Study On In Situ Synthesis Of Binderless Zeolite X Pellets And Its Adsorption Performance For Olefins

Binderless zeolite is considered to be a promising alternative for binder-containing zeolite involved in the industrial separation process due to its higher adsorption capacity,adsorption affinity and adsorption/desorption rate as well as lower coking tendency.Binderless zeolite X pellets were in situ synthesized by modifying the method we reported earlier to prepare zeolite A.and then the pellets were modified by post-synthesis ion-exchange using a series of metal nitrate solutions.Furthermore,the adsorption performance of 1-hexene on synthesized zeolite X samples was investigated by means of both experiments and simulated calculations.The binderless zeolite X pellets were in situ synthesized by hydrothermal treatment of silica gel precursors which were preformed by moulded silica sol in the sodium aluminate alkaline solution.Based on the synthesis experiments,the optimized synthesis conditions are found as follows:n(SiO2):n(NaAlO2):n(NaOH):n(H2O)=1:1:2:48,aging temperature of 40?,aging time of 6 h,crystallization temperature of 100? and crystallization time of 24 h.Under the optimum conditions,the as-synthesized binderless zeolite X pellets show a diameter of 2?3 mm,specific surface area of 582 m2/g and pore volume of 0.313 cm3/g.Compared with the commercial binder-containing zeolite X pellets,the synthesized binderless sample exhibits higher adsorption capacity and adsorption rate.Specifically,the gas phase adsorption capacity of 1-hexene on the binderless zeolite X pellets is 178.2 mg/g,which is 26?51%higher than that on the binder-containing ones at 25?.The binderless sample has the 21-49%higher liquid phase apparent adsorption capacity than that on the binder-containing ones.Additionally,the apparent diffusion coefficient of 1-hexene on the binderless zeolite X pellets is 2.85 ×10-6 cm2/s,which are 1.5?2.3 times that on the binder-containing samples.Furthermore,the underlying conversion mechanism were investigated using multiple characterization teniques including XRD.SEM.TEM,UV-Raman and NMR.First,in the low temperature aging period,the precursors containing silica nanoparticles are converted to amorphous aluminosilicate aggregates by reacting with the A1 species to form the primary building units of[SiO4]and[AlO4].in which the Al species diffuse from the bulk solution to the silica gel precursors along with the formation of second building units(four-membered rings and six-membered rings).During the crystallization process,the double six-membered rings are created by interconnection between four-membered rings and six-membered rings,while the ? cages are developed via self-assembly of the second building units.Finally,the zeolite X framework is constructed through the reorganization of ? cages with double six-membered rings.In addition,the in situ crystallization kinetics can be described as ?=[1-exp(-(0.16x(t-0.81))1.93)]×100%.The static and dynamic adsorption performance of 1-hexene on ion-exchanged binderless zeolite X pellets were evaluated and compared.Under the adsorption temperature of 25 ?,NaX,Mg-NaX,Ca-NaX,Co-NaX,Ni-NaX,Cu-NaX,Zn-NaX and Ag-NaX have the gas phase adsorption capacities ranging from 119.4 to 196.2 mg/g,the liquid phase apparent adsorption capacities ranging from 54.9 to 88.2 mg/g,and the Thomas rate constants ranging from 0.00323 to 0.00624 mL/(min·mg),respectively.It is suggested that Co2+,Ni2+,Cu2+,Zn2+and Ag+-exchanged samples exhibit better adsorption separation performance for 1-hexene/hexane mixture.At last,the adsorption of 1-hexene on ion-exchanged zeolite X unit cell structures was simulated based on the GCMC method.According to the calculation results,the calculated gas phase adsorption capacities are in good accordance with the experimental data for all ion-exchanged samples at 25?.Specifically,NaX,Mg-NaX,Ca-NaX,Co-NaX,Ni-NaX,Cu-NaX,Zn-NaX and Ag-NaX have the simulated results ranging from 123.7 to 187.3 mg/g.Meanwhile,the adsorption energies are observed ranging from 50.97 to 56.66 kJ/mol.Based on the simulated results of the effect of ion type and ion-exchange degree on adsorption performance,it is indicated that incorporation of Mg2+and Ca2+into zeolite X enables higher adsorption capacity while introduction of Ag+enhances interaction energy.In addition,the incorporation of Co2+,Ni2+,Cu2+and Zn2+boosts both adsorption capacity and interaction energy.Generally,the impacts of ion-exchange degree on both adsorption capacity and interaction energy tend to be less significant as the degree of ion-exchange is higher than 80%.