Physicochemical and catalytic properties of nano-nickel in the process of conversion of isopropyl alcohol to acetone
nickel-kieselguhr catalyst in the conversion
of isopropyl alcohol in acetone showed
that the process is accompanied by the loosening of initial metal structure and the formation of nanoclusters of nickel. Catalytic properties of the nickel-kieselguhr catalyst are connected with change of morphology, phase, electronic, magnetic state and the nature of surface-active nickel clusters like homogeneous spinel.
Earlier in this direction were carried out researches of the applying of industrial nickel-kieselguhr catalyst in the conversion of aliphatic alcohols C1-C4 [9-12], which allowed to establish that this catalyst has a high selectivity of ketone. The purpose of this study was to establish the relationship between morphological, structural, magnetic and electronic properties of nickel-kieselguhr catalyst and its catalytic activity in the process of conversion of isopropyl alcohol to acetone.
Methods of research
Researches of activity of the catalyst in reaction of gas-phase dehydrogenation of alcohol were conducted on laboratory installation of continuous flow at temperature of 150-350 °C and with the speed of supply of alcohol 600 h-1. The analysis of raw materials and products of reaction was performed using the chromatographic method according to the technique described in [9-12].
Morphological features of samples of the nickel-kieselguhr catalyst was studied by scanning electron microscope Philips 515 with energy of primary electron beam 30 kV.
For X-ray phase study of samples of the nickel-kieselguhr catalyst was used diffractometer DRON-3M using CuKα-radiation, equipped with a graphite monochromator on the diffracted beam.
Conductometric studies were performed using teraohmmeter E6-13A (range of measured resistance from 10 to 1014 Ohms; the main error doesn't exceed +2.5%). A sample of catalyst was placed in a measuring chamber, which simultaneously served as a catalytic flow reactor. The sample was pressed in the squared beam, which area of cross-section is known. In the bar cross section were inserted platinum probes. The sample was fixed in the clip, which was also equipped with two probes attached to the platinum probes on the sample surface.
Measurement of a magnetic susceptibility of the nickel-kieselguhr catalyst carried out according to Faraday's method .
The effect of temperature on the process
of conversion of isopropyl alcohol
Fig.1 and 2 shows the effect of temperature on the performance of the process of conversion of isopropyl alcohol in the presence of industrial nickel-kieselguhr catalyst. These results allow to characterize the catalyst as highly active and selective for the acetone (at 250°C the yield of up to 82.4%). Apart from the last, products of reaction also include isopropyl ether and propylene, the maximum output of which, in terms of missed alcohol is 20% and 42%, respectively. The formation of aldehyde is not observed.
At about 150 ºC, the conversion degree of the alcohol reaches 12%, the main part of the formed products are dipropyl ether (6%) and equal amounts of acetone and propylene (3 %). With increasing temperature from 150ºC to 200ºC the conversion of alcohol increases to 52%. There has been a sharp increase in the yield of acetone to 27%, and ether to 20%, which indicates a sequential process of formation of ether: dehydrogenation of alcohol and condensation of the resulting ketone in ether.
Increasing of the reaction temperature up to 250ºC leads to a sharp decrease in the quantity of ether in the condensate, and increases the conversion of alcohol to 93%, mainly due to the formation of acetone. The yield of the last amounted to 82.4%, while there has been a slight increase in the yield of propylene – for 2%.
Further increase in temperature up to 300ºC accompanied by an increase in conversion to 97% and a reduction of yield of acetone to 68%. Reduction of formation of acetone at almost full conversion of alcohol is apparently caused by decomposition of the formed carbonyl compounds, and may also be associated with deactivating role of seal products, formed in conditions of high temperatures and reducing the area of the active catalyst surface.
With increasing temperature up to 350ºC, the conversion value is still high, and the reaction products contains: acetone (51%), propylene (42% ) and ether (3%).
These reactions are accompanied by a wide range of topochemical processes on the surface of the nickel-kieselguhr catalyst: transition of the second kind (ferro- and paramagnetism) and structural transformations [9-12].
The formation of surface nanoclusters
Scanning electron microscopy allowed to reveal the formation of surface nanoscale clusters (fig.3). So, if before carrying out catalytic experiments the original sample has the structure of a typical metal system, smooth with chips, representing set of agglomerates of the irregular shape, then the reaction of dehydrogenation of isopropyl alcohol is accompanied by a significant development of the sample surface under the influence of the reaction medium and the advent of nanoscale entities (fig.3b). The process takes place against the background of loosening of the original homogeneous globular structure of the catalyst with the formation of nanoclusters of nickel with a predominant size of 7 nm. On the surface of the newly formed agglomerates the nucleus of a new phase are visible. According to X-ray phase analysis [9, 10], it can be attributed to carbon deposits formed in thermodynamically favorable conditions of the dehydrogenation of isopropyl alcohol. Formation of the surface carbon compounds, probably leads to the destruction of large agglomerates of nickel, the so-called "carbide cracking", and, ultimately, to the formation of nanoclusters of nickel.
The heterogeneous nature of the clusters, when one of the atoms of nickel is in a metallic state, and the other (on the surface of the clusters) – in a chemical bond with oxygen, apparently, plays a significant role in the activation of nickel-kieselguhr catalyst.
Magnetic properties of the catalyst
It is known, that nanoscale systems have a number of new physical effects [13-16]. The results of the study of the magnetic properties of the initial and exhaust samples (fig.4) indicate that their magnetic susceptibility has a complex temperature dependence in the area of 25-350°C, which is different from the Curie-Weiss law. Magnetic susceptibility of the initial sample initially is very high, but with increasing temperature up to 50°C it decreases more sharply. This pattern of changes associated with the presence of metallic nickel, in which electron spins are parallel – ideally ordered due to the interaction with the spins of neighboring ions in the crystal lattice. With further increase in temperature, the sample begins to lose its ferromagnetic properties and turn into the antiferromagnet, to which the Curie-Weiss is also not applicable. Magnetic susceptibility of antiferromagnetic component (NiO, electronic spins are built "disordered " and cancel each other out) raises during heating, which is associated with the tendency to increase the structural disordering at high temperatures [15-17]. 240°C is the Neel temperature for the studied sample, when magnetic susceptibility takes the maximum value in 25.76∙106. Above the Neel temperature, the sample loses its magnetic properties and becomes paramagnetic, and magnetic susceptibility decreases by Curie-Weiss law, reaching 12.88∙106 at 350°C.
Studies of the magnetic properties of the sample, involved in the catalytic process, has allowed to establish lack of observed earlier exchange interaction of the spins, resulting in the transition from ferromagnetism to antiferromagnetism. At temperature of 25-160°C, the sample of catalyst is a antiferromagnet, and with increasing of temperature above 160°C enters the paramagnetic state. Thus phase transition of the second kind antiferromagnetic-paramagnetic observed already at 170°C [17,18]. It can be assumed that at this temperature occur topochemical changes, associated with the emergence of nano-nucleation phase of metallic nickel, embedded into oxide matrix, which form the active microheterogeneous system Ni/NiO. The properties of the active centers of this system are determined by the combination of magnetic properties of atoms of a nickel cluster like homogeneous spinel: cubic close-packed lattice of oxygen ions, in the voids of which in tetrahedral positions are ions Ni+1, and in the octahedral positions – Ni+2. The latter have a magnetic spins, directed antiparallel to the spins of the ions, occupying the tetrahedral knots.
Electronic and catalytic properties
Direct connection of electronic and catalytic properties of nickel catalyst illustrates fig.5. During the dehydrogenation of isopropyl alcohol, the yield of acetone and the conductivity, change together with temperature, starting to rise sharply after 200°C. In this temperature range in the presence of hydrogen nickel-kieselguhr system has the lowest activation energy. The number of free carriers will grow exponentially with temperature, determined by the bandgap. Here is realized a hopping mechanism of conduction, in which the charge transfer is carried out by quantum tunneling transitions ("hops") of charge carriers between different localized states of nickel [19-23]. It is obvious, that the more free valences, the more intensive is the process of formation of acetone, as is confirmed experimentally.
The hydrogen, which was formed during reaction, is adsorbed on a surface of nickel-kieselguhr system, capturing the free valencies, presented by holes in the conduction band, that significantly increases conductivity and efficiency of the catalyst.
Fig.6 presents a schematic mechanism of the dehydrogenation of isopropyl alcohol for industrial nickel-kieselguhr catalyst. The process is carried out by an associative mechanism involving the adsorption of reactants [24,25]. If we exclude non-specific van der Waals adsorption, the first stage of chemical interaction is the break of O-N connection when a molecule of alcohol occurs with a free hole in the surface of the catalyst. Break of this connection leads to the formation of surface compounds with "strong" ((CH3)2CHO-surface) and weak (H-surface) homo-polar bonds, as a result of reversible processes of the electronic exchange. The radicals stay certain time in each of two types of a adsorption link and accordingly change their reactivity.
In the second stage, the hydrogen atom recombines with the formation of the hydrogen molecule with hydrogen secondary carbon atom, and the remaining adsorbed radical (CH3)2CO desorbed, forming a molecule of acetone.
Thus, in the course of the research it was found high catalytic activity of industrial nickel-kieselguhr catalyst in the process of conversion of isopropyl alcohol to acetone. The application of modern research methods, such as scanning electron microscopy, magnetic and conductivity measurements, have allowed to establish the formation of nickel nanoclusters like homogeneous spinel. The mechanism of their action consists in ability of ions of nickel quite easily to move from one condition of oxidation into another. Participation of the catalyst in process leads to ordering of the active centers: structures of the spinel type Ni+[Ni2+O], nickel oxide (II) and metallic nickel. ■