2.1. Structure and Acidity
shows the XRD characterization result of the zeolite samples. The five strongest peaks centred at 7.9°, 8.9°, 23.1°, 23.3°, and 23.9°, corresponding to JCPDS card No. 44-0003, were found in all samples, which confirmed the MFI structure of the HZSM-5 zeolite.
From the SEM images shown in Figure 2
, the average particle size of all HZSM-5 zeolites was about 1.3 μm by the agglomeration of smaller primary crystals, the average primary crystal size was about 90 nm, and showed a similar spherical morphology. Further XRF confirmed the as-synthesized zeolites have SiO2
molar ratios of 194, 281, and 365, respectively. In addition, the measured SiO2
molar ratios were lower than the corresponding recipe values, and the difference between measured values and recipe values increased with the SiO2
molar ratio, which indicated that the utilization of silicon source decreased with the increase of SiO2
molar ratio. Two desorption peaks of all samples around 165 °C and 350 °C were observed over the NH3
-TPD profile in Figure 3
, corresponds to NH3
desorption from weak acid sites and strong acid sites, respectively [22
]. As the existence of Al is the origin of HZSM-5 acidity, the total acid sites of the sample generally increase with the decrease of SiO2
molar ratio, as shown in Figure 3
To check the SiO2
molar ratio changes of the inner layer and outer shell of the HZSM-5, the crystals were crushed to expose the interior part of zeolites and the as-synthesized zeolites were analyzed by XPS technology, the results of which are shown in Figure 4
and Figure 5
. The XPS results only give information about the very outermost surface layer (10 nm) elements of the samples, while the overall element composition of the sample was given by the XRF results. As shown in Figure 4
, the SiO2
molar ratios of the as-synthesized ZSM-5 determined by XPS were obviously lower than the XRF results with the SiO2
molar ratio increasing, disclosing that the exterior shell had more abundant Al with the increase of SiO2
. Meanwhile, the XPS results of the as-synthesized and crushed ZSM-5 showed that the low SiO2
molar ratio (200, 300) slightly increased after crushing, while the high SiO2
molar ratio (400) greatly increased after crushing, as shown in Figure 4
and Figure 5
. Al distribution in Z-400 was significantly heterogeneous and its surface was distinctly richer in Al than its bulk phase [23
]. Therefore, more acid sites existed on the exterior shell of Z-400 than the interior.
2.2. Catalytic Performance
Catalyst evaluation results of HZSM-5 zeolites for MTPB reaction were shown in Table 1
. In the initial stage, 100% methanol conversion was observed over all samples. As the reaction progressed, methanol conversion decreased gradually, and the lifetime of Z-200, Z-300, and Z-400 was 20 h, 28 h, and 32 h, respectively. In the whole life span of the catalyst, the weight of methanol that each gram of catalyst can process was 60, 84, and 96 g for Z-200, Z-300, and Z-400, respectively. This indicated that the higher SiO2
molar ratio HZSM-5 had the capability of converting methanol to light olefins more efficiently and was more durable than low SiO2
molar ratio samples.
It was also noticed with the increase of SiO2
molar ratio, which always means fewer acid sites for the reaction (see Figure 3
), that the selectivity for propylene increased from 29.5% to 42.9% and the selectivity for butene (including t-2-C4
, and c-2-C4
) increased from 14.5% to 21.7%. In addition, the pentene selectivity and C6+
hydrocarbon selectivity increased, while the ethylene selectivity, ethane selectivity, propane selectivity, and butane selectivity decreased with the SiO2
molar ratio increase. These results were considered to be a result of the decrease of acid site density in high SiO2
molar ratio samples, which may restrain the hydrogen transfer, oligomerization and aromatization reactions [25
]. Therefore, the Z-400 showed the highest selectivity for propylene and butene, the highest methanol/catalyst ratio, and the longest lifetime.
2.3. Coke Formation on HZSM-5
TEM images of fresh and spent Z-400 with different magnifications are shown in Figure 6
. By contrast, one can easily find the spent Z-400 (Figure 6
b) showing less ordered carbonaceous deposits with a space around 0.33 nm at the edge of the crystal, indicating the formation of near-graphite carbonaceous species. EDS images further confirmed the existence of these graphite species, as shown in Figure 6
d. Before the reaction, the external surface of fresh Z-400 was free of coverage. However, after the reaction, it is obvious the external surface of the sample was covered by the graphite carbon, which led to the deactivation of zeolite.
Li et al. [26
] reported that owing to the passivation of external surface, the modified HZSM-5 zeolite showed higher aliphatic selectivity, but lower aromatic selectivity, than the parent HZSM-5 zeolite in methanol-to-hydrocarbon reactions, suggesting the occurrence of higher olefin aromatization on the external surface of HZSM-5. In addition, it was also discovered that the selectivity to olefins significantly increased over the modified HZSM-5 zeolite, while that of alkanes decreased, which indicated that the external surface was an important place for the formation of alkanes [26
]. Regarding the formation of near-graphite carbonaceous deposits on the external surface, we believed it was a result of the coke precursor that was not easy to continuously react due to the poor internal Al distribution. The products diffused from the micropores and re-adsorption occurred on the Z-400 external surface with a large number of acid sites, where continuous oligomerization, cyclization dehydrogenation to aromatics, and condensation reactions occurred to form polycyclic aromatics, and eventually led to the formation near-graphite carbonaceous deposit.
adsorption-desorption isotherms of fresh and coked Z-400 are shown in Figure 7
. The N2
adsorption-desorption isotherms of fresh Z-400 combined type I and type IV, indicating the formation of intercrystalline mesopores in the sample. Over the spent Z-400 catalyst, the N2
adsorption-desorption isotherms changed to type I with the absence of hysteresis rings, as well as the decrease of micropore volume, suggesting the deposition of coke in the micropores and intercrystalline mesopores. The specific surface area and pore volume data of fresh and coked Z-400 were listed in Table 2
, after the reaction, the obvious decreases of SBET
, and Vmeso
of Z-400 were observed. It is believed that the coke formed in the channels of HZSM-5, which occupied/blocked part of the channel space and, hence, the decrease of the micropore surface and volume, as shown in Table 2
. About a 50% and 30% decrease in mesopore volume and micropore volume, respectively, was observed. As HZSM-5 is a microporous zeolite and its mesopores are created by the aggregate of tiny crystals, this indicated that coke formation on the external surface was more serious than in that in micropores. In addition, Z-200 and Z-300 showed a similar phenomenon. Meanwhile, the decreasing Vmeso
values of Z-200 and Z-300 were lower than Z-400, while the decreasing Vmicro
values of Z-200 and Z-300 were higher than Z-400. These results further suggested greater coke deposition on the external surface of Z-400 than on Z-200 and Z-300. Nevertheless, greater coke deposition in the micropores of Z-200 and Z-300 than Z-400. These results could be attributed to the coke precursor being easy to continuously react to form coke in the micropores of Z-200 and Z-300 due to a large amount of internal Al, while the coke precursor was easy to continuously react to form coke on the external surface of Z-400 due to a large amount of external Al with little internal Al.
The pore size distribution of fresh and spent Z-400 is shown in Figure 8
. After the reaction, it was clearly seen the narrowing of mesopore from 3.25 to 4 nm to 2.5 to 3.25 nm, and a significant mesopore volume decrease. These results further indicated that a certain amount of coke deposited in the intercrystalline mesopores, where the mesopores’ surface is the external surface of very small primary crystals.
The thermogravimetric analysis-derivative thermogravimetric analysis (TGA-DTG) profiles of deactivated Z-200, Z-300, and Z-400 are shown in Figure 9
. Approximately 10%, 10%, and 8% weight loss of Z-200, Z-300 and Z-400, respectively, between 420 °C and 680 °C was observed, which was ascribed to the coke combustion of the deactivated HZSM-5. The higher DTG peak temperature of Z-400 than Z-200 and Z-300 showed the coke species were difficult to combust, which was consistent with the near-graphite carbonaceous species on the external surface of Z-400 as discerned by TEM. The amounts of coke on the external surface and in the micropores were calculated by the method in [27
]. The coke formation inside the micropores was calculated from a decrease in micropore volume, assuming coke has a density 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. As shown in Table 3
, coke deposited on the external surface accounted for 1.8%, 9.8%, and 67.8% of the total coke of Z-200, Z-300, and Z-400, respectively, while coke deposited in the micropores accounted for 98.2%, 90.1%, and 32.2% of the total coke of Z-200, Z-300, and Z-400, respectively. These results further indicated that coke preferred to form in the micropores of Z-200 and Z-300, while coke preferred to form on the external surface and intercrystalline mesopores of Z-400. The higher coke content in the micropore of Z-200 and Z-300 than Z-400 might be due to the coke precursor continuously reacting on these acid sites to form coke in the micropores. While coke preferred to form on the external surface of Z-400, this was attributed to the distinctly rich Al shell with a poor Al core, resulting in the non-shape-selective side reactions of product diffusion from the micropores on the large number of acid sites on the external surface.
showed the 13
C MAS NMR spectrum of spent Z-400. There were a major signal around 128 ppm and minor signals around 32 ppm and 20 ppm, which were attributed to a mixture of C6
aromatics and paraffins, such as polymethylbenzenes (134 ppm, 129 ppm, and 21 ppm), and 2,3-hexadiene (132–134 ppm, 126–128 ppm, and ca. 18 ppm) [28
]. These molecules were proposed to be the main components of carbonaceous deposits over HZSM-5.