Transition metal (Mn, Fe, or Ni) incorporated SAPO-34 (MeAPSO-34) nanocatalysts were synthesized using a hydrothermal method to improve the catalytic lifetime in the conversion of dimethyl ether to light olefins (DTO). The structures of the synthesized catalysts were characterized using several methods including XRD, SEM, BET, 29Si-MAS NMR, and NH3-TPD techniques. Although the structure of the MeAPSO-34 catalysts was similar to that of the SAPO-34 catalyst, the amount of weak acid sites in all MeAPSO-34 catalysts was markedly increased and accompanied by differences in crystallinity and structural arrangement. The amount of weak acid sites decreased in the following order: NiAPSO-34 > FeAPSO-34 > MnAPSO-34 > SAPO-34 catalyst. The MeAPSO-34 catalysts, when used in the DTO reaction, maintained DME conversion above 90% for a longer time than the SAPO-34 catalyst, while also maintaining the total selectivity above 95% for light olefins. In addition, the NiAPSO-34 catalyst showed the longest catalytic lifetime; the lifetime was extended approximately 2-fold relative to the SAPO-34 catalyst. Therefore, the increase in the catalytic lifetime is related to the amount of weak acidic sites, and these sites are increased in number by incorporating transition metals into the SAPO-34 catalyst.
As a microporous material, aluminophosphates (AlPO) nanomolecular sieves have been used in many processes as a shape-selective catalyst [
The DTO process is an important technique used to produce light olefins starting from alternative energy sources, such as coal, natural gas, and biomass. Among the SAPO molecular sieves, the SAPO-34 catalyst has high selectivity for light olefins production due to its shape selectivity during the methanol to light olefins (MTO) and DTO reactions; numerous studies have been conducted to improve the lifetime of this catalyst [
In this work, the MeAPSO-34 catalysts by incorporating various metals (Mn, Fe, or Ni) were prepared using a hydrothermal method. Physicochemical properties of the prepared catalysts were studied using SEM, XRD, and BET. The variations in the structure and acidity of molecular sieves were characterized using 29Si magic-angle spinning (MAS) NMR and ammonia temperature-programmed desorption (NH3-TPD). At this point, we have focused on the acidic site of catalyst that influences the efficiency of DTO process. DTO reactions were conducted using the MeAPSO-34 catalysts to evaluate the catalytic performance and lifetime, as well as the selectivity for light olefins.
The SAPO-34 and MeAPSO-34 (Me = Mn, Fe, or Ni) catalysts were prepared based on a procedure in the literature [
The catalysts were prepared using a hydrothermal synthesis process in an autoclave. DEA and deionized water were added to aluminum isopropoxide, and phosphoric acid was subsequently added dropwise over 2 h with stirring. A solution of LUDOX AS-40 and TEAOH was added to the mixture over a period of 1 h. For the MeAPSO-34 catalysts, a metal salt was added to the mixture with stirring for 1 h. The final gel was transferred into an autoclave and heated at 200°C for 72 h. After crystallization, the prepared samples were filtered, washed, dried at 100°C and calcined at 600°C for 6 h to obtain the final catalyst. The obtained samples were designated MnAPSO-34, FeAPSO-34 and NiAPSO-34 for Mn, Fe and Ni, respectively.
The crystallinity and composition of the catalysts were characterized using X-ray diffraction (XRD, Rigaku D/max III-B) with Cu K
29Si MAS NMR spectra were recorded with a Varian 500 and a Bruker solid-state NMR spectrometer. The Larmor frequency was 79.488 MHz. The acidity of the catalyst was measured using NH3-TPD. Before analysis, approximately 0.2 g of the prepared catalyst was activated at 600°C for 2 h in helium flowing at 30 mL/min. NH3 gas was injected at 100°C to saturate the catalysts for 1 h. After adsorbing the NH3, the catalysts were kept under flowing helium until the TCD signal indicating the NH3 was absent. Afterward, the NH3-TPD was operated within 100–700°C under a helium flow at 30 mL/min.
The DTO reaction was conducted in a fixed bed reactor under atmospheric pressure. The catalyst (0.2 g) was charged into the center of the quartz reactor (O.D. = 1.1 cm), and the reaction temperature was measured with the thermocouple inside the catalyst bed. Before the reaction, a pretreatment proceeded for 1 h under flowing N2 at atmospheric pressure at 400°C. After the pretreatment, the DME and N2 were introduced into the reactor at the desired flow rate using a mass flow controller (MFC). The volume ratio of the DME and N2 mixed gas was fixed at 1 : 3. The weight hourly space velocity (WHSV) of DME was 3.54 h−1. The DTO reaction products were analyzed online with a gas chromatograph (GC, HP 5890 plus) equipped with a capillary column (HP-plot Q, L 30 m × I.D. 0.320 mm) and a flame ionization detector (FID).
Figure
Textural properties of the SAPO-34 and MeAPSO-34 catalysts.
Catalyst | BET surface area (m2/g) | Total pore volume (cm3/g) | Adsorption average pore width (nm) |
---|---|---|---|
SAPO-34 | 538.79 | 0.39 | 2.92 |
MnAPSO-34 | 543.30 | 0.25 | 2.42 |
FeAPSO-34 | 533.27 | 0.26 | 2.62 |
NiAPSO-34 | 564.31 | 0.26 | 2.34 |
XRD patterns of the SAPO-34 and MeAPSO-34 catalysts: (a) SAPO-34, (b) MnAPSO-34, (c) FeAPSO-34, and (d) NiAPSO-34.
SEM images of the SAPO-34 and MeAPSO-34 catalysts: (a) SAPO-34, (b) MnAPSO-34, (c) FeAPSO-34, and (d) NiAPSO-34.
The variation in the framework starting from the AlPO molecular sieves and leading to MeAPSO-34 catalyst is illustrated in Figure
Scheme demonstrating the possible ways to introduce Si into the AlPO framework and a transition metal into the SAPO framework (oxygen atoms are not represented for clarity): (a) AlPO, (b) SAPO, and (c) MeAPSO.
The 29Si MAS NMR data for the SAPO-34 and NiAPSO-34 catalysts are shown in Figure
29Si MAS NMR spectra of the (a) SAPO-34 and (b) NiAPSO-34 catalysts.
The acidity and strength of acid sites on the SAPO-34 and MeAPSO-34 catalysts are shown in Figure
Measured acidity during the ammonia temperature-programmed ammonia desorption.
Catalyst | Acidity: NH3 desorption amounts (mmol/g) | ||
---|---|---|---|
Weak |
Strong |
Total amount | |
SAPO-34 | 0.3426 | 0.4466 | 0.7891 |
MnAPSO-34 | 0.3795 | 0.5013 | 0.8809 |
FeAPSO-34 | 0.4460 | 0.5100 | 0.9560 |
NiAPSO-34 | 0.5638 | 0.4959 | 1.0598 |
NH3-TPD profiles of the SAPO-34 and MeAPSO-34 catalysts.
The DTO reaction performed over the MeAPSO-34 catalysts to identify the catalytic performances. For all of the catalysts, 100% conversion of DME was obtained during the initial stages of the reaction, and the DME conversion decreased during the reaction as shown in Figure
DME conversion over the SAPO-34 and MeAPSO-34 catalysts.
Figures
Selectivity for light olefins over the SAPO-34 and MeAPSO-34 catalysts: (a) SAPO-34, (b) MnAPSO-34, (c) FeAPSO-34, and (d) NiAPSO-34.
Selectivity for saturated hydrocarbons over the SAPO-34 and MeAPSO-34 catalysts.
Catalytic lifetime of the SAPO-34 and MeAPSO-34 catalysts (catalytic lifetime: the reaction time until the DME conversion drops below 90%).
From the results of DTO reaction, it was observed that the MeAPSO-34 catalysts show long catalytic lifetimes and high selectivities toward light olefins compared with the SAPO-34 catalyst. Among the MeAPSO-34 catalysts, the NiAPSO-34 catalyst demonstrated the longest catalytic lifetime. This result may be explained by the acidity difference between the SAPO-34 catalyst and the MeAPSO-34 catalysts, as observed in the NH3-TPD results. As described above, the difference in the weak acidic sites between the SAPO-34 catalyst and the MeAPSO-34 catalysts is larger than the difference in strong acidic sites. Although the strong acidic sites improve the reactivity, an excess of strong acidic sites induces deactivation due to coke deposition. However, the weak acidic sites are active sites but also delay the coke deposition because they induce mild reaction condition. Therefore, the higher increase in weak acidity resulted in the improvement of catalytic lifetime in the reaction. Consequently, we concluded that metal incorporation is effective in delaying the deactivation of the catalyst during DTO reaction.
The physical properties of MeAPSO-34 catalysts, including the relative crystallinity and surface area, were similar regardless of the incorporated metal (Mn, Fe, or Ni). Specifically, metal incorporation improved the crystallinity of catalyst but barely affected the cubic morphology. The MeAPSO-34 catalysts have a relatively high acidity, and the acidity decreased in the order of NiAPSO-34, FeAPSO-34, and MnAPSO-34 catalyst. The catalytic performance of the SAPO-34 and MeAPSO-34 catalysts in the DTO reaction was experimentally investigated. The DME conversion over all of the catalysts decreased during the reaction, and this result is attributed to the conversion of intermediates, such as PAH, formed in the catalytic pores. The selectivity for the light olefins decreased and that for the hydrocarbons increased with increasing time on stream due to the deactivation by coke deposition. The time needed to maintain the total selectivity above 95% for light olefins of the SAPO-34, MnAPSO-34, FeAPSO-34 and NiAPSO-34 catalysts is 185, 208, 254, and 254 min, respectively. The difference in weak acidic sites between the SAPO-34 catalyst and MeAPSO-34 catalysts is higher than the difference in strong acidic sites, and therefore the catalytic lifetime of MeAPSO-34 catalysts is higher than that of SAPO-34 catalyst. The catalytic lifetimes of the SAPO-34, MnAPSO-34, FeAPSO-34, and NiAPSO-34 catalysts were 79, 86, 118, and 142 min, respectively. It was found that the high increase in weak acidity improves the catalytic lifetimes of the MeAPSO-34 catalysts. Therefore, we concluded that the incorporation of transition metal is effective in improving the catalytic lifetime during the DTO reaction.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the research fund of the Chungnam National University.