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Acidity characterization of H-ZSM-5 catalysts modified by pre-coking and silylation
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
2269
ACIDITY C H A R A C T E R I Z A T I O N OF H-ZSM-5 C A T A L Y S T S M O D I F I E D BY P R E - C O K I N G AND S I L Y L A T I O N Wen-Hua Chen, 1 Frank Bauer 2, Evelin Bilz 2, Annette Freyer 2, Shing-Jong Huang l, Chiun-Shen Lai 1 and Shang-Bin Liul* 1Institute of Atomic and Molecular Sciences, Academia Sinica, PO Box 23-166, Taipei 106, Taiwan, R.O.C. 2Institut f'tir Oberfl~ichenmodifizierung,University of Leipzig, Permoserstral3e 15, D-04303 Leipzig, Germany.
ABSTRACT A comprehensive study on the acidic and coking properties of selectivated H-ZSM-5 catalysts during xylene isomerization has been made. The effects of surface modification by pre-coking and silylation on the nature and location of carbonaceous deposits and on related acidic features of the spent catalysts were respectively characterized by 129Xe NMR of adsorbed Xe and solid-state 31p MAS NMR of adsorbed phosphine oxide probe molecules.
INTRODUCTION Xylene isomerization, together with toluene disproportionation and transalkylation of toluene and benzenes are the major selective reactions commonly applied in the petrochemical industry to balance the strongest market demand for para-xylene [1-3]. The principle of the xylene isomerization is applicable to a number of toluene derivatives carrying functional groups such as hydroxyl, amino, nitrilo and halogen, zeolites of ZSM-5 type are particularly suited for such equilibrations. In principle, the para-isomers selectivity enhancement can achieved by increasing crystal size of the zeolite and reduction of surface acid sites by modification techniques, such as adsorption of voluminous amines, chemical vapor deposition of silanes, modification by P, B, Zn or Mg oxides, halogen-based treating agents and deposition of coke. Various modification techniques have been suggested to enhance the selectivity of H-ZSM-5 in aromatic hydrocarbon processing [4-7]. Chemical vapor/liquid deposition of organosilicon compounds (such as tetraethoxysilane or TEOS) and/or pre-coking treatment are known to effectively passivate the non-selective acid sites present on the external surface of zeolite crystallites. However, the modification procedures can affect the diffusion phenomenon of the reactants and products. Thus, further understanding of the effects of surface modification on acidity and coking of the zeolitic catalyst is crucial in optimizing industrial selectivation procedures. Application of 31p MAS NMR of adsorbed organic phosphine oxides has been shown to be especially useful in the characterization of zeolite acidity, since it provides information on the overall acid properties (namely nature, location, strength and concentration) [8-12]. Zhao et al. demonstrated a new method for concurrent qualitative and quantitative characterization of internal and external acid sites in H-ZSM-5 zeolite, which can be determined by a combination of elemental analysis and 31p MAS NMR of adsorbed phosphorous probe molecules [12]. Moreover, 129Xeis a nonreactive spherical molecule with a diameter of 0.44 nm and a 1/2-spin nucles. Polarization of its spherical electronic shell, during adsorption or due to collisions with other Xe atoms or with a solid, affects the NMR chemical shift. The techinque has been used to monitor the pore structure of porous materials. In this study, liquid deposition of TEOS and pre-coking by methanol decomposition were used to modify H-ZSM-5 catalysts. The aim of investigation is to discern and quantify the internal and external acid sites on the parent and modified H-ZSM-5 catalysts by mean of 31p MAS NMR using different adsorbed phosphine oxides in conjunction with elemental analysis. Whereas the variations of sorption capacity and the pore structure in the catalyst were monitored by xenon isotherm and 129XeNMR.
2270
EXPERIMENTAL
Surface modification methods The pre-coked sample (0.3 wt.-% carbonaceous deposits; pre-coking/ZSM-5) was prepared by heating a methanol loaded HZSM-5 (Si/A1 = 25; TRICAT, Germany; ZSM-5) up to 773 K followed by hydrogen flushing. For liquid deposition of TEOS (4 wt.-% SiO2; Si-CLD/ZSM-5), the sample was dispersed in acetone, after adding TEOS and a stoichiometric amount of water refluxed for 1 h, dried and finally calcined at 773 K. Catalytic studies with a p a r a - x y l e n e depleted industrial feedstock were conducted at 400~ in a fixed-bed microreactor [13]. Reaction products were analyzed on a Bentone 34/Didecyl-phthalate SCOT column. Characterization methods The variations of A1 coordination in the structural framework of parent and modified samples were determined by solid-state 27A1 MAS NMR. The changes in the acidity after modification were characterized solid-state 31p MAS NMR of two adsorbed probe molecules with different molecular size, namely, trimethylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO). Moreover, the adsorption capacity and pore structure of the samples were measured by xenon isotherms and t29Xe NMR experiments. All 27A1 and 3]p MAS NMR experiments were carried out on a Bruker MSL-500P at frequency 130.32 and 202.46 MHz and spinning rate 10-12 kHz, using 1 M AI(H20)6 +3 and 85% H3PO4 solution as the chemical shift reference, respectively. All 129Xe NMR spectra were obtained by a NMR spectrometer (Bruker MSL-300P) operating at the Larmor frequency of 83.012 MHz, using diluted xenon gas as the chemical shift reference. Detailed description of the related experimental set-ups and procedures of the above experiments have been described earlier [12, 14]. To afford the quantitative (i.e. concentration) determination of the acid sites, each adsorbate-loaded sample was also subjected to elemental analyses by ICP-MS. RESULTS AND DISCUSSION
Solid-state 27A! M A S N M R study It is well known that 27A1 MAS NMR can provide information about the coordination numbers of the alumina atoms in the zeolite catalyst. Figure 1 displays the 27A! MAS NMR spectra of hydrated H-ZSM-5 samples. In general, all samples show two main signals, one at ca. 55ppm is attributed to tetrahedral-coordinated aluminum, and the other at ca. 0ppm is attributed to octahedral-coordinated aluminum. (Figure 1) It is found that the framework A1 still remains although the peak at 0 ppm corresponding to extra-framework AI became broad after modification.
~
~.~
ZSM-5
~-~
460 12 ~i-G LI~'Zi'i-~
M-5
I
~
140
G, 0,1 0
P.
X
it 120 gaC.l.l~'l~'d- ~,
o
"tJo" eb " do" ~ "-do "-do" C~emical skiff Opm) Figure l. 27A1MAS NMR spectra of hydrated ZSM-5 samples before and after surface modification.
o
4oo
soo
Pressure (Ton")
|
o
t2
Xe loaling (• 2~ a w ~ / g )
Figure 2. Xenon adsorption isotherms and the variations of ]29XeNMR chemical shifts with xenon density for the parent and modified HZSM-5 samples.
2271
Room temperature xenon isotherm and 129Xe NMR studies Xenon adsorption isotherms and the variations of 129XeNMR chemical shifts with xenon density for the parent and modified zeolite samples are presented in Figure 2. As shown in Figure 2, the isotherms show the presence of Langmuir-type adsorption within the pressure range covered. Only slight decreases in adsorption capacity as well as increase in 129Xe NMR chemical shifts are observed for both Si-CLD and pre-coking modified samples. In general, the observed 129Xe NMR chemical shift 8(9) of adsorbed xenon can be described by the sum of three contributions [15]:
(1)
8(J3 ) = 8 0 -k- 8 S ( p = 0 ) -t- (YXeP
Where 80 = 0 is the chemical shift reference and p represents the xenon loading density. The term 8s (p=0) represents the chemical shift at zero loading and can be divide into the sum of the interaction between Xe and the zeolite walls or coke residues or silane molecules. The value of 8s (p=0) can readily be obtained by extrapolating the straight line to the chemical shift axis. The last term is characteristic of the Xe-Xe interactions and is proportional to the density of the adsorbed Xe. The value of CrXewhich may be obtained from the slope of the 8(9) vs. p plot at high xenon loading. In this study, the related parameters obtained from 129XeNMR studies corresponding to xenon loading greater than 4 • 1020 atoms g-1 are listed in Table 1. It is generally accepted that the value of CYxeis inversely proportional to the effective pore free volume of zeolite. [ 15] Accordingly, the relative internal free volume accessible to xenon gas per gram of catalyst can be expressed as (2)
V/V0 = (~Xe) parent/ ((YXe) modified
Where V and V0 are the internal volumes of the modified and the parent samples, respectively. Additional information about the dimensions of the zeolite channels can be obtained by estimating the mean-free-path (A) of the adsorbed xenon from following empirical equation [16]: 8(p) = 243 •
(3)
The calculated values of V/V0 and A from Eq. (2) and (3) for the parent and the modified samples are listed in Table 1. Table 1. List of NMR parameters obtained from 129XeNMR studies. Sample
8s (p =0 )(ppm) ~xe(ppm/atom.g-1) x 1020
V/Vo(%) A (nm)
ZSM-5
95.9
4.3
100
0.315
Si-CLD/ZSM-5
99.2
4.5
95.6
0.298
Pre-coking/ZSM-5
99.8
4.6
93.5
0.295
The extrapolated values 8s (p=0) for the parent, silylated and pre-coking samples are 95.9, 99.2 and 99.8, respectively. Moreover, assuming that the internal volume of the parent sample is 100, together with the observed values of 95.6 and 93.5 for the silylated and pre-coking samples. These results show that a slight decrease in relative free volume (4-6 %) for the modification samples. Moreover, the free zeolite volume decreases to a slightly higher extent by pre-coking compared to silylation. It indicates that the coke reagents predominately located on the external surface of the H-ZSM-5 crystalline, only a few hydrolysis coke regents entering the internal channels. In the case of silylation sample, the pore-narrowing phenomenon was observed.
Solid-state 31p MAS NMR study 31p MAS NMR was utilized to investigate the effects of the modification procedures on acidity properties. Accordingly, the nature, strength and the distribution of acid sites (internal vs. external surface) in all samples were monitored by the changes in changes 31p MAS NMR chemical shifts arising from the adsorbed trimethylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO) probes. The size of TMPO (ca. 0.55 nm) is smaller than the pore aperture of H-ZSM-5, is accessible to both internal and external acid sites of the
2272 zeolite. Whereas the size of TBPO (ca. 0.82 nm) is too big to penetrate into the channels and hence can only be adsorbed on the external surface of HZSM-5. The 31p MAS NMR spectra obtained from the parent and modified samples loaded with TMPO and TBPO are shown in Fig. 3 along with the simulated spectra obtained by Gaussian deconvolution method. As shown in Figure 3(a), up to five characteristic peaks located at 85, 77, 66, 56 and 44 ppm (error + 1 ppm) were observed for the parent sample (HZSM-5), whereas only three resonance peaks (90, 72 and 55 ppm) were observed for TBPO. The peaks at 44 and 55 ppm can be unambiguously assigned to physisorbed TMPO and TBPO, respectively, whereas the other peaks observed at lower field are ascribed as due to interaction of the phosphine oxides probe molecules with acid sites on the H-ZSM-5 zeolite. In other word, there are four different acid sites (85, 77, 66 and 56 ppm) can be identified from 31p MAS NMR spectra of adsorbed TMPO and only two different acid sites (90 and 72 ppm) on the external surface of the H-ZSM-5. Early study [12] suggested that the chain length of the alkyl group on the phosphine oxide has nearly no effect on its corresponding difference in 31p chemical shift (AS) between the adsorbed and crystalline (39 ppm for TMPO and 47 ppm for TBPO). In other word, the change in electron density surrounding of 31p nuclei in TMPO and TBPO probe molecules is similar at a given acid site. Accordingly, in view of the similar A~5 values, it clearly indicates that the peaks at 85 and 66 ppm obtained from TMPO/HZSM-5 systems corresponding respectively to the peaks at 90 and 72 ppm from TBPO/HZSM-5 systems.
(a)
0~)
ZSM-5
,
A .
$i-CLD/ZSM-5 ,&
9
,&
ere-eoki~/ZSM-5
.
150
"
11~1]
Chemical
" shift
5'0 (ppm)
"
.
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.
.
.
.
.
.
.
150
.
.
.
.
.
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..
11J0
Chernical
-
~O
-
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shill: (pprn)
Figure 3. The 3,p MAS NMR spectra of (a) TMPO and (b) TBPO adsorbed on the parent (ZSM-5), silyated (Si-CLD/ZSM-5) and pre-coking (Pre-coking/ZSM-5) samples. The asterisks in the spectrum represent spinning sidebands.
Comparing the 31p MAS NMR spectra of adsorbed TMPO on the parent and modified samples, all the peaks are still presented in both modified samples, but the relative concentration of the peaks are changed (Figure 3(a)). For the silylated sample (Si-CLD/HZSM-5), an additional strong sharp peak at 30 ppm was observed, which can be attributed to "mobile" TMPO that either is attached in the intercrystalline voids or is weakly adsorbed near the opening of the channel pores of zeolites. This peak was also observed for another silica surface deposition samples. [8, 10] On the other hand, the 31p MAS NMR spectrum of adsorbed TBPO on the pre-coking sample (Figure 3(b)) reveals that the relative concentration of the peak at 90 ppm (external surface acid sites) is markedly reduced. The characteristics of 31p MAS NMR resonance observed using TMPO and TBPO molecules adsorbed on all samples can be quantified by correlation with the analytic results of Si, A1 and P elemental concentrations using ICP-MS. Together with information on the relative peak areas derived by Gaussain deconvolution of the 31p MAS NMR spectrum, the distribution of each acid site on all sample can be estimated by taking the total peak area responsible for acid sites as 100% and the results show in Table 2.
2273
Table 2.3~p MAS NMR chemical shift assignments and distribution of acid sites of the parent and modified HZSM-5 zeolites loaded with TMPO and TBPO probe molecules. (A& ppm)
a
Probe Molecule
44
~8
27
17
Physisorbed
TMPO
Chemical Shift (ppm) 85
77
66
56
,
ZSM-5 b
8.8 (0.040, 0.011)c
14.2 (0.083, --)
54.4 (0.284, 0.033)
22.6 % (0.131,--)
44 (P)
Si-CLD/ZSM-5 b
2.6 (0.004, 0.009)
13.1 (0.066,--)
60.9 (0.271, 0.033)
23.4 % (0.117,--)
43 (P)
32.8 (0.081,--)
50.0 (0.089, 0.033)
4.8% (0.012,--)
49 (P)
-
73 74.3
-
55 (P)
78.8
-
54 (P)
40 (C)
96.6
-
55 (P)
45 (C)
31 (P)
,
Precoking/ZSM-5 b
12.4 (0.029, 0.001)
Probe Molecule
TBPO
Chemical Shift (ppm) !90
,
ZSM-5 c
25.7
~! I
Si-CVD/ZSM-5 c
21.2
i
Precoking/ZSM-5 c
3.4
-
"Data (AS + 2 ppm) refer to the corresponding chemical shift differences with respect to crystalline TMPO (39ppm) or TBPO (47 ppm). bValues on top represent relative concentration of acid sites (%); data in parenthesis (int, ext) give the amounts of internal and external acid sites (+ 0.002 mmol/g cat.), respectively. CValues specifically represent relative concentration of external acid sites (%). Moreover, the mechanism of xylene isormerization has been proposed that the alkyl isomerization takes place through a monomolecular mechanism including a 1,2-methyl shift. On the other hand, a possible transalkylation of the methyl group leading to toluene and TMB is a bimolecular reaction [2,17]. In this text, compared to the parent catalyst, both modified samples yield higher xylene selectivity, i.e. a lower xylene loss. A substantial reduction in xylene loss was observed for the pre-coked sample, whereas the silylation treatment results in a slightly preferred formation of para-xylene. As shown in Table 2, the overall amounts of acid sites for the parent, silyated and pre-coking samples were respectively determined as 0.582, 0.470 and 0.245 mmol/g-zeolite. In addition, considering the internal surface acid sites, the silyated sample (Si-CLD/ZSM-5) shows notable decrease in the amount of strongest internal acid site (85 ppm for TMPO), but almost no or slightly change in the other weak acid sites. It should be noticed that a large portion of the deposited polymeric SiO2 frequently leads to a decrease in the pore openings of the channels or even plugging of a part of the channel entrance due to the formation of the silica after silylation treatment. Furthermore, the stronger acid site should locate near the pore mouths of the zeolites. Accordingly, it can be explained that a lots of the polymeric SiO2 are deposited on the stronger acid sites, which located near the pore opening resulted in the pore narrowing. In the case of pre-coking sample (pre-coking/ZSM-5), decrease in the amount of the internal acid sties except for the peak at 77 ppm for TMPO. Moreover, considering the external surface acid sites, the amount of strong acid site at 90 ppm (corresponding to 85ppm for TMPO) notable decreases for the pre-coking sample and slightly decrease for silyated sample, whereas the weak acid site at 73 ppm (corresponding to 66ppm for TMPO) shows no change for both samples. It is implied that although most carbonaceous deposits are located on the external surface of the H-ZSM-5 sample, but a portion of coke regents entering the internal channels to vanish some internal acid sites. Base on the above results from various characterization techniques, it was found that the high proportion of strong acid sites remaining on the external of the silyation sample (Si-CLD/ZSM-5), implies the undesired transalkylation activity after silylation. The observed slightly more selective formation of para-xylene on the silylated sample mainly due to the diffusion limitation. On the other hand, most carbonaceous deposits are located on strong acid sites of the external surface after pre-coking treatment. Hence, a substantial reduction in xylene loss was observed for the pre-coked sample. The deposition of external coke effectively modifies
2274 the surface acid site of the zeolite crystalline, in turn, inhibits the secondary isomerization pathways of para-xylene and thus enhances the para-selectivity. CONCLUSIONS The effects of the silylation and pre-coking on the acidity and pore structure of H-ZSM-5 during xylene isomerization have been investigated. It was found that the coke regents mainly deposited on the stronger external acid sites of H-ZSM-5, resulting in reduction of the xylene loss while maintaining the desired activity and improving the catalyst selectivity in xylene isomerization after pre-coking modification treatment. Furthermore, liquid deposition of silica proves to be insufficient in external acid sites coverage and deposited SiO2 seems to be located near to the channel openings of the H-ZSM-5. Overall, we can concluded that the para-xylene selectivity enhancement of silyated sample is found due to the effect of diffusion limitations, whereas those of pre-coking sample is ascribed to the effect of inactivation of external active sites. ACKNOWLEDGEMENTS The support of this work by National Science Council, Taiwan, ROC (NSC 91-2113-M-001-030) is gratefully acknowledged. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Tsai, T.C., Liu, S.B., Wang, I., Appl. Catal., AI81 (1999), 355-398. (~ejka, J., Wichterlovfi, B., Catal. Rev., 44 (2002), 375-421. Corma, A., Catal. Lett., 22 (1993), 33-52. Weber, R W., Fletcher, M611er, K.P., O'Connor, C.T., Microporous Mater., 7 (1996) 15-25. Kim, J. H., lshida, A., Okajima, M., Niwa, M., J. Catal., 161 (1996), 387-392. Pradhan, A.R., Lin, T.S., Chen, W. H., Jong, S. J., Wu, J.F., Chao, K.J., Liu, S.B., J. Catal., 184 (1999), 29-38. Kunieda, T., Kim, J. H., Niwa, M., J. Catal., 188 (1999), 431-433. Lunsford, J.H., Rothwell, W. P., Shen, W., J. Am. Chem. Soc., 107 (1985), 1540- 1547. Lunsford, J.H., Top. Catal., 4 (1997), 91- 98. Rakiewicz, E.F., Peters, A.W., Wormsbecher, R.F., Sutovich, K.J., Mueller, K.T., J. Phys. Chem. B, 102 (1998) 2890- 2896. Osegovic, J.P, Drago, R.S., J. Phys. Chem. B, 104 (2000) 147-154. Zhao, Q., Chen, W.H., Huang, S.J., Wu, Y.C., Lee, H.K., Liu, S.B., J. Phys. Chem. B, 106 (2002) 44624469. Bauer, F. Chen. W.H., Zhao, Q., Freyer, A., Liu, S.B., Microp. Mesop. Mater., 47 (2001), 67-77. Chen, W.H., Tsai, T.C., Jong, S.J., Zhao, Q., Tsai, C.T., Wang, I., Lee, H. K, Liu, S.B., J. Mol. Catal. A, 181 (2002) 41-45. Ito, T., Fraissard. J, J. Chem. Phys., 76 (1982), 5225-5229. Demarquay, J., Fraissard. J, Chem. Phys. Lett., 136 (1987), 314-318. Mirth, G., (~ejka, J., Lercher, J.A., J. Catal. 139 (1993), 24-33.
2269
ACIDITY C H A R A C T E R I Z A T I O N OF H-ZSM-5 C A T A L Y S T S M O D I F I E D BY P R E - C O K I N G AND S I L Y L A T I O N Wen-Hua Chen, 1 Frank Bauer 2, Evelin Bilz 2, Annette Freyer 2, Shing-Jong Huang l, Chiun-Shen Lai 1 and Shang-Bin Liul* 1Institute of Atomic and Molecular Sciences, Academia Sinica, PO Box 23-166, Taipei 106, Taiwan, R.O.C. 2Institut f'tir Oberfl~ichenmodifizierung,University of Leipzig, Permoserstral3e 15, D-04303 Leipzig, Germany.
ABSTRACT A comprehensive study on the acidic and coking properties of selectivated H-ZSM-5 catalysts during xylene isomerization has been made. The effects of surface modification by pre-coking and silylation on the nature and location of carbonaceous deposits and on related acidic features of the spent catalysts were respectively characterized by 129Xe NMR of adsorbed Xe and solid-state 31p MAS NMR of adsorbed phosphine oxide probe molecules.
INTRODUCTION Xylene isomerization, together with toluene disproportionation and transalkylation of toluene and benzenes are the major selective reactions commonly applied in the petrochemical industry to balance the strongest market demand for para-xylene [1-3]. The principle of the xylene isomerization is applicable to a number of toluene derivatives carrying functional groups such as hydroxyl, amino, nitrilo and halogen, zeolites of ZSM-5 type are particularly suited for such equilibrations. In principle, the para-isomers selectivity enhancement can achieved by increasing crystal size of the zeolite and reduction of surface acid sites by modification techniques, such as adsorption of voluminous amines, chemical vapor deposition of silanes, modification by P, B, Zn or Mg oxides, halogen-based treating agents and deposition of coke. Various modification techniques have been suggested to enhance the selectivity of H-ZSM-5 in aromatic hydrocarbon processing [4-7]. Chemical vapor/liquid deposition of organosilicon compounds (such as tetraethoxysilane or TEOS) and/or pre-coking treatment are known to effectively passivate the non-selective acid sites present on the external surface of zeolite crystallites. However, the modification procedures can affect the diffusion phenomenon of the reactants and products. Thus, further understanding of the effects of surface modification on acidity and coking of the zeolitic catalyst is crucial in optimizing industrial selectivation procedures. Application of 31p MAS NMR of adsorbed organic phosphine oxides has been shown to be especially useful in the characterization of zeolite acidity, since it provides information on the overall acid properties (namely nature, location, strength and concentration) [8-12]. Zhao et al. demonstrated a new method for concurrent qualitative and quantitative characterization of internal and external acid sites in H-ZSM-5 zeolite, which can be determined by a combination of elemental analysis and 31p MAS NMR of adsorbed phosphorous probe molecules [12]. Moreover, 129Xeis a nonreactive spherical molecule with a diameter of 0.44 nm and a 1/2-spin nucles. Polarization of its spherical electronic shell, during adsorption or due to collisions with other Xe atoms or with a solid, affects the NMR chemical shift. The techinque has been used to monitor the pore structure of porous materials. In this study, liquid deposition of TEOS and pre-coking by methanol decomposition were used to modify H-ZSM-5 catalysts. The aim of investigation is to discern and quantify the internal and external acid sites on the parent and modified H-ZSM-5 catalysts by mean of 31p MAS NMR using different adsorbed phosphine oxides in conjunction with elemental analysis. Whereas the variations of sorption capacity and the pore structure in the catalyst were monitored by xenon isotherm and 129XeNMR.
2270
EXPERIMENTAL
Surface modification methods The pre-coked sample (0.3 wt.-% carbonaceous deposits; pre-coking/ZSM-5) was prepared by heating a methanol loaded HZSM-5 (Si/A1 = 25; TRICAT, Germany; ZSM-5) up to 773 K followed by hydrogen flushing. For liquid deposition of TEOS (4 wt.-% SiO2; Si-CLD/ZSM-5), the sample was dispersed in acetone, after adding TEOS and a stoichiometric amount of water refluxed for 1 h, dried and finally calcined at 773 K. Catalytic studies with a p a r a - x y l e n e depleted industrial feedstock were conducted at 400~ in a fixed-bed microreactor [13]. Reaction products were analyzed on a Bentone 34/Didecyl-phthalate SCOT column. Characterization methods The variations of A1 coordination in the structural framework of parent and modified samples were determined by solid-state 27A1 MAS NMR. The changes in the acidity after modification were characterized solid-state 31p MAS NMR of two adsorbed probe molecules with different molecular size, namely, trimethylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO). Moreover, the adsorption capacity and pore structure of the samples were measured by xenon isotherms and t29Xe NMR experiments. All 27A1 and 3]p MAS NMR experiments were carried out on a Bruker MSL-500P at frequency 130.32 and 202.46 MHz and spinning rate 10-12 kHz, using 1 M AI(H20)6 +3 and 85% H3PO4 solution as the chemical shift reference, respectively. All 129Xe NMR spectra were obtained by a NMR spectrometer (Bruker MSL-300P) operating at the Larmor frequency of 83.012 MHz, using diluted xenon gas as the chemical shift reference. Detailed description of the related experimental set-ups and procedures of the above experiments have been described earlier [12, 14]. To afford the quantitative (i.e. concentration) determination of the acid sites, each adsorbate-loaded sample was also subjected to elemental analyses by ICP-MS. RESULTS AND DISCUSSION
Solid-state 27A! M A S N M R study It is well known that 27A1 MAS NMR can provide information about the coordination numbers of the alumina atoms in the zeolite catalyst. Figure 1 displays the 27A! MAS NMR spectra of hydrated H-ZSM-5 samples. In general, all samples show two main signals, one at ca. 55ppm is attributed to tetrahedral-coordinated aluminum, and the other at ca. 0ppm is attributed to octahedral-coordinated aluminum. (Figure 1) It is found that the framework A1 still remains although the peak at 0 ppm corresponding to extra-framework AI became broad after modification.
~
~.~
ZSM-5
~-~
460 12 ~i-G LI~'Zi'i-~
M-5
I
~
140
G, 0,1 0
P.
X
it 120 gaC.l.l~'l~'d- ~,
o
"tJo" eb " do" ~ "-do "-do" C~emical skiff Opm) Figure l. 27A1MAS NMR spectra of hydrated ZSM-5 samples before and after surface modification.
o
4oo
soo
Pressure (Ton")
|
o
t2
Xe loaling (• 2~ a w ~ / g )
Figure 2. Xenon adsorption isotherms and the variations of ]29XeNMR chemical shifts with xenon density for the parent and modified HZSM-5 samples.
2271
Room temperature xenon isotherm and 129Xe NMR studies Xenon adsorption isotherms and the variations of 129XeNMR chemical shifts with xenon density for the parent and modified zeolite samples are presented in Figure 2. As shown in Figure 2, the isotherms show the presence of Langmuir-type adsorption within the pressure range covered. Only slight decreases in adsorption capacity as well as increase in 129Xe NMR chemical shifts are observed for both Si-CLD and pre-coking modified samples. In general, the observed 129Xe NMR chemical shift 8(9) of adsorbed xenon can be described by the sum of three contributions [15]:
(1)
8(J3 ) = 8 0 -k- 8 S ( p = 0 ) -t- (YXeP
Where 80 = 0 is the chemical shift reference and p represents the xenon loading density. The term 8s (p=0) represents the chemical shift at zero loading and can be divide into the sum of the interaction between Xe and the zeolite walls or coke residues or silane molecules. The value of 8s (p=0) can readily be obtained by extrapolating the straight line to the chemical shift axis. The last term is characteristic of the Xe-Xe interactions and is proportional to the density of the adsorbed Xe. The value of CrXewhich may be obtained from the slope of the 8(9) vs. p plot at high xenon loading. In this study, the related parameters obtained from 129XeNMR studies corresponding to xenon loading greater than 4 • 1020 atoms g-1 are listed in Table 1. It is generally accepted that the value of CYxeis inversely proportional to the effective pore free volume of zeolite. [ 15] Accordingly, the relative internal free volume accessible to xenon gas per gram of catalyst can be expressed as (2)
V/V0 = (~Xe) parent/ ((YXe) modified
Where V and V0 are the internal volumes of the modified and the parent samples, respectively. Additional information about the dimensions of the zeolite channels can be obtained by estimating the mean-free-path (A) of the adsorbed xenon from following empirical equation [16]: 8(p) = 243 •
(3)
The calculated values of V/V0 and A from Eq. (2) and (3) for the parent and the modified samples are listed in Table 1. Table 1. List of NMR parameters obtained from 129XeNMR studies. Sample
8s (p =0 )(ppm) ~xe(ppm/atom.g-1) x 1020
V/Vo(%) A (nm)
ZSM-5
95.9
4.3
100
0.315
Si-CLD/ZSM-5
99.2
4.5
95.6
0.298
Pre-coking/ZSM-5
99.8
4.6
93.5
0.295
The extrapolated values 8s (p=0) for the parent, silylated and pre-coking samples are 95.9, 99.2 and 99.8, respectively. Moreover, assuming that the internal volume of the parent sample is 100, together with the observed values of 95.6 and 93.5 for the silylated and pre-coking samples. These results show that a slight decrease in relative free volume (4-6 %) for the modification samples. Moreover, the free zeolite volume decreases to a slightly higher extent by pre-coking compared to silylation. It indicates that the coke reagents predominately located on the external surface of the H-ZSM-5 crystalline, only a few hydrolysis coke regents entering the internal channels. In the case of silylation sample, the pore-narrowing phenomenon was observed.
Solid-state 31p MAS NMR study 31p MAS NMR was utilized to investigate the effects of the modification procedures on acidity properties. Accordingly, the nature, strength and the distribution of acid sites (internal vs. external surface) in all samples were monitored by the changes in changes 31p MAS NMR chemical shifts arising from the adsorbed trimethylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO) probes. The size of TMPO (ca. 0.55 nm) is smaller than the pore aperture of H-ZSM-5, is accessible to both internal and external acid sites of the
2272 zeolite. Whereas the size of TBPO (ca. 0.82 nm) is too big to penetrate into the channels and hence can only be adsorbed on the external surface of HZSM-5. The 31p MAS NMR spectra obtained from the parent and modified samples loaded with TMPO and TBPO are shown in Fig. 3 along with the simulated spectra obtained by Gaussian deconvolution method. As shown in Figure 3(a), up to five characteristic peaks located at 85, 77, 66, 56 and 44 ppm (error + 1 ppm) were observed for the parent sample (HZSM-5), whereas only three resonance peaks (90, 72 and 55 ppm) were observed for TBPO. The peaks at 44 and 55 ppm can be unambiguously assigned to physisorbed TMPO and TBPO, respectively, whereas the other peaks observed at lower field are ascribed as due to interaction of the phosphine oxides probe molecules with acid sites on the H-ZSM-5 zeolite. In other word, there are four different acid sites (85, 77, 66 and 56 ppm) can be identified from 31p MAS NMR spectra of adsorbed TMPO and only two different acid sites (90 and 72 ppm) on the external surface of the H-ZSM-5. Early study [12] suggested that the chain length of the alkyl group on the phosphine oxide has nearly no effect on its corresponding difference in 31p chemical shift (AS) between the adsorbed and crystalline (39 ppm for TMPO and 47 ppm for TBPO). In other word, the change in electron density surrounding of 31p nuclei in TMPO and TBPO probe molecules is similar at a given acid site. Accordingly, in view of the similar A~5 values, it clearly indicates that the peaks at 85 and 66 ppm obtained from TMPO/HZSM-5 systems corresponding respectively to the peaks at 90 and 72 ppm from TBPO/HZSM-5 systems.
(a)
0~)
ZSM-5
,
A .
$i-CLD/ZSM-5 ,&
9
,&
ere-eoki~/ZSM-5
.
150
"
11~1]
Chemical
" shift
5'0 (ppm)
"
.
I)
.
.
.
.
.
.
.
150
.
.
.
.
.
"
..
11J0
Chernical
-
~O
-
I)
shill: (pprn)
Figure 3. The 3,p MAS NMR spectra of (a) TMPO and (b) TBPO adsorbed on the parent (ZSM-5), silyated (Si-CLD/ZSM-5) and pre-coking (Pre-coking/ZSM-5) samples. The asterisks in the spectrum represent spinning sidebands.
Comparing the 31p MAS NMR spectra of adsorbed TMPO on the parent and modified samples, all the peaks are still presented in both modified samples, but the relative concentration of the peaks are changed (Figure 3(a)). For the silylated sample (Si-CLD/HZSM-5), an additional strong sharp peak at 30 ppm was observed, which can be attributed to "mobile" TMPO that either is attached in the intercrystalline voids or is weakly adsorbed near the opening of the channel pores of zeolites. This peak was also observed for another silica surface deposition samples. [8, 10] On the other hand, the 31p MAS NMR spectrum of adsorbed TBPO on the pre-coking sample (Figure 3(b)) reveals that the relative concentration of the peak at 90 ppm (external surface acid sites) is markedly reduced. The characteristics of 31p MAS NMR resonance observed using TMPO and TBPO molecules adsorbed on all samples can be quantified by correlation with the analytic results of Si, A1 and P elemental concentrations using ICP-MS. Together with information on the relative peak areas derived by Gaussain deconvolution of the 31p MAS NMR spectrum, the distribution of each acid site on all sample can be estimated by taking the total peak area responsible for acid sites as 100% and the results show in Table 2.
2273
Table 2.3~p MAS NMR chemical shift assignments and distribution of acid sites of the parent and modified HZSM-5 zeolites loaded with TMPO and TBPO probe molecules. (A& ppm)
a
Probe Molecule
44
~8
27
17
Physisorbed
TMPO
Chemical Shift (ppm) 85
77
66
56
,
ZSM-5 b
8.8 (0.040, 0.011)c
14.2 (0.083, --)
54.4 (0.284, 0.033)
22.6 % (0.131,--)
44 (P)
Si-CLD/ZSM-5 b
2.6 (0.004, 0.009)
13.1 (0.066,--)
60.9 (0.271, 0.033)
23.4 % (0.117,--)
43 (P)
32.8 (0.081,--)
50.0 (0.089, 0.033)
4.8% (0.012,--)
49 (P)
-
73 74.3
-
55 (P)
78.8
-
54 (P)
40 (C)
96.6
-
55 (P)
45 (C)
31 (P)
,
Precoking/ZSM-5 b
12.4 (0.029, 0.001)
Probe Molecule
TBPO
Chemical Shift (ppm) !90
,
ZSM-5 c
25.7
~! I
Si-CVD/ZSM-5 c
21.2
i
Precoking/ZSM-5 c
3.4
-
"Data (AS + 2 ppm) refer to the corresponding chemical shift differences with respect to crystalline TMPO (39ppm) or TBPO (47 ppm). bValues on top represent relative concentration of acid sites (%); data in parenthesis (int, ext) give the amounts of internal and external acid sites (+ 0.002 mmol/g cat.), respectively. CValues specifically represent relative concentration of external acid sites (%). Moreover, the mechanism of xylene isormerization has been proposed that the alkyl isomerization takes place through a monomolecular mechanism including a 1,2-methyl shift. On the other hand, a possible transalkylation of the methyl group leading to toluene and TMB is a bimolecular reaction [2,17]. In this text, compared to the parent catalyst, both modified samples yield higher xylene selectivity, i.e. a lower xylene loss. A substantial reduction in xylene loss was observed for the pre-coked sample, whereas the silylation treatment results in a slightly preferred formation of para-xylene. As shown in Table 2, the overall amounts of acid sites for the parent, silyated and pre-coking samples were respectively determined as 0.582, 0.470 and 0.245 mmol/g-zeolite. In addition, considering the internal surface acid sites, the silyated sample (Si-CLD/ZSM-5) shows notable decrease in the amount of strongest internal acid site (85 ppm for TMPO), but almost no or slightly change in the other weak acid sites. It should be noticed that a large portion of the deposited polymeric SiO2 frequently leads to a decrease in the pore openings of the channels or even plugging of a part of the channel entrance due to the formation of the silica after silylation treatment. Furthermore, the stronger acid site should locate near the pore mouths of the zeolites. Accordingly, it can be explained that a lots of the polymeric SiO2 are deposited on the stronger acid sites, which located near the pore opening resulted in the pore narrowing. In the case of pre-coking sample (pre-coking/ZSM-5), decrease in the amount of the internal acid sties except for the peak at 77 ppm for TMPO. Moreover, considering the external surface acid sites, the amount of strong acid site at 90 ppm (corresponding to 85ppm for TMPO) notable decreases for the pre-coking sample and slightly decrease for silyated sample, whereas the weak acid site at 73 ppm (corresponding to 66ppm for TMPO) shows no change for both samples. It is implied that although most carbonaceous deposits are located on the external surface of the H-ZSM-5 sample, but a portion of coke regents entering the internal channels to vanish some internal acid sites. Base on the above results from various characterization techniques, it was found that the high proportion of strong acid sites remaining on the external of the silyation sample (Si-CLD/ZSM-5), implies the undesired transalkylation activity after silylation. The observed slightly more selective formation of para-xylene on the silylated sample mainly due to the diffusion limitation. On the other hand, most carbonaceous deposits are located on strong acid sites of the external surface after pre-coking treatment. Hence, a substantial reduction in xylene loss was observed for the pre-coked sample. The deposition of external coke effectively modifies
2274 the surface acid site of the zeolite crystalline, in turn, inhibits the secondary isomerization pathways of para-xylene and thus enhances the para-selectivity. CONCLUSIONS The effects of the silylation and pre-coking on the acidity and pore structure of H-ZSM-5 during xylene isomerization have been investigated. It was found that the coke regents mainly deposited on the stronger external acid sites of H-ZSM-5, resulting in reduction of the xylene loss while maintaining the desired activity and improving the catalyst selectivity in xylene isomerization after pre-coking modification treatment. Furthermore, liquid deposition of silica proves to be insufficient in external acid sites coverage and deposited SiO2 seems to be located near to the channel openings of the H-ZSM-5. Overall, we can concluded that the para-xylene selectivity enhancement of silyated sample is found due to the effect of diffusion limitations, whereas those of pre-coking sample is ascribed to the effect of inactivation of external active sites. ACKNOWLEDGEMENTS The support of this work by National Science Council, Taiwan, ROC (NSC 91-2113-M-001-030) is gratefully acknowledged. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
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