With the depletion of oil resource, the methanol to propene (MTP) process provides an alternative route to produce propene beyond the conventional steam cracking of naphtha and fluid catalytic cracking process [1-4]. The MTP reaction has aroused significant interest in both the academic and industrial fields since it was originally developed by Lurgi company [1]. It has been recognized that the methanol conversion over zeolite catalysts is a typical acid catalyzed process and coke deposition is the main cause of the catalyst deactivation [5-8]. ZSM-5 zeolite, with a 3D interconnected pore system composed of 10-membered rings, is currently the most effective catalyst, which has found application in the industrial fixed bed MTP process.
Due to the space limitation in the micropores of ZSM-5 (0.51 nm × 0.55 nm, 0.53 nm × 0.56 nm), the coke deposition in the crystal interior can be greatly avoided during the MTP reaction. And the deactivation on ZSM-5 is closely related to the external surface acidity [9]. Therefore, attempts have been made to reduce the external surface acidity of ZSM-5, aiming to retard the coke deposition rate and improve the catalytic stability. The strategies developed to passivate the external surface acid sites of zeolites can be mainly classified as macromolecule deposition [10-19], synthesis of zeolites with core-shell structure [20-23], metal or non-metal modification [24-28] and so on.
The macromolecules reported for the passivation/modification of external surface acid sites of zeolites include bulky organophosphorus compounds [10, 11], bulky nitrogen compounds [12] and various siloxane [14-19]. Losch et al. [29] once investigated the effect of surface passivation of nano-sized ZSM-5 on the methanol conversion reaction by a chemical liquid deposition (CLD) technique using tetraethylorthosilicate. The resultant catalyst exhibited an enhanced selectivity to lower olefins, but a significant decrease in the stability, which was due to the partial blocking of micropores (the coking process exclusively occurred within the pores and fastened the catalyst deactivation). Phosphorus modification of HZSM-5 zeolite by wet impregnation has long been known to decrease aromatic formation and improve catalytic stability in methanol conversion chemistry [26, 30, 31]. Impregnation of HZSM-5 with phosphoric acid leads to the hydrolysis of framework Al and the condensation of Brønsted acid sites with POH groups, resulting in higher framework Si/Al ratio and the decrease of strong acid sites [31, 32]. However, phosphoric acid can deposit both in the inner pore and external surface of ZSM-5 due to its smaller size. Recently, Liu et al. [24] reported an effective method to prepare P-modified H-ZSM-5 with the preferential covering of the external surface acid sites by adding ethanol during the impregnation. This occurred because bulky organic phosphates were likely formed from the reaction of phosphoric acid and ethanol, which deposited on the external surface of HZSM-5 crystals. The obtained material gave high p-xylene selectivity along with a relatively high catalytic activity in toluene disproportionation reaction.
Zeolites with core-shell structure are generally synthesized by epitaxial growth of inert shell outside zeolite core. In principle, the degree of surface coverage and shell thickness are the key factors influencing the catalytic performance. Luo et al. [33] successfully synthesized ZSM-5@S-1 material, which gave comparable propene selectivity with the ZSM-5 precursor and a prolonged lifetime from 74 to 139 h in the MTP reaction. The change in lifetime should result from the reduced number of external acid sites and the attenuated occurrence of side reactions. Yin et al. [34] also demonstrated that the coverage of surface acid sites as a result of contiguous silicalite-1 coating was the reason for the high stability and selectivity of ZSM-5 in CH3Br conversion reaction.
Recently, selective dealumination methods were reported to remove the external surface acid sites of ZSM-5. Inagaki et al. [35] found that post-synthetic HNO3 treatment of ZSM-5 zeolite synthesized in the absence of organic structure-directing agent can selectively remove framework Al on the external surface, producing a unique ZSM-5 zeolite catalyst that has very few acid sites on its external surface. The treated ZSM-5 showed high resistance to coke formation during the cracking of hexane or other paraffin molecules.
In this contribution, a facile post-synthetic treatment method was developed to remove/modify the external surface acidity of ZSM-5 by utilizing as-made ZSM-5 containing organic structure directing agent (OSDA) as the precursor. The existence of OSDA in the as-made ZSM-5 crystals may effectively block the channels and confine the modification on the crystal exterior without altering the internal acidity and integrity of the crystals. Two common reagents, Na2H2EDTA and H3PO4, herein were explored for the post-synthetic treatment. It is mentioned that large molecule EDTA, as a powerful chelating agent, has been widely used for the dealumination of Y zeolite [36]. But its use for the dealumination of ZSM-5 is quite rare. The variations of structure integrity, surface acid density and catalytic performance of the modified ZSM-5 were investigated. Moreover, the H3PO4 concentration was optimized to further prolong the lifetime in the MTP reaction.
The chemical reagents used in synthesis and modification of ZSM-5 zeolites include: tetraethyl orthosilicate (TEOS, Tianjin Kemiou Chemical Reagent Co), aluminium iso-propoxide (Al(OPri)3, Sinopharm Chemical Reagent Co.), tetrapropylammonium hydroxide (TPAOH, 10 wt% Tianjin Jingrun Chemical Co), urea (Tianjin Damao Chemical Co), ethylenediamine tetraacetic acid disodium salt (Na2H2EDTA, Tianda Chemical Reagent Co. Ltd.), phosphoric acid (85.0 wt%, Sichuan Xianfeng Chemical Co.).
The synthesis of ZSM-5 zeolite was according to the reported literature [37]. The gel molar composition was 300SiO2:1Al2O3:90TPAOH:6000H2O:450urea. Detained synthesis procedure was carried out as follows: 104.2 g TEOS was added dropwise into 210.5 g TPAOH solution under stirring. After 12 h, 0.681 g Al(OPri)3 was added into the mixture and stirred for 12h. After further addition of 45.0 g urea and stirred for 1 h, the resulting mixture was transferred into an autoclave. The crystallization was carried out at 180 ℃ for 48 h under tumbling 60 r/min. The as-made sample was centrifuged, washed with distilled water, dried at 120 ℃ overnight.
Typical post-synthetic modification treatment process with H3PO4 solution was as follows: 1.5 g as-made sample was added to 20.0 mL solution (1 mol/L) in a Teflon-lined stainless steel autoclave. After treatment at 90 ℃ for 12 h under tumbling 60 r/min, the slurry was filtered, washed with distilled water for three times until pH=7. All products were dried at 120 ℃ overnight and calcined at 550 ℃ for 8 h to remove the organic template. In the case of Na2H2EDTA, the solution pH was adjusted to 1–2 by HCl and the final slurry was filtrated, followed by washing with large amount of hot water due to the low solubility of Na2H2EDTA. The ammonium exchange of calcined sample was carried out at 80 ℃ with NH4NO3 solution (1.0 mol/L) for three times to remove the possible residual sodium during the Na2H2EDTA treatment. The calcined precursor and samples modified by Na2H2EDTA and H3PO4 were named as ZSM-5, ZSM-5-E-x, ZSM-5-P-x respectively, where x represent the solution concentration.
The powder XRD pattern was recorded on a PANalytical X'Pert PRO X-ray diffractometer with Cu-Kα radiation (λ = 0.15418 nm), operating at 40 kV and 40 mA. The chemical composition of solid samples was determined with a PhilipsMagix-601 X-ray fluorescence (XRF) spectrometer. XPS measurements were performed using a Thermo ESCALAB 250Xi spectrometer with Al Kα radiation as the excitation source. The surface charge of the sample was calibrated by referencing to the Al 2p peak of Al2O3 at 74.7 eV. The crystal morphology was observed using a scanning electron microscopy (Hitachi SU8020). N2 adsorption-desorption isotherms at –196 ℃ were determined on a Micromeritics ASAP2020. Prior to the measurement, samples were degassed at 350 ℃ under vacuum for 4 h. The total surface area was calculated based on the BET equation. The micropore volume and micropore surface area were evaluated using the t-plot method. 27Al solid state NMR experiments were performed on a Varian Infinity plus 600 WB spectrometer with BBO MAS probe operating at magnetic field strength of 14.1 T. The resonance frequencies were 104.2 MHz and chemical shifts were referenced to 1.0 mol/L Al(NO3)3.The spinning rate of the samples at the magic angle was 4 kHz. Temperature-programmed desorption of ammonia (NH3-TPD) was measured on a chemical adsorption instrument of Micrometric 2920. Each sample (40–60 mesh, 0.20 g) was loaded into a quartz U-shaped reactor and pretreated at 600 ℃ for 1 h in flowing He. After the pretreatment, the sample was cooled down to 100 ℃ and saturated with NH3 gas. Then NH3-TPD was carried out in a constant flow of He (20 mL/min) from 100 to 600 ℃ at a heating rate of 10 ℃/min.
Fourier transform infrared (FT-IR) spectra after pyridine adsorption were obtained on a Bruker Tensor 27 instrument with a resolution of 4 cm-1. Samples were pressed into a self-supporting wafer (R = 13 mm, 20 mg) and evacuated (10-2 Pa) in an IR cell at 723 K for 30 min prior to each measurement. Adsorption of pyridine was conducted at room temperature for 1 min to ensure saturated loading and then evacuation was carried out at 350 ℃ for 30 min and cooled down to room temperature prior to acquisition of IR spectra. The densities of Br nsted and Lewis acid sites were calculated from the IA values of the difference spectra at 1540 and 1450 cm-1, respectively, using the extinction coefficients reported by Emeis [38].
0.3 g calcined catalyst was pressed, sieved to 40–60 mesh and loaded in a fixed-bed quartz tubular reactor with inner diameter of 8 mm. Prior to the reaction, the catalyst was activated at 550 ℃ for 60 min, and then the temperature was adjusted to reaction temperature. Methanol was fed by passing the carrier gas (40.0 mL/min) through a saturator containing methanol at 35 ℃, which gave a WHSV of 4.0 h–1. Methanol conversion was performed under atmospheric pressure. The effluent products from reactor were kept at 200 ℃ and analyzed by an online Agilent 7890A GC equipped with a PONA capillary column (100 m × 0.25 mm × 0.5 mm) and a FID detector. The conversion and selectivity were calculated on CH2 basis. Dimethyl ether (DME) was considered as reactant in the calculation. For the 1, 3, 5-triisopropylbenzene (TIPB) cracking reaction, similar condition was employed except that the reaction temperature was 360 ℃.
The coke percent of discharged catalysts was calculated by thermogravimetric analysis (TGA) and the loss between 300 to 700 ℃ was used to estimate coke content. Considering that the different amounts of methanol processed on each catalyst, the average coke deposition rate are defined as followed:
where: Rcoke (mg/(gcat·h)): the average coke deposition rate, C (%): the coke percent in discharged catalysts determined by TGA, T (h): reaction time, M (g): the catalyst amount.
The coke formation inside micropores (internal coke) was calculated from the decrease in the micropore volume determined by N2 adsorption at –196 ℃, assuming the coke density (dcoke) of 1.22 g/cm3. The coke content deposited on the external surface was calculated by subtracting the internal coke content from the total coke content determined by TGA. The amounts of internal coke and external coke were calculated as follows [39-41]:
where Winternal coke, Wexternal coke, and Wtotal coke are the amounts (gcoke/gcat) of internal, external, and total coke, respectively. Vmicro, fresh and Vmicro, after reaction are the micropore volumes (cm3/g) of the catalysts before and after the reaction. dcoke is the coke density (1.22 g/cm3).
The XRD patterns of ZSM-5 treated by Na2H2EDTA and H3PO4 solutions are shown in Fig. S1. It can be seen that ZSM-5-E-1 and ZSM-5-P-1 have a typical diffraction pattern of MFI topology, showing the high stability of as-made ZSM-5 to the post-synthetic treatment. The relative crystallinity calculated based on the intensity of the characteristic peaks at 7.8°, 8.8° and 22.4° are listed in Table S1. A decrease in relative crystallinity was observed for ZSM-5-E-1. The SEM images of the samples are displayed in Fig. S2, showing a thin flakiness morphology with thickness of about 100 nm. No clear difference in the surface smoothness and morphology can be distinguished for the crystals before and after post-synthetic treatment. The N2 adsorption-desorption isotherms and pore distributions of the samples are given in Fig. S3. The corresponding textural properties are listed in Table S1. It can be seen that the micropore surface area and micropore volume of the treated samples display a little increase, while the external surface area and mesopore volume slightly decrease. It is speculated that there may exist small amounts of amorphous materials on the shell region of the parent crystals, which was removed by the post-synthetic treatment.
Solid-state 27Al MAS NMR spectra were measured to investigate the change of local atomic environments in the samples and are displayed in Fig. 1. One strong broad resonance centered around 56 ppm is observed for the calcined ZSM-5, which is attributed to tetrahedral Al species [42]. It implies that only framework Al atoms exist in the zeolite precursor. No obvious change can be found in the 27Al MAS NMR spectra after post-synthetic treatment, indicating that the present strategy could effectively protect the framework Al atoms from dealumination by preventing access of the modifier into the crystal interior.
Table 1 lists the bulk and surface Si/Al ratios of the calcined samples. It can be seen that the bulk Si/Al ratio increases slightly for ZSM-5-E-1 and ZSM-5-P-1. On the other hand, the ZSM-5 precursor gives a surface Si/Al ratio of 60, which is obviously lower than the value of the bulk, implying an Al enrichment phenomenon on the crystal surface. The surface Si/Al ratios of the treated samples differ from each other; the value of ZSM-5-P-1 is close to the precursor, whereas a great increase is observed for ZSM-5-E-1. It is speculated that the as-made ZSM-5 has high resistance to the acid treatment, and Na2H2EDTA is more effective than acids for the dealumination of high-Si ZSM-5. Moreover, there is small amounts of P species left on ZSM-5-P-1 (0.17 wt%). The bulk and surface P/Al ratios of ZSM-5-P-1 are given in Table 1. An obviously higher surface P/Al ratio can be observed, implying that the P atoms are mainly located on the exterior of ZSM-5 crystals.
The acid properties of the samples were examined by NH3-TPD and pyridine-adsorbed FT-IR. The results are summarized in Fig. 2 and Table 1. There exist two distinct desorption peaks centered around 180 and 380 ℃ in the NH3-TPD curves (Fig. 2(a)), corresponding to weak and strong acid sites in the samples. In comparison with ZSM-5 precursor, ZSM-5-E-1 shows little change in the desorption temperature and peak height. However, the intensity of high-temperature desorption peak of ZSM-5-P-1 drops slightly, indicating the decrease of strong acid sites. The acid density calculated based on the high-temperature desorption peak is listed in Table 1. The pyridine-adsorbed FT-IR spectra are displayed in Fig. 2(b). The peaks at 1540 and 1455 cm–1 are attributed to the vibrations associated with pyridine adsorption on Br nsted acid sites and Lewis acid sites, respectively [43]. The corresponding quantitative results are given in Table 1, which indicate that the treated samples possess a slightly higher Br nsted acid sites and lower Lewis acid sites than the precursor. The variations occur possibly due to the removal of the amorphous materials on the external surface and are consistent with the results in the part of textural characterization. The acidity characterization results together with the above paragraphs imply that the structural integrity in the crystal interior of ZSM-5 remains intact after the post-synthetic treatment.
TIPB cracking reaction was employed to investigate the external surface acidity of the samples (Fig. 3). ZSM-5-P-1 gives the lowest TIPB conversion among the samples. This is reasonable because the strong interactions between P and Al atoms on the P-rich surface can effectively decrease the number and strength of acid sites [44]. However, unexpectedly, ZSM-5-E-1 shows the highest catalytic activity. The TIPB conversions on ZSM-5-E-1 are more than twice as high as those of the precursor (the TIPB cracking experiment has been repeated over repeatedly prepared ZSM-5-E-1 and almost similar results are obtained). It seems contrary to the high surface Si/Al ratio of ZSM-5-E-1 (Table 1). Possibly, silanol nests [45] are formed on the external surface of ZSM-5-E-1 crystals following the removal of framework Al atoms during the post-synthetic treatment. The silanol groups interacting through extended hydrogen bonding have moderate acidity and contribute to the enhanced TIPB cracking activity [47].
Catalytic tests of methanol to olefins reaction over the samples were carried out at 480 ℃ with a methanol WHSV of 4.0 h–1. The methanol conversion as a function of TOS is presented in Fig. 4. Both ZSM-5-E-1 and ZSM-5-P-1 show an improved catalytic lifetime as compared with the ZSM-5 precursor. Given that the crystal interiors of ZSM-5-E-1 and ZSM-5-P-1 are little influenced as revealed by the above characterizations, the improvement of catalytic lifetime should be closely related to the change of the surface properties.
The coke content of discharged catalysts was determined by TG analysis (Fig. S4). The coke distributions in the micropores and on the external surface were estimated according to the results of N2 physisorption and TG analysis. As shown in Fig. 4, the coke is mainly distributed on the external surface of ZSM-5 (at least 75% of the total coke content), which is consistent with the previous reports [9, 39-41]. The treated samples possess more external coke than the precursor, indicating an increased tolerance to the external coke deposition. The corresponding coke deposition rate is shown in Fig. 4. ZSM-5-P-1 possesses lower external coke deposition rate, whereas ZSM-5-E-1 gives higher external coke deposition rate than the precursor. The variations of external coke deposition rate on the treated samples are consistent with the TIPB cracking results. On the other hand, both ZSM-5-E-1 and ZSM-5-P-1 give lower internal coke content and deposition rate than the precursor, which evidences an enhanced mass transfer in both samples (the micropores are more open after the treatment). It is inferred that the improved catalytic lifetime of ZSM-5-P-1 is due to a combination of the increased external tolerance to the external coke deposition and the depressed coking rate (reduced side reactions); but for ZSM-5-E-1, the increased tolerance to the external coke should be the major reason. Likely, the post-synthetic treatment removes the amorphous materials around the pore openings and helps improve the tolerance to the external coke deposition.
The products selectivities on the samples as a function of TOS are shown in Fig. 5. Moreover, detailed product distributions at TOS = 60 h are listed in Table 2. It can be seen that the ZSM-5 precursor gives propene as the most abundant product, followed by butene, C5+N and ethene. The selectivities to ethene, propene and butene drop as the reaction proceeds, while those of C5+N and aromatics increase. It is generally accepted that the reaction on ZSM-5 follows a dual-cycle mechanism: ethene mainly origins from the aromatics-based cycle, while propene and higher alkenes are formed to a considerable extent from the olefins-based cycle [46-48]. The depression of lower olefins as the increase of TOS implies a dropping activity of both cycles. The evolution trend of product selectivity on ZSM-5-E-1 resembles that on the ZSM-5 precursor, which suggests similar surface properties of the two samples despite their different surface Si/Al ratios. However, for ZSM-5-P-1, the selectivity of propene is maintained at high level until the deactivation; the selectivities of butene and C5+N are relatively stable at the first dozens of hours, while the ethene selectivity displays a continuous dropping trend. The higher propene selectivity on ZSM-5-P-1 should be attributed to the decreased external acid density, which abates the consecutive reaction of light olefins on the external surface and consists with its lower selectivities to C9+ aromatics (formed on the external acid sites) and alkanes (C2-40). These results demonstrate that the reduction of external surface acid sites can effectively improve both the catalytic lifetime and the stability of propene selectivity.
Given the higher propene selectivity and longer lifetime observed on ZSM-5-P-1, the H3PO4 concentration in the post-synthetic treatment was further adjusted to optimize the catalytic performance. The product Si/Al ratios and P contents of the samples are summarized in Table 3. No regular change in the Si/Al ratio can be observed following the increase of H3PO4 concentration, and the samples have similar bulk and surface P contents. The insignificant effect of H3PO4 solution on the dealumination of high-Si as-made ZSM-5 is understandable, since high-Si zeolites are generally acknowledged to be resistant to the acid solution. Indeed, we have carried out post-synthetic HCl treatment of the same precursor and almost unchanged Si/Al ratios are obtained (Table S2).
The TIPB conversions of the samples are also listed in Table 3. A gradual drop of the TIPB conversion can be observed following the increase of H3PO4 concentration, which implies the reduction of external acid density from ZSM-5-P-1 to ZSM-5-P-5. The TIPB cracking on pure Si silicalite-1 is also tested under the same conditions, which gives a conversion of 1.8% and confirms the existence of small amount of acid sites on the H3PO4-treated samples. The reduction in the external surface acid density as the increasing H3PO4 concentration may be caused by the slight degradation of surface structural integrity when ZSM-5 was treated under higher concentration acid solution.
Fig. 6 shows the methanol conversion results over the H3PO4-treated samples. The catalytic lifetime reaches a maximum of 118 h on ZSM-5-P-3 and reduces to 92 h on ZSM-5-P-5. The improvement in catalytic lifetime of ZSM-5-P-3 is apparently bigger than that of high-Si ZSM-5 with similar P loading (obtained by impregnation of HZSM-5 with H3PO4) in the previous report [49]. The coke content in the discharged catalysts was evaluated by TG analysis (Fig. S4) and the coke distributions in the crystal interior and on the external surface are also presented in Fig. 6. Clearly, ZSM-5-P-1 and ZSM-5-P-3 have similar external coke content, which is obviously higher than that on ZSM-5-P-5. The external coke deposition rate shows a reducing trend from ZSM-5-P-1 to ZSM-5-P-5, in accordance with the external acid density of the samples revealed by TIPB cracking. On the other hand, the internal coke content and deposition rate show a sequence of ZSM-5-P-3 < ZSM-5-P-5 < ZSM-5-P-1 < ZSM-5. The abnormal order between ZSM-5-P-3 and ZSM-5-P-5 suggests a reduced mass transfer in ZSM-5-P-5, which may be caused by the degradation of surface structural integrity of ZSM-5-P-5. This speculation is also consistent with the lower external acid density (TIBP cracking ability) and lower tolerance to the external coke deposition on ZSM-5-P-5. It implies that suitable H3PO4 concentration in the post-synthetic treatment of as-made ZSM-5 is important to maximize the catalytic lifetime.
The product selectivities as a function of TOS over the samples are shown in Fig. S5. Detailed product distributions (TOS = 60 h) are presented in Table 2. Similar to ZSM-5-P-1, ZSM-5-P-3 and ZSM-5-P-5 display similar evolution trend of product selectivity as the reaction proceeds. The selectivities of propene, butene and C5+N are relatively stable at the first dozens of hours, while the selectivity to ethene drops gradually. From Table 2, the selectivities towards heavy aromatic and alkanes decrease gradually from ZSM-5-P-1 to ZSM-5-P-5, which implies the restrained side reactions and consist with their dropping external acid density. This also explains well the highest propene selectivity observed over ZSM-5-P-5 with the lowest external acid density.
In summary, we have presented a facile strategy to reduce the external surface acidity of high-silica ZSM-5 by utilizing the as-made zeolite as a precursor. The existence of OSDA in the as-made ZSM-5 crystals effectively blocks the channels and confines the modification on the crystal exterior without altering the internal acidity and integrity of the crystals. H3PO4 treatment cannot remove surface Al atoms, but the small amount of P left on the external surface effectively decreases the acid density. Na2H2EDTA treatment selectively removes the surface Al atoms. However, new acid sites (likely silanol nests) are generated on the external surface after Na2H2EDTA treatment. Both Na2H2EDTA and H3PO4 treatments can prolong the catalytic lifetime of ZSM-5 in the MTP reaction. The reduced external surface acid density on the H3PO4 -treated ZSM-5 has a lower external coke deposition rate (reduced side reactions), which helps the sample show higher propene selectivity. Under the optimized H3PO4 concentration, the treated ZSM-5 extends the catalytic lifetime from 80 h of the precursor to 118 h.
2018,
Vol. 39 Issue (8): 1418-1426 
PDF
2018,
Vol. 39 Issue (8): 1418-1426 PDF