CC BY
5Технология неорганических веществ
УДК 544.478, 665.652.4
D.Yu. Murzin1, S.V. Myakin2, E.A. Vlasov3, M.M. Sychov4, A, Yu. Postnov5, N.V . Mal'tseva6, A.O. Dolgashev7, Sh.O. Omarov8
ACID-BASE PROPERTIES OF SULFATED HETEROGENEOUS CATALYSTS FOR ISOBUTANE CONVERSION
Abo Akademi University FI-20500, Turku/Abo, Finland St-Petersburg State Institute of Technology (Technical University) Moskovsky Pr. 26, St-Petersburg, 190013, Russia e -mail: ap1804@yandex.ru
Currently, a promising direction of development of the alkyla-tion of isobutane is the use of heterogeneous catalysis. A series of catalysts for isobutene conversion are synthesized by the sulfation and chlorination of y-Al2O3, ZrO2 and their mixtures) under different conditions and characterized by different methods, including pycnometry, IR spectroscopy, programmed ammonia thermodesorption and the adsorption of acid-base indicators affording the distribution of functional groups on their surface according to their acid-base properties. It is shown, that the sulfation generates Br0ensted acidic centers the average force on the surface of y-AhO3 and significantly activates the acid sites of different strength of ZrO2, and the use of sulfated alumina or Zirconia in the synthesis of Al-S-Zr-catalyst provides a 91-98 % degree of conversion of isobutane at 44-50 % selectivity.
Keywords: alumina, zirconia, porosity, surface acid-base properties, i-butane alkylation
Д.Ю. Мурзин, С.В. Мякин, Е.А. Власов, М.М. Сычев, А.Ю. Постнов, Н.В. Мальцева, А.О. Долгашов, Ш.О. Омаров
КИСЛОТНО-ОСНОВНЫЕ СВОЙСТВА ZrS и AlSZr -КАТАЛИЗАТОРОВ АЛКИЛИРОВАНИЯ ИЗОБУТАНА
Университет АБО, FI-20500, Турку, Финляндия Санкт-Петербургский государственный технологический институт (технический университет) Московский пр., д.26, Санкт-Петербург, 190013, Россия e -mail: ap1804@yandex.ru
В настоящее время перспективным направлением развития процесса алкилирования изобутана является применение гетерогенного катализа. В статье представлены результаты исследования сульфатированных и хлорированных катализаторов алкилирования изобутана, на основе y-AhO3 и ZrO2, приготовленных методом пропитки. Проведён анализ структурно-прочностных и кислотно-основных свойств с применением методов пикнометрии, ИК-спектроскопии, термопрограммиру-емой десорбции аммиака и адсорбции кислотно-основных индикаторов. Показано, что сульфатирование генерирует кислотные центры Бренстеда средней силы на поверхности y-Al2O3 и существенно активирует кислотные центры разной силы на ZrO2, а использование сульфатированных оксидов алюминия или циркония в синтезе Al-S-Zr-катализаторов обеспечивает 91-98 % степень конверсии изобутана при 44-50 % селективности.
Ключевые слова: оксиды алюминия и циркония, пористость, кислотно-основные свойства поверхности, алкилирование изобутана
1 Murzin Dmitry Yu., Dr Sci (Chem.), Professor, Head, Laboratory of Industrial Chemistry, Faculty of Chemical Engineering, Abo Akademi, Head , Laboratory of Catalytic Technology SPbGTI(TU), e -mail: dmurzin@abo.ti
Мурзин Дмитрий Юрьевич, Д-р хим. наук, профессор, зав. лаб. промышленной химии и реакционной техники Университета Або и лаборатории каталитических технологий СПбГТИ(ТУ), e -mail: dmurzin@abo.ti
2 Myakin Sergey V., PhD (Eng.), Associate Professor, Department of theoretical foundations of Materials Science SPbGTI(TU), e-mail: sergey_mjakin@mail.ru Мякин Сергей Владимирович, канд. техн. наук, доцент каф. теоретических основ материаловедения СПбГТИ(ТУ), ), e-mail: sergey_mjakin@mail.ru
3 Vlasov Evgeny A., Dr Sci (Chem.), Professor, Head, Department of general chemical technology and catalysis, leading researcher, Laboratory of Catalytic Technology SPbGTI(TU), e -mail: ap1804@yandex.ru
Власов Евгений Александрович, д-р хим. наук, профессор, зав каф. общей химической технологии и катализа, вед. науч. сотр. лаб. каталитических технологий СПбГТИ(ТУ), e -mail: ap1804@yandex.ru
4 Sychev Maxim M., Dr Sci (Chem.), Head, Department of theoretical foundations of Materials Science SPbGTI(TU), e-mail: msychov@yahoo.com Сычев Максим Максимович, д-р техн. наук, зав. каф. . теоретических основ материаловедения СПбГТИ(ТУ), ), e-mail: msychov@yahoo.com
5 Postnov Arkady Yu. PhD (Eng.), Associate Professor, Department of general chemical technology and catalysis, leading researcher, Laboratory of Catalytic Technology SPbGTI(TU), e -mail: ap1804@yandex.ru
Постнов Аркадий Юрьевич, канд. техн. наук, доцент каф. общей химической технологии и катализа, вед. науч. сотр. лаб. каталитических технологий СПбГТИ(тУ), e -mail: ap1804@yandex.ru
6 Mal'tseva Natalia V., Associate Professor, Department of general chemical technology and catalysis, leading researcher, Laboratory of Catalytic Technology SPbGTI(TU), e -mail: ap1804@yandex.ru
Мальцева Наталья Васильевна, канд. техн. наук, доцент каф. общей химической технологии и катализа, вед. науч. сотр. лаб. каталитических технологий СПбГТИ(ТУ), e -mail: ap1804@yandex.ru
7 Dolgashev Andrey O., student Department of general chemical technology and catalysis SPbGTI(TU), e-mail:andrey125dolgashov@yandex.ru Долгашов Андрей Олегович, студент каф. общей химической технологии и катализа СПбГТИ(ТУ), e-mail:andrey125dolgashov@yandex.ru
8 Omarov Shamil O., student Department of general chemical technology and catalysis, laboratory assistant, Laboratory of Catalytic Technology SPbGTI(TU), e -mail: sham-omarov@mail.ru
Омаров Шамиль Омарович, студент каф. общей химической технологии и катализа, лаборант лаб. каталитических технологий СПбГТИ(ТУ), e -mail: sham-omarov@mail.ru
Received November, 21 2014
Дата поступления 21 ноября 2014 года
DOI: 10.15217/issn1998984-9.2014.27.11
Isobutane (IB) alkylation with alkenes results in obtaining alkylates with a high octane rating (OR more than 94) used for both for synthetic motor fuel production and for compounding of low OR gasolines [1-6]. A promising direction in the development of alkylation process is the transition from a liquid-phase catalysts such as HF and H2SO4 to a heterogeneous catalysis thus avoiding problems relating to chemical corrosion of equipment, toxicity of waste acids and their disposal. For this purpose catalysts based on sulfated oxides of Zr, Al, Si, Ti, Sn can be used, particularly sulfated zirconium (S-Zr) and aluminum-zirconium (Al-S-Zr) compositions featuring with a high activity in alkylation. However, the main problem of broad application of S-Zr and Al-S-Zr-catalysts is their relatively fast deactivation in the course of an alkylation, which is most often associated with side formation of high molecular hydrocarbons, and coke which block of the pore structure and the surface active sites. Therefore, the search for advanced technology and optimal structure of Zr-containing catalysts with reduced coking capacity is an essential target.
Similar to the desirably low Hammet acidity function (Ho= -8...-11) for such homogeneous catalysts as HF and H2SO4, for heterogeneous alkylation catalysts used various approaches (particularly chlorination or sulfation of the used alumina and zirconia) were applied to increase the concentration of Braensted (BAC) and Lewis acidic centers (LAC) on the catalyst surface. However, so far there are no data on the distribution of acid-base centers according to their acid-base characteristics and concentration, as well as on the effect of a certain sulfation procedure upon the changes in such distributions.
This work is devoted to addressing this problem for S-Zr-and Al-S-Zr-catalysts for isobutene alkylation with isobutene.
Table 1. Structural and strength properties AlS, ZrS and AlSZr-samples
Samples Vz Vmeso Vma Ss, m2/g Hoi Pa MPa
cm3/g
Y-AI2O3; chlorinated Y-AI2O3; sulfated Y-AI2O3 и ZrO2
Al 0,70 0,48 0,22 195 8,0 1,6
AlCl 0,70 0,48 0,23 157 4,8 2,3
AlS 0,69 0,47 0,22 154 5,0 2,1
ZrS 0,45 0,25 0,20 102 5,1 ---
Impregnation Al-Zr and AlS-Zr solution (NH)2SO4
Al-Zr-3 0,68 0,43 0,25 127 4,3 1,3
Al-Zr-4 0,67 0,43 0,24 135 4,9 1,7
Al-Zr-5 0,68 0,41 0,27 162 7,8 1,9
Al-Zr-6 0,65 0,43 0,22 150 7,5 1,7
Impregnation AlS solution ZrO(NO3)2
AlS-Zr-3 0,68 0,39 0,29 --- 4,7 3,0
AlS-Zr-4 0,68 0,43 0,25 154 4,8 3,8
AlS-Zr-5 0,70 0,45 0,25 163 4,7 3,1
AlS-Zr-6 0,70 0,43 0,27 153 4,6 4,2
Impregnation Al-Zr and AlS-Zr solution (NH4)2SO4
Al-Zr-4-S 0,61 0,39 0,22 104 5,8 2,1
Al-Zr-6-S 0,62 0,39 0,23 104 5,9 2,0
AlS-Zr-4-S 0,64 0,41 0,23 85 4,6 3,4
AlS-Zr-6-S 0,64 0,40 0,24 98 4,8 3,5
Mixing PB with ZrO2
Al-ZrO2-1 0,38 0,24 0,14 145 7,7 1,8
Al-ZrO2-2 0,40 0,19 0,21 73 7,4 1,3
Mixing PB with Zr(SO4)2
Al-ZrS 0,34 0,02 0,32 6 4,8 ---
The catalyst samples (Table 1) were obtained using different methods:
1. Sulfated alumina (AlS) was prepared by impregnation of granules of 2,0-3,0 mm in size y-Ah03 brand A-64 (Al) by solution H2SO4 from calculation 10 wt. % SO42-, by drying at 110 °C and calcination in air flow at 400 °C for 3 hour; chlorination of alumina (AlCl) was carried out in solution of hydrochloric acid (concentration 1,5 M; the weight ratio of liquid: solid is 3) with constant stirring for 1 hours; heat treatment was similar to a sulphating stage.
Sulfatedzirconia (SZr) was prepared from zirconium dioxide hydrate besieged from solution ZrO (N03)2-2H2O by ammonia at 20 °C and pH > 8 and heat treatment at 150 and 400 °C for 3 hour with the subsequent impregnation of granules 2,0-3,0 mm in size by solution (NH4)2SO4 from calculation 10 wt. % SO42- at 70 °C, by drying at 150 °C and heat treatment in air flow at 400 °C for 3 hour;
2. series of Al-Zr and AlS-Z samples were prepared by impregnation of y-Al203 granules (brand A-64) (Al-Zr series) and Al-S granules (A-lS-Zr) with ZrO (N03)2 solution (by calculation according to 10 wt. % ZrO2) followed by drying at 110 °C and calcination in air flow at 300 (3), 400 (4), 500 (5) 600 °C (6) for 3 hour;
3. sulfation of Al-Zr- 4(6)-S and AlS-Zr-4(6)- S by impregnation of Al-Zr or AlS-Zr 2.0-3.0 mm sized granules with (NH4)2SO4 solution (calculated according to 10 wt. % SO42-) at 70 °C followed by drying at 150 °C and calcination in air flow at 400 °C for 3 hour;
4. samples of Al-ZrO2 -1 and Al-ZrO— series were prepared by mixing pseudoboehmite (PB) with ZrO2 in the ratios Al203/ZrO2 = 84/16 (1) and 30/70 (2), and AlS-Zr samples were prepared by mixing PB and Zr(SO4)2 in the ratio Al203/ZrO2 = 14/86, followed by drying at 150 °C, tableting and calcination at 400 °C within 3 h.
The porous structure of the synthesized samples was studied by pycnometry (measuring true density d with benzene and apparent density S with mercury) and BET (by measuring specific surface area S using thermal desorption of nitrogen) methods. The total pore volume was calculated by the equation:
Vs= 1/S-1/d
Mesopore volume Vmeso was determined by desiccation over pairs of benzene at 18 °C and p/ps = 0.98. The macropore volume was calculated as
Vma= Vs- V meso-
Compression strength of the granules was measured by end face (P^) technique using a MP-2C installation. Integral Hammet functions (Hoi) were determined from pH-metric curves. Radiographic phase analysis of the samples was carried out using a DRON-3M diffractometer with CuKa-radiation and Ni-filter. IR-spectra were recorded using a SHIMADZU FTIR-8400S spectrometer. Functional composition of the surface of the samples was investigated by adsorption of acid-base indicators with different pKa values ranging from -4.4 to 14.2, which selectively adsorbed on the surface active sites with the corresponding values a pKa according to the method described in [7]. Contents of adsorption centers was determined by the change in optical density of aqueous solutions of the indicators on SF-46 spectro-photometer. The temperature-programmed flash-desorption (TPD) of ammonia from the surface of the samples was performed using a Micromeritics Autochem 2910 installation with the heating rate 10°/min. NH3 adsorption capacity
was calculated according to the desorbed ammonia amount (QNH3, meqv/g) at a given temperature.
The catalytic activity of the synthesized samples was determined using a flowing mode installation. Butane and hydrogen were pre-dried on y-Al2O3. The process parameters were as follows: pressure - 6 atm; temperature - 80 °C; volumetric flow of butane and isobutene - 3.0 and 0.15 cm3/sec correspondingly; catalyst volume 15 cm3.The hydrocarbon composition of the gas mixtures and liquid phases were analyzed at 0.5 and 1.0 hours using the software and hardware systems «Crystal Lux 4000» with the packed column «heptadecane on diatomite carrier, 6 m» and "Chro-matec-Crystal 5000" on the capillary column «SGEBP1-PO-NA 100m-0.25mm-0.5|jm». Isobutane conversion degree was determined according to the concentration changes and integral selectivity was determined according to C5-C8 and «C9 and higher» fractions in the alkylate.
The results of catalysts characterization summarized in Table 1 revealed the following:
1. almost no changes in the pore volumes of Y-Al2O3 (AlS and AlCl samples) were observed in the course of sulfation and chlorination (Vx = 0,69-0,70; Vmezo = 0,47-0,48; cm3/g; Vma = 0,22) while their specific area markedly decreased from 195 to 154-157 m2/g and HoI value decreased from 8.0 to 4.8-5.0 as expected after acidic processing with the formation of SO42- and Cl- groups on the pore surface.
IR spectra showed in Figure 1 shows various straining fluctuations including a pronounced band at about 16301650 cm -1 corresponding to H2O molecules, absorption maximum at 3455-3461 cm-1 relating to asymmetric fluctuations of O-H groups as well as bands at 602 cm-1. In the sulfat-ed sample stretching vibrations are observed at 1090 and 1142 cm-1 relating to S6+ in SO42- group as well as a slight band at 1120 cm-1 corresponding to the brachium of the line 1215 cm-1 suggesting the presence of hydrosulfate complex-
nent growth of medium acidic centers (Tm = 580 °C) probably corresponding to Braensted centers formed by proton-donating sulfuric acid groups. On the contrary, chlorination (Al-Cl samples) leads to a drastic decrease in the content of all adsorption centers probably due to their partial replacement by chloride groups.
Figure 1. IR spectrums y-AlJOz (1); sulfated (2) and chlorinated (3) Y-AI2O3
The data of temperature-programmed flash-desorption (TPD) of ammonia (Fig. 2) indicate:
a) y-AI203 surface is featured with the presence of two types of acidic centers significantly differing in the adsorption energy, including the predominant weak Lewis acidic centers (LACw) with the maximum desorption at Tm = 245 °C and less amount of strong Broensted acidic centers BACs with Tm = 707 °C. Sulfation of y-Al203 (AlS samples) results in a promi-
Figure 2. Change of TPD of a signal from ammonia desorption temperature
on ZrS (1), AlS (2), Al (3), AlCl (4) and Zr (5)-samples
b) the surface of zirconia is featured with a reduced content of acidic centers compared with Y-AI2O3 that is likely determined by its predominant coverage with basic oxygen atoms. Sulfation (ZrS samples) resulted in the formation of different centers including a large amount of relatively weak LAC with Tm = 252 u 346 °C as well as a lower amount of medium (412 and 507 °C) and strongly acidic (564 u 738 °C). The observed versatility of acidic centers on sulfated Zr02 surface can be attributed to either sulfuric acid groups (BAC) or Zr atoms in different coordination (various LAC).
Generally, the formation of large amounts of various acidic centers on the surface of Y-AI2O3 and ZrO2 upon sulfation opens promising for their use (including a combined complementary application) as catalysts for alkylation processes.
2. Regardless of the annealing temperature (300600 °C), incorporation of Zr02 into Y-AI2O3 pores (Al-Zr samples) resulted in a certain decrease of V and Vmeso (from 0.70 to 0.65-0.67 and from 0.48 to 0.41-0.43 cm3/g correspondingly). The surface area of these samples drops from 195 to 127-135 m2/g only at sintering temperatures up to 400 °C (samples Al-Zr-3 and 4, Table 1) while the subsequent temperature growth to 600 °C led to the increase of specific area to 150-162 m2/g probably due to the crystallization of X-ray amorphous Zr02 inside y- Al203 pores. The dehydration of Y-AI2O3 surface, improvement of the crystal lattice perfectness and increase of the monoclinic Zr02 phase at 400-600 °C resulted in the change of the surface state to almost neutral (from H0| = 4,3-4,9 to 7,5-7,9 for the samples Al-Zr-5 and 6).
IR spectroscopy characterization revealed that a sufficient decomposition of ZrO(NO3)2 salt proceeds only at 400 °C (Fig. 3, curve 2) as shown by the almost complete disappearance of the band relating to NO3 - group (1383 cm -1) and appearance of bands at 490, 576 and 738 cm-1 corresponding to straining fluctuations of Zr and O atoms in the tetrahedral modification of ZrO2. H2O molecules are found on zirconia surface up to 600 °C (Fig. 3, curve 4) as suggested by bands at 1510-1539 and 1629-1639 cm-1 corresponding to straining fluctuations of atoms in H20 molecules as well as a diffused absorption band at 3424 cm-1. IR spectra of Al-Zr-4 and Al-Zr-6 samples (Fig. 4, curves 1, 3) are almost identical to the spectra relating to ZrO(NO3)2 decomposition at 400 and 600 °C
(Fig. 3, curves 2, 4). Al-Zr-4 sample is featured with the appearance of an absorption band at 1394 cm1 that disappears upon the temperature increase to 600 °C (Al-Zr-6) and corresponds to zirconium hydride compounds.
T% £
40U0 3600 3100 2800 2400 2(1110 18(1(1 I ((00 140(1 1200 1000 8(10 MO 4(10
Figure 3. IR spectrums of ZrO(NO3)2, heat treated at 300, 400, 500 and 600 °C (1-4)
4000 300(1 3200 2Ш 1400 2000 1800 I6II0 I4O0 1201! 1IIII0 Ш 000 41«!
Figure 4. IR spectrums of samples of Al-Zr-4 (1), Al-Zr-4-S (2), Al-Zr-6 (3), Al-Zr-6-S (4)
3. Impregnation of the sulfated y-AI203 (AlS) with a ZrO (NO3)2 solution (AlS-Zr series) indicates a predominance of monoclinic modification of ZrO2 in both mesopores (with the volume changing from 0.47 to 0.43 cm3/g for AlS-Zr-4 sample) and macropores (0.22 ^ 0.25 cm3/g), with its volume slightly increasing due to the crystallization of amorphous ZrO2. Regardless of the annealing temperature, the deposition of ZrO2 onto AlS samples provided alkylation catalysts featuring with a stability of porous structure parameters (V = 0.68-0.70; Vmes0 = 0.43-0.45; V™ = 0.25-0.29 cm3/g; S = 154-163 m2/g) and acid-base properties (H0i = 4.6-4.8) in combination with a high compression strength of the granules (Pc = 3.8-4.2 MPa).
The analysis of obtained IR spectra also revealed that annealing at 500 °C (Fig. 5, curve 3) results in a complete formation of the surface structure involving SO42- ions (bands at 1151 u 1071 cm-1), thermally stable H2O molecules (1642 and 1510 cm-1) and O-H groups in different coordination (diffused band at 3466 cm-1);
Figure 5. IR spectrums of samples of AlS-Zr, heat treated at 300, 400, 500 and 600 °C (1-4)
4. Sulfation of Al-Zr granules (samples Al-Zr-4(6)-S) resulted in the decrease in all the porous structure parameters (for the sample Al-Zr-4-S: Vz dropped from 0.67 to 0.61 cm3/g; Vmeso - from 0.43 to 0.39 cm3/g; Vma - from 0.24 to 0.22 cm3/g; S - from 135 to 104 m2/g) while the whole surface remained weakly acidic (Hoi = 5.8). Similar results were also obtained for the sample Al-Zr-6-S (Table 1).
Regardless of the annealing temperatures, IR spectra of the sulfated samples Al-Zr-(4)6-S (Figure 4, curves 2, 4) involve peaks 1146-1152 and 1090 cm-1 relating to stretching fluctuations of SO42- ions and retain the spectral features of hydrated species (Figure 3) intrinsic to Zr02 obtained by thermal decomposition of ZrO (NO3)2.
5. Additional sulfation of AlS-Zr samples (AlS-Zr-4(6)- S) provided results similar to those observed at the formation of sulfate groups on Al-Zr samples: Vz = decreased from 0.68-0.70 to 0.64; Vmeso - from 0.43 to 0.40-0.41; Vma - from 0.25-0.27 to 0.23-0.24 cm3/g; S - from 154 to 8598 m2/g, whereas HoI maintained at the level 4.6-4.8.
The additional sulfation (with additional 10 % wt. SO42-) did not provide any considerable changes in IR spectra (compare curves 1 and 2 in Fig. 6) suggesting a predomi-
nant sorption of sulfate-ions on basic centers in Y-AI2O3 featuring with a 2-4-fold increased specific area compared with Zr02 formed in pores of alumina. Generally, the obtained results indicate an essential difference between the sulfation of Zr02 in bulk (Fig. 2) and upon deposition onto Y-AfeOs.
Figure 6. IR spectrums of samples of AlS-Zr-4-S (1) and AlS-Zr-6-S (2) received after additional sulphatization
Figure 7. Distribution of adsorption centers on the surface of the initial (Al) (♦) and sulfated (b) y-AhOs (AlS)
6. The combined application of mixed PB and Zr02 leads to significant changes in the porous structure of Al-Zr02 catalysts: including the decrease of A^Os: Zr02 ratio from 84: 16 (Al-Zr02-1) to 30: 70 (Al-Zr02-2), reduction of S from 145 to 73 m2/g and Vmeso from 0.24 to 0.19 cm3/g, whereas Vma increased from 0.14 to 0.21 cm3/g (Table 1). However, taking into account high Hoi values (7.4-7.7) for these samples determined by basic centers in Y-A12O3 and Zr02 the selectivity of such catalysts is expected to be low. On the contrary, the application of Zr02 obtained from Zr(SO4)2 in the synthesis of Al-ZrS-catalysts allowed the preparation of catalysts with a macroporous structure (S = 6 m2/g; Vmeso = 0.02 cm3/g u Vma = 0,32 cm3/g) with the surface acidity Hoi = 4.8-4.9.
The study of acid-base indicator adsorption revealed the following effects of the applied modification upon the surface chemical functionality of the studied catalysts:
a) sulfation of y-Al2O3 (AlS) results in an almost complete disappearance of weakly acidic Broensted centers with pKa 5.0 (predominant on the initial surface) and formation of more acidic Braensted centers with pKa 2.5 (probably corresponding to OH-groups in hydrosulfates) and Lewis basic centers with pKa - 4.4 corresponding to oxygen atoms in SO42- groups (Figure 7). The latter is in agreement with a significant increase in the ammonia thermodesorption peak relating to medium acidic centers (Figure 2) and suggests that alumina reacts with sulfuric acid involving OH-groups with pKa ~ 5. Furthermore, sulfation of y-AhOs leads to the formation of a large amount of Lewis basic centers (LBC) with pKa - 4.4 formed by aprotic oxygen atoms in SO42- (Figure 1).
b) the temperature of ZrS- and AlS-Zr-catalysts annealing is found to significantly affect their surface functional composition. For catalysts prepared by ZrO2 deposition onto y-AhOs (Al-Zr series) a general trend is observed towards the decrease in the amount of most active centers with the growth of temperature from 300 to 600 °C (Fig. 8). At 300 °C a prominent increase in the content of BAC with pKa 5.0, 3.5, 2.5 as well as LBC with pKa -0.3 (formed by oxygen atoms in bridging groups such Zr-O-Zr) and especially LBC with pKa - 4.4 formed by oxygen atoms in such groups as S=O:. The temperature increase results in a drastic decrease in the content of LBC with pKa < 0 probably determined by sintering of the particles and blocking of electronegative Broensted basic centers (BBC) due to the interaction with electron accepting groups as well as by the formation of new types of Broensted centers (particularly groups with pKa 1.3 at 500 °C) due to water chemisorption via strained bonds after thermal treatment;
Figure 8. Distribution of adsorption centers on the surface of y-Al2Os ( ♦ ) and Al-Zr catalysts prepared by annealing at 300 (▲), 400 ( ◊ ), 500 ( □ ) and 600°C (•)
c) the post-preparation treatment temperature also provides a significant effect upon the distribution of surface centers of the samples obtained by ZrO2 deposition on the sulfated Y-Al2O3 (AlS-Zr series) as shown in Fig. 9. An abrupt decrease in the content of BAC with pKa 2.5 predominant on the initial surface of AlS samples suggests that ZrO2 immobilization on their surface proceeds according to the donor-acceptor mechanism involving the corresponding centers, i.e. oxygen in ZrO2 and protons in acidic OH groups.
Qs pmol/g
Figure 9. Distribution of adsorption centers on the surface of y-Al2O3 (♦) and AlS-Zr catalysts prepared by annealing at 300 (▲), 400 (◊), 500 ( □ ) and 600°C (•)
The changes in the content of certain centers on the surface of AlS-Zr catalysts depending on the processing temperatures are featured with an interrelated behavior (Fig. 10). Particularly, a similar behavior is observed for pairs of centers with pKa -4.4 and -0.9 (featuring with a linear correlation coefficient 0.99), -0.3 and 2.5 as well as 14.2 and 5.0, with the changes of the former two pairs are evidently opposite to each other.
Q, ^mol/g
Figure 10. Changes in the content of centers with pKa -4.4 (1), -0.9 (2), -0.3 (3), 2.5 (4), 14.2 (5) and 5.0 (6) on the surface of AlS-Zr catalysts depending on the processing temperature (Note: the concentration of centers with pKa -0.9, -0.3 and 2.5 is increased 10-fold for presentation convenience)
This fact suggests a similar nature and/or synchronous interrelated formation mechanisms of the centers relating to the above pairs, particularly:
- LBC with pKa - 4.4 and - 0.9 corresponding to oxygen atoms such as = O;
- oxygen atoms in bridging groups (responsible for centers with pKa - 0.3) and acidic hydroxyls with pKa 2.5 resulting from the hydroxylation of a certain part of such oxygen bridges;
- LAC with pKa 14.2 (corresponding to coordi-natively unsaturated Zr4+ cations) and hydroxyls with pKa 5.0 formed by hydroxylation of a part of these LAC proportional to its overall content.
Similar interrelated changes in the content of different functional groups on the surface determined by alternating hydroxylation-dehydroxylation reactions were observed in our earlier studies for the processing of various oxides by mechanical dispersion as a function of milling time [1] and by electron beam processing depending on the absorbed dose [2]. In this case such reactions can be accounted for the features of zirconia and alumina particles sintering in the surface layer at different temperatures followed by hydroxylation due to water chemisorption on strained bonds on the surface.
The sulfated AlS-Zr-4 catalyst featuring with a highly developed porous structure (Table 1) and stable surface acid-base performances (Figure 9, 10) provides isobutene conversion over 90 % in combination with selectivity towards C5-C8 products almost 50 % (Table 2). Non-sulfated Al-Zr-cat-alysts prepared by mixing 70 % wt. ZrO2 with PB (30 % wt. relating to AbOs) provide almost 98-99 % isobutene conversion (Table 2), however the target C5-C8 fraction is only 10.56 %. This is in agreement with IR spectra (Figure 11) indicating straining fluctuations of H2O molecules and ammonia desorption data (Figure 2) demonstrating a low content of acidic centers and high Hoi values (7.4-7.7). On the contrary, zirconium sulfate based catalysts (Al-ZrS samples) featuring with the presence of various acidic centers on the surface (Figure 2) maintain an almost 98 % isobutane conversion and selectivity 43.5 % even after 1 h of the process.
Table 2. Isobutane conversion and C5-C8 production selectivity of AlS-Zr-, Al-Zr- and Al-ZrS catalysts
Samples Conversion of isobutane, % Selectivity, %
after 0,5 h after 1,0 h С5-С8 С9 and more
90,46 89,78 49,45 50,55
Al-ZrO2-2 98,74 98,92 10,56 89,44
А^гё 98,32 97,80 43,51 56,49
% т
Figure 11. IR spectrums of samples of Al-Z.O2-1 (1), Al-Zr02-2 (2) nad Al-ZrS(3)
Conclusions:
A series of catalysts for isobutene conversion are synthesized by sulfation and chlorination of Y-Al2O3, ZrO2 and their mixtures (particularly including pseudobo-hemite) under different conditions and characterized by different methods, including pycnometry, IR spectroscopy, programmed ammonia thermodesorption and adsorption of acid-base indicators affording the distribution of functional groups on their surface according to their pKa values. Generally, chlorination is shown to provide a drastic decrease in the content of all active centers on the
surface of the studied samples while sulfation results in the formation of Braensted acidic centers (proton donating sulfuric acid groups) on the surface of Y-AhO3 and in a significant activation of acidic centers on the surface of ZrO2. Sulfated Al-Zr catalysts featuring with the presence of versatile acidic centers on the surface are found to provide more than 90 % isobutene conversion and almost 50 % selectivity towards C5-Cs products. On the contrary, non-sulfated catalysts with a more basic surface provide about 98-99 % isobutene conversion, however their selectivity selectivity towards the target C5-C8 fractions is about 5 times lower compared with the sulfated counterparts. The effect of post-preparation annealing temperature on the surface functionality of the catalysts is studied. The combined application of programmed thermodesorp-tion and adsorption of acid-base indicators is shown to be an effective approach to the precise characterization of the catalyst surface functionality and its changes as a function of various factors.
Work is performed within the State contract N 14.Z50.31.0013 from March 19, 2014.
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