Sun Lijie; Fan Yaming; Dong Songtao; Xian Ce; Long Xiangyun; Li Dadong

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Abstract: Three different zeolite catalysts with different pore sizes (MFI-type, BEA-type, and FAU-type zeolites) have been prepared. The influence of different zeolite catalysts on reactivity and product shape selectivity of tetralin is investigated.Clear differences are observed in the reactivity of tetralin and distribution of products achieved by different catalysts.The diffusion and adsorption behavior of the reactant tetralin and its intermediates, n-butylbenzene and 1-methylindane under the reaction conditions are simulated using molecular simulation methods. Upon combining simulation results and experimental observations, it is shown that the difference in diffusion coefficient and competitive adsorption capacity can explain the reactivity of tetralin and the selectivity of products. The steric hindrance of the MFI-type zeolite mainly limits the key step of ring opening of tetralin, leading to lower selectivity of ring-opening products. n-Butylbenzene molecules can diffuse sufficiently fast in the large pores of FAU-type zeolite and the weak adsorption capacity of n-butylbenzene leads to its insufficient cracking. In addition, it also explains the reason that the BEA-type zeolite has the best BTX selectivity,because it can satisfy both good ring-opening activity and sufficient butylbenzene cracking depth.

Key words: hydrocracking; tetralin; molecular simulation; diffusion; competitive adsorption.

1 Introduction

In recent years, the global consumption of diesel fuel has gradually become stable. At the same time, the related environmental protection regulations have become continually stringent, which requires higher efficiency in diesel products processing[1]. Light cycle oil (LCO), derived from the fluid catalytic cracking unit, is a very poor-quality blending component in the diesel pool due to its high sulfur(up to 4.0%), nitrogen (up to 600 μg/g), and aromatic contents[2]. The chemical composition of LCO is dominated by aromatic hydrocarbons. Taking a refinery as an example,the content of aromatic compounds in LCO reaches more than 80%, and most of them are di-aromatic hydrocarbons.Accordingly, it has become the focal point of intense researches on converting potential aromatic resources of LCO into high value-added chemical products, such as benzene, toluene and xylene (BTX)[3-4]. The hydrocracking(HYC) technology is used to partially hydrogenate diaromatic hydrocarbons, and then the saturated naphthenic ring can be opened selectively and cracked into BTX, which can realize the full utilization of aromatic resources in LCO and build a bridge between refining and chemical industry.Zeolite is an important support of most hydrocracking(HYC) catalysts, which can provide the active centers for cracking reaction. As discussed in the previous reports, HYC catalysts supported on different zeolites were investigated for producing BTX from LCO[5],naphthalene[6], and tetralin[7-9]. Upare, et al.[10]obtained a desired monocyclic aromatics yield of 62.2% coupled with 99.5% of tetralin conversion over CoMo/Beta catalyst.In the study performed by Laredo, et al.[11], different proportions of NiMo/Al2O3and ZSM-5 zeolite mixtures were employed as catalysts. The result showed that the accessible, well-dispersed and strong Brönsted acid sites eased the hydrocracking of tetralin to BTX and the metallic hydrogenation functions of nickel-molybdenum catalysts were also required to minimize the deactivation. Previous studies[12-14]on the reaction path of tetralin hydrocracking have consistently shown that the main reaction path for the conversion of tetralin to BTX is a typical series of consecutive reactions, as shown in Figure 1.

Figure 1 The main reaction path for the conversion of tetralin to BTX[14]

For a consecutive reaction series, the diffusion and adsorption of reactants and intermediates in the zeolite support is one of the important factors affecting reactivity and product distributions[15]. The reactivity could be affected due to the confinement created by the zeolite pores, which can inhibit the adsorption and/or diffusion of reactants to the acidic sites[16]. Moreover, if the mobility of reactants is limited in the zeolite, reactants will stay longer at the active sites and the probability for consecutive reactions to convert these molecules would increase[17-18], thereby affecting the product distribution.So far, most studies on zeolite supports for tetralin HYC catalysts were focused on the fields of acidity or the synergistic effect of acid and hydrogenation active centers.Due to the strict experimental requirement and limitations of traditional experimental methods, there are few studies on the diffusion and adsorption of reactive molecules under reaction conditions. Smit, et al.[15]systematically discussed the relationship between adsorption, diffusion and shape selectivity through the molecular simulation method, and then concluded that molecular simulation has progressed to the extent that reliable predictions can be obtained by simulating “ideal” reference systems.Therefore, it is meaningful and feasible to use molecular simulation methods to explain the performance of HYC catalysts from the perspective of diffusion and adsorption capacity of reactants over zeolite support. The combination of traditional experiments and molecular simulation is helpful to understand the structure-activity relationship of zeolite supports and provide theoretical guidance for the design of industrial catalysts.

In this study, a 10-MR pore zeolite (ZSM-5) and two kinds of 12-MR pore zeolites (Beta, HY) were selected as supports of HYC catalysts. Tetralin was used as the model compound, and a hydrocracking micro-reactor was used to evaluate the reaction performance of tetralin on various HYC catalysts. Material Studio software was used to simulate the diffusion behavior and competitive adsorption of reactive molecules on various zeolites under reaction conditions, and the diffusion coefficients of reactive molecules on various zeolites were calculated.Then the simulation and calculation results were used to explain the difference in the reaction performance of tetralin on various HYC catalysts.

2 Experimental and Numerical

2.1 Experimental section

2.1.1 Catalysts preparation

ZSM-5, Beta, and HY zeolites were provided by the Sinopec Research Institute of Petroleum Processing(RIPP). A mixture composed of 40% of zeolites and 60% of high-purity mesoporous alumina was extruded into strips and calcined at 550 ℃ for 5 h. After that, Ni and Mo were respectively loaded on the obtained bar solid by equal volume impregnation with an appropriate concentration of NiCO3·2Ni(OH)2·4H2O and MoO3. All the impregnated catalysts were dried in air at 120 ℃overnight and calcined at 450 ℃ for 4 h. The prepared catalysts were named NiMo/ZSM-5, NiMo/Beta, and NiMo/HY, respectively.

2.1.2 Catalysts characterizations

The powder X-ray diffraction patterns were recorded with a Phillips PW 1830 diffractometer using the CuKα radiation.The N2adsorption-desorption isotherms were recorded at -196 °C using a Micromeritics ASAP2020 automated instrument. Surface areas were estimated according to both the Langmuir and BET models. The Si/Al ratio of zeolites and metal content of catalysts were determined by X-ray fluorescence spectroscopy (XRF) using a Philips PW 2404 instrument. The acid properties of catalysts were evaluated by Pyridine (Py)-FTIR spectra with an IMPACT-410(Germany) infrared spectrophotometer. The topological properties and elemental composition characterization results of catalysts are shown in Table 1.

Table 1 The topological properties and elemental composition of catalysts

2.2 Catalysts measurement

All evaluation experiments were carried out in a fixed-bed stainless steel micro-reactor, using 1.0 g of catalyst with a particle size of 40―60 mesh. Tetralin andn-butylbenzene were purchased from the Tokyo Chemical Industry Co.and Aladdin, respectively. The sulfided oil was composed of 2.5% of H2S and 97.5% of cyclohexane. The catalysts were sulfided in hydrogen atmosphere (100 mL/min, 4 MPa) at 360oC for 4 h, while the pre-sulfided oil flow rate was 0.4 mL/min. The reactant was fed by means of a high-pressure double plunger pump at a rate of 0.1 mL/h.The molar ratio of H2to substrate was 9/1, the hydrogen partial pressure was 4 MPa, the reaction temperature range was 320―400 ℃, and the space velocity was 5.0 h-1. The products were analyzed by on-line sampling with an Agilent GC 6850, equipped with the TCD and FID detectors using a HP-1 (60 m×0.32 mm×0.5 μm)column.

In order to facilitate the analysis of reaction results, the conversion rate of tetralin (%) andn-butylbenzene (%)were calculated by the following equations:

wherewTandwTorepresent the mass fraction of tetralin in feed and product, respectively; andwNandwNorepresent the mass fraction ofn-butyl benzene in feed and product,respectively.

In the hydrocracking reaction of tetralin,Siwas defined as the selectivity of hydrocarbon, which was calculated by the following equation:

whereXiis the mass concentration of each hydrocarbon at the outlet.

In order to further study the selectivity of the reaction path,SBTXandSROare defined as the selectivity of BTX and the selectivity of the selective ring-opening reaction,respectively, which can be calculated by the following equations:

In order to intuitively describe the conversion depth of alkyl benzene side chain cracking reaction, theKSCcoefficient is defined, which can be calculated by the following equation:

2.3 Molecular simulation method

The molecular simulation studies were carried out using the Material Studio provided by the Molecular Simulations Inc.,USA. Molecular construction of tetralin, 1-methylindane,n-butylbenzene, and benzene was completed by the visualizer module, and the geometric optimization was carried out by the Dmol3 module using the consistent value force field (CVFF)[19-20]. The cluster models of various zeolites were obtained from the International Zeolite Association (IZA) database. On this basis, the cluster models of ZSM-5 (Si91Al5O192), Beta (Si58Al6O128)and HY (Si171Al42O171) zeolites were established by replacing silicon atom with aluminum atom according to the actual Si/Al ratio of zeolites, as shown in Figure 2. It should be pointed out that some additional factors,such as the effect of hydroxyl radicals generated after ion exchange on the diffusion and adsorption of reaction molecules, were not taken into account in this study.The simulation results of XRD spectra were obtained by Reflex tools. The geometry was set as Bragg-Brentano, the angle range was 5oto 35oand the step size was set to 0.05o.The grand canonical Monte Carlo (GCMC) method and sorption module were used to calculate the adsorption of molecules over various zeolites[21-22]. The electrostatic interaction was treated by the van der Waals method, and the interaction cut-off distance was set at 1.25 nm. The low energy snapshots obtained from GCMC adsorption calculation were used for molecular dynamics (MD)calculation with the dynamic function of forcite module. The NVT ensemble[23]was selected and the simulated temperature was set at 320 ℃. The MD calculation can simulate the diffusion trajectory of hydrocarbons in zeolites to obtain the mean square displacement (MSD) curve[24]. If there is a good linear relationship between MSD and diffusion time,the Einstein’s method (Eq. 7) can be simplified to Eq. 8. The diffusion coefficient (Dc) of hydrocarbons in zeolites can be calculated by the following equations:

Figure 2 Cluster models of different zeolites topologies

whereKMSDis the linear part of the slope of MSD to time curve.

The comparison between the actual XRD spectra and the simulated XRD spectra is shown in Figure 3. The positions and intensities of the characteristic peaks in the simulated XRD spectra can basically correspond to the actual XRD spectra, proving the accuracy of the zeolite modeling work.

Figure 3 Comparison of actual and simulated XRD spectra of various zeolites

3 Results and Discussion

3.1 Evaluation of tetralin hydrocracking catalyst

3.1.1 Reactivity of tetralin hydrocracking

Under the same reaction conditions, the conversion of tetralin (CT) over three catalysts all increased naturally with an increasing temperature, as shown in Figure 4. And as the reaction temperature increased from 320 °C to 400 °C,a marked increment ofCTwas found over NiMo/Beta (from 41% to 85%) and NiMo/HY (from 29% to 74%), while theCTover NiMo/ZSM-5 only increased from 14% to 31%.The research carried out by Chen, et al.[16]showed that the diffusion behavior of molecules in zeolite supports have an obvious effect on the conversion rate. Considering that NiMo/ZSM-5 also has considerable acid properties and metal loading compared with the other two catalysts (as shown in Table 1), the difference in increments ofCTover NiMo/ZSM-5 might be attributed to the hindered diffusion of molecules in the narrow 10-MR channels of ZSM-5 zeolite. In order to further study the reactivity difference between the three catalysts, it is meaningful to quantify the diffusion capacity of reaction molecules in various zeolite supports by using the molecular simulation methods.

Figure 4 The reactivity of tetralin over NiMo/ZSM-5,NiMo/Beta and NiMo/HY

3.1.2 Selectivity of tetralin hydrocracking

The comparison of the selectivity of tetralin hydrocracking reaction needs to control the approximate conversion rate of tetralin on various catalysts. By changing the WHSV, the conversion rate of tetralin on three catalysts was controlled at about 30%, with the selectivity of all kinds of products shown in Table 2.As shown in Table 2, NiMo/Beta has the highest BTX selectivity, up to 44.9%, which is consistent with the results of other researchers[4,25-26]. The selectivity of naphthalene is the highest on NiMo/ZSM-5 to reach 36.9%, which is consistent with the experimental results of Xin, et al[27]. Xin, et al. believed that tetralin as a typical hydrogen donor would be adsorbed on the pore mouths of ZSM-5 and could undergo hydrogen transfer reaction to form naphthalene. The selectivity of >C10products on NiMo/HY is 9.6%, which is much higher than the other two catalysts. As regards the intermediates of consecutive reactions, there is no significant difference in the selectivity of methylindane on these three catalysts, but the selectivity of butylbenzene on NiMo/HY (15.8%) is much higher than the other two catalysts.With reference to the main reaction path of tetralin to BTX (Figure 1), in order to obtain high BTX selectivity of tetralin, the catalyst needs to have ideal ring-opening ability and high cracking depth of butylbenzene. TheSROandSCC,which were defined in the previous paper, can reflect the ring opening capability of tetralin and cracking depth of butylbenzene, respectively, on various catalysts,as shown in Figure 5. TheSROon NiMo/Beta and NiMo/HY was 46.6% and 43.9%, respectively, which was higher than 26.4% achieved by NiMo/ZSM-5. It indicates that the catalyst with 12-MR zeolite as the support has higher ring-opening capability than the catalyst supported by 10-MR zeolite, which is consistent with the research of Corma, et al[28]. Corma, et al. proposed that the ring-opening reaction of tetralin mostly occurs on the external surface of ZSM-5 crystallites. However,theKsccoefficient of NiMo/ZSM-5 and NiMo/Beta is both satisfactory, at 0.94 and 0.95, respectively. But theKsccoefficient of NiMo/HY is only 0.64, indicating that some butylbenzene molecules have been subjected to a relatively insufficient cracking reaction on NiMo/HY.Considering the difference in pore size and topological structure of 3 kinds of zeolite supports, the diffusion and competitive adsorption of reactive molecules in various zeolite supports might be one of the important factors that can affect the selectivity of products. Therefore, it is necessary to simulate the diffusion and competitive adsorption behavior of reactive molecules in zeolite supports by using the method of molecular simulation.

Table 2 The selectivity of tetralin hydrocracking productsover various catalysts

Figure 5 The SBTX, SRO and KSC coefficient of NiMo/ZSM-5, NiMo/Beta and NiMo/HY

3.2 Molecular simulation and calculation

Tetralin,n-butylbenzene, 1-methylindane and benzene were used as probe molecules respectively, and the diffusion coefficients of probe molecules in various zeolites were calculated by the forcite module. The results are shown in Figure 6. According to Figure 6,diffusion coefficients have a good correlation with the pore size of zeolites. The diffusion coefficients of four probe molecules in 12-MR Beta and HY zeolites are all by about an order of magnitude higher than that of the 10-MR ZSM-5 zeolite. This indicates that the restriction effect caused by the ZSM-5 zeolite pores may inhibit the adsorption and diffusion of tetralin to the reaction site, leading to the inferior reactivity of NiMo/ZSM-5 compared with NiMo/Beta and NiMo/HY. Therefore, one of the main reasons for the different activity of tetralin on various catalysts may be the different diffusion ability of tetralin in zeolite supports. Miao, et al.[29]designed the diffusion experiment and proved that tetralin had poor accessibility to the acidic sites of ZSM-5 zeolite, which also confirmed our point of view. Moreover, there is a correlation between the diffusion coefficient of tetralin in different zeolites and theSRO. Ferraz, et al.[14,30]proposed that the acidic sites on zeolite supports can promote the protonation of tetralin, leading to isomerization,cycloalkane ring opening and cracking reactions to obtain higher yields of aromatic products. The higherSROon NiMo/Beta and NiMo/HY may benefit from the good acidic site accessibility of tetralin in its zeolite supports.Besides, the diffusion coefficients of four probe molecules in HY zeolite are all slightly larger than that in Beta zeolite, which should be ascribed to the fact that the HY zeolite (0.74 nm ×0.74 nm) has larger 12-MR pores than Beta zeolite (0.66 nm×0.67 nm). Comparing the diffusion coefficients of different probe molecules in HY, Beta and ZSM-5 zeolites, tetralin and 1-methylindane are probe molecules with low diffusion coefficients. However, the diffusion coefficient ofn-butylbenzene in three zeolites is by about an order of magnitude higher than that of tetralin and 1-methylindane. This simulation result is worthy of attention, because the three probe molecules all have 10 carbon atoms, but the diffusion ofn-butylbenzene into the three zeolites is obviously easier. In order to verify this simulation results,n-butylbenzene was selected as a model compound to evaluate its activity on the three catalysts, as shown in Figure 7. It can be seen from Figure 7 that with the reaction temperature increasing from 320°C to 400 °C, theCNon NiMo/ZSM-5 increases from 32.1% to 82%, which is much higher than the increment ofCT. In addition, the activity ofn-butylbenzene over NiMo/Beta and NiMo/HY is also slightly higher than that of tetralin. Based on the simulation and evaluation results,it is speculated thatn-butylbenzene may have a better coupling effect with the zeolite channels, which makes it easier to diffuse in the zeolites than tetralin and 1-methyl indane.

Figure 6 The diffusion coefficient of NiMo/ZSM-5, NiMo/Beta and NiMo/HY

Figure 7 The comparison of reactivity of n-butylbenzene and tetralin over NiMo/ZSM-5, NiMo/Beta and NiMo/HY

Figure 8 The competitive adsorption probability and position of tetralin, 1-methylindane and n-butylbenzene on various zeolites

Figure 8 shows the competitive adsorption probability and position of tetralin, 1-methylindane andn-butylbenzene on various zeolites under reaction conditions (320 ℃,4 MPa). In the ZSM-5 and Beta zeolites, the reaction molecules are mainly distributed in the cross channels,while for the HY zeolite, the reaction molecules are mainly distributed in the super cages. The results of the competitive adsorption and their respective proportions of the three probe molecules on different zeolites are shown in Figure 9. The quantity of adsorbed molecules and the adsorption ratio of three C10probe molecules in various zeolites are obviously different. Due to the existence of the super cages, the HY zeolite can adsorb 50 reaction molecules, which are much higher than the 16 reaction molecules adsorbed by the ZSM-5 zeolite and the 25 reaction molecules adsorbed by the Beta zeolite. And tetralin is in a dominant position on HY zeolite in the competitive adsorption with 1-methylindane andn-butyl benzene, accounting for 52%. Sato, et al.[12]proposed the same conjecture, denoting that the super cages of HY can provide sufficient reaction space to promote the bimolecular reaction of tetralin. So the NiMo/HY zeolite has higher selectivity of >C10products.

Meanwhile, the adsorption amount of tetralin on different zeolites is also related to its diffusion coefficient. The higher adsorption capacity of tetralin on HY is ascribed to its higher diffusion coefficient.Judging from the perspective of kinetics, more adsorbed tetralin can increase the concentration of tetralin in the confined reaction space of HY zeolite, leading to higher SORof tetralin on NiMo/HY. Similar to the case with HY zeolite, the adsorption and diffusion of tetralin on Beta zeolite are also considerable, resulting in higher reactivity and ring-opening activity of NiMo/Beta. The competitive adsorption ratio of tetralin on the ZSM-5 zeolite only accounts for 12.5%, which is consistent with the lower reactivity of tetralin and poor ringopening ability over NiMo/ZSM-5. However, the better diffusion and adsorption capacity ofn-butylbenzene on ZSM-5 zeolite and the evaluation results show that once the cycloalkane ring of tetralin is opened to formn-butylbenzene, the subsequent reaction can take place smoothly. Therefore, the ring opening of tetralin on NiMo/ZSM-5 is the key step of the reaction, because the narrow channels of ZSM-5 restrict the diffusion of tetralin and make tetralin disadvantageous in competitive adsorption.

In order to obtain satisfactory BTX selectivity, in addition to higherSRO, a sufficient butylbenzene cracking depth is also required. As evidenced by the experimental results shown in Figure 6, the cracking depth of butylbenzene(SCC) is the least ideal on NiMo/HY. This might occur,because the competitive adsorption ofn-butylbenzene on HY zeolite is disadvantageous, accounting for only 12%. In contrast,n-butylbenzene can have considerable adsorption capacity and moderate diffusion coefficient hindrance of 10-MR ZSM-5 zeolite limits the diffusion and adsorption of tetralin and 1-methylindane, resulting in a relatively low activity and ring-opening selectivity of tetralin. Beta zeolite with a pore size between ZSM-5 zeolite and HY zeolite can satisfy both the ringopening activity of tetralin and the cracking depth of the intermediate butyl benzene, thus having the highest BTX selectivity. This may be ascribed to the fact that Beta zeolite can provide tetralin and its reaction intermediates with good diffusion capacity and approximate adsorption capacity. Moreover, introducing mesopores into ZSM-5 zeolite or mixing small-pore zeolite with HY zeolite may be possible to meet the ability of ring opening and alkylbenzene cracking depth at the same time.on Beta and ZSM-5 zeolites, which may be one of the reasons why butyl benzene can be subject to cracking reaction sufficiently on NiMo/Beta and NiMo/ZSM-5.

Figure 9 The competitive adsorption number and proportion of probe molecules on ZSM-5, Beta, and HY zeolites

4 Conclusions

In the present study, three zeolites (ZSM-5, Beta and HY) with different pore sizes were selected as supports respectively to prepare HYC catalysts. The hydrocracking reactivity and product selectivity of tetralin over various catalysts were evaluated in a fixed-bed stainless steel micro-reactor and were investigated from the perspective of diffusion and adsorption by molecular simulation methods. On the basis of the study results, the following conclusions are obtained.

The larger pore size of HY zeolite can facilitate the accessibility of tetralin and 1-methylindane to acidic sites, and the existence of super cages makes tetralin and 1-methylindane advantageous in competitive adsorption,and these factors make NiMo/HY attain higher tetralin activity and ring-opening reaction selectivity. However,NiMo/HY has low selectivity for alkylbenzene side chain cracking reaction. The NiMo/ZSM-5 catalyst can convert most of butylbenzene into BTX, but the steric