Silicoaluminophosphate (SAPO) molecular sieves, first reported by the Union Carbide Corporation in the 1980s, are important microporous materials often used in industrial catalytic processes [1-4]. The incorporation of Si atoms into neutral aluminophosphate frameworks results in a moderate acidity and high thermal and hydrothermal stabilities, which make them suitable for many important applications. For example, SAPO-34 with a CHA structure is an excellent catalyst for the methanol-to-olefin (MTO) reaction [5, 6] and its Cu²⁺ ion-exchanged counterpart shows a superior catalytic performance and stability for the selective catalytic reduction (SCR) of NO_x [7, 8]. Further, Pt/SAPO-11 was used as a catalyst for hydro-isomerization due to its outstanding isomerization activity and lower cracking activity [9, 10].

The impressive properties and consequent important applications of SAPO molecular sieves are mainly controlled by their Si distribution and morphology. Considerable efforts have been put in over the past few decades to develop synthetic strategies for SAPO molecular sieves. Generally, the synthesis of SAPO molecular sieves is influenced by a number of parameters, such as the type of initial sources, solvent, crystallization temperature, and time. The structure-directing agent (SDA) is one of the most important factors; it plays complicated roles in the synthesis process, such as dissolving the raw materials, orienting the structure, balancing the charges, and filling the skeleton space [11]. Consequently, the crystalline phase, morphology, composition, and acidity of the final SAPO products can be modified by template selection. Taking the synthesis of SAPO-34 as an example, low silicon SAPO-34 can only be synthesized using a large organic amine template, such as trimethylamine (TEA) or tetraethylammonium hydroxide (TEAOH), because only one protonated template molecule can be accommodated in one CHA cage to balance the negative charge of the framework. Meanwhile, SAPO-34s synthesized by small organic amines, such as diethylamine (DEA), piperazine (PIP), and morpholine (MOR), have relatively high Si contents due to their small volume and high charge density [12-14]. More interestingly, the SAPO-34 product directed by TEAOH presents a nano-sized morphology, which exhibits an improved MTO catalytic performance. The strong alkalinity of TEAOH may induce the dissolution of the silica source, which promotes nucleation and small-crystal formation [15, 16]. Hence, discovering novel SDAs is an effective way to modify the properties of SAPO molecular sieves.

DNL-6 is an SAPO molecular sieve with an RHO topology (composed of α-cages linked via double 8-rings), which was first reported by our laboratory. It was found accidentally during an investigation on the phase-transformation processes of SAPO molecular sieves [17]. Thereafter, the synthesis of DNL-6 was accomplished by various strategies including surfactant-assisted hydrothermal synthesis [18], dry-gel conversion [19], and aminothermal crystallization [20]. It is noteworthy that DNL-6 possesses strong Brønsted acid sites comparable to those of HZSM-5 or even stronger [21], over which heptamethylbenzenium cations (heptaMB⁺) were stabilized and directly observed for the first time during a real MTO reaction [22]. Moreover, the material can accommodate high concentrations of single Si(OAl)₄ species, indicating huge potential for CO₂/CH₄ and CO₂/N₂ adsorption/separation [19]. Optimized synthesis of DNL-6 is thus highly desired; it may provide opportunities to develop new applications.

Thus far, diethylamine (DEA) is the only known template to synthesize DNL-6. Moreover, the use of cationic surfactants is important for the synthesis of DNL-6. The strong adsorption of cationic surfactant molecules on SAPO species protects DNL-6 from dissolving, and inhibits its transformation to SAPO-34 [23]. In this work, we report a new template, N, N'-dimethylethylenediamine, to synthesize DNL-6 molecular sieves. Notably, a cationic surfactant is not necessary anymore. The synthesized DNL-6s were characterized by various techniques including X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), N₂ adsorption, and solid-state magic-angle spinning nuclear magnetic resonance (solid-state MAS NMR). The sample with an ultrahigh silica content and acidity shows excellent catalytic activity and a remarkable methylamine and dimethylamine selectivity in the methanol amination reaction.

A typical hydrothermal synthesis procedure for DNL-6 is as follows. Orthophosphoric acid (85 wt%) and tetraethyl orthosilicate were mixed with a solution of aluminum isopropoxide and deionized water. After the synthesis solution was stirred for 2 h, the template agent, N, N'-dimethylethylenediamine, was added and it was continually stirred for 4 h. Later, the synthesis gel was transferred into a stainless steel autoclave and heated at 200 ℃ for 12 h under rotation. After equilibrating for a certain time period at this temperature for crystallization, the autoclave was cooled down and the solid product was recovered by filtration, washed three times with deionized water, and dried at 110 ℃ overnight. Finally, the product was calcined at 550 ℃ for 4 h to remove organic species.

Powder XRD patterns were recorded on a PANalytical X'Pert PRO X-ray diffractometer with Cu K_α radiation (λ = 1.54059 Å), operating at 40 kV and 40 mA. SEM images were generated on a Hitachi SU8020 electron microscope at 25 kV. The bulk and surface compositions of samples were determined using a Philips Magix-601 XRF spectrometer and a VG ESCALAB MkII XPS. N₂ adsorption measurements were carried out on a Micromeritics 2010 analyzer at -196 ℃ after the sample was degassed at 350 ℃ under vacuum. All solid-state NMR experiments were performed on a Bruker Avance Ⅲ 600 spectrometer equipped with a 14.1 T wide-bore magnet and a 4 mm MAS probe. The resonance frequencies at this field strength were 150.9, 156.4, 119.2, and 242.9 MHz for ¹³C, ²⁷Al, ²⁹Si, and ³¹P, respectively. ¹³C cross polarization (CP)/MAS NMR spectra were recorded with a contact time of 3 ms and recycle delay of 2 s at a spinning rate of 12 kHz. ²⁷Al MAS NMR spectra were recorded using one pulse sequence at a spinning rate of 12 kHz. A total of 200 scans were accumulated with a π/8 pulse width of 0.75 μs and a 2 s recycle delay. Chemical shifts were referenced to (NH₄)Al(SO₄)₂·12H₂O at -0.4 ppm. ²⁹Si and ³¹P MAS NMR spectra were recorded by high-power proton decoupling. ²⁹Si MAS NMR spectra were recorded with a π/4 pulse width of 2.5 μs and a recycle delay of 10 s at a spinning rate of 8 kHz. Chemical shifts were referenced to kaolinite at -91.5 ppm. ³¹P MAS NMR spectra were recorded with a π/4 pulse width of 2.25 μs and a 10 s recycle delay. Chemical shifts were referenced to 85% H₃PO₄ at 0 ppm. Two-dimensional (2D) ²⁷Al multiple quantum magic angle spinning (MQ-MAS) NMR analysis was performed on a 4 mm H-X WVT probe at a spinning rate of 13 kHz. An rf field of 49.1 kHz was used for the creation (0Q → ±3Q) and first conversion (±3Q → 0Q) pulses. The central transition selective soft 90° pulse used for the last conversion step (0Q → ±1Q) was 3.47 kHz. A shearing transformation was used to achieve 2D Fourier transformation and obtain a pure absorption-mode 2D contour plot. The quadrupolar interaction product, P_Q (P_Q is identical to the ''second order quadrupole effect (SOQE)'' parameter used in various publications), and isotropic chemical shift (δ_iso) values were calculated according to procedures reported previously [24]. The ¹H MAS NMR spectra were recorded using a 4 mm MAS probe. The pulse width was 2.2 μs for a π/4 pulse and 32 scans were accumulated with a 4 s recycle delay. The samples were spun at 12 kHz and chemical shifts were referenced to adamantine at 1.74 ppm. Thermogravimetric analyses (TG-DTG) were performed on a TA Q600 analyzer. The sample was heated at a rate of 10 ℃/min to 900 ℃. Temperature-programmed desorption by NH₃ (NH₃-TPD) experiments were conducted on an automated characterization system (AutoChem 2920, Micromeritics) with a TCD detector. The calcined sample (200 mg) was compressed into pellets (40-60 mesh) and packed into a U-tube reactor. The packed column was initially purged with He flow and outgassed at 600 ℃. After pretreatment, the sample was cooled to 100 ℃ and saturated with ammonia. Physically adsorbed ammonia was swept by purging with He at a flow rate of 40 mL/min for 30 min. Finally, the packed bed was heated at a rate of 10 ℃/min to 600 ℃ under He flow (20 mL/min).

Methanol amination experiments were performed in a continuous flow fixed-bed reactor at atmospheric pressure. A 0.3 g portion of the calcined sample (40-60 mesh) was loaded into the reactor and pretreated in He (40 mL/min) at 500 ℃ for 1 h prior to the reaction. The temperature of the catalyst bed was cooled to reaction temperature (260 or 300 ℃) in He. A NH₃ flow of 6.2 mL/min was fed into the reactor by a mass-flow controller. Methanol was introduced into the system by passing He (47 mL/min) through a methanol-filled saturator (10 ℃), which resulted in a weight hourly space velocity (WHSV) of 0.813 h⁻¹ and methanol/ammonia = 0.5 (mole). The products were analyzed on-line using an Agilent 6890 gas chromatograph with a flame ionization detector (FID) using a CP-Volamine column.

The detailed synthesis conditions including gel composition, crystallization time, product crystalline phases, and composition are listed in Table 1. The corresponding XRD patterns of these products are shown in Fig. 1. When TEOS is absent (SP-D1), AlPO₄-21 with an AWO topology is the final product. The AWO phase decreases and RHO phase increases with an increase in the amount of TEOS. Pure DNL-6 molecular sieves cannot be synthesized unless the molar ratio of TEOS/Al₂O₃ is 1. Further, an amorphous phase appears along with DNL-6 when the TEOS/Al₂O₃ molar ratio is greater than 1.7. Pure-phase DNL-6s synthesized at TEOS/Al₂O₃ ratios of 1.0, 1.2, and 1.7 were denoted as L-DNL-6, M-DNL-6, and H-DNL-6, respectively. Their Si contents range from 22.4% to 36.4%. It should be noted that SAPO molecular sieves with such high Si contents are rarely reported. Upon further prolonging the crystallization time, the RHO phase gradually converts to a dense GIS phase (68 h for SP-D4 and 143 h for SP-D5), indicating the metastable characteristics of DNL-6. Meanwhile, the Si content of solid products is higher than that of M-DNL-6.

All pure DNL-6 samples are present in the form of micron-sized particles, as shown in Fig. 2. The product morphology changes with an increase in the Si content; L-DNL-6 is sphere-like (Fig. 2(a) and (b)) and M-DNL-6 exhibits a truncated rhombic dodecahedron morphology (Fig. 2(c) and (d)). H-DNL-6 exhibits a spherical morphology with an average particle size of about 4 μm, whose outer layer consists of small cubic crystals of 50 nm size (Fig. 2(e) and (f)).

The textural properties of the obtained DNL-6 samples were characterized by N₂ adsorption and are shown in Fig. 3 and Table 2. The samples exhibit Type Ⅰ adsorption-desorption isotherms with little mesopore volume, indicating that the inner regions of the particles are solid. All DNL-6 samples exhibit large Brunauer-Emmett-Teller (BET) micropore surface area and volume, which verifies their high crystallinity. In particular, owing to the rough surfaces of the particles, the external surface area of H-DNL-6 is larger than that of its analogs.

The chemical environments of the as-synthesized DNL-6s were analyzed by solid-state MAS NMR, as shown in Fig. 4. The ¹³C/CP MAS NMR spectra (Fig. 4(a)) show two strong resonance peaks at 44.1 and 32.9 ppm, corresponding to the methylene and methyl groups of N, N'-dimethylethylenediamine template, respectively. The two weak split signals at about 55 ppm are caused by strong interactions between the methylene groups of templates and the molecular sieve framework [25, 26]. These results verify the intact status of the organic amine in the synthesized DNL-6 crystals. Fig. 4(b) displays the ²⁷Al NMR spectra of the samples, all of which show a broad peak at 39 ppm; it can be assigned to the tetrahedrally coordinated Al species of the framework. The ³¹P NMR spectra (Fig. 4(c)) show a peak at -29 ppm attributable to the tetrahedral PO₄ group. The ²⁹Si NMR spectra become complicated with increasing Si content in DNL-6, as shown in Fig. 4(d). L-DNL-6 shows a strong signal at -92 ppm and a very weak peak at -95 ppm indicating that Si(4Al) is the main environment for L-DNL-6. Meanwhile, M-DNL-6 shows quadruple signals at -92, -95, -99, and -104 ppm, indicating the appearance of a large number of Si(3Al), Si(2Al) and Si(1Al) species. Regarding H-DNL-6, a quintuplet of signals appears at -92, -95, -99, -104, and -109 ppm. The signals at -95, -99, and -104 ppm are as strong as the signal at -92 ppm, suggesting that the amount of Si(3Al), Si(2Al), and Si(1Al) increased. In addition, the chemical shift at -109 ppm indicates the existence of Si islands. The surface compositions of DNL-6 products were evaluated by XPS, as shown in Table 1. As expected, all the DNL-6 samples contain high Si contents on the crystal surface [27]. As the degree of Si enrichment increases, the surface P and Al contents show a corresponding decrease. Combining the XPS results and ²⁹Si NMR spectra, it is believed that the outer surfaces of DNL-6 crystals have more complex Si environments (more Si islands) than the inner parts.

In order to further understand the atomic chemical environment of H-DNL-6, ²⁷Al MQ-MAS NMR analysis was conducted on calcined H-DNL-6 because Al atoms are the bridges for Si and P atoms. As ²⁷Al is a quadrupolar nucleus, the single-pulse MAS NMR spectral line shape is often markedly broadened, owing to residual second-order quadrupolar interactions. ²⁷Al MQ-MAS NMR can effectively eliminate the second-order quadrupolar interactions experienced by half-integer quadrupolar nuclei and thus narrow down asymmetric line shapes considerably [28]. The ²⁷Al MQ-MAS NMR spectrum of calcined H-DNL-6 is shown in Fig. 5. The indirect dimension (F1) displays five peaks at 34, 42, 48, 59, and 67 ppm, which are preliminarily attributed to Al(OP)₄ ( IV_a), Al(OSi)_n(OP)_(4−n) (n = 1, 2, and 3) ( IV_b and IV_c), and Al(OSi)₄ ( IV_d and IV_e), respectively [21, 29]. The peak at 42 ppm is the strongest, showing that Al(OSi)_n(OP)_(4−n) (n = 1, 2, and 3) corresponding to Si atoms at the borders of Si islands, is a dominant chemical environment in H-DNL-6. On the basis of these results, the distributions of Si concentration and Si environments in the H-DNL-6 crystals are proposed and illustrated in Fig. 6. Si content increases gradually from the inner part to the exterior part of the crystals. The shell region of the crystals is enriched with large Si islands and the region adjacent to the shell contains a number of relatively small Si islands, both of which provide Si(nAl) (n = 1, 2, and 3) environments. The interior of H-DNL-6 crystals, which has a relatively low Si content, is enriched with Si(4Al) species.

Thermal analysis was carried out to investigate the content and existing status of the N, N'-dimethylethylenediamine template. The TG curves are shown in Fig. 7. The first weight loss stage of ca. 5 wt % from room temperature to 200 ℃ corresponds to the desorption of physically adsorbed water. The second weight loss of around 20 wt % between 200 and 800 ℃ could be attributed to the decomposition of the template molecules occluded within the crystals. The detailed weight loss data of the three DNL-6 samples are listed in Table 3. The number of N, N'-dimethylethylenediamine molecules per α cage is calculated to be about 4.4, very similar to that of DNL-6 directed by DEA (4.28 DEA per α cage) [18]. However, N, N'-dimethylethylenediamine has an obviously higher charge density than DEA due to its low C/N ratio (2:1 vs. 4:1). This means that it is possible for one N, N'-dimethylethylenediamine molecule to carry double positive charges, which can balance more negative charges induced by greater Si incorporation. This result is consistent with the high Si concentration of DNL-6 samples.

Methylamines, especially methylamine (MMA) and dimethylamine (DMA), are important industrial chemicals for producing pesticides, pharmaceuticals, surfactants, and solvents. Up to now, almost all methylamines are being produced by the reaction of methanol and ammonia on an amorphous silica-alumina-based catalyst [31]. The synthesis route of methylamines from methanol and ammonia is illustrated in Scheme 1. It is mentioned that dimethyl ether, obtained by the dehydration of methanol, can undergo the same reaction as methanol with NH₃, MMA, and DMA to produce methylamines [31].

One marked drawback of current industrial catalysts is that the product distribution of the reaction obeys a thermodynamic equilibrium, producing trimethylamine (TMA) predominantly instead of MMA and DMA. Thus, small-pore molecular sieve catalysts, such as RUB-1 [32], RHO, SSZ-16, and SAPO-34 [33, 34], have been investigated in order to optimize reaction selectivity and meet commercial demands. The lack of strong acid sites is one of the main drawbacks hindering the application of SAPO materials as amination catalysts, resulting in a lower methanol conversion and much higher selectivity for dimethyl ether than the corresponding aluminosilicate analogs [33]. According to literature, DNL-6 possesses very strong Brønsted acid sites, similar to the aluminosilicate HZSM-5. Moreover, the pore dimensions of DNL-6 (3.6 × 3.6 Å) are smaller than the shortest dimension of TMA (3.9 Å), which may help to suppress the formation of bulk TMA.

The catalytic performance of DNL-6 samples was evaluated in the methanol amination reaction at reaction temperatures of 260 and 300 ℃, methanol WHSV of 0.813 h⁻¹, and n(ammonia):n(methanol) = 2:1. The steady-state catalytic results are illustrated in Table 4. The main products are MMA and DMA with all DNL-6 catalysts. Increasing the reaction temperature from 260 to 300 ℃ could greatly improve the catalytic activity. However, the selectivity for MMA decreased obviously accompanied by an increase in DMA and TMA, due to an enhanced methylation reaction at elevated temperatures. H-DNL-6, with its ultrahigh silica content, induces the highest methanol conversion and selectivity for MMA plus DMA. Fig. 8 illustrates the changes in methanol conversion and product selectivity with time on stream for H-DNL-6 at 300 ℃. It is clear that the reaction is stable during the investigated period; further, methanol conversion is higher than 88% and selectivity for MMA plus DMA is greater than 85%.

Considering that acidity plays an important role in the methanol amination reaction [33], NH₃-TPD was conducted to investigate the acidity of DNL-6 samples in order to better understand the catalytic results. As shown in Fig. 9, there are two desorption peaks corresponding to NH₃ desorbed from weak and strong acid sites. All the samples exhibit similar acid strength, despite the different acid concentrations. A gradual drop in strong acid sites can be observed following an increase in Si concentration in the samples. Moreover, H-DNL-6 possesses a small amount of super-strong acid sites (the corresponding desorption temperature is higher than 550 ℃), which are absent in L-DNL-6 and M-DNL-6. Because the acidity of different Si environments in SAPO molecular sieves follows the order of Si(1Al) > Si(2Al) > Si(3Al) > Si(4Al), it is speculated that the super-strong acid sites are located at the borders of larger Si islands in H-DNL-6 [6]. These acid characterization results suggest that acid strength is more important than acid density for the methanol amination reaction activity. Furthermore, the Brønsted acid density of H-DNL-6 is determined by ¹H MAS NMR and a value of 0.69 mmol/g is obtained. This value amounts to 67% of the total acid concentration (1.03 mmol/g), as determined by NH₃-TPD (NH₃ desorption between 325-600 ℃), implying the existence of Lewis acid sites with strong acidity in H-DNL-6. This result is indeed consistent with our previous work on the acidity of DNL-6 with a composition of Si_0.182Al_0.490P_0.328O₂ [21]. The generation of Lewis acid sites in the structure is a result of the dehydroxylation of adjacent Brønsted acid sites during the calcination process. Herein, considering that amorphous acidic oxides (Al₂O₃ or SiO₂-Al₂O₃) can act as effective catalysts in the industrial methanol amination process, we speculate that the Lewis acid sites in DNL-6 may also contribute to the reaction. For comparison, the results of methanol amination reaction on high-Si SAPO-34 from our previous report [34] are also listed in Table 4. High-Si SAPO-34 contains a larger number of acid sites, but has a lower acid strength than H-DNL-6; the position of the high-temperature NH₃ desorption peak in the NH₃-TPD profile of SAPO-34 was located at 460 ℃, whereas it was located at 492 ℃ for H-DNL-6. From Table 4, it can be understood that a comparable methanol conversion (82%) can only be achieved at a high reaction temperature of 380 ℃ when SAPO-34 was used as the catalyst. This confirms the effect of acid strength on the methanol amination reaction activity.

H-DNL-6 exhibits a high selectivity for MMA plus DMA. This may be related to the extremely high Si enrichment at the outer shell of H-DNL-6 crystals, which causes a relatively low density of strong acid sites. Therefore, for the product molecules, the probability of consecutive reaction with methanol is reduced when they diffuse out of the inner region to reach the outer surface of the crystals.

DNL-6 was hydrothermally synthesized using N, N'-dimethylethylenediamine as a novel structure-directing agent. It is found that the crystalline region of the resultant DNL-6 products is wider than previously reported. About 4.4 amines are confined in one α-cage, which implies that more than 8 positive charges can exist simultaneously in one cage to balance the negative charge framework caused by Si incorporation; this explains the relatively high Si content of DNL-6. DNL-6 with an ultrahigh silica content (36.4% Si per mole) shows a remarkable catalytic performance for the methanol amination reaction with a high methanol conversion (> 88%) and good selectivity for MMA plus DMA (> 85%), implying that DNL-6 is a promising candidate for the synthesis of methylamines.