CN115646437B - Preparation method of high-silicon ZSM-5 molecular sieve adsorbent suitable for adsorbing styrene - Google Patents
Preparation method of high-silicon ZSM-5 molecular sieve adsorbent suitable for adsorbing styrene Download PDFInfo
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Abstract
The invention discloses a preparation method of a high-silicon ZSM-5 molecular sieve adsorbent suitable for adsorbing styrene, belonging to the technical field of ultra-high silicon nano molecular sieves, and the preparation method comprises the following steps: the invention has the beneficial effects that the MFI crystal seed synthesis, the ultra-high silicon nanometer molecular sieve raw powder synthesis and the preparation of the high silicon ZSM-5 molecular sieve adsorbent are as follows: the n-butyl alcohol induces the MFI molecular sieve crystal seed to be synthesized and plays a role in supporting a crystal seed pore channel; citric acid can selectively remove aluminum atoms of an MFI crystal seed framework through chelation, so that defects are formed in a pore structure, and MFI is used as a crystal seed to induce a silicon source to enrich and grow on the surface and defect positions of the crystal seed to synthesize an ultrahigh silicon ZSM-5 molecular sieve crystal, namely a crystal seed dissolving and regrowing strategy; nanometer silica flour, because nanometer silica specific surface is big, can produce strong cross-linking effect with organic matter binder molecule, improve the adhesive force of high wet blank greatly, simultaneously because nanometer silica flour size is less, can fill the hole in the molecular sieve, show promotion honeycomb type adsorbent intensity.
Description
The technical field is as follows:
the invention belongs to the technical field of ultra-high silicon nano molecular sieves, and particularly relates to a method for synthesizing an ultra-high silicon nano molecular sieve by an organic template-free system.
Background art:
the ZSM-5 molecular sieve is an aluminosilicate with a unique three-dimensional cross channel and ten-membered ring pore structure. Because of its good hydrothermal stability, framework thermal stability and strong acidity, it is an ideal material for catalysis, so far, ZSM-5 zeolite has been largely applied to important industrial processes such as aromatic alkylation, catalytic cracking, light hydrocarbon aromatization of methanol to gasoline, etc. The nanocrystalline ZSM-5 has a large external specific surface area to provide more external surface active sites, and a short intragranular pore channel to achieve a high intragranular diffusion rate, so that reactant molecules can easily reach the catalytic active sites, and the generated product can be quickly diffused out of the pore channel. Therefore, the nano-crystalline ZSM-5 has excellent performances in the aspects of improving the utilization rate of the catalyst, enhancing the conversion capability of macromolecules, reducing the deep reaction of products, reducing the coking inactivation rate of the catalyst and the like.
The framework silica-alumina ratio is the key of the acid property of the zeolite molecular sieve, the low-silica ZSM-5 has better application prospect in the field of petroleum catalytic cracking due to more acid amount and acid sites, and the high-silica ZSM-5 has better hydrophobicity and weak acid sites and is more suitable for being used as a VOCs adsorbent for styrene easy polymerization.
In order to obtain a high-silicon ZSM-5 molecular sieve with uniform and regular crystal morphology, and then research on synthesis of the ZSM-5 molecular sieve for decades, organic amines (such as primary amine, secondary amine, tertiary amine, quaternary ammonium base, quaternary ammonium salt and the like) are mostly adopted as template agents for guided synthesis. However, organic amine is used as a template agent to synthesize the ZSM-5 molecular sieve, and the discharge of wastewater containing the organic template agent in the synthesis process can cause water body pollution; the organic template agent needs to be removed at high temperature in the open pore passage of the molecular sieve, so that the energy consumption cost is increased, and NO is increased x And CO 2 The emission of (2) causes air pollution; in addition, the price of the organic template agent is relatively expensive, and the use of a large amount of the organic template agent can greatly increase the production cost of the molecular sieve. The ZSM-5 molecular sieve synthesized by using non-organic amine as a template agent has irregular crystal appearance and poor performance, and has the problems of high energy consumption and environmental pollution caused by difficult crystallization for a long time. Therefore, the synthesis of the ZSM-5 molecular sieve with high silica-alumina ratio by a template-free system is always the goal pursued by people and the hot spot of the synthesis research of the molecular sieve.
Chinese patent CN102642847B discloses a synthesis method of a template-free submicron ZSM-5 zeolite molecular sieve, which is to prepare Na with the molar ratio of 10-30 2 O:100SiO 2 :1.25～5.0Al 2 O 3 :12SO 4 2- :2000～6000H 2 Adding 10-30 Na into the pre-crystallized liquid of O 2 O:100SiO 2 :1.25～5.0Al 2 O 3 :12SO 4 2- :2000～6000H 2 O synthesis mother liquor, and controlling addition of alkali and H for multiple times 2 The proportion of O and the step crystallization are carried out to obtain the submicron molecular sieve with the grain diameter of 400-500 nm, but the method has the defects that the addition amount of the pre-crystallization liquid is large and reaches more than 10wt percent), the dispersibility of the molecular sieve is poor, the crystal grains are in a cluster shape, the appearance is irregular, the operation is more complicated, and the industrialization is not easy to realize.
CN102671693A discloses a synthesis method of a high silica-alumina ratio nano ZSM-5 molecular sieve, which adopts an organic template method to directly add organosilane into an original solution of the molecular sieve, graft silane on a ZSM-5 molecular sieve seed crystal under the condition of condensation reflux, and remove the organic template and the organosilane by roasting. The crystal size of the nano ZSM-5 molecular sieve synthesized by the method is between 0.1 and 500nm, and the ratio of silicon to aluminum is SiO 2 /A1 2 O 3 Not less than 30, and the molecular sieve has 0.5-2 nm micropores, but the ZSM-5 molecular sieve synthesized by the method has uneven grain size, lower silicon-aluminum ratio range and harmfulness of the adopted organic template agent.
Chinese patent CN101054186A discloses a method for preparing nano zeolite by adopting a micro-channel reactor, which synthesizes molecular sieve with required silicon source, aluminum source, template agent, sodium hydroxide and water according to 1SiO 2 :0～0.02Al 2 O 3 :0.1～0.5TPA + :0～0.1Na 2 O:20～80H 2 Preparing reaction mixture according to the proportion of O, injecting the mixture into a microchannel reactor with the inner diameter of 0.5-3.0 mm after alcohol removal and concentration, controlling the reaction materials to stay in the microchannel reactor for 1-60 min under the conditions of the reaction temperature of 120-180 ℃ and the reaction pressure of 3-15 atm, and centrifuging, washing and drying the obtained suspension to obtain the nano zeolite. But the organic template adopted by the method can generate pollution in the roasting process, and the cost of the raw materials is higher.
In conclusion, the method and other related similar methods can effectively reduce or avoid the use of organic templates, but most of the technologies still have the problems of long synthesis time, irregular crystal grains of the obtained ZSM-5 molecular sieve, low silica-alumina ratio and the like, and the formed molecular sieve adsorbent prepared by taking the molecular sieve synthesized by the method as raw powder has the defect that the adsorption capacity is greatly influenced by humidity, so that the application of the molecular sieve prepared by the synthesis method in the aspect of VOCs treatment is greatly limited
The invention content is as follows:
in order to solve the problems and overcome the defects of the prior art, the method for preparing the high-silicon ZSM-5 molecular sieve adsorbent suitable for adsorbing styrene by honeycomb molding by using inorganic silica gel as an adhesive is provided, wherein a small amount of molecular sieve seed crystals are added into a synthesis system without an organic template in order to overcome the defects of large use of a large amount of organic template, large influence of water vapor on the adsorption capacity of the adsorbent, low high-temperature regeneration service life of the adsorbent and the like in the preparation technology of the ZSM-5 molecular sieve for adsorbing VOCs at present.
The specific technical scheme for solving the technical problems comprises the following steps: the preparation method of the high-silicon ZSM-5 molecular sieve adsorbent suitable for adsorbing styrene comprises the following steps:
step (I) MFI seed crystal synthesis:
the method comprises the following steps of taking a silicon source, an aluminum source, a reaction auxiliary agent I, sodium hydroxide and water as raw materials, wherein the molar ratio of the raw materials is NaOH: siO 2 2 ：Al 2 O 3 : reaction auxiliary agent I: h 2 O is 0.05-0.2;
and (II) synthesizing the raw powder of the ultrahigh silicon nano molecular sieve:
mixing a silicon source, a reaction auxiliary agent II and water, wherein the molar ratio of various substances is SiO 2 Reaction assistant II is H 2 Adding a proper amount of MFI seed crystal prepared in the step (I) into the mixture with the ratio of O =1: 0.02-0.1, uniformly mixing the mixture, placing the mixture in a hydrothermal kettle at 120-180 ℃ for reaction for 12-48 h, performing dealuminization, dissolution and recrystallization growth on the mixture, and performing conventional filtration, washing and drying on the obtained suspension to obtain the ultrahigh silicon nano molecular sieve powder; wherein the silicon-aluminum ratio is 800-1200;
step (III) preparation of the high-silicon ZSM-5 molecular sieve adsorbent:
and (3) mixing the ultrahigh silicon nano molecular sieve powder obtained in the step (II) with silica sol, nano silicon powder and an organic adhesive, and performing honeycomb molding to obtain the ultrahigh silicon ZSM-5 molecular sieve adsorbent suitable for adsorbing styrene.
Further, the reaction auxiliary agent I is n-butyl alcohol.
Further, the reaction auxiliary agent II is citric acid.
Further, the silicon source in step (i) is one of water glass, silica sol, white carbon black, and tetraethyl orthosilicate; the aluminum source is one of sodium metaaluminate, sodium aluminate, aluminum sulfate and aluminum isopropoxide.
Further, the pH value of the sol obtained in the step (I) is 11-12.
Further, in the step (ii), the silicon source is one of water glass, silica sol, white carbon black, and tetraethyl orthosilicate.
Further, the step (iii) specifically includes: and (3) fully stirring the ultrahigh silicon nano molecular sieve powder obtained in the step (II) with sesbania powder, hydroxypropyl methyl cellulose, silica sol and nano silicon powder in a pug mill, and kneading into a wet uniform plastomer, wherein the mass ratio of fed materials is as follows: ultra-high silicon nano molecular sieve powder: sesbania powder: hydroxypropyl methylcellulose: silica sol: the nano silicon powder = 33; the prepared uniform plastic body is molded and extruded in a high-pressure vacuum extruder, the extrusion temperature is 30-45 ℃, the extrusion rate is 20-50 mm/s, the extrusion pressure is 20-50 MPa, the extrusion die is in a square specification of 40-200 mm, the aperture is 2mm, the wall thickness is 1mm, and the cutting length is 100mm; drying the honeycomb-shaped wet-based adsorbent at 100-120 ℃ for 6-12 h, and then roasting at 450-650 ℃ for 6-12 h to obtain the honeycomb type molecular sieve adsorbent.
Further, the particle size of the nano silicon powder in the step (III) is 20-500 nm;
the invention has the beneficial effects that:
(1) N-butyl alcohol is added in MFI molecular sieve crystal seed synthesis, and is used as a pore canal filler, and the electronegativity of alcohol molecular hydroxyl is enhanced under alkaline conditions, so that the method is favorable for the synthesis of MFI molecular sieve crystal seed and Al 3+ Forming a competitive relationship, adjusting the position change of Al atoms in the pore channel, inducing MFI molecular sieve seed crystal synthesis and simultaneously playing a role in supporting the pore channel of the seed crystal;
(2) Citric acid is added in the synthesis of the ultra-high silicon ZSM-5 molecular sieve, the citric acid can selectively remove aluminum atoms of an MFI seed crystal framework through chelation in the hydrothermal synthesis process of the citric acid, so that defects are formed in a pore structure, and meanwhile, MFI is used as a seed crystal to induce a silicon source to enrich and grow on the surface and defect positions of the silicon source to synthesize the ultra-high silicon ZSM-5 molecular sieve crystal, namely a seed crystal dissolving and regrowing strategy;
(3) Nanometer silica flour has been added in the honeycomb forming process, because nanometer silica specific surface is big, can produce strong cross-linking effect with organic matter binder molecule, improve the adhesive force of high wet blank greatly, because nanometer silica flour size is less simultaneously, can fill the hole in the molecular sieve, show promotion honeycomb type adsorbent intensity.
Description of the drawings:
FIG. 1 is a schematic XRD spectrum of examples 1-3 of the present invention and commercial standard ZSM-5;
FIG. 2 is a SEM pictorial illustration of examples 1-3 of the present invention;
FIG. 3 is a schematic representation of a ZSM-5-A embodiment of the invention;
the specific implementation mode is as follows:
in the description of the invention, specific details are given only to enable a full understanding of the embodiments of the invention, but it should be understood by those skilled in the art that the invention is not limited to these details for the implementation. In other instances, well-known structures and functions have not been described or shown in detail to avoid obscuring the points of the embodiments of the invention. The specific meanings of the above terms in the present invention can be understood in a specific case to those of ordinary skill in the art.
The specific implementation mode of the invention is as follows:
in order to better understand the present invention, the specific embodiments are specifically illustrated, it is emphasized that the effects of the embodiments are not substantially different from the various embodiments within the scope of the present invention, including the respective reagents and the content ratios of the reagents, which can achieve the described effects of the present invention and solve the above problems, and other combinations are not described herein;
example 1
(1) Synthesis of MFI seed crystals
Water glass is used as a silicon source, aluminum sulfate is used as an aluminum source, n-butyl alcohol, sodium hydroxide and water are used as raw materials, and the molar ratio of the substances is 0.2Na 2 O:100SiO 2 :0.8Al 2 O 3 :20butanol:1000H 2 O, preparing uniform sol, wherein the pH of the sol is =11, placing the sol in a hydrothermal kettle at 120 ℃ for crystallization for 48h, filtering and washing to obtain MFI molecular sieve seed crystal, and Si is Al =60;
(2) Synthesis of ultra-high silicon nano molecular sieve raw powder
Mixing silica sol as silicon source, citric acid and water in the molar ratio of SiO 2 :citric acid:H 2 Adding the MFI seed crystal prepared in the step 1) into a reactor with the ratio of O =1 2 100 (mass ratio), uniformly mixing, placing in a hydrothermal kettle at 120 ℃ for 24 hours, performing crystal seed dealumination, dissolution and recrystallization growth, filtering and washing the obtained suspension, and placing the obtained powder in an oven at 120 ℃ for drying for 12 hours to obtain the ultrahigh-silicon nano molecular sieve, wherein the molecular sieve is marked as ZSM-5-1;
(3) Preparation of high-silicon ZSM-5 molecular sieve adsorbent
Fully stirring the high-silicon molecular sieve powder obtained in the step (2), sesbania powder, hydroxypropyl methyl cellulose, silica sol and nano silicon powder in a pug mill, kneading into a wet uniform plastomer, and feeding the materials according to the following mass ratio: molecular sieve powder: sesbania powder: hydroxypropyl methylcellulose: silica sol: nano silicon powder = 33; the prepared uniform plastic body is molded and extruded in a high-pressure vacuum extruder, the extrusion temperature is 35 ℃, the extrusion rate is 25mm/s, the extrusion pressure is 30MPa, the extrusion die is 100mm square, the aperture is 2mm, the wall thickness is 1mm, and the cutting length is 100mm; drying the honeycomb-shaped wet-based adsorbent at 120 ℃ for 10h, and then roasting at 550 ℃ for 6h to obtain the honeycomb-type molecular sieve adsorbent, wherein the index zeta of the honeycomb-type molecular sieve adsorbent is recorded as ZSM-5-A and is 2.46.
Example 2
(1) The synthesis of MFI seed crystals is the same as that in example 1, the only difference is that the synthesis is carried out by hydrothermal crystallization for 48 hours at the temperature of 100 ℃ to obtain MFI seed crystals;
(2) Synthesis of high-silicon nano molecular sieve raw powder
Tetraethoxysilane is taken as a silicon source to be mixed with citric acid and water, and the molar ratio of various substances is SiO 2 :citric acid:H 2 O =1 2 100 (mass ratio), uniformly mixing, placing in a hydrothermal kettle at 120 ℃ for 24 hours, performing crystal seed dealumination, dissolution and recrystallization growth, filtering and washing the obtained suspension, and placing the obtained powder in an oven at 120 ℃ for drying for 12 hours to obtain the ultrahigh-silicon nano molecular sieve, wherein the molecular sieve is marked as ZSM-5-2;
(3) The preparation of the high-silicon ZSM-5 molecular sieve adsorbent is the same as that of the example 1, and the only difference is that the mass ratio of the materials is as follows: molecular sieve powder: sesbania powder: hydroxypropyl methylcellulose: silica sol: nano silicon powder = 33.
Example 3
(1) The synthesis of the MFI seed is the same as that in example 1, the only difference is that the hydrothermal crystallization treatment is carried out for 72 hours at the temperature of 120 ℃ to obtain the MFI seed;
(2) Synthesis of high-silicon nano molecular sieve raw powder
Tetraethoxysilane is taken as a silicon source to be mixed with citric acid and water, and the molar ratio of various substances is SiO 2 :citric acid:H 2 Adding the MFI seed crystal prepared in the step 1) into a reactor with the ratio of O =1 2 100 (mass ratio), uniformly mixing, placing in a hydrothermal kettle at 120 ℃ for reaction for 48h, performing crystal seed dealumination, dissolution and recrystallization growth, filtering and washing the obtained suspension, and placing the obtained powder in an oven at 120 ℃ for drying for 12h to obtain the ultrahigh-silicon nano molecular sieve, wherein the molecular sieve is marked as ZSM-5-3;
(3) The preparation of the high-silicon ZSM-5 molecular sieve adsorbent is the same as that of the example 1, and the only difference is that the mass ratio of the materials is as follows: molecular sieve powder: sesbania powder: hydroxypropyl methylcellulose: silica sol: nano silicon powder = 33.
Examples 1-3, examination of the range of honeycomb type molecular sieve adsorbent index ζ prepared by honeycomb molding described in step (3), wherein the honeycomb type molecular sieve adsorbent index ζ satisfies the following formula:
ζ＝(ζ 1 -ζ 2 ) (1-P) +S 0
ζ 1 ＝[3.1(5μ) λ -2γ 0.75 ](1-Z -0.025T )+2S 0 0.75
ζ 2 ＝W 1.3 /Z 0.25 *120
wherein, W is the numerical part of the molding water-powder ratio, wt%;
z is the numerical part of the proportion of the molecular sieve, and is wt%;
lambda is the forming pH value;
p is the numerical part of the extrusion molding pressure, MPa;
t is the numerical portion of the extrusion temperature, DEG C;
μ is the numerical portion of the extrusion speed, mm/s;
gamma is the numerical part of the porosity of the honeycomb-formed molecular sieve,%;
S 0 is the silicon-aluminum ratio of the molecular sieve.
Comparative example 1
Synthesizing ZSM-5 molecular sieve raw powder by an organic template system: using sodium silicate as silicon source, aluminium sulfate as aluminium source, sodium hydroxide and water as raw materials, and using tetraethylammonium chloride as molecular sieve synthesis template agent to synthesize mother liquor, wherein the molar ratio of the materials is Na 2 O:SiO 2 :Al 2 O 3 :H 2 The method comprises the following steps of 1, 0.04. Filtering the obtained product, washing until the filtrate is neutral, and drying at 120 deg.C8h, roasting at 550 ℃ for 6h to obtain the nano molecular sieve raw powder, marked as ZSM-5-4;
in order to more intuitively show the process advantages of the invention, the preparation method of the high-silicon ZSM-5 molecular sieve adsorbent is compared with the alternative method adopted by the same process
Comparative example 2
The synthesis of ZSM-5 molecular sieve raw powder is the same as that of comparative example 1, and the only difference is that no n-butanol is added to obtain molecular sieve raw powder which is marked as ZSM-5-5;
comparative example 3
The synthesis of ZSM-5 molecular sieve raw powder is the same as that of comparative example 1, and the only difference is that no citric acid is added to obtain molecular sieve raw powder which is marked as ZSM-5-6;
comparative example 4
The synthesis of ZSM-5 molecular sieve raw powder is the same as that of the comparative example 1, and the only difference is that no n-butanol is added, and no citric acid is added to obtain molecular sieve raw powder which is marked as ZSM-5-7;
comparative example 5
(1) The synthesis of ZSM-5 molecular sieve raw powder is the same as that of comparative example 1;
(2) The preparation of ZSM-5 zeolite adsorbent was the same as that of example 1, except that no silica nanopowder was added, and the index Zeta of the honeycomb type zeolite adsorbent was 8.56, which is denoted as ZSM-5-B.
The molecular sieves prepared in examples 1-3 of the present invention and comparative examples 1-5 were tested for their effects by the following methods:
(1) the samples prepared in the invention are analyzed by using an X-ray diffractometer (XRD) of D8-Advance type of Bruker company in Germany to obtain crystal forms: cuK α Ray, tube voltage 40kV, tube current 30mA, scanning range 5-50 degrees, scanning step size 0.05 degrees, dwell time 0.5s.
The XRD patterns of the samples ZSM-5-1, 2, 3 raw powder and ZSM-5-A honeycomb prepared in examples 1-3 in FIG. 1 were matched with the standard pattern of commercial standard ZSM-5, and showed good MFI pattern.
(2) The method for testing the micro-morphology of the sample prepared by the invention comprises the following steps: samples were dispersed in ethanol, pipetted onto copper grids, and the zeolite was topographically analyzed using a philips em420 Scanning Electron Microscope (SEM) at an accelerating voltage of 100kV.
The SEM spectrogram result of FIG. 2 shows that the ZSM-5 prepared in examples 1-3 has good molecular sieve dispersion, uniform appearance, flat benzene ring crystal shape, particle size of about 600-700 nm, and honeycomb formation does not affect the microstructure. FIG. 3 is a drawing of a sample of the adsorbent of the honeycomb molecular sieve of example 1, wherein ZSM-5-A is structured and has a particle size of about 300 mm.
(3) The Si/Al ratio of the sample of the present invention was analyzed by using an inductively coupled plasma emission spectrometer (ICP) model OPTMA20000V from Perkin Elmer, USA for Si and Al contents in the synthesis solution.
(4) The styrene adsorption capacity of the sample of the invention is measured by a dynamic adsorption method: the whole adsorption system consists of a generator, an adsorption bed, a detector and the like. The nitrogen is used as carrier gas, the gas with certain styrene concentration and relative humidity is simulated by adjusting and controlling the flow of the gas to enter the adsorption device, and the styrene concentration in the tail gas is analyzed and measured by a gas chromatograph. Drying the molecular sieve sample in a muffle furnace at 550 ℃ for 2h to remove impurities in the adsorbent, taking 5g of granular sample which is sieved and formed into 20-30 meshes, putting the granular sample into an adsorption bed layer, and placing an adsorption tube in a constant-temperature water bath kettle at 35 ℃ for adsorption performance evaluation. The styrene adsorption amount is obtained by integral calculation of an adsorption curve, and is shown in a calculation formula. The adsorbent regeneration mode is that the adsorption saturated adsorbent is placed in a muffle furnace for high-temperature regeneration at 300 ℃ for 5h, and then the adsorption capacity test is carried out, wherein the adsorption regeneration times are 10 times.
C0-mass concentration of toluene in the feed gas, mg. M -3 ；
Ci-mass concentration of toluene in the exhaust gas after i minutes of adsorption, mg · m -3 ；
F-total flow of gas, mL. Min -1 ；
t-adsorption time, min;
ts-adsorption equilibrium time, i.e. outlet gas concentration equals inlet gas concentration, min;
w is the loading of the adsorbent, g;
q-equilibrium adsorption amount of organic matter, g.g -1 ；
TABLE 1 parameters of samples of examples 1-3 and comparative examples 1-2
As can be seen from Table 1, the Si/Al ratio of the samples obtained in examples 1 to 3 is 800 to 1000, the relative crystallinity is more than 85%, and the saturated adsorption capacity of the dynamic styrene is more than or equal to 0.047g/g -1 The adsorption capacity of the nano molecular sieve is 2.6 to 5.8 times of the saturated water absorption capacity of the nano molecular sieve, the high silicon-aluminum ratio of the nano molecular sieve has good hydrophobicity, the surface has no catalytic center, the adsorption and regeneration are carried out for 10 times, and the saturated adsorption capacity of the styrene is only reduced to be more than 94 percent of the original adsorption capacity, so that the preparation method of the nano molecular sieve effectively solves the problems of high cost and environmental pollution caused by the large amount of organic template used in the traditional synthesis process; the honeycomb type molecular sieve adsorbent prepared by taking the ultrahigh-silicon molecular sieve as raw powder and matching with the nano silicon powder has proper porosity and mechanical strength, and is suitable for adsorbing VOCs (volatile organic compounds) which are easy to polymerize in styrene.
Comparative example 1, the prepared sample has a Si/Al ratio of 320, a relative crystallinity of 83%, and a dynamic styrene saturated adsorption capacity of 0.034g/g or more -1 The silicon-aluminum ratio is 1.6 times of the saturated water absorption capacity of the catalyst, and the catalyst has certain hydrophobicity due to the high silicon-aluminum ratio, but acid active sites still exist on the surface, so that the polymerization carbon deposition of styrene is promoted during high-temperature regeneration, and the adsorption capacity is remarkably reduced.
In comparative example 2, the silica alumina ratio of the prepared sample was 407 and the relative crystallinity was 36%, and since n-butanol was not added as a pore filler in the seed crystal synthesis process, neither the silica alumina ratio nor the relative crystallinity was very low compared to examples 1-3, no high quality ZSM-5 molecular sieve was formed.
In comparative example 3, the silica to alumina ratio of the sample obtained was 368, the relative crystallinity was 54%, and no high quality high silica to alumina ratio ZSM-5 molecular sieve was formed because it was synthesized by seed induction, but no citric acid was added for seed dealumination, regardless of the silica to alumina ratio or relative crystallinity relative to examples 1-3.
Comparative example 4, the silica alumina ratio of the prepared sample is 258, the relative crystallinity is 21%, and no high-quality ZSM-5 molecular sieve with high silica alumina ratio is formed because n-butanol is not added as a pore channel filler in the seed crystal synthesis process, and citric acid is not added for seed crystal dealumination, and the silica alumina ratio and the relative crystallinity are both very low compared with those of examples 1-3.
Comparative example 5, the prepared sample has a Si/Al ratio of 973, a relative crystallinity of 72%, and a saturated adsorption capacity of dynamic styrene of not less than 0.038g/g -1 The adsorption capacity of the porous silica gel is 4.75 times of the saturated water absorption capacity of the porous silica gel, the porous silica gel has certain hydrophobicity due to the high silica-alumina ratio, the adsorption capacity of the porous silica gel is not obviously affected by humidity under the high humidity condition, but the comprehensive performance is poorer compared with that of the porous silica gel which is not added in the forming process of the embodiment 1-3, the adsorption performance is obviously reduced after the porous silica gel is regenerated, and the index zeta of the honeycomb type molecular sieve adsorbent is not in the empirical range of the invention and is not suitable for the treatment of industrial VOCs.
In summary, the following steps: (1) N-butyl alcohol is added in MFI molecular sieve crystal seed synthesis, and is used as a pore canal filler, and the electronegativity of alcohol molecular hydroxyl is enhanced under alkaline conditions, so that the synthesis of the MFI molecular sieve crystal seed and Al is facilitated 3+ Forming a competitive relationship, adjusting the position change of Al atoms in the pore channel, inducing the MFI molecular sieve seed crystal to synthesize and simultaneously playing a role in supporting the pore channel of the seed crystal;
(2) Citric acid is added in the synthesis of the ultra-high silicon ZSM-5 molecular sieve, the citric acid can selectively remove aluminum atoms of an MFI seed crystal framework through chelation in the hydrothermal synthesis process of the citric acid, so that defects are formed in a pore structure, and meanwhile, MFI is used as a seed crystal to induce a silicon source to enrich and grow on the surface and defect positions of the silicon source to synthesize the ultra-high silicon ZSM-5 molecular sieve crystal, namely a seed crystal dissolving and regrowing strategy;
(3) Nanometer silica flour has been added in the honeycomb forming process, because nanometer silica specific surface is big, can produce strong cross-linking effect with organic matter binder molecule, improve the adhesive force of high wet blank greatly, because nanometer silica flour size is less simultaneously, can fill the hole in the molecular sieve, show promotion honeycomb type adsorbent intensity
(4) The MFI molecular sieve seed crystal is adopted to replace an organic template agent, and the high-quality ultrahigh-silicon ZSM-5 molecular sieve (Si: al = 800-1200) is induced and synthesized by a seed crystal dealuminization, dissolution and recrystallization growth strategy, so that a series of problems of high cost, environmental pollution and the like caused by the large use of the organic template agent in the traditional synthesis are effectively overcome;
(5) The honeycomb type molecular sieve adsorbent prepared by taking the ultrahigh silicon molecular sieve as raw powder and matching with the silicon-based adhesive has no acidic active sites on the surface, solves the problems that the adsorption capacity of the traditional molecular sieve adsorbent is greatly influenced by humidity and the service life is reduced due to high-temperature regeneration carbon deposition, has proper porosity and strength, and is suitable for adsorbing VOCs (volatile organic chemicals) which are easy to polymerize at high temperature, such as styrene and the like.
Claims (6)
1. A preparation method of a high-silicon ZSM-5 molecular sieve adsorbent suitable for adsorbing styrene is characterized by comprising the following steps:
step (I) MFI seed crystal synthesis:
the method is characterized in that a silicon source, an aluminum source, a reaction auxiliary agent I, sodium hydroxide and water are used as raw materials, and the molar ratio of the raw materials is NaOH: siO 2 2 ：Al 2 O 3 : reaction auxiliary agent I: h 2 O is 0.05-0.2;
step (II) synthesis of the raw powder of the ultra-high silicon nano molecular sieve:
mixing a silicon source, a reaction auxiliary agent II and water, wherein the molar ratio of various substances is SiO 2 Reaction assistant II is H 2 O =1, 0.02-0.1, adding a proper amount of MFI seed crystal prepared in the step (I), uniformly mixing, placing in a hydrothermal kettle at 120-180 ℃ for reaction for 12-48 h, performing dealuminization, dissolution and recrystallization growth on the seed crystal, and performing conventional filtration, washing and drying on the obtained suspension to obtain the ultrahigh silicon nano molecular sieve powder;
step (III) preparation of the high-silicon ZSM-5 molecular sieve adsorbent:
mixing the ultrahigh silicon nano molecular sieve powder obtained in the step (II) with silica sol, nano silicon powder and an organic adhesive, and carrying out honeycomb molding to obtain the high silicon ZSM-5 molecular sieve adsorbent suitable for adsorbing styrene,
the reaction auxiliary agent I is n-butyl alcohol, and the reaction auxiliary agent II is citric acid.
2. The method of claim 1, wherein the silicon source in step (i) is one of water glass, silica sol, silica white, tetraethyl orthosilicate; the aluminum source is one of sodium metaaluminate, sodium aluminate, aluminum sulfate and aluminum isopropoxide.
3. The method for preparing the high-silica ZSM-5 molecular sieve adsorbent of claim 1, wherein the pH of the sol in step (I) is 11-12.
4. The method of claim 1, wherein the silicon source in step (ii) is one of water glass, silica sol, silica white, and tetraethyl orthosilicate.
5. The method of preparing a high-silicon ZSM-5 molecular sieve adsorbent as claimed in claim 1, wherein step (iii) specifically comprises: and (3) fully stirring the ultrahigh silicon nano molecular sieve powder obtained in the step (II) with sesbania powder, hydroxypropyl methyl cellulose, silica sol and nano silicon powder in a pug mill, and kneading into a wet uniform plastomer, wherein the mass ratio of fed materials is as follows: ultra-high silicon nano molecular sieve powder: sesbania powder: hydroxypropyl methylcellulose: silica sol: the nano silicon powder = 33; the prepared uniform plastic body is molded and extruded in a high-pressure vacuum extruder, the extrusion temperature is 30-45 ℃, the extrusion rate is 20-50 mm/s, the extrusion pressure is 20-50 MPa, the extrusion die is in a square specification of 40-200 mm, the aperture is 2mm, the wall thickness is 1mm, and the cutting length is 100mm; drying the honeycomb-shaped wet-based adsorbent at 100-120 ℃ for 6-12 h, and then roasting at 450-650 ℃ for 6-12 h to obtain the honeycomb type molecular sieve adsorbent.
6. The preparation method of the high-silicon ZSM-5 molecular sieve adsorbent according to claim 5, wherein the nano-silica powder in step (III) has a particle size of 20-500 nm.
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