Photodecomposition of chloroform catalyzed by unmodified MCM-41 mesoporous silica

Article abstract of DOI:10.1111/php.12245

Unactivated MCM-41 mesoporous silica catalyzes the photodecomposition of chloroform to phosgene and hydrogen chloride under near-UV (λ > 360 nm) irradiation. The rate of photodecomposition increases toward an asymptotic limit as the O₂ partial pressure is increased. Deuterochloroform does not decompose under the same experimental conditions. Low concentrations of both cyclohexane and ethanol quench the photodecomposition, whereas water, up to its solubility limit, does not. Dissolved tetraalkylammonium salts suppress photodecomposition. The data are consistent with a mechanism in which light absorption by an SiO₂ defect yields an electron-deficient oxygen atom, which then abstracts hydrogen from chloroform. The resulting CCl ₃ radicals react with oxygen to form a peroxy radical that decomposes, eventually yielding phosgene and hydrogen chloride. Unmodified MCM-41 silica catalyzes the photodecomposition of chloroform under near-UV irradiation. It is proposed that decomposition is initiated through hydrogen abstraction from chloroform at a photoactive SiO₂ defect site.

Full text of DOI:10.1111/php.12245

Photochemistry and Photobiology, 2014, 90: 760–766
Photodecomposition of Chloroform Catalyzed by Unmodified MCM-41
Mesoporous Silica
~
Laura A. Pena, Alissa M. Chan, Larissa R. Cohen, Karen Hou, Brent M. Harvey and Patrick E. Hoggard*
Department of Chemistry, Santa Clara University, Santa Clara, CA
Received 11 September 2013, accepted 16 January 2014, DOI: 10.1111/php.12245
ABSTRACT
When irradiated at wavelengths below approximately 250 nm,
chloroform absorbs light and decomposes through C–Cl bond
fission, yielding phosgene and hydrogen chloride. The mecha-
nism has been reasonably well worked out in the gas phase, and
is thought to occur in substantially the same fashion in the liquid
phase (47).
Unactivated MCM-41 mesoporous silica catalyzes the photo-
decomposition of chloroform to phosgene and hydrogen chlo-
ride under near-UV (k > 360 nm) irradiation. The rate of
photodecomposition increases toward an asymptotic limit as
the O2 partial pressure is increased. Deuterochloroform does
not decompose under the same experimental conditions. Low
concentrations of both cyclohexane and ethanol quench the
photodecomposition, whereas water, up to its solubility limit,
does not. Dissolved tetraalkylammonium salts suppress pho-
todecomposition. The data are consistent with a mechanism
in which light absorption by an SiO2 defect yields an elec-
tron-deficient oxygen atom, which then abstracts hydrogen
from chloroform. The resulting CCl3 radicals react with oxy-
gen to form a peroxy radical that decomposes, eventually
yielding phosgene and hydrogen chloride.
hm
CHCl3ꢀ! ꢁCHCl2 þ Clꢁ
ꢁCHCl2 þ CHCl3 ! CH2Cl2 þ ꢁCCl3
Cl ꢁ þ CHCl3 ! HCl þ ꢁCCl3
ꢁCCl3 þ O2 ! CCl3OOꢁ
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
2 CCl3OOꢁ ! 2 CCl3Oꢁ þ O2
CCl3Oꢁ ! COCl2 þ Clꢁ
INTRODUCTION
The key points are the formation of trichloromethylperoxy
radicals (48–50), their self-termination (51) and the dissociation
of chlorine atoms from the resulting trichloromethoxy radicals
(52). The radical termination products observed in highest con-
centration during vapor phase decomposition are C2Cl6 and
C2HCl5, with lesser amounts of CH2Cl2 and C2H2Cl4 and no
detectable CCl4 (47). As Eqs. (3)–(6) constitute a radical chain,
a photocatalyst can effect photodecomposition by generating,
directly or indirectly, any of the radicals within the chain.
If unmodified MCM-41 were to catalyze the photodecomposi-
tion of chloroalkanes, this would imply that careful controls are
required when specific photocatalytic species are heterogenized
on MCM-41, to distinguish the relative contribution of the
adsorbed species from that of the mesoporous silica itself. In
fact, MCM-41 does catalyze the photodecomposition of chloro-
form, and this is reported herein.
Mesoporous silica has been widely used as a support for a vari-
ety of catalysts (1–4); catalytic activity most often being
achieved either by adsorbing a catalytic species on the silica sur-
face (5–8) or by incorporating one or more metal oxides during
the synthesis of the mesoporous silica (9–11). Mesoporous silica
has also been extensively used for photocatalytic reactions
(12,13), in some cases unmodified, except for activation at high
temperature (14–21), but quite often by supporting the actual
catalytic species (22–26) or by modifying the silica matrix
(27,28).
Much of the recent work in our laboratory has been on the
catalyzed decomposition of chloroalkanes (29–36); thus, we have
followed with special interest the published work on the photo-
decomposition of chloroalkanes and chloroarenes by modified
mesoporous silica (27,37–44). These studies have incorporated a
wide variety of modifications. To our knowledge, there are no
reports of photodegradation of chlorinated hydrocarbons cata-
lyzed by unmodified mesoporous silica.
MATERIALS AND METHODS
Photocatalysis by unmodified mesoporous silica has generally
been found to require wavelengths shorter than 370 nm (45),
which corresponds quite well with the diffuse reflectance spec-
trum of MCM-41, which can be represented as absorbing only
below approximately 370 nm (46).
Chemicals
Chloroform (J. T. Baker, ACS Grade) was washed five times with an
equal volume of water to remove the ethanol stabilizer, after which
4A molecular sieves were added to remove dissolved water. Water-sat-
urated chloroform was prepared by omitting the last step. MCM-41
mesoporous silica, with pore sizes between 2.3 and 2.7 nm, was
obtained from Sigma-Aldrich and was not activated at high tempera-
ture before use.
*Corresponding author email: phoggard@scu.edu (Patrick E. Hoggard)
© 2014 The American Society of Photobiology
760

Photochemistry and Photobiology, 2014, 90 761
one-to-one ratio, as would be expected from the stoichiometry of
Eqs. (3)–(6), and the development of products with time is
shown in Fig. 1. After irradiating for 50 min, the concentration
of both species had risen to approximately 0.1 M. The rate of
formation accelerated with time and the data in Fig. 1 were fit to
an exponential growth function.
Table 1 shows concentrations of all products detected by
GC-MS in a typical irradiation. The chloroalkyl termination
products found were CCl4 and C2Cl6, the latter in much smaller
amounts. None of the termination products expected from CHCl2
radicals was detected by GC-MS, i.e. C2H2Cl4, C2HCl5 and
CH2Cl2. This rules out radical formation by either C–Cl bond
homolysis in chloroform or the reduction in chloroform to yield
CHCl2 and Clꢀ.
Photochemical reactions
A 1.0 mL portion of chloroform was added to a weighed quantity of
MCM-41 in a fused silica spectrophotometer cuvette with a stirbar. The
cuvette was capped loosely, leaving approximately 3 mL of head space.
Light from an Oriel Q Series 100 W mercury lamp was passed through a
360-nm high-pass filter and focused on the sample. The irradiance was
0.45 mW cmꢀ2, as measured by an Oriel Goldilux UV meter. To vary
the O2 partial pressure in contact with the reaction mixture, balloons were
filled with predetermined amounts of air and nitrogen or air and oxygen
and attached to the cuvette. A fan was used to maintain the temperature
in the cuvette at 22 ꢂ 2°C.
Analytical procedures
A 500 lL portion of the final photolysate was taken, combined with
50 lL of a solution of o-xylene in ethanol as an internal standard and
subjected to gas chromatography/mass spectrometry. This was carried out
with a Shimadzu QP-5000 GC-MS with an Agilent DB624 column
(30 m, 0.32 mm 9 1.8 lm cyanopropylphenyl/dimethyl polysiloxane
film). The oven start temperature was 40°C and a linear temperature gra-
dient of 20°C minꢀ1 was applied to 240°C. Injection was carried out
with a 10:1 split ratio. All species were identified from their mass spec-
tra, and peak areas were used to determine concentrations. While CCl4
and C2Cl6 solutions with o-xylene were used to develop standards for
their determinations, phosgene concentrations were measured by the con-
densation of COCl2 with ethanol to make ethyl chloroformate. The etha-
nol in the 50 lL o-xylene solution was sufficient for complete
conversion; hence, ethyl chloroformate was used as the standard. In the
absence of ethanol, phosgene was readily identifiable in the gas chro-
matogram and its complete disappearance in favor of ethyl chloroformate
was readily verified. GC-MS analysis had to be undertaken within a few
hours, otherwise ethyl chloroformate began to condense further to build
diethyl carbonate.
HCl production was tracked by periodically removing aliquots of the
photolysate, diluting with CHCl3 and adding a portion to a 3.00 mL solu-
tion of tetraphenylporphyrin (H2TPP) in CHCl3. The absorbance at
446 nm was used to determine the amount of H4TPP2+ formed, by use
of the extinction coefficients for H2TPP and H4TPP2+ (53,54), taking
note that the intermediate H3TPP+ species can be neglected (55).
In addition, a chemical analysis for phosgene was conducted by tak-
ing aliquots from the photolysate during and following irradiation and
mixing them with 3.00 mL solutions of approximately 0.01 M Bu4NI in
CHCl3 from which ethanol had been removed. After 20 min, the absor-
bance at 365 nm was used to determine the concentration of I3ꢀ, arising
from the reaction (56)
Mechanistic hypothesis
Because carbon–chlorine bond fission occurs during the photocata-
lyzed decomposition of carbon tetrachloride in ethanol (57), one
might expect an analogous bond fission to occur when chloroform
is photocatalytically decomposed, yet this clearly does not occur
with MCM-41 as the catalyst because if it did, the termination
products from CHCl2 radicals would have been found (35).
Light absorption by silica, and any consequent photocatalytic
activity, arises from a number of point defect sites, the optical
properties of which have been extensively studied (58–63) and
characterized (64). Two of these surface defect sites can be
excited by radiation near 360 nm, one characterized as a non-
bridging oxygen hole center (NBOHC), with an absorption maxi-
mum at approximately 260 nm, the other a surface dioxasilyrane
(essentially a bidentate peroxo group on a single silicon), with a
weaker absorption maximum at approximately 310 nm (64). The
NBOHC may be represented as ≡Si–Oꢁ (≡ indicating three-single
bonds to oxygen atoms) (65), and the dioxasilyrane as = Si(O2)
(= indicating two-single bonds to oxygen atoms) (66). It should
be noted that Inaki et al. have proposed, on the basis of IR evi-
dence, that the NBOHC defect is actually a strained O–Si–O
COCl2 þ 2 Iꢀ ! COðgÞ þ I2 þ 2 Clꢀ
ð7Þ
ꢀ
and the subsequent formation of I3 from the excess of iodide ion. Iodide
also reduces Cl2, peroxides and other oxidizing species; thus, the true
concentration of COCl2 could be overestimated by this measure.
RESULTS AND DISCUSSION
Control experiments
Broadband irradiation of chloroform, in the absence of mesopor-
ous silica, at wavelengths above 360 nm produced no detectable
products within 1 h. Neither did stirring a suspension of MCM-
41 in chloroform for 2 h in the dark. Broadband irradiation
(k > 360 nm) of a suspension of ordinary silica gel in chloro-
form for 40 min produced no detectable decomposition products.
Photodecomposition products in MCM-41/CHCl3 suspensions
Figure 1. Growth of HCl (open circles) and COCl2 (filled squares) dur-
ing the irradiation (k > 360 nm) of 1.0 mL of chloroform in which
12 mg of MCM-41 mesoporous silica was suspended. Solid (HCl) and
dashed (COCl2) lines are least squares fits to the function y = a(ebt–1);
R2(HCl) = 0.97, R2(COCl2) = 0.94.
Under broadband (k > 360 nm) irradiation, chloroform under-
went photodecomposition in a suspension with MCM-41, yield-
ing primarily phosgene and hydrogen chloride in a nearly

~
762 Laura A. Pena et al.
Table 1. Product concentrations determined by GC-MS measurements
following a 20-min irradiation (k > 360 nm) of a suspension of 40 mg
of MCM-41 mesoporous silica in 1.0 mL of CHCl3.
CCl4, which can be made by the reaction of HCl with trichloro-
methanol, but not with the trichloromethoxy radical.
CCl3OH þ HCl ! CCl4 þ H2O
ð12Þ
Product
Concentration (mM)
The buildup of CCl4 in turn, may explain the acceleration in
the rate of chloroform photodecomposition. Carbon tetrachloride
may to some extent undergo dissociative adsorption on MCM-
41, as it does on TiO2 (70). The Cl and CCl3 radicals could then,
through the chain process shown in Eqs. (4) through (7), induce
further chloroform decomposition.
COCl2*
CCl4
C2Cl6
17
5.1
0.2
*After conversion to ethyl chloroformate.
linkage (45). Photoexcitation of the dioxasilyrane can be pre-
sumed to yield Si–Oꢁ units with homolysis of the weak O–O
bond, while the strained siloxane bridge is proposed to break
upon photoexcitation to yield adjacent Siꢁ and Si–Oꢁ sites (45).
Thus, whatever the nature of photoactive defect, it should present
as an electron-deficient Si–Oꢁ unit upon excitation.
Effect of oxygen
Assuming that CCl3 radicals are formed as in Eq. (8), with subse-
quent formation of CCl3OO radicals leading to degradation
products, and assuming a bimolecular termination step for trichlo-
romethyl radicals, the decomposition rate would be expected to be
directly proportional to the dioxygen concentration. This was
tested by varying the atmosphere above the reaction mixture from
100% N2 to 100% O2, the results from which are shown in Fig. 2.
The data are not consistent with direct proportionality, suggesting
that some degree of quenching by dioxygen takes place.
The data can be reasonably well fit by assuming that O2 is
required to form products, as in Eq. (4), but also quenches the
SiO2 defect excited state. Denoting the light-absorbing mesopor-
ous silica unit by C, we have for this simplified model
Given the oxygen-centered radical at the photoactive site,
abstraction of hydrogen should be favorable, i.e.
ꢃ Si ꢀ Oꢁꢄþ CHCl3 ! ꢃ Si ꢀ OH þ ꢁCCl3
ð8Þ
Reaction of the trichloromethyl radical with oxygen and the
self-reaction of the subsequent trichloromethylperoxy radicals, as
in Eqs. (4) and (5), yield trichloromethoxy radicals. In the gas
phase, the direct decay to phosgene and chlorine atoms, as in
Eq. (6), is the primary fate of CCl3Oꢁ radicals (52). In the con-
densed phase, however, hydrogen abstraction from CHCl3 may
be competitive, although leading to the same products, and a
fraction of the trichloromethoxy radicals may abstract hydrogen
from silanol groups, regenerating the ≡ Si–Oꢁ defects.
hm
C ꢀ Cꢄ
ð13Þ
k1
k2
Cꢄ þ O2 ꢀ! Products
ð14Þ
ð15Þ
k3
CCl3O ꢁ þ CHCl3 ! CCl3OH þ ꢁCCl3
CCl3O ꢁ þ ꢃ Si ꢀ OH ! CCl3OH þ ꢃ Si ꢀ Oꢁ
CCl3OH ! COCl2 þ HCl
ð9Þ
ð10Þ
ð11Þ
Cꢄ þ O2 ꢀ! C þ Oꢄ2
Expressing the rate of formation of the excited state as I0f/V,
where I0 is the incident light intensity, f the fraction of light
absorbed and V the sample volume, and assuming photostationary
The proposed sequence of steps is shown in Scheme 1 .
Scheme 1
The formation of trichloromethanol replaces a weak C–H
bond (393 kJ molꢀ1) (67) with a strong O–H bond (calculated to
be 465 kJ molꢀ1 (68)). It should be noted that the trichloro-
methylhydroperoxy radical, from which CCl3O is generated,
Eq. (5), would not itself be expected to abstract hydrogen from
chloroform because the O–H bond energy in CCl3OOH is only
370–380 kJ molꢀ1 (69).
Figure 2. Variation in COCl2 yield with the fraction of O2 in N2/O2
mixtures in contact with a suspension of 12 mg of MCM-41 mesoporous
silica in 1.0 mL of CHCl3 after 20 min of irradiation (k > 360 nm). Solid
line is a fit to Eq. (16), R2 = 0.94.
The establishment of a photostationary state concentration of
CCl3OH provides an answer to the puzzle of high yields of

Photochemistry and Photobiology, 2014, 90 763
state conditions for C*, the rate of product formation is
Product yield as a function of irradiation wavelength range
dP
dt
ðk2 ꢀ k3ÞI0f
a½O2ꢅ
The product yields were measured with different cutoff filters
(Table 2), showing a marked drop between cutoff wavelengths
of 360 and 375 nm, which can be ascribed to the steep decline
in absorptivity of the NBOHC defects with peak maxima at 260
and 310 nm that are implicated in the photochemical reactivity.
¼
½O2ꢅ ¼
ð16Þ
Vðk1 þ ðk2 ꢀ k3Þ½O2ꢅÞ
1 þ b½O2ꢅ
The data in Fig. 2 were fit to this function, yielding a satisfac-
tory result.
Decomposition rate in deuterochloroform
Rate as a function of catalyst load
Neither HCl nor phosgene was detected in the photolysate when
10 mg of MCM-41 was suspended in 1.0 mL of deuterochloro-
form and irradiated (k > 360 nm) for 20 min. This result is com-
patible with the proposed mechanism in that hydrogen
abstraction from chloroform at the photoactive site, Eq. (8),
requires an additional 25 kJ molꢀ1 in CDCl3 (73); thus, the
effect on the rate of homolysis can be very large.
The product yield varied greatly with the amount of mesoporous
silica employed, passing through a maximum, as can be seen in
Fig. 3. This behavior is expected from considerations of the light
absorbed by a weakly absorbing suspended material, the light
striking which may be transmitted through the suspension unab-
sorbed, reflected back from the front face, or scattered, with a
particular probability of eventual absorption (71,72). This analy-
sis predicts that the fraction of incident light absorbed by the
suspension, to which the photocatalytic product yield will be
proportional, can be expressed as
Decomposition rate in the presence of cyclohexane, ethanol
and water
f ¼ 1 ꢀ eꢀbm ꢀ cð1 ꢀ eꢀdm
Þ
ð17Þ
Following the addition of 5% cyclohexane or ethanol by volume
to chloroform, no decomposition was observed to take place after
irradiating a suspension (20 mg of MCM-41 in 1 mL of solu-
tion) for 20 min (k > 360 nm). Both cyclohexane and ethanol
would be expected to retard photodecomposition by radical scav-
enging. Reacting with the trichloromethyl radical, as in Eq. (18),
for example, would be quite effective in eliminating chain carri-
ers.
where m is the mass of catalyst, c represents the probability that
light scattered within the solution will be reflected rather than
absorbed, and b and d are phenomenological constants represent-
ing the relative probabilities of transmission and reflectance (71).
C6H12 þ ꢁCCl3 ! ꢁC6H11 þ CHCl3
ð18Þ
This would, however, generate termination products from the
cyclohexyl radical in solution, particularly bicyclohexyl, but no
such products were found by GC-MS. Likewise, none of the rad-
ical products from the 1-hydroxyethyl radical were found,
namely acetaldehyde, acetal (57) or 2,3-butanediol (74).
We conclude, therefore, that the cyclohexyl and 1-hydroxy-
ethyl radicals remain on the silica surface at other defect sites,
and that cyclohexane and ethanol retard photodecomposition
by intercepting the photoactive defect site, hindering the reac-
tion shown in Eq. (8). The binding of cyclohexyl radicals to
silica has been demonstrated (75), and there is ample experi-
mental evidence for the dissociative addition of ROH (76) and
RNH2 (45) species to silica surfaces, even in thermal reac-
tions.
The photodecomposition rate did not change when the chloro-
form was saturated with water, which can be attributed to the
much higher O–H bond dissociation energy in H2O.
Figure 3. COCl2 yield as a function of the mass of MCM-41 mesopor-
ous silica suspended in 1.0 mL of CHCl3 after 20-min irradiation
(k > 360 nm). Solid line is a fit to Eq. (17).
The solid line in Fig. 3 was fit to this function.
Effect of added ions
Chloroform was irradiated in suspensions of mesoporous silica
with several different tetrabutylammonium salts dissolved
(50 mg in 1.0 mL CHCl3). Bromide, chloride, perchlorate and
hexafluorophosphate salts essentially stopped the photodecompo-
sition. No degradation products were detected. When photode-
composition is initiated through the photodissociation of a
chlorine atom (57,71), chloride ion has been shown to acceler-
ate considerably the catalyzed photodecomposition of chlori-
nated hydrocarbons, stabilizing the radical as Cl2ˉ and retarding
Table 2. Product yield (in lmol) as a function of bandpass filter cutoff
wavelength. Exptl conditions: 10 mg MCM-41 suspended in 1.0 mL
CHCl3, irradiated 30 min, 100-W Hg lamp.
Cutoff wavelength
COCl2
CCl4
C2Cl6
345 nm
360 nm
375 nm
90.1
89.5
4.6
2.7
3.7
0
0.1
0.15
0

~
764 Laura A. Pena et al.
4. Fajula, F. and D. Brunel (2001) Unique surface and catalytic proper-
ties of mesoporous aluminosilicates. Microporous Mesoporous
Mater. 48, 119–125.
recombination within the solvent cage (77–80). Thus, these
experiments serve as additional evidence against a photodissocia-
tion mechanism.
5. Ahn, W. S., G. J. Kim and G. Seo (2004) Catalysis involving mesopor-
ous molecular sieves. Series on Chemical Engineering 4, 649–693.
6. Crosman, A., C. Schuster, H.-H. Wagner, M. Batorfi, J. Cubillos and
W. Hoelderich (2007) Immobilization of transition metal complexes
and their application to enantioselective catalysis. In Asymmetric
Synthesis with Chemical and Biological Methods, (Edited by D.
Enders and K.-E. Jaeger), pp. 277–297. Wiley-VCH, Weinheim,
Germany.
The completely opposite behavior exhibited in this system
cannot reasonably be ascribed to reductive quenching of the sil-
ica defect excited state, which is observed with some SiO2 do-
pants (81), as PF6ꢀ, in particular, should not be oxidizable in
these circumstances. This may simply be an electrostatic effect,
that is, the attraction of the anions (as part of an ion pair when
in chloroform) to the positively charged defects, inhibiting access
by chloroform.
7. Decyk, P. (2006) States of transition metal ions in modified meso-
porous MCM-41 and in microporous ZSM-5 studied by ESR spec-
troscopy. Catal. Today 114, 142–153.
8. Woo, S. I., H. Y. Cho, L. Cui, N. H. Tarte and D. S. Hong (2004)
Technology of advanced immobilized single site catalysts. Current
Trends in Polymer Science 9, 99–107.
CONCLUSIONS
9. Ziolek, M., I. Nowak, I. Sobczak, A. Lewandowska, P. Decyk and J.
Kujawa (2000) Physico-chemical and catalytic properties of MCM-
41 mesoporous molecular sieves containing transition metals (Cu,
Ni, and Nb). Stud. Surf. Sci. Catal. 129, 813–822.
Unmodified MCM-41 mesoporous silica acts as an effective cata-
lyst for the photodecomposition of chloroform under near-UV
light. The most likely mechanism for its photocatalytic activity is
through hydrogen abstraction by excited state defect sites from
chloroform and a radical chain initiated by the resulting trichlo-
romethyl radicals ending in phosgene and hydrogen chloride. In
the presence of water, which does not retard the reaction, phos-
gene can be converted into carbon dioxide, so this has the poten-
tial to serve as an environmentally friendly remediation process
for chlorinated hydrocarbons, using sunlight and generating
relatively benign final products.
An obvious question from the results obtained is why ordinary
silica gel showed no photocatalytic activity while mesoporous sil-
ica was even more active than other heterogeneous catalysts for
chlorocarbon photodegradation that we have studied (82,83). It is
unlikely that photophysical differences are responsible for this, as
the two types of silica surface should have similar photophysical
properties. We suggest that the photochemical reactivity in the
presence of mesoporous silica results from the small size of the
pores, ca. 2.5 nm (84), so that they effectively act as reaction
chambers, serving to retain CCl3O radicals near the silica surface
to permit the deprotonation of silanol functionalities (Eq. 10) and
the reestablishment of NBOHC sites. The faster rate of hydrogen
abstraction from Si–OH groups by CCl3O compared with the rate
of decomposition to COCl2 and Cl, which is the primary reaction
channel for COCl3 in the gas-phase photodecomposition of chlo-
roform (52), would then be responsible for the retention of cata-
lytic activity by the mesoporous silica surface.
€
10. Schuth, F., A. Wingen and J. Sauer (2001) Oxide loaded ordered
mesoporous oxides for catalytic applications. Microporous Mesopor-
ous Mater. 44–45, 465–476.
11. Chen, Y.-W. and Y.-H. Lu (1999) Characteristics of V-MCM-41 and
its catalytic properties in the oxidation of benzene. Ind. Eng. Chem.
Res. 38, 1893–1903.
12. Maldotti, A., A. Molinari and R. Amadelli (2002) Photocatalysis
with organized systems for the oxofunctionalization of hydrocarbons
by O2. Chem. Rev. 102, 3811–3836.
13. Maldotti, A. and A. Molinari (2011) Design of heterogeneous photo-
catalysts based on metal oxides to control the selectivity of chemical
reactions. Top. Curr. Chem. 303, 185–216.
14. Badr, Y., M. G. Abd El-Wahed and M. A. Mahmoud (2008) Photo-
catalytic degradation of methyl red dye by silica nanoparticles.
J. Hazard. Mater. 154, 245–253.
15. Itoh, A., T. Kodama, Y. Masaki and S. Inagaki (2006) Oxidative
photo-decarboxylation in the presence of mesoporous silicas. Chem.
Pharm. Bull. 54, 1571–1575.
16. Karunakaran, C., R. Dhanalakshmi, G. Manikandan and P. Gomathi-
sankar (2011) Photodegradation of carboxylic acids on Al2O3 and
SiO2 nanoparticles. Indian J. Chem., Sect A 50A, 163–170.
17. Tada, N., Y. Matsusaki, T. Miura and A. Itoh (2011) Oxidative
photodecarboxylation of a-hydroxycarboxylic acid derivatives with
FSM-16 under visible light irradiation of fluorescent lamp. Chem.
Pharm. Bull. 59, 906–908.
18. Yuliati, L., M. Tsubota, A. Satsuma, H. Itoh and H. Yoshida (2006)
Photoactive sites on pure silica materials for nonoxidative direct
methane coupling. J. Catal. 238, 214–220.
19. Ogata, A., A. Kazukawa and M. Enyo (1986) Photoactivation of sil-
ica gel with UV light during the reaction of carbon monoxide with
oxygen. J. Phys. Chem. 90, 5201–5205.
20. Yoshida, H., T. Tanaka, M. Yamamoto, T. Yoshida, T. Funabiki and
S. Yoshida (1997) Epoxidation of propene by gaseous oxygen over
silica and Mg-loaded silica under photoirradiation. J. Catal. 171,
351–357.
21. Palmisano, G., E. Garcia-Lopez, G. Marci, V. Loddo, S. Yurdakal,
V. Augugliaro and L. Palmisano (2010) Advances in selective con-
versions by heterogeneous photocatalysis. Chemical Communications
(Cambridge) 46, 7074–7089.
Acknowledgement—This work was supported in part by the National
Science Foundation through Grant CHE-074968. Acknowledgement is
also made to the donors of the American Chemical Society Petroleum
Research Fund for support of this research through grant 49334-UR3.
22. Mori, K., K. Watanabe, M. Kawashima, M. Che and H. Yamashita
(2011) Anchoring of Pt(II) pyridyl complex to mesoporous silica
materials: Enhanced photoluminescence emission at room tempera-
ture and photooxidation activity using molecular oxygen. J. Phys.
Chem. C 115, 1044–1050.
23. Xu, Y. and C. H. Langford (1997) Photoactivity of titanium dioxide
supported on MCM41, Zeolite X, and Zeolite Y. J. Phys. Chem. B
101, 3115–3121.
REFERENCES
1. Koch, H., A. Liepold, K. Roos, M. Stocker and W. Reschetilowski
(1999) Comparative study of the acidic and catalytic properties of
the mesoporous material H-MCM-41 and zeolite H-Y. Chem. Eng.
Technol. 22, 807–811.
2. Lin, H.-P., Y.-R. Cheng, C.-R. Lin, F.-Y. Li, C.-L. Chen, S.-T.
Wong, S. Cheng, S.-B. Liu, B.-Z. Wan, C.-Y. Mou, C.-Y. Tang and
C.-Y. Lin (1999) The synthesis and application of the mesoporous
molecular sieves MCM-41 - a review. Journal of the Chinese Chemi-
cal Society (Taipei) 46, 495–507.
24. Molinari, A., G. Varani, E. Polo, S. Vaccari and A. Maldotti (2007)
4ꢀ
Photocatalytic and catalytic activity of heterogenized W10O32 in
the bromide-assisted bromination of arenes and alkenes in the pres-
ence of oxygen. J. Mol. Catal. A: Chem. 262, 156–163.
3. Van Bekkum, H. and K. R. Kloetstra (1998) New organic chemical
conversions over MCM-41-type materials. Stud. Surf. Sci. Catal.
117, 171–182.
25. Molinari, A., A. Bratovcic, G. Magnacca and A. Maldotti (2010)
Matrix effects on the photocatalytic oxidation of alcohols by

Photochemistry and Photobiology, 2014, 90 765
[nBu4N]4W10O32 incorporated into sol-gel silica. Dalton Trans. 39,
7826–7833.
48. Cooper, R., J. B. Cumming, S. Gordon and W. A. Mulac (1980)
The reactions of the halomethyl radicals CCl3 and CF3 with oxygen.
Radiat. Phys. Chem. 16, 169–74.
49. Hautecloque, S. (1980) On the photooxidation of gaseous trichlorom-
ethane and chlorosyl radical formation. J. Photochem. 14, 157–65.
50. Mosseri, S., Z. B. Alfassi and P. Neta (1987) Absolute rate constants
for hydrogen abstraction from hydrocarbons by the trichloromethyl-
peroxy radical. Int. J. Chem. Kinet. 19, 309–17.
51. Catoire, V., R. Lesclaux, W. F. Schneider and T. J. Wallington
(1996) Kinetics and mechanisms of the self-reactions of CCl3O2 and
CHCl2O2 radicals and their reactions with HO2. J. Phys. Chem. 100,
14356–14371.
52. Lesclaux, R., A. M. Dognon and F. Caralp (1987) Photooxidation of
halomethanes at low temperature: The decomposition rate of CCl3O
and CFCl2O radicals. J. Photochem. Photobiol. A 41, 1–11.
53. Du, H., R.-C. A. Fuh, J. Li, L. A. Corkan and J. S. Lindsey (1998)
Photochem CAD: A computer-aided design and research tool in pho-
tochemistry. Photochem. Photobiol. 68, 141–142.
26. Molinari, A., A. Maldotti, A. Bratovcic and G. Magnacca (2011) Fe
(III)-porphyrin heterogenized on MCM-41: Matrix effects on the
oxidation of 1,4-pentanediol. Catal. Today 161, 64–69.
27. Davydov, L., E. P. Reddy, P. France and P. G. Smirniotis (2001)
Transition-metal-substituted titania-loaded MCM-41 as photocatalysts
for the degradation of aqueous organics in visible light. J. Catal.
203, 157–167.
28. Liu, S.-H. and H. P. Wang (2002) Photocatalytic generation of
hydrogen on Zr-MCM-41. Int. J. Hydrogen Energy 27, 859–862.
29. Doyle, K. J., H. Tran, M. Baldoni-Olivencia, M. Karabulut and P. E.
Hoggard (2008) Photocatalytic Degradation of Dichloromethane by
Chlorocuprate(II) Ions. Inorg. Chem. 47, 7029–7034.
~
30. Seidl, A. J., L. R. Cohen, L. A. Pena and P. E. Hoggard (2008)
Chlorochromate ion as a catalyst for the photodegradation of chloro-
form by visible light. Photochem. Photobiol. Sci. 7, 1373–1377.
~
31. Cohen, L. R., L. A. Pena, A. J. Seidl, K. N. Chau, B. C. Keck, P. L.
Feng and P. E. Hoggard (2009) Photocatalytic degradation of chloro-
form by bis (bipyridine)dichlororuthenium(III/II). J. Coord. Chem.
62, 1743–1753.
54. Lindsey, J. S., PhotochemCAD, spectra recorded by Junzhong Li
and Richard W. WagnerAvailable at: http://omlc.ogi.edu/spectra/Pho-
tochemCAD/html/index.html. Accessed on 1 April 2013.
55. Stone, A. and E. B. Fleischer (1968) The molecular and crystal
structure of porphyrin diacids. J. Am. Chem. Soc. 90, 2735–48.
56. Olsen, J. C., G. E. Ferguson, V. J. Sabetta and L. Scheflan (1931)
Quantitative determination of phosgene. Ind. Eng. Chem. 3, 189–
191.
57. Hoggard, P. E. and A. Maldotti (2010) Catalysis of the photodecom-
position of carbon tetrachloride in ethanol by an amberlite anion
exchange resin. J. Catal. 275, 243–249.
58. Weeks, R. A. (1994) The many varieties of E’ centers: A review.
J. Non-Cryst. Solids 179, 1–9.
~
32. Cohen, L. R., L. A. Pena, A. J. Seidl, J. M. Olsen, J. Wekselbaum
and P. E. Hoggard (2009) The photocatalytic decomposition of chlo-
roform by tetrachloroaurate(III). Monatsh. Chem. 40, 1159–1165.
~
33. Pena, L. A., A. J. Seidl, L. R. Cohen and P. E. Hoggard (2009) Fer-
rocene/ferrocenium ion as a catalyst for the photodecomposition of
chloroform. Transition Met. Chem. (Dordrecht, Neth.), 34, 135–141.
34. Gilbert, R. M., M. Karabulut and P. E. Hoggard (2010) Photocataly-
sis of chloroform degradation by l-dichlorotetrachloropalladate(II).
Inorg. Chim. Acta 363, 1462–1468.
~
35. Pena, L. A. and P. E. Hoggard (2010) Photocatalysis of chloroform
decomposition by hexachloroosmate(IV). Photochem. Photobiol. 86,
467–470.
59. Nishikawa, H., E. Watanabe, D. Ito and Y. Ohki (1994) Decay
kinetics of the 4.4-eV photoluminescence associated with the two
states of oxygen-deficient-type defect in amorphous SiO2. Phys. Rev.
Lett. 72, 2101–2104.
60. Skuja, L. (1994) Direct singlet-to-triplet optical absorption and lumi-
nescence excitation band of the twofold-coordinated silicon center in
oxygen-deficient glassy SiO2. J. Non-Cryst. Solids 167, 229–238.
61. Nelson, C. M. and R. A. Weeks (1960) Trapped electrons in irradi-
ated quartz and silica: I, Optical Absorption. J. Am. Ceram. Soc. 43,
396–399.
62. Friebele, E. J., D. L. Griscom, M. Stapelbroek and R. A. Weeks
(1979) Fundamental defect centers in glass: The peroxy radical in
irradiated, high-purity, fused silica. Phys. Rev. Lett. 42, 1346–1349.
63. Stapelbroek, M., D. L. Griscom, E. J. Friebele and G. H. Jr Sigal
(1991) Oxygen-associated trapped-hole centers in high-purity fused
silicas. J. Non-Cryst. Solids 32, 313–326.
64. Skuja, L. (1998) Optically active oxygen-deficiency-related centers
in amorphous silicon dioxide. J. Non-Cryst. Solids 239, 16–48.
65. Skuja, L., K. Tanimura and N. Itoh (1996) Correlation between the
radiation-induced intrinsic 4.8 eV optical absorption and 1.9 eV pho-
toluminescence bands in glassy SiO2. J. Appl. Phys. 80, 3518–3525.
66. Raghavachan, K. and G. Pacchioni (2001) Photoabsorption of dioxsi-
lyrane and silanone groups at the surface of silica. J. Chem. Phys.
114, 4657–4662.
~
36. Pena, L. A. and P. E. Hoggard (2010) Hexachlororhodate(III) and
the photocatalytic decomposition of chloroform. J. Mol. Catal. A:
Chem. 327, 20–24.
37. Chien, Y.-C., H. P. Wang, S.-H. Liu, T. L. Hsiung, H.-S. Tai and
C.-Y. Peng (2008) Photocatalytic decomposition of CCl4 on Zr-
MCM-41. J. Hazard. Mater. 151, 461–464.
38. Zanjanchi, M. A., A. Ebrahimian and M. Arvand (2010) Sulphonated
cobalt phthalocyanine-MCM-41: An active photocatalyst for degrada-
tion of 2,4-dichlorophenol. J. Hazard. Mater. 175, 992–1000.
39. Rodrigues, S., K. T. Ranjit, S. Uma, I. N. Martyanov and K. J.
Klabunde (2005) Visible-light photooxidation of trichloroethylene by
Cr-Al-MCM-41. J. Catal. 230, 158–165.
40. Orlov, A., Q. Z. Zhai and J. Klinowski (2006) Photocatalytic proper-
ties of the SBA-15 mesoporous silica molecular sieve modified with
titanium. J. Mater. Sci. 41, 2187–2193.
41. Qiao, W.-T., G.-W. Zhou, X.-T. Zhang and T.-D. Li (2009) Prepara-
tion and photocatalytic activity of highly ordered mesoporous TiO2-
SBA-15. Mater. Sci. Eng., C 29, 1498–1502.
42. Sun, B., E. P. Reddy and P. G. Smirniotis (2006) TiO2-loaded Cr-
modified molecular sieves for 4-chlorophenol photodegradation
under visible light. J. Catal. 237, 314–321.
43. Yang, Y.-H., N. Ren, Y.-H. Zhang and Y. Tang (2009) Nanosized
cadmium sulfide in polyelectrolyte protected mesoporous sphere:
A stable and regeneratable photocatalyst for visible-light-induced
removal of organic pollutants. J. Photochem. Photobiol., A 201,
111–120.
67. Hudgens, J. W., R. D. III Johnson, R. S. Timonen, J. A. Seetula and
D. Gutman (1991) Kinetics of the reaction trichloromethyl + bromine
and thermochemistry of trichloromethyl radical and cation. J. Phys.
Chem. 95, 4400–5.
44. Cao, S., K. L. Yeung and P.-L. Yue (2007) An investigation of tri-
chloroethylene photocatalytic oxidation on mesoporous titania-silica
aerogel catalysts. Appl. Catal. B 76, 64–72.
45. Inaki, Y., H. Yoshida, T. Yoshida and T. Hattori (2002) Active sites
on mesoporous and amorphous silica materials and their photocata-
lytic activity: An investigation by FTIR, ESR, VUV-UV and
68. Brudnik, K., J. T. Jodkowski, A. Nowek and J. Leszczynski (2007)
Kinetics of the formation reactions of trichloro- and tribromomethyl
hypohalites and alcohols in the gas-phase: Theoretical study. Chem.
Phys. Lett. 435, 194–200.
69. Kondo, O. and S. W. Benson (1984) Kinetics and equilibria in the
system atomic bromine + methyl hydroperoxide (MeOOH) = hydro-
gen bromide + methylperoxy radical (MeOO). An upper limit for the
heat of formation of the methylperoxy radical. J. Phys. Chem. 88,
6675–80.
photoluminescence spectroscopies. J. Phys. Chem.
9098–9106.
B
106,
46. Zanjanchi, M. A. and S. Asgari (2004) Incorporation of aluminum
into the framework of mesoporous MCM-41: The contribution of
diffuse reflectance spectroscopy. Solid State Ionics 171, 277–282.
47. Dickey, L. C. and R. F. Firestone (1970) Radiolysis of chloroform
vapor. Effects of phase on the Arrhenius parameters of the hydro-
gen-atom abstraction reaction of dichloromethyl radicals with chloro-
form. J. Phys. Chem. 74, 4310–4313.
70. Lin, Y.-S., M.-T. Chen, Y.-F. Lin, S.-J. Yang and J.-L. Lin (2006)
Investigation of chemical decomposition of CCl4 on TiO2 near room
temperature. Appl. Surf. Sci. 252, 5892–5899.
~
71. Hoggard, P. E., L. R. Cohen, L. A. Pena, B. M. Harvey and A. M.
Chan (2013) The dependence of photocatalytic reaction yields on
catalyst mass in solid-liquid suspensions. Current Catalysis 2, 2–6.

~
766 Laura A. Pena et al.
72. Loddo, V., M. Addamo, V. Augugliaro, L. Palmisano, M. Schiavello
and E. Garrone (2006) Optical properties and quantum yield determi-
nation in photocatalytic suspensions. AlChE J. 52, 2565–2574.
73. Tschuikow-Roux, E. and S. Paddison (1987) Bond dissociation ener-
gies and radical heats of formation in chloromethane, dichlorome-
thane, bromomethane, dibromomethane, chlorofluoromethane, and
dichlorofluoromethane. Int. J. Chem. Kinet. 19, 15–24.
74. Zhang, X., J. Wu and Y. Zhou (1994) The reaction of a-hydroxyeth-
yl radical in aqueous solution and ethyl alcohol. Radiat. Phys. Chem.
43, 335–8.
75. Nasuto, R. (2000) The influence of the concentration of cyclohexane
radicals bonded with an Si100 silica gel surface upon the retention
of some hydrocarbons in gas chromatography. Adsorpt. Sci. Technol.
18, 43–53.
76. Sindorf, D. W. and G. E. Maciel (1981) 29 Si CP/MAS NMR stud-
ies of methylchlorosilane reactions on silica gel. J. Am. Chem. Soc.
103, 4263–4265.
77. Glebov, E. M., V. F. Plyusnin, V. P. Grivin, Y. V. Ivanov, N. V.
Tkachenko and H. Lemmetyinen (1998) Mechanism of Br 2- and
Cl2- radical anions formation upon IrCl62- photoreduction in metha-
nol solutions containing free Br- and Cl- ions. J. Photochem. Photo-
biol., A 113, 103–112.
79. Nagarajan, V. and R. W. Fessenden (1985) Flash-photolysis of tran-
sient radicals. 1. Cl2-, Br 2-, I2-, and SCN2-. J. Phys. Chem. 89,
2330–2335.
80. Yu, X. Y. (2004) Critical evaluation of rate constants and equilib-
rium constants of hydrogen peroxide photolysis in acidic aqueous
solutions containing chloride ions. J. Phys. Chem. Ref. Data 33,
747–763.
81. Hiller, D., M. Jivanescu, A. Stesmans and M. Zacharias (2010) P0
centers at the Si-nanocrystal/SiO2 interface as the dominant
photoluminescence quenching defect. J. Appl. Phys. 107, 84309–
84310.
82. Chan, A. M., B. M. Harvey and P. E. Hoggard (2013) Photodecom-
position of dichloromethane catalyzed by tetrachloroferrate(III) sup-
ported on a Dowex anion exchange resin. Photochem. Photobiol.
Sci. 12, 1680–1687.
~
83. Pena, L. A., A. M. Chan, A. S. Cohen, K. Hou, B. M. Harvey and
P. E. Hoggard (2013) Catalysis of the photodecomposition of chloro-
form by a polystyrene anion exchange resin. Current Catalysis 2,
79–87.
84. Sigma-Aldrich, Mesostructured silica, MCM-41 type (hexago-
nal).Available
at:
http://www.sigmaaldrich.com/catalog/product/
aldrich/643645?lang=en&region=US. Accessed on 11 Sept 2013.
78. Michalski, R., A. Sikora, J. Adamus and A. Marcinek (2010) Diha-
lide and pseudohalide radical anions as oxidizing agents in nonaqu-
eous solvents. J. Phys. Chem. A 114, 861–866.