A and 3.09 A of CuO model were used. The experimental fit results in k and R spectra (not shown) and fitted parameters. The peak at 1 .97(1 ) was seen, that represents the first shell of Cu-0 scattering. The Cu-O-Cu contribution from second and third shell were seen at 2.86(2) A and 3.06(2) A respectively. The fitting result indicates that a major portion of Cu exists in the form of oxide nanoparticles. The Cu shell catalysts exhibit a prominent peak at 1 .91 (3), which is assigned to the Cu-0 bond. Notably another peak was seen at 2.54(3) A, which corresponds to Cu-0 scattering in Cu(OH)2.

[0077] Fourier transformed k2weighted EXAFS spectra were also calculated (not shown). In all catalysts, a prominent peak was observed between 1-2 A that originates from the O atoms surrounding the Cu in the first shell. The wavelettransform (WT) of the k2-weighted EXAFS spectra of Cu standard materials and catalysts was performed using HAMA Fortran software package, which reflects the structural information in the kand R space. Cu foil exhibits wavelet transform at k ~6.6 A-1and R ~2.2 A (not shown) before phase correction, which is assigned to first shell Cu-Cu scattering path. CuO standard materials exhibit a lobe centered at k ~4.9 A-1and R ~1 .5 A corresponding to the first shell Cu-0 coordination environment. Higher shell scattering paths were also seen. Cu(OH)2 shows first shell Cu-0 in the region of k space ~4.8 A-1and R ~1 .4 A. The contribution from higher shells of Cu-O-Cu scattering paths were seen at k ~7.5 A-1and R ~2.5 A. CU2O presents two major lobes, a first one centered at ~5.0 A-1and R ~1 .4 A and a laterone at ~4.8 A-1and R ~2.8 A. The back scattering amplitude F(k) strongly depends on the atomic number Z. The region below 6 A-1is usually populated with lighter elements such as O, Si, Mg, and so on, while the high-Z elements belong to the region above 6 A-1. Back scattering amplitude of scattering path of elements for O, Mg, and Cu were also determined. The WT contour plot of CuO / MgO catalyst shows the first shell Cu-0 scattering path in the region of R ~1 .5 A and k ~3.6 A-1.Besides, another lobe was seen at k ~5.0 A-1and R-2.5 A, which corresponds to the Cu-Mg scattering path. The WT plot of Cu-Core catalyst has two prominent peaks corresponding to Cu-0 and Cu-Mg scattering paths as seen in the CuO / MgO system. However, the intensity of Cu-Mg scattering path is found to be lesser than that of CuO / MgO, suggesting the reduction of Mg neighbors around Cu sites. During Cu-Core synthesis, there is a high probability of having Cu sites in the vicinity of SiO2 / MgO interphase (Mg-O-Si), which can be a plausible reason for less intensive Cu-Mg peak. In addition, a sub lobe is extended to the higher / (corresponds to the Cu-Cu contribution.

[0078] To investigate the coordination environment of Cu atoms, the XANES spectra of CuO / MgO was simulated with the Cu atom used as a dopant in the crystal lattice of MgO (MgO space group fm3m, a = 421 .1 pm). The XANES features of simulated and experimental spectra are quite similar which is evident for the Cu solid solution. EXAFS data fitting can give an insight into the structural details of CuO / MgO catalysts. The aforementioned Cu-dopped MgO model used two scattering paths of Cu-0 and Cu-Mg for fitting. A radial distance value at 1 .99(2) represents the interatomic distance between the Cu and O atoms in the first shell.Compared with Cu foil, which shows a Cu-Cu scattering path at 2.5 A, no Cu-Cu peak was detected, indicating the absence (or negligible) amount of metallic Cu nanoparticles.

[0079] Another prominent peak was seen at 2.99(2) A which originates from the Cu-Mg scattering in the second shell. The average coordination number (CN) for the first (Cu-O) and second (Cu-Mg) shell was determined as 3.4(5) and 6.9(8) respectively. There are reports in the art that Cu in MgO lattice with a coordination number less than 6 is assigned to static disorder due to Jahn-Teller distortion generally occurs in Cu2+ions. The XANES simulation and EXAFS fitting results verify that the Cu atoms are atomically dispersed on the lattice of MgO.

[0080] The catalytic performance of the Cu functionalized catalysts 11 10 and 1 120 was investigated as now discussed. The temperature varied from 300 to 475 °C. The conversion increases with temperature. The product distribution is quite different from sample to sample, showing the influence of the metal sites, which is performing the dehydrogenation function to convert ethanol to acetaldehyde, the rate determining step of the ethanol-to-butadiene. It is noted that the butadiene selectivity increases with temperature for a no-metal catalyst, while the Cu promoted samples (Cu-Core 11 10 and Cu-Shell 1 120) showed a maxima of butadiene selectivity for the varied temperature. The core-shell structured catalysts with copper showed better catalytic activity. The conversion order is slightly different from sample to sample: Cu-Core > Cu-Shell > no-metal. However, the product distribution differs. This indicates that the proximity of the Cu to the surface of the catalysts matters. The Cu- Core 1 110 showed higher activity than Cu-Shell 1120.

[0081] The activity of the core-shell structured materials with different Cu location with dual bed configurations (i.e. , two different catalysts being used in the reactor) were also tested. The reference catalysts such as CuO / SiC>2 and CuO / MgO were also included. The dual-bed configuration of catalysts was provided here, to resemble the Cu-incorporated core-shell structured catalysts (Cu-Core and Cu- Shell). It should be noted that CuO on SiC>2 and CuO on MgO showed different catalytic activity. Compared to non-metal promoted catalysts, the metals are reported to provide the hydrogenolysis function. For Cu / MgO, it was observed the production of butenes (~8% selectivity), indicating C-C coupling of ethanol (and / or intermediates) can occur on MgO sites. Meanwhile, for Cu / SiO2, more ethylene is formed at higher temperatures above 450 °C, due to the acidic nature of the surface silanol groups. For Cu modified MgO and SiO2, acetaldehyde is the main product, without formation of butadiene. This indicates the importance of the existence of multifunctional active sites in the catalysts for remaining reaction steps to butadiene such as aldol condensation, dehydration, which is responsible for magnesium silicates.

[0082] For Cu / MgO based catalysts, for the dual bed (cs-Si / Mg- 0.78+Cu / MgO), the ethanol conversion and the ethylene selectivity increase substantially. This is also observed for the dual-bed system of CuO / SiC>2 + cs-Si / Mg- 0.78 and Cu / MgO + cs-Si / Mg-0.78, where butadiene was produced more selectively. The dual-bed configuration with cs-Si / Mg-0.78 + Cu / MgO showed improved ethanol conversion, suggesting the Cu / MgO is still active. Meanwhile, Cu / MgO + cs-Si / Mg- 0.78 showed not much improved ethanol conversion, while the butadiene selectivityis higher, suggesting that (i) the acetaldehyde formed from Cu / MgO catalyst layer can be further utilized to form butadiene on the cs-Si / Mg-0.78 catalyst, (ii) the catalytic sites (Mg-O-Si) are being (preferentially) occupied and involved in butadiene formation, and (iii) not available for remaining ethanol from the first Cu / MgO catalyst layer.

[0083] Compared to the Cu / MgO related dual-bed configurations, Cu-Core promoted not only ethanol conversion, but also the BD formation, indicating the proximity of dehydrogenation and acid / base sites (Cu and Mg-O-Si) plays an unexpected role in the ETB process. For Cu / SiO2, the ethylene yield is higher at high temperature due to thermodynamics. The Cu / SiO2 showed higher activity than Cu / MgO, as expected from H2-TPR. The results indicate that Cu / SiO2 + cs-Si / Mg- 0.78 is not a good combination. Cu / SiO2 already fully converted ethanol to acetaldehyde around 425 C. This shows the important role of the remaining ethanol for the ETB process.

[0084] The above discussed embodiments indicate that the location of the metal promoter (Cu in this case) is relevant not only for the butadiene yield, but also for the catalyst lifetime in the one-step ethanol-to-butadiene process.

[0085] A method for transforming ethanol into butadiene using a core-shell structured catalyst 502 / 11 10 / 1 120 is now discussed with regard to FIG. 13. The method includes a step 1300 of placing the core-shell structured catalyst into a reactor to form a bed, a step 1302 of passing the ethanol through the core-shell structured catalyst bed to generate butadiene through a Lebedev process (i.e. , a single step process), and a step 1304 of collecting the generated butadiene. Thecore-shell structured catalyst includes a core made of MgO, an interface of Mg-O-Si, and a shell made of Si, and optionally Cu particles are distributed either within the interface or within the shell or on the surface of the shell.

[0086] The above discussed embodiments demonstrate that: (i) the modification of conventional wet-kneading can selectively produce Si / MgO catalyst, by removal of the parts related to the Mg / SiO2. This modification can improve butadiene selectivity while reducing byproduct yield (ethylene); (ii) the modification of the wet-kneaded method uses ‘complete dissolution of SiC ’ to remove SiC>2 nanoparticles, and thus, the use of a TEOS Si source makes a difference. In this regard, the inventors prepared Si / MgO as core-shell structured catalyst as a second generation of Lebedev catalyst (core-shell MgO-SiC ), where the Si subunits were selectively deposited on the MgO nanoparticles as surface magnesium silicates, while avoiding the formation of SiO2 aggregations. It should be noted that the magnesium silicates (Mg-O-Si) can only be formed at the interface between core- MgO and shell-SiO2. Compared to the conventional wet-kneaded silica-magnesia catalyst system, the embodiments discussed above reduce the amount of Si source by about 40 times with the enhanced catalytic performance towards butadiene; and / or (iii) to boost butadiene yield even more, typically transition metals are impregnated to silica-magnesia catalyst. For example, Cu has been mostly studied as a promoter. However, it suffers from fast deactivation during the ethanol-to- butadiene process due to (a) the agglomeration of Cu, and (b) the coke species. To overcome these problems, the inventors prepared the Cu impregnated core-shell structured MgO-SiC catalyst by (1 ) placing atomically dispersed Cu species on Mgsupport and (2) covering the Cu species on the MgO core by the SiC -shell. It is noted that, by placing Cu species at the interface between core-MgO and shell— SiO2 (Cu-Core 11 10), the formation of butadiene occurs more efficiently, showing the initial butadiene yield of 45%, compared to the reference samples (35% and 25% for Cu-Shell and no-metal core-shell MgO-SiO2 samples).

[0087] The term “about” is used in this application to mean a variation of up to 20% of the parameter characterized by this term.

[0088] It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first object or step could be termed a second object or step, and, similarly, a second object or step could be termed a first object or step, without departing from the scope of the present disclosure. The first object or step, and the second object or step, are both, objects or steps, respectively, but they are not to be considered the same object or step.

[0089] The terminology used in the description herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used in this description and the appended claims, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and / or" as used herein refers to and encompasses any possible combinations of one or more of the associated listed items. It will be further understood that the terms "includes," "including," "comprises" and / or "comprising," when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Further, as used herein, the term "if" may be construed to mean "when" or "upon" or "in response to determining" or "in response to detecting," depending on the context.

[0090] The disclosed embodiments provide a (functionalized) core-shell structured catalyst for an ETB process. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0091] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0092] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may includeother examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.ReferencesThe entire content of all the publications listed herein is incorporated by reference in this patent application.

[0001] Pomalaza, G., Arango Ponton, P., Capron, M. & Dumeignil, F. Ethanol-to- butadiene: the reaction and its catalysts. Catal. Sci. Technol. 10, 4860-491 1 (2020).[2] Cabello Gonzalez, G. M. etal. Ethanol conversion into 1 ,3-butadiene over a mixed Hf-Zn catalyst: Effect of reaction conditions and water content in ethanol. Fuel Process. Technol. 193, 263-272 (2019).[3] Taifan, W. E. & Baltrusaitis, J. In Situ Spectroscopic Insights on the Molecular Structure of the MgO / SiO2 Catalytic Active Sites during Ethanol Conversion to 1 ,3- Butadiene. J. Phys. Chem. C 122, 20894-20906 (2018).[4] Makshina, E. V etal. Review of old chemistry and new catalytic advances in the on- purpose synthesis of butadiene. Chem. Soc. Rev. 43, 7917-7953 (2014).[5] Huang, X., Men, Y., Wang, J., An, W. & Wang, Y. Highly active and selective binary MgO-SiO2 catalysts for the production of 1 ,3-butadiene from ethanol. Catal. Sci. Technol. 7, 168-180 (2017).[6] Tonelli, M. etal. Structural characterization of magnesium silicate hydrate: towards the design of eco-sustainable cements. Dalt. Trans. 3294-3304 (2016) doi:10.1039 / C5DT03545G.[7] D. Niu, Z. Ma, Y. Li, J. Shi, Synthesis of core-shell structured dual-mesoporous silica spheres with tunable pore size and controllable shell thickness, J Am Chem Soc, 132 (2010) 15144-15147.

Claims

WHAT IS CLAIMED IS:1 . A core-shell structured catalyst (502) for an ethanol to butadiene process, the catalyst (502) comprising: a core (506) made of MgO particles; a shell (504) made of Si particles; and an interface (508) between the core (506) and the shell (504), the interface being made of magnesium silicate Mg-O-Si, wherein a molar ratio of the Si to Mg is equal to or larger than 0.08.

2. The catalyst of Claim 1 , wherein the molar ratio of the Si to Mg is equal to or larger than 0.24 and the shell fully encapsulates the core.

3. The catalyst of Claim 1 , wherein acid and basic sites on the catalyst are 5 A or less apart from each other.

4. The catalyst of Claim 1 , wherein a thickness of the shell is 7 nm or higher.

5. The catalyst of Claim 1 , further comprising:Cu particles.

6. The catalyst of Claim 5, wherein the Cu particles are distributed only over an exterior surface of the shell.

7. The catalyst of Claim 5, wherein the Cu particles are distributed only within the shell.

8. The catalyst of Claim 5, wherein the Cu particles are distributed only within the interface between the core and the shell.

9. The catalyst of Claim 8, wherein a size of the Cu particles is 2 nm or less.

10. A method for making a core-shell structured catalyst (502) for an ethanol to butadiene process, the method comprising: providing (602) Mg(OH)2 platelets; providing (604) tetraethyl orthosilicate (TEOS) as a source of Si particles; adding (606) cetyltrimethylammonium bromide (CTAB) to the Mg(OH)2 platelets; mixing (608) the TEOS, CTAB, and the Mg(OH)2 platelets to form a mixture; and calcinating (614) particles extracted from the mixture to obtain the core-shell structured catalyst (502).1 1 . The method of Claim 10, wherein a core of the catalyst is made of MgO particles, a shell is made of the Si particles, and an interface between the core and the shell is made of magnesium silicate Mg-O-Si.

12. The method of Claim 10, wherein a molar ratio of Si to Mg in the mixture is equal to or larger than 0.08.

13. The method of Claim 10, wherein the TEOS is the only source of Si particles.

14. The method of Claim 10, wherein the step of providing Mg(OH)2 platelets includes providing Cu loaded Mg(OH)2 platelets.

15. The method of Claim 10, further comprising: separating the particles from the mixture before calcinating the particles.

16. The method of Claim 15, further comprising: loading the particles with Cu before calcinating the particles.

17. The method of Claim 10, wherein the molar ratio of the Si to Mg is equal to or larger than 0.24 and the shell fully encapsulates the core.

18. The method of Claim 10, wherein acid and basic sites on the catalyst are 5 A or less apart from each other.

19. The method of Claim 10, wherein a thickness of the shell is 7 nm or higher.

20. A method of transforming ethanol into butadiene using a core-shell structured catalyst (502 / 1110 / 1120), the method comprising: placing (1300) the core-shell structured catalyst (502 / 1110 / 1120) into a reactor to form a bed; and passing (1302) ethanol through the core-shell structured catalyst bed to generate butadiene with a Lebedev process, wherein the core-shell structured catalyst (502 / 1110 / 1120) includes a core made of MgO, an interface of Mg-O-Si, and a shell made of Si, and wherein Cu particles are distributed either within the interface or within the shell.

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