An alumina-coated, egg-shell Pd/α-Al₂O₃@SiC catalyst with enhanced ethylene selectivity in the selective hydrogenation of acetylene

Abstract

For the selective hydrogenation of acetylene to ethylene, the selectivity of the palladium (Pd) catalyst is crucial. Pd is present as a skin distributed on the surface of Al₂O₃ supports and Pd active sites dispersed on the skin exhibit high activity for acetylene removal, which also inhibits over-hydrogenation of ethylene to ethane and the formation of oligomeric species on the catalyst. Therefore, it is significant to accurately control the distribution range of Pd active sites on the support by a feasible route. Herein, we demonstrate a facile approach to precisely control the Pd active-site distribution range on the support. An α-Al₂O₃ layer was prepared directly on the outside surface of a ring-like SiC substrate by one-step spray-coating, to obtain an egg-shell α-Al₂O₃@SiC support. The thickness of the α-Al₂O₃ layer could be freely tuned from 50 μm to 300 μm. The newly designed α-Al₂O₃@SiC support was used as the catalyst body in the selective hydrogenation of acetylene. As expected, the rational design of a Pd catalyst with this α-Al₂O₃@SiC composite material as the support has the ability to achieve higher ethylene selectivity than a conventional extrusive pellet Pd catalyst. In this work, the ethylene selectivity of the egg-shell Pd–Ag/α-Al₂O₃@SiC catalyst was 16% greater than that of a conventional pellet Pd–Ag/α-Al₂O₃ catalyst when achieving complete removal of acetylene in an ethylene-rich stream.

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1. Introduction

Materials science and catalysis have been the focus of attention for the last three decades. Furthermore, the worldwide revolutionary developments in materials science have provided a driving force and begun to significantly influence developments in conventional porous catalysis materials, especially for key components of important heterogeneous catalysts and industry relevant catalysts for chemical processes.^1–3 In fact, catalytic materials research is crucial and important to the petrochemical, environmental and chemistry industries, being involved in more than 85% of products for all fuels and chemicals. Therefore, novel materials research is not only offering many useful approaches, but is also applicable in many important practical fields.^4–10

In the current petrochemical industry, ethylene is an important chemical intermediate for the polymer industry (for example for polyethylene). However, it is inevitable that an ethylene-rich feed stream contains a trace amount of acetylene (0.5–2% by volume). Moreover, the small traces of acetylene can poison polymerization catalysts (such as Ziegler–Natta). Therefore, it is necessary to remove the acetylene impurity in an ethylene feed stream by selective hydrogenation. In general, the acetylene impurity must be reduced to less than 5 ppm.^11–13 For the selective hydrogenation of acetylene, supported Pd catalysts are well-used in industrial processes. These supported Pd catalysts have attracted much attention and extensive work has been undertaken on them.^14–24 There are two major issues with Pd catalysts, namely the addition of promoters and the modification of supports. A number of other metals have been intensively studied as promoters to improve the performance of the Pd catalysts, such as Ag,^25–27 Ti,^17,28 Ni,²⁹ Sn,³⁰ Au,³¹ Cu,³² K³³ and Ga.^22,34 Some metals or metal oxides such as Zn, Ti, La₂O₃ and Nb₂O₅ have been utilized to modify Al₂O₃ or SiO₂ supports.^35–38 For conventional industrial catalysts for acetylene selective hydrogenation, these methods have proved fruitful in improving the performance of Pd catalysts. However, palladium is present as a skin distributed on the surface of Al₂O₃ supports. Pd active sites dispersed on the skin exhibit high activity for acetylene removal, which also inhibits over-hydrogenation of ethylene to ethane and the formation of oligomeric species on the catalyst.^39,40 Therefore, it is significant to investigate the effect of the Pd active-site distribution range on the catalytic performance of Pd catalysts.

Among the many catalyst supports so far reported (such as Al₂O₃, SiO₂, ZrO₂, CeO₂, carbon, pumice, sepiolite, MCM-41, Y zeolite, polymers and so on), only the Al₂O₃ support has been utilized to develop a commercial catalyst for the selective hydrogenation of acetylene in the modern petrochemical industry.^11,39,40 In order to achieve a feasible route to control precisely the Pd active-site distribution range on the support, silicon carbide (SiC) was used as a substrate to prepare α-Al₂O₃@SiC as the catalyst support for the selective hydrogenation of acetylene to ethylene. SiC has admirable mechanical strength, high thermal conductivity, excellent chemical inertness and high resistance towards oxidation. It is also a simple and inexpensive material compared to Al₂O₃. Moreover, SiC seems to be a potentially excellent functional material for various catalytic reactions, such as electrocatalytic reactions,^41,42 photocatalytic reactions,^43–46 Fischer–Tropsch synthesis^47–49 and Pt catalysis for fuel cell applications.^50,51

Recently, there has been widespread interest in the development of useful functional coating layer catalysts with enhanced catalytic performance. Y. Liu et al.⁵² systematically investigated a Co-based catalyst for Fischer–Tropsch synthesis. The Co-based catalyst was prepared using silicon carbide coated with TiO₂ such that the thin layer of TiO₂ enhanced cobalt active phase dispersion. This catalyst exhibited an excellent and stable catalytic activity for the Fischer–Tropsch synthesis with high C₅₊ selectivity. W. Song et al.⁵³ reported that carbon-coated, methanol-tolerant platinum/graphene catalysts had an excellent long-term performance for oxygen reduction reactions and the carbon coating was found to be effective in protecting susceptible Pt nanoparticles from direct exposure to the corrosive environment and minimizing the negative effect of methanol crossover. B. Liu et al.⁵⁴ synthesized a washcoating catalyst of cordierite honeycomb monoliths coated with vanadia–tungsta–titania mixed oxides. The coating process created a high dispersion of active species and the coated monolithic catalyst exhibited desirable catalytic performance in the selective catalytic reduction of NO with NH₃. F. Ding et al.⁵⁵ prepared SiO₂-coated FeK/Al₂O₃ catalysts and the SiO₂-coating improved the synthesis of hydrocarbons from CO₂ hydrogenation. In addition, silica coated ZSM-5 composites exhibiting excellent catalytic performances have been reported by Y. Deng et al.⁵⁶ and X. Qian et al.⁵⁷

The ethylene selectivity of supported Pd catalysts is a crucial issue for the selective hydrogenation of acetylene in ethylene-rich feed stock. Although much progress has been made, it is unlikely that further great improvements in the ethylene selectivity for conventional pellet Pd/Al₂O₃ catalysts can be realized in a fixed-bed reactor. Therefore, it is compelling to synthesize a Pd/α-Al₂O₃@SiC catalyst with an alumina-coated egg-shell α-Al₂O₃@SiC composite material as the support. Moreover, the α-Al₂O₃@SiC composite material can facilitate the development of coated materials for catalysis in the optimized structures. To the best of our knowledge, no work has yet been documented on the α-Al₂O₃@SiC composite material as a functional support for a Pd catalyst for the selective hydrogenation of acetylene to ethylene. In this work, the α-Al₂O₃@SiC support was prepared by spray-coating using a newly designed alumina slurry. An α-Al₂O₃ layer was prepared directly on the outside surface of a ring-like SiC substrate by one-step coating, and good adhesion of the Al₂O₃ layer on SiC was achieved after calcination. Meanwhile, the thickness of the Al₂O₃ layer could be tuned from 50 μm to 300 μm. When used to form supported Pd catalysts, the SiC substrate could resist Pd precursor penetration, and therefore the Pd active sites were fully distributed on the Al₂O₃ layer.

2. Experimental

2.1 One-step coating of an Al₂O₃ layer on SiC

The coating slurry was prepared by dispersing α-Al₂O₃ powder (BET surface area 25–35 m² g⁻¹) in 6.25 wt% boehmite sol by milling the α-Al₂O₃ powder with a stirring ceramic ball mill (JM-5, Changsha Tencan Powder Technology Co., Ltd, China) at room temperature for 8 h. Determination by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd, UK) showed that 90% of the powder diameters of the slurry were less than 4 μm, which was necessary to achieve good adhesion of the Al₂O₃ layer to the SiC substrate.

The outer surface of the SiC substrate (a ring-like SiC, 8 mm in length and 6/3 mm in OD/ID) was coated once by using a high-efficiency spray coating machine (BG1-5, AVIC Beijing Aeronautical Manufacturing Technology Research Institute, China). The weight of the Al₂O₃ loading on the SiC substrate can be controlled according to the spray coating time at 120 °C. Then the α-Al₂O₃@SiC support was calcined at 1020 °C in air for 4 h. The thickness of the Al₂O₃ layer was tuned by the weight of the Al₂O₃ loading on the SiC substrate (Fig. 1). The adhesion strength of the Al₂O₃ layer was measured by an impact test in which the Pd/α-Al₂O₃@SiC catalyst was dropped along a 4.2 meter metal tubule, simulating the catalyst packing mode in the reactor. The weight loss was defined as follows:

wherein W₁ is the weight of the Pd/α-Al₂O₃@SiC catalyst before the impact test, W₂ is the weight of the Pd/α-Al₂O₃@SiC catalyst after the impact test and W₃ is the weight of Al₂O₃ loading on the SiC substrate (see ESI†).

Fig. 1 Schematic illustration of the synthesis procedures for the Pd/α-Al₂O₃@SiC catalyst.

2.2 Preparation of catalysts

A 500.0 g α-Al₂O₃@SiC support was put in a laboratory coating machine (rotating speed: 80 rpm), and 35 mL Pd(NO₃)₂ solution (Pd²⁺: 5 mg mL⁻¹) was mist-sprayed onto the support using a home-made glass sprayer actuated by compressed air (pressure: 0.1 MPa). The Pd/α-Al₂O₃@SiC catalyst was subsequently dried at 120 °C for 8 h and calcined in air at 450 °C for 8 h.

For comparison, conventional extrusive α-Alumina (α-Al₂O₃) pellets (average diameter 3.5 mm, BET surface area 28.4 m² g⁻¹) used as a support material were produced by the Sinopec Beijing Research Institute of Chemical Industry. A pellet Pd//α-Al₂O₃ catalyst was also prepared using the mist-spraying technique described above. Meanwhile, a Pd–Ag/α-Al₂O₃ catalyst and a Pd–Ag/α-Al₂O₃@SiC catalyst (14.1 wt% Al₂O₃ loading on SiC) were prepared by the same method as the Pd/α-Al₂O₃@SiC catalyst with Ag as a promoter. The four types of catalysts were analyzed by inductively coupled plasma mass spectrometry (ICP-MS 7500CX, Agilent) and the results are shown in Table 1.

Table 1 The characterization results of the four types of catalyst (determined by ICP)

Sample	Pd-loading (wt%)	Ag-loading (wt%)
Pd/α-Al2O3@SiC	0.033	—
Pd/α-Al2O3	0.035	—
Pd–Ag/α-Al2O3@SiC	0.031	0.169
Pd–Ag/α-Al2O3	0.035	0.176

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2.3 Catalyst characterization

The catalysts were characterized by mercury porosimetry, X-ray powder diffraction (XRD), temperature programmed desorption of ammonia (NH₃-TPD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The pore size distributions of the catalysts were analyzed by mercury porosimetry (Quantachrome PoreMaster 60GT, USA). XRD analysis was performed on a Bruker AXS D8 Advance instrument (λ = 1.5406 Å; tube voltage, 40 kV; tube current, 300 mA). The NH₃-TPD was performed to determine the acidic properties of the Al₂O₃ supports. Prior to NH₃ adsorption, 0.1 g sample was pretreated in a U-tube quartz reactor at 600 °C for 60 min with a flow of He (30 mL min⁻¹) and the sample was cooled down to 120 °C in a stream of He. After cooling, 30 mL min⁻¹ 10% NH₃ in He was introduced into the reactor for 1 h. After NH₃ adsorption, the gas was switched to He and purging was performed at 120 °C for 1 h and then the temperature was ramped from 120 °C to 600 °C with a heating rate of 10 °C min⁻¹ under the flow of He (30 mL min⁻¹). The NH₃-TPD experiments were carried out using an AutoChem II 2920 instrument (Micromeritics Instruments, USA). TEM images were recorded by a Philips TECNAI 20 (200 kV) and SEM observations were performed on a FEI XL30 ESEM-FEG. BET surface area was determined by an ASAP 2020C instrument (Micromeritics Instruments, USA). Pd dispersion was measured using CO pulse chemisorption by an AutoChem II 2920 instrument (Micromeritics Instruments, USA).

2.4 Catalytic test

For tests of catalytic performance, the selective hydrogenation of acetylene in an ethylene-rich stream was performed in an 8 mm (i.d.) stainless steel tube microreactor with 1 mL catalyst at atmospheric pressure. For comparison with the conventional pellet Pd–Ag/α-Al₂O₃ catalyst, an industrial side-line experiment for the selective hydrogenation of acetylene was carried out using a fixed-bed reactor (i.d. 14 mm) with 100 mL catalyst (reaction pressure: 2 MPa). Before the reaction, the catalyst was reduced by H₂ (gas hourly space velocity (GHSV): 300 h⁻¹) at 180 °C for 4 h. The typical feed stream consisted of 0.71% H₂, 0.45% C₂H₂, 7.15% C₂H₆, and balanced C₂H₄. The total GHSV was 10 000 h⁻¹ and the products were analyzed by gas chromatography (Agilent 7890) with a flame ionization detector (FID) and thermal conductivity detector (TCD). According to the literature, the acetylene conversion can be calculated by:^22,25

The ethylene selectivity was calculated using moles of ethylene produced per mole of acetylene converted according to the following equation:^22,35

3. Results and discussion

In order to investigate the crystalline phases of the catalyst supports, XRD measurements were carried out. Fig. 2 shows the XRD patterns of the Al₂O₃ layer of the α-Al₂O₃@SiC support (14.1 wt% Al₂O₃ loading on SiC) and the conventional pellet α-Al₂O₃ support, which have the same characteristic α-Al₂O₃ peaks (2θ = 25.58, 35.15, 37.78, 43.35, 52.55, 57.5, 66.52 and 68.21, ICDD file no. 82-1468). Besides the characteristic peaks of α-Al₂O₃, θ-Al₂O₃ peaks (2θ = 31.47, 32.78, 36.68, 38.92, 44.83 and 67.42, ICDD file no. 35-0121) were also found. Fig. 2 also compares the XRD patterns of the Al₂O₃ layer of the α-Al₂O₃@SiC support before calcination and after calcination. The maximum peak of Fig. 2a is located at 2θ = 35.15 before calcination, in comparison to that for the Al₂O₃ layer sample after calcination and the conventional pellet α-Al₂O₃ support (2θ = 43.35, Fig. 2b and c). This result indicates that the boehmite sol crystal-phase transformation into α-Al₂O₃ or θ-Al₂O₃ was completed after the heat treatment.

Fig. 2 XRD patterns of (a) the Al₂O₃ layer of the α-Al₂O₃@SiC support before calcination, (b) the Al₂O₃ layer of the α-Al₂O₃@SiC support after calcination and (c) the conventional pellet α-Al₂O₃ support.

Fig. 3 shows SEM images of the surface and cross-section morphologies of the Al₂O₃ layer of the α-Al₂O₃@SiC support (14.1 wt% Al₂O₃ loading on SiC) and the conventional pellet α-Al₂O₃ support. The surface and cross-section of the Al₂O₃ layer of the α-Al₂O₃@SiC support were smooth, homogeneous and showed dense coating before calcination (Fig. 3A and a). The particles of the Al₂O₃ layer are small and uniform and the particles were compact using boehmite sol as the binder. The surface and cross-section morphologies of the Al₂O₃ layer of the α-Al₂O₃@SiC support (14.1 wt% Al₂O₃ loading on SiC) after calcination at 1020 °C in air for 4 h are shown in Fig. 3B and b, respectively. Some conspicuous changes can be observed. It can be seen that the surface and cross-section morphologies of the Al₂O₃ layer were very different. The calcination resulted in the densification and shrinkage of the alumina particles. It appeared that the nanosized boehmite promoted the sintering process effectively, which is in accordance with the results reported in the literature.^58,59 Fig. 3C and c show the surface and cross-section morphologies, respectively, of the conventional pellet α-Al₂O₃ support. It is noted that the Al₂O₃ support produced by the conventional extrusive process has a higher bonding strength astringing the alumina particles as compared with the Al₂O₃ layer produced by the one-step spray coating process. Therefore, the particles do not remain in a highly dispersed state as the surface porosity decreases.

Fig. 3 SEM images of the Al₂O₃ layer of the α-Al₂O₃@SiC support: (A) surface before calcination, (a) cross-section before calcination, (B) surface after calcination and (b) cross-section after calcination. SEM images of the conventional pellet α-Al₂O₃ support: (C) surface and (c) cross-section.

Fig. 4 shows the NH₃ temperature programmed desorption profiles of the Al₂O₃ layer of the α-Al₂O₃@SiC support (14.1 wt% Al₂O₃ loading on SiC) and the conventional pellet α-Al₂O₃ support. Compared with the desorption peak area of the Al₂O₃ layer of the α-Al₂O₃@SiC support before calcination, that of the Al₂O₃ layer of the α-Al₂O₃@SiC support after calcination at 1020 °C in air for 4 h was relatively low. This was due probably to the dramatic sintering process of boehmite sol because the specific surface area of the Al₂O₃ layer of the α-Al₂O₃@SiC support was decreased from 82.0 m² g⁻¹ to 28.0 m² g⁻¹ after calcination at high temperature and the quantity of total NH₃ uptake was also decreased from 0.252 mmol g⁻¹ to 0.081 mmol g⁻¹ at the same time. For the conventional pellet α-Al₂O₃ support, the desorption peak area was nearly the same as that of the Al₂O₃ layer of the α-Al₂O₃@SiC support calcined at 1020 °C in air for 4 h. However, as shown in Fig. 4, all the catalyst support samples exhibited a broad peak. The NH₃ desorption peak presented at 215–245 °C, which is assigned to weak acidic sites. NH₃ desorption peaks corresponding to medium acidic sites and strong acidic sites were not observed. The heat treatment at high temperature reduced the acidic sites of the catalyst supports and this result is in good agreement with those reported by other researchers.^60,61

Fig. 4 NH₃-TPD profiles of (a) the Al₂O₃ layer of the α-Al₂O₃@SiC support before calcination, (b) the Al₂O₃ layer of the α-Al₂O₃@SiC support after calcination and (c) the conventional pellet α-Al₂O₃ support.

For the one-step spray coating process, the composition of the slurry suspension is important for preparing a crack-free Al₂O₃ layer on the outside of the ring-like SiC substrate. In this work, α-Al₂O₃ powder (BET surface area 25–35 m² g⁻¹) was added into 6.25 wt% boehmite sol and the powder was milled with a stirring ceramic ball mill for 8 h. The distribution of particle sizes of the milled α-Al₂O₃ powder was measured by a laser particle size analyzer. Some 90% powder diameters of the slurry suspension were less than 4 μm, which was very effective in preventing crack formation and achieving good adhesion of the Al₂O₃ layer on the SiC substrate.

As shown in Fig. 5a, the Pd/α-Al₂O₃@SiC catalyst was prepared with 14.1 wt% Al₂O₃ loading on the ring-like SiC substrate and the thickness of the Al₂O₃ layer on the outside of the substrate was 190 ± 10 μm (Fig. S1, ESI†). The successive uniform Al₂O₃ layer can be observed on the outside of the ring-like SiC substrate. It also can be noted that no cracks occurred in the Al₂O₃ layer. The TEM image (Fig. 5b) showed that the Pd particles of the Pd/α-Al₂O₃@SiC catalyst were uniformly dispersed on the Al₂O₃ layer with diameters ranging from 4 to 10 nm. Only a few aggregations were observed; the value of Pd dispersion was 23.5%. In fact, the conventional extrusive pellet α-Al₂O₃ support is not generally desirable because the narrow, long pores of the pellet α-Al₂O₃ support could invite Pd precursor penetration. Although at least 80% Pd active sites were distributed on the surface of the pellet Pd/α-Al₂O₃ catalyst, the depth of Pd precursor penetration was still in the range of 1–1000 μm (Fig. 5c). The size of the Pd particles on the pellet Pd/α-Al₂O₃ catalyst was less than 10 nm (Fig. 5d) and the value of Pd dispersion was 19.2%. However, the depth of Pd active sites dispersed on the pellet α-Al₂O₃ support was more than 300 μm which is undesirable for the selective hydrogenation of acetylene to ethylene. Based on the literature, Pd active sites are present as a skin distributed on the surface of Al₂O₃ pellet supports which exhibit high activity for acetylene removal, and inhibit over-hydrogenation of ethylene to ethane and the formation of oligomeric species on the catalyst.^39,40 Therefore, it is very significant to control the Pd active site distribution range by hindering Pd precursor penetration using the alumina-coated egg-shell α-Al₂O₃@SiC composite material as a support.

Fig. 5 (a) SEM image of the Pd/α-Al₂O₃@SiC catalyst, (b) TEM image of Pd particles on the Al₂O₃ layer of the Pd/α-Al₂O₃@SiC catalyst, (c) SEM image and EDS analysis of the pellet Pd/α-Al₂O₃ catalyst and (d) TEM image of Pd particles on the Al₂O₃ support of the pellet Pd/α-Al₂O₃ catalyst.

On the other hand, α-Al₂O₃ is commonly used as a support for the selective hydrogenation of acetylene due to its suitable macroporous nature. In addition, the surface area of α-Al₂O₃ is smaller than that of γ-Al₂O₃ and the α-Al₂O₃ can provide lower dispersion of Pd active sites. According to the literature, it is desirable for the selective hydrogenation of acetylene that the Pd/α-Al₂O₃ catalyst possesses fewer Pd active sites for direct ethane formation than the Pd/γ-Al₂O₃ catalyst, which has higher ethylene selectivity⁶² and it was reported that α-Al₂O₃ was the proper material to be used as the support due to its sufficiently large pore size.^39,62 Herein, mercury porosimetry was employed to determine the pore size distribution of the SiC substrate, α-Al₂O₃@SiC support (14.1 wt% Al₂O₃ loading on SiC) and conventional pellet α-Al₂O₃ support (Fig. 6).

Fig. 6 Pore size distribution curves for the samples: (a) SiC substrate, (b) α-Al₂O₃@SiC support and (c) conventional pellet α-Al₂O₃ support. The dV/dlog D value was shifted by 0.25 and 0.10 for the curves of data sets (a) and (b), respectively.

As shown in Fig. 6, the Al₂O₃ layer of the α-Al₂O₃@SiC support has a similar pore size distribution to the conventional extrusive pellet α-Al₂O₃ support, but the SiC substrate has no such pores in the range of 30–300 nm. Moreover, for the Al₂O₃ layer on SiC substrate, the main pore size distribution is in the range of 100 nm to 300 nm. Meanwhile, the pore size distribution is broadened compared with the conventional extrusive pellet α-Al₂O₃ support. The largest pore sizes of the α-Al₂O₃@SiC support are increased from 200 nm to 300 nm. The increase of the pore size of α-Al₂O₃@SiC support might offer a benefit for the selective hydrogenation of acetylene to ethylene.

Developing a facile approach to precisely control the Pd active-site distribution range on Al₂O₃ supports is of significance for supported Pd catalysts. Herein, the α-Al₂O₃ layer was prepared directly on the outside surface of the ring-like SiC substrate by one-step spray-coating, which resulted in an egg-shell α-Al₂O₃@SiC support. The thickness of the α-Al₂O₃ layer could be freely tuned from 50 μm to 300 μm (Fig. S1, ESI†). For the alumina-coated egg-shell Pd/α-Al₂O₃@SiC catalyst, the thickness of the Al₂O₃ layer was determined by the amount of Al₂O₃ loading on the SiC substrate.

As seen in Fig. 7, we explored the catalytic performances of the Pd/α-Al₂O₃@SiC catalysts with different Al₂O₃ loadings in acetylene selective hydrogenation. The performances of the Pd/α-Al₂O₃@SiC catalysts for the selective hydrogenation of acetylene to ethylene were generally excellent overall. The acetylene conversion increased as the Al₂O₃ loading on SiC increased, however, the tendency was not significant when the Al₂O₃ loading on SiC was more than 9.0 wt%. This means that Pd active sites are well dispersed on the Al₂O₃ layer when the thickness of the Al₂O₃ layer on the SiC substrate is more than 110 ± 10 μm. Moreover, the ethylene selectivity gradually decreased as the acetylene conversion increased, consistent with previous reports for acetylene selective hydrogenation catalyzed by supported Pd catalysts.³⁵ This is because the reaction of acetylene hydrogenation is a typical consecutive reaction and ethylene is produced as an intermediate.

Fig. 7 Performances of the egg-shell Pd/α-Al₂O₃@SiC catalysts at atmospheric pressure. Acetylene conversion: (a) 7.3 wt% Al₂O₃ loading, (c) 9.0 wt% Al₂O₃ loading, (e) 12.0 wt% Al₂O₃ loading and (g) 14.1 wt% Al₂O₃ loading. Ethylene selectivity: (b) 7.3 wt% Al₂O₃ loading, (d) 9.0 wt% Al₂O₃ loading, (f) 12.0 wt% Al₂O₃ loading and (h) 14.1 wt% Al₂O₃ loading. (H₂/C₂H₂ = 1.6, 0.71% H₂, 0.45% C₂H₂, 7.15% C₂H₆, and balanced C₂H₄.)

Generally, for supported Pd catalysts, Ag has been used to modify the catalyst as an excellent promoter for achieving high ethylene selectivity.^25–27 According to the results of an adhesion test and an abrasion resistance test, the Al₂O₃ layer of a Pd–Ag/α-Al₂O₃@SiC catalyst (14.1 wt% Al₂O₃ loading on SiC) has good adhesion and abrasion resistance (Tables S1 and S2, ESI†). Therefore, we chose the 14.1 wt% Al₂O₃ loading on the SiC substrate to prepare a Pd–Ag/α-Al₂O₃@SiC catalyst for enhancing ethylene selectivity. The results of a selectivity comparison of different Pd catalysts based on similar conversion levels are shown in Table 2. Industrially, in the tail-end hydrogenation process, acetylene must be completely removed from ethylene-rich streams. Therefore, a slight excess of hydrogen is added to the gas feed streams and the quantity of the H₂/C₂H₂ (mole ratio) is larger than one. The selectivity of the catalyst beds is mainly controlled by the value of the H₂/C₂H₂ ratio and the temperature.¹² In general, ethylene selectivity will decrease as the reaction temperature increases at the same value of H₂/C₂H₂. However, the α-Al₂O₃@SiC composite material has a better thermal stability than a conventional pellet α-Al₂O₃ support. Entries 1–4 in Table 2 show a comparison between the α-Al₂O₃@SiC composite material and a conventional pellet α-Al₂O₃ support. For identical Pd loading, the ethylene selectivities of the Pd/α-Al₂O₃@SiC catalyst and the Pd–Ag/α-Al₂O₃@SiC catalyst at 65 °C were respectively higher than those of a Pd/α-Al₂O₃ catalyst and a Pd–Ag/α-Al₂O₃ catalyst at 55 °C when the value of H₂/C₂H₂ was 1.6. The α-Al₂O₃@SiC support was effective in enhancing ethylene selectivity when the acetylene conversions were at similar levels. For different Pd loadings, the literature describes a Pd catalyst supported on Zn-modified α-Al₂O₃ for the selective hydrogenation of acetylene in ethylene feed stock which was investigated by Chinayon et al.³⁶ The value of H₂/C₂H₂ was 1.1 and the Pd loading was 0.3 wt%. Its ethylene selectivity was 64% when the acetylene conversion was 82% (Table 2, entry 7). In contrast, for the Pd/α-Al₂O₃@SiC catalyst, when the value of H₂/C₂H₂ was 1.6 and the Pd loading was 0.035 wt%, the ethylene selectivity was 65% and the acetylene conversion was 83% (Table 2, entry 6). For another example, the catalytic performance of a Pd–Ga/MgO–Al₂O₃ catalyst was reported by He et al.²² The value of H₂/C₂H₂ was 2.0 and the Pd loading was 0.05 wt%. The acetylene conversion and ethylene selectivity were respectively 99% and 54% at 55 °C (Table 2, entry 5). For the Pd–Ag/α-Al₂O₃@SiC catalyst, the value of H₂/C₂H₂ was 1.6 and the Pd loading was 0.035 wt%. Its ethylene selectivity was 57% at 65 °C when the acetylene conversion was 99% (Table 2, entry 3). Thus the ethylene selectivity of the Pd–Ag/α-Al₂O₃@SiC catalyst was a little higher than that of the Pd–Ga/MgO–Al₂O₃ catalyst.

Table 2 Ethylene selectivity comparison based on similar acetylene conversion levels for Pd catalysts

Entry	T/°C	Catalyst	Pd (wt%)	Conversion (%)	Selectivity (%)
a Egg-shell Pd catalyst with 14.1 wt% Al2O3 loading on SiC, H2/C2H2 = 1.6, 0.71% H2, 0.45% C2H2, 7.15% C2H6, and balanced C2H4, reaction pressure of 2 MPa.b Pd catalyst prepared with conventional extrusive pellet α-Al2O3 support, H2/C2H2 = 1.6, 0.71% H2, 0.45% C2H2, 7.15% C2H6, and balanced C2H4, reaction pressure of 2 MPa.c Egg-shell Pd catalyst with 7.3 wt% Al2O3 loading on SiC, H2/C2H2 = 1.6, 0.71% H2, 0.45% C2H2, 7.15% C2H6, and balanced C2H4, atmospheric pressure.d Literature data.^22e Literature data.^36
1^a	65	Pd/α-Al2O3@SiC	0.035	95	48
2^b	55	Pd/α-Al2O3	0.035	99	36
3^a	65	Pd–Ag/α-Al2O3@SiC	0.035	99	57
4^b	55	Pd–Ag/α-Al2O3	0.035	100	41
5^d	55	Pd–Ga/MgO–Al2O3	0.05	99	54
6^c	130	Pd/α-Al2O3@SiC	0.035	83	65
7^e	100	Pd/Zn-modified-α-Al2O3	0.3	82	64

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For assessing the initial performance, a side-line experiment for the selective hydrogenation of acetylene to ethylene was performed to contrast the Pd–Ag/α-Al₂O₃@SiC catalyst and the conventional pellet Pd–Ag/α-Al₂O₃ catalyst under industrial reaction conditions. A condition of this experiment is to thoroughly remove acetylene in an ethylene-rich stream and contrast ethylene selectivity with time on stream. In order to achieve this objective, the inlet temperature of the reactor for the Pd–Ag/α-Al₂O₃@SiC catalyst was kept at 65 °C and the inlet temperature of the reactor for the conventional pellet Pd–Ag/α-Al₂O₃ catalyst was kept at 55 °C. As shown in Fig. 8, the acetylene conversion of the conventional pellet Pd–Ag/α-Al₂O₃ catalyst was kept at 100% within the first 60 h which showed the stable initial activity of the catalyst. Its selectivity to ethylene was around 41%. The performance of the Pd–Ag/α-Al₂O₃@SiC catalyst was also stable within the first 60 h at 65 °C. Its acetylene conversion was kept at almost 99% and its selectivity to ethylene was around 57%, which was 16% more than that of the conventional pellet Pd–Ag/α-Al₂O₃ catalyst (Fig. 8). As can be seen in Fig. 8, this indicates that the Pd–Ag/α-Al₂O₃@SiC catalyst had a good initial performance without any noticeable deactivation within the first 60 h. The fluctuation of ethylene selectivity can be observed due to the dynamic change of the value of H₂/C₂H₂ between 1.5 and 1.6. In our present work, although the performance of the Pd–Ag/α-Al₂O₃@SiC catalyst still needs to be improved further, it is a promising candidate for the selective hydrogenation of acetylene to ethylene.

Fig. 8 Side-line experiment results, in acetylene conversion and ethylene selectivity data show percentage of error bar: (a) acetylene conversion of Pd–Ag/Al₂O₃ catalyst at 55 °C, (b) ethylene selectivity of Pd–Ag/α-Al₂O₃ catalyst at 55 °C, (c) acetylene conversion of Pd–Ag/α-Al₂O₃@SiC catalyst at 65 °C and (d) ethylene selectivity of Pd–Ag/α-Al₂O₃@SiC catalyst at 65 °C. (14.1 wt% Al₂O₃ loading on SiC, H₂/C₂H₂ = 1.6, 0.71% H₂, 0.45% C₂H₂, 7.15% C₂H₆, and balanced C₂H₄, reaction pressure of 2 MPa.)

4. Conclusions

An alumina-coated, egg-shell Pd/α-Al₂O₃@SiC catalyst – comprising a thickness-controlled Al₂O₃ layer on SiC – was studied by XRD, NH₃-TPD, mercury porosimetry, SEM and TEM. In summary, we demonstrated a facile approach for preparing the egg-shell α-Al₂O₃@SiC composite material using a one-step spray coating method. The egg-shell α-Al₂O₃@SiC composite material was utilized as a support for the selective hydrogenation of acetylene to ethylene. The thickness of the α-Al₂O₃ layer could be freely tuned from 50 μm to 300 μm. The newly designed α-Al₂O₃@SiC support could very effectively control the Pd precursor penetration range, to achieve uniform distribution of Pd active sites on the Al₂O₃ layer. As compared with a conventional pellet Pd–Ag/α-Al₂O₃ catalyst, the Pd–Ag/α-Al₂O₃@SiC catalyst had a higher ethylene selectivity based on the same acetylene conversion, and is therefore a promising candidate for the selective hydrogenation of acetylene to ethylene.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51404097), the Sinopec Beijing Research Institute of Chemical Industry (11-08ZS0442), the Program for Innovative Research Team (in Science and Technology) in the University of Henan Province (16IRTSTHN005) and the Doctoral Foundation of Henan Polytechnic University (B2015-15 and B2015-12).

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Footnote

† Electronic supplementary information (ESI) available: SEM photographs of the Al₂O₃ layer at the different Al₂O₃ loadings on SiC, detailed analysis of the adhesion test and abrasion resistance test. See DOI: 10.1039/c6ra08320j

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