Keggin Structure and Surface Acidity of 12-Phosphotungstic Acid Grafted Zr-MCM-48 Mesoporous Molecular Sieves

A zirconium modified MCM-48 mesoporous material was synthesized by surfactant-templated method. Surface grafting Zr-MCM-48 with tungstophosphoric acid led to a great enhancement of both the number of the Brønsted acid sites and acidity strength in comparison with the bare support. At 100 ̊C, the 30 wt% H3PW12O40/Zr-MCM-48 contained 174 mol/g Brønsted acid sites which were 21.7 times greater than that of Zr-MCM-48. The Keggin structure of the grafted heteropolyacid was rather stable after calcination at 400 ̊C for 2 h, approximately 93.3% of Keggin structure in the dispersed heteropolyacid were remained without destruction but slightly distorted in some degree, as evidenced by FTIR characterization and P NMR-MAS analysis. This H3PW12O40/Zr-MCM-48 solid with three dimensional mesoporous system, large surface area and very strong Brønsted acidity will be a promising catalyst for acid catalytic reactions.


Introduction
In the history of porous materials discovery, the synthesis of M41S mesoporous materials reported by Mobil Company in 1992 occupies an important position because they bridge microporous and macroporous materials [1].The unique properties of these mesoporous materials like large surface area, great porosity, ordered pore system and narrow pore size distribution make them very attracttive in the fields like pharmaceutical and fine chemical industries, petrochemical and refinery, adsorption and separation processes and heterogeneous catalysis 2-6.
One member of this novel M41S family, Si based cubic type MCM-48 solid, has a structure consisting of separate three-dimensional channel systems [7,8].It is recognized that MCM-48 is superior to MCM-41 which has a one-dimensional pore system in the catalytic applications because the three-dimensional interwoven channels in MCM-48 could allow reactants and products faster diffusion, and thus enhance the mass transfer kinetics and avoid pore blocking 9.However, the pure siliceous mesoporous materials have some disadvantages like low hydrothermal stability, neutral framework and lack of active sites in the pore wall which limit their special applications.It is impossible, for example, to directly use Si-based MCM-48 as catalyst in the reactions like alkylation and isomerization of alkanes owing to lack of Brønsted acidity; it is impossible to use it as adsorbent for wastewater treatment due to its hydrothermal instability in an aqueous solution.
The thermal and hydrothermal stability of Si-based MCM-48 could be improved by framework modification.Zhang and coworkers recently report that zirconium modified MCM-48 could remain its cubic mesoporous structure even after calcination at 800˚C for 4 h or after hydrothermal treatment in water at 100˚C for 24 h [10].Incorporations of rare metals such as La [11] and Ce [12] and transition metals like Fe [12], Mn [13], Ti [14], Sn [15], W [16], V [17], Cu [18] and Zn [18] etc. into the framework of the MCM-48 have been also reported.These incorporated metal ions may serve as active sites or structural stabilizers of the MCM-48 solid.
The surface acidity of mesoporous materials can be effectively enhanced through multifunctionalization approach.It is reported that by grafting some strong acid sites like sulfate ions 19 or heteropolyacids 20,21, onto the surface of mesoporous materials MCM-41, its surface Brønsted acidity can be generated and the total number of acid sites is significantly increased.Salas et al. once reported the modification of thermal stability and surface acidity of MCM-41 by using surface sulfating and zirconium substitution in the framework [22].However, their results show that the surface acidity enhancement seems not significant.Late on, Wang and coworkers simultaneously introduced Zr 4+ ions into the framework and grafted heteropoly acid on the surface of MCM-41, both thermal stability and surface acidity were improved [23].Unfortunately, investigation on surface acidity of the mesoporous materials was tensely concentrated on the MCM-41.Up to date, little attention was paid on the surface acidic property of pure or modified MCM-48 [16].Owing to the importance of the acidity of MCM-48 material in the acid catalyzed reactions, studies on the enhancement of surface acidity of MCM-48 will be of interesting.
In the present work, we describe an approach for double modifications of Si-based MCM-48 by framework substitution by Zr ions and surface grafting with 12-phosphotungstic acid.Introducing zirconium ions into the framework of MCM-48 aims to enhancing its hydrothermal stability, while surface grafting with heteropoly acid is for creating acid sites in Zr-MCM-48 for acidic catalysis reactions.Because the hydrothermal stability of the Zr-MCM-48 has been studied by other researchers [10], herein we will mainly focus our attention on the Keggin structure and surface acidity properties of H 3 PW 12 O 40 /Zr-MCM-48 materials.
According to this composition, a given amount of CATCl surfactant, NaOH and water was mixed at 50˚C with magnetic stirring for 0.5 h.Then TEOS and zirconyl chloride octahydrate were slowly added into the above solution with vigorous agitation for 2 h.The product was aged at 110˚C for 72 h in a sealed Teflon bottle under autogenous pressure.The resultant slurry, containing a white precipitate, was filtered and extensively washed 4 times with 500 ml deionized water, then dried at ambient temperature for 12 h.Finally, the resultant solid was calcined at 600˚C for 6 h in a flowing air (50 ml/min).Care was taken in the calcination by controlling the temperature increasing rate at 1˚C/min in order to avoid any collapsing of the mesoporous structure of the products due to the temperature rapid increasing.
The 30 wt% H 3 PW 12 O 40 /Zr-MCM-48 catalyst was prepared by impregnating the Zr-MCM-48 sample with a proper amount of H 3 PW 12 O 40 (noted as HPW) in 50 ml methanol.The solvent was evaporated at 40˚C in a rotary.The dried solids were calcined in air at 400˚C in air for 2 h.

Characterization
The X-ray diffraction patterns of the calcined sample were measured in a D-500 SIEMENS diffractometer with a monochromatic CuK radiation.The scanning was made for a 2 angle from 1.5˚ to 10˚, with a step size of 0.02˚ and a step time of 2 s.Position correction was made using the NIST standard reference material 675.
The surface area, pore volume and pore size distribution of the samples were measured with a Digisorb 2600 using N 2 adsorption-desorption isotherms method.Before the measurement, the sample was treated at 300˚C under vacuum to remove the surface impurities.The surface area was computed by using the multi-point Brunauer-Emmett-Teller (BET) method based on the adsorption data in the partial pressure P/P o range from 0.01 to 0.2.The value of 0.1620 nm 2 was taken for the cross-section area of the adsorbed N 2 molecule.The pore diameters were determined by using the (BJH) method.
Solid state 31 P MAS-NMR spectra were recorded with a Bruker 400 MHz spectrometer at room temperature at a frequency of 79.49MHz, spinning of 7.5 kHz, emtting pulses at 90 s intervals and using 4 mm zirconia rotors.85% H 3 PO 4 solution (0 ppm) was used as reference to obtain the chemical shift.The spectral deconvolution was performed with the Unity spectrometer software.
High Resolution TEM was carried out at room temperature in a JEOL 4000 EX electron microscope equipped with a pole piece having a spherical aberration coefficient of C s = 1.00 mm.The powder sample was softly grounded in an agate mortar and dispersed in isopropyl alcohol within an ultrasonic bath for several minutes.A few drops were then deposited on 200 mesh copper grids covered with a carbon film.Mesostructures of the solid were imaged directly by a conventional method.
The surface acidity of the samples was analyzed by FTIR method of pyridine adsorption on a 170-SX FTIR spectrometer in a temperature range between 25 and 400˚C.Before pyridine adsorption, the sample was heated to 400˚C under vacuum, then cooled down to room temperature.Afterwards, the solid wafer was exposed to pyridine by breaking, inside the spectrometer cell, a capillary containing 50 l of liquid pyridine.The FTIR spectra were recorded under various conditions by increasing the IR cell temperature from 25˚C to 400˚C.The quantitative calculation of Lewis and Brønsted acid sites was made with respect to the area of the adsorption bands at 1450 cm −1 and 1540 cm −1 , respectively [24,25].The areas of the IR absorption bands at 1450 and 1540 cm −1 at different temperatures were integrated by using the extinction coefficients E B = 1.0086 mol/cm 2 for Brønsted acid sites and E L = 0.9374 mol/cm 2 for Lewis acid sites.The acid strength was determined according to the variation of the number of acid sites as a function of the temperature.

Textural Properties
The textural characteristics of Zr-MCM-48 and H 3 PW 12 O 40 / Zr-MCM-48 samples were determined from nitrogen adsorption-desorption isotherms.The surface area, pore volume and average pore diameter are reported in Table 1.Zr-MCM-48 has a surface area 797.8 m 2 /g, pore volume 0.75 cm 3 /g and an average pore diameter 3.8 nm.After grafting with 30 wt% H 3 PW 12 O 40 , the surface area diminishes to 674.1 m 2 /g, pore volume and average pore diameter to 0.66 cm 3 /g and 3.2 nm, respectively.This is due to the surface coverage and partial pore blocking by  the deposited heteropolyacid which was confirmed by high resolution TEM observation shown in the following section.

High-Resolution TEM Observation
Figure 2 shows the morphologies of the Zr-MCM-48 and H 3 PW 12 O 40 /Zr-MCM-48 solids observed by high resolution TEM.The Zr-MCM-48, Figure 2(A), shows a very ordered pore system with pore diameter between 2.5 and 4 nm.In some areas, surface structural defects are observed.The structural imprefects may be formed in the calcination procedure due to collapse of the walls between adjact pores resulted from the removal or combustion of the synthetic template, herein the surfactant. 2 nm, which is smaller than the dimension of the pores (2.5 -4 nm).Therefore, the heteropolyacid species can locate inside the internal pore of the support.However, it is also possible that the H 3 PW 12 O 40 molecules forms clusters by assembling several molecules, leading to most of particles having diameter 2 -3 nm, and some of these larger than the dimension of the pores.Therefore, heteropolyacid may locate inside the internal pores, or disperse on the external surface of the support.These TEM image reveals that heteropolyacids dispersion is very homogeneous and large heteropolyacid aggregates with diameter larger than 6 nm are not formed in the H 3 PW 12 O 40 /Zr-MCM-48 sample.

Keggin Structure Stability of the H 3 PW 12 O 40 /Zr-MCM-48
In the H    ), where M is Zr or Si ions, by Keggin units interacting with surface hydroxyl groups, which is slightly different from the species (M-OH + ) ( 2 1 2 4 0 ) as indicated by the NMR resonance at −13.48 ppm 29.
The results of FTIR and 31 P MAS-NMR characterizations confirm that the Keggin structure of H 3 PW 12 O 40 is rather stable; it is only slightly deformed after impregnation and calcination at 400˚C.This conclusion is important because the Keggin structure at 400˚C has a great influence on the surface property, i.e. surface acidity of the H 3 PW 12 O 40 /Zr-MCM-48.

Surface Acidity
Although the goal to introduce zirconium ion into the framework of MCM-48 is to improve the thermal and hydrothermal stability, it certainly influences the surface acidity.In order to study the surface acidity property, both Zr-MCM-48 and H 3 PW 12 O 40 /Zr-MCM-48 samples were comparatively measured by in situ FT-IR spectroscopy of pyridine adsorption.Figure 6 shows a set of in   As shown in Figure 6, the Brønsted acidity of the Zr-MCM-48 is not sufficient and the acidity strength is still week, enhancement of both the number and strength of the Brønsted acid sites becomes necessary if Zr-MCM-48 is considered as catalyst for acid catalyzed reactions like n-alkanes hydroisomerization.
The FTIR spectra of pyridine adsorption on the H 3 PW 12 O 40 /Zr-MCM-48 catalyst are shown in Figure 7.The quantitative acidity data derived from the FTIR spectra of Figures 6 and 7 are reported in Tables 2 and 3, respectively.Both Brønsted (B) and Lewis (L) acid sites coexisted on the solid as indicated by the adsorption bands at 1450 cm −1 (L), 1610 cm −1 (L), 1540 cm −1 (B) and 1637 cm −1 (B).The bands at 1595 and 1578 cm −1 are assigned to the hydrogen-bonded pyridine and they were very weak and rapidly disappeared after thermal treatment at 200˚C 30.In comparison with the bare Zr-MCM-48 support, the number of the Brønsted acid sites of the heteropolyacid grafted H 3 PW 12 O 40 /Zr-MCM-48 catalyst was greatly enhanced by 21.7 times at 100˚C (Tables 2 and 3).Also, the acidity strength was significantly improved as evidenced by the acid nature even after 400˚C of treatment, 36 mol/g B sites and 28 mol/g L sites still remain in the H 3 PW 12 O 40 /Zr-MCM-48.
The Brønsted acidity strength is closely correlate with the Keggin sturcture and the dispersion of the loaded heteropoly anions which are determined by the interacttion between heteropoly acid and the support.The strong acidity of the H 3 PW 12 O 40 /Zr-MCM-48 may relate to the three dimensional pore system of the Zr-MCM-48 support.These hydroxyls may have a moderately strong interaction with the heteropolyacid, producing a big number of Brønsted acid sites.This H 3 PW 12 O 40 /Zr-MCM-48 solid with three dimensional mesoporous system, large surface area and very strong Brønsted acidity will be a  promising catalyst for acid catalytic reactions.The catalytic evaluation will be carried out in the near future.

Conclusion
Incorporation of zirconium ions into the framework of the mesoporous silica generates Brønsted acidity which would be remarkably enhanced by more than 21.7 times after surface grafting with 30 wt% 12-tungstophosphoric acid.The highly dispersed heteropolyacid was uniformly distributed within the pores or pore openings of Zr-MM-48 solid, and after thermal treatment at 400˚C for 2 h, the Keggin structure in the heteropoly anions of the H 3 PW 12 O 40 /Zr-MCM-48 was only deformed in some degree without destruction as evidenced by FTIR, TEM and 31 P MAS-NMR characterizations.

Figure 2 (
Figure2shows the morphologies of the Zr-MCM-48 and H 3 PW 12 O 40 /Zr-MCM-48 solids observed by high resolution TEM.The Zr-MCM-48, Figure2(A), shows a very ordered pore system with pore diameter between 2.5 and 4 nm.In some areas, surface structural defects are observed.The structural imprefects may be formed in the calcination procedure due to collapse of the walls between adjact pores resulted from the removal or combustion of the synthetic template, herein the surfactant.Figure 2(B) shows high resolution TEM image of the H 3 PW 12 O 40 /Zr-MCM-48 sample.Most of the regions are covered with nanoparticles with diameter 2 -3 nm, indicating a high dispersion of the supported heteropolyacid clusters.The diameter of a single H 3 PW 12 O 40 molecule is estimated to be 1 -1.2 nm, which is smaller than the dimension of the pores (2.5 -4 nm).Therefore, the heteropolyacid species can locate inside the internal pore of the support.However, it is also possible that the H 3 PW 12 O 40 molecules forms clusters by assembling several molecules, leading to most of particles having diameter 2 -3 nm, and some of these larger than the dimension of the pores.Therefore, heteropolyacid may locate inside the internal pores, or disperse on the external surface of the support.These TEM image reveals that heteropolyacids dispersion is very homogeneous and large heteropolyacid aggregates with diameter larger than 6 nm are not formed in the H 3 PW 12 O 40 /Zr-MCM-48 sample.
3 PW 12 O 40 /Zr-MCM-48 solid, the dispersion and structural stability of Keggin unit in the H 3 PW 12 O 40 are very important because they determine the surface acidity and thermal stability of the solid.In this work, two spectroscopic techniques, FTIR and 31 P MAS-NMR, were applied to characterize the dispersion and structural stability of Keggin structurre in the grafted H 3 PW 12 O 40 .Figure 3 shows the FTIR spectrum of the H 3 PW 12 O 40 / Zr-MCM-48.Four bands around 1080, 982, 896 and 819

Figure 2 .Figure 4 Figure 3 .
Figure 2. High Resolution TEM micrographs (A): Zr-MCM-48; (B): H 3 PW 12 O 40 /Zr-MCM-48 solid.cm −1 are observed; these bands correspond to the IR absorption vibrations of the heteropolyanions and they are very similar to the fingerprint absorption bands observed in the pure H 3 PW 12 O 40 [26,27].In the Keggin unit, the P-O symmetric stretching is characterized by the vibrational transition at 1080 cm −1 .The normal W = O d stretching mode is indicated by IR band at 982 cm −1 .The bands at 896 and 819 cm −1 are associated with the stretching motion of W-O-W bridges.The band at 896 cm −1 is assigned to the W-O b -W stretching mode (interbridges between corner sharing octahedral) and the band at 819 cm −1 is attributed to W-O c -W stretching mode (intra-bridges between edge sharing octahedral) [26,27].Appearance of these fingerprints of IR absorption bands indicates that the primary structure of Keggin unit in the H 3 PW 12 O 40 is completely retained in the H 3 PW 12 O 40 / Zr-MCM-48 solid after thermal treatment at 400˚C for 2 h.Figure4shows the spectrum of 31 P MAS-NMR spectroscopy of H 3 PW 12 O 40 /Zr-MCM-48 solid.One strong signal is present at approximately -13.48 ppm.In addi-

Figure 6 .
Figure 6.A set of FTIR spectra of pyridine adsorption-desorption of Zr-MCM-48 sample at different temperatures.

Table 1 . Textural data of Zr-MCM-48 and HPW/ZrMCM- 48 solids.
peak appears at around −14.93 ppm, Figure 5, a peak position shift (∆δ = −1.45ppm) is observed in the H 3 PW 12 O 40 /Zr-MCM-48 sample.The main peak position shift indicates that the Keggin structure is deformed in certain degree.Because no peaks around −4, −8, −11 ppm were observed in the H 3 PW 12 O 40 /Zr-MCM-48 sample, the formation of highly fragmented or a partially fragmented or defective Keggin unit can be completely excluded.The major peak at −13.48 ppm indicates that the more than 93.3% of Keggin structure in the dispersed heteropolyacid compound of H 3 PW 12 O 4 /Zr-MCM-48 were remained without destruction but slightly distorted.The weak signal at −14.94 ppm (3.6%) can be assigned to large H 3 PW 12 O 40 particles similar to bulk 12phosphotungstic acid crystals having very weak interaction with MCM-48 support; while, the one at −12.11 ppm (3.1%) is due to the formation of the species ( 2 ) 2 (