CN103663489B - A kind of SAPO-44 molecular sieve and synthetic method thereof - Google Patents

Abstract

The present invention relates to a kind of SAPO-44 molecular sieve and synthetic method thereof.It is characterized in that comprising template hexamethylene imine in this microporous molecular sieve, the slight Silicon-rich in molecular sieve crystal surface, the ratio of outside surface silicone content and the body phase silicone content of crystal is 1.50 ~ 1.01.This molecular sieve can be used as the catalyzer of acid catalyzed reaction and oxygen-containing compound conversion to produce olefine reaction after roasting in 400 ~ 700 DEG C of air.

Description

A kind of SAPO-44 molecular sieve and synthetic method thereof

technical field

The invention belongs to the field of SAPO molecular sieves, in particular to a SAPO-44 molecular sieve and a synthesis method thereof.

Background technique

In 1984, United Carbide Corporation (UCC) developed the SAPO molecular sieve series of silicoaluminophosphate (USP4440871). The molecular sieve is a kind of crystalline silicoaluminophosphate, and its three-dimensional skeleton structure is composed of PO ₂ ⁺ , A1O ₂ ^- and SiO ₂ tetrahedra. Among them, SAPO-34 is a chabazite-like structure, the main channel is composed of eight rings, and the orifice is 0.38nm×0.38nm. Due to its suitable acidity and pore structure, SAPO-34 molecular sieve has attracted much attention because of its excellent catalytic performance in the reaction of methanol to light olefins (MTO).

SAPO-34 molecular sieves are generally synthesized by hydrothermal method, using water as solvent, in a closed autoclave. Synthesis components include aluminum source, silicon source, phosphorus source, templating agent and deionized water. Silica sol, activated silica and orthosilicate can be used as the silicon source. The aluminum source includes activated alumina, pseudoboehmite and aluminum alkoxide. The ideal silicon and aluminum sources are silica sol and pseudoboehmite. Bauxite; Phosphorus source generally adopts 85% phosphoric acid. Common templates include tetraethylammonium hydroxide (TEAOH), morpholine (MOR), piperidine (Piperidine), isopropylamine (i-PrNH2), triethylamine (TEA), diethylamine (DEA), di Propylamine etc. and their mixtures. SAPO-44 and SAPO-34 have a similar skeleton structure (CHA), but there are some differences in the XRD spectra between the two. SAPO-44 is usually synthesized using cyclohexylamine as a template.

Hexamethyleneimine (HMI) is generally used as a structure-directing agent for the synthesis of SAPO-35 molecular sieves in the synthesis of SAPO molecular sieves. Chinese patent 200710175273.X reported the synthesis of SAPO-35 using HMI as a template. The initial synthesis mixture needs to be gelled at 35-100°C, and the synthesis ratio is (0.5-1.8)R:(0.05-2)SiO ₂ :1Al ₂ O ₃ :(0.5-1.5)P ₂ O ₅ :(10-150 )H ₂ O, crystallized at 150-210°C for 0.5-500h. SAPO-35 molecular sieve belongs to the LEV structure, which is formed by stacking double six-membered rings in the order of AABCCCABBC. The CHA structure is formed by stacking double six-membered rings in AABBCC order. It can be seen that there is a big difference in the structure between the two. Generally, the synthesis of SAPO molecular sieves requires organic amine/ammonium as a structure-directing agent. An organic amine can synthesize molecular sieves with various structures under different conditions. Similarly, a molecular sieve can be synthesized using a variety of different organic amines. But until now, the relationship between the structure of organic amines and the structures of the resulting molecular sieves is not very clear. Although many researchers have done a lot of research and attempts in this area, and have made some progress, it is still very difficult to predict the structure of the structure-directing agent and the molecular sieve generated by it. The organic amines required for the synthesis of most molecular sieves have been discovered through experiments.

In the synthesis of SAPO molecular sieves, many researchers have reported that the synthesized molecular sieves have the characteristics of silicon-rich surface. This is mainly due to the fact that the initial gel system of SAPO molecular sieves is generally acidic or nearly neutral. As the crystallization proceeds, phosphoric acid is gradually consumed (crystallization to form molecular sieves), resulting in an increase in the pH value of the synthesis system. The silicon source usually exists in the form of polymerization in the initial stage of crystallization. Because of its low isoelectric point, as the pH value of the synthesis system increases, silicon oxide will gradually depolymerize to form oligomeric silicon species, so that silicon can participate in The proportion of the SAPO molecular sieve skeleton is increased, resulting in the silicon-rich phenomenon on the surface of the molecular sieve grains. For example, in our previous research on the synthesis of SAPO-34 using diethylamine, we found that silicon is unevenly distributed in the SAPO-34 molecular sieve crystal, and its content increases from the core to the shell, and the silicon content on the outer surface (molar ratio Si/(Si+ The ratio of Al+P)) to the bulk silicon content of the crystal is 1.41 (Microporous and Mesoporous Materials, 2008, 114(1-3): 4163). In the study of SAPO-44 by Akolekar et al., it was found that the ratio of surface silicon content to bulk silicon content was as high as 6-10. (Colloids and Surfaces A: Physicochemical and Engineering Aspects 146 (1999) 375-386). Generally speaking, SAPO molecular sieves generally show the characteristics of silicon-rich crystal grain surface, but it is worth pointing out that even for the same SAPO molecular sieve, its surface element composition and bulk phase composition will vary with the synthesis conditions and the template used. There is a big difference in the change.

Usually, as the silicon content in SAPO molecular sieves increases, the coordination environment of silicon will also transition from the initial simple Si(4Al) to the coexistence of Si(nAl) (n=0-4) in a variety of silicon environments (the skeleton of different SAPO molecular sieves The maximum single-silicon dispersion allowed to exist is different, see J.Phys.Chem., 1994, 98, 9614). Changes in the coordination environment of silicon lead to greater changes in its acid concentration and acid strength, and the acid strength has the following order Si(Al)>Si(2Al)>Si(3Al)>Si(4Al). On the other hand, with the appearance of silicon islands in the SAPO molecular sieve framework, the number of acid centers generated per silicon atom decreases (1 for Si(4Al), less than 1 for multiple silicon environments), that is, the acid density reduce. It can be imagined that SAPO molecular sieve as an acid catalyst, if the distribution of silicon in the molecular sieve grains is uneven, its acid properties will also be uneven, which will inevitably have an important impact on the catalytic performance of the molecular sieve. If the surface of molecular sieve grains is rich in silicon, it means that the silicon coordination environment near the outer shell of the grains is relatively more complex than that inside. Weckhuysen et al. have reported that in the methanol-to-olefins reaction (MTO), the reaction first proceeds near the outer surface of the SAPO-34 grains. As the reaction progresses, larger carbon deposits gradually form and block the pores, making the interior of the grains The difficulty of product diffusion increases (Chemistry-A European Journal, 2008, 14, 11320-11327; J. Catal., 2009, 264, 77-87). This also shows that the acidic environment on the outer surface of molecular sieve grains is particularly important for catalytic reactions. It is of great significance to find a method to effectively control the silicon-rich degree on the surface of molecular sieves.

Contents of the invention

The object of the present invention is to provide a SAPO-44 molecular sieve, the anhydrous chemical composition of the molecular sieve can be expressed as: _{mSDA ·} ( _SixAlyPz ) _O2 , wherein: SDA is hexamethyleneimine; m represents each Mole (SixAlyPz) _O2 corresponds to the number of moles of organic amine, m=0.1～0.5; _x , _y , _z represent the mole fractions of Si, Al and P respectively, and the ranges are x=0.01～0.60 respectively, y=0.2˜0.60, z=0.2˜0.60, and x+y+z=1. The crystal surface of the molecular sieve is slightly rich in silicon, and the ratio of the silicon content on the outer surface (Si/(Si+Al+P) molar ratio) to the bulk silicon content of the crystal is 1.50-1.01, preferably 1.40-1.02, more preferably 1.35-1.03, More preferably, it is 1.30 to 1.03. The increasing content of silicon in SAPO-44 molecular sieve crystals from core to shell can be uniform or non-uniform.

Another object of the present invention is to provide a synthetic method of SAPO-44 molecular sieve.

Another object of the present invention is to provide an acid-catalyzed reaction catalyst or an oxygen-containing compound conversion-to-olefin reaction catalyst prepared by synthesizing SAPO-44 molecular sieve by the above method.

Another object of the present invention is to provide a SAPO-44 molecular sieve synthesized by the above method and a gas adsorbent prepared therefrom.

The technical problem to be solved in the present invention is to directly use hexamethyleneimine (hereinafter referred to as HMI) as a structure-directing agent, and use the phosphorus source, silicon source and aluminum source used in the synthesis of conventional molecular sieves as raw materials to synthesize under hydrothermal conditions. Pure-phase SAPO-44 molecular sieve, and the crystal surface of the synthesized molecular sieve is slightly rich in silicon, and the ratio of the silicon content on the outer surface (Si/(Si+Al+P) molar ratio) to the bulk silicon content of the crystal is 1.45-1.01. The present inventors found through experiments that by adding a small amount of surfactant to the synthesis system and adopting the temperature-variable crystallization method, the surface silicon-rich degree of the synthesized SAPO-44 molecular sieve can be effectively reduced.

The present invention is characterized in that the preparation process is as follows:

a) Mix silicon source, aluminum source, phosphorus source, deionized water, surfactant and SDA to form an initial gel mixture with the following molar ratio:

SiO ₂ /Al ₂ O ₃ =0.01~1;

P ₂ O ₅ /Al ₂ O ₃ ＝0.5～1.5;

_H2O / _Al2O3 =30～130 _;

SDA/Al ₂ O ₃ =2.0～6;

BM/Al ₂ O ₃ =0.01～0.10;

Wherein SDA is hexamethyleneimine, and BM is a surfactant;

b) Put the initial gel mixture obtained in step a) into the synthesis kettle, seal it, raise the temperature to 190-230° C., and crystallize under autogenous pressure for 1-15 hours;

c) reduce the crystallization temperature to 160-180° C. and crystallize under autogenous pressure for 1-15 hours;

d) After the crystallization is complete, the solid product is centrifuged, washed with deionized water until neutral, and dried to obtain SAPO-44 molecular sieve.

Said step a) the silicon source in the initial gel mixture is one or any mixture of silica sol, activated silica, orthosilicate, metakaolin; the aluminum source is aluminum salt, activated alumina, One or any mixture of aluminum alkoxide and metakaolin; the phosphorus source is one or any mixture of orthophosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, organic phosphide or phosphorus oxide The surfactant is dodecyltrimethylammonium chloride, tridecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, pentadecyltrimethylammonium chloride, tendecyltrimethylammonium chloride, Hexaalkyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tridecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, pentadecyltrimethylammonium bromide One or any mixture of ammonium bromide and cetyltrimethylammonium bromide.

The molar ratio of SDA to Al ₂ O ₃ in the initial gel mixture in step a) is SDA/Al ₂ O ₃ =2.5-5.0, more preferably SDA/Al ₂ O ₃ =3.0-4.5.

The molar ratio of H ₂ O and Al ₂ O ₃ in the initial gel mixture in step a) is H ₂ O/Al ₂ O ₃ =35-100.

The molar ratio of BM to Al ₂ O ₃ in the initial gel mixture in step a) is BM/Al ₂ O ₃ =0.03˜0.08.

The crystallization temperature in the step b) is 195-225° C., and the crystallization time is 1-12 hours. The preferred crystallization temperature is 211-225° C., and the crystallization time is 1-10 hours.

The crystallization temperature in the step c) is 165-175° C., and the crystallization time is 3-12 hours.

The present invention also relates to a catalyst for acid-catalyzed reaction, which is obtained by calcining the above-mentioned SAPO-44 molecular sieve or the SAPO-44 molecular sieve synthesized according to the above-mentioned method in air at 400-700°C.

The present invention also relates to a catalyst for converting oxygen-containing compounds into olefins, which is obtained by calcining the above-mentioned SAPO-44 molecular sieve or the SAPO-44 molecular sieve synthesized according to the above-mentioned method in air at 400-700°C.

The present invention also relates to a gas adsorbent, which is obtained by calcining the above-mentioned SAPO-44 molecular sieve or the SAPO-44 molecular sieve synthesized according to the above-mentioned method in air at 400-700°C.

The beneficial effects that the present invention can produce include:

(1) Obtain a SAPO-44 molecular sieve using hexamethyleneimine as a template, and has the characteristics of slightly rich silicon on the grain surface, and the silicon content on the outer surface (molar ratio Si/(Si+Al+P)) The ratio of the bulk silicon content to that of the crystal is 1.50 to 1.01.

(2) The prepared SAPO-44 molecular sieve showed excellent catalytic performance and gas adsorption and separation performance in the catalytic reaction.

Description of drawings

Fig. 1 is a scanning electron micrograph (SEM) of a product synthesized in Example 1 of the present invention.

Detailed ways

Bulk phase element composition was determined by PANalyticalX’PertPROX-ray diffractometer (XRF), Cu target, Kα radiation source (λ=0.15418nm), voltage 40KV, current 100mA.

Surface element composition XPS was measured by X-ray photoelectron spectrometer ThermoESCALAB250Xi (monochromatic AlKα was used as the excitation source), and Al2p=74.7eV of Al2O3 on the sample surface was used as the internal standard to correct the charge on the sample surface.

The present invention is described in detail below by examples, but the present invention is not limited to these examples.

Example 1

The molar proportions and crystallization conditions of each raw material are shown in Table 1. The specific batching process is as follows, mix _16.4g of phosphoric acid (85% by mass _of H3PO4) with 30g of deionized water, stir evenly, then add 5.7g of silica sol (30% by mass of _SiO2 ), and stir vigorously 1h. 21.5 g of hexamethyleneimine HMI (99% by mass) was added to the previous mixture, sealed and stirred for 30 min to obtain a homogeneous mixture, which was marked as A. In addition, 10g of pseudo _- boehmite ( _Al2O3 mass percentage 72.5%), 1.29g of cetyltrimethylammonium bromide (CTAB) and 20.9g of deionized water were mixed and stirred, and added to the mixture A After stirring for 30 minutes in an airtight container to make it evenly mixed, the gel was transferred to a stainless steel reaction kettle. The molar ratio of each component in the synthesis system is 3.0HMI:0.4SiO ₂ :1Al ₂ O ₃ :1P ₂ O ₅ :0.05CTAB:50H ₂ O.

Raise the temperature of the synthesis kettle to 230°C for dynamic crystallization for 2 hours, then lower the temperature to 170°C for 10 hours for crystallization. After the crystallization, the solid product was centrifuged, washed, and dried in air at 100°C to obtain the original powder. The samples were analyzed by XRD, and the results showed that the synthesized product had the characteristics of CHA structure, and the XRD data are shown in Table 2. The XRD results of Examples 2-10 are close to Example 1, that is, the peak positions are the same, and the relative peak intensity of each peak varies slightly with the synthesis conditions and the feed ratio, fluctuating in the range of ± 10%, indicating that the synthetic product is SAPO-44 Molecular sieve. The SEM results of the samples are shown in Figure 1.

The surface and bulk phase element compositions of the molecular sieve products were analyzed by XPS and XRF, and the ratio of the silicon content on the outer surface to the bulk phase silicon content is listed in Table 1. The bulk phase element of the sample in Example 1 is Al _0.49 P _0.40 Si _0.11 .

CHN elemental analysis was carried out on the raw powder sample of Example 1, which showed that the C/N molar ratio was 6.05. The CHN elemental analysis results were normalized with the inorganic element composition determined by XRF, and the composition of the molecular sieve raw powder was 0.17HMI·(Si _0.11 Al _0.49 P _0.40 )O ₂ .

The ¹³ CMASNMR analysis of the original powder sample only found the carbon resonance peak belonging to HMI, but no characteristic carbon resonance peak belonging to CTAB was observed. These results indicated that CTAB did not enter into the final synthetic product.

Table 1 Molecular sieve synthesis ingredients and crystallization conditions table*

*: The aluminum source is pseudoboehmite (72.5% by mass of Al ₂ O ₃ ), the source of phosphorus is phosphoric acid (85% by mass of H ₃ PO ₄ ), the source of silicon is silica sol (100% by mass of SiO ₂ content of 30%); a: the aluminum source is γ _- alumina, and the mass percentage of _Al2O3 is 93%; b: tetraethoxysilane is the silicon source; c: the aluminum source is aluminum isopropoxide; d: Silicon source is fuming silica (SiO ₂ mass percentage content 93%); e: BM is surfactant, and the BM of embodiment 1 to 5 is CTAB, and the BM of embodiment 6-9 is respectively dodecamethyl Trimethylammonium Chloride, Trimethyltrimethylammonium Bromide, Tetradecyltrimethylammonium Chloride, Pentamethyltrimethylammonium Chloride, and Hexadecylmethyltrimethylammonium Chloride , the BM of Example 10 is a mixture of dodecamethyltrimethylammonium bromide and CTAB, and the molar ratio is 1/1.

The XRD result of table 2 embodiment 1 sample

Example 2-10

The specific batching ratio and crystallization conditions are shown in Table 1, and the specific batching process is the same as in Example 1.

The synthetic sample is analyzed by XRD, and the result shows that the product synthesized in Example 2-10 has the structural characteristics of SAPO-44, and the XRD data result is close to Table 2, that is, the peak position and shape are the same, and the relative peak intensity is within ± fluctuate within 10%.

The surface and bulk phase element compositions of the molecular sieve products were analyzed by XPS and XRF, and the ratio of the silicon content on the outer surface to the bulk phase silicon content is listed in Table 1. The bulk element of the sample in Example 10 is Al _0.50 P _0.42 Si _0.08 .

The CHN elemental analysis of the raw powder samples of Examples 2-10 showed that the C/N molar ratio fluctuated at 6.0±0.05. The CHN elemental analysis results were normalized with the inorganic element composition determined by XRF, and the composition of the molecular sieve raw powder was 0.12HMI·(Si _0.07 Al _0.49 P _0.43 )O ₂ , 0.13HMI·(Si _0.10 Al _0.55 P _0.35 ) O ₂ , 0.14HMI·(Si _0.05 Al _0.53 P _0.42 )O ₂ , 0.15HMI·(Si _0.14 Al _0.47 P _0.39 )O ₂ , 0.16HMI·(Si _0.21 Al _0.40 P _0.39 )O ₂ , 0.20HMI·(Si _0.28 Al _0.38 P _0.34 )O ₂ , 0.18HMI·(Si _0.10 Al _0.48 P _0.42 )O ₂ , 0.16HMI·(Si _0.11 Al _0.49 P _0.40 )O ₂ , 0.17HMI·(Si _0.08 Al _0.50 P _0.42 )O ₂ .

The raw powder samples of Examples 2-10 were analyzed by ¹³ CMASNMR, and only the carbon resonance peak belonging to HMI was found, but no characteristic carbon resonance peak belonging to CTAB was observed. These results indicated that CTAB did not enter into the final synthetic product.

Example 11

Take 3g of the synthetic samples of Examples 1-10, put them into a plastic beaker, add 3ml of 40% hydrofluoric acid solution to dissolve the molecular sieve skeleton under ice-water bath conditions, and then add 15ml of carbon tetrachloride to dissolve the organic matter therein. The composition of the organic matter was analyzed by GC-MS to show that the organic matter contained therein was hexamethyleneimine.

Example 12

The synthetic sample of Example 1 was taken, the epoxy resin was cured, and then polished on a polishing machine. Using the line scan mode of SEM-EDX, the crystal plane close to the crystal core was selected for composition analysis from the core to the shell. The results show that the atomic ratio of Si/Al in the core region of the crystal is about 0.19, and the atomic ratio of Si/Al in the region near the surface is about 0.28.

Get the synthetic sample of embodiment 10 (SEM shows as rhombohedral morphology, grain size 1-5 μm), epoxy resin is solidified, polishes on the polishing machine then, utilizes the line scan mode of SEM-EDX, selects the near crystal core The compositional analysis from the core to the shell was carried out on the crystal plane. The results show that the atomic ratio of Si/Al in the core region of the crystal is about 0.14, and the atomic ratio of Si/Al near the surface is about 0.21.

Comparative example 1

Concrete batching ratio and batching process are with embodiment 10. The crystallization condition was changed to 215°C for 13h.

The synthesized sample was analyzed by XRD, and the result showed that it was close to Table 2, that is, the peak position and peak shape were the same, indicating that the synthesized product had the characteristics of SAPO-44 structure. The relative crystallinity of the sample is 89% compared with the sample of Example 1 (the crystallinity of the sample of Example 1 is defined as 100%).

Relative crystallinity=(I ₁ +I ₂ +I ₃ )*100%/(I ₁ '+I ₂ '+I ₃ ') (I ₁ , I ₂ and I ₃ are the most in the XRD spectrum of the sample of Comparative Example 1 Strong three diffraction peak heights, I ₁ ', I ₂ ' and I ₃ ' are the strongest three diffraction peak heights in the sample XRD spectrogram of Example 1.)

XPS and XRF were used to analyze the surface and bulk element composition of the molecular sieve product, and the bulk element of the sample in Comparative Example 1 was Al _0.50 P _0.40 Si _0.10 . The ratio of the silicon content on the outer surface to the silicon content in the bulk phase is Si _{outer surface} /Si _{bulk phase} =2.5.

Comparative example 2

The specific batching ratio and batching process are the same as in Example 10, and the addition of surfactant is omitted.

The synthesized sample was analyzed by XRD, and the result showed that it was close to Table 2, that is, the peak position and peak shape were the same, indicating that the synthesized product had the characteristics of SAPO-44 structure. The relative crystallinity of the sample is 95% compared with the sample of Example 1 (the crystallinity of the sample of Example 1 is defined as 100%).

XPS and XRF were used to analyze the surface and bulk phase element composition of the molecular sieve product, and the ratio of the silicon content on the outer surface to the silicon content in the bulk phase was Si _{outer surface} /Si _{bulk phase} =2.0.

Comparative example 3

The specific batching ratio and batching process are the same as in Example 10, except that the addition of surfactant is omitted, and the crystallization process changes to crystallization at 215° C. for 5 hours.

The synthesized sample was analyzed by XRD, and the result showed that it was close to Table 2, that is, the peak position and peak shape were the same, indicating that the synthesized product had the characteristics of SAPO-44 structure. The relative crystallinity of the sample is 75% compared with the sample of Example 1 (the crystallinity of the sample of Example 1 is defined as 100%).

XPS and XRF were used to analyze the surface and bulk phase element composition of the molecular sieve product, and the ratio of the silicon content on the outer surface to the silicon content in the bulk phase was Si _{outer surface} /Si _{bulk phase} =1.8.

Example 13

The sample obtained in Example 9 was used as a propylene adsorbent. The adsorption isotherm of the sample was determined on the ASAP2020 of Micromeritics, USA. Adsorbed gases were propylene (99.99%), and propane (99.99%). In order to avoid the influence of the physically adsorbed water in the molecular sieve on the adsorption test, the sample was roasted in air at 600°C for 4 hours before the isotherm test, and then further processed in ASAP2020 under the conditions of extremely low vacuum Temperature (5×10-3mmHg), the temperature was raised to 350°C at a rate of 1°C/min and kept for 8 hours. Use a constant temperature water bath (accuracy: plus or minus 0.05°C) to control the gas adsorption temperature, and the adsorption temperature is 298K. The results showed that the adsorption capacity of the sample to propylene and propane were 2.0 and 1.0 mmol/g (when the pressure was 101 kPa). The adsorption selectivity calculated by this is propylene/propane=2.0.

The sample after the adsorption experiment was vacuumized on the ASAP2020 device at room temperature for 30 minutes, and then the adsorption isotherm was measured again. The adsorption capacity of the sample to propylene and propane were 2.05 and 1.1 mmol/g (at a pressure of 101 kPa), respectively. It shows that the sample has good regeneration performance and can be regenerated under very mild conditions.

Example 14

The samples obtained in Example 10 and Comparative Example 1 were calcined at 600° C. for 4 hours, and then pressed into tablets and crushed to 20-40 meshes. Weigh 1.0g sample and put it into a fixed bed reactor to evaluate the ethanol dehydration reaction. Activate at 550°C for 1 hour with nitrogen gas, and then lower the temperature to 260°C for reaction. The ethanol is carried by nitrogen, the nitrogen flow rate is 60ml/min, and the ethanol weight space velocity is 2.0h ^-1 . The reaction products were analyzed by online gas chromatography (Varian3800, FID detector, capillary column PoraPLOTQ-HT). The results showed that the conversion rate of the sample of Example 10 was 100%, and the ethylene selectivity was 100%. The conversion rate of the sample of Comparative Example 1 is 72%, the ethylene selectivity is 89%, and the product contains hydrocarbon by-products such as methane.

Example 15

The samples obtained in Example 10 and Comparative Example 1 were calcined at 600° C. for 4 hours, and then pressed into tablets and crushed to 20-40 meshes. Weigh 1.0g sample and load it into a fixed-bed reactor for MTO reaction evaluation. Activate at 550°C for 1 hour with nitrogen gas, and then lower the temperature to 450°C for reaction. A 60wt% methanol aqueous solution is fed with a pump, and the methanol weight space velocity is 2.5h ^-1 . The reaction products were analyzed by online gas chromatography (Varian3800, FID detector, capillary column PoraPLOTQ-HT). The results are shown in Table 3.

Table 3 Sample Methanol Conversion to Olefins Reaction Results

*Highest (ethylene+propylene) selectivity at 100% methanol conversion.

Claims (18)

1. A SAPO-44 molecular sieve, characterized in that the anhydrous chemical composition of the molecular sieve is expressed as: _mSDA ( _{SixAlyPz ) O2} , wherein: SDA is hexamethyleneimine; m represents the number of moles of organic amine per mole of ( _SixAlyPz ) _O2 , _{m =} 0.1～0.5; x, y, and z respectively represent the mole fractions of Si, Al, and P, and their ranges are x=0.01~0.60, y=0.2~0.60, z=0.2~0.60, and x+y+z=1; The surface of the molecular sieve crystal is slightly rich in silicon, and the ratio of the silicon content on the outer surface to the bulk silicon content of the crystal is 1.50-1.01, wherein the silicon content is the molar ratio of Si/(Si+Al+P). 2. The SAPO-44 molecular sieve according to claim 1, characterized in that the surface of the molecular sieve crystal is slightly rich in silicon, and the ratio of the silicon content on the outer surface to the bulk silicon content of the crystal is 1.40 to 1.02, wherein the silicon content is Si /Molar ratio of (Si+Al+P). 3. The SAPO-44 molecular sieve according to claim 1, characterized in that the surface of the molecular sieve crystal is slightly rich in silicon, and the ratio of the silicon content on the outer surface to the bulk silicon content of the crystal is 1.35 to 1.03, wherein the silicon content is Si /Molar ratio of (Si+Al+P). 4. The SAPO-44 molecular sieve according to claim 1, characterized in that the surface of the molecular sieve crystal is slightly rich in silicon, and the ratio of the silicon content on the outer surface to the bulk silicon content of the crystal is 1.30 to 1.03, wherein the silicon content is Si /(Si+Al+P) molar ratio. 5. SAPO-44 molecular sieve according to claim 1, is characterized in that, silicon content progressively is uniform from core to shell in SAPO-44 molecular sieve crystal. 6. The SAPO-44 molecular sieve according to claim 1, characterized in that, the content of silicon in the SAPO-44 molecular sieve crystal increases from the core to the shell inhomogeneously. 7. a method for synthesizing the described molecular sieve of claim 1, described method comprises the following steps: a) Mix silicon source, aluminum source, phosphorus source, deionized water, surfactant and SDA to form an initial gel mixture with the following molar ratio: SiO ₂ /Al ₂ O ₃ =0.01~1; P ₂ O ₅ /Al ₂ O ₃ ＝0.5～1.5; _H2O / _Al2O3 =30～130 _; SDA/Al ₂ O ₃ =2.0～6; BM/Al ₂ O ₃ =0.01～0.10; Wherein SDA is hexamethyleneimine, and BM is a surfactant; b) Put the initial gel mixture obtained in step a) into the synthesis kettle, seal it, raise the temperature to 190-230° C., and crystallize under autogenous pressure for 1-15 hours; c) reduce the crystallization temperature to 160-180° C. and crystallize under autogenous pressure for 1-15 hours; d) After the crystallization is complete, the solid product is centrifuged, washed with deionized water until neutral, and dried to obtain SAPO-44 molecular sieve. 8. according to the described method of claim 7, it is characterized in that, described step a) silicon source in the initial gel mixture is one or any in silica sol, active silicon dioxide, orthosilicate, metakaolin A mixture of several kinds; the aluminum source is one or any mixture of aluminum salts, activated alumina, aluminum alkoxide, metakaolin; the phosphorus source is orthophosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, organic phosphorus One or any mixture of compounds or phosphorus oxides; surfactants are dodecyltrimethylammonium chloride, tridecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride Amine, pentadecyltrimethylammonium chloride, hexadecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tridecyltrimethylammonium bromide, tetradecane One or any mixture of trimethylammonium bromide, pentadecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide. 9 . The method according to claim 7 , wherein the molar ratio of SDA to Al ₂ O ₃ in the initial gel mixture in step a) is SDA/Al ₂ O ₃ =2.5˜5.0. 10. The method according to claim 7, characterized in that the molar ratio of SDA to Al ₂ O ₃ in the initial gel mixture in step a) is SDA/Al ₂ O ₃ =3.0˜4.5. 11. The method according to claim 7, characterized in that the molar ratio of H ₂ O to Al ₂ O ₃ in the initial gel mixture in step a) is H ₂ O/Al ₂ O ₃ =35-100. 12. The method according to claim 7, characterized in that the molar ratio of BM to Al ₂ O ₃ in the initial gel mixture in step a) is BM/Al ₂ O ₃ =0.03˜0.08. 13. The method according to claim 7, characterized in that, the crystallization temperature in the step b) is 195-225°C, and the crystallization time is 1-12h. 14. The method according to claim 7, characterized in that, the crystallization temperature in the step b) is 211-225° C., and the crystallization time is 1-10 h. 15. The method according to claim 7, characterized in that, the crystallization temperature in the step c) is 165-175° C., and the crystallization time is 3-12 hours. 16. A catalyst for acid-catalyzed reactions, characterized in that, the SAPO-44 molecular sieve according to any one of claims 1-6 or the synthetic SAPO-44 molecular sieve according to any one of claims 7-15 through 400 It can be obtained by roasting in air at ~700°C. 17. A catalyst for the conversion of oxygenated compounds into olefins, characterized in that, the SAPO-44 molecular sieve according to any one of claims 1-6 or the synthetic SAPO-44 molecular sieve according to any one of claims 7-15 44 molecular sieves are obtained by roasting in air at 400-700 °C. 18. A gas adsorbent, characterized in that the SAPO-44 molecular sieve according to any one of claims 1-6 or the SAPO-44 molecular sieve synthesized according to any one of claims 7-15 is passed through 400-700 °C obtained by roasting in air.