Methanol converting to propylene on weakly acidic and hierarchical porous MFI zeolite

ZHAO Yan-nan ，F(xiàn)AN Su-bing ，MA Qing-xiang ，ZHANG Jian-li ，ZHAO Tian-sheng

（State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China）

Abstract: H-[B,Al]-ZSM-5 zeolites were synthesized with glucose as assistant template to catalyze methanol converting toward propylene. The superior catalytic performance in terms of the propylene selectivity and the activity longevity was related to high ratio of weak acid to strong acid for favorable production of propylene and to high mesoporosity for improved diffusion of reactants and prevention from fast coking. More framework Al siting in the straight or sinusoidal channels of the MFI zeolite could also enhance the propylene/ethylene ratio due to the promotional effect on propylene formation. Low weak acid density was conducive to the production of high propylene/ethylene ratio. With the B/Al ratio of 2 and the (Al2+B2)/Si ratio of 0.01, HZ5-G-2B was applied in the methanol to propylene reaction at CH3OH/H2O (1∶1.2) WHSV of 1.8 h?1 and 480 °C. Propylene selectivity of 51.6%, the C=2 ?4 selectivity of 83.7% and complete conversion of methanol were achieved.The propylene/ethylene ratio was 2. The catalytic activity kept stable for 580 h.

Key words: H-[B；Al]-ZSM-5；weak acidity；mesoporosity；MTP

Selective transformation of methanol toward hydrocarbons is of significance since methanol is now regarded as a bridge between fossil or biomass carbon resources and petroleum chemicals. For the methanolto-hydrocarbons (MTH) catalysis, the influences of both the topology and the acidity of microporous zeolites on the product distribution have been widely recognized[1]. For example, earlier Ni-SAPO-34 catalyst of the CHA structure showed prominent selectivity of ca. 90% toward ethylene[2]. Element modified HZSM-5 of the MFI structure with adjusted acidity and porosity also exhibited high propylene selectivity and propylene/ethylene (P/E) ratio[3].

To improve both the propylene selectivity and catalyst durability in the MTH reaction, ZSM-5-based zeolite catalysts have been extensively studied. For example, Lee et al. reported that the acid strength of the MFI zeolite could be optimized by incorporation of Al3+and Fe3+into the framework of ZSM-5 for the maximization of the propylene selectivity in the MTH reaction[4]. Additionally, research suggested that the MFI zeolite with strong acidity could initiate the methanol dehydration and then enhance the subsequent formation of hydrocarbons, but it could also promote coke formation, thus causing the catalyst deactivation.More importantly, the MFI zeolite with weak acidity is beneficial not only to the production of propylene from methanol, but also to enhance its coking resistance[5].Therefore, fine-tuning the acidic property of the MFI zeolite has been proved to be a crucial factor to improve the catalytic performance for methanol to propylene (MTP) reaction.

Among various element modified HZSM-5 catalysts, H-[B]-ZSM-5 exhibited less activity than H-[Al]-ZSM-5 in methanol reactions[6], but studies on the catalytic performance of ZSM-5 catalysts with both Al and B incorporation for the MTH reaction have been widely reported in recent years. One consideration is that the incorporation of boron into the framework of MFI structure could give rise to weak acid sites, thus changing the overall acidity of MFI type zeolite. Chu et al. reported that the isomorphous substitution of Si in the MFI framework by boron could induce the formation of bridge hydroxyl groups and negative charges, which were the weak acid sites[7]. On the other hand, regulating the acidity of the MFI zeolite just through altering its Si/Al2ratio might not match the acid site requirement well for the MTP catalysis. Yang et al. reported that the B-incorporated high silica ZSM-5 (Si/Al2=400) with B/Al=1 exhibited more the weak acid sites, which were proved to show good anti-coking capability, thus enhancing catalytic stability for the MTP reaction[8]. Meanwhile, Yaripour et al. reported that propylene selectivity did not change significantly with the isomorphous substitution of B for the HZSM-5(Si/Al2=400) with B/Al=20, but the activity lifetime of the modified zeolite increased remarkably at WHSV of 0.9 h?1[9]. More importantly, Liang et al. reported that the framework Al (AlF) siting in the MFI zeolite was highly related to the catalytic pathway during the MTH reaction[10]. To be exact, the AlFin the channel intersections exhibit higher selectivity to ethene and aromatics via the aromatic-based cycle, whereas those in the straight and the sinusoidal channels display higher selectivity to propene and higher olefins via the alkene-based cycle. Very recently, it was found that the framework Al distribution in ZSM-5 with Si/Al2of 200[11]or 400[12]could be regulated by boronmodification, thus affecting their catalytic performance.The activity, however, was not lasting. All in all, clear understanding the structure-activity relationship of Aland boron-containing MFI zeolite for the MTP reaction is still under the way.

In addition, hierarchically porous HZSM-5 normally exhibits short diffusion path lengths for reactants and coke precursors in the MTH reaction,which favors the activity durability as well as the propylene selectivity[13?15].

In this work, the H-[B,Al]-ZSM-5 catalysts with mesopores and varied weak acidity were synthesized successfully using glucose as assistant template, which showed high catalytic performance toward the MTP reaction. Additionally, the correlation between the structure and the activity of the MFI zeolite was also elucidated.

1 Experimental

1.1 H-[B,Al]-ZSM-5 synthesis

The molar composition of synthetic gel is Al2O3∶SiO2∶NaOH∶TPAOH∶glucose∶H2O∶B2O3=1∶200∶36∶36∶60:12000∶x, wherexis 1, 2, or 9.Typically, 0.48 g NaOH was first added into 81 g deionized water to make a clear solution under fast stirring for 0.5 h. Then, 4.5 g glucose, 0.14 g aluminum isopropoxide, 15.69 g tetraethyl orthosilicate, 9.7 g TPAOH (tetrapropylammonium hydroxide), desired amount of H3BO3and 1.37 g HZSM-5 (seed crystal,Nankai Catalyst Factory, Si/Al2=81) were added into above solution under stirring for 0.5 h in turns.Afterwards, the obtained mixture was further stirred at 1000 r/min. for 16 h to form a homogeneous gel and then was transferred into a 100 mL stainless-steel autoclave followed by first crystallization at 110 °C for 24 h and second crystallization at 180 °C for 48 h.After cooling down, the solid was filtered, washed,dried, calcined at 550 °C for 6 h. Finally, the obtained product was ion-exchanged in an aqueous solution of NH4NO3(1 mol/L) at liquid-to-solid ratio of 10 under stirring at 90 °C for 1 h and repeated for 3 times,followed by filtration, washing, and calcination. The synthetic samples were denoted as HZ5-G-xB withx=1, 2, or 9.

1.2 Characterization

Morphology of synthetic samples was observed on a ZEISS EVO18 tungsten filament scanning electron microscope (SEM) operated at 15 kV. Crystallographic phases were analyzed on a Bruker D8 Advance X-ray diffractometer with CuKα radiation operated at 40 mV and 40 mA. The actual Si/Al2and B/Al ratios were calculated based on element analysis using an Aglient 5110 inductively coupled plasma-optical emission spectrometry (ICP-OES). Textural properties were measured by means of N2physical adsorption on a JWBK132F surface area and pore size analyzer. 100 mg sample was vacuum-pretreated at 350 °C for 3 h. The micropore and mesopore volumes were calculated using SF model and BJH model, respectively. The siting and coordination of Al and boron in the framework were analyzed on a Bruker 400 MHz nuclear magnetic resonance (NMR) instrument using a 4 mm probe and a rotation speed of 13 kHz. Carbon deposit was analyzed in an air flow (30 mL/min) at a heating rate of 10 °C/min on a Netzsch STA409-QMS403C thermogravimetric (TG) analyzer.

The surface acidity analysis by NH3temperatureprogrammed desorption (NH3-TPD) was carried out on a Xianquan TP-5080 instrument. In the TPD experiments, 100 mg sample (20?40 mesh) was loaded in the quartz tube and pretreated at 550 °C for 1 h in a He flow. After saturated by NH3at ambient temperature, the sample was purged by the He flow at 120 °C for 0.5 h. Finally, the NH3desorption was synchronously recorded from 20 to 650 °C with a heating rate of 5 °C/min. Acid type analysis by means of pyridine adsorption was performed on a Bruker Tensor-70 FT-IR spectrometer equipped with a TOPS in-situ cell, according to a procedure in previous literature. In a typical experiment, a self-supported sample wafer (ca 25 mg) placed in the cell was first pretreated at 500 °C for 1 h under the vacuum condition, and then pyridine vapor was adsorbed at 150 °C for 10 min. After the sample was heated at 350 °C in a He flow (20 mL/min) for 0.5 h, the FT-IR spectrum was recorded. The acid amount was calculated referring to literature[16].

1.3 Catalytic performance evaluation

The activity test of synthetic samples for the MTP reaction was carried out in a micro fixed bed reactor(i.d. 14 mm, o.d. 16 mm, L 400 mm). In a typical run,1 g catalyst sample (20?40 mesh) was loaded.Methanol-water mixture (molar ratio of 1∶1.23) with a WHSV of 1.8 h?1was pumped into the line and carried into the reactor by a N2flow (20 mL/min). Reaction temperature and pressure was 480 °C and 0.1 MPa,respectively. The products preserved at 180 °C were on-line analyzed in a sampling interval of 2 h on a GC-9560 gas chromatography equipped with a capillary separation column (HP-PLOT/Q, 30 m × 0.32 mm ×20 μm) and FID detector. Methanol conversion and product selectivity were calculated as follows, whereAis the GC peak area andfis the relative molar correction factor.

Hydrogen transfer index (HTI) was defined as(C3°+C4°)/(C3+C4), which is a measure of hydrogen transfer activity during the reaction.

2 Results and discussion

2.1 Properties of synthetic samples

Figure1 shows the SEM and TEM images of various synthetic samples. For all HZ5-G-xB (H-[B,Al]-ZSM-5) samples, uniform coffin-shaped particles with a spatial size of about 2.0 μm × 1.1 μm ×0.2 μm could be clearly observed, and the thickness of those particles along the b-axis direction was thin. In contrast, HZ5-G (H-[Al]-ZSM-5) sample consisted of uniform microsphere particles with a diameter of about 0.5 μm. Additionally, obvious bright spots were observed for the HZ5-G-2B and HZ5-G samples from the TEM images, suggesting the presence of mesopores using glucose as assistant template.

Figure2 is the XRD patterns of various synthetic samples. All samples exhibited the typical diffraction patterns for MFI structure at 2θof 7.8°, 8.8°, 23.1°,23.9° and 24.3°, ascribed to the (101), (020), (051),(303) and (313) crystal planes, respectively.Additionally, compared with HZ5-G without B modification, the peak intensity of HZ5-G-9B with the B/Al ratio of 9 decreased significantly.

N2sorption isotherms of various synthetic samples are shown in Figure3. Both HZ5-G and HZ5-G-xB samples exhibited the typical type-IV isotherm with an obvious hysteresis loop atp/p0=0.8?1, further confirming the presence of mesopores. More importantly, HZ5-G-xB samples showed larger hysteresis loops in the highp/p0region compared with HZ5-G sample without B modification, and the hysteresis loop for HZ5-G-2B with the B/Al ratio of 2 was the largest. Additionally, all samples had uniform mesopores with a pore width of about 3.5 nm. Among them, HZ5-G-2B and HZ5-G-9B also presented mesopores with a diameter range of 7.5?20 nm and 6.2?15 nm, respectively, which might be attributed to the intercrystalline piled pores. In comparison, the isotherm of HZ5-B sample showed typical type-I profile, indicating the main microporous structure.

Table1 lists the chemical compositions of various synthetic samples. Compared with the charged ones,the measured Si/Al2molar ratios of synthetic samples became higher, but the measured B/Al ratios became lower, implying the incomplete of crystallization. The(Al2+B2)/Si also increased with the increase of B usage for various HZ5-G-xB samples. Moreover, the textural properties of various synthetic samples are also summarized in Table1. HZ5-G-1B showed much higher mesoporous volume than HZ5-1B, indicating the presence of glucose is beneficial to the formation of mesopores. With an increase in B/Al ratio, the BET total surface area and micropore volume of synthetic samples tended to decrease. Among them, HZ5-G-2B had the lowest micropore surface area but largest mesopore one. Furthermore, HZ5-G-2B showed the largest mesopore volume and the highest mesoporosity with glucose assistant template. For HZ5-G-9B with higher B/Al ratio, both the mesopore volume and mesoporosity decrease significantly, suggesting the adverse effect of high boron content. For example,parts of its mesopores might be blocked by noncrystalline impurities (see Figure2). The change of mesopore induced by using the assistant template and boron incorporation might be related to the different crystal sizes of various synthetic samples.

To investigate the siting and coordination of Al and boron in the MFI framework, the11B and27Al MAS NMR analysis was performed. The11B MAS NMR spectra are shown in Figure4. For various HZ5-G-xB samples, the peak centered at ?3.2 was assigned to the tetracoordinated framework boron[17], demonstrating the existence of the framework boron. For HZ5-G-9B sample, there was a shoulder peak centered at 7.1,which could be ascribed to the presence of tricoordinated extraframework boron. The27Al MAS NMR spectra are shown in Figure5. All of them exhibited a strong peak at 55 and a weak peak at 0,corresponding to the four-coordination framework Al(AlF) and the six-coordination extra-framework Al[18],respectively. Furthermore, for all samples, the peak area of the four-coordination Al was always much higher than that of the six-coordination Al. To clarify the Al siting, the chemical shift peak between 40?70 was deconvoluted into five peaks at 52, 53, 54, 56 and 58. The fitting curves and their relative percentages are shown in Figure6 and Table2. The peak at 54 was attributed to the AlFin the channel intersections, and the peak at 56 corresponded to the AlFin the straight and sinusoidal channels[19,20]. More importantly, with the increase in B/Al ratio, HZ5-G-xB samples showed an evident increase in the percentage of Al (56) while a decrease in the percentage of Al (54) compared with HZ5-G. In other words, the ratio of Al56/Al54increased with the B/Al ratio, indicating more AlFwould be located in the straight and the sinusoidal channels for HZ5-G-xB with high B/Al ratio. The main reason for this phenomenon is that boron might preferentially occupy the channel intersections of the MFI zeolite[10].

To investigate the influence of B-modification on the surface acidity properties of synthetic samples,NH3-TPD and Py-FTIR analysis were carried out. As shown in Figure7, all samples showed two NH3desorption peak regions at 170–174 and 384–400 °C, corresponding to weak and strong acid sites,respectively[21]. With the increase of B/Al ratio, the peak areas belonged to weak acid sites increased whereas that assigned to strong acid sites decreased evidently. The quantitative results are summarized in Table3. Obviously, regardless of B ‐ incorporation content, all synthetic samples exhibited the similar strong acid amount. However, both the weak acid amount and the W/S ratio increased with the increasing of B ‐ incorporation content. These results might be ascribed to the fact that the incorporated boron in the MFI zeolite produces only weak acid site[22]. The FT-IR bands at 1543, 1487 and 1455 cm–1by pyridine adsorption at 350 °C on synthetic samples were characteristic of Br?nsted (B), B+L and Lewis (L) acid sites, respectively (Figure8). The corresponding B acid and L acid amounts are shown in Table3. The B acid amounts tended to increase with the increasing of B/Al ratio, but the L acid amounts for all samples were similar, which is in line with previous report[12].Considering the increased (Al2+B2)/Si from HZ5-G to HZ5-G-9B (Table1), both Al and boron incorporation could generate the B acid sites, and the increase in the B acid amount might be contributed mainly to boron with the increase of B/Al ratio.

Table3 Surface acidity of synthetic samples

2.2 Catalytic performance for MTP

The catalytic activity of synthetic samples for the MTP reaction versus time-on-stream (TOS) is shown in Figure9 and Table4. The activity durability of all samples was quite different. To be exact, for all HZ5-G-xB (H-[B,Al]-ZSM-5) samples, the methanol conversion (100%) and the product selectivity could remain stable within 230 h. Among various HZ5-G-xB samples, the activity stability could reach up to 580 h for HZ5-G-2B with the highest propylene selectivity of 50% and the lowest HTI. However, further increasing in B/Al ratio to 9 led to an obvious decline in the activity stability. For all HZ5-G-xB samples, the ethylene selectivity reached 22%?26%, and the C=2?4selectivity, the methane selectivity, and the C5+selectivity were above 80%, below 4.5%, and below 8%, respectively. In addition, the propylene/ethylene(P/E) ratio increased with the rise of B/Al ratio, and reached the highest (2.2) at B/Al ratio=2. The lower P/E ratio for HZ5-G-9B sample was inferred to be related to the higher weak acid density of synthetic samples as shown in Table3. When the weak acid density was controlled at lower level[9,12], the P/E ratio or the propylene selectivity tended to increase.Furthermore, HZ5-G-2B showed more AlFsiting in the straight and sinusoidal channels (Table2), which is more encouraging for the olefins-based cycle mechanism during the MTH reaction[10], thus possessing the highest P/E ratio.

Table4 Catalytic activity of synthetic samples for MTP

For the samples without B-incorporation, the activity durability of HZ5-G (H-[Al]-ZSM-5) was 80 h,longer than that of HZ5 (23 h), but they had the similar propylene selectivity (about 40%). Meanwhile, they also had similar C5+selectivity (14%–16%), much higher than that of HZ5-G-xB samples. In addition, the methane selectivity for traditional HZ5 was above 5%,demonstrating that the presence of mesopores endowed by using glucose assistant template might be in favor of enhancing the MTP activity.

Coke deposition rate is an important evaluation for the catalytic performance. The weight loss curves of the spent catalysts obtained by TG analysis are shown in Figure10. The HZ5-G-2B after the MTP TOS reaction for 640 h showed the highest weight loss(19.15%) from 482 to 723 °C, but the lowest rate of coke deposition (4.8×10?3mg/h). For the HZ5-G merely using glucose assistant template without Bincorporation, the coking rate was 1.5×10?2mg/h after the MTP TOS reaction for 96 h. By contrast, the weight loss of HZ5 after the MTP TOS reaction for 23 h was 3.48% from 441 to 717 °C, and the rate of coke deposition for fresh HZ5 was up to 1.7×10?2mg/h.These results indicated clearly that HZ5-G-2B possesses the highest anti-coking capacity due to its high weak acid/strong acid ratio as well as high mesoporosity (Table1).

From the foresaid experimental data, synthetic HZ5-G-2B (H-[B,Al]-ZSM-5) exhibited the superior catalytic performance toward the MTP reaction in terms of the propylene selectivity and the activity longevity. There are three interwoven factors contributing to the superior catalytic performance.Firstly, high ratio of weak acid to strong acid, mainly the B acid sites[9], could raise the propylene selectivity,although low weak acid density of H-[B, Al]-ZSM-5 or high Si/Al2of H-[Al]-ZSM-5[23]was conducive to high P/E ratio. This might be resulted from the fact that hydrogen transfer activity on the weak acid sites became much less[24], thus enhancing the propylene selectivity. Secondly, for HZ5-G-2B sample, there were much more AlFsitting in the straight and the sinusoidal channels, which could further promote the propylene formation via the alkene-based cycle mechanism[25]. Thirdly, the main reason for the remarkable activity durability might be ascribed to the high mesoporosity of HZ5-G-2B, which is beneficial for the diffusion of reactants and coke precursors, for example, polymethylbenzene intermediates, thus endowing it with excellent anti-coking capability.

3 Conclusions

H-[B,Al]-ZSM-5 zeolite with advantageous weak acidity and incremental mesopores were synthesized and applied to the MTP reaction. Through varying the(Al2+B2)/Si ratio, the regulation of mesoporosity, weak acid amount, weak acid density, and framework Al distribution was successfully realized. When the B/Al ratio was 2 and the (Al2+B2)/Si ratio was 0.01, H-[B,Al]-ZSM-5 displayed superior catalytic performance for the methanol to propylene reaction at CH3OH/H2O (1∶1.2) WHSV of 1.8 h?1and 480 °C in terms of the propylene selectivity and the activity longevity. These results demonstrated that appropriate weak acid sites and mesoporosity were required for effective MTP reaction.