Utilization of Metal-Functionalized ZSM-5 for Methanol and Low-Carbon Hydrocarbon Coupling Aromatization

Abstract

Aromatics assume a paramount role as indispensable organic chemical feedstock within diverse industrial domains. Simultaneously, the global aromatics market is scarce, particularly with the exorbitant demand for high-value aromatics. Generating aromatics via coal-based methanol and low-carbon hydrocarbon coupling reactions has become a novel green and sustainable development trajectory. In this study, HZSM-5 catalysts featuring different Si/Al ratios and active metal-functionalized modifications were utilized to explore the aromatization effect in light of the Si/Al ratio, types of active components, and metal-loading content in a fixed-bed reactor. The outcomes were that the conversion ratios for methanol and n-pentane attained 99.9% and 83.1%, respectively. Remarkably, an oil phase yield of 32.1% was accomplished, along with an aromatic content of approximately 74.2%, while xylene selectivity reached approximately 37.6% for the 1.0%-ZnO/ZSM-5 (50) catalyst. Ultimately, a reaction mechanism for the coupling of methanol and n-pentane to yield aromatics using a 1.0%-ZnO/ZSM-5(50) catalyst is postulated.

1. Introduction

Aromatics play a pivotal role as crucial organic chemical feedstock across diverse industries. The global aromatics market is currently facing a shortage, with particularly high demand for BTX. More than 90% of aromatic hydrocarbons are obtained via naphtha cracking and catalytic reforming processes. However, excessive dependence on oil for aromatic hydrocarbon production is encountering significant challenges, such as raw material scarcity and escalating costs attributed to the depletion of global oil resources [1,2,3]. Given the abundant coal reserves worldwide, the development of the coal chemical industry has facilitated the expansion of coal-based methanol production. It has also contributed to reducing dependency on naphtha-derived aromatics. So far, methanol-to-aromatics (MTA) is widely acknowledged as an environmentally sustainable and relatively stable production route.

In the 1970s, the HZSM-5 molecular sieve was first utilized to catalyze the conversion of methanol to aromatics in a one-stage fixed-bed methanol process developed by the U.S. Mobile Company. Research revealed that Cu/ZSM-5-catalyzed products exhibited improved aromatics selectivity and aromatization performance [4,5]. In the 1980s, fluidized bed continuous reaction regeneration technology was developed, achieving nearly the complete conversion of methanol reactants. The selectivity of C₂⁼ and C₃⁼ reactive low-carbon olefins in products ranged from 75 to 80%. MTA fluidized bed technology was initially proposed by Tsinghua University, which used a modified ZSM-5 catalyst as the aromatization catalyst [6]. In this process, hydrogen, methane, C₈ aromatics, and C₉⁺ aromatics were discharged from the products. Subsequently, C₂⁺ low carbon hydrocarbons, C₇ aromatics, and the remaining C₉⁺ aromatics were refluxed to further promote alkane and olefin conversion to aromatics while increasing the xylene content in the products. In addition, methanol-to-aromatics (MTA) industrialization successfully commenced production in China in 2012 [7]. However, a significant disparity exists between the actual aromatic hydrocarbon yield from existing aromatization processes and the theoretical yield. The microporous HZSM-5 catalyst was usually adopted in this process due to its short pore size and uneven acidity distribution, which quickly led to coking and deactivation of the catalyst, severely restricting the development of the MTA process.

In recent years, significant research has focused on developing and optimizing MTA catalysts to enhance reactive activation and, correspondingly, suppress coke formation. Techniques such as active metal modification and pore size adjustment have been widely employed to tailor textural properties [8,9] or enhance the MTA catalyst’s Lewis acid sites [10,11]. Yang [12] observed that ordered HZSM-5 (with particle sizes of 20 nm) exhibited excellent catalytic effects, achieving high selectivity for light aromatics at 60%, with no significant deactivation over 51 h. Ni [13] indicated that Ga-modified ZSM-5 catalysts improved methanol aromatization and selectivity toward benzene and toluene. It was also found [14] that the catalyst’s activity is primarily determined by the initial methanol conversion reaction rate, which is related to the volume of micropores, with more micropores leading to higher activity. Despite these advancements, existing studies have primarily focused on preparing and modifying MTA catalysts to improve aromatics selectivity and catalytic stability. However, little attention has been given to studying the coupling aromatization reaction between methanol and low-carbon hydrocarbons under mild conditions.

In this study, mesoporous ZSM-5 catalysts modified with active metals were prepared as MTA catalysts using methanol and low-carbon hydrocarbon coupling aromatization, and their reaction properties were evaluated in a fixed-bed reactor. The performance of MTA catalysts for aromatization reactions was investigated in terms of the Si/Al ratio, type of active metal, and active metal content of ZSM-5 catalysts. In addition, the preliminary investigation focused on the reaction mechanism of the methanol and n-pentane coupling aromatization reaction to produce aromatics using a 1.0%-ZnO/ZSM-5 catalyst.

2. Materials and Methods

2.1. Catalyst Preparation

Different Si/Al ratios (Si/Al = 25, 50, 80, and 120) and Na-ZSM-5 zeolite (50 g) were placed in a NH₄NO₃ solution (300 mL) and stirred at 80 °C for 2 h. Subsequently, the mixtures were washed and filtered at least three times. Then, the filtered samples were dried at 100 °C for 10 h and calcined at 550 °C for 4 h. The prepared HZSM-5 with different Si/Al ratio catalysts are denoted as HZSM-5(25), HZSM-5(50), HZSM-5 (80), and HZSM-5 (120), respectively.

Table 1. Purity and brand of experimental materials.

Materials	Purity	Production Units
ZSM-5	Normal	Catalyst Plant, Nankai University
NH4NO3	AR	Macklin Biochemical Co., Ltd. (Shanghai, CN)
Zn (NO3)2	AR	Macklin Biochemical Co., Ltd. (Shanghai, CN)
Ni (NO3)2	AR	Macklin Biochemical Co., Ltd. (Shanghai, CN)
Cu (NO3)2	AR	Macklin Biochemical Co., Ltd. (Shanghai, CN)
Co (NO3)2·6H2O	AR	Macklin Biochemical Co., Ltd. (Shanghai, CN)
(NH4)2MoO4	AR	Macklin Biochemical Co., Ltd. (Shanghai, CN)
Methanol	AR	Macklin Biochemical Co., Ltd. (Shanghai, CN)
n-Pentane	AR	Macklin Biochemical Co., Ltd. (Shanghai, CN)

shows the purity and brand of experimental materials.

An active metal modification catalyst was prepared using the equal volume impregnation method. The saturated water absorption of molecular sieves was first measured before loading the active metal. The metal nitrate solution was added to the molecular sieve drop by drop, and the solution was stirred until it was pasted, and then, it was impregnated for 10.0 h at room temperature. Secondly, the obtained catalysts were dried for 10.0 h at 100 °C and calcined for 4.0 h at 550 °C. A M₂O_n/ZSM-5 catalyst with a loading capacity of X% is denoted as X%-M₂O_n/ZSM-5. The prepared catalysts were compressed in 15 MPa for 3.0 min and crushed through a 20–40 mesh before further use.

2.2. Characterization and Analysis

An X-ray diffractometer operating at 40 kV and 40 mA was used to identify the physicochemical properties and crystal structures and was provided by D8 ADVAHCL (Bruker Corp., Karlsruhe, Germany). Textural properties were characterized by N₂ adsorption/desorption at 77 K with the ASAP20 apparatus (Micromeritics Corp., Norcross, GA, USA).

The temperature-programmed desorption of ammonium (NH₃-TPD) was applied to test the acidity of these catalysts using a ChemiSorb Model 2750 (Micromeritics Corp., USA). Typically, 0.03 g of each sample was preheated under a He stream of 30 mL∙min⁻¹ at 500 °C for 1 h. Afterward, the sample was treated under a He flow of 30 mL∙min⁻¹ at 50 °C for 30 min. Finally, the desorption of NH₃ was measured by a TCD from 50 °C to 800 °C at a rate of 10 °C∙mim⁻¹.

The acid distribution of the catalyst samples was characterized by Fourier Transform-Infrared spectra of the catalysts with pyridine adsorption (resolution 4 cm⁻¹, scanning range 400~4000 cm⁻¹), which was provided by a Nicolet 501P spectrophotometer. Each sample (20~60 mg) was pressed into a self-supporting wafer (diameter of 13 mm) and degassed under vacuum at 450 °C for 3 h. Then, the wafer was cooled to 50 °C and absorbed in pyridine vapor for 0.5 h. Finally, the IR spectra were recorded at 200 °C and 350 °C.

2.3. Aromatization Reaction Test and Product Analysis

In this test, methanol and n-pentane were adopted as raw materials, and the effect of the aromatization reaction was performed in a fixed-bed device with an inner diameter of 10 mm (as shown in Figure 1). The catalyst was placed in the center of the fixed-bed reactor, and quartz sand was put above and below the aromatization catalysts. For this test, 2.5 ± 0.1 g of each catalyst was adopted. Firstly, the preheater was heated up to 200 °C. Secondly, the reactor temperature was heated to a preset temperature (400 °C) with a stream of nitrogen gas (90 mL/min); the aromatization reaction was typically 8.0 h in duration. Moreover, the reaction temperature and pressure were recorded every 30 min. The products were collected after being separated by a condensing tank. The gas phase, liquid phase, and oil phase of the products were detected by offline analysis. During this period, a GC equipped with dual TCD detectors was regularly determined online. The liquid phase was condensed in the trap and analyzed offline in a GC equipped with a capillary column. The oil phase product’s content was analyzed using a 7890B-5977B GC/MS gas chromatograph and a 7890B gas chromatograph (Agilent, Santa Clara, CA, USA) with a PONA column (50 m × 0.20 mm × 0.50 µm).

The methanol and n-pentane conversion, aromatic hydrocarbon, and xylene selectivity and yield are defined as follows:

X(methanol) = [(methanol mass in feed − methanol mass in the product) × 100%]/(methanol mass in feed)

(1)

X(n-pentane) = [(n-pentane mass in feed − n-pentane mass in product) × 100%]/(n-pentane mass in feed)

(2)

Y(oil) = [(oil phase mass in product) × 100%]/(n-pentane and methanol mass in feed)

(3)

S(product) = [(product mass) × 100%]/(oil phase mass in product)

(4)

where X is the conversion of methanol and n-pentane, S is the selectivity of aromatic hydrocarbon and xylene, and Y is the yield of the oil phase.

3. Results and Discussion

3.1. Catalyst Characterization

Table 2. Textural properties of HZSM−5 with different Si/Al ratios and modified catalysts.

Catalyst	ABET/m2·g−1	Vtotal/cm3·g−1	Pore Size/nm
Na-ZSM-5	282	0.33	2.27
HZSM-5 (25)	290	0.62	3.10
HZSM-5 (50)	300	2.80	2.43
HZSM-5 (80)	310	3.26	2.79
HZSM-5 (120)	319	3.64	3.28
0.5%-ZnO/ZSM-5	263	3.11	2.57
1.0%-ZnO/ZSM-5	259	2.98	2.41
1.5%-ZnO/ZSM-5	254	2.97	2.40
2.0%-ZnO/ZSM-5	250	2.91	2.61
2.5%-ZnO/ZSM-5	240	2.81	2.49

presents the textural properties of HZSM-5 catalysts with varying Si/Al ratios and modified catalysts.

As depicted in Table 2, the internal structure of the HZSM-5 catalysts displayed discernible trends. The pore size of the HZSM-5 catalysts experienced minimal variation, while the specific surface area and pore volume exhibited gradually increased with an increasing Si/Al ratio. This could be ascribed to a decrease in the quantity of skeleton Al-bridged hydroxyl and an increase in the Si-O bonds, combined with the smaller radius of Si compared to that of Al [15,16]. Upon elevating Zn loading from 0.5% to 2.5%, a decline in specific surface area from 263 m²·g⁻¹ to 240 m²·g⁻¹, a reduction in pore volume from 3.11 cm³·g⁻¹ to 2.81 cm³·g⁻¹, and an average pore diameter decrease from 0.16 nm to 0.15 nm was observed. The reason could be that the decomposition of Zn could decrease the S_BET of the tested ZSM-5 (50) catalysts. Meanwhile, there is a significant impact on the internal structure of the catalysts caused by the addition of active metal oxides (the other metal-modified catalysts exhibit similar textural properties as those observed for the 1.0%-ZnO/ZSM-5, and these are not listed here). The framework rearrangement occurred in the Zn-modified catalyst, which partially impedes the N₂ uptake in the zeolite micropores. One part of the active component was distributed on the outer surface of the catalyst. It provided the required active center for the dehydrogenation of cycloalkanes. In contrast, the other part entered the molecular sieve pores and combined with the molecular sieve skeleton to form a Si-Zn-Al structure. With an escalation in the loading content of the active metal, there was a progressive accumulation of active substances on the surface of the catalyst, resulting in a continuous reduction in the effective specific surface area of the catalyst. Concurrently, there was an augmentation in active components infiltrating the structure of the zeolite, leading to a gradual diminution of pore volume and rendering the catalyst susceptible to carbon deposition and deactivation phenomena [17,18].

Figure 2 shows the XRD spectrum of the catalyst at 1.0% metal oxide loading. It can be seen that all the samples exhibited a typical MFI topology, and the peak angles were the same as those of the unmodified HZSM-5 catalyst, indicating the crystal structure of HZSM-5 did not change after introducing the active component [19,20]. Meanwhile, loading the appropriate amount of metal oxides had no obvious performance on the internal microstructure of the HZSM-5 catalyst. This indicates that the active components are uniformly dispersed in the structure of the catalyst, which provides the active sites required for the aromatization reaction [21].

Figure 3 shows the acidity amount, and NH₃-TPD analyzed the strength of the samples. The number of acid sites is summarized in

Table 3. Acidity properties of the catalysts.

Catalyst	Strong Acid/μmol·g−1	Weak Acid/μmol·g−1	Strong/Weak
HZSM-5(25)	23	168	0.13
HZSM-5(50)	27	90	0.30
HZSM-5(80)	4	85	0.05
HZSM-5(120)	10	32	0.31
1.0%-MoO3/ZSM-5(50)	6	41	0.15
1.0%-CuO/ZSM-5(50)	89	114	0.78
1.0%-CoO/ZSM-5(50)	53	39	1.35
1.0%-NiO/ZSM-5(50)	119	97	1.23
1.0%-ZnO/ZSM-5(50)	161	200	0.81

. It is clear that all samples exhibited two obvious desorption peaks at about 200 and 400 °C, commonly defined as the desorption of NH₃ molecules from weak and strong acid sites, respectively [22]. Figure 3a shows the desorption peak of HZSM-5(50) at low temperatures shifted to a higher temperature with a decreasing Si/Al ratio. The reason is attributed to part of the active components in the HZSM-5(50) channels that had a certain effect on NH₃ desorption, and the NH₃ desorption peak at high temperatures was slightly shifted. From Table 3, the acid amount decreased with increasing the Si/Al ratio of ZSM-5, which probably affected the aromatization performance of the catalysts [23]. It is acknowledged that Al in the zeolitic framework presented strong acid sites. Increasing the Si/Al ratios resulted in increasing strong acid sites. Figure 3b and Table 3 show that the acid amount of MoO₃/HZSM-5(50) and CoO/HZSM-5(50) decreased compared to the unmodified HZSM-5 (50). This is attributed to Mo and Co species that were diffused into the internal pores of HZSM-5 (50) and covered part of the surface active sites [24]. Additionally, free Cu, Ni, and Zn formed stable-state metal salts in the molecular sieve, and the CuO, NiO, and ZnO species easily interacted with Si-OH in the skeleton, enhancing the strong and weak acid sites on the catalyst surface [25].

The Py-IR of HZSM-5 with different Si/Al ratios and modified catalysts in the range of 1400–1600 cm⁻¹ are shown in Figure 4.

Figure 4 shows the adsorption peaks at about 1540 cm⁻¹ and 1450 cm⁻¹, which are assigned to pyridinium ions generated from the Brønsted acid (B acid) sites and pyridine coordinatively bonded to Lewis acid (L acid) sites, respectively. It is generally believed that the surface acidity of zeolite was reflected by the [AlO₄]⁻ group via the brig hydroxyl content of skeleton aluminum. B acid sites were mainly provided by skeleton aluminum Si-O-Al, and L acid was reflected by the defect site of non-skeleton aluminum. In the HZSM-5 catalyst, the acid content of B acid was higher due to the high content of skeleton aluminum. Thus, the amount of both L acid and B acid gradually decreased with the increase in the Si/Al ratio [26]. For the modified catalysts, this was consistent with the adsorption peaks of the unmodified HZSM-5 catalyst, suggesting the incorporation of active components into the catalysts had not disrupted the site of the HZSM-5 adsorption peaks.

As shown in Table 3, both L and B acid amounts of HZSM-5 gradually decrease with the increasing Si/Al ratio. However, the amount of B acid consistently exceeded that of L acid, maintaining an L/B value of about 0.5. Changes in the Si/Al ratio resulted in different pore sizes in the molecular sieve, further affecting the diffusion and mass transfer rates of the reactants. Narrow pores can easily block large molecular hydrocarbon substances, exacerbating carbon buildup. Additionally, the surface acidity of the HZSM-5 catalyst was influenced by the aggregation of these large molecules, thereby impacting the aromatization activity effect [27,28,29].

From

Table 4. Acidic properties of different HZSM-5 and modified catalysts.

Catalyst	LAS/μmol·g−1	BAS/μmol·g−1	L/B
HZSM-5(25)	218	436	0.50
HZSM-5(50)	190	391	0.49
HZSM-5(80)	173	340	0.51
HZSM-5(120)	130	290	0.45
1.0%-MoO3/ZSM-5(50)	227	416	0.55
1.0%-CuO/ZSM-5(50)	217	450	0.48
1.0%-CoO/ZSM-5(50)	211	412	0.51
1.0%-NiO/ZSM-5(50)	230	461	0.50
1.0%-ZnO/ZSM-5(50)	331	439	0.75
0.5%-ZnO/ZSM-5(50)	260	412	0.63
1.5%-ZnO/ZSM-5(50)	366	418	0.88
2.0%-ZnO/ZSM-5(50)	312	401	0.78
2.5%-ZnO/ZSM-5(50)	234	342	0.68

, the amount of L acid in the modified catalysts increased significantly, and B acid increased slightly. This may be due to the metal oxides readily interacting with L acids to generate M-L acid sites, and M-L acid sites covered the B acid sites. The catalyst 1.0%-NiO/HZSM-5 increased the B acid amount by 70 μmol·g⁻¹, and 1.0%-ZnO/HZSM-5 increased the L acid amount by 140 μmol·g⁻¹ compared to HZSM-5(50). B acid centers primarily facilitate olefin cyclization and condensation, whereas the L acid accelerates the dehydrogenation and hydrogen transfer processes of cycloalkanes. Both B and L acids influence the distribution of aromatization products and can be loaded with suitable metal oxides for the reaction process based on practical application requirements [30,31]. Additionally, introducing ZnO species enhances the presence of the Zn-L acidic site on the surface of the HZSM-5(50) catalyst, expediting high-carbon olefin dehydrogenation. The L-acid content increased and then decreased with the increase in ZnO loading. The maximum value of the surface L acid content was 366 μmol·g⁻¹ at 1.5% loading, and the amount of L acid was significantly reduced at 2.0% and 2.5% ZnO loadings. This could be described as a reduction in force between the molecular sieve surface and reactants, resulting in a slower reaction rate that impacts the catalyst’s acidic center quantity.

3.2. Reaction Performance of HZSM-5 with Different Si/Al Ratios

The aromatization reaction effect of HZSM-5 molecular sieves with different Si/Al ratios was investigated. Methanol was reacted with n-pentane for 8.0 h at 400 °C, 0.25 MPa, and WHSV = 2.0 h⁻¹.

The experimental results show that the n-pentane conversion is close to 100%, the methanol conversion stays around 90%, and the oil phase yield reaches a maximum value of 22.63% at Si/Al = 50. As the conversion of methanol to dimethyl ether is affected by L acid, low Si/Al was more favorable for the aromatization reaction. The high B-acid content of the HZSM-5(25) catalyst led to the inhibition of the olefin conversion reaction, and the aggregation of macromolecular hydrocarbons blocked the pores and accelerated the catalyst deactivation, resulting in a decrease in the oil phase yield of the product. Catalysts with Si/Al ≥ 80 exhibit weak acidity, which is not conducive to the dehydrogenation of alkane molecules to obtain olefins; thus, aromatization is poorly achieved.

As depicted in

Table 5. Distribution of products catalyzed by HZSM−5 with different Si/Al ratios.

Distribution/%	HZSM-5(25)	HZSM-5(50)	HZSM-5(80)	HZSM-5(120)
H2	1.18	1.78	1.06	1.41
CO	0.51	0.47	0.48	0.54
CO2	0.46	0.54	0.31	0.47
CH4	1.14	1.74	1.64	1.64
C2 hydrocarbon	1.84	1.04	1.45	2.84
C3 hydrocarbon	5.14	5.43	6.01	2.62
C4 hydrocarbon	2.38	3.41	4.41	9.78
C5+ alkanes	10.52	9.39	18.68	13.22
C5+ olefin	2.26	1.51	3.63	8.81
C5+cycloalkanes	18.06	8.23	8.88	9.28
C5+ cyclic olefin	3.45	5.94	3.06	0.46
Aromatic hydrocarbon	52.74	60.13	50.31	48.26
N-pentane conversion	99.1	99.2	99.2	99.1
Methanol conversion	87.7	92.9	94.0	92.3
Oil phase yield	18.8	22.6	18.8	16.7
Aromatics Distribution/%
Toluene	19.68	25.42	17.26	16.33
Ethylbenzene	2.22	2.75	2.28	2.28
Xylene	27.61	23.68	21.51	26.21
p-Methylethylbenzene	1.92	1.82	2.01	2.24
o-Toluene	0.32	4.54	5.14	0.32
Tetramethylbenzene	0.34	1.93	0.85	0.87

, the product distribution is as follows: aromatics > alkanes > light olefins. When the Si/Al ratio was 50, the aromatic content of the product was 60.13%, and the selectivity of toluene and xylene was 25.42% and 23.68%, respectively. It was significantly higher than the HZSM-5 catalysts with other Si/Al ratios. The high content of B acid in HZSM-5(25) hinders olefin reforming, resulting in a substantial accumulation of hydrocarbons that obstruct the catalyst pores and lead to catalyst deactivation [32]. For HZSM-5(80) and HZSM-5(120), weak catalyst acidity (shown in Table 2) and reduced HZSM-5 pore size were not conducive to alkane dehydrogenation for obtaining olefins, while the mass transfer and diffusion of C₅⁺ hydrocarbons within the pore channels were limited, leading to pore blockage and catalysts deactivation [33]. The co-conversion reaction of methanol and pentane into aromatics is a strongly acid-catalyzed process. Thus, the HZSM-5(50) catalyst is suitable for the aromatization reaction based on the results from the catalyst evaluation.

3.3. Reaction Performance of 1.0%-M₂O_n/HZSM-5(50) Catalysts

Metal-functionalized catalysts were examined for their aromatization effect under the same reaction conditions.

Methanol conversion of 100% was reached by all the 1.0%-M₂O_n//HZSM-5 (50) catalysts, the n-pentane conversions were NiO (92.2%) > CuO (89.3%) > ZnO (83.1%) > CoO (79.5%) > MoO₃ (72.1%), and the oil phase yields were ZnO (32.1%) > CoO (22.8%) > MoO₃ (17.2%) > CuO (16.0%) > NiO (11.8%). The introduction of Cu and Ni led to the enhancement of the strong acidic sites of the catalyst, and the dehydroalkylation of n-pentane was accelerated. The conversion of alkanes and olefins to aromatics requires the catalytic action of L acid, and the amount of L acid on the surface of MoO-, CoO-, and ZnO-modified catalysts was significantly increased, resulting in higher oil phase yields than those of CuO and NiO.

As depicted in

Table 6. Distribution of products catalyzed by 1.0%-M₂O_n/HZSM-5(50).

Distribution/%	MoO3	CuO	CoO	NiO	ZnO
H2	0.62	0.21	0.51	1.21	0.42
CO	0.54	0.32	0.32	1.05	0.42
CO2	0.41	0.24	0.43	1.31	0.3
CH4	2.45	1.56	2.31	3.41	0.15
C2 hydrocarbon	4.57	3.74	3.52	5.61	1.02
C3 hydrocarbon	5.24	3.55	1.4	4.23	0.46
C4 hydrocarbon	1.34	2.72	2.41	2.29	0.52
C5+ alkane	17.14	14.79	22.51	5.22	12.26
C5+ olefin	11.04	4.24	5.98	0.91	4.11
C5+ cycloalkane	6.84	8.24	6.68	8.74	9.47
C5+ cyclic olefin	1.84	4.85	2.88	0.14	3.4
Aromatic hydrocarbon	47.96	55.48	51.04	65.78	67.46
N-pentane conversion	72.1	89.3	79.5	92.2	83.1
Methanol conversion	99.9	99.8	99.9	99.7	99.9
Oil phase yield	17.2	16.0	22.8	11.8	32.1
Aromatics Distribution/%
Toluene	4.16	20.76	13.13	26.14	22.73
Ethylbenzene	1.16	2.28	2.22	3.19	3.15
Xylene	13.92	24.47	17.79	31.73	32.33
p-MEB	2.67	2.55	3.14	4.52	3.53
o-Toluene	3.36	0.25	2.12	0.15	0.14
Tetramethylbenzene	2.3	2.24	0.98	2.28	1.36

, the distribution of products was as follows: aromatic > C₅⁺ alkanes > cycloalkanes > C₅⁺ olefins > cycloolefins. Different metals exert distinct effects on the acid distribution of the catalyst surface, thereby determining the product distribution. Referring to the oil phase yields in Table 6, it is evident that the ZnO/HZSM-5 (50) catalyst exhibits superior catalytic efficacy, characterized by a significantly higher amount of L acid and B acid than HZSM-5(50). The presence of L acid on the catalyst surface expedites the conversion of low-carbon olefins into high-carbon olefins, while Zn-L acidic sites accelerate the dehydrogenation of cycloalkanes into aromatics, resulting in an increased aromatic yield [34]. The alkane content of C₅⁺ indicates a poor synergistic effect between B and L acids for the MoO₃/HZSM-5(50) and CoO/HZSM-5(50) catalysts. The content of cycloalkanes and cycloolefin under each component load is slightly different. The aromatics content was 47.96% for MoO₃, 55.48% for CuO, 51.04% for CoO, 65.78% for NiO, and 67.46% for ZnO. The selectivity of aromatics was xylene > toluene > p-methylethylbenzene > ethylbenzene and tetramethylbenzene. Both catalyst acidity and acid distribution greatly influenced the co-conversion of methanol and n-pentane into aromatics [35]. The introduction of Ni and Zn notably enhanced B and L acids within the catalysts, accelerating methanol and n-pentane conversion to low-carbon olefins while further converting aggregated cycloalkanes into aromatics and thus facilitating easier conversion from trimethylbenzene benzene to xylene via hydrocarbon-methyl group removal [36,37,38].

3.4. Reaction Performance of X%-ZnO/HZSM-5(50) Catalyst

In this section, methanol was reacted with n-pentane under the same conditions.

Methanol conversion reached 100% by the ZnO/HZSM-5 (50) catalyst. The n-pentane conversion was almost constant after increasing from 76% to 83%, with the maximum oil phase yield (32%) at 1% ZnO loading.

From

Table 7. Distribution of products catalyzed by X%-ZnO/HZSM-5(50).

Distribution/%	0.5%-ZnO	1.0%-ZnO	1.5%-ZnO	2.0%-ZnO	2.5%-ZnO
H2	0.12	0.42	0.21	0.32	0.31
CO	0.04	0.42	0.35	0.22	0.26
CO2	0.02	0.3	0.32	0.34	0.37
CH4	0.18	0.15	0.63	0.75	0.91
C2 hydrocarbon	0.21	1.02	0.26	0.41	0.42
C3 hydrocarbon	1.06	0.46	0.33	0.73	0.38
C4 hydrocarbon	0.42	0.52	0.42	0.67	0.27
C5+ alkane	12.01	12.26	17.97	20.09	21.50
C5+ olefin	1.79	4.11	3.82	4.27	11.03
C5+ cycloalkane	12.31	9.47	11.10	17.37	19.75
C5+ cyclic olefin	5.57	3.40	2.63	3.09	1.97
Aromatic hydrocarbon	66.51	67.46	61.96	51.84	42.83
N-pentane conversion	76.1	83.1	82.5	82.8	81.9
Methanol conversion	99.9	99.9	99.9	99.9	99.8
Oil phase yield	15.8	32.1	24.8	21.1	21.4
Aromatics Distribution/%
Toluene	30.23	22.73	21.98	25.67	17.02
Ethylbenzene	2.66	3.15	2.78	4.00	2.32
Xylene	28.69	32.33	30.26	28.23	24.13
p-MEB	3.44	3.53	4.54	7.01	4.16
o-Toluene	0.28	0.14	0.16	0.27	0
Tetramethylbenzene	1.12	1.36	1.53	2.50	2.45

, the aromatic content enhancement of the products catalyzed by ZnO/HZSM-5 compared to HZSM-5 is certain. The content of components in the oil phase was aromatics > alkanes > cycloalkanes > olefins > cycloolefins. Aromatic content increases and then decreases with increasing ZnO loading. This was because the acidic center promotes the cyclization of olefins to produce cycloalkanes and the dehydrogenation of cycloalkanes to aromatics. However, an excess of ZnO may cause the acidic sites on the catalyst surface to be covered and the dehydrogenation and hydrogen transfer process of cyclic hydrocarbons to be inhibited [39]. In addition, Py-IR data reveal that both 1.0% and 1.5%-ZnO/HZSM-5(50) catalysts exhibited higher acid amounts and a stronger synergistic performance between L and B acids in line with the reaction evaluations. Additionally, the gaseous hydrocarbon (C₁-C₄) decreases significantly compared to HZSM-5(50). This is because the cracking reaction of large hydrocarbons is inhibited by the increase in the amount of acid in the catalyst loaded with ZnO [40]. The selectivity of the aromatic hydrocarbons was in the following order: xylene > toluene > methylene-ethylbenzene > ethylbenzene > tetramethylbenzene. The content of xylene in the product catalyzed by 1%-ZnO/HZSM-5 was the largest (32.33%). Excessive loading of active species will cause an imbalance in the ratio of L acid to B acid. On the other hand, the distribution of active components in the pores was unfavorable to the diffusion of hydrocarbons, which makes it difficult for the reactants to enter into the internal process of the catalyst. The aggregation of the reaction products will be extremely prone to carbon buildup, which will seriously affect the service life of the catalyst [41].

3.5. Investigation of Catalyst Stability

The aromatization yield and xylene selectivity of the 1.0%-ZnO/ZSM-5 catalyst were the highest. Therefore, the optimal conditions of the process were selected to investigate the catalytic stability, and the products were collected every 6 h for determination. After 42 h of reaction, the sampling was stopped, and the data were recorded.

As can be seen from Figure 5, the conversion rate of methanol and n-pentane decreased significantly with the extension of reaction time. When the reaction was run for 42 h, the conversion rate of n-pentane and the oil phase yield decreased to 74.97% and 22.12%, respectively, and the oil phase yield in the product reached the maximum of 31.24% when the reaction time was 12 h. With the aggravation of carbon deposition on the catalyst, more and more product macromolecules are gathered in the catalyst pore, which limits the mass transfer between molecules and destroys the catalyst activity, and the number of acidic sites on the surface is gradually covered by carbon deposition molecules, and the oil phase content in the product is greatly reduced. In a certain reaction time (<12 h), the catalyst performance is better, and the reactant conversion and oil phase yield are higher, indicating that the 1.0%-ZnO/ZSM-5 catalyst has good aromatization reaction stability.

4. Reaction Mechanism of Aromatization

The methanol molecule undergoes dealkylation with dimethyl ether via protonation by B and L acids on the catalyst surface, resulting in the production of unsaturated low-carbon olefins and methyl radicals. Dehydrogenation of n-pentane to reactive olefins was facilitated by H⁺ generated from methyl and aluminum-bridged hydroxyl groups in the catalyst backbone. In experiments investigating the arylation performance of Si/Al carriers, it was observed that a strong surface acidity of the catalyst led to accelerated conversion rates of methanol and n-pentane in the initial stages of the reaction, yielding the ideal oil phase product. The introduction of an active component could evidently strengthen the dehydrogenation of macromolecule hydrocarbons, obviously increasing the content of the low-carbon olefin in the products while reducing the early-stage hydrocarbon product formation. The induction performance of methyl radicals primarily contributed to the generation of reactive olefinic substances [42]. The aromatization reaction process was as follows ((1)–(4)):

As hydrocarbon substances permeate through the catalyst’s pores, they interact with methanol molecules, giving rise to a considerable amount of methyl radicals. Once the dealkylation reaction within the catalyst attains a stable equilibrium state, alkenes embark on their conversion into the product of aromatics. Following the introduction of the active component Zn, there was a remarkable increase in both cycloalkanes and low-carbon alkenes in the product compared to the situation without loading active metal. Consequently, this led to an elevated aromatic content and enhanced selectivity for aromatics. The surface acidity of the catalyst exerts a significant impact on the dehydrogenation process of cycloalkanes to form alkenes. A considerable portion of low-carbon alkenes produced during the initial coupled reactions was directly converted into high-carbon alkenes by the polymerization processes, while a smaller fraction underwent alkylation to generate branched or straight-chain low-carbon alkanes. High-carbon alkenes were subsequently converted into cycloalkanes via dehydrogenation and ring formation mechanisms, which were then further transformed into polycyclic aromatic hydrocarbons via proton transfer and hydrogenation routes. Notably, among these polycyclic aromatic hydrocarbons, toluene constituted a relatively large proportion of the final product.

Multi-methylbenzene undergoes transformation into low-carbon olefins by means of dealkylation and elimination reactions. In the context of a highly acidic catalyst, the proportion of low-carbon alkanes within the product escalates as the low-carbon olefins facilely undergo conversion to low-carbon alkanes under the B acid catalysis process. Throughout this reactional process, interconversion between low-carbon and high-carbon olefins transpires via polymerization and cracking reaction mechanisms. The concentration of the aromatic compounds within the resultant product was increased via the reaction (3).

The methanol and n-pentane coupling aromatization reaction mechanism over 1.0%-ZnO/ZSM-5(50) is shown in Figure 6. In summary, this process can be divided into two steps. Firstly, methanol and n-pentane underwent the process of dehydrogenation and alkylation to form the active alkene species. Secondly, these highly reactive species were readily transformed into aromatic products facilitated by hydrogen transfer from protonated acid sites on the surface of the aromatization catalyst. A comparison of the reaction product types pre- and post-loading active components reveals that the novel L acid formed by the metal active center bonded to the catalyst surface accelerated the rate of active alkene generation, leading to a significant increase in aromatic product content. As the reaction progressed, poly-methylbenzene was cleaved into alkane and alkene-like substances under protonic acid action. These large molecules diffused to reduce surface acidity and block pores on the catalyst surface, accelerating the carbon deposit formation and reducing the catalyst activity. Based on this mechanism, it can be inferred that high surface acidity can promote the production of the alkene product while treating the internal structure of the catalyst. This involved expanding pores to facilitate the diffusion of large aromatic molecules, thereby slowing down carbon deposit formation.

5. Conclusions

Aromatics play a pivotal role as crucial organic chemical feedstock across diverse industries. In this study, the optimal process parameters for the methanol coupling reaction with low-carbon hydrocarbons to produce aromatics with the reaction condition of 400 °C, 1.0 MPa, and WHSV of 2.0 h⁻¹ were investigated. Under these test conditions, methanol conversion reached 100%, n-pentane conversion was 83.06%, the oil phase yield was 32.12%, and the aromatics content was 74.21%, with xylene selectivity reaching about 37.63%. The stability of the catalysts was evaluated by monitoring changes in reactant conversion, target product yield, and component content during the reaction process. A catalyst comprising 1.0%-ZnO/ZSM-5(50) was selected for methanol and low-carbon hydrocarbon coupling aromatization under identical conditions due to its cost-effectiveness, ease of acquisition, and favorable economic benefits suitable for industrial application. The analysis of the products from the methanol and n-pentane coupling aromatization reaction process indicates that the reaction mechanism is more closely linked with the ‘hydrocarbon pool mechanism’.