Synthesis and characterization of various MgO and related systems M. A. Aramendia," V. Borau, C. JimCnez, J. M. Marinas, A. Porras and F. J. Urbano Department of Organic Chemistry, Faculty of Sciences, University of Cbrdoba, Avda San Albert0 Magno sln, E-14004 Cbrdoba, Spain Various magnesium oxide catalysts have been prepared by thermal treatment of two different precursors: Mg(OH), and Mg5(OH),(C03),. An additional system based on MgO and doped with B203 was also prepared. The textural and acid-base properties of the catalysts were investigated. The synthesized solids were characterized from adsorption isotherms, X-ray diffraction (XRD), 'H MAS NMR, diffuse reflectance IR Fourier transform spectroscopy (DRIFT) and temperature-programmed desorption-mass spectrometry (TPD-MS) of adsorbed probe molecules (pyridine, 2,6-dimethylpyridine and carbon dioxide).The surface properties of the solids were found to be strongly influenced by the magnesium oxide preparation conditions (uiz.the precursor and calcination method used). Use of Mg(OH), as the precursor and in uucuo calcination provided highly basic solids (those with the highest proportions of strong basic sites). Various types of Lewis-acid sites were observed in the catalysts prepared in uacuo, probably as the result of the presence of Mg2+ cations of low coordination after the calcination. MgO, CaO and BaO were once regarded as catalytically inert materials, but are currently known to be highly active catalysts for certain base-catalysed reactions if properly activated.High- temperature heat treatment is required to obtain highly active catalysts. Currently, all these materials are typically basic solid catalysts, particulary MgO, which can be considered as a reference material among solid base catalysts.' Mixed oxides containing MgO were thoroughly studied re~ently.~.~The mixtures are often prepared in the hope of finding synergetic effects, i.e. to produce a material with properties surpassing those of a linear combination of its constituents. In most instances, the activity and selectivity enhancement resulting from admixing of single oxides is associ- ated with the reaction of surface defects (e.g. accessible metal cations) or the production of strong Brmsted acid sites as charge balancing cation^.^ In this sense, the addition of B203 (an acid solid) to basic MgO should give solids with different acid-base properties from their precursors. MgO is obtained mainly by thermal treatment of magnesium hydroxide or carbonate, and, more recently, by the sol-gel method.' The oxide morphology depends on the preparation conditions (pH, gelling agent, calcination rate and tempera- ture); however, the formation mechanism remains obscure, particulary as regards oxide hydroxylation.The thermal pretreatment gives rise to H,O and CO, evolution. Water evolution begins at about 400°C when Mg(OH)2 is heat-treated in uacuo. Carbon dioxide starts to evolve at a slightly higher temperature.' Basic sites appear upon heat treatment above 400"C, where the oxide surface is revealed by removal of H,O and CO,.However the stronger basic sites can only be produced by calcination in uacuo. Surface areas vary with the heat treatment conditions. Outgassing results in high surface areas relative to calcination under atmospheric conditions.' It therefore seems that in uucuo calcination leads to materials with enhanced properties: high surface area, stronger basic sites as well as greatly enhanced Lewis-basic sites. The presence of basic sites on MgO is attributed to surface 0,-ions. Magnesium oxide has a highly defective surface structure including steps, kinks, corners, etc., which provide O2-sites of low coordination.6 These differently coordinated 0,-sites must be responsible for basic sites of different strengths.The lower the coordination number of the 0,-site, the higher the strength of the basic sites.7 In this work we describe the synthesis of various magnesium oxide catalysts from two different precursors using two different calcination procedures (in a static atmosphere and in uacuo). A material based on magnesium oxide mixed with boron oxide was also prepared in the dilute binary oxide mode, i.e. it consisted of two components with the major oxide component (MgO) controlling the structure. All the catalysts were thor- oughly characterized by determining their textural properties, X-ray diffraction patterns, 'H MAS NMR spectra, diffuse reflectance IR spectra and acid-base properties in the first part of a comprehensive research program including a study of the activity of the catalyst in the dehydration/dehydrogen- ation of propan-2-01.Experimenta1 Catalyst synthesis The magnesium oxides used were prepared from two differ- ent precursors, namely: (a) MgS(OH),(CO3), * 4H@ (Merck Art. 5827), which yielded the catalysts MgO(1)AIR and MgO(1)VAC; and (b) Mg(OH), (Merck Art. 5870), which provided the solid MgO(I1)AIR. All the solids tested were obtained by calcination in a ceramic crucible, either in the air or in uucuo (hence AIR or VAC labels), by heating from room temperature to 600°C at a rate of 4°C min-' and then maintaining the final temperature for 2 h, after which the solids were allowed to cool to room temperature.Catalyst BM50 was prepared by suspending 29.0g of Mg(OH), and 0.35 g of B203 in 200 ml of distilled water and sonicating the mixture for 1h. The resulting solid was dried in a stove at 120°C for 2 h and calcined by using the above- described temperature programme. The final Mg:B ratio thus obtained was 50: 1. Thermal analysis of the precursors The thermal analysis of the precursors was performed on a Micromeritics temperature-programmed desorption-tempera- ture-programmed reaction instrument (TPD-TPR 2900) that was fitted to a VG Sensorlab quadrupole mass spectrometer from Fisons Instruments plc/VG quadrupole (East Sussex, UK) operating in the multiple ion monitoring (MIM) mode. A portion (ca. 50 mg) of precursor was placed in the middle of the reactor (1cm id, 20 cm length) and stabilized at 50 ml min-' at room temperature for 15 min.Then, the temperature was raised to 700°C at a rate of 5°C min-'. Water (m/z 18), J. Muter. Chem., 1996, 6(12), 1943-1949 1943 carbon dioxide (m/z 44) and carbon monoxide (m/z 28) were monitored in all instances Adsorption isotherms and surface areas The textural properties of the solids (specific surface area, pore volume and mean pore radius) were determined from nitrogen adsorption-desorption isotherms at liquid-nitrogen tempera- ture by using a Micromeritics ASAP-2000 instrument Surface areas were calculated by the Brunauer-Emmett-Teller (BET) method' while pore distributions were determined by the Barret-Joyner-Halenda (BJH) method' (adsorption branch, cylindrical pores open on one side only and adsorbed layer thickness calculated by the Halsey method) All samples were degassed at 350°C to 0 1 Pa prior to measurement X-Ray diffraction and 'H MAS NMR spectroscopy X-Ray diffraction patterns for the solids were recorded on a Siemens D-500 diffractometer equipped with an automatic control and data acquisition system (DACO-MP) PatterFs were run from nickel-filtered copper radiation (A= 1 5405 A) at 35 kV and 20 mA, the diffraction angle 28 being scanned at 2" min-l Room-temperature 'H MAS NMR measurements were car- ried out on a Bruker ACP 400 spectrometer Samples were heated in a nitrogen flow (50 ml min-') for 4 h at the reported temperature, then cooled in the flow and transferred to the sample holder in an environmental chamber (in nitrogen atmosphere) Spectra were acquired with a 90" pulse (5 ps) and the repetition time was 10s The rotation frequency was 3 5 kHz The chemical shifts were expressed relative to tetra- methylsilane (SiMe,) with the usual conventions Diffuse reflectance IR spectroscopic experiments Diffuse reflectance IR (DRIFT) experiments were carried out for probe molecules pre-adsorbed on the solids In these experiments, each catalyst was cleaned by passing an Ar stream at 50 ml min-l at 100°C for 30 min The solids were then saturated with the probe molecule and subsequently flushed with a stream of pure Ar (50ml min-') at the saturation temperature for 2 h in order to avoid physisorption Spectra were obtained at four different temperatures (50, 100, 200 and 300 "C) with a temperature levelling time of 20 min Full details of the procedure are described elsewhere DRIFT expenments were conducted on a Bomen MB-100 instrument with an environmental chamber from Spectra-Tech The instrument was operated at a resolution of 8 cm-' over the range 4000-400 cm-' to gather 256 scans Temperature-programmed desorption-mass spectrometry (TPD-MS) experiments TPD-MS expenments were carried out on the above-described Micromeritics TPD/TPR 2900-VG Sensorlab quadrupole mass spectrometer The optimum TPD conditions were as follows heating rate 10°C min-' and Ar flow-rate 50ml min-l The mass spectrometer, which was operated in the MIM mode, was programmed to perform 6 scans min-l The amines used as probe molecules in order to determine the acid properties of the solids were pyridine (pK =5 25) and 2,6-dimethylpyridine (pK =7 3) In a previous DRIFT study of the bands for the two amines in the region 1400-1700 cm-l, Marinas and co-workers" found pyridine to be adsorbed at Brarnsted- and Lewis-acid sites, and 2,6-dimethylpyridine to be adsorbed on the former site type only owing to the steric hindrance of its two methyl substituents The peaks used to quantify pyridine were the base peak (m/z79) and the secondary peak at m/z 52 (80% abundance) The MS peaks chosen for 2,6-dimethylpyridine were the base peak (m/z 107) and the secondary peak at m/z 66 (60% abundance) Calibration was 1944 J Mater Chem, 1996,6(12), 1943-1949 performed by injecting pulses of variable size (1-10 pl) of a 10-5-10-6 mol dm-3 amine solution in cyclohexane Carbon dioxide was the probe molecule used to determine the basic properties of the catalysts The gases used in the CO, TPD-MS experiments, CO, and 5% CO, in argon, were both supplied by Sociedad Espaiiola de Oxigeno S A (> 99 999%) Carbon dioxide was quantified by its base (m/z 44) and secondary peaks (m/z 12, 10% abundance) Calibration was performed by injecting variably sized pulses of pure CO, or 5% CO, in Ar In this way, two calibration graphs were obtained one encompassing the range 1-20 e-' by using pulses of 1-10 ml of 5% CO, in argon and the other spanning from 15 to 40 e-' with pulses of 0 5-1 ml of pure CO, Correlation coefficients better than 0 99 were always obtained Several repetitions of each experiment provided a mean error of 2% in peak areas Moreover, the activation energy for the desorption of the chemical species formed at each solid was calculated from the Kissinger equation12 where A and C are constants TPD-MS experiments were carried out at a variable heating rate (b)that shifted the peaks in the profile (Tmax)Activation energies were calculated from the slope of a plot of In (b/Tmax)us l/Tmax Prior to adsorption of any probe molecule, each catalyst was cleaned by passing an Ar stream at 110 "C at 50 ml min-l for 30 min The solids were then saturated by passing an amine-N, or C02-Ar stream (50ml min-') at 25 or 50"C, respectively Subsequently, a pure N, or Ar stream (50 ml min-l) was passed at the saturation temperature for 2 h in order to remove any physisorbed molecules Once a stable baseline was obtained, chemisorbed amine or CO, was desorbed by heating from the saturation temperature to 600°C in a programmed fashion The selected mass peaks were monitored throughout the process Each experiment used ca 100 mg of fresh catalyst Full details of the TPD-MS method and equipment are given elsewhere l3 UV-VIS experiments The acid-base properties of the materials used in this work were also studied by using a spectroscopic method based on the determination of the amount of adsorbate retained in monolayer form by the solid This determination was carried out by measuring, by UV-VIS spectroscopy, the amount of adsorbate that remained in the overlaying solution after equilibrium was reached The adsorbates used were pyridine and 2,6-dimethylpyridine in cyclohexane solutions of known concentration for determining acid sites, and benzoic acid for basic sites Full details are given elsewhere l5 Results and Discussion Thermal analysis of the precursors Fig 1 shows the thermal analysis of the precursors used in the synthesis of the solids For Mg,(OH),(CO,), .4H20 (A), water started to evolve at a low temperature (ca 200"C), with a maximum at 265°C Carbon dioxide evolution was detected in the gas phase at ca 400"C, and peaked at 442°C On the other hand, carbon monoxide was never detected in the gas phase For Mg(OH), (B), the main peak corresponds to water at ca 388°C A small carbon dioxide peak was detected at the same temperature, which suggests that magnesium hydroxide was slightly carbonated As for the hydroxycarbonate, no traces of carbon monoxide were detected As the calcination temperature used in the synthesis was [ IAl 0 II iI"1 I, \E 100 200 300 400 500 600 700d 009? 00 Ba M I L II 1 100 200 300 400 500 600 700 temperature/"C Fig.1 Thermal behaviour of the precursors Mg,(OH)2(C03)4 -4H,O (A) and Mg(OH)2 (B). Monitored masses correspond to water (m/z 18), carbon dioxide (m/z 44) and carbon monoxide (m/z 28). 600"C, we can expect the final solids to consist primarily of MgO, without any traces of the precursors, as confirmed by XRD measurements (see below).Adsorption isotherms and surface areas Nitrogen adsorption-desorption isotherms obtained for all solids are shown in Fig. 2. All exhibit closed hysteresis loops. The isotherms are type IV in the Brunauer, Demming, 150 150 -100100 -50 -50 r Lh I-0 0v) a, 0.0 0.5 1.o 0.0 0.5 1.o w 5 \ 43 225 ba225 150 150 75 75 0 0 0.0 0.5 1.0 0.0 0.5 1.o PIP, Fig. 2 Nitrogen adsorption-desorption isotherms for the solids at liquid-nitrogen temperature Demming and Teller (BDDT) classification for mesoporous solids.I6 The surface area, pore volume and mean pore radii for the solids are given in Table 1.The catalysts prepared from magnesium hydroxide had a higher surface area (almost double) than those obtained from magnesium hydroxycarbon- ate. That for solid BM50 was slightly lower than that for MgO(II)AIR, probably owing to the presence of a small amount of B,03. On the other hand, the solid calcined in uucuo had a higher surface area than that calcined in air, consistent with reported re~u1ts.l~ X-Ray diffraction (XRD) Fig. 3 shows the X-ray diffraction patterns for the four solids studied in this work. They are all similar and exhibit three characteristic peaks for MgO (periclase variety) at d =2.44, 2.11 and 1.49 nm (20= 36.7, 42.8 and 62.2", respectively). The pattern for MgO(I1)AIR included a small peak at d= 3.05 nm (28=29.3") that was identified as a different variety of MgO that can be obtained by dehydration of Mg(OH)2, and gives mixed periclase and spinel-type patterns.No peaks from the Mg(OH)2 diffraction pattern were detected. Acid-base properties The acid-base properties of the catalysts studied were determined by two different procedures, viz. temper-ature-programmed desorption-mass spectrometry of probe molecules adsorbed on the solids, and UV-VIS spectroscopy. The probe molecules used to determine acid sites were pyridine (py, pKb =5.25) and 2,6-dimethylpyridine (dmpy, pKb=7.3). Dmpy is known to be selectively adsorbed on Brrnsted-but not on Lewis-acid sites because of steric hindrance by two methyl groups; on the other hand, sterically unhindered py is adsorbed on both Brrnsted- and Lewis-acid sites.The difference between both profiles (py -dmpy) accounts Table 1 Textural properties of MgO-based catalysts. Specific surface area (SBET),pore volume (V,) and average pore radius (r,) for the solids obtained in this work catalyst precursor sBET/m2g-' b/ml g-' r,/A ~ MgO (1)AIR Mgo(l)VAC MgO(I1)AIR BM50 Mg, (OH )2(C03 )4 Mg5(oH)2(Co3)4 Mg(OH), Mg(oH kB203 60 69 119 104 0.22 0.22 0.39 0.34 148 118 94 100 BM50 * A t I= ,MgO(I)AIR \F 21 p! .--lu-MgO(1)VAC e 10 20 30 40 50 60 70 2eldegrees Fig. 3 X-Ray diffraction patterns for the solids J. Muter. Chern., 1996, 6(12), 1943-1949 1945 for Lewis sites Since the catalytic properties of Brmsted-acid sites may well be different from those of Lewis-acid sites, it is important to understand the factors that determine the strength and type of sites in metal oxide catalysts Py and dmpy TPD-MS profiles were obtained for all the the catalysts, the results are shown in Fig 4 and 5, respectively, and summarized in Table 2 Pyridine TPD-MS profiles (Fig 4) were similar for all the solids they included two desorption peaks at low temperatures (103 and 145°C) except for MgO( II)AIR, which exhibited a single, broad peak (T,,, = ~II -1.. 1 . . . 1 . . _I, _-., . . . 0 100 200 300 400 500 600 temperaturePC Fig. 4 Temperature-programmed desorption-mass spectrometry of pre-adsorbed pyndine over the catalysts studied in this work 0 100 200 300 400 500 600 ternperature/*C Fig.5 Temperature-programmed desorption-mass spectrometry of pre-adsorbed 2,6-dimethylpyndine over the catalysts studied in this work 122°C) In addition, the catalyst calcined zn uacuo, MgO(I)VAC, exhibited an additional desorption peak at a high temperature (Tmax=381 "C) that was not observed in the other catalysts As far as dmpy TPD-MS profiles are concerned, all catalysts had broad desorption peaks at low temperatures Thus, the solids prepared from magnesium hydroxycarbonate exhibited a desorption peak at 72 "C, while those prepared from magnesium hydroxide exhibited a desorption peak at 101"C No high-temperature desorption peak was detected for MgO(I)VAC, however The results obtained by integrating the areas under the desorption peaks lead to the Brsnsted, Lewis and overall acidity values for the solids given in Table 2, which also includes the overall acidity as calculated from UV-VIS spectra by titration of acid sites with py The results provided by the two methods are seemingly inconsistent However, because all the catalysts exhibit predominantly basic rather than acid properties, the differences can be considered negligible Moreover, since B203 is an acid solid, the results provided by the TPD-MS technique, which gives a slightly higher total acidity for BM50 than for the other solids, seem more reason- able Finally, recording of DRIFT spectra for adsorbed amines was attempted However, because of the low acidity of these catalysts, the results were rather poor, so their DRIFT spectra were excluded The most salient result of this study was the py desorption peak for MgO( 1)VAC at high temperature (Fig 4), which was absent from the corresponding dmpy TPD-MS profile (Fig 5) This peak is related to Lewis-acid sites of high strength It has been reported that, on a thoroughly degassed MgO surface, coordinatively unsaturated Mg2+ cations are exposed18 which exhibit Lewis-acidic character l9 Such Mg2+ cations of low coordination may be responsible for the new Lewis-acid sites present after calcination zn uacuo to produce MgO(1)VAC Base properties were determined, by using C02 (the seem- ingly most suitable choice on account of its acidic nature) as the probe molecule in TPD-MS experiments In addition, a UV-VIS spectroscopic method" was also used to determine the basicity of the solids using benzoic acid as the titrant In studying adsorbed carbon dioxide, carbonate species of different types such as unidentate carbonate, bidentate carbon- ate, carbonate ions and hydrogen carbonates were found to be formed depending on the adsorption conditions and surface structure For unidentate carbonate, the adsorbed species was bound to the surface through a bond between the C in C02 and surface O2 For bidentate carbonate, the adsorbed species was bound to the surface via two bonds one between the C in CO, and surface 02-and the other between an 0 in C02 and a surface Mg2+ ion The presence of hydrogen carbonates suggest that hydroxy groups on MgO also act as bases toward C02 In the TPD-MS experiments with adsorbed COz, the con- centration of basic sites was reflected in the peak area of the TPD-MS profile, and their strength in the temperature at which the CO, desorption peak appeared l4 Fig 6 shows the COz TPD-MS profiles obtained for all the catalysts As can be seen, up to three different peaks were obtained two small Table 2 Determination of the acidity of the catalysts obtained in this work by temperature-programmed desorptlon-mass spectrometry (TPD-MS) and UV-VIS spectroscopy total acidity Bransted acidity Lewis acidity PYIPOl g catalyst dmPY/Pmol g (TPD-MS) PY -dmPY/Pmol g(TPD-MS) uv-VISTPD-MS MgO(1)AIR MgO( 1I)AIR MgO(1)VAC BM50 25 30 27 28 4 5 4 8 29 47 35 37 31 46 36 28 1946 J Mater Chem ,1996, 6(12), 1943-1949 0.1 0.0 MgO( 1)VAC MgO( 1)AIR 1 I 1 I I I 100 200 300 400 500 600 700 temperature/"(= Fig.6 Temperature-programmed desorption-mass spectrometry of pre-adsorbed carbon dioxide over the catalysts studied in this work peaks at low and high temperatures (Tma,=139 and 598"C), and the main peak at about 355"C, shifted to a higher temperature in the MgO(1)VAC profile.The desorption peak detected at high temperature (T,,, =598 "C) was only present in those solids prepared from magnesium hydroxide [MgO( 1I)AIR and BM50l. Therefore, magnesium hydroxide yields solids with stronger basic sites. The appearance of three regions in the TPD-MS profile suggests the presence of three different types of adsorption sites differing in their adsorption strength for C02.Similar results were reported previously.20,21 Thus, Tsuji et aL2' ana-lysed the isotopic distribution of CO, desorbed from an MgO surface containing adsorbed C1*O, and assigned the peak at the lower temperature to bidentate carbonate. They also suggested that, for peaks 2 and 3, processes other than simple adsorption-desorption of CO, on one pair of Mg2+ 0,-sites were involved and concluded that the fraction of C02adsorbed as unidentate carbonate was rather small. Table 3 shows the results obtained by integration of the area under each carbon dioxide desorption peak, as well as the total basicity as determined by CO, TPD-MS and by UV-VIS spectroscopy by using benzoic acid as the titrant.The results Table 3 Determination of the basicity of the catalysts obtained in this work by C02temperature-programmed desorption-mass spectrometry (TPD-MS). Comparison with results provided by UV-VIS spec-troscopy using benzoic acid as the probe molecule TPD-MS/pmol g-' UV-VIS/pmol g -catalyst peak 1 peak 2 peak 3 total total MgO(1)AIR 31 169 -200 156 MgO(1)VAC 25 436 -436 407 MgO(I1)AIR 135 326 45 506 495 BM50 25 354 61 440 42 1 provided by both methods are quite consistent. The solids prepared from magnesium hydroxide are more basic than those prepared from magnesium hydroxycarbonate. In this sense, MgO(I1)AIR was found to have 2.5 times more basic sites than MgO( 1)AIR (its analogue reprepared from magnesium hydroxycarbonate).The presence of B,O, in BMSO made the final material slightly less basic than MgO(I1)AIR. Finally, calcination in uacuo produced a material with enhanced basicity as revealed by a comparison between MgO(1)AIR and MgO(1)VAC: the solid prepared in uacuo was 2.1 times more basic than that calcined in the air. Table 4 shows the CO, desorption activation energies obtained from the Kissinger equation.', The activation energy can be related to the basic strength of the site type.14 The results are quite revealing: once again, the effect of in uucuo calcination is clearly apparent when the activation energies obtained for the solid MgO(1)AIR are compared with those for MgO(1)VAC.Thus, MgO(1)AIR was not only the solid containing the fewest basic sites, but also the weakest (Ea=4 and 14 kJ mol-' for peaks 1 and 2, respectively). In uacuo calcination produces a larger number of stronger basic sites (Ea=45 kJ mol-I). However, the precursor also has a clear effect that is apparent on comparing the results obtained for MgO( 1)AIR (magnesium hydroxycarbonate as precursor) and MgO( 1I)AIR (magnesium hydroxide as precursor). Solids pre- pared from magnesium hydroxide exhibited a larger number of basic sites that were also of a higher strength (Table 4). It is very interesting to note that catalyst BM50 had the strongest basic sites (Ea=108 kJ mol-I). In addition to the C02 TPD-MS profile, DRIFT spectra were recorded for carbon dioxide pre-adsorbed on all samples.Fig. 7 and 8 show the pre-adsorbed C02 DRIFT spectra for MgO(1)AIR and MgO(I1)AIR respectively, in the range 2000-1000 cm-l. Adsorbed C02 gave strong bands in the region 1400-1550cm-', with a shoulder at an even higher I I 1800 1600 1400 1200 wavenumbedcm-' Fig. 7 DRIFT spectra of pre-adsorbed carbon dioxide over MgO( 1)AIR at variable temperatures Table 4 C02 desorption activation energies (E,) for the catalysts studied as determined from the Kissinger equation, and correlation coefficients (r) peak 1 peak 2 peak 3 catalyst E,/kJmol- r E,/kJ mol - r E, /k J mol - Y MgO( 1)AIR MgO( 1)VAC 4 45 0.985 0.997 14 - 0.995 - -- -- MgO( 1I)AIR BM50 34 26 0.997 0.987 41 39 0.999 0.970 51 108 0.989 0.945 J.Mater. Chem., 1996, 6(12), 1943-1949 1947 h gE cBE cE c 1800 1600 1400 1200 wavenurnberkm-' Fig. 8 DRIFT spectra of pre-adsorbed carbon dioxide over MgO( 1I)AIR at variable temperatures wavenumber Weak bands at ca 1070 and 1220cm-' were also observed The DRIFT spectra for pre-adsorbed C02 on solids prepared from the same precursor were similar However, the materials made from different precursors also gave different DRIFT spectra While some bands (Al, A2, B2, A3 and B3) appeared in both spectra, others only appeared in the MgO(I1)AIR DRIFT spectra (B1 and E2) Table 5 shows all possible interactions between CO, and the MgO surface, and assigned IR bands in the DRIFT spectra Bands between 1400 and 1550 cm-' and that appearing at ca 1070 cm-' are ascribed to surface unidentate carbonates According to Philipp et al, unidentate carbonate seems to be the dominant adsorption state of CO, at room temperature," which is consistent with our DRIFT spectra (Fig 7 and 8) The band shoulder at 1630-1660 cm-' is assigned to surface hydrogencarbonate 600°C 300°C 120°C I I I I I I I , 20 10 0 -10 6 Fig.9 'H MAS NMR spectra of the solid MgO(I1)AIR after heating for 4h at 120, 300,600 and 900 "C The strong bands in the DRIFT spectra (Fig 7 and 8) corresponding to unidentate carbonate indicate that, under our conditions, the main adsorption mode of CO, on MgO is unidentate carbonate, as reported previously by Philipp et a1 21 On the other hand, TSUJ~ et a1 ,20 based on TPD experiments, found not unidentate, but bidentate carbonate to be adsorbed on MgO However, most of the C02 desorbed in their experi- ments was in a low-temperature peak, while most of that in our experiments was desorbed over the second peak at moder- ate temperature (peak maximum at ca 350 "C)In the adsorbed state, the symmetry of adsorbed carbonate species is lowered and those species formed generally present two v(C0) bands to either side of 1415 cm-' (corresponding to free carbonate) It has been considered that the Av splitting characterizes the structures of the species formed ca 100, 300 and 400 cm-' for unidentate, bidentate and bridged species, respectively 22 23 Table 5 C02 adsorption sites and species formed over magnesium oxides Vibration modes and wavenumbers ~~ adsorption mode unidentate carbonate hydrogencarbonate bridged carbonate ?I1 bidentate carbonate IR bands A1 (C-0,, stretching) A2 (0,-C-0, sym stretching) A3 (OI-C-O, asym stretching) B1 (COH bending) B2 (01-C-O1 sym stretching) B3 (0,-C-0, asym stretching) C1 (C=O,, stretching) El (C=OlI stretching) E2 (O,-C-OI asym stretching) cm 1075 1393 1526 1220 1419 1650 1776 1625 1300 1948 J Muter Chem, 1996, 6(12), 1943-1949 However, there is some confusion in the literature about such a relation By comparing X-ray structural and IR spectral data, it was shown that bidentate carbonate species generally have Av >250 cm- and bridged carbonate species have Av <250 cm-', the only unidentate carbonate structurally characterized has Av =80 cm-' 24 The Av found for our solids (Fig 7 and 8) agree with values reported in the literature corresponding to unidentate carbonate species [Av =70 and 140 cm-' for MgO(1)AIR and MgO(II)AIR, respectively] In addition to unidentate carbonate, hydrogencarbonate bands were found in both solids, the clearer in the MgO(I1)AIR DRIFT spectra at 1220 cm-' The formation of hydrogen- carbonate species is related to the presence of OH sites acting as bases against C02 'H MAS NMR spectra recorded after heating the catalyst MgO( 1I)AIR at several temperatures for 4 h (Fig 9) show that even after heating at 900°C the band due to the surface OH is still present The presence of these OH sites may be crucial in some types of catalysts, as they can enhance metal-support interaction^^^ or facilitate some types of catalysed organic reactions such as the aldol addition of acetone 26 As far as BM50 is concerned, we have no experimental evidence of the physical state of boron in the solid However, solids BM50 and MgO(I1)AIR show different reactivity and selectivity patterns in propan-2-01 decomposition 27 Derouane et a1 and McKenzie et a1 ,from 27Al NMR studies of A120, doped Mg028 29 suggest that, although aluminium is probably well distributed throughout the lattice, calcination produces a concentration of A1 at the surface, however, this A13+ ion is in a tetrahedrally coordinated oxidic environment which is unfavourable for acid-catalysed reactions 28 29 Additional experiments are being carried out to reveal the role of boron in this catalyst Conclusions The above results allow one to draw some interesting con- clusions (2) The precursor used to prepare an MgO catalyst is crucial in order to obtain the desired textural and acid-base characteristics Thus, solids prepared from Mg(OH), exhibited higher BET surface areas and greater basicity than those obtained from magnesium hydroxycarbonate In addition, basic sites involving OH species are present in both solids even after calcination at 900°C This type of basic site is crucial for some types of catalysed organic reactions (zz) For the same precursors, the calcination method also influences the production of the desired solid Thus, zn uacuo calcination leads to solids with a slightly higher surface area and greatly enhanced basicity It also gives rise to a new type of Lewis-acid site that probably arises from low coordinated Mg2+ (m)Finally, the catalyst doped with B203 (BM50) exhibits a lower surface area than its related system MgO(II)AIR, as well as a slightly lower basicity due to the primarily acid nature of pure B20, The authors gratefully acknowledge funding of this research by the Consejeria de Educacibn y Ciencia de la Junta de Andalucia and the Direccion General de Investigacibn Cientifica y Tecnica (DGICyT) in the framework of project PB92-0816 The staff at the Mass Spectrometry and Nuclear Magnetic Resonance Services and Inorganic Department of the University of Cordoba are also acknowledged for their kind 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