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1,4-Butanediol conversion routes over bifunctional supported Co-Zn catalyst
Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All rights reserved.
641
1,4-Butanediol conversion routes over bifunctional supported Co-Zn catalyst L.Leite^, S.Kruc^, Zh.Yuskovets^, V.Stonkus^, M.Fleisher^, E.Lukevics^, J.Stoch^ and M.Mikolayczyk^ ^Latvian Institute of Organic Synthesis, 21 Aizkraukles Str. Riga, LV-1006, Latvia Institute of Catalysis and Surface Chemistry PoUsh Acad. Sci., ul.Niezapominajek, PL-30239 Cracow, Poland 1,4-Butanediol conversion has been performed on Co-Zn/porcelain catalysts and supports alone. The effect of the support calcination and the pretreating with hydrochloric acid has been established. The possible reaction steps and intermediates have been defined by the quantum-chemical calculations.
1. BSTRODUCTION Nowadays the 1,4-butanediol (1,4-BD) conversion in the presence of CoZn/porcelain catalyst (elaborated in our laboratory) is used in the production of 2,3-dihydrofuran (2,3-DHF), the starting material for one of the most effective antitimaor medicines, Ftorafur [1]. Up to now the main products of 1,4-BD conversion in the presence of the cobalt containing catalysts are described and the parallel-consecutive conversion mechanism is offered [2,3]. The aim of the present work is to elucidate the role of Co-Zn/porcelain catalyst support in 1,4-BD conversion and the nature of active SiOg (the main component of the support) surface sites needed for 2,3-DHF and tetrahydrofiiran (THF) formation as well as the possible intermediates.
2. EXPERIMENTAL 2.1. Catalyst preparation Catalyst precursors were prepared by a precipitation method with NagCOg from aqueous Co(N03)2 and Zn(N03)2 solution in the presence of the suspended support. The starting material (Si02:Al203=73:27) used in the porcelain production pretreated in air at 600-1050 ^C for 6 h was used as support. After calcina-
642
tion the support was treated with hot cone. HCl and washed with distilled water till pH 6. After such processing Si02/Al203 molar ratio was 6:1. The t3TDical procedure involved: Co(N03)2H20 (lOg), zinc salt (Co:Zn molar ratio 18:1) and support (2g) (Co:support 1:1 by weight) were mixed in 80 ml of distilled water. Na2C03 (3.8 g) solution in 40 ml of water was added to this mixture at room temperature for 5 min im.der vigorous stirring. The separated precipitate was aged for 20 min under gentle stirring, filtered off, washed with distilled water (3x120 ml) and dried at 110 °C for 12 h. Then the catalyst was calcined in air at 320 °C for 7 h. Before reaction a catalyst precursor was reduced in a Hg stream of 10 1/h at 530 °C for 20 min. After reduction the sample was cooled to ambient temperature in a N2 stream and quickly put into the reactor. 2.2. Analysis BET surface areas of supports (0.2-0.5 mm) were 9-78 mVg, and those of catalysts - 62-110 m^/g. XPS measurements were performed on a VG Scientific ESCA-3 spectrometer using AlKaj2 radiation (1486.6 eV) from an X-ray source operating at 13 kV and 10 mA [4]. Binding energies were referenced to the C l s peak at 284.8 eV. Chemical analysis was carried out on a RF-510 Hitachi atomic absorption spectrometer. GLC analysis was performed using a column (2.5 m x 3 mm) packed with 25% dioctyl phthalate + 3%K0H on Chromosorb W. The columns used for GC analysis contained diatomite with 30% vaseline oil (6 m), zeolite NaX (1.5 m) and active charcoal SKT (1 m). The ^H NMR spectra were recorded on a Bruker WH-90/DS spectrometer in DMSO-dg, TMS - as an internal standard. The mass-spectra were registered on a Kratos MS-25 chromatograph-mass-spectrometer with ionizing energy of 70 eV. Quantiun-chemical calculations were carried out using the LabVision Software Package on a Silicon Graphics Iris Indigo workstation. The molecular orbital semiempirical method was performed using the AMI Hamiltonian. 2.3. Catalytic testing The reaction was carried out in a glass reactor equipped with a dropping funnel, a stirrer, a thermometer, a port for introduction of nitrogen. The reaction was conducted at 170-230 °C in the N2 atmosphere. During the 1,4-BD conversion the products formed were distilled off* and collected in a water-cooled condenser and dry-ice cooled vapour trap.
3. RESULTS AND DISCUSSION 1,4-BD conversion on Co-Zn/porcelain catalyst includes dehydrogenation, dehydration, intra- and intermolecular condensation, C-C bond destruction processes. The reaction products - 2,3-DHF, THF, 4-hydroxybutanal (4-HB) (1-2%), l-(2-tetrahydrofiiranoxy)-2-propanol, a small amount (1-5%) of the y-butjrrolac-
643 tone and l-(2-tetrahydrofuranoxy)-4-pentanol are detected in the reaction mixture. The structures of these compounds are confirmed by chromatomass- and ^H NMR spectrometry. The presence of alkanes C^ - C3 and Hg in the gaseous products shows that 1,4-BD molecule destruction also occurs under these reaction conditions and the tetrahydrofiiranoxy derivatives mentioned above could be formed from the destruction fragments on the catalyst surface. Perhaps 2,3-DHF and THF are formed also from the tetrahydrofuranoxy derivatives which were detected at 170-200 °C in the significant amount. The possibility of 2,3-DHF formation from l-(2-tetrahydrofuranoxy)-4-butanol was shown in [5]. It has been established that 2,3-DHF was hydrogenated to THF under the similar reaction conditions [2]. To decrease the consecutive conversion of the reaction products and 1,4-BD intermolecular condensation to the polycondensation products the influence of the support calcination temperature on catalyst activity was examined during the simultaneous gradual 1,4-BD charging and products distillation away. The reaction results are summarized in Table 1. Table 1. Influence of the support calcination temperature on 1,4-butanediol conversion in the presence of Co-Zn/porcelain catalysts^
Sample No 1 2 3 4 5 6
Support calcination temperature, °C 2,3-DHF 600 800 950 950^ 1000 1050
THF
32 40 37 2 trace
3 4 3 trace trace
-
-
Selectivity, % l-(2-TetraPolycondenhydrofuransation and oxy)-2-propanol gaseous products 10 40 31 7 34 6 48 40 21 76 20 76
Conversion,
% 98 98 98 89 69 34
^ h e reaction carried out at 190-245 °C for 4h. support was not treated with hydrochloric acid.
As shown in Table 1 the activity of the catalyst depends on the support calcination temperature. The highest selectivity of 2,3-DHF was reached in the case of support calcined at 800-950 °C but of the tetrahydrofuranoxy derivative - at 950 °C. These supports and the catalysts on them have the largest surface areas 60-74 and 100-110 mVg, respectively. When the support was not treated with hydrochloric acid the catalyst lost its ability to form 2,3-DHF and THF but it was extremely favourable for 1,4-BD conversion into the tetrahydrofuranoxy derivatives of alkanols.
644
The calcined at 600-1000 °C and HCl-treated porcelain alone gave THF as the only product of the 1,4-BD intramolecular dehydration (Table 2). Table 2. 1,4-Butanediol conversion on support^ Support calcination temperature, °C 600 600^ 800 950 950^ 1000 1050
THF yield, %
38
75
Catalytic activity, mmol THF Scat n 214
60 74 18 23 12
44 44
'62 60
Surface area,
-
-
15
19
-
-
Conversion, % 100 4 72 69 5 19 3
*The reaction temperature 230 *^C, reaction time 4 h; l,4-butanediol:support weight ratio 50. Support was not treated with hydrochloric acid.
The catalyst calcined at 600 *^C possesses the highest catalytic activity of one gram of support in relation to THF. This support imtreated with hydrochloric acid is completely inactive. The reduced Co-Zn supported catalysts are air unstable. It is the matter why only the surface of the catalyst precursors (catalysts after calcination at 320 °C) are studied by the XPS method. The comparison of XPS data of catalyst samples 3 and 5 (active and inactive catalysts in relation to 2,3-DHF formation, notation as in Table 1) shows that the binding energies (BE) of Si and Al in both cases are similar to those in the pure oxides (Si2p 103.3 and 103 eV, A12p 74.7 and 74.6 eV, respectively). It points out the lack of strong interaction between Si and other elements as well as the absence (or few) of tj^ical aluminosilicate network. The surface of active sample 3 contains more of Si than the inactive catalyst 5 (Si/A12.6 and 2.1, respectively). The Co2p spectra (Fig. 1) show that both samples after calcination at 320 °C do not contain metal cobalt. As follows from the spectra decomposition cobalt at the surface is in the form of Co(III)-ions in the octahedral coordination (the same as in ZnCogO^ with BE about 780.1 eV [4]) and Co(II)-ions in the tetrahedral coordination (similar to CoAlgO^ with BE above 781.5 eV [4]). The ratio of Co(III):Co(II) in samples 3 and 5 is 1.4 and 1.7, respectively, i.e. Co(III) content in the inactive catalyst is about 30% higher than in the active one. The 0 1 s band of samples 3 and 5 is composed of two easily resolved contributory peaks: at lower BE (about 530.5 eV) corresponding to Co, Zn-oxide phases
645
Co2p 7663
J 4358
5679 j
3695
1171
-273 771
780
789
798
807 BE[eV]
775
782
789
796
803 BE [eV]
Fig.l. Co2p XP spectra for Co-Zn/porcelain catalyst precursor after calcination at 320 °C. (a) sample 3 and (b) sample 5. Notations as in Table 1. and at higher BE (about 532.5 eV) due to the silica-rich carrier. The 0(1):0(2) ratio reflects the relative content of these phases.
01s 53491
3947
2545
1143
-259 528
531
534
537 BE [©V]
525
534
BE [eV]
Fig.2. 0 1 s JOP spectra for Co-Zn/porcelain catalyst precursor after calcination at 320 °C. (a) sample 3 and (b) sample 5. Notation as in Table 1.
646
As seen (Fig. 2) the metal (Co, Zn)-phase is much better spread over the support surface in the inactive catalyst 5. The (Co+Zn):Si ratio characterizing the covering of the support is 1.25 in the sample 3 and 1.64 in the sample 5. This effect proves that the deposit of the active component covers and eliminates the catalytic active centres at the bare surface of the support. Thus, the support calcination at temperature higher than 950 *^C leads to the decrease of the surface area and content of SiOg on the surface due to the interphase diffusion and consequently to the decrease of SiOg accessibility to a reactant. In the quantum-chemical calculation the active surface of support was modelled by the cluster consisting of an acidic (H"*^) and basic sites - ^Si-0" and =Si=0 fragments of SiOg (similar to [6]). The data obtained confirmed the step-wise cyclodehydration mechanism of 1,4-BD conversion into THF suggested in [7] for the Afunctional modified ZSM-5 catalysts. This mechanism includes the 1,4-BD interaction with support H* centre leading to the dehydration and, subsequently, to the formation of linear and cyclic (I) cations as intermediates (Scheme 1): Hp-(CH2)4-OH
" +CH2—(CH2)3-OH
o
o
I
I H Scheme 1. THF desorbed afl:er the deprotonation of a cyclic cation on the basic SiOg surface centres. The comparison of the stabilization energy values (7.680 and 1.977 eV) showed that the probabiUty of the basic centre ^Si-0" participation in cation I deprotonation being higher than that of =Si=0 centre. On the other hand, according to the quantum-chemical calculations 1,4-BD cyclodehydration on SiOg surface seems to* be accounted for in terms of the concerted mechanism (Scheme 2): H0-(CH2).-~0H
c
H+
OI Si
)
/|\
/
°
\
+ H2 0
OH i Si
/|\,,,, Scheme 2.
647
The initial metal-catalyzed transformation of 1,4-BD (as reported for other diols [8]) on the reduced Co-Zn/porcelain catalyst leads to 4-HB formation. As it has been found by quantima-chemical calculation the further 4-HB conversion occurs on the SiOg active sites via step-wise mechanism similar to 1,4-BD (Scheme 3): CHj-CH^-OH I CHg-CH^-OH Co "•^2
\ ~Si=0,
CH2-CH2-OH ^+H+^ CH2~CH2+ CH2-CHO
-H2O
^
CH2-CHO
"2p~^\"2 I "2Cs^/CH
^Si-O"^ _H+
/
^
X Q ^
II Scheme 3. The stabilization energy values of cation II on basic centres 3Si-0" and =Si=0 characterizing deprotonation reaction were 6.952 and 0.926 eV, respectively. The basic centre =Si=0 interacts with cation II giving a stable surface complex, too, with the chemosorption energy 2.654 eV. It can result in the accumulation of the surface intermediates transforming to 2,3-DHF and it coincides with the observations under the given experimental conditions: the formation of 2,3-DHF was observed during approximately 1 h after finishing 1,4-BD supply. Thus, one could assume (analogously to [9] in the relation to n-butanol dehydration reaction) that during the initial stages of 1,4-BD conversion the accimiulation of the surface intermediates occurs which later slow down the transformation to 2,3DHF. Earlier the transformation of 4-HB into 2,3-DHF via 2-hydroxytetrahydrofuran (as well as other hydroxyoxo compound conversion into 2,3-DHF derivatives) was discussed [2, 3, 10]. Quantum-chemical calculation showed that the concerted mechanism with the participation of Lewis acidic and basic site of the SiOg surface in the case of 4-HB may be present in the following way (Scheme 4). CH2—CH2-OH CH2-CHO
0 /
0 0 0 \ / \ Scheme 4.
OH OH I I Si—O—Si 0 0 0 0 / \ / \
648
4. CONCLUSIONS 1,4-Butanediol conversion on Co-Zn/porcelain catalyst is a paralell-consecutive process including dehydrogenation, inter- and intramolecular dehydration as well as cyclization and cracking reactions. The studied catalyst support pretreated with HCl activates the dehydration and cyclization reactions of 1,4-butanediol and 4-hydroxybutanal. These reactions result in the formation of tetrahydrofuran and 2,3-dihydrofuran, respectively. According to the quantum-chemical calculations using cluster models the 1,4butanediol cyclodehydration and deprotonation of cyclic intermediates proceed via the stagewise or sjmchronous mechanism with participation of SiOg Br0nsted acidic and basic (=Si-0', =Si=0) centers. The interaction with ;Si-0' centers is more preferable energetically. The structures of cyclic surface intermediates in the case of 2,3-dihydrofuran and tetrahydrofuran are different. Sxirface cluster including both Lewis acidic and basic (=Si-0") centers participates in the transformation of 2-hydroxytetrahydrofuran into 2,3-dihydrofuran.
REFERENCES 1. S.A.ffiller, R.A.Zhuk, M.JuXidak, Dokl.Akad.Nauk SSSR 176, (1967) 332. 2. P.Dimroth, H.Pasedach, Angew.Chem., 72 (1960) 865. 3. LI.Geiman, L.F.Bulenkova, A.A.Lazdinsh, A.K.Veinberg, V.A.Slavinska and A.A.Avots, Khim.heterocykl.soed. 4 (1981) 448. 4. J.Stoch, Izv.Khim., Bulg.Acad.Sci., 22 (1983) 213 . 5. H.E.Petree, J.B.Nabors. Process for the preparation of cyclopropylmethyl-Npropylamine. U S Patent No 4,275,238 (1981). 6. Yu. L Gorlov, V.A. Zaets, A.A. Tshuiko, Theoretical and experimental chemistry, N 4(1988)407. 7. Y.V. Subba Rao, S.J. Kulkami, M. Subrahmanyam, A.V. Rama Rao, J. Org. Chem., 59 (1994) 3998. 8. M. Bartok, A. Molndr. The Chemistry of Ethers, Crown Ethers, Hydroxyl Groups and their Sulphur Analogues, Suppl. E, Chapter 16. Dehydration of diols. Ed.: S. Patai, J. Wiley & Sons: New York, 1980, p. 721. 9.M.A.Makarova, E.A.Paukshtis, J.M.Thomas, C.WiUiams and K.LZamaraev, J.Catal., 149 (1994) 36. 10. M. Bartok, A. M o l n ^ . Acta Chim. Acad. Sci. Hung., 100 (1979) 203.
641
1,4-Butanediol conversion routes over bifunctional supported Co-Zn catalyst L.Leite^, S.Kruc^, Zh.Yuskovets^, V.Stonkus^, M.Fleisher^, E.Lukevics^, J.Stoch^ and M.Mikolayczyk^ ^Latvian Institute of Organic Synthesis, 21 Aizkraukles Str. Riga, LV-1006, Latvia Institute of Catalysis and Surface Chemistry PoUsh Acad. Sci., ul.Niezapominajek, PL-30239 Cracow, Poland 1,4-Butanediol conversion has been performed on Co-Zn/porcelain catalysts and supports alone. The effect of the support calcination and the pretreating with hydrochloric acid has been established. The possible reaction steps and intermediates have been defined by the quantum-chemical calculations.
1. BSTRODUCTION Nowadays the 1,4-butanediol (1,4-BD) conversion in the presence of CoZn/porcelain catalyst (elaborated in our laboratory) is used in the production of 2,3-dihydrofuran (2,3-DHF), the starting material for one of the most effective antitimaor medicines, Ftorafur [1]. Up to now the main products of 1,4-BD conversion in the presence of the cobalt containing catalysts are described and the parallel-consecutive conversion mechanism is offered [2,3]. The aim of the present work is to elucidate the role of Co-Zn/porcelain catalyst support in 1,4-BD conversion and the nature of active SiOg (the main component of the support) surface sites needed for 2,3-DHF and tetrahydrofiiran (THF) formation as well as the possible intermediates.
2. EXPERIMENTAL 2.1. Catalyst preparation Catalyst precursors were prepared by a precipitation method with NagCOg from aqueous Co(N03)2 and Zn(N03)2 solution in the presence of the suspended support. The starting material (Si02:Al203=73:27) used in the porcelain production pretreated in air at 600-1050 ^C for 6 h was used as support. After calcina-
642
tion the support was treated with hot cone. HCl and washed with distilled water till pH 6. After such processing Si02/Al203 molar ratio was 6:1. The t3TDical procedure involved: Co(N03)2H20 (lOg), zinc salt (Co:Zn molar ratio 18:1) and support (2g) (Co:support 1:1 by weight) were mixed in 80 ml of distilled water. Na2C03 (3.8 g) solution in 40 ml of water was added to this mixture at room temperature for 5 min im.der vigorous stirring. The separated precipitate was aged for 20 min under gentle stirring, filtered off, washed with distilled water (3x120 ml) and dried at 110 °C for 12 h. Then the catalyst was calcined in air at 320 °C for 7 h. Before reaction a catalyst precursor was reduced in a Hg stream of 10 1/h at 530 °C for 20 min. After reduction the sample was cooled to ambient temperature in a N2 stream and quickly put into the reactor. 2.2. Analysis BET surface areas of supports (0.2-0.5 mm) were 9-78 mVg, and those of catalysts - 62-110 m^/g. XPS measurements were performed on a VG Scientific ESCA-3 spectrometer using AlKaj2 radiation (1486.6 eV) from an X-ray source operating at 13 kV and 10 mA [4]. Binding energies were referenced to the C l s peak at 284.8 eV. Chemical analysis was carried out on a RF-510 Hitachi atomic absorption spectrometer. GLC analysis was performed using a column (2.5 m x 3 mm) packed with 25% dioctyl phthalate + 3%K0H on Chromosorb W. The columns used for GC analysis contained diatomite with 30% vaseline oil (6 m), zeolite NaX (1.5 m) and active charcoal SKT (1 m). The ^H NMR spectra were recorded on a Bruker WH-90/DS spectrometer in DMSO-dg, TMS - as an internal standard. The mass-spectra were registered on a Kratos MS-25 chromatograph-mass-spectrometer with ionizing energy of 70 eV. Quantiun-chemical calculations were carried out using the LabVision Software Package on a Silicon Graphics Iris Indigo workstation. The molecular orbital semiempirical method was performed using the AMI Hamiltonian. 2.3. Catalytic testing The reaction was carried out in a glass reactor equipped with a dropping funnel, a stirrer, a thermometer, a port for introduction of nitrogen. The reaction was conducted at 170-230 °C in the N2 atmosphere. During the 1,4-BD conversion the products formed were distilled off* and collected in a water-cooled condenser and dry-ice cooled vapour trap.
3. RESULTS AND DISCUSSION 1,4-BD conversion on Co-Zn/porcelain catalyst includes dehydrogenation, dehydration, intra- and intermolecular condensation, C-C bond destruction processes. The reaction products - 2,3-DHF, THF, 4-hydroxybutanal (4-HB) (1-2%), l-(2-tetrahydrofiiranoxy)-2-propanol, a small amount (1-5%) of the y-butjrrolac-
643 tone and l-(2-tetrahydrofuranoxy)-4-pentanol are detected in the reaction mixture. The structures of these compounds are confirmed by chromatomass- and ^H NMR spectrometry. The presence of alkanes C^ - C3 and Hg in the gaseous products shows that 1,4-BD molecule destruction also occurs under these reaction conditions and the tetrahydrofiiranoxy derivatives mentioned above could be formed from the destruction fragments on the catalyst surface. Perhaps 2,3-DHF and THF are formed also from the tetrahydrofuranoxy derivatives which were detected at 170-200 °C in the significant amount. The possibility of 2,3-DHF formation from l-(2-tetrahydrofuranoxy)-4-butanol was shown in [5]. It has been established that 2,3-DHF was hydrogenated to THF under the similar reaction conditions [2]. To decrease the consecutive conversion of the reaction products and 1,4-BD intermolecular condensation to the polycondensation products the influence of the support calcination temperature on catalyst activity was examined during the simultaneous gradual 1,4-BD charging and products distillation away. The reaction results are summarized in Table 1. Table 1. Influence of the support calcination temperature on 1,4-butanediol conversion in the presence of Co-Zn/porcelain catalysts^
Sample No 1 2 3 4 5 6
Support calcination temperature, °C 2,3-DHF 600 800 950 950^ 1000 1050
THF
32 40 37 2 trace
3 4 3 trace trace
-
-
Selectivity, % l-(2-TetraPolycondenhydrofuransation and oxy)-2-propanol gaseous products 10 40 31 7 34 6 48 40 21 76 20 76
Conversion,
% 98 98 98 89 69 34
^ h e reaction carried out at 190-245 °C for 4h. support was not treated with hydrochloric acid.
As shown in Table 1 the activity of the catalyst depends on the support calcination temperature. The highest selectivity of 2,3-DHF was reached in the case of support calcined at 800-950 °C but of the tetrahydrofuranoxy derivative - at 950 °C. These supports and the catalysts on them have the largest surface areas 60-74 and 100-110 mVg, respectively. When the support was not treated with hydrochloric acid the catalyst lost its ability to form 2,3-DHF and THF but it was extremely favourable for 1,4-BD conversion into the tetrahydrofuranoxy derivatives of alkanols.
644
The calcined at 600-1000 °C and HCl-treated porcelain alone gave THF as the only product of the 1,4-BD intramolecular dehydration (Table 2). Table 2. 1,4-Butanediol conversion on support^ Support calcination temperature, °C 600 600^ 800 950 950^ 1000 1050
THF yield, %
38
75
Catalytic activity, mmol THF Scat n 214
60 74 18 23 12
44 44
'62 60
Surface area,
-
-
15
19
-
-
Conversion, % 100 4 72 69 5 19 3
*The reaction temperature 230 *^C, reaction time 4 h; l,4-butanediol:support weight ratio 50. Support was not treated with hydrochloric acid.
The catalyst calcined at 600 *^C possesses the highest catalytic activity of one gram of support in relation to THF. This support imtreated with hydrochloric acid is completely inactive. The reduced Co-Zn supported catalysts are air unstable. It is the matter why only the surface of the catalyst precursors (catalysts after calcination at 320 °C) are studied by the XPS method. The comparison of XPS data of catalyst samples 3 and 5 (active and inactive catalysts in relation to 2,3-DHF formation, notation as in Table 1) shows that the binding energies (BE) of Si and Al in both cases are similar to those in the pure oxides (Si2p 103.3 and 103 eV, A12p 74.7 and 74.6 eV, respectively). It points out the lack of strong interaction between Si and other elements as well as the absence (or few) of tj^ical aluminosilicate network. The surface of active sample 3 contains more of Si than the inactive catalyst 5 (Si/A12.6 and 2.1, respectively). The Co2p spectra (Fig. 1) show that both samples after calcination at 320 °C do not contain metal cobalt. As follows from the spectra decomposition cobalt at the surface is in the form of Co(III)-ions in the octahedral coordination (the same as in ZnCogO^ with BE about 780.1 eV [4]) and Co(II)-ions in the tetrahedral coordination (similar to CoAlgO^ with BE above 781.5 eV [4]). The ratio of Co(III):Co(II) in samples 3 and 5 is 1.4 and 1.7, respectively, i.e. Co(III) content in the inactive catalyst is about 30% higher than in the active one. The 0 1 s band of samples 3 and 5 is composed of two easily resolved contributory peaks: at lower BE (about 530.5 eV) corresponding to Co, Zn-oxide phases
645
Co2p 7663
J 4358
5679 j
3695
1171
-273 771
780
789
798
807 BE[eV]
775
782
789
796
803 BE [eV]
Fig.l. Co2p XP spectra for Co-Zn/porcelain catalyst precursor after calcination at 320 °C. (a) sample 3 and (b) sample 5. Notations as in Table 1. and at higher BE (about 532.5 eV) due to the silica-rich carrier. The 0(1):0(2) ratio reflects the relative content of these phases.
01s 53491
3947
2545
1143
-259 528
531
534
537 BE [©V]
525
534
BE [eV]
Fig.2. 0 1 s JOP spectra for Co-Zn/porcelain catalyst precursor after calcination at 320 °C. (a) sample 3 and (b) sample 5. Notation as in Table 1.
646
As seen (Fig. 2) the metal (Co, Zn)-phase is much better spread over the support surface in the inactive catalyst 5. The (Co+Zn):Si ratio characterizing the covering of the support is 1.25 in the sample 3 and 1.64 in the sample 5. This effect proves that the deposit of the active component covers and eliminates the catalytic active centres at the bare surface of the support. Thus, the support calcination at temperature higher than 950 *^C leads to the decrease of the surface area and content of SiOg on the surface due to the interphase diffusion and consequently to the decrease of SiOg accessibility to a reactant. In the quantum-chemical calculation the active surface of support was modelled by the cluster consisting of an acidic (H"*^) and basic sites - ^Si-0" and =Si=0 fragments of SiOg (similar to [6]). The data obtained confirmed the step-wise cyclodehydration mechanism of 1,4-BD conversion into THF suggested in [7] for the Afunctional modified ZSM-5 catalysts. This mechanism includes the 1,4-BD interaction with support H* centre leading to the dehydration and, subsequently, to the formation of linear and cyclic (I) cations as intermediates (Scheme 1): Hp-(CH2)4-OH
" +CH2—(CH2)3-OH
o
o
I
I H Scheme 1. THF desorbed afl:er the deprotonation of a cyclic cation on the basic SiOg surface centres. The comparison of the stabilization energy values (7.680 and 1.977 eV) showed that the probabiUty of the basic centre ^Si-0" participation in cation I deprotonation being higher than that of =Si=0 centre. On the other hand, according to the quantum-chemical calculations 1,4-BD cyclodehydration on SiOg surface seems to* be accounted for in terms of the concerted mechanism (Scheme 2): H0-(CH2).-~0H
c
H+
OI Si
)
/|\
/
°
\
+ H2 0
OH i Si
/|\,,,, Scheme 2.
647
The initial metal-catalyzed transformation of 1,4-BD (as reported for other diols [8]) on the reduced Co-Zn/porcelain catalyst leads to 4-HB formation. As it has been found by quantima-chemical calculation the further 4-HB conversion occurs on the SiOg active sites via step-wise mechanism similar to 1,4-BD (Scheme 3): CHj-CH^-OH I CHg-CH^-OH Co "•^2
\ ~Si=0,
CH2-CH2-OH ^+H+^ CH2~CH2+ CH2-CHO
-H2O
^
CH2-CHO
"2p~^\"2 I "2Cs^/CH
^Si-O"^ _H+
/
^
X Q ^
II Scheme 3. The stabilization energy values of cation II on basic centres 3Si-0" and =Si=0 characterizing deprotonation reaction were 6.952 and 0.926 eV, respectively. The basic centre =Si=0 interacts with cation II giving a stable surface complex, too, with the chemosorption energy 2.654 eV. It can result in the accumulation of the surface intermediates transforming to 2,3-DHF and it coincides with the observations under the given experimental conditions: the formation of 2,3-DHF was observed during approximately 1 h after finishing 1,4-BD supply. Thus, one could assume (analogously to [9] in the relation to n-butanol dehydration reaction) that during the initial stages of 1,4-BD conversion the accimiulation of the surface intermediates occurs which later slow down the transformation to 2,3DHF. Earlier the transformation of 4-HB into 2,3-DHF via 2-hydroxytetrahydrofuran (as well as other hydroxyoxo compound conversion into 2,3-DHF derivatives) was discussed [2, 3, 10]. Quantum-chemical calculation showed that the concerted mechanism with the participation of Lewis acidic and basic site of the SiOg surface in the case of 4-HB may be present in the following way (Scheme 4). CH2—CH2-OH CH2-CHO
0 /
0 0 0 \ / \ Scheme 4.
OH OH I I Si—O—Si 0 0 0 0 / \ / \
648
4. CONCLUSIONS 1,4-Butanediol conversion on Co-Zn/porcelain catalyst is a paralell-consecutive process including dehydrogenation, inter- and intramolecular dehydration as well as cyclization and cracking reactions. The studied catalyst support pretreated with HCl activates the dehydration and cyclization reactions of 1,4-butanediol and 4-hydroxybutanal. These reactions result in the formation of tetrahydrofuran and 2,3-dihydrofuran, respectively. According to the quantum-chemical calculations using cluster models the 1,4butanediol cyclodehydration and deprotonation of cyclic intermediates proceed via the stagewise or sjmchronous mechanism with participation of SiOg Br0nsted acidic and basic (=Si-0', =Si=0) centers. The interaction with ;Si-0' centers is more preferable energetically. The structures of cyclic surface intermediates in the case of 2,3-dihydrofuran and tetrahydrofuran are different. Sxirface cluster including both Lewis acidic and basic (=Si-0") centers participates in the transformation of 2-hydroxytetrahydrofuran into 2,3-dihydrofuran.
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