Synthesis Of Zirconium-containing Mesoporous Molecular Sieves: Characterization And Super Acidic Performance After Sulfated

Mesoporous molecular sieves can be able to make huge moleculers or groups enter into their pores because of their high specific surface area, large pore volume and pore diameter as well as narrow pore size distribution. So they can be widely used in the area which microporous molecular sieves are impracticable to perform such as the process of catalysis, adsorption or separation which require macromolecules, especially for heavy oil or residual oil catalytic cracking process. But pure silicon mesoporous molecular sieves lack in acidity and stability. So synthesis of mesoporous molecular sieves with strong acidity and high stability is the key aim which researchers go in for. And synthesis of mesopocous molecular sieves with super acidity has more important value for petrochemical processing. Zirconia is the only transition metal oxide with acid sites and alkali sites as well as oxidoreduction ability and good ion-exchange capacity. And as a p-type semiconductor, it rich in surface oxygen no-bit. In recent years, zirconia can form a solid super acid after sulfated, which have excellent acid catalysis characterization. Thereby zircenia is ocused attention upon in catalysis field. So synthesis of mesoporous zirconia or zirconium-doping meroporous molecular sieves with super-acidity has great meaning on theory and practical.In this study basing on sol-gel method and self-assembly theory, using Zr(NO₃)₄·3H₂O as zirconium source, a series of mesostructure zirconia and Zr-SBA-15 mesoporous molecular sieves with high zirconium loading were synthesized with impregnating crystallization method, one-step crystallization method or two-step crystallization method, and characterized with XRD, TEM, N₂ adsorption-desorption, TG-DTA, UV-vis, NMR and element analysis et al. The as-synthesized samples were prepared as super acid molecular sieves after sulfated with impregnating in sulfuric acid solution or adding sulfuric acid during the synthesis of samples. And the acid catalysis capacity and acidity of as-synthesized super acid molecular sieves were characterized with isomerization of n-pentane at low temperature (35℃), pyridine-IR and NH₃-TPD et al. The results obtained from experiments and studies were given in following. (1) Using cheaper Zr(NO₃)₄·3H₂O as precursor, cetyl trimethyl ammonium bromide (CTAB) as structure template agent, mesoporous zirconia molecular sieves were synthesized with two-step crystallization method in alcohol phase. The main factors were the dosage of template agent, pre-crystallization temperature and time, pH value, dosage of complexing agent triethanolamine, and crystallization temperature and time. Well ordered misoporous zirconia with high surface area had been synthesized at n_CTAB/n_Zr molar ratio 0.18-0.48, pH value 8-11, pre-crystallization temperature 90℃and time 4h, crystallization temperature 120℃and time 48h. BET specific surface area of the sample synthesized at optimum condition was 197m²/g after calcinated at 450℃, and mesostructure of pure mesoporous zirconia was still existed after calcinated at 550℃that shows the as-synthesized samples had good thermal stabilities as well as good hydrothermal stabilities. The thermal stabilities of samples were improved at various degrees after doped with aluminum, zinc, yttrium, lanthanum, cerium or thorium. Mesostructure of samples doped with Al, La, TR could exist after calcinated at 650℃. BET surface area of sample doped with Al was 251m²/g after calcinated at 450℃because of samples nanolization. At the same time, thermal stability and hydrothermal stability of samples doped with Al were improved. The results show that samples were crystallized continuously during calcinations process. This might be the key factor affecting the stabilities of samples. The stability of tetragonal phase of zirconia doped with Al was improved, and so the stability of mesostructrure zirconia was advanced too. But taking one with another, the stability of zirconia as a structural component of self-assembly material was rather poor, which was determinate by its crystal performance.(2) A series of Zr-SBA-15 mesoporous molecular were synthesized with impregnation-crystallization method by impregnating calcinated SBA-15 in zirconium nitrate alcoholic solution and then crystallizing at ethanol vapour. The results show: mesostructure Zr-SBA-15 can be synthesized when the Zr/Si molar ratio was less than 2.1. The element analysis results show: the zirconia content in Zr-SBA-15 could be 50.54% as Zr/Si molar ratio was 1.34, and the BET specific surface area was still as high as 356m²/g after calcinated at 600℃, which was advantage over traditional impregnating method. The degree of order could be improved by adding an amount of CTAB or triethanolamine in during synthesis process. TEM and UV-vis analysis results show: zirconium component in Zr-SBA-15 was mainly in the channel of SBA-15 and at high dispersed condition, and was mostly incorporated into the framework.(3) At hydrothermal system using Zr(NO₃)₄·3H₂O as zirconium source, Zr-MCM-41 and Zr-SBA-15 mesoporous molecular sieves had been synthesized with one-step crystallization method. The results show:â‘ Zr/Si molar ratio should been less than 0.25 for synthesis of Zr-MCM-41. The peak indexed to 100 in XRD pattern was decreased with Zr/Si molar ratio increasing, and got higher after calcinated at 600℃for 6h comparing with uncalcinated one. No characteristic peaks belonging to Zr compounds at 20=20-70 could be observed. This proved that Zr is fully dispersed in MCM-41 structure.â‘¡The maximum Zr/Si molar ratio was 2.3 for synthesis of Zr-SBA-15. The characteristic peaks belonging to mesostructure were gradually fall-off in intensity with increasing of Zr/Si molars ratio, and BET surface area decreased too. When Zr/Si molar ratio was less than 1.5, samples were with high surface area (BET specific surface area was over 398m²/g after calcinated at 600℃) and pore diameter about 7.4nm, which were belong to mesoporous material. TEM images showed that the new materials possessed thick hole wall. But element analysis results showed that zirconia loading was not so high with Zr/Si molar ratio increasing, and less than 30% which came short of zirconia content in theory for forming super acid.(4) Using Zr(NO₃)₄·3H₂O as zirconium source, well-ordered Zr-SBA-15 mesoporous molecular sieves with high zirconium loading were synthesized successfully with two-step crystallization method and neutral template self-assembly route. The results show:â‘ The maximum Zr/Si molar ratio was 2.3 for synthesis Zr-SBA-15 at only hydrochloric acid system. The intensity of characteristic peak belonging to SBA-15 was decreasing with Zr/Si molar ratio increasing, and BET specific surface area decreasing too. But pore size of samples were 5-6.4nm belonging mesopore. TEM image showed that the new materials possessed thick hole wall which indicated they had good thermal stabilities and hydrothermal stabilities. Element analysis results showed the samples with high zirconium loading. Zircomia content in sample was as high as 54.18% at Zr/Si molar ratio 0.7, and virtual rating of zirconium loading was 91.9%, and BET area was 431m²/g after calcinated at 600℃. Zirconia content in sample was 63.28% when Zr/Si molar ratio was 1.3, and virtual rating of zirconium loading was still 88.9%, and BET surface area was 226.8m²/g after calcinated at 600℃. These results were better than that of as-synthesized samples prepared with on-step crystallization method or impregnation-crystallization method. TEM, NMR and UV-vis results showed that zirconium was fully dispersed in SBA-15 structure when Zr/Si molar ratio less than 0.7. After that, the single particles of tetragonal ZrO₂ phase appeared in the samples. XRD and pore size distribution indicated that as-synthesized Zr-SBA-15 possessed two types mesopore, one 3.5nm and another 5-6.4nm.â‘¡The maximum Zr/Si molar ratio for synthesis of Zr-SBA-15 was 3.0 at adding sulfuric acid system. The variation regulars of characteristics of samples were similar to that of only hydrochloric acid system when added sulfuric acid in during synthesis process. At the same Zr/Si molar ratio, the BET surface area of the sample was bigger than that of only hydrochloric acid system, especially Zr/Si molar ratio at 3.0, and pore size down to 3.74nm which still belong to mesostructure. At the same Zr/Si molar ratio, the BET surface areas of samples were different with sulfuric acid to hydrochloric acid molar ratio changing. The samples synthesized at only sulfuric acid system had the biggest surface area. But pore sizes of samples changed little with sulfuric acid to hydrochloric acid molar ratio changing. This proved the thermal stability of sample was improved after adding sulfuric acid in during synthesis process. TEM, NMR and UV-vis results indicated that zirconium was fully dispersed in sample, and mostly incorporated into the framework. Even Zr/Si molar ratio over 0.7 no bulk particles of ZrO₂ appeared that proved sulfuric acid had good stabilization to zirconium compounds. Being similar to only hydrochloric acid system, XRD and pore size distribution indicate that the as-synthesized Zr-SBA-15 had two types mesopore, one 3.5nm and the other one 5-6nm. More important, Zr-SBA-15 synthesized at adding sulfuric acid system could form SO₄^2-/Zr-SBA-15 super acid after cacinated directly without impregating with sulfuric acid solution and drying procedure, which established bases for industrial application. During synthesis of Zr-SBA-15 process with two-step crystallization method, pre-crystallization process had great influence up on the self-assembly progress of formation of mesoporous material.(5) As-synthesized mesostructure SO₄^2-/ZrO₂ and SO₄^2-/Zr-SBA-15 super acid molecular sieves had high catalytic activities on isomerization of n-peutare at low temperature (35℃). The catalytic activities were changed with preparing methods or conditions.â‘ Using mesostructure SO₄^2-/ZrO₂ as a catalyst, conversion of n-pentane was 47.3% after reacting for 3h which was less than that (50.2%) of bulk SO₄^2-/ZrO₂, and increased a little after doped with Al or Zn. NH₃-TPD results showed mesoporous SO₄^2-/ZrO₂ had strong acid site.â‘¡The catalytic activities of SO₄^2-/Zr-SBA-15 prepared in only hydrochloric acid system on isomerization of n-pentane were influenced by removing method of template agent, concentration of impregnating sulfuric acid solution, calcination temperature, activation temperature and Zr/Si molar ratio. When Zr/Si molar ratio was 1.1-1.3, Zr-SBA-15 impregnated with 1M sulfuric acid solution without pre-removing of template agent, calcination temperature was 600℃, and activation temperature was 300℃, the catalytic activity of SO₄^2-/Zr-SBA-15 on n-pentane reaction was the highest, conversion of n-pentane were 52.9%-57.6%, and S/Si molar ratio of catalysis were 0.28-0.35 which was much higher than that (about 0.061) of bulk SO₄^2-/ZrO₂ prepared with precipitation method.â‘¢The catalytic activities of SO₄^2-/Zr-SBA-15 prepared in adding sulfuric acid system on isomerization of n-pentane were higher too, and influenced by dosage of sulfuric acid during synthesis process, calcinations temperature, activation temperature and Zr/Si molar ratio of samples. When Zr/Si molar ratio was 1.5-2.5, calcinations temperature was 600℃, and activation temperature was 300℃, the conversions of n-pentane were 30.8%-50.6%. At the same Zr/Si molar ratio, the catalytic activities were influenced greatly by sulfuric acid to hydrochloric acid molar ratio, and raised with increasing of dosage of sulfuric acid after Zr/Si molar ratio over 1.5. And S/Zr molar ratio were more than 0.06.â‘£The catalytic activities of SO₄^2-/Zr-SBA-15 prepared with impregnating crystallization method on isomerization of n-pentane were good too, and influenced by concentration of impregnating sulfuric acid, calcinations temperature, activation temperature, and Zr/Si molar ratio. When Zr/Si molar ratio was 1.96-2.15, Zr-SBA-15 impregnated with 1M sulfuric acid solution, calcination temperature was 600℃and activation temperature was 300℃, the conversion of n-pentane were 40.0%-46.6% after reacting 3h.⑤Pyridine-IR analysis results showed that SO₄^2-/Zr-SBA-15 possessed L-acid and B-acid at the same time, and the quantity of L-acid was more than that of B-acid.â‘¥XPS analysis results show that SO₄^2- was mostly compaginate with zirconium components on the surface of Zr-SBA-15 to form strong acid sites, and was less concern with zirconium and silicon in body. Carbon deposition formed during isomerization reaction. XPS curves of Zr3d, S2p, O1s and Si2p of samples show that the binding energies of Zr, S, O and Si were increased after calcinated, which indicated the strong interaction of components in samples influenced the binding energies.