The influence of straight pore blockage on the selectivity of methanol to aromatics in nanosized Zn/ZSM-5: an atomic Cs-corrected STEM analysis study

Abstract

Here we report the direct atomic scale observation of ZSM-5 pores and the influence of extra-framework Al on methanol to aromatic (MTA). We synthesized a nano sized ZSM-5 catalyst with a short b-axis, and fully opened straight channels. A 98% ultrahigh aromatic selectivity, more than 300 hours long life time and high anti-hydrothermal ability can be achieved in the MTA process.

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Zeolites are widely used as acidic catalysts in the chemical and petrochemical industries due to their well-defined channels, high surface area and tunable acidity.^1,2 Almost all the zeolite catalysts suffer from deactivation in industrial processes via coke deposition³ or hydrothermal irreversible deactivation.⁴ The activity can be regenerated by burning off the coke, but the high temperature vapor generated from burning coke leads to hydrothermal deactivation. The loss of framework acid sites is considered to be the primary cause of permanent deactivation.^5–7 Many efforts have been made to overcome catalytic deactivation for decades.^8–10

ZSM-5 is an acidic catalyst used in the petroleum and coal-to-chemical industry. A higher hydrothermal stability can be obtained by using phosphorus modified ZSM-5,¹¹ Hydrothermal stability higher than 700 °C is difficult to achieve, but is inevitable to coke burning off. The unique molecular-size, micro pores with straight channels of 0.56 × 0.53 nm in [010] (b-axis) is the important characteristic of ZSM-5 exhibiting excellent shape selectivity in many aromatization reactions, such as methanol to aromatic (MTA). MTA is a very important process for coal/natural gas to chemicals. Our group has successfully developed and commercialized a fluidized process with a capacity of 30 kt per a.¹² Catalysts with improved high selectivity and long life time are still needed to be developed, for instance, by using different impregnation agents,^13,14 changing morphology by introducing various structure directing agents,^6,15 or creating mesoporous and macroporous to enhance transport.^16–19 Low aromatics selectivity, around 75%, of micron order ZSM-5 and short life time, about 2 hours, with high methanol conversion seem to be inevitable.^13,20 Shen and his co-workers^21,22 reported that nano-sized ZSM-5 with ball shape-like could improve aromatics selectivity and increase life-time exhibiting a much lower coking rate than micron sized ones. The high amount of external surface acid sites improved aromatic generation. However, the favorite crystal growth direction was not clear. Tetrapropylammonium hydroxide (TPAOH) as structure directing agent results in a preferable location of acid site on external surface. Ryoo⁶ found that ZSM-5 with short b-axis would have a rather long life-time because of low diffusion resistance. A combination of short b-axis, strong external surface acidity and nano size straight channel together may improve both the aromatic selectivity and the life-time of ZSM-5 catalyst. Based to this consideration, we synthesized a nano sized ZSM-5 with high uniform crystallization and short b-axis. The obtained catalyst (labeled as pristine) has highest aromatics selectivity, lowest coking rate and longest life time. When hydrothermally treated at 760 °C for 4 hours, the catalyst (denoted as aging) demonstrated strong anti-hydrothermal deactivation ability. With the iDPC STEM, we found hydrothermal deactivation mainly caused by blockage of channels by extra-framework alumina.

We use urea as additive to hinder the growth along the b-axis.²³ By increasing the crystallization temperature and adjusting pH value using NaOH we can control the crystal size, high crystallization degree and atomic smooth (010) surface. Many samples with different morphology had been synthesized by changing synthesis conditions, four of them are displayed in Fig. S1.† The detailed synthesis procedure is described in the ESI.† The as-synthesized sample (pristine) had remarkable performance (ESI Fig. S2†). Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of pristine in Fig. 1 and S4† revealed that pristine had a uniform coffin shape with short b-axis around 60 nm, consecutive crystal lattices. We obtained a clear template to investigate MTA process without the interference of complex crystal planes. ZSM-5 are usually characterized by means of AFM,^24,25 XPS,²⁶ XRD. We can hardly obtain atomic resolution information from almost all these technics, except AFM. Recently developed Cs-corrected HRTEM is technology to obtain atomic resolution images and elements distribution, which may solve above problems. But highly porous zeolites are very sensitive to electron irradiation.^6,27 For zeolites, beam damage is mainly caused by ionization²⁸ which can be decreased with increasing accelerating voltage as electrons are faster and spend less time traversing the sample. With 300 kV scanning transmission electron microscope (STEM) technics, Cs-corrected HRTEM STEM and iDPC-STEM images can be obtained shown in Fig. S5 and 1d,† respectively. It seemed Cs-corrected iDPC-STEM had a higher resolution than Cs-STEM. They proved that all straight channels were fully opened. This is the basic requirement of low aromatic diffusion resistance. It should be mentioned here, the MTA performance of H-pristine was around 30%, which was similar to normal ZSM-5. Most Lewis acid sites were generated by impregnation of zinc. While at any condition of impregnation, no ZSM-5 reported before had aromatic selectivity higher than 75%. All finely crystallized nanosized catalysts synthesized in this work, shown in Fig. S1–S3,† had higher than 90% aromatic selectivity. Due to the shorter b-axis and perfect crystallization, pristine had the highest aromatic selectivity and longest life time. Under above consideration, the ultrahigh aromatic selectivity should be contributed to the short b-axis and fully open channels for Zn/ZSM-5.

Fig. 1 (a) SEM image of pristine, (b) Cs-corrected TEM image of pristine along b-axis, (c) detailed view of Cs-corrected TEM image of pristine along b-axis, (d) iDPC-STEM image of pristine sample along b-axis.

The catalytic activities of pristine and aging in MTA reaction were shown in Fig. 2a. Overall aromatic selectivity was higher than 98% with nearly 100% methanol conversion in about 300 h (ESI Fig. S3†). After induction time, the (100) plane of pristine was covered with 10 nm layer of coke which blocked the zig-zag channels of ZSM-5 to prevent paraffin and olefins diffusing out of zeolite channels and (010) plane coke thickness was much smaller which gain higher aromatics selectivity (Fig. 2e and f). The much higher total weight loss of used pristine sample than that of aging (Fig. S6†) combination with the huge difference of micro pore volume loss between these two samples (Table 1) indicated that large amount of coke can be formed inside the micro-pore of pristine rather than aging. The low coke selectivity (the percent of carbon conversed to coke from methanol) and long life time of pristine should be attributed to short b-axis. About 70% aromatic product was trimethyl benzenes and heavier multi-methylbenzene, less than 2% double ring aromatics were detected. This is assigned to strong external surface acidity of the zeolite (Fig. S7†), which transformed p-xylene yielded from straight channels into heavier components.²⁹

Fig. 2 (a) Methanol conversion and aromatic selectivity of pristine and aging samples as a function of time-on-stream (reaction temperature: 475 °C, reaction pressure: atmospheric pressure, WHSV: 0.8 h⁻¹), (b) turnover frequencies (TOFs) to different products versus acid density, (c) Py-IR profile of pristine and aging samples, (d), ²⁷Al NMR spectrum of pristine and aging samples (e and f) TEM images of used pristine sample along b-axis and a-axis, respectively.

Table 1 Physical and chemical characters of pristine and aging samples

Samples	Si/Al ratio^a	BET specific area (m^2 g^−1)	External surface specific area^b (m^2 g^−1)	Total pore volume^b (mL g^−1)	Micro pore volume^b (mL g^−1)	Stacking pore volume^b (mL g^−1)
a Determined by ICP.b Calculated by NLDFT method according to adsorption branch.
Pristine	64.5	427	280	0.54	0.20	0.34
Aging	58.2	403	269	0.38	0.19	0.19
Pristine-used	—	178	131	0.39	0.06	0.33
Aging-used	—	266	148	0.34	0.17	0.17

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After hydrothermal aging for 4 hours at 760 °C, the conversion and selectivity as well as life time of the ZSM-5 decreased significantly. Aging still exhibits an excellent MTA activity. The selectivity was still 75–87% for aging sample, even a little higher than that of micro size ZSM-5 catalysts without aging treatment reported in literature.^13,20 It is interesting that, after aging, the coke was likely to deposit on the external surface of b-axis straight channels (Fig. S8†). The coke hindered the aromatics diffusion which induced a lower aromatic selectivity and methanol conversion. The acid density of pristine and 660, 760 °C for 4 hours aging temperature samples decreases with aging temperature. The calculated TOFs of different products are shown in Fig. 2b. We can see that TOFs of methanol and aromatics increase several times and TOFs of paraffin and olefin increase hundreds fold with the decrease of acidity. This is the apparent reason for the significant drop of aromatic selectivity by hydrothermal treatment. The declined activity of aging samples is also partly due to the acid loss shown in Fig. 2c & S9,† total acid amount dropped to 1/10 of that of pristine. ²⁷Al NMR spectrum revealed that framework Al content declined by only 20%, which was much smaller than the detected result from NH₃-TPD. More extra-framework Al (EFA) was formed after hydrothermal treatment. Channel blockage caused by EFA should be the reason of above difference of acidic tests.

In order to confirm the effect of channel blockage, we used ammonium hexafluorosilicate (AHFS) to remove part of the inert extra-framework Al species of pristine and aging,³⁰ denoted as pristine-AHFS and aging-AHFS, respectively. By comparing the iDPC-STEM of pristine, aging and corresponding AHFS treated samples (Fig. 3a) along [010] direction, AHFS could remove few framework Al due to the same performance of pristine and pristine-AHFS (Fig. 3b). The species found in the aging straight channels are highly disordered alumina (new peak at 35 ppm for aging sample in Fig. 2d) with irregular structures compared with pristine. Straight channels were narrower even fully blocked by these newly formed EFA to prevent the generation of aromatic products and significantly increase, light paraffin selectivity. After aging treatment, the newly formed EFA occupied the straight channels. And they hindered aromatics diffusion out of the channels. Besides iDPC-STEM, we also found other evidences for the present of EFA as blockage from pore volume distribution of pristine, aging, and coked samples shown in Fig. 3c and d & Table 1. The micro pore volume loss caused by the formation of coke was up to 70% before aging. After aging, micro pore volume loss was only 10% which suggested aromatization and coking reaction couldn't happen in seriously blocked micro pores, although the micro pore can be detected by Ar BET measurement (Fig. S10†). Weight loss of pristine sample was 30% higher than aging one is another evidence (Fig. S6†). It indicates that micro pore of used pristine sample is full of coke. It seems Ar can freely access in the blocked micro pore of zeolite but aromatics and NH₃ can't. After aging, the total acid amount decreased to 10% of that of pristine, more than 85% of straight channels are blocked with highly disordered alumina, which consist with the 90% acidity loss during aging for TPD analysis. From NH₃-TPD, we may obtain an exaggerated acid amount decreasing result. We found that there was no obvious difference of catalysis performance between pristine and pristine-AHFS. In the induction period, both pristine and pristine-AHFS had low selectivity and then gradually increased after 2–4 hours. After removal of part of the blockage, the aromatic selectivity of aging-AHFS was recovered about 10% (Fig. 2a and 3b). It proved the mechanism of blockage caused performance decrease again. All above interference factor disturbed our previous research focus. Under above findings, methanol can be adsorbed in the micro pores through some fully or partly open channels as well as zig-zag channels. Further generated aromatics can't diffuse out these channels, only smaller size compounds, paraffin and olefin, can diffuse out of these channels. This mechanism significantly promotes paraffin, olefin but decreases the aromatics selectivity.

Fig. 3 (a) iDPC-STEM images of aging-AHFS sample along b-axis, (b) methanol conversion and aromatic selectivity of pristine-AHFS and aging-AHFS samples as a function of time-on-stream (reaction temperature: 475 °C, reaction pressure: atmospheric pressure, WHSV: 0.8 h⁻¹), (c and d) Ar adsorption/desorption isotherms and NLDFT pore size distribution of pristine, aging, pristine-coked, and aging-coked, (e) the illustration of the mechanism of B/L synergy and channel blockage.

The discussion above gives us a picture of high selectivity, long life time and high stability of hydrothermal zeolite (Fig. 3e). Due to the TOFs of aromatics and methanol are almost the same, short fully opened straight channels result in low coke selectivity as well as long life time of the catalyst. With hydrothermal treatment of the ZSM-5, the blocked straight channels hundreds fold increase the paraffin and olefin generation activity and significantly decrease aromatic selectivity. High hydrothermal stability should be contributed to the single crystal with large external surface area and high external surface acid amount.

Conclusions

In summary, a nano size b-axis ZSM-5 catalyst was synthesized using urea and structure directing agent (TPAOH). Cs-corrected atomic resolution iDPC STEM, HRTEM study reveals that the short fully opened straight channel is the reason of excellent performance. This ZSM-5 catalyst exhibited highest aromatic selectivity, long life-time and high hydrothermal stability. After aging, the newly formed EFA blocks the straight channels leading a hundreds fold increase of paraffin and olefin formation and a decrease of aromatic selectivity and life time of the catalyst. With the new atomic scale tool, Cs-iDPC-STEM for zeolite channel analysis, the obtained exaggerated acid loss data by hydrothermal treatment was needed to revise. We need to reconsider the contribution of acid loss on the performance decrease after hydrothermal treatments. The complex surface pore blockage on the zeolite catalysis can be solved and the mechanism of huge increase of paraffin formation activity can be revealed. This may guide the development of next generation zeolite catalysts in many different reactions giving high selectivity and life time. We believe that this will be significantly benefit for more green and selective chemical process.

Acknowledgements

Financial support was provided by the NSFC program (21376135, 91434122 and 51236004) and CNPC Innovation Foundation of 2014D-5006-0506.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19073a

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