Study of the alkylation of benzene with methanol for the selective formation of toluene and xylene over Co₃O₄–La₂O₃/ZSM-5

Abstract

HZSM-5 zeolite was prepared by alkali treatment and NH₄⁺-exchange, followed by modification with cobalt nitrate and lanthanum nitrate via impregnation. The samples were characterized by XRD, SEM, FT-IR, nitrogen adsorption and NH₃-TPD. The catalytic performance of Co₃O₄–La₂O₃/ZSM-5 for benzene alkylation with methanol under different conditions was investigated. Alkali-treated ZSM-5 zeolite showed a high catalytic activity but a low selectivity for xylene in the reaction. However, the performance of HZSM-5 loaded with Co₃O₄ and La₂O₃ was excellent; the total selectivity of toluene and xylene reached 89.02%, and the benzene conversion reached 47.99%. The optimum conditions for benzene alkylation over Co₃O₄–La₂O₃/ZSM-5 were also determined. The best results were achieved at 723 K under a pressure of 0.1 Mpa for a feed ratio of 1 : 1 benzene–methanol at a space velocity of 3 h⁻¹. The total selectivity of toluene and xylene was 90.66%, xylene selectivity was 38.07%, and the benzene conversion was 51.36%.

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1 Introduction

p-Xylene is an important raw material for the synthesis of terephthalic acid, which is used in the synthesis of nylon and other polyesters.¹ The huge demand from the chemical fiber industry means that more countries are planning to set up p-xylene production facilities, and new p-xylene production reached 3.6 Mtons per annum in China in 2012.² Selective production of p-xylene using zeolite catalysts has attracted much interest in recent years. The ZSM-5 zeolite catalyst is of particular interest owing to its high activity and shape selectivity.^3–5 The selective preparation of p-xylene by alkylation of toluene with methanol has been developed. However, direct alkylation of benzene with methanol to prepare p-xylene is a more economical process, although it generates a mixture of toluene and xylene isomers.

Because of the wide use of ZSM-5 zeolite in the petrochemical industry, it is increasingly important commercially.^6,7 The activity and selectivity of ZSM-5 in organic reactions is mainly affected by zeolite acidity, and the size and dimensionality of the zeolite channels.^8–11 ZSM-5 zeolite catalysts show excellent catalytic performance in the alkylation of benzene. The use of ZSM-5 zeolite catalysts for producing high value-added aromatics is an environmentally friendly method and controlling the pores greatly improves product selectivity.¹²

Various modifications of ZSM-5 zeolite to increase its catalytic activity and selectivity of the target product have been studied, such as impregnation with metallic or non-metallic compounds,^13–15 pre-coking,¹⁶ CVD^17,18 and CLD.^19,20 Bjørgen et al.²¹ found that NaOH treatment reduced the Si/Al ratio, increased the specific surface area, generated mesopores, and improved the diffusion properties of ZSM-5. Niwa et al.²² found that moderate hydrothermal treatment changed the B acid strength, and the number of acid sites decreased with increasing water vapor partial pressure, although the acid strength increased. Gao et al.²³ observed that zinc salt had a great effect on ZSM-5 synthesis; the lower the SiO₂/ZnO ratio of the gel, the higher the Lewis/Brönsted ratio. Chandawar et al.²⁴ modified ZSM-5 with P or B to eliminate the strong acid sites on the surface and achieve higher ethylbenzene selectivity. Sidhpuria et al.²⁵ found that the total acid site concentration and acid site density per unit area of Rh/ZSM-5 were lower than those for the corresponding pure ZSM-5 sample, which suggested that significant numbers of acid sites were shielded by the metal particles after Rh loading on ZSM-5 samples. Li et al.²⁶ prepared Mo/ZSM-5 by impregnation and found that B acid sites were covered with [Mo₅O₁₂]⁶⁺, which was formed after Mo entered the zeolite pores and decreased the amount of acid B and increased the amount of L acid.

In previous studies of alkylation of toluene or benzene with methanol, Aboul-Gheit and colleagues^16,27 reported that the para-selectivity increased with the increase of the Pt or Pd content compared with ZSM-5 by narrowing the pore openings, but the overall selectivity for p-xylene was lower than 40%. Inagaki et al.²⁸ concluded that the high selectivity of p-xylene in MCM-22 zeolite was caused by shape selectivity originating from the interlayer 10MR micropores with 12MR supercages, as well as the intralayer sinusoidal 10MR micropores in the MWW structure. Smirniotis and Ruckenstein²⁹ observed that β zeolite favored secondary alkylation reactions and disproportionation reactions of the generated alkylaromatics compared with ZSM-5, because of the pore structure of β zeolite. Yashima et al.^30,31 reported that para-selectivity of 45–55% in toluene alkylation by methanol can be achieved by the ion exchange of Y type zeolite with different metal cations. Zhao and colleagues^32,33 found that a nanosized ZSM-5 catalyst modified by the SiO₂, P₂O₅ and MgO exhibited high para-selectivity of 98%.

Although previous studies have investigated the catalytic properties of discrete modified ZSM-5 zeolite for the alkylation of benzene with methanol, the overall catalytic performance is not yet satisfactory. This paper is intended to clarify the performance of benzene alkylation with methanol over a Co₃O₄–La₂O₃/ZSM-5 catalyst. The structure of the channels and the acidity of ZSM-5 were changed to improve its catalytic performance by alkali treatment and loading of metal oxides. In addition, the effect of temperature, feed ratio and space velocity over Co₃O₄–La₂O₃/ZSM-5 on the conversion and selectivity were discussed. Xylene is the primary target product, and toluene is the secondary target product.

2 Experiment

2.1 Catalyst preparation

Commercial grade ZSM-5 purchased from Shandong Li Yuan Chemical Group with SiO₂/Al₂O₃ ratio of 45 was treated with 0.5 mol L⁻¹ NaOH solution at 348 K for 2 h. The samples were separated by vacuum filtration, washed thoroughly with deionized water, dried at 384 K for 12 h, and then calcined in air at 823 K for 4 h. To obtain hierarchical HZSM-5 zeolite, the samples were ion-exchanged with 0.5 mol L⁻¹ NH₄NO₃ solution at 358 K three times, dried at 383 K for 12 h, and then calcined in air at 813 K for 4 h. The hierarchical HZSM-5 zeolite was soaked in solutions of Co(NO₃)₂ (0.24 M) and La(NO₃)₃ (0.19 M) at room temperature overnight. The zeolite was dried in an oven at 383 K for 12 h and calcined in air at 823 K for 4 h to obtain zeolites loaded with Co₃O₄ and La₂O₃ by the incipient-wetness impregnation method. These samples were named hierarchical Co₃O₄–La₂O₃/ZSM-5.

2.2 Catalyst characterization

The crystalline phases of the samples were investigated by XRD (XRD-6000, Bruker) using Cu Kα radiation (α = 1.5418 Å) in the 2θ range from 3 to 50°, with a step size of 0.02° s⁻¹. The morphology of the crystalline phase and particle size was determined by SEM (JEM 2000FX, JEOL) with an acceleration voltage of 100 kV. FT-IR spectra were recorded to characterize the zeolite framework vibrations and silanol groups at room temperature (Nicolet 380, Thermo Fisher Scientific) in the range of 400–4000 cm⁻¹ on a KBr pellet. Pore structure was determined by an adsorption instrument (3H-2000PS4, Bei Shide). Samples were subjected to N₂ adsorption–desorption tests after pretreatment at 30 °C under vacuum conditions for 5 h. The specific surface area was determined from the linear portion of the BET plot. Pore volume and pore size were calculated by the HK equation. The acidic properties of the catalysts were calculated by NH₃ temperature-programmed desorption (NH₃-TPD; Pulse Chemisorb 2720, Micromeritics).

2.3 Catalytic testing

Catalytic activity of the samples was determined in a fixed-bed continuous flow reaction system under different conditions. The catalyst (1 g) was loaded into the reactor, which was 200 mm long and had an actual inside diameter of 30 mm. Benzene and methanol at different molar ratios were introduced directly into the reactor at a series of weight space velocities, and the temperature and pressure were varied. High purity (99.99%) nitrogen was used as the carrier gas at a rate of 40 mL min⁻¹.

The conversion of benzene (C_B), the selectivity of toluene (S_T) and the selectivity of xylene (S_X) are defined by the following equations.

3 Results and discussion

3.1 Characterization of the catalyst

Fig. 1 shows the XRD pattern of ZSM-5, HZSM-5 andCoO-La₂O₃/ZSM-5 zeolite. All the patterns show diffraction peaks at 2θ of 7.92°, 8.80°, 14.78°, 23.18°, 23.90°, and 24.40°, which are characteristic of the MFI topology, indicating that the zeolite structure was retained after treatment. No crystalline Co₃O₄ and La₂O₃ were detected in the XRD spectra for Co₃O₄ loading of 3 wt% and La₂O₃ loading of 3 wt%, which shows that Co₃O₄ and La₂O₃ are well dispersed.

Fig. 1 XRD patterns of different samples.

Fig. 2 shows the SEM images of ZSM-5, HZSM-5 and Co₃O₄–La₂O₃/ZSM-5 zeolite. ZSM-5 zeolite had rectangular crystals with smooth, angular surfaces, and a long-axis length of 5 μm and a short-axis length of 2 μm. Both of the H-ZSM-5 zeolites prepared by alkali treatment and the Co₃O₄–La₂O₃/ZSM-5 exhibited an anomalistic ellipsoidal shape with rough surfaces and ill-defined particles. This morphology arose from the alkali treatment to achieve partial desilication, which decreased the crystallinity.

Fig. 2 SEM images of catalysts (a) ZSM-5, (b) HZSM-5, (c) Co₃O₄–La₂O₃/ZSM-5, and (d) Co₃O₄–La₂O₃/ZSM-5.

Fig. 3 shows the FT-IR spectra of ZSM-5, HZSM-5 and Co₃O₄–La₂O₃/ZSM-5 zeolite. The spectra were characteristic of MFI zeolites. For all samples, in the FT-IR spectra of the framework absorption region (1600–400 cm⁻¹), absorption bands were observed at 1250–1050 cm⁻¹ (external or internal asymmetric stretch), 800 cm⁻¹ (external symmetric stretch), 550 cm⁻¹ (double ring), and 450 cm⁻¹ (T–O bend).^23,34,35 FT-IR spectra demonstrated that the addition of Co₃O₄ and La₂O₃ did not affect the skeleton framework of the ZSM-5 zeolite. The SEM results are consistent with the XRD test results.

Fig. 3 FT-IR spectra of the samples.

Nitrogen sorption isotherms are shown in Fig. 4. The parent ZSM-5 zeolite exhibited type I nitrogen sorption isotherms typical of zeolites, whereas both the modified products of HZSM-5 and Co₃O₄–La₂O₃/ZSM-5 produce type IV isotherms associated with mesoporous materials. The textural properties of samples are listed in Table 1, which compares the surface area and pore volume of ZSM-5, HZSM-5, and Co₃O₄–La₂O₃/ZSM-5 zeolite samples. Compared with the original ZSM-5 zeolite, the HZSM-5 and Co₃O₄–La₂O₃/ZSM-5 zeolite samples showed a slight decrease in the pore volume (V_Micro) of the micropores. Micropore volumes are often used as an indicator for the crystallinity of MFI zeolites. The volumes indicated that the sample was well crystallized and retained the enriched micropores and mesopores within the particles. The BET surface area of Co₃O₄–La₂O₃/ZSM-5 decreased substantially, because rare earth supported oxides were deposited in the channels of ZSM-5. In contrast, the external surface area increased monotonically, whereas a thin layer of Co₃O₄–La₂O₃ formed on the surface.

Fig. 4 Nitrogen adsorption/desorption isotherm and DFT pore-size distribution of (a and b) ZSM-5, (c and d) HZSM-5, and (e and f) Co₃O₄–La₂O₃/ZSM-5.

Table 1 Textural properties of the samples

Sample	SBET (m^2 g^−1)	SMicro (m^2 g^−1)	SEXT^a (m^2 g^−1)	VMicro^a (cm^3 g^−1)
a Determined by a t-plot according to the method of Lippens and de Boer.
ZSM-5	317.7	299.9	17.8	0.124
HZSM-5	303.2	213.2	90.1	0.093
Co3O4–La2O3/ZSM-5	258.9	179.2	79.7	0.078

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The results of NH₃-TPD characterization shown in Fig. 5 show that Co₃O₄–La₂O₃/ZSM-5 have typical profiles comprising weak and strong acid sites (peaks at 210 °C and 380–390 °C). Alkali treatment reduced the acid amount and strength effectively. The combination of Co₃O₄ and La₂O₃ increased the strong acid strength to an extent, but it did not increase the acid amount. Therefore, compared with the original ZSM-5, Co₃O₄–La₂O₃/ZSM-5 had a lower weak acid strength, similar strong acid strength and fewer strong acid sites, which is associated with the degree of further alkylation.

Fig. 5 NH₃-TPD patterns of ZSM-5 catalysts.

3.2 Comparison of benzene alkylation with methanol over different zeolites

The results in Fig. 6 show the benzene conversion and desired product selectivity during alkylation with different catalysts at 693 K, 1.0 h⁻¹ WHSV, and a benzene–methanol molar ratio of 1/1 using a continuous nitrogen flow at atmospheric pressure. Xylene selectivity was in the order Co₃O₄–La₂O₃/ZSM-5 > HZSM-5 > ZSM-5.

Fig. 6 Comparison of benzene alkylation over different zeolites.

The total selectivity of toluene and xylene, and the benzene conversion were in the same order. Over Co₃O₄–La₂O₃/ZSM-5, the total selectivity of toluene and xylene reached 89.02%, the selectivity of xylene was 36.58%, and the benzene conversion also reached 47.99%. The total selectivity of toluene and xylene, the selectivity of xylene and benzene conversion over ZSM-5 were just 78.38%, 23.33% and 37.92%, respectively. Hence, the experimental results clearly indicate alkali treatment improved the open framework. This helps to increase benzene conversion and selectivity of xylene. Co₃O₄ and La₂O₃ incorporation in the zeolite has a significant role in activating the catalysts, which together with the acidic sites of ZSM-5 produce the best acidic environment. This change in the acidity is beneficial to xylene production. Overall, the results showed that most of La₂O₃ entered the channels in ZSM-5 and covered some of the acid sites. Because La₂O₃ has a high basicity, it is preferentially attracted to strong acid sites, thus preventing further alkylation. However, more Co₃O₄, which is weakly alkaline, was dispersed on the surface of ZSM-5 and adjusted the surface acidity.³⁶ This could stop the isomerization of the generated xylene. In addition, the metal oxide can narrow the channels, which prevents unintended damage to channels by alkali treatment. All the changes to ZSM-5 helped to improve the selectivity of xylene considerably. Therefore, the modified Co₃O₄–La₂O₃/ZSM-5 showed good catalytic performance.

The data in Table 2 shows that while the alkylation time was prolonged, the catalytic properties of HZSM-5 and Co₃O₄–La₂O₃/ZSM-5 remained relatively stable. Benzene conversion and xylene selectivity varied slightly with no sharp decline. Benzene conversion decreased gradually after 10 h. This difference is caused by alkali treatment. The large number of mesoporous structures prevented carbon deposition and product blockage, which reduced the barrier between reactants and products in channels. It improved mass transfer efficiency, helping to maintain catalytic activity.

Table 2 Effect of time on stream

Time/h	Benzene conversion/%			Xylene selectivity/%
	ZSM-5	HZSM-5	Co3O4–La2O3/ZSM-5	ZSM-5	HZSM-5	Co3O4–La2O3/ZSM-5
2	37.67	45.54	47.35	22.84	31.35	37.45
4	37.92	45.31	47.99	23.33	31.06	36.58
8	38.32	44.97	47.07	23.65	31.19	36.92
10	36.98	45.39	48.40	22.71	30.12	37.43
12	35.16	46.03	47.89	24.09	31.77	36.70
14	34.53	45.44	48.03	23.87	30.52	37.12

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3.3 Effects of temperature on alkylation over Co₃O₄–La₂O₃/ZSM-5

The effect of temperature (623–823 K) on benzene alkylation with methanol is shown in Fig. 7. The total conversion of benzene increased with an increase of temperature between 623–823 K, and reached 51.31% at 823 K. The active sites of the catalyst may not have been activated fully because of the low temperature, meaning that some active centers were not catalytically active. Active sites were all activated at 723 K. Further increases in temperature did not improve the catalytic performance, but also made the catalyst vulnerable to coking deposition and deactivation. Toluene selectivity increased slowly as the temperature increased. Xylene selectivity increased with increasing temperature, and then decreased, reaching a maximum of 36.58% at 723 K. Overheating caused over-alkylation of benzene. The temperature had a significant effect on the reaction; the optimum temperature of the catalytic reaction was 723 K.

Fig. 7 Effect of temperature on alkylation over Co₃O₄–La₂O₃/ZSM-5.

3.4 Effects of pressure on alkylation over Co₃O₄–La₂O₃/ZSM-5

To study the effect of pressure on the alkylation over Co₃O₄–La₂O₃/ZSM-5, the pressure was varied from 0.1 to 0.4 MPa at a reaction temperature of 723 K, a benzene–methanol ratio of 1, and a space speed of 1.0 h⁻¹. Fig. 8 shows that within the pressure range investigated, conversion of benzene over Co₃O₄–La₂O₃/ZSM-5 decreased from 47.99% at 0.1 MPa to 44.85% at 0.4 MPa. However, xylene selectivity increased as the pressure increased from 0.1 to 0.2 MPa, a maximum xylene selectivity of 37.69% was achieved at 0.2 MPa, and then it decreased slightly. Accordingly, the selectivity of toluene decreased slightly with the increase in pressure, indicating that increased pressure promoted the over-alkylation of benzene on Co₃O₄–La₂O₃/ZSM-5. When the pressure was increased, ethylbenzene selectivity increased. This behavior also shows that pressure does not act as an important promoter for benzene conversion. Co₃O₄–La₂O₃/ZSM-5 achieved the best catalytic activity and excellent selectivity at 0.1 MPa.

Fig. 8 Effects of pressure on alkylation over Co₃O₄–La₂O₃/ZSM-5.

3.5 Effects of space velocity on alkylation over Co₃O₄–La₂O₃/ZSM-5

Fig. 9 clearly shows that when the space velocity increased from 1 to 5 h⁻¹, benzene conversion increased until it reached a maximum of 52.12% at 4 h⁻¹, and then gradually reduced. Toluene selectivity remained constant. Xylene selectivity first increased, reaching a maximum of 38.07% at a space velocity of 3 h⁻¹, and then decreased. This means that the space velocity determined the contact time of the reactants with the catalyst, which directly affected the degree of further alkylation. The alkylation reaction of benzene and methanol decreased as the contact time of the reactants with the catalyst decreased at a high space velocity, which meant that the reaction only generated toluene. This result in a high selectivity for toluene and a low selectivity for xylene.

Fig. 9 Effects of space velocity on alkylation over Co₃O₄–La₂O₃/ZSM-5.

3.6 Effects of methanol–benzene ratio on alkylation over Co₃O₄–La₂O₃/ZSM-5

The feed ratio of methanol and benzene was increased from 0.5 to 2 to examine the effect on the performance of the Co₃O₄–La₂O₃/ZSM-5 catalyst. Fig. 10 shows that increases in the methanol–benzene molar ratio increased the conversion of benzene substantially and gradually increased the selectivity of the desired product, xylene. The benefit of increasing the target product by increasing the ratio addition to 1 was clear, considering the correlation between the selectivity and ratio of methanol and benzene. In addition, methanol promoted further alkylation of benzene to produce xylene. However, methanol partial pressure in the system was high, which led to the dehydration reaction and over-alkylation. A benzene–methanol ratio of 1 : 1 was the best process conditions for the reaction.

Fig. 10 Effects of methanol–benzene ratio on alkylation over Co₃O₄–La₂O₃/ZSM-5.

4 Conclusions

We investigated benzene alkylation with methanol as an alkylating agent over Co₃O₄–La₂O₃/ZSM-5 for the selective formation of toluene and xylene isomers, particularly p-xylene. We can draw the following conclusions from our results. All the synthesized ZSM-5 catalysts were characterized by XRD, SEM, surface area, FT-IR and NH₃-TPD, and were similar. The open framework was improved after alkali treatment to increase the shape selectivity of the catalysts. Mesopores increased the efficiency of mass transfer greatly, which aided the conversion of benzene. The activity of HZSM-5 with Co₃O₄ and La₂O₃ at concentrations of 3% was effective for the alkylation of benzene with methanol by changing the number of acid sites and the acid strength. The total selectivity of toluene and xylene reached 89.02%, the selectivity of xylene was 36.58%, and the benzene conversion reached 47.99%. The catalyst also improved the stability of the catalyst significantly. The temperature, pressure, space velocity, and benzene–methanol ratio may be responsible for achieving the maximal values of benzene conversion and the desired product selectivity in benzene alkylation with methanol. The best results were achieved at 723 K under a pressure of 0.1 Mpa when the feed was a ratio of 1 : 1 benzene–methanol at a space velocity of 3 h⁻¹. The total selectivity of toluene and xylene, the selectivity of xylene, and the benzene conversion were 90.66%, 38.07%, and 51.36%, respectively.

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