Ethanol Dehydration to Ethylene over High-Energy Facets Exposed Gamma Alumina

Abstract

Highly efficient and stable catalysts are among the key factors in industrial ethanol dehydration to ethylene. Among the widely studied catalysts, alumina is the most suitable for industrial application. In this study, novel gamma alumina was synthesized by solvent protection and a hydrothermal procedure. HRTEM, XRD, FT-IR, NH₃-TPD, H-D exchange, and ²⁹Si MAS NMR were employed to compare the difference in physicochemical properties between the novel gamma alumina and commercial alumina. Characterization results show that the as-synthesized novel gamma alumina mainly exposes the high-energy crystal plane (111) while the commercial alumina mainly exposes the thermostatically stable (110) crystal plane. The dominating (111) plane, according to the characterizations, endows the novel gamma alumina with a higher density of surface hydroxyl groups, higher acid content, and higher surface energy compared to the commercial alumina. The catalytic performance of the two catalysts for industrial ethanol dehydration to ethylene was studied. The novel (111) plane-exposed alumina showed a higher yield of ethylene than commercial alumina under the same reaction conditions. This could be related to the difference in atomic arrangement and the unsaturated aluminum coordination of different crystal planes. Stability testing under severe reaction conditions (450 °C, 1 MPa, 4 h⁻¹) indicates that novel gamma alumina shows better stability (catalyst life cycle increased by 50%) and produces less acetaldehyde as a byproduct. The effects of steam treatment on the catalytic performance were further investigated. The surface acidity and the catalytic performance of novel gamma alumina present a volcanic curve with the increase in steam treatment temperature. Under the optimal water vapor treatment temperature of 650 °C, the conversion of ethanol and selectivity of ethylene were both higher than 99%.

1. Introduction

Ethylene is one of the most important raw materials in the petrochemical industry, and its production has been considered an indicator to measure the development level of national industry. According to China’s 14th Five-Year plan, China will become the world’s largest ethylene producer, with its production capacity increasing from 35.2 Mt in 2020 to 73.5 Mt by 2025 [1]. In China, ethylene is commonly produced by naphtha cracking (74%) and coal or the methanol-to-olefins process (20%), while ethane dehydrogenation accounts for the largest proportion of ethylene production in the Middle East and the United States [2]. Dwindling fossil fuels and the uneven distribution of petroleum in regions have driven the urgent need to develop alternative renewable feed stocks. In addition, the carbon neutrality policy [3] committed to by the Chinese government also calls for an ethylene production process that is less energy-intensive and creates fewer carbon emissions.

In the past decades, ethanol production from renewable biomass has been well developed; examples include the production of bioethanol from cyanobacteria [4], industrial waste gas utilization to produce ethanol by biological fermentation [5], hydrogenation of CO₂ or CO to produce alcohols [6,7], and the break-through of coal-derived ethanol by the methanol dehydrogenation–DME carbonylation–methyl acetate hydrogenolysis pathway [8,9]. These technologies significantly reduce the price of ethanol and thus endow the process of ethanol dehydration to ethylene with great economic competitiveness (especially compared with the methanol-to-olefins route) and new application prospects [10]. Dehydration of ethanol to ethylene is an acid-catalyzed endothermic reaction with certain products and reactants. Theoretically, increasing the reaction temperature and decreasing the reaction pressure can effectively improve the conversion rate. Therefore, the key point for industrial ethanol dehydration to ethylene is to obtain higher selectivity and reaction stability through catalyst development [11,12]. Extensive studies have been carried out to develop heterogeneous catalysts for the dehydration of ethanol, including alumina [13,14,15,16], zeolites [17,18,19], metal oxides [20,21], and heteropoly acids [22,23]. Outstanding catalysts such as zeolite and heteropoly acid present high selectivity and lower reaction temperature when applied to ethanol dehydration at the laboratory scale, yet problems such as the short catalyst life and the narrow range of reaction temperatures need to be solved. Alumina, due to its impressive selectivity and stability, as well as its low cost, is regarded as the most suitable catalyst for industrial ethanol dehydration to ethylene [24]. Previous studies have been conducted on the preparation and characterization of γ-Al₂O₃ [25,26,27,28]; a few of these focus on crystal plane regulation of industrial gamma alumina and the relationship between surface properties and catalytic performance.

In this work, high-energy plane (111)-exposed gamma alumina was synthesized by solvent protection and a hydrothermal procedure. The differences in structural properties and surface acidity between the above alumina and conventional alumina, which mainly exposes the thermodynamically stable (110) crystal plane, were studied. Catalytic performance of the two catalysts for industrial ethanol dehydration to ethylene in a fixed-bed reactor was compared. Experimental results demonstrated that the novel (111) plane-exposed γ-Al₂O₃ shows higher activity, higher selectivity of ethylene, and higher reaction stability. Various characterizations were employed to describe the relationship between crystal plane exposure and the physicochemical properties of alumina, which inspired a novel strategy to design highly efficient catalysts by manipulating crystal planes.

2. Results and Discussion

2.1. Characterizations

2.1.1. Morphology and Textural Properties

High-energy plane (111)-exposed gamma alumina (HT-γ-Al₂O₃) was synthesized and compared with common commercial alumina (C-γ-Al₂O₃).

A high-resolution transmission electron microscope (HRTEM) was employed to detect the morphology of the two alumina samples. As shown in Figure 1a, the as-synthesized HT-γ-Al₂O₃ presents uniform nanorods with diameters of 7–8 nm and lengths of 60–100 nm. In our previous study [29,30,31], a reasonable three-dimensional model of gamma alumina was constructed according to theoretical calculations, in which the (111) crystal plane and (100) crystal plane alternately formed an octagonal structure at the cross section. The lattice spacing on the section was 0.460 nm, and the plane that presented an angle of 60° with the lattice fringe was discriminated as the (111) crystal plane. It can be deduced that the synthesized HT-γ-Al₂O₃ displayed the (111) crystal plane on the external surface, which increases the ratio of the (111) plane on alumina. This is due to the existence of the surfactant oleic acid, which effectively protects the high-energy (101) plane of AlOOH (see Table S1), and subsequently transforms to the corresponding (111) plane of gamma alumina by calcination (scheme shown in Figure S1). By comparison, as shown in Figure 1b, conventional alumina C-γ-Al₂O₃ showed an irregular porous structure formed by stacking of small particles. In addition, selected area electron diffraction (SAED) were carried out and the results were shown in Figure S2. Diffraction rings of (400) and (440) planes are observable in the SAED patterns, and spots ascribed to (222) can be discriminated in Figure S2a.

X-ray diffraction (XRD) was used to analyze the crystal phase of the two samples, and the results are shown in Figure 2. Diffraction peaks at 37.6°, 45.8°, and 67.0° can be discriminated, corresponding to the characteristic peaks of gamma alumina (JCPDS No. 10-0425). In addition, HT-γ-Al₂O₃ has a narrow half-peak breadth and higher diffraction peak intensity than C-γ-Al₂O₃, indicating a higher crystallization degree and more regular morphology. This has good consistency with the HRTEM results.

It is reasonable to infer that the different morphologies of the two samples are closely related to the synthesis procedure, because HT-γ-Al₂O₃ underwent extra hydrothermal treatment before calcination. In order to investigate the effect of hydrothermal treatment on HT-γ-Al₂O₃, textural properties were detected by N₂ adsorption and are listed in Table 1. Both of the N₂ adsorption–desorption isotherms possess type IV sorption curves according to the IUPAC classification, as is typical of mesoporous materials. There were H2 hysteresis loops within the range of relative pressure of P/P₀ ≈ 0.6–0.9. The high relative pressure of capillary condensation could be associated with larger mesoporous pore size. The desorption branch shows a steep drop at medium pressure, reflecting ink-bottle-shaped pores with small orifices and large cavities. According to Table 1, the pore size of alumina after hydrothermal treatment increases (7.6 nm → 8.7 nm), while the specific surface area decreases (213 m²/g → 208 m²/g), indicating that alumina grows gradually during hydrothermal treatment. It is worth pointing out that HT-γ-Al₂O₃ still maintains a narrow pore distribution after hydrothermal treatment, and shows good thermal stability and potential for catalytic reaction.

2.1.2. Surface Properties

In order to investigate surface functional groups on alumina, Fourier transform infrared spectroscopy (FT-IR) was performed. Generally speaking, low wavenumbers of alumina IR spectra could be assigned to Al-O vibration bands, and high wavenumbers (3850–3550 cm⁻¹) were caused by surface hydroxyl groups. The spectra of the two catalysts in the region of surface hydroxyl groups are presented in Figure 3.

In the range of 3850–3550 cm⁻¹ there is a complex of 4–6 closely overlapped bands, which have been variously attributed to free or quasi-free surface OH groups. The different intensities usually imply the distinction in morphology (different exposed planes, corners, and edges), structure, or the quantity and type of impurities [32,33]. According to the spinel structure model of alumina and the hydroxyl model proposed by Knözinger et al. [34], there are five types of hydroxyl groups (types III, III, IIa, IIb, Ia and Ib) on alumina. Among the above OH groups, type III and type IIa (band at 3730–3710 cm⁻¹) are characteristic hydroxyl groups on the (111) crystal plane [35], representing hydroxyl HO-µ₂-Al_VI and HO-µ₁-Al_VI (where Al_X stands for aluminum atoms surrounded by X oxygen atoms and HO-µ_n stands for OH groups linked to n aluminum atoms) on the (111) crystal plane, respectively. In addition, the characteristic hydroxyl groups on the (111) plane have the strongest charges among all hydroxyl groups. Therefore, the presence of type III and IIa hydroxyl groups predicts stronger surface Brønsted acidity.

Various studies have been carried out on the surface structure (hydroxyl group type, density, etc.) and surface properties of alumina [27,36,37]. For the low exponential surface (111), (110), and (100) planes of gamma alumina, the atomic arrangement and coordination number have a remarkable influence on the surface properties. According to the structural model proposed by Digne et al. [38,39], O and Al atoms appear alternately on the (111) crystal plane, penta-coordinated Al atoms and O atoms with triple coordination exist on the (100) crystal plane, while Al atoms with both quadruple and triple coordination, and two-coordinated and tri-coordinated O atoms appear on the (110) crystal plane. Theoretical calculation results (see Table S2) show that the order of both the number of Al-O bonds per unit area and energies of each Al-O bond is (111) > (110) > (100), and the surface energies of the contracted surfaces are 4480, 2590, and 1430 mJ/m², respectively; that is, among the low exponential planes of γ-Al₂O₃, the (111) crystal plane has the highest acid density and surface energy.

²⁹Si MAS NMR measurements were carried out to study the dispersion and interaction on the alumina surface, as shown in Figure 4. Before the characterization, 4 wt% SiO₂ (lower than the dispersion capacity of the alumina monolayer) was loaded on the two alumina samples. The resonance at −116 ppm was attributed to the crystallographically equivalent site of the Si(OSi)₄ (Q4) group, and resonance at −78 and −83 ppm can be assigned to Si(OSi)₂(OH)₂ and Si(OAl)₃OH, respectively [40]. As shown in Figure 4, Si species on the HT-γ-Al₂O₃ surface are mainly Si(OAl)₃OH groups, reflecting less contact between Si species, whereas there are three kinds of Si species on the surface of C-γ-Al₂O₃, implying Si species are in the aggregation state; that is, HT-γ-Al₂O₃ has stronger interaction with surface species and better dispersion on the surface than conventional alumina C-γ-Al₂O₃. According to the above analysis, HT-γ-Al₂O₃ preferentially exposes the (111) crystal plane; hence, the strong interaction can be reasonably attributed to the higher surface energy of (111) crystal plane. In summary, Si species tend to form aggregates on the (110) crystal plane, while on the (111) crystal plane they tend to form monodisperse Si species, as shown in Figure 4b.

2.1.3. Acidity

The type and quantity of coordinative unsaturated aluminum on the surface are strongly related to the surface acidity. The coordinative unsaturated Al (Lewis acid) on the surface can provide a vacant orbital for the OH group, and the unsaturated O atom (Lewis base) with a lone-pair electron can accept a proton to form the OH group. H₂O absorbed on the alumina surface dissociates and is subsequently hydroxylated. The Lewis acid receives the OH group to form a Brønsted base, while the Lewis base receives a proton (from H₂O dissociation) to form a Brønsted acid. The hydroxylation and dehydration on the alumina surface are reversible. Generally, thermodynamically stable (less-active) crystal planes tend to be exposed first during crystal growth. For gamma alumina, the preferential crystal plane is (110). Tetra-coordinated Al atoms (layer C in Figure 5b) on the (110) crystal plane are dehydroxylated to form quasi-tri-coordinated Al. However, it will spontaneously enter the octahedral vacancy of the next layer to form hexa-coordinated Al, due to its high energy density and unstable geometry. For the (111) crystal plane, tetra-coordinated Al atoms (labeled A in Figure 5a) also form quasi-tri-coordinated Al by OH removal, but it can exist because of the relatively stable geometric configuration. In addition, the Al vacancy on the (111) crystal plane helps reduce the energy, making the quasi-tri-coordinated Al atoms stable. Under normal conditions, the lower the coordination number of aluminum, the stronger the Lewis acidity. In general, tri-coordinated Al on the alumina surface represents a strong acid center, tetra-coordinated Al means a medium-strong acid center, and penta-coordinated Al means a weak acid center. In summary, the (111) crystal plane has stronger Lewis acidity than the (110) crystal plane due to the presence of quasi-tri-coordinated aluminum.

D₂-OH isotope thermally driven exchange was used to further characterize the surface hydroxyl density and Brønsted acid intensity of alumina. The activation energy for the exchange reaction D₂ + OH → OD + HD can be correlated with the O-H bond energy and the exchange is easier with a weaker O-H bond, as supported by DFT calculations [41], implying stronger acidic OH groups exchange with D₂ at lower temperatures. Hydroxyl density can be quantitatively determined after all H-D exchange is completed. The obtained H-D exchange curve is shown in Figure 6a. From the above analysis, the peak at the lower temperatures corresponds to the D₂ exchange with the acidic hydroxyl groups and the peak at the higher temperatures is the D₂ exchange with the non-acidic hydroxyl groups. HT-γ-Al₂O₃ possesses a larger number of acidic hydroxyl groups than C-γ-Al₂O₃, in accordance with the above structure analysis that shows that HT-γ-Al₂O₃ mainly exposes the (111) plane. According to mass spectrum results, the hydroxyl density of HT-γ-Al₂O₃ and C-γ-Al₂O₃ is 0.90 × 10¹⁵ and 0.62 × 10¹⁵ cm⁻², respectively. That means the hydroxyl density of alumina increases after hydrothermal treatment. In addition, the HT-γ-Al₂O₃ curve started to exchange at a lower temperature (250 °C), indicating that hydrogen in the hydroxyl group on its surface was more easily dissociated. This further proves the stronger acidity of HT-γ-Al₂O₃, which is in good agreement with the FT-IR results.

NH₃ temperature-programmed desorption (NH₃−TPD) was employed to characterize the acid strength and acid content distribution on the surface of the two catalysts, as shown in Figure 6b. Both HT-γ-Al₂O₃ and C-γ-Al₂O₃ show two NH₃ desorption peaks that represent two kinds of acid center, where the low-temperature desorption peak represents a weak acid center, and the middle temperature desorption peak represents a medium-strong acid center. Considering the shoulder peak of ammonia desorption at ~350 °C, HT-γ-Al₂O₃ has higher acid density and acidity than the other samples. This verifies that the exposed surfaces of HT-γ-Al₂O₃ possess more dense acidic OH groups, which is a property of the (111) crystal plane. According to the peak area of NH₃ desorption, the weak acid content of the two alumina samples is basically the same, while the medium-strong acid content of HT-γ-Al₂O₃ (with hydrothermal treatment) increases significantly and shifts to a lower temperature, indicating an increase in total acid content and relatively constant acid strength. This kind of increase in non-strong acid will, in principle, improve the performance of HT-γ-Al₂O₃ as a catalyst for ethanol dehydration.

2.2. Ethanol Dehydration

2.2.1. Catalytic Performance

The two alumina catalysts were applied to the ethanol dehydration reaction to study their catalytic performance. In addition to the main product of ethylene, the reaction of ethanol dehydration also generates a small quantity of byproducts, such as diethyl ether, acetaldehyde, and hydrocarbons (methane, ethane, propylene, and C4). Extensive experiments [42,43] show that the byproduct is mainly diethyl ether at the lower conversion of ethanol, while it becomes C4 at high ethanol conversion. The ethanol dehydration reaction routes mainly involve three reversible reactions, namely, ethanol intramolecular dehydration to ethylene, ethanol intermolecular dehydration to diethyl ether, and diethyl ether dehydration to ethylene. As can be seen from Figure 7a, the yield of ethylene over the two catalysts increases with the increase in the reaction temperature, reaching complete conversion to ethylene at about 350 °C. Ethanol dehydration is a reversible reaction, of which the forward reaction is endothermic. Therefore, when raising the reaction temperature, the reaction equilibrium moves in the direction of the forward reaction, thus increasing the conversion rate of ethanol. In addition, reaction temperature has a significant influence on the product distribution. It favors ethanol intermolecular dehydration to form diethyl ether at a relatively low reaction temperature, while high temperature is conducive to the intramolecular dehydration of ethanol to form ethylene. At a low reaction temperature, not only is conversion of ethanol lower, but more diethyl ether is also produced. The two aspects together lead to a low yield of ethylene. Therefore, adequate temperature is necessary for ethanol dehydration on alumina.

It is worth noting that HT-γ-Al₂O₃ after hydrothermal treatment shows better low-temperature activity at the same LHSV, due to the stronger acidity on its surface. For example, at 300 °C, the yield of ethylene on HT-γ-Al₂O₃ is about 70%, while that on C-γ-Al₂O₃ is only 42%. Low-temperature activity is of great significance for industrial ethanol dehydration catalysts because the deactivation of the ethanol dehydration catalyst is usually caused by carbon deposits, which are significantly affected by the reaction temperature. On the acid center of alumina, the ethanol molecule is eliminated to intermediately form carbon, and then rapidly loses β-H to form alkenes. In the meantime, the series reaction of ethylene polymerization takes place and generates carbon deposits [11]. With an increase in the reaction temperature, the rate of polymerization series reaction increases and the carbon accumulation rate accelerates. As a result, the surface area, pore volume, and acidity of catalysts will decrease, leading to final deactivation. It can be deduced that if one catalyst possesses excellent low-temperature activity, it means that the catalyst could operate at a lower reaction temperature, thus slowing down carbon deposit and extending the catalyst’s life. Therefore, HT-γ-Al₂O₃ shows greater potential as a catalyst for industrial ethanol dehydration due to its excellent low-temperature activity.

The key step of ethanol dehydration to ethylene on an alumina catalyst is ethanol’s interaction with the surface Brønsted acid site (Al-OH) to form ethoxy intermediates [44], in which the surface OH group plays an important role. An in situ infrared experiment of ethanol dehydration showed that in the initial stage of the reaction, the disappearance of OH groups and emergence of ethoxy groups occurred simultaneously [26]. A part of ethanol formed ethoxy groups and H₂O by replacing OH groups on the alumina surface, while another part of ethanol tended to form ethoxy groups and new OH groups by dissociation and adsorption on Lewis acid–base pairs. Surface ethoxy groups are the intermediate stage to producing both ethylene and diethyl ether.

Typical product distributions on alumina at different conversion levels are listed in Table 2. There was almost no diethyl ether at high conversion (~99%), but a certain amount of diethyl ether (9.14%) appeared at the medium conversion level. Since the reaction order of diethyl ether synthesis is higher than that of ethanol dehydration to ethylene [45], the partial pressure of reactant ethanol becomes an important issue. It is generally believed that diethyl ether is formed through nucleophilic substitution reaction, in which the ethoxy group acts as a nucleophilic reagent and interacts with another ethanol molecule to increase the electrophilicity of the alcohol carbon atom, and further nucleophilic substitution to form diethyl ether (reversible reaction). When fewer ethanol molecules are available (i.e., high conversion level of ethanol), the reversible reaction disappears and the reaction of ethylene production dominates.

The effect of different space velocities on catalytic performance (reaction temperature 350 °C) was further investigated, as shown in Figure 7b. The yield of ethylene decreased significantly with the increase in space velocity. On one hand, the short residence time of ethanol and the limited contact with active sites at a high space velocity are responsible for decreasing conversion. On the other hand, as the conversion level decreases, it produces more diethyl ether, as mentioned above, which, overall, decreases the yield of ethylene.

In addition, the yield of ethylene decreases more slowly on HT-γ-Al₂O₃ than C-γ-Al₂O₃ with the increase in space velocity, making HT-γ-Al₂O₃ a potential catalyst for industrial ethanol dehydration at a high space velocity. This is because HT-γ-Al₂O₃ mainly exposes the (111) crystal plane and thus presents more edge sites than C-γ-Al₂O₃, which mainly exposes the (110) crystal plane [46]; in addition, OH groups and coordinated unsaturated aluminum on different crystal planes result in different acid strengths [34]. The two aspects together affect product distribution on the HT-γ-Al₂O₃ catalyst. According to H-D exchange and NH₃-TPD, the HT-γ-Al₂O₃ catalyst possesses higher density of OH groups and stronger acidity, leading to the stronger adsorption strength of ethanol on the surface and forming more ethoxy intermediates, which are conducive to the formation of ethylene.

2.2.2. Stability

In addition to reactivity and selectivity, long period stability is another important index of catalysts for industrial ethanol dehydration to ethylene. The stability of the two catalysts was tested under severe conditions (450 °C, 1 MPa, 4 h⁻¹) with industrial ethanol (95%) as reactant. Changing curves of product ethylene and the main byproduct acetaldehyde are shown in Figure 8. HT-γ-Al₂O₃ shows better stability (catalyst life cycle increased by 50%), and produces less acetaldehyde byproduct. In addition, as previously mentioned, HT-γ-Al₂O₃ has greater low-temperature activity and can operate stably at a lower temperature, resulting in an improvement in catalyst life of more than 50%.

The adsorption–desorption of H₂O and ethanol is competitive. When the reaction temperature is fixed, theoretically, the difference in the degree of hydroxylation between two alumina surfaces will gradually weaken or even disappear as the desorption of reactant H₂O and the re-adsorption of the generated H₂O reaches an equilibrium. However, based on results in Figure 8, the selectivity of ethylene and acetaldehyde on the two catalysts varies significantly, which is caused by the different acid distributions on the surface. According to mechanism studies [47], ethanol dehydrogenates to acetaldehyde at basic sites and dehydrates directly or via diethyl ether to ethylene at acidic sites. Because of the strong acidity and higher total acid content of HT-γ-Al₂O₃, the reaction of ethanol dehydration to ethylene always dominates on the surface.

It is generally believed that the byproduct C4 is produced through ethylene dimerization. On one hand, strong acid sites of the catalyst should be strengthened in order to decelerate C4 production, due to the lower polymerization rate of ethylene at strong acid sites. On the other hand, however, excessive strong acid will accelerate deactivation, because the strong acid center is the active center of the carbon deposit. Therefore, in order to design selective and stable catalysts for ethanol dehydration, the appropriate amount of medium-strong acid is an important consideration. The above experimental results are consistent with NH₃-TPD. The amount of medium-strong acid on HT-γ-Al₂O₃ after hydrothermal treatment increases, endowing HT-γ-Al₂O₃ with higher product selectivity and reaction stability.

2.2.3. Effect of Steam Treatment

Steam treatment is considered an effective way to adjust the physicochemical properties of the alumina surface. In order to further improve the catalytic performance, the aforementioned selective and stable HT-γ-Al₂O₃ was treated with water vapor at different temperatures for 3 h. The treated alumina catalysts were applied to a 95% ethanol dehydration reaction at a low temperature (320 °C), and the catalytic performance is shown in Table 3. When HT-γ-Al₂O₃ was treated in steam at 650 °C, the catalytic performance significantly improved, resulting in conversion of ethanol and selectivity of ethylene that were both higher than 99%. The catalytic performance of HT-γ-Al₂O₃ compared with catalysts from the literature is listed in Table 4. Comprehensively considering the effect of reaction temperature and ethanol purity, catalysts in this work show the highest activity for ethanol dehydration to ethylene.

The effect of steam treatment on physicochemical properties and Al atom migration of alumina was investigated by XRD. For comparison, C-γ-Al₂O₃ was also treated with water vapor under the same conditions, and the XRD patterns of the two catalysts after steam treatment are shown in Figure 9.

As shown in the XRD patterns, the crystal size of both types of alumina did not change significantly after high-temperature treatment. With the increase in the steam treatment temperature, the crystal structure of HT-γ-Al₂O₃ changes obviously. For example, δ-phase occurred after the 800 °C treatment and θ-phase was detected after the 900 °C treatment, while C-γ-Al₂O₃ did not show a phase transformation. This phenomenon could be ascribed to different surface energies of different crystal planes. C-γ-Al₂O₃ with the (110) crystal plane shows a lower surface energy, and will remain stable at high temperature. HT-γ-Al₂O₃ mainly exposes the high-energy (111) crystal plane and tends to transfer into a more stable state at high temperature, resulting in the phase transformation during steam treatment. In the environment of water vapor, OH groups around the Al atom form a large number of intramolecular and intermolecular hydrogen bonds with H₂O. The bond energy of the Al-O bond is weakened due to the existence of these hydrogen bonds, and breaks at high temperature, causing migration or removal of hexa-coordinated Al. As a result, the stacking mode between Al-O atoms changes and thus affects the distribution of surface acidity. According to BET characterization results, the specific surface area showed a decreasing trend with the increase in steam treatment temperature, but the pore structure of alumina did not change significantly. This is because only Al³⁺ local migration occurred during the transformation from γ-phase to θ-phase, while the O^2- did not rearrange.

To analyze surface acidity of HT-γ-Al₂O₃ treated at different temperatures, NH₃-TPD experiments were performed and results are shown in Figure 10. With the increase in the steam treatment temperature from 550 °C to 650 °C, the acid density per unit mass ought to decrease because of the decreasing specific surface area, but the acid density (peak area) actually increases in Figure 10. Based on the theoretical calculation of OH groups’ desorption on different alumina crystal planes, the binding energy of the OH groups on the (111) plane is higher than that on other planes. Therefore, desorption of OH groups on the (111) crystal plane requires a higher temperature. Only if the steam treatment temperature rises to 650 °C will a large number of OH groups remove from the surface, thus resulting in increased acid content. When the steam treatment temperature further increases to a high temperature (900 °C), total acid content decreases due to the decreasing surface area, and the catalytic performance of ethanol dehydration obviously decreases (Table 3). Based on the above results, the optimal steam treatment temperature was determined as 650 °C.

3. Materials and Methods

3.1. Materials

Oleic acid (C₁₈H₃₄O₂, AR, Sigma-Aldrich, St. Louis, MO, USA), boehmite colloidal particulates (AlOOH, CP, Zhejiang Yuda Chemical Co., Ltd., Ningbo, China), ammonia (NH₃·H₂O, AR, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), absolute ethanol (C₂H₆O, AR, Aladdin Holdings Group Co., Ltd., Shanghai, China), and ethanol (C₂H₆O, 95%, Aladdin Holdings Group Co., Ltd., Shanghai, China) were all purchased from commercial sources.

3.2. Catalyst Synthesis

The typical procedure to synthesize novel alumina was described in detail in our previous work [29,30,31]. Quantities of 5 g of oleic acid and 60 g of boehmite colloidal particulates were dispersed in deionized water under magnetic stirring at 80 °C. An appropriate amount of ammonia was added drop-wise into the mixture to adjust pH to ~8.5. After stirring overnight, the mixture was transferred into a Teflon-lined stainless steel autoclave and heated to 150 °C for 48 h. After cooling to room temperature, the solid precipitate was collected by centrifugation, rinsed with absolute ethanol several times, and dried at 80 °C. The precursor powder was then calcined at 550 °C for 4 h to obtain the final product, denoted as HT-γ-Al₂O₃ (where HT stands for hydrothermal). For comparison, industrial γ-Al₂O₃ was acquired from SINOPEC Shanghai Research Institute of Petrochemical Technology Co., Ltd. and denoted as C-γ-Al₂O₃ (where C stands for commercial). The C-γ-Al₂O₃ sample was also calcined at 550 °C for 4 h.

3.3. Characterizations

The morphology of the samples was examined using a JEOL JEM-2010 high-resolution transmission electron microscope (HRTEM, Tokyo, Japan), operated at an accelerating voltage of 200 kV.

X-ray powder diffraction (XRD) was performed on a Philips X’pert Pro diffractometer (Amsterdam, Netherlands) using Cu-Kα radiation (λ = 0.15418 nm) generated at 40 kV and 40 mA, with a scanning range from 10° to 80° at a speed of 0.02°/s.

The specific surface area and pore size distribution of the samples were measured by nitrogen adsorption using a Micromeritics ASAP-2020 analyzer (Norcross, GA, USA). Before the analysis, the samples were degassed for sample preparation at 120 °C in a vacuum for 1 h. The specific surface area was calculated according to the Brunauer–Emmett–Teller (BET) model. The total pore volume and pore size distribution were evaluated using the Barrett–Joyner–Halenda (BJH) method.

NH₃-TPD profiles were obtained using a BEL-CAT-B-82 apparatus (Osaka, Japan) equipped with a thermal conductivity detector (TCD) and mass spectrum (MS). Prior to the adsorption of NH₃, a 0.1 g sample was pretreated in He flow at 600 °C (20 °C/min) for 30 min and then cooled to 100 °C. The sample was then saturated with a 5% NH₃/He stream at 100 °C for 30 min. Then the excess ammonia was removed by He. Ammonia gradually desorbed by heating from 100 °C to 600 °C at a rate of 10 °C/min.

Fourier transform infrared spectroscopy (FT-IR) was undertaken using a Nicolet 5700 spectrometer (Waltham, MA, USA) equipped with a KBr beam splitter. A quantity of 50 mg of sample powder (dried, 200 mesh) was pressed into a transparent sheet on a tablet press for measurement. Before the characterization, samples were pretreated under vacuum at 300 °C for 0.5 h. The FT-IR setup and procedures used were detailed in the literature [48,49]. The spectra were collected with a 4 cm⁻¹ resolution in 4000–1300 cm⁻¹ and 32 scans were accumulated for each spectrum.

H-D exchange was used to characterize the surface hydroxyl density. The catalyst samples (400 mg) were pretreated in He at 600 °C (10 °C/min) for 1 h and then cooled to room temperature. The deuterium (6%D₂/Ar) exchanged with protonium presented in the samples was measured by increasing the temperature to 700 °C (10 °C/min). The signal of HD molecules was monitored by mass spectrometry. Ar was used as an internal standard to calculate the formation of HD during the exchange between D₂ and hydroxyls in the samples. The density of hydroxyl groups on the surface was calculated by detecting the final exchanged H-D amount at equilibrium and according to the quantitative relation of D₂ + OH → OD + HD. The H-D exchange setup and procedures used were detailed in the literature [50,51].

²⁹Si MAS NMR was determined using a Bruker Avance Type III pulsed solid state NMR apparatus (Karlsruhe, Germany) with a dual-channel 7.0 mm probe. The powder sample was placed in a ZrO₂ rotor with a rotation speed of 6 kHz. The ²⁹Si chemical shift was corrected using an external standard, referenced by DSS (4,4-Dimethyl-4-silapentane-1-sulfonic acid sodium salt) to 1.534 ppm.

3.4. Catalytic Test

Catalytic experiments of ethanol dehydration to ethylene were carried out in a fixed-bed reactor. A quantity of 10 mL of catalyst (40–60 mesh) was placed in a stainless steel tube with an inner diameter of 10 mm. Reactant ethanol (absolute ethanol or 95% ethanol) was injected into the tube by a metering pump at a constant speed, and the reaction temperature was 260–420 °C. Output products were passed through a gas–liquid separator to obtain liquid phase products (including water, ethanol, diethyl ether, and acetaldehyde) and gas phase products (including ethylene, C3 and C4). The product composition was analyzed using a HP-6890 gas chromatograph. Conversion of ethanol, selectivity, and yield were calculated based on the carbon atomic mole number.

4. Conclusions

Among the widely studied catalysts, alumina is the most suitable for industrial ethanol dehydration to ethylene. In order to investigate the influence of crystal surface regulation on surface properties and microstructure of industrial γ-Al₂O₃, γ-Al₂O₃ with the high-energy (111) crystal plane exposed (i.e., HT-γ-Al₂O₃) was synthesized by solvent protection and hydrothermal treatment, and the differences in structural properties between HT-γ-Al₂O₃ and industrial alumina with the thermodynamically stable (110) crystal surface exposed (i.e., C-γ-Al₂O₃) were studied. Catalytic performance of the two catalysts for industrial ethanol dehydration to ethylene in a fixed-bed reactor was compared. The novel (111) plane-exposed γ-Al₂O₃ showed higher activity, higher selectivity of ethylene, and higher reaction stability. Characterization results indicate that the types and densities of surface OH groups, acid content, and surface interactions are different due to the differences in atomic arrangement and coordination of unsaturated aluminum of different crystal planes. Afterwards, effects of reaction temperature, space velocity, and steam treatment on the performance of HT-γ-Al₂O₃ were investigated. Under the optimal conditions, both the conversion of ethanol and selectivity of ethylene on the HT-γ-Al₂O₃ catalyst were higher than 99%. This work shows the possibility of changing alumina surface properties by crystal plane manipulation and provides a new strategy for further improving the catalytic performance of catalysts for industrial ethanol dehydration to ethylene.

Author Contributions

Conceptualization, J.L., D.W. and W.Y.; Data curation, X.G.; Investigation, X.G.; Methodology, J.L., D.W. and W.D.; Resources, L.P., W.D. and W.Y.; Supervision, W.Y.; Writing—original draft, J.L.; Writing—review and editing, D.W. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. HRTEM images of the two alumina samples (a) HT-γ-Al2O3 and (b) C-γ-Al2O3.

Figure 2. XRD patterns of the two alumina samples.

Figure 3. FT−IR spectra (the region of surface hydroxyl groups) of alumina catalysts.

Figure 4. Dispersion on alumina surface. (a) 29Si MAS NMR of 4%SiO2/γ−Al2O3 and (b) dispersion of Si species on different crystal facets.

Figure 5. Diagram of surface atoms on (a) (111) facet (b) (110) facet of γ-Al2O3, where red spheres represent O atoms, and blue and green spheres represent Al atoms.

Figure 6. (a) H−D revolution profiles and (b) NH3−TPD results of the two alumina catalysts.

Figure 7. Catalytic performance of the two catalysts for absolute alcohol dehydration at (a) different reaction temperatures and (b) different LHSV.

Figure 8. Stability test under severe conditions (450 °C, 1 MPa, 4 h−1) with industrial ethanol as reactant: selectivity of (a) ethylene and (b) byproduct acetaldehyde.

Figure 9. XRD patterns of (a) HT-γ-Al2O3 and (b) C-γ-Al2O3 treated at different temperatures in steam.

Figure 10. NH3-TPD of HT-γ-Al2O3 catalyst treated at different temperatures in steam.

Table 1

Textural properties of the two alumina samples.

Sample	Specific Surface Area (m2/g)	Average Pore Diameter (nm)	Pore Volume (m3/g)
C-γ-Al2O3	213	7.6	0.410
HT-γ-Al2O3	208	8.7	0.453

Table 2

Typical product distribution (%) at different conversion levels.

	Ethylene	Ethane	Propylene	Acetaldehyde	C4	Ethanol	Diethyl Ether	Others
High conversion	99.77	0.12	0.01	0.01	0.07	0.00	0.01	0.00
Medium conversion	90.20	0.29	0.04	0.02	0.10	0.16	9.14	0.03

Table 3

Effect of steam treatment temperature on catalytic performance of HT-γ-Al₂O₃.

	HT-γ-Al2O3	Steam Treatment Temperature/°C
		500	650	800	900
Conversion of ethanol %	96.0	98.2	99.9	97.2	98.0
Selectivity of ethylene %	94.0	96.4	99.6	95.3	95.1

Table 4

Comparison with catalysts from references.

Catalyst	HT-γ-Al2O3	HT-γ-Al2O3−650 °C Treated	Industrial Alumina	Silica-Alumina	SiO2-ZrO2	PhosphomolybdicAcid-PILC	ZSM-5 Nanosheets
Reactant ethanol	absolute	95%	94%	absolute	99.5%	absolute	bioethanol
Temperature (°C)	350	320	400	400	450	300	300–350
Conversion%	100	99.9	97.5	100	99.2	93.2	>90
Selectivity of ethylene%	99.8	99.6	95.0	96.1	96.7	99.1	85%
Reference	This work	This work	24	13	21	22	17

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13060994/s1, Table S1: Calculated surface energies of AlOOH planes. Table S2: Calculated surface energies of γ-alumina planes. Figure S1: Schematic diagram of the corresponding crystal plane changes during the transition of Boehmite to alumina. Figure S2: Selected area electron diffraction (SAED) of (a) HT-γ-Al₂O₃; (b) C-γ-Al₂O_3.

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