A small eggshell Ni/SFC3R catalyst for C5 petroleum resin hydrogenation: preparation and characterization

Abstract

A small eggshell Ni-based catalyst, supported in a fluid catalytic cracking catalyst residue (SFC3R) with distinct outer shell regions, was synthesized by incipient wetness impregnation using n-heptane and nickel nitrate solution and was applied to C5 petroleum resin hydrogenation. For comparison, a uniform Ni/SFC3R catalyst was prepared by conventional impregnation. The eggshell Ni/SFC3R catalyst exhibited higher activity and better stability than the uniform Ni/SFC3R catalyst for C5 petroleum resin hydrogenation because of its nickel component distribution in the outer region of the particles, small NiO particles, and interaction of NiO–SFC3R. The morphology and microstructure of the micron-sized Ni/SFC3R particles were determined through a FIB (focused ion beam)-cutting technique combined with SEM-EDX. The results showed that the eggshell thickness was approximately 1.8 μm.

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1. Introduction

C5 petroleum resin is a thermoplastic polymer made from the polymerization of the C5 fraction, which is isolated from the production of ethylene from a naphtha-based petroleum source, accounting for approximately 10–14% of the produced ethylene.¹ Current ethylene production has reached approximately 1.5 million tons per year worldwide.² Thus, effectively utilizing the low-grade C5 fractions and developing high-end downstream products are the priorities in developing the petrochemical industry. Polymerization combined with the formation of petroleum resin is one of the main methods to recycle the by-products of pyrolysis plants.³ C5 petroleum resin, referred to as a “tackifying resin”, is an oligomer with molecular weights ranging from 300–5000. C5 petroleum resin without any post-treatment normally contains unsaturated bonds, particularly ethylenic C=C bonds, and fewer aromatic rings, resulting in properties such as a dark color, poor thermal stability, and poor oxidation resistance.⁴ Hydrogenation is a conventional modification method to lighten the resin color. Hydrogenated petroleum resin has a low bromine value, watery white color, high thermal stability, and good light resistance. A high-quality resin is particularly important in manufacturing sanitary napkins, pressure-sensitive adhesive tape, food packaging, and clear sealants. Traditionally, hydrogenated petroleum resin is obtained by hydrogenating the resin at 3–15 MPa, 1–18 h, and 200–330 °C in the presence of a noble metal catalyst, i.e., Pd supported on alumina or carbon.^5,6 However, this method presents certain disadvantages. For example, Pd-based catalysts are susceptible to poisoning by sulfur impurities in the petroleum resin, and the Pd noble metal is too expensive. Recently, the Ni catalyst is usually used to produce hydrogenated petroleum resins.^7,8 However, all of the details of the catalyst utilized in the patents are not mentioned.

On one hand, nickel-based catalysts have received significant attention for many reactions because of their lower price, resistance to poison, and moderate activity.^9–13 On the other hand, an eggshell catalyst is superior to a uniform catalyst for processes influenced by inner diffusion^14,15 and heat release¹⁶ limitation because of the shortened diffusion path of the reactant and heat transfer. Moreover, an eggshell structure has been theoretically and experimentally proven to improve the activity and poisoning resistance of the catalyst.¹⁷ The molecular weight of the C5 petroleum resin with a S-compound is relatively large, and the hydrogenation of functional groups is an exothermic process.¹⁸ In other words, the hydrogenation of petroleum resin is not only a desulfurated process but also an exothermal process, which is accompanied by internal diffusion resistance¹⁹ within the pores of the catalysts. Thus, an eggshell Ni-based catalyst can be suitable for the hydrogenation of petroleum resins. However, few researchers have investigated eggshell Ni-based catalysts for hydrogenating petroleum resins. Furthermore, extremely small (50–100 μm) eggshell catalyst has received very little attention in the literature when compared to the vast amount of published work for eggshell catalysts with large size such as Al₂O₃ (1.5–3.0 mm).^10,14,15 As generally accepted, smaller particles exhibit relatively larger outer surface areas and shorter diffusion path.

The fluid catalytic cracking catalyst residue (SFC3R, also called the spent FCC catalyst) with heavy metals (i.e., typically nickel, vanadium, and ferrum)^20,21 has a high specific surface area.²² It consists of small particles with diameters of approximately 50–100 μm (ref. 23) and is mainly structured with SiO₂ and Al₂O₃.²⁰ Oil refineries all around the world demand 300 thousand tons of FCC catalyst annually.²⁴ As a consequence, 150–170 thousand tons of SFC3R are generated each year.²⁵ Currently, it is usually handled by means of being disposed in landfills,²⁶ chemical and magnetic separations,^20,25,27,28 and being used as construction material.^29–31 However, in the above research, no study has been conducted on the direct reuse of the heavy ions (mainly Ni) in SFC3R. SFC3R can serve as an excellent carrier^32,33 due to its high surface area, high Ni metal concentration, and zeolite structure which is active for hydrodesulfurization.^34,35 In the present study, we utilize SFC3R for the first time as a carrier to hydrogenate the C5 petroleum resin and the catalyst life is increased. Hence, an environmentally friendly and cost-effective method to recycle SFC3R is developed.

So far, no specific work has discussed eggshell catalysts for petroleum resin hydrogenation, in particular the eggshell Ni/SFC3R catalyst. In this work, we present a small eggshell SFC3R-supported Ni catalyst with distinct outer shell regions, which was successfully employed in the catalytic hydrogenation of the C5 petroleum resin. The properties of the eggshell Ni/SFC3R catalyst were investigated through FIB/SEM-EDX, ICP, XRD, BET, and H₂-TPR. As comparison, a uniform Ni/SFC3R catalyst was prepared and characterized. The hydrogenated C5 petroleum resin (HPR) was characterized in terms of the color, bromine number, softening point, ICP, and GPC. In addition, we investigated the effect of catalyst preparation conditions on the catalytic performance of eggshell catalysts.

2. Experimental

2.1. Materials

SFC3R (PetroChina Guangxi Tiandong Petrochemical Co., Ltd.) was pretreated by air oxidation and then was used as the support. The basic chemical elements of the SFC3R sample were characterized by SEM-EDX, as presented in Table 1. The nickel loading was 1.89 wt% and there was no carbon deposition. Ni(NO₃)₂ 6H₂O (Guanghua Sci-Tech Co., Ltd, China, 98 wt%) was utilized as a precursor of the active phase. n-Heptane (Guanghua Sci-Tech Co., Ltd, China, 98 wt%) was used as a pre-soak solvent for the support to fill the pores of SFC3R. C5 petroleum resin (bromine number of 27 g Br/100 g, Gardner color grade no. 9, softening point of 98 °C, M_w of 4613, M_w/M_n of 2.25, acid value of 0.28 mg KOH g⁻¹, sulfur content of 56.0 mg kg⁻¹) was obtained from PetroChina Lanzhou Petrochemical Company. No. 200 industrial grade solvent oil (200^# solvent oil) was purchased from Jiangsu Hualun Chemical Industry Co., Ltd., China, and was used as a solvent to dissolve the C5 petroleum resin. Both n-heptane and 200^# solvent oil can be recycled and reused. Activated clay was purchased from Hangzhou Yongsheng Catalyst Co., Ltd., China, and was utilized as a sorbent to adsorb the impurities (mainly gel) in the C5 petroleum resin.

Table 1 Chemical element analysis of SFC3R using SEM-EDX (unit: %)

Element	O	Al	Si	Ca	Fe	Ni	Ce	Na	V
Content	48.46	24.74	19.48	2.73	1.18	1.89	0.79	0.53	0.20

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2.2. Catalyst preparation

Firstly, the SFC3R was pretreated by air oxidation. A total of 30 g of SFC3R (with a diameter of approximately 50–100 μm, specific surface area of 75.7 m² g⁻¹, and pore size of 8.2 nm) was calcined in air at 773 K (heating rate of 20 °C min⁻¹) for 4 h to remove the coke deposited on the surface. The eggshell Ni/SFC3R catalyst was then prepared through the following steps. The calcined SFC3R was soaked in n-heptane for 30 min. Then, the upper excess n-heptane solution was removed, and the n-heptane in the outer layer of SFC3R was evaporated at room temperature for a certain amount of time (30 s, 1 min, 3 min, 6 min, or 9 min). Subsequently, the moist SFC3R was impregnated by incipient wetness with a specified amount of aqueous solution of Ni(NO₃)₂ 6H₂O (Ni loading amounts of 1.89, 5, 10, 15, or 20 wt%). In this step, the water content (including the crystal water in Ni(NO₃)₂ 6H₂O) was approximately 14.70 g. The mixtures were stirred with a magnetic stirring apparatus for 30 s and dipped for several minutes (30 s to 8 min). Then, the SFC3R and solution were layered. SFC3R was in the lower layer, and n-heptane was in the upper layer which was colorless. After removing the upper layer (n-heptane layer), the sample was dried at 373 K for 16 h to remove the n-heptane in the interior completely and the water in the exterior. The catalyst was calcined at 723 K (heating rate of 20 °C min⁻¹) for 4 h in air to obtain the eggshell NiO/SFC3R catalyst. The Ni/SFC3R catalyst was obtained by reducing the NiO/SFC3R catalyst at 723 K for 3 h in the flowing H₂/N₂ at a flow rate of 30 ml min⁻¹.

As a comparison, a uniform NiO/SFC3R and Ni/SFC3R catalyst was prepared through conventional incipient wetness impregnation. The preparation routes of the uniform NiO/SFC3R catalyst and the eggshell NiO/SFC3R catalyst are illustrated in Fig. 1. The catalysts labeled as Egg-10/SFC3R and IM-10/SFC3R indicate the eggshell NiO/SFC3R catalyst (with 10 wt% Ni loading, n-heptane evaporation time of approximately 3 min, and dipping time of 2 min) and the uniform NiO/SFC3R catalyst with 10 wt% Ni loading, respectively. The other conditions are identical. The Egg-10/SFC3R and IM-10/SFC3R sample particles were cut in half using a dual beam focused ion beam (FIB). The eggshell thickness was determined by SEM-EDX.

Fig. 1 Preparation process for the uniform and eggshell NiO/SFC3R catalysts.

2.3. Analysis and characterization

A FIB-cutting technique was employed to obtain the cross-sectional structure of the small micron-sized Ni-based catalyst. The FIB-cutting was conducted by FEI 200XP instrument. The cross-sectional images were obtained by SEM at 15 kV and WD 10. The line distribution profile of the Ni element in the cross section of SFC3R was obtained through energy-dispersion X-ray spectroscopy (EDX) attached on the Hitachi S-3400N SEM. Ni loading was determined by using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Perkin Elmer Optima 8000, PE, America). To study the crystalline phases of the catalyst, X-ray powder diffraction patterns were obtained by a Rigaku Smartlab X-ray diffraction instrument with a Cu Kα source (k = 1.541 Å) at 40 kV and 200 mA in the air. The sample was scanned over Bragg angles ranging from 10° and 90° at a scan rate of 10° min⁻¹. Temperature programmed reduction (TPR) analyses were conducted by a Micromeritics Chemisorb 2720 instrument. Approximately 100 mg of the sample was loaded to a quartz reactor and cleaned at 400 °C for 0.5 h. The sample was reduced in a 10 vol% H₂/Ar flow (40 ml min⁻¹) with a heating rate of 10 K min⁻¹ up to 900 °C. The BET specific surface areas were determined by N₂ adsorption in a Gemini VII 2390 surface area analyzer at −196 °C with an evacuation rate of 30 ml min⁻¹.

2.4. Catalyst performance tests

The hydrogenation of C5 petroleum resin was performed in a 2 dm³ autoclave equipped with a double-tiered paddle agitator (Dalian Jingyi Autoclave Co., Ltd., China). The reaction temperature was controlled by an external electrically heating jacket and an internal water cooling coil. Before the reaction, the C5 petroleum resin was dissolved in 200^# oil (C5 concentration was 30 g/100 g) and pretreated with 9.6 g of activated clay to adsorb the hydrogel in the C5 petroleum resin. Approximately 650 ml of the C5/200^# oil mixture and 18.01 g of the eggshell Ni/SFC3R catalyst were introduced into the autoclave. The first-stage reaction was performed under mild hydrogenation conditions (210 °C, 20–30 bar, and 2 h). Subsequently, all gases in the autoclave were removed to eliminate the H₂S generated in the first stage. The second-stage reaction was carried out at 270 °C and 70–80 bar for 4 h. After the reaction, the HPR was obtained by vacuum distillation. The sulfur contents of the C5 petroleum resin and HPR were determined by an inductively coupled plasma mass spectrometer (ICP-MS, Model PerkinElmer Elan DRC-e). The bromine number was measured by the coulometry method³⁶ with the bromine valence and bromine index apparatus (denoted BVBI, Model BR-1, made in China GuoRui). Generally, the lower the bromine value, the higher the degree of hydrogenation and the lighter the color.³⁷ The color was determined through the Gardner color number method (50 wt% solution in toluene) with a Lovibond Gardner scale 3000 comparator. The softening point was measured according to ASTM D 6493-11. The average molecular weight (M_w) and molecular weight distribution (M_w/M_n) were determined by gel permeation chromatography (GPC, Waters 1525-2414, based on polystyrene standards). A schematic of the experimental setup in this study is shown in Fig. 2.

Fig. 2 Schematic diagram of the experimental setup used in this study.

3. Results and discussion

3.1. Characterization of Egg-10/SFC3R and IM-10/SFC3R

3.1.1. FIB/SEM-EDX results. To evaluate the eggshell thickness and nickel line distribution profiles of the cross section of the small Ni-based catalysts, SEM-EDX was conducted after the catalyst was cut by FIB. The cross-sectional SEM images and nickel line distribution profiles of IM-10/SFC3R and Egg-10/SFC3R are shown in Fig. 3. Remarkably, the nickel distributions are very uniform along the cross section of the IM-10/SFC3R catalyst (Fig. 3a and b). However, nickel is mainly distributed on the outer surface layer over the Egg-10/SFC3R catalyst (Fig. 3c and d), which exhibits an eggshell-type distribution. The average eggshell thickness is approximately 1.8 μm. This distribution difference mainly results from the evaporation of n-heptane in the outer layer of SFC3R and the varying dipping times of the two types of catalysts. The appearance of large pores in the cross section is caused by the high energy of FIB. The high energy of FIB cutting only leads to the structural damage at the center and does not collapse the entire structure. The SFC3R support has been proven to exhibit excellent physical strength from the side. The layer thickness of the catalytically active phase plays a key role in enhancing the catalytic activity of the eggshell catalysts by addressing intraparticle diffusion limitation and improving the utilization efficiency of the active components.^10,38 Liu et al.^15,17 determined that the egg-shell CoMoS catalyst showed higher hydrodesulfurization activity than the uniform catalyst because of the shorter diffusion path of the reactant. Song et al.³⁹ demonstrated that the egg-shell nickel catalyst exhibited higher activity and selectivity than conventional catalysts for CO hydrogenation. Thus, the eggshell Ni/SFC3R catalyst may achieve high hydrogenation activity and sulfur resistance for the hydrogenation of the C5 petroleum resin because of its eggshell distribution of the nickel component. In addition, the Ni contents of the Egg-10/SFC3R catalyst and IM-10/SFC3R catalyst are 9.13 wt% (referring to the total weight of the catalyst) and 9.18 wt%, respectively, according to ICP. This result indicates that the Ni loading of the Egg-10/SFC3R catalyst and IM-10/SFC3R catalyst are comparable. The Ni content of Egg-10/SFC3R is agreement with the theoretical content of 10 wt% by once impregnation, indicating that the Ni loading content is controllable. Moreover, the preparation of an eggshell catalyst overcomes the drawback of multiple impregnation.

Fig. 3 Representative SEM profile of the cross section of (a) IM-10/SFC3R and (c) Egg-10/SFC3R. The corresponding Ni line distribution profiles for (b) IM-10/SFC3R and (d) Egg-10/SFC3R by EDX analysis (NiKα) are shown.

3.1.2. X-ray power diffraction results. The NiO and Ni crystalline phases in the catalysts were examined through XRD analysis. The XRD diffractograms of the studied samples (SFC3R, unreduced and reduced IM-10/SFC3R, unreduced and reduced Egg-10/SFC3R) are shown in Fig. 4. Furthermore, the reduced IM-10/SFC3R and Egg-10/SFC3R catalysts were stored in air for two months. Fig. 4a shows that the SFC3R contains different crystalline zeolite phases (mainly Y zeolite and ZSM-5 zeolite) and Al₂O₃ identifiable through XRD, which is consistent with the literature,³³ thereby verifying that this material can be utilized as an excellent support. The XRD pattern of IM-10/SFC3R shown in Fig. 4d is not significantly different from that of Egg-10/SFC3R (Fig. 4b). Both patterns show characteristic peaks visibly identified as NiO at 2θ = 37.3°, 43.3°, 62.8°, 75.4°, and 79.4°, which correspond to the (111), (200), (220), (311), and (222) planes, respectively.⁴⁰ The peak at 79.4° is too weak to distinguish in Fig. 4b. The diffraction peaks of IM-10/SFC3R (Fig. 4d) are intense and sharp, whereas the peaks of Egg-10/SFC3R (Fig. 4b) are weak and broad, suggesting that the crystallite diameter of NiO in IM-10/SFC3R is larger. The average crystallite diameter NiO was calculated according to the Scherrer formula based on the NiO (200) plane. The crystallite size of NiO in Egg-10/SFC3R is approximately 20.3 nm, whereas that of IM-10/SFC3R is approximately 37.1 nm. The NiO in Egg-10/SFC3R showed a smaller crystal size compared with that in IM-10/SFC3R because the presence of organic substance in the eggshell preparation enhanced the metal dispersion. Fig. 4c and e show that the Ni characteristic peaks can be confirmed in the XRD patterns of the reduced IM-10/SFC3R (Fig. 4e) and Egg-10/SFC3R (Fig. 4c) catalysts. In contrast, no characteristic peaks of NiO are observed, demonstrating that the catalytic active Ni component is stable. The characteristic peaks representing metallic Ni appear in Fig. 4e and c at 2θ = 44.5°, 51.8°, and 76.3°, which correspond to the Ni (111), Ni (200), and Ni (220) planes, respectively.⁴¹ Fig. 4b and d show that the crystallite sizes of Ni were difficult to be determined precisely because of the partial overlap of Ni peaks and Al₂O₃ peaks at approximately 2θ = 44.5°. In addition, as illustrated in Fig. 4b–e, the characteristic diffraction peaks of the defect spinel phase NiAl₂O₄ (ref. 42) appear approximately at 2θ = 37°, 45°, 60°, and 66°, suggesting that the NiAl₂O₄ spinel forms on both catalysts. Moreover, the peaks of NiAl₂O₄ partially overlap with the NiO peak (at approximately 2θ = 37°) and Al₂O₃ peaks (at approximately 2θ = 45° and 66°).

Fig. 4 XRD patterns of (a) SFC3R, (b) unreduced Egg-10/SFC3R, (c) reduced Egg-10/SFC3R, (d) unreduced IM-10/SFC3R, and (e) reduced IM-10/SFC3R.

3.1.3. H₂ TPR results. To investigate the reducibility of the two types of catalysts, TPR analyses were conducted. The TPR profiles of the Egg-10/SFC3R and IM-10/SFC3R catalysts are presented in Fig. 5 for comparison. Evidently, the trace of IM-10/SFC3R (Fig. 5b) shows two peaks, whereas the trace of Egg-10/SFC3R (Fig. 5a) indicates four peaks. The peaks with a T_max of 844 °C are close to the reduction temperature of spinel NiAl₂O₄ (ref. 43) (the TPR result is consistent with that of the XRD analysis), whereas the peaks below 750 °C can be attributed to the reduction of Ni²⁺ in the NiO phase. The sharp and intense peak detected at approximately 400 °C in Fig. 5b can be assigned to the reduction temperature of the “free state” bulk NiO,⁴⁴ which shows no interaction with SFC3R. In Fig. 5a, the peak with a T_max of 370 °C is attributed to the reduction of pure NiO.⁴⁵ The peak at approximately 442 °C corresponds to the reduction of the NiO crystallites supported on SFC3R.⁴⁶ Fig. 5a also shows a low-broad peak with a T_max in TPR located at 676 °C, which hinders the migration and aggregation of the reduced Ni particle.

Fig. 5 TPR profiles of Egg-10/SFC3R and IM-10/SFC3R catalysts.

3.1.4. BET results. To acquire the specific surface area, BET was employed. The BET surface area and average NiO crystal size of the catalysts are summarized in Table 2. The results indicate that the specific surface area of Egg-10/SFC3R is larger than that of IM-10/SFC3R. This outcome can be attributed to the smaller average particle sizes of NiO in eggshell catalyst and the absence of blockage within the internal pores. At the same time, this result is also verified by the XRD results, which indicate that the average NiO crystallite diameter in the Egg-10/SFC3R is 20.3 nm and that in the IM-10/SFC3R catalyst is 37.1 nm because small particle sizes are derived from a large BET surface.

Table 2 Textural properties of IM-10/SFC3R and Egg-10/SFC3R catalysts

Catalyst	BET surface area (m^2 g^−1)	NiO average crystal size^a (nm)
a Determined by XRD.
IM-10/SFC3R	55.3	37.1
Egg-10/SFC3R	68.5	20.3

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3.2. Catalyst testing over Egg-10/SFC3R and IM-10/SFC3R catalyst

3.2.1. Catalyst activity testing. In this paper, a two-stage hydrogenation process was performed mainly to prevent resin degradation which could lead to an unacceptably low softening point under severe conditions,⁴⁷ as well as remove part of the sulfur during the first stage. To assess the properties of the C5 petroleum resin at each stage, the C5 petroleum resin was hydrogenated in an autoclave over the Egg-10/SFC3R catalyst. The properties of the C5 petroleum resin during the first stage (C5 HPR-1) and second stage (C5 HPR-2) were analyzed and compared. The results are shown in Table 3. Table 3 shows that the bromine number of C5 HPR-1 was reduced down to 20.36 g Br/100 g, and the sulfur content was reduced down to 24.7 mg kg⁻¹ from 56.0 mg kg⁻¹. This illustrates that the hydrogenation reaction (mainly the hydrogenation of ethylenic C=C bonds) and hydrodesulphurization reaction were carried out in the first stage. The bromine number of C5 HPR-2 dropped to 0.98 g Br/100 g sharply, indicating that most of the hydrogenation of unsaturated bonds took place in the second stage. There were little changes in molecular weight (M_w), molecular weight distribution (M_w/M_n) and softening point because the resin was stable under the mild reaction conditions of the first stage. The molecular weight (M_w) of C5 HPR-1 was increased approximately 0.4%, which was partly due to the hydrogenation of ethylenic C=C bonds. The molecular weight (M_w) and molecular weight distribution (M_w/M_n) of C5 HPR-2 were 3537 and 1.67, respectively. Obviously, the molecular weight (M_w) was decreased, and the molecular distribution (M_w/M_n) became narrower because the large molecular weight resin was degraded to a small molecular weight resin under the severe conditions of the second stage of reaction. Therefore, the softening point of the resin was slightly decreased from 371.0 K to 363.5 K as the molecular weight was decreased.

Table 3 Properties of C5 petroleum resin at first stage and second stage over the Egg-10/SFC3R catalyst

Items	Bromine value^a, g Br/100 g	Mw^b	Mw/Mn^b	Softening point, K^c	Sulfur content^d, mg kg^−1
a Determined by bromine valence & bromine index apparatus measured.b Determined by GPC measurement.c Determined by ASTM D 6493-11.d Determined by ICP-MS.
C5 petroleum resin	27	4613	2.25	371.0	56.0
C5 HPR-1	20.36	4810	2.31	371.1	24.7
C5 HPR-2	0.98	3537	1.67	363.5	<10

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To assess the performance of the Egg-10/SFC3R and IM-10/SFC3R catalyst samples, the C5 petroleum resin was hydrogenated in an autoclave. As shown in Table 4, the characteristics of the C5 petroleum resin and HPR were illustrated and compared. The results indicate that the performance of Egg-C5 HPR was improved significantly. Its bromine number and Gardner color no. decreased to 0.98 g Br/100 g and 1, respectively, coupled with a sulfur content reduction to <10 mg kg⁻¹. However, the bromine number and Gardner color no. of IM-C5 HPR with a sulfur content of 26.3 mg kg⁻¹ decreased only to 12 g Br/100 g and 4, respectively. Both softening points were slightly decreased by approximately 7 °C due to the degradation of the resin. These results indicate the excellent hydrogenation activity and high sulfur removal capacity of the Egg-10/SFC3R catalyst. In addition, the low bromine number and watery white color of Egg-C5 HPR suggest that the hydrogenation of the C5 petroleum resin was complete during the second stage.

Table 4 Properties of C5 petroleum resin and hydrogenated C5 petroleum resin over the Egg-10/SFC3R and IM-10/SFC3R catalysts

Items	Bromine value^a, g Br/100 g	Gardner color no.^b	Softening point, K^c	Sulfur content^d, mg kg^−1
a Determined by bromine valence & bromine index apparatus measured.b Determined by Gardner color number method (50 wt% solution in toluene).c Determined by ASTM D 6493-11.d Determined by ICP-MS.e The hydrogenated C5 petroleum resin which catalyzed by the Egg-10/SFC3R catalyst.f The hydrogenated C5 petroleum resin which catalyzed by the IM-10/SFC3R catalyst.
C5 petroleum resin	27	9	371.0	56.0
Egg-C5 HPR^e	0.98	1	363.5	<10
IM-C5 HPR^f	12	4	364.0	26.3

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All of the data above show that the Egg-10/SFC3R catalyst exhibited a higher initial activity for C5 petroleum resin hydrogenation than the IM-10/SFC3R catalyst. This finding indicates that the eggshell catalyst was more beneficial for C5 petroleum resin hydrogenation because of the presence of its eggshell distribution at the outer layer, smaller NiO particles, and stronger NiO-SFC3R interaction compared with the uniform catalyst, as characterized in Section 3.1. The molecular weight of the C5 petroleum resin, which contains S-compound, is relatively high (M_w = 4613). The hydrodesulfurization of the S-compound is limited by internal diffusion, and the hydrogenation process is an exothermic reaction. The hydrogenation of the C5 petroleum resin may be limited by the exothermicity of the reaction and the internal diffusion resistance¹⁹ within the tiny pores of the catalysts. The active component of the Egg-10/SFC3R catalyst is distributed with the eggshell layer on the outer surface, thereby enhancing the catalytic activity by reducing intraparticle diffusion limitation, facilitating exothermal dissipation, and improving the utilization efficiency of the active layer due to the decrease in the diffusion distance. On the contrary, the active component of the IM-10/SFC3R catalyst is mainly concentrated in the interior, resulting in the inability of the active nickel to contact with the reactant efficiently. In addition, the small NiO particles and the interaction between NiO and SFC3R are beneficial for the hydrogenation.³² Thus, the eggshell catalyst is superior to the uniform catalyst for C5 petroleum resin hydrogenation in terms of the activity. Moreover, the good activity and the sharply decreased sulfur content in the resin prove that the eggshell catalyst possesses a high total sulfur removal activity and resistance to poisoning. This result is consistent with the literature, which show that the reduction of internal diffusion inhibition can significantly enhance the overall hydrodesulfurization reaction rate.⁴⁸

3.2.2. Catalyst stability testing. The Egg-10/SFC3R catalyst showed higher initial activity for C5 petroleum resin hydrogenation than the IM-10/SFC3R catalyst. Testing the stability of the Egg-10/SFC3R and IM-10/SFC3R catalysts is crucial. This can be evaluated by the running times because the autoclave is a batch reactor. The analyses of the catalyst stability of the Egg-10/SFC3R and IM-10/SFC3R catalysts were conducted as follows. After the first cycle of hydrogenation, the reactant was allowed to settle down until the temperature decreased to 80 °C, and then the supernatant product mixture was removed from the reactor. A fresh charge of reactants was added to the catalyst residue in the reactor, and a subsequent run was performed.³² This procedure was followed by an appropriate number of runs, and the results are shown in Fig. 6. The Egg-10/SFC3R catalyst could maintain its catalytic activity (bromine number = 3.02 g Br/100 g) even after 5 runs, and the bromine number over the Egg-10/SFC3R catalyst changed slightly after each running time, implying the stability of the catalyst appreciates. After two cycles of being reused, the bromine number over the IM-10/SFC3R catalyst increased to 18.04 g Br/100 g. This increase was evident with the running time, indicating poor stability of the IM-10/SFC3R catalyst. The varying stability between the Egg-10/SFC3R catalyst and IM-10/SFC3R catalyst can also be mainly attributed to the different structure of the catalyst. Firstly, the eggshell catalyst is suitable for heat dissipation, which can slow down the sintering of the Ni component at a high temperature. Moreover, the eggshell catalyst can maximize the use of active Ni because of the decrease in the diffusion distance, and the eggshell catalyst possesses excellent sulfur resistance, as shown in Section 3.2.1, which makes it less likely to be poisoned and deactivated. Hence, the eggshell catalyst is stable even after five runs, whereas the uniform catalyst is clearly deactivated.

Fig. 6 Stability of catalysts for C5 petroleum resin hydrogenation. Reaction condition: resin, 30% C5 solution 650 ml; catalyst, 18.01 g; one stage: H₂ pressure, 2–3 MPa; temperature, 210 °C; time 2 h; two stage: H₂ pressure, 7–8 MPa; temperature, 270 °C, time 4 h.

3.3. Effect of the preparation conditions of the catalyst on the performance of the eggshell Ni/SFC3R catalyst

The eggshell Ni/SFC3R catalyst was obtained by pre-soaking the material with n-heptane which filled the SFC3R pores, and then by adjusting the n-heptane evaporation time and controlling the impregnation time. The distinct difference in hydrophilicity between the Ni²⁺ aqueous solution (hydrophilic) and n-heptane (hydrophobic) prevented the internal access of the Ni²⁺ aqueous solution quickly. In this section, we investigated the effect of nickel loading, n-heptane evaporation time and impregnation time during the catalyst preparation on the performance of the eggshell catalysts. In this process, the images of the color reduction of the C5 petroleum resin after hydrogenation with various bromine numbers are shown in Fig. 7. The hydrogenated C5 petroleum resin with a bromine number of 0.98 exhibited a near watery or watery white color.

Fig. 7 Color reduction of the C5 petroleum resin after hydrogenation with various bromine numbers over the eggshell Ni/SFC3R catalyst.

3.3.1. Effect of nickel loading. Fig. 8a presents the effect of nickel loading on the performance of the eggshell catalysts. A blank experiment over SFC3R (with Ni loading of 1.89 wt%) was conducted, and the result reveals that the bromine number decreased from 27 to 24, indicating that the nickel metal deposited on SFC3R was active for C5 petroleum resin hydrogenation. Fig. 8a shows that the bromine number decreased from 24 to 0.98 as the Ni content increased from 1.89 wt% to 10 wt%. It was because that the hydrogenation reaction rate was enhanced with an increase in the availability of catalytically active sites. However, the bromine number increased as the Ni loading increased from 10 wt% to 20 wt%. This result was attributed to the partial aggregation of the active component. Massive active component was accumulated in the eggshell layer when the Ni loading increased from 10 wt% to 20 wt%, resulting in the aggregation of the active component and the decrease in dispersion. In order to characterize the intrinsic performance of catalysts, TOFs and specific activity of the catalysts were determined. The TOFs at different Ni loading (1.89–20 wt%) were 9.82 h⁻¹, 23.16 h⁻¹, 16.08 h⁻¹, 9.78 h⁻¹ and 6.22 h⁻¹, respectively. The specific activities at different Ni loading (1.89–20 wt%) were 0.14 g m⁻², 0.33 g m⁻², 0.23 g m⁻², 0.16 g m⁻² and 0.10 g m⁻², respectively. The TOFs and specific activities increased first and then decreased with increasing Ni loading. The increase of TOF can be attributed to a higher specific activity. The TOF reached a maximum value of 23.16 h⁻¹ at Ni loading of 5 wt%.

Fig. 8 Bromine number versus various preparation conditions for eggshell catalysts. Preparation conditions: (a) evaporation time: 3 min; impregnation time: 2 min; (b) Ni loading: 10 wt%; impregnation time: 2 min; (c): Ni loading: 10 wt%; evaporation time: 3 min.

3.3.2. Effect of n-heptane evaporation time and impregnation time. Fig. 8b and c illustrate the effects of varying the n-heptane evaporation time and impregnation time on C5 petroleum resin hydrogenation. As shown in Fig. 8b and c, the bromine number decreased to a minimum value and then increased with the increase of the evaporation time and impregnation time. The eggshell thickness became thicker with the increase in the evaporation time and impregnation time. The eggshell thickness could be controlled by adjusting both the parameters of n-heptane evaporation time and impregnation time.^15,17 The longer the evaporation time, the thicker the eggshell thickness.¹⁵ And the longer the impregnation time, the thicker the eggshell thickness.¹⁷ The thicker the eggshell thickness, the higher the nickel dispersion. In other words, the longer the evaporation time and impregnation time, the thicker the eggshell thickness and the higher the dispersion. Therefore, the bromine number decreased at first with the increased of the evaporation time and impregnation time. However, the bromine number increased with the continued increase of evaporation time and impregnation time because the effective active component was decreased due to the diffusion limitation when the eggshell thickness was too thick.⁴⁹ When the n-heptane evaporation time was 3 min and the impregnation time was 2 min, the bromine number was at its minimum (Fig. 8b and c). This finding indicates that the thickness was appropriate at these conditions for C5 petroleum resin hydrogenation. Accordingly, the eggshell thickness of Ni at these conditions was approximately 1.8 μm, as shown in Fig. 3c.

4. Conclusions

In this work, a small eggshell Ni/SFC3R catalyst for C5 petroleum resin hydrogenation was successfully prepared. The morphology and microstructure of the micron-sized eggshell and uniform particles were determined by a FIB-cutting technique combined with SEM-EDX. The properties of IM-10/SFC3R and Egg-10/SFC3R catalysts were characterized through XRD, ICP, BET, and H₂-TPR. FIB/SEM-EDX measurements of the catalyst indicate that the nickel was mainly distributed in the outer layers of Egg-10/SFC3R catalyst and the average eggshell thickness was approximately 1.8 μm. The eggshell catalyst exhibited a superior catalytic hydrogenation performance and stability for C5 petroleum resin hydrogenation, which can be attributed to its eggshell Ni distribution, small NiO crystallite size, and interactions between NiO and SFC3R. The hydrogenation results over the eggshell Ni/SFC3R catalyst showed that the bromine number was reduced from 27 g Br/100 g (Gardner color grade no. 9) to 0.98 g Br/100 g (Gardner color grade no. 1) and that the sulfur content in petroleum resin was reduced from 56.0 mg kg⁻¹ to <10 mg kg⁻¹. The eggshell Ni/SFC3R catalyst was more beneficial for the hydrogenation of bulky C5 petroleum resin than a traditional hydrogenation catalyst, when SFC3R (in which zeolite structure is active for hydrodesulfurization) was used as the catalyst support. The Ni deposited on SFC3R was utilized maximally, thereby providing significant economic benefits. Our investigation provides a simple method to prepare small eggshell catalysts in which the loading content of the active component can be controlled, and a feasible route for disposing spent solid catalysts can be obtained.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 31560241, 51563001), the Guangxi Natural Science Foundation (Grant No. 2014GXNSFDA118010), Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (Grant No. 2014Z006, 2015Z011).

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