1 Introduction

With the abundant reserves and desirable carbon-hydrogen ratio, methane (CH4), the major component of shale gas, is a promising starting material to synthesize important industrial chemical intermediates such as light olefins(C2–C4) and aromatics (benzene, toluene and xylene). Currently the only industrial approach to convert methane is through the syngas route, which demand high operating temperature and capital cost. On the other hand, the direct methane conversion processes are potentially more cost-effective and environmentally friendly than the syngas route. These methods include pyrolysis [1, 2], oxidative coupling of methane [3], superacid catalysis [4, 5], halogenation [6, 7], etc. Among all these processes, the conversion of methane to chloromethane (CH3Cl) has been drawing increasing attention since the temperature requirement is much less aggressive comparing to the traditional syngas approaches. In this route, methane is converted to chloromethane by reacting with chlorine using oxychlorination [7,8,9] or light/thermal initiation [10,11,12]. The reaction temperature for the oxychlorination process is between 350 and 450 °C; and for light initiation, methane chlorination can be conducted even at room temperature.

The main product of methane halogenation, i.e. chloromethane (CH3Cl), has limited direct applications [13,14,, 14]. Nevertheless, CH3Cl can serve as a versatile chemical intermediates to form ethylene, propylene, and aromatics (Eq. 1). Moreover, chlorine can be regenerated from the HCl byproduct via the Deacon process (Eq. 2), and can eventually be recycled for methane chlorination.

$${\text{nCH}}_{3} {\text{Cl}} \to {\text{C}}_{n} {\text{H}}_{2n} + {\text{ nHCl}}$$
(1)
$$2{\text{HCl }} + \, \frac{1}{2}{\text{O}}_{2} \to {\text{ H}}_{2} {\text{O }} + {\text{ Cl}}_{2}$$
(2)

The catalysts used in chloromethane conversion are typically bifunctional acid–base catalysts and several shape-selective microporous zeolites [15,16,17,18,19]. Su et al. [18, 20,21,22,23,24] studied the chloromethane conversion over SAPO-34, which has been proved to be the most excellent MTO catalyst [25]. A conversion of 70% was reached at 500 °C, with the C2 products dominating the product distribution (approximately 40% C2=, excluding coke). Other zeolites, such as MCM-22, ZSM-11 or SAPO-5 were also tested for chloromethane conversion. However, the performances of these zeolites are not satisfying due to low chloromethane conversion, low olefin selectivity, high coking rate, or short catalyst lifetime.

Here we report a series of metal-promoted H-ZSM-5 zeolites exhibiting superior performance on the conversion of chloromethane to olefins or aromatics. The catalysts exhibited above 90% CH3Cl conversion over 3 h of time on stream, and product selectivity towards 90% olefin or 40% aromatics, depending on the choice of the promoters and reaction conditions. Various types of zeolite in combination with different metal loadings were investigated and optimized for the chloromethane conversion. The reaction conditions, including temperature, space velocity, catalyst regeneration procedure, etc. were also optimized to improve the catalyst selectivity and extend the lifetime of the catalyst.

2 Experimental

2.1 Catalyst Preparation

NH4-ZSM-5(Alfa Aesar, Si:Al = 30, specific surface area = 400 m2/g) and SAPO-34 (ACS Materials LLC, specific surface area ≥ 550 m2/g) were used in the study. H-ZSM-5 was obtained by calcining NH4-ZSM-5 at 550 °C in static air for 3 h. For metal-impregnated catalysts, Ni(NO3)2·6H2O, Mg(NO3)2·6H2O, and Mn(NO3)2·xH2O were used as metal precursors. To obtain different wt% of metal in H-ZSM-5 zeolite, a desired amount of Ni, Mg or Mn nitrates were dissolved in water, and H-ZSM-5 was slowly added into the solution under vigorous stirring. The slurry was stirred for 12 h, followed by heating to 80 °C to evaporate the water. The sample was then dried at 120 °C and calcined at 550 °C for 3 h. All the catalysts were crushed and sieved to obtain 50 mesh particles prior to catalyst testing.

2.2 Catalyst Testing

A continuous-flow fixed-bed reactor with an 8 mm i.d. quartz tube was used for CH3Cl to olefins and aromatics reaction. The reactor was operated at different temperatures ranging from 325 to 475 °C, and the space velocity for CH3Cl varied from 1.3 to 3.3 h−1. In each test, a mixed flow of CH3Cl:N2 = 1:3 was flowed into the reactor, and the weight of catalyst was adjusted to yield different space velocity. The effluent gas was analyzed by an Agilent 7890 gas chromatograph (GC) that is equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The tubing after the reactor was heated to 180 °C to eliminate condensation of the products. After each run, the quartz tube with the spent catalyst inside was detached from the reactor and the total weight of the quartz tube + catalyst was measured, which is named as w1. Then the catalyst was calcined at 800 °C, and the total weight was measured again, named as w2. The weight of coke accumulated during the reaction is the difference of w1 and w2. The mass balance in all experiments were 95–100% based on carbon.

2.3 Ammonia Temperature-programmed Desorption (NH3-TPD)

NH3-TPD was carried out on an Altamira-300 reactor system (Altamira Instruments Inc.) in conjunction with an Ametek LC-D Mass Spectrometer. In a typical test, 50 mg of catalyst was loaded into a U-shape quartz tube reactor. The catalyst was firstly heated at 120 °C under He flow for 30 min, then cooled down to 50 °C. Ammonia adsorption was performed at 50 °C by flowing 40 sccm of NH3 through the reactor for 1 h. After the adsorption, 25 sccm of He was flowed for 40 min to remove residual ammonia. The reactor was then heated to 600 °C with a ramp rate of 10 °C/min to desorb the chemisorbed ammonia. Mass spectrometer was turned on during the ramping step to detect the desorbed ammonia in the effluent gas.

2.4 BET Surface Area

Specific surface areas of the catalysts were examined by a 3H-2000PS1 (BeiShiDe Instrument Technology) according to the Brunauer–Emmett–Teller (BET) method. The pressure range for N2 adsorption/desorption is between 0.04 and 0.32. The micropore volume were analyzed using the T-plot method.

2.5 X-ray Diffraction

XRD analysis was performed on a PANalytical Empyrean XRD Diffractometer with Cu Kα radiation in the 5°–50° 2θ range. Step width and scanning speed were set at 0.02° and 2°/min, respectively.

3 Results and discussion

3.1 Chloromethane Conversion on Unmodified Zeolites

The chloromethane conversion reaction and the MTO/MTA reaction share immense similarity since the CH3Cl and CH3OH are structurally alike. Thus, similar reaction conditions and catalysts for MTO/MTA were often adapted to catalyze the chloromethane conversion. In this work, we also started with plain, untreated zeolites, including H-ZSM-5 and SAPO-34, to lay down the groundwork and find out the ideal reaction conditions for chloromethane conversion. The performance of the unprompted H-ZSM-5 and SAPO-34 at different reaction temperatures are summarized in Table 1. Coke selectivity is not included in this section since the temperature was continuously changing during the analysis. For H-ZSM-5 zeolite, the temperature ramped from 375 to 525 °C with a 50 °C step size, and the reaction data was recorded after 30 min of reaction at that temperature. On H-ZSM-5, propylene and butylene dominated the product distribution at 375 °C, while ethylene and aromatics (toluene, C8 and C9 components) selectivity increased significantly when the temperature is above 425 °C. A further increase of the reaction temperature yields even higher ethylene and aromatics selectivity, this could be understood from both the thermodynamics and surface reaction perspectives. The Calculation on the reaction enthalpy change of the reactions (Table 2) show that chloromethane to ethylene and styrene reactions are highly endothermic while the reactions for other components are exothermic or less endothermic. Thus, these components are more favorable in the high temperature region. In a surface reaction perspective, Ibáñez et al. [26] claimed that the chloromethane to olefin reaction undergoes the same mechanism with MTO reaction, which is a hydrocarbon pool system with two cycles running simultaneously [27]: Cycle I involves aromatics and ethene formation from the lower methylbenzenes followed by re-methylation, and Cycle II includes C3 + alkenes formation by methylation and cracking. In addition, On H-ZSM-5, Cycle I and II are connected since the higher alkenes formed in the methylation/cracking cycle can be further converted to methylbenzenes via cyclization and hydrogen transfer reactions. The cyclization of alkene is especially favored at high temperatures [27, 28]. Therefore, as the reaction increases, additional higher alkenes are converted into methylbenzenes, which are circulated in Cycle I and generate ethene and aromatics as the final products.

Table 1 Chloromethane conversion over H-ZSM-5 and SAPO-34 at different temperatures
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Table 2 Thermodynamic constants and heat of reaction for relevant compounds in chloromethane conversion
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SAPO-34 was also tested following the similar procedure with the temperature ranging from 325 to 475 °C. The catalyst also showed higher ethylene selectivity as the temperature increases. At the end of the experiment, a significant amount of coke were founded on the SAPO-34 catalyst.

Another test was carried out at constant reaction temperature to evaluate the coke selectivity and catalyst deactivation rate. A direct comparison of H-ZSM-5 and SAPO-34 at 425 °C is shown in Table 3. After 60 min of TOS, the main products on H-ZSM-5 were C2–C4 olefins, along with an 18.06% total aromatics selectivity and a 2.09% coke selectivity. SAPO-34 showed ethylene and propylene as the dominating products, and a substantial amount of coke (12.19%) was detected. In terms of the conversion, the two zeolites had comparable conversions in the first hour of TOS, but SAPO-34 deactivated at a much faster rate than H-ZSM-5, according to Figure S1. Since SAPO-34 showed comparable selectivity results to H-ZSM-5 but more rapid deactivation, chloromethane conversion on SAPO-34 requires either frequent regeneration as is in the MTO process, or further modification of the zeolite with promoters to mitigate coke formation.

Table 3 Chloromethane conversion over unmodified zeolites
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The space velocity is also examined in this study as various WHSVs were used for chloromethane conversion before. For example, Su et al. [20, 21, 30] carried out chloromethane conversion at WHSV = 3.17 gCH3Cl/g/h for both H-ZSM-5 and SAPO-34 zeolites, which was inherited from conventional MTO reaction conditions. Lersch and Bandermann [31] performed the reaction at WHSV = 1 gCH3Cl/g/h for their enlarged lab scale aging test on H-ZSM-5 and obtained over 90% of CH3Cl conversion for their first 20 h of reaction. Figure 1 shows the CH3Cl conversion and product selectivity for CH3Cl conversion on H-ZSM-5 zeolite at 425 °C, with different WHSVs ranging from 1.3 to 3.3 gCH3Cl/g/h. The ideal WHSV that maximizes both chloromethane conversion and aromatics selectivity was found at 1.87 gCH3Cl/g/h. At higher WHSVs, the diffusion of reactants becomes the main limitation, causing the decrease of CH3Cl conversion. On the other hand, a lower WHSV yields more coke products (10.5% coke selectivity when WHSV = 1.3 gCH3Cl/g/h), which will reduce the lifetime of the catalysts.

Fig. 1
figure 1

CH3Cl conversion and product distribution on H-ZSM-5 at different WHSVs. The black line and ⬛ mark denote the CH3Cl conversion rate after 60 min TOS

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The product distribution of chloromethane conversion over H-ZSM-5 is similar to the results that used methanol as the feed [32,33,34]. On the other hand, SAPO-34 zeolites deactivate at a much faster rate than that in MTO synthesis under similar reaction conditions, as MTO using SAPO-34 usually retains > 90% conversion for more than 2 h of operation [35,36,37]. In order to address the deactivation issue, metal oxide promoters were impregnated into the zeolites to evaluate if a higher catalyst reactivity and a lower coke rate could be achieved.

3.2 Impregnating Metal Oxide Promoters into SAPO-34

Previous studies on SAPO-34 for MTO reaction reported that modifying the zeolite with metal oxides could increase the conversion, inhibit the coke formation, and extend catalyst life time [38,39,40]. In this study, a variety of metal oxide promoters that were used on SAPO-34 for MTO reaction, including Mg, Mn and Ca, were impregnated into SAPO-34 for chloromethane conversion testing. The weight % of the promoters were set at 1%, and the catalysts were denoted as 1% Me-SAPO-34 (Me=Mg, Mn or Ca). For Mg, 0.5 wt% and 5 wt% of Mg were also impregnated into SAPO-34 zeolites and the samples were denoted as 0.5% Mg-SAPO-34 and 5% Mg-SAPO-34, respectively. The CH3Cl conversion and product distribution is summarized in Fig. 2, and the change of conversion rate is shown in Figure S3. Metal impregnation seems to have limited effect, if not detrimental, on improving the catalyst activity: Only Mn promoted SAPO-34 showed higher (> 90%) initial CH3Cl conversion, nevertheless the activity for 1% Mn-SAPO-34 decreases rapidly over the 160 min of time on stream. The deactivation rate was almost identical for blank and metal oxide promoted SAPO-34 zeolites. For the SAPO-34 zeolite impregnated with different Mg loading in SAPO-34, the activity changed from 64% on 0.5% Mg-SAPO-34 to 60% on 1% Mg-SAPO-34, then to 54% on 5% Mg-SAPO-34. According to the BET surface area and pore volume analysis in Table 4, the promoted catalysts have less surface area and smaller micropore volume comparing to the blank SAPO-34 zeolite. As is proven by X-ray Diffraction in Figure S4, impregnating metal nitrate precursors into zeolite materials followed by calcination creates amorphous metal oxide on the zeolite surface. The metal oxide accumulated on the surface and block the pores on SAPO-34, which leads to lower chloromethane conversion while the deactivation rate and product distribution remains the same.

Fig. 2
figure 2

Catalytic performance for SAPO-34 impregnated with different metal oxides, including the product distribution and CH3Cl activity after 60 min of reaction. Catalyst impregnated with X wt% of metal Y is denoted as X%YSAPO-34. For instance, 1% MgSAPO-34 is the SAPO-34 sample impregnated with 1 wt% of Mg. Reaction conditions: 400 °C, WHSV = 1.87 h−1

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Table 4 BET surface area and micropore volume parameters of promoted SAPO-34 zeolite
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3.3 Impregnating Metal Oxide Promoters into H-ZSM-5

In methanol conversion, H-ZSM-5 were also modified by metal oxide promoters to increase methanol conversion and extend the life time [41, 42]. These metal oxide promoters include several alkaline earth and transition metals, such as zinc, iron, manganese and magnesium, with metal content up to 5% in the catalyst. Furthermore, according to Lersch et al. [31], in chloromethane conversion, the selectivity of aromatics is strongly correlated with the Sanderson electronegativity. Hence the aromatics selectivity should follow the sequence of Mg < Mn < Ni < Cu < Fe < Zn. Unfortunately, we observed that when chloromethane was used as the feed, hydrogen chloride could react with zinc, copper or iron oxides and form volatile metal chlorides which would be leached out of the zeolite. The volatile metal chlorides would crystallize and accumulate in the cooler zones in the reactor system. Therefore, given the proper reaction temperature, the metal element for impregnation needs to be non-volatile in their metal chloride form at the reaction temperature. Based on the characteristics above, Mn, Mg and Ni were selected as metal oxide promoters and impregnated into H-ZSM-5.

Figure 3 summarized the product distribution and the CH3Cl conversion of the metal-impregnated H-ZSM-5 catalysts. The weight percentage of metallic Mn, Mg and Ni impregnated in the zeolite were controlled at two levels: 1 wt% and 5 wt%. With the optimized temperature and space velocity, all the catalysts showed 85–97% CH3Cl conversion after 60 min on stream. At 1% metal loading, 1% Mn–H-ZSM-5 and 1% Ni-H-ZSM-5 gave higher C5 alkene and C6+ aromatics selectivity than H-ZSM-5 samples, while on 1% Mg-H-ZSM-5 the aromatics selectivity was suppressed. The electronegativities for divalent Mg, Mn and Ni are 1.32 eV, 1.66 eV and 1.94 eV, respectively [43]. Thus, the Sanderson electronegativity of the metal oxide promoters is highly correlated with the aromatics selectivity. The electronegativity of the metal ions on a solid surface is a rough measure of its Lewis acidity [44, 45], and it also indicates the tendency of an atom to attract electrons [31, 46, 47]. Adding metal cations with higher electronegativity into the zeolites, such as Mn and Ni, will increase the acidity of the zeolite and enhance its ability to attract electrons. As a consequence, the alkene dehydrogenation and cyclization reactions are improved on 1% Mn-H-ZSM-5 and 1% Ni-H-ZSM-5 and more aromatics were obtained.

Fig. 3
figure 3

Change in hydrocarbon distribution for chloromethane conversion over H-ZSM-5 promoted by different metals. Reaction conditions: 425 °C, WHSV = 1.87 h−1, time on stream = 180 min

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At 5% metal loading, the aromatics selectivity for 5% Mn-H-ZSM-5 and 5% Mg-H-ZSM-5 were only at 5.5% and 6.5%, respectively. In comparison, on 5% Ni-H-ZSM-5, the total aromatics selectivity was 40%. To sum up, the 5% Mn-H-ZSM-5 and 5% Mg-H-ZSM-5 catalysts have high yield to olefins, while 5% Ni-H-ZSM-5 is more prone to produce aromatics. The trend of aromatics selectivity on the 5% metal-oxide-promoted catalysts does not match the promoters’ Sanderson electronegativity of the metallic promoters. The different behavior of aromatics selectivity on 5% Ni vs 5% Mn and 5% Mg promoted zeolites are explained by NH3-TPD and XRD, which are discussed in the following sections.

3.4 Acidic Properties for Metal Oxide Promoted H-ZSM-5

As shown in previous reports on MTA conversion, the site acidity of H-ZSM-5 can be roughly correlated with the aromatics formation rate [32, 48]. In this study, H-ZSM-5 modified by 5% metal oxide promoters were characterized by NH3-TPD measurement. Figure 4 shows the NH3-TPD profiles for the H-ZSM-5 and the 5% metal modified H-ZSM-5 zeolites. The temperatures at the peak maximum for weak, medium and strong adsorption, as well as the NH3 adsorption amount, are summarized in Table 5. Promoting the zeolites with 5% Mg and 5% Mn had reduced the overall acidity of the zeolite, and eliminated the medium/strong acid sites that desorbs at ~ 400 °C. On 5% Ni H-ZSM-5, the zeolite also showed fewer medium/strong acid sites in the 400 °C ~ 550 °C region, whereas a broad shoulder at 330 °C and a small peak that desorb at 515 °C were detected. The overall acidity on nickel oxide promoted zeolite is slightly higher than un-promoted H-ZSM-5 zeolites. It has been reported that the weak acid sites(150–300°C) are correlated with the Si–OH in the H-ZSM-5 zeolite, whereas the strong acid sites(400–550°C) corresponds to the protonic (Si–OH–Al) and Lewis acid sites [49, 50]. Impregnating 5% Ni into the H-ZSM-5 zeolite caused a significant amount of strong acid sites to transform into the medium strength acid sites. These new acid sites could be ascribed to the interactions between nickel oxide and the protonic or Lewis acid sites in the H-ZSM-5 zeolite. Comparing with blank H-ZSM-5, the 5% Ni-H-ZSM-5 sample contains equal amount of weak acid sites and more medium strength acid sites. The high selectivity to aromatics on 5% Ni-H-ZSM-5 is consistent with the consensus that higher acidity of H-ZSM-5 favors the formation of aromatics [32, 48, 51, 52].

Fig. 4
figure 4

NH3-TPD pattern of H-ZSM-5 and the metal-promoted H-ZSM-5

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Table 5 NH3-TPD results for various metal-promoted H-ZSM-5
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As is shown in the NH3-TPD tests, the H-ZSM-5 zeolite loses acidity when impregnated with 5% Mg and 5% Mn, but gained more acidity when impregnated with 5% Ni. It turns out that the excess coverage of the metallic promoters and the blockage of microchannels could have played a key role in affecting the acidity of the zeolites. A series of XRD were carried out on the blank and 5% metal oxide promoted H-ZSM-5 catalysts. The catalysts were tested once after fresh calcination, and once again after the reaction. In Fig. 5, the catalysts promoted with 5% Mg and 5% Mg showed no extra feature on the XRD pattern than the blank H-ZSM-5 zeolite, indicating that these two promoters are amorphously distributed on the zeolite surface. On the other hand, the freshly calcined 5% Ni-H-ZSM-5 sample showed extra peaks at 37.2° and 43.2°, corresponding to the diffraction peaks for NiO crystallite (PDF#47-1049) [53]. Based on the calculation from the Scherrer equation, the grain size of the NiO crystallite on the 5% Ni-H-ZSM-5 catalyst is around 35 nm. When the H-ZSM-5 zeolite was impregnated with 5% Mg and 5% Mn, a significant amount of amorphous metal oxides accumulated on the catalyst surface, blocking the microchannels and the acid sites, and reducing the surface acidity. On the contrary, for the H-ZSM-5 impregnated with 5% Ni, bulk crystallites were formed on the surface, which means only a fraction of nickel oxide could diffuse into the pores and interact with the acidic sites. By impregnating the zeolite with 5% Ni, the weak acid sites remain unchanged while some of the strong acid sites were affected by nickel oxide promoters and transformed to medium strength acid sites. Therefore, the zeolite acidity could be retained and the catalyst is more selective to aromatics. After 5% Ni-H-ZSM-5 was used for CH3Cl conversion, the diffraction peaks corresponding to NiO crystallites had reduced in size, which implies that the nickel oxide crystallites may have been leached by the HCl. Reduction in NiO crystallite size also affected the reaction performance. 5% Ni-H-ZSM-5 is not as robust as other metal oxide promoted catalysts in terms of product selectivity. The aromatics selectivity reduced from 46.3 to 27.6% during the 4 h of time on stream (Fig. 6).

Fig. 5
figure 5

X-ray Diffraction for calcined and reacted H-ZSM-5 and 5% Me-H-ZSM-5 zeolite

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Fig. 6
figure 6

CH3Cl Conversion and Aromatics Selectivity of 5% Ni-H-ZSM-5 vs Time On Stream. Reaction Conditions: 425 °C, WHSV = 1.87 h−1

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The formation of aromatics and coke is also affected by the Si/Al ratio of the ZSM-5 used in the experiment [27, 54], since the catalytic performance is mostly determined by the number of Brönsted sites in the zeolite. In addition, the anchoring site for metal oxides on H-ZSM-5 may be different when the Si/Al ratio varies [55]. The catalysts tested above were synthesized from the same ZSM-5 zeolite with a Si/Al ratio of 30, which has low silica content and consequently more Brönsted sites. Additional reaction testing were carried out on H-ZSM-5 with a Si/Al ratio of 80 to further study the effect of silica content. As is summarized in Table 6, on blank H-ZSM-5, the zeolite showed lower aromatics selectivity and higher propylene selectivity when the Si to Al ratio was increased from 30:1 to 80:1. Similar behavior was also discovered on samples impregnated with 5% Ni and 5% Mg. In particular, on 5% Mg-H-ZSM-5 with Si/Al = 80, the catalyst displayed < 3% selectivity to aromatics and 65% to propylene. The results have confirmed that the selectivity is concurrently affected by the choice of metal promoters and the Si:Al ratio. A maximized propylene selectivity could be achieved on an Mg promoted H-ZSM-5 with high silica content, and a high aromatics selectivity can be achieved on Ni promoted H-ZSM-5 with low Si:Al ratio.

Table 6 Chloromethane conversion over 30:1 and 80:1 H-ZSM-5
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3.5 Catalyst Regeneration

As is shown above, given the optimized space velocity and temperature, H-ZSM-5 and metal oxide promoted H-ZSM-5 zeolites could retain ~ 90% CH3Cl conversion over 180 min of reaction. After each run, the catalysts were regenerated by keeping at 800 °C for 1 h under 20% O2/N2 flow. A comparison of the reaction performance for 1% Ni, 5% Mg and 5% Ni promoted H-ZSM-5 catalysts and their corresponding regenerated catalysts is shown in Fig. 7. For the regenerated 1% Ni-H-ZSM-5 sample and 5% Ni-H-ZSM-5, which is denoted as “regened 1% Ni” and “regened 5% Ni”in Fig. 7, the chloromethane conversion is identical to the fresh catalyst, except that the regenerated 5% Ni-H-ZSM-5 exhibited less aromatics selectivity. For 5% Mg-H-ZSM-5, the regenerated sample showed slightly higher propylene selectivity and lower C5+ selectivity but the conversion remains the same. In summary, the metal oxide promoted H-ZSM-5 catalysts could be regenerated by air calcination and the catalytic performance is mostly retained after the regeneration.

Fig. 7
figure 7

Catalytic Performance of metal oxide promoted H-ZSM-5 before and after the regeneration. Reaction conditions: 425 °C, WHSV = 1.87 h-1, time on stream = 180 min

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4 Conclusion

The chloromethane conversion reaction can be performed on the zeolite catalysts under the conditions similar to the methanol to olefins conversion. However, H-ZSM-5 excels in CH3Cl activity and catalyst lifetime. In order to maximize the yield to olefins and reduce the coke formation on H-ZSM-5, the reaction should be conducted when the temperature is set at 425 °C and the WHSV is less than 3 h−1. Impregnating the H-ZSM-5 with metal oxide promoters alters the acidity of the zeolites and the aromatics selectivity increases linearly with the acidity of the metal-promoted H-ZSM-5 zeolite. When the H-ZSM-5 is impregnated with 1wt% metal oxide promoters, the aromatics selectivity follows the electronegativity of the metals impregnated. However, when the concentration of the metal oxide reaches 5%, the promoters could accumulate as amorphous phase on the catalyst surface and block the acid sites, thus the aromatics selectivity is significantly lower on the zeolites promoted with 5% Mg and 5% Mn. On the contrary, 5% Ni-H-ZSM-5 showed higher aromatics selectivity than blank H-ZSM-5 since most nickel oxide exhibited as bulk crystallites, thus the overall acidity could be retained. Nickel oxide promoters diffused into the H-ZSM-5 cages helped transform strong acid sites into medium strength acid sites, which also contributed to the formation of aromatics. The aromatics selectivity also decreases when the silica content of ZSM-5 zeolite was increasing. In summary, the product distribution of CH3Cl reaction on H-ZSM-5 can be adjusted by using different metal oxide promoters. A high aromatics yield was achieved on 5% Ni-H-ZSM-5 while on 5% Mg and 5% Mn promoted H-ZSM-5 the catalyst favors propylene and butylene.