Boronation and galliation of zeolites β in an alkaline medium

Materials Chemistry and Physics 63 (2000) 55–66

Boronation and galliation of zeolites ␤ in an alkaline medium Chun Yang a,∗ , Qinhua Xu b , Cheng Hu c a

c

Department of Chemistry, Nanjing Normal University, Nanjing, 210097, China b Department of Chemistry, Nanjing University, Nanjing, 210093, China National Key Laboratory of Solid Microstructure, Nanjing University, Nanjing, 210093, China Received 13 April 1999; received in revised form 9 July 1999; accepted 17 August 1999

Abstract Zeolite ␤ (with a Si/Al ratio of 15.20) was boronated or galliated in an alkaline solution containing boron or gallium species. The boronated or galliated samples were characterized by XRD, FTIR, MAS NMR and N2 sorption measurement and the process for boronation/galliation was understood upon the characterization. By comparing with the alumination process, a difference between boronation/galliation and alumination was shown, i.e., boron or gallium atoms insert into the framework with more difficulty than aluminum atoms, resulting in a limited degree of introduction of the heteroatoms and a modification of the porous property of zeolite under the conditions similar to those for alumination. In combination with the deformability of beta framework, the state and the stability of boron atom in the framework were also discussed. The poor stability of boron or gallium atom in the framework and their unsuitable atomic sizes should be responsible for the low level of boronation or galliation. ©2000 Elsevier Science S.A. All rights reserved. Keywords: Zeolites ␤; Boronation; Galliation; State and stability of boron

1. Introduction It is well known that the elements in the framework of zeolite molecular sieves greatly influence their properties and behaviors. For example, B-containing and Ga-containing zeolites possess weaker acidity than their aluminosilicate analogue and exhibit better catalytic activities and selectivities in some reactions [1–5]. The introduction of boron or gallium atoms into the framework of zeolites can be realized by means of direct hydrothermal synthesis [6–13] or so-called post-synthesis [2,14–16]. Up to now, the post-syntheses of B-containing or Ga-containing zeolites are mainly focused on ZSM-5, Y and mordenite, whereas B- or Ga-containing zeolites ␤ were generally synthesized directly. Liu et al. [16] incorporated gallium atoms into zeolite ␤ by treating the zeolite with NaGaO2 solution and investigated the mechanism for the galliation; however, as far as we know, no post-synthesis of B-containing zeolites ␤ is reported till now. In the previous work [17], we successfully aluminated zeolites ␤ with NaAlO2 solution, enhancing the content of aluminum in the framework of zeolite ␤ dramatically (the number of aluminum atoms in unit cell (uc) was increased from 2.8 to 14.3 per unit cell). It was found that the aluminum content of sample was associated with the conditions ∗

Corresponding author.

for alumination. In a given range, increasing the amount of aluminum species added in the solution, the temperature, the alkalinity of solution or the repeated alumination times was favorable to the insertion of aluminum atoms into the framework. The investigation of alumination process and mechanism showed the existence of transient-state aluminum species, non-framework aluminum species and structural vacancies in the parent zeolite ␤. During the alumination, the transient-state aluminum was converted back into tetrahedrally coordinated framework aluminum upon Na+ ion-exchange, the non-framework aluminum species were dissolved into the solution and reinserted back into the framework under the alkaline conditions. Upon the action of base, a part of silicon atoms in the lattice was extracted out of the framework, and the aluminum species added in the solution entered the framework by occupying the structural vacancies and substituting the framework silicon atoms. In the present paper, the boronation and the galliation of zeolites ␤ are performed in a way similar to alumination. It is found that the process for boronation or galliation is very different from that for alumination. In substance, this difference reflects whether different trivalent atoms enter the framework with the same ease or not, and whether the stability of these atoms in the framework is identical or not.

0254-0584/00/$ – see front matter ©2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 2 0 5 - 9

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C. Yang et al. / Materials Chemistry and Physics 63 (2000) 55–66

2. Experimental 2.1. Materials and boronation/galliation procedure The zeolite ␤, synthesized using hydrothermal method, was kindly provided by zeolite plant of NanKai University and labeled as TP-␤. It was calcined at 550◦ C for 9 h in an oxygen flow to decompose the template. The zeolite ␤ after calcination, with a Si/Al ratio of 15.20 (chemical analysis value), was labeled as dTP-␤-1 and used as the parent material for boronation and galliation. NaBO2 solution was prepared by dissolving a certain amount of B2 O3 in a NaOH aqueous solution. NaGaO2 solution was prepared as follows: a certain amount of metal gallium was dissolved in a concentrated HCl solution, NH4 OH was then added to precipitate Ga(OH)3 , filtrating this sediment and dissolving it in a NaOH aqueous solution to obtain the NaGaO2 solution. The dTP-␤-1 was boronated with the NaBO2 solution (0.1 M) under stirring. The ratio of the weight of the zeolite to the volume of the solution (g ml−1 ) was 1:30. The suspension was heated to 70◦ C and maintained at that temperature for 6.5 h, and the pH value of the system was adjusted and retained at 12.0–12.3 with a concentrated solution of NaOH. The solid product was then separated from its mother liquor by filtration, washed with a solution of 0.05 M NaOH until no boron species in the filtrate was detected, and washed again with distilled water until the pH of the filtrate was 6–7, then dried at 120◦ C. The sample thus obtained was divided into two parts, one of them was designated as [B]-Na␤-1; the other was treated once again by the above-mentioned procedure and was then called [B]-Na␤-2. The dTP-␤-1 was galliated with the NaGaO2 solution (0.05 M). The ratio of the weight of zeolite to the volume of the solution (g ml−1 ) was 1 : 20. Other parts of the procedure were similar to those for boronation, and the obtained samples were labeled as [Ga]-Na␤-1 and [Ga]-Na␤-2. For comparison, several aluminated zeolite ␤ samples were also prepared with NaAlO2 aqueous solution and dTP-␤-2 obtained by calcining TP-␤ at 550◦ C for 12 h. A detailed procedure was described in our previous work [17]. In the aluminated samples labeled as [Al]-Na␤, the amount of introduced aluminum atoms was controlled by adjusting the alkalinity and the temperature of solution as well as the amount of NaAlO2 added in the solution.

6 mg cm−2 ), used in hydroxyl vibration region studies, were heated to 450◦ at a rate of 5◦ C min−1 under vacuum, and evacuated at that temperature to 1.3 × 10−2 Pa, then cooled to ambient temperature for recording the IR spectra. IR spectra for studying the effect of pyridine and water on the state of boron were also recorded on the Nicolet 510P FTIR spectrometer. Self-supporting wafers (ca. 6 mg cm−2 ) were heated to 450◦ C at a rate of 5◦ C min−1 under vacuum, contacted with O2 at that temperature for 20 min and further evacuated at the same temperature for 3 h. The surface-cleaned samples were cooled to room temperature and exposed to pyridine vapor or atmosphere for 10 min, followed by evacuation at a given temperature for 0.5 h, then the spectra were recorded at room temperature. Solid MAS NMR spectra were obtained from a Bruker MSL-300 NMR spectrometer (7.0 T magnetic field). The rotor was spun at 2.5–3.5 kHz. The samples were hydrated by exposing them in atmosphere for a long time before the experiment. Other experimental conditions were for 11 B: 96.25 MHz spectra frequency, 0.5 s recycle time, spectra collected over 5000 scans with BF3 ·Et2 O as external reference; for 71 Ga: 91.49 MHz spectra frequency, 0.6 s recycle time, spectra collected over 100 000 scans with Ga(H2 O)3+ 6 as external reference; for 27 Al: 78.17 MHz spectra frequency, 0.1 s recycle time, spectra collected over 4000 scans 29 with Al(H2 O)3+ 6 as external reference; for Si: 59.60 MHz spectra frequency, 10 s recycle time, spectra collected over 2000–4000 scans with TMS as external reference. Sorption measurements for studying the microporous properties were conducted with N2 as adsorbate at liquid N2 temperature on an automated physisorption instrument (ASAP 2000, Micromeritics Corporation). Before measurement, the samples were degassed at 350◦ C for 4–5 h until the vacuum of system was better than 0.67 Pa. The data for micropore and mesopore were calculated by t-plot and BJH method (using desorption curve), respectively. The single point total pore volume at high relative pressure was taken as the total volume. The Si/Al ratios of samples were determined by conventional chemical analysis, and the contents of boron, gallium and sodium were analyzed by using the inductively coupled plasma (ICP) technique.

3. Results and discussion

2.2. Characterization

3.1. Characterization of the boronated/galliated samples

XRD determination was carried out on a Rigaku D/Max-␥A X-ray diffractometer at 30 kV and 50 mA with Cu K␣ radiation, using NaCl as an internal standard for the measurement of unt cell parameters. IR spectra for the framework vibrations were recorded on a Nicolet 510P FTIR instrument with a resolution of 2 cm−1 using the KBr wafer technique. Self-supporting wafers (ca.

3.1.1. Analysis on the compositions of samples The Si/Al and other atomic ratios of boronated and galliated samples obtained from chemical analysis are listed in Table 1. If aluminum in the samples are not lost during the boronation or the galliation (i.e., the non-framework aluminum species produced in template-removing process are reinserted into the framework in alkaline medium, which

C. Yang et al. / Materials Chemistry and Physics 63 (2000) 55–66 Table 1 Compositions of the boronated and galliated samples

57

Table 3 Mean compositions of unit cells assumed to possess no structural vacancy

Samples

Si/Al 1(Si/Al)a Si/B(Ga) B(Ga)/Al B(Ga)/Mb Si/Mb

Samples

Compositions of unit cell

dTP-␤-1 [B]-Na␤-1 [B]-Na␤-2 [Ga]-Na␤-1 [Ga]-Na␤-2

15.20 10.36 8.10 11.30 10.51

[B]-Na␤-1 [B]-Na␤-2

Na7.02 Al5.50 B1.52 Si57.00 O128 Na8.39 Al6.87 B1.52 Si55.61 O128

[Ga]-Na␤-1 [Ga]-Na␤-2

Na8.46 Al4.92 Ga3.54 Si55.55 O128 Na9.14 Al5.22 Ga3.92 Si54.86 O128

a b

– 4.84 7.10 3.90 4.69

– 37.60 36.60 15.68 14.03

– 0.28 0.22 0.72 0.75

– 0.22 0.18 0.42 0.43

15.20 8.12 6.63 6.57 6.01

1(Si/Al) = 15.20-Si/Al. M = trivalent elements.

has been proved to be possible in our previous work [17]), the decrease in Si/Al ratios of the boronated and the galliated samples should be caused by the removal of silicon from the framework. The values of 1(Si/Al) in Table 1 represent the number of dissolved silicon (expressed as Si atom/Al atom). For the boronated or the galliated samples, 1(Si/Al) ratios are much greater than B(Ga)/Al ratios, indicating that the number of boron or gallium atoms inserted into the framework is much less than that of removed silicon atoms, i.e., a number of vacancies resulted from silicon removal are not filled by trivalent elements and remain in the framework, consistent with our observation that the masses of the samples decrease after the boronation or the galliation. It can also be deduced that the vacancies are more in the boron-containing samples than in the gallium-containing samples because the 1(Si/Al) values of the former are greater than those of the latter. On the supposition that the total number of unit cells keeps invariable and no aluminum atoms are lost during the boronation or the galliation, the composition of the unit cell and the population of vacancies can be estimated as listed in Table 2. It can be seen that the vacancies occupy about 30–50% of total T sites after the boronation or the galliation. Can the framework be stable in the case of so many vacancies existing? Here, it should be noted that the population of vacancies thus obtained by chemical analysis is only a bulk average result. The composition on the surface of crystallites is actually different from that in the bulk because the dissolution of silicon starts first from the surface, so that the vacancies on the surface are much more than those in the interior of crystallites. Such a large number of vacancies on the surface will result in collapse and dissolution of the surface parts of crystal particles. Therefore, the number of unit

cells in the treated samples is actually less than that in the parent sample, whereas aluminum atoms in each unit cell of treated samples should be more than those in parent sample. Boron or gallium atoms in each unit cell should also be more than those showed in the unit cell composition in Table 2, especially for the sample [B]-Na␤-2 in which silicon are removed more severely. On the other hand, if all the 64 T sites are occupied by silicon and trivalent atoms, we can give another set of unit cell compositions as shown in Table 3. The real composition of a unit cell should be between these two sets of compositions, i.e., the 64 T sites are neither occupied completely nor vacated so severely that the collapse of the framework occurs. It is also found from Table 2 or Table 3 that the introduction of boron or gallium atoms, particularly boron atoms, into the framework is very limited as compared with the insertion of aluminum atoms in alumination. The number of boron or gallium atoms does not increase significantly even though the secondary boronation or galliation is performed. For [B]-samples, even no increase of boron atoms is observed after the secondary treatment, and more severe removal of silicon from the framework takes place, indicating the treatment at twice is hardly effective, very different from the case in alumination. 3.1.2. X-Ray diffraction The X-ray diffraction patterns of samples before and after the borontion or the galliation are shown in Fig. 1. It can be seen that the boronated or the galliated samples are pure zeolites ␤ with no other phases, and their zeolite structure is essentially retained. The change of unit cell parameters is usually given as an evidence for the incorporation of heteroatoms into the framework since the introduction of T atoms with longer

Table 2 Mean compositions of unit cells assumed to possess the largest structural vacancy numbers Samples

Compositions of units cellsa

Vacancies/total T sites (%)b

[B]-Na␤-1 [B]-Na␤-2

Na4.41 Al3.46 B0.95 Si35.85 O128 [ ]23.74 H94.96 Na4.23 Al3.46 B0.77 Si28.03 O128 [ ]31.74 H126.96

37 50

[Ga]-Na␤-1

Na5.95 Al3.46 Ga2.49 Si39.10 O128 [ ]18.95 H75.80

30

[Ga]-Na␤-2

Na6.05 Al3.46 Ga2.59 Si36.36 O128 [ ]21.59 H86.36

34

a b

[ ] refer to structure vacancy in the framework. Total T sites in unit cell = 64.

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of galliation is greater. Nevertheless, whether this increase results from the incorporation of boron or gallium atoms or not should be concluded after a careful analysis because the reinsertion of the non-framework aluminum species into the framework is accompanied during the boronation or the galliation. Comparing the increment of unit cell volume (1V) of aluminated sample listed at the bottom in Table 4 with that of boronated or galliated sample, we find that the 1V of boronated or galliated sample is close to or greater than that of aluminated sample although the Si/M ratio of the former is greater than or close to that of the latter. This suggests that the increase in the size of unit cell results not only from the reinsertion of aluminum but also from the incorporation of boron or gallium atoms with a longer T–O bond (or larger O–T–O angle), i.e., boron or gallium atoms are introduced into the framework of zeolite ␤. Since Ga–O bond (0.182 nm) is longer than Si–O bond (0.161 nm), there is no question of the increase in the size of unit cell after the galliation. However, B–O bond is generally considered to be shorter than Si–O bond (e.g., the length of tetrahedral B–O bond in NaBSi3 O8 is 0.1443–0.1478 nm [18]). It is even reported that the unit cell sizes of the hydrothermally synthesized borosilicates with MFI structure [7,19] or Nu-1 structure [8] are smaller than those of their pure siliceous analogues, and their unit cell volumes decrease with the increase of the boron content in the framework [7,19]. Thus, the unexpected increase in unit cell sizes of our boronated samples probably arises from their BEA structure. Anyway, from the change of unit cell dimensions, we are impressed that the B–O bonds or O–B–O angles in our [B]-zeolites ␤ are actually larger than those expected. Fig. 1. X-ray diffraction pattern of samples: (a) dTP-␤-1, (b) [Ga]-Na␤-1, (c) [Ga]-Na␤-2, (d) [B]-Na␤-1 and (e) [B]-Na␤-2.

T–O bond or larger O–T–O bond angle will result in the increase in the size of unit cell. The data listed in Table 4 are mean values of four set of unit cell parameters calculated from the diffraction peaks of (008), (302), (306). From these data, it is evident that both the boronation and the galliation lead to the increase in the unit cell size, and the effect

3.1.3. IR spectra in framework region The IR spectra in framework vibration region for the boronated and the galliated samples are shown in Fig. 2. The frequency for T–O asymmetric stretching vibration (ν a(OTO) ), which is sensitive to the Si/M ratio in the framework, is listed in Table 4. The bathochromic shift of ␯a(OTO) caused by the introduction of boron or gallium atoms into the framework has been reported by many other authors

Table 4 Lattice parameters and T–O asymmetric stretching frequencies of samples Samples

Si/Ma

B(Ga)/Ma

Lattice parameter (nm) a

c

Vuc (nm3 )

4 Vuc (%)b

ν a(OTO) (cm−1 )

dTP-␤-1 [B]-Na␤-1 [B]-Na␤-2 [Ga]-Na␤-1 [Ga]-Na␤-2

15.20 8.12 6.63 6.57 6.01

– 0.22 0.18 0.42 0.43

1.2478 1.2491 1.2508 1.2566 1.2551

2.6319 2.6313 2.6331 2.6409 2.6631

4.098 4.105 4.119 4.170 4.195

– 0.2 0.5 1.8 2.4

1089 1071 1066 1065 1063

dTP-␤-2 [Al]-Na␤-1 [Al]-Na␤-2

15.20 6.69 5.39

– – –

1.2463 1.2466 1.2486

2.6378 2.6453 2.6501

4.097 4.111 4.132

– 0.3 0.9

1090 1076 1073

a b

M = trivalent elements. 4 Vuc = [(Vuc −Vuc(dTP-␤) )/Vuc(dTP-␤) ]·100

C. Yang et al. / Materials Chemistry and Physics 63 (2000) 55–66

Fig. 2. IR spectra in framework region for boronated/galliated samples: (a) dTP-␤-1, (b) [B]-Na␤-1, (c) [B]-Na␤-2, (d) [Ga]-Na␤-1 and (e) [Ga]-Na␤-2.

[6,9,10,12]. We also find this frequency to shift towards lower wavenumber after the boronation or the galliation, and the ν a(OTO) of boronated or galliated sample is lower than that of aluminated one with a lower or close Si/M ratio (see the bottom in Table 4). This indicates that the incorporation of boron or gallium atoms occurs during the boronation or the galliation, and the contribution to the shift of ν a(OTO) from the incorporation of boron or gallium atoms is greater than that from the incorporation of aluminium atoms. The shift of ν a(OTO) to lower wavenumber caused by the galliation is due to the substitution of weaker Ga–O bond for Si–O bond and of more weighty gallium atom for silicon atom; whereas the bathochromic shift resulting from the boronation should be solely contributed by the less force constant of B–O bond, i.e., the weaker B–O bond, coincident with the impression made by XRD results that the B–O bond in [B]-zeolite ␤ is not as short as expected. In the IR spectra for the framework vibration, the band at ∼ 950 cm−1 is assigned to the vibration of Si–O in the structural vacancies [20]. For the parent sample (dTP-␤-1), a very weak band at ∼ 950 cm−1 (see Fig. 2a) suggests no severe dealumination occurring during the removal of template. After the galliation, this band almost completely disappears (see Fig. 2d, e), suggesting that this type of vacancies have been filled by gallium and non-framework aluminum species or transformed into other type of structural defect with a variant micro-surrounding (e.g., larger pore caused by a local collapse). Different from the galliated samples, a weak shoulder band seems to appear in the range of 900–1000 cm−1 in the spectra of boronated samples (see Fig.

59

Fig. 3. IR spectra in hydroxyl vibration region for boronated/galliated samples: (a) dTP-␤-1, (b) [Ga]-Na␤-1, (c) [Ga]-Na␤-2, (d) [B]-Na␤-1 and (e) [B]-Na␤-2.

2b, c). It has been reported [9,10] that there is a framework Si–O–B vibration band in the same range for [B]-ZSM-5; but the specific location of this band has not reached unanimity yet. Someone reported it at 970 cm−1 [9], the others thought it at 905 cm−1 [10], while the B(OH)− 4 species in solution was reported at 950 cm−1 [21]. In any case, the occurrence of shoulder band in this region for the boronated sample suggests the insertion of boron atoms into the framework. 3.1.4. IR spectra in hydroxyl vibration region Fig. 3 shows the IR spectra in OH vibration region for the samples before and after the boronation or the galliation. At least four bands present in the spectrum of dTP-␤-1 (parent sample) at 3610, 3667, 3745 and 3784 cm−1 (Fig. 3a). They were attributed to bridging OH group (3610 cm−1 ), non-framework AlOH group (3667 cm−1 ), terminal SiOH group (3745 cm−1 ) and transient-state AlOH group (3784 cm−1 ), respectively [17,22]. The broad 3745 cm−1 band contains a contribution from internal SiOH band located at 3738 cm−1 , which is related to the transient-state SiOH group [17,22]; moreover, some internal SiOH associated with (SiOH)4 nest at structure vacancy is also reported to hind in this band [23–25]. After the boronation or the galliation (Fig. 3b–e), the bridging OH band disappears because of the exchange of Na+ for H+ . The bands for the non-framework AlOH, as well as the transient-state AlOH and SiOH, also disappear, indicating that these species are reinserted into the framework or restore the tetrahedral configuration, similar to the cases in alumination [17]. The band at 3676 cm−1 , which was

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observed for the aluminated samples and cannot be definitely assigned for the time being [17], also exists in the spectra of all the boronated or the galliated samples. This suggests, as least from an angle of view, that this band should not be attributed to the non-framework species of aluminum, gallium or boron because the band should be at different location of frequency for the non-framework species associated with different trivalent atoms instead of at the same location. Combining with the Na/M ratios of boronated or galliated samples approximate to 1, we can infer that the trivalent elements in the samples are located in the lattice. Nevertheless, a distinct difference is found between the spectra of boronated/galliated samples and aluminated samples. The intensity of SiOH band at 3745 cm−1 pronouncedly decreases after the alumination [17], but it is not obviously changed for the galliated samples and even slightly enhanced for the boronated samples (Fig. 3b–e). This suggests that few of boron or gallium species react with SiOH groups on the surface or at the vacancies, and the structure defects even increase duo to the dissolution of silicon in the framework, especially for the boronated samples.

3.1.5. 11 B, 71 Ga, 27 Al and 29 Si MAS NMR A more direct evidence for the incorporation of boron or gallium atoms into the framework and for their coordination states is given by 11 B or 71 Ga MAS NMR spectrum of boronated or galliated sample (Fig. 4). For the boronated sample, 11 B spectrum (Fig. 4a) exhibits only one narrow line at −3.5 ppm from BF3 ·Et2 O as external reference, assigned to BO4 tetrahedrons in zeolite framework [9,26,27]. No signals are observed for BO4 units in an amorphous structure (∼ −0.3 ppm [28,29]). Therefore, it can be concluded that the boronation in an alkaline medium leads to boron species in a fully crystalline structure. Since the sample has been hydrated before the NMR investigation, we cannot find from Fig. 4a the coordination of boron in corresponding dehydrated sample. However, it should not be excluded that the other coordination state of boron appears after dehydration occurs (see infra) because the configuration of boron in framework has been reported to depend strongly on the degree of hydration of samples for several types of zeolites [9,26,27]. For the galliated sample, the typical signal of a tetrahedral Ga species at ∼157 ppm [13,30] is observed in 71 Ga NMR spectrum (Fig. 4b) even though the signal-to-noise ratio is low. No resoluble non-framework octahedral Ga is found at around 0 ppm. 27 Al NMR spectra are shown in Fig. 5. For the parent sample (Fig. 5a), two lines are clearly exhibited at ∼54 and 0 ppm, respectively. The former is characteristic of four-coordinated aluminum atoms in framework, the latter is corresponding to octahedral non-framework aluminum resulted from the dealumination during the removal of template, consistent with the observation from IR spectrum in hydroxyl region. For the boronated and galliated sample

Fig. 4. 11 B and 71 Ga MAS NMR spectra: (a) 11 B spectrum of [B]-Na␤-1, (b) 71 Ga spectrum of [Ga]-Na␤-2 and (*) spinning side-bands.

Fig. 5. 27 Al MAS NMR spectra: (a) dPT-␤-1, (b) [B]-Na␤-1, (c) [Ga]-Na␤-2 and (*) spinning side-bands.

C. Yang et al. / Materials Chemistry and Physics 63 (2000) 55–66

Fig. 6.

29 Si

61

MAS NMR spectra: (a) dTP-␤-1, (b) [B]-Na␤-1 and (c) [Ga]-Na␤-2.

(Fig. 5b,c), only tetrahedral framework aluminum is found. This further verifies that the non-framework aluminum is reinserted into the framework during the treatment, and no non-framework trivalent elements exist in boronated or galliated samples. 29 Si MAS NMR technique has been widely used to study the modification of chemical surrounding of silicon atom in zeolite framework. For zeolite ␤ with a moderate Si/Al ratio, the 29 Si spectrum consists of framework lines of 2Si(0Al), Si(1Al) and Si(2Al) centered at ∼ −115, ∼ −110, ∼ −104 and ∼ −98 ppm, respectively [13,24]. The experimental 29 Si NMR profiles of our samples are shown in Fig. 6. Using Gaussian line-shapes, several lines characteristic of different chemical surrounding of silicon were deconvoluted from the spectra of parent and boronated/galliated sample. Two lines centered at ∼ −110 and ∼ −114 ppm are ascribed to Si(0M) of different crystallographic sites. The line of Si(2M) is hardly observed on the parent sample (Fig. 6a). However, accompanied by the increase of intensity of Si(1M)

line, Si(2M) and even Si(3M) lines, centered at ∼ −97 and ∼ −91 ppm, respectively, appear in the spectra of boronated and galliated samples (Fig. 6b,c), indicating the introduction of trivalent elements into the framework upon the boronation or the galliation. It should be noted that the contribution of Si(3Si, 1OH) species, which was reported to locate at ∼ −102 ppm [13,24], is included in Si(1M) line. The omission of this defect signals from deconvoluted spectra causes an overestimation of intensity of Si(1M) line and must influence the accuracy of (Si/Al)NMR ratio. It can be seen from Table 5 that the (Si/Al)NMR is lower than the corresponding value determined by chemical analysis, especially for dTP-␤-1 with more Si(3Si, 1OH) species, which come from the inherent structure vacancies and the vacancies formed by dealumination. As the trivalent elements are incorporated, Si(3Si, 1OH) species decrease due to both its extraction from the framework under the action of base and the insertion of trivalent elements into the vacancies. Thus, the

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Table 5 Si/Al ratios of samples Samples

(Si/Al)CA a

(Si/Al)NMR b

dTP-␤-1 [B]-Na␤-1 [B]-Na␤-2 [Ga]-Na␤-1 [Ga]-Na␤-2

15.20c 8.12 6.63 6.57 6.01

11.01 8.08 – 5.96 5.76

a

Determined by chemical analysis. Calculated with the line intensities P in 29 Si NMR spectra according to P the following equation: (Si/Al)N MR = 4n=0 ISi(nM) / 4n=0 0.25nISi(nM) . c The framework Si/Al ratio of this sample is 17.18, determined by T–O asymmetric stretching frequency in framework IR spectrum. b

Table 6 Pore volumes of samples Samples

Si/M

Vt (ml/g)

Vmicro (ml/g)

Vmeso (ml/g)

dTP-␤-1 [B]-Na␤-1 [B]-Na␤-2 [Ga]-Na␤-1 [Ga]-Na␤-2

15.20 8.12 6.63 6.57 6.01

0.404 0.663 0.632 0.457 0.509

0.164 0.113 0.146 0.131 0.114

0.319 0.607 0.513 0.388 0.444

dTP-␤-2 [Al]-Na␤-3 [Al]-Na␤-4

15.20 6.37 3.46

0.549 0.298 0.209

0.163 0.093 0.087

0.403 0.282 0.170

difference between (Si/Al)NMR and (Si/Al)CA is decreased for the boronated or galliated sample. This variation in (Si/Al)NMR also suggests us that the SiOH groups shown in the OH-IR spectra of boronated/galliated samples little include Si(3Si, 1OH) species. 3.1.6. Microporosity and mesoporosity The porous volumes measured by N2 adsorption are listed in Table 6, the data for aluminated samples are also given in the bottom lines for comparison. After the boronation or the galliation, the total porous volumes (Vt ) of the samples increase, contrary to the case after alumination. This change in total porous volume is also demonstrated by the measurement of benzene adsorption capacity (not shown here). The difference between the porosity modification caused by the alumination and that caused by the boronation/galliation may be understood as follows: (1) A number of aluminum atoms and Na+ cations are introduced into the sample during the alumination, leading to the increase in the average masses of crystal particles and the decrease in the number of particles in unit weight of sample, thus, the total porous volume (ml g−1 ) reduces. Conversely, the average masses of particles decreases during the boronation or the galliation owing to a limited introduction of trivalent atoms and Na+ cations and a severe dissolution of silicon, thereby, the total porous volume (ml g−1 ) increase. (2) During the alumination, Na+ cations introduced in large amount take up considerable space in the channels or pores, resulting in the decrease of total porous volume. But this effect hardly exists in the boronation or the galliation because the introduction of Na+ cations is limited in these processes. (3) The trans-

Fig. 7. Mesopore distribution for boronated/galliated samples (–··–·) dTP-␤-1, (——) [Ga]-Na␤-1, (·········) [Ga]-Na␤-2, (- - - - ) [B]-Na␤-1, (–·–·) [B]-Na␤-2.

formation of pore size during the alumination is just contrary to that during the boronation or the galliation. As can be seen from Table 6, although both the mesoporous volume and microporous volume decrease after the alumination, the former changes more than the latter. This suggests that the decrease of mesoporous volume is caused not only by the reduction in the number of particles in unit weight of sample and by the occupation of partial space in the pore by Na+ , but also by the degradation of some intracrystalline mesopores into micropores owing to the filling of vacancies and defects belonging to these mesopores by aluminum species. As a result of this transformation of mesopores into micropores, the number and the volume of micropores do not decrease too much after the alumination. However, an inverse conversion between micropores and mesopores occurs upon boronation or galliation. It is seen from Table 6 that the mesoporous volumes increase and the microporous volumes decrease after the boronation/galliation, meaning that not only the intracrystalline mesopores are not decreased, but also some micropores are developed into mesopores due to the removal of silicon from the framework. Thereby, the total porous volume increases. The distributions of pore diameter in 2–100 nm region, from which the modification of mesoporosity of the boronated/galliated samples can be investigated, are shown in Fig. 7. Two main peaks appear for all the samples, one located in the region bellow 10 nm (peak 1), the other is in the region above 10 nm (peak 2). The former should be ascribed to the intracrystalline mesopores while the latter may be associated with the interparticle mesopores. From this figure the following facts can be found: (1) After the boronation or the galliation, the porous volumes of the intracrystalline mesopores (peak 1) increase, in accordance with above observation that some of the micropores are

C. Yang et al. / Materials Chemistry and Physics 63 (2000) 55–66

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developed into the mesopores. The mesoporous volumes of boronated samples larger than those of galliated samples indicate that silicon atoms are extracted more severely during the boronation. (2) After the secondary boronation or galliation, the diameters of the intracrystalline mesopores increase (peak 1 shifts right), i.e., the further dissolution silicon takes place during the secondary treatment. (3) An obvious decrease in the diameters of the interparticle mesopores is observed after the boronation or the galliation (peak 2 shifts left), meaning that the corrosion and dissolution of the surface of crystallite occur, reducing the size of crystallite. For [B]-Na␤-2, the dissolution of the surface is so severe that the diameter of interparticle mesopore decreases significantly and even approximates to that of the enlarged intracrystalline mesopore, giving rise to almost continuous change in the diameters of the intracrystalline mesopores and the interparticle mesopores (see Fig. 7). The above results obtained from N2 sorption further support the conclusion drawn from the composition analysis and the IR spectra in hydroxyl region.

3.2. Comparison of boronation/galliation process with alumination process According to the foregoing characterization, we can describe the boronation or the galliation process of zeolites ␤ as follows: boron or gallium atoms are inserted into the framework of zeolite ␤ by treating the zeolite sample with an alkaline solution containing boron or gallium species. However, their insertion is very limited, especially for boron atoms. Accompanied by the boronation or the galliation of the framework, a small amount of non-framework aluminum and transient-state aluminum species in the parent sample is reinserted into the framework or restores tetrahedral configuration. Meanwhile, silicon atoms are dissolved from the lattice in a considerable amount, especially for the boronated samples, resulting in that the outer layer of crystallite is corroded and the size of crystalline particle is reduced. Although the experimental conditions for borontion or galliation are similar to those for alumination, a significant difference in the amount of inserted trivalent elements and of extracted silicon has been exhibited, except that the non-framework aluminum and the transient-state aluminum behave similarly in these processes. This difference mainly results from that boron or gallium atoms insert into the framework with more difficulty than aluminum atom because the atomic size of the former is smaller or larger than that of the latter, unsuitable to the size of the structural vacancy. Thus, the speed of dissolution of silicon is enhanced and more silicon atoms are extracted. This situation can be explained more clearly with Fig. 8. The arrows in Fig. 8 refer to the Si(3Si, 1OH) sites in the framework, they disappear when a trivalent atom inserts into the vacancy lying at the center site. In the case shown by Fig. 8a, one Si(3Si, 1 OH) disappears after the vacancy is filled; two Si(3Si, 1

Fig. 8. Relationship between structural vacancies and Si(3Si, 1OH) sites (⊕) vacancies, (O) Si, (䊉) Al.

OH) disappear in the case in Fig. 8b,. . . . . . on the analogy of this. Therefore, the number of the Si(3Si, 1OH) greatly depends on the number of the inserted trivalent atoms. Since Si(0Al) species are dissolved most easily among the tetrahedral framework silicon [31], whereas the dissolution of Si(3Si, 1OH) is easier owing to lack of a bond connecting to the framework, the less the trivalent atoms insert (i.e., the more the Si(3Si, 1OH) species remain), the more the silicon atoms are extracted, and the defects in lattice are enlarged to form larger secondary mesopores. Another reason why boron or gallium atoms insert into the framework with more difficulty than aluminum atoms may be their poor stability in the framework, especially boron atom, so that they escape easily from the lattice even after having entered the framework. Moreover, when investigating the hydrothermal condensation mechanism of B-containing zeolite with MFI structure, Ruiter et al. [21] proposed that the framework network was built by the condensation of silicon building units with boron units through the boron nuclei being nucleophilically attacked by Si–O− groups. Because the tetrahedrally coordinated boron species, B(OH)− 4 , cannot form the 5-coordinated sp3 d transition state required for SN 2-type reaction, it cannot condensed with Si–O− group, only B(OH)3 building unit can react with Si–O− group. But − Al(OH)− 4 or Ga(OH)4 species is not subject to this limit. Under our experimental conditions, B(OH)− 4 are predominant boron species [21] since the pH value of the solution is higher than 11, i.e., the connection of boron species with Si–O− groups cannot occur. This is also probably responsible for the insertion of boron atoms at a low level. In a word, the difference in character and atomic size between boron/gallium atom and aluminum atom makes the

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Fig. 10. The change of 1385 cm−1 band (on [B]-Na␤-2): (a) dehydrated at 450◦ C, (b) adsorbing pyridine at room temperature, (c) evacuated at 150◦ C after adsorption of pyridine, (d) evacuated at 250◦ C after adsorption of pyridine and (e) evacuated at 350◦ C after adsorption of pyridine.

Fig. 9. The change of 1385 cm−1 band (on [B]-Na␤-1): (a) dehydrated at 450◦ C, (b) rehydrated at room temperature, (c) evacuated at room temperature, (d) evacuated at 120◦ C and (e) evacuated at 250◦ C.

boronation or the galliation under alkaline condition not so efficient as the alumination. 3.3. The states and the stability of boron in B-containing zeolites ␤ In our previous work [22], the states and the stability of aluminum in zeolits ␤ were investigated. It was indicated that, since the framework of zeolite ␤ was of a deformable character, the states of aluminum in the zeolite intensely depended on the nature of compensating cation. When pro-

tons, which possess high electron affinity, were located at cationic sites, the distortion and the tension in zeolite lattice occurred, leading to the breakage of Al–O bond and the removal of aluminum from the framework via transient-states. When H+ was replaced by other cation (e.g., Na+ ) or converted to ammonium, pyridinium and oxonium ion by adsorbing NH3 , pyridine and methanol, respectively, its high electron affinity was lost. Thus, the distorted lattice was relaxed so that the transient-state aluminum and even the non-framework aluminum went back into the framework. From the foregoing characterization, an impression is made on us that the B–O bond in zeolite ␤ is weaker and longer than that expected, and boron exists in the framework more unstably than aluminum and even gallium. Moreover, we find that a band appears at 1385 cm−1 in the IR spectrum of our boronated sample after evacuating this sample at high temperature (see Fig. 9a and Fig. 10a). This band has been ascribed to the tri-coordinated boron in the framework [9,10,19]. After the sample is rehydrated or adsorb pyridine, this band disappears (see Fig. 9b and Fig. 10b), but emerges again when the sample is dehydrated or pyridine is desorbed (see Fig. 9c–e and Fig. 10c–e). These phenomena imply some problems relevant to the states and the stability of boron in zeolite ␤, which will be discussed here.

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We begin the discussion with a summary and analysis of previous reports about B-containing HZSM-5 and H␤ to find out some regular pattern for state transformation. It has been reported that, after the template was removed from B-containing pentasil-type zeolites [9] or dehydration took place in hydrated H-boralite with ZSM-5 structure [10,19,26], a part of framework tetrahedral boron, which gave a narrow signal at ∼ −3.6 ppm in 11 B MAS NMR, transformed into trigonal boron in the lattice. But the tetrahedrally coordinated configuration could be restored when electron donor molecule, such as H2 O, NH3 , methanol or pyridine, was adsorbed on the samples [9,19]. The signal for trigonal boron are a doublet line in 11 B NMR spectrum [26] and a band at 1380 cm−1 in IR spectrum characteristic of B–O vibration in BO3 unit, respectively [9,10,19]. This process can be shown as follows:

65

size of boron atom. The poor stability of tetrahedral boron has been reported also in other types of B-containing zeolites [8,27]. B-containing zeolites ␤ have been less investigated. The studies [33,34] of the removal procedure of template in zeolites ␤ showed that, after as-synthesized sample was calcinated, the tetrahedral framework boron could partially convert to tri-coordinated boron species with a broad and complex 11 B NMR signal. These species could not restore to the tetrahedral configuration after being rehydrated but could be removed from the sample by an ion-exchange with NH4 NO3 solution, meaning that these tri-coordinated boron species have completely left off the framework to become the non-framework species:

This situation is very similar to the dealumination of H␤ as discussed in our previous work [22]:

A theoretical calculation made by Stave and Nicholas [32] further verifies and interprets the fact that the tri-coordinated boron exists in [B]-HZSM-5. For anhydrous ZSM-5 with H+ as counter cation, the B–O bond in B–O–Si bridge is much longer than other three bonds in BO4 unit. The O–B–O angle constituted by the three shorter B–O bonds is far from the tetrahedral angle (109◦ ) formed by sp3 hybridization but near to the sp2 hybridized trigonal angle (120◦ ). Then, the boron in anhydrous [B]-HZSM-5 exists almost in trigonally coordinated state as shown in following pattern:

This is the reason why the bridge OH band (∼3720 cm−1 ) in IR spectrum of B-containing HZSM-5 is close to the frequency of SiOH group, and the [B]-zeolites show only a very weak acidity. However, in the case without H+ as counter cation, the four B–O bonds in BO4 tetrahedron is not very different from each other, and the O–B–O angle is approximate to 109◦ . This indicates once again that the proton of high electron affinity deforms the framework, being unfavorable to the retention of tetrahedral configuration of the trivalent elements. This effect being greater on boron than on other trivalent elements may be in relation to the smaller

Evidently, there are some differences between H␤ and HZSM-5: (1) The trivalent elements in the framework of zeolites ␤ are more unstable than those in ZSM-5. Upon the same treatment, the trivalent elements (especially boron) in HZSM-5 suffer only from modification of coordination state (i.e., partial disconnection from the framework), while a considerable number of trivalent elements in H␤ are completely extracted out of the framework. We think that the high deformability of framework should be responsible for the easy breakage of T–O link and the poorer stability of trivalent elements in zeolites ␤. (2) For H␤, NH3 or pyridine or methanol can restore the non-tetrahedrally coordinated trivalent elements to tetrahedral configuration [35,36], but H2 O cannot. Thus, the signals corresponding to both non-tetrahedral and tetrahedral trivalent elements are simultaneously displayed in the NMR spectrum of hydrated H␤ [33,35]. For HZSM-5, however, even H2 O can restore trigonal boron in the framework to tetrahedrally coordinated state so that no noticeable signal for trigonal boron is observed in NMR spectrum and IR spectrum of hydrated HZSM-5 [9,26]. This difference is also caused by the greater deformability of framework of zeolites ␤ than that of ZSM-5. For a framework of greater deformability, counter proton distorts TO4 tetrahedron more severely. This greater tension can be released by adsorption of only molecules such as NH3 , pyridine and so on, which

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possess stronger electron-offering ability than H2 O, whereas for a less deformable framework, a slight distortion can be relaxed even by adsorption of H2 O. Now, let us discuss the case of boronated samples investigated here. As shown in Figs. 9 and 10, the state of boron can convert between tetrahedral and trigonal configuration, and this reversible transformation is promoted not only by adsorption–desorption cycles of pyridine but also by hydration–dehydration cycles, similar to the case of B-containing HZSM-5:

Here, two problems should be explained: (1) As indicated in our previous work [22], the distorted framework can be relaxed and the dealumination can be restrained by the substitution of Na+ for H+ . However, for our boronated samples, Na+ as compensating cation cannot stop the transformation of tetrahedral boron into trigonal boron owing to the poorer stability of boron. (2) In Figs. 9 and 10, 1385 cm−1 band characteristic of trigonal boron in the lattice reveals that boron does not completely disconnect from the framework upon desorption of pyridine and water. The trigonal boron can be converted to the tetrahedral configuration even by adsorption of H2 O, different from the case of H␤. This may be due to that the electron affinity of Na+ is much lower than that of proton. Thus, the tension of the framework is so limited that boron is not extracted from the framework and the distortion can be easily relaxed by the adsorption of H2 O with very weak electron-offering ability.

4. Conclusion Zeolites ␤ can be boronated or galliated by treating them with an alkaline solution containing boron or gallium species. However, the amount of inserted heteroatoms, especially boron, is limited, quite different from the incorporation of a number of aluminum atoms in alumination under the similar conditions. This is due to that the size of boron or gallium atom is unsuitable to that of structural vacancies, and the stability of boron or gallium atoms in the framework, especially in the deformable beta framework, is very poor. This poor stability also transforms the tetrahedral framework boron in Na forms of zeolites ␤ into the trigonal configuration upon the dehydration of samples. Since the trivalent heteroatoms are difficult to enter the framework, a number of silicon atoms are extracted out of the lattice, so that part of micropores transform into mesopores within the zeolite crystal and the outer surface of crystallite is corroded and dissolved.

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