$str = 'KMnO4 + HCl = KCl + MnCl2 + Cl2 + H2O';
$list = explode('+', explode('=', str_replace(' ', '', $str))[1]);
print_r($list);
Array
(
[0] => KCl
[1] => MnCl2
[2] => Cl2
[3] => H2O
)SELECT `type`
FROM `table1`
WHERE `date` > :date
UNION SELECT `type`
FROM `table2`
WHERE `date` > :date
...