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The Influence of Alcohols in Driving the Morphology of MgCl
2Nanocrystals
K. S. Thushara, Maddalena D’Amore*, Alessandro Piovano, Silvia Bordiga and Elena Groppo*
Abstract: MgCl2 nano-crystals prepared in the presence of methanol
and ethanol have been characterized by complementing surface science tools with a systematic quantum-mechanical investigation of the stability order (in terms of Gibbs free energy) of different MgCl2
surfaces in the presence of the two alcohols. Both the alcohols drastically change the overall stability and the stability order of the exposed surfaces, mainly as a consequence of the entropic contribution to the Gibbs free energy, hence inhibiting or promoting crystal growth in certain directions. The environment-dependent
occurrence of the MgCl2 surfaces may influence the structure and
the properties of the supported TiClx sites in the MgCl2/TiCl4
pre-catalyst, with important implications in the design of morphologically controlled Ziegler-Natta catalysts.
Many efficient and selective catalytic materials for a wide range of energy and environmental applications are composed of nanometer-sized particles exposing different surface sites.[1-5]
Identifying the correlation between the particle structure at the nano-scale and their activity and selectivity is fundamental to develop tailored catalysts, a challenge that up to now has been almost a privilege of homogeneous catalysts. However, the experimental characterization of the surface and structural properties of nano-catalysts is problematic,[6-10] especially in
reaction conditions, and in most of the cases the obtained information is averaged over the whole ensemble of sites. Recently, a huge effort has being devoted to synthetize nano-structures with controlled morphology, where well-defined
exposed surfaces display a specific catalytic behaviour.[11-13] The
use of surfactants to stabilize certain surfaces with respect to others is a well-established and efficient method to drive the morphology of nanomaterials in general, ranging from metal-oxides to metal nanoparticles.[11,14-19] The design of
nano-catalysts with a defined morphology requires the optimization of the synthetic protocol, i.e. of the thermodynamic and environmental conditions of operation. In this process, the theoretical development of accurate adsorption models may help. In this respect, the computational design of heterogeneous catalysts is becoming feasible, thanks to the recent advances in Density Functional Theory (DFT), which makes it possible to
accurately describe the surface processes.[20]
In this work we show the potential of combining an accurate surface characterization with state-of-the-art DFT methods for
the prediction of the structural and surface properties of MgCl2
nano-crystals prepared in the presence of different alcohols,
which are relevant components in Ziegler-Natta (ZN) catalysis. Modern heterogeneous ZN catalysts for olefin polymerization may be seen as stereotypes of complex nanometre-sized materials. Regarding the complexity, it mainly derives from their multi-component nature,[21,22] consisting of MgCl
2, TiCl4, an
aluminium alkyl (e.g. AlEt3), and organic Lewis Bases (LB, e.g.
aromatic mono- and di-esters), the latter introduced to achieve a high stereo-selectivity in propene polymerization. These systems can be definitely labelled – and treated with all the related techniques – as nanometer-sized materials, since the MgCl2
activated support is featured with a relevant structural disorder along the c axis and nanometric crystalline dimensions.[23-26]
Disordered MgCl2 is usually synthesized in the presence of TiCl4
following either mechanical or chemical routes.[27-30] In
mechanical methods, MgCl2 and TiCl4 are co-milled with or
without the solvent to generate the active catalyst. In chemical
routes, highly crystalline MgCl2 (α form) is interacted with Lewis
bases (mainly alcohols), then de-alcoholated or reacted with TiCl4 to form disordered and nano-sized MgCl2 ( form). The
influence of the synthesis protocol in determining the structural, morphological and surface properties of the final catalyst is well known in the industrial practice.[31] It is recognized that the nature
of the molecular adduct and the alcohol employed in the synthesis of MgCl2 drastically influence the porosity of the
catalyst and hence its activity and the properties of the obtained polymer.[31-33] “Super active” catalyst supports have been
prepared from molecular adducts between MgCl2 and alcohols.
[34,35] However, a clear correlation between the synthesis
variables and the properties of the catalyst at the nano-scale is still missing.
Since the 1980s,[36,37] ZN catalysts have been traditionally
investigated by means of molecular mechanics and quantum mechanical computations, aimed at identifying possible reaction mechanisms for olefin polymerization on what were supposed to
be the most stable MgCl2 surfaces in the presence of LB and
TiClx. In the last decade, the application of DFT and DFT
methods including dispersion (DFT-D) to elucidate the energetic of interaction and the local structure of TiCl4 (or TixCl3x) and LB
at these surfaces[25,38-46] substantially renewed the seminal vision
of catalysis on MgCl2. Recent simulations performed on clusters
of different size and morphology[41,42] demonstrated that the
relative stability of MgCl2 surfaces drastically changes in the
presence of donors. This holds for several small donor molecules (H2O,[44] NH3,[44] methanol,[42,44] ethanol[44]), as well as
for the industrially employed donors.[44,46-50] Interestingly, these
theoretical studies validated the experimental results obtained years before by Thune and co-workers on a planar model of MgCl2,[51,52] when it was visualized for the first time the effect of
an internal donor in controlling the morphology of the MgCl2
crystals formed during the preparation of a ZN catalyst. Although so far all the computational studies were performed only on the (110) and (104) surfaces (the most plausible ones in naked
MgCl2 crystals), these results clearly indicate that the reaction
environment affects the free energy of the exposed MgCl2
Dr. K. S. Thushara, Dr. M. D’Amore, A. Piovano, Prof. S. Bordiga, Dr. E. Groppo
Department of Chemistry, INSTM and NIS Centre University of Torino
Via Quarello 15, 10135 Torino, Italy
E-mail: elena.groppo@unito.it, maddalena.damore@unito.it
Supporting information for this article is given via a link at the end of the document.
surfaces. Hence, the MgCl2 nano-crystals are expected to have
a dynamic behaviour (i.e. to change the overall morphology) during the catalyst synthesis, and likely also during the catalyst function.
Recently, we accomplished the first systematic
computational investigation of the most stable surfaces of MgCl2
aiming at obtaining their stability order in terms of Gibbs free energy both for naked crystals and in the presence of methanol.
[53] Our computational morphological analysis revealed that the
surfaces exposing highly acidic Mg2+ cations (namely the (012),
(015), and (110) surfaces) are highly stabilized by methanol, especially at the temperature typically adopted in the preparation of the pre-catalysts. We also demonstrated that FT-IR spectroscopy of CO adsorbed at 100 K, coupled with vibrational analysis performed on adducts CO/surface (hkl), is a powerful method to characterize the surface of structurally disordered MgCl2.[53] In the present work we complete our previous
systematic study by investigating the effect of ethanol in driving the morphology of MgCl2 nano-crystals, in the attempt to
rationalize the role of alcohols as shape-directing agents for
MgCl2 crystals. We wish to underline that our work constitutes a
step further most of the previous theoretical works describing the
adsorption of donors on the MgCl2 surfaces, that do concentrate
only on the “classical” (104) and (110) surfaces (whose presence was inferred on the basis of the pure crystallographic perfection) and give an estimation of the adsorption enthalpy only, neglecting the entropic term.
Briefly, the disordered MgCl2 nano-crystals were obtained
upon controlled de-alcoholation of the corresponding MgCl2·6ROH adducts,[53-55] where R = CH3 (Me) or C2H5 (Et),
resulting in MgCl2-MeOH and MgCl2-EtOH samples, respectively.
According to the thermogravimetric data shown in Figure 1a, all the alcohol molecules are removed below 200 °C, although the de-alcoholation process proceeds with different steps in the two
cases. Particularly, the MgCl2·6EtOH adduct loses only four out
of the six ligands below 150 °C, and the last two in the 150-200 °C range, in very good agreement with previously
Figure 1. Part a): Thermogravimetric (full lines) and differential
thermogravimetric (dotted lines) analysis of the two alcohol precursors, MgCl 2-6EtOH (black) and MgCl2-6MeOH (grey). Part b): XRPD patterns of the resulting disordered MgCl2 nano-crystals: MgCl2-EtOH (black) and MgCl2-MeOH (grey). The two patterns are vertically translated for clarity. The broad peak centered around 2θ = 20° is due to the glass capillary.
published data.[56] In contrast, at 150 °C the MgCl
2·6MeOH
adduct has already lost five out of the six ligands, in agreement with the higher volatility of methanol with respect to ethanol. In
both cases, a layered MgCl2 phase characterized by an
extensive structural disorder is obtained, as demonstrated by the X-Ray powder diffraction (XRPD) patterns shown in Figure 1b and in agreement with literature.[55,57-60] The two diffraction
patterns are very similar, irrespective of the starting precursor. This is a clear limit of the XRPD technique, which is unable to reveal subtle structural differences for nano-sized and disordered materials.
The surface properties of the two MgCl2 samples were
determined by combining N2 physisorption measurements with
FT-IR spectroscopy of CO adsorbed at 100 K. The former technique gives indication on the total specific surface area, while the latter discriminate the type of exposed surface sites. Indeed, we have recently demonstrated that this technique can
be successfully used to probe the exposed MgCl2 surfaces in
terms of coordinatively unsaturations and polarizing ability of the
exposed Mg2+ cations, and hence of their Lewis acidity.[53] Both
samples show adsorption isotherms of Type IV (Figure 2a) and are characterized by a pronounced hysteresis loop, which indicates the presence of inter-particles mesopores. The specific surface area, evaluated by means of the BET model, is of about
40 m2/g and 100 m2/g for MgCl
2-EtOH and MgCl2-MeOH, respectively.
The FT-IR spectra, in the ν̃(CO) region, of CO adsorbed at
100 K on the MgCl2-EtOH and MgCl2-MeOH as a function of the CO
coverage (θCO) are reported in Figure 2b and Figure 2c,
respectively. Two absorption bands are observed in both cases, having the same behavior as a function of θCO, but with a
different total and relative intensity. These two bands have been assigned in our previous work based on a detailed vibrational analysis.[53] Briefly, the absorption band located at 2182 cm-1 for
maximum CO and shifting to 2194 cm-1 for CO → 0, was
interpreted as the superposition of three contributions, namely CO adsorbed on the (110), (015) and (012) surfaces. On these
surfaces the Mg2+ cations are either tetra-coordinated (on the
(110) face) or penta-coordinated but with a high polarizing ability due to the proximity of chlorine ligands belonging to a
neighbouring layer. Hereafter we will refer to these Mg2+ sites
with the name of “strongly acidic Mg2+ sites”. The second
absorption band, located at 2163 cm-1 at maximum CO and
much less sensitive to the decrease of coverage, was assigned to CO adsorbed on the (104) and (107) penta-coordinated
families of surfaces. We will call these sites “weakly acidic Mg2+
sites”. No absorption bands due to CO adsorbed on defects
(expected around 2200 cm-1) are observed, likely a
consequence of the persistence of a minor amount of alcohol molecules (or some by-products)[61,62] on the most energetic
surface sites. The absolute intensity of the ν̃(CO) absorption
bands at the maximum θCO is smaller for MgCl2-EtOH (Figure 2b)
than for MgCl2-MeOH (Figure 2c), in fair agreement with the
specific surface area determined by the BET analysis (Figure 2a). More important, the relative intensity of the two absorption
bands is reversed in the two cases. For CO adsorbed on MgCl
2-EtOH (Figure 2b), the absorption band assigned to weakly acidic
Figure 2. Part a): N2 adsorption isotherms at 77 K for MgCl2-EtOH (black) and MgCl2-MeOH (grey). Parts b) and c): FT-IR spectra (in the ν̃(CO) region) of CO adsorbed at 100 K on MgCl2-EtOH and MgCl2-MeOH, respectively, as a function of the CO coverage (θmax in bold). Part d): as part c) but for a MgCl2-MeOH sample de-alcoholated at 300 °C. All the FT-IR spectra are normalized to the thickness of the pellet and subtracted to the spectrum collected before CO dosing.
strongly acidic Mg2+ cations, while the opposite behavior is
observed for CO adsorbed on MgCl2-MeOH (Figure 2c). Hence,
despite MgCl2-EtOH and MgCl2-MeOH are almost indistinguishable by
XRPD, they clearly expose different surfaces, at a different extent. This is the first experimental proof of the role of alcohol in
driving the morphology of MgCl2 at the nano-scale, so far only
predicted by theoretical calculations.[43] It is also interesting to
notice that the relative intensity of the two ν̃(CO) absorption bands does not change when the MgCl2 nano-crystals are
obtained at higher de-alcoholation temperature. For example, Figure 2d shows the FT-IR spectra of CO adsorbed at 100 K on a MgCl2-MeOH samples obtained upon de-alcoholation of the
corresponding adduct at 300 °C. With respect to MgCl2-MeOH
de-alcoholated at 200 °C, only the absolute intensity of the spectra decreases, while the relative intensity of the two ν̃(CO) absorption bands remains almost the same. Similar results were obtained for MgCl2-EtOH. This clearly indicates that the
morphology of the MgCl2 nano-crystals is strictly dependent on
the type of alcohol. In contrast, the activation temperature seems to influence the total surface area (i.e. the dimension of the crystals), in that a high activation temperature likely induces sintering phenomena.
In the attempt to rationalize the behaviour of MgCl2 in the
presence of different alcohols we extended our systematic computational study on the stability of the exposed surfaces, previously performed in the presence of methanol, to the ethanol case. The results are summarized in Table 1. A graphical representation of the computed surfaces in the presence of methanol can be found in our previous work.[53] Those with
ethanol, except for longer alkyl chains, are practically indistinguishable,. Briefly, we considered the most stable surfaces of α-MgCl2 and for all of them we evaluated the
variation of the total electron energy due to the adsorption of
alcohol molecules (ΔEads), as well as the Gibbs free energy of
adsorption (ΔGads) at 298 K. The calculations were performed
considering an alcohol coverage ROH = 1 for all the surfaces,
except for the (110) one exposing tetra-coordinated Mg2+
cations, where also ROH = 2 was considered. Table 1 reports
also the Gibbs free energy (which is a measure of the stability)
of each surface in the presence of alcohol in the gas (Gs-ROH(g))
and liquid (Gs-ROH(l)) phase (in J/m2), calculated by adding ΔGads to
the formation energy of each naked surface starting from bulk crystal (Gs), i.e. Gs-ROH = Gs + ΔGads. Finally, the calculations were
performed also at 373 K. Indeed, the de-alcoholation of the
MgCl26ROH adduct and the formation of the activated MgCl2
nano-crystals occurs in the 298 – 473 K temperature range. Also
the typical processes for the preparation of MgCl2 with improved
controlled morphology starting from MgCl2 alcoholate adducts
are conducted around 373 K.
According to DFT-D calculations, in the presence of adsorbates a huge stabilization of all the surfaces occurs. In particular, both alcohols selectively stabilize the (110) surface with respect to naked MgCl2, in agreement with the recent
literature.[40-44] Our systematic computational approach allows
appreciating relevant differences between methanol and ethanol.
In particular, the Gibbs free energy of adsorption (ΔGads in Table
1) of ethanol on the (012), (101) and (110) surfaces are substantially lower than those of methanol on the same surfaces, which means that the (012), (101) and (110) families of surfaces are less stabilized by ethanol with respect to methanol. More in details, for the (012) and (101) surfaces, the enthalpic contribution already disfavours the ethanol adsorption with
respect to methanol (lower ΔEads), while for the (110) surface
entropic reasons hinder the adsorption of ethanol (similar ΔEads
for methanol and ethanol). At the same time, ΔGads of ethanol on
the (104) and (107) families of surfaces are higher than those of methanol, indicating that these surfaces are stabilized more by ethanol than by methanol. In this case the phenomenon is driven mainly by the entropic contribution. Finally, the stability of the (015) surface is the same, irrespective of the alcohol. The
stability (Gs) of the different surfaces is a balance between the
enthalpic and the entropic contributions, and follows a different order in the presence of the two alcohols.
Table 1. Predicted adsorption energies (ΔEads and ΔGads) for ethanol and methanol on different MgCl2 surfaces, and predicted stability (in terms of Gibbs free energy, in J/m2) of each surface in the presence of ethanol and methanol in the gas phase (Gs-ROH(g)) and in the liquid phase (Gs-ROH(l)) at 298 K and 373 K.
Surface (hkl)
R-OH ΔEads
(kJ/mol, 298K) ΔGads (kJ/mol, 298K) Gs-ROH(g) (J/m2, 298 K) Gs-ROH(l) [a] (J/m2, 298 K) Gs-ROH(g) (J/m2,373.15 K)
EtOH MeOH EtOH MeOH EtOH MeOH EtOH MeOH EtOH MeOH
(001) 1 -34.9 - 35.4 - 0.0498 0.0498 0.0429 (101) 1 -113.5 -122.1 -48.0 -51.3 0.236 0.210 0.283 0.243 0.408 0.381 (012) 1 -116.6 -136.6 -50.3 -63.1 0.188 0.0947 0.233 0.126 0.340 0.249 (104) 1 -83.8 -83.8 -20.0 -15.2 0.556 0.586 0.595 0.613 0.730 0.773 (015) 1 -101.8 -100.9 -34.4 -34.8 0.259 0.257 0.294 0.282 0.388 0.370 (107) 1 -84.2 -87.1 -24.2 -18.9 0.271 0.297 0.300 0.317 0.383 0.410 (110) 1 -116.7 -118.2 -60.6 -52.8 0.637 0.672 0.665 0.717 0.813 0.850 (110) 2 -109.5 -111.5 -27.4 -42.8 0.513 0.479 0.586 0.518 0.655 0.713
[a] Thermodynamic data of normal alcohols as reported in references [63-65].
The computational results summarized in Table 1 allow explaining the different relative intensity of the two ν̃(CO) absorption bands observed in the FT-IR spectra of CO adsorbed at 100 K on MgCl2-EtOH and MgCl2-MeOH. As a matter of fact,
calculations predict that ethanol, more than methanol, stabilizes
the surfaces exposing weakly acidic Mg2+ cations (i.e. the (104)
and (107) surfaces), which are responsible of the absorption band at 2163 cm-1. At the same time, most of the surfaces
exposing strongly acidic Mg2+ sites (i.e. the (110) and (012)),
which account for the absorption band at 2182 cm-1, are less
stabilized by ethanol than by methanol. On the contrary, this computational approach does not allow predicting the total intensity of the FT-IR spectra in the ν̃(CO) region, that means the total specific surface area of MgCl2 nano-crystals. In this
respect it is important to notice that our approach allows determining which are the possible surfaces in the chosen experimental conditions (donors, temperature). However, the degree of expression of the possible surfaces depends on the crystallization kinetic, which clearly influences also the
dimension of the MgCl2 nano-crystals.
The experimental and computational results reported above demonstrate that a careful vibrational analysis conducted at DFT-D level, coupled with FT-IR spectroscopy of CO adsorbed at 100 K, is a very powerful tool for predicting and validating the morphology of disordered MgCl2 nano-crystals formed in the
presence of different alcohols as Lewis bases. We have demonstrated that the alcohol drastically changes the overall stability and the stability order of the exposed surfaces, mainly as a consequence of the entropic contribution to the Gibbs free energy. Our results definitely prove that long chain aliphatic alcohols would stabilize those surfaces exposing weakly acidic Mg2+ cations, while methanol favors the surfaces exposing
strongly acidic Mg2+ sites, irrespective of their crystallographic
definition in terms of (hkl) indexes, which is sometimes criticized when dealing with nano-sized materials. Since the distribution
and the extension of the MgCl2 surfaces influence the structure
and the properties of the supported TiClx sites in the MgCl2/TiCl4
pre-catalyst, these results are of primary interest in the design of morphologically controlled ZN catalysts.
Experimental and Computational Details Section
The MgCl2·6ROH adducts were synthesized as reported in refs. [53-55].
The active MgCl2-ROH phases was obtained from the corresponding
adducts through a controlled de-alcoholation in dynamic vacuum at 473 K for a prolonged time, as described in ref. [53]. The thermal gravimetric
analysis (TGA) was carried out on a TAQ600 instrument, at a heating ramp rate of 1 °C/min from room temperature to 300 °C in nitrogen flow, after an equilibration in N2 at room temperature at 30 °C for 30 minutes.
X-ray Diffraction (XRPD) patterns were collected in the capillary configuration with a PW3050/60 X’Pert PRO MPD diffractometer from PANalytical working in Debye−Scherrer geometry, using as a source the high power ceramic tube PW3373/10 LFF with a Cu anode, equipped with a Ni filter to attenuate Kβ and focused by X-ray mirror PW3152/63. Scattered photons were collected by a RTMS (real time multiple strip) X’celerator detector. The surface area of de-alcoholated MgCl2 samples
was determined by the Brunauer–Emmett–Teller (BET) method via nitrogen adsorption ay 77 K performed on a ASAP 2020 instrument. In all the cases, care was taken to avoid exposure of the samples to air. A Bruker Vertex70 spectrophotometer equipped with a MCT detector was used to collect in-situ FT-IR spectra of disordered MgCl2-ROH in the
presence of CO at 100 K, as a function of CO coverage. The standard procedure is described in reference [53].
Density Functional calculations were performed with the CRYSTAL14 code employing a Gaussian type basis set.[66] The hybrid Becke, three
parameters, Lee-Yang-Parr (B3LYP) functional was adopted with a parametrized damped 1/r6 term added to the energy obtained with the
standard density functional methods to account for dispersion. The DFT-D2 version proposed by Grimme[67] with a new parametrization for
crystals was adopted.[68] The inclusion of dispersion terms proper of
DFT-D formulations allows to overcome the intrinsic limit of standard DFT-DFT functionals lacking long-range contributions. All - electron Gaussian-type Split valence triple- basis sets plus polarization (TZVP) functions were applied to describe all the elements (Mg, Ti and Cl atoms).[69] An Ahlrichs
VTZ[70] plus polarization quality was adopted for the adsorbed alcohol
molecules, whereas the coefficients of the polarization Gaussian functions (αpol) were optimized in preliminary studies, thus enabling more
accurate calculations of the adsorption energy and vibrational frequencies. The choice was aimed at reducing the Basis Set Superposition Error (BSSE), that may become quite large in particular when the dispersion correction is included during the optimization process, i.e. when the molecule and the surface become closer. In a previous paper by us on methanol adsorption at the same level of computations we computed the BSSEs for all the surfaces and their values fell in the range 9.4%÷12% of the computed total adsorption energy, indicating that the ‘customized’ Ahlrichs basis sets adopted are adequate. Due to the low values of the correction in the adsorption energies (both ΔEads and ΔGads), in this work we did not included the
BSSE corrections. The Gauss-Legendre quadrature and Lebedev schemes were used to generate angular and radial points of a pruned grid consisting of 75 radial points and maximum number of 974 angular points over which electron density and its gradient were integrated. Values of the tolerance that control the Coulomb and exchange series in periodical systems were set to 7 7 7 7 18. All the bielectronic integrals, Coulomb and exchange, were evaluated exactly. For all B3LYP-D calculations, 10 K points have been adopted. For all the surfaces, calculations were performed considering a methanol coverage of θMeOH =
1, and for the (110) and (015) surfaces also θMeOH = 2 was considered,
although a second methanol molecule at this level of computation seems to be stable only on the (110) surface. By means of the “Planes” tool implemented in CRYSTAL code, the analysis of the -MgCl2 planes was
performed to locate the most stable surfaces of the crystals following the Bravais' law, since the surface free energy is inversely proportional to the directional density and thus proportional to dhkl (i.e the distance between
the planes).The {hkl} planes that have the interplanar distance in the range (� �hkl, �hklmax) have been analysed, (where �hklmax is the maximum
interplanar distance for a given plane {hkl}) and f is a factor between 0 and 1, in our case all plane families were considered up to f=0.3.
Acknowledgements
Adriano Zecchina and Gabriele Ricchiardi are kindly acknowledged for the useful discussion. The computational work was performed on both the Abel Cluster, owned by the University of Oslo and the Norwegian meta-centre for High Performance Computing (NOTUR), and operated by the Department for Research Computing at USIT, the University of Oslo IT-department (http://www.hpc.uio.no/) and at the HPC centre CINECA, thanks to the Galileo resource accessed through the POLCAT ISCRA C Project. This work has been supported by the Progetto di Ateneo/CSP 2014 (Torino_call2014_L1_73).
Keywords: MgCl2• nanocrystals • DFT-D calculations • FT-IR spectroscopy• Ziegler-Natta catalysts
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Calculations at DFT-D level, coupled with FT-IR spectroscopy of CO adsorbed at 100 K allows predicting and validating the morphology of disordered MgCl2
nano-crystals formed in the presence of different alcohols as Lewis bases.
K. S. Thushara, Maddalena D’Amore,* Alessandro Piovano, Silvia Bordiga and Elena Groppo*
Page No. – Page No.
The Influence of Alcohols in Driving the Morphology of MgCl2
